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

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

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SIMPLE COMPLEXES:
SYNTHESIS AND PHOTOPHYSICAL STUDIES OF LUMINESCENT, MONOVALENT,
2-COORDINATE CARBENE-COINAGE METAL COMPLEXES AND HIGHER
COORDINATION GEOMETRIES

By
Rasha Hamze

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
May 2019

Copyright November 2018 Rasha Hamze

2

Acknowledgements
I would like to extend my gratitude and appreciation, first and foremost, to my research advisor
Prof. Mark Thompson, whose support, caring, and mentorship were invaluable over the course of
my graduate school career. Prof. Thompson encouraged me to pursue new ideas and collaborative
projects, providing the perfect environment for me to grow as an independent scientist.
I would also like to thank Prof. Brent Melot, Prof. Noah Malmstadt, Prof. Surya Prakash, Prof.
Ralf Haiges, Prof. Richard Brutchey, Prof. Andrea Armani, and Prof. Robert Baker for the valuable
time and insights they gave while serving on my thesis defense committee, my qualifying exam
committee, and my screening exam committee. I am especially thankful to have worked with Prof.
Peter Djurovich, whose intimate knowledge of the literature and challenging questions inspired
me to tackle long-standing problems more creatively.
I am very grateful for the fruitful collaborations with our colleagues at the University of California
at San Diego: Prof. Guy Bertrand, Dr. Michele Soleilhavoup, Dr. Rodolphe Jazzar, and Jesse
Peltier. I benefited and learned a great deal from their work on carbenes and their expertise in air-
sensitive chemistry.
I would like to express my sincere thanks to USC-based collaborators and coordinators who have
helped me in my graduate career: Prof. Brent Melot for his willingness to guide my venturing into
the world of magneto-photoluminescence, and Joanna Milam Guerrero and Laura Estergreen for
making those adventures a reality; to Prof. Travis Williams and Dr. Zhiyao Lu for incorporating
my bulky ligands into efficient catalysts and for their NMR expertise; to Prof. Ralf Haiges for his
invaluable help in solving my most difficult crystal structures; to Allan Kershaw, whose support
was instrumental in my temperature-dependent NMR and photophysical studies.
3

My colleagues at the MET labs provided an ideal work environment that fostered cooperation and
helped alleviate the stress of graduate school, for which I am very grateful: to Dr. Shuyang Shi,
who was up until this year my sole partner in team Cu; to Daniel Sylvinson, for his insightful work
on computational modeling of my chemistry, and his patience in explaining that work to me; to
Moon-Chul Jung, for his diligence in making OLEDs employing my Cu(I) emitters; to Muazzam
Idriss, for his valuable help in making necessary precursors for my carbenes; to Savannah Kapper
and Narcisse Ukwitegetse for always offering a helping hand when needed; to Patrick Saris, Becky
Wilson, and Dr. Niki Bayat for being a great team of scientists with whom I started my graduate
career at USC. Special thanks to previous lab members who have offered their help and advice
over various points in my time at USC: Dr. Tyler Fleetham, Dr. John Chen, Dr. Matthew Jurow,
and Dr. Jonathan Sommers. Min-Ju Kim, Juan Martinez-Galvan, Benny Char, and Jose Cardenas
were the best undergraduate researchers a graduate student can hope for, and for that I am very
grateful. Lastly at USC, I am deeply appreciative of the invaluable administrative help and support
provided by Judy Hom and Michele Dea.
I would also like to acknowledge Prof. Maryanne Collinson, Prof. Lara Halaoui, and all my
undergraduate research colleagues at the American University of Beirut and Virginia
Commonwealth University.
Lastly, I would like to acknowledge the invaluable support of my family: my loving parents Walid
and Hoda Hamze, and my wonderful siblings, Lara, Bahaa, and Hadi. I would not be the person I
am today if it were not for their love and encouragement, especially when I most needed it. I am
also indebted to my extended family in Lebanon, who welcomed me during my time there and
offered their warmest always. I am especially thankful for my caring grandparents, and for the
precious memories I have of my late grandfather and uncle from my time spent in Lebanon. My
4

host family in Richmond, Virginia offered me a place to call home, and for that I am eternally
grateful. My friends and loved ones in LA made a big city feel like home: Natty Clucas, whose
thoughtfulness, support, and understanding helped me get through the most challenging times, Dr.
Nadia Korovina, with engaging and fun conversations over tea time, and Dr. John Facendola, for
always keeping it real. My friends from AUB, VCU, and TEDxBeirut who kept in touch
throughout my time in grad school helped make my transition to a new city and a new chapter in
my life a lot easier.

5

Contents
Acknowledgements ......................................................................................................................... 2
List of Tables .................................................................................................................................. 8
Chapter 1. Introduction ............................................................................................................. 22
1.1. Spin-orbit coupling and the triplet state .................................................................... 22
1.2. Phosphorescence of Cu(I) complexes.......................................................................... 25
Tetrahedral Cu(I) complexes ............................................................................... 26
Three Coordinate Cu(I) complexes ..................................................................... 31
Two-coordinate Cu(I) complexes: less is more ................................................... 34
TADF ...................................................................................................................... 36
1.3. References ..................................................................................................................... 42
Chapter 2. Cyclic alkyl amino carbenes (CAACs) as an efficient ligand system for red-
shifting the luminescence of 2-, 3-, and 4-coordinate Cu(I)-carbene complexes ................... 48
2.1 Introduction .................................................................................................................. 48
2.2 Results and Discussion ................................................................................................. 50
2.1. Synthesis .................................................................................................................... 50
2.2. XRay analysis ............................................................................................................ 51
2.3. Photophysical characterization ............................................................................... 52
2.4. Computational analysis ............................................................................................ 60
2.3 Experimental Methods ................................................................................................. 62
2.4 References ..................................................................................................................... 66
Chapter 3. Molecular dynamics of 4-coordinate carbene-Cu(I) complexes employing
tris(pyrazolyl)borate ligands ...................................................................................................... 69
2.1. Introduction .................................................................................................................. 69
2.2. Results and Discussion ................................................................................................. 70
2.1. Synthesis .................................................................................................................... 70
2.2. XRay Analysis ........................................................................................................... 71
2.3. NMR Analysis ........................................................................................................... 72
2.4. Photophysical Characterization .............................................................................. 81
2.5. DFT calculations ....................................................................................................... 85
2.3. Conclusions ................................................................................................................... 86
2.4. Experimental Methods ................................................................................................. 87
2.5. References ..................................................................................................................... 87
6

Chapter 4. Iridium-like luminescence of 2-coordinate Cyclic alkyl amino carbene-Cu(I)
complexes ..................................................................................................................................... 89
4.1. Introduction .................................................................................................................. 89
4.2. Results and Discussion ................................................................................................. 92
4.2.1. Synthesis................................................................................................................. 92
4.2.2. XRay Analysis ....................................................................................................... 93
4.2.3. Electrochemistry ................................................................................................... 95
4.2.4. DFT calculations ................................................................................................... 96
4.2.5. Photophysical Characterization .......................................................................... 99
4.2.6. TDDFT and molecular dynamics ...................................................................... 108
4.2.7. OLED characterization ...................................................................................... 111
4.3. Conclusion ................................................................................................................... 114
4.4. Experimental methods ............................................................................................... 116
4.5. References ................................................................................................................... 124
Chapter 5. Luminescent, 2-coordinate, d
10
coinage metal complexes: A systematic study of
the role of the metal as an electronic conduit ......................................................................... 128
5.1. Introduction ................................................................................................................ 128
5.2. Results and Discussion ............................................................................................... 131
5.2.1. Synthesis............................................................................................................... 131
5.2.2. XRay analysis ...................................................................................................... 134
5.2.3. Electrochemistry ................................................................................................. 136
5.2.4. Photophysical characterization.......................................................................... 137
5.2.5. OLED characterization ...................................................................................... 149
5.2.6. Computational analysis ...................................................................................... 152
5.3. Conclusions ................................................................................................................. 157
5.4. Experimental Methods ............................................................................................... 158
5.5. References ................................................................................................................... 164
Appendix 1. Spiro-center containing compounds with high T1 energy for applications as
hosts in blue OLEDs ................................................................................................................. 168
A1.1. Introduction ............................................................................................................ 168
A1.2. Results and Discussion ........................................................................................... 168
A1.2.1. Synthesis ........................................................................................................... 168
A1.2.2. Photophysical characterization ...................................................................... 170
7

A1.3. References ................................................................................................................ 174

8

List of Tables
Table 1. 1. Values for the spin-orbit coupling parameter, ξ1, of common atoms provide a
measure of the strength of the SOC interaction.
10
........................................................................ 23
Table 1. 2. Frontier molecular orbitals and solution photophysics of select examples from
references 35 and 36. Both complexes are reported to be brightly luminescent in the solid state,
with the one in ref. 35 having ΦPL = 0.9. ...................................................................................... 29

Table 2. 1. Photophysical properties of 1-Cl and 2-Cl in MeCy and 2-MeTHF at different
concentrations. .............................................................................................................................. 57
Table 2. 2. Photophysical properties of microcrystalline powders of all luminescent complexes
in this chapter. ............................................................................................................................... 60

Table 3. 1. Crystallographic information of complexes appearing in this chapter. ..................... 71
Table 3. 2. Photophysical properties of complexes 1 – 6. ............................................................ 85

Table 4. 1. Crystallographic information for complexes 1a – 1d, 2a, 2b, 3, 4, and 5. ................ 95
Table 4. 2. Redox potentials of complexes 1a, 3 - 5 as well as KCz, the parent amines,
CAAC
Men
-CuCl precursor, and Cu host. ...................................................................................... 96
Table 4. 3. Frontier MO surfaces and energies as well as ground state dipoles (μ GS) of complexes
1a – 1d, 2a, 2b, 3 – 5, and Cu host computed at the B3LYP/LACVP** level. .......................... 97
Table 4. 4. Photophysical properties of complexes 1a – 1d, 2b, 3, 4, 5 and Cu host in various
media. .......................................................................................................................................... 107
9

Table 4. 5. Calculated singlet and triplet excited state energies for the complexes in this chapter
obtained through TDDFT performed at the CAM-B3LYP/LACVP** level. ............................ 109

Table 5. 1. Crystallographic information for complexes C1 – C3 and B1 – B3........................ 135
Table 5. 2. Redox potentials of complexes C1 – C3 and B1 – B3, and the associated
experimental frontier orbital energies. ........................................................................................ 136
Table 5. 3. Photophysical properties of complexes C1 – C3 and B1 – B3 in various media at
room temperature and 77 K. ....................................................................................................... 149
Table 5. 4. Frontier orbital surfaces and energies for C1 – C3 and B1 – B3 and the permanent
dipole moment μ GS obtained through DFT calculations performed at the B3LYP/LACVP**
level. ............................................................................................................................................ 153
Table 5. 5. Calculated singlet and triplet excited state energies for the complexes in this chapter
obtained through TDDFT performed at the CAM-B3LYP/LACVP** level. ............................ 157

Table A1. 1. Photophysical properties of SPINACH 2 in 2-MeTHF at RT and 77 K. ............. 173
Table A1. 2. Calculated and experimental parameters for SPINACH 2. ................................. 173

10

Figure 1. 1. Jablonski diagram depicting photophysical processes that follow electron excitation
in molecules: internal conversion (IC, purple) to the lowest-lying excited state in a manifold,
which can then decay radiatively via fluorescence (blue arrow) or non-radiatively through
vibronic coupling to the ground state (n. r., red) or through inter-system crossing (ISC, green)
between the excited singlet and triplet manifolds. T1 can also decay radiatively via
phosphorescence (orange arrow) or non-radiatively through vibronic deactivation. ................... 24
Figure 1. 2. Jahn-Teller flattening distortion established in four-coordinate Cu(I) complexes
following excitation. ..................................................................................................................... 27
Figure 1. 3. Ligands most commonly employed in 4-coordinate cationic and neutral Cu(I)
complexes, including the earliest bisphenanthroline,
27, 42
bisphosphine,
26, 43
and
triphenylphosphine
26, 36, 44
ligands investigated by McMillin and coworkers as well as the anionic
amidophosphines
36
explored by Peters and coworkers and bispyrazolylborates employed by
Yersin et. al and Osawa et. al.
35, 40, 45
............................................................................................ 28
Figure 1. 4. 3-coordinate Cu(I) complexes with commonly-employed ligands. ......................... 32
Figure 1. 5. Y to T distortion in the triplet excited state (MLCT in nature) of trigonal planar
Cu(I) complexes. ........................................................................................................................... 33
Figure 1. 6. Luminescent, 2-coordinate, linear Cu(I) complexes. ............................................... 35
Figure 1. 7. Excited state reorganization that is possible in linear, 2-coodinate Cu(I) complexes.
....................................................................................................................................................... 36
Figure 1. 8. Published reports relating to TADF over a 10-year period, per SciFinder. Plot
copied from ref 77. ........................................................................................................................ 37
Figure 1. 9. Jablonski diagram depicting the different states and processes involved in TADF. 39

11

Figure 2. 1. Electronic properties of NHC's (left, center) and CAAC’s (right). In NHC’s, the two
nitrogens exert a push-pull effect, where one nitrogen donates mesomerically into the vacant 2pz
orbital of the Ccarbene, and the other nitrogen is electron-withdrawing from the filled sp
2
orbital of
Ccarbene. Contrastingly, in asymmetric CAAC’s, only one nitrogen is mesomerically donating into
the 2pz of Ccarbene, which makes the orbital overall more π-electrophilic than in NHC’s.
Additionally, the tertiary carbon is inductively donating into the filled sp
2
orbital, resulting in
CAACs being stronger σ-donors than their NHC counterparts. ................................................... 49
Figure 2. 2. Synthetic scheme and molecular structures of the complexes in this chapter. ......... 50
Figure 2. 3. Crystal structures of complexes 1-Tp, 2-Tp, 3-Tp and 5-Tp showing the Oak Ridge
Thermal Ellipsoid Plots (ORTEPs). The isopropyl units on the 2,6-diisopropylphenyl (Dipp)
moiety and the hydrogen atoms were removed for clarity. .......................................................... 52
Figure 2. 4. Absorption spectra of precursors 1-Cl and 2-Cl showing their extinction coefficients
in CH2Cl2 (left) and their observed negative solvatochromism. ................................................... 53
Figure 2. 5. Emission spectra of microcrystalline powders of precursors 1-Cl, 2-Cl, and 5-Cl at
room temperature (solid lines) and 77K (dashed lines). ............................................................... 54
Figure 2. 6. Emission spectra of 1-Cl (left) and 2-Cl (right) in microcrystalline solid form (SS)
and in 2-MeTHF solutions at RT and 77K. .................................................................................. 55
Figure 2. 7. Absorption and emission spectra of 1-Cl and 2-Cl in MeCy showing concentration-
dependent low-energy emission bands. The inset shows the PL decay traces of both high-energy
and low-energy bands. .................................................................................................................. 56
Figure 2. 8. Modified Stern-Volmer plot of self-quenching kinetics for 1-Cl (left) and 2-Cl
(right) in MeCy. Due to poor solubility limits, the concentrations were determined using the
extinction coefficients of the high-energy absorption peak of the complex in CH2Cl2. For 1-Cl,
12

the equation displayed in the figure corresponds to a kinetic model where the monomeric species
(M) forms an aggregate species (A), with a rate constant of formation, 𝑘 𝑓 𝐴 = 1.8 x 10
10
s
-1
. The
emission decay rate of the monomer at infinite dilution has a rate constant of 𝑘 𝑑 𝑀 = 9.5 x 10
5
s
-1
,
as given by the intercept of the linear fit. Excitation spectra corresponding to emission at 650 nm
of the more concentrated samples show a clear peak at 528 nm. The PL decays at 650 nm for all
the concentrations examined show no discernable rise time. These observations, together with 𝑘 𝑓 𝐴
= 1.8
10
s
-1
, indicate the formation of an aggregate species of 1-Cl in MeCy. For 2-Cl, the
equation displayed in the figure corresponds to a kinetic model where the monomeric species
(M) forms an excimer (E), with a rate constant of formation, 𝑘 𝑓 𝐸 = 7.4 x 10
9
s
-1
. The emission
decay rate of the monomer at infinite dilution has a rate constant of kdM = 8.3 x 10
5
s
-1
, as given
by the intercept of the linear fit. Excitation spectra corresponding to emission at 650 nm are
identical to those corresponding to emission at 430 nm. The PL decay at 650 nm shows a rise
time that is concentration-dependent. These observations, together with 𝑘 𝑓 𝐸 = 7.4 x 10
9
s
-1
,
indicate a diffusion-limited excimer formation process for 2-Cl in MeCy. ................................. 56
Figure 2. 9. Absorption spectra of complexes 1-Tp, 2-Tp, 3-Tp, and 5-Tp in CH2Cl2. ............. 58
Figure 2. 10. Emission spectra of microcrystalline powders of complexes 1-Tp - 5-Tp at RT
(solid lines) and 77K (dashed lines), showing the color tuning to lower emission energies via
employing CAACs (left) and across the NHC series via π extension and aza-substitution (right).
....................................................................................................................................................... 60
Figure 2. 11. HOMO (solid) and LUMO (mesh) frontier orbital surfaces and energies of
complexes 1-Tp to 5-Tp. .............................................................................................................. 62

Figure 3. 1.Synthetic scheme depicting the structures of the complexes in this chapter. ............ 70
13

Figure 3. 2. Crystal structures of complexes 1, 2, 3, 5, and 6 showing the Oak Ridge Thermal
Ellipsoid Plots (ORTEPs). The isopropyl units on the 2,6-diisopropylphenyl (Dipp) moiety are
depicted in wireframe mode and the hydrogen atoms are removed for clarity. ........................... 71
Figure 3. 3. Scheme depicting the tumbling mechanism of the Tp ligand adapted for 4-
coordinate carbene-Cu(I) complexes.
18
........................................................................................ 72
Figure 3. 4. Variable temperature (VT)
1
H-NMR of complex 1 in acetone d6. ........................... 73
Figure 3. 5. VT
1
H-NMR spectrum of complex 2 in acetone d6. ................................................. 74
Figure 3. 6. VT
13
C-NMR spectra of complex 2 in acetone d6, showing the signal from the
carbene carbon (c) appearing at low temperatures. The multiple peaks, especially in the aliphatic
region, appearing at low temperatures are due to an instrument artifact: effective C-H decoupling
is compromised at low temperatures giving rise to C-H splitting. ............................................... 74
Figure 3. 7. VT
1
H-NMR of complex 2 in CDCl3. ...................................................................... 76
Figure 3. 8. Mechanism for the 1,2-borotropic shift, adapted for carbene-Cu(I) Tp complexes.
22
....................................................................................................................................................... 76
Figure 3. 9. VT
13
C-NMR spectra of complex 2 in CDCl3. Defective C-H decoupling at low
temperatures gives rise to C-H splitting........................................................................................ 77
Figure 3. 10. VT
1
H-NMR spectra of complex 3 in acetone d6. .................................................. 77
Figure 3. 11. VT
13
C-NMR spectra of complex 3 in acetone d6, showing the signal from the
carbene carbon (c) appearing at low temperatures. ...................................................................... 79
Figure 3. 12. VT
1
H-NMR spectrum of complex 5 in acetone d6. ............................................... 80
Figure 3. 13. VT
1
H-NMR spectra of complex 6 in acetone d6. .................................................. 81
14

Figure 3. 14. a) Room temperature and 77 K emission spectra of microcrystalline powders of 1
and 2, and of 2 in MeCy. b) Room temperature and 77 K emission spectra of microcrystalline
powders of 3 and 4, and of 4 in 2-MeTHF. .................................................................................. 82
Figure 3. 15. Emission spectra of microcrystalline powders of 5, and 6 compared with 2, at room
temperature and 77 K. ................................................................................................................... 84
Figure 3. 16. Frontier orbital surfaces and energies of complexes 1 and 2 (highest occupied
molecular orbital, HOMO, solid; lowest unoccupied molecular orbital, LUMO, mesh). ............ 86

Figure 4. 1. Synthetic scheme followed in the preparation of complexes 1a - 1d, 2a, 2b, 3, 4, and
5..................................................................................................................................................... 92
Figure 4. 2. Oak Ridge thermal ellipsoid plot (ORTEP) representation of 1a – 1d, 2a, 2b, and 3 –
5. Thermal ellipsoids shown at 50% probability. Hydrogens removed for clarity. ...................... 93
Figure 4. 3. Crystal structures of the decomposition products of complex 2a (left) and 2b
(middle) in ambient air, showing a bridging𝐶𝑂
3
2−
moiety and two 1,8-dimethyl-9-H-carbazole
moieties within hydrogen-bonding distances from two of the carboxylate oxygens. On the right is
a general representation of both structures. .................................................................................. 94
Figure 4. 4. Variable temperature (VT)
1
H NMR spectra of 1a in CDCl3................................... 95
Figure 4. 5. (Top) HOMO (solid) and LUMO (mesh) surfaces of complex 1a. (Bottom)
Simplified picture of the HOMO and LUMO of this complex. .................................................... 97
Figure 4. 6. Extinction coefficients of 1a – 1d (a) 1a, 3, 4, 5 (b), and 1b and 2b (c) in THF. The
inset in the a) shows a linear relation between the energy of the CT absorption band in
methylcyclohexane, (MeCy) and the oxidation potential of the complexes. ................................ 99
15

Figure 4. 7. a) Negative solvatochromism of the ICT absorption band observed in complexes 1a
(top) and 5 (bottom) (2-MeTHF: 2-methyltetrahydrofuran, AcN: acetonitrile). b) Absorption
spectra of complex 1a at room temperature and 77K, showing a blue-shift in the ICT band at low
T. c) Absorption spectra of solutions of 1a in 2-MeTHF at three concentrations (C1 = 2.2 x 10
-4

M, C2 = 4.2 x 10
-5
M, C3 = 2.2 x 10
-5
M), collected at 298 K, 195 K, and 77K. 195K was attained
in a medium of dry-ice/acetone. Hence, the spectra are normalized to the peak at 370 nm
(marked with an asterisk), beyond the absorption cut-off of acetone. ........................................ 101
Figure 4. 8. a) RT and 77K emission spectra of microcrystalline powders and a neat thin film of
1a compared. b) RT and 77K emission spectra of the coplanar 1a and the orthogonal 2b
compared. c) Excitation (dashed lines) and emission (solid lines) spectra of microcrystalline
powders of complex 5 at RT and 77K. ....................................................................................... 102
Figure 4. 9. Emission spectra of 1a (a) and 5 (b) in solvents of increasing polarity. c) RT and
77K emission spectra of 1b and 2b compared. ........................................................................... 103
Figure 4. 10. RT and 77K emission spectra of 1a – 1d in 2-MeTHF (a) and of 1a, 3 – 5 in MeCy
(b), 2-MeTHF (c), and 1 wt% PS films (d). ................................................................................ 104
Figure 4. 11. Emission spectra of complex 3 in MeCy (top) and toluene (bottom) at two different
concentrations. The inset (top, right corner) shows the PL decay traces recorded at 440 nm and
600 nm in MeCy. While the decay at 440 nm is monoexponential, at 600 nm it is biexponential
and shows a rise time, consistent with excimer formation and emission. .................................. 105
Figure 4. 12. Temperature-dependent PL profiles (a) and decays (b) for 1 wt% PS films of 1a
and 5. ........................................................................................................................................... 106
16

Figure 4. 13. a) State diagram depicting
I,3
CT/
3
LE ordering in the reported complexes. The
relative energies of the states are based on emission spectra. b) Jablonski diagram depicting the
different processes operating in various media at room temperature and 77K........................... 108
Figure 4. 14. Top: Solvation effects operating in a polar medium, 2-MeTHF, on the ground and
ICT excited state dipoles (μ-GS and μ-CT respectively) at room temperature (fluid medium) and
77K (frozen glass). Bottom: Solute-solute interactions operating in non-polar fluid and frozen
media that can explain the observed destabilization in of the ICT state at low temperatures. ... 111
Figure 4. 16. a) OLED device architecture for 1a-based OLEDs after optimization of EML and
ETL thickness. b) EQE traces of devices employing different hosts; the inset is a picture of a Cu
host device. c) L-V and d) J-V traces of the Cu-host device. .................................................... 113
Figure 4. 17. a) EL and b) PL spectra of 1a in various hosts. c) Molecular structures of the
different hosts used in OLEDs employing 1a. ............................................................................ 113
Figure 4. 18. a) Extinction coefficient of Cu host in THF. b) top: emission of microcrystalline
powder of Cu host at RT and 77K compared with the emission of 1a doped into Cu host (20
wt%); bottom: RT and 77K emission spectra of Cu host and 1a in MeCy. ............................... 114

Figure 5. 1. Synthetic scheme (top) depicting the preparation of the two coordinate coinage
metal complexes C1 – 3 and B1 – 3 shown below. .................................................................... 133
Figure 5. 2. Synthetic route used in the preparation of BzI-M-Cl precursors. ........................... 134
Figure 5. 3. Oak Ridge Thermal Ellipsoid Plots (ORTEPs) representing the crystal structures of
complexes C1 – C3, B1 – B3 including both conformers of complex B2: B2a and B2b. ......... 134
Figure 5. 4. Extinction coefficients of complexes C1 – C3 in THF (a) and complexes B1 – B3 in
2-MeTHF (b). .............................................................................................................................. 137
17

Figure 5. 5. Scaled absorption spectra of complexes C1 – C3 (a – c) and B1 – B3 (d – f) in
various solvents. .......................................................................................................................... 138
Figure 5. 6. Emission spectra of complexes C1 – C3 (top) and B1 – B3 (bottom) in various
solvents. ...................................................................................................................................... 140
Figure 5. 7. Qualitative energy diagram representing the ground state (GS) and both excited state
energy surfaces (CT and
3
Cz) as a function of solvent coordinate. a) non-polar solvents induce
electronic transitions with small reorganization energies and result in vibronically-structured
3
Cz-dominant emission; b) polar solvents induce a blue-shift in absorption which leads to broad
and featureless CT-dominant emission reflecting the high reorganization energies. Blue arrows
represent CT absorption and red arrows represent emission in non-polar (solid) and polar
solvents (dotted). Green arrows represent rapid thermalization into the lowest vibrational energy
level of the manifold. .................................................................................................................. 143
Figure 5. 8. Emission profiles of complexes C1 (a), C2 (b), and C3 (c) at room temperature
(top) and frozen glassy or polymeric matrices (bottom). ............................................................ 144
Figure 5. 9. Temperature-dependent PL decays (red dots) of complexes C1 (a), C2 (b), and C3
(c) in thin PS films and their respective fits (black line) to the modified Boltzmann equation. The
insets show the excited state models obtained from the fits. ...................................................... 145
Figure 5. 10. Emission profiles of complexes B1 (a), B2 (b), and B3 (c) at room temperature
(top) and frozen glassy or polymeric matrices (bottom). ............................................................ 147
Figure 5. 11. State energy diagram representing room temperature and 77 K emission in
complexes B1 – B3. .................................................................................................................... 148
Figure 5. 12. a) architecture of devices employing C2 as an emitter doped at 10 vol% in mCBP
and mCP; b) EQE curves of the devices; c) J-V-L curves; d) EL spectra. ................................. 150
18

Figure 5. 13. a) Device architectures for OLEDs with C1 and C2 emitters doped at 20 vol% in
mCBP; b) EQE curves; c) characteristic J-V-L curves; d) EL spectra. ...................................... 151

Figure A1. 1. Synthetic route for bSF (top) and SPINACH (bottom). ...................................... 170
Figure A1. 2. Emission spectra of 2,2'-bSF in 2-MeTHF (left) and powdered form (right). .... 171
Figure A1. 3. Absorption and emission spectra of SPINACH 1 (left) and 2 (right) at RT and 77
K. ................................................................................................................................................. 172

19

Abstract
Phosphorescent organometallic complexes have been applied in fields ranging from photocatalysis
to solar fuels, chemo- and biosensing, organic light emitting diodes (OLEDs), and solid state
lighting (SSL). State-of-the-art phosphors in these applications employ rare-earth, heavy metals
such as Ru(II), Os(II), Pt(II), and Ir(III). Among the more abundant, first-row transition metals,
phosphorescent Cu(I) complexes have been studied most extensively. However, the vast majority
of organocopper complexes investigated are 4-coordinate, bearing two bidentate ligands.
Compared to their heavier metal analogues, the phosphorescence of tetrahedral Cu(I) complexes
is either inefficient, characterized by long radiative lifetimes, or both. This work explores Cu(I)
complexes with different geometries that can circumvent common non-radiative deactivation
pathways. In particular, we highlight that 2-coordinate carbene-Cu(I) complexes with redox active
ligands that display highly-efficient, Ir-like luminescence. Their Ag(I) and Au(I) analogues
possess remarkable photophysical properties as well.
Chapter 1 introduces the basics of phosphorescence in organometallic complexes, highlighting key
differences between the luminescence of common organocopper(I) emitters and state-of-the-art
organoiridium(III) and organoplatinum(II) phosphors. It provides an overview of the most
impactful advances in Cu(I) luminescence, from poor and inefficient phosphorescence to
thermally-activated delayed fluorescence (TADF).
2-, 3-, and 4-coordinate Cu(I) complexes employing N-Heterocyclic (NHC) and Cyclic
(alkyl)(amino) carbenes (CAACs) are studied in Chapter 2. Completing the coordination sphere is
a chloride or a trispyrazolyl borate (Tp) as the anionic ligand. Emission is tuned by modulating the
electrophilicity of the carbene: through benzannulation and aza-substitution of the NHC, and
20

through employing CAAC as a better electron acceptor. The steric substituents on the carbene
control the binding mode of Tp: η
3
Tp gives complexes with more efficient phosphorescence than
η
2
.
Chapter 3 explores the solution equilibration between the structural isomers formed as a result of
the different binding modes of Tp. XRay crystallography is used to examine the solid state
structures of series of complexes bearing varying steric groups on the Tp ligand and the carbene.
Variable temperature (VT)
1
H- and
13
C NMR experiments help elucidate the mechanism behind
isomeric equilibration, which is hampered and not curbed at low temperatures. Higher hapticity of
Tp is found to enhance the photoluminescence quantum yields (ΦPL) of the Cu(I) complexes in
microcrystalline powder form. Increased steric encumbrance of Tp results in improved ΦPL in
solution.
Simple, 2-coordinate Cu(I) complexes pairing CAACs with various amides were examined in
Chapter 4. The redox properties of these linear complexes are largely determined by the ligands:
oxidation by the electron-rich amides and reduction by the electron-deficient carbene. Density
functional theory (DFT) calculations reveal a picture of the frontier molecular orbitals (MO’s) that
mirror the electrochemistry: the highest occupied MO (HOMO) comprises primarily the amide N
2pz orbital, and the lowest unoccupied MO (LUMO) localizes on the carbene C 2pz orbital. The
metal’s d-orbitals contribute only weakly to both orbitals, allowing for a representation of these
complexes as donor-bridge-acceptor structures, with the Cu center acting as an efficient electronic
bridge. The coplanar ligand orientation and the weak d-orbital overlap facilitate highly allowed
amide-to-carbene charge transfer (ligand to ligand CT) transitions characterized by strongly
absorbing CT bands and radiative rate constants (kr > 10
5
s
-1
) that rival state-of-the-art Ir(III)
phosphors. This chapter registers the first 100% efficient Cu(I) emitters in fluid and polymeric
21

media. Emission color is tuned via electronic substitution on the amide, and low temperature
studies reveal a rich excited state manifold comprising
1
CT,
3
CT, and a closely-lying amide-
centered triplet state with ms photoluminescent (PL) decay times. Temperature-dependent studies
show that efficient TADF occurs within the CT manifold, and one of the complexes is employed
as an emitter in blue OLED devices.
Chapter 5 comprises the CAAC-Ag(I) and Au(I) congeners of the Cu(I) complexes studied in
chapter 4. The heavier metals enforce a larger ligand separation, contributing to smaller energy
gaps between
1
CT and
3
CT. In this series, the Ag(I) complex shows the longest carbene-amide
distance, and thus the lowest exchange energy. The radiative rates are found to increase down the
periodic table; however, the Ag(I) complex stands out with the highest kr = 2 x 10
6
s
-1
, making it
the fastest triplet-based emitter. Temperature-dependent studies reveal highly-efficient TADF
between
1
CT and
3
CT, as well as strong zero-field splitting of the
3
CT sublevels, which signifies
strong spin-orbit coupling interactions. Ag(I)-based OLEDs are found to be more efficient that
their Cu(I)-based counterparts. Chapter 5 also reports the preparation of a bulky
benzo[d]imidazole-2-ylidene, a long-standing synthetic target, and its metalation onto Cu(I),
Ag(I), and Au(I). These highly-efficient emitters (kr > 10
5
s
-1
) show a different ordering of their
excited state manifolds and a higher extent of coupling between the CT and amide-localized states.

22

Chapter 1. Introduction

The long excited state lifetimes and tunable energy levels offered by the lowest-lying triplet excited
state (T1) in organic and organometallic molecules have made it instrumental in major
advancements in photocatalysis and photochemistry,
1
chemo-
2
and biosensing,
3
dye-sensitized
solar cells (DSSC’s)
4, 5
and solar fuels,
6
and organic electronics.
7
Understanding transitions that
culminate in and emanate from this excited state is therefore essential for designing systems with
T1 properties tuned for the desired application.
1.1. Spin-orbit coupling and the triplet state
In general, transitions between singlet states and triplet states involve a change in the electron spin,
making them formally forbidden. However, the likelihood or allowedness of these transitions can
be enhanced through spin-orbit coupling (SOC), a relativistic quantum mechanical phenomenon
that couples a change in electron spin angular momentum (associated with spin quantum number,
mS) with a change in its orbital angular momentum, (associated with orbital quantum number, mL).
8

Viewed from a fixed-electron frame of reference, a positively-charged nucleus can be pictured as
orbiting a fixed electron, thereby generating a magnetic field B. The magnetic moment created by
electron spin thereby aligns itself with B, and the resultant interaction is termed spin-orbit
coupling. It is the Columbic dependence of B on the magnitude of nuclear charge Z that originates
the term “heavy atom effect”,
9
whereby the presence of an external heavy atom can affect efficient
SOC.

23

Table 1. 1. Values for the spin-orbit coupling parameter, ξ1, of common atoms provide a measure
of the strength of the SOC interaction.
10

Element Atomic number Atomic mass (u
-1
) ξ1(cm
-1
)
C 6 12.0107 32
N 7 14.0067 78
Cu 29 63.546 857
Os 76 190.23 3381
Ir 77 192.217 3909
Pt 78 195.078 4481
Au 79 196.9665 5104
Ag 47 107.8682 1779
I 53 126.9045 5069

In addition to the external heavy atom effect, SOC can be invoked by an “internal” effect that
depends on the parentage of the excited states and is best described by the El-Sayed rules.
11

In polyatomic molecules, the spin orbit coupling Hamiltonian is defined as follows:
∑ ∑ 𝜉 (𝑟 𝑖𝐴
)𝒍 𝑖 𝐴 .𝒔 𝑖 𝑖 +
𝐴 2-e
-
terms (Equation 1. 1)
12

where 𝜉 (𝑟 𝑖𝐴
) is describes the separation of the ith electron from the nucleus, and thus has higher
magnitudes with increasing nuclear charge Z; 𝒍 𝑖 𝐴 and 𝒔 𝑖 are the electron’s orbital angular
momentum and spin angular momentum respectively. While the first part represents the one-
electron component that describes the coupling between 𝒍 𝑖 𝐴 and 𝒔 𝑖 for a single electron, the second
part is a 2-electron component that describes interactions between 𝒍 𝑖 𝐴 and 𝒔 𝑗 for a different
electron, j.
12
The one particle component has a direct dependence on the nuclear charge, making
it the dominant component, especially in heavy-metal complexes. Thus, the SOC Hamiltonian can
be reliably approximated as a one-electron operator. In effect, this signifies that the SOC operator
cannot couple a single electronic orbital with itself, or put differently, it cannot couple two excited
states that share the same orbital parentage. ĤSO can only couple states that differ in one spin-
24

orbital and that satisfy the Slater-Condon selection rule: ΔMS = 0, ±1.
13

In organic molecules bearing heteroatoms such as O and N, SOC acts as follows: it cannot couple
singlet and triplet states that result from π → π* transitions. However, coupling between the
π → π* singlet
1
(ππ*) and one of the n → π* triplet sublevels
3
(nπ*)1 is effective, where n
represents the orbital occupied by the lone pair of the heteroatom.
11
In organometallic complexes
with optically-active metal-to-ligand charge transfer (MLCT) transitions, SOC is most efficient in
coupling singlet and triplet substates that differ in one d-orbital, such as
1
(dxyπ*) and
3
(d yzπ*)-1.
7

The large SOC factor of the metal (Table 1. 1) further aids in maximizing this interaction, which
ultimately enhances non-radiative S1 → T1 ISC and radiative T1 → S0 phosphorescence in these
complexes.

Figure 1. 1. Jablonski diagram depicting photophysical processes that follow electron excitation
in molecules: internal conversion (IC, purple) to the lowest-lying excited state in a manifold, which
can then decay radiatively via fluorescence (blue arrow) or non-radiatively through vibronic
coupling to the ground state (n. r., red) or through inter-system crossing (ISC, green) between the
excited singlet and triplet manifolds. T1 can also decay radiatively via phosphorescence (orange
arrow) or non-radiatively through vibronic deactivation.

To put into perspective the enhancement offered by SOC to otherwise forbidden transitions, we
introduce the electronic energy level diagram, commonly known as the Jablonski diagram
14

25

(Figure 1. 1). The complete development of this state diagram into its more recognizable, modern
form, was made by Lewis and Kasha, whose work elucidated spin multiplicities of the depicted
states.
15, 16
Following instantaneous electronic excitation into the singlet manifold, rapid internal
conversion (IC) dissipates the energy of the higher lying singlet states through vibrational
relaxation, leading to the lowest excited singlet state S1. Kasha assigned IC processes a rate
constant ranging between 10
11
– 10
14
s
-1
.
17
S1 can then decay radiatively, through fluorescence (kr,f
~ 10
8
– 10
9
s
-1
), and/or non-radiatively through vibronic coupling to the ground state. Additionally,
singlet excited states (or excitons) can undergo intersystem crossing (ISC) into quasi-resonant
excited triplets. In organic systems where SOC is operant, kISC for Sn → Tn (n ≥ 1) can be as large
as 10
7
s
-1
,
18, 19
thus competing with fluorescence and a representing a dramatic improvement for a
formally-forbidden process. In organometallic complexes featuring heavy metals such as Ru (II),
Ir (III), Pt (II), and Os (II), strong SOC affects quantitative ISC between S n and Tn, with kISC as
high as 10
13
– 10
14
s
-1
.
20, 21
Once in the triplet manifold, and following IC down to T1, SOC can
further enhance radiative ISC from T1 → S0, with phosphorescence rate constants, kr,p, ranging
from 10
2
– 10
4
s
-1
in optimized organic systems to 10
5
s
-1
in organometallic complexes comprising
the heavy metals enumerated above.
1.2. Phosphorescence of Cu(I) complexes
Among the lighter and more abundant first row transition metals, copper is the metal whose
complexes show the richest history of phosphorescence.
22-24
This can be understood in light of the
Cu nucleus being the second to heaviest among the first-row transition metals, and more
importantly, the stability of the Cu(I) oxidation state with its d
10
electron configuration. In
complexes with other first row transition metals having unfilled d-orbitals, excited states involving
triplets rapidly decay to low-lying dark states emanating from d-to-d transitions. These properties
26

have motivated numerous research efforts investigating Cu(I)-based phosphors as cheaper, more
abundant alternatives to their heavy metal-based counterparts.
However, the lighter Cu nucleus induces significantly weaker SOC effects that result in slower
electronic transitions involving the triplet manifold: kISC and kr,p in Cu(I) complexes most
commonly range between 10
10
– 10
11
s
-1
and 10
3
– 10
4
s
-1
respectively; orders of magnitude slower
than organo-Ir(III) and Pt(II) phosphors.
25

This chapter thus aims at providing an overview of the features and challenges presented by
phosphorescent Cu(I) complexes: their different geometries; most common ligand sets; the
electronic configuration of their excited states; common non-radiative decay processes; recent
forays into circumventing the metal’s inherently weak SOC; culminating with their applications
in organic electronics.
Tetrahedral Cu(I) complexes
The earliest studied phosphorescent Cu(I) complexes by McMillin were cationic, four-coordinate,
homoleptic ones employing two phenanthroline moieties as ligands.
26, 27
Upon excitation, these
complexes, henceforth referred to as [Cu(N^N)2]
+
where N^N denotes the bidentate bis-imine
ligand, experience an MLCT transition where an electron is promoted from the Cu-centered
HOMO to the N^N-centered (π*) LUMO. The orange-to-red luminescence exhibited by these
complexes is weak and only observed in the microcrystalline solid state and in frozen glassy
matrices. At room temperature in aerated fluid media, phosphorescence is almost completely
quenched, with the photoluminescence (PL) quantum yield (ΦPL) < 0.01 and emission lifetimes
ranging from 10’s of ns to a few μs in dichloromethane (CH2Cl2)
27
or alcoholic solvents like
ethanol/methanol. PL quenching is exacerbated in coordinating solvents such as acetonitrile
27

(AcN), and the mechanism underlying this non-radiative deactivation process has been extensively
studied.

