Synthesis and photovoltaic properties of novel C60 bisadducts based on benzo[2,1,3]-thiadiazole

Article abstract of DOI:10.1016/j.tet.2014.01.026

A novel C₆₀ solar cell acceptor (BTOQC, benzo[2,1,3]-thiadiazole-o-quinodimethane-C₆₀ bisadducts) based on benzo[2,1,3]thiadiazole has been synthesized as model to study how the thiadiazole group will affect the device performance in bulk heterojunction organic photovoltaics (BHJ-OPV) with poly(3-hexylthiophene) (P3HT) as donor. The optoelectronic, electrochemistry, and photovoltaic properties of the novel bisadduct BTOQC have been fully investigated. The best device performance of this fullerene derivative in our research was obtained as 2.50% with a high V_oc of 0.74 V.

Full text of DOI:10.1016/j.tet.2014.01.026

Tetrahedron xxx (2014) 1e5
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and photovoltaic properties of novel C60 bisadducts based
on benzo[2,1,3]-thiadiazole
,
Wangqiao Chen a b, Qian Zhang c, Teddy Salim a, Sandy Adhitia Ekahana d,
Xiangjian Wan c, Tze Chien Sum d, Yeng Ming Lam a, Alfred Hon Huan Cheng d,
,
a,b,
*
Yongsheng Chen c , Qichun Zhang
*
a School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
b Institute for Sports Research, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
c Institute of Functional Polymer Materials, Center for Nanoscale Science & Technology, Nankai University, Tianjin 300071, PR China
d School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel C60 solar cell acceptor (BTOQC, benzo[2,1,3]-thiadiazole-o-quinodimethane-C60 bisadducts)
based on benzo[2,1,3]thiadiazole has been synthesized as model to study how the thiadiazole group will
affect the device performance in bulk heterojunction organic photovoltaics (BHJ-OPV) with poly(3-
hexylthiophene) (P3HT) as donor. The optoelectronic, electrochemistry, and photovoltaic properties of
the novel bisadduct BTOQC have been fully investigated. The best device performance of this fullerene
derivative in our research was obtained as 2.50% with a high Voc of 0.74 V.
Received 19 November 2013
Received in revised form 6 January 2014
Accepted 13 January 2014
Available online xxx
Keywords:
Photovoltaic
C60
Ó 2014 Elsevier Ltd. All rights reserved.
Solar cell acceptor
Benzo[2,1,3]thiadiazole
1. Introduction
relatively slow. Although [6,6]-phenyl-C61-butyric acid methyl es-
ter (PC61BM) has been widely used in OSCs due to its high electron
Organic solar cells (OSCs) have become one of the most active
research areas due to their promising advantages, such as simple
structure, clean and renewable energy source, low cost, light
weight, and mechanical flexibility.1e4 Normally, an OSC consists of
a photoactive blend layer (this layer is typically made of conjugated
polymers as donors and fullerene derivatives or other molecules as
acceptors), which is sandwiched between a transparent indium-
mobility and compatibility with various conjugated polymer/small
molecule donor materials,19 there are several limitations associated
with PC61BM compound: (a) PC61BM has weak absorption in the
visible region;20 (b) the Lowest-Unoccupied Molecular Orbital
(LUMO) energy level of PC61BM is relatively low when it is used in
the OSCs based on the conjugated donor with a higher Highest-
Occupied Molecular Orbital (HOMO) energy level; (c) PC61BM has
high crystallinity, which may readily promote inter-molecular
aggregation;21e23 and (d) the interaction between PC61BM and
conjugated donors is typically weak due to the lack of any func-
tional groups on C60. To address these limitations, one strategy is to
modify C60 acceptor with functional groups to widen the absorp-
tion in the visible range and to enhance the whole absorption of the
device. For instance, PC71BM-based OSCs usually exhibits higher
PCE than PC61BM in the same condition due to the stronger ab-
sorption in the visible region.24 Recently, Mikroyannidis et al.25,26
replaced the methoxy (eOMe) group of the PC61BM with 4-nitro-
tin-oxide (ITO) electrode and
a low work function metal
electrode. Although several strategies, such as control of the mor-
