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

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Novel Pyrimidine-based Hole Blocking Materials for Long-lived and Highly Efficient Organic
Light Emitting Diodes

by

Brenda Ontiveros

A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CHEMISTRY)

December 2020

Copyright 2020 Brenda Ontiveros
ii

Acknowledgements
I would like to express my sincere gratitude and appreciation, first and foremost, to my Ph. D.
advisor and mentor, Prof. Mark E. Thompson, for giving me the opportunity to be a part of an inclusive and
supportive research group. I thank him for his encouragement and care during my graduate school, and for
establishing a nurturing environment for scientific, intellectual, and personal growth in his laboratory. I am
grateful for his patience and motivation. Prof. Thompson and his research group have helped me grow into
a creative and independent scientist.
I would also like to thank Dr. Peter Djurovich for his guidance and assistance throughout my
graduate studies at USC. His vast knowledge of literature and invaluable suggestions have been imperative
for the development of my research projects. His challenging questions have helped expand my scientific
perspective and critical thinking skills.
I extend my appreciation to Prof. Brent Melot, Prof. Barry Thompson, Prof. Andrea Armani, and
Prof. Megan Feiser. I would like to thank Prof. Feiser for being an exemplary role model, as well for her
continuous efforts in creating supportive environments for students and women in the Department of
Chemistry.
My colleagues at USC have played an imperative role throughout my graduate career; I thank them
for all their knowledge, comfort, and motivation. I would like to thank Dr. Muazzam Idris for his patience
and care as my mentor in the Thompson group. I thank my senior and former colleagues Dr. Jessica Golden
and Taylor Hodgkins for being exemplary scientific female role models. I would also like to thank the
current members of the Thompson group; Daniel Sylvinson, Moon Chul Jung, Jie Ma, and Abegail Tadle
who I collaborated and exchanged ideas on projects. I extend my appreciation to the rest of the group
members, past and present, for helping me with my studies and research, as well as for their comfort and
emotional support. I would like to thank JoAnna Milam-Guerrero who helped me with thermogravimetric
analysis for my project. A special thanks to Daniel Sylvinson and Cindy Tseng for their friendship and
kindness.
I am grateful for all my professors throughout my graduate coursework, and Dr. Richard Brutchey
for allowing me to rotate in his laboratory and giving me access to funding opportunities. I thank the USC
Department of Chemistry, Dornsife College, Women in Science and Engineering, and Universal Display
Corporation for funding my graduate studies and research project. Thank you to the Department of
Chemistry administration, Michelle Dea, Magnolia Benitez, and especially Judy May Fong, for their
administrative help and assistance. I would also like to the thank The Haven at USC for their care
throughout my time as a graduate student.
iii

My endless gratitude goes to my family and friends for their encouragement, patience, and support
throughout this journey. I am honored to have had such an invaluable support system, at home, in Los
Angeles, and at USC.

iv

Table of Contents
Acknowledgements ..................................................................................................................................... ii
List of Figures .............................................................................................................................................. v
List of Tables ............................................................................................................................................ viii
Abstract ....................................................................................................................................................... ix
Chapter 1 – Introduction ............................................................................................................................ 1
1.1.1. Photophysics of Luminescent Materials ..................................................................................... 2
1.1.2. Fluorescent and Phosphorescent Emission ................................................................................ 3
1.1.3. Thermally Activated Delayed Fluorescence (TADF) Emission ............................................... 6
1.1.4. OLED Efficiency ........................................................................................................................... 7
1.2. OLED Architecture ......................................................................................................................... 8
1.2.1. Materials used in OLEDs ...................................................................................................... 9
1.2.1.1. Host Materials ....................................................................................................................... 9
1.2.2 Hole and Electron Transport Materials ................................................................................ 13
1.2.3 Hole and Electron Blocking Layers ....................................................................................... 16
1.2.4. Emitting Materials/Dopants .................................................................................................. 20
1.2.5. Exciton-Exciton and Exciton-Polaron Annihilation ........................................................... 23
References .................................................................................................................................................. 25
Chapter 2 – Pyrimidine-based Hole Blocking Materials for Blue OLEDs ................................ 31
2.1. Introduction ................................................................................................................................... 31
2.2. Results and Discussion .............................................................................................................. 33
2.2.1. Design and Synthesis ............................................................................................................. 33
2.2.2 Electrochemical and Thermal Properties ............................................................................. 34
2.2.3. Photophysical Properties ....................................................................................................... 40
2.2.4. Performance of Blue Phosphorescent OLEDs ..................................................................... 44
2.3. Conclusion ...................................................................................................................................... 50
2.4. Experimental .............................................................................................................................. 51
2.4.1. General .................................................................................................................................... 51
2.4.2. DFT and TD-DFT Calculations ............................................................................................ 53
2.4.3. OLED Fabrication and Characterization ............................................................................. 53
2.4.4. Synthesis .................................................................................................................................. 54
2.4. Experimental .............................................................................................................................. 56
References .................................................................................................................................................. 57
v

List of Figures
Chapter 1
Figure 1. 1. Jablonski diagram illustrating the photophysical mechanisms upon electrical
excitation. ........................................................................................................................................ 2

Figure 1. 2. Schematic diagram displaying electroluminescence excitation processes for organic
and organometallic transition metal emitters, respectively, and explains the effects of triplet and
singlet harvesting.
9
.......................................................................................................................... 3

Figure 1. 3. Schematic diagram of selected molecular orbitals (MOs) for a (pseudo)octahedral
complex with low lying MLCT transitions.
9
................................................................................. 5

Figure 1. 4. Jablonski diagram depicting the excited states and processes involved in TADF. ..... 6

Figure 1. 5. Simplified structure of a double heterostructure device. ............................................. 8

Figure 1. 6. Schematic representation of Förster energy transfer (a) and Dexter energy transfer
(b); energy transfer (c) and charge trapping (d) alignment for dopant emission in host–dopant
systems.
23
........................................................................................................................................ 9

Figure 1. 7. Schematic energy-level alignment of the S1 and T1-excited states and S0 ground
states, as well as the energy transfer and emission processes in host–dopant systems.
23
............. 10

Figure 1. 8. Hole-transport type host materials............................................................................. 11

Figure 1. 9. Electron-transport type host materials. ...................................................................... 11

Figure 1. 10. Bipolar-type host materials...................................................................................... 12

Figure 1. 11. Typical hole-transporting materials containing triarylamine and carbazole. .......... 14

Figure 1. 12. Representative electron-transport materials containing various EWGs. ................. 15

Figure 1. 13. Emission spectra of PtOEP-CBP devices with and without hole/exciton-blocking
layer, BCP. (a) Device without BCP exhibits significant Alq3 emission at λmax ∼520 nm. (b) No
Alq3 emission demonstrates the effect of the BCP layer. ............................................................. 16

Figure 1. 14. Schematic diagram of the energy levels of an efficient PHOLED with a hole-
blocking layer. Excitons are depicted as starburst patterns. ......................................................... 17
vi

Figure 1. 15. Schematic diagram of the energy levels of an efficient PHOLED with an electron-
blocking layer. Excitons are depicted as starburst patterns. ......................................................... 18

Figure 1. 16. Spiro-linked derivatives of common electron-blocking/hole-transporting materials.
....................................................................................................................................................... 19

Figure 1. 17. Molecular structures of widely investigated triplet emitters. .................................. 20

Figure 1. 18. Strategies to blue shifting the emission of cyclometalated Ir complexes.
79
............ 21

Figure 1. 19. Examples of blue TADF emitters. ........................................................................... 22

Figure 1. 20. Configuration diagrams of (a) TTA, and (b) TPQ process. .................................... 24

Chapter 2
Figure 2. 1. Pyridine-based HBMs commonly used in blue PhOLEDs........................................ 32

Figure 2. 2. Multisubstituted pyrimidines presented in this work. ............................................... 33

Figure 2. 3. Synthesis of (a) trisubstituted pyrimidines and (b) tetraphenylsubstituted pyrimidine.
....................................................................................................................................................... 34

Figure 2. 4. CV curves of trisubstituted pyrimidines TP, DPD, and TPD. ................................... 35

Figure 2. 5. DPV curves of trisubstituted pyrimidines TP, DPD, and TPD. ................................ 36

Figure 2. 6. DSC Curves of TP depicting its crystallization and melting temperatures. .............. 37

Figure 2. 7. Cyclic voltammetry curves of TP, MTP, TPP, TP2. ................................................. 38

Figure 2. 8. DPV curves of TP, MTP, TPP, and TP2. .................................................................. 39

Figure 2. 9. TP2 DPV reduction curves (left) and CV reduction curves (right) referenced to
Fc
+
/Fc. ........................................................................................................................................... 39

vii

Figure 2. 10. (a) Absorption spectra of TP, MTP, TP2, and DPP in 2-MeTHF. .......................... 41

Figure 2. 11. Normalized emission spectra of TP (a), MTP (b), TPP (c), and TP2 (d), in 2-
MeTHF and in solid-state at 298 K and 77 K. (e) Comparison of T1 energies at 77 K for all
compounds. ................................................................................................................................... 42

Figure 2. 12. Optimized structures and frontier molecular orbital representation of pyrimidine
HBMs. ........................................................................................................................................... 43

Figure 2. 13. EL spectra of TP, MTP, TPP, and TP2 in the solid-state at 77 K. .......................... 44

Figure 2. 14. Device architecture and chemical structures of the OLED materials. ..................... 46

Figure 2. 15. Device characteristics of blue PhOLED using TP as HBL. (a) EQE vs. current
curves, (b) EL spectrum, (c) J-V curves, and (d) luminance vs. current curves. ......................... 46

Figure 2. 16. Device architecture and chemical structures of the OLED materials for comparative
studies. .......................................................................................................................................... 47

Figure 2. 17. Device characteristics of blue PhOLED using BCP, TP, and MTP as HBLs. (a) EL
spectra, (b) EQE vs. current curves, (c) luminance vs. current curves, and (d) J-V curves. ........ 48

Figure 2. 18. Energy level diagram and materials used in devices for lifetime studies. ............... 49

Figure 2. 19. Luminance of blue PhOLED using fac-Ir(tppz)3 as the emitter and MTP and T2T as
HBLs. ............................................................................................................................................ 50

viii

List of Tables
Table 2. 1. Properties of the hole-blocking materials in Figure 2.1.............................................. 32

Table 2. 2. Summarized electrochemical properties of TP, DPD, and TPD. ................................ 35

Table 2. 3. Summarized electrochemical properties of TP, MTP, TP2, and DPP. ....................... 40

Table 2. 4. Summarized electrochemical and photophysical properties of selected pyrimidine
HBMs. BCP is listed here as a reference. ..................................................................................... 45

ix

Abstract

Organic light-emitting diodes (OLEDs) have emerged as an important technology for attractive, high
efficiency flat-screen displays and solid-state lighting applications. Particularly, red and green
phosphorescent OLEDs (PhOLEDs) have garnered significant attention due to their 100% internal quantum
efficiencies and long operational lifetimes of T95>100,000, which is sufficient for display and lighting
applications.
1
On the other hand, achieving high efficiencies and long lifetimes in blue OLEDs remains a
significant challenge.
2
Their inefficiency and lack of stability is caused by the degradation of organic layers
and emissive materials caused by the formation of high energy excited states upon electrical excitation. One
of the important factors for achieving high efficiency PhOLEDs is confining these excited states in the
emitting layer. For this reason, four novel hole-blocking materials (HBMs) materials based on 1,3,5-tri-
and 1,3,4,5-tetrasubstituted pyrimidine were synthesized. TP, MTP, TPP, and TP2 were found to possess
high T 1 and deep HOMO energy levels, as well as high T g values for their use as HBMs in blue PhOLEDs.
The thermal, electrochemical, and optical properties of the pyrimidine compounds were systematically
investigated and are discussed in Chapter 2.

