Synthesis of near-infrared absorbing and fluorescing thiophene-fused BODIPY dyes with strong electron-donating groups and their application in dye-sensitised solar cells

Article abstract of DOI:10.1039/c8nj04672g

Thiophene-fused BODIPY dyes with two diethylaminophenyl groups as strong donors demonstrated near-infrared (NIR) absorption (λ_max: 783-812 nm, ?: 119500-145900) and fluorescence (F_max: 862-916 nm, Φ_f: 0.02-0.12) in dichloromethane. When applied to dye-sensitised solar cells, these dyes exhibited a panchromatic incident photon-to-current efficiency (IPCE) response from 400 to 850 nm, with the onset of the IPCE response at ～950 nm.

Full text of DOI:10.1039/c8nj04672g

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Synthesis of near-infrared absorbing and
fluorescing thiophene-fused BODIPY dyes with
strong electron-donating groups and their
application in dye-sensitised solar cells†
Cite this: New J. Chem., 2019,
43, 1156
a
Yasuhiro Kubota, *a Kosei Kimura,a Jiye Jin,b Kazuhiro Manseki,
Kazumasa Funabiki
a
and Masaki Matsui*a
Thiophene-fused BODIPY dyes with two diethylaminophenyl groups as strong donors demonstrated near-
infrared (NIR) absorption (lmax: 783–812 nm, e: 119 500–145 900) and fluorescence (Fmax: 862–916 nm,
Ff: 0.02–0.12) in dichloromethane. When applied to dye-sensitised solar cells, these dyes exhibited
a panchromatic incident photon-to-current efficiency (IPCE) response from 400 to 850 nm, with the
onset of the IPCE response at B950 nm.
Received 13th September 2018,
Accepted 23rd November 2018
DOI: 10.1039/c8nj04672g
rsc.li/njc
(NIR) photosensitisers; however, photoelectric conversion in
the NIR region is still limited. The development of an efficient
Introduction
Because of environmental issues and the exhaustion of petro- NIR photosensitiser is a critical, challenging problem that must
leum resources, solar cells have attracted considerable atten- be solved.
tion as renewable and environmentally friendly energy sources.
Boron-dipyrromethene (BODIPY) is one of the most pro-
Dye-sensitised solar cells (DSSCs) have many advantages over minent fluorescent dyes.13 Its absorption wavelength can be
conventional silicon cells, such as low production costs, flexi- controlled from the visible to the NIR region by structural
bility, and multiple colour choices.1 Ruthenium–pyridyl com- modification. Because of their excellent absorption properties
plexes are one of the candidates of photosensitisers for DSSCs.2 and easily controllable molecular structures, BODIPY dyes have
Ruthenium–pyridyl complexes with a solar-to-electrical power also attracted considerable research interest for application as
conversion efficiency (Z) exceeding 11% have been reported. photosensitisers in DSSCs.14 The application of BODIPY dyes to
However, Ru complexes have some disadvantages, such as the DSSCs was first reported by Fukuzumi and coworkers in 2005.15
high cost of the rare metal Ru and low molar extinction However, the energy conversion efficiency (Z) was very low
coefficients. Therefore, ruthenium-free organic dyes have been (0.13–0.16). There have been many attempts to modify the
aggressively investigated for use as photosensitisers.3 Efficient structure of BODIPY photosensitizers, which can be roughly
ruthenium-free organic dyes such as porphyrin,4 indoline,5 divided into two types: 3,5,8-modified BODIPY dyes (vertical
perylene,6 tetrathienoacene,7 phenothiazine,8 and others9 have design)16 and 2,6-modified BODIPY dyes (horizontal design).17
been reported. In particular, zinc porphyrin complex (SM315: Ziessel and coworkers reported that 3,5,8-modified BODIPY dye
Z = 13%),4d indoline-based dye (YA422: Z = 10.65%),5e perylene- having two 3-(2-methoxyethoxy)prop-1-ynyl groups on the boron
based dye (C275: Z = 12.5%),6e tetrathienoacene-based dye atom exhibited a high Z value (5.75%) along with a high
(TTAR-b8: Z = 11.18%),7b oligothiophene with alkoxysilyl-anchor incident photon-to-current conversion (IPCE) efficiency response
