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Development of N-type chromophores for organic photovoltaics, and thermally activated delayed fluorescence NHC complexes for organic light-emitting diodes

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Development of N-type chromophores for organic photovoltaics, and thermally activated delayed fluorescence NHC complexes for organic light-emitting diodes

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Content Development of N-type Chromophores for Organic
Photovoltaics, and Thermally Activated Delayed
Fluorescence NHC Complexes for Organic
Light-Emitting Diodes

by

Narcisse Ukwitegetse

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
CHEMISTRY

December 2020

Copyright 2020 Narcisse Ukwitegetse
ii

DEDICATION

Unreservedly, I would like to dedicate this Doctoral Degree accomplishment to my so dearly missed Father,
Pierre Claver Habagakunzi, who was an Educator but sadly passed away too soon to see me finish (nor
even start) school.
iii

ACKNOWLEDGEMENTS

“If I have seen further, it is by standing on the shoulders of giants” – Isaac Newtown.
First, I would like to enormously thank my research advisor, Prof. Mark E. Thompson (a.k.a MET), under
whose guidance and supervision I am completing this doctorate program. MET research lab is full of so
many opportunities and learning experiences, and it has been a great privilege to be part of it. Prof
Thompson permitted me to explore several chemistry projects with his unyieldingly supportive guidance
and mentorship. I thank him for pushing me to become a great chemist and exposing me to the “hottest”
and important topics in today’s Chemistry and Engineering communities: OLEDs and OPVs.
I would like to thank Prof Peter Djurovich, a research professor in MET lab. He has provided me with very
rich scientific (and non-scientific) ideas/discussions, including his constant updates on the most relevant
research topics whether from new or old literature. He always challenged me to go the extra mile and do
chemistry, not only because it is useful, but also interesting.
I would like to extend my appreciations to Prof. Barry Thompson, Prof. Andrea Armani, Prof. Richard
Brutchey, and Prof. Sri Narayan who served as the committee members on my Ph.D Defense and/or
Qualifying & Screening exams. Their support, advice, and availability are precious and highly
appreciated. I also thank Prof. Brent Melot and Prof. Megan Fieser, for I benefited from working next
to their research labs by using chemicals/equipment from their labs or just benefiting from their expertise
in sciences, not to mention their help in troubleshooting or building lab equipment. I also thank Prof.
Lincoln Hall who worked in MET research group and was always sharing his chemistry knowledge and
wisdom with me.
I am eternally grateful for MET group members, as well as other collaborators, who became good friends
of mine and made my graduate work atmosphere so friendly and fun. Special THANKS to: Dr. Patrick
Saris and Dr. Jonathan Sommer who contributed to the tetra-aza-pentacene project; Daniel Sylvinson who
iv

played a huge role in all computational-related work; Dr. Rasha Hamze and Dr. Shuyang, good friends of
mine who pioneered the coinage metal NHC complexes projects that I latter picked up; Mattia Di Niro, my
close and hard-working project partner; Dr. Tia-Yi Li, Dr. Muazzam Idris, Jie Ma, and Collin Muniz who
contributed to the progress of the carbene projects; Savannah Kapper, my awesome hood-mate who would
watch and monitor my reactions in my absence; Dr. John Chen and Konstatin Mallon, my chemical safety
co-officers; Moon Chul Jung, my Vapor deposition system trainer; Abegail Tadle and Taylor Hodgekins,
my “go-to” friends for crystal structure experiments or break mingling; Ariel Vaughn, a great friend of
mine and my job-hunt mock interviewer; Ali Akil and Dr. Karim El Roz, my fun co-workers and Lebanese
language teachers; Mahsa Rezaiyan and Niki Bayat, my Farsi Language teachers; Austin Menke and Brenda
Ontiveros, my contagiously happy co-workers who would offer their help any time I needed their hand. I
very much thank all new great lab-mates for all kinds of support: Dr. Sunil Kandappa, Dr. Eric McClure,
Allen Darius Shariaty, Gemma Goh, Megan Cassingham, and Anton Razgoniaev.
I would like to acknowledge our collaborators from UCLA, Prof. Neil Garg’s group (Dr. Robert Susick,
Katie Spence, and Jason Chari) who shared with us their coolest and newly developed ligands to try in our
two-coordinated TADF complexes for OLED application. Also, I thank Prof Ralf Haiges who would offer
help for X-ray crystallographic measurements whenever we needed assistance.
I wish to express my sincere, deep appreciations to my undergraduate Chemistry professors, especially
Prof. David Hales (my academic advisor), Prof. Thomas Goodwin, and Prof. Liz Gron. They inspired me
to truly love Chemistry and equipped me with all it takes to embark on and face graduate school.
A great deal of gratitude goes to Judy Fong, Magnolia Benitez, and Michele Dea for their incredible
administrative assistance. They made every step of my Ph.D program smoother by handling all necessary
administrative paper-works and documents. Judy has particularly been available and supportive in all sorts
of businesses, whether lab-related or personal life mentorship. I am very grateful for her kindness on top of
her sacrifice she makes for MET group and the Department in general.
v

Importantly, I wish to honor and thank my family and friends for their unmeasurable love and support
throughout. My lovely Wife, Lily Umutoniwase, has been my rock throughout the whole PhD struggle and
has never ceased to believe in me and cheered for me every single day, all the years; Her patience,
encouragement, and LOVE have been superb despite being physically thousands of miles away. Similarly,
I cannot express enough gratitude and love to my family-in law who have been constantly praying and
blessing me and my wife for this accomplishment to happen. We love and cherish them so dearly.
I am eternally grateful to my amazing mother, Bertha, for her indescribable sacrifice and support during the
hardest financial conditions to make sure I stay in and keep up with school. I am so thankful for my lovely,
amazing siblings: Appoline, Mado, Gabriel, Salome, and Bosco for taking care of our mother while I was
physical absent doing school abroad.
Many thanks to an amazingly fun and caring Land-lady, Tsae Askey, who went from being just a landlord
to a “grand-mother” to me and Dr. Robert Nshimiyimana, whom I also appreciate very much for being an
awesome room-mate. It is hard to believe that we stayed with Tsae for our entire PhD careers; we deeply
thank Her for taking care of us and making our graduate school fight go so smoothly.
I am thankful for a wonderful family of the Butuye’s (Alex, Gisele, Alexson & Alexis) who would welcome
me into their home during many weekends and holidays to sing, dance and relax with them whenever the
research stress was getting uncontrollable. Their kindness and hospitality have been priceless during my
PhD battle.
I would never thank enough my host families from Arkansas: The LEWIS’s (Mark, Tory, kids and
grand-parents) and the BUTLER’s (Dempsey, Jeanne, Karis & Rusty) who not only offered me a home
when I came in the US, but also treated me like their very own, and have made my academic journey
smooth and doable. Their love and kindness to me have been incredible throughout this journey.
vi

Lastly, I would like to acknowledge the Government of Rwanda for giving me the Presidential Scholarship
to pursue my undergraduate studies in the US, which later opened the doors for me to pursue this PhD
program.
To all my friends, families, and acquaintances I crossed paths with along this journey, you have been an
unwavering source of support, love and encouragement that lifted me up and led me to this accomplishment.
TO ALL OF YOU, THANK YOU, AND GOD BLESS YOU!

vii

Table of Contents
DEDICATION .............................................................................................................................................. ii
ACKNOWLEDGEMENTS ......................................................................................................................... iii
List of Figures ............................................................................................................................................... x
List of Tables .............................................................................................................................................. xv
Abstract ..................................................................................................................................................... xvii
1 CHAPTER I: Introduction .................................................................................................................... 1
1.1 Organic Photovoltaics ................................................................................................................... 1
1.1.1 Overview on Global Energy Demand ................................................................................... 1
1.1.2 Fundamentals of organic photovoltaics ................................................................................ 2
1.1.3 Recent Successes in non-fullerene acceptors development .................................................. 4
1.1.4 Challenges of synthetic complexity of today’s NFAs ........................................................... 5
1.2 Organic Light-emitting diodes ...................................................................................................... 5
1.2.1 Spin-orbit coupling (SOC) and phosphorescence ................................................................. 6
1.2.2 Thermally activated delayed fluorescence: TADF ............................................................... 8
1.2.3 Davydov splitting and multipole interactions in bimetallic TADF phosphors ................... 12
1.3 Other molecular semiconductor investigations ........................................................................... 13
1.4 Recapitulation of Chapters’ Layout. ........................................................................................... 14
1.5 References ................................................................................................................................... 15
2 CHAPTER II: One-pot Friedländer synthesis of tetra-aza-pentacenes: N-type small molecules for
OPVs ........................................................................................................................................................... 19
2.1 Introduction ................................................................................................................................. 19
2.2 Results and Discussion ............................................................................................................... 21
2.2.1 Synthesis ............................................................................................................................. 21
2.2.2 Photophysical Properties ..................................................................................................... 25
2.2.3 Electrochemical Properties ................................................................................................. 27
2.2.4 X-ray analysis ..................................................................................................................... 28
2.3 Electronic properties: Device testing .......................................................................................... 30
2.4 Conclusion .................................................................................................................................. 31
2.5 Experimental ............................................................................................................................... 31
2.5.1 General information ............................................................................................................ 31
2.5.2 Synthesis of precursors ....................................................................................................... 32
2.5.3 Synthesis of Tetra-aza-pentacenes (tAPs) ........................................................................... 36
viii

2.5.4 Single crystal structure data collection ............................................................................... 39
2.5.5 Anaerobic electrochemical cell set-up ................................................................................ 42
2.5.6 NMRs spectra ...................................................................................................................... 43
2.6 References ................................................................................................................................... 53
3 CHAPTER III: Synthesis and characterization of Zinc(II) complexes bearing 4-acridinol and
1-phenazinol ................................................................................................................................................ 57
3.1 Introduction ................................................................................................................................. 57
3.2 Results and Discussions .............................................................................................................. 58
3.2.1 Synthesis ............................................................................................................................. 58
3.2.2 Electrochemical properties .................................................................................................. 61
3.2.3 Photophysical properties ..................................................................................................... 62
3.2.4 DFT Calculations ................................................................................................................ 66
3.3 Conclusion .................................................................................................................................. 68
3.4 Experimental ............................................................................................................................... 69
3.4.1 General information ............................................................................................................ 69
3.4.2 Synthesis of complexes ....................................................................................................... 70
3.4.3 NMR spectra ....................................................................................................................... 73
3.5 References ................................................................................................................................... 77
4 CHAPTER IV: Two-coordinate N-heterocyclic carbene gold(I) complexes bearing
phenanthrocarbazolyl donor ligand ............................................................................................................. 79
4.1 Introduction ................................................................................................................................. 79
4.2 Results and Discussion ............................................................................................................... 79
4.2.1 Synthesis ............................................................................................................................. 79
4.2.2 Electrochemical properties .................................................................................................. 80
4.2.3 Computational studies ......................................................................................................... 83
4.2.4 Photophysical properties ..................................................................................................... 85
4.3 Conclusion .................................................................................................................................. 92
4.4 Experimental ............................................................................................................................... 93
4.4.1 General information ............................................................................................................ 93
4.4.2 Synthesis of Ligands ........................................................................................................... 93
4.4.3 Synthesis of Complexes ...................................................................................................... 95
4.4.4 Measurements information ................................................................................................. 97
4.5 References ................................................................................................................................... 98
5 CHAPTER V: Two-coordinate coinage metal NHC complexes bearing phenanthrimidazolyl amide
donor ......................................................................................................................................................... 100
ix

5.1 Introduction ............................................................................................................................... 100
5.2 Results and Discussion ............................................................................................................. 100
5.2.1 Synthesis ........................................................................................................................... 100
5.2.2 Electrochemical properties ................................................................................................ 101
5.2.3 DFT/TDDFT Studies ........................................................................................................ 103
5.2.4 Photophysical properties ................................................................................................... 105
5.3 Conclusion ................................................................................................................................ 110
5.4 Experimental ............................................................................................................................. 110
5.4.1 Synthesis of complexes ..................................................................................................... 110
5.4.2 Other measurements’ information .................................................................................... 112
5.5 References ................................................................................................................................. 114
6 APPENDIX: Binuclear N-heterocyclic Carbene Complexes............................................................ 116
6.1 APPENDIX A: Synthesis of benzo-bis(imidazolylidene) carbenes for bimetallic NHC
complexes ............................................................................................................................................. 117
6.1.1 Results and Discussion...................................................................................................... 117
6.1.2 Photophysical properties of unidentified polymeric complexes of BBI ........................... 120
6.1.3 Theoretical studies ............................................................................................................ 121
6.1.4 Conclusion ........................................................................................................................ 122
6.1.5 Experimental ..................................................................................................................... 123
6.2 APPENDIX B: A case study of a bis(amide) NHC binuclear complex featuring
diketopyrrolopyrrole ............................................................................................................................. 130
6.2.1 Results and Discussion...................................................................................................... 130
6.2.2 Conclusion ........................................................................................................................ 132
6.2.3 Experimental ..................................................................................................................... 133
6.2.4 References ......................................................................................................................... 134

x

List of Figures
Chapter I
Figure 1. 1: Verified high research cell efficiency chart from NREL. Today’s OPV efficiency record has
surpassed 17% since 2018..................................................................................................................... 2
Figure 1. 2: Architectures of OPVs. A) Planar heterojunction (bilayer structure). B) Bulk heterojunction
structure. C) Mechanism of light conversion: (a) Light absorption → exciton formation, (b) exciton
diffusion toward D/A interface. (c) charge transfer ( → exciton dissociation). (d) charge transport and
collection. .............................................................................................................................................. 3
Figure 1. 3: Example of A-D-A non-fullerene acceptor, IEIC, and its multi-step synthesis. ...................... 5
Figure 1. 4: Luminescence processes governing present generations of OLEDs. Fluorescence ( 1
st
Gen),
phosphorescence (2
nd
Gen), and thermally activated delayed fluorescence – TADF (3
rd
Gen). ......... 6
Figure 1. 5: Simplified kinetics of (left) phosphorescence in conventional heavy-metal organometallic
emitters (e.g. Ir(ppy) 3); and (right) TADF kinetics of organic vs. inorganic molecules. In the present
TADF kinetics, the three triplet sublevels are assumed to be degenerate; the emission efficiency is
assumed to be close to unity, and hence non-radiative rates are considered relatively insignificant and
not shown. The simplified expressions of  ph and  TADF are given. ...................................................... 9
Figure 1. 6: Examples of recently reported coinage metal(I) TADF phosphors with high quantum yield
(~100%) an sub-microsecond lifetimes. (Bottom) Phenanthrocarbazolyl and phenanthrimidazolyl are
new N-donor ligands that will be explored in Chapter 4&5. .............................................................. 11
Figure 1. 7: Model of Davydov splitting and dipole-dipole interaction by considering either bis-carbene or
bis-amide as a binuclear supporting core. Radiative rates can be increased in TADF systems adopting
this design. (See Appendix for further development). ........................................................................ 13
Chapter II
Figure 2.1: (A) Retrosynthesis of tetra-aza-pentacenes comprised of pyridine moieties. (B)
Litterature-know, not-fully aromatized Friedländer product from base-catalyzed conditions with ring
fusion at the 2,3-positions of the dione. (C) Our developed one-pot Friedländer synthesis of
tetra-aza-pentacenes. ........................................................................................................................... 20
xi

Figure 2.2: Proposed mechanism for linear and/or bent tAP formation. The two mechanistic routes from
1,4-cyclohexanedione depend on the base utilized: piperidine (weak base) and NaOH (strong base). A
weak base-catalyzed Friedlander reaction solely yields bent products, whereas strong base-catalyzed
reaction yields a mixture of linear and bent isomers. .......................................................................... 22
Figure 2.3: Proposed in-situ aromatization mechanism of linear and bent tAP in the presence of strong base.
............................................................................................................................................................ 23
Figure 2.4: NOESY NMR analysis for atropisomers of compound l 4. The two singlets were selected for
excitation: 4.85 ppm (top NOESY) and 5.08 ppm (middle NOESY). The lack of coupling through
space indicates that the two protons are non-geminal, but that there exist two atropisomers (syn and
anti). .................................................................................................................................................... 24
Figure 2.5: Photophysical properties of tAPs: Absorption (solid lines) and emission (dotted lines) spectra
of linear tAPs (A) and bent tAPs (B) in DMSO. (C) Aerobic photooxidation of tAP 3l monitored via
absorption spectra. Note that the sample C profiles is a mixture of linear and bent isomers (3l & 3b).
............................................................................................................................................................ 26
Figure 2.6: Cyclic voltammograms of l 5 and b 5 at 100 mV/s scan rate. In the CV of b 5, two reversible
reduction peaks are observed at -1.65 V and -1.99 V corresponding to E LUMO of -2.88 eV. The linear
tAP isomer l 5 shows irreversible behavior with a reduction peak appearing at -0.95 V (i.e
E LUMO = -3.71 eV.) The bent isomer is, as expected, harder to reduce compared to the linear
counterpart, supporting Clar’s argument.
9
No oxidation waves were observed in DMSO due to limited
solvent redox window. ........................................................................................................................ 28
Figure 2.7: X-ray single crystal structures of l 1, l 4 and b 5, along with molecular packings. Nitrogen atoms
are in blue; grey and green colors are used in crystal packing for a clearer view. Hydrogen atoms are
omitted for clarity. .............................................................................................................................. 29
Figure 2.8: Current–voltage (I–V) curves for a bilayer devices with structure ITO/pentacene (or TAP l 1)
(250 Å)/C60 (400 Å)/BCP (100 Å)/Al (A), and calculated schematic energy level diagram for the
active materials (B). (BCP = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline). ................................ 30
Figure 2.9: Anaerobic electrochemical cell used to measure CVs. ............................................................. 42
xii

Chapter III
Figure 3.1: (Left) Metal chelates of MQX type (e.g. AlQ3 and ZnQ2) are well-studied and use 8-quinolinol
(Q) as ligand. (Right) p-extended quinolinol-like ligands (acridinol and phenazinol derivatives) are
used in the present work to investigate their zinc (II) chelates. .......................................................... 57
Figure 3.2: Synthesis of homoleptic (ZnX2) and heteroleptic (ZnX1) complexes by varying the equivalents
of Zn(acac) 2. ........................................................................................................................................ 58
Figure 3.3: Partial
1
H NMR spectra (aromatic region) for homoleptic (ZnA 2) vs heteroleptic (ZnA 1)
products formed as a function of Zn(acac) 2 equivalents. From bottom to top spectrum,
Zn(acac) 2:acridinol ratio is 0.5:1, 3:1, and 8:1 (top stacked spectra). In the middle spectrum, hash-tags
(#) and stars (*) represent ZnA 1 and ZnA 2 peaks, respectively. Similar results were observed for ZnP 1
and ZnP 2 (bottom stacked spectra). .................................................................................................... 59
Figure 3.4: Cyclic voltammograms of free ligand (acridinol, phenazinol, and benzophenazinol) and
homoleptic complexes (ZnA 2, ZnP 2, and ZnbP 2) in DMF. Glassy carbon (working electrode), Pt
counter electrode, Ag/Ag
+
(reference electrode), ferrocene (internal reference), 0.1M
tetrabutylammonium hexafluorophosphate (nBu 4NPF 6) were used. All CV were referenced to
ferrocene/ferrocenium couple (Fc/Fc
+
= 0.0 V). ................................................................................. 61
Figure 3.5: Absorption and emission spectra of ligands in dichloromethane. Photoluminescence quantum
yield and lifetimes are insetted. ........................................................................................................... 63
Figure 3.6: UV-vis absorption (and emission for ZnA 2) spectra of all homoleptic Zn(II) complexes.
Extinction coefficient (, in M. cm
-1
) of charge-transfer band maximum is annotated for each complex.
Emission of ZnP 2, ZnxP 2 and ZnbP 2 is too poor to be recorded on our fluorometer ( PL < 0.001). . 64
Figure 3.7: Comparison of emission intensities between ZnA 2 vs ZnA 1 when excited at the same
wavelength (=450 nm) with the same optical density. ..................................................................... 65
Figure 3.8: HOMO and LUMO of anionic ligands of acridin-4-ol, phenazin-1-ol, benzo[b]phenazin-1-ol,
and 6-xylyl-phenazin-1-ol. .................................................................................................................. 67
Figure 3.9: Frontier orbitals (HOMO-1, HOMO, LUMO, and LUMO+1) of homoleptic complexes (top)
and representative heteroleptic complexes (bottom) from DFT calculations. .................................... 68

xiii

Chapter IV
Figure 4.1 : Synthesis of N-H pCz ligand and its NHC complexes, pCz-AuBZI and pCz-AuMAC ....... 80
Figure 4.2: Differential pulse voltammograms (left column) and cyclic voltammograms (right column) of
free pCz ligand, pCz-AuBZI, and pCz-AuMAC in 0.1 M DMF. We used tetrabutylammonium
hexafluorophosphate (TBAF) electrolytes, glassy carbon working electrode, Pt wire counter electrode,
and silver wire (Ag/Ag+) pseudo reference electrode, and ferrocene/ferrocenium internal reference
(Fc/Fc+ = 0.0 V). Redox potentials are extracted from DPV. ............................................................ 82
Figure 4.3: HOMO (blue)/LUMO (red) of optimized structure pCz-CuBZI and pCz-CuMAC (first
column). Natural transition orbitals (NTOs) of T 1, T 2, and S 1 from TD-DFT calculations performed on
optimized structures (2
nd
, 3
rd
, 4
th
column). Green contours represent hole; yellow contours represent
electron. Isovalue = 0.09 ..................................................................................................................... 84
Figure 4.4: Qualitative energetic depiction theoretical results of pCz-CuBZI and pCz-CuMAC from
TD-DFT calculations. In pCz-CuBZI,
3
pCz–
1
ICT separation is larger (0.88 eV) whereas in
pCz-CuMAC the separation is smaller (0.45 eV). With solvent effects that can stabilize the
1/3
ICT
manifold closer to
3
pCz, TADF events are more probable in pCz-AuMAC than in pCz-AuBZI. These
computational values might not correlate well with experimental values. ......................................... 84
Figure 4.5: Absorption (A) and emission (B) spectra of pCz-AuMAC in MeTHF, MeCy, and polystyrene
film. Comparison of pCz-AuMAC vs Cz-AuMAC: molar absorptivity (C) and emission spectra (D)
in MeTHF............................................................................................................................................ 87
Figure 4.6: Qualitative potential energy surface diagram of states in pCz-AuMAC vs Cz-AuMAC
illustrating solvatochromism behaviors observed in the complexes. Absorption in polar solvent is
hypsochromically shifted relative to that in non-polar solvent. Emission in polar solvent is
bathochromically shifted relative to that in non-polar solvent. More distortions of excited states in
MeTHF are illustrate by the shift of the potential energy surface well to the left, corresponding to the
poorer emission quantum yield in the polar solvent. .......................................................................... 88
Figure 4.7: Qualitative representation of states and photophysical events taking place in pCz-AuMAC vs
Cz-AuMAC at room temperature and 77 K. Both complexes undergo TADF at room temperature, and
phosphorescence at 77 K. The
3
pCz is lower than that of
3
Cz. The separation between
3
LE and
1/3
CT
manifold is greater in the phenanthrocarbazolide complex. ............................................................... 89
xiv

Figure 4.8: Absorption (E) and emission (F) spectra of pCz-AuBZI compared to Cz-AuBZI (G). ......... 90
Figure 4.9: Qualitative representation of photophysical events taking place in pCz-AuBZI vs Cz-AuBZI
at room temperature and 77 K.
3
Cz and
1/3
CT of Cz-AuBZI are near degenerate and hence show rt
TADF emission stemming from the states’ mixing. pCz-AuBZI is characterization by inefficient
phosphorescence at room temperature and 77 K. ............................................................................... 91
Chapter V
Figure 5. 1: Synthesis of complexes pI-AuBZI and pI-CuMAC .............................................................. 101
Figure 5.2: Differential pulse voltammograms (left column) and cyclic voltammograms (right column) of
free pI ligand and pI-AuMAC in 0.1 M DMF. Measurements were performed by using
tetrabutylammonium hexafluorophosphate (TBAF) electrolytes, glassy carbon working electrode, Pt
wire counter electrode, and silver wire (Ag/Ag+) pseudo reference electrode, and
ferrocene/ferrocenium internal reference (Fc/Fc+ = 0.0 V). See Table 5.1 for values extracted from
DPV................................................................................................................................................... 103
Figure 5.3: Qualitative states energy diagram of pI-AuBZI, and pI-CuMAC from TDDFT calculations in
gas phase. In pI-AuBZI, S 1-T 1 separation is larger (1.10 eV) heralding conventional phosphorescence,
whereas in pI-CuMAC the separation is smaller (0.67 eV), which promises delayed fluorescence
emissions. .......................................................................................................................................... 105
Figure 5.4: Absorption (A) and emission (B) spectra of pI ligand and pI-AuBZI in MeTHF. Extinction
coefficient (C) and emission spectra (D) of pI-CuMAC in MeTHF vs MeCy. See Table 5.3 for
tabulated absorption and emission properties. .................................................................................. 106
Figure 5.5: Qualitative potential energy surface diagram of states in pI-AuMAC illustrating the
solvatochromism behavior observed in the complex. Absorption in polar solvent is hypsochromically
shifted relative to that in non-polar solvent. Emission in polar solvent is bathochromically shifted
relative to that in non-polar solvent. More distortions of excited states in MeTHF are illustrate by the
shift of the potential energy surface well to the left, corresponding to the poorer emission quantum
yield in the polar solvent. .................................................................................................................. 107
Figure 5. 6: Qualitative representation of states and emission processes taking place in pI-CuMAC vs pI-
AuBZI at rt and 77 K. pI-CuMAC undergoes TADF at room temperature, and phosphorescence at
xv

77 K. pI-AuBZI undergoes inefficient conventional phosphorescence at rt and 77 K, regardless of
solvent polarity, because
3
pI is always the low-lying, emitting state. ............................................... 109
Appendix A
Figure A.1: Synthesis of bulky benzobis(imidazolium) salts (I) and quinobis(imidazolium) salts (II) for
binuclear NHC complexes. ............................................................................................................... 118
Figure A.2: Proton NMR analysis of single aryl amination (2a) vs double aryl amination (2b) products.
.......................................................................................................................................................... 119
Figure A.3: Absorption and emission spectra of unidentified polymeric complexes of BBI in MeTHF vs.
toluene. .............................................................................................................................................. 120
Figure A.4:Frontier orbitals (HOMO-1, HOMO, and LUMO) of BBI(CuCz) 2........................................ 122
Appendix B
Figure B.1: Synthesis of the dinuclear complex: D(CuIPr) 2. .................................................................. 130
Figure B.2: HOMO and LUMO of optimized structure of D(CuiPr) 2 from DFT calculations at
B3LYP/LACVP** level of theory. Isovalue = 0.09. ........................................................................ 131
Figure B.3: Absorption and emission spectra of ligand D vs. complex D(CuIPr) 2 in MeTHF. Emission
photos of ligand (green-yellowish) and complex (orange) under black light are provided. ............. 132
List of Tables
Table 2.1: Absorption & emission maxima of linear and bent tAPs. ......................................................... 26
Table 2.2: Crystal structure data collection for l 1 (CCDC# 1866946) ........................................................ 39
Table 2.3: Crystal structure data collection for DB-tAP l 4 (CCDC# 1866945) .......................................... 40
Table 2. 4: Crystal structure data collection for DTBA-tAP b 5 (CCDC# 1866947) ................................... 41
Table 3.1: Redox potential values from electrochemical measurements of ligands and complexes in N,N-
dimethylformamide. All values are referenced to Fc/Fc
+
. HOMO is calculated from formula
1.15(E ox) + 4.79 and LUMO from 1.18(E red) - 4.83, according to literature.
11
HOMO/LUMO values in
parentheses are from DFT calculations. .............................................................................................. 60
xvi

Table 4.1: Redox values extracted from electrochemical measurements (DPV). HOMO and LUMO are
calculated according literature formulae.
5

a
From reference.
1

b
From reference.
3
................................. 81
Table 4.2: Tabulated photophysical properties of pCz-AuMAC compared to Cz-AuMAC.
c
Triplet energy
from 77K emission onset (500 nm) corresponds to 2.50 eV. .............................................................. 86
Table 4.3: Tabulated emission properties of pCz-AuBZI compared to Cz-AuBZI.
d
Triplet energy from
77K emission onset (498/504 nm) is 2.47 eV. .................................................................................... 90
Table 5.1: Redox values extracted from electrochemical measurements (DPV). HOMO and LUMO are
calculated according literature formulae.
7

a
From reference [3].
3

b
From reference [1].
1
.................. 102
Table 5.2: DFT/TDDFT calculations: HOMO (blue)/LUMO (red) of optimized structure pI ligand,
pI-CuBZI, and pI-CuMAC (first column). Hole (green) and electron (yellow) contours of natural
transition orbitals (NTOs) for T 1, T 2, and S 1 are given in 2
nd
, 3
rd
, and 4
th
column, respectively. ..... 104
Table 5.3: Absorption and emission properties of pI ligand, pI-AuBZI and pI-CuMAC.
c
pI triplet energy
77K emission onset is 2.87 eV. ......................................................................................................... 108
Table A.1: Tabulated emission properties of BBI polymeric complexes.
a
Triplet energy from 77K emission
peak is 2.90 eV. ................................................................................................................................. 120

xvii

Abstract

The global energy demand is increasing at an alarming rate amid the fast-growing world of electronics. In
the light of this concern, Professor Mark Thompson’s research group uses chemistry and engineering to
develop new materials for optoelectronic devices, particularly organic photovoltaics (OPVs) and organic
light-emitting diodes (OLEDs). This dissertation embodies a great contribution for these two technologies
whose fundamentals and overview are introduced in Chapter I. As OPV technologies await
commercialization, challenges of expensive and inefficient materials – compared to inorganic PV
counterparts – need to be tackled. One of the key components of an OPV device that have hampered high
performance is an electron-acceptor layer, which has been relying on using expensive and unstable fullerene
derivatives for quite long. In this regard, we disclose (in Chapter II) a one-pot synthesis of non-fullerene
n-type materials of a simple molecular structure: tetra-aza-pentacenes. Presented in Chapter III are
chromophores of Zinc(II) complexes bearing acridinol/phenazinol ligands. The study of these complexes
shed light on further understanding and future design of active materials for OPVs and OLEDs. The last
Chapters (IV, V, and Appendix) are dedicated to thermally-activated delayed fluorescence (TADF)
complexes of (amide)-metal-(carbene) structure. In these two-coordinated N-heterocyclic carbene (NHC)
TADF phosphors, we have explored phenanthrocarbazolyl amide-based complexes (Chapter IV),
phenanthrimidazolyl-containing complexes (Chapter V) , and lastly, binuclear NHC complexes featuring
benzobis(imidazolylidene) Janus type carbenes, as well as bis(amide) donors. The latter families of emitters
aim to circumvent the conundrum of long-lived excitons, or slow radiative rates, in existing OLED emitters
which are detrimental to devices’ lifetime and efficiency. The work presented herein is indeed of a great
contribution to the future research and technology advancements in the field of (opto)electronic devices.
Further investigation into these families of materials is continuing in Prof Thompson’s research group.
1 CHAPTER I: Introduction

Organic semiconductors have been revolutionizing the world of electronics for a number of decades.
1
These
materials have gained even more popularity due to their many advantages including low cost, environmental
friendliness, large scale accessibility through organic synthesis, tunable properties (optical, electrical,
photoelectric, magnetic, etc.), flexibility, and others. Organic solids consist of molecules interacting
through weak forces allowing versatile molecular level designs compared to inorganic solids where ionic
bonds between atoms are dominant. The development of photo-active and electro-active organic materials
has, therefore, seen tremendous progress to in many (opto)electronic devices such as solar cells,
electroluminescent devices, field-effect transistors, sensors, etc. The contribution of the work presented
herein focuses on the development of materials for organic photovoltaics (OPVs) and organic light-emitting
diodes (OLEDs) applications.
1.1 Organic Photovoltaics
1.1.1 Overview on Global Energy Demand
The global energy demand is predicted to increase by nearly 50% from 2018 to 2050 where the total energy
consumption will reach an astounding 30 TW.
2
The lion’s share of today’s energy consumption comes from
fossil fuels with a consumption rate increase of around 2.1% per year.
3
This raises serious concerns
regarding future energy security, not to mention pollutant gas emissions that only exacerbates the effects
of global warming. Thankfully, renewable energy alternatives (e.g. solar, wind, geothermal, and
hydroelectric power) account for 15% of total energy (as of 2018) and are projected to continually increase
in use to 28% by 2050.
4
Solar energy, in particular, draws much attention because of its abundance and
non-intermittency nature, and hence, can solely supply the whole global energy demand if the right
technologies were developed.
2

2

Figure 1. 1: Verified high research cell efficiency chart from NREL. Today’s OPV efficiency record has surpassed 17% since 2018.

