Synthesis of triphenylamine-based thiophene-(N-aryl)pyrrole-thiophene dyes for dye-sensitized solar cell applications

Article abstract of DOI:10.1016/j.tet.2012.04.104

Two new triphenylamine-based metal-free organic dyes (TPTDYE-1 and TPTDYE-2) containing 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole as a new π-conjugated chromophore were synthesized for dye-sensitized solar cell (DSSC) applications. TPTDYE-1 conta

Full text of DOI:10.1016/j.tet.2012.04.104

Tetrahedron 68 (2012) 5890e5897
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of triphenylamine-based thiophene-(N-aryl)pyrrole-thiophene dyes
for dye-sensitized solar cell applications
,
a,c,
*
Vellaiappillai Tamilavan a, Nara Cho b, Chulwoo Kim b, Jaejung Ko b , Myung Ho Hyun
*
a Department of Chemistry, Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic of Korea
b Department of Material Chemistry, Korea University, Jochiwon, Chungnam 339-700, Republic of Korea
c Division of High Technology Materials Research, Busan Center, Korea Basic Science Institute (KBSI), Busan 618-230, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new triphenylamine-based metal-free organic dyes (TPTDYE-1 and TPTDYE-2) containing 1-(2,6-
diisopropylphenyl)-2,5-di(2-thienyl)pyrrole as a new -conjugated chromophore were synthesized for
dye-sensitized solar cell (DSSC) applications. TPTDYE-1 containing three donor groups around the ac-
Received 20 March 2012
Received in revised form 24 April 2012
Accepted 25 April 2012
p
ceptor group was found to show relatively narrow absorption band from 300 nm to 470 nm while
Available online 7 May 2012
TPTDYE-2 having extended
pep delocalization between the donor and acceptor group showed broad
absorption band from 300 nm to 550 nm. The electrochemical studies indicate that the HOMOeLUMO
energy gap of TPTDYE-1 is considerably wider than that of TPTDYE-2. The dye-sensitized solar cell
performance of each dye was investigated, and the TPTDYE-2-sensitized cell was found to show
a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 75%, a short-
circuit photocurrent density (Jsc) of 13.50 mA/cm2, an open-circuit voltage (Voc) of 0.72 V, and a fill
factor (FF) of 0.69, corresponding to an overall conversion efficiency of 6.71% under simulated AM 1.5
irradiation (100 mW/cm2). Under the same condition the TPTDYE-1-sensitized cell showed the same
IPCE value of 75% with a promising conversion efficiency of 6.00%, a Jsc of 11.11 mA/cm2, a Voc of 0.76 V,
and a FF of 0.71.
Keywords:
Dye-sensitized solar cells
1-(2,6-Diisopropylphenyl)-2,5-di(2-thienyl)
pyrrole
Triphenylamine-based dyes
Metal-free organic dyes
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
conversion efficiency in the range of 7e10%.6e26 Among them,
triphenylamine (TPA)-based metal-free organic dyes displayed
6e9
Dye-sensitized solar cells (DSSCs) are considered as the most
attractive research area, because of their high performance in
converting solar energy to electric energy at low cost and easy
production.1 In DSSCs so far two classes of dyes such as organo-
metallic complexes and metal-free organic dyes were employed in
the electrical energy production from sunlight. At present, DSSCs
based on organometallic complexes (Ru(II)epolypyridyl com-
promising performances up to 9e10%.
TPA has nonplanar
structure and suppresses the dye aggregation when they are used
in DSSC applications.27 Previously, various thiophene or benzene
derivatives were used as a bridging
the triphenylamine donor and cyanoacrylic acid acceptor group.
Whereas pyrrole-based -conjugated systems were rarely studied
in DSSC applications. The DSSC performances of dyes in-
corporating N-alkylpyrrole as bridging -conjugation unit
p-conjugation unit between
p
plexes) showed maximum energy conversion efficiency (
h
) of 11%
a
p
under simulated AM 1.5 irradiation (100 mW/cm2).2e5 On the
other hand, DSSCs based on metal-free organic dyes have many
advantages in lower cost, easier structural modification, and
higher molar extinction coefficient of the dyes, and consequently,
they have been considered as an excellent alternative for organ-
ometallic complex-based DSSCs. The DSSCs made from metal-free
organic dyes showed maximum solar energy-to-electrical energy
showed maximum conversion efficiency around 4e6%,28 but at the
same time the N-arylpyrrole-based dyes showed promising per-
formances up to 7.2%, which is 91% of the standard cell from
N719.29 This inspires us to develop new N-arylpyrrole containing
dyes for DSSC applications. There are two major factors influenc-
ing the performances of the DSSCs, the light harvesting ability of
the dye and photo excited electron injection from the lowest
unoccupied molecular orbital (LUMO) of the dye to the conduction
band of the semiconductor in the anode. In order to enhance
the DSSC performance finding new N-arylpyrrole-based
gated chromophore having higher molar extinction coefficient is
essential.