Figure 1. 2. Jahn-Teller flattening distortion established in four-coordinate Cu(I) complexes
following excitation.
Since the excitation of a four-coordinate Cu(I) complex represents an MLCT transition, it
effectively entails a formal oxidation of the metal center from d
10
to d
9
electron configuration,
thereby leading to a Jahn-Teller desymmetrization from pseudo-tetrahedral geometry to pseudo-
square planar (Figure 1. 2).
28, 29
This ‘flattening” distortion is associated with large reorganization
energies and leaves an open coordination site on the metal center. A combination of these two
features results in non-radiative deactivation of the excited state, through vibronic coupling to the
ground state and/or exciplex formation with a coordinating group, usually the solvent acting as a
Lewis base.
30-32
In more rigid media such as the crystalline solid matrix or a frozen glassy one,
structural reorganization in the excited state is suppressed, leading to the recovery of luminescence,
however weak, in such media. Alternately, the addition of sterically-demanding groups on the 1,9-
positions of the phenanthroline moiety has been shown to have a similar mitigating effect on the
extent of structural reorganization, yielding complexes with increasing ΦPL and emission
lifetimes.
28, 33, 34

Heteroleptic complexes employing a combination of bisimine and bisphosphine ligands have also
been extensively studied, as the phosphino ligands introduced new handles on steric and electronic
modulation of four-coordinate Cu(I) complexes (Figure 1. 3). Employing asymmetric ligands has
28

afforded overall neutral complexes, as well as cationic ones. Over the years, heteroleptic, neutral
Cu(I) emitters have been prepared that have remarkably high ΦPL and long excited state lifetimes:
ΦPL = 0.90; τ = 13 μs in the microcrystalline solid state,
35
and ΦPL = 0.56; τ = 20.2 μs in benzene
36

for example. Such advancements in the photophysical properties have enabled the application of
similar Cu(I) complexes as sensors
37
and emitters in light-emitting electrochemical cells
(LEEC’s),
38, 39
and organic light-emitting diodes (OLEDs).
40, 41

Figure 1. 3. Ligands most commonly employed in 4-coordinate cationic and neutral Cu(I)
complexes, including the earliest bisphenanthroline,
27, 42
bisphosphine,
26, 43
and
triphenylphosphine
26, 36, 44
ligands investigated by McMillin and coworkers as well as the anionic
amidophosphines
36
explored by Peters and coworkers and bispyrazolylborates employed by Yersin
et. al and Osawa et. al.
35, 40, 45

However, with the exception of the complexes reported by Peters et. al,
36
four-coordinate Cu(I)-
based emitters show a dramatic decrease in their ΦPL in non-rigid matrices, due to non-radiative
quenching processes that relate to the large reorganization energies associated with MLCT
transitions. In fact, it appears that key to the high solution ΦPL achieved by Peters et al. is the low
contribution from the Cu center to the frontier molecular orbitals. To illustrate, we compare the
former example with one of the most efficient solid state emitters reported by Yersin et. al
35
(Table
1. 2). The calculated surfaces of the highest occupied molecular orbital (HOMO) paint a different
picture for both complexes. Whereas in the Yersin complex, the HOMO is largely comprised of
29

the Cu d-orbitals, the Peters complex shows only a minor contribution from the metal. The lowest
unoccupied molecular orbital (LUMO) in both complexes comprises of the π* orbitals of the
phenylphosphino ligands, showcasing the CT nature of the optically active transitions: accurately
described as MLCT in the Yersin complex, and metal-perturbed intraligand charge transfer (ILCT)
Table 1. 2. Frontier molecular orbitals and solution photophysics of select examples from
references 35 and 36. Both complexes are reported to be brightly luminescent in the solid state,
with the one in ref. 35 having ΦPL = 0.9.

Yersin et. al
35

Peters et. al
36

HOMO

LUMO

ΦPL / τ(μs) (solution) 0.08 / 1.8 (CH2Cl2) 0.21 / 22.3 (C6H6)
kr / knr (s
-1
) 4.4 x 10
4
/ 5.1 x 10
5
9.4 x 10
3
/ 3.5 x 10
4

30

in the Peters example. The solution photophysics of the two complexes also show stark differences,
especially in terms of their non-radiative rates. Whereas the Yersin complex exhibits efficient non-
radiative quenching of its luminescence in CH2Cl2 (knr = 5.1 x 10
5
s
-1
), the Peters one shows a less
dramatic drop in ΦPL in benzene (knr = 3.5 x 10
4
s
-1
). In fact, optimizing the steric encumberance
of the Peters emitters results in a complex with record solution ΦPL = 0.7. This behavior can very
well be attributed to the difference in the parentage of the radiative transitions in both families of
complexes: the example with the reduced Cu contribution in its HOMO experiences a lesser extent
of structural reorganization in the excited state, which is associated with a formal oxidation at the
metal center upon excitation. Nevertheless, a certain extent of Cu involvement in the frontier
orbitals is necessary to maintain the “internal heavy atom effect.” The impact of the different Cu
contribution to the MO’s can also be seen in the different radiative rates of these complexes: the
MLCT Yersin complex has a radiative rate constant (kr) nearly five times higher than that of the
ILCT Peters one. Hence, it appears that a balance must be struck between minimizing the
reorganization energies incurred with MLCT excitations involving the Cu center (suppressing knr),
while maintaining sufficient metal involvement to increase kr. This seeming dilemma in Cu
luminescence will be expanded on in chapter 4 of this work. It is also worth noting that the efficient
solid state luminophores reported by Yersin and coworkers, emits via a process termed thermally-
activated delayed fluorescence, or TADF.
46
In fact, a majority of the best-performing tetrahedral,
mono- and polynuclear Cu(I) emitters undergo TADF, which is touched upon in a later section of
this chapter.
More recently, carbenes have emerged as a new class of chromophoric ligands
47
that have made
possible novel geometries, including 4-coordinate ones with C3 symmetry that are immune to the
aforementioned flattening distortion.
48
Chapter 2 in this work introduces this new structural class
31

of Cu(I) complexes and explores their photophysical properties as a function of carbene
electrophilicity. In addition to the carbene, the tridentate ligand employed in these complexes to
complete the 4-coordinate geometry is tris-pyrazolylborate, Tp. This ligand set allows for
systematic steric modulation and thorough investigation of molecular dynamics, which are
explored in depth in chapter 3.
Three Coordinate Cu(I) complexes
A three-coordinate, Y-shaped, and planar motif for Cu(I)-based emitters has been introduced in
more recent years, motivated in part by the need to counter the flattening distortion which occurs
in the excited state of tetrahedral Cu(I) complexes.
1.2.2.1. Phosphines, amides, and halides
In 2010, the Peters lab reported a series of neutral, 3-coordinate Cu(I) emitters bearing a
bisphosphine ligand (or two, monodentate phosphine ligands) and a arylamide ligand (Figure 1.
4).
49
The solution luminescence of these complexes (in methylcyclohexane) showed the viability
of Cu(I) complexes with low coordination number, given the proper ligand design: ΦPL up to 0.24
and τ ranging from 1.7 μs to 11.7 μs. Examining the frontier molecular orbitals of these complexes
reveals a picture similar to that observed in Peters’ 4-coordinate complexes, with the charge
transfer occurring over a longer range: the HOMO is amide-based, with minimal contribution from
the metal’s d-orbitals, and the LUMO is primarily localized on the π* orbitals of the aryl-phosphine
ligand. Hence, the lowest-lying HOMO → LUMO transition is best described as a metal-perturbed
ligand-to-ligand charge transfer (LLCT).
32

Figure 1. 4. 3-coordinate Cu(I) complexes with commonly-employed ligands.
Osawa and coworkers reported related neutral, 3-coordinate Cu(I) complexes with bulky
bisphoshine chelating ligands and different halides as the anionic ligands.
50
In solutions of CH2Cl2,
these complexes exhibit remarkably-efficient luminescence, with ΦPL = 0.6, and τ = 6.5 μs for the
iodide analogue. The halide contributes in large part to the HOMO, as well as σ-orbitals
corresponding to P-Cu bonds. The LUMO on the other hand is comprised of the antibonding
orbitals on the arylphosphine. As such, the lowest energy HOMO → LUMO transition is termed
(X + σ) → π* charge transfer. Based on the halide’s contribution to the HOMO, it is understandable
then that the iodide complex shows the fastest kr (9.2 x 10
4
s
-1
compared to 8.8 x 10
4
s
-1
and 8.7 x
10
4
s
-1
for the chloride and bromide analogues respectively), as it benefits from a secondary heavy
atom effect. These complexes were successfully employed as emitters in OLEDs with high
external quantum efficiencies (EQE): 2.13% for the bromide complex, relative to a theoretical
maximum EQE = 25%. However, as was the case for Peters et al. OLED utilizing a dinuclear
emitter, the EQE curve is characterized by steep roll-off at high current densities – a result of
second-order quenching processes such as triplet-triplet annihilation (TTA) or triplet-polaron
annihilation (TPA).
51, 52
This observation in the case of Cu(I) emitters can be attributed to their
longer excited state lifetimes compared to Ir(III) and Pt(II)-based phosphors, their poor charge
33

trapping properties (namely hole trapping), and the large reorganization energies associated with
their MLCT transitions.
To illustrate a possible mode of excited state reorganization operant in 3-coordinate Cu(I)
complexes, we turn to a report by Barakat et. al
53
who modeled the optimized triplet excited state
structure in a trigonal planar Au(I) complex employing PR3 ligands. The described Y – T distortion
(Figure 1. 5) was also reported by Osawa et. al
50
for the Cu(I) analogue of the original Au(I)
complex. The authors note that significant MLCT character is needed in the excited state to induce
this type of Jahn-Teller distortion.

Figure 1. 5. Y to T distortion in the triplet excited state (MLCT in nature) of trigonal planar Cu(I)
complexes.
1.2.2.2. NHC’s and other carbenes
N-heterocyclic carbenes (NHC) have also expanded the library of stable, luminescent Cu(I)
complexes. Since the isolation of the first stable free NHC by Arduengo in 1991,
54
this class of
carbenes has developed a rich history as a remarkable ligand set in transition metal catalysis.
55
An
attractive feature of NHC-metal complexes is the strong metal-carbene bonds owing to the NHC’s
strong σ- donation as well as π-accepting properties.
56
Employed as ligands for Cu(I), NHC’s give
complexes with varying coordination geometries
57
and a wide scope of catalytic reactivity.
58

The luminescence of NHC-Cu(I) complexes was not examined until the late 2000’s/early 2010’s,
with a report from Tsubomura and coworkers
59
on luminescent dinuclear, cationic complexes, and
the work of Thompson et. al
60
that showcased a highly-efficient, 3-coordinate, neutral emitter
34

bearing a bulky NHC and an anionic bidentate ligand. A following report by the same group
showed the ability to use NHC’s as chromophoric ligands by tuning their electrophilicity through
benzannulation and aza-substitution (Figure 1. 4). In this series of 3-coordinate complexes, the
luminescence is tuned from blue to orange by employing more electron-accepting carbenes, which
affects a stabilization in the LUMO energy. The lowest-energy transition is indeed a metal/metal-
ligand to carbene charge transfer. Despite the efficient luminescence observed in the crystalline
form of these complexes, ΦPL is found to drops significantly in solution, likely owing to
reorganization processes described above (Figure 1. 5). Additionally, the non-radiative rates are
found to increase with decreasing emission energies: a consequence of the energy gap law.
More recent work by Thompson et. al focused on more electron rich
bis(amino)cyclopropenylidene carbene (BAC) as monodentate ligands for three-coordinate Cu(I)
complexes (Figure 1. 4).
61
Unlike the imidazole-based NHC’s, these complexes showed no
contribution to the LUMO from the vacant 2pz orbital on the carbene carbon, corroborating the
poor π-accepting ability of these carbenes. Crystal structures of the complexes showed multiple
conformers, including Y-shaped and T-shaped ones, indicating a low barrier for a Y-to-T distortion
in BAC-Cu(I) compounds.
Two-coordinate Cu(I) complexes: less is more
Motivated in part by the reduced excited state reorganization energies observed in three-coordinate
Cu(I) complexes relative to 4-coordinate ones, investigating Cu(I) emitters with lower
coordination numbers has attracted much attention recently. A report by Thompson, Whittlesey,
and coworkers showed a series of linear, homo- and heteroleptic Cu(I) complexes bearing
dialkylamido carbenes (DACs) as the strongly π-accepting moiety.
62
The symmetric, cationic
[DAC2-Cu]
+
complex showed remarkable photophysics in both rigid and fluid environments. The
35

efficient photoluminescence displayed by the microcrystalline powders is only slightly reduced in
a CH2Cl2 solution: ΦPL = 0.85 in the former matrix and 0.65 in the latter. This feature is likely due
to the effective suppression of excited state reorganization, owing to the high steric demands
imposed by the carbene groups. Additionally, the authors note the high ΦPL observed in a solution
sparged with O2 and attribute that unusual behavior to the steric encumbrance around the metal
center, which protects against O2 diffusion and the ensuing Dexter energy transfer, as well as to
the high oxidation potential of the cationic complex. Gernert et. al
63
explored a similar linear,
cationic, and symmetric Cu(I) complex employing a cyclic alkylamino carbene (CAAC) (Figure
1. 6). The efficient phosphor (ΦPL = 0.65; τ = 10.6 μs in the solid state) was employed in a proof-
of-concept, solution processed OLED.

Figure 1. 6. Luminescent, 2-coordinate, linear Cu(I) complexes.
Since their initial discovery by Bertrand and coworkers in 2005,
64, 65
CAACs have witnessed an
explosion in their application as potent ligands for transition catalysis.
66
Their unique properties
(explained in more detail in chapter 2) can be attributed to their strong σ-donating properties
67
as
well as their increased electrophilicity relative to NHC’s, which makes them even better π-
acceptors when paired with transition metals.
68
In Cu(I) complexes, CAAC’s have enabled the
isolation of stable, 2-coordinate mono- and binuclear complexes, cationic and neutral. Stable
neutral Cu(I) complexes include halides, alkoxides, and acetylides as the anionic ligand. However,
as with NHC’s, their luminescent properties were only recently explored. 2-coordinate CAAC-Cu-
36

halides were found to exhibit efficient phosphorescence in the solid state,
48, 69
with ΦPL up to 0.96
and τ = 23 μs. In solution, however, luminescence was largely quenched by exciplex and excimer
formation in coordinating and non-coordinating solvents respectively.
48
Calculations reveal the
lowest-lying singlet and triplet excited states to be metal + halide to CAAC charge transfer
(XMLCT) in nature., and the triplet excited state geometry to exhibit a small bending along the
carbene-Cu-halide axis (Figure 1. 7).
48
Neutral, 2-coordinate CAAC-Cu(I) complexes (as well as
their Au(I) analogues) bearing sulfides, amides, and alkoxides have also shown moderate to high
ΦPL in their microcrystalline powder form, but their solution photophysics was not reported.
70

More recently, Di, Romanov and coworkers reported a series of CAAC-Au(I)-amides, and one
Cu(I) analogue, employed in OLEDs with remarkably high EQE’s and low roll-off at high
brightness.
71
While the molecular photophysical properties were not examined in detail, and the
proposed mechanisms for luminescence remain hotly-contested,
72, 73
this work highlighted the
remarkable potential of this 2-coordinate, linear, donor-metal-acceptor design motif.

Figure 1. 7. Excited state reorganization that is possible in linear, 2-coodinate Cu(I) complexes.

TADF
As mentioned earlier in this chapter, an expanding amount of work on Cu(I) luminescence has
focused on complexes that emit via TADF.
46, 74
This process enhances k ISC through bringing S1
and T1 into near-resonance, thereby achieving a small energy separation ∆𝐸 𝑆 1
−𝑇 1
and facilitating
rapid thermal equilibration between the two manifolds. Optimizing TADF is especially important
37

in organic emitters that lack a heavy atom, and consequently exhibit weak SOC that makes for
poor phosphorescence.
Delayed fluorescence was first observed in eosin by Boudin,
75
examined in fluorescein by Lewis
and co-workers who identified the thermal component of the delayed fluorescence,
76
and studied
methodically by Parker and Hatchard,
77
who termed the process E-Type delayed fluorescence (E
for eosin). However, it was not until recently that interest in delayed fluorescence has surged,
78

motivated in large part by the application of such emitters in OLEDs, and the quest for alternatives
rare-earth metal based luminophores that utilize triplet excitons formed by electroluminescence
79-
81
(Figure 1. 8). As such, to offer a comprehensive explanation of TADF, one must differentiate
between the processes and design principles that underlie it in purely organic systems versus
organo-copper ones.

Figure 1. 8. Published reports relating to TADF over a 10-year period, per SciFinder. Plot copied
from ref 77.
38

In systems optimized for TADF, S1, populated by photoexcitation to the singlet manifold and rapid
internal conversion, can decay radiatively to S0 through prompt fluorescence or non-radiatively to
T1 through enhanced intersystem crossing. This last process is favored by minimizing the energy
separation between the two states, ∆𝐸 𝑆 1
−𝑇 1
. The small delta also promotes thermal up-ISC (UISC,
also frequently referred to as reverse ISC: rISC) back to S 1, which can then decay radiatively via
delayed fluorescence (Figure 1. 9). Bringing S1 and T1 into resonance is typically accomplished
by the spatial separation of the wavefunctions that comprise the transitions contributing to both
states. This often entails the partitioning of the HOMO and the LUMO surfaces on separate parts
of the molecule in an orthogonal conformation that ensures minimal orbital overlap. This is
frequently accomplished by a donor-acceptor (D-A) motif with the lowest energy transitions being
charge transfer (HOMO → LUMO) in character. A small ∆𝐸 𝑆 1
−𝑇 1
(typically < 1000 cm
-1
) signifies
minimized exchange energy K, which is also known as the Fermi correlation energy. This
parameter represents the energy with which T1 is stabilized and S1 is destabilized, owing to the
way a pair of electrons in the former state are better correlated than in the latter, and as such
experience less electronic repulsion.
39

Figure 1. 9. Jablonski diagram depicting the different states and processes involved in TADF.
Like most processes involving a thermal equilibrium, TADF represents a competition of decay
rates of the excited states involved – the population of which is governed by a Boltzmann
distribution. The fastest radiative process is prompt fluorescence (pf) from S1 → S0, with kpf ~ 10
7

– 10
9
s
-1
. In organic systems that undergo TADF, improved kISC (up to 10
7
s
-1
)
82
is still too slow to
effectively quench S1; as a result, prompt fluorescence is always observed, and bi/multiexponential
PL decays are recorded at all temperatures, corresponding to kpf, kTADF, and kphos. This is in stark
contrast with Cu-based TADF emitters, where strong SOC affects fast k ISC (~ 10
12
s
-1
)
25, 83
that
quantitively depopulates S1. Consequently, prompt fluorescence is not observed in Cu(I) TADF
complexes, and PL decays are monoexponential at all temperatures. Another contrasting element
between organic and organocopper TADF is the nature of T1 itself. In organic systems, direct
phosphorescence out of T1 is weak and slow, and as such outpaced by kUISC. However, in Cu-based
TADF emitters, T1 is highly-luminescent with radiative rates that dominate the thermally-driven
kUISC at low temperatures.
83
As such, a hallmark of organocopper TADF is an increase in the PL
decay lifetime and a red-shift in the emission profile upon cooling, both corresponding to T1-based
phosphorescence in the low temperature regime. When plotted against temperature, the increase
40

in excited state lifetimes can be fitted to a modified Boltzmann equation 𝜏 𝑇𝐴𝐷𝐹 =

3 + exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
3 𝑘 𝑇 1
+ 𝑘 𝑆 1
exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
(Equation 1. 2), which gives the magnitude of the energy separation
∆𝐸 𝑆 1
−𝑇 1
as well as 𝑘 𝑇 1
and 𝑘 𝑆 1
, the decay rate constants of phosphorescence (kphos) and prompt
fluorescence (kpf) respectively.
𝜏 𝑇𝐴𝐷𝐹 =
3 + exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
3 𝑘 𝑇 1
+ 𝑘 𝑆 1
exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
(Equation 1. 2)
Two conundrums arise as a result of the discussion above: one has to do with the strength of the
SOC interaction between S1 and T1, and the second pertains to the oscillator strength of the S1 →
S0 transition. Both issues are addressed in the following section. First, by structuring D-A type
molecules in a way that optimizes TADF, the lowest-lying states, charge-transfer type S1 and T1
(
1
CT and
3
CT) end up having largely the same orbital parentage. As discussed in the first section
of this chapter, the one-electron spin orbit operator operating on two states that stem from identical
transition orbitals has a matrix element of zero. As such,
1
CT and
3
CT can only be effectively
coupled if they differ even slightly in their orbital parentage. This discrepancy can be introduced
through second-order, nonadiabatic coupling with closely-lying states of the same multiplicity,
namely the lowest donor- or acceptor-localized triplet state,
3
LE. In this regard,
3
CT can couple
with
3
LE through configuration interaction (CI)
7
or through vibronic coupling,
84, 85
resulting in
effective SOC with
1
CT, which enhances kISC and kUISC up to four-fold.
84
Indeed, most efficient
organic TADF emitters rely on second-order perturbations operating within their excited state
manifolds.
The second conundrum presented by TADF has to do with the requirement of minimal energy
separation, delta, and the way this is accomplished by ensuring minimal orbital overlap. An
41

unintended result of the increased spatial separation of HOMO and LUMO is the reduced
allowedness of transitions involving these orbitals. This is reflected in a small extinction
coefficient of the HOMO → LUMO charge transfer absorption band in and a reduced radiative
rate of the ensuing S1 → S0 emission. Nevertheless, as McGlynn, Azumi, and Kinoshita note, this
last transition is not the rate limiting event,
8
and as such optimizing the rates of ISC and UISC
takes precedence in ensuring efficient TADF. In this sense, Cu(I)-based TADF emitters
outperform their purely organic counterparts, as they benefit from stronger SOC which aids in the
aforementioned spin-forbidden transitions.
Just as most commonly-studied Cu(I) phosphors are tetrahedral and four-coordinate around the
metal, the majority of Cu(I)-based TADF emitters has the same geometry.
46
Typically in such
complexes, the HOMO is Cu(I)-based and the LUMO comprises the π* orbitals on the electron-
deficient ligand, resulting in effective HOMO/LUMO separation and extensive CT character of
the lowest energy excited states (namely MLCT). Nevertheless, as mentioned in earlier sections
of this chapter, the tetrahedral geometry and high Cu d-orbital contribution to the HOMO are both
associated with large reorganization energies detrimental to efficient luminescence. With two-
coordinate Cu(I) complexes coming into prominence recently and offering reduced non-radiative
rates and improved phosphorescence rates (including the fastest Cu(I) phosphor to date
63
), we
devote chapter 4 of this thesis to exploring complexes having this simple linear geometry and
optimized for TADF. The two ligands are redox active, donor/acceptor in character, and the
metal’s d-orbitals act as an efficient electronic conduit, with reduced contribution to the electron
configurations to the lowest CT transitions. Chapter 5 explores the role of metal as bridge by
systematically investigating analogous Ag(I) and Au(I) complexes. Functioning OLED devices
42

employing these complexes as emitters are discussed. Ultimately, this work outlines design rules
for obtaining Ir-like luminescence from Cu(I) complexes with simple geometries.

1.3. References

1. Hoffmann, N., Photochemical Reactions as Key Steps in Organic Synthesis. Chemical
Reviews 2008, 108 (3), 1052-1103.
2. Keefe, M. H.; Benkstein, K. D.; Hupp, J. T., Luminescent sensor molecules based on
coordinated metals: a review of recent developments. Coordination Chemistry Reviews 2000,
205 (1), 201-228.
3. Lo, K. K.-W.; Louie, M.-W.; Zhang, K. Y., Design of luminescent iridium(III) and
rhenium(I) polypyridine complexes as in vitro and in vivo ion, molecular and biological probes.
Coordination Chemistry Reviews 2010, 254 (21), 2603-2622.
4. Grätzel, M., Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells. Inorganic
Chemistry 2005, 44 (20), 6841-6851.
5. Kuciauskas, D.; Freund, M. S.; Gray, H. B.; Winkler, J. R.; Lewis, N. S., Electron
Transfer Dynamics in Nanocrystalline Titanium Dioxide Solar Cells Sensitized with Ruthenium
or Osmium Polypyridyl Complexes. The Journal of Physical Chemistry B 2001, 105 (2), 392-
403.
6. Kalyanasundaram, K., Photophysics, photochemistry and solar energy conversion with
tris(bipyridyl)ruthenium(II) and its analogues. Coordination Chemistry Reviews 1982, 46, 159-
244.
7. Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T., The triplet state of
organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient
OLEDs. Coordination Chemistry Reviews 2011, 255 (21-22), 2622-2652.
8. McGlynn, S. P.; Azumi, T.; Kinoshita, M., Molecular Spectroscopy of the Triplet State.
Prentice-Hall, Inc.: Englewood Cliffs, N. J., 1969.
9. McGlynn, S. P.; Daigre, J.; Smith, F. J., EXTERNAL HEAVY-ATOM SPIN-ORBITAL
COUPLING EFFECT .4. INTERSYSTEM CROSSING. Journal of Chemical Physics 1963, 39
(3), 675-&.
10. Hamze, R.; Djurovich, P. I.; Thompson, M. E., Organic Light Emitting Devices. In The
WSPC Reference on Organic Electronics: Organic Semiconductors, Bredas, J.-L.; Marder, S. R.,
Eds. World Scientific Publishing Co. Pte. Ltd.: Singapore, 2016; Vol. 2.
11. El‐Sayed, M. A., Spin—Orbit Coupling and the Radiationless Processes in Nitrogen
Heterocyclics. The Journal of Chemical Physics 1963, 38 (12), 2834-2838.
12. Baková, R.; Chergui, M.; Daniel, C.; Vlček, A.; Záliš, S., Relativistic effects in
spectroscopy and photophysics of heavy-metal complexes illustrated by spin–orbit calculations
of [Re(imidazole)(CO)3(phen)]+. Coordination Chemistry Reviews 2011, 255 (7), 975-989.
43

13. Szabo, A.; Ostlund, N. S., Modern Quantum Chemistry: Introduction to Advanced
Electronic Structure Theory McGraw-Hill: New York, 1989.
14. Jablonski, A., Efficiency of Anti-Stokes Fluorescence in Dyes. Nature 1933, 131, 839.
15. Lewis, G. N.; Kasha, M., Phosphorescence and the Triplet State. Journal of the American
Chemical Society 1944, 66 (12), 2100-2116.
16. Lewis, G. N.; Kasha, M., Phosphorescence in Fluid Media and the Reverse Process of
Singlet-Triplet Absorption. Journal of the American Chemical Society 1945, 67 (6), 994-1003.
17. Kasha, M. Characterization of electronic transitions in complex molecules Discussions of
the Faraday Society [Online], 1950, p. 14-19.
18. Tao, Y.; Chen, R.; Li, H.; Yuan, J.; Wan, Y.; Jiang, H.; Chen, C.; Si, Y.; Zheng, C.;
Yang, B.; Xing, G.; Huang, W., Resonance-Activated Spin-Flipping for Efficient Organic
Ultralong Room-Temperature Phosphorescence. Advanced Materials 2018, 0 (0), 1803856.
19. Baba, M., Intersystem Crossing in the 1nπ* and 1ππ* States. The Journal of Physical
Chemistry A 2011, 115 (34), 9514-9519.
20. Marian, C. M., Spin–orbit coupling and intersystem crossing in molecules. Wiley
Interdisciplinary Reviews: Computational Molecular Science 2011, 2 (2), 187-203.
21. Kleinschmidt, M.; van Wüllen, C.; Marian, C. M., Intersystem-crossing and
phosphorescence rates in fac-IrIII(ppy)3: A theoretical study involving multi-reference
configuration interaction wavefunctions. The Journal of Chemical Physics 2015, 142 (9),
094301.
22. Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A., Photochemistry and Photophysics
of Coordination Compounds: Copper. In Photochemistry and Photophysics of Coordination
Compounds I, Balzani, V.; Campagna, S., Eds. Springer Berlin Heidelberg: Berlin, Heidelberg,
2007; pp 69-115.
23. Ford, P. C.; Cariati, E.; Bourassa, J., Photoluminescence properties of multinuclear
copper(I) compounds. Chemical Reviews 1999, 99 (12), 3625-3647.
24. McMillin, D. R.; McNett, K. M., Photoprocesses of Copper Complexes That Bind to
DNA. Chemical Reviews 1998, 98 (3), 1201-1220.
25. Bergmann, L.; Hedley, G. J.; Baumann, T.; Bräse, S.; Samuel, I. D. W., Direct
observation of intersystem crossing in a thermally activated delayed fluorescence copper
complex in the solid state. Science Advances 2016, 2 (1).
26. Buckner, M. T.; McMillin, D. R., Photoluminescence from copper(I) complexes with
low-lying metal-to-ligand charge transfer excited states. Journal of the Chemical Society,
Chemical Communications 1978, (17), 759-761.
27. Blaskie, M. W.; McMillin, D. R., Photostudies of copper(I) systems. 6. Room-
temperature emission and quenching studies of bis(2,9-dimethyl-1,10-phenanthroline)copper(I).
Inorganic Chemistry 1980, 19 (11), 3519-3522.
28. Everly, R. M.; Ziessel, R.; Suffert, J.; McMillin, D. R., Steric influences on the
photoluminescence from copper(I) phenanthrolines in rigid media. Inorganic Chemistry 1991, 30
(3), 559-561.
29. Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K., Structure-Dependent
Photophysical Properties of Singlet and Triplet Metal-to-Ligand Charge Transfer States in
Copper(I) Bis(diimine) Compounds. Inorganic Chemistry 2003, 42 (20), 6366-6378.
30. Everly, R. M.; McMillin, D. R., CONCENTRATION-DEPENDENT LIFETIMES OF
Cu(NN)+2 SYSTEMS: EXCIPLEX QUENCHING FROM THE ION PAIR STATE.
Photochemistry and Photobiology 1989, 50 (6), 711-716.
44

31. Stacy, E. M.; McMillin, D. R., Inorganic exciplexes revealed by temperature-dependent
quenching studies. Inorganic Chemistry 1990, 29 (3), 393-396.
32. Crane, D. R.; DiBenedetto, J.; Palmer, C. E. A.; McMillin, D. R.; Ford, P. C., Pressure
effects on copper(I) complex excited-state dynamics. Evidence supporting an associative
nonradiative deactivation mechanism. Inorganic Chemistry 1988, 27 (21), 3698-3700.
33. Eggleston, M. K.; McMillin, D. R.; Koenig, K. S.; Pallenberg, A. J., Steric Effects in the
Ground and Excited States of Cu(NN)2+ Systems. Inorganic Chemistry 1997, 36 (2), 172-176.
34. Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J.-P.; Kirchhoff, J. R.; McMillin,
D. R., Bis(2,9-diphenyl-1,10-phenanthroline)copper(I): a copper complex with a long-lived
charge-transfer excited state. Journal of the Chemical Society, Chemical Communications 1983,
(9), 513-515.
35. Czerwieniec, R.; Yu, J.; Yersin, H., Blue-Light Emission of Cu(I) Complexes and
Singlet Harvesting. Inorganic Chemistry 2011, 50 (17), 8293-8301.
36. Miller, A. J. M.; Dempsey, J. L.; Peters, J. C., Long-Lived and Efficient Emission from
Mononuclear Amidophosphine Complexes of Copper. Inorganic Chemistry 2007, 46 (18), 7244-
7246.
37. Smith, C. S.; Branham, C. W.; Marquardt, B. J.; Mann, K. R., Oxygen Gas Sensing by
Luminescence Quenching in Crystals of Cu(xantphos)(phen)+ Complexes. Journal of the
American Chemical Society 2010, 132 (40), 14079-14085.
38. Costa, R. D.; Tordera, D.; Ortí, E.; Bolink, H. J.; Schönle, J.; Graber, S.; Housecroft,
C. E.; Constable, E. C.; Zampese, J. A., Copper(i) complexes for sustainable light-emitting
electrochemical cells. Journal of Materials Chemistry 2011, 21 (40), 16108-16118.
39. Elie, M.; Sguerra, F.; Di Meo, F.; Weber, M. D.; Marion, R.; Grimault, A.; Lohier, J.-
F.; Stallivieri, A.; Brosseau, A.; Pansu, R. B.; Renaud, J.-L.; Linares, M.; Hamel, M.; Costa,
R. D.; Gaillard, S., Designing NHC–Copper(I) Dipyridylamine Complexes for Blue Light-
Emitting Electrochemical Cells. ACS Applied Materials & Interfaces 2016, 8 (23), 14678-14691.
40. Igawa, S.; Hashimoto, M.; Kawata, I.; Yashima, M.; Hoshino, M.; Osawa, M., Highly
efficient green organic light-emitting diodes containing luminescent tetrahedral copper(i)
complexes. Journal of Materials Chemistry C 2013, 1 (3), 542-551.
41. Dumur, F., Recent advances in organic light-emitting devices comprising copper
complexes: A realistic approach for low-cost and highly emissive devices? Organic Electronics
2015, 21, 27-39.
42. Blasse, G.; Breddels, P. A.; McMillin, D. R., The luminescence of solid bis(2,9-
diphenyl-1,10-phenanthroline) copper(I). Chemical Physics Letters 1984, 109 (1), 24-26.
43. Cuttell, D. G.; Kuang, S.-M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A., Simple
Cu(I) Complexes with Unprecedented Excited-State Lifetimes. Journal of the American
Chemical Society 2002, 124 (1), 6-7.
44. Buckner, M. T.; Matthews, T. G.; Lytle, F. E.; McMillin, D. R., Simultaneous emissions
including intraligand emission and charge-transfer emission from
[bis(triphenylphosphine)(phenanthroline)copper](1+). Journal of the American Chemical Society
1979, 101 (19), 5846-5848.
45. Czerwieniec, R.; Yersin, H., Diversity of Copper(I) Complexes Showing Thermally
Activated Delayed Fluorescence: Basic Photophysical Analysis. Inorganic Chemistry 2015, 54
(9), 4322-4327.
45

46. Yersin, H.; Czerwieniec, R.; Shafikov Marsel, Z.; Suleymanova Alfiya, F., TADF
Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI
Complexes. ChemPhysChem 2017, 18 (24), 3508-3535.
47. Krylova, V. A.; Djurovich, P. I.; Conley, B. L.; Haiges, R.; Whited, M. T.; Williams,
T. J.; Thompson, M. E., Control of emission colour with N-heterocyclic carbene (NHC) ligands
in phosphorescent three-coordinate Cu(i) complexes. Chemical Communications 2014, 50 (54),
7176-7179.
48. Hamze, R.; Jazzar, R.; Soleilhavoup, M.; Djurovich, P. I.; Bertrand, G.; Thompson ,
M. E. Phosphorescent 2-, 3- and 4-coordinate cyclic (alkyl)(amino)carbene (CAAC) Cu(I)
complexes Chemical Communications [Online], 2017, p. 9008-9011.
49. Lotito, K. J.; Peters, J. C., Efficient luminescence from easily prepared three-coordinate
copper(i) arylamidophosphines. Chemical Communications 2010, 46 (21), 3690-3692.
50. Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M., Highly
Efficient Green Organic Light-Emitting Diodes Containing Luminescent Three-Coordinate
Copper(I) Complexes. Journal of the American Chemical Society 2011, 133 (27), 10348-10351.
51. 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.
52. Kondakov, D. Y.; Pawlik, T. D.; Hatwar, T. K.; Spindler, J. P., Triplet annihilation
exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes.
Journal of Applied Physics 2009, 106 (12), 124510.
53. Barakat, K. A.; Cundari, T. R.; Omary, M. A., Jahn-Teller distortion in the
phosphorescent excited state of three-coordinate Au(I) phosphine complexes. Journal of the
American Chemical Society 2003, 125 (47), 14228-14229.
54. Arduengo, A. J.; Harlow, R. L.; Kline, M., A stable crystalline carbene. Journal of the
American Chemical Society 1991, 113 (1), 361-363.
55. Díez-González, S.; Marion, N.; Nolan, S. P., N-Heterocyclic Carbenes in Late Transition
Metal Catalysis. Chemical Reviews 2009, 109 (8), 3612-3676.
56. Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L., Understanding the
M(NHC) (NHC=N-heterocyclic carbene) bond. Coordination Chemistry Reviews 2009, 253 (5),
687-703.
57. Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J.
B., Coinage Metal−N-Heterocyclic Carbene Complexes. Chemical Reviews 2009, 109 (8), 3561-
3598.
58. Díez-González, S.; Nolan, S. P., N-Heterocyclic Carbene-Copper(I) Complexes in
Homogeneous Catalysis. Synlett 2007, 2007 (14), 2158-2167.
59. Matsumoto, K.; Matsumoto, N.; Ishii, A.; Tsukuda, T.; Hasegawa, M.; Tsubomura, T.,
Structural and spectroscopic properties of a copper(I)-bis(N-heterocyclic)carbene complex.
Dalton Transactions 2009, (34), 6795-6801.
60. Krylova, V. A.; Djurovich, P. I.; Whited, M. T.; Thompson, M. E., Synthesis and
characterization of phosphorescent three-coordinate Cu(i)-NHC complexes. Chemical
Communications 2010, 46 (36), 6696-6698.
61. Shi, S.; Djurovich, P. I.; Thompson, M. E., Synthesis and characterization of
phosphorescent three-coordinate copper(I) complexes bearing bis(amino)cyclopropenylidene
carbene (BAC). Inorganica Chimica Acta 2018, 482, 246-251.
46

62. Shi, S.; Collins, L. R.; Mahon, M. F.; Djurovich, P. I.; Thompson, M. E.; Whittlesey,
M. K., Synthesis and characterization of phosphorescent two-coordinate copper(i) complexes
bearing diamidocarbene ligands. Dalton Transactions 2017, 46 (3), 745-752.
63. Gernert, M.; Müller, U.; Haehnel, M.; Pflaum, J.; Steffen, A., A Cyclic
Alkyl(amino)carbene as Two-Atom π-Chromophore Leading to the First Phosphorescent Linear
CuI Complexes. Chemistry A European Journal 2017, 23 (9), 2206-2216.
64. Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.; Bertrand, G., Stable cyclic (alkyl)
(amino) carbenes as rigid or flexible, bulky, electron-rich ligands for transition-metal catalysts: A
quaternary carbon atom makes the difference. Angewandte Chemie-International Edition 2005,
44 (35), 5705-5709.
65. Lavallo, V.; Canac, Y.; DeHope, A.; Donnadieu, B.; Bertrand, G., A Rigid Cyclic
(Alkyl)(amino)carbene Ligand Leads to Isolation of Low-Coordinate Transition-Metal
Complexes. Angewandte Chemie International Edition 2005, 44 (44), 7236-7239.
66. Melaimi, M.; Jazzar, R.; Soleilhavoup, M.; Bertrand, G., Cyclic (Alkyl)(amino)carbenes
(CAACs): Recent Developments. Angewandte Chemie International Edition 2017, 56 (34),
10046-10068.
67. Rao, B.; Tang, H. R.; Zeng, X. M.; Liu, L.; Melaimi, M.; Bertrand, G., Cyclic
(Amino)(aryl)carbenes (CAArCs) as Strong sigma-Donating and pi-Accepting Ligands for
Transition Metals. Angewandte Chemie-International Edition 2015, 54 (49), 14915-14919.
68. Back, O.; Henry-Ellinger, M.; Martin, C. D.; Martin, D.; Bertrand, G., (PNMR)-P-31
Chemical Shifts of Carbene-Phosphinidene Adducts as an Indicator of the pi-Accepting
Properties of Carbenes. Angewandte Chemie-International Edition 2013, 52 (10), 2939-2943.
69. Romanov, A. S.; Di, D.; Yang, L.; Fernandez-Cestau, J.; Becker, C. R.; James, C. E.;
Zhu, B.; Linnolahti, M.; Credgington, D.; Bochmann, M., Highly photoluminescent copper
carbene complexes based on prompt rather than delayed fluorescence. Chemical
Communications 2016, 52 (38), 6379-6382.
70. Romanov Alexander, S.; Becker Ciaran, R.; James Charlotte, 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.
71. Di, D. W.; 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.
72. 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.
73. Taffet, E. J.; Olivier, Y.; Lam, F.; Beljonne, D.; Scholes, G. D., Carbene–Metal–Amide
Bond Deformation, Rather Than Ligand Rotation, Drives Delayed Fluorescence. The Journal of
Physical Chemistry Letters 2018, 9 (7), 1620-1626.
74. 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.
75. Boudin, S., Phosphorescence des solutions glycériques d'éosine influence des iodures. J.
Chim. Phys. 1930, 27, 285-290.
47

76. Lewis, G. N.; Lipkin, D.; Magel, T. T., Reversible Photochemical Processes in Rigid
Media. A Study of the Phosphorescent State. Journal of the American Chemical Society 1941, 63
(11), 3005-3018.
77. Parker, C. A.; Hatchard, C. G., Triplet-singlet emission in fluid solutions.
Phosphorescence of eosin. Transactions of the Faraday Society 1961, 57 (0), 1894-1904.
78. 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.
79. 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.
80. Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C., Design of
Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light
Emitting Diodes. Journal of the American Chemical Society 2012, 134 (36), 14706-14709.
81. Nakanotani, H.; Higuchi, T.; Furukawa, T.; Masui, K.; Morimoto, K.; Numata, M.;
Tanaka, H.; Sagara, Y.; Yasuda, T.; Adachi, C., High-efficiency organic light-emitting diodes
with fluorescent emitters. Nature Communications 2014, 5, 4016.
82. Fernando, B. D. a. T. J. P. a. A. P. M., Photophysics of thermally activated delayed
fluorescence molecules. Methods and Applications in Fluorescence 2017, 5 (1), 012001.
83. Hofbeck, T.; Monkowius, U.; Yersin, H., Highly Efficient Luminescence of Cu(I)
Compounds: Thermally Activated Delayed Fluorescence Combined with Short-Lived
Phosphorescence. Journal of the American Chemical Society 2015, 137 (1), 399-404.
84. Gibson, J.; Monkman, A. P.; Penfold, T. J., The Importance of Vibronic Coupling for
Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence
Molecules. ChemPhysChem 2016, 17 (19), 2956-2961.
85. Etherington, M. K.; Gibson, J.; Higginbotham, H. F.; Penfold, T. J.; Monkman, A. P.,
Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence.
Nature Communications 2016, 7, 13680.