Research in solar cell technologies has made impressive advances as summarized by the chart of
high-record research-cell efficiencies reported by National Renewable Energy Laboratory (NREL)
(Figure 1.1).
5
Cells with higher performance are notably inorganic semiconductor-based, where the
efficiencies of single-junction GaAs and silicon-based solar cells, as one example, surpass 26%. The
economics of these devices, however, remain unfavorable. For this reason, solar power has not yet become
overwhelmingly adoptable to compete with conventional electrical grid power. In this regard, the
development of organic photovoltaics (OPVs) is becoming a center of attention due to a number of
advantages over inorganic counterparts, such as light weight, lower cost, flexibility, semi-transparency,
tunable properties, eco-friendliness, etc. In fact, the NREL-verified highest performance OPV has exceeded
17% as of 2018 (Figure 1.1).
1.1.2 Fundamentals of organic photovoltaics
An organic photovoltaic device consists of at least one organic semiconductor, also known as active
material, to convert light (photons) into electricity. Generally, electron-donor (D) and electron-acceptor (A)

3

materials are paired to make up the active layer, which is sandwiched between two electrodes (anode and
cathode), Figure 1. 2. Figure 1.2-C shows a typical relative energy level diagram of HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the active materials, and
the mechanism of OPV working principles. Light absorption must take place in one of the active layer
chromophores. Here, a photon is absorbed by a donor material leading to exciton (bound hole-electron pair)
formation. The exciton diffuses toward the D-A interface, where a charge-transfer state is formed leading
to charge separation (or exciton dissociation). A hole and electron now become free carriers and migrate
respectively toward anode and cathode for charge collection.

Figure 1. 2: Architectures of OPVs. A) Planar heterojunction (bilayer structure). B) Bulk heterojunction structure. C) Mechanism
of light conversion: (a) Light absorption → exciton formation, (b) exciton diffusion toward D/A interface. (c) charge transfer ( →
exciton dissociation). (d) charge transport and collection.

Two common architectures of OPV devices are planar heterojunction where an acceptor is layered over a
donor material to form a bilayer structure (Figure 1.2-A). When the donor and acceptor are mixed, on the
other hand, they constitute a bulk heterojunction architecture (Figure 1.2-B). The later architecture has more
advantages over the bilayer structure such as efficient exciton diffusion events owing to shortened diffusion
lengths.
6
Active materials can be polymeric or small molecule semiconductors. Examples of electron-donor
active materials in early developments of OPVs include polymer P3HT (poly(3-hexylthiophene)]
7
and
molecular metallophthalocyanines.
8
For a long time, the dominantly used acceptor materials have been

4

fullerene derivatives due to their attractive properties including high electron affinities, high charge carrier
mobilities, anisotropic electron transport events, etc.
9
However, the progress in fullerene-acceptor (FA)
based OPVs appears to have plateaued with a PCE of around 13%. The emergence of
non-fullerene-acceptors (NFA) have begun to overshadow FA-based counterparts
10
due to their limitations
including poor absorption in the visible region of the solar spectrum, limited synthetic and energetic
tunability, poor cell voltage, and morphological device instability, not to mention high costs.
1.1.3 Recent Successes in non-fullerene acceptors development
Among successful molecular NFAs, perylene diimide (PDI) derivatives constitute a family that has been
given much attention due to their desirable optoelectronic properties. PDI building blocks intrinsically
inherit high electron affinities and mobilities (>1 cm
2
V
−1
s
−1
). Due to undesirable aggregations in
unfunctionalized PDI, molecular engineering was adopted to develop dimers, trimer, and even tetramers
PDI derivatives. The latter development improved morphological and optoelectronic properties that
permitted the fabrication of devices with impressive power-conversion efficiencies (PCE) at the time. For
example, a single junction device that employed SF-PDI 2 as an acceptor benchmarked a PCE of 9.5%.
11

Recently, a family of fused-ring NFA materials structurally consisting of electron-deficient unit (A) and
electron-rich unit (D) caught the attention of the OPV researchers’ community. The best performance NFA
semiconductors to date follows the A-D-A design strategy, and the power-conversion efficiency (PCE) of
devices employing this family of materials have surpassed 16% for single junction
12
and 17% for tandem.
13

The remarkable performance of these A-D-A NFA materials are attributed to the versatile molecular
engineering allowing tunability of energetics, improved absorption properties, great charge transfer events
due to push-pull design, mitigated self-aggregation, etc.
10
Additionally, these NFAs are
solution-processable, eliminating the unfavorable energy-costly vacuum vapor-deposition techniques.
5

1.1.4 Challenges of synthetic complexity of today’s NFAs
While research in NFA-based OPVs has achieved significant milestone in their performance relative to
their analogous fullerene-based OPVs, synthetic complexity is one of the key challenges that must be
addressed for commercialization of these types of OPVs. Figure 1.3 shows a structure of an early-developed
A-D-A acceptor, IEIC, and its synthetic routes.
14
The requirement of multi-step synthesis is due to the
complexity of its molecular structure, and unfortunately most of A-D-A NFAs suffer the same long,
multi-step syntheses with harsh conditions and low yields.
15-16
These fused-ring NFAs are unsurprisingly
more expensive compared to fullerene acceptors. Therefore, a discovery of new NFAs with simple
structures that can be accessible by means of inexpensive syntheses is crucial for the future of organic
photovoltaics.

Figure 1. 3: Example of A-D-A non-fullerene acceptor, IEIC, and its multi-step synthesis.

1.2 Organic Light-emitting diodes
Organic light-emitting diodes (OLEDs) have known different stages of development to improve efficiency,
stability, and economics. The early technology began with fluorescence-based OLEDs (1
st
generation),
where only singlet excitons (25% internal quantum efficiency – IQE) are utilized, leaving the other 75% of
triplet excitons unharvested (Figure 1.4).
17
According to the Pauli exclusion principle, the radiative decay
of triplet excitons from T 1 to the ground state S 0 in pure organic emitters is formally forbidden due to the
requirement of spin flip, leading to high non-radiative decay rates to the ground state. While the stability of
the conventional fluorescent OLELDs (red, green & blue) are remarkable, the inherited low efficiency
remains a major downside, as an external quantum efficiency (EQE) at best ranges from 5 to 7.5%, even

6

with outcoupling efficiencies reaching 30%.
18
Significant research efforts have been made to boost the
exciton utilization to 100% IQE and this resulted in the formation of 2
nd
and 3
rd
generations of today’s
OLEDs.
19
The key phenomena that govern the latter two generations are phosphorescence and thermally
activated delayed fluorescence (TADF), respectively (Figure 1.4). These processes are elaborated further
in the following sections.

Figure 1. 4: Luminescence processes governing present generations of OLEDs. Fluorescence ( 1
st
Gen), phosphorescence (2
nd

Gen), and thermally activated delayed fluorescence – TADF (3
rd
Gen).

1.2.1 Spin-orbit coupling (SOC) and phosphorescence
The formally forbidden transition between singlet (S 1) and triplet (T 1) states can be turned on in emitters
by introducing what is known as the spin-orbit coupling (SOC) effect. SOC is a relativistic quantum
mechanical effect that promotes an electron spin change when coupled to a change in angular momentum
of its orbital in an given nucleus, hence the “spin-orbit coupling” terminology.
20
This coupling originates
from an alignment of an electron spin magnetic moment with a magnetic field generated by the orbiting
nucleus of positive charge number Z. Therefore, atoms with larger nuclear charge provides a greater SOC
impact in complexes containing heavier atoms, hence the “heavy atom effect ”.
21
The spin-orbit parameter

7

(ξ) for a given atom determines the strength of SOC effect; for example, from carbon (mass = 12.01 u), to
copper (63.55 u), iridium (192.22 u) and platinum (195.08 u), the parameter ξ increases from 32, to 857,
3909, and 4481 cm
-1
, respectively. That explains why organometallic emitters containing heavy metals such
as Ir(III),
22-23
Pt(II),
24
and Au(III)
25
can theoretically achieve 100% emission quantum yields by harvesting
all possible excitons through phosphorescence. SOC allows intersystem crossing (ISC) from S 1 to T 1 and
T 1 to S 0 (phosphorescence). In purely organic emitters, SOC is significantly weaker with S 1 →T 1 rates
reaching 10
7
s
-1
at best, while rates in heavy metal-containing phosphors can be 6 to 7 orders of magnitude
greater (10
13
– 10
14
s
-1
).
26-28
Excitons in the lowest emitting triplet state, T 1, radiatively decay (or
phosphoresce) to S 0 at rates between 10
2
– 10
4
s
-1
in SOC-driven purely organic emitters and up to 10
5
s
-1

in heavy-metal containing emitters. The poor phosphorescence in organic emitters is therefore caused by
slower ISC processes because they are outpaced by prompt fluorescence.
Red and green phosphorescent complexes [e.g. Ir(piq) 3), Ir(ppy) 3)] are efficient, stable, and widely used in
OLEDs industry.
29
However, the poor stability of blue phosphorescent OLEDs remain a challenge. The
instability of blue phosphors originates from high energy excitons that trigger detrimental bimolecular
processes known as triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA).
30-31
TTA is
realized when two triplet excitons fuse together to generate higher energy (“hot”) singlet exciton (S n) and
a ground state singlet exciton S o (i.e. 2T 1 → S 0 + S n). For TPA, a hot polaron is formed once a triplet comes
together with a polaron A
-*
(i.e. T 1 + A
-
→S 0 + A
-*
). These “hot”, unstable species can have energy greater
than 7 eV and readily break chemical bonds, hence short operational lifetimes of these devices. Such
detrimental processes are less common in singlet excitons because these have relatively short excited state
lifetimes. In other words, the longer lifetime (>1 s) of triplet excitons in heavy metal-containing phosphors
is the core problem, as TTA and TPA events find enough time to take place well before excitons in question
can radiatively recombine.
Significant efforts to develop conventional organo-transition metal phosphors with short-lived excitons
have mostly failed to shorten the lifetimes down to the sub-microsecond regime.
32
For example, the widely
8

used standard green phosphor, Ir(ppy) 3, has an average phosphorescence lifetime of 2 s.
23, 33
The
unsuccessful decrease of lifetime of such phosphors is due to the compromising effect of SOC on triplet
zero-field splitting (ZFS). SOC-induced ZFS in
3
MLCT-based organo-transition phosphors prompt
sublevels splitting such that E I,II << E I,III = E(ZFS).
32
In other words, T I and T II are closely spaced while
T III lies above them with a relatively considerable energy gap, as depicted in Figure 1.5. For example
Ir(ppy) 3 and Ir(piq) 3 have E(ZFS) of 170 cm
-1
and 64 cm
-1
, respectively, meaning that SOC is larger in
the former complex.
32
The emission lifetime of T III in such phosphors with large E(ZFS) is much shorter
than the emission lifetimes of T I,II. That is,  III >>  I,  II. And since the E I,II << E I,III = E(ZFS), then the
averaged phosphorescence lifetime ( ph) of the triplet sublevels at room temperature can be roughly
approximated to: 𝜏 𝑝 ℎ
= 2(𝜏 𝐼𝐼𝐼
) exp (
∆𝐸 (𝑍𝐹𝑆 )
𝑘 𝑏 𝑇 ) (Eq. 1), where k b is the Boltzmann constant and T is
temperature. From this crude approximation, it is obvious that plot of  ph as a function of E(ZFS) will
reach a point where it appears asymptotic, and that is exactly what is observed in the plot summary of
conventional, well studied heavy-metal phosphors by Yersin, H. et al; the asymptotic lifetime value is about
1 s. This is due to the fact that large SOC will greatly increase intersystem crossing events, but at some
point, it will concomitantly induce excessive E(ZFS) that will slow down the equilibration between T I,II
and T III. Therefore, this relationship has hampered the development of such organo-transition metal
phosphors with lifetimes in sub-microsecond regime. In this regard, TADF strategy in metal-free organic
emitters was introduced to give birth to the 3
rd
generation of OLEDs.
1.2.2 Thermally activated delayed fluorescence: TADF
The 3
rd
generation of OLEDs takes advantage of thermally activated delayed fluorescence (TADF) to
harvest triplet and singlet excitons, hence promising 100% internal quantum efficiency (IQE). The “crude”
concept of TADF mechanism was introduced in 1929 by Perrin et al., followed by other sporadic related
reports,
34-35
but TADF was not adopted in OLED application until 2012 by Chihaya Adachi’s group.
36

Delayed fluorescence stems from emission from the thermally populated S 1 (from T 1) as shown in TADF
9

kinetics, Figure 5. The key design approach is to bring singlet and triplet states close enough in energy that
the forward and reverse intersystem crossing (ISC and RISC) processes are fast and in equilibrium above
given temperatures (e.g. 300 K). In principle, the singlet-triplet separation, E ST, should be <0.12 eV for
efficient TADF emission, albeit the process can still be observed for E ST of up to 0.25 eV. This is usually
achieved by spatially and/or orthogonally separating donor-acceptor (D-A) molecular moieties to promote
charge-transfer (CT) electronic transitions, due to minimized electronic coupling between the highest
occupied molecular orbital (HOMO) of the donor, and lowest unoccupied molecular orbital (LUMO) of the
acceptor unit.

Figure 1. 5: Simplified kinetics of (left) phosphorescence in conventional heavy-metal organometallic emitters (e.g. Ir(ppy)3); and
(right) TADF kinetics of organic vs. inorganic molecules. In the present TADF kinetics, the three triplet sublevels are assumed to
be degenerate; the emission efficiency is assumed to be close to unity, and hence non-radiative rates are considered relatively
insignificant and not shown. The simplified expressions of ph and TADF are given.

Simple kinetics of the TADF mechanism is shown in Figure 5. For simplicity k I, k R, and k S1 represent the
rates for S 1 →T 1, T 1 →S 1 and S 1 →S 0 events, respectively. The TADF emission efficiency is assumed to be

10

close to unity, and hence other non-radiative rates are considered relatively small and insignificant. With
both k I and k R being >> k S1, the kinetic scheme below allows us to apply pre-equilibrium approximation to
deduce TADF emission lifetime as  TADF =  S1/𝐾 𝑒𝑞
𝑇𝐴𝐷𝐹 , where 𝐾 𝑒𝑞
𝑇 𝐴 𝐷𝐹
is the equilibrium constant between
S 1↔T 1.
37

To decrease  TADF, one can increase the equilibrium constant 𝐾 𝑒𝑞
𝑇𝐴𝐷𝐹 by minimizing E ST, or (2) decrease the
singlet lifetime  S1 or do both. However, E ST and  S1 have an inverse relationship that makes this tricky.
That is, when E ST is reduced,  S1 is increased. This is because when decoupling HOMO and LUMO to
achieve small E ST, the oscillator strength (and hence prompt fluorescence rate) is compromised and the
TADF lifetime ( TADF) becomes longer. Original TADF materials applied in the 3
rd
generation of OLEDs
are purely organic D-A type molecules in which ISC processes are inherently slow (k I of up to 10
7
s
-1
)
meaning that prompt fluorescence compete with the intersystem crossing/equilibrium.
38
The ultimate
outcome are bi/multi-exponential emissions characterized with fast emission (prompt fluorescence) and
delayed emission. As a result, TADF lifetimes of organic materials fall in the range of 1-100 s.
36

To speed up ISC events, SOC-driven inorganic TADF systems were developed, notoriously 4-coordinated
organo-copper(I) complexes reported by Yersin’s group.
39
ISC rates in such inorganic TADF materials now
become drastically improved to reach k I of 10
12
s
-1
, five orders of magnitude higher than those in organic
TADF counterparts.
40
Prompt fluorescence is overwhelmed by ISC processes, and hence only
mono-exponential emission is observed, corresponding to delayed fluorescence. One conundrum with such
4-coordinated organo-Cu(I) phosphors is that they are well known to undergo structural distortions in
excited states due to MLCT transitions, rendering their solution emission quantum yields very poor.
39
Plus,
these systems still suffer the inverse relationship between E ST and  S1, as observed for organic TADF
molecules.
11

Figure 1. 6: Examples of recently reported coinage metal(I) TADF phosphors with high quantum yield (~100%) an
sub-microsecond lifetimes. (Bottom) Phenanthrocarbazolyl and phenanthrimidazolyl are new N-donor ligands that will be explored
in Chapter 4&5.

In this regard, our lab has recently reported a new family of two-coordinated TADF complexes of
N(amide)-M-C(carbene) structure, where the metal M is Cu(I), Ag(I), or Au(I) ions.
41-44
As shown in
Figure 6, the N-amide donor (HOMO residence) and N-heterocyclic carbene acceptor (LUMO residence)
ligands are co-planar and sufficiently distanced from each other through the bridging metal ion to fulfill the
HOMO-LUMO separation principle for small E ST. The distance between the N of the amide and the C of
the carbene ranges from 3.7 to 4.2 Å, with silver(I) complexes having the largest separation due to the
weaker bonding nature of the metal. The co-planarity is crucial for efficient electronic coupling between
HOMO and LUMO, resulting in inter-ligand charge transfer (ICT). The metal ion has a minimum
contribution to the frontier orbitals; in other words, it does not contribute to the CT transition, but rather
acts like an electronic bridge to ensure the donor and acceptor ligands are kept distant and provides
sufficient SOC for high intersystem crossing rates. Organo-Cu(I) TADF phosphors like those reported by

12

Yersin, H. et al.
39
and organic TADF emitters
38
all have lifetimes greater than 1 s; however the
two-coordinated TADF phosphors of C: →M-N type show improved TADF lifetimes as low as 400 ns
(Figure 6). Therefore, the success of our design is ascribed to the TADF design combined with SOC while
ensuring optimized D-A electronic coupling to increase 𝐾 𝑒𝑞
𝑇𝐴𝐷𝐹 and radiative decay rates from S 1 to S 0.
One of the advantages of the C: →M-N systems is that the donor and acceptor moieties can be tuned
individually through easy and quick molecular engineering. This is particularly advantageous for OLED
materials pairing, for example host and emissive materials; in other words, if a given C: →M-N TADF
phosphor has a HOMO suitable for a selected host material but with unfavorable LUMO, it is easy to swap
carbenes and obtain a desirable LUMO energy; the same can be applied for unsuitable HOMO. While the
reported two-coordinate phosphors employed various types of NHC carbenes to investigate their properties,
the N-amide donors remain only limited to carbazolyl-based ligands.
41-43
It is, therefore, of a great interest
to explore properties of C: →M-N complexes using different types of donor ligands. The contribution of
this work, in this regard, is to explore other two-coordinate TADF phosphors of C: →M-N type by using
different donors: (1) phenanthrocarbazolyl and (2) phenanthrimidazolyl (Figure 6). Synthesis,
photophysics, and electrochemistry of the complexes will be given a bulk discussion.
1.2.3 Davydov splitting and multipole interactions in bimetallic TADF phosphors
In addition to the amid-M-carbene design toward fast TADF, Davydov splitting and multipole interaction
opens up an avenue toward TADF phosphors with even higher radiative rates. Davydov model states that
the transition dipole moments of two symmetry-related molecules can couple by adding up or subtracting
their dipole vectors, and hence engendering two new transition dipole moments.
45
The magnitude of one
new dipole moment vector is increased by two-folds and leads to a gerade (allowed) state transition or
S 0 →S 1, while another new transition dipole moment is of a zero magnitude and corresponds to ungerade
(forbidden) transition or S 0 →S 2 (Figure 7). Since radiative rate is directly proportional to the squared
magnitude of transition dipole moment, (|2|
2
), then k r associated with S 0 →S 1 transition will be increased
by a factor of four. Fluorescence radiative decay rates can now be increased without having to compromise
13

the small E ST. Application of this model for TADF materials design was first demonstrated by Yersin’s
group who reported bimetallic 4-coordinated Cu(I) TADF emitters that, indeed, showed radiative decays
six-times faster than those in monometallic counterparts
46
. We sought to design Janus bis-carbenes that can
be used to design bimetallic TADF phosphors of N-M (biscarbene) →M-N structure thatare expected to
undergo Davydov splitting and dipole-dipole interactions, and hence higher radiative rates (Figure 7). A
similar approach can be applied for bis-amides to design carbene →M-(bisamide)-M carbene bimetallic
systems. The preliminary work on this design strategy will be presented in the Appendix; related future
work will be carried on by our research group.

Figure 1. 7: Model of Davydov splitting and dipole-dipole interaction by considering either bis-carbene or bis-amide as a binuclear
supporting core. Radiative rates can be increased in TADF systems adopting this design. (See Appendix for further development).

1.3 Other molecular semiconductor investigations
The discovery of a variety of new semiconductors is an ongoing quest for the betterment of electronic
devices. As our research group mainly focuses on OLEDs and OPV materials, we explore and understand
structure-property relationship of molecular semiconductor of different families. In this dissertation, we
also present other investigated materials that will greatly contribute to the future design and synthesis of
molecular semiconductors of targeted electronic properties. Chapter 3 will particularly unveil the synthesis
and characterization of homoleptic and heteroleptic Zinc(II) complexes bearing acridinol/phenazinol

14

ligands (ZnX 2). These complexes are worthy of investigation as they reminisce symmetry-breaking charge
transfer (SBCT) chromophores employed in OPV,
47-48
not to mention their resemblance of
zinc(II)-bis(8-hydroxyquinoline) used in electroluminescent devices. Appendix will be allocated for other
miscellaneous material studies.
1.4 Recapitulation of Chapters’ Layout.
Presented in this dissertation are five chapters and an Appendix section. After Chapter 1 embodying the
main introduction, will follow Chapter 2 disclosing the synthesis and characterization of
tetra-aza-pentacenes (TAPs) as molecular non-fullerene acceptors. Chapter 3 will be dedicated to Zn(II)
bis(acridinol/phenazinol) complexes. Synthesis and characterization of mononuclear NHC complexes
employing phenanthrocarbazolyl (pCz-M-carbene) and phenanthrimidazolyl (pI-M-carbene) will be
elaborated in Chapters 4 and 5, respectively. An Appendix section will lay out preliminary work on (A)
benzobis(imidazolylidene) Janus-type carbenes for bimetallic NHC complexes and (B) a case study of
pyrrolopyrrolyl-based binuclear complex bearing a high energy carbene, IPr-CuCl.
15

1.5 References

1. Handbook of Organic Materials for Optical and (Opto)Electronic Devices: Properties and
Applications. Handbook of Organic Materials for Optical and (Opto)Electronic Devices: Properties and
Applications 2013, (39), 1-804.
2. Darling, S. B.; You, F. Q., The case for organic photovoltaics. Rsc Advances 2013, 3 (39), 17633-
17648.
3. Azad, K.; Rasul, M. G.; Khan, M. M. K.; Sharma, S. C., Introduction to sustainable and
alternative ecofuels. Advances in Eco-Fuels for a Sustainable Environment 2019, 1-14.
4. EIA: U.S Energy Information Administration.
https://www.eia.gov/todayinenergy/detail.php?id=41433 (accessed April 1
st
2020).
5. NREL Best Research-Cell Efficiency Chart. https://www.nrel.gov/pv/cell-efficiency.html.
6. Siegmund, B.; Sajjad, M. T.; Widmer, J.; Ray, D.; Koerner, C.; Riede, M.; Leo, K.; Samuel, I. D.
W.; Vandewal, K., Exciton Diffusion Length and Charge Extraction Yield in Organic Bilayer Solar Cells.
Advanced Materials 2017, 29 (12), 1604424.
7. Dang, M. T.; Hirsch, L.; Wantz, G., P3HT:PCBM, best seller in polymer photovoltaic research.
Adv Mater 2011, 23 (31), 3597-3602.
8. Yuen, A. P.; Jovanovic, S. M.; Hor, A. M.; Klenkler, R. A.; Devenyi, G. A.; Loutfy, R. O.;
Preston, J. S., Photovoltaic properties of M-phthalocyanine/fullerene organic solar cells. Solar Energy
2012, 86 (6), 1683-1688.
9. He, Y. J.; Li, Y. F., Fullerene derivative acceptors for high performance polymer solar cells.
Physical Chemistry Chemical Physics 2011, 13 (6), 1970-1983.
10. Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K. Y.; Marder, S. R.; Zhan, X., - Non-fullerene
acceptors for organic solar cells. 2018.
11. Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.;
Ade, H.; Inganäs, O.; Gundogdu, K.; Gao, F.; Yan, H., Fast charge separation in a non-fullerene organic
solar cell with a small driving force. Nature Energy 2016, 1 (7), 16089.
12. Cui, Y.; Yao, H.; Zhang, J.; Zhang, T.; Wang, Y.; Hong, L.; Xian, K.; Xu, B.; Zhang, S.; Peng, J.;
Wei, Z.; Gao, F.; Hou, J., Over 16% efficiency organic photovoltaic cells enabled by a chlorinated
acceptor with increased open-circuit voltages. Nat Commun 2019, 10 (1), 2515.
13. Meng, L.; Zhang, Y.; Wan, X.; Li, C.; Zhang, X.; Wang, Y.; Ke, X.; Xiao, Z.; Ding, L.; Xia, R.;
Yip, H. L.; Cao, Y.; Chen, Y., Organic and solution-processed tandem solar cells with 17.3% efficiency.
Science 2018, 361 (6407), 1094-1098.
16

14. Lin, Y. Z.; Zhang, Z. G.; Bai, H. T.; Wang, J. Y.; Yao, Y. H.; Li, Y. F.; Zhu, D. B.; Zhan, X. W.,
High-performance fullerene-free polymer solar cells with 6.31% efficiency. Energy & Environmental
Science 2015, 8 (2), 610-616.
15. Wang, H. T.; Cao, J. R.; Yu, J. S.; Zhang, Z. H.; Geng, R. Y.; Yang, L. Q.; Tang, W. H.,
Molecular engineering of central fused-ring cores of non-fullerene acceptors for high-efficiency organic
solar cells. Journal of Materials Chemistry A 2019, 7 (9), 4313-4333.
16. Wadsworth, A.; Moser, M.; Marks, A.; Little, M. S.; Gasparini, N.; Brabec, C. J.; Baran, D.;
McCulloch, I., Critical review of the molecular design progress in non-fullerene electron acceptors
towards commercially viable organic solar cells. Chem Soc Rev 2019, 48 (6), 1596-1625.
17. Zou, S. J.; Shen, Y.; Xie, F. M.; Chen, J. D.; Li, Y. Q.; Tang, J. X., Recent advances in organic
light-emitting diodes: toward smart lighting and displays. Materials Chemistry Frontiers 2020, 4 (3), 788-
820.
18. Su, S. J.; Gonmori, E.; Sasabe, H.; Kido, J., Highly Efficient Organic Blue-and White-Light-
Emitting Devices Having a Carrier- and Exciton-Confining Structure for Reduced Efficiency Roll-Off.
Advanced Materials 2008, 20 (21), 4189-+.
19. Cai, X. Y.; Su, S. J., Marching Toward Highly Efficient, Pure-Blue, and Stable Thermally
Activated Delayed Fluorescent Organic Light-Emitting Diodes. Advanced Functional Materials 2018, 28
(43).
20. Marian, C. M., Spin–orbit coupling and intersystem crossing in molecules. WIREs Computational
Molecular Science 2012, 2 (2), 187-203.
21. McGlynn, S. P.; Daigre, J.; Smith, F. J., External Heavy‐Atom Spin—Orbital Coupling Effect.
IV. Intersystem Crossing. The Journal of Chemical Physics 1963, 39 (3), 675-679.
22. Huckaba, A. J.; Nazeeruddin, M. K., Strategies for Tuning Emission Energy in Phosphorescent
Ir(III) Complexes. Comments on Inorganic Chemistry 2017, 37 (3), 117-145.
23. Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.;
Thompson, M. E., Synthesis and characterization of facial and meridional tris-cyclometalated iridium(III)
complexes. J Am Chem Soc 2003, 125 (24), 7377-87.
24. Strassner, T., Phosphorescent Platinum(II) Complexes with C(wedge)C* Cyclometalated NHC
Ligands. Acc Chem Res 2016, 49 (12), 2680-2689.
25. Tang, M. C.; Chan, A. K.; Chan, M. Y.; Yam, V. W., Platinum and Gold Complexes for OLEDs.
Top Curr Chem (Cham) 2016, 374 (4), 46.
26. Kleinschmidt, M.; van Wullen, C.; Marian, C. M., Intersystem-crossing and phosphorescence
rates in fac-Ir(III)(ppy)3: a theoretical study involving multi-reference configuration interaction
wavefunctions. J Chem Phys 2015, 142 (9), 094301.
17

27. Tao, Y.; Chen, R.; Li, H.; Yuan, J.; Wan, Y.; Jiang, H.; Chen, C.; Si, Y.; Zheng, C.; Yang, B.;
Xing, G.; Huang, W., Resonance-Activated Spin-Flipping for Efficient Organic Ultralong Room-
Temperature Phosphorescence. Advanced Materials 2018, 30 (44), 1803856.
28. Baba, M., Intersystem Crossing in the 1nπ* and 1ππ* States. The Journal of Physical Chemistry
A 2011, 115 (34), 9514-9519.
29. Wei, Q.; Fei, N.; Islam, A.; Lei, T.; Hong, L.; Peng, R.; Fan, X.; Chen, L.; Gao, P.; Ge, Z., Small-
Molecule Emitters with High Quantum Efficiency: Mechanisms, Structures, and Applications in OLED
Devices. Advanced Optical Materials 2018, 6 (20), 1800512.
30. Giebink, N. C.; D’Andrade, B. W.; Weaver, M. S.; Mackenzie, P. B.; Brown, J. J.; Thompson, M.
E.; Forrest, S. R., Intrinsic luminance loss in phosphorescent small-molecule organic light emitting
devices due to bimolecular annihilation reactions. Journal of Applied Physics 2008, 103 (4), 044509.
31. Kondakov, D. Y.; Pawlik, T. D.; Hatwar, T. K.; Spindler, J. P., Triplet annihilation exceeding
spin statistical limit in highly efficient fluorescent organic light-emitting diodes. Journal of Applied
Physics 2009, 106 (12), 124510.
32. Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T., The triplet state of organo-
transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs. Coordination
Chemistry Reviews 2011, 255 (21-22), 2622-2652.
33. Hofbeck, T.; Yersin, H., The triplet state of fac-Ir(ppy)3. Inorg Chem 2010, 49 (20), 9290-9.
34. Parker, C. A.; Hatchard, C. G., TRIPLET-SINGLET EMISSION IN FLUID SOLUTIONS -
PHOSPHORESCENCE OF EOSIN. Transactions of the Faraday Society 1961, 57 (11), 1894-&.
35. Lewis, G. N.; Lipkin, D.; Magel, T. T., Reversible Photochemical Processes in Rigid Media. A
Study of the Phosphorescent State. Journal of the American Chemical Society 1941, 63 (11), 3005-3018.
36. Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly efficient organic light-
emitting diodes from delayed fluorescence. Nature 2012, 492 (7428), 234-8.
37. Ravinson, D. S. M.; Thompson, M. E., Thermally assisted delayed fluorescence (TADF):
fluorescence delayed is fluorescence denied. Materials Horizons 2020.
38. Liu, Y.; Li, C.; Ren, Z.; Yan, S.; Bryce, M. R., All-organic thermally activated delayed
fluorescence materials for organic light-emitting diodes. Nature Reviews Materials 2018, 3 (4), 18020.
39. Czerwieniec, R.; Leitl, M. J.; Homeier, H. H. H.; Yersin, H., Cu(I) complexes – Thermally
activated delayed fluorescence. Photophysical approach and material design. Coordination Chemistry
Reviews 2016, 325, 2-28.
18