1

CHAPTER 1 – Introduction

Light-emitting diodes (LEDs) are solid-state devices built from noncarbon-based materials such as
silicon or In xGa 1–x, GaN, etc.
1
Light emission occurs when current flows through the device causing an
electrical excitation of electrons. Upon relaxation, the electrons recombine with holes in the semiconductor
device releasing energy in the form of photons. Organic LEDs (OLEDs) promote electrons and emit light
through the same excitation mechanism, however, the electroluminescent material in the active layer of the
diode is composed of organic compounds. Unlike conventional LEDs consisting of an array of individual
bulbs, OLEDs use a series of thin, light emitting films wherein each pixel directly produces light. This
property allows OLEDs to be fabricated into light and flexible panels while produce brighter light and
consuming less energy than LED technologies. Over the past few years, OLEDs have been implemented in
a wide range of applications including full color displays for mobile phones and flat screen TVs, wearable
technology such as watches, and solid-state lighting.
2-4

1.1.1. Photophysics of Luminescent Materials

Understanding the photophysics of light emission in organic and organometallic complexes is
crucial in tuning their photophysical and electrochemical properties to fit the demands of a device.
Electronic transitions between ground and excited states can be illustrated using the Jablonski diagram
shown in Figure 1.1.
1, 5
In both organic and organometallic complexes, an excited state is generated when
visible light is absorbed by a chromophore, promoting an electron from an occupied molecular orbital to a
higher lying unoccupied orbital of the molecule. This process is equivalent to promoting an electron from
a singlet ground state (S 0) to a higher laying excited state (S n). The electron in the excited S n state can relax
via three main pathways. Interconversion (IC), which is a process where the excited electron in an S n state
non-radiatively relaxes to the lower lying S 1 state, also referred to as thermal deactivation. In the S 1 state,
the electron can either (1) non-radiatively relax back to the S 0 where the energy is released as heat, (2)
radiatively relax back to S 0 (fluorescence), or (3) non-radiatively decay to a triplet excited state T n, a process
referred to as inner system crossing (ISC). The electron in the T n then undergoes interconversion to the T 1
2

state. Direct relaxation from the T 1 state to S 0 (phosphorescence) involves the spin flip of an electron and
is thus a symmetry forbidden, low probabilistic event. For this reason, phosphorescence is slow and weak
for pure organic materials with large energy differences between the S 1 and T 1 ( E ST) states, often giving
luminescent lifetimes of milliseconds to minutes, as opposed to nanoseconds for fluorescent transitions.
1

Figure 1. 1. Jablonski diagram illustrating the photophysical mechanisms upon electrical excitation.

The probability of ISC from the S 1 →T 1 state can be increased by making the E ST gap significantly
smaller ( E ST < 0.1 eV)
6
or by incorporating organometallic complexes with heavy metals to facilitate the
transition through spin-orbit coupling (SOC). SOC is a physical phenomenon that allows the mixing of S 1
and T 1 states when the magnetic field generated by the nucleus of an atom interacts with the dipole moment
of its electron spin. The relative orientations of the atom’s spin axis and it’s orbital angular momentum axis
determines the energy levels of the system.
7
Because of the coulombic nature of the magnetic field, the
SOC factor is typically dependent on the magnitude of the atom’s nuclear charge. Therefore, the larger
nuclei of “heavier” metal atoms increase the probability of the S 1 →T 1 transition. Organometallic complexes
with heavy metal atoms such as Ir, Pt, and Os undergo fast radiative decay, often <100 μs,
8
leading to high-
emission efficiencies and phosphorescence at room temperature.
3

1.1.2. Fluorescent and Phosphorescent Emission

Figure 1.2. compares the efficiency obtainable with a purely organic molecule to that of a transition
metal complex, assuming both molecules exhibit equal photoluminescent quantum yields (PLQY).
9
As
mentioned in the previous section, two different electron-transfer pathways are possible upon formation of
an exciton and its subsequent radiative relaxation in the form of fluorescence or phosphorescence. Because
hole and electrons have equal population of positive and negative spins (m s = +1/2 and m s = -1/2), it is
possible that electron-hole recombination may result in identical or opposite spin pairs, i.e. as a singlet or a
triplet exciton. Based on spin-statistics, the ratio of singlets to triplets formed upon recombination is 1:3.
This splitting of the exciton into singlet and triplet fractions has a significant impact on the efficiency of
electroluminescence. If only the singlet-excited states in organic molecules emit through fluorescence, the
upper limit on OLED efficiency is 25%. On the other hand, organometallic compounds with heavy metal
centers exhibit fast ISC to the lowest triplet state via phosphoresce. In principle, a triplet emitter can achieve
almost 100% internal quantum efficiencies because of strong SOC.

Figure 1. 2. Schematic diagram displaying electroluminescence excitation processes for organic and
organometallic transition metal emitters, respectively, and explains the effects of triplet and singlet
harvesting.
9

4

Iridium and platinum organometallic complexes bearing mostly organic ligands are usually
employed as emitters in OLEDs.
1
The ligands in these complexes can form different geometries around the
metal center and have a significant impact on the orbital splitting of the complex. Excitation in
organometallic complexes involve transitions between π and d-orbitals resulting in the formation of excited
states. Figure 1.3 is used to represent orbitals for a pseudo-octahedral, d 6 organometallic complex, such as
Ir(ppy) 3,
10
with three chelating ligands that have three π and π* orbitals.
9
Three d-orbitals, representing the
split t 2g set, are also shown. Transition between the t 2g and the π* orbitals lead to the formation of four
energy states, one
1
MLCT and three substates of
3
MLCT. Inter- or intra-ligand π-π* transitions lead to
singlet and triplet excited states (
1
LC and
3
LC). Multiple studies on cyclometalated organometallic
complexes have demonstrated that luminescence for these complexes originates from the T 1 state, which is
a
3
MLCT 1 admixed with mostly
1
MLCT 2 and
3
LC states due to SOC.
11-13
The lowest energy excited state
will vary depending on the energies and amount of mixing between the
3
MLCT and
3
LC states. Research
has shown that the radiative decay rate in luminescent complexes is significantly increased when a small
amount of
1
MLCT 2 character is mixed into the
3
MLCT 1 state. As a result, the luminescent lifetimes
drastically decrease, and an increase in phosphorescence efficiency occurs simultaneously.
14, 15
In this
example, strong σ-donation of a formally anionic ligand stabilizes the
3
MLCT state, decreases the
3
MLCT−
3
LC (ΔE) energy gap, and allows for a higher degree of mixing between states resulting in an increase of
phosphorescence. For this reason, organometallic complexes bearing heavy atoms with cyclometalated
ligands are ideal dopants in OLEDs.
5

Figure 1. 3. Schematic diagram of selected molecular orbitals (MOs) for a (pseudo)octahedral complex
with low lying MLCT transitions.
9

1.1.3. Thermally Activated Delayed Fluorescence (TADF) Emission
Another strategy to increase the ISC is to drastically reduce the S 1-T 1 energy gap extremely small
(ΔE ST < 0.1 eV).
16
Thermal equilibration of the electron between the S 1 and T 1 states can occur if the ΔE ST
of an organic chromophore falls in the order of available thermal energy in surroundings (≈25.6 meV at
298 K).
17
When the S 1 state is populated after photoexcitation, it can radiatively decay through prompt
fluorescence, or it can nonradiatively decay to the T 1 state. From the T 1 state, it will thermally repopulate
back to the S 1 through reverse intersystem crossing (RISC), also referred to as up-ISC (UISC), where it
ultimately decays to S 0 followed by “delayed fluorescent” emission (Figure 1.4.). Though the emission
spectra of prompt fluorescence and thermally activated delayed fluorescence (TADF) commonly have
similar wavelength characteristics, the lifetime of TADF is comparable to that of T 1 states. The latter is
shown by the Arrhenius equation below (1), where K TADF is the rate constant for emission and is temperature
dependent, where A is the frequency factor and R is the gas constant. It can be rearranged to show the ratio
of quantum yields of TADF ( Ф TADF) and phosphorescence ( Ф p) (2), where Ф f and k p are fluorescence
quantum yield and the phosphorescence rate constant, respectively.
18
This indicates that the ratio is
independent of T 1 formation yield and any T 1 quenching process.
6

(1) 𝑘 TADF
= 𝐴 exp (−
Δ
𝑆𝑇
𝑅𝑇
)
(2)
Φ
TADF
Φ
P
= Φ
F
𝐴 𝑘 𝑝 exp (−
Δ𝐸 𝑆𝑇
𝑅𝑇
)

TADF emitters have recently garnered attention due to their exciton harvesting mechanism for their
application in OLEDs, as both S 1 and T 1 can be utilized for electroluminescence from organic
chromophores. It is particularly important in organic emitters that lack heavy atoms where phosphorescence
is inefficient due to weak SOC. As such, TADF materials have the potential to be utilized as alternatives to
rare-earth metal emitters that utilize T 1 excitons,
18-20
and makes devices susceptible to degradation due to
bimolecular quenching processes, a topic which is further elaborated in Section 1.2.5.

Figure 1. 4. Jablonski diagram depicting the excited states and processes involved in TADF.

