Synthesis and applications of new triphenylamine dyes with donor-donor-(bridge)-acceptor structure for organic dye-sensitized solar cells

Article abstract of DOI:10.1039/c2nj40374a

Four novel asymmetric triphenylamine dyes configured with donor-donor-acceptor (D-D-A) and donor-donor-bridge-acceptor (D-D-π-A) structures were synthesized and applied to organic dye-sensitized solar cells. The terminal methoxy group in the donor part and the furane-bridge unit contributed to the wider absorption region and enhanced electron life time, resulting in a higher photocurrent density (J_sc), open-circuit voltage (V_oc), and overall conversion efficiency (η). Among the synthesized dyes, the SK2-based DSSC showed the highest conversion efficiency of 5.57% (J_sc = 10.41 mA cm^-2, V_oc = 729 mV, and FF = 0.73) caused by the synergetic effect of the methoxy group and the furane-bridge unit.

Full text of DOI:10.1039/c2nj40374a

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PAPER
Synthesis and applications of new triphenylamine dyes with
donor–donor–(bridge)–acceptor structure for organic
dye-sensitized solar cellsw
Chun Sakong,a Hae Joong Kim,a Se Hun Kim,a Jin Woong Namgoong,a
Jong Ho Park,a Jang-Hyun Ryu,a Boeun Kim,b Min Jae Kob and Jae Pil Kim*a
Received (in Montpellier, France) 9th May 2012, Accepted 16th July 2012
DOI: 10.1039/c2nj40374a
Four novel asymmetric triphenylamine dyes configured with donor–donor–acceptor (D–D–A) and
donor–donor–bridge–acceptor (D–D–p–A) structures were synthesized and applied to organic
dye-sensitized solar cells. The terminal methoxy group in the donor part and the furane-bridge
unit contributed to the wider absorption region and enhanced electron life time, resulting in a
higher photocurrent density (Jsc), open-circuit voltage (Voc), and overall conversion efficiency (Z).
Among the synthesized dyes, the SK2-based DSSC showed the highest conversion efficiency of
5.57% (Jsc = 10.41 mA cmÀ2, Voc = 729 mV, and FF = 0.73) caused by the synergetic effect of
the methoxy group and the furane-bridge unit.
p-conjugated bridge units, such as methine, furane and thiophene.
Introduction
Many groups have worked on the development of organic dyes
with D–p–A structure and published results on triphenylamine,
indoline, coumarin, cyanine, perylene and phenothiazine.3,4
Among them, indoline5 and triphenylamine6 based organic dyes
showed good conversion efficiencies above 9%, which were close
to those of the Ru complex dyes.
Dye-Sensitized Solar Cells (DSSCs) as some of the promising
alternatives for the photovoltaic conversion of solar energy have
attracted considerable attention because of their low production
cost, easy preparation and relatively high solar-to-electricity
conversion efficiency.1 Sensitizers play a key role in the DSSCs’
high conversion efficiency and have been studied extensively by
researchers throughout the world. The most typical sensitizers
used in DSSCs are the ruthenium(II) polypyridyl complex series,
which are known as N3 and N719. These Ru-complex dyes have
shown high conversion efficiencies (more than 11%) due to the
broad absorption band and good long-term stability.2 However,
they have the disadvantages of a relatively high production cost
and difficulties in purification.
On the other hand, organic dyes with D–D–A or D–D–p–A
structure are reported by several groups.7 They suggested that
organic dyes based on D–D–p–A structure can achieve the
enhanced photovoltaic performance through the modifications of
molecular structure compared to the organic dyes with simple
D–p–A structure. Furthermore, introducing an additional donor
group into the dye molecule can increase the electron donating
ability of the donor and enhance charge separation and electron
injection, which resulted in high overall conversion efficiency.8
Based on these strategies, we designed and synthesized four novel
triphenylamine-based organic dyes with D–D–A and D–D–p–A
molecular structures. Additional electron donor groups, benzene
and methoxy benzene, were connected by a methine chain
(–CHQCH–) in order to increase the molar extinction coefficient.
And furane and thiophene moieties were used as a p-conjugated
bridge unit for the effective conjugation and red-shifted absorption
spectrum. Photovoltaic cells were fabricated using the synthesized
dyes and their photovoltaic properties were measured and analyzed.
