Synthesis of new-type donor-acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole fluorescent dyes and their photovoltaic performances of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2007.10.107

New-type donor-acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole fluorescent dyes with various lengths of non-conjugated alkyl chains containing a carboxyl group at the end position have been developed and their photovoltaic performances of dy

Full text of DOI:10.1016/j.tetlet.2007.10.107

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Tetrahedron Letters 48 (2007) 9167–9170
Synthesis of new-type donor–acceptor p-conjugated
benzofuro[2,3-c]oxazolo[4,5-a]carbazole fluorescent dyes and
their photovoltaic performances of dye-sensitized solar cells
Yousuke Ooyama, Yoshihito Shimada, Yusuke Kagawa, Yuuki Yamada,
Ichiro Imae, Kenji Komaguchi and Yutaka Harima*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
Received 3 August 2007; revised 19 October 2007; accepted 22 October 2007
Available online 25 October 2007
Abstract—New-type donor–acceptor p-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole fluorescent dyes with various lengths of
non-conjugated alkyl chains containing a carboxyl group at the end position have been developed and their photovoltaic perfor-
mances of dye-sensitized solar cells are investigated. It is found that in spite of the lengths of the alkyl chains, due to flexibility
of alkyl chain, the cyano group of the dyes is located in close proximity to TiO2 surface and thus a good electron communication
between the dyes and TiO2 surface is established.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Dye-sensitized solar cells (DSSCs) based on nanocrys-
talline TiO2 electrodes have received considerable atten-
tion because of high incident solar light-to-electricity
conversion efficiency and low cost of production.1–8
Recently, much research has focused on the development
of new metal-free organic dyes. In particular, donor–
acceptor p-conjugated organic dyes possessing broad
and intense spectral features are useful as sensitizers.
Many donor–acceptor p-conjugated dyes with carboxyl
group, acting as not only the anchoring group for
attachment on metal oxide but also the electron accep-
tor, have been synthesized and used as sensitizers of
DSSCs.2–6 A number of studies have suggested that a
carboxyl group can form an ester linkage with TiO2
surface to provide a strongly bound dye and a good
electron communication between them. However,
development of new donor–acceptor p-conjugated dyes
for DSSCs is limited because a carboxyl group is
required to combine with p-conjugation system or
electron acceptor moiety of dyes for the above reasons.
mophore skeleton on photovoltaic performances of
DSSCs,9 we have designed and synthesized novel
donor–acceptor p-conjugated benzofuro[2,3-c]oxazol-
o[4,5-a]carbazole-type fluorescent dyes 1a, 3c, 4a and
5a (Scheme 1), where the electron acceptor is a carboxyl
group for 1a and a cyano group for 3c, 4a and 5a. The
photovoltaic performance of 3c having a non-conju-
gated linkage between a carboxyl group and a chromo-
phore is similar to that of 1a and higher than those of 4a
and 5a. To understand the differences in the photovol-
taic performance, it was assumed that the dye 3c is
standing perpendicularly to the TiO2 substrate as shown
in Figure 1, and that the dye 3c can efficiently inject elec-
trons from the phenylcyano group to the conduction
band of TiO2 electrode through an intermolecular
hydrogen bonding between a cyano nitrogen of the
dye and a hydroxyl proton of the TiO2 surface. To
provide a further confirmation for this view, we have
designed and synthesized new-type donor–acceptor
p-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole
fluorescent dyes 3a–f with different lengths of non-con-
jugated alkyl chains containing a carboxyl group at
the end position.
In our previous study, to clarify the influence of the
position of a carboxyl group attached to the same chro-
In this Letter, we report photovoltaic performances of
DSSCs based on the fluorescent dyes 3a–f and discuss
a relationship between chemical structures of the dyes
and their photovoltaic properties.
Keywords: Heterocycles; Dye-sensitized solar cells; Fluorescence;
Donor–acceptor p-conjugated fluorophores.
Corresponding author. Tel.: +81 82 424 6534; fax.: +81 82 424
*
5494; e-mail: harima@mls.ias.hiroshima-u.ac.jp
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.107