1.1.4 OLED Efficiency
The external electroluminescence (EL) quantum efficiency (EQE) is a key parameter used to define
the performance of an OLED. It is described by the equation
(1) 𝜂 EQE
= 𝜂 int
× 𝜂 out
= (𝛾 × 𝜂 𝛾 × Φ
PL
) × 𝜂 out

where 𝜂 EQE
is the EQE, 𝜂 int
is the internal EL quantum efficiency, and 𝜂 out
is the light-out-coupling
efficiency. According to equation (1), 𝜂 int
is limited by the following three factors: (i) charge balance of
7

injected holes and electrons (𝛾 ), (ii) efficiency of the radiative exciton production 𝜂 𝛾 and (iii)
photoluminescence (PL) quantum yield (PLQY) of the emitter molecules Φ
PL
. The ideal 𝛾 can be achieved
by careful design of OLED structures with the appropriate selection of charge-transport layers, host–dopant
system, and anode/cathode materials. In addition, based on a proper molecular design for light emission,
Φ
PL
of nearly 100% has been demonstrated in a wide variety of fluorescent and phosphorescent materials.
19

𝜂 𝛾 severely limits the 𝜂 int
of fluorescent dopants, and hence EQE, of fluorescent OLEDs since only 25%
of generated excitons formed in S 1 states are harvested. P-and TADF-OLEDs, on the other hand, can
theoretically achieve EQEs of <100% through exciton harvesting of both S 1 and T 1 states.

1.2. OLED Architecture
An OLED can contain single or multiple layers of organic material between an anode and a cathode
(e.g. indium tin oxide and aluminum, respectively), totaling no more than 2,000 Å in thickness.
1
One of
the electrodes is supported on a glass, metal, or plastic substrate, and the organic materials are deposited as
smooth thin films. The basic structure of an OLED device is depicted in Figure 1.5 When a voltage is
applied to the device, molecules at the anode are oxidized and reduced at the cathode, forming holes and
electrons. These charge carriers then migrate either through the hole transport layer (HTL) or the electron
transport layer (ETL) towards the center of the film. Subsequent recombination of holes and electron occur
in the emissive layer (EML) forming excitons (hole-electron pairs). Depending on the pairing of the two
electrons, A S 1 or T 1 excited state is formed, and the exciton finally decays to the S 0 state emitting light of
a specific wavelength unique to the bandgap of the material comprising the EML.

8

Figure 1. 5. Simplified structure of a double heterostructure device.

With a few exceptions, the excited states generated in the EL are the same as those formed by the
photoexcitation of a chromophore through photoluminescence (PL). PL is a particularly useful tool for
screening active layer materials in terms of color and potential efficiency for emission in an OLED. PL
efficiencies are measured in solution whereas EL efficiencies for OLEDs are measured in the solid-state.
The dynamic quenching processes present in solution will differ from those in the solid state, and thus, PL
measurements are not suitable for estimating the potential EL efficiency. To do so would require the
emitting material to be analyzed under identical experimental conditions. However, both measurements can
be used congruently to understand the excited state and mobility of organic chromophores, which are crucial
for improving the lifetime and efficiency of OLEDs. Because PL measurements are more convenient and
less tedious than EL measurements, PL measurements are typically first utilized to characterize the
photophysical properties of OLED materials. Materials that show the desired properties for a specific device
are then used to fabricate OLEDs and their EL is characterized.
1.2.1. Materials used in OLEDs
1.2.1.1. Host Materials
OLEDs are double charge injection devices and thus require the simultaneous supply of both holes
and electrons to the emissive layer.
21
An OLED generally requires facile and balanced charge transport, as
well as a high conversion efficiency (EQE) of excitons to light.
22
To fulfill these requirements, most highly
efficient OLEDs tend to have multilayer device configurations containing; a hole transport layer (HTL), an
Substrate
Anode
HTL
EML
ETL
Cathode
Host & Dopant
V
-
+
9

electron transport layer (ETL), an emissive layer (EML), and a host material. Some also include carrier
injection and blocking layers. The charge transport/injecting layers are used to transport and provide holes
and electrons to the EML, where the charges recombine and form the excitons. Subsequently, the OLED
emits light and its emission spectrum is determined by the choice of the emitting material that is used in the
EML. The use of a luminescent material in the form of a neat film is rarely used due to self-quenching
processes caused by the molecular aggregation of formed excitons. Bimolecular processes often result in
molecular degradation which significantly reduce the lifetime and efficiency of OLEDs. These processes
are further discussed in Section 1.2.5. For this reason, luminescent materials are often doped into a host
material which is used to disperse the emitter molecules and increase the EQE of a device.

Figure 1. 6. Schematic representation of Förster energy transfer (a) and Dexter energy transfer (b); energy
transfer (c) and charge trapping (d) alignment for dopant emission in host–dopant systems.
23

In these host/dopant systems, three routes generally lead to emission, as shown above in Figure
1.6.
23-25
(1) S 1 excitons formed in the host are transferred to the emitter via Förster and Dexter energy
transfer. The excitons can then either relax to S 0 (fluoresce) or converted to T 1 excitons by efficient ISC
(phosphorescence) Figure 1.7.
23
(2) The T 1 excitons formed in the host can be transferred to the phosphor
10

through Dexter energy transfer, then radiatively decay producing phosphorescent emission. (3) Holes and
electrons injected from the electrodes can directly recombine on the phosphor, where the generated T 1
excitons by charge trapping of the host, relax through phosphorescence to the ground state.
26
Förster energy
transfer, directly from dopant to host, is a fast (∼10−12 s) and long-range process (up to 10 nm). Dexter
energy transfer is an electron-exchange interaction between the host and dopant, which is a short distance
process (1.5–2.0 nm).
23
For Förster transfer, the emission spectrum of the host matrix needs to overlap
significantly with the absorption spectrum of the dopant. On the other hand, Dexter transfer requires the
energies of the S 1 and T 1 for host and dopant to match. A significant offset of the HOMO and LUMO
energies between the host and dopant is required for the direct charge trapping on the phosphor, as
schematically depicted in Figure 1.6. (d).
27
Dexter energy transfer between the host and dopant is dominant
in PhOLEDs, but unfortunately the doping concentrations required to maximize EQE are much higher than
those devices doped with fluorescent dyes, which occur by long-range Förster transfer.

Figure 1. 7. Schematic energy-level alignment of the S1 and T1-excited states and S0 ground states, as well
as the energy transfer and emission processes in host–dopant systems.
23

To achieve efficient electrophosphorescence, the host should possess higher T 1 energies than those
of the emitter to prevent reverse energy transfer back to the host leading to energy loss, as well as confining
11

T 1 excitons in the emissive layer. The hosts must also have balanced charge carrier transport properties to
maximize recombination in the EML, in addition to having a good thermal and morphological stability to
prolong the lifetime of the device. Hosts usually incorporate hole and/or electron transport units and bulky,
sterically hindered moieties to enhance the latter, respectively. Host materials can be classified as hole-
transport-type (HT-type), electron-transport-type (ET-type), and bipolar transport hosts according to their
charge carrier transporting feature. Hole transport type materials feature electron donating groups for hole
trapping and transport. The holes trapped at the HTL interface on the host molecules can diffuse across the
EML before they attract electrons to form excitons that will eventually be transferred to the dopant.
Examples of hole-transporting host materials are depicted in Figure 1.8.

Figure 1. 8. Hole-transport type host materials.

E-type host materials on the other hand, contain electron-withdrawing groups in their molecular
structures and mainly transport electrons. These host materials often contain electron deficient heteroarene
moieties like oxadiazoles, imidazoles, triazoles, triazines, phenanthroline and diaryl phosphine oxide
groups (Figure 1.9.). Additionally, most E-type host materials demonstrate good hole-blocking properties
and thus are used as hole-blocking materials.

Figure 1. 9. Electron-transport type host materials.

12

Unbalanced charges in the EML can be detrimental to OLEDs, which can occur when using E-type
and H-type host materials because they shift the recombination zone according to their interface with the
EML.
28
Often devices fabricated using these materials result in a narrow charge recombination zone which
can expedite bimolecular processes, resulting in efficiency roll-offs due to local accumulation of high
density of high-energy excitons, especially under high current densities.
29
For this reason, bipolar-transport
host materials have aroused recent attention in the area of PhOLEDs because they can provide more balance
in electron and hole fluxes and simplify device structures. One difficulty in bipolar hosts is being able to
acquire high T 1 energies because EWG and EDG on one molecule lower the band gap of the material
through intramolecular charge transfer. To address this issue, molecular designs (Figure 1.10.) of bipolar
host materials focus on the interruption of the π-conjugation between EWG and EDG, which delocalizes
triplet energies. Strategies to achieve the latter include: introducing a fluorene or methyl steric group
between D and A units,
30, 31
connecting the two A-A through meta-and /or ortho-linkages,
32, 33
and by
incorporating flexible, non-conjugated σ*-linkage.
29

Figure 1. 10. Bipolar-type host materials.

Hosts for red and green emitters have extensively been developed compared to hosts for blue triplet
emitters. It is a considerable challenge for a blue phosphorescent host to meet the requirements of a
high T 1 (≥ 2.75 eV) to effectively confine triplet excitons on the dopant molecules. As shown in Figure
1.8. and Figure 1.9., conjugated organic molecules are the most common host materials for PhOLEDs.
However, because there is a trade-off between the triplet energy and the π-conjugation length of host
material,
34
host materials for blue PhOLEDs are limited to nonconjugated molecules. Biphenyl and
carbazole systems are the most commonly used host materials for blue emitters due to their higher
T 1 energies.
35, 36
Recently, fluorene, dibenzofuran, dibenzothiophene derivatives have been employed as
13

hosts for deep blue.
17
Additionally, naphthalene, triphenyl, and phenanthrene have been utilized for
developing host materials for sky-blue PhOLEDs, with T 1 energies around 2.60 eV.
27

1.2.2 Hole and Electron Transport Materials
As was emphasized in the previous section, hosts incorporate EWGs and EDGs to increase charge-
carrier transport ability. Hole and electron-transport materials follow the same strategy. Like host materials,
it is necessary for transport materials to have good charge-carrier mobility, as well as T 1 energies higher
than that of the phosphorescent emitter to confine the generated T 1 excitons in the emissive layer. The
structures of hole-transport materials usually contain electron-donating moieties, such as triarylamine,
diphenylamine, and carbazole. HTMs should have appropriate HOMO levels to ensure low energy barriers
for hole injection from the anode, and a suitable LUMO level to block electron injection from the EML to
the HTL. Figure 1.11. displays the chemical structures of the most frequently used HTMs in PhOLEDs.
TPD is commonly used as a HTL, especially for those based on organolanthanide complexes as emitters,
37,
38
however it suffers from having a low T g (≤ 65 °C), resulting in a lack of thermal and morphological
stability which makes the film susceptible crystallization. NPB is a structural modification of TPD through
the replacement of the tolyl group by the bulky naphthyl group, showing an improved T g (95 °C).
39

However, its T 1 energy slightly lowers to 2.29 eV.
40
NPB is the most widely used hole transport material
either in fluorescent or phosphorescent OLEDs, however, due to its low T 1 energy, it cannot be solely used
as a HTL in blue and white PhOLEDs. TCTA can be used concurrently with NPB due to its high T 1 energy
(2.76 eV),
36
serving as an exciton blocker and a “transitional” HTL to reduce the hole barrier injection from
NPB and the EML. TAPC exhibits the highest T 1 energy ((2.87 eV)
41
and is frequently used as an HTL for
highly efficient blue PhOLEDs, however, its morphological instability(T g = 78 °C) makes it undesirable
for long lifetime OLEDs.
42

14

Figure 1. 11. Typical hole-transporting materials containing triarylamine and carbazole.

