Asymmetric design of bipolar host materials with novel 1,2,4-oxadiazole unit in blue phosphorescent device

Article abstract of DOI:10.1021/ol5002494

The intrinsic asymmetry of 1,2,4-oxadiazole was utilized to synthesize three isomers, DCzmOXD-1, DCzmOXD-2, and mCzmOXD, and high triplet energies over 2.80 eV made them good candidates for host materials in blue OLEDs. The best efficiencies of 23.0 cd A<

Full text of DOI:10.1021/ol5002494

Letter
pubs.acs.org/OrgLett
Asymmetric Design of Bipolar Host Materials with Novel 1,2,4-
Oxadiazole Unit in Blue Phosphorescent Device
Qian Li,† Lin-Song Cui,† Cheng Zhong,‡ Zuo-Quan Jiang,*,† and Liang-Sheng Liao*,†
†Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials
(FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123,
P.R. China
‡Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072,
P.R. China
S
* Supporting Information
ABSTRACT: The intrinsic asymmetry of 1,2,4-oxadiazole was
utilized to synthesize three isomers, DCzmOXD-1,
DCzmOXD-2, and mCzmOXD, and high triplet energies
over 2.80 eV made them good candidates for host materials in
blue OLEDs. The best efficiencies of 23.0 cd A−1/20.5 lm
W1−/11.2% in CE/PE/EQE were achieved by DCzmOXD-1
with derivation at the β-carbon position of 1,2,4-oxadiazole.
uring the past decades, there has been growing interest in
developing new organic π-conjugated materials for
In this regard, the conjugation size of a host molecule should be
confined and the meta-linkage is undoubtedly a good choice.8
Previously, we wanted to utilize the meta-linked configuration
of 1,2,4-oxadiazole to increase the triplet energy (as compared
to its 1,3,4-isomer) in our design which was successful; the
resulting mCzmOXD could gain a triplet energy of 2.81 eV and
achieve a current efficiency of 11.5 cd A−1 in blue
phosphorescent OLEDs. During this research process, we
gradually realize that the asymmetry of 1,2,4-oxadiazole could
be another factor to influence the material property. In this
work, we rearranged the building blocks and synthesized two
new host materials DCzmOXD-1 and DCzmOXD-2 in
asymmetrical design, which could achieve enhanced current
efficiencies of 22.5 and 19.9 cd A−1, respectively.
Synthetic routes of the three compounds are outlined in
Scheme 1. The 1,2,4-oxadiazole rings are formed from the
corresponding amidoximes and acylchlorides. Then the resulted
1,2,4-oxadiazole derivatives were reacted with carbazole or 9H-
3,9′-bicarbazole to obtain the final products by Buchwald−
Hartwig reaction.9 All the materials are fully characterized, and
their thermal properties were determined by differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) measurements (Supporting Information (SI)). As
shown in Table 1 and Figure S7, all the materials exhibited
high glass-transition temperatures over 100 °C, which indicated
good thermal and morphological stability under device
fabrication.
D
optoelectronic devices, such as organic light-emitting diodes
(OLEDs), organic field-effect transistors (OFETs), and organic
photovoltaics (OPVs).1 In this development, organic synthesis
plays a pivotal role in constructing various p-type or n-type
functional materials.2 Molecular engineering gave organic
chemists many means such as the modification of backbones,
side chains, substituents, linking positions, etc. to modulate the
material properties.3 In this way, numerous classic π-conjugated
units and their derivatives (for instance, fluorene, carbazole,
naphthalene, pentacene, perylene, thiophene, 1,3,4-oxadiazole,
etc.) have been investigated and applied in different areas.4 It
should be noted that most of these classic units are symmetrical
units. An additional option in molecular engineering could be
employed if people can develop some asymmetry units, because
the properties will vary with the change of substituted position
or direction. For example, G. Bazan’s group systematically
studied an asymmetrical unit 4,7-dibromo[1,2,5]thiadiazolo-
[3,4-c]pyridine and and found that the critical HOMO/LUMO
levels of the resulted small molecules could be finely tuned by
adopting a different configuration with the same substituents.5
Recently, we also reported a new asymmetrical π-conjugated
unit 1,2,4-oxadiazole, an isomer of 1,3,4-oxadiazole, and its
application as host materials in phosphorescent OLEDs
(PHOLEDs).6 It is known that the PHOLEDs have great
potential in achieving a high internal quantum efficiency of
∼100% by harvesting both singlet and triplet excitons.7 The
phosphor emitters should be doped uniformly in the host
materials to suppress the multiparticle quenching effects. The
first design principle of host materials is that the triplet energy
of the host should be higher than that of the emitting phosphor.
