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Molecular approaches to solve the blue problem in organic light emitting diode display and lighting applications

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MOLECULAR APPROACHES TO SOLVE THE BLUE PROBLEM IN ORGANIC LIGHT
EMITTING DIODE DISPLAY AND LIGHTING APPLICATIONS

By
Muazzam Idris

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)
May 2020

Copyright 2020 Muazzam Idris
ii

Acknowledgements
I would like to express my sincere gratitude and appreciation, first and foremost, to my
Ph.D. advisor and mentor, Prof. Mark E. Thompson for giving me the opportunity to be part of his
research group and for the continuous support and care during my graduate school, for his patience
and motivation. Prof. Thompson helped me to grow as a creative and an independent scientist.
I would also like to extend my appreciation to Prof. Surya Prakash, Prof. Andrea Armani,
Prof. Chao Zhang, Prof. Malancha Gupta and Ralf Haiges for their time and guidance while serving
in my screening and qualifying exam and defense committee.
I would like to thank Prof. Dr. Peter Djurovich for his invaluable help throughout my
graduate study at USC. His vast knowledge of literature and challenging questions helped me to
tackle challenging problems in my research. I am very thankful to have him during my PhD studies.
My colleagues at USC offered me great help and comfort throughout my stay at USC. First,
I would like to thank my seniors and former colleagues Dr. Thilini Batagoda, Dr. Rasha Hamze,
Dr. Shuyang Shi, Dr. Tyler Fleetham, Dr. Patrick Saris, Dr. John Facendola and Dr. John Chen
who trained me on several instruments and helped me to settle in and collaborated on several
projects in Thompson’s group. I would also like to thank the current members of Thompson’s
group, Moon Chul Jung, Daniel Sylvinson, Jie Ma, Savannah Kapper, Brenda Ontiveros, Narcisse
Ukwitegetse, Abegail Tadle, Dr. Karim Elroz who I collaborated with and exchanged ideas on
several projects and comfort me as friends. I would also like to thank my collaborator at USC,
Sahar Roshandel, who helped me to synthesize necessary precursors for the host materials project,
JoAnna Milam-Guerrero helped with thermogravimetric analysis and Laura Estergreen helped me
with temperature dependent photophysical measurements and set-up.
iii

I would also like to extend my appreciation to USC administration, Judy May Fong,
Michelle Dea and Magnolia Benitez who provided me with administrative help and support at
USC. I would also like to thank Allan Kershaw who helped me on variable temperature studies for
my papers and thesis.
Special thanks to my collaborators at University of Michigan Ann Arbor: to Prof. Stephen
R. Forrest who contributed a lot on the ideas I worked on in my research and helped me edited my
papers. I would like to also thank his students: to Caleb Coburn, Chan Ho So, Jongchan Kim and
Chanyeong Jeong who helped me with device fabrication and involved in writing my papers.
I want to thank my family for their great support throughout this long journey. Their
immense help, support and patience helped to keep me going in stressful moments. I would also
like to thank my friends in Los Angeles, Ekraam Sabir, Daniel Rojas, Ahsan Javid, Rizwan Saeed
and many others that helped me to settle and live well in this great city of Los Angeles.

iv

Table of Contents
Acknowledgements ....................................................................................................................... ii
List of Tables ............................................................................................................................... vii
List of Figures ............................................................................................................................. viii
Abstract ....................................................................................................................................... xiii
CHAPTER 1 − Introduction ........................................................................................................ 1
1.1. Excited States of Luminescent Materials ..................................................................... 1
1.1.1. Fluorescent Materials ................................................................................................. 3
1.1.2. Phosphorescent Emitters (Triplet Emitters) ............................................................ 3
1.1.3. Thermally Activated Delayed Fluorescence (TADF) Emitters .............................. 8
1.2. OLED Structure ........................................................................................................... 11
1.2.1. Materials in the Emissive Layer .............................................................................. 12
1.2.1.1. Host Materials for OLEDs ................................................................................... 13
1.2.1.2. Tuning Emission Energy of Phosphorescent and TADF Emitters ................... 17
1.2.1.3. Blue Phosphorescent Emitters ............................................................................. 20
1.3. Exciton-Exciton and Exciton-Polaron Annihilation ................................................. 23
1.4. References ..................................................................................................................... 25
CHAPTER 2 − Phenanthro[9,10-d]-triazole and imidazole as High Triplet Energy Host
Materials for Blue OLEDs ......................................................................................................... 28
2.1. Introduction .................................................................................................................. 28
2.2. Results and Discussion ................................................................................................. 29
2.2.1. Synthesis .................................................................................................................... 29
2.2.2. Photophysical and Thermal Properties .................................................................. 30
2.2.3. Frontier Orbital Energies ........................................................................................ 38
2.2.4. Electroluminescence Properties .............................................................................. 43
2.3. Conclusion ..................................................................................................................... 50
2.4. Experimental................................................................................................................. 51
2.4.1. General ...................................................................................................................... 51
2.4.2. DFT and TD-DFT Calculations: ............................................................................. 52
2.4.3. OLED Fabrication and Characterization: ............................................................. 52
2.4.4. Synthesis .................................................................................................................... 53
2.5. References ..................................................................................................................... 65
v

CHAPTER 3 − Photophysical Study of Deep Blue and Sky-blue fac/mer-Iridium (III) NHC
Carbene Complexes and their Application in OLEDs ............................................................ 66
3.1. Introduction .................................................................................................................. 66
3.2. Results and discussion .................................................................................................. 67
3.2.1. Synthesis .................................................................................................................... 67
3.2.2. Computational Studies ............................................................................................. 69
3.2.3. Electrochemical and Photophysical Properties ..................................................... 70
3.2.4. Electroluminescence Properties .............................................................................. 86
3.3. Conclusion ..................................................................................................................... 90
3.4. Experimental................................................................................................................. 93
3.4.1. Synthesis .................................................................................................................... 93
3.4.2. NMR Data ................................................................................................................. 99
3.4.3. Mass Spectra Data .................................................................................................. 107
3.5. References ................................................................................................................... 109
CHAPTER 4 − Electrochemical and Photophysical Studies of Highly Efficient Deep Blue
Luminescence 2-coordinate Coinage Metal Complexes Bearing Bulky NHC Benzimidazolyl
Carbene ...................................................................................................................................... 111
4.1. Introduction ................................................................................................................ 111
4.2. Results and Discussion ............................................................................................... 112
4.2.1. Synthesis and x-ray Analysis ................................................................................. 112
4.2.2. Electrochemistry ..................................................................................................... 113
4.2.3. Computational Analysis ......................................................................................... 115
4.2.4. Photophysical Characterization ............................................................................ 116
4.2.5. OLED Characterization ......................................................................................... 127
4.3. References ................................................................................................................... 129
CHAPTER 5 − Tuning Singlet and Triplet Excited State Energies and Frontier Orbitals of
Imidazole Host/Emitter for Hybrid White OLEDs ............................................................... 131
5.1. Introduction ................................................................................................................ 131
5.2. Results and Discussion ............................................................................................... 135
5.2.1. Synthesis .................................................................................................................. 135
5.2.2. Computational Modelling ...................................................................................... 136
5.2.3. Photophysical and Electrochemical Properties ................................................... 137
5.2.4. Thermal Properties ................................................................................................. 142
vi

5.2.5. Electroluminescence Properties ............................................................................ 143
5.3. Conclusion ................................................................................................................... 149
5.4. Experimental............................................................................................................... 150
5.4.1. Synthesis .................................................................................................................. 150
5.5. References ................................................................................................................... 153
Appendix − High Triplet Energy N2-phenanthro[9,10-d]-triazoles with Bulky Substituents .
............................................................................................................................................. 155
References .............................................................................................................................. 159

vii

List of Tables
Table 2.1. Summary of properties for selected phenanthro-triazoles and -imidazoles. ............... 43
Table 2.2. PLQY and EL properties of host materials. ................................................................ 48
Table 3.1. Calculated energy levels and frontier orbitals of Ir-carbene complexes ..................... 69
Table 3.2. Electrochemical properties of Ir(C^C:)3 complexes. ................................................... 72
Table 3.3. Photophysical properties of Ir(C^C:)3 complexes in 2-MeTHF, PS and PMMA ....... 75
Table 3.4. Photophysical properties of fac-Ir(pmpz)3 in various solvents at 298K and 77K. ...... 76
Table 3.5. Photophysical properties of fac-Ir(tpz)3 in various solvents at 298K and 77K. .......... 77
Table 3.6. Photophysical properties of mer-Ir(pmpz)3 in various solvents at 298K and 77K...... 78
Table 3.7. Photophysical parameters obtained from Boltzmann fits to temperature-dependent
lifetimes using the TADF model .................................................................................................. 86
Table 3.8. Photophysical parameters obtained from Boltzmann fits to temperature-dependent
lifetimes using the triplet model .................................................................................................. 86
Table 4.1. Redox potentials of complexes M
BZI
and the associated experimental frontier orbital
energies. ...................................................................................................................................... 113
Table 4.2. Calculated singlet and triplet excited state energies and dipole moments for the
complexes obtained through TDDFT performed at the CAM-B3LYP/LACVP** level. (Relative
contributions of localized Cz and CT character of the transitions are given in parentheses). .... 116
Table 4.3. Luminescence properties of complexes M
BZI
in various media. .............................. 120
Table 4.4. Luminescence properties of Au
BZI
in various CH2Cl2:MeCy mixtures. ................... 122
Table 4.5. Photophysical properties of Au
BZI
in various solvents. ............................................ 123
Table 4.6. Photophysical properties of Au
BZI
in CH2Cl2 at various temperatures ..................... 125
Table 4.7. Photophysical properties of Au
BZI
in MeCy at various temperatures. ...................... 125
Table 5.1. Calculated energy levels and frontier orbitals of I1–I6 ............................................ 136
Table 5.2. PLQY data of Ir(ppy)3, Ir(bt)2acac and PQIr in I1–I6 films, solution and as neat
materials. ..................................................................................................................................... 140
Table 5.3. Electrochemical properties of I1–I6 ......................................................................... 142
Table 5.4. WOLED device metrics of I1 ................................................................................... 147
Table 5.5. WOLED device metrics of I5 ................................................................................... 149
Table A1.1. Summarized properties of N2-phenanthro-triazoles .............................................. 157

viii

List of Figures
Figure 1.1. Jablonski diagram depicting photophysical processes following electron excitation in
molecules ........................................................................................................................................ 3
Figure 1.2. Schematic diagram of selected molecular orbitals (MOs) for a (pseudo)octahedral
complex with low lying MLCT transitions. Adapted from reference
9
........................................... 4
Figure 1.3. Energy level diagram depicting the mixing between
3
LC,
1
MLCT and
3
MLCT states
in d
6
cyclometalated transition metal complexes. ........................................................................... 5
Figure 1.4. Emission decay time av (300 K) versus the total zero-field splitting E(ZFS).
Adapted from reference
9
................................................................................................................. 7
Figure 1.5. Jablonski diagram depicting the different states and processes involved in TADF. ... 9
Figure 1.6. Basic structure of OLED device ................................................................................ 12
Figure 1.7. Commonly used building blocks for developing high triplet energy host materials . 14
Figure 1.8. Hole-transport type host materials ............................................................................. 15
Figure 1.9. Electron-transport type host materials ....................................................................... 16
Figure 1.10. Bipolar type host materials ...................................................................................... 17
Figure 1.11. HOMO and LUMO plots of Ir(ppy)3 and 3 adapted from reference 49 and 33,
respectively ................................................................................................................................... 19
Figure 1.12. Methods of blue shifting the emission. Adapted from reference 50. ...................... 22
Figure 1.13. Energy level diagram for fac-Ir(ppz)3 and fac-Ir(C^C:)3 depicting non radiative
states. Adapted from reference 57 and 58. .................................................................................... 23
Figure 1.14. Common blue emitting phosphors ........................................................................... 23
Figure 1.15. Triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TTA) processes.
Adapted from reference 59. .......................................................................................................... 24
Figure 2.1. Building blocks with high triplet energies ................................................................. 29
Figure 2.2. (a) Absorption spectra in 2-MeTHF at 298K. Normalized emission spectra in
2-MeTHF at 298K (b) and 77K (c), and as a neat solid at 77K (d). ............................................. 32
Figure 2.3. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K. ............................................... 32
Figure 2.4. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K. ............................................... 34
Figure 2.5. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 77K. (b)
Normalized emission spectra in neat solid at 77K. ....................................................................... 34
Figure 2.6. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K. ............................................... 35
Figure 2.7. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K. ............................................... 35
Figure 2.8. (a) Normalized emission spectra of tpI in 2-MeTHF and solid (b) Normalized
emission spectra of mCBP measured in 2-MeTHF and solid. ...................................................... 36
Figure 2.9. DSC Curves of phenanthro[9,10-d]triazoles/imidazoles depicting their glass
transition temperatures. ................................................................................................................. 36
Figure 2.10. DSC Heating curves of phenanthro[9,10-d]triazoles/imidazoles depicting their
crystallization and melting temperatures. ..................................................................................... 37
ix

Figure 2.11. DSC Heating curves of phenanthro[9,10-d]triazoles/imidazoles depicting their
crystallization and melting temperatures. ..................................................................................... 37
Figure 2.12. TGA curves of phenanthro[9,10-d]triazoles/imidazoles ......................................... 38
Figure 2.13. Cyclic voltammetry curves of phenanthro[9,10-d]triazoles. ................................... 39
Figure 2.14. Cyclic voltammetry curves of phenanthro[9,10-d]imidazoles ................................ 39
Figure 2.15. (a) Cyclic voltammetry curves of phenanthro[9,10-d]triazoles (b) DPV data of
selected phenanthro[9,10-d]triazoles/imidazoles .......................................................................... 40
Figure 2.16. Molecular orbital representation of phenanthro[9,10-d]triazoles and
phenanthro[9,10-d]imidazoles ...................................................................................................... 41
Figure 2.17. Molecular orbital representation of phenanthro[9,10-d]triazoles (H = HOMO, L =
LUMO). ........................................................................................................................................ 42
Figure 2.18. Molecular orbital representation of phenanthro[9,10-d]imidazoles (H = HOMO, L =
LUMO). ........................................................................................................................................ 42
Figure 2.19. Device architecture and chemical structures of the OLED materials ...................... 45
Figure 2.20. Device characteristics of OLEDs using phenanthro[9,10-d]-triazole hosts. (a) J–V
curves. (b) EL spectra. (c) Efficiency versus current curves. (d) Luminance versus current
curves. ........................................................................................................................................... 45
Figure 2.21. OLED device characteristics of phenanthro[9,10-d]imidazole and reference hosts.
(a) J–V curves. (b) EL spectra. (c) Efficiency versus current curves. (d) Luminance versus
current curves. ............................................................................................................................... 46
Figure 2.22. Current Efficiency plot of phenanthro[9,10-d]triazoles and phenanthro[9,10-
d]imidazoles. ................................................................................................................................. 46
Figure 2.23. Comparative OLED testing for devices with phenanthro-triazole (mxT) and
phenanthro-imidazole (mxI) host materials. The device structure used here was (70 nm ITO/5
nm HATCN/40 nm TAPC/10 nm mCP/20 nm8 vol% FIrpic:Host/45 nm BP4mPy/1.5 nm
LiQ/100 nm Al). While the J-V characteristics show only a minor shift on repeated cycling, the
EQE values for the triazole drop markedly on the 2
nd
and 3
rd
J-V scan of the devices. The
significantly higher roll-off in device efficiency for the triazole based device at J > 10 mA/cm
2
is
likely due to device degradation at the higher current density. The electroluminescence (EL)
spectra are nearly independent of the host material, with minor spectral differences likely caused
by cavity effects on the position of the exciton formation zone. .................................................. 48
Figure 2.24. (a) Hole only Structure: 20 min UV Ozone 70 nm ITO / 5 nm HATCN / 20 nm
TAPC / 10 nm mCP / 20 nm 8vol% FIrpic:host or neat host / 10 nm HATCN / 100 nm Al. (b)
Electron only Structure: no UV Ozone 70 nm ITO / 30 nm BP4mPy / 20 nm 8 vol% Firpic:host
or neat host / 30 nm BP4mPy / 1.5 nm LiQ / 100 nm Al ............................................................. 49
Figure 2.25. Polarized emission spectra. a–c, Cross-sections of the measurements and
simulations of the angle-dependent p-polarized photoluminescence emission spectra (considering
an emission in the x–z plane) for films of 20 nm fxI doped with 8 vol% FIrpic (at 484 nm) (a),
20 nm txI doped with FIrpic (at 483 nm) (b) and 20 nm mCBP doped with FIrpic (at 482 nm) (c)
....................................................................................................................................................... 49
Figure 3.1. DFT (singlet and triplet, spin density, HOMO and LUMO surfaces) TDDFT (orbital
contributions). S1 and T1 are HOMO to LUMO transition. ........................................................... 70
x

Figure 3.2. Cyclic voltammetry curves and differential pulse voltammetry data of Ir(C^C:)3
complexes. .................................................................................................................................... 71
Figure 3.3. Photophysical properties of Ir(C^C:)3 complexes (a) UV-visible spectra in 2-MeTHF
at 298K, (b) PL spectra in 2-MeTHF at 298K, (c) PL spectra in PS at 298K (d) PL spectra in 2-
MeTHF at 77K (e) PL spectra in PMMA at 298K ....................................................................... 73
Figure 3.4. Photophysical properties of fac-Ir(pmpz)3 (a) UV-visible spectra in various solvents
at 298K (*fac-Ir(pmpz)3 is highly insoluble in MeCy), (b) PL spectra in various solvents at 298K,
(c) PL spectra in MeCy and 2-MeTHF at 298K and 77K (d) Energy gap law plot at room
temperature ................................................................................................................................... 76
Figure 3.5. Photophysical properties of fac-Ir(tpz)3 (a) UV-visible spectra in various solvents at
298K, (b) PL spectra in various solvents at 298K, (c) PL spectra in MeCy and 2-MeTHF at 298K
and 77K (d) Energy gap law plot at room temperature. ............................................................... 77
Figure 3.6. Photophysical properties of mer-Ir(pmpz)3 (a) UV-visible spectra in various solvents
at 298K, (b) PL spectra in various solvents at 298K, (c) PL spectra in MeCy and 2-MeTHF at
298K and 77K (d) Energy gap law plot at room temperature. ...................................................... 78
Figure 3.7. Emission spectra of 2 wt% fac- and mer-Ir(pmpz)3 and fac-Ir(tpz)3 in PS at various
temperatures. ................................................................................................................................. 79
Figure 3.8. Emission lifetime versus temperature of all the complexes fitted to a TADF model.
....................................................................................................................................................... 81
Figure 3.9. Emission lifetime versus temperature of all the complexes fitted to a triplet model.
....................................................................................................................................................... 85
Figure 3.10. Device architecture of fac-Ir(tpz)3 as a dopant in OLEDs targeted for efficiency
along with the electroluminescence properties of the OLED device. ........................................... 89
Figure 3.11. Device architecture of fac-Ir(tpz)3 as dopant in OLED targeted for stability along
chemical structures of hole blocking materials. ............................................................................ 89
Figure 3.12. Electroluminescence properties of fac-Ir(tpz)3 as a dopant in OLEDs targeted for
stability. ......................................................................................................................................... 90
Figure 3.13.
1
H-NMR of 3 in Chloroform-d. ............................................................................... 99
Figure 3.14.
13
C-NMR of 3 in Chloroform-d. .............................................................................. 99
Figure 3.15.
1
H-NMR of 4 in DMSO-d6. ................................................................................... 100
Figure 3.16.
13
C-NMR of 4 in DMSO-d6. .................................................................................. 100
Figure 3.17.
1
H-NMR of mer-Ir(pmpz)3 in Chloroform-d. ........................................................ 101
Figure 3.18.
13
C-NMR of mer-Ir(pmpz)3 in Chloroform-d. ....................................................... 101
Figure 3.19.
1
H-NMR of fac-Ir(pmpz)3 in Chloroform-d. ......................................................... 102
Figure 3.20.
13
C-NMR of fac-Ir(pmpz)3 in Chloroform-d. ........................................................ 102
Figure 3.21.
1
H-NMR of 5 in DMSO-d6. ................................................................................... 103
Figure 3.22.
13
C-NMR of 5 in DMSO-d6. .................................................................................. 103
Figure 3.23.
1
H-NMR of 6 in Chloroform-d. ............................................................................. 104
Figure 3.24.
13
C-NMR of 6 in Chloroform-d. ............................................................................ 104
Figure 3.25:
1
H-NMR of fac-Ir(tpz)3 in Chloroform-d. ............................................................. 105
Figure 3.26.
13
C-NMR of fac-Ir(tpz)3 in Chloroform-d. ............................................................ 105
Figure 3.27. Variable temperature
1
H-NMR of fac-Ir(tpz)3 in Benzene-d6. .............................. 106
Figure 3.28. Variable temperature
1
H-NMR of fac-Ir(tpz)3 in Acetone-d6. .............................. 106
xi

Figure 3.29. MALDI spectrum of mer-Ir(pmpz)3. ..................................................................... 107
Figure 3.30. MALDI spectrum of fac-Ir(pmpz)3. ...................................................................... 107
Figure 3.31. MALDI spectrum of 5. .......................................................................................... 108
Figure 3.32. MALDI spectrum of 6. .......................................................................................... 108
Figure 3.33. MALDI spectrum of fac-Ir(tpz)3. .......................................................................... 109
Figure 4.1. The structures of M
BZI
complexes. .......................................................................... 112
Figure 4.2. Cyclic voltammetry data of M
BZI
. ........................................................................... 114
Figure 4.3. Differential pulse voltammetry data of M
BZI
and CH2Cl2. ...................................... 114
Figure 4.4. HOMO (E = 4.22 eV, solid) and LUMO (E = 1.44 eV, mesh) surfaces of complex
Au
BZI
. .......................................................................................................................................... 115
Figure 4.5. Absorption (dashed lines) and photoluminescence (solid line) spectra of M
BZI

complexes in MeCy, 2-MeTHF and polystyrene at room temperature (RT) and 77 K. ............. 117
Figure 4.6. Qualitative energy diagram representing the ground state (S0) and both excited state
potential energy surfaces (
3
Cz and
1,3
ICT) as a function of nuclear coordinate in MeCy (blue) and
CH2Cl2 (red) solution along with absorption (solid) and emission (dashed) transitions. ........... 119
Figure 4.7. Absorption and emission spectra of complex Au
BZI
in CH2Cl2, MeCy and in various
solvent ratios. .............................................................................................................................. 121
Figure 4.8. Change in PL and radiative and nonradiative rate constants of Au
BZI
as a function
of solvent polarity. ...................................................................................................................... 122
Figure 4.9. Scaled absorption and emission spectra of complexes Au
BZI
in various solvents
(DMF = N,N-Dimethyl formamide). .......................................................................................... 123
Figure 4.10. Emission spectra of Au
BZI
in CH2Cl2 and MeCy at various temperatures. ........... 124
Figure 4.11. (a) Kinetic scheme for the emission from Au
BZI
. Arrhenius plot of the temperature-
dependent lifetime data for Au
BZI
in MeCy (b) and in CH2Cl2 (c) recorded from 180 to 300 K,
along with fits to the data according to Equation 1. ................................................................... 126
Figure 4.12. Variable temperature
1
H NMR spectra of Au
BZI
in CD2Cl2. ................................. 127
Figure 4.13. Devices with 5 and 10% doping without (WO) and with UGH3 blocker (4 devices)
..................................................................................................................................................... 128
Figure 5.1. Exciton energy transfer in hybrid fl/ph WOLED (a) using a blue fluorescent dopant
and a separate host, (b) using a blue fluorescent material as a neat emitter and as a host. ......... 132
Figure 5.2. Commonly used blue fluorescent emitters for hybrid fl/ph WOLED ..................... 134
Figure 5.3. Molecular orbital representation of I1 – I6 (H = HOMO, L = LUMO). ................. 137
Figure 5.4. Absorption spectra of I1–I6. ................................................................................... 138
Figure 5.5. Photoluminescence spectra of I1 – I6. .................................................................... 139
Figure 5.6. Summarized S1/T1 of I1–I6 measured in solution and in the solid-state. S1 and T1 are
obtained from the onset of their individual emission spectra at 0.2 intensity............................. 140
Figure 5.7. Normalized emission spectra of Ir(ppy)3, Ir(bt)2acac and PQIr in phenanthro-
imidazole and CBP Hosts. .......................................................................................................... 141
Figure 5.8. Cyclic voltammetry curves of II – I6. ..................................................................... 142
Figure 5.9. DSC Curves of I1 – I6. ............................................................................................ 143
Figure 5.10. TGA curves of I1 – I6. .......................................................................................... 143
Figure 5.11. Monochromatic OLED characteristics of phenanthro[9,10-d]imidazoles as neat blue
fluorescent emitters. .................................................................................................................... 144
xii

Figure 5.12. I1 WOLED device architecture and characteristics. ............................................. 146
Figure 5.13. I5 WOLED device architecture and characteristics. ............................................. 148
Figure A1.1. Structures of N1- and N2-phenanthro[9,10-d]-triazolesalong with their bond
dissociation energies (BDE) calculated using DFT .................................................................... 156
Figure A1.2. Photophysical properties of 2-pT, 2-xT and 2-mxT ............................................. 156
Figure A1.3. MALDI data of 2-mxT ......................................................................................... 157
Figure A1.4. OLED device characteristics of phenanthro[9,10-d]imidazole and reference hosts.
(top) Device architecture and EL spectra. (bottom) Efficiency versus current curves, J–V curves
and Luminance versus current curves. ........................................................................................ 159

xiii

Abstract
Over the past several years, organic light-emitting diodes (OLEDs) have attracted
considerable attention for full color display and solid-state lighting applications owing to their high
electroluminescence quantum efficiency. Despite their high efficiency, major deficiencies remain
in OLEDs. Blue PHOLEDs in particular, are inefficient and unstable due to degradation of host
materials and phosphorescent emitters (dopants) in the emissive layer. Most commonly used host
materials and dopants feature weak C−N/Ir−N bonds which have been attributed to molecular
degradation during device operation. For this reason, alternative host materials and phosphorescent
emitters are strategically designed and developed here with the aim of achieving both high
efficiency and stability in blue OLEDs. Following computational screening of a large library of
host materials, phenanthro[9,10-d]triazoles and imidazoles were found to have higher C−N bond
dissociation and triplet energies (BDE = 92 kcal/mol, ET = 2.97 eV) than commonly used
carbazole-based host materials (BDE = 83 kcal/mol, ET = 2.93 eV) for blue OLEDs. Synthesis,
electrochemical, photophysical and electroluminescence characterization of these materials as host
materials for blue PHOLEDs are discussed in Chapter 2.
The second class of materials, tris-iridium (III) based blue phosphorescent emitters
featuring NHC pyrydino and pyrazinoimidazolyl carbene ligands, are discussed in Chapter 3.
These materials are synthesized as alternative sky blue dopants to the current unstable sky blue
phosphorescent dopants. They are chosen due to their stronger Ir−C bond compared to commonly
used phosphors with weaker Ir−N bonds. Computational and photophysical studies were carried
out to understand their extraordinary photophysical properties, including fast radiative rates
(1.3×10
6
s
−1
). Optimized blue PHOLED devices using the host materials and the phosphorescent
xiv

emitters developed in Chapter 2 and 3 achieved excellent electroluminescence efficiency (21%),
high brightness (35 000 cd/m
2
) at low current density and moderate device lifetime (T80 = 16 h).
In Chapter 4, two-coordinate coinage metal (i.e. Cu, Ag, Au) complexes bearing a bulky
benzoimidazolyl carbene and carbazolyl as the anion ligand are discussed. These materials emit
via thermally activated delayed fluorescence (TADF) and exhibit efficient and deep blue narrow
emission, crucial for display applications. The electrochemical and photophysical properties of
these complexes were studied to understand the origin of their interesting properties, including
efficient and deep blue narrow emission in nonpolar medium and abrupt decrease in efficiency in
polar medium. OLED devices incorporating these materials as dopants achieved 12% EQE with
narrow and deep blue emission.
In Chapter 5, neat blue fluorescent emitters are designed and developed to reduce the
number of stacked layers of hybrid fluorescent/phosphorescent (F/P) white organic light-emitting
diodes (WOLEDs), which is aimed at reducing costs and lowering driving voltages. Several
phenanthro[9,10-d]imidazole-based materials featuring high fluorescence efficiencies in neat
films were developed. Their singlet and triplet excited states were engineered to serve as both neat
fluorescent emitters and hosts for green and red phosphors in hybrid WOLEDs. A highly efficient
(20%) hybrid F/P WOLED with a high luminous efficacy (65 lm/W) at low driving voltages (< 5
V) was demonstrated using a single emissive layer with these new blue fluorescent emitters along
with conventional green and red phosphors.
1

1CHAPTER 1 − Introduction
Light-emitting diodes (LEDs) are solid-state devices built from noncarbon-based materials
such as silicon, gallium or arsenide that generate light upon electrical excitation. Similar to LEDs,
organic LEDs (OLEDs) emits light in response to electric field but utilizes organic materials in the
active part of the device. Over the past few years, OLEDs have attracted considerable attention for
full color displays (e.g. mobile and television), wearables and solid-state lighting.
1-3
This is
because OLEDs can be made into thin and flexible panels, have high efficiency (do not require
backlight), are bright, and have fast response times (in the microsecond regime).
Solid-state lighting is another area where OLED is gaining interest. Currently, LED lighting is
leading the solid-state lighting market due to their low cost, long lifetime, high brightness, high
efficacy and high color rendering index (CRI).
4
However, White OLEDs (WOLEDs) offer other
advantages over LED lighting. This includes, diffuse area lighting, flexible, light weight, thin and
transparent panel, and color-tunability to achieve both warm and cool white light. For these
reasons, WOLEDs have great potential as solid state lighting source. For example, IKEA sells an
OLED lamp, the Vitsand, a chandelier that uses 7 OLED panels to provide 700 lumens at 2700
K.
5
Another commercial application of WOLEDs is their use as display panels in LG TVs.
6

1.1. Excited States of Luminescent Materials
Understanding the excited states of organic and organometallic complexes is crucial in
tuning their photophysical and electrochemical properties. Electronic transitions between ground
and excited states can be illustrated using a Jablonski diagram shown in Figure 1.1.
7
In both
organic and organometallic complexes, an excited state is generated when a photon is absorbed by
a molecule to promote an electron from an occupied molecular orbital to a higher lying unoccupied
2

molecular orbital of the molecule. This process is equivalent to promoting an electron from a
singlet ground state (S0) to a higher laying excited state (Sn). The electron in the excited Sn state
can decay in three main pathways. By non-radiatively decaying back to the lowest singlet excited
state (S1) in a process called internal conversion (IC). The electron in the S1 excited state can non-
radiatively decay back to the ground state, radiatively decay back to the ground state (fluorescence)
or non-radiatively decay to a triplet state excited state Tn (intersystem cross (ISC)). The electron
in Tn state decays in similar manner as the Sn state. The radiative decay of the electron in Tn to the
S0 is termed phosphorescence. The Sn to Tn transition or vice-versa involves spin flip and is
symmetry forbidden and thus pure organic materials with large energy difference between the S1
and T1 (EST) typically have long phosphorescent radiative lifetimes (milliseconds to seconds).
The “allowedness” of S1 to T1 can however be increased by making the S1 − T1 gap extremely
small (EST < 0.1 eV) or by incorporating heavy metals to organic molecules (organometallic
complexes) to facilitate transition between singlet and triplet through spin-orbit coupling (SOC).
SOC is a physical phenomenon that allows mixing of singlet and triplet. This phenomenon
occurs when the magnetic field generated by nucleus of an atom, B interacts with the dipole
moment of electron spin, M. This interaction makes the electron spin to align itself with the
magnetic field and the resultant interaction is termed SOC. The relative orientations of the spin
axis and the orbital angular momentum axis determine the energy levels of the system.
8
Because
of the coulombic nature of the magnetic field, SOC factor is dependent on the magnitude of the
nuclear charge. Generally, the larger “heavier” the nucleus of an atom, the larger the SOC matrix
3

and hence the more allowedness of S1−T1 transition. Thus, organometallic complexes bearing
heavy atoms like Ir and Pt have efficient phosphorescence at room temperature.

Figure 1.1. Jablonski diagram depicting photophysical processes following electron excitation in
molecules
1.1.1. Fluorescent Materials
The radiative S1 − S0 transition (fluorescence) in pure organic materials is usually efficient
and is in the order of nanoseconds. However, the transition from S1 to T1 is slow, on the order of
microseconds in pure organic materials with large S1 − T1 gap (> 0.3 eV), making the S1 − T1
intersystem crossing inefficient. Therefore, phosphorescence in pure organic compounds is often
inefficient at ambient conditions due to long excited lifetimes, allowing the excited state to decay
through non-radiative processes.
1.1.2. Phosphorescent Emitters (Triplet Emitters)
Organometallic complexes bearing heavy atoms (e.g. iridium and platinum) with mostly,
but not limited to organic ligands are usually employed as emitters in OLEDs. The ligands in these
complexes can form different geometries around the metal atom (e.g. linear, square planar and
4

octahedral) which has a significant effect on the orbital splitting of the complex. The formation of
the excited state in organometallic complexes involves transition between π and d-orbitals. Taking
a distorted octahedral low spin d
6
organometallic emitter with three bidentate π-ligands as an
example, the orbital splitting of the complex can approximately be described in Figure 1.2.
9
For
simplicity, we will focus on transitions between the t2g (dxy, dxz, dyz) the π and π* orbitals of the
ligand. The singlet and triplet metal to ligand charge transfer (
1
MLCT and
3
MLCT) excited states
result from the transition between the t2g (dxy, dxz, dyz) and the π* orbitals. Ligand centered singlet
and triplet (
1
LC and
3
LC) excited states stem from inter- or intra-ligand π-π* transition. d-d*
transitions are not considered here since is highly unlikely in low spin d
6
octahedral complexes.

Figure 1.2. Schematic diagram of selected molecular orbitals (MOs) for a (pseudo)octahedral
complex with low lying MLCT transitions. Adapted from reference
9

Several studies on cyclometalated transition metal complexes have shown and established that the
luminescence of these type of complexes originates from the T1, which is a
3
MLCT1 admixed with
mostly but not limited to
1
MLCT2 and
3
LC states through SOC.
10-13
As mentioned above, heavy
5

atoms like Ir and Pt induce high SOC which results in spin flip and thus transition between S1 and
T1 that was formally forbidden process has now a certain allowedness. Depending on the energies
of
3
MLCT and
3
LC states, the amount of mixing and the predominant character of the lowest
energy excited state will vary.
14-16
Figure 1.3 illustrates the energy level mixing in 4d
6
and 5d
6

complexes.

