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Studies of molecular orientation using iridium phosphors and integration of corannulene into organic light emitting diodes (OLEDs)
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i
Studies of Molecular Orientation using Iridium Phosphors
and Integration of Corannulene into Organic Light Emitting
Diodes (OLEDs)
by
John Facendola
___________________________________________________________________
A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA in Partial Fulfillment of the Requirements for the
Degree DOCTOR OF PHILOSOPHY (CHEMISTRY)
December 2017
Copyright 2017 John Facendola
ii
Dedication
To my mother Katherine who always put her children first
iii
Acknowledgments
I want to first and foremost thank and express my extreme gratitude to my advisor Professor Mark
Thompson for giving me such a great opportunity to work for you. You have given me so many
opportunities and so much guidance in both chemistry and the real world to help me grow as a both
a scientist and a person.
I am very thankful to have had Prof. Richard Brutchey, Prof. Moh El-Naggar, Prof. Smaranda
Marinescu, Prof. G. K Surya Prakash, and Prof Chao Zhang for serving on my PhD Guidance
Committee. Thank you for your time and valuable feedback. Special thanks to Dr. Peter Djurovich
for always being there to assist me whenever I looked for a second opinion or needed someone to
run my ideas. I am very grateful for your help.
I have been lucky to work in the Thompson lab. I’d like to thank all past and present Thompson
group members that helped me for being there in the trenches, helping me in the lab and friendship.
I especially want to thank Dr. Jonathan Sommer, Dr. Lincoln Hall, Dr. Matt Jurow, Dr. Nadia
Korovina, Dr. Thilini Batagoda and future doctors Rasha Hamze, Martin Seifrid, Mu'azzam Idris,
and Moonchul Jung as well as Gabriel Mandel. I would never have made it out alive without your
valuable assistance and fruitful conversations.
I must say a special thank you to Dr. Ralf Haiges for all the X-ray crystallography assistance over
the years. I would also like to thank Prof. Travis Williams for his NMR assistance.
I must also thank Prof. Jay Siegel and Dr. Daphne Diemer for giving me the opportunity to work at
the University of Tianjin and for assisting me survive the culture shock. I am truly grateful.
I would like to express my greatest gratitude and thanks to Judy Fong. Without you the lab would
not run and I am so thankful for the tireless hours you put in and for the emotional support you give.
I’m so grateful to my parents and brothers for supporting me in various ways and for sacrificing so
much to help me get to where I am. Thank you for always supporting me and giving me love when
I needed it
And last but not least I want to thank my friends in LA, for becoming a second family for me during
my time in LA: Sam Oglesby, Jack Meighan, Pavel Gordeyev, Kimberly Ndombe, Renée Reizman,
Anthony Tamborino, Matt Rickett, Abbey Neer, Betsy Melenbrink, Stephanie McKean and Kate
Lewis. Thank you for picking me up when I was down, supporting me in ways I can’t begin to
describe and for helping me become who I am today. This would not have been possible without
you.
iv
TABLE OF CONTENTS
Dedication ......................................................................................................................................... ii
Acknowledgments............................................................................................................................ iii
List of Tables .................................................................................................................................. vii
List of Figures ................................................................................................................................ viii
Abstract .......................................................................................................................................... xiii
CHAPTER 1. Introduction ............................................................................................................ 1
1.1 Organic Light Emitting Diodes .................................................................................................. 1
1.1.1 Electroluminescence of OLEDs ....................................................................................... 1
1.1.2 Photophysics of Chromophores used in OLEDs .............................................................. 1
1.2 Composition of an OLED .......................................................................................................... 3
1.2.1 Basic Steps in Electroluminescence ................................................................................. 3
1.2.2 Single Organic Layer OLEDs .......................................................................................... 4
1.2.3 Single Heterojunction OLEDs .......................................................................................... 4
1.2.4 Double Heterojunction OLEDs ........................................................................................ 5
1.3 Efficiency of OLEDs................................................................................................................... 6
1.3.1 External Quantum Efficiency ............................................................................................ 6
1.3.2 Photoluminescent Quantum Efficiency ............................................................................. 7
1.3.3 Carrier Recombination Efficiency ..................................................................................... 7
1.3.4 Consideration of Spin Statistics within an OLED ............................................................. 8
1.4 Phosphorescent Complexes ....................................................................................................... 9
1.4.1 Advantages of Organometallic Complexes in OLEDs ...................................................... 9
1.4.2 Characteristics of Iridium Phosphors in OLEDs ............................................................. 10
1.5 Tuning of Iridium Complexes ................................................................................................... 11
1.5.1 Modifying of the Ligand π System .................................................................................. 11
1.5.2 Modification by Addition of Substituents to the Ligand System .................................... 12
Chapter 1 References ...................................................................................................................... 13
CHAPTER 2. Synthesis and Characterization of Phosphorescent Platinum and Iridium
Complexes Containing Corannulene .......................................................................................... 18
2.1. Introduction .............................................................................................................................. 18
2.2. Synthesis of Complexes .......................................................................................................... 21
2.3 Crystal Structures ...................................................................................................................... 22
2.3.1 (corpy)Pt(dpm) Crystal Structure ................................................................................... 22
2.3.2 (corpy)Ir(ppz)2 Crystal Structure ..................................................................................... 24
2.4 NMR and Dynamic Behavior ................................................................................................... 25
2.4.1 Room Temperature NMR ............................................................................................... 25
2.4.2 Variable Temperature NMR and Analysis ..................................................................... 26
2.5 Electrochemical Properties ....................................................................................................... 32
v
2.6 Photophysical Properties ........................................................................................................... 34
2.6.1 Absorption Spectra........................................................................................................... 34
2.6.2 Emission Spectra .............................................................................................................. 35
2.6.3 DFT Calculations ............................................................................................................. 39
2.7 Conclusion ................................................................................................................................ 41
2.8 Experimental ............................................................................................................................ 42
Chapter 2 References ...................................................................................................................... 47
CHAPTER 3. Synthesis and Characterization of Phosphorescent Iridium Complexes with
Multiple Cyclometalated Corannulenes ..................................................................................... 52
3.1 Introduction ............................................................................................................................... 52
3.2 Synthesis of Complexes ............................................................................................................ 53
3.3 Crystal Structures ...................................................................................................................... 54
3.4 NMR and Dynamic Behavior ................................................................................................... 57
3.4.1 Room Temperature NMR ................................................................................................ 57
3.4.2 Variable Temperature NMR and Analysis ...................................................................... 59
3.5 Electrochemical Properties ....................................................................................................... 63
3.6 Photophysical Properties ........................................................................................................... 65
3.6.1 Absorption Spectra .......................................................................................................... 65
3.6.2 Emission Spectra .............................................................................................................. 66
3.6.3 DFT Calculations ............................................................................................................. 69
2.7 Conclusion ................................................................................................................................ 72
2.8 Experimental ............................................................................................................................. 73
Chapter 3 References ...................................................................................................................... 76
CHAPTER 4. Development and Characterization of Transport Materials Containing
Corannulene .................................................................................................................................. 80
4.1 Introduction .............................................................................................................................. 80
4.1.1 Hole Transport Materials ................................................................................................. 80
4.1.2 Electron Transport Materials ........................................................................................... 81
4.1.3 Ambipolar Transport Materials Figure ............................................................................ 82
4.1.4 Electronic Properties of Corannulene .............................................................................. 83
4.2 Synthesis of CPD ..................................................................................................................... 85
4.3 Electrochemical Properties ....................................................................................................... 86
4.4 Photophysical Properties ........................................................................................................... 88
4.5 OLED Device Performance ...................................................................................................... 91
4.6 Conclusion ................................................................................................................................ 95
4.7 Experimental ............................................................................................................................. 96
Chapter 4 References ...................................................................................................................... 98
CHAPTER 5. Studies of Structural Effects on Molecular Orientation of Organometallic
Iridium Phosphors in Organic Light Emitting Diodes ............................................................ 103
5.1 Introduction ............................................................................................................................ 103
5.1.1 Factors that Influence Outcoupling Efficiency .............................................................. 103
vi
5.1.2. Methods to Engineer Increased Outcoupling Efficiency .............................................. 104
5.1.3 Improving Outcoupling by Non-Isotropic Dipole Orientation ...................................... 106
5.1.4 Measurement of the Degree of Orientation ................................................................... 107
5.1.5 Determination of Molecular Alignment in a Doped Thin Film .................................... 108
5.1.6 Determination of the Transition Dipole ......................................................................... 109
5.1.7 Examples of Orientation of Iridium Complexes ............................................................ 110
5.1.8 Proposed Mechanisms of Dopant Orientation in Thin Films ........................................ 111
5.2 Synthesis of the Complexes .................................................................................................... 117
5.3 Electrochemical Properties ..................................................................................................... 119
5.4 Photophysical Properties ......................................................................................................... 121
5.4.1 Absorption Spectra......................................................................................................... 121
5.4.2 Emission Spectra ............................................................................................................ 122
5.5 Angle Dependent Photoluminescence ................................................................................... 124
5.6 OLED Device Performance .................................................................................................... 127
5.7 Conclusion ............................................................................................................................. 129
5.8 Experimental .......................................................................................................................... 130
Chapter 5 References .................................................................................................................... 135
Bibliography ................................................................................................................................. 140
vii
List of Tables
Table 2.1. Redox data for the (corpy)Pt(dpm), (corpy)Ir(ppz)2, (phenpy)Ir(ppz)2 complexes.
Ligand definitions are given in Figure 2.3 ...................................................................................... 34
Table 2.2. Absorption data for the (corpy)Pt(dpm), (corpy)Ir(ppz)2, (phenpy)Ir(ppz)2
complexes ....................................................................................................................................... 35
Table 2.3. Photoluminescence (PL) data for the (corpy)Pt(dpm), (corpy)Ir(ppz)2, (phenpy)Ir(ppz)2
complexes ....................................................................................................................................... 37
Table 2.4. Orbital contributions calculated for S0 → T1 transition of (corpy)Pt(dpm) .................. 40
Table 2.5. Orbital contributions calculated for S0 → T1 transition of (corpy)Ir(ppz)2 .................. 41
Table 3.1. Redox data for (copy)2Ir(dpm) and Ir(corpy)3 .............................................................. 65
Table 3.2. Absorption data for the (corpy)2Ir(dpm) and Ir(corpy)3 complexes ............................. 65
Table 3.3. Photoluminescence (PL) data for copy)2Ir(dpm) and Ir(corpy)3................................... 67
Table 3.4. Orbital contributions calculated for S0 → T1 transition of (corpy)2Ir(acac) ................. 70
Table 4.1. Redox data for Corannulene Phenyl Diamine (CPD) ................................................... 87
Table 4.2. Absorption data for CPD ............................................................................................... 88
Table 4.3 Photoluminescence (PL) data for CPD .......................................................................... 90
Table 4.4. Device Characteristics for Selected OLEDs ................................................................. 93
Table 5.1. Comparison of Iridium emitters known to align and their molecular orientation....... 111
Table 5.2. Redox data for the Ir complexes Ir(pim)3, Ir(pimF)3, Ir(pimp)3. Ligand definitions are
given in Figure 5.10 ...................................................................................................................... 120
Table 5.3. Absorption data for the Ir complexes Ir(pim)3, Ir(pimF)3, Ir(pimp)3 .......................... 122
Table 5.4. Photoluminescence (PL) data for the Ir complexes Ir(pim)3, Ir(pimF)3, Ir(pimp)3 .... 124
viii
List of Figures
Figure 1.1. Jablonski diagram depicting excitation, intersystem crossing (ISC), internal
conversion (IC), and radiative decay processes such as fluorescence or phosphorescence for a
given chromophore ........................................................................................................................... 2
Figure 1.2. Basic Principle of a working OLED with a single organic layer .................................. 3
Figure 1.3 Standard device architecture for single heterostructure and double heterostructure
OLED ................................................................................................................................................ 5
Figure 1.4. Jablonski diagram depicting spin statistics of carrier recombination and emission
within an OLED for both a fluorescent and phosphorescent emitter. Reprinted from Coordination
Chemistry Reviews 2011, 255 ........................................................................................................... 8
Figure 1.5. Plot of Current Density vs EQE of an OLED with an Ir(ppy)3 emitter at various
doping percentages. Reprinted from Apl. Phys. Lett., 1999, 75 ..................................................... 10
Figure 1.6. Jablonksi Diagram of Triplet-Triplet Annihilation .................................................... 11
Figure 1.7. Red shift of the emission of iridium complexes upon extending π-conjugation ........ 12
Figure 1.8. HOMO-LUMO energy diagram depicting a phenyl pyridine iridium species and the
changes in the frontier orbitals through the addition of various substituents ................................. 13
Figure 2.1. Corannulene structure as a 2D (left) and 3D representation (right) ............................ 18
Figure 2.2. Energy diagram of bowl inversion (top); Diagram showing relationship between
substituents and bowl inversion (bottom) ....................................................................................... 19
Figure 2.3. Structures of (corpy)Pt(dpm), (corpy)Ir(ppz)2, and (phenpy)Ir(ppz)2 ......................... 20
Figure 2.4. a.) Crystal structure of (corpy)Pt(dpm); b.) unit cell of (corpy)Pt(dpm) (methyl
groups omitted for clarity); c.) top and d.) side view of (corpy)Pt(dpm) dimers. All hydrogens
omitted for clarity. The atoms are colored blue (N) black (C), red (O) and gray (Pt) .................... 23
Figure 2.5. Crystal structure of (corpy)Ir(ppz)2 (left) and unit cell (right). All hydrogens omitted
for clarity. The atoms are colored by blue (N) black (C), and dark blue(Ir) .................................. 24
Figure 2.6. (a)
1
H NMR spectrum of (corpy)Pt(dpm) in CDCl3, (b)
1
H NMR spectrum of
(corpy)Ir(ppz)2 in 2:1 CD2Cl2: acetone-d6. The labelling scheme for the corpy ligand is the same
in both spectra. Resonances marked with ‘are for the ppz ligand trans to pyridyl ........................ 25
Figure 2.7. Variable temperature
1
H NMR spectra of (corpy)Ir(ppz)2 in 2:1
CD2Cl2: acetone-d6.......................................................................................................................... 27
ix
Figure 2.8. Erying analysis of VT NMR of (corpy)Ir(ppz)2 .......................................................... 28
Figure 2.9. Structural representations of (corpy)Ir(ppz)2. Protons m and m’ are shown in red and
n and n’ in green. A view down the N–Ir–N axis is shown at the left. To the right is a view
roughly down the N–Ir–C axis showing the -P (left) and -M (right) diastereomers of
(corpy)Ir(ppz)2 ................................................................................................................................ 29
Figure 2.10. Low temperature VT NMR of (phenpy)Ir(ppz)2 in CD2Cl2 ...................................... 31
Figure 2.11. High temperature VT NMR of (phenpy)Ir(ppz)2 in (CD3)2SO ................................. 31
Figure 2.12. Cyclic Voltammetry of (corpy)Pt(dpm) vs Fc/Fc
+
.................................................... 32
Figure 2.13. Cyclic Voltammetry of (corpy)Ir(ppz)2 vs Fc/Fc
+
..................................................... 33
Figure 2.14. Absorption (in CH2Cl2) and emission (in 2-MeTHF and PMMA) spectra of
(corpy)Pt(dpm), left, and (corpy)Ir(ppz)2, right .............................................................................. 35
Figure 2.15. Absorption (in CH2Cl2) and emission (in 2-MeTHF and PMMA) spectra of
(phenpy)Ir(ppz)2 .............................................................................................................................. 37
Figure 2.16. Emission Decay Spectra of (corpy)Ir(ppz)2 various wavelengths ............................. 38
Figure 2.17. Molecular orbitals for S0 → T1 transition of (corpy)Pt(dpm) .................................. 40
Figure 2.18. Molecular orbitals for S0 → T1 transition of (corpy)Ir(ppz)2 -P ............................. 41
Figure 3.1. Structures of (corpy)Ir(ppz)2, (corpy)2Ir(dpm), and Ir(corpy)3 ................................... 53
Figure 3.2. a.) Crystal structure of (corpy)2Ir(dpm); b.) view of (corpy)2Ir(dpm) down the N-Ir-N
bond axis c.) packing of (corpy)2Ir(dpm) molecules within the unit cell. Two molecules of
(corpy)2Ir(dpm) were omitted for clarity. All hydrogens were also omitted for clarity. The atoms
are colored light blue (N) black (C), red (O) and dark blue (Ir) ..................................................... 55
Figure 3.3. (top)
1
H NMR spectrum of (corpy)2Ir(dpm) in CDCl3, (bottom)
1
H NMR spectrum of
Ir(corpy)3 in CD2Cl2. The labelling for the corpy ligand is the same in both spectra ................... 57
Figure 3.4. Variable low temperature
1
H NMR spectra of (corpy)2Ir(dpm) in CDCl3 .................. 59
Figure 3.5. Variable low temperature
1
H NMR spectra of Ir(corpy)3 in CD2Cl2 .......................... 60
Figure 3.6. Top: Structural representation of (corpy)2Ir(acac). The view is roughly down the N-Ir-N
axis showing the -MM (left) and -PP (right) diastereomers of (corpy)2Ir(acac). Bottom:
Structural representation of Ir(corpy)3. The view is roughly down the N-Ir–C axis showing the
x
-MMM (left) and -PPP (right) diastereomers of Ir(corpy)3. Proton a is shown in blue and l in
green and k in red ........................................................................................................................... 62
Figure 3.7. Cyclic Voltammetry of (corpy)2Ir(dpm) vs Fc/Fc
+
..................................................... 63
Figure 3.8. Cyclic Voltammetry and Differential Pulse Voltammetry of Ir(corpy)3 vs Fc/Fc
+
..... 64
Figure 3.9. Absorption and emission (in 2-MeTHF) spectra of (corpy)2Ir(dpm), left, and
absorption (in CH2Cl2) and emission (in 2-MeTHF and PMMA) spectra of Ir(corpy)3, right ....... 66
Figure 3.10. Molecular orbitals for S0 → T1 transition of (corpy)2Ir(acac) -MM ....................... 71
Figure 3.11. Molecular orbitals for S0 → T1 transition of (corpy)2Ir(acac) -PP ......................... 71
Figure 4.1 Chemical structures of common materials used in hole transport layers. Reprinted
from Comprehensive Organometallic Chemistry ........................................................................... 80
Figure 4.2. Chemical structures of common materials used in electron transport layers. Reprinted
from Comprehensive Organometallic Chemistry ........................................................................... 81
Figure 4.3. Cyclic voltammetric curves of corannulene in0.08M TMAB/DMF solution. Reprinted
from Journal of Physical Chemistry B 2009, 113........................................................................... 83
Figure 4.4. Molecular Structure of Corannulene Phenyl Diamine (CPD) ..................................... 84
Figure 4.5 Cyclic Voltammetry & Differential Pulse Voltammetry of CPD vs Fc/Fc
+
in DMF ... 86
Figure 4.6. Comparison of HOMO and LUMO levels of common hole and electron transport
materials with CPD ......................................................................................................................... 88
Figure 4.7. Absorption (in CH2Cl2) and emission (in 2-MeTHF and as a neat thin film) spectra of
CPD ................................................................................................................................................. 89
Figure 4.8. a.) Device architecture for selected OLEDs varying the HTL. b.) Electroluminescence
spectra of NPD/BCP (black) and CPD/BCP (red); c.) Current density versus voltage for all
OLEDs; d.) EQE versus current density for all OLEDs. ............................................................... 91
Figure 4.9. a.) Device architecture for selected OLEDs varying the ETL and thickness of BCP.
b.) Electroluminescence spectra of NPD/Alq3 (black), NPD/Alq3/BCP (red), NPD/CPD (green),
NPD/CPD/BCP (blue); c.) Current density versus voltage for all OLEDs; d.) EQE versus current
density for all OLEDs ..................................................................................................................... 93
Figure 5.1. A diagram of an OLED showing the various outcoupling pathways of light produced
in a device. .................................................................................................................................... 104
xi
Figure 5.2. Schematic depiction of different scattering structures to improve outcoupling
efficiency by use of (a) Microlens array, (b) Scattering particles, (c) Index grating. Reprinted from
Phys. Status Solidi A 2013, 1 ........................................................................................................ 105
Figure 5.3. Simulation of power dissipation for (a) horizontally oriented transition dipole, (b)
vertically oriented transition dipole as a function of ETL thickness. Reprinted from Phys. Status
Solidi A 2013, 1 ............................................................................................................................. 106
Figure 5.4. (Left) Experimental setup of angle dependent PL, (right) Sample angle dependent PL
spectra demonstrating relationship between vertical (pz) dipoles and horizontal (px) dipoles.
Reprinted from Phys. Status Solidi A 2013, 1 ............................................................................... 107
Figure 5.5. A family of carbazole dyes where the conjugation is increasing. The transition dipole
lies along the long axis of the molecule. As the phenyl chain increases, so does the degree of
horizontal orientation .................................................................................................................... 109
Figure 5.6. Transition dipole vector (TDV) mapped onto (ppy)Re(CO)4 and an Ir(ppy) fragment.
The angle δ describes the angle of the Metal-Nitrogen-TDV ....................................................... 110
Figure 5.7. 3D representation of Ir(piq)3 (left), and Ir(chpy)3 (right) .......................................... 111
Figure 5.8. Chemical structures & calculated electrostatic surface potentials for (bppo)2Ir(acac),
(bppo)2Ir(ppy), and (ppy)2Ir(bppo). Reprinted from Nature Materials 2016, 15 ......................... 112
Figure 5.9. Illustration of a vapor deposited film with (bppo)2Ir(acac) as the dopant. The
vaccum/organic interface allows the dopant to rearrange to have a favorable interaction with the
host ................................................................................................................................................ 113
Figure 5.10. Space filling models of Ir(ppy)3 and Ir(ppy)2(acac) The colored circles are to
emphasize a chemical deviation from spherical Ir(ppy)3 circle by the addition of the aliphatic
diketonate moiety .......................................................................................................................... 114
Figure 5.11 Diagram detailing synthetic modifications to the “parent” complex Ir(pim)3 to probe
molecular orientation. Included are space-filling models of each complex from a top side view
and top view illustrating geometrical changes .............................................................................. 116
Figure 5.12. The angle between the iridium-nitrogen bond and the calculated transition dipole
vector for the iridium complexes .................................................................................................. 117
Figure 5.13. Cyclic Voltammetry and Differential Pulse Voltammetry in MeCN of a.) Ir(pim)3 in
DcFc/DcFc
+
, Ir(pimF)3 in Fc/Fc
+
, Ir(pimp)3 in DcFc/DcFc
+
........................................................ 119
Figure 5.14. Absorption spectra of Ir complexes Ir(pim)3, Ir(pimF)3, Ir(pimp)3 ......................... 121
Figure 5.15. Emission spectra of: (top) Ir(pim)3; (middle) Ir(pimF)3; (bottom) Ir(pimp)3 in
2-MeTHF ...................................................................................................................................... 123
xii
Figure 5.16 Polarized emission spectra. a–c, Cross-sections of the measurements and simulations
of the angle-dependent p-polarized photoluminescence emission spectra for films of mCBP doped
with Ir(pim)3 (a), Ir(pimF)3 (b) and Ir(pimp)3 (c) .......................................................................... 125
Figure 5.17. Diagram of a thin film during deposition illustrating how a.) Ir(pim)3, b.) Ir(pimF)3
and c.) Ir(pimp)3 will orient at the vacuum/organic interface ....................................................... 127
Figure 5.18. a.) Device architecture for all OLEDs; b.) Electroluminescence spectra of Ir(pim)3
(red), Ir(pimp)3 (blue), and Ir(pimF)3; c.) EQE versus current density for all iridium complexes;
d.) Current density versus voltage for all iridium complexe ........................................................ 128
xiii
Abstract
Organic light emitting diode (OLED) displays have made considerable headway into
commercial display products from cell phones, televisions and wearable products, to use in solid
state lighting applications, that there is significant interest in creating materials to improve device
performance. Phosphorescent iridium (III) organometallic complexes have demonstrated great
success within OLEDs as they not only can have high quantum efficiencies, but also their
photophysical properties can be tuned through synthetic modification. To date, significant research
has been focused on both understanding the fundamentals of the emission properties of certain
classes of iridium complexes, as well as improving the device performance of OLEDs through
material design with certain properties. A new class of iridium complexes containing the polycyclic
aromatic hydrocarbon corannulene to probe non-radiative decay pathways is presented in this work.
Additionally, research looking into device performance improvements of OLEDs is explored
through both the development of a carrier transport material incorporating corannulene and the
examination of the structure-property relationship between the shape of iridium complexes and
molecular orientation. Broad potential for variation in design make these materials potentially
useful for a variety of photophysical applications.
Chapter 1 gives an overview of OLED technology and the integration of iridium organometallic
complexes and how these complexes can be synthetically modified to tune their photophysical
properties.
