Synthesis of new bipolar host materials based on 1,2,4-oxadiazole for blue phosphorescent OLEDs

Article abstract of DOI:10.1016/j.dyepig.2013.09.029

Two novel bipolar host materials, namely 3,5-bis(4-(9H-carbazol-9-yl) phenyl)-1,2,4-oxadiazole (pCzmOXD) and 3,5-bis(3-(9H-carbazol-9-yl)phenyl)-1,2, 4-oxadiazole (mCzmOXD) were designed and synthesized, by incorporating a new block 1,2,4-oxadiazole as th

Full text of DOI:10.1016/j.dyepig.2013.09.029

Dyes and Pigments 101 (2014) 142e149
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of new bipolar host materials based on 1,2,4-oxadiazole
for blue phosphorescent OLEDs
,
a
a
Qian Li a, Lin-Song Cui a, Cheng Zhong b , Xiao-Dong Yuan , Shou-Cheng Dong ,
***
,
a,
**
Zuo-Quan Jiang a , Liang-Sheng Liao
*
a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University,
Suzhou, Jiangsu 215123, China
b Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel bipolar host materials, namely 3,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,2,4-oxadiazole
(pCzmOXD) and 3,5-bis(3-(9H-carbazol-9-yl)phenyl)-1,2,4-oxadiazole (mCzmOXD) were designed and
synthesized, by incorporating a new block 1,2,4-oxadiazole as the n-type moiety and changing its linking
pattern with carbazole. As expected, high triplet energy (over 2.81 eV) for mCzmOXD was achieved due
to the intrinsic meta-linkage of 1,2,4-oxadiazole. When both materials were applied in blue phospho-
rescent organic light-emitting diodes, good performance of 13.0 cd Aꢀ1/16.0 cd Aꢀ1 were 0achieved for
Received 20 August 2013
Received in revised form
13 September 2013
Accepted 16 September 2013
Available online 4 October 2013
pCzmOXD and mCzmOXD in iridium(III) bis½ð4; 6 ꢀ difluorophenylÞpyridinato ꢀ N; C2
ꢁ
picolinate
Keywords:
OLEDs
1,2,4-Oxadiazole
Bipolar
(FIrpic)-based devices, respectively.
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Host material
Electrophosphorescence
High triplet energy
1. Introduction
polymeric solar cells (PSCs) [6e9] and organic light-emitting di-
odes (OLEDs) [10e14].
There are four different regioisomeric forms of the classic five-
membered heterocycles oxadiazoles, namely 1,2,5-oxadiazole,
1,3,4-oxadiazole, 1,2,4-oxadiazole and 1,2,3-oxadiazole. Their de-
rivatives have attracted a wide attention of chemist in searching for
the new therapeutic molecules and in developing new optoelec-
tronic materials especially with the rise of organic semiconductors.
That is mainly because of their electron-deficient nature which is
suitable for designing n-type semiconductor in material research
[1e4]. Among the four oxadiazoles (Scheme 1), the 1,2,3-isomer is
unstable and easily reverts to the diazoketone tautomer [5]; the
1,2,5-isomer incorporating with one phenyl ring to form 2,1,3-
benzooxadiazole and 1,3,4-isomer appending with two phenyl
rings to form diphenyl-1,3,4-oxadiazole are two common blocks in
synthesizing optoelectronic materials, which could be used in
In comparison with the 1,2,5-isomer and 1,3,4-isomer, the 1,2,4-
oxadiazole has received very little attention in the study of opto-
electronics. By analyzing the structure difference of these three
isomers, we found that they correspond well with three kinds of
conjugation modes-ortho, meta and para in substitution (Scheme
1). Similar theoretical studies on meta-linked phenylene com-
pounds suggest that these groups interrupt the conjugation by
introducing kinks into the backbone which will lead to a helical
conformation [15]. Accordingly, the introduction of meta-linkages
could also influence both the electron delocalization and the
conjugation degree of oxadiazole-based derivatives. Thus, the
meta-linkage, e.g. the derivation based on 1,2,4-oxadiazole, will
result in the highest S1 and T1 energies among the isomers [16e18].
This merit could be utilized in designing high triplet energy host
materials in phosphorescent OLEDs (PHOLEDs) [19e22]. It is well
known that the PHOLEDs could achieve 100% internal external ef-
ficiency by harvesting both the singlet/triplet excitons in device and
have made significant progress in the past decade [23]. This prog-
ress is closely related to the development of host materials, which is
used to disperse the phosphor emitter’s concentration to deter the
tripletetriplet annihilation or tripletepolaron annihilation [24].
