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Molecular design of pyridine and quinoline based compounds for organic optoelectronics

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Molecular design of pyridine and quinoline based compounds for organic optoelectronics

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Content Copyright 2021 Abegail Cardenas Tadle
Molecular Design of Pyridine and Quinoline Based Compounds for Organic Optoelectronics
by
Abegail Cardenas Tadle
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the degree
DOCTOR OF PHILOSOPHY
(Chemistry)
December 2021
ii
1. Dedication
To my family, this one is for all your sacrifices and hard work.
iii

2. Acknowledgements
I would like to thank my adviser, Dr. Mark E. Thompson. I have had a great experience
through my PhD training and a lot of it is due to the support you have given me in my scientific
and personal growth. I have made great friendships and collaborations in graduate school and a
lot of this is due to the welcoming and encouraging environment that you’ve maintain in your
lab. You have set a great example, and I am grateful I had the opportunity to work with you.
I would like to thank all the professors who took the time to be a part of my screening,
qualification and thesis committee. You all have had helped me through my scientific journey
and have been a guiding hand through all this. It is a pleasure to be surrounded by well-respected
and successful scientists. Dr. Sri Narayan, Dr. Richard Brutchey, Dr. Andrea Armani, Dr.
Smaranda Marinescu, Dr. Ralph Haiges and Dr. Brent Melot, thank you.
Thank you to all my previous and current MET lab mates who I had the opportunity to work
with. It has made graduate school an experience to remember. I have gained great friendships I
will treasure. I have learned from every single one of you and you all have contributed to my
positive experience in grad school. I look forward to traveling all over the world to visit every
single one of you at some point! I want to especially thank those who helped me with some of
the work presented in this book; Dr. Karim El Roz, Daniel Sylvinson, Moon Chul Jung, and
Konstantin Mallon. I appreciate the time spent to help me move forward with my project.
To our collaborators at University of Michigan, Dr. Stephen Forrest and Chan Ho Soh, I am
grateful I had the opportunity to work with you. Your work complemented my project well and
gave it a complete story which we can share to the scientific community. Thank you for your
efforts! My collaborators at USC who were mentioned in different chapters of this book, thank
iv

you for all your help! You all are great colleagues and I have a lot of respect for every single one
of you.
Michele Dea, Magnolia Benitez and Alan Kershaw have helped me navigate through
departmental and/or instrumental responsibilities. You have made these obligations a lot easier
with your assistance. Thank you for having my best interest in mind. Dr. Peter Djurovich has
been a great resource to have in lab and is always willing to share his knowledge to everyone. I
appreciate your excitement for science and the efforts to guide MET lab members throughout our
scientific journey. Thank you for your guidance!
Judy Fong, I don’t know what we would all do without you in the lab! You have been a
great friend and I am so glad I had a chance to work with you while I was at USC. You have
extended such kindness and a friendship that I treasure. I am truly grateful for all the time and
effort you’ve shared with me.
To all my USC friends, you all know who you are! Thanks to all the great memories and
sharing all the up and downs of grad school together. It has made this experience much more
exciting especially with such creative minds. You all are sincere human beings and I look
forward to our future hang outs together.
To my family, this has been a journey. I am so thankful for all the love and support you
have given me throughout all these years. Dr. Guia Tadle and Conrado Tadle Jr – you both have
set a great example. Your work ethic and drive has led me down this path and I have so much
love and respect for both of you, mom and dad. Angelica, Aaron and Adrian, you all are my
younger siblings, but you’ve always challenged me to be the best version of myself. You all are
v

always there to support me, and I couldn’t have asked for better best friends. I am so happy I can
share this big accomplishment in my life with you all.

vi

3. Table of Contents
Dedication ................................................................................................................................. ii
Acknowledgements .................................................................................................................. iii
List of Tables .............................................................................................................................x
List of Figures......................................................................................................................... xii
List of Schemes .......................................................................................................................xvi
Abstract ............................................................................................................................. xvii
Chapter 1: Introduction ............................................................................................................1
1.1 Organic Light Emitting Diode (OLED) ..................................................................3
1.1.2 Dopant Emission Mechanisms ....................................................................................6
1.1.2.1 Simple Jablonksi Diagram ....................................................................................6
1.1.2.1 Phosphorescence ..................................................................................................7
1.1.2.2 Fluorescence ...................................................................................................... 10
1.1.3 Optimize Internal Quantum Efficiency of Fluorescent Molecules in WOLEDs ......... 13
1.1.4 Designing Molecules with small Singlet-Triplet Energy Gap (E ST) ......................... 15
1.2 Organic Photovoltaics ......................................................................................................... 15
1.2.1 Methods to Achieve a Bathochromic Shift in Organic Molecules .............................. 19
1.3 Chapter 1 References ........................................................................................................... 22
Chapter 2: Developing Blue Fluorescent Materials with Small Singlet Triplet Energy Gap ..
................................................................................................................................ 26
2.1 Introduction ......................................................................................................................... 26
2.2 Synthesis of azaDIPYR compounds .................................................................................... 29
2.3 Electrochemistry of azaDIPYR compounds ......................................................................... 30
2.4 Photophysical Characterization Observed for azaDIPYR Family ........................................ 34
2.5 TDDFT Calculations ........................................................................................................... 39
2.6 Thermogravimetric Analysis of azaDIPYR, -azaDIPYR, DIPYR and -DIPYR .............. 42
2.7 Monochromatic Blue OLED using -aD ............................................................................ 44
2.8 WOLED Device Using -azaDIPYR (-aD) as blue dopant ............................................... 51
2.9 Lifetime tests for the blue OLEDs ....................................................................................... 57
2.10 aD Expansion: Destabilizing the HOMO energy and Addition of Steric Bulk on the
azaDIPYR core ......................................................................................................................... 59
2.11 aD Expansion: Destabilizing the T2 Energy of aD to minimize intersystem crossing ......... 66
2.12 Conclusion ....................................................................................................................... 71
2.13 Experimental .................................................................................................................... 73
vii

2.13.1 Synthesis ................................................................................................................ 73
2.13.2 Electrochemical Measurements ............................................................................... 73
2.13.3 Photophysical Measurements .................................................................................. 73
2.13.4 Molecular Modeling ............................................................................................... 74
2.13.5 Photophysical Characterization ............................................................................... 75
2.13.6 OLED Devices ........................................................................................................ 76
2.13.7 Thermogravimetric Analysis ................................................................................... 76
2.14 Chapter 2 References ................................................................................................. 77
Chapter 3: Developing Host Materials with Triplet Energies Lower than azaDIPYR ........ 88
3.1 Introduction ......................................................................................................................... 88
3.2 H2P as a Host Material ........................................................................................................ 93
3.2.1 Photophysical Properties of 3,8 H2P ......................................................................... 93
3.2.2 Monochromatic Device for 3,8 H2P .......................................................................... 96
3.3 Fluorene-based Host Material .............................................................................................. 98
3.3.1 Quenching Studies of host with -aD ....................................................................... 99
3.3.2 TDDFT of Fluorenone-based material .................................................................... 100
3.3.3 Photophysical Properties of MFL and MFLO in Solution and Film ......................... 103
3.4 Quinoline-Based Host Material ......................................................................................... 107
3.4.1 Photophysical Properties of 2BQ and 4BQ .............................................................. 108
3.5 Napthalene-Bridged Quinoline and Isoquinoline Compounds as Host Materials ................ 111
3.5.1 Synthesis of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ. ............................................................ 112
3.5.2 Electrochemistry of 1,4Q, 1,4 IQ, 1,5 Q and 1,5 IQ. ................................................ 113
3.5.3 Photophysical Properties of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ...................................... 121
3.5.4 TGA and DSC of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ. .................................................... 127
3.5.5 Single Crystal XRD and Thin Film XRD ........................................................... 130
3.5.6 Electroluminescence of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ. ...................................... 134
3.5.7 TDDFT and MD Simulations: Addition of bridging naphthalene to produce 1,4Q, 1,4
IQ, 1,5Q and 1,5 IQ ......................................................................................................... 136
3.5.8 Expansion: Addition of Steric Bulk in the Napthalene-Based Systems .................... 141
3.6 Conclusion ........................................................................................................................ 141
3.7 Experimental ..................................................................................................................... 144
3.7.1 Synthesis ................................................................................................................ 144
3.7.2 Nuclear Magnetic Resonance .................................................................................. 148
viii

3.7.3 Matrix Assisted Laser Desorption Ionization (MALDI) Data .................................. 155
3.7.4 Photophysical Characterization ............................................................................... 158
3.7.5 Device Fabrication .................................................................................................. 159
3.7.6 Thermogravimetric Analysis and Differential Scanning Calorimetry....................... 159
3.7.7 Electrochemistry ..................................................................................................... 160
3.7.8 Molecular Modeling ............................................................................................... 160
3.8 References ......................................................................................................................... 163
Chapter 4: Structure-Property Relationship, Non-Radiative Decay Pathways and
Bathochromic Shift of DIPYR to Near-Infrared ................................................................. 168
4.1 Introduction ....................................................................................................................... 168
4.2. Structure Property Relationship: Photophysical Properties of Modified DIPYRs .............. 168
4.3 Non-Radiative Pathways and Design Principles of DIPYR for Application in Optoelectronics
............................................................................................................................................... 173
4.3.1 Photophysical Properties of DIPYR-Ph and DIPYR-Tol ......................................... 176
4.3.2 TDDFT for DIPYR-Ph and DIPYR-Tol .................................................................. 179
4.4 Increased Resonance in DIPYR Molecules Similar to Fluoresceine or Rhodamine ............ 181
4.5 Experimental ..................................................................................................................... 186
4.5.1 Photophysical Characterization ............................................................................... 186
4.6 Chapter 4 References ......................................................................................................... 187
Chapter 5: (aza)DIPYR Salts and their pH Dependence..................................................... 189
5.1 Introduction ....................................................................................................................... 189
5.2 Synthesis of Cationic and Rigid DIPYRs and azaDIPYRs ................................................. 190
5.3 Photophysical Properties of (aza)DIPYR salts ................................................................... 191
5.4 TDDFT of CARDIPYRs ................................................................................................... 196
5.5 Neutral CARDIPYR .......................................................................................................... 199
5.6 Crystal Structure................................................................................................................ 202
5.7 Conclusion ........................................................................................................................ 204
5.8 Experimental ..................................................................................................................... 205
5.7.1 Molecular Modeling ............................................................................................... 205
5.7.2 Electrochemistry ..................................................................................................... 205
5.7.3 Photophysical Characterization ............................................................................... 205
5.9 Chapter 5 References ......................................................................................................... 207
Appendix A ............................................................................................................................ 209
6.1 Synthesis ........................................................................................................................... 210
ix

6.2 Characterization ................................................................................................................ 212
6.3 New Isomer for hybrid structure ........................................................................................ 216
6.4 Appendix A References ..................................................................................................... 217
x

4. List of Tables
Table 1.1. Photophysical Properties of DIPYR systems ............................................................. 21
Table 2.1. Electrochemical potentials of 1a-1c, 2a-2d, 3 and 4. ................................................. 31
Table 2.2. HOMO and LUMO energies (in eV) for 1a-1b, 2a-2d, 3 and 4. ................................. 32
Table 2.3. Molecular orbital representation of compounds 1a-1c, 2a-2d, 3 and 4 at the
B3LYP/6-311G** level ............................................................................................................. 33
Table 2.4. Summary of the photophysical parameters for 1a-1b, 2a-2d, 3 and 4. ........................ 37
Table 2.5. Calculated and Experimental S1 and T1 energies for 1a-1b, 2a-2d, 3 and 4, where the
difference between the two energies is represented by E(S1-T1). ............................................. 39
Table 2.6. Calculated excited state energies (in eV) at S0 and S1 optimized geometries.
(B3LYP/6-311G**) for 2a-2d, 3 and 4. ..................................................................................... 42
Table 2.7. Extent of overlap calculation using TDDFT with experimental comparison. ............. 42
Table 2.8. Properties of OLEDs doped with 1 wt % 2a into CBP and 26DCzPPy hosts. ............ 49
Table 2.9. Calculation and Experimental Results for the S1, T1 and E ST ................................. 62
Table 2.10. Photophysical properties of 1a,1b and 1c. ............................................................... 62
Table 2.11. DFT (B3LYP/6-31G**) calculated properties for -aD derivatives (DIPYR CORE
STRUCTURE). Compounds with each substituent type (OMe, ipr and Mes) are arranged by
HOMO energy. ......................................................................................................................... 64
Table 2.12. DFT (B3LYP/6-31G**) calculated properties of prospective candidates based on the
aD core-structure. ...................................................................................................................... 67
Table 2.13. Calculation Results for AD1 and AD2 derivatives .................................................. 70
Table 2.14. TDDFT Results which include S1, T1 and T2 valuers of Push-Pull Compounds for
T2 destabilization ...................................................................................................................... 71
Table 3.1 3,8-H2P-tBu and -aD HOMO, LUMO, Redox, and Triplet Energies. ...................... 94
Table 3.2. Photoluminescence efficiencies and lifetime for neat film and spincoated or vapor
deposited doped films of H2P.................................................................................................... 95
Table 3.3. Solution Photophysics of MFL................................................................................ 104
Table 3.4. Photophysical properties of MFLO at room-temperature and at 77K. ...................... 104
Table 3.5. Photophysical properties of 1% α-aD in PMMA as a polymer matrix and MFLO as a
host. ........................................................................................................................................ 105
Table 3.6. S1 and T1 Energies in eV and HOMO/LUMO values of 2BQ, 3BQ and 4BQ from
B3LYP/631G** ...................................................................................................................... 108
Table 3.7. Lifetime and Photoluminescence Quantum Yield for 2BQ Film Studies ................. 110
Table 3.8. HOMO and LUMO energies (in eV) for 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ ................ 114
Table 3.9. Summary of the photophysical parameters for 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ. ..... 122
Table 3.10. Spincoated Films .................................................................................................. 126
Table 3.11. Experimental and TDDFT S1 and T1 Energies ....................................................... 127
Table 3.12. XRR thickness and surface roughness. .................................................................. 131
Table 3.13. Calculated Reorganization Energy ........................................................................ 140
Table 3.14 TDDFT Results for substituted naphthalene bridged biquinoline systems .............. 142
Table 4.1. Photophysical properties of DIPYR with benzannulated or donor substituents. ....... 169
xi

Table 4.2. Photophysical Properties of BrD and MeOD in methylcyclohexane and
methyltetrahydrofuran. ............................................................................................................ 170
Table 4.3. Photophysical Properties of BODIPY: Addition of Phenyl and Mesityl in the
5-position and Effect on Non-Radiative Pathway. ................................................................... 174
Table 4.4. Photophysical properties of DIPYR, DIPYR-Ph and DIPYR-Tol ............................ 178
Table 5.1. Photophysical properties of aCARD, aCOD and COD. ........................................... 196
Table 5.2 Photophysical Properties of CARDIPYR family ...................................................... 196
Table 5.3. ROKS calculation on CARDIPYRs ........................................................................ 198
Table 5.4. HOMO and LUMO orbital contributions of CARDIPYR salts ................................ 198
Table 5.5. Oxidation/Reduction Potential of aCARD, aCOD and COD. Data taken relative to
ferrocene in acetonitrile ........................................................................................................... 199
Table 5.6. Photophysical Properties of aCOOH compared to -aCARD .................................. 201
Table 5.7. Photophysical Properties of aCOOH in different pH ............................................... 202

xii

5. List of Figures
Figure 1.1. OLED structure (A) OLED Emission perpendicular to the device surface (B) OLED
Energy Diagram. ........................................................................................................................4
Figure 1.2. Jablonski Diagram .....................................................................................................7
Figure 1.3. Simplified Jablonski for Phosphorescence Emission ..................................................8
Figure 1.4. Simplified Jablonski diagram of (a)conventional fluorescence and (b) TTF. ............ 10
Figure 1.5. Simplified Jablonski of TADF. ................................................................................ 13
Figure 1.6. General Proposed Energy Transfer Mechanisms in the hybrid
fluorescent/phosphorescent WOLED. ........................................................................................ 15
Figure 1.7. Contributions of solar spectrum ............................................................................... 18
Figure 1.8. Examples of organic small molecules which have benzannulation or donor-acceptor
motifs ........................................................................................................................................ 20
Figure 1.9. Length of resonance decreases the E in conjugated alkenes. .................................. 20
Figure 1.10. Extension of π-system in BODIPY molecules results in a bathochromic shift
relative to the non-extended core. .............................................................................................. 20
Figure 2.1. Atomic transmutation of meso-position in DIPYR from methene-bridge to
imine-bridge. ............................................................................................................................. 28
Figure 2.2. Structures of aza-boron-dipyridylmethene (aD). ...................................................... 29
Figure 2.3. General synthetic scheme to make substituted aza-boron-dipyridylmethene
derivatives. Precursors can be pyridyl, quinolyl or isoquinolyl. ................................................ 29
Figure 2.4. Cyclic Voltammetry for compounds in Acetonitrile vs. ferrocene (100mV/s) .......... 31
Figure 2.5. Normalized emission spectra at room temperature and gated phosphorescence
emission recorded after 500 µs delay time at 77 K. . ................................................................ 35
Figure 2.6. Normalized Spectra of absorption, fluorescence at 298K, and phosphorescence
emission at 77K in 2-MeTHF for 2a, 2b, 2c, 2d, 3 and 4. ........................................................... 36
Figure 2.7. Thermogravimetric analysis curves for (A) DIPYR, (B) azaDIPYR, (C) -
azaDIPYR, (D) -DIPYR ......................................................................................................... 44
Figure 2.8. Normalized absorbance and emission of 2a in three different solvents;
2-methyltetrahydrofuran (2-MeTHF), toluene and methylcyclohexane (Mch). .......................... 45
Figure 2.9. DPEPO Host Doped with 2a.. .................................................................................. 46
Figure 2.10. 1 wt. % and 10 wt. % 2a spin-coated films doped in NPD show exciplex formation.
................................................................................................................................................. 47
Figure 2.11. PL emission of neat 2a, 1% and 5% 2a dopant in CBP and 26DCzPPY host
materials. .................................................................................................................................. 47
Figure 2.12. (A) Device architecture and energy levels of an OLED with CBP host and 2a
dopant. (B) Electroluminescence spectra with increasing current using CBP and 26DCzPPy
hosts. ......................................................................................................................................... 48
Figure 2.13. OLED data for 1% 2a doped film with CBP as host.. ............................................. 50
Figure 2.14. OLED data for 1% 2a doped film with 26DCzPPY as host.. .................................. 51
Figure 2.15. Hybrid WOLED with -aD fl-dopant and CBP host. ............................................. 52
Figure 2.16. Emission spectrum of sensing devices using BCzVBi and 2awith sensors implanted
in similar positions. ................................................................................................................... 53
xiii

Figure 2.17. Triplet sensing with an -aD fluorescent dopant in CBP, using experiment scheme
of Figure 2.15. ........................................................................................................................... 54
Figure 2.18. Device Performance of -aD in Imidazole-based Host .......................................... 55
Figure 2.19. Device structure and characteristics of Imidazole host (I5) based hybrid WOLED,
Structure 1................................................................................................................................. 56
Figure 2.20. Device structure and characteristics of Imidazole host (I5) based hybrid WOLED,
Structure 2................................................................................................................................. 56
Figure 2.21. Lifetime Studies of Blue OLED with Alq3 ETL .................................................... 58
Figure 2.22. Spectrum of -aD Blue OLED device with Alq3 ETL ........................................... 58
Figure 2.23. Synthetic Route to obtain boron substituted derivatives of aD ................................ 61
Figure 2.24. Normalized Spectra of absorption, fluorescence at 298K, and phosphorescence
emission at 77K in 2-MeTHF for 1a, 1b and 1c ......................................................................... 61
Figure 2.25. Liquid chromatography-mass spectrometry trace and UV-visible spectrum for
currently synthesized compounds intended for mesityl derivatization. ....................................... 65
Figure 2.26. Target Compounds with T2 energy higher than the S1 energy based on TDDFT. .... 67
Figure 2.27. NTO of electron and hole pair using B3LYP/631G** of non-benzannulated aD
core. .......................................................................................................................................... 69
Figure 2.28. Combination of Donor-Acceptor Substitution (push-pull) which further destabilize
the T2 Energy ............................................................................................................................ 70
Figure 3.1. Simple representation of the energetic alignment target for WOLED devices. ......... 90
Figure 3.2. Gated Phosphorescence PL of -aD and CBP in 2-methyltetrahydrofuran. .............. 91
Figure 3.3. Blue monochromatic device of -aD with CBP host. .............................................. 92
Figure 3.4. Photoluminescence Emission of neat 3,8-H2P-tBu, 1% -aD and 5% Ir(ppy)3 doped
spincoated films. ....................................................................................................................... 94
Figure 3.5. PL Emission of -aD doped in CBP and 3,8 H2P (spin coated and vapor deposited)
doped films. .............................................................................................................................. 95
Figure 3.6. Blue Monochromatic OLED Device Data for -aD doped 3,8 H2P. ........................ 97
Figure 3.7. Absorbance and emission spectra of concentration dependent doping of fluorenone in
-aD solution. ......................................................................................................................... 100
Figure 3.8. HOMO and LUMO Orbital Contributions for Fluorene and 9-Fluorenone
(631G**/B3LYP). ................................................................................................................... 102
Figure 3.9. HOMO and LUMO Orbital Contributions for MFL and MFLO (631G**/B3LYP).
............................................................................................................................................... 102
Figure 3.10. Photophysical properties of 2,7-dimesitylfluorene (MFL) in
2-methyltetrahydrofuram. ........................................................................................................ 103
Figure 3.11. Absorbance and Emission of MFLO in solution and powder. ............................... 104
Figure 3.12. Film studies using 2,7-dimesitylfluorenone (MFLO) as host material for 1% α-aD
doped film. PMMA was used as a reference for the experiment. .............................................. 106
Figure 3.13. PL emission of -aD doped in SAS. ................................................................... 107
Figure 3.14. Molecular Orbitals of 2BQ, 3BQ and 4BQ from B3LYP/631G** using QChem.. 108
Figure 3.15. Photophysical properties of 4,4’biquinoline and 2,2’biquinoline. ......................... 109
xiv

Figure 3.16. Film Studies using 2BQ as a host with 1% doping of -aD, 5% doping of Ir(ppy)3
and 5% doping of PQIr.. .......................................................................................................... 110
Figure 3.17. Structures Synthesized as Host Material for aD compounds ................................. 113
Figure 3.18. Cyclic Voltammetry of 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ in Acetonitrile and
Dichloromethane referenced with Ferrocene. .......................................................................... 114
Figure 3.19. HOMO and LUMO orbitals of 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ compared to
ternapthalene derivatives. ........................................................................................................ 120
Figure 3.20. Absorbance and PL Emission in Solution and Solid of 1,4 Q, 1,4 IQ, 1,5 Q, and 1,5
IQ. .......................................................................................................................................... 123
Figure 3.21. Film Studies: Neat Film and -aD Doped Films .................................................. 125
Figure 3.22. Spin coated films of 1,5 Q with -aD and -5OD ................................................ 126
Figure 3.23. Differentials Scanning Calorimetry (DSC) for 1,4Q, 14, IQ, 1,5 Q, and 1,5 IQ. ... 128
Figure 3.24. Thermogravimetric Analysis of 1,4Q, 1,4IQ, 1,5 Q, and 1,5 IQ. .......................... 129
Figure 3.25. Measured melting point of the non-aza substituted counterparts of 1,4 IQ and 1,5
IQ. .......................................................................................................................................... 130
Figure 3.26. Thin Film XRD Measurements for 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ. Compounds
were referenced to CBP and a blank quartz. ............................................................................ 131
Figure 3.27. SXRD for 1,4 Q and 1,5 Q. .................................................................................. 132
Figure 3.28. SXRD Planes for 1,4 Q and 1,5 Q. Images shows how the molecules pack. ......... 133
Figure 3.29. 1,4 Q measured angle between naphthalene and quinoline plane is 54.78 while 1,5
Q is measured to be 42.18. ..................................................................................................... 133
Figure 3.30. Monochromatic devices made for 1% blue dopant in 1,5 Q host and 10% red dopant
in 1,5Q. ................................................................................................................................... 135
Figure 3.31. Triplet Spin Density obtained using Geometry Optimization of multiplicity =3 with
B3LYP/631G*** .................................................................................................................... 137
Figure 3.32. Dipole moments associated with the host materials using TDDFT. ...................... 139
Figure 3.33. Host materials analyzed for efficiency in WOLEDs. (a) N6BL. (b) N6BO. (c) 2-
Biquinoline-6-Binaphthalene. (d) 3-Biquinoline-6-Binaphthalene. (e) 4,4′-Bis(N-carbazolyl)-
1,1′-biphenyl (CBP). ............................................................................................................... 141
Figure 4.1. DIPYR derivatives which include a mixture of methoxy donor compounds and
benzannulated systems. ........................................................................................................... 169
Figure 4.2. Absorbance and Emission Spectra of BrD, MeOD and -3,6-MeO DIPYR in
methylcyclohexane and methyltetrahydrofuran........................................................................ 172
Figure 4.3. Absorption and emission spectra in methylcyclohexane of -Bt ............................ 172
Figure 4.4. Illustrates the orbital delocalization based on the orientation of the phenyl ring and
the effect in the dihedral angles in phenyl and mesityl derivatives. .......................................... 175
Figure 4.5. Synthetic route
1
used to produce targeted molecules (blue) is similar to those
followed to make DIPYR compounds. .................................................................................... 176
Figure 4.6. Absorbance and Emission Spectra of DIPYR, DIPYR-Ph and DIPYR-Tol. ........... 177
Figure 4.7. Solvent dependence of DIPYR-Ph with increasing polarity using methylcyclohexane,
dichloromethane, and acetonitrile. ........................................................................................... 177
Figure 4.8. LUMO orbital contribution of BODIPY, DIPYR, DIPYR-Ph and DIPYR-Tol. ..... 180
Figure 4.9. Change in fluorescence quantum yield when varying the charge on fluorescein. .... 182
xv

Figure 4.10. Fluoresceine derivatives with high fluorescence quantum yield. .......................... 183
Figure 4.11. Rhodamine and Rhodamine B Structures with rhodamine inspired DIPYR .......... 185
Figure 4.12. Ligand desired to use for rhodamine-modified DIPYR ........................................ 186
Figure 4.13. Phosphorous based atoms mimicking rhodamine. ................................................ 186
Figure 5.1. Carbon chelated and carbonyl chelated compounds synthesized for photophysics
studies. The molecules highlighted in blue have been previously synthesized in our lab. ......... 190
Figure 5.2. Previously synthesized compounds in our lab which will be compared to the
molecules presented in this chapter. Data were taken in 2-MeTHF .......................................... 192
Figure 5.3 -aCARD absorbance and emission properties in 2:1 Ethanol to methanol mixture
and 2-MeTHF. 2-MeTHF spectra of -aD is included for comparison. .................................... 193
Figure 5.4. aCARD absorbance and emission properties in 2:1 Ethanol to methanol mixture and
2-MeTHF. 2-MeTHF spectra of aD is included for comparison. .............................................. 194
Figure 5.5. aCOD and COD absorbance and emission spectra in 2-MeTHF............................. 195
Figure 5.6. -aCARD CarbAcid (aCOOH) compared to -aCARD absorbance and emission
spectra in 2:1 ethanol to methanol mixture. ............................................................................. 200
Figure 5.7. Display of pH dependence of aCOOH in PBS buffer. ............................................ 202
Figure 5.8. Single Crystal XRD for aCARD ............................................................................ 203
Figure 5.9 aCARD incorporated in lead iodide XRD- (aCARD)3Pb2I8
.
2I ................................ 203
Figure 5.10. Diffuse Reflectance of azaCARD and azaCARD in lead iodide. .......................... 204
Figure 6.1 MDA and DPy1 compounds as organic components in hybrid systems .................. 209
Figure 6.2. Nuclear Magnetic Resonance of DPy1 (
1
HNMR) .................................................. 210
Figure 6.3. Figure 4. Liquid Chromatography- Mass Spectrometry (LCMS) for DPy1. The data
suggests that the mass for DPy1 was found at m/z=199.1 ........................................................ 211
Figure 6.4. Diffuse Reflectance of hybrid material. DPy1 only absorbs from two to three electron
volts (eV), however, the hybrid material is able to absorb beyond 3 eV. Also, the peak shape at
2.5 eV is different in both spectra. ........................................................................................... 212
Figure 6.5. Powder X-Ray diffraction (PXRD).. ...................................................................... 213
Figure 6.6. Absorbance spectra of DPy1. ................................................................................. 214
Figure 6.7. Absorbance of DPy1 in water. Addition of acid decreases and leads to doubly
protonated DPy1 which is colorless. ........................................................................................ 214
Figure 6.8 Hybrid Structure with DPy1 and Lead Iodide ......................................................... 215
Figure 6.9. Synthesis of new isomer to compare with DPy1 and MDA. ................................... 216

xvi

6. List of Schemes
Scheme 2.1. Synthesis of two ligands for borylation to produce aD derivatives where the T 2
energy is destabilized relative to the S1 energy .......................................................................... 67
Scheme 3.1. General Synthetic Scheme to produce 1,4Q, 1,4IQ, 1,5Q, and 1,5IQ ................... 113
Scheme 4.1. Synthetic routes for fluoresceine inspired target molecule. .................................. 184
Scheme 5.1. General Synthesis of Cationic DIPYRs and azaDIPYRs ...................................... 191
Scheme 6.1. Synthesis of DPy1. .............................................................................................. 210

xvii

7. Abstract
Exploration of the stability and synthetic ease of modification in the structure of
dipyridyl-methane (DIPYR) compounds were the focus in Chapter 2, 4 and 5. DIPYR systems
are particularly interesting because of their orbital pattern where nodes bisect every other carbon
atom in the HOMO and LUMO orbitals, leading to intense and narrow absorption and emission
spectra with very small Stokes’ shift. The modification around the core structure provided a
range of materials which can be used for both organic light emitting diodes and organic
photovoltaics.
Chapter 2 focuses on the aza substitution in the meso position of the compound and the
effects of donating groups around the periphery of the material. A series of substituted boron aza-
dipyridylmethene (aD) compounds were explored for their photophysical and electrochemical
properties as fluorescent emitters for blue OLEDs. Previous work in our lab have demonstrated
the synthetic ease of dipyridylmethene monomers
4
and their interesting photophysical properties
as homoleptic meso-linked dimers
4a
. Using Density Functional Theory (DFT) modeling, we
combine our theoretical and experimental efforts to synthesize a library of aD compounds by
replacing the meso carbon with nitrogen to destabilize the HOMO energy. The synthesized aD
derivatives emit blue light with max between 400-460 nm and internal quantum efficiencies above
85 %. A small singlet and triplet energies were deduced from fluorescence and phosphorescence
emission, respectively, which shows a ΔE ST ≤ 0.4 eV. The EQEmax = 4.5 % for the fabricated
monochromatic device reaches close to the maximum theoretical limit of fluorescent (fl) OLED
of 5 %. Rigorous device studies suggests that triplet exciton trapping is observed and a new set of
host materials with stabilized triplet energies is necessary to further optimize these materials in
White OLEDs.
xviii

Chapter 3 explores aromatic core structures which were used to study with aD as host
material for White OLEDs. The focus deviates from the understanding of the structure-property
relationship of DIPYRs but it explores its utility in optoelectronics. A wide range of material were
predicted to have triplet energies below of the aD family using TDDFT; fluorenone, spirofluorene,
H2P, biquinolines and naphthalene-bridged biquinolines. All materials were synthesized and
characterized to have triplet energies ranging between 2.4-2.6 eV with gated phosphorescence
measurements at 77K in 2-methyltetrahydrofuran. The best material found to be most ideal for aD
are the naphthalene-bridged-biquinolines (1,4 Q, 1,5 Q, 1,4 IQ and 1,5 IQ). The material was
observed to be more conductive than other commonly used hosts in OLEDs such as mCBP and
CBP. Additionally, these have high sublimation temperatures (>300C) and melting points
(>200). The napthelene bridged systems does not contain any heteroatom bond which can
minimize bond cleavage observed in OLED devices due to high energy photons generated in the
emission layer. The physical, electrochemical and photophysical properties of these materials were
studied in completion. Preliminary blue monochromatic device produced an EQE max ~ 1.5% but
further optimization is needed to reach a more competitive EQE.
Chapter 4 and 5 illustrates the potential of DIPYR dyes as NIR absorber in Organic
Photovoltaic. The synthesis of cyanine-like dyes, systems such as boron dipyrromethene
(BODIPY), is commonly used for application in organic photovoltaic. However, synthetic
modification is not easily achieved due to polymerization and porphyrin restrictions to prevent
non-radiative decay pathways. The parent DIPYR molecule has shown to be a straightforward
synthesis with desirable and tunable photophysical properties, leading to libraries of interesting
fluorophores. Benzannulation of DIPYR has shown improvement in minimizing non-radiative
decay pathways by eliminating the symmetry allowed transition through intersystem crossing to
xix

the low-lying T2 state in DIPYR but these materials are not a well-studied system. Substitution in
the meso-position of DIPYR with tolyl and phenyl suggests that the “free rotor” movement of
these aromatic systems does not affect the non-radiative decay rate (~2 ns) which is observed in
BODIPY. The PLQY of DIPYR and meso-substituted DIPYR remain unchanged (=0.18).
Addition of substituents with non-bonding electrons such as halides or methoxy resulted in a low
PLQY due to a heavy atom effect and n- transition which increases the allowedness of
intersystem crossing in these systems. The structure-property relationship learned in these systems
provides knowledge of how to further design these systems for optoelectronics.

1

1. Chapter 1: Introduction
Renewable energy refers to naturally occurring resources such as biofuels, wind,
geothermal and solar energy. The use of this energy is practically infinite and is only limited by
energy available at a given time. For example, wind speeds are weather dependent, and the light
intensity of the sun is dependent on the time of day. However, these resources are attractive due
to the unlimited energy that can be harvested especially when used parallel with multiple
renewable energy resources. Use of the latter can help minimize the global demand for fossil
fuels which has long-term environmental effects including CO2 and other greenhouse gas
emissions.
In the United States, use of fossil fuels which include coal, petroleum and natural gas
have been the main sources of energy, even today. In 2020, Petroleum (35%), natural gas (34%)
nuclear power (9%), and coal (10%) supply more than 75% of energy sources.
1
Hydropower and
wood were used as the main renewable energy resources until the 1990s but expansion to solar,
hydrothermal, geothermal, wind and biomass has increased energy resources today to 12%.
However, this is still a small fraction of the overall energy resources. There is a large interest in
the research community to find alternative energies to fossil fuels and meet future global energy
needs. Solar energy is one pathway towards utility of renewable energy where photovoltaics
technology allows to harvest the sun’s rays and turn it into energy. The latter is the model system
used as an inspiration for the molecular designs proposed here to harvest most of the sun’s
energy, especially in the near infrared region.
Furthermore, the growing energy consumption due to the rise of technological
advancement can also be decreased by developing materials which require less energy to
function for an extended period. Display technologies has been incorporated in a widespread of
2

products including but not limited to cellphones, televisions, computers, jumbotrons, and other
display materials that take part of everyday energy consumption. Additionally, artificial lighting
for illumination (ex. streetlamps) or for signals (ex. street signs) also contribute to the overall
energy consumption and costs. In the United States, artificial lighting account for ~8% energy or
22% of electricity and are only expected to increase by 3 to 4 fold.
2
The high energy
consumption in lighting is attributed to use of incandescent light bulb and fluorescent lighting
since most of the power input in these materials are wasted as heat, at least ~ 90% in
incandescent light bulbs. The push for solid state lighting (SSL) began in the early 2000s and is
still of interest to make lighting energy efficient. Inorganic light emitting diodes have helped
replace energetically wasteful lighting sources such as mercury lamps, incandescent light and
fluorescent lighting in homes, cars, streets, and other lighting sources. However, LEDs still
suffer from inhomogeneities in light output, color and operating voltages which require binning
that leads to increased costs of the technology. Organic Light Emitting Diodes (OLEDs) display
help eliminate the need for backlight and color filters typically found in liquid crystal display
(LCDs) and LEDs, minimizing energy, material, and manufacturing costs. In particular, white
OLEDs (WOLEDs) are attractive for white lighting because it can be flexible and have diffuse
illumination current LEDs are unable to achieve. WOLEDs have great potential to surpass the
energy savings in current LEDs and be more versatile for lighting placements. OLEDs offer an
alternative technology to current LEDs that can further develop the efficiency of LEDs at high
luminance, advance the efficiency and lifetime of emitter materials, and improve light extraction
efficiency and optical control. The use of OLEDs and WOLEDs are attractive for solid state
lighting and were an inspiration in the fundamental questions explored here.
3

1.1 Organic Light Emitting Diode (OLED)
A simple organic light emitting diodes with proven high efficiency consist of the
emission layer, the hole transport layer, the electron transport layer, the anode and the
cathode layer, shown in Figure 1.1. Often times, to move charges more effectively, electron
and hole blocking layers are added to prevent any charge leakage in a device which leads to
low efficiencies and charge imbalance. The active layers are stacked in between electrodes to
allow charge carrier injection from an external bias which proceeds in three main processes
for electroluminescence: charge injection, transport, and recombination. The charges are
injected to the electrodes (anode and cathode) which are transported to the active layer (also
known as the emissive layer) and the holes and electrons recombine there to emit light.
Charge carriers move perpendicular to the electrodes and the active area which has contact
with the electrode is the only place were light emission is observed.
2
The advantage of
OLEDs relative to its inorganic counterpart is its low charge carrier mobility since these
materials can be made thin (~100 nm) and low-voltage devices can have high electric fields
(10
8
V/m).
2
However, this same reason is what leads to a large contribution of generated
photons to be trapped in a device due to the refractive index mismatch in OLED films and
only 20-30% are extracted from the emitted light.
3

4

Figure 1.1. OLED structure (A) OLED Emission perpendicular to the device surface (B) OLED
Energy Diagram. Adapted from literature
4

1.1.1 Emission Layer (EML) in OLEDs
The emission layer is an essential part of the device as this consists of the host matrix and
the dopant, where electrical energy leads to the production of light. Neat fluorescent emitters (or
aggregated induced emitters) with high photoluminescence quantum yield can be used as the
only component in the emissive layer but these materials are not commonly used as they are
limited by low internal quantum efficiencies (IQE) and not easily designed.
5
It is difficult to
predict if the designed molecule will have a high fluorescence quantum yield in the film. The
emissive layer is believed to be the site that contributes largely to the device efficiency of
OLEDs. The emitter (dopant) is typically diluted in a host matrix to minimize aggregation and
reduce bimolecular quenching processes.
6
Chapter 2 and Chapter 3 is focused on the
development of the host and dopant materials which are intended for organic light emitting diode
application.
In order to have uniformity and have direct comparison between different stacked or colored
OLED devices, performance parameters are well established. Typically, current efficiency
5

(cd/A) and the luminous efficacy (lumens/watt) of devices are reported and both provide an
understanding of how well a light source produces light. The drawback of directly comparing
current efficiency and luminous efficacy between devices is that it accounts for the human eye
response function which makes comparison between different regions of the electroluminescence
(EL) spectra challenging. The more useful parameters to compare different devices in varying
EL spectral region is the external quantum efficiency (EQE or ext) value which is representative
of the ratio of extracted photons over injected charges.
4, 7
The EQE is the product of internal
quantum efficiency (int) and optical extraction efficiency (out or opt), where int is dependent
on the competition between the radiative and nonradiative rates of the emitting state of the
emitter (effective radiative quantum efficiency or photoluminescence quantum yield), the
excitons that decay radiatively due to quantum-mechanics spin selection rules (spin factor or
fraction of radiative excitons), and an optimized OLED device (electrical electricity or electron-
hole recombination).
3-4
The optical extraction efficiency, also known as the outcoupling
efficiency, refers to the optical environment and includes the orientation of the transition dipole
moment and the molecular orientation of the organic molecule to the surface of the device.
Studies for optimization of this parameter are explored in literature and in our laboratory to
understand in a molecular level how organic molecules can be designed to optimize the EQE.
Additionally, increasing light extraction using device engineering methods is also used.
8

Although some of the materials presented in Chapter 2 could have an optimized molecular
orientation in a device, this topic was not explored in the shared chapters and will not be directly
addressed here.
IQE is a parameter that is predominantly dependent on the photophysical properties of
the emitting material. Understanding radiative and non-radiative rates, photoluminescence
6

quantum yields, structure-property relationships and the emissive state of an organic molecule
can help optimize the IQE when the material is incorporated in an OLED device. Molecules can
have different inherent emissive properties which can significantly increase or decrease the
device EQE. Some device engineering can be conducted to optimize the IQE
8-9
but the approach
taken in Chapter 2 and 3 takes on both the material and device optimization inspired from work
produced in our lab in 2006.
10
However, in order to understand the different known mechanisms
and utilize ~100% of the excitons to radiatively decay, the different emission pathways will be
explained; conventional fluorescence, triplet-triplet fusion, phosphorescence, and thermally
activated delayed fluorescence (TADF). It is worth noting there are also many other types of
emitters such as hybridized local and charge transfer emitters, doublet emitters and sensitized
fluorescent emitters but the three that will be discussed are some of the main explored emitters
for OLEDs.
1.1.2 Dopant Emission Mechanisms
1.1.2.1 Simple Jablonksi Diagram
A simplified Jablonski diagram is shown in Figure 1.2 where the energy absorption from
(S0→S1) is shown in black and the blue transitions refer to the radiative pathways such as
fluorescence (S1→S0) and phosphorescence (T1→T0). The dotted lines refer to the non-radiative
pathways which are typically expelled as heat. In this system, the radiative pathways produce
light emission and can be categorized as fluorophores if it emits from the singlet excited state to
the ground state and a phosphor if it emits from the triplet state to the ground state. In the lab,
there are ways to measure the emission efficiency of these lumiphores and this provides an
understanding about the ground state and excited state nature if paired with other photophysical
parameters and optical properties. One of the measurements which quantifies how effective the
7

absorbed photons are emitted as photons is the photoluminescence quantum yield (PLQY or )
of a material. A ratio of 1 for the photons emitted to the photons absorbed suggests that the
material used 100% of the excitons and emitted as light. In order to obtain the radiative decay
rate (kr) of the material, the value can be extrapolated from the  and the lifetime () values
which are shown in Equation 1 and 2.

Figure 1.2. Jablonski Diagram

Equation 1:Φ=
𝑘 𝑟 𝑘 𝑟 +𝑘 𝑛𝑟

Equation 2: τ=
1
𝑘 𝑟 +𝑘 𝑛 𝑟

1.1.2.1 Phosphorescence
Phosphorescent emitters showed great promise for OLEDs in the late 90’s with use of
heavy atoms with organic ligands, such as Os(II) and Pt(II).
11
Luminescence from the T1 state is
8

attributed to
3
MLCT1 admixed with
1
MLCT2 and
3
LC, and a mixture of other states through spin
orbit coupling
12
. The addition of heavy metals such as Ir(III), Pt(II), Os(II), Re(I), Ru(II)
promotes intersystem crossing from a formally forbidden process with some allowedness
(S1→T1). The T1→S0 transition becomes an efficient process which leads to room temperature
phosphorescence.
13
In this scenario, the 25% singlet excitons through ISC, combines with the
75% triplet excitons and leads to radiative decay of 100% of the generated excitons typically in
the 1-5 s lifetime (Figure 1.3) The combination of excitons in the process of phosphorescence
leads to 100% IQE, with ~20% EQE in a standard flat device.