In contrast to HTMs, electron transport materials (ETM) normally contain EWGs in their molecular
structures. Like HTMs, an appropriate ETM in PhOLEDs should possess good electron mobility, suitable
HOMO/LUMO levels to block holes and facilitate electron injection, appropriate triplet energies and good
thermal stability. Alq 3 (tris(8-hydroxyquinoline)aluminium) is one of the most commonly used ETMs
owing to its high LUMO level (3.0 eV) and T g (172 °C).
43
Because of its rather low triplet energy (2.0 eV)
and relatively high HOMO (5.8–6.0 eV),
25
Alq 3 cannot be used as an independent ETL in PhOLEDs.
Usually, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) with a deeper HOMO level of 6.5–6.7 eV
and T 1 of 2.5 eV is used as an ETL together with Alq 3 in red and green PhOLEDs.
44
Due to their high
T 1 energies, benzimidazole-based 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (T 1 = 2.74 eV)
45

and 1,2,4-triazole-based 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (t-Bu-TAZ)
(T 1 = 2.7 eV)
46
are widely used as ETLs in blue PhOLEDs. Recently, Kido’s group reported a series of
pyridine-based ETMs, such as 1,3,5-tri(m-pyrid-3-ylphenyl)benzene (Tm3PyPB),
47
which has a high T 1
energy of 2.78 eV making it desirable for blue OLEDs. Triazine-based compounds, such as 2,4-diphenyl–
6-(4′-triphenylsilanylbiphenyl-4-yl)-1,3,5-triazine (DTBT), have demonstrated their ability as ETLs for
highly efficient green PhOLEDs.
48
The structures of the selected ETMs are shown in Figure 1.12. It is
15

important to note that most of these ETMs have also been used as hole-blocking layers (HBLs), which are
further elaborated in the following section.

Figure 1. 12. Representative electron-transport materials containing various EWGs.

1.2.3 Hole and Electron Blocking Layers
Although PhOLEDs have demonstrated superior performance over their fluorescent counterparts,
49

they require more complex device architectures. Singlet excitons have short diffusion lengths (10-100 Å),
whereas triplet states can diffuse >1000 Å due to their long lifetimes.
50, 51
Thus, it is essential to use device
configurations that confine the excitons within the EML. In theory, triplet exciton confinement in a triple
layer, single heterostructure is possible if the HTL and ETL have higher optical gaps than the exciton
binding energy, but it is not always achieved with common OLED materials.
1
A solution to the problem of
exciton leakage in PhOLEDs was first proposed by O’Brien et al. Prior to their work, a red phosphorescent
emitter, PtOEP, was doped into a CBP (4,4′-N,N′-dicarbazolebiphenyl) host, giving a double
heterostructure device (ITO/NPD/CBP-PtOEP/Alq 3/Mg–Ag) with an EQE of 4.2%.
51
To increase the
efficiency of device, O’Brien et al. introduced an exciton blocking layer between the CBP and Alq 3, the
ETL (i.e. ITO/NPD/CBP–PtOEP/BCP/Alq 3/Mg–Ag, BCP=bathocuproine).
50
Because BCP has a T 1 energy
16

significantly higher than PtOETP (2.5 vs. 1.9 eV), the BCP layer effectively blocks the diffusion of PtOEP
excitons into the Alq 3, which would have resulted in energetic losses due to nonradiative recombination as
seen in the original device. Additionally, because BCP has a deep HOMO level (6.5 eV), it efficiently
blocks the diffusion of holes from the EML into the Alq 3 layer. Thus, BCP acts as a combined hole/exciton
blocking layer. The result is a marked improvement in the device efficiency, from an EQE of 4.2% for the
device without hole-blocking to 5.6% with BCP. The effect of the BCP layer can be seen in the EL spectra
of the devices (Figure 1.13.)
50
The BCP blocked device gives a spectrum consistent with only PtOEP
emission, whereas the unblocked device displays Alq 3 emission due to hole leakage in to the PtOEP layer.

Figure 1. 13. Emission spectra of PtOEP-CBP devices with and without hole/exciton-blocking layer, BCP.
(a) Device without BCP exhibits significant Alq 3 emission at λ max ∼520 nm. (b) No Alq 3 emission
demonstrates the effect of the BCP layer.

A similar example can be seen in devices made with the green phosphor, fac-tris(2-
phenylpyridine)iridium (Ir(ppy) 3) doped into CBP (ITO/NPD/CBP : Ir(ppy) 3/BCP/ Alq 3/Mg-Ag) by Baldo
et al.
52
Here, they also use BCP as the HBL and obtain an EQE of 8% in the device, as opposed to only
0.2% when the BCP layer was omitted from the device. These examples demonstrate that, for a compound
17

to act as an ideal HBM, it must have a HOMO level deeper than that of the dopant and host, as well as a
T 1 energy high enough to prevent triplet excitons from diffusing out of the EML. The role of a HBM can
be seen in the energy level diagram shown below in Figure 1.14.

Figure 1. 14. Schematic diagram of the energy levels of an efficient PHOLED with a hole-blocking layer.
Excitons are depicted as starburst patterns.

The two most common hole-blocking materials are BCP
52-55
and BAlq (4-biphenyloxolato
aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate)
56-58
A number of other organic materials
have also been used as hole-blocking materials, specifically heteroaromatic molecules with high
electronegativity such as pyridine, triazine, and oxadiazoles.
59, 60
The ETMs in Figure 1.12. are examples
of the latter, which are also frequently used as HBMs.
While several hole-blocking materials had been investigated in the past, the need for an electron
blocking layer (EBL) only became apparent with the development of blue and white PhOLEDs. Commonly
used hosts for PhOLEDs are also hole-transporters (i.e. carbazole derivatives), and because holes are
typically more mobile than electrons, these factors result in the buildup of holes at the EL/ETL interface,
but no electron buildup at the HTL/EL interface. However, when HOMO and LUMO energies approach
18

those of the host material, electron leakage can become significant, as is the case when shifting dopant
energy levels to higher levels (i.e. into the blue). As the host and dopant energies become similar, the hole-
injection barrier of the EML also approaches those of the electron-injection barrier of the HTL, leading to
recombination in the HTL.
44
Therefore, the addition of an EBL prevents electron leakage into the HTL. In
2002, Adamovich et al. developed efficient white OLEDs (WOLEDs) using blue phosphors based of a
series of platinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C
2
′] β-diketonates (FPt1 & FPt2) doped into
CBP and mCP (dicarbazolyl-3,5-benzene) (i.e. NPD/Irppz/mCP : FPt(1, 2) /BCP/Alq3/LiF/Al).
61
The best
efficiencies and color stabilities were achieved when an EBL is inserted between the HTL and EML of
these devices. The material used as an EBL was fac-tris(1-phenylpyrazolato-N,C2′)iridium(III) (Irppz).
Without the EBL, NPD (HTL) emission is observed in the EL spectra. Pure phosphor emission, on the other
hand, is observed when Irppz is used as the EBL, effectively preventing electrons and excitons from passing
through the emissive layer into the hole transporting NPD layer.

Figure 1. 15. Schematic diagram of the energy levels of an efficient PHOLED with an electron-blocking
layer. Excitons are depicted as starburst patterns.

Blocking electron leakage can improve efficiency by balancing out charge-carrier injection at the
HTL/EL interface, however, it is also important for the EBL to have a higher T 1 energy to prevent loss of
19

excitons into the HTL. Thus, an EBL must have several characteristics to be an efficient electron/exction
blocker: a high T 1 energy, a LUMO energy sufficiently high to prevent electron leakage from the EL into
the HTL, and a HOMO energy that is close to that of the HTL, as shown in Figure 1.15. Hole transfer
materials commonly possess these characteristics in addition to having good hole mobility and density,
therefore, they are frequently used as EBMs. Common EBMs are those HTMs consisting of triarylamines
and benzidines, such as those shown in Figure 1.11.,
36, 62, 63
however some of these molecules (excluding
TCTA) suffer long-term morphological instability due to their low T g.
36-41
An approach used to prevent
crystallization induced degradation is by raising the T g of amorphous materials. Spiro-linked derivatives of
these EB/HTMs have been developed demonstrating increased morphological stability to their parent
counterparts (Fig XX: USE REF W spiro-TPD, spiro-2NPB, spiroTAD).
64, 65
The most extensively used
EBL for blue PhOLEDs, WOLEDs, and TADF PhOLEDs, is TCTA due to its excellent hole transport and
mobility properties, as well as its high Tg (151 °C).
66-68

Figure 1. 16. Spiro-linked derivatives of common electron-blocking/hole-transporting materials.

1.2.4. Emitting Materials/Dopants
Tang and Van Slyke developed the first OLED in 1987 incorporating Alq 3 as a green fluorescent
emitter.
69
Further advancement of fluorescent, or first-generation, OLEDs has led to devices exhibiting high
color purity EL and long operational lifetimes.
19
However, their 25% IQE limit makes them inefficient for
industrial applications. For this reason, phosphorescent dyes offer a means of achieving improved light
emission efficiencies, since emission can result from both S 1 and T 1 states. The use of heavy metal
organometallic complexes as emitters has proven to be a successful strategy for increasing the EQEs of
OLEDs, which is caused by the mixing of S 1 and T 1 states through SOC, and hence accelerated radiative
20

decay from the T 1 state through electrophosphorescence. The triplet emitter, platinum octaethylporphyrin
(PtOEP), was successfully used in 1998 as a red dopant in a PHOLED, giving an IQE of 23% and 4%.
EQE.
23
Many phosphorescent emitters of heavy-metal complexes have since been developed.
Cyclometallated octahedral Ir(III) complexes are the most commonly used emitters in PhOLEDs,
particularly the blue bis[(4′,6′-difluorophenyl)pyridinato-N,C
2
′]iridium(III) picolinate (FIrpic),
70, 71
green
fac-tri(2-phenylpyridinato-N,C
2
′)iridium(III) (Ir(ppy) 3),
52, 72
and bis(2-phenylpyridinato-N,C
2
′)iridium(III)
acetylacetonate [(ppy) 2Ir(acac)],
55
and red bis(1-phenylisoquinolinato)(acetylacetonate)iridium
[(piq) 2Ir(acac)].
73
Their chemical structures are shown in Figure 1.17.

