Photovoltaic performance of dye-sensitized solar cells based on donor-acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes with a carboxyl group at different positions of the chromophore skeleton

Article abstract of DOI:10.1039/b705694j

Donor-acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes 3a, 3b, 8a, and 8b with a carboxyl group at different positions of the chromophore skeleton have been designed and synthesized. The absorption and fluorescence spect

Full text of DOI:10.1039/b705694j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Photovoltaic performance of dye-sensitized solar cells based on donor–
acceptor p-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type
fluorescent dyes with a carboxyl group at different positions of the
chromophore skeleton
Yousuke Ooyama, Yoshihito Shimada, Yusuke Kagawa, Ichiro Imae and Yutaka Harima*
Received 16th April 2007, Accepted 9th May 2007
First published as an Advance Article on the web 25th May 2007
DOI: 10.1039/b705694j
Donor–acceptor p-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes 3a, 3b, 8a,
and 8b with a carboxyl group at different positions of the chromophore skeleton have been designed
and synthesized. The absorption and fluorescence spectra and cyclic voltammograms of the fluorescent
dyes agree very well, showing that the position of the carboxyl group has a negligible influence on the
photophysical and electrochemical properties of these dyes. When these dyes are used in dye-sensitized
solar cells, however, their photovolatic performances are considerably different. The short-circuit
photocurrents and energy conversion efficiencies under a simulated solar light increase in the order: 3a
(2.12 mA cm−2, 1.00%) ≈ 3b (2.10 mA cm−2, 1.06%) > 8b (1.50 mA cm−2, 0.67%) > 8a (0.84 mA cm−2,
0.34%). Based on semi-empirical molecular orbital calculations (AM1 and INDO/S) together with
spectral analyses and their photovolatic performance, the relationships between the observed
photovolatic properties and the chemical structures of the benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type
fluorescent dyes are discussed. It is found that strong interaction between a TiO2 surface and the
electron accepting moiety of the dye leads to a high photovoltaic performance.
development of new donor–acceptor p-conjugated dyes for DSSCs
Introduction
is limited because a carboxyl group is required to combine with
Donor–acceptor p-conjugated dyes are useful as sensitizers of
the p-conjugation system or the electron accepting moiety of dyes
dye-sensitized solar cells (DSSCs)1 and emitters of organic light
for the above reasons. To obtain new and efficient sensitizers for
emitting diodes (OLEDs)2 because of their strong absorption
and emission properties. Many donor–acceptor p-conjugated dyes
DSSCs, a novel molecular design such as forming a strong
interaction between the electron accepting moiety of sensitizers
with a carboxyl group, acting not only as the anchoring group
and a TiO2 surface is necessary. Therefore, we have designed and
for attachment to metal oxides but also as the electron acceptor,
are synthesized and used as sensitizers of DSSCs.3–5 A number
synthesized novel donor–acceptor p-conjugated benzofuro[2,3-
c]oxazolo[4,5-a]carbazole-type fluorescent dyes 3a, 3b, 8a, and 8b
of researchers have suggested that a carboxyl group can form an
with a carboxyl group at different positions of the chromophore
ester linkage with a TiO2 surface to provide a strongly bound
dye and good electronic communication between them. However,
skeleton (Fig. 1). In this paper, we report the photovoltaic
performance of DSSCs based on benzofuro[2,3-c]oxazolo[4,5-
a]carbazole-type fluorescent dyes 3a, 3b, 8a, and 8b. To elucidate
Department of Applied Chemistry, Graduate School of Engineer-
the main differences in photovolatic performance among the
ing, Hiroshima University, Higashi-hiroshima 739-8527, Japan. E-mail:
fluorescent dyes 3a, 3b, 8a, and 8b, we performed semi-empirical
harima@mls.ias.hiroshima-u.ac.jp; Fax: (+81) 82-424-5494
Fig. 1 Chemical structures of benzofuro[2,3-c]oxazolocarbazole-type fluorescent dyes.
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molecular orbital calculations (AM1 and INDO/S). On the basis
of the results of calculations, spectral analyses, and photovolatic
performance, the relationships between the observed photovoltaic
properties and the chemical structures of the benzofuro[2,3-
c]oxazolo[4,5-a]carbazole-type fluorescent dyes are discussed.
oxazole compounds using ammonium acetate. In this reaction,
NH3 resulting from CH3COONH4 in the initial stage is acting as
the nucleophilic reagent to the 6- and/or 7-carbonyl carbon. In
this case, NH3 preferentially attacks the 7-carbonyl carbon rather
than the 6-carbonyl in spite of the similar steric reactivity of the
two carbonyls. It was considered that the conjugated linkage of
the dialkylamino group to the 6-carbonyl group would make the
6-carbonyl carbon less electrophilic than the 7-carbonyl carbon,
so that the nucleophilic reagents (NH3) preferentially attack
the electrophilic 7-carbonyl carbon. As the result, this reaction
afforded preferentially the compounds 6. The compounds 8a and
8b were prepared by hydrolysis of their precursors 6a and 6b,
respectively.
