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

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

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Content i

Synthesis, Photo- and Electroluminescence of Three- and Two-coordinate
coinage metal Complexes Featuring Non-N-Heterocyclic Carbene and
Non-conventional N-Heterocyclic Carbene Ligands

By
Shuyang Shi

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 2019 Shuyang Shi
ii

Acknowledgements
First, I would like to thank my graduate advisor and mentor, Prof. Mark Thompson for giving
me the opportunity to be a part of his research group and constant caring and support. I would not
have been able to start this dissertation without him, let alone finished it. His guidance continues
to reverberate far beyond these pages.
I am greatly appreciative of my committee members, Prof. Surya Prakash, Prof. Ralf Haiges,
Prof. Edward Goo, Prof. Barry Thompson and Prof. Smaranda Marinescu. Aside from their
intellectual contributions, each has demonstrated generosity and kindness at every turn.
I owe a special debt of gratitude to all my collaborators for providing data in this dissertation.
I thank Lee Collins and Prof. Michael Whittlesey for proving materials, Moon Chul Jung and
Caleb Coburn for all the device work, Abegail Tadle for X-ray crystallography and Daniel
Sylvinson for theoretical calculations. I would like to thank Rasha Hamze for her inspiration on
the Cu projects. Special thanks to Prof. Peter Djurovich for proving invaluable advice and
encouragement.
I would like to thank everyone in my research group who shares their experiences and
friendship. I am greatly appreciative of their kindness and mental support. I have been very luck
to work in such wonderful research environment. I am sincerely grateful for Judy Fong’s help.
Thank you for encouraging me when I was stressed by job hunting. Thank you for being a genuine
friend.
I would like to thank my parents and my brother for supporting me in various ways. Thank
my twin nephews for enlightening my life. Your smile is my biggest encouragement. Above all, I
would like to thank my love Bo Shen. This would not have been possible without you.
iii

Table of Contents

Acknowledgements ................................................................................................................... ii
List of Figures ............................................................................................................................ vi
List of Tables .............................................................................................................................. ix
Abstract ......................................................................................................................................... x
Chapter 1. Introduction ............................................................................................................. 1
1.1. Photophysics of mononuclear phosphorescent Cu(I) complexes ............................. 1
1.1.1. Four-coordinate Cu(I) complexes ...................................................................... 1
1.1.2. Three-coordinate Cu(I) complexes .................................................................... 4
1.1.3. Two-coordinate Cu(I) complexes ...................................................................... 5
1.1.4. Thermally-activated-delayed fluorescence (TADF) from Cu(I) complexes ..... 6
1.2. Cu(I) complexes as emitters in Organic Light Emitting Diodes (OLEDs) ............... 9
1.2.1. Mononuclear Four-coordinate Cu(I) complexes ............................................... 9
1.2.2. Mononuclear Three- and two- coordinate Cu(I) complexes ........................... 13
1.2.3. Multinuclear Cu(I) complexes ......................................................................... 15
1.3. Persistent carbene ligands ....................................................................................... 16
1.3.1. Conventional and Non-conventional N-Heterocyclic Carbene (NHC) ligands
………………………………………………………………………………..17
1.3.2. Non-N-Heterocyclic carbene ligands .............................................................. 19
Chapter 1 References ......................................................................................................... 20
Chapter 2. Synthesis and Characterization of Phosphorescent Three-Coordinate
Copper(I) Complexes Bearing Bis(amino)cyclopropenylidene carbene (BAC)...... 25
2.1. Results and Discussion ........................................................................................... 25
2.1.1. Synthesis and X-ray structures ........................................................................ 26
2.1.2. Photophysical Properties ................................................................................. 29
2.1.3. Theoretical Calculations .................................................................................. 31
2.2. Conclusion .............................................................................................................. 34
2.3. Experimental ........................................................................................................... 35
2.3.1. Synthesis .......................................................................................................... 35
2.3.2. X-ray crystallography ...................................................................................... 37
2.3.3. Density Functional Calculations (DFT) .......................................................... 37
iv

Chapter 2 References ......................................................................................................... 38
Chapter 3. Synthesis and Characterization of Phosphorescent Two-Coordinate
Copper(I) Complexes Bearing Diamidocarbene Ligands .............................................. 41
3.1. Results and Discussion ........................................................................................... 41
3.1.1. Synthesis and X-ray structures ........................................................................ 42
3.1.2. Photophysical properties.................................................................................. 44
3.1.3. DFT and TD-DFT Calculations ....................................................................... 52
3.2. Conclusion .............................................................................................................. 55
3.3. Experimental ........................................................................................................... 55
3.3.1. Synthesis .......................................................................................................... 55
3.3.2. X-ray crystallography ...................................................................................... 57
Chapter 3 References ......................................................................................................... 58
Chapter 4. Highly Efficient Photo- and Electroluminescence from Two-Coordinate
Cu(I) Complexes Featuring Non-conventional N-heterocyclic Carbenes ................. 62
4.1. Results and discussion ............................................................................................ 62
4.1.1. Synthesis and characterization ......................................................................... 63
4.1.2. Photophysical properties.................................................................................. 67
4.1.3. Electroluminescence ........................................................................................ 75
4.2. Conclusion ............................................................................................................. 78
4.3. Experimental ........................................................................................................... 79
4.3.1. Synthesis .......................................................................................................... 79
4.3.2. Electrochemical measurements ....................................................................... 87
4.3.3. Density Functional Calculations ...................................................................... 87
4.3.4. OLED fabrication and characterization ........................................................... 87
Chapter 4 References ......................................................................................................... 88
Chapter 5. Highly Efficient photo- and Electroluminescence from Two-Coordinate
Ag(I) and Au(I) complexes .................................................................................................... 92
5.1. Introduction to luminescent two-coordinate Ag(I) and Au(I) complexes .............. 92
5.2. Results and Discussion ........................................................................................... 94
5.2.1. Synthesis and characterization ......................................................................... 94
5.2.2. Photophysical properties.................................................................................. 97
5.2.3. Electroluminescence ...................................................................................... 104
5.3. Conclusion ............................................................................................................ 106
5.4. Experimental ......................................................................................................... 107
v

5.4.1. Synthesis ........................................................................................................ 107
Chapter 5 References ....................................................................................................... 110
Chapter 6. Conclusion ........................................................................................................... 113

vi

List of Figures
Figure 1.1. Examples of diimine and phosphine ligands used to prepare luminescent mononuclear
four-coordinate Cu(I) complexes. ................................................................................................... 2
Figure 1.2. Examples of luminescent four-coordinate carbene-Cu complexes. ............................ 3
Figure 1.3. Examples of luminescent three-coordinate Cu(I) complexes. ..................................... 4
Figure 1.4. Examples of luminescent two-coordinate Cu(I) complexes. ....................................... 6
Figure 1.5. Schematic diagram to illustrate the molecular mechanism of thermally activated
delayed fluorescence (TADF). ........................................................................................................ 7
Figure 1.6. Examples of four-coordinate Cu(I) complexes used as dopants in OLEDs. ............. 10
Figure 1.7. Examples of three- and two-coordinate Cu(I) complexes used as dopants in OLEDs.
....................................................................................................................................................... 14
Figure 1.8. Molecular structures of [Cu(PNP)]2 and of [Cu(PNP-
t
Bu)]2. .................................... 16
Figure 1.9. Ground-state electronic structure of imidazole-2-ylidenes.. ..................................... 17
Figure 1.10. Structures of some of the most common stable NHCs.. .......................................... 19
Figure 1.11. Examples of stable non-heterocyclic carbenes. ....................................................... 20
Figure 2.1. Molecular structures of complexes 1 and 2 and their IPr analogues. ........................ 26
Figure 2.2. Crystal structures of complexes 1-2, IPr1 and IPr2.. ................................................. 27
Figure 2.3. Two views of the four substructures of complex 2 superimposed with four different
colors.. ........................................................................................................................................... 28
Figure 2.4. Absorption and emission spectra of complexes 1 and IPr1 (a), 2 and IPr2 (b) at room
temperature (RT) and 77 K.. ......................................................................................................... 29
Figure 3.1. Molecular structures of complexes 1–4. .................................................................... 41
Figure 3.2. Molecular structures of the cation in 1 and of complexes 2–4.. ................................ 43
Figure 3.3. UV-visible absorption spectra of complexes 1–4 at room temperature in CH2Cl2.. . 44
vii

Figure 3.4. Emission spectra of complexes 1–3 in the solid state and in solution (1 in CH2Cl2 at
RT and ethanol at 77 K, 2 and 3 in CH2Cl2 at RT and 2-MeTHF at 77 K). ................................. 46
Figure 3.5. Emission spectra of complexes 1-3 in different solvents. (a) Complex 1 in CH2Cl2,
CH3CN and ethanol. (b) Complex 2 in CH2Cl2 and benzene. (c) Complex 3 in CH2Cl2 and benzene.
....................................................................................................................................................... 47
Figure 3.6. (a) Stern-Volmer plot of 1 by acetonitrile in CH2Cl2. (b) Space filling model of
complex 1.. .................................................................................................................................... 49
Figure 4.1. Molecular structures and frontier orbitals. (a) Molecular structures of complexes 1-6.
(b) LUMO (left) and HOMO (right) of complex 3 (top) and 6 (bottom). .................................... 63
Figure 4.2. Absorption spectra of complexes 1–6 at room temperature. (a) Spectra of complexes
1–6 in 2-MeTHF. (b) The energy of the low energy band (obtained from onset) versus the redox
gaps. (c) Spectra of complex 3 in methylcyclohexane (MeCy), toluene, 2-MeTHF and acetonitrile.
....................................................................................................................................................... 67
Figure 4.3. Emission spectra of complexes 1–6 in fluid solution at room temperature. (a) Spectra
of complexes 1–5 in 2-MeTHF. (b) Spectra of complex 3 in methylcyclohexane, toluene,
2-MeTHF and acetonitrile. ............................................................................................................ 69
Figure 4.4. Emission spectra of complexes 1-4 (top to bottom) at room temperature (RT) and 77 K
in 2-MeTHF. ................................................................................................................................. 71
Figure 4.5. (a) Emission spectra of complex 3 at 78-280 K in 2-MeTHF. (b) Schematic energy
diagram depicting the effect of rigidochromism on the ordering of the ICT/
3
Cz states. .............. 72
Figure 4.6. Photophysics of complexes 1–6 in polystyrene films. (a) The emission spectra at room
temperature and 77 K. (b) Energy gap law plot at room temperature. ......................................... 73
viii

Figure 4.7. (a) Emission lifetime versus temperature of complex 3 in the PS film. (b) Energy level
diagram for complex 3. ................................................................................................................. 74
Figure 4.8. Electroluminescent device characteristics containing complex 3 at doping
concentrations of 10%, 40% and 100%. (a) Electroluminescent spectra. (b) Current
density-voltage-luminance (J-V-L). (c) External quantum efficiency. ......................................... 76
Figure 4.9. Electroluminescent device characteristics containing complex 3 at doping
concentrations of 10%, 40% and 100%. (a) Electroluminescent spectra. (b) Current
density-voltage-luminance (J-V-L). (c) External quantum efficiency. ......................................... 76
Figure 5.1. Molecular structures of complexes 1-3. .................................................................... 94
Figure 5.2. Frontier orbitals of complexes 2 (top) and 3 (bottom). ............................................ 96
Figure 5.3. (a) Absorption spectra of complexes 1-3 in 2-methylTHF. (b) Absorption spectra of
complex 2 and (c) absorption spectra of complex 3 in methylcychohexane (MeCy), toluene,
2-MeTHF and acetonitrile (ACN). ............................................................................................... 98
Figure 5.4. Emission spectra of complexes 1-3 in 2-methylTHF (a) and methylcyclohexane (b) at
room temperature (RT) and 77 k. ................................................................................................. 99
Figure 5.5. Emission spectra of complexes 2 (a) and 3 (b) in methylcychohexane (MeCy), toluene,
2-MeTHF and acetonitrile (ACN). ............................................................................................. 100
Figure 5.6. Emission spectra of complexes 1-3 in polystyrene films at RT (a) and 77 K (b). .. 102
Figure 5.7. Top: Emission lifetime versus temperature of complexes 2 (a) and 3 (b). Bottom:
Energy level diagram for complexes 2 (c) and 3 (d). ................................................................. 103
Figure 5.8. Electroluminescent device characteristics containing complexes 2 and 3 at doping
concentrations of 40% for 2, and 40% and 100% for 3. (a) Electroluminescent spectra. (b) Current
density-voltage-luminance (J-V-L). (c) External quantum efficiency. ....................................... 105
ix

List of Tables
Table 2.1. Summary of selected bond lengths [Å] and angles [°] in 2. ....................................... 28
Table 2.2. Photophysical properties of complexes 1, 2, IPr1 and IPr2 in the solid state ............. 30
Table 2.3. Selected frontier orbitals and triplet spin density of complexes 1, 2, IPr1 and IPr2. .. 32
Table 3.1. Absorption data for complexes 1–4. ........................................................................... 45
Table 3.2. Luminescent properties of complexes 1–3 in the solid state and solution.
a
................ 48
Table 3.3. Frontier orbitals and triplet spin densities calculated for complexes 1–4. .................. 52
Table 3.4. Lowest vertical energy transitions for complexes 1–4 determined from TD-DFT
calculations. .................................................................................................................................. 54
Table 4.1. Redox data for complexes 1–6 and carbene-CuCl and dipole moments for 1–6 in the
ground state (S0), lowest ligand-localized excited state (Cz) and charge transfer excited states
(ICT). ............................................................................................................................................ 66
Table 4.2. Luminescent properties of complexes 1–6 in different media. ................................... 70
Table 4.3. Turn-on voltage (VT, defined at brightness of 0.1 cd/m
2
), maximum external quantum
efficiency (EQEmax), maximum brightness (Bmax), and emission maximum (λmax) of the OLEDs
with complex 3 at different doping concentration. ....................................................................... 78
Table 5.1. Redox data for complexes 1-3 and dipole moments for 1–3 in the ground state (S0),
lowest ligand-localized excited state (Cz) and charge transfer excited states (ICT). ................... 95
Table 5.2. Photophysical properties of complexes 1-6 in 2-MeTHF, 1% polystyrene (PS) films
and neat solid. ............................................................................................................................. 101

x

Abstract
Phosphorescent Cu(I) complexes have received a great amount of attention due to their
applications in organic emitting diodes (OLEDs), solar cell conversion and sensors. In the case of
OLEDs, Cu(I) complexes have been considered as potential alternatives to the successful
phosphorescent emitters using noble- metals due to the low cost of copper relative to such elements
as iridium and platinum. The most extensively studied mononuclear luminescent Cu(I) complexes
are four-coordinate tetrahedral homo- and heteroleptic complexes bearing diimine and
organophosphine ligands. Three-coordinate Cu(I) complexes bearing N-heterocyclic carbenes
have also been reported. Interestingly, while the catalytic properties of two-coordinate (NHC)Cu(I)
complexes have been investigated extensively, reports of their luminescent properties have only
appeared recently. This is due to the common belief that three and four-coordinate geometries at
the copper center are required for efficient luminescence.
Chapter 1 introduces the state-of-the-art luminescent Cu(I) complexes and their applications
in organic light emitting diodes (OLEDs). The properties of conventional and non_conventional
N-heterocyclic carbene (NHC) as well as non NHC ligands have also been described in this chapter.
The synthesis, crystal structures and photophysical properties of Cu(I) complexes bearing
non-NHC carbenes is described in Chapter 2. While luminescent carbene-Cu(I) complexes have
been well investigated, research to date has focused principally on Cu(I) complexes bearing
N-heterocyclic carbenes (NHC). In this work, we have investigated two three-coordinate
mononuclear Cu(I) complexes bearing the non-NHC ligand BAC
(bis(di-isopropylamino)cyclopropenylidene carbene) and compare their photophysical properties
to those of the reported NHC analogues bearing the Ipr
(1,3-bis(2,6-di-isopropylphenyl)imidazole-2-ylidene ligand. The X-ray structure of one of the
xi

BACCu complexes has four unique molecules in an asymmetrical unit with different distortion
angles at the copper center (from relatively more Y-shaped to more T-shaped geometry), which
demonstrates a shallow potential energy barrier of Y- to T-shaped distortion. The (BAC)Cu
compounds have photophysical properties comparable to their NHC analogues. We demonstrate
the possibility of using non-NHC carbenes for making efficient mononuclear luminescent copper(I)
complexes.
Chapter 3 focuses on the rarely studied system – two-coordinate Cu(I) complexes. The
cationic biscarbene Cu complex [(DAC)2Cu][BF4] shows a quantum efficiency of 65% in solution
which is one of the brightest mononuclear Cu(I) complexes reported so far. We demonstrate that
two coordination at the Cu center can also achieve high quantum yield if the ligands can provide
enough steric hindrance. Interestingly, the phosphorescence of this compound in CH2Cl2 solution
shows negligible quenching by oxygen in CH2Cl2 solution. This insensitivity to quenching is
attributed to the excited state redox potential being insufficient for electron transfer to oxygen as
well as the steric hindrance of the DAC ligands.
While the non-radiative rate (knr = 10
3
- 10
4
s
-1
) of [(DAC)2]Cu[BF4] is smaller compared to
most of other luminescent Cu complexes, the radiative rate (kr = ~10
4
s
-1
) is still at least an order
of magnitude smaller than the successful Ir and Pt emitters in fluid solution. The synthesis and
photophycial propertis of a series of neutral two-coordinate carbene-Cu(I)-carbazole complexes
bearing non-conventional N-heterocyclic carbene ligands DAC (diamidocarbene) and MAAC
(monoamidoamino carbene) have been introduced in Chapter 4. These complexes show almost
100% quantum yield with extremely short decay lifetime (~1 μs). The nature of the radiative
transition is charge transfer from the electron rich carbazole to the electron-deficient carbene, with
little metal contribution. The extremely fast radiative rate (kr) is due to thermally-activated-delayed
xii

fluorescence (TADF). Variations of both the donor groups (carbazole) and the acceptors (carbenes)
lead to emission color tuning over 270 nm from deep blue to red which covers the entire visible
region. Organic light-emitting diodes (OLEDs) fabricated with (MAC*)Cu(Cz) as a green
emissive dopant have high external quantum efficiencies (EQE = 19.4%) and brightness of (54000
cd/m
2
) with alleviated roll-off. The complex can also be used as a neat emissive layer to make
highly efficient OLEDs (EQE = 16.3%).
These linear Cu complexes is then extended to complexes bearing other coinage metals
including Ag and Au. In chapter 5, we report a series of linear luminescent, 2-coordinate Cu(I) (1),
Ag(I) (2), and Au(I) (3) complexes bearing MAC* and carbazole ligands. The complexes were
found to exhibit high quantum efficiency up to 90% in fluid and polymeric matrices with radiative
rates on the order of 10
5
– 10
7
s
-1
, which are extraordinary for monovalent coinage metal complexes
and comparable to state of the art organoiridium and organoplatinum phosphors. Especially
remarkable is the Ag(I) complex that is not only highly luminescent but also has sub-microsecond
radiative lifetimes. The CT state was found to have a small energy splitting between its singlet and
triplet manifolds, with ΔEST of 155 cm
-1
for the Ag analogue and 377 cm
-1
for the Au analogue,
priming the reported complexes for highly-efficient thermally activated delayed fluorescence
(TADF). OLED devices with complexes 2 and 3 as dopants and neat emitters were fabricated
through vapor deposition and show high external quantum efficiency up to 18.9% and alleviated
efficiency roll-off.
1

Chapter 1. Introduction

Photophysics of mononuclear phosphorescent Cu(I) complexes
Phosphorescent Cu(I) complexes have received a great deal of attention for their use in
applications including organic light emitting diodes (OLEDs), solar-energy conversion, sensors,
and biological systems. In the case of OLEDs, Cu(I) complexes have been considered as potential
alternatives to the successful phosphorescent emitters using noble-metals due to the low cost of
copper relative to such elements as iridium and platinum. The most extensively studied
mononuclear luminescent Cu(I) complexes are four-coordinate tetrahedral homo- and heteroleptic
complexes bearing diimine and organophosphine ligands. Recently, a variety of three-coordinate
luminescent Cu(I) complexes bearing N-heterocyclic carbene (NHC) ligands have also been
reported. Interestingly, while the catalytic properties of two-coordinate (NHC)Cu(I) complexes
have been investigated extensively, reports of their luminescent properties have only appeared
recently.
Four-coordinate Cu(I) complexes
The photophysical properties of mononuclear four-coordinate Cu(I) complexes bearing diimine
and organophosphine ligands have been studied since late 1970s.
1-3
As mentioned, the
photoluminescence originates from the low-lying metal-to-ligand charge transfer state. The
four-coordinate Cu(I) complexes are known to go through Jahn-Tellar distortion in the excited
state from tetrahedral to square planar geometry, leading to increase of non-radiative decay rates.
Therefore, for most of these Cu(I) complexes, even though the quantum efficiency could be very
high in the solid state, the emission is almost completely quenched in solution. This quenching
process has been proposed by Chen etc. to be due to the formation of a five-coordinate exciplex
2

involving direct coordination of the solvent to the metal center.
4-7
However, recent work by
Chergui etc. has questioned the strength and nature of this Cu-solvent interaction and has instead
attributed the luminescent quenching to the effect of outer-sphere solvation on the
3
MLCT energy.
8,
9
The quantum efficiency of mononuclear four-coordinate Cu(I) complexes have been improved
by introducing more sterically demanding ligands (Figure 1.1), such as bidentate phosphine
ligands DPEphos (DPEphos = bis[2-(diphenylphosphino)phenyl]ether). The ether linkage in
DPEphos imposes a larger bite angle than the monodentate phosphine ligand PPh3 (PPh3 =
triphenylphosphine) and inhibits an increase in the coordination number of the Cu(I) complex.
Introduction of alkyl or aryl groups on the 2,9 position of the dimine ligands has also been reported
to improve the quantum efficiency of the Cu(I) complexes by minimizing the distortion in the
excited state.
5, 10-12

Figure 1.1. Examples of diimine and phosphine ligands used to prepare luminescent
mononuclear four-coordinate Cu(I) complexes.
3

It is worth noting that cationic and neutral four-coordinate Cu(I) complexes bearing
N-heterocyclic-carbene (NHC) or cyclic (alkyl)(amino)carbene (CAAC) ligands have also been
reported (Figure 1.2).
13-16
The cationic four-coordinate Cu(NHC-N)(POP) complexes (POP =
bis[2-(diphenylphosphino)phenyl]ether) show metal-to-ligand charge transfer transition (MLCT)
with relatively long decay lifetimes (30-80 s).
1
H NMR spectra of the neutral carbene-Cu-Tp
complexes
13
(Tp = a trispyrazolylborate) show only three resonances corresponding to the three
unique protons on the pyrazolyl rings. This pattern is characteristic of C3 symmetry, indicating
equilibration of all three groups on the NMR time scale at room temperature. The emission of the
CAAC analogue is red-shifted compared the NHC analogue due to the better -accepting ability
of the CAAC ligand.

Figure 1.2. Examples of luminescent four-coordinate carbene-Cu complexes.
4

Three-coordinate Cu(I) complexes
As mentioned before, the four-coordinate mononuclear Cu(I) complexes undergo flattering
distortion in the excited state from tetrahedral to planar geometry. Three-coordinate Cu(I)
complexes, which starts with planar geometry, has the potential to reduce this type of structural
rearrangement in the excited state(Figure 1.3).
17-26
In terms of structure, either a bidentate N^N or

Figure 1.3. Examples of luminescent three-coordinate Cu(I) complexes.
5

P^P ligand is employed with a monodentate neutral or cationic ligand (NHC, halide, amide,
thiolate). Color tuning of the emission could be achieved by modifying either the bidentate ligands
(e.g., substituting the light main group elments to sulfur
27
or adding electron-donating or
withdrawing groups) or the monodentate ligands (e.g., extending the conjugation system of the
NHC ligands
18
). Interestingly, photophysics of P2Cu-carbazole complexes with -SiH-Cu motif
was also reported which demonstrates that -complexation can be used a strategy for designing
Cu-based light emitters.
28
Luminescent (phen)Cu complexes with bulky monodenate phosphine
ligands have also been reported. Among them, the complex bearing
2-(di-tert-butylphosphino)biphenyl) ligand is considerably emissive both in solution and solid
states.
20

The three-coordinate Cu(I) complexes are also reported to undergo Jahn-Tellar induced, Y-
to T-shape distortion upon excitation.
29, 30
Interestingly, the three-coordinate Cu complexes
bearing phosphine and amide ligands show high quantum efficiency (>20%) in fluid solution.
Organic light emitting diodes with these complexes have been fabricated and showed high EQE
with a significant roll-off.
Two-coordinate Cu(I) complexes
The photophysical properties of an interesting class of mononuclear two-coordinate Cu(I)
complexes have only been investigated recently, by attaching either an NHC
31
or CAAC
13, 25, 32, 33

ligands (Figure 1.4). Bochmann has recently published a report on a two-coordinate
CAAC-Cu-carbazole complex
32
which shows high efficient thermally-activated-delayed
fluorescence (TADF). It was claimed that the singlet-triplet energy splitting is strongly dependent
on the molecular configuration. The external quantum efficiency (EQE) of the organic light
6

emitting diode (OLED) is 9% with an alleviated roll-off. However, the photophysical properties
of the Cu complex utilized in the OLED devices were relatively unexplored. In another two
separate accounts, Romanov, Bochmann et al. reported that emission from CAAC–CuCl
complexes was exclusively from prompt fluorescence, with subnanosecond
33
and nanosecond
34

lifetimes. Such poor intersystem crossing and subsequent absence of phosphorescence is highly
unusual in luminescent Cu(I) complexes. Furthermore, the abovementioned results are not in
agreement with data obtained by Gernert
25
and Hamze,
13
who reported typical microsecond
lifetimes for similar complexes.