(ADEKA-1: Z = 12.8%),9j and 9,19-dihydrobenzo[10,100]phenanthro- of over 60% from 560 to 760 nm.16c In 2,6-modified BODIPY
[30,40:4,5]thieno[3,2-b]benzo[1,10]phenanthro[3,4-d]thiophene-based dyes, MPBTTCA showed a higher Z value (6.4%).17g In addition,
dye (R6: Z = 12.6%)9o show remarkable performances. Squarine,10 benzene-fused BODIPY dye showed a high Z value (6.06%) as
cyanine,11 and phthalocyanine12 have been reported as near-infrared well as a high IPCE response of over 70% in the long-wavelength
region from 600 to 720 nm, with the onset of the IPCE response
at B800 nm.18a Moreover, benzene-fused BODIPY dye conju-
a Department of Materials Science and Technology, Faculty of Engineering,
gated with a phenothiazine moiety as an anchoring group
showed the highest Z value (7.69%).18b Therefore, BODIPY dyes
Gifu University, Yanagido, Gifu, 501-1193, Japan. E-mail: kubota@gifu-u.ac.jp
b Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1, Asahi,
are good candidates for use as efficient NIR photosensitisers
for DSSCs. However, the development of a BODIPY dye with
efficient photoelectric conversion at wavelengths greater than
Matsumoto, Nagano, 390-8621, Japan
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of new compounds. See DOI: 10.1039/c8nj04672g
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800 nm is an intractable problem because adjusting the HOMO of 4 with 4-formylbenzoic acid gave the corresponding dipyrrole-
and LUMO energy levels is very difficult.19
methane. Without isolation, dipyrrolemethane was oxidised with
On the other hand, annelation of an aromatic ring to the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to give the
BODIPY core is an efficient strategy to achieve a redshift of the corresponding dipyrromethene. This was followed by boron com-
absorption maximum.20 Since furan-fused BODIPY dyes were firstly plexation using triethylamine and boron trifluoride diethyl ether
reported by Umezawa and coworkers in 2008,21 they have received complex to give thiophene-fused BODIPY dye 5 in 16% yield.
attention for their application in NIR fluorescence probes22 and A similar procedure using 5-formyl-2-thiophenecarboxylic acid
electron donors for organic solar cells (OSCs).23 Furthermore, gave thiophene-fused BODIPY dye 6 in 31% yield.
thiophene-fused BODIPY24 and azaBODIPY25 dyes have been used
Optical properties
as NIR photosensitisers in photodynamic therapy (PDT) and NIR
absorbing dyes. To the best of our knowledge, there has been no
report on the application of thiophene-fused BODIPY dyes as
photosensitisers in DSSCs. In this paper, we report the application
of (D)2–p–A type (vertical design) thiophene-fused BODIPY dyes with
strong donor (D) groups for use as NIR photosensitisers in DSSCs.
The UV-vis-NIR absorption spectra of BODIPY dyes 5 and 6 in
dichloromethane were recorded (Fig. 1a). Interestingly, both
BODIPY dyes showed absorption in the NIR region; the absorp-
tion maximum (lmax) of thiophene derivative 6 (813 nm) was
red-shifted compared with that of phenylene derivative 5
(783 nm) (Table 1). In the absorption spectra of 5 and 6, not
Synthesis
only the first absorption band but also the second (5: lmax
=
=
The previously reported compound 2 was synthesised in three 565 nm, 6: lmax = 586 nm) and third (5: lmax = 447 nm, 6: lmax
steps from ethyl bromoacetate (Scheme 1). Ethyl azidoacetate26 456 nm) absorption bands were observed. Such multiple absorp-
was obtained through the reaction of sodium azide and ethyl tion bands are advantageous for wide-range solar light harvest-
bromoacetate. Aldol-type condensation of ethyl azidoacetate ing, which leads to panchromatic sensitisation in DSSCs. These
with 5-bromo-2-formylthiophene in the presence of sodium dyes also showed high molar extinction coefficients (e: 5: 145 900,
ethoxide gave (Z)-ethyl 2-azido-3-(5-bromothiophen-2-yl)acrylate 6: 119 500).