Figure 1.3. Energy level diagram depicting the mixing between
3
LC,
1
MLCT and
3
MLCT states
in d
6
cyclometalated transition metal complexes.
The lowest triplet excited state of 4d
6
and 5d
6
organometallic complexes can be defined by first
order perturbation theory shown below.
𝛹 𝑇 1
= √1 − 𝛼 2
|
3
𝐿𝐶 ⟩ + 𝛼 |
3
𝑀𝐿𝐶𝑇 (1.1)
In this equation, the ΨT1 is the wave function of the lowest triplet excited state of 4d
6
and
5d
6
organometallic complexes and the coefficient α gives an estimate of the degree of mixing of
MLCT character mixed into the unperturbed
3
LC state.
11-14, 17
The value α can be approximated
with the following formula:
𝛼 =
⟨
3
𝐿𝐶 |𝐻 𝑆𝑂
|
3
𝑀𝐿𝐶𝑇 ⟩
𝛥𝐸
(1.2)
6

where ⟨
3
LC|H
SO
|
3
MLCT⟩ is the SOC matrix element, characterizing the strength of SOC between
3
LC and
3
MLCT, and E is the energy difference between the
3
LC and
3
MLCT transitions.
14
Thus,
mixing of these states into what is principally
3
MLCT1 state has significant effects on the optical
properties of those complexes.
10
The radiative decay rate in luminescent metal complexes is
significantly increased when a small amount of
1
MLCT2 character is mixed into the lowest-excited
state. Consequently, a large decrease in the luminescence lifetimes and concomitant increase in
phosphorescence efficiency occurs.
14, 18
The strong σ-donation of C- in a formally anionic
cyclometalating ligand also stabilizes the
3
MLCT state and hence decreases the
3
MLCT−
3
LC (ΔE)
energy gap. This further increases the value of α and hence increases the phosphorescence
efficiency. Thus, organometallic complexes bearing heavy atoms like Ir and Pt with
cyclometalated ligands are ideal dopants in OLEDs. Therefore, phosphorescent emitters can in
principle harvest all the excitons to achieve 100% internal OLED efficiency.
The T1 state has three substates (i.e.
I
T1,
II
T1 and
III
T1), and the energy difference between
the highest (
III
T1) and the lowest substates (
I
T1) commonly known as zero field splitting (E(ZFS)),
is directly related to the strength of SOC. Increasing the strength of SOC i.e. increasing coupling
between the emitting triplet state and higher lying singlet states, is associated with an increase in
E(ZFS) and a decrease in emission decay time. Figure 1.4 shows several examples of
organometallic complexes with their E(ZFS) and corresponding emission lifetimes. Complexes
on the right-hand side predominantly emit from the
3
MLCT and have large E(ZFS) (> 50 cm
-1
)
due to strong SOC and thus their emission lifetime is short (< 2 s). For example, Ir(ppy)3, one of
the most efficient phosphorescent emitters, have E(ZFS) value of 170 cm
−
and a radiative decay
time of only 1.6 s) [40]). In contrast, complexes on the far left have predominantly
3
LC emission
and their SOC is weak and therefore their E(ZFS) is small (< 1 cm
−
). The radiative decay times
7

of these complexes can be as long as several milliseconds. Complexes in the middle are
characterized with
3
LC emission admixed with
3
MLCT and therefore their E(ZFS) is in the
intermediate region, ranging from several cm
−
up to several ten cm
−
. The radiative decay times
for these complexes are longer than for complexes that emit predominantly from
3
MLCT states
and are usually in the range − s.

Figure 1.4. Emission decay time av (300 K) versus the total zero-field splitting E(ZFS).
Adapted from reference
9

The zero-field splitting parameter for a triplet emitter can be obtained from an analysis of
the temperature dependence of the emission lifetime. Under the assumption of a fast
thermalization, the averaged decay time av for a system of three excited substates I, II, and III can
be written as
19-21

𝜏 (𝑇 ) =
1+ 𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 + 𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 1
𝜏 (𝐼 )
+
1
𝜏 (𝐼𝐼 )
𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 +
1
𝜏 (𝐼𝐼𝐼 )
𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 (1.3)
8

where (I), (II), and (III) represent the individual lifetimes of the triplet substates I, II, and III,
respectively. E(II−I) and E(III−I) are the energy separations between II−I and III−I substates
and kB is the Boltzmann constant.
1.1.3. Thermally Activated Delayed Fluorescence (TADF) Emitters
Another strategy to enhance ISC is by making the S1 − T1 gap extremely small (0.1 eV) to
facilitate rapid thermal equilibration of the electron between S1 and T1 states that ultimately decays
to ground state from the S1 manifold in a process known as delayed fluorescence. Delayed
fluorescence was first observed in organic material (Eosin) by Boudin
22
and later in fluorescein by
Lewis and co-workers who identified the thermal component of the delayed fluorescence,
23
and
then later named the process as E-type (E for eosin) delayed fluorescence by Parker and Hatchard
after their detailed study on the process.
24
Recently, thermally activated delayed fluorescence
(TADF) emitters have gained significant attention due in large part to their promising application
as dopants in OLEDs. In OLEDs, TADF is particularly important in pure organic emitters that lack
heavy atoms where phosphorescence is inefficient due to weak SOC. As such, materials that
undergo TADF materials can harvest all the excitons in OLED devices and can therefore be
utilized as alternatives to rare-earth metal based emitters that utilize triplet excitons for
electroluminescence.
25-27

In TADF systems, the S1 is populated after photoexcitation which can radiatively decay to
the ground state through prompt fluorescence or nonradiatively decay to the T1 state through ISC
and then thermally populate back to the S1 through a process called up-ISC (UISC, also frequently
referred to as reverse ISC: rISC), which can then decay radiatively via delayed fluorescence
(Figure 1.5). The ISC and rISC in pure organic TADF materials have fast rates of intersystem
crossing (kISC up to 10
7
s
-1
)
28
owing to their small ∆𝐸 𝑆 1
−𝑇 1
. However, this rate is still too slow to
9

effectively quench the prompt fluorescence S1 (kpf ~ 10
7
– 10
9
s
-1
) and as a result
bi/multiexponential photoluminescence (PL) decays recorded at all temperatures are always
observed, corresponding to prompt fluorescence and TADF. However, this is not the case in most
commonly-studied Cu(I) TADF emitters, where strong SOC induces fast k ISC (~ 10
12
s
-1
)
29, 30
that
rapidly depopulates S1. As a result, prompt fluorescence is not observed in Cu(I) TADF complexes,
and PL decays measured at all temperatures are usually monoexponential. Additionally, unlike
organic TADF emitters, where phosphorescence out of T1 is weak and slow, phosphorescence in
Cu(I) TADF is effective with radiative rates that dominate the thermally-driven kUISC at low
temperatures.
29
Therefore, an increase in the PL decay lifetime and a red-shift in the emission
spectrum at low temperatures, both corresponding to T1-based phosphorescence in the low
temperature regime are signatures for organometallic TADF emitters.

Figure 1.5. Jablonski diagram depicting the different states and processes involved in TADF.
Similar to the parameters controlling triplet systems, the magnitude of the energy
separation ∆𝐸 𝑆 1
−𝑇 1
as well as 𝑘 𝑇 1
and 𝑘 𝑆 1
, the decay rate constants of phosphorescence (kphos) and
prompt fluorescence (kpf) respectively in TADF systems are usually obtained by measuring the
10

TADF emission lifetime at various temperatures. The values of these parameters can be extracted
by plotting the measured lifetime against temperature, and then fitted to a modified Boltzmann
equation (below).
𝜏 (𝑇 ) =
2+ 𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 + 𝑒𝑥𝑝 −∆𝐸 (𝑆 1
−𝑇 1
)
𝑘 𝐵 𝑇 2
𝜏 (𝐼 ,𝐼𝐼 )
+
𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 𝜏 (𝐼𝐼𝐼 )
+
𝑒𝑥𝑝 −∆𝐸 (𝑆 1
−𝑇 1
)
𝑘 𝐵 𝑇 𝜏 (𝑆 1
)

(1.4)
where kB is the Boltzmann constant.
Earlier work on Cu(I) TADF emitters is mostly based on four and three-coordinate around the
copper metal.
31
In these complexes, the HOMO is usually Cu(I)-based whereas the LUMO is
usually localize on the π* orbitals of the electron-deficient ligand, resulting in overlap between the
HOMO and the LUMO that results in metal to ligand charge transfer (MLCT) lowest energy
excited states. The nonradiative rate of these complexes is high owing to their large reorganization
energies as a result of distortion in the excited state in fluid environment. Recently linear two-
coordinate Cu(I) TADF complexes have gained significant attention owing to their low excited
state reorganization energies and non-radiative rates and high radiative rates.
32-35
These complexes
have redox active carbene (acceptor) and amide (donor) ligands connected by a coinage metal (Cu,
Ag, Au) in a linear fashion i.e. (carbene)M
(I)
(amide). They emit via TADF mechanism with the
lowest energy transition being the internal charge transfer from the amide to the carbene (HOMO
→ LUMO) and the metal’s d-orbitals acting as an efficient electronic conduit. The fast radiative
lifetimes of these complexes are due to two factors; the small energy separation between their
lowest singlet and triplet excited states (∆𝐸 𝑆 1
−𝑇 1
) and spin orbit coupling via the metal ion.
Together these two parameters lead to rapid endothermic intersystem crossing from the long-lived
triplet to the faster radiating singlet state.
34
Chapter 4 is devoted to the study of structural,
11

photophysical and electrochemical properties of three luminescent 2-coordinate coinage metal (i.e.
M = Cu, Ag, Au) complexes bearing a sterically bulky carbene, and carbazolide (Cz) as the anionic
ligand.
1.2. OLED Structure
The basic structure of OLED device is depicted in Figure 1.6. It consists of a substrate
(usually a glass), an anode (e.g. indium tin oxide), a hole transport layer (HTL), an emissive layer
(EML), a hole transport layer (ETL) and a cathode (e.g. aluminum). When voltage is applied to
the device, holes and electrons are injected from the anode and the cathode, respectively to the
organic transport layers where the charges are transported. The holes and the electrons recombine
in the emissive layer (active layer) to generate excitons (hole-electron pair, excited state). The two
unpaired electrons of the exciton have possible magnetic quantum numbers of M S +1, 0, and −1.
The values of MS = +1 and − correspond to two substates of the triplet and the third triplet substate
is given by a positive linear combination of the two possible configurations with M S = 0. The
negative linear combination with MS = 0 corresponds to the singlet state.
9
Therefore, according to
spin statistics, the ratio of singlets to triplets formed during recombination is 1:3. For this reason,
OLEDs with organic emitters that have large S1 − T1 gap can only achieve maximum internal
OLED efficiency of 25% since their phosphorescence is inefficient, whereas organic TADF
emitters and organometallic complexes bearing heavy atoms can harvest all the excitons to achieve
100% OLED internal efficiency. The excitons formed after recombination decay to ground state
to emit light of a specific wavelength unique to the bandgap of the emitting material. This process
of generating light by exciting a chromophore using electrical energy is called electroluminescence
(EL). Another method of generating light from a chromophore using light as the source of energy
is photoluminescence (PL). Both PL and EL measurements form the same excited states of
12

molecules and therefore can both be used to understand the excited state and transport levels of
organic chromophores, which are crucial in improving the efficiency and device lifetime of
OLEDs. PL measurements are more convenient and less tedious than EL measurements, and for
this reason PL measurements were first utilized to characterize the photophysical properties of
OLED materials developed in this thesis. Materials that show desired properties were then used to
fabricate OLED devices followed by EL characterization.

Figure 1.6. Basic structure of OLED device
1.2.1. Materials in the Emissive Layer
In a basic OLED device, the emissive layer consists of an emitter doped into a host matrix.
The host material disperses the emitter molecules to prevent aggregation of the emitter molecules,
which consequently reduces bimolecular exciton-exciton quenching processes. Undoped
phosphorescent emitters in OLEDs are uncommon, but fluorescent emitters with high efficiency
13

as neat materials are seldom utilized as nondoped emitters in OLEDs. In fact, nondoped blue
fluorescent emitters are developed and utilized in white OLEDs in Chapter 5.
1.2.1.1.Host Materials for OLEDs
The primary role of a host material in OLEDs is to disperse dopants to prevent aggregation
of the dopant and to reduce bimolecular quenching processes. For phosphorescent OLEDs
(PHOLEDs) and TADF OLEDs, the host material needs to have a triplet energy (ET) higher than
that of the dopant to ensure exclusive emission from the dopant.
Due to their excellent transport, thermal properties and their low cost, conjugated organic
molecules are the most common host materials for PHOLEDs. Increasing the conjugation length
of conjugated systems delocalizes the triplet spin density and hence decreases the triplet energy.
36

For this reason, host materials for deep blue phosphors are limited to nonconjugated systems and
to biphenyl for sky blue emitting phosphors. For example, host materials for the deep blue
phosphor, fac-Ir(pmp)3 is limited to TSPO1 type nonconjugated host materials.
37
Conjugated
materials with biphenyl (ET = 2.78 eV) as their longest conjugation moiety are employed as host
materials for sky blue phosphors. Among them, carbazole based host materials are the most
commonly used hosts for sky blue PHOLEDs.
38
These materials are designed to localize the triplet
densities of these materials on the carbazole or the biphenyl moiety, to ensure high triplet energies
above 2.75 eV.
In addition to carbazole based host materials, other materials with high triplet energies like
fluorene, dibenzofuran, dibenzothiophene derivatives are also employed as hosts for sky blue
emitting phosphors. Because of their lower triplet energy than blue phosphors, designing hosts for
green and red phosphors are relatively easier than for blue phosphors. This requirement allows
14

additional moieties like naphthalene, triphenyl, phenanthrene, phenanthroline to be utilized for
developing host materials with triplet energies below 2.60 eV.

Figure 1.7. Commonly used building blocks for developing high triplet energy host materials
Host materials with large HOMO−LUMO gaps can host the transport levels of dopants
thereby ensuring direct trapping of excitons on the dopant. This direct trapping decreases the
possibility of energy losses stemming from ineffective energy transfer from the host to the dopant.
Despite these advantages, nesting the transport levels of the dopant by the host requires the emitter
to be doped at high concentration to achieve acceptable charge conduction across the emissive
layer. To increase charge conductivity in the emissive layer, bipolar, hole, or electron transport
type host materials are utilized in PHOLEDs.
Hole transport type host materials feature electron donating groups for hole trapping and
transport. The holes trapped at the HTL interface on the host molecules hop across the EML before
they attract electrons to form excitons that will eventually be transferred to the dopant. Examples
of hole conducting hosts materials are depicted in Figure 1.8. They feature electron donating
groups like triphenylamines and carbazoles. Carbazole based materials are common hole transport
15

type host materials due to their high triplet energies and high hole mobilities. For example,
PHOLEDs that utilized CBP as host materials for green and red dopants achieve efficiencies as
high as 57.2 cd A
−
for green Ir(ppy)3,
39
and 5.82 cd A
−
for a dendritic deep red iridium complex.
40

Many other host materials were derived from CBP to increase its triplet energy and glass transition
temperature (Tg). For example, Tokito et al.
41
reported that introducing two methyl groups in 2-
and 2’-positions of biphenyl in CBP structure increased the triplet energy from 2.50 to 3.0 eV. A
drastic increase in the external quantum efficiency from 5.1 % to 10.4% was observed when the
new host (CDBP) was utilized instead of CBP as a host for sky blue emitting phosphor (FIrpic).
Another important carbazole based host material for blue emitting dopants is mCBP. The carbazole
and the phenyl of mCBP are connected in meta fashion to break conjugation, keeping the triplet
energy high (ET = 2.93 eV). Blue PHOLEDs with mCBP as host and a sky blue dopant (FIrpic)
achieved EQE as high as 17%.
42

Figure 1.8. Hole-transport type host materials
In contrast to hole transporting type host materials, electron transport type (E-type) host
materials feature electron deficient heteroarene moieties like oxadiazoles, imidazoles, triazoles,
triazines, phenanthroline and diaryl phosphine oxide (Figure 1.9). These kinds of moieties can be
easily reduced and therefore lower electron injection barrier to reduce the driving voltages.
PHOLEDs with E-type hosts materials are reported to achieve high external quantum efficiencies
(EQEs) and conductivities with blue, green and red dopants. For example, a high EQE (14.4±0.1%)
and deep blue PHOLED with TSPO1 host material was reported by Jaesang et. al.
37
The same
16

group reported a high EQE of 19±1% and a power efficiency of 60±5 lmW
-1
using NTAZ doped
with a green dopant (ppy)2Ir(acac).
43
Similarly, a high EQE of 15.1% was achieved with a red
dopant doped into TPBI.
44

Figure 1.9. Electron-transport type host materials
Bipolar type host materials contain a combination of both electron rich and deficient
moieties for both holes and electron trapping and transportation, respectively. The functional
groups are usually connected through a spacer to break conjugation between the functional groups.
Bipolar type host materials are employed in PHOLEDs to achieve effective charge balance in the
emissive layer, to lower hole and electron injection barriers and to increase charge transport in the
emissive layer. Examples of hosts materials with bipolar functional groups are shown in Figure
1.10. These materials contain a combination of electron donors (carbazoles or triarylamine) with
electron acceptors such as the ones mentioned above connected together or through phenyl spacers.
The main drawback of bipolar type host materials is their spectral broadening emanating from a
charge transfer transition from the donor to the acceptor. This broadening usually leads to low
singlet and triplet energies, thus liming the use of bipolar type host materials in blue PHOLEDs.
Despite, this challenge, few research groups successfully designed and developed high triplet
energy bipolar type host materials that are compatible with sky-blue emitting phosphors. For
example, Kido’s group reported two bipolar host materials compounds (26DCzPPy and
35DCzPPy) with triplet energies as high as 2.71 eV. Utilizing these host materials in blue
17

PHOLEDs, high external quantum and power efficiencies of 19.1% and 34.6 lm W
−
and 24.3%
and 46.1 lm W
−1
were achieved for the 35DCzPPy-hosted and 26DCzPPy-hosted blue PHOLEDs,
respectively.
45

Figure 1.10. Bipolar type host materials
In addition to the energetic requirement, host materials should have good thermal and
morphological stabilities, in order to reduce the likelihood of phase separation during device
operation, which could lead to low device operational lifetime. In general, high molecular weight
materials with bulky and sterically hindered substituents are preferred to enhance the glass
transition (Tg) and crystallization temperatures to form morphologically stable and uniform
amorphous films. Host materials with glass transition temperatures as high as 100
o
C are reported
to make stable and uniform films.
42
Regardless of the type of host materials, materials with good
thermal properties and good morphological properties in thin films are always desired host
materials.
1.2.1.2.Tuning Emission Energy of Phosphorescent and TADF Emitters
Phosphorescent emitters are the most widely used emitters in OLEDs due to their high
electroluminescence efficiency. Phosphorescent emitters bearing Cu, Pt and Ir metals with organic
ligands have all been reported to give moderate OLED efficiencies.
37, 46-48
Among them,
cyclometallated octahedral Ir(III) complexes are the most commonly used in OLEDs due to their
high phosphorescence efficiencies, short radiative lifetimes and molecular stabilities emanated
18

from their high SOC. Most of these homoleptic complexes contain bidendate phenyl pyridine C^N
type cyclometallating ligands with emission color ranging from deep blue to deep red.
48

Heteroleptic tris-Ir (III) complexes with two bidendate phenyl pyridine C^N type cyclometallating
ligands and one ancillary ligand are also utilized in OLEDs. Since the emission of these compounds
has significant
3
LC character, the emission energies of these complexes can be tuned by modifying
the cyclometalling ligands. The extent of metal and ligand character in the HOMO and LUMO is
indicative of the amount of
3
MLCT/
3
LC mixing in the excited state.
Density Functional Theory (DFT) and Time Dependent (TD-DFT) calculations are useful
tools in understanding and predicting the electronic properties of these complexes. For example,
the HOMO, LUMO and the triplet spin density of a green triplet emitter Ir(ppy) 3
49
and a TADF
emitter (3)
33
calculated using DFT are shown in Figure 1.11. For Ir(ppy)3, the electron density of
the HOMO is localized on the iridium and phenyl, whereas the LUMO is mainly localized on the
pyridine with very small metal contribution. The triplet spin density is localized on iridium and
phenylpyrine ligand. In the case of 3, the HOMO is localized on the donor (carbazole), whereas
the LUMO is localized on the acceptor (carbene). Therefore, the emission energy of these
complexes can be tuned by modifying the HOMO and the LUMO of the chromophore ligands. In
the case of Ir(ppy)3 type complexes, the HOMO is usually modified by adding substituents to the
phenyl ring, whereas the LUMO is modified by adding substituent groups to the pyridine ring.
Addition of electron withdrawing on the phenyl ring stabilizes the HOMO and consequently results
in blue shift of the emission energy. Incorporating electron donating group will have the opposite
effect. In contrast, addition of electron withdrawing groups on the pyridine ring stabilizes the
LUMO and consequently results in red shift of the emission energy, whereas addition of electron
donating groups destabilizes the LUMO and results in a hypsochromic shift. Introduction of one
19

or more fluorine atoms or aza substitution of the ligand skeleton are the main strategies in
stabilizing the HOMO and LUMO energies. A single fluoride substituent on the node (4’or 6’-
position) of the phenyl ring can lead to a 12 nm blue shift whereas difluoro substitution on the
4’and 6’-positions can lead to a 30 nm blue shift. Substitution of a single fluoride on the electron
dense position (5’-position) induces a lesser blue shift since weak π-donation by the fluoride
offsets the withdrawing effect of the fluorine.
49
Aza substitution on similar Ir(III) cyclometalling
ligands have similar effects.
50

Figure 1.11. HOMO and LUMO plots of Ir(ppy)3 and 3 adapted from reference 49 and 33,
respectively
Strategies to redshift the emission of tris-cyclometallated iridium (III) complexes include
addition of strong donors like MeO- or R2N- to the phenyl ring of the cyclometalling ligand to
destabilize the HOMO of the complex. Replacing the phenyl ring with groups that are more
susceptible to oxidation, also destabilizes the HOMO. For example, the emission energy of
Ir(ppy)2acac (λem= 516 nm) is redshifted when the phenyl group is replaced by benzothiophene
20

(btp)2Ir(acac) (λem= 610 nm).
51
Another strategy is by extending the π-conjugation of the
cyclometallating ligand. For example, the emission spectra of (ppy)2Ir(acac) (λem= 516 nm) is
redshifted by extending its π-system through addition of a bridging vinyl group to form
(bzq)2Ir(acac) (λem= 548 nm).
51

Similar approaches mentioned above can be utilized to tune the emission color of the linear
2-coordinate metal(I) TADF complexes introduced in the previous section. Tuning the emission
energy of these complexes can be achieved by independently tuning the electronic properties of
the donor and the acceptor since the donor and acceptor are completely separated. Addition of
electron withdrawing groups on the donor stabilizes the HOMO and leads to blue shift, whereas
addition of electron donating groups on the donor will lead to redshifts.
32
For example, addition of
two cyano groups on carbazole in complex 3 lead to a 74 nm blue shift.
33
In contrast, addition of
electron withdrawing groups on the acceptor is expected to stabilize the LUMO and leads to
redshifts, whereas addition of electron donating groups on the acceptor is expected to lead to
blueshifts.
1.2.1.3.Blue Phosphorescent Emitters
Due to their high efficiencies and long device lifetimes, PHOLEDs with green and red
phosphorescent emitters are used in commercial applications. However, the lifetime of blue
PHOLEDs is too short for practical applications in display and solid-state lighting. For this reason,
research on stable blue phosphorescent emitters has been going on for more than a decade. Blue Ir
(III) phosphors are obtained by following one of the strategies mentioned above. For example, one
of the efficient blue phosphors (FIrpic) is obtained by addition of two fluorines into the 4 and 6
positions of the phenyl in Ir(ppy)3. Despite their high efficiency, blue emitting phosphors
containing fluorines are reported to have short device lifetime due to cleavage of F−C bond during
21

device operation.
52
An alternative strategy to obtain blue phosphors is by replacing one of the
chromophoric ligands with an ancillary ligand.
53, 54
This replacement results in stabilization of the
HOMO energy leaving the LUMO unaffected. Although this strategy can result in hypsochromic
shift, increase separation between the
3
MLCT and
3
LC energies as a result of the decrease in
HOMO energy results in decrease in radiative rate constants of the resulting complexes,
consequently decreasing the luminous efficiency.
Another alternative strategy of obtaining blue phosphors is by replacing the pyridine in
Ir(ppy)3 with more electron rich heterocycles like pyrazole, imidazole, triazole, e.t.c. This
substitution results in destabilization of the LUMO and hence blue shift in the emission energy.
For example, tris iridium complexes with phenylimidazole as their cyclometallating ligands (e.g.
Ir(N-Mepim)2acac and fac-Ir(dmp)3) have sky blue emission with high PL efficiencies (> 0.9).
55,
56
To achieve deep blue emission, the pyridine in Ir(ppy)3 is replaced with more electron rich
heterocycle like pyrrazole to form fac-Ir(ppz)3 which has a blue shifted emission of 80 nm.
57

Unfortunately the PL efficiency of fac-Ir(ppz)3 is very low at room temperature and the efficiency
only increases at low temperatures (77K) which is not practical in OLED applications. It has been
found that this temperature dependence of the PL efficiency of fac-Ir(ppz)3 is due to thermal
population of the non-radiative metal centered ligand field states (
3
LF) (d-d transition) from the
T1 at room temperature (Figure 1.13).
57, 58
One approach to avoid thermalization into the
3
LF from
the T1 is to use ligands that can raise the nonradiative
3
LF states, while maintaining high triplet
energy of the complex. Nonradiative states with metal localized ligand field can be destabilized
by strengthening the metal ligand bonds since the
3
LF is an antibonding orbital of the metal-ligand
bond. Phenylimidozolium ligands (C^C:), commonly known as N-heterocyclic carbenes (NHC)
are ideal for this purpose owing to their strong metal-carbene bond. The carbene heterocycle is a
22

neutral, two electron donor and a good electron-acceptor from the metal to the carbene empty p-
orbital resulting in strong metal-carbene bonds (Figure 1.13). Lepport et al reported the earliest
Ir(C^C)3 (Ir-Lappert) in 1980 and since then other blue emitting tris iridium cyclometalated
carbene complexes like Ir(pmi)3 and Ir(pmb)3 have been reported (Figure 1.14). The efficiencies
of Ir(pmi)3 and Ir(pmb)3 are also low (0.02−0.05) at room temperature but are better than their fac-
Ir(ppz)3 counterparts. Red-shifting the emission energy of the carbene complexes lead to higher
efficiencies. For example, fac-and mer-Ir(pmp)3 with emission maxima at 410nm and 430nm,
respectively, have efficiencies >70%.
37

Figure 1.12. Methods of blue shifting the emission. Adapted from reference 50.
23

Figure 1.13. Energy level diagram for fac-Ir(ppz)3 and fac-Ir(C^C:)3 depicting non radiative
states. Adapted from reference 57 and 58.

Figure 1.14. Common blue emitting phosphors
1.3.Exciton-Exciton and Exciton-Polaron Annihilation
Even though OLEDs offer many advantages in display and lighting applications, they suffer
from many drawbacks. The main drawback of OLEDs is their short device lifetime, especially in
blue PHOLEDs. This short lifetime is attributed to the degradation of materials during device
operation. The degradation is even more severe in the emissive layer where excitons are
generated.
52
Materials in OLED can degrade in many ways, two of them that are shown to be
important mechanisms in OLED degradation are discussed below.
52, 59

24

Exciton-exciton or triplet-triplet annihilation (TTA) is a bimolecular annihilation process that
occurs at high exciton concentration. In the case of phosphorescent OLEDs, when two triplets
interact with one another, they annihilate each other by promoting one of the triplets to a higher
excited state “hot state” and the other to the ground state. The hot excited state can either internally
convert to the radiative excited state or can cause molecular degradation if concentrated on weak
bonds. Another mechanism that results in molecular degradation in PHOLEDs is triplet-polaron
(TPA) annihilation. Similar to TTA, TPA is another bimolecular process where a triplet and a
polaron interact to create hot excited state, which can also cause molecular degradation if localized
on weak bonds. Because of their long-excited state lifetimes (typically > 1s), phosphorescent and
TADF emitters suffer more from TTA and TPA than fluorescent emitters. The molecular
degradation caused by TPA or TTA is more detrimental in blue PHOLEDs since the hot excited
state for blue materials is higher in energy than that of green and red materials. Therefore, in this
thesis we focus on developing alternative host materials and phosphorescent and TADF emitters
with short excited state lifetimes aiming to improve the lifetime and efficiency of blue PHOLEDs.

Figure 1.15. Triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TTA) processes.
Adapted from reference 59.

25

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28

2CHAPTER 2 − Phenanthro[9,10-d]-triazole and imidazole as High Triplet
Energy Host Materials for Blue OLEDs
2.1.Introduction
Hosts materials for blue OLEDs are limited to rigid, non-conjugating building blocks with triplet
energies approaching 3 eV, such as fluorene, carbazole (cz), dibenzothiophene (dbt) and
dibenzofuran (dbf) (Figure 2.1). In addition to these moieties, we identified phenanthro[9,10-d]-
triazole and imidazole as high triplet energies (~3 eV) moieties despite their large π-system. Our
study of the properties of these phenanthrene-based materials started with simple structures, aimed
at characterizing the electronic properties of the phenanthro-triazole and -imidazole core
structures. While our theoretical modeling study identified phenanthro-triazole and -imidazole
based materials with high triplet energies, they also showed that it was not possible to predict
which compounds would make the best host materials without detailed and time consuming
modeling in the solid state.
1
Moreover, these modeling studies did not give insight into the bulk
thermal properties of the materials, which is important in making stable OLEDs. Here we prepared
and studied a number of phenanthro-triazole and -imidazole based materials, to find the structures
that have the desired electronic and thermal properties to be viable host materials for blue
phosphorescent OLEDs. Optimized syntheses of these materials, their photophysical and
electrochemical properties, and a strategy to inhibit aggregation induced red-shifts to retain high
triplet energies in a neat solid are discussed. The most promising materials were incorporated as
host materials in high efficiency, blue phosphorescent OLEDs.
29

Figure 2.1. Building blocks with high triplet energies
2.2.Results and Discussion
2.2.1. Synthesis
A wide variety of phenanthro[9,10-d]triazole and phenanthro[9,10-d]imidazole based materials
were prepared to investigate the electronic and thermal properties as a function of the substituents
on the triazole and imidazole rings (Scheme 2.1). Alkyl triazoles were prepared from 9-bromo
phenanthrene in two steps. The benzyne click chemistry (first step) is reported elsewhere.
14
The
subsequent alkylation step was carried out in a one pot reaction of hT with K2CO3 and methyl
iodide. This reaction yielded predominantly the N2-isomer (2-MeT, 44%) and a minor amount of
the N1 isomer (1-MeT, 25%), presumably due to steric interactions between the methyl group and
the neighboring proton of the phenanthrene. Aryl-substituted triazoles were prepared using
modified procedures of Ueda et al. and Taillefer et al. who reported Ullman conditions for arylation
of benzo-triazole.
2
Ullman conditions using hT and phenyl halide resulted in 2-pT as the only
product in 60% yield. The same result with slightly lower yield was obtained when Buchwald
conditions employed.
2
The larger size of the phenyl ring is the likely reason that only the aryl N2
isomer is obtained. N2-xylyl and mesityl substituted triazoles were obtained using catalyst-free
regioselective N2 arylation of 1,2,3-Triazoles with diaryl iodonium salts.
3
N1 aryl substituted
triazoles were prepared via a modified fluoride-induced elimination to generate benzyne under
mild conditions.
4
This procedure produced N1 aryl substituted triazoles 1-pT, mT, mxT, fxT and
30

txT in 40–50% yields. A key modification from the literature procedure is preparation of 9-
hydroxyphenanthrene using n-BuLi instead of Mg tunings,
5
which gives the product in higher
yield, with fewer byproducts and shorter reaction times. Phenanthro[9,10-d]imidazoles MeI, pI,
mI, mxI, fxI, txI and tpI were prepared from inexpensive starting material (9,10-
phenanthrenequinone) from two high yielding steps.
6

Scheme 2.1. i: NaN3, K-tBuO, DMSO rt. 12 h. ii: MeI, K2CO3, DMF, rt. iii: PhI, Fe(acac)3, CuO,
DMF, 100
o
C, 30 h. iv: v: RN3, CsF, MeCN, rt. 30 h. vi: R1NH2, Formaldehyde, NH2OAc, Acetic
acid, 100
o
C, 4 h. vii: ArylB(OH)2, K2CO3, Toluene/Water (2/1), 110
o
C, 12-48 h.
2.2.2. Photophysical and Thermal Properties
Absorption spectra of the methyl substituted phenanthro-triazole and -imidazole materials were
recorded in 2-MeTHF, Figure 2.2a. The lowest energy band of the phenanthro-triazoles are
blue-shifted by about 15 nm compared to a similar band of the phenanthro-imidazoles.
31

Substitution with phenyl leads to different properties depending on the site of substitution. Phenyl
substituents at the 1-position of either the triazole- or imidazole-based materials do not lead to a
red shift, relative to the methyl substituted analogs (Figure 2.3). Steric interactions force the
phenyl ring at the 1-position to be out of plane with phenanthro- core and thus disrupt conjugation
between the two aromatic ring systems. In contrast, a phenyl substituent at the 2-position can
adopt a coplanar conformation with the phenanthro[9,10-d]triazole core, leading to conjugation
and a red-shift of ca. 30 nm relative to the analogous 1-phenyl substituted compounds. The same
red shift has been observed for analogous phenyl substitution on phenanthro[9,10-d]imidazole.
1

Thus, substitution at the 1-position in these phenanthro-based materials was chosen to maintain
high exciton energy.
The S1 (fluorescent) and T1 (phosphorescent) transitions of the triazoles are blue-shifted relative
to their imidazole analogs (e.g. compare 1-MeT to MeI, Figure 2.2b and c). The triplet energies
of 1-MeT and MeI in solution are high (T1 = 2.97 and 2.89 eV, respectively); however, the planar
structures of 1-MeT and MeI lead to aggregation in thin films, lowering their triplet energies to
2.66 eV and 2.70 eV, respectively (Figure 2.2d). The PL efficiency of FIrpic, a blue
phosphorescent emitter (ET = 2.70 eV),
7
doped into 1-MeT films is low (PLQY < 20%). A higher
efficiency is observed for FIrpic doped into MeI films (PLQY = 60%), but both hosts fall short of
the 80% efficiency observed for FIrpic doped into a polystyrene film (ET = 3.2 eV).
8