Thermally-activated-delayed fluorescence (TADF) from Cu(I) complexes
In the emission layer of an OLED device, the electron-hole recombination produces 75%
triplet excitons and 25% excitons (Figure 1.5)
35
. For luminescent materials to be applied in OLEDs,
it is essential that both singlet and triplet excitons be harvested in the emission layer to achieve

Figure 1.4. Examples of luminescent two-coordinate Cu(I) complexes.
7

high efficiency. Organometallic complexes with noble metals such as Ir(III), Pt(II), Re(I) and Ru(II)
can induce efficient spin-orbit coupling (SOC) between T1 and S1 and can achieve high radiative
rate constants from T1 to S0. Thermally-activated-delayed fluorescence (TADF) is another
mechanism to harvest all excitons, which can be realized by purely organic molecules or Cu
complexes that emit from charge-transfer(CT) states. Thermal activation, from the lowest triplet
state T1 to the higher lying singlet state S1 requires a relatively small energy separation ΔE(S1-T1)
between these states. At ambient temperature, efficient thermal activation with fast intersystem
crossing (ISC) is not expected to occur for ΔE(S1-T1) distinctly above 1000 cm
-1
(130 meV).

Figure 1.5. Schematic diagram to illustrate the molecular mechanism of thermally activated delayed
fluorescence (TADF). Reprinted with permission from Ref. 35. Copyright 2016 Coordination
Chemistry Reviews.
8

There are three crucial photophysical requirements for developing efficient TADF materials.
36

First, excited state distortion should be suppressed to achieve small non-radiative rate and hence
higher quantum efficiency. Upon an electronic excitation, Cu(I) complexes undergo distinct
distortions with respect to the ground state geometries, leading to an increase of non-radiative
deactivation. These shortcomings may be suppressed by using sterically hindered ligands on Cu,
which has been frequently discussed in the literature for the four-coordinate and three-coordinate
complexes[REF]. Surprisingly, rigidifying the molecular structures can also be achieved in
two-coordinate Cu(I) complexes, which we will address in Chapter 3. Secondly, ΔE(S1-T1) must
be small. This means the frontier orbitals, HOMO (highest occupied molecular orbital) and LUMO
(lowest unoccupied molecular orbital), must be spatially largely separated. This leads to a small
exchange interaction between the involved electrons and hence, to the required small splitting
between the singlet S1 and triplet T1. Lastly, the allowedness of the S1 to S0 transition (radiative
rate constant kr) should be as large as possible to obtain a short TADF decay time, because the
radiative rate constant kr (S1→S0) governs the TADF decay lifetime. However, kr and ΔE(S1-T1)
are correlated. Small splitting ΔE(S1-T1) requires a small exchange interaction between the
unpaired electrons resulting from the small HOMO-LUMO overlap. In the meantime, the small
HOMO-LUMO overlap leads to a small oscillator strength (small allowedness) of the the S 1→S0
transition. This correlation exists for a large amount of four-coordinate Cu(I) complexes that show
MLCT transitions. However, we found out that the two-coordinate carbene-Cu(I)-amide system
can break this correlation and hence, small ΔE(S1-T1) and high oscillator strength can be achieved
at the same time. The details will be discussed in Chapter 4.

9

Cu(I) complexes as emitters in Organic Light Emitting Diodes (OLEDs)
1.2.1. Mononuclear Four-coordinate Cu(I) complexes
For the four-coordinate Cu(I) OLEDs, a series of structural modification strategies have been
applied to achieve color tuning and high device efficiency and stability (Figure 1.6). These
strategies include introduction of bulky groups on the P^P or N^N ligands or incorporation of
electron-donating or withdrawing groups, extension of the -system of the ligands, etc.
37
The first
mononuclear four-coordinate Cu OLED was reported in 2004 by Wang et al.
38
Photophysics of a
series of Cu(NN)(PPh3) and Cu(NN)(DPEphos) have been reported in this work. All complexes
are green-emissive in PMMA films and the DPEphos analogues show higher quantum efficiency
than the PPh3 analogues. Incorporation of bulky groups on the phenanthroline ligands further
improve the efficiency with n-butyl substitution on the 2,9 positions of phenanthroline (dnbp)
giving the highest PLQY in PMMA film ( = 0.69%). Considering its promising properties, this
complex (Cu(dnbp)(DPEphos) was selected for OLED fabrication.
ITO/PEDOT:PSS/Cu(I):PVK/BCP/Alq 3/LiF/Al was employed as the device structure (ITO =
indium tin oxide; PEDOT:PSS = poly(3,4-ethylenedioxythiophene):polystyrene sulfonate; PVK =
poly(9-vinylcarbazole); BCP = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; Alq3 = tris(8-
hydroxyquinolinato) aluminum(III); LiF = lithium fluoride; Al = aluminum). The best
performance was achieved at a 16% of doping concentration, giving Lmax of 1663 cd/m
2
at 28 V,
CEmax of 10.5 cd/A at 1.0 mA/cm
2
and PEmax of 1.6 lm/W at 105 cd/m
2
. However, the efficiency
roll-off was severe likely attributed to the long exciton decay time of the complex (20 s), which
will promote triplet-triplet and triplet-polaron annihilation that are often the causes of lower
efficiencies at higher brightness/current.
10

Color tuning of the OLED devices have been achieved by extending the p-system of the N^N
ligands on Cu. Wang et al.
39
synthesized and fully characterized a series of orange-red to red
phosphorescent heteroleptic Cu(I) complexes (the first ligand: 2,2′-biquinoline (bq),
4,4′-diphenyl-2,2′-biquinoline (dpbq) or 3,3′-methylen-4,4′-diphenyl-2,2′-biquinoline (mdpbq);
the second ligand: PPh3 or DPEphos). With highly rigid bulky biquinoline-type ligands,
complexes [Cu(mdpbq)(PPh3)2](BF4) and [Cu(mdpbq)(DPEphos)](BF4) emit efficiently in 20 wt%
PMMA films with photoluminescence quantum yield of 0.56 and 0.43 and emission maximum of
606 nm and 617 nm, respectively. The complex [Cu(mdpbq)(DPEphos)](BF4) exhibits the best
device performance. With the device structure of ITO/PEDOT/TCCz:Cu(I)

Figure 1.6. Examples of four-coordinate Cu(I) complexes used as dopants in OLEDs.
.

11

(10 wt %)/TPBI/LiF/Al (III) (TCCz = N-(4-(carbazol-9-yl)phenyl)-3,6-bis(carbazol-9-
yl)carbazole; TPBI = 2,20,200-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole)), a external
quantum efficiency (EQE) up to 4.9% (CEmax = 8.0 cd/A) with max at 618 nm.
Li et al.
40
has demonstrated the color tunability by introducing electron-withdrawing group
(cyano) to the phen ligand. OLED devices based on [Cu(Dicnq)(DPEphos)]BF4 doped into CBP
(Dicnq = 6,7-Dicyanodipyrido[2,2-d:2’, 3’-f] quinoxaline; CBP = 4,4-N,N-dicarbazole-biphenyl)
has been fabricated by vapor deposition technique. Color-tuning EL emissions can be observed
from green-yellow to orange-red region as the doping concentration increases from 2% to 25%. A
low turn-on voltage of 4 V, a maximum current efficiency up to 11.3 cd/A, and a peak brightness
of 2322 cd/m
2
were achieved, respectively.
Osawa et al has synthesized and fully characterized a series of four-coordinate neutral Cu(I)
complexes Cu(dppb)(pz2Bph2), Cu(dppb-F)(pz2Bph2), and Cu(dppb-CF3)(pz2Bph2) (dppb =
1,2-bis(diphenylphosphino)benzene, dppb-F = 1,2-bis[bis(3,5-difluorophenyl)phosphino]benzene,
and dppb-CF3 = 1,2-bis[bis[3,5-bis(trifluoromethyl)phenyl]phosphino]benzene, pz2Bph2 =
diphenyl-bis(pyrazol-1-yl)borate). The thermal stability was improved by introducing F and CF3
substituents to the meta-positions of the four peripheral phenyl rings in the P^P ligands. The
improved thermal stability allowed the fabrication of their OLED devices by the vacuum
deposition approach. The best device performance was recorded for Cu(dppb-CF3)(pz2Bph2) with
EQEmax of 17.7% and CEmax of 54.1 cd/A.
As mentioned, luminescent four-coordinate Cu(I) complexes normally fall into two catogaries:
(i) the neutral complexes with a neutral bidentate ligand and a negative ligand and (ii) cationic
complexes with a neutral bidentate ligand and a neutral ligand. The former class of Cu compelxes
has good volatility and is suitable for fabricating efficient OLEDs via vapor deposition.
12

Nevertheless, cationic compounds generally have high sublimation temperature over their
degradation temperature, and decomposition easily happens during thermal evaporation. Therefore,
the emitters of OLEDs based cationic Cu complexes were mostly fabricated using a spin-coated
method. It is worth noting that spin-coat method will accelerate ligand dissociation of cationic
complexes in solution during the fabrication process.
39, 41
Therefore, it is highly desirable to design
cationic complexes that can be deposited by vapor evaporation. To improve the sublimability of
cationic Cu(I) complexes, two solutions have been employed, either by introducing bulky counter
anions
42
or bulky ligands.
43
These findings indicated that introducing bulky ligands or counter
anions can weaken lattice energies and electrostatic interaction and tend to improve volatilities of
these cationic complexes. By following these guidelines, a series of bipolar
[Cu(N^N)(DPEphos)]PF6 complexes containing both 4,5-diazafluorene and bis(carbazole) ligands
was synthesized by Zhang et al for realizing sublimable yellowish-green OLEDs.
44
These three Cu
complexes [Cu(DPEphos)(ECAF)]PF6 (ECAF = 9,9-bis(9ethylcarbazol-3-yl)-4,5-diazafluorene),
[Cu(DPEphos)(EHCAF)]PF6 (EHCAF = 9,9-bis(9-ethylhexylcarbazol-3-yl)-4,5diazafluorene),
and [Cu(DPEphos)(PCAF)]PF6 (PCAF = 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene),
feature several unique features compared with other cationic POP-based cuprous compounds,
which always contain small sized diamine ligands. First, bulky ligands ECAF, EHCAF, and PCAF
can weaken lattice energies and electrostatic interaction of their parental complexes and make them
easier to form film by vacuum deposition. Second, these complexes possess bipolar
charge-transporting abilities due to introducing hole-transporting carbazole and
electron-transporting 4,5-diazafluorene moieties, which can balance charge carriers in the
light-emitting layer in OLEDs to improve the efficiency of the device. OLED devices with the
structure of ITO/PEDOT: PSS (40 nm)/TCTA (15 nm)/1, 2, or 3 (10 wt %):mCP (30 nm)/ TmPyPb
13

(50 nm)/LiF (0.5 nm)/Al (100 nm) were fabricated. [Cu(DPEphos)(ECAF)]PF6 -based device
achieved a maximum luminance of 11010 cd/m
2
, a current efficiency of 47.03 cd/A, and an
external quantum efficiency of 14.81%. The high electroluminescence efficiencies of these
complexes are assumed to be due to their good thermal stabilities as well as the TADF feature that
captures both singlet and triplet excitons. Despite the good EQE, efficiency roll-off was observed
in all devices.
1.2.2. Mononuclear Three- and two- coordinate Cu(I) complexes
Osawa et al. have reported a series of three-coordinate (dtpb)CuX complexes by the
combination of 1,2-bis[bis(2-methylylphenyl)phosphino]benzene ligands (dtpb) with either Cl, Br
or I atoms to study the effect of the halide (Figure 1.7).
21, 23
DFT calculations showed that the
HOMOs were localized on the halogen atom and orbitals involving -character between the Cu(I)
and P atoms, whereas the LUMO was distributed on the o-phenylene group in the P^P ligand. All
three complexes showed green EL peaks which were slightly blue-shifted from Cl to I (527–513
nm) spectra. The heavy atom effect arising from the halogen atom was found to enhance the
radiative decay in the solid state. The best device was achieved for (dtpb)CuBr, with maximum
EQE of 21.3% and CE of 65.3 cd/A. Such a high device efficiency was attributed to the bulky P^P
ligand, leading to the suppressed non-radiative decay. By altering the ortho-substituents from
methyl to ethyl to isopropyl in the phosphine ligand, respectively,
21
a negligible effect of the alkyl
substituents on the electronic states of the three ligands was revealed. The crystal structures show
that the ortho-substituents were pointed towards the central metal atoms, giving rise to strong steric
hindrance and preventing the formation of a stable binuclear Cu(I) complex. It is worth noting that
the bidentate phosphine ligand without the ortho-substituents form halide-bridged dimer. Among
14

these three complexes, the best performance was achieved for (LEt)CuBr with EQEmax of 22.5%
and CEmax of 69.4 cd/A.
The electroluminescence properties of two-coordinate Cu complexes have recently been
investigated (Figure 1.7). Bochmann et al
32
has fabricated OLED devices based on a
two-coordinate neutral (CAAC)Cu(9-carbazole) complex, which showed a sub-microsecond
emission at 300 K, and such a lifetime was considerably shorter than other efficient Cu-based
TADF emitters (>3.3 s). It was claimed that the dihedral angle between the CAAC and the
carbazole ligands can somehow achieve low exchange energies and retain appreciable oscillator
strength at the same time. Although the proposed mechanism is controversial, this kind of emission
behavior may avoid the degradation pathway arising from bimolecular annihilation events. In fact,
we re-investigated this linear system by replacing the CAAC ligand with different carbenes and
found very interesting photophysical properties of this type of Cu complexes. More importantly,
highly efficient OLED devices with alleviated roll-off was obtained. Details will be given in
Chapter 4. Steffan et al.
25
has also demonstrated the use of a CAAC complex (CAAC)CuCl in
OLED devices. However, the performance of the devices was poor and showed inefficient energy
transfer from the host to the Cu dopant.

Figure 1.7. Examples of three- and two-coordinate Cu(I) complexes used as dopants in OLEDs.
15

1.2.3. Multinuclear Cu(I) complexes
Apart from mononuclear Cu(I) complexes, photophysical properties of multinuclear Cu(I)
complexes have also been investigated with applications in OLEDs. As what we learnt from the
mononuclear four-coordinate Cu(I) system, incorporation of rigid bidentate bisphosphine ligand
could (i) suppress solvent-induced exciplex formation due to steric factors and (ii) limit
problematic ligand distortion from the excited state.
45
The polynuclear system was also developed
to increase the rigidity of the complexes. In 2005, Peters et al investigated an amido-bridged
bemetallic copper system, [Cu(PNP)]2 (Figure 1.8), derived from a chelating bis(phosphine)amide
ligand ([PNP]
-
= bis(2-(diisobutylphosphino)phenyl)amide).
45
The quantum yield of this complex
are above 65% in cyclohexane and THF with a long decay lifetime (10.2 s). The high quantum
efficiency may be due to several factors. First, the relatively low structural reorganization energy
induced by the bulky PNP ligand effectively suppressed the structural distortion in the excited
state. This assertion is consistent with the relatively narrow full width at half-maximum (FWHM)
of its emission band (2400 cm
-1
). Second, the steric protection afforded by the bulky PNP ligand
removed the possibility of forming exciplex with the solvent molecules. In 2010, the same group
has investigated a close analogue of the originally reported complex, [Cu(PNP-
t
Bu)]2
46
(Figure
1.8), which shows efficient TADF. The quantum efficiency (57%) and decay lifetime (11.5 s) are
16

similar to the previously reported [Cu(PNP)]2. Vapor-deposited OLEDs doped with the complex
in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that
triplet excitons can be harvested very efficiently through the delayed fluorescence channel.
Liu et al.
47
demonstrated a new approach for utilizing CuI coordination complexes as emissive
layers in organic light-emitting diodes that involves in situ codeposition of CuI and
3,5-bis(carbazole-9-yl)pyridine (mCPy). The quantum yields of codeposited films were up tp 64%
depending on the molar ratio of CuI and mCPy. It was determined that the emitting species in
CuI:mCPy thin films is the dimeric complex [CuI(mCPy)2]2. A maximum luminance and external
quantum efficiency (EQE) of 9700 cd/m
2
and 4.4% of the OLED devices were achieved.
Persistent carbene ligands
Carbene is defined as neutral compounds containing a divalent carbon atom with a
six-electron valence shell. Their incomplete electron octet renders free carbenes inherently
unstable and they have been considered as highly reactive intermediates in organic
transformations.
48
The first isolable carbene was reported by Bertrand and co-workers in 1988,
which was stabilized by favorable interactions with adjacent phosphorus and silicon substituents.
49

Figure 1.8. Molecular structures of [Cu(PNP)] 2 and of [Cu(PNP-
t
Bu)] 2. [Reprinted with
permission from Ref. 45 and 46. Copyright 2005 and 2010 American Chemical Society.]
17

In 1991, Arduengo et al. reported an isolable carbene incorporated into an nitrogen heterocycle,
which is the first N-heterocyclic carbene (NHC), 1,3-di(adamantyl)imidazol-2-ylidene (IAd)
(Figure 1.10). The remarkable stability of this NHC led to an explosion of experimental and
theoretical studies with libraries of novel NHCs being synthesized and analysed.
48

1.3.1. Conventional and Non-conventional N-Heterocyclic Carbene (NHC) ligands
NHCs such as IAd exhibit a singlet ground-
state electronic configuration with the highest
occupied molecular orbital(HOMO) and the
lowest unoccupied molecular orbital (LUMO)
best described as a formally sp2-hybridized lone
pair and an unoccupied p-orbital at the C2 carbon,
respectively (Figure 1.9). The adjacent -
electron-withdrawing and p-electron-donating
nitrogen atoms stabilize this structure both inductively by lowering the energy of the occupied -
orbital and mesomerically by donating electron density into the empty p-orbital. The cyclic nature
of NHCs also helps to favor the singlet state by forcing the carbene carbon into a bent, more sp2-
like arrangement.
48

Conventional NHCs are the imidazolylidene that is a five-member ring with two N atoms
adjacent to the C2 atom.
50
This type of NHC also benefits from a greater degree of stabilization by
virtue of their partial aromaticity. However, stable NHCs without aromaticity (SIMes and SIPr)
have also been reported (Figure 1.10).
51
In addition, NHCs bearing alternative heteroatoms such

Figure 1.9. Ground-state electronic structure
of imidazole-2-ylidenes. [Reprinted with
permission from Ref. 48. Copyright 2014
Nature].
18

as sulfur and oxygen have also been isolated such as thiazolylidene and triazolylidene.
52
Bertrand
et al. have also synthesized a series of stable NHCs with only one heteroatom (nitrogen) in the
five-membered ring, cyclic(alkyl)(amino)carbene (CAAC).
53
The catalytic and photophysical
properties of organometallic complexes bearing CAAC ligands have been well-investigated.
13, 25,
32, 53

A class of non-conventional NHC, “abnormal” or mesoionic carbenes, where the carbene
carbon is at alternative positions to C2, has also been reported. For mesoionic carbenes, the
canonical resonance structures with the carbene depicted cannot be drawn without adding
additional charges. They are generally more electron-donating than their “normal” analogues and
can display very different properties.
54

While five-membered rings still make up the largest class of NHCs, examples containing
larger ring size such as monoamidoaminocarbene (MAC) and diamidocarbene (DAC) have also
been reported.
55, 56
The six-membered ring leads to increased steric shielding owing to the greater
N-C2-N bond angle, which effectively pushes the nitrogen substituents closer to the carbene center.
19

In addition, the carbonyl groups incorporated into MACs and DACs increase the -accepting
properties of the carbene and concomitantly lower the overall energy.
1.3.2. Non-N-Heterocyclic carbene ligands
Several classes of stable non-N-Heterocyclic carbenes have also been reported including
acyclic and cyclic derivatives featuring different heteroatoms or different ring sizes (Figure 1.11).
Bertrand et al. have isolated stable (phosphino)(silyl)carbene,
57
whereas Alder et al have reported
isolable carbenes stabilized by one dialkylamino group and an alkoxy, aryloxy, or thioaryloxy
groups.
58
(amino)(phosphino)carbene and positively charged (amino)(phosphonio)carbene have
also been synthesized.
59
Bertrand et al. have also expanded the variety of stable carbenes to acyclic
monoheteroatom-substituted carbenes such as
(phosphanyl)[2,6-bis(trifluoromethyl)phenyl]carbene.
60
Another family of carbenes is based on
a cyclopropenylidene core, a three-carbon ring with a double bond between the two atoms adjacent

Figure 1.10. Structures of some of the most common stable NHCs. Ad: adamantyl; Mes: mesityl;
tBu: tert-butyl; dipp: diisopropylphenyl.
20

to the carbenic one. This family is exemplified by bis(diisopropylamino)cyclopropenylidene
(BAC).
61
Interestingly, the catalytic properties of BAC-Cu(I) complexes have been studied, the
photophysical properties of these complexes were only recently reported by our group. The details
will be given in Chapter 2.

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25

Chapter 2. Synthesis and Characterization of Phosphorescent
Three-Coordinate Copper(I) Complexes Bearing
Bis(amino)cyclopropenylidene carbene (BAC)

2.1. Results and Discussion
As mentioned in section 2.3.2, trigonal three-coordinate luminescent Cu(I) complexes bearing
NHC ligands have been well investigated.
1-12
Barakat et al. used density functional theory (DFT)
calculations to optimize the triplet excited state of a three-coordinate Au complex and identified a
Jahn-Teller induced, Y- to T-shaped distortion in the excited state of three-coordinate Au(I)
complexes.
13
Krylova et. al. have observed a related luminescent quenching of three-coordinate
NHC-Cu(I) complexes in both coordinating and non-coordinating solvents and tentatively
assigned the process to a similar type of distortion in the excited triplet state.
5

While luminescent carbene-Cu(I) complexes have been well investigated, research to date has
focused principally on Cu(I) complexes bearing N-heterocyclic carbenes (NHC). In this chapter,
we have investigated two three-coordinate mononuclear Cu(I) complexes bearing the non-NHC
ligand BAC (bis(di-isopropylamino)cyclopropenylidene carbene) and compare their
photophysical properties to those of the reported NHC analogues bearing the IPr
(1,3-bis(2,6-di-isopropylphenyl)imidazole-2-ylidene) ligand
2, 5
(Figure 2.1). The X-ray structure
of complex 2 has four unique molecules in an asymmetrical unit with different distortion angles at
the copper center (from relatively more Y-shaped to more T-shaped geometry), which
demonstrates a shallow potential energy barrier of Y- to T-shaped distortion. We also find that
26

the (BAC)Cu compounds have photophysical properties comparable to their NHC analogues (IPr1
and IPr2).
Synthesis and X-ray structures
The copper compounds were obtained in good yields from the precursor (BAC)Cu(Cl).
Reaction of a stoichiometric amount of sodium di(2-pyridyl)dimethylborate and (BAC)Cu(Cl) in
dry THF gave 1 as a beige solid. Complex 2 was obtained as a yellow solid by deprotonation of
2-(2-pyridyl)benzimidazole with sodium hydride in THF followed by addition of the (BAC)Cu(Cl)
precursor. Both compounds are stable to air in the solid state and stable in solution under anaerobic
conditions.

Figure 2.1. Molecular structures of complexes 1 and 2 and their IPr analogues.

27

Single crystals of 1 and 2 suitable for X-ray diffraction studies were grown from THF/hexane
to afford the molecular structures in Figure 2.2. The compounds are monomeric, three-coordinate
structures with a planar geometry around copper (sum of bond angles around Cu = 359°). The
C-Cu bond lengths in 1 [1.870(2) Å) and 2 (1.853(6)-1.885(6) Å] are comparable to values
reported for their IPr-Cu(I) analogues (1.895(2) Å in (IPr)Cu(py2BMe2) and 1.877(2) Å in
(IPr)Cu(pybim))
2, 5
as well as in other NHC-Cu(I) complexes.
14-16
The plane defined by the Npy1,
Npy2 and Cu atoms in complex 1 is nearly perpendicular to the cyclopropenylidene ring of the BAC
ligand (torsion angle = 89°), whereas the imidazole ring of IPr1 is coplanar with the Npy1–Cu–Npy2
plane (torsion angle = 3.8°).
2
The coordination geometry of 1 is a distorted Y-shape (C–Cu–Npy
angles of 134.84(7)° and 127.04(8)° respectively), unlike the near ideal Y-shaped geometry found
in (IPr)Cu(py 2BMe2).
2
The two Cu–Npy distances in 1 bonds are also different (1.997(2) Å and
2.037(2) Å).

Figure 2.2. Crystal structures of complexes 1-2, IPr1 and IPr2. Only one of the substructures of 2
is shown (substructure a). The structures shown for IPr1 and IPr2 are taken from references 26 and
29, respectively. Thermal ellipsoids are shown at the 50% probability. All hydrogens are omitted
and all isopropyl groups are shown as wireframes for clarity. The atom colors are: C (grey), N (blue),
Cu (red) and B (green).
28

The plane defined by the Nim, Npy and Cu atoms of the pybim ligand in complex 2 is also
perpendicular to the cyclopropenylidene ring of the BAC ligand (torsion angles between 86–90°),
whereas the imidazole ring and pybim ligand in IPr2 are nearly coplanar (torsion angle = 8.9°).
5

Interestingly, there are four unique molecules in the asymmetrical unit of 2 (Figure 2.3). Selected
bond angles and bond lengths are shown in Table 2.1. The four substructures in 2 have different
coordination metrics at the copper center. All four substructures are distorted from the ideal
Y-shaped geometry but the degree of distortion increases from a to d. Structure 2a has the smallest
difference between C–Cu–Nim and C–Cu–Npy and smallest bond length difference between Cu–
Nim and Cu–Npy, whereas 2d has the largest difference in these same parameters. The differing
degree of distortion in these four substructures indicates a shallow potential energy barrier for the
Y- to T-shaped distortion in this three-coordinate Cu complex.