(1).27 Intramolecular annulation of the nitrene intermediate
BODIPY dyes 5 and 6 exhibited NIR fluorescence in dichloro-
obtained by heating azido 1 led to the formation of 2-bromo- methane (Fig. 1b). The fluorescence maximum (Fmax) of 6
4H-thieno[3,2-b]pyrrole-5-carboxylate (2).27 The Suzuki–Miyaura (916 nm) was also bathochromically shifted compared with
cross-coupling reaction between bromothienopyrrole 2 and that of 5 (862 nm). The fluorescence quantum yield (Ff) of 5
4-(diethylamino)phenylboronic acid in the presence of Pd(PPh3)4 (0.12) was slightly higher than that of 6 (0.02). Time-resolved
afforded ethyl 2-(40-diethylaminophenyl)-4H-thieno[3,2-b]pyrrole- fluorescence spectroscopy was performed to obtain the fluores-
5-carboxylate (3) in 87% yield. Heating an ethylene glycol solution cence lifetimes (ts) of these dyes. The fluorescence decays were
of 3 in the presence of potassium hydroxide resulted in hydrolysis, fitted to a monoexponential function (5: w2 = 1.10, 6: w2 = 1.07)
followed by decarboxylation, to give 2-(40-diethylaminophenyl)-4H- (Fig. S1, ESI†). The ts values of 5 and 6 were 1.24 and 0.26 ns,
thieno[3,2-b]pyrrole (4) in 69% yield. The Friedel–Crafts reaction respectively (Table 1). The radiative rate constant (kf) of
Scheme 1 Synthesis of thiophene-fused BODIPY dyes.
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Fig. 1 (a) UV-vis-NIR absorption spectra and (b) normalised fluorescence spectra of 5 and 6 in dichloromethane (1.0 ꢀ 10ꢁ5 M).
Table 1 Optical properties of thiophene-fused BODIPY dyes in dichloro-
methanea
d
f
Compd
lmax (e) (nm)
Fmax (nm)
ffb,c
ts
kfe
knr
5
783 (145 900)
565 (26 100)
447 (18 200)
812 (119 500)
586 (22 700)
456 (13 200)
862
0.12
1.24
0.26
0.97
7.1
6
916
0.02
0.77
37.7
a
b
Measured at a concentration of 1.0 ꢀ 10ꢁ5 M. The excitation
c
wavelength (lex) was as follows: 5 (720 nm) and 6 (770 nm). Measured
using an integrating sphere method. Measured using a single photon-
counting method. Radiative rate constant (kf = Ff/ts). Non-radiative
rate constant (knr = (1 ꢁ Ff)/ts).
d
e
f
5 (0.97 ꢀ 108 sꢁ1) was comparable to that of 6 (0.77 ꢀ 108 sꢁ1).
However, the non-radiative rate constant (knr) of 6 (37.7 ꢀ 108 sꢁ1
)
was significantly higher than that of 5 (7.1 ꢀ 108 sꢁ1). Thus, the
main reason for the lower Ff value of 6 was the susceptibility to
non-radiative processes. According to the energy gap law, knr
decreases exponentially with increasing energy gap.28 Thus, one
of the reasons for the smaller knr value of 5 is the higher energy
gap (5: 1.51 eV, 6: 1.44 eV). Additionally, thiophene derivative 6 is
expected to easily lead to intersystem crossing (ISC) compared to
phenylene derivative 5 because of the heavy atom effect of the
sulfur atom in the meso-positioned thiophene moiety.24h There-
fore, the smaller knr value of 5 is probably due to both the higher
energy gap and suppressed ICT of 5.
Fig. 2 Molecular orbital energy diagram and isodensity surface plots of
the HOMOꢁ1, HOMO, LUMO, and LUMO+1 states of BODIPY dyes.
Table 2 Calculated absorption maximum (lmax), oscillator strength (f) and
the main orbital transition calculated by TDDFT at the B3LYP/6-31G(d,p)
level
Compd
transition lmax (nm)
Main orbital transition
f
5
S0 to S1
S0 to S2
S0 to S5
S0 to S1
S0 to S2
S0 to S5
654
524
443
682
544
466
HOMO to LUMO (0.71)
HOMOꢁ1 to LUMO (0.68)
HOMO to LUMO+1 (0.70)
HOMO to LUMO (0.71)
HOMOꢁ1 to LUMO (0.68)
HOMO to LUMO+1 (0.70)
1.47
0.34
0.16
1.34
0.38
0.23
Theoretical calculations
6
Density functional theory (DFT) calculations were performed
using the Gaussian 09 package for a deeper insight into
the absorption properties of the synthesised BODIPY dyes.29
Geometry optimisation and time-dependent DFT (TDDFT) transitions of 5 and 6 were intramolecular charge transfer (ICT)
calculations were performed using the B3LYP/6-31G(d,p) transitions from the diethylaminophenyl moiety (donor) to the
method in a vacuum. Thiophene derivative 6 tended to have a anchor (acceptor) moiety (Fig. 2). Therefore, thiophene-fused
