University of Southern California Dissertations and Theses

Molecular design strategies for blue organic light emitting diodes

(USC Thesis Other)

Molecular design strategies for blue organic light emitting diodes

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content Molecular Design Strategies for
Blue Organic Light Emitting Diodes (OLEDs)
by
Jie Ma
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
(MATERIALS SCIENCE)
August 2023
Copyright 2023 Jie Ma
ii

Dedication

This dissertation is dedicated to my parents:
Lailin Ma and Mei Yang

iii

Acknowledgements
I would like to express my deepest appreciation to Prof. Mark Thompson, my supervisor,
for his exceptional guidance, unwavering support, and constant encouragement throughout my
research. I am grateful to have been offered a room at the MET lab, and my journey here has
been immensely rewarding thanks to the unparalleled expertise and invaluable insights of Prof.
Mark Thompson. Moreover, I am grateful for the nurturing environment that he provided, which
enabled me to develop as an independent scientist.
Furthermore, I would like to extend my sincere gratitude to my committee members, as
well as my prior qualifying committee members, namely Prof. Andrea Armani, Prof. Steve Nutt,
Prof. Barry Thompson, Prof. Jayakanth Ravichandran, and Prof. Stephen Cronin, for their
valuable time and invaluable contributions. Their insightful feedback and constructive criticism
have been instrumental in shaping my research and helping me achieve my academic goals.
I would like to express my gratitude to Prof. Peter Djurovich for his exceptional expertise,
profound insights, and encouragement in deepening my understanding of scientific concepts and
tackling challenging questions. He is like a walking science-GPT in the lab. Whenever I want to
engage in a no-time-limit one-on-one science lecture, I always find myself drawn to his office. I
am grateful for his generosity in sharing his time, knowledge, and sometimes sweet fruits.
Furthermore, I would like to extend my sincere gratitude to all the professors who have taught
me during my graduate and undergraduate studies. Each one of them has played a significant role
in shaping my academic and professional growth. Special thanks go to Prof. Anupam Madhukar,
who taught me how to simplify complex scientific concepts and present them in a clear and
concise manner, helped me develop critical thinking skills and nurtured my addition to Italian
roasted coffee.
I would also like to express my gratitude for the opportunity to collaborate with Prof.
Stephen Forrest and his student, Haonan Chen, Xinjing Huang, Dr. Yongxi Li and Dr. Jongchan
Kim during my doctoral studies. Their invaluable contributions were essential to the successful
completion of my research. I am also grateful for the joyful collaboration with Dr. Francisco
Rodella, who impressively adhered to our scheduling times. He attributed this skill to his
education in Germany. Furthermore, I would like to extend my appreciation to Prof. Barry
Thompson and Qingpei Wan for their help on the solution thin films preparation, to Allan
Kershaw for the temperature-dependent NMR training, to Phil at USC glass shop for his constant
effort in sealing my broken glassware, to Joe for his exceptional technical expertise in fixing
broken instruments, and to unwavering administrative support from Michele Dea, Andy Chen,
and Anthony Tritto.
I am also grateful for the supportive work environment provided by my colleagues at the
MET labs, which helped ease the stress of graduate school. I want to express my gratitude to
Jonas Schaab, who worked a lot with me on both the binuclear and device projects. His
enthusiastic attitude, along with his powerful voice, motivated us to overcome many challenges
iv

during difficult times. I am also thankful for Dr. Narcisse Ukwitegetse, who previously worked
on the binuclear project and his optimistic spirit that always encouraged me. James Fortwengler
was an instrumental and enthusiastic collaborator on my new favorite MRTADF project. I am
truly grateful for his contributions and dedication to the project. Dr. Tyler Fleetham inspired me
to start on the host work, and Dr. Anton Razgoniaev gave me a fluffy host material, and the
contributions from Konstantin Mallon, Junru Su, and Darius Shariaty who generously offered me
a wealth of host materials to work with, far more than I could possibly finish in the last year.
Plus, I would like to thank Konstantin Mallon for his valuable and not-so-valuable suggestions
on synthesis, Junru Su has been an amazing friend, sharing many common hobbies with me,
including but not limited to organic light emitting diodes, discussing beauty tips, and cycling
between overeating and overexercising, and Darius Shariaty for cute dishwashing sponges. I
would like to thank Dr. Savannah Kapper, Aamani Ponnekanti and Luna were helpful in my
thiazole project, and I would like to express my gratitude to Dr. Marsel Shafikov BECAUSE his
unwavering patience in providing insightful explanations on photophysics and for adding a touch
of humor with his jokes, and Dr. Eric McClure for his invaluable support in cluster calculations,
despite my tendency to forget commands. I would like to express my thanks to my hood/office
desk neighbor, Austin Menke, for his unwavering patience and willingness to explain scientific
and non-scientific concepts to me as much as his occasional loan of the flask light, and to Dr.
Collin Muniz, his sweet flower plant and with whom I engaged in numerous insightful
discussions on cMa complexes, and to Mattia Di Niro, his invaluable efforts on CHNS
measurements, and Nina Baluyot-Reyes for her efforts in promoting binuclear complexes at
NREL. I want to extend a special thanks to Dr. Sunil Kumar Kandappa, Frances Yau, Gemma
Goh, Megan Cassingham, Mahsa Rezaiyan, Kelly Biv, and Ao Jiao for being a great team
member at USC. Lastly at MET, I am deeply appreciative of the invaluable administrative help
and support provided by Judy Hom. I would like to express my sincere thanks to her for
everything.
I would want to thank the previous MET members, my previous hood-mate and close
friend, Dr. Abegail Tadle, who also helped me start in the lab as well. When I was working with
her, I never had to worry about not being able to open any lids in the lab, and to Dr. Shuyang Shi,
for engaging in fruitful scientific discussion especially for cMa complexes with me and other
topics such as gossip, and to Dr. Muazzam Idris, who helped me get started in the host project
and who always claimed I was his best friend in the lab, although he moved me down to the third
name in the acknowledgement tier, and to Dr. Tianyi Li for his active involvement in fruitful
experiments discussions and encouragement by his working efficiency, to Dr. Daniel Sylvinson
for his dedicated efforts in teaching me computational modeling and for engaging in insightful
discussions on a wide range of scientific topics, and to Dr. Rasha Hamze's work on cMa
compelxes laid a foundation for me to continue my work on it, and to Dr. Moon Chul Jung was
instrumental in making OLEDs. I want to extend a special thanks to Dr. Thilini Batagoda, Dr.
Niki Bayat, Dr. Patrick Saris, Dr. Jessica Golden and Dr. Becky Wilson.
v

I am also thankful for all my previous collogues at AMI especially Dr. Jonathan Lasch,
Dr. Cesar Blanco, Winn Hong, Nan Chen who is a very close friend (and a life GPT) from my
undergraduate years, Dr. David Tyvoll, Dr, Fikret Kirkbir, Susan Cooper and William Hacker. I
am truly appreciative of their friendship and mentorship throughout my professional journey.
Finally, I would like to express my deepest gratitude to my parents, Lailin Ma and Mei
Yang, my uncle Chaoyang Jiang, and my aunt Hong Yang, who have been my constant source of
support and motivation throughout my graduate school journey. Their unwavering love and
encouragement have given me the strength to overcome the challenges I faced. I would also like
to thank my boyfriend, Dr. Zeyu Chen, for his Schrodinger’s support and absence in the past
three years due to the pandemic, which greatly leveled up my productivity in the lab.
Furthermore, I am deeply grateful to Dr. Xin Miao, who has been a great source of inspiration
throughout my graduate life. I would also like to express my appreciation to Dr. Mao Wang, who
has been a caring and supportive friend who drove to comfort me when I needed. Additionally, I
would like to thank Dr. Jiefei Zhang for being a role model to me, as well as Wei Wang,
Menglan Jiang, Yufei Hu, Shiyue Peng and Dr. Xi Wang, my best college friends. I am also
grateful to Dan Wu and Dr. Xinyi Lu, my childhood friends who have always been welcoming
hosts in Seattle. Lastly, I would like to thank Tong Tong, Yin Hua, Xichao Wang, Xinyu Yan,
Dr. Shanyuan Niu, Dr. Mengzhe Wang, Dr. Xuan Cao, Dr. Lang Shen, Dr. Amanda Kate, Dr.
Haotian Shi, Dr. Huandong Chen, and Dr. FengCan Bao for keeping in touch throughout my
time in graduate school. Their friendship and support have meant the world to me.

vi

Table of Contents

Dedication ………………………………………………………………………………………. ii
Acknowledgements …………………………………………………………………………….. iii
List of Tables ………………………………………………………………………………….. viii
List of Figures …………………………………………………………………………………... ix
Abstract …………………………………………………………………………………………. xi
Chapter 1 Introduction .................................................................................................................... 1
1.1 Organic semiconductors ....................................................................................................... 1
1.2 OLEDs development and status ............................................................................................ 2
1.3 OLEDs degradation .............................................................................................................. 5
1.4 TADF molecules in OLEDs ................................................................................................. 7
1.5 References ........................................................................................................................... 13
Chapter 2 Symmetric “Double Spiro” Hosts for Blue Phosphorescent OLED Devices .............. 18
2.1 Introduction ......................................................................................................................... 18
2.2 Results and Discussion ....................................................................................................... 21
2.2.1 Synthesis and structures ............................................................................................... 21
2.2.2 Photophysical, electrochemical and thermal properties............................................... 23
2.2.3 Electroluminescent properties ...................................................................................... 26
2.3 Conclusion .......................................................................................................................... 39
2.4 Experimental Methods ........................................................................................................ 40
2.5 References ........................................................................................................................... 45
Chapter 3 Dynamics of Rotation in Thiazolyl Copper (I) Carbazolyl Complexes ....................... 51
3.1 Introduction ......................................................................................................................... 51
3.2 Results and Discussion ....................................................................................................... 53
3.2.1 Synthesis ...................................................................................................................... 53
3.2.2 Crystal Structure .......................................................................................................... 54
3.2.3 NMR studies ................................................................................................................ 56
3.2.4 Computational Studies ................................................................................................. 58
3.2.5 Photophysical properties .............................................................................................. 60
3.3 Conclusion .......................................................................................................................... 65
3.4 Experimental Methods ........................................................................................................ 65
3.5 References ........................................................................................................................... 69
vii

Chapter 4 Luminescent Binuclear Gold (I) Complexes Utilizing Janus Carbenes ....................... 73
4.1 Introduction ......................................................................................................................... 73
4.2 Results and Discussion ....................................................................................................... 75
4.2.1 Synthesis ...................................................................................................................... 75
4.2.2 Crystallographic Analysis ............................................................................................ 77
4.2.3 Computational Results ................................................................................................. 78
4.2.4 Electrochemistry .......................................................................................................... 84
4.2.5 Photophysical properties .............................................................................................. 86
4.2.6 OLED Devices ............................................................................................................. 97
4.3 Conclusion ........................................................................................................................ 102
4.4 Experimental Methods ...................................................................................................... 102
4.5 References ......................................................................................................................... 107

viii

List of Tables

Table 1.1 Several properties of organic and inorganic semiconductor ........................................... 2
Table 2.1 Summary of properties of SAS and XAX .................................................................... 26
Table 2.2 Summary of photoluminescence properties for Ir(tpz)3 doped into ............................. 28
Table 2.3 OLED performance parameters for Ir(tpz)3 based OLEDs.
a)
....................................... 39
Table 3.1 Selected geometric data from X-ray single crystal measurements. .............................. 55
Table 3.2 Summary of photophysical properties of complexes 1-H, 1-Me and 1-iPr .................. 63
Table 3.3 Photophysical data of 1-Ph in 2-MeTHF, MeCy, and 1 wt% in PS film...................... 68
Table 4.1 Selected crystal data. ..................................................................................................... 78
Table 4.2 Electrochemical data. .................................................................................................... 85
Table 4.3 Photophysical data for mono- and binuclear cMa complexes ...................................... 92
Table 4.4 Energy and rate data from variable temperature ........................................................... 95
Table 4.5 Center of h
+
,e
-
for S1 NTOs for cMa complexes. ......................................................... 96
Table 4.6 Photoluminenscence data of binuclear complexes in different host materials. ............ 97
Table 4.7 Performance parameters for BCzAu
BBI
AuBCz and BCzAu
BAZ
AuBCz based OLEDs ....... 102
Table 4.8 Photophysical Data for CzAu
BAZ
AuCz based compounds ............................................ 106

ix

List of Figures

Figure 1.1 Configuration of the first OLED (taken from reference
6
). ............................................ 3
Figure 1.2 PL spectra of Ir phosphors in 2Me-THF (taken from reference
12
). ............................... 4
Figure 1.3 Simplified illustration of an OLED stack (taken from reference
21
). ............................. 5
Figure 1.4 The processes of EL in an OLED (taken from reference
21
). ......................................... 6
Figure 1.5 TTA and TPA mechanism (taken from reference
27
). .................................................... 7
Figure 1.6 Emission decay time versus the zero-field splitting (taken from reference
28
). ............. 8
Figure 1.7 Selected organic TADF molecules (adapted from reference
36
). ................................... 9
Figure 1.8 Selected cMa complexes (taken from reference
32
). ..................................................... 10
Figure 1.9 Simplified kinetic scheme for emission via TADF (taken from reference
45
). ............ 11
Figure 2.1 Commonly used rigid aromatic moieties for high triplet host materials.
40
................. 19
Figure 2.2 Crystal structures of SAS and XAX with thermal ellipsoids at 50%. ......................... 22
Figure 2.3 Crystal packing of SAS (left) and XAX (right)........................................................... 22
Figure 2.4 Frontier molecular orbitals and triplet spin density calculated for SAS and XAX. .... 23
Figure 2.5 Absorption and emission spectra of SAS and XAX. ................................................... 24
Figure 2.6 CV (black) and DPV curves (red: oxidation, blue: reduction) of compounds. ........... 25
Figure 2.7 TGA curves (scan rate: 10K/min) of SAS and XAX. ................................................. 26
Figure 2.8 Normalized PL spectra of blend films of SAS or XAX hosts. .................................... 27
Figure 2.9 Doping concentration-controlled devices with SAS host material. ............................. 29
Figure 2.10 Doping concentration-controlled devices with XAX host material. ........................ 30
Figure 2.11 Doping concentration-controlled devices with mCBP host material. ....................... 31
Figure 2.12 TAPC thickness-controlled devices with SAS host material. ................................... 33
Figure 2.13 TmPyPb thickness-controlled devices with SAS host material. ............................... 34
Figure 2.14 TAPC thickness-controlled devices with XAX host material. .................................. 35
Figure 2.15 TmPyPb thickness-controlled devices with XAX host material. .............................. 36
Figure 2.16 OLED device characteristics of SAS, XAX and mCBP. ........................................... 38
Figure 2.17 CV (black) and DPV curves (red: oxi, blue: red) of decamethylferrocene ............... 42
Figure 3.1 (left) Recently reported (carbene)Metal(carbazolyl) emitters; .................................... 53
Figure 3.2 Single crystal X-ray structure of complexes 1-H, 1-Me and 1-iPr with thermal
ellipsoids at 50%. Hydrogens were omitted for clarity. ................................................................ 54
Figure 3.3 Crystal packing of 1-Me (left) and 1-iPr (right) .......................................................... 55
Figure 3.4 (top) (Thia)Cu(X-Cz) with protons labelled; .............................................................. 57
Figure 3.5 Calculated
1
H NMR spectra for the anti- and syn-conformers of 1-iPr. ..................... 58
Figure 3.6 (top) HOMO (solid) and LUMO (mesh) orbitals of complexes .................................. 59
Figure 3.7 (top) Space-filling diagrams of (Thia)Cu(XCz) complexes ........................................ 60
Figure 3.8 (a) Molar absorptivity of (Thia)Cu(X-Cz) complexes in toluene. .............................. 62
Figure 3.9 Emission spectra of (Thia)Cu(X-Cz) complexes ........................................................ 64
Figure 3.10 Emissions of 1-H in MeCy normalized to 425 nm carbazolyl peak.......................... 64
Figure 3.11 Emission spectra of 1-Ph in 2-MeTHF, MeCy, and 1 wt% PS. ................................ 68
Figure 4.1 Crystal structures of (a) BCzAu
BAZ
AuBCz and (b) bimAu
BAZ
Aubim ................................. 78
Figure 4.2 Top: frontier molecular orbitals for CzAu
BBI¢
AuCz (left) and CzAu
BZI¢
(right). ............ 79
Figure 4.3 The NTOs for wavefunctions of CzAu
BZI¢
(left) and CzAu
BBI¢
AuCz (right). ................. 80
x

Figure 4.4 Potential energy surface scan of complexes. ............................................................... 82
Figure 4.5 Calculated S1, S2 (with oscillator strength) and T1, T2 energies .................................. 83
Figure 4.6 CV (black) and DPV (red: oxidation, blue: reduction) traces of complexes ............... 85
Figure 4.7 Absorption and emission spectra of mono- and binuclear cMa complexes ................ 87
Figure 4.8 Absorption and emission spectra of BCzAu
BAZ
AuBCz in various solvents. ................... 90
Figure 4.9 OLED Devices with 20% BCzAu
BBI
AuBCz in TCTA as host material. ......................... 98
Figure 4.10 Doping concentration-contolled OLED devices with BCZAu
BBI
AuBCz ...................... 99
Figure 4.11 OLED device characteristics of BCzAu
BBI
AuBCz and BCzAu
BAZ
AuBCz OLEDs. ....... 101
Figure 4.12 Absorption and Emission Spectra of CzAu
BAZ
AuCz ................................................. 106

xi

Abstract
Conventional organometallic complexes, such as Os(II), Pt(II), and Ir(III), have found
widespread application in various fields, including photocatalysis, organic light-emitting diodes
(OLEDs), and biosensors. Among these complexes, Ir-based ones have made significant inroads
in the commercialization of OLEDs, serving as phosphorescent emitters. However, a long-
standing issue with blue phosphorescent emitters is their short lifespan in devices, commonly
referred to as the "blue gap" problem in OLEDs. The blue gap is one of the most challenging
problems in OLEDs because the devices comprise several tens to hundreds of materials in layers,
and each layer could contribute to their decomposition. To simplify this question, this work
focuses on the most vulnerable layer, which is the emissive layer (EML), comprising host and
emitter materials. Here, the design of these materials from a molecular level is discussed, with
the ultimate goal of extending the lifespan of blue phosphorescent emitters in OLEDs.
Chapter 1 provides a brief introduction to OLEDs and discusses the degradation
mechanisms responsible for the short lifespan of blue phosphors in devices, which serves as the
foundation for molecule design in this work. The chapter then delves into one type of emitter
used in OLEDs, namely thermally activated delay fluorescence (TADF) molecules, highlighting
their differences compared to the common phosphors based on Pt(II) and Ir(III). The kinetic
scheme and decay rates of TADF molecules are also briefly discussed to provide a physical
understanding of the design of emitters. Overall, Chapter 1 lays the groundwork for the
subsequent chapters, providing a foundation for the design and optimization of efficient and
stable materials for blue OLEDs.
Chapter 2 focuses on the design of robust host materials with high triplet energy to
suppress degradation in blue OLEDs. The chapter discusses the importance of strong bonds and
minimized conjugation in designing hosts, especially in discussing the moieties that can be
chosen as the backbone for hosts and how to link them together to avoid triplet decrease. Two
wide energy gap hosts, free of weak bonds, are introduced, with large energy gaps (≥5.0 eV)
between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO), and correspondingly high triplet energies in the solid state (ET ~ 3.0 eV). The
chapter also includes an analysis of devices using these host materials with a blue
xii

phosphorescent dopant, showing that charges are transported and trapped by the dopant, which
subsequently form excitons directly on the phosphor. This suppresses luminescence quenching
pathways, leading to blue phosphorescent devices with high (~18%) external quantum efficiency.
Overall, Chapter 2 provides a detailed exploration of the design of robust host materials, paving
the way for the development of efficient and stable blue OLEDs.
Another approach to limit degradation is to design TADF dopants with high luminescent
efficiency and fast radiative rates. If the dopant can efficiently undergo radiative decay into the
ground state before engaging in bimolecular processes, the damage caused by “hot states” can be
suppressed. Chapter 3 focuses on how to suppress the non-radiative decay of TADF dopants,
while Chapter 4 focuses on designing dopants with high efficiency and fast radiative lifetimes.
In Chapter 3, three two-coordinate Cu(I) complexes with carbazolyl ligands featuring
substituents of varying steric bulk ortho to N are investigated. The impact of these substituents
on the luminescence energies of the complexes is negligible, but they serve to modulate the
rotation barriers along the metal-ligand coordinate bond. The geometric arrangement of ligands
in complexes with alkyl substituents was found to differ, with syn conformers in the solid state
versus anti in solution, as revealed by crystallographic analysis and nuclear magnetic resonance
spectroscopy, respectively. Potential energy surface scan calculations were also performed on
different conformations of the three complexes to provide a theoretical evaluation of the rotation
barriers around the metal-ligand bond axis. The study demonstrates that rates for nonradiative
decay decrease with increasing bulk of the substituents on the carbazolyl ligand, indicating the
potential to suppress non-radiative decay in TADF dopants through steric modulation.
Chapter 4 introduces a binuclear model for designing TADF emitters with high efficiency
and fast radiative decay lifetimes. The chapter describes the synthesis and analysis of a series of
binuclear carbene-metal-amide (cMa) complexes with bridging biscarbene ligands. The
complexes exhibit solvation-dependent absorption and emission, with the molar absorptivity of
the binuclear complexes found to be correlated with the energy barrier to rotation of the metal-
ligand bond. The binuclear cMa complexes also exhibit short emission lifetimes ( = 200-300 ns)
with high photoluminescence efficiencies (ΦPL > 95%). The radiative rates of binuclear cMa
complexes are 3-4 times faster than those of the corresponding mononuclear complexes.
xiii

Analysis of temperature-dependent luminescence data shows that the lifetime for the singlet state
(τS1) of binuclear cMa complexes is around 12 ns with a singlet-triplet splitting (ΔEST) of
40-50 meV. These findings provide a general design strategy for cMa complexes to achieve
small values for ΔEST while retaining high radiative rates. Additionally, solution-processed
OLED devices incorporating two of the complexes as luminescent dopants exhibited low roll-off
at high luminance. This chapter highlights the potential of binuclear cMa complexes in pushing
the radiative lifetime to a few hundreds of nanosecond range, making them a promising
candidate for improving the lifespan of blue OLEDs.
1

Chapter 1 Introduction
1.1 Organic semiconductors
The definition of organic semiconductors is solids whose building blocks are -bonded
molecules or polymers made up by carbon and hydrogen atoms and at times heteroatoms such as
nitrogen, sulfur and oxygen. The molecules in organic solids are held together by weak van der
Waals force giving a random arrangement of the molecules in the bulk. Inorganic
semiconductors are made by the Ⅲ, Ⅳ, Ⅴ main group atoms such as silicon, arranged in a highly
ordered structure via covalent bonds. As a result, the electronic properties of organic and
inorganic semiconductors differ fundamentally due to the nature of their binding. The valence
electrons in the organic semiconductors are confined to individual molecules or small groups of
molecules, leading to discrete electronic states can be well approximated by the tight binding
model. In contrast, in the inorganic crystalline array, valence electrons interact with each other
and form a continuous band of electronic energies described by the band theory. Therefore, the
charges are transported through the organic solids by hopping between localized electronic states
accompanied by the formation of polarons. The mobility of charges in organic solids is about
three orders of magnitude lower than that in the inorganic solids (Table 1.1), in which charges
are transported through a continuous band. In the practical applications of organic
semiconductors, the use of thin film structure in the electronics is a common strategy to
overcome their low charge mobility.
Even though organic semiconductor is not supreme at the mobility, it outcompetes
inorganic semiconductor in other properties which make their own way to the electronics market.
Compared to inorganic semiconductors, the refractive index of organic semiconductors is low
(Table 1.1). This is an advantage when used in the OLEDs, as the light can be more efficiently
extract out from the glass and air interface compared to inorganic materials with high refractive
index. Plus, because the intermolecular force is van der Waals, the Young’s modulus of organic
semiconductors are much lower than inorganic (Table 1.1), which enables them to deposit on
curved substrates or even foldable substrates. The flexibility and low sublimation temperature
2

makes organic semiconductor compatible with manufacturing requests, which makes them
attractive in large-area electronics and flexible displays.
Table 1.1 Several properties of organic and inorganic semiconductor
Property Organic Semiconductor Inorganic Semiconductor
Bond van der Waals covalent
Electronic State discrete continuous
Charge Mobility < 1 cm
2
/Vs ~ 1000 cm
2
/Vs
Refractive Index 1.5-2 3-5
Young’s Modulus ~10 GPa 10~400 GPa

1.2 OLEDs development and status
The conversion of electrical energy into visible light dates has been an important concern
since the latter half of the 19
th
century and continues to be so today. Among the various
technologies that enable light generation, organic light emitting diodes (OLEDs) are considered
to be one of the most significant. OLEDs are widely employed in a range of applications such as
flexible, transparent, and high-resolution displays, as well as in large-area solid lighting.
1-4

The first demonstration of electroluminescence (EL) in organic molecular crystals was
carried out in 1963 by M. Pope et al. using anthracene single crystals.
5
However, it required a
high voltage above 1000 V to observe EL because anthracene single crystal was several
millimeters thick. In 1987, C. W. Tang and S. A. VanSlyke made a significant advancement by
utilizing a thin-film OLED employing a fluorescent emitter, tris(8-hydroxyquinolinato)
aluminum (Alq3), which enabled low driving voltages (as shown in Figure 1.1).
6

3

Figure 1.1 Configuration of the first OLED (taken from reference
6
).
Although the first thin-film based OLED achieved decent luminance with lowing driving
voltages, the efficiency of device is limited by the fluorescence nature of Alq 3. When electrons
and holes are injected into organic layers with random spin orientation, the formation of singlet
and triplet excitons are in a 1:3 statistical ratio. In most organic molecules, the relaxation from
the lowest triplet state (T1) to the ground state (S0) occurs slowly due to weak spin coupling.
Consequently, 75% of the excitons are lost to non-radiative pathways.
7

To maximize the internal EL efficiency, it is crucial to harvest triplet excitons through
electro-phosphorescence. In this regard, in late 1990s, Thompson and Forrest reported on the use
of heavy metal complexes, such as those containing Ir(III) and Pt(II) in OLEDs, achieving a
potential internal quantum efficiency (IQE) of 100%.
8-11
These complexes (Figure 1.2) exhibit
strong spin-orbital coupling (SOC), which results in the mixing of triplet and singlet states,
allowing the triplet states to acquire some singlet character.
12
The mixing process significantly
enhances the phosphorescent radiative decay rate of emitters to 10
4
to 10
6
s
-1
with close to unit
quantum efficiency (QE). Nowadays, phosphorescent Ir(III) complexes are extensively used as
emitting dopants in the OLED industry.
13-15

4

Figure 1.2 PL spectra of Ir phosphors in 2Me-THF (taken from reference
12
).
Even though Ir(III) complexes can harness all injected carries by fast intersystem
crossing (ISC), this efficiency improvement comes at a cost. The price paid for four-fold
increase in IQE is the extended excited state lifetime (~s) that two to three orders of magnitude
longer compared to conventional fluorescent dyes (~ns). The extended lifetime combined with
the high energy of blue phosphors (>2.7 eV) have hindered their commercialization into mass
production in display and lighting. Despite decades of research and development, blue
phosphors have yet to be made that can offer both a proper color point and stability comparable
to commercial green or red phosphors.
16-19
This hardest challenge in OLED technology is
commonly known as the “blue gap problem”, as there is a significant trade-off between the
efficiency and stability in blue phosphors-based OLEDs. Currently, commercial OLED displays
and solid-state lighting only could use fluorescent materials for blue pixels, which have a lower
IQE than green or red phosphors. Plus, our human eyes are less sensitive to blue light compared
to outher colors. To overcome these limitations, fluorescent blue pixels occupy up to 52% of the
total pixel area.
20
Therefore, if a highly efficient and long-lived blue phosphors were to be
developed, it could enable for the creation of full phosphors-based display with lower power
consumption and reduced cost.

5

1.3 OLEDs degradation
To solve the blue gap problem in OLED technology, it is essential to first understand how
OLEDs work and degradation mechanism of blue OLEDs. Generally, an OLED has multiple
functional organic layers sandwiched between two electrodes (as shown in Figure 1.3).
21
The
anode, which is located at the bottom of the device, is made of highly transparent indium tin
oxide (ITO). This material consists of a nonstoichiometeric composite of SnO2 (10-20%) and
In2O3 (90-80%). Materials used for the cathode are low work function metals like calcium or
aluminum, which are covered a glass for preventing reaction with oxygen and moisture. A thin
layer of LiF (1-3 nm) is usually deposited on to the cathode which help lowering the working
function.
22
The main organic layers between the two electrodes are the hole transport layer
(HTL), emission layer (EML) and electron transport layer (ETL). The HTL facilitates the
transport of holes through the organic layers, thus it utilizes easily oxidized moieties such as
amine groups. For example, compounds like 1,1-bis[(di-4-tolylamino) phenyl] cyclohexane
(TAPC) and N,N’-bis-(1-naphthyl)-N,N’-diphenyl benzidine (α-NPD) are usually employed in
the HTL.
23, 24
The EML is a blend of host and chromophore emitting light in a specific
wavelength. Commonly used ETL materials include Alq3, 1,3,5-tri(phenyl-2-benzimidazole)
benzene (TPBi) and 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPb) which have low reduction
potential.
25
In some cases, electron or hole blocking layers are added to the side of EML to
prevent charges leakage. Despite the multiple layers present in an OLED, the entire device has a
thickness of only a few hundred nanometers.