48

Chapter 2. Cyclic alkyl amino carbenes (CAACs) as an efficient ligand system
for red-shifting the luminescence of 2-, 3-, and 4-coordinate Cu(I)-carbene
complexes

2.1 Introduction
Developing paradigms for tuning the chemical and electronic properties of Cu(I)-based
luminophores is of increasing importance. The commonly-used set of ligands has been rather
limited, comprising traditionally of bidentate, chelating N^N,
1
P^P,
2, 3
N^P,
4
and mixtures of both
N^N and P^P type ligands,
5
as well as different halides.
6-8
Color tuning in these systems is typically
achieved by ligand modification such as incorporating electron-donating or withdrawing groups
and aza-substitutions onto the ligands supporting the frontier orbitals
9-12
as well as increasing the
size of the chromophoric ligand.
8, 13
Yellow to red emitters obtained by the aforementioned
electronic modulations generally exhibited poorer photoluminescence (PL) efficiencies than their
higher-energy counterparts, due to, in large part, the energy gap law.
12-14
More recently,
N-heterocyclic carbenes (NHC’s) have emerged as a class of chromophoric ligands for Cu(I)
complexes
15-17
with desirable properties such as strong metal-carbon bonds and amenability to
color-tuning through chemical modification of the carbene itself.
16
Here too, ligand modifications
such as benzannulation and aza-substituion of imidazole-based NHC’s were used to shift the
emission energies to lower wavelengths.
16
These modifications aimed at stabilizing the
chromophoric, carbene-based LUMO, leading to increased contribution from the vacant 2pz orbital
on the carbene carbon. However, the larger, NHC-based chromophores also yielded phosphors
with reduced photoluminescent (PL) efficiencies and increased non-radiative decay rates.
16
With
this knowledge, we turned our attention to cyclic alkyl amino carbenes (CAACs) as an alternate
class of highly electrophilic carbenes,
18
known for their superior π-accepting and σ-donating
properties compared to NHC’s (Figure 2. 1).
19

49

Figure 2. 1. Electronic properties of NHC's (left, center) and CAAC’s (right). In NHC’s, the two nitrogens
exert a push-pull effect, where one nitrogen donates mesomerically into the vacant 2p z orbital of the C carbene,
and the other nitrogen is electron-withdrawing from the filled sp
2
orbital of C carbene. Contrastingly, in the
asymmetric CAAC’s, only one nitrogen is mesomerically donating into the 2p z of C carbene, which makes the
orbital overall more π-electrophilic than in NHC’s. Additionally, the tertiary carbon is inductively donating
into the filled sp
2
orbital, resulting in CAACs being stronger σ-donors than their NHC counterparts.
In recent years, a number of reports have highlighted the chromophoric properties of the neutral,
2-coordinate CAAC-CuCl complexes.
20-23
In particular, two separate accounts from Romanov et.
al. reported unprecedented prompt fluorescence, with subnanosecond
20
and nanosecond
22

lifetimes, from CAAC-CuCl complexes with varying steric bulk in their substituents. However,
their results are not in agreement with those reported by Gernert et al, who measured typical
microsecond lifetimes for similar complexes.
21
In this chapter, we firstly reexamine the
photophysical properties of two of the previously-reported 2-coordinate CAAC-CuCl complexes,
1-Cl and 2-Cl, in different media. Secondly, we employ trispyrazolylborate, Tp, as a stronger
chelating ligand with higher hapticity than chloride. The resultant complexes, 1-Tp and 2-Tp,
have 4- and 3-coordinate geometry, respectively, around the metal center. To determine the extent
and the efficiency of the red-shift in emission afforded by the CAAC ligand, we also examine three
related NHC-CuTp complexes, 3-, 4- and 5-Tp.
50

2.2 Results and Discussion
2.1. Synthesis

Figure 2. 2. Synthetic scheme and molecular structures of the complexes in this chapter.
Complexes 1-Cl, 2-Cl, 4-Cl, and 5-Cl were synthesized following reported literature procedures.
16,
20, 24, 25
In the case of less sterically encumbered 1-Cl, we found that the asymmetric CAAC-CuCl
disproportionates into the asymmetric metathesis product,[(CAAC)
2
Cu]
+
,CuCl
2
−
, in a polar,
coordinating solvent such as tetrahydrofuran (THF) upon prolonged standing. This can also occur
during the reaction to prepare the CAAC-CuCl, and the desired product is then precipitated out
with the metathesis product as an impurity, which can cause errors in the photophysical
measurements. Slow recrystallization under an inert atmosphere allowed for successful separation
of the two complexes. Under inert atmosphere, simple addition of a stoichiometric amount of
potassium trispyrazolyl borate (KTp) to the carbene-CuCl precursors in THF yielded complexes
1-Tp – 5-Tp in high yields (90-100%; Figure 2. 2). The isolated white (3-Tp, 4-Tp) and yellow
51

to orange powders (1-Tp, 2-Tp, 5-Tp) have moderate stability under ambient atmosphere, whereas
in the solutions were less stable, showing decomposition over a few hours to a several days. 1-Tp
was also isolated from the cationic [(CAAC)
2
Cu]
+
,CuCl
2
−
following the same method, showing
that the anionic Tp ligand can displace the carbene.
2.2. XRay analysis
XRay diffraction analyses of the single crystals of 1-Cl obtained therein and of 2-Cl confirmed
the metal center as having 2-coordinate geometry and revealed bond lengths and angles similar to
previous reports. Attempts to grow crystals of 5-Cl were unsuccessful in our hands, and
consistently gave narrow needles not suited for single crystal XRay crystallography. The Tp
complexes 1-, 3-, and 5-Tp were found to be 4-coordinate in geometry around the Cu center, with
the Tp ligated in an η
3
fashion (Figure 2. 3). The exception to this motif is complex 2-Tp that
assumes 3-coordinate geometry (Tp is η
2
) in its crystalline matrix, likely due to the steric demands
of the adamantyl unit on the CAAC. In the The Ccarbene-Cu bond lengths are comparable across all
Tp complexes: 1.904 (1) Å, 1.886 (1) Å, 1.898 (4) Å, 1.893(6) Å for 1-Tp, 2-Tp, 3-Tp and 5-Tp
respectively -values comparable to those reported in CAAC-Cu complexes
21, 22
and our NHC-Cu
ones.
15, 16
In all complexes except 2-Tp, the tridentate ligand binds in its typical scorpionate
fashion, with one pyrazole (pz) ring chelating at a slightly longer Cu-Npz bond length than the
other two: 2.151 (1) Å, 2.123 (1) Å, 2.138 (1) Å for 1-Tp; 2.145 (3) Å, 2.109 (3) Å and 2.093 (4)
Å for 3-Tp; 2.133 (5) Å, 2.061 (5) Å, 2.085 (5) Å for 5-Tp. In the bulkier 2-Tp, the two
coordinating pz rings have shorter Npz-Cu bond lengths than those in 1-Tp at 1.970 (1) Å and
2.053 (1) Å, whereas the deligated pz ring is situated at 3.655 (2) away from the Cu center.
52

Figure 2. 3. Crystal structures of complexes 1-Tp, 2-Tp, 3-Tp and 5-Tp showing the Oak Ridge Thermal
Ellipsoid Plots (ORTEPs). The isopropyl units on the 2,6-diisopropylphenyl (Dipp) moiety and the
hydrogen atoms were removed for clarity.

Despite the asymmetric binding nature of Tp in all complexes, and the asymmetry of the CAAC
ligand itself,
1
H NMR spectra of both complexes show only three sets of signals corresponding to
the three different types of protons on each of the pz rings. This signal pattern is characteristic of
C3 symmetry, indicating the equilibration of all three groups on the NMR time scale at room
temperature. An underlying mechanism for this behavior was proposed by Trofimenko in 1969.
26

In addition to the tumbling of the pz rings (which involves the dissociation of a pz group) the Tp
ligand itself was found to rotate around the boron-metal axis. 2-Tp, with its 3-coordinate geometry
in the crystalline form and its C3 symmetry in solution, is a good representative of this mechanism.
Variable temperature NMR experiments needed to understand the nature of the underlying
exchange processes are discussed in detail in the next chapter.
2.3. Photophysical characterization
Complete photophysical characterization was carried out on single crystals as well as fine powders
of both 1-Cl and 2-Cl, and the results were found to be identical. Safe for typical oxidation of the
metal center in the dissolved samples with prolonged exposure to air, no signs of decomposition
products due to photoexcitation were observed. The UV-Vis absorption spectra of both chloride
precursors obtained in dichloromethane (CH2Cl2) solutions (Figure 2. 4) show high energy bands
53

(at 273 nm for 1-Cl, 281 nm for 2-Cl), which correspond to spin-allowed π-π* transitions on the
pendant 2,6-diisopropylphenyl (Dipp) groups of the CAAC. Additionally, lower-lying bands at
328 nm and 352 nm, for 1-Cl and 2-Cl respectively are assigned to halide/metal-to-ligand charge
transfer (XMLCT) transitions. DFT and TDDFT calculations performed with a LACVP
**
basis set
and B3LYP as the functional are in agreement with the published literature,
20-22
and confirm the
XMLCT nature of the lowest triplet state T1.
250 300 350 400 450
0
2
4
6
8
10

 x 10
3
(M
-1
. cm
-1
)
Wavelength (nm)
1-Cl
2-Cl
CH
2
Cl
2
250 300 350 400 450
0.0
0.5
1.0

Scaled absorption (a. u.)
Wavelength (nm)
CH
2
Cl
2
2-MeTHF
MeCy
1-Cl
250 300 350 400 450
0.0
0.5
1.0 2-Cl
Scaled absorption (a. u.)
Wavelength (nm)
CH
2
Cl
2
2-MeTHF
MeCy

Figure 2. 4. Absorption spectra of precursors 1-Cl and 2-Cl showing their extinction coefficients in CH 2Cl 2
(left) and their observed negative solvatochromism.
1-Cl, 2-Cl, and 5-Cl are luminescent in their crystalline and powder form and exhibit broad and
featureless emission profiles indicative of the CT nature of their emissive state. 3-Cl has been
recently found to exhibit UV phosphorescence in its microcrystalline powder form and in
solution.
27
CAAC complexes 1-Cl and 2-Cl display highly-efficient blue luminescence with
photoluminescence quantum yield (PLQY, ΦPL) = 0.87 at λmax = 424 nm for the former, and ΦPL
= 0.96 at λmax = 436 nm for the latter. The excited state lifetimes of the 1-Cl and 2-Cl show
monoexponential decays of 24 μs and 29 μs respectively, indicating that the origin of luminescence
in both complexes is phosphorescence. This is in contrast to the report by Romanov et al, who
recorded lifetimes of 3.2 ns for 1-Cl
22
and 0.2 – 0.3 ns for 2-Cl.
20
The unusually fast signal
reported in the former study likely stems from the instrument response function (irf), which the
authors note to have a width of 0.2 ns, nearly identical to the reported lifetime of 2-Cl and its -Br
54

and -I analogues.
20
In their investigation of the luminescence of 1-Cl, the authors also recorded a
long-lived component with a decay lifetime of 25.7 μs
22
which they attribute to a
photodecomposition product. We were able to reproduce this decay lifetime (24 μs), for both fine
powders and crystalline forms of 1-Cl, before and after UV irradiation, which led us to conclude
that this is the true excited state lifetime of the phosphorescent complex. However, in agreement
with authors’ data, we observed that the solid state luminescence of 2-Cl appears to be independent
of temperature, with the lifetime and emission profiles virtually unchanged from room temperature
to 77K (τ = 28 μs and 29 µs; λmax = 432 and 436 nm at 77K and RT respectively, Figure 2. 5).
Interestingly, 1-Cl shows a pronounced red-shift in emission upon cooling (Δλmax = 52 nm).
However, since this is not accompanied by a significant increase in excited state lifetimes (24 μs
at RT and 32 μs at 77K), TADF is ruled unlikely, and the red-shift is attributed to changes in the
solid state packing upon cooling (Table 2. 1). The bright yellow luminescence of 5-Cl is also
phosphorescent in origin (ΦPL = 0.42; τ = 11.3 μs). Upon cooling, the broad and featureless spectral
line resolves slightly, likely revealing vibronic structure (Δν = 3095 cm
-1
). This is verified by the
decay lifetimes as well as excitation traces recorded at both peaks being nearly identical (11.3 μs
and 13.6 μs at 550 nm and 670 nm respectively).
400 500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
1-Cl, RT
1-Cl, 77K
2-Cl, RT
2-Cl, 77K
5-Cl, RT
5-Cl, 77K

Figure 2. 5. Emission spectra of microcrystalline powders of precursors 1-Cl, 2-Cl, and 5-Cl at room
temperature (solid lines) and 77K (dashed lines).
55

In solutions of 2-MeTHF, a good coordinating solvent, both 1-Cl and 2-Cl at room temperature
show orange phosphorescence that is independent of concentration (Figure 2. 6). As reported by
Romanov et. al
20
and our lab, this is likely due to exciplex formation. Such excited state
interactions are broken upon freezing the solution, and both complexes regain blue emission with
longer lifetimes, more consistent with their behavior in the solid state.
400 500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
SS, RT
SS, 77K
2-MeTHF, RT
2-MeTHF, 77K
1-Cl

400 500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
SS, RT
SS, 77K
2-MeTHF, RT
2-MeTHF, 77K
2-Cl

Figure 2. 6. Emission spectra of 1-Cl (left) and 2-Cl (right) in microcrystalline solid form (SS) and in
2-MeTHF solutions at RT and 77K.
Interestingly, solutions of 1-Cl and 2-Cl in non-coordinating MeCy exhibit concentration
dependent emission with two peaks: one blue peak, similar to the solid state (λmax = 442 nm for 1-
Cl and 476 nm for 2-Cl), and the other peak red shifted (λmax = 570 nm for 1-Cl and 614 nm for
2-Cl), an observation which is indicative of excimer or aggregate formation. This is verified upon
dilution, as the intensity of the low energy band is reduced relative to the peak at higher energy,
whereas the absorption spectra remain unchanged (Figure 2. 7). Furthermore, in 2-Cl the excited
state decays recorded for the band at low energy display nanosecond rise times that are displays a
rise time that is concentration dependent, unlike 1-Cl where the decay does not show a rise time.
Congruently, the low energy emission is less intense in 2-Cl than in 1-Cl, likely due to the steric
bulk in the former complex hindering aggregate formation. Orange-coloured, μ-bridging chloride
56

dimers of analogous diamidocarbene (DAC) copper-chloride complexes have been isolated by
Collins et al.
28
A similar type of dimer formed in the ground state for 1-Cl and excited state for
2-Cl may be responsible for the low energy emission observed in MeCy solution. In fact, excimer
formation in 2-Cl is diffusion limited in MeCy, with the formation rate constant 𝑘 𝑓 𝐸 = 7.4 x 10
9
s
-1

(Figure 2. 8). In an analogous manner to 1-Cl, CH2Cl2 solutions of 5-Cl are colorless and
luminesce deep blue, indicating that the yellow luminescence of the yellow powder likely stems
from a μ-bridged dimer that is broken apart upon dissolution.
300 400 500 600 700 800
0
1

Em, conc.
Em, dilute
Em, 77K
Wavelength (nm)
Absorbance (a. u.)
0
Abs, conc.
Abs dilute (x 5),
PL Intensity (a. u.)
1-Cl

300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
Em, conc.
Em, dilute
Em, 77K
Wavelength (nm)
Absorbance (a. u.)
0
1
0 2 4 6 8
0
1
475 nm
650 nm
Counts (a. u.)
Time (s)
Abs, conc.
Abs. dilute x 4
Normalized Intensity (a. u.)
2-Cl

Figure 2. 7. Absorption and emission spectra of 1-Cl and 2-Cl in MeCy showing concentration-dependent
low-energy emission bands. The inset shows the PL decay traces of both high-energy and low-energy bands.

0.0 2.0x10
-4
4.0x10
-4
6.0x10
-4
0.0
2.0x10
6
4.0x10
6
6.0x10
6
8.0x10
6
1.0x10
7
1.2x10
7
K
obs
= k
M
d
+ k
A
f
[1-Cl]

Linear fit, R
2
= 0.78
K
obs
(s
-1
)
[1-Cl] (M)
K
obs
= 9.5 x 10
5
+ 1.8 x 10
10
[1-Cl]

0.0 5.0x10
-5
1.0x10
-4
1.5x10
-4
2.0x10
-4
2.5x10
-4
1.2x10
6
1.6x10
6
2.0x10
6
2.4x10
6
2.8x10
6
K
obs
= k
M
d
+ k
E
f
[2-Cl]
K
obs
= 8.3 x 10
5
+ 7.4 x 10
9
[2-Cl]
Linear fit, R
2
= 0.94
K
obs
(s
-1
)
[2-Cl] (M)

Figure 2. 8. Modified Stern-Volmer plot of self-quenching kinetics for 1-Cl (left) and 2-Cl (right) in MeCy.
Due to poor solubility limits, the concentrations were determined using the extinction coefficients of the
high-energy absorption peak of the complex in CH 2Cl 2. For 1-Cl, the equation displayed in the figure
corresponds to a kinetic model where the monomeric species (M) forms an aggregate species (A), with a
57

rate constant of formation, k
f
A
= 1.8 x 10
10
s
-1
. The emission decay rate of the monomer at infinite dilution
has a rate constant of k
d
M
= 9.5 x 10
5
s
-1
, as given by the intercept of the linear fit.
Excitation spectra corresponding to emission at 650 nm of the more concentrated samples show a clear
peak at 528 nm. The PL decays at 650 nm for all the concentrations examined show no discernable rise
time. These observations, together with k
f
A
= 1.8
10
s
-1
, indicate the formation of an aggregate species of 1-
Cl in MeCy. For 2-Cl, the equation displayed in the figure corresponds to a kinetic model where the
monomeric species (M) forms an excimer (E), with a rate constant of formation, k
f
E
= 7.4 x 10
9
s
-1
. The
emission decay rate of the monomer at infinite dilution has a rate constant of k
d
M
= 8.3 x 10
5
s
-1
, as given
by the intercept of the linear fit.
Excitation spectra corresponding to emission at 650 nm are identical to those corresponding to emission at
430 nm. The PL decay at 650 nm shows a rise time that is concentration-dependent. These observations,
together with k
f
E
= 7.4 x 10
9
s
-1
, indicate a diffusion-limited excimer formation process for 2-Cl in MeCy.

Table 2. 1. Photophysical properties of 1-Cl and 2-Cl in MeCy and 2-MeTHF at different concentrations.
RT 77K
Complex Solution τ (μs) (nm)
*
Φ PL λ max
**

(nm)
Solution τ (μs) (nm)
*
λ max
(nm)
1-Cl MeCy;
conc.
• 0.52 (451 nm)
• 5.0 (20%);
18 (80%) (600 nm)
0.19 442;
570
MeCy;
conc.
32 (475 nm) 456
MeCy;
5x dilution
• 0.41 (451 nm)
• 8.1 (57%);
22.8 (43%) (600 nm)
0.17 MeCy;
5x dilution
30 (475 nm)
2-MeTHF;
conc.
4.2 (600 nm) 0.08 594 2-MeTHF;
conc.
33 (550 nm) 482
2-MeTHF;
10x dilution
4.6 (600 nm) 0.08 2-MeTHF;
10x
dilution
33 (550 nm)
2-Cl MeCy,
conc.
• 0.54 (475 nm)
• 0.42 (-29%);
1.34 (129%) (650
nm)
0.03 476;
614
MeCy;
conc.
32 (500 nm) 434
MeCy;
4x dilution
• 1.23 (475 nm)
• 0.93 (-124%);
1.33 (224%) (650
nm)
0.04 MeCy;
4x dilution
28 (450 nm)
2-MeTHF;
conc.
0.58 (600 nm) 0.015 610 2-MeTHF;
conc.
28 (475 nm) 426
2-MeTHF;
10x dilution
0.69 (600 nm) 0.016 2-MeTHF;
10x
dilution
28 (475 nm)
*
The lifetimes (μs) are reported with the wavelengths (nm) at which they were recorded.
**
In MeCy at RT, λ max values are reported for both, the high-energy peak as well as the low-energy one observed.
58

Looking to further red-shift the luminescence of the CAAC-Cu precursors, Tp was incorporated
for its stronger σ-donating properties than -Cl, and its ability to afford robust, stable complexes.
29,
30
The UV-Vis absorption spectra of both 1-Tp, 2-Tp, 3-Tp and 5-Tp in dichloromethane are
shown below. The absorption cut-offs for the CAAC- complexes are strongly red-shifted with
respect to their colorless chloride precursors: at 460 nm for 1-Tp and 480 nm for 2-Tp, compared
with 370 nm and 400 nm respectively (Figure 2. 9).
300 400 500 600
0
5
10
15
20

 x 10
4
(M
-1
. cm
-1
)
Wavelength (nm
1-Tp
2-Tp
3-Tp
5-Tp x 0.25

Figure 2. 9. Absorption spectra of complexes 1-Tp, 2-Tp, 3-Tp, and 5-Tp in CH 2Cl 2.
Upon photoexcitation, crystals of 1-Tp exhibits bright yellow luminescence (λmax = 550 nm), with
ΦPL = 0.46, and a phosphorescence decay τ = 12 μs. On the other hand, crystals of the adamantyl
decorated, 3-coordinate 2-Tp display weak orange phosphorescence (λmax = 616 nm) with a much
reduced PLQY = 0.02 (Figure 2. 10). Accordingly, its excited state is short-lived and exhibits
biexponential decay at 0.4 μs and 1.0 μs (at 40% and 60% contributions respectively), indicating
different quenching processes. However, upon cooling the sample down to 77K, the excited state
lifetime of the complex increases tenfold (τ = 4.2 μs; 14 μs at 75% and 25% contributions
respectively, Table 2. 1), concomitant with an increase in the luminescence intensity. This reflects
an incomplete suppression of the molecular distortions responsible for non-radiative quenching.
The dramatic variation in PLQY between the two CAAC-CuTp complexes highlights the impact
59

of the hapticity of the Tp ligand on the photophysics.
The luminescence of 1-Tp and 2-Tp is significantly red-shifted relative to the NHC-Cu complex
with a similar size carbene ring, 3-Tp. Microcrystalline powders of the latter display sky-blue
phosphorescence with ΦPL = 0.25 and τ = 29 μs. These properties are not altered much upon
cooling to 77 K, in contrast with the benzannulated NHC complexes 4-Tp and 5-Tp. Powders of
the latter two complexes also display a red-shift in their luminescence relative to 3-Tp, with 4-Tp
and 5-Tp giving green and orange luminescence respectively (ΦPL = 0.33 and 0.29 respectively).
Upon cooling to 77K, both complexes show a significant increase in their excited state lifetimes
(2-fold for 4-Tp and 3-fold for 5-Tp) as well as a red-shift in their emission profiles (12 nm for 4-
Tp and 28 nm for 5-Tp). Both observations are consistent with TADF operating in these
benzannulated NHC complexes, unlike what is observed for the CAAC analogues. Notably, the 4-
coordinate complex 2-Tp with its CAAC ligand exhibits a red-shift in emission relative to 3-Tp,
stronger than what is observed for the benzannulated NHC complexes 4-Tp and 5-Tp. Upon
moving the emission to lower energies in the NHC series 3-Tp to 5-Tp, we see a dramatic increase
in the non-radiative decay rates from 2.6 x 10
4
s
-1
to 1.4 x 10
5
s
-1
respectively (a five-fold increase)
– a feature of the energy gap law. This effect is less pronounced when comparing the CAAC-
bearing 1-Tp to 3-Tp: despite showing a red-shift in luminescence comparable to what is observed
upon benzannulation, the non-radiative decay rate of 1-Tp is only double that of 3-Tp. These
results highlight the CAAC’s chromophoric properties of tuning emission to lower energies while
mitigating non-radiative losses through the energy gap law.
60

400 500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
1-Tp, RT
1-Tp, 77K
2-Tp, RT
2-Tp, 77K
3-Tp, RT
3-Tp, 77K

400 500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
3-Tp, RT
3-Tp, 77K
4-Tp, RT
4-Tp, 77K
5-Tp, RT
5-Tp, 77K

Figure 2. 10. Emission spectra of microcrystalline powders of complexes 1-Tp - 5-Tp at RT (solid lines)
and 77K (dashed lines), showing the color tuning to lower emission energies via employing CAACs (left)
and across the NHC series via π extension and aza-substitution (right).

Table 2. 2. Photophysical properties of microcrystalline powders of all luminescent complexes in this
chapter.

emission at room temperature
b
emission at 77 K
b
λ max
(nm)
τ (μs) Φ PL
k r x 10
4

(s
-1
)
k nr x 10
4

(s
-1
)
λ max
(nm)
τ (μs)
1-Cl 424 22 0.86 3.9

0.64 496 ---
2-Cl 436 29 0.96 3.3 1.4 432 28
1-Tp 550 12 0.46 3.9 4.6 562
9.6 (0.2);
24 (0.8)
2-Tp 616
0.4 (0.4);
1.0 (0.6)
0.02 2.6
c
130
c
614
4.2 (0.75);
14 (0.25)
3-Tp 468 29 0.25 0.86 2.6 468 21
4-Tp 528
1.8 (0.2)
7.2 (0.8)
0.33 5.3 11 540
5.2 (0.3)
14.6 (0.7)
5-Tp 608 4.9 0.29 5.9 14 636 14
a
In dichloromethane.
b
In solid state.
c
Calculated from the weighted average of the two contributions to τ.

2.4. Computational analysis
To gain a better understanding of the ground and excited state properties of these complexes, we
turned to density functional theory (DFT) and time-dependent DFT (TDDFT) calculations
61

performed at the B3LYP, LACVP
**
level. We found that the optimized structures reproduced the
experimental crystal structures, with Tp binding in an asymmetric η
3
-fashion in 1-Tp, 3-Tp, 4-Tp
and 5-Tp, and η
2
in 2-Tp. Apart from this structural difference, all complexes share similar frontier
orbital contributions, with the HOMO being principally Cu-based, and the LUMO being largely
localized on both, the empty 2pz orbital on the carbene carbon. An exception to the LUMO
localization is complex 3-Tp, with its poorly electrophilic carbene. In this complex, the LUMO
comprises the π* orbitals of the Dipp moiety, and the 2pz orbital of Ccarbene constitutes the
LUMO + 4 (Figure 2. 11). TDDFT calculations revealed that the lowest singlet and triplet states
are comprised of mainly HOMO → LUMO vertical transitions, except for 3-Tp where S1 and T1
are comprised of HOMO → LUMO+1 and HOMO → LUMO+4 transitions. The calculated energy
splitting separating S1 and T1, a key parameter for TADF, was found to be smaller in the NHC
complex 3-Tp than in the CAAC complexes 1-Tp and 2-Tp (∆𝐸 𝑆 1
−𝑇 1
= 0.27 eV, 0.47 eV, and
0.51 eV respectively), indicating that while TADF can be ruled out in the CAAC complexes, it is
more likely to be operating in the NHC complexes with their increased frontier orbital separation.
The optimized T1 geometry shows minimal distortions from the ground state structure in all
complexes, with the spin density residing primarily over the Cu-Ccarbene moieties.
In conclusion, a series of luminescent 2- , 3- and 4- coordinate CAAC- and NHC-Cu complexes
utilizing either chloride or trispyrazolylborate as ancillary ligands were examined. In non-
coordinating MeCy, the chloro-derivatives display concentration dependent, orange emission
consistent with excimer formation. This effect is more pronounced in the case of the less sterically
encumbered 1-Cl. Upon displacement of the chloride with Tp, 4 -and 3-coordinate complexes are
obtained that exhibit blue to orange phosphorescence, depending on the electrophilicity of the
carbene, with the 4-coordinate complexes being more efficient than the 3-coordinate one. The
62

emission energies in both CAAC-Cu and benzannulated NHC-Cu complexes are red-shifted
relative to 3-Tp, owing to LUMO stabilization in both series. Notably, the CAAC complex 1-Tp
showed reduced non-radiative losses with its lower-energy emission than the benzannulated NHC
complexes. These attributes make CAACs a highly promising ligand set for attaining efficient,
yellow and orange phosphorescence while mitigating losses due to the energy gap law.

Figure 2. 11. HOMO (solid) and LUMO (mesh) frontier orbital surfaces and energies of complexes 1-Tp
to 5-Tp.
2.3 Experimental Methods
Synthesis. All reactions were performed under nitrogen atmosphere in oven dried glassware. Potassium
dihydrotris(1-pyrazolyl)borate K[pz 3BH] (KTp) and Chloro[1,3-bis(2,6-di-i-propylphenyl)imidazol-2-
ylidene]copper(I) (IPr)CuCl (3-Cl) and were purchased from Sigma-Aldrich. KTp was washed with cold
toluene before use to remove traces of excess pyrazole. Tetrahydrofuran and dichloromethane were purified
by Glass Contour solvent system by SG Water USA, LLC. (1-(2,6-diisopropylphenyl)-3,3-diethyl-5,5-
dimethylpyrrolidin-2-ylidene)copper(I) chloride (1-Cl)
20, 25
, ((1R,3S,5r,7r)-1'-(2,6-diisopropylphenyl)-
63

4',4'-dimethylspiro[adamantane-2,3'-pyrrolidin]-2'-ylidene)copper(I) chloride (2-Cl)
31
, (1,3-bis(3,5-
dimethylphenyl)benzo[d]imidazol-2-ylidene)copper(I) chloride (4-Cl)
16
and (1,3-bis(3,5-
dimethylphenyl)imidazo[4,5-b]pyrazin-2-ylidene)copper(I) chloride (5-Cl)
16
were synthesized according
to literature procedures. Dry, air-free methylcyclohexane (MeCy) and 2-methyltetrahydrofuran (2-MeTHF)
were purchased from Sigma-Aldrich and used without further purification.
1
H and
13
C NMR spectra were
recorded on a Varian Mercury 400. The chemical shifts are given in units of ppm and referenced to the
residual proton resonance of acetone ((CD 3) 2CO) at 2.05 ppm. Elemental analyses were performed at the
University of Southern California, CA.
CAAC
Et
-CuTp (1-Tp). In the glovebox, CAAC
Et2
-CuCl 1-Cl (78 mg, 0.196 mmol) and KTp (54
mg, 0.216 mmol) were mixed in THF and stirred at room temperature for 2 hours. The reaction
mixture was then filtered under inert atmosphere through a plug of celite, and the solvent was
removed under reduced pressure. The obtained yellow powder was dried under vacuum for 3 days
(95 mg, 85%). Single crystals were grown by layering dry hexane over a concentrated solution of
the complex in THF under inert atmosphere.
1
H NMR (400 MHz, Acetone-d6, δ) 7.51 (t, J = 8.2
Hz, 1H), 7.49 (d, J = 2.7 Hz, 3H), 7.39 (d, J = 7.9 Hz, 2H), 6.77 (s, 3H), 5.91 (d, J = 2.0 Hz, 3H),
3.20 (sept, J = 6.76 Hz, 2H), 2.14 (s, 6H), 1.43 (s, 6H), 1.31 (d, J = 6.8 Hz, 6H), 1.19 (t, J = 7.48
Hz, 6H), 0.94 (d, J = 6.7 Hz, 6H).
13
C NMR (101 MHz, acetone-d6, δ) 147.11, 140.24, 138.26,
133.98, 129.34, 103.62, 80.64, 64.79, 41.08, 32.66, 29.99, 29.95, 27.37, 23.13. Carbene carbon
not observed. Elemental Analysis: Anal. Cacld. for C30H43BCuN7 + 0.5 H2O: C, 62.2; H, 7.74; N,
16.4. Found: C, 62.3; H, 7.46; N, 16.4.
CAAC
Ad
-CuTp (2-Tp). Same method as 1-Tp. CAAC
Ad
-CuCl 2-Cl (100 mg, 0.223 mmol); KTp
(62 mg, 0.245 mmol). Yellow powder product (132 mg, 83%). Single crystals grown by same
method as 1-Tp.
1
H NMR (400 MHz, Acetone-d6, δ) 7.48 (t, J = 7.7 Hz, 1H), 7.39 (d, J = 2.0 Hz,
3H), 7.33 (d, J = 7.7 Hz, 2H), 6.97 (d, J = 1.4 Hz, 3H), 5.94 (t, J = 2 Hz, 3H), 3.78 (d, J = 13.6 Hz,
64

3H), 3.09 (sept, J = 6.68 Hz, 2H), 2.40 (s, 2H), 2.23 – 2.09 (m, 5H), 1.95 – 1.73 (m, 8H), 1.65 (d,
J = 12.6 Hz, 2H), 1.38 (s, 6H), 1.28 (d, J = 6.7 Hz, 6H), 0.82 (d, J = 6.7 Hz, 5H).
13
C NMR (101
MHz, acetone-d6, δ) 146.97, 140.59, 138.02, 134.51, 129.50, 125.64, 103.95, 77.72, 66.35, 48.31,
39.44, 37.98, 36.07, 35.28, 29.88, 29.58, 28.36, 27.00, 23.69. Carbene carbon not observed.
Elemental Analysis: Anal. Cacld. for C36H50BCuN7 + 0.5 H2O: C, 65.2; H, 7.60; N, 14.8. Found:
C, 65.0; H, 7.48; N, 14.7.
IPr-CuTp (3-Tp). Same method as 1-Tp. IPrCuCl (150 mg, 0.308 mmol); KTp (85 mg, 0.338
mmol). White powder product (188 mg, 92%). Single crystals grown by same method as 1-Tp.
1
H
NMR (400 MHz, Acetone-d6, δ) 7.53 (s + t, 2H + 2H (J = 7.82 Hz) respectively), 7.37 (d, J = 7.8
Hz, 4H), 7H), 7.34 (d, J = 2 Hz, 3H), 6.27 (d, J = 1.5 Hz, 3H), 5.77 (t, J = 2Hz, 3H), 3.08 (t, J =
6.9 Hz 4H), 1.22 (d, J = 6.9 Hz, 12H), 1.05 (d, J = 6.9 Hz, 12H).
13
C NMR (101 MHz, acetone-d6,
δ) 147.21, 140.34, 138.46, 133.77, 130.34, 124.69, 124.14, 29.21, 24.85, 24.14. Carbene carbon
not observed. Elemental Analysis: Anal. Cacld. for C36H46BCuN8 + H2O: C, 63.3; H, 7.08; N,
16.4. Found: C, 63.2; H, 6.80; N, 16.8.
BzI-CuTp (4-Tp). Same method as 1-Tp. BzICuCl (60 mg, 0.141 mmol); KTp (34 mg, 0.141
mmol) were mixed in 15mL THF and stirred at room temperature for 3 hours. The reaction mixture
was then filtered through a plug of Celite® and the solvent was evaporated under vacuum. The
product was obtained as a white powder.
1
H NMR (400MHz, acetone-d6, δ) 7.61 (s, 4H), 7.54-
7.56 (m, 2H), 7.49-7.50 (d, 3H), 7.39-7.42 (m, 2H), 7.09 (s, 2H), 6.65-6.66 (d, 1H), 5.89 (s, 3H),
2.77 (s, 6H), 2.20 (s, 12H).
PzI-CuTp (5-Tp). Same method as 1-Tp. PzICuCl (157 mg, 0.37 mmol); KTp (93 mg, 0.37
mmol) were mixed in 25mL THF and stirred at room temperature for 3 hours. The reaction mixture
was then filtered through a plug of Celite® and the solvent was evaporated under vacuum. The
65

product was obtained as a yellow powder that decomposes into a green powder (likely due to
oxidation to Cu
2+
) if stored under ambient conditions for days.
1
H NMR (400 MHz, Acetone-d6) δ
8.48 (s, 2H), 7.77 (s, 4H), 7.53 (d, J = 2.1 Hz, 3H), 7.09 (s, 2H), 6.66 (d, J = 1.3 Hz, 3H), 5.91 (t,
J = 1.9 Hz, 3H), 2.18 (s, 12H).
XRay Crystallography. The single-crystal X-ray diffraction data for compounds 1 – 3-Tp and 5-
Tp were collected on a Bruker SMART APEX DUO three-circle platform diffractometer with the
χ axis fixed at 54.745° and using Mo Kα radiation (λ = 0.710 73 Å) monochromated by a
TRIUMPH curved-crystal monochromator. The crystals were mounted in Cryo-Loops using
Paratone oil. Data were corrected for absorption effects using the multiscan method (SADABS).
The structures were solved by direct methods and refined on F2 using the Bruker SHELXTL
software package. All non-hydrogen atoms were refined anisotropically.
Photophysical characterization. The UV-visible spectra were recorded on a Hewlett-Packard
4853 diode array spectrometer. Steady state emission measurements were performed using a
QuantaMaster Photon Technology International spectrofluoremeter. All reported spectra are
corrected for photomultiplier response. Phosphorescence lifetime measurements were performed
using an IBH Fluorocube instrument equipped with 281 nm and 331 nm LED excitation sources
and a 405 nm laser source using time-correlated single photon counting method. Quantum yields
at room temperature were measured using a Hamamatsu C9920 system equipped with a xenon
lamp, calibrated integrating sphere and model C10027 photonic multichannel analyzer (PMA). All
solution samples were deaerated by bubbling N2 in a quartz cuvette fitted with a Teflon stopcock.
DFT Calculations. All calculations were performed using Jaguar
32
9.1 software package on the
Schrodinger Material Science Suite (v2016-4). Gas phase geometry optimization was calculated
using B3LYP
33, 34
functional with the LACVP**
35
basis set as implemented in Jaguar. Geometric
66

parameters obtained from XRD analyses were used as a starting point for single point calculations
on the ground state and geometry optimization was performed on the triplet state.

2.4 References
1. Cuttell, D. G.; Kuang, S.-M.; Fanwick, P. E.; McMillin, D. R.; Walton, R. A., Simple
Cu(I) Complexes with Unprecedented Excited-State Lifetimes. Journal of the American Chemical
Society 2002, 124 (1), 6-7.
2. Czerwieniec, R.; Kowalski, K.; Yersin, H., Highly efficient thermally activated
fluorescence of a new rigid Cu(I) complex Cu(dmp)(phanephos). Dalton Transactions 2013, 42
(27), 9826-9830.
3. Osawa, M.; Kawata, I.; Ishii, R.; Igawa, S.; Hashimoto, M.; Hoshino, M., Application
of neutral d(10) coinage metal complexes with an anionic bidentate ligand in delayed fluorescence-
type organic light-emitting diodes. Journal of Materials Chemistry C 2013, 1 (28), 4375-4383.
4. Hofbeck, T.; Monkowius, U.; Yersin, H. Highly Efficient Luminescence of Cu(I)
Compounds: Thermally Activated Delayed Fluorescence Combined with Short-Lived
Phosphorescence Journal of American Chemical Society [Online], 2014, p. 399-404.
5. Linfoot, C. L.; Leitl, M. J.; Richardson, P.; Rausch, A. F.; Chepelin, O.; White, F. J.;
Yersin, H.; Robertson, N., Thermally Activated Delayed Fluorescence (TADF) and Enhancing
Photoluminescence Quantum Yields of [CuI(diimine)(diphosphine)]+ Complexes—
Photophysical, Structural, and Computational Studies. Inorganic Chemistry 2014, 53 (20), 10854-
10861.
6. Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M., Highly
Efficient Green Organic Light-Emitting Diodes Containing Luminescent Three-Coordinate
Copper(I) Complexes. Journal of the American Chemical Society 2011, 133 (27), 10348-10351.
7. Zhang, X.; Song, L.; Hong, M.; Shi, H.; Xu, K.; Lin, Q.; Zhao, Y.; Tian, Y.; Sun, J.;
Shu, K.; Chai, W., Luminescent dinuclear copper(I) halide complexes double bridged by
diphosphine ligands: Synthesis, structure characterization, properties and TD-DFT calculations.
Polyhedron 2014, 81, 687-694.
8. Khatri, N. M.; Pablico-Lansigan, M. H.; Boncher, W. L.; Mertzman, J. E.; Labatete, A.
C.; Grande, L. M.; Wunder, D.; Prushan, M. J.; Zhang, W.; Halasyamani, P. S.; Monteiro, J.
H. S. K.; Bettencourt-Dias, A. d.; Stoll, S. L., Luminescence and Nonlinear Optical Properties in
Copper(I) Halide Extended Networks. Inorganic Chemistry 2016, 55 (21), 11408-11417.
9. Liu, Z.; Djurovich, P. I.; Whited, M. T.; Thompson, M. E., Cu4I4 Clusters Supported by
P∧N-type Ligands: New Structures with Tunable Emission Colors. Inorganic Chemistry 2012, 51
(1), 230-236.
10. Lotito, K. J.; Peters, J. C., Efficient luminescence from easily prepared three-coordinate
copper(i) arylamidophosphines. Chemical Communications 2010, 46 (21), 3690-3692.
11. Liu, W.; Fang, Y.; Wei, G. Z.; Teat, S. J.; Xiong, K.; Hu, Z.; Lustig, W. P.; Li, J., A
Family of Highly Efficient CuI-Based Lighting Phosphors Prepared by a Systematic, Bottom-up
Synthetic Approach. Journal of the American Chemical Society 2015, 137 (29), 9400-9408.
12. Wei, F.; Qiu, J.; Liu, X.; Wang, J.; Wei, H.; Wang, Z.; Liu, Z.; Bian, Z.; Lu, Z.; Zhao,
Y.; Huang, C., Efficient orange-red phosphorescent organic light-emitting diodes using an in situ
67

synthesized copper(i) complex as the emitter. Journal of Materials Chemistry C 2014, 2 (31),
6333-6341.
13. Ni, T.; Liu, X.; Zhang, T.; Bao, H.; Zhan, G.; Jiang, N.; Wang, J.; Liu, Z.; Bian, Z.;
Lu, Z.; Huang, C., Red emissive organic light-emitting diodes based on codeposited inexpensive
CuI complexes. Journal of Materials Chemistry C 2015, 3 (22), 5835-5843.
14. Gneu; Leitl, M. J.; Finger, L. H.; Rau, N.; Yersin, H.; Sundermeyer, J., A new class of
luminescent Cu(i) complexes with tripodal ligands - TADF emitters for the yellow to red color
range. Dalton Transactions 2015, 44 (18), 8506-8520.
15. Krylova, V. A.; Djurovich, P. I.; Whited, M. T.; Thompson, M. E., Synthesis and
characterization of phosphorescent three-coordinate Cu(i)-NHC complexes. Chemical
Communications 2010, 46 (36), 6696-6698.
16. Krylova, V. A.; Djurovich, P. I.; Conley, B. L.; Haiges, R.; Whited, M. T.; Williams,
T. J.; Thompson, M. E., Control of emission colour with N-heterocyclic carbene (NHC) ligands in
phosphorescent three-coordinate Cu(i) complexes. Chemical Communications 2014, 50 (54),
7176-7179.
17. Marion, R.; Sguerra, F.; Di Meo, F.; Sauvageot, E.; Lohier, J.-F.; Daniellou, R.; Renaud,
J.-L.; Linares, M.; Hamel, M.; Gaillard, S., NHC Copper(I) Complexes Bearing Dipyridylamine
Ligands: Synthesis, Structural, and Photoluminescent Studies. Inorganic Chemistry 2014, 53 (17),
9181-9191.
18. Back, O.; Henry-Ellinger, M.; Martin, C. D.; Martin, D.; Bertrand, G., - 31P NMR
Chemical Shifts of Carbene–Phosphinidene Adducts as an Indicator of the π-Accepting Properties
of Carbenes. 2013.
19. Rao, B.; Tang, H.; Zeng, X.; Liu, L.; Melaimi, M.; Bertrand, G., - Cyclic
(Amino)(aryl)carbenes (CAArCs) as Strong σ-Donating and π-Accepting Ligands for Transition
Metals. 2015.
20. Romanov, A. S.; Di, D.; Yang, L.; Fernandez-Cestau, J.; Becker, C. R.; James, C. E.;
Zhu, B.; Linnolahti, M.; Credgington, D.; Bochmann, M., Highly photoluminescent copper
carbene complexes based on prompt rather than delayed fluorescence. Chemical Communications
2016, 52 (38), 6379-6382.
21. Gernert, M.; Müller, U.; Haehnel, M.; Pflaum, J.; Steffen, A., - A Cyclic
Alkyl(amino)carbene as Two-Atom π-Chromophore Leading to the First Phosphorescent Linear
CuI Complexes. 2017.
22. Bochmann, M.; Romanov, A. S.; Becker, C. R.; James, C. E.; Di, D.; Credgington, D.;
Linnolahti, M., - Copper and gold cyclic (alkyl)(amino)¬carbene complexes with sub-microsecond
photoemissions: Structure and substituent effects on redox and luminescence properties. 2017.
23. Hamze, R.; Jazzar, R.; Soleilhavoup, M.; Djurovich, P. I.; Bertrand, G.; Thompson , M.
E. Phosphorescent 2-, 3- and 4-coordinate cyclic (alkyl)(amino)carbene (CAAC) Cu(I) complexes
Chemical Communications [Online], 2017, p. 9008-9011.
24. Hu, X.; Soleilhavoup, M.; Melaimi, M.; Chu, J.; Bertrand, G., - Air-Stable (CAAC)CuCl
and (CAAC)CuBH4 Complexes as Catalysts for the Hydrolytic Dehydrogenation of BH3NH3.
2015.
25. Frey, G. D.; Dewhurst, R. D.; Kousar, S.; Donnadieu, B.; Bertrand, G., Cyclic
(alkyl)(amino)carbene gold(I) complexes: A synthetic and structural investigation. Journal of
Organometallic Chemistry 2008, 693 (8–9), 1674-1682.
68

26. Trofimenko, S., Stereochemical nonrigidity in some transition metal complexes containing
tris(1-pyrazolyl) borate ligands. Journal of the American Chemical Society 1969, 91 (12), 3183-
3189.
27. Vogler, A., Luminescence of (NHC)Cu(I)Cl with NHC=1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene under ambient conditions. UV phosphorescence in solution
and in the solid state. Inorganic Chemistry Communications 2017, 84, 81-83.
28. Collins, L. R.; Lowe, J. P.; Mahon, M. F.; Poulten, R. C.; Whittlesey, M. K., Copper
Diamidocarbene Complexes: Characterization of Monomeric to Tetrameric Species. Inorganic
Chemistry 2014, 53 (5), 2699-2707.
29. Trofimenko, S., Boron-Pyrazole Chemistry. Journal of the American Chemical Society
1966, 88 (8), 1842-1844.
30. Trofimenko, S., Recent advances in poly(pyrazolyl)borate (scorpionate) chemistry.
Chemical Reviews 1993, 93 (3), 943-980.
31. Hu, X. B.; Soleilhavoup, M.; Melaimi, M.; Chu, J. X.; Bertrand, G., Air-Stable (CAAC)
CuCl and (CAAC)CuBH4 Complexes as Catalysts for the Hydrolytic Dehydrogenation of
BH3NH3. Angewandte Chemie-International Edition 2015, 54 (20), 6008-6011.
32. Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood, J. R.; Braden, D. A.; Philipp,
D. M.; Rinaldo, D.; Halls, M. D.; Zhang, J.; Friesner, R. A., Jaguar: A high-performance quantum
chemistry software program with strengths in life and materials sciences. International Journal of
Quantum Chemistry 2013, 113 (18), 2110-2142.
33. Becke, A. D., DENSITY-FUNCTIONAL THERMOCHEMISTRY .3. THE ROLE OF
EXACT EXCHANGE. Journal of Chemical Physics 1993, 98 (7), 5648-5652.
34. Lee, C. T.; Yang, W. T.; Parr, R. G., DEVELOPMENT OF THE COLLE-SALVETTI
CORRELATION-ENERGY FORMULA INTO A FUNCTIONAL OF THE ELECTRON-
DENSITY. Physical Review B 1988, 37 (2), 785-789.
35. Hay, P. J.; Wadt, W. R., ABINITIO EFFECTIVE CORE POTENTIALS FOR
MOLECULAR CALCULATIONS - POTENTIALS FOR K TO AU INCLUDING THE
OUTERMOST CORE ORBITALS. Journal of Chemical Physics 1985, 82 (1), 299-310.

