Structure and photovoltaic properties of (E)-2-cyano-3-[4-(diphenylamino) phenyl] acrylic acid substituted by tert-butyl groups

Article abstract of DOI:10.1246/cl.2010.864

(E)-3-{ 4-[Bis(4-tert-butylphenyl)amino]phenyl ) -2-cyanoacrylic acid and its related compounds were synthesized by the Knoevenagel condensation of the corresponding 4-aminobenzaldehydes with cyanoacetic acid. Cyclic voltammetry measurements showed that t

Full text of DOI:10.1246/cl.2010.864

doi:10.1246/cl.2010.864
864
Published on the web July 10, 2010
Structure and Photovoltaic Properties of (E)-2-Cyano-3-[4-(diphenylamino)phenyl]acrylic
Acid Substituted by tert-Butyl Groups
Katsuhiko Ono,*1 Tomoya Yamaguchi,1 and Masaaki Tomura2
1Department of Materials Science and Engineering, Nagoya Institute of Technology,
Gokiso, Showa-ku, Nagoya, Aichi 466-8555
2Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585
(Received May 10, 2010; CL-100455; E-mail: ono.katsuhiko@nitech.ac.jp)
(E)-3-{4-[Bis(4-tert-butylphenyl)amino]phenyl}-2-cyano-
acrylic acid and its related compounds were synthesized by the
Knoevenagel condensation of the corresponding 4-aminobenz-
aldehydes with cyanoacetic acid. Cyclic voltammetry measure-
ments showed that these organic dyes exhibited reversible
oxidation waves. The tert-butyl substituents also decreased
molecular stacking in the crystals, thus affecting the photo-
voltaic properties of the dyes. A solar cell using the tert-butyl-
substituted dye exhibited higher performance than that for the
analog without tert-butyl group substitution.
The fabrication of low cost and high performance dye-
sensitized solar cells (DSCs) has attracted considerable attention.1
DSCs using ruthenium sensitizers exhibited a solar energy-to-
electricity conversion efficiency (©) above 11% at standard AM
1.5 solar light.2 Recently, DSCs using metal-free organic dyes
have been intensely studied. A number of organic dyes, such as
triphenylamine,3-5 carbazole,6,7 coumarin,8 indoline,9 heteroan-
thracene,10 and phthalocyanine dyes11 have been synthesized and
investigated. DSCs incorporating these dyes exhibit conversion
efficiencies ranging within 4-10%. The major factors decreasing
conversion efficiencies are the formation of dye aggregates on the
TiO2 electrodes and the recombination of excited electrons.4a,6b In
this regard, we introduced tert-butyl groups into (E)-2-cyano-3-
[4-(diphenylamino)phenyl]acrylic acid (CDPA)12 to generate dye
1 and its analogs 2 and 3 (Scheme 1). These dyes have simple
donor-³-acceptor structures, which are easily modified into
extended ³-conjugation systems.3,7,13 Therefore, significant
information is obtained about the tert-butyl substituent effects.
In this paper, we report the synthesis, properties, and crystal
structures of organic dyes 1-3 and their application to DSCs.
Figure 1. (a) UV-vis absorption spectra and (b) cyclic
voltammograms of 1-3 in acetonitrile.
Table 1. UV-vis absorption properties and oxidation potentials
of dyes 1-3 in acetonitrile
Dye
-
max/nm (¾)
Egap/eV
E1/2ox/Va
1
2
3
426 (36900)
406 (17600)
403 (31400)
422 (36200)
b
2.54
2.67
2.73
2.60
ox
+1.17
+1.40
+0.88
+1.29b
CDPA
aV versus NHE, ref. 15. Irreversible: Ep ¹ 0.03 V.
The synthesis of organic dyes 1-3 is illustrated in
Scheme 1.14 The dyes were prepared in 56-80% yields by the
Knoevenagel condensation of 4-aminobenzaldehydes 4a-4c
with cyanoacetic acid in the presence of piperidine. Structural
determinations of the dyes were performed using spectroscopic
and elemental analyses. The dyes were obtained as orange
crystals or a yellow solid, and their melting points were above
200 °C. UV-vis absorption spectra of 1-3 in acetonitrile are
shown in Figure 1a. The absorption maximum of 1 was slightly
red-shifted compared to that of CDPA, and the HOMO-LUMO
energy gap (Egap) of 1 was smaller than for CDPA (Table 1).
