Dye-sensitized solar cells based on D-π-A fluorescent dyes with pyridine ring forming strong interaction with nanocrystalline TiO2

Article abstract of DOI:10.1246/bcsj.20100098

New-type donor-acceptor π-conjugated (D-π-A) fluorescent dyes OH11, OH14, and OH15 with pyridine, 4-phenylpyridine, and quinoline rings, respectively, as electron-accepting groups have been developed and their photovoltaic performance as dye-sensitized solar cells (DSSCs) are investigated. The short-circuit photocurrent densities (J _sc)of DSSCs using OH11, OH14, and OH15 are 4.88, 3.91, and 1.72mAcm-2, and the solar energy-to-electricity conversion yields (n) are 1.62, 1.30, and 0.56%, respectively, under a simulated solar light (AM 1.5, 100mW cm-2). The IR spectra of OH11 and OH14 adsorbed on TiO₂ indicate the formation of hydrogen bonding between the pyridine ring of dyes and the hydroxy group of TiO ₂ surface and the formation of strong coordinate bonding between the pyridine ring of dyes and the Lewisacidsite of TiO₂ surface. On the other hand, in the IR spectra of OH15 adsorbed on TiO₂, only the formation of weak hydrogen bonding was observed. This work suggests that the pyridine rings forming a strong coordinate bonding with the Lewisacidsite of TiO₂ surface are promising candidates as electron-accepting groups of new D-π-A dye sensitizers for DSSCs.

Full text of DOI:10.1246/bcsj.20100098

© 2010 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 83, No. 9, 1113–1121 (2010) 1113
Dye-Sensitized Solar Cells Based on D-³-A Fluorescent Dyes with
Pyridine Ring Forming Strong Interaction with Nanocrystalline TiO2
Yousuke Ooyama, Tomoya Nagano, Shogo Inoue, Ichiro Imae, Kenji Komaguchi, and Yutaka Harima*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527
Received April 5, 2010; E-mail: harima@mls.ias.hiroshima-u.ac.jp
New-type donor-acceptor ³-conjugated (D-³-A) fluorescent dyes OH11, OH14, and OH15 with pyridine,
4-phenylpyridine, and quinoline rings, respectively, as electron-accepting groups have been developed and their
photovoltaic performance as dye-sensitized solar cells (DSSCs) are investigated. The short-circuit photocurrent densities
(Jsc) of DSSCs using OH11, OH14, and OH15 are 4.88, 3.91, and 1.72 mA cm¹2, and the solar energy-to-electricity
conversion yields (©) are 1.62, 1.30, and 0.56%, respectively, under a simulated solar light (AM 1.5, 100 mW cm¹2). The
IR spectra of OH11 and OH14 adsorbed on TiO2 indicate the formation of hydrogen bonding between the pyridine ring
of dyes and the hydroxy group of TiO2 surface and the formation of strong coordinate bonding between the pyridine ring
of dyes and the Lewis acid site of TiO2 surface. On the other hand, in the IR spectra of OH15 adsorbed on TiO2, only
the formation of weak hydrogen bonding was observed. This work suggests that the pyridine rings forming a strong
coordinate bonding with the Lewis acid site of TiO2 surface are promising candidates as electron-accepting groups of new
D-³-A dye sensitizers for DSSCs.
COOH
Donor-acceptor ³-conjugated (D-³-A) dyes which have
both the electron-donating (D) and electron-accepting (A)
groups linked by a ³-conjugated bridge, are one of the most
promising organic sensitizers in dye-sensitized solar cells
(DSSCs). Most of the D-³-A dyes such as coumarin dyes,1
polyene dyes,2 thiophene dyes,3 and indoline dyes4 have
dialkylamine or diphenylamine moiety as electron donor, and
carboxylic acid, cyanoacrylic acid, or rhodanine-3-acetic acid
moiety which acts as electron acceptor as well as anchoring
group for attachment on a TiO2 surface. The carboxyl group can
form an ester linkage with a TiO2 surface to provide a strongly
bound dye and good electron communication between them.
On the other hand, in our previous study, a novel fluorescent
dye OH11 with pyridine ring as an electron-accepting group, as
a new D-³-A dye sensitizer, has been designed and synthe-
sized (Scheme 1).5 The dye OH11 has a non-conjugated alkyl
chain containing a carboxyl group at the end position, so that
the carboxyl group as an anchoring group is separated from the
electron acceptor moiety. The photovoltaic performance of
OH11 is similar to that of OH1 having a carboxyl group acting
as not only anchoring group for attachment on TiO2 surface but
also the electron acceptor. It was suggested that the pyridine
ring acting as electron acceptor is located in close proximity to
the TiO2 surface by the formation of a strong interaction such
as the hydrogen bonding between the pyridine ring of OH11
and the TiO2 surface. Consequently, the dye OH11 can inject
efficiently electrons from the pyridine ring to the conduction
band of TiO2 electrode through their electronic interaction.
However, it is necessary to give reliable evidence for the inter-
action between the pyridine ring of dye and the TiO2 surface.
