5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole-bridged dyes for dye-sensitized solar cells with high open-circuit voltage performance

Article abstract of DOI:10.1002/ejoc.201201424

Three new metal-free dyes with a 5,6-bis(octyloxy)benzo[c][1,2,5] thiadiazole core (DOBT-I-III) have been designed and synthesized for use as DSSCs. Their absorption properties and electrochemical and photovoltaic performances have been investigated syste

Full text of DOI:10.1002/ejoc.201201424

FULL PAPER
DOI: 10.1002/ejoc.201201424
5,6-Bis(octyloxy)benzo[c][1,2,5]thiadiazole-Bridged Dyes for Dye-Sensitized
Solar Cells with High Open-Circuit Voltage Performance
Long Chen,[a][‡] Xin Li,[b][‡] Weijiang Ying,[a] Xiaoyu Zhang,[a] Fuling Guo,[a] Jing Li,*[a] and
Jianli Hua*[a]
Keywords: Heterocycles / Dyes/pigments / Electrochemistry / Photovoltaic properties / Solar cells
Three new metal-free dyes with a 5,6-bis(octyloxy)benzo-
[c][1,2,5]thiadiazole core (DOBT-I–III) have been designed
and synthesized for use as DSSCs. Their absorption proper-
ties and electrochemical and photovoltaic performances have
been investigated systematically. The DSSCs based on
DOBT-I–III show high open-circuit voltages (Voc) of 829, 818,
and 784 mV, respectively. Of the three dyes, DOBT-III, which
contains a thiophene-bridging linker, exhibits the best photo-
voltaic performance: a short-circuit photocurrent density (Jsc)
of 12.74 mAcm–2 and a fill factor (FF) of 0.73, which corre-
sponds to an overall conversion efficiency of 7.29% under
standard global AM 1.5 solar conditions.
Introduction
Remarkably, the strong electron-withdrawing benzothia-
diazole unit with the small band gaps in these new com-
pounds are highly suited to optimizing energy levels and
improving light-harvesting and efficiencies.[16] In 2008, Ko
and co-workers[17] reported that DSSCs with benzothia-
diazole had conversion efficiencies of 6.61%. By introduc-
ing two alkyl chains onto benzothiadiazole, the dye effec-
tively prevents the redox mediator from approaching the
TiO2 surface. Zhu et al.[18] used indoline as a powerful elec-
tron donor, which achieved conversion efficiencies of
9.04%, even though the dyes have low Voc values. It is well
known that a low voltage is one of the most important fac-
tors that limit the efficiency of DSSCs. In this work, we
introduced two longer alkyl chains onto benzothiadiazole
units to obtain three new dyes, DOBT-I–III, in an attempt
to improve Voc and optimize their efficiency in DSSCs. We
also synthesized a benzothiadiazole-containing sensitizer
BT without alkyl groups for the purpose of comparison.
In recent decades, dye-sensitized solar cells (DSSCs) have
attracted considerable scientific and industrial interest as
promising candidates for new renewable energy sources be-
cause of their relatively high efficiencies and low cost com-
pared with conventional inorganic photovoltaic devices.[1]
In the past few years, such solar cells have witnessed tre-
mendous progress. For example, a power conversion effi-
ciency (PCE) of 12.0% has been reached with Ru complex
dyes under standard AM 1.5 solar light irradiation[2] and a
high performance efficiency of about 12.3% has also been
achieved very recently for Zn–porphyrin dyes.[3] The sensi-
tizer is always a crucial element in DSSCs, exerting a signifi-
cant influence on the power conversion efficiency (η) as well
as on the device stability. Metal-free organic dyes[4] have
become more promising as a result of their facile structure
modification, high extinction coefficients, and low cost
compared with classical ruthenium polypyridine complexes.
Recently, novel organic dyes based on ethylenedioxy-
thiophene,[5] dithienosilole,[6] 2,2-bithiazole,[7] quinoxal-
ine,[8] benzotriazole,[9] diketopyrrolo-pyrrole,[10] iso-
indigo,[11] pyrimidine,[12] pyridine,[13] and squaraine[14] have
been investigated as sensitizers for DSSCs and considerable
progress has been made in this field.[15]
[a] Key Laboratory for Advanced Materials and the Institute
of Fine Chemicals, East China University of Science and
Technology,
Shanghai 200237, China
Fax: +86-21-64252758
E-mail: jlhua@ecust.edu.cn
lijhy@ecust.edu.cn
Homepage: http://hyxy.ecust.edu.cn
[b] Department of Theoretical Chemistry and Biology, School of
Biotechnology, KTH Royal Institute of Technology,
10691 Stockholm, Sweden
[‡] These authors contributed equally to this work
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201424.
Scheme 1. Molecular structures of DOBT-I–III and the reference
dye BT.
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Benzothiadiazole-Bridged Dyes for Dye-Sensitized Solar Cells
The molecular structures of the four dyes are shown in duced the disubstituted side-products as well, but the
Scheme 1. Finally, the three new sensitizers were applied monosubstituted compound can be easily separated by col-
successfully to the sensitization of nanocrystalline TiO2- umn chromatography. In the next step, Suzuki coupling re-
based solar cells and the corresponding photovoltaic per- action of 7 with (4-formylphenyl)-, (5-formylfuran-2-yl)-, or
formances and electronic and optical properties were (5-formylthiophen-2-yl)boronic acid afforded aldehyde pre-
studied.
cursors 8–10, respectively. Finally, the target compounds
DOBT-I–III were synthesized by Knoevenagel condensa-
tion of the corresponding aldehydes with cyanoacetic acid
in the presence of ammonium acetate. All the key interme-
diates and the three new organic octyloxy-benzothiadia-
zole-based sensitizers DOBT-I–III were confirmed by 1H
and 13C NMR spectroscopy and HRMS.
