Substituent effects in a double rhodanine indoline dye on performance of zinc oxide dye-sensitized solar cell

Article abstract of DOI:10.1016/j.dyepig.2009.12.009

The solar-light-to-electricity conversion efficiency of substituted indoline dyes depended on short-circuit photocurrent density which is affected by maximum incident photon-to-current efficiency. The oxidation potential of the double rhodanine indoline dyes needed to be >～0.2?V vs Fc/Fc⁺ in acetonitrile to show high incident photon-to-current efficiency in a zinc oxide, dye-sensitized solar cell. To make a molecular design of highly efficient indoline dyes, the HOMO energy level should be more negative than ～-4.9?eV by the DFT calculations.

Full text of DOI:10.1016/j.dyepig.2009.12.009

Dyes and Pigments 86 (2010) 143e148
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Substituent effects in a double rhodanine indoline dye on performance
of zinc oxide dye-sensitized solar cell
,
a
a
a
b
Masaki Matsui a , Tomoki Fujita , Yasuhiro Kubota , Kazumasa Funabiki , Jiye Jin ,
*
Tsukasa Yoshida c, Hidetoshi Miura d
a Department of Materials Science and Technology, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan
b Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan
c Environmental and Renewable Energy System Division, Graduate School of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan
d Chemicrea Co., Ltd., 2-1-6 Sengen, Tsukuba, Ibaragi 305-0047, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The solar-light-to-electricity conversion efficiency of substituted indoline dyes depended on short-circuit
photocurrent density which is affected by maximum incident photon-to-current efficiency. The oxidation
potential of the double rhodanine indoline dyes needed to be >w0.2 V vs Fc/Fcþ in acetonitrile to show
high incident photon-to-current efficiency in a zinc oxide, dye-sensitized solar cell. To make a molecular
design of highly efficient indoline dyes, the HOMO energy level should be more negative than wꢀ4.9 eV
by the DFT calculations.
Received 7 October 2009
Received in revised form
7 December 2009
Accepted 10 December 2009
Available online 4 January 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Dye-sensitized solar cell
Indoline dyes
Sensitizers
DFT calculations
Oxidation potential
1. Introduction
electrolyte. This paper concerns the relationship among these
energy levels in double rhodanine indoline dyes.
Recently, the structures of highly efficient organic sensitizers for
dye-sensitized solar cells have been focused on arylamine [1,2],
dimethylfluorene [3], carbazole [4], and indoline dyes [5e10].
Among these organic sensitizers, a double rhodanine indoline dye
named D149 has been reported to show highest solar-to-electricity
2. Results and discussion
2.1. Synthesis
conversion efficiency (h) 9.0% on titanium oxide [11]. More recently,
Indoline dyes 21e26 were synthesized as shown in Scheme 1.
Compound 1 was allowed to react with aromatic bromides 2e7 to
give N-aryl derivatives 8e13, followed by formylation to afford
14e19, which were allowed to react with a double rhodanine 20 in
the presence of piperidine to give 21e26.
D205, the N-octyl derivative of D149 was reported to show even
higher efficiency of 9.4% [12]. In the case of zinc oxide, indoline dyes
also displayed high conversion efficiency among the organic
sensitizers. In order to exhibit high conversion efficiency, it is
important to obtain high incident photon-to-current efficiency
(IPCE), which can reflect high short-circuit photocurrent density
(Jsc). In principle, the reduction potential of sensitizers should be at
a greater negative level than the conduction band level of the
semiconductor, whilst, at the same time, the oxidation potential
level should be more positive than the Iꢀ/I3ꢀ redox level of the
2.2. UVevis absorption and fluorescence spectra
The UVevis absorption and fluorescence spectra of 21e26 are
shown in Fig. 1. The results are also listed in Table 1. The absorption
maxima (lmax) were observed in the range of 516e555 nm with
molar absorption coefficients (3) in the range of 56 200e78 300. As
the double rhodanine indoline dyes show intramolecular charge-
transfer chromophoric system from indoline to rhodanine moiety,
electron-donating substituent such as dimethylamino, diphenyla-
mino, and methoxy groups, on the indoline moiety can cause
* Corresponding author. Tel.: þ81 (0) 58 293 2601; fax: þ81 (0) 58 293 2794.
E-mail address: matsuim@gifu-u.ac.jp (M. Matsui).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.12.009

144
M. Matsui et al. / Dyes and Pigments 86 (2010) 143e148
Br
R
CHO
2-7
i
ii
N
N
N
H
1
R
R
14-19
8-13
S
O
Et
S
Et
R
N
Compd
S
N
N
O
O
S
CH2COOH
S
NMe2
NPh2
OMe
H
CF3
CN
2, 8, 14, 21
3, 9, 15, 22
4, 10, 16, 23
5, 11, 17, 24
6, 12, 18, 25
7, 13, 19, 26
N
CH2COOH
S
iii
20
O
N
R
21-26
Scheme 1. i) 2e7 (1.0 eq), K2CO3 (3.0 eq) or t-BuOK (1.3 eq), Pd(OAc)2 (cat), (t-Bu)3P (0.1 eq), ii) POCl3 (2.0 eq), DMF, ꢀ10 ꢁC to rt (1 day) or ꢀ10 ꢁC to 60 ꢁC (3 h), iii) 20 (1.0 eq),
piperidine (cat), BuOH, reflux, 3 h.
