Photochromism of new diarylethenes bearing both thiazole and benzene moieties

Article abstract of DOI:10.1016/j.tetlet.2010.05.004

A new class of photochromic diarylethenes bearing both thiazole and benzene moieties has been developed, and the effects of substitution on their properties, including photochromism, fatigue resistance, and fluorescence properties have been investigated.

Full text of DOI:10.1016/j.tetlet.2010.05.004

Tetrahedron Letters 51 (2010) 3575–3579
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photochromism of new diarylethenes bearing both thiazole and
benzene moieties
*
Shouzhi Pu , Hui Li, Gang Liu, Weijun Liu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of photochromic diarylethenes bearing both thiazole and benzene moieties has been devel-
oped, and the effects of substitution on their properties, including photochromism, fatigue resistance, and
fluorescence properties have been investigated. They exhibited good photochromism and functioned as a
fluorescence switch upon alternating irradiation with UV and visible light both in solution and in PMMA
film. The electron-donating substituents could significantly enhance the cyclization quantum yield and
depress the cycloreversion quantum yield whereas the electron-withdrawing groups functionalized an
inverse action for these diarylethene derivatives. Relatively big differences exist among the properties
of these diarylethenes which may be attributed to the different substituent effects.
Received 30 March 2010
Revised 4 May 2010
Accepted 5 May 2010
Available online 8 May 2010
Keywords:
Photochromism
Diarylethene
Thiazole and benzene moieties
Ó 2010 Elsevier Ltd. All rights reserved.
There has been considerable interest in photochromic com-
pounds because of their potential ability for optical memories,
photo switches, and display devices.1 During the past several dec-
ades, photochromic diarylethenes have been extensively explored
for optoelectric applications because of their high conversion effi-
ciency in reversible photochromic reactions, excellent thermal sta-
bility of both isomers, and remarkable fatigue resistance.2 Upon
photoirradiation, diarylethene derivatives can undergo photochro-
mic cyclization/cycloreversion reactions either in solution or in the
solid state. The reversible photochromic cyclization/cycloreversion
reactions can lead to changes in their absorption spectra and other
characteristics such as fluorescence, oxidation/reduction poten-
tials, refractive indices, etc.3 The most important difference is that
are thermally stable and do not return to the open-ring isomers in
the dark.6b Maybe for this reason, among diarylethenes hitherto re-
ported, most of the heteroaryl moieties have been thiophene or ben-
zothiophene rings, with just a few reports concerning other
heteroaryl moieties such as furan,8 indene,9 indole,10 benzofuran,11
pyrrole,12 pyrazole,13 etc. Thiazole is a heterocyclic ring which has
low aromatic stabilization energy,6b and its structure is similar to
that of thiophene and pyrrole. Therefore, introduction of a thiazole
ring into the heteroaryl moieties of diarylethene derivatives can be
expected to undergo excellent photochromic reactions.
Up to date, there are a few examples of photochromic diary-
lethene derivatives bearing thiazole rings. Irie et al. reported a
series of diarylethenes bearing thiazole rings. They demonstrated
that thiazole moieties could significantly shift the kmax of the
closed-ring isomer to shorter wavelengths.6b,14 Hasegawa and
Kawai and co-workers developed a new type of photochromic
terarylene system bearing thiazole rings and elucidated some
special properties of these derivatives.15 Tanaka et al. synthesized
some bis(thiazolyl)ethene derivatives and revealed their good
thermal stability.16 From these reports, it can be easily concluded
that the hexatriene backbone of all photochromic systems having
thiazole moieties is composed of five-membered heterocyclic
rings or a combination of a five-membered aryl ring and a vinyl
group. The results have contributed to a broad understanding of
the effects of thiazole rings on the properties of photochromic
diarylethenes, and they have also given us valuable insight to de-
sign some new photochromic diarylethene systems. To the best
of our knowledge, photochromic hybrid diarylethene derivatives
bearing both thiazole and six-membered moieties have not hith-
erto been reported.