Therefore, in order to provide further confirmation for the
effects of the interaction between the pyridine ring and the TiO2
surface on the photovoltaic performances of DSSCs, we have
N
O
N
O
N
O
HO
O
O
HN
N
NBu2
NBu2
OH1
OH11
Scheme 1. Chemical structures of fluorescent dyes OH1 and
OH11.
designed and synthesized new D-³-A fluorescent dyes OH14
and OH15 with 4-phenylpyridine and quinoline rings, respec-
tively, as electron-accepting groups. Similarly to OH11, the
two dyes have a non-conjugated alkyl chain containing a
carboxyl group as an anchoring group at the end position. In
this paper, on the basis of the IR spectra of the dyes adsorbed
on TiO2 particles, it was demonstrated that the dyes OH11 and
OH14 can inject electrons efficiently from the pyridine ring to
the conduction band of the TiO2 electrode through the strong
coordinate bonding rather than the hydrogen bonding between
the pyridine ring of dye and the TiO2 surface.
Results and Discussion
Synthesis of D-³-A Fluorescent Dyes OH11, OH14, and
OH15. As shown in Scheme 2, 3-dibutylaminobenzofuro[2,3-
c]-6,7-carbazoledione (1)10 was used as a starting material. The
reaction of quinone 1 with 4-(4-pyridinyl)benzaldehyde in the
presence of ammonium acetate gave the dye 2. On the other
hand, the quinone 1 was allowed to react with 4-quinoline-
Published on the web September 2, 2010; doi:10.1246/bcsj.20100098

1114 Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010)
Dye-Sensitized Solar Cells
N
OH11
OH14
O
O
O
OHC
N
N
O
Br
OEt
O
HN
6
OH15
O
HN
CH3COONH4
CH3COOH
NaH, DMF
NBu2
NBu2
1
2
17%
N
N
NaOH aq.
N
O
O
N
N
O
O
EtO
HO
O
N
O
EtOH/CH2Cl2
6
6
NBu2
NBu2
3
80%
OH14 86%
Figure 1. Absorption (®) and fluorescence (*) spectra of
OH11, OH14, and OH15 in THF.
N
N
Table 1. Spectroscopic Properties of OH11, OH14, and
OH15 in THF
O
O
N
O
O
N
OHC
N
O
HN
-
abs/nm
O
O
HN
HN
max
a)
fl/nm ΦF
SSb)/nm
CH3COONH4
CH3COOH
Dye
-
max
¹1
(¾max/dm3 mol¹1 cm
)
NBu2
NBu2
NBu2
OH11 349 (31000), 412 (26600)
OH14 350 (24200), 413 (20400)
OH15 358 (21500), 446 (13300)
533
556
593
0.86
0.55
0.61
121
143
147
1
4
40%
5
8%
N
N
a) The Φ values were determined by using a calibrated
integrating sphere system (-ex = 325 nm). b) Stokes shift
value.
O
N
O
O
N
O
O
Br
NaOH aq.
EtO
HO
EtOH/CH2Cl2
NBu2
OEt
6
O
4
N
O
N
6
6
NaH, DMF
NBu2
OH15 70%
OH11
OH14
6
78%
Scheme 2. Synthesis of fluorescent dyes OH14 and OH15.
OH15
carboxaldehyde in the presence of ammonium acetate to give
the structural isomers of 4 and 5. The reaction of 2 and 4 with
ethyl 7-bromoheptanoate using sodium hydride yielded 3 and
6, respectively. The D-³-A fluorescent dyes OH14 and OH15
were obtained by hydrolysis of 3 and 6, respectively.
Spectroscopic Properties of OH11, OH14, and OH15 in
Solution and Adsorbed on TiO2 Particles. The absorption
and fluorescence spectra of OH11, OH14, and OH15 in THF
are shown in Figure 1 and their spectral data are summarized in
Table 1. The three dyes show two absorption maxima: a sharp
band at 350 nm is ascribable to ³ ¼ ³* transition and another
band at 410-450 nm to intramolecular charge transfer (ICT)
excitation from the dibutylamino group to pyridine ring for
OH11, to phenylpyridine for OH14, and to the quinoline ring
for OH15. The ICT band of OH15 occurs at a longer
wavelength than those of OH11 and OH14. The corresponding
fluorescence bands for OH11, OH14, and OH15 are observed
at around 533, 556, and 593 nm, respectively. The fluorescence
quantum yield increases in the order of OH14 (ΦF = 0.55) <
OH15 (ΦF = 0.61) < OH11 (ΦF = 0.86).
Figure 2. Absorption spectra of OH11, OH14, and OH15
adsorbed on TiO2 particles with CDCA (®) and without
CDCA (*) as coadsorbent.
for OH15 compared with those in THF. Chenodeoxycholic
acid (CDCA) has been employed as coadsorbent to prevent dye
aggregation on the TiO2 surface. When CDCA is coadsorbed
with OH11 and OH14 on TiO2, the absorption peak wave-
lengths are blue-shifted by 8 nm for OH11 and 16 nm for
OH14, compared to without CDCA. The peak wavelengths are
Absorption spectra of the dyes adsorbed on TiO2 particles
are shown in Figure 2. The absorption peak wavelengths are
red-shifted by 38 nm for OH11, 38 nm for OH14, and 40 nm

Y. Ooyama et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010) 1115
still red-shifted compared with those in THF. In contrast, the
absorption peak wavelength of OH15 adsorbed on TiO2 with
coadsorption of CDCA is similar to that in THF. These results
show that the red-shiftings of OH11 and OH14 by adsorption
on TiO2 are due to the interaction between the dye and TiO2
surface, which will be discussed later again along with the IR
spectra of the dyes adsorbed on TiO2.
IR Spectra of OH11, OH14, and OH15 Adsorbed on TiO2
Particles.