bathochromic shift [13]. The fluorescence maximum (Fmax) was
observed in the range of 578e712 nm. Interestingly, the fluores-
cence intensity drastically changed by the kinds of substituent.
oxidative wave (I) at þ0.38 V was due to the oxidation of the
internal standard of ferrocene, whereas the second wave (II) near
þ0.71 V vs AgQRE was due to the electrochemical oxidation of 23,
respectively. The third oxidative wave (III) at 1.25 V may arise from
the further oxidation of 23. The oxidation peak potential (Epa) for 23
can therefore be calculated to be þ0.33 V vs Fc/Fcþ in acetonitrile.
Although the standard Eox are usually not easily obtained experi-
mentally, it can be approximately estimated from the cyclic vol-
tammetric peak potential, which equals to (Epa e 29 mV) if the
electrochemical oxidation is a reversible step [14]. Hence, the Eox
for 23 was estimated to be þ0.30 V vs Fc/Fcþ in this study. The Iꢀ/Iꢀ3
redox level was estimated as ꢀ0.05 V vs Fc/Fcþ.
2.3. Electrochemical properties
The oxidation potential (Eox) of 21e26 was measured in aceto-
nitrile by cyclic voltammetry. Fig. 2 shows the voltammogram of 23
in the presence of ferrocene as an internal standard. There are three
distinct redox waves observed in the voltammogram. The first
No reduction peak was observed for indoline dyes. Therefore,
the EoxeE0e0 level, where E0e0 represents the intersection of
normalized absorption and the fluorescence spectra in chloroform,
was calculated. This is considered to correspond to the reduction
potential [15]. The E0e0 of 23 was observed at 590 nm, corre-
sponding to 2.10 eV. Therefore, the EoxeE0e0 level of 23 is calculated
to be ꢀ1.80 V vs Fc/Fcþ in acetonitrile. The Eox and EoxeE0e0 levels of
the other indoline dyes were obtained in the same way. The results
are also listed in Table 1.
3000
21
0.8
22
23
24
0.6
0.4
0.2
0
2000
1000
0
25
26
4
2
400
500
600
700
800
Wavelength / nm
Fig. 1. UVevis absorption and fluorescence spectra of 21e26 in chloroform. Measured
on 1.0 ꢂ 10ꢀ5 mol dmꢀ3 of substrate at 25 ꢁC.
0
-2
Table 1
Physical properties of indoline dyes.
II
I
-4
-6
Compd lmax
(3
)a/nm
Fmaxa/nm RFIb Eoxc/V
E
oxeE0e0c/V
21
22
23
24
25
26
396 (24 300), 555 (56 200) 712
386 (26 500), 549 (66 500) 700
387 (27 500), 546 (78 300) 633
385 (21 800), 538 (65 600) 608
381 (24 000), 521 (68 800) 584
383 (27 700), 516 (68 200) 578
5
ꢀ0.11 ꢀ2.13
III
1.2
20 þ0.15 ꢀ1.88
217 þ0.30 ꢀ1.80
100 þ0.40 ꢀ1.75
48 þ0.51 ꢀ1.73
40 þ0.53 ꢀ1.73
0.8
0.4
0
Potential / V vs. AgQRE
Fig. 2. Electrochemical measurement of 23. Measured in acetonitrile (2 ml) containing
tetrabutylammonium perchlorate (0.1 mol dmꢀ3) and ferrocene (ca. 1 mmol dmꢀ3).
AgQRE was used as a reference electrode. Platinum wire was used as the working and
a
Measured on 1 ꢂ 10ꢀ5 mol dmꢀ3 of substrate in chloroform at 25 ꢁC.
Relative fluorescence intensity.
b
c
vs Fc/Fcþ in acetonitrile.
counter electrodes. The scan rate was 100 mV sꢀ1
.

M. Matsui et al. / Dyes and Pigments 86 (2010) 143e148
145
Table 2
2.4. Photoelectrochemical properties of indoline
dyes and MO calculations
Photoelectrochemical properties of indoline dyes.a
Compd lmax
21 529
22 517
23
24
25
26
(3
)/nm Absorbance IPCE/% Jsc/mA cmꢀ2 Voc/V ff
h/%
In all cases, addition of cholic acid (CA) during dye-adsorption
process improved the cell performance. The UVevis absorption and
photon-to-current action spectra on zinc oxide in the presence of
CA are shown in Figs. 3 and 4, respectively. Fig. 3 shows that the
order of bathochromicity of indoline dyes on zinc oxide is similar to
that in chloroform. Indoline dyes substituted with electron-
donating group were more bathochromic than those with electron-
withdrawing group. To discuss the substituent effect, IPCE was used
because of similar absorbance (2.33e2.50) of dyes on zinc oxide.