Results and discussion
Synthesis of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type
fluorescent dyes
The synthetic pathway is shown in Schemes 1–3. Com-
pound 3a was synthesized by hydrolysis of 7-(4-cyanophenyl)-3-
dibutylaminobenzofuro[2,3-c]oxazolo[4,5-a]carbazole6 (1a) under
basic conditions (Scheme 1). Compound 3b was also synthesized
by hydrolysis of N-alkylated 2b obtained by reaction of 1a with
ethyl 4-bromobutyrate using sodium hydride (Scheme 2). Synthesis
of 8a and 8b is outlined in Scheme 3. Carbazole-1,2-dione was pre-
pared according to the published procedure.7 The compounds 4a
and 4b were obtained by the reaction of carbazole-1,2,-dione with
4-[butyl-(3-hydroxyphenyl)amino]butyric acid ethyl ester and 4-
[(3-ethoxycarbonylpropyl)-(3-hydroxyphenyl)amino]butyric acid
ethyl ester in the presence of CuCl2, respectively. Next, intramolec-
ular oxidative-cyclization of 4a and 4b using Cu(OCOCH3)2
gave the quinones 5a and 5b, respectively. The quinones 5 were
allowed to react with p-cyanobenzaldehyde to give the structural
isomers of oxazolocarbazole-type fluorophores 6 and 7. This
is the first report of the preparation of this isomeric pair of
Spectroscopic properties of benzofuro[2,3-c]oxazolo[4,5-a]-
carbazole-type fluorescent dyes in solution and on TiO2 film
The absorption, excitation, and fluorescence spectra of 3a, 3b,
8a, and 8b in 1,4-dioxane are shown in Fig. 2 and their spectral
data are summarized in Table 1. The absorption and fluorescence
spectra of all these fluorescent dyes agree very well, showing that
the effect of the position of the carboxyl group attached to the
same chromophore skeleton on the photophysical properties of
3a, 3b, 8a, and 8b is negligible. All these fluorescent dyes exhibit
intense absorption bands at around 420 and 350 nm, and an
intense fluorescence at around 530 nm. The fluorescence quantum
yields (U) of these dyes in 1,4-dioxane were close to 100%. From
the measurement of the fluorescence excitation spectra of 3a,
3b, 8a, and 8b, it was found that two absorptions lead to the
same efficiency for the emission (Fig. 2b). Absorption spectra
of dyes adsorbed on TiO2 film are shown in Fig. 3, where the
amounts of adsorbed dyes on TiO2 film are 6.8 × 1016, 4.4 × 1016,
3.9 × 1016, and 6.6 × 1016 molecules cm−2 for 3a, 3b, 8a, and 8b,
Table 1 Optical properties of the dyes in 1,4-dioxane
a
Dye kmaxabs /nm (emax/dm3 mol−1 cm−1
)
kmaxfl/nmb
U
SSc/nm
3a
3b
8a
8b
416 (25700), 347 (25400)
425 (24400), 349 (27300)
429 (21500), 353 (25500)
424 (22200), 348 (23900)
525
537
534
541
0.97 109
0.99 112
0.97 105
0.97 117
a 2.0 × 10−5 M. b 2.0 × 10−6 M. c Stokes shift value.
Scheme 1 Synthesis of fluorophore 3a.
Scheme 2 Synthesis of fluorophore 3b.
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Scheme 3 Synthesis of fluorophores 8a and 8b.
respectively. No significant differences in the absorption properties
Electrochemical properties of benzofuro[2,3-c]oxazolo[4,5-a]-
are noted among the four dyes. Although the absorption peak
wavelengths of adsorbed dyes on TiO2 films are similar to those
in 1,4-dioxane, the onset of absorption bands are red-shifted by
80–100 nm. Such a red-shift is attributable to dye aggregation on
the TiO2 electrode.5,8 Further studies on the possible influence of
the molecular arrangement of dyes adsorbed on TiO2 surface, on
the photovoltaic performance of DSSCs are now in progress in our
group.
carbazole-type fluorescent dyes and their HOMO and LUMO
energy levels
The electrochemical properties of 3a, 3b, 8a, and 8b were
determined by cyclic voltammetry (CV) in acetonitrile containing
0.1 M Et4NClO4. As an example, the CV of 8a is shown in Fig. 4.
CV curves of these compounds showed three redox waves at similar
potentials irrespective of the compounds, though the peaks of
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Fig. 4 Cyclic voltammogram of compound 8a in acetonitrile containing
0.1 M Et4NClO4 at a scan rate of 20 mV s−1. The arrow denotes the
direction of the potential scan.
0.25–0.31 V for the first redox step. The first redox waves are clearly
observed and the peak separations are ca. 60 mV, suggesting that
the first oxidized states of the dyes are stable. On the other hand, ill-
defined waves for the second and third oxidation processes indicate
that the second and third oxidized states of the dyes are less stable
than the first ones.
Fig. 2 a) Absorption and fluorescence spectra and b) excitation and
fluorescence spectra of 3a, 3e, 8a, and 8b in 1,4-dioxane.
On the basis of the spectral analyses and CVs, we estimated
the HOMO and LUMO energy levels of the four dyes. The
HOMO energy levels for 3a, 3b, 8a, and 8b were evaluated to
be 0.87, 0.91, 0.93, and 0.91 V with respect to NHE, respectively.
The LUMO energy levels of the dyes were estimated from the
first oxidation potential and an intersection of absorption and
fluorescence spectra [480 nm (2.58 eV), 486 nm (2.55 eV), 485 nm
3a were shifted in a negative direction compared with those of
the other compounds (Table 2). Three oxidation peaks of these
compounds are determined to be 0.31–0.37, 0.82–0.88, and 0.98–
1.02 V vs. Ag/Ag+. The corresponding reduction peaks appear at
Fig. 3 Absorption spectra of the dyes adsorbed on TiO2 film and the IPCE spectra for DSSCs based on the dyes: a) 3a, b) 3b, c) 8a, and d) 8b. The
amount of adsorbed dyes on TiO2 film is 6.8 × 1016, 4.4 × 1016, 3.9 × 1016, and 6.6 × 1016 molecules cm−2for 3a, 3b, 8a, and 8b, respectively.
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Table 2 Electrochemical properties of 3a, 3b, 8a, and 8b
a
Compound
E
pa/V vs. Ag/Ag+
Epc/V vs. Ag/Ag+b
DEp/mV
E
1/2/V vs. Ag/Ag+
3a
3b
8a
8b
0.31, 0.82, 0.98
0.35, 0.84, 0.98
0.37, 0.85, 1.02
0.35, 0.88, 0.99
0.25, –, –
60, –, –
70, 60, –
60, 50, –
50, –, –
0.28, –, –
0.28, 0.78, –
0.31, 0.80, –
0.30, –, –
0.32, 0.81, –
0.34, 0.83, –
0.33, –, –
a Epa and Epc are the anodic and cathodic peak potentials in acetonitrile (0.1 M Et4NClO4). b Epa and Epc are the anodic and cathodic peak potentials in
acetonitrile (0.1 M Et4NClO4).