32

300 350 400 450
0.0
0.3
0.5
0.8
1.0
0.0
0.3
0.6
0.9

PL Intensity
Wavelength (nm)
PL soln @ 298K

Absorbance (a.u)
1-MeT
2-MeT
2-pT
MeI
Abs soln @ 298K
(a)
(b)
400 450 500 550 600 650
0.0
0.3
0.6
0.9
0.0
0.3
0.6
0.9

PL Intensity
Wavelength (nm)
(c)
(d)
PL solid @ 77K

PL Intensity
PL soln @ 77K

Figure 2.2. (a) Absorption spectra in 2-MeTHF at 298K. Normalized emission spectra in
2-MeTHF at 298K (b) and 77K (c), and as a neat solid at 77K (d).
350 400 450
0.0
0.2
0.4
0.6
0.8
1.0
Absorbance (a.u)
Wavelength (nm)
1-pT Absorption
pI Absorption
1-pT PL soln 298K
pI PL soln 298K
(a)
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
1-pT Soln 77K
pI Soln 77K
1-pT Solid 77K
pI Solid 77K
(b)

Figure 2.3. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K.
Aryl substituents at the 1-position in the triazole and imidazole compounds lead to higher triplet
energies in the solid-state (ETsolid) as the out-of-plane conformation of the aryl groups hinders −
stacking in the condensed phase. The triplet energy of the triazole material with a phenyl group at
33

the N1-position (1-pT, ETsolid = 2.75 eV) is higher than that of 1-MeT (ETsolid = 2.67 eV), whereas
the ETsolid for the imidazole analogs, pI and MeI, are the same (ETsolid = 2.70 eV). To further inhibit
aggregation-induced redshifts, mesityl groups were incorporated at the N1-positions of the triazole
and imidazole compounds. The resultant materials, mT and mI, have high triplet energies (ETsolid
= 2.77 eV and 2.74 eV, respectively) and the PL efficiency of FIrpic doped into films of both
compounds is high (PLQY = 80%).
In addition to maintaining a high triplet energy in the solid state, it is important for a host material
to have high sublimation (Ts) and glass transition (Tg) temperatures to fabricate stable OLEDs.
Thermogravimetric analysis showed that mT and mI have Ts < 300 °C, due to their low molecular
weight (Figure 2.12). No Tg was observed for mT, whereas the value for mI is too low (Tg = 72
o
C) for OLED applications (Figure 2.9). Therefore, derivatives with a mesityl-o-xylyl group at the
N1-position (mxT and mxI) were prepared to improve the thermal properties of these materials
(Figure 2.9−Figure 2.12). Both mxT and mxI give values of Ts of 326 °C and Tg of 94 °C. As
expected, mxT and mxI have the same energies for the S1 and T1 states (Figure 2.5) as mT and
mI (Figure 2.4). The mesityl-o-xylyl group also raises the triplet energy of mxT in the solid-state
(ETsolid = 2.77 eV) relative to 1-MeT (ETsolid = 2.67 eV) due to inhibited − interactions, although
not for mxI relative to MeI (Figure 2.5a, b).
34

300 350 400 450 500
0.0
0.1
0.2
0.3
mT Absorption
mI Absorption
mT PL soln 298K
mI PLsoln 298K
Wavelength (nm)
e (
4
M
−
cm
−1
)
(a)
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity

400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
mT Soln 77K
mI Soln 77K
mT Solid 77K
mI Solid 77K
(b)

Figure 2.4. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K.
300 350 400 450 500
0.0
0.1
0.2
0.3
mxT Absorption
mxI Absorption
mxT PL soln 298K
mxI PLsoln 298K
Wavelength (nm)
e (
4
M
−
cm
−1
)
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
(a)
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
mxT Soln 77K
mxI Soln 77K
mxT Solid 77K
mxI Solid 77K
(b)

Figure 2.5. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 77K. (b)
Normalized emission spectra in neat solid at 77K.
Dibenzofuran (dbf) and dibenzothiophene (dbt) groups are often used in the construction of host
materials with high triplet energies. Here, these groups were used to further improve the thermal
properties of the phenanthro-triazole and phenanthro-imidiazole compounds. The triplet energies
for fxT, txT, fxI and txI are similar to mxT and mxI, but the former compounds have higher
sublimation (Ts > 390
o
C) and glass transition (Tg = 125–130
o
C) temperatures (Table 2.1).
Coupling the dbf and dbt groups to the phenanthro-triazole or -imidiazole core through a p-xylyl
group proved to be important. If the p-xylyl ring is replaced with a phenyl group, i.e. tpI,
conjugation between dbt and phenanthro-imidiazole substantially lowers the triplet energy (ETsolid
35

= 2.64 eV, Figure 2.8). However, placing the p-xylyl spacer between dbt or dbf on fxT, txT, fxI
and txI allowed these materials to retain a high triplet energy in solution and solid-state (ETsolid >
2.75 eV, Figure 2.6 and Figure 2.7). To compare the triplet energies of these host materials and
literature host materials, the triplet energies of mCBP is measured in solution and in the solid state.
The triplet energy of isolated mCBP (ET = 2.93 eV) is similar to the isolated triplet energies of
these host materials. However, the solid-state triplet energy of mCBP is redshifted by only 70 meV
compared to ~300 meV redshift for the new host materials (Figure 2.8). This larger redshift in the
new materials is due to their large π-system.
350 400 450 500
0.0
0.1
0.2
0.3
fxT Absorption
fxI Absorption
fxT PL soln 298K
fxI PLsoln 298K
Wavelength (nm)
e (
4
M
−
cm
−1
)
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
(a)

400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
fxT Soln 77K
fxI Soln 77K
fxT Solid 77K
fxI Solid 77K
(b)

Figure 2.6. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K.
350 400 450 500
0.0
0.1
0.2
0.3
0.4
0.5
txT Absorption
txI Absorption
txT PL soln 298K
txI PLsoln 298K
Wavelength (nm)
e (
4
M
−
cm
−1
)
(a)
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
txT Soln 77K
txI Soln 77K
txT Solid 77K
txI Solid 77K
(b)

Figure 2.7. (a) Absorption spectra and normalized emission spectra in 2-MeTHF at 298K. (b)
Normalized emission spectra in 2-MeTHF and neat solid at 77K.
36

350 400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0

PL Intensity
Wavelength (nm)
tpI Soln 298K
tpI Soln 77K
tpI Solid 77K
(a)
400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0

PL Intensity
Wavelength (nm)
mCBP Soln 77K
mCBP Solid 77K

(b)

Figure 2.8. (a) Normalized emission spectra of tpI in 2-MeTHF and solid (b) Normalized
emission spectra of mCBP measured in 2-MeTHF and solid.
60 80 100 120 140 160 180 200
2
3
4
5
6
7
8
Heat Flow (a.u)
Temperature (
o
C)
mT
mI
mxT
mxI
60 80 100 120 140 160 180 200
2
3
4
5
6
7
8
9

Heat Flow (a.u)
Temperature (
o
C)
fxT
fxI
txT
txI

Figure 2.9. DSC Curves of phenanthro[9,10-d]triazoles/imidazoles depicting their glass
transition temperatures.
0 50 100 150 200 250 300
-20
-10
0
10
20
30

Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
mT
0 50 100 150 200 250 300
0
10
20
30
Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
mI

37

0 50 100 150 200 250 300
0
10
20
30
Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
mxT
0 50 100 150 200 250 300
0
10
20
30
Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
mxI

Figure 2.10. DSC Heating curves of phenanthro[9,10-d]triazoles/imidazoles depicting their
crystallization and melting temperatures.
0 50 100 150 200 250 300
0
10
20
Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
fxT
0 50 100 150 200 250 300
0
10
20
Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
fxI
0 50 100 150 200 250 300
0
10
20
Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
txT
0 50 100 150 200 250 300
0
10
20
Heat Flow (a.u)
Temperature (
o
C)
20 to 300
300 to 20
20 to 300
300 to 20
o
C
txI

Figure 2.11. DSC Heating curves of phenanthro[9,10-d]triazoles/imidazoles depicting their
crystallization and melting temperatures.
38

0 100 200 300 400 500 600
0
20
40
60
80
100

Weight Loss (%)
Temperature (
o
C)
mT
mI
mxT
mxI
fxT
fxI
txT
txI

Figure 2.12. TGA curves of phenanthro[9,10-d]triazoles/imidazoles
2.2.3. Frontier Orbital Energies
The electrochemical properties of the phenanthro-triazole and -imidiazole compounds were
characterized using cyclic voltammetry (CV) and differential pulse voltammetry (DPV); data for
selected compounds are given in Table 2.1. Phenanthro-triazoles show irreversible oxidation and
quasi-reversible reduction (scan rates of 0.1 V/s and 10 V/s) with the exception of 1-MeT and
2-pT, which show reversible reductions (Figure 2.13 and Figure 2.15a). Phenanthro-imidazoles
have quasi-reversible oxidation and reversible reductions (Figure 2.14). The phenanthro-triazoles
oxidize in the range of 1.42–1.45 V vs. Fc
+
/Fc, whereas the phenanthro-imidazoles are cathodically
shifted by roughly 400 mV (Eox = 1.0–1.05 V). The reduction potentials for the phenanthro-
triazoles are similarly shifted by 250 mV relative to those of the imidazole-based materials (Ered
= -2.69 and -2.96 V, respectively). The anodic shift for both oxidation and reduction of triazoles
relative to their imidazole-based counterparts is due to replacement of carbon with more
electronegative nitrogen atom in the triazoles, stabilizing both the HOMO and the LUMO. The
HOMO and LUMO energies in Table 2.1 were estimated from the measured oxidation and
39

reduction potentials.
9
The wide HOMO/LUMO gaps of these materials make them suitable for
hosting blue phosphorescent dopants such as FIrpic (Eox = 0.92 V, Ered = -2.29 V).
10

-3.0 -2.5 -2.0 -1.5 0.5 1.0 1.5 2.0
-0.00016
-0.00008
0.00000
0.00008
0.00016
0.00024

Current (A)
Potential (V vs Fc/Fc+)
mT
mxT
fxT
txT

Figure 2.13. Cyclic voltammetry curves of phenanthro[9,10-d]triazoles.
-3.0 -2.5 -2.0 -1.5 0.5 1.0
-0.00016
-0.00008
0.00000
0.00008
Current (A)
Potential (V vs Fc/Fc+)
mI
mxI
fxI
txI

Figure 2.14. Cyclic voltammetry curves of phenanthro[9,10-d]imidazoles

40

-2.8 -2.4 -2.0 0.8 1.2 1.6 2.0
-2.8 -2.4 -2.0 0.8 1.2 1.6 2.0

Potential (V)
1-MeT

Current (a.u)
2-MeT

1-pT

2-pT
(a)
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Voltage (V) vs Fc
+
/Fc
fxT
Fc
+
/Fc

fxI

Current (a.u)
txT

txI
(b)

Figure 2.15. (a) Cyclic voltammetry curves of phenanthro[9,10-d]triazoles (b) DPV data of
selected phenanthro[9,10-d]triazoles/imidazoles
The electronic properties of the phenanthro-triazoles and -imidazoles were also investigated
theoretically using density functional theory and time dependent density functional theory.
Geometry optimization in the gas phase was performed using B3LYP functional with LACVP**
basis set. Contours representative for the valence molecular orbitals (MOs) of phenanthro-triazoles
and -imidazoles are shown in Figure 2.17 and Figure 2.18. The HOMO is localized on the
phenanthro-triazole/imidazole ring for all the compounds, consistent with the small variation in
Eox within each phenanthro-triazole and phenanthro-imidazole series. The LUMO is localized on
the phenanthro-triazole/imidazole ring for mxT and mxI. In contrast, the LUMO is localized on
the dbf moiety in fxT and fxI or dbt moiety in txT and txI whereas, the LUMO+1 is localized on
the phenanthro-triazole or -imidazole moiety. The electrochemical data for fxT, fxI, txT and txI
however, suggest that the first reduction occurs on the phenanthro-triazole or -imidazole moiety
41

and the second reduction on the dbf or dbt moiety (Table 2.1, Figure 2.15b). These contrasting
results is likely due to close energy of the calculated LUMO and LUMO+1 of fxT, fxI, txT and
txI. (see Figure 2.17 and Figure 2.18).
The triplet energies calculated for the phenanthro-triazoles are 3.05 eV and 2.95 eV for
phenanthro-imidazoles. These values are similar to the experimental values of 2.97 eV for the
triazoles and 2.90 eV for the imidiazoles. The triplet spin density for the N1-substituted
phenanthro-triazoles and -imidazoles is localized on the phenanthro-triazole/imidazole core
(Figure 2.16−Figure 2.18). Exceptions are 2-pT and fxT. In 2-pT the triplet spin density
delocalizes over the entire phenyl-phenanthro-triazole, stabilizing the triplet to 2.80 eV (measured
= 2.75 eV). The triplet density of fxT is localized on the dbf moiety (Figure 2.17) due to the
similar energies of dbf (T1 = 3.13 eV) and phenanthro-triazole core (T1 = 3.06 eV).

1-MeT 2-MeT 1-pT 2-pT MeI pI
Spin
Density

LUMO

HOMO

Figure 2.16. Molecular orbital representation of phenanthro[9,10-d]triazoles and
phenanthro[9,10-d]imidazoles

42

Spin
Density
HOMO-1 HOMO LUMO LUMO+1 LUMO+2 S1 Transition
mT

S 0 -S 1 = 308 nm (f =
0.0215)
H → L (58 %)
H-1 → L+1 (17 %)
H → L+1 (14 %)
mxT

S 0 -S 1 = 308 nm (f =
0.0207)
H → L (58%)
H-1 → L+1 (17%)
H → L+1 (15%)
fxT

S 0 -S 1 = 308 nm (f =
0.0191)
H → L+1 (58%)
H-1 → L+2 (17%)
H → L+2 (14%)
txT

S 0 -S 1 = 309 nm (f =
0.0190)
H → L+1 (57%)
H-2 → L+2 (17%)
H → L+2 (15%)
Figure 2.17. Molecular orbital representation of phenanthro[9,10-d]triazoles (H = HOMO, L =
LUMO).

Spin
Density
HOMO-1 HOMO LUMO LUMO+1 LUMO+2
S1
Transition
mI

S 0 -S 1 = 318 nm (f =
0.0233)
H → L (79%)
H-1 → L+1 (16%)
H → L+1 (4%)
mxI

S 0 -S 1 = 319 nm (f =
0.0220)
H → L (77%)
H-1 → L+1 (15%)
H → L+1 (4%)
fxI

S 0 -S 1 = 319 nm (f =
0.0185)
H → L+2 (62%)
H → L+1 (17%)
H-1 → L+3 (10%)
txI

S 0 -S 1 = 319 nm (f =
0.0177)
H → L+2 (79%)
H-1→ L+3 (10%)
H-1→ L+1 (7%)
Figure 2.18. Molecular orbital representation of phenanthro[9,10-d]imidazoles (H = HOMO, L =
LUMO).

43

Table 2.1. Summary of properties for selected phenanthro-triazoles and -imidazoles.
Compound S1 (eV)
a
ET (eV) Eox (V)
d
Ered (V)
d
HOMO/LUMO (eV)
e
Tg/Tm/Ts (
o
C)
f

Soln.
b
Solid
c

1-MeT 3.65 2.97 2.66 +1.61 -2.80 -6.64/-1.53 -
2-MeT 3.75 2.97 2.64 +1.68 -2.81 -6.72/-1.51 -
MeI 3.45 2.91 2.70 - - - -
mT 3.65 2.97 2.77 +1.44 -2.69 -6.45/-1.66 -/263/293
mxT 3.65 2.97 2.77 +1.42 -2.73 -6.42/-1.61 94/247/326
fxT 3.65 2.97 2.75 +1.45 -2.67, -3.03 -6.46/-1.68 126/237/398
txT 3.65 2.97 2.70 +1.45 -2.64, -3.00 -6.46/-1.71 128/219/398
mI 3.45 2.89 2.74 +1.00 -2.96 -5.94/-1.34 72/171/294
mxI 3.45 2.89 2.73 +1.05 -2.98 -6.00/-1.31 94/196/326
fxI 3.45 2.89 2.71 +1.00 -2.95, -3.06 -5.94/-1.35 126/-/376
txI 3.45 2.89 2.74 +1.01 -2.93, -3.04 -5.95/-1.37 130/212/396
tpI 3.45 2.88 2.64 - - - -
mCBP 3.60 2.93 2.86 +0.88 -2.84 -5.80/-1.48 -

a
Measured in 2-MeTHF at 298K and
b
at 77K.
c
Onset of the triplet emission for the neat powder at 77K.
d
Obtained from differential
pulse voltammetry (DPV) in acetonitrile vs. Fc
+
/Fc.
e
Calculated from redox values according to reference
9
.
f
Tg = glass transition
temperature, Tm = melting point, Ts = sublimation temperature under nitrogen.
2.2.4. Electroluminescence Properties
The photophysical, electrochemical and thermal properties of selected phenanthro-triazoles and -
imidazoles are summarized in Table 2.1. All of the listed compounds have suitable electronic
properties to host blue phosphorescent OLEDs. The thermal properties of mT and mI are
unsuitable for OLED applications, so they were not considered further. Materials with high triplet
energies in the solid-state and good thermal properties were incorporated as host materials in blue
OLED. Preliminary devices were fabricated at University of Southern California before
44

transferring the materials to University of Michigan Ann Arbor. All the devices shown in this
chapter were fabricated at University of Michigan Ann Arbor by Caleb Coburn.
In the first set of experiments, a direct comparison was made of OLEDs with phenanthro-triazole
and -imidazole hosts, i.e. mxT and mxI, doped with FIrpic (Figure 2.19−Figure 2.22). While the
triazole and imidazoles based devices exhibit similar current density–voltage (J–V) characteristics
and electroluminescence (EL) spectra, devices using mxT hosts degrade rapidly under EL
operation, with second and third J-V scans (0.01-100 mA/cm
2
) showing a drop in efficiency of >
75% for the triazole based devices (Figure 2.23). For comparison the EQE for the imidazole-
based device drops < 10% with the same current cycling. This rapid drop in efficiency of the mxT
based device is probably due to degradation of the phenanthro[9,10-d]triazoles which could be due
to irreversibility of the phenanthro[9,10-d]triazoles upon oxidation or reduction under electrical
excitation. All device testing was done with devices packaged under dry nitrogen. Rapid decay in
EQE at high current density is observed with the other phenanthro-triazole devices as well (fxT
and txT, Figure 2.20). It should be noted that this sort of cycling to high current is not a direct
indicator of the device lifetime. Maximum drive currents normally employed in a display are 10
mA/cm
2
, yet the behavior of mxT clearly shows an inherent instability for the triazole based
materials. For this reason, we choose to not further pursue N1-phenanthro-triazole based hosts
OLED studies, but instead focused on imidazole-based materials.
45

Figure 2.19. Device architecture and chemical structures of the OLED materials
0 2 4 6 8 10 12
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
Current Density (mA/cm
2
)
Voltage (V)
txT
fxT
mxT
(a)
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
EL Intensity
Wavelength (nm)
txT
fxT
mxT
(b)
0.01 0.1 1 10 100
0
5
10
15
20
EQE (%)
Current Density (mA/cm
2
)
txT
fxT
mxT
(c)
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
Luminance (x10
4
cd/m
2
)
Current Density (mA/cm
2
)
txT
fxT
mxT
(d)

Figure 2.20. Device characteristics of OLEDs using phenanthro[9,10-d]-triazole hosts. (a) J–V
curves. (b) EL spectra. (c) Efficiency versus current curves. (d) Luminance versus current
curves.
46

0 2 4 6 8 10 12
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
Current Density (mA/cm
2
)
Voltage (V)
mCBP
26DCzppy
txI
fxI
mxI
(a)
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
EL Intensity
Wavelength (nm)
mCBP
26DCzppy
txI
fxI
mxI
(b)

0.01 0.1 1 10 100
0
5
10
15
20
EQE (%)
Current Density (mA/cm
2
)
mCBP
26DCzppy
txI
fxI
mxI
(c)
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Luminance (x10
4
cd/m
2
)
Current Density (mA/cm
2
)
mCBP
26DCzppy
txI
fxI
mxI
(d)

Figure 2.21. OLED device characteristics of phenanthro[9,10-d]imidazole and reference hosts.
(a) J–V curves. (b) EL spectra. (c) Efficiency versus current curves. (d) Luminance versus
current curves.
0.5 1.0 1.5 2.0
0
10
20
30
40
50
CE (cd/A)
Luminance (x10
4
cd/m
2
)
txT
fxT
mxT
0.5 1.0 1.5 2.0 2.5 3.0
0
10
20
30
40
50

CE (cd/A)
Luminance (x10
4
cd/m
2
)
mCBP
26DCzppy
txI
fxI
mxI

Figure 2.22. Current Efficiency plot of phenanthro[9,10-d]triazoles and phenanthro[9,10-
d]imidazoles.

47

In Figure 2.21, the performance of OLEDs using the three phenanthro-imidazole based host
materials are compared, as well as analogous devices with two conventional host materials (mCBP
and 26DCzppy), see Table 2.2 for device metrics. The device structure is illustrated in Figure
2.21. The mCBP and phenanthro-imidazole derivatives have similar J–V characteristics, while the
26DCzppy shows a shift to higher voltage, Figure 2.21a. The electroluminescence (EL) spectra,
shown in Figure 2.21b, are independent of the host material and are all stable under repeated
electrical excitation. The transport properties of these materials were further investigated using
hole- and electron-only devices (Figure 2.24). The J–V characteristics of these single carrier
devices are largely independent of the host, with mCBP devices showing slightly higher hole and
electron conductivities than the phenanthro-imidazole host materials. The phenanthro-imidazole
and reference hosts devices exhibit similar brightness at low current densities with slight
differences at higher current densities (Figure 2.21d). OLEDs with the txI host give the highest
efficiency (peak EQE = 20.2±0.3%, CE = 43.9 cd/A), while OLEDs utilizing fxI and mxI hosts
give slightly lower efficiencies (19%) (Figure 2.21c and Table 2.2). The higher device efficiency
of devices employing txI than fxI and mxI is due to higher triplet energy of txI in the solid-state
than that of mxI and txI. This could be due to slightly larger size of dbt unit in txI than dbf unit in
fxI or mesityl unit in mxI which helps to reduce aggregation-induced redshift of txI in the solid
state. The mCBP host device efficiency is marginally lower with a peak EQE = 16.7±0.5% and CE
= 39.0 cd/A. The difference in EQE of the phenanthro-imidazole based OLEDs relative to mCBP
is not due to differences in PLQY of FIrpic in the host materials (Table 2.2). Improved molecular
alignment of the emitter in phenanthro-imidazole based host OLEDs, which affects outcoupling
efficiency,
11
was ruled out by angular PL measurements of FIrpic in these host materials (Figure
48

2.25). Thus, improved charge balance for the phenanthro-imidazole host OLEDs is the likely
reason why the phenanthro-imidazole based devices outperform mCBP based OLEDs.
Table 2.2. PLQY and EL properties of host materials.
Host PLQY (%)
a
EQE max (%) V on (V)
b
CE (cd/A)
@100 cd/m
2
@1000 cd/m
2
@10000 cd/m
2

mxT 81 21 3.3 49.3 44.6 24.4
mxI 77 19 3.2 43.1 40.3 33.2
fxI 66 19 3.1 42.1 39.4 33.7
txI 77 20 3.1 46.8 43.9 37.2
mCBP 87 17 3.3 40.2 39.0 34.9
26DCzppy 88 17 4.1 40.2 38.1 32.4
a
PLQY of spin coated film doped with 10 wt% FIrpic in host, measured using an integrating sphere.
b
Measured at 1 cd/m
2
.
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0 mxI
mxT
EL Intensity
Wavelength (nm)
0 2 4 6 8 10 12
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
Current Density (mA/cm
2
)
Voltage (V)
mxT 1
st
mxT 2
nd
mxT 3
rd
mxI 1
st
mxI 2
nd
mxI 3
rd
0.01 0.1 1 10 100
0
5
10
15
20
EQE (%)
Current Density (mA/cm
2
)

Figure 2.23. Comparative OLED testing for devices with phenanthro-triazole (mxT) and
phenanthro-imidazole (mxI) host materials. The device structure used here was (70 nm ITO/5 nm
HATCN/40 nm TAPC/10 nm mCP/20 nm8 vol% FIrpic:Host/45 nm BP4mPy/1.5 nm LiQ/100 nm
Al). While the J-V characteristics show only a minor shift on repeated cycling, the EQE values for
the triazole drop markedly on the 2
nd
and 3
rd
J-V scan of the devices. The significantly higher roll-
off in device efficiency for the triazole based device at J > 10 mA/cm
2
is likely due to device
49

degradation at the higher current density. The electroluminescence (EL) spectra are nearly
independent of the host material, with minor spectral differences likely caused by cavity effects
on the position of the exciton formation zone.
10
-1
10
0
10
1
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2

Current Density (mA/cm
2
)
Voltage (V)
mCBP
26DCzppy
txI
fxI
FIrpic:mCBP
FIrpic:26DCzppy
FIrpic:txI
FIrpic:fxI
Hole Only
(a)

10
-1
10
0
10
1
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
Current Density (mA/cm
2
)
Voltage (V)
mCBP
26DCzppy
txI
fxI
FIrpic:mCBP
FIrpic:26DCzppy
FIrpic:txI
FIrpic:fxI
Electron Only
(b)

Figure 2.24. (a) Hole only Structure: 20 min UV Ozone 70 nm ITO / 5 nm HATCN / 20 nm
TAPC / 10 nm mCP / 20 nm 8vol% FIrpic:host or neat host / 10 nm HATCN / 100 nm Al. (b)
Electron only Structure: no UV Ozone 70 nm ITO / 30 nm BP4mPy / 20 nm 8 vol% Firpic:host
or neat host / 30 nm BP4mPy / 1.5 nm LiQ / 100 nm Al

Figure 2.25. Polarized emission spectra. a–c, Cross-sections of the measurements and simulations
of the angle-dependent p-polarized photoluminescence emission spectra (considering an emission
in the x–z plane) for films of 20 nm fxI doped with 8 vol% FIrpic (at 484 nm) (a), 20 nm txI doped
with FIrpic (at 483 nm) (b) and 20 nm mCBP doped with FIrpic (at 482 nm) (c)
50

2.3.Conclusion
A series of non-carbazole phenanthro[9,10-d]imidazole/triazole host materials were
developed as alternative hosts for blue PHOLEDs. Synthesis, photophysics and electrochemical
properties of N2 and N1-aryl substituted phenanthro[9,10-d]triazoles are reported. The newly
developed host materials exhibited wide energy gap and triplet energies as high as 2.97 eV which
are crucial for hosting blue emitters. Their triplet energies, however, undergo a marked redshift of
ca. 0.30 eV in solid state due to their planar structure. To partially overcome this aggregation
induced shift, bulky substituent groups were incorporated into the 1-position, reducing the redshift
in the solid state to 0.1 eV. This improvement highlights the importance of evaluating triplet
energy of host materials in the solid state for use with phosphorescent (or thermally activated
delayed fluorescence, TADF) emitters in OLEDs. The triplet energy of these materials in the solid
state is high enough to host FIrpic and achieve a PLQY of near unity. Furthermore, these materials
exhibited moderate glass transition temperatures with no decomposition observed before
sublimation.
Optimized materials were incorporated as host materials for blue OLEDs. The host materials have
similar transport properties as mCBP. Devices using phenanthro-triazole host materials exhibited
a maximum EQE of 21%, albeit with a high roll-off that is likely due to decomposition of the host
during device operation. In contrast, devices using phenanthro-imidazole hosts showed a
maximum EQE of 20% and a roll-off similar to mCBP hosted device, suggesting improved
stability of the phenanthro-imidazole materials. Therefore, the phenanthro-imidazole hosts
developed here can serve as alternatives to carbazole-based host materials for OLEDs using blue
phosphorescent and thermally activated delayed fluorescent (TADF) emitters. Methods to improve
the stability of the phenanthro-triazoles are given in the Appendix.
51

2.4.Experimental
2.4.1. General
Nuclear magnetic resonance (NMR) spectra were recorded on Varian 400 NMR
spectrometer and referenced to residual protons in the deuterated chloroform (CDCl 3) solvent.
UV–visible spectra were recorded on a Hewlett–Packard 4853 diode array spectrometer. Steady
state photoluminescent spectra were measured using a QuantaMaster Photon Technology
International phosphorescence/fluorescence spectrofluorometer, whereas gated phosphorescence
was measured on the same instrument using a Xe flash lamp with 40 µs delay. Photoluminescent
quantum yield (PLQY) measurements were carried out using a Hamamatsu C9920 system
equipped with a Xe lamp, calibrated integrating sphere and model C10027 photonic multi-channel
analyzer (PMA). Photophysical measurements were carried out in 2-methyltetrahydrofuran
(2-MeTHF). Samples were deoxygenated by bubbling N2 in a quartz cuvette fitted with a Teflon
stopcock. Spin-coated films were prepared on quartz substrates, and photoluminescence quantum
yield (PLQY) measurements were done under nitrogen atmosphere. Cyclic voltammetry and
differential pulse voltammetry were performed using a VersaSTAT 3 potentiostat. Anhydrous
acetonitrile (Aldrich) solvent was used under nitrogen atmosphere with 0.1 M
tetra(n-butyl)-ammonium hexafluorophosphate (TBAF) as the supporting electrolyte. A Ag wire
was used as the pseudo reference electrode, a Pt wire as the counter electrode, and a glassy carbon
rod working electrode. The redox potentials are based on the values from differential pulsed
voltammetry measurements and are reported relative to the ferrocenium/ferrocene (Cp2Fe
+
/Cp2Fe)
redox couple used as an internal reference, whereas electrochemical reversibility was studied using
cyclic voltammetry. Thermogravimetric analysis (TGA) measurements were performed on a
NETZSCH STA 449F3 thermogravimeter by measuring weight loss while heating at a rate of 10
o
C
52

min
-1
under nitrogen. Differential scanning calorimetry (DSC) measurements were performed on
a Perkin Elmer DSC 8000 with CLN2 instrument at a heating rate of 10
o
C min
-1
under nitrogen
atmosphere.
2.4.2. DFT and TD-DFT Calculations:
Calculations were performed using Jaguar 8.4 (release 12) software package on the
Schrödinger Material Science Suite (v2017-2). Gas phase geometry optimization was obtained
using B3LYP functional with the LACVP** basis set. The HOMO and LUMO energies were
determined using minimized singlet geometries to approximate the ground state, whereas the
triplet excited state is calculated using self-consistent field method (∆SCF) by taking the difference
between lowest singlet and triplet excited states.
2.4.3. OLED Fabrication and Characterization:
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 / 5 nm dipyrazino[2,3,-f:20,30-h]quinoxaline
2,3,6,7,10,11-hexacarbonitrile (HATCN) / 40 nm 4,4′-cyclohexylidenebis
[N,N-bis(4-methylphenyl)benzenamine] (TAPC) / 10 nm N,N’-dicarbazolyl-3,5-benzene (mCP)/8
vol% FIrpic:Host / 45 nm BP4mPy / 1.5 nm 8-hydroxyquinolinato lithium (LiQ) / 100 nm Al. The
host is either 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP),
2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzppy) or one of the
53

phenanthro-imidazole/triazole compounds. 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.
2.4.4. Synthesis
All commercial reagents and solvents are purchased from Sigma Aldrich, Matrix Scientific and
Ark Pharm and are used without further purification.
Synthesis of Phenanthro[9,10-d]triazole (hT)
To a solution of 9-bromophenanthrene (15.0g, 58.4mmol) in 265 ml of dry dimethyl sulfoxide
were added dry potassium t-butoxide (19.6g, 175.0mmol) and sodium azide (7.58g, 116.7mmol).
The reaction mixture was stirred at room temperature under nitrogen atmosphere for 12 hours. The
reaction mixture was poured into 500 ml of 1M HCl to form off-white precipitate. The precipitate
is filtered and further washed with cold acetone. The residue is dried under vacuo to yield off-
white solid (7.05 g, 32 mmol, 55%).
Synthesis of 1-methyl-phenanthro[9,10-d]triazole (1-MeT) and 2-methyl-phenanthro[9,10-
d]triazole (2-MeT)
To solution of hT (4.46g, 20.3mmol) in DMF (15 mL) was added K2CO3 (5.62g, 40.8 mmol)
followed by methyl iodide (2.89g, 20.3 mmol). The resulting mixture was stirred for 12 hours at
room temperature. Upon the completion of the reaction, DCM (50 mL) and water (50 mL) were
added, and the layers were separated. The aqueous layer was further extracted with
dichloromethane. The organic layer was dry over Na2SO4 and concentrated to dryness. The residue
was purified by chromatography on silica gel (eluting with a mixture of ethyl acetate and hexane)
to give 2-MeT as yellow solid (2.1 g, 9.00 mmol, 44%):
1
H NMR (400 MHz, Chloroform-d) δ
54

8.61 – 8.56 (m, 1H), 8.50 – 8.45 (m, 1H), 7.69 – 7.58 (m, 2H), 4.52 (d, J = 1.0 Hz, 2H).
13
C NMR
(101 MHz, cdcl3) δ 141.16, 130.30, 127.60, 127.57, 124.50, 123.66, 123.42, 77.29, 77.17, 76.97,
76.65, 42.66. (Found: C, 77.12; H, 4.73; N, 17.59. Calc. for C15H11N3: C, 77.23; H, 4.75; N,
18.01%).
and 1-MeT as brown solid (1.2 g, 5.14 mmol, 25%):
1
H NMR (400 MHz, Chloroform-d) δ 8.74
(ddd, J = 7.8, 2.9, 1.5 Hz, 1H), 8.70 – 8.62 (m, 1H), 8.58 – 8.51 (m, 1H), 8.30 – 8.23 (m, 1H), 7.73
– 7.61 (m, 4H), 4.63 (d, J = 5.9 Hz, 3H);
13
C NMR (101 MHz, cdcl3) δ 142.05, 130.80, 128.67,
128.36, 128.07, 127.50, 127.28, 126.92, 125.01, 124.41, 123.23, 122.87, 122.30, 120.74, 77.32,
77.00, 76.68, 38.24. (Found: C, 76.81; H, 4.82; N, 17.14. Calc. for C15H11N3: C, 77.23; H, 4.75;
N, 18.01%).
Synthesis of 2-phenyl-2H-phenanthro[9,10-d][1,2,3]triazole (2-pT)
In a 3-necked round bottomed flask Fe(acac)3 (966 mg, 2.74 mmol), CuO (72.56 mg, 0.2 mmol),
phenanthro[9,10-d]triazole (2.00 g, 9.12 mmol), and Cs2CO3 (5.94 g, 18.24 mmol). Iodobenzene
(1.86 g, 9.12 mmol) and anhydrous DMF (10 mL) were added to the flask. The reaction mixture
was heated to 90
o
C and stirred for 30 h under nitrogen. After cooling to room temperature, the
mixture was diluted with dichloromethane and filtered. The filtrate was washed twice with water,
and the combined aqueous phases were extracted twice with dichloromethane. The organic layers
were combined, dried over Na2SO4, and concentrated to yield the crude product, which was further
purified by silica gel chromatography (7:3 hexanes/EtAc) to yield 2-phenyl-2H-phenanthro[9,10-
d][1,2,3]triazole (1) as puffy solid (2.10g g, 7.11 mmol, 78%):
1
H NMR (400 MHz, Chloroform-
d) δ 8.64 – 8.52 (m, 4H), 8.42 – 8.36 (m, 2H), 7.71 – 7.62 (m, 4H), 7.60 – 7.52 (m, 2H), 7.45 –
7.38 (m, 1H).
13
C NMR (101 MHz, Chloroform-d) δ 142.14, 140.40, 130.77, 129.35, 128.07,
55

