Self-assembly of novel fluorescent silole derivatives into different supramolecular aggregates: Fibre, liquid crystal and monolayer

Article abstract of DOI:10.1039/b925746b

Two novel organogelators based on 2,3,4,5-tetraphenylsilole functionalized with long-chain alkoxydiacylamido platforms (1a and 1b) were synthesized. The silole derivatives induced gelation of only hydrocarbon solvents and showed aggregation-induced emissi

Full text of DOI:10.1039/b925746b

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PAPER
www.rsc.org/softmatter | Soft Matter
Self-assembly of novel fluorescent silole derivatives into different
supramolecular aggregates: fibre, liquid crystal and monolayer†
a
Jun-Hua Wan, Lin-Yan Mao, Yi-Bao Li, Zhi-Fang Li, Hua-Yu Qiu, Chen Wang and Guo-Qiao Lai
a
b
a
a
b
a
*
*
Received 8th December 2009, Accepted 30th March 2010
First published as an Advance Article on the web 20th May 2010
DOI: 10.1039/b925746b
Two novel organogelators based on 2,3,4,5-tetraphenylsilole functionalized with long-chain
alkoxydiacylamido platforms (1a and 1b) were synthesized. The silole derivatives induced gelation of
only hydrocarbon solvents and showed aggregation-induced emission (AIE) in the gel state, in contrast
to very weak emission in solution. The polarized optical microscopic (POM) and field emission
scanning electron microscopy (FE-SEM) studies exhibited that the xerogels formed fibrous structures.
Hydrogen bonding and p-stacking interactions were the main driving forces for the formation of
organogels, based on the FT-IR and absorption investigations. Differential scanning calorimetry
(DSC) and POM studies indicated that both compounds 1a and 1b exhibited stable liquid crystalline
(LC) phases over a wide temperature range. It is interesting that uniform and well-ordered monolayers
were also obtained for both compounds on the HOPG surface. The new silole derivatives are quite
unique because they can self-assemble into one-dimensional (fibres), three-dimensional (liquid crystal)
and even two-dimensional (molecular monolayer) aggregates.
sensors,7 light-emitting diodes,8 photovoltaic cells,9 and field-
effect transistor.10 The unique electronic properties of siloles
arise from s*–p* conjugation between an exocyclic s* orbital on
the silicon atom and the p* LUMO orbital of the butadiene
moiety, which gives rises to the low-lying LUMO of the silole
molecule and increases its electron affinity.11 The calculated
LUMO level of a silole ring is even lower than those of electron
deficient heterocycles, such as 1,3,4-oxadiazole and pyridine.12 It
is well known that most chromophores exhibit aggregation-
induced quenching of emission, which results in decreasing
emission intensity by aggregation in poor solvents or in solid-
state devices.13 These difficulties have compromised the devel-
opment of efficient optoelectronic devices based on them. In
contrast, silole molecules such as 2,3,4,5-tetraphenylsilole
derivatives and hexaphenylsilole (HPS) exhibit an unusual
characteristic, that is, aggregation-induced emission (AIE)
behavior.14 Many 2,3,4,5-tetraphenylsilole derivatives show
extremely high photoluminescence (PL) quantum yields (up to
100%) in the solid state, while they are nonemissive in solution.15
Our interest in fluorescent organogels and liquid crystals based
on organic functional molecules inspires us to design a series of
self-assembly materials having 2,3,4,5-tetraphenylsilole as
a chromophore. However, it is well known that 2,3,4,5-tetra-
phenylsiloles have non-coplanar geometry in both solution and
solid states, which prevents aggregation through intermolecular
p-stacking.14c,d Very recently, Zhang et al. described the prepa-
ration of two new organic gelators based on silole with urea and
cholesterol moieties.16 Herein, we designed other novel silole-
derived gelators containing long-chain amide moieties (1a and
1b, see Scheme 1) and extended our investigation from organo-
gels to liquid crystals and molecular monolayers. Besides gel, the
2,3,4,5-tetraphenylsilole moiety can self-assembly into liquid
crystals and molecular monolayers with the aid of long alkyl
chains.
