Versatile benzimidazole/triphenylamine hybrids: Efficient nondoped deep-blue electroluminescence and good host materials for phosphorescent emitters

Article abstract of DOI:10.1002/asia.201000206

Two new bipolar compounds, N, N, N′, N′-tetraphenyl-5′- (1-phenyl-1H-benzimidazol-2-yl)-1, 1′:3′, 1′-terphenyl-4, 4′-diamine (1) and N, N, N′, N′-tetra-phenyl-5′-(1- phenyl-1H-benzimidazol-2-yl)-1, 1′:3′, 1′-terphenyl-3, 3′-diamine ( 2), were synthesized

Full text of DOI:10.1002/asia.201000206

DOI: 10.1002/asia.201000206
Versatile Benzimidazole/Triphenylamine Hybrids: Efficient Nondoped Deep-
Blue Electroluminescence and Good Host Materials for Phosphorescent
Emitters
Shaolong Gong,[a] Yongbiao Zhao,[b] Meng Wang,[a] Chuluo Yang,*[a] Cheng Zhong,[a]
Jingui Qin,[a] and Dongge Ma*[b]
Abstract: Two new bipolar compounds,
N,N,N’,N’-tetraphenyl-5’-(1-phenyl-1H-
benzimidazol-2-yl)-1,1’:3’,1’’-terphenyl-
4,4’’-diamine (1) and N,N,N’,N’-tetra-
phenyl-5’-(1-phenyl-1H-benzimidazol-
2-yl)-1,1’:3’,1’’-terphenyl-3,3’’-diamine
(2), were synthesized and character-
ized, and their thermal, photophysical,
and electrochemical properties were in-
vestigated. Compounds 1 and 2 possess
good thermal stability with high glass-
transition temperatures of 109–1298C
and thermal decomposition tempera-
tures of 501–5318C. The fluorescence
quantum yield of 1 (0.52) is higher
than that of 2 (0.16), which could be at-
tributed to greater p conjugation be-
tween the donor and acceptor moieties.
A nondoped deep-blue fluorescent or-
ganic light-emitting diode (OLED)
using 1 as the blue emitter displays
high performance, with a maximum
current efficiency of 2.2 cdAÀ1 and a
maximum external efficiency of 2.9%
at the CIE coordinates of (0.17, 0.07)
that are very close to the National Tel-
evision System Committeeꢀs blue stan-
dard (0.15, 0.07). Electrophosphores-
cent devices using the two compounds
as host materials for green and red
phosphor emitters show high efficien-
cies. The best performance of a green
phosphorescent device was achieved
using 2 as the host, with a maximum
current efficiency of 64.3 cdAÀ1 and a
maximum
power
efficiency
of
68.3 lmWÀ1; whereas the best perfor-
mance of a red phosphorescent device
was achieved using 1 as the host, with a
maximum
current
efficiency
of
11.5 cdAÀ1, and a maximum power ef-
ficiency of 9.8 lmWÀ1. The relationship
between the molecular structures and
optoelectronic properties are discussed.
Keywords: benzimidazole
· host–
guest systems · materials science ·
phosphorescence · triphenylamine
Introduction
thermore, the development of a deep-blue emission, which
is defined as having Commission Internationale de
a
As compared with the superior device performances of
green and red emitters, blue-light-emitting materials and de-
vices still need to be improved in terms of efficiency and
color purity. Highly efficient blue organic light-emitting
diodes (OLEDs) are a pressing concern for high-resolution
full-color displays and solid-state lighting applications.[1] Fur-
L’Eclairage (CIE) y coordinate value of less than 0.15,[2] is
of special significance because such an emitter can not only
effectively reduce the power consumption of a full-color
OLED but also be utilized to generate light of other colors
by energy cascade to a lower-energy fluorescent or phos-
phorescent dopant.[3,4] Although the dopant system as an
emitting layer has been proven to improve the device per-
formance,[3,5] the fabrication process became complicated
and expensive during the mass production of OLEDs,[5a] and
therefore, the use of a nondoped emitting layer is of practi-
cal significance. Charge transporting and charge balancing
are two crucial factors for high-efficiency OLEDs. To bal-
ance the electron–hole recombination, one promising strat-
egy is to incorporate an electron-donating moiety and an
electron-withdrawing moiety into a single emitter;[4,6] how-
ever, this donor–acceptor system could cause a remarkable
bathochromic effect, and thereby the color purity of the
[a] S. Gong, M. Wang, Prof. C. Yang, C. Zhong, Prof. J. Qin
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Wuhan University
Fax : (+86)276-875-6757
E-mail: clyang@whu.edu.cn
[b] Y. Zhao, Prof. D. Ma
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun 130022 (P. R. China)
E-mail: mdg1014@ciac.jl.cn
Chem. Asian J. 2010, 5, 2093 – 2099
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2093

FULL PAPERS
blue emission might be impaired. To date, deep-blue light-
emitting materials with bipolar transporting properties
remain a great challenge.