Table 3 Calculated absorption spectra for 3a, 3b, 8a, and 8b
(2.56 eV), and 487 nm (2.54 eV) for 3a, 3b, 8a, and 8b, respectively]
corresponding to the energy gap between HOMO and LUMO. The
LUMO energy levels for these dyes were −1.71, −1.64, −1.63, and
−1.63 V for 3a, 3b, 8a, and 8b, respectively. Evidently, they are
higher than the energy level of TiO2 conduction band (−0.5 V),
showing that these dyes can efficiently inject electrons into the
conduction band of the TiO2 electrode. The results of absorption
and fluorescence spectra and CV for these dyes demonstrate that
all these dyes have similar HOMO and LUMO energy levels.
Absorption (calc.)
Compound
kmax/nm
f a
CI componentb
3a
402
328
0.92
0.59
HOMO → LUMO (75%)
HOMO → LUMO + 1 (43%)
HOMO-1 → LUMO (13%)
HOMO → LUMO (77%)
HOMO → LUMO + 1 (47%)
HOMO-1 → LUMO (16%)
HOMO → LUMO (78%)
HOMO → LUMO + 1 (45%)
HOMO-1 → LUMO (13%)
HOMO → LUMO (79%)
HOMO → LUMO + 1 (46%)
HOMO-1 → LUMO (12%)
3b
8a
8b
404
329
0.90
0.64
398
327
0.95
0.61
Semi-empirical MO calculations (AM1, INDO/S)
394
325
0.96
0.62
To understand the photophysical and electrochemical properties
of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes,
we have carried out semi-empirical molecular orbital (MO)
calculations of 3a, 3b, 8a, and 8b using the INDO/S method9 after
geometrical optimizations using the MOPAC/AM1 method.10
The calculated absorption wavelengths and the transition char-
acters of the first absorption bands are collected in Table 3. The
observed and calculated absorption spectra of the compounds
(Tables 1 and 3) compare well with each other with respect to
both the absorption wavelength and the absorption intensity,
although the calculated absorption wavelengths are blue-shifted.
The deviation of the INDO/S calculations, giving transition
energies greater than the experimental values, has been generally
observed.11 The calculations showed that the longest excitation
bands of the dyes were mainly assigned to the transition from
HOMO to LUMO, where HOMOs were mostly localized on the 3-
dibutylaminobenzofuro[2,3-c]oxazolo[4,5-a]carbazole moiety for
all dyes, and LUMOs were mostly localized on the carboxyphenyl
moiety for 3a and the cyanophenyl moiety for 3b, 8a, and 8b.
The changes in the calculated electron density accompanying
the first electron excitation are shown in Fig. 5, which reveals
a Oscillator strength. b The transition is shown by an arrow from one
orbital to another, followed by its percentage CI (configuration interaction)
component.
a strong migration of intramolecular charge transfer from the
3-dialkylaminobenzofuro[2,3-c]oxazolo[4,5-a]carbazole moiety to
the carboxylphenyl or cyanophenyl moiety for all dyes. The
calculations showed that the second excitation bands of the
dyes were mainly assigned to the transition from HOMO to
LUMO + 1, where a moderate migration of charge from the
3-dialkylaminobenzofuro[2,3-c]oxazolocarbazole moiety to the
carboxyphenyl moiety for 3a and the cyanophenyl moiety for 3b,
8a, and 8b is also observed. These calculation results suggest that
the effects of the cyano and carboxyl groups and N-alkylation
of the carbazole ring on the photophysical and electrochemical
properties of these dyes are negligible, so that all dyes show similar
absorption and fluorescence spectra and CV.
Fig. 5 Calculated electron density changes accompanying the first electronic excitation of 3a, 3b, 8a, and 8b. The black and white lobes signify decrease
and increase in electron density accompanying the electronic transition respectively. Their areas indicate the magnitude of the electron density change.
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Photovoltaic performance of DSSCs based on
Relationship between photovolatic performance and chemical
benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dye
structure
The incident photon-to-current conversion efficiency (IPCE)
spectra for DSSCs are shown in Fig. 3. The IPCE is represented
by the following equation:
To understand the differences in Jsc, we assumed that the dye
molecule is standing perpendicular to the TiO2 substrate as shown
in Fig. 6. In 3a, a carboxyl group can form an ester linkage with the
TiO2 surface, so that electrons will be injected from the dye to TiO2
through the carboxyl group. For 3b, 8a, and 8b, on the other hand,
the carboxyl group acts as an anchoring group for attachment to
the TiO2 surface, but it cannot be the electron acceptor. From the
molecular structure of 3b, it was suggested that the phenylcyano
group acting as electron acceptor is located in close proximity to
the TiO2 surface by interactions such as intermolecular hydrogen
bonding between the cyano nitrogen of the dye and the hydroxyl
proton of the TiO2 surface. Consequently, dye 3b can efficiently
inject electrons from the phenylcyano group to the conduction
band of the TiO2 electrode through intermolecular hydrogen
bonding. On the other hand, we presume that free rotation of
the butyl group for 8a and rigid linkages with the TiO2 surface
for 8b prevent the phenylcyano moiety of the dye from being in
close proximity to the TiO2 surface. Fukuzumi et al. reported that
for DSSC based on fluorescein derivatives, the xanthen moiety
acting as an electron acceptor, which can inject electrons into
the conduction band of the semiconductor, is located in close
proximity to the semiconductor surface.12 On the other hand, it is
reported that the Fermi level of TiO2 is affected by the adsorption
of dyes with a permanent dipole.13 If our model for dye adsorption
is correct, the direction of the dipole moments of 3a and 3b in the
adsorption state will be opposite to those of 8a and 8b. Thus, this
will cause the difference in Voc.