Figure 1.3 Simplified illustration of an OLED stack (taken from reference
21
).
The operation of an OLED can be divided into four steps as illustrated in Figure 1.4
21
:
6

i. Injection of electrons and holes at the electrodes.
ii. Transport of charge carriers through the ETL and HTL. Both charge carries are injected
from opposite electrodes under an electric field of a few volts. Charges drift toward each
other and fill the frontier orbitals.
iii. Formation of excitons in the EML. When initially free charges are close to each other,
they strongly bind together by Coulombic attraction, forming of a bound state known as
exciton.
iv. Radiative exciton decay and emission of light.

Figure 1.4 The processes of EL in an OLED (taken from reference
21
).
Even though chromophores are evenly distributed in the host of EML, excitons tend to
form in a thin slab in close proximity to an interface with an adjacent material layer. This is
because the unequal rates of electron and hole transport. The general width of the recombination
zone is less than 5 nm, resembling a delta distribution.
18, 26
Because of the high density and
microseconds lifetime of triplet excitons, two nonlinear quenching processes triplet-triplet
annihilation (TTA) and triplet-polaron annihilation (TPA) could happen (Figure 1.5).
27
TTA
occurs when two triplet excitons annihilate each other, leading to promote one of the triplets to a
higher excited state known as the “hot state”, which is larger than 5 eV in blue OLEDs. The
high energy of “hot state” either relax to the first excited state by a vibronic pathway or dumping
energy to localized bonds. The probability of TTA increases significantly at high current density
due to its dependence on the square of exciton density. TPA is another bimolecular process
where a triplet and a polaron interact to form a hot excited state that decay similarly to the “hot
7

state” in TTA. These high energy states not only quench efficiency of devices, more importantly,
the ruptured bonds act as defects that trap excitons and charges. Therefore, making robust host
materials (as discuss in Chapter 2), or by reducing the exciton density or the lifetime of triplet
excitons (as discussed in Chapter 3 and 4), it is possible to decrease the occurrence of
bimolecular processes and extend the operational lifetime of blue OLEDs.

Figure 1.5 TTA and TPA mechanism (taken from reference
27
).
1.4 TADF molecules in OLEDs
As discussed in section 1.2, the mononuclear Ir(Ⅲ) or Pt(Ⅱ) complexes shorten the
radiative lifetime of the T1 state to a range of 1 to 100 s by the heavy atom effect.
8, 9, 11, 28

However, the fastest radiative decay lifetime of these complexes has remained at 1.5 s after
decades (see Figure 1.6).
28
Hence, there is a growing interest in developing alternative types of
emitters that can break the microsecond barrier. Thermally activated delayed fluorescence
(TADF) emitters have emerged as the most successful alternatives to achieve sub-microsecond
lifetime with high luminescence efficiency.
29-32

8

Figure 1.6 Emission decay time versus the zero-field splitting (taken from reference
28
).
TADF molecules mix singlet and triplet states by T1→S1 up-conversion and subsequently
radiative decay from S1 to S0. The first observation of delayed fluorescence was in 1930s when
Boudin dissolved Eosin Y in a glycerol solution at room temperature.
33
About 40 years after,
delay fluorescence was also observed in a series of metal complexes.
34
However, TADF
molecules were rarely considered for practical application at that time due to their low efficiency
until Adachi et al. demonstrated a series of molecules with donor-acceptor design boosting
photoluminance efficiency to 94% in 2012.
35
The key to achieving this improvement is
decreasing S1/T1 gap of the molecule by adjusting the dihedral angle between the donor (D) and
acceptor (A) to spatially separate the electrons in the excited state. Since then, a plethora of
organic TADF molecules have been reported, however, the fastest τTADF for all-organic TADF
compounds is 2-5 s.
36

9

Figure 1.7 Selected organic TADF molecules (adapted from reference
36
).
On the other hand, the metal based TADF emitters have been reported in recent years
while they have been limited to copper complexes. The initial Cu(I) complexes are
photoluminescence inefficient because of the John-Teller distortion upon excitation.
37-40
The
lowest energy transition state in these Cu (I) complexes is a metal-to-ligand charge transfer
(MCLT) state which oxidizes Cu and induces large reorganization energy. Nevertheless, a
turning point was reached with the discovery of highly efficient two-coordinate linear
carbene-metal-amide (cMa) complexes by Di et al,
41
Hamze et al,
30
and Shi et al.
42
These cMa
complexes feature carbene as an acceptor and amide as a donor, connected via a metal in a linear
fashion (as shown in Figure 1.8). These complexes have demonstrated remarkable potential as
luminance efficiently emitters in OLEDs with a fast lifetime around 0.5-3 s.
31, 32

10

Figure 1.8 Selected cMa complexes (taken from reference
32
).
Compared to pure organic TADF molecules, the intersystem rate (kISC) and the S1
relaxation rate (kS1) are faster in cMa complexes. The kISC value is determined by Fermi’s
golden rule, which is described as follows:
𝑘 𝐼𝑆𝐶 =
2𝜋 ℎ
𝜌 𝐹𝐶
|〈
𝑇 1
|𝐻 𝑆𝑂𝐶 |
𝑆 1
〉|
2
𝑒𝑞 . 1.1

Where 
T1
and 
S1
are the electronic wavefunctions of the S1 and T1 states respectively, FC is
the Frank Condon weighted vibrational density of states and HSOC is the SOC operator. The
heavy metal in cMa complexes significantly boost the ISC rate to as high as10
10
s
-1
since HSOC is
proportional Z
4
.
31
Besides, the degenerate or near-degenerate metal d orbitals aid in maximizing
coupling of the S1 and T1 by providing different orbital parentage. In contrast, in pure organic
TADF molecules, without a heavy metal atom, the ISC rate remains relatively slow (10
6
-10
7
s
-1
)
since the SOC only can be increased by introducing vibronic-spin orbital coupling or/and
interacting with energetically close-lying states of different orbital parentage.
35, 43, 44
Moreover,
in cMa complexes, a principal role of the metal is to effectively couple the donor and acceptor
moieties in the excited state, markedly enhancing the oscillator strength of the S1 relaxation,
which leads to TADF of cMa complexes breaking the sub-microsecond lifetime barrier. In
contrast, the weak overlap between the donor and acceptor in pure organic TADF molecules
leads to slow kS1.
The kinetics scheme of cMa TADF complexes with a near unity luminescence efficiency
could be simplified by ignoring the nonradiative pathways. The triplet sublevels in TADF
complexes are considered degenerate because the splitting of sublevels is smaller than the singlet
11

triplet gap (EST). The radiative rate of TADF complexes (kTADF) is determined by the product
of the radiative rate from the S1 state (𝑘 𝑟 𝑆 1
) and the equilibrium constant between S1 and T1 states
Keq (T1 ⇄ S1),
45

𝑘 𝑟 𝑇𝐴𝐷𝐹 = 𝑘 𝑟 𝑆 1
∙ 𝐾 𝑒𝑞
𝑒𝑞 . 1.2

Figure 1.9 Simplified kinetic scheme for emission via TADF (taken from reference
45
).
The equilibrium
45
between singlet and triplet states is determined by the ratio of reserve
ISC (k1) and ISC (k-1) rate, which can be described as follows:
𝐾 𝑒𝑞
=
𝑘 1
𝑘 −1
=
1
3
𝑒 (−
𝐸 𝑆𝑇
𝑘 𝐵 𝑇 )
𝑒𝑞 . 1.3
where EST is energy gap between S1 and T1, kB is Boltzmann's constant and T is absolute
temperature. EST is determined by the exchange integral K,
46
which is described as follows:
𝛥𝐸
𝑆𝑇
= 2𝐾 = 2 ∬ 
𝐻 (𝑟 1
)
𝐿 (𝑟 2
)
1
|𝑟 2−
𝑟 1
|

𝐻 (𝑟 1
)
𝐿 (𝑟 2
)𝑑 𝑟 1
𝑑 𝑟 2
𝑒𝑞 . 1.4
Where r1 and r2 denote the coordinates of electrons in HOMO and LUMO. Most of the S1 and T1
states in TADF molecules are pure HOMO→LUMO transitions. As implied in the equation, the
exchange energy stems from Coulombic repulsion of two electrons. Therefore, the ΔEST can be
reduced by spatially separated HOMO and LUMO (or increased |r2-r1|). The rate of S1→S0 is
proportional to the square of transition dipole moment 𝜇 𝑆 0
𝑆 1
,
46
which is described as follows:
𝑘 𝑆 1
∝ (𝜇 𝑆 0
𝑆 1
)
2
= |〈
𝐻 (𝑟 )|𝒓 |
𝐿 (𝑟 )〉|
2
= |∫ 
𝐻 (𝑟 ) 𝒓 
𝐿 (𝑟 )𝑑 𝑟 |
2
𝑒𝑞 . 1.5

Where r denotes the coordinate of shared electrons between HOMO and LUMO and operator r is
coupling operator of S0 and S1 state. Therefore, there is a trade-off between EST and kS1. A
small EST can be achieved by a large separation of HOMO and LUMO, while it leads to a small
12

rate of S1→S0. Thus, to achieve a short-lived TADF emitter, it is critical to satisfy two
contradictory factors of small EST and large kS1 simultaneously.

13

1.5 References

[1] Kim, S.; Kwon, H.-J.; Lee, S.; Shim, H.; Chun, Y.; Choi, W.; Kwack, J.; Han, D.; Song,
M.; Kim, S.; Mohammadi, S.; Kee, I.; Lee, S. Y., Low-Power Flexible Organic Light-Emitting
Diode Display Device. Advanced Materials 2011, 23 (31), 3511-3516.

[2] Han, J.-H.; Moon, J.; Cho, D.-H.; Shin, J.-W.; Joo, C. W.; Hwang, J.; Huh, J. W.; Chu,
H. Y.; Lee, J.-I., Transparent OLED Lighting Panel Design Using Two-Dimensional OLED
Circuit Modeling. ETRI Journal 2013, 35 (4), 559-565.

[3] Jung, Y. K.; Choi, H. S.; Ahn, S. Y.; Kim, S.; Choi, H.; Han, C. W.; Kim, B.; Kim, S.
J.; Kim, J.; Choi, J. H.; Yoon, S.; Tak, Y. H.; Choi, H. C.; Ahn, B. C.; Kang, I. B., 52-3:
Distinguished Paper: 3 Stacked Top Emitting White OLED for High Resolution OLED TV. SID
Symposium Digest of Technical Papers 2016, 47 (1), 707-710.

[4] Lee, J.; Slootsky, M.; Lee, K.; Zhang, Y.; Forrest, S. R., An electrophosphorescent
organic light emitting concentrator. Light: Science & Applications 2014, 3 (6), e181-e181.

[5] Pope, M.; Kallmann, H. P.; Magnante, P., Electroluminescence in Organic Crystals. The
Journal of Chemical Physics 1963, 38 (8), 2042-2043.

[6] Tang, C. W.; VanSlyke, S. A., Organic electroluminescent diodes. Applied Physics Letters
1987, 51 (12), 913-915.

[7] Yersin, H.; Rausch, A. F.; Czerwieniec, R., Organometallic Emitters for OLEDs: Triplet
Harvesting, Singlet Harvesting, Case Structures, and Trends. In Physics of Organic
Semiconductors, 2012; pp 371-424.

[8] Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Nearly 100% internal
phosphorescence efficiency in an organic light-emitting device. Journal of Applied Physics 2001,
90 (10), 5048-5051.

[9] 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. Journal of the American Chemical Society 2003, 125
(24), 7377-7387.

[10] Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.;
Forrest, S. R., Highly efficient phosphorescent emission from organic electroluminescent devices.
Nature 1998, 395 (6698), 151-154.

[11] Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.;
Burrows, P. E.; Forrest, S. R.; Thompson, M. E., Highly Phosphorescent Bis-Cyclometalated
14

Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light
Emitting Diodes. Journal of the American Chemical Society 2001, 123 (18), 4304-4312.

[12] Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.;
Holmes, R. J.; Forrest, S. R., Blue and Near-UV Phosphorescence from Iridium Complexes with
Cyclometalated Pyrazolyl or N-Heterocyclic Carbene Ligands. Inorganic Chemistry 2005, 44
(22), 7992-8003.

[13] Xia, S. C.; Kwong, R. C.; Adamovich, V. I.; Weaver, M. S.; Brown, J. J. In OLED
Device Operational Lifetime: Insights and Challenges, 2007 IEEE International Reliability
Physics Symposium Proceedings. 45th Annual, 15-19 April 2007; 2007; pp 253-257.

[14] Hack, M.; Weaver, M. S.; Brown, J. J., 15-2: Invited Paper: Status and Opportunities for
Phosphorescent OLED Lighting. SID Symposium Digest of Technical Papers 2017, 48 (1), 187-
190.

[15] Lee, J.-H.; Chen, C.-H.; Lee, P.-H.; Lin, H.-Y.; Leung, M.-k.; Chiu, T.-L.; Lin, C.-F.,
Blue organic light-emitting diodes: current status, challenges, and future outlook. Journal of
Materials Chemistry C 2019, 7 (20), 5874-5888.

[16] Kondakov, D. Y.; Lenhart, W. C.; Nichols, W. F., Operational degradation of organic
light-emitting diodes: Mechanism and identification of chemical products. Journal of Applied
Physics 2007, 101 (2), 024512.

[17] So, F.; Kondakov, D., Degradation Mechanisms in Small-Molecule and Polymer Organic
Light-Emitting Diodes. Advanced Materials 2010, 22 (34), 3762-3777.

[18] Song, W.; Lee, J. Y., Degradation Mechanism and Lifetime Improvement Strategy for
Blue Phosphorescent Organic Light-Emitting Diodes. Advanced Optical Materials 2017, 5 (9),
1600901.

[19] Kim, S.; Bae, H. J.; Park, S.; Kim, W.; Kim, J.; Kim, J. S.; Jung, Y.; Sul, S.; Ihn, S.-
G.; Noh, C.; Kim, S.; You, Y., Degradation of blue-phosphorescent organic light-emitting
devices involves exciton-induced generation of polaron pair within emitting layers. Nature
Communications 2018, 9 (1), 1211.

[20] Volz, D., Review of organic light-emitting diodes with thermally activated delayed
fluorescence emitters for energy-efficient sustainable light sources and displays. Journal of
Photonics for Energy 2016, 6 (2), 020901.

[21] Brütting, W.; Frischeisen, J.; Schmidt, T. D.; Scholz, B. J.; Mayr, C., Device efficiency
of organic light-emitting diodes: Progress by improved light outcoupling. physica status solidi
(a) 2013, 210 (1), 44-65.

15

[22] Bory, B. F.; Gomes, H. L.; Janssen, R. A. J.; Leeuw, D. M. d.; Meskers, S. C. J.,
Electrical conduction of LiF interlayers in organic diodes. Journal of Applied Physics 2015, 117
(15), 155502.

[23] Cai, M.; Xiao, T.; Hellerich, E.; Chen, Y.; Shinar, R.; Shinar, J., High-Efficiency
Solution-Processed Small Molecule Electrophosphorescent Organic Light-Emitting Diodes.
Advanced Materials 2011, 23 (31), 3590-3596.

[24] Yook, K. S.; Lee, J. Y., Organic Materials for Deep Blue Phosphorescent Organic Light-
Emitting Diodes. Advanced Materials 2012, 24 (24), 3169-3190.

[25] Jou, J.-H.; Kumar, S.; Agrawal, A.; Li, T.-H.; Sahoo, S., Approaches for fabricating high
efficiency organic light emitting diodes. Journal of Materials Chemistry C 2015, 3 (13), 2974-
3002.

[26] Kobayashi, T.; Ide, N.; Matsusue, N.; Naito, H., Temperature Dependence of
Photoluminescence Lifetime and Quantum Efficiency in Neat fac-Ir(ppy)3 Thin Films. Japanese
Journal of Applied Physics 2005, 44 (4R), 1966.

[27] 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.

[28] 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), 2622-2652.

[29] Di, D.; Romanov, A. S.; Yang, L.; Richter, J. M.; Rivett, J. P.; Jones, S.; Thomas, T.
H.; Abdi Jalebi, M.; Friend, R. H.; Linnolahti, M.; Bochmann, M.; Credgington, D., High-
performance light-emitting diodes based on carbene-metal-amides. Science 2017, 356 (6334),
159-163.

[30] 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.

[31] 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.

16

[32] Muniz, C. N.; Schaab, J.; Razgoniaev, A.; Djurovich, P. I.; Thompson, M. E., π-
Extended Ligands in Two-Coordinate Coinage Metal Complexes. Journal of the American
Chemical Society 2022, 144 (39), 17916-17928.

[33] Clarkson, R. G.; Gomberg, M., SPIRANS WITH FOUR AROMATIC RADICALS ON
THE SPIRO CARBON ATOM1. Journal of the American Chemical Society 1930, 52 (7), 2881-
2891.

[34] Adachi, C., Third-generation organic electroluminescence materials. Japanese Journal of
Applied Physics 2014, 53 (6), 060101.

[35] Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly efficient organic
light-emitting diodes from delayed fluorescence. Nature 2012, 492 (7428), 234-238.

[36] 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.

[37] To, W.-P.; Cheng, G.; Tong, G. S. M.; Zhou, D.; Che, C.-M., Recent Advances in Metal-
TADF Emitters and Their Application in Organic Light-Emitting Diodes. Frontiers in Chemistry
2020, 8.

[38] Ma, Y.; Zhang, H.; Shen, J.; Che, C., Electroluminescence from triplet metal—ligand
charge-transfer excited state of transition metal complexes. Synthetic Metals 1998, 94 (3), 245-
248.

[39] Yersin, H.; Czerwieniec, R.; Shafikov, M. Z.; Suleymanova, A. F., TADF Material
Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes.
ChemPhysChem 2017, 18 (24), 3508-3535.

[40] Leitl, M. J.; Zink, D. M.; Schinabeck, A.; Baumann, T.; Volz, D.; Yersin, H., Copper(I)
Complexes for Thermally Activated Delayed Fluorescence: From Photophysical to Device
Properties. Topics in Current Chemistry 2016, 374 (3), 25.

[41] Di, D.; Romanov, A. S.; Yang, L.; Richter, J. M.; Rivett, J. P. H.; Jones, S.; Thomas, T.
H.; Jalebi, M. A.; Friend, R. H.; Linnolahti, M.; Bochmann, M.; Credgington, D., High-
performance light-emitting diodes based on carbene-metal-amides. Science 2017, 356 (6334),
159-163.

[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.

17

[43] Inomata, H.; Goushi, K.; Masuko, T.; Konno, T.; Imai, T.; Sasabe, H.; Brown, J. J.;
Adachi, C., High-Efficiency Organic Electrophosphorescent Diodes Using 1,3,5-Triazine
Electron Transport Materials. Chemistry of Materials 2004, 16 (7), 1285-1291.

[44] Wada, Y.; Nakagawa, H.; Matsumoto, S.; Wakisaka, Y.; Kaji, H., Organic light emitters
exhibiting very fast reverse intersystem crossing. Nature Photonics 2020, 14 (10), 643-649.

[45] Ravinson, D. S. M.; Thompson, M. E., Thermally assisted delayed fluorescence (TADF):
fluorescence delayed is fluorescence denied. Materials Horizons 2020, 7 (5), 1210-1217.

[46] Marian, C. M., Understanding and Controlling Intersystem Crossing in Molecules. Annual
Review of Physical Chemistry 2021, 72 (1), 617-640.

18

Chapter 2 Symmetric “Double Spiro” Hosts for Blue
Phosphorescent OLED Devices
2.1 Introduction
Phosphorescent organic light emitting diodes (PHOLEDs) have gained increasing
acceptance in flat-panel, flexible displays and solid-state lighting applications upon the
realization of 100% internal quantum efficiency and versatile color tunability.
1-3
Even though
red and green phosphors have made their way into mass production for displays, short
operational lifetimes for blue PHOLEDs have hindered their commercialization.
4-6
Among the
multiple layers used to construct a PHOLED, the emissive layer, consisting of an emissive
dopant and host matrix, plays the most crucial role in determining device performance.
7-10
While
attention has been paid to the development of stable blue phosphorescent emitters, there is a
dearth of stable materials necessary to host blue phosphors.
11-15
The challenge to create such
hosts is the stringent prerequisites required for these materials, which include: (i) a triplet energy
(ET) high enough to confine excitons onto the dopant by preventing energy transfer back to the
host (ET >2.8 eV),
16-18
(ii) a large energy gap between the highest occupied molecular orbital
(HOMO)-lowest unoccupied molecular orbital (LUMO) to promote charge recombination on the
dopant,
19
(iii) strong chemical bonds that not easily ruptured by energy of excitons or polarons
formed in blue PHOLEDs,
20, 21
and (iv) thermal and morphological stability during device
operation.
22, 23
To the best of our knowledge, no reported host materials satisfy all four criteria
for blue PHOLEDs.
Common molecular building blocks for hosts with triplet energies greater than 2.8 eV are
shown in Figure 2.1.
24
Among these fragments, carbazoles have been widely employed as the
core electron donating moiety in hosts for blue PHOLEDs.
25
The carbazole unit is often
substituted with electron accepting groups such as triazine,
26-29
pyridine,
30
triazole,
31, 32

phosphine oxide
33-38
and sulfone
39
to balance hole and electron transport in the emissive layer.
However, intramolecular charge transfer between the electron donor and acceptor units can
lower the triplet energy level, despite being separated by poorly conjugating spacers such as
arylsilane, phenylene and fluorene in the molecular backbones.
19

Figure 2.1 Commonly used rigid aromatic moieties for high triplet host materials.
40

Both high triplet energy and wide HOMO-LUMO gaps in the host play a crucial role in
forcing charge recombination to occur principally on the dopant. Achieving proper HOMO and
LUMO energies in the host is important as most blue phosphors are formed by stabilizing the
HOMO or destabilizing the LUMO energy of a green phosphor, making hosts used for green
PHOLEDs impractical for blue PHOLEDs. For example, LUMO energies in blue Ir dopants
with carbene
40-42
and five-membered heterocyclic rings
43, 44
are destabilized by 0.5 eV or more
from values in green phosphors. Such large changes in the HOMO and/or LUMO energies can
promote the formation of unwanted exciplexes between the dopants and many of the
conventional hosts used in blue PHOLEDs. Therefore, a host with a wide HOMO-LUMO gap is
needed to frustrate exciplex formation between the blue dopant and host.
An additional weakness in existing host materials for blue dopants is that chemical bonds
in the hosts are susceptible to rupture during device operation. Energies of excitons formed in
blue PHOLEDs are between 2.8-3.0 eV. Common building blocks in existing hosts, such as
carbazole, phosphine oxide or sulfone, have C-N, C-P, or C-S bonds, and their homolytic bond
dissociation energies (BDE) tend to be close to or lower than 3.0 eV.
45
Therefore, cleavage of
C-N, C-P, or C-S bonds in the excited state leads to formation of nonradiative recombination
centers and/or luminescence quenchers which degrade the device performance.
To mitigate the possibility of C-X bond rupture, research groups have investigated
hydrocarbon host materials based on spiro fluorene oligomers
46-50
and polymers
51, 52
that utilize
C-C linkages with a BDE of ca. 3.6 eV.
53
Unfortunately, their triplet energies are relatively low
(ET  2.8 eV) in solution owing to π conjugation between covalently linked phenyl rings. As
shown in Figure 1, fluorene has a triplet energy of 2.94 eV. Therefore, linking the spirofluorene
20

units together into oligomers and polymers, while increasing the glass transition temperature (Tg),
also causes a corresponding decrease in triplet energies.
46-52

To obtain compounds with high triplet energies, our previous work bypassed molecules
with direct phenyl-phenyl linkages and instead employed materials, termed ultrawide gap hosts
(UGH), where individual phenyl rings are bound to a tetravalent silicon core atom, e.g. 1,4-
(Ph3Si)C6H4(SiPh3).
54
This approach electronically isolates the arene rings in the molecule and
leads to high triplet energies (ET > 3.2 eV). Exciton formation in UGH-based OLEDs occurs by
charge recombination at the phosphorescent emitter, achieving high external efficiency, while
avoiding exciplex formation between the guest and host.
19
Unfortunately, UGH-type materials
often have low glass transition temperatures, which limit the stability of OLEDs that incorporate
them as hosts. Replacing the central phenylene group in UGHs with a biphenyl linkage increases
the Tg, however, the triplet energy of such modified hosts drops to 2.7 eV in solution.
55
These
trade-offs between enhancing the glass transition temperature and minimizing electronic
conjugation presents another challenge to the design of host materials for blue phosphors.
In this work, we aim to build host molecules with high energy gaps, strong covalent
linkages and good thermal stability. Here we focus in spiro-based materials to achieve high
thermal stability.
56
Our study also involves a comparison of the properties of a host with
biphenylene groups to one with isolated phenyl rings. To that end, two building blocks, fluorene
and benzene (Figure 2.1), are linked together via double spiro centers on a dihydroanthracene
core to form dispiro[fluorene-9,9'-anthracene-10',9''-fluorene] (SAS) and dispiro[xanthene-9,9'-
anthracene-10',9''-xanthene] (XAX). As the fluorene and phenyl units are isolated by spiro
centers, SAS and XAX maintain high triplet energies not only in solution (ET = 2.92 and
3.44 eV, respectively) but also in solid state (ET = 2.77 and 3.08 eV, respectively). Both
compounds also have large energy separations between their respective HOMOs and LUMOs (
5.0 eV). Furthermore, SAS and XAX only have C-C or comparably strong C-O bonds and are
stable up to 450 C. SAS and XAX have been successfully used as host materials to fabricate
blue PHOLEDs with a low turn-on voltage (ca. 2.9 V) and high external quantum efficiency
(EQE = 18% and 16% at 0.01 mA-cm
-2
, respectively).
21

2.2 Results and Discussion
2.2.1 Synthesis and structures
SAS and XAX were synthesized from readily available starting materials in high yields
using a two-step sequence (Scheme 2.1). The first step in either SAS or XAX synthesis is
lithiation of 2-bromobiphenyl and 1-bromo-2-phenoxybenzene with n-butyllithium, respectively,
followed by nucleophilic addition of the anion to the anthraquinone, which gives the desired
intermediates (1a and 1b) in 85% yield. The next step involves acid mediated Friedel−Crafts
cyclization of the hydroxyl precursors, giving the desired products in 80% yield.

Scheme 2.1 Synthesis of SAS and XAX.
The crystal structures of SAS and XAX are shown in Figure 2.2. The spiro linkages
prevent electronic interaction between the -systems on either side of the linkage in both SAS
and XAX. The spirofluorene planes of SAS are nearly perpendicular to the plane of
dihydroanthracene (dihedral angle = 87). Unlike the spirofluorene groups of SAS, the two
arenes of the diaryl-ether moiety in XAX are not coplanar. The dihedral angle between the two
arene rings, illustrated by the two-colored planes in Figure 2.2, are 11 and 18 for the two
independent XAX molecules in the unit cell. The closest intermolecular contacts observed in
crystals involve edge-to-face packing, with shortest spacings of 3.70 Å and 3.80 Å between
dihydroanthracene aryl planes and those of the spirofluorene in SAS or diaryl-ether planes in
XAX, respectively (see Figures 2.3). No close face-to-face contacts are observed between the
-systems of adjacent molecules in crystals of either compound.
22

SAS XAX
Figure 2.2 Crystal structures of SAS and XAX with thermal ellipsoids at 50%.
(Hydrogen atoms were omitted for clarity.  is the angle between yellow and blue colored arene
rings in XAX.)

Figure 2.3 Crystal packing of SAS (left) and XAX (right).
The electronic structure, valence molecular orbital compositions and energies, along with
the triplet excited state energies (Figure 2.4) were examined theoretically using density
functional theory (DFT) at the B3LYP/6-31G** level of theory. The structural parameters of the
geometry optimized compounds compare well with data obtained from the single crystal X-ray
analysis. The dihedral angle between spirofluorene and dihydroanthracene in SAS is 90 in the
optimized structure, close to the value observed in the single crystal. The dihedral angle between
the planes of the two flanking aromatic rings in XAX is 15 intermediate between those

23

observed in the crystal structure. The HOMO/LUMO contours of SAS are primarily localized
on the biphenyl moieties resulting in a large energy gap, whereas the triplet spin density is
localized on a single biphenyl moiety (the HOMO, LUMO and triplet spin density were
predicted using DFT calculations, see the Experimental section for details). The HOMO of XAX
is mainly localized on aryl-ether moieties whereas the LUMO is delocalized over each aromatic
ring in the entire molecule. The calculated HOMO and LUMO of XAX are similar in energy to
that of SAS. The triplet state has a spin density that is distributed principally over one aromatic
ring in XAX and has a high energy (ET = 3.54 eV).