Results and Discussion
Synthesis and Characterization
Over the past decade, considerable progress has been
made in phosphorescent organic light-emitting diodes
(PHOLEDs).[7–10] For PHOLEDs, the phosphorescent
dopant of a heavy metal complex has to be doped in the
host materials to reduce concentration quenching and trip-
let–triplet annihilation.[11] Because of the intrinsically wide
band gap of blue emitters, they could be utilized as host ma-
terials for phosphorescent emitters.
The new compounds, N,N,N’,N’-tetraphenyl-5’-(1-phenyl-
1H-benzimidazol-2-yl)-1,1’:3’,1’’-terphenyl-4,4’’-diamine (1)
and N,N,N’,N’-tetraphenyl-5’-(1-phenyl-1H-benzimidazol-2-
yl)-1,1’:3’,1’’-terphenyl-3,3’’-diamine (2), were synthesized
through Suzuki cross-coupling reactions of 2-(3,5-dibromo-
phenyl)-1-phenyl-1H-benzimidazole (BI-Br2) with 4-(diphe-
nylamino)phenylboronic acid and 3-(diphenylamino)phenyl-
boronic acid, respectively (Scheme 1).[15] Their structures
1
In our previous work, we reported two simple triphenyl-
were fully characterized by H and 13C NMR spectroscopy,
A
mass spectrometry, and elemental analysis (see the Experi-
mental Section).
ters and hosts for red PHOLEDs and we realized two-color-
based white OLEDs.[12] Howev-
er, the CIE coordinates of y>
0.15 for the new blue emitters
were not satisfied. Herein, we
report the design and synthesis
of two new benzimidazole/tri-
phenylamine hybrids through
the Suzuki cross-coupling reac-
tion. The benzimidazole unit
was introduced as an electron-
accepting component owing to
its good electron mobility.[4,13]
Combined with the good hole
mobility of arylamines,[14] we
decided to link the benzimida-
zole unit and the diphenyl-
ACHTUNGTRENNUNGamine unit with an m-terphenyl
spacer. We anticipate that the
combination of benzimidazole
and diphenylamine by means of
the rigid m-terphenyl linkage
can alleviate the intramolecular
charge transfer from the donor
to the acceptor, and this would
Scheme 1. Synthesis and molecular structures of 1 and 2.
therefore not only have little
effect on the color purity of the
Thermal Properties
blue-emissive materials with bipolar character, but would
also avoid lowering the triplet energies of the hybrids to
some extent. The proper triplet energies of the hybrids
would ensure that they could host a phosphorescent emitter.
A device that incorporates 1 as the emitter exhibits a maxi-
mum current efficiency of 2.2 cdAÀ1, a maximum external
efficiency of 2.9%, together with satisfactory CIE coordi-
nates (0.17, 0.07). When 1 and 2 were used as host materials
doped with phosphorescent emitters, efficient green and red
PHOLEDs were obtained, respectively. Additionally, some
relationship between the molecular structures and optoelec-
tronic properties will be addressed in this paper.
The thermal properties of the compounds were character-
ized using thermogravimetric analysis (TGA) and differen-
tial scanning calorimetry (DSC; Table 1). TGA measure-
ments reveal their high thermal-decomposition temperatures
(Td, which corresponds to 5% weight loss) of 531 (1) and
5018C (2). The DSC trace exhibits distinct glass-transition
temperatures (Tg) of 129 (1) and 1098C (2) during the
second heating scans (Figure 1). Compound 2, with a meta
disposition between 1,2-diphenyl-1H-benzimidazole and the
diphenylamine units, has a lower Tg value than 1 with a para
disposition, which can be attributed to the greater energetic
disorder in the meta derivatives.[16] The high Td and Tg
values suggest that they could form morphologically stable
and uniform amorphous films upon thermal evaporation,
which is highly important to improve the efficiency and life-
time of OLEDs.
2094
www.chemasianj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 2093 – 2099

Benzimidazole/Triphenylamine Hybrids
Table 1. Physical properties of 1 and 2.
2 is almost completely suppressed owing to the minimizing
conjugation between the donor and acceptor. For the same
reason, the PL emission maximum of 2 (411 nm) is blue-
shifted by 10 nm with respect to that of 1 (421 nm). The
fluorescence quantum yield of 1 in dichloromethane (0.52)
is substantially higher than that of 2 (0.16). This is attributed
to the extended p conjugation for para-triphenylamine
linked 1. The high quantum yield of 1 also makes it a candi-
date for an efficient blue-light-emitting material in OLEDs.
