Bipolar anthracene derivatives containing hole- and electron-transporting moieties for highly efficient blue electroluminescence devices

Article abstract of DOI:10.1039/c0jm03300f

A series of bipolar anthracene derivatives containing hole-transporting triphenylamine and electron-transporting benzimidazole moieties were synthesized and characterized. These compounds possess high thermal properties and quantum yield in toluene solution but their photoluminescence spectra are sensitive to the polarity of the solvent. Three types of devices were fabricated to investigate their electron-transporting and hole-transporting as well as light-emitting properties. The electroluminescence spectra for the three derivatives B1, B3 and B4 show that they emit blue light, while the B2-based devices produced white light at a low current density because of the electromer formation. Moreover, a single layer OLED device for B4 exhibited a current efficiency of 3.33 cd A^-1 with a pure blue color of CIE _(x,y) of (0.16, 0.16) at 20 mA cm^-2 and a maximum brightness of 8472 cd m^-2 at 8.7 V. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm03300f

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PAPER
Bipolar anthracene derivatives containing hole- and electron-transporting
moieties for highly efficient blue electroluminescence devices†
Jinhai Huang,a Jian-Hua Su,a Xin Li,a Mei-Ki Lam,b Ka-Man Fung,b Hai-Hua Fan,b Kok-Wai Cheah,b
a
Chin H. Chenbc and He Tian
*
Received 1st October 2010, Accepted 22nd November 2010
DOI: 10.1039/c0jm03300f
A series of bipolar anthracene derivatives containing hole-transporting triphenylamine and electron-
transporting benzimidazole moieties were synthesized and characterized. These compounds possess
high thermal properties and quantum yield in toluene solution but their photoluminescence spectra are
sensitive to the polarity of the solvent. Three types of devices were fabricated to investigate their
electron-transporting and hole-transporting as well as light-emitting properties. The
electroluminescence spectra for the three derivatives B1, B3 and B4 show that they emit blue light, while
the B2-based devices produced white light at a low current density because of the electromer formation.
Moreover, a single layer OLED device for B4 exhibited a current efficiency of 3.33 cd Aꢀ1 with a pure
blue color of CIE(x,y) of (0.16, 0.16) at 20 mA cmꢀ2 and a maximum brightness of 8472 cd mꢀ2 at 8.7 V.
overall cost and accelerate its ongoing commercialization
Introduction
activities. Considerable progress has been reported to apply
Organic light-emitting diodes (OLEDs) have drawn great scien-
the bipolar molecules in the OLEDs.8–15 However, the donor–
tific and commercial attention, due to their potential applications
acceptor system will bring out some serious disadvantages of the
in full-color, flat-displays as well as solid-state lighting.1–3
A
bipolar materials in OLEDs, such as a huge bathochromic
effect,16 low quantum yield due to the dipolar quenching,17 easy
crystallization due to large dipolar moment,17 as well as the low
band gap of the materials owing to the intermolecular charge
transfer.18 Hence, compared to the efficient and stable red and
green device, sufficiently wide band gap of highly efficient and
stable bipolar blue luminescent materials are still rare.19,20 In
addition, blue emission materials play a particularly important
role in the development of OLEDs, which can not only be used as
a blue light source in their own right,21 but also can be utilized to
generate light of green and red color by energy cascade to
a suitable emissive dopant.22
typical trilayer electroluminescent (EL) device consists of a light-
emitting layer, an electron-transporting layer, as well as a hole-
transporting layer. Therefore, the EL performance significantly
depends on the recombination efficiency of the holes and elec-
trons injected from the anode and cathode, respectively.4 Large
efforts are devoted to optimizing the charge flux balance for
improving the device efficiency and limiting the energy
consumption, because the hole-transporting material has
a higher carrier drift than the electron-transporting material in
most OLEDs materials.5,6
Bipolar materials with emission characteristics incorporating
both hole- and electron-transporting segments can effectively
stabilize exciton formation and balance the hole and electron
charge in the emitting layer.7 Moreover, the utilization of bipolar
compounds will significantly simplify the device structure with
double layers or even a single layer, which will largely limit the
Anthracene has been intensively studied as an attractive
building block and starting material in OLEDs, due to its
unusual photoluminescence and electroluminescence properties
and excellent electrochemical properties, as well as easier modi-
fication. The anthracene derivatives imply multifunctional
properties through introducing different groups, such as
light-emitting layer,23 hole-transporting layer,23f,24 and electron-
transporting layer.23e,25 As the light-emitting layer, the fluores-
cence spectra color of anthrance derivatives can be easily tuned
from blue to green by importing different bulks at the C-9 and 10
positions.23 Moreover, the white OLEDs based on the anthra-
cene derivatives were also achieved in a single component
system.26
aKey Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Shanghai, 200237, China.
E-mail: tianhe@ecust.edu.cn; Fax: (+86) 21-64252288
bCentre for Advanced Luminescence Materials, Department of Physics,
Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
cDisplays & Lighting Center, National Engineering Lab of TFT-LCD
Materials and Technologies, Shanghai Jiao Tong University, Shanghai,
200240, China
† Electronic supplementary information (ESI) available: The absorption
spectra and fluorescence spectra of B2 and B3, the TGA and DSC curves
of target compounds, the device performance of B1 and B2, and the EL
spectra of device I-III for B2. See DOI: 10.1039/c0jm03300f
In this paper, we synthesized a series of bipolar blue lighting
materials containing a hole-transporting triphenylamine moiety
and an electron-transporting benzimidazole moiety at the C-9
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Scheme 1 The synthetic routes of the bipolar compounds.
and C-10 positions of the anthracene. The color and the energy
Results and discussions
levels were modified by the phenyl bridge between the func-
tionalized moieties and the anthracene core. The thermal prop-
erties and photophyscial properties were fully investigated. The
HOMO and LUMO values were measured by cyclic voltamme-
try. In addition, a series of devices were fabricated to systemat-
ically study the hole-transporting and electron-transporting and
the emitting properties of these bipolar anthracene derivatives.
