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

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

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Content Ir(III) AND Pt(II) PHOSPHORESCENT EMITTERS IN ORGANIC LIGHT EMITTING
DIODES: FROM MATERIALS DEVELOPMENT TO LIGHT OUT-COUPLING.

by

Batagoda Kankanamalage Thilini Batagoda

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

ii

Dedication

Dedicated to my loving parents B. K. D. Chandrasena, R.M. Bandaramenike and my loving
husband Evan Pathiratne.

iii

Acknowledgements
I would like to acknowledge and express my special appreciation to my advisor Professor
Mark Thompson for giving me the opportunity to be a part of his research group and work
in such a wonderful environment. He has always encouraged me to pursue my own ideas
and has always been there for guidance. Thank you for being a great mentor and sharing
his extensive knowledge.
I am fortunate to have had Prof. Barry Thompson, Prof. Edward Goo, Prof. Surya Prakash
and Prof. Ralf Heiges on my PhD Guidance Committee. Thank you for your time and
valuable feedback. Special thanks to Prof. Peter Djurovich for motivation, encouragement
and for willing to direct your insightful intellect towards my research problems.
I would like to thank my former and present colleagues for all the times we spent together
in the lab bouncing ideas off each other and talking about chemistry. I have been lucky to
work in such a dynamic and stimulating research environment. I would like to convey
special thanks to few individuals Muazzam Idris, Tyler Fleethem, Daniel Sylvinson Sarah
Rodney, and Dr. Sam Chen who has been extremely helpful with all my research work. I
I would like to thank Dr. Jaesang Lee, Jongchan Kim and Prof. Steve Forrest at University
of Michigan for fruitful collaboration.
I must say special thank you to Dr. Ralf Haiges for X-ray crystallography classes and
training. Thank you, Judy, Hom, Micheal Dea and Magnolia Benitez for tactical support.
I am thankful to my family, specially to my parents Mr. B.K.D. Chandrasena, Mrs.
R.M. Bandaramenike, my loving husband Evan Pathiratne and my brother Sajith Batagoda,
without them, none of this would have been possible. I would also like to express my
iv

gratitude to my in-laws Prof. Saman Pathiratne and Prof. Asoka Pathiratne for their
guidance and understanding. I would like to take this opportunity to thank all my friends
in Los Angeles who helped me settle down here and help me during difficult times. Also,
acknowledge my friends at USC, all present and past MET group members with whom I
have had the pleasure to work with during 5 years.
v

Table of Contents
Dedication…………………………………………………………………………………ii
Acknowledgements………………………………………………………………………iii
List of Tables……………………………………………………………………….……vii
List of Figures……………………………………………………………………………vii
Abstract……………………………………………………………………………...…..xiv
CHAPTER 1 - Introduction……..………………………………………………………...1
1.1. Phosphorescent emitters for OLEDs……………………………………………….....1
1.1.1. Excited state of cyclometalated Pt and Ir complexes…………………….……….3
1.1.2. Tuning emission energy of phosphorescent emitters…………………………… 6
1.1.3. Non-radiative processes of Ir and Pt emitters……………………………………12
1.2. Blue emitters for OLEDs……………………………………………......…………..14
1.3. OLEDs general introduction and light out-coupling…………………………………17
References - Chapter 1…..……………………………………………………………….24
CHAPTER 2 - Phosphorescent Ir(III) and Pt(II) cyclometalated complexes with
cyclophane based chelates……………………………………………………………….29
2.1. Introduction………………………………………………………………………….29
2.2. Experimental………………………………………………………………………...31
2.2.1. Equipment……………………………………………………………………...42
2.2.2. DFT calculations……………………………………………………………….43
2.3. Results and Discussion………………………………………….…………………..43
2.3.1. Synthesis of Ir and Pt cyclometalates…………………………………………..43
2.3.2. Crystal structure………………………………………………………………..45
2.3.3. Electrochemistry……………………………………………………………….46
2.3.4. Photophysical properties……………………………………………………….49
2.3.5. DFT calculations……………………………………………………………….55
2.4. Conclusion………………………….…………………………………………… …57
References - Chapter 2……………..…………………………………………………….58
CHAPTER 3 - Highly efficient deep blue emission from Iridium cyclometalates
with N-heterocyclic carbene ligands……………………………………………………..60
3.1. Introduction…………………………………………………………………….......60
3.2. Experimental…………………………………………………………………….....63
3.2.1. Device fabrication and characterization………………………………………..65
vi

3.2.2. Probing the recombination zone……………………………………………….66
3.3. Results and Discussion……………………………………………………………..67
3.3.1. Synthesis……………………………………………………………………….67
3.3.2. Electrochemistry……………………………………………………………….69
3.3.3. Photophysical characterization………………………………………………...70
3.3.4. DFT calculations……………………………………………………………….75
3.3.5. Device design and performances………………………………………………77
3.3.6. Optimizing the emitter for better performance…………………………………81
3.4. Conclusion………………………………………………………………………. ...85
References - Chapter 3…………………..……………………………………………….86
CHAPTER 4 - Molecular degradation in blue OLEDs and higher energy
excited state management using Ir(pmp)3…………………………....…………………..88
4.1. Introduction………………………………………………………………………...88
4.2. Theory……………………………………………………………………………...91
4.2.1. Bimolecular annihilation processes…………………………………………….91
4.2.2. Excited state manager…………………………………………………………..93
4.3. Results and Discussion……………………………………………………………..96
4.4. Conclusion………………………………………………………………………..107
References - Chapter 4………………………………………………………………….109
CHAPTER 5 - Structure-property relationship of heteroleptic Pt phosphors
for understanding the orientation and related light outcoupling in OLEDs…………….110
5.1. Introduction……………………………………………………………………….110
5.2. Results and Discussion……………………………………………………………122
5.2.1. Design strategy…………………………………………………………….... 122
5.2.2. Synthesis……………………………………………………………………..127
5.2.3. Crystal structure analysis…………………………………………………….130
5.2.4. Angle dependent photoluminescent results………………………………….132
5.3. Conclusions…………………………………………………………………….....139
References - Chapter 5………….………………………………………………………140

BIBLIOGRAPHY………………………………………………………………………142

vii

List of Tables
Table 1.1. Emission of heteroleptic Pt complexes………………………………………..11
Table 1.2. Emission of heteroleptic Ir complexes……………………………………….11
Table 2.1: Selected bond lengths and angles of (pCpz)2Ir-C1…………………………....46
Table 2.2. Electrochemical data for Pt and Ir complexes………………………………..48
Table 2.3. Photophysical data for Pt and Ir complexes………………………………….55
Table 3.1. Electrochemical data of complexes and host material………………………..70
Table 3.2. Photophysical properties of fac- and mer-Ir(pmp)3…………………………...74
Table 3.3. Photophysical properties of mer-Ir(pmb-N2)3 in solution…………………….84
Table 3.4. Photophysical properties mer-Ir(pmb-N2)3 in PMMA……………………….84
Table 5.1. Summary of angle dependent PL data for complexes 1-9……………………139

viii

List of Figures
Figure 1.1. Electro-luminescence process for organic and organo-transition
metal emitters……………………………………………………………………………...2
Figure 1.2. Energy level diagram depicted the mixing between
3
LC and
3
MLCT
states in d
6
complexes…………………………………………………………………......4
Figure 1.3. DFT calculated molecular orbital surfaces of (ppy)2Ir(acac)
and (ppy)Pt(acac)………………………………………………………………………….7
Figure 1.4. Series of heteroleptic Ir (C^N)2Ir(L^X) and Pt (C^N)Pt(L^X)
complexes with varying ligand design…………………………………………………….8
Figure 1.5. Change in photoluminescence spectra of Ir complexes with a
varying ligand design………………………………………………………………….......9
Figure 1.6. Jablonski diagram of the deactivation pathways of lowest energy triplet
state by temperature dependent and temperature independent non-radiative pathways….13
Figure 1.7. Methods of blue shifting the emission……………………………………….15
Figure 1.8. Energy level diagram for fac-Ir(ppz)3 and fac-Ir(C^C:)3 depicting
nonradiative states..............................................................................................................17
Figure 1.9. Schematic representation of a basic OLED architecture…………………….18
Figure 1.10. Schematic of different optical channels for an OLED……………………..21
Figure 2.1 Structural parameters of [2.2]-paracyclophane……………………………….30
Figure 2.2. Structures of (pCpy)Pt(acac), (pCpz)Pt(acac), (pCpy)2Ir(acac)
and (pCpz)2Ir(acac)............................................................................................................31
Figure 2.3. Synthesis of pCpzH and pCpyH……………………………………………..32
Figure 2.4. Synthesis of Pt and Ir cyclometalated complexes……………………………34
ix

Figure 2.5.
1
H NMR spectrum of (pCpy)Pt(acac)………………………………………..35
Figure 2.6.
1
H NMR spectrum of (pCpz)Pt(acac)………………………………………..36
Figure 2.7.
1
H NMR spectrum of (ppz)Pt(dpm)…………………………………………37
Figure 2.8.
1
H NMR spectrum of (pCpy)2Ir(acac)……………………………………….39
Figure 2.9.
1
H NMR spectrum of (pCpz)2Ir(acac) - C1 isomer……..…………………….40
Figure 2.10.
1
H NMR spectrum of (pCpz)2Ir(acac) – C2 isomer…………………………41
Figure 2.11. R and S configuration of substitutions on the cyclophane core……………44
Figure 2.12. ORTEP representation of the 
RS
enantiomer of (pCpz)2Ir-C1……………...45
Figure 2.13. CV traces of (pCpz)2Ir-C1, (pCpz)2Ir-C2 and (ppz)2Ir(acac)………………48
Figure 2.14. Photoluminescence spectra of (pCpz)Pt and (ppy)Pt………………………50
Figure 2.15. Photoluminescence spectra of (pCpy)2Ir and (ppy)2Ir……………………...52
Figure 2.16. Photoluminescence spectra of Pt and Ir-pCpz/ppz cyclometalates…………53
Figure 2.17. Density functional theory calculation (DFT) of HOMO, LUMO
and triplet spin density for (pCpz)2Ir(acac)-C1…………………………………………..56
Figure 3.1. Nature of M-C: bond………………………………………………………..61
Figure 3.2. Structure of fac-Ir(pmb)3, fac and mer-Ir(pmp)3…………………………….62
Figure 3.3. Synthesis of fac- and mer-Ir(pmp)3…………………………………………..63
Figure 3.4. Device architecture of Dfac and Dmer devices with fac-Ir(pmp)3
and mer-Ir(pmp)3…………………………………………………………………………66
Figure 3.5. Exciton recombination zone sensing scheme in EML……………………….67
Figure 3.6.
1
H NMR spectrum of fac-Ir(pmp)3 in acetone-d6……………………………68
Figure 3.7.
1
H NMR spectrum of mer-Ir(pmp)3 in acetone-d6…………………………..69

x

Figure 3.8. Absorption spectra of fac- and mer-Ir(pmp)3 with fac-Ir(pmb)3
in solution at room temperature………………………………………………………….71
Figure 3.9. Photoluminescence spectra of fac- and mer-Ir(pmp)3 in Me-THF
at room temperature and 77 K……………………………………………………………72
Figure 3.10. Transient phosphorescent decay of dilute fac- and mer-Ir(pmp)3
in 2-MeTHF obtained at room temperature and 77 K…………………………………….73
Figure 3.11. Frontier MOs and triplet spin densities……………………………………..76
Figure 3.12. Charge transportation and exciton formation mechanism in
uniformly and graded doped EML……………………………………………………….78
Figure 3.13. Triplet density distribution of uniformly doped and
graded doped EMLs in Dmer……………………………………………………………….79
Figure 3.14. Device performance of Dfac and Dmer………………………………………80
Figure 3.15. Structures of proposed Ir-NHC complexes…………………………………81
Figure 3.16. HOMO - LUMO energies of Ir-NHC complexes and triplet energies………82
Figure 3.17. Synthetic scheme for Ir(pmb-N2)3…………………………………………82
Figure 3.18.
1
H NMR spectrum of mer-Ir(pmb-N2)3 in acetone-d6……………………...83
Figure 3.19.
1
H NMR spectrum of fac-Ir(pmb-N2)3 in acetone-d6………………………83
Figure 4.1. Performance of commercially available PHOLEDs ………………………...88
Figure 4.2. Configurational diagram for exciton-exciton annihilation mechanism………91
Figure 4.3. Configurational diagram for exciton-polaron annihilation mechanism……..92
Figure 4.4. Jablonski diagram of the EML containing manager molecule………………94
Figure 4.5. Molecules used in the EML, blue dopant, manager, and host………………..96
Figure 4.6. Identified byproducts from CBP degradation inside OLEDs……………….98
xi

Figure 4.7. Homolytic bond dissociation energies for all the exocyclic bonds of mCBP..99
Figure 4.8 Higher energy singlet (S1-S100) and triplet excited states (T1-T100)
for the molecules used in the analysis……………….......................................................100
Figure 4.9. Energy level diagram of the doped EML along with the energies
of QA and QB…………………………………………………………………………...102
Figure 4.10. Calculated energies of possible fragments and byproducts
after degradation…………………………………...……………………………………103
Figure 4.11. DFT calculated HOMO-LUMO energy levels and triplet energies
for the host, dopant, manager and the respective degradation products…….………….105
Figure 4.12. Device architecture and the energy level diagram for the blue PHOLEDs..106
Figure 5.1. Schematic illustration of an OLED showing different optical channels……111
Figure 5.2. Schematic of light outcoupling enhancement in HI substrate………………112
Figure 5.3. Schematic of external outcoupling structures………………………………113
Figure 5.4. Schematic of internal scattering structure by a periodic grating
placed between the glass substrate and the thin film stack………………………………114
Figure 5.5. Layer design of a top-emitting device with first, second and
the third cavity order…………………………………………………………………….115
Figure 5.6. Simulation of power dissipation for exclusively horizontal or
vertical dipole orientation in a prototypical OLED……………………………………..116
Figure 5.7. Three basic dipoles and corresponding emission patterns…………………..116
Figure 5.8. Experimental set up for angle dependent PL………………………………..118
Figure 5.9. Scale of Ɵ with corresponding orientation of TDVs………………………...119
Figure 5.10. Transition dipole moment measured for (ppy)Re(CO)4...............................119
xii

Figure 5.11. Proposed mechanism for molecular orientation in
(C^N)2Ir(acac) type complexes…………………………………………………………121
Figure 5.12. Schematic of TDV relative to the molecular frame of (ppy)Pt(dpm)……...122
Figure 5.13. Direction of TDV of (dbq)Pt(dpm)………………………………………..123
Figure 5.14. Photoluminescence spectra of thin film with (dbq)Pt(dpm)
and angle dependent PL spectra of same film…………………………………………...124
Figure 5.15. Schematic representation of the chromophoric ligand and the
ancillary ligand of the heteroleptic Pt complexes……………………………………….124
Figure 5.16. Structures of all the Pt heteroleptic complexes (1-9)………………………126
Figure 5.17. Two routes for the synthesis of Pt cyclometalated complexes…….………127
Figure 5.18.
1
H NMR spectrum of (dbx)Pt(Dmes) in CDCl3…………………………...129
Figure 5.19.
1
H NMR spectrum of (ppy)Pt(Dmes) in CDCl3…………………………...130
Figure 5.20. ORTEP drawing of complex (dbx)Pt(dpm)……………………………….131
Figure 5.21. Angle dependent PL spectrum for complexes 1 and 2 and
their corresponding emission spectra…………………………………………………...132
Figure 5.22. Angle dependent PL spectrum for complex 3 and
its corresponding emission spectra……………………………………………………...133
Figure 5.23. Electrostatic distribution map of (dbx)Pt(dpm) and
(dbx)Pt(Dmes)…………………………………………………………………………..134
Figure 5.24. Angle dependent PL spectrum for complexes (ppy)Pt(dpm)
7 and (ppy)Pt(Dmes) 8 and their corresponding emission spectra………………………135
Figure 5.25. Electrostatic distribution map of (ppy)Pt(dpm) and (ppy)Pt(Dmes)………136

xiii

Figure 5.26. Angle dependent PL spectrum for complexes (dbq)Pt(acac) 6
and (dbx)Pt(acac) 2 and their corresponding emission spectra………………………….137
Figure 5.27. Angle dependent PL spectrum for complex 9 and its
corresponding emission spectra………………………………………………………...138

xiv

Abstract
Since the breakthrough by Kodak in 1987, Organic light emitting devices (OLEDs)
have attracted a great deal of attention as a promising technology for future display
applications and solid-state lighting. The lower power dissipation, high brightness, light
weight and high color quality of OLEDs make them ideal for many applications. Despite
intense research efforts during the last decade there are still improvements to be made in
OLED-lifetime and OLED-outcoupling when it comes to commercializing these products.
Among the three primary Red, Green, and Blue (RGB) colors, green and red phosphors
that meet the necessary lifetime requirement has already been well established, but the
design and fabrication of stable blue phosphorescent OLEDs is still an ongoing challenge.
Therefore, my graduate research primarily focuses on developing stable blue
phosphorescent emitters and apply them in efficiently engineered devices.
In addition, OLED light outcoupling has also become an enormous challenge in the
industry. Simple ray-optics allows to estimate the external quantum efficiency of a standard
OLED to 20% of the initially generated light. Therefore 80% of the light generated, will
be trapped inside the device due to waveguides and surface plasmons. To overcome this
problem many approaches have been introduced and controlling the alignment of the
emitting molecules used as dopants in organic light emitting diodes has been identified as
an effective strategy to improve the outcoupling efficiency of OLED devices. In this study,
I will demonstrate my involvement in designing of stable blue OLEDs and structure
property related light outcoupling of these phosphorescent emitters in making stable and
highly efficient OLEDs to be used in future display and lighting technologies.
1

CHAPTER 1 - Introduction
1.1. Phosphorescent Emitters for OLEDs
Organometallic complexes bearing heavy metals like Ir, Pt, Os and Ru find an
increasing interest due to their extensive potential for various photophysical and
photochemical applications. These applications include photocatalysis,
1-4

photoelectrochemistry,
5-7
and optoelectronics.
3, 8-13
The distinction among them highly
governed by the excited state properties of these individual organometallic complexes as
they impose different properties based on the transition metal and the organic ligands
involved in formation of the complex. Among these organometallic complexes specially
Ir(III) and Pt(II) complexes possess many attractive properties that make them useful for
optoelectronic applications including emitters in Organic Light Emitting Diodes
(OLEDs).
11, 14
High luminescent efficiency, microsecond lifetime, chemical stability, and
most importantly tunable ground and excited states include those beneficial properties
which make them suitable candidates for aforementioned optoelectronic applications.
In electroluminescence process, the population of the excited state occurs via
formation of an exciton by recombination of positively charged hole and a negatively
charged electron through Coulomb interaction in the emissive layer of the OLED. Since
both hole and electrons have spins, four different spin combinations including one singlet
configuration and three triplet configurations are possible for this electrically generated
exciton. Therefore, in statistical limits, 25% of the excitons represent singlets and 75%
represent triplets.

2

Figure 1.1. Electro-luminescence process for organic and organo-transition metal emitters
(adapted from ref 15).
The organic molecules can exhibit an efficient and fast decaying of S1→S0 with
lifetimes of the order of nanoseconds(ns). On the other hand, since the probability for the
radiative T1→S0 transition is also very small, the deactivation of the T1 state (75% of
excitons generated) occurs normally non-radiatively at ambient temperature mostly as heat.
In contrast, the organo-transition metal complexes bearing heavy atoms like Ir and Pt
induces this special phenomenon called heavy atom assisted Spin-Orbit Coupling (SOC)
which promotes significant mixing between singlet and triplet states.
15, 16
This physical
phenomena SOC occurring in an atom, is a combined effect of the electronic spin angular
momentum and electronic orbital angular momentum. The typical picture of an atom is a
fixes-nucleus frame of reference, however, in a fixed-electron frame of reference, the
3

positively charged, orbiting nucleus creates a magnetic field, B. The electron spin which
can be viewed as a magnetic dipole, in turn result in a dipole moment, M. When acted upon
by a magnetic field, this magnetic dipole moment experiences a torque acting to align it
with the direction of B. The interaction of the two magnetic fields is termed ’SOC’, and
the resultant possible relative orientations of the spin axis and the orbital angular
momentum axis determine the energy levels of the system.
16
The coulombic nature of the
magnetic field implies a dependence of the SOC factor on the magnitude of the nuclear
charge. This is the origin of SOC and this in-turn facilitates intersystem crossing (ISC)
from lowest energy singlet to lowest energy triplet state and leads to efficient
phosphorescence at room temperature, which make them efficient dopant for OLED
applications.
9, 10, 17
This process is called triplet harvesting in Phosphorescent Organic
Light Emitting Diodes (PHOLEDs). The energy of these singlet (Sn) and triplet (Tn) states
relative to ground state dependent on the various factors including the coordination
geometry, coordination number of the central metal and the ligand field splitting of the
chelating ligands.
9

1.1.1. Excited state of cyclometalated Pt and Ir complexes
With the aid of majority of the studies done on the excited state properties of these
cyclometalated organometallic complexes, it has been established that luminescence from
these complexes originates from a triplet excited state comprised of a triplet ligand centered
(
3
LC) state mixed with a triplet metal-to-ligand charge transfer (
3
MLCT) state that has
enhanced singlet character due to spin-orbit coupling with higher energy
1
MLCT states.
18-
23
Depending on the energy of
3
MLCT and
3
LC state, the extent of energy level mixing and
the predominant character of the lowest energy excited state will vary.
24-26
A schematic
4

representation of the energy level mixing in 4d
6
and 5d
6
complexed shown in Figure 1.2.
.

Figure 1.2. Energy level diagram depicted the mixing between
3
LC and
3
MLCT states in
d
6
complexes.
As the energy of
3
LC state is lower in energy than the
3
MLCT state, the
luminescence that shows will have more vibronic structure characteristic similar to the
phosphorescence from the free ligand. In contrast, emission from complexes where the
3
MLCT state is lowest in energy is typically broad and featureless indicative of more
charge transfer character. Mixing between
3
LC and
3
MLCT states could be treated by
application of the first-order perturbation theory will result in the following formula that
defines the lowest energy excited state for these cyclometalated complexes.