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Y. Ooyama et al. / Tetrahedron Letters 48 (2007) 9167–9170
The reaction of 1 with alkylhalide using sodium hydride
yielded 2a–f. The compounds 3a–f were obtained by
hydrolysis of 2a–f.
The absorption and fluorescence spectra of 3a–f in 1,4-
dioxane are shown in Figure 2 and their spectral data
are summarized in Table 1. All these fluorescent dyes
exhibit intense absorption bands at around 430 and
350 nm, and an intense fluorescence at around 530 nm.
The fluorescence quantum yields (U) of these dyes in
1,4-dioxane are close to 100%. Absorption spectra of
dyes adsorbed on TiO2 film are shown in Figure 3,
where the amounts of adsorbed dyes are 4.4 · 1015,
6.6 · 1016, 5.5 · 1016, 7.8 · 1016, 7.2 · 1016, and 7.4 ·
1016 molecules cmꢀ2 for 3a–f, respectively. The absor-
bance and absorption peak wavelengths of adsorbed
dyes on TiO2 films are very similar to one another, but
only the absorbance of 3a is lower than those of the
other dyes. Undoubtedly, the low absorbance of 3a is
ascribable to the small amount of 3a adsorbed on
TiO2 film. In all the dyes except 3a, the onsets of absorp-
tion bands of adsorbed dyes are red-shifted by 50–80 nm
relative to those in 1,4-dioxane. Such a red-shift is
attributable to aggregation of the dyes on TiO2
electrode.5c,10,11
Scheme 1. Chemical structures of benzofuro[2,3-c]oxazolocarbazole-
type fluorescent dyes.
The electrochemical properties of 3a–f were determined
by cyclic voltammetry (CV) in acetonitrile containing
0.1 M Et4NClO4. CV curves of these compounds
showed three redox waves at similar potentials irrespec-
tive of the sort of the dyes. The first oxidation peaks for
Figure 1. Plausible configuration of 3c on TiO2 surface. Light blue,
green, blue, and red balls correspond to hydrogen, carbon, nitrogen,
and oxygen atoms, respectively.
The synthetic pathway of benzofuro[2,3-c]oxazolo[4,5-a]-
carbazole-type fluorescent dyes 3a–f is shown in
Scheme 2. We used compound 19 as a starting material.
Figure 2. (a) Absorption (—) and (b) fluorescence (---) spectra of 3c in
1,4-dioxane.
Scheme 2. Synthesis of fluorescent dyes 2a–f and 3a–f. Reagents and conditions: (a) NaH, alkylhalide, acetonitrile, rt, 1–12 h, 72% for 2a, 39% for 2b,
63% for 2c, 74% for 2d, 76% for 2e, and 69% for 2f; (b) NaOH aq, ethanol, 60 ꢁC, 2–12 h, 54% for 3a, 25% for 3b, 89% for 3c, 39% for 3d, 63% for 3e,
and 77% for 3f.