40. Bergmann, L.; Hedley, G. J.; Baumann, T.; Bräse, S.; Samuel, I. D. W., Direct observation of
intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state.
Science Advances 2016, 2 (1), e1500889.
41. Hamze, R.; Shi, S.; Kapper, S. C.; Muthiah Ravinson, D. S.; Estergreen, L.; Jung, M.-C.; Tadle,
A. C.; Haiges, R.; Djurovich, P. I.; Peltier, J. L.; Jazzar, R.; Bertrand, G.; Bradforth, S. E.; Thompson, M.
E., “Quick-Silver” from a Systematic Study of Highly Luminescent, Two-Coordinate, d10 Coinage Metal
Complexes. Journal of the American Chemical Society 2019, 141 (21), 8616-8626.
42. Shi, S.; Jung, M. C.; Coburn, C.; Tadle, A.; Sylvinson M. R, D.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E., Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I)
Complexes Featuring Nonconventional N-Heterocyclic Carbenes. Journal of the American Chemical
Society 2019, 141 (8), 3576-3588.
43. Hamze, R.; Peltier, J. L.; Sylvinson, D.; Jung, M.; Cardenas, J.; Haiges, R.; Soleilhavoup, M.;
Jazzar, R.; Djurovich, P. I.; Bertrand, G.; Thompson, M. E., Eliminating nonradiative decay in Cu(I)
emitters: >99% quantum efficiency and microsecond lifetime. Science 2019, 363 (6427), 601-606.
44. Hamze, R.; Idris, M.; Muthiah Ravinson, D. S.; Jung, M. C.; Haiges, R.; Djurovich, P. I.;
Thompson, M. E., Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes
Bearing Bulky NHC Benzimidazolyl Carbene. Frontiers in Chemistry 2020, 8 (401).
45. Ghosh, P. N., Davydov splitting and multipole interactions. Solid State Communications 1976, 19
(7), 639-642.
46. Schinabeck, A.; Chen, J.; Kang, L.; Teng, T.; Homeier, H. H. H.; Suleymanova, A. F.; Shafikov,
M. Z.; Yu, R.; Lu, C.-Z.; Yersin, H., Symmetry-Based Design Strategy for Unprecedentedly Fast
Decaying Thermally Activated Delayed Fluorescence (TADF). Application to Dinuclear Cu(I)
Compounds. Chemistry of Materials 2019, 31 (12), 4392-4404.
47. Trinh, C.; Kirlikovali, K.; Das, S.; Ener, M. E.; Gray, H. B.; Djurovich, P.; Bradforth, S. E.;
Thompson, M. E., Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems: Zinc
Dipyrrins. Journal of Physical Chemistry C 2014, 118 (38), 21834-21845.
48. Bartynski, A. N.; Gruber, M.; Das, S.; Rangan, S.; Mollinger, S.; Trinh, C.; Bradforth, S. E.;
Vandewal, K.; Salleo, A.; Bartynski, R. A.; Bruetting, W.; Thompson, M. E., Symmetry-Breaking Charge
Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics.
Journal of the American Chemical Society 2015, 137 (16), 5397-5405.
19

2 CHAPTER II: One-pot Friedländer synthesis of tetra-aza-pentacenes:
N-type small molecules for OPVs

2.1 Introduction
Pentacene derivatives are promising candidates for the development of high performance organic
optoelectronic devices, such as photovoltaics (OPVs) and organic field effect-transistors, due to their strong
optical absorptivities and high charge carrier mobilities.
1-3
In most cases, pentacene and its derivatives have
been used as p-type semiconductors in these devices; however, n-type pentacene derivatives have been
formed through cyclopentannulation strategies or by the introduction of peripheral electron-withdrawing
groups such as chlorine, fluorine, trifluoromethyl, and nitrile substituents.
2, 4-8
Peripheral electron
withdrawing groups are effective in lowering HOMO and LUMO energy levels, but increase the molecular
volume, perturbing the favorable bulk morphology of pentacene. Aza-substitution is an alternative method
of lowering the frontier orbital energy levels that does not impact the molecular volume of pentacene.
Houk and Winkler used DFT calculations to identify a series of aza-pentacenes comprised of pyridine
moieties as potentially promising n-type materials.
10
We have also carried out a theoretical study on a
combinatorial library of tetra-aza-pentacenes.
11
In our study, we determined that incorporating four
nitrogen atoms into the pentacene framework lowers the energy level of the LUMO to approximately that
of the commonly used fullerene acceptor, C 60, making these aza-acenes attractive candidates as OPV
electron-acceptor materials. Of the 135 possible substitution patterns for tetra-aza-pentacene, only 3 have
been reported, all containing pyrazine moieties derived from condensations of orthodiaminoaryls.
12-14

Moreover, synthetic routes to prepare aza-acenes with pyridine rather than pyrazine moieties remain scarce.
Herein, we disclose a base-catalyzed Friedländer synthesis that provides access to these promising
materials, previously thought to be inaccessible.
13-14

20

Figure 2.1: (A) Retrosynthesis of tetra-aza-pentacenes comprised of pyridine moieties. (B) Litterature-know, not-fully aromatized
Friedländer product from base-catalyzed conditions with ring fusion at the 2,3-positions of the dione. (C) Our developed one-pot
Friedländer synthesis of tetra-aza-pentacenes.

This report focuses on 1,7,8,14-tetraazapentacene (tAP), which is predicted to have several properties, i.e.
LUMO energy (3.37 eV), dipole moment (μ = 0.00 D), and electron reorganization energy (λ = 0.15 eV),
that closely resemble values found in C 60. Our retrosynthetic analysis (Figure 2.1, A) of the target tAP
involves the construction of two naphthyridines by ring fusion at 2,5-positions of cyclohexadione with
substituted amino-pyridines. Similar acid-catalyzed Friedländer syntheses of di-aza-pentacenes starting
from 1,4-cyclohexanedione and 2-aminobenzaldehyde derivatives have been reported to yield linear

21

dihydro-di-aza-pentacene adducts, which were then oxidized to fully aromatized products.
15-16
Several
reports claim that o-amino-aryl-aldehydes and 1,4-cyclohexanedione exclusively give products formed by
fusion at the 2,3-positions of the dione under basic conditions, Figure 2.1 (B).
17-20
In our hands, we observe
adducts resulting from the cyclohexanedione-ring fusion at the 2,5- and 2,3-positions (Figure 2.1, C), which
will be hereafter referred to as “linear” (l) and “bent” (b) products, respectively.
2.2 Results and Discussion
2.2.1 Synthesis
The R-substitutions on tAP is made possible by pre-functionalizing the starting materials.
2-amininonicotinaldehyde (a1) is commercially available. Its brominated intermediate (a2) allows
palladium catalyzed arylation with phenyl (a2) and 2,6-di-tert-butylanisole (DTBA) (a3).
21

1,4-cyclohexanedione (c1) is also commercially available, while the 2,5-substituted
cyclohexane-1,4-diones can be synthesized by SN2 alkylation reaction of dicarboxylate ester (c3) followed
by decarboxylation in HCl/AcOH mixture. We were able to put hands on dimethylated (c2) and
dibenzylated (c4) cyclohexane-1,4-diones. Detailed synthesis of tAP precursors is laid out in Experimental
section.
The reaction between 1,4-cyclohexanedione (c1) and 2-amino-nicotinaldehyde (a1) in refluxing ethanol,
promoted by 2M aqueous NaOH (0.5 eq), yielded a dark purple suspension containing both the linear (l 1)
and bent (b 1) Friedländer products (Figure 2.1, C). Compound b 1 is air-stable and relatively soluble in
dichloromethane, which facilitates purification and characterization. However, the air-sensitivity of l 1 – like
regular pentacene
22
– made purification challenging, and no acceptable NMR data could be recorded due to
its poor solubility. Nevertheless, we were able to obtain crystals of l 1 suitable for X-ray crystallography in
low yields (<1%) by sublimation at 290
o
C and 10
-6
torr.
22

Figure 2.2: Proposed mechanism for linear and/or bent tAP formation. The two mechanistic routes from 1,4-cyclohexanedione
depend on the base utilized: piperidine (weak base) and NaOH (strong base). A weak base-catalyzed Friedlander reaction solely
yields bent products, whereas strong base-catalyzed reaction yields a mixture of linear and bent isomers.

The formation of linear Friedländer aza-pentacenes from the same starting materials (1,4-cyclohexanedione
and 2-amininonicotinaldehyde) was hinted at by Majewicz and Caluwe.
18
They reported the formation of
unidentified “intractable and multi-colored products” when KOH was used as a catalyst. When piperidine
base was used in similar syntheses, bent Friedländer products were exclusively isolated, illustrating the
importance of the catalyst choice.
19-20, 23
From Figure 2.2, the presumptive intermediate after the first
Friedländer condensation is a naphthyridocyclohexanone, which has inequivalent alpha positions with
respect to the ketone. Deprotonation of the more acidic alpha proton leads to the bent isomer, while
deprotonation at the less acidic site would lead to the linear isomer, meaning that the bent isomer is the
product of the more favored thermodynamic intermediate enolate. In previous reports using weak base
conditions (piperidine), a rapid acid-base equilibrium quickly tautomerizes all the intermediate to the
thermodynamic enolate, leading to exclusive formation of the bent isomer. Our conditions employing
strong base (NaOH) trap some of the less favored kinetic enolate, leading to formation of the linear isomer,
although the bent isomer is still the major product.
The in-situ aromatization of the synthesized tAPs was somewhat surprising due to the lack of an added
oxidizing agent in the reaction. Reactions were carried out in rigorously oxygen-free environments, which
eliminates the possibility of O 2 as an oxidant. One plausible mechanism for the aromatization of tAPs is
deprotonation-hydride elimination, as proposed by the mechanism in Figure 2.3. Reetz and Eibach

23

demonstrated a similar method to aromatize dihydro-acenes into their corresponding acenes in good yields,
with potassium fencholate or n-butyl lithium as the catalyzing base.
24

Figure 2.3: Proposed in-situ aromatization mechanism of linear and bent tAP in the presence of strong base.
The poor solubility of the unsubstituted linear tAP (l 1) hinders purification and characterization, and hence
we were compelled to attach solubilizing groups. Using appropriate precursors and similar conditions
already established in the synthesis of l 1 and b 1, we synthesized 6,13-subsituted tAPs decorated with methyl
(l 2), methyl carboxylate (l 3), and benzyl (l 4). Similarly, phenyl, DTBA, and bromide groups were substituted
at the 2,9-positions of tAPs to obtain l 6, l 5 and l 7, respectively. Of all substituted tAPs, only l 5 (with
2,6-di-tert-butylanisole: DTBA) showed significantly improved solubility. Therefore, NMR
characterization of the insoluble tAP was not successful, except b 1, b 5, l 4, and l 5. The formation of tAP
products were primarily confirmed by MALDI analysis and characteristic absorption/emission of tAPs.
Taking advantage of appreciable solubility of DTBA-substituted tAPs, we were able to neatly separate the
linear (l 5) and bent (b 5) isomers by using anaerobic column chromatography. Given molar absorption
spectra of individual pure components, as well as the UV-vis absorption spectrum of the crude product
mixture, we used a mixture analysis technique described by Harris
25
to determine the l 5:b 5 product ratio to
be 1:7. The reaction yields both the linear and bent tetra-aza-pentacenes because 1,4-cyclohexanedione has
only hydrogen substituents on the alpha carbons. Therefore, all tAPs formed from unsubstituted
1,4-cyclohexanedione comprises a mixture of bent and linear products.

24

To promote formation of solely linear tAP, 2,5-dibenzylcyclohexane-1,4-dione (c4) was used in place of
the unsubstituted cyclohexanedione, and as expected, only the linear 6,13-dibenzylated tAP (l 4) was
isolated. NMR analysis of l 4 indicates the presence of two atropisomers (syn and anti). The NOESY
1
H
NMR spectrum (Figure 2.4) was acquired and confirmed that there is no through-space coupling between
the two singlet peaks at 4.87 ppm and 5.10 ppm. In other words, the two protons are non-geminal, meaning
that they are not attached to the same carbon of the benzylic methylene, but rather they are born out of two
atropisomers of debenzylated tAP due to different geometric orientations of two benzyl groups.

Figure 2.4: NOESY NMR analysis for atropisomers of compound l4. The two singlets were selected for excitation: 4.85 ppm (top
NOESY) and 5.08 ppm (middle NOESY). The lack of coupling through space indicates that the two protons are non-geminal, but
that there exist two atropisomers (syn and anti).

25

2.2.2 Photophysical Properties
The UV-visible absorption and emission spectra of the linear tAPs in DMSO are shown in
Figure 2.5, A&B. The compounds exhibit well-defined vibronic progressions observed in other known
tetra-aza-pentacenes
26
and acene molecules in general.
27-28
TAPs l 1—l 4 show almost identical absorption
and emission maxima (Table 2.1) These linear tAPs are substituted at the 6,13-positions with hydrogen,
methyl, CO 2Me, and benzyl groups, all of which show no drastic electronic impact relative to the parent
tAP (l 1). However, compounds l 5 , l 6 and l 7 are characterized by a substantial redshift both in absorption (9,
16, and 20 nm shift difference relative to l 1) and emission (5, 14, 23 nm shift difference relative to l 1). The
redshift brought about by phenyl and DTBA groups are indicative of conjugation between the substituents
and the aza-acene core, with phenyl groups having the more impactful conjugation as its derived tAP
redshifts further. The most red-shifted of all is the dibromo-dimethyl-tAP (l 7). The strong
electron-withdrawing effect of bromide lowers the LUMO, and hence narrows down the optical gap.
Compared to 6,13-Bis(triisopropylsilylethynyl)tetraazapentacene, or TIPS-tAP ( abs = 780 nm), reported
by Bunz, et al.,
26
the absorption spectra of our tAPs l 1—l 4 are hypsochromically shifted, while those of l 5-l 7
are of comparable maxima.
On the other hand, absorption and emission spectra of the bent derivatives, b 1 and b 5, are considerably
blue-shifted relative to the linear counterparts (Figure 2.5, C). According to Clar’s rule,
9
the bent isomers
have more aromatic π-sextets (i.e. two versus one) which results in a wider HOMO – LUMO gap, hence
the observed hypsochromic shift. A greater red-shifting observed from b 1 to b 5 is indicative of more
pronounced conjugation of the b 5 tAP core with DTBA groups. The charge transfer character in the
absorption and emission spectra of b 5 signifies the involment of the methoxy groups in the electronic CT
transitions.
26

Figure 2.5: Photophysical properties of tAPs: Absorption (solid lines) and emission (dotted lines) spectra of linear tAPs (A) and
bent tAPs (B) in DMSO. (C) Aerobic photooxidation of tAP 3l monitored via absorption spectra. Note that the sample C profiles
is a mixture of linear and bent isomers (3l & 3b).
Table 2.1: Absorption & emission maxima of linear and bent tAPs.
l 1 l 2 l 3 l 4 l 5 l 6 l 7 b 1 b 5
 abs (nm) 568 567 569 567 577 584 588 330 384
 em (nm) 583 585 586 579 588 598 606 364 490

To demonstrate how fast the linear tetra-aza-pentacenes photo-oxidize, absorption spectrum of the mixture
of l 5/b 5 was recorded anaerobically and then exposed to air/light to monitor the change in absorption over
time (Figure 2.5,D). A 1-minute exposure to air resulted in more than 50% photo-decomposition of the
linear isomer as indicated by a decrease in the band maximum at 577 nm. After 20 minutes of air exposure,
475 500 525 550 575 600 625
0.0
0.4
0.8
1.2
Norm. Absorbance (a.u)
Wavelength (nm)
l
1
l
2
l
3
l
4
l
5
l
6
l
7
Absorption of linear TAPs
A)

550 600 650 700
0.0
0.4
0.8
1.2
Norm. PL Intensity (a.u)
Wavelength (nm)
l
1
l
2
l
3
l
4
l
5
l
6
l
7
Emission of linear TAPs
B)

300 400 500 600 700
0.0
0.4
0.8
1.2
PL. intensity (a.u.)

Norm. Abs. (a.u)
Wavelength (nm)
b
1 (abs)
b
1 (em)
b
5 (abs)
b
5 (em)
Bent TAPs C)
300 400 500 600
0.0
0.4
0.8
1.2

Absorbance (a.u.)
Wavelength (nm)
Air-free
1 min in air
5 min in air
10 min in air
20 min in air
Photo-oxidation of l
5
D)

27

the characteristic bands were completely bleached indicating full decomposition of the products. The
photochemical stability of pentacenes and larger acenes have previously been investigated where the
degradation byproducts include endoperoxide derivatives.
22, 29
The present tetra-aza-pentacenes are
expected to follow similar degradation mechanism in the presence of light and oxygen. As for the linear
tAPs reported here, they all show same level of instability. Bent isomers are however stable and all
measurements of b 1 and b 5 were performed in ambient atmosphere.
2.2.3 Electrochemical Properties
Cyclic voltammetry measurements of the soluble tAPs (b 5 and l 5) were carried out under anaerobic
conditions in DMSO (Figure 2.6). In the measurement set-up, tetrabutylammonium hexafluorophosphate
(nBu 4NPF 6) served as electrolytes, ferrocene as an internal reference, glassy carbon as a working electrode,
Pt wire as a counter electrode, and silver wire as reference electrode. The linear isomer exhibits an
irreversible reduction wave with a reduction peak at -0.95 V versus Fc/Fc
+
. This value is similar to the first
reduction potential of fullerene C 60 (E
red
= -0.97 V vs. Fc/Fc
+
).
30
The 6,13-ethynylated tetraazapentacene
reported by Miao et. al. showed the first reduction at a half-wave potential of -0.79 V vs. Fc/Fc
+
.
31
Using
the conversion equation by Sworakowski and co-workers (LUMO = 1.18 E
red
– 4.83),
32
the reduction
potential of l 5 corresponds to LUMO of -3.71 eV. Hence, from purely hydrocarbon pentacene
(E
red
= -1.76 V, i.e. LUMO = -2.75 eV)
33
to l 5, the LUMO was lowered by ca. 1 eV, emphasizing the effect
of incorporating inductively electron-withdrawing atoms in the framework. The cyclic voltammogram of
bent DTBA-tAP counterpart (b 5) exhibits two reversible reduction peaks at -1.65 V and -1.99 V. The more
negative reduction potentials in the bent isomer are consistent with Clar’s argument referred to above.
Oxidation waves of l 5 and b 5 were not observed due to the limited redox window of DMSO solvent.
28

Figure 2.6: Cyclic voltammograms of l5 and b5 at 100 mV/s scan rate. In the CV of b5, two reversible reduction peaks are observed
at -1.65 V and -1.99 V corresponding to ELUMO of -2.88 eV. The linear tAP isomer l5 shows irreversible behavior with a reduction
peak appearing at -0.95 V (i.e ELUMO = -3.71 eV.) The bent isomer is, as expected, harder to reduce compared to the linear
counterpart, supporting Clar’s argument.
9
No oxidation waves were observed in DMSO due to limited solvent redox window.

2.2.4 X-ray analysis
Single crystal structures for l 1, l 4 and b 5 are shown in Figure 2.7 (Cambridge Crystallographic Data Center
numbers – CCDC – are 1866946, 1866945 and 1866947, respectively). Structures b 5 and l 4 belong to the
triclinic space group P1
̅
, and l 1 belongs to the monoclinic C2/c(Table 2.2,2,3). Carbon–carbon bond lengths
in all three tAP cores fall in the range of 1.349(4) Å – 1.454(2) Å, and follow a pattern of bond length
alternation observed in pentacene (short perimeter versus long internal C-C bonds).
34
The carbon-nitrogen
bonds are short, reminiscent of the bond length pattern observed in acridine.
35
The structure core of l 1 and
l 4 are planar, whereas that of b 5 is twisted with an offset angle of 17.4
o
between planes of the two terminal
naphthyridines.
The arrangement of pentacene in the crystal lattice is well studied and known for its edge-to-face
herringbone packing motif.
34, 36-37
Neighboring molecules in l 1 are organized into ribbons with adjacent
nitrogen atoms aligned opposite to each other at a distance close to the sum of van der Waals radii (N···N
spacing = 3.005(2) Å, Figure 2.7, A). The molecules in these ribbons are arranged in a shallow staircase
with a step height of 0.8 Å. The series of slip-stacked ribbons form a sheet propagating in the
crystallographic ab plane. The molecules in these sheets are not co-facially stacked alongside adjacent
-1.5 -1.0 -0.5 0.0 0.5
-4
0
4
8

Current (A)
Potential (V) vs. Fc/Fc+
CV for l
5
in THF
Fc/Fc
+
A)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
-10
-5
0
5
10
15
Fc/Fc
+

Current (µA)
Potential (V) vs. Fc/Fc
+
CV of b
5
in DMSO
B)

29

sheets, but rather are disposed edge-to-face at an angle of ~53°. The unit cell contains two sheets, related
by a glide plane, that alternate along the crystallographic c axis in a fashion reminiscent of herringbone
packing. Thus, l 1 has co-facial -stacking in two dimensions, and herringbone-like packing in the third. In
contrast, the herringbone arrangement in the crystal packing of pentacene (edge-to-face angle = 52°)
propagates in two dimensions, with collinear end-to-end packing in the third.

Figure 2.7: X-ray single crystal structures of l1, l4 and b5, along with molecular packings. Nitrogen atoms are in blue; grey and
green colors are used in crystal packing for a clearer view. Hydrogen atoms are omitted for clarity.

Isoda and co-workers reported different packing behaviors in the crystal of 5,6,13,14-tetraazapentacene due
to induced CH···N hydrogen bonds between molecules from neighboring columns.
38
Similarly, Campbell
et.al. proposed that packing in multi-nitrogen-substituted pentacene structures should be dominated by
flattened herringbone or sheet-like arrangements due to intermolecular CH···N interactions.
39
For the case
of l 1, however, CH···N hydrogen bonds are overshadowed by N···N interactions.

30

Compound b 5 has a “V-shape” molecular structure, rendering ts packing peculiar in a way that two adjacent
molecules super-impose in an X-like arrangement. That is, two superimposed molecules face in opposite
directions and exhibit a slipped π-π-stacking between their naphthyridine rings with an averaged
plane-to-centroid distance of 3.447 Å. The molecular packing of l 4 follows a “stairs-like” arrangement by
π-π stacking a naphthyridine ring of one molecule to that of the next molecule above it. The average distance
of this π-π interaction is measured to be 3.517 Å. Molecules from two adjacent staircase columns also show
some degree of π-π interaction between their side benzyl rings as represented by a 3.701 Å distance. Such
molecular arrangement is more likely to hinder rotation of benzyl groups which supports the earlier
reasoning that two atropisomers of l 4 must be present as suggested by NMR analysis (vide supra).
2.3 Electronic properties: Device testing

Figure 2.8: Current–voltage (I–V) curves for a bilayer devices with structure ITO/pentacene (or TAP l1) (250 Å)/C60
(400 Å)/BCP (100 Å)/Al (A), and calculated schematic energy level diagram for the active materials (B). (BCP = 2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline).

Simple bilayer devices of structure ITO/pentacene (or TAP l 1) (250 Å)/C60 (400 Å)/BCP (100 Å)/Al were
fabricated using vapor deposition techniques (Figure 2.8, B). The electron-acceptor layer material was kept

31

the same, C 60, and two devices separately employing parent pentacene TAP l 1 were compared. A device
made with pentacene/C60 displays the asymmetric I–V curve expected for a diode due to the 1 eV offset
between the frontier orbital energies of the two materials, whereas a similar device made with l 1/C60
displays a near symmetric I–V trace. The electrical response from the latter device is indicative of a near
barrierless flow of electrons between the two materials thus indicating a close match in HOMO/LUMO
energies for the two compounds.
2.4 Conclusion
In conclusion, a Friedländer reaction has been used to form fully aromatized tetra-aza-pentacene products
in the absence of an exogenous oxidant. Peripheral substitution yields more soluble materials and allows
for the isolation and characterization of pure tetra-aza-pentacene molecules. Unsubstituted
1,4-cyclohexanedione permits the formation of both linear and bent Friedländer products, whereas blocking
its 2,5-positions enforces exclusive formation of the linear isomer. The photophysical properties of the
linear tAPs are similar to pentacene, whereas their crystal structures differ. The electrochemistry of
dibenzylated tAP agrees well with computational results, with reduction potentials comparable to that of
fullerene acceptors. Therefore, this class of tAPs are promising as potential n-type semiconductors to
contribute to the fast-growing research on optoelectronic devices, such as OPVs and/or OFETs, and the
adopted synthetic strategy opens up avenues to access other aza-substituted acenes of the same family
2.5 Experimental
2.5.1 General information
All purchased reagents were used as received without further purification. 2-Aminonicotinaldehyde (a1)
and dimethyl 2,5-dioxocyclohexane-1,4-dicarboxylate (c3) were purchased from Matrix Scientific and Alfa
Aesar, respectively. Purchased from Sigma Aldrich are 1,4-cyclohexanedione and 4-bromo-2,6-
di-tert-butylphenol (i). Purification by column chromatography was performed on a Teledyne ISCO
Combiflash RF200 UV/VIS for intermidates. Manual silica-gel column chromatography was used for the
separation of tAP l 5 from b 5. NMR spectra were recorded in CDCl 3 (for soluble intermidates) or TFA-d 1
32

(for poorly soluble tAPs) on Varian 400-MR. The UV-visible spectra were recorded on a Hewlett-Packard
4853 diode array spectrometer, and emission spectra on a QuantaMaster Photon Technology International
spectrofluoremeter. Bruker APEX II CCD Diffractometer was used to acquire X-ray single crystal
structures of tetra-aza-pentacenes. Cyclic voltammetry (CV) measurements of DB-tAP l 5 from b 5 were
performed anaerobically on VersaSTAT 3 electrochemical analyzer (Princeton Applied Research) using
glassy carbon working electrode, Pt counter electrode, Ag/Ag
+
reference electrode, and ferrocene as an
internal reference; electrolytes consist of tetrabutylammonium hexafluorophosphate (nBu 4NPF 6) in DMSO.
2.5.2 Synthesis of precursors

2-amino-5-bromonicotinaldehyde (a2). A previously reported procedure was slightly modified to
synthesize a1.
21
Under a nitrogen gas atmosphere, a two-neck round-bottom flask charged with dry THF
(40 mL) solution of 2-aminonicotinaldehyde (2.50 g, 2.47 mmol) was cooled to 0
o
C in the dark.
N-bromosuccinimide (3.64 g, 20.47 mmol) was slowly added and stirred for 12 hours. Cold water (30 mL)
was added into the mixture and it was extracted with ethyl acetate (x4). The combined organic layers were
washed with brine, dried over Na 2SO 4, and concentrated under reduced pressure. Pure yellow crystals were
grown by layering hexane over a concentrated DCM solution of the crude product. (Yield: 3.10 g, 75%).
1
H NMR (400 MHz, CDCl 3): δ = 9.81 (s, 1H, –CHO), 8.29 (d, J = 2.4, 1H), 7.89 (d, J = 2.4 Hz, 1H), 7.26
(s, 1H), 6.71 (broad, 2H, –NH 2).
13
C NMR (101 MHz, CDCl 3): δ = 191.45, 156.60, 155.44, 145.35, 114.66,
106.04. Elem. anal. for C 6H 5BrN 2O; calcd: C, 35.85%; H, 2.51; N, 13.94%; O, 796; Br, 39.75. Found:
35.90% C; 2.53% H; 13.89% N.
33

5-Bromo-1,3-di-tert-butyl-2-methoxybenzene (ii). Referring to a previously reported procedure,
40
barite
(30.16 g, 95.59 mmol) and MeI (14.77 mL, 237.18 mmol) were successively added into a nitrogen-purged
solution of i (20.50 g, 71.87 mmol) in DMF (230 mL) and the mixture was stirred overnight. The insoluble
salts were filtered off and washed with Et 2O. The filtrate was sequentially washed with water (250 mL), aq.
1M NaOH (200 mL) and brine (200 mL) before drying it over Na 2SO 4. The concentrated crude product
was purified by column chromatograph to obtain yellow oil ii, which solidifies at room temperature. Yield:
20.70 g (96.0 %).
1
H NMR (400 MHz, CDCl 3): δ 7.33 (s, 2H, aromatic H), 3.67 (s, 3H, –OCH 3), 1.40 (s,
18H, –CCH 3).
13
C NMR (101 MHz, CDCl 3): δ = 158.90, 146.18, 129.67, 116.53,64.54, 36.10, 32.03.
Elem. anal. for C 15H 23BrO; calcd: 60.21% C, 7.75% H, 26.70% Br, 5.35% O. Found: 60.15% C; 7.79% H.
(3,5-di-tert-butyl-4-methoxyphenyl)boronic acid (iii)
40
. Under N 2 atmosphere, a solution of tBuLi (1.5 M
in heptane, 16.04 mL, i.e. 40.10 mmol) was slowly added into a solution of ii (8.0 g, 26.73 mmol) in dry
THF (60 mL) cooled at –78
o
C. After 2 hours of stirring, triisopropyl borate (6.48 mL, 28.07 mmol) was
added drop-wise and the mixture was stirred another 15 minutes before being warmed up to room
temperature. The acidification with aq. 1M HCl was followed by extraction with ethyl acetate (80 mL, x3).
The organic layers were combined, washed with brine, dried over Na 2SO 4, and concentrated in vacuo.
Colorless crystals of iii (4.5 g, 63%) were obtained by layering hexane over a concentrated solution of
DCM+crude product to allow slow mixing.
1
H NMR (400 MHz, CDCl 3): δ = 8.16 (s, 2H, aromatic - H),
3.76 (s, 3H, – OCH 3), 1.52 (s, 18H, –CCH 3).
13
C NMR (101 MHz, CDCl 3): δ = 163.93, 143.44, 134.29,
132.03, 64.53, 35.89, 32.15. Elem. anal. for C 15H 25BO 3; calcd: 68.20% C, 9.54% H, 4.09% B, 18.17% O.
Found: 68.13% C; 8.81% H.
34

Compounds a3 / a4: A three- neck round bottom flask was charged with an appropriate boronic acid aryl
(5.50 mmol), a2 (5.50 mmol), potassium carbonate (1.09 g, 7.89 mmol), and PdCl 2(PPh 3) 2 (0.370 g,
0.526 mmol), and was pumped down and back-filled with nitrogen gas (x3). A purged mixture of DMF/H 2O
(30 mL/6 mL) was cannula-transferred into the reaction flask, which was then refluxed for 24 hours. The
completed reaction mixture was cooled to room temperature. Ethyl acetate (40 mL) was added and
insoluble residues were filtered off on a pad of celite. The organic filtrate was washed with saturated sodium
bicarbonate (70 mL, x3), dried over sodium sulfate, and the solvents were removed on rotary evaporator.
The crude product was purified by using column chromatography (hexanes/ethyl acetate, 80:20%) to
obtain:
a3 (yield, 82%):
1
H NMR (400 MHz, Chloroform-d) δ = 9.97 (s, 1H), 8.55 (d, J = 2.5 Hz, 1H),
8.02 (d, J = 2.5 Hz, 1H), 7.56 – 7.52 (m, 2H), 7.50 – 7.46 (m, 2H), 7.41 – 7.34 (m, 1H).
a4 (yield: 1.28 g, 71%):
1
H NMR (400 MHz, CDCl 3): δ = 9.97 (s, 1H, –CHO), 8.48 (s, 1H, pyridine
H), 7.94 (s, 1H, pyridine H), 7.37 (s, 2H, aromatic H), 3.74 (s, 3H, –OCH 3), 1.49 (s, 18H, –CCH 3).
13
C NMR (101 MHz, CDCl 3): δ = 192.99, 159.47, 157.26, 153.48, 144.64, 142.18, 131.56, 127.66,
124.79, 113.53, 64.52, 36.10, 32.24. Elem. anal. for C 21H 28N 2O 2; calcd: 74.08% C, 8.29% H,
8.23% N, 9.40% O. Found: 74.10% C; 8.26% H; 8.23% N.