In recent years, a range of organic dyes have been developed as
alternatives to the Ru-complex dye due to the many advantages
such as diversity of the molecular structures, high molar extinction
coefficient, simple synthesis and low cost.3 Most organic dyes have
the push-pull structure of donor–conjugated bridge–acceptor
(D–p–A). Amine moieties are generally used as the electron donor,
and cyanoacrylic acid or a rhodanine moiety is used as the electron
acceptor and anchoring group. These two parts are linked by
a Department of Materials Science and Engineering, Seoul National
University, Daehak-dong, Kwanak-gu, Seoul 151-744, Korea.
E-mail: jaepil@snu.ac.kr; Fax: +82-2-885-1748;
Tel: +82-2-880-7187
b Solar Cell Research Center, Materials Science and Technology
Division, Korea Institute of Science and Technology (KIST), Seoul
136-791, Korea. E-mail: mjko@kist.re.kr; Fax: +82-2-958-6649;
Tel: +82-2-958-5518
Results and discussion
(1) Synthesis of the dyes
The molecular structures and synthetic routes of the triphenyl-
amine dyes with an additional donor group are shown in Fig. 1
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj40374a
c
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Fig. 2 Absorption spectra of the dyes in CH2Cl2 (a) and adsorbed on
TiO2 (b).
Fig. 1 Molecular structures of synthesized dyes.
two major absorption bands, appearing at below 400 nm and
420–500 nm, respectively. The former is due to a localized
p–p* transition and the latter is attributed to the intra-
molecular charge transfer transition (ICT) from the triphenyl-
amine donor containing an additional donor group to the
cyanoacrylic acid acceptor group.9
and Scheme 1, respectively. 4-((4-Bromophenyl)(phenyl)amino)
benzaldehyde as a starting material was synthesized according
to the published procedure.8d The triphenylamine donor was
extended to the donor–donor intermediates 1–3 all in 75 to 85%
yield, using the efficient Horner–Wadsworth–Emmons olefination
methodology with the appropriate phosphonates, diethyl-4-methyl-
benzylphosphonate and diethyl-4-methoxybenzylphosphonate.
The donor–donor intermediates 1 and 2 were formylated via
Vilsmeier–Haack reaction in moderate yield to form intermediates
4 and 5. Subsequently, the carbaldehydes were condensed with
cyanoacetic acid by the Knoevenagel reaction in the presence of
piperidine and converted to SP1 and SK1. For the preparation of
SK2–3, the intermediate 3 was allowed to react with appropriate
boronic acids via Suzuki coupling to form intermediates 6 and 7.
Subsequent reactions of the intermediates with cyanoacetic acid and
piperidine afforded the desired compounds. The structures of all
synthesized intermediates and dyes were characterized by NMR
and mass spectrometry.
A comparative examination of the absorption spectra
between SP1 and SK1 indicates that the maximum absorption
(2) Photophysical properties of the dyes in solution and TiO2 film
The absorption spectra of the synthesized dyes in a 5 Â 10À6 M
dichloromethane solution are shown in Fig. 2(a) and the
corresponding data are collected in Table 1. All dyes exhibited
Fig. 3 Cyclic voltammograms (C–V curves) of the synthesized dyes.
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Scheme 1 Synthetic routes; (a) BuOtK, THF, 0 1C - room temp.; (b) DMF, POCl3, reflux; (c) cyanoacetic acid, piperidine, reflux; (d) Pd(PPh3)4,
K2CO3, THF, 5-formyl-2-furylboronic acid or 5-formyl-2-thienylboronic acid, reflux.
wavelength is unaffected by the introduction of the methoxy
group into the additional donor part. On the other hand,
maximum absorption wavelengths of SK2 and SK3 showed a
considerable red shift of 46 nm by the introduction of
p-conjugated bridge units, furane and thiophene. This is due
to the extension of p-conjugation length by the introduction of
bridge units and the red shifts in the absorption spectra
are desirable for light harvesting. The molar extinction
coefficients, e, of the synthesized dyes ranged from 23 000 to
55 880 MÀ1 cmÀ1 and were relatively higher than those of the
conventional triphenylamine dyes due to methine chains. Such
a red shift in the absorption spectrum and a higher molar
extinction coefficient could increase the amount of light
harvested and enhance the photocurrent generation of DSSCs.