127.91, 127.69, 124.45, 123.96, 123.72, 119.72, 77.30, 76.98, 76.66. (Found: C, 81.64; H, 4.56;
N, 13.78. Calc. for C20H13N3: C, 81.34; H, 4.44; N, 14.23%).
Synthesis of 9-Hydroxyphenanthrene
In a dry 1000 ml one-necked round bottom flask, 9-bromophenanthrene (15.0g, 58.3 mmol) was
dissolved in 120 ml tetrahydrofuran and cooled down to -78
o
C. n-butyl lithium (4.48g, 70.0mmol,
2.5 M) was added dropwise to the reaction mixture under nitrogen. The reaction mixture was
stirred at -78
o
C for 30 minutes followed by dropwise addition of trimethyl borate (7.27g,
70.0mmol) in 50 ml tetrahydrofuran. The reaction mixture is stirred at -5
o
C for 1 hour and then
recooled to -10
o
C. Glacial acetic acid (5.3 ml) was added followed by a solution of 30% aqueous
hydrogen peroxide (6.6 ml) in water (6.0 ml) while maintaining the solution at -10
o
C. The resulting
solution was allowed to warm to room temperature while stirring for an additional 40 min.
Saturated aqueous ammonium chloride solution was added (100 ml) followed by tetrahydrofuran
(50 ml), and the organic layer was separated. The organic layer was washed with saturated aqueous
sodium bicarbonate solution (100 ml), water (100 ml), and brine (3 × 100 ml), dried with sodium
sulfate, and filtered. The residue was purified by chromatography on silica gel (eluting with a
mixture of ethyl acetate and hexane) to give white powder (9.5 g, 49 mmol, 84%):
1
H NMR (400
MHz, DMSO-d6) δ 10.29 (s, 1H), 8.69 (dddd, J = 41.0, 8.1, 1.3, 0.6 Hz, 2H), 8.29 – 8.15 (m, 1H),
7.76 – 7.54 (m, 3H), 7.44 (dddd, J = 28.7, 8.3, 7.0, 1.4 Hz, 2H), 7.04 (s, 1H).
Synthesis of 10-Bromo-9-phenanthrol: is prepared from literature procedure without
modification
4

A solution of NBS (5.6 g, 31.7 mmol) in methylene chloride (280 mL) was added dropwise over
1 hour to a solution of 9-phenanthrol (5.6 g, 28.8 mmol) and i-Pr2-NH (0.4 ml, 2.88 mmol) in
methylene chloride. After addition was complete, the mixture was stirred at room temperature for
56

1 h, poured on H2O, and acidified to pH 1 by careful addition of concentrated H2SO4. The resulting
mixture was extracted with CH2Cl2, and the combined organic layers were dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. Column chromatography of the residue
(SiO2, 1:3 AcOEt/hexane) afforded a white solid (5.9 g, 75%):
1
H NMR (400 MHz, Chloroform-
d) δ 8.70 – 8.53 (m, 2H), 8.37 (ddd, J = 7.8, 1.6, 0.6 Hz, 1H), 8.19 – 8.07 (m, 1H), 7.78 – 7.45 (m,
4H), 6.29 (s, 1H).
Synthesis of 10-Trimethylsilylphenanthryl 9-Trifluoromethanesulfonate: is prepared from
literature procedure without modification
4

n-BuLi (11.2 mL, 2.50 M, 27.80 mmol) was added dropwise to a solution of 10-Bromo-9-
phenanthrol (5.10 g, 18.6 mmol) in THF (250 mL) cooled to -78 °C. After stirring at -78 °C for 15
min, TMSCl (3.78 mL, 29.8 mmol) was added, the cooling bath was removed, and stirring was
kept for 10 min at room temperature. The mixture was again cooled to -78 °C, n-BuLi (7.44 mL,
2.50 M, 18.6 mmol) was added dropwise, stirring was kept up at -78 °C for 15 min, and TMSCl
(3.78 mL, 29.8 mmol) was added. Stirring at room temperature was kept up overnight, H2O (300
mL) was added, and the resulting mixture was extracted with Et2O. The combined organic layers
were dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to afford a
crude intermediate, which was taken to the next step without further purification. To a solution of
this crude residue in Et2O (250 mL) at 0 °C, n-BuLi (16.80 mL, 2.50 M, 42.0 mmol) was dropwise
added. The mixture was stirred at room temperature for 4 h and cooled again to 0 °C, and then
Tf2O (13.4 mL, 79.7 mmol) was added. Stirring was kept for 40 min, and then saturated aqueous
NaHCO3 (300 mL) was added. The mixture was extracted with Et2O, and the combined organic
layers were dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The
residue was purified by column chromatography (SiO2; hexane) to afford yellow viscous oil (9.6
57

g, 46%):
1
H NMR (400 MHz, Chloroform-d) δ 8.76 – 8.51 (m, 1H), 8.27 – 8.00 (m, 1H), 7.86 –
7.43 (m, 2H), 0.59 (d, J = 1.5 Hz, 4H).
Synthesis of aryl azides: Aryl azides are from literature procedure without modification
12

Azidobenzene, 2-azido-1,3,5-trimethylbenzene, 1-azido-2-methylbenzene, 2-azido-1,3-
dimethylbenzene, 2-azido-5-bromo-1,3-dimethylbenzene have all been reported and are prepared
from reference 2 without modification.
12-15

1-azido-4-bromo-2,5-dimethylbenzene
Yellow liquid/solid (1.58 g, 7.00 mmol, 70%):
1
H NMR (400 MHz, Acetone-d6) δ 7.40 (s, 1H),
7.17 (s, 1H), 2.37 (s, 3H), 2.14 (s, 3H).
13
C NMR (101 MHz, acetone) δ 137.70, 136.56, 134.17,
128.86, 120.32, 119.42, 21.46, 15.55.
[3+2] Cycloaddition of Aryl Azides to benzynes and product characterization
General procedure: to a solution of benzyne precursor (5.02 mmol, 1 equiv) and azide (6.02 mmol,
1.2 equiv) in dry MeCN (30 mL) was added CsF (1.52 mmol, 2.0 equiv). The reaction vial was
sealed, and the reaction mixture stirred at room temperature for 24 h before being poured into
saturated aqueous NaHCO3. The resulting mixture was extracted with EtOAc or DCM, and the
combined organic layers were dried over MgSO4 and evaporated. The residue was purified by
silica gel chromatography (hexane/EtOAc, 70/30).
1-phenyl-1H-phenanthro[9,10-d][1,2,3]triazole: (1-pT)
White solid (200 mg, 0.68 mmol, 62%):
1
H NMR (400 MHz, Chloroform-d) δ 8.90 (ddd, J = 7.9,
1.5, 0.7 Hz, 1H), 8.74 (ddd, J = 8.3, 1.2, 0.6 Hz, 1H), 8.66 (ddd, J = 8.2, 1.3, 0.7 Hz, 1H), 7.83 –
7.74 (m, 1H), 7.78 – 7.62 (m, 7H), 7.63 (ddd, J = 8.2, 1.4, 0.7 Hz, 1H), 7.50 – 7.36 (m, 1H).
13
C
58

NMR (101 MHz, Chloroform-d) δ 141.55, 138.04, 131.17, 130.46, 129.89, 129.03, 128.99,
128.21, 127.84, 127.15, 127.01, 126.93, 124.94, 124.32, 123.34, 123.14, 122.86, 120.30.
1-(o-tolyl)-1H-phenanthro[9,10-d][1,2,3]triazole (1-tT)
White solid (490 mg, 1.58 mmol, 50%):
1
H NMR (400 MHz, Chloroform-d) δ 8.92 – 8.87 (m,
1H), 8.73 (ddd, J = 8.3, 1.1, 0.6 Hz, 1H), 8.66 (ddt, J = 8.2, 1.1, 0.6 Hz, 1H), 7.78 (ddd, J = 8.0,
7.2, 1.2 Hz, 1H), 7.71 (ddd, J = 8.5, 7.1, 1.5 Hz, 1H), 7.68 – 7.56 (m, 2H), 7.54 – 7.48 (m, 3H),
7.41 – 7.32 (m, 2H), 2.01 (s, 3H).
13
C NMR (101 MHz, Chloroform-d) δ 141.28, 137.14, 136.15,
131.50, 131.04, 130.88, 129.27, 128.96, 128.21, 127.84, 127.82, 127.45, 127.35, 127.09, 125.05,
124.22, 123.36, 123.07, 122.09, 120.42, 77.29, 76.97, 76.66, 17.34.
1-(2,6-dimethylphenyl)-1H-phenanthro[9,10-d][1,2,3]triazole (xT)
White solid (250 mg, 0.77 mmol, 74%):
1
H NMR (400 MHz, Chloroform-d) δ 8.91 (dt, J = 7.9,
1.9 Hz, 1H), 8.73 (d, J = 8.4 Hz, 1H), 8.67 (d, J = 8.3 Hz, 1H), 7.82 – 7.61 (m, 3H), 7.48 (td, J =
7.8, 1.9 Hz, 1H), 7.42 – 7.22 (m, 4H), 1.94 (d, J = 1.8 Hz, 6H).
13
C NMR (101 MHz, Chloroform-
d) δ 141.38, 136.43, 136.35, 130.98, 130.60, 128.98, 128.91, 128.83, 128.23, 127.88, 127.64,
127.09, 125.13, 124.20, 123.39, 123.00, 121.51, 120.47, 77.30, 76.98, 76.66, 17.51.
1-mesityl-1H-phenanthro[9,10-d][1,2,3]triazole (mT)
White solid (1.2 g, 3.56 mmol, 71%):
1
H NMR (400 MHz, Chloroform-d) δ 8.92 – 8.87 (m, 1H),
8.72 (dd, J = 8.4, 1.0 Hz, 1H), 8.66 (dd, J = 8.2, 1.1 Hz, 1H), 7.78 (ddd, J = 8.0, 7.2, 1.2 Hz, 1H),
7.68 (dddd, J = 21.8, 8.4, 7.1, 1.5 Hz, 2H), 7.40 (ddd, J = 8.1, 6.9, 1.1 Hz, 1H), 7.33 (dd, J = 8.1,
1.4 Hz, 1H), 7.13 (s, 2H), 2.46 (s, 3H), 1.89 (s, 6H).
13
C NMR (101 MHz, Chloroform-d) δ 141.33,
140.61, 135.97, 133.80, 130.93, 129.59, 128.96, 128.91, 128.19, 127.80, 127.59, 127.02, 125.18,
124.14, 123.37, 122.98, 121.56, 120.58, 77.30, 76.98, 76.66, 21.34, 17.42. (Found: C, 81.80; H,
5.72; N, 12.16. Calc. for C23H19N3: C, 81.87; H, 5.68; N, 12.45%).
59

1-(4-bromo-2,6-dimethylphenyl)-1H-phenanthro[9,10-d][1,2,3]triazole (2a)
White solid (1.4 g, 3.48 mmol, 68%):
1
H NMR (400 MHz, Chloroform-d) δ 8.89 (ddt, J = 7.9, 1.5,
0.7 Hz, 1H), 8.74 (dd, J = 8.2, 1.2 Hz, 1H), 8.70 – 8.62 (m, 1H), 7.79 (ddt, J = 7.9, 7.0, 1.0 Hz,
1H), 7.71 (dddt, J = 16.8, 8.2, 7.1, 1.0 Hz, 2H), 7.51 (q, J = 0.8 Hz, 2H), 7.44 (ddt, J = 8.1, 7.1,
1.0 Hz, 1H), 7.33 – 7.27 (m, 1H), 1.92 (q, J = 0.8 Hz, 6H), 1.56 (s, 3H).
13
C NMR (101 MHz,
Chloroform-d) δ 138.57, 135.49, 131.85, 131.06, 128.98, 128.32, 128.09, 127.78, 127.23, 124.99,
124.53, 124.32, 123.40, 123.01, 121.36, 120.20, 77.29, 77.18, 76.97, 76.65, 17.42.
1-(4-bromo-2,5-dimethylphenyl)-1H-phenanthro[9,10-d][1,2,3]triazole (2b)
White solid (3.22 g, 8.00 mmol, 56%):
1
H NMR (400 MHz, Chloroform-d) δ 8.91 – 8.85 (m, 1H),
8.73 (dt, J = 8.4, 0.7 Hz, 1H), 8.66 (ddd, J = 8.0, 1.3, 0.7 Hz, 1H), 7.78 (ddt, J = 7.9, 7.0, 1.0 Hz,
1H), 7.74 – 7.63 (m, 3H), 7.47 – 7.37 (m, 3H), 2.52 – 2.36 (m, 3H), 2.01 – 1.87 (m, 3H).
13
C NMR
(101 MHz, Chloroform-d) δ 141.30, 137.52, 136.15, 135.10, 134.94, 131.07, 129.65, 128.95,
128.27, 127.99, 127.48, 127.22, 127.18, 124.94, 124.30, 123.38, 123.08, 122.02, 120.23, 77.29,
77.18, 76.97, 76.65, 22.50, 16.70.
General Procedure for Condensation reaction
A mixture of phenanthrene-9,10-dione (48.03 mmol, 1 equiv), mesityl amine (57.63 mmol, 1.2
equiv), formaldehyde (48.03 mmol, 1 equiv.) and ammonium acetate (96.05 mmol, 2 equiv) in
glacial acetic acid (550 mL) was refluxed for 3 h. The solvent is evaporated to dryness and the
crude is extracted with saturated aqueous NaHCO3 and CH2Cl2. The organics were combined and
dried over Na2SO4. The crude was purified by silica gel chromatography (hexane/EtOAc, 60/40)
followed by sublimation to give pure products.
60

1-methyl-1H-phenanthro[9,10-d]imidazole (MeI)
Yellow solid (600 mg, 0.95 mmol, 59%):
1
H NMR (400 MHz, Chloroform-d) δ 8.81 – 8.74 (m,
1H), 8.67 (dtd, J = 7.7, 1.4, 0.6 Hz, 2H), 8.37 – 8.29 (m, 1H), 7.89 – 7.77 (m, 1H), 7.69 (ddd, J =
8.0, 7.0, 1.2 Hz, 1H), 7.66 – 7.55 (m, 3H), 4.28 (d, J = 0.5 Hz, 3H).
13
C NMR (101 MHz,
Chloroform-d) δ 142.04, 138.55, 129.00, 127.93, 127.48, 127.29, 126.64, 125.95, 125.38, 125.05,
124.98, 124.30, 123.51, 123.06, 122.31, 120.59, 109.98, 77.30, 77.19, 76.98, 76.66, 35.60. (Found:
C, 82.72; H, 5.22; N, 11.87. Calc. for C16H12N2: C, 82.73; H, 5.21; N, 12.06%).
1-phenyl-1H-phenanthro[9,10-d]imidazole (pI)
White solid (220 mg, 0.75 mmol, 5%):
1
H NMR (400 MHz, Chloroform-d) δ 8.78 (dd, J = 8.0, 1.7
Hz, 1H), 8.72 (d, J = 8.4 Hz, 1H), 8.66 (d, J = 8.2 Hz, 1H), 7.98 (d, J = 2.0 Hz, 1H), 7.77 – 7.69
(m, 1H), 7.66 – 7.56 (m, 4H), 7.55 – 7.46 (m, 3H), 7.40 (dd, J = 8.4, 1.7 Hz, 1H), 7.32 – 7.25 (m,
1H).
13
C NMR (101 MHz, Chloroform-d) δ 141.70, 137.86, 137.72, 129.93, 129.79, 129.59,
129.19, 128.29, 127.39, 127.22, 126.32, 126.29, 125.66, 125.22, 124.08, 123.15, 122.69, 122.53,
121.16, 77.39, 77.08, 76.76. (Found: C, 86.01; H, 4.98; N, 9.53. Calc. for C21H14N2: C, 85.69; H,
4.79; N, 9.52%).
1-mesityl-1H-phenanthro[9,10-d]imidazole (mI)
White solid (10.2g , 30.32 mmol, 71%):
1
H NMR (400 MHz, Chloroform-d) δ 8.80 – 8.73 (m,
2H), 8.72 – 8.68 (m, 1H), 7.83 (s, 1H), 7.74 (ddd, J = 8.0, 7.0, 1.1 Hz, 1H), 7.64 (ddd, J = 8.4, 7.0,
1.5 Hz, 1H), 7.52 (ddd, J = 8.4, 7.0, 1.3 Hz, 1H), 7.31 (ddd, J = 8.2, 7.0, 1.2 Hz, 1H), 7.19 (dt, J =
8.2, 0.9 Hz, 1H), 7.13 – 7.08 (m, 2H), 2.45 (s, 3H), 1.95 (s, 6H).
13
C NMR (101 MHz, Chloroform-
d) δ 140.68, 139.69, 137.88, 136.25, 133.40, 129.54, 128.96, 128.20, 127.55, 127.36, 126.90,
125.53, 125.46, 125.17, 123.91, 123.26, 123.16, 122.30, 119.65, 77.32, 77.21, 77.00, 76.69, 21.25,
17.66. (Found: C, 85.91; H, 5.99; N, 8.28. Calc. for C24H20N2: C, 85.68; H, 5.99; N, 8.33%).
61

1-(4-bromo-2,6-dimethylphenyl)-1H-phenanthro[9,10-d]imidazole (2c)
White solid (1.25 g, 3.11 mmol, 22%):
1
H NMR (400 MHz, Chloroform-d) δ 8.82 – 8.68 (m, 3H),
7.92 (s, 1H), 7.77 (ddd, J = 8.0, 7.0, 1.2 Hz, 1H), 7.67 (ddd, J = 8.4, 7.1, 1.5 Hz, 1H), 7.57 (ddd, J
= 8.4, 7.1, 1.3 Hz, 1H), 7.52 – 7.47 (m, 2H), 7.37 (ddd, J = 8.2, 7.1, 1.2 Hz, 1H), 7.17 (dd, J = 8.2,
1.3 Hz, 1H), 1.98 (d, J = 0.6 Hz, 6H).
13
C NMR (101 MHz, Chloroform-d) δ 175.43, 140.27,
138.77, 137.48, 135.04, 131.84, 129.16, 128.35, 127.61, 127.18, 127.04, 125.86, 125.58, 125.24,
124.11, 123.70, 123.22, 122.83, 122.46, 119.40, 20.91, 17.65, 17.42.
1-(4-bromo-2,5-dimethylphenyl)-1H-phenanthro[9,10-d]imidazole (2d)
White solid (5.50 g, 13.71 mmol, 46%):
1
H NMR (400 MHz, Chloroform-d) δ 8.82 – 8.74 (m,
2H), 8.71 (ddt, J = 8.9, 1.2, 0.6 Hz, 1H), 8.00 (s, 1H), 7.77 (ddd, J = 8.0, 7.0, 1.1 Hz, 1H), 7.68 (s,
1H), 7.73 – 7.63 (m, 1H), 7.58 (ddt, J = 8.2, 7.1, 1.2 Hz, 1H), 7.43 – 7.33 (m, 2H), 7.31 – 7.23 (m,
1H), 2.49 (s, 3H), 1.97 (s, 3H).
13
C NMR (101 MHz, cdcl3) δ 175.70, 141.04, 137.50, 137.11,
135.77, 135.51, 134.88, 129.89, 129.15, 128.34, 127.58, 126.94, 126.90, 126.41, 126.16, 125.84,
125.53, 124.09, 123.20, 122.76, 122.55, 120.08, 22.50, 16.83.
1-(4-bromophenyl)-1H-phenanthro[9,10-d]imidazole (2e)
White solid (10.05 g, 26.9 mmol, 56
1
H NMR (400 MHz, Chloroform-d) δ 8.77 (d, J = 10.0 Hz,
1H), 8.71 (dd, J = 8.1, 6.0 Hz, 2H), 8.01 (d, J = 0.8 Hz, 1H), 7.76 (m, 3H), 7.66 (dd, J = 10.8, 0.8
Hz, 1H), 7.56 (t, J = 0.0 Hz, 1H), 7.45 (m, 3H), 7.37 (t, J = 7.6 Hz, 1H).
13
C NMR (101 MHz,
cdcl3) δ 141.66, 137.88, 136.73, 133.22, 132.88, 129.30, 128.77, 128.36, 127.52, 127.02, 126.45,
125.85, 125.45, 124.22, 123.67, 123.17, 122.55, 122.45, 121.03.
General Procedure for Suzuki Coupling:
In a one-necked RBF, add Pd(PPh3)4 in glovebox the bromide, boronic acid and K2CO3 are added
outside glovebox quickly to minimize the amount of time Pd(PPh3)4 is exposed to air. The flask is
62

equipped with condenser and stirrer bar. The flask is then filled and evacuated 3 times.
Toluene/Water is added via cannula transfer and the reaction mixture is stirred at 110
o
C for 48 h.
The reaction mixture is extracted with water and the organic layer is dried with Na2SO4. The crude
residue was purified by column chromatography (silica), using gradient of ethyl acetate in hexane,
yielding 1
1-(2',3,4',5,6'-pentamethyl-[1,1'-biphenyl]-4-yl)-1H-phenanthro[9,10-d][1,2,3]triazole
(mxT)
White solid (1.00 g, 2.26 mmol, 91%):
1
H NMR (400 MHz, Chloroform-d) δ 8.95 – 8.90 (m, 1H),
8.75 (d, J = 8.4 Hz, 1H), 8.71 – 8.66 (m, 1H), 7.80 (ddd, J = 8.1, 7.1, 1.1 Hz, 1H), 7.70 (dddd, J =
22.1, 8.4, 7.0, 1.6 Hz, 2H), 7.38 (ddt, J = 7.8, 6.9, 0.9 Hz, 1H), 7.35 – 7.28 (m, 1H), 7.12 (s, 2H),
7.00 (s, 2H), 2.36 (s, 3H), 2.18 (s, 3H), 2.14 (s, 3H), 1.97 (s, 6H).
13
C NMR (101 MHz, cdcl3) δ
143.86, 141.46, 137.85, 137.01, 136.58, 135.79, 135.26, 134.81, 131.06, 129.84, 129.01, 128.88,
128.27, 128.11, 127.88, 127.50, 127.12, 125.16, 124.27, 123.42, 123.01, 121.37, 120.51, 21.05,
20.86, 20.60, 17.58. (Found: C, 84.64; H, 6.36; N, 9.55. Calc. for C31H27N3: C, 84.32; H, 6.16; N,
9.52%).
1-(2',3,4',5,6'-pentamethyl-[1,1'-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (mxI)
White solid (1.20 g, 2.72 mmol, 90%)
1
H NMR (400 MHz, Chloroform-d) δ 8.81 (t, J = 9.1, 8.0
Hz, 2H), 8.73 (d, J = 9.1 Hz, 1H), 7.78 (ddd, J = 8.1, 7.0, 1.1 Hz, 1H), 7.67 (ddd, J = 8.4, 7.0, 1.5
Hz, 1H), 7.56 (ddd, J = 8.4, 7.0, 1.3 Hz, 1H), 7.33 – 7.24 (m, 2H), 7.18 (dd, J = 8.2, 1.3 Hz, 1H),
7.11 (s, 2H), 7.01 (d, J = 6.5 Hz, 2H), 2.38 (s, 3H), 2.20 (s, 3H), 2.16 (s, 3H), 2.03 (s, 6H).
13
C
NMR (101 MHz, cdcl3) δ 143.06, 140.26, 137.90, 137.01, 136.76, 135.57, 135.47, 134.40, 129.82,
129.12, 128.29, 128.21, 127.48, 127.4126.77, 125.62, 125.40, 125.27, 124.04, 123.21, 123.15,
63

122.39, 119.51, 21.06, 20.85, 20.59, 17.84. (Found: C, 88.10; H, 6.43; N, 6.36. Calc. for C32H28N2:
C, 87.24; H, 6.41; N, 6.36%).
1-(4-(dibenzo[b,d]furan-4-yl)-2,5-dimethylphenyl)-1H-phenanthro[9,10-d][1,2,3]triazole
(fxT)
White solid (1.20 g, 2.45 mmol, 82%):
1
H NMR (400 MHz, Chloroform-d) δ 8.95 – 8.90 (m, 1H),
8.79 – 8.74 (m, 1H), 8.71 – 8.66 (m, 1H), 8.06 – 8.00 (m, 2H), 7.80 (ddd, J = 8.0, 7.1, 1.2 Hz, 1H),
7.72 (tdd, J = 8.3, 7.1, 1.5 Hz, 2H), 7.68 – 7.63 (m, 1H), 7.60 (dt, J = 8.3, 0.9 Hz, 1H), 7.55 – 7.44
(m, 6H), 7.39 (td, J = 7.5, 1.0 Hz, 1H), 2.32 (s, 3H), 2.03 (s, 3H).
13
C NMR (101 MHz, cdcl3) δ
156.21, 153.43, 141.30, 139.28, 136.61, 136.58, 133.32, 132.98, 131.07, 129.36, 129.20, 128.99,
128.23, 128.18, 127.89, 127.51, 127.38, 127.09, 125.12, 124.97, 124.47, 124.24, 124.19, 123.39,
123.10, 122.90, 122.39, 120.78, 120.54, 120.26, 111.87, 77.30, 77.19, 76.98, 76.66, 19.81, 16.96.
(Found: C, 83.68; H, 4.79; N, 8.48; O, 3.05. Calc. for C34H23N3O: C, 83.41; H, 4.74; N, 8.58; O,
3.27%).
1-(4-(dibenzo[b,d]furan-4-yl)-2,5-dimethylphenyl)-1H-phenanthro[9,10-d]imidazole (fxI)
White solid (1.52 g, 3.11 mmol, 83%)
1
H NMR (400 MHz, Chloroform-d) δ 8.88 – 8.78 (m, 3H),
8.74 (d, J = 8.8 Hz, 2H), 8.10 (s, 1H), 8.08 – 8.01 (m, 3H), 7.78 (ddd, J = 15.2, 12.0, 7.4 Hz, 1H),
7.69 (ddd, J = 8.4, 7.0, 1.5 Hz, 1H), 7.65 – 7.57 (m, 2H), 7.54 – 7.37 (m, 5H).

13
C NMR (101 MHz, cdcl3) δ 156.20, 153.42, 141.06, 138.47, 137.43, 136.53, 136.51, 136.27,
133.46, 133.23, 129.50, 129.12, 128.25, 127.46, 127.41, 127.39, 127.28, 126.83, 126.40, 125.66,
125.34, 124.93, 124.49, 124.21, 124.03, 123.18, 123.09, 123.07, 122.94, 122.54, 120.80, 120.43,
120.26, 111.86, 19.82, 17.10. (Found: C, 86.48; H, 4.91; N, 5.65; O, 2.96. Calc. for C35H24N2O:
C, 86.04; H, 4.95; N, 5.73; O, 3.27%).
64

1-(4-(dibenzo[b,d]thiophen-4-yl)-2,5-dimethylphenyl)-1H-phenanthro[9,10-d][1,2,3]triazole
(txT)
White solid (1.26 g, 3.13 mmol, 82%):
1
H NMR (400 MHz, Chloroform-d) δ 8.96 – 8.89 (m, 1H),
8.79 – 8.74 (m, 1H), 8.72 – 8.67 (m, 1H), 8.23 (dt, J = 7.9, 1.3 Hz, 2H), 7.90 – 7.84 (m, 1H), 7.80
(ddd, J = 8.0, 7.0, 1.2 Hz, 1H), 7.76 – 7.67 (m, 2H), 7.64 – 7.40 (m, 8H), 2.26 (s, 3H), 2.02 (s, 3H).
13
C NMR (101 MHz, cdcl3) δ 142.69, 141.32, 136.74, 135.95, 135.89, 135.80, 135.53, 133.41,
132.41, 131.10, 129.52, 129.37, 129.01, 128.24, 127.92, 127.53, 127.12, 127.10, 126.95, 125.09,
124.79, 124.54, 124.27, 123.41, 123.10, 122.81, 122.24, 121.88, 120.83, 120.50, 100.38, 77.30,
77.19, 76.99, 76.67, 19.40, 16.91. (Found: C, 80.95; H, 4.54; N, 8.05; S, 6.31. Calc. for C34H23N3S:
C, 80.76; H, 4.59; N, 8.31; S, 6.34%).
1-(4-(dibenzo[b,d]thiophen-4-yl)-2,5-dimethylphenyl)-1H-phenanthro[9,10-d]imidazole
(txI)
White solid (1.58 g, 3.13 mmol, 84%):
1
H NMR (400 MHz, Chloroform-d) δ 8.89 (dd, J = 8.0, 1.4
Hz, 1H), 8.80 (d, J = 8.3 Hz, 1H), 8.73 (d, J = 8.3 Hz, 1H), 8.27 – 8.16 (m, 3H), 7.86 (t, J = 4.2
Hz, 1H), 7.79 (ddd, J = 8.0, 7.1, 1.0 Hz, 1H), 7.69 (ddd, J = 8.5, 7.0, 1.4 Hz, 1H), 7.62 (td, J = 7.8,
2.7 Hz, 2H), 7.54 – 7.37 (m, 7H), 2.19 (s, 3H), 2.04 (s, 3H).
13
C NMR (101 MHz, cdcl3) δ 142.35,
139.53, 136.13, 135.92, 135.81, 135.37, 133.67, 132.47, 129.62, 129.37, 128.51, 127.76, 127.06,
127.00, 126.21, 125.81, 124.84, 124.60, 124.13, 123.21, 122.93, 122.77, 122.62, 121.92, 120.89,
120.34, 77.30, 77.19, 76.99, 76.67, 19.41, 17.02. (Found: C, 83.70; H, 4.82; N, 5.49; S, 6.45. Calc.
for C35H24N2S: C, 83.30; H, 4.79; N, 5.55; S, 6.35%).
1-(4-(dibenzo[b,d]thiophen-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (tpI)
White solid (10.20 g, 21.4 mmol, 80 %):
1
H NMR (400 MHz, Chloroform-d) δ 8.81 (ddd, J = 10.6,
8.1, 4.4 Hz, 2H), 8.74 (dd, J = 9.1, 5.2 Hz, 2H), 8.29 – 8.21 (m, 2H), 8.13 (s, 1H), 8.06 – 8.01 (m,
2H), 7.92 – 7.86 (m, 2H), 7.81 – 7.57 (m, 6H), 7.56 – 7.48 (m, 2H), 7.43 (ddd, J = 8.3, 7.0, 1.2 Hz,
65

1H).