To be an appropriate host material, the host should have
higher triplet energy (ET) than the phosphorescent guest
emitter to prevent reverse energy transfer from the guest
back to the host. The triplet energies (ET) of 1 and 2, deter-
mined by the highest-energy vibronic sub-band of the phos-
phorescence spectra in the film state at 77 K (Figure 3), are
2.41 and 2.55 eV, respectively; these values are high enough
to host green or red phosphorescent emitters. Additionally,
the triplet energy of 2 is significantly higher than that of 1,
which is associated with the decrease of p conjugation in the
meta configuration.
1
2
Tg/Tm/Td [8C]
129/257/531
309, 340
421
109/212/501
298
411
[a]
labs [nm]
lem,max[b] [nm]
F[c] [%]
52
16
HOMO/LUMO[d] [eV]exptl
5.20/2.06
5.22/1.48
2.41
5.27/1.82
5.28/1.50
2.55
HOMO/LUMO[e] [eV]calcd
[f]
ET [eV]exptl
DE (T1ÀS0)[e] [eV]calcd
2.69
2.87
[a] Measured in dichloromethane. [b] Measured in the film state. [c] Mea-
sured in dichloromethane with an integrating sphere. [d] Determined
from cyclic voltammetry and the absorption onset. [e] Obtained from
DFT calculations. [f] Measured in the film state at 77 K.
Figure 1. DSC traces of 1 and 2 recorded at a heating rate of 108CminÀ1
.
Photophysical Properties
The photophysical properties of the compounds were inves-
tigated by obtaining the UV/Vis absorption and photolumi-
nescence (PL) spectra (Figure 2 and Table 1). The absorp-
tion bands around 300 nm for both 1 and 2 could be as-
signed to the triphenylamine-centered n–p* transition. The
longer wavelength absorption band around 340 nm for 1
could be attributed to the charge transfer (CT) p–p* transi-
tion from the electron-donating triphenylamine moiety to
the electron-accepting benzimidazole moiety. It is worth
noting that the intramolecular charge-transfer transition for
Figure 3. Phosphorescence spectrum of 1 and 2 in the film state at 77 K.
Electrochemical Properties
Cyclic voltammetry (CV) was performed to investigate the
electrochemical properties of the compounds. As shown in
Figure 4, 1 and 2 exhibit one quasi-reversible, one-electron
oxidation process, which can be assigned to the oxidation of
the arylamine moiety. The HOMO energy levels of the com-
Figure 2. UV/Vis absorption of 1 and 2 in dichloromethane or films, and
PL spectra of 1 and 2 in films.
Figure 4. Cyclic voltammograms of 1 and 2 in CH2Cl2 for an oxidation
scan.
Chem. Asian J. 2010, 5, 2093 – 2099
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2095

D. Ma, C. Yang et al.
FULL PAPERS
pounds were determined from the onset of the oxidation po-
tentials with regard to the energy level of ferrocene (4.8 eV
below vacuum). The estimated HOMO levels are 5.20 (1)
and 5.27 eV (2), which are similar to most triphenylamine
derivatives. In CH2Cl2, no reduction waves were detected
for the two compounds owing to the benzimidazole struc-
ture.[4] The LUMO energy levels were deduced from
HOMO energy levels and energy gaps determined by the
onset of absorption (Table 1). The deduced LUMO levels
are 2.06 (1) and 1.82 eV (2).
Electroluminescent Devices
To evaluate the applicability of 1 as the blue-emitting mate-
rial, we fabricated nondoped blue-emitting device A with
the configuration: ITO/MoO3 (10 nm)/1,4-bis[(1-naphthyl-
phenyl)amino]biphenyl (NPB, 80 nm)/1 (20 nm)/1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene
(TPBI,
40 nm)/LiF
(1 nm)/Al (100 nm). MoO3 is used to promote hole injection
and NPB serves as the hole-transporting material; TPBI and
LiF act as the electron-transporting and electron-injecting
layer, respectively. Device A exhibits deep-blue emission
that peaks around 428 nm, with a narrow FWHM (full-
width at half-maximum) of 51 nm at 7 V (Figure 6). It is
noteworthy that the CIE chromaticity coordinates of (0.17,
0.07) are very close to the standard blue emission recom-
mended by the National Television System Committee
(NTSC, 0.15, 0.07)[18] (inset of Figure 6).
Theoretical Calculations
To understand the structure–property relationship of the
compounds at the molecular level, the geometric and elec-
tronic properties of the compounds were studied by density
functional theory (DFT) and time-dependent DFT (TD-
DFT) calculations using B3LYP hybrid functional (for de-
tails, see the Experimental Section). According to DFT cal-
culations (Figure 5), the HOMO orbitals are mainly located
Figure 7 shows current density–voltage–brightness (J–V–
L) characteristics, and the current efficiency versus current
density curve for device A. Device A displays satisfactory
Figure 6. EL spectrum of device A operated at 7 V. Inset: CIE coordi-
nates of device A.