This indicates that tert-butyl groups slightly enhance the donor-
³-acceptor character of the CDPA skeleton. On the other hand,
the absorption maxima of 2 and 3 were blue-shifted compared to
that of 1, and the HOMO-LUMO energy gaps of 2 and 3 were
larger than that of 1. Therefore, bis(4-tert-butylphenyl)amine
groups facilitate absorption at long wavelengths.
Bu
Bu
t
t
NC
NCCH2CO2H
CO2H
N
CHO
N
piperidine
CH3CN
Bu
Bu
X
Bu
Bu
X
t
t
t
t
4a
80%
1
NC
NCCH2CO2H
CO2H
N
CHO
N
piperidine
CH3CN
Bu
Bu
t
t
4b X = bond
4c
77%
56%
2
3
X = bond
X = S
X = S
Cyclic voltammetry (CV) measurements in acetonitrile were
Scheme 1. Synthesis of organic dyes 1-3.
performed to investigate the redox properties of 1-3.15 The
Chem. Lett. 2010, 39, 864-866
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

865
Figure 2. Electrochemical potential diagram of dyes 1-3 in
volts versus NHE: the oxidation potentials of ground states (
and excited states ( ).
)
voltammograms displayed reversible oxidation waves for 1-3
(Figure 1b). CDPA showed an irreversible oxidation wave at
+1.32 V versus normal hydrogen electrode (NHE), suggesting
that the tert-butyl groups stabilize the cation radical species
generated by CV scanning. The oxidation potential of 1 was
slightly negative compared to that of CDPA. On the other hand,
oxidation potentials varied depending on the nature of the
amines. The potential of 1 was more negative than that for 2, but
was in the positive when compared to 3 (Table 1).
Figure 3. X-ray crystallography: molecular structures (ellip-
soid) of CDPA (a) and 1 (b, c) and packing modes (ball and
stick) of CDPA (d) and 1 (e, f).
Figure 2 shows the electrochemical potential diagrams of
dyes 1-3 in volts versus NHE. The oxidation potentials of the
excited dyes were calculated from the difference between their
halfwave oxidation potential (E1/2ox) and their HOMO-LUMO
ox
4a
energy gap (Egap), E1/2 ¹ Egap
.
The oxidation potentials of
excited dyes 1-3 are sufficiently more negative than the potential
of the TiO2 conduction band (¹0.5 V),1b suggesting an effective
electron injection from the excited dyes to the TiO2 electrodes in
DSCs. The oxidation potentials of 1-3 in the ground states are
¹
¹
more positive than the I /I3 redox potential (+0.4 V), suggest-
ing the effective reduction of the oxidized dyes by iodine.
The molecular structures of 1 and CDPA16 were investigated
by X-ray crystallography.14,17 Single crystals of 1 and CDPA
were grown by recrystallization from their acetonitrile solutions.
Compound 1 gave two crystal forms; one with and one without
acetonitrile,18 and the structure of the crystal obtained without
acetonitrile has been successfully elucidated. Two crystallo-
graphically independent molecules were observed in the crystal.
Bond distances and angles were almost identical for 1 and
CDPA (Figures 3a-3c).14 The (E)-2-cyano-3-phenylacrylic acid
moieties were found to be planar and to form hydrogen-bonded
carboxylic acid dimers. The dimer units stacked into columnar
structures which featured intermolecular distances of 3.42 ¡ for
CDPA (Figure 3d) and 3.61 ¡ for 1 (Figure 3e). The stacking
distance between CDPA molecules was similar to the sum of the
van der Waals radii for carbon atoms (C£C = 3.40 ¡), suggest-
ing the presence of intermolecular interactions in the column. On
the other hand, the stacking distance was much larger than the
sum of the van der Waals carbon atom radii for 1, indicating that
the tert-butyl groups sterically hinder molecular stacking in the
crystal. In addition, the second molecular structure of 1 was not
columnar (Figure 3f). Therefore, the introduction of tert-butyl
groups in a CDPA backbone may prove significantly effective in
inhibiting molecular aggregation on TiO2 electrodes.
Figure 4. Photovoltaic performance for cells 1-3: (a) current
density-voltage characteristics and (b) IPCE spectra.