Figure 2.3. Two views of the four substructures of complex 2 superimposed with four different
colors (a: green, b: yellow, c: blue, d: red). Isopropyl groups are omitted and the pybim ligands are
truncated for clarity.
Table 2.1. Summary of selected bond lengths [Å] and angles [°] in 2.
Substructure C–Cu–N im C–Cu–N py N py–Cu–N im C–Cu (Å) Cu–N py (Å) Cu–N im (Å)
a 149.5(3) 130.3(2) 79.6(2) 1.885(6) 2.194(6) 1.933(5)
b 157.2(3) 122.5(3) 79.7(2) 1.873(7) 2.215(6) 1.934(6)
c 160.1(3) 119.4(3) 79.9(2) 1.870(7) 2.226(6) 1.932(6)
d 164.5(3) 114.9(2) 79.7(2) 1.853(6) 2.251(6) 1.931(5)

29

Photophysical Properties
Absorption spectra for complexes 1 and 2, as well as for IPr1 and IPr2, in CH2Cl2 are shown
in Figure 2.4. An intense band at 300 nm in 1 and 350 nm in 2 is assigned to spin-allowed
intraligand π to π* transitions. Transitions at lower energy ( > 300 nm in 1 and  > 350 nm in 2)
can be ascribed to charge transfer (CT) transitions because they are absent in the absorption spectra
of the ligand precursors.
2, 5
The slight red-shift in the lowest energy absorption band of the
(BAC)Cu compounds compared to their IPr analogues is likely due to destabilization of the
HOMO by the strong donating carbene (BAC) ligand.
Emission spectra for complexes 1, 2, IPr1 and IPr2 in the solid state are shown in Figure 2.4.
Photophysical data are summarized in Table 2.2. The complexes are virtually non-emissive in
solution. Complex 1 displays bright blue emission (max = 458 nm) with quantum efficiency of
72%, which is comparable to the IPr analogue, whereas 2 displays yellow-orange emission (max
= 558 nm) with quantum efficiency of 4%. Emission from both compounds can be fit to
single-exponential decays in the solid state. The microsecond lifetimes are indicative of
300 400 500 600 700
0.0
2.0k
4.0k
6.0k 1
IPr1
Wavelength (nm)
Molar absorptivity (M
-1
cm
-1
) (a)
0.0
0.5
1.0
1.5
2.0
Emission intensity (AU)
x20
RT
77 K
300 400 500 600 700
0.0
4.0k
8.0k
12.0k
16.0k
x20
Molar absorptivity (M
-1
cm
-1
)
2
IPr2
Wavelength (nm)
(b)
0.0
0.5
1.0
1.5
2.0
Emission intensity (AU)
RT
77 K

Figure 2.4. Absorption and emission spectra of complexes 1 and IPr1 (a), 2 and IPr2 (b) at room
temperature (RT) and 77 K. The absorption spectra are for CH 2Cl 2 solutions and the emission are for
microcrystalline powders. An expanded region of the absorption spectra is shown as well (x20). The
spectra of IPr1 and IPr2 are from references 26 and 29.
30

luminescence from a triplet state. There is a significant shift between the absorption and emission
bands, which is consistent with spin-allowed CT absorption and emission from IL triplet states
(supported by DFT calculations in the following section).
5
The only exception is the absorption
spectrum of 1 which shows a small energy shift between absorption and emission, due to a weak
absorption band at 450 nm, which is likely attributed to the S0 →T1 transition. The radiative and
non-radiative rate constants of 1 are similar to values found for IPr1. However, for complex 2, the
radiative rate constant is comparable to the IPr analogue, whereas the non-radiative rate is higher
by almost two orders of magnitude. This difference in the non-radiative rate is likely due to energy
migration to trap sites in 2.
17
These traps could be impurities or defects in the crystalline lattice or
since there are four distinct molecular structures in the unit cell one of them could be non-emissive
and act as a trap. At 77 K, the emission spectra of the neat samples of 1 and 2 shift to lower energy
(Table 2) and the decay lifetimes increase only a modest amount ( = 12 to 28 μs for 1 and  = 4.2
to 16 μs for 2). The relatively small increase in emission lifetime at 77 K indicates that
luminescence at room temperature is principally phosphorescence, not thermally activated delayed
fluorescence (TADF). TADF luminescence from Cu complexes typically displays an order of
magnitude or more increase in emission lifetime upon cooling to 77 K.
18-21
The excitation spectra
for both complexes match their respective absorption spectra.

Table 2.2. Photophysical properties of complexes 1, 2, IPr1 and IPr2 in the solid state
Emission at room temperature Emission at 77 K
 max (nm)  (s)
a
 PL
b
k r (10
4
s
-1
) k nr (10
4
s
-1
)  max (nm) τ (s)
a

1 458 12 0.72 6.0 2.3 470 28
IPr1
c
476 11 0.80 7.2 1.8 492 36
2 558 4.2 0.04 0.95 23 570 16
Ipr2
d
568 33 0.58 1.8 1.3 558 36

a
Error in τ is ±5%.
b
Error in  PL is ±10%.
c
Data from

reference 14.
d
Data from

reference 17.

31

Theoretical Calculations
Density functional theory and time-dependent DFT (TD-DFT) calculations were carried out
for all the complexes using geometric parameters obtained from X-ray analyses (substructure 2a
for 2) as initial inputs for optimization. The frontier molecular orbital (MO) surfaces calculated
for 1, 2, IPr1 and IPr2 are shown in Table 2.3. The triplet spin density calculated at both the
optimized triplet state and ground state (GS) geometry are shown in Table 2.3 as well. The
structures obtained from DFT for the optimized singlet state show that both the complexes have
trigonal geometry around copper center and dihedral angles of ca. 89°, which agree well with
values obtained by X-ray diffraction analysis. The C–Cu bond lengths of both the complexes are
slightly longer in the calculated structures (1: 1.870 Å → 1.913 Å, 2: 1.885 Å → 1.907 Å). The
same overestimation is observed for the Cu–N bond lengths. For complex 1, the Cu–N bond
lengths increase from 1.997 Å and 2.037 Å to 2.049 Å and 2.089 Å, whereas in 2 the bond
distances increase from Cu–Nim = 1.933 Å and Cu–Npy = 2.193 Å to Cu-Nim = 1.945 Å and Cu–
Npy = 2.331 Å. The py 2BMe2 ligand of complex 1 differs by ca. 10° in plane in the optimized single
32

geometry (C–Cu–N1: 134.84° → 123.99°, C–Cu–N2: 127.04 → 137.32° respectively). The bond
angles in the optimized structure of complex 2 (C–Cu–Nim = 161.2° and C–Cu–Npy = 119.4°) lie
between values found for the four unique structures in the crystal structure.
A comparison of the frontier orbitals in the (BAC)Cu and (IPr)Cu complexes reveals that
variation of the carbene ligands has a pronounced effect on HOMO (highest occupied molecular
orbital) and LUMO (lowest unoccupied molecular orbital) compositions. The HOMO of 1 consists
predominately of d orbitals on copper admixed with orbitals on py 2BMe2 ligand and the lone pair
electrons of the N atoms of amino groups on the carbene ligand. The HOMO of 1 is destabilized

Table 2.3. Selected frontier orbitals and triplet spin density of complexes 1, 2, IPr1 and IPr2.
Triplet Spin Density
HOMO LUMO Optimized Geometry
Ground-state
geometry
1

-4.71 eV -0.20 eV 3.20 eV (387 nm) 3.62 eV (342 nm)
IPr1

-4.87 eV -0.56 eV 3.11 eV (399 nm) 3.83 eV (323 nm)
2

-4.60 eV -0.76 eV 2.53 eV (496 nm) 2.91 eV (426 nm)
IPr2

-4.64 eV -0.85 eV 2.50 eV (496 nm) 2.91 eV (426 nm)
The frontier orbitals and triplet spin density were calculated using B3LYP functional with the
LACVP** basis set.

33

by the carbene ligand compared to the HOMO of the IPr analogue, which is dominated by the d
orbitals on copper and the py 2BMe2 ligand.
2
The LUMO of 1 is mainly localized on the py 2BMe2
ligand, whereas the LUMO of the IPr analogue is primarily localized on the NHC ligand.
2
The
lowest unoccupied MO with contribution from BAC ligand of 1 is LUMO+2 (+0.98 eV), which is
significantly higher than the energy of the IPr carbene. The MO with contribution from the
py 2BMe2 ligand of IPr1 is first found on LUMO+4 (-0.21 eV), which has similar energy as that of
the LUMO in 1. In complex 2, the HOMO and HOMO-1 are composed principally orbitals on the
benzimidazolate ligand with a minor contribution from the copper atom. The LUMO of 2 is
predominantly on the pyridyl moiety whereas the LUMO of IPr2 consists of orbitals from both the
carbene and the pyridyl moieties.
5

The geometries of the ligands around the copper center in the optimized triplet state are
notably twisted from what is observed in the crystal structures. For complex 1, the dihedral angels
between the py2BMe2 and BAC ligands (69°) decrease by 20° relative to the crystal structure. The
C–Cu–N1 and C–Cu–N2 angles also change by ~30° (134.84° → 103.35° and 127.04° → 160.24°
respectively). For complex 2, the dihedral angle between the pybim and BAC ligands (87°) is close
to what is found in the crystal structure, whereas the C-Cu–Nim (141.03°) and C–Cu–Npy (136.1°)
angles lie outside the ranges of the angles in the four unique molecules in the asymmetric unit of
2. Since the photophysical properties of these compounds were measured in the solid state, we
assume that the triplet spin density and energy calculated at optimized ground-state geometry will
better reflect the electronic structure in the solid state.
In order to better understand the nature of the absorptive and emissive transitions in these
materials, TD-DFT calculations were carried out. Both B3LYP and PBE0 functionals were used
for these calculations, with the latter designed for systems with long range coulombic
34

interactions.
22, 23
The TD-DFT results were similar for the two functionals and agree well with
absorption spectra obtained experimentally (estimated S0→S1 energies are within 0.2 eV of the
experimental values for both functionals). The lowest vertical singlet and triplet excitations
calculated by TD-DFT are mainly HOMO → LUMO transitions for 1 and HOMO and HOMO-1
→ LUMO transitions for 2. Therefore, the lowest lying triplet transitions for 1 can be ascribed as
(M+L)LʹCT (metal + BAC ligand) to py2BMe2 charge transfer) admixed with intra-ligand
(py2BMe2) charge transfer (ILCT) transitions. The lowest lying triplet transitions for 2 is
principally ILCT with a small amount of MLCT character, similar to what is found in IPr2.
The calculated triplet spin density further supports the TD-DFT assignment. For complex 1,
the spin contour is localized mainly on the py 2BMe2 ligand, whereas that of IPr1 is delocalized on
the whole complex. For both 2 and IPr2, the triplet spin density is on the pybim ligand with almost
no contribution from the metal. The triplet energy calculated for 2 and IPr2 (2.91 eV) are identical,
which correlates well the absorption and emission spectra. However, the calculated triplet energy
of 1 is about 200 meV lower than that of IPr1, which does not follow data from the emission
spectra. This mismatch between the experimental and computational results could be due to the
fact that the calculations were performed for complexes in the gas phase, whereas the emission
spectra of the compounds were measured in the crystalline solid.
2.2.Conclusion
The photophysical properties of two mononuclear three-coordinate copper(I) complexes
bearing BAC carbene have been investigated. The compounds are stable to air in the solid state
and in anaerobic solution. Surprisingly, even though the BAC carbene is more electron-donating
than IPr, the photophysical properties of these compounds are not dramatically different. Both
complexes show emission spectra and radiative rates similar as their IPr analogues. The quantum
35

efficiency of complex 1 is comparable to the IPr analogue. Complex 2 has four distinct structures
in an asymmetrical unit with different degrees of Y- to T-shaped distortion resulting from the
shallow potential energy barrier of this type of distortion for this three-coordinate compound.
Considering the electronic properties of these compounds in comparison with their NHC analogues,
we demonstrate the possibility of using non-NHC carbenes for making efficient mononuclear
luminescent copper(I) complexes.
2.3. Experimental
Synthesis
All reactions were carried out using standard Schlenk and glovebox techniques using dried
and degassed solvents. The (BAC)Cu(Cl)
24
precursor and sodium dimethyldi(2-pyridyl)borate
(Na(py2BMe2))
25, 26
were synthesized by following the literature procedures.
2-(2-Pyridyl)benzimidazole was purchased from Sigma-Aldrich. NMR spectra were recorded on
a Varian 400 NMR spectrometer and referenced to the residual proton resonance of acetone
((CD3)2CO) solvent at 2.05 ppm or acetonitrile (CD3CN) at 1.94 ppm. The UV-visible spectra
were recorded on a Hewlett-Packard 8453 diode array spectrometer. Photoluminescent emission
measurements were performed using a Photon Technology International QuantaMaster Model
C-60 fluorimeter. Phosphorescent lifetimes were measured by time-correlated single-photon
counting using an IBH Fluorocube instrument equipped with an LED excitation source. Quantum
yield measurements were carried out using a Hamamatsu C9920 system equipped with a xenon
lamp, calibrated integrating sphere and model C10027 photonic multi-channel analyer (PMA).
Syntheis of (BAC)Cu(py2BMe2) (1)
In a 100 ml Schlenk flask, (BAC)Cu(Cl) (60 mg, 0.18 mmol) and sodium
di(2-pyridyl)dimethylborate (40 mg, 0.18 mmol) were mixed in 20 mL of THF and stirred for 1 h
36

at RT under N2 atmosphere. The resulting yellow solution was filtered through a pad of Celite.
The filtrate was concentrated to ~3 mL and pentane was added resulting in yellow precipitate. The
precipitate was filtered and dried in vacuo. Single crystals suitable for X-ray diffraction studies
were grown from THF/hexane. Yield: 70 mg (79%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K):
0.27 (br s, 6H, BCH3), 1.46 (br s, 24H, CH3-CH), 4.04 (sept, J = 6.8 Hz, 4H, CH-CH3), 6.85 (ddd,
J = 7.4, 5.2, 1.5 Hz, 2H, Hpy), 7.41 (td, J = 7.6, 1.8 Hz, 2H, Hpy), 7.58 (d, J = 7.8 Hz, 2H, Hpy), 8.41
(ddd, J = 5.2, 1.8, 1.0 Hz, 2H, Hpy). Decoupled
13
C NMR δC (acetone-d6, 101 MHz, 298 K): The
chemical shifts for the py2BMe2 ligand are 14.75, 118.73, 127.75, 134.58, 147.73, 209.94 based
on spectra reported for Napy2BMe2 [
26
] and IprCupy 2BMe2 [
2
]. The chemical shifts for the BAC
ligand are 22.64, 50.09, 149.60 based on spectra reported in BACCuCl [
24
]. The carbene carbon is
not discernible. Analysis found for C27H42BCuN4: C, 65.15; H, 8.50; N, 11.15. Requires: C, 65.25;
H, 8.52; N, 11.27.
Synthesis of (BAC)Cu(pybim) (2)
In a 100 mL Schlenk flask, 2-(2-pyridyl)benzimidazole (35 mg, 0.18 mmol) and sodium
hydride (4.7 mg, 0.20 mmol) were dissolved in THF under N2 atmosphere. The reaction mixture
was stirred for 1 h followed by addition of (BAC)Cu(Cl) (60 mg, 0.18 mmol). The reaction mixture
was stirred for 3 hours, filtrated through Celite and the solvent was removed in vacuo. Single
crystals suitable for X-ray diffraction studies were grown by slow diffusion of hexane into THF.
Yield: 60 mg (68%). Resonances in the
1
H NMR of the compound are broad and featureless
indicating fluxional behavior in solution.
1
H NMR δH (CD3CN, 400 MHz, 298 K): 1.40 (br s, 24H,
CH3-CH), 3.92 (br s, 4H, CH-CH3), 6.99 (br s, 2H, Hbim), 7.34 (br, 1H, Hpy), 7.54 (br, 2H, Hbim)
7.92 (br s, 1H, Hpy), 8.38 (br s, 2H, Hpy). Decoupled
13
C NMR δc (CD3CN, 101 MHz, 298 K): The
chemical shifts for the BAC ligand are 22.59, 23.33, 48.66, 148.59 based on spectra reported in
37

BACCuCl [
24
]. The chemical shifts for the pybim ligand are 119.88, 120.95, 122.66, 123.51,
124.56, 126.22, 127.66, 129.19, 129.88, 139.01, 145.91, 147.35 based on spectra reported in
IPrCupybim [
5
]. The carbene carbon is not discernible. Analysis found for C27H36CuN5: C, 65.50;
H, 7.41; N, 14.07. Requires: C, 65.63; H, 7.34; N, 14.17.
X-ray crystallography
The single crystal X-ray diffraction data for both 1 and 2 were collected on a Bruker SMART
APEX DUO 3-circle platform diffractometer. The structures were solved by direct methods and
refined on F2 using the Bruker SHELXTL software package. The diffraction images show that the
compound adopts a larger unit cell with four symmetry independent molecules in the asymmetric
unit (Z’ = 4). Structure solution and refinement proceeded well using the large unit cell in space
group Pca21 with Z = 16, Z’ = 4. Structure solution and refinement was attempted in the smaller
unit cell in space group Pca21 with Z = 8, Z’ = 2 resulting in a large number of atoms with
non-positive displacement factors. Refinement in space group Pbca with Z = 8, Z’ = 1 resulted in
a structure with strong positional disorder, essentially resulting from the superposition of four
independent molecules of the larger unit cell. Therefore, we chose to use the X-ray structures from
the larger unit cell. All non-hydrogen atoms were refined anisotropically. CCDC 1819804 and
CCDC 1819805 contain the supplementary crystallographic data for complex 1 and 2. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.
Density Functional Calculations (DFT)
All calculations were performed using the Jaguar 9.6 (release 14) software pacakge on the
Schrodinger Material Science Suite (v2017-2). Gas phase geometry optimization was calculated
using B3LYP functional with the LACVP** basis set as implemented in Jaguar. Geometric
38

parameters obtained from single crystal X-ray analyses were used as a starting point for geometry
optimization.

Chapter 2 References
1. 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.
2. 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.
3. Visbal, R.; Gimeno, M. C., N-heterocyclic carbene metal complexes: photoluminescence
and applications. Chemical Society Reviews 2014, 43 (10), 3551-3574.
4. 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.
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. Hupp, B.; Schiller, C.; Lenczyk, C.; Stanoppi, M.; Edkins, K.; Lorbach, A.; Steffen, A.,
Synthesis, Structures, and Photophysical Properties of a Series of Rare Near-IR Emitting Copper(I)
Complexes. Inorganic Chemistry 2017, 56 (15), 8996-9008.
7. 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 2016, 23 (9), 2206-2216.
8. Elie, M.; Weber Michael, D.; Di Meo, F.; Sguerra, F.; Lohier, J. F.; Pansu Robert, B.;
Renaud, J. L.; Hamel, M.; Linares, M.; Costa Rub én, D.; Gaillard, S., Role of the Bridging Group
in Bis ‐Pyridyl Ligands: Enhancing Both the Photo ‐ and Electroluminescent Features of Cationic
(IPr)CuI Complexes. Chemistry – A European Journal 2017, 23 (64), 16328-16337.
9. Elie, M.; Renaud, J. L.; Gaillard, S., N-Heterocyclic carbene transition metal complexes
in light emitting devices. Polyhedron 2018, 140, 158-168.
10. Weber Michael, D.; Fresta, E.; Elie, M.; Miehlich Matthias, E.; Renaud, J. L.; Meyer,
K.; Gaillard, S.; Costa Rub én, D., Rationalizing Fabrication and Design Toward Highly Efficient
39

and Stable Blue Light ‐Emitting Electrochemical Cells Based on NHC Copper(I) Complexes.
Advanced Functional Materials 2018, 28 (17), 1707423.
11. 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.
12. 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.
13. 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.
14. Díez-González, S.; Stevens, E. D.; Scott, N. M.; Petersen, J. L.; Nolan, S. P., Synthesis
and Characterization of [Cu(NHC)2]X Complexes: Catalytic and Mechanistic Studies of
Hydrosilylation Reactions. Chemistry – A European Journal 2008, 14 (1), 158-168.
15. Bantu, B.; Wang, D.; Wurst, K.; Buchmeiser, M. R., Copper (I) 1,3-R2-3,4,5,6-
tetrahydropyrimidin-2-ylidenes (R=mesityl, 2-propyl): synthesis, X-ray structures, immobilization
and catalytic activity. Tetrahedron 2005, 61 (51), 12145-12152.
16. Kleeberg, C.; Cheung, M. S.; Lin, Z.; Marder, T. B., Copper-Mediated Reduction of CO2
with pinB-SiMe2Ph via CO2 Insertion into a Copper–Silicon Bond. Journal of the American
Chemical Society 2011, 133 (47), 19060-19063.
17. Büchner, R.; Field, J. S.; Haines, R. J.; Cunningham, C. T.; McMillin, D. R.,
Luminescence Properties of Salts of the [Pt(trpy)Cl]+ and [Pt(trpy)(MeCN)]2+ Chromophores:
Crystal Structure of [Pt(trpy)(MeCN)](SbF6)2. Inorganic Chemistry 1997, 36 (18), 3952-3956.
18. Blasse, G.; McMillin, D. R., On the luminescence of bis (triphenylphosphine)
phenanthroline copper (I). Chemical Physics Letters 1980, 70 (1), 1-3.
19. 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. Journal of the American Chemical Society 2010, 132 (27), 9499-9508.
20. Leitl, M. J.; Küchle, F.-R.; Mayer, H. A.; Wesemann, L.; Yersin, H., Brightly Blue and
Green Emitting Cu(I) Dimers for Singlet Harvesting in OLEDs. The Journal of Physical Chemistry
A 2013, 117 (46), 11823-11836.
21. 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.
40

22. Perdew, J. P.; Ernzerhof, M.; Burke, K., Rationale for mixing exact exchange with density
functional approximations. The Journal of Chemical Physics 1996, 105 (22), 9982-9985.
23. Adamo, C.; Barone, V., Toward reliable density functional methods without adjustable
parameters: The PBE0 model. The Journal of Chemical Physics 1999, 110 (13), 6158-6170.
24. Bidal, Y. D.; Lesieur, M.; Melaimi, M.; Cordes, D. B.; Slawin, A. M. Z.; Bertrand, G.;
Cazin, C. S. J., A simple access to transition metal cyclopropenylidene complexes. Chemical
Communications 2015, 51 (23), 4778-4781.
25. Hodgkins, T. G.; Powell, D. R., Derivatives of the Dimethylbis(2-pyridyl)borate(1−) Ion:
Synthesis and Structure. Inorganic Chemistry 1996, 35 (7), 2140-2148.
26. Khaskin, E.; Zavalij, P. Y.; Vedernikov, A. N., Facile Arene C−H Bond Activation and
Alkane Dehydrogenation with Anionic LPtIIMe2- in Hydrocarbon−Water Systems (L =
Dimethyldi(2-pyridyl)borate). Journal of the American Chemical Society 2006, 128 (40), 13054-
13055.

41

Chapter 3. Synthesis and Characterization of Phosphorescent
Two-Coordinate Copper(I) Complexes Bearing Diamidocarbene Ligands

Results and Discussion
while the catalytic properties of two-coordinate (NHC)Cu(I) complexes have been
investigated extensively,
1-8
reports of their luminescent properties have only appeared recently.
9,
10
This oversight may be due to a previous belief that three and four-coordinate geometries at the
copper center are required for efficient luminescence.
11-18
In this chapter, we have investigated
four linear diamidocarbene Cu(I) complexes (Figure 3.1), the previously reported
[(DAC)2Cu][BF4] (1)
3
and (DAC)Cu(2,4,6-Me3C6H2) (4),
19
and two new compounds,
(DAC)CuOSiPh3 (2) and (DAC)CuC6F5 (3) (DAC =
1,3-bis(2,4,6-trimethylphenyl)-5,5-dimethyl-4,6-diketopyrimidinyl-2-ylidene). Diamidocarbenes
display a combination of reduced σ-donor and greater π-acceptor properties relative to their
diamino counterparts.
20-23
We show that the bis(diamidocarbene) complex 1 exhibits high
photoluminescence quantum efficiency in CH2Cl2 solution and its phosphorescence is only weakly
quenched under aerobic conditions, which differs from most other luminescent Cu(I) complexes.
24-
26
(Figure 3.1).

Figure 3.1. Molecular structures of complexes 1–4.
42

3.1.1. Synthesis and X-ray structures
Complexes 1 and 4 were synthesized according to literature procedures.
3, 19
Complexes 2 and
3 were formed by protonolysis of either (DAC)CuO
t
Bu or 4 with Ph3SiOH or C6F5H and isolated
in 86% and 53% yield, respectively. Whereas the formation of 2 took place rapidly (<1 h) at room
temperature, protonolysis with pentafluorobenzene required heating to 333 K for ca. 12 h,
reflecting the higher acidity of the silanol (Ph3SiOH, pKa = 10.8;
27
C6F5H, pKa = 24.2).
28
Complex
1 is indefinitely stable to air, whereas 2–4 are each air- and moisture-sensitive in solution and the
solid state.
Single crystals of the compounds suitable for X-ray diffraction studies were grown from
CH2Cl2/hexane (1), toluene/hexane (2) or by slow evaporation of arene/hexane solutions (3 and 4)
to afford the molecular structures shown in Figure 3.2. The crystal structure of [(DAC)2Cu]
+
is for
[(DAC)2Cu][PF6].
23
The photophysical studies discussed below were carried out with
[(DAC)2Cu][BF4], but we do not expect the structure of the (DAC)2Cu
+
ion to be dependent on the
identity of the counter ion. Compounds 1–4 all have monomeric, two-coordinate structures with a
linear geometry around the copper center (1: 178.4(1);
23
2: 178.8(1); 3: 175.4(1); 4:
179.7(1)).
19
The Cu–CNHC bond lengths in 1 (1.926(2) Å and 1.927(2) Å)
23
, 2 (1.858(2) Å), 3
(1.902(3) Å), and 4 (1.905(3) Å)
19
are comparable to the values reported for diaminocarbene Cu(I)
complexes.
8, 17, 29-31
The significantly longer Cu–CNHC distances in 1, 3 and 4 compared to that of
2 and (DAC)CuCl (1.886(2) Å)
23
are consistent with added steric repulsion arising from the
presence of a second carbene ligand or a 2,6-disubstituted aryl group.
The torsion angle between the planes of the two carbene ligands in complex 1 (defined as the
N–C–N plane of the DAC ligand = 71°) is close to values found in other [(NHC)2Cu]
+
complexes
bearing bulky N-aryl-substituted carbenes.
3, 8, 29, 32
For complex 2, the Cu–O–Si angle (133.5(1)°)
43

and the torsion angle between the carbene ligand and Cu–O-Si plane (76°) are both similar to those
found for (IPr)CuOSiMe2Ph (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene).
30
In
contrast to the approximately perpendicular ligand planes in 1 and 2, the copper bound aryl groups
in complexes 3 and 4 are nearly coplanar with the diamidocarbenes , having respective dihedral
angles of 12° and 7°. The Cu–C6F5 distance (1.922(3) Å) in complex 3 is significantly longer than
that in (py)CuC6F5 (Cu–C6F5 = 1.891(2) Å),
33
but shorter than in three-coordinate
(IPr)Cu(tfppy) (Cu–C6F5 = 1.969(5) Å, tfppy = 2-(2,3,4,5-tetrafluorophenyl)pyridine).
12
The Cu–
C(mesityl) distance of 1.927(3) Å in complex 4 is comparable to the values found in
(I
i
Pr2Me2)Cu(2,4,6-Me3C6H2) (1.922(4) Å, I
i
Pr2Me2 =
1,3-di-isopropyl-4,5-dimethylimidazol-2-ylidene)
34
and (IPr)Cu(2-MeOC6H4) (1.9155(18) Å).
35

Figure 3.2. Molecular structures of the cation in 1 and of complexes 2–4. The structures shown for
1 and 4 are taken from references 32 and 28, respectively. Thermal ellipsoids are shown at the 30%
probability and all hydrogens are omitted for clarity. The atom colors are: C (grey), N (blue), O
(red), F (yellow) and Cu (green).