* Corresponding author. Tel.: þ86 521 65880093; fax: þ86 521 658808220.
** Corresponding author. Tel.: þ86 521 65880945; fax: þ86 521 658808220.
*** Corresponding author. Tel.: þ86 27 68752330; fax: þ86 27 68756757.
E-mail addresses: zhongcheng@whu.edu.cn (C. Zhong), zqjiang@suda.edu.cn
(Z.-Q. Jiang), lsliao@suda.edu.cn (L.-S. Liao).
0143-7208/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.029

Q. Li et al. / Dyes and Pigments 101 (2014) 142e149
143
a Thermo ISQ mass spectrometer using a direct exposure probe.
UVeVis absorption spectra were obtained by a Perkin Elmer
Lambda 750 spectrophotometer. Photoluminescence (PL) spectra
and phosphorescent spectra were recorded on a Hitachi F-4600
fluorescence spectrophotometer. For thermal properties, differen-
tial scanning calorimetry (DSC) was performed on a TA DSC 2010
unit at a heating rate of 10 ꢂC/min under a nitrogen atmosphere to
determine the glass transition temperatures (Tg) from the second
heating scan.
Electrochemical measurements were made using a CHI600
voltammetric analyzer. A conventional three-electrode configura-
tion consisting of a platinum working electrode, a Pt-wire counter
electrode, and an Ag/AgCl reference electrode was used. The sol-
vent in all measurements was CH2Cl2, and the supporting electro-
lyte was 0.1 M [Bu4N]PF6. Ferrocene was added as a calibrant after
each set of measurements, and all potentials reported were quoted
with reference to the ferroceneeferrocenium (Fc/Fcþ) couple at a
scan rate of 100 mV sꢀ1. Theoretical calculations based on density
functional theory (DFT) approach at the B3LYP level were per-
formed with the use of Gaussian 09 program.
Scheme 1. Four different regioisomeric forms of oxadiazole.
The first principle of host materials is that the triplet energy of host
must be higher than that of guest [25e27]. This requirement is
critical for high device efficiency by confine the excitons in the
emitting layer. Another issue to achieve high efficiency is balancing
the carrier transport and thus broadening the combination zone.
For this purpose, the concept of bipolar host materials was pro-
posed and the 1,3,4-oxadiazole bearing the electron-transport
property had played a major role in this topic. For example,
Yang’s group have synthesized a series of bipolar host materials
based on 1,3,4-oxadiazole and achieved good results in green and
red emission [28e30]. However, there is still a dilemma between
pursuing high triplet energy and reducing the interplay from hole-
transport moiety to electron-transport moiety, especially for the
most important blue phosphorescence [31,32]. That is because a
strong donoreacceptor (DeA) interaction will lower the T1 state,
owing to the formation of intramolecular charge-transfer (CT)
states [33]. There are two alternatives for this quandary: inter-
rupting the conjugation between hole-transport/electron-
transport moieties by insulating linkage, such as sp3 hybridized Si
or C atoms, or developing new blocks with higher triplet energies.
Thus in this article, we firstly utilized 1,2,4-oxadiazole as the
electron-withdrawing group in the design and synthesis of bipolar
2.2. General procedures for the preparation of pCzmOXD and
mCzmOXD
2.2.1. Preparation of 3,5-bis(4-fluorophenyl)-1,2,4-oxadiazole
(pFmOXD)
4-Fluorobenzonitrile (3.33 g, 27.47 mmol), hydroxylamine hy-
drochloride (4.20 g, 60.44 mmol) and triethylamine (6.40 g, 8.77 ml,
63.37 mmol) were dissolved by 40 ml ethanol and 2 ml water in a
100 ml flask, then the mixture was heated to 75 ꢂC for 12 h. After
cooled to room temperature added 65 ml water in the solution,
then evaporated the ethanol in the solution with a rotary evapo-
rator to get white powder precipitated out. Dried the white powder
4-fluoro-N0-hydroxybenzimidamide after filtered, then they can be
used directly in the next step.