Results and Discussion
Synthesis
The synthetic route to the dyes DOBT-I–III is depicted
in Scheme 2. The octyloxy substituents on the benzothiadi-
azoles can improve their solubility and form a tightly
Photophysical Properties
–
packed insulating monolayer preventing I3 or cations ap-
proaching the TiO2, hence increasing the electron lifetime
The absorption spectra of DOBT-I–III in CH2Cl2 and
and open circuit voltage (Voc).[19] 1,2-Bis(octyloxy)benzene BT in CH2Cl2/methanol (low solubility in CH2Cl2) are
(2) was prepared by a standard alkylation of catechol with shown in Figure 1 and the corresponding data are summa-
bromooctane in EtOH at 80 °C.[20]
rized in Table 1. The dyes exhibit two major prominent
Electrophilic aromatic nitration of 2 afforded the substi- bands at 310–317 and 423–468 nm. The band with the ab-
tuted 1,2-dinitro-4,5-bis(octyloxy)benzene (3).[21] Reduction sorption maximum below 400 nm exhibits shoulders, which
of the nitro groups with tin(II) chloride[22] gave the diamine have been attributed to a more localized π–π* transition.
as its hydrochloride salt 4, which had to be used directly The origin of this absorption band will be further discussed
because of its unstable nature. Treatment of the diamine in connection with theoretical simulations. The absorption
with thionyl chloride afforded 5, which was brominated at around 423–468 nm can be ascribed to an intramolecular
with molecular bromine to give monomer 6 in excellent charge transfer (ICT) between the triphenylamine donor
yield.
and the cyanoacetic acceptor, the absorption maxima for
The Suzuki coupling reaction of
6
with [4-(di- BT and DOBT-I–III appearing at 460, 423, 463, and
phenylamino)phenyl]boronic acid afforded the monosubsti- 468 nm, respectively. The effect of the conjugation bridge
tuted compound 7. Unfortunately, the reaction also pro- on the absorption peak is quite distinct. A hypochromic
Scheme 2. Synthetic routes to the three benzothiadiazole dyes DOBT-I–III.
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Table 1. Optical and electrochemical properties of the benzothiadiazole dyes.
[a]
[b]
[d]
Dye
λmax [nm]
λmax
HOMO[c]
[V] (vs. NHE)
E0–0
LLUMO[e]
[V] (vs. NHE)
(ε [104 M–1 cm–1])
[nm]
[eV]
BT
460 (1.10)
423 (0.76)
463 (1.79)
468 (1.61)
448
412
457
457
1.00
0.99
1.00
1.01
2.16
2.49
2.22
2.23
–1.16
–1.50
–1.22
–1.22
DOBT-I
DOBT-II
DOBT-III
[a] Absorption maximum of BT in CH2Cl2/methanol and DOBT-I–III in CH2Cl2 (3ϫ 10–5 m). [b] Absorption maximum on a TiO2 film
[without chenodeoxycholic acid (CDCA)]. [c] The HOMO was internally calibrated with ferrocene (0.4 V versus NHE) in CH2Cl2. [d]
E0–0 derives from the wavelength at absorption thresholds in the absorption spectra of dye-loaded 4 μm TiO2 films. [e] The LUMO was
calculated by subtracting E0–0 from the HOMO.
shift of 37 nm is observed when replacing the benzothiadi-
azole unit (BT) with the 5,6-bis(octyloxy)benzo[c][1,2,5]-
thiadiazole unit (DOBT-I) in the π bridge. The two dyes
DOBT-II and DOBT-III, also with the 5,6-bis(octyloxy)-
benzo[c][1,2,5]thiadiazole unit as the π bridge, have similar
absorption properties. Compared with DOBT-I, DOBT-II
and DOBT-III exhibit redshifted absorptions of about 40
and 45 nm, respectively. This can be explained by the fact
that the replacement of the benzene moiety by thiophene
or furan leads to a strong enhancement in the coplanarity
between the 5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole unit
and the cyanoacetic anchoring moiety.
Figure 2. Normalized absorption spectra of BT and DOBT-I–III
on 4 μm transparent TiO2 films.
Electrochemical Properties
To evaluate the possibility of electron injection and dye
regeneration in the conduction band of TiO2, cyclic voltam-
metry was performed in DCM with the ferrocene/ferrocen-
ium (Fc/Fc+) redox couple as an external standard. The
voltammograms are shown in Figure 3. The highest-occu-
pied molecular orbitals (HOMOs) of BT and DOBT-I–III,
which correspond to their first redox potentials, are 1.00,
0.99, 1.00, and 1.01 V versus the normal hydrogen electrode
(NHE), respectively. The band gap energies (E0–0) of the
four dyes (BT and DOBT-I–III) are 2.16, 2.49, 2.22, and
2.23 eV, respectively, which were calculated from the ab-
sorption thresholds of the dye-adsorbed TiO2 films. The es-
Figure 1. Absorption spectra of BT in CH2Cl2/methanol and
DOBT-I–III in CH2Cl2.
The absorption spectra of BT and DOBT-I–III on 4 μm timated excited-state potentials corresponding to the low-
TiO2 films are shown in Figure 2. The maximum absorption est-unoccupied molecular orbital (LUMO) levels, calculated
peaks for BT and DOBT-I–III on TiO2 films are at 448, from EHOMO – E0–0, are –1.16, –1.50, –1.22, and –1.22 V
412, 457, and 457 nm, respectively. Compared with the versus NHE, respectively. The HOMO and LUMO levels
solution spectra, the absorption bands of the dyes on the of the four dyes are presented in Table 1. Thus, we find the
TiO2 films are blueshifted by 12, 11, 6, and 11 nm, respec- LUMO levels of BT and DOBT-I–III are more negative
tively. Generally, the sensitizers have a tendency to aggre- than the conduction band (Ecb) of the TiO2 electrode
gate at solid/liquid interfaces when they are adsorbed onto (–0.5 V vs. NHE), which indicates that electron injection
TiO2 films.[23] The experimental results in this work indicate from the LUMO orbital into the conduction band of TiO2
that those dyes form weaker blueshifted hydrogen aggre- is energetically permitted.[24] The HOMO levels of BT and
–
gates, which suggest that they reduced the aggregation of DOBT-I–III are sufficiently more positive than the I–/I3
sensitizers. This can be explained by the long alkoxy chains redox couple (0.4 V) to ensure the thermodynamic regener-
reducing the aggregation of sensitizers.
ation of the dyes. This screening strategy has helped us to
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Benzothiadiazole-Bridged Dyes for Dye-Sensitized Solar Cells
optimize the TiO2/dye/hole-transporting materials in terms efficiency of DOBT-I is higher than that of BT in the region
of the balance between photovoltage, driving forces, and of 395–510 nm, although the IPCE value of DOBT-I is sig-
spectral response for future preparation of highly efficient nificantly lower than BT in the remaining visible region (see
dyes by matching suitable donor–spacer–acceptor construc- Figure 4), which suggests that DOBT-I has a higher cell
tions.[25]
performance. Compared with DOBT-I, DOBT-II and
DOBT-III have broader action spectra, which means that
they can generate greater conversion efficiency. In addition,
although the IPCE efficiencies of DOBT-III are lower than
those of DOBT-II in the region 450–550 nm, the IPCE val-
ues of DOBT-III are higher and broader than those of
DOBT-II in the remaining region, which indicates that the
DOBT-III-sensitized TiO2 electrode would generate the
highest conversion yield among the four dyes.