p-conju-
* Corresponding authors. Tel.: þ82 41 860 1337; fax: þ82 41 867 5396 (J.K.);
tel.: þ82 51 510 2245; fax: þ82 51 516 7421 (M.H.H.); e-mail addresses: jko@
korea.ac.kr (J. Ko), mhhyun@pusan.ac.kr (M.H. Hyun).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.04.104

V. Tamilavan et al. / Tetrahedron 68 (2012) 5890e5897
5891
The molar extinction coefficient of 2,5-bis(2-thienyl)-N-2,6-
diisopropylphenyl pyrrole (N-arylpyrrole TPT) was found to be rel-
atively higher than 2,5-bis(2-thienyl)-N-4-octylphenyl pyrrole and
2,5-bis(2-thienyl)-N-dodecyl pyrrole.30 Recently, we reported the
use of N-arylpyrrole TPT-based polymers for bulk heterojunction
(BHJ) solar cell applications.30e33 The photovoltaic studies of the
polymers clearly indicated that N-arylpyrrole TPT unit harvests the
sunlight effectively.30e33 From this result we expect that dyes uti-
lizing the N-arylpyrrole TPT unit will show promising performance
in DSSCs. In this study, we synthesized two new dyes incorporating
N-arylpyrrole TPT unit namely TPTDYE-1 and TPTDYE-2 (Scheme
1). In TPTDYE-1, donor triphenylamine groups were attached on
both side of the N-arylpyrrole TPT unit and cyanoacrylic acid ac-
ceptor bridged by a methine fragment was introduced on pyrrole
back bone in N-arylpyrrole TPT unit. On the other hand, TPTDYE-2
was constructed by serially connecting the acceptor group with
donor groups such as benzene, N-arylpyrrole TPT unit, and triphe-
nylamine. In terms of conjugation TPTDYE-2 is expected to show
organic donoreacceptor dyes (TPTDYE-1 and TPTDYE-2). In addi-
tion, we also briefly discussed the influence of -conjugation and
electron donating strength on the energy conversion efficiency of
p
the DSSCs.
2. Results and discussions
2.1. Synthesis and characterization of the dyes
The synthetic procedure for TPTDYE-1 and TPTDYE-2 is shown
in Scheme 1. Compounds 1 and 2 were synthesized according to the
known procedure reported from our group.30 Vilsmeier for-
mylation of compound 2 gave compound 3. Suzuki coupling re-
action of compound 3 with 4-(diphenylamino)phenylboronic acid
yielded compound 4. By treating compound 1 with 1 equiv of N-
bromosuccinimide (NBS) yielded a mixture of mono and dibromi-
nated products. The mono-brominated product (5) was separated
and treated with 4-formylphenylboronic acid via Suzuki coupling
reaction to afford compound 6. The bromination of compound 6
with 1 equiv of NBS gave compound 7. Suzuki coupling reaction of
compound 7 with 4-(N,N-diphenylamino)phenylboronic acid yiel-
ded compound 8. The Knoevenagel condensation reaction of
compounds 4 and 8 with cyanoacetic acid in the presence of pi-
peridine yielded TPTDYE-1 and TPTDYE-2, respectively. All
better
pep conjugation than TPTDYE-1. In terms of electron do-
nating ability from the donor to the acceptor group, TPTDYE-1 is
expected to be more effective than TPTDYE-2 because two triphe-
nylamine groups and one phenyl group are placed around the ac-
ceptor group. In this paper, we reported the detailed synthesis,
characterization, and photovoltaic studies of two new metal-free
Scheme 1. Synthetic route for the synthesis of dyes TPTDYE-1 and TPTDYE-2.

5892
V. Tamilavan et al. / Tetrahedron 68 (2012) 5890e5897
compounds prepared newly in this study were well characterized,
and the spectral data are summarized in the Experimental Section.
The two dyes were found to be well soluble in chloroform,
dichloromethane, tetrahydrofuran, and ethanol.
2.2. Optical properties
The UVevisible absorption spectra of dyes in tetrahydrofuran
(THF, 1.0ꢀ10ꢁ5 M) solution are shown in Fig. 1. The absorption
spectrum of the TPTDYE-1 was quite sharp covering the region
from 300 nm to 470 nm, whereas that of TPTDYE-2 was relatively
broad from 300 nm to 550 nm. Though there are several electron
donor groups around the acceptor group, the relatively short
p-
conjugation length of TPTDYE-1 between the triphenylamine and
cyanoacrylic acid groups leads the blue shifted absorption maxi-
mum (418 nm,
3
¼5.9ꢀ104) compared to that (455 nm,
3
¼5.8ꢀ104)
of TPTDYE-2. The extended pep delocalization between the donor
(triphenylamine) and acceptor (cyanoacrylic acid) group in
TPTDYE-2 results 37 nm red-shifted absorption maximum com-
pared to that of TPTDYE-1. We expect that the absorption band of
TPTDYE-1 is derived from the localized
other hand the broad absorption band of TPTDYE-2 is expected to
be originated from the overlap of the transition in donor
pep transition. On the
*
pep
*
units and the donoreacceptor intramolecular charge transfer (ICT)
between the donor and the cyanoacrylic acid anchoring moi-
ety.24,28,29 The solvent dependent absorption spectra of dyes
TPTDYE-1 and TPTDYE-2 presented in Fig. 2 might support the
possibility of the overlap of the pep transition in donor units and
*
the donoreacceptor ICT between the donor and the cyanoacrylic
acid anchoring moiety. The UVevisible absorption spectra of dyes
TPTDYE-1 and TPTDYE-2 adsorbed on TiO2 are also presented in
Fig. 1. The absorption maximum of TPTDYE-1 or TPTDYE-2
adsorbed on TiO2 was found to be 463 nm or 507 nm, respectively.
Compared to the spectra in THF solution, the red-shifted absorp-
tion maxima and broad absorption band of the dyes adsorbed on
TiO2 might to be attributed to the formation of J-type
aggregate.28,34e36 Fig. 3 shows the normalized absorption and
photoluminescence (PL) spectra of dyes TPTDYE-1 and TPTDYE-2
in THF solution. The photoluminescence maximum was observed
at 521 nm and 622 nm, respectively, and the optical band gaps
(E0e0) of dyes TPTDYE-1 and TPTDYE-2 were estimated to be
2.65 eV and 2.27 eV, respectively, from the intersection of ab-
sorption and emission spectra in solution. The photo-physical
properties of the dyes TPTDYE-1 and TPTDYE-2 are summarized
in Table 1.
Fig. 2. Comparison of UVevisible absorption spectra of dyes TPTDYE-1 and TPTDYE-2
in methylene chloride (CH2Cl2), THF, and methanol (MeOH) (1.0ꢀ10ꢁ5 M).
Fig. 3. Normalized absorption and photoluminescence spectra of dyes TPTDYE-1 and
TPTDYE-2 in THF.