35

Dimethyl 1,4-dibenzyl-2,5-dioxocyclohexane-1,4-dicarboxylate (c-Bn). Under N 2 atmosphere, benzyl
bromide (6.25 mL, 52.60 mmol) was added to a suspension of dimethyl
2,5-dioxocyclohexane-1,4-dicarboxyalte (4.0 g, 17.53 mmol) and potassium carbonate (9.75 g,
70.54 mmol) in dry THF (80 mL). The mixture was refluxed until all the starting material c3 is consumed
(monitored by TLC). The reaction mixture was cooled to room temperature before an addition of 70 mL of
ice-water and extraction with ethyl acetate (x3). The organic layer was washed with brine, dried over
sodium sulfate, and concentrated on rotary evaporator. Pure white solid c-Bn was obtained by column
chromatography purification using ethyl acetate/hexane as an eluent on ISCO. Yield: 4.11 g, 57.4 %. Note
the presence of cis and trans isomers from NMR, but the peaks of the cis isomer are very miniature and are
not reported here.
1
H NMR (400 MHz, CDCl 3): δ = 77.24-7.20 (m, 6H), 6.99-6.93 (m, 4H), 3.69 (s, 6H),
3.25 (d, J = 13.9 Hz, 2H), 3.13 (d, J = 13.7 Hz, 2H), 2.91 (d, J = 16.4, 0.7 Hz, 2H), 2.26 (d, J = 16.4, 0.7
Hz, 2H).
13
C NMR (101 MHz, CDCl 3): δ = 201.85, 169.71, 134.88, 130.13, 128.78, 127.62, 60.31, 53.28,
44.68, 41.40. Elem. anal. for C 24H 24O 6; calcd: 70.58% C, 5.92% H, 23.50% O. Found: 70.38 % C; 5.98% H.
Compounds c2/c4: In a round-bottom flask equipped with a water condenser, a suspension of c-Me or c-Bn
(8.84 mmol) in aq. 6M HCl/glacial AcOH (15 mL/30 mL) was heated at 120
o
C for 15 hours. Cooling the
reaction mixture to room temperature led to the formation of white precipitate which was collected by
vacuum filtration and washed by cold water (ca. 50 mL). Recrystallization in layered hexane/DCM yielded:
c3 (78%, white solid):
1
H NMR (400 MHz, Chloroform-d) δ 2.90 (dp, J = 12.9, 6.6 Hz, 1H),
2.83– 2.70 (m, 3H), 2.36 (dd, J = 17.9, 12.8 Hz, 1H), 1.15 (d, 6H).
c4 (yield: 2.01 g, 71%, white solid):
1
H NMR (400 MHz, CDCl 3): δ = 7.35-7.17 (m, 7H),
7.15-7.06 (m, 4H), 3.17 (dd, J = 13.8, 4.7 Hz, 2H), 2.89-2.78 (m, 2H), 2.68-2.60 (m, 4H), 2.50
(ddd, J = 15.1, 11.2, 0.8 Hz, 2H).
13
C NMR (101 MHz, CDCl 3): δ = 208.33, 138.14, 129.31,
128.77, 126.84, 49.00, 43.54, 36.40. Elem. anal. for C 20H 20O 2; calcd: 82.16 % C, 6.90% H,
10.94% O. Found:82.35% C; 10.57% H.
36

2.5.3 Synthesis of Tetra-aza-pentacenes (tAPs)

General procedures
Under nitrogen atmosphere in the dark (cover with Al foil), a 100-mL three-neck flask equipped with a
water condenser was charged with a cyclohexanedione derivative (1.0 eq.) and 2-amino-nicotinaldehyde
derivative (2.1 eq.). Nitrogen-purged ethanol (ca. 5 – 15 mL per mmol of the cyclohexanedione) was
cannula-transferred into the oxygen-free reaction flask, and the mixture was stirred for 10 minutes or till
dissolution of starting materials. Aqueous solution of 2M NaOH (0.5 eq) was injected in, followed by an
overnight reflux. A dark purple precipitate was collected by air-free filtration using Schleck filter and
washed with a copious amount of nitrogen-purged ethanol. The formation of l 2, l 3, l 6, and l 7 were only
confirmed by MALDI and photophysical analyses, no NMR characterization due to poor solubility.
Isolation and characterization of l 1, l 4, l 5 , b 1 and b 5 are laid out below.
1,7,8,14-tAP: bent (b 1) and linear (l 1). 1,4-cyclohexanedione (0.7 g, 6.24 mmol), 2-aminonicotinaldehyde
(1.56 g, 12.80 mmol), 2M aq. NaOH (1.6 mL, 3.20 mmol), and 40 mL of ethanol were used. Following the
general procedure, the precipitate was washed with plenty of nitrogen-purged dichloromethane (~200 mL)
to collect the more soluble isomer (b 1) in the DCM wash, which was concentrated, and re-precipitated out
by layering over hexane to obtain 115 mg of b 1 (%yield not applicable). The purification of the poorly

37

soluble tAP (l 1) was attempted via sublimation at 290
o
C and 10
-6
torr, but it poorly sublimes in <1% yield.
Nevertheless, single crystals suitable for X-ray crystallographic analysis were obtained. No acceptable
NMR data was acquired due to poor solubility and challenging purification.
tAP b 1:
1
H NMR (400 MHz, TFA-d 1) δ = 0.60 (s, 2H), 9.72 (d, J = 8.4 Hz, 2H), 9.67 (d, J = 5.4
Hz, 2H), 8.76 (s, 2H), 8.47 (ddd, J = 7.9, 5.5, 2.0 Hz, 2H).
13
C NMR (101 MHz, TFA-d 1) δ =
152.66, 150.82, 150.01, 145.36, 138.29, 136.65, 126.15, 123.57, 123.19. MALDI, calcd m/z:
282.09; found [MH]
+
= 283.24.
tAP l 1:
1
H NMR (400 MHz, TFA-d 1): No NMR nor CHNS analyses due to intractable purification.
MALDI: Calcd m/z: 282.09; found Calcd m/z: 282.09; found [MH]
+
= 283.20. Also see analysis by
single crystal structure and photophysical measurement in the Results & Discussion section.
3,10-DTBA-1,7,8,14-tAP: bent (b 5) and linear (l 5). 1,4-cyclohexanedione (125 mg, 1.11 mmol),
2-amino-5-(3,5-di-tert-butyl-4-methoxyphenyl)nicotinaldehyde (797.08 mg, 2.34 mmol), 2M NaOH
(0.28 mL, 0.560 mmol), and 45 mL of ethanol were used. Following the general procedure, the filtrate was
washed with hexanes. The separation of l 5 from b 5 was accomplished by manual column chromatography
on silica gel (THF/hexane (60:40 v/v)) in a glove box to obtain 35 mg (4% yield) of l 5 and 114 mg (14%
yield) of b 5. Using the mixture analysis technique described by Harris,
25
the l 5: b 5 product formation ratio
was measured to be 1:7.
tAP b 5:
1
H NMR (400 MHz, CDCl 3): δ = 9.61 (s, 2H), 9.59 (d, J = 2.6 Hz, 2H), 8.57 (d, J = 2.6 Hz,
2H), 8.44 (s, 2H), 7.69 (s, 4H), 3.81 (s, 6H), 1.55 (s, 36H).
13
C NMR (101 MHz, CDCl 3):
δ = 160.70, 155.73, 154.72, 151.16, 145.22, 135.82, 135.56, 133.12, 132.21, 131.26, 126.05,
124.53, 121.29, 64.66, 36.27, 32.27. Elem. anal. for C 48H 54N 4O 2: 80.19 % C, 7.57 % H, 7.79 % N,
4.45 % O. Found: 80.61 % C; 7.93 % H; 7.56 % N. MALDI, calcd m/z: 718.42; found [MH]
+
=
719.37.
38

tAP l 5:
1
H NMR (400 MHz, CDCl 3): δ = 9.82 (d, J = 30.0 Hz, 2H), 9.22 (s, 2H), 8.38 (d,
J = 27.7 Hz, 2H), 7.86 (s, 4H), 7.60 (s, 2H), 3.94 (s, 6H), 1.55 (s, 38H). MALDI, calcd m/z: 718.42;
found [MH]
+
= 719.31.
1,13-dibenzyl-1,7,8,14-tAP (l 4). 2,5-dibenzylcyclohexane-1,4-dione (1.20 g, 4.09 mmol),
2-aminonicotinaldehyde 1.02 g, 8.35 mmol), 2M NaOH (1.0 mL, 2.05 mmol), and 45 mL of ethanol were
used. Following the general procedure, the filtered precipitate was sequentially washed with degassed
ethanol (40 mL) and DCM (30 mL), and it was dried to yield 252 mg (14% yield) of a dark purple solid l 4.
Single crystals for X-ray analysis were grown by recrystallization in THF in a pressurized tube heated at
130
o
C and slowly allowed to cool down.
1
H NMR (400 MHz, TFA-d1) for two isomers:

δ = 9.38–9.35 (m,
2H), 9.25-9.21 (m, 2H), 8.15-8.08 (m, 4H), 8.05-8.00 (m, 2H), 7.39-7.16 (m, 26H), 5.10 (s, 4H), 4.87 (s,
4H).
13
C NMR (101 MHz, TFA-d1): δ = 151.05, 147.95, 147.50, 146.10, 144.44, 142.98, 136.22, 135.61,
135.12, 134.60, 133.57, 132.98, 129.00, 128.78, 128.53, 127.40, 127.27, 126.93, 126.62, 121.97, 121.38,
120.18, 117.91, 117.23, 31.59, 30.59, (two carbon peaks are indiscernible). MALDI, calcd m/z: 462.18;
found [MH]
+
= 463.20
39

2.5.4 Single crystal structure data collection

Table 2.2: Crystal structure data collection for l1 (CCDC# 1866946)
Diffractometer Bruker SMART APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 2.65
o
- 30.52
o

Index ranges -25 ≤ h ≤ 24; -8 ≤ k ≤ 8; -20 ≤ l ≤ 19
Reflections collected 29850
Independent reflections 1924
Absorption correction Multi-scan, 0.91≤ T ≤ 0.99
Structure solution technique Direct methods
Refinement method Full-matrix least-squares on F
2

Data/restraints/parameters 1924/0/100
Goodness-of-fit on 1.040
Chemical formula C 18H 10N 4
Formula weight 282.30
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.210 x 0.181 x 0.124 mm
Crystal habit “Lustrous dark red-green” prism
Crystal system monoclinic
Space group C1 2/c 1

Unit cell dimensions
a = 17.6422(16); α = 90
o

b = 5.8299(5); ß = 119.1460(10)
o

c = 14.0724(12); γ = 90
o

Cell volume 1264.11(19) Å
3

Z 2
Density (calculated) 1.486 g/cm
3

Absorption coefficient 0.092 mm
-1

F(000) 584

40

Table 2.3: Crystal structure data collection for DB-tAP l4 (CCDC# 1866945)
Diffractometer Bruker SMART APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 2.44
o
- 25.84
o

Index ranges -9 ≤ h ≤ 9; -10 ≤ k ≤ 10; -11 ≤ l ≤ 11
Reflections collected 10011
Independent reflections 2561
Absorption correction Multi-scan, 0.619 ≤ T ≤ 0.746
Structure solution technique Direct methods
Refinement method Full-matrix least-squares on F
2

Data/restraints/parameters 2127/0/163
Goodness-of-fit on 1.068
Chemical formula C 32H 22N 4
Formula weight 462.53
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.100 x 0.100 x 0.100 mm
Crystal habit “lustrous dark purple” crystalline
Crystal system triclinic
Space group P-1
Unit cell dimensions a = 8.003(3); α = 100.812(5)
o

b = 8.385(3); ß = 110.820(5)
o

c = 9.133(3); γ = 90.330(5)
o

Cell volume 560.9(3) Å
3

Z 1
Density (calculated) 1.369 g/cm
3

Absorption coefficient 0.082 mm
-1

F(000) 242

41

Table 2. 4: Crystal structure data collection for DTBA-tAP b5 (CCDC# 1866947)
Diffractometer Bruker SMART APEX DUO
Radiation source fine-focus tube, MoKα
Theta range for data collection 2.36
o
- 19.96
o

Index ranges -16 ≤ h ≤ 16; -18 ≤ k ≤ 18; -30 ≤ l ≤ 30
Reflections collected 60378
Independent reflections 18499
Absorption correction Multi-scan, 0.82 ≤ T ≤ 0.99
Structure solution technique Direct methods
Refinement method Full-matrix least-squares on F
2

Data/restraints/parameters 18499/6/1026
Goodness-of-fit on 1.026
Chemical formula C 48H 54N 4O 2
Formula weight 718.95
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.577 x 0.389 x 0.149 mm
Crystal habit “colorless” crystalline
Crystal system Triclinic
Space group P-1

Unit cell dimensions
a = 7 13.235(4); α = 91.771(4)
o

b = 14.229(5); ß = 103.340(4)
o

c = 24.230(8); γ = 99.564(5)
o

Cell volume 4367.(2) Å
3

Z 4
Density (calculated) 1.094 g/cm
3

Absorption coefficient 0.067 mm
-1

F(000) 1544

42

2.5.5 Anaerobic electrochemical cell set-up
To ensure anaerobic conditions, the sample is prepared in a glovebox, and the electrodes are pierced through
rubber septa, plunged into the sample solution (DMSO ~ 1 x 10
-5
M), and the cell is sealed before taking it
out of glovebox. Tetrabutylammonium hexafluorophosphate (nBu4NPF6) are used as electrolytes,
ferrocene as an internal reference, glassy carbon as a working electrode, Pt as a counter electrode, and
Ag/Ag+ as reference electrode. All CVs are recorded at 100 mV/s scan rate.

Figure 2.9: Anaerobic electrochemical cell used to measure CVs.

43

2.5.6 NMRs spectra

1
H and
13
C NMR spectra of compound 2a (CDCl 3, 400 MHz and 101 MHz, respectively).

44

1
H and
13
C NMR spectra of compound ii (CDCl 3, 400 MHz and 101 MHz, respectively).

45

1
H and
13
C NMR spectra of compound iii (CDCl 3, 400 MHz and 101 MHz, respectively)

46

1
H and
13
C NMR spectra of compound 4a (CDCl 3, 400 MHz and 101 MHz, respectively).

47

1
H and
13
C NMR spectra of compound c-Bn (CDCl 3, 400 MHz and 101 MHz, respectively).

48

1
H and
13
C NMR spectra of compound c4 (CDCl 3, 400 MHz and 101 MHz, respectively).

49

1
H and
13
C NMR spectra of compound b1 (TFA-d1, 400 MHz and 101 MHz, respectively). Due to
extremely low intensity of analyte peaks in
13
C NMR spectrum, TFA-d 1 solvent peaks (two quartets at
161.82 and 114.17 ppm) are cut out for clarity.

50

1
H and
13
C NMR spectra of compound b5 (CDCl 3, 400 MHz and 101 MHz, respectively).

51

1
H and
13
C NMR spectra of compound l4 (TFA-d 1, 400 and 101 MHz, respectively. Note that the
13
C
NMR spectrum shows a zoomed-in inset with TFA-d 1 peaks cut out for clarity (Two protons and two
carbons are indiscernible).

52

1
H NMR spectrum of compound l5 (TFA-d 1, 400 MHz).
13
C NMR was not acquired due to solubility
issues.

.

53

2.6 References

1. Zhou, T. L.; Jia, T.; Kang, B. N.; Li, F. H.; Fahlman, M.; Wang, Y., Nitrile-Substituted QA
Derivatives: New Acceptor Materials for Solution-Processable Organic Bulk Heterojunction Solar Cells.
Advanced Energy Materials 2011, 1 (3), 431-439.
2. Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S.,
Perfluoropentacene: High-Performance p-n Junctions and Complementary Circuits with Pentacene. J. Am.
Chem. Soc. 2004, 126 (26), 8138-8140.
3. Reiss, H.; Ji, L.; Han, J.; Koser, S.; Tverskoy, O.; Freudenberg, J.; Hinkel, F.; Moos, M.;
Friedrich, A.; Krummenacher, I.; Lambert, C.; Braunschweig, H.; Dreuw, A.; Marder, T. B.; Bunz, U. H.
F., Bromination Improves the Electron Mobility of Tetraazapentacene. Angewandte Chemie-International
Edition 2018, 57 (30), 9543-9547.
4. Tang, M. L.; Oh, J. H.; Reichardt, A. D.; Bao, Z., Chlorination: A General Route toward Electron
Transport in Organic Semiconductors. J. Am. Chem. Soc. 2009, 131, 3733-3740.
5. Kuo, M.-Y.; Chen, H.-Y.; Chao, I., Cyanation: Providing a Three-in-One Advantage for the
Desing of n-Type Organic Field-Effect Transistors. Chem. Eur. J. 2007, 13, 4750-4758.
6. Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony, J. E.; Mayer, A. C.; Malliaras, G. G.,
Synthesis and Characterization of Electron-Deficient Pentacenes. Org. Lett. 2005, 7 (15), 3163-3166.
7. Tang, M. L.; Bao, Z., Halogenated Materials as Organic Semiconductors. Chem. Mater. 2011, 23,
446-455.
8. Bheemireddy, S. R.; Ubaldo, P. C.; Rose, P. W.; Finke, A. D.; Zhuang, J. P.; Wang, L. C.;
Plunkett, K. N., Stabilizing Pentacene By Cyclopentannulation. Angewandte Chemie-International
Edition 2015, 54 (52), 15762-15766.
9. Sola, M., Forty years of Clar's aromatic pi-sextet rule. Frontiers in Chemistry 2013, 1.
10. Winkler, M.; Houk, K. N., Nitrogen-Rich Oligoacenes: Candidates for n-Channel Organic
Semiconductors. J. Am. Chem. Soc. 2007, 129 (6), 1805-1815.
11. Halls, M. D.; Djurovich, P. J.; Giesen, D. J.; Goldberg, A.; Sommer, J.; McAnally, E.; Thompson,
M. E., Virtual screening of electron acceptor materials for organic photovoltaic applications. New J. Phys.
2013, 15, 105029-105044.
12. Bunz, U. H. F., N-Heteroacenes. Chem. Eur. J. 2009, 15, 6780-6789.
13. Bunz, U. H. F., The Larger N-Heteroacenes. Pure Appl. Chem. 2010, 82 (4), 953-968.
54

14. Bunz, U. H. F., Large N-Heteroacenes: New Tricks for Very Old Dogs? Angew. Chem. Int. Ed.
2013, 52, 3810-3821.
15. Kitahara, K.; Nishi, H., New Heterocyclic Compounds Derived from Diethyl 2,5-Dioxo-1,4-
cyclohexanedicarboxylate and 2-Aminobenzophenone. J. Heterocyclic Chem. 1988, 25, 1063-1065.
16. Gellerman, G.; Rudi, A.; Kashman, Y., THE BIOMIMETIC SYNTHESIS OF MARINE
ALKALOID RELATED PYRIDO 2,3,4-KL ACRIDINES AND PYRROLO 2,3,4-KL ACRIDINES.
Tetrahedron 1994, 50 (45), 12959-12972.
17. Thummel, R. P., 2-Aminonicotinaldehyde. e-EROS Encyclopedia of Reagents for Organic
Synthesis 2001.
18. Majewicz, T. G.; Caluwe, P., CHEMISTRY OF ORTHO-AMINO ALDEHYDES -
REACTIONS OF 2-AMINONICOTINALDEHYDE AND CYCLOHEXANEDIONES. Journal of
Organic Chemistry 1975, 40 (23), 3407-3410.
19. Fenlon, E. E.; Murray, T. J.; Baloga, M. H.; Zimmerman, S. C., Convenient Synthesis of 2-
Amino-1,8-naphthyridines, Building Blocks for Host-Gues and Self-Assembling Systems. J. Org. Chem.
1993, 58, 6625-6628.
20. Hu, Y. Z.; Xiang, Q.; Thummel, R. P., Bi-1,10-phenanthrolines and their mononuclear Ru(II)
complexes. Inorganic Chemistry 2002, 41 (13), 3423-3428.
21. Koizumi, T. A.; Kato, S.; Yamamoto, T., Synthesis and Characterization of New pi-Conjugated
Polymers Containing 1,8-Naphthyridine in the Main Chain: Role of the 1,8-Naphthyridine Unit in pi-
Conjugated Polymers. Journal of Polymer Science Part a-Polymer Chemistry 2011, 49 (19), 4204-4212.
22. Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T., Photochemical stability of
pentacene and a substituted pentacene in solution and in thin films. Chemistry of Materials 2004, 16 (24),
4980-4986.
23. Tanaka, H.; Ikeno, T.; Yamada, T., Efficient Oxidative Aromatization of 9,10-
Dihydroanthracenes with Molecular Oxygen Catalyzed by Ruthenium Porphyrin Complex. Synlett 2003,
4, 576-578.
24. Reetz, V. M. T.; Eibach, F., Deprotonierung-Hydrideliminierung als Methode zur Dehydrierung.
Angewantde Chemie 1978, 90 (4), 285-286.
25. Harris, D. C., Quantitative chemical analysis. 6
th
ed.; W.H. Freeman and Company: New York,
2003; p 434-435.
26. Bunz, U. H. F., The Larger Linear N-Heteroacenes. Accounts of Chemical Research 2015, 48 (6),
1676-1686.
55

27. Nijegorodov, N.; Ramachandran, V.; Winkoun, D. P., The dependence of the absorption and
fluorescence parameters, the intersystem crossing and internal conversion rate constants on the number of
rings in polyacene molecules. Spectrochimica Acta Part a-Molecular and Biomolecular Spectroscopy
1997, 53 (11), 1813-1824.
28. Pramanik, C.; Miller, G. P., An Improved Synthesis of Pentacene: Rapid Access to a Benchmark
Organic Semiconductor. Molecules 2012, 17 (4), 4625-4633.
29. Mondal, R.; Tonshoff, C.; Khon, D.; Neckers, D. C.; Bettinger, H. F., Synthesis, stability, and
photochemistry of pentacene, hexacene, and heptacene: a matrix isolation study. J Am Chem Soc 2009,
131 (40), 14281-9.
30. Xie, Q. S.; Perezcordero, E.; Echegoyen, L., ELECTROCHEMICAL DETECTION OF C-60(6-)
AND C-70(6-) - ENHANCED STABILITY OF FULLERIDES IN SOLUTION. Journal of the American
Chemical Society 1992, 114 (10), 3978-3980.
31. Miao, S.; Appleton, A. L.; Berger, N.; Barlow, S.; Marder, S. R.; Hardcastle, K. I.; Bunz, U. H.
F., 6,13-Diethynyl-5,7,12,14-tetraazapentacene. Chem. Eur. J. 2009, 15, 4990-4993.
32. Sworakowski, J.; Lipinski, J.; Janus, K., On the reliability of determination of energies of HOMO
and LUMO levels in organic semiconductors from electrochemical measurements. A simple picture based
on the electrostatic model. Organic Electronics 2016, 33, 300-310.
33. Ruoff, R. S.; Kadish, K. M.; Boulas, P.; Chen, E. C. M., THE RELATIONSHIP BETWEEN
THE ELECTRON-AFFINITIES AND HALF-WAVE REDUCTION POTENTIALS OF FULLERENES,
AROMATIC-HYDROCARBONS, AND METAL-COMPLEXES. Journal of Physical Chemistry 1995,
99 (21), 8843-8850.
34. Campbell, R. B.; Trotter, J.; Robertson, J. M., CRYSTAL AND MOLECULAR STRUCTURE
OF PENTACENE. Acta Crystallographica 1961, 14 (7), 705-&.
35. Mei, X. F.; Wolf, C., Formation of new polymorphs of acridine using dicarboxylic acids as
crystallization templates in solution. Crystal Growth & Design 2004, 4 (6), 1099-1103.
36. Mattheus, C. C.; Dros, A. B.; Baas, J.; Oostergetel, G. T.; Meetsma, A.; de Boer, J. L.; Palstra, T.
T. M., Identification of polymorphs of pentacene. Synthetic Metals 2003, 138 (3), 475-481.
37. Nabok, D.; Puschnig, P.; Ambrosch-Draxl, C.; Werzer, O.; Resel, R.; Smilgies, D. M., Crystal
and electronic structures of pentacene thin films from grazing-incidence x-ray diffraction and first-
principles calculations. Physical Review B 2007, 76 (23).
38. Isoda, K.; Nakamura, M.; Tatenuma, T.; Ogata, H.; Sugaya, T.; Tadkoro, M., Synthesis and
Characterization of Electron-accepting Nonsubstituted Tetraazaacene Derivatives. Chem. Lett. 2012, 41
(9), 937-939.
56

39. Campbell, J. E.; Yang, J.; Day, G. M., Predicted energy-structure-function maps for the
evaluation of small molecule organic semiconductors. Journal of Materials Chemistry C 2017, 5 (30),
7574-7584.
40. Diemer, V.; Chaumeil, H.; Defoin, A.; Fort, A.; Boeglin, A.; Carre, C., Syntheses of sterically
hindered zwitterionic pyridinium phenolates as model compounds in nonlinear optics. European Journal
of Organic Chemistry 2008, (10), 1767-1776.

57

3 CHAPTER III: Synthesis and characterization of Zinc(II) complexes
bearing 4-acridinol and 1-phenazinol

3.1 Introduction
Metal chelates of quinolin-8-ol (Q) derivatives have been widely investigated (Figure 3.1).
1-3
These
complexes of MQ n formula (where n is the number of oxidation state of the metal M) find applications
mostly in organic electronic devices. Tris(8-hydroxyquinolinato)aluminum (AlQ 3) and
bis(8-hydroxyquinolinato)zinc chelates (ZnQ 2) are often used in organic light-emitting diodes (OLEDs) as
emissive, electron transporting, and light-outcoupling materials.
4-6
Compelled by the detrimental higher
operating voltage requirements of AlQ 3, researchers demonstrated that ZnQ 2 can be employed at lower
voltages and still produce high device efficiencies.
7-8

Figure 3.1: (Left) Metal chelates of MQX type (e.g. AlQ3 and ZnQ2) are well-studied and use 8-quinolinol (Q) as ligand. (Right)
p-extended quinolinol-like ligands (acridinol and phenazinol derivatives) are used in the present work to investigate their zinc (II)
chelates.

While zinc chelates of quinoline-8-ol are well studied, -extended quinolinol-like chelates have rarely
investigated. Phenazin-1-ol and acridin-4-ol are examples of -extended (N,O) donor ligands that contain
the coordination moiety of quinolinol. Only one report on related complexes, with minimum
characterization, uses acridinol ligand to coordinate to Al(III), Ga(III) ionic metals to yield heteroleptic
complexes.
9
Here we sought to explore the synthesis, electrochemical and photophysical properties of
homoleptic complexes of ZnX 2 type, where X represents the (N,O) donor ligand: phenazin-1-ol (P),

58

benzo[b]phenazin-1-ol (bP), 6-xylyl-phenazin-1-ol (xP), and acridin-4-ol (A) (Figure 3.1). Representative
heteroleptic zinc(II) chelates of acridinol (ZnA 1) and phenazinol (ZnP 1) will be synthesized and compared
to their homoleptic counterparts. Density functional theory (DFT) calculations on the complexes are
performed to predict energetics of frontier orbitals.

Figure 3.2: Synthesis of homoleptic (ZnX2) and heteroleptic (ZnX1) complexes by varying the equivalents of Zn(acac)2.

3.2 Results and Discussions
3.2.1 Synthesis
Acridinol is commercially available while phenazinol and benzophenazinol are synthetically accessible.
10

The homoleptic zinc(II) chelates (ZnA 2, ZnP 2, ZnxP 2 and ZnbP 2) are synthesized in good yields
(66 – 90%) by subjecting half equivalent of zinc acetylacetonate monohydrate, Zn(acac) 2, to a methanolic
solution of appropriate (N,O) donor ligand at room temperature overnight (Figure 3.2). Zinc acetate,
Zn(OAc) 2, can also be used instead of Zn(acac) 2 under similar conditions to synthesize the homoleptic
complexes in comparable yields. These homoleptic complexes are stable as solids with no sign of
decomposition after several months of storage in a desiccator.

59

Figure 3.3: Partial
1
H NMR spectra (aromatic region) for homoleptic (ZnA2) vs heteroleptic (ZnA1) products formed as a function
of Zn(acac)2 equivalents. From bottom to top spectrum, Zn(acac)2:acridinol ratio is 0.5:1, 3:1, and 8:1 (top stacked spectra). In the
middle spectrum, hash-tags (#) and stars (*) represent ZnA1 and ZnA2 peaks, respectively. Similar results were observed for ZnP1
and ZnP2 (bottom stacked spectra).

The formation of heteroleptic products (ZnA 1 and ZnP 1), on the other hand, required use of Zn(acac) 2 in
excess, i.e. an 8:1 equivalent ratio of Zn(acac) 2 to N,O-ligand. To favor the formation of heteroleptic
products, a solution of acridinol or phenazinol in dichloromethane (DCM) is added dropwise to a stirring

--------------------------------------------------------------------------------------------------

60

DCM/MeOH solution of excess Zn(acac) 2, and the reaction was stopped after 15 minutes to remove
volatiles. The heteroleptic products required the presence of excess of Zn(acac) 2 to prevent intermolecular
rearrangement to form the more stable homoleptic zinc(II) products. Thus, NMR and UV-vis measurements
of ZnA 1 and ZnP 1 were performed on samples containing an excess of Zn(acac) 2.
Figure 3.3 shows the proton NMR analysis of acridinol- and phenazinol-based chelates used to analyze the
ratio of heteroleptic/homoleptic product formed as the amount of Zn(acac) 2 was varied from 0.5 to 8
equivalents. A ratio of 0.5 molar equivalent of Zn(acac) 2 versus 1.0 equivalent of acridinol ligand yielded
ca. 100% ZnA 2. Increasing the equivalents of the zinc(II) source to 1:1, and then to 3:1, changed the
ZnA 1/ZnA 2 product ratio from 0.8:1 to 3:1. For Zn(acac) 2 equivalents greater than or equal to 8.0, the
products are ca. 100% heteroleptic (ZnA 1). The same results were observed for phenazinol-based
homoleptic/heteroleptic chelates.
Table 3.1: Redox potential values from electrochemical measurements of ligands and complexes in N,N-dimethylformamide. All
values are referenced to Fc/Fc
+
. HOMO is calculated from formula 1.15(Eox) + 4.79 and LUMO from 1.18(Ered) - 4.83, according
to literature.
11
HOMO/LUMO values in parentheses are from DFT calculations.
COMPOUND E
ox (V)
E
red (V)
HOMO
exp

(HOMO
DFT
)/eV
LUMO
exp

(LUMO
DFT
)/eV
E
HO/LU

(exp.)
Homoleptic Complexes

ZnA
2
0.42 -2.0 -5.27 (-5.03) -2.47 (-2.28) 2.8
ZnP
2
0.52 -1.43 -5.38 (-5.39) -3.14 (-2.83) 2.24
ZnbP
2
0.41 -1.39 -5.27 (-5.22) -3.19 (- 3.05) 2.08
Ligands

Acridinol 0.49 -1.86 -5.35 (-5.44) -2.64 (-2.09) 2.71
Phenazinol 0.57 -1.51 -5.45 (-5.74) -3.05 (-2.53) 2.40
bPhenazinol 0.48 -1.35 -5.34 (-5.44) -3.24 (-2.83) 2.10

61

Figure 3.4: Cyclic voltammograms of free ligand (acridinol, phenazinol, and benzophenazinol) and homoleptic complexes (ZnA2,
ZnP2, and ZnbP2) in DMF. Glassy carbon (working electrode), Pt counter electrode, Ag/Ag
+
(reference electrode), ferrocene
(internal reference), 0.1M tetrabutylammonium hexafluorophosphate (nBu4NPF6) were used. All CV were referenced to
ferrocene/ferrocenium couple (Fc/Fc
+
= 0.0 V).