69

Chapter 3. Molecular dynamics of 4-coordinate carbene-Cu(I) complexes
employing tris(pyrazolyl)borate ligands

2.1. Introduction
Most commonly studied luminescent organocopper(I) compounds are four-coordinate, tetrahedral
(or pseudo-tetrahedral) ones prone to excited state flattening distortions.
1-4
This has motivated the
exploration of new coordination geometries that circumvent such distortions. Recent years have
seen the development of luminescent, 3-coordinate, planar Cu(I) complexes bearing
N-heterocyclic carbenes (NHC’s).
5-7
In this chapter, we examine a new class of luminescent
4-coordinate carbene-Cu complexes with pseudo-C3 symmetry introduced by a trispyrazolylborate
(Tp) ligand. Such a geometry is highly tunable, with the carbene providing control over electronic
modulation, and the Tp affording control over the sterics. The rich literature on
trispyrazolylborates highlights their highly dynamic binding modes and their feasibility as
spectroscopic probes to examine solution dynamics (through IR as well as
1
H,
13
C,
11
B, and
19
F
NMR spectroscopy).
8-10

While previous studies have looked at the relation between the solution dynamics and
photophysics of organoiridium(III) and organoplatinum(I) complexes,
11
similar investigations into
Cu(I) emitters are lacking. Such research efforts are of special importance in Cu(I) complexes,
which are known for their dramatically reduced photoluminescence efficiencies (PLQY, ΦPL) in
fluid environments. In addition to structural reorganization in the excited state, commonly
examined using ultrafast optical spectroscopy
3, 12
and pump-probe XRay spectroscopy,
13
studies
of the solution dynamics of Cu(I) complexes can lend insight into additional channels of non-
radiative deactivation of their excited states. In this chapter, we investigate the structural properties
and solution dynamics of luminescent, 4-coordinate carbene-Cu-Tp complexes. Variable
temperature (VT) NMR studies help elucidate the underlying mechanisms of isomeric
70

equilibration. Photophysical studies allow us to establish the role played by the steric encumbrance
and binding mode of Tp in limiting non-radiative decay processes.
2.2. Results and Discussion
2.1. Synthesis

Figure 3. 1.Synthetic scheme depicting the structures of the complexes in this chapter.
The Tp complexes were synthesized following the route outlined in Figure 3. 1. We also found
that complexes 1 and 2 can be prepared via a solid state reaction: simply grinding the precursors
using a mortar and pestle yields the desired, blue-luminescent complexes. CAAC
Ad
-CuCl
14
and
the asymmetric Tp
3Me
and Bp
3Me15
ligands were prepared following published procedures. All the
complexes bearing the high-energy NHC ligand were isolated as white microcrystalline powders,
whereas those bearing the more electrophilic CAAC
Ad
- ligand were recovered as bright yellow
powders. In their crystalline form, the complexes are stable to ambient conditions, with the least
sterically-encumbered complex 1 developing a greenish tint over time, characteristic of Cu(II)
formation.
71

2.2. XRay Analysis
Single crystals suited for XRay structural analysis were grown by slow diffusion of pentane into a
concentrated CH2Cl2 solution or by slow evaporation of a toluene solution. The Cu center in Tp
complexes 1 and 2 was found to be 4-coordinate, with the chelating ligand binding in a scorpionate
fashion: two Cu-Npz bonds with similar bond lengths, and a longer one representing the scorpion
tail (Figure 3. 1, Table 3. 1). In contrast, complexes 3 and 5 reveal a 3-coordinate metal center
and a η
2
chelating ligand. In complex 3, the increased steric demands imposed by the adamantyl
group hinder the chelation of the third pz ring. The denticity of Tp
3Me
in complex 5 is interesting,
as it presents the same steric hindrance around the metal center as Tp* in complex 2, yet the overall
complex is 3-coordinate around Cu. Complex 6 with its bidentate Bp
3Me
ligand shows Ccarbene-Cu
and Cu-Npz bond lengths slightly shorter than complexes 1, 2, and 5 (Table 3. 1).

Figure 3. 2. Crystal structures of complexes 1, 2, 3, 5, and 6 showing the Oak Ridge Thermal
Ellipsoid Plots (ORTEPs). The isopropyl units on the 2,6-diisopropylphenyl (Dipp) moiety are
depicted in wireframe mode and the hydrogen atoms are removed for clarity.
Table 3. 1. Crystallographic information of complexes appearing in this chapter.
Complex Space
group
Ccarbene-Cu
(Å)
Cu-Npz
(Å)
Cu-Npz
(Å)
Cu-Npz

(Å)
Σ angles
around Cu
dha
**

1 P1 1.898(4) 2.093(4) 2.109(3) 2.145(3) 345.7° (2) 87.38°
2 P21/c 1.921(2) 2.091(1) 2.090(2) 2.292(2) 355.53° (10) 78.89°
3 P21/c 1.886(1) 1.970(1) 2.053(1) 3.655(2)
‡
359.22° (7) 76.41°
5 P212121 1.912(2) 2.062(2) 2.042(2) 5.846(2)
‡
357.18° (13) 73.46°
6 P21 1.895(1) 2.052(1) 2.056(1) --- 357° (10) ---
*
marks the longest distance (or bond length) between the Cu center and the N of the socrpionate pyrazolyl moiety.
**
marks the diheadral angle formed between the plane comprising the scorptionate pyrazolyl moiety at the longest
separation from Cu and the plane of the carbene ligand.
‡
marks the distance between Cu and N pz, since that N is
not chelated to the Cu center in these complexes.

72

2.3. NMR Analysis
Despite the asymmetric binding fashion of the scorpionate pyrazolyl borate ligand in complexes
1 – 5, as described above,
1
H-NMR spectra of the complexes in solution show only one set of
resonances for all three pyrazolyl (pz) rings. This observed equivalence of the three pz groups is
consistent with C3 symmetry in solution, and likely results from a rapid exchange of the pz rings
on the NMR time scale. In fact, since their early reports by Trofimenko,
8, 16
complexes employing
η
3
pyrazolyl borate ligands have been known to undergo a tumbling mechanism in solution, which
renders the different pz groups equivalent.
9, 17-19
(Figure 3. 3) With that in mind, we turned to
variable temperature (VT)
1
H- and
13
C- NMR, to probe the dynamic equilibration processes
operating in solution samples of our complexes.

Figure 3. 3. Scheme depicting the tumbling mechanism of the Tp ligand adapted for 4-coordinate
carbene-Cu(I) complexes.
18

1
H-NMR spectra of complex 1 in acetone d6 shows signals that correspond to three equivalent pz
groups at room temperature down to -50°C, signifying that the dynamic process equilibrating
between the different isomers persists on a faster timescale than the NMR’s even upon cooling.
The only observed change at low temperatures is a better resolution of the peaks appearing between
7.38 – 7.67 ppm, allowing their assignment to the protons on the 2,6-diisopropylphenyl (Dipp)
moiety, one of the protons (H3) on the pz ring, and the two protons on the imidazole respectively
(Figure 3. 4).
73

Figure 3. 4. Variable temperature (VT)
1
H-NMR of complex 1 in acetone d6.
We expected the more sterically encumbered complex 2, with its bulkier Tp* ligand, to have
slower equilibration dynamics as noted by Trofimenko for some Tp ligands with larger substituents
in the 3-position.
20
Nevertheless, VT
1
H-NMR spectra of this complex show that it too undergoes
rapid pz tumbling at -50°C in acetone d6 (Figure 3. 5). The dynamic process occurring along the
C3 axis, which is parallel to the Ccarbene-Cu bond also manifests in the absence of the signature
Ccarbene signal in
13
C-NMR spectra at room temperature. However, VT
13
C-NMR experiments show
the signal growing in within the temperature range of 0°C to -50°C, at which point it becomes
unambiguous with a chemical shift at 190.5 ppm (Figure 3. 6). The discrepancy observed between
the VT NMR experiments carried out on both
1
H and
13
C nuclei is likely a result of the much
longer T2 (spin-spin) relaxation times of the
13
C nucleus.
74

Figure 3. 5. VT
1
H-NMR spectrum of complex 2 in acetone d6.

Figure 3. 6. VT
13
C-NMR spectra of complex 2 in acetone d6, showing the signal from the carbene
carbon (c) appearing at low temperatures. The multiple peaks, especially in the aliphatic region,
appearing at low temperatures are due to an instrument artifact: effective C-H decoupling is
compromised at low temperatures giving rise to C-H splitting.
75

Performing the same VT
1
H-NMR experiments in the more polar, poorly-coordinating solvent
CDCl3 yields similar results to those observed in acetone d6: rapid Tp* equilibration giving rise to
equivalent resonances between all three pz rings that persists at -50 °C. However, one key
difference between the temperature-dependent spectra recorded in CDCl3 and in acetone d6 is the
near coalescence of the 3- and 5-methyl groups on the pz rings observed upon cooling. The
dependence of dynamic equilibrium processes governing Tp complexes on solvent polarity was
noted by Venanzi, Younger, and coworkers.
21
In this case, we note that the two magnetically
distinct methyl groups of Tp* becoming equivalent is consistent with a 1,2-borotropic shift
reported by Albinati et al. in Ir(I)-Tp complexes
22
and shown in (Figure 3. 8). The authors
proposed mechanism for this type of isomerization requires an intermediate where the Tp ligand
binds in η
2
fashion.
13
C-NMR studies of complex 2 in CDCl3 at different temperatures reveal an
intriguing set of results. While the aliphatic carbons corresponding to the 3,5-dimethyl groups
maintain the same chemical shifts down to -50 °C, it is the pz ipso carbons they’re appended to
that show a near convergence in their chemical shifts upon cooling (Figure 3. 9). Further in this
chapter, we introduce complexes 5 and 6, bearing asymmetric Tp
3Me
and Bp
3Me
ligands, that aim
at elucidating whether a 1,2-borotropic shift is indeed responsible for the trends observed in VT
1
H-NMR of complex 2 in CDCl3.
76

Figure 3. 7. VT
1
H-NMR of complex 2 in CDCl3.

Figure 3. 8. Mechanism for the 1,2-borotropic shift, adapted for carbene-Cu(I) Tp complexes.
22

77

Figure 3. 9. VT
13
C-NMR spectra of complex 2 in CDCl3. Defective C-H decoupling at low
temperatures gives rise to C-H splitting.

Figure 3. 10. VT
1
H-NMR spectra of complex 3 in acetone d6.
78

We turned to complex 3 for its asymmetric cyclic alkyl amino carbene (CAAC), its increased steric
demands imposed by the adamantyl (Ad) group, and its red-shifted luminescence as discussed in
chapter 2 and in an earlier published report.
23
Despite the η
2
binding fashion of the Tp ligand in
crystalline samples of this complex, room temperature
1
H-NMR studies showed 3 only pyrazolyl
resonances corresponding to equivalent pz groups. This result indicates that complex 3 also
undergoes the rapid exchange mechanism highlighted in Figure 3. 3.
1
H-NMR spectra recorded
at lower temperatures reveal that the rapid equilibration mechanism is rendered more sluggish,
leading to baseline coalescence of the signals from the pz protons at different points: H 1, closest
to the metal center, experiences that first at -50 °C, followed by H2 at -78 °C, and lastly H3, farthest
from the Cu center at -83.5 °C. Further cooling to -88 °C does not result in the resolution of the
peaks to reflect the different isomers obtained at the slow exchange regime. Attempts to carry out
the experiments at temperatures lower than -88 °C were thwarted by limitations in the instrument
cooling threshold and the freezing point of acetone d6 (-94 °C).
In a similar fashion to complex 2, VT-NMR experiments performed on the
13
C nucleus were able
to capture the complex in the slow exchange regime, where the signal from the carbene carbon
becomes apparent at -70°C (Figure 3. 11)– the temperature at which the Tp tumbling becomes
sufficiently sluggish at the
13
C-NMR timescale. This effective rotation around the Ccarbene-Cu bond
axis was reported for related 3-coordinate NHC-Cu complexes.
6
Notably, the chemical shift of the
CAAC’s more electrophilic Ccarbene (265.5 ppm) is significantly downfield relative to the NHC
Ccarbene.
Complex 4, bearing the same CAAC
Ad
carbene and the methylated Tp* ligand also showed
equivalent pz groups in its room temperature
1
H-NMR spectra (in CDCl3). The rapid exchange
process was found to persist at -60°C, just above the solvent freezing point (-64 °C). Unfortunately,
79

limited solubility of this complex in acetone prevented further VT NMR studies at temperatures
lower than -60 °C.

Figure 3. 11. VT
13
C-NMR spectra of complex 3 in acetone d6, showing the signal from the
carbene carbon (c) appearing at low temperatures.
Next, we turned to complexes 5 and 6, Tp
3Me
and Bp
3Me
ligands, with an asymmetric pz substitution
pattern, to examine a possible 1,2-borotropic shift occurring. Interestingly, the Cu center in
complex 5 is 3-coordinate, unlike the related complex 2 where the metal experiences a similar
degree of steric encumbrance by the 3-methyl group on the pz rings. Nevertheless, at room
temperature, all three pz groups are found to be equivalent on the
1
H-NMR time scale.
1
H-NMR
spectra show no change in peak intensity or chemical shift upon cooling to -35 °C in acetone d6,
indicating rapid Tp equilibration at low temperatures, and disfavoring a 1,2-borotropic shift
process (Figure 3. 12). As reported by Albinati et al., the 1,2-borotrtopic shift mechanism occurs
via an intermediate with η
2
-binding pyrazolyl borate ligand. As such, we investigated complex 6,
80

which is truly 3-coordinate owing to its Bp
3Me
ligand. VT
1
H-NMR experiments of the complex in
acetone d6 show no signs of the formation of the rearranged isomer, even at -53 °C, allowing us to
conclude that a 1,2-borotropic shift is not likely to be occurring in our carbene-Cu complexes
(Figure 3. 13). Hence, the observed coalescence between the 3- and 5-methyl signals in complex
2 (CDCl3, low T) is attributed to small, irregular changes in the magnetic environment around the
metal center that appear upon cooling, and not to a dynamic exchange process. In this context, the
sensitivity of such minor changes to the solvent polarity is understood.

Figure 3. 12. VT
1
H-NMR spectrum of complex 5 in acetone d6.
81

Figure 3. 13. VT
1
H-NMR spectra of complex 6 in acetone d6.
2.4. Photophysical Characterization
As highlighted in chapter 2, the photoluminescence efficiencies of microcrystalline powders of
carbene-Cu complexes bearing Tp ligands were found to depend on the denticity of the latter:
compounds with an η
3
binding Tp showed higher PLQY and lower knr than ones with Tp in η
2

binding mode. Here too, we study the photophysical properties of the complexes in their solid state
and examine the effects of the binding mode and steric encumbrance of the pyrazolylborate.
Additionally, we investigate the ways in which solution dynamics influence the photophysics in
fluid matrices such as 2-methyltetrahydrofuran (2-MeTHF) and MeCy.
All IPr-Cu complexes are blue-phosphorescent (kr 10
4
– 10
5
s
-1
) in their microcrystalline form,
with varying ΦPL values that correlate with the denticity of the pyrazolylborate ligand and the steric
demands it imposes: complex 2 with its bulky Tp* ligand shows a higher ΦPL and a lower non-
82

radiative rate constant (knr) compared to 1 bearing the unsubstituted Tp (Table 3. 2). Despite their
room temperature emission profiles being nearly superimposable (Figure 3. 14 a), their radiative
rates are starkly different, with 2 registering a radiative rate constant that is more than an order of
magnitude larger than 1. The origin of this discrepancy is touched upon briefly in the
computational section of this chapter. However, at this point we can surmise that while Tp
*
does
not alter the energies of the optical transitions relative to its Tp analogue, it perturbs the electronic
makeup of those transitions. At 77 K, non-radiative deactivation modes are somewhat suppressed,
resulting in more intense and slightly longer-lived phosphorescence in complex 1. Complex 2, on
the other hand, registers a red-shift of 500 cm
-1
and a 3-fold increase in its PL lifetime. This more
significant change can be attributed to thermally-activated delayed fluorescence (TADF), which is
hampered by low temperatures.
0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
77 K

Normalized Intensity (a. u.)
1, SS
2, SS
2, MeCy
RT
a)

Wavelength (nm)
1, SS
2, SS
2, MeCy

0.0
0.5
1.0
500 600 700 800
500 600 700 800
0.0
0.5
1.0
77 K

Normalized Intensity (a. u.)
3, SS
4, SS
4, 2-MeTHF
RT
b)

Wavelength (nm)
3, SS
4, SS
4, 2-MeTHF

Figure 3. 14. a) Room temperature and 77 K emission spectra of microcrystalline powders of 1
and 2, and of 2 in MeCy. b) Room temperature and 77 K emission spectra of microcrystalline
powders of 3 and 4, and of 4 in 2-MeTHF.
83

In MeCy, the emission of 2 is drastically red-shifted (2000 cm
-1
), indicating large structural
reorganization in this fluid matrix. The extent of this reorganization is reflected in the sharp drop
in ΦPL (from 0.77 to 0.12) and increase in knr (from 6.9 x 10
5
s
-1
to 3.5 x 10
6
s
-1
). However, kr is
nearly doubled in MeCy, likely signifying emission from a different excited state accessible in
fluid environments. At 77 K, the frozen solvent matrix results in emission that more closely
resembles that of the microcrystalline powder.
Complexes 3 and 4 bearing the more electrophilic CAAC show phosphorescence (kr ~ 10
4
s
-1
) that
is red-shifted relative to the IPr complexes. Here too, microcrystalline solids of 4 with the more
sterically-encumbered Tp* registers a higher ΦPL than 3. This result is somewhat puzzling when
one considers that 3 is in fact a 3-coordinate complex in its crystalline form, which exacerbates
losses to knr. It’s challenging to imagine how the bulkier Tp
*
in complex 4 can attain η
3
binding.
Unfortunately, attempts to isolate crystals of 4 were not successful in our hands. Yet, it is possible
that crystal packing exerts a rigidifying effect that mitigates excited state reorganization in this
complex. This is seen somewhat in the way 4 exhibits a blue-shift in its emission spectrum relative
to 3. At 77 K, both complexes experience a rigidochromic effect, where emission profiles are
narrowed, and excited state lifetimes are slightly increased. The suppression of non-radiative decay
pathways is most notable for complex 3, where the recorded PL decay time is increased a full order
of magnitude.
The emission spectrum of complex 4 in a 2-MeTHF solution also shows a 2000 cm
-1
red shift
compared to that in the solid state (Figure 3. 14 b, Table 3. 2). This is accompanied by an increase
in the non-radiative decay rate as well as a dramatic drop in the radiative rate. VT
1
H- NMR
experiments show that 4 undergoes rapid fluxional behavior, adopting an overall equilibrium
geometry between its η
2
and η
3
isomers. It is likely then that one of those structures offers access
84

to a different excited state with lower kr. Alternately, an open coordination site on the η
2
isomer
can promote exciplex formation with the weakly-coordinating 2-MeTHF. Exciplex quenching is a
well-known phenomenon in Cu(I) complexes
24, 25
and is disrupted in frozen glass matrices.
23

Indeed, in frozen 2-MeTHF at 77 K, the emission profile resembles that of the solid state, and the
long PL decay is recovered (17.2 μs). Solutions of 3 in 2-MeTHF do not show luminescence.
0.0
0.5
1.0
400 500 600
400 500 600
0.0
0.5
1.0
77 K

Normalized Intensity (a. u.)
2, SS
5, SS
6, SS
RT

Wavelength (nm)
2, SS
5, SS

Figure 3. 15. Emission spectra of microcrystalline powders of 5, and 6 compared with 2, at room
temperature and 77 K.
Lastly, we compare the photophysical properties of blue-luminescent microcrystalline powders of
complexes 5 and 6 to that of complex 4 (Figure 3. 15). All three complexes bear the same carbene
and a Tp ligand with a methyl substituent in the 3-position, most proximal to the metal center. Yet,
while 2 is 4-coordinate in its crystalline form, 5 adopts a 3-coordinate geometry with a free,
uncoordinated pz group. More fair comparisons can be thus drawn on to elucidate the effects of
hapticity on the photophysics. Firstly, we note that the ΦPL of both 5 and 6 is reduced relative to
85

2, indicating that η
3
binding mode of the Tp ligand results in more efficient luminescence than η
2
.
Secondly, among the two 3-coordinate complexes, 6 shows higher ΦPL and diminished knr. This is
likely due to the presence of an uncoordinated pz ring in complex 5, which introduces more modes
of non-radiative deactivation. The radiative rates of both 5 and 6, however, are an order of
magnitude slower than 2.
Table 3. 2. Photophysical properties of complexes 1 – 6.
Complex λmax, RT
(nm)
ΦRT τRT
(μs)
kr, RT
(10
4
s
-1
)
knr, RT
(10
4
s
-1
)
λmax, 77K
(nm)
τ77K
(μs)
1 SS 468 0.22 13.6 1.6 5.7 468 22.6
2 SS 466 0.77 0.84 (0.5);
5.6 (0.5)
23
*
6.9
*
478 0.86 (0.2);
12.0 (0.8)
MeCy 514 0.12 0.25 48 350 456 15.8
3 SS
616 0.02
0.4 (0.4);
1.0 (0.6)
2.6
*
1.3
*
614
4.2 (0.75);
14 (0.25)
4 SS 547 0.25 13 1.9 5.7 556 24
2-MeTHF 614 0.02 8.36 0.19 12 550 17.2
5 SS 430 0.12 9.6 1.2 9.2 440 26.1
6 SS 422 0.5 15 3.3 3.3 --- ---
*
Calculated from the weighted averages of both contributions.

2.5. DFT calculations
To further probe the origin of the fast radiative rates in complex 2, we turned to computational
analysis using density functional theory (DFT). The calculated energies of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are comparable
in complexes 1 and 2, indicating that the substituents on Tp
*
do not have a direct impact on the
electronic structure. However, the makeup of the frontier orbitals are significantly different: the
HOMO wavefunction in 2 comprises a greater contribution from the pyrazolyl nitrogens compared
to 1. More importantly, the LUMO in 2 resides primarily on the carbene 2pz carbon, whereas in 1
it localizes on the π* orbitals of the Dipp moiety. Thus, the spatial distribution of the frontier orbital
wavefunctions appears to depend on ligand conformation: in 1, the boat structure formed by Cu,
86

the four pyrazolyl nitrogens, and B is shallower than in 2 (Ccarbene-Cu-B = 175° in the former and
159° in the latter). It seems that the carbene’s 2pz contribution to the LUMO is essential to the high
kr values observed. A key literature report on TADF in Cu(I) complexes with different
conformational isomers lends insight into the importance of the steric control exerted by the
ligands in facilitating efficient and strongly-allowed luminescence.
26

Figure 3. 16. Frontier orbital surfaces and energies of complexes 1 and 2 (highest occupied
molecular orbital, HOMO, solid; lowest unoccupied molecular orbital, LUMO, mesh).
2.3. Conclusions
A series of 4-coordinate Cu(I) complexes was prepared employing a NHC or CAAC carbene and
a scorpionate Tp ligand. Despite the scorpionate binding fashion of Tp, NMR spectra of the
complexes correspond to geometries with C3 symmetry. XRay crystallography as well as VT
1
H-
and
13
C-NMR spectroscopy help elucidate the dynamic process that equilibrates the different
structural isomers. In our complexes, rapid Tp tumbling was only hampered but not fully curbed
at -88 °C. Lastly, we show how the hapticity of the Tp moiety impacts the solid state photophysics,
and how the steric requirements it introduces affect luminescence in solution.
87

2.4. Experimental Methods
1
H,
11
B, and
13
C spectra were collected using a Varian VNMRS 500 or a Varian VNMRS 600
spectrometer. The
1
H and
13
C chemical shifts were referenced to the residual solvent signals. All
variable temperature (VT) NMR studies were performed starting at the lowest temperatures,
warming up by 20 – 25 °C intervals, and culminating with the room temperature experiments.
2.5. References
1. Everly, R. M.; Ziessel, R.; Suffert, J.; McMillin, D. R., Steric influences on the
photoluminescence from copper(I) phenanthrolines in rigid media. Inorganic Chemistry 1991, 30
(3), 559-561.
2. Eggleston, M. K.; McMillin, D. R.; Koenig, K. S.; Pallenberg, A. J., Steric Effects in the
Ground and Excited States of Cu(NN)2+ Systems. Inorganic Chemistry 1997, 36 (2), 172-176.
3. Iwamura, M.; Takeuchi, S.; Tahara, T., Real-Time Observation of the Photoinduced
Structural Change of Bis(2,9-dimethyl-1,10-phenanthroline)copper(I) by Femtosecond
Fluorescence Spectroscopy: A Realistic Potential Curve of the Jahn−Teller Distortion. Journal
of the American Chemical Society 2007, 129 (16), 5248-5256.
4. Iwamura, M.; Takeuchi, S.; Tahara, T., Ultrafast Excited-State Dynamics of Copper(I)
Complexes. Accounts of Chemical Research 2015, 48 (3), 782-791.
5. Krylova, V. A.; Djurovich, P. I.; Whited, M. T.; Thompson, M. E., Synthesis and
characterization of phosphorescent three-coordinate Cu(i)-NHC complexes. Chemical
Communications 2010, 46 (36), 6696-6698.
6. Krylova, V. A.; Djurovich, P. I.; Aronson, J. W.; Haiges, R.; Whited, M. T.;
Thompson, M. E., Structural and Photophysical Studies of Phosphorescent Three-Coordinate
Copper(I) Complexes Supported by an N-Heterocyclic Carbene Ligand. Organometallics 2012,
31 (22), 7983-7993.
7. Krylova, V. A.; Djurovich, P. I.; Conley, B. L.; Haiges, R.; Whited, M. T.; Williams,
T. J.; Thompson, M. E., Control of emission colour with N-heterocyclic carbene (NHC) ligands
in phosphorescent three-coordinate Cu(i) complexes. Chemical Communications 2014, 50 (54),
7176-7179.
8. Trofimenko, S., Boron-pyrazole chemistry. II. Poly(1-pyrazolyl)-borates. Journal of the
American Chemical Society 1967, 89 (13), 3170-3177.
9. Trofimenko, S., Recent advances in poly(pyrazolyl)borate (scorpionate) chemistry.
Chemical Reviews 1993, 93 (3), 943-980.
10. Northcutt, T. O.; Lachicotte, R. J.; Jones, W. D., 11B NMR: A New Tool for the
Determination of Hapticity of Tris(pyrazolyl)borate Ligands. Organometallics 1998, 17 (23),
5148-5152.
11. Facendola, J. W.; Seifrid, M.; Siegel, J.; Djurovich, P. I.; Thompson, M. E., Synthesis
and characterization of phosphorescent platinum and iridium complexes with cyclometalated
corannulene. Dalton Transactions 2015, 44 (18), 8456-8466.
12. Iwamura, M.; Takeuchi, S.; Tahara, T., Substituent effect on the photoinduced structural
change of Cu(i) complexes observed by femtosecond emission spectroscopy. Physical Chemistry
Chemical Physics 2014, 16 (9), 4143-4154.
88

13. Chen, L. X.; Shaw, G. B.; Novozhilova, I.; Liu, T.; Jennings, G.; Attenkofer, K.;
Meyer, G. J.; Coppens, P., MLCT State Structure and Dynamics of a Copper(I) Diimine
Complex Characterized by Pump−Probe X-ray and Laser Spectroscopies and DFT Calculations.
Journal of the American Chemical Society 2003, 125 (23), 7022-7034.
14. Hu, X. B.; Soleilhavoup, M.; Melaimi, M.; Chu, J. X.; Bertrand, G., Air-Stable
(CAAC) CuCl and (CAAC)CuBH4 Complexes as Catalysts for the Hydrolytic Dehydrogenation
of BH3NH3. Angewandte Chemie-International Edition 2015, 54 (20), 6008-6011.
15. Niedenzu, K.; Niedenzu, P. M.; Warner, K. R., Boron-nitrogen compounds. 105. Boron
derivatives of 3-methylpyrazole. Inorganic Chemistry 1985, 24 (10), 1604-1606.
16. Trofimenko, S., Boron-Pyrazole Chemistry. Journal of the American Chemical Society
1966, 88 (8), 1842-1844.
17. Jesson, J. P.; Trofimenko, S.; Eaton, D. R., Spectra and structure of some transition
metal poly(1-pyrazolyl) borates. Journal of the American Chemical Society 1967, 89 (13), 3148-
3158.
18. Keyes, M. C.; Young, V. G.; Tolman, W. B., Diastereoselective Intramolecular C−H
bond Activation by Optically Active Tris(pyrazolyl)hydroborate Complexes of Rhodium.
Organometallics 1996, 15 (20), 4133-4140.
19. Rheingold, A. L.; White, C. B.; Trofimenko, S., Hydrotris(3-mesitylpyrazol-1-yl)borate
and hydrobis(3-mesitylpyrazol-1-yl)(5-mesitylpyrazol-1-yl)borate: symmetric and asymmetric
ligands with rotationally restricted aryl substituents. Inorganic Chemistry 1993, 32 (16), 3471-
3477.
20. Rheingold, A. L.; Zakharov, L. N.; Trofimenko, S., The Effect of a 3-Benzyl Group on
the Coordination Chemistry of Homoscorpionate Ligands. Inorganic Chemistry 2003, 42 (3),
827-833.
21. Bucher, U. E.; Currao, A.; Nesper, R.; Rueegger, H.; Venanzi, L. M.; Younger, E.,
.kappa.2-.kappa.3 Isomerism in Rhodium(I) Tris(pyrazolyl)borate Complexes of the Type
Tp3R,4R,5RRh(LL) (LL = 2CO, COD, and NBD) and Their Dynamic Behavior in Solution. X-
ray Crystal Structure of TpMeRh(NBD). Inorganic Chemistry 1995, 34 (1), 66-74.
22. Albinati, A.; Bovens, M.; Rüegger, H.; Venanzi, L. M., Hydrotris(pyrazolyl)borato
Cycloocta-1,5-diene Complexes of Iridium(I): Synthetic Studies and Equilibria in Solution. X-
ray Crystal Structures of a Four- and a Five-Coordinate Iridium(I) Hydrotris(pyrazolyl)borato
Complex. Inorganic Chemistry 1997, 36 (26), 5991-5999.
23. Hamze, R.; Jazzar, R.; Soleilhavoup, M.; Djurovich, P. I.; Bertrand, G.; Thompson ,
M. E. Phosphorescent 2-, 3- and 4-coordinate cyclic (alkyl)(amino)carbene (CAAC) Cu(I)
complexes Chemical Communications [Online], 2017, p. 9008-9011.
24. Everly, R. M.; McMillin, D. R., CONCENTRATION-DEPENDENT LIFETIMES OF
Cu(NN)+2 SYSTEMS: EXCIPLEX QUENCHING FROM THE ION PAIR STATE.
Photochemistry and Photobiology 1989, 50 (6), 711-716.
25. Stacy, E. M.; McMillin, D. R., Inorganic exciplexes revealed by temperature-dependent
quenching studies. Inorganic Chemistry 1990, 29 (3), 393-396.
26. Leitl, M. J.; Krylova, V. A.; Djurovich, P. I.; Thompson, M. E.; Yersin, H.,
Phosphorescence versus Thermally Activated Delayed Fluorescence. Controlling Singlet-Triplet
Splitting in Brightly Emitting and Sublimable Cu(I) Compounds. Journal of the American
Chemical Society 2014, 136 (45), 16032-16038.

89

Chapter 4. Iridium-like luminescence of 2-coordinate Cyclic alkyl amino
carbene-Cu(I) complexes

4.1. Introduction
It has been a long sought-after goal to obtain luminescence out of first-row transition metal
complexes, such as Cu, comparable to that from heavy metals such as Ir, Pt, and Ru, which have
been applied in fields from photocatalysis to energy conversion and organic light emitting diodes
(OLEDs). Achieving that would require overcoming the weak spin-orbit coupling (SOC) imposed
by the light metal as well as limiting the high reorganization energies typical in the excited states
of Cu(I) complexes. The strong SOC parameters in organoiridium and organoplatinum complexes
enhances the rate of intersystem crossing (ISC) between the singlet and triplet manifolds (k ISC ~
10
13
– 10
14
s
-1
)
1, 2
and the rate of phosphorescence from T1 → S0 (kr,p > 10
5
s
-1
).
3, 4
For context,
Cu(I) complexes typically show kISC and kr,p on orders of 10
10
– 10
11
s
-1 5, 6
and 10
3
– 10
4
s
-1

respectively. Moreover, the lowest-energy metal to ligand charge transfer (MLCT) transitions in
Cu(I) luminophores involve formal oxidation at the d
10
metal center. This induces Jahn–Teller
distortions
7
that not only increase nonradiative decay rates but also lead to ISC rates even slower
than expected based only on SOC considerations.
8
While long excited state lifetimes are beneficial
in applications such as photosensitization
9
, photocatalysis,
10
and sensing,
11
they can lead to
deleterious second-order quenching processes that limit their applicability as phosphors in organic
light emitting diodes (OLEDs) and light emitting electrochemical cells (LEECs).
12-15

Over the last decade, efforts in designing and optimizing Cu(I) complexes that undergo thermally
activated delayed fluorescence (TADF) have resulted in efficient luminescence with faster
radiative rates (larger radiative rate constants, kr).
16
Compared to purely organic TADF molecules,
Cu systems present a major difference: enhanced k ISC in the latter quantitatively depopulates the
lowest excited singlet state S1, outpacing prompt fluorescence, and resulting in monoexponential
90

decays at all temperatures. In contrast, organic TADF is marked by bi- or multi-exponential
decays, due to S1-based emission prior to ISC. In a similar fashion, up-ISC (UISC) from T1 to the
closely-lying S1, a process also tied to SOC interactions,
17
is enhanced in Cu TADF systems
relative to organic ones. This helps in circumventing the commonly-posited TADF conundrum.
The latter contrasts the TADF requirement for the partitioning of the HOMO (highest occupied
molecular orbital) and LUMO (lowest unoccupied molecular orbital) wavefunctions with
achieving high radiative rates, as the minimized orbital overlap reduces the probability of
transitions between them. However, we note that since the radiative transition in TADF is S1 →
S0, it is not in fact the limiting factor. To illustrate, we turn to TADF in a Cu complex with the
smallest recorded ∆𝐸 𝑆 1
−𝑇 1
= 33 meV and radiative rate (kr) of 2 x 10
5
s
-1
in the solid state at room
temperature (RT).
18

16
Fitting the temperature-dependent photoluminescent decay of this complex
to the modified Boltzmann Equation 𝜏 =
3 + exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
3 𝑘 𝑇 1
+ 𝑘 𝑆 1
exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
(𝑘 𝑆 1
is the rate constant for
radiative decay from the S1 state, kB is Boltzmann’s constant and T is temperature) gives a rate of
prompt fluorescence (emission from S1 prior to ISC, 𝑘 𝑆 1
= 2 x 10
6
s
-1
)
19
that is a full order of
magnitude faster than the TADF rate at RT. This demonstrates that even a weakly-allowed S1 →
S0 transition radiates faster than its corresponding T1 → S0 transition. Nevertheless, even the most
efficient TADF-based Cu emitters have low photoluminescent quantum yields ( ΦPL) in fluid or
polymeric matrices. High rates of non-radiative decay (knr) are observed in non-rigid environments
due to significant distortions in the excited state that are common in frequently studied tetrahedral
motifs.
18, 20, 21

Reports of linear Cu complexes with high PLQY in non-rigid matrices highlight the appeal of low
coordination in limiting excited state reorganization.
22, 23
Unfortunately, the MLCT nature of the
91

radiative transitions in these derivatives lead to excited state lifetimes that are relatively long (kr =
10
4
s
-1
).
24
A recent paper has reported highly-efficient OLED devices employing 2-coordinate
carbene-copper and -gold complexes.
25
Here, we examine a related family of 2-coordinate, neutral
cyclic (alkyl)(amino)carbene,
26-28
CAAC-Cu(I)-amide complexes with remarkable photophysical
properties. By modifying the steric bulk of the carbene, we achieve 100% efficient luminescence
from Cu(I) complexes in fluid and rigid media, with fast emission lifetimes (τ = 2–3 μs). The
emission color of these compounds can be varied from violet to the orange part of the visible
spectrum through suitable choice of the amide ligand. Electrochemical and computational analysis
of the complexes reveal a picture of ligand-based frontier orbitals with minimal metal contribution.
We investigate the excited state manifold in this family of compounds and describe, through
solvent- and temperature-dependent photophysical studies, an interplay between an interligand
charge transfer (ICT) state and a ligand centered state (carbazolide-based triplet,
3
Cz). In order to
probe the ICT manifold exclusively, and to determine whether a closely-lying ligand-centered state
is needed for fast radiative rates, we examine a complex where the ICT has been preferentially
stabilized. Temperature dependent photoluminescence (PL) experiments show that
uniquely-efficient TADF is indeed occurring within the
1
ICT and
3
ICT states. Additionally, we
consider the role of the coplanar orientation of the donor (D, carbazole) and acceptor (A, carbene)
ligands in determining the unique photophysical properties of these D–Cu–A systems. To this end,
we prepared a complex where the carbazole and carbene are constrained to an orthogonal
conformation. We find that while this results in poorer overlap between the HOMO and LUMO, a
markedly lower kr is obtained. Molecular dynamic calculations replicate the experimentally
observed solvato- and rigidochromic effects. Doping a blue emitter into a wide bandgap, Cu-based
host gave OLEDs with EQE > 9.5%.
92

4.2. Results and Discussion
4.2.1. Synthesis
The 2-coordinate CAAC-Cu amide complexes were prepared in high yields (80 – 90%) following
a literature procedure
25
(Figure 4. 1). Complexes 1a–d and 2b with increasing steric bulk on the
CAAC and carbazole ligands were isolated as white to yellow powders. The complexes display
varying degrees of O2 and moisture stability depending on the steric bulk around the carbene and
the nature of the amide. The bulkier CAACs in 1a and 1b lead to complexes that are indefinitely
air stable in the solid state and in solution. Complex 5, with its highly electron-rich amide,
decomposes in solutions under aerobic conditions; however, single crystals of the complex stored
in a desiccator are indefinitely stable to air. Complexes 2a and 2b, bearing 1,8-dimethyl
carbazolide (CzMe2), was found to be highly sensitive to H2O and CO2 in ambient air as a solid
and in solution. All four CAAC-CuCl
29-32
precursors as well as CzMe2
33
and CzCN
34
were
synthesized following published reports.
Complex CAAC NR
2
1a A Cz
1b B Cz
1c C Cz
1d D Cz
2a A CzMe
2
2b B CzMe
2
3 A CzCN
4 A CzOMe
5 A NPh
2

Figure 4. 1. Synthetic scheme followed in the preparation of complexes 1a - 1d, 2a, 2b, 3, 4, and 5.
93

4.2.2. XRay Analysis
X-ray diffraction analysis were grown via was performed on single crystals of the complexes 1a,
1c, 1d, 2a, 2b, 3, 4, and 5. The complexes all display a near-linear coordination geometry around
the Cu center (174° – 179°), with 3.73 Å – 3.77 Å separation between the carbene carbon (CCarbene)
and the amide nitrogen (Namide) due to similar C–Cu (1.88 – 1.89 Å) and Cu–N (1.85 – 1.87 Å)
bond lengths (Figure 4. 2, Table 4. 1). Dihedral angles between the ligands are < 9° in complexes
1a, 3–5 whereas complex 2b has a near orthogonal conformation (dihedral angle = 83°). Complex
2a, on the other hand shows a highly unusual ligand conformation where Namide is pyramidalized:
the sum of angles around Namide = 326.6°, a far deviation from the planar 360°. Despite the
asymmetry of the CAAC, only one set of
1
H NMR resonances for the carbazolide is observed in
all complexes. This observation indicates rapid exchange on the NMR time scale, likely caused by
rotation around the Ccarbene-Namide bond axis.