Figure 1.3. Simplified Jablonski for Phosphorescence Emission. Adapted from literature
14

Green and red phosphorescent OLEDs are now used in the market and the luminance
decay to 95% of its initial luminance (T95) are longer than 10,000 hours.
14
. However, blue
OLEDs can have poor color gamut with poor emission efficiency and long excited state lifetimes
which requires further improvements. The current challenge in phosphorescent OLEDs is finding
a stable blue phosphor since these can generate high energy excitons and polarons that can lead
to bond breaking mechanisms in the molecule, leading to unstable and low efficiency devices. It
is also important that 100% of the excitons are utilized in a radiative process which can be
9

measured, and these can be quantified through the photoluminescence quantum yield (PLQY) of
the material. One of the drawbacks, especially in the beginning stages of blue phosphor
development, is that the emissive state is close to the metal-centered dd* state and is prone to
quenching by the repulsive dd state through the potential energy surface (PES) with respect to
the ground state. The latter leads to a decrease in the emission efficiency and photostability of
these materials.
15
A way to circumvent this is by the addition of stronger ligand field and metal-
to-ligand charge transfer character that increases the coupling of orbital angular momentum to
the electron spin that the T1→S0 transition and lead to a large first order spin-orbit coupling term.
As a result, a faster radiative lifetime and higher PLQY can be achieved which is ideal for
OLEDs.
15-16
However, one of the larger challenges in blue phosphors is the large energy gap in
these materials. A good balance between the coordinating ligands around the heavy atom is
required such that the dd excited states does not lead to radiationless decay or bond dissociation
in the weakest metal-ligand site.
17
The lowest lying excited state (ILCT and MLCT) mixing with
a thermally accessible ligand to ligand charge transfer (LLCT) transitions must be minimized
since these can lead to potential energy surface (PES) intersection or thermal population of dd
excited state resulting in a poor PLQY. The long-lived excited state of these blue lumiphores
compared to fluorescent materials still require further research to prevent some of the triplet-
triplet annihilation and triplet-polaron annihilation mechanism found in these high energy
materials. Phosphorescent OLEDs have a proven record especially this has made its way to
everyday technology but the “blue problem” in phosphors is still a large research problem that is
yet to be solved. In the meantime, blue fluorescent materials are utilized and paired with green
and red phosphorescent emitters for application in OLED technology.
10

1.1.2.2 Fluorescence
Spin statistics suggest that upon electrogeneration of a fluorescent material, 25% of
singlet and 75% of triplet excitons are formed, shown in Figure 1.4. In a conventional
fluorescence material, the radiative transition from S1-S0 occurs in a nanosecond time scale and
is usually an efficient transition. The S 1→T1 transition is commonly a spin forbidden process for
organic dyes and a large singlet and triplet excited state energies are typically observed for these
materials (>0.3 eV). The relaxation of triplet excitons (T1→S0) is an inefficient process due to
the long-excited state lifetimes (>5 s) which leads to the non-radiative decay losses in
fluorescence. The fast excited state lifetimes with high photoluminescence quantum yield are
found to be the most effective emitters for OLED but its limited internal quantum efficiency
(25%) leads to a maximum theoretical external quantum efficiency of only 5% in fluorophores.
One of the early fluorescent OLEDs by Tang and Slyke had an EQE of 1%, but eventually the
theoretical limit was reached.
18

Figure 1.4. Simplified Jablonski diagram of (a)conventional fluorescence and (b) TTF. Adapted
from Literature
14

Interest in development of conventional fluorescent materials for OLED has been limited
after the second (phosphorescence) and third generation (TADF) of OLED emitters showed more
11

promise in increasing the IQE, but conventional fluorophores are still explored to make its IQE
competitive. Some ways triplet excitons have been harvested is through triplet fusion (TF), also
known as triplet-triplet annihilation (TTA). Upconverting two triplets into a singlet increases the
theoretical maximum to ~62.5% but this has a strict material restriction such that the dopant 2 x
T1 must be higher than the S1.
9, 19
Additionally, a high triplet concentration is necessary for the
bimolecular process to be efficient and molecular packing is essential for this to occur.
20

Harvesting the triplet excitons in monochromatic OLED for blue fluorescent materials can be
challenging especially with newer generations of OLED emitters such as phosphorescence and
thermally activated delayed fluorescence (TADF) material that can achieve ~100% IQE.
However, current technology still uses blue fluorescent emitters and the efficiency roll-off at
high current densities in fluorescent OLEDs are still less than those found in phosphorescent blue
OLEDs due to the shorter exciton lifetime and less bimolecular quenching observed in
fluorescent emitters. The device lifetime for blue phosphorescent based OLEDs (LT 50% ~100
hrs.) is still much shorter than fluorescent based OLEDs (LT 50% ~11,000 hrs.) which makes the
latter still a viable option.
Another form of fluorescence which falls in the third generation of OLED emitters are
thermally activated delayed fluorescence (TADF). Organic TADF in OLED became of interest
to the research community after first reports of highly efficient OLEDs using carbazolyl
dicyanobenzene (CDCB)
21
which has close to unity PLQY ( ~95%), 5s delayed fluorescence
and stable EQE > 10% .
21
Boron,
22
nanographene,
23
metal
24
based TADF and many other TADF
systems have exhibited that delayed fluorescence can be achieved in a wide range of materials.
The theoretical IQE of TADF material are the same as phosphorescent emitters (~100%) and are
facilitated through the rapid thermal equilibration of the electron between the S1 and T1 states
12

such that prompt and delayed fluorescence from the S 1→S0 are observed. Delayed fluorescence
was initially observed in eosin dyes, and resulted for some to refer to this type of emission as E-
type delayed fluorescence. In order to enhance the ISC in these systems, the S 1 and T1 energy
gap must be small ( E ST < 0.5 eV) such that upon photoexcitation, 25% of the singlet excitons
can radiatively decay to the ground state as prompt fluorescence (kpf ~ 10
7
-10
9
s
-1
) and 75% non-
radiatively decay to the T1 state (Figure 1.5). The small singlet-triplet energy gap (E ST) allows
for the 75% triplet excitons remaining to thermally populate back to the S1 energy (~10
7
s
-1
)
through up intersystem crossing, also known as reverse intersystem crossing, and radiatively
decay as delayed fluorescence. TADF materials have proven to be a great alternative to
phosphorescent emitters and have been shown to reach close to the theoretical maximum of
19.4% EQE in OLEDs.
24c
However, TADF materials have shown to still suffer from TPA and
TTA mechanisms in devices due to the long lived excited state lifetimes of delayed fluorescence.
The same issue are observed for phosphorescent emitters since delayed fluorescence and
phosphorescent emission excited state lifetime is in the s regime and success in OLED
performance were found to have lifetimes below 5s for TADF and Phosphorescent emitters.
TADF and phosphorescent emitters have an advantage over conventional fluorescence material
since both can achieve a 100% IQE which leads to a higher EQE. However, if an alternative
route can be realized to harvest 100% of generated excitons in conventional fluorescence, this
will be useful for OLEDs due to the nanosecond lifetimes of fluorophores. The fast excited state
lifetimes will be key to minimizing the degradation pathways that lead to loss in color purity and
poor device performance.
13

Figure 1.5. Simplified Jablonski of TADF. Adapted from literature
14

1.1.3 Optimize Internal Quantum Efficiency of Fluorescent Molecules in WOLEDs
In WOLEDs, combination of blue, red and green or blue and yellow emitters are used to
produce white light. Use of electrophosphorescent WOLEDs have exhibited
25
high quantum (60-
70%) and luminous power efficiencies (~150 lm/W) at 500-1000 cd/m
2
in the past decade.
However, the all-phosphor-doped WOLEDs have short operational and decrease the color
stability limited by the blue electrophosphorescent material.
26
In 2006, a hybrid
fluorescent/phosphorescent device (Figure 1.6) were developed by Sun et al.
10
where external
quantum efficiencies (18.7%) and power efficiency (3.6%) at high luminance (500 cd/m
2
) were
achieved without outcoupling efficiency. During that time, an all phosphorescent WOLED had
quantum efficiencies (5-12%) and luminous power efficiencies (6-20 lm/W) at brightness less
than 100 cd/m
2
. The device harvested 25% of the electrogenerated excitons of the singlets (s)
for emission and the red and green phosphor emit the remaining lower-energy triplet exciton (t).
The triplet energies on the host have long diffusion lengths
27
(~100nm) such that it can migrate
into the center of the EML and be transferred onto the phosphors to emit. An undoped host
spacer larger ~3nm was deposited to prevent Förster energy transfer from the blue fluorophore to
the phosphors. The advantage of this structure is that the singlets are harvested by the blue
dopant and the triplet excitons are harvested by the phosphors such that transfer from
host→dopant through the singlet and triplet pathways minimize energy losses and maintains the
14

high IQE. TTA is a loss mechanism which leads to high device roll-off in phosphorescence-
based WOLEDs but since triplet exciton density is lower in the hybrid device, a more stable
EQE with increasing current density is observed.
The advantage of the device published in 2006 is that it can achieve high efficiency,
doubling those found in WOLEDs with all phosphorescent dopants or other hybrid white
devices. However, the limitation was due to the 25% IQE in the BCzCi dopant material with low
triplet energy (1.8 eV) and this is well below 4,4’-bis(N-carbazolyl)biphenyl (2.6 eV). The triplet
energy of the fluorescent dopant being well below those of the host materials leads to trapping of
the triplets away from the phosphor. As a result, the overall efficiency decreases since these do
not contribute to light emission on a device. The approach taken to harvest triplet excitons of
fluorescent dopant and increase the IQE while maintaining all the benefits in this structure is to
design a blue fluorescent dopant with small exchange energy between the singlet and triplet
states that the fluorescent dopant triplets have enough driving force to transfer efficiently and
rapidly to the host materials. The design of small E ST emitters and application of these materials
are shared in Chapter 2 and 3.

S
S
T
HOST
Exciton
formation
zone
RED and
GREEN
phosphorescent
dopants
T
S
T
χ
s
= 0.25
χ
t
= 0.75
BLUE
fluorescent
dopant
Förster transfer
Diffusive transfer
T
Energy
15

Figure 1.6. General Proposed Energy Transfer Mechanisms in the hybrid
fluorescent/phosphorescent WOLED. Adapted from literature
10

1.1.4 Designing Molecules with small Singlet-Triplet Energy Gap (E ST)
In order to find a good candidate, it would seem that TADF materials are ideal for this
project since a small singlet-triplet energy gap is a requirement for organic TADF to have
delayed fluorescence which allows for conversion of 100% of excitons to emitted light. The
small exchange energy between the singlet and triplet excited states (E ST) in these charge
transfer (CT) molecules is small (0.1 eV) and can be achieved by decreasing the coupling
between the donor and acceptor. However, one of the problems in these materials is the low
oscillator strength for fluorescence which leads to the high ISC state to a non-emissive triplet and
long radiative lifetime. It is necessary that fluorescent dyes have sufficient donor-acceptor
coupling to give high oscillator strengths for the S 0 S1 transitions. The latter will allow for a
shorter radiative lifetime and maintain the small exchange energy in these systems. Azlactone
and coumarin compounds are some examples of these dyes but one that were more interesting
are boron aza-dipyridylmethene (aD) compounds which are further discussed in Chapter 2. The
aD family has sufficient donor-acceptor coupling that by looking at the HOMO-LUMO orbitals,
the orbital density is localized in an alternating fashion, similar to those found in multi-resonance
TADF, cyanine dyes and bodipy dyes but have fast nanosecond lifetimes and smaller exchange
energy between the singlet and triplet excited states.
1.2 Organic Photovoltaics
Organic Photovoltaics (OPV) research aims to help decrease the energy consumption
from energy sources such as fossil fuels, which contribute to global warming.
28
The low-energy-
production from using earth abundant resources will allow for better sustainability and large
16

decreases in CO2 generation. Harvesting solar energy is attractive since it is the most available
resource. The sun produces about 96,000 TW of energy, far greater than the demand of over 27
TW by 2040. With an average power conversion energy (PCE) of 12% and 2% land coverage, at
minimum 67 TW can be obtained. Thus, solar energy is one of the renewable energies resources
capable of contributing to the global energy demand without compromising environmental
health.
28-29

Silicon solar cells are commercialized with efficiencies above 20% PCE. However, there
are drawbacks with these materials in that they are large and require labor for installation, which
adds to the costs of these materials. Development of alternative PV technologies could
supplement the current silicon solar cell technology and help address some of the drawbacks
found in these systems such as flexibility, versatility, and accessibility.
30
A system which has
potential for these applications are organic photovoltaics (OPV), also known as organic solar
cells (OSC), since use of these materials could allow for manufacturing of thinner and
transparent materials. Although organic solar cells have advantages to silicon-based solar cells,
the low power conversion efficiencies and short device lifetimes are still insufficient compared
to the inorganic counterpart.
The mechanism of OPVs is slightly different than inorganic photovoltaics, in that when
photons are absorbed, photogenerated electron-hole pairs (exciton) need to be separated to move
charges. Thus, a built-in internal field is necessary to break up the exciton into free charge
carriers. For this reason, this is one of the challenges presents in OPVs. Efforts to overcome
these challenges include design of donor-acceptor (D-A) type dyads, symmetry breaking charge
transfer (SBCT) compounds and other molecular modifications to help with voltage losses,
17

energy transfer losses and stability issues.
31
Inorganic solar cells do not have this same issue
because it produces free charge carriers upon light exposure.
The advantage of organic solar cells to their inorganic counterparts is the capability of
modification of the HOMO and LUMO gap using donor-acceptor substituents, due to the
modular framework and easily predicted singlet energy of small organic molecules. Inorganic
systems have discrete conduction and valence bands that make tuning these not typically
straightforward. The problem with organic solar cells is the small exciton diffusion lengths and
the low carrier mobility which leads to the low efficiency and low operational lifetimes.
32
In
order to improve the absorption efficiency for these systems, development of materials with
wider spectral coverage is needed to achieve devices with higher current densities.
32

Additionally, a system which can absorb in the near infrared region (  >750 nm) is ideal since
52% of solar energy distribution belongs to wavelengths ranging between 750-2500 nm, which
are not harvested due to limitations of currently available absorbers (Figure 1.7).
29b

18

Figure 1.7. Contributions of solar spectrum: 52% near infrared, 43% visible region, 5% Ultra-
violet.
33

19

1.2.1 Methods to Achieve a Bathochromic Shift in Organic Molecules
Boron-dipyridylmethene (DIPYR) molecules have very comparable properties to those of
cyanine dyes and BODIPY dyes, such as their extended conjugation structures which can tune
the optical properties of the system based on the conjugation length, such as benzannulation, or
by addition of donor-acceptor pairings designed around the core structure. Since these materials
are high absorbing materials, the optical gap is very similar to the HOMO and LUMO gap such
that molecular modeling of these orbitals allows for better predictive trends in the system. By
looking at extension of benzene (255 nm) to naphthalene (286 nm) and into anthracene (375 nm),
an increase in the absorbance wavelength maxima is observed as π extension is added onto
benzene (Figure 1.8). The same particle in the box concept has been evident in extended
conjugated structures in alkene (Figure 1.9), cyanine and BODIPY dyes relative to their core
structure.
34
Figure 1.10 shows that π-extension around the core of BODIPY can achieve a max
absorbance shift ranging between 605 nm to 841 nm, which is a bathochromic shift of ~345 nm
from the core structure.
35
The push-pull system where addition of donating groups such as
groups with amine or methoxy, or withdrawing groups such as nitriles or nitro groups, result in a
similar bathochromic shift by increasing the length or inducing resonance in the core structure.
36

Figure 5 shows that π extension around the core of BODIPY can achieve a max absorbance shift
ranging between 605 nm to 841 nm which is a bathochromic shift of ~345 nm from the core
structure.
35

20

Figure 1.8. Examples of organic small molecules which have benzannulation or donor-acceptor
motifs, which bathochromically shift by addition of resonance through the compound.

Figure 1.9. Length of resonance decreases the E in conjugated alkenes.

Figure 1.10. Extension of π-system in BODIPY molecules results in a bathochromic shift relative to
the non-extended core.

21

Previous studies in our group suggest that benzannulation in the alpha and gamma
position of these boron dipyridylmethene (DIPYR) systems are interesting to explore as these are
not well-established in literature and were reported to be non-emissive. It was observed that upon
the fluorescence quantum yield of DIPYR is only 17% but upon benzannulation, an increase in
PLQY (~80%) was observed, as shown in Table 1.1. Additionally, an increase in the lifetime (~
4 ns) and a bathochromic shift was observed going from DIPYR (λmax = 481 nm) to -DIPYR
(λmax = 500 nm) and -DIPYR (λmax = 520 nm).
37
In addition to this work, meso-linked dimers in
these types of systems were observed for symmetry breaking charge transfer
38
but understanding
of this system is very limited. Thus, these materials are interesting to explore and understanding
of how it can be modified to shift in the near-infrared is of interest. The synthesized derivatives
of DIPYR appears to be more stable and synthetically accessible than most bodipy dyes. Chapter
4 and 5 explores the structure property relationships explored in these systems and how it has
been developed to apply for OPVs.

Table 1.1. Photophysical Properties of DIPYR systems
37

DIPYR α-DIPYR γ-DIPYR
Quantum yield (𝜱 𝒇𝒍
) 0.17 0.80 0.77
Absorbance maxima ( max
l
) 481 nm 520 nm 500 nm

22

1.3 Chapter 1 References
1. Energy Consumption: Total Energy. 2020.
2. Phillips, J. M.; Burrows, P. E.; Davis, R. F.; Simmons, J. A.; Malliaras, G. G.; So, F.;
Misewich, J. A.; Nurmikko, A. V.; Smith, D. L.; Tsao, J., Basic Research Needs for Solid-State
Lighting. Report of the Basic Energy Sciences Workshop on Solid-State Lighting, May 22-24,
2006. 2006.
3. Zou, S.-J.; Shen, Y.; Xie, F.-M.; Chen, J.-D.; Li, Y.-Q.; Tang, J.-X., Recent advances in
organic light-emitting diodes: toward smart lighting and displays. Materials Chemistry Frontiers
2020, 4 (3), 788-820.
4. Will, P.-A.; Reineke, S., 21 - Organic light-emitting diodes. In Handbook of Organic
Materials for Electronic and Photonic Devices (Second Edition), Ostroverkhova, O., Ed.
Woodhead Publishing: 2019; pp 695-726.
5. Rosenow, T. C.; Furno, M.; Reineke, S.; Olthof, S.; Lüssem, B.; Leo, K., Highly efficient
white organic light-emitting diodes based on fluorescent blue emitters. Journal of Applied
Physics 2010, 108 (11), 113113.
6. Kawamura, Y.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C., Intermolecular
Interaction and a Concentration-Quenching Mechanism of Phosphorescent Ir(III) Complexes in a
Solid Film. Physical Review Letters 2006, 96 (1), 017404.
7. (a) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K.,
White organic light-emitting diodes with fluorescent tube efficiency. Nature 2009, 459; (b)
Reineke, S.; Walzer, K.; Leo, K., Triplet-exciton quenching in organic phosphorescent light-
emitting diodes with Ir-based emitters. Physical Review B 2007, 75 (12), 125328.
8. Kim, J.; Batagoda, T.; Lee, J.; Sylvinson, D.; Ding, K.; Saris, P. J. G.; Kaipa, U.; Oswald,
I. W. H.; Omary, M. A.; Thompson, M. E.; Forrest, S. R., Systematic Control of the Orientation
of Organic Phosphorescent Pt Complexes in Thin Films for Increased Optical Outcoupling.
Advanced Materials 2019, 31 (32), 1900921.
9. Chiang, C.-J.; Kimyonok, A.; Etherington, M. K.; Griffiths, G. C.; Jankus, V.; Turksoy,
F.; Monkman, A. P., Ultrahigh Efficiency Fluorescent Single and Bi-Layer Organic Light
Emitting Diodes: The Key Role of Triplet Fusion. Advanced Functional Materials 2013, 23 (6),
739-746.
10. Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R.,
Management of singlet and triplet excitons for efficient white organic light-emitting devices.
Nature 2006, 440 (7086), 908-912.
11. (a) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.;
Forrest, S. R., Highly Efficient Phosphorescent Emission from Organic Electroluminescent
Devices. Nature 1998, 395, 151; (b) Ma, Y.; Zhang, H.; Shen, J.; Che, C., Electroluminescence
from triplet metal—ligand charge-transfer excited state of transition metal complexes. Synthetic
Metals 1998, 94 (3), 245-248.
12. (a) Colombo, M. G.; Guedel, H. U., Synthesis and high-resolution optical spectroscopy of
bis[2-(2-thienyl)pyridinato-C3,N'](2,2'-bipyridine)iridium(III). Inorganic Chemistry 1993, 32
(14), 3081-3087; (b) Strouse, G. F.; Guedel, H. U.; Bertolasi, V.; Ferretti, V., Optical
Spectroscopy of Single Crystal [Re(bpy)(CO)4](PF6): Mixing between Charge Transfer and
Ligand Centered Excited States. Inorganic Chemistry 1995, 34 (22), 5578-5587; (c) Schmidt, J.;
Wiedenhofer, H.; von Zelewsky, A.; Yersin, H., Time-resolved vibrational structures of the
triplet sublevel emission of Pd(2-thpy)2. The Journal of Physical Chemistry 1995, 99 (1), 226-
229; (d) Wiedenhofer, H.; Schuetzenmeier, S.; von Zelewsky, A.; Yersin, H., Characterization of
23

triplet sublevels by Highly resolved vibrational satellite structures. Application to Pt(2-thpy)2.
The Journal of Physical Chemistry 1995, 99 (36), 13385-13391.
13. Yang, X.; Zhou, G.; Wong, W. Y., Functionalization of Phosphorescent Emitters and
Their Host Materials by Main-Group Elements for Phosphorescent Organic Light-Emitting
Devices. Chem. Soc. Rev. 2015, 44, 8484.
14. Lee, J.-H.; Chen, C.-H.; Lee, P.-H.; Lin, H.-Y.; Leung, M.-k.; Chiu, T.-L.; Lin, C.-F.,
Blue organic light-emitting diodes: current status, challenges, and future outlook. Journal of
Materials Chemistry C 2019, 7 (20), 5874-5888.
15. Fu, H.; Cheng, Y.-M.; Chou, P.-T.; Chi, Y., Feeling blue? Blue phosphors for OLEDs.
Materials Today 2011, 14 (10), 472-479.
16. Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.;
Chen, P.-Y., Systematic Investigation of the Metal-Structure–Photophysics Relationship of
Emissive d10-Complexes of Group 11 Elements: The Prospect of Application in Organic Light
Emitting Devices. Journal of the American Chemical Society 2011, 133 (31), 12085-12099.
17. Yeh, Y.-S.; Cheng, Y.-M.; Chou, P.-T.; Lee, G.-H.; Yang, C.-H.; Chi, Y.; Shu, C.-F.;
Wang, C.-H., A New Family of Homoleptic Ir(III) Complexes: Tris-Pyridyl Azolate Derivatives
with Dual Phosphorescence. ChemPhysChem 2006, 7 (11), 2294-2297.
18. (a) Zhen, C.-G.; Dai, Y.-F.; Zeng, W.-J.; Ma, Z.; Chen, Z.-K.; Kieffer, J., Achieving
Highly Efficient Fluorescent Blue Organic Light-Emitting Diodes Through Optimizing
Molecular Structures and Device Configuration. Advanced Functional Materials 2011, 21 (4),
699-707; (b) Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R., Excitonic singlet-
triplet ratio in a semiconducting organic thin film. Phys Rev B Condens Matter 1999, 60; (c)
Brown, A. R.; Pichler, K.; Greenham, N. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B.,
Optical spectroscopy of triplet excitons and charged excitations in poly(p-phenylenevinylene)
light-emitting diodes. Chemical Physics Letters 1993, 210 (1), 61-66.
19. Lakatos, A. I., Introduction. Journal of the Society for Information Display 2000, 8 (1), 1-
1.
20. (a) Di, D.; Yang, L.; Richter, J. M.; Meraldi, L.; Altamimi, R. M.; Alyamani, A. Y.;
Credgington, D.; Musselman, K. P.; MacManus-Driscoll, J. L.; Friend, R. H., Efficient Triplet
Exciton Fusion in Molecularly Doped Polymer Light-Emitting Diodes. Advanced Materials
2017, 29 (13), 1605987; (b) Tierce, N. T.; Chen, C.-H.; Chiu, T.-L.; Lin, C.-F.; Bardeen, C. J.;
Lee, J.-H., Exciton dynamics in heterojunction thin-film devices based on exciplex-sensitized
triplet–triplet annihilation. Physical Chemistry Chemical Physics 2018, 20 (43), 27449-27455;
(c) Lin, B.-Y.; Easley, C. J.; Chen, C.-H.; Tseng, P.-C.; Lee, M.-Z.; Sher, P.-H.; Wang, J.-K.;
Chiu, T.-L.; Lin, C.-F.; Bardeen, C. J.; Lee, J.-H., Exciplex-Sensitized Triplet–Triplet
Annihilation in Heterojunction Organic Thin-Film. ACS Applied Materials & Interfaces 2017, 9
(12), 10963-10970.
21. Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly Efficient Organic
Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234.
22. Lee, H.; Karthik, D.; Lampande, R.; Ryu, J. H.; Kwon, J. H., Recent Advancement in
Boron-Based Efficient and Pure Blue Thermally Activated Delayed Fluorescence Materials for
Organic Light-Emitting Diodes. Frontiers in Chemistry 2020, 8 (373).
23. Madayanad Suresh, S.; Hall, D.; Beljonne, D.; Olivier, Y.; Zysman-Colman, E.,
Multiresonant Thermally Activated Delayed Fluorescence Emitters Based on Heteroatom-Doped
Nanographenes: Recent Advances and Prospects for Organic Light-Emitting Diodes. Advanced
Functional Materials 2020, 30 (33), 1908677.
24

24. (a) To, W.-P.; Cheng, G.; Tong, G. S. M.; Zhou, D.; Che, C.-M., Recent Advances in
Metal-TADF Emitters and Their Application in Organic Light-Emitting Diodes. Frontiers in
Chemistry 2020, 8 (653); (b) Hamze, R.; Kapper, S. C.; Sylvinson Muthiah Ravinson, D.;
Haiges, R.; Djurovich, P. I.; Thompson, M. E., Molecular dynamics of four-coordinate carbene-
Cu(I) complexes employing tris(pyrazolyl)borate ligands. Polyhedron 2020, 180, 114381; (c)
Hamze, R.; Shi, S.; Kapper, S. C.; Muthiah Ravinson, D. S.; Estergreen, L.; Jung, M.-C.; Tadle,
A. C.; Haiges, R.; Djurovich, P. I.; Peltier, J. L.; Jazzar, R.; Bertrand, G.; Bradforth, S. E.;
Thompson, M. E., “Quick-Silver” from a Systematic Study of Highly Luminescent, Two-
Coordinate, d10 Coinage Metal Complexes. Journal of the American Chemical Society 2019,
141 (21), 8616-8626; (d) Shi, S.; Jung, M. C.; Coburn, C.; Tadle, A.; Sylvinson M. R, D.;
Djurovich, P. I.; Forrest, S. R.; Thompson, M. E., Highly Efficient Photo- and
Electroluminescence from Two-Coordinate Cu(I) Complexes Featuring Nonconventional N-
Heterocyclic Carbenes. Journal of the American Chemical Society 2019, 141 (8), 3576-3588; (e)
Li, T.-y.; Djurovich, P. I.; Thompson, M. E., Phosphorescent monometallic and bimetallic two-
coordinate Au(I) complexes with N-heterocyclic carbene and aryl ligands. Inorganica Chimica
Acta 2021, 517, 120188; (f) Li, T.-y.; Muthiah Ravinson, D. S.; Haiges, R.; Djurovich, P. I.;
Thompson, M. E., Enhancement of the Luminescent Efficiency in Carbene-Au(I)-Aryl
Complexes by the Restriction of Renner–Teller Distortion and Bond Rotation. Journal of the
American Chemical Society 2020, 142 (13), 6158-6172.
25. (a) Cagan, D. A.; Garcia, A. C.; Li, K.; Ashen-Garry, D.; Tadle, A. C.; Zhang, D.; Nelms,
K. J.; Liu, Y.; Shallenberger, J. R.; Stapleton, J. J.; Selke, M., Chemistry of Singlet Oxygen with
a Cadmium–Sulfur Cluster: Physical Quenching versus Photooxidation. Journal of the American
Chemical Society 2019, 141 (1), 67-71; (b) D’Andrade, B. W.; Forrest, S. R., Effects of exciton
and charge confinement on the performance of white organic p−i−n electrophosphorescent
emissive excimer devices. Journal of Applied Physics 2003, 94 (5), 3101-3109; (c) Tokito, S.;
Iijima, T.; Tsuzuki, T.; Sato, F.; J., K.; M., K.; K., N., High-efficiency white phosphorescent
organic light-emitting devices with greenish-blue and red-emitting layers. Applied Physics
Letters 2003, 83 (12), 2459-2461; (d) D’Andrade, B. W.; Forrest, S. R.; C., A.; A., B. M.; E., T.
M.; R., F. S., Effects of exciton and charge confinement on the performance of white organic
p−i−n electrophosphorescent emissive excimer devices. Journal of Applied Physics 2003, 94 (5),
3101-3109.
26. Tung, Y.-J.; Ngo, T.; Hack, M.; Brown, J.; Koide, N.; Nagara, Y.; Kato, Y.; Ito, H., 5.2:
A High Efficiency Phosphorescent White OLED for LCD Backlight and Display Applications.
SID Symposium Digest of Technical Papers 2004, 35 (1), 48-51.
27. Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R., Excitonic singlet-triplet
ratio in a semiconducting organic thin film. Physical Review B 1999, 60 (20), 14422-14428.
28. Mazzio, K. A.; Luscombe, C. K., The future of organic photovoltaics. Chemical Society
Reviews 2015, 44 (1), 78-90.
29. (a) Duan, L.; Uddin, A., Progress in Stability of Organic Solar Cells. Advanced Science
2020, 7 (11), 1903259; (b) Darling, S. B.; You, F., The case for organic photovoltaics. RSC
Advances 2013, 3 (39), 17633-17648.
30. (a) Gebeyehu, D.; Maennig, B.; Drechsel, J.; Leo, K.; Pfeiffer, M., Bulk-heterojunction
photovoltaic devices based on donor–acceptor organic small molecule blends. Solar Energy
Materials and Solar Cells 2003, 79 (1), 81-92; (b) Brabec, C. J., Organic photovoltaics:
technology and market. Solar Energy Materials and Solar Cells 2004, 83 (2), 273-292; (c)
Bundgaard, E.; Krebs, F. C., Low band gap polymers for organic photovoltaics. Solar Energy
25

Materials and Solar Cells 2007, 91 (11), 954-985; (d) Li, G.; Zhu, R.; Yang, Y., Polymer solar
cells. Nature Photonics 2012, 6 (3), 153-161; (e) Chen, L. X., Organic Solar Cells: Recent
Progress and Challenges. ACS Energy Letters 2019, 4 (10), 2537-2539; (f) Nelson, J.,
Polymer:fullerene bulk heterojunction solar cells. Materials Today 2011, 14 (10), 462-470.
31. Riede, M.; Spoltore, D.; Leo, K., Organic Solar Cells—The Path to Commercial Success.
Advanced Energy Materials 2021, 11 (1), 2002653.
32. Schlenker, C. W.; Thompson, M. E., Current Challenges in Organic Photovoltaic Solar
Energy Conversion. In Unimolecular and Supramolecular Electronics I: Chemistry and Physics
Meet at Metal-Molecule Interfaces, Metzger, R. M., Ed. Springer Berlin Heidelberg: Berlin,
Heidelberg, 2012; pp 175-212.
33. Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.; Youngblood, W. J.; Calder, M. E.;
Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.; Birge, R. R.; Lindsey, J. S.; Holten, D.,
Structural control of the photodynamics of boron-dipyrrin complexes. The journal of physical
chemistry. B 2005, 109 (43), 20433-43.
34. Bessette, A.; Hanan, G. S., Design, synthesis and photophysical studies of
dipyrromethene-based materials: insights into their applications in organic photovoltaic devices.
Chemical Society Reviews 2014, 43 (10), 3342-3405.
35. (a) Shen, Z.; Röhr, H.; Rurack, K.; Uno, H.; Spieles, M.; Schulz, B.; Reck, G.; Ono, N.,
Boron–Diindomethene (Bdi) Dyes and Their Tetrahydrobicyclo Precursors—En Route to a New
Class of Highly Emissive Fluorophores for the Red Spectral Range. Chem. —Eur. J. 2004, 10
(19), 4853; (b) Chen, J. J.; Conron, S. M.; Erwin, P.; Dimitriou, M.; McAlahney, K.; Thompson,
M. E., High-Efficiency BODIPY-Based Organic Photovoltaics. ACS Applied Materials &
Interfaces 2015, 7 (1), 662-669; (c) Trinh, C.; Kirlikovali, K. O.; Bartynski, A. N.; Tassone, C.
J.; Toney, M. F.; Burkhard, G. F.; McGehee, M. D.; Djurovich, P. I.; Thompson, M. E., Efficient
Energy Sensitization of C60 and Application to Organic Photovoltaics. Journal of the American
Chemical Society 2013, 135 (32), 11920-11928.
36. (a) Xuan, S.; Zhao, N.; Ke, X.; Zhou, Z.; Fronczek, F. R.; Kadish, K. M.; Smith, K. M.;
Vicente, M. G. H., Synthesis and Spectroscopic Investigation of a Series of Push–Pull Boron
Dipyrromethenes (BODIPYs). The Journal of organic chemistry 2017, 82 (5), 2545-2557; (b)
Wang, H.; Fronczek, F. R.; Vicente, M. G. H.; Smith, K. M., J. Org. Chem. 2014, 79, 10342; (c)
Wang, H.; Fronczek, F. R.; Vicente, M. G. H.; Smith, K. M., Functionalization of 3,5,8-
Trichlorinated BODIPY Dyes. The Journal of organic chemistry 2014, 79 (21), 10342-10352.
37. Golden, J. H.; Facendola, J. W.; Sylvinson M. R, D.; Baez, C. Q.; Djurovich, P. I.;
Thompson, M. E., Boron Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based
Chromophores. The Journal of organic chemistry 2017, 82 (14), 7215-7222.
38. Golden, J. H.; Estergreen, L.; Porter, T.; Tadle, A. C.; Sylvinson M. R, D.; Facendola, J.
W.; Kubiak, C. P.; Bradforth, S. E.; Thompson, M. E., Symmetry-Breaking Charge Transfer in
Boron Dipyridylmethene (DIPYR) Dimers. ACS Applied Energy Materials 2018, 1 (3), 1083-
1095.

26

2. Chapter 2: Developing Blue Fluorescent Materials with Small Singlet
Triplet Energy Gap

2.1 Introduction
Organic light emitting diodes (OLEDs) have been commercialized for use in the displays
of mobile phones, tablets, televisions and wearable technologies, as well as in solid-state lighting
panels. The electroluminescent process involves hole/electron recombination that leads to a
mixture of singlet and triplet excitons.
1
In the late 1990’s phosphorescent emitters were
incorporated into OLEDs, making it possible to harvest both types of excitons and achieve 100%
internal efficiency for conversion of electrical charges into photons. OLEDs with green and red
phosphorescent emitters have achieved both high quantum efficiencies and long device lifetimes,
and have thus become standard emissive dopants in commercial OLED displays.
2
While the
internal quantum efficiencies of OLEDs utilizing blue phosphorescent dopants have also reached
the theoretical limit of 100%, the operational lifetimes of these devices have thus far been short
and can be improved for practical application in displays.
3
The stability of blue phosphorescent
OLEDs is limited by degradation of the host and/or dopant materials via bimolecular decay
processes, i.e. exciton-exciton or exciton-polaron annihilation.
3j, 4
These second order processes
are exacerbated by long excited state decay lifetimes of phosphorescent emitters, typically a few
microseconds. Fluorescent blue emitters are less efficient in OLEDs, but their markedly shorter
excited state lifetimes (nanoseconds) dramatically reduce the rate of bimolecular decay, thereby
increasing the device operational lifetime. Thus, fluorescence-based blue dopants are
conventionally used for OLEDs in commercial displays.
Blue fluorescent materials also have utility in white OLEDs (WOLEDs) for solid state
lighting.
5
Our interest in blue fluorophores stems from a device architecture that splits the
27

singlet and triplet excitons spatially within the WOLED, allowing for the singlet excitons to be
harvested on a blue fluorescent dopant and the triplets on red and green phosphorescent
dopants.
5-6
This hybrid fluorescent/phosphorescent WOLED has the potential to give high color
quality with an internal quantum efficiency of 100%, without the need for blue phosphors.
However, aside from highly efficient blue luminescence, the energy of the triplet state of the
fluorescent dopant in this architecture needs to be high enough to enable endothermic energy
transfer to the green-to-red phosphorescent dopant. This requirement places a restriction on the
most common structural motifs used to create fluorescent blue lumiphores (stilbenes,
anthracenes, etc.), as energies for the triplet state in these materials is typically too low (ET <
2 eV) for effective energy transfer to the phosphor. Moreover, the hybrid WOLED puts a further
restriction on the fluorophore in that it needs to have a blue emissive singlet and a high triplet
energy, thus requiring a small energy difference between the singlet and triplet excited states
(EST), preferably with EST < 400 meV.
Here, we focus on demonstrating a DIPYR (boron dipyridylmethene, Figure 1) family of
dyes
7
to achieve highly efficient blue fluorescence in OLEDs. DIPYR dyes are related to the
more widely studied BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) chromophores, dyes
that have high photoluminescent efficiencies ( PL > 0.8), short emission lifetimes ( < 10 ns) and
narrow emission linewidths (full width half maxima, FWHM < 50 nm). However, shifting the
emission color of BODIPY into the blue is difficult, and these compounds have intrinsically low
triplet energies.
8
These drawbacks make DIPYR motifs attractive alternatives for fluorescent
blue dopants for use in WOLEDs.
28

Figure 2.1. Atomic transmutation of meso-position in DIPYR from methene-bridge to imine-bridge
alters the optical properties of the molecule. Photoluminescence quantum yields were obtained in
methylcyclohexane.

A simple transmutation from methene carbon to nitrogen converts the green-emissive
DIPYR to a blue emissive azaDIPYR (aD) (Figure 2.1). This structural modification stabilizes
the energy of the highest occupied molecular orbital (HOMO) but leaves the lowest unoccupied
molecular orbital (LUMO) relatively unperturbed, thereby inducing a hypsochromic shift in the
emission energy.
9
The emission lifetimes of  < 10 ns of DIPYRs are suitable for use in OLEDs,
but the low  PL limits the external quantum efficiency (EQE). Previous work on DIPYR
compounds suggests that benzannulation of the molecular core can improve photoluminescence
efficiency, while maintaining the short emission lifetime and narrow linewidths.
9b, 10
We have
examined benzannulation along with substitution around the core structure to modify the
photophysical properties of a set of aD molecules shown in Figure 2.2. Heterocyclic ligands
conjugated with boron fluoride, analogous to the aD core, have been previously investigated as
dyes,
9a, 11
aggregation-induced emitters,
12
and pH sensors,
13
but few studies have been reported
on aD materials as emitters, aside from a citation in the patent literature.
14
Benzannulated
derivatives of the aD core are potentially useful as blue fluorescent dopants due to their narrow
emission profile, nanosecond lifetime, high thermal stability, and high  PL. The development of
these organic blue-emitting materials is described, including their synthesis, electrochemical and
photophysical characterization, and performance of 2a in blue OLEDs.
29

Figure 2.2. Structures of aza-boron-dipyridylmethene (aD) in this work.

2.2 Synthesis of azaDIPYR compounds
The synthesis of the aD dyes is similar to a previously reported procedure (Figure 2.3).
9a,
13a
A palladium catalyzed coupling reaction of 2-amino and 2-bromo substituted heteroaryl
compounds was used to form the desired ligand. The ligand was deprotonated with Hunig’s base
and treated with BF3·OEt2 to give the aD dye. Aryl substituted derivative was prepared by
treating the ligand with 2-aminoethoxydiphenyl borate. The products were obtained as
microcrystalline solids, which are white-to-yellow for 1a-1b and bright yellow for 2a-d, 3 and 4.

Figure 2.3. General synthetic scheme to make substituted aza-boron-dipyridylmethene derivatives.
Precursors can be pyridyl, quinolyl or isoquinolyl.

30

2.3 Electrochemistry of azaDIPYR compounds
The electrochemical properties of the aD compounds were analyzed by cyclic
voltammetry (CV), as seen in Table 2.1 and Figure 2.4. Oxidation is irreversible for all the
compounds, whereas reduction is irreversible for 1a-1b and reversible or quasi-reversible in the
benzannulated derivatives, i.e. 2a-2d, 3, 4. The oxidation potentials of the aD dyes span a range
of 0.72-1.15 V (Eredox ~ 400 meV). The reduction potentials span a larger range of -1.91 to -
2.59 V (Eredox ~ 700 meV). The potentials of the benzannulated derivatives 2a-2d, 3 and 4 are
anodically shifted relative to 1a-1b, suggesting stabilization of both the filled and vacant frontier
molecular orbitals, similar to what is observed in the DIPYR system.
9b
Addition of substituents
such as isopropyl (2b) or methoxy (2c, 2d, 4) groups leads to the cathodic shifts in both
oxidation and reduction potentials. For example, the electrochemical potentials of 2c are shifted
relative to 2a by 0.21 V for oxidation and 0.12 V for reduction. Adding donor groups to the
periphery of the aD dye lowers the oxidation potential of the dye, as seen when shifting from H
to iPr to OMe in 2a, 2b and 2c, respectively. The HOMO and LUMO energies were extrapolated
from experimental redox potentials and shown in Table 2.2. These values will be important
when designing the OLED device for these systems. It is worth noting that when looking at the
HOMO and LUMO orbitals of these systems (Table 2.3), it has an atomic localization in their
orbital contribution every for every other atom in the HOMO and LUMO orbitals which are
similarly found in cyanine dyes, BODIPY, DIPYR and MR-TADF molecules which allows for a
selective tunability of these compounds in their redox potential.

31

Table 2.1. Electrochemical potentials of 1a-1c, 2a-2d, 3 and 4.
a

Eox (V)
a
Ered (V)
a
Eredox (V)
1a +0.92 -2.30 3.22
1b +0.72 -2.59 3.31
2a +1.15 -1.91 3.06
2b +1.08 -1.98 3.06
2c +0.94 -2.03 2.97
2d +1.09 -2.07 3.16
3 +1.10 -2.09 3.19
4 +1.04 -2.14 3.18
a
Redox potentials obtained from cyclicvoltammetry in
acetonitrile with ferrocenium/ferrocene as an internal standard.

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Fc/Fc
+

Current (A)
Volts vs Fc/Fc
+
CV in ACN, 100 mV/s
aD (1a)
-aD (2a)
-aD (3)

-2 -1 0.5 1.0 1.5
-aD (2a) -aID (2b) -5OD (2c)
-OD (2d) -OD (4)

Current (A)
Volts vs Fc/Fc
+

Figure 2.4. Cyclic Voltammetry for compounds in Acetonitrile vs. ferrocene (100mV/s)

32

Table 2.2. HOMO and LUMO energies (in eV) for 1a-1b, 2a-2d, 3 and 4.

HOMO
a
LUMO
a
HOMO
b
LUMO
b

1a -5.85 -2.12 -5.82 -1.85
1b -5.62 -1.77 -5.52 -1.71
2a -6.11 -2.58 -5.88 -2.34
2b -6.03 -2.49 -5.71 -2.26
2c -6.04 -2.39 -5.80 -2.23
2d -5.87 -2.44 -5.71 -2.10
3 -6.06 -2.36 -5.82 -2.23
4 -5.99 -2.31 -5.74 -2.15
a
HOMO/LUMO energies extrapolated from experimental redox potentials. HOMO = -1.15
(Eox) + 4.79; LUMO = -1.18 (Ered) - 4.83.
b
HOMO/LUMO values extrapolated from calculations
(B3LYP/6-311G
**
).