The absorption spectra of the dyes adsorbed on transparent
TiO2 films are shown in Fig. 2(b) and the corresponding data
are collected in Table 1. All the dyes showed broad and blue-
shifted absorption spectra on the TiO2 surface compared to those
in solution. Either a polar interaction of the dyes with TiO2
through the deprotonation of carboxylic acid or H-aggregation
of the dyes has been suggested accountable for such broadening
and blue-shift of the absorption spectrum.10
energy of the dyes estimated from the intersection between the
normalized absorption and emission spectra. The electroche-
mical properties of the synthesized dyes are listed in Table 1
and the HOMO–LUMO level diagram is shown in Fig. 4. The
HOMO levels of all synthesized dyes ranged from 1.15 eV to
0.96 eV (vs. NHE) and became more negative by the introduc-
tion of the methoxy group into the additional donating group
and p-conjugated bridge units. The HOMO levels of the dyes
were positive enough compared to the redox potential of the
electrolyte, ca. 0.4 V (vs. NHE), indicating that the oxidized
dyes could be regenerated effectively by the electrolyte. The
LUMO levels of the dyes ranged from À1.29 eV to À1.43 eV
(vs. NHE) and became more positive by the introduction of
p-conjugated bridge units. The LUMO levels of the dyes were
sufficiently more negative than the conduction band of
TiO2, ca. À0.5 eV (vs. NHE), indicating that the excited
electrons of the dyes could be effectively injected into the
conduction band of TiO2.
(4) Photovoltaic properties of DSSCs based on the synthesized
dyes
DSSCs were fabricated using the synthesized dyes according
to the methods described in Experimental section (3). The
incident monochromatic photon-to-current conversion
efficiencies (IPCEs) of the DSSCs are shown in Fig. 5. The
DSSCs based on the synthesized dyes as sensitizers exhibited a
high maximum IPCE value. The maximum IPCE value of the
DSSC based on SK2 reached 80.3% at 480 nm, and the values
of DSSCs based on SP1, SK1 and SK3 reached 77.4%, 77.7%
(3) Electrochemical properties of the dyes
The first oxidation potential (Eox) corresponding to the
HOMO level of the dye was measured in dichloromethane
by cyclic voltammetry (CV), and the C–V curves of the dyes
are shown in Fig. 3. The LUMO levels of the dyes were
calculated by Eox À E0–0, where E0–0 is the zeroth–zeroth
c
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Table 1 Photophysical and electrochemical properties of the synthe-
sized dyes
Potentials and energy levels
b
c
lmaxa/nm
lmax on lemissiona/ Eox
/
E0–0
V
/
Eox
E0–0c/V
À
Dye (e/MÀ1 cmÀ1) TiO2/nm nm
V
SP1 448 (50 950) 438
SK1 448 (55 880) 438
SK2 494 (40 630) 456
SK3 494 (23 000) 458
548
548
593
598
1.15
1.00
0.97
0.96
2.58
2.54
2.27
2.25
À1.43
À1.54
À1.30
À1.29
a
Absorption and emission spectra were measured in CH2Cl2 solution.
The oxidation potentials (Eox) of the dyes were obtained by cyclic
b
voltammetry in CH2Cl2 with 0.1 M tetrabutylammonium tetrafluoro-
borate (TBATFB) with a scan rate of 100 mV sÀ1 and converted to
c
NHE. The zeroth–zeroth transition E0–0 values were estimated from
the intersection of the normalized absorption and emission spectra
(Fig. S1, ESI).
Fig. 5 Incident photon-to-current conversion efficiencies spectra for
the DSSCs based on the synthesized dyes.
and 78.7%, respectively. These high maximum IPCE values of
the dyes may be tentatively attributed to efficient electron
injection and charge separation due to the D–D–A and
D–D–p–A molecular structures.7,8 The IPCE spectrum of
the DSSC based on SK1 was slightly broadened and red
shifted compared to the spectrum of SP1 due to the introduc-
tion of the methoxy group into the donor part. The IPCE
spectra of the DSSCs based on SK2–3 containing p-conju-
gated bridges remarkably red shifted and the onsets of the
IPCE spectra were extended to 680 nm. Among the dyes, the
onset of the IPCE spectrum of the DSSC based on SK2
was 670 nm, and the best IPCE performance (479%) was
observed in the range from 420 nm to 500 nm. These higher
and broader IPCE spectra of the DSSCs based on SK2–3 are
responsible for the higher Jsc and superior Z.
short-circuit photocurrent density (Jsc), open-circuit voltage
(Voc), fill factor (FF), and conversion efficiency (Z), are listed
in Table 2. The Jsc of the DSSCs based on the synthesized dyes
increased gradually in the order of SP1 o SK1 o SK2 o SK3.
It could be attributed to the wider absorption spectra and the
broadening of IPCE spectra towards a longer wavelength
region by the introduction of the methoxy group into the
donor part and p-conjugated bridge units.