Introduction
Self-assembly represents a process for controlling the supramo-
lecular packing and orientation of molecules to obtain ordered
aggregates in materials science.1 The optical and electronic
properties of organic functional materials, which depend to
a certain extent on the molecular packing arrangements through
excitonic interactions, can be effectively modulated by self-
assembly.2 Recently, low molecular weight organogelators
(LMOGs) as a novel class of self-assembled materials have
attracted much attention.3 It is found that specific noncovalent
interactions including hydrogen bonds, hydrophobic interaction,
p–p interactions, and van der Waals forces play a very important
role in the formation of the organogels.4 During organogelation,
a very subtle balance between gelator-gelator interaction and
gelator-solvent interaction controls the local packing arrange-
ments of organic molecules as well as the self-assembled supra-
molecular nanostructures.5 It is very interesting that many
organogelators can also form liquid crystalline phases, another
kind of ordered and dynamic molecular aggregation.6
Increasing interest in silole-containing p-conjugated small
molecules and polymers is to a large extent due to their unique
electronic properties and unusual photoluminescence character-
istics, which make them intriguing candidates for applications in
aKey Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, Hangzhou Normal University, Hangzhou, 310012,
P. R. China. E-mail: gqlai@hznu.edu.cn; Fax: +86-571-28868081; Tel:
+86-571-28868081
bNational Center for Nanoscience and Technology, Beijing, 100080, China.
E-mail: wangch@nanoctr.cn; Fax: +86-10-82545561
† Electronic supplementary information (ESI) available: Details of
synthesis, images of the thermoreversible transition between sol and
gel, experimental details, synthesis and characterization, photographic
images, IR spectra, additional optical spectra, additional POM and
STM images, NMR spectra. See DOI: 10.1039/b925746b
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Table 1 Gelation properties of 1a and 1b in various solventsa
1a
1b
Solvent
Statea
CGCb (mg/mL)
State
CGC (mg/mL)
hexane
G
G
G
S
12
9
12
G
P
G
S
14
13
cyclohexane
n-decane
chloroform
CH2Cl2
S
S
THF
S
S
1,4-dioxane
toluene
acetone
ethanol
ethyl acetate
S
S
P
P
P
S
S
P
P
P
a
G ¼ gel, S ¼ solution, P ¼ solution when heated but precipitation after
cooling. b Measured at 20 ꢀC.
the ambient temperature. Gelation typically occurred during the
cooling step. Among various solvents investigated here, only
hydrocarbon solvents, such as hexane, cyclohexane and
n-decane, were induced into gelation by the above method (see
Table 1). Siloles 1a and 1b thermoreversibly induced gelation of
hexane. Compound 1a formed stable organogel in cyclohexane
within five minutes after cooling the hot transparent solution to
room temperature, while compound 1b did not form organogel
in the same solvent. In the same way, the organogel of 1a in
cyclohexane also exhibits thermally reversible sol–gel transitions.
The data of the critical gelation concentration (CGC) for
different solvents of the two compounds are shown in Table 1.
Scheme 1 The synthesis of 1a and 1b.
Results and discussion
Synthesis
Generally, the synthesis of the silole derivatives (1a and 1b)
involves three procedures, i.e., synthesis of 3,4,5-trialkylox-
ybenzoyl chlorides (4),17 synthesis of the 2,5-bis(4-amidophenyl)-
3,4-diphenylsilole (6), and coupling reaction (see Scheme 1).
A key intermediate compound, 2,5-bis(4-amidophenyl)-3,4,-
diphenylsilole (6), required to access to the target silole deriva-
tives has been recently synthesized by Okada.18 However, the
reported synthesis sequence is rather complicated and involves
two steps: 2,5-bis(4-nitrophenyl)-3,4-diphenylsilole was firstly
synthesized using bis(phenylethynyl)dimethylsilane (5) as the
starting material, then was reduced with zinc and acetic acid. In
this paper, we developed a one-pot synthetic route for compound
6 starting from 5. After reductive cyclization of 5 with excessive
LiNaph, the reaction mixture is treated with two equivalents of
[ZnCl2(TMEDA)] (TMEDA ¼ N,N,N0,N0-tetramethylethylene-
diamine) to form 3,4-diphenylsilole-2,5-di(zinc chloride), which
is allowed to react with 4-iodoaniline in the presence of
[Pd(PPh3)2Cl2] to afford 6 in an acceptable yield (28%). The
consecutive procedures from the cyclization to the cross-coupling
reaction could be carried out under one-pot conditions.