the
p-systems of two aryl rings are separated in the open-ring iso-
mer, while the
p
-conjugation is delocalized throughout the mole-
cule in the closed-ring isomer.4 Till now, most of diarylethene
compounds colored in magenta or cyan have been well studied,
yellow color photochromic diarylethene derivatives are more less
than them.5
Theoretical consideration based on a molecular orbital theory re-
vealed that the thermal stability of both isomers of the diarylethene
compounds can be improved by introducing aryl groups that have
low aromatic stabilization energy,6 and it was demonstrated by
the synthesis of diarylethenes with various types of aryl groups.7
When the aryl groups are thiophene or benzothiophene rings that
have low aromatic stabilization energy, the closed-ring isomers
* Corresponding author. Tel./fax: +86 791 3831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.004

3576
S. Pu et al. / Tetrahedron Letters 51 (2010) 3575–3579
In this Letter, we designed a new class of diarylethenes bearing
F
F
F
F
Br
both thiazole and benzene moieties. The synthesized diarylethenes
are 1-(2,4-dimethyl-5-thiazolyl)-2-(2-methoxyphenyl)perfluoro-
cyclopentene (1o), 1-(2,4-dimethyl-5-thiazolyl)-2-(2-methyl-
phenyl)perfluorocyclo-pentene (2o), 1-(2,4-dimethyl-5-thiazolyl)-
2-(2-cyanophenyl)perfluorocyclopentene (3o), and 1-(2,4-dimethyl-
5-thiazolyl)-2-(2-trifluoromethylphenyl)perfluorocyclopentene
(4o). All of these diarylethene derivatives showed good photochro-
mism both in solution and in PMMA amorphous film. The photo-
chromic scheme of 1o–4o is shown in Scheme 1.
The synthetic route for diarylethenes 1o–4o is shown in
Scheme 2. First, compound 6 was prepared in 80% yield by bromi-
nating compound 5 in carbon disulfide at 0 °C. Lithiation of
compound 6 followed by the addition of excess octafluorocycl-
opentene generated compound 7 in 44% yield at À78 °C. Finally,
compounds 8a–d were separately lithiated and then coupled with
7 to give diarylethenes 1o–4o. The structures of 1o–4o were con-
firmed by elemental analysis, NMR, and IR.17
S
S
n-BuLi, C5F8
195K
Br2, CS2
Ice Bath
F
F
S
N
5
F
N
6
7
N
F
F
F
F
Br
OCH3
CH3
CN
8a: R =
8b: R =
8c: R =
8d: R =
F
F
S
CF3
R
R
N
n-BuLi,THF
195K
OCH3
CH3
CN
1o: R =
2o: R =
3o: R =
4o: R =
CF3
Scheme 2. Synthesis of diarylethenes 1o–4o.
0.25
Diarylethenes 1–4 showed good photochromism and could be
toggled between the colorless open-ring isomers (1o–4o) and the
colored closed-ring isomers (1c–4c) by alternating irradiation with
UV light and appropriate wavelength visible light both in hexane
and in PMMA film. The absorption spectral change of diarylethene
1 and the color changes of diarylethenes 1–4 are shown in Figure 1.
The absorption maximum of diarylethene 1o was observed at
Vis
UV
0.20
0.15
0.10
0.05
0.00
294 nm in hexane (
e
, 1.08 Â 104 L molÀ1 cmÀ1), which arose from
UV
Vis
18
*
p?p
transition. Upon irradiation with 297 nm light, the color-
less solution of 1o turned yellow, in which the absorption maxi-
mum was observed at 469 nm (
e
, 2.51 Â 103 L molÀ1 cmÀ1). The
yellow color was due to the formation of the closed-ring isomer
1c. Alternatively, the yellow solution turned colorless upon irradi-
ation with visible light (k >400 nm) because 1c reverted to the ini-
tial state 1o. As with diarylethene 1, compounds 2–4 also show
good photochromism in hexane. Their color changes upon photoir-
radiation are shown in Figure 1B. When arriving at the photosta-
tionary state, the absorption maxima of diarylethenes 2–4
appeared at 449, 441, and 444 nm, respectively. In PMMA films,
diarylethenes 1–4 showed similar photochromism as in hexane.
Compared to those in hexane, the absorption maxima of both the
open-ring and the closed-ring isomers of diarylethenes 1–4 in
PMMA films are at longer wavelengths. The red-shift phenomena
may be attributed to the polar effect of the polymer matrix and
the stabilization of molecular arrangement in solid medium.19
The absorption spectral parameters of these compounds are sum-
marized in Table 1.