phology of active layer,5e8 modification of the electrode9,10 and
adjustment of donor-to-acceptor ratio,11 have been strongly
addressed in the past decades, the fundamental driving force to
obtain higher PCE (Power Conversion Efficiency) is still laid on the
design and synthesis of novel donors and acceptors.12
For donor materials, greater successes have been witnessed in
the syntheses of novel materials with broad light absorptions, low
bandgaps, high hole mobilities, and finely-tuned electronic energy
levels.13e18 However, the research progress in new acceptors is
40-hydroxy-
a-cyanostilbene and found that the PCE could be in-
creased from 2.93% to 5.25%. Another way is to increase the LUMO
energy level of the acceptor materials through multi-
functionalization, which can desirably increase the open-circuit
* Corresponding authors. E-mail address: qczhang@ntu.edu.sg (Q. Zhang).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.01.026
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2
W. Chen et al. / Tetrahedron xxx (2014) 1e5
voltage (Voc) of OSCs device because Voc is proportionally de-
termined by the difference between the LUMO energy level of the
acceptor and the HOMO energy level of the donor.27 Recently,
a series of fullerene derivatives with higher LUMO levels than
PC61BM have been employed as acceptors and poly(3-
hexylthiophene) (P3HT) as the donor under 1 sun illumination in
OSCs to give promising results. For instances, Lenes et al.28,29 used
a new [60]PCBM bisadduct (bis-PCBM) (whose LUMO level is 0.1 eV
higher than that of PC61BM) as acceptor and P3HT as donor in OSCs,
resulting in Voc and PCE of 0.73 V and 4.5%, respectively, which are
much higher than Voc (0.58 V) and PCE (3.4%) of the P3HT:PC61BM-
based solar cell prepared under the same conditions. He et al.30 also
reported a new indene-functionalized C60 bisadduct (ICBA), whose
LUMO energy level is 0.17 eV higher than PC61BM. The optimized
OSCs based on P3HT:ICBA have a high Voc (0.84 V) with a PCE as
high as 6.48%. Other successful examples, such as dihydronaphthyl-
C60 bisadduct (NCBA), dihydronaphthyl-C70 bisadduct (NC70BA),
thieno-o-quinodimethane-C60 (bis-TOQC) and di(4-methylphenyl)-
methano-C60 bisadduct (DMPCBA) have also been reported.31e35
Although the remarkable progress in bisadduct fullerene system
has been made, there is still plenty of room for developing new
efficient acceptors through reasonable chemical functionalization
of fullerene. One promising strategy is to introduce the special
functional groups to generate the suitable interaction (e.g., S/S
interaction) between the donor and acceptor to adjust the packing
or to render the separation of electron and hole more readily within
the interface.36e39 In this work, we design and synthesize a novel
benzo[2,1,3]thiadiazole-containing fullerene derivative (benzo
The UVevis spectra of BTOQC and the reference PC61BM are
shown in Fig. 1. Both fullerene derivatives display a strong UV ab-
sorption from 200 to 350 nm in chloroform, while BTOQC shows an
additional enhanced absorption in the 380e500 nm range. Much
same as the UVevis absorption of P3HT:PC61BM (Fig. S5), the solid-
state absorption of P3HT:BTOQC film shows the features of both
components. After annealing, the maximum absorption peak of the
P3HT:BTOQC blend red-shifts for about 25 nm and shoulders at 545
and 600 nm becomes more pronounced, indicating that the heat
treatment enhances P3HT crystallinity. The red-shifted absorption
of the blend may contribute to the improvement of the device
performance.
Fig. 1. UVevis absorption spectra for: (a) BTOQC and PC61BM in chloroform (10ꢁ5 mol/
L); (b) thin film of mixture of P3HT and BTOQC.
In Fig. 2, The DSC measurement indicates that there is no crys-
tallization transition below 350 ꢀC for BTOQC, while PC61BM has
crystallization peaks at ca. 245 ꢀC and 283 ꢀC. Thus it is expected that
this new acceptor is able to overcome thermally driven crystalliza-
tion and achieve high thermal stability in the as-fabricated OSCs.