Aggregation structure of the gels
The morphology of xerogels of 1a and 1b, which are prepared by
slow evaporation of hexane from the corresponding organogels,
was investigated by polarized optical microscopy (POM) and
field emission scanning electron microscopy (FE-SEM). As
shown in Fig. 1, the xerogels of both compounds 1a and 1b are
Gelation
The gelation properties of 1a and 1b were evaluated in various
organic solvents at a moderate concentration. The ‘‘stable to
inversion of a test tube’’ method was adopted to test their gela-
tion properties.19 Generally, the gelation process involves three
steps. The first is mixing an organogelator with a solvent at
ambient temperature. The second is warming the mixture to
produce a clear solution. Finally, the solution is cooled slowly to
Fig. 1 POM images (a and b) and FE-SEM images (c and d) of xerogel
formed from silole derivatives in hexane (14 mg/mL), a and c for 1a, b and
d for 1b.
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composed of fibrous structures. The POM images exhibited
coexistence of intertwisted fibres and ordered fibres for the
xerogels of 1a (Fig. 1a). The FE-SEM investigations provided the
details of morphology. As shown in Fig. 1c, the intertwisted and
interlocked fibres formed the 3D network structure. Being
different from 1a, the xerogel of 1b obtained from the same
solvent had a threadlike structure based on the POM images
(Fig. 1b). The further investigations provided by FE-SEM indi-
cated that long and threadlike fibres were intertwisted (Fig. 1d).
On the basis of the above results, it is obvious that the
morphologies of the xerogels greatly depend on the length of
alkyl chains incorporated in tetraphenylsilole. In order to reveal
how the silole moiety aggregated into the fibrils, FT-IR spectra
and absorption were recorded for the organogels.
previously reported silole derivatives, the former is attributed to
the p–p* transition of the aryl groups and the latter is due to
the p–p* transition of the silole ring.21 The absorption band at
lmax ¼ 360–390 nm extends into the visible region and tails off at
about 450 nm, which can account for the yellow-green color of
1a. Compared with the absorption of CH2Cl2 solution, the
absorption of gel for 1a exhibits a slight red shift with two
absorption bands at 290 nm and 382 nm, respectively (Fig. 2).
The slight red-shift for absorption with spectrum broadening,
not the typical features of J-aggregates,22 suggests the existence of
intermolecular p–p interactions in the gel state. Therefore, the
p-stacking as another driving force may also play a key role in
the formation of organogels.
The fluorescent spectra of 1a in solution (Fig. 2) and in gel
states (Fig. 3) were recorded using a fluorescence spectropho-
tometer. The dilute solution of 1a exhibited only one fluorescence
maximum with lmax at 503 nm. However, the obvious difference
is detected for the fluorescence spectra between the gel and
solution. The emission for gel with lmax at 519 nm exhibited
a remarkable red shift (16 nm) compared with that of the solu-
tion. The red-shift of emission for 1a after gelation may be due to
the internal emission reabsorption increase and coplanarisation
of its peripheries and core.14a,23e Furthermore, with the increasing
of the temperature, a slight blue shift (4 nm) was observed for the
emission of gel with lmax at 515 nm at 50 ꢀC but 519 nm at 0 ꢀC.
Similarly, diluting the gels also resulted in a blue shift (5 nm) of
the emission maxima with lmax at 514 nm (see the ESI†). It is
likely that both heating and diluting the gels cause partial
dissolution of the molecular aggregates.
FT-IR spectroscopy
IR spectroscopy is a powerful tool for the study of hydrogen
bonds in various states of matter. Generally, for the free amides,
the nNH and nCO stretching vibrations lie in the 3500–3400 cmꢁ1
and around 1680 cmꢁ1 ranges, respectively.20 While in the xero-
gel, both 1a and 1b showed a nNH stretching vibrations band at
around 3200 cmꢁ1 (3273 cmꢁ1 for 1a, 3280 cmꢁ1 for 1b) and nCO
stretching vibrations band at around 1650 cmꢁ1 (1642 cmꢁ1 for
1a, 1644cmꢁ1 for 1b) (see the ESI†). For comparison, the IR
spectra of 1a in solid and solution states were also investigated
with the free nNH stretching vibration band at 3444 cmꢁ1 for the
former and 3436 cmꢁ1 for the latter. Decrease of wavenumbers of
both nNH and nCO of 1a and 1b in gel states is a good indication
of the intermolecular hydrogen bonding between amides in the
gels.