300
350
400
450
500
550
600
Wavelength/nm
A
1c
2c
3c 4c
1o 2o
3o 4o
B
The cyclization and cycloreversion quantum yields of diaryleth-
enes 1–4 were measured in hexane at room temperature, and the
results are also summarized in Table 1. The results indicated that
different substituents at 2-position of the benzene ring had a sig-
nificant effect on the photochromic features of diarylethenes 1–4,
including the absorption maxima, molar absorption coefficients,
and quantum yields. The absorption maxima of 1c–4c showed a
notable red-shift with the increase in electron-donating ability
both in hexane and in PMMA film. On the one hand, the absorption
maximum of 1c was much longer than that of 2c, as a result of the
electron-donating ability of methoxy group which was larger than
that of the methyl group.4a,20 The molar absorption coefficients of
Figure 1. Absorption spectral changes of diarylethene 1 (A) and the color changes
of diarylethenes 1–4 (B) by photoirradiation in hexane (2.0 Â 10À5 mol LÀ1) at room
temperature.
diarylethenes bearing an electron-donating substituent (methoxy
or methyl group such as in compound 1c or 2c) were much larger
than those of compounds bearing an electron-withdrawing group
(cyano or trifluoromethyl group such as in compound 3c or 4c).
The cyclization quantum yields enhanced significantly when the
electron-donating groups were substituted at the 2-position of
the benzene ring of these diarylethenes, while the cycloreversion
quantum yields were improved when the electron-withdrawing
groups were substituted at the same position. Therefore, the cycli-
zation quantum yields of diarylethenes 1 and 2 bearing electron-
donating groups were higher than those of diarylethenes 3 and 4
bearing electron-withdrawing groups, but a reverse case for the
cycloreversion quantum yields. This indicated that the electron-
donating substituents could significantly enhance the cyclization
quantum yield and depress the cycloreversion quantum yield,
while the electron-withdrawing groups functionalized an inverse
F
F
F
F
F
F
F
F
OCH3
CH3
CN
OCH3
CH3
CN
1c: R =
2c: R =
3c: R =
4c: R =
1o: R =
2o: R =
3o: R =
4o: R =
UV
Vis
F
F
F
F
S
S
CF3
CF3
R
N
R
N
Scheme 1. Photochromism of diarylethenes 1o–4o.

S. Pu et al. / Tetrahedron Letters 51 (2010) 3575–3579
3577
Table 1
Absorption spectral properties of diarylethenes 1–4 in hexane (2.0 Â 10À5 mol LÀ1) and in PMMA film (10% w/w) at room temperature
Compound
ko,max/nma
(e
/L molÀ1 cmÀ1
)
kc,max/nmb
(e
/L molÀ1 cmÀ1
)
Uc
Hexane
PMMA
Hexane
PMMA
Uo–c
Uc–o
1
2
3
4
294(1.08 Â 104)
293(9.28 Â 103)
307(8.72 Â 103)
296(1.09 Â 104)
304
303
308
301
469(2.51 Â 103)
449(2.72 Â 103)
441(2.27 Â 103)
444(2.11 Â 103)
472
462
446
445
0.46
0.42
0.37
0.35
0.095
0.052
0.19
0.16
a
b
c
The maximum absorption peak of the open-ring isomers.
The maximum absorption peak of the closed-ring isomers.
Quantum yields of cyclization (Uo–c) and cycloreversion (Uc–o), respectively.
action for these diarylethene derivatives. The result is contrary to
that of diarylethenes bearing both thiophene and benzene moie-
ties.21 When replacing the thiazole moiety with the thiophene unit,
the molar absorption coefficients and the absorption maxima in-
creased significantly, but the changing trends of the cyclization
and cycloreversion quantum yields are reversed compared with
thiazole moiety.21 On the side, the photochromic features of diary-
lethenes 1–4 were also remarkably different from those of other
diarylethenes bearing a thiazole unit when replacing the benzene
ring with other groups.14,22 Compared to the symmetrical/unsym-
metrical analogs bearing thiazole ring(s),6b the cyclization/cyclore-
version quantum yields of 1–4 are greater, but the molar
absorption coefficients of their closed-ring isomers are much low-
er. Compared to those bearing two six-membered rings,23 the pho-
tochromic features of 1–4 are much more excellent.
