Simple CBP isomers with high triplet energies for highly efficient blue electrophosphorescence

Article abstract of DOI:10.1039/c1jm14903b

Two simple CBP isomers, namely m-CBP and o-CBP, were designed and synthesized by finely tuning the linking topology between the carbazole and the central biphenyl units of CBP, and their thermal, photophysical and electrochemical properties were investigated. Such simple modification of the linking topology endows the CBP isomers with high triplet energy and relative high thermal and morphological stability. The high triplet energies of m-CBP and o-CBP ensure efficient energy transfer from the host to the phosphor and triplet exciton confinement on the phosphor, as indicated by the transient photoluminescence decay of 3 wt% FIrpic doped into m-CBP and o-CBP. Blue phosphorescent devices employing FIrpic as guest and the two CBP isomers as hosts exhibit high efficiencies. The best EL performance is achieved for the o-CBP-based device, with a maximum current efficiency of 29.9 cd A^-1, and a maximum external quantum efficiency of 14.2%, which are over 2 times higher than those of CBP.

Full text of DOI:10.1039/c1jm14903b

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PAPER
Simple CBP isomers with high triplet energies for highly efficient blue
electrophosphorescence†
a
a
b
a
a
b
Shaolong Gong, Xun He, Yonghua Chen, Zuoquan Jiang, Cheng Zhong, Dongge Ma, Jingui Qina
*
a
*
and Chuluo Yang
Received 30th September 2011, Accepted 8th November 2011
DOI: 10.1039/c1jm14903b
Two simple CBP isomers, namely m-CBP and o-CBP, were designed and synthesized by finely tuning
the linking topology between the carbazole and the central biphenyl units of CBP, and their thermal,
photophysical and electrochemical properties were investigated. Such simple modification of the
linking topology endows the CBP isomers with high triplet energy and relative high thermal and
morphological stability. The high triplet energies of m-CBP and o-CBP ensure efficient energy transfer
from the host to the phosphor and triplet exciton confinement on the phosphor, as indicated by the
transient photoluminescence decay of 3 wt% FIrpic doped into m-CBP and o-CBP. Blue
phosphorescent devices employing FIrpic as guest and the two CBP isomers as hosts exhibit high
efficiencies. The best EL performance is achieved for the o-CBP-based device, with a maximum current
efficiency of 29.9 cd Aꢀ1, and a maximum external quantum efficiency of 14.2%, which are over 2 times
higher than those of CBP.
The carbazole-based compound N,N0-dicarbazolyl-4,40-
Introduction
biphenyl (CBP) has been widely utilized as the host material for
efficient green and red PhOLEDs,13,14 but it is not an efficient
host for blue PhOLEDs because of its lower triplet energy (2.56
eV) relative to that of blue phosphor iridium(III) bis(4,6-
Phosphorescent organic light-emitting diodes (PhOLEDs) have
attracted intense interest because of their merit of high quantum
efficiency as compared to conventional fluorescent OLEDs
through utilizing both singlet and triplet exitons for emission.1–5
To avoid concentration quenching and triplet–triplet annihila-
tion,6 phosphorescent emitters of heavy-metal complexes are
usually doped into a suitable host material. Therefore, the design
and synthesis of host materials are vital to the realization of
efficient PhOLEDs.7 It is desirable that a host material should
have high enough triplet energy to suppress triplet energy back
transfer from the guest to the host and confine the triplet exciton
on the guest molecule.8 Recently, green and red PhOLEDs with
100% internal quantum efficiency have been reported,9–12 but
highly efficient and stable blue PhOLEDs remain to be developed
because of the lack of suitable host materials possessing higher
triplet energy levels than blue phosphors.