44

3.1.2. Photophysical properties
Absorption spectra for complexes 1–4 in CH2Cl2 are shown in Figure 3.3, data is given in
Table 3.1. Strong bands in the UV region ( < 300 nm) are assigned to -* transitions on the
ligands. Absorption bands at lower energy are assigned to charge transfer (CT) transitions as they
are absent in the free ligands. Relatively intense bands between 300–400 nm in 1 ( = 1.4 x 10
3

M
-1
cm
-1
) and 2 and 3 ( > 3 x 10
3
M
-1
cm
-1
), including those between 350–450 nm in 4, are
tentatively assigned to singlet metal-to-ligand charge transfer (
1
MLCT) transitions. Bands with
lower intensity ( < 1 x 10
3
M
-1
cm
-1
) at lower energy are assigned to CT states with
ligand-to-ligand character. Weak shoulders ( < 2 x 10
2
M
-1
cm
-1
) at the lowest energies of these
CT bands likely correspond to triplet CT states admixed with states having significant singlet
character (Figure 3.3, inset). The bands at low-energy bands between 400-550 nm for complexes
2–4 can be assigned to charge transfer (CT) transitions involving the non-carbene ligands since no
equivalent low energy absorption features are present in the bis-carbene complex 1.
300 400 500 600
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
400 500 600
0
500
1000

Molar absorbance (M
-1
cm
-1
)
Wavelength (nm)
1
2
3
4

Figure 3.3. UV-visible absorption spectra of complexes 1–4 at room temperature in CH 2Cl 2.
An expanded region of the visible spectra is shown in the inset.
45

Emission spectra for complexes 1–3 in the solid state and in solution are shown in Figure 3.4;
the photophysical data are summarized in Table 3.2. In the solid state, complex 1 gives bright blue
emission, 2 and 3 display yellow-green emission and 4 is non-emissive. The full width half
maximum (fwhm) value for the emission band of solid 1 is narrower (fwhm = 2300 cm
-1
) than that
of 2 (fwhm = 2900 cm
-1
) and 3 (fwhm = 3700 cm
-1
). The long lifetimes ( > 10 μs) found for the
three compounds as either neat solids or in frozen solution can be fit to single exponential decays
and are indicative of emission (phosphorescence) from triplet excited states. The broad emission
bands of 2 and 3 are consistent with luminescence from a triplet charge transfer (
3
CT) state,
whereas the narrower profile in 1 is indicative of a greater degree of intraligand (
3
IL) character in
the excited state. There is a less than two-fold increase in emission lifetimes upon cooling the solid
samples to 77 K, suggesting that thermally activated delayed fluorescence (TADF) does not
contribute significantly to the luminescent properties.
36

Table 3.1. Absorption data for complexes 1–4.
λ max(nm) (ε, 10
3
M
-1
cm
-1
)
1 325sh (3.58), 375 (1.37), 385 (1.42)
2 343 (7.96), 447 (0.38)
3 317 (7.50), 420 (0.79)
4 361 (3.86), 410sh (2.73), 510sh (0.52)
a
Absorption spectra recorded in CH 2Cl 2.
46

The photoluminescent quantum efficiency ( PL) of complexes 1–3 are quite high in the solid
state, reaching 0.85 for 1 and going down to 0.18 for 3. The variation in PL follows the trend in
radiative rate constants (kr), which decrease in the order 1 > 2 > 3, whereas the nonradiative rate
constants (knr) increase in the order 1 < 2 < 3. The trend in kr indicates that 1 has the greatest
amount of perturbing singlet character in its triplet state, whereas 3 has the lowest amount. The
increase in knr from 1 to 2 can be a consequence of following Energy Gap Law behavior;
37, 38

however, the larger increase in 3 appears to be caused to additional nonradiative decay processes
in the compound. The emission profiles for 1 and 3 in the solid state do not change markedly upon
0.0
0.5
1.0
0.0
0.5
1.0
400 500 600 700 800
0.0
0.5
1.0

Solid RT
Solid 77K
Solution RT
Solution 77K
1

Normalized intensity (a.u.)
2

Wavelength(nm)
3

Figure 3.4. Emission spectra of complexes 1–3 in the solid state and in solution (1 in CH 2Cl 2 at
RT and ethanol at 77 K, 2 and 3 in CH 2Cl 2 at RT and 2-MeTHF at 77 K).
47

cooling to 77 K, whereas the spectrum for 2 broadens and undergoes a 28 nm red-shift. In contrast,
the emission spectra for all three compounds broaden and undergo large bathochromic shifts
(∆λmax) in CH2Cl2 solution at room temperature (∆λmax = 34 nm, 1500 cm
-1
for 1, ∆λmax = 102 nm,
3000 cm
-1
for 2 and ∆λmax = 72 nm, 2200 cm
-1
for 3). The emission spectra are weakly
solvatochromic, compound 1 displays a 10 nm hypsochromic shift in polar CH3CN, whereas 2 and
3 undergo bathochromic shifts of 16 nm and 22 nm, respectively, in non-polar benzene (Figure
3.5). The hypsochromic shifts in solvents with high polarity (CH3CN for 1, CH2Cl2 for 2 and 3)
indicate that the excited state is destabilized, and thus less polar, than the ground state. Solution
studies at low temperature were performed in ethanol for 1 as the compound was insoluble in
2-methyltetrahydrofuran (2-MeTHF), whereas 2 and 3, while non-emissive in 2-MeTHF at room
400 500 600 700
0.0
0.5
1.0

Normalized Intensity (a.u.)
Wavelength (nm)
1-CH
2
Cl
2
1-CH
3
CN
1-ethanol
(a)
500 600 700 800
0.0
0.5
1.0

Normalized intensity (a.u.)
Wavelength (nm)
2-CH
2
Cl
2
2--Benzene
(b)
500 600 700 800
0.0
0.5
1.0

Normalized intensity (a.u.)
Wavelength (nm)
3-CH
2
Cl
2
3-Benzene
(c)

Figure 3.5. Emission spectra of complexes 1-3 in different solvents. (a) Complex 1 in CH 2Cl 2,
CH 3CN and ethanol. (b) Complex 2 in CH 2Cl 2 and benzene. (c) Complex 3 in CH 2Cl 2 and benzene.

48

temperature, are strongly emissive in the same solvent at 77 K. The emission spectra in solution
for both 1 and 2 display large hypsochromic shifts (50 nm for 1 and 140 nm for 2) upon cooling
to 77 K, whereas the spectrum for 3 is relatively unchanged. The rigidochromic shifts in 1 and 2
indicate that both compounds undergo large conformation changes in frozen media. In contrast,
the structure of compound 3 is effectively unchanged in both fluid and rigid solvent. A structural
change that can account for these differing behaviors is rotation of the ligand around the Cu–ligand
bond axis. Examination of the X-ray crystal structures (Figure 3.2) shows the aryl rings of adjacent
ligands in 1 and 2 can come within close contact if both ligands are allowed to freely rotate.
Therefore, significant structural changes caused by steric conflicts between opposing ligands can
be expected when either complex is dissolved in fluid media. However, the C 6F5 group in 3
presents a small degree of steric hindrance with respect to the adjacent DAC ligand, and thus only
minor distortion is likely to occur amongst the various rotational conformers in the complex. The
luminescent efficiency for both complexes 2 and 3 are also much lower in solution (2; PL = 0.03,
3;  PL < 0.01) compared to 1, which surprisingly remains highly emissive (PL = 0.65). Copper(I)
complexes generally have a significantly higher quantum efficiency as crystalline solids than in
fluid solution.
11, 14, 16, 39
The geometry of the compound is held rigidly in place in a crystalline
sample, whereas structural relaxation in the excited state can occur more easily in fluid solution
Table 3.2. Luminescent properties of complexes 1–3 in the solid state and solution.
a

Solid at room temperature
Solid at
77 K
Solution at room
temperature
b

Solution at
77 K
c

λ max
(nm)
τ
(μs)
d
 PL
e
k r
(10
4
s
-1
)
k nr
(10
4
s
-1
)
λ max
(nm)
τ
(μs)
d
λ max
(nm)
τ
(μs)
d
 PL
e
λ max
(nm)
τ
(μs)
d
1 456 18 0.85 4.7 0.83 460 19 490 18 0.65 440 20
2 534 16 0.62 3.9 2.4 562 20 636 1.2 0.03 496 16
3 534 12 0.18 1.5 6.8 543 13 606 0.37 <0.01 606 17
a
Complex 4 is non-emissive in either solid state or solution.
b
Recorded in CH 2Cl 2.
c
Complex 1
recorded in ethanol, 2 and 3 in 2-MeTHF.
d
Error in τ is ±5%.
e
Error in  PL is ±10%.
49

leading to both red shifted emission and enhanced non-radiative decay. Mononuclear Cu
complexes having PL > 0.40 in fluid solution are rare with only a few examples being previously
reported.
40, 41

To obtain a better understanding of the outstanding photophysical behavior of complex 1 in
fluid solution, a space filling model of the geometry optimized structures for 1 is shown in Figure
3.6a. The steric encumbrance imposed by the 1,3,5-Me3C6H2 rings on the N-substituents of the
carbene ligands in 1, in particular the ortho methyl groups, “lock” the aryl rings into positions
orthogonal to the N–C–N plane of the diamidocarbene, effectively minimizing excited state
deactivation caused by librational motion of the aryl rings. The rigidity of 1 leads to only a minor
decrease in non-radiative decay in fluid solution and a small red-shift in emission.
Further evidence of the significant steric crowding in 1 is provided by the luminescent
quenching behaviour in MeCN solution. While the emission efficiency and lifetime are strongly
diminished in MeCN (PL = 0.04,  = 1.1 s), the quenching rate constant determined by
Stern-Volmer analysis for MeCN in CH2Cl2 (Figure 3.6b) is extremely small (kq = 3.8 x 10
4
M
-1
s
-1
)
compared to the value found for the four-coordinate Cu(I) complex [Cu(dmp)2]
+
(kq =
0 5 10 15 20
0
4
8
12
16


0
/
ACN conc. (M)

0
/ = 1 + 

k
q
[ACN], k
q
= 3.8  10
4
M
-1
S
-1

(b)

Figure 3.6. (a) Stern-Volmer plot of 1 by acetonitrile in CH 2Cl 2. (b) Space filling model of
complex 1. The model is obtained from geometry optimization using DFT calculations. Atom
colors are: C (grey), H (white), N (blue), O (red) and Cu (green).
50

1.8 x 10
7
M
-1
s
-1
, (dmp = 2,9-dimethyl-1,10-phenanthroline).
42
The high quenching rate constants
of bis(1,10-phenanthroline) Cu(I) complexes in Lewis basic solvents such as MeCN has been
proposed to be due to formation of an exciplex involving direct coordination of the solvent to the
metal center, which is expected to be precluded in 1.
16, 39, 43, 44
However, recent work has
questioned the strength and nature of this copper-MeCN interaction and has instead attributed the
luminescent quenching to the effect of outer-sphere solvation on the
3
MLCT energy.
45, 46

Regardless, the roughly thousand fold smaller value for the quenching rate constant of 1 by MeCN
relative to that of [Cu(dmp)2]
+
implies effective steric protection of the copper complex by the
DAC ligands.
A commonly cited application for phosphorescent copper complexes is as oxygen sensors,
due to the high propensity for oxygen to quench their emission.
24-26
Thus, a decrease in the
luminescent efficiency or lifetime of the complex in a given environment relative to the same
complex under anaerobic conditions can be used to quantify the amount of oxygen present.
Surprisingly, phosphorescence for 1 is only slightly decreased ( PL = 0.50) when a CH2Cl2
solution is sparged with O2. Similarly, the luminescent lifetime under nitrogen ( = 18 s) is only
slightly diminished in oxygenated CH2Cl2 ( = 14 s). This relative insensitivity of the emission
intensity and lifetime to oxygen is highly unusual for phosphorescent compounds. There are two
possible quenching mechanisms of the triplet excited state by oxygen, involving either electron
transfer or energy transfer.
47
For luminescent quenching by electron transfer (eq 1a) to be
thermodynamically feasible, the excited state of 1 has to have a sufficient potential for oxidative
quenching to be exergonic (eq 1b):
1
+
* + O2 → 1
2+
+ O2
-
1a
G = -nF[E(O2
0/-1
) -E(1
2+/1+
*)] 1b
51

Work terms needed to account for coulombic attraction/repulsion between the products and
reactants in eq 1b should also be considered, but these values are typically small in solvents with
high dielectric constants and for purposes of discussion here will be neglected. The value for
E(1
2+/1+
*) in eq 1b is usually obtained by subtracting the spectroscopic excited state energy from
the oxidation potential i.e. [E(1
2+/1+
*) = E(1
2+/1+
) - E0-0]. Unfortunately, we were unable to obtain
a value for E(1
2+/1+
) as we could not observe a discernible oxidation wave for 1 using cyclic
voltammetry in MeCN. However, two distinct reversible reduction waves are present at
E
1/2
= -1.48 V and -1.78 V versus Fc
+
/Fc (Fc = ferrocene), thus allowing a limiting value for the
thermodynamic potential for quenching by electron transfer to oxygen to be approximated using
eqs 2a-b:
1
0
+ O2 → 1
+
+ O2
-
2a
G = -nF[E(O2
0/-1
) -E(1
1+/0
)] 2b
Using the reported value of E(O2
0/-1
) in MeCN (-1.29 V vs Fc
+
/Fc)
48
in eq 2b gives an exergonic
free energy (G = -0.21 V). However, a substantial increase in this free energy will be present
when quenching 1* since a significant coulombic attraction needs to be accounted for in the
removal of an electron from 1
+
(eq 1b) as opposed to 1
0
(eq 2b). This change in coulombic
interaction can be estimated from the difference between the standard redox couples Cu(II)/Cu(I)
and Cu(I)/Cu(0) (E = +0.36 V).
49
Upon adding this value to eq 2b, electron transfer becomes
endergonic and thus, quenching of 1* by O2 (eq 1a) will be a thermodynamically unfavorable
process. This leaves Dexter energy transfer from the triplet state to oxygen (forming singlet oxygen)
as a potential quenching pathway. Efficient Dexter energy transfer requires good overlap between
the frontier molecular orbitals of both species,
47
which is expected to be severely constrained due
to the steric demands of the DAC. Therefore, we can conclude that the relative insensitivity of
52

phosphorescent quenching of 1 by O2 is due to the combined effects of the high oxidation potential
of the complex along with steric protection of the metal center by the DAC ligands.
3.1.3. DFT and TD-DFT Calculations
Density functional theory (DFT) calculations were carried out for all of the diamidocarbene
complexes using geometric parameters obtained from X-ray analyses as starting structures for 1–
4. The frontier molecular orbital (MO) surfaces calculated for 1–4 are shown in Figure 3.7. The
lowest singlet and triplet vertical energies determined by time-dependent DFT (TD-DFT)
calculations are given in Table 3.3. The optimized ground state structures of 1–4 have a linear

Table 3.3. Frontier orbitals and triplet spin densities calculated for complexes 1–4.
HOMO LUMO Spin density
1

-8.85 eV -4.97 eV
2

-5.90 eV -2.60 eV
3

-5.85 eV -2.66 eV
4

-4.86 eV -2.49 eV

53

coordination geometry at the copper center, with bond lengths that correlate well to the values
from the X-ray structures. The torsion angles from the crystal structure and the optimized geometry
in complex 1 are similar (71 and 79 respectively), reflecting the steric constraints of the two
DAC ligands. In contrast, the DFT optimized geometry of complex 2 fails to reproduce the large
ligand-ligand torsion angle in the crystal structure (X-ray: 76; DFT: 56). This mismatch between
experimental and computational structures is also seen for complexes 3 and 4 and is attributed to
steric repulsion between the phenylene and DAC ligands, leading to larger torsion angles in the
optimized geometries when compared to the experimental values measured in the crystal structures
(3: 12→24, 4: 7→60). The optimized structures of the T1 states in 1, 3 and 4 retain a linear
coordination geometry at the copper center (S0→T1: 1: 178→179, 3: 175→179, 4:
180→178). Compound 2 has a smaller angle around copper center in the T1 state (S0→T1:
179→156). The Cu–CNHC bond distances decrease ca. 0.05 Å in T1 state of 1, 3 and 4 and
increase by 0.03 Å in 2. The bond lengths to the other ligand get either similarly longer (1 and 3)
or remain unchanged (2 and 4). The torsion angles between the ligands in the T1 state remain
unchanged in 1 and 4, whereas the torsion angles decrease in 3 (78→ 56) and 4 (32→ 24).
Interestingly in 4, despite having a linear CNHC-Cu-CMes coordination geometry, the mesityl ring
is no longer linearly coordinated to Cu. Instead, the aryl ring is bent with a Cu-CMes-centroidMes
angle of 160.
54

For complexes 1–4, the calculated LUMOs have essentially identical orbital character,
consisting predominantly of π* orbitals on the diamidocarbene ligands mixed with d-orbitals on
copper. However, variation of the non-carbene ligand has a pronounced effect on HOMO
composition and orbital energy. For the three heteroleptic Cu(I) complexes, the HOMOs are
mainly localized on the metal and the non-carbene ligands. The HOMO energies of 2 and 3 are
similar, but that of 4 is destabilized by 1.0 eV due to the strong electron-donating ability of the
mesityl group. TD-DFT calculations of 1–4 show that the calculated wavelength of the S0→S1
transitions correlate well with the solution absorption onsets (Table 3.3). The calculations indicate
that the lowest lying triplet transitions for complex 1 is intra-ligand charge transfer (ILCT)
admixed with metal-to-ligand charge transfer (MLCT) transitions. The lowest lying triplet
transitions for 2–4 are principally MLCT admixed with ligand-to-ligand charge transfer (LLCT)
character. The calculated spin density surfaces for the triplet electronic configuration further reflect

Table 3.4. Lowest vertical energy transitions for complexes 1–4 determined from TD-DFT
calculations.
Complex transitions
a
λ (nm) 𝑓
1
S0→S1 396 0.0058
S0→T1 430 0
2
S0→S1 510 0.0032
S0→T1 561 0
3
S0→S1 494 0.0021
S0→T1 540 0
4
S0→S1 645 0.0049
S0→T1 703 0
a
Orbital contributions to each transition are given in the supplementary information.
55

these same assignments for emissive state showing contours that are principally localized on the
DAC ligand and metal center (Figure 3.7).
Conclusion
The photophysical properties of a series of four linear, two-coordinate diamidocarbene
copper(I) have been investigated. Complex 1 is stable to air and moisture, whereas 2–4 are air- and
moisture sensitive. The bis(diamidocarbene) complex 1 displays narrow emission band relative to
the other three diamidocarbene species and has a high photoluminescence quantum yield in both
the solid state and CH2Cl2 solution (PL = 0.85 and 0.65, respectively). The phosphorescence of 1
is only weakly quenched by O2, which is remarkable for a Cu phosphor with an 18 sec lifetime.
Complex 1 contains a sterically demanding ligand, suggests that the steric bulk of the ligands
around Cu is an important factor in designing systems with increased photoluminescence
efficiency and suppressed quenching by oxygen. These results echo observations on mononuclear
four-coordinate copper complexes,
11, 40
where increasing the steric bulk of the ligands bound to
copper limits the structural changes that occur in the excited state, thereby increasing the
luminescence efficiency.
Experimental
Refer to section 2.2.2 for the methods used for Photophysical Characterization and section 2.2.4
for Density Functional Calculations.
3.3.1. Synthesis
All manipulations were carried out using standard Schlenk, high vacuum and glovebox
techniques using dried and degassed solvents. Hexane and toluene (purified using an MBraun SPS
solvent system) and benzene (refluxed over sodium dispersion) were all dried further over 3 Å
56

molecular sieves and stored over potassium mirrors. THF was refluxed over sodium wire and
stored over 3 Å molecular sieves. C6D6 was dried over potassium and vacuum transferred. NMR
spectra were recorded on a Bruker Avance 500 MHz NMR spectrometer and referenced to  7.16
(
1
H) and  128.0 (
13
C).
19
F spectra were referenced to CFCl3 at  = 0.0. IR spectra were recorded
as KBr discs on a Nicolet Nexus spectrometer. Elemental analyses were performed by the
Elemental Analysis Service, London Metropolitan University, London, UK and Elemental
Microanalysis Limited, Okehampton, Devon, UK. DAC,
22, 50
[(DAC)2Cu][BF4] (1),
3

(DAC)CuO
t
Bu,
19
[Cu(2,4,6-Me3C6H2)]n
51
and (DAC)Cu(2,4,6-Me3C6H2) (4)
19
were prepared
according to literature methods.
Synthesis of (DAC)CuOSiPh3 (2). A benzene (20 mL) solution of (DAC)CuO
t
Bu (0.455 g,
0.886 mmol) and Ph3SiOH (0.273 g, 0.989 mmol) was stirred at room temperature for 1 h, with a
yellow precipitate being generated very early in the reaction. The solvent was removed under
reduced pressure, the yellow residue dissolved in a minimum amount of toluene and reprecipitated
by addition of hexane. The solid was cannula filtered, washed with hexane (20 mL) and dried in
vacuo. Single crystals suitable for X-ray diffraction studies were grown from toluene/hexane.
Yield: 0.543 g (86%).
1
H NMR: δH (C6D6, 500 MHz, 298 K) 7.63 (m, 6H, o-SiArH), 7.19 (m, 9H,
m-SiArH and p-SiArH), 6.68 (s, 4H, m-NArH), 2.05 (s, 6H, p-NArCH3), 1.93 (s, 12H, o-NArCH3),
1.30 (s, 6H, C(CH3)2).
13
C{
1
H} NMR: δC (C6D6, 126 MHz, 298 K) 216.0 (s, NCN), 171.2 (s, CO),
142.7 (s, i-SiAr), 139.8 (s, p-NAr), 136.0 (s, i-NAr), 135.5 (s, o-SiAr), 134.1 (s, o-NAr), 130.4 (s,
m-NAr), 128.2 (s, p-SiAr), 127.3 (s, m-SiAr), 51.3 (s, OC(CH3)2), 24.3 (s, OC(CH3)3), 21.1 (s,
p-NArCH3), 18.0 (s, o-NArCH3). IR (cm
-1
): 1759 (CO), 1729 (CO). Analysis found: C, 70.45; H,
6.19; N, 4.00. C42H43N2O3SiCu requires: C, 70.51; H, 6.06; N, 3.92.
57

Synthesis of (DAC)CuC6F5 (3). C6F5H (0.150 mL, 1.35 mmol) was added to a benzene
(10 mL) solution of (DAC)Cu(2,4,6-Me3C6H2) (0.498 g, 0.89 mmol) in a rigorously flame dried
ampoule and the mixture heated at 60 °C for 21 h. After cooling to room temperature, the solvent
was removed, and the dull orange residue dried in vacuo. This was washed with 20 mL of 1:4 v:v
benzene/hexane mixture and then with hexane (3 x 10 mL) to give a bright orange powder after
drying. Yield: 0.285 g (53%). Single crystals suitable for X-ray diffraction were grown by slow
evaporation of a benzene/hexane solution (1:4 v:v).
1
H NMR: δH (C6D6, 500 MHz, 298 K) 6.78 (s,
4H, m-NArH), 2.06 (s, 12H, o-NArCH3), 2.05 (s, 6H, p-NArCH3), 1.34 (s, 6H, C(CH3)2).
13
C{
1
H}
NMR: δC (C6D6, 126 MHz, 298 K; signals for the C6F5 ligand were not observed) 216.8 (s, NCN),
171.3 (s, CO), 140.4 (s, p-NAr), 135.2 (s, i-NAr), 134.2 (s, o-NAr), 130.4 (s, m-NAr), 51.7 (s,
C(CH3)2), 24.3 (s, C(CH3)2), 21.0 (s, p-NArCH3), 18.2 (s, o-NArCH3).
19
F NMR: δF (C6D6,
470 MHz, 298 K) -112.5 (m, 2F, o-C6F5), -159.4 (t,
3
JFF = 20 Hz, 1F, p-C6F5), -163.0 (m, 2F,
m-C6F5). IR (cm
-1
): 1763 (CO), 1732 (CO). Analysis found: C, 59.21; H, 4.72; N, 4.64.
C30H28N2O2F5Cu requires: C, 59.35; H, 4.65; N, 4.61.
3.3.2. X-ray crystallography
An Agilent Supernova diffractometer equipped with Cu(K) X-rays was used for data
collection on 2, while a Nonius kappaCCD diffractometer equipped with Mo(K) X-rays was
employed for data acquisition on 4. Both experiments were conducted at 150 K. Crystal structure
solution and refinement was unremarkable in both cases. CCDC 1480899 and 1480900 contain
the supplementary crystallographic data for 2 and 4, respectively. These data can be obtained free
of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html.

