Donor-acceptor organic sensitizers assembled with isoxazole or its derivative 3-oxopropanenitrile

Article abstract of DOI:10.1016/j.tet.2010.03.085

A series of new donor-acceptor organic sensitizers LJ5-LJ7 assembled with isoxazole or its derivative 3-oxopropanenitrile was synthesized and used in dye-sensitized solar cells to study the effect of both π-spacers and anchoring groups on device performance. X-ray crystal structure analyses show planar arrangement of dithiophene and isoxazole fragments, providing an excellent π conjugation and hence the large electronic coupling between the triphenylamine donor and electron acceptor units. For the DSSC applications, the derived photophysical and photovoltaic properties revealed a lower η value of 1.06% (LJ7) and 2.41% (LJ5), and 3.30% (LJ7) for the dyes with rhodanine-3-acetic acid, cyanoacetyl group, and vinyl cyanoacetic acid as the anchor. Electrochemical impedance spectroscopy was used to determine the interfacial charge transfer process occurring in solar cells that employed different dyes. Crown Copyright

Full text of DOI:10.1016/j.tet.2010.03.085

Tetrahedron 66 (2010) 4223–4229
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Donor–acceptor organic sensitizers assembled with isoxazole or its derivative
3-oxopropanenitrile
,
b
b
Yi-Tsung Li a, Chao-Ling Chen a, Yu-Yen Hsu a, Hui-Chu Hsu a, Yun Chi a , Bo-So Chen , Wei-Hsin Liu ,
*
,
Cheng-Hsuan Lai b, Tsung-Yi Lin b, Pi-Tai Chou b
*
a Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan, People’s Republic of China
b Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new donor–acceptor organic sensitizers LJ5–LJ7 assembled with isoxazole or its derivative
Received 28 January 2010
Received in revised form 16 March 2010
Accepted 23 March 2010
3-oxopropanenitrile was synthesized and used in dye-sensitized solar cells to study the effect of both
p
-spacers and anchoring groups on device performance. X-ray crystal structure analyses show planar
arrangement of dithiophene and isoxazole fragments, providing an excellent conjugation and hence
the large electronic coupling between the triphenylamine donor and electron acceptor units. For the
DSSC applications, the derived photophysical and photovoltaic properties revealed a lower value of
p
Available online 27 March 2010
h
Keywords:
Dye-sensitized solar cell
Dithiophene
Isooxazole
Cyanoacetic acid
1.06% (LJ7) and 2.41% (LJ5), and 3.30% (LJ7) for the dyes with rhodanine-3-acetic acid, cyanoacetyl group,
and vinyl cyanoacetic acid as the anchor. Electrochemical impedance spectroscopy was used to
determine the interfacial charge transfer process occurring in solar cells that employed different dyes.
Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction
may also serve as an antenna, the lower lying electron transfer
absorption bands could be broad, intense in the visible region,
The dye-sensitized solar cells (DSSCs) have attracted consider-
able attention as a promising photovoltaic technology during the
past two decades owing to their prospect of high energy conversion
efficiency and low production costs.1 Upon optimization, a few
photosensitizers based on ruthenium metal complexes have now-
which is pivotal for producing a large photocurrent response. The
resulting electron (or charge) transfer to the acceptor group may
subsequently facilitate rapid electron injection from the sensitizers
into the conduction band of a wide band-gap semiconductor, such
as TiO2, the process of which is essential for making efficient DSSCs.
adays achieved solar-to-electric power conversion efficiency
h
, over
Among various compositions that constitute these D–p-A organic
10–11% under AM 1.5 irradiation.2 Due to the relatively high cost of
ruthenium element, numerous research groups have expended
endeavors to develop metal free organic dyes as the alternative
sensitizers.3 As a result, impressive photovoltaic performances have
been reported on various tailor-made organic dyes, which include
coumarin,4 indoline,5 oligoene,6 merocyanine,7 hemicyanine,8
sensitizers, it has been demonstrated that triphenylamine (TPA),13
thiophene based aromatics,14 and cyanoacetic acid15 (or even
rhodanine-3-acetic acid)16 are the effective electron donating
moiety,
p-conjugated spacer and electron acceptor (anchoring
group), respectively. Notable bathochromatic shift and increase of
the molar extinction coefficient of the absorption spectra could be
further achieved by functionalization of these units such as to de-
perylene,9 and xanthene,10 showing promising
range of 5–9.8%.
h values in the
velop an extended
p-framework, showing direct influence of the
Majority of the aforementioned organic sensitizers could be
considered as electron donor– -conjugated-acceptor (D– -A)
compounds.11 Due to the
-orbital overlapping, the coupling ma-
molecular architecture on the DSSCs performances.