3.2.2 Electrochemical properties
Electrochemical studies of the free ligands (A, P, and bP) and associated homoleptic complexes (ZnA 2,
ZnP 2, and ZnbP 2) were conducted in N,N-dimethylformamide (DMF) solution of 0.1 M
tetrabutylammonium hexafluorophosphate (nBu 4NPF 6) by using glassy carbon working electrode, Pt
counter electrode, silver wire reference electrode, and ferrocene as an internal reference. Table 3.1
summarizes the redox potentials and derived HOMO/LUMO values extracted from differential pulse
voltammetry (DPV). Reversibility was judged from cyclic voltammograms (Figure 3.4). The acridinol
ligand and complex are characterized by irreversible redox waves whereas phenazinol derivatives are
quasi-reversible. Of the three ligands, acridinol is the hardest to reduce (E red = -1.88 V) and
benzophenazinol the easiest (E red = -1.35 V). Phenazinol has the most positive oxidation potential (i.e.
harder to oxidize, E ox = 0.57 V) whereas acridinol and benzophenazinol have comparable oxidation
-3 -2 -1 0 1
-2
-1
0
1
2
3
Current (x10
-4
A)
Potential (V) vs. Fc/Fc+
Acridinol
-3 -2 -1 0 1
-20
-10
0
10
20
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
Phenazinol
-3 -2 -1 0 1
-8
-4
0
4
8
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
benzophenazinol
-3 -2 -1 0 1
-4
0
4
8
Current (x10
-5
A)
Potential (V) vs Fc/Fc
+
ZnA
2
-3 -2 -1 0 1
-8
-4
0
4
8
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
ZnP
2
-3 -2 -1 0 1
-8
-4
0
4
8
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
ZnbP
2

62

potentials. The more positive oxidation potentials reflect the electron-withdrawing effect of two nitrogen
in phenazinol derivatives (as opposed to one N in acridinol), which stabilizes the energy of the frontier
orbitals of phenazinol and benzophenazinol. Further narrowing of the HOMO-LUMO gap in
benzophenazinol agrees with the effect of extending the -conjugation in the core.
Upon reduction and oxidation of ligands, follow-up chemical reactions form new unidentified electroactive
products that show redox behaviors on the reverse scan. This new electroactive region ranges from -1.2 V
to 0.5 V for acridinol, -1.0 V to 0.0 V for phenazinol, and -0.8 V to -0.1 V for benzophenazinol. Similar
electrochemical observations have been reported for 8-quinolinol.
3

The redox potentials for the homoleptic complexes resemble those of their corresponding ligands. For
example, the reduction and oxidation potential difference between ZnA 2 and free acridinol is 140 mV and
70 mV, respectively (Table 3.1). Additionally, the trend in redox potentials observed in the ligands is the
same as that in the complexes, leading to the comparable HOMO/LUMO gaps between ligands and
complexes. Aluminum (III) and zinc(II) chelates of 8-hydroxyquinoline were also reported to have
ligand-centered electrochemical behaviors with the metal remaining redox innocent.
3

3.2.3 Photophysical properties
The optical properties of the ligands and complexes were examined using UV-visible and emission
spectroscopy. Absorption and emission spectra of methoxy analogs of the ligands (methoxy-acridine
A-OMe, methoxyphenazine P-OMe, xylyl-methoxyphenazine xP-OMe, and methoxybenzophenazine
bP-OMe) were measured in dichloromethane (Figure 3.5) and are compared to the corresponding
homoleptic complexes in Figure 3.6. Methoxy derivatives of the hydroxy ligands were chosen for their
relatively greater and detectable luminescence. All the ligands displayed a low-energy absorption shoulder
attributed to intramolecular charge transfer (CT) from the phenoxy ring moiety to the rest of the aromatic
skeleton. The sharp and/or structured band at higher energies corresponds to -

electronic transitions.
63

Figure 3.5: Absorption and emission spectra of ligands in dichloromethane. Photoluminescence quantum yield and lifetimes are
insetted.

Extinction coefficients () of the more intense -

transitions of the ligands fall in the range of
4.3-8.6x10
3
M
-1
cm
-1
, with methoxy-benzophenazine having the highest and methoxy-acridine the lowest
value. Note that the two bands are overlapped in the absorption spectrum of A-OMe and become
distinguishable in P-OMe, xP-OMe, and bP-OMe. The CT absorption and emission maxima of Acr-OMe
are blue-shifted compared to the phenazinoxy derivatives. Among the phenazinoxy-based ligands,
bPhen-OMe has the most red-shifted profiles agreeing with its more -extended core.
The lower photoluminescence quantum yields of methoxyphenazine ( PL = 0.11) and
methoxybenzophenazine ( PL = 0.04) reflect the consequence of high nonradiative rates brought about by
the lower energy emission, i.e. the energy gap law. Methoxyacridine that has a blue-shifted emission leads
to an improved quantum yield of  PL = 0.56. Notably, the quantum yields correspond with a decrease in
excited state lifetimes from acridinol to phenazinol and benzophenazinol, confirming an increase in
non-radiative rates. Surprisingly, among the phenoxy-derivatives, xP-OMe has a relatively higher quantum
300 400 500 600 700
0
2
4
6
8
10
300 400 500 600
0
1
2
3
4
5
300 400 500 600 700
0
2
4
6
8
10
300 400 500 600 700
0
2
4
6
8
10
Ext. coef. (x10
3
M
-1
cm
-1
)
Wavelength (nm)
Phen-OMe (DCM)
Absorption
0.0
0.4
0.8
1.2
Emission, RT
Norm. PL intensity (a.u)
Φ = 0.11
 = 5.74 ns
Ext. coef. (x10
3
M
-1
cm
-1
)
Wavelength (nm)
Acr-OMe (DCM)
Absorption
0.0
0.4
0.8
1.2
Emission, RT
Norm. PL intensity (a.u)
Φ = 0.56
 = 15.5 ns
Ext. coef. (x10
3
M
-1
cm
-1
)
Wavelength (nm)
bPhen-OMe (DCM)
Absorption
Φ = 0.04
 = 2.43 ns
0.0
0.4
0.8
1.2
Emission, RT
Norm. PL intensity (a.u)
Ext. coef. (x10
3
M
-1
cm
-1
)
Wavelenght (nm)
xPhen-OMe (DCM)
Absorption
0.0
0.4
0.8
1.2
Emission, RT
Norm. PL intensity (a.u)
Φ = 0.37
 = 17.4 ns

64

yield ( PL = 0.37) despite the side xylyl group that was expected to introduce non-radiative pathways
through rotations.

Figure 3.6: UV-vis absorption (and emission for ZnA2) spectra of all homoleptic Zn(II) complexes. Extinction coefficient (, in
M. cm
-1
) of charge-transfer band maximum is annotated for each complex. Emission of ZnP2, ZnxP2 and ZnbP2 is too poor to be
recorded on our fluorometer (PL < 0.001).

UV-vis absorption and emission spectra of homoleptic zinc(II) chelates (ZnA 2, ZnP 2, ZnxP 2 and ZnbP 2)
were acquired in dichloromethane (DCM) and 2-methycylohexane (MeCy), Figure 3.6. Like their precursor
ligands, absorption spectra of the complexes in both solvents are characterized by two well-resolved, but
red-shifted, transition bands (-

and CT). The lower intensity in the CT bands of ZnP 2, ZnxP 2 and ZnbP 2
might be attributed to the decrease in HOMO-LUMO separation brought about by an additional nitrogen
in the phenazinol core. The values for  of ZnX 2 complexes are approximately two times greater than those
of their free ligands, ranging from  CT = 5.4– 8.8 x10
3
M
-1
cm
-1
consistent with the homoleptic nature of the
300 400 500 600 700 800
0.0
0.4
0.8
1.2
300 400 500 600 700 800
0.0
0.4
0.8
1.2
300 400 500 600 700 800
0.0
0.4
0.8
1.2
300 400 500 600 700 800
0.0
0.4
0.8
1.2
Wavelength (nm)
ZnbP
2
Abs (MeCy)
Abs (DCM)
 = 7.5 x 10
3
1wt% PS film
Norm. intensity (a.u)
MeCy
DCM
ZnA
2
 = 8.8 x 10
3
Φ = 0.012
 = 13.5 ns
Norm. intensity (a.u)
Wavelength (nm)
ZnP
2
Abs (MeCy)
Abs (DCM)
 = 5.4 x 10
3
ZnxP
2
Abs (MeCy)
Abs (DCM)
 = 6.2 x 10
3

65

complexes. Monzon, L. M. A, et al. described the same photophysical behavior in metal chelates of 8-
hydroxyquinoline (MQ 2 and MQ 3).
3

Figure 3.7: Comparison of emission intensities between ZnA2 vs ZnA1 when excited at the same wavelength (=450 nm) with
the same optical density.

Complexes ZnP 2, ZnxP 2 and ZnbP 2 are so poorly emissive to detect emission ( PL < 0.001) regardless of
the solvent. Emission of ZnA 2, however, could be recorded in 2-methylcyclohexane ( PL = 0.014),
dichloromethane ( PL = 0.010), and 1%wt of polystyrene film ( PL = 0.015). From non-polar to polar
solvent, absorption and emission spectra are characterized by a positive solvatochromism behavior. This
indicates that dipole moments in the ground state and excited state are in the same direction, with the latter
being larger. Therefore, in a polar solvent the excited CT states are stabilized at a greater extent during
absorption and emission events causing bathochromism. The poor emission efficiency of ZnP 2, ZnxP 2, and
ZnbP 2 is paralleled by the weakly emissive phenazinol-based ligands (vide supra). Additional reasons for
the decrease in  PL include (1) more potential distortions introduced in the four-coordinated complexes
(compared to free ligands), (2) accentuated CT transitions in the complexes, and (3) energy band-gap law
since the complexes are more redshifted relative to free ligands. Lastly, symmetry-breaking charge-transfer
(SBCT) is a phenomenon known to suppress radiative emission in homoleptic complexes
12-13
To investigate
a potential contribution of SBCT, emission intensities of ZnA 2 and ZnA 1 excited at the same optical density
0.0
0.4
0.8
1.2
Optical density (a.u)
Wavelength (nm)
ZnA
2
vs ZnA
1
(MeTHF)
ZnA
2
ZnA
1
300 400 500 600 700 800
0
2
4
6
8
10
PL intensity (Counts x10
-5
)

66

wavelength were compared (supporting information); the homoleptic complex showed a greater emission
intensity than the heteroleptic counterpart by roughly a factor of two, ruling out the SBCT taking place. In
other words, if SBCT events were taking place, the intensity of ZnA 2 should not be higher than that of the
heteroleptic counterpart.
3.2.4 DFT Calculations
Density functional theory (DFT) calculations were performed on the ligands and complexes at
B3LYP/LACVP** level of theory,
14
and frontier orbitals (HOMO and LUMO) of optimized structures of
anionic ligands are summarized in Figure 3.8. The HOMO of the free ligand is localized on the phenoxy
ring moiety whereas the LUMO is almost evenly distributed over the rest of the aromatic core. This
electronic distribution is reminiscent of the CT and -

observed in the absorption spectra of all free
ligands, where the two bands start to become more distinct in the phenazinol-based ligands. The localization
of HOMO is more evident in the -extended ligand, benzophenazinoxy, explaining the more resolved
CT - -

bands in the ligand. Worthy of note is the little or no contribution of 2,6-dimthylphenyl group in
the frontier orbitals of xylyl-phenazinoxy ligand, which agrees with the small difference in the absorption
and emission spectra of Phen-OMe and xPhen-OMe (Figure 3.5).
The HOMO-LUMO separation becomes more apparent in the zinc(II) complexes as shown in Figure 3.9,
where the HOMO distribution over the quinoline moiety is even more insignificant. Consequently, the CT
and − bands of complexes are more resolved. The zinc(II) center has little electronic contribution to the
frontier orbitals, agreeing with the electrochemical measurements where the redox processes do not involve
the metal.
67

Figure 3.8: HOMO and LUMO of anionic ligands of acridin-4-ol, phenazin-1-ol, benzo[b]phenazin-1-ol, and
6-xylyl-phenazin-1-ol.

The DFT calculations find that the HOMO-1 and HOMO (similarly LUMO and LUMO+1) of each complex
are near degenerate in energy. For example, values for ZnA 2 are HOMO-1 = -5.06 eV and
HOMO = -5.03 eV, whereas both LUMO and LUMO+1 = -2.28 eV. The involvement of more orbitals from
two ligands, and hence more states, are consistent with the measured higher extinction coefficients
– roughly doubled – in the complexes relative to their free ligands. A distorted tetrahedral geometry is
determined for the optimized complexes. This distortion is due to the bite angle (O-Zn-N, on the same
ligand), which happens to be less than a common tetrahedral bond angle (109.5
o
) in all our complexes. For
example, the O-Zn-N bite angle in ZnA 2 is 85
o
.

68

Figure 3.9: Frontier orbitals (HOMO-1, HOMO, LUMO, and LUMO+1) of homoleptic complexes (top) and representative
heteroleptic complexes (bottom) from DFT calculations.

3.3 Conclusion
To conclude, we have disclosed the synthesis and characterization of homoleptic Zn(II) and heteroleptic
chelates of 4-acridinol and 1-phenazinol derivatives. Either Zn(acac) 2 or Zn(OAc) 2 can be used as a source
of zinc(II) ion in the synthesis of the homoleptic complexes. Excessive amount of Zn(acac) 2 is required to
drive the heteroleptic products formation. Electrochemical and photophysical studies reveal that the

69

electronic properties of the complexes are ligand-centered. Unfortunately, the  PL values for the zinc
complexes are very low, < 0.02, making them poor candidates as OLED emitters. The charge-transfer
character is accentuated from free ligands to Zn(II)-coordinated chelates. DFT calculations were performed
to support the experimental observations. This study provides a great contribution in further understanding,
design and synthesis of metal chelates of MX n type, particularly those for electronic device applications.
3.4 Experimental
3.4.1 General information
Commercially available starting materials were purchased and used as received without further purification,
unless noted. Purchased chemicals include zinc (II) acetylacetonate monohydrate [Zn(acac)2], zinc(II)
acetate [Zn(OAc) 2], and acridin-4-ol (A), all from Sigma Aldrich. Phenazinol-derived ligands
[phenazin-1-ol (P), benzo[b]phenazin-1-ol (bP), and 6-xylyl-phenazin-1-ol (xP) ] were synthesized
according to literature (Journal of Medicinal Chemistry 2010, 53 (24), 8688-8699). NMR spectra (
1
H and
13
C) were recorded on Varian 400-MR in chloroform-d 1 for ligands [7.26 ppm (
1
H), 77.06 ppm (
13
C)] or
DMSO-d 6 for complexes [2.50 ppm (
1
H), 39.53 ppm (
13
C)]. UV-visible spectra were recorded by using
Hewlett-Packard 4853 diode array spectrometer. Steady state emission spectra were recorded on
QuantaMaster Photon Technology International spectrofluorometer. Emission quantum yields were
measured by using Hamamatsu C9920 system with a xenon lamp, integrating sphere and a model C10027
photonic multichannel analyzer (PMA). Emission lifetimes were acquired on IBH Fluorocube instrument
equipped with a 405-nm laser excitation source using time-correlated single photon counting (TCSPC)
method. Electrochemical studies (cyclic voltammograms and differential pulse voltammograms) were
recorded on VersaSTAT 3 potentiostat electrochemical analyzer (Princeton Applied Research) by using
glassy carbon (working electrode), Pt counter electrode, Ag/Ag
+
(reference electrode), ferrocene (internal
reference), 0.1 M tetrabutylammonium hexafluorophosphate (nBu 4NPF 6) in N,N-dimethyformamide. All
DFT calculations reported here were performed using the Q-Chem 5.1 program.
14
Ground state
optimization was performed at the B3LYP/LACVP** level of theory and Time-dependent density
70

functional theory (TDDFT) calculations were then performed on the optimized ground state geometries at
the CAM-B3LYP/LACVP** level.
3.4.2 Synthesis of complexes
General procedure for synthesis of heteroleptic complexes: ZnX 1
A DCM/methanol solution of acridinol or phenazinol derivative (1 eq., ~25 mol/mL) was added
drop-wise into a stirring solution of zinc acetylacetonate monohydrate (8 eq.,  mol/mL) in
DCM/methanol (1:1). The reaction mixture was stirred for another 15 minutes at room temperature
before removing the volatiles on rotary evaporator and then drying the solid under vacuum. The
resulting powder was collected and used for NMR and photophysical studies without removal of
excess Zn(acac)2 to prevent intermolecular rearrangement of heteroleptic complexes into stable
homoleptic structures.
General procedure for synthesis of homoleptic complexes: ZnX 2
Zinc acetylacetonate monohydrate (1 eq.) was added, in one portion, into a reaction flask
containing a stirring solution of acridinol, or phenazinol, or benzophenazinol (2 eq.,
~25 mol/mL) in methanol. The mixture was stirred at room temperature overnight. The formed
precipitate was collected by vacuum filtration, washed with methanol, and dried under vacuum
overnight.
ZnA2: 50 mg (256.1 mol) of 4-acridinol and 37.9 mg (134.5 µmol) of Zn(acac)2 was used to yield
48 mg (82%) of a red-orange powder.
1
H NMR (400 MHz, DMSO-d6) δ = 9.21 (s, 2H), 8.97 –
8.87 (m, 2H), 8.31 – 8.21 (m, 2H), 7.96 (ddd, J = 8.8, 6.6, 1.4 Hz, 2H), 7.69 (ddd, J = 8.4, 6.6, 1.1
Hz, 2H), 7.46 (dd, J = 8.4, 7.5 Hz, 2H), 7.23 (dd, J = 8.6, 1.1 Hz, 2H), 6.83 (dd, J = 7.5, 1.1 Hz,
2H).
13
C NMR (101 MHz, DMSO-d6) δ = 161.65, 143.99, 143.00, 138.62, 130.54, 129.52, 128.43,
71

128.06, 127.95, 126.99, 125.88, 110.06, 109.93. Elem. anal. for C26H16N2O2Zn; calcd: C, 68.81%;
H, 3.55%; N, 6.17%; O, 7.05%; Zn, 14.41%. Found: C, 68.52%; H, 3.56%; N, 6.15%.
ZnA1: 50 mg (256.1 µmol) of 4-acridinol and 0.6 g (2.1 mmol) of Zn(acac)2 was used to yield a
100% of ZnA1. Note that the NMR spectra of ZnA1 show peaks (not listed here) of the excess
Zn(acac)2.
1
H NMR (400 MHz, DMSO-d6) δ = 9.12 (s, 1H), 8.49 (d, J = 8.9 Hz, 1H), 8.18 (d, J =
8.4 Hz, 1H), 7.83 (ddd, J = 8.7, 6.7, 1.4 Hz, 1H), 7.61 (dd, J = 8.4, 6.7 Hz, 1H), 7.42 (t, J = 8.0
Hz, 1H), 7.14 (d, J = 8.3 Hz, 1H), 6.74 (dd, J = 7.5, 1.1 Hz, 1H), 5.25 (s, 36H, overlap with excess
Zn(acac)2 protons), 1.83 (s, 225H, overlap with excess Zn(acac)2 protons).
13
C NMR (101 MHz,
DMSO-d6) δ = 191.58, 161.63, 143.71, 142.80, 138.45, 130.15, 129.48, 128.32, 127.86, 127.71,
126.92, 125.67, 109.64, 109.49, 98.94, 27.82. No elemental analysis due to excess Zn(acac)2.
ZnP2: 50 mg (254.8 µmol) of 1-phenazinol and 36.6 mg (130 µmol) of Zn(acac)2 were used to
yield a dark purple powder (45 mg, 77.5%).
1
H NMR (400 MHz, DMSO-d6) δ = 9.14 – 8.97 (m,
2H), 8.29 – 8.20 (m, 2H), 7.97 (dq, J = 6.7, 4.5, 3.4 Hz, 4H), 7.87 – 7.81 (m, 2H), 7.31 (d, J = 8.4
Hz, 2H), 6.93 (d, J = 7.7 Hz, 2H). C NMR (101 MHz, DMSO-d6) δ =162.39, 145.79, 144.02,
137.36, 136.13, 135.27, 130.80, 130.44, 128.91, 128.42, 110.57, 109.65. Elem. anal. for
C24H14N4O2Zn: C, 63.25%; H, 3.10%; N, 12.29%; O, 7.02%; Zn, 14.34. Found: C, 63.44%; H,
3.24%; N, 12.24%.
ZnP1: 50 mg (254.8 µmol) of 4-acridinol and 588 mg (2.1 mmol) of Zn(acac)2 was used to yield
a 100% of ZnP1. Note that the NMR spectra of ZnP1 show peaks (not listed here) of the excess
Zn(acac)2.
1
H NMR (400 MHz, DMSO-d6) δ = 8.66 – 8.54 (m, 1H), 8.29 – 8.18 (m, 1H), 7.93 (dq,
J = 6.6, 4.4, 3.1 Hz, 2H), 7.77 (t, J = 8.2 Hz, 1H), 7.25 (d, J = 8.7 Hz, 1H), 6.77 (d, J = 7.7 Hz,
1H), 5.25 (s, 13H, overlap with excess Zn(acac)2 protons), 1.83 (s, 89H, overlap with excess
Zn(acac)2 protons). No elemental analysis due to excess Zn(acac)2.
72

Zn(xP)2: 40 mg (203.0 µmol) of benzophenazin-1-ol and 28.6 mg (101.5 µmol) of Zn(acac)2 were
used to yield 58 mg (90%) of a dark purple solid.
1
H NMR (400 MHz, DMSO-d6) δ 8.99 (d,
J = 7.9 Hz, 2H), 8.09 (d, J = 8.7 Hz, 2H), 8.00 (t, J = 7.7 Hz, 2H), 7.90 (t, J = 7.4 Hz, 2H), 7.55
(d, J = 7.9 Hz, 2H), 7.25 – 7.16 (m, 6H), 6.97 (d, J = 7.6 Hz, 2H), 1.92 (s, 12H).
Zn(bP)2: 50 mg (133.17 µmol) of xylyl-phenazin-1-ol and 20.0 mg (71.0 µmol) of Zn(acac)2 were
used to yield 62 mg (66%) of a black solid.
1
H NMR (400 MHz, DMSO-d6) δ = 9.76 (s, 2H), 8.85
(s, 2H), 8.22 – 8.06 (m, 4H), 7.83 (t, J = 8.2 Hz, 2H), 7.64 – 7.51 (m, 4H), 7.20 (d, J = 8.6 Hz,
2H), 7.04 (d, 2H). Carbon NMR spectrum was not acquired due to poor solubility of the compound.
Elem. anal. for C34H20N2O2Zn; Calcd: C, 69.14%; H, 3.26%; N, 10.08%; O, 5.76%; Zn, 11.76%.
Found: C, 69.09%; H, 3.26%; N, 9.99%.
73

3.4.3 NMR spectra

1
H (top) and
13
C (bottom) NMR spectra of compound ZnA2 (DMSO, 400 MHz, 101 MHz).
74

1
H (top) and
13
C (bottom) NMR spectra of compound ZnP2 (DMSO, 400 MHz, 101 MHz).
75

1
H (top) and
13
C (bottom) NMR spectrum of compound ZnA1 (DMSO, 400 MHz, 101 MHz). Note
that sample contains excess of Zn(acac)2 to prevent the rearrangement from heteroleptic to
homoleptic complex.
76

1
H NMR spectrum of compound ZnP1 (DMSO). The sample contains excess of Zn(acac)2 to
prevent the rearrangement from heteroleptic to homoleptic complex.
.
1
H NMR spectrum of compound ZnbP2 (DMSO, 400 MHz).
13
C NMR was not acquired due to
poor solubility.
77

3.5 References

1. Tsuboi, T.; Nakai, Y.; Torii, Y., Photoluminescence of bis(8-hydroxyquinoline) zinc (Znq(2)) and
magnesium (Mgq(2)). Central European Journal of Physics 2012, 10 (2), 524-528.
2. Shen, L.; Li, F. Y.; Sha, Y. W.; Hong, X. Y.; Huang, C. H., Synthesis of fluorescent dendritic 8-
hydroxyquinoline ligands and investigation on their coordinated Zn(II) complexes. Tetrahedron Letters
2004, 45 (20), 3961-3964.
3. Monzon, L. M. A.; Burke, F.; Coey, J. M. D., Optical, Magnetic, Electrochemical, and Electrical
Properties of 8-Hydroxyquinoline-Based Complexes with Al3+, Cr3+, Mn2+, Co2+, Ni2+, Cu2+, and
Zn2+. Journal of Physical Chemistry C 2011, 115 (18), 9182-9192.
4. Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K., ORGANIC
ELECTROLUMINESCENT DEVICES WITH 8-HYDROXYQUINOLINE DERIVATIVE-METAL
COMPLEXES AS AN EMITTER. Japanese Journal of Applied Physics Part 2-Letters 1993, 32 (4A),
L514-L515.
5. Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.; Riccio, K. K. C.; Fogarty, D.;
Kohlmann, H.; Ferris, K. F.; Burrows, P. E., Electroluminescent zinc(II) bis(8-hydroxyquinoline):
Structural effects on electronic states and device performance. Journal of the American Chemical Society
2002, 124 (21), 6119-6125.
6. Dumur, F., Zinc complexes in OLEDs: An overview. Synthetic Metals 2014, 195, 241-251.
7. Chen, C. H.; Shi, J., Metal chelates as emitting materials for organic electroluminescence.
Coordination Chemistry Reviews 1998, 171, 161-174.
8. Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.; Riccio, K. K. C.; Fogarty, D.;
Kohlmann, H.; Ferris, K. F.; Burrows, P. E., Electroluminescent Zinc(II) Bis(8-hydroxyquinoline):
Structural Effects on Electronic States and Device Performance. Journal of the American Chemical
Society 2002, 124 (21), 6119-6125.
9. Aiello, I.; Aiello, D.; Ghedini, M., Aluminum(III), gallium(III), and indium(III) 4-
hydroxyacridinato complexes. Journal of Coordination Chemistry 2009, 62 (20), 3351-3365.
10. Conda-Sheridan, M.; Marler, L.; Park, E. J.; Kondratyuk, T. P.; Jermihov, K.; Mesecar, A. D.;
Pezzuto, J. M.; Asolkar, R. N.; Fenical, W.; Cushman, M., Potential Chemopreventive Agents Based on
the Structure of the Lead Compound 2-Bromo-1-hydroxyphenazine, Isolated from Streptomyces Species,
Strain CNS284. Journal of Medicinal Chemistry 2010, 53 (24), 8688-8699.
11. Sworakowski, J., How accurate are energies of HOMO and LUMO levels in small-molecule
organic semiconductors determined from cyclic voltammetry or optical spectroscopy? Synthetic Metals
2018, 235, 125-130.
78

12. Trinh, C.; Kirlikovali, K.; Das, S.; Ener, M. E.; Gray, H. B.; Djurovich, P.; Bradforth, S. E.;
Thompson, M. E., Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems: Zinc
Dipyrrins. Journal of Physical Chemistry C 2014, 118 (38), 21834-21845.
13. Golden, J. H.; Estergreen, L.; Porter, T.; Tadle, A. C.; Sylvinson, M. R. D.; Facendola, J. W.;
Kubiak, C. P.; Bradforth, S. E.; Thompson, M. E., Symmetry-Breaking Charge Transfer in Boron
Dipyridylmethene (DIPYR) Dimers. Acs Applied Energy Materials 2018, 1 (3), 1083-1095.
14. Shao, Y. H.; Gan, Z. T.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange, A.
W.; Behn, A.; Deng, J.; Feng, X. T.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.; Kaliman, I.;
Khaliullin, R. Z.; Kus, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard, R. M.; Rohrdanz, M.
A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.; Zuev, D.; Albrecht, B.; Alguire,
E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.; Brandhorst, K.; Bravaya, K. B.; Brown, S.
T.; Casanova, D.; Chang, C. M.; Chen, Y. Q.; Chien, S. H.; Closser, K. D.; Crittenden, D. L.;
Diedenhofen, M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.; Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.;
Ghysels, A.; Golubeva-Zadorozhnaya, A.; Gomes, J.; Hanson-Heine, M. W. D.; Harbach, P. H. P.;
Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.; Jagau, T. C.; Ji, H. J.; Kaduk, B.; Khistyaev, K.; Kim,
J.; King, R. A.; Klunzinger, P.; Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.;
Lawler, K. V.; Levchenko, S. V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar,
P.; Manzer, S. F.; Mao, S. P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.;
Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O'Neill, D. P.; Parkhill, J. A.; Perrine, T. M.;
Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.; Small, D. W.;
Sodt, A.; Stein, T.; Stuck, D.; Su, Y. C.; Thom, A. J. W.; Tsuchimochi, T.; Vanovschi, V.; Vogt, L.;
Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel, J.; White, A.; Williams, C. F.; Yang, J.; Yeganeh, S.;
Yost, S. R.; You, Z. Q.; Zhang, I. Y.; Zhang, X.; Zhao, Y.; Brooks, B. R.; Chan, G. K. L.; Chipman, D.
M.; Cramer, C. J.; Goddard, W. A.; Gordon, M. S.; Hehre, W. J.; Klamt, A.; Schaefer, H. F.; Schmidt, M.
W.; Sherrill, C. D.; Truhlar, D. G.; Warshel, A.; Xu, X.; Aspuru-Guzik, A.; Baer, R.; Bell, A. T.; Besley,
N. A.; Chai, J. D.; Dreuw, A.; Dunietz, B. D.; Furlani, T. R.; Gwaltney, S. R.; Hsu, C. P.; Jung, Y. S.;
Kong, J.; Lambrecht, D. S.; Liang, W. Z.; Ochsenfeld, C.; Rassolov, V. A.; Slipchenko, L. V.; Subotnik,
J. E.; Van Voorhis, T.; Herbert, J. M.; Krylov, A. I.; Gill, P. M. W.; Head-Gordon, M., Advances in
molecular quantum chemistry contained in the Q-Chem 4 program package. Molecular Physics 2015, 113
(2), 184-215.

79

4 CHAPTER IV: Two-coordinate N-heterocyclic carbene gold(I) complexes
bearing phenanthrocarbazolyl donor ligand

4.1 Introduction
Recent developments of TADF phosphors of N(amide)-metal-C(carbene) structure are attractive due to
their promising high efficiencies and fast radiative rates, which are key parameters for high performance
OLEDs. The amide and carbene act as donor and acceptor, respectively, while the coinage metal (Cu, Ag,
and Au) acts as an electronic bridge. Our lab (Thompson’s group) has explored a number of exciting
amide-metal-carbene molecules, but mainly employing carbazolyl as the amide donor.
1-4
In other words,
we have manipulated these systems by exploring different coinage metals and carbenes (e.g. CAAC-M-Cl,
MAC-M-Cl, DAC-M-Cl, etc.) with little variation on the donor unit. It is therefore important to explore
other amide donors for this family of TADF phosphors. In this regard, this chapter is focused on the systems
featuring a -extended carbazolyl donor, phenanthrocarbazole pCz, to understand the photophysical and
electrochemical effect brought about by the extended conjugation in core of the donor. We present the
synthesis, computational, electrochemical, and photophysical findings on the featured NHC complexes:
pCz-Au-carbene. Parent carbazolyl analogues will be used for comparison.
4.2 Results and Discussion
4.2.1 Synthesis
The heterocyclic aryne, 10H-phenanthro[9,10-b]carbazole, pCz, was synthesized by Neil Garg’s group at
UCLA via a palladium-catalyzed annulation synthesis they developed as summarized by Figure 4.1.
(Detailed synthesis procedures are given in Experimental section). Pd (5 mol%)-catalyzed annulation
between 2-bromobiphenyl and N-Boc-protected carbazolyne precursor were carried out in refluxing
co-solvents acetonitrile/toluene (1:1) with 10 equivalents of cesium fluoride (CsF) , followed by Boc
deprotection in trifluoroacetic acid (TFA)/dichloromethane at room temperature to yield 65% of the N-H
amide, pCz ligand, ready to use in the NHC complexation.
80

The syntheses of complexes are executed in dry and anaerobic conditions by using Schlenk line and/or
glove-box techniques. The metal-carbene chloride precursors (BZI-AuCl and MAC-AuCl) were
synthesized according to the literature.
1, 3
The deprotonation of pCz N-H amide is realized by using sodium
tert-butoxide in THF solvent over the course of 30 minutes, after which an appropriate NHC carbene source
(here MAC-AuCl or BZI-AuCl) was added into the mixture to form the linear N-Au-Carbene complexes
overnight. Pure yellow complexes pCz-AuBZI and pCz-AuMAC were isolated in 78% and 60% yields,
respectively. Analogous syntheses employing N-H carbazole amide (Cz) were reported by our research
group to prepare Cz-AuBZI
1
and Cz-AuMAC
3
which will be compared to the new pi-extended complexes
in the following sections.