33

Table 2.3. Molecular orbital representation of compounds 1a-1c, 2a-2d, 3 and 4 at the B3LYP/6-
311G** level

HOMO -1 HOMO LUMO LUMO +1
1a

1b

2a

2b

2c

2d

3

4

34

2.4 Photophysical Characterization Observed for azaDIPYR Family
The UV-Visible absorption spectra of 1a-1b, 2a-2d, 3 and 4 are shown in Figure 2.5.
All of the aD compounds have high molar absorptivities ( ~ 10
4
-10
5
M
-1
cm
-1
), similar to dyes
such as fluorescein, BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) and porphyrin. The
aD compounds display vibronically structured -

absorption bands between 300-445 nm. The
lowest energy absorption bands in compounds 1a-1b (max = 395-410 nm) are broader than the
same transitions in 2a-2d, 3 and 4 (max = 420-445 nm). The decrease in absorption energy in
the benzannulated derivatives follows a related decrease in the redox gap, as seen in Table 2.1.
The full width half maximum (fwhm) for the 0-0 transition in both 2a-2d (fwhm = 310 cm
-1
) and
3, 4 (fwhm = 515 cm
-1
) is narrow. Narrow linewidths are similarly observed for the structurally
related DIPYR dyes.
9b
The intensity ratios for the 0-0 to 0-1 transitions in 1a-1b are also smaller
than in the benzannulated derivatives. The narrow linewidths along with the large ratio in 0-0 to
0-1 transition intensity suggest that the benzannulated compounds undergo minimal structural
change in their excited states.
Photoluminescence spectra of 1a-1b, 2a-2d, 3 and 4 are shown in Figure 2.5 and their
photophysical data summarized in Table 2.4. The photophysical properties of PMMA films
doped at 1 % with the aD dyes are similar to those in solution (Table 2.4). The aD series give
fluorescent emission between em = 400-450 nm. Compounds 1a-1b exhibit violet-to-blue
fluorescence spectra that are mirror images of their absorption bands. The Stokes shift increases
from 6 nm for 1a to 20 nm upon addition of phenyl groups in 1b. The emission profiles of the
benzannulated derivatives are bathochromically shifted compared to the non-benzannulated
analogs, yet they retain similar vibrational features with an average Stokes shift of ~ 4 nm.
Phosphorescence spectra for aD compounds taken in 2-MeTHF at 77 K have emission maxima
that range from 463 nm for 1a-1b and 484 nm to 502 nm for 2a-2d, 3 and 4 (Figure 2.5 and
35

Figure 2.6). The E0-0 energies for the lowest excited singlet (S1) and triplet (T1) states
determined from the peak maxima of the fluorescence and phosphorescence emission spectra,
respectively, are given in Table 2.4. The material shows a cyanine-like property where there is
relatively little orbital overlap between the HOMO and LUMO, as these orbitals are distributed
on different atoms in the molecule (vide infra). Thus, Franck-Condon factors are
correspondingly small, which minimizes vibronic coupling and structural relaxation in the
excited state leading to a narrow emission line shape.
15
In addition, this orbital configuration
gives rise to the small singlet-triplet gap of these materials.
0
0.5
1
400 440 480 520 560 600
0
0.5
1
Wavelength (nm)
Phos. at 77 K
1a
1b
Normalized emission intensity (a.u.)
Fl. at 298 K
1a
1b

0
0.5
1
420 450 480 510 540 570 600
0
0.5
1
Phos. at 77 K
2a
2b
2c
2d
Wavelength (nm)
Normalized emission intensity (a.u.)

Fl. at 298 K
2a
2b
2c
2d

0.0
0.5
1.0
420 450 480 510 540 570 600
0.0
0.5
1.0
Phos. at 77 K
3
4
Wavelength (nm)
Normalized emission intensity (a.u.)
Fl. at 298 K
3
4

Figure 2.5. Normalized emission spectra at room temperature (upper plots), and gated
phosphorescence emission (bottom plots) recorded after 500 µs delay time at 77 K. Measurements
were performed in 2-methyltetrahydrofuran (2-MeTHF).

36

300 350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
300 350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
Abs

Normalized Abs/PL
Wavelength (nm)
1a in MeTHF
Fl at RT
Gated Phos
at 77K
350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1

Normalized Abs/PL
Wavelength (nm)
1b in MeTHF
Abs
Fl at RT
Gated Phos
at 77K

350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Abs/PL
Wavelength (nm)
2a in MeTHF
Abs
Fl at RT
Gated Phos
at 77K
350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
2b in MeTHF
Abs
Fl at RT
Gated Phos
at 77K

Normalized Abs/PL
Wavelength (nm)

350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
2c in MeTHF
Abs
Fl at RT
Gated Phos
at 77K

Normalized Abs/PL
Wavelength (nm)
350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Abs/PL
Wavelength (nm)
2d in MeTHF
Abs
Fl at RT
Gated Phos
at 77 K
300 350 400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
3 in MeTHF
Abs
Fl at RT
Gated Phos
at 77K

Normalized Abs/PL
Wavelength (nm)
350 400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0 4 in MeTHF
Abs
Fl at RT
Gated
Phos at 77K

Normalized Abs/PL
Wavelength (nm)

Figure 2.6. Normalized Spectra of absorption (dash), fluorescence (solid) at 298K, and
phosphorescence emission at 77K in 2-MeTHF for 2a, 2b, 2c, 2d, 3 and 4.
37

Table 2.4. Summary of the photophysical parameters for 1a-1b, 2a-2d, 3 and 4.
a

 abs
(nm)
 em max
(nm)
b

 PL 
(ns)
k r
(10
8
s
-1
)
c

k nr
(10
8
s
-1
)
d

 em max
(nm)
e

E ST (eV)
1a 398 404 0.42 2.1 2.0 27 464 0.44
1a
f
- 402 0.48 2.34 2.05 22.2 - -
1b 409 429 0.30 2.1 1.4 34 463 0.34
2a 433 434 0.86 3.3 2.7 4.3 484 0.30
2a
f
- 432 0.86 3.37 2.55 4.15 - -
2b 440 444 0.87 3.8 2.3 3.4 494 0.30
2c 442 446 0.84 3.5 2.4 4.6 496 0.30
2d 441 442 0.84 3.3 2.6 4.9 488 0.27
3 422 432 0.87 3.2 2.8 4.1 498 0.44
3
f
- 430 0.90 2.92 3.00 3.4 - -
4 433 437 0.90 2.8 3.2 3.6 502 0.39
a
Recorded in 2-MeTHF.
b
Fluorescence measured at 298 K.
c
kr =  PL/.
d
knr = (1- PL)/.
e
Phosphorescence measured at 77 K.
f
Recorded in PMMA (1% doping)

The photoluminescence quantum yields of the benzannulated compounds in solution and
in doped PMMA films are high ( PL > 0.80). Polymer films doped at high concentrations
(> 1 wt %) display bathochromic shifts, broadened emission spectra and lower quantum yields
due to self-absorption, as expected for fluorophores with small Stokes shifts.
16
The excited state
lifetimes (

= 2 to 4 ns) and radiative rates [kr = (0.93 - 3.2) x 10
8
s
-1
] are similar across the series,
which aligns with common organic fluorophores.
17
However, the non-radiative rates are an
order of magnitude higher for the non-benzannulated compounds (knr = 10
8
s
-1
) relative to the
benzannulated derivatives (knr = 10
7
s
-1
). The higher non-radiative rates in the
non-benzannulated derivatives are attributed to the faster rates for intersystem crossing (ISC) in
these systems.
9b

38

The singlet and triplet excited state energies were calculated using TDDFT (B3LYP functional,
6-311G** basis set; seen in Table 2.5). Previous studies with BF2-pyridylmethene and BODIPY
dyes have shown that these DFT calculations tend to overestimate the singlet energy but give
acceptable values for the triplets.
8l, 9b, 18
Thus, a correction factor of -0.44 eV is needed for the
calculated singlet energies to align with the experimental values. The corrected S1 and
uncorrected T1 state energies predicted by these modeling studies fall within 0.2 eV of
spectroscopically determined values. Intersystem crossing transitions between the S1 and T1
states are symmetry forbidden, hence a comparatively slow rate for ISC is expected for S 1→T1.
However, the S1→T2 transition is symmetry allowed, so it is important for the T 2 state to be
higher in energy than the S1 state to prevent ISC via S1→T2 from being competitive with
fluorescence. The T2 state is lower in energy than S1 in 1a-1b, but is calculated to be higher than
S1 in 2a-2d, 3 and 4. Thus, low quantum yields (PL ≤ 42%) for derivatives 1a-1b are attributed
to exergonic ISC between the S1 and T2 states.
9b
Benzannulation in aD dyes stabilizes the S1
state more than the T2 state, thereby making the S1→T2 transition thermodynamically
unfavorable.
39

Table 2.5. Calculated and Experimental S 1 and T 1 energies for 1a-1b, 2a-2d, 3 and 4, where the
difference between the two energies is represented by E(S 1-T 1).
aD
series
 (10
5
M
-1
cm
-
1
)
a

Calculated results
b
Experimental results
c
S1 (eV/nm)
T1
(eV/nm)
E(S1-T1)
S1
(eV/nm)
T1 (eV/nm) E(S1-T1)
1a 0.31 3.14/395 2.66/467 0.49 3.11/398 2.67/464 0.44
1b - 2.85/436 2.51/495 0.34 3.02/409 2.68/463 0.34
2a 0.83 2.73/455 2.40/517 0.33 2.86/433 2.56/484 0.30
2b 0.96 2.65/469 2.35/529 0.30 2.81/440 2.51/494 0.30
2c 1.30 2.79/444 2.48/500 0.31 2.80/442 2.50/496 0.30
2d 0.88 2.76/450 2.44/509 0.32 2.81/441 2.54/488 0.27
3 0.50 2.78/447 2.41/515 0.37 2.93/422 2.49/498 0.44
4 0.78 2.78/447 2.43/510 0.35 2.86/433 2.47/502 0.39
a: Molar absorptivity values measured in tetrahydrofuran (THF)
b: TD-DFT B3LYP/6-311G** with 0.44 eV correction applied to the S1 energies
c: Singlet energies are extrapolated from peak max of fluorescence (298 K) and triplet energies
from gated phosphorescence (77 K) in 2-methyltetrahydrofuran (2-MeTHF)

2.5 TDDFT Calculations
The experimental S1-T1 gaps fall in a small range within the aD series
(E ST = 0.20-0.45 eV). The largest gap is observed for 1a (E ST = 0.44 eV), where the singlet
and triplet gap is similar to that of DIPYR (E ST = 0.42 eV) and the benzannulated DIPYR
derivatives (E ST = 0.43-0.48 eV).
9b
Interestingly, the aD benzannulated derivatives have
singlet-triplet gaps smaller than the parent aD compound (1a). Quinoline-based systems (2a-2d)
maintain a E ST ~ 0.30 eV, whereas isoquinoline systems (3 and 4) have a larger gap (E ST ~
0.40).
40

To determine the origin of the small S 1-T1 gaps in the aD compounds, the extent of
spatial overlap (Λ) between the hole and electron natural transition orbitals (NTOs) was
calculated for transitions associated with the first excited states (S 1/T1) (see SI for details). The
value of Λ is near unity for strongly localized excitations such as in -

transitions (where the
hole and electron involve the same orbitals), giving rise to a large E ST, and Λ = 0 for purely CT
transitions with little or no spatial overlap, and thus a small E ST. The computed Λ values and
experimental S1-T1 gaps of the aD series are intermediate between those of a localized transition
(anthracene)
19
and a nearly pure CT state (4CzIPN)
20
. Both S1 and T1 states in the aD
compounds show similar degrees of spatial overlap (1a-1b, Λ = 0.64-0.68; 2a-2d, 3 and 4, Λ =
0.61-0.68). Λ is 0.84 for anthracene (E ST = 1.46 eV) and 0.29 for 4CzIPN (E ST = 0.10 eV). It
is evident that the small Λ range, with ~ 0.15 eV difference between the highest and lowest
value, is responsible for the relatively invariant E ST ~ 0.30 eV found in the benzannulated
derivatives.
All calculations reported in this work were performed using the Q-Chem 5.1 program
21
.
Ground-state optimization calculations were performed using the B3LYP functional and the 6-
311G** basis set. Time dependent density functional theory (TDDFT) calculations were used to
obtain the excitation energies and optimized geometries of the S 1 state at the same level, shown
in Table 2.6. The S1 energies were corrected by subtracting 0.44 eV to offset the large errors
commonly associated with cyanine-like dyes.
9b

The extent of overlap (Λ) associated with the electronic transition from the ground state
to the S1 state was computed using the natural transition orbitals (NTOs) according to the
following expression:
41

Λ=
∑ 𝜎 𝑘 𝑘 ∫| 𝜙 𝑘 𝑒 | | 𝜙 𝑘 ℎ
| 𝑑𝜏 ∑ 𝜎 𝑘 𝑘
where, 𝜙 𝑘 𝑒 and 𝜙 𝑘 ℎ
are the electron and hole NTO pairs and 𝜎 𝑘 is the amplitude of the
corresponding NTO pair. The value of Λ would be bounded below by 0 for purely CT transitions
with no spatial overlap and bounded above by ≈1 for strongly localized excitations. The
computed Λ values and experimental S1-T1 gaps of the aD series were compared with those of
anthracene
19
and 4CzIPN
20
and are shown in Table 2.7.

42

Table 2.6. Calculated excited state energies (in eV) at S 0 and S 1 optimized geometries.
(B3LYP/6-311G**) for 2a-2d, 3 and 4.
Compound S1/T2 energies S1-T2
At S0 geometry At S1 geometry At S0 geometry At S1 geometry
2a 2.73/2.92 2.40/2.71 -0.19 -0.31
2b 2.65/2.90 2.38/2.71 -0.26 -0.33
2c 2.79/2.81 2.48/2.60 -0.01 -0.13
2d 2.76/2.79 2.41/2.71 -0.03 -0.30
3 2.78/2.95 2.87/2.49 -0.18 -0.21
4 2.78/2.81 2.84/2.47 -0.03 -0.32

Table 2.7. Extent of overlap calculation using TDDFT with experimental comparison.
Compound Λ Exp S1/T1 energies (eV) Exp S 1-T1 gap
anthracene 0.84 3.10/1.78 1.32
1a 0.68 3.11/2.67 0.44
1b 0.65 3.02/2.68 0.34
2a 0.63 2.86/2.56 0.30
2b 0.61 2.81/2.56 0.30
2c 0.66 2.80/2.51 0.30
2d 0.66 2.81/2.50 0.27
3 0.66 2.93/2.49 0.44
4 0.68 2.86/2.47 0.39
4CzIPN 0.29 2.49/2.43 0.06

2.6 Thermogravimetric Analysis of azaDIPYR, -azaDIPYR, DIPYR and -DIPYR
Thermal gravimetric analysis (TGA) was performed on a TGA Q50 instrument and
samples were run in an alumina crucible under a flowing nitrogen atmosphere with a heating rate
of 10 C/min. Traces are shown in Figure 2.7. azaDIPYR (1a) sublimes cleanly and completely.
The sublimation starts at ca 200°C and is complete by 280°C. The DIPYR compound sublimes
43

over a broader temperature range than 1a and the sublimation is not completed by 400°C. The
change in slope at ~300°C at a weight of 45% suggest that the complex may decompose at
elevated temperatures, but this is well below the temperature used to sublime (for purification) or
deposit DIPYR. It is important to point out that this sample could also contain some impurity and
the decomposition observed is attributed to a different species. One thing is evident in these two
traces, the onset for 1a and DIPYR begins at very similar temperatures for sublimation. -
azaDIPYR (2a) starts sublimation at a higher temperature (~275°C) than azaDIPYR (1a) but has
a similar sublimation temperature range as found in -DIPYR. Benzannulation of the azaDIPYR
(1a) core to -azaDIPYR (2a) is enough to increase the sublimation properties by ~75°C.
Purification of 2a and other aD derivatives were carried out by sublimation at ~200 C and
1.6x10
-6
Torr. Decomposition was not observed during sublimation (for purification) or
deposition of azaDIPYR materials for devices.
44

100 200 300 400
20
40
60
80
100
A
Mass (wt %)
Temperature (C)
DIPYR
100 200 300 400 500
0
20
40
60
80
100
Mass (wt %)
Temperature (C)
azaDIPYR (1a)
B

100 200 300 400 500 600
0
20
40
60
80
100
Mass (wt %)
Temperature (C)
-DIPYR
C
100 200 300 400 500 600
0
20
40
60
80
100
Mass (wt %)
Temperature (C)
-azaDIPYR (2a)
D

Figure 2.7. Thermogravimetric analysis curves for (A) DIPYR, (B) azaDIPYR, (C) -azaDIPYR,
(D) -DIPYR
9b
.
2.7 Monochromatic Blue OLED using -aD
OLEDs were fabricated using compound 2a as an emissive dopant since its frontier
orbital energies and photophysical properties are representative of the aD series. The
photoluminescence properties of 2a are also not significantly affected by solvent polarity (see
Figure 2.8), suggesting that a wide range of host materials with different dielectric constants can
be employed to equal effect. Compounds reported to be effective hosts for fluorescent blue
dopants in OLEDs, such as N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine
(NPD),
22
bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO)
23
, 2,6-bis(3-(carbazol-9-
45

yl)phenyl)pyridine (26DCzPPy)
24
and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP),
25
were thus
doped with 2a.
340 360 380 400 420 440
0
0.5
1
Normalized Absorbance (a.u.)
Wavelength (nm)
2-MeTHF
toluene
Mch
400 450 500 550
0
0.5
1

Normalized Emission Intensity (a.u.)
Wavelength (nm)
2-MeTHF
toluene
Mch

Figure 2.8. Normalized absorbance and emission of 2a in three different solvents;
2-methyltetrahydrofuran (2-MeTHF), toluene and methylcyclohexane (Mch).

NPD and DPEPO both proved to be poor host materials. NPD films doped at 1 and
10 wt% 2a exhibit a broad photoluminescence between 470 nm to 750 nm, whereas OLEDs with
an emissive layer of 2a doped in DPEPO displayed featureless electroluminescence between
550 nm to 750 nm, which is attributed to emission from an exciplex (see Figure 2.9 and Figure
2.10). Fortunately, photoluminescence spectra of 2a doped at 1 and 5 wt % in CBP and
26DCzPPy hosts retain the sharp vibronic emission bands observed in solution. However, the
small Stokes shift of 2a leads to the reabsorption of emitted photons resulting in self-quenching
of the fluorophore when doped at higher concentrations. The intensity of the (0-0)
photoluminescent emission peak of 2a (max = 453 nm) decreases markedly in 5 wt % films and
neat films (Figure 2.11). The PL efficiency of CBP and 26DCzPPy films doped at 1 wt % were
higher ( PL = 0.71 and 0.66, respectively) than those doped at 5 wt % ( PL= 0.39 and 0.43,
respectively). OLEDs with a 15 nm thick emissive layer (EML) were fabricated with 2a doped at
46

1 wt % into CBP or 26DCzPPy. The hole transport layer (HTL) consisted of 10 nm of
dipyrazino[2,3,-f:20,30-h]quinoxaline 2,3,6,7,10,11-hexacarbonitrile (HATCN) and 45 nm of
4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC), whereas the electron
transport layer (ETL) comprised 45 nm 4,7-diphenyl-1,10-phenanthroline (Bphen) and 1.5 nm
(8-quinolinolato)lithium (LiQ) (1.5 nm). ITO was used as the anode and aluminum as the
cathode.

Figure 2.9. DPEPO Host Doped with 2a. ITO (70nm)/HATCN (10nm)/TAPC
(40nm)/mCP(10nm)/DPEPO: 1% vol. a (15nm)/ TSPO1 (35nm)/TPBi (35nm)/ LiQ (1.5nm)/ Al
(100nm). (A) A broad exciplex emission is observed between 550 nm-750 nm. (B) It is evident that
the excimer formation results in a rapid decline in the EQE with increasing current. (C) The
DPEPO device is more resistive than CBP and 26DCzPPy with a turn-on voltage of 5 V.

400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Intensity (a.u.)
Wavelength (nm)
100mA/cm2
10mA/cm2
0.01 0.1 1 10 100 1000
0
1
2
3
4
5
EQE (%)
J
OLED
(mA/cm
2
)
0 2 4 6 8 10 12 14 16 18
1E-7
1E-5
1E-3
0.1
10
1000
V
OLED
(V)
J
OLED
(mA/cm
2
)
1
10
100
1000
10000
100000
Luminance (cd/m
2
)
A B
C
47

400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
1.2

Normalized Intensity (a.u.)
Wavelength (nm)
2a doped in NPD
1 % -aD
10 % -aD

Figure 2.10. 1 wt. % and 10 wt. % 2a spin-coated films doped in NPD show exciplex formation.
400 450 500 550 600 650 700 750
0.0
0.4
0.8
1.2
1.6
2.0
1% 2a in CBP
5% 2a in CBP
Normalized PL Intensity
Wavelength (nm)
400 450 500 550 600 650 700 750
0.0
0.4
0.8
1.2
1.6
2.0
1% 2a in 26DCzPPy
5% 2a in 26DCzPPy
Normalized PL Intensity
Wavelength (nm)

400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
1.2
Normalized Intensity (a.u.)
Wavelength (nm)
2a Neat Film

Figure 2.11. PL emission of neat 2a, 1% and 5% 2a dopant in CBP and 26DCzPPY host materials.

48

0.01 0.1 1 10 100 1000
0
1
2
3
4
5
400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
1.2

100 mA/cm
2
10 mA/cm
2
1 mA/cm
2
Normalized Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
1E-7
1E-5
0.001
0.1
10
1000

V
OLED
(V)
J
OLED
(A/cm
2
)
1
10
100
1000
10000
100000
Luminance (cd/m
2
)
CBP (open)
current
luminance
26DCzPPy (filled)
current
luminance
-6
-5
-4
-3
-2
-1
Energy (eV)
TAPC
CBP
2a BPhen
HATCN
-5.5 -5.5
-1.2
-2.6
-6.1
-5.8
-1.7
-2.0
-6.5
(B)
(C)
(A)
(D)

1% 2a doped in
CBP
26DCZPPY
EQE (%)
J
OLED
(mA/cm
2
)

Figure 2.12. (A) Device architecture and energy levels of an OLED with CBP host and 2a dopant.
(B) Electroluminescence spectra with increasing current (1-100 mA/cm
2
) (C) Current vs voltage
plots and (D) EQE vs current plots for devices using CBP and 26DCzPPy hosts.

49

Figure 2.12A shows the device architecture employing CBP.
26
The electroluminescent
emission spectrum in Figure 2.12B retains the sharp and narrow vibronic structure with
increasing current density (J = 1-100 mA/cm
2
); similar EL spectra are observed in devices using
26DCzPPy (see SI). The turn-on voltage of CBP (Von = 3.5 V; Figure 2.12C and Table 2.8) is
lower than 26DCzPPy (Von = 4.5 V). Additionally, the maximum EQE for CBP (4.5 ± 0.2%) is
higher than 26DCzPPy (3.5 ± 0.2%), and closer to the theoretical maximum of ~ 5% in a
fluorescent OLED on a glass substrate (Figure 2.12D). One drawback in these devices is the
steep roll-off in EQE at high current densities, likely caused by hole leakage since the HOMO
energy in 2a (-6.11 eV) is lower than that of either host (CBP, -5.80 eV; 26DCzPPy, 6.05 eV).
The small peak observed between wavelengths of 380 and 410 nm with increasing current is
attributed to emission from Bphen owing to the hole leakage in these devices (Figure 2.13 and
Figure 2.14). Additionally, the very weak emission could also be a very weak CBP host
emission, indicating that the dopant is easily saturated. This could also help explain the high
EQE roll off. Increasing the concentration to reduce saturation, however, will result in emission
quenching by reabsorption. Therefore, a trade-off exists between quantum yield and dopant
saturation with this particular dopant. Thus, a selection of substituted 2a derivatives is worth
exploring in devices, particularly 2b and 2c.
Table 2.8. Properties of OLEDs doped with 1 wt % 2a into CBP and 26DCzPPy hosts.

Host  max EL (nm) V on (V) % EQE max % EQE
(100 cd/m
2
)
% EQE
(1000 cd/m
2
)
CIE
coordinate

CBP 445 3.0 4.5 4.1 2.7 (0.15, 0.14)
26DCzPPy 445 3.7 3.5 3.5 2.7 (0.15, 0.14)
50

400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
1.2
380 400 420 440
0.001
0.01
0.1
1
0.01 0.1 1 10 100 1000
0
1
2
3
4
5
100 mA/cm
2
10 mA/cm
2
1 mA/cm
2
Normalized Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
1E-7
1E-5
0.001
0.1
10
1000

V
OLED
(V)
J
OLED
(A/cm
2
)
1
10
100
1000
10000
100000
Luminance (cd/m
2
)
D
B
C
A
100mA/cm2
10mA/cm2
1mA/cm2
Normalized Intensity (a.u.)
Wavelength (nm)
EQE (%)
J
OLED
(mA/cm
2
)

Figure 2.13. OLED data for 1% 2a doped film with CBP as host. (A) Electroluminescence vs.
wavelength data illustrates that the electroluminescence emission is maintained with increasing
current scans in the device (B) The graph depicts the current vs. voltage and the luminance vs.
voltage of the OLED. The device has a turn on voltage of 3 V. (C) There is a small growing peak
with increasing current which suggest charge leakage in the device (D) EQE vs current data
suggests that 1% -aD doped film with CBP has an EQE max of 4.5%.

51

400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
1.2
380 400 420 440
0.001
0.01
0.1
1
0.01 0.1 1 10 100 1000
0
1
2
3
4
5
Normalized Intensity (a.u.)
Wavelength (nm)
100mA/cm
2
10mA/cm
2
1 mA/cm
2
0 2 4 6 8 10 12
1E-7
1E-5
0.001
0.1
10
1000
V
OLED
(V)
J
OLED
(A/cm
2
)
1
10
100
1000
10000
100000
Luminance (cd/m
2
)
C
D
B
Normalized Intensity (a.u.)
Wavelength (nm)
100mA/cm2
10mA/cm2
1mA/cm2
A
EQE (%)
J
OLED
(mA/cm
2
)

Figure 2.14. OLED data for 1% 2a doped film with 26DCzPPY as host. (A) Electroluminescence vs.
wavelength data illustrates that the electroluminescence emission is maintained with increasing
current scans in the device (B) The graph depicts the current vs. voltage and the luminance vs.
voltage of the OLED. The device has a turn on voltage of 3.7 V. (C) There is a small growing peak
with increasing current which suggest charge leakage in the device (D) EQE vs current data
suggests that 1% -aD doped film with CBP has an EQE max of 3.5%.

2.8 WOLED Device Using -azaDIPYR (-aD) as blue dopant
The goal is to incorporate the new fluorescent dopant into a standard hybrid WOLED
structure and test the performance of these materials with small singlet-triplet materials. Hybrid
WOLEDs with two different host materials were explored and closely studied using the Sun et
al. device structure.
5
(4P-NPD) host material was used; however, 2a (-aD hereafter) emission
was quenched by the host due to a low energy exciplex expected for this combination (-
aD
-
/4P-NPD
+
). From an emission standpoint, using CBP host is ideal due to the higher exciplex
52

energy, as compared to the -aD singlet energy with emission efficiency of 80%. The WOLED
structure, efficiency and spectra are shown in Figure 2.15.

Figure 2.15. Hybrid WOLED with -aD fl-dopant and CBP host.

The efficiency is far less than that of the analogous device with a BCzVBi fluorescent
dopant in a CBP host.
5
There is also a much larger blue component to the WOLED’s emission
spectrum than in the BCzVBi based device (see Figure 2.16). The higher blue emission
contribution and low EQE is due to the higher S1 energy of CBP (0.35 eV) than -aD, which
leads to the singlet exciton trapping in the blue dopant, while the higher T 1 energy in CBP (0.20
eV) leads to triplet trapping at the dopant. As a result, the singlet and triplet excitons are trapped
0.1 1 10 100 1000
0
2
4
6
8
10
EQE (%)
J
OLED
(mA/cm
2
)
400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1mA/cm2
1mA/cm2
10mA/cm2
100mA/cm2
Normalized Intensity (a.u.)
Wavelength (nm)
Glass
ITO 150nm
Bphen 40nm
Al 100 nm
CBP: -azaDiPYR 1% 10nm
CBP 30nm
CBP: -azaDiPYR 1% 15nm
NPD 40nm
LiQ 1.5nm
EML
CBP
(55nm)
0 10 20 30 40 50 55
0.5 PQIr
0.5 Ir(ppy)
3
EML
53

close to the interface, leading to insufficient energy transfer to the phosphorescent dopants. This
results in the low device efficiency and emission contribution of green and red emission in CBP-
based WOLED. In order to identify the site of trapping, a triplet sensing experiment (Figure
2.17) was conducted.
27
The result suggests that triplet density is highest at the interface between
the emission layer and electron transport layer at all current densities, unlike when BCzVBi is
used as a blue fluorescent dopant. These device studies illustrate the limitations of the
established materials for hybrid WOLEDs, and suggest that better host materials with a lower
triplet energy than the fluorescent dopant are necessary for this fluorescent/phosphorescent
hybrid WOLED design.

Figure 2.16. Emission spectrum of sensing devices using BCzVBi (left) and 2a (right) with sensors
implanted in similar positions.
54

Figure 2.17. Triplet sensing with an -aD fluorescent dopant in CBP, using experiment scheme of
Figure 2.15.
Three different materials were designed to serve as a host for -aD. The device
performance of the hosts doped with -aD is summarized in Figure 2.18. The three devices
show external quantum efficiencies of 2.5 to 3%. This can be later improved by optimizing the
transport layer thicknesses for better charge balance and outcoupling efficiency. A coarse exciton
profiling experiment was conducted using PQIR as a sensor to design hybrid WOLEDs using
these hosts. I4 and I5 were the materials used due to their potential as neat emitters.
28
Results
showed the exciton formation zone is at the HTL/EML interface, indicating that the two
materials are primarily electron conducting.
Figure 2.19 and 2.20 shows the structure and performance of the WOLED using I5 as a
host, which showed higher efficiency compared to I4. Delta doped phosphor layers were used to
minimize electron trapping that may quench blue emission. However, the spectrum showed lack
55

of green emission which led to a low color rendering index of 74. The emission contribution
from the green component is lacking and can be resolved by increasing the doping concentration
of the green dopant to induce more trapping. The efficiency roll-off was an issue in the device
where a poor charge balance in the device exist or there is a high triplet-triplet annihilation rate.
It is evident in the spectrum that the red emission diminishes with increasing current density,
which supports the high rate of triplet-triplet annihilation. Overall, the device showed a luminous
power efficacy of 27lm/W at low current density. Further device optimization is needed to
reduce the roll-off in these devices in order to maintain high efficiency.
400 450 500 550 600 650 700 750
0
0.5
1
Normalized Intensity (a.u.)
Wavelength (nm)
I2
I5
I4
0 2 4 6 8 10 12
1E-7
1E-5
0.001
0.1
10
1000
I2
I5
I4
J
OLED
(mA/cm
2
)
V
OLED
(V)
0.1 1 10 100 1000
0
1
2
3
4
5
I2
I5
I4
EQE (%)
J
OLED
(mA/cm
2
)

Figure 2.18. Device Performance of -aD in Imidazole-based Host
56

Figure 2.19. Device structure and characteristics of Imidazole host (I5) based hybrid WOLED,
Structure 1.

Figure 2.20. Device structure and characteristics of Imidazole host (I5) based hybrid WOLED,
Structure 2.
150nm ITO
40nm TAPC
10nm Host: -aD 1 vol%
50nm BPhen
100nm Al/LiQ
5nm Host
0.05nm Ir(ppy)
3
4nm Host 4nm Host
0.05nm PQIr
6nm Host
400 450 500 550 600 650 700 750
0
1
2
3
4
1mA/cm2
10mA/cm2
100mA/cm2
Normalized Intensity (a.u.)
Wavelength (nm)
0.01 0.1 1 10 100
0
5
10
15
20
0.01 0.1 1 10 100
0
5
10
15
20
25
30
J
OLED
(mA/cm
2
)
EQE (%)
LE (lm/W)
0 2 4 6 8 10
1E-7
1E-5
1E-3
0.1
10
1000
J
OLED
(mA/cm
2
)
V
OLED
(V)
400 450 500 550 600 650 700 750
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Normalized Intensity (a.u.)
Wavelength (nm)
1mA/cm2
10mA/cm2
100mA/cm2
150nm ITO
40nm TAPC
10nm Host: -aD 1 vol%
50nm BPhen
100nm Al/LiQ
5nm Host
4nm Host:Ir(ppy)
3
2 vol%
0.05nm PQIr
6nm Host
0 2 4 6 8
1E-7
1E-5
1E-3
0.1
10
1000
J
OLED
(mA/cm
2
)
V
OLED
(V)
0.01 0.1 1 10 100
0
5
10
15
20
0.01 0.1 1 10 100
0
5
10
15
20
25
30
J
OLED
(mA/cm
2
)
EQE (%)
LE (lm/W)
57

2.9 Lifetime tests for the blue OLEDs
Lifetime tests for the blue OLEDs fabricated using -aD has been a challenge in this
study due to difficulty finding a stable hole blocking material. Alq 3 is used as an electron
transporting layer in lifetime structures due to its reported high stability; however, its’ shallow
HOMO energy (5.8 eV) relative to the blue dopant requires the use of hole blocking materials.
As shown in Figure 2.21, a new peak appears between 500-650 nm with increasing current
density due to hole leakage in the ETL layer. Other hole blocking materials such as BPhen,
BP4mPy, BCP and TPBi either crystallize or decompose during device operation (cite), which
will greatly affect the lifetime measurements. Photoluminescence lifetime of -aD films were
measured using mCBP as a host and Alq3 as hole blocking layer with 1 vol% and 10 vol% -aD
were prepared. The films were degraded using a UV LED lamp of 340nm wavelength and
130mW/cm
2
intensity. Figure 2.22 shows that both the 1% and 10% -aD doped films degraded
similarly. Lifetime of the films were fitted using the empirical equation: 𝐿 /𝐿 0 =exp[−(𝑡 /𝜏 )
𝛽 ] with
fitting values of 𝜏 =196hrs and 𝛽 =1.4. The measured T70 of the films was 95 hours. The
lifetime can be potentially improved by using a more stable host, since mCBP has a higher
singlet energy than its own bond dissociation energy.

58

400 450 500 550 600 650 700 750
0.0
0.5
1.0
1.5
2.0
Normalized Intensity (a.u.)
Wavelength (nm)
1mA/cm2
10mA/cm2
100mA/cm2

Figure 2.21. Spectrum of -aD Blue OLED device with Alq 3 ETL

Figure 2.22. Lifetime Studies of Blue OLED with Alq3 ETL

59

2.10 aD Expansion: Destabilizing the HOMO energy and Addition of Steric Bulk on
the azaDIPYR core

Benzannulated derivatives such as 2a and 3 have an experimental singlet-triplet energy
gap of ΔS1/T1 ≤ 0.3 eV which meets the criteria of developing blue fluorescent dopants for
hybrid WOLED. Both compounds exhibit strong  to * absorption bands and intense
fluorescent emission in the blue part of the visible spectrum, associated with high quantum yields
in solution and doped thin films,  PL = 80-86%. However, the experimental HOMO energies of
3 (6.14 eV) and 2a (6.11 eV) deviated from the calculated HOMO energies by 0.63-0.70 eV.
Common hosts such as mCBP, CBP and DPEPO have shallower HOMO energies which do not
nest our fluorescent dopants. As a result, excimer formation is observed when doped in the
different host materials. New types of host materials were developed (Chapter 3) which are
targeted to nest the HOMO and LUMO energies of 2a with lower triplet energies than CBP to
address this issue. Additionally, adding substituents around the aD core is another way to
optimize some of our WOLED structures by making the HOMO energy shallower for the blue
dopant than the host. As a result, designing compounds which target values closer to HOMO
energies around 5.6-5.8 eV (HOMO energy ≤ 6.0eV) are of interest.
From a material perspective, it is interesting to understand the structure-property
relationships of these aD molecules since these are not well explored in literature. Additionally,
it provides an opportunity to tune the photophysical and electrochemical properties of these
compounds for application both in optoelectronics and other areas which require good absorbers
and/or emitters. From DFT modeling, our data suggests that by decorating the aD compounds
with electron donating functional groups in selected positions, we can destabilize the HOMO
energy, thus making it shallower. Following this approach, the HOMO/LUMO energies will be
60

suitable for common host materials. The non-benzannulated compounds have HOMO/LUMO
energies that are shallower than the benzannulated 2a derivative, causing them to be suitably
nested in a host matrix and making them ideal fluorescent blue emitters for the hybrid WOLED
structure. Due to the synthetic complexity of the substituted 2a core (6-7 synthetic steps), it is
worth exploring other substitution sites which could effectively provide us similar destabilization
of the HOMO energy, while increasing the steric bulk of the molecule to minimize any potential
self-quenching when doped at high concentrations.
Synthesis of aD with phenyl (1b) or tolyl (1c) substituents in place of the fluorine atoms on the
boron center are outlined in Figure 2.23. The photophysical characterization is shown in Figure
2.24, Table 2.9 and Table 2.10, which show the newly synthesized aD-substituted systems along
with their photophysical parameters and redox potentials. Based on these studies, substitution in
the boron position of aD is worth exploring because of the ease in synthesizing bulkier versions
of the aD core. There are two ways to make an aryl substitution on the boron center. One method
is by using aD as a precursor and reacting it with an aryl grignard. Note that a mono-substituted
molecule is a possible side product for this reaction. Another synthetic option is reacting the
desired aD ligand with a diaryl boronic anhydride precursor under reflux to form an exclusive
disubstituted boron atom. The calculated values for a tolyl or phenyl substitution are shown in
Table 2.9. DFT calculation suggests that small singlet-triplet energy gaps (ΔS1/T1 ≤ 0.4 eV) are
maintained. As shown in Table 2.10, the HOMO of 1b obtained from electrochem is -5.62 eV,
which is shallower than the -6.21 eV value of α-aD. The triplet of 1b is also well above 2.50 eV,
making them suitable as blue dopant materials that can shuttle triplet excitons to green and red
phosphors in WOLED.

61

Figure 2.23. Synthetic Route to obtain boron substituted derivatives of aD
300 350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
300 350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
Abs

Wavelength (nm)

Normalized Abs/PL
Wavelength (nm)
1a in MeTHF
Fl at RT
Gated Phos
at 77K
350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Abs/PL
Wavelength (nm)
1b in MeTHF
Abs
Fl at RT
Gated Phos
at 77K
300 350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
1c in MeTHF
Abs
Fl at RT
Gated Phos
77K

Normalized Abs/PL
Wavelength (nm)

Figure 2.24. Normalized Spectra of absorption (dash), fluorescence (solid) at 298K, and
phosphorescence emission at 77K in 2-MeTHF for 1a, 1b and 1c
62

The drawback in these materials is the fast nonradiative rates and low quantum yields of
1b and 1c compared to 1a, suggesting that the deactivation pathway observed in aD is enhanced
by the addition of the bulky groups. The latter could be a result in the free rotation of the phenyl
or tolyl groups and the geometry distortion that occurs between the ground state and the excited
state, as discussed earlier in the chapter. Alternative structures with or without benzannulation
are proposed later in this chapter to circumvent the low photoluminescence efficiencies observed
in these materials.
Table 2.9. Calculation and Experimental Results for the S1, T1 and E ST
aD
series
Calculated results
b
Experimental results
c
S 1 (eV/nm) T 1 (eV/nm) E ST S 1 (eV/nm) T 1 (eV/nm) E ST
1a 3.14/395 2.66/467 0.49 3.07/404 2.67/464 0.40
1b 2.85/436 2.51/495 0.34 2.89/429 2.68/463 0.21
1c 2.82/441 2.48/500 0.33 2.82/440 2.66/466 0.16
b: TD-DFT B3LYP/6-311G** with 0.44 eV correction applied to the S1 energies

Table 2.10. Photophysical properties of 1a,1b and 1c.

 abs
(nm)
 em max
(nm)
b

 PL

(ns)
k r
10
8
(s
-1
)
c

k nr
10
7
(s
-1
)
d

 em max
(nm)
e

E ST
(eV)
1a 398 404 0.42 2.1 2.0 27 464 0.40
1b 409 429 0.30 2.1 1.4 34 463 0.21
1c 398 440 0.22 2.4 0.93 33 466 0.16

An approach taken to destabilize the HOMO energy of the 2a system is to add a weakly
donating group in the core structure where there is significant orbital contribution in the HOMO
orbital. Substitutions around the 2a core for synthetic feasibility were considered. DFT
calculations suggest that substitution in the 3,4 or 6 position of 2a can slightly destabilize the
63

HOMO energies (Table 2.11) yet maintain the favorable photophysics and the small singlet-
triplet energy gap. Our recent studies suggest that by adding a mesityl- or an isopropyl- group in
the 4 and 6 positions, respectively, the redox-oxidation potential will have a shallower HOMO
energy that retains the high fluorescence quantum yield (PLQY ≥ 86 %). Addition of R-groups
into the 2a core is used as a strategy to tune the molecular/photophysical properties of the system
in favor of desired values.
Synthesis of mesityl substitution in the 3- and 4- position is close to completion as shown
by the liquid chromatography-mass spectrometry trace, along with the associated UV-visible
spectrum in Figure 2.25. The -aM3 (2a-(Mes)2-3) and -aBr4 (precursor to make 2a-(Mes)2-
4) are ligands for target compounds and have been characterized using NMR, LCMS and UV-vis
analysis. The m/z value of 508.40 (-aM3) and 430.00 (-aBr4) suggests that we are only a
couple of steps away from our target molecules -aM3D and -aM4D, respectively. These
compounds would be interesting to borylate and explore their photophysical and electrochemical
properties, especially to confirm if self-quenching is observed at higher doping concentration. It
is worth noting that the synthesis require optimization to have enough material for the borylation
step.

64

DIPYR core structure.

Table 2.11. DFT (B3LYP/6-31G**) calculated properties for -aD derivatives (DIPYR CORE
STRUCTURE). Compounds with each substituent type (OMe, ipr and Mes) are arranged by
HOMO energy.

S1 (eV) S1 (nm) T1 (eV) T1 (nm)
ΔS1/T1
(eV)
HOMO
(eV)
LUMO
(eV)
2a 2.73 454 2.39 519 0.34 -5.62 -2.07
OMe
2a-(OMe) 2-6 2.49 498 2.19 567 0.31 -5.25 -1.95
2a-(OMe) 2-3 2.64 470 2.25 550 0.38 -5.31 -1.90
2a-(OMe) 2-7 2.65 468 2.36 525 0.29 -5.36 -1.90
2a-(OMe) 2-4 2.92 425 2.67 465 0.25 -5.41 -1.62
2a-(OMe) 2-5 2.79 444 2.47 502 0.33 -5.45 -1.81
ipr
2a-(ipr)2-4 2.73 454 2.43 511 0.31 -5.47 -1.90
2a-(ipr)2-6 2.65 467 2.34 530 0.31 -5.47 -1.99

2a-(ipr)2-7 2.69 460 2.39 520 0.31 -5.49 -1.96
2a-(ipr)2-5 2.71 457 2.39 518 0.32 -5.52 -1.96
2a-(ipr)2-3 2.75 452 2.42 512 0.32 -5.55 -1.99
Mes
2a-(Mes)2-4 2.71 457 2.41 515 0.30 -5.52 -1.96
2a-(Mes)2-5 2.71 458 2.40 517 0.31 -5.55 -2.01
2a-(Mes)2-7 2.63 472 2.38 522 0.25 -5.55 -2.07
2a-(Mes)2-3 2.70 459 2.38 521 0.32 -5.58 -2.04
2a-(Mes)2-6 2.66 466 2.36 525 0.30 -5.58 -2.07

65

Figure 2.25. Liquid chromatography-mass spectrometry trace and UV-visible spectrum for
currently synthesized compounds intended for mesityl derivatization.
66

2.11 aD Expansion: Destabilizing the T2 Energy of aD to minimize intersystem
crossing
It was discussed earlier in the chapter that the boron center of aD was replaced with tolyl
and phenyl groups to produce bulkier aD compounds with shallower HOMO energies. The small
singlet-triplet energy gaps (ΔS1/T1 ≤ 0.4 eV) are maintained in the system. The HOMO energy
of 1b obtained from differential pulse voltammetry is -5.62 eV, which is shallower than 2a. The
triplet of 1b is also well above 2.50 eV, making them suitable as blue dopant materials that can
shuttle triplet excitons to green and red phosphors in WOLED. However, the fast nonradiative
rates and low quantum yields of 1b and 1c are not ideal characteristics for our dopants. The low
PLQY can be attributed to a low-lying T2 state in near resonance with the S1 state in these
systems facilitating non-radiative decay.
DFT calculation suggests that candidates aD-5N and aD-6N are starting points (Figure
2.26) to explore this finding, especially since ligands could be synthetically accessed (Scheme
2.1). Based on the DFT results, unlike other aD-based systems explored so far, these candidates
are expected to feature T2 states that are destabilized relative to the S1 state (Table 2.12).
Therefore, they should exhibit higher PLQYs since this intersystem crossing pathway contributes
to the non-radiative pathways. The HOMO energies are more stabilized than our target values (>
-6.0 eV) but this is still worth exploring due to the reordering of triplet states which could lead to
high PL efficiencies. The ligands for these compounds have been reported in literature but
borylation of these have not been successful, when attempted in the lab. The regular borylation
conditions used for other DIPYRs produced no reaction and changing the base strength did not
lead to production of desired product. It is likely that the deprotonation step works but finding
67

the right solvent, boron source and boron equivalence is necessary.
1
Unfortunately, borylation of
these ligands has been a challenge.

Figure 2.26. Target Compounds with T 2 energy higher than the S 1 energy based on TDDFT.

Scheme 2.1. Synthesis of two ligands for borylation to produce aD derivatives where the T 2 energy
is destabilized relative to the S 1 energy. Synthesis are adapted from literature.
29

Table 2.12. DFT (B3LYP/6-31G**) calculated properties of prospective candidates based on the aD
core-structure.