In this study, we initiated a project via seminally exploiting the
electron-deficient isoxazole fragment as part of the p-spacer suited
p
p
p
trix between neutral and charge transfer (zwitterion) species is
normally>>kT. Thus, upon electronic excitation, fast, barrierless
type of intramolecular charge transfer takes place, which is com-
monly named as adiabatic type of optical electron transfer.12 Upon
for DSSCs. Despite the fact that isoxazole is well-known for its great
electron transporting property in OLED applications,17 it has never
been tested and utilized for preparation of DSSC sensitizers. One of
our aims is to gain a better understanding of the relationship be-
tween molecular structures and elemental properties of organic
sensitizers in DSSC applications. Herein, we report on the synthesis
and characterization of both cyanoacetic acid and rhodanine-3-
acetic acid derivatives with unprecedented bithiophene–isoxazole
ingenious design of D/A chromophores as well as the p-spacer that
* Corresponding authors. E-mail address: ychi@mx.nthu.edu.tw (Y. Chi).
0040-4020/$ – see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.03.085

4224
Y.-T. Li et al. / Tetrahedron 66 (2010) 4223–4229
spacer (compounds 1–4). Single crystal X-ray diffraction study on
one intermediate compound 3 was also conducted to reveal the
group are arranged in a nearly perpendicular fashion, the result of
which renders maximum steric hindrance to block the
approaching of other molecules. In fact, careful examination of the
crystal-packing diagram of 3 indicates the adaptation of a head-
to-tail placement with an average separation of 3.8 Å between
skeletal arrangement of the bithiophene–isoxazole unit. This
p-
conjugated-acceptor part is then linked with TPA as the electron
donor unit,13,18 due to its wider applicability to establish an in-
tegrated donor–p-acceptor framework, i.e., compounds LJ6 and LJ7,
the p spacer of adjacent molecules. This nearly non-bonding
suited for DSSCs. Amid the study, we also accidentally generated
a 3-oxopropanenitrile anchoring group (LJ5) during a futile attempt
to synthesize the carboxyl substituted isoxazole fragment upon
employing base catalyzed ester hydrolysis. DSSCs with fair to good
performance characteristics were successfully fabricated using
these organic sensitizers.
distance could explain the absence of pp stacking interaction in
its crystal lattices, and serve as a model to eliminate similar in-
teraction on the TiO2 surface after the impregnation and de-
position of relevant dyes.
2. Results and discussion
The organic dyes LJ5, LJ6, and LJ7 were prepared by a synthetic
protocol illustrated in Scheme 1. The targeted dyes are synthesized
in multi-step procedures from a bithiophene derivative, namely 1-
(50-bromo-2,20-bithiophen-5-yl) ethanone. Compound 1 was syn-
thesized employing Claisen condensation with diethyl oxalate to
form
Compound 2 was next obtained by treatment of 1 with lithium tri-
tert-butoxyaluminohydride to afford hydroxymethyl in-
b-diketone, followed by the hydroxylamine cyclization.
a
termediate, followed by selective oxidation employing pyridinium
chlorochromate to form the anticipated carbaldehyde. After then,
both compounds 1 and 2 were treated with 4-(diphenylamino)
phenylboronic acid in the presence of Pd(PPh3)4 to afford TPA
substituted derivatives 3 and 4, respectively. Organic dye LJ5 was
then synthesized from 3 upon treatment of NaOH in methanol,
while LJ6 and LJ7 were successfully synthesized via condensation
of 4 with cyanoacetic acid under Knoevenagel condition and with
rhodanine-3-acetic acid, respectively.
Figure 1. Crystallographic structure of compound 3.
Identification of the organic dyes LJ5–LJ7 were next achieved
according to their spectroscopic data and microanalysis. Particu-
larly noteworthy is the generation of LJ5 dye via the NaOH cata-
lyzed decarboxylative ring opening and cleavage of isoxazole
Scheme 1. Synthetic route to the sensitizers LJ5, LJ6, and LJ7; conditions: (i) (CO2Et)2, NaOEt, (ii) H2NOH$HCl, (iii) LiAl(OtBu)3H, (iv) PCC, RT, (v) DPBA, Pd(PPh3)4, K2CO3, reflux, (vi)
NaOH, MeOH, 12 h, (vii) CAA, NH4OAc, HOAc, (viii) RAA, NH4OAc, HOAc.
Single crystal X-ray diffraction study on compound 3 was ex-
amined in an aim to unravel the relative geometry between
dithiophene and isoxazole fragments. As shown in Figure 1, the
side view clearly shows the minimal deviation of torsional angles
from planarity (ꢀ2ꢁ). In theory, such planar configuration is
fragment of 3.19 Note that LJ5 possesses a unique cyanoacetyl
group, which is also suited for anchoring on TiO2 (vide infra).
Identification of the LJ5 dye was provided by the observation of
a methylene singlet at d
3.93 in the 1H NMR spectrum as well as the
detection of
spectrum.
n
(ChN) stretching band at 2254 cmꢂ1 in its IR
expected to provide excellent
p conjugation and hence the large
electron coupling between the donor and the acceptor units.