4-fluorobenzoic acid (2.80 g, 20.0 mmol) and sulfurous
dichloride (4.76 g, 2.92 ml, 40.0 mmol) were added in a 100 ml
flask, then the mixture was stirred at room temperature for 10 min,
76 ꢂC refluxed for 30 min, stirred at 80 ꢂC for 3 h. Redundant sul-
furous dichloride was removed by reduced pressure distillation,
then 4-fluorobenzoyl chloride was achieved. Then 4-fluoro-N0-
hydroxybenzimidamide (3.00 g, 19.46 mmol) afforded before and
20 ml DMF were added into the flask, and heated to 120 ꢂC stirred
for 30 min. After cooling to room temperature, the reaction mixture
was dumped into ice water and filtered, washed with water. The
crude material was purified by column chromatography with 1:3
(v/v) dichloromethane/petroleum ether as the eluent to give a
white solid (4.29 g).
host
materials.
3,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,2,4-
oxadiazole (pCzmOXD) and 3,5-bis(3-(9H-carbazol-9-yl)phenyl)-
1,2,4-oxadiazole (mCzmOXD) were prepared and their thermal,
photophysical, and electrochemical properties were fully investi-
gated. The two materials were used as host materials of blue phos-
phorescent OLEDs and the device performances were evaluated.
When pCzmOXD and mCzmOXD were employed as the host for th0e
phosphor iridium(III) bis½ð4; 6 ꢀ difluorophenylÞpyridinato ꢀ N; C2 ꢁ
picolinate (FIrpic) in PHOLEDs, current efficiencies of 13.0 and
16.0 cd Aꢀ1, with external quantum efficiencies of 5.6% and 6.8%,
were achieved.
83.2% yield. 1H NMR (400 MHz, CDCl3)
d
(ppm): 8.23e8.21(m,
(ppm):
2H), 8.19e8.15(m, 2H), 7.27e7.18(m, 4H). 13C NMR (CDCl3)
d
174.91, 168.21, 166.81, 165.91, 164.27, 163.41, 132.87, 130.68, 130.59,
129.74, 129.65, 123.08, 120.62, 116.43, 116.19, 115.97, 115.61. MS (EI):
m/z 258.29 [Mþ]. Anal. calcd for C14H8F2N2O (%):C 65.12, H 3.12, N
10.85; found: C 64.32, H 3.23, N 10.39.
2. Experimental
2.1. Chemicals and instruments
2.2.2. Preparation of 3,5-bis(3-bromophenyl)-1,2,4-oxadiazole
(mBrmOXD)
All chemicals purchased were used without further purification,
and intermediates were purified and dried before using. Solvents
used in the synthetic routes were purified by PURE SOLV (Innova-
tive Technology) purification system. All the other reagents were
used as received from commercial sources.
mBrmOXD was synthesized according to the same procedure as
for pFmOXD by using 3-bromobenzonitrile (5.00 g, 27.47 mmol)
and 3-bromobenzoic acid (4.02 g, 20.0 mmol). mBrmOXD (6.42 g)
was afforded as a white solid.
84.5% yield. 1H NMR (400 MHz, CDCl3)
d
(ppm): 8.39e8.33 (m,
2H), 8.16e8.14 (d, J ¼ 8.0 Hz, 1H), 8.12e8.10 (d, J ¼ 8.0 Hz,1H), 7.75e
7.65 (m, 2H), 7.47e7.38 (m, 2H). 13C NMR (CDCl3)
(ppm): 174.64,
167.99, 135.90, 134.34, 131.11, 130.74, 130.56, 130.48, 128.65, 126.68,
1H NMR and 13C NMR spectra were measured on a Varian Unity
Inova 400 spectrometer at room temperature, and the chemical
shifts were quoted relative to SiMe4. Mass spectra were recorded on
d

144
Q. Li et al. / Dyes and Pigments 101 (2014) 142e149
126.05, 125.88, 123.23, 123.01. MS (EI): m/z 380.15 [Mþ]. Anal. calcd
for C14H8Br2N2O (%):C 44.25, H 2.12, N 7.37; found: C 44.26, H 2.03,
N 7.20.
dipalladium (0) were added to the reaction mixture. The mixture
was stirred at 110 ꢂC for 48 h. After the reaction was finished, the
organic layer was separated and washed with dichloromethane and
water, then the organic layer was dried over by anhydrous Na2SO4,
filtered and evaporated under reduced pressure. The crude material
was purified by column chromatography with 1:3 (v/v) dichloro-
methane/petroleum ether as the eluent to give a white solid state
mCzmOXD (3.85 g).