We achieved a current efficiency of 3.33 cd Aꢀ1 with a pure blue
CIE(x,y) of (0.16, 0.16) at 20 mA cmꢀ2 and a maximum brightness
of 8472 cd mꢀ2 at 8.7 V in a single layer device for B4, which has
one of the best pure blue electroluminescence performances
among the reported single-layer devices.8–20
Synthesis
The synthetic routes to the bipolar compounds are illustrated in
Scheme 1. Some starting materials were synthesized according to
the literatures in good yield. The 9-anthracene boronic acid was
obtained by reacting 9-bromoanthracene and n-BuLi and
triisopropylborate in good yield. Suzuki aryl-aryl coupling
reactions and bromination reactions were employed in the
synthesis of the intermediates with a yield of over 70%. The target
compounds were also achieved by the boronate compounds
reacting with bromides through Suzuki aryl-aryl coupling reac-
tions. The compounds were availably purified with the silica
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similar absorption bands in the range of 350–400 nm, which were
assigned to the p–p* transition of the characteristic vibrational
structures of the isolated anthracene groups. Moreover, the
absorption was not affected by the phenyl bridge length, this was
due to the lack coplanarity of biphenyl group and a twisting
angle of about 60ꢁ caused by the steric interactions of
9,10-phenyl rings with hydrogen atoms in the peri-positions of
the anthracene unit.21,27
Because the twist between the anthracene and phenyl rings can
effectively reduce the charge transfer from the triphenylamine
moiety to the benzimidazole moiety, these compounds emit light
in the blue region. In comparison with the absorption spectra, the
emission spectra are sensitive to the polarity of the solvent
(see Fig. 1 and S1†). Especially, pronouncedly red-shifted and
broadened spectra were found in CH2Cl2 for all the compounds.
This phenomenon can be explained by the solvent stabilization of
the excited state and dipolar interactions between the bipolar
compounds and the polar solvents.28 The emission spectra in the
film for all the materials are located between that for toluene and
THF solutions in the blue region, implying that no significant
intermolecular interactions occur in the ground state. These
derivatives indicate a high quantum yield in toluene because of
the presence of the anthracene moiety. It is noteworthy that
quantum yield increase with the increase of the phenyl rings,
because less coplanarity and less p-electron conjugation of
phenyl ring could reduce energy wastage in charge transfer and
even nonradiative decay.
Fig. 1 The absorption spectra and fluorescence spectra (excited at 380
Thermal properties
nm) of B1 and B4.
The thermal properties of these compounds were determined by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The key data were listed in Table 2. TGA,
column chromatographic method or recrystallization in the
common solvents except the B4, which can be obtained by simple
filtration from the reaction mixture, because of whose poor
solubility. The compounds were verified by 1H NMR, 13C NMR
and high-resolution mass spectrometry. Compound B4 was
confirmed by MALDI-TOF-MS spectrum with only one peak at
842.3582.
with a heat rate of 20 C minꢀ1 under N2, indicates the decom-
ꢁ
position temperatures (Td; corresponding to 5% weight loss) for
B1, B2, B3, and B4 with as high as 460.4 ꢁC, 481.0 ꢁC, 486.0 ꢁC,
and 519.7 ꢁC, respectively, increasing as the phenyl rings increꢁase
(see Fig. S2†). The melting points of B1, B2, and B4 (310.6 C,
349.8 ꢁC and 398.8 ꢁC, respectively) are higher than that for B3
ꢁ
(276.1 C), detected by the DSC, since the molecular symmetry
Photophysical properties
and weight are effectively beneficial to the melting point. The
Tg values for B1, B2 and B3 are 144.1 ꢁC, 152.7 ꢁC, and 161.5 ꢁC,
respectively. The Tg of B4 was not found in the DSC curves.
Fig. S3 showed the repeated DSC heating scan for Tg of B1.† The
high thermal properties of these compounds are of importance to
retain stable film morphological properties during the device
operation, which are also desirable for good performance of
OLEDs with high stability and efficiency.
The UV-vis and PL spectra of the bipolar anthracene derivatives
in three different solvents (toluene, THF, and dichloromethane)
as well as in the solid thin film spin-coated with PMMA on
quartz plates were measured as shown in Fig. 1 and Fig. S1.† A
summary of the photophysical data of these compounds is also
given in Table 1. There is no obvious solvatochromism in the
absorption spectra. These four compounds exhibit the rather
Table 1 The photophysical data for B1–4
labs (nm)
a
lem (fwhm)(nm)
Toluene
CH2Cl2
THF
Toluene
CH2Cl2
THF
Film
Ffb
B1
B2
B3
B4
304, 359, 379, 398
309, 358, 377, 398
308, 357, 378, 398
320, 357, 377, 397
303, 360, 378, 398
306, 357, 377, 397
309, 357, 378, 397
315, 357, 377, 397
303, 359, 378, 397
306, 356, 376, 396
309, 357, 378, 396
315, 357, 377, 397
459 (63)
440 (56)
457 (62)
439 (57)
496 (80)
496 (84)
493 (79)
498 (118)
481 (75)
461 (93)
477 (73)
454 (86)
475 (65)
452 (69)
472 (66)
444 (68)
0.55
0.73
0.60
0.82
a
Excited at 380 nm. b PL quantum yield relative to 9,10-diphenylanthracene(DPA) in cyclohexane (Ff ¼ 0.90).