Conversely, terminal phenyl substituents of the triphenylamine
The UV–vis absorption and emission spectra of LJ5–LJ7 dyes in
N,N-dimethyformamide (DMF) are shown in Figure 2, and the

Y.-T. Li et al. / Tetrahedron 66 (2010) 4223–4229
4225
O
HO
N
N
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
S
S
1.5
1.2
0.9
0.6
0.3
0.0
LJ5
LJ6
LJ7
S
S
400
500 600
wavelength (nm)
Ph2N
Ph2N
Scheme 2. Proposed keto-enol tautomerization involving the sensitizer LJ5.
To further study the possibilities of electron injection from the
excited state dye to conduction band (CB) of semiconductor and the
dye regeneration, the CV of these dyes was performed in acetoni-
trile to measure the redox potentials. The first oxidation potentials
(Eox) corresponding to the HOMO levels of the dyes were summa-
rized in Table 1. The HOMO levels of LJ5, LJ6, and LJ7 dyes are
calculated to be 0.75, 0.82, and 0.79 V, respectively, which are
positive enough comparing with that of iodine/iodide (0.4 V),22
indicating that the oxidized dyes could be reduced effectively by
electrolyte and then regenerated. Moreover, the HOMO level is
mainly affected by electron donor. Thus, from the HOMO level data
of these dyes, it is not too surprising for the observation of only
a minor change in HOMO level among these dyes.
300
400
500
wavelength (nm)
600
700
Figure 2. The UV/Vis and emission spectra of LJ5, LJ6, and LJ7 dyes in DMF solution.
Sample is excited at the lowest energy absorption maximum. Insert: The film UV–vis
absorption on 6 mm thickness TiO2 thin film.
On the other hand, the LUMO levels can be estimated from their
respective HOMO level and the S0/S1 (0,0) vibronic onset energy
of the dyes; the latter is calculated from the intersection between
the absorption and emission spectra (E0–0) obtained in solution.
These obtained HOMO and LUMO energy levels of dyes are also
summarized in Table 1. It is notable that, for effectively injecting the
electron into the conduction band of TiO2, the LUMO levels of the
dyes must be sufficiently more negative than the conducing band
energy (Ecb) of TiO2 semiconductor, (ꢂ0.5 V vs NHE).22 The corre-
sponding values of these organic dyes are calculated to be ꢂ1.98,
ꢂ1.76, and ꢂ1.76 V for LJ5, LJ6, and LJ7, respectively. Furthermore,
the larger energy gaps between the LUMO of the organic sensitizers
and Ecb of semiconductor are not only sufficient for efficient elec-
tron injection, but also allowing the employment of 4-tert-butyl-
pyridine (TBP) as the additive, which shift the Ecb of the TiO2
negatively and improve the open-circuit voltage and total conver-
sion efficiency consequently.23
corresponding data were collected in Table 1, particularly the ab-
sorption peak wavelengths on the TiO2 surface, which are essential
for identifying the respective bonding interaction. In general, LJ5–
LJ7 showed the maximum absorption wavelength (labs) at 412, 416,
and 414 nm, respectively, which were corresponding to HOMO
(highest occupied molecular orbital)/LUMO (lowest unoccupied
molecular orbital) type of pp transition. When LJ5–LJ7 were an-
*
chored on the TiO2 surface, the absorption maxima of these dyes
were only blue-shifted by 5, 17, and 12 nm in comparison to those
in solution, respectively, a result generally attributed to the
deprotonation of the carboxylic acid.20 However, the onset of the
lowest lying absorption for three dyes all tails down to w600 nm.
This is plausibly due to the electronic interaction with electron
deficient TiO2, lowering the corresponding energy gap.21,6b In DMF,
both LJ6 and LJ7 exhibited distinct emission band with peak
wavelength centered at 570 and 559 nm, respectively. The large
stokes shift of >8000 cmꢂ1 in DMF between absorption and
emission maxima manifests the strong charge transfer character,
being expected via their original design strategy. Dual emission was
resolved for LJ5, consisting of a band maximized at w490 nm and
a distinct shoulder at w600 nm. Upon monitoring at emission
wavelength at, e.g., 500 nm, an excitation peak wavelength of
400 nm was resolved, while a peak wavelength of w440 nm was
observed upon monitoring at, e.g., 650 nm (see Fig. S1 in
Supplementary data). Since LJ5 has an acidic methylene unit, that
is, linked by the electron withdrawing carbonyl and cyano groups,
the occurrence of keto-enol tautomerization is plausible (see
The photovoltaic properties of the solar cells fabricated with
these organic dyes were measured under simulated AM 1.5 G irra-
diation (100 mW cmꢂ2). A 12þ4
mm thickness of TiO2 film was used
for fabrication of DSSCs, where the first and second digits indicate
the thickness of dye adsorption and light scattering layers, re-
spectively. The sensitization was conducted employing a 3ꢃ10ꢂ4
M of dye solution in DMF and 1 mM of deoxycholic acid (DCA) for
a period of 10 h. The electrolyte contains 0.6 M 1-butyl-3-methyl-
imidazolium iodide (BMII), 0.1 M LiI, 0.05 M I2, 0.5 M 4-tert-butyl-
pyridine (TBP), 0.1 M guanidinium thiocyanate in dry acetonitrile.