S
1
(eV)* T
1
(eV) T
2
(eV)
Osc st.
(S
1
)
HOMO
(eV)
LUMO
(eV)
ΔS 1-T 1
(eV)
ΔS 1-T 2
(eV)
aD-5N 2.70 2.40 2.91 0.012 -6.21 -2.42 0.30 -0.21
aD-6N 2.89 2.55 2.91 0.013 -6.61 -2.62 0.34 -0.02
* -0.44 eV correction factor applied based on internal benchmarks.

68

Our attempt to borylate these pyrimidines, pyridazines or any other ligands with more than three
aza-substitutions in the aD core leads to recovery of starting ligand, using similar borylation
conditions for BODIPY, DIPYR and aD synthesis.
Compounds with substitution around the core with donating and withdrawing groups
rather than atomic transmutation around the core were explored to diversify the targeted
molecules. NTO orbitals, using the electron and hole pairs, were used to identify the atomic sites
which have orbital contribution in the T 2 state but is absent in the T1 state. Figure 2.27
illustrates the site meta to the pyridine nitrogen as one of the main sites which can lead to
reordering singlet and triplet state energies, such that the S 1 state energy is less destabilized than
the T2 state, through addition of electron donating groups.
Table 2.13 shows the DFT calculation results when adding electron donating groups to
the AD1 and AD2 core structures. The S1-T2 difference range is between 0.04-0.20, where the T2
is lower than the S1 energies, suggesting that these core structures and substitution patterns do
not achieve our target structures. As a result, this led to developing donor-acceptor (push-pull)
structures, providing structures 1 and 2 as promising candidates with S1-T2 difference of -0.18
and -0.15 eV, respectively. Structures and TDDFT results conducted with structures 1 and 2 are
shown in Figure 2.28 and Table 2.14. Although these compounds have not been synthesized and
characterized, these are promising candidates to test the hypothesis that one of the main
contributors in the non-radiative pathway of these aD compounds is the ISC into the triplet state.

69

A
B

Figure 2.27. (A) NTO of electron and hole pair using B3LYP/631G** where the red circles
represent the site where the orbital contribution is different for T 1 and T 2. (B) Illustrates the site
chosen and the donor molecules used (represented by R) to understand which substitution pattern,
if any, can stabilize the T 2 energy in a non-benzannulated aD core.
70

Table 2.13. Calculation Results for AD1 and AD2 derivatives
S 1 (eV) T 1 (eV) T 2 (eV)
Osc st.
(S 1)
HOMO
(eV)
LUMO
(eV)
S 1-T 1 (eV) S 1-T 2 (eV)
AD1-H 3.01 2.34 2.84 0.31 -5.80 -2.07 0.67 0.17
AD1-Me 2.95 2.29 2.81 0.31 -5.62 -1.97 0.66 0.14
AD1-OMe 2.91 2.16 2.84 0.42 -5.51 -1.98 0.75 0.07
AD1-NMe2 2.76 2.16 2.72 0.36 -5.28 -1.81 0.60 0.04
AD2-H 2.84 2.24 2.64 0.21 -5.33 -1.73 0.60 0.2
AD2-Me 2.89 2.27 2.71 0.30 -5.24 -1.62 0.62 0.18
AD2-OMe 2.80 2.08 2.68 0.40 -5.08 -1.64 0.72 0.12
AD2-NMe2 2.71 2.09 2.59 0.34 -4.91 -1.50 0.62 0.12

Figure 2.28. Combination of Donor-Acceptor Substitution (push-pull) which further destabilize the
T 2 Energy
71

Table 2.14. TDDFT Results which include S1, T1 and T2 valuers of Push-Pull Compounds for T 2
destabilization
S 1 (eV) T 1 (eV) T 2 (eV) S 1-T 1 (eV) S 1-T 2 (eV)
1 2.81 2.58 2.99 0.23 -0.18
2 2.77 2.48 2.92 0.29 -0.15
3 2.74 2.35 2.82 0.39 -0.08
4 2.86 2.62 2.91 0.24 -0.05
5 2.87 2.62 2.92 0.25 -0.05
6 2.85 2.56 2.90 0.29 -0.05
7 2.87 2.55 2.90 0.32 -0.03
8 2.89 2.31 2.91 0.58 -0.02

2.12 Conclusion
Substituted aza-boron-dipyridylmethenes (aD) were explored as candidates for fluorescent blue
dopants in OLEDs. The synthetic flexibility of these materials makes them easy to modify with
different substituents to alter their energetics, while also maintaining the high quantum
efficiency, small S1-T1 gap and small Stokes’ shift. Eight substituted aD compounds were
synthesized to study their photophysical and electrochemical properties. All the compounds
display blue fluorescence (em = 400 - 500 nm) with quantum efficiencies > 85%. Minimal
overlap between the HOMO and LUMO leads to the small singlet-triplet energy gaps of these
materials (ΔE ST ≤ 0.4 eV). OLEDs prepared using one of these derivatives (2a) have low turn-
on voltages (3 V) and high efficiency (EQEmax = 4.5 ± 0.2%), approaching the maximum
theoretical limit of fluorescent OLEDs on glass substrates (EQE = 5%). These studies suggest
that 2a and the other compounds in the aD series can serve as fluorescent blue dopants in both
monochromatic and white OLEDs. Furthermore, their small single-triplet energy gaps present an
72

opportunity to harvest the triplet excitons to increase the internal quantum efficiency in hybrid
fluorescent/phosphorescent white light emitting diodes. WOLED studies suggest that
development and exploration of new host materials with increased triplet energy and HOMO
energy stabilization, as compared to 2a and other compounds in the aD series, will provide an
opportunity to achieve systems with high IQE for the blue dopant in the hybrid
fluorescent/phosphorescent WOLEDs structure.

73

2.13 Experimental
2.13.1 Synthesis
Precursors for 1a-1b were purchased from Sigma-Aldrich. Aza-boron-dipyridylmethene
(aD) synthesis for 1a-1b, 2a-2d, 3 and 4 were prepared using similar coupling reaction synthesis
with pyridine, quinoline or isoquinoline core.
13b
The detailed synthesis and characterization of
each of the compounds are given in the Supporting Information.
2.13.2 Electrochemical Measurements
Cyclic voltammetry and differential pulsed voltammetry were performed using a
VersaSTAT potentiostat measured at 100 mV/s scan. Anhydrous acetonitrile (DriSolv) from
Sigma Aldrich was used as the solvent under nitrogen environment, and 0.1 M
tetra(n-butyl)ammoniumhexafluorophosphate (TBAF) was used as the supporting electrolyte. A
glassy carbon rod was used as the working electrode; a platinum wire was used as the counter
electrode, and a silver wire was used as a pseudoreference electrode. The redox potentials are
based on values measured from differential pulsed voltammetry and are reported relative to a
ferrocenium/ferrocene (Cp2Fe
+
/Cp2Fe) redox couple used as an internal reference;
electrochemical reversibility was determined using cyclic voltammetry.
2.13.3 Photophysical Measurements
All samples in fluid solution were dissolved in 2-methyltetrahydrofuran (2-MeTHF) with
absorbance between 0.05-0.15 to prevent reabsorption when performing photoluminescence
measurements due to the small stoke-shift in the aD series. Doped poly(methyl methacrylate)
thin films were prepared from a solution of poly(methyl methacrylate) (PMMA) in
dichloromethane. Samples of 1a, 2a and 3 (1 vol%) were dissolved in the PMMA solution and
spin coated on a quartz substrate (2 cm x 2 cm) rotating at 700 rpm for 45 seconds. The UV-
visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Steady state
74

fluorescence emission measurements were performed using a QuantaMaster Photon Technology
International spectrofluorometer. Gated phosphorescence measurements were carried on the
fluorimeter using a 500 microsecond delay on samples at 77 K. All reported spectra are
corrected for photomultiplier response. Fluorescence lifetime measurements were performed
using an IBH Fluorocube instrument equipped with 331 nm LED and 405 nm laser excitation
sources using a time-correlated single photon counting method. Photoluminescence quantum
yields were obtained using the C9920 Hamamatsu integrating sphere system.
2.13.4 Molecular Modeling
All calculations reported in this work were performed using the Q-Chem 5.1 program
21
.
Ground-state optimization calculations were performed using the B3LYP functional and the 6-
311G** basis set. Time dependent density functional theory (TDDFT) calculations were used to
obtain the excitation energies and optimized geometries of the S 1 state at the same level. The S1
energies were corrected by subtracting 0.44 eV to offset the large errors commonly associated
with cyanine-like dyes.
9b

The extent of overlap (Λ) associated with the electronic transition from the ground state to the S1
state was computed using the natural transition orbitals (NTOs) according to the following
expression:
Λ=
∑ 𝜎 𝑘 𝑘 ∫| 𝜙 𝑘 𝑒 | | 𝜙 𝑘 ℎ
| 𝑑𝜏 ∑ 𝜎 𝑘 𝑘
where, 𝜙 𝑘 𝑒 and 𝜙 𝑘 ℎ
are the electron and hole NTO pairs and 𝜎 𝑘 is the amplitude of the
corresponding NTO pair. The value of Λ would be bounded below by 0 for purely CT transitions
with no spatial overlap and bounded above by ≈1 for strongly localized excitations. The
computed Λ values and experimental S1-T1 gaps of the aD series were compared with those of
anthracene
19
and 4CzIPN
20
. The integrals are computed numerically for each NTO pair using the
75

ORBKIT
30
and Cubature
31
python libraries. An in-house python code was used to compute Λ
from the NTOs generated by Q-Chem in Molden format. The source code is available on GitHub
(https://github.com/danielsylvinson/OverlApp) and the pre-built binaries (for Windows only) can
be downloaded from SourceForge (https://sourceforge.net/projects/overlapp). The summary of
the calculated excited state energies (in eV) at S0 and S1 optimized geometries (B3LYP/6-
311G**). The molecular orbital representation of compounds 1a-1c, 2a-2d, 3 and 4 at the
B3LYP/6-311G** level optimized at S0 geometry.
2.13.5 Photophysical Characterization
All samples in fluid solution were dissolved in 2-methyltetrahydrofuran with absorbance
between 0.05-0.15 to prevent reabsorption when using the integrating sphere for  PL
measurements due to the small stoke-shift in the aD series. Doped poly(methyl methacrylate)
thin films were prepared from a solution of poly(methyl methacrylate) (PMMA). 0.1 g of
PMMA pellets were mixed with 1mL of dichloromethane. After all pellets have dissolved, 1
volume percent samples were made with 1, 2 and 3. 1 mg of chosen aD derivative was
dissolved in the PMMA solution and 1mL was spin coated on a quartz substrate (2cm x 2 cm)
using a pipet with the substrate rotating at 700 rpm for 45 seconds. The film was left to air dry.
The UV-visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer.
Steady State fluorescence emission measurements were performed using a QuantaMaster Photon
Technology International spectrofluorometer. Gated phosphorescence measurements were
carried on the fluorimeter with 500 microsecond delay where the sample is in 77 K temperature.
All reported spectra are corrected for photomultiplier response. Fluorescence lifetime
measurements were performed using an IBH Fluorocube instrument equipped with 331 nm LED
76

and 405 nm laser excitation sources using a time-correlated single photon counting method.
Quantum yield values were obtained using a C9920 Hamamatsu integrating sphere system.
2.13.6 OLED Devices
OLEDs were fabricated and tested by Glass substrates with pre-patterned, 1 mm wide
indium tin oxide (ITO) stripes were cleaned by sequential sonication in tergitol, deionized water,
acetone, and isopropanol, followed by 15 min UV ozone exposure. Organic materials and metals
were deposited at rates of 0.5-2 Å/s through shadow masks in a vacuum thermal evaporator with
a base pressure of 10-7 Torr. A separate shadow mask was used to deposit 1 mm wide stripes of
100 nm thick Al films perpendicular to the ITO stripes to form the cathode, resulting in 2 mm
2

device area. The device structure is: glass substrate/70 nm ITO/10 nm
dipyrazino[2,3,-f:20,30-h]quinoxaline 2,3,6,7,10,11-hexacarbonitrile (HATCN)/45 nm 4,4′-
cyclohexylidenebis [N,N-bis(4-methylphenyl)benzenamine] (TAPC)/1 vol% -aD:
4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) host or 1 vol% -aD:
N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD)/45 nm 4,7-Diphenyl-
1,10-phenanthroline (BPhen)/1.5 nm (8-Quinolinolato)lithium (LiQ) /100 nm Al. A
semiconductor parameter analyzer (HP4156A) and a calibrated large area photodiode that
collected all light exiting the glass substrate were used to measure the J-V-luminance
characteristics. The device spectra were measured using a fiber-coupled spectrometer.
2.13.7 Thermogravimetric Analysis
Thermal gravimetric analysis (TGA) was performed on a TGA Q50 instrument and
samples were run in an alumina crucible under a flowing nitrogen atmosphere with a heating rate
of 10C/min.
77

2.14 Chapter 2 References
1. (a) Yersin, H., Triplet Emitters for OLED Applications. Mechanisms of Exciton Trapping
and Control of Emission Properties. In Transition Metal and Rare Earth Compounds: Excited
States, Transitions, Interactions III, Springer Berlin Heidelberg: Berlin, Heidelberg, 2004; pp 1-
26; (b) Pfeiffer, M.; Forrest, S. R.; Leo, K.; Thompson, M. E., Electrophosphorescent p–i–n
Organic Light-Emitting Devices for Very-High-Efficiency Flat-Panel Displays. Advanced
Materials 2002, 14 (22), 1633-1636.
2. (a) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R.
C., High-efficiency red electrophosphorescence devices. Applied Physics Letters 2001, 78 (11),
1622-1624; (b) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M.
E.; Forrest, S. R., Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998, 395 (6698), 151-154; (c) Brooks, J.; Babayan, Y.; Lamansky, S.;
Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E., Synthesis and Characterization of
Phosphorescent Cyclometalated Platinum Complexes. Inorganic Chemistry 2002, 41 (12), 3055-
3066; (d) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.;
Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E., Synthesis and Characterization of
Phosphorescent Cyclometalated Iridium Complexes. Inorganic Chemistry 2001, 40 (7), 1704-
1711; (e) Lamansky, S.; Kwong, R. C.; Nugent, M.; Djurovich, P. I.; Thompson, M. E.,
Molecularly doped polymer light emitting diodes utilizing phosphorescent Pt(II) and Ir(III)
dopants. Organic Electronics 2001, 2 (1), 53-62.
3. (a) Shih-Wen, W.; Meng-Ting, L.; Chen, C. H., Recent development of blue fluorescent
OLED materials and devices. Journal of Display Technology 2005, 1 (1), 90-99; (b) Lee, J.;
Jeong, C.; Batagoda, T.; Coburn, C.; Thompson, M. E.; Forrest, S. R., Hot excited state
management for long-lived blue phosphorescent organic light-emitting diodes. Nature
Communications 2017, 8 (1), 15566; (c) Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich,
V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Endothermic energy transfer: A mechanism
for generating very efficient high-energy phosphorescent emission in organic materials. Applied
Physics Letters 2001, 79 (13), 2082-2084; (d) Zhuang, J.; Li, W.; Su, W.; Liu, Y.; Shen, Q.;
Liao, L.; Zhou, M., Highly efficient phosphorescent organic light-emitting diodes using a
homoleptic iridium(III) complex as a sky-blue dopant. Organic Electronics 2013, 14 (10), 2596-
2601; (e) Klubek, K. P.; Dong, S.-C.; Liao, L.-S.; Tang, C. W.; Rothberg, L. J., Investigating
blue phosphorescent iridium cyclometalated dopant with phenyl-imidazole ligands. Organic
Electronics 2014, 15 (11), 3127-3136; (f) Kang, Y. J.; Lee, J. Y., High triplet energy electron
transport type exciton blocking materials for stable blue phosphorescent organic light-emitting
diodes. Organic Electronics 2016, 32, 109-114; (g) Zhang, L.; Zhang, Y.-X.; Hu, Y.; Shi, X.-B.;
Jiang, Z.-Q.; Wang, Z.-K.; Liao, L.-S., Highly Efficient Blue Phosphorescent Organic Light-
Emitting Diodes Employing a Host Material with Small Bandgap. ACS Applied Materials &
Interfaces 2016, 8 (25), 16186-16191; (h) Seo, J.-A.; Jeon, S. K.; Gong, M. S.; Lee, J. Y.; Noh,
C. H.; Kim, S. H., Long lifetime blue phosphorescent organic light-emitting diodes with an
exciton blocking layer. Journal of Materials Chemistry C 2015, 3 (18), 4640-4645; (i) Jeon, S.
K.; Lee, J. Y., Four times lifetime improvement of blue phosphorescent organic light-emitting
diodes by managing recombination zone. Organic Electronics 2015, 27, 202-206; (j) Giebink,
N.; Dandrade, B.; Weaver, M.; Brown, J.; Forrest, S., Direct evidence for degradation of polaron
78

excited states in organic light emitting diodes. Journal of Applied Physics 2009, 105, 124514-
124514.
4. (a) Reineke, S.; Walzer, K.; Leo, K., Triplet-exciton quenching in organic
phosphorescent light-emitting diodes with Ir-based emitters. Physical Review B 2007, 75 (12),
125328; (b) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E.; A., B. M.; F., O. B. D.;
Y., Y.; A., S.; S., S.; E., T. M.; R., F. S., High-efficiency organic electrophosphorescent devices
with tris(2-phenylpyridine)iridium doped into electron-transporting materials. Applied Physics
Letters 2000, 77 (6), 904-906; (c) Wang, Q.; Aziz, H., Degradation of Organic/Organic
Interfaces in Organic Light-Emitting Devices due to Polaron–Exciton Interactions. ACS Applied
Materials & Interfaces 2013, 5 (17), 8733-8739; (d) Kim, S.; Bae, H. J.; Park, S.; Kim, W.; Kim,
J.; Kim, J. S.; Jung, Y.; Sul, S.; Ihn, S.-G.; Noh, C.; Kim, S.; You, Y., Degradation of blue-
phosphorescent organic light-emitting devices involves exciton-induced generation of polaron
pair within emitting layers. Nature Communications 2018, 9 (1), 1211; (e) Giebink, N.;
D’Andrade, B.; Weaver, M.; Mackenzie, P.; Brown, J.; Thompson, M.; Forrest, S., Intrinsic
luminance loss in phosphorescent small-molecule organic light emitting devices due to
bimolecular annihilation reactions. Journal of Applied Physics 2008, 103, 044509-044509; (f)
Patel, N. K.; Cina, S.; Burroughes, J. H., High-efficiency organic light-emitting diodes. IEEE
Journal of Selected Topics in Quantum Electronics 2002, 8 (2), 346-361.
5. Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R.,
Management of singlet and triplet excitons for efficient white organic light-emitting devices.
Nature 2006, 440 (7086), 908-912.
6. (a) Sun, J. X.; Zhu, X. L.; Peng, H. J.; Wong, M.; Kwok, H. S., Bright and efficient white
stacked organic light-emitting diodes. Org Electron 2007, 8; (b) Sun, N.; Wang, Q.; Zhao, Y.;
Chen, Y.; Yang, D.; Zhao, F.; Chen, J.; Ma, D., High-Performance Hybrid White Organic Light-
Emitting Devices without Interlayer between Fluorescent and Phosphorescent Emissive Regions.
Advanced Materials 2014, 26 (10), 1617-1621; (c) Cho, Y. J.; Yook, K. S.; Lee, J. Y., Cool and
warm hybrid white organic light-emitting diode with blue delayed fluorescent emitter both as
blue emitter and triplet host. Sci Reports 2015, 5; (d) Farinola, G. M.; Ragni, R.,
Electroluminescent materials for white organic light emitting diodes. Chem Soc Rev 2011, 40; (e)
Farinola, G. M.; Ragni, R., Organic emitters for solid state lighting. Journal of Solid State
Lighting 2015, 2 (1), 9; (f) Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R., Recent advances in
white organic light-emitting materials and devices (WOLEDs). Adv Mater 2010, 22; (g) Mazzeo,
M.; Vitale, V.; Della Sala, F.; Anni, M.; Barbarella, G.; Favaretto, L.; Sotgiu, G.; Cingolani, R.;
Gigli, G., Bright white organic light-emitting devices from a single active molecular material.
Adv Mater 2005, 17; (h) Nishide, J.; Nakanotani, H.; Hiraga, Y.; Adachi, C., High-efficiency
white organic light-emitting diodes using thermally activated delayed fluorescence. Appl Phys
Lett 2014, 104; (i) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.;
Leo, K., White organic light-emitting diodes with fluorescent tube efficiency. Nature 2009, 459;
(j) Reineke, S.; Thomschke, M.; Lüssem, B.; Leo, K., White organic light-emitting diodes: Status
and perspective. Rev. Mod. Phys. 2013, 85 (3), 1245-1293.
7. Peumans, P.; Yakimov, A.; Forrest, S. R., Small molecular weight organic thin-film
photodetectors and solar cells. J. Appl. Phys. 2003, 93 (7), 3693-3723.
8. (a) Loudet, A.; Burgess, K., Bodipy Dyes and Their Derivatives: Syntheses and
Spectroscopic Properties. Chem. Rev. 2007, 107 (11), 4891; (b) Chen, J. J.; Conron, S. M.;
79

Erwin, P.; Dimitriou, M.; McAlahney, K.; Thompson, M. E., High-Efficiency BODIPY-Based
Organic Photovoltaics. ACS Applied Materials & Interfaces 2015, 7 (1), 662-669; (c) Benstead,
M.; Mehl, G. H.; Boyle, R. W., 4,4 ′-Difluoro-4-Bora-3a,4a-Diaza-S-Indacenes (Bodipys) as
Components of Novel Light Active Materials. Tetrahedron 2011, 67 (20), 3573; (d) Bura, T.;
Leclerc, N.; Fall, S.; Lévêque, P.; Heiser, T.; Retailleau, P.; Rihn, S.; Mirloup, A.; Ziessel, R.,
High-Performance Solution-Processed Solar Cells and Ambipolar Behavior in Organic Field-
Effect Transistors with Thienyl-Bodipy Scaffoldings. J. Am. Chem. Soc. 2012, 134 (42), 17404;
(e) Deng, Y.; Cheng, Y.-y.; Liu, H.; Mack, J.; Lu, H.; Zhu, L.-g., Asymmetrical aza-boron-
dipyridomethene derivatives with large Stokes shifts: synthesis and spectroscopic properties.
Tetrahedron Letters 2014, 55 (28), 3792-3796; (f) Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew,
L. V.; Chung, L. Y.; Burgess, K., BODIPY Dyes In Photodynamic Therapy. Chemical Society
reviews 2013, 42 (1), 10.1039/c2cs35216h; (g) Karolin, J.; Johansson, L. B. A.; Strandberg, L.;
Ny, T., Fluorescence and Absorption Spectroscopic Properties of Dipyrrometheneboron
Difluoride (BODIPY) Derivatives in Liquids, Lipid Membranes, and Proteins. Journal of the
American Chemical Society 1994, 116 (17), 7801-7806; (h) Kim, B.; Ma, B.; Donuru, V. R.; Liu,
H.; Frechet, J. M. J., Bodipy-Backboned Polymers as Electron Donor in Bulk Heterojunction
Solar Cells. Chem. Commun. 2010, 46 (23), 4148; (i) Kolemen, S.; Bozdemir, O. A.; Cakmak,
Y.; Barin, G.; Erten-Ela, S.; Marszalek, M.; Yum, J. H.; Zakeeruddin, S. M.; Nazeeruddin, M.
K.; Gratzel, M.; Akkaya, E. U., Optimization of Distyryl-Bodipy Chromophores for Efficient
Panchromatic Sensitization in Dye Sensitized Solar Cells. Chem. Sci. 2011, 2 (5), 949; (j)
Kowada, T.; Yamaguchi, S.; Fujinaga, H.; Ohe, K., Near-Infrared Bodipy Dyes Modulated with
Spirofluorene Moieties. Tetrahedron 2011, 67 (17), 3105; (k) Kurata, S.; Kanagawa, T.;
Yamada, K.; Torimura, M.; Yokomaku, T.; Kamagata, Y.; Kurane, R., Fluorescent quenching-
based quantitative detection of specific DNA/RNA using a BODIPY(®) FL-labeled probe or
primer. Nucleic Acids Research 2001, 29 (6), e34-e34; (l) Momeni, M. R.; Brown, A., Why Do
TD-DFT Excitation Energies of BODIPY/Aza-BODIPY Families Largely Deviate from
Experiment? Answers from Electron Correlated and Multireference Methods. Journal of
Chemical Theory and Computation 2015, 11 (6), 2619-2632; (m) Orte, A.; Debroye, E.; Ruedas-
Rama, M. J.; Garcia-Fernandez, E.; Robinson, D.; Crovetto, L.; Talavera, E. M.; Alvarez-Pez, J.
M.; Leen, V.; Verbelen, B.; Cunha Dias de Rezende, L.; Dehaen, W.; Hofkens, J.; Van der
Auweraer, M.; Boens, N., Effect of the substitution position (2, 3 or 8) on the spectroscopic and
photophysical properties of BODIPY dyes with a phenyl, styryl or phenylethynyl group. RSC
Advances 2016, 6 (105), 102899-102913; (n) Schmitt, A.; Hinkeldey, B.; Wild, M.; Jung, G.,
Synthesis of the Core Compound of the BODIPY Dye Class: 4,4 ′-Difluoro-4-bora-(3a,4a)-
diaza-s-indacene. Journal of Fluorescence 2009, 19 (4), 755-758; (o) Sutter, A.; Retailleau, P.;
Huang, W.-C.; Lin, H.-W.; Ziessel, R., Photovoltaic performance of novel push-pull-push
thienyl-Bodipy dyes in solution-processed BHJ-solar cells. New Journal of Chemistry 2014, 38
(4), 1701-1710; (p) Ulrich, G.; Ziessel, R.; Harriman, A., The Chemistry of Fluorescent Bodipy
Dyes: Versatility Unsurpassed. Angewandte Chemie International Edition 2008, 47 (7), 1184-
1201; (q) Zhang, X.-F.; Zhu, J., BODIPY parent compound: Excited triplet state and singlet
oxygen formation exhibit strong molecular oxygen enhancing effect. Journal of Luminescence
2019, 212, 286-292; (r) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F., The triplet excited
state of Bodipy: formation, modulation and application. Chemical Society Reviews 2015, 44 (24),
8904-8939.
9. (a) Bañuelos, J.; Arbeloa, F. L.; Martinez, V.; Liras, M.; Costela, A.; Moreno, I. G.;
Arbeloa, I. L., Difluoro-boron-triaza-anthracene: a laser dye in the blue region. Theoretical
80

simulation of alternative difluoro-boron-diaza-aromatic systems. Physical Chemistry Chemical
Physics 2011, 13 (8), 3437-3445; (b) Golden, J. H.; Facendola, J. W.; Sylvinson M. R, D.; Baez,
C. Q.; Djurovich, P. I.; Thompson, M. E., Boron Dipyridylmethene (DIPYR) Dyes: Shedding
Light on Pyridine-Based Chromophores. The Journal of organic chemistry 2017, 82 (14), 7215-
7222; (c) Boyer, J. H.; Haag, A. M.; Sathyamoorthi, G.; Soong, M.-L.; Thangaraj, K.;
Pavlopoulos, T. G., Pyrromethene–BF2 complexes as laser dyes: 2. Heteroatom Chemistry 1993,
4 (1), 39-49; (d) Sathyamoorthi, G.; Soong, M.-L.; Ross, T. W.; Boyer, J. H., Fluorescent
tricyclic β-azavinamidine–BF2 complexes. Heteroatom Chemistry 1993, 4 (6), 603-608.
10. Kondo, Y.; Yoshiura, K.; Kitera, S.; Nishi, H.; Oda, S.; Gotoh, H.; Sasada, Y.; Yanai, M.;
Hatakeyama, T., Narrowband deep-blue organic light-emitting diode featuring an organoboron-
based emitter. Nature Photonics 2019.
11. Araneda, J. F.; Piers, W. E.; Heyne, B.; Parvez, M.; McDonald, R., High Stokes Shift
Anilido-Pyridine Boron Difluoride Dyes. Angewandte Chemie International Edition 2011, 50
(51), 12214-12217.
12. (a) Quan, L.; Chen, Y.; Lv, X. J.; Fu, W. F., Aggregation-induced photoluminescent
changes of naphthyridine-BF2 complexes. Chemistry (Weinheim an der Bergstrasse, Germany)
2012, 18 (46), 14599-604; (b) Yang, Y.; Su, X.; Carroll, C. N.; Aprahamian, I., Aggregation-
induced emission in BF2–hydrazone (BODIHY) complexes. Chemical Science 2012, 3 (2), 610-
613.
13. (a) Wang, D.; Liu, R.; Chen, C.; Wang, S.; Chang, J.; Wu, C.; Zhu, H.; Waclawik, E. R.,
Synthesis, photophysical and electrochemical properties of aza-boron-diquinomethene
complexes. Dyes and Pigments 2013, 99 (1), 240-249; (b) Zhu, X.; Huang, H.; Liu, R.; Jin, X.;
Li, Y.; Wang, D.; Wang, Q.; Zhu, H., Aza-boron-diquinomethene complexes bearing N-aryl
chromophores: synthesis, crystal structures, tunable photophysics, the protonation effect and
their application as pH sensors. Journal of Materials Chemistry C 2015, 3 (15), 3774-3782.
14. Kondakova, M. E.; Deaton, J. C.; Pawlik, T. D.; Giesen, D. J.; Kondakov, D. Y.; Young,
R. H.; Royster, T. L.; Comfort, D. L.; Shore, J. D.; R., D. A.; J., S. J.; M., H. C.; F., F. D., Highly
efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes.
Journal of Applied Physics 2010, 107 (1), 014515.
15. (a) Kondo, Y.; Yoshiura, K.; Kitera, S.; Nishi, H.; Oda, S.; Gotoh, H.; Sasada, Y.; Yanai,
M.; Hatakeyama, T., Narrowband deep-blue organic light-emitting diode featuring an
organoboron-based emitter. Nature Photonics 2019, 13; (b) Oda, S.; Kawakami, B.; Kawasumi,
R.; Okita, R.; Hatakeyama, T., Multiple Resonance Effect-Induced Sky-Blue Thermally
Activated Delayed Fluorescence with a Narrow Emission Band. Organic Letters 2019, 21 (23),
9311-9314.
16. (a) Ding, H.; Yu, H.; Dong, Y.; Tian, R.; Huang, G.; Boothman, D. A.; Sumer, B. D.;
Gao, J., Photoactivation Switch from Type II to Type I Reactions by Electron-Rich Micelles for
Improved Photodynamic Therapy of Cancer Cells Under Hypoxia. Journal of controlled release
: official journal of the Controlled Release Society 2011, 156 (3), 276-280; (b) Stopel, M. H. W.;
Blum, C.; Subramaniam, V., Excitation Spectra and Stokes Shift Measurements of Single
Organic Dyes at Room Temperature. The Journal of Physical Chemistry Letters 2014, 5 (18),
3259-3264; (c) Sissa, C.; Painelli, A.; Terenziani, F.; Trotta, M.; Ragni, R., About the origin of
81

the large Stokes shift in aminoalkyl substituted heptamethine cyanine dyes. Physical Chemistry
Chemical Physics 2020, 22 (1), 129-135.
17. (a) Berezin, M. Y.; Achilefu, S., Fluorescence lifetime measurements and biological
imaging. Chemical reviews 2010, 110 (5), 2641-2684; (b) Ostroverkhova, O., Organic
Optoelectronic Materials: Mechanisms and Applications. Chemical Reviews 2016, 116 (22),
13279-13412.
18. Charaf-Eddin, A.; Le Guennic, B.; Jacquemin, D., Excited-states of BODIPY–cyanines:
ultimate TD-DFT challenges? RSC Advances 2014, 4 (90), 49449-49456.
19. Schols, S.; Kadashchuk, A.; Heremans, P.; Helfer, A.; Scherf, U., Triplet excitation
scavenging as method to control the triplet concentration. Proceedings of SPIE - The
International Society for Optical Engineering 2009, 7415.
20. (a) Nakanotani, H.; Masui, K.; Nishide, J.; Shibata, T.; Adachi, C., Promising operational
stability of high-efficiency organic light-emitting diodes based on thermally activated delayed
fluorescence. Scientific reports 2013, 3, 2127-2127; (b) Cho, Y. R.; Cha, S. J.; Suh, M. C., Ideal
combination of the host and dopant materials showing thermally activated delayed fluorescent
behavior. Synthetic Metals 2015, 209, 47-54.
21. (a) Zhang, X. H.; Liu, M. W.; Wong, O. Y.; Lee, C. S.; Kwong, H. L.; Lee, S. T.; Wu, S.
K., Blue and white organic electroluminescent devices based on 9,10-bis(2 ′-
naphthyl)anthracene. Chemical Physics Letters 2003, 369 (3), 478-482; (b) Slyke, S. A. V.;
Chen, C. H.; Tang, C. W., Organic electroluminescent devices with improved stability. Applied
Physics Letters 1996, 69 (15), 2160-2162; (c) Guo, F.; Ma, D.; Wang, L.; Jing, X.; Wang, F.,
High efficiency white organic light-emitting devices by effectively controlling exciton
recombination region. Semiconductor Science and Technology 2005, 20 (3), 310-313.
22. Kumar, M.; Pereira, L., Effect of the Host on Deep-Blue Organic Light-Emitting Diodes
Based on a TADF Emitter for Roll-Off Suppressing. Nanomaterials (Basel, Switzerland) 2019, 9
(9), 1307.
23. Su, S.-J.; Sasabe, H.; Takeda, T.; Kido, J., Pyridine-Containing Bipolar Host Materials
for Highly Efficient Blue Phosphorescent OLEDs. Chemistry of Materials 2008, 20 (5), 1691-
1693.
24. (a) Bagnich, S. A.; Athanasopoulos, S.; Rudnick, A.; Schroegel, P.; Bauer, I.; Greenham,
N. C.; Strohriegl, P.; Köhler, A., Excimer Formation by Steric Twisting in Carbazole and
Triphenylamine-Based Host Materials. The Journal of Physical Chemistry C 2015, 119 (5),
2380-2387; (b) Bagnich, S. A.; Rudnick, A.; Schroegel, P.; Strohriegl, P.; Köhler, A., Triplet
energies and excimer formation in meta- and para-linked carbazolebiphenyl matrix materials.
Philos Trans A Math Phys Eng Sci 2015, 373 (2044), 20140446.
25. Yoshida, H.; Yoshizaki, K., Electron affinities of organic materials used for organic light-
emitting diodes: A low-energy inverse photoemission study. Organic Electronics 2015, 20, 24-
30.

82

1. (a) Yersin, H., Triplet Emitters for OLED Applications. Mechanisms of Exciton Trapping
and Control of Emission Properties. In Transition Metal and Rare Earth Compounds: Excited
States, Transitions, Interactions III, Springer Berlin Heidelberg: Berlin, Heidelberg, 2004; pp 1-
26; (b) Pfeiffer, M.; Forrest, S. R.; Leo, K.; Thompson, M. E., Electrophosphorescent p–i–n
Organic Light-Emitting Devices for Very-High-Efficiency Flat-Panel Displays. Advanced
Materials 2002, 14 (22), 1633-1636.
2. (a) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R.
C., High-efficiency red electrophosphorescence devices. Applied Physics Letters 2001, 78 (11),
1622-1624; (b) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M.
E.; Forrest, S. R., Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998, 395 (6698), 151-154; (c) Brooks, J.; Babayan, Y.; Lamansky, S.;
Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E., Synthesis and Characterization of
Phosphorescent Cyclometalated Platinum Complexes. Inorganic Chemistry 2002, 41 (12), 3055-
3066; (d) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.;
Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E., Synthesis and Characterization of
Phosphorescent Cyclometalated Iridium Complexes. Inorganic Chemistry 2001, 40 (7), 1704-
1711; (e) Lamansky, S.; Kwong, R. C.; Nugent, M.; Djurovich, P. I.; Thompson, M. E.,
Molecularly doped polymer light emitting diodes utilizing phosphorescent Pt(II) and Ir(III)
dopants. Organic Electronics 2001, 2 (1), 53-62.
3. (a) Shih-Wen, W.; Meng-Ting, L.; Chen, C. H., Recent development of blue fluorescent
OLED materials and devices. Journal of Display Technology 2005, 1 (1), 90-99; (b) Lee, J.;
Jeong, C.; Batagoda, T.; Coburn, C.; Thompson, M. E.; Forrest, S. R., Hot excited state
management for long-lived blue phosphorescent organic light-emitting diodes. Nature
Communications 2017, 8 (1), 15566; (c) Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich,
V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Endothermic energy transfer: A mechanism
for generating very efficient high-energy phosphorescent emission in organic materials. Applied
Physics Letters 2001, 79 (13), 2082-2084; (d) Zhuang, J.; Li, W.; Su, W.; Liu, Y.; Shen, Q.;
Liao, L.; Zhou, M., Highly efficient phosphorescent organic light-emitting diodes using a
homoleptic iridium(III) complex as a sky-blue dopant. Organic Electronics 2013, 14 (10), 2596-
2601; (e) Klubek, K. P.; Dong, S.-C.; Liao, L.-S.; Tang, C. W.; Rothberg, L. J., Investigating
blue phosphorescent iridium cyclometalated dopant with phenyl-imidazole ligands. Organic
Electronics 2014, 15 (11), 3127-3136; (f) Kang, Y. J.; Lee, J. Y., High triplet energy electron
transport type exciton blocking materials for stable blue phosphorescent organic light-emitting
diodes. Organic Electronics 2016, 32, 109-114; (g) Zhang, L.; Zhang, Y.-X.; Hu, Y.; Shi, X.-B.;
Jiang, Z.-Q.; Wang, Z.-K.; Liao, L.-S., Highly Efficient Blue Phosphorescent Organic Light-
Emitting Diodes Employing a Host Material with Small Bandgap. ACS Applied Materials &
Interfaces 2016, 8 (25), 16186-16191; (h) Seo, J.-A.; Jeon, S. K.; Gong, M. S.; Lee, J. Y.; Noh,
C. H.; Kim, S. H., Long lifetime blue phosphorescent organic light-emitting diodes with an
exciton blocking layer. Journal of Materials Chemistry C 2015, 3 (18), 4640-4645; (i) Jeon, S.
K.; Lee, J. Y., Four times lifetime improvement of blue phosphorescent organic light-emitting
diodes by managing recombination zone. Organic Electronics 2015, 27, 202-206; (j) Giebink,
N.; Dandrade, B.; Weaver, M.; Brown, J.; Forrest, S., Direct evidence for degradation of polaron
excited states in organic light emitting diodes. Journal of Applied Physics 2009, 105, 124514-
124514.
83

4. (a) Reineke, S.; Walzer, K.; Leo, K., Triplet-exciton quenching in organic
phosphorescent light-emitting diodes with Ir-based emitters. Physical Review B 2007, 75 (12),
125328; (b) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E.; A., B. M.; F., O. B. D.;
Y., Y.; A., S.; S., S.; E., T. M.; R., F. S., High-efficiency organic electrophosphorescent devices
with tris(2-phenylpyridine)iridium doped into electron-transporting materials. Applied Physics
Letters 2000, 77 (6), 904-906; (c) Wang, Q.; Aziz, H., Degradation of Organic/Organic
Interfaces in Organic Light-Emitting Devices due to Polaron–Exciton Interactions. ACS Applied
Materials & Interfaces 2013, 5 (17), 8733-8739; (d) Kim, S.; Bae, H. J.; Park, S.; Kim, W.; Kim,
J.; Kim, J. S.; Jung, Y.; Sul, S.; Ihn, S.-G.; Noh, C.; Kim, S.; You, Y., Degradation of blue-
phosphorescent organic light-emitting devices involves exciton-induced generation of polaron
pair within emitting layers. Nature Communications 2018, 9 (1), 1211; (e) Giebink, N.;
D’Andrade, B.; Weaver, M.; Mackenzie, P.; Brown, J.; Thompson, M.; Forrest, S., Intrinsic
luminance loss in phosphorescent small-molecule organic light emitting devices due to
bimolecular annihilation reactions. Journal of Applied Physics 2008, 103, 044509-044509; (f)
Patel, N. K.; Cina, S.; Burroughes, J. H., High-efficiency organic light-emitting diodes. IEEE
Journal of Selected Topics in Quantum Electronics 2002, 8 (2), 346-361.
5. Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R.,
Management of singlet and triplet excitons for efficient white organic light-emitting devices.
Nature 2006, 440 (7086), 908-912.
6. (a) Sun, J. X.; Zhu, X. L.; Peng, H. J.; Wong, M.; Kwok, H. S., Bright and efficient white
stacked organic light-emitting diodes. Org Electron 2007, 8; (b) Sun, N.; Wang, Q.; Zhao, Y.;
Chen, Y.; Yang, D.; Zhao, F.; Chen, J.; Ma, D., High-Performance Hybrid White Organic Light-
Emitting Devices without Interlayer between Fluorescent and Phosphorescent Emissive Regions.
Advanced Materials 2014, 26 (10), 1617-1621; (c) Cho, Y. J.; Yook, K. S.; Lee, J. Y., Cool and
warm hybrid white organic light-emitting diode with blue delayed fluorescent emitter both as
blue emitter and triplet host. Sci Reports 2015, 5; (d) Farinola, G. M.; Ragni, R.,
Electroluminescent materials for white organic light emitting diodes. Chem Soc Rev 2011, 40; (e)
Farinola, G. M.; Ragni, R., Organic emitters for solid state lighting. Journal of Solid State
Lighting 2015, 2 (1), 9; (f) Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R., Recent advances in
white organic light-emitting materials and devices (WOLEDs). Adv Mater 2010, 22; (g) Mazzeo,
M.; Vitale, V.; Della Sala, F.; Anni, M.; Barbarella, G.; Favaretto, L.; Sotgiu, G.; Cingolani, R.;
Gigli, G., Bright white organic light-emitting devices from a single active molecular material.
Adv Mater 2005, 17; (h) Nishide, J.; Nakanotani, H.; Hiraga, Y.; Adachi, C., High-efficiency
white organic light-emitting diodes using thermally activated delayed fluorescence. Appl Phys
Lett 2014, 104; (i) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.;
Leo, K., White organic light-emitting diodes with fluorescent tube efficiency. Nature 2009, 459;
(j) Reineke, S.; Thomschke, M.; Lüssem, B.; Leo, K., White organic light-emitting diodes: Status
and perspective. Reviews of Modern Physics 2013, 85 (3), 1245-1293.
7. Peumans, P.; Yakimov, A.; Forrest, S. R., Small molecular weight organic thin-film
photodetectors and solar cells. J. Appl. Phys. 2003, 93 (7), 3693-3723.
8. (a) Loudet, A.; Burgess, K., Bodipy Dyes and Their Derivatives: Syntheses and
Spectroscopic Properties. Chem. Rev. 2007, 107 (11), 4891; (b) Chen, J. J.; Conron, S. M.;
Erwin, P.; Dimitriou, M.; McAlahney, K.; Thompson, M. E., High-Efficiency BODIPY-Based
Organic Photovoltaics. ACS Applied Materials & Interfaces 2015, 7 (1), 662-669; (c) Benstead,
84