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Table 2 The physical data for B1–4
a
b
Tg (ꢁC) a
Tm (ꢁC)
Td (ꢁC)
HOMO (eV) c
LUMO (eV)c
Eg (eV) d
HOMO (eV) e
LUMO (eV) e
Eg (eV) e
B1
B2
B3
B4
144.1
152.7
161.5
not available
310.6
349.8
276.1
398.8
460.4
481.0
486.0
519.7
5.33
5.29
5.33
5.28
2.40
2.32
2.41
2.32
2.93
2.97
2.92
2.96
4.97
4.95
4.97
4.95
1.62
1.64
1.63
1.62
3.35
3.31
3.34
3.33
a
Determined by DSC measurement with a heating and cooling rate of 20 ꢁC minꢀ1 under N2. b Determined by TGA with a heating rate of 20 ꢁC minꢀ1
under N2. Obtained by CV in CH2Cl2. All the potentials were calculated relative to ferrocence. LUMO energy was estimated based on the HOMO
c
levels and Eg. d Determined from the absorption threshold. e Theoretical calculations with B3LYP/6-31G.
Electrochemical properties
Theoretical calculations
The electrochemical properties of the bipolar compounds,
studied by the cyclic voltammetry (CV) analyses in CH2Cl2
solution, are revealed in Table 2. The HOMO levels for B1–B4
are around 5.3 eV, which are higher than that for other anthra-
cene derivatives, due to the electron-donating triphenylamine
moiety. The higher HOMO values are close to the work function
of ITO, which can effectively decrease the energy barrier of hole
injection and improve the device efficiency. The CV curves of
these materials with a quasi-reversible two-electron oxidation are
attributable to the triphenylamine and anthracene group. The
wide optical energy band gaps (Eg) were found about 2.95 eV,
determined from the threshold of UV-vis absorption spectra, and
then used to calculate the LUMO levels. The electrochemical
properties of these four compounds are almost identical, which
are in agreement with the behavior of the photophysical
properties.
The three-dimensional geometries and the frontier molecular
orbital energy levels of these bipolar anthracene derivatives were
calculated using density functional theory (DFT). The calcula-
tions of the HOMO and LUMO levels were depicted in Table 2.
The theoretical values of the HOMO and LUMO levels for the
four compounds are almost identical. Fig. 2 shows the calculated
optimized geometries and spatial distributions of the compounds
B1–4. The presence of large dihedral angles (about 79–88ꢁ
between the anthracene and the phenyl rings and about 36ꢁ
between the biphenyl linkage according to the theoretical
calculations) will be useful to keep the unique properties of each
functional segments and decrease their interaction. As shown in
Fig. 2, the electron densities of HOMO for B1 and B3 are mostly
localized on the triphenylamine and anthracene units, while
those for B2 and B4 are mainly populated over the triphenyl-
amine moieties. Compared to the separation of spacial distri-
butions of B2 and B4 between the triphenylamine and
anthrancene, those of B1 and B3 partially encompass the central
anthracene, which will promote B1 and B3 to emit longer
wavelength light than B2 and B4. Moreover, based on the
especial electron densities, we can easily explain why the HOMO
level of B1 and B3 is about 0.04 eV lower than that of B2 and B4.
However, the LUMOs for all compounds are localized on the
anthracene units. The spacial distribution of HOMO and LUMO
is desirable to improve the charge transport.
Electroluminescent properties
To investigate the hole-transporting properties, electron-trans-
porting properties, and light-emitting properties, we fabricated
three types of devices, with the device configurations: device I:
ITO/NPB (40 nm)/B1–4 (40 nm)/LiF (1 nm)/Al (100 nm); device
II: ITO/B1–4 (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm); and
device III: ITO/NPB (40 nm)/B1–4 (40 nm)/TPBI (20 nm)/LiF
(1 nm)/Al (100 nm). 4,40-bis[N-(1-naphthyl)-N-phenyl-l-amino]-
biphenyl (NPB) was used as the hole-transporting layer, TPBI
was used as the electron-transporting layer, and LiF and Al were
used as the electron injecting layer and cathode, respectively.
The device performances for the bipolar compounds were
listed in Table 3 and Table S1.† The current density-voltage-
luminance (I–V–L) and current density-current efficiency char-
acteristics for the compound B4 were shown in Fig. 3 and Fig. 4.
Compared to other compounds, the compound B1 showed the
lowest device performance. For example, the current efficiency of
the device III at a current density of 20 mA cmꢀ2 for B2, B3, and
Fig. 2 Calculated optimized geometries and spatial distributions of
B1–4.
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Table 3 Electroluminescence performance for B3 and B4
i
l
Device Von (V)a Lmax(Voltage) (cd mꢀ2) (V)b hp (lm Wꢀ1)c/d hc (cd Aꢀ1)e/f hext (%)g/h L (cd mꢀ2
)
V (V)j lem(FWHM) (nm)k CIE
(x,y)
B3
B4
I
2.6
7429(7.4)
12250(8.6)
10750 (8.8)
5610(8.0)
8656(7.6)
8612(7.5)
8472(8.7)
2.47/2.68
3.17/3.50
2.60/2.83
1.76/1.79
3.52/4.52
2.88/3.50
2.64/2.83
2.72/2.72
4.29/4.29
3.63/3.63
1.98/1.98
4.58/4.72
3.54/3.60
3.33/3.33
1.90/1.91 543.1
2.64/2.66 857.8
2.41/2.42 725.0
1.92/1.93 396.3
3.87/3.94 915.8
3.54/3.60 707.5
2.64/2.66 666.6
3.5
4.2
4.4
3.5
4.1
3.9
4.3
468(60)
468(64)
468(60)
452(60)
456(60)
456(60)
456(60)
(0.14,0.19)
(0.15,0.23)
(0.14,0.21)
(0.15,0.12)
(0.15,0.14)
(0.15,0.14)
(0.16,0.16)
II 2.8
III 2.7
I
II 2.7
III 2.6
IV 2.6
2.5
a
b
c
Von: turn-on voltage. Lmax: maximum luminance. Voltage: voltage at the maximum luminance. hp: power efficiency measured at 20 mA cmꢀ2
.
d
e
f
g
Maximum power efficiency. hc: current efficiency measured at 20 mA cmꢀ2
.