[2,1,3]thiadiazole-o-quinodimethane-C60
bisadducts-(BTOQC))
through the DielseAlder reaction to study how the hetero-atom
(sulfur atom) on the acceptor will affect the final device perfor-
mance. Meanwhile, the investigation on the synergistic influence of
the LUMO energy levels and the absorbance on the final photo-
voltaic performance of the fullerene acceptors are also discussed.
Our OSCs based on P3HT:BTOQC can show a reasonable PCE of
2.50% with a high Voc of 0.74.
2. Results and discussion
The DielseAlder reaction of fullerene provides a powerful
method for fullerene functionalization.40,41 The synthetic route of
BTOQC is shown in Scheme 1. The 5,6-bis(bromomethyl)benzo
[2,1,3]thiadiazole was used as the precursors for benzo[2,1,3]thia-
diazole-o-quinodimethane dienes and prepared according to the
literature.42,43 BTOQC was prepared through the [2þ4] cycloaddi-
tion reaction of C60 and the in situ generated benzo[2,1,3]
thiadiazole-o-quinodimethane dienes by a published method.44,45
We employed 2 equiv of precursor for the reactions in order to
render the BTOQC as the main product. The yield for bisadduct
BTOQC is 33%.
Fig. 2. Differential scanning calorimetry (DSC) measurement of BTOQC.
The electrochemical properties of BTOQC were studied by cyclic
voltammetry (CV). Fig. 3 shows the CV of BTOQC as well as that of
PC61BM for comparison. Both BTOQC and PC61BM exhibit three
quasi-reversible reduction waves in the negative potential range
from 0 to ꢁ2.0 V versus Ag/Agþ. Table 1 lists the half-wave po-
tentials (defined as E¼0.5[Ep,cþEp,a], where Ep,c is the cathodic peak
potential and Ep,a the corresponding anodic peak potential) of the
reduction processes of the fullerene derivatives BTOQC and PC61BM
for comparison. It can be clearly observed that the first (E1), second
(E2) and third (E3) reduction potentials of BTOQC all shifted nega-
tively compared to those of PC61BM, and the onset reduction po-
tential of BTOQC (ꢁ0.75 V) is also negatively shifted compared to
PC61BM (ꢁ0.54 V). Moreover, the LUMO energy levels of the ful-
lerene derivatives can be calculated from the onset reduction po-
tentials (Eorend) according to the following equation,46
À
Á
on
ELUMO ¼ ꢁe E þ 4:71 ½eVꢂ
Scheme 1. Synthetic route of BTOQC.
red
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W. Chen et al. / Tetrahedron xxx (2014) 1e5
3
As shown in Table 2, the optimized device performance was
achieved at weight ratio (P3HT:BTOQC) of 1.0:0.6. The compared
P3HT:PC61BM device (1.0:0.6) is also included in the table for
comparison (entries 1, 2). The as-cast 1.0:0.6 device that was not
subjected to any heat treatment resulted in low power conversion
efficiency (PCE) of 0.777% (entry 3), with open-circuit voltage (Voc
)
of 0.96 V, short-circuit current (Jsc) of 2.55 mA/cm2 and fill factor
(FF) of 0.317. The effect of higher LUMO level (compared to that of
PC61BM) can be seen in the high Voc of the device (close to 1 V).
Thermal annealing at 150 ꢀC for 10 min significantly improved the
PCE by more than threefold to 2.50% (entry 4). Similar improve-
ment was also observed for the device with blend ratio of 1.0:0.8,
although the PCE increase was only twofold (entries 5, 6). The in-
crease in Jsc can be attributed to the more red-shift absorption
(Fig. 1) of the blend film after the heat treatment as well as the
enhancement in charge transport. Heat treatment is expected to
induce self-organization of both P3HT and BTOQC molecules,
resulting in more efficient charge percolation pathways for both
holes and electrons.47,48 The improved charge transport can be
attributed to the increased FF, which suggests a decrease in device
series resistance. Thermal annealing also resulted in a decrease in
Voc of the devices from 0.96 to 0.74 V. This possibly associates with
the upward shift of P3HT HOMO level49 or may be resulted from the
property difference of the acceptor in the active layers. In addition,
although we believe that regioisomers could have some impacts on
PCE, it is impossible for us to separate these isomers with column
because of their similar polarities. Due to our current limited ex-
perimental conditions, we can’t give more comments on the re-
lationship between regioisomers and their impact on PCE.