Chapters 2 and 3 explore the impact of cyclometalated corannulene on platinum and iridium
organometallic complexes. Corannulene has been shown to undergo fluxional behavior in solution
on a time scale similar to the radiative rates of iridium and platinum phosphors that a family of
complexes was synthesized attaching one, two and three corannulenes on iridium. Variable
xiv
temperature
1
H NMR shows that dynamic behavior can be observed for these complexes and this
process can be best described as an inversion of the corannulene bowl creating distinct
diastereomers that rapidly interconvert at room temperature. Additionally, non-first order decay
behavior is observed from the mono-corannulene complex but is absent in the decay from the bis-
and tris-complexes, suggesting that interconversion between diastereomers responsible for the
mono-complex’s unusual luminescent decay does not apply to all organometallic complex with
fluxional diastereomers.
Chapter 4 investigates the use of corannulene as a carrier transport material in OLEDs. The
ambipolar molecule corannulene phenyl diamine (CPD) was synthesized and employed as both an
HTL and ETL in devices. CPD as an HTL does not give any major improvements over the common
HTL material NPD, while substitution of CPD in the ETL for Alq 3 allows for moderate
improvements in the operational voltage and EQE of CPD devices over Alq3 devices, provided a
hole blocking layer is included in device architecture.
Chapter 5 probes the structure-relationship between ligand modification of phenyl imidazole
iridium complexes and orientation of the transition dipole in a thin film. Previous studies have
shown that if the transition dipole of an emitting material is oriented such that it lies parallel the
substrate in an OLED, the outcoupling efficiency will increase, improving the overall external
quantum efficiency of the device itself. By modifying the geometric shape of these iridium
complexes by synthetic means from spherical to oblate to pseudo-rod-like, the orientation of the
transition dipole parallel to the substrate in a thin film increases, which was shown to correlate to
an increase in the overall external quantum efficiency of OLEDs integrating such complexes.
1
CHAPTER 1. Introduction
1.1 Organic Light Emitting Diodes
1.1.1 Electroluminescence of OLEDs
Within the current display market, there has been an increasing demand for novel and more
energy efficient display technologies, specifically those that involve Organic Light Emitting Diodes
(OLEDs). Over the past few years, OLED displays have made considerable headway into
commercial display products from cell phones, televisions and wearable products, to integration of
OLED technology for use in solid state lighting applications.
1, 2
OLEDs function by electrically
promoting an organic chromophore, such as a small molecule or polymer, into its excited state.
3
This excited species can then relax to the ground state by emitting light, and then electrically repeat
the cycle again through a process called electroluminescence (EL). The formation of an excited
state species can also occur through photoexcitation of an organic chromophore, which relaxes and
emit light through a process called photoluminescence (PL). For a given chromophore, the excited
state formed in EL is the same one as the excited state formed in PL.
3
PL measurements in the solid
state are also very useful in the determination of the color of light emitted as well as the efficiency
of OLEDs.
3
Therefore, the photophysics of these chromophores from PL can be used in order to
understand the nature of the excited states formed by EL.
1.1.2 Photophysics of Chromophores used in OLEDs
A useful way to visualize the relationship between the ground state and the excited state
energies is through the use of a Jablonksi diagram as shown in Figure 1.1.
3, 4
The Jablonski
diagrams considers the electronic states of a molecule and any transitions between these states,
taking into account the spin multiplicity of a particular state.
4
Due to the Pauli exclusion principle,
a molecule with a filled orbital in the ground state will have electrons with antiparallel spins,
2
corresponding to a singlet ground state. Photoexcitation of an electron through absorption of a
photon promotes an electron in the ground state, S 0, to the lowest energy singlet excited state, S1.
The singlet excited state could radiatively decay, emitting a photon through a process known as
fluorescence or undergo thermal deactivation to the singlet ground state through a process known
as internal conversion.
4
Alternatively, the singlet excited state could undergo intersystem crossing
to transfer the electron to the triplet state, where the spins of both electrons are parallel. The species
could then undergo internal conversion from the triplet state or emit a photon through a process
called phosphorescence.
4
Direct emission from the triplet state to the singlet ground state is
symmetry forbidden, resulting in luminescent lifetimes that are significantly longer for phosphors.
Decay lifetime for phosphors range from microseconds for organometallic complexes with heavy
metals to seconds long for organic molecules, versus fluorophores, which have lifetimes of
nanoseconds.
3
Figure 1.1. Jablonski diagram depicting excitation, intersystem crossing (ISC), internal
conversion (IC), and radiative decay processes such as fluorescence or phosphorescence for a
given chromophore.
3
1.2 Composition of an OLED
1.2.1 Basic Steps in Electroluminescence
Figure 1.2. Basic Principle of a working OLED with a single organic layer.
The most basic representation of an OLED involves an organic material sandwiched
between an anode, typically indium-tin oxide (ITO) and a cathode, which ideally is a low work
function metal such as magnesium, though other metals such as silver or aluminum are also
commonly-used.
5, 6
The basic principle of a working OLED is illustrated in Figure 1.2. First, an
electric field is applied to the device, oxidizing molecules at the anode/organic interface, creating
the absence of an electron and leaving a net positive charge in what is referred to as “holes”.
3, 7
4
Similarly, at the cathode/ organic interface electrons are injected in the device. After charge
injection, the carriers migrate through the organic material in response to the applied electric field.
The mechanism for this carrier migration can be approximately described as an outer-sphere self-
exchange reaction dictated by Marcus theory.
8, 9
When the hole and electron migrate to a position
within the device such that they are physically located on molecules closely-spaced, the charges
then attract coulombically. Once both charges reside on the same molecule, they undergo hole-
electron recombination to form an exciton.
3, 7
In a perfect OLED device, the exciton will then
radiatively decay back to its ground state producing a photon by electroluminescence, with the
process then repeating itself.
1.2.2 Single Organic Layer OLEDs
The first reported electroluminescent devices contained a single organic layer and were very
inefficient.
10
Organic materials used in OLEDs conduct either holes or electrons more efficiently,
suggesting that a single organic layer OLED will not effectively trap charge to recombine. Instead,
transport of one type of carrier occurs from one electrode to the other, causing an imbalance of
charge carriers, leading to poor EL. Recombination requires that there be a balance of holes and
electrons in the device which requires both charges to be trapped within the organic material.
1.2.3 Single Heterojunction OLEDs
Tang and Van Slyke resolved this charge balance issue by devising a single heterostructure
OLED with two organic layers.
11
Their OLED used a triarylamine to transport holes and a
tris-aluminum quinolate complex to both transport electrons and act as the emitting layer. Each
layer within the OLED stack is optimized separately, with a carefully-tailored material that best
serves the function of that layer. Materials with good charge transport properties can be used in
either a hole transport layer (HTL) or an electron transport layer (ETL).
5
The determination of HOMO and LUMO energies is integral to the evaluation of a new
material set.
12, 13
These energies are chosen such that there is an energetic barrier for holes migrating
from the HTL to the ETL, and a barrier for electrons from the ETL to the HTL.
3, 11, 14
These energies
can be determined directly from UV-photoelectron spectroscopy (UPS) for the HOMO and inverse
photoemission spectroscopy (IPES) for the LUMO or indirectly through methods such as
electrochemistry.
15-18
A further description on specific materials used for HTL and ETL is found in
Chapter 4. Separating carrier transport into HTL and ETL layers allows for well-balanced hole and
electron currents at the organic interface.
19
1.2.4 Double Heterojunction OLEDs
Figure 1.3 Standard device architecture for single heterostructure and double
heterostructure OLED.
Additionally, a separate emissive layer (EML) can be introduced into the device structure,
sandwiched between the HTL and ETL in a device architecture referred to as a double
heterostructure. Creation of an EML independent of charge transport allows for holes and carriers
to be effectively trapped in the emissive region and allowing for devices to have higher efficiencies
than single heterojunction devices.
20
The emissive material can consist of a neat layer of a single
6
species or a mixed dopant-host system, where the exciton recombines on the doped layer. The
dopant ideally should have energy levels favorable to trap either holes or electrons in order to
facilitate recombination solely on the dopant.
21, 22
More complex device architectures also include
blocking layers to further improve device efficiency, by confining charges as well as excitons within
the emissive layer.
20, 23
Figure 1.3 shows a basic device architecture of some common OLED stacks.
1.3 Efficiency of OLEDs
1.3.1 External Quantum Efficiency
Equation 1.1
In characterizing the performance of an OLED, one important parameter is the external
quantum efficiency, or EQE, which describes the ratio of the number of emitted photons to the
injected charge carriers.
24, 25
The different factors used to determine EQE can be calculated as
shown in Equation 1.1, where ΦEL is the external quantum efficiency, ΦPL is the photoluminescent
quantum efficiency, χ is the fraction of excitons allowed to decay radiatively by spin statistics, ηr is
the carrier recombination efficiency, and ηe is the outcoupling efficiency. A detailed description on
outcoupling efficiency can be found in Chapter 5.
1.3.2 Photoluminescent Quantum Efficiency
The photoluminescent quantum efficiency, or quantum yield (PLQY) is defined as a ratio
of the number of emitted photons to the total number of absorbed photons. PLQY can be also be
stated as a relationship between the rate of radiative decay (kr) from the excited state to the ground
state and the rate of non-radiative decay (knr) as shown in Equation 1.2. It has been shown that there
e r PL EL
7
are multiple pathways that contribute to non-radiative decay, such as deactivating vibrational modes
or non-emissive energy states of compounds, that can impact the photoluminescent quantum
efficiency.
26-28
Ideally, an emitting compound in an OLED should have a high radiative rate and a
fast decay lifetime. This latter requirement is essential to reducing the likelihood of second-order
quenching processes of the excited state, such as triplet-triplet annihilation and triplet-polaron
annihilation.
21, 29, 30
nr r
r
PL
k k
k
Equation 1.2
1.3.3 Carrier Recombination Efficiency
The carrier recombination efficiency, ηr, also referred to as the charge carrier balance factor,
is a measure of the number of holes and electrons injected, and what fraction of them are consumed
by recombination.
25
If there is an imbalance of injected electrons and holes or inefficient
recombination, then an excess of charge carriers will not contribute to photon generation, which
reduces the efficiency of the OLED. A single organic layer will transport charge from one electrode
to the other due to improper charge balance in the device, decreasing ηr. This factor can be
maximized by confining the holes and electrons either through a dopant within a distinct emissive
layer, or introducing carrier blocking layers such as the hole blocking layer bathocuproine.
14, 31
8
1.3.4 Consideration of Spin Statistics within an OLED
Figure 1.4. Jablonski diagram depicting spin statistics of carrier recombination and emission
within an OLED for both a fluorescent and phosphorescent emitter. Reprinted from Coordination
Chemistry Reviews 2011, 255.
33
The fraction of excitons allowed to decay radiatively by spin statistics, or the singlet-triplet
factor, χ, depends on the nature of the excited state that emits within the OLED.
7
The spins of both
the hole and electron must be considered upon recombination of the carriers for exciton
formation.
32, 33
In the formation of an exciton, two unpaired electrons from the hole and electron
can result in possible magnetic quantum numbers of MS are +1, 0, and −1. The values of MS = ±1
correspond to two substates of the triplet.
33
The third triplet substate is given by a positive linear
9
combination of the two possible configurations with MS = 0. The respective negative linear
combination describes the corresponding singlet state.
33
Due to spin statistics, the ratio of singlets
to triplets formed during recombination is 1:3. Electroluminescence, unlike photoluminescence,
follows these spin statistics, with the recombination probability of excitons being 75% triplet and
25% singlet. For most organic compounds, phosphorescence is often inefficient in a device due to
the long decay lifetime allowing the exciton to decay through other nonradiative pathways. A
Jablonksi diagram describing fluorescence and phosphorescence within an OLED is shown in
Figure 1.4.
Phosphorescent Complexes
1.4.1 Advantages of Organometallic Complexes in OLEDs
In order to obtain a device that harvests both singlets and triplets, thereby maximizing χ, a material
set that efficiently emits from the triplet state is needed. Cyclometalated iridium and platinum
complexes have been studied extensively due to their efficient phosphorescence.
33-37
The strong
spin-orbit coupling of the heavy metal allows for significant mixing between singlet and triplet
excited states, facilitating intersystem crossing and promoting phosphorescence as shown in Figure
1.4. This intersystem crossing rapidly converts any singlet excitons trapped on the phosphor to
triplet excitons, allowing for harvesting of both singlets and triplets. These cyclometalated
complexes are also highly suited for use in OLEDs due to their short decay lifetimes in the
microsecond regime and high quantum efficiencies.
38, 39
Cyclometalated complexes have been used
as dopants within the EML and may trap both holes and/or electrons as well as be the site of exciton
formation. Ideally, this phosphorescent dopant will have an excited-state energy that is lower than
the host material of the EML.
2, 20
10
1.4.2 Characteristics of Iridium Phosphors in OLEDs
Figure 1.5. Plot of Current Density vs EQE of an OLED with an Ir(ppy)3 emitter at various
doping percentages. Reprinted from Apl. Phys. Lett., 1999, 75.
Within the field of phosphorescent emitters for OLEDs, more work has been done with iridium
complexes, due to faster decay lifetimes than platinum complexes.
38, 40, 41
This can be attributed to
the higher degree of ligand centered character (
3
LC) in the triplet for platinum complexes versus
more metal-to-ligand charge transfer character (
3
MLCT) in the triplet for iridium complexes and
can be observed in the emission spectra of these complexes.
38, 41
For certain iridium complexes, the
literature has shown that there is an optimal doping percentage of six percent as shown in Figure
1.5. Too low a doping percentage will not allow for percolation pathways for the charges to
recombine.
21, 29
At high doping concentrations, iridium complexes have been shown to self-quench
reducing the quantum yield.
42, 43
Higher doping concentrations can also increase the probability of
non-radiative decay pathways such as triplet-triplet annihilation. Triplet-triplet annihilation occurs,
11
when two excitons in their excited state combine to form a singlet excited state and a ground state
molecule (Figure1.6), greatly reducing EQE.
Figure 1.6. Jablonksi Diagram of Triplet-Triplet Annihilation.
1.5 Tuning of Iridium Complexes
1.5.1 Modifying of the Ligand π-System
The photophysical properties of iridium and complexes strongly depend on the chemical structures
of the cyclometalated (C^N) ligands.
26, 38, 41
The emission energies of such complexes are closely
related to the nature of the chromophoric C^N ligand and there have been numerous in depth studies
probing this relationship.
34, 44, 45
One example is by extending the π-conjugation in the C^N ligand,
to induce a bathochromic shift in emission, (Figure 1.7).
3, 33, 40
By extending the size of the aromatic
π-system, from phenyl pyridine (ppy) to phenyl isoquinoline (piq), the energy of the ligand-centered
transition decreases relative to the ppy ligand and for the iridium complex as well.
3
Addition of
softer atoms such as sulfur into the aromatic ring can also lower the
3
LC energy, causing a red shift
of the complex.
3
12
Figure 1.7. Red shift of the emission of iridium complexes upon extending π-conjugation.
Blue shifting the complexes can be achieved by modifying the aromatic ligand itself. Changing the
phenyl pyridine ligand to either a phenyl pryrazole or phenyl imidazole ligand produces a blue shift
in the spectrum due a destabilization of the LUMO relative to the ppy ligand.
26
Changing the nature
of the C^N ligand can also impact radiative and non-radiative pathways of the complexes and
provide valuable insight of the nature of the excited state.
36, 46
For example, blue shifting of iridium
complexes with phenyl pyrazole ligands will destabilize the LUMO and lead to population of metal
centered triplet states (
3
MC), which are symmetry-forbidden and decay non-radiatively reducing
the quantum yield.
36
1.5.2 Modification by Addition of Substituents to the Ligand System
Addition of electron donating or accepting groups can produce either a bathochromic shift
or a hypsochromic shift depending where the substituent is located on the ligand. When looking at
the tris-phenylpyridine iridium(III) complex (Ir(ppy)3), the HOMO is located on the metal center
and the phenyl ring while the LUMO is situated on the pyridine ring. Addition of an electron
withdrawing group such as fluorine to the phenyl ring will withdraw electron density from the
phenyl ring and stabilize the HOMO of the complex.
3
This causes the HOMO-LUMO gap to
increase, resulting in a hypsochromic shift. Similarly, addition of an electron donor, such as a
13
methoxy substituent, on the phenyl ring destabilizes the HOMO, shrinking the HOMO-LUMO gap,
producing a bathochromic shift. Addition of an electron donor on the pyridyl group will donate
electron density into the pyridine ring, destabilizing the LUMO of the complex, leading to an
increase of the HOMO-LUMO gap, causing a hypsochromic shift. Addition of alkyl substituents
is shown to have only a small effect on the energetics of the complex, giving shifts of 3-10 nm in
the emission spectrum.
3
A summary of how to tune the HOMO-LUMO gap by adding substituents
is shown in Figure 1.8.
Figure 1.8. HOMO-LUMO energy diagram depicting a phenyl pyridine iridium species and the
changes in the frontier orbitals through the addition of various substituents.
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18
CHAPTER 2. Synthesis and Characterization of
Phosphorescent Platinum and Iridium Complexes
Containing Corannulene
2.1. Introduction
The study of fullerenes has inspired chemists to understand and probe the structural and
electronic properties of these compounds as well as the applications of these molecules.
1, 2, 3, 4
Corannulene, shown in Figure 2.1, is nicknamed “buckybowl” and has been described as the
smallest fullerene fragment that retains curvature.
5, 6,
7
Corannulene was first reported in the
literature in 1966 by a multistep organic synthesis by Barth and Lawton.
8
They surmised that this
new aromatic compound has a unique polar resonance form containing two charged concentric
conjugated systems.
8, 9
The system contains the “core” 6 electron cyclopentadiene anion and the
outer 14 electron “annulene” cation; hence the name “corannulene”. The initial paper argued that
this polar resonance form satisfied the requirements of Hückel's rules but other theoretical papers
dispute this rationale.
8, 10
Figure 2.1. Corannulene structure as a 2D (left) and 3D representation (right).
In 1991, Scott published a new procedure for synthesis of corannulene involving flash
vacuum pyrolysis.
11
Although flash vacuum pyrolysis reduces the overall number of steps from the
19
original procedure, it has modest yields with poor functional group tolerance, limiting its possibility
for scale-up for practical uses.
11, 12
Recently this shortcoming was overcome by Siegel, who
demonstrated the ability to produce corannulene on a kilogram scale, allowing for the viability of
use in many applications.
12
As a result of its unique structure, corannulene has been proposed for
use in many promising applications ranging from charge transport to end caps for nanotubes.
13, 14,
15
Figure 2.2. Energy diagram of bowl inversion (top); Diagram showing relationship between
substituents and bowl inversion (bottom).
One interesting property exhibited by corannulene as a result of its curvature is dynamic
bowl-to-bowl inversion in solution. The kinetics of bowl inversion was first probed by Scott, noting
that bowl inversion occurs rapidly room at room temperature in solution.
16
. Siegel further probed
this fluxional behavior noting a relationship between bowl depth and the rate of inversion.
17
Substituents were placed in the peri positions on corannulene that are repulsive, such as bulky
phenyl groups will flatten the bowl and increase the rate of inversion. Annelation or smaller
substituents with decreased repulsion cause the bowl depth to be deeper, slowing the rate inversion
20
as shown in Figure 2.2 The inversion process of corannulene has been intensively studied with
various substitutions, additional fused rings, as well η
2
and η
6
ligand coordinated to a transition
metal.
16,18,17,19,20,21
The energy barrier to bowl inversion has been determined using variable
temperature NMR, showing that substituents on corannulene can directly affect the rate of
inversion.
16, 17
The rate of inversion for the parent corannulene falls within the microsecond regime,
the same time scale as radiative decay for some phosphorescent heavy transition metal complexes,
particularly those of iridium and platinum.
22,
23,24
Previously in the literature, a report has appeared of 2-(corannulene)pyridine (corpy-H)
ligand precursor cyclometalated onto Pd(II).
25
Both corpy-H and a [(corpy)Pd(ACN)2]
+
(ACN =
acetonitrile) complex were structurally characterized and a columnar bowl-bowl packing
arrangement of the corpy ligand was observed in the Pd species. In addition, a (corpy)Pd complex
with an optically active ligand was also prepared; however, dynamic bowl inversion of corannulene
could not be observed in this derivative. Moreover, aside from analysis of the UV-visible
absorption spectrum of [(corpy)Pd(ACN)2]
+
, no other photophysical characterization was given for
the Pd complexes.
Figure 2.3. Structures of (corpy)Pt(dpm), (corpy)Ir(ppz)2, and (phenpy)Ir(ppz)2.
21
Herein, Pt and Ir complexes containing a cyclometalated corpy ligand are synthesized and
their dynamic properties probed. The Pt complex is (corpy)Pt(dpm), where dpm =
2
-dipivolylmethane and the Ir complex is (corpy)Ir(ppz)2, where ppz = 1-phenyl-1H-pyrazolyl
(Figure 2.3). A third compound, (phenpy)Ir(ppz)2 (phenpy = 2-(5-phenanthryl)pyridyl)) was also
synthesized to mimic the steric bulk of (corpy)Ir(ppz)2 and probe the effect of atropisomerism the
dynamic behavior in solution. Variable temperature NMR is used to examine any dynamic
processes that these complexes undergo in fluid solution. Both (corpy)Pt(dpm) and (corpy)Ir(ppz)
are phosphorescent with the photophysical properties dictated by the corpy ligand.
2.2. Synthesis of Complexes
Scheme 2.1
The corpy-H and phenpy-H ligand precursors were prepared by Suzuki cross coupling as
described for other C^N ligands.
23
The Pt(II) and Ir(III) complexes containing corpy were prepared
by routes similar to those described for other C^N complexes (Scheme 2.1).
22, 23, 26
The
(corpy)Pt(dpm) complex was synthesized by first preparing a cyclometalated (corpy)PtCl
22
intermediate in a reaction between corpy-H and K2PtCl4, followed by addition of dipivalylmethane
and base to displace the Cl ligand. The (corpy)Ir(ppz)2 and (phenpy)Ir(ppz)2 complexes were
synthesized by treating the [(ppz)2Ir( -Cl)]2 dimer with the correspond ligand precursor in the
presence of base. The orange colored complexes were isolated as single species that are stable in
air as neat solids and in fluid solution. In addition, the Ir complexes are photolytically stable as
determined by monitoring the UV-visible absorption spectra in MeCN solution before and after
prolonged irradiation with 365 nm light.
2.3. Crystal Structures
2.3.1 (corpy)Pt(dpm) Crystal Structure
X-ray diffraction analysis was performed on crystals of (corpy)Pt(dpm) and (corpy)Ir(ppz)2
grown by slow diffusion of hexanes into dichloromethane of the metal complex. The structure of
(corpy)Pt(dpm) is shown in Figure 2.4a. The unit cell of (corpy)Pt(dpm) is comprised of 8
molecules in a monoclinic C 2/c space group (Figure 2.4b) and contain both enantiomers of the
corannulene bowl, designated P or M using the stereodescriptor system for chiral buckybowls.
27, 28
The complex has a distorted square planar geometry around the metal center, with deviations from
ideality due to chelate bite angles (C(1)-Pt-N(1) = 81.1(2)° and O(1)-Pt-O(2) = 89.85 (18)°) that
are comparable to values found in other cyclometalated Pt(β-diketonato) complexes.
26, 29
The
corannulene and pyridyl fragments in the corpy ligand are twisted with a dihedral angle of 10.6°
between the two rings. There are no metal-metal interactions as the closest Pt···Pt distance is 5.65
Å. The bond lengths to the metal (Pt-C(1) = 1.984(6) Å, Pt-N(1) = 1.993(5) Å, Pt-O(1) = 2.086(4)
Å and Pt-O(2) = 2.021(4) Å) are also comparable to values reported for (ppy)Pt(dpm)
30
and other
Pt(β-diketonato) derivatives with cyclometalated ligands.
22, 26, 29
The bowl depth of corannulene
23
(dbowl = 0.895 Å) is similar to values in unsubstituted corannulene (dbowl = 0.87 Å) and corpy-H
(dbowl = 0.89 Å) but deeper than found in [(corpy)Pd(ACN)2]
+
( dbowl = 0.82 Å).
17, 31
Figure 2.4. a.) Crystal structure of (corpy)Pt(dpm); b.) unit cell of (corpy)Pt(dpm) (methyl
groups omitted for clarity); c.) top and d.) side view of (corpy)Pt(dpm) dimers. All hydrogens
omitted for clarity. The atoms are colored blue (N) black (C), red (O) and gray (Pt).
Previous crystallographic studies of corannulene derivatives have shown that the molecules
can preferentially -stack in an ordered arrangement, colloquially referred to as bowl-to-bowl
packing.
25, 32
For example, the [(corpy)Pd(ACN)2]
+
complex is stacked in a columnar arrangement
displaying corannulene-corannulene -interactions (3.3–3.4 Å) between adjacent molecules.
However, no such bowl-to-bowl stacking is present in the crystal of (corpy)Pt(dpm). Instead,
complexes are arranged into antiparallel pairs of enantiomers with the convex faces of the
corannulene moieties pointing towards each other (Figures 2.4c and d).
25, 32
The corpy ligands are
a.)
b.)
c.)
d.)
b.)
a.)
24
situated atop one another with the closest -interaction (~3.5 Å) between the aromatic ring of one
corannulene and a pyridyl group from a neighboring molecule.