Maximum current efficiency. hext: external quantum efficiency
j k
h
i
measured at 20 mA cmꢀ2
emission wavelength at 20 mA cmꢀ2. FWHM: full width at half maximum at 20 mA cmꢀ2
.
Maximum external quantum efficiency. L: luminance measured at 20 mA cmꢀ2
.
V: voltage at 20 mA cmꢀ2
.
.
lem:
l
.
Values at 20 mA cmꢀ2
B4 are 3.70, 3.63, and 3.54 cd Aꢀ1, respectively, all of which are
higher than that of B1 (2.34 cd Aꢀ1). The shortest phenyl rings
bridge might promote the bipolar quench to affect the device
performance, agreeing with data of the quantum yield in the
toluene. Generally, the performance of the device II for all the
compounds is better than those of the device I. This was attrib-
uted to the closer HOMO energy level between B1–4 and NPB,
the small energy barrier between the B1–4 and anode would
facilitate the hole injection. In contrast, the larger energy level
between bipolar compounds and the cathode will increase the
difficulty of electron injection from the cathode to the anthracene
derivatives. In device II, an electron-transporting layer (TPBI)
was inserted between the B1–4 and cathode, therefore, the energy
barrier between the cathode and B1–4 will be effectively reduced,
which will increase the electron injection and improve the device
performance. Although another hole-transporting layer (NPB)
was added in the device III, device II exhibited better perfor-
mance than that of device III, this maybe due to the interface
effect between layers. The result also displays that the blue
emitters contain the excellent hole-transporting and electron-
transporting properties, which is one important condition of the
simplification of OLEDs.
Fig. 4 The current density-current efficiency characteristics of device
I–IV for B4.
of B1 and B3 showed a little red-shift compared to that of B4,
because the shorter phenyl bridge could increase the coplanarity
between the electron-donating triphenylamine and the light-
emitting anthracene core. It is interesting that an orange emission
peak at around 550 nm was observed in the EL spectra of B2
(see in Fig. S4†). For three devices, at a low current density
(<5 mA cmꢀ2), the orange emission peak was too strong to make
the devices emit near white light. As increasing the current
density, the orange peak decreases to reach a steady value about
For compounds B1, B3, and B4, all three types of devices emit
the light in blue region with a narrow FWHM (full width at half
maximum), and the EL spectra are almost identical with PL
spectra in the film. Fig. 5 illustrated the EL spectra of the device
II for the compounds B1, B3 and B4 at 20 mA cmꢀ2. The spectra
Fig. 3 Current density–voltage–luminance (I–V–L) characteristics of
Fig. 5 EL spectra of the device II for the compounds B1, B3 and B4 at 20
mA cmꢀ12
.
devices I–IV for B4.
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a half of the blue peak in intensity. This phenomenon should be
ascribed to the electromer formation of B2, since there is no
evidence of excimer formation from the film and solution PL
spectrum.
a heating and cooling scan rate of 20 ꢁC/min. Thermogravimetric
analysis was undertaken using a TGA instrument under
a nitrogen atmosphere with a heating scan rate of 20 ꢁC/min.
Cyclic voltammetric (CV) measurements were carried out in
a conventional three electrode cell using a Pt button working
electrode of 2 mm in diameter, a platinum wire counter electrode,
and a SCE reference electrode on a computer-controlled EG&G
Potentiostat/Galvanostat model 283 at room temperature.
Reduction CV of all compounds was performed in dichloro-
methane containing Bu4NPF6 (0.1 M) as the supporting elec-
trolyte. The E1/2 values were determined by (Epa + Epc)/2, where
Epa and Epc are the anodic and cathodic peak potentials,
respectively. Ferrocene was used as an external standard. Elec-
trochemistry was done at a scan rate of 100 mV/s. Density
functional theory (DFT) was calculated at the B3LYP/6-31G*
level.
Importantly, a single-layer device based on B4 with the
sturcture of ITO/B4 (80 nm)/LiF (1 nm)/Al (100 nm) was also
fabricated to exhibit its excellent bipolar characteristic and light-
emitting properties. The EL performance and spectra were also
displayed in the Table 3 and Fig. 3, 4, and 5, which were
comparable with those of the multilayer devices. These good
results show that B4 can be used as the promising blue light-
emitting material in the single layer OLEDs. As a result, we
achieved a current efficiency of 3.33 cd Aꢀ1 with a CIE(x,y) of
(0.16,0.16) at 20 mA cmꢀ2 and a maximum brightness of 8472 cd
mꢀ2 at 8.7 V, which is better than the previous reports about the
single-layer devices.8–20
Conclusions
Synthesis
In summary, we have synthesized a series of bipolar anthracene
derivatives containing hole-transporting triphenylamine and
electron-transporting benzimidazole moieties. They possess good
thermal properties with the Tg higher than 140 ꢁC. The PL
spectra were largely affected by the polarity of the solvent and
the phenyl bridge length. A series of devices based on these
derivatives exhibit their excellent bipolar properties and low
operating voltage. Due to the electromer formation of B2, we can
get the white emission OLEDs with a single component at a low
current density (< 5 mA cmꢀ2), while others devices emit blue
light with a narrow FWHM about 60 nm. Moreover, an
un-optimized single layer device for B4 exhibited a current
efficiency of 3.33 cd Aꢀ1 with a pure blue color of CIE(x,y) of
(0.16, 0.16) at 20 mA cmꢀ2 and a maximum brightness of 8472 cd
mꢀ2 at 8.7 V. This EL performance is the best among reports for
the pure blue single-layer OLEDs in the previous literatures.