58

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Milne, C. J.; Chergui, M., Solvent-Induced Luminescence Quenching: Static and Time-Resolved
X-Ray Absorption Spectroscopy of a Copper(I) Phenanthroline Complex. The Journal of Physical
Chemistry A 2013, 117 (22), 4591-4601.
46. Capano, G.; Rothlisberger, U.; Tavernelli, I.; Penfold, T. J., Theoretical Rationalization
of the Emission Properties of Prototypical Cu(I)–Phenanthroline Complexes. The Journal of
Physical Chemistry A 2015, 119 (27), 7026-7037.
47. Djurovich, P. I.; Murphy, D.; Thompson, M. E.; Hernandez, B.; Gao, R.; Hunt, P. L.;
Selke, M., Cyclometalated iridium and platinum complexes as singlet oxygen photosensitizers:
quantum yields, quenching rates and correlation with electronic structures. Dalton Transactions
2007, (34), 3763-3770.
48. Singh, P. S.; Evans, D. H., Study of the Electrochemical Reduction of Dioxygen in
Acetonitrile in the Presence of Weak Acids. The Journal of Physical Chemistry B 2006, 110 (1),
637-644.
49. Bard, A. J.; Parsons, R.; Jordan, J., Standard Potentials in Aqueous Solutions. Marcel
Dekker, Inc.: New York, 1985.
50. Hudnall, T. W.; Moerdyk, J. P.; Bielawski, C. W., Ammonia N-H activation by a
N,N[prime or minute]-diamidocarbene. Chemical Communications 2010, 46 (24), 4288-4290.
51. Eriksson, H.; Håkansson, M., Mesitylcopper: Tetrameric and Pentameric.
Organometallics 1997, 16 (20), 4243-4244.

62

Chapter 4. Highly Efficient Photo- and Electroluminescence from
Two-Coordinate Cu(I) Complexes Featuring Non-conventional
N-heterocyclic Carbenes

4.1. Results and discussion
In Chapter 3, we introduced an two-coordinate cationic Cu carbene complex
[(DAC)2Cu][BF4]
1
(DAC = 1,3-bis(mesityl)-5,5-dimethyl-4,6-diketopyrimidinyl-2-ylidene) that
shows a quantum efficiency of 65% in solution, which is one of the brightest mononuclear
Cu(I) complexes reported.
2-4
The high quantum efficiency of this complex in solution
demonstrates that a two-coordinated Cu center can achieve high quantum yield if the ligands can
provide sufficient steric hindrance to prevent structural distortion in the excited state or exciplex
formation. Although the non-radiative rate constant (knr = 10
3
–10
4
s
-1
) of [(DAC)2]Cu[BF4] in
solution is small compared to most of other luminescent Cu complexes,
5, 6
the radiative rate
constant (kr = ~10
4
s
-1
) is still at least an order of magnitude smaller than Ir and Pt based emitters
(kr = 10
5
–10
6
s
-1
).
7

In this chapter, we introduce a series of two-coordinate neutral Cu complexes (Figure 4.1a)
bearing non-conventional cyclic amidocarbenes
8-11
including (MAC*)Cu(CzCN2) (1),
(MAC*)Cu(CzCN) (2), (MAC*)Cu(Cz) (3), (DAC*)Cu(CzCN2) (4), (DAC*)Cu(CzCN) (5) and
(DAC*)Cu(Cz) (6) (MAC = cyclic monoamido-aminocarbene, DAC = cyclic diamidocarbene, *
indicates that the aryl group bound to N is 2,6-diisopropylphenyl, Cz = carbazole). The carbonyl
groups incorporated into MACs and DACs increase the -accepting properties of the carbene and
concomitantly lower the energy of the LUMO. The diisopropylphenyl group on the carbene
ligands provides sufficient steric hindrance to suppress the formation of an exciplex between the
63

complexes and the solvent. All the complexes show highly efficient thermally-activated-delayed
fluorescence (TADF) with short decay lifetime in polystyrene films and solution. The radiative
rates (kr = 10
5
–10
6
s
-1
) are comparable to those state-of-the-art emitters bearing noble metal like Ir
and Pt. OLED devices with complex 3 as the dopant and neat emitter were fabricated through
vapor deposition and show high external quantum efficiency up to 19.4% and alleviated efficiency
roll-off.
4.1.1. Synthesis and characterization
The pyrimidinium salt precursor to MAC* was synthesized by condensation of
N,N’-bis(2,6-diisopropylphenyl) formamidine and 3-chloropivaloyl chloride in the presence of
excess triethylamine at 0 °C, followed by intramolecular cyclization in refluxing in toluene for
16 h. The DAC*Cl salt was synthesized following the literature procedure.
12
The (carbene)CuCl
precursors were synthesized from the free MAC* and DAC* carbenes generated in situ, followed
by addition of CuCl. MAC*CuCl was isolated as a white solid, whereas the DAC* analogue is a
deep red solid. The red color for the DAC* complex is likely due to the formation of a dimeric
species [(DAC*)Cu(Cl)2Cu(DAC*)], as reported for the mesityl-substituted DAC analog.
13
The

Figure 4.1. Molecular structures and frontier orbitals. (a) Molecular structures of complexes 1-6. (b)
LUMO (left) and HOMO (right) of complex 3 (top) and 6 (bottom).

64

(carbene)CuCz complexes were synthesized from reaction of the (carbene)CuCl with sodium
carbazolide and isolated in 68–85% yield. Complexes 1 and 2 are white solids, whereas complexes
3–6 vary in color from yellow, orange-yellow, orange and purple, respectively. Complexes 1–5
are air- and moisture-stable and can be sublimed under vacuum, whereas complex 6 reacts with
ambient moisture to generate free CzH and a yellow solid, presumably DAC*CuOH.
Single crystals of the 1–4 suitable for X-ray diffraction studies were grown by slow
evaporation of CH2Cl2/pentane solutions. The complexes show a near linear geometry at the
copper center (CNHC–Cu-NCz = 173–180). Bond lengths to the ligands (Cu–CNHC =
1.868(2)-1.902(2) Å, Cu–NCz = 1.852(1) –1.855(2) Å) are comparable to the values reported for
mononuclear Cu–CNHC and Cu–Cz complexes.
1, 5, 14-16
Dihedral angles between ligand planes are
in the range between 1–16 and lead to a near parallel orientation of 2pz orbitals on CNHC and NCz.
This geometric arrangement, in light of the close distance between the 2pz orbitals on the ligated
atoms (CNHC···NCz = 3.719(2)–3.762(2) Å), suggests that a significant  interaction should be
present across the metal center.
17

The electrochemical properties of the complexes and their precursors (carbene-CuCl) were
examined by cyclic voltammetry and differential pulse voltammetry in acetonitrile solution (Table
4.1). The redox potentials are referenced to an internal ferrocene (Fc
+
/Fc) couple, and converted
to HOMO and LUMO energies.
18
All the complexes display quasi-reversible reduction and
irreversible oxidation waves. The reduction potentials are similar in complexes with the same
carbene ligand and are largely unchanged from the precursors MAC*CuCl and DAC*CuCl.
Reduction potentials for the DAC* analogues are anodically shifted relative to the MAC*
analogues by ca. 0.90 V due to the second carbonyl group in DAC*. The oxidation potentials
increase from complexes 3 to 1 (and 6 to 4) consistent with the donor strength of the Cz ligands
65

decreasing with addition of the nitrile groups. Oxidation potentials of chloride precursors (Eox =
0.91 V and 1.03 V for MAC*CuCl and DAC*CuCl) are markedly larger than the Cu-Cz complexes.
The redox behavior suggests that the reduction (oxidation) potential is determined by the carbene
(carbazolyl) ligands.
Theoretical calculations on these compounds support the assignment of the electrochemistry
results. The contours for the LUMO and HOMO obtained from the Density Functional Theory
(DFT) calculations of the (carbene)Cu(Cz) complexes (for example, see Figure 4.1b) show the
LUMO is mainly localized on the carbene ligand, whereas the HOMO is predominately on the Cz
ligand. Both LUMO and HOMO have little metal character. DFT and Time Dependent DFT
(TD-DFT) calculations were carried out at the CAM-B3LYP/LACVP** level to predict the lowest
energy excited states for these complexes.
19
The CAM-B3LYP functional was found to provide
an accurate description of both the charge transfer (CT) and ligand-localized (Cz) states (vide infra).
Compounds 1 and 2 show two closely spaced, but quite different excited states. One state
corresponds to an intramolecular charge transfer (ICT) involving the carbazolyl (donor) and the
carbene (acceptor). The singlet and triplet (
1
ICT and
3
ICT) levels for these configurations are
predicted to be close in energy. The second excited state corresponds to a triplet localized on the
carbazolyl ligand (
3
Cz). Both the
3
Cz and
1,3
ICT states are present in compounds 3–6, but the ICT
states fall well below the energy of the
3
Cz state.
66

Ground (gs) and excited state ( es) dipole moments for compounds 1–6 were estimated using
DFT and TD-DFT calculations and given in Table 4.1. Dipole moments were calculated at the
CAM-B3LYP/LACVP** level for the
3
Cz,
1
ICT and
3
ICT states using optimized structures of the
ground state.
19
The combination of linear geometry and long donor-acceptor separation imparts a
large gs to all the complexes. Addition of a carbonyl group decreases gs by ~2 D upon going
from MAC* to DAC* analogs. The ICT transitions redistribute electron density away from
carbazolyl towards the carbene, which effectively leads to a  es that is much smaller in 1 and 4, or
even in the direction opposite to  gs in 2, 3, 5 and 6. The singlet and triplet ICT states have slightly
different dipole moments because the
1
ICT is essentially a single configuration (HOMO-LUMO),
whereas the
3
ICT is configurationally mixed with other triplet states that alter  es. For example,
the
1
ICT state of 2 is predominantly a HOMO–LUMO transition (94%) whereas the
3
ICT state has
significant contribution from
3
Cz transitions (20%). In all of compounds except 6,  es for the
3
Cz
state is similar to that of gs since the transition involves only the redistribution of electron density
Table 4.1. Redox data for complexes 1–6 and carbene-CuCl and dipole moments for 1–6 in the ground
state (S 0), lowest ligand-localized excited state (Cz) and charge transfer excited states (ICT).
Potentials Dipole moment ()
c

Complex
E ox
a

(V)
E red
a

(V)
HOMO
b

(eV)
LUMO
b

(eV)
 gs
S 0
 es
3
Cz
 es

1
ICT/
3
ICT
1 0.60 -2.45 -5.48 -1.94 19.1 19.1 5.5/4.5
2 0.38 -2.45 -5.23 -1.94
14.9 12.5 -8.6
d
/1.2
3 0.17 -2.50 -4.98 -1.88 10.6 9.9 -13.7
d
/-11.6
d

4 0.60 -1.60 -5.48 -2.94 17.2 16.7 8.9/6.6
5 0.38 -1.56 -5.23 -2.99 13.7 11.4 -13.5
d
/-11.8
d

6 0.17 -1.60 -4.98 -2.94 8.2 0.60 -17.0
d
/-15.4
d

(MAC*)CuCl 0.91 -2.50 -5.84 -1.88
(DAC*)CuCl 1.03 -1.60 -5.97 -2.94
a
Redox potentials obtained in acetonitrile with 0.1 M TBAPF6 versus internal Fc+/Fc. b HOMO =
1.15(Eox) + 4.79; LUMO = 1.18(Ered) – 4.83.38 c Obtained from TD-DFT calculations
(CAM-B3LYP/LACVP**) using geometry optimized structures. d Dipole moments are opposite in
direction from the ground state moments.
67

within the carbazolyl ligand. However, in 6, the
3
Cz state was found to contain a significant
contribution from the HOMO-LUMO ICT transition (27%) leading to a marked reduction in  es.
4.1.2. Photophysical properties
Absorption spectra of complexes 1–6 in 2-methyltetrahydrofuran (2-MeTHF) are shown in
Figure 4.2a. Structured absorption bands between 300–380 nm are assigned to allowed transitions
localized on the carbazolyl ligand (
1
Cz) by comparison to similar absorption features found in
potassium carbazolide. Strong ( = 4000–6000 M
-1
cm
-1
), featureless absorption bands at lower
energy are distinct in 2–6. A similar band present in 1 is obscured by the structured
1
Cz transitions
and has an absorption edge at 400 nm. The bands red shift in the series 1→6 as the donor strength
300 400 500 600 700
0k
2k
4k
6k
8k
10k
12k
14k
16k
18k
20k

Molar absorptivity (10
3
M
-1
cm
-1
)
Wavelength (nm)
1
2
3
4
5
6
(a)

1.8 2.0 2.2 2.4 2.6 2.8 3.0
1.8
2.0
2.2
2.4
2.6
2.8
3.0
5
6
4
3
2
R
2
= 0.97

E
CT
(eV)
Eox - Ered (V)
1
(b)
300 350 400 450 500
0
1
2

Normalized absorption (AU)
Wavelength (nm)
MeCy
Toluene
MeTHF
CH
3
CN
(c)

Figure 4.2. Absorption spectra of complexes 1–6 at room temperature. (a) Spectra of complexes
1–6 in 2-MeTHF. (b) The energy of the low energy band (obtained from onset) versus the redox
gaps. (c) Spectra of complex 3 in methylcyclohexane (MeCy), toluene, 2-MeTHF and
acetonitrile.
68

of the carbazolyl ligand increases (from Cz(CN)2 to Cz) and the electrophilicity of the carbene
increases (from MAC* to DAC*). A plot of redox gaps versus the onset energy of the low energy
bands show a linear trend (Figure 4.2b). These bands are assigned to allowed ICT transitions on
the basis of this data and TD-DFT calculations. A hypsochromic shift of 57 nm (2478 cm
-1
) is
observed in the onset of the
1
ICT band upon varying the solvent from non-polar
methylcyclohexane to polar acetonitrile (Figure 4.2c). In contrast, the absorption bands for the
localized
1
Cz transitions barely shift in the same solvents. The
1
ICT bands display negative
solvatochromism due to the large change in dipole moment of the excited state relative to the
ground state (Table 4.1).
Emission spectra of complexes 1–5 in 2-MeTHF at room temperature are shown in Figure
4.3a, photophysical data are summarized in Table 4.2. Complex 6 is non-emissive in 2-MeTHF.
Complexes 1–5 show broad and featureless bands indicative of emission from the ICT states. The
emission color ranges over 220 nm, from deep blue to red, by varying either the Cz donor or the
carbene acceptor ligands. The quantum efficiency ( PL) ranges from 2% to 100% with short decay
lifetimes ( = 0.052–2.3 μs). The radiative rate constants of complexes 1–5 are high (kr =
10
5
-10
6
s
-1
) and comparable to values for efficient phosphorescent emitters with noble metals such
as Ir(III) and Pt(II). Emission spectra of the complexes are also solvatochromic. For example, the
peak maximum of complex 3 undergoes a bathochromic shift of 47 nm (1618 cm
-1
) (Figure 4.3b).
The shift with increasing the solvent polarity is due to stabilization of the ICT state through local
interactions between solvent molecules and the excited-state dipole moment of the complex. The
solvatochromic shifts are smaller in emission than in absorption due to the smaller dipole of the
complexes in the excited state than in the ground state.
69

The emission spectra of the complexes undergo marked rigidochromic shifts in frozen media
as shown for complexes 1–4 in 2-MeTHF at RT and 77 K in Figure 4.4. The spectral shifts upon
cooling the fluid solutions come as a result of destabilization of the ICT states in the rigid
environment since the dipole moment of the excited state is smaller or even in the opposite
direction to the ground state (Table 4.1). Thus, a solvation shell that stabilizes the ground state
will destabilize the excited state. The solvent organizes around the large ground state dipole at
room temperature and is locked into this arrangement on freezing, thereby shifting the ICT energy
to higher energy. Varying ratios of
3
Cz/ICT emission are observed from complexes with different
carbene and/or Cz ligands. Complexes 1 and 2 display vibronic fine structure from the lower lying
3
Cz state at 77 K in 2-MeTHF and have decay lifetimes at 430 nm in the range of milliseconds.
The ICT state in these derivatives is destabilized to the point that emission is exclusively from the
3
Cz state. Both
3
Cz and ICT emission are observed in complex 3 in 2-MeTHF at 77 K as both
states are similar in energy. Complex 4 shows only broad ICT emission in 2-MeTHF at 77 K. The
ICT emission in 4 blue-shifts by 125 nm (4300 cm
-1
) upon cooling to 77 K; however, this change
is insufficient to raise the energy of the ICT state above that for the
3
Cz state.
400 500 600 700 800
0.0
0.5
1.0
5 4 3 2

Normalized intensity (AU)
Wavelength (nm)
1
(a)
450 500 550 600 650 700 750
0.0
0.5
1.0

Normalized emission (AU)
Wavelength (nm)
MeCy
Toluene
MeTHF
ACN
(b)

Figure 4.3. Emission spectra of complexes 1–6 in fluid solution at room temperature. (a)
Spectra of complexes 1–5 in 2-MeTHF. (b) Spectra of complex 3 in methylcyclohexane,
toluene, 2-MeTHF and acetonitrile.
70

To further probe the nature of the rigidochromic shift, emission spectra of 3 in 2-MeTHF
were measured at temperatures between 77 and 295 K (Figure 4.5a). The emission spectrum at
77 K displays features from both the
3
Cz ( = 2.1 ms at 430 nm) and
3
ICT ( = 180 μs at 520 nm)
states. Negligible change in the emission profile is observed until warming to 93 K, which is close
to the glass transition temperature of 2-MeTHF (Tg = 90-91 K).
20-22
At temperatures above Tg the
dielectric relaxation rate of the solvent increases and the ICT state is stabilized through solvation
Table 4.2. Luminescent properties of complexes 1–6 in different media.
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,77
K
(nm)
τ 77K (μs)
solution in 2-MeTHF
1 448 0.24 2.3 1.0 3.3 424 9300
2 492 1.00 1.2 8.3 < 0.083 428 2200
3 542 0.55 1.1 5.0 4.1 432
430 nm:
2100 (62%)
342 (38%)
520 nm: 181
4 602 0.05 0.080 6.2 119 492 18
5 666 0.02 0.052 3.8 180 536 213
1 wt% in polystyrene film
1 432 0.80
2.6 (47%)
14 (53%)
0.9
a
0.23
a
424 3200
2 468 1.00 1.3 7.7 < 0.077 464 99
3 506 0.90 1.4 6.4 0.71 502 227
4 548 0.78 1.2 6.5 1.8 544 153
5 616 0.30 0.75 4.0 9.3 612 408
6 704 0.03 0.19 1.6 51 682 146
neat solid
1 438 0.05
0.37(33%)
1.8 (67%)
0.38
a
7.2
a
438 7100
2 474 0.76 0.75 10.0 3.2 468 91
3 492 0.53 0.84 6.3 5.5 482 164
4 550 0.68 1.0 6.8 3.2 558 280
5 616 0.15 0.33 4.5 26 598 180
6 658 0.12 0.39 3.1 22 634 306
a
Calculated from weighted average of the two contributions to .
71

effects. The softening of the matrix causes the ICT band to red-shift and increase in the intensity
between 93 K and 105 K. The change in lowest state energy from
3
Cz to ICT as a function of
temperature is schematically illustrated in Figure 4.5b. Emission from the
3
Cz state disappears at
temperatures above 105 K and luminescence comes exclusively from the ICT state. The ICT state
is fully stabilized even before the melting point of 2-MeTHF (Tm = 137 K) as the emission profile
is nearly unchanged from 105 K to room temperature.
0.0
0.5
1.0
0.0
0.5
1.0
0.0
0.5
1.0
400 500 600 700
0.0
0.5
1.0

1
4
3
2

Normalized emission (AU)
2-MeTHF RT
2-MeTHF 77 K

Wavelength (nm)

Figure 4.4. Emission spectra of complexes 1-4 (top to bottom) at room temperature
(RT) and 77 K in 2-MeTHF.
72

It is common for luminescent Cu(I) complexes to have low quantum efficiencies in fluid
solution due to high non-radiative decay rates caused either by unimolecular distortion of the
complexes in the excited state or bimolecular formation of excimers or exciplexes with the
solvent.
23, 24
The impact of these processes have on the PL can be ameliorated in the solid state.
Indeed, complexes 2–6 display comparable or even higher  PL as neat solids than in 2-MeTHF
solution (Table 4.2). High values of  PL are also observed for the complexes when doped into
rigid films of polystyrene (PS). Complexes 1–6 doped at 1 wt% in PS show broad featureless ICT
emission at room temperature (Figure 4.6a). The kr values of the complexes in the thin films are
similar to those in 2-MeTHF, whereas values for knr are much smaller. The magnitude of knr
increases across the series 1→6. This trend is a consequence of the energy gap law relationship.
25

In a series of materials with similar excited state character, the energy of the transition (E) is
logarithmically dependent on the non-radiative rate constant. For complexes 2-6, a plot of log(knr)
vs. E gives a linear fit (Figure 4.6b). However, complex 1 falls well off the line, suggesting a
different orbital composition in the excited state for 1. It is also worth noting that the kr of 1 is
considerably smaller than that of complexes 2–6 in both 2-MeTHF and PS films. These deviations
400 500 600 700
0.0
0.5
1.0

Emission intensity (AU)
Wavelength (nm)
78 K
93 K
95 K
98 K
100 K
105 K
120 K
140 K
280 K
(a)

Figure 4.5. (a) Emission spectra of complex 3 at 78-280 K in 2-MeTHF. (b) Schematic energy diagram
depicting the effect of rigidochromism on the ordering of the ICT/
3
Cz states.
73

in kr and knr from complexes 2–6 are consistent with an increased contribution from the
3
Cz state
in emission from 1.
The emission properties of complexes 1–6 in PS films were measured at 77 K (Figure 4.6a)
and decay lifetimes were found to be strongly temperature-dependent. For complex 1, the emission
is well-resolved and shows a characteristic vibronic structure of Cz at 77 K. The lifetime increases
from a few s at room temperature to 3.2 ms at 77 K. The vibronic structure along with the long
lifetime at 77 K is consistent of emission from the locally-excited
3
Cz state. For complexes 2–6,
the emission spectra remain broad and featureless at 77 K indicative of emission from ICT states.
The emission spectra show a minimal blue-shift at 77 K; however, the decay lifetimes markedly
increase by two orders of magnitude. The large increase in emission lifetime on cooling to 77 K
suggests that the luminescence at room temperature is due to thermally-activated-delayed
fluorescence (TADF), which requires a small energy separation between
1
ICT and
3
ICT.
400 500 600 700 800
0.0
0.5
1.0
6 5 4 3 2

Normalized emission (AU)
Wavelength (nm)
RT
77 K
1
(a)
1.6 2.0 2.4 2.8
10
4
10
5
10
6
10
7
1
6
5
4
3
k
nr
(s
-1
)
E
CT
(eV)
(b)
2
R
2
= 0.98

Figure 4.6. Photophysics of complexes 1–6 in polystyrene films. (a) The emission spectra at room
temperature and 77 K. (b) Energy gap law plot at room temperature.
74

To obtain parameters governing the temperature dependent emission properties, the emission
lifetime of complex 3 in PS film was measured between 5 K and 320 K (Figure 4.7a). The lifetime
increases gradually until near 150 K, where the increase becomes more pronounced, before rising
steadily below 50 K. The increase in lifetime upon cooling is attributed to successive depopulation
of states at high energy that have radiative rate constants faster than the lowest lying state. At
temperatures above 150 K, emission is dominated by a higher-lying S1 state, whereas at
temperatures below 50 K, thermal activation between triplet substates is observed. Under an
assumption of a fast thermalization, the temperature dependent decay curve can be fit to the
Boltzmann distribution equation (equation 1).
𝜏 =
2 + 𝑒 −
∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 + 𝑒 −
∆𝐸 (𝑆 1
−𝐼 )
𝑘 𝐵 𝑇
2(
1
𝜏 𝐼 ,𝐼𝐼
)+(
1
𝜏 𝐼𝐼𝐼
)𝑒 −
∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 +(
1
𝜏 𝑆 1
)𝑒 −
∆𝐸 (𝑆 1
−𝐼 )
𝑘 𝐵 𝑇 (1)
Here, S1 represents the lowest singlet state, whereas I, II and III represent the triplet substates
I
T1,
II
T1 and
III
T1, and kB is the Boltzmann constant. Substates I and II are treated as being degenerate
since the energy splitting between these two states are normally very small (<10 cm
-1
) in an axially
distorted structure.
7
Fits of the experimental lifetime data to equation 1 reveal the decay rate of
0 50 100 150 200 250 300
0
50
100
150
200

experiment
fit
Lifetime (s)
Temperature (K)
(a)

Figure 4.7. (a) Emission lifetime versus temperature of complex 3 in the PS film. (b) Energy level
diagram for complex 3.
75

each state and the energy separation between them (Figure 4.7b). The exchange energy is
characterized by ΔE(S1-
III
T1), which is determined to be 415 cm
-1
(51.5 meV). This energy
separation is among the smallest separation in Cu-based TADF complexes.
4, 26-29
The decay
lifetime of the S1 state (S
1
= 73.4 ns) is among the fastest values of S
1
for Cu complexes
26
and
consistent with the high kr as mentioned above. The decay lifetimes are 58.2 s and

206 s for
III
T1
and
I
T1/
II
T1 substates. ΔE (III–II/I), which corresponds to the zero-field splitting (ZFS), is 74 cm
-1
.
The value for ZFS is exceptionally large for Cu(I) complexes,
30
and is induced by the effective
spin orbit coupling (SOC).
4.1.3. Electroluminescence
OLED devices with complex 3 as the dopant were fabricated through vapor deposition. The
device structure was: glass substrate / 70 nm ITO / 5 nm hexaazatriphenylene hexacarbonitrile
(HATCN) / 40 nm 4,4′-cyclohexylidenebis [N,N-bis(4-methylphenyl)benzenamine] (TAPC) /
10 nm N,N’-dicarbazolyl-3,5-benzene (mCP) / 25 nm EML / 65 nm 2, 2,
2′′-(l,3,5-benzenetriyl)-tris(L-phenyl-l-H-benzimidazole) (TPBi) / 1.5 nm 8-hydroxyquinolinato
lithium (LiQ) / 100 nm Al. Here, the EML is compound 3 doped into
3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) at 10 or 40 vol%, or used as a neat material
(100%). Frontier orbital energies and molecular structures of materials used in the devices are
shown in inset of Figure 4.8.