Figure 5. Optimized geometry and spatial distributions of the HOMO
and LUMO levels of 1 and 2.
device performance, with a maximum current efficiency
(hc.max) of 2.2 cdAÀ1, a maximum power efficiency (hp.max) of
1.6 lmWÀ1, and a maximum external efficiency (hext.max) of
2.9%, which are comparable with the best device perfor-
mance in the literature for the nondoped deep-blue OLEDs
with a CIE coordinate of y<0.10.[19,20] For example, we re-
cently reported a series of fluorine-based oligomers with spi-
on the electron-donating triphenylamine moiety, whereas
the LUMO orbitals are mainly distributed on the electron-
accepting benzimidazole moiety. Compounds 1 and 2 have
almost complete separation of the HOMO and LUMO at
the hole- and electron-transporting moieties, respectively,
which most likely arise from the weak intramolecular charge
transfer from the triphenylamine to the benzimidazole unit.
Since the separation between HOMO and LUMO levels is
preferable for efficient hole- and electron-transporting prop-
erties and the prevention of reverse energy transfer,[17] 1 and
2 could be suitable as host materials for green or red phos-
phorescent emitters. The calculated triplet energy gaps DE
(T1ÀS0) are 2.69 eV for 1 and 2.87 eV for 2, which are in
good agreement with the experimental results (Table 1). As
shown in Figure 5, the optimized dihedral angles at the m-
terphenyl spacer increase from 38.2 and 35.28 (1) to 40.2
and 37.68 (2), which suggests a decrease in the coupling be-
tween the electron donor and acceptor for 2.
Figure 7. Current density–voltage–brightness characteristics of device A.
Inset: Current efficiency versus current density curve for device A.
2096
www.chemasianj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 2093 – 2099

Benzimidazole/Triphenylamine Hybrids
roannulated triarylamine as stable deep-blue emitters, and a
maximum current efficiency of 1.91 cdAÀ1 at the CIE coor-
dinates of (0.16, 0.07) was achieved.[19c] Chien et al. reported
a high-efficiency deep-blue OLED with a maximum current
efficiency of 2.9 cdAÀ1 at the CIE coordinates of (0.15, 0.07)
by utilizing a bipolar compound as the emitter.[20c]
To investigate the utility of the two compounds as bipolar
host materials for phosphors, we first fabricated green phos-
phorescent devices B and C by using 1 and 2 as hosts for
green phosphorescent emitter [IrACHTUNRGTNEUNG(ppy)3] (ppy=2-phenylpyri-
dine), respectively. The configurations of the devices are as
follows: ITO/MoO3 (10 nm)/NPB (80 nm)/1 or 2: 9 wt% [Ir-
ACHTUNGTRENNUNG(ppy)3] (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). The
current density–voltage–brightness (J–V–L) characteristics,
and the efficiency versus current density curves of the devi-
ces are shown in Figure 8. Electroluminescence (EL) data of
the devices are summarized in Table 2. Devices B and C
show typical green emission around 530 nm that originates
from the guest [IrACHTUNGTRENNUNG(ppy)3] (Figure 8d). Device B hosted by
para-linked 1 shows a maximum luminance of 48428 cdmÀ2
at 11.9 V, a maximum current efficiency of 59.0 cdAÀ1, and
a maximum power efficiency of 48.7 lmWÀ1. Better EL per-
formance is achieved in device C hosted by meta-linked 2
(40702 cdmÀ2 at 10.9 V, 64.3 cdAÀ1, and 68.3 lm WÀ1). The
triplet energy of 2 (2.55 eV) is higher than that of the guest
[Ir
ACHTUNGTRENNUNG
(2.41 eV) is close to that of [IrAHCNUTGTRENNUNG
gies between the host and guest could allow for reverse
energy transfer from the guest back to the host, and conse-
quently decrease the efficiency of the device.
Finally, we also fabricated red phosphorescent devices D
and E by doping 1 or 2 with 6 wt% bis(1-phenylisoquinoli-
nato-N,C2’)iridium(acetylacetonate) ([(piq)2IrACTHNUTRGNEG(NU acac)]) (ET =
2.0 eV)[22] in the same device structures as devices B and C.