Figure 1. 17. Molecular structures of widely investigated triplet emitters.

High efficiencies and long device lifetimes have been achieved for red and green PhOLEDs and as
a result, are used in various commercial applications.
24, 73-75
However, because of their wide bandgap (∼3
eV) and long lifetimes (∼μs), materials used for blue are susceptible to degradation caused by high-energy
excited states (∼6 eV) as a result of bimolecular processes.
76
Thus, the lifetime of blue PhOLEDs is too
short for practical applications in display and solid-state lighting. Most research in the OLED field has been
devoted towards developing stable emitters and materials for blue OLEDs for over a decade. The most
straightforward strategy towards designing blue emitters has been to shift the emission wavelength of a
21

stable green emitter, such as tris[2-phenylpyridinato-C
2
,N]iridium(III)] (Ir(ppy) 3), to a higher frequency by
modifying the chelating ligands.
52
In the case of Ir(ppy) 3, adding EWGs (i.e. fluorine) to the phenyl ring of
pyridine will deepen the HOMO level, as shown in Figure 1.18. (left). An example of this is bis[2-(4,6-
difluorophenyl)pyridinato-C
2
,N](picolinato)-iridium(III) (FIrpic), which is one of the first phosphors with
sky-blue emission and high efficiencies. However, due to the poor electrochemical stability of the fluoro
substituents, the operation lifetime is limited.
77
Likewise replacing the pyridine ring of Ir(ppy) 3 with a more
electron donating ligand (i.e. imidazole) raises the LUMO energy Figure 1.18. (right). Blue OLEDs with
an EQE of approximately 30% have been demonstrated using fac‐tris(mesityl‐2‐phenyl‐1H‐
imidazole)iridium(III) (fac‐Ir(mpim) 3) as and emitter.
78
Thus, the emission energy can be tuned by
modifying the HOMO and the LUMO of the chromophore ligands.

Figure 1. 18. Strategies to blue shifting the emission of cyclometalated Ir complexes.
79

As discussed in Section 1.1.3., the most important feature in TADF systems is the energy difference
between S 1 and T 1 (ΔE ST), which is determined by the electron exchange energy. The gap decreases
concurrently with the HOMO-LUMO wavefunction integral.
80
When ΔE ST < 0.1 eV, the T 1 exciton RISCs
to the S 1 via thermal energy (Figure 1.4.). To construct a TADF emitter, electron donor (D) and acceptor
(A) moieties are used with a small overlap of the HOMO/LUMO wavefunction. This can be achieved with
a suitable central bridge and/or a large dihedral angle between the D and A units. There are lots of
combinations with various D and A materials, numerous electron D and A moieties, as well as numerous
22

molecular configurations. Therefore, there are many possible blue TADF emitters.
80
By incorporating two
strong electron donors (i.e. acridan) and a weak electron acceptor (i.e. diphenylsulfone), a blue emitter with
a large band gap and short lifetime, such as DMAC-DPS (3.1 μs), can be designed (Fig. 1.19.) .
81
With
suitable tuning of the position, number, and substituents weak D and strong A moieties (carbazole and
cyano, respectively), blue emission can be achieved by 2CzPN.
18
Connecting three carbazole and one
triazole (with moderate A and D characteristics) units on the central phenyl ring, TCzTrz shows efficient
blue emission.
82
The main issue with TADF materials, is that a small overlap between the HOMO and
LUMO wavefunctions contradicts the requirement of high PLQY. However, this can be overcome by
increasing the number of functional units and precisely controlling the dihedral angle.
83
In recent years,
TADF OLEDs have been posed to be the most promising exciton harvesting mechanism for OLEDs.
17

These OLEDs exhibit greater flexibility in molecular design without the need to incorporate heavy metal
atoms, as is the case with PhOLEDs. However, TADF OLEDs suffer from the same constraint for the
lifetime of PhOLEDs caused by bimolecular side reactions due to long exciton lifetime and high T 1 energy
of blue emitters.

Figure 1. 19. Examples of blue TADF emitters.

1.2.5. Exciton-Exciton and Exciton-Polaron Annihilation
The mechanisms responsible for the degradation of OLEDs, particularly in PH/TADF OLEDs, are
triplet-triplet (TTA) and triplet-polaron annihilation (TPA). Because of their long-excited state lifetimes
(typically > 1 s), phosphorescent and TADF emitters suffer more from TTA and TPA than fluorescent
emitters. Long lifetimes allow for their accumulation with device operation, and the greater their
23

concentration, the likelihood of TTA and TPQ increases. Both processes involve the energy of two excited
states fusing together and forming a higher energy S 1 and T 1 state, or a high-energy polaron. These high
energy or “hot” excited states can cause molecular bond disassociation if concentrated on weak bonds,
which degrades the material and shortens the lifetime. TTA and TPA reduce the efficiency of a device
under high current densities, also referred to as the efficiency roll-off.
84
For commercial applications such
as small-area displays (mobile phones) and solid-state lighting, OLEDs must have brightness levels in the
range of 100-400 cd m
-2
and 5000 cd m
–2
,
62, 81
respectively. Achieving higher brightness requires increased
current levels, thus TTA and TPA must be suppressed for the realization of OLEDs in useful applications.
Figure 1.20. below shows the configuration diagrams of the TTA and TPQ processes to describe
in detail the hot excited state. The interaction of excitons in the S 1 or T 1 state can generate a hot exciton in
S n or T n states (n > 1) or a hot polaron (D n, n > 1). The R curve (blue dashed line) represents the pre-
disassociation potentials. The hot excited state will either dissociate via a direct reaction (route 1) to yield
radical fragments that result in molecular bond rupture, or via vibronic relaxation (route 2) to create a hot
material. For a blue emitter with a triplet exciton (∼2.8 eV) interacting with a polaron (∼3.3 eV), the newly
formed hot excited state is over 6 eV. This is higher than the bond dissociation energies (BDEs) of most
organic materials.
81
Hence, hot excited states are the root causes of the short lifetime of blue OLEDs. The
latter is not a problem for their green and red counterparts due to their lower T 1 state energy. Managing
these bimolecular processes is currently one of the most important issues in OLEDs research. To minimize
the degradation caused by the dissipation of hot states from the EML to the adjacent layers, this thesis
focuses on the development of hole blocking materials used to improve the lifetime and efficiency of blue
OLEDs.
24

Figure 1. 20. Configuration diagrams of (a) TTA, and (b) TPQ process.

25

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31

Chapter 2 – Pyrimidine-based Hole Blocking Materials for Blue OLEDs

2.1. Introduction
The development of high-performance PhOLEDs with long-lifetimes and stability must be realized
for their largescale commercialization. Unlike its red and green counterparts, satisfactory efficiencies and
lifetimes have not been achieved by blue PhOLEDs. To increase their performance, triplet excitons must
be confined to the EML and prevented from dissipating onto adjacent layers to circumvent molecular
degradation. Introduction of a hole blocking layer (HBL) between the emissive layer (EML) and celectron-
transport layer (ETL) has proven to be a useful strategy for effective confinement.
1-11
An ideal HBM for
blue PhOLEDs must demonstrate (i) a HOMO level deeper than that of the dopant and host to prevent
triplet exciton and hole diffusion, (ii) a T 1 energy higher than that of the emitting material (≥ 2.75 eV),
12, 13

and (iii) a high T g (∼120 °C) to confer morphological stability and prevent thin-film crystallization during
device operation.
Heteroaromatic molecules with high electronegativities such as pyridine,
14, 15
triazine,
16, 17
and
oxadiazole segments,
16, 17
are used as building blocks for HBLs. Particularly, the pyridine segment is
frequently employed because of its ease in synthetic modification (Figure 2.1.).
18-20
BCP
16, 21-23
and BAlq
3,
7
are the two most common HBLs for PhOLEDs, however, their low T 1 energies (<2.4 eV) limits their
efficiency in blocking blue excitons. Table 2.1. demonstrates the challenge in finding a material that meets
all the requirements of a blue HBL. It is difficult to obtain a HBM that has both a high T 1 and T g since there
is usually a trade-off between E T and thermal stability.
24

32

Figure 2. 1. Pyridine-based HBMs commonly used in blue PhOLEDs.

HOMO (eV) LUMO (eV) T 1 (eV) T g (°C)
BCP -6.50 -2.90 2.50 89
BPhen -6.40 -3.00 2.50 62
BAlq -5.90 -2.90 2.18 99
Tm2PyPB -6.43 -2.74 2.75 77
Tm3PyPB -6.68 -2.90 2.75 79
Tm4PyPB -6.68 -2.94 2.75 99

Table 2. 1. Properties of the hole-blocking materials in Figure 2.1.

Herein, we designed and synthesized novel HBMs based on tri- and tetra-substituted pyrimidine
frameworks. The pyrimidine segment was chosen because it is highly modular like pyridine, but also more
electronegative for realizing deeper HOMO levels and higher electron transporting properties. Pyrimidine
has a higher thermal stability as well, which is useful for achieving a high T g. Despite these advantages,
only a few HBMs with pyrimidine segments have been reported.
24
Four of these HBMs are presented and
33

investigated in this work (Figure 2.2). Optimized synthesis of these materials, their electrochemical,
photophysical, and thermal properties, as well as a strategy to inhibit aggregation induced red-shifts for
maintaining high T 1 energies and minimizing thin film crystallization are discussed.

Figure 2. 2. Multisubstituted pyrimidines presented in this work.

2.2. Results and Discussion
2.2.1. Design and Synthesis
The molecular structures of TP, MTP, and TP2 consist of trisubstituted, and TPP of
tetrasubstituted, pyrimidine frameworks. Initial studies included pyridine and tolyl derivatives of TP, PDP
and TDP, respectively, to investigate the effects electron-donating and electron-withdrawing substituents
would have on the HOMO/LUMO and T 1 energies of the pyrimidine system. The trisubstituted pyrimidines
were prepared according to the base-mediated, three-component tandem reaction presented by Liu, et. al.
from amidines, aryl alkynes, and aldehydes in a one-pot matter.
25
Benzamidine hydrochloride and phenyl
acetylene were used in the synthesis of all trisubstituted compounds. Only the stoichiometry and
corresponding aldehyde were modified from the original procedure (Figure 2.3.a.). Mesityl-1-phenyl-4-
carbaldehyde was prepared using the palladium-catalyzed Suzuki reaction. The synthetic yields of the
trisubstituted compounds are within the range of 27-68%. The low yield of TP2 is likely affected by an
accumulation of side-products due to a two-fold increase in stoichiometry (for the alkyne and benzamidine),
and the intricacy of two simultaneous cyclization reactions occurring on a single substrate
(isopthalaldehyde). Tetrasubstituted pyrimidine, or TPP (71%), was synthesized by formal cycloaddition
34

of cyclopentylbenzamide and diphenylacetylene in the presence of triflic anhydride, as reported by Stopka,
et. al. (Figure 2.3.b.).
26
Cyclopentylbenzamide was prepared according to their procedure.