Figure 4. 2. Oak Ridge thermal ellipsoid plot (ORTEP) representation of 1a – 1d, 2a, 2b, and 3 – 5. Thermal
ellipsoids shown at 50% probability. Hydrogens removed for clarity.
94

Variable temperature NMR experiments performed on complex 1a down to -60°C show no signs
of coalescence, which suggests a low energy barrier for the rotation in question (Figure 4. 4).
Complex 2b, also shows one set of carbazolide resonances in its
1
H-NMR spectrum, consistent
with an orthogonal conformation for the ligands. Attempts to recrystallize complexes 2a and 2b in
air gave bright yellow-crystals that luminesce green. X-Ray diffraction analysis revealed structures
containing two CAAC-Cu moieties, bridged by a 𝐶𝑂
3
2−
, which in turn is hydrogen-bonded to two
the NH groups on both carbazole units (Figure 4. 3). This surprising result signifies the potential
for application of our CAAC-Cu-amide complexes in small molecule activation, namely CO2 and
H2O. The applicability of this chemistry under ambient conditions, as opposed to 1 atm of CO2, is
especially interesting.
35
Viewed in this light, and with the donor/acceptor character of the ligands
in mind, one can consider these donor-Cu-acceptor molecules as a special class of frustrated Lewis
pairs, π-type FLP’s with: the filled N orbital comprising the Lewis Base, the empty π-orbital on
the CAAC comprising the Lewis acid, and the Lewis pair overall frustrated by the metal (at 3.7 Å
separation). The orthogonal ligand conformation, which weakens the π-FLP interaction, appears
to further activate these complexes for small molecule activation.

Figure 4. 3. Crystal structures of the decomposition products of complex 2a (left) and 2b (middle) in
ambient air, showing a bridging 𝐶𝑂
3
2−
moiety and two 1,8-dimethyl-9-H-carbazole moieties within
hydrogen-bonding distances from two of the carboxylate oxygens. On the right is a general representation
of both structures.
95

Figure 4. 4. Variable temperature (VT)
1
H NMR spectra of 1a in CDCl 3.
Table 4. 1. Crystallographic information for complexes 1a – 1d, 2a, 2b, 3, 4, and 5.
Complex C–Cu (Å) Cu–N (Å) C–N (Å) dha° C-M-N° Space
group
Z per
unit cell
1a 1.881 (2) 1.859 (2) 3.735 (3) 1.3 (2) 173.51 (9) P21/c 1
1b 2.069 (7) 2.057 (7) 4.12 (1) 0.1 (8) 174.6 (3) P21 1
1c 1.885 (1) 1.862 (1) 3.743 (2) 7.8 (2) 174.34 (7) P21/n 1
1d 1.879 (1) 1.864 (1) 3.739 (1) 16.6 (1) 174.58 (5) P21/c 2
2a 1.900 (1) 1.909 (2) 3.802 (3) 60.2 (2) 172.86 (9) P212121 1
2b 1.907 (1) 1.897 (1) 3.792 (2) 83.2 (2) 170.68 (6) P21/c 1
3 1.895 (2) 1.875 (2) 3.769 (3) 2.0 (2) 177.03 (8) P212121 2
4 1.882 (2) 1.856 (2) 3.734 (3) 6.2 (2) 174.88 (8) P65 1
5 1.882 (5) 1.854 (4) 3.736 (7) 8.7 (5) 179 (2) P212121 2
Cu host 1.896 (4) 1.917 (4) 3.813 (5) 21.6 (5) 178.5 (2) P21 1
C–M and M–N refer to Carbene–metal and metal–carbazole bond lengths respectively. dha
refers to the carbene-carbazole dihedral angle.

4.2.3. Electrochemistry
The electrochemical properties of the Cu complexes 1a, 3 – 5, precursors (CAAC
Men
-CuCl and the
protonated amines) and potassium carbazolide (KCz) were examined (Table 4. 2). The Cu
complexes undergo irreversible oxidation at potentials that vary over a 1 V range, depending on
the donor strength of the amide ligand. Relative to their parent amines, the oxidation potentials of
the Cu-amides are decreased by 0.6–0.7 V. All potentials fall well below the Cu
+/2+
potential of
96

CAAC
Men
-CuCl. The oxidation potential of 1a is anodically shifted by 0.73 V compared to that
of KCz, consistent with metalation. Reduction potentials are quasi-reversible with values that are
unchanged from the parent CAAC
Men
–CuCl. The data shows that the redox potentials are
independently controlled by the ligands: oxidation is primarily at the amide, reduction at the π-
accepting carbene.
Table 4. 2. Redox potentials of complexes 1a, 3 - 5 as well as KCz, the parent amines, CAAC
Men
-
CuCl precursor, and Cu host.
Complex Eox (V) Ered (V) ΔEredox (V) EHOMO (eV) ELUMO (eV)
1a 0.239 (ir) -2.84 (q) 3.08 -5.06 -1.48
3
a
0.69 (ir) -2.79 (q) 3.48 -5.58 -1.53
4 -0.2 (ir) -2.85 (q) 2.65 -4.56 -1.47
5 -0.29 (ir) -2.89 (q) 2.60 -4.46 -1.42
KCz -0.49 (ir) --- --- --- ---
Cz
a
0.84 (ir) --- --- --- ---
CzCN
a
1.38 --- --- --- ---
CzOMe 0.44 --- --- --- ---
HNPh2 0.45 --- --- --- ---
CAAC
Men
-CuCl > 1.5
b
-2.85 --- --- ---
Cu host 0.9 -2.7 3.6 -5.82 -1.64
Potentials versus ferrocenium/ferrocene obtained in DMF with 0.1 M TBAPF6.
a
Potentials obtained in acetonitrile.
b
Oxidation not observed in the DMF solvent window.
ir = irreversibie, q = quasi-reversible. The redox peaks were taken from differential pulse
voltammograms (DPV) and converted to HOMO/LUMO energies using the equations in
ref.
36

4.2.4. DFT calculations
The redox non-innocent nature of the ligands is also captured by Density Functional Theory
(DFT, B3LYP/LACVP**). As shown in Figure 4. 5, the HOMO is principally amide-based, with
significant electron density residing on the filled p-orbital of Namide. The LUMO is localized
largely on the unfilled p-orbital of Ccarbene. The nature of the frontier molecular orbitals as well as
the coplanar orientation of the ligands allow for a simplified depiction of the frontier orbitals of
the complexes (Figure 4. 5). This representation is a donor-bridge-acceptor linear system, where
the metal d-orbitals act as a weak electronic bridge between the parallel donor (Namide 2pz) and
97

acceptor (Ccarbene 2pz) orbitals, thereby illustrating the potential for long-range π-interaction.
37,
38
The ground state of these complexes is marked by a large permanent dipole, μg ~11.3 D, in
close agreement with the report by Föller et al. for the isoelectronic Au complex.
39
The
optimized geometries of 2a and 2b reproduce the crystallographic structures.

Figure 4. 5. (Top) HOMO (solid) and LUMO (mesh) surfaces of complex 1a. (Bottom) Simplified picture
of the HOMO and LUMO of this complex.
Table 4. 3. Frontier MO surfaces and energies as well as ground state dipoles (μ GS) of complexes 1a – 1d,
2a, 2b, 3 – 5, and Cu host computed at the B3LYP/LACVP** level.
HOMO (eV) LUMO (eV) μGS (D)
1a

11.80
-4.16 -1.50
1b

11.76
-4.13 -1.52
1c

11.62
-4.16 -1.58
98

1d

11.45
-4.13 -1.52
2a

---
-4.13 -1.50
2b

13.80
-4.05 -1.60
3

12.64
-5.14 -1.96
4

10.43
-3.75 -1.50
5

10.43
-3.84 -1.36
Cu
host

10.29
-6.01 -1.55
99

4.2.5. Photophysical Characterization
Absorption spectra of complexes 1a, 1b, 2b, and 3–5 in THF (Figure 4. 6) show high-energy bands
(λ < 350 nm) corresponding to π-π* transitions of the ligands. Broad, low energy bands apparent
in these complexes are assigned to singlet interligand charge transfer (
1
ICT) from the electron rich
amide to the electron accepting carbene. The onsets of the ICT bands for 1a, 3, 4 and 5 fall in the
order expected based on the oxidation potentials of their amide ligands (Figure 4. 6 a, inset). A
striking feature of the ICT transitions in these complexes is their high extinction coefficients
(εICT > 10
3
M
-1c
m
-1
), which is surprising considering the ~3.7 Å separation between the HOMO
and the LUMO. These values are greater than what is typically observed for MLCT transitions in
organo-copper complexes (10
2
M
-1c
m
-1
). We tentatively attribute the strong allowedness of the ICT
transitions in these complexes to the small but non-negligible contribution of the Cu d-orbitals
acting as an effective electronic bridge between the donor and the acceptor components. In
addition, the coplanarity of the ligands contributes further to the oscillator strength of the ICT
absorption, as seen in Figure 4. 6 c, where the orthogonal conformer 2b shows a significantly
reduced εICT relative to 2a: 1300 and 4000 M
-1
cm
-1
, respectively.
300 350 400 450 500
0
1
2

 (x10
4
M
-1
.cm
-1
)
Wavelength (nm)
1a
1b
1c
1d
a)
300 350 400 450 500 550
0
1
2
b)
-0.4 0.0 0.4 0.8
2.6
2.8
3.0
3.2
3
1a
4
5


CT
(eV)
E
ox
(V)

 (x 10
4
M
-1
.cm
-1
)
Wavelength (nm)
1a
3 (x 0.25)
4
5
300 350 400 450 500
0
1
2

 (x 10
4
M
-1
. cm
-1
)
Wavelength (nm)
1b
2b
c)

Figure 4. 6. Extinction coefficients of 1a – 1d (a) 1a, 3, 4, 5 (b), and 1b and 2b (c) in THF. The inset in the
a) shows a linear relation between the energy of the CT absorption band in methylcyclohexane, (MeCy)
and the oxidation potential of the complexes.
A characteristic feature of the ICT band in these complexes is pronounced hypsochromic
solvatochromism in solvents of increasing polarity (Figure 4. 7 a). The absorption onset of the
100

ICT band undergoes a blue-shift of 43 nm (2400 cm
-1
) in complex 1a, from MeCy to CH2Cl2, and
62 nm (2600 cm
-1
) in complex 5 from MeCy to AcN. The influence of the solvent polarity on the
energy of the ICT band reflects a strong change in the electronic dipole moment upon excitation.
The direction of the trend, i.e., hypsochromic solvatochromism, is a consequence of the ground
state dipole being much larger in magnitude and opposite in orientation relative to its excited state
counterpart.
40
Similar hypsochromic shifts of the ICT absorption band are observed upon freezing
the solvent matrix, and are pronounced in MeCy relative to 2-MeTHF (Figure 4. 7 b). The blue-
shift in 2-MeTHF at 77 K is likely due to the solvent dipoles being frozen around the large solute
dipole, stabilizing the ground state (relaxing the potential energy surface) and destabilizing the
ICT excited state. The blue-shift recorded in MeCy at 77K, where the solubility of 1a is reduced,
can be brought about by long-range dipole-dipole interactions between the solute molecules.
41
In
both instances, the hypsochromic shift is reversed smoothly when the solvent glass is thawed
(Figure 4. 7 c). In 2-MeTHF, the hypshchromic trend observed is concentration-independent.
Unfortunately, solubility limitations of the complex in MeCy thwarted similar studies over
meaningful concentration ranges.
101

0
1
350 400 450 500 550
350 400 450 500 550
0
1

MeCy
Toluene
2-MeTHF
CH
2
Cl
2
a)

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
Toluene
2-MeTHF
CH
2
Cl
2
AcN

0
1
300 350 400 450
300 350 400 450
0
1

Normalized Intensity (a. u.)
RT
77K
MeCy
b)
2-MeTHF

Wavelength (nm)
RT
77K

350 400 450
0
1
2-MeTHF

Normalized Intensity (a. u.)
Wavelength (nm)
298 K, C1
298 K, C2
298 K, C3
195 K, C1
195 K, C2
125 K, C3
77 K, C1
77 K, C2
77K, C3
*
c)

Figure 4. 7. a) Negative solvatochromism of the ICT absorption band observed in complexes 1a (top) and
5 (bottom) (2-MeTHF: 2-methyltetrahydrofuran, AcN: acetonitrile). b) Absorption spectra of complex 1a
at room temperature and 77K, showing a blue-shift in the ICT band at low T. c) Absorption spectra of
solutions of 1a in 2-MeTHF at three concentrations (C 1 = 2.2 x 10
-4
M, C 2 = 4.2 x 10
-5
M, C 3 = 2.2 x 10
-5

M), collected at 298 K, 195 K, and 77K. 195K was attained in a medium of dry-ice/acetone. Hence, the
spectra are normalized to the peak at 370 nm (marked with an asterisk), beyond the absorption cut-off of
acetone.

Powdered samples of the carbazolide complexes 1a-d, and 3 are poorly emissive, whereas 2b, 4,
and 5 exhibit stronger luminescence in their microcrystalline forms than in solution (Figure 4. 8).
Aggregation of the planar carbazolide moieties likely quenches luminescence in the solid state,
giving rise to interesting emission spectra. The two narrow, vibrationally structured peaks
observed are assigned to a carbazolide-localized state,
3
Cz and a lower-energy aggregate.
Processing into neat thin films breaks up the aggregation, resulting in broad and featureless
emission likely ICT in origin. The orthogonal ligand conformation in 2b also disrupts the
aggregation, resulting in similar ICT emission. Lastly, 5 bearing an amide with higher degrees of
freedom exhibits similar ICT emission at lower energy than the Cz complexes owing to the more
102

electron-rich NPh2. The red-shift recorded in the ICT emission of all three complexes upon cooling
is one of the two hallmarks of TADF, which is studied further in a later section of this chapter.
400 450 500 550 600 650 700
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
SS, RT
SS, 77K
neat film, RT
neat film, 77K
a)
400 500 600 700
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
(1a), RT
(1a), 77K
(2b), RT
(2b), 77K
b)

300 400 500 600 700
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
Em, RT
Em, 77K
Ex, RT
Ex, 77K
2
nd
harmonic
c)

Figure 4. 8. a) RT and 77K emission spectra of microcrystalline powders and a neat thin film of 1a
compared. b) RT and 77K emission spectra of the coplanar 1a and the orthogonal 2b compared. c)
Excitation (dashed lines) and emission (solid lines) spectra of microcrystalline powders of complex 5 at RT
and 77K.
Solution samples of the complexes studied show broad, unstructured ICT-based emission with kr
> 10
5
s
-1
and ΦPL increasing as steric encumbrance on the carbene is increased in the series 1d <
1c < 1b < 1a ( ΦPL = 1.0, 0.7, 0.6, 0.1 respectively, Table 4. 4). This makes complex 1a the first
100% efficient Cu(I) emitter in fluid matrices. Since the radiative rate constants of 1a – d are
similar (kr = 2.0–4.3 x 10
5
s
-1
), the principal effect of increasing steric bulk is to decrease the rates
of non-radiative decay. However, unlike the strong solvatochromic trends observed in absorption,
emission spectra of 1a and 5 do not show clear trends with increasing solvent polarity (Figure 4.
9). This is likely due to the dipole of the ICT excited state (μICT) being weaker than the permanent
dipole of the ground state (μ GS), as shown in the TDDFT section of this chapter. At room
temperature, emission spectra of 1b and 2b are similar, albeit with 2b displaying a minor red-shift
in its λmax owing to its more electron-rich Me2Cz. Yet, both ΦPL and kr are reduced in the latter
complex (PL = 0.12 and kr = 1.1 x 10
5
s
-1
for 2b compared to 0.68 and 3 x 10
5
s
-1
for 1b), mirroring
the reduction in the ICT oscillator strength observed in absorption. Hence, a coplanar ligand
conformation is required for maintaining the Ir-like radiative rates observed in these complexes.
103

400 500 600 700
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
a)
500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
AcN
b)
400 500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
(1b), RT
(1b), 77K
(2b), RT
(2b), 77K
c)

Figure 4. 9. Emission spectra of 1a (a) and 5 (b) in solvents of increasing polarity. c) RT and 77K emission
spectra of 1b and 2b compared.
Complexes 4 and 5 show emission that is red-shifted relative to complex 1a, with radiative rates
comparable to that of 1a (Figure 4. 10). The sparingly soluble complex 3 shows narrow, structured
emission centered at 426 nm in MeCy, toluene, and 2-MeTHF solutions, with a much lower
radiative rate constant (kr = 1.5 x 10
3
s
-1
) compared to the other complexes, features we attribute
to
3
Cz-based emission, vide infra (Table 4. 4, Figure 4. 13 a). In addition, complex 3 displays a low
energy ( ~ 600 nm) concentration-dependent emission band in solution, characterized by a rise
time in its PL decay traces (Figure 4. 11), which is consistent with a diffusional process to form a
luminescent excimer.
To eliminate the complications of aggregation and excimer formation in the photophysical studies,
we examined doped thin films of the complexes (1 wt% in polystyrene, PS), where excimer
formation is suppressed. At room temperature, samples in the rigid matrix exhibit a blue-shift in
their emission relative to spectra recorded in solution and suppressed rates of non-radiative decay
(Figure 4. 10 d). Complexes 1a and 4 display broad emission with near unit luminescence
efficiency in PS (PL = 1.0), whereas complex 5 is less efficient (ΦPL = 0.78) (Table 4. 4). Thin
films of complex 3 give narrow, structured emission at room temperature with a biexponential
decay lifetime of 240 s and 1.3 ms. The long lifetimes are consistent with emission from a state
with substantial triplet ligand character, due to the highly destabilized ICT. Notably, the high ΦPL
104

and kr values for complexes 1a, 4 and 5 in solution and thin films are comparable to phosphors
containing heavy metals, such as Ir and Pt.
0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
RT

1a
1b
1c
1d
77K
a)

Normalized Intensity (a. u.)
Wavelength (nm)
1a
1b
1c
1d

0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
a)

Normalized Intensity (a. u.)
1a
3
4
5
*
*excimer
b)

Wavelength (nm)
1a
3
4
5
RT
77 K
0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
RT

Normalized Intensity (a. u.)
1a
3
4
5
77 K
*
* excimer
c)

Wavelength (nm)
1a
3
4
5
KCz

0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0 77K

Normalized Intensity (a. u.)
1a
3
4
5
RT
d)

Wavelength (nm)
1a
3
4
5

Figure 4. 10. RT and 77K emission spectra of 1a – 1d in 2-MeTHF (a) and of 1a, 3 – 5 in MeCy (b),
2-MeTHF (c), and 1 wt% PS films (d).
The photoluminescent properties are dramatically altered on cooling to 77 K. A vibronically
structured, long-lived emission ( = ms – seconds, kr < 10
3
s
-1
) is observed for 1a-d, 3, and 4 in
frozen glasses of 2-MeTHF and MeCy (Figure 4. 10, Table 4. 4). The emission at 77 K is assigned
105

to a
3
Cz, as the phosphorescence spectrum of KCz in frozen 2-MeTHF replicates the same profile
as 1a (Figure 4. 10 c). The rigidochromic blue-shift in emission observed in non-polar MeCy at
77 K is consistent with the hypsochromic shift observed in ICT absorption at that temperature,
since destabilizing the ICT transition leaves
3
Cz as the lowest-lying emissive state. Emission from
5 is broad and featureless in frozen glassy matrices, with long excited state lifetimes ( = 560 s
in 2-MeTHF and MeCy), consistent with the triplet of diphenylamide (
3
NPh2) being significantly
higher in energy than
3
Cz.
0.0
0.5
1.0
400 500 600 700 800
400 500 600 700 800
0.0
0.5
1.0

Normalized Intensity (a. u.)
MeCy
conc
5x dilute
0 40 80
0
1
440 nm
600 nm
Normalized counts (a. u.)
time (s)

Wavelength (nm)
Toluene
conc
2x dilute

Figure 4. 11. Emission spectra of complex 3 in MeCy (top) and toluene (bottom) at two different
concentrations. The inset (top, right corner) shows the PL decay traces recorded at 440 nm and 600 nm in
MeCy. While the decay at 440 nm is monoexponential, at 600 nm it is biexponential and shows a rise time,
consistent with excimer formation and emission.
The luminescent properties of 1a and 5 in thin PS films were examined as function of temperature
to probe the ICT manifold whilst avoiding complications of the dominant
3
Cz excited state in
frozen solvents (Figure 4. 13 b). Temperature-dependent emission of thin PS films of both
complexes show broad, featureless ICT spectra at all temperatures between 10 K and 300 K
106

(Figure 4. 12 a) and excited state lifetimes that increase with decreasing temperatures (Figure 4. 12
b). Fits of the temperature-dependent PL decay curve to the modified Boltzmann equation above
give ∆𝐸 𝐼𝐶𝑇 1
− 𝐼𝐶𝑇 3 = 0.054 eV (435 cm
-1
) for 1a and 0.071 eV (573 cm
-1
) for 5. The derived decay
rate of
1
ICT is found to be lower in 1a (kr = 1.8 x 10
6
s-
1
, 𝜏 𝐼𝐶𝑇 1 = 537 ns) than in 5 (kr = 4.9 x 10
6

s
-1
, 𝜏 𝐼𝐶𝑇 1 = 206 ns). This result is consistent with 1a having a smaller ∆𝐸 𝐼𝐶𝑇 1
− 𝐼𝐶𝑇 3 than 5. The
derived radiative lifetimes of
3
ICT and
1c
T are comparable to those reported by Brase, et al. and
Yersin, et al. in the fastest Cu(I) TADF emitters reported to date.
18, 42
Lastly, we note that
preparation of 1 wt% PS films of 1a sometimes gave emission spectra with mixed
3
Cz/
3
ICT
emission at low temperatures. Selectively exciting the ICT manifold at 400 nm eliminated the
3
Cz
contribution, allowing for straight-forward TADF studies.
0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
5

9.6 K
60 K
100 K
140 K
180 K
220 K
260 K
298 K
a)
1a

Normalized Intensity (a. u.)
Wavelength (nm)
10 K
50 K
90 K
130 K
170 K
210 K
250 K
298 K
310 K

0
25
50
75
50 100 150 200 250 300
50 100 150 200 250 300
0
100
200
5

Lifetime (s)
1a
b)
E
S-T
= 0.071 eV

T1
= 250 s

S1
= 210ns
E
S-T
= 0.054 eV

T1
= 64 s

S1
= 540 ns

Temperature (K)

Figure 4. 12. Temperature-dependent PL profiles (a) and decays (b) for 1 wt% PS films of 1a and 5.

107

Table 4. 4. Photophysical properties of complexes 1a – 1d, 2b, 3, 4, 5 and Cu host in various media.
Complex λmax, RT
(nm)
ΦRT τRT
(μs)
kr, RT
(10
5
s
-1
)
knr, RT
(10
5
s
-1
)
λmax, 77K
(nm)
τ77K
(μs)
1a MeCy 468; 486 0.92 2.3 4.0 0.35 430 6700
Toluene 488 1.0 2.5 4.0 < 0.04 --- ---
2-MeTHF 492 1.0 2.5 3.9 < 0.08 430 7300
CH 2Cl 2 482 0.40 1.6 2.5 3.8 --- ---
PS film 474 1.0 2.8 3.5 < 0.04 480 61
Neat film 474 0.65 1.3 4.9 2.6 482 70.4
1b 2-MeTHF 510 0.68 2.3 3.0 1.7 430 3000 (430 nm);
48 (550 nm)
PS film 480 0.99 4.9 2.0 0.02 490 48
1c 2-MeTHF 500 0.56 1.8 3.1 2.4 430 7000
PS film 462 0.99 3.4 2.9 0.03 464 72
1d 2-MeTHF 510 0.11 0.54 2.0 1.6 430 5000
PS film 468 0.83 3.1 2.7 0.55 464 75
2b SS 490 0.61 2 (0.3);
11.2 (0.7)
0.72* 0.46* 508 34 (at 550 nm)
2-MeTHF 510 0.12 0.86 (0.79);
2.3 (0.21)
1.0 7.6 430 2400 (430 nm);
38 (500 nm)
3 MeCy 482; 586
a
0.19 450 nm: 5.9
600 nm:
a
2.3
(-7%); 20.5
(107%)
--- --- 422 5300
2-MeTHF 428; 590
a
0.11 450 nm: 8.3
600 nm:
a
0.3
(-2%); 8.0
(102%)
--- --- 422 120000
PS film 426 0.82 240 (0.7);
1300 (0.3)
0.015
*
0.003 424 6900
4 MeCy 542 0.62 1.12 5.6 3.4 472 seconds
2-MeTHF 558 0.25 0.87 8.9 27 740 seconds
PS 518 1.0 2.3 4.3 < 0.04 490 550
5 MeCy 556 0.55 2.38 2.3 1.9 530 193
Toluene 468 0.41 1.43 2.9 4.1 --- ---
2-MeTHF 580 0.16 0.87 1.8 97 500 215
CH2Cl2 568 0.24 0.88 2.7 8.6 --- ---
AcN 584 0.11 0.47 2.3 19 --- ---
SS 496 1.0 4.1 2.4 < 0.02 518 87
PS film 532 0.78 2.6 3.0 0.85 536 264
Cu
host
MeCy 440 0.17 3.6 0.47 2.3 422 14
SS 424 0.92 15 0.39 0.05 418 14
*Calculated from the weighted averages of both contributions.
a
excimeric peak.

108

a) b)
Figure 4. 13. a) State diagram depicting
I,3
CT/
3
LE ordering in the reported complexes. The relative energies
of the states are based on emission spectra. b) Jablonski diagram depicting the different processes operating
in various media at room temperature and 77K.
4.2.6. TDDFT and molecular dynamics
Time dependent DFT (TDDFT) was used to model the main electronic transitions of the excited
states in these complexes. It has been found that using common hybrid functionals, such as B3LYP
and PBE0 severely underestimate the energies of charge transfer states,
43-45
whereas long-range
corrected functionals, such as CAM-B3LYP, PBE and B97xD, better match the observed
energies.
46-49
In this work, TDDFT calculations performed using the CAM-B3LYP functional were
found to offer good agreement with experimental values. For all CAAC-Cu-amide complexes, the
first two singlet excited states can be characterized as interligand charge-transfer, from the amide
to the carbene ligand, i.e.
1
ICT. These states are characterized by high oscillator strengths (f > 0.1).
In agreement with the recent reports from Föller et al.
39
and Tafett et al.
50
,
3
ICT, which lies within
0.25 eV of
1
ICT and shares the same orbital parentage (Table 4. 5), is the lowest energy triplet state
in all complexes except 3. Additionally, we note that
3
Cz is only 0.03 – 0.1 eV higher in energy
than
3
ICT in all the carbazolide-based complexes except for 3, where it is the lowest triplet state.
In 5, the triplet state localized on the diphenylamide,
3
LE (i.e.
3
NPh2), is destabilized relative to
the lower energy
3
ICT by 0.5 eV. Notably, 2b is calculated to have a vanishingly-small ∆𝐸 1
𝐼𝐶𝑇 −
3
𝐼𝐶𝑇
relative to its coplanar analogue 2a, in agreement with the reduced HOMO/LUMO overlap ensured
by the orthogonal ligand conformation. However, this does not translate in increased k r, which
109

further highlights the importance of the coplanar ligand conformation in facilitating efficient
TADF in these complexes.
Table 4. 5. Calculated singlet and triplet excited state energies for the complexes in this chapter obtained
through TDDFT performed at the CAM-B3LYP/LACVP** level.

E (eV)
fS
1

T
1
T
2

T3
S
1
S
2

S3 ∆𝐸 1
𝐼𝐶𝑇 −
3
𝐼𝐶𝑇 ∆𝐸 3
𝐶𝑧 −
3
𝐼𝐶𝑇
1a
2.99
(CT)
3.02
(LE)
3.23
(LE)
3.25
(CT)
3.89
(CT)
4.11
(LE)
0.26 0.03 0.123
μES (D) 4.25 11.27
1b
2.99
(CT)
3.12
(LE)
3.19
(CT)
3.23
(CT)
3.66
(CT)
4.18
(CT)
0.24 0.13 0.112
μES (D) 5.47 11.18
1c
2.91
(CT)
3.02
(LE)
3.23
(LE)
3.17
(CT)
3.87
(CT)
4.04
(CT)
0.26 0.11 0.128
μES (D) 5.11 11.62
1d
2.97
(CT)
3.02
(LE)
3.22
(LE)
3.23
(CT)
3.90
(CT)
4.20
(CT)
0.26 0.05 0.125
μES (D) 4.50 11.45
2b
2.98
(CT)
3.04
(LE)
3.09
(CT)
3.06
(CT)
3.57
(CT)
4.06
(CT)
0.08 0.06 0.005
μES (D) 2.46 13.8
3
2.91
(LE)
3.14
(LE)
3.38
(LE)
3.79
(CT)
4.06
(CT)
4.11
(LE)
--- < -0.47 0.144
μES (D) 20.37
4
2.67
(CT)
2.80
(LE)
2.95
(LE)
2.94
(CT)
3.70
(LE)
4.02
(CT)
0.27 0.13 0.129
μES (D) 0.99 12.64
5
2.68
(CT)
3.04
(LE)
3.33
(CT)
2.96
(CT)
3.75
(CT)
4.09
(CT)
0.28 0.36 0.102
μES (D) 6.66 10.43
Cu host
3.27
(CT)
3.38*
(LE)
3.48
(LE)
3.78
(CT)
4.75
(CT)
4.82
(CT)
0.51 0.11 0.011
μES (D) 6.80 10.29

110

We further modeled the effects of solvation on the excited states of complex 1a at 77 K and 300
K using a multi-scale hybrid approach that employed classical Molecular Dynamics (MD)
simulations in conjunction with TDDFT (ωPBEh/6-31G**) with  = 0.263 bohr
-1
tuned to satisfy
the global density-dependent (GDD) criterion to get a balanced description of CT and LE states
51,
52
as detailed below. A simulation cell was built with 128 2-MeTHF solvent molecules around the
copper complex. To approximate the response of the solvent molecules to the
3
ICT excited state
at 300 K, the atomic charges of the forcefield for the complex were replaced with the electrostatic
potential-fitted (esp) point charges of the
3
ICT state derived from Unrestricted DFT. A 200 ns NPT
MD simulation (P = 1 atm, T = 300 K) was performed using the OPLS2005 forcefield.
53
. 50
snapshots were extracted from the last 100 ns of the MD run, and TDDFT (ωPBEh/6-31G**)
calculations were performed on each snapshot with atoms of all the solvent molecules replaced by
the corresponding esp-fitted point charges to serve as a polarizing influence on the complex. It was
thus found that the lowest triplet state in all cases was
3
ICT due to stabilization by the solvent
molecules. Next, to study the effect of solvation at 77 K, a similar procedure was followed where
the atomic charges of the OPLS2005 forcefield were replaced by the ground state esp-fitted
charges computed at the ωPBEh/6-31G** level. A series of 10 ns NVT runs were performed on
the 300 K equilibrated cell in steps of decreasing temperatures (300–200–100–77 K) followed by
a 200 ns NPT simulation (P = 1 atm, T = 77 K). The simulation resulted in a frozen rigid
equilibrated cell which was used to perform a single point TDDFT calculation, as done in the
previous case. Under these conditions, it was found that the
3
Cz state becomes the lowest lying
triplet in accordance with the experimental observation of
3
Cz emission in 2-Me-THF at 77 K.
The destabilization of the
3
ICT state can be attributed to its corresponding dipole (4.25 D) which
is opposite in direction to the larger ground state dipole (𝜇 𝐺𝑆
=11.8 𝐷 ). In contrast, the molecular
111

dipole moment of the
3
Cz is similar to that of ground state (𝜇 3
𝐶𝑧
=11.27 𝐷 ). Hence, the solvent
molecules in a frozen matrix are expected to be arranged in such a configuration so as to stabilize
the large ground state dipole, whereas the dipole of the
3
CT state, being in the opposing direction,
would be destabilized (Figure 4. 14). Negative solvatochromic effects observed in absorption can
be explained using a similar rationale.

Figure 4. 14. Top: Solvation effects operating in a polar medium, 2-MeTHF, on the ground and ICT
excited state dipoles (μ-GS and μ-CT respectively) at room temperature (fluid medium) and 77K (frozen
glass). Bottom: Solute-solute interactions operating in non-polar fluid and frozen media that can explain
the observed destabilization in of the ICT state at low temperatures.
4.2.7. OLED characterization
OLED devices incorporating 1a as an emitter were fabricated by vapor deposition, following the
general architecture outlined in Figure 4. 15 a, and only changing the emissive layer (EML) to
screen different wide bandgap host materials (Figure 4. 16 c). In addition to hosts commonly used
in blue OLEDs (mCBP, mCP, and UGH3), a 2-coordinate CAAC-Cu(C6F5) complex (Cu host),
with 𝐸 𝑇 1
higher than both carbazolyl-based hosts, was examined. All devices exhibit blue EL,
112

similar to the PL for 1a in a PS film. This is in contrast to the green OLEDs reported by Di et al.,
which employed the analogous 1b as the emitter and poly(9-vinylcarbazole), PVK, as the host.
25

Indeed, we observed green PL from solution-processed thin films of 1a doped into PVK at 20
wt%. The red-shifted luminescence observed in PVK matrices is likely due to exciplex formation
with the weakly coordinating PVK host. In our case, devices prepared with 1a doped into Cu host
at 20 vol% give the highest EQEmax = 9.5% at 0.2 mA/cm
2
, marking the first Cu(I) OLED doped
into a Cu(I) host. However, the Cu host devices exhibit steep, irreversible roll-off at higher current
densities (J), which we postulate is a result of host instability. OLEDs with mCBP, mCP, and
UGH3 as hosts exhibit less severe EQE roll-off at higher J, with the UGH3 device registering
EQEmax = 9.1%. These results corroborate our hypothesis on the instability of the Cu host to
higher charge densities. However, devices employing UGH3 showed increased crystallinity within
an hour of fabrication, which severely undermined their performance. While the low EQE max of
the mCBP device can be explained by a mismatch in the host/guest triplet energies and
HOMO/LUMO energies, the case with the mCP devices is unexpected. 40 nm thick films of 1a
doped into the various hosts (at 20 vol%) show similar trends with ΦPL in Cu host > mCBP > mCP
(1.0, 0.6, 0.3 respectively, Figure 4. 16 a). Hence it appears that mCP is simply not well suited as
a host for 1a, likely due to poor solvation in the solid state. In fact, the persistent high-energy
feature in PL spectra of mCP films shows incomplete energy transfer from the host to the dopant.
We expect that more stable, high triplet energy host materials for OLEDs employing emitter 1a
should positively impact both efficiency and device stability.
113

10
0
10
1
10
2
1
10
Cu host
mCBP
mCP
UGH3

EQE (%)
Current density (mA/cm
2
)
b)

0 2 4 6 8 10 12
10
-1
10
0
10
1
10
2
10
3
10
4

Luminance (cd/m
2
)
Voltage (V)
mCBP
mCP
UGH3
Cu host
c)
0 2 4 6 8 10 12
0
100
200
300
400
500
600

Current Density (mA/cm
2
)
Voltage (V)
mCBP
mCP
UGH3
Cu host
d)

Figure 4. 15. a) OLED device architecture for 1a-based OLEDs after optimization of EML and ETL
thickness. b) EQE traces of devices employing different hosts; the inset is a picture of a Cu host device.
c) L-V and d) J-V traces of the Cu-host device.
400 450 500 550 600 650
0.0
0.5
1.0 mCBP
mCP
Cu host
UGH3
Normalized Intensity (a.u.)
Wavelength (nm)
a)
400 450 500 550 600 650
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
20 vol% in Cu host
20 vol% in mCBP
20 vol% in mCP

PL, Cu host
= 100%

PL, mCBP
= 60%

PL, mCP
= 34%
b)

Figure 4. 16. a) EL and b) PL spectra of 1a in various hosts. c) Molecular structures of the different hosts
used in OLEDs employing 1a.
Lastly, we highlight the photophysical properties of the high-energy emitter: the 2-coordinate Cu
host. DFT calculations reveal a largely Cu-based HOMO, courtesy of the poor donor C6F5 moiety,
while the LUMO is carbene-localized as is the case with the other complexes. As such, the lowest
energy transition in this complex is more aptly described as MLCT in nature. This transition, with
114

absorption λmax = 350 nm, results in highly-efficient phosphorescence in the solid state (ΦPL =
0.92, τRT = 15 μs). In addition to the reduced radiative rates compared to the amide complexes, the
phosphorescence is largely quenched non-radiatively in MeCy solutions (ΦPL = 0.17, τRT = 3.6 μs).
Emission profiles and lifetimes are temperature-independent, indicating that TADF is not operant
within the MLCT manifold of this complex. These results are further proof that MLCT transitions
in 2-coordinate Cu(I) complexes give overall higher knr and lower kr compared to 2-coordinate
Cu(I) complexes with redox active, coplanar ligands and ICT transitions.
250 300 350 400
0.0
2.5
5.0
7.5
10.0

 (x 10
4
M
-1
.cm
-1
)
Wavelength (nm)
Cu host, THF
a)

0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
microcrystalline powder

Normalized Intensity (a. u.)
Cu host, RT
Cu host, 77K
1a in Cu host, 20 wt%
b)

Wavelength (nm)
Cu host, RT
Cu host, 77K
1a, RT
1a, 77K
MeCy

Figure 4. 17. a) Extinction coefficient of Cu host in THF. b) top: emission of microcrystalline powder of
Cu host at RT and 77K compared with the emission of 1a doped into Cu host (20 wt%); bottom: RT and
77K emission spectra of Cu host and 1a in MeCy.