In the above equation, the Ψ T1 is the wave function of the lowest excited state for these
triplet emitters and α is the coefficient that gives an estimate of the degree of MLCT
5

character mixed into the unperturbed
3
LC state.
19, 24
The value α can be approximated with
the following formula:

Term ⟨3LC|CI|3MLCT⟩ is the configuration interaction matrix element,
characterizing the extent of configuration interaction between
3
LC and
3
MLCT states, and
ΔE is the energy difference between the
3
LC and
3
MLCT transitions.
24
The mixing of an
3
MLCT state with enhanced singlet character due to spin-orbit coupling with the higher
lying
1
MLCTstates, into
3
LC state which was principally forbidden now introduces
dramatic outcome on the photophysical properties of the overall complex.
18
The oscillator
strength and radiative decay rate of the
3
LC state of these cyclometalated complexes are
significantly increased when a small amount of
1
MLCT character is mixed into the lowest
triplet state. As a result, a large decrease in phosphorescence lifetime compared to their
free ligand occurs along with a concomitant increase in phosphorescence quantum
efficiency.
23, 24
In addition to the strong spin orbit coupling of these heavy atoms like Ir
and Pt which brings out efficient intersystem crossing to the triplet manifold, the
cyclometalating ligand also plays a major role in the exited state properties of these
complexes. The strong σ-donation from C
-
in a formally anionic cyclometalating ligand
also stabilizes the
3
MLCT state and decreases the energy separation to the
3
LC state. This
leads to a smaller ΔE term in the denominator in mixing coefficient equation, that further
increase the value of α. Consequently, organometallic complexes with ligands
cyclometalated into heavy atoms like Ir or Pt are ideally suited to serve as phosphorescent
dopants in OLEDs.
6

1.1.2. Tuning emission energy of phosphorescent emitters
There are various strategies to tune the emission energy of these organometallic
complexes by wisely changing the ligands around the metal center. These effects are
readily observed in the PL spectra from a series of heteroleptic Pt and Ir (C^N)M(L^X)
type complexes where C^N implies the chromophoric ligand which directly contributes to
the
3
LC energy of the complex and L^X is usually a β-diketonate type ancillary ligand.
8,
11, 14
Most efficient organometallic Ir and Pt phosphores commonly contain the ppy-like
cyclometalation motif where the carbon γ to nitrogen is deprotonated and bound to a metal
ion to form a stable five-membered metalacycle. Since the lowest triplet state of these
organometallic complexes have a considerable
3
LC character, the excited state energy
could be varied over a wide spectral range by employing different cyclometalating ligands.
However most of the structure-property relationship studies of these ligand variations were
initiated by having ppy ligand containing Ir and Pt complexes as the reference.
11, 27
The
Density Functional Theory (DFT) predictions of the frontier molecular orbitals of this
molecule help us to understand photophysical characteristics of these interesting green
emitters. The HOMO and LUMO of the heteroleptic (ppy)2Ir(acac) and (ppy)Pt(acac)are
given in the Figure 1.3. In both the complexes more than half of the HOMO electron
density is located on the central metal atom and the other half is located on the phenyl ring
of the ppy ligand. The LUMO is mainly localized on the pyridine ring of the ppy ligand
and the metal contribution on the LUMO is negligible. The ancillary ligand acac does not
contribute much for either of the Frontier MOs in (ppy)2Ir(acac) while it has a very little
HOMO character in (ppy)Pt(acac) complex. The triplet spin densities of both these
complexes are located on the metal and the cyclometallating ligand while there is no
7

contribution from the ancillary ligand. The extent of metal and ligand character in the
HOMO and LUMO is indicative of the amount of
3
MLCT/
3
LC mixing in the excited state.
Accordingly, (ppy)Pt(acac) with less metal character on the HOMO is expected to have
more ligand centered character in the excited state than that of (ppy)2Ir(acac). This could
be visualized clearly when comparing the emission spectra for these two complexes in
ref 28.
28

Figure 1.3. DFT calculated molecular orbital surfaces of (ppy)2Ir(acac) (left)
and (ppy)Pt(acac) (right)
(ppy)2Ir(acac)
(ppy)Pt(acac)
HOMO
LUMO
8

The emission energy variation upon ligand modification could be clearly seen in
the PL spectra from a series of heteroleptic Ir and Pt complexes developed by our lab
(Figure 1.4).
8, 11, 14, 29-31

Figure 1.4. Series of heteroleptic Ir (C^N)2Ir(L^X) and Pt (C^N)Pt(L^X) complexes with
varying ligand design.

9

The emission wavelengths of the complexes in the Figure 1.4 are summarized in
Table 1.1 and 1.2, and they have either acac, dpm or picolenate as the ancillary ligand.
Both acac and dpm have same effect on the electronics of these heteroleptic complexes
despite their structural variation. When looking at ppy and tpy containing Pt and Ir
complexes, there is no change induce by the alkyl substitution on the chromophoric ligand.

Figure 1.5. Change in photoluminescence spectra of Ir complexes with a varying ligand
design.
32

The strategies to blue shift the emission energy include, the incorporation of an
electronegative atom in the chromophoric ligand. This comprises introducing one or more
fluorine atoms or aza substitution on the ligand skeleton.
33, 34
A single fluoride substituent
in the 6’-position on the (6’Fppy)Pt(dpm) (λem=468 nm) leads to a 12 nm blue shift
10

compared to (ppy)Pt(dpm) (λ em=486 nm). Difluoro substitution induce even more (30 nm)
hypsochromic shift in emission in 4’6’F2ppy system while the 4’5’F2ppy substitution
induce a smaller shift. The electron density in HOMO is centered at the 5’ position of the
phenyl ring and the nodes exist at the 4’ and 6’ positions. Therefore, week π-donation into
this molecular orbital from the 5’-fluoro group raises the HOMO level and offsets the
electron-withdrawing effect from the 4’-fluoro group. Another strategy to blue-shift the
emission is by using a tryphenylene type chromophoric ligand with one or more aza
substitutions. Most recent example for such a complex is (bzp)Pt(pic) which emits sky-
blue (λ em=466 nm) at room temperature similar to (4’6’-F2ppy)Pt(dpm).
35

Similarly, substitution with a strong electron-donating groups such as MeO- group
on the phenyl ring will lower the HOMO energy causing a pronounced bathochromic shift
in emission. Among the strategies to red shifting the emission, extending the π-conjugation
in the chromophoric ligand also of great interest. Well known example for this is
(bzq)2Ir(acac) (λ em=548 nm) which has an extended π-system with a bridging vinyl group,
is redshifted by 32 nm from its ppy analogue (ppy)2Ir(acac) (λ em=516 nm).
8
If the π-system
of the pyridal fragment is enlarged using a 2-quinolyl moiety, e.g., (pq)2Ir(acac)
(λ em=597 nm) the emission will red shift further. A similar redshift occurs when ppy moiety
is replaced with a benzothiophene group (btp)2Ir(acac) (λ em=610 nm) which contains soft,
polarizable atom like sulfur incorporated in a five-membered ring system.
8

11

Table 1.1. Emission of heteroleptic Pt complexes.

Table 1.2. Emission of heteroleptic Ir complexes.

(C^N)Pt(LX) Emission E0-0 (nm)
C^N L^X 77 K 298 K
ppy dpm 477 486
6’fppy dpm 468 476
4’6’F2ppy dpm 458 466
4’5’F2ppy acac 476 484
4’MeOppy dpm 480 490
5’MeOppy dpm 525 -
pq dpm 555 -
btp acac 600 -
(C^N)2Ir(LX) Emission
C^N L^X 298 K
ppy acac 516
tpy acac 512
bzq acac 548
bt acac 557
bt pic 541
pq acac 597
12

1.1.3. Non-radiative processes of the excited state of Ir and Pt phosphorescent
emitters
While fac-Ir(ppy)3 have almost unity PL quantum efficiency, the fac-Ir(ppz)3
remains almost non-emissive at room temperature.
32, 36
The loss of luminance efficiency
could result due to uni-molecular quenching or bi-molecular quenching. Bi-molecular
quenching involve self-quenching or quenching by impurities like oxygen. Self-quenching
could be systematically reduced by performing the measurement in very dilute conditions
(≤10
-5
M) or doping the emitter in to a host matrix in the emissive layer of the OLED.
Quenching from oxygen could be minimized by taking the measurements in a solution that
is vigorously deoxygenated and tightly sealed or encapsulating the OLED. Quenching by
oxygen in more problematic specially in these triplet emitters as they tend to quench at near
diffusion-controlled rates.
1
The rapid quenching is a consequence of deactivation by both
energy and electron transfer process in which the electron transfer is particularly effective
as their high triplet energy and ease of oxidation make then potent reducing agents in the
excited state.
1, 37
The quenching by oxygen is not so problematic for measurements done
in frozen glasses or rigid matrices as the rate of diffusion is substantially low or negligible
in such circumstances. Even though, these bi-molecular quenching processes could give
rise to low quantum yields and potentially avoidable, the temperature dependent and
temperature independent unimolecular deactivation pathways are unique to a given
molecule and should be understood well to develop efficient and robust phosphors for
photophysical applications.
13

Figure 1.6. Jablonski diagram of the deactivation pathways of lowest energy triplet state
by temperature dependent and temperature independent non-radiative pathways.
The decay of the lowest excited state to the ground state of these phosphorescent
emitters could follow one of the three pathways, radiative process kr(T) or one of the two
nonradiative processes knr or knr(T) as in Figure 1.6. The temperature dependence of kr
comes from thermal population of individual triplet sublevels (T1, T II and TIII) of the T1,
and each of which has a unique radiative rate.
23
At higher temperatures, due to rapid
thermalization between the triplet sublevels leads to emission which characteristics of a
single radiative state.
13
This is true only for the complexes with larger zero-field splitting
(zfs). Zero-field splitting could be defined as the energy difference between two states in
the absence of an external magnetic field and the zfs values for the metal complexes are
enhanced (zfs>200 cm
-1
) due to the mixing of singlet and triplet due to SOC effect
compared to organic materials (zfs<1 cm
-1
). Therefore, the thermal equilibration of the
triplet sublevels for most of the cyclometalated Ir and Pt complexes require temperatures
well above 77 K.
36

14

The non-radiative deactivation could be temperature independent and/or
dependent. The temperature independent non-radiative decay could occur through two
different pathways: vibrational coupling to the ground state and/or direct surface crossing
from T1 and S0 states. The vibrational coupling to the ground state is governed by the
energy gap law and the compounds that follow the EGL show a linear decrease in ln(knr)
with increasing energy. The direct surface crossing from T1 and S0 states are not very
common for these Ir and Pt cyclometalates as we can see vibronic feature in the emission
spectra which indicates there is no major structural distortion occurs in the ligand-localized
excited state relative to the ground state. The temperature dependent non-radiative process,
knr, that is typically associate with an activation energy barrier Ea separating the T1 and the
non-radiative state(s) which will be discussed in detail in section 1.2.
36, 38, 39

1.2. Blue emitters for OLEDs
Since their introduction over 15 years ago, the development of a commercially
viable phosphorescence blue emitters has remained insufficient and this in turn limits the
practical use of these PHOLEDs in displays and solid state lighting applications. Before
going in to the drawbacks of these blue emitters it is worthwhile to discuss the strategies
involve in blue shifting the emission of these Ir and Pt organometallic complexes. One can
achieve increase in emission energy either by stabilizing the HOMO or destabilizing the
LUMO relative to one another. Strategies involving stabilizing the HOMO include addition
of electron withdrawing groups to the phenyl ring of the ppy type ligand.
33, 34
The most
common electron withdrawing group used for this purpose is fluoride. This increase in
energy upon fluorine substitution could be clearly seen in fac-Ir(F2ppy)3 in Figure 1.6 and
also (4’6’-F2ppy)Pt(dpm) in Table 1.1. Nevertheless, this F-substitution in the emitter has
15

not given promising results in term of device stability. An alternate approach to alter the
HOMO energy involves careful selection of ancillary ligand which could bring about a
blue shift in emission in heteroleptic (C^N)2Ir(L^X) type complexes.
40, 41
Although this
method could result in a significant increase in energy, it also decreases the radiative rates
relative to non-radiative rates and in turn decline the overall luminance efficiency. The
lowering of radiative rate is due to an increased separation between the
1
MLCT and
3
LC
energies as a result of the decreased HOMO energy.
40

Figure 1.7. Methods of blue shifting the emission. Adapted from ref 35.
Another effective approach to increase the emission energy of these phosphorescent
emitters, involves replacing the pyridyl functionality of ppy type ligands, with moieties
that destabilize the LUMO. Examples of such nitrogen containing five membered
heterocycles include, pyrazoles (pz), imidazoles (imid) and triazoles (triaz). They have
higher
3
LC energies and poor electron affinities than that of pyridines. The triplet energy
of fac-Ir(ppz)3 (E0-0= 414 nm, 77 K) is significantly greater than that of fac-Ir(ppy)3
(E0-0= 494 nm, 77 K).
32
Beside from the greater potential of these five-membered N-
heterocycles to blue shift the emission, they suffer from increased non-radiative
deactivation of the excited state as the energy increases. This effect is evident when
16

comparing the PL behavior of fac-Ir(ppy)3 and fac-Ir(ppz)3. It has been found out that the
PL efficiency of fac-Ir(ppz)3 is strongly temperature dependent as it produces a weak
emission at room temperatures (<230 K), while intense emission occurs at 77 K.
36

Evidently this fluctuation in PL efficiency is due to the thermal population of higher lying
excited states at high temperature (~230 K), which then undergo subsequent non-radiative
decay. Extensive research has been carried out to study the thermally activated decay
processes in these luminescent transition metal complexes, in particular, Ru(II) and Os(II)
tris-diimine complexes which are isoelectronic with cyclometalated Ir(III) complexes.
38,
42
It has been found out the temperature dependent luminescence in these complexes are
characteristic of thermal population to metal centered ligand field states(
3
LF) which could
also describe as
3
d-d states.
43
These states are antibonding in nature and highly
dissociative.
44
As the T
1
energy of these fac-Ir(ppz)3 is really high the energy barrier
between the T
1
and the
3
LF states become accessible at the room temperature. Thus, the
fac-Ir(ppz)3 could decay nonradiatively through population of higher energy
3
LF states. In
addition, theoretical calculations of the potential energy surface of the triplet state show a
preference for the dissociation of the weakest bond of the complex which is the Ir-N bond
upon populating these
3
LF states. Therefore, in order to achieve efficient blue
phosphorescence from cyclometalated ppz-based Ir complexes at room temperature, it is
necessary to retard or eliminate these thermally activated non-radiative processes that
deactivate the excited state. One approach to achieve this is to use ligands that raise the
energy of the non-radiative states, while retaining the high triplet energy of the overall
complex. If the non-radiative or deactivating state is a metal localized ligand field state,
strengthening the metal-ligand bond will raise its energy since that state is comprised of
17

antibonding counterparts to the metal-ligand bonding orbital. This strategy could be
employed to make stable and highly efficient blue phosphors by phenyl-imidazolium
moieties commonly referred to as N-heterocyclic carbenes (NHC) to make Ir
cyclometalated complexes.
32, 45-47
The carbene moiety is a neutral, two electron donor,
which makes the cyclometalated ligand a bidentate monoanionic ligand (C^C:). These
NHCs form very strong bonds with transition metal complexes which will shift the metal-
carbene antibonding orbitals to high energy. This will decrease the tendency of
nonradiative decay through the ligand field states (Figure 1.8).

Figure 1.8. Energy level diagram for fac-Ir(ppz)3 and fac-Ir(C^C:)3 depicting non radiative
states.
32

1.3. OLEDs general introduction and light out-coupling
Organic light emitting devices have attracted a great deal of attention due to their
potential use in display and solid state lighting applications. The lower power dissipation,
18

high brightness, light weight and high color quality of OLEDs make them ideal for many
applications. The basic structure of an OLED consists of a stack two or more organic layers
sandwiched between a transparent anode and a metallic cathode. the organic layers consist
of hole transporting layer (HTL), an emissive layer containing dopant and the host
material (EML), and an electron transporting layer (ETL). As voltage is applied across the
device, molecules at or near the anode/organic interface are oxidized and they are referred
to as holes since they lack and electron in the filled molecular orbitals. At the same time
the molecules at or near the cathode/organic interface are reduced and these reduced
molecules are referred as electrons, as they carry an extra electron. The holes and electrons
which are injected from the anode and the cathode respectively will migrate through the
organic material towards the center of the emissive layer. Subsequent recombination of a
hole and an electron leads to formation of either S1 or T1 state, which ultimately relaxes to
give observed Electroluminescence (EL). With the proper choice of material and proper
engineering, the color and the efficiency of the EL could be maximized in the OLED
device.

Figure 1.9. Schematic representation of a basic OLED architecture.
transparent conductor as anode (ITO)
Glass substrate
Metal cathode
} organic layers ~ 100
nm
(-)
(+)
19

Apart from the spectral characteristics, the most important parameter that describes
the efficiency of the OLED is its external electroluminescence quantum efficiency (EQE).
EQE describes the ratio between the number of emitter photons to the injected charge
carriers. The factors contributing to the EQE of the OLED includes:
48, 49

𝜂 𝐸𝑄𝐸 = 𝛾 𝜂 𝑆 /𝑇 𝑞 eff
𝜂 out

In this equation, γ is the charge carrier balance factor, describing whether or not
equal amounts of electrons and holes are injected and what fraction of them recombines to
form excitons. Under ideal conditions the charge recombination factor will be unity,
meaning none of the holes or electrons injected from the opposite electrodes do not leave
the device without recombination to form an exciton. In small molecular multilayer
OLEDs, the charge carrier balance can be tuned to unity by using suitable transport layers,
additional selective carrier and exciton blocking layers.
50, 51

The next factor is singlet/triplet factor (ηS/T) that describes the probability for the
formation of singlet and triplet excitons. This directed by spin orientation of an electrically
generated exciton. According to this spin statistics, the probability of forming triplet is
three-fold larger than that of forming singlets. Therefore, the ηS/T for conventional OLEDs
with just fluorescent emitters is limited to 25% because they do not have a mechanism to
harvest 75% of the triplets generated inside the emissive layer, but the OLEDs with
phosphorescent emitters can harvest both singlets as well as triplets eventually leading to
100% in ηS/T.
9, 52, 53
The third term, effective radiative quantum efficiency (qeff) of the
emitter dependent on two factors. Specifically, the intrinsic radiative quantum
efficiency (q), and the modification of the radiative decay rate by the optical cavity
20

environment around the emitter. If the intrinsic quantum yield (q) of the emitter measured
in the same medium as it is in the OLED, then qeff and q become comparable. Per definition
q is given by,
𝑞 =
𝑘 r
𝑘 r
+ 𝑘 nr

Where kr is the radiative decay rate of the excited state and the knr is the sum of all
competing non-radiative decay rates. Most of the phosphorescent emitters used in
commercial OLED devices have reached the limit of q=1.
51
. The γ, ηS/T and qeff,
collectively known as the internal quantum efficiency ( ηIQE) of a given device. With the
above said, ηIQE could be brought up towards the theoretical limit of 100% with (i) proper
management of charge injection and transportation, (ii) use of phosphorescent emitters and
(iii) if the non-radiative exciton quenching processes are suppressed.
51
The only factor that
restrict the ηEQE being high as its theoretical value is the out coupling efficiency ( ηout).
The light generated inside the device find it hard to escape to the outside due to
coupling to various optical modes and the main reason for this coupling is the differences
in the refractive indices of different layers of the OLED stack. When viewed from the
emitter position the light escape cone has an opening angle of about 30˚ with respect to the
surface normal and the amount of light contained in it is typically less than 20% of the total
light generated inside.
49
In order to address this point, it is very important to get a clear
understanding what these optical loss channels are (Figure 1.10).

21

Figure 1.10. Schematic of different optical channels for an OLED.
Among these optical loss channels, close to 20-30% light get trapped in substrate
modes. Due to refractive index mismatch between the glass substrate (n ≈ 1.5)/air (n=1.0)
planar interface, light impinging with an angle larger than the critical angle is subject to
total internal reflection. It is then guided in the so called substrate modes, damped due to
the limited reflectivity of the underlying stack, and can escape through the edges of the
device. The light trapped in the substrate modes could be fully extracted if an
index-matched microscopic lenses, prisms or microstructured substrates are used in the
case where active pixel area is not that large. However, if the application requires unique
foam factor to be maintained e.g.: thin and flat, this method is not practical for large area
devices.
Due to the difference in refractive index between the glass substrate and the organic
layers (n ≈ 1.7-2.0), another considerable fraction of light get trapped in waveguided modes
which propagating within the organic layer and the transparent electrode.
++ ̶ ̶ ++ ̶ ̶ ++ ̶ ̶ ++
Normal to
surface/
Emission to air
Partially reflected
Totally internally
reflected/
Substrate modes
Waveguided modes
Surface plasmons
Glass prism to extract
substrate modes
} organic layers and ITO
n ~ 1.7-2.0

Cathode
Air n = 1

Glass substrate n ~ 1.5

22

In addition, emitters located at the vicinity of the metallic cathode can either
dissipate their power through Ohmic losses by dipolar near-field coupling or via the
coupling to surface plasmon polaritons (SPPs). Several approaches have been suggested to
reduce the SPP losses, such by increasing the distance between the EML and the metal
electrode to reduce the interaction between metal surface and the emitting dipoles, or
fabricating a metal electrode free OLEDs
The rigorous computation of the fraction of light confined in the OLED stack under
the form of these three mentioned loss channels is not straightforward since microcavity
effect have to be taken into account. This impacts not only the power injected into the
different loss channels but also the spontaneous exciton decay rate which directly depends
on its optical environment via the Purcell effect.
Developing strategies to improve light coupling of OLEDs have become
challenging specially when considering light extraction from surface plasmons and
waveguided modes. Therefore, methods to extract light without affecting the planer
structure were major concerns over recent years. There are numerous studies on optimizing
the OLED cavity with respect to thickness, refractive indices or reflectivity or to use a non-
metal cathode to reduce SPP. Either way the reduction of SPP losses is mostly at the
expense of enhanced waveguided modes resulting in no gain in overall light out coupling.
Use of internal or external scattering layers, is another approach in addressing this issue.
Waveguided modes could be accessed by placing a scattering layer (random scattering
layer or a periodic grating) in between the glass substrate and the ITO anode. Although
these techniques are in general useful in light outcoupling, they need elaborate fabrication
techniques which ultimately impact the overall cost of the OLED. Therefore, instead of
23

developing tools to extract light from these loss channels, it is worthwhile to reduce
coupling into those loss channels.
Increasing the outcoupling efficiency through controlling the direction of light
emission is an alternative strategy to address this problem. The emitter molecules in these
OLEDs, usually transition metal complexes, emit light perpendicular to their transition
dipole moment vector (TDV).
54, 55
Orienting these emissive dipoles perpendicular to the
substrate would enhance the fraction of light outcoupled and minimize any potential loss
channels. There are few hypothesis have been invoked to account for the disparity in the
alignment process of different dopant molecules in amorphous host matrix. One suggests,
the large dipole moments present in the tris-cyclometalates lead to aggregation that
suppresses dopant interaction with the host matrix.
56-59
Some reports have speculated
electrostatic interaction between electronegative and electropositive regions of the host and
the emitter that give microscopic order and thus align the emitter.
60-62
In our group we have
demonstrated that neither the dopant dipole-based mechanism nor component electrostatic
adequately describe this alignment process but the emitter orientation influenced by the
inherent asymmetry at the surface of the growing films upon thermal deposition.
63
More
experimental proof for this hypothesis will be presented in chapter 5.

24

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52. Segal, M.; Baldo, M. A.; Holmes, R. J.; Forrest, S. R.; Soos, Z. G., Physical Review
B 2003, 68 (7).
53. Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R., Physical Review B
1999, 60 (20), 14422-14428.
54. Yokoyama, D., Journal of Materials Chemistry 2011, 21 (48), 19187-19202.
55. Schmidt, T. D.; Setz, D. S.; Flammich, M.; Frischeisen, J.; Michaelis, D.;
Krummacher, B. C.; Danz, N.; Brutting, W., Applied Physics Letters 2011, 99 (16).
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Physics Letters 2012, 101 (25).
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of Materials Chemistry C 2014, 2 (48), 10298-10304.
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Thompson, M. E., Nature Materials 2016, 15 (1), 85-+.