Figure 2. 3. Synthesis of (a) trisubstituted pyrimidines and (b) tetraphenylsubstituted pyrimidine.

2.2.2 Electrochemical and Thermal Properties

The electrochemical properties of TP, DPD, and TPD are summarized in Table 2.2. Their cyclic
voltammetry curves are displayed in Figure 2.4. All three compounds show irreversible oxidation and
reversible reductions. Reduction potentials were acquired using differential pulse voltammetry (DPV)
measurements in acetonitrile versus Fc
+/
Fc. Their HOMO and LUMO levels were derived from redox
potentials according to methods by Sworakowski et. al.
27
The HOMO/LUMO energy levels of TP, DPD,
and TPD, are -7.00 eV/ -2.37 eV, -6.46 eV/ -1.98 eV, -6.76 eV/ -2.06 eV, respectively, indicating that all
(a)
(b)
35

three materials have sufficiently deep HOMO energy levels to block holes in a PhOLED device. Their
HOMO energy levels are much deeper than that of BCP (-6.43 eV) and mCBP (-6.0 eV), which is widely
used as a host material in blue PhOLEDs.
17
Their LUMO levels are also deep, suggesting they can
potentially be used as electron-transport materials as well.

TP TDP PDP
HOMO exp. (eV) -6.56 -6.60 -6.77
LUMO exp. (eV) -1.60 -1.58 -1.99
E° red. (V) -2.30 -2.04 -1.98
E° ox. (V) 1.54 1.57 1.72

Table 2. 2. Summarized electrochemical properties of TP, DPD, and TPD.
-2.5 -2.0 -1.5 0.5 1.0 1.5 2.0

Current (A)
Potential (V vs Fc/Fc+)
TDP
TP
PDP
CV in ACN, 100 mV/s

Figure 2. 4. CV curves of trisubstituted pyrimidines TP, DPD, and TPD.
36

Figure 2. 5. DPV curves of trisubstituted pyrimidines TP, DPD, and TPD.

The thermal properties of an OLED material are one of the most important parameters to investigate
for predicting device lifetime and stability. OLED materials with low T g values can cause thin film
crystallization during driving operation, resulting in reduced device lifetime. Thus, a material with a
sufficiently high T g value is favorable for morphological stability. The thermal properties of TP were
investigated using differential scanning calorimetry (DSC). Heat flow traces were measured from room
temperature up to 700 °C under nitrogen as shown in Figure 2.6. The T g value of TP is 110 °C and is much
higher than the T g values of the pyridine-based HBMs shown in Figure 2.1. Because their similar structures,
PDP and TDP can be expected to have T g values similar to TP.
Fc
+
/Fc
Fc
+
/Fc
37

0 100 200 300
-1
0
1
2
Heat Flow (a.u)

Temperature (°C)
20 to 300 °C
300 to 20 °C
20 to 300 °C

Figure 2. 6. DSC Curves of TP depicting its crystallization and melting temperatures.

TP, DPD, and TPD were synthesized as preliminary compounds to explore their characteristics
and potential as HBMs for blue PhOLEDs. Given their deep HOMO energy levels and sufficiently high T g
value, higher molecular-weight pyrimidine compounds were designed to ensure their thermal and
morphological stability. The structures of MTP, TP2, and DPP are shown in Figure 2.2. Their cyclic
voltammograms are shown in Figure 2.7., where TP is also shown as a reference. All compounds show an
irreversible oxidation (the oxidation of TP2 is not apparent in the CV, however, is shown in DPV (Figure
2.8.). TP and MTP demonstrate reversible reductions, whereas TPP displays only one reduction. TP2
features two reversible and one irreversible reduction, as emphasized in Figure 2.8. Their electrochemical
properties are summarized in Table 2.3.
38

Figure 2. 7. Cyclic voltammetry curves of TP, MTP, TPP, TP2.
39

Figure 2. 8. DPV curves of TP, MTP, TPP, and TP2.

Figure 2. 9. TP2 DPV reduction curves (left) and CV reduction curves (right) referenced to Fc
+
/Fc.
-3 -2 -1 0 1 2
Potential (V)
TP2
Current (A)
Fc
+
/Fc
Fc
+
/Fc
-2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4
Current (A)
Potential (V)
TP2
Fc
+
/Fc
40

TP MTP TPP TP2
HOMO exp. (eV) -6.56 -6.53 -6.44 -6.58
LUMO exp. (eV) -2.02 -2.23 -2.00 -2.08
E° red. (V) -2.38 -2.20 -2.40 -2.33
E° ox. (V) 1.54 1.51 1.43 1.55

Table 2. 3. Summarized electrochemical properties of TP, MTP, TP2, and DPP.

The HOMO values of MTP and TP2 are deeper than TP, indicating the materials will have
sufficiently deep HOMO energy levels to block holes in a blue PhOLED device. The HOMO value of TPP,
though slightly lower, is still higher than those of BCP and mCBP. Thus, TPP also suggests good hole-
blocking ability. The thermal properties of these materials where not examined, however, because of their
structural similarities and higher molecular weights than TP, high enough T g values for these compounds
can be expected.

2.2.3. Photophysical Properties

In addition to maintaining a high T g, HBMs must also have a high T 1 energy to fabricate efficient
and stable blue PhOLEDs. The photophysical properties of TP, MTP, TP2, and DPP were measured in 2-
MeTHF. Their UV-visible absorption spectra are shown in Figure 2.7. The main absorption peak of the
four compounds is around 315 nm, corresponding to the π-π* transition. The S 1 and T 1 values of the HBMs
are obtained from the onset of their respective photoluminescence spectra (Figure 2.11.) and are
summarized in Table 2.4. Fluorescence and phosphorescence spectra were obtained at 298 and 77K,
respectively, and measured in 2-MeTHF. The T 1 energies of the compounds in solution are significantly
higher than their T 1 energies in the solid-state. This redshift in energy is commonly observed in planar and
highly conjugated materials because of intramolecular π- π stacking resulting in electron delocalization in
the solid-state.
41

300 350 400 450 500 550 600 650 700
0.0
0.1
0.2
0.3
0.4
Absorbance (a.u.)
Wavelength (nm)
TP
MTP
TPP
TP2

Figure 2. 10. (a) Absorption spectra of TP, MTP, TP2, and DPP in 2-MeTHF.

42

Figure 2. 11. Normalized emission spectra of TP (a), MTP (b), TPP (c), and TP2 (d), in 2-MeTHF and in
solid-state at 298 K and 77 K. (e) Comparison of T1 energies at 77 K for all compounds.

An example of an aggregation induced redshift can be understood looking at the molecular
structures of TP and MTP. The S 1 and T 1 energies of both compounds in solution are nearly identical,
though their T 1 energies at 77 K differ (TP = 2.60 eV, MTP = 2.69 eV). The optimized molecular structures
of TP and MTP were obtained using density functional theory (DFT) calculations and simulation based on
the B3LYP method and 6-31G**, as shown in Figure 2.12. The addition of a mesityl group to one of the
aryl rings of the pyrimidine moiety provides considerable steric bulk to the system and decreases the likely
hood of π- π stacking in the solid state, hence the higher T 1 energy of MTP at 77 K.

43

Figure 2. 12. Optimized structures and frontier molecular orbital representation of pyrimidine HBMs.

The same line of reasoning can be used to explain the higher T 1 energies of TPP and TP2 (3.12
and 3.39 eV at 77 K, respectively). TPP has an additional phenyl ring in the meta position of the two
nitrogens on the pyrimidine core. The steric hinderance imposed by this ring causes it and the adjacent
phenyl substituents to slightly rotate out of plane reducing the degree of conjugation, wherein TP and MTP
the ortho phenyls would remain coplanar with the pyrimidine core. The drastic increase in T 1 energy of
TPP in the solid-state at 298 K and 77 K can be explained by examining the frontier molecular orbitals
(FMOs) of the HBMs displayed in Figure 2.12. A significant portion of the HOMO and LUMO density is
located on the pyrimidine moiety for TPP. The ortho and meta phenyl rings perpendicular to the nitrogen
atoms are inductively electron withdrawing which stabilize the HOMO and causes a blue shift in energy.
The adjacent rings, on the other hand, are electron donating on the pyrimidine, stabilizing the LUMO. The
stabilization of both HOMO and LUMO orbitals results in a sum total blue shift and higher energy T 1. The
44

T 1 energy of TPP in solution is higher than that of TP2 because TP2 has its HOMO distributed among the
two pyrimidine moieties, and hence the T 1 spin density is more delocalized. However, in the solid-state the
T 1 energy of TP2 is greater because of the overall steric bulk provided by two additional peripheral aryl
rings which minimize aggregation. The T 1 energies of the pyrimidine compounds at 77 K are compared in
Figure 2.13.

Figure 2. 13. EL spectra of TP, MTP, TPP, and TP2 in the solid-state at 77 K.

Compound

S 1 (eV)
a

E T (eV)
E ox (V)
d

E red (V)
d

HOMO/LUMO (eV)
e
Soln.
b
Solid
c

TP 3.72 2.91 2.60 1.32 -2.30 -6.56/-2.02
MTP 3.75 2.90 2.69 1.48 -2.21 -6.53/-2.23
TPP 3.85 3.80 3.12 1.68 -2.41 -6.44/-2.00
TP2 3.93 3.54 3.39 1.54 -2.42 -6.58/-2.08
BCP - - 2.50 1.43 -2.56 -6.43/-1.80
a
Measured in 2-MeTHF at 298K and at
b
77K.
c
Onset of the triplet emission for the neat powder at 77K.
d
Obtained from differential
pulse voltammetry (DPV) in acetonitrile vs. Fc
+
/Fc.
e
Calculated from redox values according to reference.
27

45

Table 2. 4. Summarized electrochemical and photophysical properties of selected pyrimidine HBMs.
BCP is listed here as a reference.