4.3. Conclusion

We have prepared a series of 2-coordinate CAAC-Cu-amide complexes with emission tunable
across the visible spectrum, high ΦPL in non-rigid media, and kr > 10
5
s
-1
. Emission stems from a
strongly-allowed amide to carbene ICT transition, with the coplanar ligand conformation and
115

coupling through the metal d-orbitals ensuring strong 𝜀 𝐼𝐶𝑇 and resultant kr. In contrast, a near-
orthogonal arrangement of ligands leads to low 𝜀 𝐼𝐶𝑇 and decrease in kr due to poor orbital overlap.
In the Cu-carbazolide complexes, there exists a closely-lying
3
Cz-centered state that dominates
emission in frozen solvent glasses, due to the destabilization of the ICT manifold in such media.
Within the ICT manifold, we find that efficient TADF is operant with ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 < 75 meV,
among the smallest values recorded for mononuclear Cu(I) based TADF systems.
21

The 𝜏 𝐼𝐶𝑇 1 values we obtain are typical in Cu(I) TADF-based emitters, however, they are far longer
than the prompt fluorescence rates recorded in pure organic TADF systems, whose 𝜏 𝑆 1
values are
on the order of 10 ns.
54
The rate of emission at low temperature attributed to
3
ICT-based
phosphorescence is faster for 1a than for 5, due to the close-lying
3
Cz in 1a, which can enhance
SOC by mixing with
3
ICT through configuration interaction
2, 55
. Here too we note a distinction
from organic TADF systems which show much longer lived (millisecond to seconds)
phosphorescence at low temperatures. The discrepancy between organometallic and organic
TADF suggests that a spin-pure treatment of the CT manifold is inadequate in organometallic
emitters, due to stronger SOC effects in metal complexes.
56
The slow
1
ICT decay in Cu-based
systems is likely due to significant mixing with
3
ICT via SOC. The opposite is true for
3
ICT,
where strong singlet character in the nominally triplet state leads to markedly faster decay than
expected for a spin-pure triplet. The acronym TADF is an imprecise description for what is
observed in copper-based complexes of the type described here, since neither the singlet or triplet
states are spin-pure. Lifetimes as high as 0.5 μs for the
1
ICT state suggest that this state has
significant triplet contribution, thus fluorescence is not the best description for this type of
emission.
56, 57
The process observed here is thus a special subset of TADF, where both the lower
116

energy and higher energy states are highly emissive, albeit with the lower state having a much
longer radiative lifetime.
The extent of Cu involvement in the electronic properties of these complexes appears to be the
answer to the Cu-TADF conundrum we posited: the metal contribution is large enough to induce
high exo- and endothermic kISC, yet low enough to ensure small reorganization energies. This
work shows that one can indeed obtain photophysical properties from the first-row transition metal
Cu comparable to those of heavy metal containing complexes such as Ir, Pt, Ru, Os, and Re. These
properties can be realized simply by maintaining a 2-coordinate Cu center that employs redox
active ligands while ensuring their orientation is coplanar. These results therefore open the door to
the investigation of these complexes in fields where traditional heavy metal-based phosphors have
been used, from photoredox catalysis to solar fuel generation and sensing to name a few.
4.4. Experimental methods
Synthesis. All reactions were performed under nitrogen atmosphere in oven dried glassware.
Carbazole (Cz), diphenylamide (NPh2), and polystyrene beads (PS, average Mw ~ 192,000) were
purchased from Sigma-Aldrich. 3,5-dimethoxy-9-H-carbazole (CzOMe) was purchased from Ark
Pharm. 3,5-dicyano-9-H-carbazole (CzCN)(1) and 1,8-dimethyl-9-H-carbazole (MeCz)(2) were
prepared following literature procedures. CAAC-CuCl precursors and complex 1b(3) were
synthesized following published reports.(4-7) Dry, air-free methylcyclohexane (MeCy) and
2-methyltetrahydrofuran (2-MeTHF) were purchased from Sigma-Aldrich. MeCy was used
without further purification. Dichloromethane (CH2Cl2), and toluene (tol.) were purified by Glass
Contour solvent system by SG Water USA, LLC. Tetrahydrofuran (THF) and 2-MeTHF were
distilled over Na/benzophenone under N2 atmosphere for use with complexes 2b and 5 that are
ultrasensitive to trace moisture. Drisolv acetonitrile (AcN) and dimethylformamide (DMF) were
117

purchased from EMD Millipore.
1
H and
13
C NMR spectra were recorded on a Varian Mercury 400,
a Bruker Advance 300 (complex 2b), and a Varian Inova 500 (Cu host).
19
F NMR spectrum for
complex “Cu host” was recorded on a Bruker Avance 300 MHz. The chemical shifts are given in
units of ppm and referenced to the residual proton resonances of acetone ((CD3)2CO) at 2.05 ppm,
chloroform (CDCl3) at 7.26 ppm., or dichloromethane CD2Cl2 at 5.32 ppm. Elemental analyses
were performed at the University of Southern California, CA.
General procedure. CAAC-CuCl, carbazole ligand, and sodium or potassium tert-butoxide
(NaO
t
Bu or KO
t
Bu)) were dissolved in THF and stirred for overnight at RT. The resulting mixture
was filtered through Celite and the solvent was removed under reduced pressure to afford a solid.
The solid was washed copiously with cold pentane and filtered. The filtrate was dried under
vacuum for 24 h.
CAAC
Men
-CuCz (1a). The complex was prepared from CAAC
Men
-CuCl (200 mg, 0.42 mmol),
Cz (73 mg, 0.44 mmol) and NaO
t
Bu (42 mg, 0.44 mmol) and isolated as a yellowish
microcystalline solid. Yield: 228 mg (90%). Crystals suitable for an X-ray diffraction study were
grown from a saturated solution of CH2Cl2 layered with pentane. Tsublimation = 250 °C at 2 x 10
-6

Torr.
1
H NMR (400 MHz, Chloroform-d)
1
H NMR (400 MHz, Chloroform-d) δ 7.96 (d, J = 7.7
Hz, 2H), 7.68 (t, J = 8.5 Hz, 1H), 7.46 (dd, J = 16.2, 7.8 Hz, 2H), 7.05 (t, J = 7.6 Hz, 2H), 6.92 (t,
J = 7.7 Hz, 2H), 6.42 (d, J = 8.2 Hz, 2H), 3.19 – 2.86 (m, 5H), 2.41 (d, J = 13.5 Hz, 2H), 2.19 –
1.96 (m, 4H), 1.87 (d, J = 14.4 Hz, 1H), 1.51 (dd, J = 26.1, 2.5 Hz, 11H), 1.34 (t, J = 7.2 Hz, 7H),
1.16 (dd, J = 14.5, 6.1 Hz, 6H), 1.07 (t, J = 6.0 Hz, 6H), 0.93 (d, J = 6.4 Hz, 3H).
13
C NMR (101
MHz, CDCl3) δ 252.22, 150.14, 146.51, 146.03, 136.26, 129.73, 125.57, 125.49, 124.48, 123.22,
119.39, 115.26, 114.45, 77.91, 65.73, 35.99, 31.60, 30.10, 29.76, 29.59, 29.41, 28.00, 27.64, 26.44,
118

25.63, 24.43, 23.12, 22.95, 22.86, 19.90. Elemental Analysis: Anal. Cacld. for C39H51cuN2: C,
76.62%; H, 8.41%; N, 4.58%. Found: C, 76.77%; H, 9.26%; N, 4.70%.
CAAC
Et2
-CuCz (1c). The complex was prepared from CAAC
Et2
-CuCl (150 mg, 0.36 mmol),
carbazole (64 mg, 0.38 mmol) and NaO
t
Bu (37 mg, 0.38 mmol) and isolated as an off-white
microcrystalline solid. Yield: 182 mg (92%). Crystals suitable for an X-ray diffraction study were
grown by slow evaporation of a saturated solution of acetone.
1
H NMR (400 MHz, Chloroform-d)
δ 7.97 (d, J = 7.6 Hz, 2H), 7.64 (t, J = 7.8 Hz, 1H), 7.44 (d, J = 7.8 Hz, 2H), 7.09 (t, J = 8.2 Hz,
2H), 6.93 (t, J = 7.3 Hz, 2H), 6.65 (d, J = 8.1 Hz, 2H), 2.98 (hept, J = 6.8 Hz, 3H), 2.20 – 1.92 (m,
7H), 1.34 (d, J = 6.8 Hz, 7H), 1.27 – 1.18 (m, 12H).
13
C NMR (101 MHz, CDCl3) δ 252.56, 149.91,
145.80, 135.45, 129.65, 125.16, 124.29, 123.11, 119.37, 115.18, 114.37, 80.54, 62.59, 43.40,
31.50, 29.34, 26.78, 22.60, 9.77. Elemental Analysis: Anal. Cacld. for C37H43CuN2 + 2 H2O: C,
70.49%; H, 8.18%; N, 4.84%. Found: C, 70.26%; H, 7.80%; N, 4.84%.
CAAC
Me2
-CuCz (1d). The complex was prepared from CAAC
Me2
-CuCl (250 mg, 0.65 mmol),
carbazole (114 mg, 0.68 mmol) and NaO
t
Bu (66 mg, 0.68 mmol) and isolated as a beige
microcystalline solid. Crystals suitable for an X-ray diffraction study were grown by slow
evaporation of a saturated solution of Et2O. Yield: 317 mg (89%).
1
H NMR (400 MHz,
Chloroform-d) δ 7.92 (d, J = 7.4 Hz, 2H), 7.57 (t, J = 7.8 Hz, 1H), 7.37 (d, J = 7.8 Hz, 2H), 7.04
(t, J = 8.2 Hz, 2H), 6.87 (t, J = 7.7 Hz, 2H), 6.65 (d, J = 8.1 Hz, 2H), 2.88 (hept, J = 6.8 Hz, 2H),
2.10 (s, 2H), 1.60 (s, 6H), 1.40 (s, 6H), 1.28 (d, J = 6.8 Hz, 6H), 1.18 (d, J = 6.7 Hz, 6H).
13
C NMR
(101 MHz, CDCl3) δ 250.62, 149.89, 145.75, 134.80, 129.74, 125.13, 124.29, 123.14, 119.40,
115.22, 114.37, 110.55, 81.22, 77.20, 54.25, 49.84, 29.28, 28.89, 26.79, 22.67. Elemental
Analysis: Anal. Cacld. for C32H39CuN2: C, 69.72%; H, 7.08%; N, 5.08%. Found: C, 69.35%; H,
7.08%; N, 4.87%.
119

CAAC
Ad
-Cu(Me2Cz) (2b). A mixture of CAAC
Ad
-CuCl (250 mg, 0.52mmol), potassium tert-
butoxide (KO
t
Bu, 59 mg, 0.53 mmol), and 1,8-dimethyl-9-H-carbazole (102 mg, 0.52 mmol) was
dissolved in dry THF (15 mL) under an argon atmosphere. After stirring for 3 hours, the volatiles
were removed under vacuum, and the light-yellow solid was washed with ether (3x3 mL).
Extraction with toluene (20 mL) followed by washing with pentane (3x3 mL), and subsequent
evaporation of the volatiles afforded the product as a pale-yellow powder (311 mg, 93 % yield).
Crystals suitable for an X-ray diffraction study were grown from a saturated solution of THF
layered with TMS2O.
1
H NMR (CD2Cl2, 300 MHz): δ = 7.84 (d, J = 7.4 Hz, 2H), 7.42 (t, J = 7.7
Hz, 1H), 7.30 (d, J = 7.7 Hz, 2H), 7.02 (d, J = 7.4 Hz, 2H), 6.89 (t, J = 7.4 Hz, 2H), 3.50 (br d, J
= 12.2 Hz, 2H), 3.06 (sept, J = 6.7 Hz, 2H) 2.56 (s, 6H), 2.34 (s, 2H), 2.10-1.78 (m, 12H), 1.46 (s,
6H), 1.34 (d, J = 6.7, 6H), 1.24 (d, J = 6.7 Hz, 6H);
13
C{
1
H} NMR (CD2Cl2, 76 MHz): δ = 257.4,
149.7, 145.5, 136.6, 129.8, 125.8, 125.2, 124.6, 122.6, 117.2, 115.7, 79.3, 66.5, 47.5, 38.5, 37.7,
36.4, 34.6, 30.3, 29.4, 28.2, 27.5, 26.8, 24.0, 21.1. Elemental Analysis: Anal. Cacld. for
C40H47CuN2 + H2O: C, 75.14%; H, 8.04%; N, 4.38%. Found: C, 74.99%; H, 7.92%; N, 4.04%.
CAAC
Men
-Cu(CN2Cz) (3). The complex was prepared from CAAC
Men
-CuCl (130 mg,
0.27 mmol), CN2Cz (62 mg, 0.28 mmol) and NaO
t
Bu (27 mg, 0.28 mmol) and isolated as a white
solid. Crystals suitable for an X-ray diffraction study were grown by slow evaporation of a
saturated solution of toluene. Yield: 158 mg (88%).
1
H NMR (400 MHz, Chloroform-d) δ 8.25 (d,
J = 1.2 Hz, 2H), 7.71 (t, J = 7.8 Hz, 1H), 7.47 (dd, J = 12.9, 8.5 Hz, 2H), 7.33 (dd, J = 8.5, 1.7 Hz,
2H), 6.36 (d, J = 8.5 Hz, 2H), 3.03 – 2.88 (m, 4H), 2.79 (bs, 1H), 2.43 (d, J = 13.5 Hz, 1H), 2.35
(d, J = 13.2 Hz, 1H), 2.21 – 2.12 (m, 1H), 2.09 – 1.96 (m, 3H), 1.90 (d, J = 13.6 Hz, 1H), 1.61 –
1.46 (m, 10H), 1.36 (t, J = 6.8 Hz, 8H), 1.15 – 1.04 (m, 10H), 1.02 (d, J = 6.9 Hz, 3H), 0.95 (d, J
= 6.4 Hz, 3H).
13
C NMR (101 MHz, CDCl3) δ 251.25, 152.76, 146.47, 146.02, 136.29, 129.99,
120

127.94, 125.77, 125.63, 125.04, 123.95, 121.69, 115.34, 99.15, 78.53, 65.74, 52.87, 51.36, 48.43,
35.86, 32.06, 30.14, 29.72, 29.58, 29.40, 27.91, 27.65, 26.47, 25.85, 24.29, 23.03, 22.89, 22.77,
19.84. Elemental Analysis: Anal. Cacld. for C41H49CuN4 + H2O: C, 72.48%; H, 7.57%; N, 8.25%.
Found: C, 72.29%; H, 7.60%; N, 7.91%.
CAAC
Men
-Cu(OMe2Cz) (4). The complex was prepared from CAAC
Men
-CuCl (200 mg,
0.42 mmol), OMe2Cz (95 mg, 0.42 mmol) and NaO
t
Bu (44 mg, 0.46 mmol) and isolated as a
microcrystalline yellow solid; bright green luminescent. Crystals suitable for an X-ray diffraction
study were grown by slow evaporation of a saturated solution of Et2O. Yield: 305 mg (109%) due
to residual THF. The powder was further dried under vacuum for 4 days.
1
H NMR (400 MHz,
Chloroform-d) δ 7.65 (t, J = 7.8 Hz, 2H), 7.47 – 7.42 (m, 4H), 7.41 (d, J = 2.6 Hz, 5H), 6.72 (dd,
J = 8.8, 2.6 Hz, 4H), 6.28 (d, J = 8.8 Hz, 4H), 3.88 (s, 11H), 3.13 – 2.90 (m, 5H), 2.39 (d, J = 13.5
Hz, 3H), 2.17 – 1.95 (m, 2H), 1.84 (d, J = 3.8 Hz, 2H), 1.52 (dd, J = 13.7, 4.4 Hz, 4H), 1.46 (d, J
= 4.6 Hz, 14H), 1.38 – 1.28 (m, 10H), 1.15 (dd, J = 15.9, 6.7 Hz, 11H), 1.07 (t, J = 7.3 Hz, 12H),
0.92 (d, J = 6.4 Hz, 6H).
13
C NMR (101 MHz, CDCl3) δ 252.22, 151.04, 146.52, 146.02, 136.30,
129.69, 125.58, 125.48, 123.92, 115.20, 113.37, 101.70, 68.12, 65.67, 56.41, 53.00, 51.51, 48.62,
35.98, 31.59, 30.07, 29.71, 29.56, 29.39, 27.97, 27.60, 26.43, 25.76, 25.66, 24.42, 23.09, 22.93,
22.84, 19.88. Elemental Analysis: Anal. Cacld. for C41H55CuN2O2 + H2O: C, 71.43%; H, 8.33%;
N, 4.06%. Found: C, 71.69%; H, 8.69%; N, 4.12%.
CAAC
Men
-Cu(NPh2) (5). The complex was made from CAAC
Men
-CuCl (950 mg, 1.98 mmol),
NPh2 (351 mg, 2.08 mmol) and NaO
t
Bu (200 mg, 2.08 mmol) as a bright yellow solid. Yield: 1.1
g (93%). The complex is exceedingly sensitive to moisture, especially in solution. Single crystals
suited for structural determination were grown by vacuum sublimation, Tsub = 175 °C, P = 1.8 x
10
-6
mTorr.
1
H NMR (400 MHz, Chloroform-d) δ 7.56 (t, J = 7.8 Hz, 1H), 7.44 – 7.31 (m, 3H),
121

7.31 – 7.21 (m, 3H), 7.08 (dd, J = 8.6, 1.1 Hz, 1H), 6.94 (dt, J = 7.3, 1.2 Hz, 1H), 6.92 – 6.83 (m,
4H), 6.47 (t, J = 7.2 Hz, 2H), 6.33 (dd, J = 8.5, 1.1 Hz, 3H), 2.98 – 2.78 (m, 4H), 2.68 (dd, J =
13.3, 9.6 Hz, 1H), 2.53 (bs, 1H), 2.34 – 2.20 (m, 3H), 2.13 (d, J = 11.4 Hz, 1H), 1.99 – 1.63 (m,
7H), 1.51 (dd, J = 14.0, 3.5 Hz, 1H), 1.43 – 1.18 (m, 29H), 1.04 (dd, J = 11.2, 6.9 Hz, 4H), 0.94
(d, J = 6.9 Hz, 3H), 0.89 (dd, J = 6.7, 3.1 Hz, 5H), 0.76 (d, J = 6.5 Hz, 3H).
13
C NMR (101 MHz,
CDCl3) δ 252.00, 156.13, 146.23, 145.72, 145.25, 136.23, 129.84, 129.49, 128.61, 125.31, 124.90,
121.40, 117.96, 115.97, 65.39, 52.82, 51.43, 51.24, 48.97, 48.63, 36.88, 35.66, 31.23, 30.97, 30.08,
29.70, 29.48, 29.33, 29.18, 27.89, 27.31, 27.13, 26.11, 24.65, 24.43, 23.19, 23.08, 22.81, 22.73,
22.60, 20.18, 19.72, 2.95, 2.92, 2.89, 2.88, 2.82, 2.81, 2.79, 2.68. Elemental Analysis: Anal. Cacld.
for C39H53CuN2 + 0.75 H2O: C, 74.72%; H, 8.76%; N, 4.47%. Found: C, 74.83%; H, 8.48%; N,
4.18%.
CAAC
Men
-Cu(C6F5) (Cu host). LiC6F5 was prepared by the addition of nBuLi (1.05 mmol, 2.5
M in hexanes) to bromopentafluorobenzene (131 μL, 1.05 mmol) in ether (30 mL) at -78 °C for
45 minutes (Caution: LiC6F5 is known to explode at temperatures above -40 °C). To the crude
solution of LiC6F5, an ether solution of (MenthylCAAC)CuCl (500 mg, 1.04 mmol, 0.017 M) was
slowly added at -78 °C over 30 minutes. The reaction mixture was then stirred for one hour and
slowly warmed to room temperature for another 12 hours. Subsequently, the mixture was filtered
through celite, and the volatiles were removed under vacuum to afford a pale yellow solid.
Afterwards, the solid was washed with pentane (3x2 mL) to obtain a white powder (497 mg, 78%
yield). Crystals suitable for an X-ray diffraction study were grown from a saturated solution of
benzene layered with pentane.
1
H NMR (C6D6, 500 MHz): δ = 7.18 (t, J = 7.80 Hz, 1H), 7.06 (d,
J = 7.80 Hz, 1H), 7.04 (d, J = 7.80 Hz, 1H), 3.01-2.93 (m, 2H), 2.83 (sept, J = 6.10 Hz, 2H), 2.18
(br d, J = 12.42 Hz, 1H), 1.92 (d, J = 12.42 Hz), 1.84-1.76 (m, 3H), 1.38 (d, J = 6.59 Hz, 3H), 1.29
122

(d, 13.16 Hz, 1H), 1.20 (d, J = 12.42 Hz, 1H), 1.14-1.08 (m, 7H), 1.05-0.99 (m, 1H), 0.98 (d, J =
4.1 Hz, 6H), 0.94 (d, J = 6.23 Hz, 3H) 0.90 (d, J = 6.43 Hz, 3H) ppm;
13
C NMR (C6D6, 125 MHz):
254.6, 150.2 (d, JF = 222.5 Hz), 150.0 (d, JF = 222.5 Hz), 145.8, 145.3, 139.1 (d, JF = 243.3 Hz),
136.6 (d, JF = 252.3 Hz), 135.8, 130.0, 125.8 (t, 2JF = 74.2 Hz), 125.2, 125.0, 77.7, 65.9, 53.1,
51.6, 48.0, 36.1, 31.4, 29.6, 29.5, 29.2, 29.0, 28.0, 27.5, 26.6, 25.2, 24.2, 23.1, 22.8, 22.6, 20.0
ppm;
19
F NMR (C6D6, 282 MHz): δ = -111.9 – -112.2 (m), -160.4 (t, J = 19.8 Hz), -162.9 – -163.2
(m) ppm. Elemental Analysis: Anal. Cacld. for C33H43CuF5N: C, 64.74%; H, 7.08%; N, 2.29%.
Found: C, 64.31%; H, 7.12%; N, 1.90%.

DFT/TDDFT Calculations and Molecular Dynamics. All DFT and TDDFT calculations
reported in this chapter were performed using the Q-Chem 5.0 package.(8) Geometry optimization
for all complexes was performed at the B3LYP/LACVP** level. Single point TDDFT calculations
were performed on the ground state optimized structures at the CAM-B3LYP/LACVP** level to
compute excited state properties. The electrostatic potential-fitted (esp) atomic charges for 2-
MeTHF and complex 1a used to replace the partial charges of the OPLS2005 forcefield for the
MD simulations, were computed at the B3LYP/6-31G** and ωPBEh( = 0.263 bohr
-1
) /6-31G**
levels respectively. The esp charges of the triplet (
3
ICT) state of 1a was computed using the
unrestricted DFT scheme (UDFT). TDDFT calculations on the solvated clusters derived from the
snapshots of the MD simulations were performed at the ωPBEh( = 0.263 bohr
-1
) /6-31G** level
by replacing the atoms of all solvent molecules with their corresponding esp charges to act as a
polarizing influence on the complex. The value of the range separation parameter () in the ωPBEh
functional was tuned to satisfy the global density-dependent (GDD) criterion to get a balanced
description of CT and LE states and in the case of complex 1a, the tuned value was found to be
123

0.263 bohr
-1
.(9) All MD simulations reported here were performed using the Desmond program
available within Schrӧdinger’s Materials Science Suite.(10)
OLED Fabrication and Optimization. OLED devices were fabricated on pre-patterned ITO-
coated glass substrates (20 ± 5 Ω cm
-2
, Thin Film Devices, Inc.). Prior to deposition, the substrates
were cleaned with soap, rinsed with deionized water and sonicated for 15 minutes. Afterwards,
two subsequent rinses and 12-minute sonication baths were performed in acetone and isopropyl
alcohol sequentially. All organic layers as well as the Al cathode were deposited in a vacuum
thermal evaporator, EVO Vac 800 deposition system from Angstrom Engineering, at 6 x 10
-7
Torr.
Current-voltage-luminescence (J-V-L) curves were using a Keithley power source meter model
2400 and a Newport multifunction optical model 1835-C, PIN-220DP/SB blue-enhanced silicon
photodiodes (OSI optoelectronics Ltd.). The sensor was set to measure power at an energy of 520
nm, followed by correcting to the average electroluminescence wavelength for each individual
device during data process. Electroluminescence (EL) spectra of OLEDs were measured using the
fluorimeter (model C-60 Photon Technology International QuantaMaster) at several voltages.

124

4.5. References
1. Bergmann, L.; Hedley, G. J.; Baumann, T.; Bräse, S.; Samuel, I. D. W., Direct
observation of intersystem crossing in a thermally activated delayed fluorescence copper
complex in the solid state. Science Advances 2016, 2 (1).
2. Penfold, T. J.; Gindensperger, E.; Daniel, C.; Marian, C. M., Spin-Vibronic Mechanism
for Intersystem Crossing. Chem. Rev. 2018.
3. 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
iridium complexes: Synthesis, photophysical characterization, and use in organic light emitting
diodes. J. Am. Chem. Soc. 2001, 123 (18), 4304-4312.
4. Lamansky, S.; Kwong, R. C.; Nugent, M.; Djurovich, P. I.; Thompson, M. E.,
Molecularly doped polymer light emitting diodes utilizing phosphorescent Pt(II) and Ir(III)
dopants. Org. Electron. 2001, 2 (1), 53-62.
5. Hedley, G. J.; Ruseckas, A.; Samuel, I. D. W., Ultrafast luminescence in Ir(ppy)3. Chem.
Phys. Lett. 2008, 450 (4), 292-296.
6. Kleinschmidt, M.; van Wüllen, C.; Marian, C. M., Intersystem-crossing and
phosphorescence rates in fac-IrIII(ppy)3: A theoretical study involving multi-reference
configuration interaction wavefunctions. The Journal of Chemical Physics 2015, 142 (9),
094301.
7. Everly, R. M.; Ziessel, R.; Suffert, J.; McMillin, D. R., Steric influences on the
photoluminescence from copper(I) phenanthrolines in rigid media. Inorg. Chem. 1991, 30 (3),
559-561.
8. Siddique, Z. A.; Yamamoto, Y.; Ohno, T.; Nozaki, K., Structure-Dependent
Photophysical Properties of Singlet and Triplet Metal-to-Ligand Charge Transfer States in
Copper(I) Bis(diimine) Compounds. Inorg. Chem. 2003, 42 (20), 6366-6378.
9. McCusker, C. E.; Castellano, F. N., Design of a Long-Lifetime, Earth-Abundant,
Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects. Inorg. Chem. 2013,
52 (14), 8114-8120.
10. Le, C.; Chen, T. Q.; Liang, T.; Zhang, P.; MacMillan, D. W. C., A radical approach to
the copper oxidative addition problem: Trifluoromethylation of bromoarenes. Science 2018, 360
(6392), 1010.
11. Smith, C. S.; Branham, C. W.; Marquardt, B. J.; Mann, K. R., Oxygen Gas Sensing by
Luminescence Quenching in Crystals of Cu(xantphos)(phen)+ Complexes. J. Am. Chem. Soc.
2010, 132 (40), 14079-14085.
12. Elie, M.; Sguerra, F.; Di Meo, F.; Weber, M. D.; Marion, R.; Grimault, A.; Lohier, J.-
F.; Stallivieri, A.; Brosseau, A.; Pansu, R. B.; Renaud, J.-L.; Linares, M.; Hamel, M.; Costa,
R. D.; Gaillard, S., Designing NHC–Copper(I) Dipyridylamine Complexes for Blue Light-
Emitting Electrochemical Cells. ACS Applied Materials & Interfaces 2016, 8 (23), 14678-14691.
13. Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen,
D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C., E-Type Delayed
Fluorescence of a Phosphine-Supported Cu2(μ-NAr2)2 Diamond Core: Harvesting Singlet and
Triplet Excitons in OLEDs. J. Am. Chem. Soc. 2010, 132 (27), 9499-9508.
14. Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M., Highly
Efficient Green Organic Light-Emitting Diodes Containing Luminescent Three-Coordinate
Copper(I) Complexes. J. Am. Chem. Soc. 2011, 133 (27), 10348-10351.
125

15. Zhang, Q.; Komino, T.; Huang, S.; Matsunami, S.; Goushi, K.; Adachi, C., Triplet
Exciton Confinement in Green Organic Light‐Emitting Diodes Containing Luminescent Charge‐
Transfer Cu(I) Complexes. Adv. Funct. Mater. 2012, 22 (11), 2327-2336.
16. Yersin, H.; Czerwieniec, R.; Shafikov Marsel, Z.; Suleymanova Alfiya, F., TADF
Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI
Complexes. ChemPhysChem 2017, 18 (24), 3508-3535.
17. Samanta, P. K.; Kim, D.; Coropceanu, V.; Brédas, J.-L., Up-Conversion Intersystem
Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of
the Nature of Singlet vs Triplet Excited States. Journal of the American Chemical Society 2017,
139 (11), 4042-4051.
18. Volz, D.; Zink, D. M.; Bocksrocker, T.; Friedrichs, J.; Nieger, M.; Baumann, T.;
Lemmer, U.; Bräse, S., Molecular Construction Kit for Tuning Solubility, Stability and
Luminescence Properties: Heteroleptic MePyrPHOS-Copper Iodide-Complexes and their
Application in Organic Light-Emitting Diodes. Chem. Mater. 2013, 25 (17), 3414-3426.
19. Czerwieniec, R.; Yu, J.; Yersin, H., Blue-Light Emission of Cu(I) Complexes and
Singlet Harvesting. Inorg. Chem. 2011, 50 (17), 8293-8301.
20. Leitl, M. J.; Kuechle, F.-R.; Mayer, H. A.; Wesemann, L.; Yersin, H., Brightly Blue
and Green Emitting Cu(I) Dimers for Singlet Harvesting in OLEDs. J. Phys. Chem. A 2013, 117
(46), 11823-11836.
21. Osawa, M.; Kawata, I.; Ishii, R.; Igawa, S.; Hashimoto, M.; Hoshino, M., Application
of neutral d(10) coinage metal complexes with an anionic bidentate ligand in delayed
fluorescence-type organic light-emitting diodes. Journal of Materials Chemistry C 2013, 1 (28),
4375-4383.
22. Shi, S. Y.; Collins, L. R.; Mahon, M. F.; Djurovich, P. I.; Thompson, M. E.;
Whittlesey, M. K., Synthesis and characterization of phosphorescent two-coordinate copper(I)
complexes bearing diamidocarbene ligands. Dalton Transactions 2017, 46 (3), 745-752.
23. Gernert, M.; Müller, U.; Haehnel, M.; Pflaum, J.; Steffen, A., A Cyclic
Alkyl(amino)carbene as Two-Atom π-Chromophore Leading to the First Phosphorescent Linear
CuI Complexes. Chemistry A European Journal 2017, 23 (9), 2206-2216.
24. Hamze, R.; Jazzar, R.; Soleilhavoup, M.; Djurovich, P. I.; Bertrand, G.; Thompson ,
M. E. Phosphorescent 2-, 3- and 4-coordinate cyclic (alkyl)(amino)carbene (CAAC) Cu(I)
complexes Chem. Commun. [Online], 2017, p. 9008-9011.
25. Di, D. W.; 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.
26. Back, O.; Henry‐Ellinger, M.; Martin Caleb, D.; Martin, D.; Bertrand, G., 31P NMR
Chemical Shifts of Carbene–Phosphinidene Adducts as an Indicator of the π‐Accepting
Properties of Carbenes. Angewandte Chemie 2013, 125 (10), 3011-3015.
27. Back, O.; Henry-Ellinger, M.; Martin, C. D.; Martin, D.; Bertrand, G., 31P NMR
Chemical Shifts of Carbene–Phosphinidene Adducts as an Indicator of the π-Accepting
Properties of Carbenes. Angew. Chem. Int. Ed. 2013, 52 (10), 2939-2943.
28. Melaimi, M.; Jazzar, R.; Soleilhavoup, M.; Bertrand, G., Cyclic (Alkyl)(amino)carbenes
(CAACs): Recent Developments. Angewandte Chemie International Edition 2017, 56 (34),
10046-10068.
126

29. Frey, G. D.; Dewhurst, R. D.; Kousar, S.; Donnadieu, B.; Bertrand, G., Cyclic
(alkyl)(amino)carbene gold(I) complexes: A synthetic and structural investigation. Journal of
Organometallic Chemistry 2008, 693 (8–9), 1674-1682.
30. Hu, X. B.; Soleilhavoup, M.; Melaimi, M.; Chu, J. X.; Bertrand, G., Air-Stable
(CAAC) CuCl and (CAAC)CuBH4 Complexes as Catalysts for the Hydrolytic Dehydrogenation
of BH3NH3. Angewandte Chemie-International Edition 2015, 54 (20), 6008-6011.
31. Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.; Bertrand, G., Stable cyclic (alkyl)
(amino) carbenes as rigid or flexible, bulky, electron-rich ligands for transition-metal catalysts: A
quaternary carbon atom makes the difference. Angewandte Chemie-International Edition 2005,
44 (35), 5705-5709.
32. Romanov, A. S.; Di, D.; Yang, L.; Fernandez-Cestau, J.; Becker, C. R.; James, C. E.;
Zhu, B.; Linnolahti, M.; Credgington, D.; Bochmann, M., Highly photoluminescent copper
carbene complexes based on prompt rather than delayed fluorescence. Chemical
Communications 2016, 52 (38), 6379-6382.
33. 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.
34. Yang, Y.; Xue, M.; Marshall, L. J.; de Mendoza, J., Hydrogen-Bonded Cyclic Tetramers
Based on Ureidopyrimidinones Attached to a 3,6-Carbazolyl Spacer. Organic Letters 2011, 13
(12), 3186-3189.
35. Sanz-Pérez, E. S.; Murdock, C. R.; Didas, S. A.; Jones, C. W., Direct Capture of CO2
from Ambient Air. Chemical Reviews 2016, 116 (19), 11840-11876.
36. 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.
37. Hansmann, M. M.; Melaimi, M.; Munz, D.; Bertrand, G., Modular Approach to Kekulé
Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes. J. Am. Chem. Soc. 2018, 140 (7),
2546-2554.
38. Rosenfeld, D. C.; Wolczanski, P. T.; Barakat, K. A.; Buda, C.; Cundari, T. R., 3-
Center-4-Electron Bonding in [(silox)2MoNtBu]2(μ-Hg) Controls Reactivity while Frontier
Orbitals Permit a Dimolybdenum π-Bond Energy Estimate. J. Am. Chem. Soc. 2005, 127 (23),
8262-8263.
39. 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.
40. Reichardt, C., Solvatochromic Dyes as Solvent Polarity Indicators. Chemical Reviews
1994, 94 (8), 2319-2358.
41. Bulović, V.; Deshpande, R.; Thompson, M. E.; Forrest, S. R., Tuning the color emission
of thin film molecular organic light emitting devices by the solid state solvation effect. Chem.
Phys. Lett. 1999, 308 (3), 317-322.
42. Czerwieniec, R.; Leitl, M. J.; Homeier, H. H. H.; Yersin, H., Cu(I) complexes -
Thermally activated delayed fluorescence. Photophysical approach and material design. Coord.
Chem. Rev. 2016, 325, 2-28.
43. Dreuw, A.; Weisman, J. L.; Head-Gordon, M., Long-range charge-transfer excited states
in time-dependent density functional theory require non-local exchange. J. Chem. Phys. 2003,
119 (6), 2943-2946.
127

44. Dreuw, A.; Head-Gordon, M., Failure of time-dependent density functional theory for
long-range charge-transfer excited states: The zincbacteriochlorin-bacterlochlorin and
bacteriochlorophyll-spheroidene complexes. J. Am. Chem. Soc. 2004, 126 (12), 4007-4016.
45. Tozer, D. J.; Amos, R. D.; Handy, N. C.; Roos, B. O.; Serrano-Andres, L., Does density
functional theory contribute to the understanding of excited states of unsaturated organic
compounds? Mol. Phys. 1999, 97 (7), 859-868.
46. Chai, J. D.; Head-Gordon, M., Long-range corrected double-hybrid density functionals. J.
Chem. Phys. 2009, 131 (17).
47. Jacquemin, D.; Perpete, E. A.; Scuseria, G. E.; Ciofini, I.; Adamo, C., TD-DFT
performance for the visible absorption spectra of organic dyes: Conventional versus long-range
hybrids. J. Chem. Theory Comput. 2008, 4 (1), 123-135.
48. Jacquemin, D.; Wathelet, V.; Perpete, E. A.; Adamo, C., Extensive TD-DFT
Benchmark: Singlet-Excited States of Organic Molecules. J. Chem. Theory Comput. 2009, 5 (9),
2420-2435.
49. Tsuneda, T.; Hirao, K., Long-range correction for density functional theory. Wiley
Interdisciplinary Reviews-Computational Molecular Science 2014, 4 (4), 375-390.
50. Taffet, E. J.; Olivier, Y.; Lam, F.; Beljonne, D.; Scholes, G. D., Carbene–Metal–Amide
Bond Deformation, Rather Than Ligand Rotation, Drives Delayed Fluorescence. The Journal of
Physical Chemistry Letters 2018, 9 (7), 1620-1626.
51. Modrzejewski, M.; Rajchel, L.; Chalasinski, G.; Szczesniak, M. M., Density-Dependent
Onset of the Long-Range Exchange: A Key to Donor-Acceptor Properties. J. Phys. Chem. A
2013, 117 (45), 11580-11586.
52. Modrzejewski, M.; Rajchel, Ł.; Chalasinski, G.; Szczesniak, M. M., Density-Dependent
Onset of the Long-Range Exchange: A Key to Donor–Acceptor Properties. The Journal of
Physical Chemistry A 2013, 117 (45), 11580-11586.
53. Banks, J. L.; Beard, H. S.; Cao, Y.; Cho, A. E.; Damm, W.; Farid, R.; Felts, A. K.;
Halgren, T. A.; Mainz, D. T.; Maple, J. R.; Murphy, R.; Philipp, D. M.; Repasky, M. P.;
Zhang, L. Y.; Berne, B. J.; Friesner, R. A.; Gallicchio, E.; Levy, R. M., Integrated Modeling
Program, Applied Chemical Theory (IMPACT). J. Comput. Chem. 2005, 26 (16), 1752-1780.
54. Santos, P. L.; Ward, J. S.; Data, P.; Batsanov, A. S.; Bryce, M. R.; Dias, F. B.;
Monkman, A. P., Engineering the singlet–triplet energy splitting in a TADF molecule. Journal of
Materials Chemistry C 2016, 4 (17), 3815-3824.
55. 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. Coord. Chem. Rev. 2011, 255 (21-22), 2622-2652.
56. Baková, R.; Chergui, M.; Daniel, C.; Vlček, A.; Záliš, S., Relativistic effects in
spectroscopy and photophysics of heavy-metal complexes illustrated by spin–orbit calculations
of [Re(imidazole)(CO)3(phen)]+. Coord. Chem. Rev. 2011, 255 (7), 975-989.
57. Turro, J. N.; Ramamurthy, V.; Scaiano, J. C., Principles of Molecular Photochemistry:
An introduction. University Science Book: Sausalito, California, 2009.

128

Chapter 5. Luminescent, 2-coordinate, d
10
coinage metal complexes: A
systematic study of the role of the metal as an electronic conduit

5.1. Introduction
The luminescence of monovalent coinage metal complexes employing identical sets of ligands is
an under-explored topic of interest. Investigations into this field have been limited by the structural
and electronic constraints imposed by the metals themselves: second- and third-row group 11
metals tend to adopt lower coordination numbers and higher oxidation potentials. Thus, MLCT
metal-to-ligand charge transfer (MLCT) transitions, most common in Cu complexes, are largely
destabilized in Ag(I) and Au(I) complexes, thereby making comparisons across the coinage metal
series challenging. A 1993 account from Ford and Vogler investigated luminescent Cu(I), Ag(I),
and Au(I) clusters, noting the varying natures of the emissive states depending on the metal core.
1

Whereas tetranuclear Cu4X4L4 clusters showed emission originating from an excited state with
admixed halide-to-ligand charge (XLCT) and halide to metal CT (XMCT) contributions,
luminescence of Ag4X4L4 clusters was found to have metal-centered d-s character, with a small
LMCT contribution. This latter contribution is minimized further in tetranuclear Au(I) clusters,
Au4X4L4, where the emissive state is described as metal-resident d-s in character. A more recent
systematic study by Hsu, Chi, and coworkers examined a series of mononuclear, 4-coordinate
Cu(I) and Ag(I) complexes and similar 3-coordinate Au(I) ones.
2
The findings highlight the Cu(I)
complexes as the most efficient MLCT-based phosphors, with the highest photoluminescent
quantum yields (ΦPL) and the fastest radiative rates (with rate constants, kr ~ 10
4
s
-1
). In contrast,
Ag(I) and Au(I) congeners showed mixed fluorescence and phosphorescence for the former (10
9

s
-1
and 10
1
– 10
2
s
-1
) and slow phosphorescence in the latter (kr ~ 10
2
– 10
3
s
-1
), reflecting slower
intersystem crossing (ISC) rates between S1 → T1 and T1 → S0. The lower-lying d-orbitals of Ag(I)
and Au(I) increase the energies of MLCT transitions, making the lowest-energy triplet excited
129

states predominantly ligand-centered, π → π* in origin. This leaves the metal centers in these
complexes to exert a weaker “external heavy atom effect,” in contrast with Cu(I) which exerts a
stronger “internal heavy atom effect” with the direct involvement of its d-orbitals in low-energy
MLCT excited states. Employing judicious ligand design, the authors prepared a Cu(I) complex
where the lowest energy triplet is π → π* in origin having little d-orbital contribution. Comparing
this complex with Ag(I) and Au(I) congeners shows a trend in ISC rates that fits well with
increasing atomic number Z, supporting the external heavy atom effect purview: the Cu(I)
3
ππ*
phosphors have the slowest rates, followed by Ag(I), and Au(I) as the fastest. A seminal report
from Osawa and coworkers in 2013 centered on a series of neutral, monovalent, coinage metal
complexes that undergo thermally-activated delayed fluorescence (TADF).
3
In addition to a
commonly-employed bisphosphine ligand that acts as an electron acceptor, tetrahedral
coordination around the metal core was fulfilled by a bidentate, anionic phosphinothiolate ligand
acting as a strong electron donor. As such, the lowest-energy transitions across the group 11 series
are ligand to ligand charge transfer in character (LLCT), with the reduced metal contribution aiding
in the alleviation of excited state structural reorganization. As a result of the partitioning of the
electron and hole wavefunctions, all three Cu(I), Ag(I), and Au(I) complexes show efficient
TADF, with kr ~ 10
5
s
-1
in their microcrystalline powders. Solution samples of the complexes show
a precipitous drop in ΦPL in fluid environments as well as increased thermal instability of the Au(I)
complex. Interestingly, the energy separation between the lowest excited S 1 and T1, ∆E
S
1
-T
1
, was
calculated to be the smallest for the Ag(I) complex and was not tested experimentally. More recent
work on a cationic, tetrahedral Ag(I) TADF emitter showed promising ΦPL and remarkable kr =
10
5
s
-1
.
4, 5
However, the dominant MLCT character of the emissive state induces large structural
reorganization in the excited state, which effectively quenches luminescence in non-rigid media.
130

Upon comparison to Cu(I) congeners reported by a different group in 2015,
6
we find that the solid
state ΦPL and kr values are higher for the Ag(I) complexes reported by Yersin and coworkers.
5
This
is likely related to the experimental ∆𝐸 𝑆 1
−𝑇 1
which is smaller in the Ag(I) complex than its Cu(I)
analogue: 650 cm
-1
in the former and 921 cm
-1
in the latter. However, with S1 and T1 both being
largely MLCT in origin, non-radiative decay back to the ground state dominates in fluid
environments, effectively quenching luminescence. These results further motivate the
development of complexes with simpler geometries that show reduced excited state
reorganization.
The luminescence efficiency of most commonly reported 2-coordinate Au(I) complexes depends
strongly on the matrix: high ΦPL is achieved through tenuous solid state interactions such as Au-
Au,
7
Au-π, π-π,
8
H-bonding,
9
and in frozen solutions at 77 K.
10
The first report of N-heterocyclic
carbene (NHC)–Au(I) amides examined the weak, ligand-centered phosphorescence of the
microcrystalline powders.
11
A subsequent report of luminescent NHC (namely IPr)-Au amides
assigned their weak emission to fluorescence.
12
Luminescent 2-coordinate NHC-Ag(I) complexes
have been reported that are cationic
13, 14
and/or part of a large macrocycle
13
with Ag-Ag
interactions.
15
The expanding library of carbenes as strong σ-donating and π-accepting ligand set
has allowed for the isolation of stable linear Ag(I) and Cu(I) complexes, in addition to their Au(I)
analogues. Over the last two years, monovalent neutral and cationic coinage metal complexes
bearing electrophilic cyclic alkyl amino carbenes (CAACs) have been reported with high ΦPL and
fast phosphorescence rates.
16-19
However, the MLCT nature of the excited states in these
complexes result in poor ΦPL in fluid, dilute environments. Recent reports of neutral, 2-coordinate
Cu(I) and Au(I) complexes pairing CAACs with electron-rich amides as the donor/acceptor
131

ligands highlight their interesting photophysics and their potential for use as dopants in efficient
organic light emitting diodes (OLEDs).
20, 21

In this chapter, the photophysical properties of a series of 2-coordinate, monovalent coinage metal
complexes bearing donor/acceptor ligands are explored. The electron-accepting ligands
investigated are two types of carbenes: CAAC and benzo[d]imidazol-2-ylidene (BzI), and
carbazolide is the electron-donating ligand. The rich excited state composition of these complexes
was probed as a function of solvent and temperature. For the CAAC series, new Ag(I) and Au(I)
complexes were compared against the model Cu(I) complex examined in chapter 4. The BzI series
showed unique photophysics, courtesy of the aromatic carbene and the different sterics it imposes.
Lastly, OLEDs employing CAAC-Cu(I) and Ag(I) emitters were compared.
5.2. Results and Discussion
5.2.1. Synthesis
All six carbazolide complexes appearing in this chapter were synthesized in high yields (80 – 90%)
via the one-pot reaction outlined in Figure 5. 1, allowing for the isolation of complexes C1 – C3
as light-yellow powders and complexes B1 – B3 as greyish white powders. The CAAC-M-Cl
precursors for complexes C1 – C3 were prepared following modified literature procedures.
18, 22-24

The synthesis of the bulky benzo[d]imidazol-3-ium chloride, the precursor for the
benzo[d]imidazol-2-ylidene carbene in the BzI complexes has eluded organometallic chemists in
fields ranging from transition metal (TM) catalysis
25-27
to luminescent materials.
28
Following a
Buchwald-Hartwig amination to the N,N’-bis(aryl)benzene-1,2-diamine,
29
the cyclization reaction
to yield the benzo[d]imidazol-3-ium salt has proven to be particularly challenging in the case of
substrates having a 2,6-ortho substitution patterns on both N,N’- aryl groups.
30
Attempts
employing reactive electrophiles such as triethyl orthoformate or chloromethyl pivalate
31
in the
presence of HCl/HBF4 or AgOTf respectively only returned the starting diamine.
30
In our hands,
132

reactions with triethyl orthoformate and HBF4 gave insoluble fine powders of a dark green material
that was difficult to separate from a bright-blue luminescent product.
1
H-NMR spectra of the latter
showed a characteristic aldehyde signal at ~ 9.5 ppm, in addition to other intractable signals. An
alternate strategy involved increasing the oxidation state of the 1,2-diamine to 1,2-dimine, inspired
by an early report by Bielawski and coworkers
32
on the oxidation of 1,2,4,5-tetraaminobenzene to
its corresponding 2,5-diamino-1,4-benzoquinonediimine and its subsequent cyclization to the
benzobis(imidazolium) salt. Nevertheless, oxidation of N,N’-bis(aryl)benzene-1,2-diamine
resulted in a sigmatropic rearrangement yielding dihydrophenazine. The authors report the
synthesis of N, N’-mesitylbenzoimidazoliun 𝐵𝐹
4
−
salt from the dihydrophenazine substrate;
however, this was not reproducible in our hands. Additionally, imidazolium 𝐵𝐹
4
−
salts are more
challenging to deprotonate than their halide analogues, further complicating the isolation of the
free carbene.
33
Inspired by a more recent report that highlighted the role of chlorotrimethylsilane
(TMSCl) as a Lewis acid to activate the strong electrophile triethylorthoformate,
34
we set out to
reproduce this synthesis and adapt it for our purposes. The optimized route is described in the
experimental methods section (We were also able to isolate the less bulky 1,3-bis(2,6-
dimethylphenyl)-1H-benzo[d]imidazol-3-ium chloride salt following this same procedure). The
benzo[d]imidazol-3-ium salt, isolated as a grey powder, can be deprotonated by bases such as
potassium bis(trimethylsilyl)amide (KHMDS) or sodium tert-butoxide (NaOtBu), yielding the free
carbene, or by silver(I) oxide, Ag2O, yielding the complex BzIAgCl. Transmetallation of this latter
complex with copper(I) chloride (CuCl) or chloro(dimethylsulfide) gold(I) (Me2S-AuCl) gives
BzICuCl and BzIAuCl respectively (Figure 5. 2). All three BzI-M-Cl precursors were isolated as
greyish white powders. Upon prolonged exposure to air, solutions of the Ag(I) and Au(I)
complexes can turn into a purplish color, indicating the formation of metal nanoparticles – a
133

product of photoreduction. However, within the short period of sample preparation for
photophysical analysis, the complexes are stable. All photophysical characterization are performed
under a N2 atmosphere, eliminating risks of photoreduction.