Table 1
Optical and electrochemical properties of TPTDYE-1 and TPTDYE-2
b
e
f
g
Sensitizer Abs max in
solutiona (nm)
3
Abs max on PL maxd E0e0 Eox ELUMO
TiO2c (nm) (nm)
(eV) (V) (V)
TPTDYE-1 418
TPTDYE-2 455
5.9 ꢀ 104 463
5.8 ꢀ 104 507
521
622
2.65 1.00 ꢁ1.65
2.27 0.83 ꢁ1.44
a
Measurements from dye dissolved in THF (1ꢀ10ꢁ5 M).
b
c
Molar absorptivity (Mꢁ1 cmꢁ1) of the dyes measured in THF (1ꢀ10ꢁ5 M).
Measurements from the dye adsorbed on TiO2.
d
e
Measurements from dye dissolved in THF (1ꢀ10ꢁ5 M).
The optical band gap (E0e0) estimated at the intersection of absorption and
emission spectra in solution.
f
Fig. 1. UVevisible absorption spectra of dyes TPTDYE-1 and TPTDYE-2 in THF solution
(1.0ꢀ10ꢁ5 M) and on TiO2 film (the spectra on TiO2 film are normalized to 6ꢀ104).
The onset oxidation potentials were estimated from cyclic voltammetry analysis.
g
The ELUMO level was calculated using the following equation, ELUMO¼EoxꢁE0e0
.

V. Tamilavan et al. / Tetrahedron 68 (2012) 5890e5897
5893
ensured. In addition, the HOMO levels of the dyes were sufficiently
lower than that of electrolyte (Iꢁ3 /Iꢁ) redox couple (0.4 V), ensuring
a smooth electron flow from the electrolyte to the dyes. Conse-
quently, newly synthesized dyes were expected to show promising
photovoltaic properties as sensitizers in DSSC applications.
2.4. Computational studies
The structure of the dyes was optimized by using B3LYP hybrid
functional and 6-31G basis sets for the better understanding of the
*
correlation between the structure and the photophysical property
as well as the device performance.6e36 The optimized structures of
dyes TPTDYE-1 and TPTDYE-2 are shown in Fig. 5. The geo-
metrically optimized structure of the dyes clearly indicates that the
coplanarity between the donor and acceptor group is greater in
TPTDYE-2 than in TPTDYE-1. In TPTDYE-1, two big nonplanar tri-
phenylamine groups, sterically hindered isopropyl groups in the
phenyl ring on the pyrrole nitrogen and the presence of acceptor
group increase the twist angles between the aryl groups. The twist
angles between the triphenylamine and thiophene were found to be
17.8ꢂ and 19.0ꢂ, respectively, and that between the less hindered
thiophene and pyrrole plane was found ꢁ17.1ꢂ. However, the pres-
ence of acceptor group on the pyrrole ring decreases the coplanarity
quite much, the twist angle being ꢁ38.1ꢂ between the thiophene
and pyrrole in the other side of TPTDYE-1. In contrast, the sterically
hindered isopropyl groups play a vital role in the formation of planar
structure of TPTDYE-2. The isopropyl groups prevent the bond ro-
tation between the thiophene and pyrrole units and, consequently,
enhance the coplanarity between the donor and acceptor group in
TPTDYE-2. The twist angle between the triphenylamine and thio-
phene was calculated to be 16.2ꢂ, and those between the other ar-
omatic groups were found to be less than 1.6ꢂ. Consequently, in
TPTDYE-2, all aromatic groups and the acceptor group are laid al-
most in same plane except for the triphenylamino group. The
electronic distributions in the HOMO and LUMO of the dyes are
displayed in Fig. 6. At the ground state (HOMO) of the dyes, electron
density is homogeneously distributed on the electron donor groups
Fig. 4. Cyclic voltammograms of dyes TPTDYE-1 and TPTDYE-2, platinum working
electrode in 0.1 M Bu4NBF4/acetonitrile at 100 mV/s, potential versus Ag/AgCl.
2.3. Electrochemical properties
Fig. 4 shows the cyclic voltammetry (CV) diagrams of dyes
TPTDYE-1 and TPTDYE-2. The onset oxidation potential (Eox),
which reflects the highest occupied molecular orbital (HOMO) of
the dye, was calculated to be 1.00 V and 0.83 V, respectively, and
the lowest unoccupied molecular orbital (LUMO) levels estimated
from the values of Eox and E0e0 were ꢁ1.65 V and ꢁ1.44 V, re-
spectively. The electrochemical properties of dyes TPTDYE-1 and
TPTDYE-2 are included in Table 1. In order to utilize the synthesized
dyes as sensitizers in DSSCs, it is necessary to ensure their energy
levels are suitable. For the efficient electron injection from dye to
TiO2, the conduction band energy level of the dye should be posi-
tioned above the conduction band of TiO2.35 According to the lit-
erature the conduction band of TiO2 is positioned at ꢁ0.5 V, which
is located 1.15 V and 0.94 V, respectively, lower than the LUMO level
of TPTDYE dyes. Consequently, the possibility of excited electron
injection from the LUMO of the dye to the LUMO of TiO2 can be
Fig. 5. Optimized structures of dyes TPTDYE-1 and TPTDYE-2.

5894
V. Tamilavan et al. / Tetrahedron 68 (2012) 5890e5897
Fig. 6. The frontier orbital plots of the HOMO and LUMO of dyes TPTDYE-1 and TPTDYE-2.
Table 2
such as triphenylamine and N-aryl TPT. At the excited state (LUMO)
of the dyes, the electron density is distributed on the electron ac-
ceptor group (cyanoacrylic acid). The electronic distribution over
the N-aryl TPT unit in both HOMO and LUMO suggests that an ef-
fective intramolecular charge transfer occurs from the donor to
acceptor through N-aryl TPT unit. The frontier molecular orbital of
the dyes reveals that HOMOeLUMO excitation moves the electron
density distribution from donor moiety to cyanoacrylic acid.
