Metal-free organic dyes with benzothiadiazole as an internal acceptor for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2013.08.075

We synthesized three metal-free organic dyes (H11-H13) consisting of a 3,6-disubstituted carbazole, benzothiadiazole, and cyanoacrylic acid. All the dyes exhibited high molar extinction coefficients and suitable energy levels for electron transfer from the electrolyte to the TiO₂ nanoparticles. Under standard AM 1.5G solar irradiation, the device using dye H13 with co-adsorbed chenodeoxycholic acid (CDCA) displayed the best performance: an open-circuit voltage (V_oc) of 0.71 V, a short-circuit current density (J_sc) of 12.69 mA cm^-2, a fill factor (FF) of 0.71, and a power conversion efficiency (PCE) of 6.32%. The PCE was ～79% of that for commercially available N719 cells (8.02%) under the same conditions.

Full text of DOI:10.1016/j.tet.2013.08.075

Tetrahedron 69 (2013) 9175e9182
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Tetrahedron
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Metal-free organic dyes with benzothiadiazole as an internal
acceptor for dye-sensitized solar cells
*
Woochul Lee, Ji yeon Seng, Jong-In Hong
Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2013
Received in revised form 26 August 2013
Accepted 30 August 2013
Available online 5 September 2013
We synthesized three metal-free organic dyes (H11eH13) consisting of a 3,6-disubstituted carbazole,
benzothiadiazole, and cyanoacrylic acid. All the dyes exhibited high molar extinction coefficients and
suitable energy levels for electron transfer from the electrolyte to the TiO2 nanoparticles. Under standard
AM 1.5G solar irradiation, the device using dye H13 with co-adsorbed chenodeoxycholic acid (CDCA)
displayed the best performance: an open-circuit voltage (Voc) of 0.71 V, a short-circuit current density
(Jsc) of 12.69 mA cmꢀ2, a fill factor (FF) of 0.71, and a power conversion efficiency (PCE) of 6.32%. The PCE
was w79% of that for commercially available N719 cells (8.02%) under the same conditions.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Dye-sensitized solar cells (DSCs)
Metal-free organic dyes
Benzothiadiazole
D-A-p-A structures
1. Introduction
efficient light harvesting. Second, appropriate HOMO and LUMO
energy levels are required in the dyes for efficient electron injection
In recent years, remarkable increases in energy consumption
have focused interest on renewable natural energy sources. Among
these, solar energy is very attractive as a clean and readily available
from the excited dye to the conduction band of TiO2, and rapid dye
regeneration by electron injection into the oxidized dye from the
redox couple. Third, the oxidized center in the donor part of the dye
must be spatially separated from the TiO2 film surface to minimize
charge recombination between the injected electrons and the
resulting oxidized dye.6
€
resource. Since the first report in 1991 by Michael Gratzel et al.,
dye-sensitized solar cells (DSCs) have received much attention
because of their low production costs and relatively high energy
conversion efficiencies.1 Conventional DSCs generally consist of
a nanocrystalline titanium oxide (TiO2) layer, a sensitizer, and an
electrolyte. Of these, a highly efficient photosensitizer is an es-
sential factor for the commercialization of DSCs. To date, ruthenium
(Ru)-based dyes have achieved high power conversion efficiencies
of up to w11%2 under air mass 1.5 global (AM 1.5G) irradiation.
However, Ru dyes have some drawbacks, such as low molar ex-
tinction coefficients and difficult synthesis and purification. As al-
ternatives to Ru dyes, organic dyes exhibit various advantages, such
as high molar extinction coefficients, facile structural modification,
relatively low material costs, and environmental friendliness.3 Re-
markable advances have been achieved in metal-free organic dyes,4
and the highest power conversion efficiency of about 12% has been
achieved by using a porphyrin-based dye.5 As a key factor in the
fabrication of highly efficient solar cells, dyes must possess several
properties. First, dyes must have a wide absorption overlap with the
solar spectrum and exhibit high molar extinction coefficients for
The general configuration of most reported organic dyes is
a D-p-A structure because of its effective photoinduced intra-
molecular charge transfer (ICT) properties.7 To improve the effi-
ciency in DSCs, the sensitizers should have good light harvesting
ability, which is related to the molecular extinction coefficient and
absorption region.8 In this regard, the highly efficient organic dyes
with D-A-p-A structures recently have been described; these in-
corporate an additional electron-withdrawing moiety, which af-
fords several advantages, such as easy tuning of the molecular
energy levels and bathochromic shifts of the absorption region,
which improve photovoltaic performance.9 Typical donor molec-
ular structures are arylamine derivatives, such as triphenylamine or
carbazole, which are well known for their strong electron-donating
abilities.10 Benzothiadiazole,11 benzotriazole,12 quinoxaline,13
diketopyrrolopyrrole,14 and nitro-cyanostilbene15 are used as
internal electron acceptor units. Thiophene or benzene units are
often used as p-bridges. Cyanoacrylic acid is commonly used as an
acceptor moiety. Phosphinic acid16 and the rhodanine moiety17
have also been reported as acceptor units.