Devices D and E emit deep red light with CIE chromaticity
coordinates of (0.68, 0.32). Device E hosted by 2 shows a
maximum luminance of 11446 cdmÀ2 at 12.7 V, a maximum
current efficiency of 6.5 cdAÀ1, and a maximum power effi-
ciency of 5.1 lmWÀ1; whereas device D hosted by 1 exhibits
much better EL performance (23616 cdmÀ2 at 13.7 V,
11.5 cdAÀ1, and 9.8 lmWÀ1). Why does the device hosted by
1 display better red phosphorescent device performance
than the device hosted by 2? Conversely, why does the
device hosted by 2 display better green phosphorescent
device performance than the device hosted by 1? This could
be elucidated as follows: 1) for the green phosphorescent
device, the discrepancy of triplet energy between the host
and guest is the main factor affecting the device efficiency;
and 2) for the red phosphorescent device, both 1 and 2 have
a high enough triplet energy to host a red phosphor guest,
and therefore the triplet energy of host does not determine
the device efficiency. In this situation, the charge-transport
ability of the host may become important for the device effi-
ciency, and as a consequence, the lower charge-transport
ability of 2 that owes itself to the meta-linkage may result in
the inferior red-phosphorescent device efficiency. We note
that all the phosphorescent devices (B–E) display low turn-
Figure 8. a) Current density–voltage–brightness characteristics for devices
B–E. b) Current efficiency and power efficiency versus current density
curves for devices B and C. c) Current efficiency and power efficiency
versus current density curves for devices D and E. d) EL spectra for devi-
ces B–E.
on voltages of 2.7–2.9 V, which could be attributed to their
matching HOMO levels (5.2–5.3 eV) with that of the hole-
transport NPB layer (ꢀ5.4 eV).
Conclusion
In summary, we have synthesized and characterized two
new bipolar molecules that combine triphenylamine and
Chem. Asian J. 2010, 5, 2093 – 2099
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2097

D. Ma, C. Yang et al.
FULL PAPERS
Table 2. EL performances of devices A–E.
Device[a] EML Von[b] [V] Lmax [cdmÀ2],[c] hc.max[d] [cdAÀ1
background current of the cyclic vol-
tammogram. The starting material 2-
(3,5-dibromophenyl)-1-phenyl-1H-ben-
zimidazole (BI-Br2) was prepared ac-
cording to the literature procedures.[23]
[e]
[f]
]
hp.max [lmWÀ1
]
hext.max [%] CIE[g] [x, y]
Voltages [V]
A
B
C
D
E
1
3.5
4448, 13.5
2.2
59.0
64.3
11.5
6.5
1.6
48.7
68.3
9.8
2.9
15.5
17.0
14.2
8.3
0.17, 0.07
0.34, 0.62
0.32, 0.64
0.68, 0.32
0.68, 0.32
1-IrG 2.7
2-IrG 2.7
1-IrR 2.7
2-IrR 2.9
48428, 11.9
40702, 10.9
23616, 13.7
11446, 12.7
Computational Details
The geometric and electronic proper-
ties were performed with the Gaussi-
an 03 program package.[24] The calcula-
tion was optimized by means of
5.1
[a] Devices configuration: ITO/MoO3 (10 nm)/NPB (80 nm)/EML (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al
(100 nm). [b] Turn-on voltages at 1 cdmÀ2. [c] Maximum luminance. [d] Maximum current efficiency. [e] Maxi-
mum power efficiency. [f] Maximum external quantum efficiency. [g] Commission International de I’Eclairage
coordinates.
B3LYP
(Becke
three-parameter
hybrid functional with Lee–Yang–
Perdew correlation functionals)[25] with
the 6-31G(d) atomic basis set. Then
the electronic structures were calculat-
benzimidazole moieties. A nondoped deep-blue fluorescent
ed at B3LYP/6-311g(d) level. The triplet states DE (T1ÀS0) were calculat-
ed using time-dependent density functional theory (TD-DFT) calcula-
tions with B3LYP/6-311g(d).
OLED using 1 as the blue emitter reveals high performance,
with a maximum current efficiency of 2.2 cdAÀ1, and a max-
imum external efficiency of 2.9% at the CIE coordinates of
(0.17, 0.07). Electrophosphorescent devices using the two
compounds as host materials for green and red phosphor
emitters show high efficiencies. The best performance of the
green phosphorescent device was achieved when using 2 as
the host, with a maximum current efficiency of 64.3 cdAÀ1
and a maximum power efficiency of 68.3 lmWÀ1; whereas
the best performance of red phosphorescent device was ach-
ieved using 1 as the host, with a maximum current efficiency
Device Fabrication and Measurement
The hole-injection material MoO3, hole-transporting material NPB (1,4-
bis(1-naphthylphenylamino)biphenyl), and electron-transporting materi-
als 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) were commer-
cially available. Commercial indium tin oxide (ITO)-coated glass with
sheet resistance of 10 WsquareÀ1 was used as the starting substrate.
Before device fabrication, the ITO glass substrates were precleaned care-
fully and treated by oxygen plasma for 2 min. Then the sample was trans-
ferred to the deposition system. MoO3 (10 nm) was first deposited on the
ITO substrate, followed by NPB, the emissive layer, and TPBI (40 nm).