Figure 2.4 Frontier molecular orbitals and triplet spin density calculated for SAS and XAX.
2.2.2 Photophysical, electrochemical and thermal properties
Absorption spectra of SAS and XAX recorded in 2-methyltetrahydrofuran (2-MeTHF)
are shown in Figure 2.5(a). SAS and XAX display absorption bands between  and  nm,
where the peak at lowest energy is attributed a  to 

transitions on the flanking arene rings.
The fluorescence spectra of SAS and XAX in 2-MeTHF at room temperature are featureless and
exhibit a Stokes shift of ~1600 cm
-1
(Figure 2.5(b)). Singlet energies for SAS (ES = 3.96 eV) and
XAX (ES = 4.26 eV) were determined from the onset of the fluorescence spectra. The
phosphorescence spectra of the compounds were measured in 2-MeTHF (Figure 2.5(c)) and as
neat solids (Figure 2.5(d)) at 77 K. The triplet energy of SAS estimated from the onset of the
phosphorescent spectrum (ET = 2.92 eV) is redshifted in the solid state (ET = 2.77 eV). Isolation
of aromatic rings in XAX leads to triplet energies in solution (ET = 3.44 eV) and in the solid state
(ET = 3.08 eV) that are markedly higher values found for SAS (Table 2.1). Triplet energies of
24

SAS and XAX measured in solution agree with calculated values, whereas the triplet energies
measured in solid state are redshifted due to effects from aggregation.
260 280 300 320 340 360 380 400
0.0
0.5
1.0
260 280 300 320 340 360 380 400
0.0
0.2
0.4
0.6

PL Intensity (a.u.)
Wavelength (nm)

Absorbance (a.u.)
SAS
XAX
(a)
(b)

350 400 450 500 550 600 650
0.0
0.5
1.0
350 400 450 500 550 600 650
0.0
0.5
1.0

PL Intensity (a.u.)
Wavelength (nm)
(d)
SAS
XAX
solution
(77K)
powder
(77K)

PL Intensity (a.u.)
(c)

Figure 2.5 Absorption and emission spectra of SAS and XAX.
(a) and (b) in 2-MeTHF at 298 K. Gated emission (phosphorescence) spectra of SAS and XAX
at 77 K are shown for samples in 2-MeTHF solution (c) and as neat solids (d). The spectra for (c)
and (d) were collected with a time delay of 200 ms. SAS emission spectra were excited at 290
nm and XAX emission spectra were excited at 275 nm.
Electrochemical properties of SAS and XAX were determined by cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) (Figure 2.6). Oxidation potentials of compounds
were determined by using decamethylferrocene (DMFc) as an internal reference and are reported
relative to the ferrocenium/ferrocene (Fc
+
/Fc) redox couple (see the Experimental Section). SAS
and XAX display irreversible oxidation waves near .59 V versus decamethylferrocenium/
decamethylferrocene (DMFc
+
/DMFc) in acetonitrile (MeCN). HOMO energies estimated from
their respective oxidation potentials (-6.0 eV for both SAS and XAX) agree well with values
obtained using ultraviolet photoelectron spectroscopy (UPS, -5.9 eV for SAS and -6.3 eV for
25

XAX). The reduction potentials of both SAS and XAX lie beyond the potential of the MeCN
solvent (-3.0 V), indicating a LUMO level for these materials shallower than -1 eV (LUMO
= -1.18 × Ered – 4.83).
57
Therefore, the HOMO-LUMO gaps of SAS and XAX derived from
UPS and electrochemical studies are greater than 5.0 eV. Overall, compared to the widely used
host 3,3'-bis(carbazol-9-yl) biphenyl (mCBP), SAS and XAX have higher triplet energies and
larger HOMO-LUMO gaps (Table 1.1).

Figure 2.6 CV (black) and DPV curves (red: oxidation, blue: reduction) of compounds.
(CV and DPV curves of SAS and XAX are recorded in MeCN (scan rate:0.1 V/s) with TBAF as
electrolyte. Decamethylferrocene (DMFc) is used as internal reference. The average of the
oxidation and reduction peak values of ferrocene is set to 0 V.)
The thermal properties of SAS and XAX were investigated using thermogravimetric
analysis (TGA). Both compounds are thermally stable up to 450 C. No decomposition was
observed before sublimation of these materials in TGA experiments (Figure 2.7). Likewise, no
glass transition or melting temperature was observed for SAS and XAX solids upon analysis
using differential scanning calorimetry (DSC). The high thermal and morphological stability of
the compounds is ascribed to the rigid double spiro configuration in the molecular structure.

-3 -2 -1 0 1 2
Current (A)
Volts (V) vs Fc
+
/Fc
DMFc
+
/DMFc
20 A
20 A
SAS

-3 -2 -1 0 1 2
Current (A)
Volts (V) vs Fc
+
/Fc
DMFc
+
/DMFc
XAX
20 A
20 A
26

0 100 200 300 400 500 600
0
20
40
60
80
100
Mass%
Temperature (°C)
SAS
XAX

Figure 2.7 TGA curves (scan rate: 10K/min) of SAS and XAX.
Table 2.1 Summary of properties of SAS and XAX

Abs
(eV)
a)

S1
(eV)
a)

T1
(eV)
b)

T1
(eV)
c)

Eox

(V)
d)

HOMO
(eV)
e)

HOMO
(eV)
f)

Ts
(C)
g)

SAS 4.00 3.96 2.92 2.77 1.51 -6.0 -5.9 409
XAX 4.27 4.26 3.44 3.08 1.49 -6.0 -6.3 417
mCBP
h)
- 3.60 2.93 2.86 0.88 -5.8 - -
a)
Peak of the absorption band and onset of the fluorescence measured in 2-MeTHF at 298 K.
b)

Onset of the phosphorescence band measured in 2-MeTHF at 77 K.
c)
Onset of the
phosphorescence band for the neat powder at 77 K.
d)
Obtained using DPV in acetonitrile vs.
DMFc
+
/DMFc.
e)
Calculated from equation (HOMO = -1.15 × Eox - 4.79) according to
reference
57
with redox potentials adjusted versus ferrocene as 0 V. The redox potential measured
for decamethylferrocene relative to ferrocene can be found in the experimental section.
f)

Obtained using UPS.
g)
Ts = sublimation temperature under nitrogen. h) Data from reference
32
.

2.2.3 Electroluminescent properties
The performance of SAS and XAX was investigated by fabricating vacuum deposited
films (80 nm thick) using a blue emitting phosphor we recently reported, fac-tris(N,N-di-p-tolyl-
pyrizinoimidazol-2-yl)iridium(III) (Ir(tpz)3),
58
as a dopant across a range of concentrations.
This dopant was chosen for study because it has high chemical and thermal stability and
excellent photophysical properties, parameters which are crucial for fabricating efficient
PHOLEDs. Moreover, the ligand in Ir(tpz)3 is a cyclometalated N-heterocyclic carbene,
27

Ir(C^C:)3. Blue phosphors using these types of ligands have an advantage over traditional Ir
complexes using C^N: ligands as they do not have datively bound nitrogen groups such as Ir-
pyridyl, which are prone to bond rupture in the excited state.
59-61

The photoluminescence quantum yields (PL), emission lifetimes () and decay rates of
the films as a function of Ir(tpz)3 doping level in SAS and XAX are summarized in Table 2.2.
The films give sole emission from Ir(tpz)3 at doping levels ≥ 10 vol% (Figure 2.8). The PL of
SAS films containing 20-30 vol% Ir(tpz)3 are close to 100% and have non-radiative rates (knr) an
order of magnitude lower compared to 10 vol% film. In contrast, the PL in XAX films drops as
Ir(tpz)3 concentration increases to 30 vol% due to an increase in knr. These results suggest that
both SAS and XAX confine excitons on the blue phosphor, with the dopant being less effectively
dispersed in XAX at high concentration.
300 400 500 600 700
0.0
0.5
1.0
300 400 500 600 700
0.0
0.5
1.0

Normalized PL Intensity (a.u.)
10% doping
20% doping
30% doping
(a) excite
SAS

Wavelength (nm)
(b) excite
fac-Ir(tpz)
3
in SAS
300 400 500 600 700
0.0
0.5
1.0
300 400 500 600 700
0.0
0.5
1.0

Normalized PL Intensity (a.u.)
10% doping
20% doping
30% doping
(c) excite
XAX

Wavelength (nm)
(d) excite
fac-Ir(tpz)
3
in XAX

Figure 2.8 Normalized PL spectra of blend films of SAS or XAX hosts.
(a) SAS films excited at 310 nm (b) fac-Ir(tpz)3 in SAS films excited at 380 nm, (c) XAX films
excited at 290 nm and (d) fac-Ir(tpz)3 in XAX films excited at 380nm.

28

Table 2.2 Summary of photoluminescence properties for Ir(tpz)3 doped into
SAS and XAX films.
concentration (%) PL (%,)  (μs, )
c)
kr (10
5
s
-1
) knr (10
5
s
-1
)
in SAS
10 87, 2
a)
1.37, 0.06 6.4 1.0
20 97, 2
a)
1.53, 0.05 6.3 0.2
30 98, 2
a)
1.47, 0.02 6.7 0.1
in XAX
10 93, 2
b)
1.43, 0.07 6.5 0.5
20 91, 2
b)
1.42, 0.04 6.4 0.6
30 86, 2
b)
1.27, 0.05 6.8 1.1
a)
Measured with excitation energy at 310 nm.
b)
Measured with excitation energy at 290 nm.
Quantum yield is the average of four measurements, listed with their standard deviation ()
c)

Measured at emission at 490 nm. Lifetime is the average of three measurements, listed with their
standard deviation ().

PHOLEDs using Ir(tpz)3 as a dopant were fabricated in SAS and XAX hosts. The
performance of these devices was compared to reference devices fabricated with a commonly
used host in blue OLEDs, mCBP. In the first set of experiments devices were analyzed using
different concentrations of Ir(tpz)3 doped in SAS, XAX and mCBP (see Figures 2.9, 2.10 and
2.11 for the OLED performance). In SAS-based PHOLEDs, the current density (J) increased as
the doping level was raised from 10% to 30%. The turn-on voltage (Von, defined at brightness of
1 cd-m
-2
) dropped from 3.15 V to 2.80 V over the same range. The increase in current density
with doping concentration is consistent with charges being injected directly onto and carried by
the dopant, as expected since the energies of the HOMO and LUMO for Ir(tpz)3 (-5.6 eV and -
2.0 eV, respectively) are nested within those of SAS. The same trend of current density
increasing with doping concentration is observed in XAX-based devices. However, J decreases
with increasing doping concentration in devices using the mCBP host. This difference is likely
due to mCBP carrying both holes and electrons at low doping concentration since the energies of
its HOMO (-5.8 eV) and LUMO (-1.6 eV) are close to those of Ir(tpz)3.
62-64
In contrast, charges
are exclusively trapped and transported by Ir(tpz)3 in SAS and XAX films since both materials
have deeper HOMO and shallower LUMO levels.
29

400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1 10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3
Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
100
200
300
400
500
600

10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3
Current density (mAcm
-2
)
Voltage (V)
0.01 0.1 1 10 100
0
5
10
15
20
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3
External Quantum Efficiency (%)
Current Density (mAcm
-2
)
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3

Luminance (cdm
-2
)

Voltage (V)

Figure 2.9 Doping concentration-controlled devices with SAS host material.
EML = 10, 20 or 30% fac-Ir(tpz)
3
in SAS
5nm
HATCN
-8.0
40nm
TAPC
-5.5
-1.3
20 nm
EML
-2.0
-5.6
40nm
TmPyPb
-6.7
-2.6
ITO
LiF/Al
-5.5
30

400 500 600 700
0
0.2
0.4
0.6
0.8
1
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3
Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
0
50
100
150
200
250
300
350

10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3
Current density (mAcm
-2
)
Voltage (V)

10
-2
10
-1
10
0
10
1
10
2
0
5
10
15
20

10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3
EQE (%)
Current density (mAcm
-2
)
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3

Luminance (cdm
-2
)

Voltage (V)

Figure 2.10 Doping concentration-controlled devices with XAX host material.
EML = 10, 20 or 30% fac-Ir(tpz)
3
in XAX
5nm
HATCN
-8.0
40nm
TAPC
-5.5
-1.3
20 nm
EML
-2.0
-5.6
60nm
TmPyPb
-6.7
-2.6
ITO
LiF/Al
-5.5
31

400 500 600 700
0
0.2
0.4
0.6
0.8
1
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3
Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
100
200
300
400
Current density (mAcm
-2
)
Voltage (V)
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3

0.01 0.1 1 10 100
0
5
10
15
20
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3 External Quantum Efficiency (%)
Current Density (mAcm
-2
)
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10% Ir(tpz)
3
20% Ir(tpz)
3
30% Ir(tpz)
3

Luminance (cdm
-2
)

Voltage (V)

Figure 2.11 Doping concentration-controlled devices with mCBP host material.
Also, thickness-controlled experiments of hole transport layer TAPC and electron transport
layer TmPyPb (Figure 2.12-2.15) have been done to optimize the device performance for SAS
and XAX. By increasing TAPC layer thickness from 20 nm to 60 nm, the current density
decreases and luminescence proportionately drops as well, resulting in minimal impact on the
EQE of all devices. To further optimize SAS and XAX devices, TmPyPb thickness-controlled
EML = 10, 20 or 30% fac-Ir(tpz)
3
in mCBP
5nm
HATCN
-8.0
40nm
TAPC
-5.5
-1.3
20 nm
EML
-2.0
-5.6
60nm
TmPyPb
-6.7
-2.6
ITO
LiF/Al
-5.5
32

experiment was performed. As TmPyPb thickness increase from 20 nm to 60 nm in the SAS
devices, the current density slightly dropped whereas the highest luminescence increases
dramatically from 8043 cdm
-2
to 16712 cdm
-2
. Hence the device maximum EQE increase from
10% to 18%. The XAX devices follow the same trend. The thickness-controlled experiments
performed on the ETL and HTL suggests that outcoupling is the major factor that determines the
device efficiency instead of charge balance.
33

400 500 600 700
0
0.2
0.4
0.6
0.8
1
TAPC 20 nm
TAPC 40 nm
TAPC 60 nm
Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
0
100
200
300
400
500

TAPC 20 nm
TAPC 40 nm
TAPC 60 nm
Current density (mAcm
-2
)
Voltage (V)

0.01 0.1 1 10 100
0
5
10
15
20
TAPC 20 nm
TAPC 40 nm
TAPC 60 nm
External Quantum Efficiency (%)
Current Density (mAcm
-2
)
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
TAPC 20 nm
TAPC 40 nm
TAPC 60 nm

Luminance (cdm
-2
)

Voltage (V)

Figure 2.12 TAPC thickness-controlled devices with SAS host material.
EML = 30% fac-Ir(tpz)
3
in SAS
TAPC: 20, 40 or 60 nm
5nm
HATCN
-8.0
TAPC
-5.5
-1.3
20 nm
EML
-2.0
-5.6
60 nm
TmPyPb
-6.7
-2.6
ITO
LiF/Al
-5.5
34

400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1 TmPyPb 20 nm
TmPyPb 40 nm
TmPyPb 60 nm
Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
50
100
150
200
250
300

Current density (mAcm
-2
)
Voltage (V)
TmPyPb 20 nm
TmPyPb 40 nm
TmPyPb 60 nm

0.01 0.1 1 10 100
0
5
10
15
20
TmPyPb 20 nm
TmPyPb 40 nm
TmPyPb 60 nm
External Quantum Efficiency (%)
Current Density (mAcm
-2
)
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
TmPyPb 20
TmPyPb 40
TmPyPb 60

Luminance (cd/m
-2
)

Voltage (V)

Figure 2.13 TmPyPb thickness-controlled devices with SAS host material.
EML = 30% fac-Ir(tpz)
3
in SAS
TmPyPb: 20, 40 or 60 nm
5nm
HATCN
-8.0
40nm
TAPC
-5.5
-1.3
20 nm
EML
-2.0
-5.6
TmPyPb
-6.7
-2.6
ITO
LiF/Al
-5.5
35

400 500 600 700
0
0.2
0.4
0.6
0.8
1
TAPC 20 nm
TAPC 40 nm
TAPC 60 nm
Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
0
100
200
300
400
500
600

TAPC 20 nm
TAPC 40 nm
TAPC 60 nm
Current density (mAcm
-2
)
Voltage (V)

10
-2
10
-1
10
0
10
1
10
2
0
5
10
15
20

TAPC 20 nm
TAPC 40 nm
TAPC 60 nm
EQE (%)
Current density (mAcm
-2
)
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
TAPC 20 nm
TAPC 40 nm
TAPC 60 nm

Luminance (cdm
-2
)

Voltage (V)

Figure 2.14 TAPC thickness-controlled devices with XAX host material.
EML = 30% fac-Ir(tpz)
3
in XAX
TAPC: 20, 40 or 60 nm
5nm
HATCN
-8.0
TAPC
-5.5
-1.3
20 nm
EML
-2.0
-5.6
60 nm
TmPyPb
-6.7
-2.6
ITO
LiF/Al
-5.5
36

400 500 600 700
0
0.2
0.4
0.6
0.8
1
TmPyPb 20 nm
TmPyPb 40 nm
TmPyPb 60 nm
Intensity (a.u.)
Wavelength (nm)
0 2 4 6 8 10 12
0
200
400
600
800

TmPyPb 20 nm
TmPyPb 40 nm
TmPyPb 60 nm
Current density (mAcm
-2
)
Voltage (V)

10
-2
10
-1
10
0
10
1
10
2
0
5
10
15
20

TmPyPb 20 nm
TmPyPb 40 nm
TmPyPb 60 nm
EQE (%)
Current density (mAcm
-2
)
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
TmPyPb 20 nm
TmPyPb 40 nm
TmPyPb 60 nm

Luminance (cdm
-2
)

Voltage (V)

Figure 2.15 TmPyPb thickness-controlled devices with XAX host material.

EML = 30% fac-Ir(tpz)
3
in XAX
TmPyPb: 20, 40 or 60 nm
5nm
HATCN
-8.0
20nm
TAPC
-5.5
-1.3
20 nm
EML
-2.0
-5.6
TmPyPb
-6.7
-2.6
ITO
LiF/Al
-5.5
37

Based on previous controlled devices, the optimized devices were fabricated with the
structure ITO/HATCN (5 nm)/30% Ir(tpz)3 in hosts (20 nm)/TmPyPb (60 nm)/LiF (1 nm)/Al
(100 nm). The electroluminescence (EL) spectra of Ir(tpz)3 in SAS, XAX and mCBP hosts
match the PL spectra of Ir(tpz)3 (Figure 2.16(a)). The EL spectrum is similar in all host materials,
with minor differences presumably due to optical cavity effects. The SAS and XAX based
devices have the same turn-on potential (ca. 2.9 V) and have similar current-voltage (J-V)
characteristics, whereas the SAS device is slightly more conductive (Figure 2.16(b)). The XAX
and reference host mCBP devices exhibit similar brightness at low current densities, with slight
differences at higher current densities. PHOLEDs with SAS give the highest efficiency (EQE =
18%) whereas the XAX device has a slightly lower efficiency (EQE =16%) (Figure 2.16(c)).
The SAS based devices remain efficient (EQE ~10%) at high brightness (10000 cd-m
-2
), whereas
the EQEs of XAX based devices drop dramatically at >1000 cd-m
-2
. This difference suggests
that SAS avoids aggregation-induced quenching of the long-lived triplet excitons as is observed
in UGH-type hosts.
54

38

400 450 500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
SAS
XAX
mCBP
EL Intensity (a.u.)
Wavelength (nm)
(a)
0 2 4 6 8 10 12
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
Current density (mAcm
-2
)
Voltage (V)
SAS
XAX
mCBP
(b)

10
0
10
1
10
2
10
3
10
4
0
5
10
15
20

EQE (%)
Luminance (cdm
-2
)
SAS
XAX
mCBP
(c)

Figure 2.16 OLED device characteristics of SAS, XAX and mCBP.
(top) Device architecture and molecular structure of materials. (a) EL spectra. (b) J-V curves. (c)
Efficiency versus luminance curves. (d) Luminance versus voltage curves.
0 2 4 6 8 10 12
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
SAS
XAX
mCBP

Luminance (cdm-
2
)

Voltage (V)
(d)
39

Table 2.3 OLED performance parameters for Ir(tpz)3 based OLEDs.
a)

Host

Von
(V)

EQEmax
(%,)
Efficiency at 1000 cd-m
-2

λmax (nm)
(CIE)
EQE
(%)
current
density
(mA-cm
-2
)
CE
(cd-A
-1
)
PE
(lm-W
-1
)
SAS 2.9 17.9, 0.2
b)
14 2.62 35 30
488
(0.19, 0.40)
XAX 2.9 15.7, 0.4
b)
14 3.19 31 23
488
(0.17, 0.37)
mCBP 2.8 17.1, 0.1
c)
15 2.79 35 26
488
(0.17, 0.38)
a)
Von = voltage at 1 cd-m
−2
, EQEmax= EQE at 0.01 mA-cm
-2
, L = luminance, CE = current
efficiency, PE = power efficiency.
b)
Maximum EQE is the average of four devices, listed with
their standard deviation ().
c)
Maximum EQE is the average of two devices.

2.3 Conclusion
We report two wide energy gap hosts without heteroatomic exocyclic bonds. SAS and
XAX have been prepared in high yields, from readily available precursors. The double spiro
structure in SAS and XAX interrupts conjugation between aromatic  systems by holding the
spirofluorene or diphenylether moieties orthogonal to the dihydroanthracene core. As a result,
both SAS and XAX have large HOMO-LUMO gaps ( 5.0 eV) and, more importantly, retain
high triplet energies (ET = 2.77 and 3.08 eV, respectively) in the solid state, parameters which
are crucial in hosting blue phosphors. XAX has a higher triplet energy than that of SAS due to
isolated phenyl rings in diphenylether moieties, showing that the use of even the high triplet
energy, fluorene group in a host limits the applicability of the host for deep blue phosphors. The
high thermal stabilities of SAS and XAX are attributed to double spiro centers on the
dihydroanthracene core. We utilized these wide energy gap materials as hosts for a blue
phosphor to fabricate bright, efficient blue PHOLEDs. The SAS and XAX compounds act as
inert matrices that enable guest dopants to directly transport and trap charges that subsequently
form excitons, which lead to high-performance devices. These compounds can serve as
platforms on which to build other high energy host materials.
40

2.4 Experimental Methods
Synthesis and Characterization: All commercial reagents and solvents are purchased from
Sigma Aldrich or Matrix Scientific and used without further purification. All reactions were
carried out using standard Schlenk line techniques, using dried and degassed solvents. The
synthesis of SAS and XAX are modified from a literature method for related compounds.
65
A
key modification from the literature procedure is preparation of 1a using n-Butyllithium (n-BuLi)
instead of Grignard reagent which gives the product in higher yield. Iridium
(III)N,N-di-p-tolyl-pyrizinoimidazol-2-yl was prepared as previously.
58

1
H NMR spectra were
recorded on a Varian 400 instrument.
13
C NMR spectrum was recorded on a Varian 600
instrument. Mass spectra were recorded on a Bruker Auto Flex Speed Laser Desorption
Ionization (LDI) Mass Spectrometer. Elemental analyses were performed using a Thermo
Scientific FlashSmart CHNS elemental analyzer.
9,10-di([1,1'-biphenyl]-2-yl)-9,10-dihydroanthracene-9,10-diol (1a) ： Dry and degassed
THF was canula transferred into a nitrogen purged 250 mL round bottom flask.
2-Bromobiphenyl (2.48 ml, 14.4 mmol, 2 eq.) was added and the solution was cooled to -78C.
n-BuLi (6.34 mL, 2.5 M, 2.2 eq.) was added dropwise. After 1 hour of stirring at -78C, a
solution of anthraquinone (1.5 g, 7.2 mmol, 1.00 eq.) in 15 ml of THF was added to the mixture
over 5 mins. The reaction mixture warmed up to room temperature over a period of 8 hours and
stirred overnight. The resulting mixture was quenched with water (30 mL), yielding an off-white
solid. The mixture was transferred to a Büchner funnel and vacuum filtered. The residue was
washed with ether resulting in a white powder. Yield: 3.2 g, 86%.
1
H NMR (400 MHz,
Chloroform-d, δ): 8.45 (dd, J = 8.1, 1.3 Hz, 2H; Ar H), 7.48 (ddd, J = 8.1, 1.5 Hz, 2H; Ar H),
7.24 (dd, J = 7.4, 1.3 Hz, 2H; Ar H), 7.17 – 7.10 (m, 6H; Ar H), 6.95 (dd, J = 6.1, 3.4 Hz, 4H; Ar
H), 6.87 (tt, J = 8.1, 1.5 Hz, 4H; Ar H), 6.70 (dd, J = 7.5, 1.5 Hz, 2H; Ar H), 5.93 (dd, J = 8.1,
1.3 Hz, 4H; Ar H). [M-2OH] calcd for C38H26, 482.2; found, 482. 6
9,10-bis(2-phenoxyphenyl)-9,10-dihydroanthracene-9,10-diol (1b) ： Dry and degassed
THF was canula transferred into a nitrogen purged 250 mL round bottom flask.
1-bromo-2-phenoxybenzene (4.79 g, 19.21 mmol, 2 eq.) was added and the solution was cooled
down to -78C. n-BuLi (8.45 mL, 2.5 M, 2.2 eq.) was added dropwise. After 1 hour of stirring
at -78C, a solution of anthraquinone (2 g, 9.61 mmol, 1.00 eq.) in 20 ml of THF was added to
41

the mixture over 5 mins. The reaction mixture warmed up to room temperature over a period of
8 hours and stirred overnight. The resulting mixture was quenched with water (30 mL), yielding
an off-white solid. The mixture was transferred to a Büchner funnel and vacuum filtered. The
residue was washed with ether resulting in a white powder. Yield: 4.5 g, 85%.
1
H NMR
(400 MHz, Chloroform-d, δ): 8.36 (dd, J = 7.8, 1.8 Hz, 2H; Ar H), 7.23 – 7.10 (m, 12H; Ar H),
7.05 (tdd, J = 7.8, 1.8 Hz, 4H; Ar H), 6.94 (tt, J =7.2, 1.8 Hz, 2H; Ar H), 6.47 (dd, J = 8.0, 1.3
Hz, 2H; Ar H), 6.17 (dd, J = 8.7, 1.3 Hz, 4H; Ar H). MS: [M-2OH] calcd for C38H26O2, 514.2;
found, 514.5.
Dispiro[fluorene-9,9'-anthracene-10',9''-fluorene] (SAS): The resulting solid 1a (3.2 g,
6.19 mmol, 1 eq) was dissolved in a mixture of solution of 106 mL (1.86 mol, 300 eq.) glacial
acetic acid and 15.5 ml (12 M, 30 eq.) hydrochloric acid. The reaction was stirred for 12 hours
at 110 C under reflux. The reaction mixture was cooled to room temperature, filtered and
washed with DI water yielding a white solid. Yield: 2.5 g, 84%. The compound was further
purified by sublimation at 270 C and 10
-6
torr.
1
H NMR (400 MHz, Acetone-d6, δ): 8.07 (ddd, J
= 7.5, 1.0 Hz, 4H; Ar H), 7.47 (ddd, J = 7.5, 1.5 Hz, 4H; Ar H), 7.36 – 7.28 (m, 8H; Ar H),
6.83(dd, J = 6.1, 3.4 Hz, 4H; Ar H), 6.35 (dd, J = 6.1, 3.4 Hz, 4H; Ar H).
13
C NMR (600 MHz,
Chloroform-d, δ):157.51, 140.67, 136.53, 128.95, 128.68, 127.66, 126.91, 125.63, 120.24, 58.13.
Anal. calcd for C38H24: C 94.97, H 5.03; found: C 94.95, H 5.06. MS: [M] calcd for C38H24,
480.2; found, 480.4.
Dispiro[xanthene-9,9'-anthracene-10',9''-xanthene] (XAX): The resulting solid 1b (4.5 g,
8.2 mmol, 1 eq) was dissolved in a mixture of solution of 140 mL (2.46 mol, 300 eq.) glacial
acetic acid and 20.5 ml (12 M, 30 eq.) hydrochloric acid. The reaction mixture was stirred for 12
hours at 110 C under reflux. The reaction mixture was cooled to room temperature, filtered and
washed with DI water yielding a white solid. Yield: 3.5 g, 83%. The compound was further
purified by sublimation at 290 C and 10
-6
torr.
1
H NMR (400 MHz, Chloroform-d, δ): 7.29 (dd,
J = 8.2, 1.5 Hz, 4H; Ar H), 7.23 (ddd, J = 8.2, 1.5 Hz, 4H; Ar H), 7.11 (dd, J = 6.1, 3.4 Hz, 4H;
Ar H), 6.98 (dd, J = 6.1, 3.4 Hz, 4H; Ar H), 6.93 (ddd, J = 8.0, 1.5 Hz, 4H; Ar H), 6.87 (dd, J =
8.2, 1.5 Hz, 4H; Ar H). Anal. calcd for C38H24O2: C 89.04, H 4.72; found: C 88.68, H 4.77. MS:
[M] calcd for C38H24O2, 514.2; found, 514.5.
42

Electrochemical, physical and photophysical measurements. Cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) were performed in MeCN using a VersaSTAT 3
potentiostat with a 0.1 M tetra-n-butyl ammonium hexafluorophosphate (TBAF) as the
supporting electrolyte, an Ag wire was used as the pseudo reference electrode, a Pt wire as the
counter electrode, and a glassy carbon rod as the working electrode. Decamethylferrocene is
employed as an internal reference. To determine the relative redox potential of
decamethylferrocene compared to ferrocene, CV and DPV scans are performed with these two
references only as shown in Figure 2.9. Two references present reversible oxidation peaks as
shown in CV plots. According to the DPV results, when the redox potentials of ferrocene are
fixed to 0.0 V, those of decamethylferrocene are around -0.54 V. Thus, by setting the decamethyl
ferrocene reference peaks at -0.54 V, all the samples’ redox potentials are reported relative to 0.0
V for ferrocene. The redox potentials of SAS and XAX are based on the values from differential
pulsed voltammetry measurements and are reported relative to the Fc
+
/Fc redox couple, whereas
cyclic voltammetry was measured to look at if any electrochemical reversibility is inherent to
these materials in order to obtain more accurate redox potentials.