lower energy level compared with that of phenylene derivative BODIPY dyes 5 and 6 appeared to be effective for electron
5, and the decrease of the LUMO level was especially noticeable injection from the excited state to the TiO2 conduction
(5: ꢁ2.63 eV, 6: ꢁ2.74 eV) (Fig. 2). The calculated lmax, the main band. The observed second (5: 565 nm, 6: 586 nm) and third
orbital transition, and the oscillator strength ( f ) of these dyes (5: 447 nm, 6: 456 nm) absorption bands were mainly attributed
are shown in Table 2. The TDDFT results indicated that the to the HOMOꢁ1–LUMO and HOMO–LUMO+1 transitions,
experimentally observed first absorption bands of 5 (783 nm) respectively. The calculated lmax values were in good agreement
and 6 (813 nm) are attributed to the S0 - S1 transition and are with the experimental values. A spectral red shift of 6 compared
mainly due to the HOMO–LUMO transitions. The HOMO–LUMO with 5 is suggested. In the optimised structure, the dihedral
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angles between the BODIPY core and the meso-aryl moieties of 5
and 6 were 55.81 and 48.91, respectively. Therefore, the red-
shifted lmax of 6 was probably due to the smaller dihedral angle,
which led to more effective p-conjugation and ICT transition.30
Electrochemical properties
The cyclic voltammograms of 5 and 6 were measured in aceto-
nitrile (Fig. 3). BODIPY dye 5 underwent two successive one-
electron oxidation processes. The first and second half-wave
potentials for oxidation (E1/2 (ox)) were 0.12 and 0.26 (vs. Fc/Fc+),
respectively. In acetonitrile, the conversion of E1/2 (vs. Fc/Fc+) to
E1/2 (vs. NHE) is known to be as follows: E1/2 (vs. NHE) = E1/2
(vs. Fc/Fc+) + 0.63 V.31 Therefore, the first and second E1/2 (ox)
values of 5 were estimated to be 0.75 and 0.89 V (vs. NHE)
(Table 3). Similarly, the E1/2 (ox) values of 6 were determined to
be 0.77 and 0.89 V (vs. NHE). Therefore, the first Eox values
corresponding to the HOMO energy levels of 5 (0.75 vs. NHE)
Fig. 4 Energy level diagrams for thiophene-fused BODIPY dyes.
ELUMO values of 5 and 6 were estimated to be ꢁ0.76 and ꢁ0.67 V
vs. NHE, respectively. Because the ELUMO values of 5 and 6 were
more negatively shifted compared with that of the conduction
band level of TiO2 (ꢁ0.5 V vs. NHE), electron transfer from the
excited BODIPY dyes to the conduction band of TiO2 is expected
to be thermodynamically allowed.33
and 6 (0.77 vs. NHE) were more positive than the Iꢁ/I3 redox
ꢁ
potential (0.4 vs. NHE), indicating that regeneration of the
oxidised dyes by electron transfer from the Iꢁ/I3 redox in the
ꢁ
electrolyte is considered to be thermodynamically possible32
(Fig. 4).
Photovoltaic performance
The LUMO energy level (ELUMO) was obtained by subtracting
the zero–zero excitation energy (E0–0) (5: 823 nm, 1.51 eV, 6:
861 nm, 1.44 eV) determined by the intersection of the normal-
ised absorption and fluorescence spectra in dichloromethane
from the corresponding E1/2 (ox) values (Fig. S2, ESI†). Thus, the
The photovoltaic characteristics of 5 and 6 as sensitisers were
evaluated in DSSCs comprising a TiO2 photoelectrode (trans-
parent layer: 8 mm, scattering layer: 4 mm), chenodeoxycholic
acid (CDCA, 5: 40 equiv., 6: 60 equiv.) as a co-adsorbent in
acetonitrile/t-butyl alcohol (v : v = 1 : 1), a Pt-sputtered FTO glass
plate, and a 3-methoxypropionitrile solution containing I2 (0.1 M),
LiI (2.0 M), 1,2-dimethyl-3-propylimidazolium iodide (DMPII,
0.6 M), and 4-tert-butylpyridine (0.5 M) as a liquid electrolyte.
For the sake of comparison, the well-known photosensitizer
D1495b was evaluated under the same fabrication conditions
(Fig. S3, ESI† and Table 4). The IPCE spectra of 5 and 6 are
shown in Fig. 5. The IPCE values of 5 and 6 were higher than
5% in the range of 400 to 850 nm, and the onsets of the IPCE
response were at B950 nm. The maximum IPCE of 6 (12.3% at
810 nm) was red-shifted and lower than that of 5 (18.1% at
790 nm) (Table 4).