Fig. 3. Cyclic voltammograms of PC61BM and BTOQC in o-dichloro-benzene-
eacetonitrile (5:1 v/v) with 0.1 M Bu4NPF6 at 100 mV/s.
Table 1
Electrochemical properties of BTOQC and PC61BM
on
Acceptor
E1 [V]
E2 [V]
E3 [V]
E
[V]
LUMO
red
PC61BM
BTOQC
ꢁ0.60
ꢁ0.82
ꢁ1.00
ꢁ1.15
ꢁ1.50
ꢁ1.60
ꢁ0.54
ꢁ0.75
ꢁ4.17
ꢁ3.96
on
where the unit of E is V versus Ag/Agþ. With this equation, the
red
LUMO energy levels of BTOQC and PC61BM are calculated as ꢁ3.96
and ꢁ4.17 eV, respectively. Obviously, the LUMO level of BTOQC is
increased by ca. 0.21 eV in comparison with that of PC61BM. Un-
doubtedly, the higher LUMO energy level of BTOQC is desirable for
its application as an acceptor in P3HT-based OSCs because it is
expected to increase the open-circuit voltage of the devices.
To evaluate the new BTOQC acceptor, bulk heterojunction solar
cells based on the ITO/PEDOT:PSS/P3HT:Acceptor/LiF/Al configu-
ration were fabricated and characterized under simulated 100 mW/
cm2 AM 1.5 G illumination. The energy level diagram and current
densityevoltage (JeV) curves of the devices are shown in Fig. 4,
including acceptor PC61BM as the comparison group. The corre-
sponding device characteristics with the optimal blending ratio are
shown in Table 2.
The surface morphology of the blends was investigated using
tapping-mode atomic force microscopy (AFM) (Fig. 5). The height
image of the non-annealed (NA) P3HT:BTOQC blend is smooth and
featureless. Upon heat treatment, the blend surface becomes
rougher where the root-mean-square roughness (rms) increases
from 0.95 nm to 1.29 nm (Fig. S6). The surface of the non-annealed
(NA) P3HT:BTOQC blend reveals the nanodomains of the active
materials. After the thermal annealing (TA), these domains become
Fig. 4. (a) Schematic energy level diagram and (b) current densityevoltage charac-
teristics of ITO/PEDOT:PSS/P3HT:Acceptor/LiF/Al devices under illumination of AM
1.5 G, 100 mW/cmꢁ2
.
Table 2
Photovoltaic performance of the P3HT-based OSCs with BTOQC
Entry
Acceptor
Weigh ratioa
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
1
PC61BM
PC61BM
BTOQC
BTOQC
BTOQC
BTOQC
1.0:0.6
1.0:0.6
1.0:0.6
1.0:0.6
1.0:0.8
1.0:0.8
0.72
0.56
0.96
0.74
0.90
0.74
3.60
9.78
2.55
7.11
3.61
6.17
0.35
0.56
0.317
0.476
0.338
0.492
0.906
3.04
0.777
2.50
1.10
2.24
2b
3
4b
5
6b
Fig. 5. Tapping-mode atomic force microscopy (AFM) phase images: (a, b)
P3HT:BTOQC (1.0:0.6, w/w) and (c, d) P3HT:PC61BM (1.0:0.6, w/w) with (a, c) non-
a
Weight ratio of P3HT:Acceptor.
Annealed devices.
b
annealed and (b, d) thermally annealed. The scan size is 2
mmꢃ2
mm.
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4
W. Chen et al. / Tetrahedron xxx (2014) 1e5
less obvious and the blend surface becomes rougher, suggesting
possible domain rearrangement.50,51 The presence of the fibre-like
structures may contribute to the improvement in the charge
transport,52 which eventually increase the device performance in
the thermally annealed device.
lower polaron signal than PCBM. This relative value indicates the
number of the generated excitons, which have undergone charge
separation into the acceptor side to be negative polarons.55 Thus,
from this relative value, it can be inferred that P3HT:BTOQC (1.0:0.6,
TA) may perform slightly worse than P3HT:PCBM (1.0:0.6, TA) and
it agrees with the performance result as discussed before.