To optimize the solar cell performance of DOBT-I–III,
the effect of chenodeoxycholic acid (CDCA) content as co-
sensitizer was investigated and the data are reported in
Table 2. The values of Voc for DOBT-I–III increase from
768 to 796 mV, 638 to 775 mV, and 658 to 795 mV, respec-
tively, as the concentration of CDCA increases (from 0 to
30 mm), whereas the values of Jsc for DOBT-I–III initially
increase and then decrease to the starting value. It is known
that the addition of CDCA could reduce the aggregation of
the dye on the TiO2 surface, thereby reducing the deactiva-
tion due to quenching of intermolecular excited-electron ag-
gregation and increasing the injection efficiency of the ex-
cited-state photoelectron to the TiO2 conduction band.[26]
On the other hand, dyes and CDCA are adsorbed competi-
tively by the TiO2 surface, which will reduce the amount of
coverage of the dye. Thus, excess CDCA (30 mm) reduces
the performance of the solar cells based on DOBT-I–III.
Figure 3. Cyclic voltammetry curves of BT and DOBT-I–III in
CH2Cl2.
Photovoltaic Performances of DSSCs
Figure 4 shows the incident photon-to-current conver-
sion efficiency (IPCE) as a function of incident wavelength
for DSSCs based on BT and DOBT-I–III. The dye-coated
TiO2 film was used as the working electrode, platinized
FTO glass as the counter electrode, and 0.5 m 1,2-dimethyl-
3-propylimidazole iodide (DMPII), 0.05 m I2, 0.1 m LiI, and
0.5 m 4-tert-butylpyridine (TBP) in acetonitrile as the redox
electrolyte. Generally, the solar cells sensitized by DOBT-II
and DOBT-III show better performances than BT and
DOBT-I due to their broader photocurrent action spec-
trum.
Table 2. Influence of CDCA on the performance of DSSCs.
Dye
CDCA
Jsc
Voc
[mV]
FF
η
[%]
[molL–1] [mAcm–2]
DOBT-I
0
6.54
6.62
6.67
5.74
9.14
10.03
9.23
9.14
10.20
11.53
10.49
10.11
768
794
796
796
638
729
757
775
658
748
790
795
0.67
0.76
0.74
0.76
0.64
0.68
0.74
0.76
0.63
0.60
0.73
0.74
3.37
3.97
3.93
3.78
3.71
4.95
5.20
5.39
4.24
5.18
6.07
5.81
10
20
30
0
10
20
30
0
DOBT-II
DOBT-III
10
20
30
To improve the Voc of the dyes, the effect of TBP content
in the dye solution on the performance of solar cells was
also investigated and the data are presented in Table 3. The
values of Voc for DOBT-I–III increase from 814 to 852 mV,
726 to 828 mV, and 765 to 816 mV, respectively, with in-
creasing concentration of TBP (0.25–0.75 m). This can be
explained by the adsorption of TBP on the TiO2 surface
preventing the dye molecules from reaching the porous
Figure 4. Incident photon-to-current conversion efficiencies of BT
and DOBT-I–III.
The IPCE of DOBT-II and DOBT-III exceeds 75.0% in membrane on TiO2 on which recombination takes place. In
the spectral range of 430–550 nm, whereas the BT- and contrast, Jsc values decrease progressively with increasing
DOBT-I-sensitized cells exhibit IPCE values above 70.0% concentrations of TBP (0.25–0.75 m) because the addition
in the range of 430–510 nm. On the other hand, the IPCE of TBP can reduce the photoinduced electron injection
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power and the photogenerated electron concentration in the AM 1.5 solar light conditions and with 0.5 m DMPII,
porous film as well as extending electronic transmission 0.05 m I2, 0.1 m LiI, and 0.5 m 4-tert-butylpyridine in aceto-
time and reducing the short-circuit current density. Thus, nitrile as the redox electrolyte. The short-circuit current
photoelectric conversion efficiency can be improved by add- (Jsc) is related to the molar extinction coefficient of the dye
ing appropriate concentrations of 4-tert-butylpyridine. molecule, a higher molar extinction coefficient showing
When the concentration of 4-tert-butylpyridine was in- good light-harvesting ability and yielding a higher short-
creased to 0.50 m, the DSSCs based on DOBT-II and circuit current. The DOBT-II and DOBT-III dyes have
DOBT-III gave the best performance.
higher light-harvesting efficiencies and consequently an im-
proved Jsc due to their larger molar extinction coefficients
compared with DOBT-I. Compared with DOBT-II, DOBT-
III has a slightly lower molar extinction coefficient, but the
highest photovoltaic performance among the four dyes.
This is probably due to the fact that DOBT-III has a
broader high value of IPCE than DOBT-II. Note that
DOBT-I–III have higher open-circuit voltages (784–
829 mV) than BT (622 mV) and N719 (759 mV) measured
under the same conditions (Table 4); the introduction of
two long alkyl chains into the benzothiadiazole ring can
inhibit the charge recombination and improve Voc. DSSCs
based on DOBT-III exhibit the best overall light-to-elec-
tricity conversion efficiency of 7.29% (Jsc = 12.74 mAcm–2,
Voc = 784 mV, FF = 0.73) under AM 1.5 irradiation
(100 mWcm–2), which reached 92% that of an N719-based
device fabricated under similar conditions.
Table 3. Influence of TBP on the performance of DSSCs.
Dye
TBP
Jsc
Voc
[mV]
FF
η
[%]
[molL–1] [mAcm–2]
DOBT-I
0.25
0.50
0.75
0.25
0.50
0.75
0.25
0.50
0.75
7.44
7.22
7.63
11.60
9.85
8.30
12.49
11.38
11.03
814
830
852
726
808
828
765
808
816
0.72
0.75
0.73
0.72
0.74
0.75
0.70
0.75
0.74
4.36
4.47
4.78
6.08
6.08
5.15
6.66
6.90
6.63
DOBT-II
DOBT-III
To further optimize the Jsc values of the dyes, the effect
of DMPII content in the dye solution on the solar cell per-
formance was also investigated and the data are summa-
rized in Table 4. The values of Jsc for DOBT-I–III increase
from 6.81 to 7.86 mA/cm2, 10.82 to 12.10 mA/cm2, and
12.06 to 12.74 mA/cm2, respectively, with increasing con-
centration of DMPII (0.1–0.5 m). This is because the in-
crease in the concentration of DMPII leads to an increase
in the electrical conductivity and a decrease in the interface
transfer resistance Rct. However, when the concentration of
DMPII was increased from 0.50 to 0.60 mm, the value of
Jsc declined due to the higher solution viscosity and lower
–
diffusion coefficient of I3 .
Table 4. Influence of DMPII on the performance of DSSCs.
Dye
DMPII
Jsc
Voc
[mV]
F
η
[%]
[molL–1]
[mA cm–2]
Figure 5. Current–voltage curves for BT and DOBT-I–III.