The Voc of the DSSCs based on SP1 and SK1–2 increased in
the order of SP1 o SK1 o SK2 as the methoxy group and
the bridge unit were introduced. However, the Voc of the
SK3-based DSSC is lower than all the other dyes, although
it has both the methoxy group and the bridge unit. To
elucidate the correlation of Voc with the dyes, electrochemical
impedance spectroscopy (EIS) was measured in the dark under
a forward bias of À0.70 V with a frequency range of 0.1 Hz
to 100 kHz. The Nyquist plots and Bode phase plots of
the DSSCs based on SP1 and SK1–3 are shown in Fig. 7.
The corresponding photocurrent density–voltage (J–V)
curves of DSSCs based on the synthesized dyes are shown in
Fig. 6 and the detailed photovoltaic parameters, including
Fig. 4 Energy diagrams of synthesized dyes and the frontier molecular orbitals of the HOMO and LUMO calculated with DFT on a B3LYP/
6-31+G (d,p) level.
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Table 2 Photovoltaic performances of DSSCs based on SP1 and
SK1–3
Dye
Jsc/mA cmÀ2
Voc/mV
FF
Za (%)
SP1
SK1
SK2
SK3
N719
6.85
7.40
10.41
10.67
15.13
696
718
729
689
757
0.74
0.73
0.73
0.73
0.71
3.54
3.89
5.57
5.37
8.10
a
The concentration was maintained at 5 Â 10À4 M in CH2Cl2, with
0.5 M 1-methyl-3-propylimidazolium iodide (PMII), 0.2 M LiI,
0.05 M I2 and 0.5 M 4-tert-butylpyridine (TBP) in ACN-VN
(85 : 15) solution. The efficiencies of DSSCs were an average value
obtained from measurements of three cells with a 0.24 cm2 working
area.
Fig. 6 Photocurrent density–voltage (J–V) curves for the DSSCs
based on the synthesized dyes under AM 1.5G simulated solar light
(100 mW cmÀ2).
Fig. 7 Electrochemical impedance spectra of DSSCs based on the
synthesized dyes measured in the dark under À0.70 V bias; (a) Nyquist
plots, (b) Bode phase plots.
In the Nyquist plot, the large semicircle at the intermediated
frequency is assigned to the charge transfer impedance at the
TiO2–dye–electrolyte interface.11 The charge recombination
resistance at the TiO2 surface, Rrec, can be estimated from
the radius of the middle semicircle in the Nyquist plot and the
larger Rrec means a slower charge recombination rate. As
shown in Fig. 7(a), the Rrec values increased in the order of
SK3 o SP1 o SK1 o SK2. These results are consistent with
the tendency of Voc in the DSSCs based on the synthesized
dyes. The increased Rrec values imply the retardation of charge
recombination between the injected electron into the TiO2 and
oxidized species (I3À) in the electrolyte. Correspondingly, the
frequency of the characteristic frequency peaks in the Bode phase
plots increased in the order of SK3 o SP1 o SK1 o SK2. The
characteristic frequency in the Bode plot is related to the
charge recombination rate, and its reciprocal is associated
with the electron lifetime.12 Therefore, these results suggest
that the methoxy group in the donor part and furane-bridge
unit contributes to the enhanced electron lifetime due toÀa
the terminal methoxy group and the bridge unit (thiophene).
This is consistent with published results for PTZ and TPA-
based dyes,4g,12b,13 but the reasons for the results were not
explained clearly. We postulate that the thiophene-bridge
unit can lead to an increased intermolecular p–p interaction,
which can explain the higher amount of SK3 adsorbed on
TiO2 compared to the other dyes. The amounts of dye
adsorbed increased in the order of SP1 E SK1 o SK2 {
SK3 (SP1, 5.20 Â 10À8; SK1, 5.24 Â 10À8; SK2, 7.81 Â 10À8
;
SK3, 1.12 Â 10À7 mol cmÀ2). This thiophene-bridge unit can
increase the charge recombination due to the higher aggrega-
tion and result in a lower Voc of SK3.8b,14
Among the synthesized dyes, the DSSC based on SK2 exhibited
the best conversion efficiency of 5.57% with Jsc = 10.41 mA cmÀ2
,
Voc = 729 mV, and FF = 0.73 under simulated AM 1.5G
irradiation conditions.