The morphology of as-cast P3HT:PC61BM blend looks similar to
that of the P3HT:BTOQC. However, thermal annealing at 150 ꢀC
greatly enhanced the phase separation in the P3HT:PC61BM blend.
Consequently, needle-like PC61BM crystals (which are a few mi-
crons in length) appeared in the heat-treated blend, which is
common in PC61BM system. The lack of such structures in the heat-
treated P3HT:BTOQC blend may account for the improvement of
the device performance after thermally annealing. Nevertheless,
the lackness of the ability for the P3HT:BTOQC to phase separation
sufficiently could also limit its performance because the preferen-
tial interaction between BTOQC and P3HT molecules may interrupt
the self-organization of both donor and acceptor molecules and,
hence, lower the effectiveness of the charge transport in the device.
In the next section, the charge dynamics of the P3HT:BTOQC will be
discussed to further understand the interaction between the donor
and acceptor.
The analysis was extended further with the fs-TAS spectra at
near infrared (NIR) to probe the dynamics of the polarons, which
will elucidate the behaviour of the charge transfer process from
P3HT to the acceptor. The spectra at this range are shown in Fig. 6(a,
b) where the polaron signal is the long lived broad signal around
1000 nm. The first few picoseconds of the signal at this NIR range is
dominated by the exciton dynamics, which involves the exciton
recombination from P3HT domain, germinate pairs and non ger-
minate pairs, which will last around 100 ps.53 Thus, by subtracting
the pure contribution of exciton dynamics (1200 nm) from the
signal at 1000 nm, we can reveal the pure polaron dynamics, which
is a negative polaron in this NIR region.54
3. Conclusion
We have successfully synthesized a new thiadiazole-o-quino-
dimethane-C60 bisadducts (BTOQC), which have been explored as
an acceptor in P3HT-based PSCs. This material shows device per-
formance of 2.50% with a high Voc 0.74 V when the weight ratio of
P3HT:BTOQC is 1.0:0.6. Although the PCE is moderate due to the
phase separation, however, the benzo[2,1,3]thiadiazole is a good
motif for further introduction of various functional groups. Thus, it
may have great potential for further applications on optoelectronic
devices based on various
materials.
p-conjugated conducting organic
4. Experimental section
4.1. Materials
All chemicals were purchased from commercial sources Sig-
maeAldrich etc. and used without further purification and all sol-
vents were purified and freshly distilled prior to use.
4.2. General measurement and characterization
1H NMR and 13C NMR spectra were measured on Bruker DMX-
300 spectrometers. Chemical shifts of NMR are reported in parts
per million relative to the singlet of TMS at 0 ppm for 1H NMR
spectroscopy. Absorption spectra were taken on a SHIMADZU UV-
2501PC UVeVis spectrophotometer. Electrochemical cyclic vol-
tammetry (CV) was conducted on a CHI 660C Electrochemical
Workstation. In the CV measurement, a Pt disk was used as the
working electrode, Pt wire as the counter electrode, and Ag/Agþ
electrode (Satured KCl solution) as the reference electrode were
used in a mixed solution of o-dichlorobenzene: acetonitrile (5:1 v/
v) with 0.1
M
tetrabutylammonium hexafluorophosphate
(NBu4PF6) at 100 mV sꢁ1. The differential scanning calorimetry
(DSC) analysis of fullerene derivatives was performed under a ni-
trogen atmosphere on an Instrument TA Q-10 at heating rates of
10 ꢀC minꢁ1. Tapping-mode atomic force microscopy (AFM) mea-
surement was done with MFP-3D (Asylum Research). White light
pump probe measurement was done with custom built white light
pump probe (WLPP) setup. The pump laser used was generated
from TOPAS (Coherent, Inc.) with laser pulse of 100 fs, repetition
1 kHz. The probe laser was generated from a fundamental wave-
length of 800 nm (1 kHz) by a white light generation from a sap-
phire crystal with thickness of 2 mm.