Finally, a cathode composed of lithium fluoride (1 nm) and aluminum
(100 nm) was sequentially deposited onto the substrate in a vacuum of
10À6 Torr. The L–V–J of the electroluminescent (EL) devices was mea-
sured with a Keithley 2400 Source meter and a Keithley 2000 Source
multimeter equipped with a calibrated silicon photodiode. The EL spec-
tra were measured with a JY SPEX CCD3000 spectrometer. All meas-
urements were carried out at room temperature under ambient condi-
tions.
of 11.5 cdAÀ1 and
a maximum power efficiency of
9.8 lmWÀ1. The results reveal that the bipolar compound 1
has the potential to be a multifunctional material as both
deep-blue emitter and host for the phosphorescent dopant
in OLEDs.
Experimental Section
Synthesis of Compound 1
General Information
Dry THF (35 mL) was added to a mixture of 2-(3,5-dibromophenyl)-1-
phenyl-1H-benzimidazole (0.86 g, 2.00 mmol), 4-(diphenylamino)phenyl-
boronic acid (1.73 g, 6.00 mmol), K2CO3 (2m in H2O, 6.0 mL, 12.0 mmol),
1H and 13C NMR spectra were measured with a MERCUYR-VX300
spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen
were performed with a Vario EL III microanalyzer. Liquid chromatogra-
phy (LC) mass spectra were measured with a Waters ZQ-Mass ESI. UV/
Vis absorption spectra were recorded with a Shimadzu UV-2500 record-
ing spectrophotometer. Photoluminescence (PL) spectra were recorded
with a Hitachi F-4500 fluorescence spectrophotometer. The PL quantum
yields were measured from dilute dichloromethane solution (around
10À6 molLÀ1) with the Edinburgh F-900 Instruments integrating sphere
excited with an Xe lamp. Differential scanning calorimetry (DSC) was
performed with a NETZSCH DSC 200 PC unit at a heating rate of 108C
minÀ1 from 30 to 3508C under argon. The glass-transition temperature
(Tg) was determined from the second heating scan. Thermogravimetric
analysis (TGA) was undertaken with a NETZSCH STA 449C instrument.
The thermal stability of the samples under a nitrogen atmosphere was de-
termined by measuring their weight loss while heating at a rate of
158CminÀ1 from 25 to 6008C. Cyclic voltammetry (CV) was carried out
in nitrogen-purged dichloromethane (oxidation scan) at room tempera-
ture with a CHI voltammetric analyzer. Tetrabutylammonium hexafluor-
ophosphate (TBAPF6) (0.1m) was used as the supporting electrolyte. The
conventional three-electrode configuration consisted of a platinum work-
ing electrode, a platinum wire auxiliary electrode, and an Ag wire pseu-
doreference electrode with ferrocenium–ferrocene (Fc+/Fc) as the inter-
nal standard. Cyclic voltammograms were obtained at a scan rate of
100 mVsÀ1. Formal potentials were calculated as the average of cyclic
voltammetric anodic and cathodic peaks. The onset potential was deter-
mined from the intersection of two tangents drawn at the rising and
and [PdACHTNUGTRNEGNU(PPh3)4] (92 mg, 0.08 mmol). The mixture was heated at reflux for
48 h under argon. After cooling to room temperature, the mixture was
poured into water and extracted with CH2Cl2. The organic extracts were
collected and dried with anhydrous Na2SO4. After removal of the solvent,
the residue was purified by column chromatography on silica gel using
CHCl3/petroleum (1:1 v/v) as the eluent to give 1 as a white powder.
Yield: 56%; m.p.: 256–2588C; 1H NMR (300 MHz, CDCl3): d=7.94 (d,
J=7.8 Hz, 1H), 7.74–7.72 (m, 3H), 7.58–7.55 (m, 5H), 7.43–7.40 (m,
5H), 7.30–7.28 (m, 10H), 7.14–7.03 ppm (m, 16H); 13C NMR (75 MHz,
CDCl3): d=152.28, 147.54, 147.44, 142.94, 141.11, 137.23, 137.14, 134.18,
130.47, 129.99, 129.26, 128.54, 127.78, 127.61, 126.26, 125.87, 124.41,
123.70, 123.40, 122.97, 119.86, 110.42 ppm; MS (ESI): m/z: 757 [M++1];
elemental analysis calcd (%) for C55H40N4: C 87.27, H 5.33, N 7.40;
found: C 87.33, H 4.91, N 7.06.
Synthesis of Compound 2
Compound 2 was prepared according to a similar procedure as 1. Yield:
62%; m.p.: 211–2128C; 1H NMR (300 MHz, CDCl3): d=7.90 (d, J=
7.8 Hz, 1H), 7.64–7.60 (m, 3H), 7.35–7.24 (m, 18H), 7.14–7.09 (m, 12H),
7.04–6.99 ppm (m, 6H); 13C NMR (75 MHz, CDCl3): d=152.37, 148.62,
148.16, 143.30, 141.94, 141.83, 137.53, 137.25, 130.92, 130.25, 130.13,
129.66, 129.13, 127.84, 127.61, 127.34, 124.56, 124.28, 123.86, 123.62,
123.43, 123.06, 122.46, 120.27, 110.85 ppm; MS (ESI): m/z: 757 [M++1];
elemental analysis calcd (%) for C55H40N4: C 87.27, H 5.33, N 7.40;
found: C 87.52, H 5.23, N 7.47.