Conclusion
slower recapture of the conduction band electrons by I3
,
leading to a higher Voc. The DSSC based on SK2 consequently
showed the highest Voc, which is presumably caused by the
synergetic effect of the methoxy group and the furane-bridge
unit. On the other hand, the SK3-based DSSC showed a lower
Voc compared to that based on SK2, although SK3 also had
A new series of D–D–A and D–D–p–A structured organic
dyes containing additional donor groups connected by
methine chains and heterocyclic p-conjugated bridge units
were designed and synthesized to improve the Jsc and Voc
of DSSCs. The terminal methoxy group in the donor part and
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1
p-conjugated bridge units led to the higher and broader IPCE
spectra of the DSSCs, resulting in an improved Jsc and Z. An
electrochemical impedance study was conducted to elucidate
the correlation of Voc with the dye structure. The result
indicates that the terminal methoxy group and the furane-
bridge unit contribute to the enhanced electron lifetime due to
the retardation of charge recombination between the injected
electron into the TiO2 and oxidized species in the electrolyte,
leading to a higher Voc. The DSSC based on SK2 showed the
compound 2 (1.43 g, 78%) as a pale yellow solid. H NMR
(CDCl3, 500 MHz), d: 7.36 (d, J = 8.7 Hz, 2H), 7.29 (d, J =
8.6 Hz, 2H), 7.19 (t, J = 7.8 Hz, 4H), 7.04 (d, J = 8.5 Hz, 4H),
6.97–6.93 (m, 4H), 6.87 (d, J = 6.4 Hz, 2H), 6.82 (d, J =
8.7 Hz, 2H), 3.74 (s, 3H).
(E)-4-Bromo-N-(4-(4-methoxystyryl)phenyl)-N-phenylaniline (3).
This compound was synthesized using the method established for
compound 1. The crude product was purified by column
chromatography on silica gel using ethyl acetate : hexane
(1 : 12, v/v) afforded compound 3 (4.2 g, 80%) as a pale yellow
solid. 1H NMR (CDCl3, 500 MHz), d: 7.43 (d, J = 8.6 Hz, 2H),
7.37 (d, J = 8.5 Hz, 2H), 7.34 (d, J = 8.7 Hz, 2H), 7.28 (t, J =
7.8 Hz, 2H), 7.09 (d, J = 7.8 Hz, 2H), 7.06–7.02 (m, 3H),
6.98–6.93 (m, 4H), 6.89 (d, J = 8.5 Hz, 2H), 3.82 (s, 3H).
highest conversion efficiency of 5.57% (Jsc = 10.41 mA cmÀ2
,
V
oc = 729 mV, and FF = 0.73) caused by the synergetic effect
of the methoxy group and the furane-bridge unit.
Experimental section
(1) Materials and general procedures
(E)-4-((4-(4-Methylstyryl)phenyl)(phenyl)amino)benzalde-
hyde (4). POCl3 (0.7 mL, 7.5 mmol) was added dropwise to ice-
cold DMF (3 mL) and the mixture was stirred for 30 min at
0 1C. A solution of 1 (1.08 g, 3 mmol) in dichloroethane
(20 mL) was added and the resulting mixture was stirred for
14 h at 80 1C. The reaction mixture was cooled to room
temperature and poured into ice water. The mixture was
neutralized with 20% NaOH aqueous solution and extracted
with dichloromethane, washed with water and brine, and dried
over anhydrous MgSO4. The solvent was removed by rotary
evaporation and subsequently dried in a vacuum oven. The
crude product was purified by column chromatography on
silica gel using dichloromethane : hexane (1 : 1, v/v) afforded
compound 4 (0.63 g, 54%) as a yellow solid. 1H NMR (CDCl3,
500 MHz), d: 9.84 (s, 1H), 7.71 (d, J = 8.7 Hz, 2H), 7.46 (d,
J = 8.0 Hz, 4H), 7.38 (t, J = 7.9 Hz, 2H), 7.21 (d, J = 7.6 Hz,
3H), 7.16 (d, J = 8.5 Hz, 2H), 7.09–6.94 (m, 4H), 6.92 (d, J =
8.7 Hz, 2H), 2.30 (s, 3H).
4-(Diphenylamino)benzaldehyde, 4-bromo-N,N-diphenylaniline,
diethyl-4-methylbenzylphosphonate, diethyl-4-methoxybenzyl-
phosphonate and 5-formyl-2-furanylboronic acid purchased
from Sigma-Aldrich, 5-formyl-2-thienylboronic acid from TCI
were used without further purification. All chemicals used in this
1
study were of synthesis-grade. H NMR and 13C NMR spectra
were recorded on a Bruker Avance 500 at 500 MHz using CDCl3
and DMSO-d6 as the solvent and TMS as the internal standard.
Mass spectra were measured using a JEOL JMS-600W Agilent
6890 Series. UV-Visible absorption spectra were measured with
an HP 8452A spectrophotometer and emission spectra were
recorded on a QuantaMastert UV-VIS spectrofluorometer.