13
C NMR (101 MHz, cdcl3) δ 141.99, 141.72, 139.32, 138.46, 138.00, 137.25, 136.57, 135.67,
135.42, 129.81, 129.31, 128.38, 127.56, 127.47, 127.23, 127.11, 127.03, 126.44, 126.35, 125.76,
125.35, 125.32, 124.65, 124.18, 123.18, 122.71, 122.68, 122.60, 121.87, 121.31, 121.22.
2.5.References
1. D. Sylvinson M. R, H.-F. Chen, L. M. Martin, P. J. G. Saris and M. E. Thompson, ACS
Applied Materials & Interfaces, 2019, 11, 5276-5288.
2. S. Ueda, M. Su and L. Buchwald Stephen, Angewandte Chemie International Edition,
2011, 50, 8944-8947.
3. S. Roshandel, M. J. Lunn, G. Rasul, D. S. Muthiah Ravinson, S. C. Suri and G. K. S.
Prakash, Organic Letters, 2019, 21, 6255-6258.
4. Q. Chen, H. Yu, Z. Xu, L. Lin, X. Jiang and R. Wang, The Journal of Organic
Chemistry, 2015, 80, 6890-6896.
5. D. W. Norman, C. A. Carraz, D. J. Hyett, P. G. Pringle, J. B. Sweeney, A. G. Orpen, H.
Phetmung and R. L. Wingad, Journal of the American Chemical Society, 2008, 130,
6840-6847.
6. S. Chen, Y. Wu, Y. Zhao and D. Fang, RSC Advances, 2015, 5, 72009-72018.
7. A. F. Rausch, M. E. Thompson and H. Yersin, The Journal of Physical Chemistry A,
2009, 113, 5927-5932.
8. G. A. George and D. K. C. Hodgeman, Eur. Polym. J., 1977, 13, 63-71.
9. J. Sworakowski, J. Lipiński and K. Janus, Org. Electron., 2016, 33, 300-310.
10. E. Baranoff, B. F. E. Curchod, F. Monti, F. Steimer, G. Accorsi, I. Tavernelli, U.
Rothlisberger, R. Scopelliti, M. Grätzel and M. K. Nazeeruddin, Inorganic Chemistry,
2012, 51, 799-811.
11. T. D. Schmidt, T. Lampe, D. Sylvinson M. R, P. I. Djurovich, M. E. Thompson and W.
Brütting, Physical Review Applied, 2017, 8, 037001.
12. S. W. Kwok, J. R. Fotsing, R. J. Fraser, V. O. Rodionov and V. V. Fokin, Organic
Letters, 2010, 12, 4217-4219.
13. N. Faucher, Y. Ambroise, J.-C. Cintrat, E. Doris, F. Pillon and B. Rousseau, The Journal
of Organic Chemistry, 2002, 67, 932-934.
14. M. Kitamura, S. Kato, M. Yano, N. Tashiro, Y. Shiratake, M. Sando and T. Okauchi,
Organic & Biomolecular Chemistry, 2014, 12, 4397-4406.
15. D. G. Brown, N. Sanguantrakun, B. Schulze, U. S. Schubert and C. P. Berlinguette,
Journal of the American Chemical Society, 2012, 134, 12354-12357.
66

3CHAPTER 3 − Photophysical Study of Deep Blue and Sky-blue fac/mer-
Iridium (III) NHC Carbene Complexes and their Application in OLEDs
3.1. Introduction
Developing stable blue phosphorescent emitters is crucial in improving device lifetime of
blue PHOLEDs and all phosphorescent WOLEDs. Among the Ir (III) based blue phosphorescent
emitters reported, NHC based complexes show promising OLED performance for blue color
display application.
1
As mentioned in Chapter 1, the cyclometalated NHC ligand (C^C:) is overall
an anionic ligand, donating two electrons from the neutral carbene carbon and the other two
electrons from the anionic phenyl carbon. The carbene carbon is sp
2
hybridized with the sp
2
and p
orbitals orthogonal to each other. The ground state of the carbene is usually a singlet with σ
2

electronic configuration. However, the carbene can afford triplet configuration (σ
1
pπ
1
) if the energy
difference between the singlet state and the triplet state is small. In their triplet configuration,
carbenes are highly reactive and they usually react with each other to form dimers. Carbenes with
singlet-triplet gap above 65 kcal/mol are reported to be mostly in their singlet state, and thus are
mostly stable. Singlet carbenes form sigma bond with transition metals by donating two electrons
from the sp
2
orbitals to the d-orbital of the metal.
2-4
This bond is further strengthened when the
metal donates back π-electrons from the filled metal d-orbitals into the empty pπ orbital of the
carbene carbon.
This strong carbene-metal bond is among the reasons deep blue NHC-based Ir(III)
complexes could achieve high efficiencies. For example, our group reported highly efficient deep
blue facial and meridional isomers of Ir(pmp)3.
1
Unlike C^N type Ir-complexes, which undergo
photoisomerization in the excited state, the meridional isomer of Ir(pmp)3 do not undergo
photoisomerization due to strong carbene-Ir bond. OLED devices employing fac- and mer-
Ir(pmp)3 as emitters achieved high efficiencies (15%). However, due to their high emission
67

energies, these emitters could only be hosted by phosphine oxide type unstable host materials,
leading to short device lifetimes. Additionally, the LUMO energies of these complexes are
shallow, making electron injection and transport in these materials difficult. To address these
issues, we developed new Ir(III) complexes employing pyrazinoimidazolyl carbene ligands with
deeper LUMO levels and lower emission energies than Ir(pmp)3 complexes. Syntheses of these
new materials, their unique and interesting photophysical, electrochemical and
electroluminescence properties materials as emitters in OLED devices are discussed in this
chapter.
3.2. Results and discussion
3.2.1. Synthesis
Synthesis of fac- and mer-Ir(pmpz)3 is reported briefly by Batagoda.
5
Here we report the
detailed and modified synthesis of these materials. Synthesis of mer-Ir(pmpz)3 is accomplished in
four steps. The first two steps are high yielding steps (yield > 80%, Scheme 3.1). Methylation of
the pyrazino imidazole in the third step yielded two isomers. The first methylation occurs on the
nitrogen in the imidazole ring whereas the second methylation occurs on the nitrogen of the
pyrazine ring. The mixture was taken to the next step without further purification. The subsequent
step is cyclometallation of the carbene (C^C:) ligand on iridium using Ir(COD) 2Cl2 to obtain the
meridional isomer (mer-Ir(pmpz)3) in 20% yield . The facial isomer is obtained in a separate
reaction through acid induced isomerization or by heating the meridional isomer under vacuum.
In both methods, the conversions of the reactions are 50%. The facial isomer sublimes cleanly with
sublimation yield over 90%. The facial isomer is separated from the meridional isomer through
column chromatography.
68

Because of the long steps and low yield reaction of fac-Ir(pmpz)3, fac-Ir(tpz)3 was
synthesized. Similar to the synthesis of Ir(pmpz)3, the first two steps of Ir(tpz)3 synthesis steps are
ligand synthesis and are obtained in high yields (Scheme 3.1). However, the cyclometallation of
tpz ligand on iridium yielded only the facial isomer in 45%. The meridional isomer did not form
during the reaction, which is probably due to steric clash between the tolyl groups in the meridional
isomer. fac-Ir(tpz)3 is found to be remarkably stable under sublimation, with sublimation yields as
high as 95% with no decomposition products. From the mass spectroscopy study, the ligand on
fac-Ir(tpz)3 is found to be tightly bonded to the metal with no ligand loss observed in the mass
spectrum (Figure 3.33). In addition, unlike the mer-Ir(pmpz)3 where it reacts with acids, fac-
Ir(tpz)3 showed remarkable stability in malonic and acetic acid.

Scheme 3.1. Synthesis of fac- and mer-Ir(pmpz)3

Scheme 3.2. Synthesis of fac-Ir(tpz)3
69

3.2.2. Computational Studies
As reported in earlier work, the LUMO density of Ir(pmp)3 complexes is localized on the
pyridinoimidazole ring.
1
Our initial study showed that, addition of nitrogen on the pyridine moiety
stabilizes the LUMO more than the HOMO, consequently redshifting the emission spectrum
(Table 3.1). The triplet energies of Ir(pmpz)3 are ideal for blue emission and low enough to be
hosted by stable host materials like mCBP. Additionally, their LUMO level is around 2.0 eV,
similar to those of common electron transport materials, ideal for electron injection and transport
in OLEDs. For these reasons, Ir(pmpz)3 complexes were chosen for further studies. The HOMO,
LUMO and spin density surfaces of these complexes are depicted in Figure 3.1. Interestingly, the
HOMO−LUMO overlap of the meridional isomers is minimal, resulting in a very small S1−T1 gap
and a weaker oscillator strength than the facial isomers. The facial isomers have larger
HOMO−LUMO overlap than the meridional isomers, giving the facial isomers larger S1−T1 gap
and stronger oscillator strengths. The methyl group in fac-Ir(tpz)3 destabilizes its HOMO more
than its LUMO, consequently redshifting its singlet and triplet energies compared to those of fac-
Ir(pmpz)3.
Table 3.1. Calculated energy levels and frontier orbitals of Ir-carbene complexes
HOMO LUMO S 1 T 1 Osc (S 1) ΔS 1-T 1
fac-Ir(pmp) 3 -4.97 -1.14 3.23 3.04 0.0413 0.19
fac-Ir(pmpz) 3 -5.28 -1.80 2.90 2.69 0.0355 0.21
fac-Ir(tpz) 3 -5.03 -1.61 2.85 2.65 0.0386 0.20
mer-Ir(pmp) 3 -4.84 -1.28 2.99 2.93 0.0045 0.06
mer-Ir(pmpz) 3 -5.13 -1.92 2.65 2.60 0.0027 0.05
B3LYP/LACVP**, *All energies are in eV
70

HOMO LUMO Triplet Spin Density
mer-Ir(pmp)3

fac-Ir(pmp)3

mer-Ir(pmpz)3

fac-Ir(pmpz)3

fac-Ir(tpz)3

Figure 3.1. DFT (singlet and triplet, spin density, HOMO and LUMO surfaces) TDDFT (orbital
contributions). S1 and T1 are HOMO to LUMO transition.
3.2.3. Electrochemical and Photophysical Properties
The electrochemical properties of the complexes were determined using cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) (Figure 3.2). All the complexes
71

show reversible reduction, whereas their oxidation is quasi-reversible. The greater
electronegativity of the nitrogen atom versus methene (CH) lowers the reduction potential of fac-
Ir(pmpz)3/fac-Ir(tpz)3 to -2.21±0.05 V relative to -2.77±0.05 V for fac-Ir(pmp)3, and their
oxidation potentials are differed by 300 mV (Eox = +0.80±0.05 V and Eox = 0.47±0.05 V,
respectively). As was observed with Ir(pmp)3, the oxidation potential of mer-Ir(pmpz)3 is about
200 mV lower than that of fac-Ir(pmpz)3 or fac-Ir(tpz)3 but their reduction potentials are the same.
The HOMO and LUMO energies are calculated from the measured Eox and Ered (Table 3.2).
Properties of Ir(pmp)3 complexes are included to directly compare them with the properties of
Ir(pmpz)3 complexes.
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

mer-Ir(pmpz)
3
Fc/Fc+

Current (a.u)
fac-Ir(pmpz)
3

Potential (V vs Fc/Fc+)
fac-Ir(tpz)
3
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

mer-Ir(pmpz)
3
Fc/Fc+

Current (a.u)
fac-Ir(pmpz)
3

Voltage (V vs Fc
+
/Fc)
fac-Ir(tpz)
3

Figure 3.2. Cyclic voltammetry curves and differential pulse voltammetry data of Ir(C^C:)3
complexes.

72

Table 3.2. Electrochemical properties of Ir(C^C:)3 complexes.
Complex
DMF
E ox(V) E red(V) ΔE redox (V) HOMO (eV) LUMO (eV)
fac-Ir(pmp) 3 +0.47
a
-2.77
a
3.24 -5.3 -1.6
fac-Ir(pmpz) 3 +0.80 -2.21 3.01 -5.7 -2.2
fac-Ir(tpz) 3 +0.66 -2.14 2.80 -5.6 -2.3
mer-Ir(pmp) 3 +0.23
a
-2.80
a
3.03 -5.1 -1.5
mer-Ir(pmpz) 3 +0.55 -2.21 2.76 -5.4 -2.2
a: Obtained from reference 1. HOMO and LUMO are calculated from their Eox and Ered,
respectively according reference
The absorption spectra of mer-Ir(pmpz)3, fac-Ir(pmpz)3 and fac-Ir(tpz)3 in Figure 3.3a
show that their spin-allowed metal-to-ligand charge-transfer (
1
MLCT) transitions onsets are lower
in energy (400–425 nm) than similar transitions in mer- and fac-Ir(pmp)3 (350–380 nm).
1
The
observed redshift for the absorption spectrum of the pyrizine analogs is consistent with their
smaller energy gaps compared to the pyridine analogs inferred from their Ered and Eox. The
1
MLCT
transitions of the facial isomers have remarkably high extinctions, indicating high oscillator
strengths of their singlets. Their absorption spectra are independent of solvent polarity (Figure
3.4−Figure 3.6).
The photoluminescence (PL) spectra of all the complexes in 2-MeTHF, polystyrene (PS)
and poly(methyl methacrylate) (PMMA) are shown in Figure 3.3. The emission spectra of fac-
Ir(pmpz)3 is redshifted (max = 476 nm) compared to the emission spectra of fac-Ir(pmp)3 (max =
418 nm) in all media.
1
Similar to mer-Ir(pmp)3, the PL spectrum of mer-Ir(pmpz)3 is broad and
exhibits a large room-temperature bathochromic shift (max = 534 nm) relative to the fac-isomer.
The lower emission energy of mer-Ir(pmpz)3 compared to the facial isomer is due to its lower
oxidation potential and nearly identical reduction potentials, which result in a correspondingly
73

reduced energy gap. The emission in 2-MeTHF for both Ir(pmpz)3 isomers undergoes a
pronounced rigidochromic shift at T = 77 K and unlike fac-Ir(pmp)3, the emission line shape of
250 300 350 400 450 500 550 600
0
1
2
3
4
5
6
7
8

e (10
4
M
-1
. cm
-1
)
Wavelength (nm)
fac-Ir(pmp)
3
fac-Ir(pmpz)
3
fac-Ir(tpz)
3
mer-Ir(pmp)
3
mer-Ir(pmpz)
3
(a)
400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
2-MeTHF 298K
Normalized PL (a.u)
Wavelength (nm)
fac-Ir(pmp)
3
fac-Ir(pmpz)
3
fac-Ir(tpz)
3
mer-Ir(pmp)
3
mer-Ir(pmpz)
3
(b)
400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
2wt% in PS 298K
Normalized PL (a.u)
Wavelength (nm)
fac-Ir(pmp)
3

fac-Ir(pmpz)
3

fac-Ir(tpz)
3

mer-Ir(pmp)
3

mer-Ir(pmpz)
3

(c)
400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL (a.u)
Wavelength (nm)
fac-Ir(pmp)
3
fac-Ir(pmpz)
3
fac-Ir(tpz)
3
mer-Ir(pmp)
3
mer-Ir(pmpz)
3
(d)
2-MeTHF 77K
400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL (a.u)
Wavelength (nm)
fac-Ir(pmp)
3
298K
fac-Ir(pmpz)
3
298K
fac-Ir(tpz)
3
298K
mer-Ir(pmp)
3
298K
mer-Ir(pmpz)
3
298K
2wt% in PMMA 298K
(e)

Figure 3.3. Photophysical properties of Ir(C^C:)3 complexes (a) UV-visible spectra in 2-MeTHF
at 298K, (b) PL spectra in 2-MeTHF at 298K, (c) PL spectra in PS at 298K (d) PL spectra in 2-
MeTHF at 77K (e) PL spectra in PMMA at 298K
74

fac-Ir(pmpz)3 shows no vibronic fine structure in its emission at 77K (Figure 3.3d). fac-Ir(tpz)3
exhibits similar PL properties with fac-Ir(pmpz)3, but with a slight redshift of 5 nm.
The emission lifetimes, , were obtained from mono-exponential fits to the data at room
temperature. Radiative (kr) and non-radiative (knr) rate constants are calculated using the
relationship kr = PL/, where PL = kr / (kr + knr). The knr of all the complexes is suppressed in
rigid matrices (PMMA and PS), with quantum yields as high as 92%. The kr of the meridional
isomers in PMMA and PS are remarkably high. As observed with mer-Ir(pmp)3 in PS film, mer-
Ir(pmpz)3 has a shorter emission lifetime ( = 0.93 s) than its facial isomer (2.50 s), which
results in its higher kr = 9.16×

s
−
(versus 3.44×

s
−
for the facial isomer), and knr = 1.62×10
5

s
−
(versus knr = 0.56×10
5
s
−
for the facial isomer). At T = 77 K, the emission lifetimes of mer-
and fac-Ir(pmpz)3 in PS increased to  = 1.66 s , and 5.03 s, respectively. The emission lifetime
of fac-Ir(tpz)3 is slightly shorter than that of fac-Ir(pmpz)3 at room temperature and 77K. Table
3.3 summarizes the photophysical parameters of all the complexes.
Figure 3.4−Figure 3.6 show the PL spectra of diluted fac- and mer-Ir(pmpz)3, and fac-
Ir(tpz)3 in media of different polarity; methylcyclohexane (MeCy), toluene, 2-methyl
tetrahydrofuran (2-MeTHF), dichloromethane (CH2Cl2), and acetonitrile (MeCN). The PL spectra
of all the complexes exhibit bathochromic shift with increasing solvent polarity. mer-Ir(pmpz)3
shows the strongest solvatochromism and its nonradiative rate constant (knr) increases significantly
with increasing solvent polarity due to decrease in energy of transition (ΔE), consistent with energy
gap law (Figure 3.6d).
6
Similarly, the knr of the facial isomer increases from MeCN to 2-MeTHF,
albeit to a lesser degree. However, the knr increases from 2-MeTHF to MeCy in the case of fac-
Ir(pmpz)3, whereas the knr of fac-Ir(tpz)3 only increased from toluene to MeCy. This increase in knr
75

in toluene and MeCy of the facial isomers could be due to the increase in emission energies in
these solvents that could result in thermal population of the non-radiative metal centered ligand
field states (LF) (d-d transition) from their emitting states at room temperature.
7
The radiative rate
constant of all the complexes remains nearly the same regardless of the solvent media and hence
decrease in quantum yield is observed in polar solvents (Table 3.4−Table 3.6).
Table 3.3. Photophysical properties of Ir(C^C:)3 complexes in 2-MeTHF, PS and PMMA
2-MeTHF
298 K 77 K
 max (nm)  (%)  (s) k r (s
-1
) x10
5
k nr (s
-1
) x10
5
 max (nm)  (s)
fac-Ir(pmp) 3 417 76 1.2 6.4 2.0 393 3.9, 9.2
fac-Ir(pmpz) 3 475 87 2.5 3.5 0.52 447 7.5
fac-Ir(tpz) 3 481 98 2.0 4.9 0.10 458 4.36
mer-Ir(pmp) 3 465 78 0.8 10 2.7 413 1.0
mer-Ir(pmpz) 3 530 27 0.44 6.2 17 490 2.0
2wt% Polystyrene
fac-Ir(pmp) 3 420 78 1.10 7.09 2.00 --- 3.32
fac-Ir(pmpz) 3 460 86 2.50 3.44 0.560 --- 5.03
fac-Ir(tpz) 3 480 94 2.08 4.52 0.288 --- 3.97
mer-Ir(pmp) 3 440 80 0.637 12.6 3.14 --- 1.05
mer-Ir(pmpz) 3 490 85 0.928 9.16 1.62 --- 1.66
2wt% PMMA
fac-Ir(pmp) 3 420 54 1.34 4.03 3.43 --- ---
fac-Ir(pmpz) 3 470 82 1.80 4.6 1.0 --- ---
fac-Ir(tpz) 3 478 92 1.91 4.8 0.42 --- ---
mer-Ir(pmp) 3 445 66 0.704 9.38 4.83 --- ---
mer-Ir(pmpz) 3 500 74 0.47 16 5.5 --- ---

76

300 400 500 600
0
2
4

e (10
4
M
-1
cm
-1
)
Wavlength (nm)
MeCy
Toluene
2-MeTHF
CH
2
Cl
2
MeCN
*
*
(a)

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0

Normalized PL (a.u.)
Wavelength (nm)
MeCy
Toluene
2-MeTHF
CH
2
Cl
2
MeCN
(b)

400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL (a.u)
Wavelength (nm)
MeCy 298K
2-MeTHF 298K
MeCy 77K
2-MeTHF 77K
(c)

2.3 2.4 2.5 2.6 2.7 2.8
10
4
10
5
10
6
k
nr
(s
-1
)
E
00
(eV)
MeCN
CH
2
Cl
2
2-MeTHF
Toluene
MeCy
(d)

Figure 3.4. Photophysical properties of fac-Ir(pmpz)3 (a) UV-visible spectra in various solvents at
298K (*fac-Ir(pmpz)3 is highly insoluble in MeCy), (b) PL spectra in various solvents at 298K, (c)
PL spectra in MeCy and 2-MeTHF at 298K and 77K (d) Energy gap law plot at room temperature
Table 3.4. Photophysical properties of fac-Ir(pmpz)3 in various solvents at 298K and 77K.
Solvent (298 K)  (μs)  k r (s
-1
) x10
5
k nr (s
-1
) x10
5

MeCy 1.55 0.67 4.32 4.13
Toluene 1.85 0.73 3.95 1.46
2-MeTHF 2.5 0.87 3.48 0.52
CH 2Cl 2 0.793 0.29 3.66 8.95
MeCN 0.763 0.27 3.54 9.57
MeCy 77K 5.54 --- --- ---
2-MeTHF 77K 7.5 --- --- ---
77

300 400 500 600
0
2
4

e (10
4
M
-1
cm
-1
)
Wavlength (nm)
MeCy
Toluene
2Me-THF
CH
2
Cl
2
MeCN
(a)
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0

Normalized PL (a.u.)
Wavelength (nm)
MeCy
Toluene
2-MeTHF
CH
2
Cl
2
MeCN
(b)

400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL (a.u)
Wavelength (nm)
MeCy 298K
2-MeTHF 298K
MeCy 77K
2-MeTHF 77K
(c)

2.3 2.4 2.5 2.6 2.7 2.8
10
4
10
5
10
6
k
nr
(s
-1
)
E
00
(eV)
MeCN
CH
2
Cl
2
2-MeTHF
Toluene MeCy
(d)

Figure 3.5. Photophysical properties of fac-Ir(tpz)3 (a) UV-visible spectra in various solvents at
298K, (b) PL spectra in various solvents at 298K, (c) PL spectra in MeCy and 2-MeTHF at 298K
and 77K (d) Energy gap law plot at room temperature.
Table 3.5. Photophysical properties of fac-Ir(tpz)3 in various solvents at 298K and 77K.
Solvent (298 K)  (μs)  k r (s
-1
) x10
5
k nr (s
-1
) x10
5

MeCy 2.2 0.76 3.5 1.1
Toluene 2.1 0.89 4.2 0.52
2-MeTHF 2.0 0.98 4.9 0.98
CH 2Cl 2 1.1 0.45 4.1 5.0
MeCN 0.86 0.31 3.6 8.0
MeCy 77K 4.0 --- --- ---
2-MeTHF 77K 4.4 --- --- ---
78

300 350 400 450 500 550 600
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0

e (10
4
M
-1
. cm
-1
)
Wavelength (nm)
MeCy
Toluene
2-MeTHF
CH
2
Cl
2
MeCN
(a)

400 450 500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL (a.u)
Wavelength (nm)
MeCy
Toluene
2-MeTHF
CH
2
Cl
2
MeCN
(b)

400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL (a.u)
Wavelength (nm)
MeCy 298K
2-MeTHF 298K
MeCy 77K
2-MeTHF 77K
(c)
2.1 2.2 2.3 2.4 2.5
10
5
10
6
10
7
10
8
k
nr
(s
-1
)
E
00
(eV)
MeCN
CH
2
Cl
2
2-MeTHF
Toluene
MeCy
(d)

Figure 3.6. Photophysical properties of mer-Ir(pmpz)3 (a) UV-visible spectra in various solvents
at 298K, (b) PL spectra in various solvents at 298K, (c) PL spectra in MeCy and 2-MeTHF at
298K and 77K (d) Energy gap law plot at room temperature.
Table 3.6. Photophysical properties of mer-Ir(pmpz)3 in various solvents at 298K and 77K.
Solvent (298 K)  (μs)  k r (s
-1
) x10
5
k nr (s
-1
) x10
5

MeCy 0.89 76 8.50 2.68
Toluene 0.60 49 8.19 8.53
2-MeTHF 0.44 27 6.21 16.8
CH 2Cl 2 0.02 1.2 6.03 496
MeCN 0.015 0.8 5.44 675
MeCy 77K 1.61 --- --- ---
2-MeTHF 77K 2.00 --- --- ---

79

To obtain parameters governing the temperature dependent emission properties, emission
lifetimes of all the complexes in PS film were measured between 5 K and 300 K. The emission
spectra of mer-Ir(pmpz)3 slightly increase in intensity and display no changes in line shape with
decreasing temperature (Figure 3.7). In contrast, the emission spectra of fac-Ir(pmpz)3 and fac-
Ir(tpz)3 increase in intensity with decreasing temperature and show vibronic features, more
pronounced in fac-Ir(pmpz)3 at low temperatures (5−100 K). The onset of the emission spectra of
fac-Ir(tpz)3 blue shifts with increasing temperature, indicating thermal population to a higher lying
state at higher temperatures. Similar shift is observed in the emission spectra of fac-Ir(pmpz)3 and
mer-Ir(pmpz)3, but to a lesser degree. The PLQY of fac-Ir(tpz)3 at temperatures below 200 K
calculated from their integrated area are above 100%. Careful measurements of the temperature
dependent emission of fac-Ir(tpz)3 need to be carried out resolve this issue.
400 450 500 550 600 650 700
0
5
10
15
20
PL Intensity (a.u)
Wavelength (nm)
5K
10K
15K
20K
30K
40K
50K
60K
70K
80K
100K
150K
200K
250K
300K
fac-Ir(pmpz)
3
400 450 500 550 600 650 700
0
5
10
15
20
PL Intensity
Wavelength (nm)
7.5K
10K
20K
30K
40K
50k
60k
70K
80K
100K
150K
200K
300K
fac-Ir(tpz)
3

400 450 500 550 600 650 700
0
1
2
3
4
5
6
7
8
PL Intensity (a.u)
Wavelength (nm)
5K
10K
20K
50K
100K
200K
300K
mer-Ir(pmpz)
3

Figure 3.7. Emission spectra of 2 wt% fac- and mer-Ir(pmpz)3 and fac-Ir(tpz)3 in PS at various
temperatures.
80

The emission lifetime of fac-Ir(pmpz)3 and fac-Ir(tpz)3 increase abruptly upon decreasing
temperature from 300 K, until near 180 K, where the increase becomes more gradual, before
becoming nearly constant below 40 K. For fac-Ir(pmp)3, mer-Ir(pmp)3 and mer-Ir(pmpz)3, there is
a steep rise in the emission lifetime from 300 K to 70 K, 50 K and 40 K, respectively, before
becoming nearly constant below 30 K (Figure 3.8 and Figure 3.9). The increase in emission
lifetime upon cooling is attributed to depopulation of states at high energy that have radiative rate
constants faster than the lowest lying state. Under an assumption of a fast thermalization, the
temperature dependent decay curves were first fitted to the Boltzmann distribution equation with
a TADF model (see below).
𝜏 (𝑇 ) =
2 + 𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 + 𝑒𝑥 𝑝 −∆𝐸 (𝑆 1
−𝑇 1
)
𝑘 𝐵 𝑇 2
𝜏 (𝐼 , 𝐼𝐼 )
+
𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 𝜏 (𝐼𝐼𝐼 )
+
𝑒𝑥𝑝 −∆𝐸 (𝑆 1
−𝑇 1
)
𝑘 𝐵 𝑇 𝜏 (𝑆 1
)

All the temperature dependent decay curves fit well with the TADF model with R
2
value
> 0.98. The TADF parameters obtained from the fitting using this model are tabulated in Table
3.7. Among the facial isomers, fac-Ir(pmp)3 has the smallest energy difference between the
radiative singlet and the lowest triplet (EST) and fastest singlet lifetime (𝜏 (𝑆 1
) = 49 ns). These
observations agree with the fastest radiative rate observed for fac-Ir(pmp)3 (kr = 7.09×10
5

s
−
) among the facial isomers. The EST of fac-Ir(tpz)3 is larger (888 cm
−
) than that of fac-
Ir(pmpz)3 (746 cm
−
), but with faster singlet (𝜏 (𝑆 1
) = 50 ns for fac-Ir(tpz)3 and 𝜏 (𝑆 1
) = 74 ns for
fac-Ir(pmpz)3), giving the fac-Ir(tpz)3 overall faster radiative rate at room temperature (kr =
4.52×10
5
s
−
for fac-Ir(tpz)3 and kr = 3.44×10
5
s
−
for fac-Ir(pmpz)3). The emission lifetime of the
highest triplet substates of the facial isomers are faster than their lower lying substates (see Table
3.7), which is commonly observed for organometallic complexes. Although, the emission lifetime
81

of the lower lying states (e.g. 𝜏 (𝐼 , 𝐼𝐼 ) = 6.21 s for fac-Ir(pmpz)3) is smaller than highest triplet
substates, these values are surprisingly very small compared to the values reported for most
mononuclear organometallic emitters (e.g. 𝜏 (𝐼 ) = 116 s, 𝜏 (𝐼𝐼 ) = 6.4 s, 𝜏 (𝐼𝐼𝐼 ) = 0.2 s for
10 100
1.5
2.0
2.5
3.0
3.5
4.0
Data: Book1_B
Model: Boltzmann3_Ir
Weighting:
y No weighting

Chi^2/DoF = 0.00102
R^2 = 0.99852

P1 0.0208 ±0.00194
P2 0.0844 ±0.00898
P3 3.5917 ±0.01064
P4 0.8831 ±0.14095
P5 0.04929 ±0.01423

tau (us)
fit
Lifetime (s)
Temperature (K)
fac-Ir(pmp)
3
10 100
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Data: Book1_B
Model: Boltzmann3_Ir
Weighting:
y No weighting

Chi^2/DoF = 0.00139
R^2 = 0.9995

P1 0.01122 0.00036
P2 0.09248 0.0029
P3 6.21249 0.01466
P4 1.85705 0.04915
P5 0.07443 0.00664
tau (us)
fit
Lifetime (s)
Temperature (K)
fac-Ir(pmpz)
3
10 100
0.6
0.7
0.8
0.9
1.0
Data: Book1_B
Model: Boltzmann3_Ir
Weighting:
y No weighting

Chi^2/DoF = 0.00018
R^2 = 0.98879

P1 0.00169 ±0.01105
P2 0.01926 ±0.00267
P3 0.94195 ±0.01581
P4 0.9627 ±0.05075
P5 0.21095 ±0.00998
tau (us)
fit
Lifetime (s)
Temperature (K)
mer-Ir(pmp)
3
10 100
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Data: Book1_B
Model: Boltzmann3_Ir
Weighting:
y No weighting

Chi^2/DoF = 0.00015
R^2 = 0.99769

P1 4.5703E-6 0.00474
P2 0.01298 0.00365
P3 1.28642 268.84688
P4 3.24873 3440.76069
P5 0.34687 0.09004
tau (us)
fit
Lifetime (s)
Temperature (K)
mer-Irpmpz)
3

10 100
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0

Data: Book1_C
Model: Boltzmann3_Ir
Weighting:
y No weighting

Chi^2/DoF = 0.00217
R^2 = 0.99822

P1 0.01125 ±0.00061
P2 0.11011 ±0.00809
P3 4.72617 ±0.01826
P4 1.51697 ±0.06258
P5 0.05002 ±0.01251
tau (us)
fit
Lifetime (s)
Temperature (K)
fac-Ir(tpz)
3

Figure 3.8. Emission lifetime versus temperature of all the complexes fitted to a TADF model.
82

Ir(ppy)3).
8
Recently though, very short emission lifetimes of the lower lying triplet substates is
observed in a dinuclear Ir(III) complex (𝜏 (𝐼 ) = 8.2 s, 𝜏 (𝐼𝐼 ) = 7 s, 𝜏 (𝐼𝐼𝐼 ) = 0.05 s).
9
The value
of the zero field splitting (ZFS) obtained for fac-Ir(pmp)3 ((−) =  cm
−
) is largest among
the facial isomers, indicating stronger SOC in fac-Ir(pmp)3 that suggests faster forward and reverse
ISC rates, leading to the fastest TADF rate in this series of compounds. Similarly, the larger ZFS
of fac-Ir(tpz)3 ((−) =  cm
−
) compared to that of fac-Ir(pmpz)3 ((−) =  cm
−
) is
consistent with the faster TADF rate of fac-Ir(tpz)3 compared to that of fac-Ir(pmpz)3. These values
are comparable to the Ir(III) based organometallic emitters (e.g. (−) =  cm
−
for
Ir(ppy)3).
8

The EST of the meridional isomers is exceedingly small, smaller than those of the facial
isomers and most of the efficient TADF emitters, which could be the reason why the meridional
isomers have extraordinary fast radiative rate (1.3×10
6
s
−
) at ambient conditions. Small EST (200
cm
−
) values and fast radiative rates (2×10
6
s
−
) were also observed in TADF linear 2-coordinate
silver complexes.
10
The value of EST obtained from the fit is larger for mer-Ir(pmp)3 (EST = 156
cm
−
) than for mer-Ir(pmpz)3 (EST = 105 cm
−
) which is in disagreement with their radiative rates
measured at room temperature. The fluorescence lifetimes of the meridional isomers are 211 and
347 ns for mer-Ir(pmp)3 and mer-Ir(pmpz)3, respectively. These values are significantly long for
materials with high extinctions (> 10 000 M
−
cm
−
). Additionally, the fits show that the highest
triplet substates of the meridional isomers are slower (e.g. 𝜏 (𝐼𝐼𝐼 ) = 3.25 s for mer-Ir(pmpz)3) than
their lower lying substates (e.g. 𝜏 (𝐼 , 𝐼𝐼 ) = 1.29 s for mer-Ir(pmpz)3), which is extremely rare for
organometallic complexes. The ZFS of the meridional isomers obtained using this model are small
((−)   cm
−
). These values are expected to be larger considering the large SOC factor of
83

iridium atom. Because of the abnormalities in the TADF parameters of the meridional isomers
obtained using the TADF model, emission out of the triplet of these materials cannot be
disregarded. To check whether the emission of these materials is governed by the lowest triplet
state, the temperature dependent decay curves were fitted to the Boltzmann distribution equation
with a triplet model (see equation below).
𝜏 (𝑇 ) =
1 + 𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 + 𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 1
𝜏 (𝐼 )
+
1
𝜏 (𝐼𝐼 )
𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 +
1
𝜏 (𝐼𝐼𝐼 )
𝑒𝑥𝑝 −∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇
All the temperature dependent decay curves fit well with the triplet model with R
2
value >
0.98 (Figure 3.9). Using this model, the values of the ZFS for the meridional complexes are more
reasonable for mononuclear Ir(III) based emitters than for the values obtained using the TADF
model above. The highest triplet substates of the meridional isomers are faster than their lower
laying substates which is more reasonable than what is observed in the TADF model above. The
larger ZFS ((−) =  cm
−
) and faster triplet substates of mer-Ir(pmp)3 relative to those of
mer-Ir(pmpz)3 ((−) =  cm
−
) is consistent with the faster radiative rate of mer-Ir(pmp)3
over mer-Ir(pmpz)3 at room temperature. The values for the 𝜏 (𝐼𝐼𝐼 ) (𝜏 (𝐼𝐼𝐼 ) = 0.268 s for mer-
Ir(pmp)3 and 𝜏 (𝐼𝐼𝐼 ) = 0.440 s, for mer-Ir(pmpz)3) are commonly observed for mononuclear
Ir(III) emitters, whereas the lower lying substates are extremely fast (𝜏 (𝐼 ) = 0.94 s, 𝜏 (𝐼𝐼 ) = 0.95
s) for mer-Ir(pmp)3 and 𝜏 (𝐼 ) = 1.17 s, 𝜏 (𝐼𝐼 ) = 2.25 s for mer-Ir(pmpz)3) which is surprising
for mononuclear Ir(III) organometallic emitters (See Table 3.8). The small values of the lower
laying triplet substates are probably the reason for the fast radiative rate of the meridional isomers
observed at room temperature.
84

The temperature dependent decay curves of the facial isomers were also fitted to the
Boltzmann distribution equation with a triplet model (Figure 3.9). The ZFS of the facial isomers
are exceptionally large and are in increasing order fac-Ir(pmp)3 ((−) =  cm
−
) < fac-
Ir(pmpz)3 ((−) = 4 cm
−
)  fac-Ir(tpz)3 ((−) =  cm
−
) Their triplet substates are
extremely fast with fac-Ir(pmp)3 having the fastest (𝜏 (𝐼 ) = 3.59 s, 𝜏 (𝐼𝐼 ) = 1.30 s, 𝜏 (𝐼𝐼𝐼 ) = 0.074
s) followed by fac-Ir(tpz)3 (𝜏 (𝐼 ) = 4.72 s, 𝜏 (𝐼𝐼 ) = 2.06 s, 𝜏 (𝐼𝐼𝐼 ) = 0.080 s) and then fac-
Ir(pmpz)3 (𝜏 (𝐼 ) = 6.21 s, 𝜏 (𝐼𝐼 ) = 2.56 s, 𝜏 (𝐼𝐼𝐼 ) = 0.114 s) (Table 3.8). The order in ZFS values
disagrees with the fastest radiative rate of fac-Ir(pmp)3, but the fastest emission lifetime of the
triplet substates of fac-Ir(pmp)3 could be the reason why it has the fastest radiative rate at room
temperature followed by fac-Ir(tpz)3 and fac-Ir(pmpz)3. The energy gaps between the lowest triplet
substate and the middle substate are also surprisingly large for the facial isomers. This along with
the extraordinary large ZFS of the facial isomers questions whether the emission of the facial
isomers is out of their triplet states. Detailed computational study and magnetic measurements at
variable temperature need to be conducted to check whether the emission of the facial isomers at
room temperature is from their triplet or singlet manifold. Additionally, the initial assumption that
under a fast thermalization, the temperature dependent decay curves were fitted to the Boltzmann
distribution equation with a TADF and triplet models without taking account of the changes in
quantum yields might not be holding true. This is because of the small changes in emission
lifetimes between 300 K and 5 K, so small changes in quantum yields might affect the outcome of
the parameters using these models.