0
(difluorophenyl)pyridine-N,C2 ) picolinate (FIrpic, 2.65 eV).8 To
address the issue, a series of structurally modified CBP-deriva-
tives have been developed. Through replacing the biphenyl unit
by a single benzene group in combination with meta-position
instead of para-position between the 9-position of carbazole and
phenyl ring, N,N0-dicarbazolyl-3,5-benzene (mCP) with a high
triplet energy of 2.9 eV has been reported as host material for
blue PhOLEDs.8 However, problems still remain in terms of
relative low thermal and morphological stability due to the
decreased molecular weight.15 Another strategy to increase the
triplet energies of CBP-based materials is to incorporate steric
group or non-conjugated linkage between the two carbazole
moieties. For example, 4,40-bis(9-carbazolyl)-2,20-dimethyl-
biphenyl (CDBP, 3.0 eV) with two methyl substitution in the 2-
and 20-position of the biphenl unit,16 and bis(4-(9-carbazolyl)
phenyl)diphenylsilane (CPSiCBP, 3.0 eV) with the tetraphe-
nylsilane group as the linkage have been prepared to
realize efficient blue electrophosphorescence.17 Although these
materials have high enough triplet energies to host the
blue phosphor FIrpic, their complicated multi-step synthesis
and moderate to low yields will increase the overall cost of
the practical devices. In this study, we designed and synthesized
two CBP isomers by finely tuning the linking topology
between the two carbazole units and the biphenyl group from
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan, 430072, People’s
Republic of China. E-mail: clyang@whu.edu.cn
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, People’s Republic of China. E-mail: mdg1014@ciac.
jl.cn
† Electronic supplementary information (ESI) available: Crystal data of
o-CBP, TGA traces of m-CBP and o-CBP, and UV-vis absorption of
CBP, m-CBP and o-CBP in film state. CCDC reference number
846417. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1jm14903b
2894 | J. Mater. Chem., 2012, 22, 2894–2899
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di-para-position to di-meta- or di-ortho-positions. We anticipate
that the simple modification of linking topology would endow the
material with relative high triplet energies. We will present
a comprehensive investigation that encompasses the thermal,
photophysical, and electrochemical properties of the compounds,
and demonstrate the applicability of these materials as hosts for
blue PhOLEDs.
the Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Computational details
The geometrical and electronic properties were performed with
the Gaussian 09 program package.18 The calculation was opti-
mized by means of the B3LYP (Becke three parameters hybrid
functional with Lee-Yang-Perdew correlation functionals) with
the 6-31G(d) atomic basis set.19,20 Then the triplet states DE (T1–
S0) were calculated using time-dependent density functional
theory (TD-DFT) calculations with B3LYP/6-311+g(d).
Experimental
General information
1H NMR and 13C NMR spectra were measured on
a MERCURY-VX300 spectrometer. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a Vario EL
III microanalyzer. GC-Mass spectra were measured on a Thermo
Trace DSQ II GC-MS. UV-Vis absorption spectra were recorded
on a Shimadzu UV-2500 recording spectrophotometer. Photo-
luminescence (PL) spectra were recorded on a Hitachi F-4500
fluorescence spectrophotometer. The PL lifetimes were measured
by a single photon counting spectrometer from Edinburgh
Instruments (FLS920) with a hydrogen-filled pulse lamp as the
excitation source. The data were analyzed by iterative convolu-
tion of the luminescence decay profile with the instrument
response function using the software package provided by
Edinburgh Instruments. Differential scanning calorimetry (DSC)
was performed on a NETZSCH DSC 200 PC unit at a heating
Device fabrication and measurement
The hole-transporting material 1,4-bis[(1-naphthylphenyl)
amino]biphenyl (NPB), and electron-transporting material 3-(4-
biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ)
were commercially available. Commercial ITO (indium tin
oxide) coated glass with sheet resistance of 10 U per square was
used as the starting substrates. Before device fabrication, the ITO
glass substrates were pre-cleaned carefully and treated by oxygen
plasma for 2 min. Then the sample was transferred to the
deposition system. NPB (80 nm) was firstly deposited to ITO
substrate, followed by emissive layer (20 nm), and TAZ (40 nm).