DOBT-I
0.1
0.3
0.4
0.5
0.6
0.1
0.3
0.4
0.5
0.6
0.1
0.3
0.4
0.5
0.6
0.5
6.81
7.01
7.09
7.86
7.42
10.82
11.10
11.72
12.10
10.12
12.06
12.08
12.35
12.74
12.13
10.41
770
803
815
829
802
778
810
780
818
722
781
799
785
784
777
622
0.72
0.75
0.72
0.74
0.74
0.72
0.72
0.71
0.73
0.73
0.73
0.71
0.72
0.73
0.73
0.69
3.97
4.25
4.13
4.82
4.44
6.07
6.44
6.48
7.19
5.31
6.86
6.90
6.96
7.29
6.89
4.48
Electrochemical Impedance Spectroscopy (EIS)
EIS was employed to study the electron recombination
in DSSCs based on BT and DOBT-I–III dyes under a
–0.70 V bias applied voltage in the dark. The Nyquist and
Bode plots are shown in Figure 6 along with the equivalent
circuits. Some important parameters can be obtained by fit-
ting the EIS spectra with an electrochemical model.[27] RS,
Rrec, and RCE represent the series resistance and the charge-
transfer resistance at the TiO2/dye/electrolyte interface and
counter electrode, respectively. The reaction resistances of
the DSSCs consisting of Pt/electrolyte and TiO2/dye/elec-
trolyte interfaces based on BT and DOBT-I–III were tested
DOBT-II
DOBT-III
BT
Figure 5 shows the optimized current–voltage character- by using the ZSimpWin software with an equivalent circuit.
istics of DSSCs fabricated with benzothiadiazole dyes BT The series resistance (RS) and charge transport resistance at
and DOBT-I–III as sensitizers under standard global the counter electrode/electrolyte (RCE) corresponding to the
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Benzothiadiazole-Bridged Dyes for Dye-Sensitized Solar Cells
first semicircle show almost the same value for the four dye- 54.1 ms, respectively. Paradoxically, DOBT-I has a lower
based DSSCs as a result of the same electrode material and photoelectric conversion efficiency, which may be caused by
same electrolyte, and Rrec corresponds to the middle semi- poor electron transport in the dye molecules.
circle (Figure 6). From the EIS measurements, the electron
lifetime (τe) expressing the electron recombination between
Theoretical Approach
the electrolyte and TiO2 was calculated in accord with the
literature.[28]
Density functional theory (DFT) and time-dependent
density functional theory (TDDFT) calculations were car-
ried out with the Gaussian 09 package[30] to further investi-
gate the electron distribution in the frontier molecular or-
bitals and the electronic transition processes upon photoex-
citation. The ground-state geometries of the dyes BT and
DOBT-I–III were optimized in the gas phase by DFT using
the hybrid functional B3LYP[31] and the standard 6-31G(d)
basis set. For the TDDFT calculations, performed on the
B3LYP-optimized ground-state geometries, the hybrid func-
tional BH and HLYP[30,32] was used with the 6-31G(d) basis
set, which is a feasible choice for investigating excitations
with charge-transfer character. The solvation effect was
taken into account in the TDDFT calculations in CH2Cl2
with the nonequilibrium version of the C-PCM model[33]
implemented in Gaussian 09.[30]
Figure 7 shows the ground-state geometries of the dyes
with the dihedral angles between two neighboring conju-
gated segments indicated. There is a significant twist of the
benzo[2,1,3]thiadiazole and 5,6-bis(octyloxy)benzo[2,1,3]-
thiadiazole entities from coplanarity with the phenyl entity,
and the dihedral angle of the former (BT) is 8.0° smaller
than that of the latter (DOBT-I). In contrast, the dihedral
angles between the furyl or thienyl entity and the 5,6-bis-
(octyloxy)benzo[2,1,3]thiadiazole entity in DOBT-II and
DOBT-III are only 9.2 and 4.2°, respectively. The frontier
orbitals of the dyes are also shown in Figure 7.
Figure 6. Electron impedance spectra: (a) Nyquist plots, (b) Bode
phase plots, and (c) equivalent circuits for DSSCs based on dyes
measured at a –0.70 V bias in the dark. The lines of (a) and (b)
show theoretical fits using the equivalent circuits (c).
Figure 6b shows the Bode plots of DSSCs based on BT
and DOBT-I–III. Each of the EIS Bode plots exhibit two
peaks. The higher-frequency peak corresponds to charge
transfer at the Pt/electrolyte interface and the lower-fre-
quency peak corresponds to the charge transfer at the TiO2/
dye/electrolyte interface, which is related to the charge re-
combination rate, the reciprocal of which is associated with
the electron lifetime.[29] In Figure 6, the low-frequency peak
of DOBT-I is at a lower frequency than those of DOBT-II
and DOBT-III, which indicates that DOBT-I-based DSSCs
have a longer electron lifetime, which leads to a lower rate
of charge recombination and a higher Voc. The electron life-
Figure 7. Frontier orbitals of BT and DOBT-I–III (isodensity: 0.04
a.u.) Hydrogen atoms have been omitted for clarity. [a] The dihe-
dral angles between the benzothiadiazole rings and π spacer are
times of BT and DOBT-I–III are 4.7, 173.0, 78.3, and presented.
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The trend in the calculated HOMO energies (Table 5), be seen that –ΔGinject is positively and linearly correlated to
DOBT-III (0.94 eV) = DOBT-II (0.94 eV) = BT (0.94 eV) the HOMO–LUMO gap for all the organic dyes investi-
Ͼ DOBT-I (0.91 eV), is in qualitative agreement with that gated in this work. In contrast, the magnitude of the oscil-
observed in the electrochemical studies. The presence of the lator strength is negatively and linearly correlated to the
octyloxy chains introduces greater steric hindrance that HOMO–LUMO gap. Taking all these key parameters into
leads to a larger dihedral angle between the benzothia- account, we predict that DOBT-II and DOBT-III are the
diazole ring and π spacer, which results in diminished π best candidate dyes for DSSC applications. Furthermore,
conjugation and a slightly enhanced HOMO–LUMO en- previous experience[35] indicates that the photovoltaic con-
ergy gap compared with those of previously reported com- version efficiency is slightly enhanced when the cyano-
pounds.[18] For all the dyes, the lowest-energy excitations to acrylic acid group is covalently bonded to a thiophene ring
the S1 state mainly originate from coupling between the instead of a furan. Therefore DOBT-III is expected to exhi-
HOMO and LUMO orbitals (Figure 7). The differences be- bit the best performance in photovoltaic conversion, in
tween the electron density distributions in the HOMO and agreement with experimental measurements (see Tables 2–
LUMO orbitals strongly indicate a predominantly intramo- 4).
lecular charge-transfer (ICT) character. The oscillator
strengths (f = 0.94–1.49) of these dyes also reflect the mea-
sured absorption coefficients.