Photovoltaic properties of TPTDYE-1 and TPTDYE-2
Jsc (mA/cm2)
FFc (%)
h
d (%)
IPCEe (%)
a
b
Sensitizer
Voc (V)
TPTDYE-1
TPTDYE-2
0.76
0.72
11.11
13.50
0.71
0.69
6.00
6.71
75
75
a
Open-circuit voltage.
Short-circuit current density.
Fill factor.
Power conversion efficiency.
Incident photon-to-collected electron efficiency.
b
c
d
e
2.5. Photovoltaic performance of DSSCs
the other hand, TPTDYE-2-sensitized cell showed enhanced pho-
tocurrent and energy conversion efficiency than TPTDYE-1-sensi-
tized cell. The photocurrent is mainly correlated with sunlight
absorption ability of sensitizer. The optical studies indicate that both
dyes TPTDYE-1 and TPTDYE-2 have similar absorption coefficient,
but the broader absorption ability of TPTDYE-2 enhances the light
harvesting, and consequently offered higher Jsc in their DSSCs
studies. The relatively higher Jsc value of the TPTDYE-2-based DSSC
is expected to induce higher performance up to 6.71%.
Fig. 7 represents the currentedensity/voltage (JeV) character-
istics of the DSSCs obtained with the two dyes, and the detailed
photovoltaic performance parameters (Jsc, Voc, FF, and h) are listed in
Table 2. The photovoltaic performances of the DSSCs were measured
at 100 mW/cm2 under simulated AM 1.5 G solar light conditions.
The TPTDYE-1-sensitized cell gave a short-circuit photocurrent
density (Jsc) of 11.11 mA/cm2, an open-circuit voltage (Voc) of 0.76 V,
and a fill factor (FF) of 0.71, corresponding to an overall conversion
efficiency (
sensitized cell gave a Jsc value of 13.50 mA/cm2, a Voc of 0.72 V, and
an FF of 0.69, corresponding to a value of 6.71%. The Voc and FF of
h) of 6.00%. Under the same conditions, the TPTDYE-2-
Fig. 8 shows the action spectra of monochromatic IPCEs (In-
cident Photon-to-electron Conversion Efficiencies) for the sand-
wiched DSSCs. The IPCE value of TPTDYE-1-sensitized cell exceeds
h
the TPTDYE-sensitized cells were found to be almost identical. On
Fig. 7. JeV Characteristics of DSSCs made from TPTDYE-1 and TPTDYE-2 under AM 1.5
irradiation (100 mW/cm2).
Fig. 8. IPCE characteristics for DSSCs made from TPTDYE-1 and TPTDYE-2.

V. Tamilavan et al. / Tetrahedron 68 (2012) 5890e5897
5895
60% in a spectral range from 390 nm to 560 nm and reaches its
maximum of 75% at 490 nm, and the DSSC made from TPTDYE-2
gave the maximum IPCE value of 75% at 470 nm with over 60% in
a range from 370 nm to 590 nm. The IPCE maximum values of the
TPTDYE-sensitized cells were identical, though the 60% energy
conversion region was found to be quite broad for TPTDYE-2-sen-
sitized cell. The broader IPCE spectrum of TPTDYE-2-sensitized cell
evidently supports its higher energy conversion efficiency com-
pared to that of TPTDYE-1-sensitized cell.
water and ethanol. Then, the plates were immersed in 40 mM TiCl4
(aqueous) at 70 ꢂC for 30 min and washed with water and ethanol.
A transparent nanocrystalline layer was prepared on the FTO glass
plates by using a doctor blade printing TiO2 paste (Solaronix, Ti-
Nanoxide T/SP), which was then dried for 2 h at 25 ꢂC. The TiO2
electrodes were gradually heated under an air flow at 325 ꢂC for
5 min, at 375 ꢂC for 5 min, at 450 ꢂC for 15 min, and at 500 ꢂC for
15 min. The thickness of the transparent layer was measured by
using an Alpha-step 250 surface profilometer (Tencor Instruments,
San Jose, CA). A paste containing 400 nm sized anatase particles
(CCIC. PST-400C) was deposited by means of doctor blade printing
to obtain the scattering layer, and then dried for 2 h at 25 ꢂC. The
TiO2 electrodes were gradually heated under an air flow at 325 ꢂC
for 5 min, at 375 ꢂC for 5 min, at 450 ꢂC for 15 min, and at 500 ꢂC for
3. Conclusions
Two new metal-free organic dyes incorporating 1-(2,6-
diisopropylphenyl)-2,5-di(2-thienyl)pyrrole were synthesized and
used as sensitizers in DSSCs. The dye (TPTDYE-1) containing three
donor groups around the acceptor group showed narrow absorp-
tion band in the range from 300 nm to 470 nm. The maximum
electrical energy conversion efficiency of the DSSC based on
TPTDYE-1 was 6.00% with a Jsc of 11.11 mA/cm2, a Voc of 0.76 V, and
15 min. The resulting film was composed of a 6
mm thick trans-
parent layer and a 4 m thick scattering layer. The TiO2 electrodes
m
were treated again with TiCl4 at 70 ꢂC for 30 min and sintered at
500 ꢂC for 30 min. Then, they were immersed into the dye
(TPTDYE-1 or TPTDYE-2) solution and kept at room temperature
for 12 h. FTO plates for the counter electrodes were cleaned in an
ultrasonic bath in H2O, acetone, and 0.1 M aqueous HCl, sub-
sequently. The counter electrodes were prepared by placing a drop
of an H2PtCl6 solution (2 mg Pt in 1 mL ethanol) on an FTO plate and
heating it (at 400 ꢂC) for 15 min. The dye adsorbed TiO2 electrodes
and the Pt counter electrodes were assembled into a sealed
sandwich-type cell by heating at 80 ꢂC using a hot-melt ionomer
film (Surlyn) as a spacer between the electrodes. A drop of the
electrolyte solution was placed in the drilled hole of the counter
electrode and was driven into the cell via vacuum backfilling. Fi-
nally, the hole was sealed using additional Surlyn and a cover glass
(0.1 mm thickness). The electrolyte was then introduced into the
cell, which is composed of 0.6 M 3-propyl-1,2-dimethyl imidazo-
lium iodide, 0.05 M iodine, 0.1 M LiI and 0.5 M 4-tert-butylpyridine
in acetonitrile.