25
2.3.2 (corpy)Ir(ppz)2 Crystal Structure
Figure 2.5. Crystal structure of (corpy)Ir(ppz)2 (left) and unit cell (right). All hydrogens omitted
for clarity. The atoms are colored by blue (N) black (C), and dark blue (Ir).
The unit cell of (corpy)Ir(ppz)2 contains two molecules of the complex as well as two
dichloromethane solvate molecules in a triclinic P1 ̅ space group (Figure 2.5). The ligands are
arranged in a meridional (mer) configuration around the pseudo-octahedral metal center with the
pyrazolyls in a trans disposed arrangement. Both enantiomers of a single diastereomer ( -P and
-M) are present in the unit cell, where and describe the stereochemistry at the metal center.
27,
33
The bond angles for the atoms trans-disposed around the metal (C(9)-Ir(1)-C(25) = 170.90(6)°,
N(1)-Ir(1)-N(3) = 174.20(5)° and N(5)-Ir(1)-C(10) = 169.71(6)°) are comparable to values in other
mer-Ir(C^N)3 complexes.
23
The dihedral angle between the two fragments of the corpy ligand is
14.2°. The bond length for Ir(1)-N(5) (2.1222(13) Å) is longer than for Ir(1)-N(1) (2.0137(13) Å)
and Ir(1)-N3) (2.0154(13) Å) but comparable to distances for the Ir-N(pyridyl) trans to phenyl in
other mer-Ir(C^N)3 complexes.
23
Similarly, the bond length for Ir(1)-C(25) (2.1023(15) Å) is longer
25
than for either Ir(1)-C(9) (2.0804(16) Å) or Ir(1)-C(10) (2.0173(16) Å) bonds of the ppz ligands.
The bowl depth of corannulene (dbowl = 0.837 Å) is shallower than in (corpy)Pt(dpm). No
bowl-to-bowl packing is present as the corannulenyl moiety is nested with a pyrazolyl ligand from
a neighboring complex.
17, 31
2.4. NMR and Dynamic Behavior
2.4.1 Room Temperature NMR
Figure 2.6. (a)
1
H NMR spectrum of (corpy)Pt(dpm) in CDCl3, (b)
1
H NMR spectrum of
(corpy)Ir(ppz)2 in 2:1 CD2Cl2: acetone-d6. The labelling scheme for the corpy ligand is the same
in both spectra. Resonances marked with ‘ are for the ppz ligand trans to pyridyl.
The Pt and Ir complexes were characterized using
1
H,
13
C, 1D and 2D NOESY, and gCOSY
NMR spectroscopy (Figure 2.6). Sharp, distinct resonances are observed in aromatic regions of the
1
H NMR spectra measured at 298 K. Peak assignments for (corpy)Pt(dpm) (Figure 2.6a), were
made by analysis of the gCOSY spectrum. The two resonances furthest downfield are assigned to
26
the proton a ( = 9.25 ppm) adjacent to nitrogen on the pyridyl ring and proton l ( = 8.92 ppm) on
the corannulene ring. Both protons are deshielded due to close proximity (~2.36 Å) to the carbonyl
oxygens on the dpm ligand. The remaining protons on the pyridyl ring ( = 8.51, 7.92 and 7.14
ppm) were assigned using gCOSY. The 1D-NOESY spectrum supported assignment of the
resonance at = 8.20 ppm to proton e; this proton is coupled to the remaining aromatic resonances
on the corannulenyl ring located between = 7.70–7.85 ppm.
The
1
H NMR spectrum for (corpy)Ir(ppz)2 in 2:1 CD2Cl2:acetone-d6 displays series of well-
resolved resonances integrating to 26 protons that is consistent with a single species, even though
multiple conformers are possible. The proton assignments shown in Figure 2.6b were confirmed
on the basis of gCOSY and 1D-NOESY spectroscopy. Resonances for protons d ( = 8.87 ppm)
and e ( = 8.32 ppm) on the corpy ligand are now the furthest downfield as protons a ( = 8.22
ppm) and l ( = 7.47 ppm) are shifted upfield due to shielding by the ring currents of the adjacent
ppz ligands. Resonances for protons f-j on corranulene are distinct between = 7.7–7.8 ppm as is
proton k ( = 7.27 ppm). The remaining protons on the pyridyl and ppz ligands are likewise clearly
identified and assigned, with protons on the pyrazolyl ligands being furthest upfield (n’, = 6.34
ppm; n, = 6.19 ppm).
2.4.2 Variable Temperature NMR and Analysis
The (corpy)Ir(ppz)2 complex undergoes a dynamic process in fluid solution that was studied
using variable temperature (VT)
1
H NMR spectroscopy (Figure 2.7). A solvent mixture of 2:1
CD2Cl2: acetone-d6 was used to provide good separation of all proton resonances and to maintain
adequate solubility of the complex at low temperatures. Proton resonances of the corpy ligand, as
well as specific resonances on the ppz ligands, broaden in stages when a sample is cooled
27
Figure 2.7. Variable temperature
1
H NMR spectra of (corpy)Ir(ppz)2 in 2:1 CD2Cl2: acetone-d6.
below room temperature. Initially, the resonance for proton l ( = 7.47 ppm) broadens and merges
into the baseline at 245 K. This change is concurrent with broadening of signals from protons k,
m’, m, n’ and n that achieve coalescence at 231 K. It is readily apparent in the spectra measured
at 245 K and 231 K that signals for protons n’ and n broaden at different rates. Additional
broadening occurs for most of the remaining protons on corpy and the ppz ligands between 231 K
and 198 K. However, some resonances, specifically from the phenyl protons p-s on ppz, and
surprisingly q’ on ppz and d on pyridyl, remain sharp at all temperatures. It should also be noted
that weak, broad signals grow in near the baseline at the lowest temperature reached during the
experiment (198 K). These new resonances, in a molar ratio of approximately 1:4.5, are tentatively
assigned to a second diastereomer of (corpy)Ir(ppz)2. Unfortunately, definitive identification of
28
this new species cannot be made as we were unable to obtain clearly resolved signals due to the
inability of our spectrometer to collect data to lower temperatures.
Figure 2.8. Erying analysis of VT NMR of (corpy)Ir(ppz)2.
The dynamic behavior displayed by (corpy)Ir(ppz)2 in the VT
1
H NMR spectra is consistent
a bowl-to-bowl inversion process occurring on the corannulene moiety of corpy. Corannulene has
an intrinsic permanent dipole moment (2.07 D) with enhanced electron density localized in the base
of the bowl.
34
The dipolar flip that accompanies inversion of the bowl should strongly affect protons
closest to corpy ligand. Protons that are especially useful for interpretation of the fluxional process
are labeled on molecular models of the two interconverted structures shown in Figure 2.9. These
protons (l, m, m’, n and n’) have anisotropic chemical shifts that depend on the orientation of the
corannulene bowl. The relation of the bowl (concave or convex) to the pyrazolyl protons will
dictate whether their resonances are shielded or deshielded by the direction of the dipole. The
fluxional behavior is manifested as an average chemical shift at room temperature. Kinetic analysis
of the VT
1
H NMR spectra was carried out using the resonance at = 6.3 ppm assigned to the
pyrazolyl proton n (Figures 2.6b and 2.9). The rate of inversion was determined through Eyring
29
analysis of the line width (∆ ) as a function of temperature as shown in Figure 2.8.
35
Values of ∆H
‡
(4.9 kcal mol
-1
) and ∆S
‡
(-27 kcal mol
-1
) were determined from curve fitting and the energy barrier,
∆G
‡
, was found to be 13 kcal mol
-1
, corresponding to a rate for inversion of 2.5 x 10
3
s
-1
at room
temperature. On the basis of studies on corannulenes substituted at the peri-positions, Siegel and
coworkers established a correlation between the rate of inversion and bowl depth. While the
substitution pattern of the cyclometalated corpy in (corpy)Ir(ppz)2 is at the ortho-positions, the
value determined for ∆G
‡
matches the one estimated using Siegel’s data and the experimental bowl
depth (dbowl = 0.837 Å). The close correspondence of the present result to data from Siegel’s study
suggests that peri- and ortho-substitution patterns affect the stability of the corannulene in a similar
manner.
Figure 2.9: Structural representations of (corpy)Ir(ppz)2. Protons m and m’ are shown in red and
n and n’ in green. A view down the N–Ir–N axis is shown at the left. To the right is a view
roughly down the N–Ir–C axis showing the -P (left) and -M (right) diastereomers of
(corpy)Ir(ppz)2.
Inversion of the corannulene bowl is likely not the only mechanism that can account for the
fluxional behavior observed for (corpy)Ir(ppz)2. Another process to consider is interconversion of
conformers created by the twist between the corannulene and pyridyl groups of the corpy ligand.
To investigate this possibility, the (phenpy)Ir(ppz)2 complex was used to provide an (C^N)Ir(ppz)2
analog that can still undergo conformational isomerism but not bowl inversion. Molecular models
show that the torsion angle for the bonds labelled in red in Scheme 2.2 (37.7°) are comparable to
n
m
m'
n'
n
m
m'
n'
m
m'
n
P
M
30
values for the equivalent bonds in (corpy)Ir(ppz)2 (34.1°). Likewise, the
1
H NMR spectrum of
(phenpy)Ir(ppz)2 at room temperature displays 26 resonances consistent with presence of a single
species undergoing rapid interconversion on the NMR timescale. Proton resonances remain sharp
at all temperatures up to 342 K in (CD3)2SO and down to 233 K in CD2Cl2 (Figures 2.10 & 2.11),
indicating that atropisomerism is rapid even at low temperature. Therefore, the dominant process
responsible for the fluxional behavior observed in (corpy)Ir(ppz)2 (Figure 2.7) is likely not due to
a related atropisomerism. However, the current model assumes that a simple unimolecular
inversion is the only dynamic process causing decoalescence of proton resonances in
(corpy)Ir(ppz)2. The fact that Eyring analysis gives a large contribution for ∆S
‡
suggests that the
dynamic behavior is more complex. While bowl inversion is the primary mechanism, other
secondary molecular distortions may participate in the fluxional process. Another feature to
consider is how the difference between the dipole moments of diastereomers formed during bowl
inversion affects the solvation of the complex. As (corpy)Ir(ppz)2 undergoes inversion, not only
does the stereochemistry of the corannulene bowl change but so does the orientation of the dipole
with respect to the Ir(ppz)2 fragment. The magnitude of dipole moments calculated for the two
diastereomers of (corpy)Ir(ppz)2 ( -P = 5.27 D, -M = 5.23 D) are larger than that of corannulene
(2.07 D). A change in the dipole of his magnitude could cause the solvent to reorganize around the
complex, leading to a large ∆S
‡
for the inversion.
Scheme 2.2
31
Figure 2.10. Low temperature VT NMR of (phenpy)Ir(ppz)2 in CD2Cl2.
Figure 2.11. High temperature VT NMR of (phenpy)Ir(ppz)2 in (CD3)2SO.
32
2.5 Electrochemical Properties
The redox properties of the complexes were examined by cyclic voltammetry and
differential pulse voltammetry in DMF solution with 0.1 M TBAF (Table 2.1). All potential values
were referenced to an internal ferrocene couple (Fc/Fc
+
). The oxidative properties of the complexes
are similar to related derivatives with cyclometalated ligands. The (corpy)Pt(dpm) complex
displays an irreversible oxidative wave at E
1/2
= 0.51 V (Figure 2.12). The potential and
irreversibility of this process is comparable to oxidative behavior seen in other (C^N)Pt(dpm)
complexes with expanded -systems.
30
The Ir complexes display reversible couples at potentials
(E
1/2
= 0.30 V for (corpy)Ir(ppz)2 (Figure 2.13) and 0.27 V for (phenpy)Ir(ppz)2) that are slightly
lower than mer-(ppy)Ir(ppz)2 (E
1/2
= 0.37 V).
36
Oxidation in all of these cyclometalated complexes
is typically assigned to an orbital with mixed metal-aryl character.
23, 37
A second, irreversible
oxidation wave is observed in the iridium complexes at higher potentials.
-3 -2 -1 0 1
-30
-20
-10
0
10
20
-2.05
+0.57
Current ( A)
Potential (V vs. Fc
+
/Fc)
Pt(corpy)(dpm)
+
+
Fc
+
/Fc
-2.46
Figure 2.12. Cyclic Voltammetry of (corpy)Pt(dpm) vs Fc/Fc
+
.
33
-3 -2 -1 0 1
-150
-100
-50
0
50
Current ( A)
Potential (V vs. Fc
+
/Fc)
Ir(corpy)(ppz)
2
Fc
+
/Fc
Figure 2.13. Cyclic Voltammetry of (corpy)Ir(ppz)2 vs Fc/Fc
+
.
The complexes show reversible reduction waves in DMF solution. The (corpy)Pt(dpm)
complex displays two reversible waves at -2.02 V and -2.42 V. For (corpy)Ir(ppz)2 reversible
reduction occurs at -2.26 V while a second quasireversible wave appears at -2.66 V. The cathodic
waves in both of these complexes are assigned to reduction of the corpy ligand. These potentials
are markedly less negative than the first reduction potential of (phenpy)Ir(ppz)2 (E
1/2
= -2.55 V).
Additional irreversible reduction waves beyond -3.0 V in the Ir complexes are assigned to reduction
of the ppz ligands. The lower potential for (corpy)Ir(ppz)2 compared to (phenpy)Ir(ppz)2 is
consistent with the corannulene being a better electron acceptor than phenanthrene.
38, 39
Similarly,
the second reduction for (corpy)Pt(dpm) and (corpy)Ir(ppz)2 are assigned to corpy since
corannulene has been shown to undergo up to three reductions in solution.
38, 40, 41
The 400 mV
separation between the first and second reduction waves in the Pt and Ir complexes is smaller than
that found in corrannulene (700 mV). The difference in the metal complexes indicates a decrease
34
in coulombic repulsion in the radical anion due to the electron being delocalized onto the pyridyl
ring of the corpy ligand.
Table 2.1. Redox data for the Pt and Ir complexes.
a
Compound Eox1 Ered1 Ered2 Ered3
(corpy)Pt(dpm) 0.51 V
b
-2.06 V -2.46 V --
(corpy)Ir(ppz)2 0.30 V -2.27 V -2.66 V
c
-3.13 V
b
(phenpy)Ir(ppz)2 0.27 V -2.55 V -3.04 V
b
--
a
Redox potentials were recorded in 0.1 M TBAF/DMF solution and referenced to an internal
Fc
+
/Fc couple.
b
Irreversible.
c
Quasireversible.
2.6 Photophysical Properties
2.6.1 Absorption Spectra
The absorption and emission spectra of the complexes were recorded at room temperature
and 77 K as well as in a rigid PMMA matrix at room temperature (Figures 2.14 & 2.15). The
absorption data is listed in Table 2.2 and emission data in Table 2.3. The absorption spectra for the
complexes show intense bands ( < 360 nm, > 10
4
M
-1
cm
-1
) assigned to the ligand centered π→π*
transitions on the cyclometalated ligands. In particular, the bands between = 300–360 nm are
assigned to π→π* transition on corpy on the basis of comparison with spectra from the free corpy-
H ligand. Less intense bands at lower energy ( = 350–500 nm, ≈ 5 x 10
3
M
-1
cm
-1
) are assigned
to allowed metal-to-ligand charge transfer (MLCT) transitions. Much weaker absorptions ( > 500
nm, < 10
2
M
-1
cm
-1
) are assigned to triplet MLCT transitions that are partially allowed due to spin-
orbit coupling with the singlet states by the heavy atom metal center.
35
Table 2.2. Absorption data for the Pt and Ir complexes 1–3.
λmax(nm) (ε, 10
3
M
-1
cm
-1
)
(corpy)Pt(dpm) (1) 302 (44.8), 331 (33.8), 387 (7.90), 441 (5.82), 465 (5.76)
(corpy)Ir(ppz)2 (2) 299 (51.4), 339 (30.2), 425 (5.93), 469 (sh, 3.00)
(phenpy)Ir(ppz)2 (3) 300 (34.3), 403 (6.15), 450 (sh, 3.44)
a
Absorption spectra recorded in CH2Cl2.
2.6.2. Emission Spectra
300 400 500 600 700 800
0
10
20
30
40
50
60
(mM
-1
cm
-1
)
Wavelength (nm)
0
1
RT
77 K
PMMA (RT)
PL intensity (a.u.)
300 400 500 600 700 800
0
10
20
30
40
50
60
70
(mM
-1
cm
-1
)
Wavelength (nm)
0
1
RT
77K
PMMA (RT)
PL intensity (a.u.)
Figure 2.14. Absorption (in CH2Cl2) and emission (in 2-MeTHF and PMMA) spectra of
(corpy)Pt(dpm), left, and (corpy)Ir(ppz)2, right.
All three complexes display broad, featureless red luminescence at room temperature in 2-
MeTHF solution. At 77 K, the spectra of (corpy)Pt(dpm) and (corpy)Ir(ppz)2 shows distinct
vibronic structure, whereas emission from (phenpy)Ir(ppz)2 remains broad and relatively
featureless. The emission lifetimes at 77 K are single exponential and fall in the range = 9.4–15
s consistent with phosphorescence. The vibronic structure displayed by the corpy complexes
indicate that emission originates from a triplet state with significant
3
LC character. However, the
energy of the triplet state in these complexes is over 0.2 eV lower than that of the
3
- state in the
free ligand corpy-H (E0-0 = 525 nm, 2.36 eV). The decrease in energy shows that a substantial
36
stabilization of the excited state occurs upon cyclometalation of the ligand. Since the emission
lifetimes of (phenpy)Ir(ppz)2 and (corpy)Ir(ppz)2 are comparable, as are the energies of the
3
-
states in the free ligands (see Figure 2.13), the absence of distinct vibronic features in the former
complex is likely due significant distortion in the excited state, as opposed to there being greater
MLCT character in (phenpy)Ir(ppz)2 than in (corpy)Ir(ppz)2.
The photoluminescent quantum yields of the complexes in fluid solution are relatively low
( = 0.02-0.09). The radiative decay rate constants (kr = 1.1–1.9 x 10
4
s
-1
) are roughly an order of
magnitude lower than values reported for highly efficient red Pt and Ir phosphors with
cyclometalated ligands.
22, 42
and indicates that the excited state has significant
3
LC character.
However, the low quantum efficiency is mainly a consequence of rapid non-radiative decay (knr >
2 x 10
5
s
-1
). These rates are nearly two orders of magnitude greater than what is found in efficient
cyclometalated phosphors.
23
The luminescent spectra blue-shift upon dispersing the complexes in
rigid media (polymethylmethacrylate, PMMA) and display vibronic features comparable to spectra
recorded at 77 K. The quantum efficiencies also increase to = 0.20–0.32. The higher efficiency
in PMMA is mainly due to a two to four-fold decrease in knr from values measured in fluid solution.
The ridigochromic shifts and decrease in non-radiative rate constants indicate that large structural
changes present in the excited state are suppressed in the rigid PMMA media. The fact that the
emission lifetimes in PMMA at room temperature are comparable to values measured in 2-MeTHF
at 77 K implies that vibrational deactivation remains the principal mechanism for non-radiative
decay at low temperature (weak coupling limit).
43-45
37
300 400 500 600 700 800
0
10
20
30
40
50
60
70
(mM
-1
cm
-1
)
Wavelength (nm)
0
1
RT
77K
PMMA (RT)
PL intensity (a.u.)
Figure 2.15. Absorption (in CH2Cl2) and emission (in 2-MeTHF and PMMA) spectra of
(phenpy)Ir(ppz)2.
Table 2.3. Photoluminescence (PL) data for the Pt and Ir complexes 1–3.
solution
a
PMMA (1% doped)
298 K
c
k r
d
k nr
e
77 K 298 K τ
c
k r k nr
λ max (nm)
[ ]
b
(µs) 10
4
s
-1
10
4
s
-1
λ 0-0 (nm)
[τ (µs)]
c
λ max (nm)
[ ]
b
(µs) 10
4
s
-1
10
4
s
-1
1 660
[0.05]
5.1 1.0 ± 0.2 19 ± 3 592
[14.1]
654
[0.20]
12.0 1.7 ± 0.2 6.7 ±
1.5
2 678
[0.02]
1.7
f
1.2 ± 0.2 58 ± 8 582
[9.4]
632
[0.20]
3.3 (14%),
8.2 (86%)
-- --
3 668
[0.09]
4.8 1.9 ± 0.3 19 ± 3 580
[15.0]
612
[0.32]
15 2.1 ± 0.3 4.5 ±
0.7
a
Emission spectra recorded in 2-MeTHF.
b
Photoluminescent quantum yield. Error is ± 10%.
c
Error is ± 5%.
d
Derived using = krτ.
e
Derived using = kr/(kr + knr).
f
Measured at 800 nm. See
text.
An additional feature observed in the luminescent spectrum of (corpy)Ir(ppz) 2 is that the
emission lifetime is wavelength dependent as shown in Figure 2.16. The emission lifetime is
distinctly non-first order for wavelengths <700 nm in spectra measured at room temperature with a
longer decaying component appearing at higher energy. Spectra measured at 650 nm can be fit to a
biexponential decay with lifetime values of =1.6 µs (64%), 9.4 µs (36%) in 2-MeTHF and = 3.3
µs (14%), 8.2 µs (86%) in PMMA. In contrast, the emission lifetimes for the other two complexes
38
remain first-order for all wavelengths under the same conditions. The lifetime data suggests that
two or more different states are emitting for (corpy)Ir(ppz)2. The presence of a fac-isomer impurity
can be excluded since no evidence of photoisomerization is observed in the UV-visible spectrum
of the complex after photolysis in acetonitrile for 8 hours with 254 nm light. The
Figure 2.16. Emission Decay Spectra of (corpy)Ir(ppz)2 various wavelengths.
dynamic process observed in the
1
H NMR spectra for (corpy)Ir(ppz)2 (Figure 2.7) gives credence
for presence of two or more different species in solution, which could lead to the non-first order
decay. Note that the exchange rate found by VT
1
H NMR at room temperature (kexchange = 2.5 x 10
3
s
-1
) is much slower than either emission decay rate (k = 6.1 and 1.1 x 10
-5
s
-1
). Therefore, any
diastereomers that form by bowl inversion are not expected to interconvert during the lifetime of
the excited state and will thus emit independently. The (corpy)Pt(dpm) complex cannot show this
behavior since the bowl inversion does not lead to diastereomers, but instead to enantiomers that
will decay from excited states with identical rates. The fact that excited state decay from the
0 2 4 6 8 10 12 14 16 18
0.00248
0.00674
0.01832
0.04979
0.13534
0.36788
1
Normalized ln(decay)
Time ( s)
(corpy)Ir(ppz)
2
575 nm
(corpy)Ir(ppz)
2
678 nm
(corpy)Ir(ppz)
2
800 nm
39
(phenpy)Ir(ppz)2 complex is wavelength independent indicates atropisomerism is not the principal
cause of non-first order decay seen from (corpy)Ir(ppz)2.
2.6.3 DFT calculations
A possible origin for the biexponential emission decay of (corpy)Ir(ppz)2 could be related
to the different ligand-orbital overlaps expected in the two diastereomers, Figure 2.9. The excited
states of organometallic Ir and Pt complexes are typically described as being mixtures of MLCT,
LC and LLCT excited states (LLCT = ligand-to-ligand charge transfer).
46
Significant spectral
changes in both of the Pt and Ir complexes with corpy ligands are seen on comparing room
temperature and 77K luminescence (Figure 2.14), involving substantial sharpening and blue
shifting on cooling. These spectral changes have been termed rigidochromism
47
and are due to
changes in the ratios of MLCT:LC:LLCT, favoring LC states at low temperatures for the complexes
reported here. This change suggests that mixing between the two electronic configurations is
sensitive to the solvation environment around the complexes. The emission lifetime is effected by
the MLCT:LC:LLCT ratio, with greater MLCT character generally giving faster radiative rates.
48
We have modeled the excited state properties of the two diastereomers of (corpy)Ir(ppz)2 using
time-dependent density functional theory (TD-DFT), shown in Tables 2.4 2.5 and illustrated in
Figures 2.17 & 2.18. The complexes show the S0-T1 transitions for the two isomers are comprised
of different amounts MLCT, LC and LLCT character, with the transition for the -P isomer being
74% MLCT and that of the -M isomer being 85% MLCT (see Figure 2.17). Thus, since the S 0-
T1 transitions of the two isomers have different compositions it is not unreasonable to assume that
40
the two diastereomers of (corpy)Ir(ppz)2 will have unequal radiative rates, as observed. For
comparison, the S0-T1 transition for (corpy)Pt(dpm) has > 91% MLCT character.
Table 2.4. Orbital contributions calculated for S0 → T1 transition of (corpy)Pt(dpm).
( = 581 nm)
transition % assignment
120 => 121 88.9 MLCT
120 => 122 2.8 MLCT
119 => 121 3.0 LC
119 => 122 1.9 LC
118 => 121 3.4 ML’LCT
91.7% MLCT, 4.9% LC, 3.4% ML’LCT
Figure 2.17. Molecular orbitals for S0 → T1 transition of (corpy)Pt(dpm).
121 (-2.14 eV) 122 (-1.68 eV)
120 (-5.57 eV) 119 (-5.99 eV) 118 (-6.06 eV)
41
Table 2.5. Orbital contributions calculated for S0 → T1 transition of (corpy)Ir(ppz)2.