Figure 2.17 CV (black) and DPV curves (red: oxi, blue: red) of decamethylferrocene
Ultraviolet photoelectron spectroscopy was carried out with a He I UV source that has a
photon energy of 21.2 eV under high vacuum (10
−8
torr). The spectra were collected by a
hemispherical electron energy analyzer (Thermal VG) with a – 8.0 V bias voltage.

-1 -0.5 0 0.5
Current (A)
Volts (V) vs Fc
+
/Fc
5 A
Fc
+
/Fc
DMFc
+
/DMFc
50 A
43

Thermogravimetric analysis (TGA) measurements were performed on a NETZSCH STA 449F3
thermogravimeter under nitrogen at a heating rate of 10 C min
-1
.
UV-visible absorption spectra were recorded using a Hewlett-Packard 8453 diode array
spectrometer. Steady-state photoluminescent emission spectra were performed using a Photon
Technology International QuantaMaster model C-60 fluorimeter, whereas gated
photoluminescent emission spectra were measured on the same instrument using a Xe flash lamp
with 200 s delay. Photoluminescent quantum yields were determined using a Hamamatsu
C9920 system equipped with a Xe lamp, calibrated integrating sphere and model C10027
photonic multichannel analyzer (PMA). Solution samples were deoxygenated by bubbling N2 in
a quartz cuvette fitted with a Teflon stopcock. Powder samples were measured in a quartz NMR
tube. Films were prepared by vacuum deposition (10
-7
Torr) on quartz substrates. Emission
lifetimes were measured by time-correlated single-photon counting using an IBH Fluorocube
instrument. Radiative rates are obtained from the equation 𝑘 𝑟 =

𝑃𝐿
𝜏 and non-radiative rates are
obtained from the equation 𝑘 𝑛𝑟
=
1−
𝑃𝐿
𝜏 .
The single crystals were obtained through sublimation. See the synthesis section for
details of sublimation. Single crystal structures were determined at 100K with Bruker X-ray
diffractometer, equipped with an APEX II CCD detector and an Oxford Cryosystems 700 low
temperature apparatus, using Mo K radiation. Details of the data collection and structure
solution are given in the SI. CCDC 1978365 (SAS) and 1978366 (XAX) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Computational modeling. All calculations reported in this work were performed using
the Q-Chem 5.1 software package. Ground state (S0) and triplet state (T1) geometries
optimization were performed for all structures at the B3LYP/6-31G** level of theory. The
optimized geometries of ground state geometries have been examined by frequency analysis at
the B3LYP/6-31G** level of theory. The energies of optimized geometries are local minimum
energies as no negative values where found. The energies for the T1 state shown in Figure 3
were determined from the difference in energies between the optimized S0 and T1 geometries
(SCF method).doi.org/10.1016/j.ccr.2006.05.021
44

OLED Fabrication and Testing. Glass substrates with prepatterned, 2 mm wide indium
tin oxide (ITO) stripes were cleaned by sequential sonication in deionized water, acetone, and
isopropanol, followed by 10 mins UV ozone exposure. Organic materials and metals were
deposited at rates of 0.− Å/s through shadow masks in a vacuum thermal evaporator with a
base pressure of 10
-7
Torr. A separate shadow mask was used to deposit 1 mm wide stripes of
100 nm thick Al films perpendicular to the ITO stripes to form the cathode, resulting in 4 mm
2

device area. The device structure is: glass substrate / 70 nm ITO / 5 nm dipyrazino [2,3-f:2',3'-h]
quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) / 40 nm 4,4 ′ cyclohexylidene-bis [N,N
bis(4 methylphenyl)benzenamine] (TAPC) / 30 vol% fac-iridium(III)N,N-di-p-tolyl-
pyrizinoimidazol-2-yl (Ir(tpz)3):Host / 60 nm 1,3,5-Tri(m-pyridin-3-ylphenyl)benzene (TmPyPb)
/ 1 nm lithium fluoride (LiF) / 100 nm Al. The host is either 3,3’-di(9H-carbazole-9-yl)-1,1’-
biphenyl (mCBP), or one of the SAS and XAX compounds.
A semiconductor parameter analyzer (HP4156A) and a calibrated large area photodiode
that collected all light exiting the glass substrate were used to measure the current density-
voltage-luminance (J-V-L) characteristics. The device spectra were measured using a fiber
coupled spectrometer.

45

2.5 References
[1] Tang, C. W.; VanSlyke, S. A., Organic electroluminescent diodes. Applied Physics Letters
1987, 51 (12), 913-915.

[2] Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R., Very high-
efficiency green organic light-emitting devices based on electrophosphorescence. Applied
Physics Letters 1999, 75 (1), 4-6.

[3] Hack, M.; Weaver, M. S.; Brown, J. J., 15-2: Invited Paper: Status and Opportunities for
Phosphorescent OLED Lighting. SID Symposium Digest of Technical Papers 2017, 48 (1), 187-
190.

[4] Murawski, C.; Leo, K.; Gather, M. C., Efficiency Roll-Off in Organic Light-Emitting
Diodes. Advanced Materials 2013, 25 (47), 6801-6827.

[5] Xia, S. C.; Kwong, R. C.; Adamovich, V. I.; Weaver, M. S.; Brown, J. J. In OLED Device
Operational Lifetime: Insights and Challenges, 2007 IEEE International Reliability Physics
Symposium Proceedings. 45th Annual, 15-19 April 2007; 2007; pp 253-257.

[6] So, F.; Kondakov, D., Degradation Mechanisms in Small-Molecule and Polymer Organic
Light-Emitting Diodes. Advanced Materials 2010, 22 (34), 3762-3777.

[7] Coburn, C.; Forrest, S. R., Effects of Charge Balance and Exciton Confinement on the
Operational Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes. Physical Review
Applied 2017, 7 (4), 041002.

[8] Kim, S.; Bae, H. J.; Park, S.; Kim, W.; Kim, J.; Kim, J. S.; Jung, Y.; Sul, S.; Ihn, S.-G.;
Noh, C.; Kim, S.; You, Y., Degradation of blue-phosphorescent organic light-emitting devices
involves exciton-induced generation of polaron pair within emitting layers. Nature
Communications 2018, 9 (1), 1211.

[9] Kondakov, D. Y., Characterization of triplet-triplet annihilation in organic light-emitting
diodes based on anthracene derivatives. Journal of Applied Physics 2007, 102 (11), 114504.

[10] Giebink, N. C.; D’Andrade, B. W.; Weaver, M. S.; Mackenzie, P. B.; Brown, J. J.;
Thompson, M. E.; Forrest, S. R., Intrinsic luminance loss in phosphorescent small-molecule
organic light emitting devices due to bimolecular annihilation reactions. J. Appl. Phys. 2008, 103
(4), 044509.

[11] Yook, K. S.; Lee, J. Y., Organic Materials for Deep Blue Phosphorescent Organic Light-
Emitting Diodes. Advanced Materials 2012, 24 (24), 3169-3190.

46

[12] Song, W.; Lee, J. Y., Degradation Mechanism and Lifetime Improvement Strategy for
Blue Phosphorescent Organic Light-Emitting Diodes. Advanced Optical Materials 2017, 5 (9),
1600901.

[13] Lee, J.-H.; Chen, C.-H.; Lee, P.-H.; Lin, H.-Y.; Leung, M.-k.; Chiu, T.-L.; Lin, C.-F.,
Blue organic light-emitting diodes: current status, challenges, and future outlook. Journal of
Materials Chemistry C 2019, 7 (20), 5874-5888.

[14] Wang, Y.; Yun, J. H.; Wang, L.; Lee, J. Y., High Triplet Energy Hosts for Blue Organic
Light-Emitting Diodes. Advanced Functional Materials 2021, 31 (12), 2008332.

[15] Poriel, C.; Rault-Berthelot, J., Designing Host Materials for the Emissive Layer of Single-
Layer Phosphorescent Organic Light-Emitting Diodes: Toward Simplified Organic Devices.
Advanced Functional Materials 2021, 31 (24), 2010547.

[16] Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.;
Thompson, M. E., Blue organic electrophosphorescence using exothermic host–guest energy
transfer. Applied Physics Letters 2003, 82 (15), 2422-2424.

[17] Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Nearly 100% internal
phosphorescence efficiency in an organic light-emitting device. Journal of Applied Physics 2001,
90 (10), 5048-5051.

[18] O’Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R., Improved energy transfer
in electrophosphorescent devices. Applied Physics Letters 1999, 74 (3), 442-444.

[19] Holmes, R. J.; D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.; Thompson, M. E.,
Efficient, deep-blue organic electrophosphorescence by guest charge trapping. Applied Physics
Letters 2003, 83 (18), 3818-3820.

[20] Schmidbauer, S.; Hohenleutner, A.; König, B., Chemical Degradation in Organic Light-
Emitting Devices: Mechanisms and Implications for the Design of New Materials. Advanced
Materials 2013, 25 (15), 2114-2129.

[21] Kondakov, D. Y.; Lenhart, W. C.; Nichols, W. F., Operational degradation of organic
light-emitting diodes: Mechanism and identification of chemical products. Journal of Applied
Physics 2007, 101 (2), 024512.

[22] Tokito, S.; Tanaka, H.; Noda, K.; Okada, A.; Taga, Y., Thermal stability in oligomeric
triphenylamine/tris(8-quinolinolato) aluminum electroluminescent devices. Applied Physics
Letters 1997, 70 (15), 1929-1931.

[23] Adamovich, V. I.; Weaver, M. S.; Kwong, R. C.; Brown, J. J., High temperature
operation and stability of phosphorescent OLEDs. Current Applied Physics 2005, 5 (1), 15-18.

47

[24] Murov, S. L., Handbook of photochemistry M Dekker: New York 1993.

[25] van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.; Langeveld, B. M. W.; Rothe,
C.; Monkman, A.; Bach, I.; Stössel, P.; Brunner, K., Carbazole Compounds as Host Materials
for Triplet Emitters in Organic Light-Emitting Diodes: Polymer Hosts for High-Efficiency
Light-Emitting Diodes. Journal of the American Chemical Society 2004, 126 (24), 7718-7727.

[26] Inomata, H.; Goushi, K.; Masuko, T.; Konno, T.; Imai, T.; Sasabe, H.; Brown, J. J.;
Adachi, C., High-Efficiency Organic Electrophosphorescent Diodes Using 1,3,5-Triazine
Electron Transport Materials. Chemistry of Materials 2004, 16 (7), 1285-1291.

[27] Zeng, L.; Lee, T. Y. H.; Merkel, P. B.; Chen, S. H., A new class of non-conjugated
bipolar hybrid hosts for phosphorescent organic light-emitting diodes. Journal of Materials
Chemistry 2009, 19 (46), 8772-8781.

[28] Rothmann, M. M.; Haneder, S.; Da Como, E.; Lennartz, C.; Schildknecht, C.; Strohriegl,
P., Donor-Substituted 1,3,5-Triazines as Host Materials for Blue Phosphorescent Organic Light-
Emitting Diodes. Chemistry of Materials 2010, 22 (7), 2403-2410.

[29] Son, K. S.; Yahiro, M.; Imai, T.; Yoshizaki, H.; Adachi, C., Analyzing Bipolar Carrier
Transport Characteristics of Diarylamino-Substituted Heterocyclic Compounds in Organic Light-
Emitting Diodes by Probing Electroluminescence Spectra. Chemistry of Materials 2008, 20 (13),
4439-4446.

[30] Su, S.-J.; Sasabe, H.; Takeda, T.; Kido, J., Pyridine-Containing Bipolar Host Materials
for Highly Efficient Blue Phosphorescent OLEDs. Chemistry of Materials 2008, 20 (5), 1691-
1693.

[31] Kim, M. K.; Kwon, J.; Kwon, T.-H.; Hong, J.-I., A bipolar host containing 1,2,3-triazole
for realizing highly efficient phosphorescent organic light-emitting diodes. New Journal of
Chemistry 2010, 34 (7), 1317-1322.

[32] 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.

[33] Ding, J.; Wang, Q.; Zhao, L.; Ma, D.; Wang, L.; Jing, X.; Wang, F., Design of star-
shaped molecular architectures based on carbazole and phosphine oxide moieties: towards
amorphous bipolar hosts with high triplet energy for efficient blue electrophosphorescent devices.
Journal of Materials Chemistry 2010, 20 (37), 8126-8133.

[34] Son, H. S.; Seo, C. W.; Lee, J. Y., Correlation of the substitution position of
diphenylphosphine oxide on phenylcarbazole and device performances of blue phosphorescent
organic light-emitting diodes. Journal of Materials Chemistry 2011, 21 (15), 5638-5644.
48

[35] Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y., Phenylcarbazole-Based Phosphine Oxide
Host Materials For High Efficiency In Deep Blue Phosphorescent Organic Light-Emitting
Diodes. Advanced Functional Materials 2009, 19 (22), 3644-3649.

[36] Hsu, F.-M.; Chien, C.-H.; Shih, P.-I.; Shu, C.-F., Phosphine-Oxide-Containing Bipolar
Host Material for Blue Electrophosphorescent Devices. Chemistry of Materials 2009, 21 (6),
1017-1022.

[37] Cai, X.; Padmaperuma, A. B.; Sapochak, L. S.; Vecchi, P. A.; Burrows, P. E., Electron
and hole transport in a wide bandgap organic phosphine oxide for blue electrophosphorescence.
Applied Physics Letters 2008, 92 (8), 083308.

[38] Chou, H.-H.; Cheng, C.-H., A Highly Efficient Universal Bipolar Host for Blue, Green,
and Red Phosphorescent OLEDs. Advanced Materials 2010, 22 (22), 2468-2471.

[39] Chen, R.-F.; Xie, G.-H.; Zhao, Y.; Zhang, S.-L.; Yin, J.; Liu, S.-Y.; Huang, W., Novel
heterofluorene-based hosts for highly efficient blue electrophosphorescence at low operating
voltages. Organic Electronics 2011, 12 (10), 1619-1624.

[40] Holmes, R. J.; Forrest, S. R.; Sajoto, T.; Tamayo, A.; Djurovich, P. I.; Thompson, M.
E.; Brooks, J.; Tung, Y.-J.; D’Andrade, B. W.; Weaver, M. S.; Kwong, R. C.; Brown, J. J.,
Saturated deep blue organic electrophosphorescence using a fluorine-free emitter. Applied
Physics Letters 2005, 87 (24), 243507.

[41] Lee, J.; Chen, H.-F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.;
Forrest, S. R., Deep blue phosphorescent organic light-emitting diodes with very high brightness
and efficiency. Nature Materials 2016, 15 (1), 92-98.

[42] Lu, K.-Y.; Chou, H.-H.; Hsieh, C.-H.; Yang, Y.-H. O.; Tsai, H.-R.; Tsai, H.-Y.; Hsu,
L.-C.; Chen, C.-Y.; Chen, I.-C.; Cheng, C.-H., Wide-Range Color Tuning of Iridium
Biscarbene Complexes from Blue to Red by Different N⁁N Ligands: an Alternative Route for
Adjusting the Emission Colors. Advanced Materials 2011, 23 (42), 4933-4937.

[43] 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. Journal of the American Chemical Society 2003, 125
(24), 7377-7387.

[44] Lo, S.-C.; Shipley, C. P.; Bera, R. N.; Harding, R. E.; Cowley, A. R.; Burn, P. L.;
Samuel, I. D. W., Blue Phosphorescence from Iridium(III) Complexes at Room Temperature.
Chemistry of Materials 2006, 18 (21), 5119-5129.

[45] Lin, N.; Qiao, J.; Duan, L.; Wang, L.; Qiu, Y., Molecular Understanding of the Chemical
Stability of Organic Materials for OLEDs: A Comparative Study on Sulfonyl, Phosphine-Oxide,
49

and Carbonyl-Containing Host Materials. The Journal of Physical Chemistry C 2014, 118 (14),
7569-7578.

[46] Jiang, Z.; Yao, H.; Zhang, Z.; Yang, C.; Liu, Z.; Tao, Y.; Qin, J.; Ma, D., Novel Oligo-
9,9′-spirobifluorenes through ortho-Linkage as Full Hydrocarbon Host for Highly Efficient
Phosphorescent OLEDs. Organic Letters 2009, 11 (12), 2607-2610.

[47] Sicard, L. J.; Li, H.-C.; Wang, Q.; Liu, X.-Y.; Jeannin, O.; Rault-Berthelot, J.; Liao, L.-
S.; Jiang, Z.-Q.; Poriel, C., C1-Linked Spirobifluorene Dimers: Pure Hydrocarbon Hosts for
High-Performance Blue Phosphorescent OLEDs. Angewandte Chemie International Edition
2019, 58 (12), 3848-3853.

[48] Wong, K.-T.; Liao, Y.-L.; Lin, Y.-T.; Su, H.-C.; Wu, C.-c., Spiro-Configured
Bifluorenes: Highly Efficient Emitter for UV Organic Light-Emitting Device and Host Material
for Red Electrophosphorescence. Organic Letters 2005, 7 (23), 5131-5134.

[49] Wang, Q.; Lucas, F.; Quinton, C.; Qu, Y.-K.; Rault-Berthelot, J.; Jeannin, O.; Yang,
S.-Y.; Kong, F.-C.; Kumar, S.; Liao, L.-S.; Poriel, C.; Jiang, Z.-Q., Evolution of pure
hydrocarbon hosts: simpler structure, higher performance and universal application in RGB
phosphorescent organic light-emitting diodes. Chemical Science 2020, 11 (19), 4887-4894.

[50] Luo, Y.; Liu, Z.; Yang, G.; Wang, T.; Bin, Z.; Lan, J.; Wu, D.; You, J., Iridium(III)-
Catalyzed Diarylation/Annulation of Benzoic Acids: Facile Access to Multi-Aryl
Spirobifluorenes as Pure Hydrocarbon Hosts for High-Performance OLEDs. Angewandte
Chemie International Edition 2021, 60 (34), 18852-18859.

[51] Wu, C.-c.; Liu, T.-L.; Hung, W.-Y.; Lin, Y.-T.; Wong, K.-T.; Chen, R.-T.; Chen, Y.-
M.; Chien, Y.-Y., Unusual Nondispersive Ambipolar Carrier Transport and High Electron
Mobility in Amorphous Ter(9,9-diarylfluorene)s. Journal of the American Chemical Society
2003, 125 (13), 3710-3711.

[52] Wei, W.; Djurovich, P. I.; Thompson, M. E., Properties of Fluorenyl Silanes in Organic
Light Emitting Diodes. Chemistry of Materials 2010, 22 (5), 1724-1731.

[53] Moldoveanu, S. C., 1 - General information about pyrolysis. In Analytical Pyrolysis of
Natural Organic Polymers (Second Edition), Moldoveanu, S. C., Ed. Elsevier: 2021; Vol. 20, pp
3-27.

[54] Ren, X.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E.,
Ultrahigh Energy Gap Hosts in Deep Blue Organic Electrophosphorescent Devices. Chemistry of
Materials 2004, 16 (23), 4743-4747.

[55] Lin, J.-J.; Liao, W.-S.; Huang, H.-J.; Wu, F.-I.; Cheng, C.-H., A Highly Efficient
Host/Dopant Combination for Blue Organic Electrophosphorescence Devices. Advanced
Functional Materials 2008, 18 (3), 485-491.
50

[56] Liu, S.; Xia, D.; Baumgarten, M., Rigidly Fused Spiro-Conjugated π-Systems.
ChemPlusChem 2021, 86 (1), 36-48.

[57] Sworakowski, J.; Lipiński, 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.

[58] Idris, M.; Kapper, S. C.; Tadle, A. C.; Batagoda, T.; Muthiah Ravinson, D. S.;
Abimbola, O.; Djurovich, P. I.; Kim, J.; Coburn, C.; Forrest, S. R.; Thompson, M. E., Blue
Emissive
fac
/
mer
‐Iridium (III) NHC Carbene Complexes and their Application in OLEDs. Advanced Optical
Materials 2021.

[59] Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.; Thompson, M.
E., Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes. Journal
of the American Chemical Society 2009, 131 (28), 9813-9822.

[60] Zhou, X.; Powell, B. J., Nonradiative Decay and Stability of N-Heterocyclic Carbene
Iridium(III) Complexes. Inorganic Chemistry 2018, 57 (15), 8881-8889.

[61] Arroliga-Rocha, S.; Escudero, D., Facial and Meridional Isomers of Tris(bidentate) Ir(III)
Complexes: Unravelling Their Different Excited State Reactivity. Inorg. Chem. 2018, 57 (19),
12106-12112.

[62] Tonnelé, C.; Stroet, M.; Caron, B.; Clulow, A. J.; Nagiri, R. C. R.; Malde, A. K.; Burn,
P. L.; Gentle, I. R.; Mark, A. E.; Powell, B. J., Elucidating the Spatial Arrangement of Emitter
Molecules in Organic Light-Emitting Diode Films. Angewandte Chemie International Edition
2017, 56 (29), 8402-8406.

[63] Gao, M.; Lee, T.; Burn, P. L.; Mark, A. E.; Pivrikas, A.; Shaw, P. E., Revealing the
Interplay between Charge Transport, Luminescence Efficiency, and Morphology in Organic
Light-Emitting Diode Blends. Advanced Functional Materials 2020, 30 (9), 1907942.

[64] Coehoorn, R.; Bobbert, P. A., Effects of Gaussian disorder on charge carrier transport and
recombination in organic semiconductors. physica status solidi (a) 2012, 209 (12), 2354-2377.

[65] Clarkson, R. G.; Gomberg, M., SPIRANS WITH FOUR AROMATIC RADICALS ON
THE SPIRO CARBON ATOM1. Journal of the American Chemical Society 1930, 52 (7), 2881-
2891.

51

Chapter 3 Dynamics of Rotation in Thiazolyl Copper (I)
Carbazolyl Complexes
3.1 Introduction
Dr. Savannah Kapper was responsible for synthesizing the substituted carbazole, as well
as performing the partial final complexes synthesis and conducting photophysics measurements.
Meanwhile, I contributed to the project by synthesizing the carbene precursor, performing
calculations, conducting photophysics experiments (remeasuring all complexes due to the
mistakenly set lifetime windows). Additionally, I was also involved in the partial final
complexes’ synthesis.
Linear, two-coordinate (carbene)Metal(amide) (cMa) complexes have been recently
investigated as possible alternatives to state-of-the-art iridium-based phosphors for organic light
emitting diodes (OLEDs).
1-15
These emitters are composed of a carbene acceptor and an amide
donor ligand bridged by the monovalent coinage metal ion. These two-coordinate complexes
emit via thermally activated delayed fluorescence (TADF), with efficient luminescence in
microsecond to sub-microsecond time scale.
2, 3, 5, 16-18
In TADF, a small singlet-triplet splitting
energy (EST) favors intersystem crossing (ISC) from the lowest energy triplet (T1) to singlet
(S1) states, followed by emission from S1. The cMa complexes give high ISC rates (10
10
~10
11
s
-
1
), markedly outpacing TADF or emission from S1, manifesting in a mono exponential decay at
room temperature.
18
This fast ISC rate is due to the strong spin orbital coupling (SOC)
19, 20

provided by the central metal ion. The high ISC rate leads to a simplification of the kinetic
scheme, such that the TADF radiative lifetime is given by 𝜏 𝑇𝐴𝐷𝐹 =
𝜏 𝑆 1
𝐾 𝑒𝑞
(𝑇 1
⇄ 𝑆 1
)
⁄
. Thus the
high rates of emission in cMa complexes are due to both a short S 1 lifetime and a comparatively
large Keq due to a small EST.
21

The small EST in the TADF compounds is achieved by spatially separating highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). In
organic TADF molecules this spatial separation is typically accomplished by steric interactions
between the donor and acceptor moieties that hold the HOMO and LUMO in an orthogonal
52

relationship, minimizing overlap between the two unpaired electrons in the S 1 and T1 states.
22-26

The ligands, and therefore HOMO and LUMO, are coplanar in most of the reported cMa
complexes, and in a case where both coplanar and orthogonal ligand relationships are present, a
three-fold lower radiative rate was found for the orthogonal relationship compared to its coplanar
analog.
27
Consequently, one design strategy effective in highly emissive cMa emitters has been
to employ sterically bulky carbene ligands to constrain carbene (acceptor) and carbazole (donor)
to a coplanar orientation and limit reorganization in the excited state.
Rotation and bending around the metal-ligand bond are considered to be main
deactivation pathways of the excited states in cMa molecules.
6, 10, 15, 27-31
This notion is based on
photophysical studies of cMa molecules, which have shown that non-radiative decay rates are
largely suppressed in a rigid matrix like polystyrene (PS) when compared to fluid solution.
Therefore, cMa complexes typically use carbene ligands flanked on each side with bulky
moieties such as diisopropyl phenyl (dipp),
5, 6, 18, 28
or bulky alkyl groups (Figure 3.1, left). The
steric bulk of these groups confines the adjacent carbazolyl ligand within a “pocket” that
enforces a coplanar orientation with respect to the carbene. In a previous study we showed that
decreasing the steric bulk of alkyl groups of cyclic (alkyl)(amino)carbene (CAAC) leads to a
marked increase in the nonradiative rate for emission in fluid solution, consistent with the need
for bulky groups to hinder the carbazolyl rotation.
27
Thus, sterically bulky carbene ligands are
considered to be an essential component to prevent rotation and bending in the excited state in
luminescent cMa complexes.
In contrast to previously published work, the present study examines cMa complexes
coordinated with an asymmetric thiazolyl carbene and various substituted carbazolyl ligands to
probe rotational deactivation of the excited state. To that end, we synthesized a series of
complexes (Thia)Cu(Cz) (1-H), (Thia)Cu(Me-Cz) (1-Me) and (Thia)Cu(iPr-Cz) (1-iPr) as shown
in Figure 3.1, right. Alkyl substituents ortho to N on the carbazolyl (position 1) were chosen to
provide varying degrees of steric hindrance while also having a minimal impact on the
luminescence energy of the complexes. By keeping the emission energy constant in the
complexes, secondary effects due to the energy gap law can be eliminated in the observed
nonradiative decay rates. The alkyl groups on the carbazolyl donor serve to hinder rotation about
53

the metal-ligand bond axis. We anticipated that by increasing the size of the alkyl substitution,
increased barriers to rotation would suppress non-radiative decay pathways upon excitation.
Single crystal X-ray diffraction data reveal an unexpected preference for the syn-conformation in
solid state. Nuclear magnetic resonance (NMR) spectroscopic studies show dynamic equilibria
in solution between the syn- and anti-conformers that varies with increasing steric bulk of the
substituent at the 1-position of the carbazolyl. The barrier to rotation around the metal-ligand
bond axis was theoretically evaluated for the complexes using potential energy surface (PES)
scans. Photophysical characterization determined that the non-radiative rates of this series of
complexes decreased with increasing steric bulk of the substituent on the carbazolyl ligand.

3.2 Results and Discussion
3.2.1 Synthesis
Substituted carbazoles, X-Cz (X = H, Me, iPr), were synthesized using a literature
procedure modified to be performed in a pressure flask heated to 150 °C overnight instead of a
microwave reactor for 3.5 h.
32
The carbene precursor, ThiaBF4, was synthesized following
literature procedure.
33
ThiaCuCl was synthesized following a modified procedure by Shi, et al.
5

Dropwise addition of potassium bis(trimethylsilyl)amide base to a solution of excess CuCl
(3~4 equiv) and ThiaBF4 mixture to prevent polymerization of thiazolyl carbene. Finally, the
(Thia)Cu(X-Cz) complexes were synthesized following a similar prep to that by Shi, et al.
5

X-Cz was deprotonated using NaOtBu and then treated with (Thia)CuCl to form the respective

Figure 3.1 (left) Recently reported (carbene)Metal(carbazolyl) emitters;
(right) The complexes presented in this paper.
54

(Thia)Cu(X-Cz) complexes in high yield (72-82%). Complexes 1-H and 1-Me are isolated as
yellow powders whereas 1-iPr is a pale white powder. These compounds are considerably more
air sensitive than the (CAAC)Cu(Cz) analogs. Samples will oxidize in air over a period of
weeks in the solid state and within ca. 10 minutes in fluid solution.
3.2.2 Crystal Structure
X-ray structures for single crystals of complexes 1-H, 1-Me and 1-iPr grown in layered
CH2Cl2/pentane are shown in Figure 3.2, selected geometric data is listed in Table 3.1.
Surprisingly, the crystal structures obtained for both 1-Me and 1-iPr are in the syn-conformation
despite the expectation that the anti-conformer would be favored energetically (vide infra).
Powder X-ray diffraction analysis of microcrystalline powders confirmed that the
syn-conformation is maintained in bulk samples of 1-Me and 1-iPr. The preference of the
syn-conformation is due to more efficient packing in the crystals. As shown in Figure 3.3, the
molecules stack in a head-to-tail fashion which enables the dipp and alkyl to flank both sides of
the dimeric unit.