Triphenylamine boronic acid (BA1), 4-(diphenylamino)-
biphenyl-40-boronic acid (BA2),29 4-(1-phenyl-1H-benzimidazol-
2-yl)-phenylboronic acid (BA3)20d and 1-phenyl-1H-2-(4-bro-
mophenyl)-benzimidazole (BA4)20d were prepared according to
the literatures. The bipolar compounds B1–B4 were synthesized
following the similar procedures with Suzuki aryl-aryl coupling
reactions.
Synthesis of 9-anthracene boronic acid (1). Under the atmo-
sphere of nitrogen, 9-bromoanthracene (2.57 g, 10 mmol) was
added to 30 ml of dry THF solution and stirred for 30 min at ꢀ78
ꢁC, then n-BuLi (8 ml, 15 mmol, 1.6 M solution in hexane) was
added. The mixture was stirred for 2 h. Triisopropylborate (2.9 g,
8.6 mmol) was added to the reaction mixture. After the addition
was completed, the solution was warmed slowly to room
temperature and stirred overnight. The reaction mixture was
added to an aqueous solution of HCl (2 N) and extracted with
dichloromethane. The organic extracts were washed with brine
and H2O, and dried with anhydrous MgSO4 and filtered. After
the solvent was evaporated, the crude product was crystallized
from dichloromethane and n-hexane to afford 1.47 g of white
solid (66.2%). 1H NMR: (400 MHz, DMSO, d): 8.81 (s, 2 H), 8.52
(s, 1 H), 8.05–8.08 (m, 2 H), 7.97–8.00 (m, 2 H), 7.47–7.52 (4 H).
Experimental
General information
Unless otherwise specified, all reactions and manipulations were
performed under nitrogen atmosphere using standard Schlenk
techniques. All chemical reagents were used as received from
commercial sources without further purification. And the
solvents were dried using standard procedures. 1H and 13C NMR
Synthesis of 9-(4-bromophenyl)-anthracene (2). 9-anthracene
boronic acid (1) (0.11 g, 0.5 mmol) and 4-bromo-iodobenzene
(0.154 g, 0.55 mmol) were mixed in 10 ml of THF. K2CO3 (2.0
M, 10 ml) was added, and the mixture was stirred with
magnetic stirring. Then tetrakis (triphenylphsosphine) palla-
dium (10 mg, 0.0075 mmol) was added to the mixture. The
reaction solution was heated under reflux for 12 h under the
atmosphere of nitrogen. After the mixture cooled, the solvent
was evaporated and the product was extracted with dichloro-
methane. The organic was washed with brine and water, and
then dried by anhydrous MgSO4. The solvent was evaporated,
and the residue was purified by using column chromatography
with DCM : n-Hexane ¼ 1 : 6 eluent to afford a white solid
€
spectra were recorded on Bruker AV-400, spectrometer tetra-
methylsilane (TMS) as the internal standard. High resolution
mass spectrometric measurements were carried out using
€
a Bruker autoflex MALDI-TOF mass spectrometer. UV-vis
spectra were obtained on a Varian Cary 200 spectrophotometer.
Fluorescence spectra were obtained on a Perkin Elmer LS55
luminescence spectrometer with the excitation at 380 nm. To
measure the fluorescence quantum yields (FF), degassed solu-
tions of the compounds in toluene were prepared. The concen-
tration was adjusted so that the absorbance of the solution would
be lower than 0.1. The excitation was performed at 380 nm, and
9,10-diphenylanthracene (DPA) in the solution of cyclohexane,
which has Ff ¼ 0.9, was used as a standard. The differential
scanning calorimetry (DSC) analysis was performed under
a nitrogen atmosphere on a TA Instruments DSC 2920 with
1
(0.14 g, 84.3%). H NMR: (400 MHz, CDCl3, d): 8.51 (s, 1 H),
8.04–8.06 (d, J ¼ 8.4 Hz, 2 H), 7.71–7.73 (d, J ¼ 6.4 Hz, 2 H),
7.61–7.64 (d, J ¼ 8.8 Hz, 2 H), 7.45–7.49 (m, 2 H), 7.35–7.39
2962 | J. Mater. Chem., 2011, 21, 2957–2964
This journal is ª The Royal Society of Chemistry 2011

View Online
(m, 2 H), 7.30–7.24 (m, 2 H). 13C NMR (400 MHz, CDCl3, d):
137.69, 135.42, 132.95, 131.61, 131.26, 130.02, 128.41, 126.96,
126.41, 125.61, 125.17, 121.70. MALDI-TOF-MS (m/z): [M+]
calcd for C20H13Br, 334.0187; found, 334.0170.
a yellow solid (0.783 g, 71.5%). 1H NMR: (400 MHz, CDCl3, d):
8.50 (s, 1 H), 8.03–8.05 (d, J ¼ 8.4 Hz, 2 H), 7.92–7.94 (d, J ¼ 8.0
Hz, 1 H), 7.79–7.81 (d, J ¼ 8.0 Hz, 2 H), 7.69–7.71 (m, 6 H), 7.43–
7.56 (m, 7 H), 7.33–7.40 (m, 5 H), 7.28–7.29 (m, 2 H). 13C NMR
(400 MHz, CDCl3, d): 152.10, 143.08, 141.59, 139.08, 138.33,
137.31, 137.05, 136.40, 131.78, 131.33, 130.15, 129.95, 128.93,
128.63, 128.37, 127.48, 126.94, 126.69, 125.42, 125.12, 123.39,
123.05, 119.84, 110.46. MALDI-TOF-MS (m/z): [M+] calcd for
C39H26N2, 522.2101; found, 522.2123.
Synthesis of 9-[4-(1-phenyl-1H-benzimidazol-2-yl)-phenyl]-
anthracene (3). 1-phenyl-1H-2-(4-bromophenyl)-benzimidazole
(BA4) (1.0 g, 2.87 mmol) and 9-anthracene boronic acid (1) (0.64
g, 2.87 mmol) were mixed in 10 ml of THF. K2CO3 (2.0 M, 10 ml)
was added, and the mixture was stirred with magnetic stirring.
