Three asymmetrical conjugated D-π-D′ sulfur-containing chromophores with a focus on two-photon absorption

Article abstract of DOI:10.1071/CH10229

Three novel asymmetrical D-π-D′ type sulfur-containing chromophores, (E)-4-(4-(benzylthio)styryl)-N,N-diethylbenzenamine (S1), (E)-4-(4-(tert-butylthio)styryl)-N,N-diethylbenzenamine (S2) and (E)-4-(4-(thio)styryl)-N,N-diethylbenzenamine (S3), were synthe

Full text of DOI:10.1071/CH10229

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2011, 64, 174–179
www.publish.csiro.au/journals/ajc
Three Asymmetrical Conjugated D-p-D0 Sulfur-Containing
Chromophores with a Focus on Two-Photon Absorption
A B
,
A
A
A
Zhang-Jun Hu,
Rui-Long Zhang, Ping-Ping Sun, Lin Li,
A E
,
A C
,
A C E
, ,
Jie-Ying Wu, Jia-Xiang Yang,
Yu-Peng Tian,
D
and Chuan-Kui Wang
A
School of Chemistry and Chemical Engineering, Key Laboratory of Functional Inorganic
Materials of Anhui Province, Anhui University, 3 Feixi Road, Hefei 230039, PR China.
State Key Laboratory of Pollution Control and Resource Reuse, College of
Environmental Science and Engineering, Tongji University,
B
1239 Siping Road, Shanghai 200092, PR China.
C
State Key Laboratory of Crystal Materials, Shandong University, 27 Shanda Nanlu,
Jinan 250100, PR China.
D
Department of Physics, Shandong Normal University, 1 Daxue Road,
Jinan 250014, PR China.
E
Corresponding authors. Email: jywu1957@163.com; yptian@ahu.edu.cn
Three novel asymmetrical D-p-D0 type sulfur-containing chromophores, (E)-4-(4-(benzylthio)styryl)-N,N-diethylbenze-
namine (S1), (E)-4-(4-(tert-butylthio)styryl)-N,N-diethylbenzenamine (S2) and (E)-4-(4-(thio)styryl)-N,N-diethylbenze-
namine (S3), were synthesized and characterized. Two kinds of substituents and hydrogen were introduced into the three
different chromophores to investigate the influence of electron distribution on the sulfur atom. Meanwhile, a simple
synthetic strategy of p-conjugated 4-(thio)styrene derivatives was performanced successfully. Linear and non-linear
optical properties of S1, S2 and S3 were investigated both experimentally and theoretically. The optical properties indicate
that they all have obvious characteristics of asymmetrical dipole molecules. The measured maximum two-photon cross-
sections of S1–3 are 79, 57 and 19 GM (Goeppert-Mayer), respectively. It shows that the different substituents on the sulfur
atom in molecules S1–3 lead to different electronic structures, which affect the optical properties. In these structures, the
aromatic benzyl substituent (in S1) is superior for optimizing two-photon activity in the designed molecular framework.
Manuscript received: 5 June 2010.
Manuscript accepted: 19 October 2010.
Introduction
molecules.[11] In terms of the dipole molecules, D-p-A is a
typical representative; a large number of D-p-A molecules with
TPA have been developed in recent years, and their structure and
properties have been investigated theoretically or experimen-
tally in detail.[12] However, to date, there has been relatively
little research on D-p-D0-type molecules, which is also one
of the useful molecular models to achieve desired TPA
materials.[13] As is well known, many organic groups with
electron-donating capacity are available, as opposed to electron-
accepting ones,[7–12] which would provide an easier way to
fabricate more species of TPA materials.
