2,1,3-Benzothiadiazole-containing donor-acceptor-acceptor dyes for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2012.05.052

A series of organic dyes with a donor-acceptor-acceptor (D-A-A) configuration, in which various diarylthienylamine donors and a cyanoacrylic acid acceptor are bridged by a low-band-gap 2,1,3-benzothiadiazole acceptor, have been synthesized, characterized, and employed as photosensitizers for dye-sensitized solar cells (DSSCs). The adoption of 2,1,3-benzothiadiazole as the bridging acceptor endowed these tailor-made dyes with superior light-harvesting capabilities in comparison to their previously reported pyrimidine-based analogs. After fine-tuning the fabrication conditions, DSSCs based on these dyes showed solar spectral responses extending to the near-IR region and achieved power conversion efficiencies (PCEs) of up to 3.16% (OHexDPTB) under simulated AM 1.5G irradiation (100 mW cm^-2).

Full text of DOI:10.1016/j.tet.2012.05.052

Tetrahedron 68 (2012) 7509e7516
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
2,1,3-Benzothiadiazole-containing donoreacceptoreacceptor dyes
for dye-sensitized solar cells
,
Li-Yen Lin a, Chih-Hung Tsai b, Francis Lin a, Tsung-Wei Huang b, Shu-Hua Chou a, Chung-Chih Wu b
,
*
,
Ken-Tsung Wong a
*
a Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
b Department of Electrical Engineering, Graduate Institute of Photonics and Optoelectronics, and Graduate Institute of Electronics Engineering, National Taiwan University,
Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 March 2012
Received in revised form 27 April 2012
Accepted 15 May 2012
Available online 23 May 2012
A series of organic dyes with a donoreacceptoreacceptor (DeAeA) configuration, in which various dia-
rylthienylamine donors and cyanoacrylic acid acceptor are bridged by low-band-gap 2,1,3-
a
a
benzothiadiazole acceptor, have been synthesized, characterized, and employed as photosensitizers for
dye-sensitized solar cells (DSSCs). The adoption of 2,1,3-benzothiadiazole as the bridging acceptor en-
dowed these tailor-made dyes with superior light-harvesting capabilities in comparison to their previously
reported pyrimidine-based analogs. After fine-tuning the fabrication conditions, DSSCs based on these
dyes showed solar spectral responses extending to the near-IR region and achieved power conversion
efficiencies (PCEs) of up to 3.16% (OHexDPTB) under simulated AM 1.5G irradiation (100 mW cmꢀ2).
Ó 2012 Published by Elsevier Ltd.
Keywords:
Dye-sensitized solar cells
Donoreacceptoreacceptor molecules
1. Introduction
DSSCs employing a De
peA porphyrin dye in conjunction with
a Co(II/III)tris(bipyridyl)-based redox electrolyte when co-sensitized
Dye-sensitized solar cells (DSSCs) have been regarded as one of
with another organic DepeA dye, demonstrating a bright future
the most promising candidates for next-generation solar cells since
for organic dye-based DSSCs.31
1
€
their invention by Gratzel and co-workers in 1991 because of their
Previously, we reported a series of efficient organic dyes, where
various diarylthienylamine donors are connected to a cyanoacrylic
acid acceptor (also TiO2-anchoring group) through an electron-
ease of production, potential to allow low-cost manufacturing, and
ability to be fabricated onto light-weight flexible substrates. During
the last two decades, combined developments of new photo-
sensitizers2e6 and electrolytes7e12 have contributed to significant
improvements both in power conversion efficiencies (PCEs) and
long-term stability. At present, DSSCs based on ruthenium sensi-
tizers have exhibited PCEs in the range of 11e12%.13e17 However,
the rarity and high cost of the ruthenium metal may impede their
further development for large-scale applications. Accordingly,
metal-free organic sensitizers that are generally modeled on the
deficient pyrimidine
such structural arrangement as the donoreacceptoreacceptor
(DeAeA) molecular architecture. In comparison to a De eA coun-
terpart that adopts phenylene as a -spacer, these dyes showed red-
p
-spacer (Scheme 1).32 We later referred to
p
p
shifted absorption and hence gave larger photocurrent as employed
in DSSCs. From the point of view of energy levels, both frontier mo-
lecular orbital levels of the DeAeA dyes were lower than those of the
DepeA counterpart, yet the degree of the shift on the lowest un-
donore(
p
-spacer)eacceptor (De
p
eA) system have also being in-
occupied molecular orbital (LUMO) level was larger than that on the
highest occupied molecular orbital (HOMO) level, thus leading to
narrower energy gaps for the DeAeA dyes. On the basis of the ex-
perimental observations mentioned above, we argued that such
DeAeA architecture shows a potential ability to simultaneously en-
hance the short-circuit current density (Jsc) and open-circuit voltage
(Voc) as utilized in the design of molecular donors for small-molecule
organic solar cells (SMOSCs). Recently, two DeAeA donor molecules
were practicallysynthesized andemployed in SMOSCs, which indeed
successfully demonstrated remarkable achievements with PCEs near
or exceeding 6%.33,34 Remarkably, the donor molecule, DTDCTB,
featuring a low-band-gap 2,1,3-benzothiadiazole (BT) exhibited
tensely explored in DSSCs due to several predominant advantages
over their ruthenium counterparts, such as larger molar extinction
coefficients, flexibility in structural modification for manipulation
of optoelectronic properties, facile synthesis, and relatively lower
cost.18e30 Although the performance of organic sensitizers has
lagged behind those of ruthenium sensitizers over a long period of
time, a record-high PCE of 12.3% has recently been realized for
* Corresponding author. E-mail addresses: chungwu@cc.ee.ntu.edu.tw (C.-C.
