Highly efficient bipolar host material based-on indole and triazine moiety for red phosphorescent light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2015.09.011

Two novel indole-based, bipolar, host materials were designed and synthesized by introducing the triazine unit to the 5-position of indole moiety with a meta-linking strategy. The two host materials exhibited excellence bipolar transport abilities and the

Full text of DOI:10.1016/j.dyepig.2015.09.011

Accepted Manuscript
Highly Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red
Phosphorescent Light-Emitting Diodes
Yi Chen, Jingwei Xie, Zixing Wang, Jin Cao, Hongwei Chen, Jinhai Huang, Jianhua
Zhang, Jianhua Su
PII:
S0143-7208(15)00358-7
10.1016/j.dyepig.2015.09.011
DYPI 4922
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 20 August 2015
Revised Date: 9 September 2015
Accepted Date: 10 September 2015
Please cite this article as: Chen Y, Xie J, Wang Z, Cao J, Chen H, Huang J, Zhang J, Su J, Highly
Efficient Bipolar Host Material Based-on Indole and Triazine Moiety For Red Phosphorescent Light-
Emitting Diodes, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.011.
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ACCEPTED MANUSCRIPT
Highly Efficient Bipolar Host Material Based-on Indole and Triazine
Moiety For Red Phosphorescent Light-Emitting Diodes
a§
b§
b
b
a
Yi Chen, Jingwei Xie, Zixing Wang, Jin Cao, Hongwei Chen, Jinhai
a*
b*
a
Huang, Jianhua Zhang and Jianhua Su
Graphical abstract

ACCEPTED MANUSCRIPT
Highly Efficient Bipolar Host Material Based-on Indole and Triazine
Moiety For Red Phosphorescent Light-Emitting Diodes
a§
b§
b
b
a
Yi Chen, Jingwei Xie, Zixing Wang, Jin Cao, Hongwei Chen, Jinhai
a*
b*
a
Huang, Jianhua Zhang and Jianhua Su
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East
China University of Science & Technology, Shanghai 200237, P. R. China. Fax:
(
86) 21-64252288; Tel: (86) 21-64252288; E-mail: dele12@163.com.
b
Key Laboratory of Advanced Display and System Applications, Ministry of
Education, Shanghai University, 149 Yanchang Rd, Shanghai, 200072, China.
E-mail: jhzhang@staff.shu.edu.cn
1

ACCEPTED MANUSCRIPT
Abstract
Two novel indole-based, bipolar, host materials were designed and synthesized
by introducing the triazine unit to the 5-position of indole moiety with a meta-linking
strategy. The two host materials exhibited excellence bipolar transport abilities and
o
the meta-linking analog presented a high T value (>120 C). Furthermore, their
g
electrochemical and photo-physical properties were also fully characterized and all
the results exhibited that the meta-linking strategy made a significant effect on the
physical properties of the two hosts. Moreover, the PHOLED devices using
9
-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)
phenyl)-9H-carbazole
-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazo
and
9
le as hosts were also fabricated to evaluate the practical utilities of host materials. The
devices achieved the maximum external quantum efficiencies of 17.53 % for
para-linking compound and 14.53 % for meta-linking analog, respectively. In
particular, para-linking compound based device revealed the maximum external
quantum
efficiency
was
twice
that
of
the
device
using
4
,4′-bis(N-carbazolyl)-1,10-biphenyl as the host.
2

ACCEPTED MANUSCRIPT
Keywords
Red phosphorescent OLEDs; Host materials; Bipolar; indole- triazine derivatives;
High efficiency; Thermal stability.
3

ACCEPTED MANUSCRIPT
1
. Introduction
Phosphorescent organic light-emitting devices (PHOLEDs) have been deemed to
be potential candidates for the next generation technology for flat-panel display and
solid-state lighting sources, as the heavy-metal phosphors can theoretically achieve
approximately 100
%
internal quantum efficiency by harvesting both
electro-generated singlet and triplet excitons. [1] To avoid efficiency roll-off induced
by the concentration quenching and triple-triple annihilation, those heavy-metal
phosphors are normally homogeneously dispersed into a host matrix. [2] Thus, the
design of host material with great performance is of equal importance to triple guest
to achieve highly device efficiency.
Generally, an appropriate host material is required to satisfy several conditions.
Firstly, the triplet energy level (E ) of host material should be higher than that of
T
dopant to facilitate the total energy transfer from host to phosphorescent emitter and
to better confine the triple exciton within the emitting layer. [3-5] To this goal, several
strategies have been put forward to develop the host materials with limited
3
π-conjugation. For example, either silicon or sp carbon was introduced into host
material as the building blocks to decrease the π-conjugation, and the method of
controlling the interconnection position between aromatics is normally implied to
increase the distortion of the backbone structure and further enhance the triplet energy
level (E ) of host materials. [6-8] Besides the appropriate triplet energy level, the
T
4