Fig. 6. (a, b): fs-TAS (transient absorption spectra) at near infrared region of
P3HT:BTOQC (1.0:0.6, TA) and P3HT:PCBM (1.0:0.6, TA) blend sample for comparison
under the same sample preparation condition; (c, d): decay profile comparison for
pure polaron level between P3HT:BTOQC (1.0:0.6, TA) and P3HT:PCBM (1.0:0.6, TA) at
short time scale and raw signal at 1000 nm, respectively, with an insert at first two
picoseconds.
4.3. Synthesis of BTOQC
A
mixture of C60 (250 mg, 0.35 mmol,
1
equiv), 5,6-
240 mg,
bis(bromomethyl)-benzo[2,1,3]thiadiazole (P2,
0.75 mmol, 2.1 equiv to C60), potassium iodide (500 mg, 3.0 mmol,
4 equiv to bis(bromomethyl)-benzo[2,1,3]-thiadiazole), and 18-
crown-6 (790 mg, 3.0 mmol, 1 equiv to potassium iodide) was
dissolved in anhydrous o-dichlorobenzene at reflux temperature
for 48 h under an argon atmosphere in dark. After cooling to room
temperature, the solvent was removed under vacuum. Afterwards,
the solid was transferred into methanol (10 mLꢃ3), sonicated and
centrifuged. The residual solid was purified by taking chloroform as
the eluent through column chromatography on silica gel. The
The pure polaron signal is shown in Fig. 6c where the
P3HT:PCBM (1.0:0.6, TA) polaron signal has a longer rise time
(10 ps) as compared to the P3HT:BTOQC (1.0:0.6, TA) (4 ps). This
faster rise time may be attributed to the higher LUMO level of
BTOQC than PCBM as discussed in the previous section. At longer
lifetime (Fig. 6d), it can be observed that both P3HT:BTOQC (1.0:0.6,
TA) and P3HT:PCBM (1.0:0.6, TA) have relatively similar polaron
level and similar polaron lifetime where the BTOQC has a slightly
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W. Chen et al. / Tetrahedron xxx (2014) 1e5
5
separated bisadduct (regioisomers) was 122 mg, yield, 33%. 1H NMR
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(300 MHz, C6D4Cl2)
d: 8.35e7.89 (m, 4H), 4.96e3.89 (m, 8H),
MALDI-TOF MS: calculated for C76H12N4S2þHþ, 1045.0582; found:
1045.0585 (Mþ). The solubility of BTOQC in o-DCB or chloroben-
zene is about 15e20 mg/mL.
4.4. Fabrication and characterization of OSCs
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A
conventional OSC structure based on ITO/PEDOT:PSS/
P3HT:BTOQC/LiF/Al was used. The ITO glass was cleaned by se-
quential ultrasonic treatment in detergent, deionized water, ace-
tone, and isopropyl alcohol under ultrasonication for 15 min,
respectively, and subsequently dried by a nitrogen blow. Then
PEDOT:PSS (poly(3,4-ethylene dioxythiophene):poly(styrene sul-
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fonate)) (Clevios PVPAI 4083) was filtered through a 0.45 mm filter
and spin-coated at 3000 rpm for 20 s on the ITO electrode, followed
by thermal annealing at 150 ꢀC for 20 min. Then the substrate was
transferred to an argon filled glove box. The blend solution of P3HT
(10 mg/mL) and BTOQC in chlorobenzene with different weight
ratio was then spin-coated on top of the PEDOT:PSS layer at
1500 rpm for 30 s. Before cathode deposition, the films were
thermally annealed at 150 ꢀC for 10 min. Finally, 1 nm LiF and 80 nm
Al layer were deposited on the active layer under high vacuum
(<2ꢃ10ꢁ4 Pa). The device active area of the OSC is ca. 4 mm2. For
comparison, P3HT/PC61BM (1.0:0.6, w/w, in chlorobenzene) based
solar cell with the same device structure was also fabricated. The
current densityevoltage (JeV) curves of the photovoltaic devices
were obtained by a Keithley 2400 source-measure unit. The pho-
tocurrent was measured under simulated illumination at 100 mW/
cm2 AM 1.5G irradiation using a xenon-lamp-based solar simulator
(Oriel 96,000 (AM 1.5G)) in an argon filled glove box. The simulator
irradiance was calibrated using a certified silicon diode.
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Please cite this article in press as: Chen, W.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.01.026