29

CHAPTER 2 - Phosphorescent Ir(III) and Pt(II) cyclometalated
complexes with cyclophane based chelates
2.1.Introduction
Heavy atom assisted spjn-orbit coupling induced efficient phosphorescence from
neutral Ir(III) and Pt(II) cyclometalated complexes have made them interesting candidates
in many optoelectronic applications during last two decades.
1-3
As described in earlier
chapter, the lowest energy triplet state for these Ir(III) and Pt(II) cyclometalates consist of
an admixture of
3
LC state that is strongly perturbed by higher-lying MLCT states.
2, 4

Therefore, the emission properties depend not only on the metal ion, but also on the
cyclometalating chromophoric (C^N) ligand. Thus, it is desirable to systematically
understand how the modifications in the C^N ligand alter the properties of Ir and Pt
cyclometalates. Even though there are many studies reported on shifting the emission
energy through bonds, there is not much done on shifting the emission energy through
space. Therefore [2.2]-paracyclophane is a good synthon to achieve this task in
chromophoric ligands used for cyclometalation.
Since the isolation of [2.2]-paracyclophane (pCp) by Brown and Farthing in 1949, and
its first synthesis by Cram and coworkers in 1951, attention was drawn to its interesting
chemical and physical properties.
5-7
Even though [2.2]-paracyclophane and its derivatives
are widely used in catalysis and polymer chemistry, applications related to optoelectronics
are still not widely developed.
8-11
The [2.2]-paracyclophane molecule consists of two
eclipsed aryl rings that are held rigidly connected to each other at the para position by
ethylene bridges. The aromatic rings are distorted into a shallow boat-like structures due
30

to the strong steric repulsion with a separation that varies from a minimum of 2.83 Å at the
bridgehead carbons to a maximum of 3.09 Å at the core.
7, 12
Even though the aromatic
systems are not conjugated to each other through bonds, they are coupled via a unique
‘transannular’ through space interaction between the two rings.
12, 13
The distortion of the
core due to high ring strain and transannular interaction between the π-systems makes this
molecule an intriguing target for various chemical and photophysical studies.

Figure 2.1 Structural parameters of [2.2]-paracyclophane
The structure of [2.2]-paracyclophane provides a platform for the design and
synthesis of substituted derivatives at various sites on the molecular frame. Although
substitution at the ethylene bridge is not very common, substitution at the aromatic ring
leads to various species that show planar chirality.
13, 14
Collectively, all these mentioned
properties of [2.2]-paracyclophane enables access to a variety of synthons with broad
tunability of their physical and chemical properties.
15
Indeed, the aforementioned
properties make [2.2]-paracyclophane an interesting molecule to introduce into
31

organometallic Pt and Ir systems so as to investigate the effect of cyclometalation on their
structural and photophysical properties.

Figure 2.2. Structures of (pCpy)Pt(acac), (pCpz)Pt(acac), (pCpy) 2Ir(acac) and
(pCpz)2Ir(acac)
Herein, we have synthesized [2.2]-paracyclophane-substituted analogs of ppy and
ppz ligands (pCpy-H and pCpz-H, respectively) and cyclometalated them to Pt and Ir.
Structures of the resultant complexes are given in Figure 2.2. The characterization of the
heteroleptic complexes with acetylacetonate (acac) ancillary ligands is reported along with
a detailed comparison of the photophysical properties to their ppy and ppz congeners. The
comparative analysis between the four complexes with their ppy and ppz analogs has
allowed us to determine how the cyclophane ring system perturbs the overall properties of
the cyclometalated complexes.
2.2.Experimental
Chemicals from commercial sources were used as received. All procedures were
carried out in inert gas atmosphere despite the air stability of the complexes, the main
concern being the oxidative and thermal stability of intermediates at the high temperatures

32

of the reactions. Both pCpyH and pCpzH were synthesized according to previously
reported procedures.
16, 17

Figure 2.3. Synthesis of pCpzH and pCpyH.
Synthesis of pCpzH- A dry Schlenk flask was charged with Pd2(dba)3 (230 mg, 0.25
mmol, 0.025 eq), t-BuX-Phos (420 mg, 1 mmol, 0.1 eq), 4-Bromo[2.2]paracyclophane
(2.87 g, 10 mmol, 1.0 eq), pyrazole (89 mg, 13.0 mmol, 1.3 eq) and NaO-t-Bu (1.44 g,
15.0 mmol, 1.5 eq). The flask was evacuated and backfilled three times with Nitrogen and
40 mL of degassed dry toluene was added. The solution was heated to 80 ˚C for 7 hrs.
When the reaction reached completion, filtered over Celite and evaporated under reduced
pressure. A white product was Isolated after flash chromatography with 19:1 Hexane:
Ethylacetate. Yield 2.53 g (9.10 mmol, 91%).
Synthesis of pCpyH
Step 1: Synthesis of 2-([2.2]paracyclophan-4-yl)pyridine N-oxide: A Schlenk flask with
a condenser attached to it was charged with 4-bromo[2.2]paracyclophane (1.50 g 5,23
mmol, 1 eq), pyridine N-oxide (1.99 g, 20.91 mmol, 4.0 eq), K2CO3 (1.44 g, 10.45 mmol,

33

2.0 eq), t-Bu3P.HBF4 (0.23 g, 0.78 mmol, 0.15 eq) and Pd(OAc)2 (60 mg, 0.26 mmol, 0.05
eq). Purged backfilled with Nitrogen three times and cannula transferred degassed toluene
(17.4 mL). Suspension was refluxed overnight and once the reaction cooled down to room
temperature, filtered through Celite and subjected to flash chromatography with
CH2Cl2:MeOH (98:2-95:5-90:10). The resulted product was washed with Et2O to remove
remaining traces of pyridine N-oxide to give 2-([2.2]paracyclophan-4-yl)pyridine N-oxide
as an off-white solid 0.705 mg, 51%
Step 2: Synthesis of 2-([2.2]paracyclophan-4-yl)pyridine: To a suspension of
2-([2.2]paracyclophan-4-yl)pyridine N-oxide (0.94 g, 3.13 mmol, 1.0 eq) in CH2Cl2 (31.3
ml) at room temperature was added Cl3SiH (4.74 ml, 46.89 mmol, 15.0 eq) and Et3N (4.35
ml, 31.26 mmol, 10.0 eq) sequentially. The suspension was heated to reflux for 3 hours
then cooled to 0 ˚C and diluted with CH2Cl2 (20 ml). NaOH (aq) (1M; 20 ml) was added
cautiously. Repeated this step once more and stirred at room temperature until
effervescence has ceased and most solid had dissolved (approx 2-3 hrs). Extracted with
CH2Cl2 and the organic layer was further dried with MgSO4. Further purified by flash
chromatography with Ethylacetate: Hexanes (3:2) followed by recrystallization from hot
CH2Cl2 to give 2-([2.2]paracyclophan-4-yl)pyridine as a white crystalline solid (142 mg,
16%).
Metal complexes were synthesized initially by reacting respective metal chlorides
with cyclometalating ligand (C^N) under Nonoyama conditions, followed by treating the
resultant intermediate with the ancillary ligand (L^X) acetylacetone and a base in refluxing
solvent.
18, 19
Platinum complexes could also be formed with the one pot synthesis starting
with [PtMe2(SMe2)]2 instead of two step synthesis with the K2PtCl4 precursor.
20
Drawbacks
34

of this method includes the selectivity of substrates and the challenging synthesis of the
starting precursor [PtMe2(SMe2)]2.
21

Figure 2.4. Synthesis of Pt and Ir cyclometalated complexes.
Synthesis of (pCpy)Pt(acac). A 3-neck flask was charged with pCpyH (200 mg, 0.70
mmol), potassium tetrachloroplatinate(II) (116 mg, 0.28 mmol) and 15 mL of 3:1 mixture
of 2-ethoxyethanol:water. All the reactants were degassed and heated to 70 ºC for 16 hrs.
The reaction was cooled to ambient temperature and the orange-yellow solid was
precipitated into water and isolated by vacuum filtration. This solid was then placed in a
new 3-neck flask charged with potassium carbonate (186 mg, 1.4 mmol) and charged with
15 mL of degassed 1,2-dichloroethane. The condenser was attached and the reaction was
heated to 75 ºC for 16 hrs after adding 2,4-pentanedione (150 µL, 0.73 mmol). The reaction
was then cooled to ambient temperature and the solvent was removed under vacuum. The
resultant solid was subjected to column chromatography on silica gel 1:1 CH2Cl2: hexanes
to give a bright yellow emissive solid (98 mg, 61%).
1
H NMR (500 MHz, CDCl3, δ) 9.10

35

(d, J = 5.80 Hz, 1H), 7.78 (d, J = 4.05 Hz, 2H), 7.02 (d, J = 8.69 Hz, 2H), 6.58 (q, J = 8.83,
13.3 Hz, 2H), 6.28 (d, J = 7.66, 1H), 6.23 (d, J = 7.67, 1H), 6.01 (d, J = 7.73 Hz, 1H), 5.51
(s, 1H), 4.38 (ddd, J = 2.70, 10.01, 13.05 Hz, 1H), 3.87 (ddd, J = 2.47, 8.76, 14.08 Hz, 1H),
3.21-3.15 (m, 1H), 3.10 (ddd, J = 2.31, 9.72, 12.62 Hz, 1H), 3.05-2.96 (m, 2H), 2.91-2.85
(m, 1H), 2.81-2.75 (m, 1H), 2.00 (d, J = 11.32 6H). Anal. For (pCpy)Pt(acac): found: C
53.97, H 4.37, N 2.41; calcd: C 53.98, H 4.36, N 2.42.

Figure 2.5.
1
H NMR spectrum of (pCpy)Pt(acac).
Synthesis of (pCpz)Pt(acac). A 3-neck flask was charged with pCpzH (500 mg, 1.82
mmol), potassium tetrachloroplatinate (II) (300 mg, 0.78 mmol) and 75 mL of 3:1 mixture
of 2-ethoxyethanol: water. The mixture was degassed and heated to 70 ºC for 16 hrs. The
reaction was cooled to ambient temperature and the pale-yellow solid was precipitated into
water and isolated by vacuum filtration. This solid was then placed in a new 3-neck flask
36

charged with potassium carbonate (410 mg, 2.9 mmol) and charged with 45 mL of
degassed 1,2-dichloroethane. The condenser was attached and the reaction was heated to
75 ºC for 16 hrs after adding 2,4-pentanedione (0.12 mL, 1.16 mmol). The reaction was
then cooled to ambient temperature and the solvent was removed under vacuum. The
resultant solid was subjected to column chromatography on silica gel 1:1 CH2Cl2: hexanes
to give a pale yellow solid (210 mg 47%).
1
H NMR (500 MHz, CDCl3, δ) 8.13 (d, J = 2.63
Hz, 1H), 7.86 (d, J = 2.08 Hz, 1H), 7.13 (d, J = 6.61 Hz, 1H), 6.59-6.57 (m, 2H), 6.45 (d,
J = 6.77 Hz, 1H), 6.27 (dd, J = 7.77, 19.0 Hz, 2H), 5.95 (d, J = 8.94 Hz, 1H), 5.51 (s, 1H),
4.39 (t, J = 10.6 Hz, 1H), 3.61 (q, J = 8.47, 14.5 Hz, 1H), 3.20-3.14 (m, 2H), 3.07 (t, J =
10.4 Hz, 1H), 2.94-2.88 (m, 1H), 2.84-2.71 (m, 2H), 1.98 (d, J = 1.32 Hz, 6H). Anal. For
(pCpz)Pt(acac): found: C 51.2, H 4.29, N 4.85; calcd: C 50.8, H 4.26, N 4.94.

Figure 2.6.
1
H NMR spectrum of (pCpz)Pt(acac).
37

Synthesis of (ppz)Pt(dpm). A 3-neck flask was charged with ppzH (0.18 mL, 1.3 mmol),
potassium tetrachloroplatinate(II) (230 mg, 0.55 mmol) and 60 mL of 3:1 mixture of 2-
ethoxyethanol:water. The mixture was degassed and heated to 70 ºC for 16 hrs. The
reaction was cooled to ambient temperature and the pale-yellow solid was precipitated into
water and isolated by vacuum filtration. This solid was then placed in a new 3-neck flask
charged with potassium carbonate (180 mg, 1.3 mmol) and charged with 30 mL of
degassed 1,2-dichloroethane. The condenser was attached and the mixture was heated to
75 ºC for 16 hrs after adding 2,2,6,6-tetramethyl-3,5-heptanedione (0.21 mL, 1.00 mmol).
The reaction was then cooled to ambient temperature and the solvent was removed under
vacuum. The resultant solid was subjected to column chromatography on silica gel 1:1
CH2Cl2: hexanes gradient to give a pale yellow solid 145 mg (51%)
1
H NMR (500 MHz,
CDCl3, δ) 7.93 (d, J = 2.85 Hz, 1H), 7.75 (d, J = 2.25 Hz, 1H), 7.60 (dd, J = 2.92, 6.68 Hz,
1H), 7.12-7.08 (m, 3H), 6.52 (t, J = 4.29 Hz, 1H), 5.81 (s, 1H), 1.26 (d, J = 8.74 Hz, 9H).
Anal. For (ppz)Pt(dpm): found: C 46.05, H 5.05, N 5.35; calcd: C 46.06, H 5.03, N 5.37.

Figure 2.7.
1
H NMR spectrum of (ppz)Pt(dpm).
38

Synthesis of (pCpy)2Ir(acac). A 3-neck flask was charged with pCpyH (200 mg, 0.70
mmol), IrCl3.H2O (116 mg, 0.28 mmol) and 15 mL of 3:1 mixture of 2-
ethoxyethanol:water. The mixture was degassed and heated to 110 ºC for 16 hrs. The
reaction was cooled to ambient temperature and the orange-yellow intermediate was
precipitated into water and isolated by vacuum filtration. This solid was then placed in a
new 3-neck flask charged with potassium carbonate (186 mg, 1.4 mmol) and charged with
15 mL of degassed 1,2-dichloroethane. The condenser was attached and reaction was
heated to 100 ºC for 16 hrs after adding 2,4-pentanedione (150 µL, 0.73 mmol). The
reaction was then cooled to ambient temperature and the solvent was removed under
vacuum. The resultant solid was subjected to column chromatography on silica gel 1:2
CH2Cl2: hexanes gradient to give a bright yellow emissive solid (98 mg, 41%).
1
H NMR
(500 MHz, CDCl3, δ) 8.65 (d, J = 5.71 Hz, 1H), 8.05 (d, J = 5.77 Hz, 1H), 8.02 (d, J = 8.51
Hz, 1H), 7.86 (d, J = 8.33 Hz, 1H), 7.72 (dt, J = 1.65, 7.35, 15.7 Hz, 1H), 7.59 (dt, J = 1.64,
7.32, 15.9 Hz, 1H), 7.05 (d, J = 7.86 Hz, 1H), 7.03 (dt, J = 1.28, 5.74, 13.0 Hz, 1H), 6.76-
6.74 (m, 3H), 6.64 (dd, J = 1.83, 7.63 Hz, 1H), 6.58 (d, J = 7.88 Hz, 1H), 6.41 (dd, J =
1.78, 7.65 Hz, 1H), 6.17 (dd, J = 1.83, 7.89 Hz, 1H), 6.03 (d, J = 7.41 Hz, 1H), 5.94 (d, J
= 7.38 Hz ,1H), 5.84 (d, J = 7.63 Hz, 1H), 5.72 (d, J = 7.41 Hz, 1H), 5.64 (d, J = 7.41 Hz,
1H), 5.04 (s, 1H), 3.90-3.82 (m, 2H), 3.65-3.59 (t, 1H), 3.46 (t, J = 8.57 Hz, 1H), 3.21-3.11
(m, 2H), 3.07-2.94 (m, 4H), 2.82 (d, J = 8.79 Hz, 2H), 2.54 (t, J = 10.2 Hz, 1H), 2.11-2.03
(m, 1H), 1.86 (s, 3H), 1.84-1.71 (m, 1H), 1.45 (s, 3H), 0.97-0.91 (m, 1H). Anal. For
(pCpy)2Ir(acac): found: C 65.63, H 5.04, N 3.26; calcd: C 65.58, H 5.10, N 3.21.
39

Figure 2.8.
1
H NMR spectrum of (pCpy)2Ir(acac).
Synthesis of (pCpz)2Ir(acac). A 3-neck flask was charged with pCpzH (195 mg, 0.70
mmol), IrCl3·H2O (116 mg, 0.28 mmol) and 15 mL of 3:1 mixture of 2-ethoxyethanol:
water. The mixture was degassed and heated to 110 ºC for 16 hrs. The reaction was cooled
to ambient temperature and the yellow intermediate was precipitated into water and
isolated by vacuum filtration. This solid was then placed in a new 3-neck flask charged
with potassium carbonate (186 mg, 1.4 mmol) and charged with 15 mL of degassed 1,2-
dichloroethane. The condenser was attached and reaction was heated to 100 ºC for 16 hrs
after adding 2,4-pentanedione (150 µL, 0.73 mmol). The reaction was then cooled to
ambient temperature and the solvent was removed under vacuum. The resultant solid was
subjected to column chromatography on silica gel 1:2 CH2Cl2: hexanes gradient to give a
two diastereomers of (pCpy)2Ir(acac) as yellow solids (134 mg, 57%). Isomer with C2
40

symmetry eluted first and as the minor product and the isomer with C 1 symmetry eluted
last as the major product in the ratio of 1:2.
(pCpz)2Ir(acac) C2-isomer. Yellow solid. Yield 20%.
1
H NMR (500 MHz, CDCl3, δ) 8.28
(d, J = 2.89 Hz, 2H), 7.73 (d, J = 2.21 Hz, 2H), 7.04 (dd, J = 1.65, 6.11 Hz, 2H), 6.69 (t, J
= 2.61 Hz, 2H), 6.48 (dd, J = 1.82, 7.73, 2H), 6.43 (dd, J = 1.72, 7.74, 2H), 5.99 (dd, J =
1.82, 7.72 Hz, 2H), 5.75 (d, J = 7.63 Hz, 2H), 5.51 (s, 1H), 5.50 (d, J = 7.63 Hz, 2H), 3.57
(ddd, J = 2.10, 8.67, 14.2 Hz, 2H), 3.07 (ddd, J = 2.24, 9.47, 12.9 Hz, 2H), 2.96-2.90 (m,
2H), 2.86-2.77 (m, 2H), 2.55 (ddd, J = 5.29, 9.56, 12.9 Hz, 2H), 2.23-2.17 (m, 2H), 2.00
(s, 6H), 1.81 (ddd, J = 5.29, 9.58, 13.4 Hz, 2H). Anal. For (pCpz)2Ir(acac)-C2: found: C
61.62, H 4.90, N 6.74; calcd: C 61.63, H 4.93, N 6.69.

Figure 2.9.
1
H NMR spectrum of (pCpz)2Ir(acac) - C1 isomer.

41

(pCpz)2Ir(acac) C1-isomer. Yellow solid. Yield 37%.
1
H NMR (500 MHz, CDCl3, δ) 8.17
(t, J = 2.87 Hz, 2H), 7.69 (d, J = 2.19 Hz, 1H), 7.27 (d, J = 2.22 Hz, 1H), 7.20 (s, 1H), 6.70
(t, J = 9.42 Hz, 2H), 6.56 (t, J = 2.35 Hz, 1H), 6.53 (dd, J = 1.92, 7.68 Hz, 1H), 6.49 (t, J
= 2.33 Hz, 1H), 6.41 (d, J = 6.81 Hz, 1H), 6.37 (dd, J = 1.87, 7.67 Hz, 1H), 6.02 (dd, J =
1.92, 7 .95 Hz, 1H), 5.98 (d, J = 7.65 Hz, 1H), 5.83 (d, J = 7.61 Hz, 1H), 5.78 (d, J = 7.63
Hz, 1H), 5.61 (d, J = 7.62 Hz, 1H), 5.24 (dd, J = 1.76, 7.96 Hz, 1H), 5.07 (s, 1H), 3.65-
3.52 (m, 2H), 3.28-3.21 (m, 2H), 3.07-2.94 (m, 6H), 2.87-2.79 (m, 2H), 2.71-2.65 (m, 1H),
2.16-2.07 (m, 2H), 1.85 (s, 3H), 1.71-1.66 (m, 1H), 1.46 (s, 3H). Anal. For (pCpz)2Ir(acac)-
C1: found: C 61.64, H 4.86, N 6.75; calcd: C 61.63, H 4.93, N 6.69.

Figure 2.10.
1
H NMR spectrum of (pCpz)2Ir(acac) – C2 isomer.

42

2.2.1. Equipment
NMR spectra were recorded on Varian 500 and Varian 400 NMR spectrometers
and referenced to residual protons in the deuterated chloroform (CDCl 3) solvent. UV-
visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer.
Photoluminescent spectra were measured using a QuantaMaster Photon Technology
International phosphorescence/fluorescence spectrofluorometer. 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). Photoluminescence lifetimes were measured by time-correlated single-photon
counting using an IBH Fluorocube instrument equipped with an LED excitation source.
UV-visible spectra were recorded in dichloromethane and all the other photophysical
measurements were carried out in 2-methyltetrahydrofuran (2-MeTHF). All the samples
were deaerated by bubbling N2 in a quartz cuvette fitted with a Teflon stopcock. Cyclic
voltammetry and differential pulse voltammetry were performed using a VersaSTAT 3
potentiostat. Anhydrous DMF (Aldrich) as used as the solvent under inert atmosphere and
0.1 M tetra(n-butyl)-ammonium hexafluorophosphate (TBAF) was used as the supporting
electrolyte. A silver wire was used as the pseudo reference electrode, and a platinum wire
was used as the counter electrode. A glassy carbon rod was used as the working electrode.
The redox potentials are based on the values measured using differential pulsed
voltammetry and are reported relative to ferrocenium/ferrocene (Cp2Fe
+
/Cp2Fe) redox
couple used as an internal reference whereas electrochemical reversibility was confirmed
by cyclic voltammetry.