Figure 3.2 Single crystal X-ray structure of complexes 1-H, 1-Me and 1-iPr with thermal
ellipsoids at 50%. Hydrogens were omitted for clarity.
55

The metal-ligand bond lengths in all three complexes are near equal (Cu-Cthia =
1.87−1.88 Å) and (Cu-Ncz = 1.86−1.87 Å). These bond distances agree with values found in
previously reported carbene-Cu-amide complexes.
5, 27
A coplanar geometry was found at the
ligated atoms (sum of angles around NCz and CThia = 360). Complex 1-H displays a near linear
two-coordination geometry (C(thia)-Cu-N(cz) = 177), whereas complexes 1-Me and 1-iPr are
slightly bent due to the steric interactions between the dipp moiety and the alkyl groups on the
carbazolyl (C(thia)-Cu-N(cz) = 172 and 166 respectively). However, the dihedral angles between
the thiazolyl and carbazolyl ligands are less affected by the alkyl-substituents and have a near
coplanar orientation.

Figure 3.3 Crystal packing of 1-Me (left) and 1-iPr (right)

Complex 1-H 1-Me 1-iPr
Cu-C(thia)
a)
(Å) 1.87 1.88 1.87
Cu-N(Cz)
b)
(Å) 1.86 1.86 1.87
bond angle (°)
C(thia)-Cu-N(Cz)
177 173 166
dihedral angle (°)
plane(thia)-plane(Cz)
13 5 13
a)
C(thia) denotes the thiazole carbene carbon.
b)
N(cz) denotes the carbazolyl nitrogen.
Table 3.1 Selected geometric data from X-ray single crystal measurements.
56

3.2.3 NMR studies
Information regarding the dynamics of ligand rotation in solution was obtained using
1
H
NMR spectroscopy. Proton resonances in aromatic region of the complexes are shown in Figure
3.4. The
1
H NMR spectrum of complex 1-H displays only four resonances for the carbazolyl
ligand (a/h, b/g, c/f and d/e) despite being exposed to the asymmetric environment of the
thiazolyl ligand (Figure 3.4). This simple pattern in the carbazolyl indicates that rotation along
the metal-ligand bond axis is sufficiently rapid on the NMR timescale to render pairs of protons
equivalent. In contrast, protons b/g, c/f and d/e in complexes 1-Me and 1-iPr are shifted relative
to each other owing to the absence of structural degeneracy in the substituted carbazolyl. The
alkyl groups on the carbazolyl ligand also hinder exchange between syn- and anti-conformers.
However, variable temperature
1
H NMR experiments showed no coalescence between
resonances upon cooling to -70°C. Therefore, the barrier to exchange between conformations is
not high enough to slow the rotation of the ligands sufficiently to observe a static structure at
these temperatures.
Previous studies of cMa complexes showed resonances for protons ortho to N on the
carbazolyl that were shifted upfield owing to shielding effects imparted by close proximity to the
ring currents from the adjacent arene in the dipp moiety.
5, 27, 28
Owing to the asymmetry of
thiazolyl carbene ligand, protons a/h in complex 1-H should appear in the
1
H NMR spectra as
two separate resonances in the absence of rotation. Therefore, the signal for these protons at  =
6.80 ppm represents the average value of an upfield and downfield shift for the hypothetical
static structure. For reference, the resonance for the same protons in the free carbazole ligand
come at 7.5 ppm. Similarly, proton a in the syn- and anti-conformers of complexes 1-Me and
1-iPr will shift upfield and downfield during rotational exchange depending on whether the
proton is directed toward the arene ring of the dipp group or not. For example, the calculated
1
H
NMR spectra of complex 1-iPr shows that when the conformation flips between anti- to syn-,
protons a, b, c and d shift downfield, whereas protons e, f and g shift upfield (Figure 3.5).
Therefore, the simple
1
H NMR spectrum for this complex is the result of dynamic equilibrium
between these two sets of chemical shifts and the chemical shift of proton a on the carbazolyl
ligand is strongly influenced by the equilibrium concentrations of syn- and anti-conformers. For
example, the resonance of proton a in complex 1-Me shifts upfield to  = 6.55 ppm and further to
57

 = 6.25 ppm in complex 1-iPr. The same trend is also observed in both complexes for proton b.
The resonances of all these protons reveal that the anti-conformer dominates in complexes 1-Me
and 1-iPr. Hence, the syn-anti equilibrium favors the anti-conformation to a greater extent in
complex 1-iPr compared to 1-Me.

Figure 3.4 (top) (Thia)Cu(X-Cz) with protons labelled;
(bottom) aromatic region of the
1
H NMR spectra for 1-H, 1-Me and 1-iPr in acetone-d6 at RT
58

Figure 3.5 Calculated
1
H NMR spectra for the anti- and syn-conformers of 1-iPr.

3.2.4 Computational Studies
Density functional theory (DFT) calculations were performed on the ground states for the
complexes at the B3LYP/LACVP* level. A near linear structure was determined in the
complexes (C(thia)-Cu-N(cz) ~180°) with anti-conformers favored in 1-Me, and 1-iPr. The highest
occupied molecular orbital (HOMO) in all complexes is principally localized on the carbazolyl
ligand whereas the lowest unoccupied molecular orbital (LUMO) is primarily localized on the
thiazolyl ligand (Figure 3.6 (top)). The alkane groups in 1-Me and 1-iPr do not significantly
perturb HOMO energies. Time dependent DFT (TD-DFT) calculations using CAM-B3LYP give
similar lowest singlet (S1) and triplet (T1) energies (Figure 3.6 (bottom)) across the (Thia)Cu(X-
Cz) series as the states are mainly comprised of a transition from HOMO to LUMO.
59

To theoretically evaluate the barrier to rotation about the metal-ligand bond, PES
calculations were performed on 1-H, 1-Me, and 1-iPr as dihedral angles between carbene and
carbazolyl were varied from 0 (anti-conformer) to 180 (syn-conformer) at the
B3LYP/LACVP* level applying a DFT-D3(BJ) dispersion correction. The results for these
calculations are shown in Figure 3.7. As expected, bulkier substituents increase the energy
barrier for rotation, which should significantly impede exchange by rotation along the
metal-ligand bond. The energy barrier maximizes at 105 for both 1-H (2 kcal/mol) and 1-Me
(4 kcal/mol), whereas 1-iPr peaks at 120 (8 kcal/mol). The larger dihedral angle reached in
1-iPr is ascribed to the need to achieve a greater distortion of the copper-carbazolyl bond to pass
the bulky alkyl substituent around the dipp group. The energy difference between anti- and
syn-conformers of 1-Me (0 and 180°, respectively) was found to be 2 kcal/mol. The equilibrium
constant between these two conformations was calculated to be ~0.034 indicating that ~3% of
the molecules will be in the syn-conformation at 300 K. The calculated energy differences for
the syn- and anti-conformers of 1-iPr (4 kcal/mol) implies an equilibrium constant of ~0.001 and
thus ~0.1% of the complex will be in the syn-conformer of the at 300 K.

Complex HOMO (eV) LUMO (eV) S1 (eV) T1 (eV)
1-H -4.14 -1.80 2.93 2.67
1-Me -4.14 -1.77 2.92 2.64
1-iPr -4.11 -1.77 2.91 2.66

Figure 3.6 (top) HOMO (solid) and LUMO (mesh) orbitals of complexes
1-H, 1-Me, and 1-iPr. Hydrogens were omitted for clarity. (bottom) Calculated values of
HOMO, LUMO, S1 and T1 energies.
60

3.2.5 Photophysical properties
UV-visible absorption spectra were recorded for all complexes in toluene (Figure 3.8(a)).
Structured absorption bands at high energy ( = 300-370 nm) are assigned to π-π
*
transitions
localized on the carbazolyl ligands. Broad, low energy bands are assigned to an intramolecular
ligand-to-ligand charge transfer (
1
ICT) from donor carbazolyl (X-Cz) to acceptor carbene
(thiazole). As shown in Figure 3.8(a), the extinction coefficient of the ICT band in toluene
increases in the order of 1-H ( = 4.8 x 10
3
M
-1
cm
-1
) < 1-Me ( = 6.8 x 10
3
M
-1
cm
-1
)  1-iPr ( =
7.2 x 10
3
M
-1
cm
-1
). The same trend was observed in the rigid matrix PS films (Figure 6b).
However, the oscillator strength (ƒ) calculated for
1
ICT transition with optimized molecular
0 30 60 90 120 150 180
0
2
4
6
8
Energy Barrier (kcal/mol)
Dihedral Angle ()
1-H
1-Me
1-iPr

Figure 3.7 (top) Space-filling diagrams of (Thia)Cu(XCz) complexes
at the maximum of energy barrier;
(bottom) Potential energy surface scan of (Thia)Cu(X-Cz) complexes
61

geometries (coplanar) have similar value: ƒ = 0.15, 0.16 and 0.13 for 1-H, 1-Me and 1-iPr,
respectively. The oscillator strength will decrease when overlap between the HOMO and LUMO
is diminished, such as caused by an increase in the dihedral angle between the ligands. Previous
work on related cMa complexes has shown that the extinction coefficient of
1
ICT band in a
complex with ligands in an orthogonal conformation is three-fold weaker relative to that of a
complex with ligands in a coplanar conformation.
3
Thus, the lower extinction coefficient
observed for the ICT transition in complex 1-H is likely attributed to conformers that have
carbene and carbazolyl ligands twisted relative to each other.
Luminescence spectra were recorded for the complexes in 2-MeTHF (3.9(a)), MeCy
(3.9(b)) and 1 wt% polystyrene (PS) (3.9(c)) at RT and 77 K. Data for the luminescence
properties are summarized in Table 3.2. The complexes have broad and featureless
1
ICT based
emission both in solution and in a PS matrix at RT. The spectra of the complexes all display
solvatochromic and rigidochromic behavior as observed for other two-coordinate coinage metal
complexes in different matrixes and temperatures.
5, 27
Emission spectra (bottom of Figures 3.9(a),
3.9(b), 3.9(c)) and lifetime data (Table 3.2) were also obtained at 77 K. A large blue shift in the
λmax of emission is due to destabilization of the
3
ICT state in the rigid media, which leads to the
triplet carbazole (
3
LE) being the lowest excited state at 77 K in 2-MeTHF and MeCy. A broad,
concentration-dependent emission band around 525 nm was also observed for complexes 1-H
and 1-Me at 77 K in MeCy and is assigned to an aggregate due to the poor solubility of these
complexes in this solvent (Figure 3.10). In PS films, emission features assigned to both
3
LE and
3
ICT states are observed as the hypsochromic shift of the
3
ICT state places it close in energy to
the triplet state on the carbazolyl ligand. Multiexponential lifetimes were observed for all
compounds in frozen media and are likely the result of the complexes being trapped in multiple
conformers at 77 K.
The photoluminescence quantum yields in solution and a rigid PS matrix range from
moderate (PL = 0.5) to near unity (Table 3.2). The radiative rates for these complexes are
similar to values found for other two-coordinate copper emitters (kr = 10
5
s
-1
) and show different
trends in solvent matrixes (Table 3.2). In 2-MeTHF, there is an increase of radiative rates in the
order of 1-H (kr = 5.310
5
s
-1
) < 1-Me (kr = 6.410
5
s
-1
) < 1-iPr (kr = 7.510
5
s
-1
). The relatively
62

low radiative decay rates for complexes 1-H and 1-Me might be due to deactivation of the
excited states by an exciplex with solvent. In toluene, radiative rates are nearly constant (kr
~810
5
s
-1
) across the series even though molar absorptivity of complex 1-H is lower than those
of complexes 1-Me and 1-iPr. An increase in the PL is observed from 1-H < 1-Me < 1-iPr in all
matrixes is the result of a significant decrease in the nonradiative decay rate. Therefore,
increasing the steric bulk on the carbazole ligand leads to greater steric hindrance to rotation, and
consequently decreased nonradiative decay in emission.
A fourth complex, ThiaCu(1-Ph), was synthesized and characterized as well, but the
photophysical properties are quite different from that of other three complexes described above.
The triplet energy of the 1-phenyl-carbazolyl ligand is close to that of the
1
ICT state, giving a
mixed excited state and a complicated decay mechanism relative to the other three complexes.
The synthesis and photophysical data for the 1-Ph complex is included in the Experimental
Section.

300 350 400 450 500
0
5
10
 (10
3
M
-1
cm
-1
)
Wavelength(nm)
1-H
1-Me
1-iPr
Toluene
(a)

300 350 400 450 500
0
0.5
1
1.5
2
2.5
Normalized Absorbance
Wavelength (nm)
1-H
1-Me
1-IPr
PS film
(b)

Figure 3.8 (a) Molar absorptivity of (Thia)Cu(X-Cz) complexes in toluene.
(b) Absorbance of 1 wt% (Thia)Cu(X-Cz) complexes in PS film normalized to the peak at
373 nm.
63

Complex  max
(nm)
 PL 
(s)
k r
(10
5
s
-1
)
k nr
(10
5
s
-1
)
 max, 77 K
(nm)
 77 K
(ms)
2-MeTHF
1-H 510 0.49 0.93 5.3 5.5 430
4.9 (35%)
11 (65%)
1-Me 510 0.73 1.14 6.4 2.4 430
8.4 (39%)
19 (61%)
1-iPr 510 0.95 1.27 7.5 0.4 430
6.7 (37%)
15 (63%)
MeCy
1-H 475 0.60 0.85 7.0 4.7 430
3.4 (55%)
9.5 (45%)
1-Me 485 0.67 0.97 6.9 3.4 430
4.5 (45%)
15 (55%)
1-iPr 490 0.76 1.30 5.8 1.8 430
2.3 (39%)
11.2 (61%)
toluene
1-H 504 0.81 0.99 8.2 1.9
1-Me 504 0.88 1.15 7.7 1.0
1-iPr 506 0.99 1.22 8.1 0.08
1 wt% in PS
1-H 470 0.87
2.1 (79%)
9.2 (21%)
2.4 0.36 460
0.3 (9%)
1.9 (48%)
6.1 (43%)
1-Me 475 0.93
1.6 (89%)
4.1 (11%)
4.9 0.37 460
0.2 (33%)
1.1 (26%)
5.6 (41%)
1-iPr 478 0.97
1.6 (92%)
5.5 (8%)
5.1 0.17 480
0.2(45%)
1.0 (26%)
4.3 (29%)

Table 3.2 Summary of photophysical properties of complexes 1-H, 1-Me and 1-iPr
in 2-MeTHF, MeCy, toluene and 1 wt% in PS. The concentration of solutions is 10
-5
M.
64

400 500 600 700
0
0.5
1
Normalized PL
Wavelength (nm)
initial solution
diluted 10 times
diluted 100 times
1-H
MeCy

Figure 3.10 Emissions of 1-H in MeCy normalized to 425 nm carbazolyl peak

400 450 500 550 600 650 700
0
0.5
1
400 450 500 550 600 650 700
0
0.5
1

1-H
1-Me
1-iPr
RT
2-MeTHF
(a)

Normalized PL Intensity
Wavelength (nm)
77 K
400 450 500 550 600 650 700
0
0.5
1
400 450 500 550 600 650 700
0
0.5
1

Normalized PL Intensity
RT
MeCy
(b)

Wavelength (nm)
77K
*
*
400 450 500 550 600 650 700
0
0.5
1
400 450 500 550 600 650 700
0
0.5
1

PS Film
RT
(c)

Normalized PL Intensity
Wavelength (nm)
77 K

Figure 3.9 Emission spectra of (Thia)Cu(X-Cz) complexes
in (a) 2-MeTHF, (b) MeCy, and (c) 1 wt% in PS films.
The emission band marked with an asterisk in MeCy at 77 K is assigned to an aggregate (see SI).
The concentration of solutions is 10
-5
M.
65

3.3 Conclusion
A series of two-coordinate carbene-copper-carbazole complexes with substituted
carbazolyl ligands (X-Cz where X = H, Me, iPr) were synthesized. The substituents on the
carbazolyl ligand were strategically designed to impede rotation about metal-ligand bond axis.
Crystallographic data indicates that the syn conformation of 1-Me and 1-iPr is preferred in the
solid state, whereas NMR spectra suggests the anti-conformation dominates in solution. The
NMR spectra elucidate the dynamic process that equilibrates syn- and anti-conformers.
Increasing the steric bulk of the group in the 1-position of the carbazolyl favors the anti-
conformer. Potential energy calculations confirmed that the barrier to rotation increases in the
order 1-H < 1-Me < 1-iPr showing that the steric bulk of the substituent considerably impacts
rotation around the metal-ligand bond axis. An increase in the photoluminescence quantum
yields of these emitters across the series from 1-H < 1-Me < 1-iPr is mainly accompanied by a
substantial decrease in the nonradiative rate. The luminescence of these complexes in solutions
and a rigid matrix demonstrate how steric bulk of the substituents can inhibit nonradiative decay
caused by bond rotation in the excited state.

3.4 Experimental Methods
Synthesis of ThiaCuCl ：ThiaBF4 (500 mg, 1.38 mmol, 1 eq) and CuCl (274 mg, 2.77
mmol, 2 eq) were added to a Schlenk flask. The flask was pumped and purged with N 2 gas three
times. THF (100 mL) was added to the flask and the mixture was allowed to stir for ~15 minutes.
KHMDS (1.98 mL, 0.7 M, 1 eq) was added dropwise to the flask and the mixture was allowed to
stir at RT overnight. The crude mixture was filtered through celite and the filtrate was rotavaped
to dryness. The resulting solid was dissolved in minimal acetone and precipitated using
hexanes/pentanes. Yield: 0.42 g, 81%.
1
H NMR (400 MHz, Acetone-d6) δ 7.59 (t, J = 7.8 Hz,
1H), 7.45 (d, J = 7.8 Hz, 2H), 2.50 (s, 3H), 2.18 (h, J = 6.9 Hz, 2H), 1.24 (d, J = 6.8 Hz, 6H),
1.19 (d, J = 6.9 Hz, 6H).
13
C NMR (101 MHz, CDCl3) δ 144.68, 141.46, 137.64, 131.0, 130.69,
124.97, 28.59, 25.43, 23.31, 12.66, 12.33.
Synthesis of (Thia)Cu(XCz): XCz (1.05 eq) was added to an oven dried flask. The flask
was pumped and purged with N2 gas three times. THF (~30 mL) was added to the flask followed
66

by NaOtBu (2.0 M, 1.05 eq). This solution was stirred for ~30 minutes. ThiaCuCl (0.5 g,
1.34 mmol, 1.00 eq) was added to the reaction flask and stirred overnight. The solution was
filtered through celite and the solvent was removed in vacuo. The solid was dissolved in
minimum DCM and precipitated with pentane. The resulting solid was washed with ether to get
pure product.
Synthesis of ThiaCuCz (1-H): Yield: 0.52 g, 76%.
1
H NMR (400 MHz, acetone) δ 7.88
(ddd, J = 7.7, 1.3, 0.8 Hz, 2H), 7.78 (t, J = 7.8 Hz, 1H), 7.59 (d, J = 7.8 Hz, 2H), 7.03 (ddd, J =
8.2, 6.9, 1.3 Hz, 2H), 6.87 – 6.80 (m, 4H), 2.60 – 2.56 (m, 3H), 2.35 (p, J = 6.8 Hz, 2H), 2.17 (s,
3H), 1.24 (dd, J = 6.8, 2.5 Hz, 12H).
13
C NMR (126 MHz, Acetone-d6) δ 150.08, 145.15, 142.07,
130.92, 125.08, 124.25, 123.17, 118.92, 115.16, 114.46. Anal. calcd for C29H31CuN2S: C 69.22,
H 6.21, N 5.57, S 6.37, found: C 69.24, H 6.16, N 5.41, S 6.39.
Synthesis of ThiaCuMeCz (1-Me): Yield: 0.57 g, 82%.
1
H NMR (400 MHz, acetone) δ
7.86 (ddd, J = 7.7, 1.4, 0.7 Hz, 1H), 7.78 (t, J = 7.3 Hz, 2H), 7.59 (d, J = 7.8 Hz, 2H), 7.01 – 6.91
(m, 2H), 6.86 – 6.77 (m, 2H), 6.56 (dd, J = 8.1, 0.9 Hz, 1H), 2.65 (s, 3H), 2.58 (s, 3H), 2.36 (p, J
= 6.8 Hz, 2H), 2.18 (s, 3H), 1.25 (d, J = 3.7 Hz, 6H), 1.23 (d, J = 3.7 Hz, 6H).
13
C NMR (101
MHz, Acetone-d6) δ 145.03, 130.91, 125.10, 124.22, 122.91, 118.84, 117.06, 115.33, 115.23,
115.03. Anal. calcd for C30H33CuN2S: C 69.67, H 6.43, N 5.42, S 6.20, found: C 69.49, H 6.29,
N 5.25, S 5.90.
Synthesis of ThiaCuiPrCz (1-iPr): Yield: 0.58 g, 79%.
1
H NMR (400 MHz, Acetone-d6)
δ 7.79 (d, J = 7.9 Hz, 2H), 7.76 – 7.71 (m, 1H), 7.57 (d, J = 7.8 Hz, 2H), 7.07 (d, J = 7.2 Hz,
1H), 6.84 (t, J = 7.5 Hz, 2H), 6.74 (t, J = 7.3 Hz, 1H), 6.20 (d, J = 8.0 Hz, 1H), 4.35 (hept, J =
7.0 Hz, 1H), 2.55 (s, 3H), 2.31 (hept, J = 7.5 Hz, 2H), 2.14 (s, 3H), 1.42 (d, J = 6.9 Hz, 6H), 1.19
(dd, J = 8.8, 6.8 Hz, 12H).
13
C NMR (101 MHz, Acetone-d6) δ 150.04, 145.15, 132.04, 130.90,
125.12, 124.63, 124.51, 122.86, 118.60, 118.59, 116.85, 115.48, 115.10, 114.97. Anal. calcd for
C 32H 37CuN2S: C 70.49, H 6.84, N 5.14, S 5.88, found: C 69.33, H 6.72, N 4.89, S 5.97.
Synthesis of ThiaCuPhCz (1-Ph): Yield: 0.59 g, 76%.
1
H NMR (400 MHz, acetone) δ
7.87 (dd, J = 7.6, 1.3 Hz, 1H), 7.83 (ddd, J = 7.2, 1.7, 0.7 Hz, 1H), 7.77 (d, J = 7.9 Hz, 1H), 7.75
– 7.71 (m, 2H), 7.58 – 7.51 (m, 4H), 7.49 – 7.44 (m, 1H), 7.03 (dd, J = 7.1, 1.3 Hz, 1H), 6.91
(dd, J = 7.6, 7.1 Hz, 1H), 6.83 – 6.72 (m, 2H), 5.75 (ddd, J = 8.0, 1.4, 0.8 Hz, 1H), 2.42 (q, J =
0.8 Hz, 3H), 2.16 (hept, J = 6.8 Hz, 2H), 2.01 (s, 3H), 1.15 (d, J = 6.9 Hz, 6H), 1.11 (d, J = 6.8
67

Hz, 6H).
13
C NMR (101 MHz, Acetone-d6) δ 150.67, 147.99, 145.11, 142.62, 141.51, 130.76,
129.20, 129.17, 128.94, 128.45, 127.44, 127.42, 127.27, 125.64, 125.35, 125.05, 124.10, 123.84,
123.06, 120.00, 119.24, 118.63, 118.52, 115.37, 115.28, 115.24, 111.26, 24.31, 22.36, 11.46,
11.38. Anal. calcd for C35H35CuN2S: C 72.57, H 6.09, N 4.84, S 5.53, found: C 71.63, H 5.68, N
4.58, S 5.69.
The single crystal structure for 1-H was determined at 100 K with Bruker X-ray
diffractometer equipped with an APEX II CCD detector and an Oxford Cryosystems 700 low
temperature apparatus using Mo K radiation. The single crystal structures for 1-Me and 1-iPr
were determined at 100 K with Rigaku Xta LAB Synergy S, equipped with an HyPix-600HE
detector and an Oxford Cryostream 800 low temperature unit, using a Cu K PhotonJet-S X-ray
radiation source. CCDC 2144503 (1-H), 2144571 (1-Me) and 2144572 (1-iPr) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
QCHEM 5.1 software package was used to calculate the properties of all complexes at
the B3LYP/LACVP* level of theory in the gas phase. Potential energy surface (PES) scans used
the same level of theory with a dispersion DFT-D3(BJ) correction. For other experimental
details, please refer to Chapter 2 section 2.4 Experimental Methods.
68

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

2Me-THF
MeCy
PS
RT

Normalized PL Intensity
Wavelength (nm)
77 K

Figure 3.11 Emission spectra of 1-Ph in 2-MeTHF, MeCy, and 1 wt% PS.

Table 3.3 Photophysical data of 1-Ph in 2-MeTHF, MeCy, and 1 wt% in PS film.
1-Ph
 max
(nm)
 PL

(s)
k r
(10
5
s
-1
)
k nr
(10
5
s
-1
)
 max, 77 K
(nm)
 77 K
(ms)
2-MeTHF 530 0.16 198 0.008 0.042 500
1.29 (28%)
3.89 (70%)
19 (2%)
MeCy 516 0.01 3.0 4.7 330 504
0.72 (62%)
2.3 (38%)
1 wt% PS 500 0.62
7.7 (7%)
38 (39%)
130 (54%)
7 4.6 464
0.29 (29%)
0.93 (44%)
3.2 (26%)

69

3.5 References
[1] Di, D.; Romanov, A. S.; Yang, L.; Richter, J. M.; Rivett, J. P.; Jones, S.; Thomas, T. H.;
Abdi Jalebi, M.; Friend, R. H.; Linnolahti, M.; Bochmann, M.; Credgington, D., High-
performance light-emitting diodes based on carbene-metal-amides. Science 2017, 356 (6334),
159-163.

[2] Romanov, A. S.; Becker, C. R.; James, C. E.; Di, D.; Credgington, D.; Linnolahti, M.;
Bochmann, M., Copper and Gold Cyclic (Alkyl)(amino)carbene Complexes with Sub-
Microsecond Photoemissions: Structure and Substituent Effects on Redox and Luminescent
Properties. Chemistry – A European Journal 2017, 23 (19), 4625-4637.

[3] 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.

[4] Romanov, A. S.; Jones, S. T. E.; Yang, L.; Conaghan, P. J.; Di, D.; Linnolahti, M.;
Credgington, D.; Bochmann, M., Mononuclear Silver Complexes for Efficient Solution and
Vacuum-Processed OLEDs. Advanced Optical Materials 2018, 6 (24), 1801347.

[5] 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.

[6] Li, T.-y.; Muthiah Ravinson, D. S.; Haiges, R.; Djurovich, P. I.; Thompson, M. E.,
Enhancement of the Luminescent Efficiency in Carbene-Au(I)-Aryl Complexes by the
Restriction of Renner–Teller Distortion and Bond Rotation. Journal of the American Chemical
Society 2020, 142 (13), 6158-6172.

[7] Gernert, M.; Balles-Wolf, L.; Kerner, F.; Müller, U.; Schmiedel, A.; Holzapfel, M.;
Marian, C. M.; Pflaum, J.; Lambert, C.; Steffen, A., Cyclic (Amino)(aryl)carbenes Enter the
Field of Chromophore Ligands: Expanded π System Leads to Unusually Deep Red Emitting CuI
Compounds. Journal of the American Chemical Society 2020, 142 (19), 8897-8909.

[8] To, W.-P.; Zhou, D.; Tong, G. S. M.; Cheng, G.; Yang, C.; Che, C.-M., Highly
Luminescent Pincer Gold(III) Aryl Emitters: Thermally Activated Delayed Fluorescence and
Solution-Processed OLEDs. Angewandte Chemie International Edition 2017, 56 (45), 14036-
14041.

[9] Conaghan, P. J.; Matthews, C. S. B.; Chotard, F.; Jones, S. T. E.; Greenham, N. C.;
Bochmann, M.; Credgington, D.; Romanov, A. S., Highly efficient blue organic light-emitting
diodes based on carbene-metal-amides. Nature Communications 2020, 11 (1), 1758.

70

[10] Romanov, A. S.; Jones, S. T. E.; Gu, Q.; Conaghan, P. J.; Drummond, B. H.; Feng, J.;
Chotard, F.; Buizza, L.; Foley, M.; Linnolahti, M.; Credgington, D.; Bochmann, M., Carbene
metal amide photoemitters: tailoring conformationally flexible amides for full color range
emissions including white-emitting OLED. Chemical Science 2020, 11 (2), 435-446.

[11] Zhou, D.; To, W.-P.; Tong, G. S. M.; Cheng, G.; Du, L.; Phillips, D. L.; Che, C.-M.,
Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF)
Emitters: Microwave-Assisted Synthesis and High-Performance OLEDs with Long Operational
Lifetime. Angewandte Chemie International Edition 2020, 59 (16), 6375-6382.

[12] Conaghan, P. J.; Menke, S. M.; Romanov, A. S.; Jones, S. T. E.; Pearson, A. J.; Evans,
E. W.; Bochmann, M.; Greenham, N. C.; Credgington, D., Efficient Vacuum-Processed Light-
Emitting Diodes Based on Carbene–Metal–Amides. Advanced Materials 2018, 30 (35), 1802285.

[13] Li, T.-y.; Shlian, D. G.; Djurovich, P. I.; Thompson, M. E., A Luminescent Two-
Coordinate AuI Bimetallic Complex with a Tandem-Carbene Structure: A Molecular Design for
the Enhancement of TADF Radiative Decay Rate. Chemistry – A European Journal 2021, 27
(20), 6191-6197.