an FF of 0.71. The dye (TPTDYE-2) having the extended
pep de-
localization between the donor and acceptor group showed quite
broader absorption band from 300 nm to 550 nm. The overall
photovoltaic performance of the DSSC based on TPTDYE-2 was
6.71% with a Jsc of 13.50 mA/cm2, Voc of 0.72 V, and an FF of 0.69.
The lower photovoltaic performance of the DSSC based on TPTDYE-
1 compared to that of the DSSC based on TPTDYE-2 might be at-
tributed to the decreased light harvesting ability of the dye. From
this study, we conclude that the effective
p-conjugation between
the donor and acceptor group of the dye is more important than the
electron donating strength from the donor to the acceptor group to
enhance the photovoltaic performances of DSSCs.
4. Experimental section
4.1. Materials and instruments
4.3. Characterization of DSSCs
All reagents were commercially available from Aldrich or TCI
chemicals and used without further purification. Solvents purified
by normal procedure were handled in a moisture-free atmosphere.
Flash column chromatography was performed on silica gel (Merck
Kieselgel 60, 70e230 mesh). 1H and 13C NMR spectra were recorded
using a 300-MHz Varian Mercury Plus spectrometer in deuterated
chloroform. The absorption spectra of dyes were recorded at 25 ꢂC
in THF solution using a JASCO V-570 spectrophotometer, and the
absorption spectra of dyes adsorbed on a TiO2 film were measured
using Cary 5 spectrophotometer. The photoluminescence spectra
were recorded using a Hitachi F-4500 fluorescence spectropho-
tometer at 25 ꢂC in THF solution. The cyclic voltammograms of dyes
TPTDYE-1 and TPTDYE-2 were recorded by using a CH Instruments
Electrochemical Analyzer. Cyclic voltammetry (CV) experiments
were conducted on a drop-cast dye film on a platinum working
electrode in acetonitrile (ACN) containing 0.1 M tetrabutylammo-
nium tetrafluoroborate (TBATFB) as the supporting electrolyte at
room temperature and ambient atmosphere at a scan rate of
100 mV/s with the use of Ag/AgCl and platinum wire as the refer-
ence and counter electrode, respectively. The CV instrument cali-
bration was performed by using the ferrocene/ferrocenium (FOC)
redox couple as an internal standard before and after the analysis.
Infrared spectra were obtained on a Nicolet 380 FTIR spectropho-
tometer with samples prepared as KBr pellets.
The cell performance was measured using 1000 W xenon light
source, whose power of an AM 1.5 Oriel solar simulator was cali-
brated by using KG5 filtered Si reference solar cell. The incident
photon-to-current conversion efficiency (IPCE) spectra for the cells
were measured on an IPCE measuring system (PV Measurements).
4.4. Synthesis of dyes
The new organic dyes, TPTDYE-1 and TPTDYE-2, were synthe-
sized as outlined in Scheme 1. Compounds 1 and 2 were synthe-
sized according to the known procedure reported from our group.30
The detailed synthetic procedure and characterization of the in-
termediates and final dyes are given below.
4.4.1. 2,5-Bis(5-bromothiophen-2-yl)-1-(2,6-diisopropylphenyl)-1H-
pyrrole-3-carbaldehyde (3). N,N-Dimethylformamide (DMF, 0.16 mL,
2.0 mmol) was added to a cooled solution (0 ꢂC) of POCl3 (0.20 mL,
2.0 mmol) in dry CH2Cl2 (10 mL). The solution was refluxed for
30 min, cooled in an ice bath. To the cooled solution was added 2,5-
bis(5-bromothiophen-2-yl)-1-(2,6-diisopropylphenyl)-1H-pyrrole
(2) (1.0 g, 1.82 mmol). The reaction mixture was refluxed with
stirring for 2 h, cooled, and then, the solution of MeCOONa$2H2O
(0.2 g, 3.0 mmol) in water (10 mL) was added. The emulsion
obtained was refluxed for 1 h, and then cooled. The organic layer
was separated, and the aqueous layer was extracted twice with
CH2Cl2. The combined organic solution was washed with a small
amount of water, dried over anhydrous NaSO4, filtered, evaporated
on a rotary evaporator to dryness, and the residue was then tritu-
rated in a small volume of hexane. The precipitated crystals of
4.2. Fabrication of dye-sensitized solar cell
Fluorine-doped tin oxide (FTO) glass plates (Pilkington TEC
Glass-TEC 8, Solar 2.3 mm thickness) were cleaned in a detergent
solution using an ultrasonic bath for 30 min and then rinsed with

5896
V. Tamilavan et al. / Tetrahedron 68 (2012) 5890e5897
product 3 were filtered off and dried. Yield 0.8 g (75%). Mp
175e176 ꢂC; 1H NMR (300 MHz, CDCl3):
(ppm) 9.97 (s, 1H), 7.56 (t,
material, dibrominated product, and expected mono-brominated
product were separated using column chromatography (silica gel,
hexane) to afford compound 5 as a sticky mass. Yield is 1.3 g (40%).
d
1H), 7.26 (d, 2H), 7.12 (s, 1H), 6.93 (d, 1H), 6.75 (d, 1H), 6.71 (d, 1H),
6.22 (d, 1H), 2.30e2.44 (m, 2H), 0.92 (d, 6H), 0.90 (d, 6H); 13C NMR
1H NMR (300 MHz, CDCl3):
d (ppm) 7.57 (t, 1H), 7.30 (d, 2H), 6.99
(75 MHz, CDCl3):
d
(ppm) 186.1, 147.8, 135.9, 135.0, 132.0, 131.6,
(dd, 1H), 6.78 (dd, 1H), 6.72 (d, 1H), 6.64 (dd, 2H), 6.42 (dd, 1H), 6.15
131.3, 131.2, 131.0, 130.2, 130.0,125.3, 125.2, 125.0,115.6,112.2, 107.8,
28.6, 24.1, 24.0; IR (KBr, cmꢁ1): 3107, 3079, 2962, 2926, 2867, 2837,
1658, 1444, 1414; HRMS (EIþ, m/z) [Mþ] calcd for C25H23Br2NOS2
574.9588, found 574.9583.