Figure 3 shows the FT-IR spectra of the dye
powders and the dyes adsorbed on TiO2. For the dye powder,
the C=O stretching band of carboxyl group was observed at
1711 cm¹1 for OH11, 1719 cm¹1 for OH14, and 1710 cm¹1 for
OH15, and the C=N or C=C stretching band due to the
¹1
oxazole ring was observed at 1634 and 1622 cm for OH11,
1623 cm¹1 for OH14, and 1623 cm¹1 for OH15. The character-
istic stretching bands for the C=N or C=C of pyridine
moiety were clearly observed at 1607, 1599, 1479, 1460, and
¹1
¹1
1412 cm for OH11, 1602, 1472, 1460, and 1412 cm for
¹1
OH14, and 1584, 1472, 1462, and 1412 cm for OH15.
When the dye was adsorbed on TiO2 surface, the C=O
¹1
stretching bands at 1711, 1719, and 1710 cm for OH11,
OH14, and OH15, respectively, disappeared and instead a new
¹1
¹1
band appeared at 1550 cm for OH11, 1545 cm for OH14,
¹1
and 1538 cm for OH15, assignable to the asymmetric mode
of carboxylate. The observations indicate that the carboxyl
groups of the dyes form a bidentate chelation with the TiO2
surface.1d On the other hand, in the IR spectra of both OH11
¹1
and OH14 adsorbed on TiO2, the band at around 1600 cm
was broadened and the bands at around 1470 and 1460 cm
¹1
became weaker, indicative of the formation of hydrogen
bonding between nitrogen of pyridine rings and hydroxy
protons of the TiO2 surface. Furthermore, a new band appeared
at 1618 cm¹1 for both OH11 and OH14, which is assignable to
pyridine rings coordinated to the Lewis acid sites of the TiO2
surface. This indicates that OH11 and OH14 are adsorbed on
the TiO2 surface by hydrogen bonding as well as coordinate
bonding. It is well known that pyridine is adsorbed on the TiO2
surface by hydrogen bonding with the hydroxy group of TiO2
surface and/or by coordinate bonding with the Lewis acid site
of TiO2 surface; when the pyridine is adsorbed on the TiO2
surface by hydrogen bonding with the hydroxy group of the
¹1
TiO2 surface, the bands at around 1580-1600 cm become
¹1
broader and the bands at around 1460-1480 cm become
weaker, on the other hand, when the pyridine is adsorbed on the
TiO2 surface by coordinate bonding with the Lewis acid site of
¹1 7,8
TiO2 surface, new bands appear at 1600-1630 cm
.
In the
IR spectra of OH15 adsorbed on TiO2, on the other hand, the
¹1
1462-cm band became weaker and no sign of coordinate
Figure 3. FT-IR spectra of the dye powders and the dyes
adsorbed on particles for (a) OH11, (b) OH14, and
(c) OH15.
bonding was observed, suggesting that OH15 is adsorbed on
TiO2 surface by weak hydrogen bonding alone. Accordingly,
the large red-shift of absorption peak for OH11 or OH14 on
TiO2 (Figure 2) is ascribable to a strong interaction of the dye
and TiO2 due to coordinate bonding.
Electrochemical Properties of OH11, OH14, and OH15
and Their HOMO and LUMO Energy Levels.
electrochemical properties of three dyes were determined by
cyclic voltammetry (CV) in DMF containing 0.1 tetraethyl-
ammonium perchlorate (Et4NClO4) for OH11 and in CH2Cl2
containing 0.1 M tetrabutylammonium perchlorate (Bu4NClO4)
for OH14 and OH15. The potentials were referred to
ferrocene/ferrocenium (Fc/Fc+) as an internal reference. A
typical CV curve of OH14 is shown in Figure 4. The CV data
are summarized in Table 2. The oxidation peaks of OH11,
OH14, and OH15 were observed, respectively, at 0.31, 0.24,
and 0.28 V vs. Fc/Fc+, whereas the corresponding reduction
peaks appeared at 0.26, 0.18, and 0.22 V. These results show
that the oxidized states of the three dyes are stable.
The

1116 Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010)
Dye-Sensitized Solar Cells
Table 3. Calculated Absorption Spectra for OH11, OH14,
and OH15
Absorption (calcd)
Dye
CI componentb)
-max/nm
f a)
OH11
OH14
OH15
420
413
426
0.74
0.98
0.75
HOMO ¼ LUMO (79%)
HOMO ¼ LUMO (71%)
HOMO ¼ LUMO (73%)
a) Oscillator strength. b) The transition is shown by an arrow
from one orbital to another, followed by its percentage CI
(configuration interaction) component.