Received: January 23, 2014
Published: March 3, 2014
© 2014 American Chemical Society
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Letter
Scheme 1. Synthetic Routes and Chemical Structures of the
Materials
Figure 1. Optimized geometry and HOMO/LUMO spatial
distributions of DCzmOXD-1, DCzmOXD-2, and mCzmOXD.
the 3,9′-bicarbazole group is appended to the phenyl ring at the
β-carbon, which has a relatively lower electron withdrawing
effect.
Figure 2 depicts the UV−vis absorption of the three
compounds in dichloromethane (CH2Cl2) and the photo-
The ambipolar behaviors of DCzmOXD-1, DCzmOXD-2,
and mCzmOXD were probed by cyclic voltammetry (see Table
1 and SI). The cyclic voltammogram curves suggest the bipolar
transporting properties in the three materials. As summarized in
Table 1, the HOMO and LUMO levels of the three compounds
are 5.99, 5.64, and 5.80 eV (HOMO), and 2.47, 2.26, and 2.48
eV (LUMO), respectively.
To further understand the structure−property relationship of
the compounds and explain the orbital energy data in Table 1,
the electronic properties of the compounds were studied by
density functional theory (DFT) calculations at the B3LYP/6-
31g(d) level.10 As shown in Figure 1, the HOMO of
mCzmOXD localized at the phenyl carbazole unit near the β-
carbon while the LUMO localized on the diphenyl oxadiazole
unit. It could be understood as follows: First, due to the meta-
substitution configuration, the conjugation between the two
groups, connected at α-carbon and β-carbon, is less effective.
Second, the central 1,2,4-oxadiazole ring exerts more of an
electron-withdrawing effect on the α-carbon than the β-carbon,
because of the more effective conjugation and larger frontier
orbital distribution on the α-carbon. This phenomenon gives
further contribution to the HOMO−LUMO separation in
mCzmOXD, which is also observed in DCzmOXD-1 and
DCzmOXD-2. DCzmOXD-1 possessed higher HOMO and
LUMO energy levels, as well as a more evident separation of
HOMO and LUMO compared to DCzmOXD-2, because of
Figure 2. (a) UV−vis absorption in dichloromethane and PL spectra
in dichloromethane and n-hexane solution at 10−5 M. (b)
Phosphorescence spectra of these compounds measured in a frozen
2-methyltetrahydrofuran matrix at 77 K.
Table 1. Physical Properties of DCzmOXD-1, DCzmOXD-2, and mCzmOXD
a
a
b
b
c
d
e
f
compd
Tg (°C)
Td (°C)
λmax,abs (nm)
λmax,f (nm)
λph (nm)
Eg (eV)
ET (eV)
HOMO/LUMO (eV)
DCzmOXD-1
DCzmOXD-2
mCzmOXD
110
114
100
341
334
362
293, 342
295, 339
291, 337
367, 385
430
435
442
3.38
3.32
3.52
2.88
2.85
2.81
5.64/2.26
5.80/2.48
5.99/2.47
340, 370, 392
385,400
a
b
Tg: glass transition temperatures, Td: decomposition temperatures; measured by DSC and TGA. UV−vis absorptions were measured in
c
d
dichloromethane solution at rt, and PL spectra were measured in n-hexane solution at rt. Measured in 2-MeTHF glass matrix at 77 K. Eg: The band
gap energy was estimated from the optical absorption edges of UV−vis absorption spectra. ET: The triplet energy was estimated from the onset peak
of the phosphorescence spectra. HOMO and LUMO estimated from the onset of oxidation potentials and the optical band gap from the absorption
e
f
spectra.