Figure 4a shows the photocurrent density-voltage charac-
teristics for DSCs using dyes 1-3 and CDPA, which are noted as
cells 1-3 and cell CDPA, respectively.14,19 The photocurrent
density of cell 1 was greater than for cell CDPA over the
observed voltage region, indicating a photocurrent density
increase caused by the introduction of tert-butyl groups. The
short-circuit photocurrent density (Jsc) and open-circuit photo-
voltage (Voc) of cell 1 were also greater than for cell CDPA
(Table 2). The solar energy-to-conversion efficiency (©) of cell 1
was measured to be 3.4%, which is 1.3 times as much as for cell
CDPA (2.7%).12 Furthermore, the photocurrent density of cell 1
was higher than that of cells 2 and 3 over the observed voltage
region. The Jsc value depends on the amine structure, although
Chem. Lett. 2010, 39, 864-866
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

866
Table 2. Photovoltaic performance of cells 1-3a
5
6
a) S. Kim, J. K. Lee, S. O. Kang, J. Ko, J.-H. Yum, S.
Fantacci, F. De Angelis, D. Di Censo, M. K. Nazeeruddin, M.
Grätzel, J. Am. Chem. Soc. 2006, 128, 16701. b) H. Qin, S.
Wenger, M. Xu, F. Gao, X. Jing, P. Wang, S. M. Zakeeruddin,
M. Grätzel, J. Am. Chem. Soc. 2008, 130, 9202. c) H. Choi, C.
Baik, S. O. Kang, J. Ko, M.-S. Kang, M. K. Nazeeruddin, M.
Grätzel, Angew. Chem., Int. Ed. 2008, 47, 327.
a) N. Koumura, Z.-S. Wang, S. Mori, M. Miyashita, E.
Suzuki, K. Hara, J. Am. Chem. Soc. 2006, 128, 14256. b)
Z.-S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi,
A. Mori, T. Kubo, A. Furube, K. Hara, Chem. Mater. 2008,
20, 3993.
C. Teng, X. Yang, C. Yuan, C. Li, R. Chen, H. Tian, S. Li, A.
Hagfeldt, L. Sun, Org. Lett. 2009, 11, 5542.
a) Z.-S. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo, K.
Hara, J. Phys. Chem. C 2007, 111, 7224. b) K. Hara, Y.
Dan-oh, C. Kasada, Y. Ohga, A. Shinpo, S. Suga, K. Sayama,
H. Arakawa, Langmuir 2004, 20, 4205.
JSC
/mA cm
VOC
/V
©
/%
Cell
FF
¹2
1
2
3
8.50
6.82
6.28
7.70
18.20
0.600
0.577
0.604
0.530
0.629
0.661
0.681
0.663
0.654
0.688
3.37
2.68
2.52
2.67
7.85
CDPA
N719
aRef. 19.
the Voc value is observed in a similar photovoltage range. This
result may be attributed to the nonplanar structure of diphenyl-
amine in 1 and the planar structures of carbazole and
phenothiazine in 2 and 3, respectively. The nonplanar bis(4-
tert-butylphenyl)amine moiety is therefore a good electron-
donating unit for metal-free organic dyes.
7
8
The incident photo-to-current conversion efficiency (IPCE)
spectra of cells 1-3 and cell CDPA were plotted as a function of
wavelength (Figure 4b). The IPCE spectra of cells 1-3 displayed
profiles similar to the absorption spectra obtained for TiO2-
supported films of 1-3 in the 350-650 nm region.14 The
maximum IPCE values for cells 1-3 and cell CDPA reached
93, 94, 84, and 91%, respectively. These values are as high as
for a DSC using N719 (cell N719) (92%). However, their IPCE
spectra were blue-shifted by about 200 nm from that of cell
N719, and their conversion efficiencies were much lower than
for cell N719 (Table 2). Therefore, more molecular design is
needed for developing high efficiency organic dyes which
absorb at long wavelength. Modification of the dyes into
extended ³-conjugation systems is in progress.
9
a) S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P.
Liska, P. Comte, P. Péchy, M. Grätzel, Chem. Commun. 2008,
5194. b) T. Horiuchi, H. Miura, K. Sumioka, S. Uchida,
J. Am. Chem. Soc. 2004, 126, 12218.
10 a) H. Tian, X. Yang, R. Chen, Y. Pan, L. Li, A. Hagfeldt, L.