Table 6. Computed ICT absorption wavelengths, oscillator
strengths, and free energies for the electron injection process.
[a]
λmax [nm]
Oscillator str.
ΔGinject [eV]
BT
453 (465)
416 (422)
478 (463)
484 (467)
1.2835
0.9429
1.4209
1.4965
–1.63
–1.89
–1.49
–1.45
DOBT-I
DOBT-II
DOBT-III
Table 5. Computed energy levels of the frontier molecular orbitals
and HOMO–LUMO energy gaps for BT and DOBT-I–III. The
computed redox potentials with respect to NHE are shown in pa-
rentheses.
[a] Experimentally measured values of λmax are shown in parenthe-
ses for comparison.
Compound
HOMO [eV]
LUMO [eV]
ΔE [eV]
BT
–5.38 (0.94)
–5.35 (0.91)
–5.38 (0.94)
–5.38 (0.94)
–2.84 (–1.60)
–2.73 (–1.71)
–2.89 (–1.55)
–2.94 (–1.50)
2.54
2.62
2.49
2.44
DOBT-I
DOBT-II
DOBT-III
As suggested by Preat et al.,[34] the oxidation potential of
the dye in the ground state [EOX(GS)] and the free-energy
change for the electron injection process (–ΔGinject) can be
theoretically computed within the framework of DFT, and
a DSSC is expected to exhibit good performance as long as
the dye has 1) a large EOX(GS), 2) a large –ΔGinject, 3) a
high light-harvesting efficiency, and 4) sufficient overlap be-
tween its absorption bands and the emission spectrum of
the sun. Our calculations suggest that the choice of π-conju-
gation bridge between benzothiadiazole and cyanoacrylic
acid significantly affects the performance of the DSSC
(Table 6). When a phenyl ring is used to connect the benzo-
thiadiazole and cyanoacrylic acid groups, the energy level
of the LUMO is increased, resulting in a larger HOMO–
LUMO gap, a larger –ΔGinject, and a smaller oscillator
strength of the ICT absorption band (e.g., in BT and
DOBT-I), which leads to a lower light-harvesting efficiency.
Figure 8. Dependence of –ΔGinject and oscillator strengths of the
ICT absorption bands on the HOMO–LUMO gap.
Conclusions
Three metal-free organic dyes (DOBT-I–III) comprising
When a thiophene or furan ring is employed as the π-conju- a triarylamine moiety as the electron donor, cyanoacrylic
gation bridge, the HOMO–LUMO gap decreases, leading acid as the anchoring group, 5,6-bis(octyloxy)benzo[c]-
to a smaller –ΔGinject and an enhanced oscillator strength. [1,2,5]thiadiazole as the bridge, and phenyl, furan, or
Based on these results, we suggest that the theoretical prin- thiophene as a linker were designed and synthesized for use
ciples for the design of DSSC dyes are 1) the choice of an as dye-sensitized solar cells (DSSCs). The results based on
appropriate donor group with large EOX(GS) and 2) the photovoltaic experiments show that dyes with a bis(octyl-
choice of an appropriate π-conjugation bridge that ensures oxy)benzothiadiazole unit exhibit a higher open-circuit
a good balance between –ΔGinject and the oscillator strength voltage (0.784–0.829 V). The power conversion efficiency
of the ICT absorption band. Figure 8 shows the depen- was shown to be sensitive to the structures of the bridge
dence of –ΔGinject and the oscillator strength of the ICT and linker units. Among the three dyes, DSSCs based on
absorption band on the HOMO–LUMO gap. It can clearly DOBT-III exhibit the best overall light-to-electricity con-
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Eur. J. Org. Chem. 2013, 1770–1780

Benzothiadiazole-Bridged Dyes for Dye-Sensitized Solar Cells
version efficiency of 7.29% (Jsc = 12.74 mAcm–2, Voc
=
Current–voltage measurements were performed with a Model 2400
Source meter (Keithley Instruments Inc., USA) and a 500 W 7IL/
PX150 xenon lamp as a white light source in conjunction with a
GRB3 filter and a 7ISW301 spectrometer. The irradiation source
for the photocurrent density–voltage (J–V) measurement was an
AM 1.5 solar simulator (91160, Newport Co., USA). The incident
light intensity was 100 mWcm–2 calibrated with a standard Si solar
cell. The tested solar cells were masked to give a working area of
0.25 cm2.
784 mV, FF
=
0.73) under AM 1.5 irradiation
(100 mWcm–2). The results reveal that optimizing the
chemical structures of dyes to achieve a good balance be-
tween –ΔGinject and the oscillator strength of the ICT ab-
sorption band should lead to the development of high-per-
formance DSSCs.
Syntheses
Experimental Section
1,2-Bis(octyloxy)benzene (2): Bromooctane (10 g, 90.8 mmol) was
Instruments and Materials: 1H and 13C NMR spectra were recorded
with a Bruker AM400 spectrometer. Elemental analyses were per-
formed with an elementar vario EL III instrument. MS were re-
corded by ESI mass spectroscopy. The UV/Vis spectra were re-
corded with a CARY 100 spectrophotometer. The cyclic voltammo-
grams of the dyes were obtained with a Versastat II electrochemical
workstation (Princeton applied research) using a normal three-elec-
trode cell with a Pt working electrode, a Pt wire counter electrode,
and a regular calomel reference electrode in saturated KCl solution.
All the commercially available reagents and solvents were used
without further purification unless otherwise stated. THF was pre-
dried with molecular sieves (4 Å) and distilled under argon from
sodium benzophenone ketyl immediately prior to use, and di-
chloromethane were distilled from calcium hydride. BT was synthe-
sized following the literature method (yield: 40%).[18]
added to
a vigorously stirred solution of catechol (35 g,
181.6 mmol) and potassium hydroxide (10 g) in ethanol (400 mL)
under nitrogen. The reaction mixture was stirred at reflux for 40 h
and then filtered through Celite with copious washings of ethanol.
The filtrate was concentrated in vacuo and subjected to column
chromatography on silica, eluting with 1:1 dichloromethane/light
petroleum ether, to give the product as a pale-yellow oil, yield
1
23.6 g (78%). H NMR (400 MHz, CDCl3): δ = 6.89 (s, 4 H), 3.95
(t, 4 H), 1.89–1.31 (m, 24 H), 0.91 (t, 6 H) ppm.