Fatigue-resistant property, that is, how many times photocycli-
zation and cycloreversion reaction cycles can be repeated without
loss of performance,2a,12b is a very important factor for practical
applications in optical devices.2a,c The fatigue resistances of diary-
lethenes 1–4 were tested both in hexane and in PMMA films by
alternatively irradiating with UV and visible light in air at room
temperature. The fatigue resistance of diarylethene 1 is shown in
Figure 2. In hexane, 88% of 1c was destroyed after 10 repeat cycles,
which may be attributed to degradation resulting from the forma-
tion of an epoxide.24 However, its fatigue resistance in PMMA film
is much stronger than that in hexane. After 10 repeat cycles, diary-
lethene 1 still showed good photochromism with only ca. 28% deg-
radation of 1c in PMMA film. Similarly, the fatigue resistances of
diarylethenes 2–4 in PMMA films are also much stronger than
those in solution. After 10 repeat cycles, they showed good photo-
chromism with only ca. 17% degradation of 2c, 6% of 3c, and 5% of
4c, respectively. But, the degradation percents of diarylethenes 2–4
in hexane are 71% of 2c, 53% of 3c, and 39% of 4c, respectively. This
remarkable improvement may result from effectively suppressing
the oxygen diffusion in the solid medium.2a In addition, the fatigue
resistances of diarylethenes with an electron-withdrawing group
(such as in diarylethenes 3 and 4) are much better than those of
diarylethenes with an electron-donating group (such as in diary-
lethenes 1 and 2) both in hexane and in PMMA films. The result
is well in agreement with that reported for analogs bearing both
thiophene and benzene moieties, where fatigue resistance mark-
edly increased with the increase of electron-withdrawing ability.21
The fluorescence spectra of diarylethenes 1o–4o both in solu-
tion (2.0 Â 10À5 mol/L) and in PMMA amorphous films (10% w/w)
were measured at room temperature. In hexane, the emission
peaks of 1o–4o were observed at 463, 452, 476, and 457 nm when
excited at 397 nm (Fig. 3). In PMMA films, the emission peaks of
diarylethenes 1o–4o were observed at 465, 465, 487, and 465 nm
when excited at 395 nm. Compared to those in hexane, the emis-
sion peaks of diarylethenes 1o–4o showed a bathochromic shift
in PMMA film consistently across their maxima absorption wave-
lengths with values of 2 nm for 1, 13 nm for 2, 11 nm for 3, and
8 nm for 4. Among compounds 1o–4o, the emission intensity of
2o is the strongest and that of 4o is the smallest in hexane, how-
ever, the emission peak of 3o is at the lowest energy both in hex-
ane and in PMMA film. The result suggested that different
substituents attached at the 2-position of the benzene ring had a
significant effect not only on the emission peak but also on the
emission intensity.
As has been observed for most of the reported diarylethenes,25
diarylethenes 1–4 exhibited a very good fluorescent switch on
changing from the open-ring isomers to the closed-ring isomers
by photoirradiation both in hexane and in PMMA. During the pro-
cess of photoisomerization, diarylethene 1 exhibited changes in its
fluorescence in hexane as shown in Figure 4. Upon irradiation with
313 nm UV light, the photocyclization reaction was carried out and
the non-fluorescent closed-ring isomer 1c was produced. Back irra-
diation of the appropriate wavelength of visible light regenerated
the open-ring isomer 1o and duplicated the original emission spec-
tra. When arriving at the photostationary state, the emission inten-
In PMMA
In Hexane
2a
3000
2500
2000
1500
1000
500
1.00
0.75
0.50
0.25
0.00
3a
1a
(%)
4a
0
/A
0
450
500
550
600
0
2
4
6
8
10
Repeat Cycles
Wavelength/nm
Figure 2. Fatigue resistance of diarylethene 1 in hexane and in PMMA film in air
atmosphere at room temperature. Initial absorptance of the sample was fixed to 1.0.
Figure 3. Fluorescence emission spectra of diarylethenes 1–4 in hexane
(2.0 Â 10À5 mol LÀ1) at room temperature when excited at 397 nm.