The open-circuit voltage (VOC), short-circuit current density (JSC), fill
factor (FF), and solar energy-to-electricity conversion efficiencies
Scheme 2). The enol-form, due to its longer p-conjugation, should
possess lower energy gap as well as stronger charge transfer
properties, resulting in the w600 nm emission in DMF. This view-
point is supported by the observation of keto-form only emission
maximized at w500 nm in cyclohexane (see Fig. S2 of
Supplementary data).
(h) were listed in Table 2. Photocurrent density–voltage (J–V)
characteristics of these dyes are depicted in Figure 3.
Depending on the dyes selected, our DSSCs showed moderate
performance data. The DSSC based on LJ7 dye gave a lower
h value
Table 1
Photophysical and electrochemical data of LJ5, LJ6, and LJ7 dyes
a
a
Dye
labs, nm (
3
, Mꢂ1 cmꢂ1
)
labs on TiO2, nm
lem
,
nm
Potentials and energy levels
b
Eox,
V
E0–0
,
c V
EoxꢂE0–0, V
LJ5
LJ6
LJ7
412 (41,553)
416 (38,820)
414 (45,673)
407
399
402
485
570
559
0.75
0.82
0.79
2.73
2.58
2.55
ꢂ1.98
ꢂ1.76
ꢂ1.76
a
Absorption and emission spectra were measured in DMF solution.
b
The oxidation potentials of dyes on TiO2 were measured in CH3CN with 0.1 M tetrabutylammonium hexafluorophosphate (TBAP) with a scan rate of 50 mV sꢂ1 versus
normal hydrogen electrode (NHE).
c
E0–0 was determined from the intersection of absorption and emission spectra.

4226
Y.-T. Li et al. / Tetrahedron 66 (2010) 4223–4229
Table 2
spectra of LJ5 and LJ6 is w600 nm, and high IPCE performance
The photovoltaic data of DSSCs based on LJ5, LJ6, and LJ7 dyes
(>60%) was observed from the region of 440–480 nm. In contrast,
the IPCE spectra of DSSC in LJ7 exhibited much lower IPCE maxima
of 39% at w445 nm. The rather low IPCE values for this dye reflect
lower photocurrent and hence inferior photovoltaic performance.
The results may further imply that rhodanine-3-acetic acid is a poor
anchor in comparison to cyanoacetic acid, at least, in these oxazole
analogs.
To verify this viewpoint, we then fabricated devices using LJ6
and LJ7 dye for electrochemical impedance measurements.
According to the Bode phase plot (see Fig. 5 below) performed with
potentiostat/glavanostat (PGSTAT 12, AUTOLAB), the Bode phase
plot of two representative devices LJ6 and LJ7 indicates that the
characteristic frequency of device LJ6 (w22 Hz) is lower than that of
device LJ7 (w126 Hz). The results provide supplementary support
in that the electron lifetime is shorter in device LJ7.26 This can be
Dye
JSC (mA cmꢂ2
)
VOC (mV)
FF
h (%)
LJ5
LJ6
LJ7
7.74
8.04
3.60
491
662
457
63.5
62.0
64.4
2.41
3.30
1.06
10
8
LJ5
LJ6
LJ7
6
4
ascribed to certain degrees of the disruption of the
*
p electron
2
conjugation between rhodanine and the carboxylic acid (due to the
addition of a methylene group in between) in LJ7, resulting in
a decrease of the electron injection efficiency.
0
0.0
0.1
0.2
0.3 0.4
Voltage (V)
0.5
0.6
0.7
30
Figure 3. The J–V curves of DSSCs based on LJ5, LJ6, and LJ7 dyes.
LJ6
LJ7
of 1.06%, together with JSC¼3.60 mA cmꢂ2, VOC¼457 mV, FF¼0.644.