1240 (eV nm) Jsc (mA cm−2)
IPCE (%) =
× 100
(1)
k (nm) U (mW cm−2)
where Jsc is the short-circuit photocurrent density generated by
monochromatic light, and k and U are the wavelength and the
intensity of the monochromatic light, respectively. When the
performances of DSSCs fabricated using these dyes as sensitizers
were examined, big differences in the IPCE spectra and the
photocurrent–voltage (I–V) characteristics were observed. As
shown in Fig. 5, the maximum IPCE value increases in the order of
3a (46%) > 3b (45%) > 8b (34%) > 8a (30%). Table 4 summarizes
the photovoltaic performances of DSSCs based on the dyes under
AM 1.5 irradiation (60 mW cm−2). The solar energy-to-electricity
conversion yield (g (%)) is expressed by the following equation:
Jsc (mA cm−2) Voc (V) ff
g (%) =
× 100
(2)
Io (mW cm−2)
where I0 is the intensity of incident white light, Voc is the open-
circuit photovoltage, and ff represents the fill factor. Interestingly,
the Jsc for 3b (1.60 mA cm−2), which has a nonconjugated linkage
between the carboxyl group and the chromophore, is similar
to that for 3a (1.30 mA cm−2) and higher than those of 8a
(0.80 mA cm−2) and 8b (0.75 mA cm−2), when comparisons are
made between similar amounts of dyes adsorbed on TiO2 (ca. 3 ×
1016 molecules cm−2). The Vocs for 3a, 3b, 8a, and 8b are 544 mV,
550 mV, 430 mV, and 485 mV, respectively, which were different
among the dyes. On the other hand, when the amount of adsorbed
dye on TiO2 film increased to 6.8 × 1016, 4.4 × 1016, 3.9 × 1016, and
6.6 × 1016 molecules cm−2 for 3a, 3b, 8a, and 8b, respectively, the Isc
and g values increased in the order: 3a (2.12 mA cm−2, 1.00%) ≈ 3b
(2.10 mA cm−2, 1.06%) > 8b (1.50 mA cm−2, 0.67%) > 8a (0.84 mA
cm−2, 0.34%). It is worth noting that the Jsc and g for 3b are similar
to those for 3a, although the amounts of adsorbed dyes on TiO2
film in 3b is less than that in 3a. This result also demonstrates that
the molecular aggregation state of the dye on the TiO2 electrode
has a great influence on the Isc and g of DSSCs. Furthermore, the
Voc values for 3a (508 mV) and 3b (530 mV) were higher than
those of 8a (435 mV) and 8b (470 mV).
Fig. 6 Plausible configurations of 3a and 3b on a TiO2 surface. Molecular
structures of 3a and 3b are optimized by the MOPAC/AM1 method.
Table 4 Photovoltaic performances of DSSCs
Conclusions
a
Dye × 1016 molecules cm−2
Jsc/mA cm−2
V
oc/mV ff
g (%)
In conclusion, novel donor–acceptor p-conjugated benzofuro[2,3-
c]oxazolo[4,5-a]carbazole-type fluorescent dyes with a carboxyl
group at different positions of the chromophore skeleton have
been designed and synthesized, and the photovoltaic performances
of DSSCs based on these dyes were investigated. It was found
that the distance between the electron acceptor moiety and TiO2
will affect the efficiency of electron injection. Further studies on
DSSCs based on these dyes are now in progress to elucidate the
influence of the molecular arrangement of dyes adsorbed on a
3a
3b
8a
8b
2.5
6.8
2.7
4.4
2.9
3.9
2.8
6.6
1.30
2.12
1.60
2.10
0.80
0.84
0.75
1.50
544
508
550
530
430
435
485
470
0.60 0.70
0.57 1.00
0.58 0.84
0.58 1.06
0.57 0.33
0.57 0.34
0.57 0.34
0.58 0.67
a Adsorption amount per unit area of TiO2 film.
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TiO2 surface on the photovoltaic performances (Jsc, Voc, and g) of
DSSCs.
(4H, m), 1.89–1.92 (2H, m), 2.46–2.50 (2H, m), 3.51–3.54 (4H, m),
4.00–4.04 (2H, m), 5.07–5.11 (2H, m), 7.00–7.04 (2H, m), 7.39–
7.46 (1H, m), 7.53–7.60 (1H, m), 7.81–7.86 (1H, m), 8.04–8.08
(2H, m), 8.51–8.58 (3H, m), 8.70–8.74 (1H, m); elemental analysis
calcd (%) for C40H40N4O4: C 74.98, H 6.29, N 8.74; found: C 74.97,
H 6.31, N 8.71.
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 (U) were determined by a Hamamatsu C9920-01 equipped
with CCD by using a calibrated integrating sphere system
(kex = 325 nm). Cyclic voltammograms (CVs) were recorded in
MeCN/Et4NClO4 (0.1 M) solution with a three-electrode system
consisting of Ag/Ag+ as the reference electrode, Pt plate as the
working electrode, and Pt wire as counter electrode, by using a
Hokuto Denko HAB-151 potentiostat equipped with a functional
generator. Elemental analyses were recorded on a Perkin Elmer
2400 II CHN analyzer. 1H NMR spectra were recorded on a JNM-
LA-400 (400 MHz) FT NMR spectrometer with tetramethylsilane
(TMS) as an internal standard. Column chromatography was
performed on silica gel (KANTO CHEMICAL, 60N, spherical,
neutral).