3578
S. Pu et al. / Tetrahedron Letters 51 (2010) 3575–3579
Tanifuji, N.; Matsuda, K.; Irie, M. Polyhedron 2005, 24, 2484–2490; (e) Kawai, T.;
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Kim, M. S.; Maruyama, H.; Kawai, T. S.; Irie, M. Chem. Mater. 2003, 15, 4539–
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Acta 2007, 360, 995–999.
450
500
550
600
Wavelength/nm
Figure 4. Change in the intensity of the fluorescence spectra of diarylethene 1 in
hexane (2.0 Â 10À5 mol LÀ1) upon photoirradiation.
sity of diarylethene 1 was quenched to ca. 20% in hexane and ca.
27% in PMMA film. Similarly, diarylethenes 2–4 also functioned
as a good fluorescent switch upon photoirradiation both in hexane
and in PMMA films. In the photostationary state, their emission
intensities were quenched to ca. 36% for 2, 37% for 3, and 39% for
4 in hexane and ca. 51% for 2, 47% for 3, and 58% for 4 in PMMA
films. The result showed that the fluorescent modulation efficien-
cies of diarylethenes 1–4 in hexane were much higher than those
in PMMA film. The result is also contrary to that reported for diary-
lethenes bearing both thiophene and benzene moieties, where the
fluorescent modulation efficiencies in hexane were much lower
than those in PMMA film.21 The incomplete cyclization reaction
and the existence of parallel conformations with relatively strong
fluorescence may be the main cause for the lower fluorescent con-
version.26 Among diarylethenes 1–4, diarylethene 1 showed the
highest fluorescent modulation efficiency both in solution and in
PMMA film, suggesting that it is the most promising candidate
for application on photoswitchable devices such as optical memory
and fluorescent modulation switches.27
In conclusion, four new unsymmetrical photochromic diary-
lethenes with both thiazole and benzene moieties have been syn-
thesized and their properties have been investigated. This new
photochromic system showed good photochromism and acted as
a remarkable fluorescent switch both in solution and in PMMA
film. The results indicated that the substituents at 2-position of
the benzene ring had a significant effect on the properties of these
diarylethene derivatives. Diarylethenes based on thiazole and ben-
zene moieties induced some new characteristics differing from
other reported diarylethenes. The results will be helpful for the
synthesis of efficient photoactive diarylethene derivatives with
new molecular skeletons and to design new photochromic systems
for further potential applications.
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16. Iwata, S.; Ishihara, Y.; Qian, C.; Tanaka, K. J. Org. Chem. 1992, 57, 3726–3730.
17. Data for 1o: Mp 55–56 °C; Calcd for C17H13F6NOS (%): Calcd C, 51.91; H, 3.33;
N, 3.56. Found C, 51.89; H, 3.37; N, 3.51; 1H NMR (400 MHz, CDCl3, ppm): d
1.90 (s, 3H, –CH3), 2.67 (s, 3H, –CH3), 3.60 (s, 3H, –OCH3), 6.88(d, 1H, J = 8.0 Hz,
benzene-H), 7.03 (t, 1H, J = 6.0 Hz, benzene-H), 7.34 (d, 1H, J = 8.0 Hz, benzene-
H), 87.40(t, 1H, J = 8.0 Hz, benzene-H); 13C NMR(100 MHz, CDCl3, ppm): d
16.12, 19.04, 55.24, 111.48, 116.88, 117.16, 120.96, 129.90, 132.07, 153.36,