Figure 4.1 : Synthesis of N-H pCz ligand and its NHC complexes, pCz-AuBZI and pCz-AuMAC.

4.2.2 Electrochemical properties
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) of the complexes and ligand were run
in 0.1 M N,N-dimethylformamide (DMF) with tetrabutylammonium hexafluorophosphate (TBAF)
81

electrolytes, glassy carbon working electrode, Pt wire counter electrode, and silver wire pseudo-reference
electrode (Figure 4.2 and). The redox values are extracted from DPV and are corrected against
ferrocene/ferrocenium couple (Fc/Fc
+
= 0.0 V) used as an internal reference. CVs of pCz ligand and
pCz-AuBZI are characterized by irreversible oxidation and reduction waves, whereas the reduction of
pCz-AuMAC is quasireversible. The oxidation potentials of pCz-AuBZI (E ox = 0.27 V) and pCz-AuMAC
(E ox = 0.29 V) are comparable, signifying an oxidation of the same entity, which must be the deprotonated
pCz ligand. The oxidation of the N-H pCz ligand (E ox = 0.61 V) is more positive than that of the complexes
because free pCz is in a protonated form, rather than anionic. On the other hand, the reduction potentials of
pCz-AuBZI (E red = -2.78 V) and pCz-AuMAC (E red = -2.41 V) are 370 mV different. As structural
difference lies in the carbene, BZI and MAC are therefore the reduction active moieties. The less negative
reduction potential of pCz-AuMAC correlate well with the higher electrophilicity (better acceptor ability)
of MAC carbene relative to BZI carbene. Note that the reduction potential of pCz-AuMAC and
Cz-AuMAC are similar as the reduced moiety (MAC) is the same. As for the electrochemical impact of
the amide donor on the complex, Cz-AuBZI is relatively harder to oxidize compared to pCz-AuBZI, which
agrees which the effect of -conjugation in the pCz system. The reverse trend in the oxidation potentials
of Cz-CuMAC (E red = 0.17 V) and pCz-AuMAC (E red = 0.27 V) might be due to the two different metals
(Au vs. Cu). Hamze, R. et al. discovered that the metal in not entirely redox innocent because Cu-based
BZI complexes were reported to be more easily oxidized compared to the same Ag- and Au-based
counterparts.
1

Table 4.1: Redox values extracted from electrochemical measurements (DPV). HOMO and LUMO are calculated according
literature formulae.
5

a
From reference.
1

b
From reference.
3

Compound E
ox (V)
E
red (V)
E
redox (V)
HOMO (eV) LUMO (eV)
pCz-AuBZI
(Cz-AuBZI)
a

0.29
(0.32)
-2.78
(-2.82)
3.07
(3.14)
-5.12
(-5.16)
-1.55
-(1.51)
pCz-AuMAC
(Cz-CuMAC)
b

0.27
(0.17)
-2.41
(-2.50)
2.68
(2.67)
-5.10
(-4.99)
-1.99
(-1.88)
pCz ligand 0.61 -2.67 3.28 -5.49 -1.68

82

Figure 4.2: Differential pulse voltammograms (left column) and cyclic voltammograms (right column) of free pCz ligand,
pCz-AuBZI, and pCz-AuMAC in 0.1 M DMF. We used tetrabutylammonium hexafluorophosphate (TBAF) electrolytes, glassy
carbon working electrode, Pt wire counter electrode, and silver wire (Ag/Ag+) pseudo reference electrode, and
ferrocene/ferrocenium internal reference (Fc/Fc+ = 0.0 V). Redox potentials are extracted from DPV.
-3 -2 -1 0 1
-6
-4
-2
0
2
4
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
Reduction
Oxidation
pCz ligand

-3 -2 -1 0 1
-10
-5
0
5
10
15
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
pCz ligand

-3 -2 -1 0 1
-3
-2
-1
0
1
2
3
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
Oxidation
Reduction
pCz-AuBZI

-3 -2 -1 0 1
-4
-2
0
2
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
pCz-AuBZI

-3 -2 -1 0 1
-3
-2
-1
0
1
2
3
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
Reduction
Oxidation
pCz-AuMAC

-3 -2 -1 0 1
-3.0
-1.5
0.0
1.5
3.0
Current (x10
-5
A)
Voltage (V) vs. Fc/Fc
+
pCz-AuMAC

83

4.2.3 Computational studies

Geometry optimization of complexes (pCz-AuBZI and pCz-AuMAC) and free anionic pCz ligand was
performed by using density functional theory (DFT) at B3LYP/LACVP** level of theory, followed by
time-depend density functional theory (TD-DFT) calculations at LACVP/CAM-B3LYP for energy
calculations (Figure 4.3). The optimized complex structures show co-planarity of the carbazolyl and NHC
carbene such that the HOMO contours are predominantly localized on pCz donor, and the LUMO on the
carbene, with a small contribution from the metal, although not insignificant. Such predictions are
consistent with the measured electrochemical properties where the oxidation occurs on the pCz ligand
(home of HOMO), and reduction on the carbene (home of LUMO).
Natural transition orbitals (NTOs) of the two lowest excited triplets (T 1 and T 2) and lowest excited singlet
(S 1) are given in Figure 4.3 with energy values in eV. The electronic transitions of the lowest triplet in both
pCz-AuBZI and pCz-AuMAC are localized within the phenanthrocarbazolyl moiety (
3
pCz) with the same
energy of 2.47 eV. The second lowest triplet transition in pCz-AuMAC is already interligand charge
transfer (
3
ICT = 2.76 eV)), whereas it remains
3
pCz-localized for pCz-AuBZI (T 2 = 2.99 eV). The lowest
1
ICT of pCz-AuBZI (3.35 eV) is higher than that of pCz-AuMAC (2.92 eV) by 0.44 eV, again, reflecting
the greater electrophilicity of MAC carbene relative to BZI. It is also important to note that the
3
pCz/
1
ICT
states for pCz-AuMAC are closer in energy by 0.45 eV, whereas for pCz-AuBZI there is larger separation
of 0.88 eV. Recall that the present theoretical calculations are performed in gas phase meaning that if
solvent effects are taken into account, the ICT manifolds will be stabilized. Therefore, TADF events are
more likely to take place in pCz-AuMAC than in pCz-AuBZI when the medium polarity is great enough
to bring the
1/3
ICT manifold at a near zero-gap separation with
3
pCz. A qualitative energetic diagram
summarizing the theoretical results is given in Figure 4.4.
84

Figure 4.3: HOMO (blue)/LUMO (red) of optimized structure pCz-CuBZI and pCz-CuMAC (first column). Natural transition
orbitals (NTOs) of T1, T2, and S1 from TD-DFT calculations performed on optimized structures (2
nd
, 3
rd
, 4
th
column). Green
contours represent hole; yellow contours represent electron. Isovalue = 0.09.

Figure 4.4: Qualitative energetic depiction theoretical results of pCz-CuBZI and pCz-CuMAC from TD-DFT calculations. In
pCz-CuBZI,
3
pCz–
1
ICT separation is larger (0.88 eV) whereas in pCz-CuMAC the separation is smaller (0.45 eV). With solvent
effects that can stabilize the
1/3
ICT manifold closer to
3
pCz, TADF events are more probable in pCz-AuMAC than in pCz-AuBZI.
These computational values might not correlate well with experimental values.

85

4.2.4 Photophysical properties
Photophysical properties of pCz-Au complexes were measured in different media and compared to Cz-Au
complex counterparts (Figure 4.5). Absorption spectra of pCz-AuMAC in methylcyclohexane (MeCy),
1%wt PS film, and 2-methyltetrahydrofuran (MeTHF) are both characterized by two major bands (Figure
4.5, A). The bands in higher region (< 375 nm) are solvent-independent and corresponds to -
*
transitions
in pCz. The broad, less intense band in the lower energy region is solvent polarity-dependent where negative
solvatochromism is observed from MeCy ( abs = 460 nm) to PS film (420 nm) to MeTHF ( abs = 417 nm).
The lower energy band in all the three media is ascribed to inter-ligand charge transfer (ICT) from pCz
donor to MAC acceptor.
Room temperature emission spectra of pCz-AuMAC in all media are characterized by feature-less, CT
bands (Figure 4.5, B). From MeTHF to MeCy to 1wt% polystyrene film, the emission peak is
hypsochromically shifted by 24 nm and 39 nm, respectively. Such opposite solvatochromic behaviors in
absorption and emission CT bands are a consequence of complex-medium dipole moments interaction
where the ground state and excited state dipole moments of molecules are opposite in direction, with the
former being larger in magnitude. A qualitative representation of this solvatochromism behaviors is
explained by the potential energy surface diagram in Figure 4.6. The well of the complex in MeTH is
shifted down-left to corresponding to the aggravated distortions (more reorganization and stabilization in
the polar solvent.
At lower temperatures (77 K), the emission spectra peaking at ~500 nm become structured and solvent
polarity-independent, evoking the localized emission from
3
pCz. In the latter case, solvent molecules are
frozen around the complex molecules restricting stabilization and reorganization of the complex ICT in the
excited states. Therefore, the triplet of pCz becomes the low-lying, and hence emissive state. Similar
behaviors were observed in complexes of the same family reported by Thompson’s group. For comparison,
absorption and emission spectra of reported Cz-AuMAC in MeTHF are given in Figure 4.5, C&D, along
with pCz-AuMAC. Absorption CT bands of these two carbazolyl-based complexes peak in the same
86

absorption region (here in MeTHF), with pCz-AuMAC having higher molar absorptivity at the maximum
CT wavelength (10,023 vs. 7,758 M
-1
. cm
-1
). The lower absorptivity in MeCy compared to MeTHF is more
likely due to the reduced solubility of pCz-AuMAC in the non-polar solvent. It is worth noting that this
molar absorptivity of pCz-AuMAC is relatively impressive, given that absorptions of the reported
complexes of this family are usually of less than 1,000 M
-1
/cm. Its higher absorptivity is consistent with the
extended -conjugation in the pCz core inducing a greater donor-acceptor orbital overlap integral. For the
same reasons, emission spectra of pCz-AuMAC are more bathochromically shifted both at room
temperature and 77 K compared to the parent Cz-based analogue. The closeness of 77K/rt emission onsets
of pCz-AuMAC (compared to Cz-AuMAC) supports the predicted small separation between
3
pCz and
1
ICT states from the gas phase DT-DFT calculations. Figure 4.7 qualitatively summarizes the comparative
photophysical outcomes between pCz-AuMAC and Cz-AuMAC at room temperature and 77 K.
Table 4.2: Tabulated photophysical properties of pCz-AuMAC compared to Cz-AuMAC.
c
Triplet energy from 77K emission
onset (500 nm) corresponds to 2.50 eV.
Complex  Abs/CT  em,RT

( em,77K) 
(%)
 RT
(s)
k r
(x10
5
s
-1
)
k nr
(x10
5
s
-1
)
 77K
(ms)
pCz-AuMAC
(MeTHF)
417

566
(498)
0.39 2.8 1.39 2.18 2.36 (72%)
5.26 (28%)
pCz-AuMAC
(MeCy)
460 544
(500)
c

0.81 3.3 2.45 0.58 1.77 (59%)
3.09 (41%)
pCz-AuMAC
(1% PS film)
420 529
(500)
c

0.74 2.27 3.26 1.15 2.96
Cz-AuMAC
(MeTHF)
412 544
(428)
0.50 0.79 6.30 6.30 0.260

87

Figure 4.5: Absorption (A) and emission (B) spectra of pCz-AuMAC in MeTHF, MeCy, and polystyrene film. Comparison of
pCz-AuMAC vs Cz-AuMAC: molar absorptivity (C) and emission spectra (D) in MeTHF.
Photoluminescence quantum yield of pCz-AuMAC in MeCy is 0.81, twice that in MeTHF (0.39), Table
4.2. This drop in efficiency can be ascribed to (a) higher reorganization energy in the polar solvent and/or
(b) mixing of localized triplet emission and CT emission (Figure 4.6). Radiative rates of pCz-AuMAC are
relatively slower than those of Cz-AuMAC. In MeTHF, the later has radiative rates of 6.30 x 10
5
/s, whereas
the former radiates at 1.39 x 10
5
/s. The slower emission in the phenanthrocarbazolide complex might be
due to the smaller separation between
3
LE and
1/3
CT which can induce some phosphorescence from the LE
state. This might also be the same reason why the lifetimes of pCz-AuMAC are relatively longer (by a
factor of >3 in MeTHF). Non-radiative rates of pCz-AuMAC are, however, lower than those of
Cz-AuMAC, indicating alleviated distortions in the former complex.
300 350 400 450 500 550
0.0
0.3
0.6
0.9
1.2
500 550 600 650 700 750
0.0
0.4
0.8
1.2
300 350 400 450 500
0.0
0.5
1.0
1.5
2.0
400 500 600 700
0.0
0.4
0.8
1.2
 (x10
4
. cm
-1
)
Wavelength (nm)
pCz-AuMAC
MeCy
MeTHF
A)
pCz-AuMAC
Norm. PL intensity (a.u.)
Wavelength (nm)
Em RT (MeCy)
Em RT (MeTHF)
Em 77K (MeCy)
Em 77K (MeTHF)
PS film RT
PS film 77K
B)
 (x10
4
. cm
-1
)
Wavelength (nm)
pCz-AuMAC
Cz-Cu-MAC
C)
D)
(in MeTHF)
Norm. PL intensity (a.u.)
Wavelength (nm)
pCz-AuMAC (RT)
pCz-AuMAC (77K)
Cz-AuMAC (RT)
Cz-AuMAC (77K)
0.0
0.3
0.6
0.9
1.2
Abs. of PS Film (a.u)

88

Figure 4.6: Qualitative potential energy surface diagram of states in pCz-AuMAC vs Cz-AuMAC illustrating solvatochromism
behaviors observed in the complexes. Absorption in polar solvent is hypsochromically shifted relative to that in non-polar solvent.
Emission in polar solvent is bathochromically shifted relative to that in non-polar solvent. More distortions of excited states in
MeTHF are illustrate by the shift of the potential energy surface well to the left, corresponding to the poorer emission quantum
yield in the polar solvent.

The short-lived excited states of these systems, in general, are due to thermally activated delayed
fluorescence (TADF) phenomenon which is more efficient when the excited state singlet-triplet separation,
E ST, is small (<1000 cm
-1
), not to mention the contribution of SOC to increase singlet-triplet states mixing,
as well as great overlap integral between the donor and acceptor entities.
1-4, 6
The previous reports revealed
that E ST of carbazolyl-based NHC TADF complexes fall in the regime of 10’s of meV. Among the three
coinage metal in Cz-M-MAC, silver(I) and Au(I) derivatives demonstrated the narrowest E ST due to the
longer N–Ag–C bond or greater SOC effect, respectively.
2
The same outcome is expected for pCz-M-MAC
coinage metal complexes.

89

Figure 4.7: Qualitative representation of states and photophysical events taking place in pCz-AuMAC vs Cz-AuMAC at room
temperature and 77 K. Both complexes undergo TADF at room temperature, and phosphorescence at 77 K. The
3
pCz is lower than
that of
3
Cz. The separation between
3
LE and
1/3
CT manifold is greater in the phenanthrocarbazolide complex.

Worthy of attention is also the difference in emission properties of pCz-AuMAC in MeCy vs PS film. The
rt emission spectrum in PS is blue-shifted by 15 nm relative to MeCy, despite the comparable polarity of
the two media. The unexpected hypsochromic shift can be explained by the more rigidity of molecules in
polystyrene that can hinder stabilization, and hence partially turns on
3
LE emissions to mix with the
dominant ICT emission. As a result, the 77K and RT onsets of MeCy emissions are energetically closer,
whereas in PS they are further apart (Figure 4.5, B). The same explanation can be based upon to understand
why the quantum yield in PS ( = 74%) is lower than that in MeCy ( = 81%), which is opposite of what
is generally observed in other reported TADF phosphors of this family.
3-4
The larger stokes shift in PS is
indicative of larger reorganization energy, and hence higher non-radiative rates and poorer  PL in PS film
relative to MeCy, as supported by experimental values (Table 4.3).

90

Table 4.3: Tabulated emission properties of pCz-AuBZI compared to Cz-AuBZI.
d
Triplet energy from 77K emission onset
(498/504 nm) is 2.47 eV.
Complex  Abs, CT  Em, RT
(
77K
)

(%)

RT

(
77K
)
k
r
(s
-1
)

k
nr

(s
-1
)
pCz-AuBZI
(MeTHF)
382
( = 18,285)
504
(498)
d

2.40 57.1 s
(5.05 ms)
4.20 x 10
2
1.71 x 10
4

pCz-AuBZI
(MeCy)
416 502
(504)
d

2.80 30.0 s
(5.75 ms)
9.36 x 10
2
3.25 x10
4

Cz-AuBZI
(MeTHF)
366
( = 14,942)
452
(426)
0.79 2.63 s
(650 s)
3.0 x 10
5
0.8 x 10
5

Figure 4.8: Absorption (E) and emission (F) spectra of pCz-AuBZI compared to Cz-AuBZI (G).

Absorption and emission spectra of pCz-AuBZI are shown in Figure 4.8. Like in Cz-AuBZI,
1
Absorption
spectra of pCz-AuBZI are also characterized by polarity-independent high energy bands and
solvent-dependent lower energy bands attributed to -
*
and ICT transitions, respectively. The CT
300 350 400 450 500
0.0
0.4
0.8
1.2
Norm. Absorbance (a.u)
Wavelength (nm)
pCz-AuBzI
Abs (MeCy)
Abs (MeTHF)
 = 18,285
E)

475 500 525 550 575 600 625
0.0
0.4
0.8
1.2
Norm. PL intensity (a.u)
Wavelength (nm)
pCz-AuBzI
Em RT (MeCy)
Em RT (MeTHF)
Em 77K (MeCy)
Em 77K (MeTHF)
F)

300 350 400 450 500 550 600
-0.5
0.0
0.5
1.0
1.5
MeCy
2-MeTHF
PS
Wavelength (nm)
 (10
4
M
-1
cm
-1
)
Cz-AuBZI
G)
-0.5
0.0
0.5
1.0
PL Intensity
RT
77K

91

extinction coefficient of the pCz complex derivative is greater than that of Cz-based complex (18,285 vs.
14,942 M
-1
.cm
-1
). Due to more -conjugation in BZI relative to MAC acceptor, the associated complexes
of BZI present higher absorptivity. In either solvent, the lower-energy band of the Cz-AuBZI is blue-shifted
compared to pCz-AuMAC, again, indicative of the -conjugation effect in the pCz derivative.

Figure 4.9: Qualitative representation of photophysical events taking place in pCz-AuBZI vs Cz-AuBZI at room temperature and
77 K.
3
Cz and
1/3
CT of Cz-AuBZI are near degenerate and hence show rt TADF emission stemming from the states’ mixing.
pCz-AuBZI is characterization by inefficient phosphorescence at room temperature and 77 K.

Emissions of pCz-AuBZI in all solvents regardless of temperature are
3
pCz-localized, corresponding to
conventional phosphorescence rather than delayed fluorescence (Figure 4.8, F). The triplet energy
determined from the 77K peak is 2.47 eV, matching the theoretical value. Poor luminescence quantum
yields (<3%) and long lifetimes of tens of microseconds in both MeCy and MeTHF are measured (Table
4.3). Additionally, the rt radiative rates of the present BZI-based complex are three orders of magnitude
slower than those of the MAC-derived complex (e.g. 4.2 x 10
2
s
-1
vs. 1.39 x 10
5
s
-1
in MeTHF). Such
photophysical properties agree with the proposed energetic diagram from DFT prediction (Figure 4.4),

92

where the
3
pCz/
1/3
CT separation in pCz-AuBZI was calculated to be larger. Therefore,
1/3
CT manifold is
always higher than
3
pCz even with polar solvent effects, allowing exclusively phosphorescence as the only
radiative emission pathway at room temperature and 77 K. On the other hand, parent carbazolyl complex
counterpart, Cz-AuBZI, exhibits mixed
1/3
CT/
3
Cz emissions in non-polar solvent, and pure CT emissions
in polar solvent, indicative of near degenerate
1/3
CT/
3
Cz manifold configuration. Consequently, TADF is
turned on in Cz-AuBZI with high quantum yield of 79% and lifetime of 2.63 s, as opposed to inefficient
phosphorescence of pCz-AuBZI.
From the photophysical studies above, it is evident that the experimental properties of pCz-Au complexes
relate to the theoretical results from the gas phase DFT calculations (Figure 4.3 and Figure 4.4). It was
predicted that the
1/3
ICT manifold is energetically farther above the
3
LE in pCz-AuBZI (0.88 eV) compared
to pCz-AuMAC energetics (0.45 eV). In the later complex, solvent polarity effects clearly bring down the
1/3
ICT states below
3
LE, triggering TADF events at rt. The larger
3
LE-
1/3
ICT separation cannot be overcome
by solvent effects in pCz-AuBZI, resulting in a poor phosphorescence (
3
LE) emission at all temperatures
and in all solvents.
4.3 Conclusion
Extending -conjugation from carbazole to phenanthrocarbazole has a remarkable impact on the
photophysical properties of the metal-BZI and metal-MAC based complexes. From DFT calculations, it
was predicted that the lowest
3
LE and
1/3
ICT states of pCz-AuMAC are closely separated (0.44 eV)
promising TADF emissions to take place, whereas the
3
LE/
1/3
ICT separation in pCz-AuBZI is large enough
(0.88 eV) to keep
3
LE always the lowest emitting state for conventional phosphorescence events.
pCz-AuMAC show polarity-dependent ICT emissions (TADF) at room temperature, and
3
pCz-localized
emission at 77K. Due to the higher triplet energy of Cz, even the poor electrophilic carbene BZI induces
TADF emission in the corresponding complex (Cz-AuBZI), which is different from what is observed in
the pCz analogue. Electrochemical properties are also consistent with computational outcomes, where the
93

reduced moiety is pCz or Cz (residence of HOMO) and the oxidized moiety is the carbene, BZI or MAC
(residence of LUMO).
The -extended conjugation was, therefore, proven to be another robust strategy to manipulate the NHC
TADF phosphors. More conjugation induces greater overlap integral between the N-donor and carbene
acceptor, which results in higher oscillator strength and hence higher molar absorptivity as observed in the
phenanthrocarbazolyl systems compared to carbazolyl’s. With higher oscillator strengths, S 1 emission
lifetime is also increased, which shortens the TADF lifetime. Additionally, the -conjugation helps in
designing imides (N-donors) with different HOMOs (and LUMOs), which is beneficial for a quick design
and synthesis of amid-M-carbene emitters whose energetics are suitable for a chosen host material. In other
words, if the host material requires a dopant with a specific HOMO energy, we can achieve this by
manipulating the amid donor unit through -conjugation while keeping high or improving the oscillator
strength. The same benefit applies for independent manipulation of LUMO by changing the carbene. This
work greatly contributes to the nascent study of exciting two-coordinate NHC TADF phosphors of
amide-metal-carbene family for the advancements of OLEDs.
4.4 Experimental
4.4.1 General information
All reactions were carried out in dry and anerobic atmosphere by using Schlenk line or glovebox techniques,
unless noted. Dry and degassed solvents were used in reactions and measurements. MAC-AuCl and
BZI-AuCl were synthesized according to literature.
1-2
pCz ligand (5.27) was synthesized by our
collaborators, Prof Neil Garg’s group at UCLA, according to the procedures detailed below.
1
H and
13
C NMR spectra were recorded on a 400 MHz or 600 MHz Varian NMR instrument. Chloroform-d 1 (
1
H
at 7.26 ppm,
13
C at 77.2 ppm). Acetone-d6 (
1
H at 2.05 ppm,
13
C at 206.7 & 29.9 ppm).
4.4.2 Synthesis of Ligands

94

N-Boc-Carbazole Silyl Triflate: To a 50 mL roundbottom flask was added carbazole silyl triflate 5.23
(1.00 g, 2.58 mmol, 1 equiv), THF (13 mL, 0.2 M), 4-diemthylaminopyridine (63 mg, 0.513 mmol, 0.2
equiv), and di-tert-butyl dicarbonate (839 mg, 3.84 mmol, 1.5 equiv) which stirred under positive nitrogen
pressure at 23 °C for 2 h. The mixture was quenched with deionized water (10 mL). The layers were
separated, and the aqueous layer was extracted with CH 2Cl 2 (3 x 10 mL). The organic layers were combined,
dried over Na 2SO 4, filtered, and concentrated under reduced pressure to afford a yellow solid. The crude
material purified by flash chromatography (100:1 hexanes:EtOAc) to yield N-Boc-carbazole silyl triflate
5.25 (1.17 g, 93% yield) as a white solid. N-Boc-carbazole silyl triflate 5.25: R f 0.63 (9:1 hexanes:EtOAc);
1
H NMR (400 MHz, CDCl 3): δ 9.38 (s, 1H), 8.86 (d, J = 8.2, 1H), 8.81 (d, J = 7.8, 1H), 8.68 (td, J = 8.6,
1.5, 2H), 8.54 (s, 1H), 8.31 (dt, J = 7.8, 0.9, 1H), 7.72–7.60 (m, 4H), 7.63 (td, J = 7.8, 1.1, 1H), 7.58 (td, J
= 7.8, 1.1, 1H), 7.47 (d, J = 8.1, 1H), 7.33 (td, J = 7.4, 0.8, 1H), 4.03 (s, 3H);
13
C NMR (100 MHz, CDCl 3):
(27 of 28 signals observed) δ 142.9, 141.3, 131.1, 130.6, 130.0, 128.85, 128.78, 127.4, 127.1, 127.0, 126.8,
126.1, 123.9, 123.6, 123.5, 123.4, 123.1, 123.0, 122.9, 120.8, 119.2, 114.8, 108.5, 101.1, 29.3; IR (film):
3049, 2923, 2854, 1638, 1603, 1500, 1443, 1258, 754 cm
–1
; HRMS-APCI (m/z) [M]
+
calcd for C 25H 17N
+
,
331.13555; found 331.13609.

Carbazole (5.27): A 2-dram vial was charged with Pd(dba) 2 (7.1 mg, 0.012 mmol, 5 mol%). Next, toluene
(1.3 mL), P(o-tolyl) 3 (3.8 mg, 0.012 mmol, 5 mol%), 2-bromobiphenyl (5.7, 58.0 mg, 0.248 mmol, 1.0
equiv), silyl triflate 5.25 (241 mg, 0.495 mmol, 2.0 equiv), and acetonitrile (1.6 mL) were added, followed
95

by an oven-dried magnetic stirbar and then CsF (376 mg, 2.48 mmol, 10.0 equiv). The vial was sealed with
a Teflon-lined screw cap and stirred at 110 °C for 24 h. After allowing cooling to 23 ºC, the mixture was
transferred with dichloromethane (20 mL) and H 2O (10 mL) to a separatory funnel containing brine (15
mL). The layers were separated and the aqueous layer was extracted with dichloromethane (3 x 30 mL).
The combined organic layers were dried over Na 2SO 4, filtered, and concentrated under reduced pressure to
yield a brown residue that was carried forward without further purification.
The crude material was dissolved in 10:1 CH 2Cl 2:TFA (24.2 mL, 0.01 M) and stirred at 23 °C for 5 h. The
reaction was then slowly transferred to a separatory funnel containing sat. aq. sodium bicarbonate (30 mL).
The mixture was further diluted with CH 2Cl 2 (10 mL) and the layers were separated. The organic phase
was dried over Na 2SO 4 and filtered. To the filtrate was added silica (500 mg), which was then dried under
reduced pressure until a free-flowing solid was obtained. The crude material purified by flash
chromatography (100% hexanes, then 9:1 hexanes/EtOAc, then 1:1 hexanes/benzene) to yield carbazole
5.27 (51 mg, 65% yield) as an off-white solid. Carbazole 5.27. R f 0.63 (4:1 hexanes:EtOAc);
1
H NMR (600
MHz, CDCl 3):  9.41 (s, 1H), 8.88 (d, J = 8.0, 1H), 8.75–8.62 (m, 4H), 8.22 (d, J = 9.0, 1H), 8.19 (s, 1H),
7.73–7.62 (m, 4H), 7.53 (d, J = 2.8, 2H), 7.28 (m, 1H); HRMS-APCI (m/z) [M + H]
+
calcd for C 24H 16N
+
,
318.1277; found 318.12843.
4.4.3 Synthesis of Complexes

pCz-AuBzI: Sodium tert-butoxide (6.4 mg, 331mmol) was added into a solution of pCz ligand (20mg, 63
mmol) in THF (10 mL). BzI-AuCl (44.4 mg, 66.16 mmol)) was added into the reaction flask in one portion
96

and the mixture was left stirring under inert gas at room temperature for 12 hours. The solution was filtered
through a plug of celite and volatiles were removed under reduced pressure. A minimum amount of
dichloromethane was added into the residue, followed by 10 mL of dry hexane to precipitate out an
off-white solid which was collect by vacuum filtration and washed by more hexane to yield (47 mg, 78%).
1
H NMR (400 MHz, Chloroform-d) δ 9.25 (s, 1H), 8.77 (dd, J = 8.5, 1.3 Hz, 1H), 8.60 (ddd, J = 16.7, 8.4, 1.4 Hz,
2H), 8.28 (dd, J = 8.5, 1.4 Hz, 1H), 8.17 (dd, J = 7.6, 1.0 Hz, 1H), 8.13 (s, 1H), 7.86 (t, J = 7.8 Hz, 2H), 7.72 (ddd, J
= 8.2, 6.9, 1.3 Hz, 1H), 7.65 – 7.56 (m, 6H), 7.52 – 7.47 (m, 3H), 7.29 – 7.26 (m, 2H), 7.16 (ddd, J = 8.2, 7.0, 1.3 Hz,
1H), 7.01 (ddd, J = 7.9, 7.0, 1.0 Hz, 1H), 6.67 (dd, J = 8.1, 0.9 Hz, 1H), 2.60 (hept, J = 6.9 Hz, 4H), 1.40 (d, J = 6.9
Hz, 12H), 1.19 (d, J = 6.8 Hz, 12H).

pCz-AuMAC: Sodium tert-butoxide (7.3 mg, 76.09µmol) was added into a solution of pCz ligand (23 mg,
72.47µmol) in THF (10 mL). MAC-AuCl (50.2 mg, 73.92µmol) was added into the reaction flask in one
portion and the mixture was left stirring under inert gas at room temperature for 12 hours. The solution was
filtered through a plug of celite and volatiles were removed under reduced pressure. A minimum amount
of dichloromethane was added into the residue, followed by 15 mL of dry hexane to precipitate out a solid
which was collected by vacuum filtration and washed by more hexane to yield pure yellow solid (42 mg,
60%).
1
H NMR (400 MHz, Chloroform-d) δ = 9.17 (s, 1H), 8.72 (d, 1H), 8.57 (ddd, J = 18.5, 8.3, 1.1 Hz,
2H), 8.39 (d, 1H), 8.08 (d, J = 7.4, 1.5, 0.7 Hz, 1H), 7.82 – 7.70 (m, 3H), 7.65 – 7.44 (m, 8H), 7.04 – 6.91
(m, 2H), 5.81 (d, 1H), 3.86 (s, 2H), 3.36 (hept, J = 6.5 Hz, 2H), 3.08 (hept, J = 6.8 Hz, 2H), 1.45 – 1.33 (m,
18H), 1.26 (d, J = 6.8 Hz, 6H).
97