M.; Mehl, G. H.; Boyle, R. W., 4,4 ′-Difluoro-4-Bora-3a,4a-Diaza-S-Indacenes (Bodipys) as
Components of Novel Light Active Materials. Tetrahedron 2011, 67 (20), 3573; (d) Bura, T.;
Leclerc, N.; Fall, S.; Lévêque, P.; Heiser, T.; Retailleau, P.; Rihn, S.; Mirloup, A.; Ziessel, R.,
High-Performance Solution-Processed Solar Cells and Ambipolar Behavior in Organic Field-
Effect Transistors with Thienyl-Bodipy Scaffoldings. J. Am. Chem. Soc. 2012, 134 (42), 17404;
(e) Deng, Y.; Cheng, Y.-y.; Liu, H.; Mack, J.; Lu, H.; Zhu, L.-g., Asymmetrical aza-boron-
dipyridomethene derivatives with large Stokes shifts: synthesis and spectroscopic properties.
Tetrahedron Letters 2014, 55 (28), 3792-3796; (f) Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew,
L. V.; Chung, L. Y.; Burgess, K., BODIPY Dyes In Photodynamic Therapy. Chemical Society
reviews 2013, 42 (1), 10.1039/c2cs35216h; (g) Karolin, J.; Johansson, L. B. A.; Strandberg, L.;
Ny, T., Fluorescence and Absorption Spectroscopic Properties of Dipyrrometheneboron
Difluoride (BODIPY) Derivatives in Liquids, Lipid Membranes, and Proteins. Journal of the
American Chemical Society 1994, 116 (17), 7801-7806; (h) Kim, B.; Ma, B.; Donuru, V. R.; Liu,
H.; Frechet, J. M. J., Bodipy-Backboned Polymers as Electron Donor in Bulk Heterojunction
Solar Cells. Chem. Commun. 2010, 46 (23), 4148; (i) Kolemen, S.; Bozdemir, O. A.; Cakmak,
Y.; Barin, G.; Erten-Ela, S.; Marszalek, M.; Yum, J. H.; Zakeeruddin, S. M.; Nazeeruddin, M.
K.; Gratzel, M.; Akkaya, E. U., Optimization of Distyryl-Bodipy Chromophores for Efficient
Panchromatic Sensitization in Dye Sensitized Solar Cells. Chem. Sci. 2011, 2 (5), 949; (j)
Kowada, T.; Yamaguchi, S.; Fujinaga, H.; Ohe, K., Near-Infrared Bodipy Dyes Modulated with
Spirofluorene Moieties. Tetrahedron 2011, 67 (17), 3105; (k) Kurata, S.; Kanagawa, T.;
Yamada, K.; Torimura, M.; Yokomaku, T.; Kamagata, Y.; Kurane, R., Fluorescent quenching-
based quantitative detection of specific DNA/RNA using a BODIPY(®) FL-labeled probe or
primer. Nucleic Acids Research 2001, 29 (6), e34-e34; (l) Momeni, M. R.; Brown, A., Why Do
TD-DFT Excitation Energies of BODIPY/Aza-BODIPY Families Largely Deviate from
Experiment? Answers from Electron Correlated and Multireference Methods. Journal of
Chemical Theory and Computation 2015, 11 (6), 2619-2632; (m) Orte, A.; Debroye, E.; Ruedas-
Rama, M. J.; Garcia-Fernandez, E.; Robinson, D.; Crovetto, L.; Talavera, E. M.; Alvarez-Pez, J.
M.; Leen, V.; Verbelen, B.; Cunha Dias de Rezende, L.; Dehaen, W.; Hofkens, J.; Van der
Auweraer, M.; Boens, N., Effect of the substitution position (2, 3 or 8) on the spectroscopic and
photophysical properties of BODIPY dyes with a phenyl, styryl or phenylethynyl group. RSC
Advances 2016, 6 (105), 102899-102913; (n) Schmitt, A.; Hinkeldey, B.; Wild, M.; Jung, G.,
Synthesis of the Core Compound of the BODIPY Dye Class: 4,4 ′-Difluoro-4-bora-(3a,4a)-
diaza-s-indacene. Journal of Fluorescence 2009, 19 (4), 755-758; (o) Sutter, A.; Retailleau, P.;
Huang, W.-C.; Lin, H.-W.; Ziessel, R., Photovoltaic performance of novel push-pull-push
thienyl-Bodipy dyes in solution-processed BHJ-solar cells. New Journal of Chemistry 2014, 38
(4), 1701-1710; (p) Ulrich, G.; Ziessel, R.; Harriman, A., The Chemistry of Fluorescent Bodipy
Dyes: Versatility Unsurpassed. Angewandte Chemie International Edition 2008, 47 (7), 1184-
1201; (q) Zhang, X.-F.; Zhu, J., BODIPY parent compound: Excited triplet state and singlet
oxygen formation exhibit strong molecular oxygen enhancing effect. Journal of Luminescence
2019, 212, 286-292; (r) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F., The triplet excited
state of Bodipy: formation, modulation and application. Chemical Society Reviews 2015, 44 (24),
8904-8939.
9. (a) Bañuelos, J.; Arbeloa, F. L.; Martinez, V.; Liras, M.; Costela, A.; Moreno, I. G.;
Arbeloa, I. L., Difluoro-boron-triaza-anthracene: a laser dye in the blue region. Theoretical
simulation of alternative difluoro-boron-diaza-aromatic systems. Physical Chemistry Chemical
Physics 2011, 13 (8), 3437-3445; (b) Golden, J. H.; Facendola, J. W.; Sylvinson M. R, D.; Baez,
85

C. Q.; Djurovich, P. I.; Thompson, M. E., Boron Dipyridylmethene (DIPYR) Dyes: Shedding
Light on Pyridine-Based Chromophores. The Journal of organic chemistry 2017, 82 (14), 7215-
7222; (c) Boyer, J. H.; Haag, A. M.; Sathyamoorthi, G.; Soong, M.-L.; Thangaraj, K.;
Pavlopoulos, T. G., Pyrromethene–BF2 complexes as laser dyes: 2. Heteroatom Chemistry 1993,
4 (1), 39-49; (d) Sathyamoorthi, G.; Soong, M.-L.; Ross, T. W.; Boyer, J. H., Fluorescent
tricyclic β-azavinamidine–BF2 complexes. Heteroatom Chemistry 1993, 4 (6), 603-608.
10. Kondo, Y.; Yoshiura, K.; Kitera, S.; Nishi, H.; Oda, S.; Gotoh, H.; Sasada, Y.; Yanai, M.;
Hatakeyama, T., Narrowband deep-blue organic light-emitting diode featuring an organoboron-
based emitter. Nature Photonics 2019.
11. Araneda, J. F.; Piers, W. E.; Heyne, B.; Parvez, M.; McDonald, R., High Stokes Shift
Anilido-Pyridine Boron Difluoride Dyes. Angewandte Chemie International Edition 2011, 50
(51), 12214-12217.
12. (a) Quan, L.; Chen, Y.; Lv, X. J.; Fu, W. F., Aggregation-induced photoluminescent
changes of naphthyridine-BF2 complexes. Chemistry (Weinheim an der Bergstrasse, Germany)
2012, 18 (46), 14599-604; (b) Yang, Y.; Su, X.; Carroll, C. N.; Aprahamian, I., Aggregation-
induced emission in BF2–hydrazone (BODIHY) complexes. Chemical Science 2012, 3 (2), 610-
613.
13. (a) Wang, D.; Liu, R.; Chen, C.; Wang, S.; Chang, J.; Wu, C.; Zhu, H.; Waclawik, E. R.,
Synthesis, photophysical and electrochemical properties of aza-boron-diquinomethene
complexes. Dyes and Pigments 2013, 99 (1), 240-249; (b) Zhu, X.; Huang, H.; Liu, R.; Jin, X.;
Li, Y.; Wang, D.; Wang, Q.; Zhu, H., Aza-boron-diquinomethene complexes bearing N-aryl
chromophores: synthesis, crystal structures, tunable photophysics, the protonation effect and
their application as pH sensors. Journal of Materials Chemistry C 2015, 3 (15), 3774-3782.
14. Kondakova, M. E.; Deaton, J. C.; Pawlik, T. D.; Giesen, D. J.; Kondakov, D. Y.; Young,
R. H.; Royster, T. L.; Comfort, D. L.; Shore, J. D.; R., D. A.; J., S. J.; M., H. C.; F., F. D., Highly
efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes.
Journal of Applied Physics 2010, 107 (1), 014515.
15. (a) Kondo, Y.; Yoshiura, K.; Kitera, S.; Nishi, H.; Oda, S.; Gotoh, H.; Sasada, Y.; Yanai,
M.; Hatakeyama, T., Narrowband deep-blue organic light-emitting diode featuring an
organoboron-based emitter. Nature Photonics 2019, 13; (b) Oda, S.; Kawakami, B.; Kawasumi,
R.; Okita, R.; Hatakeyama, T., Multiple Resonance Effect-Induced Sky-Blue Thermally
Activated Delayed Fluorescence with a Narrow Emission Band. Organic Letters 2019, 21 (23),
9311-9314.
16. (a) Ding, H.; Yu, H.; Dong, Y.; Tian, R.; Huang, G.; Boothman, D. A.; Sumer, B. D.;
Gao, J., Photoactivation Switch from Type II to Type I Reactions by Electron-Rich Micelles for
Improved Photodynamic Therapy of Cancer Cells Under Hypoxia. Journal of controlled release
: official journal of the Controlled Release Society 2011, 156 (3), 276-280; (b) Stopel, M. H. W.;
Blum, C.; Subramaniam, V., Excitation Spectra and Stokes Shift Measurements of Single
Organic Dyes at Room Temperature. The Journal of Physical Chemistry Letters 2014, 5 (18),
3259-3264; (c) Sissa, C.; Painelli, A.; Terenziani, F.; Trotta, M.; Ragni, R., About the origin of
the large Stokes shift in aminoalkyl substituted heptamethine cyanine dyes. Physical Chemistry
Chemical Physics 2020, 22 (1), 129-135.
86

17. (a) Berezin, M. Y.; Achilefu, S., Fluorescence lifetime measurements and biological
imaging. Chemical reviews 2010, 110 (5), 2641-2684; (b) Ostroverkhova, O., Organic
Optoelectronic Materials: Mechanisms and Applications. Chemical Reviews 2016, 116 (22),
13279-13412.
18. Charaf-Eddin, A.; Le Guennic, B.; Jacquemin, D., Excited-states of BODIPY–cyanines:
ultimate TD-DFT challenges? RSC Advances 2014, 4 (90), 49449-49456.
19. Schols, S.; Kadashchuk, A.; Heremans, P.; Helfer, A.; Scherf, U., Triplet excitation
scavenging as method to control the triplet concentration. Proceedings of SPIE - The
International Society for Optical Engineering 2009, 7415.
20. (a) Nakanotani, H.; Masui, K.; Nishide, J.; Shibata, T.; Adachi, C., Promising operational
stability of high-efficiency organic light-emitting diodes based on thermally activated delayed
fluorescence. Scientific reports 2013, 3, 2127-2127; (b) Cho, Y. R.; Cha, S. J.; Suh, M. C., Ideal
combination of the host and dopant materials showing thermally activated delayed fluorescent
behavior. Synthetic Metals 2015, 209, 47-54.
21. Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange,
A. W.; Behn, A.; Deng, J.; Feng, X.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.;
Kaliman, I.; Khaliullin, R. Z.; Kuś, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard,
R. M.; Rohrdanz, M. A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.;
Zuev, D.; Albrecht, B.; Alguire, E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.;
Brandhorst, K.; Bravaya, K. B.; Brown, S. T.; Casanova, D.; Chang, C.-M.; Chen, Y.; Chien, S.
H.; Closser, K. D.; Crittenden, D. L.; Diedenhofen, M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.;
Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.; Ghysels, A.; Golubeva-Zadorozhnaya, A.; Gomes, J.;
Hanson-Heine, M. W. D.; Harbach, P. H. P.; Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.;
Jagau, T.-C.; Ji, H.; Kaduk, B.; Khistyaev, K.; Kim, J.; Kim, J.; King, R. A.; Klunzinger, P.;
Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.; Lawler, K. V.;
Levchenko, S. V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar, P.;
Manzer, S. F.; Mao, S.-P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.;
Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O’Neill, D. P.; Parkhill, J. A.; Perrine, T.
M.; Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.;
Small, D. W.; Sodt, A.; Stein, T.; Stück, D.; Su, Y.-C.; Thom, A. J. W.; Tsuchimochi, T.;
Vanovschi, V.; Vogt, L.; Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel, J.; White, A.;
Williams, C. F.; Yang, J.; Yeganeh, S.; Yost, S. R.; You, Z.-Q.; Zhang, I. Y.; Zhang, X.; Zhao,
Y.; Brooks, B. R.; Chan, G. K. L.; Chipman, D. M.; Cramer, C. J.; Goddard, W. A.; Gordon, M.
S.; Hehre, W. J.; Klamt, A.; Schaefer, H. F.; Schmidt, M. W.; Sherrill, C. D.; Truhlar, D. G.;
Warshel, A.; Xu, X.; Aspuru-Guzik, A.; Baer, R.; Bell, A. T.; Besley, N. A.; Chai, J.-D.; Dreuw,
A.; Dunietz, B. D.; Furlani, T. R.; Gwaltney, S. R.; Hsu, C.-P.; Jung, Y.; Kong, J.; Lambrecht, D.
S.; Liang, W.; Ochsenfeld, C.; Rassolov, V. A.; Slipchenko, L. V.; Subotnik, J. E.; Van Voorhis,
T.; Herbert, J. M.; Krylov, A. I.; Gill, P. M. W.; Head-Gordon, M., Advances in molecular
quantum chemistry contained in the Q-Chem 4 program package. Molecular Physics 2015, 113
(2), 184-215.
22. (a) Zhang, X. H.; Liu, M. W.; Wong, O. Y.; Lee, C. S.; Kwong, H. L.; Lee, S. T.; Wu, S.
K., Blue and white organic electroluminescent devices based on 9,10-bis(2 ′-
naphthyl)anthracene. Chemical Physics Letters 2003, 369 (3), 478-482; (b) Slyke, S. A. V.;
87

Chen, C. H.; Tang, C. W., Organic electroluminescent devices with improved stability. Applied
Physics Letters 1996, 69 (15), 2160-2162; (c) Guo, F.; Ma, D.; Wang, L.; Jing, X.; Wang, F.,
High efficiency white organic light-emitting devices by effectively controlling exciton
recombination region. Semiconductor Science and Technology 2005, 20 (3), 310-313.
23. Kumar, M.; Pereira, L., Effect of the Host on Deep-Blue Organic Light-Emitting Diodes
Based on a TADF Emitter for Roll-Off Suppressing. Nanomaterials (Basel, Switzerland) 2019, 9
(9), 1307.
24. Su, S.-J.; Sasabe, H.; Takeda, T.; Kido, J., Pyridine-Containing Bipolar Host Materials
for Highly Efficient Blue Phosphorescent OLEDs. Chemistry of Materials 2008, 20 (5), 1691-
1693.
25. (a) Bagnich, S. A.; Athanasopoulos, S.; Rudnick, A.; Schroegel, P.; Bauer, I.; Greenham,
N. C.; Strohriegl, P.; Köhler, A., Excimer Formation by Steric Twisting in Carbazole and
Triphenylamine-Based Host Materials. The Journal of Physical Chemistry C 2015, 119 (5),
2380-2387; (b) Bagnich, S. A.; Rudnick, A.; Schroegel, P.; Strohriegl, P.; Köhler, A., Triplet
energies and excimer formation in meta- and para-linked carbazolebiphenyl matrix materials.
Philos Trans A Math Phys Eng Sci 2015, 373 (2044), 20140446.
26. Yoshida, H.; Yoshizaki, K., Electron affinities of organic materials used for organic light-
emitting diodes: A low-energy inverse photoemission study. Organic Electronics 2015, 20, 24-
30.
27. Coburn, C.; Forrest, S. R., Effects of Charge Balance and Exciton Confinement on the
Operational Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes. Physical Review
Applied 2017, 7 (4), 041002.
28. Idris, M.; Coburn, C.; Fleetham, T.; Milam-Guerrero, J.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E., Phenanthro[9,10-d]triazole and imidazole derivatives: high triplet energy host
materials for blue phosphorescent organic light emitting devices. Materials Horizons 2019, 6 (6),
1179-1186.
29. Garnier, E.; Audoux, J.; Pasquinet, E.; Suzenet, F.; Poullain, D.; Lebret, B.; Guillaumet,
G., Easy Access to 3- or 5-Heteroarylamino-1,2,4-triazines by SNAr, SNH, and Palladium-
Catalyzed N-Heteroarylations. The Journal of organic chemistry 2004, 69 (23), 7809-7815.
30. Hermann, G.; Pohl, V.; Tremblay, J. C.; Paulus, B.; Hege, H.-C.; Schild, A., ORBKIT: A
modular python toolbox for cross-platform postprocessing of quantum chemical wavefunction
data. Journal of Computational Chemistry 2016, 37 (16), 1511-1520.
31. Castro, S. G. P.; Loukianov, A.; scku208; Dewing, M.; Bernardo Oliveira, J.; Arnold, D.;
Eljarrat, A. Python wrapper for Cubature: adaptive multidimensional integration, v0.14.5;
Zenodo: 2020.
88

3. Chapter 3: Developing Host Materials with Triplet Energies Lower
than azaDIPYR

3.1 Introduction
Significant advances in OLED technology have been made through the introduction of
phosphorescent based emitters, which improved electroluminescence efficiency to a possible
IQE of 100%, leading to higher EQE than fluorescent-based OLEDs. In order to have an
effective OLED device, balanced charge transport and high conversion efficiency of generated
excitons to produce light is ideal. Most highly efficient OLEDs follow a multilayer device
structure comprised of hole transport layer (HTL), electron transport layer (ETL), hole and
injection layer and emission layer (EML). However, much research has been focused on
developing the emission layer of OLEDs since it has been hypothesized that host and/or dopant
highly contribute to the decreased efficiencies and short operational lifetimes from molecular
degradation. The formation of high energy radical species from bond cleavage of organic
molecules limits device performance by non-radiative recombination centers, luminescence
quenchers and deep charge traps.
1
Additionally, a high density of localized excitons and charges
also leads to device degradation.
2
The long-term intrinsic loss of electroluminescent brightness
cannot be visually identified until tested for device operation.
1e, 3
Efforts to develop red, green
and blue emitters are a major focus in academia, where different generations of dopant emitters
are developed to improve device performance including fluorescent, phosphorescent, and
thermally activated delayed fluorescence mechanisms.
4

In the EML layer, it has been shown that organic neat emitters can be used as the only
component in the emission layer. However, use of organic host matrices minimizes concentration
quenching, triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA).
89

Additionally, it helps with the electrical transport of the emission layers. Furthermore, dilution of
these dopant materials in a host environment presents an opportunity to expand the material set
with planar structures prone to aggregation and self-quenching in OLEDs. Polar and non-polar
host matrices can also help tune the photoluminescence and electroluminescence of a
solvatochromic dopant material and can be influenced by the polarity of its surrounding
environment. Exploration of host materials in the past decade or so has been in the field of
phosphorescent dopants with wide energy band gap, due to the high IQE achieved with these
emitters. However, development of host materials is not as common as dopant material in
literature, but it seems that further development of host compounds is much needed for
optoelectronics. The rising interest of TADF material, for example, involves the development of
a dopant with a small singlet-triplet energy gap needed in order to achieve 100% IQE. However,
energetic alignment of most host materials developed are for phosphorescent dopants which
typically have a high energy gap. A variety of host materials which are less prone to degradation
and can accommodate the needs of both fluorescent and phosphorescent based emitters are
needed, especially when being used for hybrid fluorescent/phosphorescent WOLEDs.
In Chapter 2, the need for new host materials for aD lumiphores with small singlet-triplet
energy gap was discussed since common host matrices such as NPD, DPEPO, mCBP,
26DCzPPy and CBP were not energetically aligned with the blue dopant for WOLED studies.
Figure 3.1 shows the target values for the S1 and T1 energies of the fluorescent dopant, host and
phosphors. Ideally, upon electrogeneration, the singlet excitons generated on the host material
will follow a Förster energy transfer into -aD. This requires that the host material have a higher
S1 energy than the dopant material. Due to the low doping concentration of the dopant, direct trap
formation of the exciton is minimized and both Förster and Dexter mechanisms are suppressed
90

for non-radiative host triplets.
5
The long diffusion lengths of triplet excitons, typically ~100 nm,
can migrate to the center of the EML layer to transfer onto the phosphors. An undoped host
spacer of ~3 nm, which is larger than the Förster radius, will be incorporated between the blue
fluorophore and the phosphors to prevent direct energy transfer from the dopant to the red
phosphor. Since the energy of S1(host) > S1 (fl dopant), the host’s frontier energy levels (HOMO
and LUMO) are expected to be nested inside the HOMO/LUMO levels of the fl dopant which
can prevent charge trapping. In previously reported devices, triplet trapping at the fluorescent
dopant led to a net loss in the overall efficiency which is similar to what was observed in devices
shared in Chapter 2.
6
Thus, there is a decrease in fluorescence efficiency especially at high
brightness.

Figure 3.1. Simple representation of the energetic alignment target for WOLED devices.

2.90
2.60
2.55
>2.90
2.50
T1R
T1G
2.30
T1flB
S1flB
T1H
S1H
S0
Goal Structure
host
green phosphor
red phosphor
blue fluorophore
2.86
2.55
2.61
3.08
2.50
T1R
T1G
2.30
T1flB
S1flB
T1H
S1H
S0
Actual: CBP as Host
CBP host
Ir(ppy)
3
dopant
PQIR dopant
-aD dopant
2.90
2.60
2.55
>2.90
2.50
T1R
T1G
2.30
T1flB
S1flB
T1H
S1H
S0
Goal Structure: Single Stack New structure: Two Stack
2.86
2.56
>2.30
>2.86
2.50
T1R
T1G
~2.30
T1flB
S1flB
T1H
S1H
S0
blue fluorophore
host
green phosphor
red phosphor
blue fluorophore
host
red phosphor
green phosphor
91

In Chapter 2, the WOLED results clearly show that device optimization and development of new
host materials are needed to achieve a more efficient transfer of triplet excitons and minimize
charge trapping in the blue dopant. The triplet energy of CBP (2.61 eV) is higher than that
observed for -aD, as shown in Figure 3.2. In the beginning of this project, a single stack
WOLED was designed such that all three RGB emitters are in the same stack; however, the
triplet energies of the synthesized blue dopants were close to those of the green phosphor
(Ir(ppy)3), introducing an additional trapping mechanism in the device. As a result, a simpler
way to approach this is to separate the green phosphor from the blue and red component which
provides a wider range of triplet energies (2.56 eV > x >2.30 eV) that can be developed to use
for the WOLEDs. It lessens the energy alignment obstacle by changing the design from a single
to a two-stack structure, where the green dopant will be in a separate layer from the blue and red
emitters, as shown in Figure 3.1.
450 500 550 600
0
0.2
0.4
0.6
0.8
1
450 500 550 600
0
0.2
0.4
0.6
0.8
1
CBP

gated PL at 77K
Wavelength (nm)
2-MeTHF
-aD

Figure 3.2. Gated Phosphorescence PL of -aD and CBP in 2-methyltetrahydrofuran.
92

In order to set a standard device for these studies, the same -aD monochromatic device
with CBP host in Chapter 2 was remade in the laboratory, as shown in Figure 3.3. The turn-on
voltage (~3.0 V) and the EL spectrum (max = 450 nm) are similar to devices produced from
collaborators in University of Michigan (UM).
7
The EQE of these devices (~2.5%) is lower than
what was produced in UM but this is expected since their device fabrication is better optimized
for these studies. The latter monochromatic device data will be used as a baseline to compare the
host materials shared in this chapter.
0.01 0.1 1 10 100 1000
0
5
-aD in CBP
device 1
device 2
device 3
External Quantum Efficiency (%)
Current Density (mA/cm
2
)
EQE= 2.48%
0 1 2 3 4 5 6 7 8 9 10 11 12
1E-6
1E-5
1E-4
0.001
0.01
0.1
1
10
100
1000
2.65V
-aD in CBP
device 1
device 2
device 3
Current Density (mA/cm
2
)
Voltage (V)
2.95V
0 1 2 3 4 5 6 7 8 9 10 11 12
0.01
0.1
1
10
100
1000
10000
-aD in CBP
device 1
device 2
device 3
Luminance (cd/m
2
)
Voltage (V)
400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
Electroluminescence
Wavelength (nm)
7V
9V
11V
12V

Figure 3.3. Blue monochromatic device of -aD with CBP host.

93

3.2 H2P as a Host Material
In Chapter 2, it was discussed in detail why a triplet energy less than 2.56 eV for the host
is preferred for a WOLED stack when using molecules in the aD family as a blue dopant.
Previously, a family of H2P host materials were developed in our lab with the intention to use
them with blue phosphorescent dopants in OLEDs.
8
The family of molecules were designed to
have deep blue triplet energies (~2.90 eV) and a large aromatic system for efficient charge
transport in a device. The structure incorporated a rigid tetracyclic motif to mitigate the
exocyclic bond cleavage anticipated for phenylcarbazole derivatives. The H2P host materials had
experimental triplet energies in solution ~ 2.71-2.92 eV that bathochromically shifts in solid by
~0.3 eV, making it a compound worth exploring as host for -aD. The material of interest is 3,8-
H2P-tbu (3,8 H2P) which has a measured triplet energy of 2.49 eV in solid, below that of -aD
(2.56 eV). 3,8 H2P had the lowest triplet energy due to having the most - interactions (face to
face) relative to the other analogues, but it is the excited state triplet that is energetically most
suitable for our -aD dopant.
3.2.1 Photophysical Properties of 3,8 H2P
3,8 H2P has previously been characterized in our lab and the photophysical and
electrochemical properties needed for these studies are tabulated in Table 3.1. Spin coated films
of neat 3,8 H2P and 3,8 H2P doped with 1% -aD and 5% Ir(ppy)3 were made (Figure 3.4).
The neat film has a broad emission profile ranging between 400 nm to 650 nm. Doping 1% -aD
into the matrix results in a sharp emission profile (425 nm to 600 nm), while 5% Ir(ppy)3 doped
films led to a broad emission (470 nm – 700 nm) characteristic of -aD and Ir(ppy)3,
respectively. The absorbance of the blue dopant and the emission of the host has significant
overlap, suggesting there is Förster Energy Transfer between the host-dopant pair. The
94

photoluminescence quantum yield of doped -aD in 3,8 H2P ( = 0.60) is ~17% lower than in
CBP. The emission profile observed for Ir(ppy)3 doped film is representative of dopant
emission, but it also had a low  ( = 0.30), while doping these in other host matrices led to
high values ( > 0.60).
9
Although the photoluminescent efficiencies are less than 60% for -aD
and Ir(ppy)3, it is worth exploring these new materials as host materials and potentially conduct
exciton trapping studies to deduce if -aD still acts as a trap as observed in CBP. Table 3.2
tabulates the quantum yield and lifetime observed for 3,8 H2P spin coated films. The lifetime
values observed for the spin coated doped films have comparable values to their corresponding
dopant monomers. The observed emission profile and lifetime values in the spin coated doped
film suggest that there is Förster energy transfer in these materials. However, the lower PLQY is
not easily explained, so vapor deposited doped thin films were made.
Table 3.1 3,8-H2P-tBu and -aD HOMO, LUMO, Redox, and Triplet Energies.
E ox E red HOMO
(eV)
LUMO
(eV)
E T (eV)
solution solid
3,8-H2P-tbu +1.11 -2.25 -6.15 -2.10 2.71 2.47
-aD +1.15 -1.91 -6.11 -2.51 2.56 -

350 400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
PL Intensity
Wavelength (nm)
3,8-H2P-tBu films (RT)
neat film
1% -aD
5% Ir(ppy)
3

Figure 3.4. Photoluminescence Emission of neat 3,8-H2P-tBu, 1% -aD and 5% Ir(ppy) 3 doped
spincoated films.
95

Table 3.2. Photoluminescence efficiencies and lifetime for neat film and spincoated or vapor
deposited doped films of H2P.
neat film
a
1% -aD
a
5% Ir(ppy)3
a
1% -aD
b
CBP
b

fl 0.20 0.57 0.29 0.08 <0.01
 fl 3.50 ns
(80%)
2.44 ns
(92%)
0.98 ms 3.25 ns
(89%)
3.22 ns
(78%)
9.42 ns
(20%)
8.67 ns
(7%)
3.22 ms 17.4 ns (11
%)
7.32 ns
(22%)
a
Spincoated doped films
b
Vapor deposited doped films

Thermally evaporated films are more ideal than spincoated films since these typically
deposit on the surface more uniformly, where for this study ~15 nm layer of EML (1% -aD
doped in 3,8 H2P) was characterized as shown in Figure 3.5 and Table 3.2. The emission
spectra of the spincoated 3,8 H2P film compared to vapor deposited film (co-deposited) had
similar vibronic features, but the second peak ~470 nm is higher for the latter by ~50% and
bathochromically shifted by ~2 nm. The vapor deposited doped films produced low PLQY (close
to 0) and could be due to a very thin layer thickness with very dilute dopant concentration.
However, the monochromatic devices were simultaneously made with these thin films which
further confirmed that use of these host materials is not suitable for the WOLED structure.
400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
PL Intensity (a.u.)
Wavelength (nm)
1wt% -aD doped in:
CBP (co-dep)
3,8-H2P-tBu (spin coated)
3,8-H2P-tBu (co-dep)

Figure 3.5. PL Emission of -aD doped in CBP and 3,8 H2P (spin coated and vapor deposited)
doped films.

96

3.2.2 Monochromatic Device for 3,8 H2P
Blue monochromatic devices were fabricated similarly to the CBP hosted film. The
structure was kept the same and only the CBP layer was changed with 3,8 H2P, shown in Figure
3.6A.. The data obtained for these studies are shared in Figure 3.6. Device 1 is clearly an outlier
compared to Devices 2 and 3, as shown in Figure 3.6B. The turn-on voltage of these devices is
~3.0 V, which is the same as the observed value for CBP (~3.0 V) but lower than 26DCzPPY
(~3.7 V). The drop in EQE with multiple scans of the same device (Figure 3.6 D-E) along with
the formation of a new peak in the 550 nm-650 nm region in the EL spectra (Figure 3.6 F)
suggests that some decomposition is occurring which produces the new emission feature. The
HOMO and LUMO energies of the host and dopant suggests that exciplex formation is not
expected for these systems. As a result, it was determined that a different set of host materials are
needed to further optimize the WOLED stack intended for this study.

97

0.01 0.1 1 10 100 1000
0
0.5
1
1.5
2
-aD in H2P
device 1
device 2
device 3
External Quantum Efficiency (%)
Current Density (mA/cm
2
)
0.01 0.1 1 10 100
0
0.2
0.4
0.6
0.8
1
d2 first scan
d3 first scan
d2 second scan
d3 second scan
External Quantum Efficiency (%)
Current Density (mA/cm
2
)
400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
Electroluminescence
Wavelength (nm)
4V
7V
9V
11V
increasing
voltage

Figure 3.6. Blue Monochromatic OLED Device Data for -aD doped 3,8 H2P. (A) Device Structure
(B) Current Density vs Voltage (turn on voltage = 3V) (C) Luminance vs. Voltage (D) EQE vs
Current Density E (EQE) vs Current Density (multiple scan in the same device) (F) EL vs wavelength
0 1 2 3 4 5 6 7 8 9 10 11 12
1E-6
1E-5
1E-4
0.001
0.01
0.1
1
10
100
1000
-aD in H2P
device 1
device 2
device 3
Current Density (mA/cm
2
)
Voltage (V)
0 1 2 3 4 5 6 7 8 9 10 11 12
0.001
0.01
0.1
1
10
100
1000
10000
-aD in H2P
device 1
device 2
device 3
Luminance (cd/m
2
)
Voltage (V)
F
A
B
C
D
E
98

3.3 Fluorene-based Host Material
Different families of host materials were explored to use in the
fluorescent/phosphorescent hybrid WOLED including indolocarbazoles, corannulenes,
phenantroimidazoles
10
and all the developed host materials in this chapter. It is evident that one
of the issues we encountered with these materials, including H2P, indolocarbazoles and
phenantroimidazoles, is the decrease in efficiency due to bond cleavage of compounds
containing C−N, C-P or C-S donor-acceptor structures believed to expedite the device
degradation.
11
Additionally, designing close to purely hydrocarbon could lead to compounds
with thermal stability (>200C) and low triplet energies such as fluorene (2.99 eV), biphenyl
(2.84 eV), naphthalene (2.62 eV), phenanthrene (2.70 ev), and
1H-cyclopenta[l]phenanthrene (2.28 eV).
Fluorene is particularly interesting since it has been studied in OLEDs and has
historically been used to form neat blue emitters in polymer light emitting diodes (PLEDs), in
hole transport, and electron transport layers in optoelectronics. Additionally, it has been
identified to have a wide energy gap with highly efficient charge carrier mobility. Fluorene is a
tricyclic structure which has a five membered ring with two fused benzene rings on each side and
has a triplet energy of 2.99 eV. The drawback of fluorene use in OLED materials is its instability
which is attributed to aggregation, excimer formation
12
and/or fluorenone formation.
12-13

Typically, it is observed to have a low-energy emission around 540nm to 570nm in the
photoluminescence emission that leads to decrease in efficiency, stability and color purity. It has
been proposed that these low emission bands are due to exciton- and or/charge trapping keto
defects sites that can be formed in the pentacyclic region (9-position) during synthesis or photo-
/electro- oxidative degradation processes.
13b
In order to circumvent this issue, the fluorene core is
99

polymerized into extended carbon chains as polyfluorenes (PFs), as a small molecule with
substituents in the 9-position
14
or are fused together with another fluorene into spiro
compounds.
15
For our purposes, utilization of these defects either to block the 9-site with methyl
groups
14a, 15e
or produce an oxidized species (fluorenone) as the desired core structure can help
stabilize these materials in the OLED.
3.3.1 Quenching Studies of host with -aD
Quenching studies were conducted to determine if the host material quenches the dopant
through the kinetics of a photophysical intermolecular deactivation process in the host-dopant
pair (Stern-Volmer analysis). The quenching studies were conducted with fluorenone as the
quencher and -aD as the studied system. The photoluminescence quantum yield is the same
through the doped sample with values at 76%. Note that this measured value was taken with the
new integrating sphere which has a plus or minus 10% error. The photoluminescence quantum
yield for -aD taken from the old integrating sphere resulted in a value of 86% which is still
within the instrument error. However, the main takeaway from this study is that the PLQY,
lifetimes and the PL emission are consistent with the changing concentration of fluorenone.
Figure 3.7 illustrates that increasing the concentration of fluorenone from 0 M to 3.99 x 10
-5
M
led to an unchanged intensity and sharp emission profile of -aD emission suggesting that static
or dynamic quenching is not observed.
100

3.3.2 TDDFT of Fluorenone-based material
The oxidation product of fluorene is expected to have a 9-fluorenone motif which is
suggested to emit at lower energy. TDDFT calculation of 9-fluorenone was conducted to obtain
a better understanding of the optical, photophysical, and electrochemical shifts of fluorenone
relative to fluorene. TDDFT suggests that the triplet energy of fluorenone is 0.50 eV lower than
fluorene which is 0.07 eV below -aD. Addition of substituents such as phenyl, napthal, or any
aromatic systems on the fluorene core in the 2,7 position has been shown to affect the packing
and conductivity of the material but not significantly affect the photophysical properties if the
substituent does not have a significant electronic effect (i.e. simple π system).
14a
Although
fluorenone is a promising candidate, introduction of steric bulk is necessary to minimize the
possible -  stacking in these systems in solid, which can shift the triplet state to lower energies.
It also helps to guarantee a higher sublimation temperature and prevent decomposition for vapor
deposition requirements.
Fluorene has a triplet energy of 2.99 eV and if modified with mesityl in the 2 and 7
position of fluorene (MFL), TDDFT calculation using B3LYP/631G** suggests that the T1
250 300 350 400 450 500
0
0.5
1
1.5
2
2.5
3
fluorenone concentration (M)
0 M
3.99X10
-6
M
7.99X10
-6
M
3.99X10
-5
M
absorbance (AU)
wavelength (nm)
420 440 460 480 500 520 540 560
0
2x10
5
4x10
5
6x10
5
8x10
5
1x10
6
1.2x10
6
1.4x10
6
fluorenone concentration (M)
0 M
3.99X10
-6
M
7.99X10
-6
M
3.99X10
-5
M
PL Intensity
wavelength (nm)

Figure 3.7. Absorbance and emission spectra of concentration dependent doping of fluorenone
in -aD solution.
101

energy ~3.09 eV which does not have a significant shift (~0.1 eV) from the experimental value.
The calculated T1 energies for fluorene and 2,7 dimesitylfluorene were the same. Addition of 2,7
substitution pattern in 9-fluorenone (MFLO) with mesityl led to a calculated T1 = 2.42 eV, a
negligible difference from ones found in 9-fluorenone (2.49 eV). However, a triplet energy
difference of ~ 0.67 eV was observed between MFL and MFLO. The calculated S1 energy has a
~1.50 eV difference between the fluorene and the fluorenone derivative which trends with the
observed optical shift into lower energy upon oxidation.
The HOMO and LUMO orbital contributions of fluorene and fluorenone are depicted in
Figure 3.8. The HOMO orbitals are different between both molecules, where the orbitals are
concentrated in the pentacyclic structure in fluorene and in the benzene region in fluorenone,
thus resulting in the destabilization of the fluorene HOMO. The stabilization of the fluorenone
HOMO relative to the fluorene HOMO is due to the greater mobility of π electrons in the
conjugated arene which greatly increase the distribution of energy in the molecule and help
stabilize it. The LUMO energy of fluorene and 9-fluorenone has a similar orbital picture but with
the additional orbital contribution of the oxygen atom in fluorenone. The electron withdrawing
inductive effect of oxygen significantly stabilizes the LUMO energy of 9-fluorenone relative to
fluorene. Addition of mesityl in the 2 and 7 positions in both fluorene (MFL) and fluorenone
(MFLO) maintained the same HOMO and LUMO orbital contributions which supports the
unchanged HOMO/LUMO and singlet/triplet excited state energy values (Figure 3.9),
suggesting that mesityl does not electronically affect the core molecular structure. Addition of
dimethyl in the 9-position (MFLC) suggests that it will not be an ideal material as it falls
between the triplet energies of MFL and MFLO, and it is still much higher than the target (2.56
eV > T1); MFL (3.09 eV) > MFLC (2.88 eV) > MFLO (2.49 eV).
102

Figure 3.8. HOMO and LUMO Orbital Contributions for Fluorene and 9-Fluorenone
(631G**/B3LYP).

Figure 3.9. HOMO and LUMO Orbital Contributions for MFL and MFLO (631G**/B3LYP).

103

3.3.3 Photophysical Properties of MFL and MFLO in Solution and Film
Figure 3.10 and Figure 3.11 show the optical properties of 2,7-dimesitylfluorene (MFL)
and 2,7-dimesitylfluorenone (MFLO). The absorbance of MFL and MFLO range between
250nm-350nm. The absorbance in MFLO is broad in methylcyclohexane but becomes vibronic
when switching to a more polar solvent such as dichloromethane. The absorbance and emission
spectra of 2,7-dimesitylfluorenone in solution and in solid is shown in Figure 3.11. The 77K
solution and solid emission measurements have a 50 nm difference in λmax. The solid powder
room temperature and 77K emission measurements show less than a 5 nm shift from each other,
suggesting that there is not a significant shift in its optical properties in the solid form. The triplet
energy of MFLO is 2.63 eV, which is underestimated by the TDDFT calculations. MFLO can
host our red phosphor but since it is close in triplet energy of CBP, it is expected that the same
exciton trapping in the blue dopant will occur. The photophysical properties of MF and MFLO
are tabulated in Table 3.3 and 3.4, which shows low  and with faster knr than kr. Since these are
not being utilized as emitters, these properties are acceptable enough for these host materials.

250 300 350 400 450 500
0.0
0.2
0.4
0.6
0.8
1.0
Normalized Abs/PL
Wavelength (nm)
RT
abs
77K

300 350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
RT
77K Gated
77K

Figure 3.10. Photophysical properties of 2,7-dimesitylfluorene (MFL) in 2-methyltetrahydrofuram.
104

250 275 300 325 350
0
0.2
0.4
0.6
0.8
1
MeCyHx 298K
DCM 298K
Normalized Absorbance
Wavelength (nm)
A

300 350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
B
MeCyHx
298K
77K
PL Intensity
Wavelength (nm)

350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
MeCyHx
solid
solid gated
MeCyHx
gated
PL Intensity
Wavelength (nm)
C
250 300 350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
MFLO RT measurement
mch abs
mch em
solid
Abs/Emission
Wavelength (nm)
D

Figure 3.11. Absorbance and Emission of MFLO in solution and powder.

Table 3.3. Solution Photophysics of MFL.

RT 77K
 PL 0.17 -
 (ns) 1.17ns 7.50ns
kr(s
-1
) 1.45x10
8
-
knr (s
-1
) 7.09x10
8
-

Table 3.4. Photophysical properties of MFLO at room-temperature and at 77K.
MFLO RT 77K
 PL 0.07 -
(ns) 2.06 (24%) 1.86 (17%)
kr(s
-1
) 3.04x10
7
-
knr (s
-1
) 4.51x10
8
-
105

In order to confirm the latter, film studies were conducted with MFLO as the host and
were doped by 1% -aD and 1% PQIr as dopants. The PLQY of α-aD in solution and in PMMA
film is above 75%, while this value drops to 17% when doped in MFLO, suggesting -aD is
quenched in the neat film (Table 3.5). The emission spectra in Figure 3.12 also shows the
emission of 1% α-aD doped film in MFLO is the same as the emission of the solid powder
MFLO sample, which suggests that the observed emission is from the host. Looking at the
overlap of the dopant absorbance and host emission, it is evident that there is minimal to almost
no overlap between the two such that Förster energy transfer will not be present in this system
As a result, this explains the observed emission profile and low PLQY in these systems. The
comparison of these results to PMMA (Table 3.5) confirms that the low  and longer lifetime
observed in MFLO solidifies that this material is an undesirable host for WOLED studies.
Table 3.5. Photophysical properties of 1% α-aD in PMMA as a polymer matrix and MFLO as a
host.
1% -aD in PMMA MFLO
 PL 0.77 0.17
(ns) 3.79 14.87
kr(s
-1
) 2.03x10
8
1.14x10
7

knr (s
-1
) 6.07x10
7
5.58x10
7

106

400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
-aD in DCM
1% -aD-PMMA
1% -aD-Flu
PL Intensity
Wavelength (nm)
A

400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1 2,7 dimesitylefluorenone (MFLO)
solid powder MFLO
1% aaD-doped film in MFLO
PL Intensity
Wavelength (nm)
B
300 400 500 600
0
0.2
0.4
0.6
0.8
1
abs MFLO
abs -aD
Abs/PL Intensity
Wavelength (nm)
em MFLO
C

Figure 3.12. Film studies using 2,7-dimesitylfluorenone (MFLO) as host material for 1% α-aD
doped film. PMMA was used as a reference for the experiment.
107

Prior to obtaining the experimental triplet energy of a host material developed in our lab,
a spirofluorene material (SAS) was spincoated with -aD as the host material. It maintained a
sharp emission profile around 425 nm-600 nm, diagnostic of -aD emission (Figure 3.13).
However, a broad bump at 350 nm- 400nm attributed to host emission and the low PLQY
(=0.12) indicated that this material will not serve as an ideal host. Upon obtaining the triplet
energy using the onset of the neat powder PL emission, it was evident that a different family of
small-molecule hosts is needed as the T 1=2.77 eV of SAS will trap the blue fluorescent dopant
excitons in addition to the low .

350 400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
spincoated in SAS
1% -aD
Normalized PL (a.u.)
Wavelength (nm)

Figure 3.13. PL emission of -aD doped in SAS.

3.4 Quinoline-Based Host Material
Biphenyl has a triplet energy of 2.84 eV, which TDDFT suggests can be decreased by
~0.3 eV by changing the phenyl to a naphthalene (Table 3.6, Figure 3.14). Although the
calculated triplet energies of these compounds are higher than our target values (T 1<2.56 eV),
previous experience with these -extended systems with minimal steric bulk has taught us it
could lead to lower energy aggregations. TDDFT results suggests that 4,4’-biquinoline (4BQ)
108

has the highest S1 (4.15 eV) and T1 (2.79 eV) while 2,2’-biquinoline (2BQ) has the lowest S1
(3.84) eV and T1 (2.57 eV) energies. 3,3’-biquinoline (3BQ) is expected to have similar energies
to those of 2BQ, therefore 2BQ and 4BQ were chosen to obtain a representative scale by using
the two systems with the largest difference in energy if aggregation is observed for these highly
planar structures.