Synthesis of 7-(4-cyanophenyl)-3-dibutylaminobenzofuro[2,3-c]-
oxazolo[4,5-a]carbazole-9-butyric acid (3b)
To a solution of 2b (0.1 g, 0.16 mmol) in ethanol (300 ml) was added
dropwise aqueous NaOH (0.06 g, 1.6 mmol, 30 mL,) with stirring
at 60 ◦C. After further stirring for 6 h under reflux, the solution was
acidified to pH 4 with 2 M HCl, and concentrated under reduced
pressure. The residue was dissolved in CH2Cl2, and washed with
water. The organic extract was dried over MgSO4, filtered, and
concentrated. The residue was chromatographed on silica gel
(CH2Cl2–ethyl acetate = 10 : 1 as eluent) to give 3b (0.085 g, yield
89%); mp 260–262 ◦C; IR (KBr): m = 2228, 1712 cm−1; 1H NMR
(acetone-d6, TMS) d = 1.04 (6H, t), 1.46–1.53 (2H, m), 1.74–1.79
(4H, m), 1.88–1.91 (4H, m), 3.51–3.55 (2H, m), 3.61–3.64 (4H, m),
5.11–5.15 (2H, m), 6.98–7.05 (2H, m), 7.43–7.46 (1H, m), 7.56–
7.60 (1H, m), 7.85 (1H, d, J = 8.76 Hz), 8.07 (2H, d, J = 8.76 Hz),
8.54 (2H, d, J = 9.04 Hz), 8.59 (1H, d, J = 8.80 Hz), 8.73 (1H,
d, J = 8.08 Hz); elemental analysis calcd (%) for C38H36N4O4: C
74.49, H 5.92, N 9.14; found: C 74.29, H 5.83, N 9.09.
Synthesis of 7-(4-carboxyphenyl)-3-dibutylamino-
benzofuro[2,3-c]oxazolo[4,5-a]-carbazole (3a)
Synthesis of 4-{butyl-[4-(1,2-dioxo-2,9-dihydro-1H-carbazol-
4-yl)-3-hydroxyphenyl]amino}butyric acid ethyl ester (4a)
To a solution of 1a (1.0 g, 1.90 mmol) in ethanol (500 ml) was
added dropwise aqueous NaOH (0.76 g, 19 mmol, 50 mL,) with
stirring under reflux. After further stirring for 16 h under reflux,
the solution was acidified to pH 4 with 2 M HCl, and concentrated
under reduced pressure. The residue was dissolved in CH2Cl2, and
washed with water. The organic extract was dried over MgSO4,
filtered, and concentrated. The residue was chromatographed on
silica gel (CH2Cl2–ethyl acetate = 2 : 5 as eluent) to give 3a (0.91 g,
yield 88%); 296–297 ◦C (decomposition); IR (KBr): m = 3240,
1692 cm−1; 1H NMR (DMSO-d6, TMS) d = 0.97 (6H, t), 1.37–1.43
(4H, m), 1.58–1.63 (4H, m), 3.38 (4H, t) overlap peak of dissolved
water in DMSO-d6, 6.94–6.96 (1H, m), 7.03–7.05 (1H, m), 7.33–
7.38 (1H, m), 7.46–7.50 (1H, m), 7.66 (1H, d, J = 8.08 Hz), 8.20
(2H, d, J = 8.32 Hz), 8.38 (2H, d, J = 8.32 Hz), 8.46 (1H, d, J =
8.32 Hz), 8.60 (1H, d, J = 7.84 Hz), 12.50 (1H, s, –NH); elemental
analysis calcd (%) for C34H31N3O4: C 74.84, H 5.73, N 7.70; found:
C 74.72, H 5.63, N 7.67.
To a solution of 9H-carbazole-1,2-dione (3.00 g, 15.2 mmol) and
CuCl2 (2.05 g, 15.2 mmol) in DMSO (150 ml) was added 4-
[butyl-(3-hydroxyphenyl)amino]butyric acid ethyl ester (3.40 g,
15.2 mmol) with stirring at 50 ◦C. After further stirring for
1 h, the reaction mixture was poured into water. The resulting
precipitate was filtered, washed with water and dried. The residue
was chromatographed on silica gel (CH2Cl2–ethyl acetate = 3 : 1
as eluent) to give 4a (1.62 g, yield 22%); mp 141–143 ◦C; IR (KBr):
m = 3266(br), 1708, 1603 cm−1; 1H NMR (acetone-d6, TMS) d =
0.98 (3H, t), 1.23 (3H, t), 1.38–1.44 (2H, m), 1.61–1.67 (2H, m),
1.94–2.00 (2H, m), 2.37–2.44 (2H, m), 3.25–3.47 (4H, m), 4.09–
4.15 (2H, m), 5.88 (1H, s), 6.42–6.47 (2H, m), 6.99–7.03 (1H,
m), 7.20–7.52 (4H, m), 8.01 (1H, s, –OH), 11.20 (1H, s, –NH);
elemental analysis calcd (%) for C28H30N2O5: C 70.87, H 6.37, N
5.90; found: C 70.62, H 6.26, N 5.63.
Synthesis of 7-(4-cyanophenyl)-3-dibutylaminobenzofuro[2,3-c]-
oxazolo[4,5-a]carbazole-9-butyric acid ethyl ester (2b)
Synthesis of 4-{butyl-[4-(1,2-dioxo-2,9-dihydro-1H-carbazol-
4-yl)-3-hydroxyphenyl]amino}butyric acid ethyl ester (4b)
A solution of 1a (1.0 g, 1.90 mmol) in dry acetonitrile was treated
with sodium hydride (60%, 0.11 g, 2.85 mmol) and stirred for 1 h
at room temperature. Ethyl 4-bromobutyrate (1.85 g, 9.49 mmol)
was added in a dropwise manner over 20 min and the solution was
stirred at room temperature for 10 h. After concentrating under
reduced pressure, the resulting residue was dissolved in CH2Cl2,
and washed with water. The organic extract was dried over MgSO4,
filtered, and concentrated. The residue was chromatographed on
silica gel (CH2Cl2 as eluent) to give 2b (0.77 g, yield 63%); mp
209–211 ◦C; IR (KBr): m = 2228, 1738 cm−1; 1H NMR (acetone-d6,
TMS) d = 1.04 (6H, t), 1.12 (3H, t), 1.48–1.52 (4H, m), 1.71–1.76
To a solution of 9H-carbazole-1,2-dione (5.00 g, 25.4 mmol) and
CuCl2 (3.41 g, 25.4 mmol) in DMSO (200 ml) was added 4-
[(3-ethoxycarbonylpropyl)-(3-hydroxyphenyl)amino]butyric acid
ethyl ester (8.56 g, 25.4 mmol) with stirring at 50 ◦C. After further
stirring for 2 h, the reaction mixture was poured into water. The
resulting precipitate was filtered, washed with water and dried.