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Organic Letters
Letter
luminescence (PL) spectra in dichloromethane and n-hexane at
rt; all the results are summarized in Table 1. The absorption
around 296 nm for all the compounds has an affirmative
similarity to that of the unsubstituted carbazole. The optical
band gap energies of DCzmOXD-1, DCzmOXD-2, and
mCzmOXD calculated from the threshold of the absorption
spectra in CH2Cl2 solution are 3.38, 3.32, and 3.52 eV,
respectively. The emissions of the compounds DCzmOXD-1,
DCzmOXD-2, and mCzmOXD in hexane solution are in the
range 390−442 nm. Evidently, DCzmOXD-1 exhibited
hypsochromic emission by 52 nm in comparison with its
counterpart DCzmOXD-2, which is because of the fact that the
conjugation of the β-carbon with 1,2,4-oxadiazole is weaker
than the α-carbon of 1,2,4-oxadiazole. But all the compounds
showed significant red shifts in CH2Cl2 solution for the typical
photoinduced intramolecular charge transfer (Figure S11). The
phosphorescence spectra were measured in a frozen 2-
methyltetrahydrofuran matrix at 77 K, and the triplet energy
levels were estimated from the highest-energy vibronic sub-
band of the phosphorescence spectra. To our satisfaction, the
ET of all three compounds are about 2.8 eV, which is higher
than that of the most popular sky-blue emitter iridium(III)
bis(4,6-(difluorophenyl)pyridinato-N,C′)picolinate (FIrpic).
The above phenomena could be attributed to the suppression
of intramolecular charge transfer transition because of the meta-
phenyl linkage and limited conjugation via the 1,2,4-oxadiazole
ring. This can also be proved by the comparison with the 1,3,4-
oxadiazole derivatives which have a relatively low ET of ∼2.6
eV.11 Based on these results, DCzmOXD-1, DCzmOXD-2, and
mCzmOXD proved to possess sufficiently high ET to serve as
host materials for blue PHOLEDs.
Figure 3. (a) Current efficiency and power efficiency versus current
density curves; (b) Current density−voltage−luminance character-
istics.
Table 2. Electroluminescence Data of the FIrpic-Based Blue
Devices
To further investigate the electroluminescent properties of
the materials, OLEDs with the structure of indium tin oxide
(ITO)/molybdenum trioxide: 1,3-di(9H-carbazol-9-yl)benzene
(MoO3: mCP) (45 nm)/mCP (10 nm)/Host: FIrpic (25 nm,
8 wt %)/ 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)
(30 nm)/8-hydroxyquinolatolithium (Liq) (2 nm)/Al (120
nm) (Host = DCzmOXD-1: device A; DCzmOXD-2: device B;
mCzmOXD: device C) were fabricated. MoO3: mCP was used
as the hole-injection layer (HIL), mCP was used as the hole
transport layer (HTL) and electron blocking layer (EBL);
DCzmOXD-1, DCzmOXD-2, and mCzmOXD, doped with
FIrpic, were used as the emitting layers (EML); TmPyPB was
used as the hole-blocking layer (HBL); Liq was used as
electron-injecting layer (EIL); and Al was used as cathode.
Figure 3 shows the EL efficiencies and current−voltage−
luminescence (J−V−L) characteristics of the devices. The EL
spectra of the devices (see SI) show the identical spectra with a
peak at 472 nm and a shoulder at 500 nm; this means that only
the FIrpic emission peak was observed without any emission
from DCzmOXD-1, DCzmOXD-2. Such a finding strongly
indicated that the emissive excitons and charges were efficiently
confined inside the emitting layer. It could be attributed to the
high triplet energies (above 2.8 eV) of the three host materials.