(d, 1H), 2.42e2.58 (m, 2H), 0.95 (d, 6H), 0.91 (d, 6H); 13C NMR
(75 MHz, CDCl3): d (ppm) 148.2,137.1,135.1,134.1,130.8,130.7,130.0,
129.4, 127.2, 125.0, 123.6, 123.2, 122.9, 109.9, 109.5, 109.4, 28.5, 24.0,
23.9; IR (KBr, cmꢁ1): 3102, 3069, 2962, 2927, 2866,1468; HRMS (EIþ,
m/z) [Mþ] calcd for C24H24BrNS2 469.0534, found 469.0530.
4.4.2. 2,5-Bis(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-1-(2,6-
diisopropylphenyl)-1H-pyrrole-3-carbaldehyde (4). The solution of
compound 3 (0.38 g, 0.66 mmol) and 4-(diphenylamino)phenyl-
boronic acid (0.4 g, 1.4 mmol) in 30 mL THF was purged well with
nitrogen for 45 min and then Pd(PPh3)4 (2 mol %) and aqueous 2 M
K2CO3 solution (7 mL) was added to the solution. The whole mix-
ture was refluxed with vigorous stirring for 48 h under nitrogen.
The solvent was concentrated by using a rotary evaporator and the
residue was dissolved in CH2Cl2. The organic solution was washed
well with water and brine solution and then, dried over anhydrous
Na2SO4. The solvent was concentrated and the residue was purified
by column chromatography (silica gel, hexane/ethyl acetate¼80:20,
v/v) to afford compound 4 as a pure product. Yield is 0.45 g (75%).
4.4.5. 4-(5-(1-(2,6-Diisopropylphenyl)-5-(thiophen-2-yl)-1H-pyrrol-
2-yl)thiophen-2-yl)benzaldehyde (6). The solution of compound 5
(1.2 g, 2.55 mmol) and 4-formylphenylboronic acid (0.40 g,
2.65 mmol) in 30 mL THF was purged well with nitrogen for 45 min
and then Pd(PPh3)4 (2 mol %) and aqueous 2 M K2CO3 solution
(7 mL) were added to the solution. The whole mixture was refluxed
with vigorous stirring for 48 h under nitrogen. The solvent was
concentrated by using a rotary evaporator and the residue was
dissolved in CH2Cl2. The organic solution was washed well with
water and brine solution, and then dried over anhydrous Na2SO4.
The solvent was concentrated and the residue was purified by col-
umn chromatography (silica gel, hexane/ethyl acetate¼95:5, v/v) to
afford compound 6. Yield is 1.0 g (79%). Mp 157e158 ꢂC; 1H NMR
Mp 244e245 ꢂC; 1H NMR (300 MHz, CDCl3):
d (ppm) 10.11 (s, 1H),
7.53 (t, 1H), 7.21e7.33 (m, 16H), 7.00e7.11 (m, 16H), 6.90 (d, 1H),
(300 MHz, CDCl3): d (ppm) 9.94 (s,1H), 7.81 (d, 2H), 7.58 (d, 3H), 7.31
(d, 3H), 7.13 (d, 1H), 7.00 (dd, 1H), 6.78 (dd, 1H), 6.72 (dd, 2H), 6.45
6.83 (d, 1H), 6.29 (d, 1H), 2.42e2.60 (m, 2H), 0.94 (dd, 12H); 13C
NMR (75 MHz, CDCl3):
d
(ppm) 186.6, 148.1, 147.9, 147.6, 147.5, 147.2,
(dd, 1H), 6.28 (d, 1H), 2.42e2.62 (m, 2H), 0.94 (d, 6H), 0.91 (d, 6H);
143.5, 137.1, 133.0, 132.1, 131.9, 131.5, 131.1, 129.6, 129.5, 128.0, 128.0,
127.4, 126.7, 126.5, 125.7, 125.0, 124.9, 124.8, 123.7, 123.6, 123.4,
123.4, 122.4, 122.2, 107.4, 28.7, 24.2, 24.0; IR (KBr, cmꢁ1): 3064,
3032, 2961, 2928, 2925, 2867, 2839,1661,1591,1490; HRMS (EIþ, m/
z) [Mþ] calcd for C61H51N3OS2 905.3474, found 905.3476.
13C NMR (75 MHz, CDCl3):
d (ppm) 191.5, 148.2, 140.2, 139.3, 137.3,
135.0, 134.9, 134.3, 131.2, 130.8, 130.6, 129.6, 127.2, 125.5, 125.5,
125.0,123.7,123.4,110.2,109.6, 28.6, 24.0, 23.8; IR (KBr, cmꢁ1): 3068,
2959, 2924, 2864, 2818, 2732,1694,1600,1561,1439; HRMS (EIþ, m/
z) [Mþ] calcd for C31H29NOS2 495.1691, found 495.1694.
4.4.3. (E)-3-(2,5-Bis(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-1-
(2,6-diisopropylphenyl)-1H-pyrrol-3-yl)-2-cyanoacrylic acid (TPTDYE-1).