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carrier transport property is another criteria to be met, as the occurrence of charge
hopping in the emission layer would decrease the carrier recombination zone and
directly lower the device efficiency. [9] To meet this requirement, both electron
accepter moieties such as benzimidazole, [10] phosphine oxide, [11] triazine, [12] and
oxadiazole [13] and electron donors groups like carbazole, [14] amine, [15] fluorine
[
16] and dibenzothiophene [17] should be introduced to modify the bipolar carrier
abilities of hosts. In addition, a desired host material should still possess a high glass
transition temperature (T ) that could enhance the thermal and morphological stability
g
and consequently extend the operational lifetime of the device.
As known to all, 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP) is widely used as a
host material for red and green PHOLEDs due to its high triplet energy level and
excellent hole transporting property. [18] Unfortunately, this carbazole-based
derivative suffered from a low glass transition temperature, which would adversely
affect the device lifetime and restrict their commercial application. [19] Over the
years, tremendous efforts have been devoted to developing improved host materials to
replace CBP. [20-23] It was reported that, indole type materials, which also possess
high triple energy level and exceptional hole transporting properties, would be the
potential host material for PHOLEDs. [24] Grigalevicius et al. reported a series of
indole-based polymer host materials with high thermal stability. The device based on
a polyether containing 2-phenylindol-1-yl moieties achieved the best performance
2
with maximum power efficiency of 8.46 lm/W and current efficiency of 718 cd/m .
[
25] However, the indole derivatives based small molecule host materials are still rare.
5

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Taking the advantage of the indole-structure in the hosts, we designed and synthesized
two novel host materials incorporating the indole-carbazole moiety and triazine unit
with
-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)
phenyl)-9H-carbazole (MTZ) and para-linking
-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)phenyl)-9H-carbazo
a
meta-linking strategy [26] in this article. The meta-linking
9
analog
9
le (PTZ) were obtained through connecting the triazine unit to the 5-position of an
indole unit. By installing the triazine unit to the indole, the two indole-triazine
derivatives not only inherit good hole transport property, but also exhibit enhanced
electron transport ability and more balanced bipolar charge transport ability. In
o
addition, the MTZ presented a high T values above 120 C. To further evaluate the
g
EL performances of those host materials, the PHOLED devices using the
Ir(mphmq) acac as triple emitter were fabricated. The maximum external quantum
2
efficiencies of the red devices based on PTZ and MTZ as host were 17.53 % and
1
4.53 %, respectively, which show comparatively better device performances
compared to the 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP) based device. In
particular, the para-maximum EQE of PTZ based device is twice as high as that of the
device using CBP as host.
2
2
. Experimental
.1. General information
Chemicals and solvents used in the process were reagents grades and purchased
from J & K Chemical Co. and Aladdin Chemical Co. without further purification. In
6

ACCEPTED MANUSCRIPT
addition, the boronic acid pinacol esters (3 and 4) were obtained from Shanghai Taoe
chemical technology Co., Ltd. Tetrahydrofuran (THF) was purified by distillation
over sodium under a N atmosphere prior to use. All reactions and manipulations were
2
carried out under a N atmosphere. All column chromatography was performed on
2
silica gel (300-400 mesh) as the stationary phase in the column. All materials were
purified further by vacuum sublimation prior to fabrication of OLED devices.
1
13
The H and C NMR spectra were recorded on a Bruker AM 400 spectrometer
with tetramethylsilane as the internal standard. High-resolution mass spectra were
measured on a Waters LCT Premier XE spectrometer. The IR spectra were recorded in
-1
the range 4000-600 cm using the potassium bromide disk for solid samples by the
FTIR instrument. The ultraviolet–visible (UV-Vis) absorption spectra were obtained
on a Varian Cary 500 spectrophotometer. Photoluminescence (PL) spectra were
recorded on room temperature by Varian-Cary fluorescence spectrophotometer. The
cyclic voltammetry experiments were performed by a Versastat II electrochemical
work station (Princeton applied research) using a conventional three-electrode
configuration with a glassy carbon working electrode, a Pt wire counter electrode, and
a regular calomel reference electrode in saturated KCl solution. The oxidation and
reduction potentials were measured in dichloromethane/acetonitrile (7:3, v/v) solution
containing of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF ) as the
6
supporting electrolyte at a scan rate of 100 mV/s. The differential scanning
calorimetry (DSC) analysis was performed on a DSC Q2000 V24.11 Build 124
instrument with a heating scan rate of 10 °C/min from 0 °C to 250 °C under nitrogen
7