[14] Liu, Y.; Yiu, S.-C.; Ho, C.-L.; Wong, W.-Y., Recent advances in copper complexes for
electrical/light energy conversion. Coordination Chemistry Reviews 2018, 375, 514-557.

[15] Yang, J.-G.; Song, X.-F.; Wang, J.; Li, K.; Chang, X.; Tan, L.-Y.; Liu, C.-X.; Yu, F.-
H.; Cui, G.; Cheng, G.; To, W.-P.; Yang, C.; Che, C.-M.; Chen, Y., Highly Efficient
Thermally Activated Delayed Fluorescence from Pyrazine-Fused Carbene Au(I) Emitters.
Chemistry – A European Journal 2021, 27 (71), 17834-17842.

[16] Föller, J.; Marian, C. M., Rotationally Assisted Spin-State Inversion in Carbene–Metal–
Amides Is an Artifact. The Journal of Physical Chemistry Letters 2017, 8 (22), 5643-5647.

[17] To, W.-P.; Cheng, G.; Tong, G. S. M.; Zhou, D.; Che, C.-M., Recent Advances in Metal-
TADF Emitters and Their Application in Organic Light-Emitting Diodes. Frontiers in Chemistry
2020, 8.

[18] 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.

[19] Marian, C. M., Understanding and Controlling Intersystem Crossing in Molecules. Annual
Review of Physical Chemistry 2021, 72 (1), 617-640.

71

[20] Lüdtke, N.; Föller, J.; Marian, C. M., Understanding the luminescence properties of Cu(i)
complexes: a quantum chemical perusal. Physical Chemistry Chemical Physics 2020, 22 (41),
23530-23544.

[21] Ravinson, D. S. M.; Thompson, M. E., Thermally assisted delayed fluorescence (TADF):
fluorescence delayed is fluorescence denied. Materials Horizons 2020, 7 (5), 1210-1217.

[22] Nakanotani, H.; Tsuchiya, Y.; Adachi, C., Thermally-activated Delayed Fluorescence for
Light-emitting Devices. Chemistry Letters 2021, 50 (5), 938-948.

[23] Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M. P.,
Recent advances in organic thermally activated delayed fluorescence materials. Chemical Society
Reviews 2017, 46 (3), 915-1016.

[24] 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.

[25] Dias, F. B.; Penfold, T. J.; Monkman, A. P., Photophysics of thermally activated delayed
fluorescence molecules. Methods and Applications in Fluorescence 2017, 5 (1), 012001.

[26] Chen, X.-K.; Kim, D.; Brédas, J.-L., Thermally Activated Delayed Fluorescence (TADF)
Path toward Efficient Electroluminescence in Purely Organic Materials: Molecular Level Insight.
Accounts of Chemical Research 2018, 51 (9), 2215-2224.

[27] 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.

[28] 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.

[29] Chotard, F.; Sivchik, V.; Linnolahti, M.; Bochmann, M.; Romanov, A. S., Mono- versus
Bicyclic Carbene Metal Amide Photoemitters: Which Design Leads to the Best Performance?
Chemistry of Materials 2020, 32 (14), 6114-6122.

[30] Li, P.; Wang, Z.; Wang, S.; Zhou, C.; Zhang, Y.; Zheng, C.; Chen, R., Construction of
High-Performance Carbene–Metal–Amide-Like TADF Materials: A Theoretical Study. The
Journal of Physical Chemistry C 2021, 125 (48), 26770-26777.

[31] Leitl, M. J.; Krylova, V. A.; Djurovich, P. I.; Thompson, M. E.; Yersin, H.,
Phosphorescence versus Thermally Activated Delayed Fluorescence. Controlling Singlet–Triplet
72

Splitting in Brightly Emitting and Sublimable Cu(I) Compounds. Journal of the American
Chemical Society 2014, 136 (45), 16032-16038.

[32] Bedford, R. B.; Betham, M., N-H Carbazole Synthesis from 2-Chloroanilines via
Consecutive Amination and C−H Activation. The Journal of Organic Chemistry 2006, 71 (25),
9403-9410.

[33] Piel, I.; Pawelczyk, M. D.; Hirano, K.; Fröhlich, R.; Glorius, F., A Family of Thiazolium
Salt Derived N-Heterocyclic Carbenes (NHCs) for Organocatalysis: Synthesis, Investigation and
Application in Cross-Benzoin Condensation. European Journal of Organic Chemistry 2011,
2011 (28), 5475-5484.

73

Chapter 4 Luminescent Binuclear Gold (I) Complexes Utilizing
Janus Carbenes

4.1 Introduction
Jonas Schaab was responsible for crystallography and temperature-dependent
measurements, while I contributed to the project by working on echem and calculations. In
addition to our individual responsibilities, both of us contributed to the synthesis of materials,
photophysics measurements, and fabrication of devices.
The first observation of photoluminescent coinage metal (Cu, Ag, Au) complexes was in
1970,
1
although they were rarely considered for practical applications because of the poor
luminescence efficiency of compounds known at that time. A resurgence in interest in the
photophysics of coinage metal complexes came with the report of highly luminescent
two-coordinate carbene-metal-amide (cMa) complexes of copper, silver and gold.
2-19
The cMa
complexes are composed of a carbene ligand serving as an electron acceptor (A) and an amide
ligand as an electron donor (D) bridged by the monovalent metal ion. By judicious choice of the
two ligands, a metal perturbed amide-to-carbene interligand charge transfer (ICT) is the lowest
energy excited state, which undergoes thermally assisted delayed fluorescence (TADF) with high
photoluminescence efficiency (PL). The high luminescence efficiencies of cMa complexes has
enabled application in organic electronics,
15, 16
whereas the high redox potentials of the ICT
state for the cMa complexes also make them promising candidates as sensitizers for
photocatalysis.
20

Emission from TADF is observed when the lowest excited singlet (S1) and triplet (T1)
state are close enough in energy (ΔEST < 2000 cm
-1
, 250 meV) that the two states are in dynamic
equilibrium at room temperature. The cMa complexes have a large separation (4 Å) between
donors and acceptor ligands which leads to a small energy difference between the S 1 and T1
states, on the order of 100 meV.
3, 4, 19
Rates for intersystem crossing (kISC) of over 10
10
s
-1
have
been observed for cMa complexes leading to a rapid equilibrium between the S 1 and T1 states.
Considering the high  PL values for the cMa complexes, as well as the fast ISC rate, the excited
74

state lifetime is well approximated by the TADF rate. A steady-state pre-equilibrium
approximation allows the luminescence lifetime of TADF (and thus the decay rate of the excited
state) to the product of the radiative rate from the S1 state (𝑘 𝑟 𝑆 1
) and the equilibrium constant
between these states Keq(T1 ⇄ S1), equation 1.
21

𝑘 𝑟 𝑇𝐴𝐷𝐹 = 𝑘 𝑟 𝑆 1
∙ 𝐾 𝑒𝑞
(1)
Here, Keq depends on the value of ΔEST , which in turn is determined by the Coulombic repulsion
between the two unpaired spins in the S1 and T1 states. Spatial separation between the hole (h
+
)
and electron (e
-
) of natural transition orbital (NTO) in the lowest excited states will decrease the
value of ΔEST, thereby increasing Keq. However, there is a trade-off when trying to optimize
values for both EST (Keq) and 𝑘 𝑟 𝑆 1
. The rate of 𝑘 𝑟 𝑆 1
depends on the transition dipole moment
𝜇 𝑆 0
𝑆 1
, which is tied to the product of h
+
and e
-
overlap in the NTO. An increase in the h
+
and e
-

separation (and concomitant decrease in orbital overlap) will decrease EST (and consequently
Keq) but also increase 𝑘 𝑟 𝑆 1
.
Several research groups have performed experimental and theoretical investigations on
mononuclear cMa complexes having various donors and acceptors with the goal to adjust 𝑘 𝑟 𝑆 1

and EST.
11, 12, 14, 22
For example, by selectively extending the -system of the carbene or/and
amide ligands we prepared cMa complexes with lifetimes ranging from 250 ns to 1 s.
11, 12
In
contrast, photophysical studies of binuclear cMa complexes applying this approach are scarce. A
previous report from our group described a binuclear cMa complex prepared by extending a
mono-nuclear carbene by addition of a second metal-carbene, i.e. C: →Au−C: →Au-Cz where C:
is a carbene acceptor, −C: is a bridging ditopic donor-carbene, and Cz in an N-carbazolyl donor.
9

This binuclear complex has a small EST of 50 meV. However, the overlap between the donor-
carbene and the amide is poor, leading to a radiative lifetime for TADF of only 0.5 s.
To further investigate the trade-off between 𝑘 𝑟 𝑆 1
and EST, an alternative design strategy
for binuclear cMa complexes is presented in this work. The binuclear cMa complexes here
utilize Janus carbenes, facially-opposed ditopic ligands, as an acceptor where each end is
coordinated by a metal-amide donor moiety,
23
providing a general structure of
amide-metal:carbene~carbene:→metal-amide (aMccMa). The benefit of this D-A-D
75

structural motif is that 𝑘 𝑟 𝑇𝐴𝐷𝐹 can be enhanced by an increase in value of 𝑘 𝑟 𝑆 1
or a decrease in the
value of ΔEST by increasing the h
+
and e
-
separation.
24-26
Extending the excited states over two
amides and a Janus carbene moiety is expected to increase the oscillator strength of S 1 state of
the chromophore, A similar approach to increase 𝑘 𝑟 𝑇𝐴𝐷𝐹 was reported by Yersin, et al.
25, 27

Alternatively, if the molecular symmetry is broken in the excited state, the binuclear complex
can still benefit from the extended electronic -system of the Janus carbene, leading to a
decrease in ΔEST relative to the mononuclear analogs.
11
Therefore, based on the equation 1, the
overall value for 𝑘 𝑟 𝑇𝐴𝐷𝐹 will be enhanced by either mechanism.
Herein we report a series of binuclear cMa complexes using Janus carbene ligands that
have high PL efficiencies (PL > 0.95) and lifetimes for TADF ( < 300 ns) that are about one
third of their mononuclear analogs. The new design strategy leads to binuclear complexes with
small exchange energies (EST = 40 to 50 meV) and short radiative lifetimes from S1 to S0 states
(S1 12 ns). Interestingly, these binuclear complexes show solvent dependent absorption and
emission, supporting the loss of two-fold symmetry in the excited states of these materials.
Lastly, two of the binuclear complexes were used as emissive dopants in organic light emitting
devices (OLEDs), showing good quantum efficiency and small efficiency roll-off at high
brightness.

4.2 Results and Discussion
4.2.1 Synthesis
The chemical structures of two Janus carbenes used here, 1,3,6,8-tetrakis(2,6-
diisopropylphenyl)-3,4,8,9-tetrahydropyrimido[4,5-g]quinazoline-1,6-diium (BAZ) and 3,4-
dimethyl-1,7-bis(2,6-diisopropylphenyl)benzobis(imidazolium) iodide (BBI) are shown in
Scheme 4.1. The synthesis of the BAZ ligand was adapted from literature procedures,
28
whereas
BBI ligand was prepared by modifying a protocol of Bielawski, et al..
28, 29
The bulky BBI
imidazolate was prepared by treating dichlorodinitrobenzene with an excess of
2,6-diisopropylaniline (DIPA) as both reactant and solvent and heated to 150 C overnight. The
metal precursor complexes were prepared by treatment of BAZ ligand with potassium
bis(trimethylsilyl)amide followed by reaction with (MeS)2AuCl, whereas the BBI gold chloride
76

complex was prepared using a weaker base (Ag2O) to prevent polymerization of the BBI ligand.
The final cMa complexes were made using three varying donor amides, N-carbazole (Cz), 3,6-
di-tert-butylcarbazole (BCz) and N-benzo[d]benzo[4,5]imidazo[1,2-a]-imidazolyl (bim). Their
corresponding mononuclear complexes were synthesized in an analogous procedure using either
1,3-bis(2,6-diisopropylphenyl)-3,4-dihydroquinazolin-1-ium (BZAC) or 1,3-bis(2,6-
diisopropylphenyl)benzo-imidazolium (BZI) acceptor carbenes. The details of synthesis and
characterization of the complexes are given in the Experimental Section.
The binuclear complexes are significantly less soluble than their mononuclear analogs.
The solubilities of the three binuclear complexes in a range of solvents are listed in Table 4.1.
For example, complexes with unsubstituted carbazole ligands are soluble in a moderately polar
solvent like THF and insoluble in both polar solvents, such as DMSO and MeCN, as well as
non-polar solvents, such as cyclohexane and toluene. Substitution of tert-butyl groups onto
carbazolyl (BCz) markedly improves the solubility of the complexes in all solvents. Thus,
discussion of electrochemical and photophysical properties on these complexes will be focused
on the complexes with BCz and bim ligands (BCzAu
BAZ
AuBCz, BCzAu
BBI
AuBCz and bimAu
BAZ
Aubim)
and their corresponding mononuclear analogues (BCzAu
BZAC
, BCzAu
BZI
and bimAu
BZAC
).

Scheme 4.1 Synthesis of binuclear cMa complexes
Table 4.1: Solubility of binuclear complexes in various organic solvents.
Solvents MeCN DMSO DMF Acetone DCM THF Toluene Hex MeCy
77

CzAu
BAZ
Au Cz insol insol insol sol sol sol sl sol insol insol
BCzAu
BAZ
Au BCz insol insol insol sol sol sol sol sl sol sl sol
BCzAu
BBI
Au BCz sl sol sl sol sol sol sol sol sol sol sol
bimAu
BAZ
Au bim insol insol insol sol sol sol sol sl sol sl sol
sol indicates soluble > 1g/100 ml; sl sol indicates slightly soluble (0.1 to 1) g/100ml; insol indicates
insoluble < 0.1 g/100 ml

4.2.2 Crystallographic Analysis
Single-crystal X-ray structures were determined for binuclear complexes with BAZ
coordinated to Cz, BCz and bim amides, as well as for the mononuclear complexes BCzAu
BZAC

and BCzAu
BZI
. Unfortunately, X-ray quality crystals could not be obtained for binuclear
complexes using the BBI carbene. Representative structures of the compounds of
BCzAu
BAZ
AuBCz and bimAu
BAZ
Aubim are shown in Figure 4.1. The binuclear complexes crystallize
in centrosymmetric space groups with the inversion center located in the middle of the bridging
aromatic ring of the carbene. There are no significant differences in metrical parameters between
the binuclear complexes and the related bonds in the mononuclear cMa analogs.
2, 7, 11
All
complexes display near linear two-coordinate geometries (Ccarbene-Au-Namide = 175177°) along
with near equal Au-Namide and Au-Ccarbene bond lengths (1.989-2.020 Å). The sum of angles
around the ligated Namide and Ccarbene atoms are 360°, indicating a trigonal planar geometry of the
Namide and Ccarbene atoms. The binuclear complexes crystallize with both amides and the carbene
in a near coplanar conformation as dihedral angles for these ligands vary between 0° to 5°. The
amide ligands in bimAu
BAZ
Aubim are oriented antiparallel to each other, with the longer side of the
amide opposite to the methylene group of the BAZ carbene (Figure 4.1(b)). A notable S-shaped
curvature is also apparent in the molecular plane of the complex. This distortion is attributed to
crystal packing forces as a planar arrangement of carbene and amides.

(a) (b)
78

Figure 4.1 Crystal structures of (a) BCzAu
BAZ
AuBCz and (b) bimAu
BAZ
Aubim
Table 4.1 Selected crystal data.
Compound
Bond length Bond angle
C carbene-Au-
N amide (˚)
Dihedral angle
C carbene-Au-
N amide (˚)
 Angles
C carbene-Au
(Å)
Au-N amide
(Å)
C carbene / N amide
BCzAu
BAZ
Au BCz 1.989(4) 2.007(4) 177.0(2) 5.5 360 / 360
BCzAu
BZAC
2.009(4) 2.013(4) 176.8(2) 0.6 360 / 358
CzAu
BAZ
Au Cz 1.995(9) 2.020(7) 177.9(3) 0.6 360 / 358
bimAu
BAZ
Au bim 1.993(4) 2.017(3) 175.4(2) 5.4 360 / 360

4.2.3 Computational Results
The electronic structure of the mono- and binuclear complexes were examined using
density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, with
B3LYP/LACVP and cam-B3LYP/LACVP methods, respectively. Simplified carbenes
substituted with methyl groups (BBI and BZI), in place of bulky dipp moieties, were used to
streamline the calculations (Figure 4.2) The CzAu
BBI
AuCz complex was optimized by enforcing
D2h symmetry, and C2V symmetry was applied for the CzAu
BZI
complex. The results for the
related derivatives, CzAu
BAZ
AuCz and bimAu
BAZ
Aubim, optimized under Ci symmetry, along with
their corresponding mononuclear complexes, follow the same trends.
79

Figure 4.2 Top: frontier molecular orbitals for CzAu
BBI¢
AuCz (left) and CzAu
BZI¢
(right).
The isovalue are set to 0.1. The orbital contributions, energies (oscillator strength) and
magnitude of the electronic dipole moment are given for the ground and Sn states below the
structures. The direction of the dipole moment for each state is indicated with the arrow.
The symmetry of the binuclear complexes impacts the oscillator strength significantly.
The HOMO and HOMO-1 are similar in energy (-4.57 and -4.62 eV, respectively), consisting of
in phase and out of phase combinations of the carbazole -orbitals, Figure 4.2. The lowest
unoccupied molecular orbital (LUMO) of binuclear complexes is delocalized throughout the
bridging carbene ligand. The HOMO-1 for the mononuclear analog, at an energy roughly 0.6 eV
deeper than the HOMO shows no contribution from the nitrogen of the carbazole (Figure 4.2,
right). The binuclear complex exhibit two ICT transitions, which generate an S1 state with large
oscillator strength (0.6) and an S2 state with zero oscillator strength. The S1 state of the
mononuclear analog has an oscillator strength one third that of the S 1 state of the binuclear
complex. Additionally, the net electronic dipole moments for ground and excited states for
80

CzAu
BBI
AuCz complex in D2h symmetry are zero. Therefore, the hole density of the S1 and the
lowest triplet excited state (T1) states evenly distribute over two amides (Figure 4.3).

Figure 4.3 The NTOs for wavefunctions of CzAu
BZI¢
(left) and CzAu
BBI¢
AuCz (right).
Rotation about the metal-ligand bond can break the two-fold molecular symmetry in
binuclear complexes and will markedly alter their electronic properties. Thus, the geometries of
the binuclear complexes in the ground state were also optimized without symmetry. Asymmetry
was imparted by constraining one dihedral angle between the carbene and the amides (Cz or
bim) at 0 while twisting the other dihedral angle () away from 0º. The energies and
compositions for the S1 state are not significantly affected with twist angles up to 20. However,
when  increases to 30 the hole of the S1 state is localized on the twisted Cz, the oscillator
strength decreases to 0.45 and the net dipole moment of S1 state dramatically increases to 16 D.
The magnitude for the dipole moment is comparable to the value for the mononuclear analogue
and indicative of long-range charge transfer in the excited state (Figure 4.2).
3, 30
The oscillator
strength of the S1 state continues to decrease as the dihedral angle increases to 90º where the
value drops to zero In brief, a large  angle ( > 20 for the BAZ based complexes and 30 for
the BBI based complex) leads to a low oscillator strength and a large change in the net dipole
moment for the S1 state of the binuclear complexes.
To evaluate the energy barrier of rotation about the metal-ligand bond, potential energy
surface (PES) calculations were performed using the B3LYP/LACVP method including a
81

DFT-D3(BJ) dispersion correction. Both mono- and binuclear complexes with dipp groups were
examined by varying the dihedral angles between a carbene and amide from 0° to 180°. The
results for these calculations are shown in Figure 4.4. Energy barriers for rotation from 0° to
180° are similar for corresponding mononuclear and binuclear complexes. The energy barriers
for all the derivatives remain low until reaching 30° (< 1.6 kcal/mol), which will allow facile
libration about the metal-ligand bond. This geometric twisting of the metal-ligand bonds will
break the two-fold molecular symmetry, particularly in a fluid medium. The CzAu
BBI
AuCz
complex has a moderate energy barrier attributed to the absence of one dipp group on each side
of carbene and a larger angle imparted by the five-membered ring. It is noteworthy that the
energy barrier of bimAu
BAZ
Aubim is the lowest among three binuclear complexes despite the
presence of bulky dipp groups on the BAZ ligand. The energy barrier maximizes at 100° for both
BAZ ligand based binuclear complexes, while it is only 5.4 kcal/mol for CzAu
BAZ
AuCz and
1.5 kcal/mol for bimAu
BAZ
Aubim. The significant decrease in energy barriers between carbazole
and bim ligands is due to a combination of lower steric hinderance and loss of an attractive edge-
to- interaction between the C-H bonds of the carbazole and dipp moieties.
82

0 30 60 90 120 150 180
0
1
2
3
4
5
6
Energy Barrier (kcal/mol)
Dihedral Angle (°)

Cz
Au
BAZ
Au
Cz

Cz
Au
BZAC

Cz
Au
BBI
Au
Cz

Cz
Au
BZI

Cz
Au
BZI"

bim
Au
BAZ
Au
bim

bim
Au
BZAC

Figure 4.4 Potential energy surface scan of complexes.
The BZI¢¢ is the carbene substituted with a methyl group in place of one dipp moiety.
TD-DFT calculations using cam-B3LYP/LACVP were performed to study the S1, S2, T1
and T2 excited states with varying conformations (Figure 4.5). The geometries optimized in the
PES scan were used for calculations, with a 30-degree dihedral angle interval. The PES scan for
binuclear complexes was conducted by constraining one dihedral angle between the carbene and
the amides (Cz or bim) at 0 while twisting the other dihedral angle () away from 0º. As the
dihedral angle increase from 0 to 90, S1 is the transition mainly from the twisted Cz or bim to the
center carbene, while S2 is the transition mainly from the coplanar Cz or bim. Thus, the S1 is
stabilized to a greater extent by the twisted Cz or bim due to the decreasing electronic
interaction. Additionally, the oscillator strength of the S1 drops as the  increases. T1 and T2
states are slightly affected by the twisting conformations. At 90 degrees, T1 and T2 are close in
energy to the S1 state. In the mono analog, the S1 and T1 state follow the same trend, while the S2
and T2 state are barely affected and remain high in energy.

83

0 30 60 90 120 150 180
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
Energy (eV)
Dihedral Angle (°)
S1
S2
T1
T2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
f for S1
f for S2
Osillator Strength
(a)
Cz
Au
BBI
Au
Cz
0 30 60 90 120 150 180
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
Energy (eV)
Dihedral Angle (°)
S1
S2
T1
T2
(b)
BZI
Au
Cz
0.0
0.2
0.4
0.6
0.8
1.0
f for S1
f for S2
Osillator Strength

0 30 60 90 120 150 180
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
Energy (eV)
Dihedral Angle (°)
S1
S2
T1
T2
0.0
0.2
0.4
0.6
0.8
1.0
f for S1
f for S2
Osillator Strength
(c)
Cz
Au
BAZ
Au
Cz
0 30 60 90 120 150 180
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
Energy (eV)
Dihedral Angle (°)
S1
S2
T1
T2
0.0
0.2
0.4
0.6
0.8
1.0
f for S1
f for S2
Osillator Strength
(d)
BZAC
Au
Cz

0 30 60 90 120 150 180
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
Energy (eV)
Dihedral Angle (°)
S1
S2
T1
T2
(e)
bim
Au
BAZ
Au
bim
0.0
0.2
0.4
0.6
0.8
1.0
f for S1
f for S2
Osillator Strength
0 30 60 90 120 150 180
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Energy (eV)
Dihedral Angle (°)
S1
S2
T1
T2
0.0
0.2
0.4
0.6
0.8
1.0
f for S1
f for S2
Osillator Strength
(f)
BZAC
Au
bim

Figure 4.5 Calculated S1, S2 (with oscillator strength) and T1, T2 energies
with respect to their dihedral angles  of (a) CzAu
BBI
AuCz and (b)
BZI
AuCz; (c) CzAu
BAZ
AuCz and
(d)
BZAC
AuCz; (e) bimAu
BAZ
Aubim and (f)
BZAC
Aubim
84

4.2.4 Electrochemistry
The electrochemical properties of the cMa complexes were examined using cyclic and
differential pulse voltammetry (Figure 4.6), and their redox potentials relative to an internal
ferrocene reference are listed in Table 4.2. The measurements were performed in
N,N-dimethylformamide (DMF) solution except for the BAZ based complexes which were
carried out in tetrahydrofuran (THF) due to poor solubility in DMF. Unfortunately, the reduction
wave of BAZ based complexes overlaps with the THF solvent window and obscures the
reduction peak. Thus, the reduction potential for ClAu
BAZ
AuCl measured in DMF was used to
estimate the value for BCzAu
BAZ
AuBCz and bimAu
BAZ
Aubim. Previous studies have shown that
energies for the LUMO in related two-coordinated complexes are determined by the identity of
the carbene and metal ion, but independent of anionic ligand.
2

Both mono- and binuclear complexes exhibit reversible anodic waves at similar
potentials, ranging from 0.1 V to 0.2 V, that are assigned to oxidation of BCz or bim ligands.
The absence of a second oxidation wave for the binuclear complexes within the potential
window of the solvent indicates that the amide ligands are electronically uncoupled. In contrast,
reduction waves are irreversible, except for BCzAu
BZI
. The potentials for the binuclear complexes
are shifted to less negative values by 0.3 V to 0.4 V than their mononuclear counterparts (Table
4.2). This difference is due to stabilization from the extended conjugation in the -system of the
Janus carbene ligands and replicated in the LUMO energies calculated for both types of species.
85

-1.0 -0.5 0.0 0.5
-60
-40
-20
0
20
Current ( μA)
Potential (V vs. Fc
+/0
)
BCz
Au
BAZ
Au
BCz
*
-2.5 -2 -1.5 -1 -0.5
-100
-80
-60
-40
-20
0
20
40
Current ( μA)
Potential (V vs. Fc
+/0
)
Cl
Au
BAZ
Au
Cl
*

−1.0 −0.5 0.0 0.5
−40
−30
−20
−10
0
10
Current ( μA)
Potential (V vs. Fc
+/0
)
bim
Au
BAZ
Au
bim
*
-2.5 -2 -1.5 -1 -0.5 0 0.5
-60
-40
-20
0
20
Current ( μA)
Potential (V vs. Fc
+/0
)
BCz
Au
BBI
Au
BCz
*

Figure 4.6 CV (black) and DPV (red: oxidation, blue: reduction) traces of complexes
Table 4.2 Electrochemical data.
Measurements were performed using 0.1 M TBAPF6 electrolyte in DMF (except where noted),
and the potentials are listed relative to a ferrocene internal reference.
compound
oxidation
(V)
reduction
(V)
redox gap
(V)
HOMO
d

(eV)
LUMO
d

(eV)
BCzAu
BAZ
AuBCz 0.07
a

b

2.65
-4.87 -
ClAu
BAZ
AuCl - -2.58

- -1.79
BCzAu
BZAC c
0.15
c
-2.83
c
2.95 -4.96 -1.49
BCzAu
BBI
AuBCz
d
0.20 -2.44 2.64 -5.02 -1.95
BZI
AuBCz
d
0.21 -2.85 3.06 -5.03 -1.47
bimAu
BAZ
Aubim

0.22
a

b
2.80 -5.04 -1.79
86

4.2.5 Photophysical properties
The UV-visible absorption and emission spectra of the complexes were recorded in a
toluene solution (Figure 4.7a and 4.7c). The absorption spectra of all complexes display
structured bands at high energy (BCz: λ < 350 nm, bim: λ < 380 nm) that are assigned to -


transitions localized on the carbene and amide ligands. Absorption maxima at 336 and 352 nm
in BCzAu
BAZ
AuBCz are assigned to -* transitions on the carbene as absorption bands at the same
wavelength are found in the precursor complex, ClAu
BAZ
AuCl. Broad featureless absorption bands
at lower energy (λ  350 nm) are assigned to transitions from the ICT state. The ICT transitions
in the binuclear complexes are at lower energy than the mononuclear analogs consistent with the
stabilization of the LUMO observed in the reduction potentials of complexes. Luminescence at
room temperature from all complexes is broad and featureless in all solvents except MeCy,
where emission is vibronically structured as found in previous literature.
2, 6, 8

bimAu
BZAC
0.31
c
-2.69
c
3.00

-5.15

-1.66

a
in THF;
b
insoluble in DMF;
c
from reference
11
;
d
calculated using the equations HOMO = −1.15(E ox) −
4.79; LUMO = −1.18(E red) – 4.83 according to reference
31
.
87

35000 30000 25000 20000
0
1×10
4
2×10
4
3×10
4
4×10
4
5×10
4
Molar Absorptivity (M
-1
cm
-1
)
Energy (cm
-1
)

BCz
Au
BZI

BCz
Au
BBI
Au
BCz

BCZ
Au
BZAC

BCz
Au
BAZ
Au
BCz
(a)
300 350 400 450 500 550
Wavelength (nm)
25000 20000 15000
0.0
0.5
1.0
Photoluminisence (AU)
Energy (cm
-1
)

BCz
Au
BZI

BCz
Au
BBI
Au
BCz

BCz
Au
BZAC

BCz
Au
BAZ
Au
BCz
(b)
400 500 600 700 800
Wavelength (nm)