Fig. 4 indicates that the maximum IPCE was in the order of dyes:
24 (80.8), 26 (79.3), 23 (79.1), 25 (78.8) > 22 (50.8) > 21 (9.3). This
order is also similar to that of Jsc: 23 (10.29), 24 (9.80), 25 (8.36), 26
(8.31) > 22 (5.83) > 21 (0.96). The solar-light-to-electricity
2.50
2.41
2.33
2.43
2.38
2.46
9.3
50.8
79.1
80.8
78.8
79.3
0.96
5.83
10.29
9.80
8.36
8.31
0.42 0.55 0.22
0.56 0.65 2.12
0.61 0.65 4.08
0.61 0.68 4.07
0.63 0.69 3.58
0.61 0.66 3.34
516
519
479
486
a
In all cases, two molar amounts of cholic acid for dye was used during
adsorption process. Photoelectrochemical measurements were performed under
AM 1.5 irradiation (100 mW cmꢀ2).
-0.2
-0.1
0
100
90
80
70
60
50
40
30
20
10
0
24
26 25
23
conversion efficiency (h) was larger in the following order: 23
-0.11
80.8
79.3
78.8
(4.08), 24 (4.07) > 25 (3.58), 26 (3.34) > 22 (2.12) > 21 (0.22). The
values of open-circuit voltage (Voc) except for 21 and 22 were in the
range of 0.61e0.63, there being no remarkable difference. Thus, it is
79.1
22
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.15
50.8
clear that the maximum IPCE value mainly affects the
h value.
0.3
The energy level of dyes measured in solution differs from that
of adsorbed one. Unfortunately, indoline dyes and ferrocene on zinc
oxide coated ITO electrode did not give the distinct redox
responses. Therefore, the energy level of indoline dyes measured in
acetonitrile was used to discuss thermodynamic electron transfer
in the solar cell. In the case of titanium oxide, the energy gap of
0.2e0.3 V is required to proceed the electron transfer [15,16]. To
obtain high IPCE, the reduction potential should be more negative
0.4
21
0.51
0.53
9.3
-5.7 -5.6 -5.5 -5.4 -5.3 -5.2 -5.1
-5
-4.9 -4.8
HOMO / eV
Fig. 5. Relationship among maximum IPCE, Eox, and calculated HOMO level of indoline
dyes.
3
21
22
23
24
25
than the conduction band level of semiconductor. All the indoline
dyes showed sufficiently negative EoxeE0e0 level compared to the
conduction band level of zinc oxide as shown in Table 1. For
example, the EoxeE0e0 level of 23 was calculated to be ꢀ1.80 V vs
Fc/Fcþ. This level is much more negative than the conduction band
level of zinc oxide (ca. ꢀ0.95 V vs Fc/Fcþ), the energy gap being ca.
0.85 V. Even in the case of 26, whose EoxeE0e0 level was ꢀ1.73 V,
the energy gap was 0.78 V. Therefore, it is more important to
examine the Eox level of indoline dyes to obtain highly efficient
derivatives. The relationship among maximum IPCE values, Eox, and
calculated HOMO level of substituted indoline dyes are shown in
Fig. 5. High IPCE value was obtained for 23, 24, 25, and 26, whose
Eox is more positive than ca. 0.2 V vs Fc/Fcþ in acetonitrile.
2
26
1
0
400
500
600
700
wavelength / nm
To make a molecular design of indoline dyes, it is of significance
to predict the Eox level. The structures od indoline dyes were
optimized by the B3LYP/3-21G level. Then, the HOMO energy level
was calculated by the B3LYP/6-31G (d,p) level. The result is also
shown in Fig. 5. Good correlation was observed between Eox and
HOMO energy level. It was found that the HOMO energy level of
indoline dyes should be more stable than ca. ꢀ4.9 eV, which
corresponds to the Eox level more positive than ca. 0.2 V vs Fc/Fcþ, to
design high IPCE sensitizers. Table 2.
Fig. 3. UVevis absorption spectra of 21e26 on zinc oxide.
100
21
22
23
24
25
26
80
60
40
20
0
3. Conclusions
The substituent effect in double rhodanine indoline dyes was
examined to clarify the relationship among IPCE, oxidation and
reduction potential energy levels of indoline dyes, semiconductor
conduction band level, and Iꢀ/Iꢀ3 redox level. The reduction
potential level of indoline dye was sufficiently negative compared
with the conduction band level of zinc oxide. However, the Eox level
300
400
500
600
700
800
Wavelength / nm
Fig. 4. Photon-to-current action spectra of 21e26 on zinc oxide.

146
M. Matsui et al. / Dyes and Pigments 86 (2010) 143e148
was required more positive than ca. 0.2 V vs Fc/Fcþ in acetonitrile to
show high IPCE. To make a molecular design of indoline dyes, the
HOMO level should be more negative than ca. ꢀ4.9 eV by the DFT
calculations.
6.96e7.05 (m, 9H), 7.11 (d, J ¼ 7.5 Hz, 1H), 7.25e7.29 (m, 6H); EI-MS
(70 eV) m/z (rel intensity) 402 (Mþ, 100), 249 (25).