1,2-Dinitro-4,5-bis(octyloxy)benzene (3): A 65% nitric acid solution
(20 mL) was added dropwise to a three-necked round-bottomed
flask containing dichloromethane (140 mL), acetic acid (140 mL),
and 1,2-bis(octyloxy)benzene (6.68 g, 20 mmol) with cooling to
10 °C. The reaction mixture was warmed to room temperature and
stirred for 1 h. The mixture was again cooled to 10 °C and then
98% nitric acid (50 mL) was added dropwise. The mixture was
warmed to room temperature and stirred for 40 h. After comple-
tion of the reaction, the reaction mixture was poured into ice–water
and the dichloromethane layer was separated. The aqueous phase
was extracted with dichloromethane and the combined organic
phase was washed with water, satd. NaHCO3 (aq.), and brine, and
dried with MgSO4. Concentration under reduced pressure gave the
crude product. The final product was obtained as a yellow solid by
recrystallization from ethanol, yield 6.7 g (79%). 1H NMR
(400 MHz, CDCl3): δ = 7.28 (s, 2 H), 4.03 (t, J = 6.5 Hz, 4 H),
1.95–1.81 (m, 4 H), 1.58–1.12 (m, 20 H), 0.89 (t, J = 6.6 Hz, 6
H) ppm.
Preparation of the Dye-Sensitized Solar Cells: The TiO2 electrode
for DSSC was prepared by following the procedure reported in the
literature.[1a] The TiO2 colloidal dispersion was prepared according
to the literature.[1a] Commercial TiO2 (P25, Degussa AG, Ger-
many) was employed. A layer of approximately 5 μm TiO2 (13 nm
paste, T/SP) was coated onto the FTO conducting glass by screen
printing and then dried for 6 min at 125 °C. This procedure was
repeated twice (ca. 10 μm) and finally the glass was coated with a
layer (ca. 4 μm) of TiO2 paste (Ti nanoxide 300) as the scattering
layer. The trilayer TiO2 electrodes were gradually heated under a
flow of air at 275 °C for 5 min, 325 °C for 5 min, 375 °C for 5 min,
450 °C for 15 min, and 500 °C for 15 min. The sintered film was
further treated with 0.04 m TiCl4 aqueous solution at room tem-
perature for 12 h. After being washed with deionized water and
fully rinsed with ethanol, the films were heated again at 450 °C,
cooled to 50 °C, and dipped into a 3ϫ 10–4 m dye bath in CH2Cl2
solution for 12 h at room temperature. To scrutinize and optimize
the concentration of CDCA, TiO2 films were immersed in an eth-
anolic CDCA solution for 6 h before dipping into the CH2Cl2 solu-
tion of the dye. The electrode was then rinsed with toluene and
dried. The size of the TiO2 electrodes was 0.25 cm2. The dye-ad-
sorbed TiO2 photoanodes and Pt counter electrode were sealed to-
gether by heating with a 25 μm hot-melt (Surlyn 1702, Dupont) as
a spacer to produce a sandwich-type cell. The electrolyte was in-
jected through a hole in the back of the counter electrode. Finally,
the hole was sealed by using a UV meltgum and a cover glass.
Finally, the DSSCs were fabricated with the platinum sputtered
FTO used as counter electrode. The DSSCs were sealed with KC-
904 hot-melt adhesive tape (Kouuci, Shanghai) and two side holes
were left for injecting the electrolyte. The electrolyte used for the
liquid-state DSSCs consisted of 0.1 m LiI, 0–0.5 m DMPII, 0–0.5 m
4-tert-butylpyridine and 0.05 m I2 in acetonitrile solution.
4,5-Bis(octyloxy)benzene-1,2-diaminium Chloride (4): A mixture of
1,2-dinitro-4,5-bis(octyloxy)benzene (3; 1.43 g, 3.37 mmol) and
SnCl2 (5.1 g, 26.9 mmol) in ethanol (50 mL) and conc. HCl
(20 mL) was heated at 85 °C overnight. After cooling to room tem-
perature, the product was filtered and washed with water and meth-
anol. Finally, it was dried at room temp. under a stream of argon
and used directly (unstable) in the next step, yield 1.3 g (88%), off-
white solid.
5,6-Bis(octyloxy)benzo[c][1,2,5]thiadiazole (5): A solution of thionyl
dichloride (9 mL, 123.3 mmol) in dichloromethane (35 mL) was
slowly added to a mixture of 4,5-bis(octyloxy)benzene-1,2-diamin-
ium chloride (4; 7.87 g, 18 mmol) and triethylamine (26 mL,
186.4 mmol) in dichloromethane (270 mL). The mixture was then
heated at reflux for 20 h under argon. After cooling, the solution
was concentrated in vacuo followed by titration with water. After
stirring for 30 min, the mixture was filtered and purified on a silica
gel column eluting with petroleum ether, yield 1.9 g (27%), off-
1
white solid. H NMR (400 MHz, CDCl3): δ = 7.12 (s, 2 H), 4.10
(t, J = 6.5 Hz, 4 H), 2.02–1.79 (m, 4 H), 1.61–1.18 (m, 20 H), 0.93–
0.85 (m, 6 H) ppm.
Photoelectrochemical Measurements: The photocurrent action spec-
tra were recorded with a Model SR830 DSP Lock-In Amplifier and
a Model SR540 Optical Chopper (Stanford Research Corporation,
USA) and other optical systems.
4,7-Dibromo-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole (6):
Bromine (3.8 mL, 72 mmol) was added to a solution of 5,6-bis(oct-
yloxy)benzo[c][1,2,5]thiadiazole (5; 3.92 g,10 mmol) in a mixture
Eur. J. Org. Chem. 2013, 1770–1780
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L. Chen, X. Li, W. Ying, X. Zhang, F. Guo, J. Li, J. Hua
FULL PAPER
of dichloromethane (250 mL) and acetic acid (120 mL) and the re-
sulting mixture was stirred in the dark for 48 h at room tempera-
ture. The mixture was then poured into water (500 mL), extracted
with dichloromethane, sequentially washed with water, satd.
dichloromethane/petroleum ether, 1:1) to yield the product as an
orange-red solid (150 mg, 40.3%). H NMR (400 MHz, CDCl3): δ
= 9.79 (s, 1 H), 7.62 (d, J = 4.4 Hz, 3 H), 7.46 (d, J = 3.6 Hz, 1
H), 7.29 (t, J = 7.6 Hz, 4 H), 7.21–7.17 (m, 6 H), 7.07 (t, J =
1
NaHCO3 (aq.), and 1 m Na2SO3(aq.), and the solvents were evapo- 7.2 Hz, 2 H), 4.23 (t, J = 6.8 Hz, 2 H), 3.88 (t, J = 6.8 Hz, 2 H),
rated under reduced pressure. The crude product was purified on
a silica gel column eluting with petroleum ether to give needle-like
microcrystals, yield 5.06 g (92%). H NMR (400 MHz, CDCl3): δ
= 4.17 (t, J = 6.6 Hz, 4 H), 1.98–1.82 (m, 4 H), 1.65–1.49 (m, 4
H), 1.46–1.19 (m, 20 H), 0.89 (t, J = 6.6 Hz, 6 H) ppm.
