1406
Published on the web October 27, 2012
Synthesis of a Novel Ruthenium Sensitizer Bearing an ortho-Dicarboxyphenyl Group
as an Anchoring Unit for Dye-sensitized Solar Cells
Hironobu Ozawa, Shohei Oura, Ryosuke Shimizu, and Hironori Arakawa*
Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science,
12-1 Ichigaya-Funagawara, Shinjuku-ku, Tokyo 162-0826
(Received July 10, 2012; CL-120735; E-mail: h.arakawa@ci.kagu.tus.ac.jp)
A novel ruthenium sensitizer TUS-20, which is a structural
analog of black dye, bearing an ortho-dicarboxyphenylterpyri-
dine as an anchoring unit to TiO2 surface has been synthesized.
Adsorption rate of TUS-20 was much faster, and the maximum
amount of dye adsorption was larger than that of black dye. The
dye-sensitized solar cell with TUS-20 exhibited 7.3% con-
version efficiency under AM 1.5 (100 mW cm¹2) irradiation.
-
TBA OOC
-
N
N
HOOC
HOOC
NCS
Ru NCS
NCS
Ru NCS
N
SCN
N
HOOC
SCN
N
N
OOC
TBA
TUS-20
Black dye
Recently, the light-to-electrical energy conversion efficiency
of dye-sensitized solar cells (DSCs) has reached more than
11% by employing an efficient sensitizer, such as polypyridyl
ruthenium complexes1 or zinc porphyrins.2 Extensive efforts
have been thus far made to develop highly efficient sensitizers
for further improvement of the conversion efficiency of DSCs.
On the other hand, for the practical application of DSCs, long-
term stability is one of the most important requirements as well
as high conversion efficiency. Several reasons for gradual
decrease of the conversion efficiency of DSCs have been thus far
pointed out. Among them, one of the major reasons is thought to
be dye desorption from TiO2 photoelectrode.3 In this context,
various ruthenium-complex sensitizers having hydrophobic
alkyl chains, such as Z9074 and C101,1c have been synthesized
to suppress dye desorption from TiO2 photoelectrodes. The
DSCs with these types of ruthenium sensitizers actually show
superior long-term stability than those based on ruthenium dyes
without hydrophobic alkyl chains. In these cases, the binding
ability of dyes themselves are not improved by the introduction
of such functionalized substituents even though dye desorption
from the TiO2 photoelectrode was effectively suppressed.
Therefore, it is important to improve the binding ability of dyes
themselves by the structural modification of their anchoring
units. Although a little attention has been thus far paid to
improving the binding ability of dyes, it is recently reported that
DSC with zinc phthalocyanine dye having an ortho-dicarboxy-
phenyl group as an anchoring unit showed superior long-term
stability than that with zinc phthalocyanine dye bearing a
carboxyphenyl group.5 It is also reported that one carboxy group
of the ortho-dicarboxyphenyl unit binds to TiO2 surface with a
bidentate bridge coordination mode and the other carboxy group
binds with a monodentate coordination mode.6 These studies
clearly indicate that ortho-dicarboxyphenyl is an effective
anchoring unit to TiO2 surfaces.
Figure 1. Structures of TUS-20 and black dye.
2
TUS-20
Black dye
1
ε
0
300 400 500 600 700 800 900
Wavelength/nm
Figure 2. Molar absorptivity spectra of TUS-20 and black dye
in ethanol.
analog of black dye and possesses an ortho-dicarboxyphenyl
unit as an anchoring unit has been synthesized. Although two
structural analogs having an ortho-dihydroxyphenyl group as an
anchoring unit were previously synthesized,9 TUS-20 is the first
example of a ruthenium sensitizer having an ortho-dicarboxy-
phenyl unit as an anchoring unit. Here we communicate photo-
and electrochemical properties and the adsorption behavior of
TUS-20, together with the performance of DSC based on this
novel sensitizer.
A novel ortho-dicarboxyphenylterpyridine ligand was pre-
pared by the oxidation of 4¤-(3,4-dimethylphenyl)terpyridine,
which was synthesized via a Kröhnke-type reaction, using
KMnO4. The total synthetic yield for four steps was 31%.
4¤-(3,4-Dicarboxyphenyl)terpyridine was reacted with RuCl3¢
3H2O, and further reacted with [N(C4H9)4]NCS in DMF. The
crude product was purified on a silica gel column and pure
compound TUS-20 was obtained as a blue powder.
On the other hand, continuous efforts have been made to
increase the conversion efficiency of DSCs with black dye
analogs.7 More than 10.5% conversion efficiency was obtained
in black-dye-based DSCs in our group,8 however, relatively
inferior long-term stability of these DSCs compared to DSCs
with Z907 or C101 is still another subject. In this study, a novel
ruthenium sensitizer TUS-20 (Figure 1), which is a structural
TUS-20 showed similar absorption to that of black dye
although the maximum wavelengths of two MLCT bands were
shifted to shorter wavelength by about 30 nm (Figure 2). The
onset wavelength of absorption spectrum of TUS-20 was also
shifted to shorter wavelength by about 40 nm compared to that
Chem. Lett. 2012, 41, 1406-1408
© 2012 The Chemical Society of Japan