35000 30000 25000 20000
0.0
5.0×10
3
1.0×10
4
1.5×10
4
2.0×10
4
Extinction Coefficient (M
-1
cm
-1
)
Energy (cm
-1
)

bim
Au
BZAC

Bim
Au
BAZ
Au
Bim
(c)
300 400 500
Wavelegth (nm)
25000 20000 15000
0.0
0.5
1.0
Photoluminisence (AU)
Energy (cm
-1
)

bim
Au
BZAC

bim
Au
BAZ
Au
bim
(d)
400 500 600 700 800
Wavelength (nm)

Figure 4.7 Absorption and emission spectra of mono- and binuclear cMa complexes
with carbazole (a, b) and bim (c, d) in toluene.
The degree of electronic coupling between the amides and Janus carbene responsible for
the ICT transition in the binuclear complexes can be assessed from the intensity of the absorption
band.
32
For example, the molar absorptivities for the ICT transitions in the binuclear complexes
with BCz donors are larger than those in the mononuclear analogs. Integration of the ICT bands
show an increase by a factor 1.9 for BCzAu
BAZ
AuBCz and 1.3 for BCzAu
BBI
AuBCz. In contrast, the
ICT bands for bimAu
BAZ
Aubim and bimAu
BZAC
have similar molar absorptivities (Figure 4.7a, 4.7c).
The intensities follow the same order as the energy barrier to rotation calculated in the PES scan
(BCzAu
BAZ
AuBCz > BCzAu
BBI
AuBCz > bimAu
BAZ
Aubim). This correlation suggests that a high
percentage of BCzAu
BAZ
AuBCz molecules have both amides and carbene ligands in a near coplanar
geometry since the oscillator strength for the ICT transition is markedly lower for highly twisted
88

geometries. Conversely, the low energy barrier to rotation for bimAu
BAZ
Aubim leads to a high
percentage of molecules with the two amides in a random orientation, thus mitigating any
enhancement in molar absorptivity from electronic coupling.
Sovatochromism is a characteristic feature for the optical properties of mononuclear cMa
complexes owing to large differences in the magnitude and direction of the dipole moments for
the ground and excited states.
2, 3
This effect gives rise to large hypsochromic shifts in absorption
and smaller bathochromic shifts for emission with increasing polarity of solvents. This response
to solvent polarity is also displayed in binuclear complexes (Figure 4.8a). The energy of the
absorption band undergoes a larger solvatochromic shift than does the emission band. This effect
can be explained by the solvent reorganization energy being greater in the excited state upon
absorption than in the ground state upon emission (see Figure 4.8b).
33
To evaluate the
solvatochromic shift in the ground and excited states, absorption and emission maxima are
plotted in Figure 4.8c for BCzAu
BAZ
AuBCz and BCzAu
BZAC
against the solvent polarity indexed
using the ET(30) scale.
34
The absorption and emission values for each compound were fit using
a linear least squares routine and the slopes are listed in Figure 4.8d. The absorption and
emission maxima of all compounds show a linear dependence with respect to the solvent
polarity, indicating that emission originates from a common state in all solvents.
35
The absolute
slope for absorption is larger than the one for emission, which is true for all cMa complexes. The
slopes of absorption and emission of the binuclear complexes in the ET(30) plots parallel the
slopes of their mononuclear analogs, indicating the strength of the transition dipoles are similar
for both absorption and emission in mono- and binuclear complexes. Note that if the binuclear
complex remains symmetric in the ground and excited state, the modeling study described above
predicts solvatochromism to be negligible. However, the solvatochromic response indicates that
the geometry is twisted, breaking the molecular symmetry, which causes the excited state to
localize towards one half of the complex and leads to a large net dipole moment for the ICT
state, similar to those of mononuclear complexes.
Emission spectra from cMa complexes are distinguished by large rigidochromic shifts
upon going from fluid solutions to frozen media at low temperatures. Likewise, luminescence
spectra for the binuclear complexes recorded in fluid 2-MeTHF and methylcyclohexane (MeCy)
89

change markedly upon cooling solutions to frozen glass at 77 K. The featureless ICT emission
bands from BCzAu
BAZ
AuBCz and BCzAu
BBI
AuBCz blueshift at 77 K and are replaced by narrow,
highly structured bands, which are assigned to phosphorescence from a triplet excited state
localized on the BCz ligand (
3
LE). This change occurs because the immobile solvent molecules
can no longer exert a reorganization energy, thus the energy for the ICT state becomes
destabilized and rises above that for the
3
LE state. Destabilization of the ICT state occurs to a
lesser extent in a polystyrene (PS) matrix at 77 K, leading only to a blue shift in the ICT
emission band. Similarly, the bimAu
BAZ
Aubim complex displays a narrow, vibronically structured
band at 430 nm in 2-MeTHF, MeCy or PS matrix at 77 K. However, this emission band in this
derivative is assigned to a
3
LE state localized on the BAZ ligand. The energy of the
3
LE state for
the bim ligand is higher (E00 = 365 nm)
11
than BAZ and therefore the amide does not contribute
to emission.
11

90

30000 25000 20000 15000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Absorption (arb.U.)
Energy (cm
-1
)
MeCyHex
Toluene
MeTHF
CH
2
Cl
2
MeCN
(a)
BCz
Au
BAZ
Au
BCz
400 500 600 700 800
Wavelength (nm)

30 32 34 36 38 40 42 44 46
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
Abs
Em
Abs
Em
Energy (x10
4
cm
-1
)
Solvent Polarity Index
BCz
Au
BAZ
Au
BCz BCz
Au
BZAC
(b)

Figure 4.8 Absorption and emission spectra of BCzAu
BAZ
AuBCz in various solvents.
(b) Qualitative potential surfaces for ground and excited states in solvents polarities. (c)
Absorption and emission maxima vs solvent polarity (ET(30) scale) for BCzAu
BAZ
AuBCz and
BCzAu
BZAC
. (d) Slopes of absorption and emission maxima vs ET(30).
An attractive feature of the cMa complexes is high photoluminescence quantum yields
and emission lifetimes for TADF less than 1 s. The binuclear complexes, like mononuclear
complexes, can have near unity quantum yields in nonpolar solvents. Quantum yields and
emission lifetimes decrease with increasing solvent polarity (Table 4.3). Similarly, radiative rates
decrease in more polar solvents. The rate for non-radiative decay in all bimetallic complexes is
(d)
(b)
91

greater than that of the mononuclear complexes. The enhanced nonradiative decay rates for
binuclear complexes are likely due to the additional rotational degrees of freedom introduced by
the second metal-amide ligand rotor and lower energy for emission (energy gap law).
27, 36, 37

Importantly, the binuclear complexes with BCz donor ligands have radiative rates for TADF that
are significantly faster than their mononuclear analogs (see Table 4.3). Hence, BCzAu
BAZ
AuBCz
and BCzAu
BBI
AuBCz complexes have radiative rates around 34  

s
-1
, whereas the highest
radiative rates of mononuclear analogs are near 1.5  

s
-1
. Both binuclear complexes also have
near unit quantum yields in PS films (PL ≥ 0.9). In contrast, the radiative rate for bimAu
BAZ
Aubim
in toluene is only marginally faster with respect to its mononuclear analog. The
photoluminescence quantum yields in PS films for this derivative are also lower than the other
binuclear derivatives (PL = 0.76).
92

Table 4.3 Photophysical data for mono- and binuclear cMa complexes
in toluene and polystyrene films.
Complexes
Abs
 max
(nm)
PL  max
(nm)
 PL
(%)

(s)
k r
(10
6
s
-1
)
k nr
(10
6
s
-1
)
λ max
77 K
(nm)
 77 K
(s)
toluene
BCzAu
BZI
400 475 94 0.64 1.5 0.09 - -
BCzAu
BBI
Au BCz 455 535 >95 0.24 3.9 <0.01 - -
BCzAu
BZAC
410 500 >95 0.56 1.7 <0.01 - -
BCzAu
BAZ
Au BCz 450 525 90 0.33 2.7 0.3 - -
bimAu
BZAC
380 480 >95 0.43 2.3 <0.01 - -
bimAu
BAZ
Au bim 410 505 78 0.29 2.7 0.8 - -
1 wt% polystyrene film
BCzAu
BZI
- 460 94
1.58 (0.5)
8.98 (0.5)
- - 445
65 (0.3)
219 (0.7)
BCzAu
BBI
Au BCz - 515 93 0.21 4.3 0.33 505 25
BCzAu
BZAC
- 484 >95 0.72 1.4 <0.01
BCzAu
BAZ
Au BCz - 506 90 0.30 3 0.33 506 50
bimAu
BZAC
- 452 >95 0.28 3.7 <0.01 - -
bimAu
BAZ
Au bim - 475 76
0.33 (0.8)
2.85 (0.2)
- - 455
27 (0.2)
346 (0.8)

Table 4.3 (continued): Photophysical data for mono- and binuclear cMa complexes in MeCy,
2MeTHF, MeCN and CH2Cl2
Complexes
Abs
λ max
(nm)
PL λ max
(nm)
Φ PL
(%)
τ
(s)
k r
(10
6
s
-1
)
k nr
(10
6
s
-1
)
λ max
77K
(nm)
τ 77 K
(s) (%)
MeCy
BCzAu
BAZ
AuBCz 478 514 93 0.370 2.5 0.19 480 60
BCzAu
BZAC
430 495 >95 0.637 1.5 <0.01 430
92 (47)
390 (53)
BCzAu
BBI
AuBCz 495 530 >95 0.254 3.8 <0.01 475 12.84
93

To probe the origin of the fast radiative rates for TADF in the binuclear complexes,
temperature dependent photophysical measurements were carried out from 4 K to 300 K in
doped PS films. An Arrhenius model for emission decay was used to fit data in the temperature
region of 200-300 K, where TADF is the dominant mechanism emission, to extract values for kS1
BCzAu
BZI

423 452 90 0.723 1.2 0.14 438 265
BimAu
BAZ
AuBim
435 468 >95 0.265 3.7 0.11 451
790 (62)
273 (38)
BimAu
BZAC
385 454 >95 0.290 3.4 0.07 400 259
2-MeTHF
BCzAu
BAZ
AuBCz 426 544 83 0.420 2.0 0.4 442
250 (44)
530 (56)
BCzAu
BZAC
395 515 88 0.664 1.3 0.18 430
533(0.35)
875(0.65)
BCzAu
BBI
AuBCz 426 551 89 0.278 3.2 0.4 438
80 (25)
307 (75)
BCzAu
BZI

375 485 97 0.868 1.1 0.03 438 358
BimAu
BAZ
AuBim
388 522 63 0.327 1.9 1.1 442 563
BimAu
BZAC
363 494 78 0.310 2.5 0.71 400 467
MeCN
BCzAu
BAZ
AuBCz 360 550 75 0.371 2.0 0.67 - -
BCzAu
BZAC
375 525 69 0.801 0.86 0.39 - -
BCzAu
BBI
AuBCz 375 600 34 0.139 2.4 0.47 - -
BCzAu
BZI

350 510 65 2.82 0.23 0.12 - -
BimAu
BAZ
AuBim
367 540 30 2.468 0.12 0.28 - -
BimAu
BZAC
311 522 42 - - - - -
CH 2Cl 2
BCzAu
BAZ
AuBCz 410 545 89 0.374 2.3 0.29 - -
BCzAu
BZAC
375 515 81 0.664 1.2 0.29 - -
BCzAu
BBI
AuBCz 415 580 77 0.274 2.8 0.84 - -
BCzAu
BZI

370 500 66 1.80 0.36 0.19 - -
BimAu
BAZ
AuBim
375 520 80 0.581 1.4 0.34 - -
BimAu
BZAC
350 494 83 0.325 2.6 0.52 - -
94

and EST (Table 4.4). At temperature below 200 K, the emission decay at each temperature was
also fit to a three-level Boltzmann model, which gives the zero-field splitting (ZFS) and the
radiative rate for the T1 state.
2
Data for BCzAu
BZI
is not included in Table 4.4 because the
3
ICT
state and
3
LE state on the BCz ligand have energies close enough to only give biexponential
emission decays traces that change in contributions upon cooling to cryogenic temperatures.
Data for bimAu
BAZ
Aubim is included, despite also having transient decay traces in PS films that
required biexponential fits. However, as opposed to BCzAu
BZI
, a second longer lived component
contributed steadily by 20% to the overall decay trace from 200-300 K. This allowed us to
assume that the decay mechanism is invariant over this temperature range. Only the first (faster)
decay trace was thus used for data evaluation of this complex as TADF is the sole mechanism for
this component.
Comparison kS1 and EST parameters found for the mono- and binuclear complexes
allows us to determine the origin of the faster rates for TADF from the latter compounds
(Table 4.4). Both types of complexes have similar values for kS1, whereas values of EST for the
binuclear complexes are found to be much smaller (EST = 43, 54 and 70 meV for
BCzAu
BBI
AuBCz, BCzAu
BAZ
AuBCz and BCzAu
BZAC
respectively). Thus, the high radiative rate for the
binuclear complexes is mainly attributed to the significant increase in K eq induced by the small
EST imparted by the -extended Janus carbene. This conclusion is consistent with the calculated
distance between the centers of positive and negative charge (d(h
+
, e
-
)) in the S1 state. We
recently determined the relationship between d(h
+
, e
-
) and 𝑘 𝑟 𝑇𝐴𝐷𝐹 for related mononuclear cMa
complexes.
11
In general, a larger separation of h
+
and e
-
leads to a smaller EST, and therefore a
faster rate for 𝑘 𝑟 𝑇𝐴𝐷𝐹 . The d(h
+
, e
-
) values calculated for BCzAu
BAZ
AuBCz and BCzAu
BBI
AuBCz
(6.17 and 6.87 Å, respectively), are larger than for BCzAu
BZAC
(5.19 Å) (Table 4.5), which is
consistent with a decrease in EST for the binuclear complexes. The d(h
+
, e
-
) calculation on the
geometry of complexes BCzAu
BAZ
AuBCz, BCzAu
BBI
AuBCz and bimAu
BAZ
Aubim is zero because the
centers of h
+
and e
-
are overlapped because of the symmetry. To avoid this situation, these
complexes’ d(h
+
, e
-
) calculation was done by swapping one amide ligand to a Cl
-
which are
BCzAu
BAZ
AuCl, BCzAu
BBI
AuCl and bimAu
BAZ
AuCl. The value of 𝑘 𝑟 𝑇𝐴𝐷𝐹 for bimAu
BAZ
Aubim complex
is not faster than the mono-nuclear analog despite having a value for EST of 20 meV, one of the
95

smallest EST values reported to date for 2-coordinate TADF complexes.
2, 11, 13-15
The difference
in EST values is consistent with the larger value of d(h
+
, e
-
) for bimAu
BAZ
Aubim (6.29 Å) than for
bimAu
BZAC
(5.39 Å) (Table 4.5); however, the radiative rate for the S1 state in bimAu
BAZ
Aubim (S1
= 50 ns) is much slower than for bimAu
BZAC
(S1 = 19 ns). These parameters likely originate from
the low energy barrier to rotation for the bim ligand (see Figure 4.4). The presence of two
rotation centers in bimAu
BAZ
Aubim increases the probability that at least one amide ligand is
rotated out of plane in the molecule. The high population of molecules with amide ligands in
highly twisted conformations will lead to an overall increase in 𝜏 𝑆 1
. Thus, the opposing effects of
a decrease in EST and increase in 𝜏 𝑆 1
results in similar radiative rates of TADF for bimAu
BZAC

and bimAu
BAZ
Aubim.
Table 4.4 Energy and rate data from variable temperature
photophysical measurements polystyrene films (1 wt%).

𝜏 𝑆 1

(ns)
 9%
EST
(meV / cm
-1
)
 3%
𝜏 𝑇 1

(s)
 5%
ZFS
(meV / cm
-1
)
 10%
h
+
/e
-
pair distance
in S1 state
(Å)
BCzAu
BZI

a a a a a
BCzAu
BBI
AuBCz 12.6 43 / 346 33.1 1.1 / 9 6.87
BCzAu
BZAC
14.0 70 / 564 46.7 1.0 / 8 5.19
BCzAu
BAZ
AuBCz 12.7 54 / 432 32.9 0.9 / 7 6.17
bimAu
BZAC
19
b
41 / 330
b
19
b
1.2 / 10
b
5.39
bimAu
BAZ
Aubim 50
c
20 / 161
c

d

d
6.26

a
Luminescence from the
3
LE state below room temperature contributed to the lifetime and was
changing contribution upon cooling, preventing a determination of all values.
b
Data from reference
11

c
Biexponential decay is observed due to aggregation, even at concentrations <0.1 wt-%. Only
the fast component was used to determine EST and 𝜏 𝑆 1
as the relative amplitude stayed constant
at 0.65 in the measured temperature range.
d
Luminescence from the
3
LE state on BAZ at temperatures below 150 K prevented accurate fits
of the
3
ICT parameters to determine 𝜏 𝑇 1
and ZFS.

96

Table 4.5 Center of h
+
,e
-
for S1 NTOs for cMa complexes.
Complex Center of Hole Center of Electron
BCzAu
BAZ
Au BCz
S 1: 6.17 Å
T 1: 5.56 Å

BCzAu
BZAC

S 1: 5.19 Å
T 1: 4.74Å

BCzAu
BBI
Au BCz
S 1: 6.87 Å
T 1: 6.27Å

BCzAu
BZI

S 1: 5.30 Å
T 1: 4.72 Å

bimAu
BAZ
Au bim
S 1: 6.29 Å
T 1: 5.33 Å

bimAu
BZAC

S 1: 5.39 Å
T 1: 4.83 Å

97

4.2.6 OLED Devices
The high photoluminescence efficiencies and radiative rates of the binuclear complexes
makes them promising candidates as luminescent dopants for OLED devices. Unfortunately,
attempts to sublime BCzAu
BBI
AuBCz and BCzAu
BAZ
AuBCz were unsuccessful. Therefore, emissive
layers for the devices were prepared using solution processing. The host materials tris(4-
carbazoyl-9-ylphenyl)amine (TCTA) and 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC)
were chosen, due to their low hole injection barrier, good hole mobility and good dispersing
matrices for the dopants. Photoluminescence quantum yields of TCTA and TAPC films doped at
10% are over 0.85 for both dopants. Photoluminescence (PL) spectra of the films were
exclusively from the dopant (Table 4.6).
Table 4.6 Photoluminenscence data of binuclear complexes in different host materials.
20% Emitter
Host
 PL
(%)

(ns)
k r
(10
6
s
-1
)
k nr
(10
6
s
-1
)

BCzAu
BBI
Au BCz
TCTA 60 205 2.9 1.9
TAPC 75 200 3.8 1.3
BCzAu
BAZ
Au BCz
TCTA 42 242 1.7 2.4
TAPC 57 250 2.3 1.7

OLED devices were fabricated with a layer of MoOx was deposited onto the ITO rather
than the commonly used PEDOT:PSS as the MoOx was found to provide better overall
conductivity and device stability. MoOx has not been widely used in OLEDs as a HIL despite
the suitable frontier energies (-9.7 eV/-5.5 eV). A MoOx device ITO/MoOx (5 nm)/ 20%
BCzAu
BBI
AuBCz in TCTA (30 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm), was fabricated. The red
trace represents the MoOx device with the EML annealed at 110 C for 10 min under N2
atmosphere after spin coating, and the green trace is the reference device without the MoOx
layer. The black trace is the device without annealing and solvent removed under a high vacuum
at 10-7 torr for 1 hr. Pure PL spectrum (0.28, 0.61) has been observed for the device (Figure
4.9). The MoOx device exhibits much better current conduction and luminance than the PEDOT
device in literature at a certain voltage as shown in the current-luminance-voltage (J-L-V). As
450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Normalized Intensity (arb. U.)
Wavelegth (nm)
BBI in TCTA
BBI in TAPC
BAZ in TAPC
98

shown in Figure 4.9, the turn-on voltage (Von, defined at brightness of 1 cd/m
2
) is 2.8 V for the
MoOx devices. At 4.8 V, the MoOx device gives a current density of 10 mA/cm
2
and a
luminance 3423 cd/m
2
. The absence of MoOx as an HIL leads to a high high dark current and
therefore poor device performance. Furthermore, in the absence of PEDOT:PSS in the devices,
annealing is not required. The devices with the same structure were fabricated, with and without
annealing of the EML layer after spin coating. Their J-L-V characteristics are found to be similar.

Considering the energies of the HOMO and LUMO for binuclear complexes are nested
within commonly used hosts (ex. TCTA etc.), the emitter concentration-controlled devices (10%,
20% and 30% doping level) were fabricated to investigate the charge transport properties of
binuclear complex in OLED devices. The current density (J) and luminance (L) increased as the
doping level was raised from 10% to 30% (see Figure 4.10). The Von dropped from 3.7 V to
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
10
−6
10
−5
10
−4
10
−3
10
−2
10
−1
10
0
10
1
10
2
10
3
annealing at 110C
No MoO
x
no annealing
Current Density (mA/cm
2
)
Voltage (V)

10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
Luminance (cd/m
2
)
10
0
10
1
10
2
10
3
10
4
0.1
1
10
annealing at 110C
no annealing
no MoO
x
External Quantum Efficiency (%)
Current Density (mA/cm
2
)

500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
10mA Annealed
10mA not annealed

Intensity (a.u.)
Wavelength (nm)
Figure 4.9 OLED Devices with 20% BCzAu
BBI
AuBCz in TCTA as host material.
99

3.0 V over the same range. The increase in current density with doping concentration is
consistent with charges being injected directly onto and carried by the dopant. The highest
external quantum efficiency (EQE) 8.5% is achieved by the 20% doping device attributed to a
low dark current and better charge balance.

The characteristics of optimized devices are shown in Figure 4.11 and Table 4.7. The
energy levels for frontier orbitals for the materials used in the device are shown in Figure 4.11b.
The energy levels of the dopants are nested within those of hosts, so it is expected that charges
will be carried by and trapped on the dopants. Comparing the two TAPC-based OLEDs, the
current-voltage characteristics of the two devices show a larger current at a given bias for the
BCzAu
BAZ
AuBCz device (Figure 4.11c). Further, the BCzAu
BAZ
AuBCz device turns on at a lower
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
Wavelength (nm)
10%
20%
30%
0 2 4 6 8 10 12 14
10
−6
10
−5
10
−4
10
−3
10
−2
10
−1
10
0
10
1
10
2
10
3
10%
20%
30%
Current Density (mA/cm
2
)
Voltage (V)
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
Luminance (cd/m
2
)
10
0
10
1
10
2
10
3
10
4
0.1
1
10
10%
20%
30%
External Quantum Efficiency (%)
Luminance (cd/m
2
)

Figure 4.10 Doping concentration-contolled OLED devices with BCZAu
BBI
AuBCz
100

voltage (2.6 V) than the BCzAu
BBI
AuBCz device (3.0 V), likely due to differences in charge
mobility in the two devices. The device using BCzAu
BAZ
AuBCz reaches its highest external
quantum efficiency (EQE) of 8% at 4.2 V with a luminance of 610 cd/m
2
(Figure 4.11d),
whereas the BCzAu
BBI
AuBCz device reaches its highest EQE of 10% at 4.7 V with a luminance of
425 cd/m
2
(Figure 4.11c, d). We also fabricated TCTA based devices with BCzAu
BBI
AuBCz and
obtained an EQE of 11% at 4.2 V with a luminance of 740 cd/m
2
. All devices exhibited values
for brightness and EQE comparable to other cMa solution-based OLEDs.
17, 18, 38-40
It is notable
that PL efficiencies of 20% doped films are around 60-70%, limiting the maximum theoretical
EQE of 15%. Moreover, there is no significant decrease in EQE at high luminance for both
devices. The EQE of BCzAu
BBI
AuBCz with TCTA device only drops 13% from 1000 cd/m
2
to
10,000 cd/m
2
, which indicates that a fast radiative rate helps suppress TTA or triplet-polaron
annihilation (TPA) in the devices.
101

(a) (b)

0 2 4 6 8 10 12
10
−8
10
−6
10
−4
10
−2
10
0
10
2
20%
BCz
Au
BAZ
Au
BCz
in TAPC
20%
BCz
Au
BBI
Au
BCz
in TAPC
20%
BCz
Au
BBI
Au
BCz
in TCTA
Current Density (mA/cm
2
)
Voltage (V)

(c)
10
−2
10
0
10
2
10
4
10
6
10
8
Luminance (cd/m
2
)
0.01 0.1 1 10 100 1000
0.1
1
10
500 600 700
EL 1mA
EL 10mA
EL 100mA

Wavelength (nm)
External Quantum Efficiency (%)
Current Density (mA/cm
2
)

BCz
Au
BAZ
Au
BCz
in TAPC

BCz
Au
BBI
Au
BCz
in TAPC

BCz
Au
BBI
Au
BCz
in TCTA
(d)

Figure 4.11 OLED device characteristics of BCzAu
BBI
AuBCz and BCzAu
BAZ
AuBCz OLEDs.
(a) molecular structures of host and electron transport materials. (b) Device architecture with
HOMO and LUMO levels in eV and (c) Current-voltage and luminance-voltage curves, as well
as (d) efficiency curves (EQE) and EL-spectra.
102

4.3 Conclusion
In summary, we have designed and synthesized a series of luminescent binuclear cMa
complexes by using Janus carbenes to bridge Au-amide moieties. The presence of one donor on
each side of Janus carbene maintains the high oscillator strength between the donors and central
carbene acceptor. Luminescence from the binuclear complexes occurs via TADF and spans from
blue (em =  nm) to green (em =  nm) with PL efficiencies close to unity. The binuclear
complexes can have radiative rates that are 23 times faster compared to their mononuclear
analogs. Theoretical and photophysical analyses show that the fast radiative rate owes to a
decrease in the energy of the singlet-triplet gap caused by spatial extension of the ICT exciton
over the entire Janus carbene ligand. OLEDs fabricated using solution-based deposition methods
utilizing the binuclear complexes as dopants give high luminance efficiency. Moreover, MoOx is
a viable alternative to PEDOT:PSS as the HIL and the small roll-off demonstrates that fast
radiative rate of the emitter can indeed suppress the TTA and TPA in the devices.
4.4 Experimental Methods
Synthesis ClAu
BAZ
AuCl: The BAZ Ligand was synthesized according to Literature.
28
To a
100 mL Schlenk flask with bubble degassed dry THF (45 mL) 500 mg (0.444 mmol, 1.0 eq)
BAZ-Ligand was added and cooled to -78C. 1.86 mL (0.931 mmol, 2.1 eq) 0.5M KHMDS THF
solution was added dropwise over 20 min. After 2 h stirring at -78C Au(Me2S)Cl (287 mg, 0.976
mmol, 2.2 eq) was added and reaction was slowly warmed up to room temperature overnight.
Reaction was worked up by filtration through Celite, washing with dichloromethane and solvent
removal. Product was precipitated from dichloromethane by adding hexanes, yielding to the off-
white ClAu
BZAC
AuCl in 86% yield (495 mg, 0.383 mmol).
1
H NMR (400 MHz, acetone-d6) δ
Table 4.7 Performance parameters for BCzAu
BBI
AuBCz and BCzAu
BAZ
AuBCz based OLEDs
Emitter Host
V ON
(V)
EQE MAX
(%)
@ 1000 cd m
-2
@ 10,000 cd m
-2

λ max (nm),
CIE
EQE
(%)
J
(mA/cm
2
)
EQE
(%)
J
(mA/cm
2
)
BCzAu
BBI
Au BCz
TCTA 2.9 11.1 11.1 2.52 9.6 29.5 526
(0.28,0.61) TAPC 3.0 10.1 9.8 2.86 6.3 46.8
BCzAu
BAZ
Au BCz TAPC 2.6 7.7 7.7 3.73 4.8 62.8
524
(0.29,0.60)