Wu), kenwong@ntu.edu.tw (K.-T. Wong).
0040-4020/$ e see front matter Ó 2012 Published by Elsevier Ltd.
doi:10.1016/j.tet.2012.05.052

7510
L.-Y. Lin et al. / Tetrahedron 68 (2012) 7509e7516
a distinguished light-harvesting ability with absorption extending to
the near-IR region.33 The BT represents one of the most widely used
electron-accepting unitsinoptoelectronicmaterials,35e37 andseveral
BT-containing organic DSSC dyes with different configurations have
been reported.38e45 In this work, we report on a new series of organic
DeAeA dyes, where the BTacceptor is used to replace the pyrimidine
block (Scheme 2). Such structural modification is anticipated to en-
able these tailor-made dyes to show significant bathochromic shifts
in absorption as compared to their pyrimidine-based analogs. This
study also provides us with an opportunity to further assess the ap-
plicability of our DeAeA molecular design strategy on developing
photosensitizers for DSSCs.
The UVevis absorption spectra of the dyes in dichloromethane
(CH2Cl2) solutions are depicted in Fig. 1, and their pertinent pho-
tophysical parameters are summarized in Table 1. The photo-
luminescence data of these dyes are not provided here since the
dyes are almost non-luminescent. All three dyes exhibit an intense
absorption band in the long-wavelength region with a maximum
around 613e654 nm and a molar extinction coefficient higher than
3ꢁ104 Mꢀ1 cmꢀ1, which is assigned to the intramolecular charge
transfer (ICT) transition. Apparently, the absorption of this series of
dyes effectively extends the useful light-harvesting range to the
near-IR region through incorporating BT as the bridging acceptor.
The absorption bands of OMeDPTB and OHexDPTB are red-shifted
as compared to that of DPTB by ca. 40 nm because of the stronger
electron-donating nature of the dialkoxy substituents. This red shift
is profitable for photon harvesting and thus photocurrent genera-
tion in DSSCs (vide infra). It is worth mentioning that in comparison
R
to the reported DeAepeA dye incorporating thiophene as an extra
p
-spacer to connect the BT and cyanoacrylic acid acceptors (lmax at
541 nm in a tetrahydrofuran (THF) solution),38 DPTB shows an
evident bathochromic shift in absorption, corroborating again that
enhanced solar spectral responses can be realized through our
DeAeA molecular design concept (Fig. S1). Fig. 2 depicts the ab-
N
N
S
N
COOH
NC
sorption spectra of the dyes anchoring on 7
mm porous TiO2
DPTP : R = H
R
nanoparticle films. The absorption maxima of these three dyes are
blue-shifted compared to those observed in CH2Cl2 solutions. This
phenomenon can be attributed to the deprotonation of carboxylic
acid upon adsorption onto the TiO2 surface, and the resulting car-
boxylateeTiO2 unit is a weaker electron acceptor than the corre-
sponding carboxylic acid.
OMeDPTP : R = OCH3
OHexDPTP : R = OC6H13
Scheme 1. Molecular structures of the reported pyrimidine-containing DSSC dyes.
S
R
R
R
R
N
N
S
S
N
N
N
N
Br
Br
CuCN
DMF, reflux
N
CN
N
Br
N
SnBu3
PdCl2(PPh3)2, DMF, 80 o
C
S
S
S
R
R
2a : R = H
3a : R = H
1a : R = H
2b : R = OCH3
3b : R = OCH3
1b : R = OCH3
2c : R = OC6H13
3c : R = OC6H13
1c : R = OC6H13
R
R
R
S
S
N
N
N
N
1. DIBAL-H, THF, 0 o
C
NC
COOH
, NH OAc
4
N
CHO
N
2. saturated NH4Cl (aq)
glacial acetic acid, 80 oC
S
S
COOH
NC
R
4a : R = H
4b : R = OCH3
4c : R = OC6H13
DPTB : R = H
OMeDPTB : R = OCH3
OHexDPTB : R = OC6H13
Scheme 2. Synthesis of the dyes DPTB, OMeDPTB, and OHexDPTB.
2. Results and discussion
To gain more insight into the electronic structures of the dyes,
density function theory (DFT) calculations were carried out on
a B3LYP/6-31G(d) level for geometry optimization. As shown in
Fig. 3, the HOMOs of the dyes are populated over the whole mo-
lecular conjugated backbone with substantial contributions from
diarylamine and thiophene fragments, whereas LUMOs are delo-
calized through thiophene, BT, and cyanoacrylic acid blocks with
considerable contributions from the latter two. The significant
spatial overlap between the HOMO and LUMO as well as the co-
planar conformation between the thiophene and BT rings is ben-
eficial to enhance the efficiency of ICT transition, as evidenced in
absorption spectra. Moreover, the HOMOeLUMO excitation shifts
the electron density distribution from the diarylamine unit to the
The synthetic pathways of the new dyes are illustrated in Scheme
2. Stille coupling reactions of triarylamine-functionalized tributyl-
stannyl derivatives 1 and 4,7-dibromo-2,1,3-benzothiadiazole affor-
ded the mono-functionalized intermediates 2aec, which were
directly reacted with copper cyanide to yield their corresponding
cyano derivatives 3aec via the Rosenmundevon Braun reaction. Fi-
nally, the reduction of 3aec using diisopropylaluminum hydride
€
followed by Knoevenagel condensation of the resulting aldehydes
4aec with cyanoacetic acid in the presence of ammonium acetate
gave the target compounds DPTB, OMeDPTB, and OHexDPTB in
moderate overall yields.