In this study, we synthesized and characterized three metal-free
organic dyes consisting of a 3,6-bis(triphenylamine)-substituted
* Corresponding author. Tel.: þ82 2 880 6682; fax: þ82 2 889 1568; e-mail ad-
dress: jihong@snu.ac.kr (J.-I. Hong).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.08.075

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W. Lee et al. / Tetrahedron 69 (2013) 9175e9182
carbazole as an electron donor, benzothiadiazole as an internal
acceptor and -bridge, and cyanoacrylic acid as an anchoring group
carbazole and p-bridge did not contribute to the improvement in
light harvesting ability. However, the ICT absorption peak of H13
appears in the blue-shifted region, compared to that of H12, be-
cause of a more twisted structure of H13 in the p-bridge. This result
is in accordance with the calculated geometry structures of H dyes
(Section 2.3). Comparing the absorption spectra of the dyes in so-
p
to bind to TiO2 nanoparticles. By introducing a star-shaped 3,6-
bis(triphenylamine)-substituted carbazole, we were able to build
a multi-donor system, which was expected to inhibit the aggre-
gation of the dye on the surface of TiO2. Especially, a triphenylamine
moiety, which was well known for good electron-donating ability
as well as hole-transporting property, was incorporated as the
donor unit.18 In addition, a strong electron-withdrawing unit of
lution and as transparent thin films on 4 mm thick TiO2, significant
red-shifts of the absorption onsets were observed, presumably
because of the J-aggregation of the dye molecules that were an-
chored to the TiO2 films.22
benzothiadiazole in the
p-bridge would result in red-shift of ab-
sorption region and facilitating electron transfer from the electron
donor to the anchor unit.8 The photophysical and electrochemical
properties and the device performance of H11, H12, and H13 dyes
were systemically investigated. All dyes showed high molar ex-
2.3. Theoretical calculations
To understand the correlation between the molecular structures
and their photophysical properties, we performed density functional
theory (DFT) calculations at the B3LYP/6-31G* level in the Gaussian
09 package. The frontier molecular orbitals of these molecules are
shown in Fig. 2. The HOMOs of the three dyes were delocalized on
the donor groups, and the LUMOs were mainly located on the ben-
zothiadiazole, thiophene, and cyanoacrylic acid. This separated
electron density extends the absorption onset to the long wave-
length region because of the effective ICT. The dihedral angles be-
tween the benzothiadiazole and thiophene of H11 and H12 were 2.9ꢂ
and 3.5ꢂ, respectively, but the dihedral angle between the benzo-
thiadiazole and hexylthiophene of H13 was 48.7ꢂ. Consequently,
H13, which has an alkyl chain on thiophene, exhibited a blue-shifted
tinction coefficients (7ꢁ104e11ꢁ104
M
ꢀ1) and suitable energy
levels for electron injection and dye regeneration. The best per-
formance, 6.32%, was achieved by H13 dye with chenodeoxycholic
acid as a co-adsorbent.
2. Results and discussion
2.1. Synthesis
The molecular structures and synthetic procedures for the three
dyes are illustrated in Schemes
Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole 1,19 5-(7-(thiophen-2-
yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-carbaldehyde
3,20
1 and 2, respectively. 4,7-
absorption onset because of the twisted structure in the p-bridge.
and 3,6-bis-{4-[2-(4-diphenylamino-phenyl)-vinyl]-phenyl}-9H-
carbazole 7 were synthesized as described in previous reports. The
donor moiety was synthesized by the Suzuki reaction, the Heck
2.4. Electrochemical properties
reaction, and Miyaura borylation.21 The
p
-bridges were prepared
The electrochemical properties of these sensitizers were de-
termined by cyclic voltammetry in DMF containing 0.1 M tetrabu-
tylammonium hexafluorophosphate (TBAPF6) as
electrolyte. As shown in Table 1, the oxidation potentials of H11,
H12, and H13 were 0.90, 0.90, and 0.94 V (vs NHE), respectively.