Finally, a cathode composed of lithium fluoride (1 nm) and
aluminium (100 nm) was sequentially deposited onto the
substrate in a vacuum of 10ꢀ6 Torr. The J–V–L of the devices was
measured with a Keithley 2400 Source meter and a Keithley 2000
Source multimeter equipped with a calibrated silicon photo-
diode. The EL spectra were measured by JY SPEX CCD3000
spectrometer. The EQE values were calculated according to
previously reported methods.21 All measurements were carried
out at room temperature under ambient conditions.
rate of 10 C minꢀ1 from 20 to 300 C under argon. The glass
transition temperature (Tg) was determined from the second
heating scan. Thermogravimetric analysis (TGA) was under-
taken with a NETZSCH STA 449C instrument. The thermal
stability of the samples under a nitrogen atmosphere was deter-
mined by measuring their weight loss while heating at a rate of
ꢁ
ꢁ
10 C minꢀ1 from 25 to 600 C. Cyclic voltammetry (CV) was
carried out in nitrogen-purged dichloromethane (oxidation scan)
at room temperature with a CHI voltammetric analyzer. Tetra-
butylammonium hexafluorophosphate (TBAPF6) (0.1 M) was
used as the supporting electrolyte. The conventional three-elec-
trode configuration consists of a platinum working electrode,
a platinum wire auxiliary electrode, and an Ag wire pseudo-
reference electrode with ferrocenium-ferrocene (Fc+/Fc) as the
internal standard. Cyclic voltammograms were obtained at
a scan rate of 100 mV sꢀ1. The onset potential was determined
from the intersection of two tangents drawn at the rising and
background current of the cyclic voltammogram.
ꢁ
ꢁ
Synthesis of materials
9-(3-Bromophenyl)-9H-carbazole, 9-[3-(4,4,5,5-tetramethyl-1,3,
2-dioxaborolan-2-yl)phenyl]-9H-carbazole, 9-(2-bromophenyl)-
9H-carbazole, 9-[2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl]-9H-carbazole were prepared according to the reported
procedures.22
9,90-Biphenyl-3,30-diylbis-9H-carbazole (m-CBP). Dry THF
(30 mL) was added to a mixture of 9-(3-bromophenyl)-9H-
carbazole (0.97 g, 3.00 mmol), 9-[3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl]-9H-carbazole (1.47 g, 4.00 mmol),
K2CO3 (2 M in H2O, 6.0 mL, 12.0 mmol), and Pd(PPh3)4
(0.069 g, 0.06 mmol). The mixture was refluxed 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 : 3 v/v)
for the eluent to give m-CBP as a white powder. Yield: 80%. 1H
NMR (300 MHz, CDCl3) d [ppm]: 8.15 (d, J ¼ 7.8 Hz, 4H), 7.86
(s, 2H), 7.74–7.67 (m, 4H), 7.59 (d, J ¼ 6.9 Hz, 2H), 7.49–7.39 (m,
8H), 7.32–7.25 (m, 4H). 13C NMR (75 MHz, CDCl3) d [ppm]:
141.34, 140.03, 137.62, 130.00, 125.60, 125.48, 124.87, 122.62,
119.69, 119.46, 109.11. MS (EI): m/z 484.2 (M+). Anal. calcd for
X-Ray structural analysis
Single-crystal X-ray-diffraction data were obtained using
a Bruker SMART CCD area-detector diffractometer. The
measurements were performed at 292 K using graphite-mono-
ꢀ
chromated Mo-Ka radiation (l ¼ 0.71073 A). The data were
corrected for absorption using the Siemens area detector
absorption (SADABS) program. The structures were solved by
direct methods using the SHELXS-97 program. Non-hydrogen
atoms were refined with anisotropic thermal parameters by full-
matrix least-squares calculations on F2 using SHELXL-97.
Drawings were produced using Diamond 3.0 and Mercury 1.4.1
software. CCDC 846417 (o-CBP) contains supplementary crys-
tallographic data. These data can be obtained free of charge from
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C36H24N2 (%): C 89.23, H 4.99, N 5.78; found: C 89.53, H 4.64,
N 5.64.