4.3.3. 4-(4-Methoxyphenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]-
indole (10)
4. Experimental
Yield 76%; oil; 1H NMR (CDCl3)
d
¼ 1.51e2.05 (m, 6H), 3.79e3.83
(m, 1H), 3.81 (s, 3H), 4.68e4.72 (m, 1H), 6.65 (t, J ¼ 7.8 Hz, 1H), 6.71
(d, J ¼ 7.8 Hz,1H), 6.90 (d, J ¼ 8.4 Hz, 2H), 6.99 (t, J ¼ 7.8 Hz,1H), 7.09
(d, J ¼ 7.8 Hz, 1H), 7.21 (d, J ¼ 8.4 Hz, 2H); EI-MS (70 eV) m/z (rel
intensity) 265 (Mþ, 84), 278 (100).
4.1. Instruments
Melting points were measured with a Yanaco MP-13 micro-
melting-point apparatus. NMR spectra were obtained by a JEOL ECX
400P spectrometer. MS spectra were recorded on a JEOL MStation
700 spectrometer. UVevis absorption and fluorescence spectra
were recorded on Hitachi U-3500 and F-4500 spectrophotometers,
respectively. Electrochemical measurement was carried out using
an EG&G Princeton Applied Research Potentiostat/Galvanostat
(Model 263A) driven by the M270 software package.
4.3.4. 4-Phenyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (11)
Yield 74%; oil; 1H NMR (CDCl3)
d
¼ 1.49e1.59 (m, 1H), 1.61e1.70
(m, 1H), 1.80e1.96 (m, 3H), 2.00e2.09 (m, 1H), 3.82e3.86 (m, 1H),
4.73e4.78 (m, 1H), 6.73 (t, J ¼ 7.2 Hz, 1H), 6.96 (t, J ¼ 7.2 Hz, 1H),
7.02e7.05 (m, 2H), 7.12 (d, J ¼ 7.2 Hz, 1H), 7.27e7.35 (m, 4H); EI-MS
(70 eV) m/z (rel intensity) 235 (Mþ, 50), 206 (100).
4.2. Materials
4.3.5. 4-[4-(Trifluoromethyl)phenyl]-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indole (12)
1,2,3,3a,4,8b-Hexahydrocyclopenta[b]indole (1) and double
rhodanine acetic acid (20) were obtained from Chemicrea Co., Ltd.
1-Bromo-4-(dimethylamino)benzene (2), 1-bromo-4-(diphenyla-
mino)benzene (3), 1-bromo-4-methoxybenzene (4), phenyl
bromide (5), 1-bromo-4-(trifluoromethyl)benzene (6), and 1-
bromo-4-cyanobenzene (7) were purchased from Tokyo Kasei
Co., Ltd.
Yield 70%; mp 69e70 ꢁC; 1H NMR (CDCl3)
d
¼ 1.43e1.54 (m, 1H),
1.63e1.71 (m, 1H), 1.87e1.97 (m, 3H), 2.01e2.11 (m, 1H), 3.85e3.90
(m,1H), 4.69e4.74 (m,1H), 6.83 (t, J ¼ 7.1 Hz,1H), 7.08e7.17 (m, 3H),
7.32 (d, J ¼ 9.0 Hz, 2H), 7.53 (d, J ¼ 9.0 Hz, 2H); EI-MS (70 eV) m/z
(rel intensity) 303 (Mþ, 49), 274 (100).
4.3.6. 4-(4-Cyanophenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]-
indole (13)
4.3. Synthesis of 8e13
Yield 85%; mp 75e76 ꢁC; 1H NMR (CDCl3)
d
¼ 1.41e1.50 (m, 1H),
1.64e1.72 (m, 1H), 1.87e2.11 (m, 4H), 3.88e3.92 (m, 1H), 4.64e4.69
(m,1H), 6.89 (t, J ¼ 7.5 Hz,1H), 7.13 (t, J ¼ 7.5 Hz,1H), 7.16 (d, J ¼ 7.5 Hz,
1H), 7.19 (d, J ¼ 7.5 Hz,1H), 7.28 (d, J ¼ 7.2 Hz, 2H), 7.55 (d, J ¼ 7.2 Hz,
2H); EI-MS (70 eV) m/z (rel intensity) 260 (Mþ, 50), 231 (100).
In the cases of 8e11, to toluene (37 ml) were added 1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indole (1, 1.59 g, 10 mmol), aromatic
bromides 2e5 (10 mmol), potassium tert.-butoxide (1.68 g,
13 mmol), palladium acetate (12 mg, 5.4 ꢂ 10ꢀ2 mmol), and tri-
tert.-butylphosphine (caution: Pyrophoric. Spontaneously flam-
mable in air. Air sensitive; 192 mg, 1 mmol). In the cases of 12 and
13, to toluene (37 ml) was added 1,2,3,3a,4,8b-hexahy-
drocyclopenta[b]indole (1, 1.59 g, 10 mmol), aromatic bromides 6
and 7 (10 mmol), potassium carbonate (2.07 g, 30 mmol), palladium
acetate (12 mg, 5.4 ꢂ 10ꢀ2 mmol), and tri-tert.-butylphosphine
(192 mg, 1 mmol). The mixture was refluxed for 3 days under an
argon atmosphere. After the reaction was complete, the mixture
was filtered and the filtrate washed with saturated aqueous
ammonium chloride (20 ml ꢂ 3). After drying the filtrate over
anhydrous sodium sulfate, the solvent was removed in vacuo. The
crude product was purified by silica gel column chromatography
(8; CH2Cl2:AcOEt ¼ 40:1, 9, 10, 11, 12; CH2Cl2:C6H14 ¼ 1:3, 13;
CH2Cl2:C6H14 ¼ 2:1). The physical and spectral data are shown
below.