Recently, two D-p-D0 model chromophores with two weak
electronic donors of carbazole and a sulfur-containing group
were successfully prepared to give efficient blue single- and
two-photon emission.[14] Here, we present continuing work,
where the amine group, with a stronger electron donating ability,
is introduced into a sulfur-containing styryl bridge to construct
novel D-p-D0 molecules, and substituents on the sulfur atom are
changed to investigate the influence on their optical properties,
such as emission or excitation wavelengths, TPA responses and
Two-photon absorption (TPA) is a third-order non-linear optical
process and offers outstanding advantages over single-photon
absorption.[1] Since the 1990s, it has drawn significant attention
owing to its broad potential applications in various fields, such
as three-dimensional (3D) optical memory and data storage,[2]
3D optical imaging,[3] 3D lithographic microfabrication,[4] and
photodynamic therapy,[5] among many others.[6] As organic
synthesis makes it easy to shape and fine-tune molecules into
desired structures, great efforts have been made to design,
synthesize and investigate organic molecules with TPA to meet
actual requirements. Up to now, several strategies have been
employed to increase the two-photon activities of organic
molecules. Generally, there are three essential components that
are required for two-photon chromophores, namely a p-electron
donor (D), a polarizable p-bridge (p) and a p-electron
acceptor (A). Appropriate combinations and numbers of these
components can result in tailor-made molecules that are typi-
cally grouped into the following classes: dipole,[7] quadru-
pole,[8] dendrimer,[9] octupolar molecules[10] and multibranched
Ó CSIRO 2011
10.1071/CH10229
0004-9425/11/020174

D-p-D0 Sulfur-Containing TPA Chromophores
175
so on. Consequently, three novel D-p-D0 type chromophores
with tert-butyl, benzyl and hydrogen on the sulfur atom were
prepared. All three chromophores were fully characterized, and
structure–property relationships were established. The experi-
mental results confirm that the novel chromophores obtained are
provided with single- and two-photon activities. Efficient blue-
green emissions are obtained with tunable short-pulse lasers;
meanwhile, Stokes’ shifts and solvatochromism similar to those
of asymmetrical dipole D-p-A molecules are observed. Dual
emission in less polar solvents and single strongly solvent-
polarity-dependent emission in more polar solvents are present.
The phenomena are frequently reported for conjugated donor–
acceptor (D-A) or D-p-A compounds[15] and are easily inter-
preted by the widely accepted twisted internal charge transfer
(TICT) model suggested by Grabowski and coworkers.[16]
Furthermore, it was found that different substituents on the
sulfur atom had an influence on the optical properties of the
related molecules. It is notable that the maximum TPA across-
sections of the new model of the chromophores are clearly
located in the range 700–800 nm, which is easily available
from laser sources, compared with those of previous dyes we
reported.[14]
group, the similarities or differences in their absorption spectra
must arise from the terminal groups containing sulfur. It can be
interpreted in the way that the alkyl substituents (in S1–2) affect
the distribution of electronic clouds on the sulfur atom, weak-
ening the electron-donating capacity of the sulfur atom; how-
ever, the hydrogen (in S3) does little to this distribution. As
shown in Figs A1, A2 (Accessory Publication) and Table 1,
the linear absorption spectra of the three chromophores show
almost no solvatochromism, indicating that surrounding solvent
molecules have little influence on the p–p* transition energy of
the molecules.
The single-photon excited fluorescence (SPEF) spectra were
measured at the same concentration as that of the linear absorp-
tion spectra. The SPEF spectra in different solvents were also
measured (Figs 2 and A2), and the wavelength maxima are
clearly located in the blue-green spectral range. From Fig. 1, one
can see that under the same measurement conditions, SPEF
spectra showed a similar trend to linear adsorption: the spectra
S1
S2
S3
50
40
30
20
10
0
0.2
0.1
0.0
Results and Discussion
Linear Absorption and Single-photon Excited
Fluorescence Spectra
The photophysical properties of the three chromophores are
summarized in Table 1. The linear absorption spectra of the
three chromophores were measured in solvents of different
polarity at a concentration of 1 ꢀ 10ꢁ5 mol Lꢁ1. From the
spectra (Fig. 1), peaks at ,360 nm can be readily observed for
S1 and S2, and at 340 nm for S3. The absorptions are ascribed
to the p–p* transition of the p-conjugated molecular system.
Absorption profiles and maximum wavelengths are very similar
for S1–2, and S3 shows the lowest maximum wavelength.
Because the three molecules all have same amine terminal
300
400
500
600
700
800
Wavelength [nm]
Fig. 1. Linear absorption and SPEF (single-photon excited fluorescence)
spectra of S1–3 in DMF.