103

7.54 – 7.49 (m, 2H), 7.44 – 7.36 (m, 6H), 7.33 – 7.27 (m, 4H), 6.46 (s, 2H), 4.89 (s, 4H), 3.24 (p,
J = 6.7 Hz, 4H), 3.10 (p, J = 6.9 Hz, 4H), 1.39 (dd, J = 10.9, 6.8 Hz, 24H), 1.25 (d, J = 6.8 Hz,
12H), 1.16 (d, J = 6.8 Hz, 12H).
Synthesis CzAu
BAZ
AuCz: To a 50 mL Schlenkflask with bubble degassed dry THF (25 mL)
54 mg (0.325 mmol, 2.1 eq) 1H-carbazole ligand was added. 0.159 mL (0.317 mmol, 2.05 eq)
2M sodium tert-butoxide (NaOtBu) THF solution was added dropwise. After 1 h stirring at
ambient temperature, ClAu
BZAC
AuCl (200 mg, 0.155 mmol, 1.0 eq) was added and reaction was
stirred overnight. Reaction was worked up by filtration through Celite, washing with
dichloromethane and solvent removal. Product was precipitated from dichloromethane by adding
hexanes. Solid was filtered and washed with diethyl ether, yielding to the colorless
CzAu
BZAC
AuCz in 80% yield (192 mg, 0.124 mmol). Under a UV light the solid is blue
emissive.
1
H NMR (400 MHz, acetone-d6) δ 7.86 – 7.66 (m, 8H), 7.60 (d, J = 7.8 Hz, 4H), 7.51
(d, J = 7.8 Hz, 4H), 6.89 (ddd, J = 8.3, 7.0, 1.3 Hz, 4H), 6.74 (ddd, J = 7.8, 7.0, 1.1 Hz, 4H), 6.61
(s, 2H), 6.08 (dt, J = 8.2, 0.9 Hz, 4H), 5.03 (s, 4H), 3.41 (hept, J = 7.7 Hz, 4H), 3.33 – 3.20 (m,
4H), 1.37 (d, J = 6.8 Hz, 12H), 1.33 (t, J = 6.8 Hz, 24H), 1.24 (d, J = 6.9 Hz, 12H).
Synthesis BCzAu
BAZ
AuBCz: To a 50 mL Schlenk flask with bubble degassed dry THF (25
mL) 90 mg (0.325 mmol, 2.1 eq) 3,6-Di-tert-butylcarbazole ligand was added. 0.158 mL (0.317
mmol, 2.05 eq) 2M sodium tert-butoxide (NaOtBu) solution was added dropwise. After 1 h
stirring at ambient temperature, ClAu
BZAC
AuCl (200 mg, 0.155 mmol, 1.0 eq) was added and
reaction was stirred overnight. Reaction was worked up by filtration through Celite, washing
with dichloromethane and solvent removal. Product was washed with -40C hexanes and
collected the precipitations. Then repeat this step three times, yielding to the yellow powder
BCzAu
BZAC
AuBCz in 72% yield (198 mg, 0.111 mmol). Under a UV light the solid is skyblue
emissive.
1
H NMR (400 MHz, acetone-d6) δ 7.88 – 7.80 (m, 6H), 7.72 (t, J = 7.8 Hz, 2H), 7.62
(d, J = 7.8 Hz, 4H), 7.53 (d, J = 7.8 Hz, 4H), 7.00 (dd, J = 8.6, 2.0 Hz, 4H), 6.61 (s, 2H), 6.04
(dd, J = 8.6, 0.6 Hz, 4H), 5.03 (s, 4H), 3.41 (sept, J = 7.0 Hz, 4H), 3.27 (sept, J = 6.5 Hz, 4H),
1.40 (d, J = 6.9 Hz, 36H), 1.38 – 1.31 (m, 60H), 1.26 (d, J = 6.8 Hz, 12H).
13
C NMR (100 MHz,
acetone-d6) δ 229.2, 228.3, 227.8, 223.4, 223.1, 219.3, 219.2, 214.2, 211.5, 210.0, 208.8, 205.2,
202.6, 201.1, 195.5, 194.4, 192.4, 191.0, 190.1, 189.7, 185.9, 182.5, 180.6, 177.9, 177.3, 173.7,
173.6, 166.6, 164.9, 162.8, 153.1, 148.3, 148.1, 146.8, 145.7, 145.3, 137.6, 125.5, 125.3, 123.4,
104

120.4, 118.7, 114.5, 113.4, 38.4, 38.1, 37.8, 37.4, 34.0, 31.7, 24.0, 24.0, 23.6. CHN: C: 59.44%,
H: 5.93%, N: 4.97%; calculated: C: 59.68%; H: 6.35%; N: 4.13% (includes 3 CH2Cl2)
Synthesis BimAu
BAZ
AuBim: The bim ligand was synthesized according to Literature.
11
To a
50 mL Schlenk flask with bubble degassed dry THF (25 mL) 61 mg (0.293 mmol, 2.1 eq) 1H-
Bim ligand was added. 0.143 mL (0.286 mmol, 2.05 eq) 2M sodium tert-butoxide (NaOtBu)
THF solution was added dropwise. After 1 h stirring at ambient temperature, ClAu
BZAC
AuCl (180
mg, 0.139 mmol, 1.0 eq) was added and reaction was stirred overnight. Reaction was worked up
by filtration through Celite, washing with dichloromethane and solvent removal. Product was
precipitated from dichloromethane by adding hexanes. Solid was filtered and washed with
diethyl ether, yielding to the off-white BimAu
BZAC
AuBim in 76% yield (174 mg, 0.107 mmol).
Under a UV light the solid is blue emissive.
1
H NMR (400 MHz, acetone-d6) δ 7.70 – 7.57 (m,
6H), 7.57 – 7.48 (m, 4H), 7.46 – 7.39 (m, 4H), 7.30 (ddd, J = 8.0, 1.2, 0.6 Hz, 2H), 7.06 (ddd, J
= 8.0, 7.3, 1.2 Hz, 2H), 6.96 – 6.84 (m, 8H), 6.61 (s, 2H), 6.27 – 6.18 (m, 2H), 5.05 (s, 4H), 3.41
(sept, J = 6.8 Hz, 4H), 3.27 (sept, J = 6.5 Hz, 4H), 1.48 (dd, J = 14.2, 6.8 Hz, 24H), 1.32 (d, J =
6.9 Hz, 12H), 1.25 (d, J = 6.8 Hz, 12H).
13
C NMR (100 MHz, acetone-d6) δ 146.5, 145.1, 145.0,
136.7, 130.6, 129.8, 127.2, 127.1, 126.7, 126.5, 126.1, 126.0, 125.5, 125.2, 121.0, 120.9, 118.8,
118.5, 117.4, 116.7, 115.1, 113.2, 109.0, 108.8, 24.1, 23.7. CHN: C: 42.83%, H: 2.06%, N:
14.31%; calculated: C: 43.74%; H: 2.24%; N: 14.17%
Synthesis ClAu
BBI
AuCl: To a 100 mL Schlenk flask with bubble degassed dry THF (35 mL)
400 mg (0.496 mmol, 1.0 eq) BBI-Ligand was added. 1.49 mL (1.04 mmol, 2.1 eq) 0.7M
KHMDS THF solution was added dropwise. After 1h stirring at room temperature Au(Me2S)Cl
(321 mg, 1.09 mmol, 2.2 eq) was added and reaction was stirred overnight. Reaction was worked
up by filtration through Celite, washing with dichloromethane and solvent removal. Product was
precipitated from dichloromethane by adding hexanes. Solid was filtered and washed with
methanol, yielding to the off-white ClAu
BBI
AuCl in 72% yield (348 mg, 0.358 mmol).
1
H NMR
(400 MHz, acetone-d6) δ 7.54 – 7.49 (m, 2H), 7.44 – 7.36 (m, 6H), 7.33 – 7.27 (m, 4H), 6.46 (s,
2H), 4.89 (s, 4H), 3.24 (p, J = 6.7 Hz, 4H), 3.10 (p, J = 6.9 Hz, 4H), 1.39 (dd, J = 10.9, 6.8 Hz,
24H), 1.25 (d, J = 6.8 Hz, 12H), 1.16 (d, J = 6.8 Hz, 12H).
Synthesis BCzAu
BBI
AuBCz: To a 25 mL Schlenk flask with bubble degassed dry THF (10
mL) 120 mg (0.432 mmol, 2.1 eq) 3,6-Di-tert-butylcarbazole ligand was added. 0.211 mL (0.422
105

mmol, 2.05 eq) 2M sodium tert-butoxide (NaOtBu) solution was added dropwise. After 1 h
stirring at ambient temperature, ClAu
BBI
AuCl (200 mg, 0.206 mmol, 1.0 eq) was added and
reaction was stirred overnight. Reaction was worked up by filtration through Celite, washing
with dichloromethane and solvent removal. Product was purified by addition of Hexanes,
sonicating 30 min, cooling the flask slowly in a Dewar to -40C in a deep freezer. Solid was
collected at low temperature. The purification step was repeated if unreacted BCz was detected
by NMR. The bright yellow solid BCzAu
BBI
AuBCz was obtained in 73% yield (219 mg, 0.150
mmol). Under a UV light the solid is green emissive.
1
H NMR (600 MHz, acetone-d6) δ 8.65 (d,
J = 0.8 Hz, 1H), 8.00 (dd, J = 2.0, 0.7 Hz, 4H), 7.71 (t, J = 8.0 Hz, 2H), 7.52 (d, J = 8.0 Hz, 4H),
7.20 (dd, J = 8.5, 2.0 Hz, 4H), 7.13 – 7.10 (m, 5H), 4.62 (s, 6H), 2.76 (s, 2H), 2.48 (sept, J = 6.6
Hz, 4H), 1.42 (d, J = 6.6 Hz, 4H), 1.38 (s, 35H), 1.30 (d, J = 6.8 Hz, 13H), 1.04 (d, J = 6.9 Hz,
12H).
13
C NMR (151 MHz, acetone-d6) δ 148.1, 147.1, 138.1, 134.0, 132.6, 131.8, 131.1, 124.8,
123.9, 121.1, 114.9, 113.0, 110.0, 109.6, 36.3, 35.6, 34.1, 31.7, 31.5, 24.1, 23.6. CHN: C:
62.08%, H: 6.17, N: 6.44%, calculated: C: 61.17%; H: 6.58%; N: 5.49% (includes 1 THF)
Synthesis BCzAu
BZI
: ClAu
BZI
Ligand was synthesized according to Literature.
7
To a 25 mL
Schlenk flask with bubble degassed dry THF (10 mL) 83 mg (0.297 mmol, 1.05 eq) 3,6-Di-tert-
butylcarbazole ligand was added. 0.149 mL (0.297 mmol, 1.05 eq) 2M sodium tert-butoxide
(NaOtBu) solution was added dropwise. After 1 h stirring at ambient temperature, ClAu
BZI
(190
mg, 0.283 mmol, 1.0 eq) was added and reaction was stirred overnight. Reaction was worked up
by filtration through Celite, washing with dichloromethane and solvent removal. Product was
precipitated from dichloromethane by adding hexanes. Solid was filtered and washed with
diethyl ether, yielding to the colorless BCzAu
BZI
in 82% yield (212 mg, 0.232 mmol). Under a UV
light the solid is blue emissive.
1
H NMR (400 MHz, acetone-d6) δ 7.92 (dd, J = 2.1, 0.7 Hz,
2H), 7.87 – 7.78 (m, 2H), 7.68 – 7.58 (m, 6H), 7.43 (dd, J = 6.1, 3.2 Hz, 2H), 7.07 (dd, J = 8.5,
2.0 Hz, 2H), 6.69 (dd, J = 8.5, 0.6 Hz, 2H), 2.63 (sept, J = 6.8 Hz, 4H), 1.39 (d, J = 6.9 Hz,
12H), 1.34 (s, 18H), 1.19 (d, J = 6.9 Hz, 12H).
13
C NMR (100 MHz, acetone-d6) δ 148.0, 146.9,
137.9, 134.9, 131.7, 131.1, 125.8, 124.7, 123.7, 120.9, 114.8, 112.8, 112.1, 34.0, 31.7, 31.4, 24.0,
23.3. CHN: C: 67.20%, H: 6.95%, N: 5.10%, calculated: C: 67.02%; H: 6.84%; N: 4.60%

106

300 400 500 600 700
0.0
0.5
1.0
1.5
2.0
300 400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0

Absorption (normalized to 366 nm)
MeCy
2-MeTHF
Tol
PS film
Cz
Au
BAZ
Au
Cz
(a)

Emission (a.u.)
Wavelength (nm)
RT
77K
RT
77K
RT
RT
77K
(b)

Figure 4.12 Absorption and Emission Spectra of CzAu
BAZ
AuCz
Table 4.8 Photophysical Data for CzAu
BAZ
AuCz based compounds
in MeCy, 2MeTHF, MeCN and CH2Cl2

Abs
λ max
(nm)
PL λ max
(nm)
Φ PL
(%)
τ
(ms)
k r
(10
6
s
-1
)
k nr
(10
6
s
-1
)
λ max
77K
(nm)
τ 77 K
(ms)
PS - 476 98
0.33 (0.77)
1.01 (0.16)
5.6 (0.07)
1.2 0.02 476
60 (0.22)
220 (0.58)
590 (0.2)
MeCy 456 478 90
0.39 (0.97)
3.1 (0.03)
1.9 0.21 440
130 (0.44)
370 (0.56)
Tol 430 500 96
0.36
2.2 0.09 - -
2-MeTHF 405 515 81
0.45 (0.98)
3.09 (0.02)
1.6 0.38 443
380 (0.87)
760 (0.13)
107

4.5 References
[1] Ziolo, R. F.; Lipton, S.; Dori, Z., The photoluminescence of phosphine complexes of d10
metals. Journal of the Chemical Society D: Chemical Communications 1970, (17), 1124-1125.

[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] Feng, J.; Reponen, A.-P. M.; Romanov, A. S.; Linnolahti, M.; Bochmann, M.; Greenham,
N. C.; Penfold, T.; Credgington, D., Influence of Heavy Atom Effect on the Photophysics of
Coinage Metal Carbene-Metal-Amide Emitters. Advanced Functional Materials 2021, 31 (1),
2005438.

[5] Conaghan, P. J.; Matthews, C. S. B.; Chotard, F.; Jones, S. T. E.; Greenham, N. C.;
Bochmann, M.; Credgington, D.; Romanov, A. S., Highly efficient blue organic light-emitting
diodes based on carbene-metal-amides. Nature Communications 2020, 11 (1), 1758.

[6] Romanov, A. S.; Becker, C. R.; James, C. E.; Di, D.; Credgington, D.; Linnolahti, M.;
Bochmann, M., Copper and Gold Cyclic (Alkyl)(amino)carbene Complexes with Sub-
Microsecond Photoemissions: Structure and Substituent Effects on Redox and Luminescent
Properties. Chemistry – A European Journal 2017, 23 (19), 4625-4637.

[7] 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.

[8] Li, T.-y.; Muthiah Ravinson, D. S.; Haiges, R.; Djurovich, P. I.; Thompson, M. E.,
Enhancement of the Luminescent Efficiency in Carbene-Au(I)-Aryl Complexes by the
Restriction of Renner–Teller Distortion and Bond Rotation. Journal of the American Chemical
Society 2020, 142 (13), 6158-6172.

[9] Li, T.-y.; Shlian, D. G.; Djurovich, P. I.; Thompson, M. E., A Luminescent Two-
Coordinate AuI Bimetallic Complex with a Tandem-Carbene Structure: A Molecular Design for
the Enhancement of TADF Radiative Decay Rate. Chemistry – A European Journal 2021, 27
(20), 6191-6197.

108

[10] Ma, J.; Kapper, S. C.; Ponnekanti, A.; Schaab, J.; Djurovich, P. I.; Thompson, M. E.,
Dynamics of rotation in two-coordinate thiazolyl copper(I) carbazolyl complexes. Applied
Organometallic Chemistry n/a (n/a), e6728.

[11] Muniz, C. N.; Schaab, J.; Razgoniaev, A.; Djurovich, P. I.; Thompson, M. E., π-
Extended Ligands in Two-Coordinate Coinage Metal Complexes. Journal of the American
Chemical Society 2022, 144 (39), 17916-17928.

[12] Yang, J.-G.; Song, X.-F.; Wang, J.; Li, K.; Chang, X.; Tan, L.-Y.; Liu, C.-X.; Yu, F.-
H.; Cui, G.; Cheng, G.; To, W.-P.; Yang, C.; Che, C.-M.; Chen, Y., Highly Efficient
Thermally Activated Delayed Fluorescence from Pyrazine-Fused Carbene Au(I) Emitters.
Chemistry – A European Journal 2021, 27 (71), 17834-17842.

[13] Tang, R.; Xu, S.; Lam, T.-L.; Cheng, G.; Du, L.; Wan, Q.; Yang, J.; Hung, F.-F.; Low,
K.-H.; Phillips, D. L.; Che, C.-M., Highly Robust CuI-TADF Emitters for Vacuum-Deposited
OLEDs with Luminance up to 222 200 cd m−2 and Device Lifetimes (LT90) up to 1300 hours at
an Initial Luminance of 1000 cd m−2. Angewandte Chemie International Edition 2022, 61 (33),
e202203982.

[14] Chotard, F.; Sivchik, V.; Linnolahti, M.; Bochmann, M.; Romanov, A. S., Mono- versus
Bicyclic Carbene Metal Amide Photoemitters: Which Design Leads to the Best Performance?
Chemistry of Materials 2020, 32 (14), 6114-6122.

[15] Romanov, A. S.; Jones, S. T. E.; Yang, L.; Conaghan, P. J.; Di, D.; Linnolahti, M.;
Credgington, D.; Bochmann, M., Mononuclear Silver Complexes for Efficient Solution and
Vacuum-Processed OLEDs. Advanced Optical Materials 2018, 6 (24), 1801347.

[16] Conaghan, P. J.; Menke, S. M.; Romanov, A. S.; Jones, S. T. E.; Pearson, A. J.; Evans,
E. W.; Bochmann, M.; Greenham, N. C.; Credgington, D., Efficient Vacuum-Processed Light-
Emitting Diodes Based on Carbene–Metal–Amides. Advanced Materials 2018, 30 (35), 1802285.

[17] Romanov, A. S.; Jones, S. T. E.; Gu, Q.; Conaghan, P. J.; Drummond, B. H.; Feng, J.;
Chotard, F.; Buizza, L.; Foley, M.; Linnolahti, M.; Credgington, D.; Bochmann, M., Carbene
metal amide photoemitters: tailoring conformationally flexible amides for full color range
emissions including white-emitting OLED. Chemical Science 2020, 11 (2), 435-446.

[18] Di, D.; Romanov, A. S.; Yang, L.; Richter, J. M.; Rivett, J. P. H.; Jones, S.; Thomas, T.
H.; Jalebi, M. A.; Friend, R. H.; Linnolahti, M.; Bochmann, M.; Credgington, D., High-
performance light-emitting diodes based on carbene-metal-amides. Science 2017, 356 (6334),
159-163.

[19] 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.
109

[20] Kalyanasundaram, K., Photophysics, photochemistry and solar energy conversion with
tris(bipyridyl)ruthenium(II) and its analogues. Coordination Chemistry Reviews 1982, 46, 159-
244.

[21] Ravinson, D. S. M.; Thompson, M. E., Thermally assisted delayed fluorescence (TADF):
fluorescence delayed is fluorescence denied. Materials Horizons 2020, 7 (5), 1210-1217.

[22] Li, T.-y.; Schaab, J.; Djurovich, P. I.; Thompson, M. E., Toward rational design of TADF
two-coordinate coinage metal complexes: understanding the relationship between natural
transition orbital overlap and photophysical properties. Journal of Materials Chemistry C 2022,
10 (12), 4674-4683.

[23] Poyatos, M.; Peris, E., Insights into the past and future of Janus-di-N-heterocyclic carbenes.
Dalton Transactions 2021, 50 (37), 12748-12763.

[24] Telfer, S. G.; McLean, T. M.; Waterland, M. R., Exciton coupling in coordination
compounds. Dalton Transactions 2011, 40 (13), 3097-3108.

[25] 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.

[26] Sagara, Y.; Shizu, K.; Tanaka, H.; Miyazaki, H.; Goushi, K.; Kaji, H.; Adachi, C.,
Highly Efficient Thermally Activated Delayed Fluorescence Emitters with a Small Singlet–
Triplet Energy Gap and Large Oscillator Strength. Chemistry Letters 2015, 44 (3), 360-362.

[27] Shafikov, M. Z.; Martinscroft, R.; Hodgson, C.; Hayer, A.; Auch, A.; Kozhevnikov, V.
N., Non-Stereogenic Dinuclear Ir(III) Complex with a Molecular Rack Design to Afford
Efficient Thermally Enhanced Red Emission. Inorganic Chemistry 2021, 60 (3), 1780-1789.

[28] Shen, W.; Li, J.; Zhang, C.; Shi, M.; Zhang, J., Copper, Silver and Sodium Salt-Mediated
Quaternization by Arylation: Syntheses of N-Heterocyclic Carbene Precursors and 6-H-
Phenanthridine Derivatives. Chemistry – An Asian Journal 2016, 11 (13), 1883-1886.

[29] 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.

[30] 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.

110

[31] Sworakowski, J.; Lipiński, 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.

[32] Marciniak, H.; Auerhammer, N.; Ricker, S.; Schmiedel, A.; Holzapfel, M.; Lambert, C.,
Reduction of the Fluorescence Transition Dipole Moment by Excitation Localization in a
Vibronically Coupled Squaraine Dimer. The Journal of Physical Chemistry C 2019, 123 (6),
3426-3432.

[33] Mennucci, B., Modeling absorption and fluorescence solvatochromism with QM/Classical
approaches. International Journal of Quantum Chemistry 2015, 115 (18), 1202-1208.

[34] Reichardt, C., Solvatochromic Dyes as Solvent Polarity Indicators. Chemical Reviews
1994, 94 (8), 2319-2358.

[35] 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.

[36] Shafikov, M. Z.; Zaytsev, A. V.; Kozhevnikov, V. N., Cyclometalation Geometry of the
Bridging Ligand as a Tuning Tool for Photophysics of Dinuclear Ir(III) Complexes. The Journal
of Physical Chemistry C 2021, 125 (37), 20531-20537.

[37] Shafikov, M. Z.; Pander, P.; Zaytsev, A. V.; Daniels, R.; Martinscroft, R.; Dias, F. B.;
Williams, J. A. G.; Kozhevnikov, V. N., Extended ligand conjugation and dinuclearity as a route
to efficient platinum-based near-infrared (NIR) triplet emitters and solution-processed NIR-
OLEDs. Journal of Materials Chemistry C 2021, 9 (1), 127-135.

[38] Romanov, A. S.; Yang, L.; Jones, S. T. E.; Di, D.; Morley, O. J.; Drummond, B. H.;
Reponen, A. P. M.; Linnolahti, M.; Credgington, D.; Bochmann, M., Dendritic Carbene Metal
Carbazole Complexes as Photoemitters for Fully Solution-Processed OLEDs. Chem. Mater.
2019, 31 (10), 3613-3623.

[39] Zhou, D.; Wu, S.; Cheng, G.; Che, C.-M., A gold(iii)–TADF emitter as a sensitizer for
high-color-purity and efficient deep-blue solution-processed OLEDs. Journal of Materials
Chemistry C 2022, 10 (12), 4590-4596.

[40] Teng, T.; Li, K.; Cheng, G.; Wang, Y.; Wang, J.; Li, J.; Zhou, C.; Liu, H.; Zou, T.;
Xiong, J.; Wu, C.; Zhang, H.-X.; Che, C.-M.; Yang, C., Lighting Silver(I) Complexes for
Solution-Processed Organic Light-Emitting Diodes and Biological Applications via Thermally
Activated Delayed Fluorescence. Inorganic Chemistry 2020, 59 (17), 12122-12131.

Abstract (if available)

Abstract Conventional organometallic complexes, such as Os(II), Pt(II), and Ir(III), have found widespread application in various fields, including photocatalysis, organic light-emitting diodes (OLEDs), and biosensors. Among these complexes, Ir-based ones have made significant inroads in the commercialization of OLEDs, serving as phosphorescent emitters. However, a long-standing issue with blue phosphorescent emitters is their short lifespan in devices, commonly referred to as the "blue gap" problem in OLEDs. The blue gap is one of the most challenging problems in OLEDs because the devices comprise several tens to hundreds of materials in layers, and each layer could contribute to their decomposition. To simplify this question, this work focuses on the most vulnerable layer, which is the emissive layer (EML), comprising host and emitter materials. Here, the design of these materials from a molecular level is discussed, with the ultimate goal of extending the lifespan of blue phosphorescent emitters in OLEDs.
Chapter 1 provides a brief introduction to OLEDs and discusses the degradation mechanisms responsible for the short lifespan of blue phosphors in devices, which serves as the foundation for molecule design in this work. The chapter then delves into one type of emitter used in OLEDs, namely thermally activated delay fluorescence (TADF) molecules, highlighting their differences compared to the common phosphors based on Pt(II) and Ir(III). The kinetic scheme and decay rates of TADF molecules are also briefly discussed to provide a physical understanding of the design of emitters. Overall, Chapter 1 lays the groundwork for the subsequent chapters, providing a foundation for the design and optimization of efficient and stable materials for blue OLEDs.
Chapter 2 focuses on the design of robust host materials with high triplet energy to suppress degradation in blue OLEDs. The chapter discusses the importance of strong bonds and minimized conjugation in designing hosts, especially in discussing the moieties that can be chosen as the backbone for hosts and how to link them together to avoid triplet decrease. Two wide energy gap hosts, free of weak bonds, are introduced, with large energy gaps (≥5.0 eV) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and correspondingly high triplet energies in the solid state (~ 3.0 eV). The chapter also includes an analysis of devices using these host materials with a blue phosphorescent dopant, showing that charges are transported and trapped by the dopant, which subsequently form excitons directly on the phosphor. This suppresses luminescence quenching pathways, leading to blue phosphorescent devices with high (~18%) external quantum efficiency. Overall, Chapter 2 provides a detailed exploration of the design of robust host materials, paving the way for the development of efficient and stable blue OLEDs.
Another approach to limit degradation is to design TADF dopants with high luminescent efficiency and fast radiative rates. If the dopant can efficiently undergo radiative decay into the ground state before engaging in bimolecular processes, the damage caused by “hot states” can be suppressed. Chapter 3 focuses on how to suppress the non-radiative decay of TADF dopants, while Chapter 4 focuses on designing dopants with high efficiency and fast radiative lifetimes.
In Chapter 3, three two-coordinate Cu(I) complexes with carbazolyl ligands featuring substituents of varying steric bulk ortho to N are investigated. The impact of these substituents on the luminescence energies of the complexes is negligible, but they serve to modulate the rotation barriers along the metal-ligand coordinate bond. The geometric arrangement of ligands in complexes with alkyl substituents was found to differ, with syn conformers in the solid state versus anti in solution, as revealed by crystallographic analysis and nuclear magnetic resonance spectroscopy, respectively. Potential energy surface scan calculations were also performed on different conformations of the three complexes to provide a theoretical evaluation of the rotation barriers around the metal-ligand bond axis. The study demonstrates that rates for nonradiative decay decrease with increasing bulk of the substituents on the carbazolyl ligand, indicating the potential to suppress non-radiative decay in TADF dopants through steric modulation.
Chapter 4 introduces a binuclear model for designing TADF emitters with high efficiency and fast radiative decay lifetimes. The chapter describes the synthesis and analysis of a series of binuclear carbene-metal-amide (cMa) complexes with bridging biscarbene ligands. The complexes exhibit solvation-dependent absorption and emission, with the molar absorptivity of the binuclear complexes found to be correlated with the energy barrier to rotation of the metal-ligand bond. The binuclear cMa complexes also exhibit short emission lifetimes (200-300 ns) with high photoluminescence efficiencies (Φ > 95%). The radiative rates of binuclear cMa complexes are 3-4 times faster than those of the corresponding mononuclear complexes. Analysis of temperature-dependent luminescence data shows that the lifetime for the singlet state of binuclear cMa complexes is around 12 ns with a singlet triplet splitting of 40-50 meV. These findings provide a general design strategy for cMa complexes to achieve small values for singlet triplet splitting while retaining high radiative rates. Additionally, solution-processed OLED devices incorporating two of the complexes as luminescent dopants exhibited low roll-off at high luminance. This chapter highlights the potential of binuclear cMa complexes in pushing the radiative lifetime to a few hundreds of nanosecond range, making them a promising candidate for improving the lifespan of blue OLEDs.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

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

PDF

High energy hosts and blue emitters for phosphorescent organic light emitting diodes

PDF

Integration of cost effective luminescent materials into organic light emitting diodes (OLEDs) and molecular approaches to improve the device efficiency

PDF

Molecular design of pyridine and quinoline based compounds for organic optoelectronics

PDF

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

PDF

Molecular modulation to fine-tune optoelectronic properties of OLED materials

PDF

Ir(III) and Pt(II) phosphorescent emitters in organic light emitting diodes: from materials development to light out-coupling

PDF

Dopants for organic light-emitting devices

PDF

Synthesis, photo- and electroluminescence of three- and two-coordinate coinage metal complexes featuring non-N-heterocyclic carbene and non-conventional N-heterocyclic carbene ligands

PDF

Studies of molecular orientation using iridium phosphors and integration of corannulene into organic light emitting diodes (OLEDs)

PDF

Synthesis, structural and photophysical characterization of phosphorescent three-coordinate Cu(I)-N-heterocyclic carbene complexes

PDF

Simple complexes: synthesis and photophysical studies of luminescent, monovalent, 2-coordinate carbene-coinage metal complexes and higher coordination geometries

PDF

Novel pyrimidine-based hole blocking materials for long-lived and highly efficient organic light emitting diodes

PDF

Qualitative quantum chemical description of the structure-color relationships in phosphorescent organometallic complexes

PDF

Photophysical properties of luminescent iridium and coinage metal complexes

Asset Metadata

Creator Ma, Jie (author)

Core Title Molecular design strategies for blue organic light emitting diodes

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Degree Conferral Date 2023-08

Publication Date 06/16/2023

Defense Date 04/20/2023

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

Tag binuclear carbene-metal-amide (cMa) complexes,emissive layer (EML),fast radiative lifetimes,host materials,low roll-off,OAI-PMH Harvest,OLED,short lifespan of blue emitters,suppress the non-radiative decay,thermally activated delay fluorescence (TADF)

Format theses (aat)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark (committee chair), Armani, Andrea (committee member), Nutt, Steve (committee member), Thompson, Barry (committee member)

Creator Email jiem@usc.edu,majiejessica@hotmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC113170752

Unique identifier UC113170752

Identifier etd-MaJie-11967.pdf (filename)

Legacy Identifier etd-MaJie-11967

Document Type Dissertation

Format theses (aat)

Rights Ma, Jie

Internet Media Type application/pdf

Type texts

Source 20230616-usctheses-batch-1056 (batch), University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright.

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA

Repository Email cisadmin@lib.usc.edu