Cyclic voltammetry (CV) was performed with a three-electrode
electrochemical cell on a Potentostat/Gavanostat Model 273A.
Glassy-carbon, platinum wire, and Ag/Ag+ were employed as
the working, counter, and reference electrodes, respectively.
Tetrabutylammonium tetrafluoroborate (TBATFB) was used
as the supporting electrolyte and ferrocene was added as an
internal reference for calibration.
(E)-4-((4-(4-Methoxystyryl)phenyl)(phenyl)amino)benzalde-
hyde (5). This compound was synthesized using the method
established for compound 4. The crude product was purified
by column chromatography on silica gel using dichloro-
methane : hexane (2 : 1, v/v) afforded compound 5 (0.69 g,
62%) as a yellow solid.
1H NMR (CDCl3, 500 MHz), d: 9.82 (s, 1H), 7.69 (d, J =
8.7 Hz, 2H), 7.45 (d, J = 8.0 Hz, 4H), 7.36 (t, J = 7.9 Hz, 2H),
7.19 (d, J = 7.6 Hz, 3H), 7.13 (d, J = 8.5 Hz, 2H), 7.06–6.92
(m, 4H), 6.91 (d, J = 8.7 Hz, 2H), 3.83 (s, 3H).
(2) Syntheses
(E)-4-(4-Methylstyryl)-N,N-diphenylaniline (1). A solution
of potassium tert-butoxide (0.73 g, 6.5 mmol) in THF (5 mL)
was slowly added to a mixture of 4-(diphenylamino)benzalde-
hyde (1.37 g, 5 mmol), diethyl-4-methylbenzylphosphonate
(1.33 g, 5.5 mmol) and THF (60 mL) for 10 min at 0 1C. The
reaction mixture was allowed to warm to room temperature and
stirred for 8 h. The mixture was quenched into water (50 mL)
and then extracted with ethyl acetate. The organic layer was
washed with water and brine, dried over anhydrous MgSO4.
The solvent was removed by rotary evaporation and subsequently
dried in a vacuum oven. The crude product was purified by
column chromatography on silica gel using ethyl acetate : hexane
(1 : 15, v/v) afforded compound 1 (1.48 g, 82%) as a pale yellow
solid. 1H NMR (CDCl3, 500 MHz), d: 7.35 (d, J = 8.7 Hz, 2H),
7.28 (d, J = 8.6 Hz, 2H), 7.17 (t, J = 7.8 Hz, 4H), 7.01 (d, J =
8.5 Hz, 4H), 6.98–6.92 (m, 4H), 6.85 (d, J = 6.4 Hz, 2H), 6.81
(d, J = 8.7 Hz, 2H), 2.29 (s, 3H).
(E)-2-Cyano-3-(4-((4-(E)-4-methylstyryl)phenyl)(phenyl)amino)-
phenyl) acrylic acid (SP1). A mixture of 4 (0.39 g, 1 mmol),
cyanoacetic acid (0.21 g, 2.5 mmol) and piperidine (0.4 mL,
4 mmol) was dissolved in acetonitrile (80 mL) and refluxed for
6 h at 92 1C under nitrogen. The reaction mixture was cooled
to room temperature and the solvent was removed under
reduced pressure. The crude product was purified by column
chromatography on silica gel using dichloromethane : methanol
(12 : 1, v/v). The product was dissolved in dichloromethane
(150 mL) and then 0.1 M HCl aqueous solution (100 mL) was
added to this solution. The solution was vigorously stirred
for 2 h and the organic layer was dried over anhydrous
MgSO4. The solvent was removed by rotary evaporation and
subsequently dried in a vacuum oven afforded SP1 (0.35 g, 78%)
(E)-4-(4-Methoxystyryl)-N,N-diphenylaniline (2). This compound
was synthesized using the method established for compound 1.
The crude product was purified by column chromatography
on silica gel using ethyl acetate : hexane (1 : 10, v/v) afforded
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1
as an orange solid. H NMR (DMSO-d6, 500 MHz), d: 8.15
on silica gel using dichloromethane : methanol (10 : 1, v/v)
(s, 1H), 7.94 (d, J = 9.0 Hz, 2H), 7.62 (d, J = 8.5 Hz, 2H), 7.49
(d, J = 8.0 Hz, 2H), 7.45–7.42 (m, 2H), 7.27 (t, J = 7.5 Hz,
1H), 7.22–7.16 (m, 8H), 6.93 (d, J = 9.0 Hz, 2H), 2.31 (s, 3H).