Table 1. Photophysical properties of S1 3
]
E
^
Compounds
Solvent
lA [nm]
eB (ꢀ104)
lC [nm]
tD [ns]
lF [nm]
S1
DMF
364
360
360
364
360
366
362
336
366
364
345
338
335
340
340
2.18
2.34
2.57
2.31
2.27
1.83
1.98
2.09
1.96
2.04
1.64
1.76
1.91
1.56
1.60
467
0.06
–
0.07
0.05
0.08
0.12
0.15
0.08
0.06
0.07
0.08
0.10
0.06
0.02
0.07
0.04
0.05
–
Ethanol
THF
452
–
448
–
–
Toluene
Cyclohexane
DMF
426, 439
401, 423
468
0.31
–
444, 483
–
S2
S3
0.06
–
–
Ethanol
THF
451
–
448
–
–
Toluene
Cyclohexane
DMF
430, 443
400, 424
461
0.32
–
445, 484
–
0.29
–
–
–
Ethanol
THF
453
465
–
–
Toluene
Cyclohexane
423
1.58
–
470
–
403, 424
Almax of the one-photon absorption spectra.
BCorresponding molar absorption coefficient.
Clmax of the SPEF (single-photon excited fluorescence) spectra.
DLifetime of SPEF (single-photon excited fluorescence).
EFluorescence quantum yield.
Flmax of the TPEF (two-photon excited fluorescence) spectra.

176
Z.-J. Hu et al.
of S1–2 have slight red shifts relative to that of S3, and S1 shows
the most efficient emission. As shown in Figs 2 and A2, it is
obvious that there is solvatochromism for S1–3 in cyclohexane
and other solvents. This hints that the first excited state of the
three chromophores possesses larger dipole moments, causing
strong dipole–dipole interactions between the solute and sol-
vent, as solvatochromism is due to the energy lowering of the
excited state.[17] The change of solvent polarity influences not
only the energy of the emission transition but also the fluores-
cence quantum yields as well as the fluorescence lifetimes
(Table 1). The highest fluorescence quantum yield was observed
in non-polar solvents and the lowest in polar solvents. The
dependence of the fluorescence lifetime on solvent polarity
corresponds to the fluorescence quantum yield. The longest
fluorescence lifetimes were observed in toluene, whereas the
shortest was found in DMF.
As shown in Figs 2 and A2, it is notable that the obtained
chromophores show dual emissions in less polar solvents, and
single strongly solvent-polarity-dependent emissions in more
polar solvents. According to the TICT model,[16] after excitation
into a locally excited state, the dialkylamino group rotates from
the planar conformation into a lower-energy perpendicular
conformation, and the structure isomerizes into a TICT state
(as shown in Fig. 3). The TICT state possesses a larger dipole
moment compared with the ground state, so it is more stable
in more polar solvents. However, because the TICT state may
rapidly non-radiatively transit to a triplet state, it results in a
marked decrease in the fluorescence quantum yield with a rise
in solvent polarity. Clearly, the emissions of the chromophores
with D-p-D0 structure in this work agree with the TICT model,
similarly to the reported D-p-A chromophores.[18] As shown in
Scheme 1, chromophores S1–3 contain both amine and sulfur
electron-donors in a single molecule, but the sulfur group is a
weaker electron donor than the amine, so the D-p-D0 type
chromophores S1–3 show some characteristics of D-p-A mole-
cules and possess stronger solvatochromism and larger dipole
moments in comparison with those of the D-p-D0 model we
reported earlier.[14] The preliminary calculations shown later
in this work for the planar and twisted conformations of the
three novel chromophores also show that they possess dipole
moments for excited states.
125
DMF
100
Ethanol
THF
75
50
25
0
Toluene
Cyclohexane
Two-photon Excited Fluorescence Spectra and Absorption
As shown in Fig. 1, there is no linear absorption in the range
600–800 nm for S1–3. Therefore, on excitation from 600 to
800 nm, it is impossible to produce single-photon excited
fluorescence. The insert in Fig. 4 shows that the relationship
between the logarithms of the two-photon excited fluorescence
(TPEF) intensity (Iout) of S1 and the input laser intensity (Iin) is
an exponential line with a slope of 2.04, which gives experi-
mental evidence of a two-photon excitation mechanism. In order
to determine the wavelength position of maximum two-photon
absorption and the highest up-conversion efficiency, the TPEF
400
450
500
550
600
Wavelength [nm]
Fig. 2. SPEF (single-photon excited fluorescence) spectra of S1 in different
solvents.
θ
TICT
N
N
R
R
Fig. 3. Schematic illustration of twisted internal charge transfer (TICT) state.