Because the oxidation potentials of these dyes were more positive
than the redox potential of Iꢀ/Iꢀ3 (w0.4 V vs NHE), dye regeneration
would be a favorable process.23 The excited state oxidation po-
by the Suzuki reaction. The donor moiety and -bridge were cou-
p
a supporting
pled through the Ullmann or Suzuki reactions. Finally, the Knoe-
venagel condensation of the formylated compounds 10, 11, and 12
with cyanoacetic acid in the presence of piperidine provided dyes
H11, H12, and H13, respectively. Precise procedures are described in
the Experimental section.
tentials (E
*
0e0) of the dyes were calculated from the equation E
*
0-
¼EoxꢀE0e0, where the zeroezero excitation energy (E0e0) was
0
derived from the cross point of the normalized absorption and the
photoluminescence spectra. The excited state oxidation potentials
(E
spectively. The twisted molecular structure of H13 induced by the
two alkyl chains on the -bridge slightly suppressed the ICT, which
resulted in the larger band gap compared to H11 and H12. Because
the E values of the dyes were more negative than the conduc-
*
0-0) of H11, H12, and H13 were ꢀ1.46, ꢀ1.41, and ꢀ1.57 V, re-
p
*
0-0
tion band (CB) of TiO2 (ꢀ0.5 V vs NHE), the electron injection
process would also be favorable. Thus, these dyes had the requisite
energy levels for both efficient electron injection to TiO2 and the
dye regeneration process.
2.5. Photovoltaic performance
Scheme 1. Molecular structures of H11eH13.
DSCs were fabricated with all three dyes and N719 as a reference
dye. The liquid electrolyte was composed of 0.6 M 1,2-dimethyl-n-
propylimidazolium iodide (DMPII), 0.1 M LiI, 0.03 M I2, and 0.5 M
tert-butylpyridine (TBP) in acetonitrile/valeronitrile (v/v, 85:15).
Current densityevoltage curves (JeV curves, Fig. 3a) and incident
photon-to-current conversion efficiencies (IPCE, Fig. 3b) were
measured under standard global AM 1.5 solar conditions, and the
photovoltaic performance data are displayed in Table 2.
2.2. Photophysical properties
To investigate the photophysical properties of these dyes, we
measured their UVevis absorption spectra. The absorption spectra
of H11eH13 in DMF solution and in TiO2 films are shown in Fig. 1,
and their photophysical data are listed in Table 1. All the dyes
exhibited a broad absorption band from 300 to 600 nm corre-
sponding to the
p
e
p
* transition (350e450 nm), as well as an
The H13-sensitized solar cell showed the best efficiency, with an
open-circuit voltage (Voc) of 0.69 V, a short-circuit current density (Jsc)
of 11.29 mA cmꢀ2, and a fill factor (FF) of 0.68, yielding an overall
intramolecular charge transfer (ICT) band (450e600 nm). H11 and
H12 showed similar absorption regions and molar extinction co-
efficients, which indicates that an additional phenyl unit between
efficiency (h) of 5.27%. The IPCE intensity usually depends on light

W. Lee et al. / Tetrahedron 69 (2013) 9175e9182
9177
Scheme 2. Synthetic routes of H11eH13.
absorption as well as photoinduced electron injection into TiO2 and
charge recombination. Generally, the IPCE spectra appeared in the
red-shifted region compared to the absorption spectra of dye-
adsorbed TiO2 film because of light scattering in the 400 nm sized
TiO2 layer.24 As shown in Fig. 3, H11 and H12 displayed similar IPCEs,
less than 70% in all regions, but H13 showed a plateau around
400e550 nm and a higher IPCE value, about 70%, than H11 and H12.
Although the absorption onset of H13 was in the shortest region
amongthe threedyes, its photocurrenthadthehighestvalue. Itisvery
likely that both the high dye loading amount of H13 and hydrophobic
frequency peak in the Bode plot (Fig. 4b) using Eq. 1, where f is the
frequency of the peak.
s
¼ 1=ð2
p
f Þ
(1)
The injected electron lifetimes of the three H dyes were 0.1
(H11), 0.1 (H12), and 16.5 ms (H13), and these results were corre-
lated with the higher Voc value (0.69 V) of H13.27 Thus, the alkyl
chains on the
p
-bridge of H13 enhanced the photocurrent28 and
suppressed the charge recombination process, which resulted in
the improved photovoltaic performance of H13.