4.4. Synthesis of 14e19
To DMF (15 ml) was added phosphoryl chloride (caution: Reacts
violently with water. Incompatible with many metals, alcohols,
amines, phenol, DMSO, strong bases; 920 mg, 5.9 mmol) at ꢀ10 ꢁC
with stirring. To this mixture was added a DMF solution of 8e13
(3 mmol) at ꢀ5 ꢁC. The mixture was stirred for 1 h at ꢀ10 ꢁC. Then,
the mixture was stirred for 1 day at room temperature. In the cases
of 18 and 19, the mixture was heated at 60 ꢁC for 3 h. After the
reaction was completed, to the mixture was added water (50 ml).
The mixture was neutralized with 1 N hydrochloric acid. The
product was extracted with dichloromethane (150 ml ꢂ 3). After
the extract was dried over anhydrous sodium sulfate, the solvent
was removed in vacuo. The product was purified by column chro-
matography (SiO2, CH2Cl2). The physical and spectral data are
shown below.
4.3.1. 4-[4-(Dimethylamino)phenyl]-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indole (8)
Yield 85%; mp 88e90 ꢁC; 1H NMR (DMSO-d6)
d
¼ 1.44e1.45 (m,
4.4.1. 4-[4-(Dimethylamino)phenyl]-1,2,3,3a,4,8b-
1H), 1.57e1.72 (m, 4H), 1.94e2.03 (m, 1H), 2.87 (s, 6H), 3.73e3.77
(m, 1H), 4.67e4.71 (m, 1H), 6.49 (d, J ¼ 7.6 Hz,1H), 6.54 (t, J ¼ 7.6 Hz,
1H), 6.77 (d, J ¼ 8.2 Hz, 2H), 6.90 (t, J ¼ 7.6 Hz,1H), 7.03 (d, J ¼ 7.6 Hz,
1H), 7.12 (d, J ¼ 8.2 Hz, 2H); EI-MS (70 eV) m/z (rel intensity) 278
(Mþ, 100), 249 (60).
hexahydrocyclopenta[b]indole-7-carbaldehyde (14)
Yield 67%; mp 139e141 ꢁC; 1H NMR (DMSO-d6)
d
¼ 1.32e1.45 (m,
1H),1.57e1.76 (m, 4H),1.97e2.09 (m,1H), 2.91 (s, 6H), 3.79e3.83 (m,
1H), 4.87e4.91 (m,1H), 6.43 (d, J ¼ 8.3 Hz,1H), 6.79 (d, J ¼ 9.0 Hz, 2H),
7.17 (d, J ¼ 9.0 Hz, 2H), 7.48 (d, J ¼ 8.3 Hz,1H), 7.51 (s,1H), 9.58 (s,1H);
EI-MS (70 eV) m/z (rel intensity) 306 (Mþ, 100), 277 (48).
4.3.2. 4-[4-(Diphenylamino)phenyl]-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indole (9)
4.4.2. 4-[4-(Diphenylamino)phenyl]-1,2,3,3a,4,8b-
Yield 92%; mp 155e157 ꢁC; 1H NMR (DMSO-d6)
d
¼ 1.31e1.41
hexahydrocyclopenta[b]indole-7-carbaldehyde (15)
(m, 1H), 1.60e1.64 (m, 1H), 1.77e1.89 (m, 3H), 1.97e2.07 (m, 1H),
3.30e3.84 (m, 1H), 4.74e4.77 (m, 1H), 6.68 (t, J ¼ 7.3 Hz, 1H), 6.90
(d, J ¼ 7.5 Hz, 1H), 6.97e7.02 (m, 7H), 7.04 (d, J ¼ 8.7 Hz, 2H),
Yield 76%; mp 173e174 ꢁC; 1H NMR (CDCl3)
d
¼ 1.31e1.43 (m,
1H), 1.64e1.65 (m, 1H), 1.74e1.79 (m, 3H), 2.00e2.10 (m, 1H),
3.83e3.87 (m, 1H), 4.93e4.99 (m, 1H), 6.79 (d, J ¼ 8.2 Hz, 1H),

M. Matsui et al. / Dyes and Pigments 86 (2010) 143e148
147
7.62e7.07 (m, 8H), 7.29e7.33 (m, 6H), 7.54e7.57 (m, 2H), 9.65 (s,
4.5.3. Dye 23
1H); EI-MS (70 eV) m/z (rel intensity) 430 (Mþ, 100), 401 (18).