OHC
S
S
S1
S2
N
tBuOK
OHC
tBuOK
S
(1) NaBH4
S
N
CHO
CH2PPh3I
N
N
(2) KI, PPh3, HOAc
(1) Hg(ClO4)2·6H2O
(2) H2S
SH
S3
N
Scheme 1. The synthesis of chromophores S1–3.

D-p-D0 Sulfur-Containing TPA Chromophores
177
Slope ꢀ 2.04
80
60
40
20
0
3.4
S1
S2
S3
150
3.2
3.0
120
2.8
Experimental data
Fitted result
2.6
90
2.4
2.0 2.1 2.2 2.3 2.4
lg Iin
60
S1
S2
S3
30
0
400
450
500
550
600
650
700
750
800
850
Wavelength [nm]
Wavelength [nm]
Fig. 4. The TPEF (two-photon excited fluorescence) spectra of S1–3 in
toluene excited at 730 nm; the insert shows the linear dependence of lg Iout of
S1 on lg Iin (Iout: two-photon excited fluorescence intensity; Iin: the input
laser intensity).
Fig. 5. TPA (two-photon absorption) cross-sections (s, 1 GM ¼
10ꢁ50 cm4 s photonꢁ1 moleculeꢁ1) for S1–3 in toluene.
values of S3 could not be observed, but it can be expected that
larger TPA cross-section values will occur below 700 nm.
spectra (1.0 ꢀ 10ꢁ3 mol Lꢁ1 in toluene) were measured on
excitation from 700 to 850 nm and with an intensity of 100 mW.
Blue-green up-conversion fluorescence can be observed
(Fig. 4). One finds that there are similar vibronic structures in
the TPEF and SPEF spectra of the three chromophores. Some
red shifts of the peak positions of TPEF spectra relative to
their corresponding SPEF spectra are attributed to re-absorption
from the fluorescence in the concentrated solution (Table 1). In
addition to the similarities between SPEF and TPEF, the TPEF
peak positions are also independent of the laser wavelength
used. Thus, although the electrons can be pumped to the dif-
ferent excited states by linear absorption or two-photon
absorption (TPA) owing to different selection rules, they would
finally relax to the same lowest excited state via internal con-
version and/or vibration relaxation. The dual emission was also
observed for TPEF spectra in toluene, caused by a TICT process
similar to that in SPEF.
Theoretical Calculation of Charge-transfer State
The experimental results above proved that the chromophores
S1–2 have similar single- and two-photon absorption properties,
a little different from those of S3. To obtain more details about
structure and properties relationships, theoretical calculations
of the CT state were also carried out for S1–3. We used the
GAUSSIAN98 program package to optimize the molecular
geometrical structure with the hybrid density functional theory
(DFT/B3 LYP) and a basis set 6-31G. Our ab initio calculations
using time-dependent hybrid density functional theory for the
S1–3 molecules showed that the first excited state was the CT
state with the excited energy corresponding to ,350 nm. For a
better understanding of the CT process, the charge density dif-
ferences between the ground and CT states in the gas phase were
plotted for the three molecules (as shown in Fig. 6), visualized
with the MOLEKEL program.[21] From Fig. 6, it can be seen that
the substituents on the sulfur atom in S1–3 have taken part in the
CT process and affect the electron distribution on the sulfur
atom, agreeing with the experimental observations that the
substituents on the sulfur atom have an influence on the single-
or two-photon absorption of the three chromophores. As shown
in Fig. 6, the sulfur atom in S3 shows a certain ‘electron-
donating’ capacity, which is different from that in S1 and S2,
and also leads to a smaller dipole moment of the excited state.
Furthermore, it demonstrates that charge densities are deloca-
lized in the full single molecule, which is attributed to the high
planarity of S1–3.