2098
www.chemasianj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 2093 – 2099

Benzimidazole/Triphenylamine Hybrids
c) J.-L. Chen, S.-Y. Chang, Y. Chi, K. Chen, Y.-M. Cheng, C.-W. Lin,
G.-H. Lee, P.-T. Chou, C.-H. Wu, P.-I. Shih, C.-F. Shu, Chem. Asian
J. 2008, 3, 2112–2123; d) Y. Tao, L. Ao, Q. Wang, C. Zhong, C.
Yang, J. Qin, D. Ma, Chem. Asian J. 2010, 5, 278–284.
Acknowledgements
We thank the National Natural Science Foundation of China (project
nos. 90922020, 50773057, and 20621401) and the National Basic Research
Program of China (973 Program; 2009CB623602, 2009CB930603) for fi-
nancial support.
[11] M. A. Baldo, C. Adachi, S. R. Forrest, Phys. Rev. B 2000, 62, 10967–
10977.
[12] Y. Tao, Q. Wang, Y. Shang, C. Yang, L. Ao, J. Qin, D. Ma, Z. Shuai,
Chem. Commun. 2009, 77–79.
[13] Y. Q. Li, M. K. Fung, Z. Xie, S.-T. Lee, L.-S. Hung, J. Shi, Adv.
Mater. 2002, 14, 1317–1321.
[14] a) P.-I. Shih, C.-H. Chien, F.-I. Wu, C.-F. Shu, Adv. Funct. Mater.
2007, 17, 3514–3520; b) S.-y. Takizawa, V. A. Montes, P. Anzenba-
cher, Jr., Chem. Mater. 2009, 21, 2452–2458.
[15] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457–2483.
[16] Y. Shirota, J. Mater. Chem. 2000, 10, 1–25.
[17] a) Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita, M.-a. Kakimoto, Adv. Funct. Mater. 2008, 18, 584–590;
b) Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita, M.-a. Kakimoto, Org. Lett. 2008, 10, 421–424.
[1] a) C. J. Tonzola, A. P. Kulkarni, A. P. Gifford, W. Kaminsky, S. A. Je-
nekhe, Adv. Funct. Mater. 2007, 17, 863–874; b) Y. Wei, C.-T. Chen,
J. Am. Chem. Soc. 2007, 129, 7478–7479; c) S. Tao, Y. Zhou, C.-S.
Lee, S.-T. Lee, D. Huang, X. Zhang, J. Phys. Chem. C 2008, 112,
14603–14606; d) Y. Jiang, L. Wang, Y. Zhou, Y.-X. Cui, J. Wang, Y.
Cao, J. Pei, Chem. Asian J. 2009, 4, 548–553.
[2] a) M.-T. Lee, C.-H. Liao, C.-H. Tsai, C. H. Chen, Adv. Mater. 2005,
17, 2493–2497; b) S.-L. Lin, L.-H. Chan, R.-H. Lee, M.-Y. Yen, W.-J.
Kuo, C.-T. Chen, R.-J. Jeng, Adv. Mater. 2008, 20, 3947–3952; c) C.-
H. Wu, C.-H. Chien, F.-M. Hsu, P.-I. Shih, C.-F. Shu, J. Mater. Chem.
2009, 19, 1464–1470.
[18] W.-Y. Lai, R. Xia, Q.-Y. He, P. A. Levermore, W. Huang, D. D. C.
Bradley, Adv. Mater. 2009, 21, 355–360.
[3] M.-T. Lee, H.-H. Chen, C.-H. Liao, C.-H. Tsai, C. H. Chen, Appl.
Phys. Lett. 2004, 85, 3301–3303.
[19] a) C.-C. Wu, Y.-T. Lin, K.-T. Wong, R.-T. Chen, Y.-Y. Chien, Adv.
Mater. 2004, 16, 61–65; b) J. N. Moorthy, P. Venkatakrishnan, D.-F.
Huang, T. J. Chow, Chem. Commun. 2008, 2146–2148; c) Z. Jiang,
Z. Liu, C. Yang, C. Zhong, J. Qin, G. Yu, Y. Liu, Adv. Funct. Mater.
2009, 19, 3987–3995.
[4] a) M.-Y. Lai, C.-H. Chen, W.-S. Huang, J. T. Lin, T.-H. Ke, L.-Y.
Chen, M.-H. Tsai, C.-C. Wu, Angew. Chem. 2008, 120, 591–595;
Angew. Chem. Int. Ed. 2008, 47, 581–585; b) C.-H. Chen, W.-S.