Figure 5. 1. Synthetic scheme (top) depicting the preparation of the two coordinate coinage metal
complexes C1 – 3 and B1 – 3 shown below.
134

Figure 5. 2. Synthetic route used in the preparation of BzI-M-Cl precursors.

5.2.2. XRay analysis

Figure 5. 3. Oak Ridge Thermal Ellipsoid Plots (ORTEPs) representing the crystal structures of complexes
C1 – C3, B1 – B3 including both conformers of complex B2: B2a and B2b.

135

Single crystals suited for diffraction analysis were grown by slow diffusion of pentane/hexanes
into a saturated solution of the complexes in CH2Cl2/THF or by slow evaporation of a saturated
toluene solution. Crystal structures of all six complexes reveal a two-coordinate, linear geometry
around the metal center with coplanar ligand conformation (dha ~ 0.1 - 11°), except for complex
B2 which exists in two conformers, a coplanar one and an orthogonal one (dha = 11° and 95°
respectively). In all six complexes, the metal is equidistant from the carbene carbon (C carbene) and
the carbazolide nitrogen (NCz), with equivalent Ccarbene–M and M–NCz bond lengths (1.86 Å – 2.07
Å for the former and.1.86 Å – 2.06 Å for the latter). Despite having different carbenes, the Ccarbene-
M bond lengths are nearly identical among complexes bearing the same metal, which allows us to
draw proper comparisons between the two series. The most striking difference that runs across
both carbene series is the varying Ccarbene to NCz distance, which is smallest for the Cu complexes
C1 and B1 (3.74 Å and 3.73 Å respectively), followed by the Au complexes C3 and B3 (4.00 Å
for both), and largest for the Ag complex C2 and B2 (4.12 Å and 4.11 Å for C2 and B2a
respectively). Notably, the coplanar ligand conformation in complex B2b does not result in longer
Ccarbene–Ag or Ag–NCz bond lengths; the Ccarbene to NCz separation is similarly unchanged.
Table 5. 1. Crystallographic information for complexes C1 – C3 and B1 – B3.
Complex C–M
(Å)
M–N (Å) C–N (Å) dha C-M-N Space
group
Z per
unit
cell
C1 1.881 (2) 1.859 (2) 3.735 (3) 1.3° (2) 173.51° (9) P21/c 1
C2 2.069 (7) 2.057 (7) 4.12 (1) 0.1° (8) 174.6° (3) P21 2
C3 1.988 (2) 2.018 (2) 4.004 (2) 0.3° (2) 176.2° (7) P21/c 1
B1 1.863 (4) 1.869 (3) 3.726 (5) 1.5° (5) 173.9° (2) Cc 2
B2a 2.058 (2) 2.050 (2) 4.106 (3) 11.0 (2) 176.15° (7)
P21/n 2
B2b 2.063 (3) 2.063 (2) 4.125 (4) 95.0°(2) 177.66° (8)
B3 1.981 (4) 2.017 (4) 3.997 (6) 3.2 (5) 176.4 (2) C2/c 1
C–M and M–N refer to Carbene–metal and metal–carbazole bond lengths respectively. dha refers
to the carbene-carbazole dihedral angle.

136

5.2.3. Electrochemistry

Table 5. 2. Redox potentials of complexes C1 – C3 and B1 – B3, and the associated experimental
frontier orbital energies.
Complex Eox (V) Ered (V) ΔEredox (V) EHOMO
(eV)
ELUMO
(eV)
C1 0.239 -2.84 3.08 -5.06 -1.48
C2 0.19 -2.8 2.99 -5.01 -1.53
C3 0.33 -2.72 3.05 -5.17 -1.62
B1 0.13 -2.81 2.94 -4.94 -1.51
B2 0.29 -2.83 3.12 -5.12 -1.49
B3 0.32 -2.81 3.13 -5.16 -1.51
Electrochemical studies performed in acetonitrile versus Fc
+
/Fc with t-butyl
ammonium hexafluorophosphate (TBAF) as the electrolyte. The redox peaks were taken
from differential pulse voltammograms (DPV) and converted to HOMO/LUMO energies
using the equations in ref.
35

The electrochemistry of these carbene-metal-amide complexes is dominated by the ligands, as
demonstrated in chapter 4. Here too we find that changing the metal only moderately alters the
reduction potentials, with the Au complexes C3 and B3 being slightly easier to reduce than their
congeners. In a similar fashion, the minor d-orbital contribution to the HOMO of these complexes
results in complexes C3 and B3 having the highest oxidation potentials, a consequence of Au(I)
d-orbitals being the most difficult to oxidize. In the same manner, Cu(I) complex B1 has the lowest
oxidation potential, but this is not consistent with its C1 analogue. In the CAAC series, the Ag(I)
complex C2 is the easiest to oxidize. Notably, we find that the reduction potentials of the
complexes bearing CAAC and BzI as the carbene are nearly identical, around ~2.8 V. Since the
reduction is dominated primarily by the electron-deficient carbene, and given the similar Ccarbene–
metal bond lengths, it is reasonable to conclude that the CAAC and BzI ligands are comparable in
their π-accepting ability. Further spectroscopic, electrochemical, or NMR studies can be performed
to assess the electrophilicity of BzI carbenes.
36

137

5.2.4. Photophysical characterization

a)
300 350 400 450
0
1
2

 (x 10
4
M
-1
. cm
-1
)
Wavelength (nm)
C1
C1
C3
THF
a)
b)
300 350 400 450
0
1
2
3
4

 x 10
4
(M
-1
. cm
-1
)
Wavelength (nm)
B1
B2
B3
2-MeTHF
b)

Figure 5. 4. Extinction coefficients of complexes C1 – C3 in THF (a) and complexes B1 – B3 in 2-MeTHF
(b).
Absorption spectra of the complexes in THF and 2-methyltetrahydrofuran (2-MeTHF) share
common high energy signals that can be attributed to π → π* transitions on the Dipp moiety of the
carbenes (at wavelengths < 290 nm), and to π → π* and n → π* transitions on the carbazolide
ligand (sharp peaks between 300 nm and 350 nm, and ~ 370 nm respectively) (Figure 5. 4). At
lower energies, a broad, featureless band that tails into 430 nm appears in the CAAC series, most
intense in the Au complex C3, followed by the Cu complex C1, and the Ag complex C2 having
the least intense absorption. This band is ascribed to LLCT from the electron rich carbazolide to
the electron-deficient CAAC. In the BzI series, the same trend holds with the Au complex B3
having the LLCT band with the highest extinction coefficient, followed by the Cu complex B1,
and the Ag complex B2 having the weakest absorbing LLCT. In contrast with the CAAC series,
the LLCT band in the BzI series is higher energy, and barely extends out to 410 nm. This can be
rationalized by the BzI carbene being slightly less electrophilic than the CAAC, as it has a slightly
138

more negative reduction potential. The LUMO of the complexes in the BzI is thus slightly
destabilized, leading to higher energy LLCT bands.
300 350 400 450 500
0
1
2

Scaled absorbance (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
a) C1
300 350 400 450 500
0
1
2
C2 b)

Scaled absorption (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
300 350 400 450 500
0
1
2

Scaled absorbance (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
C3 c)

300 350 400 450
0
1
2
B1

Scaled absorbance (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
1 wt% PS film
d)
300 350 400 450
0
1
2
B2

Scaled absorbance (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
1 wt% PS film
e)
300 350 400 450
0
1
2
B3

Scaled absorbance (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
1 wt% PS film
f)

Figure 5. 5. Scaled absorption spectra of complexes C1 – C3 (a – c) and B1 – B3 (d – f) in various
solvents.
The negative solvatochromism exhibited by complex C1 is studied extensively in chapter 4 and
registers a dramatic 2500 cm
-1
blue shift in its LLCT band going from the mom-polar MeCy to the
polar solvent CH2Cl2. A similar trend is observed in the Ag and Au analogues, C2 and C3, which
show a blue-shift in their LLCT absorption band with a magnitude of 3000 cm
-1
and 2100 cm
-1

respectively upon increasing solvent polarity from MeCy to CH2Cl2. Unlike the LLCT band, the
sharp, narrow absorption features which correspond to carbazolide-localized transitions, do not
exhibit any solvatochromism. A shift in the energy of an absorption band typically indicates a
change in magnitude between the permanent molecular dipole and the dipole of the excited state
formed as a result of that transition. The direction of the shift, positive versus negative
solvatochromism, represents the orientation of the two dipoles relative to one another: ground and
139

excited state dipoles that are parallel show positive solvatochromism, whereas ones that are
antiparallel show negative solvatochromism. In these complexes, the permanent dipole is oriented
along the molecular z-axis (carbene-M-amide axis) towards the electron-rich amide. LLCT
absorption produces an excited state where the dipole is oriented towards the electron deficient
carbene which gains an electron as a result of the excitation.
The BzI carbene series shows similar trends in negative solvatochromism of the LLCT absorption
band in solvents of increasing polarity, with the Ag complex B2 registering the largest blue shift,
followed by B1 and B3: 2400 cm
-1
, 2200 cm
-1
, and 630 cm
-1
respectively. Interestingly, in all three
BzI complexes, the LLCT absorption band in MeCy appears to have two components and can be
fitted to two Gaussian curves. A likely origin of this observation is exciton coupling, namely J-
aggregates, occurring in MeCy in which the complexes have limited solubility. Alternately, the
energy separation between both Gaussians, ~1400 cm
-1
, is consistent with an aromatic C=C stretch.
A vibronically-structured CT absorption, however, would be highly unusual. We note that the bi-
Gaussian nature of the LLCT band can also be seen in the CAAC complexes dissolved in MeCy,
albeit to a less-pronounced extent. Overall, the LLCT bands of the BzI complexes B1 – B3 are
narrower than the CAAC complexes C1 – C3 in all solvents, indicating ground and excited state
potential energy surfaces that are more nested in the former than the latter.

140

400 500 600 700
0.0
0.5
1.0 C1

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
a)

400 500 600 700
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
b) C2

400 500 600 700
0.0
0.5
1.0

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
c) C3

400 450 500 550 600 650
0.0
0.5
1.0
B1

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
d)

400 450 500 550 600 650
0.0
0.5
1.0
B2

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
e)

400 450 500 550 600 650
0.0
0.5
1.0
B3

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
toluene
2-MeTHF
CH
2
Cl
2
f)

Figure 5. 6. Emission spectra of complexes C1 – C3 (top) and B1 – B3 (bottom) in various solvents.

Complexes C1 – C3 and B1 – B3 are highly luminescent in solutions of MeCy, toluene, 2-MeTHF,
and CH2Cl2, with the radiative rates dropping slightly upon increasing solvent polarity. In contrast,
as documented in chapter 4, the photoluminescence of the microcrystalline powders of these
complexes is red-shifted and largely quenched, likely due to the formation of aggregates. An
exception is complex B2, powders of which luminesce blue, likely owing to the presence of its
orthogonal conformer B2b which breaks apart aggregation.
The luminescence of all three CAAC complexes in all solvents is characterized by broad and
featureless emission lines, assigned as CT in origin. Unlike the strong negative solvatochromism
observed in the absorption of the CT band, CT emission of complexes C1 – C3 does not show a
clear solvatochromic trend (Figure 5. 6). However, solutions of these complexes in non-polar
MeCy consistently show the highest-energy emission, whereas solutions in the more polar,
weakly-coordinating 2-MeTHF show the biggest red-shift in emission, and the broadest spectral
141

lines. The formation of exciplexes with this solvent is largely ruled out by the high radiative rates
observed in this series (kr ~ 10
5
– 10
6
s
-1
), which is mostly unchanged across different solvents.
Within the CAAC series, we note that the radiative rate is increased upon substituting Au(I) for
Cu(I), in agreement with the heavy atom effect; the radiative rate constant k r is in fact doubled
from ~ 4 x 10
5
s
-1
in C1 to ~8 x 10
5
s
-1
in C3. An outlier in this series is the Ag(I) complex C3,
which shows a remarkable kr ~ 2 x 10
6
s
-1
in all solvents. This is seemingly counterintuitive
considering the reduced εCT observed for this complex; however, it can be rationalized in the frame
of thermally-activated delayed fluorescence (TADF), which is addressed in a later section of this
chapter.
The luminescence of complexes B1 – B3 in solution shows a stronger dependence on solvent
polarity than complexes in the CAAC series: with increasing solvent polarity, emission λ max is
shifted to longer wavelengths and spectral line widths are increased. In non-polar MeCy, emission
profiles of of all the BzI complexes are narrow and show vibronic structure (Δν ~ 1140 – 1240
cm
-1
), indicative of a ligand-centered state as dominant origin of luminescence. From our work on
complex C1 in chapter 4, we identify this state to be the carbazolide-centered triplet,
3
Cz. In
increasingly polar fluid media, emission becomes more CT-like, with broad, featureless profiles
and increasing spectral line-widths. Additionally, we note a decrease in the radiative rate constants
with increasing solvent polarity from 10
5
s
-1
in MeCy and toluene to 10
4
s
-1
in CH2Cl2. The
observed changes in spectral shape and in radiative rates indicate a change in the nature of the
emissive state as a function of solvent dipole. However, it is noteworthy that the onset of emission
remains unchanged in complexes B1 – B3 regardless of the medium, which rules out a reordering
of the excited state manifolds in different media as the underlying explanation. Investigations of
intramolecular electron transfer in organic donor-acceptor systems with closely-lying
1
CT and
142

localized excited (
3
LE) states highlight the role of solvent reorganization in changing the
topography of the excited state potential energy surfaces (PES).
37
More recent studies of
organometallic complexes with nearly resonant
3
CT and
3
LE reveal the active role the solvent
plays in of the determining the extent of vibrational coupling between the two triplets.
38, 39

However, these D-M-A complexes are unique in their ground and excited states properties. Firstly,
the ground state permanent dipole is larger than the CT excited state dipole in these complexes
(see computational section), unlike most-commonly studied organic D-A molecules. Secondly, the
CT excited state manifold in these systems is comprised of a nearly degenerate singlet and triplet,
1
CT and
3
CT, separated by a small ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 as discussed in chapter 4. Lastly, Hence, a model is
presented in Figure 5. 7 that takes into account the electronic makeup of the ground and excited
states in these systems. A polar environment has an opposite effect on the ground and CT states,
relaxing the PES of the former and narrowing it in the latter. In contrast, the localized nature of
the
3
Cz makes it largely immune to solvent dipoles. As such, upon increasing solvent polarity, a
blue-shift in the CT absorption energy is observed. In polar media, absorption into the CT manifold
is followed by thermalization to the lowest vibrational level of the
3
Cz manifold, resulting in mixed
CT/
3
Cz emission that is dominated by the locally excited state (
3
Cz). In non-polar environments,
the more rigid CT PES results in emission out of the lowest vibrational level of the
1/3
CT manifold
to the more relaxed ground state structure. Thus, this greater degree of structural reorganization
yields CT-dominant emission, with broader spectral lines and red-shifted λmax. The solvatochromic
behavior exhibited by complexes in the BzI series with the aromatic carbene thus vary significantly
from complexes in the CAAC series with the saturated carbene.
143

Figure 5. 7. Qualitative energy diagram representing the ground state (GS) and both excited state energy
surfaces (CT and
3
Cz) as a function of solvent coordinate. a) non-polar solvents induce electronic transitions
with small reorganization energies and result in vibronically-structured
3
Cz-dominant emission; b) polar
solvents induce a blue-shift in absorption which leads to broad and featureless CT-dominant emission
reflecting the high reorganization energies. Blue arrows represent CT absorption and red arrows represent
emission in non-polar (solid) and polar solvents (dotted). Green arrows represent rapid thermalization into
the lowest vibrational energy level of the manifold.
Across the BzI series, the radiative rates of the complexes in a given solvent correlate well with
the heavy atom effect, with B3 complex having the fastest radiative rate and B1 having the slowest
(Table 5. 1). This marks another divergence from the trend observed in the CAAC series, where
the Ag complex C2 shows the highest radiative rate constant. A likely explanation of the reduced
kr observed in B2 relative to C2 is the presence of a larger population of orthogonal ligand
conformers in the former, which have been shown to exhibit slower radiative rates in chapter 4.
Nevertheless, it is noteworthy that despite the fact that B2 like C2 possesses the weakest εCT, it
does not have the lowest kr – an observation that can also be attributed to more effective TADF
operating within the CT manifold of B2 with the largest D–A separation in the series.
144

Studying the photophysical properties of C1 – C3 and B1 – B3 in frozen glassy matrices and
polymeric thin films allows for the examination of rigidochromic effects as well as temperature-
dependent phenomena such as TADF and the interplay between
1/3
CT and
3
Cz.
0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0

Normalized Intensity (a. u.)
MeCy
2-MeTHF
1 wt% PS film
a)
77 K
RT
C1

Wavelength (nm)
MeCy
2-MeTHF
1 wt% PS film
(60K)

0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
RT

MeCy
2-MeTHF
1 wt% PS film
C2
77 K
b)

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
2-MeTHF
1 wt% PS film
(60 K)

0.0
0.5
1.0
400 500 600 700
400 500 600 700
0.0
0.5
1.0
77K

MeCy
2-MeTHF
1 wt% PS film
RT
C3
c)

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
2-MeTHF
1 wt% PS film
(80 K)

Figure 5. 8. Emission profiles of complexes C1 (a), C2 (b), and C3 (c) at room temperature (top) and frozen
glassy or polymeric matrices (bottom).
As with C1, complexes C2 and C3 exhibit
3
Cz emission in frozen glasses of MeCy and 2-MeTHF.
The narrow, structured emission (Δν ~ 1500 cm
-1
) is characterized by intense phosphorescence
and long decay times: ~ 7 ms for the Cu complex C1, ~ 500 μs for the Au complex C3, and ~ 18
ms for the Ag complex C2. While the enhancement of the
3
Cz radiative rates upon substituting the
heavier Au nucleus for Cu is anticipated, the phosphorescence of the Ag complex is once again
anomalous. The increased Ag–NCz bond lengths as well as the smaller contribution from the
metal’s d-orbitals to the HOMO may explain why the metal exerts a weakened external heavy
atom effect. In the more polar 2-MeTHF, freezing the solvent continuum prevents the
reorganization of solvent dipoles around the
1
CT exciton formed upon excitation, resulting in the
destabilization of
1
CT. As such, emission occurs from the now lower-lying
3
Cz. In the non-polar
145

MeCy, where solubility is low, rigidochromic effects are likely brought about by the strong
permanent dipole of the solute itself.
Thin films of complexes C1 – C3 in a polystyrene (PS) matrix, doped at 1 wt% ratio, exhibit broad
and featureless CT emission at all temperatures (from 300 K – 5 K), with negligible changes in
spectral profiles. The solvent dipoles in this more rigid matrix are restricted from reorganization,
and as such cannot as effectively destabilize the CT state at lower temperatures. However, with
decreasing temperatures, increasingly longer excited state lifetimes are recorded: 65 μs for C1, 38
μs for C2, and 43 μs for C3 at 5 K. Plots of PL decays as a function of temperature yields the
TADF signature S-shaped curves (Figure 5. 9).
0 50 100 150 200 250 300
0
20
40
60
S
0
T
1
S
1
153 ns
64μs

𝑆 1−𝑇 1 63 meV

 (s)
T (K)
C1 a)
0 50 100 150 200 250 300
10
20
30
40
S
0
I
T
1
,
II
T
1
III
T
1
S
1
31 ns
3.4 μs
38 μs

𝑆 1
−𝑇 1

𝐹𝑆 26 meV
9.2 meV

 (s)
T (K)
C2
b)

0 50 100 150 200 250 300 350
0
10
20
30
40
50
S
0
I
T
1
,
II
T
1
III
T
1
S
1
23 ns
2.4 μs
49 μs

𝑆 1−𝑇 1

𝐹𝑆 53 meV
28 meV

 (s)
T (K)
C3
c)

Figure 5. 9. Temperature-dependent PL decays (red dots) of complexes C1 (a), C2 (b), and C3 (c) in thin
PS films and their respective fits (black line) to the modified Boltzmann equation. The insets show the
excited state models obtained from the fits.
The temperature-dependent PL decay of C1 was fitted to the modified Boltzmann equation
𝜏 𝑇𝐴𝐷𝐹 =
3 + exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
3 𝑘 𝑇 1
+ 𝑘 𝑆 1
exp(
∆𝐸 𝑆 1
−𝑇 1
𝑘 𝐵 𝑇 )
, giving ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 = 62.6 meV. For C2 and C3, better fits to the
data were obtained using the modified Boltzmann equation further adjusted to include the energy
splitting between the triplet sublevels (∆𝐸 𝐹𝑆 ) as a result of zero-field splitting
𝜏 =
2+ 𝑒 −
∆𝐸 1
𝑘𝑇
+𝑒 −
∆𝐸 2
𝑘𝑇
2(
1
𝜏 1
)+(
1
𝜏 2
)𝑒 −
∆𝐸 1
𝑘𝑇
+(
1
𝜏 3
)𝑒 −
∆𝐸 2
𝑘𝑇
. The ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 values obtained herein are smallest for the Ag(I)
146

complex C2 (26 meV, 210 cm
-1
) followed by the complex with the heaviest metal C3 (53 meV,
430 cm
-1
). These values reflect well the varying donor-acceptor separation in all three complexes:
largest for C3 and smallest for C1. Interestingly, the excited state lifetimes of the lowest-lying
triplet sublevels are found to be slightly slower in C3 than in C2 (49 μs in the former and 38 μs in
the latter), which is unexpected given the heavier Au nucleus. ∆𝐸 𝐹𝑆 provides insight into the
strength of the SOC interaction in these complexes.
40
The heavier Au nucleus induces a larger
∆𝐸 𝐹𝑆 in C3 (28 meV = 226 cm
-1
) than the Ag nucleus in C2 (9.2 meV = 74 cm-1), both of which
are among the largest reported values in monovalent coinage metal complexes. The derived S1
(
1
CT) lifetime is found to be faster for C3 than C2 (23 ns and 31 ns respectively), in agreement
with the higher εCT of the former. Perhaps the most important conclusion set forth by this set of
experiments is that the long-standing TADF conundrum which posits the inherent trade-off
between minimizing ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 and maximizing kr can be circumvented in organometallic TADF
complexes with strong SOC. As noted early on by McGlynn, Azumi, and Kinoshita, the rate-
limiting process in TADF is not the radiative S1 → S0 process, but the thermally-driven up
intersystem crossing (UISC) from T1 → S1.
41
Bredas and coworkers highlight the role of SOC in
enhancing kUISC, a spin-forbidden process.
42
With that in mind, TADF organometallic complexes
such as C2 can be designed with vanishingly small ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 , rapid k UISC, and k r on the order of
10
6
s
-1
.
147

0.0
0.5
1.0
400 500 600
400 500 600
0.0
0.5
1.0
B1

Normalized Intensity (a. u.)
MeCy
2-MeTHF
1 wt% PS film
RT
77 K
a)

Wavelength (nm)
MeCy
2-MeTHF
1 wt% PS film

0.0
0.5
1.0
400 500 600
400 500 600
0.0
0.5
1.0

MeCy
2-MeTHF
1 wt% PS film
b)
77 K

Normalized Intensity (a. u.)
Wavelength (nm)
MeCy
2-MeTHF
1 wt% PS film
RT
B2

0.0
0.5
1.0
400 500 600
400 500 600
0.0
0.5
1.0

Normalized Intensity (a. u.)
MeCy
2-MeTHF
1 wt% PS film
c)
77 K

Wavelength (nm)
MeCy
2-MeTHF
1 wt% PS film
RT
B3

Figure 5. 10. Emission profiles of complexes B1 (a), B2 (b), and B3 (c) at room temperature (top) and
frozen glassy or polymeric matrices (bottom).
In frozen glassy matrices of MeCy and 2-MeTHF, complexes in the BzI series also show efficient
3
Cz emission (λmax = 424 nm – 434 nm) with long PL decays that follow the trend observed in the
CAAC series: ~6 ms for B1, ~ 19 ms for B2, and ~ 500 μs for B3. However, unlike C1 – C3, the
room temperature emission spectra of complexes B1 – B3 in MeCy, 2-MeTHF, and PS have onsets
that are at higher energies than
3
Cz (onsets: 404 nm – 410 nm). Furthermore, absorption spectra
of B1 – B3 in these three matrices reach their cut-offs at energies higher than
3
Cz (cut-offs: 400 –
425 nm). Thus, the matrix-insensitive
3
Cz state is the BzI series lies consistently below the CT
manifold in all matrices examined and at all temperatures studied. The model we present for
emission in complexes B1 – B3 involves a low-lying
3
Cz with a CT manifold that is thermally-
accessible at room temperature. Thermalization into this CT manifold is hindered at low
temperatures, resulting in emission out of the pure
3
Cz state (Figure 5. 11). The model also holds
in the more rigid PS matrix, and as such temperature-dependent studies aimed at examining TADF
within the CT manifold exclusively is hampered by the lowest-lying
3
Cz.
148

Figure 5. 11. State energy diagram representing room temperature and 77 K emission in complexes B1 –
B3.

149

Table 5. 3. Photophysical properties of complexes C1 – C3 and B1 – B3 in various media at
room temperature and 77 K.
Complex λ max, RT
(nm)
Φ RT τ RT
(μs)
k r, RT
(10
5
s
-1
)
k nr, RT
(10
5
s
-1
)
λ max, 77K
(nm)
τ 77K
(μs)
C1 MeCy 468; 486 0.92 2.3 4.0 0.35 430 6700
Toluene 488 1.0 2.5 4.0 < 0.04 --- ---
2-MeTHF 492 1.0 2.5 3.9 < 0.08 430 7300
CH 2Cl 2 482 0.40 1.6 2.5 3.8 --- ---
PS film 474 1.0 2.8 3.5 < 0.04 480 61
C2 MeCy 496 1.0 0.51 20 < 0.20 432 15000
Toluene 504 0.93 0.43 21 1.6 --- ---
2-MeTHF 523 0.71 0.37 19 7.8 432 21000
CH 2Cl 2 506 0.40 1.98 2.2 3.0 x 10
5
--- ---
PS film 496 1.0 0.50 20 < 0.20 476; 504 11.1
C3 MeCy 480 0.99 1.21 8.2 0.08 426 450
Toluene 492 0.98 1.07 9.2 0.19 --- ---
2-MeTHF 502 0.95 1.20 7.9 0.42 426 530
CH 2Cl 2 494 0.88 1.19 7.4 0.10 --- ---
PS film 478 1.0 1.14 8.8 < 0.9 480 44.6
B1 MeCy 428 0.42 1.26 3.3 4.6 428 6300
Toluene 450 0.75 1.50 5.0 1.7 --- ---
2-MeTHF 458 0.35 2.06 1.7 3.2 430 11000
CH 2Cl 2 466 0.03 1.24 0.24 7.8 --- ---
PS film 434 0.86 0.97 (36%);
4.8 (64%)
2.5
*
0.41
*
432 3000
B2 MeCy 430 0.58 1.04 5.6 4.4 432 18000
Toluene 458 0.50 3.27 1.5 1.5 --- ---
2-MeTHF 476 0.19 5.66 0.34 1.4 432 20000
CH 2Cl 2 482 0.03 1.64 0.18 5.9 --- ---
PS film 438 0.85 0.69 (26%);
5.1 (74%)
2.2
*
0.38
*
434 6600
B3 MeCy 424 0.88 1.02 8.6 1.2 424 340
Toluene 448 0.94 1.11 8.5 5.4 --- ---
2-MeTHF 452 0.79 2.63 3.0 0.8 426 640
CH 2Cl 2 458 0.21 7.07 0.30 1.1 --- ---
PS film 432 1.0 0.74 (46%);
3.6 (54%)
4.4
*
< 0.04
*
428 190
*Calculated from the weighted averages of both contributions.

5.2.5. OLED characterization
OLED devices incorporating C1 as an emitter are detailed in chapter 4. OLEDs employing C2 as
the emissive dopant were optimized in their dopant ratios and hosts (Figure 5. 12). Doped at 10
vol%, devices using mCBP as a host material showed higher EQE that those with mCP as a host,
150

reflecting the higher ΦPL observed in 10% doped thin films of C2 in mCBP (0.85) compared to
mCP (0.4): in mCBP devices, EQEmax = 5.9% at 0.4 mA/cm
2
and EQE = 3.8% at 20 mA/cm
2
; in
mCP devices, EQEmax = 1.2% at 20 mA/cm
-1
and EQE = 0.79% at 0.4 mA/cm
2
. At the time of
writing of this chapter, Romanov and coworkers reported Ag(I) OLEDs employing an analogous
CAAC-AgCz emitter with 4% EQE in mCP.
43
The device architectures studied included an
additional hole/exciton blocking layer that likely improves device efficiency through ensuring
exciton and charge confinement. Notably, electroluminescence (EL) spectra of devices with mCP
as a host reveal an extra high energy shoulder at ~ 400 nm compared to the EL of mCBP devices,
which can be attributed to incomplete energy transfer from mCP to dopant. This high-energy
feature is reproduced in PL spectra of thin films of C2 in mCP. However, mCP devices have a
lower turn-on voltage than mCBP devices, likely indicating lower charge injection barriers in the
former host.

10
-2
10
-1
10
0
10
1
10
2
10
3
0.1
1
10

mCBP
mCP
EQE (%)
Current density (mA/cm
2
)
b)

0 2 4 6 8 10 12
10
-1
10
0
10
1
10
2
10
3
10
4
mCBP
mCP
Voltage (V)
Luminance, L (cd/m
2
)
c)
0
200
400
600
Current density, J (mA/cm
2
)

400 500 600 700
0.0
0.5
1.0
mCBP
mCP
Normalized Intensity (a.u.)
Wavelength (nm)
d)

Figure 5. 12. a) architecture of devices employing C2 as an emitter doped at 10 vol% in mCBP and mCP;
b) EQE curves of the devices; c) J-V-L curves; d) EL spectra.
151

Based on the higher EQE obtained in mCBP devices of C2, further doping ratio optimization was
carried out in this host, and the devices were compared against analogous ones with C1 as the
emitter (Figure 5. 13). Despite both dopants having similar ΦPL in mCBP (0.85), devices with the
sub-microsecond emitter C2 showed higher EQE and nearly double the maximum brightness
compared to devices with their C1-based analogues: 7.1% at 3.7 mA/cm
2
; 11000 cd/m
2
at 11 V for
C2 and 4.0% at 8.9 mA/cm
2
; 5700 cd/m
2
for C1. Despite having a lower energy
3
CT than C1 and
frontier orbital levels that are better nested in mCBP, C2 devices are no more conductive than C1
devices. In fact, the OLEDs with C2 have a slightly higher turn-on voltage than C1-based OLEDs.
The reasoning behind this is not fully understood. Future studies are required to investigate the
rates of second-order processes such as triplet-triplet annihilation (TTA) or triplet-polaron
annihilation (TPA), and to determine whether shorter exciton lifetimes provided by the Ag(I)
emitter C2 enhance the operating device lifetimes.

10
-1
10
0
10
1
10
2
10
3
0.1
1
10
C1
C2

EQE (%)
Current density (mA/cm
2
)
b)

0 2 4 6 8 10 12
10
-1
10
0
10
1
10
2
10
3
10
4
C1
C2
Voltage (V)
Luminance (cd/m
2
)
c)
0
200
400
600
Current density (mA/cm
2
)
400 450 500 550 600 650
0.0
0.5
1.0 C1
C2
Normalized Intensity (a.u.)
Wavelength (nm)
d)

Figure 5. 13. a) Device architectures for OLEDs with C1 and C2 emitters doped at 20 vol% in mCBP; b)
EQE curves; c) characteristic J-V-L curves; d) EL spectra.
152

5.2.6. Computational analysis
Density Functional Theory calculations (DFT, at the B3LYP/LACVP** level) performed on the
optimized structures reproduce the experimental results obtained through electrochemistry: in all
six complexes, the highest occupied molecular orbital (HOMO) primarily resides on the NCz of the
carbazolide moiety, and the lowest unoccupied molecular orbital (LUMO) is localized on the
unfilled 2pz orbital of Ccarbene (Table 5. 4). This electronic configuration in the ground state results
in a large permanent dipole directed from the carbene towards the carbazolide, with μ GS ~ 11.6 –
13.1 D. The data obtained for C3 is in agreement with the report by Föller et al. for an analogous
Au(I) complex.
44

153

Table 5. 4. Frontier orbital surfaces and energies for C1 – C3 and B1 – B3 and the permanent dipole
moment μ GS obtained through DFT calculations performed at the B3LYP/LACVP** level.

HOMO (eV) LUMO (eV)
μ GS (D)
C1

11.8
-4.94 -1.49
C2

13.1
-4.88 -1.42
C3

11.6
-5.01 -1.50
B1

xx
-4.93 -1.51
B2a

xxx
-4.88 -1.40

B2b

xxx
xxx xxx
B3

xxx
-5.01 -1.49
154

To model the electronic transitions behind the experimental photophysics in these complexes, we
turned to time-dependent DFT (TDDFT) performed using long-range corrected functionals,
introduced in chapter 4, which constitute a better model for delocalized CT states.
45
Within the
series of CAAC complexes C1 – C3, we find that the lowest excited singlet state S1 is comprised
of a HOMO → LUMO CT transition, and is characterized by oscillator strengths that vary
depending on the metal (Table 5. 5). The heavy Au nucleus in C3 induces sufficient electronic
communication between the HOMO and LUMO wavefunctions, resulting in the highest oscillator
strength (f = 0.186) calculated for the CT transition in S1. C1 with its small Ccarbene-NCz separation
follows, with f = 0.123, and lastly C2 has the weakest calculated f = 0.092, reflecting the furthest
HOMO/LUMO partitioning observed in this complex. These results are also in agreement with the
experimentally obtained CT extinction coefficients, εCT (Figure 5. 4). For C1 and C3, the excited
triplet corresponding to S1 is T1, with HOMO → LUMO parentage; C2, in contrast, is calculated
to have a carbazolide-localized
3
Cz as T1 and
3
CT as T2. Nevertheless, the energy separation of the
CT manifold, ∆𝐸 1
𝐶𝑇 −
3
𝐶𝑇
, is calculated to be smallest for C2, followed by C1, and largest for C3:
0.18 eV, 0.26 eV, and 0.58 eV respectively. Thus, trends in
∆𝐸 1
𝐶𝑇 −
3
𝐶𝑇
obtained from TDDFT calculations and from temperature-dependent analysis are only
partially in agreement (experimental ∆𝐸 1
𝐶𝑇 −
3
𝐶𝑇
trend is Ag < Au < Cu), with the absolute values
of the calculated energy separations being significantly larger than the experimental ones. It is
noteworthy that T3 in all three CAAC complexes is an admixture of primarily
3
Cz with a prominent
3
CT contribution (70 – 90% of the former and 10 – 30 % of the latter). With T3 being close in
energy to S1 and having non-identical orbital parentage to the latter, it is reasonable that effective
ISC and UISC occur within this manifold more rapidly than between
1
CT and
3
CT (the latter being
more energetically disparate from S1 and comprised of identical orbital configuration). In all three
155

CAAC complexes, the pure 3Cz state is nearly resonant in energy with
3
CT, at ∆𝐸 3
𝐶𝑧 −
3
𝐶𝑇
~ 0.02 –
0.03 eV .
The excited state manifold of the BzI series differs from that in the CAAC series in the following
ways: firstly, the lowest-energy T1 in B1, B2, and B3 is decidedly
3
Cz in character, in agreement
with the model proposed following our low-temperature experiments. Secondly, higher-energy
excited states within the triplet manifold, namely T2 and T3, show much stronger
3
Cz–
3
CT
admixing compared to that in the CAAC series. The Cz–CT coupling is also seen in the S2 state of
the BzI series, another contrasting point with the CAAC complexes where S2 is exclusively Cz in
origin. The highly-mixed electronic makeup of the excited states in the BzI (calculated in the gas
phase) can explain the narrow, structured emission in non-polar MeCy and the strongly
solvatochromism observed, as the solvent dipole alters the extent of coupling between the Cz and
CT PES. Nevertheless, the factors contributing to this strongly coupled excited state manifold are
not fully understood. One proposed explanation involves the aromatic nature of the BzI carbene
(as opposed to the fully saturated CAAC), which introduces CT states culminating in the
benzo[d]imidazol-2-ylidene π as well as π* orbitals in addition to BzI-centered states. The higher
density of states obtained thereof can increase the extent of electronic coupling within different
closely-lying manifolds. The oscillator strengths of
1
CT (S1) mirror the CAAC series: weakest for
the Ag(I) complex B2 and strongest for the Au(I) complex B3, and the ∆𝐸 1
𝐶𝑇 −
3
𝐶𝑇
reflects the range
of HOMO/LUMO separation (smallest for B2 and largest for B1). Lastly, we compare the
electronic makeup of the orthogonal Ag(I) complex B2b and its coplanar conformer B2a. While
the calculated excited state energies are largely unchanged with the change in dihedral angle, the
electronic configuration of the excited states in B2b shows no CT/Cz admixing. One can
156

reasonably assume, therefore, that the coplanar ligand conformation is required for strong coupling
between the different excited state manifolds and for allowed transitions between them.

157

Table 5. 5. Calculated singlet and triplet excited state energies for the complexes in this chapter obtained
through TDDFT performed at the CAM-B3LYP/LACVP** level.

E (eV)
fS
1

T
1
T
2

T3
S
1
S
2

S3 ∆𝐸 1
𝐶𝑇 −
3
𝐶𝑇
∆𝐸 3
𝐶𝑧 −
3
𝐶𝑇

C1
2.99
(CT)
3.02
(Cz)
3.23
(0.9 Cz,
0.1 CT)
3.25
(CT)
3.90
(Cz)
4.11
(Cz)
0.26* 0.03 0.123
μES (D) 4.25 11.3 11.6 8.66 4.40 14.0
C2
3.08
(Cz)
3.10
(CT)
3.28
(0.7 Cz,
0.3 CT)
3.28
(CT)
4.16
(Cz)
4.19
(CT)
0.18* -0.02 0.092
μES (D) 12.4 0.96 9.06 5.50 14.0 8.22
C3
3.05
(CT)
3.07
(Cz)
3.34
(0.9 Cz,
0.1 CT)
3.63
(CT)
4.22
(Cz)
4.26
(CT)
0.58* 0.02 0.186
μES (D) 2.53 10.8 12.4 3.66 13.3 7.37
B1
3.05
(Cz)
3.10
(0.6 CT,
0.3 Cz)
3.30
(0.5 CT,
0.5 Cz)
3.37
(CT)
4.12
(0.8 Cz,
0.15 CT)
4.20
(CT)
0.27* -0.05** 0.132
μES (D)
B2a
3.05
(Cz)
3.11
(0.6 Cz,
0.3 CT)
3.36
(0.8 CT,
0.2 Cz)
3.38
(CT)
4.08
(0.8 Cz,
0.15 CT)
4.16
(CT)
0.02* -0.06** 0.078
μES (D)
B2b
3.04
(Cz)
3.14
(Cz)
3.32
(CT)
3.33
(CT)
4.04
(Cz)
4.14
(CT)
0.01 0.18 1 x 10
-6

μES (D)
B3
3.05
(Cz)
3.14
(0.5 CT,
0.4 Cz)
3.36
(0.6 CT,
0.4 Cz)
3.48
(CT)
4.15
(0.8Cz,
0.15 CT)
4.26
(CT)
0.12* 0.09** 0.200
μES (D)
*the energy difference between
1
CT (S 1) and the triplet with primarily CT contribution.
**the energy difference between
3
Cz (T 1) and the closest-lying triplet state with CT contribution.