L.-Y. Lin et al. / Tetrahedron 68 (2012) 7509e7516
7511
potential and one irreversible (or quasi-reversible) oxidation wave
at higher potential. The first waves can be ascribed to the oxidation
of the diarylthienylamine donor moieties, whereas the second
waves can be assigned to the oxidation of the conjugated back-
bones.44 The oxidation potentials of OMeDPTB and OHexDPTB are
less positive than those of DPTB due to the presence of the stronger
electron-donating dialkoxy substituents. Moreover, the in-
troduction of the dialkoxy substituents on the donor moieties in-
creases the electron density of the conjugated backbones, thus
making the second oxidation behaviors of OMeDPTB and
OHexDPTB more reversible. On the other hand, one quasi-
reversible reduction wave attributed to the reduction of the cya-
noacrylic acid block was observed in the cathodic potential regime
for the dyes, which is relatively insensitive to the structural alter-
ations of the donor moieties even though the reduction potentials
of OMeDPTB and OHexDPTB are slightly more negative than that of
DPTB. The zeroezero excitation energy (E0e0) estimated from the
onset point of the absorption spectrum and the first oxidation
potential (Eo1x) was used to calculate the excited-state oxidation
potential (Eoꢂ x). The deduced Eoꢂ x values corresponding to the LUMO
levels of these three dyes are more negative than the conduction
band edge of the TiO2 electrode (ꢀ0.5 V vs NHE),47 indicating that
the electron injection from photoexcited dyes into TiO2 would be
energetically favorable. The first oxidation potentials (Eo1x) corre-
sponding to the HOMO levels of the dyes are more positive than the
redox potential of the iodide/triiodide (Iꢀ/I3ꢀ) couple (0.4 V vs NHE).
It suggests that the oxidized dyes formed after electron injection
into TiO2 should be able to thermodynamically accept electrons
from Iꢀ ions in the electrolyte for effective dye regeneration.
In common with the photovoltaic characterization on the
pyrimidine-based dyes,32 the photovoltaic performance of DPTB,
OMeDPTB, and OHexDPTB as the photosensitizers for DSSCs was
first evaluated with a sandwich DSSC cell using 0.6 M 1-butyl-3-
methylimidazolium iodide (BMII), 0.05 M LiI, 0.03 M I2, 0.5 M 4-
tert-butylpyridine (TBP), and 0.1 M guanidinium thiocyanate in
a mixture of acetonitrileevaleronitrile (85: 15, v/v) as the redox
electrolyte (EL1) (details of the device preparation and character-
ization are described in the experimental section). The perfor-
mance parameters of the DSSCs based on these organic dyes,
including the Jsc, Voc, fill factor (FF), and PCEs, are summarized in
Table 2. The preliminary results showed that the DSSCs exhibit
extremely low PCEs (<0.06%). We attribute the poor photovoltaic
performance to the addition of TBP into the electrolyte that can
negatively shift the conduction band edge of TiO2 about 0.3 V,48,49
thus impeding efficient electron injection from the photoexcited
dyes into the TiO2 electrode.46 Very recently, a few BT-containing
sensitizers with similar structural features to our dyes (i.e., the BT
acceptor directly adjacent to the cyanoacrylic acid) were also
reported to show inferior performance to their analogs even
though the origins of the low PCEs were different.44,45 For a fair
comparison, the N719-sensitized DSSC was also fabricated under
the same conditions and reached a PCE of 8.57%.
35
30
25
20
15
10
5
DPTB
OMeDPTB
OHexDPTB
0
300 400 500 600 700 800 900
Wavelength (nm)
Fig. 1. Absorption spectra of DPTB, OMeDPTB, and OHexDPTB measured in CH2Cl2
solutions (10ꢀ5 M).
Table 1
Photophysical and electrochemical parameters of the dyes
Dye
lmax
3
lmax on TiO2 E0e0 Eo1x
Ered
(eV)c (v)d (v)e
Eoꢂ x
(nm)a (Mꢀ1 cmꢀ1
)
(nm)b
(v)f
a
DPTB
613
32,800
34,442
32,292
513
548
547
1.74 1.02 ꢀ0.66 ꢀ0.72
1.65 0.87 ꢀ0.70 ꢀ0.78
1.64 0.87 ꢀ0.69 ꢀ0.77
OMeDPTB 650
OHexDPTB 654
Absorption spectra were measured in CH2Cl2 solutions (10ꢀ5 M).
Absorption spectra on TiO2 were obtained through measuring the dyes adsor-
m TiO2 nanoparticle films in chlorobenzene solutions.
E0e0 was estimated from the onset point of the absorption spectra in CH2Cl2
solutions.
a
b
bed on 7
m
c
d
Measured in CH2Cl2 solutions with 0.1
M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as a supporting electrolyte. Scan rate: 100 mV/s; cali-
brated with ferrocene/ferrocenium (Fc/Fcþ) as an external reference and converted
to NHE by addition of 630 mV.46
e
Measured in THF solutions with 0.1 M tetrabutylammonium perchlorate (TBAP)
as a supporting electrolyte. Scan rate: 100 mV/s; calibrated with ferrocene/ferro-
cenium (Fc/Fcþ) as an external reference and converted to NHE by addition of
630 mV.46
Eoꢂ x ¼ Eo1x ꢀ E0ꢀ0
.