4.4.4 Measurements information
Electrochemical measurements:
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) of the complexes and ligand were run
on VersaSTAT 3 potentiostat electrochemical analyzer (Princeton Applied Research). We used
0.1 M N,N-dimethylformamide (DMF) with tetrabutylammonium hexafluorophosphate (TBAF)
electrolytes, glassy carbon working electrode, Pt wire counter electrode, silver wire pseudo-reference
electrode, and ferrocene as an internal reference. Reversibility was judged from CVs, and redox values
were extracted from DVP and are corrected against ferrocene/ferrocenium couple (Fc/Fc
+
= 0.0 V).
Photophysical measurements:
UV-visible spectra were recorded by using Hewlett-Packard 4853 diode array spectrometer. Steady state
emission spectra were recorded on QuantaMaster Photon Technology International spectrofluorometer.
Emission quantum yields were measured by using Hamamatsu C9920 system with a xenon lamp,
integrating sphere and a model C10027 photonic multichannel analyzer (PMA). Emission lifetimes were
acquired on IBH Fluorocube instrument equipped with SpectroLED or NanoLED excitation source using
time-correlated single photon counting (TCSPC) method.
DFT/TD-DFT Computations:
DFT Calculations was executed on Q-Chem 5.1 program
7
to obtain ground state optimized structures by
using B3LYP/LACVP** level. Time-dependent density functional theory (TD-DFT) method was
performed on the optimized ground state geometries at LACVP**/ CAM-B3LYP level of theory. Energy
values in eV were obtained by multiplying the Hartree values by 27.21.
98

4.5 References

1. Hamze, R.; Idris, M.; Muthiah Ravinson, D. S.; Jung, M. C.; Haiges, R.; Djurovich, P. I.;
Thompson, M. E., Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes
Bearing Bulky NHC Benzimidazolyl Carbene. Frontiers in Chemistry 2020, 8 (401).
2. Hamze, R.; Shi, S.; Kapper, S. C.; Muthiah Ravinson, D. S.; Estergreen, L.; Jung, M.-C.; Tadle,
A. C.; Haiges, R.; Djurovich, P. I.; Peltier, J. L.; Jazzar, R.; Bertrand, G.; Bradforth, S. E.; Thompson, M.
E., “Quick-Silver” from a Systematic Study of Highly Luminescent, Two-Coordinate, d10 Coinage Metal
Complexes. Journal of the American Chemical Society 2019, 141 (21), 8616-8626.
3. Shi, S.; Jung, M. C.; Coburn, C.; Tadle, A.; Sylvinson M. R, D.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E., Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I)
Complexes Featuring Nonconventional N-Heterocyclic Carbenes. Journal of the American Chemical
Society 2019, 141 (8), 3576-3588.
4. Hamze, R.; Peltier, J. L.; Sylvinson, D.; Jung, M.; Cardenas, J.; Haiges, R.; Soleilhavoup, M.;
Jazzar, R.; Djurovich, P. I.; Bertrand, G.; Thompson, M. E., Eliminating nonradiative decay in Cu(I)
emitters: >99% quantum efficiency and microsecond lifetime. Science 2019, 363 (6427), 601-606.
5. Sworakowski, J.; Lipinski, J.; Janus, K., On the reliability of determination of energies of HOMO
and LUMO levels in organic semiconductors from electrochemical measurements. A simple picture based
on the electrostatic model. Organic Electronics 2016, 33, 300-310.
6. Ravinson, D. S. M.; Thompson, M. E., Thermally assisted delayed fluorescence (TADF):
fluorescence delayed is fluorescence denied. Materials Horizons 2020.
7. Shao, Y. H.; Gan, Z. T.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange, A.
W.; Behn, A.; Deng, J.; Feng, X. T.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.; Kaliman, I.;
Khaliullin, R. Z.; Kus, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard, R. M.; Rohrdanz, M.
A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.; Zuev, D.; Albrecht, B.; Alguire,
E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.; Brandhorst, K.; Bravaya, K. B.; Brown, S.
T.; Casanova, D.; Chang, C. M.; Chen, Y. Q.; Chien, S. H.; Closser, K. D.; Crittenden, D. L.;
Diedenhofen, M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.; Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.;
Ghysels, A.; Golubeva-Zadorozhnaya, A.; Gomes, J.; Hanson-Heine, M. W. D.; Harbach, P. H. P.;
Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.; Jagau, T. C.; Ji, H. J.; Kaduk, B.; Khistyaev, K.; Kim,
J.; King, R. A.; Klunzinger, P.; Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.;
Lawler, K. V.; Levchenko, S. V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar,
P.; Manzer, S. F.; Mao, S. P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.;
Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O'Neill, D. P.; Parkhill, J. A.; Perrine, T. M.;
Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.; Small, D. W.;
Sodt, A.; Stein, T.; Stuck, D.; Su, Y. C.; Thom, A. J. W.; Tsuchimochi, T.; Vanovschi, V.; Vogt, L.;
Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel, J.; White, A.; Williams, C. F.; Yang, J.; Yeganeh, S.;
Yost, S. R.; You, Z. Q.; Zhang, I. Y.; Zhang, X.; Zhao, Y.; Brooks, B. R.; Chan, G. K. L.; Chipman, D.
M.; Cramer, C. J.; Goddard, W. A.; Gordon, M. S.; Hehre, W. J.; Klamt, A.; Schaefer, H. F.; Schmidt, M.
W.; Sherrill, C. D.; Truhlar, D. G.; Warshel, A.; Xu, X.; Aspuru-Guzik, A.; Baer, R.; Bell, A. T.; Besley,
N. A.; Chai, J. D.; Dreuw, A.; Dunietz, B. D.; Furlani, T. R.; Gwaltney, S. R.; Hsu, C. P.; Jung, Y. S.;
99

Kong, J.; Lambrecht, D. S.; Liang, W. Z.; Ochsenfeld, C.; Rassolov, V. A.; Slipchenko, L. V.; Subotnik,
J. E.; Van Voorhis, T.; Herbert, J. M.; Krylov, A. I.; Gill, P. M. W.; Head-Gordon, M., Advances in
molecular quantum chemistry contained in the Q-Chem 4 program package. Molecular Physics 2015, 113
(2), 184-215.

100

5 CHAPTER V: Two-coordinate coinage metal NHC complexes bearing
phenanthrimidazolyl amide donor

5.1 Introduction
Imidazoles constitute a family of N-amide donors that could be employed in the two-coordinate
(amide)-(metal)-(carbene) complexes which have been shown to be attractive TADF phosphors for
OLEDs.
1-4
These reported systems mainly focused on the carbazolyl amide donor.
Phenanthro[9,10-d]imidazoles (pI),
5
like carbazoles, are used as OLED host materials due to their high
triplet state energy, which is a key parameter for blue OLEDs. As the energetics of the associated
N-M C(carbene) complexes are impacted by the type of the N-amide donor, we sought to investigate the
phenanthrimidazolyl-based complexes (pI-AuBZI and pI-CuMAC) and compare them to the
carbazolyl-based analogues. Here we present the synthesis, electrochemical and photophysical properties
of the complexes.
5.2 Results and Discussion
5.2.1 Synthesis
All complexes reactions were carried out under dry, inert gas. Phenanthro[9,10-d]imidazole, pI, was
synthetically accessible by following literature procedures.
6
The deprotonation of pI could be realized by
using a stoichiometric amount of sodium tert-butoxide or potassium bis(trimethylsilyl)amide, (KHMDS).
To avoid potential bi-coordination (on both nitrogen atoms of pI), a carbene-MCl THF solution (BZI-AuCl
or MAC-CuCl) was added dropwise to a stirring THF solution of the deprotonated ligand for the reaction
to complete overnight. A celite-filtered solution was concentrated to yield a solid, which, after
recrystallization in toluene, yielded pure pI-AuBZI (86%) and pI-CuMAC (74%).
101

Figure 5. 1: Synthesis of complexes pI-AuBZI and pI-CuMAC.

It is important to note that if the order of mixing the two reagents was reversed (or if the carbene-MCl solid
was added in one portion to the deprotonated pI solution), bimetallic byproducts were observed, as judged
from MALDI analysis. That is, the two nitrogen of pI are individually coordinated to the metal-carbene,
forming a cationic binuclear complex. The bimetallic formation is being investigated further in MET
research group.
5.2.2 Electrochemical properties
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed on free ligand pI and
complex pI-CuMAC in N,N-dimethylformamide (DMF) with 0.1 M tetrabutylammonium
hexafluorophosphate (nBu 4NPF 6) electrolytes (Table 5.1 and Figure 5.2). We used carbon working
electrode, platinum wire counter electrode, silver wire pseudo-reference electrode, and
ferrocene/ferrocenium couple as an internal reference (adjusted to 0.0 V). From CV, pI ligand is
characterized by irreversible redox waves, with an oxidation potential E ox = 0.76 V and reduction potential
E red = -2.94 V. pI-CuMAC retains the anodic irreversibility, and the oxidation is still pI-centered, but now
with less positive oxidation potential (E ox = 0.47 V) because the coordinated ligand is anionic (as apposed

102

to the protonated free ligand). The quasi-reversible cathodic waves correspond to the reduction of MAC
carbene moiety (E red = -2.40 V) as it is reported for other carbazolyl-based NHC complexes of the same
family.
3
The irreversible small wave at -1.73 V might be due to the cationic MAC-Cu
+
of residual bimetallic
byproduct. From the latter reference, Cz-CuMAC redox values are also give in Table 5.1 for comparison.
The reduction potentials of pI-CuMAC and Cz-CuMAC are comparable with only 200 mV potential
difference. That confirms the same target in reduction, here MAC carbene. On the other hand, the carbazolyl
complex is considerably easier to oxidize (E ox = 0.17 V), indicating the shallower HOMO of carbazolyl
anion relative to that of phenanthrimidazolyl. Hence, pI (like Cz) ligand acts as a donor (HOMO residence),
and MAC carbene as an acceptor (LUMO residence).
Relating the present electrochemical properties of pI-CuMAC to those of the reported Cz-M-BZI
complexes,
1
we can safely predict the redox potentials of pI-AuBZI, where its oxidation potential is
anticipated to be comparable to that of pI-CuMAC (E ox = 0.47 V) and the reduction potential roughly
similar to that of Cz-AuBZI (E red = -2.82 V).
1
The redox potential variation induced by the metal type (Cu
vs. Ag vs. Au) on carbazolyl-based complexes was remarked, where oxidation events required slightly more
positive potentials from Cu to Ag to Au. Similar effects are anticipated for the pI-based complexes.
Table 5.1: Redox values extracted from electrochemical measurements (DPV). HOMO and LUMO are calculated according
literature formulae.
7

a
From reference [3].
3

b
From reference [1].
1

Compound E
ox
(V) E
red
(V) 
redox
(V) HOMO
(eV)
LUMO (eV)
pI ligand 0.76 -2.94 3.70 -5.66 -1.36
pI-CuMAC 0.47 -2.40 2.87 -5.33 -2.00
Cz-CuMAC
a
0.17 -2.50 2.67 -4.99 -1.88
Cz-AuBZI
b
0.32 -2.82 3.14 -5.16 -1.51

103

Figure 5.2: Differential pulse voltammograms (left column) and cyclic voltammograms (right column) of free pI ligand and
pI-AuMAC in 0.1 M DMF. Measurements were performed by using tetrabutylammonium hexafluorophosphate (TBAF)
electrolytes, glassy carbon working electrode, Pt wire counter electrode, and silver wire (Ag/Ag+) pseudo reference electrode, and
ferrocene/ferrocenium internal reference (Fc/Fc+ = 0.0 V). See Table 5.1 for values extracted from DPV.
5.2.3 DFT/TDDFT Studies
Geometry optimizations in gas phase were performed on free anionic pI ligand and the associated
complexes by using density functional theory (DFT) at B3LYP/LACVP** level of theory (Table 5.2). The
resulting optimized structures were used to perform time-depend density functional theory (TDDFT)
calculations at LACVP/CAM-B3LYP level to calculate state energies and natural transition orbitals
(NTOs). For both pI-AuBZI and pI-CuMAC, the HOMO is overwhelmingly localized on pI moiety, and
LUMO on the carbene, with a small contribution from the metal (see first column of Table 5.2). This
-3 -2 -1 0 1 2
-15
-10
-5
0
5
10
15
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
Reduction
Oxidation
pI ligand
-4 -3 -2 -1 0 1
-2
-1
0
1
2
3
Current (x10
-4
A)
Potential (V) vs. Fc/Fc
+
pI ligand

-3 -2 -1 0 1
-6
-3
0
3
6
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
Reduction
Oxidation
pI-CuMAC
-3 -2 -1 0 1
-4
0
4
8
12
Current (x10
-5
A)
Potential (V) vs. Fc/Fc
+
pI-CuMAC

104

outcome confirms the afore-discussed electrochemical properties where the oxidation occurs on the pI
ligand, and reduction on the carbene acceptor.

Table 5.2: DFT/TDDFT calculations: HOMO (blue)/LUMO (red) of optimized structure pI ligand, pI-CuBZI, and pI-CuMAC
(first column). Hole (green) and electron (yellow) contours of natural transition orbitals (NTOs) for T1, T2, and S1 are given in
2
nd
, 3
rd
, and 4
th
column, respectively.

The two lowest excited triplets (T 1 and T 2) and lowest excited singlet (S 1) are given in Table 5.2 with their
natural transition orbitals (NTOs). Green and yellow contours represent hole and electron, respectively. The
lowest triplet of pI-AuBZI is characterized by pI-localized NTOs, whereas S 1 exhibits inter-ligand charge
transfer transition. Energy separation between T 1 and S 1 is 1.10 eV. Such a big energy difference heralds
an absence of thermally activated delayed fluorescence (TADF) events as the pI triplet will always be the
low-lying and emitting state. In other words, conventional phosphorescence emission is anticipated to be
dominant in pI-AuBZI (Figure 5.3).

.
105

On the contrary, the gas phase DFT prediction of pI-CuMAC shows a smaller energy separation (0.67 eV)
between T 1 (
3
pI) and S 1. Keeping in mind that S 1 and T 2 are of ICT nature, the solvent polarity effect might
stabilize these states closer or below T 1 to turn on TADF emissions. The calculated states levels and
expected photophysical phenomena are qualitatively summarized by Figure 5.3.

Figure 5.3: Qualitative states energy diagram of pI-AuBZI, and pI-CuMAC from TDDFT calculations in gas phase. In pI-AuBZI,
S1-T1 separation is larger (1.10 eV) heralding conventional phosphorescence, whereas in pI-CuMAC the separation is smaller
(0.67 eV), which promises delayed fluorescence emissions.

5.2.4 Photophysical properties
Absorption and emission spectra of pI-AuBZI are given in Figure 5.4-A&B, and properties are tabulated
in Table 5.3. The characteristic absorption bands of the complex in MeTHF resemble those of the protonated
pI ligand except that the former is bathochromically shifted because the coordinated ligand is anionic.
Emission spectra of pI-AuBZI at room temperature and 77K are structured and similar to the triplet
emission of the associated free ligand. The slight emission spectra redshift of the complex is explained by
the anionic nature of the coordinated pI ligand. A slight blue-shift (9 nm) from rt to 77K in the emission
of the complex might be due to the matrix effect or just the presence of small pI-localized dipole moments
that causes destabilization of the excited triplet state in frozen matrix. The emission quantum yield of

106

pI-AuBZI is significantly lower ( < 2%) with longer rt lifetime ( = 23 s) and slow radiative rate
(k r = 4.2 x10
2
s
-1
). Emission properties in MeCy were recorded to be the same as those in MeTHF.
Therefore, pI-AuBZI is undergoing conventional phosphorescence as predicted from theoretical
calculations due to the large separation between lowest
3
pI and lowest
1/3
ICT. On the contrary, the free
ligand has a quantum yield as high as 78% and nanoscale emission lifetime ( = ~18 ns) with faster radiative
rates (k r = 4.4 x10
7
s
-1
) indicating prompt fluorescence.

Figure 5.4: Absorption (A) and emission (B) spectra of pI ligand and pI-AuBZI in MeTHF. Extinction coefficient (C) and
emission spectra (D) of pI-CuMAC in MeTHF vs MeCy. See Table 5.3 for tabulated absorption and emission properties.

Absorption spectra of pI-CuMAC in MeTHF vs. MeCy are given in Figure 5.4, C. Unlike other reported
TADF NHC complexes,
1-4
absorption bands of pI-CuMAC do not have distinguishable charge transfer
300 325 350 375 400
0
2
4
6
Norm Absorbance (a.u.)
Wavelength (nm)
pI ligand
pI-AuBzI
 = 3,440
(MeTHF)
A)
400 450 500 550 600
0.0
0.4
0.8
1.2
Norm. PL intensity (a.u)
Wavelegnth (nm)
pI Ligand 77K
pI-AuBzI RT
pI-AuBzI 77K
(in MeTHF)
B)
300 350 400 450 500
0.0
0.4
0.8
1.2
1.6
Ext. coef. (x10
4
M
-1
.cm
-1
)
Wavelength (nm)
pI-CuMAC
MeTHF
MeCy
C)
400 450 500 550 600 650 700
0.0
0.4
0.8
1.2
PL intensity (a.u)
Wavelength (nm)
pI-CuMAC
MeTHF RT
MeCy RT
MeTHF 77K
MeCy 77K
PS film RT
PS film 77K
D)

107

bands in polar or non-polar solvents. This outcome agrees with the theoretical small oscillator strength of
1
ICT transitions for pI-CuMAC ((f = 0.035) compared to that for Cz-CuMAC (f = 0.110),
3
Table 5.2. In
fact, the CT extinction coefficients for pI-CuMAC and pI-AuBZI (~3 x 10
3
M. cm
-1
) are roughly two
times smaller than those in Cz-based complexes. In MeTHF, the low energy absorption band of pI-CuMAC
in the 340-390 nm region surprisingly resembles that of pI-AuBZI and subsides in MeCy.

Figure 5.5: Qualitative potential energy surface diagram of states in pI-AuMAC illustrating the solvatochromism behavior
observed in the complex. Absorption in polar solvent is hypsochromically shifted relative to that in non-polar solvent. Emission in
polar solvent is bathochromically shifted relative to that in non-polar solvent. More distortions of excited states in MeTHF are
illustrate by the shift of the potential energy surface well to the left, corresponding to the poorer emission quantum yield in the
polar solvent.
Emission spectra at room temperature are of CT transition nature in 1%wt PS film, MeCy, and MeTHF,
with the latter solvent exhibiting a bathochromic shift of 30 nm relative to MeCy, and 45 nm relative to the

108

PS film (Figure 5.4, D and Table 5.3). At 77K, however, structured emissions show a negligible
solvatochromism and correspond to the pI-localized triplet emission as observed in the phosphorescence
of pI-AuBZI. The lower emission yield in the polar solvent is consistent with the expected larger
reorganization energy of excited states in polar solvents. The solvatochromism effects can be explained by
the potential energy surfaces diagram in Figure 5.5. Figure 5. 6 is also provided to illustrate the
photophysical differences between pI-AuBZI and Cz-CuMAC. The latter undergoes TADF at room
temperature, and phosphorescence at 77 K, whereas pI-AuBZI undergoes inefficient conventional
phosphorescence in all conditions.

The lifetime in MeTHF ( = 0.44 s) is the shortest among other media which is consistent with the
associated higher non-radiative rate (k nr = 1.9 x 10
6
s
-1
), which is over one order of magnitude larger than
the rates in MeCy and PS film. Radiative rates in all media are comparable and fall in the range of
1.6 – 3.4 x 10
5
s
-1
. Compared to the reported carbazole-based analogue, for example Cz-CuMAC in
MeTHF has higher quantum yield ( = 50%), improved radiative rate (k nr = 5.0 x 10
5
s
-1
), and mitigated
non-radiative rate (k nr = 4.1 x 10
5
s
-1
).
Table 5.3: Absorption and emission properties of pI ligand, pI-AuBZI and pI-CuMAC.
c
pI triplet energy 77K emission onset is
2.87 eV.


em
, rt (
em
, 77K)
c
yield,   rt
k
r
(s
-1
) k
nr
(s
-1
)
 77K
pI Ligand
(MeTHF)
(426) 0.78 17.8 ns
4.38 x 10
7
1.23 x10
7

0.64 s
pI-AuBzI
(MeTHF)
441 (432) 0.019 23.4 s
4.20 x 10
2
1.71 x 10
4

4.37 ms (79%)
8.11 ms (21%)
pI-MAC
(MeTHF) 535 (432) 0.15 0.44 s
3.41 x 10
5
1.93 x 10
6

48 ms
(MeCy) 505 (434) 0.69 2.54 s
2.72 x 10
5
1.22 x 10
5

26 ms
1% PS film 490 (442) 0.71 4.34 s
1.64 x 10
5
6.68 x 10
4

0.15 (39%
0.88 (61%)
109

Figure 5. 6: Qualitative representation of states and emission processes taking place in pI-CuMAC vs pI-AuBZI at rt and 77 K.
pI-CuMAC undergoes TADF at room temperature, and phosphorescence at 77 K. pI-AuBZI undergoes inefficient conventional
phosphorescence at rt and 77 K, regardless of solvent polarity, because
3
pI is always the low-lying, emitting state.

Experimental and theoretical results of pI-complexes show some discrepancies. The theoretical lowest
triplet was predicted to be ~2.70 eV, whereas the experimental triplet energy of the phenanthrimidazolyl
was determined to be 2.87 eV. As shown in Figure 5.3 and Table 5.2, gas phase TDDFT predicts the
3
LE
to be below the
1/3
ICT while experimental emissions reveal that
1/3
ICT is the low-lying and emitting states
at room temperature. Such a discrepancy is consistent with the fact that TDDFT calculations are performed
in gas phase, whereas the optical measurements are carried out in media of a given polarity which induces
stabilization of
1/3
ICT accordingly. The relatively more desirable emission properties (for OLED
applications) of Cz-M-carbene over the present pI-M-carbene emitters are attributed to (1) the lower triplet
energy of the pI ligand that negatively affect the required small E ST in the systems, (2) smaller orbital

110

overlap integral between the pI and carbene (i.e. smaller oscillator strength) leading to slower singlet state
emissions (or small k r). These two parameters (E ST and k r) impact TADF events in the investigated
pI-based NHC complexes. The reduced orbital overlap integral in the pI systems can be explained by the
tilted core orientation of the coordinated pI ligand in the complexes, as opposed to the favorable geometry
and accessible -system of Cz for efficient donor-acceptor orbital interaction while keeping the necessary
separation for small E ST.
5.3 Conclusion
We have explored NHC complexes bearing phenanthrol[9,9-b]imidazolyl donor (pI) with BZI and MAC
acceptor carbenes. The low-lying triplet of pI , not to mention the tilted structural core orientation –
compared to carbazolyl ligand – significantly affects the photophysical outcome of its complexes
(pI-M-BZI and pI-M-MAC). From gas phase DFT/TDDFT calculations, the lowest
3
LE is predicted to
always lie below the
1/3
ICT manifold both in pI-M-BZI and pI-M-MAC, with a larger energy separation
in the former complex which makes its photophysical emission processes to be purely phosphorescence
regardless of solvent polarity and temperature (
3
pI emission). pI-M-MAC, on the hand, inherits
1/3
ICT
manifold of lower energy (than pI triplet), allowing TADF events to take place in polar and non-polar media
at room temperature. Localized triplet emission is turned on only in frozen matrix. Such energetics
difference between pI-metal-carbene and Cz-metal-carbene explains why the latter systems showed
photophysical parameters (higher quantum yield and faster emissions) more desirable for efficient OLEDs.
Future work will explore further derivatives of imidazolyl donors (imidazolyl, benzimidazolyl, and
phenanthrimidazolyl) to understand the effect of varying -conjugation in the systems.
5.4 Experimental
5.4.1 Synthesis of complexes
General information: All reactions were carried out in dry and anerobic atmosphere by using Schlenk line
or glovebox techniques, unless noted otherwise. Dry and degassed solvents were used in reactions and
111

measurements. MAC-CuCl and BZI-AuCl were synthesized according to literature.
1-2
pI ligand was
synthesized according to literature.
6

1
H and
13
C NMR spectra were recorded on a 400 MHz or 600 MHz
Varian NMR instrument. Chloroform-d 1 (
1
H at 7.26 ppm,
13
C at 77.2 ppm). Acetone-d6 (
1
H at 2.05 ppm,
13
C at 206.7 & 29.9 ppm).

Im-AuBzI: Sodium tert-butoxide (11.6 mg, 120.27 µmol) was added into a stirring solution of pI ligand
(25 mg, 114.54 µmol) in THF (10 mL) and was allowed to react for 30 min. Dissolved BZI-AuCl (62. mg,
116.83 µmol) THF solution was added dropwise into the reaction flask containing the deprotonated pI
solution, and the mixture was left to stir for 12 hours at room temperature. The reaction solution was then
filtered through a plug of celite and volatiles were removed under reduced pressure. A minimum amount
of dichloromethane was added into the residues to make a concentrate solution; then 15 mL of dry hexane
was added to precipitate out a white solid which was collect by vacuum filtration and washed by more
hexane to yield 67 mg (86%).
1
H NMR (600 MHz, Chloroform-d) δ = 8.60 (d, J = 8.3 Hz, 1H), 8.57 (t, J =
8.1 Hz, 2H), 8.29 (d, J = 8.0 Hz, 1H), 7.72 (t, J = 7.9 Hz, 2H), 7.55 (t, J = 8.0 Hz, 1H), 7.50 – 7.46 (m =
d+dd, 6H), 7.42 (t, J = 8.4 Hz, 1H), 7.36 (t, J = 8.2 Hz, 1H), 7.33 (s, 1H), 7.22 (dd, J = 6.1, 3.1 Hz, 2H),
7.06 (t, J = 8.1 Hz, 1H), 2.53 (hept, J = 6.9 Hz, 4H), 1.34 (d, J = 6.9 Hz, 12H), 1.15 (d, J = 6.9 Hz, 12H).
13
C NMR yet to be acquired CHN
112

pI-CuMAC: Under a dry nitrogen atmosphere, sodium tert-butoxide (121 mg, 1.26 mmol) was added into
a THF solution of phenanthrimidazole (250 mg, 1.15mmol) to be stirred for 1 hour. A THF solution of
MAC-CuCl (627 mg, 1.15 mmol) prepared aside was added drop-wise into the reaction flask of the
deprotonated pI. The reaction mixture was stirred overnight before being filtered through a pad of celite.
The filtrate was concentrated to a ~4 mL before adding dry hexane to precipitate out an off-white solid. The
crude solid was recrystallized in hot toluene to yield a white product (620 mg, 74%).
1
H NMR (400 MHz,
Chloroform-d) δ 8.66 – 8.44 (m, 3H), 7.67 (t, J = 7.8 Hz, 2H), 7.57 – 7.49 (m, 1H), 7.47 – 7.35 (m, 4H),
7.34 – 7.22 (m, 1H), 7.21 – 7.11 (m, 1H), 7.04 (td, J = 7.4, 6.9, 1.1 Hz, 1H), 6.96 (dd, J = 8.0, 1.4 Hz, 1H),
6.37 (s, 1H), 3.91 (s, 2H), 3.33 (hept, J = 6.9 Hz, 2H), 3.07 (h, J = 6.9 Hz, 2H), 1.70 – 1.16 (m, 30H).
13
C NMR (101 MHz, cdcl 3) δ 210.08, 170.49, 146.57, 145.62, 144.58, 139.70, 137.31, 135.43, 131.15,
130.97, 129.01, 128.38, 128.20, 127.53, 126.84, 126.20, 126.11, 125.86, 125.14, 123.41, 123.24, 122.73,
122.41, 122.06, 120.09, 62.04, 37.99, 29.26, 28.86, 25.15, 24.74, 24.45, 24.14. CHN
5.4.2 Other measurements’ information
Electrochemical measurements:
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) of the complexes and ligand were run
on VersaSTAT 3 potentiostat electrochemical analyzer (Princeton Applied Research). Dry and degassed
DMF was used with tetrabutylammonium hexafluorophosphate (TBAF) electrolytes, glassy carbon
working electrode, Pt wire counter electrode, silver wire pseudo-reference electrode, and ferrocene as an
internal reference. Reversibility was judged from CVs, and redox values were extracted from DPV and are
corrected against ferrocene/ferrocenium couple (Fc/Fc
+
= 0.0 V).
113

Photophysical measurements:
UV-visible spectra were recorded by using Hewlett-Packard 4853 diode array spectrometer. Steady state
emission spectra were recorded on QuantaMaster Photon Technology International spectrofluorometer.
Emission quantum yields were measured by using Hamamatsu C9920 system with a xenon lamp,
integrating sphere and a model C10027 photonic multichannel analyzer (PMA). Emission lifetimes were
acquired on IBH Fluorocube instrument equipped with SpectroLED or NanoLED excitation source using
time-correlated single photon counting (TCSPC) method.
DFT/TD-DFT Computations:
DFT Calculations was executed on Q-Chem 5.1 program
8
to obtain ground state optimized structures by
using B3LYP/LACVP** level of theory. Time-dependent density functional theory (TD-DFT) method was
performed on the optimized ground state geometries at LACVP**/ CAM-B3LYP level of theory.
114

5.5 References

1. Hamze, R.; Idris, M.; Muthiah Ravinson, D. S.; Jung, M. C.; Haiges, R.; Djurovich, P. I.;
Thompson, M. E., Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes
Bearing Bulky NHC Benzimidazolyl Carbene. Frontiers in Chemistry 2020, 8 (401).
2. Hamze, R.; Shi, S.; Kapper, S. C.; Muthiah Ravinson, D. S.; Estergreen, L.; Jung, M.-C.; Tadle,
A. C.; Haiges, R.; Djurovich, P. I.; Peltier, J. L.; Jazzar, R.; Bertrand, G.; Bradforth, S. E.; Thompson, M.
E., “Quick-Silver” from a Systematic Study of Highly Luminescent, Two-Coordinate, d10 Coinage Metal
Complexes. Journal of the American Chemical Society 2019, 141 (21), 8616-8626.
3. Shi, S.; Jung, M. C.; Coburn, C.; Tadle, A.; Sylvinson M. R, D.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E., Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I)
Complexes Featuring Nonconventional N-Heterocyclic Carbenes. Journal of the American Chemical
Society 2019, 141 (8), 3576-3588.
4. Hamze, R.; Peltier, J. L.; Sylvinson, D.; Jung, M.; Cardenas, J.; Haiges, R.; Soleilhavoup, M.;
Jazzar, R.; Djurovich, P. I.; Bertrand, G.; Thompson, M. E., Eliminating nonradiative decay in Cu(I)
emitters: >99% quantum efficiency and microsecond lifetime. Science 2019, 363 (6427), 601-606.
5. Idris, M.; Coburn, C.; Fleetham, T.; Milam-Guerrero, J.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E., Phenanthro[9,10-d]triazole and imidazole derivatives: high triplet energy host
materials for blue phosphorescent organic light emitting devices. Materials Horizons 2019, 6 (6), 1179-
1186.
6. Therrien, J. A.; Wolf, M. O.; Patrick, B. O., Polyannulated Bis(N-heterocyclic carbene)palladium
Pincer Complexes for Electrocatalytic CO2 Reduction. Inorg Chem 2015, 54 (24), 11721-32.
7. Sworakowski, J., How accurate are energies of HOMO and LUMO levels in small-molecule
organic semiconductors determined from cyclic voltammetry or optical spectroscopy? Synthetic Metals
2018, 235, 125-130.
8. Shao, Y. H.; Gan, Z. T.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange, A.
W.; Behn, A.; Deng, J.; Feng, X. T.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.; Kaliman, I.;
Khaliullin, R. Z.; Kus, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard, R. M.; Rohrdanz, M.
A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.; Zuev, D.; Albrecht, B.; Alguire,
E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.; Brandhorst, K.; Bravaya, K. B.; Brown, S.
T.; Casanova, D.; Chang, C. M.; Chen, Y. Q.; Chien, S. H.; Closser, K. D.; Crittenden, D. L.;
Diedenhofen, M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.; Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.;
Ghysels, A.; Golubeva-Zadorozhnaya, A.; Gomes, J.; Hanson-Heine, M. W. D.; Harbach, P. H. P.;
Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.; Jagau, T. C.; Ji, H. J.; Kaduk, B.; Khistyaev, K.; Kim,
J.; King, R. A.; Klunzinger, P.; Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.;
Lawler, K. V.; Levchenko, S. V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar,
P.; Manzer, S. F.; Mao, S. P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.;
Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O'Neill, D. P.; Parkhill, J. A.; Perrine, T. M.;
Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.; Small, D. W.;
Sodt, A.; Stein, T.; Stuck, D.; Su, Y. C.; Thom, A. J. W.; Tsuchimochi, T.; Vanovschi, V.; Vogt, L.;
115

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

116

6 APPENDIX: Binuclear N-heterocyclic Carbene Complexes

Introduction
Davydov model constitutes a promising strategy to enhance radiative rates of molecular systems by taking
advantage of multipole interactions and subsequent states’ splitting.
1
For instance, transition dipole
moments of two symmetry-related molecular entities can couple by adding up or subtracting their dipole
vectors, and hence engendering two new transition dipole moments (Appendix Fig. 1).The end result is a
radiative rate increase by a factor of >4, as it was demonstrated by Yersin’s group on dinuclear copper(I)
complexes.
1
Building upon the mononuclear TADF phosphors developed by our lab,
2-5
we are introducing
two strategies of designing binuclear TADF systems with higher radiative rates compared to their
mononuclear counterparts. The two strategies consist of building the system from (1) bis(carbenes) or (2)
bis(amides) blocks. The resulting binuclear systems are of structures: (amide)M-[bis(carbene)]-M(amide)
and (carbene)M-[bis(amide)]-M(carbene), where M represents Cu(I), Ag(I), or Au(I). In Appendix A, we
unveil the synthesis of benzo-bis(imidazolylidene) carbenes as the center core for the first structure; in
Appendix B, we will discuss preliminary results of a case study of diketopyrrolopyrrolyl amide donor
complexing with a high energy carbene IPr-CuCl.