Then tetrakis (triphenylphsosphine) palladium (10 mg, 0.0075
mmol) was added to the mixture. The reaction solution was
heated under reflux for 12 h under the atmosphere of nitrogen.
After the mixture cooled, the solvent was evaporated and the
product was extracted with dichloromethane. The organic was
washed with brine and water, and then dried by anhydrous
MgSO4. The solvent was evaporated, and the residue was purified
by using column chromatography with ethanyl acetic : n-hexane ¼
Synthesis of 9-bromo-10-[4-(1-phenyl-1H-benzimidazol-2-yl)-
biphenyl-40-yl]-anthracene (6). 9-[4-(1-phenyl-1H-benzimidazol-
2-yl)-biphenyl-40-yl]-anthracene (5) (0.52 g, 1 mmol), N-bromo-
succinimide (NBS) (0.187 g, 1.05 mmol) was mixed iꢁn 10 ml of
chloroform. The reaction mixture was heated to 50 C for 2 h
under nitrogen. After the reaction finished, the solvent was
evaporated. The residue was crystallized from acetone and
methanol to afford a yellow solid (0.54 g, 90.0%).1H NMR: (400
MHz, CDCl3, d): 8.61–8.63 (d, J ¼ 8.8 Hz, 2 H), 7.92–7.94 (d, J ¼
8.0 Hz, 1 H), 7.80–7.82 (d, J ¼ 8.0 Hz, 2 H), 7.69–7.72 (m, 5 H),
7.46–7.62 (m, 8 H), 7.35–7.41 (m, 5 H), 7.29–7.30 (m, 2 H). 13C
NMR (400 MHz, CDCl3, d): 152.04, 142.99, 141.47, 139.41,
137.91, 137.29, 137.20, 137.02, 131.68, 130.97, 130.20, 129.99,
129.00, 128.68, 127.89, 127.49, 127.27, 127.03, 126.98, 126.93,
125.65, 123.45, 123.11, 122.89, 119.83, 110.49. MALDI-TOF-
MS (m/z): [M + H]+ calcd for C39H26BrN2, 603.1271; found,
603.1277.
1
1 : 3 eluent to afford a yellow solid (1.15 g, 89.8%). H NMR:
(400 MHz, CDCl3, d): 8.49 (s, 1 H), 8.03–8.05 (d, J ¼ 8.4 Hz, 2
H), 7.94–7.96 (d, J ¼ 8.0 Hz, 1 H), 7.78–7.80 (d, J ¼ 8.4 Hz, 2 H),
7.57–7.63 (m, 4 H), 7.44–7.54 (m, 5 H), 7.31–7.40 (m, 7 H). 13C
NMR (400 MHz, CDCl3, d): 152.23, 143.02, 140.12, 137.31,
136.99, 135.94, 131.25, 129.97, 129.93, 129.35, 129.17, 128.77,
128.38, 127.50, 126.88, 126.49, 125.51, 125.13, 123.45, 123.10,
119.88, 110.54. MALDI-TOF-MS (m/z): [M+] calcd for
C33H22N2, 446.1788; found: 446.1808.
Synthesis of 9-bromo-10-[4-(1-phenyl-1H-benzimidazol-2-yl)-
phenyl]-anthracene (4). 9-[4-(1-phenyl-1H-benzimidazol-2-yl)-
phenyl]-anthracene (3) (0.78 g, 1.76 mmol), N-bromosuccinimide
(NBS) (0.313 g, 1.76 mmol) was mixed with 10 ml of chloroform.
The reaction mixture was heated to 50 ꢁC for 2 h under nitrogen.
After the reaction finished, the solvent was evaporated. The
residue was crystallized from acetone and methanol to afford
Synthesis of 9-[4-(1-phenyl-1H-benzimidazol-2-yl)-phenyl]-10-
[4-(diphenlamino)-phenyl]-anthracene (B1). 9-bromo-10-[4-(1-
phenyl-1H-benzimidazol-2-yl)-phenyl]-anthracene (4) (0.315 g,
0.6 mmol), 4-(diphenylamino)-phenyl boronic acid (0.173 g, 0.6
mmol) 10 ml of THF. K2CO3 (2.0 M, 10 ml) was added, and the
mixture was stirred with magnetic stirring. Then tetrakis (tri-
phenylphsosphine) palladium(10 mg, 0.0075 mmol) was added to
the mixture. The reaction solution was heated under reflux for 12
h under N2. After the mixture cooled, the solvent was evaporated
and the product was extracted with dichloromethane. The
organic was washed with brine and water, and then dried by
anhydrous MgSO4. The solvent was evaporated, and the residue
was purified by using column chromatography with ethanyl
acetic : n-hexane ¼ 1 : 3 eluent to afford a yellow solid (0.353 g,
1
a yellow solid (0.80 g, 87.3%). H NMR: (400 MHz, CDCl3, d):
8.60–8.62 (d, J ¼ 8.4 Hz, 2 H), 7.94–7.96 (d, J ¼ 8.0 Hz, 1 H),
7.79–7.81 (d, J ¼ 8.0 Hz, 2 H), 7.54–7.62 (m, 7 H), 7.47–7.48 (d,
J ¼ 8.4 Hz, 2 H), 7.33–7.41 (m, 5 H), 7.32 (m, 2 H). 13C NMR
(400 MHz, CDCl3, d): 152.07, 143.06, 139.70, 137.37, 136.99,
136.73, 131.26, 130.75, 130.16, 130.03, 129.56, 129.42, 128.85,
127.93, 127.52, 127.10, 127.01, 125.76, 123.55, 123.17, 123.11,
119.95, 110.59. MALDI-TOF-MS (m/z): [M + H]+calcd for
C33H22BrN2, 527.0956; found: 527.0978.