Y. Ooyama et al. / Tetrahedron Letters 48 (2007) 9167–9170
9169
Table 1. Optical properties of 3a–f in 1,4-dioxane and their energy levels of HOMO and LUMO
a
fl
max
b
Dye
kambasx=nm ðemax=dm3 molꢀ1 cmꢀ1
Þ
k
ðnmÞ
U
SSc (nm)
HOMOd (V)
LUMOd (V)
3a
3b
3c
3d
3e
3f
427 (26,700), 350 (26,100)
425 (25,300), 350 (26,900)
427 (24,400), 349 (27,300)
430 (26,900), 354 (33,000)
430 (28,900), 354 (35,000)
431 (26,100), 354 (31,000)
540
532
537
534
534
533
0.97
0.96
0.99
0.97
0.95
0.95
113
107
112
104
104
103
0.87
0.90
0.91
0.89
0.89
0.86
ꢀ1.71
ꢀ1.70
ꢀ1.64
ꢀ1.66
ꢀ1.66
ꢀ1.75
a 2.0 · 10ꢀ5 M.
b 2.0 · 10ꢀ6
.
c Stokes shift value.
d Versus a normal hydrogen electrode (NHE).
Figure 3. Absorption spectra of 3a–f adsorbed on TiO2 film.
Figure 4. IPCE spectra of DSSCs based on 3a–f.
3a–f were determined to be 0.33–0.36 V versus Ag/Ag+.
The corresponding reduction peaks appeared at 0.25–
0.31 V for the first redox step (see Supplementary data
for details). On the bases of the spectral analyses and
CVs, we estimated the HOMO and LUMO energy levels
of the six dyes. All these dyes have similar HOMO and
LUMO energy levels (Table 2). Evidently, the LUMO
energy levels for these dyes are higher than the energy
level of TiO2 conduction band (ꢀ0.5 V), showing that
these dyes can inject efficiently electrons to conduction
band of TiO2 electrode.
The DSSCs were fabricated by using the TiO2 electrode,
Pt-coated glass as a counter electrode, and an acetoni-
trile solution with 0.05 M iodine, 0.1 M lithium iodide,
and 1,2-dimethyl-3-n-propylimidazolium iodide as elec-
trolyte. The photocurrent–voltage characteristics were
measured using a potentiostat under a simulated solar
light (AM 1.5, 61 mW cmꢀ2). The incident photon-
to-current conversion efficiency (IPCE) spectra were
measured under monochromatic irradiation with a
tungsten–halogen lamp and a monochromator. IPCE
Figure 5. Photocurrent–voltage curves of DSSCs based on 3a–f.
spectra and photocurrent–voltage curves are depicted
in Figures 4 and 5, respectively. The maximum IPCE
values are very similar except for 3a: 6% for 3a, 57%
for 3b, 58% for 3c, 56% for 3d, 52% for 3e, and 58%
for 3f. The photocurrent–voltage curves for 3b–f resem-
ble also very well. For these dyes, the short-circuit
photocurrent densities (Jsc) are 1.9–2.1 mA cmꢀ2, and
the solar energy-to-electricity conversion yield (g) values
are 0.85–1.00%. As shown in Table 2, no significant
differences are also seen for the other photovoltaic
parameters such as photovoltage (Voc) and fill factor
(ff) among the dyes except 3a. The close agreements
of all the photovoltaic features for 3b–f demonstrate
that the p-conjugated skeletons of these dyes lie on the
TiO2 surface in a similar fashion irrespective of different
lengths of alkyl chains with a carboxyl group acting as
an anchor. In view of highly efficient charge injection
probabilities as suggested by IPCE values over 50%
Table 2. Photovolatic performances of DSSCs based on 3a–f
Dye
Moleculesa
(cmꢀ2
Jsc
(mA cmꢀ2
Voc
(mV)
ff
g (%)
)
)
3a
3b
3c
3d
3e
3f
4.4 · 1015
6.6 · 1016
5.5 · 1016
7.8 · 1016
7.2 · 1016
7.4 · 1016
0.12
2.06
2.01
1.88
2.00
1.91
216
500
548
524
468
520
0.42
0.57
0.56
0.53
0.55
0.57
0.02
1.00
1.00
0.86
0.84
0.90
a Adsorption amount per unit area of TiO2 film.

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Y. Ooyama et al. / Tetrahedron Letters 48 (2007) 9167–9170
for 3b–f, it is likely that the electron-accepting phenyl-
cyano groups are attached to the TiO2 as shown in
Figure 1, so that the dyes 3b–f can efficiently inject elec-
trons from the dye skeleton to the conduction band of
the TiO2 through the phenylcyano groups. The lower
photovoltaic performance of 3a is attributable to the
small amount of 3a adsorbed on TiO2 film compared
with those for the other dyes because of a steric
hindrance between the phenylcyano moiety of 3a and
TiO2 surface. The g values of DSSCs based on the dyes
3a–f are relatively small compared with those of other
organic dyes because the p-conjugated system of 3a–f
does not have an intense absorption band over a long-
wavelength region of the solar spectrum.
Supplementary data
Supplementary data (synthetic and experimental details)
associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2007.10.107.
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In conclusion, to develop the molecular design and syn-
thetic strategy of donor–acceptor p-conjugated dyes for
DSSCs, we have designed and synthesized new-type
donor–acceptor p-conjugated fluorescent dyes 3a–f and
their photovoltaic performances of dye-sensitized solar
cells are studied. It is found that in spite of the lengths
of alkyl chains, due to the flexibility of alkyl chains,
the cyano groups of the dyes are located close to TiO2
surface and a good electron communication is estab-
lished between the cyano group and TiO2 surface. It is
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This work was supported by Grants-in-Aid for Scientific
Research (B) (19350094) and for Young Scientist (B)
(18750174) from the Ministry of Education, Science,
Sports and Culture of Japan, and by Electric Technol-
ogy Research Foundation of Chugoku.