2.2.3. Preparation of 3,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,2,4-
oxadiazole (pCzmOXD)
pFmOXD (2.58 g, 10.00 mmol), carbazole (5.01 g, 30.00 mmol)
and K2CO3 (8.28 g, 60.00 mmol) were dissolved in 30 ml DMSO. The
reaction mixture was heated to 150 ꢂC for 12 h under an argon
atmosphere. After cooling to room temperature, the mixture was
dumped into ice water and filtered, washed with water. The crude
material was purified by column chromatography with 1:3 (v/v)
dichloromethane/petroleum ether as the eluent to give a white
solid (4.48 g).
69.7% yield. 1H NMR (400 MHz, CDCl3)
d (ppm): 8.44e8.41 (d,
J ¼ 14.4 Hz, 2H), 8.29e8.25 (m, 2H), 8.16e8.14 (d, J ¼ 8.0 Hz, 2H),
7.83e7.73 (m, 4H), 7.46e7.39 (m, 8H), 7.32e7.27 (m, 4H). 13C NMR
(CDCl3)
d (ppm): 175.19, 168.56, 140.70, 140.53, 138.85, 138.50,
131.35, 130.89, 130.55, 129.87, 129.74, 129.38, 128.74, 126.91, 126.64,
126.40,126.22,126.10,125.99,123.64, 123.53,120.46,120.37,120.20.
MS (EI): m/z 552.51 [Mþ]. Anal. calcd for C38H24N4O (%): C 82.59, H
4.38, N 10.14; found: C 82.58, H 4.37, N 10.23.
81.2% yield. 1H NMR (400 MHz, CDCl3)
d (ppm): 8.52e8.50 (d,
J ¼ 8.8 Hz, 2H), 8.48e8.46 (d, J ¼ 8.4 Hz, 2H), 8.18e8.16 (d, J ¼ 8.0 Hz,
4H), 7.86e7.84 (d, J ¼ 8.4 Hz, 2H), 7.80e7.77 (d, J ¼ 8.4 Hz, 2H),
7.55e7.51 (t, 4H), 7.48e7.44 (m, 4H), 7.37e7.31 (m, 4H). 13C NMR
2.3. Device fabrication and characterization
(CDCl3)
d (ppm): 142.10, 140.55, 140.48, 140.19, 129.98, 129.24,
127.23,126.32,126.18,125.66,123.95,123.73,122.62,120.78,120.56,
120.46, 120.43. MS (EI): m/z 552.56 [Mþ]. Anal. calcd for C38H24N4O
(%): C 82.59, H 4.38, N 10.14; found: C 82.04, H 4.47, N 10.14.
PHOLEDs were fabricated with a configuration of ITO/HAT-CN
(10 nm)/TAPC (45 nm)/Host: FIrpic (20 nm, 8 wt%)/TmPyPB
(40 nm)/Liq (2 nm)/Al (120 nm), in which pCzmOXD or mCzmOXD
acted as the phosphorescent host and FIrpic acted as the dopant.
PHOLEDs mentioned in this paper were fabricated through
vacuum deposition of the materials at ca. 2 ꢃ 10ꢀ6 Torr onto ITO-
coated glass substrates. The ITO coated glass substrates were
cleaned ultrasonically with acetone, ethanol, and deionized water,
followed by dried in an oven, and exposed to UV-ozone for about
2.2.4. Preparation of 3,5-bis(3-(9H-carbazol-9-yl)phenyl)-1,2,4-
oxadiazole (mCzmOXD)
mBrmOXD (3.80 g, 10.00 mmol), carbazole (5.01 g, 30.00 mmol)
dissolved in anhydrous xylene (60 ml) under an argon atmosphere.
Sodium tert-butoxide, S-PHOS and tris(dibenzylideneaceton)
Scheme 2. Synthetic routes and chemical structures of pCzmOXD and mCzmOXD.

Q. Li et al. / Dyes and Pigments 101 (2014) 142e149
145
Table 1
30 min. Organic materials, Liq and Al were deposited in the above
Physical Properties of pCzmOXD and mCzmOXD.
ꢀ
ꢀ
ꢀ
sequence at a rate of 2e3 A/s, 0.2 A/s and 4 A/s, respectively,
through a shadow mask without breaking the vacuum between
each deposition process. After cathode deposition the devices were
encapsulated with a glass lid.