9,90-Biphenyl-2,20-diylbis-9H-carbazole (o-CBP). Prepared as
a white solid according to a similar procedure to m-CBP, from 9-
(2-bromophenyl)-9H-carbazole and 9-[2-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenyl]-9H-carbazole. Yield: 49%. 1H
NMR (300 MHz, CDCl3) d [ppm]: 7.87 (d, J ¼ 7.8 Hz, 4H), 7.72
(d, J ¼ 6.0 Hz, 4H), 7.52–7.47 (m, 4H), 7.31–7.26 (m, 4H), 7.04–
6.91 (m, 8H). 13C NMR (75 MHz, CDCl3) d [ppm]: 140.64,
138.14, 136.29, 133.68, 129.22, 129.11, 128.31, 125.24, 123.96,
119.70, 119.60, 110.31. MS (EI): m/z 484.3 (M+). Anal. calcd for
C36H24N2 (%): C 89.23, H 4.99, N 5.78; found: C 89.72, H 5.38,
N 5.53.
Fig. 1 ORTEP diagram of o-CBP.
Results and discussion
(Tg) of 97 ꢁC (m-CBP) and 82 ꢁC (o-CBP) during the secoꢁnd
heating scans (Fig. 2), which are significantly higher than 62 C
of CBP.25 In addition, further heating above Tg results in the
appearance of crystallisation temperatures (Tc) of 127 ꢁC (ꢁm-
Synthesis and characterization
The synthetic routes and chemical structures of the CBP isomers,
9,90-biphenyl-3,30-diylbis-9H-carbazole (m-CBP), and 9,90-
biphenyl-2,20-diylbis-9H-carbazole (o-CBP), are depicted in
Scheme 1. The compounds were successfully prepared through
Suzuki cross-coupling reactions of the bromide precursors with
the corresponding boronic esters.23 Detailed synthetic proce-
dures are presented in the Experimental section. The chemical
ꢁ
CBP) and 117 C (o-CBP) followed by melting peaks of 270 C
ꢁ
(m-CBP) and 248 C (o-CBP), respectively.
Photophysical properties
Fig. 3 shows the electronic absorption and fluorescence spectra
of the CBP isomers. The absorption peak around 293 nm can be
assigned to the carbazole-centered p–p* transitions, whereas the
absorptions in the range of 319–340 nm could be attributed to p–
p* transitions between the carbazole unit and the central
biphenyl unit in the molecule. The assignment can be manifested
by the fact that the intensity of p–p* transitions for m-CBP and
o-CBP are remarkably reduced in comparison with the dominant
p–p* transition for CBP, which could be ascribed to the
decreased p-conjugation for m-CBP and o-CBP owing to the
simple modification of the linking topology.
1
structures of both compounds were fully characterized by H
NMR and 13C NMR, mass spectrometry and elemental analysis.
The molecular structure of o-CBP was further determined by
single-crystal X-ray crystallographic analysis, and the crystal
data were summarized in Table S1.† As shown in Fig. 1, o-CBP
reveals a twisted molecular structure with a torsion angle of 58.4ꢁ
between the two phenyl rings of the central biphenyl unit, and
torsion angles of 65.5ꢁ and 57.8ꢁ, respectively, between the two
carbazole units and their adjacent phenyl rings. This twist in the
structure is beneficial for the reduction of the p-conjugation
between the carbazole unit and the central biphenyl group,
possibly leading to high triplet energy.24
All compounds show near-ultraviolet emissions between 349–
379 nm in dilute toluene solution. Their phosphorescence spectra
were measured in a frozen 2-methyltetrahydrofuran matrix at
77 K (Fig. 4), and the triplet energy levels estimated from the
highest-energy vibronic sub-band of the phosphorescence spectra
follow the sequence of o-CBP (3.00 eV) > m-CBP (2.84 eV) >
Thermal properties
The thermal properties of the CBP isomers were investigated by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). TGA measurement reveals their high
thermal-decomposition temperatures (Td, corresponding to 5%
ꢁ
ꢁ
weight loss) of 411 C (m-CBP) and 317 C (o-CBP) (Fig. S1).†
The DSC trace exhibits distinct glass-transition temperatures
Fig. 2 DSC traces of m-CBP and o-CBP recorded at a heating rate of
10 ꢁC minꢀ1
.
Scheme 1 Synthesis and molecular structures of m-CBP and o-CBP.
2896 | J. Mater. Chem., 2012, 22, 2894–2899
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Fig. 3 UV-vis absorption and PL spectra of CBP, m-CBP and o-CBP in
toluene solution at 5 ꢂ 10ꢀ6 M.