ACCEPTED MANUSCRIPT
atmosphere. Thermo gravimetric analysis (TGA) was carried out on the TGA
instrument by measuring weight loss of samples with a heating scan rate of 10 °C/min
from 50 °C to 800 °C under nitrogen.
2
2
9
.2. Preparation of materials
.2.1. The synthesis
-(4-(5-bromo-1H-indol-1-yl) phenyl)-9H-carbazole (2).
A mixture of 9-(4-bromophenyl)-9H-carbazole (7.66 g, 23.86 mmol) and dried
of
9-(4-iodophenyl)-9H-carbazole
(1)
and
tetrahydrofuran (100 mL) were added into a three-necked flask and bubbled with
argon stirring for 15 min under -78 °C, then 2.5 M n-butyllithium (9.5 mLꢀ23.86
mmol, n-hexane) were added to the mixture by a disposable syringe and continue to
stirring for 30 min. Subsequently, iodine (7.27 g, 28.63 mmol) dissolved in
tetrahydrofuran (50 mL) were added dropwise slowly to the reaction stirring for 1 h
and resulting mixture was reacted overnight at room temperature. After cooling to
ambient temperature, the mixture was extracted with water and dichloromethane. The
organic layer was evaporated in vacuum affording yellow solid 1, which was used in
the next reaction without future purification. To a two-necked flask added the
obtained 1, 5-bromo-1H-indole (4.92 g, 25 mmol) in mesitylene (30 mL), potassium
hydroxide (2.35 g, 19 mmol). Under argon atmosphere, cuprous iodide (0.14 g, 0.74
mmol) and phenanthrolene (0.13 g, 0.72 mmol) were added to the mixture, and the
resulting mixture was refluxed for 8 h. After cooling to room temperature, the mixture
was extracted with water and dichloromethane, and then the organic layer was dried
over anhydrous sodium sulfate. The solvent was removed in vacuum and the crude
8

ACCEPTED MANUSCRIPT
product was purified by column chromatography, affording a white solid 2 (2.56 g,
5
1
7
3
1
1
2
9
.87 mmol, 28 %). mp: 152-154 °C. IR (KBr, disk) ν 3049.22, 1515.62, 1444.97,
-1 1
230.09, 744.40, 717.91 cm . H NMR (CDCl , 400 MHz) δ 8.17 (d, J = 7.6 Hz, 3H),
3
.71 (dd, J = 8.8, 4.4 Hz, 5H), 7.49 – 7.45 (m, 5H), 7.35 – G7.30 (m, 3H), 6.69 (d, J =
1
3
.2 Hz, 1H). C NMR (CDCl , 101 MHz) δ 140.77, 138.28, 136.24, 134.58, 132.01,
3
31.15, 130.98, 128.99, 128.38, 126.13, 125.55, 125.50, 123.81, 123.53, 120.47,
20.28, 113.84, 112.48, 111.96, 109.65, 103.62.
.2.2.
The
synthesis
of
-(4-(5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)
phenyl)-9H-carbazole (PTZ).
A mixture of 2 (1.31 g, 3 mmol), boronic acidester 3 (1.57 gꢀ3.6 mmol) and 2 M
aq. K CO (10 mL) in tetrahydrofuran (20 mL) was bubbled with argon with stirring
2
3
for 30 min. Tetrakis(triphenylphosphine)palladium (0.069 g, 0.006 mmol) was added
to the mixture, and the resulting mixture was refluxed for 5 h under argon atmosphere.
The reaction mixture was cooled down to room temperature, poured into H O and
2
then extracted with dichloromethane (3*20 mL). The combined organic layer was
dried over anhydrous sodium sulfate. The solvent was removed in vacuum and the
crude product was purified by SiO column chromatography, affording yellow solid
2
PTZ (1.05 g, 1.78 mmol, 52.7 %). mp: 240-241 °C. IR (KBr, disk) ν 3052.17,
-1
1
1
603.92, 1586.32, 1515.52, 1446.92, 1230.09, 832.71 cm . H NMR (CDCl , 400
3
MHz) δ 8.88 (d, J = 8.4 Hz, 2H), 8.82 (dd, J = 8.0, 1.6 Hz, 4H), 8.19 (d, J = 7.6 Hz,
2
H), 8.08 (d, J = 1.2 Hz, 1H), 7.91 (d, J = 8.4 Hz, 2H), 7.85 – 7.74 (m, 5H), 7.70 –
9