Huang, M.-Y. Lai, W.-C. Tsao, J. T. Lin, Y.-H. Wu, T.-H. Ke, L.-Y.
Chen, C.-C. Wu, Adv. Funct. Mater. 2009, 19, 2661–2670.
[20] a) Z. Q. Gao, Z. H. Li, P. F. Xia, M. S. Wong, K. W. Cheah, C. H.
Chen, Adv. Funct. Mater. 2007, 17, 3194–3199; b) L. Wang, Y. Jiang,
J. Luo, Y. Zhou, J. Zhou, J. Wang, J. Pei, Y. Cao, Adv. Mater. 2009,
21, 4854–4858; c) C.-H. Chien, C.-K. Chen, F.-M. Hsu, C.-F. Shu, P.-
T. Chou, C.-H. Lai, Adv. Funct. Mater. 2009, 19, 560–566.
[21] K.-T. Wong, Y.-M. Chen, Y.-T. Lin, H.-C. Su, C.-C. Wu, Org. Lett.
2005, 7, 5361–5364.
[5] a) C. W. Tang, S. A. VanSlyke, C. H. Chen, J. Appl. Phys. 1989, 65,
3610–3616; b) C.-G. Zhen, Z.-K. Chen, Q.-D. Liu, Y.-F. Dai,
R. Y. C. Shin, S.-Y. Chang, J. Kieffer, Adv. Mater. 2009, 21, 2425–
2429.
[6] a) Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto, T. Ohara, J. Am.
Chem. Soc. 2000, 122, 11021–11022; b) C.-C. Chi, C.-L. Chiang, S.-
W. Liu, H. Yueh, C.-T. Chen, C.-T. Chen, J. Mater. Chem. 2009, 19,
5561–5571; c) C.-T. Chen, Y. Wei, J.-S. Lin, M. V. R. K. Moturu, W.-
S. Chao, Y.-T. Tao, C.-H. Chien, J. Am. Chem. Soc. 2006, 128,
10992–10993.
[7] a) C. W. Tang, S. A. VanSlye, Appl. Phys. Lett. 1987, 51, 913–915;
b) M. A. Baldo, D. F. OꢀBrien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151–154; c) Y.
Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S. R. Forr-
est, Nature 2006, 440, 908–912.
[8] a) Y.-J. Su, H.-L. Huang, C.-L. Li, C.-H. Chien, Y.-T. Tao, P.-T.
Chou, S. Datta, R.-S. Liu, Adv. Mater. 2003, 15, 884–888; b) S.-J.
Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho, S.-F. Hsu,
C. H. Chen, Adv. Mater. 2005, 17, 285–289; c) N. Rehmann, C. Ul-
bricht, A. Kçhnen, P. Zacharias, M. C. Gather, D. Hertel, E. Holder,
K. Meerholz, U. S. Schubert, Adv. Mater. 2008, 20, 129–133; d) C.-L.
Ho, Q. Wang, C.-S. Lam, W.-Y. Wong, D. Ma, L. Wang, Z.-Q. Gao,
C.-H. Chen, K.-W. Cheah, Z. Lin, Chem. Asian J. 2009, 4, 89–103.
[9] a) W.-Y. Wong, C.-L. Ho, Z.-Q. Gao, B.-X. Mi, C.-H. Chen, K.-W.
Cheah, Z. Lin, Angew. Chem. 2006, 118, 7964–7967; Angew. Chem.
Int. Ed. 2006, 45, 7800–7803; b) C.-L. Ho, W.-Y. Wong, Q. Wang, D.
Ma, L. Wang, Z. Lin, Adv. Funct. Mater. 2008, 18, 928–937; c) G.
Zhou, C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang, Z. Lin,
T. B. Marder, A. Beeby, Adv. Funct. Mater. 2008, 18, 499–511.
[10] a) Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin, D.
Ma, Angew. Chem. 2008, 120, 8224–8227; Angew. Chem. Int. Ed.
2008, 47, 8104–8107; b) T.-C. Lee, C.-F. Chang, Y.-C. Chiu, Y. Chi,
T.-Y. Chan, Y.-M. Cheng, C.-H. Lai, P.-T. Chou, G.-H. Lee, C.-H.
Chien, C.-F. Shu, J. Leonhardt, Chem. Asian J. 2009, 4, 742–753;
[22] C.-L. Li, Y.-J. Su, Y.-T. Tao, P.-T. Chou, C.-H. Chien, C.-C. Cheng,
R.-S. Liu, Adv. Funct. Mater. 2005, 15, 387–395.
[23] Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T.
Kajita, M.-a. Kakimoto, Chem. Mater. 2008, 20, 2532–2537.
[24] Guassian 03, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
ery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
[25] a) A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100; b) C. Lee, W.
Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
Received: March 16, 2010
Published online: July 22, 2010
Chem. Asian J. 2010, 5, 2093 – 2099
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2099