Fig. 3 Cyclic voltammograms of 5 and 6 measured in acetonitrile con-
taining tetrabutylammonium perchlorate (0.1 M) as a supporting electro-
lyte. AgQRE was used as a reference electrode. Platinum wire was used as
the working and counter electrodes. The scan rate was 50 mV sꢁ1. Fc/Fc+
was used as an external reference.
Table 4 Photovoltaic performances of 5, 6 and D149
dye
Maximum IPCE/%
Jsc/mA cmꢁ2
Voc/V
FF
Z/%
5a
18.1% (790 nm)
12.3% (810 nm)
65.2% (550 nm)
4.98
3.96
12.0
0.431
0.435
0.612
0.65
0.65
0.63
1.40
1.12
4.64
6a
D149b
Table 3 Electrochemical properties of BODIPY dyes
a
Conditions: electrode, 8 + 4 mm (transparent + scattering) layer of
TiO2; electrolyte, 0.1 M I2, 0.6 M DMPII, and 2.0 M LiI in 3-methoxy-
propionitrile; dye absorption, 0.1 mM BODIPY dye in acetonitrile/
t-butanol (v : v = 1 : 1), CDCA (5: 40 equiv., 6: 60 equiv.); dipping time,
1 h; dipping temperature, 10 1C; measurement, active area of 0.20 cm2
Compd
EHOMOa/V vs. NHE
E0–0b/eV
ELUMOc/V vs. NHE
5
6
0.75
0.77
1.51
1.44
ꢁ0.76
ꢁ0.67
using a mask under AM1.5G illumination (100 mW cmꢁ2). Conditions:
b
a
The first half-wave potentials of oxidation were measured in aceto- electrode, 8 + 4 mm (transparent + scattering) layer of TiO2; electrolyte,
nitrile with tetrabutylammonium perchlorate (0.1 M) at a scan rate of 0.05 M I2, 0.6 M 1-butyl-3-methylimidazolium iodide, 0.1 M LiI, and
50 mV sꢁ1 vs. Fc/Fc+ and then converted to vs. NHE by adding 0.63 V.31 0.05 M 4-tert-butylpyridine in acetonitrile/valeronitrile (v : v = 85 : 15);
b
The zero–zero excitation energies were estimated by the intersection dye absorption, 0.2 mM D149 in acetonitrile/t-butanol (v : v = 1 : 1),
of normalized absorption and fluorescence spectra in dichloromethane. CDCA (2.5 equiv.); dipping time, 4 h; dipping temperature, 10 1C;
c
The LUMO energy levels were obtained by subtracting the E0–0 values measurement, active area of 0.20 cm2 using a mask under AM1.5G
from the corresponding EHOMO values.
illumination (100 mW cmꢁ2).
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Fig. 7 I–V curves of 5 and 6.
Fig. 5 IPCE spectra of 5 and 6.
which facilitates the electron recombination.14d Thiophene-
fused BODIPY dyes 5 and 6 have a highly planar geometry com-
pared to common BODIPY dyes. Therefore, one of the reasons
for the relatively low performances of 5 and 6 is considered to
be the formation of aggregates. Introduction of bulky alkyl
groups into BODIPY dye is a good strategy to suppress dye
aggregates.14d Additionally, recently, we have reported that
optimization of co-adsorbents can efficiently prevent the aggre-
gation formation of dyes and significantly improve the perfor-
mance of DSSCs.35 We expect that introduction of bulky alkyl
groups and optimization of co-adsorbents will improve the
DSSC performance of thiophene-fused BODIPY dyes.
Fig. 6 Normalised UV-vis-NIR absorption spectra of 5 and 6 on TiO2 film
with CDCA (5: 40 equiv., 6: 60 equiv.).
Conclusions
Thiophene-fused BODIPY dyes with two diethylaminophenyl
groups as strong donor groups, a phenylene or thiophene group
as a spacer group, and carboxylic groups as anchor and acceptor
groups were synthesised, and their absorption, fluorescence, and
application to dye-sensitised solar cells were studied. Thiophene
derivative 6 showed a red-shifted lmax compared with the
corresponding phenylene derivative 5 due to the smaller dihe-
dral angle between the BODIPY core and the meso-substituted
spacer group, which led to the extension of p-conjugation and
promoted the ICT transition. Thiophene-fused BODIPY dyes
exhibited fluorescence in the NIR region. Thiophene-fused BOD-
IPY dyes used as sensitisers for DSSCs showed IPCE responses in
the range of 400 to 850 nm. The maximum IPCE values for 5 and
6 were 18.1% at 790 nm and 12.3% at 810 nm, respectively.