f
1.0
0.8
0.6
0.4
0.2
0.0
DPTB
OMeDPTB
OHexDPTB
To improve the poor device performance, the DSSCs were then
fabricated in a TBP-free condition by merely using 0.6 M 1,2-
dimethyl-3-propylimidazolium iodide (DMPII), 0.05 M LiI, 0.03 M
I2 in acetonitrile as the redox electrolyte (EL2). The DSSCs based on
these organic dyes exhibited remarkably improved performance,
whereas the N719-based cell showed a lower PCE than the parent
cell based on using the EL1 electrolyte, mainly due to the lower Jsc
and Voc values (Table 2). The decrease in Voc for the N719-based cell
can be partly attributed to the shift of the conduction band edge of
TiO2 toward positive potential.49 Such phenomenon also qualita-
tively verifies our aforementioned speculation on the poor photo-
voltaic performance for the DSSCs based on our organic dyes in
conjunction with the EL1 electrolyte. Fig. 5 shows the incident
monochromatic photon-to-current conversion efficiency (IPCE)
400
500
600
700
800
900
Wavelength (nm)
Fig. 2. Absorption spectra of DPTB, OMeDPTB, and OHexDPTB anchoring on 7
porous TiO2 nanoparticle films.
mm
cyanoacrylic acid block, thus facilitating efficient electron injection
from photoexcited dyes into the TiO2 electrode.
The electrochemical properties of these dyes were studied by
cyclic voltammetry (CV). Representative cyclic voltammograms are
shown in Fig. 4 and the relevant CV data are collected in Table 1. The
three dyes all exhibit one reversible oxidation wave at lower

7512
L.-Y. Lin et al. / Tetrahedron 68 (2012) 7509e7516
Fig. 3. Frontier molecular orbitals (HOMOs and LUMOs) of the dyes calculated with DFT on a B3LYP/6-31G(d) level.
70
DPTB
60
OMeDPTB
OHexDPTB
DPTB
50
OMeDPTB
40
30
OHexDPTB
20
10
1.0
0.5
0.0
-0.5 -1.0 -1.5
+
0
300
Potential (V vs. Fc /Fc)
400
500
600
700
800
900
Wavelength (nm)
Fig. 4. Cyclic voltammograms of DPTB, OMeDPTB, and OHexDPTB recorded in
solutions.
Fig. 5. IPCE spectra of DSSCs based on the EL2 electrolyte.
Table 2
Performance parameters of the DSSCs based on the organic dyesa
14
Dye
Jsc (mA cmꢀ2
)
Voc (V)
FF
PCE (%)
DPTB
DPTBb
0.31
0.42
0.44
16.12
9.12
13.06
11.10
14.05
0.28
0.25
0.30
0.76
0.41
0.39
0.45
0.57
0.45
0.45
0.45
0.70
0.62
0.62
0.63
0.67
0.039
0.045
0.059
8.57
2.30
3.13
12
10
8
OMeDPTB
OHexDPTB
OMeDPTBb
OHexDPTBb
N719b
DPTBc
OMeDPTBc
OHexDPTBc
N719c
3.16
5.39
6
a
4
The concentration of the organic dyes was maintained at 0.5 mM in a chloro-
benzene solution, with 0.5 mM deoxycholic acid (DCA) as a co-adsorbent [N719 was
fabricated in a tert-butanoleacetonitrile (1:1, v/v) solution under the same condi-
tions]. Performance of DSSCs was measured with a 0.125 cm2 working area. Irra-
diating light: AM 1.5G (100 mW cmꢀ2).
2
0
0.0
0.1
0.2
0.3
0.4
0.5
b
The composition for the EL1 electrolyte is: 0.6 M 1-butyl-3-methylimidazolium
Voltage (V)
iodide (BMII), 0.05 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine (TBP), and 0.1 M
guanidinium thiocyanate in a mixture of acetonitrile and valeronitrile (85/15, v/v).
c
Fig. 6. Photocurrent density vs voltage for DSSCs based on the EL2 electrolyte under
The composition for the EL2 electrolyte is: 0.6
M 1,2-dimethyl-3-
AM 1.5G simulated solar illumination (100 mW cmꢀ2).
propylimidazolium iodide (DMPII), 0.05 M LiI, 0.03 M I2 in acetonitrile.
spectra of the DSSCs based on the adoption of EL2 electrolyte. In
agreement with the trend in absorption spectra, the IPCE spectra of
the three cells all extend to the near-IR region, and the OMeDPTB-
and OHexDPTB-based devices show red-shifted spectral responses
as compared to the DPTB-based device. Fig. 6 shows the current
density-voltage (JeV) curves of the DSSCs using the EL2 electrolyte
under standard global AM 1.5G solar irradiation (100 mW cmꢀ2).
The Jsc values are well consistent with the IPCE spectra with
a maximum value of 13.06 mA cmꢀ2 generated in the DSSC based
on OMeDPTB, which showed both a broader spectral response and
higher IPCE values. On the other hand, the OHexDPTB-based cell
gave a higher Voc than the other two cells, benefiting from the
presence of two long aliphatic hexyloxy chains on the diary-
lthienylamine donor. It is now well known that the aliphatic alkyl
chains can act as effective blocking units between the TiO2 surface
and electrolyte to prevent the approach of hydrophilic I3ꢀ ions to the

L.-Y. Lin et al. / Tetrahedron 68 (2012) 7509e7516
7513
TiO2 surface, leading to suppressed charge recombination/dark
current and lengthened electron lifetimes and thus an enhanced
Voc (see details in our previous works32,50 and Fig. S2).51 As a whole,
the OHexDPTB-based cell showed the best PCE of 3.16% among
these three DSSCs, with a Jsc of 11.10 mA cmꢀ2, Voc of 0.45 V, and FF
of 0.63.