ACCEPTED MANUSCRIPT
1
3
7
.57 (m, 7H), 7.55 – 7.44 (m, 5H), 7.37 – 7.31 (m, 2H), 6.87 (d, J = 3.2 Hz, 1H). C
NMR (CDCl , 101 MHz) δ 171.61, 146.26, 140.83, 138.63, 136.38, 136.00, 135.72,
3
1
1
34.44, 133.33, 132.49, 130.12, 129.51, 129.00, 128.67, 128.39, 127.48, 126.14,
25.46, 123.53, 122.44, 120.47, 120.25, 120.12, 110.98, 109.70, 104.77, 100.12.
+
HRMS (ESI, m/z): [M+H] calcd for: C H N , 666.2658, found, 666.2657.
4
7
32
5
2
9
.2.3.
The
synthesis
of
-(4-(5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1H-indol-1-yl)
phenyl)-9H-carbazole (MTZ).
A mixture of 2 (1.05 g, 2.4 mmol), boronic acidester 4 (1.25 gꢀ2.9 mmol) and 2
M aq. K CO (10 mL) in tetrahydrofuran (20 mL) was bubbled with argon with
2
3
stirring for 30 min. Tetrakis(triphenylphosphine)palladium (0.055 g, 0.0048 mmol)
was added to the mixture, and the resulting mixture was refluxed for 5 h under argon
atmosphere. The reaction mixture was cooled down to room temperature, poured into
H O and then extracted with dichloromethane (3*20 mL). The combined organic
2
layer was dried over anhydrous sodium sulfate. The solvent was removed in vacuum
and the crude product was purified by SiO column chromatography, affording yellow
2
solid MTZ (0.84 g, 1.43 mmol, 52.8 %). mp: 243-244 °C. IR (KBr, disk) ν 3063.94,
-1 1
1
586.26, 1515.62, 1447.91, 1333.12, 1230.09, 800.33, 700.09 cm . H NMR (CDCl ,
3
4
00 MHz) δ 9.08 (s, 1H), 8.80 (dd, J = 8.0, 1.6 Hz, 4H), 8.75 (d, J = 7.6 Hz, 1H), 8.18
(d, J = 7.6 Hz, 2H), 8.08 (d, J = 1.2 Hz, 1H), 7.92 (d, J = 8.0 Hz, 1H), 7.86 – 7.72 (m,
5
H), 7.71 – 7.55 (m, 8H), 7.54 – 7.42 (m, 5H), 7.37 – 7.29 (m, 2H), 6.87 (d, J = 3.2
1
3
Hz, 1H). C NMR (CDCl , 101 MHz) δ 171.82, 171.69, 142.73, 140.85, 138.72,
3
1
0