Because of the higher IPCE response, 5 showed higher Jsc and
Z values compared with those of 6.
The UV-vis-NIR absorption spectra of BODIPY dyes adsorbed
on an 8 mm TiO2 film with CDCA are shown in Fig. 6. The lmax
values on the TiO2 film (5: 738 nm, 6: 765 nm) were blue-shifted
and the spectra were broadened compared with the corres-
ponding lmax values in dichloromethane (5: 783 nm, 6: 812 nm),
which was probably due to the formation of aggregates and
the interaction of the anchor group with the TiO2 surface.33 The
shape of the IPCE spectra was consistent with that of the
absorption spectra in dichloromethane and on the TiO2 film.
Compared to the absorption spectra, the IPCE response in the
shorter-wavelength region (around 550 nm) was higher than that
in the longer-wavelength region (B800 nm), which was due to
the effect of a scattering layer.34
The lower IPCE response of 6 may be due to the positively
shifted ELUMO (5: ꢁ0.76 V vs. NHE, 6: ꢁ0.67 V vs. NHE), which
makes it difficult to transfer electrons from the excited BODIPY
dyes to the conduction band of TiO2. The I–V curves of 5 and 6
are shown in Fig. 7. The short-circuit current density ( Jsc) of 5
Experimental section
General information
(4.98 mA cmꢁ2) was higher than that of 6 (3.96 mA cmꢁ2
because of the significantly higher IPCE response. The open-
)
circuit voltage (Voc) and fill factor (FF) of 5 (Voc: 0.431 V, FF: 0.65) NMR spectra were recorded on an ECX400P spectrometer. Infra-
and 6 (Voc: 0.435 V, FF: 0.65) were almost the same. Therefore, red (IR) spectra were recorded using a Shimadzu IR-Affinity
phenylene derivative 5 showed a higher power conversion effi- spectrometer. UV-vis spectra were recorded on a Hitachi U4100
ciency (Z; 1.40%) than thiophene derivative 6 (1.12%) due to the spectrophotometer. Fluorescence spectra were recorded on an
higher Jsc.
FP-8600 spectrofluorometer. Fluorescence quantum yields were
The structural planarity of BODIPY dyes is known to often determined using a Quantaurus-QY. Fluorescence lifetimes were
lead to severe dye aggregates on TiO2 nanoparticle surfaces, measured using a Quantaurus-t. Mass spectra were recorded
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on a JEOL JMS-700 spectrometer. Melting points were measured purified by column chromatography (silica gel, dichloro-
using a Yanagimoto MP-S2 micro-melting-point apparatus. methane : methanol = 20 : 1, Rf = 0.3) to afford 5 (21.3 mg,
Analytical thin-layer chromatography (TLC) was performed on Y = 16%) as a blue-brown solid. 5: mp 4300 1C; 1H NMR
pre-coated plates (Merck, silica gel 60 F254). Silica gel (Wakogel (400 MHz, CDCl3) d 1.22 (t, J = 7.4 Hz, 12H), 3.44 (q, J = 7.4 Hz,
C-200) was used for column chromatography.
8H), 6.66 (s, 2H), 6.68 (d, J = 8.7 Hz, 4H), 7.32 (s, 2H), 7.59 (d, J =
Synthesis of 2-(40-diethylaminophenyl)-4H-thieno[3,2-b]pyrrole- 8.7 Hz, 4H), 7.73 (d, J = 8.2 Hz, 2H), 8.25 (d, J = 8.2 Hz, 2H);
5-carboxylate (3). 4-(Diethylamino)phenylboronic acid (845 mg, 13C NMR (100 MHz, CDCl3) d 12.63, 44.60, 105.49, 111.45,
4.38 mmol), ethyl 2-bromo-4H-thieno[3,2-b]pyrrole-5-carboxylate 117.43, 121.49, 128.09, 130.00, 130.07, 130.95, 132.01, 136.05,
(2) (1.00 g, 3.65 mmol) and tetrakis(triphenylphosphine)- 140.21, 140.44, 148.98, 159.40, 160.11, 170.09; IR (KBr) 3082,
palladium (84.3 mg, 0.073 mmol) were added to an oven-dried 2970, 1719, 1603, 1533, 1472, 1406, 1333, 1298, 1267, 1196, 1151,
two-neck flask fitted with a condenser. The mixture was degassed 1033 cmꢁ1; MS (FAB) m/z 719 [M + H]+; HRMS (FAB) m/z calcd for
and purged with argon, then degassed toluene (100 mL) and 2 M C40H38BF2N4O2S2 [M + H]+ 719.2497, found 719.2519.