1.98–1.90 (m, 2 H), 1.34–1.26 (m, 22 H), 0.91–0.85 (m, 6 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 183.5, 160.7, 146.6, 146.0, 131.3,
129.9, 127.8, 123.4, 123.0, 118.1, 113.1, 64.5, 38.4, 36.5, 36.4, 36.1,
33.8, 33.5, 31.7, 30.9, 30.4, 29.2, 29.0, 28.7, 28.4, 21.7, 21.6,
18.7 ppm.
1
7-Bromo-4-[4-(diphenylamino)phenyl]-5,6-bis(octyloxy)benzo[c]-
[1,2,5]thiadiazole (7): A mixture of compound 6 (550 mg, 1 mmol),
[Pd(PPh3)4] (20 mg, 0.018 mmol), and K2CO3 (1.00 g, 7.25 mmol)
in THF (30 mL) and H2O (5 mL) was heated to 55 °C under nitro-
5-{7-[4-(Diphenylamino)phenyl]-5,6-bis(octyloxy)benzo[c][1,2,5]-
thiadiazol-4-yl}thiophene-2-carbaldehyde (10): Compound 7
(286 mg, 0.4 mmol), [Pd(PPh3)4] (20 mg, 0.018 mmol), and K2CO3
(1.00 g, 7.25 mmol) in THF (30 mL) and H2O (5 mL) were heated
gen for 30 min. A solution of [4-(diphenylamino)phenyl]boronic at 55 °C under nitrogen for 30 min. A solution of (5-formyl-2-thio-
acid (289 mg, 1 mmol) in THF (5 mL) was then added slowly and
the mixture was heated at reflux for a further 12 h. After cooling
to room temperature, the mixture was extracted with CH2Cl2 (3ϫ
30 mL). The organic portions were combined and dried with
MgSO4 and the solvent then removed by rotary evaporation. The
residue was purified by column chromatography (silica gel, dichlo-
romethane/petroleum ether, 1:5) to yield a yellow solid (190 mg,
phenyl)boronic acid (125 mg, 0.8 mmol) in THF (5 mL) was slowly
added and the mixture was heated at reflux for a further 12 h. After
cooling to room temperature, the mixture was extracted with
CH2Cl2 (3ϫ 30 mL). The organic portions were combined and
dried with MgSO4, and then the solvent was removed by rotary
evaporation. The residue was purified by column chromatography
(silica gel, dichloromethane/petroleum ether, 1:1) to yield the prod-
46.3%). 1H NMR (400 MHz, CDCl3): δ = 7.48 (d, J = 8 Hz, 2 H), uct as an orange-red solid (122 mg, 43.4%). H NMR (400 MHz,
7.20 (t, J = 8 Hz, 4 H), 7.10 (d, J = 8 Hz, 6 H), 6.98 (t, J = 7.2 Hz, CDCl3): δ = 10.02 (s, 1 H), 8.69 (d, J = 4.4 Hz, 1 H), 7.86 (d, J =
2 H), 4.14 (t, J = 6.8 Hz, 2 H), 4.08 (t, J = 6.8 Hz, 2 H), 3.77 (t, J 4.4 Hz, 1 H), 7.62 (d, J = 8.8 Hz, 2 H), 7.32–7.25 (m, 4 H), 7.21–
= 6.8 Hz, 2 H), 1.87–1.77 (m, 4 H), 1.51–1.42 (m, 7 H), 1.23–1.17 7.17 (m, 6 H), 7.07 (t, J = 7.2 Hz, 2 H), 4.25 (t, J = 7.2 Hz, 2 H),
1
(m, 31 H), 0.83–0.77 (m, 10 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 153.6, 152.0, 150.4, 150.3, 146.7, 146.5, 130.4, 128.3, 125.3,
123.9, 123.7, 122.3, 121.1, 103.8, 73.8, 73.7, 30.8, 29.4, 29.0, 28.4,
28.3, 28.2, 25.0, 24.9, 21.6, 13.1 ppm.
3.86 (t, J = 6.4 Hz, 2 H), 1.99–1.90 (m, 2 H), 1.26 (m, 22 H), 0.91–
0.85 (m, 6 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 183.6, 153.8,
152.8, 150.4, 147.9, 147.5, 144.3, 143.2, 135.9, 131.7, 131.1, 129.4,
126.4, 126.1, 125.0, 123.4, 122.0, 116.1, 114.1, 74.8, 74.6, 31.9, 31.8,
31.5, 30.3, 30.2, 30.1, 29.7, 29.5, 29.4, 29.3, 29.2, 29.0, 26.0, 22.7,
14.1 ppm.
4-{7-[4-(Diphenylamino)phenyl]-5,6-bis(octyloxy)benzo[c][1,2,5]-
thiadiazol-4-yl}benzaldehyde (8): A mixture of compound 7
(357 mg, 0.5 mmol), [Pd(PPh3)4] (20 mg, 0.018 mmol), and K2CO3
(1.00 g, 7.25 mmol) in THF (30 mL) and H2O (5 mL) was heated
at 55 °C under nitrogen for 30 min. A solution of (4-formylphen-
yl)boronic acid (225 mg, 1.5 mmol) in THF (5 mL) was slowly
added and the mixture was heated at reflux for a further 12 h. After
cooling to room temperature, the mixture was extracted with
CH2Cl2 (3 ϫ 30 mL). The organic portions were combined and
dried with MgSO4, and then the solvent was removed by rotary
evaporation. The residue was purified by column chromatography
(silica gel, dichloromethane/petroleum ether, 1:1) to yield the prod-
(Z)-2-Cyano-3-(4-{7-[4-(diphenylamino)phenyl]-5,6-bis(octyloxy)-
benzo[c][1,2,5]thiadiazol-4-yl}phenyl)acrylic Acid (DOBT-I): A mix-
ture of compound 8 (288 mg, 0.38 mmol) and cyanoacetic acid
(323 mg, 3.8 mmol) in acetic acid (14 mL) was heated at reflux in
the presence of ammonium acetate (120 mg, 1.56 mmol) for 12 h
under argon. After cooling the solution, water was added to
quench the reaction. The resulting precipitate was filtered and
washed with water. The residue was purified by column chromatog-
raphy (silica gel, dichloromethane/EtOH, 15:1) to yield the product
1
as an orange solid (299 mg, 97.7%). H NMR (400 MHz, THF): δ
1
uct as a deep-yellow solid (288 mg, 76.3%). H NMR (400 MHz,
= 8.27 (s, 1 H), 8.04 (s, 2 H), 7.79 (d, J = 8 Hz, 2 H), 7.31 (d, J =
8.8 Hz, 2 H), 7.16 (q, J = 7.2 Hz, 4 H), 7.04 (q, J = 8 Hz, 6 H),
6.92 (d, J = 7.2 Hz, 2 H), 3.47 (s, 4 H), 1.54–1.44 (m, 4 H), 1.29–
CDCl3): δ = 10.11 (s, 1 H), 8.04 (d, J = 8 Hz, 2 H), 7.93 (d, J =
8 Hz, 2 H), 7.03 (d, J = 8.8 Hz, 2 H), 7.30 (q, J = 7.2 Hz, 4 H),
7.20 (d, J = 8.8 Hz, 6 H), 7.07 (t, J = 7.6 Hz, 2 H), 3.91 (q, J1 = 1.09 (m, 20 H), 0.78–0.73 (m, 6 H) ppm. 13C NMR (100 MHz,
1.2, J2 = 5.2 Hz, 4 H), 1.58–1.52 (m, 2 H), 1.29–1.16 (m, 22 H),
0.89–0.84 (m, 6 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 190.7, 129.2, 127.4, 125.2, 124.6, 123.0, 122.9, 122.0, 74.4, 74.1, 31.9, 31.8,
152.7, 146.6, 146.5, 144.5, 130.6, 130.5, 129.4, 128.3, 127.0, 123.9, 30.2, 29.7, 29.4, 29.3, 26.0, 22.6, 13.5 ppm. HRMS (ESI): calcd. for
122.2, 121.2, 73.8, 73.5, 30.8, 30.7, 29.1, 28.7, 28.4, 28.2, 25.0, 24.8, C50H54N4O4S 807.3944; found 807.3943. C50H54N4O4S (807.06):
THF): δ = 153.6, 152.8, 152.2, 151.8, 147.7, 147.6, 131.8, 131.6,
21.7, 21.6, 13.1 ppm.