1
85.3%). H NMR: (400 MHz, CDCl3, d): 7.94–7.96 (d, J ¼ 8.0
Hz, 1 H), 7.81–7.85 (m, 4 H), 7.58–7.66 (m, 4 H), 7.48–7.55 (m, 3
H), 7.25–7.44 (m, 21 H), 7.06–7.10 (m, 2 H). 13C NMR (400
MHz, CDCl3, d): 152.27, 147.76, 147.17, 143.09, 140.50, 137.35,
137.05, 135.88, 132.41, 132.05, 131.45, 130.02, 129.98, 129.70,
129.38, 129.23, 128.77, 127.53, 127.13, 126.67, 125.19, 124.99,
124.69, 123.45, 123.09, 123.03, 119.92, 110.54. MALDI-TOF-
MS (m/z): [M+] calcd for C51H35N3, 689.2825; found, 689.2862.
Synthesis of 9-[4-(1-phenyl-1H-benzimidazol-2-yl)-biphenyl-40-
yl]-anthracene (5). 9-(4-bromophenyl)-anthracene (2) (0.50 g, 1.5
mmol), 4-(1-phenyl-1H-benzimidazole-2-yl)-benzene boronic
acid (BA3) (0.47 g, 1.5 mmol) were mixed in 10 ml of THF.
K2CO3 (2.0 M, 10 ml) was added, and the mixture was stirred
with magnetic stirring. Then tetrakis (triphenylphsosphine)
palladium (10 mg, 0.0075 mmol) was added to the mixture. The
reaction solution was heated under reflux for 12 h under the
atmosphere of nitrogen. After the mixture cooled, the solvent
was evaporated and the product was extracted with dichloro-
methane. The organic product was washed with brine and water,
and then dried by anhydrous MgSO4. The solvent was evapo-
rated, and the residue was purified by using column chroma-
tography with ethanyl acetic : n-hexane ¼ 1 : 3 eluent to afford
Synthesis
of
9-[4-(diphenlamino)-biphenyl-40-yl]-10-[4-(1-
phenyl-1H-benzimidazol-2-yl)-phenyl]-anthracene (B2). Yellow
solid (74.6%). 1H NMR: (400 MHz, CDCl3, d): 7.95–7.97 (d, J ¼
8.0 Hz, 2 H), 7.77–7.84 (m, 6 H), 7.59–7.68 (m, 6 H), 7.49–7.54
(m, 5 H), 7.44–7.46 (d, J ¼ 8.0 Hz, 2 H), 7.28–7.36 (m, 11 H),
7.17–7.23 (m, 6 H), 7.04–7.06 (m, 2 H). 13C NMR (400 MHz,
CDCl3, d): 152.27, 147.67, 147.37, 143.10, 140.46, 139.75, 137.36,
137.21, 137.06, 136.07, 134.61, 131.72, 131.46, 130.00, 129.88,
This journal is ª The Royal Society of Chemistry 2011
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8 (a) K. R. Justin Thomas, M. Velusamy, J. T. Lin, Y.-T. Tao and C.-
H. Chuen, Adv. Funct. Mater., 2004, 14, 387; (b) K. R. Justin Thomas,
J. T. Lin, Y.-T. Tao and C. H. Chuen, J. Mater. Chem., 2002, 12,
3516.
9 C.-T. Chen, J.-S. Lin, M. V. R. K. Moturu, Y.-W. Lin, W. Yi, Y.-
T. Tao and C.-H. Chien, Chem. Commun., 2005, 3980.
129.67, 129.42, 129.32, 128.79, 127.78, 127.54, 127.07, 126.69,
126.57, 125.22, 125.08, 124.50, 123.92, 123.46, 123.10, 123.00,
119.93, 110.55. MALDI-TOF-MS (m/z): [M+] calcd for
C57H39N2, 765.3138; found, 765.3196.
10 L.-H. Chan, H.-C. Yeh and C.-T. Chen, Adv. Mater., 2001, 13, 1637.
11 H.-C. Yeh, R.-H. Lee, L.-H. Chan, T.-Y. J. Lin, C.-T. Chen,
E. Balasubramaniam and Y.-T. Tao, Chem. Mater., 2001, 13, 2788.
12 (a) F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh, K.-
W. Wang and P.-T. Chou, Adv. Funct. Mater., 2009, 19, 2834; (b)
J. M. Hancock, A. P. Gifford, Y. Zhu, Y. Lou and S. A. Jenekhe,
Chem. Mater., 2006, 18, 4924; (c) Y. Zhu, A. P. Kulkarni, P.-T. Wu
and S. A. Jenekhe, Chem. Mater., 2008, 20, 4200; (d) F.-M. Hsu,
C.-H. Chien, P.-I. Shih and C.-F. Shu, Chem. Mater., 2009, 21, 1017.
13 S. Tao, L. Li, J. Yu, Y. Jiang, Y. Zhou, C.-S. Lee, S.-T. Lee, X. Zhang
and O. Kwon, Chem. Mater., 2009, 21, 1284.
Synthesis of 9-[4-(1-phenyl-1H-benzimidazol-2-yl)-biphenyl-40-
yl]-10-[4-(diphenlamino)-phenyl]-anthracene (B3). Yellow solid
1
(71.5%). H NMR: (400 MHz, CDCl3, d): 7.92–7.94 (d, J ¼ 8.0
Hz, 2 H), 7.83–7.87 (m, 4 H), 7.73–7.75 (m, 6 H), 7.52–7.58 (m, 5
H), 7.25–7.42 (m, 21 H), 7.07–7.10 (m, 2 H). 13C NMR (400
MHz, CDCl3, d): 152.13, 147.78, 147.16, 143.08, 141.63, 139.10,
138.70, 137.32, 137.17, 137.08, 136.34, 132.52, 132.09, 131.89,
130.07, 129.97, 129.91, 129.39, 128.94, 128.65, 127.51, 127.13,
127.01, 126.92, 126.87, 125.12, 125.00, 124.69, 123.40, 123.09,
123.07, 119.85, 110.47. MALDI-TOF-MS (m/z): [M+] calcd for
C57H39N2, 765.3138; found, 765.3109.