poured into methanol. The resulting precipitate was filtered,
washed with methanol, and dried. The crude product mixture
consisting of 2a and the disubstituted byproduct was used for the
next step without further separation. A mixture of the crude
product mixture and copper cyanide (4.48 g, 50 mmol) in N,N-
dimethylformamide (108 mL) was stirred and heated to reflux
overnight. The cooled reaction mixture was added into an icy so-
lution of concentrated hydrochloric acid (12 mL, 12 N), iron (III)
chloride (21.87 g), and water (122 mL). The mixture was then
heated at 70 ꢃC for 0.5 h, after which the mixture was extracted
with toluene and water. The combined extracts were washed with
brine, dried over anhydrous magnesium sulfate, and filtered. The
solvent of the filtrate was removed by rotary evaporation, and the
crude product was purified by column chromatography on silica gel
with dichloromethane/hexane (v/v, 1:1) as eluent to afford 3a as
3. Conclusion
In summary, three new organic dyes (DPTB, OMeDPTB, and
OHexDPTB) with a DeAeA configuration have been synthesized,
characterized, and applied in DSSCs. By making use of BT as the
bridging acceptor, these tailor-made dyes showed superior light-
harvesting capabilities to their previously reported pyrimidine-
based analogs. The incorporation of dialkoxy substituents on the
donor moieties endowed OMeDPTB and OHexDPTB with narrower
bandgaps and higher HOMO levels than DPTB. In photovoltaic
characterization, the use of the TBP-containing electrolyte (EL1)
that can negatively shift the conduction band edge of TiO2 about
0.3 V impeded efficient electron injection from the photoexcited
dyes into the TiO2 electrode because of their deep-lying LUMO
levels, thus leading to poor photovoltaic performance. In contrast,
the DSSCs based on these dyes in conjunction with the TBP-free
electrolyte (EL2) showed markedly improved performance with
solar spectral responses extending to the near-IR region, and the
best cell based on OHexDPTB reached a PCE of 3.16% under simu-
lated AM 1.5G irradiation (100 mW cmꢀ2). The current results re-
veal that although the DeAeA dyes generally show better light-
harvesting abilities than their analogs, DSSCs sensitized with
DeAeA dyes may suffer from a trade-off between photocurrent and
photovoltage owing to the possibly limited uses of TBP-containing
electrolytes.
a black solid (2.82 g, 34%). Mp 220e222 ꢃC; IR (KBr)
1584, 1531, 1492, 1461, 1370, 1206, 1054, 898 cmꢀ1; 1H NMR (CDCl3,
400 MHz)
n 3037, 2224,
d
8.15 (d, J¼4.0 Hz, 1H), 7.91 (d, J¼7.6 Hz, 1H), 7.59 (d,
J¼7.6 Hz, 1H), 7.37e7.33 (m, 4H), 7.27 (d, J¼7.2 Hz, 4H), 7.16 (t,
J¼7.2 Hz, 2H), 6.64 (d, J¼4.0 Hz, 1H); 13C NMR (CDCl3, 100 MHz)
d
157.5, 153.7, 151.0, 146.8, 135.9, 132.6, 131.0, 129.4, 127.7, 124.6,
124.2, 121.2, 117.2, 116.0, 100.7; HRMS (m/z, FABþ) Calcd for
C23H14N4S2 410.0660, found 410.0657.
4.1.3. [5-(7-Cyano-2,1,3-benzothiadiazol-4-yl)-thiophen-2-yl]-N,N-
bis(4-methoxyphenyl)amine (3b). The synthetic procedure was
similar to that of 3a, except that the eluent for column purification
was dichloromethane/hexane (v/v, 2:1). 3b was isolated as a black
solid (29%). Mp 156e158 ꢃC; IR (KBr)
n
3040, 2929, 2831, 2225,1605,
1567, 1541, 1421, 1354, 1241, 1170, 1036, 829 cmꢀ1; 1H NMR (CDCl3,
400 MHz)
8.11 (d, J¼4.4 Hz, 1H), 7.84 (d, J¼7.6 Hz, 1H), 7.46 (d,
d
J¼7.6 Hz, 1H), 7.24 (d, J¼8.8 Hz, 4H), 6.90 (d, J¼8.8 Hz, 4H), 6.37 (d,
J¼4.4 Hz, 1H), 3.83 (s, 6H); 13C NMR (CDCl3, 100 MHz)
d 159.9, 157.0,
4. Experimental section
153.8, 150.9, 139.7, 135.9, 132.8, 131.8, 126.3, 124.6, 120.2, 116.2,
114.8, 112.4, 99.4, 55.6, 55.5; HRMS (m/z, FABþ) Calcd for
C25H18N4O2S2 470.0871, found 470.0868.
4.1. Synthetic procedures and characterization
4.1.1. General methods. All chemicals and reagents were used as
received from commercial sources without purification. Solvents
for chemical synthesis were purified by distillation. All chemical
reactions were carried out under an argon or nitrogen atmosphere.
5-(N,N-diphenylamino)-2-(tri-n-butylstannyl)thiophene (1a),52
5-[N,N-bis(4-methoxyphenyl)amino]-2-(tri-n-butylstannyl)thio-
phene (1b),53,54 and 5-[N,N-bis(4-hexyloxypheny)amino]-2-(tri-n-
butylstannyl)thiophene (1c)53,54 were synthesized by a procedure
similar to that described in the next paragraph.