43

2.2.2. DFT calculations
All the calculations were performed using Jaguar 8.4 (release 12) software package
on the Schrödinger Material Science Suite (v2014-2). Gas phase geometry optimization
was calculated using B3LYP functional with the LACVP** basis set as implemented in
Jaguar. The HOMO and LUMO energies were determined using minimized singlet
geometries to approximate the ground state, whereas the geometry optimized triplet
geometries were used to approximate the triplet excited state.
2.3. Results and Discussion
2.3.1. Synthesis
The ligand precusors pCpyH and pCpzH were synthesized as racemates starting
from 4-bromo-[2.2]-paracyclophane using literature procedures. In particular, the synthesis
of pCpyH utilized a two-step process that involved a Pd catalyzed cross-coupling with
pyridine-N-oxide followed by reduction to pCpyH, whereas pCpzH was prepared directly
by a Pd catalyzed cross-coupling with pyrazole.
16, 17
The synthetic route for the metal
complexes is illustrated in Figure 2.4. The Pt complexes were synthesized using a
cyclometalated chloro intermediate prepared in a reaction between pCpyH or pCpzH with
K2PtCl4, followed by addition of 2,4-pentanedione to displace the chloride ligand.
Heteroleptic complexes of the type (C^N)2Ir(L^X) where C^N is the cyclometalating
ligand and L^X is the ancillary ligand were also prepared. Dimeric chloro-bridged Ir
complexes were synthesized by condensation of IrCl3·nH2O with the cyclometalating
ligand. The resulting dimer was then reacted with excess ancillary ligand in the presence
of a base. All cyclometalated complexes were isolated as yellow solids with yields varying
between 25–65%. Reference complexes were also prepared ligated with either
44

2-phenylpyridyl (ppy) or 1-phenylpyrazolyl (ppz) and dipivaloylmethane (dpm) as the
ancillary ligand. The dpm ligand increases the solubility of these heteroleptic complexes
and confers similar electronic properties as the acac ligand. Interestingly, two
diastereoisomers of (pCpz)2Ir(acac) were isolated in a 1:2 ratio using conventional flash
chromatography on silica gel. The minor, less polar product eluted first and was found to
have C2 symmetry by NMR spectroscopy, whereas the major product had C 1 symmetry.
The presence of diastereomers is due to the restricted rotation of the aromatic rings in
paracyclophane that gives rise to planar chirality in the pCpy and pCpz ligands. The
configuration of the ligands is defined as R or S according to Cahn-Ingold-Prelog rules and
the stereochemistry of the metal center can be either ∆ or . The configuration around the
metal center for the C2 symmetric isomer was established as 
RR
(∆
SS
) using a combination
of 2-D COSY and
1
H NOESY experiments. The major product having C1 symmetry is the

RS
( 
RS
) isomer as confirmed using both NMR and X-ray crystallography. In contrast,
only a single C1 isomer of (pCpy)2Ir(acac) was isolated from the analogous reaction of
pCpyH and IrCl3·nH2O.

Figure 2.11. R and S configuration of substitutions on the cyclophane core.
45

2.3.2. Crystal Structure
The X-ray structure of the (pCpz)2Ir-C1 crystal grown from hexane/CH2Cl2 solution
is shown in Figure 2.12, selected bond lengths and angles are summarized in Table 2.1.
The unit cell contains an enantiomeric pair of complexes along with one molecule of
adventitious H2O in a triclinic P-1 space group. The iridium center has the pseudo-
octahedral geometry where the coordinated nitrogen atoms are disposed in a trans
configuration around the Ir center. The N2–Ir–N3 angle of 177.83(7)° is comparable to
previously reported (C^N)2Ir(L^X) complexes.
4, 22-24
The chelate bite angles are N2–Ir–
C7 = 80.38(7)°, N3–Ir–C34 = 79.92(7)° and O1–Ir–O2 = 88.00(6)°. The bond lengths to
the metal center Ir-C7 = 2.041(2) Å, Ir–C34 = 2.031(2) Å, Ir–N2 = 2.006(2) Å, Ir–N3 =
2.005(2) Å, Ir-O1 = 2.140(2) Å and Ir–O2 = 2.145(2) Å are also comparable to the values
reported for (ppz)2Ir(β-diketonato) derivatives.
24
The unmetalated aromatic ring in the 
S

( 
R
) ligand is directed toward the adjacent pCpz, whereas the corresponding ring in the 
R

( 
S
) ligand is positioned toward the acac ligand.
Figure 2.12. ORTEP representation of the 
RS
enantiomer of (pCpz)2Ir-C1.
46

The bond lengths and angles of the aromatic rings preserve the inherent strain of
the cyclophane moiety and undergo only minor perturbation upon cyclometalation to Ir.
The separation between bridgehead carbons ranges between 2.77–2.81 Å and the distance
between ring centroids is 3.102 Å ( 
S
) and 3.129 Å ( 
R
). For comparison, the separation
between the bridgehead carbons in the parent cyclophane is 2.78 Å and the ring centers are
3.09 Å apart.
7

Table 2.1: Selected bond lengths and angles of (pCpz)2Ir-C1 (CCDC 1465219)

2.3.3. Electrochemistry
The electrochemical properties of all the complexes were measured using cyclic
voltammetry (CV) and differential pulse voltammetry (DPV), redox potentials relative to
an internal ferrocenium/ferrocene reference are reported in the Table 2.2. The (pCpy)Pt
Atoms Distance (Å) Bonds Angle (°)
Ir1-N2 2.006(2) C34-Ir1-N3 79.92(7)
Ir1-N3 2.005(2) C7-Ir1-N2 80.38(7)
Ir1-O1 2.140(2) O1-Ir1-O2 88.00(6)
Ir1-O2 2.145(1) C34-Ir1-O2 86.07(7)
Ir1-C34 2.031(2) C7-Ir1-O1 90.82(7)
Ir1-C7 2.041(2) C34-Ir1-N2 99.91(7)
C7-Ir1-N3 95.61(8)
N2-Ir1-N3 177.83(7)
N2-Ir1-O2 93.36(6)
47

and (pCpz)Pt complexes undergo irreversible oxidation at potentials (E
pa
= 0.61 V) that are
ca. 200 mV lower than the corresponding reference complexes with ppy and ppz ligands.
The (pCpy)Pt complex also displays a quasi-reversible reduction (E
1/2
= -2.40 V) along
with a second irreversible wave at higher potential (E
pc
= -2.98 V). These potentials are
comparable to those found in the ppy analog and are assigned to reduction of the
cyclometalated and diketonate ligand, respectively.
25
In contrast, the (pCpz)Pt complex
only shows a single irreversible reduction wave at a potential (E
pc
= -2.80 V) comparable
to what is found in (ppz)Pt(dpm). Conceivably, this process could involve either reduction
of either the C^N or O^O ligand.
The Ir derivatives with the cyclophane ligands show similar behavior in that
oxidation potentials are ca. 250 mV lower than the analogous complexes with ppy and ppz
ligands. Reduction waves from complexes with both types of ligands are quasi-reversible
and have near equal values ((pCpy)2Ir, E
1/2
= -2.68 V; (ppy)2Ir(dpm), E
1/2
= -2.64 V). A
second irreversible wave at higher potential is observed in (ppy)2Ir(acac). These processes
are assigned to reduction of the respective C^N and O^O ligands, in analogy to the redox
behavior observed in the Pt derivatives. The change in oxidation potentials upon
cyclophane substitution of these Ir-(pCpz) system is depicted in Figure 2.13. The observed
trend in oxidation potential for both Pt and Ir complexes indicates destabilization of the
HOMO upon substitution with the cyclophane moiety, whereas the LUMO is largely
unaffected relative to the analogous reference compounds.

48

Figure 2.13. CV traces of (pCpz)2Ir-C1, (pCpz)2Ir-C2 and (ppz)2Ir(acac)
Table 2.2. Electrochemical data for Pt and Ir complexes.

a
Redox measurements were carried out in anhydrous DMF solution; values were reported relative
to Fc
+
/Fc. Estimated error ±50 mV.
R
Reversible
I
Irreversible
QR
Quasi-reversible

Compound
E
ox
a
(V) E
red
a
(V)
(pCpy)Pt 0.61
I
-2.40
QR
-2.98
I

(ppy)Pt(dpm) 0.79
I
-2.41
QR
-2.95
I

(pCpz)Pt 0.60
I
-2.82
I

(ppz)Pt(dpm) 0.85
I
-2.80
I

(pCpy)2Ir 0.17
R
-2.68
QR

(ppy)2Ir(acac) 0.43
R
-2.64
R
-2.92
I

(pCpz)2Ir-C2 0.20
R
-2.97
I

(pCpz)2Ir-C1 0.15
R
-2.93
I

(ppz)2Ir(acac) 0.55
R
-3.1
R

-4 -3 -2 -1 0 1
-100
0
100
200
(pCpz)
2
Ir-C1
(pCpz)
2
Ir-C2
(ppz)2Ir(acac)
Current (a.u)
Voltage(V) vs Fc/Fc
+
Fc/Fc
+

Ir(ppz)
C
1
/C
2

49

2.3.4. Photophysical properties
The absorption and emission spectra of all the complexes were recorded at room
temperature as well as at 77 K. It has been demonstrated earlier that the extending the
π-system of the ppy ligand will lower the triplet energy of the cyclometalated ligand and
induce a bathochromic shift in the emission. By introducing the cyclophane moiety in the
phenyl position of the ppy ligand the same principle applies but now, instead of through
bond, conjugation of the π-system can occur through space, which is characteristic to
cyclophane chemistry.
26

The electronic spectra of (pCpy)Pt(acac) and (ppy)Pt(dpm) are plotted to allow for
a direct comparison of cyclophane substitution (Figure 3). Intense, high energy absorption
bands of (ppy)Pt are assigned to allowed π-π* ligand-centered (LC) transitions, whereas
the low energy, less intense transitions in the range of 375–450 nm (  < 6 x10
3
M
-1
cm
-1
)
are assigned to MLCT transitions. The same assignment can be applied to (pCpy)Pt
complexes since they exhibit a similar line shape as (ppy)Pt, but the absorption bands are
distinctly red-shifted due to the cyclophane substitution.
Both (ppy)Pt and (pCpy)Pt complexes are strongly emissive in fluid solution at
room temperature and intensively emissive at frozen glass (77 K). The emission profile of
(ppy)Pt at room temperature shows a distinct vibronic structure, whereas emission from
(pCpy)Pt is less defined suggesting greater distortion in the excited state (Figure 2.14).

50

Figure 2.14. Photoluminescence spectra of (pCpz)Pt and (ppy)Pt.
The red-shifted emission of (pCpy)Pt indicates the transannular interactions as well
as the ring strain of cyclophane core perturb the excited state of these cyclometalates
relative to ppy. The spectral shift of 0.25 eV in emission from the two complexes correlates
well with the difference in their respective electrochemical oxidation potentials. Both
complexes show well-resolved emission spectra at 77 K with a similar vibronic fine
structure. The photoluminescent quantum yield of (pCpy)Pt (  = 0.20) is lower than that
of (ppy)Pt (  = 0.29), whereas the luminescent lifetime is longer both at room temperature
(  = 5.3 µs vs.  = 2.6 µs) and at 77 K (  = 16.1 µs vs.  = 8.9 µs). The decrease in  for
(pCpy)Pt occurs despite a near two-fold decrease nonradiative rate constant (knr =
1.5 x 10
5
s
-1
vs. 2.7 x 10
5
s
-1
for (ppy)Pt) since the value of the radiative rate constant
undergoes a greater three-fold decrease (kr = 0.37 x 10
5
s
-1
vs. 1.1 x 10
5
s
-1
for (ppy)Pt).
Initially, it might be presumed that the rate of nonradiative decay for (pCpy)Pt would be
300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
(pCpy)Pt
(ppy)Pt
Wavelength (nm)
(10
4
cm
-1
M
-1
)
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
51

faster than for (ppy)Pt due to the effect of increased structural distortion in the excited state,
combined with an increase due to the red-shifted emission (energy gap law).
However, it is worth noting that these types of square planar complexes can also
undergo deactivation by thermal promotion to a metal-centered ligand field (
3
LF) state.
25,
27-29
Apparently, the increased energy separation between the emissive triplet state and
3
LF
state in (pCpy)Pt is large enough to compensate for any increase in nonradiative decay
caused by greater vibrational coupling to the ground state. Details of photophysical
characteristics of square planer Pt complexes in general will be discussed in chapter 5.
The absorption and emission spectra of (pCpy)2Ir and (ppy)2Ir are given in the
Figure 2.15. The (ppy)2Ir reference compound used in this study is a green emitter (λmax =
515 nm) with a photoluminescence quantum efficiency close to unity (  = 0.90).
Substitution of the cyclophane for the phenyl ring in ppy ligand results in a cyclometalate
that displays a similar red shift both in absorption and emission (λmax = 565 nm) as found
in the (pCpy)Pt/(ppy)Pt pair. The vibronic progression in the emission spectra from both
Ir complexes are less structured than the Pt derivatives at room temperature and 77 K,
consistent with emission originates from an excited state with mixed MLCT-LC character.
The lower photoluminescence quantum yield measured for (pCpy)2Ir (  = 0.52) relative
to (ppy)2Ir(acac) is due mainly to a five-fold increase in the non-radiative rate constant
knr = 3.2 x10
5
s
-1
) rather than the modest decrease in the radiative rate constant
(kr = 3.5 x10
5
s
-1
).

52

Figure 2.15. Photoluminescence spectra of (pCpy)2Ir and (ppy)2Ir.
1-Phenylpyrazole is an example for a ligand that possesses high triplet energy.
Thus, when cyclometalated onto heavy metals like Pt and Ir, these complexes give rise to
high triplet energy phosphors leading to emission in the blue region of the visible
spectrum.
24, 30, 31
The HOMO energies of Pt and Ir complexes with cyclometalated ppz
ligands are similar to those of the ppy analogs; however, the disproportionately higher
LUMO energies lead to an increase in the HOMO–LUMO gap and consequently, a high
triplet state energy. Notably, the photoluminescent quantum yields from these ppz
cyclometalates are highly temperature dependent, being very low at room temperature but
strongly emissive in rigid media at 77 K. This phenomenon has been ascribed to the ppz
complexes undergoing nonradiative decay by thermally population of higher energy, non-
emissive
3
LF excited states at room temperature.
32

300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
(pCpy)
2
Ir
(ppy)
2
Ir
Wavelength (nm)
(10
4
cm
-1
M
-1
)
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
53

Figure 2.16. Photoluminescence spectra of Pt (top) and Ir (bottom) pCpz and ppz
cyclometalates.
300 400 500 600 700
0.0
0.5
1.0
1.5
(pCpz)Pt
(ppz)Pt
Wavelength (nm)
(10
4
cm
-1
M
-1
)
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
300 400 500 600 700
0
1
2
3
4
(pCpz)Ir
2
-C
1
(pCpz)Ir
2
-C
2
(ppz)Ir
2
Wavelength (nm)
(10
4
cm
-1
M
-1
)
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
54

In this work, the cyclophane analog of ppz ligand was used to decrease the energy
of triplet state in these ppz systems by destabilizing the HOMO energy, leading to a more
red-shifted emission. As a consequence of this decrease, the energy separation between
the deactivating
3
LF state was anticipated to increase and thus improve the quantum
efficiency relative to that of the ppz analogs. Unfortunately, these pCpz containing Pt and
Ir cyclophanes still suffer from temperature dependent nonradiative deactivation processes
despite the decrease in triplet energies.
It is apparent that the decreasing of HOMO–LUMO gap leads to a red shift in
emission, but is insufficient to overcome the nonradiative decay pathways still active at
elevated temperatures. Since these pCpz and ppz containing Pt and Ir cyclometalates are
non-emissive at room temperature, comparisons were made using their low temperature
spectra. At 77 K these materials are blue-green emitters have relatively long lifetimes
( > 10 s). Emission from (pCpz)Pt is red-shifted from its ppz analog and shows a well
resolved vibronic fine structure at 77 K. Likewise, the E0-0 for emission from the two
diastereomers of (pCpz)2Ir is red-shifted from that of (ppz)2Ir, although the change is not
prominent at 77 K.
One possible reason for this difference could be that the broad, featureless emission
from (ppz)2Ir originates from a metal-ligand to ligand charge transfer (
3
MLLCT) state
[(ppz)2Ir to acac], rather than from a less delocalized
3
LC-MLCT state in the (pCpz)2Ir
derivatives.

55

Table 2.3. Photophysical data for Pt and Ir complexes.

a
Error in photoluminescence quantum yield ± 10%.
b
Error in lifetime measurement ± 5%.
2.3.5. DFT calculations
Density functional theory (DFT) calculations were carried out for all these
complexes using a B3LYP/LACVP** method. We will limit our discussion here on the
results of (pCpz)2Ir(acac)-C1 because the DFT results for the other Ir and Pt cyclophane
complexes follow a similar orbital description. The HOMO, LUMO and the triplet spin
density is shown in Figure 2.17.
The HOMO consists of a mixture of phenyl and Ir metal atom orbitals, whereas the
LUMO is distributed predominantly on the pyridyl ring and acac ligand, with a smaller
Compound RT 77 K
 max (nm) 
a
 ( s)
b
k r (10
5
s
-1
) k nr (10
5
s
-1
)  0-0 (nm)  ( s)
b

(pCpy)Pt 583 0.20 5.3 0.37±0.02 1.5±0.1 522 16.1
(ppy)Pt(dpm) 486 0.29 2.6 1.1±0.1 2.7±0.2 472 8.9
(pCpy) 2Ir 565 0.52 1.5 3.5±0.3

3.2±0.2 553 3.8
(ppy) 2Ir(acac) 520 0.90 1.6 5.6±0.8 0.63±0.1 510 3.2
(pCpz)Pt 468 0.016 - - - 466 28
(ppz)Pt(dpm) 438 0.015 - - - 416 8.3
(pCpz) 2Ir-C 1 - <0.001 - - - 484 11.8
(pCpz) 2Ir-C 2 - <0.001 - - - 484 11.3
(ppz) 2Ir(acac) - <0.001 - - - 455 2.4
56

mixing of the cyclometalated phenyl ring. There is little electron density on the metal center
in the LUMO. Notably the outer phenyl ring of the cyclophane core does not participate
in frontier molecular orbitals (FMOs). The triplet spin density is localized mainly on the
metal center and the cyclometalating ligand with a small contribution found on the
unmetalated phenyl ring. Even though the HOMO and LUMO distribution of these pCpy
and pCpz containing Ir and Pt cyclometalates mimic their ppy and ppz analogs, the
influence of cyclophane substitution over the red shift of emission could not be directly
explained by these FMOs. Therefore, for all these cyclophane cyclometalates, the most
probable vertical transitions of the triplet state were calculated by TD-DFT method. The
results indicates, even though the outer phenyl ring of the cyclophane core has a minimum
impact on the HOMO and LUMOs, it has a considerable electron density in other FMOs
as HOMO-1, HOMO-2, HOMO-3, LUMO+1, LUMO+2. The calculated most probable
vertical transitions and the composition of the FMOs are given in the reference. The nature
of those transitions is definitely an admixture of MLCT and some ligand, either LLCT or
ILCT (intra-ligand charge transfer) in character.

Figure 2.17. Density functional theory calculation (DFT) of HOMO (left), LUMO
(middle) and triplet spin density (right) for (pCpz)2Ir(acac)-C1.
57

2.4. Conclusion
We have synthesized [2.2]-paracyclophane-linked pyridyl and pyrazolyl ligand
systems and coordinated them onto Ir and Pt to characterize and study the photophysical
properties of four novel cyclometalated systems. The motivation for this work was to
examine the influence of cyclophane core substitution on photophysical properties of these
well-studied ppy and ppz containing Ir and Pt cyclometalating complexes. The strong
transannular effect and ring strain of cyclophane induces a bathochromic shift in
absorbance and emission compared to their ppz and ppy analogs. A relative decrease in
HOMO energies of these cyclophane containing complexes when compared to the
reference complexes, whereas the LUMO levels are relatively unchanged, ultimately leads
to a red shift in emission. The shift in frontier energy levels is a paralleled in the
electrochemical data. Both pCpy containing Ir and Pt complexes are fairly efficient
emitters at both room temperature and low temperature, whereas complexes with pCpz
ligands are poor emitters at room temperature and only highly efficient emitters at low
temperature. These ppz containing cyclometalates tend to undergo thermally activated
nonradiative decay processes due to high triplet energy that ultimately results in extremely
low quantum yield of phosphorescence under ambient conditions.