The residue was chromatographed on silica gel (CH2Cl2–ethyl
acetate = 3 : 1 as eluent) to give4b (1.58 g, yield 12%); mp 88–89 ◦C;
1
IR (KBr): m = 3264(br), 1708, 1602 cm−1; H NMR (acetone-d6,
TMS) d = 1.23 (6H, t), 1.93–2.00 (4H, m), 2.38–2.44 (4H, m),
3.40–3.48 (4H, m), 4.07–4.15 (4H, m), 5.88 (1H, s), 6.46–6.53 (2H,
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m), 6.99–7.03 (1H, m), 7.21–7.51 (4H, m), 8.50 (1H, s, –OH),
11.19 (1H, s, –NH); elemental analysis calcd (%) for C30H32N2O7:
C 67.66, H 6.06, N 5.26; found: C 67.87, H 6.02, N 5.08.
J = 8.08 Hz), 8.06 (2H, d, J = 8.80 Hz), 8.50 (2H, d, J = 8.80 Hz),
8.55 (1H, d, J = 9.04 Hz), 8.71 (1H, d, J = 8.08 Hz), 11.52 (1H, s,
–NH); elemental analysis calcd (%) for C36H32N4O4: C 73.95, H
5.52, N 9.58; found: C 74.12, H 5.24, N 9.38; 7a: mp 267–269 ◦C;
IR (KBr): m = 3276, 2230, 1702 cm−1; 1H NMR (acetone-d6, TMS)
d = 1.03 (3H, t), 1.27 (3H, t), 1.46–1.52 (2H, m), 1.70–1.75 (2H,
m), 2.05–2.09 (2H, m) overlap peak of remaining acetone, 2.48–
2.52 (2H, m), 3.52–3.69 (4H, m), 4.14–4.20 (2H, m), 7.07 (1H,
dd, J = 2.44 and 9.04 Hz), 7.18 (1H, d, J = 2.44 Hz), 7.42–7.46
(1H, m), 7.52–7.56 (1H, m), 7.77 (1H, d, J = 8.04 Hz), 8.07 (2H,
d, J = 8.80 Hz), 8.49–8.53 (3H, m), 8.74 (1H, d, J = 8.08 Hz),
11.45 (1H, s, –NH); elemental analysis calcd (%) for C36H32N4O4:
C 73.95, H 5.52, N 9.58; found: C 73.67, H 5.29, N 9.42.
Synthesis of 4-[butyl-(6,7-dioxo-7,8-dihydro-6H-5-oxa-8-aza-
indeno[2,1-c]fluoren-3-yl)amino]butyric acid ethyl ester (5a)
A solution of 4a (1.0 g, 2.11 mmol) and Cu(OCOCH3)2 (0.42 g,
2.11 mmol) in DMSO (100 ml) was stirred at 80 ◦C for 2 h. After
the reaction was completed, the reaction mixture was poured into
water. The resulting precipitate was filtered, washed with water and
dried. The residue was chromatographed on silica gel (CH2Cl2–
ethyl acetate = 3 : 1 as eluent) to give 5a (0.28 g, yield 28%);
◦
1
mp 99–101 C; IR (KBr): m = 3223, 1728, 1586 cm−1; H NMR
(acetone-d6, TMS) d = 1.00 (3H, t), 1.25 (3H, t), 1.43–1.49 (2H,
m), 1.67–1.72 (2H, m), 1.97–2.01 (2H, m), 2.46–2.50 (2H, m),
3.51–3.59 (4H, m), 4.12–4.17 (2H, m), 6.82 (1H, d, J = 2.44 Hz),
7.01 (1H, dd, J = 2.44 and 9.28 Hz), 7.26–7.31 (1H, m), 7.36–7.41
(1H, m), 7.55 (1H, d, J = 8.56 Hz), 8.18 (1H, d, J = 9.28 Hz), 8.21
(1H, d, J = 8.56 Hz), 11.21 (1H, s, –NH); elemental analysis calcd
(%) for C28H28N2O5: C 71.17, H 5.97, N 5.93; found: C 71.33, H
5.67, N 5.72.