These observations suggest that sterics may be the most determinant factor for ensuring high T 1
energies in these pyrimidine systems. Besides lowering the emission energies of materials in the solid-
state, π- π stacking increases the possibility of thin-film crystallization during device operation. Thus, in
addition to having a high T g temperature, materials must have structures that prevent aggregation and
stacking in films to achieve long-lifetimes in OLEDs. The following section emphasizes these requirements.
Blue PhOLEDs were fabricated using TP and MTP as HBLs. It was found that the highest efficiencies and
morphologically stable devices were achieved with MTP due to its higher T 1 energy and deplanarized
structure.

2.2.4. Performance of Blue Phosphorescent OLEDs

Blue phosphorescent OLEDs were fabricated to evaluate their performance as HBLs. A preliminary
device was fabricated with the following structure: ITO(150 nm)/NPD(40 nm)/mCBP:FIrpic (20 nm, 10%)/
TP(10 nm)/AlQ 3(30 nm)/Al/LiF (10 nm). The device architecture is shown in Figure 2.14. The EQE vs.
current density curves and EL spectrum of the device are shown in Figure 2.15.a. and b. The EL spectra
of the blue PhOLED device displays the characteristic emission spectra of FIrpic, which has a maximum
emission peak around 472 nm, spanning from about 450 nm to 600 nm,
28
and demonstrates that TP is
blocking holes. Hole leakage into the ETL would otherwise lead to a significant Alq 3 contribution in the
EL spectrum. The EQE of the device is merely 2.5%. Its low EQE is attributed to device degradation.
Applied voltage over time shows the formation of grain boundaries in the thin film of the device, suggesting
crystallization.
46

Figure 2. 14. Device architecture and chemical structures of the OLED materials.

Figure 2. 15. Device characteristics of blue PhOLED using TP as HBL. (a) EQE vs. current curves, (b) EL
spectrum, (c) J-V curves, and (d) luminance vs. current curves.

47

The structure of TP is highly planar and is thus prone to π-π stacking as an amorphous glass,
rendering it susceptible to crystallization. One of the peripheral aryl rings of TP was functionalized with a
sterically hindering mesityl group to minimize π-π stacking and increase the thermal stability, giving MTP.
A comparative device study was carried out to determine whether adding steric bulk to TP improves the
morphological stability and overall performance of a blue PhOLED. Three devices of the structure ITO
(150 nm)/TAPC(40 nm)/mCBP:FIrpic(20 nm, 10%)/HBL(10 nm)/AlQ 3(30 nm)/Al/LiF(10nm) where HBL
= BCP, TP, and MTP (Figure 2.16). The device with BCP as the HBL is used as a reference device. The
EL spectra in Figure 2.17.a. shows that all three materials are blocking holes in the device. The MTP-
based device exhibited an EQE of 8% and superior performance over the BCP and TP-based devices, which
both gave EQEs of 4%. The higher EQE is likely due to the T 1 energy of MTP being higher than TP and
BCP. The device performance results demonstrate that TP has comparable capabilities as hole-blocker as
BCP. Less crystallization was observed for the MTP-based device which suggest that deplanarization of
the pyrimidine system increases morphological stability and lifetime of the device.

Figure 2. 16. Device architecture and chemical structures of the OLED materials for comparative studies.
48

Figure 2. 17. Device characteristics of blue PhOLED using BCP, TP, and MTP as HBLs. (a) EL spectra,
(b) EQE vs. current curves, (c) luminance vs. current curves, and (d) J-V curves.

To further investigate the performance of MTP as a HBM for blue PhOLEDs, lifetime studies were
carried out on devices with the blue emitter, fac-Ir(ttpz) 3, and MTP as the HBL. A reference device was
fabricated with T2T as the HBL. The EL spectra of both T2T and MTP based devices indicate exciton
confinement on the dopant (Figure 2.19.). The molecular structures of the materials used in the device and
their energy level diagrams are shown in Figure 2.18. The two devices have similar J-V properties and
EQEs around 9%. The lifetime (T80) measured at 1000 nits for the T2T-based device is eight hours,
whereas the T80 for the MTP-based device was less than one hour. Both HBMs have approximately the
same HOMO energies (T2T = -6.5 eV, MTP = -6.53 eV) and the T 1 energy of T2T (2.80 eV)
29
is slightly
higher than MTP (2.69 eV). Given that both devices display similar efficiencies and characteristics, it can
be assumed that the lifetime of the MTP device is much shorter because of structural differences between
49

the HBMs. During device operation, crystallization is observed as the formation of grain boundaries at a
much faster rate than in the T2T device. T2T is less likely to aggregate in a neat film because of the steric
bulk provided by the peripheral aryl rings on the triazine core. Using a HBM such as TPP or TP2 may then
improve the lifetime of these devices relative to MTP. One of the best lifetimes (T80) for blue PHOLEDs
reported in literature is 56 hours.
30
It is clear that efficient HBMs with morphological stability are needed
to improve the lifetime of blue PhOLEDs for their practical application.

Figure 2. 18. Energy level diagram and materials used in devices for lifetime studies.

50

Figure 2. 19. Luminance of blue PhOLED using fac-Ir(tppz) 3 as the emitter and MTP and T2T as HBLs.

2.3. Conclusion
In summary, we designed and synthesized novel HBMs, TP, MTP, TPP, and TP2, consisting of
tri- and tetra-substituted pyrimidine frameworks for high-efficiency and stable PhOLEDs. The triplet
energies of TP, MTP, TPP, and TP2, were estimated to be 2.60, 2.69, 3.12, and 3.39 eV, and their HOMO
energy levels were -6.56, -6.53, -6.44, -6.58 eV, respectively. These results verify that the multisubstituted
pyrimidine materials possess high T 1 and deep HOMO energy levels for their use as HBLs in PhOLEDs.
Particularly, TPP and TP2 demonstrate the most favorable characteristics for blue PhOLEDs. The T g value
of TP was found to be 110 °C, which is sufficiently high to ensure thermal stability in devices. Because of
their similar structures and considerably higher molecular weights, MTP, TPP, and TP2 can be expected
to demonstrate high T g values as well. It was also found that deplanarizing the pyrimidine moiety by adding
51

sterically hindered substituents has a substantial effect on the T 1 energy and morphological stability of the
HBMs in the solid-state. Because of its higher T 1 and bulky structure, TP2 may demonstrate superior
performance as a HBL over its counterparts. Preliminary OLED devices were fabricated using FIrpic as the
emitter and TP/MTP as HBLs. Both HBMs proved to be effective at blocking holes, however, MTP gave
a higher EQE of 8% and less crystallization. The TP based device performed equivalently to a reference
device employing BCP as the HBL, both which gave EQEs of 4%. We anticipate that novel HBMs based
on multisubstituted pyrimidine frameworks will be key materials for the development of high-performance
blue PhOLEDs.

2.4. Experimental

2.4.1. General
Nuclear magnetic resonance (NMR) spectra were recorded on Varian 400 NMR spectrometer 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. Steady state photoluminescent spectra were
measured using a QuantaMaster Photon Technology International phosphorescence/fluorescence
spectrofluorometer, whereas gated phosphorescence was measured on the same instrument using a Xe flash
lamp with 40 µs delay. Photoluminescent quantum yield (PLQY) measurements were carried out using a
Hamamatsu C9920 system equipped with a Xe lamp, calibrated integrating sphere and model C10027
photonic multi-channel analyzer (PMA). Photophysical measurements were carried out in 2-
methyltetrahydrofuran (2-MeTHF). Samples were deoxygenated by bubbling N 2 in a quartz cuvette fitted
with a Teflon stopcock. Spin-coated films were prepared on quartz substrates, and photoluminescence
quantum yield (PLQY) measurements were done under nitrogen atmosphere. Cyclic voltammetry and
differential pulse voltammetry were performed using a VersaSTAT 3 potentiostat. Anhydrous acetonitrile
(Aldrich) solvent was used under nitrogen atmosphere with 0.1 M tetra(n-butyl)-ammonium
hexafluorophosphate (TBAF) as the supporting electrolyte. A Ag wire was used as the pseudo reference
electrode, a Pt wire as the counter electrode, and a glassy carbon rod working electrode. The redox potentials
52

are based on the values from differential pulsed voltammetry measurements and are reported relative to the
ferrocenium/ferrocene (Cp 2Fe
+
/Cp 2Fe) redox couple used as an internal reference, whereas electrochemical
reversibility was studied using cyclic voltammetry. Differential scanning calorimetry (DSC) measurements
were performed on a Perkin Elmer DSC 8000 with CLN2 instrument at a heating rate of 10 °C min-1 under
nitrogen atmosphere. Nuclear magnetic resonance (NMR) spectra were recorded on Varian 400 NMR
spectrometer and referenced to residual protons in the deuterated chloroform (CDCl3) solvent. UV–visible
spectra were recorded on a Hewlett–Packard 4853 diode array spectrometer. Steady state photoluminescent
spectra were measured using a QuantaMaster Photon Technology International
phosphorescence/fluorescence spectrofluorometer, whereas gated phosphorescence was measured on the
same instrument using a Xe flash lamp with 40 µs delay. Photoluminescent quantum yield (PLQY)
measurements were carried out using a Hamamatsu C9920 system equipped with a Xe lamp, calibrated
integrating sphere and model C10027 photonic multi-channel analyzer (PMA). Photophysical
measurements were carried out in 2-methyltetrahydrofuran (2-MeTHF). Samples were deoxygenated by
bubbling N 2 in a quartz cuvette fitted with a Teflon stopcock. Spin-coated films were prepared on quartz
substrates, and photoluminescence quantum yield (PLQY) measurements were done under nitrogen
atmosphere. Cyclic voltammetry and differential pulse voltammetry were performed using a VersaSTAT 3
potentiostat. Anhydrous acetonitrile (Aldrich) solvent was used under nitrogen atmosphere with 0.1 M
tetra(n-butyl)-ammonium hexafluorophosphate (TBAF) as the supporting electrolyte. A Ag wire was used
as the pseudo reference electrode, a Pt wire as the counter electrode, and a glassy carbon rod working
electrode. The redox potentials are based on the values from differential pulsed voltammetry measurements
and are reported relative to the ferrocenium/ferrocene (Cp2Fe+ /Cp2Fe) redox couple used as an internal
reference, whereas electrochemical reversibility was studied using cyclic voltammetry. Thermogravimetric
analysis (TGA) measurements were performed on a NETZSCH STA 449F3 thermogravimeter by
measuring weight loss while heating at a rate of 10°C 52 min-1 under nitrogen. Differential scanning
calorimetry (DSC) measurements were performed on a Perkin Elmer DSC 8000 with CLN2 instrument at
a heating rate of 10°C min
-1
under nitrogen atmosphere.
53

2.4.2. DFT and TD-DFT Calculations

Calculations were performed using Jaguar 8.4 (release 12) software package on the Schrödinger
Material Science Suite (v2017-2) and Q-Chem 5.3. Gas phase geometry optimization was obtained using
B3LYP functional with the LACVP** basis set. The HOMO and LUMO energies were determined using
minimized singlet geometries to approximate the ground state, whereas the triplet excited state is calculated
using self-consistent field method (∆SCF) by taking the difference between lowest singlet and triplet
excited states.