58

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60

Chapter 3 - Highly efficient deep blue emission from Iridium
cyclometalates with N-heterocyclic carbene ligands
3.1. Introduction
An effective approach to color shift the emission of the phosphorescent
cyclometalates in to blue region of the spectrum includes replacing the pyridyl
functionality of ppy type ligands with five-membered heterocyclic moieties as pyrazoles,
imidazoles and triazoles. Although the desired color shifting is achieved with this
technique, most of these blue phosphorescent complexes encounter an appreciable decrease
in PL efficiency due to various non-radiative decay processes including temperature
dependent deactivation.
1, 2
In order to achieve efficient luminescence from these
complexes it is crucial to retard or eliminate non-radiative processes that deactivate the
excited state. If the non-radiative state is metal localized, ligand field state, strengthening
the metal-ligand bond will raise the energy of those deactivating state since that state is
comprised of antibonding counterparts of the metal-ligand bonding orbitals.
3
A strategy to
strengthening the metal-ligand bond is to incorporate a moiety that can form a stable bond
with the central metal atom such as N-heterocyclic carbenes (NHC).
3-5

As mentioned in the Chapter 1 these NHCs are neutral, two electron donor ligands,
which makes the cyclometalated ligand a bidentate monoanionic ligand (C^C: is used as a
general abbreviation for cyclometalated NHC ligand). First crystalline stable NHC was
reported in 1991 by Arduengo et al,
6
and after that a significant amount of work was done
on understanding the properties of this valuable functionality.
7-9
The carbene carbon atom
has a sp
2
-hybridized orbital called σ and a p orbital (p
π
) that are orthogonal to each other.
61

The ground state of NHC is usually a singlet with σ
2
electronic configuration. A common
factor giving rise to higher reactivity for these carbenes is that they have a triplet state with
the configuration of σ
1
pπ
1
that is close in energy to singlet state.
10
If this energy gap is not
that large the triplet state will be favored and then the carbene could undergo dimerization.
Usually the singlet-triplet energy gap for stable carbenes is above 65 kcal/mol where
singlet carbene is the most favorable configuration.
11-13

Since these NHCs are good σ donors they tend to form strong bonds with transition
metals.
9, 14
M-NHC bond mainly results from a σ-donation of singlet carbene carbon
electron pair into the empty metal d orbital and this bond strengthen by the π-backdonation
from filled metal d orbitals into pπ orbital of the carbene carbon. In 1980 Lappert et al
reported Ir-tris-carbene(Ir(C^C:)3) complexes known as Ir-Lappert.
15
Since not much work
done on Ir-carbene complexes until Sajoto et al came up with a detailed characterization of
Ir tris-chelates containing 1-phenyl-3-methylbenzimidazolin-2-ylidene ligand.
3

Figure 3.1. Nature of M-C: bond
62

Here in we synthesized facial and meridional isomers of NHC Ir(III) complex, tris-
(N-phenyl-N-methyl-pyridoimidazol-2-yl)Ir(III), Ir(pmp)3, that is based on the near-
ultraviolet-emitting tris-(N-phenyl-N-methylbenzimidazol-2-yl) Ir(III), or Ir(pmb)3, whose
benzannulated component in the NHC ligand is replaced with a fused pyridyl ring, as
shown in Figure 3.2.
16

Figure 3.2. Structure of fac-Ir(pmb)3, fac and mer-Ir(pmp)3
The interesting energetics and the photophysical properties of these blue emitting
Ir-carbene complexes were used as the phosphorescent emitters in the emissive layer
(EML) and electron/exciton blocking layer (EBL) to fabricate deep blue emitting OLEDs.
The devices were fabricated using the graded doping technique across the EML, thereby
reducing the triplet-triplet annihilation losses at very high brightness.
16, 17
All the device
fabrication and characterization were done by Jaesang et al at Prof. Steve Forrest Group,
University of Michigan.

fac-Ir(pmp)
3

Ir
N
N
N
N
N
N
N
N
N
Ir
N
N
N
N
N
N
N
N
N
me r-Ir(pmp)
3

fac-Ir(pmb)
3

Ir
N
N
N
N
N
N
63

3.2. Experimental
Chemicals from commercial sources were used as received. All procedures were
carried out in inert 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 N-phenyl-N-methyl-pyridoimidazol-2-yl (pmp) ligand was
synthesized as a modification to a previously reported method and the complete synthetic
scheme to make fac- and mer- Ir(pmp)3 is depicted in the Figure 3.3.
18

Figure 3.3. Synthesis of fac- and mer-Ir(pmp)3
Synthesis of 2-N-Phenylamino-3-nitropyridine (a). To a three-neck flask connected to a
condenser, charged with 2-Chloro-3-nitropyridine (4.24 g, 27 mmol), Et3N (7.6 ml,
54 mmol) and aniline (32 mmol) were dissolved in n-BuOH (20 ml). After refluxed
overnight, removed n-BuOH to get blood orange colored crystals. Product was carried into
the next step without further purification.
Synthesis of 2-N-Phenylamino-3-aminopyridine (b). To a solution of 2-N-Phenylamino-
3-nitropyridine (31 mmol) in ethanol (100 ml) was added 10% palladium on charcoal

64

catalyst (10% w/w). The suspension was hydrogenated at room temperature overnight. The
Pd/C was removed by filtration through a pad of Celite and concentrated in vacuo. The
product was subjected flash column chromatography on silica.
Synthesis of N-phenyl-N-methyl-pyridoimidazol-2-yl (d). A mixture of N
3
-methyl-N
2
-
phenylpyridine-2,3-diamine (4.11 g, 20.63 mmol), triethyl orthoformate (172 mL),
concentrated HCl (0.9 mL), and 5 drops of formic acid was heated at reflux for 12 hr under
N2. The solution was cooled to room temperature, and the resulting solid (imidazolium
chloride hydrochloric salt) was collected by filtration (2.81 g, 46%).
Synthesis of fac- and mer-Ir(pmp)3. A mixture of [IrCl(COD)]2 (700 mg, 1.04 mmol),
ligand (d) (1.76 g, 6.24 mmol), silver oxide (1.45 g, 6.24 mmol), and triethylamine (635
mg, 6.24 mmol) in chlorobenzene (100 mL) was bubbled with N2 for 10 min. The solution
was heated to reflux for 12 hr under N2. The reaction mixture was cooled and
chlorobenzene was evaporated. The crude solid was coated on silica and twice purified by
column chromatography [dichloromethane (DCM) to DCM / acetone = 95 / 5] to afford
fac-Ir(pmp)3 (310 mg, 18 %) and mer-Ir(pmp)3 (890 mg, 53 %).
fac- Ir(pmp)3
1
H NMR (acetone-d6, 500 MHz)  = 8.86 (d, J = 8.0 Hz, 3H), 8.38 (dd, J = 5.0, 1.3 Hz,
3H), 7.83 (dd, J = 8.0, 1.3 Hz, 3H), 7.29 (dd, J = 8.0, 4.9 Hz, 3H), 6.99-6.94 (m, 6H), 6.60
(d, J = 3.0 Hz, 3H), 3.47 (s, 9H).
13
C NMR (CDCl3, 101 MHz)  191.74, 147.61, 146.59, 146.25, 142.83, 136.44, 128.69,
125.36, 121.49, 116.89, 115.97, 114.68, 33.67; HRMS (m/z, ESI
+
) Calcd for C39H31IrN9
65

818.2332 [M + H
+
], found: 818.2347. CHN analyses; theoretical: C 57.34, H 3.70, N 15.43;
fac-Ir(pmp)3: C 56.95, H 3.73, N 15.50
mer- Ir(pmp)3
1
H NMR (acetone-d6, 500 MHz)  = 8.93 (dd, J = 7.9, 1.3 Hz, 1H), 8.89 (dd, J = 7.7, 1.1
Hz, 1H), 8.81 (dd, J = 7.6, 1.1 Hz, 1H), 8.44-8.42 (m, 2H), 8.39 (dd, J = 5.0, 1.4 Hz, 1H),
7.86 (td, J = 8.1, 1.4 Hz, 2H), 7.84 (dd, J = 5.8, 1.4 Hz, 1H), 7.35-7.27 (m, 3H), 6.98-6.90
(m, 5H), 6.65 (td, J = 7.3, 1.3 Hz, 1H), 6.61-6.58 (m, 3H), 3.45 (s, 3H), 3.40 (s, 3H), 3.30
(s, 3H).
13
C NMR (CDCl3, 101 MHz) 190.48, 187.76, 186.63, 148.74, 148.62, 147.84,
147.15, 146.74, 146.65, 146.54, 146.50, 145.36, 143.06, 143.03, 142.79, 138.90, 138.51,
135.82, 129.18, 128.80, 125.38, 125.27, 125.15, 121.32, 120.91, 120.86, 117.12, 117.09,
116.77, 116.35, 116.18, 115.90, 115.22, 115.12, 114.56, 110.01, 33.66, 33.61, 32.97;
HRMS (m/z, ESI
+
) Calcd for C39H31IrN9 818.2332 [M + H
+
], found: 818.2313. CHN
analyses; theoretical: C 57.34, H 3.70, N 15.43; mer-Ir(pmp)3: C 57.22, H 3.52, N 15.52.
Device fabrication and characterization.
16

PHOLEDs were grown on pre-cleaned glass substrates coated with 80 nm-thick
indium tin oxide (ITO) by vacuum thermal evaporation in a chamber with a base pressure
6×10
-7
torr. The devices consist of 10 nm MoO3 doped at 15 vol% in 9-(4-tert-butylphenyl)-
3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as a hole injection layer (HIL) / 5 nm CzSi
hole transport layer (HTL) / 5 nm NHC-Ir complex electron and exciton blocking layer
(EBL) / 40 nm Ir(pmp)3 doped in TSPO-1 to form the emissive layer (EML) / 5 nm TSPO-1
hole and exciton blocking layer (HBL) / 30 nm 1,3,5-tris(1-phenyl-1H-benzimidazol-2-
yl)benzene (TPBi) electron transport layer (ETL) / 1.5 nm 8-hydroxyquinolinolato-Li
66

electron injection layer (EIL) / 100 nm Al (cathode). The EBLs used for the fac- and mer-
Ir(pmp)3 based devices were fac-Ir(pmp)3 and fac-Ir(pmb)3, respectively (Figure 3.4).

Figure 3.4. Device architecture of Dfac and Dmer devices with fac-Ir(pmp)3 (left) and
mer-Ir(pmp)3 (right).
Probing the recombination zone.
The exciton density, N(x), in the EML as a function of distance, x, from the
EBL/EML interface was determined by measuring the relative emission intensity from a
1.5 nm-thick “sensing” layer comprised of 5 vol% doped red-emitting phosphor, i.e.
Iridium (III) bis (2-phenyl quinolyl-N,C
2
’) acetylacetonate (PQIr), inserted at different
positions within the EML.
19

D
f ac

CzSi:
MoO
3

C
Z
S
i
fac-
Ir(pmp)
3

T
S
P
O
1
TPBi
TSPO1
fac-
pmp
3

6.0
1.7
5.5
6.7
6.2
1.7
6.0
1.7
HIL
100
HTL
50
EBL
50
EML
400
HBL
50
ETL
300
1.5
1.3
6.7
D

CzSi:
MoO
3

C
Z
S
i
mer-
Ir(pmp)
3

T
S
P
O
1
TPBi
TSPO1
fac-
pmb
3

6.0
1.7
5.5
6.2
1.7
6.0
1.7
1.0
1.3
5.3
D
mer

67

Figure 3.5. Exciton recombination zone sensing scheme in EML.
3.3. Results and Discussion
3.3.1. Synthesis
The structure of the cyclometalating carbene ligand used to synthesize fac- and
mer-Ir(pmp)3 was derived from the near-UV emissive tris-(N-phenyl, N-methyl-
benzimidazol-2-yl)-iridium(III); or Ir(pmb)3 whose benzannulated component in the NHC
ligand is replaced with a fused pyridyl ring as shown on the Figure 3.2.
3
NHC ligand was
synthesized as a modification to a previously reported method and [IrCl(COD)2]2 was used
as the cyclometalating agent. Previously reported Ir(pmb)3 were prepared by modification
to the method used to prepare the Ir(C^N)3 analogues where a stoichiometric amounts of
NHC ligand salt, Silver(I) oxide, and IrCl.xH2O were refluxed in a 2-ethoxyethanol
solution to give a mixture of fac- and mer-Ir(C^C:)3 complexes in a low yield along with
[(C^C:)2IrCl]2 as a byproduct and this could be further reacted with excess NHC ligand in
the presence of Silver(I) oxide to get more fac- and mer-Ir(C^C:).
PQIr sensing layer
1 2 3 4
5 6 7 8 9
10
11 12
68

However the overall yields of both the steps are limited to ~ 40%.
3
The new
[IrCl2(COD)2] method provide good yields compared to the previously reported ones and
a better separation of two isomers when purified with column chromatography. The two
isomers were eluted in 3:1 mer :fac ratio and were easily distinguishable in the NMR. More
symmetrical fac-isomer resulted in a simple NMR spectrum where the one with lower
symmetry (mer-) resulted in a complex NMR splitting pattern.

Figure 3.6.
1
H NMR spectrum of fac-Ir(pmp)3 in acetone-d6
Ir
N
N
N
N
N
N
N
N
N
69

Figure 3.7.
1
H NMR spectrum of mer-Ir(pmp)3 in acetone-d6
3.3.2. Electrochemistry
The greater electronegativity of the nitrogen atom vs methane (CH) is responsible
for the lower reduction potential of fac-Ir(pmp)3 (Ered = -2.77 V) compared to fac-Ir(pmb)3
(Ered = -3.19 V). However, the oxidation potentials of both complexes remain nearly
identical (Eox = -0.47± 0.05 and 0.45 ± 0.05 V, respectively). The reduction/oxidation
potentials of fac- and mer-Ir(pmp)3, fac-Ir(pmb)3 and TSPO1 measured by cyclic-
voltammetry (CV) is listed in Table 3.1 and the highest occupied molecular orbital
(HOMOCV) and lowest unoccupied molecular orbital (LUMOCV) are calculated based on
the CV measurement. HOMO values measured by ultraviolet photoelectron spectroscopy
(HOMOUPS) are also listed.

Ir
N
N
N
N
N
N
N
N
N
70

Table 3.1. Electrochemical data of complexes and host material.
Isomers E ox (V)
a
E red (V)
a
HOMO UPS (eV) HOMO CV (eV)
b
LUMO CV (eV)
b

fac-Ir(pmp) 3 0.47 −2.77 −5.50 ± 0.10 −5.26 ± 0.13 −1.48 ± 0.39
mer-Ir(pmp) 3 0.23 −2.80 −5.30 ± 0.10 −4.93 ± 0.10 −1.45 ± 0.39
fac-Ir(pmb) 3 0.45 −3.19 - −5.23 ± 0.13 −0.98 ± 0.43
TSPO1 - −2.91 - - −1.32 ± 0.40
a
versus Fc/Fc
+
.
b
Calculations are based on the empirical relationships as given by HOMO CV = − (1.4 ± 0.1) × E ox
− 4.6 ± 0.08 eV
2
and LUMO CV = − (1.19 ± 0.08) × E red − 4.78 ± 0.17 eV
3
.
*
Error range of the CV measurement is 50 mV.

3.3.3. Photophysical characterization
The solution absorption spectra of fac- and mer-Ir(pmp)3 were measured in
dichloromethane and compared with fac-Ir(pmb)3 in Figure 3.8. This shows that the spin-
allowed
1
MLCT transition of fac-Ir(pmp)3 and fac-Ir(pmb)3 have high energy onset at
λ = 380 nm and 320 nm, respectively. Although the two isomers have nearly identical
energies for the HOMO, the spectral red shift for fac-Ir(pmp)3 is due to the lower energy
for the LUMO. Meanwhile, the observed red shift for the absorption spectrum of
fac-Ir(pmp)3 results from its smaller energy gap compared to fac-Ir(pmb)3 inferred from
their redox potentials.

71

Figure 3.8. Absorption spectra of fac- and mer-Ir(pmp)3 with fac-Ir(pmb)3 in solution at
room temperature.
The photoluminescence (PL) spectrum of the fac-Ir(pmp)3 in Figure 3.9 is red
shifted peak wavelength (λmax = 418 nm) in the deep blue compared to the near-UV
emission of fac-Ir(pmb)3 (λmax = 380 nm). Meanwhile the PL spectrum of mer-Ir(pmp)3 is
broad and exhibits a large room-temperature bathochromic shift (λmax = 465 nm) relative
to the fac-isomer. This redshifted emission of the mer- isomer is due to its lower
oxidation potential (Eox = 0.23 ± 0.05 V) and nearly identical reduction potential
(Ered= - 2.80 ± 0.05 V) compared to the fac-isomer, which result in a correspondingly
reduced energy gap. As for many other cyclometalated Ir complexes both fac- and mer-
isomers undergo a pronounced rigidochromic shift at lower temperature (T = 77 K), with
the fac- isomer exhibiting a vibronically structured line shape.
250 300 350 400
0
2
4
6
8
10
( )
72

Figure 3.9. Photoluminescence spectra of fac- and mer-Ir(pmp)3 in Me-THF at room
temperature and 77 K.
The PL quantum yields of these two isomers at room temperature are very high
compared to previously reported Ir-carbene complexes. The PL quantum yields measured
in de-aerated Me-THF at T = 295 K for mer-Ir(pmp)3 and fac-Ir(pmp)3 were ΦPL = 78 ± 5%
and ΦPL = 76 ± 5%, respectively and those values are very high compared to the previously
reported Ir-Carbene complexes ( ΦPL of fac-Ir(pmp)3 = 37 ± 5%).
3
The difference is due to
the increased stabilization of the triplet state in Ir(pmp)3 compared to Ir(pmb)3. Another
possible explanation for the enhanced ΦPL is the decrease in torsional angle between the
phenyl and the pyridoimidazole groups in Ir(pmp)3 relative to Ir(pmb)3, which caused by
the steric interference between the H-atom at the 1,7 phenyl and benzimidazole group
positions. In Ir(pmp)3 the substitution of N instead of methane (CH), eliminates this
400 500 600 700

Wavelength(nm)
0.0
0.2
0.4
0.6
0.8
1.0
Normalized PL (a.u.)
fac-pmp (RT)
fac-pmp (77K)
mer-pmp (RT)
mer-pmp (77K)
73

conflict. The PL quantum efficiencies of both these isomers are more than 95% in frozen
solutions (77 K). The triplet lifetimes, , were obtained from a mono-exponential fit to the
transient decay data at room temperature. Thus, radiative (kr) and nonradiative (knr) rate
constants are calculated using the relationship
10
kr =  PL / , where  PL = kr / (kr + knr).
Out of the two isomers, the mer-isomer has a shorter triplet lifetime (  = 0.8 ± 0.1 s) than
the fac-isomer (  = 1.2 ± 0.1 s) which results in its higher kr = (1.0 ± 0.2) × 10
6
vs.
(6.4 ± 1.3) × 10
5
s
-1
and knr = (2.7 ± 0.4) × 10
5
vs. (2.0 ± 0.4) × 10
5
s
-1
. At T = 77 K, triplet
lifetimes for fac-Ir(pmp)3 were extracted from a multi-exponential fit (Figure 3.10).
Accordingly, fac-Ir(pmp)3 has relatively well-resolved lifetimes of 1 = 3.9 ± 0.2 s
(weighting: 45%) and 2 = 9.2 ± 0.2 s (55%). In contrast, the mer-isomer shows mono-
exponential decay at 77 K with  = 1.3 ± 0.1 s. All the photophysical parameters of both
isomers have been listed in the Table 3.2.

Figure 3.10. Transient phosphorescent decay of dilute fac- and mer-Ir(pmp)3 in 2-MeTHF
obtained at room temperature (left) and 77 K (right).
74

Table 3.2. Photophysical properties of fac- and mer-Ir(pmp)3 in de-aerated 2-MeTHF
solution at room temperature (295 K) and 77 K.
a
Photoluminescence quantum yield (  PL).
b
Mono- and multi-exponential fits are used for extracting triplet lifetimes ( τ) at temperatures of T
= 295 and 77 K, respectively.
c
Calculated by referencing the integrated emission intensity to that of fac-Ir(ppy) 3 (  PL = 100 %).
*
Errors for the model parameters (k r and k nr) are the 95% confidence interval.
The mer- isomers of the conventional red and green emitting Ir(C^N)3 type
complexes typically have a non-radiative decay rate at least an order of magnitude larger
than their fac- isomer. This difference is attributed to a more efficient thermal population
of non-radiative
3
MC states that comprise Ir-ligand antibonding orbitals in the mer-
isomer.
1, 20, 21
The asymmetric C1 molecular structure of mer-isomer leads to trans-disposed
Ir-N linkages that are more labile compared to the three equivalent Ir-N bonds in the more
symmetric (C3) fac-isomer.
22
Hence, the
3
MC states of the mer-isomer are stabilized and
thermally accessible compared to the fac-isomer. However, in Ir(pmp)3, the difference in
knr between the fac- and mer- isomers is less than a factor of two as a result of the strong
Ir-Carbene bonds destabilizing the
3
MC states for both isomers. The lack of mer- to-fac
isomerization for Ir(pmp)3 is a result of its strong metal-ligand bond nature, whereas typical
Temperature
295 K 77 K
 PL (%)
a
τ (μs)
b
k r (10
5
s
-1
) k nr (10
5
s
-1
)  (%)
c
τ (μs)
b

fac-Ir(pmp) 3 76 ± 5 1.2 ± 0.1 6.4 ± 1.3 2.0 ± 0.4 95 ± 5 3.9 ± 0.2, 9.2 ± 0.2
mer-Ir(pmp) 3 78 ± 5 0.8 ± 0.1 10 ± 2 2.7 ± 0.4 95 ± 5 1.0 ± 0.1
75

Ir(C^N)3 complexes, having weaker Ir-N bonds, allowing such conversions upon thermal
or photochemical conditions.
21
Although this is true for many Ir-carbene complexes, a
recent report come up with an acid induced mer-to-fac isomerization in Ir-Carbene
complexes with a selective group of acids.
23
At low temperatures (77 K), the quantum
yields of fac- and mer-isomers increase near unity owing to suppressed non-radiative decay
via a thermal population to
3
MC states. The relative dominance of
3
LC state over
3
MLCT
state in fac- isomer compared to the mer- is reflected in the pronounced temperature
dependence of the transient PL response. The emission spectra of both isomers measured
in different media (Dichloromethane, ploy(methyl methacrylate)(PMMA), and toluene)
with different polarities to examine the solvatochromism behavior.
16
According to the
results, both complexes exhibit stronger positive solvatochromism in a more polar medium.
Furthermore, the PL spectrum of the mer- isomer is more redshifted and broadened in polar
media than that of the fac- isomer, because the excited state of the mer- isomer is more
stabilized in the polar media. The broader emission spectrum both at 77 K and room
temperature, more pronounced bathochromic shift in a polar medium, and rigidochromic
shift in a frozen media confirm that emission from the mer- isomer originates from a polar
excited state (
3
MLCT) , rather than relatively nonpolar
3
LC-dominated states of the fac-
isomer.
24

3.3.4. DFT Calculations
The DFT calculations performed on fac- and mer-Ir(pmp)3 shows their HOMOs are
disposed on the phenyl-π and Ir-d orbitals, while their LUMOs are predominantly formed
in the methyl-pyridoimidazole ligands with a higher electron density on carbene C atom
(Figure 3.9).
16
Both HOMO and LUMO of fac-isomer are equally distributed among the
76

three ligands due to its C3 symmetry whereas for C1-symmetric mer-isomer, the phenyl π-
orbitals in the two mutually trans pyridoimidazole ligands form the HOMO, and its LUMO
is localized in π*-orbitals in the third ligand. This density distribution of mer-isomer
elongates its trans Ir-C bonds that leads to the destabilized HOMO and slightly affected
LUMO. The triplet spin density distribution of both the isomers have pronounced
3
MLCT
character. However, the greatest difference between two isomers is that triplet in a fac-
isomer is located within a single ligand leading to intra-ligand-charge-transfer (ILCT)
admixed with
3
MLCT states, whereas those in the mer-isomer are delocalized accross the
ligands represented by combined ligand-to-ligand-charge-transfer (
3
LLCT) and
3
MLCT
states. Due to the dispersed electron distribution in mer-isomer relative to the more
localized fac-isomer results in a higher transition dipole moment.
24

Figure 3.11. Frontier MOs and triplet spin densities of (a). fac- and (b). mer-Ir(pmp)3
based on TD-DFT calculations in a CH2Cl2 solvent continuum dielectric model.
a)
b)
HOMO
-5.213 eV
LUMO
-1.200 eV
triplet spin density
HOMO
-5.101 eV
LUMO
-1.271 eV
triplet spin density
77

3.3.5. Device design and performances
In collaboration with the Prof. Forrest group these interesting Ir-carbene complexes
were incorporated in to devices both as dopants as well as a neat electron/exciton blocking
layers due to the unique energetics of their frontier orbitals. In order to optimize the
recombination zone and to minimize the TTA at very high brightness, the concentration of
the deep blue emitting Ir(pmp)3 across the EML were linearly graded. The combined effect
of graded doping and introducing electron/exciton blocking layers led to marked
improvement of EQE at high current densities, i.e., J1/2 extended by more than a factor of
20. The reduced EQE roll-off is investigated by direct measurements of the triplet exciton
distribution in the EMLs.
16

The structures of the two devices using fac- and mer-Ir(pmp)3 as the dopant denoted
by Dfac and Dmer respectively are depicted in the Figure 3.4. The EBLs of Dfac consists of
fac-Ir(pmp)3 and the fac-Ir(pmb)3 is in Dmer device, where both of them have equal or
shallower LUMO energies than that for the host, as well as equal or larger triplet energy
levels than the dopant. This feature enables efficient electron transport via the host. In the
graded doped devices, the EML is linearly graded from 20% at the EBL interface to 8 vol%
at the hole blocking layer (HBL).
16, 17
Holes and the electrons injected in to the EML are
mainly transported via the dopant and the host TSPO-1 respectively. Since the dopant is
nicely nested in TSPO-1, the majority of electrons transported via the host are trapped by
the dopant and then radiatively recombine with the holes on the dopant. The TSPO-1 is a
good electron transporting material due to its diphenylphosphine oxide group.
25
Therefore
the triplets are primarily formed at the EBL/EML interface. In the graded EML, an initially
high doping concentration near the EBL/EML interface facilitates hole injection and
78

transport, which gradually reduces due to the decreasing dopant fraction at the EML/HBL
interface.