(corpy)Ir(ppz)2 -P ( = 558 nm) (corpy)Ir(ppz)2 -M ( = 565 nm)
transition % assignment
transition % assignment
168 => 169 11.4 L’LCT 168 => 169 4.5 L’LCT
168 => 170 1.2 L’LCT 167 => 169 77.7 MLCT
167 => 169 67.4 MLCT 167 => 170 7.4 MLCT
167 => 170 6.6 MLCT 166 => 169 3.4 L’LCT
166 => 169 7.4 L’LCT 165 => 169 2.1 MLCT + LC
165 => 170 1.3 MLCT + LC 165 => 170 1.3 MLCT + LC
164 => 169 4.7 MLCT +LC 163 => 169 3.5 MLCT + LC
80.0% MLCT, 20.0% L’LCT 91.1% MLCT, 8.9% LLCT
Figure 2.18. Molecular orbitals for S0 → T1 transition of (corpy)Ir(ppz)2 -P.
169 (-1.79 eV) 170 (-1.48 eV)
168 (-5.06 eV) 167 (-5.27 eV) 166 (-5.67 eV) 165 (-5.82 eV) 164 (-5.85 eV)
2.7 Conclusion
In summary, corannulene was cyclometalated onto platinum and iridium using a pendant
pyridyl group to yield phosphorescent complexes. No bowl-to-bowl stacking was seen in the crystal
42
structures of either complex. A dynamic exchange process found for (corpy)Ir(ppz) 2 examined
using VT NMR and determined to have a rate of 2.5 x 10
3
s
-1
. This process was modeled as an
inversion of the corannulene bowl creating distinct diastereomers that rapidly interconvert at room
temperature. The photophysics of the two compounds show that (corpy)Pt(dpm) and
(corpy)Ir(ppz)2 have large non-radiative rates at room temperature in solution, which decrease as
the rigidity of the surrounding solvent increases. Additionally, the decay behavior of (corpy)Ir(ppz)2
was non first-order at room temperature, differing from (corpy)Pt(dpm) as well as other common
iridium phosphors. The absence of such irregular behavior in the decay from the (corpy)Pt(dpm)
suggests that interconversion between diastereomers in (corpy)Ir(ppz)2 is responsible for the
unusual luminescent decay.
Future work could focus on dissecting the dynamics of the fluxional behavior observed in
fluid solution. One such approach will be to change the identity of the pendant coordinating ligand
with a larger, bulkier heterocycle such as a quinoline or benzoimidazole to hinder any
atropisomerism between the two ligand fragments. Another approach will be to alter the ancillary
phenyl pyrazole ligands, using bulky substituents in order to perturb the bowl inversion process.
The inherent luminescent properties of these complexes will provide an additional spectroscopic
window to help elucidate the nature of this dynamic phenomenon.
2.8 Experimental
2.8.1 Synthesis. Chemicals were received from commercial sources and used as received. All
procedures were carried out in inert N2 gas atmosphere despite the air stability of the complexes,
the main concern being the oxidative and thermal stability of intermediates at the high temperatures
of the reactions. The [(ppz)2IrCl]2 dimer was synthesized by the Nonoyama method which involves
heating IrCl3·H2O to 110 °C with 2−2.5 equivalents of ppz-H in a 3:1 mixture of 2-ethoxyethanol
43
and deionized water.
49
Corannulene was prepared as described previously.
12
The corpy-H and
phenpy-H ligand precursors were prepared by Suzuki cross coupling of their respective bromo
derivatives as previously described.
25, 50
(corpy)Pt(dpm). A 3 neck flask was charged with corpy-H (142 mg, 0.43 mmol), potassium
tetrachloroplatinate(II) (75 mg, 0.18 mmol) and 18 mL of a 3:1 mixture of 2-ethoxyethanol:water.
A condenser was attached to the flask and the mixture was degassed and heated to 100
o
C for 16
hrs. The reaction was cooled to ambient temperature, then water was added to the mixture and
filtered and an orange-yellow precipitate was isolated. This solid was then added to a new 3 neck
flask, and then charged potassium carbonate (124 mg, 0.89 mmol) and charged with
2-ethoxyethanol. A condenser was attached and the mixture was degassed after which
2,2,6,6-tetramethylheptane-3,5-dione (56 µL, 0.27 mmol) was added and the reaction was heated
to 75
o
C for 16 hr. The reaction was then cooled to ambient temperature and filtered and the
precipitate was then washed with methanol to give an orange emissive solid (52 mg, 41%).
1
H NMR
(400 MHz, CDCl3, ) 1.33 (s, 7H), 1.43 (s, 7H) 5.99 (s, 1H) 7.14 (dd, J = 7.44, 6.07 Hz, 1H), 7.79
(m, 5H), 7.92 (dd, J = 8.86, 7.45 Hz, 1H), 8.20 (d, J = 9.00 Hz, 1H), 8.51 (d, J = 8.31 Hz, 1H), 8.92
(d, J = 8.97 Hz, 1H), 9.25 (dd, J = 6.09 Hz, 1H).
13
C NMR (101 MHz, CDCl3 δ) 194.40, 169.26,
147.10, 138.28, 135.91, 135.83, 134.41, 131.68, 131.30, 130.74, 129.31, 127.94, 127.14, 126.39,
126.30, 125.41, 123.55, 121.46, 119.62, 93.64, 41.80, 41.43, 29.03, 28.39, 26.14. Anal. for
(corpy)Pt(dpm): found: C 61.08, H 4.58, N 1.98; calcd: C 61.36, H 4.43, N 1.99.
(corpy)Ir(ppz)2. A 3 neck flask was charged with corpy-H (65 mg, 0.20 mmol),
[(ppz)2Ir( -Cl)2Ir(ppz)2] (100 mg, 0.1 mmol), potassium carbonate (116 mg, 0.84 mmol) and 12
mL of 2-ethoxyethanol. A condenser was attached to the flask and the reaction was degassed and
then heated to 100
o
C for 24 hrs. The reaction mixture was then cooled to ambient temperature and
44
10 mL of deionized water was added to dissolve excess potassium carbonate. The orange-red solid
was vacuum filtered and washed with 10 mL of methanol and 10 mL hexanes, and then air dried.
Column chromatography on silica gel was performed on the resultant crude mixture (100%
methylene chloride) to give an orange-red emissive solid (36 mg, 65%).
1
H NMR (400 MHz,
acetone-d6, ) 6.30 (dd, J = 3.01, 2.10 Hz, 1H), 6.43 (dd, J = 2.88, 2.36 Hz, 1H), 6.52 (dd, J = 7.56,
1.46 Hz, 1H), 6.57 (m, 2H), 6.68 (dd, J = 2.49, 0.71, 1H), 6.80 (ddd, J = 7.97, 7.42, 1.09 Hz, 1H),
6.87 (ddd, J = 8.01, 7.24, 1.13 Hz, 1H), 6.94 (ddd, J = 8.34, 7.46, 1.35 Hz, 1H), 6.99 (ddd, J = 8.12,
7.57, 1.67 Hz, 1H), 7.13 (ddd, J = 8.24, 6.77, 1.29 Hz 1H), 7.32 (d, J = 8.66 Hz, 1H), 7.51 (dd, J =
7.97, 1.32 Hz, 1H), 7.56 (m, 2H), 7.78 (d, J = 8.67 Hz 1H), 7.84 (d, J = 8.70 Hz, 1H), 7.89 (m, 3H),
8.03 (ddd, J = 8.19,7.72, 1.83 Hz, 1H), 8.26 (dd, J = 5.38, 1.73 Hz, 1H), 8.43 (m, 2H), 8.49 (dd, J
= 2.88, 0.63 Hz, 1H), 8.99 (d, J = 8.20 Hz, 1H).
13
C NMR (101 MHz, CDCl3 δ) 185.45, 169.15,
154.49, 151.82, 150.01, 143.31, 142.89, 142.36, 142.24, 141.91, 141.42, 140.35, 136.68, 136.59,
136.57, 135.29, 134.62, 134.14, 133.76, 133.01, 132.53, 131.03, 130.46, 130.43, 129.15, 128.88,
127.57, 127.37, 127.32, 127.25, 127.23, 127.21, 127.17, 127.10, 126.92, 126.87, 126.73, 126.38,
126.09, 125.96, 125.78, 125.69, 125.19, 125.17, 125.03, 124.54, 124.11, 122.98, 122.29, 121.81,
121.75, 120.62, 119.91, 110.78, 110.75, 110.39, 107.19, 106.66, 106.31. Anal. for (corpy)Ir(ppz)2:
found: C 63.62, H 3.31, N 8.49; calcd: C 64.16, H 3.26, N 8.7.
(phenpy)Ir(ppz)2. A 3 neck flask was charged with phenpy-H (105 mg, 0.41 mmol),
[(ppz)2Ir(µ-Cl)2Ir(ppz)2] (200 mg, 0.2 mmol), potassium carbonate (215 mg, 1.56 mmol) and 26
mL of dichloroethane. A condenser was attached to the flask and the reaction was degassed and
then heated to 100
o
C for 24 hrs. The reaction mixture was then cooled to ambient temperature and
filtered through an alumina plug. The resultant mixture was then recrystallized with
dichloromethane to obtain yellow-orange emissive solid (58 mg, 20 %).
1
H NMR (400 MHz,
45
CD2Cl2, ) 6.29 (dd, J = 3.05, 2.54 Hz, 1H), 6.34 (dd, J = 7.29, 1.30 Hz, 1H), 6.40 (dd, J = 3.07,
2.47 Hz, 1H), 6.53 (dd, J = 7.20, 1.36 Hz, 1H), 6.73 (dd, J = 9.27, 2.30 Hz, 2H) 6.78 (ddd, J =
8.68,7.58, 1.10 Hz, 1H), 6.94 (m, 1H), 7.05 (ddd, J = 8.39, 7.55, 1.41 Hz, 1H), 7.29 (d, J = 7.83
Hz, 1H), 7.34 (d, 7.88 Hz, 1H), 7.42 (ddd, J = 8.35, 7.60, 1.15 Hz, 1H), 7.54 (m, 2H), 7.71 (ddd, J
= 8.87, 7.89, 1.75 Hz, 1H), 8.02 (d, J = 8.69 Hz, 1H), 8.06 (dd, J = 6.20, 2.74 Hz, 2H), 8.28 (m,
2H), 8.50 (d, J = 8.24 Hz, 1H), 8.54 (d, J = 8.24 Hz, 1H), 8.67 (d, J = 8.36 Hz, 1H).
13
C NMR (101
MHz, CDCl3, δ) 149.55, 142.88, 140.35, 132.53, 128.99, 128.44, 127.37, 127.03, 126.81, 126.77,
126.71, 126.56, 126.47, 125.78, 125.20, 122.72, 122.54, 121.75, 110.39, 106.31. Anal. for
(phenpy)Ir(ppz)2+H2O: found: C 59.66, H 3.57, N 9.15; calcd: C 59.18, H 3.76, N 9.33.
Electrochemisty. Cyclic voltammetry and differential pulsed voltammetry were performed using
an VersaSTAT 3 potentiostat. Anhydrous DMF (Aldrich) 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,
51
while electrochemical reversibility was determined using cyclic voltammetry.
NMR Measurements.
1
H NMR spectra were recorded on a Varian-500 and a Varian 400 NMR
spectrometer. Chemical shift data for each signal are reported in ppm and measured in deuterated
dichloromethane (CD2Cl2), deuterated chloroform (CDCl3), and deuterated acetone ((CD3)2CO).
Variable temperature NMR was measured in the range of 198-298 K. The temperature of the NMR
probe was calculated using a methanol temperature standard. The rate of inversion and inversion
barrier were determined by fitting the resultant data to an Eyring plot of ln(kh/kbT) vs 1/T.
35
46
X-ray Crystallography. The single-crystal X-ray diffraction data for compounds (corpy)Pt(dpm)
and (corpy)Ir(ppz)2 were collected on a Bruker SMART APEX DUO three-circle platform
diffractometer with the χ axis fixed at 54.745° and using Mo Kα radiation (λ = 0.710 73 Å)
monochromated by a TRIUMPH curved-crystal monochromator. The crystals were mounted in
Cryo-Loops using Paratone oil. Data were corrected for absorption effects using the multiscan
method (SADABS). The structures were solved by direct methods and refined on F
2
using the
Bruker SHELXTL software package. All non-hydrogen atoms were refined anisotropically.
Photophysical Measurements. Photoluminescence spectra were measured using a QuantaMaster
Photon Technology International phosphorescence/fluorescence spectrofluorometer.
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 analyzer (PMA). UV−vis spectra
were recorded on a Hewlett-Packard 4853 diode array spectrometer. Samples for transient
luminescent decay measurements were prepared in 2-MeTHF solution. The samples were deaerated
by extensive sparging with N 2.
Computational Methods. Molecular models were created and dipole moments determined
using the Jaguar 8.4 (release 17) software package on the Schrodinger Material Science Suite
(v2014-2). The molecular geometries and TD-DFT calculations were performed using a B3LYP
functional and a LACVP** basis set with a Poisson-Boltzmann (PBF) CH2Cl2 solvent dielectric
continuum as implemented in Jaguar.
47
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52
CHAPTER 3. Synthesis and Characterization of
Phosphorescent Iridium Complexes with Multiple
Cyclometalated Corannulenes
3.1 Introduction
In addition to their great success as emitters in OLEDs,
1, 2
cyclometalated iridium complexes
have also shown much versatility in other practical applications,
3
ranging from catalysis of alkane
dehydrogenation to oxidation of water.
4, 5
Cyclometalated complexes have also been used for
probes of metal ions for applications in biological sensors as well.
6, 7
One interesting molecule that
has been incorporated into organometallic complexes with resulting interesting properties is
corannulene.
8, 9
A large number of these complexes are observed to undergo rapid bowl inversion,
a behavior commonly observed in many different corannulene species.
10
Within Chapter 2, the impact of bowl inversion on the photophysics of an organometallic
iridium complex has been studied.
11
It was shown that upon introduction of a single
2-(corannulene)pyridine (corpy-H) ligand to an iridium complex, the non-radiative rates increase
significantly. This observation is attributed to bowl inversion between two distinct diastereomers.
This further spurred an interest in appending multiple corpy-H ligands to iridium, taking advantage
of the octahedral, trivalent metal’s affinity for supporting up to three cyclometallating ligands.
Within this chapter, the dynamic and photophysical properties of (corpy)2Ir(ppz) and Ir(corpy)3,
and examine the effects of the fluxional behavior of this bowl-shaped ligand.
Herein, Figure 3.1 depicts a series of iridium complexes containing one, two and three corpy-H
ligands. These complexes were chosen to represent a natural progression from the reported
(corpy)Ir(ppz)2, The bis iridum complex is (corpy)2Ir(dpm), where dpm =
2
-dipivolylmethane and
the tris iridium complex is Ir(corpy)3. The dpm ligand was chosen over the acetylacetonate (acac)
53
ligand for (corpy)2Ir(dpm) as dpm acts as an ancillary ligand just like acac and allows for greater
solubility than acetylacetonate. Variable temperature NMR is used to examine any dynamic
processes that these complexes undergo in fluid solution. Both (corpy)2Ir(dpm) and Ir(corpy)3 are
phosphorescent with the photophysical properties dictated by the corpy ligand.
Figure 3.1. Structures of (corpy)Ir(ppz)2, (corpy)2Ir(dpm), and Ir(corpy)3.
3.2 Synthesis of Complexes
The corpy-H ligand precursor was synthesized by Suzuki cross coupling as described for
other C^N ligands.
12
The bis and tris iridium (III) complexes containing corannulene were prepared
by routes similar to those described for other C^N complexes as shown in Scheme 3.1.
13, 14
The
(corpy)2Ir(dpm) complex was synthesized first by preparing the cyclometalated (corpy)2IrCl
intermediate in a reaction between corpy-H and IrCl3, followed by the addition of
dipivaloylmethane and base to displace the Cl ligands. The Ir(corpy)3 went through a different
procedure than (corpy)Ir(ppz)2 or (corpy)2Ir(dpm), where iridium(III) acetylacetonate was heated
in a ligand melt of corpy-H to obtain exclusively the facial Ir(corpy)3 isomer. It was found
previously that photoisomerization of the meridional to facial (corpy)Ir(ppz)2 does not occur after
prolonged irradiation with 254 nm light, therefore, a synthetic route with direct formation of the
facial Ir(corpy)3 isomer from the corpy-H ligand was chosen over the Nonoyama reaction.
15
Both
54
the bis and tris complexes were isolated as a single red colored species that are stable in air as neat
solids and in fluid solution.
Scheme 3.1
3.3 Crystal Structures
X-ray diffraction analysis was performed on a single crystal of (corpy)2Ir(dpm) grown by slow
diffusion of hexanes into dichloromethane of the metal complex. The structure of (corpy)2Ir(dpm)
is shown in Figure 3.2a. Although numerous methods and solvents were attempted, no successful
crystals of Ir(corpy)3 were grown. The unit cell of (corpy)2Ir(dpm) contains 4 molecules in a
monoclinic P21/c space group (Fig 3.2c). Both enantiomers of a sole diastereomer ( -PM and
-MP) are observed within the unit cell, where and symbols describe
55
Figure 3.2. a.) Crystal structure of (corpy)2Ir(dpm); b.) view of (corpy)2Ir(dpm) down the N-Ir-N
bond axis c.) packing of (corpy)2Ir(dpm) molecules within the unit cell. Two molecules of
(corpy)2Ir(dpm) were omitted for clarity. All hydrogens were also omitted for clarity. The atoms
are colored light blue (N) black (C), red (O) and dark blue (Ir).
the stereochemistry of the iridium metal center and P and M are used in describing the
stereochemistry of chiral buckybowls.
16,17
The ligands are arranged around the pseudo-octahedral
iridium center with the corannylenyl ligands in a trans-disposed arrangement. The bond angles for
the atoms that are trans-disposed around the iridium (C(6)-Ir(1)-O(1) = 174.60(7)°, N(1)-Ir(1)-N(2)
= 174.67(7)° and N(5)-Ir(1)-C(10) = 170.47(7)°) are comparable to other bis-iridium diketonate
complexes.
13
Similarly, the bond length for the Ir-C bonds (Ir(1)-C(6) =1.998(2) Å, Ir(1)-C(45) =
56
1.998(2) Å) and the Ir-N bonds (Ir(1)-N(1) = 2.0302(18) Å, Ir(1)-N(2) =2.0129(17) Å) compare
favorably with previously reported complexes. The bond lengths for Ir-O bonds (Ir(1)-O(1)
=2.1174(15) Å, Ir(1)-O(2) = 2.1383(15) Å) are slightly smaller than the comparable
(ppy)2Ir(diketonate) complex but larger than the 2.088 Å value reported for Ir-O bonds in the
Cambridge Crystallographic Database, which can be attributed to the large trans influence of the
corannulene.
13,18
Comparison of the Ir-Ncorpy
and Ir-Ccorpy bond lengths for Ir(corpy)3 and
(corpy)2Ir(dpm) complexes reveals much shorter bond lengths compared to the original
(corpy)Ir(ppz)2 complex.
11
The dihedral angle between the two fragments of the corpy ligand differ
significantly for each corpy ligand (C(4)-C(5)-C(7)-C(8) =16.1°, C(29)-C(30)-C(31)-C(32) =6.1°)
while the reported (corpy)Ir(ppz)2 complex has a dihedral angle closer to the larger dihedral (14.2°),
which could be attributed to crystal packing forces.
The corannulene moieties for the (corpy)2Ir(dpm) complex preferentially pack with a P, M
stereochemistry rather than a P, P or M, M configuration, likely a result of crystal packing forces.
The bowl depth of corannulene for each corpy ligand (dbowl = 0.835 Å, 0.818 Å) could suggest
different rates of bowl inversion. The larger bowl depth (0.835 Å) corresponds favorably to
(corpy)Ir(ppz)2 (0.837 Å) which itself is shallower than other reported corannulenes.
11,19, 20
The
corannulene moieties also do not show any bowl-to-bowl packing as is reported in the literature for
other corannulene species.
21,22
Instead, the complexes are arranged into pairs of enantiomers with
the concave faces of the corannulenes facing each other. The corranulene moieties are not
completely situated atop one another but rather two benzene rings from the extended -system of
each moiety align with the other as shown in Figure 3.2c. The distance between these -systems (~
3.5 Å) compares favorably to other -interactions seen in previously reported complexes containing
corannulene.
11, 21
57
3.4. NMR and Dynamic Behavior
3.4.1 Room Temperature NMR
Figure 3.3. (top)
1
H NMR spectrum of (corpy)2Ir(dpm) in CDCl3, (bottom)
1
H NMR spectrum of
Ir(corpy)3 in CD2Cl2. The labelling scheme for the corpy ligand is the same in both spectra.
The iridium complexes were characterized using
1
H, 1D NOESY and gCOSY NMR
spectroscopy (Figure 3.3). Distinct, sharp resonances are observed in the aromatic regions of the
proton spectra measured at 298 K. The
1
H NMR spectrum for (corpy)2Ir(dpm) in deuterated
chloroform displays a series of well resolved resonances integrating to 12 protons that is consistent
with a single species, even though multiple conformers are possible. The proton assignments for
(corpy)2Ir(dpm) were confirmed through gCOSY and 1D-NOESY. Resonances for protons d (δ =
8.91 ppm) and e (δ = 8.45 ppm) on the corpy ligand are the furthest downfield, confirmed by
58
1D-NOESY. Resonances for protons l (δ = 6.95 ppm) and k (δ = 6.31 ppm) are shifted upfield due
to the proximity of the adjacent pyridine on the opposite corpy ligand allowing these resonances to
be shielded by the pyridine ring current. Resonances for f-j on corannulene are identifiable between
δ = 7.84-7.40 ppm, while all remaining pyridine and dipivolylmethane protons are also clearly
identified and assigned. Comparison to the (corpy)Ir(ppz)2 complex shows that the pyridine proton
resonances and protons e-j are similar to the (corpy)2Ir(dpm) compound. One notable difference is
the significant shielding of resonances l and k in (corpy)2Ir(dpm) compared to the (corpy)Ir(ppz)2
complex (δ = 7.47 & 7.27 ppm) which could be explained by a stronger ring current of pyridine
over pyrazole.
The
1
H NMR spectrum of Ir(corpy)3 in deuterated dichloromethane also displays well
resolved resonances integrating to 12 protons that is not only consistent with a single species present
but also solely the facial isomer. Proton assignments shown in Figure 3.3 were confirmed on the
basis of gCOSY and 1D-NOESY spectroscopy. Similar to both (corpy)Ir(ppz)2 and
(corpy)2Ir(dpm), Ir(corpy)3 has resonances d (δ = 8.69 ppm) and e (δ = 8.21 ppm) as the furthest
downfield. Similar to (corpy)2Ir(dpm), the tris complex has resonances l (δ = 7.09 ppm) and k (δ =
6.60 ppm) significantly upfield due to the proximity to the neighboring corannulene, whose
different ring currents shield these resonances in Ir(corpy)3. One notable difference in the spectrum
of Ir(corpy)3 is the resonance a (δ = 7.34 ppm) shifting upfield relative to (corpy)Ir(ppz)2 and
(corpy)2Ir(dpm). One explanation for this is that proton a, which is proton ortho to the pyridine
nitrogen, experiences a ring current from a neighboring pyridine ring due to the facial isomer of
Ir(corpy)3. All remaining protons on corannulene and pyridine were clearly identified and assigned.
59
Figure 3.4. Variable low temperature
1
H NMR spectra of (corpy)2Ir(dpm) in CDCl3.
3.4.2 Variable Temperature NMR and Analysis
Both of the (corpy)2Ir(dpm) and Ir(corpy)3 complexes undergo a dynamic process in fluid
solution that was observed using variable temperature (VT)
1
H NMR spectroscopy (Figures 3.4 and
3.5) For (corpy)2Ir(dpm), deuterated chloroform was chosen as the solvent. It was shown to provide
good separation of all proton resonances and able to maintain solubility at lower temperatures.
Additionally, deuterated chloroform having a higher boiling point than most other common
deuterated solvents makes it advantageous for use at higher temperatures. At higher temperatures,
no changes were observed apart from the sharpening of all proton resonances in the spectrum. At
lower temperatures, a number of dynamic changes are observed. Initially, the resonance for proton
l (δ = 6.95 ppm) on corannulene and proton a (δ = 8.38 ppm) broadens and almost merges into the
baseline at 216 K. As these dynamic changes occur, broadening of protons e (δ = 8.45 ppm), h (δ
60
= 7.65 ppm), i (δ = 7.57 ppm), j (δ = 7.40 ppm), b (δ = 7.07 ppm) is observed as well. These peaks
broaden at different rates. Compared to (corpy)Ir(ppz)2 where resonance l had coalesced at 245 K,
coalescence of the corresponding peak for (corpy)2Ir(dpm) happens at a slower rate as resonance 1
does not entirely coalesce even at 216 K. The low temperature NMR studies suggests that inversion
is occurring for an iridium complex with two corannulenes and that the rate of bowl inversion is
more rapid for (corpy)2Ir(dpm) than for (corpy)Ir(ppz)2 as rate of bowl inversion is related to
coalescence temperature.