Yield 31%; mp > 300 ꢁC; 1H NMR (DMSO-d6)
d
¼ 1.19 (t,
J ¼ 7.1 Hz, 3H), 1.24e1.80 (m, 6H), 3.78 (s, 3H), 3.39e3.93 (m, 1H),
4.06 (q, J ¼ 7.1 Hz, 2H), 4.71 (s, 2H), 4.98e5.03 (m, 1H), 6.71 (d,
J ¼ 8.4 Hz, 1H), 7.01 (d, J ¼ 8.6 Hz, 2H), 7.33 (d, J ¼ 8.6 Hz, 2H), 7.41
(d, J ¼ 8.4 Hz, 1H), 7.43 (s, 1H), 7.72 (s, 1H); FABMLMS (NBA) m/z
594.1216 (MHþ), calcd for C29H28N3O5S3: 594.1191.
4.4.3. 4-(4-Methoxyphenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]-
indole-7-carbaldehyde (16)
Yield 86%; oil; 1H NMR (CDCl3)
d
¼ 1.52e1.54 (m, 2H), 1.67e1.70
(m, 2H), 1.86e1.88 (m, 2H), 2.03e2.07 (m, 1H), 3.84 (s, 3H),
4.83e4.86 (m, 1H), 6.55 (d, J ¼ 8.3 Hz, 1H), 6.95 (d, J ¼ 8.8 Hz, 2H),
7.21 (d, J ¼ 8.8 Hz, 2H), 7.48 (d, J ¼ 8.3 Hz, 1H), 7.61 (s, 1H), 9.67 (s,
1H); EI-MS (70 eV) m/z (rel intensity) 293 (Mþ, 100), 264 (94).
4.5.4. Dye 24
Yield 61%; mp 272e274 ꢁC; 1H NMR (DMSO-d6)
d
¼ 1.18 (t,
J ¼ 6.9 Hz, 3H), 1.36e1.43 (m, 1H), 1.64e1.83 (m, 5H), 3.91e3.95 (m,
1H), 4.06 (q, J ¼ 6.9 Hz, 2H), 4.78 (s, 2H), 5.06e5.09 (m, 1H), 7.00 (d,
J ¼ 8.2 Hz,1H), 7.14 (t, J ¼ 6.9 Hz,1H), 7.37e7.49 (m, 6H), 7.75 (s,1H);
FABMLMS (NBA) m/z 564.1085 (MHþ), calcd for C28H26N3O4S3:
564.1088.
4.4.4. 4-Phenyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole-7-
carbaldehyde (17)
Yield 70%; mp 120e122 ꢁC; 1H NMR (CDCl3)
d
¼ 1.49e1.55 (m,
1H), 1.65e1.78 (m, 2H), 1.86e1.91 (m, 2H), 2.04e2.08 (m, 1H),
3.81e3.86 (m, 1H), 4.91e4.94 (m, 1H), 6.82 (d, J ¼ 8.3 Hz,1H), 7.13 (t,
J ¼ 8.3 Hz, 1H), 7.28e7.31 (m, 2H), 7.37e7.39 (m, 2H), 7.51 (d,
J ¼ 8.3 Hz, 1H), 7.63 (s, 1H), 9.70 (s, 1H); EI-MS (70 eV) m/z (rel
intensity) 263 (Mþ, 57), 234 (100).
4.5.5. Dye 25
Yield 42%; mp 265e266 ꢁC; 1H NMR (DMSO-d6)
d
¼ 1.17 (t,
J ¼ 7.1 Hz, 3H), 1.31e1.41 (m, 1H), 1.66e1.70 (m, 1H), 1.72e1.77 (m,
1H), 1.83e1.86 (m, 1H), 1.89e1.98 (m, 1H), 2.09e2.18 (m, 1H),
3.94e3.99 (m, 1H), 4.05 (q, J ¼ 7.1 Hz, 2H), 4.79 (s, 2H), 5.04e5.08
(m,1H), 7.30 (d, J ¼ 9.2 Hz,1H), 7.51e7.53 (m, 2H), 7.57 (d, J ¼ 8.7 Hz,
2H), 7.72 (d, J ¼ 8.7 Hz, 2H), 7.78 (s, 1H); FABMLMS (NBA) m/z
632.0927 (MHþ), calcd for C29H25F3N3O4S3: 632.0959.
4.4.5. 4-[4-(Trifluoromethyl)phenyl]-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indole-7-carbaldehyde (18)
Yield 70%; mp 85e87 ꢁC; 1H NMR (CDCl3)
d
¼ 1.42e1.56 (m, 1H),
1.68e1.76 (m, 1H), 1.83e1.96 (m, 3H), 2.06e2.14 (m, 1H), 3.86e3.91
(m, 1H), 4.92e4.96 (m, 1H), 7.05 (d, J ¼ 8.3 Hz, 1H), 7.40 (d,
J ¼ 8.5 Hz, 2H), 7.59 (d, J ¼ 8.3 Hz, 2H), 7.63 (d, J ¼ 8.5 Hz, 2H), 7.69
(s, 1H), 9.80 (s, 1H); EI-MS (70 eV) m/z (rel intensity) 331 (Mþ, 59),
302 (100).