The TPA cross-sections were measured using the two-
photon-induced fluorescence measurement technique with the
following equation:[19]
Fref cref nref
F
s ¼ sref
ð1Þ
F
c
n Fref
Here, the subscript ref stands for the reference molecule;
s is the TPA cross-section value; c is the concentration of the
solution; n is the refractive index of the solution; F is the
integrated area of the detected two-photon-induced fluorescence
signal, and F is the fluorescence quantum yield. The sref values
for fluorescein in H2O (pH ,13) were taken from reference for
comparison.[20] It should be noted that both the largest single-
photon oscillator strength and the maximum TPA cross-section
take place on the same excited state, the charge-transfer (CT)
state, in the lower-energy region owing to the asymmetrical
characteristics of the present molecules. Fig. 5 shows the TPA
spectra of molecules S1–2 with maximum absorption from 700
to 850 nm. This is because a stronger amine donor causes an
obvious red shift in the maximum TPA compared with a
carbazol donor in the previously reported molecules.[14] Owing
to the limitations of our detecting equipment, the TPA peak
Conclusions
Three novel unsymmetrical D-p-D0 chromophores containing a
sulfur substituent donor were obtained via the solvent-free
Wittig–Horner reaction. Structure–property relationships were
established. The theoretical and experimental investigations
revealed that the three chromophores obtained present good
single- and two-photon activities. Furthermore, it was found that
dual emissions in less polar solvent, Stokes’ shifts and solva-
tochromism, which are similar to those of asymmetrical dipole
D-p-A molecules, and the linear absorption spectra were less

178
Z.-J. Hu et al.
equipped with a 450-W Xe lamp and a time-correlated single-
photon counting (TCSPC) card. All the fluorescence spectra
were collected. The SPEF quantum yields were measured using
a standard method under the same experimental conditions
for all compounds. Coumarin 307 dissolved in ethanol
(F ¼ 0.56),[22] at the same concentration as the other samples,
was used as the standard. Lifetime values were obtained by
reconvolution fit analysis of the decay profiles with the aid of
F900 analysis software. The fitting results were judged by their
values of ‘reduced chi-squared’. TPEF spectra were measured
using a mode-locked Ti:sappire femtosecond laser (Spectra-
Physics, Tsunami 3941, 700–910 nm, 80 MHz,o120 fs), which
was pumped by a compact CW ProLite diode laser (Spectra-
Physics, Millennia Pro 5S). The fluorescence signal was
recorded with a fluorescence spectrophotometer (Ocean Optics,
USB2000). All measurements were carried out in air at room
temperature. TPA cross-sections were measured using the two-
photon-induced fluorescence measurement technique.
Synthesis and Characterization
The synthetic routes for the three chromophores S1, S2 and S3
are outlined in Scheme 1. 4-(N,N-diethylamino)benzaldehyde,
(4-(diethylamino)benzyl)triphenylphosphinium iodide,[23] 4-
(benzylthio)benzaldehyde and 4-(tert-butylthio)benzalde-
hyde[24] were synthesized according to references. It is
important to note that the tert-butyl group on the sulfur atom in
S2 could be easily replaced by a hydrogen atom in the given
deprotection reaction, leading to compound S3, and the solvent-
free Wittig reaction was used to produce the trans isomer.[25]
Fig. 6. Density difference between the charge-transfer and ground states of
S1–3 in the gas phase; grey and black areas represent the electron loss and
gain, respectively, on excitation.
sensitive to the solvent polarity than the corresponding fluor-
escence spectra, which showed pronounced solvatochromic
effects. It can be concluded that different substituents on the
sulfur atom have an influence on the properties of the molecule
in the appointed framework, and D-p-D0 molecules can possess
a certain dipole moments similarly to D-p-A. The donor with
different electron-donating capacities can be employed to tune
the range of maximum TPA of D-p-D0 molecules. Therefore,
the D-p-D0-type molecules should be a supplement to D-p-A-
and D-p-D-type molecules to enrich the range of simple and
practical TPA materials.
(E)-4-(4-(Benzylthio)styryl)-N,N-diethylbenzenamine S1
t-BuOK (3.36 g, 30 mmol) was placed in a dry mortar and
milled to very tiny particles, then (4-(diethylamino)benzyl)
triphenylphosphinium iodide (5.51 g, 10 mmol) and 4-
(benzylthio)benzaldehyde (2.28 g, 10 mmol) were added and
mixed. The mixture was milled vigorously for ,15 min, and
became sticky. Anhydrous Na2SO4 (1 g) was added, and then the
mixture was continuously milled for additional 10 min. After
completion of the reaction (monitored by TLC), the mixture was
dispersed in water (100 mL). The solution was extracted with
CH2Cl2 three times. The organic layer was washed with water,
then saturated brine, and dried over anhydrous magnesium
sulfate. The solvent was removed under vacuum; the residue
was purified by silica gel chromatography,with petroleum/ethyl
acetate (50:1) as eluent. The strongly green fluorescent product
was collected to give product S1 (2.61 g, yield 70%). m/z 373.17,
268.12 (100%). m/z (ESI) [M þ 1]þ 374.2. dH ([D6]DMSO,
400 MHz) 7.43 (d, 2H, J 8.27), 7.40–7.33 (m, 4H), 7.33–7.26
(m, 4H), 7.26–7.20 (m, 1H), 7.07 (d, 1H, J 16.39), 6.87 (d, 1H,
J 16.37), 6.65 (d, 2H, J 8.66), 4.23 (s, 2H), 3.36 (dd, 4H, J 7.38,
7.37), 1.10 (t, 6H, J 6.97).