2.4.3. OLED Fabrication and Characterization

Glass substrates with pre-patterned, 1 mm wide indium tin oxide (ITO) stripes were cleaned by
sequential sonication in tergitol, deionized water, acetone, and isopropanol, followed by 15 min UV ozone
exposure. Organic materials and metals were deposited at rates of 0.5-2 Å/s through shadow masks in a
vacuum thermal evaporator with a base pressure of 10
-7
Torr. A separate shadow mask was used to deposit
1 mm wide stripes of 100 nm thick Al films perpendicular to the ITO stripes to form the cathode, resulting
in 2 mm2 device area. The first device structure is: glass substrate/ 150 nm ITO/ 40 nm N,N′-Di(1-
naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD)/ 20 nm 3,3'-di(9H-carbazol-9-yl)-1,1'-
biphenyl (mCBP) : 10 vol% Bis[2-(4,6-difluorophenyl)pyridinato-C
2
,N](picolinato)iridium(III) (FIrpic)/
10 nm TP/ 30 nm tris(8-hydroxyquinoline)aluminum(III) (Alq 3)/ 10 nm Al/LiF. The second device structure
is: glass substrate/ 150 nm ITO/ 40 nm 4,4′-cyclohexylidenebis [N,N-bis(4-methylphenyl)benzenamine]
(TAPC) / 20 nm 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (mCBP) : 10 vol% Bis[2-(4,6-
difluorophenyl)pyridinato-C
2
,N](picolinato)iridium(III) (FIrpic)/ 10 nm HBM/ 30 nm tris(8-
hydroxyquinoline)aluminum(III) (Alq 3)/ 10 nm Al/LiF. The HBM is either bathocuproine (BCP), HBMs
TP or MTP. A semiconductor parameter analyzer (HP4156A) and a calibrated large area photodiode that
54

collected all light exiting the glass substrate were used to measure the J-V-luminance characteristics. The
device spectra were measured using a fiber-coupled spectrometer.

2.4.4. Synthesis

All commercial reagents and solvents werer purchased from Sigma Aldrich, Matrix Scientific,
Oakwood Chemicals, and AK Scientific. They were used without further purification.

Synthesis of TP, PDP, and TDP are from literature procedure without modification
25

2,4,6-Triphenylpyrimidine, 2,4-Diphenyl-6-(pyridin-4-yl)pyrimidine, 2,4-Diphenyl-6-(p-tolyl)pyrimidine
have all been reported and prepared without modification.
25

2,4,6-Triphenylpyrimidine
White solid: 68% yield (1.58 g);
1
H NMR (400 MHz, CDCl 3, ppm): δ = 8.79−8.78 (m, 2H), 8.33−8.30 (m,
4H), 8.01 (s, 1H), 7.61−7.54 (m, 9H);
13
C NMR (400 MHz, CDCl 3, ppm): δ = 164.8, 164.5, 138.3, 137.6,
130.8, 130.7, 128.9, 128.6, 128.5, 127.3, 110.3.

2,4-Diphenyl-6-(pyridin-4-yl)pyrimidine
Pale yellow solid: 41% yield (0.957 g);
1
H NMR (400 MHz, CDCl 3, ppm): δ = 8.85 (d, 2H), 8.72−8.70 (m,
2H), 8.30−8.28 (m, 2H), 8.14 (d, 2H), 8.03 (s, 1H), 7.58−7.54 (m, 6H);
13
C NMR (400 MHz, CDCl 3, ppm):
δ 165.5, 165.0, 162.3, 150.6, 144.9, 137.6, 137.0, 131.2, 131.0, 129.0, 128.6, 128.5, 127.3, 121.2, 110.6

2,4-Diphenyl-6-(p-tolyl)pyrimidine
White solid: 62% yield (1.38 g);
1
H NMR (400 MHz, CDCl 3, ppm): δ = 8.76 (t, 2H), 8.30 (t, 2H), 8.21 (d,
2H), 7.98 (s, 1H), 7.59−7.54 (m, 6H), 7.37 (d, 2H), 2.47 (s, 3H);
13
C NMR (400 MHz, CDCl 3, ppm): δ
164.7, 164.6, 164.5, 141.1, 138.3, 137.7, 134.8, 130.7, 130.6, 129.7, 128.9, 128.5, 128.4, 127.3, 127.2,
109.9, 21.5
Synthesis of 2,4-diphenyl-6-(2',4',6'-trimethyl-[1,1'-biphenyl]-4-yl)pyrimidine (MTP)
55

A mixture of benzamidine hydrochloride (2.55 g, 16.39 mmol), ethynylbenzene (2.51 g, 24.58 mmol),
2',4',6'-trimethyl-[1,1'-biphenyl]-4-carbaldehyde (5.51 g, 24.58 mmol), t-BuOK (3.68 g, 32.78 mmol) was
stirred in DMSO (85 mL) at 120 °C under an N 2 atmosphere for 12 h. The reaction was cooled to room
temperature and water (85 mL) was added to reaction mixture, and the resulting mixture was extracted with
ethyl acetate. The combined organic layers were then dried over MgSO 4, filtered, and then concentrated in
vacuum. Column chromatography of the residue (SiO 2, 2.5:1 hexane/DCM) afforded a white solid (4.54 g,
65%):
1
H NMR (400 MHz, CDCl 3, ppm): δ = 2.07 (s, 6H), 2.36 (s, 3H), 6.99 (s, 2H), 7.54 (m, 6H), 8.09 (s,
1H), 8.32 (m, 4H), 8.75 (d, 2H).

Synthesis of 2',4',6'-trimethyl-[1,1'-biphenyl]-4-carbaldehyde
Pd(PPh 3) 4 (9.39 g, 8.13 mmol) was added to a mixture of mesitylboronic acid (20.00 g, 121.94 mmol), 4-
bromobenzaldehyde (15.04 g, 81.30 mmol), and K 2CO 3 (112.35 g, 812.95 mmol) in a mixture of toluene
and water (360:120 mL), and was refluxed under N 2 for 24 h. The reaction mixture was then cooled to room
temperature and the solvent was removed under pressure. The remaining mixture was dissolved in 300 mL
of dichloromethane (CH 2Cl 2) and washed three times with H 2O. The organic layer was separated and dried
over anhydrous Na 2SO 4, filtered, and concentrated under reduced pressure. Column chromatography of the
residue (SiO 2, 2:3 hexane/DCM) afforded a white solid (16.77 g, 92%):
1
H NMR (400 MHz, CDCl 3, ppm):
δ = 2.01 (s, 3H), 2.35 (s, 3H), 6.97 (s, 2H), 7.35 (d, 2H), 7.95 (d, 2H), 10.08 (s, 1H).

Synthesis of 1,3-bis(2, 6-diphenylpyrimidin-4-yl)benzene (TP2)
A mixture of benzamidine hydrochloride (5.00 g, 32.13 mmol), ethynylbenzene (4.92 g, 48.20 mmol),
isopthalaldehyde (6.47 g, 48.20 mmol), t-BuOK (7.21 g, 64.27 mmol) was stirred in DMSO (85 mL) at 120
°C under an N 2 atmosphere for 12 h. The reaction was cooled to room temperature and water (85 mL) was
added to reaction mixture, and the resulting mixture was extracted with ethyl acetate. The combined organic
layers were then dried over MgSO 4, filtered, and then concentrated in vacuum. Column chromatography of
56

the residue (SiO 2, 2.5:1 hexane/DCM) afforded a white solid (1.30 g, 15%):
1
H NMR (400 MHz, CDCl 3,
ppm): δ = 7.56 (m, 12H), 7.78 (t, 1H), 8.17 (s, 2H), 8.33 (d, 4H), 8.42 (d, 2H), 8.76 (d, 4H), 9.25 (s, 1H).

Synthesis of 2,4,5,6-Tetraphenylpyrimidine (DPP) is from literature procedure without
modification
26

White solid: 71% yield;
1
H NMR (400 MHz, CDCl 3): δ = 8.68–8.62 (m, 2H), 7.54–7.47 (m, 3H), 7.45–7.38
(m, 4 H), 7.32–7.27 (m, 3H), 7.26–7.21 (m, 3H), 7.20–7.13 (m, 3H), 7.03–6.97 (m, 2H).
13
C NMR (400
MHz, CDCl 3): δ = 165.5, 163.0, 139.0, 137.9, 136.8, 131.3, 130.7, 130.1, 129.2, 128.8, 128.6, 128.6, 128.4,
127.9, 127.4.

57

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Abstract (if available)

Abstract Organic light-emitting diodes (OLEDs) have emerged as an important technology for attractive, high-efficiency flat-screen displays, and solid-state lighting applications. Particularly, red and green phosphorescent OLEDs (PhOLEDs) have garnered significant attention due to their 100% internal quantum efficiencies and long operational lifetimes of T95>100,000, which is sufficient for display and lighting applications. On the other hand, achieving high efficiencies and long lifetimes in blue OLEDs remains a significant challenge. Their inefficiency and lack of stability is caused by the degradation of organic layers and emissive materials caused by the formation of high energy excited states upon electrical excitation. One of the important factors for achieving high-efficiency PhOLEDs is confining these excited states in the emitting layer. For this reason, four novel hole-blocking materials (HBMs) materials based on 1,3,5-tri- and 1,3,4,5-tetrasubstituted pyrimidine were synthesized. TP, MTP, TPP, and TP2 were found to possess high T₁ and deep HOMO energy levels, as well as high glass-transition temperature values for their use as HBMs in blue PhOLEDs.The thermal, electrochemical, and optical properties of the pyrimidine compounds were systematically investigated and herein discussed.

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Asset Metadata

Creator Ontiveros, Brenda Abigail (author)

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

School College of Letters, Arts and Sciences

Degree Master of Science

Degree Program Chemistry

Publication Date 12/13/2020

Defense Date 10/28/2020

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

Tag chemistry,hole blocking materials,OAI-PMH Harvest,OLEDs,organic light-emitting diodes,pyrimidine

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark (committee chair), Armani, Andrea (committee member), Fieser, Megan (committee member)

Creator Email bontiver@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-407221

Unique identifier UC11667430

Identifier etd-OntiverosB-9213.pdf (filename),usctheses-c89-407221 (legacy record id)

Legacy Identifier etd-OntiverosB-9213.pdf

Dmrecord 407221

Document Type Thesis

Rights Ontiveros, Brenda Abigail

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