The lower IPCEs obtained for LJ7 could be attributed to the result of
the LUMO being located in a rhodanine framework rather than
at the carboxylic acid group. As a result, a decrease of the electron
injection efficiency in a dynamic manner is expected.24 Next, the
25
20
15
10
5
DSSC based on LJ5 dye exhibited an improved
h value of 2.41%,
together with JSC¼7.74 mA cmꢂ2, VOC¼491 mV, FF¼0.635, despite of
the fact that the LJ5 dye contained no carboxylic acid linker. On this
basis, the adsorption of LJ5 onto TiO2 is more plausibly via its enol
group, together with the cyano group, indirectly supporting the
existence of enol-form for LJ5 in DMF (vide supra). It is obvious that
0
the nearly twofold improvement in
h value of LJ5 cell versus that of
0.1
1
10
100
frequency
1000
10000
100000
LJ7 is due to the stronger association of enolic OH group to the TiO2
semiconductor surface. Thus the electron injection could occur
from the excited state dye to the conduction band of TiO2 nano-
particles via the enolic OH group. Such an observation has been
recently reported for the kaede dye with hydroxy group as the
anchoring group.25 Finally, the best DSSC is fabricated employing
Figure 5. Bode phase plot of device LJ6 and LJ7 at open-circuit condition under 1 sun
solar irradiation.
LJ6, for which the device performance data are
h
¼3.30%,
3. Conclusions
JSC¼8.07 mA cmꢂ2, VOC¼662 mV, FF¼0.63, showing the best an-
choring capability among all three sensitizers, attributed to the
cyanoacetic acid group.
To sum up, we have demonstrated the exploitation of the
electron-deficient isoxazole fragment as portion of the
p-spacer
The incident photon-to-current conversion efficiencies (IPCEs)
of these DSSC dyes are shown in Figure 4. The onset of the IPCE
and the electron acceptor suited for DSSCs. Moderate conversion
efficiency has been achieved. The inferiority of cell performance at
current stage is due to the limit in the harvesting mainly on
ꢀ550 nm of solar photons. This obstacle should be circumvented
via the feasibility and versatility in its future derivatization.
80
LJ5
LJ6
LJ7
60
4. Experimental
4.1. General methods
40
20
0
All reactions were performed under nitrogen atmosphere and
solvents were distilled from appropriate drying agents prior to use.
Commercially available reagents were used without further puri-
fication unless otherwise stated. All reactions were monitored us-
ing pre-coated TLC plates (0.20 mm with fluorescent indicator
UV254). Mass spectra were obtained on a JEOL SX-102A instrument
operating in electron impact (EI) or fast atom bombardment (FAB)
mode. 1H and 13C NMR spectra were recorded on a Varian Mercury-
400 or an INOVA-500 instrument. Elemental analysis was carried
400
450
500
550
600
650
700
wavelength (nm)
Figure 4. IPCE spectra of DSSCs based on LJ5, LJ6, and LJ7 dyes.

Y.-T. Li et al. / Tetrahedron 66 (2010) 4223–4229
4227
out with a Heraeus CHN–O Rapid Elementary Analyzer. 1-(50-
Bromo-2,20-bithiophen-5-yl) ethanone was prepared from N-bro-
mosuccinimide (NBS) and (2,20-bithiophen-5-yl) ethanone
according to the literature procedure.27
max./min.¼0.519/ꢂ0.410 e/Å3. Crystallographic data of 3 (excluding
structure factors) was deposited in the Cambridge Crystallographic
Data Centre with the deposition numbers CCDC 769595. It can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB21EZ, UK (fax: þ44 1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk).
4.1.1. Ethyl 5-(50-bromo-2,20-bithiophen-5-yl) isoxazole-3-
carboxylate (1). A mixture of 1-(50-bromo-2,20-bithiophen-5-yl)
ethanone (500 mg, 1.74 mmol) and diethyl oxalate (500 mg,
3.48 mmol) was stirred in 5 mL of THF at rt for ten minutes, fol-
lowed by addition of sodium ethoxide (NaOEt, 240 mg, 3.52 mmol).
After stirring for 12 h, the solution was quenched by addition of 2 N
HCl solution to a pH value of 3–4. The yellow precipitate was col-
lected, rinsed with water, and dried under vacuum. This material
was then mixed with hydroxylamine hydrochloride (540 mg,
7.8 mmol) in EtOH (30 mL) and the solution was refluxed for 12 h.
After cooling to rt, solvent was then removed and the residue was
purified by silica gel column chromatography eluted with a 4:1
mixture of hexane and CH2Cl2; yield: 414 mg, 1.08 mmol, 62%. Se-
4.1.4. 5-(50-(4-(Diphenylamino)phenyl)-2,20-bithiophen-5-yl)isoxaz
ole-3-carbaldehyde (4). 4-(Diphenylamino) phenylboronic acid
(DPBA, 102 mg, 0.35 mmol) was treated with carbaldehyde 2
(100 mg, 0.29 mmol) in presence of Pd(PPh3)4 (16 mg, 5 mol %),
K2CO3 (2 N, 1.0 mL), and THF (3 mL). After the completion of the
reaction, the solvent was removed in vacuo and the residue dis-
solved into minimal amount of CH2Cl2. It was then filtered, the
solvent removed, and the residue was purified by silica gel column
chromatography eluting with CH2Cl2; yield: 89 mg, 0.18 mmol, 61%.