11, 19
No formation of new species was observed and a definitive
calculation of the rate of bowl inversion was unable to be obtained.
Figure 3.5. Variable low temperature
1
H NMR spectra of Ir(corpy)3 in CD2Cl2.
61
For Ir(corpy)3, deuterated chloroform was used as the solvent at high temperatures and
deuterated dichloromethane was used at low temperature as CDCl3 provides good separation of all
proton resonances while CD2Cl2 provides the same benefits and the ability to maintain solubility at
lower temperatures. At higher temperatures, there are no changes in the line shape of any proton
resonances except for peaks sharpening, which is comparable to (corpy)2Ir(dpm). Similar to the
other Ir-corpy complexes, resonance l (δ = 7.09 ppm) is the initial peak to broaden and disappears
into the baseline by 248 K but is not observed to remerge even at temperatures as low as 223 K.
Proton resonances b (δ = 6.67 ppm) and k (δ = 6.60 ppm) are also observed to broaden and disappear
into the baseline at 238 K and 223 K respectively as well. All other resonances exhibit broadening
at lower temperatures. Unfortunately, even though resonance l for Ir(corpy)3 disappears into the
baseline at higher temperature than corresponding proton for (corpy)Ir(ppz) 2 (245 K), no formation
of new or distinct species was observed. Lower temperatures were unable to be achieved due to
instrument limitations unable to collect data at lower temperatures.
Analysis of VT NMR of (corpy)2Ir(dpm) and Ir(corpy)3 suggests that even with the addition
of multiple corannulene moieties to the iridium complex, bowl inversion still occurs rapidly enough
on the NMR time scale that a single species is detected. For (corpy)2Ir(dpm), although lowering the
temperature slows the rate of bowl inversion between MM and PP isomers, the corannulene proton
with the most unique proton environment, peak l, does not experience a large change in chemical
shift from the MM to the PP isomers. As shown in Figure 3.6, the proton resonance l is shielded by
the ring current of a neighboring pyridine; this chemical environment does not dramatically change
from MM to PP, unlike the pyrazole protons on (corpy)Ir(ppz)2. The large dipole associated with
corannulene on the (corpy)Ir(ppz)2 complex allows for a large change in chemical shift for those
pyrazole protons, depending on which diastereomer is present as reported in Chapter 2.
23
62
Figure 3.6. Top: Structural representation of (corpy)2Ir(acac). The view is roughly down
the N-Ir–N axis showing the -MM (left) and -PP (right) diastereomers of (corpy)2Ir(acac).
Bottom: Structural representation of Ir(corpy)3. The view is roughly down the N-Ir–C axis
showing the -MMM (left) and -PPP (right) diastereomers of Ir(corpy)3. Proton a is shown in
blue and l in green and k in red.
For Ir(corpy)3, although protons l and k are shielded by a ring current of corannulene in the
PPP isomer (Figure 3.6), upon bowl inversion, the proton resonances should not experience a ring
current in the MMM isomer, suggesting one would observe a large change in chemical shift.
However, although coalescence is observed, there are no chemically distinct diastereomers
observed within the NMR spectrum, and the overall change in chemical shift is small, resulting in
no observation of multiple species, suggesting that solvation could play a role in chemical shift.
Further cooling to lower temperatures may slow bowl inversion to a rate such that multiple
diastereomers could be observed in the NMR spectra. The VT NMR also suggests that bowl
inversion on both iridium complexes likely occurs via a concerted rather than a stepwise process.
This is explained by the absence of three distinct coalescent events, which would be observed if the
63
bowl inversion of each ligand occurred sequentially. Instead, coalescence is observed only once for
proton resonances k and l .
3.5 Electrochemical Properties
Figure 3.7. Cyclic Voltammetry of (corpy)2Ir(dpm) vs Fc/Fc
+
.
The redox properties of (corpy)2Ir(dpm) and Ir(corpy)3 were examined by cyclic
voltammetry and differential pulse voltammetry in acetonitrile solution with 0.1 M TBAF as shown
in Figures 3.7 and 3.8 and summarized in Table 3.1. All potential values were referenced to an
internal ferrocene/ferrocenium couple (Fc/Fc
+
). The oxidative properties of the complexes are
similar to related complexes with cyclometalated ligands. The (corpy)2Ir(dpm) complex displays a
reversible oxidative wave with E
1/2
= 0.44 V that compares favorably with the oxidation potential
for (ppy)2Ir(acac).
24
The Ir(corpy)3 complex also displays a reversible oxidation (E
1/2
= 0.48 V) but
that value is almost 0.2 V higher than the oxidation potential for Ir(ppy)3 (E
1/2
= 0.31).
12
This
difference can be attributed to the extended -system on the corannulene moiety which stabilizes
64
the HOMO of Ir(corpy)3. Oxidation in both cyclometalated Ir-complexes is typically assigned to an
orbital with mixed metal-aryl character.
12, 25
Figure 3.8. Cyclic Voltammetry and Differential Pulse Voltammetry of Ir(corpy)3 vs Fc/Fc
+
.
The iridium complexes show multiple reduction waves in acetonitrile solution. The
(corpy)2Ir(dpm) complex displays two reversible waves at -2.21 V and -2.41 V followed a
quasireversible wave at -2.71 V and an irreversible wave at -3.04 V. The Ir(corpy)3 complex
displays five reduction waves: three reversible waves (-2.15 V, -2.35 V, -2.47 V) one
quasireversible wave (-2.84 V) and one irreversible wave (-3.07 V). The cathodic waves in both
these complexes are assigned to the reduction of the corpy ligand. The reversible reduction
potentials of both complexes compare favorably with one another and follows an approximate trend
of the first reduction potential decreasing by ~ 60 mV with addition of another corpy ligand, which
stabilizes the LUMO from (corpy)Ir(ppz)2 (-2.27 V), (corpy)2Ir(dpm) (-2.21 V), to Ir(corpy)3 (-2.15
V).
11
This trend is continued in the reduction of the second corpy ligand as shown in
(corpy)2Ir(dpm) (-2.41 V) and Ir(corpy)3 (-2.35 V). These reduction potentials are significantly
lower than the reduction potential for Ir(ppy)3 (E
1/2
= -2.70 V).
12
This difference can be attributed
to corannulene being a much better acceptor than benzene.
26, 27
The quasireversibility of the
65
reduction waves in both complexes reflects the ability of the corannulene ligand to accept up to
three electrons in solution.
26, 28
The separation (ΔEred) between the reversible and quasireversible
waves (300 mV for (corpy)2Ir(dpm), 370 mV for Ir(corpy)3) is smaller than that same difference
found in the reduction waves for corannulene (700 mV). ). This discrepancy suggests a decrease
in coulombic repulsion in the radical anion due to the electron being delocalized onto the pyridyl
ring of the corpy ligand; a feature which is not possible with unsubstituted corannulene. The final
irreversible reduction waves of both complexes can be associated with localization of charge on the
pyridyl ring in the corpy ligand. Both complexes final reduction potential compare quite favorably
with one another ( -3.04 V for (corpy)2Ir(dpm), -3.07 V for Ir(corpy)3) and with Ir(ppy)3 (-3.00
V).
12
Further reductions were not observed to due limitations of the solvent.
Table 3.1. Redox data for the Ir complexes.
a
Compound Eox1 Ered1 Ered2 Ered3 Ered4 Ered5
(corpy)2Ir(dpm) 0.44 V -2.21 V -2.41 V -2.71 V
c
-3.04 V
b
--
Ir(corpy)3 0.48 V -2.15 V -2.35 V -2.47 V -2.84 V
c
-3.07 V
b
a
Redox potentials were recorded in 0.1 M TBAF/MeCN solution and referenced to an internal
Fc
+
/Fc couple.
b
Irreversible.
c
Quasireversible.
3.6. Photophysical Properties
3.6.1 Absorption Spectra
Table 3.2. Absorption data for (corpy)2Ir(dpm) and Ir(corpy)3 complexes 1 & 2.
λmax(nm) (ε, mM
-1
cm
-1
)
(corpy)2Ir(dpm) (1)
a
306 (35.3), 351 (23.1), 438 (5.15), 482 (4.11), 513 (4.44)
Ir(corpy)3 (2)
b
309 (81.1), 340 (sh, 53.6), 430 (14.4), 502 (sh, 4.13)
a
Absorption spectra recorded in 2-methyl THF.
b
Absorption spectra recorded in CH2Cl2.
The absorption and emission spectra of the complexes were recorded at room temperature
and 77 K (Figure 3.9). Additionally, photophysics were performed in a rigid matrix for the
66
Ir(corpy)3 complex. The absorption data is listed in Table 3.2 and emission data in Table 3.3 and
has absorption band comparable to (corpy)Ir(ppz)2. The absorption spectra for both complexes
show intense bands ( < 360 nm, > 10
4
M
-1
cm
-1
) assigned to the ligand centered π→π* transitions
on the cyclometalated ligands.
11
Similarly to (corpy)Ir(ppz)2, the bands between = 300–360 nm
are assigned to π→π* transition on corpy as comparing with spectra from the free corpy-H ligand.
11
Less intense bands at lower energy ( = 350–500 nm, ≈ 5 x 10
3
M
-1
cm
-1
) are assigned to allowed
metal-to-ligand charge transfer (MLCT) transitions. Much weaker absorptions ( > 500 nm, < 10
2
M
-1
cm
-1
) are assigned to triplet MLCT transitions that are partially allowed due to spin-orbit
coupling with the singlet states by the iridium metal center.
3.6.2. Emission Spectra
Figure 3.9. Absorption and emission (in 2-MeTHF) spectra of (corpy)2Ir(dpm), left, and
absorption (in CH2Cl2) and emission (in 2-MeTHF and PMMA) spectra of Ir(corpy)3, right.
Both iridium complexes display broad featureless red luminescence at room temperature in
2-MeTHF solution suggesting that emission is from a metal-to-ligand charge transfer state (MLCT).
The emission peaks for (corpy)2Ir(dpm) (654 nm) and Ir(corpy)3 (644) exhibit a slight
67
hypsochromic shift relative to the parent (corpy)Ir(ppz)2 complex (678 nm). The emission lifetimes
for both compounds at room temperature are single exponential and fall in the narrow range of
= 3.1-3.4 μs consistent with phosphorescent emission. At 77 K, the spectra of both complexes
show distinct vibronic structure, similar to (corpy)Ir(ppz)2. Furthermore, in a frozen matrix at this
temperature, the emission lifetime is single exponential for (corpy)2Ir(dpm) (6.52 μs), while the
lifetime is double exponential for Ir(corpy)3 (τ1 = 5.6 μs (64%), τ2 = 12 μs (36%)). Solution
processed poly(methyl methacrylate) (PMMA) films (1% w/w) of the latter also showed bi-
exponential decay at room temperature (τ1 = 3.2 μs (34%), τ2 = 6.0 (66%)). The vibronic structure
and lifetimes observed by both iridium complexes suggests that emission at 77 K occurs from the
triplet state with significant ligand centered character (LC). The energy of the triplet state in these
complexes is more than 0.2 eV lower than that of the
3
- state in the free ligand corpy-H
(E0-0 = 525 nm, 2.36 eV),
11
allowing the excited state to be stabilized upon cyclometalation of the
ligand.
Table 3.3. Photoluminescence (PL) data for the Ir complexes 1 & 2.
solution
a
PMMA (1% doped)
298 K
c
k r
d
k nr
e
77 K 298 K τ
c
λ max (nm)
[ ]
b
(µs) 10
4
s
-1
10
4
s
-1
λ 0-0 (nm)
[τ (µs)]
c
λ max
(nm)
[ ]
b
(µs)
1 654
[0.21]
3.1 6.7 ± 0.2 25 ± 3 608
[6.52]
-- --
2 644
[0.13]
3.4 3.8 ± 0.3 26 ± 3 606
[5.6 (64%), 12 (36%)]
616
[0.27]
3.2 (34%),
6.0 (66%)
a
Emission spectra recorded in 2-MeTHF.
b
Photoluminescent quantum yield. Error is ± 10%.
c
Error is ± 5%.
d
Derived using = krτ.
e
Derived using = kr/(kr + knr).
The photoluminescent quantum yields of the complexes in fluid solution are relatively low
(Φ = 0.21 for (corpy)2Ir(dpm), Φ = 0.13 for Ir(corpy)3) compared to other iridium complexes used
68
in OLEDs but a significant improvement compared to the (corpy)Ir(ppz)2 complex (Φ = 0.02).
11, 12
The radiative decay rate constants (kr = 3.8-6.7 × 10
4
s
-1
) are similar to (corpy)Ir(ppz)2
(kr = 1.2 × 10
4
s
-1
) but are an order of magnitude lower than the radiative decay rate of other highly
efficient red Ir phosphors.
29
The low quantum efficiency can be attributed to the large nonradiative
decay rates (knr = 25-26 × 10
4
s
-1
) that are almost two orders of magnitude greater than nonradiative
decay rates found in more efficient iridium phosphors.
12
These large non-radiative rates are
however a significant reduction over the (corpy)Ir(ppz)2 complex, (knr = 58 × 10
4
s
-1
). This
reduction in knr can be attributed to the energy gap law as a reduced contribution of vibrational
modes from the higher energy Ir(corpy)3 and (corpy)2Ir(dpm) complexes lead to less non-radiative
decay pathways.
30, 31
The emission spectra of the Ir(corpy)3 exhibits a hypsochromic shift upon placing the
complex in a more rigid PMMA matrix, and displays vibronic features that are reminiscent of the
spectra at 77 K. The quantum efficiency is also shown to double from Φ = 0.13 to Φ = 0.27. Using
(corpy)Ir(ppz)2 as a reference, this increase in quantum yield suggests a decrease in the
non-radiative decay rate from fluid solution. The structural and rigidochromic shifts observed in all
three Ir-corpy complexes suggest that any potential structure or fluxional changes have been
suppressed in PMMA. Although any fluxional behavior of Ir(corpy)3 has been suppressed, the
emission lifetime at room temperature still has a similar magnitude, suggesting that vibrational
deactivation remains the prime mechanism for non-radiative decay at 77 K, similar behavior to
(corpy)Ir(ppz)2.
32
3.6.3 DFT Calculations
The lifetime for (corpy)2Ir(dpm) is observed to be a first order decay in both fluid and rigid
media, suggesting that there is no observable differences in the photophysics between the MM and
69
the PP diastereomers of (corpy)2Ir(dpm). A possible origin for the monoexponential emission decay
of (corpy)2Ir(dpm) could be related to the ligand-orbital overlap expected in the two diastereomers.
To understand the origin of the stark differences in excited state decay rates between the iridium
complexes, time-dependent Density Functional Theory calculations (TDDFT) were performed.
TDDFT was performed on (corpy)2Ir(acac) in order to provide a better picture of the orbital
transitions that contribute to emission and to understand the photophysics as shown in Table 3.4
and illustrated in Figures 3.10 and 3.11. The (corpy)2Ir(acac) complex can act as stand-in for
corpy 2Ir(dpm) due to the smaller diketonate moiety acting as an ancillary ligand uninvolved with
the photophysics. The excited states of organometallic Ir complexes are typically described as being
mixtures of MLCT and LC excited states.
33
Significant spectral changes in both of the Ir complexes
with corpy ligands are seen on comparing room temperature and 77K luminescence (Figure 3.9),
involving substantial sharpening and blue shifting on cooling. These spectral changes can be
attributed to rigidochromism
34
and are due to changes in the ratios of MLCT:LC states, favoring
LC states at low temperatures for the complexes reported here. Due to monoexponetial behavior in
both solution and frozen media it is argued that bowl inversion of corannulene does not heavily
impact the decay behavior. Although there are different energy levels involved in the photophysics
of each diastereomer, overall they do not have large difference in amounts of MLCT:LC character
in their S0-T1 transitions which is reflected in observation of monoexponential decay in both
solution and frozen media.
In comparison, the Ir(corpy)3 complex has a monoexponential emission decay in fluid
solution and a biexponential decay in rigid media. In rigid media, fluxional behavior would be
largely suppressed. The two isomers would be locked into the MMM or the PPP configuration,
resulting in a biexponential decay. Alternatively, it has been shown previously by Yersin et al that
70
non-first order exponential decay observed for Ir complexes in frozen media (at low temperatures)
can be attributed to the splitting of the zero field splitting of the triplet state.
35
The non-first order
decay observed in Ir(corpy)3 can be a result of decay from the T1 triplet substates.
36
Further studies
will be needed to determine why the decay behavior differs from the bis- to the tris-complex.
Table 3.4. Orbital contributions calculated for S0 → T1 transition of (corpy)2Ir(acac).
(corpy)2Ir(acac) -MM ( = 617 nm) (corpy)2Ir(acac) -PP ( = 601 nm)
Transition % assignment Transition % assignment
HOMO-1 => LUMO 5.8 LC HOMO-1 => LUMO+1 10.2 MLCT + LC
HOMO-1 => LUMO+1 12.4 LC HOMO => LUMO 88.7 MLCT + LC
HOMO => LUMO 71.7 MLCT
HOMO => LUMO+3 2.8 MLCT
82.8% MLCT, 17.2% LC 98.9% MLCT +LC
71
Figure 3.10. Molecular orbitals for S0 → T1 transition of (corpy)2Ir(acac) -MM.
LUMO (-1.91 eV) LUMO+1 (-1.88 eV) LUMO+3 (-1.49 eV)
HOMO (-4.97 eV) HOMO-1 (-5.58 eV)
Figure 3.11. Molecular orbitals for S0 → T1 transition of (corpy)2Ir(acac) -PP.
LUMO (-1.91 eV) LUMO+1 (-1.75 eV)
HOMO (-5.05 eV) HOMO-1 (-5.51 eV)
72
3.7 Conclusion
In summary, bis- and tris- cyclometalated iridium complexes containing corannulene were
synthesized to yield phosphorescent complexes. No bowl-to-bowl stacking was seen in the crystal
structures of (corpy)2Ir(dpm). Although fluxional behavior was observed in VT NMR, suggesting
the bowl inversion occurs for both (corpy)2Ir(dpm) and Ir(corpy)3, no rate of bowl inversion was
obtained due to the dynamic behavior occurring too fast on the NMR timescale at low temperatures.
This process was determined to be a concerted rather than stepwise process, creating distinct
diastereomers that rapidly interconvert at room temperature. The photophysics of the two
compounds show that both (corpy)2Ir(dpm) and Ir(corpy)3 have large non-radiative rates at room
temperature in solution, which decrease as the rigidity of the surrounding matrix increases but
overall shows an improvement upon the non-radiative decay rate of (corpy)Ir(ppz)2. Additionally,
the decay behavior of (corpy)2Ir(dpm) and Ir(corpy)3 was first-order at room temperature, differing
from the (corpy)Ir(ppz)2 complex. The absence of such irregular behavior in the decay from the
bis- and tris- suggests that interconversion between diastereomers in (corpy)Ir(ppz)2 that is
responsible for its unusual luminescent decay does not apply to all organometallic complexes with
fluxional diastereomers.
Future work could focus on further dissecting the dynamics of the fluxional behavior observed in
fluid solution. Even for (corpy)Ir(ppz)2 there was no exchange processes observed for the protons
directly on corannulene; bowl inversion was indirectly measured through the pyrazole protons. One
such approach to directly measure would be to append a diastereotopic group such as a methyl
alcohol (-CH2OH) directly onto corannulene bowl. Future work could also focus on the different
applications these corannulene complexes could be used for. In particular, the structure of
73
(corpy)2Ir(dpm) shows potential as a good candidate for molecular orientation in a thin film, the
concepts of which will be discussed in Chapter 5.
3.8 Experimental
3.8.1 Synthesis. Chemicals were received from commercial sources and used as received. All
procedures were carried out in inert N2 gas atmosphere despite the air stability of the complexes,
the main concern being the oxidative and thermal stability of intermediates at the high temperatures
of the reactions. Corannulene was prepared as described previously.
37
The corpy-H ligand precursor
was prepared by Suzuki cross coupling of the respective bromo derivative as previously
described.
21-38
(corpy)2Ir(dpm). A 3 neck flask was charged with corpy-H (82.2 mg, 0.25 mmol), IrCl3*xH2O
(30.0 mg, 0.10 mmol) and 24 mL of a 3:1 mixture of 2-ethoxyethanol: water. A condenser was
attached to the flask, the mixture was degassed and then heated to 100
o
C for 16 hrs. The reaction
was cooled to ambient temperature, after which water was added to the mixture and filtered which
afforded an orange brown precipitate (160 mg, 69 % crude yield). The precipitate was not further
purified but added to a new three neck flask (160 mg, 0.09 mmol), and charged with potassium
carbonate (126 mg, 0.9 mmol) and 21 mL of dichloroethane. A condenser was attached to the flask
and degassed, after which 2,2,6,6-tetramethylheptane-3,5-dione (57 µL, 0.27 mmol) and reaction
heated to reflux for 16 hrs. The reaction was cooled to ambient temperature, and 10 mL of deionized
water was added to dissolve excess potassium carbonate. The orange-red solid was vacuum filtered
and washed with 10 mL of methanol, and air dried. Column chromatography on silica gel was
performed on the resultant crude mixture (60% dichloromethane: 40% hexanes methylene chloride)
to give an orange-red emissive solid (55.9 mg, 38%).
1
H NMR (400 MHz, CDCl3, ) 8.91 (d, J =
8.5 Hz, 2H), 8.45 (d, J = 8.9 Hz, 2H), 8.38 (dd, J = 5.7, 1.7 Hz, 2H), 7.96 (ddd, J = 8.3, 7.4, 1.2 Hz,
74
2H), 7.84 (d, J = 8.9 Hz, 2H), 7.73 (d, J = 8.7 Hz, 2H), 7.65 (d, J = 8.7 Hz, 2H), 7.57 (d, J = 8.7
Hz, 2H), 7.40 (d, J = 8.6, Hz, 2H), 7.07 (ddd, J = 6.6, 5.7, 1.0 Hz, 2H), 6.95 (d, J = 8.8 Hz, 2H),
6.31 (d, J = 8.8 Hz, 2H), 5.44 (s, 1H), 1.57 (s, 12H).
Ir(corpy)3. A round bottom flask with was charged with iridium(III) acetylacetonate (50.0 mg,
0.102 mmol), corpy-H, (167 mg, 0.102 mmol) and tridecane (36 µL, 0.102 mmol) A condenser was
attached the the flask and the reaction heated at 240
o
C for 48 hrs. The reaction was then cooled to
ambient temperature and chromatographed on alumina in 4:1 hexanes: ethyl acetate to give a
emissive red solid. (80.0 mg, 70 %).
1
H NMR (400 MHz, CD2Cl2, ) 8.69 (d, J = 8.3 Hz, 1H), 8.21
(d, J = 8.9 Hz, 1H), 7.75 (d, J = 8.9 Hz, 1H), 7.72 (d, J = 1.5 Hz, 2H), 7.63 – 7.56 (m, 2H), 7.40 (d,
J = 8.7 Hz, 1H), 7.34 (dd, J = 5.6, 1.6 Hz, 1H), 7.09 (d, J = 9.0 Hz, 1H), 6.67 (ddd, J = 7.1, 5.6, 1.2
Hz, 1H), 6.60 (d, J = 9.0 Hz, 1H).
3.7.2 Electrochemisty. Cyclic voltammetry and differential pulsed voltammetry were performed
using an VersaSTAT 3 potentiostat. Anhydrous MeCN 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,
39
while electrochemical reversibility was determined using cyclic voltammetry.
3.7.3 NMR Measurements.
1
H NMR spectra were recorded on a Varian-600, Varian-500 and a
Varian 400 NMR spectrometer. Chemical shift data for each signal are reported in ppm and
75
measured in deuterated dichloromethane (CD2Cl2) and deuterated chloroform (CDCl3), Variable
temperature NMR was measured in the range of 216-332 K.
3.7.4 X-ray Crystallography. The single-crystal X-ray diffraction data for the
compound (corpy)2Ir(dpm) was collected on a Bruker SMART APEX DUO three-circle platform
diffractometer with the χ axis fixed at 54.745° and using Mo Kα radiation (λ = 0.710 73 Å)
monochromated by a TRIUMPH curved-crystal monochromator. The crystal was mounted in Cryo-
Loops using Paratone oil. Data was corrected for absorption effects using the multiscan method
(SADABS). The structure was solved by direct methods and refined on F
2
using the Bruker
SHELXTL software package. All non-hydrogen atoms were refined anisotropically.
3.7.5 Photophysical Measurements. Photoluminescence spectra were measured using a
QuantaMaster Photon Technology International phosphorescence/fluorescence spectrofluorometer.
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 analyzer (PMA). UV−vis spectra
were recorded on a Hewlett-Packard 4853 diode array spectrometer. All samples were deaerated
by extensive sparging with N 2.
76
Computational Methods. Molecular models were created and dipole moments determined
using the Jaguar 9.4 (release 15) software package on the Schrodinger Material Science Suite
(v2014-2). The molecular geometries and TD-DFT calculations were performed using a B3LYP
functional and a LACVP** basis set with a Poisson-Boltzmann (PBF) CH2Cl2 solvent dielectric
continuum as implemented in Jaguar.