Figure 3.12. Charge transportation and exciton formation mechanism in uniformly and
graded doped EML (ref 16).
The resulting exciton recombination profile of both graded and uniform EML layer
is giving in Figure 3.13. in the uniformly doped EML, ~ 47% of triplets are concentrated
near the EBL/EML interface which decreases to about 33% in the graded EML due to the
deeper hole penetration. This more distributed recombination profile reduces the
probability of bimolecular annihilation process in the EML.
Uniform doping, 14 wt %
Graded doping, 20 – 8 wt %
: TSPO1 : Ir(pmp)
3

EBL HBL
79

Figure 3.13. Triplet density distribution of uniformly doped (squares) and graded
doped(circles) EMLs in Dmer.
As a result of both the photophysical properties of the molecules and the novel
device architectures employed, we demonstrate that fac- and mer-Ir(pmp)3 based
PHOLEDs have Commission Internationale d’Eclairage (CIE) coordinates of [0.16, 0.09]
and [0.16, 0.15], respectively, that are independent of brightness. In fact, to our knowledge
the fac-isomer-based PHOLED is having one of the deepest blue emission among Ir
complex to date. The PHOLEDs attain maximum external quantum efficiencies of
EQE = 10.1 ± 0.2 and 14.4 ± 0.4 % at low luminance, which only decreases slightly to
9.0 ± 0.1 and 13.3 ± 0.1 % at L = 1,000 cd m
-2
, respectively. The device efficiencies are
decreased by 50% at unusually high brightness, giving L = 7,800 ± 400 and 22,000 ±
1000 cd m
-2
(corresponding to J1/2 = 160 ± 10 and 210 ± 1 mA cm
-2
), respectively.
16

80

Figure 3.12. Current density-voltage-luminance (J–V–L) characteristics of Dfac and
Dmer. (a), Electroluminescence spectra of Dfac and Dmer measured at a current density of
J =10 mA/cm
-2
(b), External quantum efficiency (EQE) vs. J for PHOLEDs (c).
16

a.
b.
c.
0 2 4 6 8 10 12
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2

fac-Ir(pmp)
3
mer-Ir(pmp)
3
Current density (mA cm
-2
)
Voltage (V)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Luminance (cd m
-2
)
x10
4
10
-2
10
-1
10
0
10
1
10
2
0
2
4
6
8
10
12
14
16
EQE (%)
Current density (mA cm
-2
)
mer, gra.
mer, uni.
fac, gra.
fac, uni.
mer, no EBL
fit
81

3.3.6. Optimizing the emitter for better performance-work in progress.
In the previous section, we have demonstrated highly efficient blue emission from
fac- and mer-Ir(pmp)3 cyclometallated NHC complexes.
16
However, these complexes have
shallow LUMO levels which makes it difficult to trap or inject electrons into LUMO levels.
Moreover, the high triplet energy of Ir(pmp)3 limits them being prospective candidates for
white OLEDs. For these reasons, we designed similar complexes where the pyridine ring
of the NHC ligand is replaced with a pyrimidine (pmpur-a, pmpur-b) or pyrazine (pmb-N2)
as shown in Figure 3.15.

Figure 3.15. Structures of proposed Ir-NHC complexes.
According to DFT calculations summarized in Figure 3.16, the extra aza
substitution in the NHC component results in lowering the LUMO relative to the pmp
counterpart which will lead to efficient exciton and charge trapping in the dopant when
doped into hosts like mCBP. The deeper LUMO levels of these complexes relative to
Ir(pmp)3 promote efficient electron injection and transportation in the device and could be
used to serve as a charge manager as well.

82

Figure 3.16 a) HOMO - LUMO energies of Ir-NHC complexes. Properties calculated
using LACVP**/B3LYP. b) extracted triplet energies from the DFT calculations.
The fac- and mer- isomer of pyrazine (Ir(pmb-N2)3) analogue was synthesized
according to a literature procedure.

Figure 3.17. Synthetic scheme for Ir(pmb-N2)3.
Ligand Isomer T1 /  (eV/nm)
pmpur-a fac 3.12 / 398

mer 2.98 / 416
pmpur-b fac 2.99 / 414

mer 2.92 / 425
pmbN2 fac 2.76 / 450

mer 2.70 / 460
b) a)

fac-pmp
mer-pmp
fac-pmpur-a
mer-pmpur-a
fac-pmpur-b
mer-pmpur-b
fac-pmb-N2
mer-pmb-N2
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5

Energy (eV)
LUMO (eV)
HOMO (eV)
83

Figure 3.18.
1
H NMR spectrum of mer-Ir(pmb-N2)3 in acetone-d6

Figure 3.19.
1
H NMR spectrum of fac-Ir(pmb-N2)3 in acetone-d6.
84

The photophysical characterization performed for mer-Ir(pmb-N2)3 and data are
summarized in the Table 3.3 and 3.4. The experimental emission values are red-shifted
compared to DFT calculated values by ~ 40 nm. The mer-isomer show enhanced radiative
rate at rigid PMMA media compared to solution as expected. The fac-isomer show
promising performance in preliminary screening and further characterization in underway.

Table 3.3. Photophysical properties of mer-Ir(pmb-N2)3 in de-aerated 2-MeTHF solution
at room temperature (295 K) and 77 K.

Table 3.4. Photophysical properties mer-Ir(pmb-N2)3 in 1% doped PMMA films.

The synthesis of pmpur-a and pmpur-b involve coupling of phenyl into purine
followed by methylation to make the NHC ligand and then cyclometalating the NHC by
reacting with [Ir(COD)Cl]2. The synthesis of the ligand and following cyclometallation is
still in progress.
Temperature
295 K 77 K
λ max (nm) 
 PL (%) τ (μs) k r (s
-1
) k nr (s
-1
)
λ max (nm) τ (μs)
mer- Ir(pmb-N2) 3 530 16 0.19 8.4x10
5
4.4x10
6
490 2.0

PMMA
λ max (nm) 
 PL (%) τ (μs) k r (s
-1
) k nr (s
-1
)
mer- Ir(pmb-N2) 3 500 74 0.47 1.6x10
6
5.5x10
5

85

3.4. Conclusion
In this study, fac- and mer-isomers of Ir(pmp)3 were successfully synthesized and
characterized. It is important to note that the mer-isomer is equally or more efficiently
luminescent than the fac-isomer in solution and in solid state opposed to conventional
Ir(C^N)3 type complexes where fac is more efficient than mer. The studies of the
photophysics of these complexes, along with their employment in the device designed with
graded doping and incorporating blocking layers, provides a solution for achieving
efficient deep blue emission at very high brightness. In particular, Dfac achieved remarkably
reduced efficiency roll-off at high current density. This leads to higher brightness (>7,800
cd m
-2
) with CIE coordinates of [0.16, 0.09], closest to the National Television System
Committee (NTSC) requirement among reported Ir-based PHOLEDs. The equally highly
emissive mer-isomer, enables even brighter PHOLED (>22,000 cd m
-2
) operating in the
blue region of the visible spectrum.
However, the shallower LUMO levels of these complexes make it difficult for charge
injection and subsequent exciton trapping. To overcome these limitations, we designed a
new class of Ir-NHC molecules where the pyridine ring of the pmp ligand was replaced
with pyrimidine or pyrazine ring. The synthesis and characterization of these complexes
are still in progress and the preliminary results reported here confirm that they could be
very useful candidates for both display and solid state lighting applications.

86

References – Chapter 3
1. Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N.
N.; Bau, R.; Thompson, M. E., Journal of the American Chemical Society 2003, 125 (24),
7377-7387.
2. Nam, E. J.; Kim, J. H.; Kim, B. O.; Kim, S. M.; Park, N. G.; Kim, Y. S.; Kim, Y.
K.; Ha, Y., Bulletin of the Chemical Society of Japan 2004, 77 (4), 751-755.
3. Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M.
E.; Holmes, R. J.; Forrest, S. R., Inorganic Chemistry 2005, 44 (22), 7992-8003.
4. Tsuchiya, K.; Yagai, S.; Kitamura, A.; Karatsu, T.; Endo, K.; Mizukami, J.;
Akiyama, S.; Yabe, M., European Journal of Inorganic Chemistry 2010, (6), 926-933.
5. Haneder, S.; Da Como, E.; Feldmann, J.; Lupton, J. M.; Lennartz, C.; Erk, P.;
Fuchs, E.; Molt, O.; Munster, I.; Schildknecht, C.; Wagenblast, G., Advanced Materials
2008, 20 (17), 3325-+.
6. Arduengo, A. J.; Harlow, R. L.; Kline, M., Journal of the American Chemical
Society 1991, 113 (1), 361-363.
7. Peris, E., N-Heterocyclic Carbenes in Transition Metal Catalysis 2007, 21, 83-116.
8. Benhamou, L.; Chardon, E.; Lavigne, G.; Bellemin-Laponnaz, S.; Cesar, V.,
Chemical Reviews 2011, 111 (4), 2705-2733.
9. Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L., Coordination
Chemistry Reviews 2009, 253 (5-6), 687-703.
10. Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M.
E.; Forrest, S. R., Nature 1998, 395 (6698), 151-154.
11. Heinemann, C.; Muller, T.; Apeloig, Y.; Schwarz, H., Journal of the American
Chemical Society 1996, 118 (8), 2023-2038.
12. Heinemann, C.; Thiel, W., Chemical Physics Letters 1994, 217 (1-2), 11-16.
87

13. Dixon, D. A.; Arduengo, A. J., Journal of Physical Chemistry 1991, 95 (11), 4180-
4182.
14. Nemcsok, D.; Wichmann, K.; Frenking, G., Organometallics 2004, 23 (15), 3640-
3646.
15. Hitchcock, P. B.; Lappert, M. F.; Terreros, P., Journal of Organometallic
Chemistry 1982, 239 (2), C26-C30.
16. Lee, J.; Chen, H. F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.;
Forrest, S. R., Nature Materials 2016, 15 (1), 92-+.
17. Zhang, Y. F.; Lee, J.; Forrest, S. R., Nature Communications 2014, 5.
18. Shi, F. Q.; Xu, X. X.; Zheng, L. Y.; Dang, Q.; Bai, X., Journal of Combinatorial
Chemistry 2008, 10 (2), 158-161.
19. Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E., Advanced Materials 2003, 15
(13), 1043-1048.
20. Deaton, J. C.; Young, R. H.; Lenhard, J. R.; Rajeswaran, M.; Huo, S. Q., Inorganic
Chemistry 2010, 49 (20), 9151-9161.
21. Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.;
Thompson, M. E., Journal of the American Chemical Society 2009, 131 (28), 9813-9822.
22. Tsuchiya, K.; Ito, E.; Yagai, S.; Kitamura, A.; Karatsu, T., European Journal of
Inorganic Chemistry 2009, (14), 2104-2109.
23. Osiak, J. G.; Setzer, T.; Jones, P. G.; Lennartz, C.; Dreuw, A.; Kowalsky, W.;
Johannes, H. H., Chemical Communications 2017, 53 (23), 3295-3298.
24. Kober, E. M.; Sullivan, B. P.; Meyer, T. J., Inorganic Chemistry 1984, 23 (14),
2098-2104.
25. Jeon, S. O.; Jang, S. E.; Son, H. S.; Lee, J. Y., Advanced Materials 2011, 23 (12),
1436-1441.

88

Chapter 4 - Molecular degradation in blue OLEDs and higher energy
excited state management using Ir(pmp)3
4.1.Introduction
The limited operational stability of blue organic light emitting diodes presents a
challenge to their widespread commercialization as large area displays and solid-state
lighting.
1-3
Although that is the case for blue OLEDs, the red and green OLEDs have
almost achieved the commercial standards in terms of efficiency and stability.
4
The shorter
operational lifetimes of these devices require a fundamental understanding of the
underlying degradation mechanisms and the issue has been focus of a significant number
of studies.
3, 5-9

Figure 4.1. Performance of commercially available PHOLEDs
Source: Universal Display Corporation (http://www.oled.com)

There are various ways of categorizing these degradation mechanisms in the OLED
device. According to the review by Leo et al, the factors affecting the stability of the
OLED device could be categorized into two main groups. Namely, intrinsic factors and
89

extrinsic factors.
9
The intrinsic factors include the electro- or photochemical reactions,
thermal behavior or interfacial effects such as cathode delamination. The extrinsic factors
include presence of impurities, insufficient encapsulation and substrate conditions.
Popovic et al categorized the OLED device degradation in to three categories,
i) Dark spot formation, ii) Catastrophic failure and iii) Intrinsic degradation.
2
Dark spot
formation occurs primarily at the device electrodes, occurs through the formation of
non-emissive regions.
10
These dark spots lead to a decrease in luminance as a result of
losses in the emissive/active area of the device. The second mode, catastrophic failure
mainly associated with the electrical shorts that result in a sudden decrease or total loss of
luminance because of large leakage current.
11
When these OLEDs are under continuous
operation for a long period of time, they are vulnerable to undergo these types of
progressive electrical shorts due to ion migration and formation of metastable thin
films.
11
The third degradation mode, intrinsic degradation, occurs without any obvious
change in the device appearance. Nevertheless, the brightness of the device degrades over
time and voltage increases. While the first two modes/extrinsic factors of degradation
could be controlled by means of proper device encapsulation and tweaking the fabricating
conditions, the intrinsic degradation has been far more challenging and continues to be an
issue for OLED commercializing specifically for blue.
Various hypothesis has been made to explain the basis for intrinsic degradation in
device efficiency and lifetime. It has been suggested that this intrinsic degradation occurs
in blue OLEDs by energetically driven formation of traps. Depending on the energy levels,
these traps can act as non-radiative recombination centers, luminescent quenchers or
charge traps. Charge traps could be further identified as deep charge traps or shallow charge
90

traps depending on the energy states of the traps generated. Luminance loss results from
the formation of non-radiative recombination centers and luminescent quenchers, and the
voltage rise caused by the presence of fixed space charge in the emissive region as a result
of filling the deep charge traps.
3
These traps are mainly formed due to the formation of
high energy excited states in bimolecular recombination processes like triplet-triplet
annihilation (TTA) and triplet-polaron annihilation (TPA) where the triplet energy of an
excited molecule could recombine with another excited molecule or a charged molecule
(polaron) respectively.
3
These high energy excited states also known as ‘hot states’ can
attain up to double the energy of the initial excited state (~ 6.0 eV) specially in TTA
process. These hot states can undergo various decay pathways when relaxing to ground
state. Most common dissociative pathway involves bond cleavage of the excited molecule
or neighboring molecules upon energy transfer. Presumably, the radicals formed upon bond
cleavage, which then participate further radical addition reactions to form even more
degraded products.
8
Therefore, these products in turn act as various kinds of traps inside
the device leading to deterioration of the efficiency and stability. Owning to their high
triplet energy, excess energy dissipated through TTA or TPA processes in blue OLEDs is
significantly higher than that for red or green OLEDs, explaining the faster degradation in
the former case. Therefore, blue fluorescent/thermally activated delayed fluorescent
(TADF) devices are no means immune to this degradation path. Thus, it is of prime
importance to analyze the mechanism in depth and to come up with a solution to overcome
this stability issue in blue OLEDs.

91

4.2.Theory
4.2.1. Bimolecular annihilation processes
In exciton-exciton annihilation process, annihilation of two singlet (S1) or triplet
(T1) excitons yield a ground state (S0), and an upper excited state (Sn* or Tn*), which may
then dissociate via a direct or pre-dissociative reaction along R to result in fragmentation
or bond rupturing (route 1). Instead of going through direct dissociation, it can also occur
via the hot-molecule mechanism where the upper excited state relaxes vibronically to
create a hot first excited state (route 2) as in Figure 4.2.

Figure 4.2. Configurational diagram for exciton-exciton annihilation mechanism.
Similarly, the annihilation of an exciton (S 1 or T1) with a polaron (D0) due to
collision will result in a ground state an excited polaron (Dn
*
), which then dissociates
along route 1 or 2 as mentioned earlier. This process could break down to two possible
reactions:
S
0

S
1/
T
1

Energy transfer
Exciton-Exciton annihilation
R
1
S
n
*
/T
n
*

S
0

2
S
1/
T
1

92

where the hot state could either reside on the dopant or the polaron.

Figure 4.3. Configurational diagram for exciton-polaron annihilation mechanism.
Since these fundamental mechanisms of degradation were revealed, there is not
much work was done towards the reduction of those processes and improving the device
lifetime. As mentioned in the chapter 3, graded doping of the emitter in the EML was
incorporated as the most recent advance to realize higher efficiencies and increased
lifetime by broadening the exciton formation rate and thereby minimizing the TTA and
TPA like bimolecular annihilation processes.
12
In this chapter another key strategy of
realizing long-lived blue phosphorescent OLEDs is introduced. There are two strategies
to prevent the energetically excited ‘hot state’ energies from ever causing molecular
dissociation. As mentioned above, the first strategy is to minimize the bimolecular
T* + H
-
→ T0 + H
-/*
(1)
T* + H
-
→ T
*/-
+ H0

(2),

R
1
D
n
*

D
0

2
S
0

S
1/
T
1

Energy
transfer
Exciton-Polaron annihilation
93

annihilation processes. The second strategy that we are going to discuss in this chapter is,
bypassing the dissociative processes inside the emissive layer.
13

4.2.2. Excited state manager
This concept was developed in Prof Steve Forrest group in University of
Michigan and all lifetime modeling and device optimization was done by Jaesang Lee et
al. Excited state manager was used to thermalize down the high-energy hot state without
damaging the dopant or the host molecules in the emissive layer. This ancillary protective
dopant should possess triplet energy higher than the emitting triplets on the EML. Thus,
the manager does not trap the excitons, but rather it efficiently returns them to the dopant
to emit photons.
13
By thermalizing the hot states into lower energy regime, the manager
reduce the probability of direct dissociation of the active material comprising the EML.
Based on the experimental results, the manager must fulfil three selection criteria to
perform this excited state management in the EML of the OLEDs.
1. The exciton energy of the manager should be higher than the lowest excited state
energy of the dopant.
2. The rate of energy transfer from the hot state to the manager must be comparable
to or higher than that for dissociation reactions leading to dissociation of the
dopant.
3. The manager should be sufficiently stable upon energy transfer from the hot state.
The Jablonski diagram below explains the excited state management process using the
manager molecule and the possible relaxation pathways for excitons.
13

94

Figure 4.4. Jablonski diagram of the EML containing manager molecule (ref 13)
Here the S0 is the ground state and T1 is the lowest energy triplet state and Sn*/Tn*
is a hot singlet/triplet manifold in the dopant or host. These hot singlet/triplet manifolds
are created because of TTA or TPA processes (TTA, process 2).
3
Then the manager can
enable the transfer of these hot states to the lowest excited state of the manager (S M/TM)
via process 3’. The SM/TM energy of the manager is greater than the dopant and the
95

excitons formed on, or transferred to the manager can be returned to the dopant for
emission of a photon. Also, exothermic transfer from dopant/host Sn*/Tn* to SM/TM is
allowed. Therefore, the damage to these molecules via dissociative reactions (process 3)
is minimized if the rate for Sn*/Tn*→ SM/TM is comparable or higher than Sn*/Tn*→D.
Since TTA can yield both hot singlets as well as triplets, the hot state resonantly transfer
via a Förster or Dexter process to the manager via process 3.
13
Förster energy transfer is a
long range energy transfer and occurs via coulombic interaction where excited donor
transfers energy to an acceptor through a non-radiative process. In contrast, Dexter
electron transfer, sometimes called as short-range, collisional or exchange energy transfer
that take place via two molecules (intermolecular) or two parts of the molecule
(intramolecular) bilaterally exchange their electrons.
In the manager molecule, these transferred singlets undergo vibrational relaxation
and Förster transfer back to the lowest dopant singlet state. Alternatively, these
thermalized singlets on the manager could undergo intersystem crossing to the triplet state
via process 4’(SM→TM), which subsequently transfers back to the dopant or the host via
process 5’(TM→T1). This leads to radiative decay of the dopant giving rise to
phosphorescence or recycled back to Sn*/Tn* by similar bimolecular recombination
process. In the manager, the process 4 is also possible where high energy S M/TM state can
result in dissociation of the manager itself via SM/TM→D process. In this case the manager
serves as a sacrificial additive to the EML where it is critical to find a stable molecule as
the manager that do not readily undergo dissociation upon energy transfer.

96

4.3.Results and Discussion
As the manager molecule for these blue OLED devices, we introduced Ir-NHC
complex namely, meridional-tris-(N-phenyl, N-methyl-pyridoimidazol-2-yl)iridium (III)
[mer-Ir(pmp)3] in the EML. The devices were fabricated by Jaeasng Lee in Prof. Steve
Forrest Group Umiversity of Michigan. The EML also consists of the highly efficient blue
dopant, Iridium (III) tris[3-(2,6-dimethylphenyl)-7-methylimidazo [1,2] phenanthridine]
Ir(dmp)3 and the host, 4,4’-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (mCBP). The
manager is characterized by relatively strong metal-ligand bond, distinct for carbenes.
14, 15

This manager fulfils first criterion by possessing triplet energy onset (E T1 ≈ 3.1 eV), that is
higher than that of the dopant Ir(dmp)3 (E T1 ≈ 2.8 eV). Although, the fulfilment of the
second and the third criteria are not studied thoroughly for this manager. The rate of energy
transfer from the hot state to the manager (process 3’) is a function of intimate orbital
overlap between manager and the dopant or host; a property controlled by the steric and
orbital characteristics of all molecules involved.