76

Figure 4.9. Electroluminescent device characteristics containing complex 3 at doping concentrations
of 10%, 40% and 100%. (a) Electroluminescent spectra. (b) Current density-voltage-luminance
(J-V-L). (c) External quantum efficiency.

450 500 550 600 650 700 750
0.0
0.5
1.0

Intensity (AU)
Wavelength (nm)
10%
40%
100%
(a)
0 2 4 6 8 10 12 14
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
Volts
lm (cd/m
2
)
0
20
40
60
80
100
120
140
J(mA/cm
2
)
(b)

0.01 0.1 1 10 100
0
2
4
6
8
10
12
14
16
18
20

EQE (%)
J (mA/cm
2
)
(c)

Figure 4.8. Electroluminescent device characteristics containing complex 3 at doping concentrations
of 10%, 40% and 100%. (a) Electroluminescent spectra. (b) Current density-voltage-luminance
(J-V-L). (c) External quantum efficiency.

77

The device characteristics are shown in Figure 4.9 and the data is summarized in Table 4.3.
The devices emit bright green light with emission peak maximum at 534-555 nm (Figure 4.9a)
which are consistent with the photoluminescence of the complex. The emission peak maximum is
slightly red-shifted upon increasing the doping concentration likely due to aggregation. The J-V-L
characteristics of the devices indicate that the turn-on voltage (defined at brightness of 0.1 cd/m
2
)
decreases from 3.5 V to 2.7 V as the doping concentration increases from 10% to neat (Figure
4.9b). The low resistance at high doping concentration suggests that the dopant has higher charge
mobility than the mCBP host. The high charge mobility is likely due to the small reorganization
energy of the dopant resulting from the small metal character in the transition. For most reported
mononuclear Cu(I) complexes, the HOMO has a significant amount of metal character.
6, 15, 31

Therefore, formal oxidation on the metal occurs upon photoexcitation (from d
10
to d
9
), leading to
Jahn-Tellar distortion, which is not the case for the Cu(I) complexes in our work. The maximum
EQE of the devices increases ranges from 15.4% to 19.4% with the highest EQE achieved at the
doping concentration of 40% (Figure 4.9c). The device efficiency is among the highest values
reported for OLEDs based on Cu(I) dopants.
2, 29, 32-34
Interestingly, the device using an neat
emissive layer also demonstrates a high efficiency (EQE = 16.3%). Indeed, the photoluminescent
efficiency of both the vapor-deposited 10% doped (ΦPL = 100%) and neat (ΦPL = 74%) film of
complex 3 are high, resulting in the high EQE of the devices. In addition, the small efficiency
roll-off at high driving currents for all the devices is unprecedented for the Cu OLEDs,
2, 29, 32-34

which is due to the short emission decay lifetime that limits triplet-triplet and triplet-polaron
annihilation at high high brightness/current.
35, 36

78

4.2. Conclusion
A series of two-coordinate Cu complexes bearing non-conventional NHCs were investigated.
All the compounds show linear geometry at the copper center with the two ligands coplanar with
each other. The compounds show efficient thermally-activated-delayed fluorescence (TADF) with
ΦPL up to 100%. The compounds emit at progressively lower energies, ranging from violet (max
= 432 nm) to deep red (max = 704 nm). The energy of this intramolecular charge transfer emission
is tied directly to the donor strength of the carbazole and acceptor strength of the carbene. The
quantum efficiency drops for 5 and 6, due to energy gap law that leads to higher non-radiative rate
constants as emission is shifted to the orange or red. The radiative rate constants are 10
5
- 10
6
s
-1
,
which is comparable to efficient Ir and Pt phosphorescent emitters. The radiative rate constant is
relatively smaller for 1 because the long-lived
3
Cz and
1,3
ICT states are close in energy at room
temperature. All complexes show strong solvatochromism and rigidochromism due to the large
dipole moment of the ground states and the relatively small dipole in the excited states.
These compounds meet all three crucial requirements to achieve efficient TADF.
26
Firstly, the
geometry changes in the excited state should be suppressed to minimize the non-radiative
deactivation. The lowest singlet and triplet transitions of these linear complexes contain little metal
character which minimizes the Jahn-Teller distortion in the excited state. Secondly, the energy
separation ΔE(S1-T1) should be relatively small which is associated with the ICT transitions that
Table 4.3. Turn-on voltage (V T, defined at brightness of 0.1 cd/m
2
), maximum external quantum
efficiency (EQE max), maximum brightness (B max), and emission maximum (λ max) of the OLEDs
with complex 3 at different doping concentration.
Doping
concentration
V T
(V)
EQE max
(%)
B max
(cd/m
2
)
λ max
(nm)
10% 3.0 17.0 41000 537
40% 2.5 19.4 54000 543
100% 2.5 16.3 41000 555

79

have spatially separated HOMO and LUMO. In this work, all complexes show strong ICT
emission at ambient temperature and the small ΔE(S1-T1) (415 cm
-1
for complex 3). The large
zero-field splitting (74 cm
-1
) is induced by the efficient spin orbit coupling. The fast prompt (S1)
(73.4 ns) is consistent with the high radiative rate (6.4 ×10
5
s
-1
in the PS film). Thirdly, the kr
should be as high as possible to obtain short TADF decay lifetime. For these linear complexes,
strong coupling between the orbitals of the donor (Cz) and acceptor (carbene) moieties through
the d orbitals of the Cu center induces high kr despite the large spatial separation between HOMO
and LUMO. Vapor-deposited OLED devices based on complex 3 show both high EQE and low
efficiency roll-off at high voltage, qualities attributed to the high quantum efficiency and the short
exciton decay time of these efficient TADF complexes. The high EQE of the host-free devices
based on 3 demonstrates the potential of these complexes used as neat emitters in the OLED
devices.
4.3. Experimental
Refer to section 2.2.2 for the methods used for photophysical characterization and section
2.2.3 for X-ray crystallography. Temperature-dependent measurements in the range of 5-300 K
were performed using an JANIS ST-100 Standard Optical Cryostat instrument equipped with an
intelligent temperature controller. CCDC 1873677, 1873680, 1873678, and 1873679 contain the
supplementary crystallographic data for 1-4, respectively.
4.3.1. Synthesis
All reactions were carried out using standard Schlenk and glovebox techniques using dried
and degassed solvents. The DAC*Cl
12
precursor, 3-cyanocarbazole,
37
3,6-dicyanocarbazole
38

were synthesized by following the literature procedure. CuCl and carbazole were purchased from
80

Sigma Aldrich. 3-chloropivaloyl chloride was purchased from Arctom Chemicals. NMR spectra
were recorded on a Varian 400 NMR spectrometer.
Synthesis of (MAC*)CuCl (1a).

3-Chloro-N-(2,6-diisopropylphenyl)-N’-((2,6-diisopropylphenylimino)methyl)-2,2-dime
thylpropanamide (1c). N,N’-bis(2,6-diisopropylphenyl) formamidine (500 mg, 1.37 mmol) and
triethylamine (287 mL, 2.06 mmol) were dissolved in dichloromethane (20 mL) and stirred at 0 ℃
for 10 min, after which 3-chloropivaloyl chloride (0.195 mL, 1.51 mmol) was added dropwise.
The solution mixture was stirred for 3 h at 0 ℃. The solvent was removed under reduced pressure
to afford a while powder, which was extracted with toluene and filtered through Celite. Removal
of the residual solvent afforded the product as a white solid. Yield: 650 mg (98%).
1
H NMR δH
(CDCl3, 400 MHz, 298 K): 1.14 (m, 18H, CH(CH3)2), 1.30 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.54
(s, 6H, C(CH3)2), 2.95 (sept, J = 6.5 Hz, 4H, CH(CH3)2), 3.77 (s, 2H, CCH2Cl), 7.08 (m, 3H,
Ar-H), 7.27 (m, 2H, Ar-H), 7.39 (m, 1H, Ar-H), 8.59 (s, 1H, NCHN).
13
C NMR δC (CDCl3,
101 MHz, 298 K): 23.17 (s, CH(CH3)2), 24,11 (s, CH(CH3)2), 24.22 (s, C(CH3)2), 24.83 (s,
CH(CH3)2), 27.35 (s, CH(CH3)2), 28.94 (s, CH(CH3)2), 46.25 (s, C(CH3)2), 53.25 (s, CCH2Cl),
123.05 (s, m-ArH), 124.01(s, m-ArH), 124.20 (s, p-ArH), 129.18 (s, p-ArH), 132.95 (s, o-NAr),
138.91 (s, o-NAr), 145.21 (ipso-NAr), 145.51 (ipso-NAr), 148.51 (s, NCN), 174.74 (s, C=O).
MALDI-TOF: m/z calculated: 447.34 [M-Cl
-
]
+
; found: 447.64 [M]
+
.
81

1,3-bis(2,6-diisopropylphenyl)-5,5-dimethyl-4-keto-tetrahydropyrimidin-1-ium
chloride (1b). 1c (650 mg, 1.34 mmol) was dissolved in toluene (20 mL) and the solution was
refluxed for 16 h at 110 ℃ during which a white precipitate formed. The reaction mixture was
cooled to RT and the white precipitate was collected by vacuum filtration and washed with cold
toluene. Yield: 450 mg (69%).
1
H NMR δH (CDCl3, 400 MHz, 298 K): 1.20 (d, J = 6.8 Hz, 6H,
CH(CH3)2), 1.30 (dd, J = 9.9, 6.8 Hz, 12 H, CH(CH3)2), 1.40 (d, J = 6.6 Hz, 6H, CH(CH3)2), 1.76
(s, 6H, C(CH3)2), 3.04 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.25 (sept, J = 6.8 Hz, 2H, CH(CH3)2),
4.66 (s, 2H, CCH2N), 7.20-7.30 (m, 4H, Ar-H), 7.45 (td, J = 7.9, 4.0 Hz, 2H, Ar-H), 9.82 (s, 1H,
N=CH-N).
13
C NMR δC (CDCl3, 101 MHz, 298 K): 24.00 (s, CH(CH3)2), 24,41 (s, CH(CH3)2),
24.59 (s, C(CH3)2), 24.77 (s, CH(CH3)2), 24.87 (s, CH(CH3)2), 28.98 (s, CH(CH3)2), 29.23 (s,
CH(CH3)2), 39.17 (s, C(CH3)2), 61.76 (s, CCH2N), 124.86 (s, m-ArH), 125.59(s, m-ArH), 131.66
(s, p-ArH), 131.97 (s, p-ArH), 129.56 (s, o-NAr), 134.48 (s, o-NAr), 144.35 (ipso-NAr), 145.98
(ipso-NAr), 159.29 (s, NCN), 169.67 (s, C=O), Analysis found for C30H43ClN2O: C: 74.31; N:
6.17; H: 8.96. Requires: C: 74.58; N: 5.80; H: 8.97.
(N,N’-bis(diisopropylphenyl)-5,5-dimethyl-4-keto-tetrahydropyrimidin-2-ylidene)-Cu(I)
chloride (MAC*CuCl) (1a). KHMDS (136 mg, 0.68 mmol) was added to a THF solution (20 mL)
of 1b (300 mg, 0.62 mmol) at RT and the solution was stirred for 1 h before CuCl (67 mg, 0.68
mmol) was added. The reaction mixture was stirred at RT for 16 h, filtered through Celite and the
solvent was concentrated to 3 mL under reduced pressure. Hexane (20 mL) was added to the
solution and a white precipitate formed. Yield: 300 mg (88%).
1
H NMR δH (acetone-d6, 400 MHz,
298 K): 1.17 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.35 (m, 18H, CH(CH3)2), 1.59 (s, 6H, C(CH3)2),
3.13 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.38 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 4.14 (s, 2H, CCH2N),
7.30 (d, J = 7.6 Hz, 2H, m-ArH), 7.37 (d, J = 7.7 Hz, 2H, m-ArH), 7.42 (t, J = 7.7 Hz, 1H, p-ArH),
82

7.47 (t, J = 7.7 Hz, 1H, p-ArH).
13
C NMR δC (acetone-d6, 101 MHz, 298 K): 23.25 (s, CH(CH3)2),
23,67 (s, CH(CH3)2), 23.69 (s, CH(CH3)2), 23.75 (s, C(CH3)2), 24.25 (s, CH(CH3)2), 28.27 (s,
CH(CH3)2), 28.58 (s, CH(CH3)2), 37.87 (s, C(CH3)2), 60.75 (s, CCH2N), 124.19 (s, m-ArH),
125.11(s, m-ArH), 129.65 (s, p-ArH), 130.05 (s, p-ArH), 136.19 (s, o-NAr), 140.14 (s, o-NAr), ,
144.55 (ipso-NAr), 145.62 (ipso-NAr), 171.16 (s, C=O), 208.89 (s, NCN). Analysis found for
C30H42ClCuN2O: C: 66.02; N: 5.43; H: 7.73. Requires: C: 66.03; N: 5.13; H: 7.76.
Synthesis of [(DAC*)Cu]2Cl2 (2a).

KHMDS (450 mg, 2.28 mmol) was added to a THF solution (20 mL) of 2b (1.39 g, 2.28 mmol)
at RT and the solution was stirred for 1 h before CuCl (230 mg, 2.28 mmol) was added. The
reaction mixture was stirred at RT for 16 h. The solvent was evaporated under reduced pressure,
and the obtained red solid was re-dissolved in toluene (20 mL) and filtered through Celite. The
filtrate was concentrated to 3 mL under reduced pressure. Hexane (20 mL) was added to the
solution and a red precipitate formed. Yield: 400 mg (31%).
1
H NMR δH (acetone-d6, 400 MHz,
298 K): 1.18 (d, J = 6.8 Hz, 24 H, CH(CH3)2), 1.33 (d, J = 6.8 Hz, 24 H, CH(CH3)2), 1.86 (s, 12H,
C(CH3)2), 3.03 (sept, J = 6.8 Hz, 8H, CH(CH3)2)), 7.38 (d, J = 7.8 Hz, 8H, m-ArH), 7.53 (d, J =
7.8 Hz, 4H, p-ArH).
13
C NMR δC (acetone-d6, 101 MHz, 298 K): 23.35 (s, CH(CH3)2), 23.58 (s,
CH(CH3)2), 24.37 (s, C(CH3)2), 28.70 (s, CH(CH3)2), 51.92 (s, C(CH3)2), 124.78 (s, m-ArH),
130.69 (s, p-ArH), 135.02 (s, o-Ar), 145.42 (ipso-N-Ar), 172.23 (s, C=O), 213.64 (s, NCN).
83

Analysis found for C60H80Cl2Cu2N4O4: C: 64.16; N: 5.29; H: 7.19. Requires: C: 64.38; N: 5.01; H:
7.19.
Synthesis of complexes 1-6

General procedure. Carbazole ligand and NaO
t
Bu were dissolved in THF and stirred for 3
hat RT. (carbene)CuCl was added to the reaction mixture and stirred for 16 h. The resulting mixture
was filtered through Celite and the solvent was removed under reduced pressure to afford a solid.
The solid was re-dissolved in dichloromethane and hexane was added to precipitate out the desired
product.
(MAC*)Cu(CzCN2) (1). The complex was made from (MAC*)CuCl (160 mg, 0.29 mmol),
[Cz(CN)2] (64 mg, 0.29 mmol) and NaO
t
Bu (29 mg, 0.30 mmol) as a white solid. Yield: 168 mg
(85%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.20-1.28 (m, 18H, CH(CH3)2), 1.43 (d, J =
6.8 Hz, 6H, CH(CH3)2), 1.68 (s, 6H, C(CH3)2), 3.30 (sept, J = 6.9 Hz, 2H, CH(CH3)2), 3.55 (sept,
J = 6.9 Hz, 2H, CH(CH3)2), 4.34 (s, 2H, CCH2N), 5.59 (d, J = 8.5 Hz, 2H, CH
1
(Cz)), 7.22 (d, J =
10.8 Hz, 2H, CH
2
(Cz)), 7.58 (d, J = 7.8 Hz, 2H, m-ArH), 7.64 (d, J = 7.8 Hz, 2H, m-ArH), 7.84
(m, 2H, p-ArH), 8.34 (s, 2H, CH
3
(Cz)).
13
C NMR δC (acetone-d6, 101 MHz, 298 K): 23.43 (s,
CH(CH3)2), 23,58 (s, CH(CH3)2), 23.63 (s, CH(CH3)2), 23.81 (s, C(CH3)2), 24.40 (s, CH(CH3)2),
28.48 (s, CH(CH3)2), 28.70 (s, CH(CH3)2), 38.06 (s, C(CH3)2), 61.08 (s, CCH2N), 99.09 (s,
CN-Cz), 115.76 (s, CH
1
(Cz)), 120.61 (s, ipso-CN(Cz)), 123.43 (s, ipso-C(Cz)), 124.74 (s,
CH
3
(Cz)), 125.02 (s, m-ArH), 125.91 (s, m-ArH), 127.16 (s, CH
2
(Cz), 130.31 (s, p-ArH), 130.69
84

(s, p-ArH), 136.51 (s, o-Ar), 140.44 (s, o-Ar), , 145.53 (ipso-N-Ar), 146.66 (ipso-N-Ar), 152.27
(s, ipso-N(Cz)), 171.18 (s, C=O), 208.74 (s, NCN). Analysis found for C44H48CuN5O: C: 72.70;
N: 9.36; H: 6.83. Requires: C: 72.75; N: 9.64; H: 6.66.
(MAC*)Cu(CzCN) (2). The complex was made from (MAC*)CuCl (200 mg, 0.37 mmol),
CzCN (71 mg, 0.37 mmol) and NaO
t
Bu (36 mg, 0.37 mmol) as a white solid. Yield: 200 mg (78%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.21-1.31 (m, 18H, CH(CH3)2), 1.43 (d, J = 6.8 Hz,
6H, CH(CH3)2), 1.67 (s, 6H, C(CH3)2), 3.30 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.55 (sept, J =
6.8 Hz, 2H, CH(CH3)2), 4.31 (s, 2H, CCH2N), 5.51 (d, J = 8.5 Hz, 1H, CH
7
(Cz)), 5.63 (d, J = 7.9
Hz, 1H, CH
1
(Cz)), 6.87 (t, J = 7.4 Hz, 1H, CH
5
(Cz)), 6.95 (t, J = 7.6 Hz, 1H, CH
6
(Cz)), 7.09 (d, J
= 8.5 Hz, 1H, CH
2
(Cz)), 7.55 (d, J = 7.8 Hz, 2H, m-ArH), 7.61 (d, J = 7.8 Hz, 2H,m- ArH),
7.79-7.84 (m, 2H, p-ArH), 7.87 (d, 1H, CH
4
(Cz)), 8.15 (s, 1H, CH
3
(Cz)).
13
C NMR δC (acetone-d6,
101 MHz, 298 K): 23.44 (s, CH(CH3)2), 23,59 (s, CH(CH3)2), 23.60 (s, CH(CH3)2), 23.82 (s,
C(CH3)2), 24.38 (s, CH(CH3)2), 28.49 (s, CH(CH3)2), 28.71 (s, CH(CH3)2), 38.01 (s, C(CH3)2),
61.12 (s, CCH2N), 96.68 (s, CN-Cz), 115.07 (s, CH
7
(Cz)), 115.16 (s, CH
1
(Cz)), 117.01 (s,
CH
5
(Cz)), 119.21 (s, CH
4
(Cz)), 121.42 (s, ipso-CN(Cz)), 123.21 (s, ipso-C(Cz)), 123.71 (s,
CH
3
(Cz)), 124.15 (s, ipso-C(Cz)), 124.48 (s, CH
6
(Cz)), 124.90 (s, m-ArH), 125.59 (s, CH
2
(Cz)),
125.81 (s, m-ArH), 130.13 (s, p-ArH), 130.50 (s, p-ArH), 136.48 (s, o-Ar), 140.47 (s, o-Ar), ,
145.45 (ipso-N-Ar), 146.56 (ipso-N-Ar), 150.50 (s, ipso-N(Cz)), 151.59 (s, ipso-N(Cz)), 171.23
(s, C=O), 209.25 (s, NCN). Analysis found for C43H49CuN4O: C: 73.45; N: 8.16; H: 6.88. Requires:
C: 73.63; N: 7.99; H: 7.04.
(MAC*)Cu(Cz) (3). The complex was made from (MAC*)CuCl (2.0 g, 3.67 mmol), Cz (613
mg, 3.67 mmol) and NaO
t
Bu (353 mg, 3.67 mmol) as a yellow solid. Yield: 2.1 g (85%).
1
H NMR
δH (acetone-d6, 400 MHz, 298 K): 1.14-1.36 (m, 18H, CH(CH3)2), 1.43 (d, J = 6.8 Hz, 6H,
85

CH(CH3)2), 1.66 (s, 6H, C(CH3)2), 3.30 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.55 (sept, J = 6.8 Hz,
2H, CH(CH3)2), 4.26 (s, 2H, CCH2N), 5.56 (d, J = 8.1 Hz, 2H, CH
1
(Cz)), 6.71 (t, J = 7.3 Hz, 2H,
CH
3
(Cz)), 6.81 (t, J = 6.9 Hz, 2H, CH
2
(Cz)), 7.53 (d, J = 7.8 Hz, 2H, m-ArH), 7.58 (d, J = 7.8 Hz,
2H, m-ArH), 7.71 (d, J = 6.9 Hz, 2H, CH
4
(Cz)), 7.73-7.79 (m, 2H, ArH).
13
C NMR δC (acetone-d6,
101 MHz, 298 K): 23.46 (s, CH(CH3)2), 23,58 (s, CH(CH3)2), 23.61 (s, CH(CH3)2), 23.82 (s,
C(CH3)2), 24.37 (s, CH(CH3)2), 28.50 (s, CH(CH3)2), 28.72 (s, CH(CH3)2), 37.97 (s, C(CH3)2),
61.16 (s, CCH2N), 114.60 (s, CH
1
(Cz)), 114,95 (s, CH
3
(Cz)), 118.39 (s, CH
4
(Cz)), 122.73 (s,
CH
2
(Cz)), 123.91( s, ipso-C(Cz)), 124.78 (s, m-ArH), 125.70 (s, m-ArH), 129.93 (s, p-ArH),
130.30 (s, p-ArH), 136.47 (s, o-Ar), 140.51 (s, o-Ar), , 145.37 (ipso-N-Ar), 146.46 (ipso-N-Ar),
149.85 (s, ipso-N(Cz)), 171.29 (s, C=O), 209.83 (s, NCN). Analysis found for C42H50CuN3O: C:
74.21; N: 6.01; H: 7.47. Requires: C: 74.58; N: 6.21; H: 7.45.
(DAC*)Cu(CzCN2) (4). The complex was made from (DAC*)CuCl (200 mg, 0.36 mmol),
[Cz(CN)2] (78 mg, 0.36 mmol) and NaO
t
Bu (35 mg, 0.36 mmol) as a yellow solid. Yield: 180 mg
(68%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.25 (dd, J = 11.3 Hz, 6.8 Hz, 24H, CH(CH3)2),
1.95 (s, 6H, C(CH3)2), 3.23 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 5.59 (d, J = 8.0 Hz, 2H, CH
1
(Cz)),
7.28 (d, J = 8.5 Hz, 2H, CH
2
(Cz)), 7.68 (d, J = 7.8 Hz, 4H, m-ArH), 7.97 (t, J = 7.8 Hz, 2H,
p-ArH), 8.36 (s, 2H, CH
3
(Cz)).
13
C NMR δC (acetone-d6, 101 MHz, 298 K): 23.34 (s, CH(CH3)2),
23.72 (s, CH(CH3)2), 24.36 (s, C(CH3)2), 28.80 (s, CH(CH3)2), 52.37 (s, C(CH3)2), 99.53 (s,
CN-Cz), 115.72 (s, CH
1
(Cz)), 120.49 (s, ipso-CN(Cz)), 123.56 (s, ipso-C(Cz)), 124.81 (s,
CH
3
(Cz)), 125.62 (s, m-ArH), 127.39 (s, CH
2
(Cz), 131.41 (s, p-ArH), 135.20 (s, o-Ar), 146.59
(ipso-N-Ar), 152.14 (s, ipso-N(Cz)), 172.11 (s, C=O), 213.90 (s, NCN). Analysis found for
C44H46CuN5O2 + 0.5H2O : C: 70.81; N: 9.26; H: 6.29. Requires: C: 70.52; N: 9.34; H: 6.32.
86