and electrochemical properties of OH11, OH14, and OH15,
we carried out semiempirical molecular orbital (MO) calcu-
lations. The molecular structures were optimized by using the
MOPAC/AM1 method,9 and then the INDO/S method10 using
the SCRF Onsager Model was used for spectroscopic calcu-
lations in THF. The calculated torsion angles between the
pyridine, 4-phenylpyridine, and quinoline rings and the oxazole
ring of OH11, OH14, and OH15 are ca. 8, 5, and 30°,
respectively. The large torsion angle for OH15 is attributed to
steric hindrance between oxygen and/or nitrogen atoms of the
oxazole ring and hydrogen atom of the quinoline ring. On the
other hand, the calculated torsion angle between the pyridine
ring and phenyl ring of OH14 was ca. 40°, which is attributed
to steric hindrance between a hydrogen atom of the pyridine
ring and a hydrogen atom of the phenyl ring. The calculated
absorption wavelengths and the transition characters of
the first absorption bands are collected in Table 3. The
longest excitation bands of OH11, OH14, and OH15 show
red-shifts in the order of OH14 (413 nm) < OH11 (420 nm) <
OH15 (426 nm), which are comparable to the observed spectra
in THF, although the calculated absorption wavelength of
OH14 is shifted to longer wavelength than that of OH11. The
calculations show that the longest excitation bands of OH11,
OH14, and OH15 are mainly assigned to the transition from
HOMO to LUMO, where HOMOs are mostly localized on the
3-dibutylaminobenzofuro[2,3-c]oxazolo[4,5-a]carbazole moie-
ty for the three dyes, and LUMOs are mostly localized on the
pyridine ring for OH11, the 4-phenylpyridine ring for OH14,
and the quinoline ring for OH15. The changes in the calculated
electron density accompanying the first electron excitation are
shown in Figure 5, which reveals an intramolecular charge
transfer (ICT) from the 3-dibutylaminobenzofuro[2,3-c]oxa-
zolo[4,5-a]carbazole moiety to the pyridine, 4-phenylpyridine,
or quinoline rings for the three dyes.
Figure 4. Cyclic voltammogram of OH14 in CH2Cl2
¹1
containing 0.1 M Bu4NClO4 at a scan rate of 50 mV s
The arrow denotes the direction of the potential scan.
.
Table 2. Electrochemical Properties of OH11, OH14, and
OH15 and Their Energy Levels of HOMO and LUMO
Dye
Epa/Va)
Epc/Va)
HOMO/Vb)
LUMO/Vb)
OH11
OH14
OH15
0.31
0.24
0.28
0.26
0.18
0.22
0.96
0.89
0.93
¹1.62
¹1.65
¹1.45
a) Epa and Epc are the anodic and cathodic peak potentials vs.
Fc/Fc+. b) vs. normal hydrogen electrode (NHE).
The HOMO and LUMO energy levels of the three dyes were
evaluated from the spectral analyses and CV data. The HOMO
energy levels for OH11, OH14, and OH15 are evaluated,
respectively to be 0.96, 0.89, and 0.93 V with respect to a
normal hydrogen electrode (NHE). It can be presumed from the
same HOMO energy levels among the three dyes that the rate
¹
of the dye regeneration by electron donation from I in redox
mediator to the oxidized dye is comparable in the three dyes.
The LUMO energy levels of the dyes are estimated from the
oxidation potential and an intersection of absorption and
fluorescence spectra (480 nm (2.58 eV) for OH11, 490 nm
(2.54 eV) for OH14, and 520 nm (2.38 eV) for OH15)
corresponding to the energy gap between HOMO and LUMO.
The LUMO energy levels of OH11, OH14, and OH15 are
¹1.62, ¹1.65, and ¹1.45 V, respectively. The LUMO level of
OH15 is lower than those of OH11 and OH14, which lead to
the decrease of the energy gap between the HOMO and LUMO
responsible for the red-shift of ICT absorption band for OH15,
relative to OH11 and OH14. The LUMO energy levels for the
three dyes are higher than the energy level of the TiO2
conduction band (¹0.5 V), suggesting that an efficient electron
injection to the TiO2 conduction band is thermodynamically
possible. On the other hand, the rate (0.1-100 ps) of the
electron injection from the excited dyes to TiO2 conduction
band is much faster than that of the fluorescence decay (1-5 ns)
of the dyes,6g which suggests that the electron injection into the
TiO2 conduction band is also kinetically possible.
Photovoltaic Performance of DSSCs Based on OH11,
OH14, and OH15. The DSSCs were prepared by using the
dye-adsorbed TiO2 electrode, Pt-coated glass as a counter
electrode, and an acetonitrile solution with 0.05 M iodine,
0.1 M lithium iodide, and 0.6 M 1,2-dimethyl-3-propylimida-
zolium iodide as electrolyte. CDCA was employed as coad-
sorbent. The photocurrent-voltage (I-V) characteristics were
measured under a simulated solar light (AM 1.5, 100
mW cm¹2). The incident photon-to-current conversion efficien-
cy (IPCE) spectra were measured under with a tungsten-
halogen lamp and a monochromator. IPCE spectra and I-V
curves are depicted in Figures 6 and 7, respectively. The IPCE
is calculated by the following equation:
Semiempirical MO Calculations (AM1, INDO/S) of
OH11, OH14, and OH15. To understand the photophysical

Y. Ooyama et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010) 1117
OH11
OH14
OH11
OH14
OH15
OH15
Figure 5. Calculated electron density changes accompany-
ing the first electronic excitation of OH11, OH14, and
OH15. The blue and red lobes signify decrease and
increase in electron density accompanying the electronic
transition, respectively. Their areas indicate the magnitude
of the electron density change (light blue, green, blue, and
red balls correspond to hydrogen, carbon, nitrogen, and
oxygen atoms, respectively.
Figure 7. Photocurrent-voltage curves of DSSCs based on
OH11, OH14, and OH15.