Figure 4.5. Molecules used in the EML, blue dopant (left), manager (middle), and host
(right).
Ir(dmp)
3

dopant
mer- Ir(pmp)
3
- manager
mCBP - host
97

In constructing a model for the excited state management of device, it is of prime
importance to identify the mechanism of these degradation processes and what byproducts
formed upon degradation of these parent molecules. There are few techniques to assess
these degraded molecules that are potentially be luminescent quenchers or charge traps in
the device. Experimentally fragments of the aged device could be extracted and subjected
to mass spectroscopic analysis or other analytical analysis.
8,9,16
Also, theoretical
calculations help identifying weakest bonds in the molecules of EML and predict possible
by products. These computational calculations also become handy and somewhat accurate
in predicting the energy of those fragments.
There are few reports on CBP
16
and mCBP
8
degradation inside the OLED under
operational conditions. Clearly the degradation occurs in those molecules associated with
chemical reactions in the bulk of the material. In most of the studies, the authors have
dissolved an electronically aged OLED device and subjected that sample to HPLC/MS and
NMR analysis to identify fragmented molecules and byproducts. The expected byproducts
could be identified by running them against commercial or synthesized fragment
molecules. The identified byproducts from such a study done on CBP is given in the Figure
4.6.
16

98

Figure 4.6. Identified byproducts from CBP degradation inside OLEDs (ref 16).
The identification of a degraded intermediate or a byproduct might shed light on
the operational degradation mechanism. This also leads to the engineering OLED materials
and devices that are less susceptible to operational degradation. However, the detection and
identification of these degraded products has been a challenge due to their lower quantities
expected to be present in the OLED device even after longer operational time. Also, the
thin layers of the OLED stack itself does not carry many molecules to begin with, which
only a small fraction may be converted into degraded products. Plausible structures of the
degraded byproducts of CBP and mCBP provide an important clue about their degradation
mechanisms, that point to the cleavage of C-N single bond between carbazolyl and aryl
radicals.
16
Not surprisingly, this is the weakest bond in the molecule with respect to
homolytic cleavage. This experimental observation could be nicely backed by the
homolytic bond dissociation energies calculated for mCBP given in Figure 4.7.
99

Figure 4.7. Homolytic bond dissociation energies for all the exocyclic bonds of mCBP.
The density functional theory calculations suggest that the heterolytic cleavage of
the C-N bond in the singlet state of the molecule of CBP can be ruled out due to their strong
endothermic nature.
16
Although there are few reports on this host molecule degradation
mechanisms, but there are almost no report in literature on dopant degradation in
operational conditions. Since the doping concentration of the emitter in the EML is quite
low (~5-20%w/w) compared to the host, it has been extremely challenging to detect dopant
degraded products or intermediates in the aforementioned analytical experiments.
We performed calculations to identify the higher energy excited states (Sn*/Tn*) of
dopant, host and the manager in this work. When TTA occurs between one or more
molecular species,
17
either the singlet or triplet state is promoted to Sn*/Tn*>5.4 eV.
13
Even
100

though most hot states rapidly relax to the lowest excited state via process 2’, those that
have sufficient energy can lead to the chemical bond dissociation via process 3. For this
dissociation to happen, the excited state energy should be larger than bond dissociation
energy of the excited molecule. The weakest bonds in the host mCBP used in this study
was calculated and shown in the Figure 4.8.

Figure 4.8 Higher energy singlet (S1-S100) and triplet excited states (T1-T100) of the
dopant, host and the manager compared with the bond dissociation energies of mCBP.

101

These degraded products could be formed in any layer of OLED device, but the
ones located in the EML play a dominant role in device performance and lifetime. The
defects generated outside the EML could give rise to increase in operating voltage. In this
study the defect generated were categorized into two groups, namely deep charge
traps(QA) and shallow charge traps (QB). Upon electrical excitation, the hot states could
be generated in blue-emitting devices and bonds with lower bond dissociation energies
could dissociate and give rise to neutral species by disproportionation, or they could
participate in radical reactions with neighboring molecules to form higher molecular mass
by-products.
9, 16
The laser desorption ionization (LDI) mass spectroscopic analysis was
done on fresh and photo-degraded materials used in this study at Prof. Stephen Forrest lab
at University of Michigan. From the results of the mass spectroscopic analysis, lower mass
products from fragmentation and higher mass products from radical addition reactions
were found. Most of the mCBP byproducts found in this study match with the previously
reported fragments.
8
Identifying the exact structure of the defect generated from the blue
dopant Ir(dmp)3 and the manager mer-Ir(pmp)3 was not straightforward and further
analysis should be carried out in addition to mass spectroscopic analysis. The deep and
shallow charge traps relative to the dopant were identified as the hole traps. Both QA and
QB are charged when filled and leading to increase in voltage. Specifically, QA could lead
to non-radiative recombination and subsequent luminance loss whereas the large energy
gap QB can capture excited states and simultaneously transfer back to dopant, thus do not
affect the luminance of the device.

102

Figure 4.9. a) Energy level diagram of the doped EML along with the energies of QA and
QB. b). Energy cascade of the triplet exciton states in the EML (ref 14).
After an extensive computational screening, following possible
degraded products were identified for host mCBP. Since the dopant degradation is quite
complicated, only ligand dissociation was assumed when determining by-products. A
detailed analysis will be done in future to address dopant degradation and additional
experimental results will be available to support this argument.

a)
b)
103

Figure 4.10. Calculated energies of possible fragments and byproducts after degradation.

HOMO: -5.93 eV
LUMO: -1.44 eV
Triplet: 3.09 eV

HOMO: 5.39 eV
LUMO: -1.41 eV
Triplet: 3.11 eV

HOMO: -5.36 eV
LUMO: -1.49 eV
Triplet: 3.05 eV

HOMO: -5.24 eV
LUMO: -1.47 eV
Triplet: 3.07 eV
HOMO: -5.51 eV
LUMO: -0.95 eV
Triplet: 3.61 eV
HOMO: -5.52 eV
LUMO: -1.01 eV
Triplet: 3.56 eV
HOMO: -5.55 eV
LUMO: -1.30 eV
Triplet: 3.09 eV
HOMO: -5.14 eV
LUMO: -1.30 eV
Triplet: 3.06 eV
HOMO: -5.72 eV
LUMO: -1.38 eV
Triplet: 3.09 eV
H1 H2
H3 H4
H5
H6
M1 D1
mCBP
104

The DFT calculated results for the defects suggests the QA and QB are both hole
traps, with QA deeper in energy than QB. In the operating device, holes could be transported
by the dopant as well as the manager, and are potentially trapped by Q A and QB. The
electrons are transported by the host and the manager. Energy level diagram in Figure 4.9
b) shows how the triplet excitons formed in the EML could transfer its energy to trap states.
there are two possible routes for the triplet excitons to be generated, i) triplet
exciplexes (ET,ex) generated between the host and the dopant, and ii) triplet excitons
directly formed on the manager (ET,M). Due to the lower triplet energy of the manager, both
these excitons could exothermically transfer to the dopant (ET,dop).
According to the model, the deep hole traps (QA) could have a low energy triplet
state that results in exciton quenching (ET,QA), while the shallow traps (QB), transfer
excitons to the lower energy sites (ET,QB). The calculated Fröntier MOs and triplet energies
of the possible fragments are depicted in the Figure 4.10. The fragments from H1-H3
consist of fission products and the H4-H6 consists of fusion products of the host mCBP.
All the energies reported here are for neutral species assuming the charge fractions undergo
neutralization by reacting with the surrounding.
The proposed fragments of the host, dopant and the manager suggest, they are likely hole
traps (QB type) but not electron traps. The Figure 4.11 a) depicts all the Fröntier MO
energy levels of the host, dopant, manager and their respective fragments. The triplet
energies of aforementioned molecules are compared in the Figure 4.11 b) and it is clear
none of the by-products act as triplet quenchers as they all possess higher triplet energies
than the dopant. It is important to note that these byproducts are mostly hypothetical and
105

the actual defects formed in the operating device might cause triplet quenching and
subsequent luminance loss.

Figure 4.11. a). DFT calculated HOMO-LUMO energy level diagram for the host, dopant,
manager and the respective degradation products. b). Calculated triplet energies of those
entities.

(a)
(b)
106

Figure 4.12. a). Device architecture and the energy level diagram for the blue PHOLEDs.
Energy levels are relative to the vacuum level. b). Doping scheme of the 50 nm-thick EML
for GRAD and managed devices.

(a)
(b)
107

The energy level diagram and the device configuration is given in the Figure
4.12 a) and the EML doping schemes of the control (GRAD) and the managed (M0)
devices are given in Figure 4.12 b). For GRAD, the concentration of the dopant is linearly
graded from 18-8 vol% from the HTL interface to the ETL interface to enable a uniform
distribution of excitons and polarons in the EML. This graded structure was previously
shown to reduce TTA and TPA processes in the device, and thereby achieve an extended
lifetime compared to the non-graded device.
12
In the managed device (M0), 3vol% of the
manager is uniformly doped across the EML and the concentration of the dopant is graded
from 15 to 5vol% to keep the doping concentration the same in both the devices. The
details of the lifetime model and the device performance could be found in the reference
13. It has been found the managed PHOLED device have increased T90 and T80 relative
to those of GRAD (TX is the time elapsed for the luminance to decrease X % of its initial
value of L0 = 1,000 cdm
-2
under constant current operation).
13
This study shows the
managed blue PHOLED attains T80= 334±5 h with a chromaticity coordinate of (0.16,
0.31), corresponding to 3.6 times improvement in a lifetime compared to conventional
and graded-EML devices of T80 = 93 ± 9 and 173 ± 3 h respectively.
12, 13
Conclusion
Since their introduction close to two decades, the operational lifetime of blue
PHOLEDs has remained insufficient for their commercial use in displays and lighting.
The reason for this instability is the bimolecular annihilation reactions (TTA and/or TPA)
of high energy excited states, producing energetically hot states that lead to molecular
dissociation specially in the EML of the device. Therefore, the key to realizing long
lifetime of blue PHOLEDs is to prevent the hot state energies from ever leading to
molecular dissociation reactions. In this study, we have by-passed the dissociation
108

reactions by introducing a molecular hot excited state manager within the EML. In a
properly designed system, hot excited states formed on the dopant or the host, transfer to
the manager and rapidly thermalize before damaging is induced on the dopant or the host.
In this study, we theorized possible degraded products from the dissociation reactions and
calculated the energies of these byproducts to model this manager concept. The device
data and theoretical calculations were used to validate the model and more details about
device performance and model parameters could be found in reference 13. By introducing
the excited state manager, our team could achieve the longest lifetime reported so far for
blue PHOLEDs to the best of our knowledge.
13
Although such approaches provide
fundamental understanding of the degradation processes going on in the devices, the
development of highly stable dopants, managers, host and transport materials remains a
challenge by the very wide energy gaps required for blue PHOLEDs.

109

References - Chapter 4
1. D'Andrade, B., Nature Photonics 2007, 1 (1), 33-34.
2. Aziz, H.; Popovic, Z. D., Chemistry of Materials 2004, 16 (23), 4522-4532.
3. Giebink, N. C.; D'Andrade, B. W.; Weaver, M. S.; Mackenzie, P. B.; Brown, J. J.;
Thompson, M. E.; Forrest, S. R., Journal of Applied Physics 2008, 103 (4).
4. Hack, M. P., NJ, US), Brown, Julia J. (Yardley, PA, US), Weaver, Michael Stuart
(Princeton, NJ, US), Premutico, Mauro (Brooklyn, NY, US) LIFETIME OLED DISPLAY.
20140077177, 2014.
5. Giebink, N. C.; D'Andrade, B. W.; Weaver, M. S.; Brown, J. J.; Forrest, S. R.,
Journal of Applied Physics 2009, 105 (12).
6. Wang, Q.; Aziz, H., Acs Applied Materials & Interfaces 2013, 5 (17), 8733-8739.
7. Reineke, S.; Walzer, K.; Leo, K., Physical Review B 2007, 75 (12).
8. Sandanayaka, A. S. D.; Matsushima, T.; Adachi, C., Journal of Physical Chemistry
C 2015, 119 (42), 23845-23851.
9. Scholz, S.; Kondakov, D.; Luessem, B.; Leo, K., Chemical Reviews 2015, 115 (16),
8449-8503.
10. Burrows, P. E.; Bulovic, V.; Forrest, S. R.; Sapochak, L. S.; McCarty, D. M.;
Thompson, M. E., Applied Physics Letters 1994, 65 (23), 2922-2924.
11. Kim, Y.; Choi, D.; Lim, H.; Ha, C. S., Applied Physics Letters 2003, 82 (14), 2200-
2202.
12. Zhang, Y. F.; Lee, J.; Forrest, S. R., Nature Communications 2014, 5.
13. Lee, J.; Jeong, C.; Batagoda, T.; Coburn, C.; Thompson, M. E.; Forrest, S. R.,
Nature Communications 2017, 8, 15566.
14. Lee, J.; Chen, H. F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.;
Forrest, S. R., Nature Materials 2016, 15 (1), 92-+.
15. Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M.
E.; Holmes, R. J.; Forrest, S. R., Inorganic Chemistry 2005, 44 (22), 7992-8003.
16. Kondakov, D. Y.; Lenhart, W. C.; Nichols, W. F., Journal of Applied Physics 2007,
101 (2).
17. Baldo, M. A.; Adachi, C.; Forrest, S. R., Physical Review B 2000, 62 (16), 10967-
10977.
110

CHAPTER 5 - Structure-property relationship of heteroleptic Pt
phosphors for understanding the orientation and related light
outcoupling in OLEDs
5.1. Introduction
Since its introduction in 1987, OLEDs have been the subject of intensive studies
and continual improvement due to their potential power efficient lighting and display
applications. As a result of these intense research efforts, the OLED efficiency has
significantly increased by the introduction of phosphorescent dopants.
1-3
This connects
back in to the original EQE equation discussed in the Chapter 1,
𝜂 𝐸𝑄𝐸 = 𝛾 𝜂 𝑆 /𝑇 𝑞 eff
𝜂 out

By using phosphorescent emitters (𝜂 𝑆 /𝑇 ) with effective radiative quantum
efficiency (𝑞 eff
) near unity and optimizing the charge balance factor (𝛾 ), internal quantum
efficiency of almost 100% could be realized for OLEDs. However, the remaining factor,
the outcoupling efficiency (𝜂 out
) due to the refractive index mismatch between layers,
resulting in various loss channels leading to trap almost 80 % of internally generated light.
As mentioned in Chapter 1, most common loss channels include surface plasmons,
waveguides and substrate emission.
4
The relative fraction of power coupled to different
optical channels are depicted in the Figure 5.1.

111

Figure 5.1. a) Schematic illustration of an OLED showing different optical channels.
b). amount of power coupled to different optical channels in a prototypical OLED (ref 4).
Recently, there has been a great progress to enhance the light outcoupling efficiency
of OLEDs by means of various internal and external outcoupling techniques. Although the
main focus of this chapter is to elaborate more on intrinsic modifications, some external
device modification techniques are listed below to give a full understanding to the reader
about the recent advances in the area of light outcoupling. Some external light outcoupling
techniques include substrate modification methods, modifying the refractive indices of
a)
b)
112

layers, use of scattering particles, micro-lens arrays, use of microcavity structure and
gratings.
4, 5

There are numerous substrate modification techniques have been implemented to
enhance the outcoupling efficiency of OLEDs and the simplest one to extract substrate
wave-guided modes is the use of rough surface. This could be done by sand blasting one
side of the glass substrate and fabricating the OLED on the other side. Due to this
roughness, waveguide modes at glass-air boundary (substrate modes) are coupled
efficiently into the air but still ITO/organic waveguide modes remain unaffected.
5

Matching the refractive indices of the EML with the surrounding layers is another
method of achieving better light outcoupling. This could achieve either by using a high-
index (HI) substrate or by bringing down the refractive index of the EML to n ~ 1. Even
though, the simulations show the increase in light outcoupling will be close to70% using
this method, realization this in actual OLED is questionable due to lack of EML materials
that result in such a low refractive index.
4
there are experimental evidence for realizing
excess of 40% in EQE by combing HI substrates with an index-matched lens but one has
to be aware that HI glass substrate would increase the overall cost of OLEDs considerably.
6

Figure 5.2. Schematic of light outcoupling enhancement in HI substrate (ref 6).
113

Use of scattering structures has also result in some promise in increased light
outcoupling. These scattering structures could be categorized as periodic and non-periodic
(random) structures or as internal or external scattering. External scattering structures can
be any modification to the backside of the glass substrate that serves to scatter out light that
would otherwise suffer from total internal reflection at the glass/air interface.
4, 7
This
includes, microlens arrays, scattering particles (Figure 5.3) or roughening of the substrate
as discussed earlier.

Figure 5.3. a) Schematic of external outcoupling structures, substrate mode extraction by
microlens arrays b) Concept of light out coupling by scattering particles in a film applied
to the backside of the substrate (ref 4).
Internal scattering structures become helpful if one wants to get access to
waveguided modes. This includes periodic gratings (photonic crystals) or random
scattering structures. The effectiveness of this technique relies on how close these
scattering structures to the emission zone of the OLED.
8
In general these photonic crystal
strucures with periodicities on a length scale comparable to optical wavelength have the
disadvantage that they required elaborate fabrication techniques. Most of these complex
114

fabricating techniques are not compatible with OLED technology as they would damage
or destroy the underline organic films.

Figure 5.4. Schematic of internal scattering structure by a periodic grating placed between
the glass substrate and the thin film stack (ref 4).
Another way to boost the light outcoupling efficiency is the use of microcavity
structures. This could be achieved by placing dielectric Bragg reflectors underneath the
ITO electrode or in top-emitting devices with a highly reflective anode on glass and
semitransparent metallic cathode, often followed by a dielectric capping layer as anti-
reflecting coating.
9
If appropriate modifications to the layer thicknesses of the OLED stack
and proper placement of the emitter in the cavity is achieved, the excitation of surface
plasmons can be minimized in such a cavity design. But usually this reduction of surface
plasmons occurs at the expense of enhanced waveguiding that result in almost no gain in
overall light outcoupling.

115

Figure 5.5. Layer design of a top-emitting device with first, second and the third cavity
order. To avoid surface plasmon losses, the emission layer is shifted away by increasing
the HTL thickness (ref 9).
Instead of developing internal and external light outcoupling techniques and tools
to extract the light lost in various loss channels, one can consider means to reduce the
excitation of those loss channels inside OLEDs. All these loss channels have a
characteristic dependence to the orientation of the transition diploe moments of the emitter.
Therefore, controlling the orientation of the emitter molecules, provide a great handle to
address this issue. Although this effect has been known for many years for polymer
OLEDs,
10
only very recently, scientific community became interested in studying this
phenomena in small molecule OLEDs fabricated by thermal evaporation.
11-17
Simulated
results combined with experimental observations clearly indicates that horizontally
oriented dipoles couples to various optical channels whereas vertical dipoles dissipate their
energy almost exclusive to surface plasmon polarizations, which makes them difficult ot
detect them in OLEDs from commonly used detection methods like angular dependent
116

emission spectra. Instead studying layer stack containing the same EML as the
corresponding OLED but not the metal layer it is possible to determine these loss channels
using angle dependent PL measurement.
4

Figure 5.6. (a) Simulation of power dissipation for exclusively horizontal or (b) vertical
dipole orientation in a prototypical OLED as a function of ETL thickness (ref 4).
The emissive process of the organometallic molecules at the core of these devices
could be describe as a dipole transition with the transition dipole moment having a certain
orientation (ϑ) with respect to the normal of the surrounding layered system. The emission
from an arbitrary oriented emitter could be decomposed into contribution of three
orthogonal dipoles, namely || TE, || TM and
┴
TM as illustrated in Figure 5.7.

Figure 5.7. Three basic dipoles and corresponding emission patterns (ref 18).
117

These orthogonal dipoles are specified according to their orientation with respect
to the interfaces of the layered architecture where parallel “||” with ϑ =   and “
┴
” with
ϑ = 0. The corresponding polarization of the emitted radiation “TE” refers to transvers-
electrical and “TM” refers to transvers magnetic.
18

Another way of expressing transition dipole moment of an emitter is by defining it
with respect to it three vectors where they could also be treated as superposition of p x, py
and pz dipoles (one third each). Materials with horizontally oriented dipoles are considered
to have one half of px and one half of py dipoles. When emission into the x-z plane is
considered, py dipoles emit only s-polarized light and px and pz dipoles are responsible for
the p-polarized emission. Therefore, the analysis of p-polarized emission can yield
information about the existence of vertical dipoles. Taking in to account, that dipoles are
radiate strongest perpendicular to their direction of oscillation, the pz diploes emit mainly
at large angles. In an actual OLED structure this corresponds to strong coupling to
waveguided modes inside the organic layers and the surface plasmons at the metal-organic
interface.
12

The orientation of these molecules in thin film structure was determined by angle
dependent photoluminescence measurements. The experimental set up is illustrated in the
Figure 5.8. the sample is attached to a fused-silica half cylinder prism by index-matching
liquid and the emission angle was changed using a rotating stage. Spectra is recorded using
a fiber optical spectrometer and a polarizing filter to distinguish between p- and s-polarized
light. The excitation of the sample will be done by an excitation source with a fixed
excitation angle of 45˚. The degree of orientation of the optical transition dipole moment
of the emitter molecules will be determined by a numerical simulation.
13, 19

118

Figure 5.8. Experimental set up for angle dependent photo luminescence measurement and
the definition of the dipole orientation on the right top corner (ref 12).
In this study, Angle dependent p-polarized emission measurements of doped films
by photoluminescent excitation are used to determine the net orientation of the transition
dipole moment vectors (TDVs) of emissive dopants. In this process, we define a value
called “anisotropy factor (Ɵ)”  as the ratio of power radiated by vertical components of the
contributing TDVs to the total power radiation. If the molecules in the sample are
isotropically oriented, it will yield a value of Ɵ = 0.33. if the emissive dipoles are aligned
perfectly parallel to the substrate, Ɵ = 0. A sample with TDVs aligned perpendicular to the
substrate will give Ɵ = 1. The equation for the anisotropy factor and the scale of Ɵ with its
corresponding orientation is illustrated in the Figure 5.9.

119

Figure 5.9. Scale of Ɵ with corresponding orientation of TDVs.
The next step is to determine the direction of TDV with respect to the molecular
frame. The orientation of TDV has been determined experimentally for a closely related
cyclometalated complex (ppy)Re(CO)4 by examining the polarization of emission obtained
from a single crystal.
20
The similarity is that, the emission of the Re complex is from
3
MLCT transition that involve Re(ppy) fragment as many Ir and Pt cyclometalated
complexes bearing (C^N) type ligands. The direction of the TDV in this Re complex is
depicted in Figure 5.10.

Figure 5.10. Transition dipole moment measured for (ppy)Re(CO)4.

120

The TDV experimentally measured for this complex lies in the plane of the ppy
ligand directed by an angle if  = 18.5˚ away from Re-N bond axis. We have developed an
accurate method of calculating the TDV of these transition metal complexes that already
gave good agreement with this experimentally measured Re complex. The calculated TDV
direction give  = 17.3 ˚ which is very similar to experimentally observed  value. Since
the direction of TDV can be calculated we can combine the TDV measured relative to the
substrate plane with this direction to identify actual molecular orientation relative to the
substrate.
There are couple of studies done on the determination of this molecular orientation
in OLED stack, and many of them contain heteroleptic and homoleptic Ir complexes. There
are very few studies on orientation of Pt emitters in OLEDs emissive layer.
19, 21
In this
study we focus on determination of structure-property related light coupling of these Pt
heteroleptic complexes and propose an orientation mechanism for these molecules in the
EML of the OLED.
So far in our group we have studied heteroleptic Ir complexes and out of them
(C^N)2Ir(acac) type complexes resulted in horizontal orientation in CBP matrix while other
heteroleptic complexes with (C^N)2Ir(C^N)’ type gave isotropic orientation. According to
these findings, we came up with a mechanism that recognizes a surface of an amorphous
(isotropic) film as inherently asymmetric during deposition, that is, organic film versus
vacuum, which leads to the alignment of molecules deposited on it. The acac group
represents an aliphatic region on the surface of the (C^N)2Ir(acac) complex, which lies
along the molecular C2 axis. In that report we proposed that the boundary created between
the organic host material on the substrate and the vacuum of the deposition chamber during
121

fabrication causes the asymmetrical (C^N)2Ir(acac) molecule to orient before it is over
coated with an amorphous layer of the host material.
13
It is important to note the molecular
rearrangement and alignment on surfaces is known to occur on timescale consistent with
the mechanism.
22
It is important that this mechanism do not require any alignment of the
host material.