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1
1
36.75, 136.30, 135.94, 135.52, 133.92, 132.55, 131.59, 130.11, 129.04, 128.68,
28.39, 127.93, 127.25, 126.14, 125.47, 123.53, 122.64, 120.48, 120.25, 120.10,
+
1
10.93, 109.72, 104.70. HRMS (ESI, m/z): [M+H] calcd for: C H N , 666.2658,
4
7
32
5
found, 666.2659.
2
.3. OLED fabrication measurement
In a general operation procedure, OLED devices were fabricated under high
-4
vacuum (~10 pa) in a chamber by thermal evaporation of organic layers onto a clean
glass substrate precoated with a 150 nm thick indium tin oxide (ITO) layer. Prior to
use, the substrate was degreased in an ultrasonic bath by the following sequence: in
detergent, de-ionized water, acetone, and isopropanol, and then cleaned in a UV-ozone
chamber for 15 min. The typical deposition rates, monitored by oscillating quartz,
were 0.5, 0.1, and 5.0 Å/s for organic materials, lithium fluoride (LiF), and aluminum
(Al), respectively. The device active area defined by the overlap between the
2
electrodes was 3*3 mm in all cases.
The current-voltage characteristics of devices were measured by a Keithley 2400
electrometer in the dark at room temperature and only the luminance from the front
face of the devices was collected with MinoltaL110 luminance meter. The
electroluminescence (EL) spectra were measured with the PR650 spectrometer. All
measurements were carried out under an ambient atmosphere without device
encapsulation, immediately after the devices have been fabricated.
3
3
. Result and discussion
.1. Synthesis and characterization
11

ACCEPTED MANUSCRIPT
Scheme 1 depicts the synthetic route to the target molecules MTZ and PTZ.
Firstly, the 9-(4-bromophenyl)-9H-carbazole was treated with n-BuLi at -78 °C,
followed by reaction with iodine affording the iodo compound 1. The iodo
intermediate 1 was used without further purification to afford the key intermediate 2
by the copper-catalyzed Ullmann reaction. The desired products (MTZ and PTZ)
could be achieved by the palladium-catalyst Suzuki–Miyaura reaction of the boronic
acid pinacol esters (3 and 4) and intermediate 2 in 53 % yield. All final products were
1
13
fully characterized by H and C NMR spectral as well as the high-resolution mass
spectrometry (HRMS). Further purification was accomplished by train sublimation
under vacuum.
3
.2. Thermal properties
The thermal properties of the compounds MTZ and PTZ were measured by the
thermalgravimetric analyses (TGA) and differential scanning calorimetry (DSC). As
shown in the Fig. 1 and Table 1, both materials exhibited the decomposition
temperatures (T , corresponding to 5 % weight loss) at 460 °C for MTZ and at 452 °C
d
for PTZ, and the glass transition temperature (T ) of MTZ was 124 ºC. However, a
g
glass transition temperature of PTZ was not observed even in the second DSC scan. It
was worth noting that MTZ showed both high T and T values, mainly owning to its
g
d
steric molecular structure. [27] The highly thermal stability of the new host materials
would be desirable for enhancing the film morphology and device lifetime by
reducing the crystallization and phase separation during device operation.
3
.3. Photophysical properties
1
2

ACCEPTED MANUSCRIPT
To investigate the photophysical properties of MTZ and PTZ, their UV-Vis
absorption spectra and photoluminescence spectra as films and in the toluene and their
phosphorescent spectra in 2-methyltetrahydrofuran at 77 K were presented in the Fig.
2
and Table 1. As shown in the UV-Vis absorption spectra, a characteristic absorption
peak around 340 nm was presented, which could be attributed to the n–π* transitions
of N-phenylcarbazole. However, a higher frequency energy band of PTZ could be
observed around 300 nm, which was assigned to the π–π* transitions of
N-phenylindole. [28] The corresponding optical band gap of MTZ was 3.5 eV,
estimated from the absorption edge of UV-vis spectra, which was higher than that of
PTZ by 0.3 eV.
It can be seen from the PL spectra that, the maximum emission peaks of MTZ
and PTZ were 407 nm and 409 nm in solution, whereas the maximum emission peaks
of MTZ and PTZ in the solid state were red shifted to 425 nm and 428 nm,
respectively, owing to the intermolecular π-stacking in the solid state. Meanwhile, the
PTZ exhibited small red-shift compared to MTZ both in toluene and in the solid state,
indicating that the para- substituent on the triazine unity would be beneficial for the
delocalization of the π electron cloud. The relative fluorescence quantum yield were
also tested with the method of using ratio value of integrated area under corrected
fluorescence spectra and the anthracene in ethanol as the reference substance. As
shown in Table 1, the fluorescence quantum yield was 0.19 for MTZ and 0.24 for
PTZ.
Additionally, the triplet energy level evaluated from the highest-energy vibronic
1
3