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80
APTER 4. Development and Characterization of Transport
Materials Containing Corannulene
4.1 Introduction
4.1.1 Hole Transport Materials
Figure 4.1 Chemical structures of common materials used in hole transport layers. Reprinted
from Comprehensive Organometallic Chemistry.
1
When designing a material for use in OLEDs, certain factors must be considered in order to
maximize device performance. One major factor is to ensure that charge is balanced within the
device. This goal can be realized by using multiple organic layers specifically designed to transport
either holes or electrons, as well as using layers designed to trap and recombine these charges. The
different layers are referred to as hole transport layer (HTL) or electron transport layer (ETL) and
emissive layer (EML) respectively.
1-3
Figure 4.1 shows common materials used in HTLs. Many
common hole transport materials found in OLEDs are various derivatives of triarylamines.
2, 4-6
The
81
triarylamine compound is efficient at transporting holes as it is nearly isostructural in both its neutral
and cationic forms.
7
This feature of minimal structural reorganization upon oxidation results in a
low barrier for intermolecular hole transport and efficient charge mobility.
1, 8
The triarylamine
group is also found to have a low dipole moment, allowing for higher carrier mobility than other
polar materials.
8
One of common materials listed in Figure 4.1, NPD, has one of the highest hole
mobilities reported for an amorphous solid and is commonly used in most OLEDs.
9
Other common
materials used in HTLs include fluorene and anthracene derivatives as well as various polymers of
these hole conducting units.
10-12
4.1.2 Electron Transport Materials
Figure 4.2. Chemical structures of common materials used in electron transport layers. Reprinted
from Comprehensive Organometallic Chemistry.
1
82
As shown in Figure 4.2, many common electron transport materials are derivatives of
heterocyclic aromatic compounds.
1
Organic heterocyclic compounds such as phenanthrolines,
oxidiazoles and triazoles have been shown to have high electron mobilites.
12, 13
However, these
compounds not only have high mobilities, but also suffer from poor electron injection from the
cathode.
1
Other compounds used in ETLs are metal coordination complexes such as aluminum(8-
hydroxyquinolate)3, Alq3, which has been used as used as an efficient ETL in many device
architectures.
2, 14
Even with the best ETL materials available, carrier mobility of holes (NPD = 10
-
4
cm
2
V
-1
s
-1
)
8
is an order of magnitude larger than electron mobility (Alq3 = 10
- 5
cm
2
V
-1
s
-1
),
highlighting the need for the design of new ETL materials with improved carrier mobilities.
4.1.3 Ambipolar Transport Materials Figure
Materials for OLEDs have also been designed to carry both holes and electrons; these
materials are termed bipolar or ambipolar materials.
7, 15-19
Ambipolar materials have been looked
into for single layer light-emitting devices as a way of eliminating exciplex formation at layer
interfaces and to simplify device fabrication.
17
Ambipolar materials have also been utilized as
emitters or as host materials for emitters in double heterojunction OLEDs in order to increase the
probability of exciton formation to occur within the emitting layer.
15
It has been argued that
utilization of “unipolar” materials as a host that transports one type of carrier risk a narrow
recombination zone due to the imbalance of “charge carrier transporting character” of these
materials which can lead to increased triplet-polaron and triplet-triplet annihilation.
15
The common
structure of these bipolar materials is to have a donor moiety such as triarylamines, fluorenes, or
carbazoles, and an acceptor moiety such as oxadiazoles, benzothiadiazoles or quinoxalines.
18
83
4.1.4 Electronic Properties of Corannulene
Figure 4.3. Cyclic voltammetric curves of corannulene in0.08M TMAB/DMF solution. Reprinted
from Journal of Physical Chemistry B 2009, 113.
One molecule with interesting electronic properties is corannulene. In addition to the
fluxional behavior of corannulene,
20, 21
previously probed in Chapters 2 and 3, the electrochemical
properties of corannulene make it a promising molecule to incorporate into devices to act as a carrier
transport layer. Shortly after its discovery, corannulene was found to readily undergo two reductions
in solution.
22
The LUMO of corannulene is low lying and is doubly degenerate, which accounts for
the stability of the dianion.
23
Corannulene is able to accept up to four electrons theoretically;
electrochemically only the trianion species of corannulene is observed due to the limitations of the
solvent window and supporting electrolytes as shown in Figure 4.3.
23
The corannulene tetraanion
can be generated and observed through NMR, though coordination with alkali metals such as
lithium is needed to stabilize the highly anionic species.
24, 25
Corannulene has also been shown in
the solid state to pack in a “bowl to bowl” formation. There is interest in the literature to take
84
advantage of these physical π-stacking interactions and the electron accepting properties of
corannulene for useful applications such as nanowires.
26, 27
There has also been interest in
incorporating corannulene into organic electronic applications
28
but with the exception of one
patent,
29
so far within the literature there have been no reports of devices employing corannulene.
Unsubstituted corannulene is not a feasible material for use in vapor deposited OLEDs as its low
molecular weight makes controlling the deposition of the material under vacuum challenging.
30
Herein, Figure 4.4 depicts an organic compound containing corannulene to probe whether
corannulene can be effective at carrier transport in OLEDs. Corannulene phenylene diamine, or
CPD, is designed to be an ambipolar compound to probe whether materials with corannulene can
act as both as hole transport layer or an electron transport layer. The phenyl diamine backbone of
CPD is expected to assist in the transport of holes while the corannulene moiety will assist in
transport of electrons. OLED device were fabricated to discern whether corannulene can
successfully transport charge and be integrated into an ambipolar scaffold.
Figure 4.4. Molecular Structure of Corannulene Phenyl Diamine (CPD).
85
4.2 Synthesis of CPD
The bromo-corannulene precursor was prepared by bromination with iodine monobromide
as previously described in the literature as shown in Scheme 4.1.
31
The corannulene phenyl diamine
material was synthesized through a double Buckwald-Hartwig cross coupling between
bromocorannulene and N,N′-diphenylbenzidine.
32
The bright yellow colored compound was
isolated as a single species that is stable in air as a neat solid and in fluid solution, but only in the
dark as it was observed to slowly decompose upon exposure to UV light. Although CPD is the only
molecule presented in this chapter, previous attempts were made to produce a material that
incorporates corannulene and increases its molecular weight for increasing feasibility in chemical
vapor deposition. These materials were unable to be isolated as a single species as they decomposed
upon sublimation and were unable to dissolve in common organic solvents; therefore, they were
not feasible for device studies. We attribute these qualities to aggregation of the material; since
corannulene efficiently packs due to π-stacking,
33
the resultant intermolecular forces are strong
enough that overcome solvation of the material. CPD presents minimal aggregation due to more
free rotation around the nitrogen-carbon bonds as well as the lone pair on nitrogen, which will
disrupt more efficient packing of the corannulene bowl.
Scheme 4.1
86
4.3 Electrochemical Properties
The redox properties of CPD were measured by cyclic voltammetry and differential pulse
voltammetry in DMF solution with 0.1 M TBAF as described in Figure 4.5 and Table 4.1. All
potential values were referenced to an internal ferrocene couple (Fc/Fc
+
). The oxidative properties
of this complex are similar to other hole transport materials. CPD displays a quasireversible
oxidative wave at E
1/2
= 0.44 V. This oxidation potential represents an improvement on common
hole transport materials such as TCTA (E
1/2
= 0.69 V)
34
and other triaryl amine derivatives but is
still slightly more difficult to oxidize than NPD (E
1/2
= 0.38 V).
34
The oxidation of CPD is assigned
to an orbital on the phenyl diamine backbone of the molecule. This moiety is the same phenyl
diamine moiety as it is for NPD, which has been previously shown to transport holes efficiently and
thus oxidize easily.
8, 9
Corannulene has been shown to be difficult to oxidize, further confirming
that corannulene does not contribute to the oxidation wave observed.
22
Figure 4.5 Cyclic Voltammetry and Differential Pulse Voltammetry of CPD vs Fc/Fc
+
in DMF.
CPD also shows multiple reduction waves that are quasi- and irreversible in acetonitrile
solution. CPD has a quaireversible reduction wave at -2.24 V and an irreversible reduction wave at
87
-3.23 V. Both reduction waves for the material can be attributed to the two corannulene moieties
on CPD due to the low-lying LUMO found on corannulene.
23
CPD has a slightly lower reduction
wave compared to reduction waves of other electron transport materials such as Alq 3
(E
1/2
= - 2.30 V) or BCP (E
1/2
= - 2.53 V).
35
The smaller reduction potential compared to other
transport materials can be due to the intrinsic nature of corannulene as a strong electron acceptor,
likely a result of a stabilized LUMO delocalized over its conjugated π-system.
23, 36
The 1 volt
separation between the first and second reduction waves in CPD is larger than that found in
corannulene. This difference could be explained due the lone pair donation on the diamine into the
corannulene moiety destabilizing the corannulene reduction potentials as the increased electron
density increases the columbic repulsion in the radical anion.
Table 4.1. Redox data for CPD.
a
Compound Eox1 Ered1 Ered2
CPD 0.44 V
b
-2.24 V
b
-3.23 V
c
a
Redox potentials were recorded in 0.1 M TBAF/DMF solution and referenced to an internal
Fc
+
/Fc couple.
b
Quasireversible.
c
Irreversible.
The HOMO and LUMO values for CPD were calculated from the oxidation and reduction
potentials determined from electrochemistry.
34, 35
A energy diagram describing the frontier orbitals
of CPD compared to other common OLED transport materials and corannulene is shown in Figure
4.6. In determining whether CPD would be an effective transport material or act as an ambipolar
material, comparison to other common transport materials is beneficial. CPD has a destabilized
LUMO relative to corannulene
23
due to the phenyl diamine substituent on the corannulene acting
as donor to corannulene, which will raise the LUMO energy. This same phenyl diamine moiety
88
helps improve the HOMO for use as a hole transport layer, as the HOMO no longer resides on the
corannulene moiety and thus is easier to oxidize. CPD has a comparable LUMO to Alq 3 and a
comparable HOMO to NPD suggesting that it has energy levels favorable to function as both an
HTL and ETL within an OLED.
34, 35
Figure 4.6. Comparison of HOMO and LUMO levels of common hole and electron transport
materials with CPD.
23, 34, 35, 37
4.4 Photophysical Properties
Table 4.2. Absorption data for CPD.
a
λmax(nm) (ε, 10
3
M
-1
cm
-1
)
CPD 324 (sh, 42.5), 340 (42.4), 414 (21.0)
a
Absorption spectra recorded in CH2Cl2
The absorption and emission spectra of CPD were recorded at room temperature and 77 K
as well as a 40 nm thin film that was created by high vacuum vapor deposition as shown in Figure
4.7. The absorption data is summarized in Table 4.2 and emission data in Table 4.3. The absorption
spectrum for CPD shows intense bands (λ < 340 nm, ε > 10
4
M
-1
cm
-1
) assigned to π→π* transitions
89
of the corannulene and the phenyl diamine backbone moieties. Another intense band at lower
energy (λ = 414 nm, ≈ 2 x 10
4
M
-1
cm
-1
) is assigned to a charge transfer transition. This transition
is believed to be an n→π* transition from the lone pair on nitrogen to the empty π* on corannulene.
This absorption spectrum differs from the corresponding analogue NPD, substituting naphthalene
for corannulene, which exhibits one intense broad peak at higher energy.
38
300 350 400 450 500 550 600 650 700
0
10
20
30
40
50
60
(mM
-1
cm
-1
)
Wavelength (nm)
0.0
0.2
0.4
0.6
0.8
1.0 2-MeTHF
77 K
Thin Film
PL Intensity (a.u.)
Figure 4.7. Absorption (in CH2Cl2) and emission (in 2-MeTHF and as a neat thin film) spectra of
CPD.
The CPD molecule exhibits broad featureless green emission at room temperature in
2-MeTHF solution. At 77 K, CPD spectrum displays distinct vibronic features with sky blue
fluorescence and yellow-orange phosphorescence. The emission lifetime at 77 K is single
exponential for the emission centered at 462 nm, with = 7.9 ns (consistent with fluorescence) and
= 1.1 s at 585 nm (consistent with phosphorescence). The vibronic features observed for
fluorescence at 77 K suggest that the emission at low temperatures is primarily a π*→π transition
rather than a charge transfer (CT) process observed at room temperature. This behavior compares
favorably with the NPD analogue, which also exhibits broad CT behavior at room temperature and
90
more vibronic features at 77 K.
39
As a neat thin film, broad featureless emission is also observed,
suggesting that at higher temperatures emission occurs from a charge transfer state and at lower
temperatures a mix of charge transfer and π*→π transitions occur.
40
The emission of CPD in the
thin film is slightly broadened and displays a bathochromic shift relative to the CPD solution
emission spectrum, suggesting that the intermolecular packing in a rigid medium causes a red shift
in the emission. The emission of CPD in a thin film compares favorably with the common ETL
material Alq3, which also displays broad featureless green luminescence.
41
The singlet (2.75 eV)
triplet state of CPD (2.12 eV) is red-shifted relative to corannulene (S1 = 3.1 eV, T1 = 2.58 eV)
suggesting that the triplet is widely delocalized along the entire compound.
42
Table 4.3 Photoluminescence (PL) data for CPD
Solution
a
Neat Thin Film
b
298 K
d
k r
d
k nr
f
77 K 298 K τ
d
λ max (nm)
[ ]
c
(ns) 10
6
s
-1
10
6
s
-1
λ 0-0 (nm)
[τ (µs)]
d
λ max (nm)
[ ]
c
(ns)
CPD 518
[0.63]
12 53 ± 3 44 ± 3 462 (S 1) 585 (T 1)
[7.9 ns] [1.1 s]
526
[0.20]
2.4 (61%),
8.0 (39%)
a
Emission spectra recorded in 2-MeTHF.
b
Thickness of film determined to be 38 nm through
ellipsometry.
c
Photoluminescent quantum yield. Error is ± 10%.
d
Error is ± 5%.
e
Derived using
= krτ.
f
Derived using = kr/(kr + knr).
The photoluminescence quantum yield of CPD in solution ( = 0.63) is relatively high
compared to the thin film quantum yield ( = 0.20). This decrease in quantum yield could be
attributed to potential self-quenching behavior as the molecule can favorably pack in such
configurations that would disrupt the charge transfer process and allow for competing non-radiative
decay pathways. The PLQY of CPD in a thin film is comparable to that of Alq 3 ( = 0.25), which
normally acts as the emitting layer in a standard NPD/Alq3 fluorescent OLED.
41
Although the
reduction in quantum efficiency from solution to amorphous thin film is not ideal if CPD was
91
designed for exclusive use as an emitter, its primary design is for carrier transport not the site of
recombination.
4.5 OLED Device Performance
Figure 4.8. a.) Device architecture for selected OLEDs varying the HTL. b.) Electroluminescence
spectra of NPD/BCP (black) and CPD/BCP (red); c.) Current density versus voltage for all
OLEDs; d.) EQE versus current density for all OLEDs.
OLEDs devices were fabricated to determine whether CPD can function within a device as
either an HTL, an ETL, or both. Figure 4.8 shows the device architecture and performance of an
OLED with CPD as an HTL, while Figure 4.9 shows the performance and device architecture with
CPD as an ETL. A summary of relevant parameters measured for all OLED devices is listed in
Table 4.4. All organic layers and aluminum were vacuum deposited onto and indium-tin oxide
coated glass substrate. The thicknesses for all OLEDs were held constant for all HTLs and ETLs.
92
The hole-blocking layer bathocuproine (BCP) was used in the fabrication of certain devices where
the ETL was varied. Although CPD was designed to be an ambipolar material, no devices were
made with CPD acting as both ETL and HTL due to inefficient charge trapping that occurs in single
layer organic OLEDs.
Hole transport properties of CPD were investigated by fabrication of single heterojunction
fluorescent OLEDs with CPD and NPD acting as the various HTLs. BCP was chosen as the ETL
due to Alq3 having a very similar emission profile as CPD, making it difficult to determine where
the recombination zone would form in a CPD/Alq3 device. The thickness of the BCP and HTL
layers was chosen to be 40 nm. The CPD/BCP devices exhibit green electroluminescence when
operated at forward bias, attributed to recombination that occurs in the CPD layer, and match the
emission observed in solution and thin film, suggesting that emission occurs solely from the HTL
and not at the interface. The reference NPD/BCP device displays sky blue emission also suggesting
recombination at the HTL interface, due to the low-lying HOMO of BCP which blocks hole
migration.
43
The J-V curves show higher current densities for the NPD devices than the CPD devices at
a given voltage. This suggests that NPD acts as a more conductive HTL than CPD device, having
a higher current density at a given voltage. The operating voltage for the NPD device (8.57 V at
1000 cd/m
2
) is much higher than that of the CPD device (5.7 Vat 1000 cd/m
2
) further supporting
that CPD can transport holes more efficiently. The CPD device has the same turn-on voltage (2.9 V)
compared to the NPD device, while the EQE plots demonstrate devices employing NPD as the HTL
have slightly higher efficiencies (0.37%) than device with a CPD HTL (0.26%). Since the PL
efficiency for a neat thin film of NPD (Φ = 0.42)
41
is slightly larger than that of CPD (Φ = 0.20),
the higher EQE for the standard device can be attributed the increased PLQY of the emitter.
93
Figure 4.9. a.) Device architecture for selected OLEDs varying the ETL and thickness of BCP.
b.) Electroluminescence spectra of NPD/Alq3 (black), NPD/Alq3/BCP (red), NPD/CPD (green),
NPD/CPD/BCP (blue); c.) Current density versus voltage for all OLEDs; d.) EQE versus current
density for all OLEDs.
Table 4.4. Device Characteristics for Selected OLEDs.
Device Architecture Von Voperational @ 1000
cd/m
2
EQEmax
NPD/BCP 2.9 V 8.6 0.37%
CPD/BCP 2.9 V 5.7 0.26%
NPD/Alq3 2.6 V 5.6 0.45%
NPD/CPD 3.0 V NA 0.05%
NPD/Alq3/BCP 2.8 V 5.1 0.59%
NPD/CPD/BCP 2.6 V 4.3 0.71%
94
Electron transport properties of CPD were investigated by fabrication of single
heterojunction fluorescent OLEDs with CPD and Alq3 acting as the various ETLs. Single
heterojunction OLEDs showed green fluorescence when operated at forward bias, attributed to
recombination that occurs in the ETL layer. The EL emission profile matches that observed in thin
films of both CPD and Alq3, suggesting that recombination occurs exclusively on the ETL and not
at the HTL/ETL interface. The CPD device transports less current than the Alq3 device for a given
voltage while having a much larger turn on voltage (3.0 V for CPD vs. 2.8 V for Alq 3). However,
the EQE of the NPD/CPD device (< 0.05%) is much weaker than that of the standard Alq 3/NPD
device (0.45%). Considering that the quantum yields of both electron transport materials are
approximately equal in a thin film, the device performance suggests that carrier recombination
efficiency is poor in a single heterojunction device of CPD as ETL. Since the HOMO levels of both
NPD and CPD are very close in energy, there is no offset in energy levels to efficiently trap holes
in the ETL. As a result, holes migrate freely through the device resulting in a leakage current,
thereby severely reducing the EQE of the device. Leakage current is not an issue present in a
standard Alq3 device.
To facilitate the trapping of holes within the CPD layer, the hole blocking layer BCP was
added, creating a double heterojunction device in order to help balance charge within the device.
43
The J-V curves of the NPD/CPD/BCP devices show increased current at a given voltage compared
to the NPD/CPD devices. Additionally, a log J vs log V plot would clearly show the space-charge-
limit regime of charge transport at low voltages.
44
For the NPD/CPD curve this region is absent,
suggesting that carriers do not populate the organic dielectric, but instead flow freely in the device,
reducing the carrier recombination efficiency. As a result of the improved hole trapping, the EQE
is dramatically increased in the NPD/CPD/BCP device (0.71%) compared to the OLED with no
95
HBL (<0.05%). The EQE of the double heterostructure CPD device is greater than the EQE of the
of the Alq3 device with the same device architecture (0.59%), suggesting that with the introduction
of a blocking layer the CPD device can outperform Alq3 with a higher carrier recombination
efficiency. The turn-on voltage (1.82 V) and operational voltage for CPD (4.30 V at 1000 cd/m
2
)
are much lower than the Alq3 turn-on voltage (2.11 V) and operational voltage (5.12 V at 1000
cd/m
2
). These improvements suggest that CPD not only can transport more charge at a given
voltage, but can run more efficiently at lower voltages, which is an important factor when
considering device lifetime. It is noteworthy that the addition of a hole injection layer (HATCN)
did not change the J-V or EQE curves, suggesting further addition of a hole-injection layer (HIL)
is not necessary for both Alq3 or CPD devices.
4.6 Conclusion
In summary, the ambipolar molecule corannulene phenyl diamine (CPD) was prepared and
investigated for the electronic properties of its corannulene moieties as well as for utilization as
both an electron- and hole-transport material (ETL and HTL respectively). OLED devices
employing CPD as HTL do not give any major improvements over the common HTL material NPD,
instead showing a slight decrease in current density and EQE. Substitution of CPD in the ETL for
Alq3 without any blocking layers does not produce efficient EL due to inefficient trapping of holes.
The addition of a blocking layer allows for moderate improvements in the operational voltage and
EQE of CPD devices over Alq3 devices
Future work would involve different OLED device architectures utilizing CPD as a host
material for both fluorescent and phosphorescent dopants. The energy levels of CPD would be
suitable for an orange or red dopant fluorophore or a red phosphor (MDQ)2Ir(acac). Due to the
96
spectral limitations of CPD as a host material, future work would also look into incorporating
corannulene into another scaffold for electron transport materials. Potential moieties include
attaching corannulene to fluorene or dibenzothiophene moieties. These materials would act solely
as ETLs, allowing for better charge trapping than CPD and potentially further improving the charge
transport properties of OLEDs.
4.7 Experimental
4.7.1 Synthesis Chemicals were received from commercial sources and used as received. All
procedures were carried out in inert N2 gas atmosphere. Corannulene was prepared as described
previously.
45
Bromo-corannulene was prepared as reported previously.
46
Corannulenyl diphenyl diamine (CPD). A three neck flask was charged was charged with
bromo-corannulene (500 mg, 1.52 mmol), N ,N ′-diphenylbenzidine (243 mg, 0.722 mmol), sodium
tert-butoxide (208 mg, 2.17 mmol), Tris(dibenzylideneacetone)dipalladium(0) (66 mg, 0.072
mmol) and 20 mL of toluene. A condenser was attached and the reaction was thoroughly
degassed, after which tri-tertbutyl phosphine (29 mg, 0.144 mmol) was added. The reaction was
then heated to reflux for 24 hrs and then cooled to ambient temperature. The reaction was then
filtered and washed with toluene and dichloromethane and resulting filtrate was concentrated in
vacuo. The crude mixture was chromatographed in hexanes: dichloromethane (5:1) to give a
bright yellow emissive solid (337 mg, 56 %)
1
H NMR (400 MHz, CD2Cl2, ) 1H NMR (400
MHz, Methylene Chloride-d2) δ 7.88 – 7.79 (m, 4H), 7.77 (d, J = 8.7, Hz, 1H), 7.67 (d, J = 8.9
Hz, 1H), 7.62 (d, J = 8.7 Hz, 1H), 7.57 (d, J = 8.8 Hz, 1H), 7.52 (m, 2H), 7.36 (s, 1H), 7.30 (m,
2H), 7.21 (m, 4H), 7.08 (m, 1H).
97
4.7.2 NMR Measurements.
1
H NMR spectra were recorded on a Varian 400 NMR spectrometer.
Chemical shift data for each signal are reported in ppm and measured in deuterated
dichloromethane (CD2Cl2).
4.7.3 Electrochemisty. Cyclic voltammetry and differential pulsed voltammetry were performed
using an VersaSTAT 3 potentiostat. Anhydrous DMF (Aldrich) 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,
47
while electrochemical reversibility was determined using cyclic voltammetry.
4.7.4 Photophysical Measurements. Photoluminescence spectra were measured using a
QuantaMaster Photon Technology International phosphorescence/fluorescence spectrofluorometer.
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 analyzer (PMA). UV−vis spectra
were recorded on a Hewlett-Packard 4853 diode array spectrometer. Samples for transient
luminescent decay measurements were prepared in 2-MeTHF solution. The samples were deaerated
by extensive sparging with N 2.
4.7.5 Device Fabrication and Characterization. Glass substrates coated with patterned ITO
(width of patterned stripes is 2 mm, thickness = 150 ± 10 nm; sheet resistance = 20 ± 5 Ω cm–2;
98
transmission 84% at 550 nm; courtesy of Thin Film Devices, Inc.) were cleaned with soap and
sonicated with water, acetone, and isopropanol (15 min each). ITO substrates were exposed to
ozone atmosphere (UVOCS T10 × 10/OES) for 10 min immediately before loading into the high-
vacuum chamber. Deposition rates for layers of neat materials: CPD (1 Å /s), NPD (1 Å /s), BCP
(1 Å /s), Alq3 (1 Å /s), LiF (0.1 Å /s), and Al (0.2 nm/s). After organic depositions, masks with 2
mm stripe width were placed on substrates under N2, and 1 nm of LiF and 100 nm of Al electrode
was deposited. Device current-voltage and light-intensity characteristics were measured using a
LabVIEW program with a Keithley 2400 SourceMeter/2000 Multimeter coupled to a Newport
1835-C Optical Meter, equipped with a UV-818 Si photocathode. The electroluminescence spectra
were measured on a PTI QuantaMasterTM model C-60 spectrofluorimeter equipped with a 820
PMT detector and corrected for detector response.