Selected spectral data of 4: 1H NMR (500 MHz, CDCl3, 298 K):
d
10.14 (1H, s), 7.46 (1H, d, JHH¼4.0 Hz), 7.43 (2H, d, JHH¼9.0 Hz),
lected spectral data of 1: 1H NMR (400 MHz, CDCl3, 298 K):
d
7.43
7.26 (4H, t, JHH¼7.8 Hz), 7.20 (1H, d, JHH¼3.6 Hz), 7.17 (1H, d,
JHH¼4.0 Hz), 7.14 (1H, d, JHH¼3.6 Hz), 7.11 (2H, d, JHH¼9.0 Hz), 7.11–
(1H, d, JHH¼4.0 Hz), 7.10 (1H, d, JHH¼4.0 Hz), 7.00 (2H, d,
JHH¼3.6 Hz), 6.75 (1H, s), 4.45 (2H, q, JHH¼7.2 Hz), 1.42 (3H, t,
7.03 (4H, m), 6.70 (1H, s). 13C NMR (125 MHz, CDCl3, 298 K):
d 184.5,
JHH¼7.2 Hz). 13C NMR (100 MHz, CDCl3, 298 K):
d
165.9, 159.7, 156.9,
166.6, 162.5, 147.8, 147.3, 144.9, 141.6, 133.9, 129.3, 128.8, 127.3,
126.5, 126.0, 125.5, 124.7, 123.9, 123.3, 123.2, 123.0, 95.6. Anal. Calcd
for C30H20N2O2S2: C, 71.40; H, 3.99; N, 5.55. Found: C, 71.28; H, 4.19;
N, 5.33.
139.8, 137.3, 130.9, 128.4, 126.7, 125.0, 124.6, 112.7, 99.6, 62.3, 14.1.
Anal. Calcd for C14H10BrNO3S2: C, 43.76; H, 2.62; N, 3.65. Found: C,
43.57; H, 2.98; N, 3.85.
4.1.2. 5-(50-Bromo-2,20-bithiophene-5-yl) isoxazole-3-carbaldehyde
(2). THF solution of lithium tri-tert-butoxyaluminohydride
(1.0 mL, 1.0 M) was added dropwisely to a THF (30 mL) solution of 1
(200 mg, 0.52 mmol) at rt. The mixture was heated at 50 ꢁC for 8 h.
After cooling to rt, solvent was evaporated in vacuo. The residue
was dissolved into CH2Cl2, washed with water, dried over anhy-
drous Na2SO4, and filtered. Concentration of filtrate gave a white
solid, which was then treated with pyridinium chlorochromate
(PCC, 168 mg, 0.78 mmol) and CH2Cl2 (20 mL). After stirring at rt
overnight, the mixture was filtered, the remaining solvent evapo-
rated in vacuo; and the residue was purified by flash column
chromatography eluting with pure CH2Cl2; yield: 159 mg,
0.47 mmol, 90%. Selected spectral data of 2: 1H NMR (400 MHz,
4.1.5. 3-(50-(4-(Diphenylamino)phenyl)-2,20-bithiophen-5-yl)-3-ox
opropanenitrile (LJ5). Compound
3 (200 mg, 0.36 mmol) was
mixed with NaOH (1 N, 1.0 mL) in MeOH (25 mL) and then refluxed
overnight. After removal of solvent in vacuo, and the orange-
residue was washed with water, dried under vacuum, and then
purified by silica gel column chromatography eluting with a 1:3
mixture of hexane and CH2Cl2; yield: 112 mg, 0.22 mmol, 60%. Se-
lected spectral data of LJ5: 1H NMR (500 MHz, CDCl3, 298 K):
d 7.65
(1H, d, JHH¼4.0 Hz), 7.43 (2H, d, JHH¼9.0 Hz), 7.30 (1H, d,
JHH¼4.0 Hz), 7.26 (4H, t, JHH¼7.8 Hz), 7.18 (1H, d, JHH¼4.0 Hz), 7.15
(1H, d, JHH¼3.6 Hz), 7.11–7.03 (8H, m), 3.93 (2H, s). MS (FAB): m/z
476 (Mþ). 13C NMR (125 MHz, CDCl3, 298 K):
d 178.6, 148.7, 148.1,
147.1,146.7,137.8,134.7,133.4,129.4,127.6,126.8,126.6,124.8,123.8,
123.5, 123.2, 123.0, 113.5, 29.0. Anal. Calcd for C29H20N2OS2: C,
73.08; H, 4.23; N, 5.88. Found: C, 72.91; H, 4.59; N, 5.68. IR (neat):
CDCl3, 298 K):
d
10.14 (1H, s), 7.45 (1H, d, JHH¼3.6 Hz), 7.11 (1H, d,
JHH¼3.6 Hz), 7.00 (2H, d, JHH¼3.6 Hz), 6.71 (1H, s). 13C NMR
(100 MHz, CDCl3, 298 K): 184.3, 166.3, 162.5, 140.1, 137.3, 130.9,
128.7, 126.5, 125.1, 124.7, 112.8, 96.0. Anal. Calcd for C12H6BrNO3S2:
C, 40.46; H, 1.70; N, 3.93. Found: C, 40.27; H, 1.88; N, 3.68.