ACCEPTED MANUSCRIPT
sub-bands of the low temperature phosphorescent spectra was 2.66 eV for MTZ and
2
.47 eV for PTZ, which qualify them to be potential hosts for red PHOLEDs.
Compared to the PTZ, the meta-disposition linkage MTZ exhibited a dramatically
higher E and a larger band gap, indicating that the meta-linking design strategy could
T
partially reduce the π conjugation between the electron-withdrawing core and
electron-deficient group. [29]
3
.4. Electrochemical properties and theoretical calculation
The electrochemical behaviors of MTZ and PTZ were investigated by cyclic
voltammetry (CV) using
tetra-n-butylammonium
a
conventional tri-electrode system with
hexauorophosphate (TBAPF6) in
dichloromethane/acetonitrile (7:3, v/v) as supporting electrolyte and the related date
are presented in the Fig. 3 and Table 1. The MTZ and PTZ exhibit the onset
oxidation potentials at 1.10 V and 1.16 V, respectively. Hence, the highest occupied
molecular orbital (HOMO) energy levels, estimated from the onset oxidation potential
by comparison to ferrocene (4.4 eV versus vacuum), [30] were calculated as -5.50 eV
for MTZ and -5.56 eV for PTZ. Meanwhile, the lowest unoccupied molecular orbital
(
LUMO) energy levels, determined by the EHOMO and optical band gap, were -2.00 eV
for MTZ and -2.36 eV for PTZ.
To better understand the electronic structure of the two materials, density
functional theory (DFT) calculations were also carried out using B3LYP hybrid
functional to simulate the HOMO-LUMO spatial distributions. It can be seen from the
Fig. 4 that, the HOMO orbital distribution of the MTZ is mainly located on the indole
1
4

ACCEPTED MANUSCRIPT
and carbazole moieties and the LUMO orbital distribution of MTZ is distributed on
the triazine unities. Compared to the electron density distribution of MTZ, the PTZ
exhibits rather dispersed over the molecule. The LUMO orbital distribution of PTZ is
mostly located on the triazine unity and slightly extended to the indole group, while
the HOMO orbitals distribution of PTZ is similar to that of MTZ. The different
LUMO orbitals distribution between MTZ and PTZ could be attributed to the
different linkage position, as the meta-linking strategy was beneficial to separate the
HOMO and LUMO orbitals of those donor-acceptor type host materials and further
improve their bipolar charge properties.
3
.5. Bipolar transporting characteristics
To assess the bipolar transporting characteristics of the two host materials,
single-carrier devices with the structure of [ITO / compounds (100 nm) / MoO (5 nm)
3
/
Al (80 nm)] for hole-only device and [ITO / LiF (1 nm) / compounds (100 nm) / LiF
1 nm) / Al (80 nm)] for electron-only device were fabricated. To better understand
(
the bipolar carrier-transport ability of the two novel host materials, single-carrier
devices based on the widely used host material CBP with the same architecture were
also fabricated for comparison. It can be seen from Fig. 5, the J-V characteristics of
these devices displayed that the current density of holes and electrons was smoothly
rising with the increase of voltage, suggesting that these host materials exhibited good
bipolar carrier-transport ability. Compared to the CBP, both PTZ and MTZ revealed
relatively balanced hole and electron transport abilities with more improved
electron-transport ability than hole-transport ability. We attributed this to the
1
5

ACCEPTED MANUSCRIPT
introduction of the electron-transport moiety, which induced the separation of HOMO
and LUMO orbitals and provided the transport channels for both holes and electrons,
thus improving the balance of bipolar carrier transportation. To further investigate
their bipolar character, the carriers mobility was also obtained by fitting the
current-voltage characteristics of the only devices to the space-charge limited current
-7
(
SCLC) model. The electron mobilities of PTZ and MTZ are around 9.88×10 and
-8
2
-1 -1
-9
-8
6
.23×10 cm V s , while the hole mobilities are around 8.61×10 and 5.70×10
2
-1 -1
cm V s , respectively, which were consistent with the single-carrier device.[31] It
was worth noting that the MTZ based devices showed more balanced bipolar carrier
transportation than that of PTZ. With the similar HOMO energy levels, this difference
could be ascribed to that the LUMO energy level of para-substituted compound PTZ
was lower than that of the meta-substituted compound MTZ. This leads to a further
separation between HOMO and LUMO energy level, which favors the balanced
transportation of holes and electrons. [32]
3
.6. Phosphorescent OLEDs
To evaluate the practical electroluminescence properties of MTZ and PTZ as
host
materials,
the
multilayer
device
configurations
of
[ITO
/
1
4
4
1
N -(naphthalen-2-yl)-N ,N -bis(4-(naphthalen-2-yl(phenyl)amino)phenyl)-N -phenylb
enzene-1,4-diamine (2-TNATA, 60 nm) / tris(4-(9H-carbazol-9-yl)phenyl)amine
(
TCTA,
10
nm)
/
host:
dopant
(8
%,
25
10
nm)
nm)
/
/
1
4
,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene
(TPBi,
,7-diphenyl-1,10-phenanthroline (BPhen, 30 nm) / LiF (1 nm) / Al (80 nm)] were
1
6