13C NMR (DMSO-d6, 500 MHz), d: 163.99, 153.15, 151.50,
145.09, 144.32, 136.98, 134.32, 134.21, 132.79, 130.00, 129.24,
128.28, 127.76, 126.49, 126.33, 126.05, 125.65, 123.22, 118.74,
116.88, 98.16, 20.76. HR-MS: m/z (%) calcd for C31H25N2O2:
457.1916; found 457.1915 [FAB+].
afforded SK2 (0.26 g, 78%) as a red solid. 1H NMR
(DMSO-d6, 500 MHz), d: 8.02 (s, 1H), 7.81 (d, J = 8.5 Hz,
2H), 7.55–7.51 (m, 5H), 7.39 (t, J = 7.8 Hz, 2H), 7.22–7.01
(m, 10H), 6.95 (d, J = 8.5 Hz, 2H), 3.77 (s, 3H). 13C NMR
(DMSO-d6, 500 MHz), d: 163.96, 159.07, 158.84, 148.50,
147.06, 146.04, 145.13, 137.35, 133.23, 129.75, 127.58,
127.40, 127.33, 126.94, 126.33, 125.39, 125.17, 124.76,
124.35, 121.51, 116.62, 114.12, 108.78, 95.91, 55.09. HR-MS:
m/z (%) calcd for C35H27N2O4: 539.1971; found 539.1973
[FAB+].
(E)-2-Cyano-3-(4-((4-((E)-4-methoxystyryl)phenyl)(phenyl)amino)-
phenyl) acrylic acid (SK1). This compound was synthesized using
the method established for compound SP1. The crude product
was purified by column chromatography on silica gel using
dichloromethane : methanol (10 : 1, v/v) afforded SK1 (0.33 g,
77%) as an orange solid. 1H NMR (DMSO-d6, 500 MHz),
d: 8.14 (s, 1H), 7.94 (d, J = 9.0 Hz, 2H), 7.60 (d, J = 8.0 Hz,
2H), 7.54 (d, J = 8.5 Hz, 2H), 7.45–7.42 (m, 2H), 7.27 (t, J =
7.5 Hz, 1H), 7.22 (d, J = 8.0 Hz, 2H), 7.18–7.10 (m, 4H),
6.96–6.91 (m, 4H), 3.77 (s, 3H). 13C NMR (DMSO-d6,
500 MHz), d: 164.03, 158.97, 153.18, 151.55, 145.11, 144.06,
134.62, 132.83, 130.01, 129.65, 128.06, 127.73, 127.58, 126.33,
126.15, 125.66, 125.22, 123.15, 118.66, 116.92, 114.16, 98.07,
55.12. HR-MS: m/z (%) calcd for C31H25N2O3: 473.1865;
found 473.1868 [FAB+].
(E)-2-Cyano-3-(5-(4-((4-((E)-4-methoxystyryl)phenyl)(phenyl)-
amino)phenyl)thiophen-2-yl)acrylic acid (SK3). This compound
was synthesized using the method established for compound
SP1. The crude product was purified by column chromato-
graphy on silica gel using dichloromethane : methanol (8: 1, v/v)
afforded SK3 (0.22 g, 75%) as a dark red solid. 1H NMR
(DMSO-d6, 500 MHz), d: 8.45 (s, 1H), 7.98 (s, 1H), 7.70 (d, J =
8.5 Hz, 2H), 7.64 (s, 1H), 7.54–7.51 (m, 4H), 7.39 (t, J = 7.8 Hz,
2H), 7.17–7.01 (m, 9H), 6.95 (d, J = 8.5 Hz, 2H), 3.77 (s, 3H).
13C NMR (DMSO-d6, 500 MHz), d: 163.65, 158.84, 152.99,
148.37, 146.33, 146.16, 145.25, 141.43, 133.57, 133.12, 129.78,
129.75, 127.60, 127.41, 127.28, 125.56, 125.41, 125.07, 124.65,
124.25, 123.97, 122.03, 116.61, 114.14, 97.61, 55.09. HR-MS:
m/z (%) calcd for C35H27N2O3S: 555.1742; found 555.1746
[FAB+].