Fig. 5 Cyclic voltammograms of CBP, m-CBP and o-CBP in CH2Cl2
for oxidation.
wave was observed within the potential window of the cyclic
voltammograms, the LUMO energy levels were deduced from
HOMO energy levels and optical band gaps determined by the
onset of absorption (Fig. S2† and Table 1). The deduced LUMO
levels are 2.23, 2.13 and 2.19 eV for CBP, m-CBP, and o-CBP,
respectively.
Phosphorescent OLEDs
To evaluate the utility of the two compounds as host materials
for blue phosphor FIrpic, we fabricated blue devices by
employing the newly designed CBP isomers as the hosts with the
configurations of ITO/NPB (80 nm)/hosts: FIrpic (8%, 20 nm)/
TAZ (40 nm)/LiF (1 nm)/Al (100 nm). NPB and TAZ were used
as the hole- and electron-transporting layers, respectively; LiF
served as electron-injecting layer; FIrpic doped in CBP isomers
was used as the emitting layer, respectively. For comparison, we
also fabricated the control device employing CBP as the host.
Fig. 6 shows the current density-voltage-brightness (J–V–L)
characteristics and efficiency versus current density curves for the
devices, and the electroluminescence (EL) data are summarized
in Table 2. The turn-on voltage of m-CBP (4.3 V) is higher than
those of the devices hosted by CBP (3.9 V) and o-CBP (3.7 V),
which is attributed to the lower HOMO level of m-CBP (5.60 eV)
relative to CBP (5.54 eV) and o-CBP (5.55 eV), leading to a large
hole-injection barrier (0.2 eV) from the hole-transport NPB layer
Fig. 4 Phosphorescence spectra of CBP, m-CBP and o-CBP in a frozen
2-methyltetrahydrofuran matrix at 77 K.
CBP (2.66 eV), which is consistent with the DFT calculated
energy order (3.07 > 2.92 > 2.77 eV). Apparently, the p-conju-
gation between the carbazole and the central biphenyl units for
m-CBP and o-CBP is decreased by incorporating meta-linkage or
twisted di-ortho-position between these units, resulting in higher
triplet energy levels. The triplet energy levels of m-CBP and o-
CBP are significantly higher than that of blue phosphor FIrpic
(2.65 eV), which implies that they could serve as host materials
for the blue triplet emitter; whereas the triplet energy level of
CBP is close to that of FIrpic, suggesting the possible triplet
energy back transfer from FIrpic to CBP in the device.
Table 1 Physical data of CBP, m-CBP and o-CBP
CBP
m-CBP
o-CBP
Electrochemical properties
a
Tg /Tm/Td/ꢁC
62a/NA/NA
293/319
366/379
5.54/2.23
2.66
97/270/411
292/328
349/363
5.60/2.13
2.84
82/248/317
293/340
361/376
5.55/2.19
3.00
labsb/nm
Cyclic voltammetry (CV) were performed to investigate the
electrochemical properties of the compounds (Fig. 5). The two
compouds exhibit irreversible oxidation waves like most carba-
zole derivatives due to the instability of their radical cations.26
The HOMO energy levels of the compounds were determined
from the onset of the oxidation potentials with regard to the
energy level of ferrocene (4.8 eV below vacuum). The estimated
HOMO levels of m-CBP and o-CBP are 5.60 and 5.55 eV, which
are similar to that of CBP (5.54 eV). Because no clear reduction
lem, maxb/nm
HOMO/LUMOc/eV
ET, expd/eV
DE (T1–S0)cale/eV
2.77
2.92
3.07
The glass-transition temperature was obtained from ref. 25. b Measured
in toluene. Estimated from the onset of oxidation potentials and the
a
c
d
optical band gap from the absorption spectra. Measured in 2-
e
methyltetrahydrofuran matrix at 77 K.
calculation.