The EL spectra, CIE coordinates and JeVeL curves of the devices
were measured with a PHOTO RESEARCH SpectraScan PR 655
photometer and a KEITHLEY 2400 SourceMeter constant current
source at room temperature. The external quantum efficiency (EQE)
values were calculated according to the previously reported
methods.
lmax,em HOMO/LUMOe [eV] ETd [eV]
Eg
[eV]
a
b
b
c
Compound Tg
[ꢂC] [nm]
lmax,abs
[nm]
exp
cal
exp cal
pCzmOXD 110 292, 341 440
mCzmOXD 100 291, 337 451
5.98/2.72 5.42/1.89 2.71 2.69 3.24
5.99/2.47 5.38/1.94 2.81 2.88 3.52
a
Tg: glass transition temperatures.
Measured in dichloromethane solution at room temperature.
Eg: The band gap energies were estimated from the optical absorption edges of
b
c
UVeVis absorption spectra.
d
ET: The triplet energies were estimated from the onset peak of the phospho-
rescence spectra which were measured in 2-MeTHF glass matrix at 77 K.
e
HOMO levels were calculated from CV data, LUMO levels were calculated from
3. Results and discussion
HOMO and Eg.
3.1. Synthesis and characterization
mCP. Therefore the two compounds can be used as stable organic
materials for OLEDs under vacuum evaporation.
The synthetic routes of pCzmOXD and mCzmOXD are outlined
in Scheme 2. For both pCzmOXD and mCzmOXD, the 1,2,4-
oxadiazole rings are formed from the corresponding amidoximes
and acylchlorides. Due to the para-positions of the two phenyl rings
adjacent to the 1,2,4-oxadiazole rings are activated by the inductive
effect of the electron-withdrawing group, pCzmOXD was synthe-
sized via aromatic nucleophilic substitution reaction between
carbazole and the fluoro-1,2,4-oxadiazole in absence of any palla-
dium catalyst. But for mCzmOXD, the fluorine at meta-position of
the phenyl ring cannot be effectively activated and the reaction
yield is quite low under the same condition as pCzmOXD. Thereby,
we chose bromo-1,2,4-oxadiazole as the precursor to react with
carbazole via BuchwaldeHartwig reaction to afford mCzmOXD in
good yield. The molecular structures of the precursors and final
products were confirmed by NMR spectroscopy, mass spectrom-
etry, and elementary analyses.
3.3. Photophysical properties
The photophysical properties of pCzmOXD and mCzmOXD
were investigated by electronic absorption and photoluminescence
(PL) measurements at room temperature in CH2Cl2. The pertinent
data are summarized in Table 1. The absorption and emission
spectra of these two molecules are shown in Fig. 2. The absorption
bands observed at about 250 and 296 nm of the two compounds are
assigned to carbazole central transitions. The absorption in the
range of 315 nme352 nm could be attributed to the intramolecular
charge transfer (ICT)
pep* transition from the electron-donating
3.2. Morphology stability
Good morphological stabilities of materials are important for
OLEDs, since the high Tg and thermal stability could withstand
inevitable Joule heat encountered during device operation. To
confirm their morphological stabilities, pCzmOXD and mCzmOXD
were investigated by differential scanning calorimetry (DSC) under
a nitrogen atmosphere at a scanning rate of 10 ꢂC/min. As shown in
Fig. 1, both materials presented glass transition above 100 ꢂC,
indicting better stability than the common used blue host material
Fig. 2. (a) UVeVis absorption and PL spectra of pCzmOXD and mCzmOXD in
dichloromethane solution at 10ꢀ5 M. (b) Phosphorescence spectra of pCzmOXD and
mCzmOXD measured in a frozen 2-methyltetrahydrofuran matrix at 77 K.
Fig. 1. DSC traces recorded at a heating rate of 10 ꢂC/min.

146
Q. Li et al. / Dyes and Pigments 101 (2014) 142e149
instead of para-linkage in pCzmOXD. At the same time, the higher
ET of pCzmOXD and mCzmOXD than the corresponding 1,3,4-
oxadiazole derivatives p-CzOXD and m-CzOXD (2.60 eV and
2.72 eV as mentioned in the previous work [34]) can be explained
by the conjugation interruption effect of the meta-linkage method
in 1,2,4-oxadiazole. Hence, we surmise mCzmOXD can be used as
potential host for blue PHOLEDs.