OH11
OH14
OH15
Table 4. Photovoltaic Performances of DSSCs Based on
OH1, OH11, OH14, and OH15
¹2
Dye
Jsc/mA cm
Voc/mV
ff
©/%
OH11
OH14
OH15
OH1
4.88
3.91
1.72
4.09
532
544
457
524
0.61
0.62
0.70
0.66
1.62
1.30
0.56
1.45
OH11 (4.88 mA cm¹2) is similar to that for OH1 (4.09
mA cm¹2) and higher than that of OH14 (3.91 mA cm¹2). On
the other hand, the Jsc for OH15 (1.72 mA cm¹2) shows a
moderate value. The © values increase in the order of OH15
(0.56%) < OH14 (1.30%) < OH1 (1.45%) < OH11 (1.62%).
The values of Voc for OH1, OH11, OH14, and OH15 are 524,
532, 544, and 457 mV, respectively, which are different among
the dyes. In addition, when 4-tert-butylpyridine (TBP) was
added to the electrolyte, the Voc was enhanced in all the cases.
For example, the DSSC based on OH11 yielded the © value of
2.51% with Jsc = 5.52 mA cm¹2, Voc = 665 mV, and ff = 0.67.
It should be noticed, however, that the Jsc value of OH15
obtained from the I-V curve does not correspond to that
obtained from the IPCE spectrum. More recently, we found that
the mismatches in the Jsc values between the I-V curve and the
IPCE spectrum are observed in some of the D-³-A dye
sensitizers with pyridine rings as electron-accepting groups.
Further studies on the DSSCs based on the D-³-A dye
sensitizers with pyridine rings as electron-accepting groups are
now in progress to elucidate some effect on the Jsc values
obtained from the I-V curve and the IPCE spectrum and will be
reported in the next paper.
Figure 6. IPCE spectra of DSSCs based on OH11, OH14,
and OH15.
IPCE=% ¼ f1240=ðeV nmÞ ꢀ Jsc=ðmA cmꢁ2Þg
=fð-=nmÞ ꢀ ꢀ=ðmW cmꢁ2Þg ꢀ 100
ð1Þ
where Jsc is the short-circuit photocurrent density generated by
monochromatic light, and - and Φ are the wavelength and
the intensity of the monochromatic light, respectively. The
maximum IPCE value increases in the order of OH15 (4% at
496 nm) ¹ OH14 (53% at 480 nm) ¯ OH11 (55% at 485 nm).
Table 4 summarizes photovoltaic performances of DSSCs
based on the OH11, OH14, and OH15, together with the one
for OH1 having a carboxyl group acting as not only anchoring
group for attachment on TiO2 surface but also the electron
acceptor. The solar energy-to-electricity conversion yield ©/%
is expressed by the following equation:
Consideration of a Relationship between Photovoltaic
Performance and Configuration of Dye on TiO2 Surface.
The FT-IR spectra of OH11 and OH14 adsorbed on TiO2
indicate the formation of hydrogen bonding between the
nitrogen of pyridine ring and the hydroxy proton of TiO2
surface as well as the formation of the strong coordinate
bonding between the nitrogen lone-pair of pyridine and Lewis
©=% ¼ fJsc=ðmA cmꢁ2Þ ꢀ Voc=Vg ꢀ ffg
=fI0=ðmW cmꢁ2Þg ꢀ 100
ð2Þ
where I0 is the intensity of incident white light, Voc is the open-
circuit photovoltage, and ff represents the fill factor. The Jsc for

1118 Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010)
Dye-Sensitized Solar Cells
accepting groups of D-³-A dye sensitizers for DSSCs, which
induces the possibility of the epoch-making molecular design
and synthesis of a variety of D-³-A dye sensitizers beyond the
traditional D-³-A dye sensitizers with a carboxyl group as
electron acceptor.
Conclusion
As a new type of D-³-A dye sensitizers for DSSCs, we
have designed and synthesized the fluorescent dyes OH11,
OH14, and OH15 with pyridine, 4-phenylpyridine, and quino-
line rings, respectively, as electron-accepting groups. The Jsc of
DSSCs using OH11 and OH14 are higher than that of OH14.
The FT-IR spectra of OH11 and OH14 adsorbed on TiO2
particles indicate the formation of hydrogen bonding between
the pyridine ring of dyes and the hydroxy group of TiO2
surface and the formation of strong coordinate bonding
between the pyridine ring of dyes and the Lewis acid site of
TiO2 surface. On the other hand, only the formation of weak
hydrogen bonding was observed in the IR spectra of OH15
adsorbed on TiO2. Consequently, it was suggested that the dyes
OH11 and OH14 can inject electrons efficiently from the
pyridine ring to the conduction band of the TiO2 electrode
through the strong coordinate bonding rather than the hydrogen
bonding. Our results demonstrate that a carboxyl group of
D-³-A dye sensitizers is necessary not as the electron
acceptor, but only as an anchoring group for attachment on
the TiO2 surface. Therefore, we propose that the pyridine rings
forming a strong coordinate bonding with the Lewis acid site of
TiO2 surface are promising candidates as electron-accepting
groups of a new type of D-³-A dye sensitizers for DSSCs.