Appendix Fig. 1: Two strategies for designing binuclear TADF NHC complexes for high radiative rates (kr), which are predicted
to increase by 4-folds according to Davydov model. N = nitrogen. M = Cu(I), Ag(I), Au(I).

117

6.1 APPENDIX A: Synthesis of benzo-bis(imidazolylidene) carbenes for bimetallic NHC
complexes

6.1.1 Results and Discussion
6.1.1.1 Synthesis
Staring from arene 1 (1,5- dichloro-2,4-dinitrobenzene), the synthesis of benzo-bis(imidazolylidene), BBI,
with less bulky substituents on N atoms were previously reported by refluxing (at 80
o
C) an ethanolic
solution of 1 and aniline to obtain “bis” aryl amination on the chloride sites.
6
Following exactly the same
protocol with a more bulky aryl, 2,6-diisopropoylaniline (Dipp), only a singly aminated product 2a is
isolated, even when excess of the aniline is used (Figure A.1). To encourage double aryl amination, we
subjected the arene 1, or the singly aminated intermediate 2a, to a copious amount of
2,6-diisopropoylaniline acting as reactant and solvent at the same time, and heated the reaction mixture at
150
o
C to obtain 2b in 77% yield. The two products, 2a and 2b, can be distinguished from NMR analysis
as shown in Figure A.2. Integrated areas of Dipp protons of 2b are clearly doubled.
The high-yielding reduction of nitro groups followed by in-situ cyclization were accomplished by reacting
2b, Pd/C and in-situ-generated sodium formate to yield a neutral benzo-bis(imidazole) product 3. The BBI
iodide salt 4 was synthesized in high yield (88%) by means of heating a mixture of 3 and iodomethane in
acetonitrile at 80
o
C in pressure flask. Due to the di-cationic nature of the salt, the solubility becomes poorer
which facilitates the isolation and purification of 4.
118

Figure A.1: Synthesis of bulky benzobis(imidazolium) salts (I) and quinobis(imidazolium) salts (II) for binuclear NHC complexes.

From bromanil, a one-pot synthesis of a symmetric quinobis(imidazolium) (QBI) dibromide salt (Q3) was
synthesized according to literature,
7
but with a bulky imidine Q2. The yield is relatively lower (47%) due
to improved solubility in the new QBI. The facile aryl amination and cyclization without a need of a catalyst
is attributed to the electron-deficiency of the bromanil. It is the same driving force behind the catalyst-free
aryl amination of the arene 1 in which the nitro groups induce great electron-deficiency. Generally, aryl
amination of halogenated benzenes requires the use of a catalyst, commonly palladium.
8

119

Figure A.2: Proton NMR analysis of single aryl amination (2a) vs double aryl amination (2b) products.

The deprotonation of BBI and QBI salts can be accomplished by using potassium (or sodium)
bis(trimethylsilyl)amide base.
7, 9
Similar free carbenes of these two types were reported to be unstable under
air and warm environment, and hence must be stored in the cold and under inert gas. In the present work,
an attempt to deprotonate BBI salt 4, followed by in-situ complexation with Me 2SAu(I)Cl and deprotonated
carbazolyl to target the bimetallic product 5, led to unidentified green-emissive polymeric complexes whose
proton shifts could not be resolved by NMR analysis. The polymeric nature of the products is confirmed
by broad and unresolved peaks. Polymers/oligomers of BBI complexes were previously reported and are
due the bitopic nature of the carbenes.
9-10
The same outcome was observed for QBI complexes. Further

120

reaction manipulations are being investigated in our lab to surmount the polymerization conundrum to form
the desirable, molecular bimetallic products (i.e. BBI(MCz) 2 and QBI(MCz) 2).

Figure A.3: Absorption and emission spectra of unidentified polymeric complexes of BBI in MeTHF vs. toluene.

Table A.1: Tabulated emission properties of BBI polymeric complexes.
a
Triplet energy from 77K emission peak is
2.90 eV.
Polymeric
complexes of 4
λ
em, RT

( λ
77K
)
Φ
PL
 rt (s) k
r, RT

(x10
5
. s
-1
)
k
nr, RT

(x10
5
. s
-1
)
 77K (ms)
MeTHF 546
(428)
a

0.75 2.3 3.2 1.10 1.72 (37%)
0.41 (59%)

Toluene
502
(428)
0.75 3.9 1.9 0.64 1.18 (28%)
0.33 (63%)

6.1.2 Photophysical properties of unidentified polymeric complexes of BBI
Photophysical measurements were conducted to analyze optical properties of the unidentified products.
Absorption and emission spectra in MeTHF and toluene are given in Figure A.3.& Table A.1. The lower
energy absorption band shoulder in both solvents corresponds to charge transfer transitions, whereas the
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
Norm. PL intensity (a.u)
Norm. Absorbance (a.u)
Wavelength (nm)
Abs (MeTHF)
Em RT (MeTHF)
Abs (Tol)
Em RT (Tol)
Em 77K (MeTHF)
Em 77K (Tol)

121

high energy band are born out of -* electronic transitions. The CT abs bands are solvatochromic by
red-shifting from MeTHF to toluene. Room temperature emissions are also of CT nature with a positive
solvatochromic shift from toluene ( em = 502 nm) to MeTHF ( em = 546 nm). At 77K, the structured,
blue-shifted emission peak at  em = 428 nm are independent of solvent polarity. The 77K emission peak
corresponds to carbazolyl triplet energy of 2.90 eV. These absorption and emission properties are indicative
of a polar complex with opposite ground and excited dipole moments. The present system is reminiscent of
mononuclear carbazolyl-metal-carbene TADF phosphors which have inter-ligand charge transfer (ICT) at
room temperature and carbazolyl-localized triplet emission in frozen matrix.
2-5

Photoluminescence quantum yield of the complex is 75% in both solvents. Weighted average lifetimes in
MeTHF and toluene are comparable: 2.3 and 3.9 s, respectively. Multiexponential decays might be due to
the potential presence of impurities or just different polymeric environments. Radiative and non-radiative
rates show no substantial difference in different solvents. This indicates that reorganization energies of
excited states in MeTHF and toluene are also comparable, which is different from the reported mononuclear
Cz-M-carbenes where reorganization energy in polar solvent is usually high and detrimental to
photoluminescence efficiency. Further reaction manipulation of these binuclear systems is underway in
MET group and will be disclosed in the near future.
6.1.3 Theoretical studies
Density functional theory (DFT) calculations were performed on BBI(CuCz) 2 to obtain optimized structure
and frontier orbitals by using B3LYP/LACVP** level of theory (Figure A.4).
11
The HOMO-1 (4.35 eV)
and HOMO (4.32 eV) are near degenerate and localized on the carbazolyl units, whereas LUMO (2.28 eV)
is localized on the BBI unit with minimum contribution of the metal. Therefore, interligand charge transfer
(ICT) is expected from carbazolyl donor to the BBI acceptor core. Two opposite dipole moments are
predicted to be in the direction from each of the Cz units to the central BBI. According to Davidov splitting
and dipole interaction theory, these systems are anticipated to have a greater oscillator strength, hence
higher extinction coefficients and radiative rates. Yersin’s group applied this model to bimetallic systems
122

which showed an improvement of oscillator strength, molar absorptivities, and radiative rates by a factor
of >4.
1
From the preliminary photophysical properties presented above, the strong solvatochromism
indicate that the unidentified complex has a substantial net dipole moment and hence not satisfying
Davydov model.

Figure A.4:Frontier orbitals (HOMO-1, HOMO, and LUMO) of BBI(CuCz)2.

6.1.4 Conclusion
The synthesis of benzo-bis(imidazolium) and quinobis(imidazolium) salts were disclosed. Previous similar
syntheses used relatively less bulky N-aryl substituents whereas herein we carried out aryl amination with
bulky substituents, 2,6-diisopropylaniline, which is very crucial for coplanarity, as well as stability, of
Q/BBI-based bimetallic TADF phosphors. The metalation of the derived bis-carbenes to form molecular
complexes is yet to be uncovered, and the investigation is underway.

123

6.1.5 Experimental
6.1.5.1 Synthesis

5-chloro-N-(2,6-diisopropylphenyl)-2,4-dinitroaniline (2a): A flask was
charged with a magnetic stir bar, EtOH (25 mL), and aniline (7.5 mL,
82.3 mmol). After 1,5-dichloro-2,4-dinitrobenzene (4.87 g, 20.6 mmol)
was added to the reaction mixture, and the flask was fitted with a water
condenser. The mixture was then stirred in an oil bath at 80 °C for two days. A slurry yellow precipitate
formed overtime. The mixture was allowed to cool and then poured into water (100 mL). Precipitated solids
were collected via vacuum filtration, rinsed cold methanol, and dried under vacuum to give 7.15 g (99%
yield) of a yellow solid.
1
H NMR (400 MHz, Chloroform-d) δ 9.52 (s, 1H), 9.11 (s, 1H), 7.46 (dd, J = 8.2,
7.3 Hz, 1H), 7.32 (d, J = 7.7 Hz, 2H), 6.52 (s, 1H), 2.91 (hept, J = 6.9 Hz, 2H), 1.20 – 1.15 (m, 12H).
13
C NMR (101 MHz, cdcl 3) δ = 147.26, 146.39, 135.86, 130.37, 129.91, 126.87, 124.83, 117.54, 28.77,
24.36, 23.07.
N,N-bis(2,6-diisopropylphenyl)-4,6-dinitrobenzene-1,3-diamine (2b): In a 500-mL two-neck flask
containing a stir bar and 5.0 g (21.1 mmol) of the
dichlorodinitrobenzene (or the singly aminated arene 2a) was equiped
with a water-condenser. The system was pumped down and back-filled
with nitrogen gas for three cycles. Nitrogen-degassed
2,6-diisopropoylaniline (23.9 mL, 126.6 mmol) was cannula-transfered into the reaction flask. The mixture
was heated at 150
o
C for 48 hours after which it was allowed to cool down to room temperature. Methanol
(~20 mL) was added and the flask was placed in a freezer (-40
o
C) to encourage precipitation. The precipitate
was collected by vacuum filtration and washed with cold methanol and dried under vacuum to yield 8.5 g
(78%) of a yellow powder.
1
H NMR (400 MHz, Chloroform-d) δ = 9.43 (s, 2H), 9.39 (s, 1H), 7.23 (t, J =
7.8 Hz, 2H), 7.04 (d, J = 7.8 Hz, 4H), 5.09 (s, 1H), 2.75 (hept, J = 6.8 Hz, 4H), 1.07 (d, J = 6.8 Hz, 12H),

124

0.85 (d, J = 6.9 Hz, 12H).
13
C NMR (101 MHz, CDCl 3) δ = 149.35, 146.30, 132.03, 129.66, 129.03, 125.15,
124.08, 94.82, 28.73, 24.93, 22.37.
1,7-bis(2,6-diisopropylphenyl)benzobis(imidazole) (3): A 500-mL round-bottom flask was charged with
a magnetic stir-bar and formic acid (98%)/water mixture (90:10
mL). NaHCO 3 (9.72 g, 115.7 mmol) was added portion-wise with
vigorous stirring. To the mixture was added Pd/C (0.616 g, 10 wt
%, 0.578 mmol Pd) and dinitroarene 2b (3.0 g, 5.78 mmol). The
flask was fitted with a water condenser and heated in an oil bath with vigorous stirring at 120 °C for 48 h.
The mixture was then allowed to cool to room temperature and filtered through a plug of celite with the aid
of 70 mL of Ethyl acetate. On a rotary evaporator, the filtrate volume was reduced to a slurry mixture. 50
mL of de-ionized water was added into the mixture, swirled, and slowly poured into a vigorously stirring
solution of saturated aqueous K 2CO 3. A beige precipitate developed and was collected via vacuum filtration,
rinsed with plenty of water H 2O and dried under vacuum to yield 2.73 g (98%).
1
H NMR (400 MHz,
Chloroform-d): δ = 8.39 (s, 1H), 7.87 (s, 2H), 7.47 (t, J = 7.8 Hz, 2H), 7.27 (d, J = 7.9 Hz, 4H), 6.52 (s,
1H), 2.26 (hept, J = 6.7 Hz, 4H), 1.06 (dd, J = 6.9, 0.7 Hz, 12H), 0.90 (dd, J = 6.8, 0.7 Hz, 12H).
13
C NMR
(101 MHz, CDCl 3) δ 147.95, 144.81, 140.84, 134.61, 130.99, 130.49, 124.38, 110.63, 89.89, 28.41, 24.88,
23.95.
3,4-dimethyl-1,7-bis(2,6-diisopropylphenyl)benzobis(imidazolium) iodide (3): In a 100-mL pressure
flask, benzobis(imidazole) 3 (500 mg, 1.04 mmol), 2-iodomethane
(0.52 mL, 8.36 mmol), and acetonitrile (5 mL) were added. The
pressure flask was sealed and heated at 90
o
C for 24 hours. After the
completion of the reaction, the mixture was cooled down to room
temperature and solvents were removed under reduced pressure. Diethyl ether was added to the resulting
crude solid, which was sonicated for 10 min, and collected by vacuum filtration to yield a beige solid (680
mg, 85%).
1
H NMR (400 MHz, Acetone-d 6) δ = 10.73 (s, 2H), 9.91 (s, 1H), 8.24 (s, 1H), 7.63 (t, 2H), 7.45

125

(d, J = 7.9 Hz, 4H), 4.73 (s, 6H), 2.45 (hept, J = 6.8 Hz, 4H), 1.14 (d, J = 6.8 Hz, 12H), 1.00 (d, J = 6.8 Hz,
12H).
13
C NMR (101 MHz, acetone) δ 146.89, 146.78, 133.35, 132.32, 127.59, 125.10, 102.47, 98.68,
36.23, 24.25, 23.15, -2.00.
1,1',3,3'-Tetra[bis(2,6-diisopropylphenyl)]quinobis(imidazolium) dibromide; Q 3: (Purged the mixture
with N 2 before heating). A solution of bromanil (1.0 g, 2.36 mmol) and
imidine Q 2 (3.44 g, 9.44 mmol) in acetonitrile (50 mL) was heated at
110
o
C for 24 hours. After the completion of the reaction, the mixture
was cooled down to r.t. The brown-purple precipitate was collected by
vacuum filtration and the solid was washed by diethyl ether. A dried solid yielded 1.1 g (47% yield).
1
H NMR (400 MHz, DMSO-d 6) δ 10.74 (s, 2H), 7.62 (t, J = 7.8 Hz, 4H), 7.46 (d, J = 7.8 Hz, 9H), 3.06
(hept, J = 6.2, 5.6 Hz, 8H), 1.09 (dd, J = 8.2, 6.7 Hz, 48H).
13
C NMR (101 MHz, dmso) δ 163.57, 145.64,
144.62, 132.70, 132.50, 128.47, 125.51, 28.60, 25.36, 23.89.

126

6.1.5.2 NMR Spectra

127

128

129

130

6.2 APPENDIX B: A case study of a bis(amide) NHC binuclear complex featuring
diketopyrrolopyrrole

6.2.1 Results and Discussion
6.2.1.1 Synthesis
Ligand D [3,6-Di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione] was synthesized according
to literature,
12
whereas IPr-CuCl is commercially available. The synthesis of the complex was carried out
in a dry N 2 atmosphere. D was deprotonated in the presence of sodium tert-butoxide in a THF solution for
two hours before adding IPr-CuCl. After 24 hours, the completed reaction mixture was filtered through a
pad of celite and concentrated to a minimum volume. The complex D(CuIPr) 2 was precipitated out by
adding hexane to obtain 65% yield (Figure B.1). It is essential to mention that the starting ligand D is poorly
soluble in THF (and other common solvents except DMSO and DMF); therefore, excess amount of solvent
is important to facilitate the deprotonation reaction.

Figure B.1: Synthesis of the dinuclear complex: D(CuIPr)2.

6.2.1.2 Theoretical calculations
The optimized structure of D(CuIPr) 2 was obtained by performing DFT calculations at B3LYP/LACVP**
level of theory.
11
The calculated HOMO and LUMO are all localized on the D ligand with negligible
contribution from the metal and absent contribution of IPr (isovalue = 0.09). These orbitals’ distribution
signals D-localized electronic transitions (LE). This agrees with the inherently high energy of the IPr

131

carbene’s LUMO, making the frontier orbitals of the binuclear complex to dominantly reside on the D
moiety. Unlike other reported amide-metal-carbene TADF phosphors,
2-5
IPr units are orthogonal to the D
core, and hence interligand charge transfer would be minimized even if it was allowed. The orthogonality
is more likely due to the side thiophene groups causing strong steric hindrance. The Davydov model is
therefore expected not to be applicable in this system since no strong opposing transition dipole moments
are present.

Figure B.2: HOMO and LUMO of optimized structure of D(CuiPr)2 from DFT calculations at B3LYP/LACVP** level of
theory. Isovalue = 0.09.

6.2.1.3 Photophysical properties
Absorption and emission spectra of the binuclear complex D(CuIPr) 2 is compared to the protonated free
ligand D in MeTHF (Figure B.3). The ligand absorption and emission maxima are at 526 nm and 534 nm,
respectively, whereas those of the complex are bathochromically shifted (593 nm and 606 nm,
respectively). The line-shapes of the ligand are very much similar to those of the complex, which signifies
that electronic transitions in both cases are born out of the same moiety, D. The redshift from free ligand to
binuclear complex is an indication of complexation the metal and D ligand. The emission color change is
also visible to the naked eye as shown by emission photos under black light (D is green-yellowish and
D(CuIPr) 2 is orange in MeTHF). The bimetallic complex retains the small stokes shift inherited from its

132

free ligand, reconfirming the D-localized electronic transitions. The photophysical properties remain the
same in all media regardless of polarity.
The photoluminescence quantum yield is 45% with a lifetime of 6 ns and radiative rate of 7.5 x 10
7
/s. These
are characteristics of conventional fluorescence emissions. The spin-orbit coupling supplied by Cu ion is
not sufficient to turn on phosphorescence. Also, it is noteworthy to point out that the quantum yield of
D(CuIPr) 2 is relatively high compared to other traditional Cu(I) complexes which are commonly known
to have poor emission efficiencies in fluids due to distortions in excited states.
13
This agrees with the
afore-discussed theoretical prediction that the metal is not participating in the frontier orbitals or in the
natural transition orbitals of involved lowest excited states.

Figure B.3: Absorption and emission spectra of ligand D vs. complex D(CuIPr)2 in MeTHF. Emission photos of ligand
(green-yellowish) and complex (orange) under black light are provided.
6.2.2 Conclusion
The investigated bimetallic complex does not meet the requirement for Davydov splitting and dipole
interactions application. Photophysical properties are ligand-centered because the carbene (IPr-Cu) has
relatively high LUMO, and hence does not participate in the NTOs of lowest excited states. DFT
calculations agree well with the measured photophysical properties. For future work, carbenes with deeper
LUMO energies, such as DAC-CuCl
4
, would be recommended for this particular donor ligand to design a
400 500 600 700
0.0
0.4
0.8
1.2
Norm. intensity (a.u)
Wavelength (nm)
D, Abs
D, Em
D(CuIPr)
2
, Abs
D(CuIPr)
2
, Em
 = 0.45
 = 6.0 ns
k
r
= 7.5 x10
7
/s
k
nr
= 9.2 x10
7
/s
D(CuIPr)
2

133

binuclear molecule that would undergo interligand charge-transfer. Although diketopyrrolopyrrole was
chosen for testing the Davydov model, it is however not ideal for OLEDs application because it is of a too
narrow band gap for display applications. In fact, D is a commonly used building block for OPV acceptor
materials. Further investigation into this topic will be continued by Mark Thompson’s research.
6.2.3 Experimental
Synthesis of D(CuiPr)2
D(CuIPr) 2: Under a dry nitrogen atmosphere, sodium tert-butoxide (33 mg, 343µmol) was added to a THF
(30 mL) solution of D ligand (50mg, 166.47µmol) and was stirred at room temperature for 2 hours.
IPr-CuCl (166.4 mg, 341.27µmol) was added into the reaction flask in one portion and the mixture was left
stirring under inert gas at room temperature for another 24 hours. The solution was filtered through a plug
of celite and concentrated to ~ 4 mL. Dry hexane was added to precipitate out a dark red solid which was
collect by vacuum filtration and washed by more hexane and methanol to yield 130 mg, 65%).
1
H NMR
(400 MHz, Acetone-d6) δ 8.47 (dd, J = 3.5, 1.4 Hz, 2H), 7.66 (s, 4H), 7.50 – 7.46 (t, 4H), 7.33 (d, J = 7.8
Hz, 8H), 6.87 (qd, J = 5.0, 2.5 Hz, 4H), 2.76 (hept, J = 6.9 Hz, 8H), 1.25 (dd, J = 14.7, 6.9 Hz, 48H).
134

6.2.4 References

1. Schinabeck, A.; Chen, J.; Kang, L.; Teng, T.; Homeier, H. H. H.; Suleymanova, A. F.; Shafikov,
M. Z.; Yu, R.; Lu, C.-Z.; Yersin, H., Symmetry-Based Design Strategy for Unprecedentedly Fast Decaying
Thermally Activated Delayed Fluorescence (TADF). Application to Dinuclear Cu(I) Compounds.
Chemistry of Materials 2019, 31 (12), 4392-4404.
2. Hamze, R.; Idris, M.; Muthiah Ravinson, D. S.; Jung, M. C.; Haiges, R.; Djurovich, P. I.;
Thompson, M. E., Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes
Bearing Bulky NHC Benzimidazolyl Carbene. Frontiers in Chemistry 2020, 8 (401).
3. Hamze, R.; Shi, S.; Kapper, S. C.; Muthiah Ravinson, D. S.; Estergreen, L.; Jung, M.-C.; Tadle, A.
C.; Haiges, R.; Djurovich, P. I.; Peltier, J. L.; Jazzar, R.; Bertrand, G.; Bradforth, S. E.; Thompson, M. E.,
“Quick-Silver” from a Systematic Study of Highly Luminescent, Two-Coordinate, d10 Coinage Metal
Complexes. Journal of the American Chemical Society 2019, 141 (21), 8616-8626.
4. Shi, S.; Jung, M. C.; Coburn, C.; Tadle, A.; Sylvinson M. R, D.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E., Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I)
Complexes Featuring Nonconventional N-Heterocyclic Carbenes. Journal of the American Chemical
Society 2019, 141 (8), 3576-3588.
5. Hamze, R.; Peltier, J. L.; Sylvinson, D.; Jung, M.; Cardenas, J.; Haiges, R.; Soleilhavoup, M.;
Jazzar, R.; Djurovich, P. I.; Bertrand, G.; Thompson, M. E., Eliminating nonradiative decay in Cu(I)
emitters: >99% quantum efficiency and microsecond lifetime. Science 2019, 363 (6427), 601-606.
6. Boydston, A. J.; Pecinovsky, C. S.; Chao, S. T.; Bielawski, C. W., Phase-tunable fluorophores
based upon benzobis(imidazolium) salts. Journal of the American Chemical Society 2007, 129 (47), 14550-
+.
7. Tennyson, A. G.; Ono, R. J.; Hudnall, T. W.; Khramov, D. M.; Er, J. A. V.; Kamplain, J. W.;
Lynch, V. M.; Sessler, J. L.; Bielawski, C. W., Quinobis(imidazolylidene): Synthesis and Study of an
Electron-Configurable Bis(N-Heterocyclic Carbene) and Its Bimetallic Complexes. Chemistry-a European
Journal 2010, 16 (1), 304-315.
8. Khramov, D. M.; Boydston, A. J.; Bielawski, C. W., Highly efficient synthesis and solid-state
characterization of 1,2,4,5-tetrakis(alkyl- and arylamino)benzenes and cyclization to their respective
benzobis(imidazolium) salts. Organic Letters 2006, 8 (9), 1831-1834.
9. Khramov, D. M.; Boydston, A. J.; Bielawski, C. W., Synthesis and study of Janus bis(carbene)s
and their transition-metal complexes. Angewandte Chemie-International Edition 2006, 45 (37), 6186-6189.
10. Su, Y. T.; Zhao, Y. X.; Gao, J.; Dong, Q. S.; Wu, B.; Yang, X. J., Alkali Metal and Zinc Complexes
of a Bridging 2,5-Diamino-1,4-Benzoquinonediimine Ligand. Inorganic Chemistry 2012, 51 (10), 5889-
5896.
135

11. Shao, Y. H.; Gan, Z. T.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange, A.
W.; Behn, A.; Deng, J.; Feng, X. T.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.; Kaliman, I.;
Khaliullin, R. Z.; Kus, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard, R. M.; Rohrdanz, M.
A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.; Zuev, D.; Albrecht, B.; Alguire,
E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.; Brandhorst, K.; Bravaya, K. B.; Brown, S. T.;
Casanova, D.; Chang, C. M.; Chen, Y. Q.; Chien, S. H.; Closser, K. D.; Crittenden, D. L.; Diedenhofen,
M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.; Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.; Ghysels, A.; Golubeva-
Zadorozhnaya, A.; Gomes, J.; Hanson-Heine, M. W. D.; Harbach, P. H. P.; Hauser, A. W.; Hohenstein, E.
G.; Holden, Z. C.; Jagau, T. C.; Ji, H. J.; Kaduk, B.; Khistyaev, K.; Kim, J.; King, R. A.; Klunzinger, P.;
Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.; Lawler, K. V.; Levchenko, S.
V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar, P.; Manzer, S. F.; Mao, S. P.;
Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.; Neuscamman, E.; Oana, C. M.; Olivares-
Amaya, R.; O'Neill, D. P.; Parkhill, J. A.; Perrine, T. M.; Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.;
Russ, N. J.; Sharada, S. M.; Sharma, S.; Small, D. W.; Sodt, A.; Stein, T.; Stuck, D.; Su, Y. C.; Thom, A.
J. W.; Tsuchimochi, T.; Vanovschi, V.; Vogt, L.; Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel, J.; White,
A.; Williams, C. F.; Yang, J.; Yeganeh, S.; Yost, S. R.; You, Z. Q.; Zhang, I. Y.; Zhang, X.; Zhao, Y.;
Brooks, B. R.; Chan, G. K. L.; Chipman, D. M.; Cramer, C. J.; Goddard, W. A.; Gordon, M. S.; Hehre, W.
J.; Klamt, A.; Schaefer, H. F.; Schmidt, M. W.; Sherrill, C. D.; Truhlar, D. G.; Warshel, A.; Xu, X.; Aspuru-
Guzik, A.; Baer, R.; Bell, A. T.; Besley, N. A.; Chai, J. D.; Dreuw, A.; Dunietz, B. D.; Furlani, T. R.;
Gwaltney, S. R.; Hsu, C. P.; Jung, Y. S.; Kong, J.; Lambrecht, D. S.; Liang, W. Z.; Ochsenfeld, C.;
Rassolov, V. A.; Slipchenko, L. V.; Subotnik, J. E.; Van Voorhis, T.; Herbert, J. M.; Krylov, A. I.; Gill, P.
M. W.; Head-Gordon, M., Advances in molecular quantum chemistry contained in the Q-Chem 4 program
package. Molecular Physics 2015, 113 (2), 184-215.
12. Chen, Z.; Li, M.; Hu, M.; Wang, S.; Miao, Z.; Xu, S.; Chen, C.; Dong, H.; Huang, W.; Chen, R.,
All-acceptor polymers with noncovalent interactions for efficient ambipolar transistors. Journal of
Materials Chemistry C 2020, 8 (6), 2094-2101.
13. Krylova, V. A.; Djurovich, P. I.; Conley, B. L.; Haiges, R.; Whited, M. T.; Williams, T. J.;
Thompson, M. E., Control of emission colour with N-heterocyclic carbene (NHC) ligands in
phosphorescent three-coordinate Cu(i) complexes. Chemical Communications 2014, 50 (54), 7176-7179.

Abstract (if available)

Abstract The global energy demand is increasing at an alarming rate amid the fast growing world of electronics. In the light of this concern, Professor Mark Thompson’s research group uses chemistry and engineering to develop new materials for optoelectronic devices, particularly organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). This dissertation embodies a great contribution for these two technologies whose fundamentals and overview are introduced in Chapter I. As OPV technologies await commercialization, challenges of expensive and inefficient materials—compared to inorganic PV counterparts—need to be tackled. One of the key components of an OPV device that have hampered high performance is an electron acceptor layer, which has been relying on using expensive and unstable fullerene derivatives for quite long. In this regard, we disclose (in Chapter II) a one-pot synthesis of non-fullerene n-type materials of a simple molecular structure: tetra-aza-pentacenes. Presented in Chapter III are chromophores of Zinc(II) complexes bearing acridinol/phenazinol ligands. The study of these complexes shed light on further understanding and future design of active materials for OPVs and OLEDs. The last Chapters (IV, V, and Appendix) are dedicated to thermally activated delayed fluorescence (TADF) complexes of (amide)-metal-(carbene) structure. In these two coordinated N-heterocyclic carbene (NHC) TADF phosphors, we have explored phenanthrocarbazolyl amide-based complexes (Chapter IV), phenanthrimidazolyl-containing complexes (Chapter V) , and lastly, binuclear NHC complexes featuring benzobis(imidazolylidene) Janus type carbenes, as well as bis(amide) donors. The latter families of emitters aim to circumvent the conundrum of long-lived excitons, or slow radiative rates, in existing OLED emitters which are detrimental to devices’ lifetime and efficiency. The work presented herein is indeed of a great contribution to the future research and technology advancements in the field of (opto)electronic devices. Further investigation into these families of materials is continuing in Prof Thompson’s research group.

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Asset Metadata

Creator Ukwitegetse, Narcisse (author)

Core Title Development of N-type chromophores for organic photovoltaics, and thermally activated delayed fluorescence NHC complexes for organic light-emitting diodes

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 04/26/2021

Defense Date 09/29/2020

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

Tag azaacenes,non-fullerene acceptors,OAI-PMH Harvest,OLED emitting materials,organic light-emitting diodes,organic photovoltaics,organic semiconductors,pentacene,phosphorescence,TADF

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark Edward (committee chair)

Creator Email ukwitege@gmail.com,unarcisse@yahoo.ca

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