3.4. Electrochemical properties
The electrochemical properties of the compounds were deter-
mined by cyclic voltammetry (CV) (Fig. 3). The two materials un-
derwent oxidation and reduction to approbate the formation of
stable cation and anion radicals, which suggests their bipolar
transporting properties. The ionization potential (which corre-
sponds to the HOMO level) of pCzmOXD and mCzmOXD were
estimated from the onset of oxidation potentials as ꢀ5.98 eV
and ꢀ5.99 eV, respectively. Their optical energy bandgaps (Eg)
were determined from the onset of the UVeVis absorption in
CH2Cl2 solution. The electron affinity (corresponding to the LUMO
level) of pCzmOXD and mCzmOXD were 2.72 eV and 2.47 eV,
respectively, which calculated by subtracting the gap energy from
the ionization potential. All the electronic data are summarized in
Table 1.
Fig. 3. Cyclic voltammograms in CH2Cl2 for oxidation and DMF for reduction.
carbazole moiety to the electron-withdrawing 1,2,4-oxadiazole
moiety in the backbone. It is clearly that the relative intensity of
para-analogue is higher than that of meta-analogue, reflecting the
larger transition dipole moment for the more effectively conjuga-
tion. Their energy gaps (Eg), calculated from the threshold of the
absorption spectra, are 3.24 and 3.52 eV for pCzmOXD and
mCzmOXD, respectively. It is not beyond the expectation that the
meta analogue should possess higher Eg, on the other hand, the
emission spectra of mCzmOXD (max at 440 nm) show a red-shift
relative to pCzmOXD (max at 450 nm), which is inconsistent
with their absorption. The larger strokes shift of mCzmOXD can be
understood by their HOMOeLUMO distributions: the HOMOe
LUMO separation in mCzmOXD is larger, indicating a larger charge
separation and a more polar excited state, which will lead to larger
strokes shift in solvent (see theoretical calculation section). The
phosphorescence of the two molecules was measured in 2-MeTHF
glass at 77 K and the spectra of pCzmOXD and mCzmOXD are
shown in Fig. 2. Their triplet energies (ET) are determined to be 2.71
and 2.81 eV for pCzmOXD and mCzmOXD, respectively, evaluated
from the first phosphorescent emission peak. The ET of mCzmOXD
is evidently higher than that of pCzmOXD and is also higher than
that of the common blue phosphorescent dopant FIrpic (2.62 eV).
The higher ET can be attributed to meta-linkage for mCzmOXD
3.5. Theoretical calculations
To further understand the structureeproperty relationship of
the compounds, the electronic properties of the compounds were
studied using density functional theory (DFT) and time-dependent
DFT (TDDFT) calculations with the B3LYP hybrid functional.
Fig. 4 shows the HOMO and LUMO distributions of mCzmOXD
and pCzmOXD. For the two compounds, the HOMO and LUMO
distributions were quite the same, except the HOMO and LUMO of
mCzmOXD were more localized than that of pCzmOXD which
could be explained by the worse conjugation of meta-linkage
compared with para-linkage. The separation between the HOMO
and LUMO levels bring benefits to the efficient charge-carrier
transport and the prevention of reverse energy transfer. As
compared with the molecular orbital distribution of p-CzOXD and
m-CzOXD [34], the separation between the HOMO and LUMO
levels of pCzmOXD and mCzmOXD are much obvious, indicating
more effective bipolarity. Fig. 4 also shows that the asymmetry of
Fig. 4. Optimized geometry and HOMOeLUMO spatial distributions of pCzmOXD and mCzmOXD.

Q. Li et al. / Dyes and Pigments 101 (2014) 142e149
147
Table 2
identical spectra with a peak at 476 nm and a shoulder at 500 nm,
which was arisen from the typical emission of the phosphor FIrpic.
As shown in Fig. 6 and Table 2, the mCzmOXD-based device has a
maximum current efficiency (CE) of 16.0 cd Aꢀ1, a power efficiency
(PE) of 16.8 lm Wꢀ1 and an external quantum efficiency (EQE) of
6.8%. Obviously, the efficiency of the pCzmOXD-based device is
lower than that of the mCzmOXD-based device. This fact can be
explained by considering in the differences of both ET and LUMO
energy level between the two compounds. As mentioned before, ET
of pCzmOXD and mCzmOXD are 2.71 and 2.81 eV respectively, the
higher ET of mCzmOXD that could confine more triplet excitons
within the emitting layers leaded to the higher device performance
in mCzmOXD-based device. On the other hand, the LUMO energy
level of mCzmOXD was estimated as ꢀ2.47 eV, which is higher than
Electroluminescence characteristics of the devices.