Figure 8. Configurations of OH11 on TiO2 surface; (a)
hydrogen bonding and (b) coordinate bonding. Light blue,
green, blue, and red balls correspond to hydrogen, carbon,
nitrogen, and oxygen atoms, respectively.
acid sites of TiO2 surface. In contrast, only the formation of
weak hydrogen bonding was observed in the IR spectra of
OH15 adsorbed on TiO2. It is inferred from the molecular
structure of OH15 that the sterically hindered quinoline ring
prevents the formation of the coordinate bonding between the
nitrogen lone-pair of quinoline ring and the Lewis acid site of
TiO2 surface. According to our previous studies,5,6 it may be
safe to assume that the dye molecules are standing perpendic-
ular to the TiO2 substrate as shown in Figure 8. Consequently,
in view of highly efficient charge injection probabilities as
suggested by IPCE values over 50%, the dyes OH11 and
OH14 can inject electrons efficiently from the pyridine ring to
the conduction band of the TiO2 electrode through the strong
coordinate bonding (Figure 8b) rather than the hydrogen
bonding (Figure 8a). The slightly low photovoltaic perform-
ance for OH14 is ascribable to the weakened ICT character-
istics by the large twisting between the pyridine ring and
phenyl ring, which leads to reduction in electron-injection
yield. On the other hand, for OH15, the absence of the
coordinate bonding between the quinoline ring and the Lewis
acid site of TiO2 surface might be responsible for lower
electron injection yield (the low Jsc value) than those of OH11
and OH14. Thus, it was suggested that the formation of strong
coordinate bonding between the pyridine ring of dyes and the
Lewis acid site of TiO2 surface leads to an efficient electron
injection owing to a good electron communication between
them. As additional evidence, the dye 2 (see Ref. 5) with
butyric acid removed from the carbazole moiety of OH11, so
that it does not have a carboxyl group as anchoring group for
attachment on TiO2 surface, can be strongly adsorbed on the
TiO2 surface. It took about two months for the dye 2 to reach
saturated adsorption, but only two days for OH11. The DSSC
based on the dye 2 without CDCAyielded the © value of 0.96%
with Jsc = 3.65 mA cm¹2, Voc = 429 mV, and ff = 0.62. The ©
value for the dye 2-DSSC is slightly lower than that (1.33%)
for OH11-DSSC without CDCA. The maximum IPCE value
for the dye 2 is 6% at 468 nm, and the mismatch in the Jsc
values between the I-V curve and the IPCE spectrum was also
observed. Therefore, our results demonstrate that the pyridine
rings are one of the most promising candidates as electron-
Experimental
Melting points were measured with a Yanaco micro melting
point apparatus MP model. IR spectra were recorded on a
Perkin-Elmer Spectrum One FT-IR spectrometer by ATR
method. Absorption spectra were observed with a Shimadzu
UV-3150 spectrophotometer and fluorescence spectra were
measured with a Hitachi F-4500 spectrophotometer. The
fluorescence quantum yields (Φ) were determined with a
Hamamatsu C9920-01 equipped with CCD by using a
calibrated integrating sphere system (-ex = 325 nm). CVs were
recorded in CH2Cl2/Bu4NClO4 (0.1 M) solution or in DMF/
Et4NClO4 (0.1 M) solution with a three-electrode system
consisting of Ag/Ag+ as reference electrode, Pt plate as
working electrode, and Pt wire as counter electrode, by using a
Hokuto Denko HAB-151 potentiostat equipped with a func-
tional generator. Elemental analyses were recorded on a Perkin-
1
Elmer 2400II CHN analyzer. H NMR spectra were recorded
on a JNM-LA400 (400 MHz) FT NMR spectrometer with
tetramethylsilane as an internal standard. Column chromatog-
raphy was performed on silica gel (KANTO CHEMICAL,
60N, spherical, neutral).
Preparation of 7-(4-Pyridin-4-ylphenyl)-3-dibutylamino-
benzofuro[2,3-c]oxazolo[4,5-a]carbazole (2). A solution of
1 (0.50 g, 1.21 mmol), 4-(4-pyridinyl)benzaldehyde (0.22 g,
1.21 mmol), and ammonium acetate (2.33 g, 28.9 mmol) in
acetic acid (50 mL) was stirred at 90 °C for 2 h. After the
reaction was complete, the reaction mixture was condensed.
The resulting precipitate was washed with water and dried. The

Y. Ooyama et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010) 1119
residue was chromatographed on silica gel (dichloromethane-
ethyl acetate = 3:1 as eluent) to give 2 (0.12 g, yield 17%) as a
(5). A solution of 1 (0.50 g, 1.21 mmol), 4-quinolinecarbox-
aldehyde (0.19 g, 1.21 mmol), and ammonium acetate (2.33 g,
28.9 mmol) in acetic acid (50 mL) was stirred at 90 °C for 2 h.
After the reaction was complete, the reaction mixture was
condensed. The resulting precipitate was washed with water
and dried. The residue was chromatographed on silica gel
(dichloromethane-THF = 3:1 as eluent) to give 4 (0.28 g, yield
40%) as an orange powder and 5 (0.06 g, yield 8%) as a red
~
yellow powder; mp 258-260 °C; IR (ATR): ¯ = 3058, 1622,
1597 cm¹1; 1H NMR (DMSO-d6, TMS): ¤ 0.96 (6H, t, J = 7.37
Hz), 1.35-1.43 (4H, m), 1.60-1.64 (4H, m), 3.40 (4H, t, J =
7.21 Hz) overlap with peak of dissolved water in DMSO-d6,
6.96 (1H, d, J = 8.80 Hz), 7.06 (1H, s), 7.37 (1H, t, J = 6.83
Hz), 7.49 (1H, t, J = 9.06 Hz), 7.68 (1H, d, J = 6.84 Hz), 7.85
(2H, d, J = 5.88 Hz), 8.14 (2H, d, J = 7.80 Hz), 8.43 (2H, d,
J = 7.80 Hz), 8.48 (1H, d, J = 8.80 Hz), 8.62 (1H, d, J = 8.80
Hz), 8.71 (2H, d, J = 5.88 Hz), 12.47 (1H, s, -NH); FABMS
m/z 579 (M+).