2254, 1653, 1591 cmꢂ1
.
4.1.6. 2-Cyano-3-(5-(50-(4-(diphenylamino)phenyl)-2,20-bithiophen-
5-yl) isoxazol-3-yl) acetic acid (LJ6). Compound (82 mg,
4
4.1.3. Ethyl 5-(50-(4-(diphenylamino)phenyl)-2,20-bithiophen-5-yl)
isoxazole-3-carboxylate (3). 4-(Diphenylamino) phenylboronic acid
(DPBA, 320 mg,1.11 mmol) was treated with 1 (300 mg, 0.92 mmol)
in presence of Pd(PPh3)4 (30 mg, 5 mol %), K2CO3 (2 N, 1.5 mL), and
THF (30 mL). After refluxing for 12 h, the solvent was removed in
vacuo and the residue dissolved into minimal amount of CH2Cl2.
The solution was then filtered, the solvent removed, and the
residue was purified by silica gel column chromatography eluting
0.16 mmol) was mixed with cyanoacetic acid (CAA, 21 mg,
0.24 mmol), ammonium acetate (19 mg, 0.06 mmol) in glacial
acetic acid (10 mL) and the solution was refluxed for 3 h. After
cooling to rt, the mixture was poured into water to yield an orange
precipitate. This precipitate was collected and washed with water
(2ꢃ5 mL) and a mixture of 1:1 mixture of hexane and diethylether
(5 mL) in sequence. Further purification was conducted by crys-
tallization from a mixture of THF and hexane; yield: 54 mg,
0.10 mmol, 60%. Selected spectral data of LJ6: 1H NMR (500 MHz,
with
0.72 mmol, 79%. Selected spectral data of 3: 1H NMR (400 MHz,
CDCl3, 298 K):
a 4:1 mixture of hexane and CH2Cl2; yield: 499 mg,
DMSO-d6, 298 K):
d
8.04 (1H, s), 7.81 (1H, d, JHH¼4.0 Hz), 7.59 (2H,
d
7.44 (1H, d, JHH¼3.6 Hz), 7.43 (2H, d, JHH¼9.0 Hz),
d, JHH¼9.0 Hz), 7.49 (1H, d, JHH¼4.0 Hz), 7.46 (1H, d, JHH¼4.0 Hz),
7.43 (1H, d, JHH¼4.0 Hz), 7.34–7.31 (5H, m), 7.10–7.04 (6H, m), 6.97
(2H, d, JHH¼9.0 Hz). MS (FAB): m/z 571 (Mþ). Anal. Calcd for
C33H21N3O3S2$H2O: C, 67.21; H, 3.93; N, 7.13. Found: C, 67.09; H,
4.16; N, 7.27.
7.27 (4H, t, JHH¼7.8 Hz), 7.19 (1H, d, JHH¼4.0 Hz), 7.16 (1H, d,
JHH¼3.6 Hz), 7.14 (1H, d, JHH¼4.0 Hz), 7.11–7.03 (8H, m), 6.74 (1H, s),
4.45 (2H, q, JHH¼7.2 Hz), 1.42 (3H, t, JHH¼7.2 Hz). MS (EI): m/z 548
(Mþ). Selected crystal data of 3: C32H24N2O3S2, M¼548.65,
monoclinic, space group C2/c, T¼150(2) K, a¼16.3161(12),
b¼8.7464(6), c¼38.003(3) Å,
b
¼93.169(2)ꢁ, V¼5415.0(7) Å3,
4.1.7. 2-(5-(5-(50-(4-Diphenylaminophenyl)-2,20-bithiophen-5-yl)
isoxazol-3-yl)methylene-4-oxo- 2-thioxothiazolidin-3-yl) acetic acid
(LJ7). Compound 4 (82 mg, 0.16 mmol) was mixed with rhodanine-
3-acetic acid (RAA, 46 mg, 0.24 mmol) and ammonium acetate
(19 mg, 0.06 mmol) in glacial acetic acid (10 mL). The solution was
Z¼8, rcalcd¼1.346 Mg/m3, F(000)¼2288,
l
(MoKa)¼0.7107 Å,
m
¼0.234 mmꢂ1
,
crystal
size¼0.30ꢃ0.25ꢃ0.05 mm3,
4759
independent reflections collected (Rint¼0.0658), GOF¼1.192,
final R1[I>2
s
(I)]¼0.0736,
uR2(all data)¼0.1733, and D-map,

4228
Y.-T. Li et al. / Tetrahedron 66 (2010) 4223–4229
2. (a) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-
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then refluxed for 3 h. After cooling to rt, the mixture was poured
into water to yield a red precipitate. This precipitate was collected
and washed with water (3ꢃ5 mL) and a mixture of 1:1 mixture of
hexane and diethylether (5 mL) in sequence. Further purification
was conducted by crystallization from a mixture of THF and hex-
ane; yield: 68 mg, 0.10 mmol, 63%. Selected Spectral data of LJ7: 1H
NMR (500 MHz, DMSO-d6, 298 K):
d 7.75 (1H, s), 7.74 (1H, d,
JHH¼4.0 Hz), 7.58 (2H, d, JHH¼8.0 Hz), 7.47 (1H, d, JHH¼4.0 Hz), 7.44
(1H, d, JHH¼3.5 Hz), 7.41 (1H, d, JHH¼3.5 Hz), 7.32 (4H, t,
JHH¼8.0 Hz), 7.21 (1H, s), 7.10–7.04 (6H, m), 6.96 (2H, d, JHH¼8.5 Hz),