Synthesis of ethoxycarbonylpropyl-{7-(4-cyanophenyl)benzofuro-
[2,3-c]oxazolo-[4,5-a]carbazole}-3-aminobutyric acid ethyl ester
(6b) and ethoxycarbonylpropyl-{7-(4-cyanophenyl)-benzofuro[2,3-
c]oxazolo[5,4-a]carbazole}-3-aminobutyric acid ethyl ester (7b)
A solution of 5b (0.20 g, 0.38 mmol), p-cyanobenzaldehyde (0.05 g,
0.38 mmol), and ammonium acetate (0.58 g, 7.54 mmol) in acetic
acid (50 ml) was stirred at 90 ◦C for 1 h. After the reaction
was completed, the reaction mixture was poured into water. The
resulting precipitate was filtered, washed with water and dried. The
residue was chromatographed on silica gel (toluene–acetic acid =
5 : 1 as eluent) to give 6◦b (0.145 g, yield 60%) and 7b (0.05 g, yield
21%); 6b: mp 252–254 C; IR (KBr): m = 3436, 2226, 1724 cm−1;
1H NMR (acetone-d6, TMS) d = 1.27 (6H, t), 2.01–2.06 (4H, m)
overlap peak of remained acetone, 2.49–2.52 (4H, m), 3.58–3.62
(4H, m), 4.15–4.20 (4H, m), 7.16 (1H, dd, J = 2.44 and 9.04 Hz),
7.27 (1H, d, J = 2.44 Hz), 7.41–7.45 (1H, m), 7.51–7.55 (1H, m),
7.81 (1H, d, J = 8.32 Hz), 8.07 (2H, d, J = 8.80 Hz), 8.52 (2H,
d, J = 8.80 Hz), 8.56 (1H, d, J = 9.04 Hz), 8.72 (1H, d, J =
7.80 Hz), 11.55 (1H, s, –NH); elemental analysis calcd (%) for
C38H34N4O6: C 71.10, H 5.33, N 8.72; found: C 71.10, H 5.14, N
Synthesis of [(6,7-dioxo-7,8-dihydro-6H-5-oxa-8-aza-indeno[2,1-
c]fluoren-3-yl)-(3-ethoxycarbonylpropyl)amino]butyric acid ethyl
ester (5b)
A solution of 4b (1.00 g, 1.88 mmol) and Cu(OCOCH3)2 (0.34 g,
1.88 mmol) in DMSO (100 ml) was stirred at 80 ◦C for 3 h. After
the reaction was completed, the reaction mixture was poured into
water. The resulting precipitate was filtered, washed with water and
dried. The residue was chromatographed on silica gel (CH2Cl2–
ethyl acetate = 3 : 1 as eluent) to give 5b (0.15 g, yield 15%);
mp 110–112 ◦C; IR (KBr): m = 3260, 1724, 1591 cm−1; 1H NMR
(acetone-d6, TMS) d = 1.25 (6H, t), 1.99–2.01 (4H, m), 2.42–2.50
(4H, m), 3.57–3.61 (4H, m), 4.12–4.17 (4H, m), 6.94 (1H, d, J =
2.20 Hz), 7.16 (1H, dd, J = 2.20 and 9.28 Hz), 7.27–7.32 (1H, m),
7.37–7.41 (1H, m), 7.55 (1H, d, J = 8.56 Hz), 8.19 (1H, d, J =
9.28 Hz), 8.22 (1H, d, J = 8.56 Hz), 11.33 (1H, s, –NH); elemental
analysis calcd (%) for C30H30N2O7: C 67.91, H 5.70, N 5.28; found:
C 68.05, H 6.00, N 5.01.
◦
8.62; 7b: mp 230–232 C; IR (KBr): m = 3344, 2228, 1727 cm−1;
1H NMR (acetone-d6, TMS) d = 1.27 (6H, t), 2.02–2.10 (4H, m)
overlap peak of remaining acetone, 2.48–2.52 (4H, m), 3.55–3.59
(4H, m), 4.15–4.20 (4H, m), 7.09 (1H, dd, J = 2.44 and 9.04 Hz),
7.22 (1H, d, J = 2.44 Hz), 7.41–7.44 (1H, m), 7.51–7.55 (1H, m),
7.74 (1H, d, J = 8.04 Hz), 8.00 (2H, d, J = 8.32 Hz), 8.39 (2H, d,
J = 8.32 Hz), 8.47 (1H, d, J = 9.04 Hz), 8.70 (1H, d, J = 8.04 Hz),
11.55 (1H, s, –NH); elemental analysis calcd (%) for C38H34N4O6:
C 71.10, H 5.33, N 8.72; found: C 71.26, H 5.14, N 8.48.
Synthesis of 7-(4-cyanophenyl)benzofuro[2,3-c]oxazolo[4,5-a]-
carbazole-3-butylaminobutyric acid ethyl ester (6a) and
7-(4-cyanophenyl)benzofuro[2,3-c]oxazolo[5,4-a]carbazole-
3-butylaminobutyric acid ethyl ester (7a)
Synthesis of 7-(4-cyanophenyl)benzofuro[2,3-c]oxazolo[4,5-a]-
carbazole-3-butylaminobutyric acid (8a)
A solution of 5a (0.15 g, 0.32 mmol), p-cyanobenzaldehyde
(0.042 g, 0.32 mmol), and ammonium acetate (0.49 g, 6.35 mmol)
in acetic acid (50 ml) was stirred at 90 ◦C for 1 h. After the reaction
was completed, the reaction mixture was poured into water. The
resulting precipitate was filtered, washed with water and dried. The
residue was chromatographed on silica gel (toluene–acetic acid =
5 : 3 as eluent) to give ◦6a (0.112 g, yield 60%) and 7a (0.03 g, yield
16%); 6a: mp 257–258 C; IR (KBr): m = 3434, 2226, 1728 cm−1; 1H
NMR (acetone-d6, TMS) d = 1.03 (3H, t), 1.27 (3H, t), 1.46–1.52
(2H, m), 1.70–1.75 (2H, m), 2.05–2.09 (2H, m) overlap peak of
remaining acetone, 2.49–2.52 (2H, m), 3.52–3.61 (4H, m), 4.15–
4.20 (2H, m), 7.09 (1H, dd, J = 2.44 and 9.04 Hz), 7.16 (1H, d,
J = 2.44 Hz), 7.41–7.44 (1H, m), 7.51–7.55 (1H, m), 7.80 (1H, d,
To a solution of 6a (0.10 g, 0.17 mmol) in ethanol (500 ml) was
added dropwise aqueous NaOH (0.034 g, 0.86 mmol, 10 mL,) with
◦
stirring at 60 C. After further stirring for 16 h under reflux, the
solution was acidified to pH 4 with 2 M HCl, and concentrated
under reduced pressure. The residue was dissolved in CH2Cl2, and
washed with water. The organic extract was dried over MgSO4,
filtered, and concentrated. The residue was chromatographed on
silica gel (CH2Cl2–ethyl acetate = 3 : 1 as eluent) to give 8a (0.048 g,
yield 49%); mp 269–271 ◦C; IR (KBr): m = 3235, 2223, 1704 cm−1;
1H NMR (acetone-d6, TMS) d = 1.03 (3H, t), 1.47–1.52 (2H, m),
1.72–1.76 (2H, m), 2.05–2.09 (2H, m) overlap peak of remained
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acetone, 2.49–2.53 (2H, m), 3.52–3.61 (4H, m), 7.11 (1H, dd,
J = 2.20 and 9.04 Hz), 7.18 (1H, d, J = 2.20 Hz), 7.40–7.45
(1H, m), 7.51–7.55 (1H, m), 7.81 (1H, d, J = 8.04 Hz), 8.08 (2H,
d, J = 8.52 Hz), 8.53 (2H, d, J = 8.52 Hz), 8.56 (1H, d, J =
9.04 Hz), 8.73 (1H, d, J = 7.80 Hz), 11.51 (1H, s, –NH); m/z (EI)
557 (M+, 30%).