Compound 4 (0.36 g, 0.4 mmol) was dissolved in the mixed solvent
of CHCl3 and acetonitrile (30 mL, 1:1, v/v) and then piperidine
(2 mmol) and cyanoacetic acid (2 mmol) were added at room
temperature. The solution was slowly heated to reflux temperature.
After 18 h, the solution was cooled to room temperature and con-
centrated by using a rotary evaporator. The residue was dissolved
into 10 mL of CH2Cl2 and poured into 100 mL water. To this stirred
mixture was added 2 N HCl solution until the reaction mixture
became acidic. The aqueous layer was extracted with CH2Cl2 and
the organic layer was washed with water and brine solution, dried
over anhydrous NaSO4, filtered, evaporated on a rotary evaporator
to dryness, and the residue was purified by column chromatogra-
phy (silica gel, CH2Cl2/CH3OH¼90:10, v/v) to afford TPTDYE-1. Yield
is 0.33 g (85%). Mp 269e270 ꢂC; 1H NMR (300 MHz, DMSO-d6):
4.4.6. 4-(5-(5-(5-Bromothiophen-2-yl)-1-(2,6-diisopropylphenyl)-
1H-pyrrol-2-yl)thiophen-2-yl)benzaldehyde (7). Compound 6 (0.75 g,
1.51 mmol) was dissolved in 10 mL of DMF. The solution was cooled
with ice bath and then NBS (0.27 g, 1.52 mmol) in 5 mL of DMF was
added drop by drop to the stirred solution at 0 ꢂC. The solution was
stirred for 6 h. The solvent was concentrated by using a rotary
evaporator and the solid was dissolved in 50 mL of CH2Cl2. The
organic solution was washed with water and brine solution, and
then dried over anhydrous Na2SO4. The solvent was concentrated
on a rotary evaporator and dried under vacuum to afford compound
7. Yield is 0.83 g (95%). Mp 149e150 ꢂC; 1H NMR (300 MHz, CDCl3):
d
(ppm) 9.95 (s, 1H), 7.81 (dd, 2H), 7.54e7.62 (m, 3H), 7.32 (d, 2H),
7.21 (t, 1H), 7.13 (d, 1H), 6.86 (dd, 1H), 6.63e6.74 (m, 3H), 6.19e6.29
(m, 1H), 2.42e2.62 (m, 2H), 0.95 (m, 12H); 13C NMR (75 MHz,
CDCl3):
d (ppm) 191.5, 148.1, 140.1, 139.6, 136.9, 136.6, 134.9, 133.9,
131.1, 130.6, 130.2, 130.0, 125.8, 125.5, 125.2, 123.9, 123.2, 110.3,
110.2,109.7, 28.6, 24.0, 23.9; IR (KBr, cmꢁ1): 3069, 2960, 2926, 2866,
2814, 2726, 1697, 1599, 1562, 1469, 1439; HRMS (EIþ, m/z) [Mþ]
calcd for C31H28BrNOS2 573.0796, found 573.0798.
d
(ppm) 8.02 (s, 1H), 7.66 (t, 1H), 7.25e7.40 (m, 16H), 7.12 (d, 1H),
6.60e7.08 (m, 11H), 6.70 (d, 1H), 6.18 (d, 1H), 2.28e2.42 (m, 2H),
0.86 (t, 12H). IR (KBr, cmꢁ1): 3433, 3061, 3029, 2962, 2927, 2866,
2210, 1590, 1491; HRMS (EIþ, m/z) [Mþ] calcd for C64H52N4O2S2
972.3532, found 972.3536.
4.4.7. 4-(5-(5-(5-(4-(Diphenylamino)phenyl)thiophen-2-yl)-1-(2,6-
diisopropylphenyl)-1H-pyrrol-2-yl)thiophen-2-yl)benzaldehyde
(8). The solution of compound 7 (0.7 g, 1.22 mmol) and 4-(diphe-
nylamino)phenylboronic acid (0.38 g, 1.32 mmol) in 30 mL THF was
purged well with nitrogen for 45 min, and then Pd(PPh3)4 (2 mol %)
and aqueous 2 M K2CO3 solution (7 mL) were added to the solution.
The whole mixture was refluxed with vigorous stirring for 48 h
under nitrogen. The solvent was concentrated by using a rotary
evaporator and the residue was dissolved in CH2Cl2. The organic
solution was washed with water and brine solution, and then dried
over anhydrous Na2SO4. The solvent was concentrated and the
residue was purified by column chromatography (silica gel, hexane/
ethyl acetate¼80:20, v/v) to afford compound 8. Yield is 0.7 g (78%).
4.4.4. 2-(5-Bromothiophen-2-yl)-1-(2,6-diisopropylphenyl)-5-(thio-
phen-2-yl)-1H-pyrrole (5). 1-(2,6-Diisopropylphenyl)-2,5-di(thio-
phen-2-yl)-1H-pyrrole (1) (2.69 g, 6.87 mmol) was dissolved in
10 mL of DMF. The solution was cooled with ice bath and then N-
bromosuccinimide (NBS, 1.23 g, 6.87 mmol) in 10 mL of DMF was
added drop by drop to the stirred solution at 0 ꢂC. The solution was
stirred for 6 h. The solvent was concentrated by using a rotary
evaporator and the residue was dissolved in 50 mL of CH2Cl2. The
organic solution was washed with water and brine solution, and
then dried over anhydrous Na2SO4. The organic solvent was evap-
orated by using a rotary evaporator and the unreacted starting

V. Tamilavan et al. / Tetrahedron 68 (2012) 5890e5897
4. Gratzel, M. J. Photochem. Photobiol., A 2004, 164, 3.
d (ppm) 9.95 (s, 1H),
5897
Mp 193e194 ꢂC; 1H NMR (300 MHz, CDCl3):
5. Yu, Q.; Wang, Y.; Yi, Z.; Zu, N.; Zhang, J.; Zhang, M.; Wang, P. ACS Nano 2010, 4,
6032.