4.69 (2H, s). MS (FAB): m/z 679 (Mþ). 13C NMR (125 MHz, CDCl3,
298 K):
d 172.0, 167.1, 165.7, 165.1, 158.2, 147.2, 146.7, 143.7, 140.1,
133.2, 129.9, 129.6, 129.1, 126.9, 126.6, 126.4, 125.1, 124.9, 124.4,
124.0, 123.6, 122.6, 118.1, 101.4, 45.1. Anal. Calcd for
C35H23N3O4S4$H2O: C, 60.41; H, 3.62; N, 6.04. Found: C, 60.76; H,
3.71; N, 6.09.
4.2. Fabrication of DSSC and photovoltaic measurements
TiO2 anatase nanoparticles of 20 nm were prepared according to
published procedures28 with slight modification. TiO2 particles
were dispersed in
TiO2 thin films of 12
on a transparent conducting oxide (F-doped SnO2, FTO). These films
were dried at 120 ꢁC for 5 min and then a 4
m thick layer of
a
-terpineol with ethyl cellulose as a binder. The
6. (a) Hara, K.; Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H.
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mm were prepared by a doctor-blade method
m
400 nm TiO2 particles (Ti–Nanoxide R/SP paste from Solaronix) was
deposited again by a doctor-blade method with a dimension of
0.5ꢃ0.5 cm2. Afterward, the double-layered films were sintered at
500 ꢁC for 30 min. After sintering, the TiO2 films were treated with
40 mM of TiCl4 solution, rinsed with water and ethanol, and sin-
tered at 500 ꢁC for 30 min. After cooling to 80 ꢁC, the TiO2 electrode
was coated with dyes by dipping into a solution of 3ꢃ10ꢂ4 M of the
targeted dye, together with 1 mM of deoxycholic acid (DCA) in DMF
solution overnight. After being rinsed with EtOH, the dye-coated
TiO2 electrode was assembled into a sandwich-type cell with
a Pt-coated FTO as counter electrode and a film (Surlyn 1702,
25 mm) as a spacer between the electrodes. The electrolyte solution
10. Nazeeruddin, M. K.; Humphry-Baker, R.; Officer, D. L.; Campbell, W. M.; Burrell,
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was then injected into the cell through a drilled hole in the back of
the counter electrode. The hole was then sealed using a hot-melt
ionomer film and a cover glass.
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(i) Zhang, G.; Bai, Y.; Li, R.; Shi, D.; Wenger, S.; Zakeeruddin, S. M.; Gra¨tzel, M.;
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The Pt counter electrode was prepared by spin-coating a 0.05 M
H2PtCl6 in isopropyl alcohol solution on FTO glass, followed by
sintering at 385 ꢁC for 15–30 min. The liquid electrolyte contained
0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.1 M LiI, 0.05 M
I2, 0.5 M 4-tert-butylpyridine (TBP), 0.1 M guanidinium thiocyanate
in dry acetonitrile. Performances of DSSCs were measured with
a 0.25 cm2 working area.29 Light-to-electricity conversion effi-
ciency values were measured using a modified light source, 450 W
Xe lamp (Oriel, 6266), an Oriel 81088 Air Mass 1.5 Global filter and
a digital source meter purchased from Keithley Instruments Inc.
The incident light intensity was calibrated by using a standard solar
cell composed of a crystalline silicon solar cell and an IR cutoff filter
(Schott, KG-5), giving the photoresponse range of amorphous sili-
con solar cell. The applied potential and cell current were measured
using a Keithley model 2400 digital source meter.
Supplementary data
Supplementary data associated with this article can be found in
online version at doi:10.1016/j.tet.2010.03.085.
15. Zang, G.; Bala, H.; Cheng, Y.; Shi, D.; Lv, X.; Yu, Q.; Wang, P. Chem. Commun.
2009, 2198–2200.
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