In order to compare the fluorescence intensity of 1a in gel state
and solution, the fluorescence of both gel and solution with the
same concentration was examined (Fig. 3). It is evident that the
fluorescence of the gel is very strong and enhanced nearly 100
times relative to that of the corresponding solution. The obvious
enhancement of fluorescence intensity after gelation can also be
distinguished by the naked eye from the photographic images
(Fig. 3 right). The remarkable fluorescence enhancement from
gels was assigned to the behavior of AIE.6c,16,23 The real reason
for the intense fluorescence in the gel state can be explained by
hindered intramolecular rotation of phenyl groups on the silole
ring around the Cphenyl-Csilole s bonds due to the aggregation
through H-bonding and p-stacking. As expected, the intensity of
the fluorescence of 1a in the gel state decreased as the
Optical absorption and fluorescence behavior
Owing to the presence of the silole ring and synthetic modifica-
tion of the chromophore, the tetraphenylsilole derivatives may
exhibit interesting photophysical properties, such as AIE. Thus,
UV-vis absorption and fluorescence characteristics of 1a were
studied in solution as well as in the gel state.
The UV-vis absorption spectra of the CH2Cl2 solution of 1a
exhibited two intense absorption bands with lmax at 286 nm and
378 nm, as shown in Fig. 2. According to the assignment for
Fig. 3 Fluorescence spectra of 1a in gel state and CH2Cl2 solution with
the same concentration (14 mg/mL) (left); photographic images of cor-
responding gel and solution observed upon UV irradiation at 365 nm
(right).
Fig. 2 UV-vis absorption spectra of 1a in gel and CH2Cl2 solution (1 ꢂ
10ꢁ4 mol/L), fluorescence of 1a in CH2Cl2 solution (1 ꢂ 10ꢁ4 mol/L). The
intensity of UV-vis absorption spectra for the gel is increased ꢂ7.
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Table 2 Phase-transition temperatures T (ꢀC) and associated enthalpy
values DH (KJ/mol) for compounds 1a–ba
temperature increased from 0 to 50 ꢀC. With the increase of
temperature, the aggregation is partially dissolved, and hence,
the intensity of the emission is decreased. The fluorescence life-
time of 1a in the gel state was also measured. A single-expo-
nential decay was observed, corresponding to s ¼ 4.5 ns. The
fluorescence lifetime of the gel is remarkably longer than the
lifetimes of dimethyl(tetraphenyl)silole (17.5 ps), hex-
aphenylsilole (20 ps) and other silole monomers.7a,16 The origin
of the increased lifetime for the gel is the restricted rotation of the
phenyl substituents due to aggregation.7a
Transition temperatures (ꢀC) [DH
(KJ/mol)]
1a
1b
Heating Cr–LC 59.3 [49.2] LC–I
206.8 [35.7]
Cooling I–LC195.0 [35.1] LC–Cr
50.7 [55.2]
Heating Cr–LC 53.6 [32.3] LC–I
175.9 [17.6]
Cooling I–LC 164.2 [15.8] LC–Cr
46.5 [35.3]
According to the above experimental results (IR and optical
spectra) and those shown in the literature,24 the following
microcosmic mechanism for the gelation of silole derivatives 1a is
proposed: The silole derivative firstly self-assembled into
a primary fibrous structure through hydrogen bonding between
amide groups as well as p-stacking between tetraphenylsilole
moieties, then assemblies into beltlike fibers as the secondary
structure of the gel through van der Waals interactions between
the outside alkyl chains. The beltlike fibers finally weave into
a 3D network and fix the solvents into network to form a gel
through surface tension.
a
Cr, Crystalline solid phase; LC, liquid crystalline phase; I, isotropic
phase.
thermograms of the compounds 1a and 1b. Their phase-transi-
tion temperatures and associated enthalpy values are given in
Table 2. The second heating curve of both compounds 1a and 1b
showed two phase transitions, one transition between crystalline
ꢀ
solid phase and the liquid crystalline (LC) phase (59 C for 1a,
54 ꢀC for 1b), and the other transition from the LC phase to the
isotropic phase (206 ꢀC for 1a, 176 ꢀC for 1b). During the cooling
process, the two phase transitions were also observed, one with
a small hysteresis at 195 ꢀC for 1a and 164 ꢀC for 1b, which
corresponds to the conversion from the isotropic phase to the LC
Mesomorphic behavior
The mesomorphic behaviors of the compounds 1a and 1b were
evaluated using differential scanning calorimetry (DSC) and
polarized optical microscopy (POM) equipped with a tempera-
ture controller as well as small-angle X-ray scattering (SAXS).