As listed in Table 2, the driving voltages at 100 cd m−2 are 3.6,
3.9, and 4.2 V for the DCzmOXD-1, DCzmOXD-2, and
mCzmOXD based devices, respectively. This declared that the
driving voltages of device A and B were about 0.6 and 0.3 eV
lower than that of device C and the lowest driving voltage can
be obtained from device A. The maximum current efficiency
and current efficiency at 100 cd m−2 of device A were 23.0 and
22.5 cd A−1, while the corresponding values obtained from
device B were 19.9 and 16.0 cd A−1. The efficiencies of the
a
ηc/ ηp/ ηext/ V
b
(cdA−1/ lmW−1/ %/
V)
ηc max/ ηp max/ ηext max/ V
host
(cdA−1/ lmW−1/%/ V)
A
B
C
DCzmOXD-1
DCzmOXD-2
mCzmOXD
22.5/19.6/8.3/3.6
19.9/16.0/8.6/3.9
11.5/8.6/5.0/4.2
23.0/20.5/11.2/3.5
20.5/17.2/9.7/3.7
12.0/9.8/5.8/3.8
a
Current efficiency (ηc), power efficiency (ηp), external quantum
b
efficiency (ηext), voltage (V) at 100 cd m−2. Maximum current
efficiency (ηc max), power efficiency (ηp max), and external quantum
efficiency (ηext max).
DCzmOXD-1 and DCzmOXD-2 devices were much higher
than those of the mCzmOXD device (11.5 cd A−1, 8.6 cd A−1@
100 cd m−2). This may be because in device A and B the hole
charge are more effectively trapped on FIrpic which can lead to
direct charge recombination.12 The resulting lower driving
voltage at the same current density observed in device A also
supported this expatiation. In addition to that, the efficiencies of
device A and B were more stable than that of device C. The
CIE coordinate for all the devices at 4 V is (0.16, 0.37), with
only small CIE shifts with the change of voltage.
In summary, we have designed and synthesized a new series
of 1,2,4-oxadiazole-based host materials DCzmOXD-1,
DCzmOXD-2, and mCzmOXD. To date, most work on
oxadiazole derivatives in OLED applications always surround-
ing the 1,3,4-oxadiazole, but the 1,2,4-isomer has its unique
merit. Benefiting from the meta-linkage of 1,2,4-oxadiazole, we
could combine it with electron-donating carbazole units as
bipolar materials and then utilize them in blue phosphorescent
organic light-emitting devices due to their high triplet energies.
Moreover, the intrinsic asymmetry of 1,2,4-oxadiazole is very
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Organic Letters
Letter
(8) (a) Jeon, S. O.; Lee, J. Y. J. Mater.Chem. 2012, 22, 4233.
(b) Dong, S.-C.; Gao, C.-H.; Yuan, X.-D.; Cui, L.-S.; Jiang, Z.-Q.; Lee,
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3967.
important in molecular design. Modification on the α- or β-
carbon position will bring different properties in energy levels.
Finally, the best performances of 23.0 cd A−1/20.5 lm W1−/
11.2% in CE/PE/EQE were achieved when DCzmOXD-1 was
used as a blue host material in device evaluation. These results
are obviously better than those of mCzmOXD, which is derived
from both sides of 1,2,4-oxadiazole.
(9) US 2009/0134784 Al 2009-5-28.
(10) Salman, S.; Kim, D.; Coropceanu, V.; Bred
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2011, 23, 5223.
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ASSOCIATED CONTENT
* Supporting Information
Synthesis, UV−vis, FL, NMR, EL, references. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
Thompson, M. E. Appl. Phys. Lett. 2003, 83, 3818.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zqjiang@suda.edu.cn.
*E-mail: lsliao@suda.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the Natural
Science Foundation of China (Nos. 21202114, 21161160446,
61036009, and 61177016). This project is also supported by
the Fund for Excellent Creative Research Teams of Jiangsu
Higher Education Institutions.
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