14 S. Takizawa, V. A. Montes and J. P. Anzenbacher, Chem. Mater.,
2009, 21, 2452.
15 (a) Z. Q. Gao, M. Luo, X. H. Sun, H. L. Tam, M. S. Wong, B. X. Mi,
P. F. Xia, K. W. Cheah and C. H. Chen, Adv. Mater., 2009, 21, 688; (b)
T. H. Lee, K. L. Tong, S. K. So and L. M. Leung, Synth. Met., 2005,
155, 116; (c) Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike,
H. Miyamoto, T. Kajita and M. Kakimoto, Org. Lett., 2008, 10, 421;
(d) Y. Sun, L. Duan, P. Wei, J. Qiao, G. Dong, L. Wang and Y. Qiu,
Org. Lett., 2009, 11, 2069; (e) T.-H. Huang, J. T. Lin, L.-Y. Chen,
Y.-T. Lin and C.-C. Wu, Adv. Mater., 2006, 18, 602.
Synthesis of 9-[4-(1-phenyl-1H-benzimidazol-2-yl)-biphenyl-40-
yl]-10-[4-(diphenlamino)-biphenyl-40-yl]-anthracene (B4). Yellow
solid (71.7%). MALDI-TOF-MS (m/z): [M + H]+ calcd for
C63H44N3, 842.3529; found, 842.3582.
16 S.-L. Lin, L.-H. Chan, R.-H. Lee, M.-Y. Yen, C.-T. Kuo and R.-
J. Jeng, Adv. Mater., 2008, 20, 3947.
17 K. R. J. Thomas, M. Velusay, J. T. Lin, C.-H. Chuen and Y.-T. Tao,
Chem. Mater., 2005, 17, 1860.
Device fabrication
Prior to the deposition of organic materials, indium-tin-oxide
(ITO)/glass was cleaned with a routine cleaning procedure and
pretreated with oxygen plasma, and then coated with a polymer-
ized fluorocarbon (CFx) film. Devices were fabricated under
about 10ꢀ6 Torr base vacuum in a thin-film evaporation coater.
The current–voltage–luminance characteristics were measured
with a diode array rapid scan system using a Photo Research
PR650 spectrophotometer and a computer-controlled, program-
mable, direct-current (DC) source. All measurements were carried
out in ambient atmosphere at room temperature.
18 (a) H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468; (b)
Y. Tao, Q. Wang, C. Yang, Z. Zhang, T. Zou, J. Qin and D. Ma,
Angew. Chem., Int. Ed., 2008, 47, 8104.
19 (a) C.-J. Kuo, T.-Y. Li, C.-C. Lien, C.-H. Liu, F.-I. Wu and M.-
J. Huang, J. Mater. Chem., 2009, 19, 1865; (b) Z. H. Li,
M. S. Wong, H. Fukutani and Y. Tao, Org. Lett., 2006, 8, 4271.
20 (a) Y.-L. Liao, C.-Y. Lin, K.-T. Wong, T.-H. Hou and W.-Y. Huang,
Org. Lett., 2007, 9, 4511; (b) M.-Y. Lai, C.-H. Chen, W.-S. Huang,
J. T. Lin, T.-H. Ke, L.-Y. Chen, M.-H. Tsai and C.-C. Wu, Angew.
Chem., Int. Ed., 2008, 47, 581; (c) C.-H. Chen, W.-S. Huang, M.-
Y. Lai, W.-C. Tsao, J. T. Lin, Y.-H. Wu, T.-H. Ke, L.-Y. Chen and
C.-C. Wu, Adv. Funct. Mater., 2009, 19, 2661; (d) Z. Ge,
T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita and M. Kakimoto, Adv. Funct. Mater., 2008, 18, 584.
21 K. Danel, T.-H. Huang, J. T. Lin, Y.-T. Tao and C. H. Chuen, Chem.
Mater., 2002, 14, 3860.
Acknowledgements
This work was financially supported by NSFC/China, National
Basic Research 973 Program and Hong Kong Baptist University.
22 (a) M.-H. Ho, Y.-S. Wu, S.-W. Wen, M.-T. Lee, T.-M. Chen and
C. H. Chen, Appl. Phys. Lett., 2006, 89, 252903; (b) C. W. Tang,
S. A. Van Slyke and C. H. Chen, J. Appl. Phys., 1989, 65, 3610.
23 (a) Y. Kan, L. Wang, L. Duan, Y. Hu, G. Wu and Y. Qiu, Appl. Phys.
Lett., 2004, 84, 1513; (b) Y.-H. Kim, H.-C. Jeong, S.-H. Kim, K. Yang
and S.-K. Kwon, Adv. Funct. Mater., 2005, 15, 1799; (c) Y.-Y. Lyu,
J. Kwak, O. Kwon, S.-H. Lee, D. Kim, C. Lee and K. Char, Adv.
Mater., 2008, 20, 2720; (d) S.-K. Kim, B. Yang, Y. Ma, J.-H. Lee
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H. Chien, F. M. Hsu, P.-I. Shih and C.-F. Shu, J. Mater. Chem.,
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J.-H. Su, C. H. Chen and H. Tian, Tetrahedron, 2010, 66, 7577.
24 M.-H. Ho, T. M,-Hsieh, K.-H. Lin, T.-M. Chen, J.-F. Chen and
C. H. Chen, Appl. Phys. Lett., 2006, 94, 023306.
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2964 | J. Mater. Chem., 2011, 21, 2957–2964
This journal is ª The Royal Society of Chemistry 2011