85

10 100
1.5
2.0
2.5
3.0
3.5
4.0
Data: Book1_B
Model: Boltzmann3_Ir_2
Weighting:
y No weighting

Chi^2/DoF = 0.00105
R^2 = 0.99848

P1 0.02168 ±0.00191
P2 0.08643 ±0.00861
P3 3.59074 ±0.01073
P4 1.30236 ±0.16671
P5 0.07443 ±0.02006
tau (us)
fit
Lifetime (s)
Temperature (K)
fac-Ir(pmp)
3
10 100
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Data: Book1_B
Model: Boltzmann3_Ir_2
Weighting:
y No weighting

Chi^2/DoF = 0.00169
R^2 = 0.99939

P1 0.01208 ±0.0004
P2 0.09288 ±0.00299
P3 6.2085 ±0.01599
P4 2.56119 ±0.05861
P5 0.11363 ±0.01024
tau (us)
fit
Lifetime (s)
Temperature (K)
fac-Ir(pmpz)
3
10 100
0.6
0.7
0.8
0.9
1.0
Data: Book1_B
Model: Boltzmann3_Ir_2
Weighting:
y No weighting

Chi^2/DoF = 0.00018
R^2 = 0.98862

P1 0.00096 ±0.01222
P2 0.01991 ±0.00527
P3 0.94066 ±0.04758
P4 0.95273 ±0.04185
P5 0.26854 ±0.01114
tau (us)
fit
Lifetime (s)
Temperature (K)
mer-Ir(pmp)
3
10 100
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Data: Book1_B
Model: Boltzmann3_Ir_2
Weighting:
y No weighting

Chi^2/DoF = 0.00017
R^2 = 0.99746

P1 5.6265E-7 ±0.00091
P2 0.0135 ±0.00279
P3 1.17456 ±519.99528
P4 2.55368 ±2462.55544
P5 0.44031 ±0.10092
tau (us)
fit
Lifetime (s)
Temperature (K)
mer-Ir(pmpz)
3
10 100
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Data: Book1_C
Model: Boltzmann3_Ir_2
Weighting:
y No weighting

Chi^2/DoF = 0.00253
R^2 = 0.99793

P1 0.01212 ±0.00066
P2 0.10894 ±0.00809
P3 4.72334 ±0.01953
P4 2.06482 ±0.07201
P5 0.08006 ±0.01975
tau (us)
fit
Lifetime (s)
Temperature (K)
fac-Ir(tpz)
3

Figure 3.9. Emission lifetime versus temperature of all the complexes fitted to a triplet model.
86

Table 3.7. Photophysical parameters obtained from Boltzmann fits to temperature-dependent
lifetimes using the TADF model
E III-I (eV, cm
-1
) E S1-I (eV, cm
-1
)  I,II (s)  III (s)  S1 (ns)
fac-Ir(pmp) 3 0.0208, 168 0.0844, 681 3.59 0.88 49
fac-Ir(pmpz) 3 0.0112, 90.3 0.0925,746 6.21 1.86 74
fac-Ir(tpz) 3 0.0125, 101 0.110, 888 4.73 1.52 50
mer-Ir(pmp) 3 0.0017, 13.6 0.0193, 156 0.94 0.96 211
mer-Ir(pmpz) 3 4.6 x 10
-6
, 0.037 0.013, 105 1.29 3.25 347

Table 3.8. Photophysical parameters obtained from Boltzmann fits to temperature-dependent
lifetimes using the triplet model
E III-II (eV, cm
-1
) E III-I (eV, cm
-1
)  I (s)  II (s)  III (ns)
fac-Ir(pmp)
3
0.0217, 175 0.0864, 697 3.59 1.30 74
fac-Ir(pmpz)
3
0.0121, 97.6 0.0929,749 6.21 2.56 114
fac-Ir(tpz)
3
0.0121, 97.6 0.109, 879 4.72 2.06 80
mer-Ir(pmp)
3
0.00096, 7.74 0.0199, 161 0.94 0.95 268
mer-Ir(pmpz)
3

5.6 x 10
-7
, 0.0045
0.0135, 109 1.17 2.55 440

3.2.4. Electroluminescence Properties
Because of the low PL efficiency of fac-Ir (pmpz)3 in mCBP (PL = 50%) and its tedious
and low yielding synthesis, fac-Ir (pmpz)3 was not considered for OLED studies. Even though mer-
Ir(pmpz)3 has moderate PL efficiency in mCBP (PL = 70%), its tedious synthesis and green
emission spectrum make it unsuitable for blue OLED studies. fac-Ir(tpz)3 was designed to slightly
lower the triplet energy of fac-Ir(pmpz)3 so it could be efficiently hosted by stable host materials.
87

The high PL efficiency of fac-Ir(tpz)3 in mCBP (PL = 90%), its sky-blue emission and scalable
and high yielding synthesis make it ideal for blue OLED studies.
To investigate the electroluminescence properties of fac-Ir(tpz)3, monochromatic devices
of fac-Ir(tpz)3 as a blue dopant were fabricated at University of Michigan Ann Arbor. Figure 3.10
shows the monochromatic device architecture of fac-Ir(tpz)3 as a dopant along with the
electroluminescence properties of the OLED devices. In these devices, fac-Ir(tpz)3 was used as a
blue dopant and mCBP and txI as hosts in the emissive layer. These devices are designed to achieve
high efficiencies, and thus high triplet energy and unstable transport materials were used.
The electroluminescence (EL) spectrum of fac-Ir(tpz)3 in all devices is the same with its
PL spectrum in thin films, indicating effective exciton confinement on the emitter. The current
density–voltage (J–V) characteristics of the device reveals that changing doping concentration do
not affect the conductivity of the device, whereas changing the host affects the conductivity. These
observations suggest that the charges are mostly carried by the host materials. Devices with txI
were not optimized and hence have lower efficiency than mCBP based devices. The maximum
efficiency (17%) was achieved by the optimized mCBP device. Devices with FIrpic in a similar
architecture
11
show similar electroluminescence (CIE = 0.15, 0.29), device efficiency,
conductivity and brightness. As a result, fac-Ir(tpz)3 can be a good replacement or alternative to
FIrpic as dopant for high efficiency sky-blue OLEDs.
To fabricate stable OLED devices with fac-Ir(tpz)3, the unstable transport materials in the
efficiency devices are replaced with more stable transport materials. And because these stable
transport materials have low triplet energies, stable and high triplet energy exciton blockers on
either side of the emissive layer were inserted. Two devices were fabricated by changing the
exciton blockers on the EML/ETL interface. The two exciton blockers, T2T and MTP have very
88

deep HOMO energies, ideal for blocking holes into the EML (Figure 3.11).
Similar to the efficiency devices, the EL spectra of both T2T and MTP based devices
indicate exciton confinement on the dopant (Figure 3.12). The two devices have the same J−V
properties and is similar to the efficiency devices. The maximum EQE achieved with the stable
devices is 12% which is lower than that of the efficiency devices. This drop in efficiency is
probably due to exciton leakage from the emissive to either one of the transport layer. The lifetime
(T80) measured at 1000 nits for T2T based device is eight hours, whereas the T80 for MTP based
device was less than one hour. One of the best lifetimes (T80) for blue PHOLEDs reported in
literature
12
is 56 hours and thus the lifetime of fac-Ir(tpz)3 needs to be improved for practical
application. One of the reasons for the short device lifetime might be due to instability of the hole
blocking materials. Both T2T and MTP have not been tested for device lifetime and therefore the
instability might be due to degradation of these materials. To our knowledge, there are no stable
blue OLED devices reported with deep HOMO (~6.7 eV) hole blocking materials. We are
currently developing stable hole blocking materials with deep HOMO levels.

89

450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Normalized EL (a.u)
Wavelength (nm)
mCBP 7%
mCBP 10%
txT 7%

0 5 10 15
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
EQE (%)
J
OLED
(mA/cm
2
)

Figure 3.10. Device architecture of fac-Ir(tpz)3 as a dopant in OLEDs targeted for efficiency
along with the electroluminescence properties of the OLED device.

Figure 3.11. Device architecture of fac-Ir(tpz)3 as dopant in OLED targeted for stability along
chemical structures of hole blocking materials.
90

450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Normalized EL (a.u)
Wavelength (nm)
T2T HBL
MTP HBL

0 5 10 15
1E-7
1E-5
0.001
0.1
10
1000
J
OLED
(mA/cm
2
)
V
OLED
(V)
T2T
MTP

0.01 0.1 1 10 100
0
5
10
15
20
EQE (%)
J
OLED
(mA/cm
2
)
T2T
MTP

Figure 3.12. Electroluminescence properties of fac-Ir(tpz)3 as a dopant in OLEDs targeted for
stability.
3.3. Conclusion
In this work, we developed three tris-Ir(III) carbene complexes featuring
pyrazinoimidazolyl cyclometalating ligand (fac- and mer-Ir(pmpz)3 and fac-Ir(tpz)3) as potential
blue dopants for OLED applications. Computational, electrochemical and photophysical studies
of these complexes along with the previously reported deep blue emitting tris-Ir(III) carbene
complexes featuring pyridinoimidazolyl ligand (fac- and mer-Ir(pmp)3) were carried out to
understand their extraordinary excited properties. The meridional isomers have minimal overlap
between their HOMO and LUMO, resulting in small calculated energy difference between their
91

singlet and triplet levels (EST) and low oscillator strengths. The frontier orbitals of the facial
isomers have larger overlap, leading to larger EST and oscillator strengths than the meridional
isomers. The HOMO (~5.4 eV) and LUMO (~2.1 eV) levels of the pyrazine analogs are ideal for
charge injection. The absorption and emission spectra of the pyazine analogs are redshifted
compared to the pyridine analogs. All the complexes have high efficiencies (> 92%) in polystyrene
matrices and non-polar solvents but decreases with increasing solvent polarity due to increase in
nonradiative rates, consistent with energy gap low. Their radiative rates are however nearly
unaffected by solvent polarity.
Variable temperature (VT) photophysical study was carried out to understand the
extraordinary fast radiative rates of the meridional isomers in polystyrene (1.3×10
6
s
−1
) at room
temperature. The decays curves obtained from the VT photophysics were fitted to Boltzmann
distribution using a TADF and a triplet model. The decay curves of the facial isomers fitted to a
TADF model give parameters that are consistent with organometallic TADF materials. This
includes small EST (e.g. ~746 cm
−
), high ZFS (e.g. ~90 cm
−
) and extremely fast singlet and
triplet emission lifetimes (e.g. S1 = 74 ns, III = 1.86 s). Fitting the decay curves of the facial
isomers with a triplet model however resulted in abnormally large ZFS (e.g. ~749 cm
−
) with
extremely fast triplet emission lifetimes (e.g. III = 0.114 s, II = 2.56 s, I = 6.21 s). The decay
curves of the meridional isomers fitted with a TADF model give extremely small EST (e.g. ~156
cm
−
), consistent with their small orbital overlaps. Their ZFS (e.g. ~14 cm
−
) is however too small
and is inconsistent with tris-Ir(III) complexes. Fitting the decay curves of the meridional isomers
with a triplet model give reasonable ZFS (e.g. ~161 cm
−
) and extremely fast emission lifetimes
of the triplet substates (e.g. III = 0.268 s, II = 0.95 s, I = 0.94 s), consistent with their fast
92

radiative rates at room temperature. This data suggests that the emission of the meridional isomers
likely originates from their triplet manifold, whereas the emission of the facial isomers likely
originates from a TADF process. Further studies, including computational and magnetic studies at
variable temperature need to be conducted to prove these hypotheses.
Finally, fac-Ir(tpz)3, the most promising complex is employed as a blue emissive dopant in
OLEDs. Optimized blue PHOLED devices using this material achieved excellent
electroluminescence efficiency (~18%), high brightness (35 000 cd/m
2
) at low current density and
moderate device lifetime (T80 = 16 h). Further OLED studies will be carried out with fac-Ir(tpz)3
to improve the operational lifetime of the OLED.

93

3.4. Experimental
3.4.1. Synthesis
Synthesis of N2-phenylpyrazine-2,3-diamine (2)

To a one-necked 500-mL round bottom flask, 3-chloropyrazin-2-amine (10.00 g, 77.19 mmol)
and aniline (7.19 g, 77.19 mmol) were added and heated for 24 hours at 100 ℃. After cooling to
room temperature, the reaction mixture was dissolved in water and brought to a pH of 7 using a
25% solution of sodium bicarbonate. The product was extracted using dichloromethane (CH2Cl2).
The resulting organic phases were further dissolved in CH2Cl2 by vigorous stirring overnight. The
resulting solution is dried over sodium sulfate and concentrated to dryness. The residue was
washed with diethyl ether resulting in a brownish yellow powder. Yield: 12.22 g, 85%. Spectra
match those reported in the literature.
13

Synthesis of 1-phenyl-1H-imidazo[4,5-b]pyrazine (3)

A solution of N2-phenylpyrazine-2,3-diamine (10.00 g, 53.70 mmol) in formic acid (324 mL,
8.60 mol) is refluxed for 24 hours. The reaction mixture is evaporated to dryness and the residue
is dissolved in CH2Cl2 and extracted with saturated sodium bicarbonate solution. The organic
phase is dried with sodium sulfate and evaporated to dryness. Yield: 9.49 g, 90%.
1
H NMR (400
94

MHz, Chloroform-d) δ 8.63 (s, 1H), 8.61 (d, J = 2.6 Hz, 1H), 8.41 (d, J = 2.6 Hz, 1H), 7.76 – 7.71
(d, 2H), 7.58 (t, 1H), 7.47 (t, 1H).
13
C NMR (101 MHz, cdcl3) δ 149.38, 145.77, 140.70, 139.71,
139.23, 134.44, 130.02, 128.46, 123.20.
Synthesis of 1-methyl-3-phenyl-1H-imidazo[4,5-b]pyrazin-1-ium iodide (4)

In a 500ml pressure flask, 1-phenyl-1H-imidazo[4,5-b]pyrazine (8.00 g, 40.77 mmol) and
methyl iodide (5.79 g, 40.77 mmol) were dissolved in 100 mL THF and stirred at 60
o
C for 72
hours. Afterwards the reaction mixture is cooled down to room temperature and the precipitate is
filtered, washed with diethyl ether and dried under vacuo. According to
1
H-NMR and
13
C-NMR,
methylation occurred on the imidazole nitrogen and one of the pyrazino nitrogens. These products
were not separated and are carried to the next step without further purification. Yield = 8.34 g,
60%.
1
H NMR (400 MHz, DMSO-d6) δ 10.74 (s, J = 0.7 Hz, 1H), 9.86 (s, 1H), 9.16 – 9.11 (m,
2H), 9.02 (d, J = 2.6 Hz, 1H), 8.97 (d, J = 2.6 Hz, 1H), 7.94 – 7.88 (m, 4H), 7.79 – 7.59 (m, 6H),
4.60 – 4.48 (s, 3H), 4.17 (s, J = 0.6 Hz, 3H).
13
C NMR (101 MHz, dmso) δ 154.08, 146.68, 146.12,
144.04, 144.03, 141.32, 138.02, 137.57, 133.36, 132.81, 132.51, 130.97, 130.58, 130.45, 130.05,
125.15, 124.60, 42.11, 32.79.
95

Synthesis of mer-tris-(N-phenyl,N-methyl-pyrizinoimidazol-2-yl)iridium (III) (mer-
Ir(pmpz)3)

To a 250 ml round bottomed one necked flask, 1-methyl-3-phenyl-1H-imidazo[4,5-b]pyrazin-
1-ium iodide (9.06 g, 26.8 mmol), bis(1,5-cyclooctadiene)diiridium(I) dichloride (3.00 g, 4.47
mmol) and silver (I) oxide (0.621 g, 2.68 mmol) were added. The reaction flask is purged with
nitrogen and evacuate three times. 100 mL of degassed chlorobenzene and triethylamine (0.271 g,
2.68 mmol) were added to the flask. The reaction mixture is stirred in the dark at 120
o
C for 24
hours. After the reaction is complete, chlorobenzene is removed under reduced pressure and the
residue is purified by column chromatography on silica using hexane and ethyl acetate (6:4) as the
eluent. Yield = 736 mg, 20%.
1
H NMR (400 MHz, Chloroform-d) δ 8.71 (ddd, J = 7.9, 1.3, 0.5
Hz, 1H), 8.67 (ddd, J = 7.9, 1.3, 0.5 Hz, 1H), 8.61 (ddd, J = 7.9, 1.3, 0.5 Hz, 1H), 8.37 (d, J = 2.8,
2H), 8.32 (d, J = 2.9 Hz, 1H), 8.29 (d, J = 2.9 Hz, 2H), 8.23 (d, J = 2.9 Hz, 1H), 7.12 – 7.01 (m,
3H), 6.95 – 6.89 (m, 2H), 6.82 (td, J = 7.3, 1.2 Hz, 1H), 6.75 (m, 2H), 6.57 (dd, J = 7.3, 1.2, 1H),
3.39 (s, 3H), 3.37 (s, 3H), 3.27 (s, 3H).
13
C NMR (101 MHz, cdcl3) δ 193.87, 190.47, 189.65,
148.00, 147.24, 146.90, 145.68, 145.50, 143.20, 142.07, 142.00, 141.85, 139.82, 139.79, 139.75,
139.04, 138.52, 137.61, 137.32, 136.69, 136.64, 136.05, 135.68, 126.14, 126.03, 125.82, 122.10,
121.61, 121.48, 115.18, 115.03, 114.56, 32.19, 32.11, 31.22.
96

Synthesis of fac-tris-(N-phenyl,N-methyl-pyrizinoimidazol-2-yl)iridium (III) (fac-
Ir(pmpz)3)

mer-tris-(N-phenyl,N-methyl-pyrizinoimidazol-2-yl)iridium (III) (0.10 g, 0.121 mmol) is
added to a 100 mL pressure tube under nitrogen atmosphere. Degassed ethyl acetate (12 mL) and
2 mL of 1M aqueous solution of malonic acid were added to the tube. The tube is sealed and stirred
for 6 days at 65
o
C. The reaction is extracted with CH2Cl2 and purified on silica with hexanes/ethyl
acetate mixture (50/50). Yield = 25 mg, 25%.
1
H NMR (400 MHz, Chloroform-d) δ 8.67 (dd, J =
7.9, 0.8 Hz, 1H), 8.33 (d, J = 2.9 Hz, 1H), 8.22 (d, J = 2.9 Hz, 1H), 7.13 (td, J = 7.9, 7.3, 1.5 Hz,
1H), 6.81 (td, J = 7.3, 1.3 Hz, 1H), 6.57 (dd, J = 8.8, 1.7 Hz, 1H), 3.41 (s, 3H).
13
C NMR (101
MHz, cdcl3) δ 194.83, 146.73, 144.56, 141.85, 139.85, 137.43, 136.37, 136.25, 126.05, 122.25,
114.66, 32.14.
Synthesis of N2,N3-di-p-tolylpyrazine-2,3-diamine (5)

97

To a 100-mL round bottom flask, 2,3-dichloropyrazine (10.00 g, 67.13 mmol) and p-
toluidine (15.82 g, 147.68 mmol) were added and heated overnight at 100 ℃. After cooling to
room temperature, the reaction mixture was dissolved in water and brought to a pH of 7 using a
25% solution of sodium hydroxide. The product was extracted using dichloromethane. The
resulting organic phases were dried over sodium sulfate and concentrated to dryness. The residue
was washed with petroleum ether resulting in an off-white powder. Yield: 10.124 g, 53%.
1
H NMR
(400 MHz, DMSO-d6) δ 9.83 – 9.49 (b, 2H), 7.60 (d, J = 8.4 Hz, 4H), 7.41 (s, 2H), 7.16 (d, J =
8.4 Hz, 4H), 2.27 (s, 6H).
13
C NMR (101 MHz, dmso) δ 141.31, 137.02, 132.75, 129.69, 127.04,
121.63, 20.96.
Synthesis of 2-ethoxy-1,3-di-p-tolyl-2,3-dihydro-1H-imidazo[4,5-b]pyrazine (6)

The 2,3-bis(N-phenylamino)pyrazine (4.00 g, 13.8 mmol) was placed in a flask with
HC(OEt)3 (70 mL, 0.415 mol) that acted as both the solvent and reactant. The reaction was stirred
overnight at 80 ℃. In the morning, the reaction was cooled to room temperature and the mixture
was filtered. A large amount of white powder was left in the filter. The filtrate was rotavaped down
and recrystallized using petroleum ether. The product was allowed to dry over the weekend to
ensure it was dry enough for the next step. Yield: 3.33 g, 70%.
1
H NMR (400 MHz, Chloroform-
d) δ 7.87 (d, J = 2.1 Hz, 4H), 7.49 (s, 2H), 7.24 (d, J = 2.1 Hz, 4H), 7.19 (s, 1H), 3.33 (q, J = 7.0
98

Hz, 2H), 2.36 (s, 6H), 1.07 (t, J = 7.0 Hz, 3H).
13
C NMR (101 MHz, cdcl3) δ 143.63, 135.36,
133.86, 130.37, 129.75, 118.90, 96.29, 55.17, 20.89, 14.51.
Synthesis of fac-tris-(N-tolyl,N-tolyl-pyrizinoimidazol-2-yl)iridium (III) (fac-Ir(tpz)3)

To a 150-mL round bottom flask, molecular sieves (3 Å , 5 g), bis(1,5-
cyclooctadiene)diiridium(I) dichloride (0.60 g, 0.89 mmol), and 2-ethoxy-1,3-di-p-tolyl-2,3-
dihydro-1H-imidazo[4,5-b]pyrazine (3.09 g, 8.93 mmol) were added to the flask. The flask was
pumped and purged three times. O-xylene (120 mL) purged with nitrogen gas was added to the
reaction flask and heated to 115 ℃ for 24 hr. The product was purified on silica using hexane and
dichloromethane. The product was obtained and recrystallized in dichloromethane. Yield: 0.43 g,
44%.
1
H NMR (400 MHz, Chloroform-d) δ 8.60 (d, J = 8.0 Hz, 1H), 8.30 (d, J = 2.9 Hz, 1H), 8.02
(d, J = 2.9 Hz, 1H), 6.96 (ddd, J = 8.0, 2.0, 0.8 Hz, 1H), 6.56 – 6.50 (m, 1H), 2.13 (s, 3H), 1.88 (s,
3H), 1.55 (d, J = 40.0 Hz, 3H).
13
C NMR (101 MHz, Chloroform-d) δ 194.57, 145.41, 144.19,
143.04, 139.76, 138.96, 137.01, 135.96, 135.16, 132.94, 128.63, 122.79, 114.55, 21.63, 20.94.
99

3.4.2. NMR Data

Figure 3.13.
1
H-NMR of 3 in Chloroform-d.

Figure 3.14.
13
C-NMR of 3 in Chloroform-d.
100

Figure 3.15.
1
H-NMR of 4 in DMSO-d6.

Figure 3.16.
13
C-NMR of 4 in DMSO-d6.
101

Figure 3.17.
1
H-NMR of mer-Ir(pmpz)3 in Chloroform-d.

Figure 3.18.
13
C-NMR of mer-Ir(pmpz)3 in Chloroform-d.
102

Figure 3.19.
1
H-NMR of fac-Ir(pmpz)3 in Chloroform-d.

Figure 3.20.
13
C-NMR of fac-Ir(pmpz)3 in Chloroform-d.
103

Figure 3.21.
1
H-NMR of 5 in DMSO-d6.

Figure 3.22.
13
C-NMR of 5 in DMSO-d6.
104

Figure 3.23.
1
H-NMR of 6 in Chloroform-d.

Figure 3.24.
13
C-NMR of 6 in Chloroform-d.
105

Figure 3.25:
1
H-NMR of fac-Ir(tpz)3 in Chloroform-d.

Figure 3.26.
13
C-NMR of fac-Ir(tpz)3 in Chloroform-d.
106

Figure 3.27. Variable temperature
1
H-NMR of fac-Ir(tpz)3 in Benzene-d6.

Figure 3.28. Variable temperature
1
H-NMR of fac-Ir(tpz)3 in Acetone-d6.
107

3.4.3. Mass Spectra Data

Figure 3.29. MALDI spectrum of mer-Ir(pmpz)3.

Figure 3.30. MALDI spectrum of fac-Ir(pmpz)3.
108

Figure 3.31. MALDI spectrum of 5.

Figure 3.32. MALDI spectrum of 6.
109

Figure 3.33. MALDI spectrum of fac-Ir(tpz)3.
3.5. References
1. J. Lee, H.-F. Chen, T. Batagoda, C. Coburn, P. I. Djurovich, M. E. Thompson and S. R.
Forrest, Nature Materials, 2016, 15, 92-98.
2. C. Heinemann and W. Thiel, Chemical Physics Letters, 1994, 217, 11-16.
3. C. Heinemann, T. Müller, Y. Apeloig and H. Schwarz, Journal of the American Chemical
Society, 1996, 118, 2023-2038.
4. D. A. Dixon and A. J. Arduengo, The Journal of Physical Chemistry, 1991, 95, 4180-
4182.
5. B. K. T. Batagoda, Doctor of Philosophy, University of Southern California, 2017.
6. J. V. Caspar and T. J. Meyer, The Journal of Physical Chemistry, 1983, 87, 952-957.
7. T. Sajoto, P. I. Djurovich, A. B. Tamayo, J. Oxgaard, W. A. Goddard and M. E.
Thompson, Journal of the American Chemical Society, 2009, 131, 9813-9822.
8. H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T. Fischer, Coordination
Chemistry Reviews, 2011, 255, 2622-2652.
9. M. Z. Shafikov, R. Daniels and V. N. Kozhevnikov, The Journal of Physical Chemistry
Letters, 2019, 10, 7015-7024.
10. R. Hamze, S. Shi, S. C. Kapper, D. S. Muthiah Ravinson, L. Estergreen, M.-C. Jung, A.
C. Tadle, R. Haiges, P. I. Djurovich, J. L. Peltier, R. Jazzar, G. Bertrand, S. E. Bradforth
and M. E. Thompson, Journal of the American Chemical Society, 2019, 141, 8616-8626.
11. M. Idris, C. Coburn, T. Fleetham, J. Milam-Guerrero, P. I. Djurovich, S. R. Forrest and
M. E. Thompson, Materials Horizons, 2019, 6, 1179-1186.
12. Y. Zhang, J. Lee and S. R. Forrest, Nature Communications, 2014, 5, 5008.
110

13. A. Freitag, P. Prajwal, A. Shymanets, C. Harteneck, B. Nürnberg, C. Schächtele, M.
Kubbutat, F. Totzke and S. A. Laufer, Journal of Medicinal Chemistry, 2015, 58, 212-
221.
111

4CHAPTER 4 − Electrochemical and Photophysical Studies of Highly Efficient
Deep Blue Luminescence 2-coordinate Coinage Metal Complexes Bearing
Bulky NHC Benzimidazolyl Carbene
4.1. Introduction
Recently, our group reported highly luminescent, neutral two-coordinate, linear d
10
metal
complexes of coinage metals i.e., Cu(I), Ag(I), Au(I).
1-6
These complexes bear redox active
carbene (acceptor) and amide (donor) ligands connected by a metal in a linear fashion i.e.
(carbene)M
(I)
(amide). Their photoluminescence efficiencies are high, approaching 100% in
solution and in the solid state, with phosphorescent lifetimes in the range 1−3 s. Because of their
excellent photophysical properties, in particular, their fast radiative lifetimes make them promising
candidates as dopants in organic light emitting diodes (OLEDs).
1-6
The emission energy of the
complexes, although nearly independent of the metal, can be altered using carbene with varied
electrophilicity, allowing the luminescence color to be varied from deep blue to deep red.
1,
4
Developing alternative blue dopants is crucial in tackling the long-standing problem of stability
in blue OLEDs. Complexes based on the CAAC ligand (M
CAAC
) are reported to have efficient blue
photoluminescence and give good efficiencies as dopants in OLEDs.
4
Unfortunately, the emission
spectra of these complexes are broad, which is not ideal for display applications. In addition, the
ability to vary the physical and electronic properties of CAAC ligands is limited and therefore
inconvenient for modifying the characteristics needed for OLEDs. Complexes based on the MAC
ligand and cyano carbazole ligand also has broad blue emission.
1
Alternatively, carbenes based
on benzoimidazoles (BZI), originally used in luminescent two-coordinate Au complexes,
7
lead to
metal complexes with LUMO energies similar to CAAC,
8, 9
suggesting that replacing CAAC with
benzoimidazolyl-carbene ligands should give similar photophysical and electrochemical
properties as M
CAAC
. Synthesis and preliminary photophysics on monovalent, linear, 2-coordinate
112

coinage metal (i.e. M = Cu, Ag, Au) complexes (M
BZI
) bearing a sterically bulky benzimidazolyl
carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (BZI), and carbazolide
(Cz) as the anionic ligand were reported in earlier thesis (Figure 4.1). Their deep blue and narrow
emission prompted us to carefully study their photophysical and electrochemical properties in
depth. Understanding their excited dynamics could give us idea how to design compounds with
similar deep blue and narrow emission which are crucial for display application. Therefore, in this
chapter we have investigated the structural and photophysical properties of the M
BZI
derivatives
to investigate the role of the carbene and the metal ion in the excited-state properties. The M
BZI

complexes have structures, redox potentials and photoluminescent efficiencies (PL = 0.8−1.0)
similar to the M
CAAC
analogs, but different excited-state dynamics. Analysis of the luminescence
at low temperature reveals that the triplet carbazole (
3
Cz) and intramolecular singlet/triplet charge
transfer (
1/3
ICT) manifolds of the M
BZI
complexes are near degenerate, resulting in photophysical
properties that are distinct from the M
CAAC
complexes. The Au
BZI
complex has also been
successfully employed as dopant to fabricate efficient blue OLEDs.

Figure 4.1. The structures of M
BZI
complexes.
4.2. Results and Discussion
4.2.1. Synthesis and x-ray Analysis
Synthesis and x-ray crystal structures of these materials have already been reported in
earlier thesis.
113

4.2.2. Electrochemistry
The electrochemical properties of the complexes were determined using cyclic voltammetry
(CV) and differential pulse voltammetry (DPV). The DPV measurements were reported, but they
were remeasured, and the curves are shown below (Figure 4.3). The copper and the silver
complexes show irreversible reduction, whereas the gold analog shows a quasi-reversible
reduction (Figure 4.2). The reduction potentials for the M
BZI
series are identical (Ered = −2.84 ±
0.02 V) and greater (more negative) than those of M
CAAC
(Ered = −2.78 ± 0.06 V) and M
MAC
(Ered
= −2.45 ± 0.06 V) complexes, consistent with greater electrophilicity of the coordinated BZI ligand
is greater than the CAAC and MAC. All the complexes undergo irreversible oxidation and, unlike
the M
CAAC
and M
MAC
complexes where the oxidation potential is the same across the series, the
potential of the M
BZI
complexes increases from Cu (Eox = 0.11 ± 0.06 V) to Au (Eox = 0.32 ±
0.06 V), suggesting participation of the metal in the oxidation process (Table 4.1). Thus, values
for the redox gap (Eredox) are greater for the silver and gold complexes than the copper derivative.
Table 4.1. Redox potentials of complexes M
BZI
and the associated experimental frontier orbital
energies.
Complex E ox (V) E red (V) ΔE redox (V) E HOMO (eV) E LUMO (eV)
Cu
BZI
0.11 -2.85 2.96 -4.92 -1.47
Ag
BZI
0.28 -2.84 3.12 -5.12 -1.48
Au
BZI
0.29 -2.82 3.14 -5.16 -1.51
CH 2Cl 2 --- -2.73 --- --- ---
Electrochemical studies of the M
BZI
complexes were performed in dimethylformamide with
tetrabutylammonium hexafluorophosphate (TBAF) as the electrolyte, using Fc
+
/Fc (reductive scans)
and Me 10Fc (oxidative scans) as an internal standard. All given values for the potentials are referenced
to Fc
+
/Fc. The redox peaks were converted to HOMO/LUMO energies using equations in reference 2.
The E
red
∗
of Au
BZI
is calculated as -2.67 V using 𝐸 𝑟𝑒𝑑 ∗
= 𝐸 𝑟𝑒𝑑 + 𝛥𝐸
0−0
.

114

-3 -2 -1 0
-3 -2 -1 0

Cu
BZI

Current (a.u)
Ag
BZI

Voltage (V) vs Fc
+
/Fc
Au
BZI

Figure 4.2. Cyclic voltammetry data of M
BZI
.
-3 -2 -1 0 1
-0.00004
-0.00002
0.00000
0.00002
0.00004
Current (A)
Voltage (V) vs Fc
+
/Fc
Ox
Red
Fc
Me
10
Fc
Cu
BZI
-3 -2 -1 0 1
-0.00004
-0.00002
0.00000
0.00002
0.00004
Fc
Current (A)
Voltage (V) vs Fc
+
/Fc
Ox
Red
Ag
BZI
Me
10
Fc

-3 -2 -1 0 1
-0.00006
-0.00004
-0.00002
0.00000
0.00002
0.00004
0.00006
0.00008
Current (A)
Voltage (V) vs Fc
+
/Fc
Ox
Red
Au
BZI
Fc
M
10
Fc
-3 -2 -1 0 1
-0.00002
0.00000
0.00002
0.00004
0.00006
0.00008
Fc
+
/Fc
Current (A)
Voltage (V) vs Fc
+
/Fc
Au
BZI
in DMF ox
Au
BZI
in DMF red
Pure DCM

Figure 4.3. Differential pulse voltammetry data of M
BZI
and CH2Cl2.
115

4.2.3. Computational Analysis
The structure calculated using density function theory (DFT) for the ground state of Au
BZI
is
shown in Figure 4.4. The HOMO density is localized largely on carbazolide (Cz), whereas the
LUMO is largely localized on the carbene ligand, with smaller contributions from the metal d-
orbitals to both MOs. Time dependent DFT (TD-DFT) calculations find that the
3
Cz state is lowest
in energy and lies less than 0.09 eV below the manifold of the
1/3
ICT states (Table 4.2).
Additionally, the oscillator strength calculated for the silver complex is the lower than the Cu and
Au derivatives (Table 4.2), consistent with the low molar absorptivity observed for the ICT
transition in the absorption spectra of the silver complex (see below). Large molecular dipole
moments calculated for the ground state are directed along the metal-ligand bond axis toward the
carbazolide ligand (calc  -12.5 debye), whereas the moments for the excited
1
ICT state is
comparable in magnitude but directed toward the BZI ligand (calc  13.5 debye).