Fig. 4 shows the first cooling and the second heating DSC
ꢀ
phase, the other also with a small hyteresis at 50 C for 1a and
46 ꢀC for 1b, which is related to the conversion from the LC
phase to the low-temperature crystalline solid phase.
As expected, with increasing the aliphatic side-chain length
from dodecyloxy to hexadecyloxy in the tetraphenylsilole
derivatives, the melting point is lowered and the temperature
ꢀ
range of the LC phase decreases (146 C for 1a, 122 ꢀC for 1b).
POM observations showed that both compounds 1a and 1b
exhibited stable LC phases over a wide temperature range
(Fig. 5). According to textures with different sizes observed by
POM (Fig. 5a and b), the mesophases formed from compound 1a
Fig. 5 Optical micrographs of compounds 1a and 1b between crossed
polarizers: liquid crystalline texture for 1a at 112.4 ꢀC (a) and at 60.6 ꢀC
(b); liquid crystalline texture for 1b at 127.2 ꢀC (c) and at 67.6 ꢀC (d).
Fig. 4 DSC curves of compounds 1a and 1b.
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supramolecular structures by epitaxial growth.25 Scanning
tunneling microscopy (STM) is a powerful method to investigate
the 2D assembly of functionalized organic molecules on surfaces,
which provides access to supramolecular structures at the single-
molecule level.26 For this purpose, silole derivatives were well
investigated at the liquid-solid interface.
Compound 1a self-assembled into well-ordered monolayers
when adsorbed from 1-phenyloctane on the HOPG (highly
oriented pyrolytic graphite) surface, as shown in Fig. 7. From the
large scale images of 1a adlayer, it is noticed that the remarkably
bright rows and dark slots cover alternately the whole area. The
details of the adlayers were seen in the high-resolution STM
images (Fig. 7b). It is clearly showed that the bright rows are
composed of many separated spots, which are attributed to the
p-conjugated backbones, that is, 2,3,4,5-tetraphenylsilole moie-
ties. The relatively darker parts corresponded to the parallel alkyl
chains with lower electronic density.27 Based on the high-reso-
lution images, all six dodecyloxy chains of 1a align perpendicular
to the 2,3,4,5-tetraphenylsilole backbone. Two piles of consecu-
tive alkyl chains at the two ends of 1a aligned with trans
conformation, and the alkyl chains are aligned in a comb-like
fashion between neighboring molecules. On the basis of the
above analysis, the unit cells of the adlayers are outlined in
Fig. 7b. The lattice parameters were determined from the STM
images to be a ¼ 3.9 ꢃ 0.2 nm, b ¼ 2.2 ꢃ 0.2 nm, a ¼ (70 ꢃ 2)ꢀ for
1a. Based on the presented high-resolution STM images, it is
difficult to determine the arrangement of 2,3,4,5-tetraphenylsi-
lole moieties in the same rows (head-to-head or head-to-tail or
maybe both). The proposed models for the molecular arrange-
ment models are illustrated in Fig. 7c. It is worthy of note that
the a value of the unit cell parameters obtained by STM
measurements is exactly the same as the distance (3.9 nm)
Fig. 6 Variable-temperature SAXS patterns for 1a.
belong to the smectic liquid crystalline phases. However, the
textures observed for 1b were obviously different from those of
1a, no uniform shape and size were observed for 1b under
polarized light (Fig. 5c, d). It is evident that the LC properties are
greatly dependent on the length of the alkyl chains. The DSC and
POM studies show that the signature of phase transitions
observed in the DSC curves of both 1a and 1b are in substance
consistent with those found by POM experiments.
Generally, organic molecules with non-planar structures are
difficult to aggregate and form LC phases. It is reasonable to
consider that the non-planar 2,3,4,5-tetraphenylsilole derivatives
1a and 1b exhibit LC properties with the aid of the six long alkyl
side-chains and amide groups in the molecules.