~
powder; 4: mp 288-291 °C; IR (ATR): ¯ = 3058, 1622, 1587
cm¹1; 1H NMR (DMSO-d6, TMS): ¤ 0.99 (6H, t, J = 7.85 Hz),
1.39-1.45 (4H, m), 1.61-1.67 (4H, m), 3.44 (4H, t, J = 9.00
Hz) overlap with peak of dissolved water in DMSO-d6, 6.99
(1H, d, J = 8.80 Hz), 7.10 (1H, s), 7.40 (1H, t, J = 7.85 Hz),
7.52 (1H, t, J = 8.76 Hz), 7.74 (1H, d, J = 7.80 Hz), 7.91-7.99
(2H, m), 8.23 (1H, d, J = 8.80 Hz), 8.45 (1H, d, J = 4.88 Hz),
8.53 (1H, d, J = 8.80 Hz), 8.65 (1H, d, J = 7.80 Hz), 9.22 (1H,
Preparation of 7-(4-Pyridin-4-ylphenyl)-3-dibutylamino-
benzofuro[2,3-c]oxazolo[4,5-a]carbazole-9-heptanoic Acid
Ethyl Ester (3). A solution of 2 (0.12 g, 0.2 mmol) in DMF
(150 mL) was treated with sodium hydride (60%, 0.01 g,
0.25 mmol) and stirred for 1 h at room temperature. Ethyl 7-
bromoheptanoate (0.27 g, 1.15 mmol) was added dropwise over
20 min and the solution was stirred at room temperature for 2 h.
After concentrating under reduced pressure, the resulting
residue was dissolved in dichloromethane, and washed with
water. The organic extract was dried over MgSO4, filtered, and
concentrated. The residue was chromatographed on silica gel
(dichloromethane-ethyl acetate = 3:1 as eluent) to give 3
d, J = 3.92 Hz), 9.73 (1H, d, J = 7.80 Hz), 12.62 (1H, s, -NH);
+
~
FABMS m/z 553 (M ); 5: mp 282-284 °C; IR (ATR): ¯ =
3064, 1622, 1586 cm¹1; H NMR (DMSO-d6, TMS): ¤ 0.99
1
(6H, t, J = 7.61 Hz), 1.39-1.45 (4H, m), 1.61-1.67 (4H, m),
3.41 (4H, t, J = 7.79 Hz) overlap with peak of dissolved water
in DMSO-d6, 6.94 (1H, d, J = 8.80 Hz), 7.11 (1H, s), 7.40
(1H, t, J = 6.77 Hz), 7.54 (1H, t, J = 8.03 Hz), 7.73 (1H, d, J =
8.80 Hz), 7.91-7.98 (2H, m), 8.23 (1H, d, J = 7.80 Hz), 8.41
(1H, d, J = 4.88 Hz), 8.47 (1H, d, J = 8.80 Hz), 8.66 (1H, d,
J = 7.80 Hz), 9.23 (1H, d, J = 3.92 Hz), 9.65 (1H, d, J =
8.80 Hz), 12.53 (1H, s, -NH); FABMS m/z 553 (M+).
(0.12 g, yield 80%) as a yellow powder; mp 188-191 °C; IR
1
(ATR): ¯ = 1732, 1622, 1595 cm¹1; H NMR (CD2Cl2, TMS):
~
¤ 0.95 (6H, t, J = 6.95 Hz), 1.09 (3H, t, J = 6.95 Hz), 1.23-
1.28 (2H, m), 1.35-1.64 (12H, m), 1.96-1.98 (2H, m), 2.16-
2.21 (2H, m), 3.36 (4H, t, J = 7.61 Hz), 3.92-4.01 (2H, m),
4.90 (2H, t, J = 6.83 Hz), 6.82 (1H, d, J = 9.76 Hz), 6.89 (1H,
s), 7.31 (1H, t, J = 7.67 Hz), 7.46 (1H, t, J = 7.31 Hz), 7.54-
7.57 (3H, m), 7.79 (2H, d, J = 8.80 Hz), 8.30-8.38 (3H, m),
8.52-8.64 (3H, m); FABMS m/z 734 (M+).
Preparation of 7-(4-Pyridin-4-ylphenyl)-3-dibutylamino-
benzofuro[2,3-c]oxazolo[4,5-a]carbazole-9-heptanoic Acid
(OH14). A solution of 3 (0.07 g, 0.10 mmol) in a mixed
solvent (ethanol 300 mL and dichloromethane 25 mL) was
added dropwise to aqueous NaOH (0.03 g, 0.7 mmol, 30 mL)
with stirring at 70 °C. After further stirring for 12 h under
reflux, the solution was acidified to pH 4 with 2 M HCl, and
concentrated under reduced pressure. The residue was dis-
solved in dichloromethane, and washed with water. The organic
extract was dried over MgSO4, filtered, and concentrated. The
resulting residue was reprecipitated from dichloromethane-
hexane to give OH14 (0.06 g, yield 86%) as a dark red
Preparation of 7-(Quinolin-4-yl)-3-dibutylaminobenzo-
furo[2,3-c]oxazolo[4,5-a]carbazole, 9-Heptanoic Acid Ethyl
Ester (6).