Figure 4.4. HOMO (E = 4.22 eV, solid) and LUMO (E = 1.44 eV, mesh) surfaces of complex
Au
BZI
.

116

Table 4.2. Calculated singlet and triplet excited state energies and dipole moments for the
complexes obtained through TDDFT performed at the CAM-B3LYP/LACVP** level. (Relative
contributions of localized Cz and CT character of the transitions are given in parentheses).

E (eV)
fS
1

Ground
state
T
1
T
2

T3
S
1
S
2

S3 ∆𝐸 1
𝐶𝑇 −
3
𝐶𝑇
∆𝐸 3
𝐶𝑧 −
3
𝐶𝑇

Cu
BZI

3.05
(Cz)
3.11
(0.6 CT,
0.3 Cz)
3.30
(0.5 CT,
0.5 Cz)
3.37
(CT)
4.12
(0.8 Cz,
0.15 CT)
4.20
(CT)
0.27* -0.05** 0.132
μES (D) -11.9 -11.4 -3.40 -0.6 12.2 -13.4 17.4
Ag
BZIa

3.06
(Cz)
3.11
(0.6 Cz,
0.3 CT)
3.36
(0.8 CT,
0.2 Cz)
3.38
(CT)
4.08
(0.8 Cz,
0.15 CT)
4.16
(CT)
0.02* -0.06** 0.078
μES (D) -13.3 -12.8 -10.4 7.8 13.8 -13.6 18.0
Ag
BZIb

3.06
(Cz)
3.14
(Cz)
3.32
(CT)
3.33
(CT)
4.04
(Cz)
4.14
(CT)
0.01 0.18 1 x 10
-6

μES (D) -13.3 -12.9 -15.3 14.3 14.4 -13.6 18.8
Au
BZI

3.05
(Cz)
3.14
(0.5 CT,
0.4 Cz)
3.36
(0.6 CT,
0.4 Cz)
3.48
(CT)
4.15
(0.8Cz,
0.15 CT)
4.26
(CT)
0.12* 0.09** 0.200
μES (D) -11.8 -11.2 -5.4 1.0 12.1 -13.3 17.1

*the energy difference between
1
CT (S 1) and the triplet with primarily CT contribution.
**the energy difference between
3
Cz (T 1) and the closest-lying triplet state with CT contribution.
Note: Positive/negative values for the dipole moments indicate that the dipole is directed in/against the Cu-N
direction.

4.2.4. Photophysical Characterization
Preliminary photophysical data of these complexes is reported in earlier thesis. Here, we
examined their excited state properties thoroughly to understand the origin of their interesting
photophysical properties. Absorption spectra of the M
BZI
complexes in polar (2-
methyltetrahydrofuran, 2-MeTHF) and nonpolar (methylcyclohexane, MeCy) solvents are shown
in Figure 4.5. Absorption bands between 300 and 375 nm of M
BZI
are assigned to -* transitions
localized on the carbazolyl ligand.
1, 3, 4
The band at lower energy (> 375 nm) is assigned to an
intramolecular ligand-to-ligand charge transfer (ICT) transition from Cz to BZI. The energy of the
ICT band of M
BZI
is higher than in the M
CAAC
and M
MAC
complexes, consistent with the order of
reduction potentials in these complexes. The ICT band extends to 410 nm in MeCy and has two
117

features separated by 1100 cm
-1
indicative of vibronic coupling. Similar to M
CAAC
and M
MAC
complexes, the ICT band in of M
BZI
displays negative solvatochromism and merges into the higher
lying ligand -* transitions in polar solvents. These shifts with solvent polarity are due to the
large change in the molecular dipole moments between the ground and excited ICT states. The
molar absorptivities of the M
BZI
complexes decrease in the order Au > Cu > Ag in all media. The
same trend was observed in the M
CAAC
and M
MAC
analogs and attributed to a decrease in the
overlap integrals between orbitals on the donor Cz and acceptor ligands mediated by the metal
center.
3

300 350 400 450 500 550 600
-0.5
0.0
0.5
1.0
1.5
MeCy
2-MeTHF
PS
Wavelength (nm)
e (10
4
M
-1
cm
-1
)
Cu
BZI
RT
77K
-0.5
0.0
0.5
1.0
PL Intensity
300 350 400 450 500 550 600
0.0
0.5
MeCy
2-MeTHF
PS
Wavelength (nm)
e (10
4
M
-1
cm
-1
)
-0.5
0.0
0.5
1.0
77K
PL Intensity
Ag
BZI
RT

300 350 400 450 500 550 600
0.0
0.5
MeCy
2-MeTHF
PS
Wavelength (nm)
e (10
4
M
-1
cm
-1
)
-0.5
0.0
0.5
1.0
77K
PL Intensity
Ag
BZI
RT

Figure 4.5. Absorption (dashed lines) and photoluminescence (solid line) spectra of M
BZI

complexes in MeCy, 2-MeTHF and polystyrene at room temperature (RT) and 77 K.
118

Emission spectra of the M
BZI
compounds in MeCy, 2-MeTHF solution and polystyrene (PS)
films at room temperature and 77 K are shown in Figure 4.5 and tabulated in Table 4.3. The
spectra from the M
BZI
complexes are blue-shifted relative to their M
CAAC
and M
MAC
counterparts,
yet display similar solvatochromic behavior, undergoing red-shifts in polar solvents. Spectra
recorded in MeCy are relatively narrow (FWHM = 44 nm, 2300 cm
-1
) and show underlying
vibronic features. The photoluminescence quantum yields (PL) of M
BZI
complexes are close to
unity in MeCy and PS films but decrease with increasing solvent polarity. Increasing solvent
polarity is correlated with increased spectral width and loss of the vibronic features, suggesting
structural distortion in the excited states. To explain the blue-shift in absorption spectra and red-
shift in emission spectra of these complexes with increasing solvent polarity, a diagram
representing the potential energy surfaces for the ground state (S0) and excited states (
3
Cz and
1,3
ICT) as a function of nuclear coordinate in MeCy and CH2Cl2 is proposed (Figure 4.6). The
vibronically structured absorption and emission spectra in nonpolar solvents (MeCy) indicate that
the potential energy surfaces are well-nested, such that ICT transitions are induced with small
reorganization energies. This is probably due to the rigid nature of the BZI ligand compared to
CAAC and MAC ligands which are not aromatic. In contrast, the blue-shifted absorption and
broad, featureless red-shifted emission observed in polar solvents (CH2Cl2) indicates that
significant reorganization occurs within the metal complex and surrounding media as a result of
the large change in dipole moment upon excitation (calc > 24 debye). Unlike M
CAAC
and M
MAC
complexes, where the radiative rate constant (kr) is fastest for the silver analog,
3, 5
Au
BZI
has the
fastest kr in accord with gold having the largest SOC constant.
119

Figure 4.6. Qualitative energy diagram representing the ground state (S0) and both excited state
potential energy surfaces (
3
Cz and
1,3
ICT) as a function of nuclear coordinate in MeCy (blue) and
CH2Cl2 (red) solution along with absorption (solid) and emission (dashed) transitions.
The emission spectra display a pronounced rigidochromic blueshift upon cooling to 77 K and
become extremely narrow and vibronically structured, with luminescence lifetimes in the
millisecond regime. The emission at low temperature is consistent with a triplet transition
localized on the carbazolide ligand (
3
Cz). This change in emission properties with temperature is
attributed to the close energy separation between the
3
Cz and
1/3
ICT manifolds, making the ICT
manifold thermally accessible at room temperature, but inaccessible in frozen MeCy and
2-MeTHF at 77 K. The fact that the M
BZI
complexes display
3
Cz emission in PS (as well as MeCy
and 2-MeTHF) at 77 K is different from behavior observed in M
CAAC
and M
MAC
complexes.
Emission from the latter complexes remains broad and featureless in a polystyrene matrix at all
temperatures, even down to 4 K.
3
. In the case of the M
CAAC
and M
MAC
complexes, the
3
ICT state
lies below the energy of the
3
Cz state in PS at all temperatures.
3
However, it is evident that the
120

lowest triplet state is localized on
3
Cz in the M
BZI
complexes in PS films at 77 K. This difference
suggests that the
3
Cz and
3
ICT states in the M
BZI
complexes are near degenerate in energy, and
TADF emission occurs via thermal activation from the
3
Cz to
1
ICT states, not within the ICT
manifold as in the case of the M
CAAC
and M
MAC
complexes.
Table 4.3. Luminescence properties of complexes M
BZI
in various media.
Complex max
(nm)
PL 
(s)
kr
(10
5

s
-1
)
knr
(10
5
s
-
1
)
max,
77K
(nm)
77K
(s)
Cu
BZI
MeCy 428 0.80 1.23 6.50 1.63 428 6300
toluene 450 0.75 1.50 5.0 1.7 --- ---
2-MeTHF 458 0.35 2.06 1.7 3.2 430 11000
CH2Cl2 466 0.03 1.24 0.24 7.8 --- ---
PS film 434 0.86 0.97
(36%);
4.8 (64%)
2.5
a
0.41
a
432 3000
Ag
BZI
MeCy 430 0.58 1.04 5.6 4.4 432 18000
toluene 458 0.50 3.27 1.5 1.5 --- ---
2-MeTHF 476 0.19 5.66 0.34 1.4 432 20000
CH2Cl2 482 0.03 1.64 0.18 5.9 --- ---
PS film 438 0.85 0.69
(26%);
5.1 (74%)
2.2
a
0.38
a
434 6600
Au
BZI
MeCy 424 0.89 1.15 7.8 0.9 424 340
toluene 448 0.94 1.11 8.5 5.4 --- ---
2-MeTHF 452 0.79 2.63 3.0 0.8 426 640
CH2Cl2 458 0.23 5.79 0.40 1.3 --- ---
PS film 432 1.0 0.74
(46%);
3.6 (54%)
4.4
a
< 0.04
a
428 190
a
Calculated from the weighted averages of both contributions.

Another difference in the properties of the M
BZI
complexes compared to the M
CAAC
and M
MAC

analogs is the pronounced decrease in luminescence efficiency with respect to solvent polarity,
particularly in CH2Cl2. For example, the quantum yield of Cu
BZI
is severely diminished in CH2Cl2
(PL = 0.03), whereas the decrease in efficiency is less for Cu
CAAC
(PL = 0.4) and Cu
MAC
(PL
121

= 0.5). To better understand the origin of this decrease in PL with solvent polarity, photophysical
properties of Au
BZI
were characterized in mixtures of MeCy and CH2Cl2 at various ratios. The
LLCT band in the absorption spectra gradually blue-shifts with increasing CH2Cl2 concentration
and the vibronic fine structure observed in MeCy disappears in mixtures with  5% CH2Cl2
(Figure 4.7). Table 4.4 and Figure 4.8 show that the radiative rate constant of Au
BZI
decreases
with increasing solvent polarity, whereas the nonradiative rate constant (knr) remains near constant,
consequently decreasing the PL. The fact that the nonradiative rate constant of Au
BZI
is
independent of CH2Cl2 concentration, despite the similar reduction potentials for the Au
BZI
excited
state (E
0/+*
= −2.67 V) and CH2Cl2 (E
0/–
= −2.73 V), indicates that there is no oxidative quenching
of excited Au
BZI
by CH2Cl2. Therefore, the decrease in PL of Au
BZI
in CH2Cl2 is due to a decrease
in the radiative rate constant in this solvent.
300 350 400 450
0.0
0.5
1.0
1.5
2.0
2.5

e (10
4
M
-1
. cm
-1
)
Wavelength (nm)
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0

PL Intensity
Wavelength (nm)
CH
2
Cl
2
CH
2
Cl
2
:MeCy 4:1
CH
2
Cl
2
:MeCy 3:2
CH
2
Cl
2
:MeCy 2:3
CH
2
Cl
2
:MeCy 1:4
CH
2
Cl
2
:MeCy 1:9
CH
2
Cl
2
:MeCy 1:19
MeCy

Figure 4.7. Absorption and emission spectra of complex Au
BZI
in CH2Cl2, MeCy and in various
solvent ratios.

122

Table 4.4. Luminescence properties of Au
BZI
in various CH2Cl2:MeCy mixtures.
CH2Cl2:MeCy ratio PL RT (μs) kr (×10
5
s
-1
) knr (×10
5
s
-1
)
1:0 0.23 5.79 0.397 1.33
4:1 0.39 6.17 0.632 0.989
3:2 0.54 5.63 0.959 0.817
2:3 0.69 4.38 1.58 0.708
1:4 0.82 2.47 3.32 0.729
1:9 0.84 1.33 6.32 1.20
1:19 0.87 1.18 7.37 1.10
0:1 0.89 1.15 7.83 0.870

30 32 34 36 38 40 42
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 PLQY
k
r
k
nr
ET (kcal/mol)
PLQY
2
4
6
8
10
Rate Constant (10
5
s
-1
)

Figure 4.8. Change in PL and radiative and nonradiative rate constants of Au
BZI
as a function
of solvent polarity.
To further confirm that the decrease in kr of Au
BZI
is not due to oxidative quenching by CH2Cl2,
N,N-dimethyl formamide (DMF) which has a higher reduction potential and a similar polarity with
CH2Cl2, is used to measure photophysical parameters of Au
BZI
. As was observed in CH2Cl2, the
absorption of Au
BZI
in DMF blueshifts, whereas the emission spectrum broadens and redshifts
123

(Figure 4.9). Furthermore, as was observed in CH2Cl2, the kr of Au
BZI
in DMF is significantly
lower than the value in MeCy and the knr is unchanged, consequently decreasing the PL (Table
4.5). This confirms that the decrease in kr in polar medium has to do with the polarity of the solvent
rather than oxidative quenching by the polar solvents.
300 350 400 450
0.0
0.5
1.0
1.5
2.0
2.5

e (10
4
M
-1
. cm
-1
)
Wavelength (nm)
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0

PL Intensity
Wavelength (nm)
MeCy
CH
2
Cl
2
DMF

Figure 4.9. Scaled absorption and emission spectra of complexes Au
BZI
in various solvents
(DMF = N,N-Dimethyl formamide).
Table 4.5. Photophysical properties of Au
BZI
in various solvents.
Solvent PL τRT (μs) kr (s
-1
) knr (s
-1
)
DMF 0.30 7.20 4.17×10
4
9.72×10
4

CH2Cl2 0.23 5.79 3.97×10
4
1.33×10
5

MeCy 0.89 1.15 7.83×10
5
8.70×10
4

Emission studies of Au
BZI
at variable temperature were conducted in MeCy and CH2Cl2 to
investigate the parameters controlling TADF (Figure 4.10). The emission spectra in MeCy
solution slightly increase in intensity and display minimal changes in line shape with increasing
temperature. In contrast, spectra in CH2Cl2 decrease in intensity with increasing temperature and
show vibronic features at low temperatures (180−230 K) that broaden abruptly at 240 K. The kr
124

values calculated from the quantum yields measured at various temperatures (Table 4.6 and Table
4.7) were fit to a two level model using Equation 1 (Figure 4.11a).
3

𝑙𝑛 (𝑘 𝑇𝐴𝐷𝐹 ) = 𝑙𝑛 (
𝑘 𝐼𝑆𝐶 𝑆 1
→𝑇 1
3
(1 −
𝑘 𝐼𝑆𝐶 𝑆 1
→𝑇 1
𝑘 𝑓𝑙
+𝑘 𝐼𝑆𝐶 𝑆 1
→𝑇 1
)) −
∆𝐸 𝑆𝑇
𝑘 𝐵 𝑇 (1)
Where, kB is the Boltzmann constant, kTADF and kfl are the radiative rate constants of the TADF and
fluorescence, respectively and 𝑘 𝐼𝑆𝐶 𝑆 1
→𝑇 1
is the intersystem crossing rate (see Figure 4.11a). The fits
give an energy difference between the triplet and emitting singlet state of Au
BZI
in MeCy as 920
cm
-1
(Figure 4.11b). However, the Arrhenius plot of radiative rate constant of Au
BZI
recorded in
CH2Cl2 at variable temperature is decidedly nonlinear (Figure 4.11b).
The fits give an energy difference between the triplet and emitting singlet state of Au
BZI
in CH2Cl2
as EST = 730 cm
-1
, below 250 K. There is also a sharp decrease in kr at 250 K in CH2Cl2, and
consequently a smaller energy barrier is observed between 250 K and 270 K (EST = 175 cm
-1
).
This change in EST coincides with the marked decrease in efficiency and shift in peak shape
observed in the PL spectrum between 240 K and 270 K.
400 450 500 550 600 650
0
2
4
6
8
10
12
14
16
18
20
PL Intensity (10
5
a.u)
Wavelength (nm)
190K
200K
210K
220K
230K
240K
250K
260K
270K
MeCy
400 450 500 550 600 650
0
2
4
6
8
10
12
PL Intensity (10
5
a.u)
Wavelength (nm)
190K
200K
210K
220K
230K
240K
250K
260K
270K

CH
2
Cl
2

Figure 4.10. Emission spectra of Au
BZI
in CH2Cl2 and MeCy at various temperatures.

125

Table 4.6. Photophysical properties of Au
BZI
in CH2Cl2 at various temperatures
T(
o
C) Integrated PL
area (×10
7
a.u)
PL τRT (μs) kr (s
-1
) knr (s
-1
)
190 7.10 0.59 95.2 6.17×10
3
4.33×10
3

200 7.00 0.58 78.9 7.35×10
3
5.32×10
3

210 6.83 0.57 62.6 9.04×10
3
6.94×10
3

220 6.88 0.57 47.9 1.19×10
4
8.98×10
3

230 7.15 0.59 36.5 1.62×10
4
1.12×10
4

240 6.64 0.55 30.5 1.80×10
4
1.48×10
4

250 4.54 0.38 25.0 1.50×10
4
2.50×10
4

260 3.89 0.32 20.8 1.55×10
4
3.26×10
4

270 2.78 0.23 14.2 1.62×10
4
5.42×10
4

Table 4.7. Photophysical properties of Au
BZI
in MeCy at various temperatures.
T(
o
C) Integrated PL
area (×10
8
a.u)
PL τRT (μs) kr (s
-1
) knr (s
-1
)
190 1.03 0.71 8.17 8.73×10
4
3.51×10
4

200 1.07 0.74 6.05 1.22×10
5
4.33×10
4

210 1.10 0.76 4.52 1.68×10
5
5.31×10
4

220 1.13 0.78 3.47 2.25×10
5
6.30×10
4

230 1.13 0.78 2.69 2.92×10
5
8.02×10
4

240 1.20 0.83 2.23 3.72×10
5
7.63×10
4

250 1.24 0.86 1.90 4.54×10
5
7.26×10
4

260 1.26 0.88 1.56 5.62×10
5
7.94×10
4

270 1.29 0.89 1.30 6.87×10
5
8.22×10
4

126

0.0036 0.0039 0.0042 0.0045 0.0048 0.0051 0.0054
1
2
3
4
5
6
7
8
MeCy
k
TADF
(10
5
s
-1
)
1/T (K
-1
)
270K
250K
240K
190K
Temp. E
ST
190-270K 920 cm
-1
(b)
0.0036 0.0039 0.0042 0.0045 0.0048 0.0051 0.0054
0.6
0.8
1
1.2
1.4
1.6
1.8
2
CH
2
Cl
2
k
TADF
(10
4
s
-1
)
1/T (K
-1
)
Temp. E
ST
190-240K 730 cm
-1
250-270K 175 cm
-1
270K
250K
240K
190K
(c)

Figure 4.11. (a) Kinetic scheme for the emission from Au
BZI
. Arrhenius plot of the temperature-
dependent lifetime data for Au
BZI
in MeCy (b) and in CH2Cl2 (c) recorded from 180 to 300 K,
along with fits to the data according to Equation 1.

Variable temperature
1
H NMR of the Au
BZI
complex in CD2Cl2 indicates that the intensity
of the residual water peak at temperatures below 240K significantly diminishes and slightly move
to downfield suggesting aquo complex formation with Au
BZI
at temperatures below 240K (Figure
4.12). This temperature range coincides with the abrupt change observed in the emission spectrum
and kr of Au
BZI
from 230 to 240K. The formation of this aquo complex could be what leads to the
anomalous behavior in CH2Cl2 at lower temperatures, making the analysis using the simple two
level model problematic.
127

Figure 4.12. Variable temperature
1
H NMR spectra of Au
BZI
in CD2Cl2.
4.2.5. OLED Characterization
The Au
BZI
complex is stable to sublimation and was thus used as dopant to fabricate OLEDs
by thermal evaporation. Considering the high triplet energy of Au
BZI
(ET = 3.1 eV), 1,3-
bis(triphenylsilyl)benzene (UGH3, ET = 3.5 eV) was employed as a host material in the device.
Devices were fabricated using different doping levels (5, 10 and 15 wt%) and the best performance
was obtained with 5 wt% Au
BZI
. Optimized devices achieved high efficiency (maximum EQE =
12%) and reasonable roll-off in efficiency with increasing current density (Figure 4.13). The
electroluminescence (EL) spectrum of Au
BZI
(max = 430 nm, FWHM = 45 nm) is identical to the
128

PL spectrum in PS demonstrating efficient exciton confinement on the complex. The color
coordinates of the EL spectrum (CIE = 0.16, 0.06) make Au
BZI
an efficient deep blue dopant for
phosphorescent OLEDs.

400 500 600
0.0
0.2
0.4
0.6
0.8
1.0
WO UGH3 5%
WO UGH3 10%
With UGH3 5%
Wtih UGH3 10%
Intensity (a.u.)
Wavelength (nm)

0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
WO UGH3 5%
WO UGH3 10%
With UGH3 5%
With UGH3 10%

Luminescence (cd/m
2
)

Voltage (V)
0
100
200
300
Current density (mA/cm
2
)
10
-2
10
-1
10
0
10
1
10
2
0.1
1
10

WO UGH3 5%
WO UGH3 10%
With UGH3 5%
With UGH3 10%
EQE (%)
Current density (mA/cm
2
)

Figure 4.13. Devices with 5 and 10% doping without (WO) and with UGH3 blocker (4 devices)
Conclusion
A series of 2-coordinate coinage metal (i.e. M = Cu, Ag, Au) complexes bearing a sterically
bulky benzimidazolyl-carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene
(BZI), and carbazolide (Cz) as the anionic ligand were investigated. The redox gap of all the
complexes is large (Eredox > 3 V), and as a result their absorption and emission, which originate
from an ICT transition between the carbazolyl and carbene ligands, occur in the blue spectral
129

region. The complexes have high luminescence efficiencies (PL >90%) and display deep blue
narrow emission in MeCy and PS films. Their absorption spectra display negative
solvatochromism, whereas their emission spectra undergo bathochromic shifts in polar solvents
that is accompanied by decrease in quantum yields (PL < 23%) and radiative rate constants (kr <
4.0×10
4
s
-1
). The nonradiative rate constants, however, are unaffected by the medium, remaining
nearly the same in polar and nonplar media (knr ~ 1×10
5
s
-1
). Temperature-dependent studies reveal
that the energy difference between the singlet and triplet excited state in methylcyclohexane is 920
cm
-1
. Vapor-deposited OLEDs fabricated using Au
BZI
as an emissive dopant have high efficiency
(EQE = 12%) and a narrow and deep blue emission (CIE = 0.16, 0.06). This narrow emission is
probably due to aromatic and rigid nature of the BZI ligand, minimizing reorganization of the
complex in the excited state in nonpolar medium. Therefore, other aromatic and rigid carbenes can
in principle be used to design two-coordinate complexes with narrow emission, which is crucial
for display applications. These two-coordinate complexes present new opportunities for use as
dopants in OLEDs, notable their deep blue and narrow emission. Lifetime studies on devices will
be carried out to determine if these gold-based emitters could serve as alternatives to the state-of-
the-art Ir(III) complexes commonly used in OLEDs.
4.3. References
1. S. Shi, M. C. Jung, C. Coburn, A. Tadle, D. Sylvinson M. R, P. I. Djurovich, S. R. Forrest
and M. E. Thompson, J. Am. Chem. Soc., 2019, 141, 3576-3588.
2. A. S. Romanov, L. Yang, S. T. E. Jones, D. Di, O. J. Morley, B. H. Drummond, A. P. M.
Reponen, M. Linnolahti, D. Credgington and M. Bochmann, Chem. Mater., 2019, 31,
3613-3623.
3. R. Hamze, S. Shi, S. C. Kapper, D. S. Muthiah Ravinson, L. Estergreen, M.-C. Jung, A. C.
Tadle, R. Haiges, P. I. Djurovich, J. L. Peltier, R. Jazzar, G. Bertrand, S. E. Bradforth and
M. E. Thompson, J. Am. Chem. Soc., 2019, 141, 8616-8626.
4. R. Hamze, J. L. Peltier, D. Sylvinson, M. Jung, J. Cardenas, R. Haiges, M. Soleilhavoup,
R. Jazzar, P. I. Djurovich, G. Bertrand and M. E. Thompson, Science, 2019, 363, 601.
5. A. S. Romanov, S. T. E. Jones, L. Yang, P. J. Conaghan, D. Di, M. Linnolahti, D.
Credgington and M. Bochmann, Advanced Optical Materials, 2018, 6, 1801347.
130

6. D. Di, A. S. Romanov, L. Yang, J. M. Richter, J. P. H. Rivett, S. Jones, T. H. Thomas, M.
Abdi Jalebi, R. H. Friend, M. Linnolahti, M. Bochmann and D. Credgington, Science,
2017, 356, 159.
7. H. M. J. Wang, C. Y. L. Chen and I. J. B. Lin, Organometallics, 1999, 18, 1216-1223.
8. R. Hamze, R. Jazzar, M. Soleilhavoup, P. I. Djurovich, G. Bertrand and M. E. Thompson
Chem. Commun., 2017, 53, 9008-9011.
9. V. A. Krylova, P. I. Djurovich, B. L. Conley, R. Haiges, M. T. Whited, T. J. Williams and
M. E. Thompson, Chemical Communications, 2014, 50, 7176-7179.
131

5CHAPTER 5 − Tuning Singlet and Triplet Excited State Energies and
Frontier Orbitals of Imidazole Host/Emitter for Hybrid White OLEDs
5.1. Introduction
White organic light-emitting diode (WOLED) have potential as next generation flat panel
display and solid-state lighting.
1-5
WOLEDs are fabricated by uniformly mixing blue, green and
red (RGB) or blue and yellow (BY) emitters in a single emissive layer, segregated into separate
emissive layers in a single device or placed side by side on the substrate as separately driven
devices. All these WOLED architectures have been shown to give high efficiencies with all
phosphorescent emitters.
2, 6
However, due to the low device lifetime of blue phosphorescent
emitters, WOLEDs incorporating all phosphorescent emitters have limited device lifetimes.
2
This
is because of the high energy and long radiative lifetime of blue phosphors leading to bimolecular
annihilation processes.
7-9
Blue fluorophores however have shorter excited lifetimes than their blue
phosphors counterparts and therefore bimolecular annihilation processes are less pronounced in
OLEDs incorporating fluorophores.
9
Thus, incorporating a blue fluorophore instead of a blue
phosphor in WOLEDs, known as hybrid fluorescent/phosphorescent (F/P) WOLED can in
principle increase the WOLED device lifetimes.
In this WOLED structure, blue fluorescent dopant (Figure 5.1a) or fluorescent host
(Figure 5.1b) will contribute to the blue emission and green and red phosphorescent emitters will
contribute to green and red emission, respectively. This concept was first demonstrated by
Thompson and Forrest group, where they utilized a fluorescent dopant and a host material with
green and red phosphorescent dopants to achieve hybrid WOLED with moderate efficacy (24
lm/W) at 500 nits and high color rendering index (CRI = 85).
1
However, this efficacy (24 lm/W)
is too low for practical applications. The low efficacy is attributed partly to the triplet exciton loss
132

on the fluorophore due to lower triplet energy of the fluorophore than the host material, resulting
in inefficient triplet energy transfer to the phosphors. To utilize all excitons in WOLEDs, the blue
fluorophore is inserted at the exciton recombination zone to ensure formation of the excitons on
the blue fluorophore. The blue fluorophore needs to be highly emissive to ensure utilization of all
the singlets for blue emission. Additionally, the triplet energy of the blue fluorophore needs to be
higher than the triplet energies of the phosphors to ensure efficient triplet energy transfer from the
blue fluorophore to the radiative triplet of green and red phosphors.
1, 10
This structure takes
advantage of the proportion dictated by spin statistics (i.e., 25% singlets vs. 75% triplets are
produced by electrical excitation
11
) to achieve ideal warm white emission where roughly 25% is
contributed by blue spectrum. Therefore 25% of excitons coming from the singlet of the blue
fluorescent material and the remaining 75% that are utilized for green and red emission conforms
naturally to the visible spectrum, allowing the hybrid WOLED to achieve high color rendering
index (CRI). Additionally, resonant energy transfer from both the host singlet and triplet energy
levels minimizes exchange energy losses for both singlet and triplet exciton transfer from the host
to each of the dopants, thereby maximizing device power efficiency while maintaining the
potential for unity internal quantum efficiency (IQE).

Figure 5.1. Exciton energy transfer in hybrid fl/ph WOLED (a) using a blue fluorescent dopant
and a separate host, (b) using a blue fluorescent material as a neat emitter and as a host.
133

Ideally, the fluorescent material will serve a dual purpose; as neat fluorescent emitter and
as host for green and red phosphorescent emitters (Figure 5.1b). Thus, the fluorescent emitter
should be highly emissive as a neat solid to utilize all the singlet excitons. Additionally, the
emission maxima of the fluorescent host should be around 450 nm to have a WOLED with warm
white emission and good color rendering index (CRI). Furthermore, the triplet energy of the
fluorescent host needs to be greater than 2.40 eV to host green and red phosphors. Green and red
emission are obtained from the sensitized triplet of the green and red phosphorescent dopants,
respectively. It is extremely difficult for a fluorescent material to exhibit all these attributes, as a
result, development of this material can be challenging.
Several fluorescent materials have been utilized in hybrid WOLEDs. The most commonly
used and promising ones are shown in Figure 5.2. Although, these materials are highly emissive
as isolated molecules, they suffer from efficiency loss in the solid-state due to aggregation. 4P-
NPD and PPI are exceptions, retaining high efficiencies as neat materials (solid > 65%). In fact,
WOLEDs with 4P-NPD as fluorescent dopant are reported to achieve efficacy as high as 45
lm/W.
6, 10
For this reason, 4P-NPD is one of the most commonly used fluorescent emitter in hybrid
WOLED. The main drawback with 4P-NPD is its low triplet energy (< 2.3 eV) due its large π-
systems like the other materials shown in Figure 5.2, thus limiting the use of 4P-NPD as host for
green phosphors. Additionally, the synthesis and purification of 4P-NPD is extremely difficult,
low yielding and tedious. 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) has high triplet
energy and high efficiency in the solid state. The main drawback with PPI is its high singlet
energy, limiting its CRI and efficacy.
134

Figure 5.2. Commonly used blue fluorescent emitters for hybrid fl/ph WOLED
Several groups reported other phenanthro[9,10-d]imidazoles derivatives with high PLQY
in the solid state.
12-15
The reported phenanthro[9,10-d]imidazoles have been utilized as
fluorescence emitters in hybrid F/P WOLED application.
16
However, the reported WOLEDs with
the phenanthro[9,10-d]imidazoles emitters either have low efficiencies due to low triplet energy
of the imidazoles resulting in inefficient triplet energy transfer from the blue fluorophore to the
phosphors or suffer from low color rendering index (CRI) due to high singlet energies.
15-21
In this
work, we identified phenanthro[9,10-d]imidazoles as potential blue fluorescent emitters and hosts
in hybrid F/P WOLED application. The photophysical and electrochemical properties of the
phenanthro[9,10-d]imidazoles are finely tuned by simple substitution of pyridine and/or phenyl at
135

the C2 or N2 position of the phenanthro[9,10-d]imidazole. This substitution resulted in six
different imidazoles (I1−I6) with varied photophysical and electrochemical properties. The
materials have appropriate singlet energies for blue emission, high radiative efficiencies as neat
solids and appropriate triplet energies for hosting green, yellow and red phosphorescent emitters.
The reported materials were incorporated as neat blue fluorescent emitters in monochromatic
fluorescent OLEDs with 5% efficiency. A highly efficient (20%) hybrid F/P WOLED with a high
luminous efficacy (65 lm/W) at low driving voltages (< 5 V) was demonstrated using a single
emissive layer with these new blue fluorescent emitters along with conventional green and red
phosphors.
5.2. Results and Discussion
5.2.1. Synthesis
I1, I4 and I5 are synthesized from commercially available starting materials. 4-pyridyl-4-
aniline is synthesized as a precursor for the syntheses of I2, I3 and I6. The aniline precursor is
obtained by following literature procedure in 80% yield (Scheme 1).
22
The phenanthro[9,10-
d]imidazoles (I1−I6) are obtained from condensation reaction of their respective aniline, aldehyde
and ammonium acetate in high yields (70−80%) (Scheme 5.1). All the materials sublimed cleanly
and their sublimation temperatures are given in the experimental section.