157.02, 167.63; IR(KBr, m
, cmÀ1): 707, 760, 790, 831, 864, 985, 1023, 1047,
1073, 1126, 1190, 1271, 1338, 1400, 1438, 1463, 1532, 1630. Data for 2o: Calcd
for C17H13F6NS (%): Calcd C, 54.11; H, 3.47; N, 3.71. Found C, 54.17; H, 3.50; N,
3.68; 1H NMR (400 MHz, CDCl3, ppm): d 1.96 (s, 3H, –CH3), 1.99 (s, 3H, –CH3),
2.53 (s, 3H, –CH3), 7.13 (d, 1H, J = 8.0 Hz benzene-H), 7.20 (t, 2H, J = 6.0 Hz,
benzene-H), 7.25–7.29 (m, 1H, benzene-H); 13C NMR(100 MHz, CDCl3, ppm): d
16.85, 19.03, 19.55, 116.40, 126.31, 127.02, 129.17, 130.16, 130.95, 136.92,
154.44, 168.42; IR (KBr, m
, cmÀ1): 586, 744, 833, 866, 989, 1072, 1132, 1193,
1276, 1342, 1400, 1456, 1627. Data for 3o: 77–78 °C; Calcd for C17H10F6N2S
(%): Calcd C, 52.58; H, 2.60; N, 7.21. Found: C, 52.63; H, 2.57; N, 7.19; 1H NMR
(400 MHz, CDCl3, ppm): d 1.97 (s, 3H, –CH3), 2.67 (s, 3H, –CH3), 7.62–7.66 (m,
2H, benzene-H), 7.73 (d, 1H, J = 8.0 Hz, benzene-H), 7.77 (t, 1H, J = 8.0 Hz,
benzene-H); 13C NMR (100 MHz, CDCl3, TMS): d 16.75, 19.17, 113.23, 115.64,
116.08, 129.93, 130.75, 131.28, 133.30, 134.01, 154.85, 169.74; IR (KBr,
m,
cmÀ1): 767, 834, 867, 949, 990, 1036, 1073, 1137, 1199, 1282, 1341, 1399,
1445, 1524, 1624, 1628, 1986, 2232, 2931, 2968, 3197. Data for 4o: Calcd for
C17H10F9NS (%): Calcd C, 47.34; H, 2.34; N, 3.25. Found: C, 47.39; H, 2.38; N,
3.60; 1H NMR (400 MHz, CDCl3, ppm): d 2.23 (s, 3H, –CH3), 2.60 (s, 3H, –CH3),
7.42 (d, 1H, J = 8.0 Hz, benzene-H), 7.64–7.70 (m, 2H, benzene-H), 7.76 (d, 1H,
J = 8.0 Hz, benzene-H); 13C NMR (100 MHz, CDCl3): d 16.90, 19.21, 115.64,
121.77, 125.20, 127.59, 127.63, 130.63, 131.25, 131.98; IR (KBr, m
, cmÀ1): 727,
771, 800, 834, 865, 950, 991, 1036, 1076, 1139, 1398, 1276, 1315, 1342, 1398,
1446, 1529, 1627, 1629, 2932. Data for 1c: 1H NMR (400 MHz, CDCl3, ppm): d
1.66 (s, 3H, –CH3), 2.35 (s, 3H, –CH3), 3.35 (s, 3H, –OCH3), 5.92 (d, 1H, J = 8.0 Hz,
benzene-H), 6.09 (t, 1H, J = 8.0 Hz, benzene-H), 6.41 (t, 1H, benzene-H), 6.52 (d,
1H, J = 8.0 Hz, benzene-H). Data for 2c: 1H NMR (400 MHz, CDCl3, ppm): d 1.25
(s, 3H, –CH3), 1.57 (s, 3H, –CH3), 2.34 (s, 3H, –CH3), 6.15 (d, 1H, J = 8.0 Hz,
benzene-H), 6.28 (t, 2H, J = 8.0 Hz, benzene-H), 6.32 (t, 1H, J = 8.0 Hz, benzene-
H). Data for 3c: 1H NMR (400 MHz, CDCl3, ppm): d 1.50 (s, 3H, –CH3), 2.45 (s,
3H, –CH3), 6.20–6.28 (m, 2H, benzene-H), 6.54 (d, 1H, benzene-H), 6.58 (t, 1H,
J = 8.0 Hz, benzene-H). Data for 4c: 1H NMR (400 MHz, CDCl3, ppm): d 1.69 (s,
3H, –CH3), 1.92 (s, 3H, –CH3), 6.07 (d, 1H, J = 8.0 Hz, benzene-H), 6.20–6.24 (m,
2H, benzene-H), 6.51 (d, 1H, benzene-H).
Acknowledgments
This work was supported by Program for the NSFC of China
(20962008), New Century Excellent Talents in University (NCET-
08-0702), Project of Jiangxi Youth Scientist, the Key Scientific Pro-
ject from Education Ministry of China (208069), National Natural
Science Foundation of Jiangxi Province (2008GZH0020), and Grad-
uated Research and Innovation Fund of Jiangxi Province.
18. Li, Z. X.; Liao, L. Y.; Sun, W.; Xu, C. H.; Zhang, C.; Fang, C. J.; Yan, C. H. J. Phys.
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