(DAC*)Cu(CzCN) (5). The complex was made from (DAC*)CuCl (200 mg, 0.36 mmol),
CzCN (69 mg, 0.36 mmol) and NaO
t
Bu (35 mg, 0.36 mmol) as an orange solid. Yield: 200 mg
(76%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.25 (dd, J = 8.3 Hz, 6.8 Hz, 24H, CH(CH3)2),
1.94 (s, 6H, C(CH3)2), 3.22 (sept, J = 6.8 Hz, 4H, CH(CH3)2), 5.51 (d, J = 8.5 Hz, 1H, CH
7
(Cz)),
5.61 (d, J = 8.1 Hz, 1H, CH
1
(Cz)), 6.91 (t, J = 7.8 Hz, 1H, CH
5
(Cz)), 7.00 (t, J = 7.6 Hz, 1H,
CH
6
(Cz)), 7.13 (d, J = 8.5 Hz, 1H, CH
2
(Cz)), 7.65 (d, J = 7.8 Hz, 4H, m-ArH), 7.89 (d, J = 7.8 Hz,
1H, CH
4
(Cz)), 7.91 (t, J = 7.8 Hz, 2H, p-ArH), 8.17 (s, 1H, CH
3
(Cz)). )).
13
C NMR δC (acetone-d6,
101 MHz, 298 K): 23.33 (s, CH(CH3)2), 23.69 (s, CH(CH3)2), 24.37 (s, C(CH3)2), 28.80 (s,
CH(CH3)2), 52.20 (s, C(CH3)2), 97.27 (s, CN-Cz), 115.06 (s, CH
7
(Cz)), 115.09 (s, CH
1
(Cz)),
117.40 (s, CH
5
(Cz)), 119.31 (s, CH
4
(Cz)), 121.24 (s, ipso-CN(Cz)), 123.33 (s, ipso-C(Cz)), 123.77
(s, CH
3
(Cz)), 124.34 (s, ipso-C(Cz)), 124.67 (s, CH
6
(Cz)), 125.51 (s, m-ArH), 125.84 (s, CH
2
(Cz)),
131.22 (s, p-ArH), 135.15 (s, o-Ar), 146.49 (ipso-N-Ar), 150.33 (s, ipso-N(Cz)), 151.49 (s,
ipso-N(Cz)), 172.17 (s, C=O), 214.12 (s, NCN). Analysis found for C43H47CuN4O2: C: 72.04; N:
7.88; H: 6.62. Requires: C: 72.19; N: 7.83; H: 6.62.
(DAC*)Cu(Cz) (6). The complex was made from (DAC*)CuCl (100 mg, 0.18 mmol), Cz
(30 mg, 0.18 mmol) and NaOtBu (18 mg, 0.19 mmol) as a purple solid. After the precipitation, the
compound was purified by sublimation. Yield: 86 mg (70%).
1
H NMR δH (acetone-d6, 400 MHz,
298 K): 1.25 (dd, J = 6.8, 5.8 Hz, 24 H, CH(CH3)2), 1.93 (s, 6H, C(CH3)2), 3.20 (sept, J = 6.8 Hz,
4H, CH(CH3)2)), 5.55 (d, J = 8.1 Hz, 2H, CH
1
(Cz)), 6.75 (t, J = 7.7 Hz, 2H, CH
3
(Cz)), 6.86 (t, J =
7.5 Hz, 2H, CH
2
(Cz)), 7.62 (d, J = 7.9 Hz, 4H, m-ArH), 7.74 (d, J = 7.6 Hz, 2H, CH
4
(Cz)), 7.86
(d, J = 7.8 Hz, 2H, p-ArH).
13
C NMR δC (acetone-d6, 101 MHz, 298 K): 23.34 (s, CH(CH3)2),
23.65 (s, CH(CH3)2), 24.38 (s, C(CH3)2), 28.84 (s, CH(CH3)2), 51.98 (s, C(CH3)2), 114.59 (s,
CH
1
(Cz)), 115.50 (s, CH
3
(Cz)), 118.40 (s, CH
4
(Cz)), 122.97 (s, CH
2
(Cz)), 124.13( s, ipso-C(Cz)),
87

125.37 (s, m-ArH), 131.01 (s, p-ArH), 135.12 (s, o-Ar), 146.39 (ipso-N-Ar), 149.71 (s,
ipso-N(Cz)),172.24 (s, C=O), 214.46 (s, NCN). MALDI-TOF: m/z calculated: 689.30 [M]
+
; found:
690.54 [M]
+
.
4.3.2. Electrochemical measurements
Cyclic voltammetry and differential pulsed voltammetry were performed using an
VersaSTAT 3 potentiostat. Anhydrous acetonitrile (DriSolv) was used as the solvent under inert
atmosphere, and 0.1 M tetra(n-butyl)ammonium hexafluorophosphate (TBAF) was used as the
supporting electrolyte. A glassy carbon rod was used as the working electrode, a platinum wire
was used as the counter electrode, and a silver wire was used as a pseudoreference electrode. The
redox potentials are based on values measured from differential pulsed voltammetry and are
reported relative to a ferrocene/ferrocenium (Cp2Fe/Cp2Fe
+
) redox couple used as an internal
reference, while electrochemical reversibility was determined using cyclic voltammetry.
4.3.3. Density Functional Calculations
Ground state geometries of all complexes reported were optimized at the B3LYP/LACVP**
level. TDDFT calculations were performed on the optimized structures at the
CAM-B3LYP/LACVP** level. These DFT and TDDFT calculations were performed
usingQ-Chem 5.1.
39
TDDFT calculations with solvent effects were performed using the IEF-PCM
solvation model in the non-equilibrium limit using the ptSS (perturbative state-specific) approach
as implemented in Q-Chem 5.1.
4.3.4. OLED fabrication and characterization
Glass substrates with 1 mm wide indium tin oxide (ITO) strips with were cleaned by sequential
sonication in tergitol, deionized water, acetone, and isopropanol, followed by 15 min UV ozone
88

exposure. Organic materials and metals were deposited in a vacuum thermal evaporator with a
base pressure of 10
-7
Torr using shadow masks, at rates of 0.2-1 Å/s. A 2 mm
2
device area was
defined using separate shadow masks to deposit 1 mm wide cathodes consisting of 100 nm Al,
aligned perpendicular to the ITO strips. The device structure was: glass substrate / 70 nm ITO / 5
nm hexaazatriphenylene hexacarbonitrile (HATCN) / 40 nm 4,4′-cyclohexylidenebis
[N,N-bis(4-methylphenyl)benzenamine] (TAPC) / 10 nm N,N’-dicarbazolyl-3,5-benzene (mCP) /
EML / 45 nm 2,2 ′,2 ′′-(l,3,5-benzenetriyl)-tris(L-phenyl-l-H-benzimidazole) (TPBi) / 1.5 nm
8-hydroxyquinolinato lithium (LiQ) / 100 nm Al. Here, the EML is 25 nm of compound 3, either
neat or doped into 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) at 10 or 40 vol%.
A HP4155B semiconductor parameter analyzer was used to source voltage and current to the
device while measuring the photocurrent from a calibrated large area Thorlabs FDS1010-CAL
photodiode that collected all light exiting the bottom of the glass substrate, without the use of any
outcoupling structures. A fiber-coupled OceanOptics USB4000-VIS-NIR spectrometer was used
to measure the output spectra.

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R. M.; Rohrdanz, M. A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.;
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92

Chapter 5. Highly Efficient photo- and Electroluminescence from
Two-Coordinate Ag(I) and Au(I) complexes

5.1. Introduction to luminescent two-coordinate Ag(I) and Au(I) complexes
To the best of our knowledge, reports of luminescent, 2-coordinate Au(I) complexes cover
dinuclear,
1-3
cationic complexes,
2, 4
heteronuclear complexes (Au/Cu),
5
and coordination
polymers
6
-all examples featuring NHC’s as the carbenes. The luminescence efficiency of most
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
or H-bonding,
9
or in frozen
solutions at 77K.
10
The first report of Au(I) amides, which employed triarylphosphines as the
L-type ligand, did not include any photophysical characterization.
11
A subsequent report of
luminescent NHC (namely IPr)-Au amides assigned their weak emission to fluorescence.
12
Most
of these reports investigated luminescent Au(I) complexes in biological applications. Recently,
Bochmann and coworkers report the mechanism of luminescence for a 2-coordinate carbene-Cu
and Au(carbazole) complex
13
as follows: molecular rotation around carbene-Cu or Cu-N bond,
which in turn allows for spin-inversion and a stabilization of the first excited singlet state below
the lowest triplet manifold. This process is termed “rotation-assisted spin inversion” (RASI). Our
studies suggest that their described mechanism is incorrect. While there are two states in
equilibrium, they are both triplets, one ligand localized and the other charge transfer in character.
Additionally, Bochmann et al. did not report PLQY or kr of their complexes. More recently, the
group published a report of a vacuum-deposited OLED with CAAC-AuCz complex as a dopant.
14

In contrast, the literature of luminescent Ag(I) complexes is scarcer. highly-luminescent
4-coordinate Ag(I) complexes have been reported with kr ~ 105 s-1.
15, 16
However, ΦPL drops to
93

< 10% in fluid environments, likely due to large excited state distortions. Luminescent 2-
coordinate NHC-Ag(I) complexes have been reported that are cationic
3, 17
and/or part of a large
macrocycle
17
with Ag-Ag interactions.
18
Most recently, Bochmann has reported the synthesis,
photo- and electroluminescence of two (CAAC)Ag(carbazole) complexes (CAAC = cyclic
alkyl(amino) carbene), with submicrosecond thermally activated radiative lifetimes.
19

Herein we report a series of linear luminescent, 2-coordinate Cu(I) (1), Ag(I) (2), and Au(I) (3)
complexes bearing non-conventional NHC monoaminoamido carbene (MAC*) and carbazole (Cz)
ligands. The detail of the photophysical properties of 1 is included in chapter 4 and is partially
shown in this chapter as a reference for the Ag and Au analogues. The Ag and Au complexes were
found to exhibit high quantum efficiency up to 85% in fluid and polymeric matrices with radiative
rates on the order of 10
6
– 10
7
s
-1
, which are extraordinary for monovalent coinage metal complexes.
Especially remarkable is the Ag(I) complex that is not only highly luminescent but also has
sub-microsecond radiative lifetimes. This constitutes an order-of-magnitude enhancement over the
radiative lifetimes of state-of-the-art phosphors employed as dopants in OLEDs, which is
important for mediating second-order quenching processes complicit in device degradation. The
nature of the radiative transition is charge transfer from the electron rich carbazole to the
electron-deficient carbene, with little metal contribution. The associated charge transfer state (CT)
is characterized by an extinction coefficient in absorption (ε ~ 1 – 6 x 10
3
M
-1
cm
-1
) that trends as:
most intense CT extinction for Au(I) complexes, followed by for Cu(I) complexes, and the weakest
for Ag(I) analogues. Furthermore, the CT state was found to have a small energy splitting between
its singlet and triplet manifolds, with ΔEST of 155 cm
-1
for the Ag analogue and 377 cm
-1
for the
Au analogue, priming the reported complexes for highly-efficient thermally activated delayed
94

fluorescence (TADF). OLED devices with complexes 2 and 3 as dopants and neat emitters were
fabricated through vapor deposition and show high external quantum efficiency up to 18.9% and
alleviated efficiency roll-off.
5.2. Results and Discussion
5.2.1. Synthesis and characterization
The pyrimidinium salt precursor to MAC* was synthesized by condensation of
N,N’-bis(2,6-diisopropylphenyl) formamidine and 3-chloropivaloyl chloride in the presence of
excess triethylamine at 0 °C, followed by intramolecular cyclization in refluxing in toluene for
16 h. The MAC*AgCl and MAC*AuCl precursors were synthesized from the free MAC* carbene
generated in situ, followed by addition of Ag2O or Au(SMe2)Cl and were isolated as white solid.
The MAC*AgCz and MAC*AuCz complexes were synthesized from reaction of the MAC*AgCl
and MAC*AuCl with sodium carbazolide and isolated in 60–95% yield. Complex 2 appears as a
beige solid, whereas 3 is a yellow-greenish solid. Both complexes are air- and moisture-stable.
The electrochemical properties of the complexes were examined by cyclic voltammetry
and differential pulse voltammetry in acetonitrile solution (Table 5.1). The redox potentials are
referenced to an internal ferrocene (Fc
+
/Fc) couple, and converted to HOMO and LUMO
energies.
20
All the complexes display quasi-reversible reduction and irreversible oxidation waves.

Figure 5.1. Molecular structures of complexes 1-3.
95

The reduction potential of the Au analogue (3) (2.51 V) is comparable to that of the Cu analogue
(2.50 V), whereas that of the Ag analogue (2) is slightly smaller (2.38 V). The oxidation potentials
of 2 and 3 are close to each other (0.27 V and 0.29 V respectively) and larger than the Cu analogue
(0.17 V). The larger oxidation potentials are comparable with the d orbitals of Ag and Au having
lower energy than the Cu analogue. The redox behavior suggests that the reduction (oxidation)
potential is determined by the carbene (carbazolyl) ligands with some metal character involved in
the oxidation.
Optimized structures of complexes 2 and 3 by Density Functional Theory (DFT) show that
both complexes have linear geometry around the metal center (the C NHC-Metal-NCz angle is 178°
for 2 and 179° for 3). The CNHC-metal distances are calculated to be 2.026 Å for both 2 and 3,
whereas the metal-NCz bond is 2.037 Å. These values are larger than the Cu analogue (The
CNHC-metal and metal-NCz are calculated to be 1.922 Å and 1.874 Å respectively). The CNHC-NCz
distance is 4.06 Å for 2 and 3, whereas that for the Cu analogue is 3.79 Å. Theoretical calculations
on these compounds support the assignment of the electrochemistry results. The contours for the
LUMO and HOMO of complexes 2 and 3 (Figure 5.2) show the LUMO is mainly localized on the
Table 5.1. Redox data for complexes 1-3 and dipole moments for 1–3 in the ground state (S 0), lowest
ligand-localized excited state (Cz) and charge transfer excited states (ICT).
Potentials Dipole moment ()
c

Complex
E ox
a

(V)
E red
a

(V)
HOMO
b

(eV)
LUMO
b

(eV)
 gs
S 0
 es
3
Cz
 es

1
ICT/
3
ICT
1 0.17 -2.50 -4.98 -1.88 10.6 9.9 -13.7
d
/-11.6
d

2 0.27 -2.38 -5.10 -2.02
12.3 11.7 -14.9/-11.9
3 0.29 -2.51 -5.12 -1.87 10.7 10.0 -13.8/-11.0
a
Redox potentials obtained in acetonitrile with 0.1 M TBAPF6 versus internal Fc+/Fc.
b
HOMO =
1.15(Eox) + 4.79; LUMO = 1.18(Ered) – 4.83.
c
Obtained from TD-DFT calculations
(CAM-B3LYP/LACVP**) using geometry optimized structures.
d
Dipole moments are opposite in
direction from the ground state moments.
96

carbene ligand, whereas the HOMO is predominately on the Cz ligand. The HOMO and LUMO
have very little spatial orbital overlap on the metal center. The orbital overlap is slightly small for
the Ag complex than the Au analogue. DFT and Time Dependent DFT (TD-DFT) calculations
were carried out at the CAM-B3LYP/LACVP** level to predict the lowest energy excited states
for these complexes.
19
The CAM-B3LYP functional was found to provide an accurate description
of both the charge transfer (CT) and ligand-localized (Cz) states (vide infra).
Ground (gs) and excited state ( es) dipole moments for compounds 2 and 3 were estimated
using DFT and TD-DFT calculations and given in Table 5.1. All of the complexes have large
ground-state dipole moment (gs). The dipole moments of the
I
CT and
3
ICT states are in the
opposite direction.  es for the
3
Cz state is similar to that of gs since the transition involves only
the redistribution of electron density within the carbazolyl ligand. It is worth mentioning that the
Ag complex displays the largest difference of  between the ground state and the excited CT states.

LUMO HOMO

Figure 5.2. Frontier orbitals of complexes 2 (top) and 3 (bottom).
97

5.2.2. Photophysical properties
Absorption spectra of complexes 1-3 are shown in Figure 5.3a. All the complexes show
structured carbazole-centered absorption at 360 nm and 370 nm as well as broad charge-transfer
(CT) band at ~ 420 nm. The extinction coefficient (εCT) of the three complexes are notably different.
Complex 2 has the weakest εCT (~1200 cm
-1
), followed by 1 (4700 cm
-1
) and 3 (7600 cm
-1
), which
suggests that complex 2 has the smallest overlap of the frontier orbitals, whereas 3 has the largest.
A hypsochromic shift of 86 nm (4029 cm
-1
) for 2 and 60 nm (2762 cm
-1
) for 3 is observed in the
onset of the
1
ICT band upon varying the solvent from non-polar methylcyclohexane to polar
acetonitrile (Figure 5.3b and 5.3c). In contrast, the absorption bands for the localized
1
Cz
transitions barely shift in the same solvents. The
1
ICT bands display negative solvatochromism
due to the large change in dipole moment of the excited state relative to the ground state (Table
5.1). The negligible shift of the absorption bands for the localized
1
Cz transitions is attributed to
similar dipole moments for the ground and excited states. It is worth noting that the hypsochromic
shift for 2 is the largest, followed by 3 and 1 (57 nm, 2478 cm
-1
), which is consistent with the
largest transition dipole moment of 2 compared to other two compounds.
Emission spectra of complexes 1-3 in 2-MeTHF and methylcyclohexane (MeCy) at room
temperature are shown in Figure 5.4a and 5.4b respectively. Photophysical data are summarized
in Table 5.2. All the complexes show broad and featureless bands indicative of emission from the
ICT states. Complexes 1 and 3 have almost identical peak maximum and emission profile at RT
in both 2-MeTHF and MeCy, whereas the emission of complex 2 is red-shifted by 26-28 nm. The
quantum efficiency ( PL) ranges from 5.6% to 55% in 2-MeTHF and 22% to 90% in MeCy. The
lower quantum yield in 2-MeTHF is likely due to formation of an exciplex with solvent molecules.
Complex 3 has similar PL to 1, whereas the PL of 2 is the lowest. The low PL of 2 is likely
98

owing to energy gap law since it is the reddest compound among the series.
21
3 has slightly faster
decay lifetime than 1 (0.79 s vs. 1.1 s in 2-MeTHF; 1.1 s vs. 1.6 s in MeCy), and hence a
slightly higher radiative decay rate constant kr, which is constant with the extinction efficient of
these two compounds. Surprisingly, complex 2, which has the smallest εCT, has the fastest decay
lifetime (36 ns in MeTHF and 120 ns in MeCy) and kr (1.5 ×10
6
s
-1
in MeTHF and 1.8 ×10
6
s
-1
in
MeCy). This kr is the highest value for luminescent Ag complexes and comparable or even faster
than Ir(III) and Pt(II) emitters. Emission spectra of the complexes are also solvatochromic. The
peak maximum of complex 2 undergoes a bathochromic shift of 58 nm (1735 cm
-1
) (Figure 5.5a),
300 350 400 450 500 550
0k
2k
4k
6k
8k
10k

Molar absorptivity (M-1 cm-1)
Wavelength (nm)
1
2
3
(a)

300 350 400 450 500 550
0.0
0.2
0.4

Absorption (AU)
Wavelength (nm)
MeCy
Toluene
MeTHF
ACN
(b)

300 350 400 450 500
0.0
0.2
0.4

Absorption (AU)
Wavelength (nm)
MeCy
Toluene
MeTHF
ACN
(c)

Figure 5.3. (a) Absorption spectra of complexes 1-3 in 2-methylTHF. (b) Absorption spectra of
complex 2 and (c) absorption spectra of complex 3 in methylcychohexane (MeCy), toluene, 2-MeTHF
and acetonitrile (ACN).
99

whereas that of complex 3 undergoes a bathochromic shift of 52 nm (1735 cm
-1
) (Figure 5.5b).
The shift with increasing the solvent polarity is due to stabilization of the ICT state through local
interactions between solvent molecules and the excited-state dipole moment of the complex.
The emission spectra of the complexes undergo marked rigidochromic shifts in frozen media
as shown for complexes 1-3 in 2-MeTHF (Figure 5.4a) and MeCy (Figure 5.4b) at RT and 77 K.
The spectral shifts upon cooling the fluid solutions come as a result of destabilization of the ICT
states in the rigid environment since the dipole moment of the excited state is in the opposite
direction to the ground state (Table 5.1). Thus, a solvation shell that stabilizes the ground state will
destabilize the excited state. The solvent organizes around the large ground state dipole at room
temperature and is locked into this arrangement on freezing, thereby shifting the ICT energy to
higher energy. Varying ratios of
3
Cz/ICT emission are observed from complexes with different
metal centers. In 2-MeTHF, complexes 2 and 3 display vibronic fine structure from the lower lying
3
Cz state at 77 K and have decay lifetimes at 430 nm in the range of milliseconds. For complex 1,
both
3
Cz and ICT emission are observed at 77 K as both states are similar in energy. The more
400 450 500 550 600 650 700 750
0.0
0.5
1.0

Normalized emission (AU)
Wavelength (nm)
1-RT
1-77 K
2-RT
2-77 K
3-RT
3-77 K
(a)
400 450 500 550 600 650 700 750
0.0
0.5
1.0

Normalized emission (AU)
Wavelength (nm)
1-RT
1-77 K
2-RT
2-77 K
3-RT
3-77 K
(b)

Figure 5.4. Emission spectra of complexes 1-3 in 2-methylTHF (a) and methylcyclohexane (b) at room
temperature (RT) and 77 k.
100

dominant
3
Cz character of the emission at 77 K for complexes 2 and 3 than that of the Cu analogue
is due to the larger difference of the dipole moments between the ground state and the excited ICT
state of 2 and 3. Therefore, the ICT state in 2 and 3 is destabilized to the point that emission is
exclusively from the
3
Cz state. In MeCy, the
3
Cz character at 77 K for all complexes is less
dominant than that in 2-MeTHF.
It is common for luminescent Cu(I) complexes to have low quantum efficiencies in fluid
solution due to high non-radiative decay rates caused either by unimolecular distortion of the
complexes in the excited state or bimolecular formation of excimers or exciplexes with the
solvent.
22, 23
The impact of these processes have on the  PL can be ameliorated in polymer films.
Complexes 1-3 doped at 1 wt% in PS show broad featureless ICT emission at room temperature
(Figure 5.6). The kr values of the complexes in the thin films are similar to those
500 550 600 650 700 750
0.0
0.5
1.0

Normalized emission (AU)
Wavelength (nm)
MeCy
Toluene
MeTHF
ACN
(a)
450 500 550 600 650 700 750
0.0
0.5
1.0

Normalized emission (AU)
Wavelength (nm)
MeCy
Toluene
MeTHF
ACN
(b)

Figure 5.5. Emission spectra of complexes 2 (a) and 3 (b) in methylcychohexane (MeCy), toluene,
2-MeTHF and acetonitrile (ACN).
101

in 2-MeTHF, whereas values for knr are much smaller. The magnitude of kr is the largest for 2,
which is consistent with what is observed in fluid solution. The emission spectra remain broad and
featureless at 77 K indicative of emission from ICT states, which is likely due to the fact that
polystyrene is less polar than 2-MeTHF. Moreover, the phenyl groups of PS are polarizable, so
they can stabilize either the ground or excited state dipoles, without the need for molecular
movement. Thus, on cooling PS films the ICT states are not destabilized enough to rise above the
energy of the
3
Cz state. The emission spectra show a minimal blue-shift at 77 K; however, the
decay lifetimes markedly increase by one to two orders of magnitude. The decay lifetime of the
Au analogue (3) at 77 K is faster than the Cu analogue due to the strong spin orbit coupling (SOC)
imparted by the heavier metal (Au). The large increase in emission lifetime on cooling to 77 K
Table 5.2. Photophysical properties of complexes 1-6 in 2-MeTHF, 1% polystyrene (PS) films and neat
solid.
Complex
λmax,RT
(nm)
ΦRT τRT (μs)
kr, RT
(10
5
s
-1
)
knr, RT
(10
5
s
-1
)
λmax,77K
(nm)
τ77K (μs)
2-MeTHF solution
1 542 0.55 1.1 5.0 4.1 432
430 nm:
2100 (62%)
342 (38%)
520 nm: 181
2 568 0.056 0.036 15 260 434 9900
3 544 0.50 0.79 6.3 6.3 428 260
Methylcyclohexane solution
1 520 0.90 1.6 5.6 0.62 480 298
2 548 0.22 0.12 18 65 438
430 nm:
5000 (38%)
321 (62%)
520 nm: 158
3 522 0.88 1.1 8.0 1.1 456 68
1 wt% in PS film
1 506 0.90 1.4 6.4 0.71 502 227
2 512 0.79 0.33 24 6.3 428 7.7
3 512 0.85 0.83 10 1.8 506 72
a
Calculated from the weighted average of the two contributions to .
102

suggests that the luminescence at room temperature is due to thermally-activated-delayed
fluorescence (TADF), which requires a small energy separation between
1
ICT and
3
ICT.
Surprisingly, the Ag analogue (2) shows the fastest lifetime at 77 K (7.7 s) among the series. The
exceptionally fast kr at RT along with the fast emission lifetime at 77 K for 2 suggest that TADF
is the most efficient for this complex.
To obtain parameters governing the temperature dependent emission properties, the emission
lifetime of complexes 2 and 3 in PS film was measured between 5 K and 320 K (Figure 5.7a and
5.7b). The lifetime of complex 3 increases gradually until near 150 K, where the increase becomes
more pronounced, before rising steadily below 30 K. The increase in lifetime upon cooling is
attributed to successive depopulation of states at high energy that have radiative rate constants
faster than the lowest lying state. At temperatures above 150 K, emission is dominated by a
higher-lying S1 state, whereas at temperatures below 30 K, thermal activation between triplet
substates is observed. For complex 2, the emission lifetime starts rising at ~ 70 K on cooling,
indicating a much smaller energy separation between the S1 state and the lower-lying states than 1
and 3. The thermal activation between triplet substates is observed at near 10 K. Under an
450 500 550 600 650 700
0.0
0.5
1.0

Normalized emission (AU)
Wavelength (nm)
1-RT
2-RT
3-RT
(a)
450 500 550 600 650 700
0.0
0.5
1.0

Normalized emission (AU)
Wavelength (nm)
1-77 K
2-77 K
3-77 K
(b)

Figure 5.6. Emission spectra of complexes 1-3 in polystyrene films at RT (a) and 77 K (b).
103

assumption of a fast thermalization, the temperature dependent decay curve can be fit to the
Boltzmann distribution equation (equation 1).
𝜏 =
2 + 𝑒 −
∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 + 𝑒 −
∆𝐸 (𝑆 1
−𝐼 )
𝑘 𝐵 𝑇
2(
1
𝜏 𝐼 ,𝐼𝐼
)+(
1
𝜏 𝐼𝐼𝐼
)𝑒 −
∆𝐸 (𝐼𝐼𝐼 −𝐼 )
𝑘 𝐵 𝑇 +(
1
𝜏 𝑆 1
)𝑒 −
∆𝐸 (𝑆 1
−𝐼 )
𝑘 𝐵 𝑇 (1)
Here, S1 represents the lowest singlet state, whereas I, II and III represent the triplet substates
I
T1,
II
T1 and
III
T1, and kB is the Boltzmann constant. Substates I and II are treated as being degenerate
since the energy splitting between these two states are normally very small (<10 cm
-1
) in an axially
distorted structure.
24
Fits of the experimental lifetime data to equation 1 reveal the decay rate of
each state and the energy separation between them (inset in Figure 5.7c and 5.7d). The exchange
energy is characterized by ΔE(S1-
III
T1), which is determined to be 155 cm
-1
(19.2 meV) for
complex 2 and 377 cm
-1
(46.8 meV) for complex 3. The energy separation of these two complexes
0 50 100 150 200 250 300
0
10
20
30
40
50

experiment
fitting
LIfetime (us)
Temperature (K)
(a)
0 50 100 150 200 250 300
0
10
20
30
40
50
60
70
80

experiment
fitting
LIfetime (us)
Temperature (K)
(b)