calcd. C 74.41, H 6.74, N 6.94; found C 74.28, H 6.69, N 6.87.
5-{7-[4-(Diphenylamino)phenyl]-5,6-bis(octyloxy)benzo[c][1,2,5]-
(Z)-2-Cyano-3-(5-{7-[4-(diphenylamino)phenyl]-5,6-bis(octyloxy)-
thiadiazol-4-yl}furan-2-carbaldehyde (9): Compound 7 (357 mg, benzo[c][1,2,5]thiadiazol-4-yl}furan-2-yl)acrylic Acid (DOBT-II): A
0.5 mmol), [Pd(PPh3)4] (20 mg, 0.018 mmol), and K2CO3 (1.00 g, mixture of compound 9 (150 mg, 0.2 mmol) and cyanoacetic acid
7.25 mmol) in THF (30 mL) and H2O (5 mL) were heated at 55 °C
under nitrogen for 30 min. A solution of (5-formyl-2-furanyl)-
boronic acid (70 mg, 0.5 mmol) in THF (5 mL) was slowly added
and the mixture was heated at reflux for a further 12 h. After cool-
ing to room temperature, the mixture was extracted with CH2Cl2
(3ϫ 30 mL). The organic portions were combined and dried with
MgSO4, and then the solvent was removed by rotary evaporation.
The residue was purified by column chromatography (silica gel,
(170 mg, 2 mmol) in acetic acid (14 mL) was heated at reflux in the
presence of ammonium acetate (120 mg, 1.56 mmol) for 12 h under
argon. After cooling the solution, water was added to quench the
reaction. The resulting precipitate was filtered and washed with
water. The residue was purified by column chromatography (silica
gel, dichloromethane/EtOH, 15:1) to yield the product as an orange
1
solid (124 mg, 75.0%). H NMR (400 MHz, THF): δ = 8.44 (d, J
= 4.4 Hz, 1 H), 7.98 (s, 1 H), 7.61 (d, J = 3.6 Hz, 1 H), 7.50 (d, J
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Benzothiadiazole-Bridged Dyes for Dye-Sensitized Solar Cells
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21, 5950–5958.
= 3.6 Hz, 1 H), 7.38 (q, J = 7.2 Hz, 4 H), 7.21 (d, J = 7.2 Hz, 4
H), 7.15 (q, J = 7.2 Hz, 2 H), 6.69 (d, J = 4 Hz, 1 H), 4.04–3.98
(m, J = 7.2 Hz, 2 H), 1.75–1.68 (m, 4 H), 1.36–1.22 (m, 20 H),
0.87–0.81 (m, 6 H) ppm. 13C NMR (100 MHz, THF): δ = 175.7,
175.1, 163.8, 160.1, 159.6, 156.0, 153.7, 152.4, 150.7, 148.7, 148.0,
146.5, 132.4, 132.2, 132.1, 131.1, 130.4, 130.0, 129.7, 125.1, 124.0,
120.1, 114.3, 114.1, 67.5, 31.2, 29.0, 28.7, 28.6, 25.5, 22.1,
13.9 ppm. HRMS (ESI): calcd. for C48H52N4O5S 797.3737; found
797.3738. C48H52N4O5S (797.02): calcd. C 72.33, H 6.58, N 7.03;
found C 72.25, H 6.67, N 7.11.
(Z)-2-Cyano-3-(5-{7-[4-(diphenylamino)phenyl]-5,6-bis(octyloxy)-
benzo[c][1,2,5]thiadiazol-4-yl}thiophen-2-yl)acrylic Acid (DOBT-
III): A mixture of compound 10 (90 mg, 0.12 mmol) and cyano-
acetic acid (102 mg, 1.20 mmol) in acetic acid (14 mL) was heated
at reflux in the presence of ammonium acetate (120 mg,1.56 mmol)
for 12 h under argon. After cooling the solution, water was added
to quench the reaction. The resulting precipitate was filtered and
washed with water. The residue was purified by column chromatog-
raphy (silica gel, dichloromethane/EtOH, 15:1) to yield the product
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as a red solid (71.5 mg, 72.0%). H NMR (400 MHz, THF): δ =
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0.74–0.72 (m, 6 H) ppm. 13C NMR (100 MHz, THF): δ = 152.7,
152.6, 150.1, 147.6, 137.7, 131.9, 129.2, 126.9, 124.6, 123.1, 121.9,
116.2, 74.2, 74.1, 31.9, 30.1, 30.0, 29.7, 29.5, 29.4, 29.3, 26.1, 26.0,
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811.3352; found 811.3351. C48H52N4O4S2 (813.08): calcd. C 70.90,
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Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (grant numbers 2116110444, 21172073 and
91233207), the National Basic Research 973 Program (grant
2013CB733700), the Fundamental Research Funds for the Central
Universities (grants WJ0913001 and WJ1114050), and the Ph. D.
Program Foundation of the Ministry of Education of China (grant
20090074110004).
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Received: October 30, 2012
Published Online: Feburary 7, 2013
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