Sun, Chem. Commun. 2007, 3741. b) H. Tian, X. Yang, J.
Cong, R. Chen, J. Liu, Y. Hao, A. Hagfeldt, L. Sun, Chem.
Commun. 2009, 6288.
11 a) S. Eu, T. Katoh, T. Umeyama, Y. Matano, H. Imahori,
Dalton Trans. 2008, 5476. b) S. Mori, M. Nagata, Y.
Nakahata, K. Yasuta, R. Goto, M. Kimura, M. Taya, J. Am.
Chem. Soc. 2010, 132, 4054. c) H. Imahori, S. Hayashi, H.
Hayashi, A. Oguro, S. Eu, T. Umeyama, Y. Matano, J. Phys.
Chem. C 2009, 113, 18406.
12 a) T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N. A.
Anderson, X. Ai, T. Lian, S. Yanagida, Chem. Mater. 2004,
16, 1806. b) W. Xu, B. Peng, J. Chen, M. Liang, F. Cai,
J. Phys. Chem. C 2008, 112, 874. c) D. P. Hagberg, T.
Marinado, K. M. Karlsson, K. Nonomura, P. Qin, G.
Boschloo, T. Brinck, A. Hagfeldt, L. Sun, J. Org. Chem.
2007, 72, 9550.
13 Z. Ning, H. Tian, Chem. Commun. 2009, 5483.
14 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
The authors are grateful to Prof. Hiroshi Segawa and Dr.
Koichi Tamaki (The University of Tokyo) for the measurements
of DSC characteristics. This work was supported by a Grant-in-
Aid for Scientific Research (No. 20550037) from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
We thank the Instrument Center of the Institute for Molecular
Science for the X-ray crystallography.
References and Notes
1
2
3
a) M. Grätzel, Inorg. Chem. 2005, 44, 6841. b) A. Hagfeldt,
M. Grätzel, Chem. Rev. 1995, 95, 49. c) B. O’Regan, M.
Grätzel, Nature 1991, 353, 737.
M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G.
Viscardi, P. Liska, S. Ito, B. Takeru, M. Grätzel, J. Am. Chem.
Soc. 2005, 127, 16835.
a) W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F.
Wang, C. Pan, P. Wang, Chem. Mater. 2010, 22, 1915. b) G.
Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P. Wang,
Chem. Commun. 2009, 2198. c) S. Hwang, J. H. Lee, C. Park,
H. Lee, C. Kim, C. Park, M.-H. Lee, W. Lee, J. Park, K. Kim,
N.-G. Park, C. Kim, Chem. Commun. 2007, 4887. d) D. P.
Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A.
Hagfeldt, L. Sun, Chem. Commun. 2006, 2245.
15 The CV measurements of 1-3 and CDPA were performed in
acetonitrile with 0.1 M n-Bu4NPF6 at a scanning rate of
¹1
100 mV s
using Pt and Ag/Ag+ electrodes. Potentials
measured versus Fc/Fc+ were converted to the NHE scale
by addition of +0.63 V.
16 A polymorph of CDPA was reported, see: S. P. Anthony, S.
Varughese, S. M. Draper, Chem. Commun. 2009, 7500.
17 CCDC-767889 and -767890 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
18 The cocrystal was identified to have a 1:1 molar ratio of 1 and
acetonitrile.
19 DSC conditions: irradiated light: AM 1.5 (100 mW cm¹2);
photoelectrode: TiO2 (16-19 ¯m thickness and 0.16 cm2 area);
electrolyte: 0.60 M 1,2-dimethyl-3-propylimidazolium iodide
(DMPII)/0.025 M iodine (I2)/0.10 M lithium iodide (LiI) in
acetonitrile.
4
a) D. P. Hagberg, J.-H. Yum, H. Lee, F. De Angelis, T.
Marinado, K. M. Karlsson, R. Humphry-Baker, L. Sun, A.
Hagfeldt, M. Grätzel, M. K. Nazeeruddin, J. Am. Chem. Soc.
2008, 130, 6259. b) Z. Ning, Q. Zhang, W. Wu, H. Pei, B.
Liu, H. Tian, J. Org. Chem. 2008, 73, 3791.
Chem. Lett. 2010, 39, 864-866
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/