4.5.6. Dye 26
Yield 58%; mp 276e278 ꢁC; 1H NMR (CDCl3)
d
¼ 1.18 (t,
J ¼ 6.9 Hz, 3H), 1.32e1.40 (m, 1H), 1.66e1.76 (m, 2H), 1.81e1.87 (m,
1H), 1.93e2.03 (m, 1H), 2.09e2.17 (m, 1H), 3.58e4.00 (m, 1H), 4.05
(q, J ¼ 6.9 Hz, 2H), 4.76 (s, 2H), 5.02e5.07 (m, 1H), 7.38 (d, J ¼ 7.3 Hz,
1H), 7.52e7.55 (m, 4H), 7.80e7.81 (m, 3H); FABMLMS (NBA) m/z
589.1072 (MHþ), calcd for C29H25N4O4S3: 589.1038.
4.4.6. 4-(4-Cyanophenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]-
indole-7-carbaldehyde (19)
Yield 74%; mp 161e163 ꢁC; 1H NMR (CDCl3)
d
¼ 1.42e1.54 (m,
1H), 1.69e1.77 (m, 1H), 1.87e2.00 (m, 3H), 2.07e2.14 (m, 1H),
3.90e3.93 (m, 1H), 4.87e4.91 (m, 1H), 7.15 (d, J ¼ 8.2 Hz, 1H), 7.38
(d, J ¼ 9.2 Hz, 2H), 7.61e7.66 (m, 3H), 7.71 (s, 1H), 9.80 (s, 1H); EI-MS
(70 eV) m/z (rel intensity) 288 (Mþ, 58), 259 (100).
4.6. Electrochemical measurements
Electrochemical measurement of indoline dyes was performed
in acetonitrile. The oxidation potential (Eox) was measured by using
small-size three electrodes. Ag quasi reference electrode (QRE) was
used as a reference. Platinum wire was used as the working and
counter electrodes. All electrode potentials were calibrated with
respect to ferrocene(Fc)/ferrocenium (Fcþ) redox couple. An
acetonitrile solution (2 ml) of dyes containing tetrabutylammo-
4.5. Synthesis of dyes 21e26
To 1-butanol (40 ml) were added 14e19 (1.0 mmol) and double
rhodanine acetic acid 20 (1.0 mmol). The mixture was refluxed for
20 min. Then, to the mixture was added piperidine (1 mg), which
was refluxed for 3 h. After the reaction was completed, the solvent
was removed in vacuo. The product was purified by column chro-
matography (SiO2, CHCl3:MeOH ¼ 10:1). The physical and spectral
data are shown below.
nium perchlorate (0.1 mol dmꢀ3) and ferrocene (ca. 1 mmol dmꢀ3
was prepared. The electrochemical measurement was performed at
the scan rate of 100 mV sꢀ1
)
.
4.7. Film preparation
4.5.1. Dye 21
An aqueous potassium chloride solution (300 ml, 0.1 mol dmꢀ3
)
Yield 30%; mp 236e238 ꢁC; 1H NMR (CDCl3)
d
¼ 1.18 (t,
was electrolyzed at ꢀ1.0 V vs SCE with bubbling an oxygen gas at
70 ꢁC for 30 min. Platinum was used as a counter electrode. To the
pre-electrolyzed film was added an aqueous solution of zinc chlo-
ride. The concentration of zinc chloride was adjusted to
5 mmol dmꢀ3. Then, the film was electro-deposited again in the
solution at ꢀ1.0 V vs SCE at 70 ꢁC for 20 min with bubbling an
oxygen gas. To the electro-deposited film was added an aqueous
J ¼ 6.9 Hz, 3H), 1.37e1.43 (m, 1H), 1.62e1.79 (m, 4H), 2.04e2.09 (m,
1H), 2.98 (s, 6H), 3.87e3.91 (m, 1H), 4.05 (q, J ¼ 6.9 Hz, 2H), 4.77 (s,
2H), 4.93e4.95 (m, 1H), 6.59 (d, J ¼ 6.7 Hz, 1H), 6.79 (d, J ¼ 8.2 Hz,
2H), 7.20 (d, J ¼ 8.2 Hz, 2H), 7.37e7.39 (m, 2H), 7.70 (s, 1H);
FABMLMS (NBA) m/z 607.1462 (MHþ), calcd for C30H31N4O4S3:
607.1507.
solution of eosin Y (50 m
mol dmꢀ3). The film was electro-deposited
4.5.2. Dye 22
at ꢀ1.0 V vs SCE at 70 ꢁC for 30 min with bubbling an oxygen gas.