5.3. Conclusions
A family of luminescent 2-coordinate, monovalent coinage metal complexes with redox-active
ligands comprising a carbene and an amide has been investigated. In conjunction with the electron-
158

rich carbazolide as the donor ligand, two different carbenes were studied as the electron-acceptors:
a bulky CAAC that afforded the first Cu(I), Ag(I), and Au(I) with 100% PLQY in fluid media, and
a bulky benzo[d]imidazole-2-ylidene carbene that gave complexes with intriguing emission.
Across both CAAC and BzI series, the Ag(I) complexes have the largest carbene–amide separation
and consequently the weakest CT absorption band. In addition to the CT emission that is dominant
at room temperature, a closely-lying
3
Cz band comprises the phosphorescence in frozen glassy
matrices. The BzI series showed stronger vibrational coupling between the CT and
3
Cz states, with
the latter dominating emission at low temperatures in frozen solvent glasses as well as polystyrene
matrices. In contrast, temperature-dependent studies of thin films of the CAAC complexes show
efficient TADF operating within the CT manifold of the Ag(I) and Au(I) congeners, with
∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 that reflects the carbene–carbazole separation: smallest for Ag(I), larger for Au(I), and
largest for Cu(I). The vanishingly small ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 in the Ag(I) complex results in the fastest
recorded radiative rate from a triplet-based emitter: kr = 2 x 10
6
s
-1
. This finding shows that the
TADF conundrum describing the trade-off between minimizing ∆𝐸 𝐶𝑇
1
− 𝐶𝑇
3 and maximizing kr
can be circumvented in organometallic complexes where strong SOC results enhances the limiting
kUSIC. Finally, OLEDs employing analogous Ag and Cu emitters were compared, with the Ag(I)-
based devices showing nearly doubled EQE and enhanced brightness. Further studies are required
to determine the impact of a sub-microsecond kr on second-order quenching processes in OLEDs
and on the overall operating device lifetimes.
5.4. Experimental Methods
Synthesis. All reactions were performed under nitrogen atmosphere in oven dried glassware.
Triethyl orthoformate, CuCl, Ag2O, (Me2S)AuCl, NaOtBu, Cz, as well as electrochemical grade
TBAF and Fc were purchased from Sigma-Aldrich. TMSCl was purchased from TCI Chemicals.
159

All chemicals were used without further purification. Tetrahydrofuran, dichloromethane, and
toluene were purified by Glass Contour solvent system by SG Water USA, LLC. Dry, air-free
methylcyclohexane (MeCy) and 2-methyltetrahydrofuran (2-MeTHF) were purchased from Sigma-Aldrich
and used without further purification.
1
H and
13
C NMR spectra were recorded on a Varian Mercury 400.
The chemical shifts are given in units of ppm and referenced to the residual proton resonance of
chloroform-d (CDCl 3) at 7.26 ppm. Elemental analyses were performed at the University of Southern
California, CA.
1,3-bis(2,6-diisopropylphenyl)-1H-benzo[d]imidazol-3-ium chloride (BzIm.HCl): In a 500
mL Schlenk flask equipped with a stir bar and a distillation apparatus, 2.09 g (4.9 mmol) of
(N1,N2-bis(2,6-diisopropylphenyl)benzene-1,2-diamine was added, and the flask was evacuated
then backfilled with N2 three times. Triethyl orthoformate (300 mL) via cannula, and the orange-
colored mixture was stirred for 2h at 150 °C. After ~ 250 mL of triethyl orthoformate was distilled,
the temperature was lowered to 50 °C (changed out oil baths), and 60 mL of fresh triethyl
orthoformate was added followed by 70 mL of TMSCl. The dark green reaction mixture was stirred
at 50 °C for 4 hours, after which the formation of a greyish precipitate was noted. The majority of
the organic solvents were removed by vacuum distillation, during which the reaction mixture
turned into a wine-red color. The heat was turned off, and the reaction mixture was left to cool
under N2. Following that, the precipitate was filtered and washed copiously with diethylether. The
filtrate was refiltered, collecting more residue. The light grey-colored product was dried under
vacuum. 1.78 g collected (76%).
1
H NMR (400 MHz, Chloroform-d) δ 13.01 (s, 1H), 7.68 (dd, J
= 6.3, 3.1 Hz, 2H), 7.64 (t, J = 7.9 Hz, 2H), 7.43 (d, J = 7.8 Hz, 4H), 7.36 (dd, J = 6.2, 3.1 Hz,
2H), 2.28 (hept, J = 6.6 Hz, 4H), 1.34 (d, J = 6.8 Hz, 12H), 1.14 (d, J = 6.8 Hz, 12H).
160

Chloro[1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene]copper(I)
(BzICuCl): In the glovebox, a 100 mL round bottom flask equipped with a stir bar was charged
with 430 mg (0.90 mmol) BzIm.HCl and CuCl (90 mg, 0.90 mmol). 30 mL THF was added,
followed by NaOtBu (91 mg, 0.95 mmol) which turned the reaction from a white suspension to a
pale, golden-colored solution. The reaction mixture was left to stir at room temperature overnight,
then filtered through a celite plug. The filtrate was collected, and the volatiles were removed under
reduced pressure. The residue was washed copiously with pentane, yielding 422 mg (87% yield)
of an off-white powder.
1
H NMR (400 MHz, Chloroform-d) δ 7.57 (t, J = 7.7 Hz, 2H), 7.38 (d, J
= 7.6 Hz, 6H), 7.08 (dd, J = 5.7, 3.0 Hz, 2H), 2.40 (hept, J = 13.4, 6.8 Hz, 4H), 1.28 (d, J = 6.8
Hz, 12H), 1.11 (d, J = 6.7 Hz, 12H).
13
C NMR (101 MHz, cdcl3) δ 187.39, 146.60, 135.03, 131.73,
131.10, 125.16, 124.77, 111.98, 29.04, 25.16, 23.88.
Chloro[1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene]silver(I) (BzIAgCl):
In the glovebox, a 200 mL round bottom flask wrapped in aluminum foil and equipped with a stir
bar was charged with 850 mg (1.79 mmol) BzIm.HCl and Ag2O (216 mg, 0.93 mmol). 80 mL THF
was added, and the light purplish-colored reaction mixture was left to stir at room temperature
overnight, then filtered through a celite plug. The filtrate was collected, and the volatiles were
removed under reduced pressure. The residue was washed copiously with pentane, yielding 945
mg of a pale purplish white powder (91% yield). Note: the pale purple tint is likely due to the
formation of Ag nanoparticles. To limit this, care must be taken to wrap vials with the solids with
aluminum foil and store in the dark under N2. The two carbene
13
C signals observed at 193 ppm
and 191 ppm correspond to the two NMR-active Ag isotopes:
107
Ag and
109
Ag.
1
H NMR (400
MHz, Chloroform-d) δ 7.58 (t, J = 7.8 Hz, 2H), 7.42 – 7.37 (m, 6H), 7.12 (dd, J = 6.2, 3.1 Hz,
2H), 2.38 (hept, J = 6.8 Hz, 4H), 1.26 (d, J = 6.9 Hz, 12H), 1.10 (d, J = 6.9 Hz, 12H).
13
C NMR
161

(101 MHz, cdcl3) δ 193.13, 190.81, 146.59, 135.16, 131.92, 131.29, 125.35, 112.27, 29.01, 25.12,
24.02.
Chloro[1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene]gold(I) (BzIAuCl):
In the glovebox, a 200 mL round bottom flask wrapped in aluminum foil and equipped with a stir
bar was charged with 620 mg (1.07 mmol) BzIAgCl and (Me2S)AuCl (330 mg, 1.12 mmol). 70
mL THF was added, and the light purplish-colored reaction mixture was left to stir at room
temperature overnight, then filtered through a celite plug. The filtrate was collected, and the
volatiles were removed under reduced pressure. The residue was washed copiously with pentane,
yielding 631 mg of a pale purplish white powder (88% yield). Note: the pale purple tint is likely
due to the formation of Au nanoparticles. To limit this, care must be taken to wrap vials with the
solids with aluminum foil and store in the dark under N2.
1
H NMR (400 MHz, Chloroform-d) δ
7.58 (t, J = 7.8 Hz, 2H), 7.40 (dd, J = 6.1, 3.1 Hz, 2H), 7.38 (d, J = 7.8 Hz, 4H), 7.08 (dd, J = 6.1,
3.1 Hz, 2H), 2.40 (hept, J = 6.6 Hz, 4H), 1.33 (d, J = 6.9 Hz, 12H), 1.09 (d, J = 6.9 Hz, 12H).
13
C
NMR (101 MHz, cdcl3) δ 181.98, 146.64, 134.73, 131.26, 125.54, 124.85, 112.22, 29.14, 24.83,
24.11.
CAAC
Men
-AgCz (C2): In the glovebox, a 100 mL round bottom flask wrapped in aluminum foil
and equipped with a stir bar was charged with 680 mg (1.3 mmol) CAAC
Men
-AgCl and Cz (216
mg, 1.3 mmol). 80 mL THF was added followed by NaOtBu (137 mg, 1.4 mmol), and the pale
orange-colored reaction mixture was left to stir at room temperature overnight, then filtered
through a celite plug. The filtrate was collected, and the volatiles were removed under reduced
pressure. The residue was washed copiously with pentane, yielding 849 mg of a white powder
(100% yield). The two carbene
13
C signals observed at 261 ppm and 259 ppm correspond to the
two NMR-active Ag isotopes:
107
Ag and
109
Ag.
1
H NMR (400 MHz, Chloroform-d) δ 8.00 (d, J
162

= 7.6 Hz, 2H), 7.63 (t, J = 7.8 Hz, 1H), 7.46 – 7.38 (m, 2H), 7.17 – 7.06 (m, 2H), 6.96 – 6.87 (m,
2H), 6.64 (d, J = 8.1 Hz, 2H), 3.04 – 2.84 (m, 2H), 2.41 (d, J = 13.5 Hz, 1H), 2.30 (d, J = 13.0 Hz,
1H), 2.11 – 1.94 (m, 4H), 1.86 (d, J = 13.6 Hz, 1H), 1.51 (d, J = 6.7 Hz, 6H), 1.35 (dd, J = 6.7, 2.9
Hz, 6H), 1.29 (d, J = 6.7 Hz, 3H), 1.22 (d, J = 6.6 Hz, 3H), 1.08 (dd, J = 6.9, 2.9 Hz, 6H), 0.93 (d,
J = 6.5 Hz, 3H).
13
C NMR (101 MHz, cdcl3) δ 261.19, 259.18, 150.56, 146.28, 145.74, 129.90,
125.64, 125.54, 124.27, 123.23, 119.58, 114.85, 114.33, 78.78, 78.66, 65.65, 65.53, 53.07, 51.68,
48.97, 48.91, 35.88, 31.30, 30.26, 29.92, 29.51, 29.28, 28.05, 27.81, 26.86, 24.96, 24.59, 23.12,
19.95.
CAAC
Men
-AuCz (C3): Same as C2. 670 mg (1.1 mmol) CAAC
Men
-AuCl, Cz (192 mg, 1.15
mmol), NaOtBu (115 mg, 1.2 mmol), 80 mL THF. 710 mg of a white powder (87% yield).
1
H
NMR (400 MHz, Chloroform-d) δ 7.99 (d, J = 7.7 Hz, 2H), 7.65 (t, J = 7.8 Hz, 1H), 7.48 – 7.38
(m, 2H), 7.17 – 7.07 (m, H), 6.95 (t, J = 7.8 Hz, H), 6.74 (d, J = 8.1 Hz, 2H), 3.41 – 3.24 (m, 2H),
3.01 – 2.85 (m, 2H), 2.46 (d, J = 13.5 Hz, 1H), 2.33 (s, 1H), 2.18 – 2.08 (m, 1H), 2.02 (dd, J =
14.9, 7.2 Hz, 2H), 1.94 (d, J = 13.6 Hz, 1H), 1.48 (d, J = 11.6 Hz, 6H), 1.38 – 1.28 (m, 12H), 1.18
(d, J = 6.9 Hz, 3H), 1.07 (d, J = 6.9 Hz, 3H), 0.92 (d, J = 6.5 Hz, 3H).

13
C NMR (101 MHz, cdcl3)
δ 242.50, 149.71, 146.44, 146.03, 136.40, 129.67, 125.47, 124.35, 123.42, 119.49, 115.91, 113.82,
64.59, 53.10, 51.62, 49.91, 35.98, 31.08, 30.66, 29.91, 29.61, 29.37, 28.25, 27.24, 26.48, 25.17,
24.82, 23.45, 23.30, 23.09, 19.99.
BzI-CuCz (B1): Same as C2. 120 mg (0.22 mmol) BzIAuCl, Cz (39 mg, 0.23 mmol), NaOtBu
(22 mg, 0.23 mmol), 30 mL THF. 135 mg of a white powder (90% yield of crude product, contains
~ 30% residual Cz and ~20% unreacted BzICuCl).
1
H NMR (400 MHz, Chloroform-d) δ 7.91 (d,
J = 8.0 Hz, 2H), 7.76 (t, J = 7.8 Hz, 2H), 7.53 (d, J = 7.9 Hz, 4H), 7.45 (m, J = 6.1, 3.1 Hz, 2H),
163

7.27 – 7.20 (m, 2H), 6.98 (t, J = 8.2 Hz, 1H), 6.87 (t, J = 7.8 Hz, 2H), 6.30 (d, J = 8.1 Hz, 2H),
2.54 (hept, J = 7.8, 7.4 Hz, 4H), 1.24 (d, J = 6.9 Hz, 12H), 1.17 (d, J = 6.9 Hz, 12H).
BzI-AgCz (B2): Same as C2. 150 mg (0.26 mmol) BzIAgCl, Cz (47 mg, 0.28 mmol), NaOtBu
(27 mg, 0.28 mmol), 30 mL THF. 130 mg of a white powder (71% yield of crude product,
including ~ 8% unreacted BzIAgCl).
1
H NMR (400 MHz, Chloroform-d) δ 8.08 (d, J = 8.0 Hz,
1H), 7.98 (d, J = 7.7 Hz, 2H), 7.69 (t, J = 7.8 Hz, 2H), 7.51 – 7.36 (m, 8H), 7.26 – 7.22 (m, 2H),
7.08 (t, J = 8.1 Hz, 2H), 6.90 (t, J = 7.3 Hz, 2H), 6.68 (d, J = 8.1 Hz, 2H), 2.53 (hept, J = 6.8 Hz,
4H), 1.31 (d, J = 6.9 Hz, 12H), 1.16 (d, J = 6.9 Hz, 12H).
BzI-AuCz (B3): Same as C2. 784 mg (1.17 mmol) BzIAuCl, Cz (195 mg, 1.17 mmol), NaOtBu
(118 mg, 1.2 mmol), 80 mL THF. 910 mg of a white powder (97% yield).
1
H NMR (400 MHz,
Chloroform-d) δ 7.95 (d, J = 7.7 Hz, 2H), 7.72 (t, J = 7.8 Hz, 7H), 7.50 (d, J = 7.8 Hz, 4H), 7.46
(dd, J = 6.1, 3.1 Hz, 2H), 7.27 – 7.19 (m, 2H), 7.06 (t, J = 8.0 Hz, 2H), 6.91 (t, J = 7.3 Hz, 2H),
6.73 (d, J = 8.1 Hz, 2H), 2.54 (hept, J = 7.1 Hz, 4H), 1.33 (d, J = 6.9 Hz, 12H), 1.15 (d, J = 6.9
Hz, 12H).

164

5.5. References
1. Ford, P. C.; Vogler, A., Photochemical and photophysical properties of tetranuclear and
hexanuclear clusters of metals with d10 and s2 electronic configurations. Accounts of Chemical
Research 1993, 26 (4), 220-226.
2. Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-
H.; Chen, P.-Y., Systematic Investigation of the Metal-Structure–Photophysics Relationship of
Emissive d10-Complexes of Group 11 Elements: The Prospect of Application in Organic Light
Emitting Devices. Journal of the American Chemical Society 2011, 133 (31), 12085-12099.
3. Osawa, M.; Kawata, I.; Ishii, R.; Igawa, S.; Hashimoto, M.; Hoshino, M., Application
of neutral d(10) coinage metal complexes with an anionic bidentate ligand in delayed
fluorescence-type organic light-emitting diodes. Journal of Materials Chemistry C 2013, 1 (28),
4375-4383.
4. Yersin, H.; Czerwieniec, R.; Shafikov Marsel, Z.; Suleymanova Alfiya, F., TADF
Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI
Complexes. ChemPhysChem 2017, 18 (24), 3508-3535.
5. Shafikov, M. Z.; Suleymanova, A. F.; Czerwieniec, R.; Yersin, H., Thermally Activated
Delayed Fluorescence from Ag(I) Complexes: A Route to 100% Quantum Yield at
Unprecedentedly Short Decay Time. Inorganic Chemistry 2017, 56 (21), 13274-13285.
6. Cheng, G.; So, G. K.-M.; To, W.-P.; Chen, Y.; Kwok, C.-C.; Ma, C.; Guan, X.;
Chang, X.; Kwok, W.-M.; Che, C.-M., Luminescent zinc(ii) and copper(i) complexes for high-
performance solution-processed monochromic and white organic light-emitting devices.
Chemical Science 2015, 6 (8), 4623-4635.
7. Forward, J. M.; Bohmann, D.; Fackler, J. P.; Staples, R. J., Luminescence Studies of
Gold(I) Thiolate Complexes. Inorganic Chemistry 1995, 34 (25), 6330-6336.
8. Chiou, J. Y. Z.; Luo, S. C.; You, W. C.; Bhattacharyya, A.; Vasam, C. S.; Huang, C.
H.; Lin, I. J. B., Gold(I) Complexes of N-Heterocyclic Carbenes and Pyridines. European
Journal of Inorganic Chemistry 2009, 2009 (13), 1950-1959.
9. Rios, D.; Pham, D. M.; Fettinger, J. C.; Olmstead, M. M.; Balch, A. L., Blue or Green
Glowing Crystals of the Cation [Au{C(NHMe)2}2]+. Structural Effects of Anions, Hydrogen
Bonding, and Solvate Molecules on the Luminescence of a Two-Coordinate Gold(I) Carbene
Complex. Inorganic Chemistry 2008, 47 (8), 3442-3451.
10. White-Morris, R. L.; Olmstead, M. M.; Jiang, F.; Tinti, D. S.; Balch, A. L., Remarkable
Variations in the Luminescence of Frozen Solutions of [Au{C(NHMe)2}2](PF6)·0.5(Acetone).
Structural and Spectroscopic Studies of the Effects of Anions and Solvents on Gold(I) Carbene
Complexes. Journal of the American Chemical Society 2002, 124 (10), 2327-2336.
11. Wang, H. M. J.; Vasam, C. S.; Tsai, T. Y. R.; Chen, S.-H.; Chang, A. H. H.; Lin, I. J.
B., Gold(I) N-Heterocyclic Carbene and Carbazolate Complexes. Organometallics 2005, 24 (4),
486-493.
12. Gómez-Suárez, A.; Nelson, D. J.; Thompson, D. G.; Cordes, D. B.; Graham, D.;
Slawin, A. M. Z.; Nolan, S. P., Synthesis, characterization and luminescence studies of gold(I)–
NHC amide complexes. Beilstein Journal of Organic Chemistry 2013, 9, 2216-2223.
13. Lu, T.; Yang, C.-F.; Steren, C. A.; Fei, F.; Chen, X.-T.; Xue, Z.-L., Synthesis and
characterization of Ag(i) and Au(i) complexes with macrocyclic hybrid amine N-heterocyclic
carbene ligands. New Journal of Chemistry 2018, 42 (6), 4700-4713.
14. Gierz, V.; Seyboldt, A.; Maichle-Mössmer, C.; Törnroos, K. W.; Speidel, M. T.;
Speiser, B.; Eichele, K.; Kunz, D., Dinuclear Coinage-Metal Complexes of Bis(NHC) Ligands:
165

Structural Features and Dynamic Behavior of a Cu–Cu Complex. Organometallics 2012, 31
(22), 7893-7901.
15. Liu, B.; Chen, W.; Jin, S., Synthesis, Structural Characterization, and Luminescence of
New Silver Aggregates Containing Short Ag−Ag Contacts Stabilized by Functionalized Bis(N-
heterocyclic carbene) Ligands. Organometallics 2007, 26 (15), 3660-3667.
16. Hamze, R.; Jazzar, R.; Soleilhavoup, M.; Djurovich, P. I.; Bertrand, G.; Thompson ,
M. E. Phosphorescent 2-, 3- and 4-coordinate cyclic (alkyl)(amino)carbene (CAAC) Cu(I)
complexes Chemical Communications [Online], 2017, p. 9008-9011.
17. Bochmann, M.; Romanov, A. S.; Becker, C. R.; James, C. E.; Di, D.; Credgington, D.;
Linnolahti, M., Copper and gold cyclic (alkyl)(amino)¬carbene complexes with sub-microsecond
photoemissions: Structure and substituent effects on redox and luminescence properties.
Chemistry A European Journal 2017, 23, 4625-4637.
18. Romanov, A. S.; Bochmann, M., Synthesis, structures and photoluminescence properties
of silver complexes of cyclic (alkyl)(amino)carbenes. Journal of Organometallic Chemistry
2017, 847, 114-120.
19. Gernert, M.; Müller, U.; Haehnel, M.; Pflaum, J.; Steffen, A., A Cyclic
Alkyl(amino)carbene as Two-Atom π-Chromophore Leading to the First Phosphorescent Linear
CuI Complexes. Chemistry A European Journal 2017, 23 (9), 2206-2216.
20. Di, D. W.; 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.
21. Conaghan Patrick, J.; Menke, S. M.; Romanov Alexander, S.; Jones Saul, T. E.;
Pearson Andrew, J.; Evans Emrys, W.; Bochmann, M.; Greenham Neil, C.; Credgington, D.,
Efficient Vacuum-Processed Light-Emitting Diodes Based on Carbene–Metal–Amides.
Advanced Materials 2018, 0 (0), 1802285.
22. Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.; Bertrand, G., Stable cyclic (alkyl)
(amino) carbenes as rigid or flexible, bulky, electron-rich ligands for transition-metal catalysts: A
quaternary carbon atom makes the difference. Angewandte Chemie-International Edition 2005,
44 (35), 5705-5709.
23. Frey, G. D.; Dewhurst, R. D.; Kousar, S.; Donnadieu, B.; Bertrand, G., Cyclic
(alkyl)(amino)carbene gold(I) complexes: A synthetic and structural investigation. Journal of
Organometallic Chemistry 2008, 693 (8–9), 1674-1682.
24. Romanov Alexander, S.; Becker Ciaran, R.; James Charlotte, 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.
25. O'Brien, C. J.; Kantchev, E. A. B.; Chass, G. A.; Hadei, N.; Hopkinson, A. C.; Organ,
M. G.; Setiadi, D. H.; Tang, T.-H.; Fang, D.-C., Towards the rational design of palladium-N-
heterocyclic carbene catalysts by a combined experimental and computational approach.
Tetrahedron 2005, 61 (41), 9723-9735.
26. Delaude, L.; Szypa, M.; Demonceau, A.; Noels, A. F., New In situ Generated
Ruthenium Catalysts Bearing N-Heterocyclic Carbene Ligands for the Ring-Opening Metathesis
Polymerization of Cyclooctene. Advanced Synthesis & Catalysis 2002, 344 (6‐7), 749-756.
166

27. Er, J. A. V.; Tennyson, A. G.; Kamplain, J. W.; Lynch, V. M.; Bielawski, C. W.,
Synthesis and Study of 5,5′-Bibenzimidazolylidenes and Their Bimetallic Complexes. European
Journal of Inorganic Chemistry 2009, 2009 (13), 1729-1738.
28. Leitl, M. J.; Krylova, V. A.; Djurovich, P. I.; Thompson, M. E.; Yersin, H.,
Phosphorescence versus Thermally Activated Delayed Fluorescence. Controlling Singlet-Triplet
Splitting in Brightly Emitting and Sublimable Cu(I) Compounds. Journal of the American
Chemical Society 2014, 136 (45), 16032-16038.
29. Wenderski, T.; Light, K. M.; Ogrin, D.; Bott, S. G.; Harlan, C. J., Pd catalyzed coupling
of 1,2-dibromoarenes and anilines: formation of N,N-diaryl-o-phenylenediamines. Tetrahedron
Letters 2004, 45 (37), 6851-6853.
30. Borguet, Y.; Zaragoza, G.; Demonceau, A.; Delaude, L., An Unexpected Synthesis of
Dihydrophenazines en Route to Benzimidazolium Salts. Advanced Synthesis & Catalysis 2012,
354 (7), 1356-1362.
31. Glorius, F.; Altenhoff, G.; Goddard, R.; Lehmann, C., Oxazolines as chiral building
blocks for imidazolium salts and N-heterocyclic carbene ligands. Chemical Communications
2002, (22), 2704-2705.
32. Khramov, D. M.; Boydston, A. J.; Bielawski, C. W., Highly Efficient Synthesis and
Solid-State Characterization of 1,2,4,5-Tetrakis(alkyl- and arylamino)benzenes and Cyclization
to Their Respective Benzobis(imidazolium) Salts. Organic Letters 2006, 8 (9), 1831-1834.
33. Wang, Y.-T.; Chang, M.-T.; Lee, G.-H.; Peng, S.-M.; Chiu, C.-W., Planar tris-N-
heterocyclic carbenes. Chemical Communications 2013, 49 (65), 7258-7260.
34. Grieco, G.; Blacque, O.; Berke, H., A facile synthetic route to benzimidazolium salts
bearing bulky aromatic N-substituents. Beilstein Journal of Organic Chemistry 2015, 11, 1656-
1666.
35. 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.
36. Back, O.; Henry‐Ellinger, M.; Martin Caleb, D.; Martin, D.; Bertrand, G., 31P NMR
Chemical Shifts of Carbene–Phosphinidene Adducts as an Indicator of the π‐Accepting
Properties of Carbenes. Angewandte Chemie 2013, 125 (10), 3011-3015.
37. Tominaga, K.; Walker, G. C.; Jarzeba, W.; Barbara, P. F., Ultrafast charge separation in
ADMA: experiment, simulation, and theoretical issues. The Journal of Physical Chemistry 1991,
95 (25), 10475-10485.
38. Goeb, S.; Rachford, A. A.; Castellano, F. N., Solvent-induced configuration mixing and
triplet excited state inversion exemplified in a Pt(ii) complex. Chemical Communications 2008,
(7), 814-816.
39. Pomestchenko, I. E.; Castellano, F. N., Solvent Switching between Charge Transfer and
Intraligand Excited States in a Multichromophoric Platinum(II) Complex. The Journal of
Physical Chemistry A 2004, 108 (16), 3485-3492.
40. Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T., The triplet state of
organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient
OLEDs. Coordination Chemistry Reviews 2011, 255 (21-22), 2622-2652.
41. McGlynn, S. P.; Azumi, T.; Kinoshita, M., Molecular Spectroscopy of the Triplet State.
Prentice-Hall, Inc.: Englewood Cliffs, N. J., 1969.
42. Samanta, P. K.; Kim, D.; Coropceanu, V.; Brédas, J.-L., Up-Conversion Intersystem
Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of
167

the Nature of Singlet vs Triplet Excited States. Journal of the American Chemical Society 2017,
139 (11), 4042-4051.
43. 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, 0 (0), 1801347.
44. 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.
45. Jacquemin, D.; Perpète, E. A.; Scuseria, G. E.; Ciofini, I.; Adamo, C., TD-DFT
Performance for the Visible Absorption Spectra of Organic Dyes: Conventional versus Long-
Range Hybrids. Journal of Chemical Theory and Computation 2008, 4 (1), 123-135.

168

Appendix 1. Spiro-center containing compounds with high T1 energy for
applications as hosts in blue OLEDs

A1.1. Introduction
Developing organic and organometallic materials with high T1 energies and long-term stability to
excitons and charges has been a long-standing problem in display and lighting technology. The
challenge is dual-fold, encompassing both the doped emitter as well as the host matrix. The former
has to meet strict requirements of color coordinates and operational stability in devices.
1, 2
The
latter should ideally have a T1 energy higher than that of the dopant, frontier orbital energy levels
that nest those of the dopant, and robust bonds that are immune to the high excited state energies
and charges present in devices.
3
Traditional hosts in blue OLEDs such as 3,3'-di(9H-carbazol-9-
yl)-1,1'-biphenyl (mCBP) and 1,3-di(9H-carbazol-9-yl)benzene (mCP) are known to have weak
exocyclic N-C bonds that are prone to rupturing as a result of triplet-polaron annihilation (TPA)
events.
4-6
Other blue OLED hosts such as ones including phosphine oxide groups with high
electron affinity are notoriously unstable in devices.
7
Reported ultrahigh energy gap phenylsilane
hosts (UGH) have low glass transition temperatures (Tg), which result in increased crystallinity
and more shorts in devices.
8
As such, the design criteria for blue OLED host materials include
bulk properties such as ensuring amorphous morphology, high Tg, and hindered aggregation which
lowers E(T1). Organic molecules with spiro-carbons satisfy all of these criteria. This appendix
examines two classes of spiro-containing materials: a pure hydrocarbon one based on bis-spiro-
9,9'-spirobi[fluorene] structural motif (bSF); and a heterocyclic one based on 8,8'-
spirobi[indolo[3,2,1-de]acridine] (SPINACH).
A1.2. Results and Discussion
A1.2.1. Synthesis

169

170

Figure A1. 1. Synthetic route for bSF (top) and SPINACH (bottom).
2,2’- and 2,4’-bSF were synthesized following Sukuzi-Miyaura coupling reactions from published
literature reports.
9
The synthesis of 3-Br-sBF was slightly more challenging,
10
and required the
preparation of the symmetric 3,6-dibromo-sBF, followed by the abstraction of one Br group and
its subsequent replacement with H. Unfortunately, standard Suzuki conditions failed to couple
3-Br-sBF and 4-B(OH)2-sBF. Homo- and hetero-coupling Suzuki conditions were also set up for
the one-pot synthesis of 2,2’-bSF and 2,4’-bSF, but only yielded 5% product in the case of the
former and none in the case of the latter. The SPINACH compounds on the other hand were easily
accessed following similar conditions used in the synthesis of 3-Br-sBF.
A1.2.2. Photophysical characterization
All four high-energy materials prepared showed bright violet luminescence in their
microcrystalline powder form and in solution. 2,2’-sBF has ΦPL = 100% ad τ = 1.3 μs in solutions
of 2-MeTHF at room temperature. The λmax of emission is only slightly red-shifted from that of
171

the microcrystalline powders, signifying a rigidochromic effect in that matrix. Rigidochromism is
also observed upon freezing the 2-MeTHF solution. No phosphorescence is observed from this
compound, even at 77 K and using gated emission techniques. This can be explained by the fully-
efficient fluorescence observed, which precludes any ISC events. Wong et al. reported E(T1) =
540 nm observed through gated phosphorescence experiments performed on thin films of the
sample at 77 K.
9
This finding is consistent with the T1 energy of the quaterphenyl obtained through
the conjugation of the para-connected SF units. This further highlights the importance of the
linkage position: the meta connectivity in 3,4’-bSF interrupts conjugation and leaves biphenyl as
the moiety with the lowest E(T1). In their earlier work on this class of host materials, Cui et. al
report phosphorescence at 77 K with T1 energies of 2.76 eV for 3,4’-bSF and 2.68 eV for
3,3’-bSF.
10

300 400 500 600
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Intensity (a.u.)
Wavelength (nm)
Ex RT
Ex 77k
Em RT
Em 77K

Figure A1. 2. Emission spectra of 2,2'-bSF in 2-MeTHF (left) and powdered form (right).
SPINACH 1 and 2 also are characterized by intense, high-energy fluorescence at room
temperature. Room temperature fluorescence of SPINACH 1 is characterized by broad, featureless
spectra that are red-shifted relative to SPINACH 2 (λem, max = 378 nm for the former and 364 nm
for the latter in 2-MeTHF). This is attributed to more CT character in the singlet excited state for
172

SPINACH 1, likely a result of its asymmetric structure. In contrast, the symmetric SPINACH 2
shows narrow, vibronically-structured fluorescence at RT. Gated phosphorescence spectra
recorded in frozen 2-MeTHF are also red-shifted for SPINACH 1 compared to SPINACH 2 (λem,
max = 430 nm for the former and 424 nm for the latter in 2-MeTHF). The microcrystalline powders
also show phosphorescence at comparable wavelengths to that recorded in dilute frozen glasses
which indicates that the presence of spiro-carbon centers disrupts efficient packing in the bulk and
maintains the high T1 energies.
250 300 350 400 450 500 550
0.0
0.5
1.0

Absorption 298K(2meTHF) Emission 298K (2meTHF)
Emission 77K (2meTHF) Emission 77K (Solid)
Normalized Intensity (a.u.)
Wavelength (nm)

350 400 450 500 550 600 650
0.0
0.5
1.0
RT, MeTHF_degassed
MeTHF, RT_N
2
Normalized Intensity (a. u.)
Wavelength (nm)
MeTHF, 77K
SS, 77K_Gated

Figure A1. 3. Absorption and emission spectra of SPINACH 1 (left) and 2 (right) at RT and 77
K.
Interestingly, we note that the fluorescence yield of SPINACH 2 is enhanced upon purging a
2-MeTHF sample of the solution. However, prolonged deareation only increases ΦPL to 42%
(Table A1. 1), indicating other modes of deactivation besides O2 quenching. Steady state 77 K
spectra show low-energy emission (λmax = 423 nm) attributed to phosphorescence, in addition to
the narrowed fluorescence line centered at 361 nm. Hence, intersystem crossing into T1 is a likely
deactivation pathway of S1 in SPINACH 2. The high T1 energy of the material aggregate makes
it a good candidate for a host material in blue OLEDs.

173

Table A1. 1. Photophysical properties of SPINACH 2 in 2-MeTHF at RT and 77 K.
SPINACH 2
(2-MeTHF)
RT 77 K
Air N2
ΦPL 28% 42% ---
τ (ns) 5.4 6.78 6.55

The frontier orbital energies of SPINACH 2 were probed using electrochemistry and modelled
using density functional theory performed at the B3LYP/6-31G** level. The results were found to
be in agreement (Table A1. 2), with the lowest unoccupied molecular orbital (LUMO) having a
sufficiently shallow energy for electrons to be preferentially trapped on the dopant in the emissive
layer (EML) of a functioning device. This favorable energetic alignment relieves host molecules,
which are significantly more abundant in the EML than dopants, from second order processes
involving high energy species, thereby likely extending device lifetimes.
Table A1. 2. Calculated and experimental parameters for SPINACH 2.
SPINACH 2 Calculated Experimental
E(HOMO) (eV) -5.53 -5.85
E(LUMO) (eV) -1.20 -1.34
E(S1) (eV) 3.77 3.49
E(T1) (eV) 2.80 2.80

174

A1.3. References
1. Song, W.; Lee Jun, Y., Degradation Mechanism and Lifetime Improvement Strategy for
Blue Phosphorescent Organic Light‐Emitting Diodes. Advanced Optical Materials 2017, 5 (9),
1600901.
2. 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.
3. May, F.; Al-Helwi, M.; Baumeier, B.; Kowalsky, W.; Fuchs, E.; Lennartz, C.;
Andrienko, D., Design Rules for Charge-Transport Efficient Host Materials for Phosphorescent
Organic Light-Emitting Diodes. Journal of the American Chemical Society 2012, 134 (33),
13818-13822.
4. 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.
5. Zhang, Y.; Lee, J.; Forrest, S. R., Tenfold increase in the lifetime of blue phosphorescent
organic light-emitting diodes. Nature Communications 2014, 5, 5008.
6. Freidzon, A. Y.; Safonov, A. A.; Bagaturyants, A. A.; Krasikov, D. N.; Potapkin, B. V.;
Osipov, A. A.; Yakubovich, A. V.; Kwon, O., Predicting the Operational Stability of
Phosphorescent OLED Host Molecules from First Principles: A Case Study. The Journal of
Physical Chemistry C 2017, 121 (40), 22422-22433.
7. Lin, N.; Qiao, J.; Duan, L.; Li, H.; Wang, L.; Qiu, Y., Achilles Heels of Phosphine Oxide
Materials for OLEDs: Chemical Stability and Degradation Mechanism of a Bipolar Phosphine
Oxide/Carbazole Hybrid Host Material. The Journal of Physical Chemistry C 2012, 116 (36),
19451-19457.
8. 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.
9. 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.
10. Cui, L.-S.; Xie, Y.-M.; Wang, Y.-K.; Zhong, C.; Deng, Y.-L.; Liu, X.-Y.; Jiang, Z.-Q.;
Liao, L.-S. Pure Hydrocarbon Hosts for ~100% Exciton Harvesting in Both Phosphorescent and
Fluorescent Light-Emitting Devices 2015.

Abstract (if available)

Abstract Phosphorescent organometallic complexes have been applied in fields ranging from photocatalysis to solar fuels, chemo- and biosensing, organic light emitting diodes (OLEDs), and solid state lighting (SSL). State-of-the-art phosphors in these applications employ rare-earth, heavy metals such as Ru(II), Os(II), Pt(II), and Ir(III). Among the more abundant, first-row transition metals, phosphorescent Cu(I) complexes have been studied most extensively. However, the vast majority of organocopper complexes investigated are 4-coordinate, bearing two bidentate ligands. Compared to their heavier metal analogues, the phosphorescence of tetrahedral Cu(I) complexes is either inefficient, characterized by long radiative lifetimes, or both. This work explores Cu(I) complexes with different geometries that can circumvent common non-radiative deactivation pathways. In particular, we highlight that 2-coordinate carbene-Cu(I) complexes with redox active ligands that display highly-efficient, Ir-like luminescence. Their Ag(I) and Au(I) analogues possess remarkable photophysical properties as well. ❧ Chapter 1 introduces the basics of phosphorescence in organometallic complexes, highlighting key differences between the luminescence of common organocopper(I) emitters and state-of-the-art organoiridium(III) and organoplatinum(II) phosphors. It provides an overview of the most impactful advances in Cu(I) luminescence, from poor and inefficient phosphorescence to thermally-activated delayed fluorescence (TADF). ❧ 2-, 3-, and 4-coordinate Cu(I) complexes employing N-Heterocyclic (NHC) and Cyclic (alkyl)(amino) carbenes (CAACs) are studied in Chapter 2. Completing the coordination sphere is a chloride or a trispyrazolyl borate (Tp) as the anionic ligand. Emission is tuned by modulating the electrophilicity of the carbene: through benzannulation and aza-substitution of the NHC, and through employing CAAC as a better electron acceptor. The steric substituents on the carbene control the binding mode of Tp: η³ Tp gives complexes with more efficient phosphorescence than η². ❧ Chapter 3 explores the solution equilibration between the structural isomers formed as a result of the different binding modes of Tp. XRay crystallography is used to examine the solid state structures of series of complexes bearing varying steric groups on the Tp ligand and the carbene. Variable temperature (VT) ¹H- and ¹³C NMR experiments help elucidate the mechanism behind isomeric equilibration, which is hampered and not curbed at low temperatures. Higher hapticity of Tp is found to enhance the photoluminescence quantum yields (ΦPL) of the Cu(I) complexes in microcrystalline powder form. Increased steric encumbrance of Tp results in improved ΦPL in solution. ❧ Simple, 2-coordinate Cu(I) complexes pairing CAACs with various amides were examined in Chapter 4. The redox properties of these linear complexes are largely determined by the ligands: oxidation by the electron-rich amides and reduction by the electron-deficient carbene. Density functional theory (DFT) calculations reveal a picture of the frontier molecular orbitals (MO’s) that mirror the electrochemistry: the highest occupied MO (HOMO) comprises primarily the amide N 2pz orbital, and the lowest unoccupied MO (LUMO) localizes on the carbene C 2pz orbital. The metal’s d-orbitals contribute only weakly to both orbitals, allowing for a representation of these complexes as donor-bridge-acceptor structures, with the Cu center acting as an efficient electronic bridge. The coplanar ligand orientation and the weak d-orbital overlap facilitate highly allowed amide-to-carbene charge transfer (ligand to ligand CT) transitions characterized by strongly absorbing CT bands and radiative rate constants (kᵣ > 10e⁵ s⁻¹) that rival state-of-the-art Ir(III) phosphors. This chapter registers the first 100% efficient Cu(I) emitters in fluid and polymeric media. Emission color is tuned via electronic substitution on the amide, and low temperature studies reveal a rich excited state manifold comprising ¹CT, ³CT, and a closely-lying amide-centered triplet state with ms photoluminescent (PL) decay times. Temperature-dependent studies show that efficient TADF occurs within the CT manifold, and one of the complexes is employed as an emitter in blue OLED devices. ❧ Chapter 5 comprises the CAAC-Ag(I) and Au(I) congeners of the Cu(I) complexes studied in chapter 4. The heavier metals enforce a larger ligand separation, contributing to smaller energy gaps between ¹CT and ³CT. In this series, the Ag(I) complex shows the longest carbene-amide distance, and thus the lowest exchange energy. The radiative rates are found to increase down the periodic table

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Asset Metadata

Creator Hamze, Rasha (author)

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

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 03/20/2019

Defense Date 11/20/2018

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

Tag carbenes,copper,electrochemistry,inorganic chemistry,luminescence,OAI-PMH Harvest,OLEDs,organometallic chemistry,phosphorescence,photochemistry,photophysics

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Malmstadt, Noah (committee member), Melot, Brent (committee member)

Creator Email hamze@usc.edu,rasha.hamze1@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-134845

Unique identifier UC11675346

Identifier etd-HamzeRasha-7169.pdf (filename),usctheses-c89-134845 (legacy record id)

Legacy Identifier etd-HamzeRasha-7169.pdf

Dmrecord 134845

Document Type Dissertation

Format application/pdf (imt)

Rights Hamze, Rasha

Type texts

Source 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 a...

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