Device Host/guest
Va (v) hcb (cd Aꢀ1
)
hpb (lm Wꢀ1
)
hextb (%)
A
B
pCzmOXD/FIrpic 4.54
mCzmOXD/FIrpic 3.35
13.0, 11.5, 9.9 10.7, 8.1, 4.4 5.6, 4.9, 4.2
16.0, 14.3, 9.9 16.8, 13.8, 5.9 6.8, 6.1, 4.2
a
Voltage (V) at 100 cd mꢀ2
Current efficiency (hc), power efficiency (hp) and external quantum efficiency
.
b
(
hext) in the order of maximum, at 100 cd mꢀ2 and at 1000 cd mꢀ2
.
the 1,2,4-oxadizole can influence the distribution of the molecular
orbitals, and the specific research is under study.
3.6. Electroluminescent properties
To assess the capability of pCzmOXD and mCzmOXD as bipolar
host materials in PHOLEDs, a typical structure consisting of multi-
ple organic layers sandwiched electrophosphorescent devices were
fabricated. Due to the comparably high ET, sky blue phosphorescent
devices were fabricated using Ir-complex FIrpic doped into the
hosts as emitting layer. Strongly electron-deficient 1,4,5,8,9,11-
hexaazatriphenylene-hexacarbonitrile (HAT-CN) was deposited
onto the precleaned ITO substrate to form the hole-injection layer
(HIL). The 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane
(TAPC) served as the hole transport layer (HTL) and electron
blocking layer (EBL). 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene
(TmPyPB) was utilized as an exciton- and hole-blocking layer (HBL)
for suppressing the blue excitons quenching at the emitting layer
(EML)/electron-transporting layer (ETL) interface. Liq served as
electron-injecting layer (EIL) and Al was utilized as cathode. These
PHOLEDs have the configuration of ITO/HAT-CN (10 nm)/TAPC
(45 nm)/Host: FIrpic (20 nm, 8 wt%)/TmPyPB (40 nm)/Liq (2 nm)/Al
(120 nm) (Host ¼ pCzmOXD: device A; Host ¼ mCzmOXD: device
B). The key parameters were summarized in Table 2. The schematic
energy level diagrams of the two devices are shown in Fig. 5.
Fig. 6 depicts the current densityevoltageeluminance (JeVeL)
characteristics, devices efficiency and EL spectra of the sky blue
devices. Table 2 summarized the detail electroluminescence (EL)
data. As shown in Fig. 6, both pCzmOXD and mCzmOXD based
devices have low operating voltages of 4.54 and 3.35 V at a
brightness of 100 cd mꢀ2, which could be attributed to the ambi-
polar transporting property and the suitable HOMO and LUMO
energy levels of the two compounds. The two devices exhibited
Fig. 6. (a) Current densityevoltageeluminance characteristics; (b) current efficiency
and power efficiency versus current density curves; (c) the EL spectra of devices at
Fig. 5. Energy level diagrams for devices.
5 mA cmꢀ2
.

148
Q. Li et al. / Dyes and Pigments 101 (2014) 142e149
(No. 2011AA03A110), and the Natural Science Foundation of Jiangsu
Province (No. BK2010003). This is also a project supported by Pri-
ority Academic Program Development of Jiangsu Higher Education
Institutions (PAPD), and by the Fund for Excellent Creative Research
Teams of Jiangsu Higher Education Institutions.
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4. Conclusions
In summary, we firstly developed one the of oxadiazole isomer-
1,2,4-oxadiazole-in designing optoelectronic materials. Bipolar host
materials pCzmOXD and mCzmOXD with this new electron-
withdrawing block were synthesized and fully characterized.
Incorporation of the 1,2,4-oxadiazole moiety could raise the triplet
energies of pCzmOXD and mCzmOXD to 2.71 and 2.81 eV which
allow the materials used in blue PHOLED. Good performance with
low operating voltage and maximum current efficiency of
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We acknowledge financial support from the Natural Science
Foundation of China (Nos. 21202114, 21161160446, 61036009, and
61177016), the National High-Tech Research Development Program

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