4.1.4. [5-(7-Cyano-2,1,3-benzothiadiazol-4-yl)-thiophen-2-yl]-N,N-
bis(4-hexyloxypheny)amine (3c). The synthetic procedure was
similar to that of 3a. The crude product was purified by column
chromatography on silica gel twice using dichloromethane/hexane
(v/v, 1:1) and then with ethyl acetate/hexane (v/v, 1:12) as eluent to
afford 3c as a black solid (34%). Mp 107e109 ꢃC; IR (KBr)
2957, 2854, 2222, 1605, 1543, 1470, 1352, 1233, 1178, 1025,
835 cmꢀ1 1H NMR (CDCl3, 400 MHz)
n 3043,
;
d
8.11 (d, J¼4.4 Hz, 1H), 7.83
(d, J¼8.0 Hz, 1H), 7.44 (d, J¼8.0 Hz, 1H), 7.23 (dd, J¼2.4, 7.2 Hz, 4H),
6.88 (dd, J¼2.4, 7.2 Hz, 4H), 6.36 (d, J¼4.4 Hz, 1H), 3.96 (t, J¼6.8 Hz,
4H), 1.82e1.78 (m, 4H), 1.50e1.46 (m, 4H), 1.39e1.34 (m, 8H),
To
a stirring solution of 2-(N,N-diphenylamino)thiophene
(5.02 g, 20 mmol) in anhydrous THF (60 mL) was dropwise added n-
BuLi (1.6 M, 16.25 mL, 26 mmol) at ꢀ78 ꢃC under an argon atmo-
sphere. The reaction mixture was stirred for 15 min at ꢀ78 ꢃC,
quickly warmed to room temperature by removing the cooling
bath, and stirred for another 15 min. Tri-n-butyltin chloride
(7.05 mL, 26 mmol) was then added to the mixture in one portion.
After stirring at ambient temperature overnight, the reaction
mixture was poured into water and extracted with diethyl ether.
The combined extracts were washed with brine, dried over anhy-
drous magnesium sulfate, and filtered. The solvent of the extracts
was removed by rotary evaporation to afford 1a as a sticky green oil,
which was used for the next step without further purification.
0.95e0.91 (m, 6H); 13C NMR (CDCl3, 100 MHz)
d 160.1, 156.6, 153.7,
150.8, 139.4, 135.8, 132.8, 131.9, 126.3, 124.2, 119.9, 116.2, 115.2, 112.1,
99.1, 68.2, 31.6, 29.3, 25.8, 22.7, 14.1; HRMS (m/z, FABþ) Calcd for
C35H38N4O2S2 610.2436, found 610.2435.
4.1.5. 7-(5-N,N-Diphenylaminothiophen-2-yl)-2,1,3-benzothiadiazole-
4-carbaldehyde (4a). To a stirring solution of 3a (616 mg, 1.5 mmol)
in anhydrous THF (30 mL) was dropwise added diisopropylalumi-
num hydride (1 M in THF, 3.75 mL, 3.75 mmol) at 0 ꢃC under an
argon atmosphere. After stirring at this temperature for 2 h, the
reaction was quenched with saturated aqueous ammonium chlo-
ride solution (15 mL), and the cooling bath was removed. After
stirring at ambient temperature for another 1 h, the reaction mix-
ture was poured into water and extracted with ethyl acetate. The
combined extracts were washed with brine, dried over anhydrous
magnesium sulfate, and filtered. The solvent of the filtrate was re-
moved by rotary evaporation, and the crude product was purified
by column chromatography on silica gel with dichloromethane/
4.1.2. [5-(7-Cyano-2,1,3-benzothiadiazol-4-yl)-thiophen-2-yl]-N,N-
diphenylamine (3a). A mixture of 1a (20 mmol), 4,7-dibromo-2,1,3-
benzothiadiazole (5.88 g, 20 mmol), and PdCl2(PPh3)2 (281 mg,
0.4 mmol) in anhydrous N,N-dimethylformamide (30 mL) was
stirred and heated at 80 ꢃC under an argon atmosphere overnight.
After cooling to room temperature, the reaction mixture was

7514
L.-Y. Lin et al. / Tetrahedron 68 (2012) 7509e7516
hexane (v/v, 2:1) as eluent to afford 4a as a black solid (200 mg,
32%). Mp 175e177 ꢃC; IR (KBr)
3035, 2839, 2729, 1690, 1587, 1537,
1490, 1369, 1265, 1154, 1058, 837 cmꢀ1; 1H NMR (CDCl3, 400 MHz)
10.59 (s, 1H), 8.16 (d, J¼4.0 Hz, 1H), 8.09 (d, J¼7.6 Hz, 1H), 7.68 (d,
HRMS (m/z, FABþ) Calcd for C28H20N4O4S2 540.0926, found
540.0933.
n
d
4.1.10. 2-Cyano-3-{7-[5-N,N-bis(4-hexyloxypheny)aminothiophen-
2-yl]-2,1,3-benzothiadiazol-4-yl} acrylic acid (OHexDPTB). The
synthetic procedure was similar to that of DPTB and afforded
J¼7.6 Hz, 1H), 7.34e7.30 (m, 4H), 7.26e7.23 (m, 4H), 7.15e7.11 (m,
2H), 6.63 (d, J¼4.0 Hz, 1H); 13C NMR (CDCl3, 100 MHz)
d 188.1, 157.4,
153.6, 151.9, 146.8, 133.3, 133.0, 130.9, 129.4, 128.4, 124.6, 124.2,
123.9, 121.6, 117.3; HRMS (m/z, FABþ) Calcd for C23H15N3OS2
413.0657, found 413.0658.