(E)-5-(4-((4-(4-Methoxystyryl)phenyl)(phenyl)amino)phe-
nyl)furan-2-carbaldehyde (6). A solution of 3 (2 g, 4.4 mmol) in
THF (120 mL) was stirred and purged with nitrogen gas for
10 min, and then tetrakis(triphenylphosphine)palladium(0)
(0.25 g, 0.22 mmol) was added followed by 5-formyl-2-
furanylboronic acid (0.67 g, 4.8 mmol) and a 2 M K2CO3
aqueous solution (6 mL). The reaction mixture was heated at
80 1C and stirred for 12 h. The reaction mixture was cooled to
room temperature and the solvent was removed under reduced
pressure. The residue was extracted with methylene chloride
and water, washed with water and brine, and dried over
anhydrous MgSO4. The solvent was removed by rotary
evaporation and subsequently dried in a vacuum oven. The
crude product was purified by column chromatography on
silica gel using dichloromethane : hexane (10 : 1, v/v) afforded
compound 6 (0.87 g, 42%) as a yellow solid. 1H NMR (CDCl3,
500 MHz), d: 9.59 (s, 1H), 7.67 (d, J = 8.7 Hz, 2H), 7.45–7.40
(m, 4H), 7.32–7.29 (m, 3H), 7.16 (d, J = 7.7 Hz, 2H), 7.12–7.08
(m, 5H), 7.6.98–6.89 (m, 4H), 6.71 (s, 1H), 3.83 (s, 3H).
(3) Fabrication of dye-sensitized solar cells
FTO glass plates (Pilkington, TEC-8, 8 O per square, 2.3 mm
thick) were cleaned with ethanol by ultrasonication for 10 min,
and then treated in a UV-O3 system for 20 min. The FTO layer
was first covered with 7.5% Ti(IV) bis(ethyl acetoacetato)-
diisopropoxide solution by a spin-coating method. For the
transparent nanocrystalline layer, TiO2 paste (230(M2331)-2T)
was coated on the FTO glass plates, which was followed by
sintering at 500 1C for 30 min. TiO2 paste (CCIC-1T) for the
scattering layer was prepared using the same method. The
resulting layer was composed of 9 mm thickness of transparent
layer and 4 mm thickness of scattering layer. The active area of
TiO2 films was approximately 0.26 cm2. The TiO2 electrodes were
immersed into the dye solution (0.5 mM in dichloromethane
containing 10 Mm CDCA) and kept at room temperature for
24 h. Counter electrodes were prepared by dropping a 0.7 mM
H2PTCl6 solution on a FTO glass and heating at 400 1C for
20 min. The dye-adsorbed TiO2 electrode and counter
electrode were sealed using 25 mm-thick surlyn film (Dupont
1702). An electrolyte solution was introduced through a drilled
hole on the counter electrode, where the electrolyte solution
consisted of 0.5 M 1-methyl-3-propylimidazolium iodide
(PMII), 0.2 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine
(TBP) in ACN/VN (85 : 15).
(E)-5-(4-((4-(4-Methoxystyryl)phenyl)(phenyl)amino)phenyl)thio-
phene-2-carbaldehyde (7). This compound was synthesized using
the method established for compound 6. The crude product
was purified by column chromatography on silica gel using
dichloromethane : hexane (8 : 1, v/v) afforded compound 7
1
(0.75 g, 39%) as a yellow solid. H NMR (CDCl3, 500 MHz),
d: 9.85 (s, 1H), 7.71 (s, 1H), 7.53 (d, J = 8.6 Hz, 2H), 7.45–7.40
(m, 4H), 7.32–7.29 (m, 3H), 7.16 (d, J = 7.8 Hz, 2H), 7.12–7.08
(m, 5H), 6.98–6.88 (m, 4H), 3.83 (s, 3H).
(4) Photovoltaic measurements
Photovoltaic measurements were performed using a Keithly model
2400 source measuring unit. A 1000 W Xe lamp (Spectra-physics)
served as a light source and its light intensity was adjusted using a
NREL-calibrated silicon solar cell equipped with a KG-5 filter to
approximate AM 1.5G of sun light intensity. IPCE was measured
(E)-2-Cyano-3-(5-(4-((4-((E)-4-methoxystyryl)phenyl)(phenyl)-
amino)phenyl)furan-2-yl)acrylic acid (SK2). This compound was
synthesized using the method established for compound SP1.
The crude product was purified by column chromatography
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 2025–2032 2031

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as a function of the wavelength from 300–800 nm using a
specially designed IPCE system for dye-sensitized solar cells
(PV Measurements, Inc.). A 75 W Xe lamp was used as the light
source to generate a monochromatic beam. Calibrations were
performed using a silicon photodiode, which was calibrated using
NIST-calibrated photodiode G425 as a standard, and IPCE
values were collected at a low chopping speed of 10 Hz.
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P. Wang, Chem. Commun., 2009, 2198; (b) Z. Ning and H. Tian,
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Acknowledgements
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This work was supported by the Korea Evaluation Institute of
Industrial Technology grant funded by the Korea government
(MKE).
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