The film was kept in a diluted aqueous potassium hydroxide solu-
tion (pH 10.5) for 24 h to remove adsorbed eosin Y. The film was
dried at 100 ꢁC for 1 h. The film was immersed in an acetoni-
Yield 61%; mp 268e270 ꢁC; 1H NMR (CDCl3)
d
¼ 1.18 (t,
J ¼ 7.3 Hz, 3H), 1.35e1.43 (m, 1H), 1.60e1.70 (m, 1H), 1.75e1.85 (m,
3H), 2.05e2.12 (m, 1H), 3.93e3.95 (m, 1H), 4.03 (q, J ¼ 7.3 Hz, 2H),
4.67 (br s, 2H), 5.00e5.04 (m, 1H), 6.95 (d, J ¼ 8.7 Hz, 1H), 7.02e7.07
(m, 8H), 7.29e7.36 (m, 6H), 7.43e7.48 (m, 2H), 7.72 (s, 1H);
FABMLMS (NBA) m/z 731.1769 (MHþ), calcd for C40H35N4O4S3:
731.1820.
trileetert.-butyl alcohol 1:1 mixed solution of dye (0.5 mmol dmꢀ3
)
containing cholic acid (1.0 mmol dmꢀ3) at room temperature. Then,
the film was washed with an acetonitrileetert.-butyl alcohol 1:1
mixed solution. Acetonitrileeethylenecarbonate (v/v
¼
1:4)

148
M. Matsui et al. / Dyes and Pigments 86 (2010) 143e148
containing tetrabutylammonium iodide (0.5 mol dmꢀ3) and iodine
(0.05 mol dmꢀ3) was used as an electrolyte.
tuning of solar-cell performance by structural modification. Chem Mater
2008;20:3993e4003.
[5] Horiuchi T, Miura H, Uchida S. Highly-efficient metal-free organic dyes for
dye-sensitized solar cells. Chem Commun 2003:3036e7.
[6] Schmidt-Mende L, Bach U, Humphry-Baker R, Horiuchi T, Miura H, Ito S, et al.
Organic dye for highly efficient solid-state dye-sensitized solar cells. Adv
Mater 2005;17:813e5.
[7] Horiuchi T, Miura H, Uchida S. Highly efficient metal-free organic dyes for dye-
sensitized solar cells. J Photochem Photobiol A Chem 2004;164:29e32.
[8] Horiuchi T, Miura H, Sumioka K, Uchida S. High efficiency of dye-sensitized
solar cells based on metal-free indoline dyes. J Am Chem Soc 2004;126:
12218e9.
[9] Dentani T, Kubota Y, Funabiki K, Jin J, Yoshida T, Minoura H, et al. Novel thio-
phene-conjugated indoline dyes for zinc oxide solar cell. N J Chem 2009;33:
93e101.
[10] Matsui M, Ito A, Kotani M, Kubota Y, Funabiki K, Jin J, et al. The use of indoline
dyes in a zinc oxide dye-sensitized solar cell. Dyes Pigments 2009;80:233e8.
[11] Ito S, Zakeeruddin SM, Humphry-Baker R, Liska P, Charvet R, Comte P, et al.
High-efficiency organic-dye-sensitized solar cells controlled by nanocrystal-
line-TiO2 electrode thickness. Adv Mater 2006;18:1202e5.
4.8. Photoelectrochemical measurements
An action spectrum was measured under monochromatic light
with a constant photon number (5 ꢂ 1015 photon cmꢀ2
s
ꢀ1). IeV
characteristics were measured under illumination with AM 1.5
simulated sun light (100 mW cmꢀ2) through a shading mast
(5.0 mm ꢂ 4.0 mm) by using a Bunko-Keiki CEP-2000 system.
Acknowledgement
This work was financially supported in part by Grants-in-Aid for
Science Research (No. 21550180) from Japan Society for the
Promotion of Science (JSPS).
[12] Ito S, Miura H, Uchida S, Takata M, Sumioka K, Liska P, et al. High-conversion-
efficiency organic dye-sensitized solar cells with a novel indoline dye. Chem
Commun 2008:5194e6.
[13] Howie WH, Claeyssens F, Miura H, Peter LM. Characterization of solid-state
dye-sensitized solar cells utilizing high absorption coefficient metal-free
organic dyes. J Am Chem Soc 2008;130:1367e75.
References
[1] Zhang G, Bala H, Cheng Y, Shi D, Lv X, Yu Q, et al. High efficiency and stable
[14] Nicholson RS, Shain I. Theory of stationary electrode polarography. Single scan
and cyclic methods applied to reversible, irreversible, and kinetic systems.
Anal Chem 1964;36:706e23.
dye-sensitized solar cells with an organic chromophore featuring a binary
conjugated spacer. Chem Commun 2009:2198e200.
[2] Ning Z, Tian H. Triarylamine, a promising core unit for efficient photovoltaic
materials. Chem Commun 2009:5483e95.
[3] Kim S, Kim D, Choi H, Kang M-S, Song K, Kang O-S, et al. Enhanced photo-
voltaic performance and long-term stability of quasi-solid-state dye-sensi-
tized solar cells via molecular engineering. Chem Commun 2008:4951e3.
[4] Wang Z-S, Koumura N, Cui Y, Takahashi M, Sekiguchi H, Mori A, et al. Hexylth-
iophene-functionalized carbazole dyes for efficient molecular photovoltaics:
p-
[15] Hara K, Sato T, Katoh R, Furube A, Ohga Y, Shinpo A, et al. Molecular design of
coumarin dyes for efficient dye-sensitized solar cells.
2003;107:597e606.
J Phys Chem B
[16] Hara K, Sato T, Katoh R, Furube A, Yoshihara T, Murai M, et al. Novel conjugated
organic dyes for efficient dye-sensitized solar cells. Adv Funct Mater 2005;15:
246e52.