Scheme 5.1. Synthesis of phenanthro[9,10-d]imidazoles.
136

5.2.2. Computational Modelling
I1–I6 are derived from PPI and are chosen based on their suitable electronic properties as
fluorescent emitters for hybrid WOLED. Based on our modeling studies (Table 5.1), the
calculated singlet energies in gas phase seem to be too high for a warm white WOLED, but as we
observed with our previously reported phenanthro-imidazoles,
23, 24
we anticipated the singlet
energies to red- shift by ~0.3 eV in the solid state to give ideal blue emission for a warm white
WOLED. Furthermore, all the calculated triplet energies of I1–I6 are high enough to host green
and red phosphors. Therefore, we synthesized six phenanthro[9,10-d]imidazoles, i.e. I1–I6 shown
in Scheme 5.1. Similar compounds were reported in literature as electron transport materials.
25

We anticipated these materials to be emissive as neat materials and thus have the potential use as
neat fluorescent emitter for hybrid F/P WOLED application.
Table 5.1. Calculated energy levels and frontier orbitals of I1–I6
S 1 T 1 HOMO LUMO f
I1 3.36 2.65 -5.15 -1.31 0.854
I2 3.08 2.64 -5.25 -1.69 0.361
I3 3.12 2.72 -5.29 -1.68 0.169
I4 3.34 2.60 -5.29 -1.56 0.502
I5 3.37 2.60 -5.30 -1.56 0.576
I6 3.12 2.60 -5.39 -1.78 0.947
f = oscillator strength of S 0–S 1 transition. All energy values are in eV. DFT/B3LYP

137

Spin Density HOMO LUMO LUMO+1 S1 Transition
I1

S 0-S 1 = 4.13 eV (f =
0.854)
H → L (49%)
H → L+1 (19%)
I2

S 0-S 1 = 3.93 eV (f =
0.361)
H → L (76%)
H → L+1 (11%)
I3

S 0-S 1 = 3.94 eV (f =
0.169)
H → L (71%)
H → L+1 (14%)
I4

S 0-S 1 = 3.97 eV (f =
0.502)
H → L (80%)

I5

S 0-S 1 = 4.00 eV (f =
0.576)
H → L (75%)
H → L+1 (5%)
I6

S 0-S 1 = 4.08 eV (f =
0.947)
H → L (34%)
H → L+1 (41%)
Figure 5.3. Molecular orbital representation of I1 – I6 (H = HOMO, L = LUMO).
5.2.3. Photophysical and Electrochemical Properties
The S1/T1 and HOMO/LUMO values of these materials are tuned by incorporating phenyl
and/or pyridyl group at the N1 or C2 positions (Scheme 5.1). All the materials absorb between
300 – 400 nm and their extinction coefficients are in close agreement with their calculated
oscillator strengths (Figure 5.4). The S1 and T1 values of these materials are obtained from the
onset of their respective photoluminescence spectra (Figure 5.5). The S1 and T1 values are shown
in Figure 5.6, whereas the HOMO and LUMO levels are summarized in Table 5.3.
138

300 325 350 375 400 425 450
0.0
0.5
1.0
1.5
2.0
2.5
3.0

e (
4
M
−
cm
−1
)
Wavelength (nm)
I1
I2
I3
I4
I5
I6

Figure 5.4. Absorption spectra of I1–I6.
The measured singlet energies (emission maxima as neat solids, 410–450nm) of I1–I6 are
ideal for blue emission in a white OLED (See Figure 5.5). Additionally, the triplet energies (ETsolid)
of I1–I3 are higher than the triplet (ET) of a green phosphor (Ir(ppy)3). Thus, moderate
photoluminescence quantum yield (PLQY) of Ir(ppy)3 is achieved in I1–I3. The ETsolid of I4–I6
however, is lower than the ET of Ir(ppy)3, resulting in low efficiencies (< 40%) of Ir(ppy)3 in these
materials (Table 2). The ETsolid of all the materials is higher than the triplet energies of orange and
red emitting phosphors, and thus high efficiencies of orange (Ir(bt)2acac) and red (PQIr) phosphors
are maintained in these hosts (Table 5.2). Furthermore, the PL spectra of the phosphors in these
hosts indicate that there is no quenching by these host materials (Figure 5.7). The
photoluminescence efficiency of the materials depends on the combination of donor/acceptor
substituent groups (phenyl and pyridyl) at the N1 and C2 positions. I1 and I4–I6 have near unity
quantum efficiencies in solution and in the solid-state (Table 5.2), whereas I2 and I3 have low
efficiencies in both solution and in the solid-state. This is because of higher overlap between the
orbitals responsible for S1 – S0 transition in I1 and I4–I6 than with orbitals responsible for S1 – S0
transition in I2 and I3 (Figure 5.3).
139

350 400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0

PL Intensity
Wavelength (nm)
Soln 298K
Solid 298K
Soln 77K
Solid 77K
I1
350 400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
Soln 298K
Solid 298K
Soln 77K
Solid 77K
I2
350 400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
Soln 298K
Solid 298K
Soln 77K
Solid 77K
I3
350 400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
Soln 298K
Solid 298K
Soln 77K
Solid 77K
I4
350 400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
Soln 298K
Solid 298K
Soln 77K
Solid 77K
I5
350 400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
Soln 298K
Solid 298K
Soln 77K
Solid 77K
I6

Figure 5.5. Photoluminescence spectra of I1 – I6.
140

1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
S
1
solution S
1
solid T
1
solution T
1
solid
Energy (eV)

I6 I5 I4 I2 I3 I1
1200
1000
800
600
400
Wavelength (nm)

Figure 5.6. Summarized S1/T1 of I1–I6 measured in solution and in the solid-state. S1 and T1 are
obtained from the onset of their individual emission spectra at 0.2 intensity.
Table 5.2. PLQY data of Ir(ppy)3, Ir(bt)2acac and PQIr in I1–I6 films, solution and as neat
materials.
PLQY I1 I2 I3 I4 I5 I6
10 wt% Ir(ppy)3 0.65 0.49 0.56 0.39 0.30 0.36
10 wt% Ir(bt)2acac 0.80 0.66 0.84 0.83 0.80 0.78
10 wt% PQIr 0.70 0.69 0.67 0.64 0.69 0.66
In 2-MeTHF ~1 0.28 0.28 ~1 ~1 ~1
As neat materials 0.95 0.10 0.10 0.55 0.81 0.83
PLQY are measured using an integrating sphere
141

450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0

PL Intensity
Wavelength (nm)
I1
I2
I3
I4
I5
I6
CBP
10 wt% Ir(ppy)
3
:Host
500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
I1
I2
I3
I4
I5
I6
CBP
10 wt% Ir(bt)
2
acac:Host
550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0

PL Intensity
Wavelength (nm)
I1
I2
I3
I4
I5
I6
CBP
10 wt% PQIr:Host

Figure 5.7. Normalized emission spectra of Ir(ppy)3, Ir(bt)2acac and PQIr in phenanthro-
imidazole and CBP Hosts.
The oxidation and reduction potentials of these materials are measured by cyclic and
differential pulse voltammetry. All the materials have reversible reductions whereas their
oxidations are irreversible (Figure 5.8). Except for I1, the LUMO density of these materials is
localized on the phenylpyridine moiety resulting in similar reduction potentials (−2.32 to −2.44
eV) across the series (Table 5.3). The LUMO density of I1 however is localized on the biphenyl
making it harder to reduce than the rest of the materials (Figure 5.3). Their HOMO density is
localized on the phenanthro-imidazole moiety with oxidation potentials of ~0.90 eV. These
materials are expected to preferentially trap and transport electrons in OLED devices because of
their low reduction potentials and high oxidation potentials.
142

-3.0 -2.5 -2.0 -1.5 0.5 1.0
0.00000
0.00008
Current (A)
Potential (V vs Fc/Fc+)
I1
I2
I3

-3.0 -2.5 -2.0 -1.5 0.5 1.0
0.00000
0.00008
Current (A)
Potential (V vs Fc/Fc+)
I4
I5
I6

Figure 5.8. Cyclic voltammetry curves of II – I6.
Table 5.3. Electrochemical properties of I1–I6
E ox (V)
a
E red (V)
a
HOMO (eV)
b
LUMO (eV)
b

I1 +0.86 -2.61 -5.81 -1.70
I2 +0.87 -2.44 -5.82 -1.89
I3 +0.90 -2.44 -5.86 -1.89
I4 +0.91 -2.32 -5.87 -2.03
I5 +0.91 -2.35 -5.87 -2.00
I6 +0.96 -2.36 -5.94 -2.00
a: reference to ferrocene b: experimental values in eV c: measured in solution using an integrating sphere d: measured as neat
material in films using an integrating sphere
5.2.4. Thermal Properties
Thermal analysis was conducted to study the thermal properties of these materials. All the
materials show good thermal properties with glass transition temperature (Tg) obtained from
differential scanning calorimetry (DSC) as high as 120
o
C (Figure 5.9). Decomposition was not
observed with these materials before sublimation in thermogravimetric analysis (TGA)
measurements (Figure 5.10). Because of their excellent properties, I1 and I4–I6 are carried further
for OLED device studies. They are incorporated as neat blue fluorescent emitters in
monochromatic OLEDs and in hybrid F/P WOLED studies.
143

50 75 100 125
-1
0
1
2
3
4
5
6
7
8
Heat Flow (a.u)
Temperature (
o
C)
I1
I2
I3
I4
I5
I6

Figure 5.9. DSC Curves of I1 – I6.
0 100 200 300 400 500 600
0
20
40
60
80
100
Weight Loss (%)
Temperature (
o
C)
I1
I2
I3
I4
I5
I6

Figure 5.10. TGA curves of I1 – I6.
5.2.5. Electroluminescence Properties
To investigate the electroluminescence properties of these materials, monochromatic
devices of I1 and I4–I6 were fabricated at University of Southern California. Figure 3 shows the
144

monochromatic device architecture of I1 and I4–I6 along with their electroluminescence
properties. In these devices, I1 and I4–I6 were used as neat blue fluorescent emitter in the emissive
layer. The electroluminescence (EL) spectra of I1 and I4–I6 are the same with their individual
photoluminescence (PL) spectrum in thin films, indicating effective singlet exciton confinement
on the emitters. The current density–voltage (J–V) characteristics of I1 and I5 devices are similar,
with I4 and I6 devices as the most conductive and the most resistive devices, respectively. Due to
their high PL efficiency (80%) in neat films, monochromatic devices with I1 and I4–I6 as neat
blue fluorescent emitters achieved close to theoretical maximum efficiency (5%).
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
Wavelength (nm)
I1
I4
I5
I6

-2 0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
I1 L-V
I4 L-V
I5 L-V
I6 L-V
I1 J-V
I4 J-V
I5 J-V
I6 L-V
Voltage (V)
Luminescence (cd/m
2
)
0
100
200
300
400
500
600
Current density (mA/cm
2
)
10
-1
10
0
10
1
10
2
0
1
2
3
4
5
6
EQE (%)
Current density (mA/cm
2
)
I1
I4
I5
I6

Figure 5.11. Monochromatic OLED characteristics of phenanthro[9,10-d]imidazoles as neat blue
fluorescent emitters.

145

Hybrid WOLED was fabricated with I1 (see Figure 5.12) as a host for green dopant and
as a neat fluorescent emitter for blue emission. From the EL spectra, it is deduced that the
recombination zone is mainly at the electron transport layer (ETL) side at low current density but
shifts to the middle zone of the device closer to the hole transport layer (HTL) side at moderate
and high current densities, respectively. I1 and the green dopant (Ir(ppy)3) were inserted at the
ETL side to harvest blue and green emission at low current density. A layer of Ir(ppy)3 on the HTL
side is used for green emission at high current density. Layers of I1 are inserted on either side of
the red dopant (PQIr) to prevent transfer of all the triplet excitons from Ir(ppy)3 to PQIr. Optimized
thin layer of PQIr (0.3 Å or 0.4 Å) was used to prevent saturation of the spectrum by the red dopant.
At low current density, the PQIr emission overwhelms the blue emission resulting in low color
rendering index (CRI). At high current density, the spectrum is well-balanced and thus maximum
CRI is achieved at 100 mA/cm
2
. Warmer white emission was obtained at lower current densities,
whereas cooler white emission is obtained at higher current densities (See Table 5.4 for device
metrics). Maximum luminance efficacy (LPE) as high as 50 lm/W was achieved at low brightness.
Similarly, external quantum efficiency (EQE) of 14% was achieved at low current density. The
roll-off in LPE and EQE are high and our collaborators at University of Michigan are working to
optimize the device and mitigate this issue.

146

0 2 4 6 8 10
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
0.4A
0.3A
Luminescence (cd/m
2
)

Voltage (V)

0
100
200
300
400
500
600
Current density (mA/cm
2
)

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
0.4A PQIr
0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0 0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
0.3A PQIr

10
-2
10
-1
10
0
10
1
10
2
0
2
4
6
8
10
12
14
16

0.4A
0.3A
EQE (%)
Current density (mA/cm
2
)

1 10 100 1000 10000
0
10
20
30
40
50
LPE(lm/W)
Luminance (cd/m
2
)
0.4A
0.3A

Figure 5.12. I1 WOLED device architecture and characteristics.

147

Table 5.4. WOLED device metrics of I1

Current density CIE CCT CRI
D1
0.1mA/cm
2

(0.50, 0.47) 2612 57
D2 (0.49, 0.47) 2802 62
D1
1mA/cm
2

(0.48, 0.46) 2732 65
D2 (0.47, 0.47) 2977 68
D1
10mA/cm
2

(0.45, 0.44) 3049 71
D2 (0.43, 0.45) 3471 73
D1
100mA/cm
2

(0.43, 0.41) 3271 73
D2 (0.39, 0.40) 3831 76
D1 = 0.4Å PQIr and D2 = 0.3Å PQIr
Due to its broad PL spectrum, I5 was utilized as a neat fluorescent emitter to fabricate
WOLED device in order to improve the CRI. This device was fabricated by our collaborators at
University of Michigan. The device architecture and proposed mechanism of exciton formation
and transfer are shown in Figure 5.13. Due to the low efficiency of Ir(ppy)3 in I5, CBP was used
to host Ir(ppy)3. Similar to I1 WOLED device, the recombination zone at low current density is
mainly at the ETL side but shifts to the HTL side at high current densities. At low current density,
the EL spectrum is dominated by the red emission resulting in low CRI and CCT (See Table 5.5).
A well-balanced spectrum is obtained at higher current densities with CRI as high as 94 and CCT
of 4015. Maximum LPE as high as 55 lm/W and EQE of 20% were achieved at low brightness.
Like devices based on I1, the roll-off in LPE and EQE in this device are high and they must be
addressed for the WOLEDs to have practical applications. Nevertheless, the efficacy and EQE
obtained with both I1 and I5 based hybrid WOLED are one of the best reported so far.
148

400 450 500 550 600 650 700 750
0
1
2
3
4
5
EL Intensity (a.u.)
Wavelength (nm)
1mA/cm2
10mA/cm2
100mA/cm2

0 2 4 6 8 10 12 14
1E-7
1E-5
0.001
0.1
10
1000
Current density (mA/cm
2
)
Voltage (V)

0.01 0.1 1 10 100
0
5
10
15
20
25
EQE (%)
Current density (mA/cm
2
)
1 10 100 1000
0
10
20
30
40
50
60
LE (lm/W)
Luminance (cd/m
2
)

Figure 5.13. I5 WOLED device architecture and characteristics.

149

Table 5.5. WOLED device metrics of I5
Current density CIE CCT CRI
0.1mA/cm
2
(0.55,0.39) 1684 68.1
1mA/cm
2
(0.52,0.40) 1979 76.4
10mA/cm
2
(0.45,0.39) 2734 87.4
100mA/cm
2
(0.38,0.39) 4015 93.6

5.3. Conclusion
In this work, we strategically designed and developed phenanthro[9,10-d]imidazole
materials as potential blue fluorescent emitters and hosts in hybrid F/P WOLED application. All
the materials were synthesized in high yields and purified via sublimation. The photophysical and
electrochemical properties of the phenanthro[9,10-d]imidazoles are finely tuned by simple
substitution of pyridine and/or phenyl at the C2 or N2 position of the phenanthro[9,10-d]imidazole.
This substitution resulted in six different imidazoles (I1−I6) with varied photophysical and
electrochemical properties. I4−I6 have blue fluorescent emission around 450 nm in the solid state
which is ideal for hybrid WOLED application. The triplet energies of I1−I3 in the solid-state are
high enough to host a green phosphor. The efficiency of the green phosphor is however quenched
in I4−I6 due to their low triplet energies in the solid state, but high enough to host an orange and
a red phosphor. I1 and I4−I6 have high efficiencies (close to unity) both in solution and in the
solid state. However, the efficiencies of I2 and I3 are low both in solution and in the solid state.
All the materials have deep HOMO and LUMO levels, making them ideal materials for electron
trapping and transport. Their thermal properties are also excellent having glass transition
temperatures around 120
o
C with no decomposition observed before sublimation. The reported
materials were incorporated as neat blue fluorescent emitters in monochromatic fluorescent
OLEDs with theoretical maximum (5%) efficiency. I1 was utilized as both neat blue fluorescent
150

material and as host for the green dopant in hybrid F/P WOLEDs. The WOLED devices achieved
excellent properties including moderate CRI (62), warm white emission (CCT ~ 2700), high LPE
(50 lm/W) and high EQE (14%) at low current densities. Similarly, WOLED device utilizing I5
as a neat blue fluorescent material is highly efficient (EQE = 20%) with a high luminous efficacy
(55 lm/W), moderate CRI (68) and warm white emission (~1690) at low voltages (< 5 V).
However, both I1 and I5 based WOLED devices exhibit steep roll-off which is probably due to
self-annihilation of the red phosphor and charge buildup at high current densities. Currently,
engineering methods are employed to alleviate the roll-off observed in these WOLED devices.
The strategy used in this work to develop these excellent materials could be extended to
develop fluorescent materials from a different class for hybrid white OLED applications.
Additionally, this work demonstrated that a highly efficient hybrid F/P WOLED could be achieved
using a single emissive layer with these new blue fluorescent emitters along with conventional
green and red phosphors.
5.4. Experimental
5.4.1. Synthesis
4-(pyridin-4-yl)aniline
In a 250 ml one-necked round bottomed flask, 4-pyridineboronic acid (14.3 g, 116.2
mmol), 4-bromoaniline (10.0 g, 58.1 mmol), tetrakis(triphenylphosphine)palladium(0) (185 mg,
0.174 mmol), and Na2CO3 (43.1 g, 406.9 mmol) were added. The flask is purged and backfilled
with nitrogen three times. 180 ml of degassed DMF/H2O (9:2) was canula transferred to the flask
and heated to reflux overnight. The reaction mixture was then cooled to room temperature and
diluted with dichloromethane (CH2Cl2). The organic layer was separated, and the solvent removed
under reduced pressure. The product was then precipitated from CH2Cl2 to yield a pale yellow
151

solid (5.30 g, 53%).
1
H NMR (Chloroform-d) 8.60 (d, J = 6.0 Hz, 2 H), 7.51 (d, J = 8.5 Hz, 2 H),
7.47 (d, J = 6.0 Hz, 2 H), 6.79 (d, J = 9.0 Hz, 2 H), 3.88 (s, 2 H).
1,2-di([1,1'-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (I1)
A mixture of phenanthrene-9,10-dione (2.00 g, 9.61 mmol), [1,1'-biphenyl]-4-amine (1.95
g 11.53 mmol), [1,1'-biphenyl]-4-carbaldehyde (1.75 g, 9.61 mmol) and ammonium acetate (1.48
g, 19.21 mmol) in glacial acetic acid (30 mL) was refluxed for 3 h. The precipitate was filtered
and washed with aqueous NaOH and deionized water. The residue is dried and sublimed at 250
o
C
and 1.2 ×10
-6
torr to give pure product. White solid (4.20 g, 8.04 mmol, 84%):
1
H NMR (400 MHz,
Acetone-d6) δ 8.93 (d, J = 8.4 Hz, 1H), 8.90 – 8.81 (m, 2H), 8.10 – 8.04 (m, 2H), 7.93 – 7.76 (m,
7H), 7.73 – 7.65 (m, 5H), 7.57 (td, J = 7.0, 1.2 Hz, 3H), 7.51 – 7.42 (m, 3H), 7.41 – 7.32 (m, 3H).
HRMS calculated for C39H26N2 [M+H]
+
522.2096, found 523.8107.
2-([1,1'-biphenyl]-4-yl)-1-(4-(pyridin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (I2)
A mixture of phenanthrene-9,10-dione (2.20 g, 10.57 mmol), 4-(pyridin-4-yl)aniline (2.16
g 12.68 mmol), [1,1'-biphenyl]-4-carbaldehyde (1.93 g, 10.57 mmol) and ammonium acetate (1.63
g, 21.13 mmol) in glacial acetic acid (70 mL) was refluxed for 3 h. The precipitate was filtered
and washed with aqueous NaOH and deionized water. The residue is dried and sublimed at 280
o
C
and 1.2 ×10
-6
torr to give pure product. White solid (2.13 g, 4.06 mmol, 39%):
1
H NMR (400 MHz,
Acetone-d6) δ 8.94 (d, J = 8.5 Hz, 1H), 8.88 (d, J = 8.3 Hz, 1H), 8.86 – 8.81 (m, 1H), 8.77 – 8.71
(m, 2H), 8.23 – 8.18 (m, 2H), 7.98 – 7.92 (m, 2H), 7.91 – 7.86 (m, 2H), 7.83 – 7.78 (m, 2H), 7.73
– 7.65 (m, 4H), 7.57 (ddd, J = 8.3, 5.5, 2.7 Hz, 1H), 7.50 – 7.42 (m, 2H), 7.41 – 7.32 (m, 2H).
HRMS calculated for C38H25N3 [M+H]
+
523.2049, found 524.8173.

152

2-phenyl-1-(4-(pyridin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (I3)
A mixture of phenanthrene-9,10-dione (1.50 g, 7.20 mmol), 4-(pyridin-4-yl)aniline (1.47
g 8.64 mmol), benzaldehyde (0.77 g, 7.20 mmol) and ammonium acetate (1.11 g, 14.41 mmol) in
glacial acetic acid (50 mL) was refluxed for 3 h. The precipitate was filtered and washed with
aqueous NaOH and deionized water. The residue is dried and sublimed at 270
o
C and 1.2 ×10
-6

torr to give pure product. White solid (1.18 g, 2.64 mmol, 37%):
1
H NMR (400 MHz, Acetone-d6)
δ 8.96 – 8.78 (m, 3H), 8.77 – 8.70 (m, 2H), 8.48 – 8.41 (m, 1H), 8.37 – 8.31 (m, 2H), 8.20 – 8.12
(m, 1H), 7.92 – 7.85 (m, 1H), 7.81 – 7.61 (m, 6H), 7.61 – 7.53 (m, 3H), 7.52 – 7.44 (m, 1H), 7.40
– 7.30 (m, 1H). HRMS calculated for C32H21N3 [M+2H]
+
447.1736, found 449.0740.
1-([1,1'-biphenyl]-4-yl)-2-(4-(pyridin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (I4)
A mixture of phenanthrene-9,10-dione (2.00 g, 9.61 mmol), [1,1'-biphenyl]-4-amine (1.95
g 11.53 mmol), 4-(pyridin-4-yl)benzaldehyde (1.76 g, 9.61 mmol) and ammonium acetate (1.48 g,
19.21 mmol) in glacial acetic acid (70 mL) was refluxed for 3 h. The precipitate was filtered and
washed with aqueous NaOH and deionized water. The residue is dried and sublimed at 270
o
C and
1.2 ×10
-6
torr to give pure product. White solid (4.20 g, 8.02 mmol, 84%):
1
H NMR (400 MHz,
Acetone-d6) δ 8.94 (d, J = 8.2 Hz, 1H), 8.90 – 8.86 (m, 1H), 8.86 – 8.81 (m, 1H), 8.66 – 8.61 (m,
2H), 8.11 – 8.05 (m, 2H), 7.93 – 7.84 (m, 6H), 7.83 – 7.76 (m, 3H), 7.74 – 7.70 (m, 1H), 7.70 –
7.63 (m, 2H), 7.58 (ddd, J = 8.1, 6.4, 1.4 Hz, 3H), 7.52 – 7.45 (m, 1H), 7.42 – 7.32 (m, 2H). HRMS
calculated for C38H25N3 [M+H]
+
523.2049, found 524.8170.
1-phenyl-2-(4-(pyridin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (I5)
A mixture of phenanthrene-9,10-dione (2.00 g, 9.61 mmol), aniline (1.07 g 11.53 mmol),
4-(pyridin-4-yl)benzaldehyde (1.76 g, 9.61 mmol) and ammonium acetate (1.48 g, 19.21 mmol)
in glacial acetic acid (70 mL) was refluxed for 3 h. The precipitate was filtered and washed with
153

aqueous NaOH and deionized water. The residue is dried and sublimed at 270
o
C and 1.2 ×10
-6

torr to give pure product. White solid (3.65 g, 8.15 mmol, 85%):
1
H NMR (400 MHz, Acetone-d6)
δ 8.95 – 8.89 (m, 1H), 8.87 (ddt, J = 8.2, 1.2, 0.6 Hz, 1H), 8.82 (ddd, J = 7.9, 1.5, 0.6 Hz, 1H),
8.68 – 8.62 (m, 2H), 7.85 – 7.74 (m, 10H), 7.71 – 7.66 (m, 3H), 7.57 (ddd, J = 8.3, 7.0, 1.3 Hz,
1H), 7.33 (ddd, J = 8.2, 7.0, 1.2 Hz, 1H), 7.22 (ddd, J = 8.3, 1.4, 0.6 Hz, 1H. HRMS calculated for
C32H21N3 [M+2H]
+
447.1736, found 449.0914.
1,2-bis(4-(pyridin-4-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (I6)
A mixture of phenanthrene-9,10-dione (2.50 g, 12.01 mmol), 4-(pyridin-4-yl)aniline (2.45
g 14.41 mmol), 4-(pyridin-4-yl)benzaldehyde (2.20 g, 12.01 mmol) and ammonium acetate (1.85
g, 24.01 mmol) in glacial acetic acid (80 mL) was refluxed for 3 h. The precipitate was filtered
and washed with aqueous NaOH deionized water. The residue is dried and sublimed at 290
o
C and
1.2 ×10
-6
torr to give pure product. White solid (4.30 g, 8.20 mmol, 68%):
1
H NMR (400 MHz,
Acetone-d6) δ 8.94 (d, J = 8.4 Hz, 1H), 8.91 – 8.85 (m, 1H), 8.83 (dd, J = 8.0, 1.5 Hz, 1H), 8.78 –
8.72 (m, 2H), 8.66 – 8.61 (m, 1H), 8.25 – 8.19 (m, 2H), 7.99 – 7.94 (m, 2H), 7.92 – 7.83 (m, 3H),
7.83 – 7.75 (m, 3H), 7.75 – 7.65 (m, 4H), 7.65 – 7.51 (m, 2H), 7.38 – 7.30 (m, 2H). HRMS
calculated for C37H24N4 [M+H]
+
524.2001, found 525.8343.
5.5. References
1. Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and S. R. Forrest, Nature, 2006,
440, 908-912.
2. C. Coburn, C. Jeong and S. R. Forrest, ACS Photonics, 2018, 5, 630-635.
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2018, 28, 1707002.
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7. S. Scholz, D. Kondakov, B. Lüssem and K. Leo, Chemical Reviews, 2015, 115, 8449-8503.
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and C.-S. Lee, ACS Applied Materials & Interfaces, 2017, 9, 7331-7338.
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155

1Appendix − High Triplet Energy N2-phenanthro[9,10-d]-triazoles with Bulky
Substituents
Because of their instability in OLED devices, N1-aryl substituted triazoles were not considered
for further OLED studies (Chapter 2). Instead, we focused on developing high triplet energy N2-
aryl substituted phenanthro-triazoles. In the early stage of this work, we couldn’t obtain bulky aryl
substituted N2-triazoles with traditional Ullman and Buckwald-Hartwig coupling reactions. The
only N2-aryl isomer (2-PT) that was obtained with traditional Ullman and Buckwald-Hartwig
coupling reactions has very low triplet energy and so it was not considered for OLED fabrication.
Recently, a new procedure for synthesizing N2-aryl triazoles with bulky groups was reported.
1
By
utilizing this procedure, we were able to obtain triazoles with bulky aryl groups at their N2-
positions (Figure A1.1). The resulting new host materials, 2-xT and 2-mxT have absorption and
emission spectra similar to their N1 aryl analogs (Figure A1.2). Their singlet and triplet energies
are higher than that of the N2-phenyl analog (2-PT). Their triplet energies in solution is 2.97 eV
and redshifted by about 0.1 eV in the solid state (ETsolid = 2.77 eV) (Figure A1.1). The PLQY of
2-xT and 2-mxT is higher (75%) than their N1-substitued analogs (<7%) suggesting higher
stability of the N2-triazoles in the excited states over the N1-triazoles. Furthermore, unlike the N1-
triazoles where they degrade in MALDI, the mass-spectra of N2-triazoles display no
decomposition products (see Figure A1.3). These new hosts have wide bandgap, similar to their
N1-analogs, ideal for hosting sky-blue emitters and as hole blocking materials.
156

Figure A1.1. Structures of N1- and N2-phenanthro[9,10-d]-triazolesalong with their bond
dissociation energies (BDE) calculated using DFT
300 350 400 450
0.0
0.1
0.2
0.3
0.4
Absorbance (a.u)
Wavelength (nm)
2-pT
2-xT
2-mxT
350 400 450 500 550
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
2-pT
2-xT
2-mxT
2-MeTHF 298K

400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
2-pT
2-xT
2-mxT
2-MeTHF 77K
450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
PL Intensity
Wavelength (nm)
2-pT
2-xT
2-mxT
Solid 77K

Figure A1.2. Photophysical properties of 2-pT, 2-xT and 2-mxT
157

Figure A1.3. MALDI data of 2-mxT
Table A1.1. Summarized properties of N2-phenanthro-triazoles

 (%) E ox (V) E red(V) HOMO (eV) LUMO (eV)
2-mxT 64 +1.57 -2.83 -6.60 -1.49
15wt% FIrpic:2-mxT 60 - - - -
2-xT 77 - - - -
2-pT 78 +1.40 -2.44 -6.40 -1.95
Prior to making devices with these materials, 15 wt% of FIrpic was doped into 2-mxT. The
efficiency of FIrpic in this host material is 60%. The efficiency could be improved by lowering
the doping concentration. Preliminary OLED devices were then fabricated at University of
Southern California. Optimized FIrpic devices with a reference hosts (mCBP) give moderate
efficiency (max EQE = 14.5%). This is done to optimize the device performance using the
reference host before using the new host material. Using the optimized mCBP based device
architecture, mCBP host is replaced with 2-mxT. The EQE of 2-mxT based device is lower than
158

that of mCBP based device. Optimizing the OLED device with 2-mxT host could improve the
efficiency. Both the mCBP and 2-mxT devices are resistive due to the use of thick transport layers
to achieve good charge balance. The EL of both devices match the PL of FIrpic indicating effective
charge recombination on the dopant (Figure A1.4). The roll-off of 2-mxT based device is better
than those of N1-aryl triazoles suggesting better stability of the N2-aryl isomer over the N1-aryl
isomers of the triazole.
Device fabrication with 2-mxT as hole blocking material is in progress. 2-mxT will be
scaled up and sent to University of Michigan Ann Arbor for device optimization. In addition,
device stability studies will be performed at University of Michigan Ann Arbor with 2-mxT and
txI (Chapter 2).

159

400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0

EL Intensity
Wavelength (nm)
mCBP
2-mxT

0.01 0.1 1 10 100
0
5
10
15
20
EQE (%)
Current Density (mA/cm
2
)
mCBP
2-mxT
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5

mCBP LV
2-mxT LV
mCBP JV
2-mxT JV
Voltage (V)
Luminescence (cd/m
2
)
0
50
100
150
200
Current density (mA/cm
2
)

Figure A1.4. OLED device characteristics of phenanthro[9,10-d]imidazole and reference hosts.
(top) Device architecture and EL spectra. (bottom) Efficiency versus current curves, J–V curves
and Luminance versus current curves.
References
1. S. Roshandel, M. J. Lunn, G. Rasul, D. S. Muthiah Ravinson, S. C. Suri and G. K. S.
Prakash, Organic Letters, 2019, 21, 6255-6258.

Abstract (if available)

Abstract Over the past several years, organic light-emitting diodes (OLEDs) have attracted considerable attention for full color display and solid-state lighting applications owing to their high electroluminescence quantum efficiency. Despite their high efficiency, major deficiencies remain in OLEDs. Blue PHOLEDs in particular, are inefficient and unstable due to degradation of host materials and phosphorescent emitters (dopants) in the emissive layer. Most commonly used host materials and dopants feature weak C-N/Ir-N bonds which have been attributed to molecular degradation during device operation. For this reason, alternative host materials and phosphorescent emitters are strategically designed and developed here with the aim of achieving both high efficiency and stability in blue OLEDs. Following computational screening of a large library of host materials, phenanthro[9,10-d]triazoles and imidazoles were found to have higher C-N bond dissociation and triplet energies (BDE = 92 kcal/mol, ET = 2.97 eV) than commonly used carbazole-based host materials (BDE = 83 kcal/mol, ET = 2.93 eV) for blue OLEDs. Synthesis, electrochemical, photophysical and electroluminescence characterization of these materials as host materials for blue PHOLEDs are discussed in Chapter 2. ❧ The second class of materials, tris-iridium (III) based blue phosphorescent emitters featuring NHC pyrydino and pyrazinoimidazolyl carbene ligands, are discussed in Chapter 3. These materials are synthesized as alternative sky blue dopants to the current unstable sky blue phosphorescent dopants. They are chosen due to their stronger Ir-C bond compared to commonly used phosphors with weaker Ir-N bonds. Computational and photophysical studies were carried out to understand their extraordinary photophysical properties, including fast radiative rates (1.3×10⁶ s⁻¹). Optimized blue PHOLED devices using the host materials and the phosphorescent emitters developed in Chapter 2 and 3 achieved excellent electroluminescence efficiency (21%), high brightness (35 000 cd/m²) at low current density and moderate device lifetime (T₈₀ = 16 h). ❧ In Chapter 4, two-coordinate coinage metal (i.e. Cu, Ag, Au) complexes bearing a bulky benzoimidazolyl carbene and carbazolyl as the anion ligand are discussed. These materials emit via thermally activated delayed fluorescence (TADF) and exhibit efficient and deep blue narrow emission, crucial for display applications. The electrochemical and photophysical properties of these complexes were studied to understand the origin of their interesting properties, including efficient and deep blue narrow emission in nonpolar medium and abrupt decrease in efficiency in polar medium. OLED devices incorporating these materials as dopants achieved 12% EQE with narrow and deep blue emission. ❧ In Chapter 5, neat blue fluorescent emitters are designed and developed to reduce the number of stacked layers of hybrid fluorescent/phosphorescent (F/P) white organic light-emitting diodes (WOLEDs), which is aimed at reducing costs and lowering driving voltages. Several phenanthro[9,10-d]imidazole-based materials featuring high fluorescence efficiencies in neat films were developed. Their singlet and triplet excited states were engineered to serve as both neat fluorescent emitters and hosts for green and red phosphors in hybrid WOLEDs. A highly efficient (20%) hybrid F/P WOLED with a high luminous efficacy (65 lm/W) at low driving voltages (< 5 V) was demonstrated using a single emissive layer with these new blue fluorescent emitters along with conventional green and red phosphors.

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Creator Idris, Muazzam (author)

Core Title Molecular approaches to solve the blue problem in organic light emitting diode display and lighting applications

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 04/21/2021

Defense Date 03/04/2020

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

Tag display,dopant,electroluminescence,fluorescent,host,OAI-PMH Harvest,organic light-emitting diode,phosphorescent,solid-state lighting,thermally activated delayed fluorescence

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Advisor Thompson, Mark E. (committee chair), Gupta, Malancha (committee member), Zhang, Chao (committee member)

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