aq. Na2CO3 (50 mL) were added. The mixture was refluxed for
Synthesis of 6. Eight drops of trifluoroacetic acid were added
24 hours, and then cooled, extracted with diethyl ether, dried over to a solution of 4 (101 mg, 0.374 mmol) and 5-formyl-2-thio-
magnesium sulfate and concentrated under reduced pressure. phenecarboxylic acid (29.1 mg, 0.186 mmol) in dichloromethane
The residue was purified by chromatography (silica gel, chloro- (50 mL), and the mixture was stirred at room temperature
form : n-hexane = 10 :1, Rf = 0.2) to afford 3 (1.09 g, Y = 87%) as a for 3 days. A dichloromethane solution of DDQ (42.2 mg,
1
yellow solid. 3: mp 177.0–179.0 1C; H NMR (400 MHz, CDCl3) 0.186 mmol) was added to the solution. After stirring the
d 1.19 (t, J = 7.3 Hz, 6H), 1.38 (t, J = 7.2 Hz, 3H), 3.39 (q, J = 7.3 Hz, mixture for 12 hours at room temperature, triethylamine
4H), 4.36 (q, J = 7.2 Hz, 2H), 6.67 (d, J = 8.7 Hz, 2H), 6.98 (s, 1H), (0.6 mL, 4.3 mmol) and boron trifluoride diethyl ether complex
7.09 (d, J = 0.9 Hz, 1H), 7.47 (d, J = 8.7 Hz, 2H), 8.94 (brs, 1H); (0.6 mL, 4.7 mmol) were added and again stirred overnight.
13C NMR (100 MHz, CDCl3) d 12.6, 14.5, 44.4, 60.5, 104.1, 107.9, Then, water was poured into the reaction mixture. The solution
111.6, 122.2, 122.7, 125.5, 127.1, 142.0, 147.6, 149.7, 161.4; IR was washed with water, extracted with dichloromethane, dried
(KBr) 3291, 2974, 2936, 2897, 1670, 1605, 1516, 1404, 1358, 1292, over Na2SO4 and concentrated. Chromatographic treatment of
1269, 1196, 1022, 810, 795, 760 cmꢁ1; MS (FAB) m/z 343 [M + H]+; the residue on silica gel (dichloromethane : methanol = 20 : 1,
anal. calcd for C19H22N2O2S: C, 66.64; H, 6.48; N, 8.18. found: Rf = 0.3) afforded 6 (42 mg, Y = 31%) as a blue-green powder. 6:
1
C, 66.72; H, 6.31; N, 7.94.
mp 4300 1C; H NMR (400 MHz, CDCl3) d 1.22 (t, J = 7.1 Hz,
Synthesis of 2-(40-diethylaminophenyl)-4H-thieno[3,2-b]- 12H), 3.44 (q, J = 7.1 Hz, 8H), 6.68 (d, J = 8.9 Hz, 4H), 6.99 (s,
pyrrole (4). A solution of 3 (500 mg, 1.46 mmol) and potassium 2H), 7.30 (s, 2H), 7.45 (d, J = 3.7 Hz, 1H), 7.60 (d, J = 8.9 Hz, 4H),
hydroxide (3.76 g, 67.1 mmol) in ethylene glycol (30 mL) was 7.95 (d, J = 3.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 12.64,
heated at 130 1C for 1 d. After cooling to room temperature, 44.60, 105.40, 109.25, 111.50, 117.32, 121.48, 127.81, 128.14,
water was added and the reaction mixture was extracted with 131.25, 132.12, 134.37, 140.26, 149.00, 159.49, 160.46, 161.67,
chloroform. The organic extract was dried over anhydrous 165.87; IR (KBr) 3088, 2970, 1692, 1603, 1530, 1470, 1408, 1335,
sodium sulfate and the solvent was evaporated under reduced 1296, 1269, 1196, 1153, 1076, 1036 cmꢁ1; MS (FAB) m/z 725
pressure. Column chromatography of the residue on silica gel [M + H]+; HRMS (FAB) m/z calcd for C38H36BF2N4O2S3 [M + H]+
(chloroform, Rf = 0.5) afforded 4 (272 mg, Y = 69%) as a white 725.2062, found 725.2048.
solid. 4: mp 146.0–148.0 1C; 1H NMR (400 MHz, CDCl3) d 1.18 (t,
Fabrication and measurement of dye-sensitized solar cells.