and 1,2-dimethyl-3-n-propylimidazolium iodide in acetonitrile as
electrolyte. The photocurrent–voltage characteristics were mea-
sured using a potentiostat under a simulated solar light (AM 1.5,
61 mW cm−2). IPCE spectra were measured under monochromatic
irradiation with a tungsten-halogen lamp and a monochromator.
The dye-coated film was immersed in a solvent mixture of THF–
DMSF–1 M NaOH aq (5 : 4 : 1), which was used to determine
the amount of dye molecule adsorbed onto the film by measuring
the absorbance. The quantification of each dye was made based
on the kmax and the molar extinction coefficient of each dye in
the above solution. Absorption spectra of the dye-adsorbed TiO2
films were measured in the diffuse-reflection mode by a JASCO
UV-VIS spectrophotometer with a calibrated integrating sphere
system ISV-469.
Synthesis of carboxypropyl-{7-(4-cyanophenyl)benzofuro[2,3-c]-
oxazolo[4,5-a]carbazole)-3-aminobutyric acid (8b)
To a solution of 6b (0.10 g, 0.16 mmol) in ethanol (500 ml) was
added dropwise aqueous NaOH (0.062 g, 1.56 mmol, 20 mL,) with
◦
stirring at 60 C. After further stirring for 22 h under reflux, the
solution was acidified to pH 4 with 2 M HCl, and concentrated
under reduced pressure. The residue was dissolved in CH2Cl2, and
washed with water. The organic extract was dried over MgSO4,
filtered, and concentrated. The residue was chromatographed on
silica gel (CH2Cl2–ethyl acetate = 3 : 1) to give 8b (0.022 g, yield
23%); mp 259–261 ◦C; IR (KBr): m = 3255, 2227, 1704 cm−1; 1H
NMR (DMSO-d6, TMS) d = 1.84–1.89 (4H, m), 3.35–3.50 (4H, t)
overlap peak of dissolved water in DMSO-d6, 6.94–6.96 (4H, m),
7.06 (1H, dd, J = 2.44 and 9.04 Hz), 7.23 (1H, d, J = 2.44 Hz),
7.36–7.40 (1H, m), 7.48–7.52 (1H, m), 7.68 (1H, d, J = 8.28 Hz),
8.11 (2H, d, J = 8.80 Hz), 8.41 (2H, d, J = 8.80 Hz), 8.45 (1H, d,
J = 9.04 Hz), 8.60 (1H, d, J = 7.80 Hz), 12.50 (1H, s, –NH); m/z
(EI) 586 (M+, 30%).
Acknowledgements
We are grateful to Dr N. Koga (Hiroshima University) for the
measurement of absorption spectra of the dyes adsorbed on TiO2
films. This work was supported by a Grant-in-Aid for Scientific
Research (B) (19350094) and a Grant-in-Aid for Young Scientist
(B) (18750174) from the Ministry of Education, Science, Sports
and Culture of Japan and by the Electric Technology Research
Foundation of Chugoku.
References
Computational methods
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All geometries were completely optimized (keyword PRECISE)
by the eigenvector following routine (keyword EF). Experimental
absorption spectra of the eight compounds were compared with
their absorption data by the semi-empirical method INDO/S
(intermediate neglect of differential overlap/spectroscopic).10 All
INDO/S calculations were performed using single excitation full
SCF/CI (self-consistent field/configuration interaction), which
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The TiO2 electrodes used for dye-sensitized solar cells were
prepared as follows. Into a powder (1.30 g) of TiO2 (P-25, d =
30–40 nm) in mortar was added water (1.9 mL) in six portions
with plenty of stirring. Then, three drops of 12 M nitric acid
and polyethylene glycol (80 mg) were successively added, and the
mixture was well-kneaded to a smooth paste. It was then applied
to a fluorine-doped tin oxide (FTO) substrate and sintered for
30 min at 500 ◦C. The 7 lm thick TiO2 electrode (0.5 × 0.5 cm2 in
the photoactive area) was immersed in a 0.3 mM tetrahydrofuran
solution of the dye for a number of hours – long enough to
adsorb the photosensitizer. The DSSCs were fabricated by using
the TiO2 electrode thus prepared, Pt-coated glass as a counter
electrode, and a solution of 0.05 M iodine, 0.1 M lithium iodide,
13 S. Ru¨hle, M. Greenshtein, S.-G. Chen, A. Merson, H. Pizem, C. S.
Sukenik, D. Cahen and A. Zaban, J. Phys. Chem. B, 2005, 109, 18907.
2054 | Org. Biomol. Chem., 2007, 5, 2046–2054
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The Royal Society of Chemistry 2007
©