3.4.1 Photophysical Properties of 2BQ and 4BQ
The absorbance and emission spectra for 2BQ and 4BQ are shown in Figure 3.15. The
absorbance of both compounds have sharp features which show some of the Franck-Condon
transitions in the compound that are slightly broadened by solvent interactions. As supported by
TDDFT, the absorbance profile of 4BQ is blue shifted (270nm-325nm) relative to 2BQ (275nm
– 375nm) in 2-MeTHF. The emission profile of 2BQ and 4BQ covers ~100 nm of the visible
spectrum where 2BQ has a broad emission profile and 4BQ has two maximum peaks at 363 nm
Table 3.6. S 1 and T 1 Energies in eV and HOMO/LUMO values of 2BQ, 3BQ and 4BQ from
B3LYP/631G**
2BQ 3BQ 4BQ
S1 3.84 3.96 4.15
T1 2.57 2.59 2.79
HOMO -6.07 -6.37 -6.37
LUMO -1.82 -1.63 -1.63

Figure 3.14. Molecular Orbitals of 2BQ, 3BQ and 4BQ from B3LYP/631G** using QChem

HOMO
LUMO
109

and 400 nm. Solution data suggests that 2BQ has more desirable S1 (3.59 eV) and T1 (2.55 eV)
energies than 4BQ (S1=3.66 eV, T1=2.71 eV) since it is bathochromically shifted. Additionally,
4BQ is an oil at room-temperature, making it challenging to work with as a host material for
vapor deposited processes. As a result, film studies were conducted on 2BQ.
300 350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
4,4'-biquinoline (4BQ)
Normalized Abs/PL
Wavelength (nm)
abs 298K
PL 298K
PL 77K
300 350 400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
2,2'biquinoline (2BQ)
Normalized Abs/PL
Wavelength (nm)
abs 298K
2-MeTHF 298K
2-MeTHF 77K
solid 298K
solid 77K

Figure 3.15. Photophysical properties of 4,4’biquinoline (left) and 2,2’biquinoline (right).

The solid powder spectra of 2BQ at room-temperature and 77K spectra does not
significantly shift in its emission profile, however its’ vibronic feature is enhanced at cooler
temperature (Figure 3.16). The spin-coated and drop-casted neat films of 2BQ have an emission
spanning in the 350 nm – 600 nm region. The doping studies were conducted using a spin-coated
method where 1% -aD was doped into 2BQ with emission representative of the dopant. 5%
Ir(ppy)3 and 5% PQIR were also doped in 2BQ but both had emission around 350 nm-500nm
attributed to the host material. Additionally, PLQY measurements suggest that the dopants are
being quenched by the host due to the low  (), as shown in Table 3.7. It is possible that the
observed emission profile of both the host and dopant is resulted from crystalline formation of
the host-dopant pair on the surface of the quartz. It is evident visually that the thin-film sample
crystallizes overtime and could be forming islands of aggregated host and dopants which leads to
110

the observed PL. Additionally, neat material of the blue dopant self-quenches due to the highly
planar structure of -aD. One way to circumvent this issue of crystallinity is by adding some
bulk to the structure to minimize the observed crystallinity on thin films.
350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
2,2' biquinoline neat film (RT)
spin coated
drop casted
350 400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
spin coated films (RT)
neat 2,2' biquinoline
1% aaD doped
5% Ir(ppy)3 doped
5% PQIR
350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
-aD doped films by weight % (RT)
1% -aD
3% -aD

Figure 3.16. Film Studies using 2BQ as a host with 1% doping of -aD, 5% doping of Ir(ppy) 3 and
5% doping of PQIr. 3% doping of -aD increases the second vibronic feature.

Table 3.7. Lifetime and Photoluminescence Quantum Yield for 2BQ Film Studies
2BQ
neat 1wt% -
aD
3wt% -aD 5wt% Ir(ppy)3 5% PQIr
 PL 0.02 0.07 0.06 0.01 0.02
(ns)
1.88 (5%)
Fast comp
(95%)
1.27(83%)
6.05(17%)
1.35(85%)
5.60(15%)
115(55%)
26.5(45%)
-

111

3.5 Napthalene-Bridged Quinoline and Isoquinoline Compounds as Host Materials
In order to add steric bulk in the biquinoline system without addition of substituents
around the quinoline core, bridging naphthalene was added which would maintain the low triplet
energies desired for WOLED structure. The molecules described are inspired by substituted
oligoquinolines used as neat emitters for blue OLEDS.
16
These materials have high Tg (>130
C), nanosecond lifetimes (1.06-1.42 ns) and high PLQY (0.73-0.94).
16a, 16i
By addition of phenyl
or biphenyl around the biquinoiline structures, the -stacking in these oligoquinolines such as
face to face or edge to face -stacking is absent. The optical gap was provided for these
oligoquinolines and due to the high PLQY and high oscillator strength of S0 → S1, the
HOMO/LUMO gap can be estimated using these values. The E g
opt
are ~3 eV which suggests that
these types of materials can be used as decent wide band gap hosts for the aD dopants and can
help facilitate electron injection and transport with improved efficiencies.
Synthesis of substituted biquinolines was explored when making the backbone of the aD
compounds. Some of these synthetic routes were at least three steps to form substituted quinoline
structures. Minimal synthetic steps with high yields are ideal for these compounds due to the
large scales required for their use as host materials in devices for WOLED studies. In order to
simplify the synthesis, while adding steric bulk and achieving comparable properties found in
these oligoquinolines, adding bridging naphthalene was proposed, which provides S 1 and T1
energy values close to the target values. TDDFT suggests that addition of benzene, naphthalene,
or anthracene results in a decreasing trend in the triplet energies which is expected due to the
increasing conjugation in the system (1.76-3.74 eV). The trend follows a similar pattern of ~1 eV
stabilization going from benzene to napthalene to anthracene, when comparing the triplet
energies of just the acenes.
17
The TDDFT calculations of these acenes deviate by ~0.10 eV in
112

these systems. The triplet densities of the benzene, naphthalene and anthracene bridge systems
suggest that each derivative had the orbital localized in the bridging acene which leads to similar
triplet energies found in the unsubstituted acenes. A total of four naphthalene bridged quinoline
and isoquinoline compounds were explored as host materials to explore for the EML layer of
WOLEDs.
3.5.1 Synthesis of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ.
The general scheme (Scheme 3.1) of the synthesized compounds (Figure 3.17) follows a
Suzuki-Miyaura coupling reaction with the respective quinoline-based boron reagent and
dibromonapthalene counterparts inspired by the synthetic route used by Alezi et al. for MOF
synthesis.
18
3-quinolineboronic acid bis(pinacol) ester and 4-isoquinolineboronic acid
bis(pinacol) ester were used to couple with either 1,4-dibromonapthalene or 1,5-
dibromonapthalene to form 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ. The product was purified by
column chromatography with hexanes and ethyl acetate. The compounds were further sublimed
at ~190-200 C to produce white solid crystals or powder. The synthesis produces a range of 10-
60% yield, which is maximized when using a combination of THF/water as the solvent.
Alternative routes were initially explored prior to this synthesis using 1,4-napthalenediboronic
acid bis(pinacol) ester and the halide counterpart of quinoline (3-bromoquinoline) or
isoquinoline (4-bromoisoquinoline), but the current synthetic route produced the least steps (1-
step) with the highest yields. The 3- position was attached to the naphthalene core instead of the
2-position because that site was reserved if further modification or addition of bulkier
substituents were desired. The 2-position of quinoline is the easiest to access once the
naphthalene bridged compound has been synthesized. Formation of the N-oxo on the quinoline
using mCPB activates the 2-position which allows for the addition of halide using POBr 3.
7

113

Lastly, a coupling reaction using Suzuki or Sonogoshira conditions can be conducted once the
halide handle has been installed.

Figure 3.17. Structures Synthesized as Host Material for aD compounds

Scheme 3.1. General Synthetic Scheme to produce 1,4Q, 1,4IQ, 1,5Q, and 1,5IQ

3.5.2 Electrochemistry of 1,4Q, 1,4 IQ, 1,5 Q and 1,5 IQ.
The electrochemical properties of the compounds were analyzed by cyclic voltammetry
(CV), see Table 3.8 and Figure 3.18. The reduction potentials have a smaller range of -2.36 to -
2.43 V (Eredox ~ 70 meV), where quinoline based systems are shown to have a slightly higher
reduction potential, as expected.
19
Oxidation is irreversible for all the compounds, whereas
reduction is reversible or quasi-reversible. The oxidation potentials of the naphthalene bridged
systems span a range of 1.18-1.56 V (Eredox ~ 400 meV), where the quinoline derivatives are
114

cathodically shifted. It is worth noting that the oxidation peaks of these compounds, especially
those of the isoquinoline derivatives, are not easily observed.
16a
Cyclic voltammetry measurements
in dichloromethane resulted in a slight increase in the current response. As a result, the oxidation
peaks found in all four compounds were deduced from the CV and DPV data in dichloromethane.
-3 -2 -1 0.5 1 1.5 2 2.5
1,4 Q

Current (A)
Volts vs Fc/Fc
+
(in acetonitrile)
1,4 IQ

1,5 Q

1,5 IQ

0 0.5 1 1.5 2
1,4 Q

Current (A)
Volts vs Fc/Fc
+
(in dichloromethane)
1,5 Q

Figure 3.18. Cyclic Voltammetry of 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ in Acetonitrile and
Dichloromethane referenced with Ferrocene.
Table 3.8. HOMO and LUMO energies (in eV) for 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ
Eox (V) Ered (V) HOMO
a
LUMO
a
HOMO
b
LUMO
b

1,4 Q 1.24 -2.39 -6.21 -2.01 -5.75 -1.71
1,4 IQ 1.51 -2.43 -6.59 -1.95 -5.89 -1.53
1,5 Q 1.18 -2.36 -6.14 -2.04 -5.75 -1.70
115

1,5 IQ 1.56 -2.43 -6.52 -1.95 -5.91 -1.49
a
HOMO/LUMO energies extrapolated from experimental redox potentials. HOMO = -1.15 (E ox) + 4.79;
LUMO = -1.18 (E red) - 4.83.
b
HOMO/LUMO values obtained from calculations (B3LYP/6-31G
**
).

The HOMO and LUMO energies were extrapolated from experimental redox potentials
and shown in Table 3.8. Comparison of the experimental data to the DFT calculations suggests
that the HOMO and LUMO energies were underestimated by an average of 0.54 eV and 0.38 eV,
respectively. The experimental HOMO values range between -6.59 eV to -6.14 eV while the
LUMO values range between -2.04 eV to -1.95 eV. The HOMO energy difference varies based on
the substituent around the naphthalene core, where the orbitals are localized more in the pyridine
motif of the isoquinoline core while it is more delocalized in the quinoline core of the 1,4 Q and
1,5 Q derivatives. However, the HOMO energies of all four compounds are well below those found
in -aD which suggests that the host will most likely carry the holes in the device. The LUMO
energies were almost unchanged which suggests that there is not a significant difference in the
electrochemical properties between the 1,4- and 1,5 or having quinoline or isoquinoline.
The calculated HOMO and LUMO values of ternapthalene analogues of 1,4 Q, 1,4 IQ, 1,5
Q, and 1,5 IQ showed very similar molecular orbital contributions, as shown in Figure 3.19. The
HOMO contours are mostly localized in the central naphthalene with some contribution from the
peripheral substituents, while the LUMO orbitals are concentrated on the periphery of the central
naphthalene core with some contribution to the central ring. Similar HOMO and LUMO molecular
orbital contributions were observed for the aza-substituted derivatives which supports the
stabilization of the HOMO and LUMO energies upon addition of a more electronegative nitrogen
atom in the quinoline and isoquinoline derivatives. The HOMO and LUMO energy values of all
host materials have a more destabilized LUMO energy and stabilized HOMO energy relative to
116

-aD, suggesting that exciplex formation can be minimized in these systems. Additionally, it is
likely the host will carry the holes and electrons in the device.
117

1,4 Q
HOMO-1 HOMO

LUMO LUMO+1

1,4 IQ
HOMO-1 HOMO

LUMO LUMO+1

118

1,5 Q
HOMO-1 HOMO

LUMO LUMO+1

1,5 IQ
HOMO-1 HOMO

LUMO LUMO+1

119

1,4 Q- Ternapthalene Analogue
HOMO-1 HOMO

LUMO LUMO+1

1,5 Q-Ternaphalene Analogue
HOMO-1 HOMO

LUMO LUMO+1

120

1,4 IQ-Ternapthalene Analogue
HOMO-1 HOMO

LUMO LUMO+1

1,5 IQ-Tetraphene analogue
HOMO-1 HOMO

LUMO LUMO+1

Figure 3.19. HOMO and LUMO orbitals of 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ compared to
ternapthalene derivatives.

121

3.5.3 Photophysical Properties of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ.
The UV-visible absorption spectra of 1,4 Q, 1,5 Q, 1,4 IQ and 1,5 IQ are shown in
Figure 3.21. All of the compounds have molar absorptivities ( ~ 10
4
M
-1
cm
-1
) which are
competitive to some organic dyes.
7, 20
. All four compounds display a broad absorbance spectrum
between 250-350 nm where 1,4 IQ and 1,5 IQ are hypsochromically shifted by ~0.1 eV relative
to its quinoline counterparts. 1,4 Q and 1,5 Q have two maxima around 283 nm and 332 nm
which are attributed to the -* transition similarly observed in oligoquinolines.
16a, 16i

Photoluminescence spectra of 1,4 Q, 1,5 Q, 1,4 IQ and 1,5 IQ are shown in Figure 3.20
and their photophysical data summarized in Table 3.9. The photoluminescence emission at room
temperature is observed em = 345-500 nm, where it emits blue to blue-greenish color. The
monomer emission of the four compounds in solution, using the onset of the spectra, had a
difference of 15 nm between 1,5 IQ and 1,4 Q. However, a larger difference is observed between
1,5 IQ (342 nm) and 1,5 Q (387 nm) of ~45 nm in solid powder PL measurements while 1,5 IQ
(360 nm) and 1,4 Q (387 nm) had ~ 27 nm difference in neat spincoated film, suggesting that the
packing of these compounds plays a role in the emission profile observed (vide infra). A
bathochromic shift is observed from solution to neat film for all compounds, where a larger shift
was observed in the quinoline derivatives (~17 nm) than in the isoquinoline (<10 nm). The
powder PL measurements are interesting in that a bathochromic shift is observed in the quinoline
derivatives but a hypsochromic shift was evident in the isoquinoline structures. Currently, there
is no explanation for the trends observed in powder and it is common to have challenges in
obtaining powder PL. There can be inconsistencies in the PL spectra shape when doing these
measurements and so the solution PL gated 77K are more reliable.

122

Table 3.9. Summary of the photophysical parameters for 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ.
a

ET
(eV)

Compound
S1
(eV)
a
Soln
b
Solid
c
Eox (V) Ered (V)
HOMO/LUMO
(eV)
e

Tg/Tm/Ts(C)
1,4 Q
3.
73
2.50 2.21 +1.24 -2.39 -6.21/-2.01 -/189/328
1,4 IQ
3.
84
2.64 2.35 +1.51 -2.43 -6.59/-1.95 -/212/325
1,5 Q
3.
75
2.53 2.22 +1.18 -2.36 -6.14/-2.04 170/232/327
1,5 IQ
3.
84
2.61 2.59 +1.56 -2.43 -6.52/-1.95 200/252/328

a
Recorded in 2-MeTHF.
b
Fluorescence measured at 298 K.
c
kr =  PL/.
d
knr = (1- PL)/.
e

Phosphorescence measured at 77 K.

123

0
1
Wavelength (nm)
absorbance (a.u.)

abs at r.t.
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ

300 400 500
0
1
Normalized PL (a.u.)

soln PL at r.t.
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ
0
1
Wavelength (nm)
Normalized PL (77K gated)

Soln. PL at 77K
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ

400 500 600 700 800
0
1

Solid at 77K
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ
350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
Normalized PL (a.u.)
Wavelength (nm)
Solid PL at RT
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ

Figure 3.20. Absorbance and PL Emission in Solution and Solid of 1,4 Q, 1,4 IQ, 1,5 Q, and 1,5 IQ.
2

2
Gated 77K measurements in 2-MeTHF is much more reliable than the powder emission. Obtaining more consistent
powder gated PL data with consistent emission profile has been challenging.
124

The photoluminescence quantum yields of these host materials in powder (0.15-0.38) and
in neat film (0.27-0.60) have a wide range of values without a straightforward trend between the
different environments and will be better explained with further molecular packing studies ( PL
> 0.15) such as powder XRD. The self-quenching observed for this system is not a limiting
factor in these studies since 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ are not being used as neat emitters.
However, 1,4 Q has potential to be used as a neat emitter based on the neat film results (( PL~
0.60). Addition of bulky groups around the periphery of these hosts can be further utilized to
transform them into potential neat blue emitters.
The solution (2-MeTHF) and powder triplet energies were estimated using the onset of
the gated phosphorescence spectrum (Figure 3.20). 1,4 IQ, 1,5 Q, and 1,5 IQ exhibited a sharp
vibronic emission ranging between 470 nm to 700 nm while 1,4 Q had the broadest and
bathochromically shifted gated phosphorescence emission observed between 500 nm to 700nm
in solution. Changing the substitution pattern of quinoline or isoquinoline around the napthalene
core, such as 1,4 Q and 1,5 Q or 1,4 IQ and 1,5 IQ, only gave a difference of 0.03 eV in the
excited state triplet energies (2.50-2.64 eV). However, a ~0.13 eV in T1 energy difference was
observed when changing the substituents on the core structure between quinoline and
isoquinoline ranging between 2.21 eV to 2.59 eV. The solution values are representative of the
target values for the WOLED hosts but the powder is much lower in energy. However, the host
material should still be sufficient to use since the red dopant (PQIR) has a lower excited state
triplet energy than the host candidates for efficient energy transfer.
The blue dopant (-aD) was doped in 1,4 Q, 1,5 Q, 1,4 IQ and 1,5 IQ at low
concentration (1 wt %), and displayed emission profiles similar to observed PL in CBP host
where the dopant is the emitting state (Figure 3.21).
7
The excited state lifetimes (

= 2 to 3 ns)
125

and radiative rates [kr = (1.2 – 2.8) x 10
8
s
-1
] are similar across the series, which align with those
of -aD in solution, PMMA and CBP doped films.
21
CBP doped film was measured as a
reference which is typically ~77%. The obtained value for 1% -aD doped in CBP film produced
a 53% PLQY, suggesting that the true  PL values range between 0.60-0.82. 1,5 IQ shows the
most contribution of host emission between 350 nm - 400 nm. The emission of these films has
three distinct peaks found at 442 nm, 470 nm, and 503 nm which are only less than 1 nm shifted
from CBP. The same quenching feature of -aD emission is observed by the lower intensity of
the 442 nm peak, previously observed in similar films.
7
In order to further understand if
crystallinity is what causes the quenching observed in spin-coated doped films, thin-film XRD
measurements were conducted to further confirm the morphology of these films which will be
discussed in a later section.
350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
Normalized Emission Intensity (a.u.)
Wavelength (nm)
Neat Film
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ
350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
Normalized PL (a.u.)
Wavelength (nm)
1% -aD spincoated in
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ
CBP

Figure 3.21. Film Studies: Neat Film and -aD Doped Films

PQIR and -aD were doped in 1,5 Q since it showed a measured solution triplet energy
lower than the blue dopant (Figure 3.22). The PL of these materials are lower than expected
especially when vapor deposited as a thin film (Figure 3.22). Crystallization of these co-doped
system was not immediately observed in this instance but future device studies lead to
126

crystallization of the device and could be what is happening in these vapor deposited EML layer,
based on the lower , shown in Table 3.10.

Figure 3.22. Spin coated films of 1,5 Q with -aD and -5OD

Table 3.10. Spincoated Films
Host: 1,5 Q Lifetimes () PLQY ()
1% -aD
1.32 ns(18%), 2.32 ns
(82%)
40%
1% -5OD
2.44 ns(89%), 9.46 ns
(11%)
52%
5% PQIR 2.01 ms 60%

5% PQIR
60%
5% PQIR
=60%
1% -aD
=40%
1% -5OD
=52%
-2.51 eV
-6.11 eV -6.14 eV
-2.04 eV
-2.39 eV
-6.04 eV
127

The singlet and triplet excited state energies were calculated using TDDFT (B3LYP
functional, 6 311G** basis set; seen in Table 3.11). The S1 experimental value deviated from
calculation values by a maximum of 0.17 eV, while the T1 energies align well with solution
experimental values (0.05 eV) and deviate by ~0.3 eV from neat film measurements.

Table 3.11. Experimental and TDDFT S 1 and T 1 Energies
compound S 1 (eV) T 1 (eV)
soln/powder
S 1
a

(eV)
T 1 (eV)
a
1,4 Q 3.73 2.50/2.21 3.58 2.53
1,4 IQ 3.84 2.64/2.35 3.76 2.65
1,5 Q 3.75 2.53/2.22 3.58 2.53
1,5 IQ 3.84 2.61/2.59 3.80 2.66
a
TDDFT: B3LYP/631G**

3.5.4 TGA and DSC of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ.
The thermal properties were characterized with differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA) which are shown in Figures 3.23-3.24. The four host
materials have fairly low molecular weight (382.15 g/mol) but 1,5 Q and 1,5 IQ showed a Tg
around 170-200 C and melting points ranged 189-252 C for all four compounds. Only 1,5 Q
and 1,5 IQ crystallize upon cooling from the melt. The sublimation onset of the compounds in
the TGA data was observed in a small 3 C window for 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ. The
host materials are stable above our purification sublimation temperatures (190-200 C) and is
ideal for using under joule heating condition in an OLED/WOLED device.
128

0 50 100 150 200 250 300 350 400
-30
-25
-20
-15
-10
-5
0
5
10
Heat Flow (Endo Down)
Temperature
o
C
1,4 Q
0 50 100 150 200 250 300 350 400
-15
-10
-5
0
5
Heat Flow (Endo Down)
Temperature
o
C
1,4 IQ
0 50 100 150 200 250 300 350 400
-30
-25
-20
-15
-10
-5
0
5
10
Heat Flow (Endo Down)
Temperature
o
C
1,5 Q
0 50 100 150 200 250 300 350 400
-30
-25
-20
-15
-10
-5
0
5
10
Heat Flow (Endo Down)
Temperature
o
C
1,5 IQ

0 50 100 150 200 250 300 350 400
-40
-35
-30
-25
-20
-15
-10
-5
0
5
Heat Flow (Endo Down)
Temperature
o
C
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ
100 150 200 250 300
1
2
3
4
5
6
7
Heat Flow (Exo Up)
Temperature
o
C
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ
(170.183, 5.8624)
(200.629, 5.2884)

Figure 3.23. Differentials Scanning Calorimetry (DSC) for 1,4Q, 14, IQ, 1,5 Q, and 1,5 IQ.
129

50 100 150 200 250 300 350 400 450 500 550
0
50
100
Mass (wt %)
Temperature (C)
1,4 Q
50 100 150 200 250 300 350 400 450 500 550
0
50
100
Mass (wt %)
Temperature (C)
1,4 IQ
50 100 150 200 250 300 350 400 450 500 550
0
50
100
Mass (wt %)
Temperature (C)
1,5 Q
50 100 150 200 250 300 350 400 450 500 550
0
50
100
Mass (wt %)
Temperature (C)
1,5 IQ

50 100 150 200 250 300 350 400 450 500 550
0
50
100
Mass (wt %)
Temperature (C)
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ

Figure 3.24. Thermogravimetric Analysis of 1,4Q, 1,4IQ, 1,5 Q, and 1,5 IQ.
130

The melting point of the naphthalene counterparts can be compared to those of 1,4 IQ
and 1,5 IQ, as shown in Figure 3.25. 1,1’:4’,1”-ternapthalene and 1,1’:5’,1”-ternapthalene
have a large melting point difference (~100C) by simply changing the substitution pattern from
1,4 to 1,5. Interestingly, addition of a nitrogen atom using isoquinoline only had a difference of
~20 C, which suggests that the simple atomic transmutation led to intermolecular forces that are
closer in strength for 1,4 IQ and 1,5 IQ than their ternapthalene counterparts.

Figure 3.25. Measured melting point of the non-aza substituted counterparts of 1,4 IQ and 1,5 IQ.

3.5.5 Single Crystal XRD and Thin Film XRD
The thin film XRD and XRR measurements were performed to obtain some
representation of “the extent of crystallinity” in these films, as well as find the thickness (nm)
and surface roughness (nm) of the spincoated films. The thin film XRD of 1,4Q, 1,5Q, 1,4
IQ and 1,5 IQ were compared to CBP and blank quartz. The gaussian shape highly matches
the diffraction pattern of a blank quartz which suggests that the samples are amorphous, ideal
for OLEDs. The surface roughness of the naphthalene bridged systems were not
significantly different to CBP which minimizes the morphological difference on the film
surface of the new host compared with the reference host (Figure 3.26 and Table 3.12).
131

10 20 30 40
0
500
1000
1500
2000
2500
Intensity (counts)
2q (degrees)
blank
1,4 Q
1,4 IQ
1,5 Q
1,5 IQ
CBP

Figure 3.26. Thin Film XRD Measurements for 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ. Compounds were
referenced to CBP and a blank quartz.

Table 3.12. XRR thickness and surface roughness.
1% -aD
in
Thickness
(nm)
Surface roughness
(nm)
1,4 Q 138 6.04
1,4 IQ 92 5.60
1,5 Q 140 5.50
1,5 IQ 98 5.80
CBP 138 7.50

In order to understand the molecular packing of these naphthalene-based compounds, single
crystal x-ray diffraction was conducted. Unfortunately, the crystal quality of the isoquinoline
films were too small and the grown crystals did not obtain good diffraction patterns to help
gather data and solve the structure. However, good single crystals for the quinoline derivatives
were grown using sublimation and were used to obtain the XRD data. Figures 3.27 show the
single crystal of 1,4 Q and 1,5 Q solved with P2(1)/c space group. The bond lengths of the C-C
and C-N bonds are very similar for both structures ranging between 1.3-1.5 Å. It appears that 1,5
132

Q has less interaction or longer distances between each molecule when compared to the 1,4 Q
compound. 1,4 Q appears to have a slip-stack orientation but slightly twisted, such that there is
no evidence of large π- π stacking between the quinoline motif, shown in Figure 3.28. The
distance between π systems ranged ~3.33-4.11 Å. 1,5 Q does not have any overlap or interaction
within its π systems. The closest distance between molecules is between the 6- and 7- position of
quinoline hydrogens with a distance of 2.61 Å. The angles between the naphthalene core and the
quinoline core were used as planes and the measured angles were found to be 54.78 and 42.18
for 1,4 Q and 1,5 Q, respectively. The measured planes are shown in Figure 3.29.

.
Figure 3.27. (left) SXRD for 1,4 Q (right) SXRD for 1,5 Q. Both compounds are in a P2(1)/c space
group with R2 values less than 7%.
133

Figure 3.28. (left images top and bottom) 1,4 Q, (right images top and bottom) 1,5 Q. Images shows
how the molecules pack.

Figure 3.29. 1,4 Q measured angle between naphthalene and quinoline plane is 54.78 while 1,5 Q is
measured to be 42.18.
134

3.5.6 Electroluminescence of 1,4Q, 1,4 IQ, 1,5Q and 1,5 IQ.
Monochromatic devices of the host materials were fabricated to demonstrate the device
performance of these new hosts with -aD. 1,5 Q was used as the host in a blue monochromatic
device fabricated similar to the OLED stack in Chapter 2. A couple of other devices has been
made with this device, but Figure 3.30 shows the best device data obtained. 1,5 Q is a good host
material for -aD (2a), maintaining its sharp vibronic emission in vacuum deposited doped films
and exhibiting photoluminescence and electroluminescence emission between 450 nm-550 nm.
Doping concentration of 1vol. % -aD was deposited with and its photoluminescence and  PL
measured. Doping PQIR resulted in a broad emission between 550 nm to 700 nm, representative
of the dopant emission. The measured  PL values for -aD and PQIR in 1,5 Q were 0.52 and
0.60, respectively. It is worth noting that these measurements could be much higher since a
reference CBP film produced a  PL=0.52 which is expected to be 0.77. Blue monochromatic
OLEDs incorporated -aD doped in 1,5 Q at 1 vol. % with 10 nm to form the emission layer
(EML) thickness. The hole transport layer (HTL) consists of HATCN (10 nm) and TAPC (40
nm), and the electron transport layer (ETL) contains LiF (1 nm) and Bphen (40 nm). ITO was
used as the anode and aluminum as the cathode. Red monochromatic device was similarly
fabricated with the same stack with a change in the EML where the red dopant (PQIR) is doped
at 10% in a 20 nm layer EML. Figure 3.30 shows the device architecture (Figure 3.30A) used
to exhibit the device capabilities of -aD and PQIR in the new host. The electroluminescent
emission profile of -aD in CBP maintains a sharp and narrow feature with increasing current
from 1-100 mA/cm
2
while PQIR maintains its broad emission spectrum without growth of any
new peaks (Figure 3.30B and 3.30C).

135

400 500 600 700 800
0
0.2
0.4
0.6
0.8
1 -aD
PQIr
Intensity (a.u.)
Wavelength (nm)
B
350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
Normalized Electroluminescence (a.u.)
Wavelength (nm)
-aD in 1,5 Q
6V
7V
8V
9V
10V
C
450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
Normalized Electroluminescence (a.u.)
Wavelength (nm)
PQIR in 1,5 Q
5V
6V
7V
8V
9V
10V
D

10
-2
10
-1
10
0
10
1
10
2
0
2
4
6
8
10
12

-aD
PQIr
EQE (%)
Current density (mA/cm
2
)
E
0 2 4 6 8 10
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
-aD
PQIr
Luminescence (cd/m
2
)

Voltage (V)

F
0
100
200
300
400
Current density (mA/cm
2
)

Figure 3.30. Monochromatic devices made for 1% blue dopant in 1,5 Q host and 10% red dopant in
1,5Q.
The difference in performance between doping in CBP or 1,5 Q is evident when looking
at the EQEmax and Von of the fabricated devices, shown in Figure 3.30 D-E. 1,5 Q is more
A
136

resistive than CBP with turn-on voltage of 6 V and 3.5 V, respectively, and could be indicative
of charge imbalance in the device and further optimization is needed. The EQEmax value of 1,5
Q is 1% less than CBP but maintains a value of 1% at 1000 mA/cm
-2
. PQIR has a peak EQE of
9% but has a large roll-off. Based on the HOMO and LUMO energies of the host and the
dopants, exciplex is not expected for these dopant-host pairs. It is worth noting that these devices
are more conduction than mCBP as shown in Figure 3.30E. Further optimization of these
devices and studies of other host material are worth exploring but our proof-of-concept device
studies suggests that -aD and PQIR can be hosted by 1,5 Q but further optimization are needed
and use of the other host materials are worth exploring. Additionally, it would be worth
exploring electron only and hole only devices and compare to MD simulations to better
understand the host-dopant interaction.
3.5.7 TDDFT and MD Simulations: Addition of bridging naphthalene to produce 1,4Q, 1,4
IQ, 1,5Q and 1,5 IQ
The experimental and calculated triplet energies of 1,4 Q, 1,4 IQ, 1,5 Q and 1,5 IQ were
compared. It was observed that the highest deviation is ~0.05 eV between the obtained solution
triplet energy and the calculated value. It is evident that the quinoline derivatives did not have a
large variation in the experimental triplet energes from the isoquinoline derivatives. However,
the difference of ~0.10 eV between the two is enough to place its triplet energy above
(isoquinoline) or below (quinoline) those of -aD. The triplet of these materials in powder,
shifts ~0.3 eV in solid but the ~0.1 eV difference is maintained. In order to understand the small
difference in the triplet energies between these materials, the triplet spin density of these
materials was calculated (Figure 3.31). Comparison of these materials suggests that most of the
137

density in all four materials are concentrated on the naphthalene core which explains the
similarity to the experimental triplet energy to naphthalene (2.63 eV).
17b

Figure 3.31. Triplet Spin Density obtained using Geometry Optimization of multiplicity =3 with
B3LYP/631G***

The four molecules presented have very similar structures but the orientation of the
nitrogen relative to the naphthalene core or the dihedral angle between the two planes can dictate
the polarity of the molecule. Benzannulated molecules in the aD family are not solvatochromic
but since host polarity is important for other dopants, the dipole moment of the molecules was
calculated to gain an understanding of any polarity differences. Figure 3.32 illustrates the
direction of the dipole in all four compounds. The dipole of 1,4 Q and 1,4 IQ are along the plane
of the aromatic naphthalene with comparable dipole moments ~ 1.56 Debye and 1.81 Debye,
respectively. 1,5 Q has the highest dipole moment perpendicular to the naphthalene plane ~3.19
Debye whereas 1,5 IQ has the smallest dipole moment close to 0 Debye. The obtained value
138

suggests that the compounds are not expected to have similar polarities with increasing polarity
going from 1,5 IQ < 1,4 Q < 1,4 IQ < 1,5 Q.
1,4 Q (1.5600 Debye)

1,5 Q (3.1896 Debye)

1,4 IQ (1.8064)

1,5 IQ (0.0015)

139

Figure 3.32. Dipole moments associated with the host materials using TDDFT.

140

The reorganization energies were obtained to gain insight in the kinetics of
intermolecular hole and electron hopping transfer and access the charge carrier conduction of the
naphthalene-bridged host materials. Additionally, these were compared to CBP and mCBP-trans
and mCBP-cis, as shown in Table 3.13. A lower value of reorganization energy decreases the
carrier hopping barrier between molecules which leads to better conductivity. The reorganization
energy for holes were comparable for 1,4 Q (0.35 eV), 1,5 Q (0.32 eV) and 1,5 IQ (0.32 eV).
CBP and mCBP (trans and cis) have a lower reorganization energy (<0.14 eV) while 1,4 IQ
(0.41 eV) had the highest. The electron reorganization energy is highest for CBP and mCBP
(trans and cis) with values above 0.51 eV while the naphthalene derivatives ranged between
0.19-0.32 eV with 1,5 IQ found to be the most conductive. The low reorganization energies for
the naphthalene-bridged family suggests efficient carrier transport in the system and are likely to
be more conductive than CBP and mCBP.
Table 3.13. Calculated Reorganization Energy

Reorganization energy (eV)
electron hole
1,4 Q 0.319 0.346
1,4 IQ 0.314 0.414
1,5 Q 0.250 0.315
1,5 IQ 0.190 0.321
CBP 0.514 0.137

mCBP-trans 0.521 0.048
mCBP-cis 0.542 0.048

141

3.5.8 Expansion: Addition of Steric Bulk in the Napthalene-Based Systems
Further modification in the presented core structures can be made to rigidify the free
rotating bonds to achieve a larger class of host material for this study. One way to modify these
is to increase the steric bulk by addition of phenyl, tolyl or mesityl groups in the quinoline and
isoquinoline core. A donor-acceptor type system can be designed with these materials with
bridging naphthalene to tune the singlet and triplet energy gap of these materials to dictate the
CT character found in these systems. The compounds shown in Figure 3.33 were analyzed using
molecular modeling to identify other potential candidates which can be synthesized in the future.
For (a) N6BL and (b) N6BO, different substituents were added to understand the structure-
property relationship in these systems. R = H (N6BL/N6BO), isopropyl (I6BL/I6BO), mesityl
(M6BL/M6BO), and phenyl (P6BL/P6BO).

Figure 3.33. Host materials analyzed for efficiency in WOLEDs. (a) N6BL. (b) N6BO. (c) 2-
Biquinoline-6-Binaphthalene. (d) 3-Biquinoline-6-Binaphthalene. (e) 4,4′-Bis(N-carbazolyl)-1,1′-
biphenyl (CBP).
Table 3.14 shows the results from the B3LYP/631G** calculation. The desired host
material T1 energy level is around 2.5 eV or lower since the blue fluorescent dopant which will
be used in these WOLEDs has a T 1 energy level around 2.56 eV. Our initial version of these
142

compounds suggests that the triplet energies obtained in solution measurement in 2-MeTHF
deviates by ~0.17 eV at most which is a small margin of error. However, neat film data gave
~0.3 eV red-shift in the triplet energies of these system. By addition of bulky groups such as
mesityl, isopropyl, phenyl and other groups which can prevent the planarity and - stacking in
these systems, it is expected that the large deviation to neat film data should be minimized. The
calculation data suggests that all the systems calculated has a triplet energy below of -aD (2.56
eV) and should be considered as the tier 2 of materials to further optimize the system Sun et. al
introduced.
22

Table 3.14 TDDFT Results for substituted naphthalene bridged biquinoline systems
COMPOUND S 1 (EV) T 1 (EV)
ΔE ST
(EV)
HOMO
(EV)
LUMO
(EV)
N6BL 3.603 2.532 1.071 -5.714 -1.654
I6BL 3.612 2.540 1.072 -5.619 -1.548
M6BL 3.524 2.514 1.010 -5.638 -1.654
P6BL 3.371 2.451 0.920 -5.586 -1.780
N6BO 3.591 2.519 1.072 -5.704 -1.657
I6BO 3.585 2.527 1.058 -5.622 -1.578
M6BO 3.535 2.515 1.020 -5.644 -1.644
P6BO 3.401 2.471 0.930 -5.603 -1.758
2-BIQUIN-6-
NAPH
3.337 2.452 0.885 -5.611 -1.872
3-BIQUIN-6-
NAPH
3.459 2.513 0.946 -5.772 -1.88
CBP 3.550 2.958 0.592 -5.314 -1.244

3.6 Conclusion
A variety of host materials were developed for WOLEDs to properly align the S1 and T1
energies to the aD fluorescent dopant and red phosphor to attain Förster energy transfer between
host → dopant and triplet exciton transfer from host → phosphor by utilizing the long-lived
diffusion of triplets. Fluorenone based structure, biquinoline systems, spirofluorene (SAS) and
H2P systems aligned with the T 1  2.56 eV target values but had problems with crystallization,
143

low , and host PL emission. Napthalene bridged quinoline or isoquinoline structures (1,4 Q,
1,5 Q, 1,4 IQ and 1,5 IQ) displayed potential as host materials with non-heteroatom bonds
which are stronger bonds than heteroatom bonds which is believed to be the degradation pathway
through bond breaking mechanisms in OLEDs. The triplet energies in solution ranged between
2.50 eV to 2.64 eV, where 1,4 Q and 1,5 Q values are below aD dopants. Doped films studies
produced  above 60% which can be further optimized with proposed structures incorporating
steric bulk. Device fabrication of blue monochromatic devices gave EQE max = 2% for -aD
doped in 1,5Q which is comparable to the 2.5% EQE obtained for CBP. The EQE max=8% but
further optimization is required which can raise the EQE values and stabilize the fast device roll-
off. The reorganization energy calculation and device data align with 1,5 Q > CBP > mCBP
trend, where 1,5 Q is the most conductive. Development of these host materials with non-
heteroatom bonds allows expands its viability as a host material for phosphorescent dopants and
small S1-T1 energy gap fluorescent dopants. The latter can have utility in TADF or MR-TADF
systems which will require further exploration. The host materials presented in this chapter may
not have been the most optimized for aD family but showed potential. Further optimizations are
needed to study the triplet exciton trapping using this host with aD for the proposed WOLED
structure, but preliminary data suggests promising results and indicate the utility of these
materials for other OLED dopants.

144

3.7 Experimental
3.7.1 Synthesis
Four compounds were synthesized with similar reaction condition with different
precursors. Synthesis of 1,4 Q and 1,5 Q required the use of 3-quinoline boronic acid pinacol
ester with 1,4-dibromonapthalene and 1,5-dibromonapthalene, respectively. 4-isoquinoline
boronic acid pinacol ester with 1,4-dibromonapthalene or 1,5-dibromonapthlene produced 1,4 IQ
and 1,5 IQ, respectively.

3-quinoline boronic acid pinacol ester and CsF was added in a round bottom flask set-up for a
condenser.