A solution of 4 (0.12 g, 0.22 mmol) in DMF
(150 mL) was treated with sodium hydride (60%, 0.01 g,
0.25 mmol) and stirred for 1 h at room temperature. Ethyl 7-
bromoheptanoate (0.26 g, 1.08 mmol) was added dropwise over
20 min and the solution was stirred at room temperature for 2 h.
After concentrating under reduced pressure, the resulting
residue was dissolved in dichloromethane, and washed with
water. The organic extract was dried over MgSO4, filtered, and
concentrated. The resulting residue was reprecipitated from
dichloromethane-hexane to give 6 (0.12 g, yield 78%) as an
~
orange powder; mp 157-159 °C; IR (ATR): ¯ = 1732, 1622,
1584 cm¹1; H NMR (acetone-d6, TMS): ¤ 1.04 (6H, t, J =
1
7.07 Hz), 1.13 (3H, t, J = 7.61 Hz), 1.19-1.24 (2H, m), 1.42-
1.74 (14H, m), 2.20-2.35 (2H, m), 3.53 (4H, t, J = 3.20 Hz),
3.97-4.09 (2H, m), 5.05 (2H, t, J = 7.25 Hz), 6.97 (1H, d,
J = 8.80 Hz), 7.02 (1H, s), 7.43 (1H, t, J = 7.67 Hz), 7.58
(1H, t, J = 7.67 Hz), 7.83 (1H, d, J = 7.84 Hz), 7.87-7.96 (2H,
m), 8.24 (1H, d, J = 8.80 Hz), 8.44 (1H, d, J = 3.92 Hz), 8.49
(1H, d, J = 8.80 Hz), 8.70 (1H, d, J = 7.80 Hz), 9.18 (1H,
d, J = 3.92 Hz), 9.78 (1H, d, J = 7.80 Hz); FABMS m/z 709
(M+).
Preparation of 7-(Quinolin-4-yl)-3-dibutylaminobenzo-
furo[2,3-c]oxazolo[4,5-a]carbazole, 9-Heptanoic Acid
(OH15). A solution of 6 (0.12 g, 0.17 mmol) in a mixed
solvent (ethanol 300 mL and dichloromethane 25 mL) was
added dropwise to aqueous NaOH (0.03 g, 0.7 mmol, 30 mL)
with stirring at 70 °C. After further stirring for 12 h under
reflux, the solution was acidified to pH 4 with 2 M HCl, and
concentrated under reduced pressure. The residue was dis-
solved in dichloromethane, and washed with water. The organic
~
powder; mp 231-234 °C; IR (ATR): ¯ = 3050, 1718, 1623,
1602 cm
;
1H NMR (DMSO-d6, TMS): ¤ 0.97 (6H, t, J =
¹1
6.89 Hz), 1.37-1.48 (12H, m), 1.60-1.63 (2H, m), 1.91-2.02
(2H, m), 2.18 (2H, t, J = 6.83 Hz), 3.44 (4H, t, J = 4.53 Hz)
overlap with peak of dissolved water in DMSO-d6, 4.95
(2H, t, J = 6.64 Hz), 6.93 (1H, d, J = 8.80 Hz), 7.02 (1H, s),
7.39 (1H, t, J = 6.77 Hz), 7.55 (1H, t, J = 6.77 Hz), 7.81 (1H,
d, J = 7.80 Hz), 7.84 (2H, d, J = 5.88 Hz), 8.10 (2H, d, J =
6.84 Hz), 8.37 (2H, d, J = 6.84 Hz), 8.44 (1H, d, J = 8.80 Hz),
8.61 (1H, d, J = 6.84 Hz), 8.71 (2H, d, J = 4.88 Hz); FABMS
m/z 706 (M+).
Preparation of 7-(Quinolin-4-yl)-3-dibutylaminobenzo-
furo[2,3-c]oxazolo[4,5-a]carbazole (4) and 7-(Quinolin-4-
yl)-3-dibutylaminobenzofuro[2,3-c]oxazolo[5,4-a]carbazole

1120 Bull. Chem. Soc. Jpn. Vol. 83, No. 9 (2010)
Dye-Sensitized Solar Cells
extract was dried over MgSO4, filtered, and concentrated. The
resulting residue was reprecipitated from dichloromethane-
hexane to give OH15 (0.08 g, yield 70%) as a dark red
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~
powder; mp 232-234 °C; IR (ATR): ¯ = 3050, 1708, 1622,
1584 cm
;
1H NMR (DMSO-d6, TMS): ¤ 0.97 (6H, t, J =
¹1
7.73 Hz), 1.38-1.62 (14H, m), 1.98-2.10 (2H, m), 2.13 (2H, t,
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(1H, d, J = 8.80 Hz), 7.04 (1H, s), 7.41 (1H, t, J = 7.79 Hz),
7.57 (1H, t, J = 6.71 Hz), 7.87 (1H, d, J = 8.80 Hz), 7.92-
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J = 3.92 Hz), 9.64 (1H, d, J = 7.80 Hz); FABMS m/z 679
(M+).
Computational Methods. The semiempirical calculations
were carried out with the WinMOPAC Ver. 3.9 package
(Fujitsu, Chiba, Japan). Geometry calculations in the ground
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unoccupied orbital, where 225 configurations were considered
[keyword CI (15 15)].
2
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