J = 6.9 Hz, 6H), 3.37 (q, J = 6.9 Hz, 4H), 6.42–6.43 (m, 1H), 6.67 Dye-sensitized solar cells were fabricated according to the
(d, J = 8.2 Hz, 2H), 6.91 (t, J = 2.8 Hz, 1H), 6.99 (s, 1H), 7.45 (d, J = previously reported method of Ito et al.34 FTO (F-doped tin
8.2 Hz, 2H), 8.15 (brs, 1H); 13C NMR (100 MHz, CDCl3) d 12.6, oxide; 4 mm thickness, 13 O &ꢁ1, Nippon Sheet Glass) glass
44.4, 101.7, 104.8, 111.8, 121.4, 121.6, 123.2, 126.7, 138.8, substrates were washed for 20 min each with an aqueous
144.0, 147.0; IR (KBr) 3368, 2970, 2928, 2889, 1605, 1531, detergent solution, distilled water, and ethanol using an ultra-
1505, 1466, 1397, 1354, 1269, 1196, 1157, 1076, 1011, 817, sonic bath. The FTO glass substrate was immersed in a 40 mM
799, 710, 505 cmꢁ1; HRMS (FAB) m/z calcd for C16H19N2S aqueous TiCl4 solution at 70 1C for 30 min, and then washed
[M + H]+ 271.1269, found 271.1260.
with distilled water and ethanol. A commercial TiO2 paste (PST-
Synthesis of 5. Eight drops of trifluoroacetic acid were added 18NR, JGC Catalysts and Chemicals Ltd.) was deposited onto
to a solution of 4 (99.9 mg, 0.369 mmol) and 4-formylbenzoic the FTO glass substrate by the squeegee method using mending
acid (27.8 mg, 0.185 mmol) in dichloromethane (50 mL), tape (4 mm thickness) twice to obtain an 8 mm transparent TiO2
and the mixture was stirred at room temperature for 3 days. film. After drying the FTO glass substrate at 130 1C in air for
A dichloromethane solution of DDQ (42.0 mg, 0.185 mmol) was 15 min, a commercial TiO2 paste (PST-400C, JGC Catalysts and
added to the solution. After stirring for 2 h at room tempera- Chemicals Ltd.) was deposited once to obtain a 4 mm scattering
ture, triethylamine (0.6 mL, 4.3 mmol) and boron trifluoride layer. The resulting substrate was sintered at 500 1C for 15 min.
diethyl ether complex (0.6 mL, 4.7 mmol) were added and stirred After cooling to room temperature, the sintered TiO2 film was
for 1 day. Then, water was poured into the reaction mixture. immersed in a 40 mM aqueous TiCl4 solution at 70 1C for
The solution was washed with water, extracted with dichloro- 30 min, and then washed with distilled water and ethanol. After
methane, dried over Na2SO4 and concentrated. The residue was drying, the TiO2 film was sintered at 500 1C for 30 min to obtain
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and S. Ito, Acc. Chem. Res., 2009, 42, 1809–1818;
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layer (8 mm thickness) and a scattering layer (4 mm thickness).
The TiO2 electrode was then immersed in a solution of
thiophene-fused BODIPY dye (0.1 mM) and chenodeoxycholic
acid (5 : 4 mM, 6 : 6 mM) in acetonitrile/t-butyl alcohol (v : v = 1 : 1).
The dye adsorption was performed at 10 1C for 1 h, and the
electrode was then washed with ethanol to remove any non-
adsorbed dye, and dried. The dye-coated TiO2 electrode and the
Pt-sputtered FTO glass counter electrode were assembled into a
sandwich type cell (active area: 5 mm ꢀ 4 mm). An electrolyte
composed of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide
(DMPII), 0.5 M 4-tert-butylpyridine, 0.1 M I2, and 2.0 M LiI in a
3-methoxypropionitrile was added to the cell. After assembling
the cell, silver paint was applied to the edges of the TiO2 photo-
electrode and the counter electrode of the cells to reduce the series
resistance. The I–V curves of the cells were measured under the
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(g) T. Higashinoa and H. Imahori, Dalton Trans., 2015, 44,
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There are no conflicts to declare.
Acknowledgements
This work was partially supported by JSPS KAKENHI Grant
Number JP16K05894 and the Yashima Environment Technology
Foundation.
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