1,4 Q: 3-quinoline boronic acid pinacol ester and CsF were added in a round bottom flask set-up
for a condenser. 70mL of THF and 30 mL of DI water were added then nitrogen gas was
bubbled into solution while stirring for 10 minutes. 1,4-dibromonapthalene and Pd(dppf)Cl2
were added to the reaction flask and allowed to stir for 15 minutes under nitrogen environment.
The reaction was taken to reflux at 60 degrees Celsius. Some formation of product was observed
after 24 hours but the reaction was stirred for a total of 48 hours to produce higher product
formation. The reaction was cooled to room temperature, added 200 mL of DI water to the
crude, and extracted using 3 x 150 mL ethylacetate. The organic phase was dried using sodium
sulfate. In order to purify the target product, it was ran in a normal phase column using hexanes
and ethylacetate as solvents. The reaction yielded ~60% of 1,4 Q which is a white solid. 1,4 Q
145

was also sublimed after column purification. Elemental analysis: calc (C: 87.93, H:4.74,
N:7.32), found (C:87.98, H:4.87, N:7.34)

1,4 IQ: 4-isoquinoline boronic acid pinacol ester and CsF were added in a round bottom flask
set-up for a condenser. 70mL of THF and 30 mL of DI water were added then nitrogen gas was
bubbled into solution while stirring for 10 minutes. 1,4-dibromonapthalene and Pd(dppf)Cl2
were added to the reaction flask and allowed to stir for 15 minutes under nitrogen environment.
The reaction was taken to reflux at 60 degrees Celsius. Some formation of product was observed
after 24 hours but the reaction was stirred for a total of 48 hours to produce higher product
formation. The reaction was cooled to room temperature, added 200 mL of DI water to the
crude, and extracted using 3 x 150 mL ethylacetate. The organic phase was dried using sodium
sulfate. In order to purify the target product, it was ran in a normal phase column using hexanes
and ethylacetate as solvents. The reaction yielded ~20% of 1,4 IQ which is a white solid.
1,4 IQ was also sublimed after column purification. Elemental analysis: calc (C: 87.93, H: 4.74,
N:7.32), found (C:87.72, H:4.93, N: 7.18)

1,5 Q: 3-quinoline boronic acid pinacol ester and CsF were added in a round bottom flask set-up
for a condenser. 70mL of THF and 30 mL of DI water were added then nitrogen gas was
146

bubbled into solution while stirring for 10 minutes. 1,5-dibromonapthalene and Pd(dppf)Cl2
were added to the reaction flask and allowed to stir for 15 minutes under nitrogen environment.
The reaction was taken to reflux at 60 degrees Celsius. Some formation of product was observed
after 24 hours but the reaction was stirred for a total of 48 hours to produce higher product
formation. The reaction was cooled to room temperature, added 200 mL of DI water to the
crude, and extracted using 3 x 150 mL ethylacetate. The organic phase was dried using sodium
sulfate. In order to purify the target product, it was ran in a normal phase column using hexanes
and ethylacetate as solvents. The reaction yielded 50-60% of 1,5 Q which is a white solid. 1,5
Q was also sublimed after column purification. Elemental analysis: calc (C: 87.93, H: 4.74,
N:7.32), found (C:87.31, H:4.83, N: 7.00)

1,5 IQ: 4-isoquinoline boronic acid pinacol ester and CsF were added in a round bottom flask
set-up for a condenser. 70mL of THF and 30 mL of DI water were added then nitrogen gas was
bubbled into solution while stirring for 10 minutes. 1,5-dibromonapthalene and Pd(dppf)Cl2
were added to the reaction flask and allowed to stir for 15 minutes under nitrogen environment.
The reaction was taken to reflux at 60 degrees Celsius. Some formation of product was observed
after 24 hours but the reaction was stirred for a total of 48 hours to produce higher product
formation. The reaction was cooled to room temperature, added 200 mL of DI water to the
crude, and extracted using 3 x 150 mL ethylacetate. The organic phase was dried using sodium
sulfate. In order to purify the target product, it was ran in a normal phase column using hexanes
and ethylacetate as solvents. The reaction yielded 55% of 1,5 IQ which is a white solid. 1,5 IQ
147

was also sublimed after column purification. Elemental analysis: calc (C: 87.93, H: 4.74,
N:7.32), found (C:87.97, H:4.55, N: 7.58)

148

3.7.2 Nuclear Magnetic Resonance

1
HNMR Spectra for 1,4 Q

149

13
CNMR Spectra for 1,4 Q

150

1
HNMR Spectra for 1,4 IQ

151

13
CNMR Spectra for 1,4 IQ
152

1
HNMR Spectra for 1,5 Q
153

13
CNMR Spectra for 1,5 Q

154

1
HNMR Spectra for 1,5 IQ

155

13
CNMR Spectra for 1,5 IQ
3.7.3 Matrix Assisted Laser Desorption Ionization (MALDI) Data

MALDI Spectra for 1,4 Q

156

MALDI Spectra for 1,4 IQ

157

MALDI Spectra for 1,5 Q

158

MALDI Spectra for 1,5 IQ
3.7.4 Photophysical Characterization
All samples in fluid solution were dissolved in chosen solvent with absorbance between
0.05-0.15 for high absorbing materials or 0.5 for those with CT bands at higher energies.
Spincoated neat films were made with 1 mg in 1mL of dichloromethane. Doped films were
dissolved in DCM and were made with 1 volume percent -aD in chosen host or 5-10% volume
percent of phosphor in the host matrix. 0.1 g of PMMA pellets were mixed with 1mL of
dichloromethane. 1mL of desired solution was spin coated on a quartz substrate (2cm x 2 cm)
using a pipet with the substrate rotating at 700 rpm for 45 seconds. The film was left to air dry.
The UV-visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer.
Steady State fluorescence emission measurements were performed using a QuantaMaster Photon
Technology International spectrofluorometer. Gated phosphorescence measurements were
159

carried on the fluorimeter with 500 microsecond delay where the sample is in 77 K temperature.
All reported spectra are corrected for photomultiplier response. Fluorescence lifetime
measurements were performed using an IBH Fluorocube instrument equipped with 331 nm LED
and 405 nm laser excitation sources using a time-correlated single photon counting method.
Quantum yield values were obtained using a C9920 Hamamatsu integrating sphere system.
3.7.5 Device Fabrication
OLEDs were fabricated and tested by Glass substrates with pre-patterned, 1 mm wide
indium tin oxide (ITO) stripes were cleaned by sequential sonication in tergitol, deionized water,
acetone, and isopropanol, followed by 15 min UV ozone exposure. Organic materials and metals
were deposited at rates of 0.5-2 Å/s through shadow masks in a vacuum thermal evaporator with
a base pressure of 10-7 Torr. A separate shadow mask was used to deposit 1 mm wide stripes of
100 nm thick Al films perpendicular to the ITO stripes to form the cathode, resulting in 2 mm
2

device area. A semiconductor parameter analyzer (HP4156A) and a calibrated large area
photodiode that collected all light exiting the glass substrate were used to measure the J-
V-luminance characteristics. The device spectra were measured using a fiber-coupled
spectrometer.
3.7.6 Thermogravimetric Analysis and Differential Scanning Calorimetry
Thermal gravimetric analysis (TGA) was performed on a TGA Q50 instrument and
samples were run in an alumina crucible under a flowing nitrogen atmosphere with a heating rate
of 10 C/min. Differential scanning calorimetry (DSC) traces were obtained using Perkin-Elmer
DSC 8000 with a scan rate of 10C/min. Sample size was ~3-5mg and samples were used as
obtained after sublimation.
160

3.7.7 Electrochemistry
Cyclic voltammetry and differential pulsed voltammetry were performed using a
VersaSTAT potentiostat measured at 100 mV/s scan. Anhydrous acetonitrile (DriSolv) from
Sigma Aldrich was used as the solvent under nitrogen environment, and 0.1 M
tetra(n-butyl)ammoniumhexafluorophosphate (TBAF) was used as the supporting electrolyte.
Dichloromethane was also used to to measure the oxidation of the host materials. A glassy
carbon rod was used as the working electrode; a platinum wire was used as the counter electrode,
and a silver wire was used as a pseudoreference electrode. The redox potentials are based on
values measured from differential pulsed voltammetry and are reported relative to a
ferrocene/ferrocenium (Cp2Fe/Cp2Fe
+
) redox couple used as an internal reference, while
electrochemical reversibility was determined using cyclic voltammetry.
3.7.8 Molecular Modeling
TDDFT Modeling
All calculations reported in this work were performed using the Q-Chem 5.1 program
23
.
Ground-state optimization calculations were performed using the B3LYP functional and the 6-
31G** basis set. Time dependent density functional theory (TDDFT) calculations were used to
obtain the excitation energies and optimized geometries of the S1 state at the same level. The
same TDDFT calculations were used to determine the partial charges on each atom within the
host and dopant material for Molecular Dynamics (MD) modeling.
Monte Carlo Modeling
Python 3.2 was used to prepare data sets for MD modeling. XenoView v.3.8.1.0
(available online at http://www.vemmer.org/xenoview/xenoview.html) was used for visualization
and assisting in molecular dynamics modeling. A random configuration of geometry-optimized
161

host and dopant molecules was created using an in-house Python script. The ratio of host to
dopant molecules was 99:1 (1% doped). All molecules are separated by 25Å per the Python
script and separation was confirmed visually using XenoView before proceeding.
Molecular Dynamics Modeling
The open source software LAMMPS, also known as the Large-scale Atomic/Molecular
Massively Parallel Simulator
24
developed at Sandia National Laboratories, USA
(http://lammps.sandia.gov) was used to carry out MD simulations. Bonds in the output files from
the in-house Monte Carlo Python 3.2 script were defined using XenoView and intial LAMMPS
files were created. Using an additional in-house Python 3.2 script, charges from TD-DFT
calculations were inserted into the LAMMPS file for all host and dopant molecules.
Once input data sets were finalized, ensembles were utilized to carry out Nose-Hoover
style non-Hamiltonian equations of motion over time, generating updated positions and
velocities for molecules in the initial data set. First, a nvt canonical ensemble (n = number of
atoms, v = volume, t = temperature) was employed at 300 K. Then, a npt (n = number of atoms,
p = pressure, t = temperature) isothermal-isobaric ensemble has not been conducted. The
combination of these two ensembles will produce the final equilibrium state for the data set.
Based on this equilibrium state, molecular orientation and arrangement can be interpreted to
evaluate potential for increasing internal quantum efficiency and illuminate causes of self-
quenching.
Hole and Electron Reorganization Energy
Density functional theory was used for ground state optimization and estimation of
HOMO/LUMO energies, T1 (triplet) state energy and hole/electron reorganization energies (
+
,
162


-
) for the molecules. Hole/electron reorganization energies were calculated per the Equations 1
and 2.
λ
+
= (𝐸 +
0
−𝐸 0
0
) + (𝐸 0
+
−𝐸 +
+
) (1)
λ
−
= (𝐸 −
0
−𝐸 0
0
) + (𝐸 0
−
−𝐸 −
−
) (2)
In the above expressions, the superscripts represent the state of the molecule (0, +, and – for
neutral, anionic and cationic states respectively) while subscripts indicate the state at which the
structure is optimized. For instance, according to this terminology 𝐸 +
0
represents the energy of
the neutral ground state at the cation-optimized geometry.
163

3.8 Chapter 3 References
1. (a) Kim, S.; Bae, H. J.; Park, S.; Kim, W.; Kim, J.; Kim, J. S.; Jung, Y.; Sul, S.; Ihn, S.-
G.; Noh, C.; Kim, S.; You, Y., Degradation of blue-phosphorescent organic light-emitting
devices involves exciton-induced generation of polaron pair within emitting layers. Nature
Communications 2018, 9 (1), 1211; (b) Kondakov, D. Y., Characterization of triplet-triplet
annihilation in organic light-emitting diodes based on anthracene derivatives. Journal of Applied
Physics 2007, 102 (11), 114504; (c) Giebink, N.; D’Andrade, B.; Weaver, M.; Mackenzie, P.;
Brown, J.; Thompson, M.; Forrest, S., Intrinsic luminance loss in phosphorescent small-molecule
organic light emitting devices due to bimolecular annihilation reactions. Journal of Applied
Physics 2008, 103, 044509-044509; (d) Giebink, N.; Dandrade, B.; Weaver, M.; Brown, J.;
Forrest, S., Direct evidence for degradation of polaron excited states in organic light emitting
diodes. Journal of Applied Physics 2009, 105, 124514-124514; (e) Aziz, H.; Popovic, Z. D.,
Degradation Phenomena in Small-Molecule Organic Light-Emitting Devices. Chemistry of
Materials 2004, 16 (23), 4522-4532; (f) Wang, D.; Cheng, C.; Tsuboi, T.; Zhang, Q.,
Degradation Mechanisms in Blue Organic Light-Emitting Diodes. CCS Chemistry 2020, 2 (4),
1278-1296.
2. (a) Giebink, N.; Forrest, S. R., Direct evidence for degradation of polaron excited states
in organic light emitting diodes. J. Appl. Phys. 2009, 105, 124514; (b) Giebink, N. C.;
D’Andrade, B. W.; Weaver, M. S.; Mackenzie, P. B.; Brown, J. J.; Thompson, M. E.; Forrest, S.
R., Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due
to bimolecular annihilation reactions. J. Appl. Phys. 2008, 103, 044509; (c) Zhang, Y.; Lee, J.;
Forrest, S. R., Ten-Fold Increase in the Lifetime of Blue Phosphorescent Organic Light Emitting
Diodes. Nature Commun. 2014, 5, 5008.
3. McElvain, J.; Antoniadis, H.; Hueschen, M. R.; Miller, J. N.; Roitman, D. M.; Sheats, J.
R.; Moon, R. L., Formation and growth of black spots in organic light‐emitting diodes. Journal
of Applied Physics 1996, 80 (10), 6002-6007.
4. (a) Rodella, F.; Bagnich, S.; Duda, E.; Meier, T.; Kahle, J.; Athanasopoulos, S.; Köhler,
A.; Strohriegl, P., High Triplet Energy Host Materials for Blue TADF OLEDs—A Tool Box
Approach. Frontiers in Chemistry 2020, 8 (657); (b) Hong, G.; Gan, X.; Leonhardt, C.; Zhang,
Z.; Seibert, J.; Busch, J. M.; Bräse, S., A Brief History of OLEDs—Emitter Development and
Industry Milestones. Advanced Materials 2021, 33 (9), 2005630; (c) Im, Y.; Byun, S. Y.; Kim, J.
H.; Lee, D. R.; Oh, C. S.; Yook, K. S.; Lee, J. Y., Recent Progress in High-Efficiency Blue-
Light-Emitting Materials for Organic Light-Emitting Diodes. Advanced Functional Materials
2017, 27 (13), 1603007; (d) Lee, J.-H.; Chen, C.-H.; Lee, P.-H.; Lin, H.-Y.; Leung, M.-k.; Chiu,
T.-L.; Lin, C.-F., Blue organic light-emitting diodes: current status, challenges, and future
outlook. Journal of Materials Chemistry C 2019, 7 (20), 5874-5888; (e) Zou, S.-J.; Shen, Y.;
Xie, F.-M.; Chen, J.-D.; Li, Y.-Q.; Tang, J.-X., Recent advances in organic light-emitting diodes:
toward smart lighting and displays. Materials Chemistry Frontiers 2020, 4 (3), 788-820; (f)
Zysman-Colman, E., Recent advances in materials for organic light emitting diodes. Beilstein
journal of organic chemistry 2018, 14, 1944-1945; (g) Lee, H.; Karthik, D.; Lampande, R.; Ryu,
J. H.; Kwon, J. H., Recent Advancement in Boron-Based Efficient and Pure Blue Thermally
Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Frontiers in
Chemistry 2020, 8 (373).
164

5. Hosokawa, C.; Higashi, H.; Nakamura, H.; Kusumoto, T., Highly efficient blue
electroluminescence from a distyrylarylene emitting layer with a new dopant. Appl. Phys. Lett.
1995, 67 (26), 3853-3855.
6. Deaton, J. C., Kondakova, Marina E., Giesen, David J., Begley, William J., Kondakov,
Denis Y. Hybrid OLED with Fluorescent and Phosphorescent Layers. 2008.
7. Tadle, A. C.; El Roz, K. A.; Soh, C. H.; Sylvinson Muthiah Ravinson, D.; Djurovich, P.
I.; Forrest, S. R.; Thompson, M. E., Tuning the Photophysical and Electrochemical Properties of
Aza-Boron-Dipyridylmethenes for Fluorescent Blue OLEDs. Advanced Functional Materials n/a
(n/a), 2101175.
8. Sylvinson M. R, D.; Chen, H.-F.; Martin, L. M.; Saris, P. J. G.; Thompson, M. E., Rapid
Multiscale Computational Screening for OLED Host Materials. ACS Applied Materials &
Interfaces 2019, 11 (5), 5276-5288.
9. Gao, M.; Lee, T.; Burn, P. L.; Mark, A. E.; Pivrikas, A.; Shaw, P. E., Revealing the
Interplay between Charge Transport, Luminescence Efficiency, and Morphology in Organic
Light-Emitting Diode Blends. Advanced Functional Materials 2020, 30 (9), 1907942.
10. Idris, M.; Coburn, C.; Fleetham, T.; Milam-Guerrero, J.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E., Phenanthro[9,10-d]triazole and imidazole derivatives: high triplet energy host
materials for blue phosphorescent organic light emitting devices. Materials Horizons 2019, 6 (6),
1179-1186.
11. Lin, N.; Qiao, J.; Duan, L.; Wang, L.; Qiu, Y., Molecular Understanding of the Chemical
Stability of Organic Materials for OLEDs: A Comparative Study on Sulfonyl, Phosphine-Oxide,
and Carbonyl-Containing Host Materials. The Journal of Physical Chemistry C 2014, 118 (14),
7569-7578.
12. Klärner, G.; Davey, M. H.; Chen, W.-D.; Scott, J. C.; Miller, R. D., Colorfast Blue-Light-
Emitting Random Copolymers Derived from Di-n-hexylfluorene and Anthracene. Advanced
Materials 1998, 10 (13), 993-997.
13. (a) List, E. J. W.; Guentner, R.; Scanducci de Freitas, P.; Scherf, U., The Effect of Keto
Defect Sites on the Emission Properties of Polyfluorene-Type Materials. Advanced Materials
2002, 14 (5), 374-378; (b) Gamerith, S.; Gadermaier, C.; Scherf, U.; List, E. J. W., The Origin of
the Green Emission Band in Polyfluorene Type Polymers. In Physics of Organic
Semiconductors, 2005; pp 153-181; (c) Lemmer, U.; Heun, S.; Mahrt, R. F.; Scherf, U.;
Hopmeier, M.; Siegner, U.; Go¨bel, E. O.; Mu¨llen, K.; Ba¨ssler, H., Aggregate fluorescence in
conjugated polymers. Chemical Physics Letters 1995, 240 (4), 373-378.
14. (a) Zhang, T.; Liu, D.; Wang, Q.; Wang, R.; Ren, H.; Li, J., Deep-blue and white organic
light-emitting diodes based on novel fluorene-cored derivatives with naphthylanthracene
endcaps. Journal of Materials Chemistry 2011, 21 (34), 12969-12976; (b) Li, G.; Zheng, J.;
Klimes, K.; Zhu, Z.-Q.; Wu, J.; Zhu, H.; Li, J., Novel Carbazole/Fluorene-Based Host Material
for Stable and Efficient Phosphorescent OLEDs. ACS Applied Materials & Interfaces 2019, 11
(43), 40320-40331; (c) Saitoh, A.; Yamada, N.; Yashima, M.; Okinaka, K.; Senoo, A.; Ueno, K.;
Tanaka, D.; Yashiro, R., 11.2: Novel Fluorene-Based Blue Emitters for High Performance
OLEDs. Sid Symposium Digest of Technical Papers 2004, 35.
165

15. (a) Jiang, Z.; Yao, H.; Zhang, Z.; Yang, C.; Liu, Z.; Tao, Y.; Qin, J.; Ma, D., Novel
Oligo-9,9′-spirobifluorenes through ortho-Linkage as Full Hydrocarbon Host for Highly
Efficient Phosphorescent OLEDs. Organic Letters 2009, 11 (12), 2607-2610; (b) Sicard, L. J.;
Li, H.-C.; Wang, Q.; Liu, X.-Y.; Jeannin, O.; Rault-Berthelot, J.; Liao, L.-S.; Jiang, Z.-Q.; Poriel,
C., C1-Linked Spirobifluorene Dimers: Pure Hydrocarbon Hosts for High-Performance Blue
Phosphorescent OLEDs. Angewandte Chemie International Edition 2019, 58 (12), 3848-3853;
(c) Saragi, T. P. I.; Spehr, T.; Siebert, A.; Fuhrmann-Lieker, T.; Salbeck, J., Spiro Compounds
for Organic Optoelectronics. Chemical Reviews 2007, 107 (4), 1011-1065; (d) Fan, C.; Chen, Y.;
Gan, P.; Yang, C.; Zhong, C.; Qin, J.; Ma, D., Tri-, tetra- and pentamers of 9,9'-spirobifluorenes
through full ortho-linkage: high triplet-energy pure hydrocarbon host for blue phosphorescent
emitter. Organic letters 2010, 12 (24), 5648-5651; (e) Wei, W.; Djurovich, P. I.; Thompson, M.
E., Properties of Fluorenyl Silanes in Organic Light Emitting Diodes. Chemistry of Materials
2010, 22 (5), 1724-1731; (f) Ren, B.-Y.; Zhong, D.-K.; Sun, Y.-G.; Zhao, X.-H.; Zhang, Q.-J.;
Liu, Y.; Jurow, M.; Sun, M.; Zhang, Z.-S.; Zhao, Y., Quinolyl functionalized spiro[fluorene-9,9′-
xanthene] host materials with bipolar characteristics for green and red phosphorescent organic
light-emitting diodes. Organic Electronics 2016, 36, 140-147.
16. (a) Tonzola, C. J.; Kulkarni, A. P.; Gifford, A. P.; Kaminsky, W.; Jenekhe, S. A., Blue-
Light-Emitting Oligoquinolines: Synthesis, Properties, and High-Efficiency Blue-Light-Emitting
Diodes. Advanced Functional Materials 2007, 17 (6), 863-874; (b) Shetty, A. S.; Liu, E. B.;
Lachicotte, R. J.; Jenekhe, S. A., X-ray Crystal Structures and Photophysical Properties of New
Conjugated Oligoquinolines. Chemistry of Materials 1999, 11 (9), 2292-2295; (c) Agrawal, A.
K.; Jenekhe, S. A., Thin-film processing and optical properties of conjugated rigid-rod
polyquinolines for nonlinear optical applications. Chemistry of Materials 1992, 4 (1), 95-104; (d)
Agrawal, A. K.; Jenekhe, S. A., Synthesis and processing of heterocyclic polymers as electronic,
optoelectronic, and nonlinear optical materials. 2. New series of conjugated rigid-rod
polyquinolines and polyanthrazolines. Macromolecules 1993, 26 (5), 895-905; (e) Agrawal, A.
K.; Jenekhe, S. A., Synthesis and processing of heterocyclic polymers as electronic,
optoelectronic, and nonlinear optical materials. 3. New conjugated polyquinolines with electron-
donor or -acceptor side groups. Chemistry of Materials 1993, 5 (5), 633-640; (f) Zhang, X.;
Shetty, A. S.; Jenekhe, S. A., Electroluminescence and Photophysical Properties of
Polyquinolines. Macromolecules 1999, 32 (22), 7422-7429; (g) Zhu, Y.; Alam, M. M.; Jenekhe,
S. A., Regioregular Head-to-Tail Poly(4-alkylquinoline)s: Synthesis, Characterization, Self-
Organization, Photophysics, and Electroluminescence of New n-Type Conjugated Polymers.
Macromolecules 2003, 36 (24), 8958-8968; (h) Kwon, T. W.; Alam, M. M.; Jenekhe, S. A., n-
Type Conjugated Dendrimers: Convergent Synthesis, Photophysics, Electroluminescence, and
Use as Electron-Transport Materials for Light-Emitting Diodes. Chemistry of Materials 2004, 16
(23), 4657-4666; (i) Tao, J.; Tretiak, S.; Zhu, J.-X., Absorption Spectra of Blue-Light-Emitting
Oligoquinolines from Time-Dependent Density Functional Theory. The Journal of Physical
Chemistry B 2008, 112 (44), 13701-13710.
17. (a) Pan, Y.; Huang, J.; Wang, Z. M.; Yu, D.; Yang, B.; Ma, Y., Computational
investigation on the large energy gap between the triplet excited-states in acenes. RSC Adv. 2017,
7, 26697-26703; (b) Baranac-Stojanović, M.; Stojanović, M.; Aleksić, J., Triplet state
(anti)aromaticity of some monoheterocyclic analogues of benzene, naphthalene and anthracene.
New Journal of Chemistry 2021, 45 (11), 5060-5074.
166

18. Alezi, D.; Belmabkhout, Y.; Suyetin, M.; Bhatt, P. M.; Weseliński, Ł. J.; Solovyeva, V.;
Adil, K.; Spanopoulos, I.; Trikalitis, P. N.; Emwas, A.-H.; Eddaoudi, M., MOF Crystal
Chemistry Paving the Way to Gas Storage Needs: Aluminum-Based soc-MOF for CH4, O2, and
CO2 Storage. Journal of the American Chemical Society 2015, 137 (41), 13308-13318.
19. Assary, R. S.; Brushett, F. R.; Curtiss, L. A., Reduction potential predictions of some
aromatic nitrogen-containing molecules. RSC Advances 2014, 4 (101), 57442-57451.
20. (a) Schramm, S.; Weiß, D., Chapter Two - Fluorescent heterocycles: Recent trends and
new developments. In Advances in Heterocyclic Chemistry, Scriven, E. F. V.; Ramsden, C. A.,
Eds. Academic Press: 2019; Vol. 128, pp 103-179; (b) Licha, K.; Resch-Genger, U., 4.08 -
Fluorescent Reporters and Optical Probes. In Comprehensive Biomedical Physics, Brahme, A.,
Ed. Elsevier: Oxford, 2014; pp 85-109; (c) Lakowicz, J. R., Principles of Fluorescence
Spectroscopy. 1999; (d) Loudet, A.; Burgess, K., Bodipy Dyes and Their Derivatives: Syntheses
and Spectroscopic Properties. Chem. Rev. 2007, 107 (11), 4891; (e) Golden, J. H.; Facendola, J.
W.; Sylvinson M. R, D.; Baez, C. Q.; Djurovich, P. I.; Thompson, M. E., Boron
Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based Chromophores. The
Journal of organic chemistry 2017, 82 (14), 7215-7222; (f) Chen, J. J.; Conron, S. M.; Erwin, P.;
Dimitriou, M.; McAlahney, K.; Thompson, M. E., High-Efficiency BODIPY-Based Organic
Photovoltaics. ACS Applied Materials & Interfaces 2015, 7 (1), 662-669.
21. (a) Berezin, M. Y.; Achilefu, S., Fluorescence lifetime measurements and biological
imaging. Chemical reviews 2010, 110 (5), 2641-2684; (b) Ostroverkhova, O., Organic
Optoelectronic Materials: Mechanisms and Applications. Chemical Reviews 2016, 116 (22),
13279-13412.
22. Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R.,
Management of singlet and triplet excitons for efficient white organic light-emitting devices.
Nature 2006, 440 (7086), 908-912.
23. Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange,
A. W.; Behn, A.; Deng, J.; Feng, X.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.;
Kaliman, I.; Khaliullin, R. Z.; Kuś, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard,
R. M.; Rohrdanz, M. A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.;
Zuev, D.; Albrecht, B.; Alguire, E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.;
Brandhorst, K.; Bravaya, K. B.; Brown, S. T.; Casanova, D.; Chang, C.-M.; Chen, Y.; Chien, S.
H.; Closser, K. D.; Crittenden, D. L.; Diedenhofen, M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.;
Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.; Ghysels, A.; Golubeva-Zadorozhnaya, A.; Gomes, J.;
Hanson-Heine, M. W. D.; Harbach, P. H. P.; Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.;
Jagau, T.-C.; Ji, H.; Kaduk, B.; Khistyaev, K.; Kim, J.; Kim, J.; King, R. A.; Klunzinger, P.;
Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.; Lawler, K. V.;
Levchenko, S. V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar, P.;
Manzer, S. F.; Mao, S.-P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.;
Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O’Neill, D. P.; Parkhill, J. A.; Perrine, T.
M.; Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.;
Small, D. W.; Sodt, A.; Stein, T.; Stück, D.; Su, Y.-C.; Thom, A. J. W.; Tsuchimochi, T.;
Vanovschi, V.; Vogt, L.; Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel, J.; White, A.;
Williams, C. F.; Yang, J.; Yeganeh, S.; Yost, S. R.; You, Z.-Q.; Zhang, I. Y.; Zhang, X.; Zhao,
Y.; Brooks, B. R.; Chan, G. K. L.; Chipman, D. M.; Cramer, C. J.; Goddard, W. A.; Gordon, M.
167

S.; Hehre, W. J.; Klamt, A.; Schaefer, H. F.; Schmidt, M. W.; Sherrill, C. D.; Truhlar, D. G.;
Warshel, A.; Xu, X.; Aspuru-Guzik, A.; Baer, R.; Bell, A. T.; Besley, N. A.; Chai, J.-D.; Dreuw,
A.; Dunietz, B. D.; Furlani, T. R.; Gwaltney, S. R.; Hsu, C.-P.; Jung, Y.; Kong, J.; Lambrecht, D.
S.; Liang, W.; Ochsenfeld, C.; Rassolov, V. A.; Slipchenko, L. V.; Subotnik, J. E.; Van Voorhis,
T.; Herbert, J. M.; Krylov, A. I.; Gill, P. M. W.; Head-Gordon, M., Advances in molecular
quantum chemistry contained in the Q-Chem 4 program package. Molecular Physics 2015, 113
(2), 184-215.
24. Plimpton, S., Fast Parallel Algorithms for Short-Range Molecular Dynamics. Journal of
Computational Physics 1995, 117 (1), 1-19.

168

4. Chapter 4: Structure-Property Relationship, Non-Radiative Decay
Pathways and Bathochromic Shift of DIPYR to Near-Infrared

4.1 Introduction
In Chapter 2, we presented a family of compounds with high molar absorptivity, high
photoluminescence quantum yields, facile synthetic routes and tunability through addition of
substituents around the core structure. The aD family is similar to compounds found in DIPYR
except the meso position is changed from a nitrogen to a carbon. Simply changing this atom can
result in a bathochromic shift of ~80-100 nm in the alpha benzannulated form while maintaining
the high molar absorptivity and high quantum yield.
1
The DIPYR material has not been
extensively explored due to previous reports that these systems are non-emissive. As a result,
these can be competitive and may be more synthetically accessible compared to pyrrole-based
systems such as BODIPY. In this section, DIPYR derivatives are explored to understand their
structure-property relationships, and maintain their high absorption properties while further red-
shifting the maxima to the near infrared spectrum.
4.2. Structure Property Relationship: Photophysical Properties of Modified DIPYRs
BrD, -2,6MeOD, MeOD and -Bt are different structures synthesized based on the
DIPYR core structure, as shown in Figure 4.1. Compounds were synthesized similar to those of
the DIPYR compound using nucleophilic aromatic substitution conditions.
1
-Bt was
synthesized using -DIPYR synthesis and an additional step using tolyl magnesium bromide and
dichloromethane as a solvent at room temperature stirred overnight. Table 4.1 shows the
photophysical properties of the synthesized compounds and suggests that addition of methoxy or
bromine groups symmetrically in the DIPYR core results in a decrease in the fluorescence
lifetime and quantum yield. This suggests that an increase in the non-radiative decay has
169

Figure 4.1. DIPYR derivatives which include a mixture of methoxy donor compounds and
benzannulated systems.
Table 4.1. Photophysical properties of DIPYR with benzannulated or donor substituents.
Solvent:MCH
2
DIPYR BrD MeOD
-3,6-Meo
DIPYR
-Bt
PLQY 0.17 <0.01 <0.01 0.25 0.75
 fl (ns)
1.90 1.1 1.68 2.90 6.84
kr (10
8
s
-1
) 0.89 - - 0.86 1.10
knr (10
8
s
-1
) 4.37 - - 2.56 0.37

occurred in these systems. The halide and methoxy both have π -π* transitions and have non-
bonding electrons which can participate in the n-π* transition. The latter transition leads to an
increase in spin-orbit coupling and increases the probability of intersystem crossing, thus, it leads
to the low photoluminescence quantum yield shown in Table 4.1.
3
Changing the polarity of the
solvent which can often stabilize CT states in compounds is not evident in these systems as the
methyltetrahydrofuran data (Table 4.2) is similar to those observed in methylcyclohexane .
Addition of methoxy group with one side π-extended led to higher ( = 0.25) quantum yield
values due to a slower lifetime and slower non-radiative decay. Previous studies with -DIPYR
and −DIPYR show very similar trends; however, it has a much higher photoluminescence
quantum yield ( = 0.77) with benzannulation on both sides of the core. Substitution of phenyl
170

Table 4.2. Photophysical Properties of BrD and MeOD in methylcyclohexane and
methyltetrahydrofuran.

Methyl THF (BrD)
MeCyHex
(BrD)
Methyl THF
(MeOD)
MeCyHex
(MeOD)
Quantum
yield (fl)
.01 .02 0.03 0.08
Lifetime (𝝉 fl)
5.2 x 10
-9
s (93)
0.4 x 10
-9
s (7)
1.1x 10
-9
s (76)
0.2 x 10
-9
s (24)
5.76 x 10
-10
s (90)
5.49 x 10
-9
s (10)
1.68 x 10
-9
s
Radiative
rate (kr)
1.92 x 10
6
s (93)
2.5 x 10
7
s (7)
1.82 x 10
7
s (76)
1.00 x 10
9
s (24)
5.21 x 10
7
s (90)
6.07 x 10
6
s (10)
4.76 x 10
7
s
Nonadiative
rate (k nr)
1.90 x 10
8
s (93)
2.48 x 10
9
s (7)
8.91 x 10
7
s (76)
4.90 x 10
9
s (24)
1.68 x 10
9
s (90)
1.98 x 10
8
s (10)

5.48 x 10
8
s

groups on the boron atom (-Bt) led to comparable  and non-radiative rates while the excited
state lifetime of -Bt (6.84 ns) is closer to those of -DIPYR (5.7 ns) than −DIPYR (3.9 ns).
The high  suggests that the ISC are spin forbidden similar to the ones observed from DIPYR
and aD systems.
1, 4
Additionally, the tolyl groups, which are typically considered as potential
free rotors in small molecules, do not contribute significantly to the structural distortions in the
excited state. This is because the high photoluminescence quantum yield is maintained, and
luminescent lifetime values in fluid solution (6.84 ns) is in the same order of magnitude to ones
measured in rigid media at 77K (3.40 ns).
The absorbance and emission spectra for BrD, MeOD and -3,6MeOD were taken in
both methylcyclohexane and methyltetrahydrofuran, while -Bt spectra were only taken in
methylcyclohexane. In the non-benzannulated compound (BrD and MeOD), the 0-1 vibronic
feature increases with increasing polarity, which suggests a second absorptive feature that is
solvent dependent. The influence of solvent polarity on the 0-1 vibronic is not observed upon
171

benzannulation of one side (-3,6MeOD) or both sides
1, 4a
of the molecules, shown in Figure
4.2 and Figure 4.3. The absorption max of BrD and MeOD is ~ 20 nm bathochromically shifted
relative to those found in DIPYR by symmetrical addition of bromine or methoxy substituents.
Previous studies in our lab have demonstrated that max of DIPYR (451 nm) shift by 20-40 nm by
alpha benzannulation (520 nm) or gamma benzannulation (500 nm). Interestingly,
benzannulation of one side of DIPYR and addition of methoxy donating group resulted in a
sharp vibronic absorption max of 526 nm, red shifted relative to the - and -DIPYR.
Additionally, modifying -DIPYR with fluorine substitution to tolyl substitution led to a red-
shifted absorbance (~50 nm) and emission (~60 nm). The Stokes’ shift of -Bt (21 nm) is 16 nm
more than those found in -DIPYR (5 nm) which suggests that there is some geometry change in
the excited state, but not significant enough to contribute to the non-radiative rate and excited
state lifetime of these systems as alluded to earlier. It would be interesting to see if the geometry
change in the excited state is enhanced by simply replacing the tolyl substitution with phenyl
groups, as this will allow for the free rotation of these systems. In these systems, it is evident that
benzannulation and addition of donor groups allow for a bathochromic shift by lengthening the
resonance structure of the material.
172

400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1

methylcyclohexane
absorbance
emission
2-methyltetrahydrofuran
absorbance
emission
Normalized Abs/PL
Wavelength (nm)
400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1

Normalized Abs/PL
Wavelength (nm)
methylcyclohexane
absorbance
emission
2-methyltetrahydrofuran
absorbance
emission
400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1

Normalized Abs/PL
Wavelength (nm)
methylcyclohexane
absorbance
emission
2-methyltetrahydrofuran
absorbance
emission

Figure 4.2. Absorbance and Emission Spectra of BrD, MeOD and -3,6-MeO DIPYR in
methylcyclohexane and methyltetrahydrofuran.
300 350 400 450 500 550 600 650
0
0.5
1

Normalized Absorbance
Wavelength (nm)
methylcyclohexane
-Bt
-DIPYR
500 550 600 650 700
0
0.2
0.4
0.6
0.8
1

Photoluminescence
Wavelength (nm)
methylcyclohexane
-DIPYR
-Bt
-DIPYR (77K)
-Bt (77K)

Figure 4.3. Absorption and emission spectra in methylcyclohexane of -Bt is shown. -Bt:  = 0.75
and  = 6.84ns (RT), 3.4ns (77K)

173

4.3 Non-Radiative Pathways and Design Principles of DIPYR for Application in
Optoelectronics
BODIPY, a pyrrole analogue of DIPYR, is a well-known dye which has been used for a
multitude of applications including but not limited to sensors, dyes, sensitizers and PV absorbers.
A contributing mechanism that leads to the non-radiative pathway of these dyes is the addition of
phenyl type groups in the meso position (5-position) of the compound, which leads to the low .
In literature, the latter is attributed to the conical intersections where the crossing between the
lowest excited singlet state and the ground state is energetically below the initial vertical
excitation.
5
Maintaining the high  in these systems is ideal for OPVs since the excitons need to
diffuse as electrical charges and not be lost to non-radiative decay pathway or to exciton
recombination.
However, it is worth noting that this same free rotating mechanism in BODIPY has been
used as fluorescent rotors to create a molecule which is environmentally sensitive by tracing the
variation in emission intensity.
5b
In order to prevent this increase in the non-radiative decay
pathway of BODIPY, addition of methyl or other bulky groups in the 3- and 7- position to
increase steric hindrance with the phenyl hydrogens have been utilized.
6
Additionally, changing
the phenyl to a mesityl or tolyl is sufficient to maintain the non-radiative rate similar to
BODIPY.
7
Table 4.3 shows the effect in the photophysical properties of the BODIPY core by
addition of phenyl and mesityl groups in the 5-position. Addition of a phenyl group in the meso
position leads to an order of magnitude faster non-radiative rate from 0.51x10
8
s
-1
to 21.0x10
8
s
-1
.
Minimizing the rotation between the BODIPY core and the aryl group by addition of steric
hindrance with tolyl or mesityl groups decreases the non-radiative rate to ~0.12 x10
8
s
-1
, thus
maintaining the high  desired for these BODIPY compounds.
174

Table 4.3. Photophysical Properties of BODIPY: Addition of Phenyl and Mesityl in the 5-position
and Effect on Non-Radiative Pathway.

solvent:toluene
10
Bodipy
8
Bodipy-Ph
7c
Bodipy-Tol
7c
Bodipy-
Mes
PLQY 0.69 0.06 0.93 0.93
 fl (ns)
5.5 0.45 5.8 6.6
kr (10
8
s
-1
) 1.25 1.3 1.6 1.4
knr (10
8
s
-1
) 0.56 21 0.12 0.11

BODIPY and DIPYR are well described with the symmetry and transition of HOMO (a2)
→ LUMO (b1) transition where these systems have a C2v symmetry, the closed-shell ground state
is A1, and the  molecular orbitals have a2 or b1 symmertry.
1, 8
In the phenyl derivative of
BODIPY shown in Figure 4.4, the HOMO is localized in the BODIPY framework irrespective
of the phenyl-group geometry. In contrast, the LUMO is localized in the BODIPY structure
when the phenyl group is orthogonal, but this orbital is delocalized into the phenyl ring upon
rotation into plane with the BODIPY framework (90 to 0). This delocalization of electron
density leads to the decrease in the LUMO energy and the transition energy upon aryl ring
rotation. As shown in Figure 4.4, the LUMO is the configurational characteristic of the strongly
allowed B2 state as illustrated by the dense orbital contribution throughout the compound. The
LUMO of the latter participates in over 90% of the configurational description.
5c
Additionally,
this could be contributed to the internal charge transfer state formed where upon excitation a
charge separation in BODIPY (with phenyl) leads to emission from the ICT state in polar
solvents.
9
The combination of all these is what leads to the solvent-dependent emission and the
175

changed non-radiative lifetimes of BODIPY relative to its phenyl derivative. In order to
understand if a similar mechanism exists in DIPYR, a systematic study of substitution in the
meso position using computational and experimental data were conducted for molecules shown
in Figure 4.5.

Figure 4.4. Illustrates the orbital delocalization based on the orientation of the phenyl ring and the
effect in the dihedral angles in phenyl and mesityl derivatives. Adapter from literature.
8

176

Figure 4.5. Synthetic route
1
used to produce targeted molecules (blue) is similar to those followed to
make DIPYR compounds.

4.3.1 Photophysical Properties of DIPYR-Ph and DIPYR-Tol
DIPYR, DIPYR-Ph and DIPYR-Tol were synthesized and their photophysical
properties are shown in Figure 4.6 and Table 4.4. The maximum absorbance of DIPYR is
480 nm, which is hypsochromically shifted compared to DIPYR-Ph (494 nm) and DIPYR-Tol
(494 nm) where both show similar sharp absorbance features. The absorbance by addition of
phenyl or tolyl group only changes ~4 nm from the BODIPY compound, but shifts ~14 nm for
DIPYRs when compared to its phenyl and tolyl counterparts. Figure 4.7 shows that the 0-0 to 0-
1 ratio of DIPYR-Ph is small for polar solvents such as dichloromethane and acetonitrile, while
it is larger for non-polar solvents such as methylcyclohexane. Similar to BrD and MeOD,
DIPYR-Ph and DIPYR-Tol have an increasing 0-1 vibronic feature with increasing polarity
going from methylcyclohexane to acetonitrile, suggesting a second absorptive feature which is
177

solvent dependent. The observed trend is expected since it has been shown that this second
absorptive feature does not exist with benzannulated derivatives but is present for all pyridine-
based systems.
300 350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1

Normalized Absorbance (AU)
Wavelength (nm)
DIPYR
DIPYR-Ph
DIPYR-Tol
450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1

PL Emission (AU)
Wavelength (nm)
DIPYR
DIPYR-Ph
DIPYR-Tol

Figure 4.6. Absorbance and Emission Spectra of DIPYR, DIPYR-Ph and DIPYR-Tol.

Figure 4.7. Solvent dependence of DIPYR-Ph with increasing polarity using methylcyclohexane,
dichloromethane, and acetonitrile.

acetonitrile
methylcyclohexane
178

Table 4.4. Photophysical properties of DIPYR, DIPYR-Ph and DIPYR-Tol

Solvent:MCH DIPYR DIPYR-Ph DIPYR-Tol
 (PLQY) 0.17 0.18 0.18
 fl (ns)
1.9 1.8 2.3
kr (10
8
s
-1
) 0.89 1.0 0.78
knr (10
9
s
-1
) 0.44 0.46 0.35

The emission profile has a red-shifted trend going from DIPYR (484 nm), DIPYR-Ph
(492 nm) and DIPYR-Tol (502 nm). DIPYR-Ph displays a solvent dependence in the emission
spectra when changing from a non-polar to polar solvent. Solvent molecules can help stabilize
and lower the energy level of the excited state by solvent relaxation around the fluorophore, thus
decreasing the energy separation between the ground and excited state. The non-polar solvent
does not significantly affect the excited state energy of the compound; however, increasing the
solvent polarity results in a larger reduction in the energy level of the excited state. In this case,
DIPYR-Ph has a larger dipole moment in the excited state than in the ground state, so the
solvent dipoles can reorient and relax around the fluorophore. Thus, the increase in solvent
polarity further lowers the energy as illustrated by the bathochromic emission trend going from
methylcyclohexane to acetonitrile. The same trend was not observed for the absorbance since
light absorption occurs in 10
-15
s which is much faster than molecular vibration and solvent
reorganization, thus it is less sensitive to solvent polarity. The solvent-fluorophore interaction
and the geometry change from the ground state to the excited state of DIPYR-Ph affects the
emission max shown in Figure 4.7.
179

4.3.2 TDDFT for DIPYR-Ph and DIPYR-Tol
One of the distinguishing differences in the molecular orbital of DIPYR from BODIPY is
the LUMO orbital contribution in the meso position. The orbital is present in BODIPY but is
absent in DIPYR. This slight modification in the frontier orbitals of these systems varies the
interaction of arene systems attached in the meso-position of these compounds. Phenyl groups
can act as a free rotor in compounds as a substituent, and in this case, planarization into the
BODIPY provides orbital density which can conjugate with the arene (Figure 4.8). As a result,
this leads to a bathochromically shifted emission and observed low . However, in DIPYR,
there is no orbital contribution in the meso position, such that planarization of arene compounds
do not greatly affect the photophysical properties since the molecular orbital distribution remains
unchanged.

180

Figure 4.8. LUMO orbital contribution of BODIPY, DIPYR, DIPYR-Ph and DIPYR-Tol, which
shows that the meso-orbital in BODIPY has an orbital contribution that is absent in DIPYR.

Increasing the steric hindrance of the substituent in the meso-position of BODIPY has an
increasing C-C bond length between the meso-carbon but when this is observed for DIPYR, the
largest difference is only 0.01 Å between phenyl, tolyl and mesityl. In BODIPY, a better
diagnostic of increasing steric bulk is the change in the dihedral angle between the BODIPY
plane and the arene substituent plane. Addition of substituents which introduce steric hindrance
to the structure lead to an increased dihedral angle for mesityl (~75) and o-tolyl (~85) relative
to phenyl (~ 60), which has freedom of rotation.
5c
In these DIPYR systems, XRD structures are
not available to compare the dihedral angles, but a geometry optimization of these structures
using B3LYP/631G** was conducted.
The geometry optimized structures do not have a trend with dihedral angles similarly to
ones found in BODIPY. It is possible that by changing from a pyrrole to pyridine, it minimizes
the distortion between the meso-linked core to its substituents. However, there is an increasing
BF2 puckering that minimizes the planarity in the structure as seen in substitution with phenyl
(1.25) < hydrogen (2.91)< tolyl (15.91) < mesityl (18.27), measured similar to Kee et al.
5c
To
compare, geometry optimized structures of -DIPYR and -DIPYR were also calculated to
obtain a trend with the displacement of boron difluoride puckering out of plane. These were
measured by assigning each pyridine as a plane and measuring the angle between the two planes.
The obtained calculated values trend with the experimental data shared here, which were ~2
higher. The single crystal data provided values as shown: DIPYR (4.73) < -DIPYR (5.68) <
-DIPYR (6.58). The TDDFT data showed that benzannulation increases deplanarization of the
181

system, but not as significantly as tolyl and mesityl substituted DIPYRs. This suggests that the
distortion of BF2 from planarity is more evident by addition of bulky substituents in the meso
position of DIPYR, such as tolyl and mesityl groups. However, this puckering observed does not
determine the non-radiative decay or excited state geometry distortion of these systems. High 
compounds such as benzannulated derivatives gave higher distortion angles than DIPYR, but
were below those of DIPYR-Ph and DIPYR-Tol. Additionally, the  and non-radiative decay
for DIPYR and DIPYR-Tol are the same despite the increase in deplanarization with increasing
steric bulk in the system.
4.4 Increased Resonance in DIPYR Molecules Similar to Fluoresceine or
Rhodamine
Structurally, DIPYR has some parallels relative to xanthene. Simple replacement of
oxygen in xanthene to boron difluoride results in the DIPYR structure. A primary theme in this
chapter is that increasing the resonance length across the DIPYR structure allows for a
bathochromic shift in its optical properties. Xanthene based dyes are an inspiration because
fluoresceine and rhodamines, for example, have a high fluorescence quantum yield (~=1) and
have absorption coefficients ~1.0x10
5
M
-1
cm
-1
which are ideal properties for OPV absorbers.
Rhodamine and Fluoresceine derivatives have been used as co-sensitizers in PV technology.
11

Similar structures with Si
12
, Ge
12
, and Se
13
have been used to structurally rigidify the derivatives
and shift the absorbance to the NIR, which makes these systems interesting to explore
fundamentally.
Fluorescein is a highly fluorescent material, but its’ highest molar absorption and
fluorescent quantum yield is observed in its dianionic form, which suggests that resonance within
the structure influences the photophysical properties of this material. There is an increase in 
182

from the cationic derivative to the dianion derivative, as shown in Figure 4.9. However, in order
to utilize these molecules for OPV and vapor deposit these materials, an overall neutral
compound is desired. Mottram et al. have demonstrated that a high  can be achieved (Figure
4.10) by changing a fluorescein structure from dianionic to anionic structure, in contrast to the
studies by Sjoback et al. shared in Figure 4.9.
14
In order to make the structure overall neutral and
potentially maintain the high , boron difluoride will replace the oxygen in the xanthene core

Type of fluorescein 𝜱 fl
dianion (1) 0.93
Anion (2) 0.37
neutral (3) 0.30
cationic (4) 0.18

Figure 4.9. Change in fluorescence quantum yield when varying the charge on fluorescein.
14a

183

Figure 4.10. Fluoresceine derivatives with high fluorescence quantum yield.
14b

The HOMO and LUMO orbitals of these DIPYRs are very similar to those of cyanine,
which was discussed in Chapter 2. The alternating atomic localization of orbitals in the HOMO
and LUMO of these systems is what leads to the intense and narrow absorbance band observed
for these materials and other cyanine-like dyes. The top half of DIPYR has similar nodal
characteristics of the C7H9 cation.
1
Tracing the resonance of the DIPYR system, we have
determined that the best location to achieve a bathochromic shift is in the meta position of the
pyridine nitrogen, which is the same position found in rhodamine and fluoresceine. In order to
synthesize these compounds, the synthetic route seen in Scheme 4.1 was the most promising
route after different synthetic routes followed. The route diverges after the second synthetic step
where route 1 explores the borylation of the formed symmetric methoxy-substituted ligand.
Afterwards, the molecule will be demethylated to the hydroxy form of the structure and
deprotonated using a base. Route 2 take the methoxy ligand and demethylating agent will be
used to form the hydroxy ligand. It is only after this step that the ligand will be borylated and
then deprotonated to form the target compound. It is important to note that the target molecule
(fluoresceine-modified DIPYR) has not been isolated. Route 1 and 2 were both explored, where
the progress for route 2 was not continued since the synthetic step to demethylate the methoxy
ligand was not successful. It also poses further challenges in the preceding steps since boron
could coordinate to the hydroxy oxygen instead or in addition to the pyridine nitrogens. Route 1
showed more promise. Demethylation from methoxy DIPYR to hydroxy DIPYR produced a
green emissive crude in solution with a mixture of the starting material. Since these were started
in a milligram scale, the 30% borylation step gives did not produced sufficient precursor for
184

optimization of the demethylation step. There is benefit in doing a much larger scale of these
reaction conditions to see if the hydroxy DIPYR is isolable. If so, formation of the fluorescein
material may be achieved. As these compounds were explored in the early stages of our DIPYR
synthetic work, their synthesis is worth revisiting now that we have learned more about DIPYR
chemistry.