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103
CHAPTER 5. Studies of Structural Effects on Molecular
Orientation of Organometallic Iridium Phosphors in Organic
Light Emitting Diodes
5.1 Introduction
5.1.1 Factors that Influence Outcoupling Efficiency
Equation 1.1
As stated in Chapter 1, without factoring in device performance lifetime or any spectral
characteristics, one significant factor in characterizing the performance of an OLED is the external
quantum efficiency (EQE). The EQE is dependent on numerous factors as shown in Equation 1.1.
It is known that the first three factors in Equation 1.1 can be maximized, depending on the device
architecture and use of an efficient phosphorescent dopant, leaving the outcoupling efficiency, η e,
as the last variable with room for significant improvement.
1-6
The outcoupling efficiency
determines which fractions of generated photons escape the device to the outside world.
5
This
limitation is due to the emitted photon being generated in a region of the OLED with a higher
refractive index than the glass substrate and ambient air.
6
Figure 5.1 shows the different modes that
light is outcoupled to after being produced internally within an OLED. When viewing emission
from the emitter position, the light that escapes the device has an opening angle of about 30° with
respect to the substrate normal. This light that escapes only accounts for approximately 20% of the
total light produced; total internal reflection at the glass-air interface limits higher angles. These
angles where total internal reflection occurs contributes to substrate modes,
1, 2
which can be
accounted for by adding an index-matched lens to the substrate, negating any issues substrate modes
may cause. At even higher angles, the light generated by the emitter will not reach the glass
e r PL EL
104
substrate, but instead is wave-guided within the organic layers and is lost by edge emission or
residual absorption.
2
Furthermore, the emitting species can couple to the field of surface plasmon
polaritons (SPPs) between the interface of the organic material and the metal cathode. Surface
plasmon polaritons are the oscillations of the electron density within the metal cathode that can
effectively couple to the light produced by an OLED.
7
Approximately 50% of the light produced
internally is lost to waveguided and surface plasmon modes.
8, 9
Figure 5.1. A diagram of an OLED showing the various outcoupling pathways of light produced
in a device.
5.1.2. Methods to Engineer Increased Outcoupling Efficiency
The literature contains numerous examples on how to engineer methods to extract
wave-guided and substrate modes. Approaches to extract these modes can be categorized into
methods using planar structures or utilizing scattering methods.
10-13
Such use of planar structures
can be as simple as optimizing layer thickness within the device or by using a substrate with a
higher refractive index than the organic layers.
11, 14
Using such a high index (HI) substrate allows
waveguided modes to go more easily into the HI substrate where light can be extracted more easily,
though such substrates would increase OLED production costs significantly.
1, 14
There have also
been proposals to match the refractive index of the emissive layer with the outside world by
105
bringing down the refractive index to n = 1, although no such material is readily available, though
by even lowering the EML refractive index below the value of the substrate would significantly
boost the EQE.
15
The use of scattering structures has also been shown to increase light extraction from a
device to improve EQE. Use of a microlens array primarily widens the “escape cone” for total
internally reflected light incident at the air–substrate interface, allowing for the extraction of light
at higher angles than the critical angle of the glass substrate.
13
Another option is to use an internal
scattering structure either as scattering particles within the substrate, or close to the emission zone
and cathode to reduce wave-guided modes and SPPs.
12, 16, 17
These scattering particles are photonic
crystals such silica microspheres that can be used as a light propagating medium to increase EQE,
but to be effective they must be distributed close to the emission zone of the OLED and require
elaborate fabrication techniques.
1, 17
Other methods have attempted mechanical roughening of the
glass substrate through a variety of means such as sandblasting, abrasion or wet etching which
improve extraction of substrate modes but does not decrease any light lost to SPPs.
1, 18
A diagram
depicting the use of these scattering structures is found in Figure 5.2.
Figure 5.2. Schematic depiction of different scattering structures to improve outcoupling
efficiency by use of (a) Microlens array, (b) Scattering particles, (c) Index grating. Reprinted from
Phys. Status Solidi A 2013, 1.
1
106
There are also examples in the literature to engineer methods to extract light coupled to
surface plasmon polaritons. One such approach to extract emission from SPPs involve periodic
grating or index coupling.
2
Periodic grating relies on the fact that there is overlap between an SPP
mode and a periodic modulation of the refractive index. This requires that the grating of the film to
be deposited near the metal layer.
1
This periodic grating allows the SPPs to gain extra momentum,
allowing light from SPP’s to be coupled out of the device, but such devices are challenging to
fabricate. Furthermore, this periodic grating creates a pronounced angular dependence on the
perceived color of light from the OLED.
1
5.1.3 Improving Outcoupling by Non-Isotropic Dipole Orientation
Figure 5.3. Simulation of power dissipation for (a) horizontally oriented transition dipole, (b)
vertically oriented transition dipole as a function of ETL thickness. Reprinted from Phys. Status
Solidi A 2013, 1.
1
While previously stated methods to increase outcoupling efficiency all show options to
engineer improvements to outcoupling efficiency, none consider intrinsic improvement of the
emitting layer to increase EQE. Taking into account the largest optical losses of light are due to
SPPs, most of the previously listed engineering methods extract energy from substrate modes and
waveguided modes.
1
The degree of surface plasmon polaritons is dependent on the transition dipole
107
of the emitting species within the OLED. The transition dipole is found to show a relationship
between its orientation within a film and power lost to SPPs as shown in Figure 5.3 for an Alq 3
device.
1, 19, 20
If one was to assume that an OLED substrate lies horizontally, at low ETL thickness,
the power dissipated by an Alq3 OLED where all transition dipoles are oriented perpendicular or
vertically relative to the substrate, almost all the power would be lost due to efficient coupling to
SPPs, significantly reducing outcoupling efficiency.
1, 21, 22
A transition dipole that is oriented
parallel or horizontally relative to the substrate, however, couples more efficiently to other optical
channels other than SPPs, suggesting that if an emitter was to be designed to maximize horizontal
orientation, or alignment, of the transition dipole, the outcoupling efficiency will improve
dramatically.
5.1.4 Measurement of the Degree of Orientation
Figure 5.4. (Left) Experimental setup of angle dependent PL, (right) Sample angle dependent PL
spectra demonstrating relationship between vertical (pz) dipoles and horizontal (px) dipoles.
Reprinted from Phys. Status Solidi A 2013, 1.
1
The degree of alignment of the transition dipole relative to the substrate can be determined
through an angle dependent photoluminescence (PL) measurement. Ellipsometric measurements
will give information pertaining to orientation of the bulk in a thin film, but is not a useful tool
when detecting low concentrations of dopant within a matrix which is needed when determining
108
orientation of phosphorescent dopants.
21, 23, 24
Bruetting devised a method shown in Figure 5.4 to
measure the orientation of phosphorescent dopants through angle dependent PL; a method in which
a doped thin film is placed on a rotation stage with a fused silica half cylinder in order to extract
light at angles greater than the critical angle of the film. The material is then excited with a laser
source and the PL intensity is measured at each angle. The emission is measured with a polarizer
to distinguish the s-polarized and p-polarized light.
21
Simulations are then used to determine both
completely horizontal orientation relative to the substrate of the transition dipole and random, or
isotropic, orientation and the experimental data is fitted accordingly. These simulations are
thickness dependent and material specific, depending on the refractive index of the material being
measured. The transition dipole orientation can be considered as a superposition of px-, py-, and pz-
dipoles. The perfectly horizontal orientation only calculates intensity for the px-dipole while the
isotropic simulation is a linear combination of both px- and pz-dipoles. These simulations then assist
in the fitting of the experimental data. The data is plotted as emission intensity as a function of
angle from the substrate normal, as shown in Figure 5.4.
21, 25
5.1.5 Determination of Molecular Alignment in a Doped Thin Film
From simulations and experimental data of angle dependent PL, a value called the
anisotropy factor, Θ, can be obtained. The anisotropy factor is used as a metric of how aligned an
emitting material is within the thin film. This metric is defined as a ratio of the pz-transition dipoles
observed versus the transition dipoles along the x, y, and z axes as shown in Equation 5.1.
21, 25
It is
important to note that the anisotropy factor is a metric of the orientation of the transition dipole
vector not of the molecule itself. If the anisotropy factor is found to be zero than the net transition
dipole of the emitting species is horizontally aligned relative to the substrate; similarly, if the
anisotropy factor is 1 then the transition dipole is vertically oriented. If the anisotropy factor is 0.33,
109
then there is an equal contribution of the dipole in the x, y, z axes, suggesting that the
alignment is isotropic and there is no preferred orientation. The anisotropy factor is the preferred
metric when determining orientation for dopants in thin films.
25
Equation 5.1
5.1.6 Determination of the Transition Dipole
Figure 5.5. A family of carbazole dyes where the conjugation is increasing. The transition dipole
lies along the long axis of the molecule. As the phenyl chain increases, does the degree of
horizontal orientation.
The anisotropy factor only determines transition dipole alignement, thus mapping where it
lies on a given molecule can help to explain the orientation of the molecule within a thin film. In
organic materials with long rod-like structures, such as carbozole-phenyl derivatives as shown in
Figure 5.5, the transition dipole vector lies along the long axis of the molecule, traversing the phenyl
backbone of the organic and the orientation can be easily discerned from the orientation
measurements.
23
Additionally, the horizontal orientation of the dye within the thin film increases as
the material elongates its phenyl backbone chain. For organometallic complexes such as iridium
and platinum species, the transition dipole vector (TDV) cannot be easily obtained by experimental
means. Instead, calculations can be peformed to map the TDV on the complex, which in conjunction
110
with the anisotropy factor, can be used to determine the orientation of the molecule. These
calculations have been compared to an known experimental TDV value of (ppy)Re(CO)4 showing
consistency between theoretical and experimetnal TDVs.
26
The calculated TDVs for iridium
complexes is found to be in the plane of the Ir-cyclometallate. This can be described as the angle
of the Ir-N bond and the TDV as shown in Figure 5.6 and can be used to help determine orientation
of the complex within a thin film. For a facial homoleptic irdium complex such as Ir(ppy) 3, if the
complex was oriented such that its C3 axis were perpendicular to the substrate, the ideal
metal-nitrogen-TDV angle δ would be 45° to have completely horizontal orientation of the
transition dipole.
27
Figure 5.6. Transition dipole vector (TDV) mapped onto (ppy)Re(CO)4 and an Ir(ppy) fragment.
The angle δ describes the angle of the Metal-Nitrogen-TDV.
5.1.7 Examples of Orientation of Iridium Complexes
There have been multiple studies of molecular alignment in the literature, showing that
numerous iridium complexes commonly used as emitters in OLEDs align as shown in Table 5.1.
4,
24, 25, 28-31
. The commonly reported green emitter Ir(ppy)3 is found to be isotropic. Most iridium
complexes reported to align are heteroleptic diketonate complexes, regardless of the chromophoric
C^N ligand, and are observed to align horizontally.
4, 27
Notably, only two homoleptic iridium
complexes, Ir(chpy)3 and Ir(piq)3 (Figure 5.7), have been reported in the literature to have dipoles
that align horizontally. Surprisingly, these homoleptic complexes also are observed to have a similar
anisotropy factor as the reported heteroleptic diketonate complexes.
28
111
Figure 5.7. 3D representation of Ir(piq)3 (left), and Ir(chpy)3 (right). Reprinted from Nature
Materials 2016, 15.
27
Table 5.1. Comparison of Iridium Emitters and their Molecular Orientation
Emitter Host Anisotropy Factor
(% vertical)
Ir(ppy)3 CBP 33%
Ir(dhfpy)2(acac) NPD 25%
Ir(ppy)2(acac) CBP 23%
Ir(ppy)2(tmd) TCTA/B3PYMPM 22%
Ir(MDQ)2(acac) NPD 24%
Ir(bt)2(acac) BPhen 22%
Ir(mphq)2(acac) NPD/B3PYMPM 23%
Ir(bppo)2(acac) CBP 22%
Ir(chpy)3 NPD 22%
Ir(piq)3 (5.3) NPD 23%
5.1.8 Proposed Mechanisms of Dopant Orientation in Thin Films
One previously reported explanation for molecular orientation was presented by Graf and
coworkers that argued that the permanent dipole moment associated with a molecule correlates
strongly with molecular orientation.
28
Graf argued that iridium complexes with a large permanent
dipole, such as Ir(ppy)3, are found to aggregate in a film due to the strong attractive potential
112
between each individual complex. These aggregates are then less affected by any interaction with
the host, and therefore more likely to randomly orient within the film.
32
Since the other heteroleptic
diketonates are reported to have much lower permanent dipoles than the homoleptic complex
Ir(ppy)3, they are less likely to aggregate and therefore more likely to interact with the host and
align. This argument was disproven when Jurow and coworkers synthesized a heteroleptic
diketonate complex, (bppo)2Ir(acac), with a large dipole.
27
The complex was doped at various levels
as high as 20% and was found to have an unchanged net horizontal alignment, suggesting that
aggregation does not impact orientation.
Figure 5.8. Chemical structures and calculated electrostatic surface potentials for
(bppo)2Ir(acac), (bppo)2Ir(ppy), and (ppy)2Ir(bppo). Reprinted from Nature Materials 2016, 15.
27
Another possible explanation for molecular alignment was proposed by Kim and coworkers,
suggesting that electrostatic dopant-host interactions lead to orientation in a thin film.
30
Kim argued
that there are strong binding energies between the heteroleptic iridium complexes with the host
molecules. These binding energies can be attributed mainly to the electron-deficient nature of the
nitrogen-containing moiety of the dopant that interacts strongly with the electron-rich region of the
113
aromatic host material. These favorable interactions allow for the creation of “binding sites” for the
heteroleptic iridium complexes, leading to supramolecular assembly between the dopant and the
host molecules, and therefore alignment.
30
This explanation, however does not hold up for all
heteroleptic iridium complexes as shown by Jurow and coworkers (Figure 5.8).
27
Jurow
demonstrated the limitations of the electrostatic theory by showing two heteroleptic complexes,
(bppo)2Ir(ppy) and (bppo)2Ir(acac), with similar electrostatic surfaces, yet only (bppo)2Ir(acac) is
observed to align while (bppo)2Ir(ppy) is found to be isotropic.
27
Figure 5.9. Illustration of a vapor deposited film with (bppo)2Ir(acac) as the dopant. The
vaccum/organic interface allows the dopant to rearrange to have a favorable interaction with the
host,
Jurow and coworkers proposed that molecular orientation is due to the deposition process
itself. When the films are fabricated, the vacuum/organic interface present during deposition helps
to induce alignment in heteroleptic diketonate iridium complexes.
27
During deposition of an
amorphous film, there is an inherent asymmetry at the surface of the growing films promotes
alignment.
33
This vacuum/organic interface induces alignment where the dopant will rearrange
114
itself an orient itself in a favorable interaction with the host before it is coated with another layer of
amorphous host material. The diketonate complexes can be thought of as having an aromatic moiety
(the chromophoric C^N ligands) and an aliphatic moiety (the diketonate). The aromatic moieties of
the dopant will interact favorably with the host material such that the diketonate ligand will stick
out of the host material giving a “discrete aliphatic surface ‘patch’” in the film as shown in Figure
5.9. The molecular rearrangement of the dopant in films is known to occur on a timescale similarly
to the timescale needed for the dopants repositioning themselves for alignment.
33
This explanation
is further supported by evidence showing that solution processed doped thin films with diketonate
iridium complexes do not show any observed molecular orientation.
34
Figure 5.10. Space filling models of Ir(ppy)3 and Ir(ppy)2(acac) The colored circles are to
emphasize a chemical deviation from spherical Ir(ppy)3 circle by the addition of the aliphatic
diketonate moiety.
The idea that the aliphatic/aromatic moieties will interact with the host and rearrange to a
favorable orientation during orientation suggests that a possible structural-property relationship
exists between overall structure and alignment. If one was to alter the dopant to increase or decrease
favorable interactions with the host, in theory this should alter the degree of molecular orientation.
As previously stated in Table 5.1, heteroleptic diketonate complexes such as (ppy)2Ir(acac) show
horizontal orientation while Ir(ppy)3 is isotropic. Taking into account the shape of Ir(ppy)3, there is
115
no preferential interaction with the host material that would lead to non-isotropic alignment (Figure
5.10). The Ir(ppy)3 molecule can be considered a relatively spherical complex with no distinct
chemical moieties. If one considers the structural differences between Ir(ppy)3 and (ppy)2Ir(acac)
or the alkyl cyclohexenyl moiety on the homoleptic Ir(chpy)3 complex (Figure 5.7), a noted
“chemical deviation” is apparent in which a chemical modification has been made to the parent
complex Ir(ppy)3 by introducing an aliphatic moiety. Other studies have shown that a “geometric
deviation” from a parent compound can also lead to a change in the anisotropy factor. J. J. Kim
looked at heteroleptic diketonate complexes and observed that if one extended the aromatic ligand
by addition of an extra methyl or phenyl groups on the chromophoric C^N ligand the degree of
orientation increased favorably.
35
Herein Figure 5.11 describes a family of homoleptic phenyl imidazole complexes where the
parent compound, Ir(pim)3, is synthetically modified in order to understand a relationship between
geometric or chemical modification and molecular orientation. The Ir(pim)3 compound was chosen
as the parent compound due to its oblate shape, which is a geometrical deviation from the more
spherical Ir(ppy)3 compound, suggesting that Ir(pim)3 would deposit and orient itself favorably to
allow transition dipole alignment to the substrate. Within the literature, OLEDs with Ir(pim)3 as the
emitter have reported EQEs of over 30%, close to the absolute maximum EQE that can be obtained
without any outcoupling modifications
36, 37
These high EQEs also were obtained with a complex
that was found to have a PLQY of only 0.6 as a doped thin film suggesting that the outcoupling
efficiency must be improved to achieve a high EQE.
37
One possible explanation for this high
performance is that the Ir(pim)3 transition dipole moment is aligned horizontally, improving
outcoupling efficiency of the device and the overall EQE.
116
Figure 5.11 Diagram detailing synthetic modifications to the “parent” complex Ir(pim)3 to
probe molecular orientation. Included are space-filling models of each complex from a top side
view and top view illustrating geometrical changes.
Further synthetic modifications to the parent compound have been chosen to understand
how a “chemical deviation” or a “geometric deviation” could impact molecular orientation. The
chemical deviation was chosen by adding a -CF3 moiety, a highly polar substituent that would not
interact with the aromatic host. The -CF3 group was chosen due the belief that the group would
behave similarly to the diketonates upon deposition, rearranging and reorienting in order to
maximize favorable interactions between the host and the dopant. A geometric deviation was
chosen by adding an additional phenyl moiety to the imidazole to further move away from a
spherical shape. This synthetic modification would maximize host-dopant interactions by orienting
itself along the C3 axis of the complex, increasing the surface area of the dopant aromatic moieties
to interact with the host.
117
Figure 5.12. The angle between the iridium-nitrogen bond and the calculated transition dipole
vector for the iridium complexes.
Additionally, the transition dipole vector has been calculated and mapped onto these iridium
complexes as shown in Figure 5.12. As explained in section 5.1.6, the location of the TDV on a
given complex can be explained as the angle between the Ir-N bond and the TDV. The TDV for all
three complexes is similar for both Ir(pim)3 and Ir(pimp)3, approximately bisecting the carbon atom
at the base of the imidazole ring. Calculations show that the angle between the C3 axis and the
TDV is 84°, close to orthogonal with one another. By this metric, if the complexes are oriented in
a thin film with the C3 orthogonal to the substrate, then the TDV is already close to parallel with
the substrate. This suggests that there should be significant horizontal alignment, and any variation
should be due to the structure of the complex.
5.2 Synthesis of the Complexes
The pim-H, pimF-H and pimp-H ligand precursors were prepared by one pot imidazole
heterocycle formation as described for other imidazole compounds as shown in Scheme 5.1. For
each corresponding ligand, either a different benzaldehyde or aniline was used to introduce either
a chemical or geometric deviation from Ir(pim)3 that would be observed in the final complex. The
118
Ir(pim)3 and Ir(pimF)3 complexes were synthesized by first preparing the cyclometalated
(ligand)2IrCl intermediate, followed by addition of the third ligand under high pressure to displace
the Cl ligand and obtain exclusively the facial iridium isomer as reported in the literature.
38, 39
The
Ir(pimp)3 complex was not synthesized by this route, but instead an alternate method was used
where iridium(III) acetylacetonate was heated in a ligand melt of corpy-H to obtain exclusively the
facial-Ir(pimp)3 isomer. All the compounds were highly emissive yellow solids that are stable in air
as neat solids and stable in most organic fluid solutions.
Scheme 5.1
119
5.3 Electrochemical Properties
Figure 5.13. Cyclic Voltammetry and Differential Pulse Voltammetry in MeCN of a.) Ir(pim)3 in
DcFc/DcFc
+
, Ir(pimF)3 in Fc/Fc
+
, Ir(pimp)3 in DcFc/DcFc
+
.
The redox properties of the iridium complexes were examined by cyclic voltammetry and
differential pulse voltammetry in acetonitrile solution with 0.1 M TBAF as seen in Figure 5.13 and
Table 5.1. The potential values for Ir(pimF)3 were referenced to an internal ferrocene couple
(Fc/Fc
+
) while Ir(pim)3 and Ir(pimp)3 were referenced to an internal decamethylferrocene couple
(DcFc/DcFc
+
) that was converted to reference ferrocene.
40
The oxidative properties of the
complexes are similar to other iridium complexes with cyclometalated ligands. Both Ir(pim) 3 and
Ir(pimp)3 show comparable reversible oxidation waves of 0.07 V and 0.10 V, respectively. These
values compare favorably with other reported tris-cyclometalated phenylimidazole iridium
120
complexes (E
1/2
= 0.10 V- 0.12 V).
41
The Ir(pimF)3 complex shows a reversible oxidation wave
much higher than Ir(pim)3 and Ir(pimp)3 with E
1/2
= 0.40 V. This increased oxidation potential can
be explained by the presence of the -CF3 group which will stabilize the HOMO relative to the other
iridium complexes. Oxidation in all three complexes can be explained by an orbital with mixed
metal aryl character.
42
Table 5.2. Redox data for the Ir complexes.
a
a
Redox potentials were recorded in 0.1 M TBAF/MeCN solution and referenced to a Fc
+
/Fc
couple.
b
Irreversible.
The complexes show differences within their reduction waves in acetonitrile solution. The
Ir(pim)3 complex shows a single irreversible reduction wave (E
1/2
= -3.32 V), while Ir(pimF)3
(E
1/2
= -2.84 V) and Ir(pimp)3 (E
1/2
= -2.86 V) show a reversible reduction wave. For all three
complexes the first reduction can be assigned to the phenyl imidazole ligand even though all three
values differ. Both Ir(pimF)3 and Ir(pimp)3 have reduction waves significantly lower than Ir(pim)3.
For Ir(pimF)3, the presence of the -CF3 moiety stabilizes not only the HOMO but also the LUMO,
resulting in a smaller reduction wave. The Ir(pimp)3 complex does not have a CF3 group to stabilize
the orbital but instead has a second phenyl group on the ligand compared to Ir(pim) 3 allowing for
the anion to delocalize across the mesityl phenyl backbone of the ligand, stabilizing the LUMO. A
Compound Eox1 Ered1 Ered2
Ir(pim)3 (5.1) 0.07 V -3.32 V
b
--
Ir(pimF)3 (5.2) 0.40 V -2.84 V --
Ir(pimp)3 (5.3) 0.10 V -2.86 V -3.10V
121
second, reversible reduction wave is also observed for Ir(pimp)3 which can be attributed to the
addition of the additional phenyl group compared to Ir(pim)3.
5.4 Photophysical Properties
5.4.1 Absorption Spectra
300 350 400 450 500
0
5
10
15
20
(mM
-1
cm
-1
)
Wavelength (nm)
Ir(pim)
3
Ir(pimF)
3
Ir(pimp)
3
Figure 5.14. Absorption spectra of Ir complexes 5.1-5.3.
The absorption and emission spectra of iridium complexes were recorded at room
temperature and 77 K as shown in Figures 5.14 and 5.15. The absorption data is summarized in
Table 5.2 and emission in Table 5.3. The absorption spectra for the complexes shows an intense
band (λ < 360 nm, ε > 10
4
M
-1
cm
-1
) assigned to a ligand centered π→π* transition on the
cyclometalated ligands. Less intense bands at lower energy (λ = 360-430 nm, > 2 x 10
3
M
-1
cm
-1
)
are assigned to allowed metal-to-ligand charge transfer (MLCT) transitions. Much weaker
absorptions ( > 450 nm, < 10
2
M
-1
cm
-1
) are assigned to triplet MLCT transitions that are partially
122
allowed due to spin-orbit coupling with the singlet states by the iridium metal center. Notable is
that each absorption band for Ir(pimF)3 displays a bathochromic shift relative to Ir(pim)3 and
Ir(pimp)3. This behavoir is supported by the electrochemical data as Ir(pimF)3 has the smaller
oxidation-reduction potential gap.