Obtained from DFT
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(ꢃ5.4 eV) to the emitting layer.27 The m-CBP-based device
achieves a maximum current efficiency (hc.max) of 18.6 cd Aꢀ1
,
a maximum power efficiency (hp.max) of 11.3 lm Wꢀ1, and
a maximum external quantum efficiency (hext.max) of 8.7%, which
are significantly higher than those for CBP (10.7 cd Aꢀ1, 8.5 lm
Wꢀ1, 5.2%). This can be attributed to the higher triplet energy
level of m-CBP compared with CBP, efficiently suppressing
triplet energy back tranfer from the guest to the host. The best
EL performance was achieved for the o-CBP-based device, with
hc.max of 29.9 cd Aꢀ1, hp.max of 25.3 lm Wꢀ1, and hext.max of 14.2%,
which are over 2 times higher than those of CBP. This drastic
improvement can be rationalized by both its high triplet energy
(3.00 eV) ensuring efficient energy transfer from the host to the
phosphor and triplet exciton confinement on the phosphor, and
its matched HOMO level (5.55 eV) facilitating efficient hole
injection to the emitting layer. The EL spectra and the
Commission International de I’Eclairage (CIE) coordinates of
devices hosted by CBP isomers are shown in Fig. 6c. All devices
show the same main peak at 475 nm with a shoulder peak at
500 nm, arising from the typical emittion of the phosphor
FIrpic.28
To further confirm the triplet exciton confinement properties
of CBP isomers and understand the different device perfor-
mance, we have measured the transient photoluminescence decay
of 3 wt% FIrpic doped into CBP, m-CBP and o-CBP. As
shown in Fig. 7, the CBP:FIrpic film exhibits a complicated
Fig. 6 (a) Current density-voltage-brightness characteristics, (b) current
efficiency and power efficiency versus current density curves, and (c) EL
spectra of blue PhOLEDs with different hosts.
Fig. 7 Decay of the PL intensity (excited at 330 nm) at room tempera-
ture at 475 nm from thin films of CBP:3 wt% FIrpic, m-CBP:3 wt%
FIrpic, and o-CBP:3 wt% FIrpic.
Table 2 Electroluminescence characteristics of the devices.a
f
hc.maxd/cd
Aꢀ1
hp.maxe/lm
Wꢀ1
hext.max
(%)
CIE (x, y)g
c
Host
Vonb/V
Lmax /cd mꢀ2, Voltages/V
CBP
m-CBP
o-CBP
3.9
4.3
3.7
10935 (12.5)
21172 (12.5)
10252 (10.3)
10.7
18.6
29.9
8.5
11.3
25.3
5.2
8.7
14.2
(0.15, 0.32)
(0.16, 0.32)
(0.16, 0.31)
a
c
Devices configuration: ITO/NPB (80 nm)/EML (20 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm). b Turn-on voltages at 1 cd mꢀ2
Maximum current efficiency. Maximum power efficiency. Maximum external quantum efficiency. Commission International de I’Eclairage
coordinates measured at 7 V.
.
Maximum luminance.
d
e
f
g
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€
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In summary, we have developed two CBP isomers by finely
tuning the linking topology between the carbazole unit and the
central biphenyl unit from di-para-position to di-meta- or
di-ortho-positions. This design strategy endows these materials
with high triplet energies, and relative high thermal and
morphological stability. We have successfully fabricated highly
efficient blue PhOLEDs by employing the simple CBP isomers as
host materials. The best EL performance was achieved for the o-
CBP-based device, with hc.max of 29.9 cd Aꢀ1, hp.max of 25.3 lm
Wꢀ1, and hext.max of 14.2%, which are over 2 times higher than
those of CBP. This can be attributed to both its high triplet
energy (3.00 eV) to confine triplet exciton on the guest and its
matched HOMO level (5.55 eV) to facilitate hole injection to the
emitting layer. These results demonstrate that simple modifica-
tion of the linking topology between the carbazole unit and the
central biphenyl group of CBP is an effective approach to design
host materials for highly efficient blue PhOLEDs.
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Acknowledgements
The authors thank the National Science Fund for Distinguished
Young Scholars of China (No. 51125013), the National Natural
Science Foundation of China (No. 90922020), the National Basic
Research Program of China (973 Program 2009CB623602 and
2009CB930603), the Fundamental Research Funds for the
Central Universities of China, and the Open Research Fund of
the State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences for financial support.
25 M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su, C.-C. Wu,
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