7.81 (d, 2H), 7.54e7.62 (m, 3H), 7.23e7.34 (m, 10H), 7.00e7.14 (m,
7H), 6.89 (d, 1H), 6.73 (dd, 2H), 6.30 (d, 1H), 6.28 (d, 1H), 2.48e2.62
(m, 2H), 0.98 (d, 6H), 0.96 (d, 6H); 13C NMR (75 MHz, CDCl3):
6. Zeng, W.; Cao, Y.; Bai, Y.; Wang, Y.; Shi, Y.; Zhang, M.; Wang, F.; Pan, C.; Wang, P.
Chem. Mater. 2010, 22, 1915.
7. Ito, S.; Miura, H.; Uchida, S.; Takata, M.; Sumioka, K.; Liska, P.; Comte, P.; Pechy,
d
(ppm) 191.5, 148.2, 147.7, 147.3, 141.9, 140.2, 139.3, 137.2, 134.9,
P.; Gratzel, M. Chem. Commun. 2008, 5194.
134.4, 133.5, 131.2, 130.9, 130.6, 129.7, 129.5, 128.5, 126.4, 125.5,
125.1, 124.7, 124.0, 123.9, 123.7, 123.3, 123.1, 122.5, 110.3, 109.6, 28.6,
24.0; IR (KBr, cmꢁ1): 3064, 3031, 2962, 2927, 2867, 2839, 2729,
1698,1598,1491,1445; HRMS (EIþ, m/z) [Mþ] calcd for C49H42N2OS2
738.2739, found 738.2742.
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2009, 2198.
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zeeruddin, M. K.; Pechy, P.; Takata, M.; Miura, H.; Uchida, S.; Gratzel, M. Adv.
Mater. 2006, 18, 1202.
11. Hara, K.; Danoh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Ara-
4.4.8. (E)-3-(4-(5-(5-(5-(4-(Diphenylamino)phenyl)thiophen-2-yl)-
1-(2,6-diisopropylphenyl)-1H-pyrrol-2-yl)thiophen-2-yl)phenyl)-2-
cyanoacrylic acid (TPTDYE-2). Compound 8 (0.59 g, 0.8 mmol) was
dissolved in a mixed solvent of CHCl3 and acetonitrile (30 mL,1:1, v/
v), and then piperidine (4 mmol) and cyanoacetic acid (4 mmol)
were added at room temperature. The solution was slowly heated
to reflux temperature. After 18 h, the solution was cooled to room
temperature and concentrated by using a rotary evaporator. The
residue was dissolved into 10 mL of CH2Cl2 and the resulting so-
lution was poured into 100 mL water. To this stirred mixture was
added 2 N HCl solution until the reaction mixture became acidic.
The aqueous layer was extracted with CH2Cl2 and the organic layer
was washed with water and brine solution, and then dried over
anhydrous NaSO4, filtered, evaporated on a rotary evaporator to
dryness. The residue was purified by column chromatography
(silica gel, CH2Cl2/CH3OH¼90:10, v/v) to give TPTDYE-2. Yield:
0.57 g (88%). Mp 289e290 ꢂC; 1H NMR (300 MHz, DMSO-d6):
kawa, H. Langmuir 2004, 20, 4205.
12. Lan, C.-H.; Wu, H.-P.; Pan, T.-Y.; Chang, C.-W.; Chao, W.-S.; Chen, C.-T.; Wang,
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2, 149.
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T.; Furube, A.; Hara, K. Chem. Mater. 2008, 20, 3993.
18. Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; DeAngelis, F.;
DiCenso, D.; Nazeeruddin, M. K.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 16701.
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Soc. 2008, 130, 6259.
d
(ppm) 7.82 (d, 4H), 7.66 (t, 1H), 7.25e7.54 (m, 12H), 6.89e7.11 (m,
24. Li, G.; Jiang, K.-J.; Li, Y.-F.; Li, S.-L.; Yang, L.-M. J. Phys. Chem. C 2008, 112, 11591.
25. Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126,
12218.
26. Kim, C.; Choi, H.; Kim, S.; Baik, C.; Song, K.; Kang, M.-S.; Kang, S. O.; Ko, J. J. Org.
Chem. 2008, 73, 7072.
27. Bonhote, P.; Moser, J.-E.; Humphry-Baker, R.; Vlachopoulos, N.; Zakeeruddin, S.
M.; Walder, L.; Gratzel, M. J. Am. Chem. Soc. 1999, 121, 1324.
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2008, 112, 12557.
8H), 6.80 (d, 2H), 6.49 (d, 1H), 6.41 (d, 1H), 2.34e2.44 (m, 2H), 0.90
(d, 12H). IR (KBr, cmꢁ1): 3438, 3062, 3030, 2960, 2925, 2866, 2214,
1627, 1591, 1491; HRMS (EIþ, m/z) [Mþ] calcd for C52H43N3O2S2
805.2797, found 805.2785.
Acknowledgements
29. Li, Q.; Lu, L.; Zhong, C.; Huang, J.; Huang, Q.; Shi, J.; Jin, X.; Peng, T.; Qin, J.; Li, Z.
Chem.dEur. J 2009, 15, 9664.
30. Tamilavan, V.; Sakthivel, P.; Li, Y.; Song, M.; Kim, C. H.; Jin, S.-H.; Hyun, M. H.
J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 3169.
31. Tamilavan, V.; Song, M.; Jin, S.-H.; Hyun, M. H. J. Polym. Sci., Part A: Polym. Chem.
2010, 48, 5514.
This research was supported by the New & Renewable Energy
program of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant (No. 20103020010050) funded by the
Ministry of Knowledge Economy, Republic of Korea.
32. Tamilavan, V.; Song, M.; Jin, S.-H.; Hyun, M. H. Polymer 2011, 52, 2384.
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Sugihara, H. J. Phys. Chem. B 2005, 109, 3907.
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