Table 5.3. Absorption data for the Ir complexes 5.1–5.3.
a
Absorption spectra recorded in CH2Cl2.
5.4.2 Emission Spectra
All three complexes display structured luminescence with vibronic features at room
temperature in 2-MeTHF solution, suggesting some
3
LC character in the emission. The Ir(pim)3
and Ir(pimp)3 complexes display sky blue emission while Ir(pimF)3 is red-shifted and exhibits green
emission which can be attributed to the -CF3 moiety stabilizing the frontier orbitals of the complex.
The Ir(pim)3 emission is slightly broader than the other iridium complexes, suggesting that there is
slightly more
3
MLCT character in the Ir(pim)3 excited state. At 77 K, the distinct vibronic features
of the emission sharpen considerably, while the Ir(pim)3 and Ir(pimp)3 spectra exhibit a slight
hypsochromic shift. The emission lifetimes at 77 K and room temperature are single exponential
and fall within the narrow range τ = 1.8-2.5 μs consistent with phosphorescence. The vibronic
structure displayed by these phenylimidazole complexes indicate that emission originates from a
λmax(nm) (ε, 10
3
M
-1
cm
-1
)
Ir(pim)3 (5.1) 304 (10.7), 350 (12.6), 380 (sh, 8.13), 413 (sh, 3.47), 459 (sh, 0.461)
Ir(pimF)3 (5.2) 353 (11.3), 386 (sh, 6.67), 428 (sh, 2.52), 473 (sh, 0.529)
Ir(pimp)3 (5.3) 340 (14.8), 380 (sh, 9.25), 415 (sh, 3.86), 459 (sh, 0.722)
123
triplet state with a large amount of
3
LC character. The emission and lifetime characteristics compare
favorably with other known homoleptic iridium (III) phenyl imidazole complexes reported.
41
Figure 5.15. Emission spectra of: (top) Ir(pim)3; (middle) Ir(pimF)3; (bottom) Ir(pimp)3 in 2-
MeTHF
The photoluminescent quantum yields of the iridium complexes in fluid solution are
exceptionally high and are close to quantitative yield (Φ = 0.91-0.99). The radiative decay rate
0.0
0.2
0.4
0.6
0.8
1.0
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
Room Temp Ir(pim)
3
77 K Ir(pim)
3
PL Intensity (a.u.)
Room Temp Ir(pimF)
3
77 K Ir(pimF)
3
Wavelength (nm)
Room Temp Ir(pimp)
3
77 K Ir(pimp)
3
124
constants (kr = 4.0–5.5 x 10
5
s
-1
) are roughly two orders of magnitude greater than the non-radiative
decay rates (knr = 4.0–11 x 10
3
s
-1
) measured. The quantum yields and radiative rate constants
compare favorably to other phenylimidazole irdium complexes as well as other top phosphors
commonly used in OLED’s.
41, 43
With short decay lifetime’s and very high radiative rates, all three
complexes would be beneficial for use within an OLED.
Table 5.4. Photoluminescence (PL) data for the Ir complexes 5.1–5.3.
PL in Solution
a
298 K
c
k r
d
k nr
e
77 K
λ max (nm)
[ ]
b
(µs) 10
5
s
-1
10
3
s
-1
λ 0-0 (nm)
[τ (µs)]
c
5.1 470
[0.91]
2.0 4.9 ± 0.2 4.9 ± 3 454
[2.7]
5.2 484
[0.99]
2.5
4.0 ± 0.2 4.0 ± 4 474
[3.2]
5.3 472
[0.98]
1.8 5.4 ± 0.2 11 ± 5 458
[2.3]
a
Emission spectra recorded in 2-MeTHF.
b
Photoluminescent quantum yield. Error is ± 10%.
c
Error is ± 5%.
d
Derived using = krτ.
e
Derived using = kr/(kr + knr).
5.5 Angle Dependent Photoluminescence
Angle dependent p-polarized emission of doped films by photoluminescent excitation was
used to determine the net orientation of the transition dipole of the iridium complexes as shown in
Figure 5.16. Films of the iridium complexes doped into mCBP and TCTA were vapor deposited at
10% weight to probe orientation of the transition dipole. All angle dependent data was collected by
the Forrest group at the University of Michigan. As stated previously in Section 5.1.5, the
anisotropy factor, Θ, is defined as the ratio of the vertical components of the transition dipole vector
to the total transition dipole vector. If the transition dipole is isotropic, the anisotropy value will
have a value of Θ = 0.33, while a transition dipole oriented horizontally and parallel to the substrate
will have a value of Θ = 0.
125
Figure 5.16 Polarized emission spectra. a–c, Cross-sections of the measurements and simulations
of the angle-dependent p-polarized photoluminescence emission spectra (considering an emission
in the x–z plane) for films of mCBP doped with Ir(pim)3 (a), Ir(pimF)3 (b) and Ir(pimp)3 (c). The
measured data have been fitted (black and red lines) to determine the degree of orientation.
Ir(pim)3 Θ = 0.25; Ir(pimF)3 Θ = 0.22; and Ir(pimp)3 Θ = 0.16.
Emission was observed to be non-isotropic for all doped thin films. Films doped with
Ir(pim)3 exhibit Θ = 0.25, which is similar to the values of other reported homoleptic iridium
complexes shown to align as well as heteroleptic diketonate iridium complexes.
4, 28
The anisotropy
factor of Ir(pimF)3 was shown to slightly decrease (Θ = 0.22) relative to Ir(pim)3, while the
anisotropy factor of Ir(pimp)3 (Θ = 0.16) showed a significant decrease from the parent Ir(pim)3
complex. This observed anisotropy factor is believed to be the lowest reported value for any
homoleptic iridium complex,
28
as well as one of the lowest anisotropy factors of any reported
iridium complex.
35
The complexes displayed almost identical anisotropy factor values in both
TCTA and mCBP, suggesting that any cause of orientation is matrix independent and depends on
the nature of the dopant.
As described in Figure 5.11, synthetic modifications were attempted to control the degree
of molecular orientation. As seen from angle dependent PL results, the distortion of the spherical
geometry of Ir(ppy)3 to the more oblate shape of Ir(pim)3 can help to preferentially orient the
126
molecule in the thin film during deposition. The observation of horizontal orientation of the
transition dipole also corroborates the findings by Kido, who reports OLEDs with EQEs above
30%.
36, 37
By distorting the shape of the complex, the dopant host interactions between aromatic
moieties are maximized by the Ir(pim)3 complex, with the molecule rearranging such that the C3
axis of the molecule is oriented perpendicular to the substrate. For a more spherical complex such
as Ir(ppy)3, the aromatic interactions between host and dopant is the same such that there would be
no preferential reorientation of the molecule immediately after deposition.
Addition of the -CF3 moiety to the Ir(pim)3 complex was used to introduce a chemical
deviation from the Ir(pim)3 complex as shown in Figure 5.11. Similar to how the diketonate
complexes rearrange to maximize aromatic moiety interactions, the polar -CF3 moiety should not
interact favorably with the aromatic host as the complex is expected to rearrange such that the -CF3
groups would face outward towards the vacuum during deposition causing the C 3 axis to be
perpendicular to the substrate. The angle dependent PL data shows that for this class of molecules
addition of the -CF3 moiety has a marginal effect on the anisotropy factor relative to Ir(pim) 3
suggesting that for this family of phenyl imidazole complexes, any chemical deviations would not
greatly improve upon the molecular orientation.
Introduction of an additional phenyl moiety to the periphery of Ir(pim)3 produces a
significant decrease in the anisotropy factor. By increasing the length of the xylyl phenyl backbone,
this geometric deviation further distorts the geometry of the oblate Ir(pim) 3 complex to an almost
propeller-like shape where 3 rod-like structures (the ligand moieties) are connected by the iridium
metal center. The elongated shape in Ir(pim)3 and further exaggerated in Ir(pimp)3 allows for more
favorable interactions between the host and ligand during deposition to help orient the C3 axis
perpendicular to the substrate, attributed to increased surface area. As shown in Figure 5.5, for rod-
127
like organic dyes, the horizontal orientation increases as the molecule becomes more elongated;
this theory also can be applied to iridium dopants in thins films as well.
23
The findings suggest that
any improvements in the anisotropy factor can be attributed to geometric deviation from spherical
to rod-like. A diagram detailing how geometrical and chemical deviations impact orientation is
summarized in Figure 5.17.
Figure 5.17. Diagram of a thin film during deposition illustrating how a.) Ir(pim)3, b.) Ir(pimF)3
and c.) Ir(pimp)3 will orient at the vacuum/organic interface.
5.6 OLED Device Performance
OLEDs devices were fabricated to determine whether the horizontal orientation observed
would impact the outcoupling efficiency of a device, improving the EQE. All device fabrication
and testing was done by the Forrest group at the University of Michigan. Figure 5.18 shows the
device architecture and performance of each iridium complex. All thickness and doping
128
concentrations were held constant. The device architecture remained constant with the dopant
varied to allow for a direct comparison of the EQE between emitters. The device architecture chosen
is similar to the device stack Kido reported in the initial Ir(pim)3 paper in order to as closely
reproduce the original work represented, with BP4mPy replacing B3PyPB as the ETL being the
only difference.
36
Figure 5.18. a.) Device architecture for all OLEDs; b.) Electroluminescence spectra of Ir(pim)3
(red), Ir(pimp)3 (blue), and Ir(pimF)3; c.) EQE versus current density for all iridium complexes;
d.) Current density versus voltage for all iridium complexes. The CIE coordinates for all OLEDs:
Ir(pim)3 (0.23, 0.53); Ir(pimp)3, (0.22, 0.53); Ir(pimF)3, (0.25, 0.58).
The EL spectra for all iridium complexes display structured luminescence with distinct
vibronic features, similar to the PL spectrum for the complexes. The spectra show no host emission
suggesting that emission occurs exclusively from the iridium dopants. The current density-voltage
curves is slightly lower for Ir(pim)3 compared to Ir(pimp)3 suggesting that the Ir(pim)3 device has
129
more traps thereby increasing the current density. The Ir(pimF)3 device has double diode
characteristic of the current density-voltage curve, which could be attributed to a non-uniform ITO-
organic interface.
The maximum EQEs for Ir(pim)3, Ir(pimF)3, and Ir(pimp)3 were found to be 25.5%, 24.8%,
and 30.5% respectively. The EQE for Ir(pimF)3 compares favorably with Ir(pim)3 and supports the
angle dependent PL data that both Ir(pim)3 and Ir(pimF)3 align to a similar degree. The device EQE
for the Ir(pimp)3 complex represents a large increase from Ir(pim)3 suggesting that the increased
horizontal orientation of Ir(pimp)3 allows for less coupling to SPPs, thereby improving the
outcoupling efficiency. The EQE of Ir(pimp)3 (30.5%) represents one of the highest device
efficiencies for a reported blue OLED.
36
The EQEs of all tested devices do not match the devices
fabricated by Kido, though is impossible to directly compare without mimicking the same
deposition conditions and data workup procedures as even similar datasets can lead to huge
differences.
44
Regardless, the device data shows a clear relationship between increasing horizontal
orientation and device efficiency.
5.7 Conclusion
In summary, a family of phenyl imidazole iridium complexes was synthesized modifying
the ligand in order to discern a structure-property relationship between ligand modification of
phosphorescent iridium complexes and horizontal orientation in a thin film. Chemical deviation
from Ir(pim)3 by introducing a -CF3 moiety on Ir(pimF)3 is shown to have a marginal impact on the
orientation of the transition dipole and on device EQE. Geometric distortion of the shape of an
iridium complex from spherical Ir(ppy)3 (Θ = 0.33), to oblate Ir(pim)3 (Θ = 0.25), to pseudo-rod-like
geometry Ir(pimp)3 (Θ = 0.16), allows for the molecule to rearrange during deposition in order to
130
maximize interaction between the aromatic moieties of the ligands and the host molecules,
decreasing the anisotropy factor and increasing horizontal orientation. This behavior is also
exhibited in device EQEs with Ir(pimp)3 (30.5%) being more efficient than Ir(pim)3 (25.5%). The
anisotropy factor and device EQE for Ir(pimp)3 is believed to be one of the highest reported values
for a blue phosphorescent emitter.
Future work could entail further modifications to the Ir(pim)3 structure scaffold such as the
extension or the removal of any external moieties to the phenyl imidazole to help quantify if the
addition or removal of a certain aromatic moiety will lead to a certain increase or decrease of the
anisotropy factor. By further distorting the complex to be more “rod-like”, one could determine if
the anisotropy factor will drop further or is there a certain minimum while the value levels off. One
other option would be to modify the ligand by addition of fluorine or nitrogen on the cyclometalated
phenyl ring. This would not only help to lower the HOMO of the complex allowing for a
hypsochromic shift, but would also shift the TDV potentially allowing for more horizontal
orientation, further improving the outcoupling efficiency.
5.8 Experimental
5.8.1 Synthesis. Chemicals were received from commercial sources and used as received. All
complexation procedures were carried out in inert N2 gas atmosphere despite the air stability of the
complexes, the main concern being the oxidative and thermal stability of intermediates at the high
temperatures of the reactions. Both the [(pim)2IrCl]2 and [(pimF)2IrCl]2 dimers were synthesized
by the Nonoyama method which involves heating IrCl3·H2O to 110 °C with 2−2.5 equivalents of
ppz-H in a 3:1 mixture of 2-ethoxyethanol and deionized water.
38
All imidazole ligands were
synthesized from a one pot procedure with differing anilines or benzaldehydes.
131
1-mesityl-2-phenyl-1H-imidazole (pim-H). A three neck flask was charged with 2,4,6-
trimethylaniline (10.0 g, 74 mmol), glyoxal (10.73 g, 74.0 mmol) and 125 mL of methanol. The
reaction mixture stirred at room temperature for 20 hours, upon which benzaldehyde (7.85 g, 74
mmol) and ammonium chloride (3.96 g, 74 mmol). A condenser was attached and the reaction was
heated to reflux. Phosphoric acid (724 mg, 7.4 mmol) was added after one hour and the reaction
was left to reflux for an additional 24 hrs. The reaction was cooled to ambient temperature and
concentrated in vacuo to remove the methanol solvent. The crude mixture was diluted with ethyl
acetate and treated with 1 M aqueous sodium hydroxide solution. The layers were then extracted
and separated with water three times, and the resultant organic layer was then washed with brine,
dried with sodium sulfate and concentrated in vacuo. The crude mixture was further purified by
column chromatography (5:1 hexanes: ethyl acetate) to yield a pale yellow solid (1.67 g, 8.6%).
1
H
NMR (400 MHz, CDCl3 δ) 7.41 – 7.37 (m, 2H), 7.29 (d, J = 1.2 Hz, 1H), 7.22 – 7.17 (m, 3H), 6.94
(d, J = 0.7 Hz, 2H), 6.86 (d, J = 1.2 Hz, 1H), 2.33 (s, 3H), 1.90 (d, J = 0.6 Hz, 6H).
1-mesityl-2-(4-(trifluoromethyl)phenyl)-1H-imidazole (pimF-H). A three neck flask was
charged with 2,4,6-trimethylaniline (7.00 g, 51.8 mmol), glyoxal (7.51 g, 51.8 mmol) and 125 mL
of methanol. The reaction mixture stirred at room temperature for 20 hours, upon which
4-(trifluoromethyl)benzaldehyde (9.01 g, 51.8 mmol) and ammonium chloride (2.77 g, 51.8 mmol).
A condenser was attached and the reaction was heated to reflux. Phosphoric acid (507 mg, 5.18
mmol) was added after one hour and the reaction was left to reflux for an additional 24 hrs. The
reaction was cooled to ambient temperature and concentrated in vacuo to remove the methanol
solvent. The crude mixture was diluted with ethyl acetate and treated with 1 M aqueous sodium
hydroxide solution. The layers were then extracted and separated with water three times, and the
resultant organic layer was then washed with brine, dried with sodium sulfate and concentrated in
132
vacuo. The crude mixture was further purified by column chromatography (5:1 hexanes: ethyl
acetate) to yield a pale yellow solid (1.56 g, 9.3%).
1
H NMR (400 MHz, Acetone-d6 δ) 7.59 (m,
5H), 7.29 (d, J = 1.2 Hz, 1H), 7.16 (d, J = 1.2 Hz, 1H), 7.06 (m, J = 0.7 Hz, 2H), 2.34 (s, 3H), 1.89
(s, 6H).
1-(3,5-dimethyl-[1,1'-biphenyl]-4-yl)-2-phenyl-1H-imidazole (pimp-H). A three neck flask was
charged with 3,5-dimethyl-[1,1'-biphenyl]-4-amine (5.00 g, 25.3 mmol), glyoxal (3.68 g, 25.3
mmol) and 100 mL of methanol. The reaction mixture stirred at room temperature for 20 hours,
upon which benzaldehyde (2.69 g, 25.3 mmol) and ammonium chloride (1.36 g, 25.3 mmol). A
condenser was attached and the reaction was heated to reflux. Phosphoric acid (248 mg, 2.53 mmol)
was added after one hour and the reaction was left to reflux for an additional 24 hrs. The reaction
was cooled to ambient temperature and concentrated in vacuo to remove the methanol solvent. The
crude mixture was diluted with ethyl acetate and treated with 1 M aqueous sodium hydroxide
solution. The layers were then extracted and separated with water three times, and the resultant
organic layer was then washed with brine, dried with sodium sulfate and concentrated in vacuo.
The crude mixture was further purified by column chromatography (4:1 hexanes: ethyl acetate) to
yield a pale yellow solid (784 mg, 9.5%).
1
H NMR (400 MHz, Acetone-d6 δ) 7.73 (m, 2H), 7.55
(m, 2H), 7.48 (m, 4H), 7.40 (m, 1H), 7.28 (d, J = 1.2 Hz, 1H), 7.25 (m, 3H), 7.17 (d, J = 1.2 Hz,
1H), 2.03 (m, 6H).
Ir(pim)3. A pressure flask was charged with [(pim)2IrCl]2 dimer (80 mg, 0.053 mmol), pim-H
ligand (70.1 mg, 0.267 mmol) and 7 mL of 50:50 dioxane water. The flask was degassed, sealed
and heated to 130
o
C for six days. The reaction was then cooled to ambient temperature and filtered,
washing the precipitate with water and cold methanol to give a pale yellow emissive solid (43.2
mg, 83 %).
1
H NMR (400 MHz, Acetone-d6 δ) 7.11 (d, J = 7.7 Hz, 2H), 6.99 (d, J = 1.5 Hz, 1H),
133
6.84 (ddd, J = 7.6, 1.3, 0.6 Hz, 1H), 6.76 (d, J = 1.5 Hz, 1H), 6.49 (ddd, J = 7.6, 7.2, 1.4 Hz, 1H),
6.35 (ddd, J = 7.7, 7.2, 1.4 Hz, 1H), 6.21 (ddd, J = 7.8, 1.4, 0.6 Hz, 1H), 2.38 (s, 3H), 2.06 (s, 3H),
1.82 (s, 3H).
Ir(pimF)3. A pressure flask was charged with [(pimF)2IrCl]2 dimer (330 mg, 0.186 mmol), pimF-H
ligand (418 mg, 1.27 mmol) and 23 mL of 50:50 dioxane water. The flask was degassed, sealed and
heated to 130
o
C for six days. The reaction was then cooled to ambient temperature, diluted in ethyl
acetate and extracted with water three times. The organic layers were washed with brine, dried with
sodium sulfate and concentrated in vacuo. Column chromatography on alumina was performed on
the resultant crude mixture (40% methylene chloride) to give an bright yellow emissive solid (65.9
mg, 30 %).
1
H NMR (400 MHz, Acetone-d6 δ) 7.20 – 7.13 (m, 3H), 6.96 (dd, J = 2.0, 0.7 Hz, 1H),
6.87 (d, J = 1.5 Hz, 1H), 6.73 (ddd, J = 8.1, 2.0, 0.8 Hz, 1H), 6.34 (d, J = 8.2 Hz, 1H), 2.40 (s, 3H),
2.11 (s, 3H), 1.77 (s, 3H).
Ir(pimp)3. A round bottom was charged with iridium(III) acetylacetonate (175 mg, 0.357 mmol),
pimp-H ligand (580 mg, 1.79 mmol), and tridecane (135 mg, 0.357 mmol). A condenser was
attached and the reaction was heated to 240
o
C for 48 hours. The reaction was cooled to ambient
temperature and column chromatography on alumina was performed on the resultant crude mixture
(20% methylene chloride) to give a pale yellow emissive solid (104 mg, 25%).
1
H NMR (400 MHz,
Acetone-d6 δ) δ 7.79 (m, 2H), 7.65 (m, 2H), 7.52 (m, 2H), 7.43 (m, 1H), 7.13 (d, J = 1.5 Hz, 1H),
6.90 (d, J = 7.0 Hz, 1H), 6.87 (d, J = 1.5 Hz, 1H), 6.54 (ddd, J = 8.4, 7.3, 1.5 Hz, 1H), 6.40 (ddd, J
= 8.4, 7.1, 1.3 Hz, 1H), 6.34 (dd, J=7.3, 1.5 Hz, 1H), 2.24 (s, 3H), 1.98 (s, 3H).
134
5.8.2 NMR Measurements.
1
H NMR spectra were recorded on a Varian 400 NMR spectrometer.
Chemical shift data for each signal are reported in ppm and measured in deuterated chloroform
(CDCl3), and deuterated acetone ((CD3)2CO).
5.8.3 Electrochemisty. Cyclic voltammetry and differential pulsed voltammetry were performed
using an VersaSTAT 3 potentiostat. Anhydrous MeCN (Aldrich) 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 for Ir(pimF)3,
45
and decamethylferrocene/decametylferrocenium (Cp
*
2Fe/Cp
*
2Fe
+
) redox
couple for Ir(pim)3 and Ir(pimp)3 which then was referenced to ferrocene/ferrocenium in the
literature.
40
Electrochemical reversibility was determined using cyclic voltammetry.
5.8.4 Photophysical Measurements. Photoluminescence spectra were measured using a
QuantaMaster Photon Technology International phosphorescence/fluorescence spectrofluorometer.
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 analyzer (PMA). UV−vis spectra
were recorded on a Hewlett-Packard 4853 diode array spectrometer. The samples were deaerated
by extensive sparging with N 2.
135
5.8.5 DFT Calculations. The ground (S0) and triplet (T1) state geometries of the complexes
reported here were optimized at the B3LYP/LACV3P** level using the Jaguar
46
(v. 9.4 release 15)
program within the Material Science suite
47
developed by Schrödinger, LLC. To compute the
TDMs for phosphorescent (T1 → S0) emission in these molecules, Time-dependent Density
Functional Theory (TDDFT) with the Zero Order Regular Approximation (ZORA) approach
48-50
(SOC-TDDFT) as implemented in Jaguar was utilized. The ZORA Hamiltonian incorporates spin-
orbit coupling (SOC) effects essential to compute TDMs associated with triplet (T1 → S0) emission.
The SOC-TDDFT calculations were performed on the T1 optimized structures using the B3LYP
functional and a mixed basis set utilizing the DYALL-2ZCVP-ZORA-J-Pt-Gen set for the Ir atoms
while the 6-31G** set was used for the rest of the atoms.
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Abstract (if available)
Abstract
Organic light emitting diode (OLED) displays have made considerable headway into commercial display products from cell phones, televisions and wearable products, to use in solid state lighting applications, that there is significant interest in creating materials to improve device performance. Phosphorescent iridium (III) organometallic complexes have demonstrated great success within OLEDs as they not only can have high quantum efficiencies, but also their photophysical properties can be tuned through synthetic modification. To date, significant research has been focused on both understanding the fundamentals of the emission properties of certain classes of iridium complexes, as well as improving the device performance of OLEDs through material design with certain properties. A new class of iridium complexes containing the polycyclic aromatic hydrocarbon corannulene to probe non-radiative decay pathways is presented in this work. Additionally, research looking into device performance improvements of OLEDs is explored through both the development of a carrier transport material incorporating corannulene and the examination of the structure-property relationship between the shape of iridium complexes and molecular orientation. Broad potential for variation in design make these materials potentially useful for a variety of photophysical applications.
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Facendola, John Warren
(author)
Core Title
Studies of molecular orientation using iridium phosphors and integration of corannulene into organic light emitting diodes (OLEDs)
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Publication Date
11/09/2017
Defense Date
09/29/2017
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Tag
corannulene,corannulene complexes,iridium,light outcoupling,molecular orientation,OAI-PMH Harvest,OLEDs,organic light emitting diodes,organometallic complexes,structure-property relationship
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English
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Thompson, Mark (
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), Brutchey, Richard (
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), El-Naggar, Moh (
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facendol@usc.edu,jwfacendola@gmail.com
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Tags
corannulene
corannulene complexes
iridium
light outcoupling
molecular orientation
OLEDs
organic light emitting diodes
organometallic complexes
structure-property relationship