Variable temperature SAXS provides complementary infor-
mation on the phase transition and liquid crystal, and the
diffraction patterns are shown in Fig. 6. At 30 ꢀC, the scattering
vector (0.159, 0.253, 0.329, 0.396) of the peaks are following
a ratio of 1 : O3 : O4 : O7, and can be assigned to (100), (110),
(200), (001) reflections from a hexagonal column structure. When
the sample was heated to 120 ꢀC, an obvious change was
observed for the scattering pattern. Two original diffractions
with q values at 0.253 and 0.329 disappeared. The other two
diffractions are more distinct apart from a slight shift. The
scattering vector (0.171, 0.363, 0.472) of the peaks at 120 ꢀC are
in a ratio of approximately 1 : 2 : 3, and can be assigned to (001),
(002), (003) strongly suggesting a lamellar structure. According
to the DSC and POM investigations, the lamellar liquid crys-
talline phase was formed at this temperature. This fact also
confirms the above hypothesis that the mesophases formed from
compound 1a belong to smectic liquid crystalline phases.
ꢀ
At 30 C, the diffraction pattern at 0.159 is corresponding to a
d-spacing of 3.9 nm. The fully extended molecular length for 1a is
estimated to be approximately 5.5 nm. It is reasonable to
consider that the terminal alkoxy chains are partially interdigi-
ꢀ
tated. The d-spacing decreases to 3.6 nm (0.171) at 120 C. It is
obvious that the 1a molecules pack more tightly in the lamellar
liquid crystalline phase.
2D self-assembly
Fig. 7 (a) Large-scale STM image (156.2 nm ꢂ 156.2 nm) of 1a. (b) A
high-resolution STM image (16.7 nm ꢂ 16.7 nm) of 1a. (c) Proposed
structural model of the ordered array (the green circles in (c) specify
2,3,4,5-tetraphenylsilole moieties).
Recently, the two-dimensional (2D) assembly has attracted
increasing interest because functionalized nanopatterns may
provide a tool for the formation of three-dimensional (3D)
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assembly of both compounds is described as follows: the 2,3,4,5-
tetraphenylsilole unites are aligned in rows, with the alkyl chains
residing between the rows.
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It is well known that the 2D self-assembly of these two mole-
cules depends on the molecule-substrate (adsorption energy) and
molecule-molecule interactions. Both silole derivatives contain
six long alkyl chains. Generally, the interactions between long
alkyl chains and substrate are very strong.28 All alkyl chains with
a full extension oriented parallel to a lattice axis within the basal
plane of graphite, thus arranged the 2,3,4,5-tetraphenylsilole
moieties into neat rows on surface. The results presented indicate
that the long alkyl chains of silole derivatives play an important
role in the formation of 2D assembly.
Conclusions
Two 2,3,4,5-tetraphenylsilole derivatives modified by long-chain
alkoxydiacylamido platforms have been synthesized as novel
materials. POM and FE-SEM studies have shown that the two
silole compounds self-assembled into fibrous aggregates in
hydrocarbon solvents, which come from the cooperation of
hydrogen bonding between amide groups and p-stacking inter-
action of the 2,3,4,5-tetraphenylsilole moieties. Furthermore,
large fluorescence enhancement was observed for silole deriva-
tives after gelation. Both silole compounds have been revealed by
POM and DCS studies to exhibit LC properties over a wide
temperature range, which were affected by the length of alkyl
chains. In addition to assembling into the gel and LC phases, the
two silole derivatives were found to form uniform and well-
ordered monolayers at the interface between the solvent and
HOPG. It is obvious that the new gelators are quite unique and
could well self-assemble into different aggregates, from one-
dimensional (fibres), two-dimensional (molecular monolayer) to
three-dimensional (liquid crystal) structures. Study of the use of
the highly fluorescent organogel-based silole derivatives for
applications in explosive and ion detection is in progress.
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Acknowledgements
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Soc., 2005, 127, 11661–11665; (b) J. C. Sanchez, S. A. Urbas,
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and K. Furukawa, Chem. Mater., 2001, 13, 2680–2683; (b)
J. H. Lee, Q.-D. Liu, M. Motala, J. Dane, J. Gao, Y. J. Kang
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W. Zhao, T. V. Timofeeva, A. Korlyukov, M. Y. Antipin,
S. Montgomery, E. Thompson, Z. S. An, B. Domercq, S. Barlow,
This Project was supported by the National Natural Science
Foundation of China (No. 20802014, No. 20974031), the
National High Technology Research and Development Program
of China (No. 2006AA050203), Zhejiang Provincial Natural
Science Foundation of China (Y4080380) and the Qianjiang
Talent Program of Zhejiang Province (2008R10018). We also
thank Prof. M. Kira (Tohoku University, Japan) for his helpful
discussion.
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