Figure 5.11. Proposed mechanism for molecular orientation in (C^N)2Ir(acac) type
complexes (ref 13).
In this analysis, we study the mechanism of molecular orientation with heteroleptic
Pt complexes to see it there are any connectivity with the hypothesis we made for Ir
cyclometalates.

122

5.2. Results and discussion.
5.2.1. Design strategy
Compared to octahedral Ir(III) complexes, Pt(II) complexes could be promising
candidates to study orientation phenomena due to their square-planar geometry. In
addition, these heteroleptic Pt complexes bear only one chromophoric ligand and one
ancillary ligand, unlike Ir complexes. Therefore, there will be only one possible TDV for
a given molecule that includes the chromophoric ligand and the Pt metal center. Theoretical
calculations reveal that this TDV resides on the plane of the chromophoric ligand with
some angle (~20˚-45˚) away from Pt-N bond vector.

Figure 5.12. a) Schematic of TDV relative to the molecular frame of (ppy)Pt(dpm). b)
triplet spin density of (ppy)Pt(dpm) calculated with B3LYP/LACV3P**. c) calculated
TDV of (ppy)Pt(dpm) with B3LYP/6-31G**[Pt: DYALL-2ZCVP-ZORA-J-Pt-GEN].

a) b)
c)
123

We selected a chromophoric ligand with a large π- system to begin the analysis and
modified the periphery of the system afterwards to see the structure -property relationship
of these emitter orientation. The initial target molecule was (dbq)Pt(dpm), platinum(II)
(dibenzo[f,h]- quinolinato-N,C12) (2,2,6,6-tetramethyl-3,5-heptadienoato-O,O) with an
extended π-system.

Figure 5.13. Direction of TDV of (dbq)Pt(dpm).
The angle dependent PL measurements performed for this molecule in 10 % (v/v)
in 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) gave an interesting observation. The
anisotropy factor measured for this system was Ɵ=54% indicating this molecule aligns
somewhat vertical relative to the substrate in CBP host matrix (Figure 5.14). at 10% (v/v)
doping concentration this molecule did not form any excimer/dimer in the doped fim,
therefore only the monomer emission was observed and that matches well with the
previously reported emission spectrum of this molecule by our group.
23
Both the Pt
complexes reported previously gave isotropic orientation (Ɵ ≈ 32%) and this directional
result gave us a handle to modify this system further to observe a change in orientation.
19

124

Figure 5.14. a) Photoluminescence spectra of thin film with (dbq)Pt(dpm) b) angle
dependent PL spectra of same film.
There are two strategies to modify the cyclometalating core of the heteroleptic
square-planar Pt(II) complexes (Figure 5.15).
1) Modify the periphery of the ancillary ligand.
2) Modify the periphery of the chromophoric ligand.

Figure 5.15. Schematic representation of the chromophoric ligand and the ancillary ligand
of the heteroleptic Pt complexes. Red arrow indicates the direction of the TDV.
Peak
=488.10nm
Peak value
used for
calculation
a)
b)
Chromophoric
ligand
Ancillary
ligand
0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
2.5
Detector Angle [Degree]
Normalized Intensity
Isotropic ( = 33.3%)
Horizontal ( = 0%)
Fit ( = 54.1%)
CBP:(dbq)Pt(dpm)(10%)
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Intensity
Wavelength [nm]
CBP:(dbq)Pt(dpm) (10%)PL
125

Both aforementioned modifications have to be done with carefully selected
functional groups without changing the direction of the overall TDV. Modifying the
ancillary group was the easiest approach to keep the TDV a constant because the emission
energy intimately related to the energy of the chromophoric (C^N) ligand and the ancillary
ligand is chosen to be “non-chromophoric” that is, to have sufficiently high singlet and
triplet energies such that the excited state properties dominated only by the (C^N) ligand
with lower triplet energy.
Modifying the ancillary ligand of our parent compound was somewhat challenging
because the chromophoric (dbq) ligand already having a high triplet energy and most of
the ancillary ligands with a π-systems like 1,3-diphenylpropane-1,3-dione (dbm) and 1,3-
dimesitylpropane-1,3-dione (Dmes) possess relatively lower triplet energies. Therefore, if
we use this dbq chromophoric ligand with such low triplet energy ancillary ligands there
is a tendency the ancillary ligand participates in the emissive process instead of the (C^N)
ligand and alter the direction of the TDV or simply quench the emission. Given those
circumstances, we picked (dbx)Pt(dpm) system as the platform for all the modifications,
simply because ease of synthesis and the low triplet energy of the dibenzo-(f,h)quinoxaline
(dbx) ligand compared to dbq which allow us to use a wide a variety of ancillary ligands.
The second strategy to modify the periphery Pt complex is to change the
chromophoric ligand by keeping the direction of TDV same. Introducing different
functional groups to dbx ligand became quite challenging because dbx is already a bulky
ligand and any additional aromatic or sterically hindered groups introducing to the system
make the overall cyclometalated complex bulkier and difficult to sublime or thermally
evaporate to fabricate thin films. Therefore, we shifted to (ppy)Pt system with a relatively
126

smaller π-system to see the effect of orientation. All the molecules synthesized and
analyzed are depicted in Figure 5.16.

Figure 5.16. Structures of all the Pt heteroleptic complexes (1-9) synthesized to analyze
the orientation effect in EML of the OLED.
127

The other important feature about dibenzo-(f,h)quinoxaline ligand is that it
possesses two coordination sites to Pt metal, leading formation of bis-Platinum complexes.
Consequently, these bis-Pt complexes are also very interesting platform to study the
molecular orientation as they induce different optical characteristics and surface effects
than mono-nuclear Pt cyclometalates.
5.2.2. Synthesis.
The synthesis and photophysical characterization of complex 1,5,7 and 9 have been
reported previously.
23-25
Complex 2 and 6 have similar photophysical properties as their
dpm analogues 1 and 5 respectively. The synthesis of complex 3, 4 and 8 was done in a
similar fashion as all the other cyclometalated Pt complexes and the (dbx)Pt system is
illustrated as the archetypal in Figure 5.17.

Figure 5.17. Two routes for the synthesis of Pt cyclometalated complexes. Conventional
Nonoyama synthesis (top), modified synthesis (bottom).

128

The synthesis of dbx ligand was done according to the literature reported
procedure.
24
The ancillary ligand 1,3-dimesityl-propane-1,3-dione (Dmes) was
synthesized via a stable intermediate (its aluminum complex) from malonyl dichloride and
mesitylene by Friedel-Craft reaction using anhydrous aluminum chloride as the catalyst.
The resultant aluminum intermediate complex was isolated, which then decomposed upon
reflux in concentrate hydrochloric acid to give the Dmes ligand as reported.
26
The ancillary
ligand (CF3-dpm) for complex 4 and the cyclometalating ligand (ppy) for complex 6 and 8
are commercially available.
(dbx)Pt(Dmes)- A 3-neck flask was charged with dbx (0.18 mL, 1.3 mmol), potassium
tetrachloroplatinate(II) (230 mg, 0.55 mmol) and 60 mL of 3:1 mixture of 2-
ethoxyethanol:water. The mixture was degassed and heated to 70 ºC for 16 hrs. The
reaction was cooled to ambient temperature and the brown solid was precipitated into water
and isolated by vacuum filtration. This solid was then placed in a new 3-neck flask charged
with potassium carbonate (180 mg, 1.3 mmol), ancillary ligand 1,3-dimesityl-propane-1,3-
dione (xx ) and charged with 30 mL of degassed 1,2-dichloroethane. The condenser was
attached and the mixture was heated to 75 ºC for 16 hrs. The reaction was then cooled to
ambient temperature and the solvent was removed under vacuum. The resultant solid was
subjected to column chromatography on silica gel 1:1 CH2Cl2: hexanes gradient to give a
orange solid 145 mg (51%)
1
H NMR (500 MHz, CDCl3, δ) 9.12 (d, J = 7.87 Hz, 1H), 9.07
(d, J = 2.63 Hz, 1H), 8.68 (d, J = 2.63 Hz, 1H), 8.60 (d, J = 8.06 Hz 1H), 8.19 (d,
J = 7.97 Hz, 1H), 7.82 (t, J = 7.31 Hz, 1H), 7.74 (t, J = 6.80 Hz, 2H), 7.59 (t, J = 7.57 Hz,
1H), 6.89 (d, J = 8.14 Hz, 4H), 5.77 (s, 1H), 2.40 (d, J = 6.55 Hz, 12H), 2.32 (d, J = 6.08 Hz,
6H).
129

Figure 5.18.
1
H NMR spectrum of (dbx)Pt(Dmes) in CDCl3.
(ppy)Pt(Dmes)- A 3-neck flask was charged with ppyH (200 mg, 0.70 mmol), potassium
tetrachloroplatinate(II) (116 mg, 0.28 mmol) and 15 mL of 3:1 mixture of
2-ethoxyethanol:water. All the reactants were degassed and heated to 70 ºC for 16 hrs. The
reaction was cooled to ambient temperature and the orange-yellow solid was precipitated
into water and isolated by vacuum filtration. This solid was then placed in a new 3-neck
flask charged with potassium carbonate (186 mg, 1.4 mmol), ancillary ligand 1,3-
dimesityl-propane-1,3-dione (xx ) and charged with 15 mL of degassed 1,2-dichloroethane.
The condenser was attached and the reaction was heated to 75 ºC for 16 hrs. The reaction
was then cooled to ambient temperature and the solvent was removed under vacuum. The
resultant solid was subjected to column chromatography on silica gel 1:1 CH2Cl2: hexanes
to give a bright yellow emissive solid (98 mg, 61%).
1
H NMR (500 MHz, CDCl3, δ) 8.96
130

(d, J = 5.50 Hz, 1H), 7.78 (t, J = 7.12 Hz, 1H), 7.63 (d, J = 7.96 Hz, 1H), 7.53 (d,
J = 7.55 Hz, 1H), 7.44 (d, J = 7.19, 1H), 7.11 (m, 2H), 7.03 (t, J = 6.80 Hz, 1H), 6.88 (d,
J = 8.26, 4H), 5.66 (s, 1H), 2.37 (d, J = 7.31 Hz, 12H), 2.30 (d, J = 5.59 Hz, 6H). Anal. For
(ppy)Pt(Dmes): found: C 58.70, H 4.79, N 2.38; calcd: C 58.53, H 4.76, N 2.13.

Figure 5.19.
1
H NMR spectrum of (ppy)Pt(Dmes) in CDCl3.
5.2.3. Crystal structure analysis.
The X-ray crystal structure of (dbx)Pt(dpm) crystal grown from the sublimation is shown
in Figure 5.20. The unit cell contains four formula units and molecules are packed favoring
d-π interaction between Pt d orbitals and π-orbitals of the dbx ligand along the z-axis of the
square-planar Pt molecule.
131

Figure 5.20 (a) ORTEP drawing of complex (dbx)Pt(dpm) (1) with ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. (b) Illustration of the packing of
two molecules inside the unit cell in single crystal.

a)
b)
132

5.2.4. Angle dependent photoluminescent results.
Films of complexes doped into CBP were vapor deposited at 10% (v/v) and were
subjected to Angle-dependent p-polarized emission measurements to determine the net
orientation of the TDVs. Emission of (dbx)Pt(dpm), (dbx)Pt(acac)-monomer, resulted in
net vertical orientation with Ɵ values ranging from 46% and 47% respectively
(Figure 5.21), indicating Pt complexes with larger π-chromophoric ligands tend to interact
more with the organic host matrix upon thermal deposition.

Figure 5.21. Angle dependent PL spectrum for complexes 1 (left) and 2 (right) and their
corresponding emission spectra in 10vol% doped films in CBP on the bottom.
0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
Isotropic ( 
ver
= 33%)
Horizontal ( 
hor
= 0%)
Normalized Intensity

Detector Angle [Degree]
Fit ( 
ver
= 46.0 0.8%)
CBP : (dbx)Pt(dbm) 10vol.%
0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
Isotropic ( 
ver
=33%)
Horizontal ( 
ver
= 0%)
Normalized Intensity
Detector Angle [Degree]
CBP: (dbx)Pt(acac) (1 vol.%)

ver
= 0.47 0.01
450 500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity
Wavelength [nm]
10vol.% CBP:PtDBQXdpm
450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Intensity
Wavelength [nm]
CBP:(dbx)Pt(acac)(10vol%)
133

In both examples, the ancillary ligand was kept more aliphatic using dpm and acac
type ligands. Therefore, molecules possess two distinguishable aliphatic and aromatic
regions. Thus, the more aromatic chromophoric ligand have more preference to interact
with the organic host matrix and the more aliphatic ancillary ligand dangle out at the
vacuum-organic boundary during thermal deposition. In order to confirm this observation,
we designed a Pt heteroleptic complex with less chemical anisotropy, meaning we made
the ancillary ligand also more aromatic while keeping the chromophoric ligand the same
(dbx). The angle dependent PL data of this complex gave us interesting result confirming
our hypothesis. The Ɵ value obtained for this complex is 27% indicating the molecule is
more towards isotropic orientation (Figure 5.22).

Figure 5.22. Angle dependent PL spectrum for complex 3 and its corresponding emission
spectra in 10vol% doped films in CBP.

0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
Isotropic ( 
ver
= 33.3%)
Horizontal ( 
ver
= 0%)
Normalized Intensity
Detector Angle [Degree]
CBP:(dbx)Pt(Dmes) (10 vol.%)
Fit ( 
ver
=0.27 0.01)
450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Intensity
Wavelength [nm]
CBP:Pt(dbx)(Dmes-acac)

(10vol.%)
134

Figure 5.23. Electrostatic distribution map of (dbx)Pt(dpm) (left) and of (dbx)Pt(Dmes)
(right) showing shape and chemical anisotropy of the two molecules. Color coordinates:
electronegativity of atoms decrease when going from red to blue.
By making the ancillary ligand more aromatic now there is less preference for only
the chromophoric dbx ligand to interact with the organic host matrix, instead the two
mesityl groups in the ancillary ligand also have the preference to interact with the host
matrix in the thermal deposition.
Support for the surface-promoted alignment of dopants is seen in the next couple
of molecules we analyzed that include (ppy)Pt(dpm) and (ppy)Pt(Dmes). In this example,
we reduce the size of the chromophoric ligand slightly by keeping the direction of TDV
almost the same. As expected by decreasing the aromatic nature, the molecule shows less
vertical orientation compared to (dbx)Pt(dpm) (Ɵ = 46%). The Ɵ values measured for
(ppy)Pt(dpm) is 38%. When introducing more aromatic Dmes as the ancillary ligand, the
orientation become perfectly isotropic indicating the three aromatic regions in the Pt
Ɵ = 46% Ɵ = 27%
135

heteroleptic complex have equal preference in interacting with the host matrix upon
thermal deposition.

Figure 5.24. Angle dependent PL spectrum for complexes (ppy)Pt(dpm) 7 (left) and
(ppy)Pt(Dmes) 8 (right) and their corresponding emission spectra in 10vol% doped films
in CBP on the bottom.
0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
Isotropic ( 
ver
= 33%)
Horizontal ( 
hor
= 0%)
CBP:(ppy)Pt(dpm) (10 vol.%)
Fit ( 
vver
=0.38 0.01)
Normalized Intensity
Detector Angle [Degree]
0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
Isotropic ( 
ver
= 33%)
Horizontal ( 
ver
= 0%)

Normalized Intensity
Detector Angle [Degree]
CBP:Pt(ppy)Dmes-acac (10 vol.%)

ver
= 0.33 0.01
450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Intensity
Wavelength [nm]
CBP:(ppy)Pt(dpm)

(10vol.%)
450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity
Wavelength [nm]
CBP:(ppy)Pt(Dmes) (10vol.%)
136

Upon introducing Dmes ligand, this shape anisotropy decreases more and the
molecule become spherical instead of planer as clearly seen in Figure 5.25 and result in
perfectly isotropic orientation.

Figure 5.25. Electrostatic distribution map of (ppy)Pt(dpm) (left) and of (ppy)Pt(Dmes)
(right) showing shape and chemical anisotropy of the two molecules.
Two of the molecules, (dbx)Pt(acac) and (dbq)Pt(acac) in the study form excimers
in the doped thin films. The formation of the excimer could clearly be observed in the CBP
doped thin film PL spectrum (Figure 5.26). The orientation of these Pt dimers was studied
earlier in a neat layer of Pt(Fppz)2 type molecules.
21
The excimer formation occurs in doped
films due to lack of sterically encumbered groups attached to the Pt complex as in
(dbx)Pt(dpm) complex. The formation of these excimers in a relatively concentrated doped
film/solution is a common observation for many Pt complexes bearing acac type ligands
as they have this open coordination site along the z-axis for to interact with a second
molecule. The angle dependent PL analysis done for films made with (dbx)Pt(acac) and
(dbq)Pt(acac) indicate that the excimers formed also aligns vertically with respect to the
Ɵ = 38%
Ɵ = 33%
137

substrate (Figure 5.26). The angle dependent emission was collected at the excimer
emission wavelength.

Figure 5.26. Angle dependent PL spectrum for complexes (dbq)Pt(acac) 6 (left) and
(dbx)Pt(acac) 2 (right) and their corresponding emission spectra.
Another interesting finding of this study is the alignment of bis-Pt complexes. The
(dbx)[Pt(dpm)]2 complex resulted in horizontal orientation upon thermal deposition in CBP
doped films. So far in literature there is no president for molecular alignment analysis for
bis-Pt complexes to the best of our knowledge. The TDV calculated for this molecule is
0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
Isotropic ( 
ver
= 33%)
Horizontal ( 
ver
= 0%)
Normalized Intensity
Detector Angle
(dbx)Pt(acac):CBP (10 vol.%)

ver
= 0.39 0.01
0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
2.0
Isotropic ( 
ver
= 33%)
Horizontal ( 
hor
= 0%)
Normalized Intensity
Detector Angle [Degree]
Fit ( 
ver
= 44% 0.01)
CBP:(dbq)Pt(acac) (10 vol%) (Excimer)
450 500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Wavelength [nm]
Normalized Intensity

CBP:(dbq)Pt(acac)(10 vol%)
500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity
Wavelength [nm]
CBP:(dbx)Pt(acac) (10 vol%)
138

along the Pt-Pt bond vector and this suggest the aromatic dbx ligand interact more with the
Organic host matrix again confirming our hypothesis.

Figure 5.27. Angle dependent PL spectrum for complex 9 (left) and its corresponding
emission spectra (red) in 10vol% doped films in CBP.
More electronegative CF3-group containing (dbx)Pt(CF3-dpm) resulted in isotropic
orientation. The duty of CF3 group in the alignment process is not well studied and that is
the reason for us to pick this molecule to get an insight in to this matter. The PL spectra
gave a broad structureless emission indicating dimer/excimer formation. Further analysis
is required to comment on the emitters with CF3 groups’ alignment. The summary of all
the Ɵ values obtained for angle dependent PL measurements are listed below.

0 10 20 30 40 50 60 70 80 90
0.0
0.5
1.0
1.5
Detector Angle [Degree]
Normalized Intensity
Isotropic ( 
ver
= 33.3%)
Horizontal ( 
ver
= 0%)

Fit ( 
ver
= 23.5 1.3%)
CBP:Pt
2
(DBQX)dpm
2
(10vol.%)
550 600 650 700 750 800 850
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity
Wavelength [nm]
CBP:Pt
2
(DBQX)dpm
2
(10vol.%)
139

Table 5.1. Summary of angle dependent PL data for complexes 1-9, measured in 10 vol%
doped into CBP.

5.3. Conclusions
We conclude that the chemical nature and the shape anisotropy play a vital role in
orienting molecules on substrate in thermal deposition. The aliphatic ligands like acac or
dpm in these heteroleptic Pt complexes induce an aliphatic patch in otherwise aromatic Pt
complexes giving them a chemical anisotropy. We reason that interaction of this
chemically anisotropic species at the boundary created between the vacuum and the organic
surface during deposition in responsible for the observed net alignment of TDVs of the
Complex θver (±0.01)
(dbx)Pt(dpm) 0.46
(dbx)Pt(acac)-monomer 0.47*
(dbx)Pt(acac)-excimer 0.39
(dbx)Pt(Dmes) 0.27
(dbq)Pt(dpm) 0.54
(dbq)Pt(acac)-monomer 0.47
(dbq)Pt(acac)-excimer 0.44
(dbx)Pt(CF3-dpm) 0.33
(ppy)Pt(dpm) 0.38
(ppy)Pt(Dmes) 0.33
(dbx)[Pt(dpm)]2 0.23
140

dopant. In this study, among all the ancillary ligands used, Dmes gave a considerable shift
in alignment towards isotropic direction as it possesses more aromatic character than acac
or dpm. This will avoid the dbx or dbq (chromophoric ligand having a larger π-system)
ligands to preferentially interact with the host material upon thermal deposition. Giving all
the aromatic substituents somewhat equal probability to interact with the host.

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Abstract (if available)

Abstract Since the breakthrough by Kodak in 1987, Organic light emitting devices (OLEDs) have attracted a great deal of attention as a promising technology for future display applications and solid-state lighting. The lower power dissipation, high brightness, light weight and high color quality of OLEDs make them ideal for many applications. Despite intense research efforts during the last decade there are still improvements to be made in OLED-lifetime and OLED-outcoupling when it comes to commercializing these products. Among the three primary Red, Green, and Blue (RGB) colors, green and red phosphors that meet the necessary lifetime requirement has already been well established, but the design and fabrication of stable blue phosphorescent OLEDs is still an ongoing challenge. Therefore, my graduate research primarily focuses on developing stable blue phosphorescent emitters and apply them in efficiently engineered devices. ❧ In addition, OLED light outcoupling has also become an enormous challenge in the industry. Simple ray-optics allows to estimate the external quantum efficiency of a standard OLED to 20% of the initially generated light. Therefore 80% of the light generated, will be trapped inside the device due to waveguides and surface plasmons. To overcome this problem many approaches have been introduced and controlling the alignment of the emitting molecules used as dopants in organic light emitting diodes has been identified as an effective strategy to improve the outcoupling efficiency of OLED devices. In this study, I will demonstrate my involvement in designing of stable blue OLEDs and structure property related light outcoupling of these phosphorescent emitters in making stable and highly efficient OLEDs to be used in future display and lighting technologies.

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Asset Metadata

Creator Batagoda, Batagoda Kankanama Thilini Sasiprabha (author)

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

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 11/06/2017

Defense Date 09/20/2017

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

Tag degradation,light outcoupling,OAI-PMH Harvest,OLEDs,organometallic complexes,phosphorescent emitters

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark (committee chair), Goo, Edward (committee member), Thompson, Barry (committee member)

Creator Email batagoda@usc.edu,thilinisb2014@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-452361

Unique identifier UC11263705

Identifier etd-BatagodaBa-5873.pdf (filename),usctheses-c40-452361 (legacy record id)

Legacy Identifier etd-BatagodaBa-5873.pdf

Dmrecord 452361

Document Type Dissertation

Rights Batagoda, Batagoda Kankanama Thilini Sasiprabha

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

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