OHexDPTB as a black solid (90%). Mp 152e154 ꢃC; IR (KBr)
n
3058,
2928, 2860, 2219, 1692, 1579, 1534, 1439, 1349, 1228, 1061,
828 cmꢀ1 1H NMR (CD2Cl2, 400 MHz)
9.05 (s, 1H), 8.66 (d,
;
d
J¼8.4 Hz, 1H), 8.08 (d, J¼4.0 Hz, 1H), 7.55 (d, J¼8.4 Hz, 1H), 7.22 (d,
J¼8.8 Hz, 4H), 6.90 (d, J¼8.8 Hz, 4H), 6.32 (d, J¼4.0 Hz, 1H), 3.97 (t,
J¼6.4 Hz, 4H), 1.83e1.76 (m, 4H), 1.48 (m, 4H), 1.38e1.36 (m, 8H),
4.1.6. 7-[5-N,N-bis(4-methoxyphenyl)aminothiophen-2-yl]-2,1,3-
benzothiadiazole-4-carbaldehyde (4b). The synthetic procedure
was similar to that of 4a. The crude product was purified by column
chromatography on silica gel twice using ethyl acetate/toluene (v/v,
1:40) and then with ethyl acetate/hexane (v/v, 1:3) as eluent to
0.93 (t, J¼6.8 Hz, 6H); 13C NMR (CD2Cl2, 100 MHz)
d 162.5, 157.9,
156.0, 151.5, 148.7, 140.0, 133.8, 133.1, 131.8, 127.4, 125.6, 121.4, 119.6,
117.0, 116.0, 112.5, 69.0, 32.2, 29.8, 26.3, 23.2, 14.4; HRMS (m/z,
FABþ) Calcd for C38H40N4O4S2 680.2491, found 680.2491.
afford 4b as a black solid (37%). Mp 150e152 ꢃC; IR (KBr)
2960, 2837, 1665, 1604, 1546, 1418, 1349, 1235, 1182, 1008,
840 cmꢀ1 1H NMR (CDCl3, 400 MHz)
10.55 (s, 1H), 8.15 (d,
n 3070,
;
d
4.2. Measurements of UVevis absorption spectra of dyes in
J¼4.4 Hz, 1H), 8.04 (d, J¼8.0 Hz, 1H), 7.56 (d, J¼8.0 Hz, 1H), 7.24 (dd,
CH2Cl2 solutions
J¼2.4, 6.8 Hz, 4H), 6.90 (dd, J¼2.4, 6.8 Hz, 4H), 6.38 (d, J¼4.4 Hz,1H),
3.83 (s, 6H); 13C NMR (CDCl3, 100 MHz)
d
187.9, 160.0, 157.0, 153.7,
UVevis absorption spectra of dyes in CH2Cl2 solutions were
151.8, 139.7, 133.7, 133.3, 131.9, 126.3, 125.3, 123.1, 120.5, 114.7, 112.6,
55.5; HRMS (m/z, FABþ) Calcd for C25H19N3O3S2 473.0868, found
473.0875.
recorded with a spectrophotometer (JASCO V-670).
4.3. Measurement of UVevis absorption spectra of dye-
loaded nanoporous TiO2 films
4.1.7. 7-[5-N,N-bis(4-hexyloxypheny)aminothiophen-2-yl]-2,1,3-
benzothiadiazole-4-carbaldehyde (4c). The synthetic procedure was
similar to that of 4a, except that the eluent for column purification
was toluene. 4c was isolated as a black solid (24%). Mp 106e108 ꢃC;
A
7-mm-thick transparent porous TiO2 nanoparticle layer
(adopting 20-nm anatase TiO2 nanoparticles) was coated on a glass
plate by the doctor-blade method. After sintering at 500 ꢃC for
30 min, the TiO2 film was immersed into a dye solution (0.5 mM dye
in chlorobenzene) at room temperature for 24 h. Then the UVevis
absorption spectrum of the dye-loaded TiO2 film was recorded on
a spectrophotometer.
IR (KBr)
n
3066, 2956, 2854,1673,1605,1538,1435,1346,1239,1179,
10.57 (s, 1H), 8.13 (d,
1051, 832 cmꢀ1; 1H NMR (CD2Cl2, 400 MHz)
d
J¼4.4 Hz, 1H), 8.05 (d, J¼7.6 Hz, 1H), 7.62 (d, J¼7.6 Hz, 1H), 7.23 (d,
J¼8.8 Hz, 4H), 6.90 (d, J¼8.8 Hz, 4H), 6.36 (d, J¼4.4 Hz, 1H), 3.97 (t,
J¼6.4 Hz, 4H), 1.83e1.76 (m, 4H), 1.50e1.45 (m, 4H), 1.38e1.36 (m,
8H), 0.93 (t, J¼6.8 Hz, 6H); 13C NMR (CDCl3, 100 MHz)
d
187.9, 160.2,
4.4. Theoretical calculation
156.6, 153.7, 151.8, 139.5, 133.7, 133.3, 132.0, 126.3, 125.1, 123.2,
120.4, 115.2, 112.4, 68.3, 31.7, 29.3, 25.8, 22.7, 14.2; HRMS (m/z,
FABþ) Calcd for C35H39N3O3S2 613.2433, found 613.2432.
Density functional theory (DFT) calculations were conducted by
using the hybrid B3LYP function for geometry optimization. The
molecular orbital levels of HOMO and LUMO were achieved with
the 6-31G(d) basis set implemented in the Gaussian 03 package.