ACCEPTED MANUSCRIPT
fabricated. The device structure and the energy levels of employed materials is
presented in the Fig. 6. For this multilayer device configurations, the red emissive
Ir(mphmq) acac [33] was doped in PTZ and MTZ with a doping concentration of
2
8
wt % to form emitting layers (EML) of device R1 and R2, respectively. To confine
the excitions within the EML, TCTA and TPBi were utilized as electron-blocking
layer (EBL) and hole-blocking layer (HBL), respectively. Moreover, to reduce the
charge injection barriers, the 2-TNATA was used as hole-injection layer / transporting
layer (HIL / HTL), while BPhen and LiF were employed as electron-transporting
layer (ETL) and electron-injection layer (EIL), respectively. For comparison, the
device R3 with the same multilayer configuration applying the commonly used red
phosphorescent host CBP was also fabricated and tested.
The current density-voltage-brightness (J-V-L) characteristics, curves of current
and power efficiencies versus luminance and the electroluminescence (EL) spectra of
devices were presented in the Fig.7 and the related data were summarized in the
Table 2. As shown in Fig. 7, devices R1 and R2 exhibited the turn-on voltage of 4.6
eV and 4.1 eV, respectively, which were higher than that of R3 (3.7 eV). We assigned
this phenomenon to higher hole transporting ability of CBP than that of the
synthesized host materials PTZ and MTZ.
The maximum external quantum, maximum current and power efficiency of
device R1 hosted by PTZ were 17.53 %, 17.33 cd/A, and 11.17 lm/W, respectively,
which were quite higher than those of MTZ based device R2 (14.53 %, 14.15 cd/A
and 8.63 lm/W). In comparison, the device R3 using CBP as host material achieved a
1
7

ACCEPTED MANUSCRIPT
maximum external quantum of 9.2 %, maximum current efficiency of 8.88 cd/A, and
maximum power efficiency of 6.09 lm/W. Both devices, R1 and R2, exhibited
significantly higher performances than device R3. Especially the maximum external
quantum of device R1 was about twice higher than that of R3, which could be
explained by the fact that the PTZ and MTZ possess a more balanced transportation
of hole and electron carriers than the CBP as mentioned in the single-carrier devices.
Therefore, a better carrier recombination within the emitting layer was achieved. [34]
Furthermore, the device R1 presented quite better performance, even though the
MTZ based single-carrier device shown a more balanced bipolar carrier-transporting
ability than that of PTZ. A possible reason was that, generally, the migration rate of
hole transport layer (HTL) was higher than that of electron transport layer (ETL) in
the devices. As thus, the carrier-transport ability of the host made a greater effect on
the device performance. The single-carrier device of PTZ revealed slightly higher
electron-transportability than that of MTZ, and lower hole-transport ability than that
of MTZ, causing the PTZ based device R1 to achieve a better carrier balance and
then higher efficiency. In addition, all of those devices exhibited pure red emission at
6
18 nm with the CIE(x,y) (0.65, 0.34) and no residual emission from the host or other
layer materials, manifesting that the electroluminescence was solely originated from
the complete energy transfer from host to dopant.
4
. Conclusion
In summary, two novel indole-based bipolar host materials have been developed
successfully via the palladium-catalyst Suzuki–Miyaura reaction for application in red
1
8