Figure 5.7. Top: Emission lifetime versus temperature of complexes 2 (a) and 3 (b). Bottom: Energy
level diagram for complexes 2 (c) and 3 (d).
104

are smaller than the Cu analogue with the Ag one having the smallest ΔE(S1-
III
T1). This
exceptionally small ΔE(S1-
III
T1) explains the fast kr of 2 at RT compared to the Cu and the Au
analogues.
The decay lifetime of the S1 state (S
1
= 57.8 ns for 2 and 71.6 ns for 3) is among the fastest values
of S
1
for Ag and Au complexes and consistent with the high kr as mentioned above. The decay
lifetimes are 4.3 s and

50.6 s for
III
T1 and
I
T1/
II
T1 substates respectively for complex 2, whereas
the values are 9.1 s and 73.3 s for complex 3. These values are much smaller than the Cu
analogue, which is attributed to the strong spin orbit coupling induced by the heavier metal (Ag
and Au). ΔE (III–II/I), which corresponds to the zero-field splitting (ZFS), are 43 and 91 cm
-1
for
complexes 2 and 3 respectively. The value for ZFS is exceptionally large for Ag(I) and Au(I)
complexes and is induced by the effective spin orbit coupling (SOC).
5.2.3. Electroluminescence
OLED devices with complex 2 and 3 as the dopants were fabricated through vapor
deposition. The device structure was: glass substrate / 70 nm ITO / 5 nm hexaazatriphenylene
hexacarbonitrile (HATCN) / 40 nm 4,4′-cyclohexylidenebis
[N,N-bis(4-methylphenyl)benzenamine] (TAPC) / 10 nm N,N’-dicarbazolyl-3,5-benzene (mCP) /
25 nm EML / 65 nm 2, 2, 2 ′′-(l,3,5-benzenetriyl)-tris(L-phenyl-l-H-benzimidazole) (TPBi) / 1.5
nm 8-hydroxyquinolinato lithium (LiQ) / 100 nm Al. Here, the EML is compound 2 doped into
3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) at 40 vol%, or compound 3 doped into mCBP at
40 vol% or used as a neat material (100%). Frontier orbital energies and molecular structures of
materials used in the devices are shown in Figure 4.8.
105

The device characteristics are shown in Figure 5.8. The devices emit bright green light with
emission peak maximum at 516-528 nm (Figure 5.8a) which are consistent with the
photoluminescence of the complexes. For complex 3, the emission peak maximum is slightly
red-shifted upon increasing the doping concentration likely due to aggregation. The J-V-L
characteristics of the devices indicate that the turn-on voltage (defined at brightness of 1 cd/m
2
) is
2.6 V for both complexes 2 and 3 at the doping concentration of 40%. The turn-on voltage
decreases to 2.5 V as the doping concentration increases to 100% for complex 3 (Figure 5.8b). The
low resistance at high doping concentration suggests that the dopant has higher charge mobility
than the mCBP host. The high charge mobility is likely due to the small reorganization energy of
440 480 520 560 600 640 680
0.0
0.5
1.0

Electroluminescence (AU)
Wavelength (nm)
2 (40%)
3 (40%)
3 (100%)
(a)

0 2 4 6 8 10
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
2 (40%)
3 (40%)
3 (100%)
Voltage (Volts)
lm (cd/m
2
)
(b)
0
100
200
300
400
500
600
J (mA/cm
2
)

0.1 1 10 100
0
2
4
6
8
10
12
14
16
18
20

EQE (%)
J (mA/cm
2
)
(c)

Figure 5.8. Electroluminescent device characteristics containing complexes 2 and 3 at doping
concentrations of 40% for 2, and 40% and 100% for 3. (a) Electroluminescent spectra. (b) Current
density-voltage-luminance (J-V-L). (c) External quantum efficiency.
106

the dopant resulting from the small metal character in the transition. Therefore, the Jahn-Tellar
distortion is strongly suppressed since formal oxidation on the metal upon photoexcitation (from
d
10
to d
9
) is minimized. The maximum EQE of the device containing 2 and 3 at doping
concentration of 40% are 10.9% and 18.9% respectively. (Figure 5.8c). The device efficiency is
among the highest values reported for OLEDs based on Au(I) dopant.
25, 26
OLED devices based
on Ag(I) complexes have never been reported. Interestingly, the device using an neat emissive
layer also demonstrates a high efficiency (EQE = 13.5%). Indeed, the photoluminescent efficiency
of both the vapor-deposited 40% doped (ΦPL = 90%) and neat (ΦPL = 60%) film of complex 3 are
high, resulting in the high EQE of the devices. In addition, the efficiency roll-off at high driving
currents for all the devices is small, which is due to the short emission decay lifetime that limits
triplet-triplet and triplet-polaron annihilation at high high brightness/current.
27, 28

5.3. Conclusion
A series of two-coordinate coinage metal complexes bearing non-conventional NHC ligands
were investigated. All the compounds show linear geometry at the copper center with the two
ligands coplanar with each other. All complexes show strong solvatochromism and
rigidochromism due to the large dipole moment of the ground states and the oppposite dipole in
the excited states. The compounds show efficient thermally-activated-delayed fluorescence
(TADF) with ΦPL ranging from 79% to 90% in polystyrene films. The emission of the Ag analogue
(2) is red-shifted compared to the other two complexes. The decay lifetime of the Ag complex is
submircoseconds (0.33 s in PS film). The radiative rate constant for the Ag analogue is
10
6
-10
7
s
-1
, which is exceptional for organometallic compounds that show TADF and is even more
efficient compared to a lot of Ir and Pt phosphorescent emitters. The fast kr for the Ag complex is
107

due to the extremely small energy separation between the lowest singlet and triplet states (155 cm
-
1
). Vapor-deposited OLED devices based on complexes 2 and 3 show both high EQE and low
efficiency roll-off at high voltage, which is attributed to the high quantum efficiency and the short
exciton decay time of these efficient TADF complexes. The high EQE of the host-free devices
based on 3 demonstrates the potential of these complexes used as neat emitters in the OLED
devices.
5.4. Experimental
Refer to section 4.3 for the methods used for photophysical characterization, electrochemical
measurements, DFT calculations and OLED fabrications.
5.4.1. Synthesis
All reactions were carried out using standard Schlenk and glovebox techniques using dried and
degassed solvents. Synthesis of the MAC*Cl and MAC*CuCl precursors and MAC*CuCz (1)
have been described in Chapter 4. Potassium bis(trimethylsilyl)amide (KHMDS), Ag2O,
Au(SMe2)Cl and carbazole were purchased from Sigma Aldrich.
Synthesis of MAC*AgCz (2)

MAC*AgCl (2a). Ag2O (176 mg, 0.76 mmol) was added to a DCM solution (30 mL) of
MAC*Cl (610 mg, 1.26 mmol) and stirred at RT for 24 h, filtered through Celite, and the filtrate
108

was reduced to 5 mL. Pentane was added to the solution to precipitate a white solid. The solid was
washed further with pentane and dried under vacuum to afford a white powder. Yield: 300 mg
(40%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.17 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.28-1.42
(m, 18H, CH(CH3)2), 1.61 (s, 6H, C(CH3)2), 3.15 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.40 (sept, J
= 6.8 Hz, 2H, CH(CH3)2), 4.25 (s, 2H, CCH2N), 7.32 (d, J = 7.4 Hz, 2H, m-ArH), 7.38 (d, J = 7.4
Hz, 2H, m-ArH), 7.43 (t, J = 7.4 Hz, 1H, p-ArH), 7.48 (t, J = 7.4 Hz, 1H, p-ArH).
13
C NMR δC
(acetone-d6, 101 MHz, 298 K): 23.34 (s, CH(CH3)2), 23,66 (s, CH(CH3)2), 23.71 (s, CH(CH3)2),
23.83 (s, C(CH3)2), 24.30 (s, CH(CH3)2), 28.24 (s, CH(CH3)2), 28.54 (s, CH(CH3)2), 37.94 (s,
C(CH3)2), 60.75 (s, CCH2N), 124.38 (s, m-ArH), 125.31 (s, m-ArH), 129.79 (s, p-ArH), 130.21
(s, p-ArH), 137.36 (s, o-NAr), 141.14 (s, o-NAr), , 144.47 (ipso-NAr), 145.60 (ipso-NAr), 170.94
(s, C=O), 217.49 (s, NCN).
MAC*AgCz (2). MAC*AgCl (257 mg, 0.44 mmol) and KCz (99 mg, 0.48 mmol) were
dissolved in THF (20 mL) and the reaction mixture was stirred at RT overnight, filtered through
Celite and the solvent was concentrated to 3 mL under reduced pressure. Pentane (20 mL) was
added to the solution and a yellow precipitate formed. The solid was dried under vacuum. Yield:
300 mg (95%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.24 (d, J = 6.8 Hz, 6H, CH(CH3)2),
1.32 (m, 12H, CH(CH3)2), 1.42 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.69 (s, 6H, C(CH3)2), 3.30 (sept,
J = 6.8 Hz, 2H, CH(CH3)2), 3.55 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 4.38 (s, 2H, CCH2N), 6.00 (d,
J = 8.1 Hz, 2H, CH
1
(Cz)), 6.72 (t, J = 6.8 Hz, 2H, CH
3
(Cz)), 6.90 (t, J = 6.9 Hz, 2H, CH
2
(Cz)),
7.52 (d, J = 7.8 Hz, 2H, m-ArH), 7.57 (d, J = 7.8 Hz, 2H, m-ArH), 7.69-7.75 (m, 2H, ArH), 7.77
(d, J = 7.6 Hz, 2H, CH
4
(Cz)).
13
C NMR δC (acetone-d6, 101 MHz, 298 K): 23.50 (s, CH(CH3)2),
23.72 (s, CH(CH3)2), 23.81 (s, CH(CH3)2), 23.91 (s, C(CH3)2), 24.32 (s, CH(CH3)2), 28.44 (s,
CH(CH3)2), 28.73 (s, CH(CH3)2), 38.12 (s, C(CH3)2), 60.87 (s, CCH2N), 114.46 (s, CH
1
(Cz)),
109

114.49 (s, CH
3
(Cz)), 118.57 (s, CH
4
(Cz)), 122.76 (s, CH
2
(Cz)), 123.82( s, ipso-C(Cz)), 124.72 (s,
m-ArH), 125.61 (s, m-ArH), 129.94 (s, p-ArH), 130.32 (s, p-ArH), 137.80 (s, o-Ar), 141.49 (s,
o-Ar), 145.13 (ipso-N-Ar), 146.18 (ipso-N-Ar), 150.33 (s, ipso-N(Cz)), 171.09 (s, C=O), 217.48
(s, NCN).

MAC*AuCl (3a). KHMDS (227 mg, 1.14 mmol) was added to a THF solution (20 mL) of
MAC*Cl (500 mg, 1.03 mmol) at RT and the solution was stirred for 1 h before AuClSMe2 (305
mg, 1.03 mmol) was added. The reaction mixture was stirred at RT overnight, filtered through
Celite and the solvent was concentrated to 3 mL under reduced pressure. Pentane (20 mL) was
added to the solution and a white precipitate formed. The solid was dried under vacuum. Yield:
480 mg (68%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.16 (d, J = 6.8 Hz, 6H, CH(CH3)2),
1.38 (m, 18H, CH(CH3)2), 1.60 (s, 6H, C(CH3)2), 3.10 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 3.35
(sept, J = 6.8 Hz, 2H, CH(CH3)2), 4.15 (s, 2H, CCH2N), 7.29 (d, J = 7.8 Hz, 2H, m-ArH), 7.36 (d,
J = 7.8 Hz, 2H, m-ArH), 7.43 (t, J = 7.8 Hz, 1H, p-ArH), 7.47 (t, J = 7.8 Hz, 1H, p-ArH).
13
C
NMR δC (acetone-d6, 101 MHz, 298 K): 23.07 (s, CH(CH3)2), 23.53 (s, CH(CH3)2), 23.68 (s,
CH(CH3)2), 23.86 (s, C(CH3)2), 23.93 (s, CH(CH3)2), 28.35 (s, CH(CH3)2), 28.65 (s, CH(CH3)2),
37.88 (s, C(CH3)2), 60.85 (s, CCH2N), 124.05 (s, m-ArH), 125.12(s, m-ArH), 129.69 (s, p-ArH),
110

130.07 (s, p-ArH), 136.36 (s, o-NAr), 140.55 (s, o-NAr), , 144.38 (ipso-NAr), 145.74 (ipso-NAr),
171.83 (s, C=O), 201.43 (s, NCN).
MAC*AuCz (3). MAAC*AuCl (140 mg, 0.21 mmol) and KCz (43 mg, 0.21 mmol) were
dissolved in THF (20 mL) and the reaction mixture was stirred at RT overnight, filtered through
Celite and the solvent was concentrated to 3 mL under reduced pressure. Pentane (20 mL) was
added to the solution and a yellow-greenish precipitate formed. The solid was dried under vacuum.
Yield: 100 mg (60%).
1
H NMR δH (acetone-d6, 400 MHz, 298 K): 1.20-1.31 (m, 18H, CH(CH3)2),
1.43 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.66 (s, 6H, C(CH3)2), 3.30 (sept, J = 6.8 Hz, 2H, CH(CH3)2),
3.55 (sept, J = 6.8 Hz, 2H, CH(CH3)2), 4.26 (s, 2H, CCH2N), 5.56 (d, J = 8.1 Hz, 2H, CH
1
(Cz)),
6.72 (t, J = 6.8 Hz, 2H, CH
3
(Cz)), 6.81 (t, J = 6.9 Hz, 2H, CH
2
(Cz)), 7.53 (d, J = 7.8 Hz, 2H,
m-ArH), 7.58 (d, J = 7.8 Hz, 2H, m-ArH), 7.73 (d, J = 7.4 Hz, 2H, CH
4
(Cz)), 7.76-7.79 (m, 2H,
ArH).
13
C NMR δC (acetone-d6, 101 MHz, 298 K): 23.47 (s, CH(CH3)2), 23,59 (s, CH(CH3)2),
23.62 (s, CH(CH3)2), 23.82 (s, C(CH3)2), 24.37 (s, CH(CH3)2), 28.50 (s, CH(CH3)2), 28.73 (s,
CH(CH3)2), 37.97 (s, C(CH3)2), 61.16 (s, CCH2N), 114.61 (s, CH
1
(Cz)), 114.96 (s, CH
3
(Cz)),
118.40 (s, CH
4
(Cz)), 122.74 (s, CH
2
(Cz)), 123.92( s, ipso-C(Cz)), 124.79 (s, m-ArH), 125.70 (s,
m-ArH), 129.94 (s, p-ArH), 130.30 (s, p-ArH), 136.48 (s, o-Ar), 140.51 (s, o-Ar), , 145.38
(ipso-N-Ar), 146.46 (ipso-N-Ar), 149.86 (s, ipso-N(Cz)), 171.29 (s, C=O), 209.83 (s, NCN).

Chapter 5 References
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111

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T. H.; Jalebi, M. A.; Friend, R. H.; Linnolahti, M.; Bochmann, M.; Credgington, D., High-
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163.
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16. 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
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17. Lu, T.; Yang, C.-F.; Steren, C. A.; Fei, F.; Chen, X.-T.; Xue, Z.-L., Synthesis and
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New Silver Aggregates Containing Short Ag−Ag Contacts Stabilized by Functionalized Bis(N-
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23. Shaw, G. B.; Grant, C. D.; Shirota, H.; Castner, E. W.; Meyer, G. J.; Chen, L. X.,
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24. Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T., The triplet state of
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performance light-emitting diodes based on carbene-metal-amides. Science 2017, 356 (6334), 159.
26. Conaghan, P. J.; Menke, S. M.; Romanov, A. S.; Jones, S. T. E.; Pearson, A. J.; Evans,
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113

Chapter 6. Conclusion

A series of three- and two-coordinate Cu(I) complexes that show a large diversity of
photophysical properties have been investigated in Chapter 2-4. The interesting photophysical
properties of two-coordinate Ag(I) and Au(I) complexes have also been introduce in Chapter 5.
These do not only open access to important applications, especially, in the field of OLEDs, but
also stimulate progress in scientific research due to the possibility of chemical tuning of electronic
energy states with respect to their energy levels, separations, oscillator strength, etc. This is
particularly interesting regarding understanding and engineering of compounds with specific
luminescence properties, such as emission colors, quantum yields and decay lifetimes.
The three-coordinate (BAC)Cu(I) complexes in Chapter 2 demonstrates the possibility of
using non-NHC ligands for synthesizing stable luminescent Cu(I) complexes. However, just as
most of the other four- and three-coordinate luminescent Cu(I) complexes introduced in Chapter
1, the quantum efficiency of these compounds drops dramatically in the fluid solution due to
Jahn-Tellar distortion in the excited state that results in the increase of the non-radiative decay rate.
One of the strategies to suppress the structural distortion in the excited state is to introduce
sterically encumbering ligands on the Cu(I) center. The fascinating photophysical properties of the
two-coordinate bis-carbene Cu(I) complex [DAC]2CuBF4 introduced in Chapter 3 demonstrates
that high quantum efficiency (65%) in fluid solution could be achieved by two-coordinate Cu(I)
complexes, which are much less investigated than their three- and four-coordinate counterparts.
The two bulky DAC ligands lock tightly around the Cu(I) center, minimizing the structural
distortion in the excited state. However, this compound shows a substantially long decay lifetime
(18 s), which could lead to triplet-triplet and triplet-polaron annihilation at high
114

brightness/current in OLED devices. More importantly, cationic compounds generally have high
sublimation temperature than their degradation temperature, and decomposition easily happens
during thermal evaporation. The emitters of OLEDs based on these cationic compounds were
mostly fabricated by spin-coating methods. However, a large majority of the small molecules are
not suitable for the solution process due to poor film morphology and easy crystallization upon
spin-casting. Thermal evaporation is still the industry-preferred fabrication process, offering
greater control and reproducibility. Therefore, it is highly demanded to develop luminescent Cu(I)
complexes with high quantum efficiency and fast emission lifetime (fast radiative rate) along with
high thermal stability for efficient and stable OLED devices.
All the aforementioned requirements for luminescent Cu(I) complexes utilized in OLEDs
are fulfilled by the neutral two-coordinate MAC*Cu(I) and DAC*Cu(I) complexes introduced in
Chapter 4. All of these complexes show highly efficient TADF (PLQY up to 100%) with short
decay lifetime (~ 1 s). The fast radiative rates (kr= 10
5
– 10
6
s
-1
) are comparable to state-of-the-art
Ir(III) and Pt(II) complexes employed as dopants in OLEDs. The nature of the radiative transition
is charge transfer from the electron rich carbazole (HOMO) to the electron-deficient carbene
(LUMO), with little metal contribution. The large spatial orbital overlap between the HOMO and
LUMO renders an extremely small ΔE(S1-T1). A small ΔE(S1-T1) is normally bought at the cost
of a small S1→S0 oscillator strength (radiative rate). However, for these linear Cu(I) complexes,
strong coupling between the orbitals of the donor (Cz) and acceptor (carbene) moieties through
the d orbitals of the Cu center induces high kr despite the large spatial separation between HOMO
and LUMO. Higher radiative rate is achieved when this linear system is applied to other coinage
metals such Ag(I) and Au(I). Especially remarkable is the Ag(I) complex that shows a
sub-microsecond emission lifetime and high kr that is in the order of 10
6
– 10
7
s
-1
. This high kr is
115

achieved by the smaller ΔE(S1-T1) compared to the Cu analogue resulting from the larger spatial
orbital separation between the HOMO and LUMO. In the meantime, the strong coupling between
the  orbitals of the donor (Cz) and acceptor (carbene) moieties through the d orbitals of the metal
center is retained. OLED devices employing these linear Cu(I), Ag(I) and Au(I) complexes as
dopants show high EQE (up to 19.4%) with alleviated roll-off, resulting from the high PLQY and
the short decay lifetime.
In conclusion, the discussed properties and trends demonstrate the high potential of coinage
metal complexes in the field of luminescence behavior, TADF properties and OLED applications.

Abstract (if available)

Abstract Phosphorescent Cu(I) complexes have received a great amount of attention due to their applications in organic emitting diodes (OLEDs), solar cell conversion and sensors. In the case of OLEDs, Cu(I) complexes have been considered as potential alternatives to the successful phosphorescent emitters using noble- metals due to the low cost of copper relative to such elements as iridium and platinum. The most extensively studied mononuclear luminescent Cu(I) complexes are four-coordinate tetrahedral homo- and heteroleptic complexes bearing diimine and organophosphine ligands. Three-coordinate Cu(I) complexes bearing N-heterocyclic carbenes have also been reported. Interestingly, while the catalytic properties of two-coordinate (NHC)Cu(I) complexes have been investigated extensively, reports of their luminescent properties have only appeared recently. This is due to the common belief that three- and four-coordinate geometries at the copper center are required for efficient luminescence. ❧ Chapter 1 introduces the state-of-the-art luminescent Cu(I) complexes and their applications in organic light emitting diodes (OLEDs). The properties of conventional and non-conventional N-heterocyclic carbene (NHC) as well as non NHC ligands have also been described in this chapter. ❧ The synthesis, crystal structures and photophysical properties of Cu(I) complexes bearing non-NHC carbenes is described in Chapter 2. While luminescent carbene-Cu(I) complexes have been well investigated, research to date has focused principally on Cu(I) complexes bearing N-heterocyclic carbenes (NHC). In this work, we have investigated two three-coordinate mononuclear Cu(I) complexes bearing the non-NHC ligand BAC (bis(di-isopropylamino)cyclopropenylidene carbene) and compare their photophysical properties to those of the reported NHC analogues bearing the Ipr (1,3-bis(2,6-di-isopropylphenyl)imidazole-2-ylidene ligand. The X-ray structure of one of the BACCu complexes has four unique molecules in an asymmetrical unit with different distortion angles at the copper center (from relatively more Y-shaped to more T-shaped geometry), which demonstrates a shallow potential energy barrier of Y- to T-shaped distortion. The (BAC)Cu compounds have photophysical properties comparable to their NHC analogues. We demonstrate the possibility of using non-NHC carbenes for making efficient mononuclear luminescent copper(I) complexes. ❧ Chapter 3 focuses on the rarely studied system—two-coordinate Cu(I) complexes. The cationic biscarbene Cu complex [(DAC)₂Cu][BF₄] shows a quantum efficiency of 65% in solution which is one of the brightest mononuclear Cu(I) complexes reported so far. We demonstrate that two coordination at the Cu center can also achieve high quantum yield if the ligands can provide enough steric hindrance. Interestingly, the phosphorescence of this compound in CH₂Cl₂ solution shows negligible quenching by oxygen in CH₂Cl₂ solution. This insensitivity to quenching is attributed to the excited state redox potential being insufficient for electron transfer to oxygen as well as the steric hindrance of the DAC ligands. ❧ While the non-radiative rate (kₙᵣ = 10³ - 10⁴ s⁻¹) of [(DAC)₂]Cu[BF₄] is smaller compared to most of other luminescent Cu complexes, the radiative rate (kᵣ = ∼10⁴ s⁻¹) is still at least an order of magnitude smaller than the successful Ir and Pt emitters in fluid solution. The synthesis and photophysicial properties of a series of neutral two-coordinate carbene-Cu(I)-carbazole complexes bearing non-conventional N-heterocyclic carbene ligands DAC (diamidocarbene) and MAAC (monoamidoamino carbene) have been introduced in Chapter 4. These complexes show almost 100% quantum yield with extremely short decay lifetime (∼1 μs). The nature of the radiative transition is charge transfer from the electron rich carbazole to the electron-deficient carbene, with little metal contribution. The extremely fast radiative rate (kᵣ) is due to thermally-activated-delayed fluorescence (TADF). Variations of both the donor groups (carbazole) and the acceptors (carbenes) lead to emission color tuning over 270 nm from deep blue to red which covers the entire visible region. Organic light-emitting diodes (OLEDs) fabricated with (MAC*)Cu(Cz) as a green emissive dopant have high external quantum efficiencies (EQE = 19.4%) and brightness of (54000 cd/m²) with alleviated roll-off. The complex can also be used as a neat emissive layer to make highly efficient OLEDs (EQE = 16.3%). ❧ These linear Cu complexes is then extended to complexes bearing other coinage metals including Ag and Au. In chapter 5, we report a series of linear luminescent, 2-coordinate Cu(I) (1), Ag(I) (2), and Au(I) (3) complexes bearing MAC* and carbazole ligands. The complexes were found to exhibit high quantum efficiency up to 90% in fluid and polymeric matrices with radiative rates on the order of 10⁵ – 10⁷ s⁻¹, which are extraordinary for monovalent coinage metal complexes and comparable to state of the art organoiridium and organoplatinum phosphors. Especially remarkable is the Ag(I) complex that is not only highly luminescent but also has sub-microsecond radiative lifetimes. The CT state was found to have a small energy splitting between its singlet and triplet manifolds, with ΔEST of 155 cm⁻¹ for the Ag analogue and 377 cm⁻¹ for the Au analogue, priming the reported complexes for highly-efficient thermally activated delayed fluorescence (TADF). OLED devices with complexes 2 and 3 as dopants and neat emitters were fabricated through vapor deposition and show high external quantum efficiency up to 18.9% and alleviated efficiency roll-off.

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Creator Shi, Shuyang (author)

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

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 08/12/2019

Defense Date 11/05/2018

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Tag carbenes,copper,high efficiency,OAI-PMH Harvest,OLED,two-coordinate

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Advisor Thompson, Mark (committee chair), Goo, Edward (committee member), Haiges, Ralf (committee member)

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