Experimental
Chemicals
All chemicals were available commercially and every solvent
was purified by conventional methods before use. tert-Butyl-
thiol, benzylthiol and 4-fluorobenzaldehyde were purchased
from Aldrich. NaOH, KI, FeS, NaBH4, HOAc, Hg(ClO4)2ꢂ
6H2O, tBuOK, triphenylphosphine and diethylamide were pur-
chased from Shanghai Reagents.
Instruments
The NMR spectra, recorded at 258C using a Bruker Avance
spectrometer, are reported in ppm relavtive to TMS (d). Cou-
pling constants J are given in Hertz. Mass spectra were deter-
mined with a Micromass GCT-MS (EI (electron ionization)
source). Electrospray ionization mass spectra (ESI-MS) were
acquired with a model LCQ ion-trap mass spectrometer
(Finnigan) equipped with a Finnigan MAT electrospray ion
source. IR spectra were recorded on a NEXUS 870 (Nicolet)
spectrophotometer in the 400–4000 cmꢁ1 region using a powder
sample in a KBr plate.
(E)-4-(4-(tert-Butylthio)styryl)-N,N-
diethylbenzenamine S2
This was prepared similarly to S1 via (4-(diethylamino)
benzyl)triphenylphosphinium iodide and 4-(tert-butylthio)
benzaldehyde to afford the solid product S2 (2.21 g, yield
65%). m/z 339.20, 268.11 (100%). dH ([D6]DMSO, 500 MHz)
7.42 (d, 2H, J 8.68), 7.52 (d, 2H, J 8.06), 7.24 (d, 2H, J 8.11),
7.16 (d, 1H, J 16.32), 6.82 (d, 1H, J 16.33), 6.66 (d, 2H, J 8.61),
3.36 (m, 4H), 1.24 (s, 9H), 1.10 (t, 6H, J 6.95).
UV-vis absorption spectra were recorded on a UV-3100
spectrophotometer. Fluorescence measurements were carried
out with an Edinburgh FLS920 fluorescence spectrometer

D-p-D0 Sulfur-Containing TPA Chromophores
179
(E)-4-(4-(Thio)styryl)-N,N-diethylbenzenamine S3
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S2 (2.04 g, 6 mmol) was dissolved in CH2Cl2 (25 mL). A
12-mmol portion of Hg(ClO4)2ꢂ6H2O in methanol (10 mL) was
added under stirring. A yellow precipitate appeared immedi-
ately. The reaction mixture was stirred for additional 30 min;
thereafter, freshly prepared H2S was bubbled into the solution
and a black precipitate of HgS appeared. The mixture was
filtered and the solid was washed with CH2Cl2. The organic
solution was washed with water (2 ꢀ 50 mL), 5% NaHCO3
(50 mL), and water (50 mL) and dried over anhydrous magne-
sium sulfate. The solvent was removed at 408C under vacuum
and the product was crystallized from hexane/2-propanol to give
S3 (1.03 g, yield 61%). m/z 284.14, 268.12 (100%). m/z (ESI)
[M þ 1]þ 340.20. nmax (KBr)/cmꢁ1 2550 (nS–H). dH ([D6]
DMSO, 500 MHz) 7.39 (d, 2H, J 8.18), 7.36 (d, 2H, J 8.66),
7.24 (d, 2H, J 8.11), 7.04 (d, 1H, J 16.36), 6.85 (d, 1H, J 16.36),
6.65 (d, 2H, J 8.33), 3.35 (m, 4H), 1.09 (t, 6H, J 6.92).
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Acknowledgements
This work was supported by grants from the National Natural Science
Foundation of China (20771001, 20875001 and 50873001), the Department
of Education of Anhui Province (KJ2010A030) and Scientific foundation of
Anhui Province for Innovation Team (2006K007TD).
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