ACCEPTED MANUSCRIPT
PHOLEDs. By installing the triazine unit to the 5-position of indole fragment, both
host materials exhibit enhanced electron transport ability and then balanced
transmission rate of holes and electrons. Meanwhile, the introduction of a
meta-linking strategy extensively limits the π-conjugation, resulting in a steric bulk
o
and high T value (>120 C) for MTZ. Furthermore, the two hosts based red
g
phosphorescent organic light-emitting devices were also fabricated to evaluate the
practical utilities of the host materials. Both devices displayed excellent performance
as the maximum EQEs of 17.53 % and 14.53 % were achieved for PTZ and MTZ,
respectively. For comparison, the maximum EQE of PTZ based device is twice as
high as that of the device using CBP as host.
Acknowledgements
Z. Wang thanks the financial support from National Natural Science Foundation
of China (No.21302122), the National Basic Research Program of China (973
program 2015CB655005), Science and Technology Commission of Shanghai
Municipality (13ZR1416600).
Notes and References
§
These authors contributed equally to this work.
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Table 1. Physical properties of the new host materials MTZ and PTZ.
Hole
Electron
mobility
abs
max
λ
em
max
λ
E
g
HOMO
LUMO
E
T
T
d
T
g
mobility
c
φ
[
nm]
b
d
e
f
g
h
h
2
-1
2
-1
[
nm]
[eV]
[eV]
[eV]
[eV]
[°C]
[°C]
[cm V
[cm V
a
-
1 i
-1 i
s ]
s ]
-
8
-
MTZ
PTZ
340
341
407,425
409,428
0.19
0.24
3.5
3.2
-5.50
-5.56
-2.00
-2.36
2.66
2.47
460
452
124
NA
5.70 *10
6.23*10
-
9
.88*10
-
9
8.61*10
7
a
c
b
Measured in toluene solution at room temperature. Measured in toluene and as film.
Relative fluorescence quantum yield were calculated using the method of ratio value
of integrated area under corrected fluorescence spectra and the anthracene in ethanol
d
Opt
as the reference substance. Estimated from onset of the absorption spectra (E
=
g
e
ox
f
1
241 / λonset). Calculated by the equation E
equation EHOMO = ELUMO - E . Calculated by the first peak of phosphorescence
spectra measured at 77 K. Measured by DSC and TGA. the bipolar carrier mobility
were calculated by fitting the current-voltage characteristics of the only devices to the
space-charge limited current (SCLC) model.
= - 4.4 - Eonset. Calculated by the
HOMO
g
g
h
i
Table 2. Electroluminescence characteristics of device.
b
b
c
d
ηce
ηpe
ηext
CIE
Device
Host
-1
-1
[
cd A ]
[lm W ]
[%]
[x,y]
R1
R2
R3
PTZ
MTZ
CBP
17.33
14.15
8.88
11.17
8.637
6.09
17.53
14.53
9.2
0.65, 0.34
0.65, 0.34
0.65,0.34
a
b
c
Efficiencies in the maxima. Maximum external quantum efficiency. Measured
from the EL spectra at 8 V.
2
6

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Fig. 1. The TGA of MTZ and PTZ.
Fig. 2. (a) UV-Vis absorption spectra of MTZ and PTZ in toluene solution. (b)
Room-temperature emission spectra of MTZ and PTZ as films and in the toluene (c)
the corresponding phosphorescence spectrum recorded in 2-methyltetrahydrofuran at
7
7 K.
Fig. 3. Cyclic voltammograms of compounds PTZ and MTZ.
Fig. 4. Spatial distributions of the calculated HOMO and LUMO energy levels of
PTZ and MTZ.
Fig. 5. J-V curves of the hole-only devices and electron-only devices for PTZ, MTZ
and CBP.
Fig. 6. Schematic energy-level diagram of the red phosphorescent OLEDs and the
molecular structures of the materials used.
Fig. 7. (a) J-V-L characteristics, (b) current efficiencies and power efficiencies versus
luminance curves and (c) electroluminescence (EL) spectra and the corresponding
CIE1931 coordinates for devices based on PTZ and MTZ at 8 V.
2
7

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1
00
MTZ
PTZ
8
6
4
2
0
0
0
0
0
2
00
300
400
500
600
700
800
o
Temperture ( C )
Fig. 1. The TGA of MTZ and PTZ.
a)
0
0
0
.10
.05
.00
MTZ
PTZ
3
00
350
400
450
Wavelength (nm)
2
8