4.1.8. 2-Cyano-3-[7-(5-N,N-diphenylaminothiophen-2-yl)-2,1,3-
benzothiadiazol-4-yl] acrylic acid (DPTB). A mixture of 4a (207 mg,
0.5 mmol), cyanoacetic acid (128 mg, 1.5 mmol), ammonium ace-
tate (27 mg) in glacial acetic acid (9 mL) was stirred and heated at
80 ꢃC for 2 h. After cooling to room temperature, the resulting
precipitate was collected by filtration and thoroughly washed with
water, methanol, and hexane to afford DPTB as a black solid
4.5. Cyclic voltammetry measurement
The electrochemical properties of dyes were investigated by
cyclic voltammetry (CV). The measurement of oxidation potentials
was carried out in anhydrous CH2Cl2 solutions (1.0 mM) containing
(225 mg, 94%). Mp 255e257 ꢃC; IR (KBr)
1533, 1490, 1377, 1235, 1153, 1053, 904, 812 cmꢀ1
tone-d6, 400 MHz)
n
3023, 2225, 1694, 1588,
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as
;
1H NMR (Ace-
a supporting electrolyte, and the measurement of the reduction
potentials was conducted in anhydrous THF solutions (1.0 mM)
containing 0.1 M tetrabutylammonium perchlorate (TBAP) as
a supporting electrolyte, purged with argon prior to conduct the
experiments. A glassy carbon electrode and a platinum wire were
used as the working and counter electrodes, respectively. All po-
tentials were recorded versus Ag/AgCl (saturated) as a reference
electrode and calibrated with the ferrocene/ferrocenium redox
d
9.13 (s, 1H), 8.77 (d, J¼8.0 Hz, 1H), 8.23 (d,
J¼4.4 Hz, 1H), 8.04 (d, J¼8.0 Hz, 1H), 7.41 (t, J¼8.0 Hz, 4H), 7.31e7.29
(m, 4H), 7.21 (t, J¼7.2 Hz, 2H), 6.66 (d, J¼4.4 Hz, 1H); 13C NMR
(DMSO-d6, 100 MHz)
d 163.3, 158.0, 154.3, 150.2, 146.5, 146.1, 131.1,
130.5, 130.0, 129.8, 127.5, 125.0, 124.2, 122.5, 120.1, 116.5, 102.5;
HRMS (m/z, FABþ) Calcd for C26H16N4O2S2 480.0715, found
480.0712.
couple. All measurement was performed at
a scan rate of
4.1.9. 2-Cyano-3-{7-[5-N,N-bis(4-methoxyphenyl)aminothiophen-2-
yl]-2,1,3-benzothiadiazol-4-yl} acrylic acid (OMeDPTB). The syn-
thetic procedure was similar to that of DPTB and afforded
100 mV sꢀ1
.
4.6. Fabrication of dye-sensitized solar cells
OMeDPTB as a black solid (80%). Mp 235e237 ꢃC; IR (KBr)
n
3047,
2951, 2219,1693, 1582, 1536,1454,1324,1213,1149,1070, 817 cmꢀ1
1H NMR (DMSO-d6, 400 MHz)
;
To prepare the DSSC working electrodes, the FTO glass plates
were first cleaned in a detergent solution under ultrasonication for
15 min, and then rinsed with water and ethanol. A layer of 20-nm-
sized anatase TiO2 nanoparticles as the transparent nanocrystalline
layer was first coated on the FTO glass plate by the doctor-blade
method. After drying the film at 120 ꢃC, another layer of 400-nm-
d
8.92 (s, 1H), 8.61 (d, J¼8.0 Hz, 1H),
8.14 (d, J¼4.4 Hz, 1H), 8.00 (d, J¼8.0 Hz, 1H), 7.29 (d, J¼8.8 Hz, 4H),
7.00 (d, J¼8.8 Hz, 4H), 6.30 (d, J¼4.4 Hz, 1H), 3.78 (s, 6H); 13C NMR
(DMSO-d6, 100 MHz)
d 163.6, 161.4, 157.2, 154.5, 150.0, 146.0, 139.2,
131.7, 130.0, 126.9, 124.1, 121.1, 118.8, 116.7, 115.1, 111.0, 101.0, 55.3;

L.-Y. Lin et al. / Tetrahedron 68 (2012) 7509e7516
7515
sized anatase TiO2 nanoparticles was then deposited as the light
scattering layer. The resulting working electrode was composed of
a 12- m-thick transparent TiO2 nanoparticle layer (particle size:
20 nm) and a 4- m-thick TiO2 scattering layer (particle size:
Supplementary data
m
These data include an absorption spectrum of DPTB in a THF
solution, electrochemical impedance spectra, and copies of 1H and
13C NMR spectra. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.tet.2012.05.052.
m
400 nm). The nanoporous TiO2 electrodes were then sequentially
heated at 150 ꢃC for 10 min, at 300 ꢃC for 10 min, at 400 ꢃC for
10 min, and finally at 500 ꢃC for 30 min. After cooling, the nano-
porous TiO2 electrodes were immersed into a chlorobenzene so-
lution containing organic dyes (0.5 mM) or acetonitrile/tert-butanol
mixture (1:1) containing N719 (0.5 mM) with deoxycholic acid
(0.5 mM, DCA) as a co-adsorbent at room temperature for 24 h.
Counter electrodes of the DSSCs were prepared by depositing 40-
nm-thick Pt films on the FTO glass plates by e-beam evaporation.
The dye-adsorbed TiO2 working electrode and a counter electrode
were then assembled into a sealed DSSC cell with a sealant spacer
between the two electrode plates. A drop of electrolyte solution
(EL1: 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.05 M LiI,
0.03 M I2, 0.5 M 4-tert-butylpyridine, 0.1 M guanidinium thiocya-
nate in a mixture of acetonitrile and valeronitrile (85/15, v/v) or
EL2: 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII),
0.05 M LiI, 0.03 M I2 in acetonitrile) was injected into the cell
through a drilled hole. Finally, the hole was sealed using the sealant
and a cover glass. An anti-reflection coating film was adhered to the
DSSC. A mask with an aperture area of 0.125 cm2 was covered on
a testing cell during photocurrentevoltage and incident photon-to-
current conversion efficiency measurements.
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