Scheme 4.1. Synthetic routes for fluoresceine inspired target molecule.

Rhodamine-modified DIPYR was also a target compound that may be further red-shifted
than the fluoresceine derivative but also more stable if the amine is protected with methyl
groups. The pH effect that is expected for the fluoresceine derivative can be minimized with the
rhodamine derivatives, shown in Figure 4.11. The ligand
15
for this system was successfully
synthesized with 50% yield (Figure 4.12) but the borylation step found to be challenging when
making these materials but more can be explored for these systems since these types of motif
show promising NIR absorbers. Figure 4.13 illustrates NIR dyes which were rigidified using
185

phosphorous atom to replace the oxygen in xanthene. The max absorbance of these materials
ranged between 666-744 nm with molar absorptivities ranging between 71,000-165,0000 M
-1

cm
-1
and  between 0.11-0.38. Further synthetic exploration to form the rhodamine modified
DIPYR is needed but literature precedence suggests these materials are promising dyes. In order
to gain better understanding of how this resonance effect contributes to boron rigidified
fluoresceine and rhodamine structures, experimental data will be needed.

Figure 4.11. Rhodamine and Rhodamine B Structures with rhodamine inspired DIPYR

186

Figure 4.12. Ligand desired to use for rhodamine-modified DIPYR

Figure 4.13. Phosphorous based atoms mimicking rhodamine. Adapted from literature
15

4.5 Experimental
4.5.1 Photophysical Characterization
All samples in fluid solution were dissolved in chosen solvent with absorbance between
0.05-0.15 for high absorbing materials The UV-visible spectra were recorded on a Hewlett-
Packard 4853 diode array spectrometer. Steady State fluorescence emission measurements were
performed using a QuantaMaster Photon Technology International spectrofluorometer. Gated
phosphorescence measurements were carried on the fluorimeter with 500 microsecond delay
where the sample is in 77 K temperature. All reported spectra are corrected for photomultiplier
response. Fluorescence lifetime measurements were performed using an IBH Fluorocube
instrument equipped with 331 nm LED and 405 nm laser excitation sources using a time-
correlated single photon counting method. Quantum yield values were obtained using a C9920
Hamamatsu integrating sphere system.
187

4.6 Chapter 4 References
1. Golden, J. H.; Facendola, J. W.; Sylvinson M. R, D.; Baez, C. Q.; Djurovich, P. I.;
Thompson, M. E., Boron Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based
Chromophores. The Journal of organic chemistry 2017, 82 (14), 7215-7222.
2. Lee, J.-H.; Chen, C.-H.; Lee, P.-H.; Lin, H.-Y.; Leung, M.-k.; Chiu, T.-L.; Lin, C.-F.,
Blue organic light-emitting diodes: current status, challenges, and future outlook. Journal of
Materials Chemistry C 2019, 7 (20), 5874-5888.
3. (a) Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.; Burgess, K.,
BODIPY Dyes In Photodynamic Therapy. Chemical Society reviews 2013, 42 (1),
10.1039/c2cs35216h; (b) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F., The triplet excited
state of Bodipy: formation, modulation and application. Chemical Society Reviews 2015, 44 (24),
8904-8939.
4. (a) Tadle, A. C.; El Roz, K. A.; Soh, C. H.; Sylvinson Muthiah Ravinson, D.; Djurovich,
P. I.; Forrest, S. R.; Thompson, M. E., Tuning the Photophysical and Electrochemical Properties
of Aza-Boron-Dipyridylmethenes for Fluorescent Blue OLEDs. Advanced Functional Materials
n/a (n/a), 2101175; (b) Golden, J. H.; Estergreen, L.; Porter, T.; Tadle, A. C.; Sylvinson M. R,
D.; Facendola, J. W.; Kubiak, C. P.; Bradforth, S. E.; Thompson, M. E., Symmetry-Breaking
Charge Transfer in Boron Dipyridylmethene (DIPYR) Dimers. ACS Applied Energy Materials
2018, 1 (3), 1083-1095.
5. (a) Prlj, A.; Vannay, L.; Corminboeuf, C., Fluorescence Quenching in BODIPY Dyes:
The Role of Intramolecular Interactions and Charge Transfer. Helvetica Chimica Acta 2017, 100
(6), e1700093; (b) Sirbu, D.; Karlsson, J. K. G.; Harriman, A., Nonradiative Decay Channels for
a Structurally-Distorted, Monostrapped BODIPY Derivative. The Journal of Physical Chemistry
A 2018, 122 (47), 9160-9170; (c) Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.;
Youngblood, W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.; Birge,
R. R.; Lindsey, J. S.; Holten, D., Structural Control of the Photodynamics of Boron−Dipyrrin
Complexes. The Journal of Physical Chemistry B 2005, 109 (43), 20433-20443; (d) Bañuelos, J.;
Arroyo-Córdoba, I. J.; Valois-Escamilla, I.; Alvarez-Hernández, A.; Peña-Cabrera, E.; Hu, R.;
Zhong Tang, B.; Esnal, I.; Martínez, V.; López Arbeloa, I., Modulation of the photophysical
properties of BODIPY dyes by substitution at their meso position. RSC Advances 2011, 1 (4),
677-684.
6. Loudet, A.; Burgess, K., BODIPY Dyes and Their Derivatives: Syntheses and
Spectroscopic Properties. Chemical Reviews 2007, 107 (11), 4891-4932.
7. (a) Benstead, M.; Mehl, G. H.; Boyle, R. W., 4,4′-Difluoro-4-Bora-3a,4a-Diaza-S-
Indacenes (Bodipys) as Components of Novel Light Active Materials. Tetrahedron 2011, 67
(20), 3573; (b) Orte, A.; Debroye, E.; Ruedas-Rama, M. J.; Garcia-Fernandez, E.; Robinson, D.;
Crovetto, L.; Talavera, E. M.; Alvarez-Pez, J. M.; Leen, V.; Verbelen, B.; Cunha Dias de
Rezende, L.; Dehaen, W.; Hofkens, J.; Van der Auweraer, M.; Boens, N., Effect of the
substitution position (2, 3 or 8) on the spectroscopic and photophysical properties of BODIPY
dyes with a phenyl, styryl or phenylethynyl group. RSC Advances 2016, 6 (105), 102899-
102913; (c) Bañuelos, J.; Arbeloa, F. L.; Martinez, V.; Liras, M.; Costela, A.; Moreno, I. G.;
Arbeloa, I. L., Difluoro-boron-triaza-anthracene: a laser dye in the blue region. Theoretical
simulation of alternative difluoro-boron-diaza-aromatic systems. Physical Chemistry Chemical
Physics 2011, 13 (8), 3437-3445.
188

8. Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.; Youngblood, W. J.; Calder, M. E.;
Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.; Birge, R. R.; Lindsey, J. S.; Holten, D.,
Structural control of the photodynamics of boron-dipyrrin complexes. The journal of physical
chemistry. B 2005, 109 (43), 20433-43.
9. Sasaki, S.; Drummen, G. P. C.; Konishi, G.-i., Recent advances in twisted intramolecular
charge transfer (TICT) fluorescence and related phenomena in materials chemistry. Journal of
Materials Chemistry C 2016, 4 (14), 2731-2743.
10. (a) Arroyo, I. J.; Hu, R.; Merino, G.; Tang, B. Z.; Peña-Cabrera, E., The Smallest and
One of the Brightest. Efficient Preparation and Optical Description of the Parent
Borondipyrromethene System. The Journal of organic chemistry 2009, 74 (15), 5719-5722; (b)
Schmitt, A.; Hinkeldey, B.; Wild, M.; Jung, G., Synthesis of the Core Compound of the
BODIPY Dye Class: 4,4′-Difluoro-4-bora-(3a,4a)-diaza-s-indacene. Journal of Fluorescence
2009, 19 (4), 755-758.
11. (a) Kazemifard, S.; Naji, L.; Afshar Taromi, F., Enhancing the photovoltaic performance
of bulk heterojunction polymer solar cells by adding Rhodamine B laser dye as co-sensitizer. J
Colloid Interface Sci 2018, 515, 139-151; (b) Pepe, G.; Cole, J. M.; Waddell, P. G.; Griffiths, J.
R. D., Molecular engineering of fluorescein dyes as complementary absorbers in dye co-
sensitized solar cells. Molecular Systems Design & Engineering 2016, 1 (4), 402-415.
12. (a) Kim, S.; Fujitsuka, M.; Miyata, M.; Majima, T., Excited-state dynamics of Si–
rhodamine and its aggregates: versatile fluorophores for NIR absorption. Physical Chemistry
Chemical Physics 2016, 18 (3), 2097-2103; (b) Grimm, J. B.; Brown, T. A.; Tkachuk, A. N.;
Lavis, L. D., General Synthetic Method for Si-Fluoresceins and Si-Rhodamines. ACS Central
Science 2017, 3 (9), 975-985.
13. Koide, Y.; Kojima, R.; Hanaoka, K.; Numasawa, K.; Komatsu, T.; Nagano, T.;
Kobayashi, H.; Urano, Y., Design strategy for germanium-rhodamine based pH-activatable near-
infrared fluorescence probes suitable for biological applications. Communications Chemistry
2019, 2 (1), 94.
14. (a) Sjöback, R.; Nygren, J.; Kubista, M., Absorption and fluorescence properties of
fluorescein. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 1995, 51
(6), L7-L21; (b) Mottram, L. F.; Boonyarattanakalin, S.; Kovel, R. E.; Peterson, B. R., The
Pennsylvania Green Fluorophore: a hybrid of Oregon Green and Tokyo Green for the
construction of hydrophobic and pH-insensitive molecular probes. Organic letters 2006, 8 (4),
581-584.
15. Zhou, X.; Lai, R.; Beck, J. R.; Li, H.; Stains, C. I., Nebraska Red: a phosphinate-based
near-infrared fluorophore scaffold for chemical biology applications. Chemical Communications
2016, 52 (83), 12290-12293.

189

5. Chapter 5: (aza)DIPYR Salts and their pH Dependence

5.1 Introduction
Cyanine dyes are optically interesting due to their molecular tunability and versatility in
applications such as molecular dyes, indicators, fluorescent labels, and many more.
1

2,2’dipyridylamine and 2,2’-dipyridylmethane were cyclized with boron difluoride in Chapter 2
and Chapter 4 to rigidify the structure which leads to high photoluminescence quantum yield and
fast nanosecond lifetimes. It seems reasonable to expect that similar effects can be achieved if
the boron difluoride is replaced with groups such as but not limited to methylene, ethylene or
carbonyl.
In 1961, a German publication reported some quinoline bridged systems where the ligand
used for DIPYR and -DIPYR were cyclized using methylene.
2
Leubner then synthesized
azaDIPYR and DIPYR ligands cyclized with methylene in the 1970s.
3
In literature these
pyridine or quinoline bridged ligands are rigidified with methylene and have been called cyclic
azacyanines
4
or azapyridocyanines.
3, 5
These molecules have been explored due to their
interesting optical properties and superconducting properties which are parallel to those observed
in cyanines. However, limited photophysical studies have been conducted for these types of
systems. As we learned from Chapters 2 and 4, we expect that upon rigidification of these
dipyridylmethene and dipyridylamine materials, positional substitution and solvent environments
can influence the photophysical properties of these systems.
4b
Additionally, rigidification of
these ligands produce high absorbing and good emitters in the visible, allowing for development
of new cationic dyes which can be used as fluorescent tags.

190

5.2 Synthesis of Cationic and Rigid DIPYRs and azaDIPYRs
The cationic molecules CarD (Me-carDIPYR) and aCARD (azaCARDIPYR), shown in
Figure 5.1, were synthesized similarly to some previously reported studies for methylene
bridged cyclization.
3-4, 5-6
The carbonyl cyclization was made using urea as the ketone source
and dichloromethane as the solvent. The general synthetic route is shown in Scheme 5.1. The
halide counterparts were replaced with hexaflurophosphate salts to better solubilize these
materials in organic solvents. The carbonyl derivatives were made to force the system to be more
planar than what was observed for the methyl- and ethyl- bridged groups of
2,2’dipyridylmethene and 2,2’ dipyridylamine rigidified ligands (based on TDDFT). As a result,
a carbonyl derivative was synthesized to better planarize these compounds which can eventually
be reduced to convert carbonyl to an alkene. By changing the bridging system from a carbonyl
(COD and aCOD) to an alkene, the overall molecular dipole will change, which will be
interesting to study photophysically and optically.

Figure 5.1. Carbon chelated and carbonyl chelated compounds synthesized for photophysics
studies. The molecules highlighted in blue have been previously synthesized in our lab.
3

3
aCOD and COD structures were helped synthesized by Dr. Caroline Black
191

Scheme 5.1. General Synthesis of Cationic DIPYRs and azaDIPYRs

5.3 Photophysical Properties of (aza)DIPYR salts
The absorption and emission spectra of aCARD, -aCARD, aCOD, and COD are
shown in Figure 5.2, Figure 5.3, Figure 5.4, and Figure 5.5. The absorption maxima of
aCARD and -aCARD in 2-MeTHF and 2:1 ethanol to methanol mixture were compared to
their respective boron difluoride counterparts. The absorbance and emission show ~10 nm shift
which is similar to those found for CARD and -CARD. aCARD absorbance has a broad and
gaussian-like profile which is different to the vibronic absorbance observed in the boron
difluoride derivative (aD). -aCARD has a similar absorbance vibronic feature but is slightly
bathochromically shifted (max = 450 nm) to its boron difluoride counterpart (-aD). It is
interesting to observe the sharp absorbance spectrum for -aCARD while aCARD appeared
much broader. In terms of the absorbance and emission spectra, aD has a similar sharp
absorbance and emission spectra that is common for currently synthesized DIPYRs but aCARD
is less vibronic. The current hypothesis is that there is a structural distortion more prominent in
the aCARD derivative, which contributes to the observed optical and photophysical properties in
these systems. It is likely that benzannulation helps rigidify the structure enough that the same
properties are not observed.
0.5
1.0
300 400 500 600 700
0.5
1.0
300 400 500 600 700
0.0
0.5
1.0

Me-carDIPYR

Et-carDIPYR

-carDIPYR
Wavelength (nm)
192

300 400 500 600 700
0.5
1
0.5
1
300 400 500 600 700
0
0.5
1

Me-carDIPYR

Et-carDIPYR

-carDIPYR
Wavelength (nm)

Figure 5.2. Previously synthesized compounds in our lab which will be compared to the molecules
presented in this chapter. Data were taken in 2-MeTHF
4

4
More information about these three compounds can be found in Dr. Jessica Golden’s thesis
193

300 350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
Normalized Abs/PL (AU)
Wavelength (nm)
-aCARD (mTHF)
abs
em RT
350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1

Normalized Abs/PL
Wavelength (nm)
-aCarD in EtOH:MeOH
abs
em @ RT (solution)

350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1

Normalized Abs/PL
Wavelength (nm)
-aD in MeTHF
Abs
Fl at RT
Gated Phos
at 77K

Figure 5.3 -aCARD absorbance and emission properties in 2:1 Ethanol to methanol mixture and
2-MeTHF. 2-MeTHF spectra of -aD is included for comparison.

194

350 400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1

Normalized Abs/PL
Wavelength (nm)
aCarD in EtOH:MeOH
abs
em @ RT (solution)
em @ RT (solid)
300 350 400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
Normalized Abs/PL
Wavelength (nm)
azaCARD
abs
em RT
gated phos 77K
300 350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
300 350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
Abs

Normalized Abs/PL
Wavelength (nm)
aD in MeTHF
Fl at RT
Gated Phos
at 77K

Figure 5.4. aCARD absorbance and emission properties in 2:1 Ethanol to methanol mixture and 2-
MeTHF. 2-MeTHF spectra of aD is included for comparison.

aCOD and COD are compared to aCarD and CarD to understand if the structure can be
rigidified by converting the methylene to a carbonyl group, and potentially planarize the core
structure of these DIPYR dyes. The absorbance spectra were taken in 2-MeTHF to directly
compare all the compounds. Unfortunately, this produced a spectrum which is not
straightforward to explain. Upon addition of carbonyl in the aza derivative (aCOD), the
absorbance profile and emission profile became broadened without vibronic features while non-
aza derivative (COD) produced sharp vibronic features, which was absent in the CARD
derivative.
195

The photophysical properties of these compounds are tabulated in Table 5.1 and Table
5.2. The excited state lifetimes of all four compounds (aCARD,aCOD,CARD and COD) are ~3
ns which aligns well with other non-benzannunlated DIPYRs presented in Chapter 2 and Chapter
4. aCARD and COD have very similar lifetimes, PLQY, radiative and non-radiative rates while
a similar relationship was observed between aCOD and CARD. It is evident that the non-
radiative rate of the latter are faster (~4 x 10
8
s
-1
) than those observed in COD and aCARD (~2.3
x 10
8
s
-1
), thereby decreasing the  by one third relative to the values found in compounds with
sharper vibronic features. It is possible that some of the similarities between aCARD (3.77
Debye) and COD (2.81 Debye) could be due to the dipole moment observed. The values in
aCARD and COD are larger than ones found in aCOD (0.65 Debye) and both have more
distinct 0-0 and 0-1 transitions and higher quantum yields. Further investigation of these
properties through excited state dynamics would be interesting, and there is still a lot to learn
from these curious photophysical characteristics of the CARDIPYR salts.
300 350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
Normalized Abs/PL
Wavelength (nm)
aCOD
abs
em RT
em 77K
gated phos em
300 350 400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
Normalized Abs/PL
Wavelength (nm)
COD
abs
em RT
gated phos em

Figure 5.5. aCOD and COD absorbance and emission spectra in 2-MeTHF

196

Table 5.1. Photophysical properties of aCARD, aCOD and COD.
azaCARDIPYR aCODIPYR CODIPYR
PLQY () 0.31 0.12 0.30
 fl (ns) 2.92 2.22 3.05
kr (10
8
s
-1
) 1.06 0.54 0.98
knr (10
8
s
-1
) 2.36 3.96 2.30

Table 5.2 Photophysical Properties of CARDIPYR family
CARDIPYRs max abs.
(nm)
max
em.
(nm)
 PL(%)  (ns) kr (x10
8
s) k nr (x10
8
s)
-CH meso
Me-carDIPYR 493 550 10 2.17 0.46 4.1
Et-carDIPYR 483 504 3.2 0.32 (95%)
/ 3.91 (5%)
0.64 19.4
-carDIPYR 515 521 77 5.0 1.5 0.46
-N meso
acarDIPYR 410 460 37 3.01 1.2 2.09
α-acarDIPYR 463 463 76 6.74 1.1 0.35

5.4 TDDFT of CARDIPYRs
A series of computational studies has reported that optical transitions in DIPYR, BODIPY,
and cyanine-type dyes typically have a double excitation character in their excited states that is
not typically accounted for in B3LYP functional.
7
Typically, a -0.44 eV correction is applied to
account for the error in the S1 energies of cyanine-type dyes. In order to circumvent this and
account for the multi-excitation in these systems, a Restricted Open-Shell Kohn (ROKS)
8

197

calculation were conducted in these CARDIPYR salts (Table 5.3). Using this technique, we have
found that the values work well for the boron difluoride derivatives, and the singlet energies are
representative of the experimental values without doing the -0.44 eV correction. However, the
ROKS calculation for CARDIPYRs can deviate ~30 nm. Nevertheless, the S1 energy trends
properly relative to experimental data. ROKS calculation shows an increasing energy going from
-aCARD (2.59 eV) < aCARD (2.79 eV) < aCOD (3.15 eV) which is the same trend observed
experimentally. Comparing -aCARD with the carboxylic acid does not affect the S1 energy. It
is only when the hydrogen is deprotonated and converted into a carboxylate that the singlet
energy decreased by 0.4 eV.
The HOMO energies obtained for these CARDIPYRs are very deep (~ -10 to -9.0 eV),
where the deepest HOMO energy is associated with aCOD and this molecule has orbital
contribution from the oxygen. The shallowest HOMO energy was observed for the -aCARD
carboxylate (-5.17 eV) and the molecular orbital suggests that most of the orbital density is
concentrated on the carboxylate. The LUMO values were also close in energy for all the
compounds listed (-6.24 to -5.90eV) except the carboxylate derivative, which has a value of 2.65
eV. Based on the molecular orbitals presented in Table 5.4, it appears that if any of the
cyclizing substituents have resonance with the system, the HOMO orbitals are concentrated on
the bridging ligand and not on the pyridine or quinoline core. The latter is observed in COD,
aCOD, alkene aCOD, and -aCARD carboxylate where it is more evident in the -aCARD
carboxylate derivative. The same calculations were conducted using B3LYP/631G** and the
HOMO and LUMO energies found in aCARD (-7.70 eV/-2.71 eV), aCOD (-6.38 eV/-3.44 eV)
and COD (-6.61 eV/-3.41 eV) deviate from the experimental value (Table 5.5) but is still closer
to these values than ones found using the ROKS calculation.
198

Table 5.3. ROKS calculation on CARDIPYRs
Compound S 1 (eV/nm) HOMO (eV) LUMO (eV)
aCARD 2.79/444 -9.6056 -5.8559
CARD 2.80/443 -9.7417 -6.2314
aCOD 3.15/392 -10.3403 -6.4219
-aCARD 2.59/479 -9.2736 -5.8695
-aCARD CarbAcid 2.54/488 -9.2682 -5.9239
-aCARD
Carboxylate
2.12/582 -5.1783 -2.6558
Alkene aCARD 2.70/459 -9.6464 -6.0382

Table 5.4. HOMO and LUMO orbital contributions of CARDIPYR salts
HOMO-1 HOMO LUMO LUMO+1
aCARD

CARD

aCOD

COD

-aCARD
CarbAcid

199

-aCARD
Carboxylat
e

EtCard

Alkene
aCARD

Alkene
CARD

-aCARD

Table 5.5. Oxidation/Reduction Potential of aCARD, aCOD and COD. Data taken relative to
ferrocene in acetonitrile
Compound E ox (V) E red (V) HOMO (eV) LUMO (eV)
aCARD 0.92 -0.67 -5.84 -4.04
aCOD 0.64 -1.1 -5.52 -3.53
COD 0.88 -1 -5.80 -3.65
5.5 Neutral CARDIPYR
Based on the compounds calculated, -aCARD CarbAcid is interesting because this
material has very similar predicted values to -aCARD but can be converted to the carboxylate
derivative. The carboxylate derivative is vastly different from all the other calculated systems,
but it is also the only neutral atom calculated. -aCARD CarbAcid will be referred to as
200

aCOOH and -aCARD carboxylate as aCOO for simplicity. The absorbance and emission
spectra of aCOOH and -aCARD are shifted by ~100 nm from each other (Figure 5.6).
aCOOH is broadened relative to -aCARD and covers a wide range of the spectrum, 350 nm-
600 nm. The  value for aCOOH (17%) decreased relative to -aCARD (76%), as shown in
Table 5.6. The lifetime is faster in aCOOH (2.22 ns) but still in the same order of magnitude as
-aCARD.
200 250 300 350 400 450 500
0
0.2
0.4
0.6
0.8
1
Normalized Abs
Wavelength (nm)
aCOOH
−aCARD
350 400 450 500 550 600 650
0
0.2
0.4
0.6
0.8
1
Normalized Abs
Wavelength (nm)
aCOOH
−aCARD

Figure 5.6. -aCARD CarbAcid (aCOOH) compared to -aCARD absorbance and emission
spectra in 2:1 ethanol to methanol mixture.
201

Table 5.6. Photophysical Properties of aCOOH compared to -aCARD
Ethanol:methanol
(2:1)
-aCARD aCOOH
 0.76 0.19

6.74 ns (87%)
3.39 ns(13%)
2.22 ns
k r
1.13 x 10
8
s (87%)
2.24 x 10
8
s (13%)
8.56 x 10
8
s
k nr
3.56 x 10
7
s (87%)
7.10 x 10
7
s (13%)
3.65 x 10
8
s

Figure 5.7 shows that by changing the pH environment of aCOOH from basic to acidic,
there is a peak that increases ~280 nm in the absorbance specrta. aCOOH in basic and pH 7.4
media led to a broad hump around 300-350nm which becomes more vibronic in acid. The
emission spectra of pH 7.4 and acid solution ranged between 350-500 nm. The basic solution is
much wider, between 320-600 nm. The low  (2%) of the latter has not been fully explained but
it could be due to a diagnostic excimer emission (~450nm-600nm), shown in Figure 5.7. Placing
aCOOH in pH 1.1 (25%) or pH 7.58 (20%) environment led to a slightly higher  (Table 5.7).
The excited state lifetime values are the same for neutral and acid conditions (~2 ns) while the
basic solution has a fast lifetime that is outside of our measuring capabilities.
202

200 250 300 350 400
0
0.2
0.4
0.6
0.8
1
Normalized Abs
Wavelength (nm)
aCOOH
pH 7.4
acidic
basic

300 350 400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
Normalized PL
Wavelength (nm)
aCOOH
pH 7.4
acidic
basic

Figure 5.7. Display of pH dependence of aCOOH in PBS buffer.

Table 5.7. Photophysical Properties of aCOOH in different pH

pH 7.58
(PBS)
pH 1.1
(PBS+HCl)
pH 12.31
(PBS+NaOH)
(eth:me)
 20% 25% 2.2% 19%
 2.49ns 2.78ns fast lifetime 2.22ns
kr 8.03 x 10
7
s 8.99 x 10
7
s - 8.56 x 10
7
s
knr 3.21 x 10
8
s 2.7 x 10
8
s -
3.65 10
8
s

5.6 Crystal Structure
The crystal structure of aCARD (Figure 5.8) has been solved in a P21/c space group and
the carbon to carbon distance between two different monomer molecules was found to be ~
3.65 Angstroms. The puckering observed in this compound (23.8) is comparable to the
DIPYR system (20.5). The compound has been incorporated in a lead halide structure and
orients itself different to the single crystal of the pure organic. Two azaCARD hosts two
iodide atoms which are oriented next to each other, as shown in Figure 5.9. The puckered
side of aCARD sandwiches the halide and they are nicely incorporated between the layered
203

lead halide structure. The diffuse reflectance on Figure 5.10 suggests that the main
component that is absorbing the energy in the hybrid structure is the organic compound.

Figure 5.8. Single Crystal XRD for aCARD

Figure 5.9 aCARD incorporated in lead iodide XRD- (aCARD) 3Pb2I 8
.
2I
5

5
The hybrid structure of aCARD with lead iodide crystal structure and the diffuse reflectance were conducted by
Taylor Hodgkins
204

Figure 5.10. Diffuse Reflectance of azaCARD and azaCARD in lead iodide.
5.7 Conclusion
A class of CARDIPYR salts were developed to structurally rigidify the ligands and compare
them to their boron difluoride counterparts. Further exploration of these systems is necessary
since some photophysical properties of these materials have not been properly explained yet due
to trends which are not currently understood. However, their tunability and pH dependence can
be useful in a lot of systems which require fluorescent tags. Finding a system which can be tuned
to be overall neutral by incorporating carboxylate within the structure may be useful for OLED
or OPVs. These materials can be expanded outside of optoelectronics and are worth further
exploring.

205

5.8 Experimental
5.7.1 Molecular Modeling
All calculations reported in this work were performed using the Q-Chem 5.1 program
9
.
Ground-state optimization calculations were performed using the B3LYP functional and the 6-
31G** basis set. Time dependent density functional theory (TDDFT) calculations were used to
obtain the excitation energies and optimized geometries of the S1 state at the same level. The
same TDDFT calculations were used to determine the partial charges on each atom within the
host and dopant material for Molecular Dynamics (MD) modeling.
5.7.2 Electrochemistry
Cyclic voltammetry and differential pulsed voltammetry were performed using a
VersaSTAT potentiostat measured at 100 mV/s scan. Anhydrous acetonitrile (DriSolv) from
Sigma Aldrich was used as the solvent under nitrogen environment, and 0.1 M
tetra(n-butyl)ammoniumhexafluorophosphate (TBAF) was used as the supporting electrolyte.
Dichloromethane was also used to to measure the oxidation of the host materials. A glassy
carbon rod was used as the working electrode; a platinum wire was used as the counter electrode,
and a silver wire was used as a pseudoreference electrode. The redox potentials are based on
values measured from differential pulsed voltammetry and are reported relative to a
ferrocene/ferrocenium (Cp2Fe/Cp2Fe
+
) redox couple used as an internal reference, while
electrochemical reversibility was determined using cyclic voltammetry.
5.7.3 Photophysical Characterization
All samples in fluid solution were dissolved in chosen solvent with absorbance between
0.05-0.15 for high absorbing materials or 0.5 for those with CT bands at higher energies.
Spincoated neat films were made with 1 mg in 1mL of dichloromethane. Doped films were
206

dissolved in DCM and were made with 1 volume percent -aD in chosen host or 5-10% volume
percent of phosphor in the host matrix. 0.1 g of PMMA pellets were mixed with 1mL of
dichloromethane. 1mL of desired solution was spin coated on a quartz substrate (2cm x 2 cm)
using a pipet with the substrate rotating at 700 rpm for 45 seconds. The film was left to air dry.
The UV-visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer.
Steady State fluorescence emission measurements were performed using a QuantaMaster Photon
Technology International spectrofluorometer. Gated phosphorescence measurements were
carried on the fluorimeter with 500 microsecond delay where the sample is in 77 K temperature.
All reported spectra are corrected for photomultiplier response. Fluorescence lifetime
measurements were performed using an IBH Fluorocube instrument equipped with 331 nm LED
and 405 nm laser excitation sources using a time-correlated single photon counting method.
Quantum yield values were obtained using a C9920 Hamamatsu integrating sphere system.
207

5.9 Chapter 5 References
1. (a) Gopika, G. S.; Prasad, P. M. H.; Lekshmi, A. G.; Lekshmypriya, S.; Sreesaila, S.;
Arunima, C.; Kumar, M. S.; Anil, A.; Sreekumar, A.; Pillai, Z. S., Chemistry of cyanine dyes-A
review. Materials Today: Proceedings 2021, 46, 3102-3108; (b) Ormond, A. B.; Freeman, H. S.,
Dye Sensitizers for Photodynamic Therapy. Materials (Basel, Switzerland) 2013, 6 (3), 817-840;
(c) Shindy, H. A., Fundamentals in the chemistry of cyanine dyes: A review. Dyes and Pigments
2017, 145, 505-513.
2. Friedrich, H. J.; Gückel, W.; Scheibe, G., Synthesen und Reaktionen der
Chinolylmethane, II. Chemische Berichte 1962, 95 (6), 1378-1387.
3. Leubner, I. H., Synthesis and properties of pyrido- and azapyridocyanines. The Journal of
Organic Chemistry 1973, 38 (6), 1098-1102.
4. (a) Tasior, M.; Bald, I.; Deperasińska, I.; Cywiński, P. J.; Gryko, D. T., An internal
charge transfer-dependent solvent effect in V-shaped azacyanines. Organic & Biomolecular
Chemistry 2015, 13 (48), 11714-11720; (b) Patra, D.; Palazzo, T. A.; Malaeb, N. N.; Haddadin,
M. J.; Tantillo, D. J.; Kurth, M. J., Cyclic Azacyanines: Experimental and Computational Studies
on Spectroscopic Properties and Unique Reactivity. Journal of Fluorescence 2014, 24 (4), 1285-
1296; (c) Huang, K. S.; Haddadin, M. J.; Olmstead, M. M.; Kurth, M. J., Synthesis and
Reactions of Some Heterocyclic Azacyanines1. The Journal of organic chemistry 2001, 66 (4),
1310-1315.
5. Leubner, I. H., Nuclear magnetic resonance spectra of pyrido- and azapyridocyanines.
Organic Magnetic Resonance 1974, 6 (5), 253-258.
6. Haddadin, M. J.; Kurth, M. J.; Olmstead, M. M., One-step synthesis of new heterocyclic
azacyanines. Tetrahedron Letters 2000, 41 (30), 5613-5616.
7. Momeni, M. R.; Brown, A., Why Do TD-DFT Excitation Energies of BODIPY/Aza-
BODIPY Families Largely Deviate from Experiment? Answers from Electron Correlated and
Multireference Methods. Journal of Chemical Theory and Computation 2015, 11 (6), 2619-2632.
8. Barca, G. M. J.; Gilbert, A. T. B.; Gill, P. M. W., Simple Models for Difficult Electronic
Excitations. Journal of Chemical Theory and Computation 2018, 14 (3), 1501-1509.
9. Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange,
A. W.; Behn, A.; Deng, J.; Feng, X.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.;
Kaliman, I.; Khaliullin, R. Z.; Kuś, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard,
R. M.; Rohrdanz, M. A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.;
Zuev, D.; Albrecht, B.; Alguire, E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.;
Brandhorst, K.; Bravaya, K. B.; Brown, S. T.; Casanova, D.; Chang, C.-M.; Chen, Y.; Chien, S.
H.; Closser, K. D.; Crittenden, D. L.; Diedenhofen, M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.;
Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.; Ghysels, A.; Golubeva-Zadorozhnaya, A.; Gomes, J.;
Hanson-Heine, M. W. D.; Harbach, P. H. P.; Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.;
Jagau, T.-C.; Ji, H.; Kaduk, B.; Khistyaev, K.; Kim, J.; Kim, J.; King, R. A.; Klunzinger, P.;
Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.; Lawler, K. V.;
Levchenko, S. V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar, P.;
Manzer, S. F.; Mao, S.-P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.;
Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O’Neill, D. P.; Parkhill, J. A.; Perrine, T.
M.; Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.;
Small, D. W.; Sodt, A.; Stein, T.; Stück, D.; Su, Y.-C.; Thom, A. J. W.; Tsuchimochi, T.;
208

Vanovschi, V.; Vogt, L.; Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel, J.; White, A.;
Williams, C. F.; Yang, J.; Yeganeh, S.; Yost, S. R.; You, Z.-Q.; Zhang, I. Y.; Zhang, X.; Zhao,
Y.; Brooks, B. R.; Chan, G. K. L.; Chipman, D. M.; Cramer, C. J.; Goddard, W. A.; Gordon, M.
S.; Hehre, W. J.; Klamt, A.; Schaefer, H. F.; Schmidt, M. W.; Sherrill, C. D.; Truhlar, D. G.;
Warshel, A.; Xu, X.; Aspuru-Guzik, A.; Baer, R.; Bell, A. T.; Besley, N. A.; Chai, J.-D.; Dreuw,
A.; Dunietz, B. D.; Furlani, T. R.; Gwaltney, S. R.; Hsu, C.-P.; Jung, Y.; Kong, J.; Lambrecht, D.
S.; Liang, W.; Ochsenfeld, C.; Rassolov, V. A.; Slipchenko, L. V.; Subotnik, J. E.; Van Voorhis,
T.; Herbert, J. M.; Krylov, A. I.; Gill, P. M. W.; Head-Gordon, M., Advances in molecular
quantum chemistry contained in the Q-Chem 4 program package. Molecular Physics 2015, 113
(2), 184-215.

209

6. Appendix A
One photophysical property of interest is charge transfer (CT) between hybrid systems;
organic and inorganic charge transfer (CT). CT has been demonstrated in (MDA)Pb2X6 (X = I,
Br, and Cl). Even though CT is present in this materials group, there is potential to incorporate a
better acceptor molecules, such as N,N-Dimethyl-4,4’-dipyridocyanine iodide (DPy1). DPy1
possesses a similar structure to MDA but moving the nitrogen from the periphery to the
conjugated ring. The two isomers are interesting to compare. The frontier orbitals of DPy1 are
lower in energy which is why CT is proposed to be stronger in the DPy1 hybrid compound.
Synthesis of DPy1 is adapted from Tolbert et al.
1
The hybrid acquired by reacting 86.8
milligrams of lead iodide (PbI2) (yellow powder) with 31.5 milligrams of DPy1 (red powder)
and 2 mL of hydrogen Iodide (HI). The mixture was heated to 80 ℃ to acquired the hybrid
crystal.

Figure 6.1 MDA and DPy1 compounds as organic components in hybrid systems
6

6
Work from Figure A.2-A.2 were done with Taylor Hodgkins and Nicolay Pacheco
210

6.1 Synthesis

Scheme 6.1. Synthesis of DPy1. Synthetic route followed from Tolbert et al.

Figure 6.2. Nuclear Magnetic Resonance of DPy1 (
1
HNMR)
211

Figure 6.3. Figure 4. Liquid Chromatography- Mass Spectrometry (LCMS) for DPy1. The data
suggests that the mass for DPy1 was found at m/z=199.1
212

6.2 Characterization

Figure 6.4. Diffuse Reflectance of hybrid material. DPy1 only absorbs from two to three electron
volts (eV), however, the hybrid material is able to absorb beyond 3 eV. Also, the peak shape at 2.5
eV is different in both spectra.

213

Figure 6.5. Powder X-Ray diffraction (PXRD). The PXRD pattern for the hybrid is less resolved
because there was not sufficient material to get a smooth baseline.
214

300 400 500 600 700 800
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Absorbance (a.u.)
Wavelength (nm)
THF
Water
Methanol

Figure 6.6. Absorbance spectra of DPy1.
300 350 400 450 500 550 600 650 700 750 800
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Absorbance (a.u)
Wavelength (nm)
1
25
50
75
100
400
500
600
1000
1400
1500
1600

Figure 6.7. Absorbance of DPy1 in water. Addition of acid decreases and leads to doubly
protonated DPy1 which is colorless.
215

Figure 6.8 Hybrid Structure with DPy1 and Lead Iodide

216

6.3 New Isomer for hybrid structure

Figure 6.9. Synthesis of new isomer to compare with DPy1 and MDA. Synthesis of this compound
was made using literature procedure
2

217

6.4 Appendix A References
1. Tolbert, L. M.; Zhao, X., Beyond the Cyanine Limit: Peierls Distortion and Symmetry
Collapse in a Polymethine Dye. Journal of the American Chemical Society 1997, 119 (14), 3253-
3258.
2. (a) Munavalli, S.; Poziomek, E. J.; Landis, W., Preparation and Properties of
Methylenebispyridinium Derivatives. ChemInform 1986, 17, 1883-1892; (b) Brenčič, J. V.;
Modec, B., The mononuclear tungsten(V) complexes: The preparation and the X-ray structures
of a series of [WOX4(H2O)]− (X=Cl, Br) salts. Inorganic Chemistry Communications 2011, 14
(9), 1369-1372.

Abstract (if available)

Abstract Exploration of the stability and synthetic ease of modification in the structure of dipyridyl-methane (DIPYR) compounds were the focus in Chapter 2, 4 and 5. DIPYR systems are particularly interesting because of their orbital pattern where nodes bisect every other carbon atom in the HOMO and LUMO orbitals, leading to intense and narrow absorption and emission spectra with very small Stokes’ shift. The modification around the core structure provided a range of materials which can be used for both organic light emitting diodes and organic photovoltaics. ❧ Chapter 2 focuses on the aza substitution in the meso position of the compound and the effects of donating groups around the periphery of the material. A series of substituted boron aza-dipyridylmethene (aD) compounds were explored for their photophysical and electrochemical properties as fluorescent emitters for blue OLEDs. Previous work in our lab have demonstrated the synthetic ease of dipyridylmethene monomers and their interesting photophysical properties as homoleptic meso-linked dimers. Using Density Functional Theory (DFT) modeling, we combine our theoretical and experimental efforts to synthesize a library of aD compounds by replacing the meso carbon with nitrogen to destabilize the HOMO energy. The synthesized aD derivatives emit blue light with λₘₐₓ between 400–460 nm and internal quantum efficiencies above 85%. A small singlet and triplet energies were deduced from fluorescence and phosphorescence emission, respectively, which shows a singlet-triplet energy gap ≤ 0.4 eV. The EQEₘₐₓ = 4.5% for the fabricated monochromatic device reaches close to the maximum theoretical limit of fluorescent (fl) OLED of 5%. Rigorous device studies suggests that triplet exciton trapping is observed and a new set of host materials with stabilized triplet energies is necessary to further optimize these materials in White OLEDs. ❧ Chapter 3 explores aromatic core structures which were used to study with aD as host material for White OLEDs. The focus deviates from the understanding of the structure-property relationship of DIPYRs but it explores its utility in optoelectronics. A wide range of material were predicted to have triplet energies below of the aD family using TDDFT; fluorenone, spirofluorene, H2P, biquinolines and naphthalene-bridged biquinolines. All materials were synthesized and characterized to have triplet energies ranging between 2.4–2.6 eV with gated phosphorescence measurements at 77K in 2-methyltetrahydrofuran. The best material found to be most ideal for aD are the naphthalene-bridged-biquinolines (1,4 Q, 1,5 Q, 1,4 IQ and 1,5 IQ). The material was observed to be more conductive than other commonly used hosts in OLEDs such as mCBP and CBP. Additionally, these have high sublimation temperatures (>300℃) and melting points (>200℃). The napthelene bridged systems does not contain any heteroatom bond which can minimize bond cleavage observed in OLED devices due to high energy photons generated in the emission layer. The physical, electrochemical and photophysical properties of these materials were studied in completion. Preliminary blue monochromatic device produced an EQEₘₐₓ ∼ 1.5% but further optimization is needed to reach a more competitive EQE. ❧ Chapter 4 and 5 illustrates the potential of DIPYR dyes as NIR absorber in Organic Photovoltaic. The synthesis of cyanine-like dyes, systems such as boron dipyrromethene (BODIPY), is commonly used for application in organic photovoltaic. However, synthetic modification is not easily achieved due to polymerization and porphyrin restrictions to prevent non-radiative decay pathways. The parent DIPYR molecule has shown to be a straightforward synthesis with desirable and tunable photophysical properties, leading to libraries of interesting fluorophores. Benzannulation of DIPYR has shown improvement in minimizing non-radiative decay pathways by eliminating the symmetry allowed transition through intersystem crossing to the low-lying T₂ state in DIPYR but these materials are not a well-studied system. Substitution in the meso-position of DIPYR with tolyl and phenyl suggests that the “free rotor” movement of these aromatic systems does not affect the non-radiative decay rate (~2 ns) which is observed in BODIPY. The PLQY of DIPYR and meso-substituted DIPYR remain unchanged (φ=0.18). Addition of substituents with non-bonding electrons such as halides or methoxy resulted in a low PLQY due to a heavy atom effect and n-π transition which increases the allowedness of intersystem crossing in these systems. The structure-property relationship learned in these systems provides knowledge of how to further design these systems for optoelectronics.

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Creator Tadle, Abegail Cardenas (author)

Core Title Molecular design of pyridine and quinoline based compounds for organic optoelectronics

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Degree Conferral Date 2021-12

Publication Date 05/02/2023

Defense Date 08/12/2021

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag blue fluorescent emitter,fluorophore,host material,near infrared absorber,OAI-PMH Harvest,OLED,optoelectronics,OPV,organic emitter,organic light emitting diode,organic photovoltaic

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