To enhance the efficiency of the DSCs, chenodeoxycholic acid
(CDCA) was co-adsorbed onto the TiO2 nanoparticles to prevent dye
aggregation. As shown in Fig. 5 and Table 2, the Jsc of the new device
using CDCA increased, and the Voc also slightly increased, yielding
alkyl chains on the p-bridge retarded charge recombination with the
oxidized components in the electrolyte (vide infra).
The Voc values of H11 and H12 were similar, but that of H13 was
higher, despite the similar oxidation potentials of the three dyes. To
discern the reason for the difference in Voc between H11eH13, we
performed electrochemical impedance spectroscopy (EIS) with the
H11eH13-based DSCs. EIS is generally used to investigate the
injected electron lifetime in a TiO2 nanocrystalline layer, which is
correlated to the Voc under open-circuit and dark conditions.25 The
three semicircles indicate the resistance of the redox reaction at the
platinum counter electrode (first semicircle), the charge re-
combination at the TiO2/dye/electrolyte (second semicircle), and
the carrier transport by ions within the electrolyte (third semi-
circle).26 As shown in Fig. 4a, the radius of the second semicircle in
the H13-based cell was larger than those of H11 and H12, which
means that the charge recombination process was retarded in H13-
an
h of 6.33%.
3. Conclusion
We synthesized three metal-free organic dyes consisting of
triphenylamine and carbazole as the donor moieties, benzothia-
diazole as an internal acceptor and p-bridge, and cyanoacrylic acid
as an anchoring group. All three dyes showed the requisite HOMO
and LUMO energy levels for dye regeneration and electron in-
jection. The alkyl chains on the
p-bridge prevented not only dye
aggregation but also charge recombination, leading to high Voc
values and high efficiencies. The best efficiency, 6.33%, was
based DSCs. The electron lifetime s can be obtained from the middle

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W. Lee et al. / Tetrahedron 69 (2013) 9175e9182
JEOL (HP 5890 Series II and JMS-AX505WA), and a matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF) mass
spectrometer from Bruker. Fluorescence spectra were recorded on
a Jasco FP-7500 spectrophotometer. Cyclic voltammetry (CV) was
performed using a CH Instruments 660 Electrochemical Analyzer
(CH Instruments, Inc., Texas) with 0.1 M TBAP in DMF at room
temperature. CV data were recorded using a Ag/Agþ reference
electrode (0.1 M AgNO3 in acetonitrile) with a platinum wire
counter electrode at a scan rate of 100 mV sꢀ1. The absolute po-
tential was calibrated against the NHE using an internal ferrocene/
ferrocenium standard. The theoretical energy levels were obtained
by DFT calculations at the B3LYP/6-31G* level in the Gaussian 09
package. The photovoltaic performances of the DSCs were mea-
sured using a 1000 W xenon light source, whose power as a Por-
table Solar Simulator PEC-L01 was calibrated using a KG5-filtered Si
reference solar cell. The IPCE spectra for the cells were measured on
an IPCE measuring system (PV Measurements). The EIS was mea-
sured on a Solar Simulator (Newport) and a potentiostat/galvano-
stat (Autolab, PGSTAT 302N).
4.2. Synthetic procedures and characterizations
4.2.1. 4,7-Bis(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (1). This in-
termediate was prepared as described in a previous report.29
4.2.2. 4,7-Bis(3-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (2). A
mixture of 4,7-dibromobenzo[c][1,2,5]thiadiazole (475 mg,
1.62 mmol), 2-(3-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (1 g, 3.40 mmol), and Pd(PPh3)4 (187 mg,
0.16 mmol) in THF (20 mL) and 2 M K2CO3 (20 mL) was refluxed for
24 h. After cooling to room temperature, the reaction mixture was
extracted with dichloromethane (MC), and the organic phase was
washed with water, and dried over Na2SO4. The solvent was evap-
orated to give an orange liquid product (580 mg, 77%), which was
Fig. 1. (a) Absorption and fluorescence spectra of H11eH13 and (b) normalized ab-
sorption spectra of dye-loaded TiO2.
Table 1
Photophysical and electrochemical data for H11eH13
Dye
H11
lmax
(
)a/nm
Eoxb/V
E0e0c/V
E
*
d/V
HOMOe/eV
LUMOe/eV
Relative dye loading amount on TiO2 film
0-0
378 (84,980)
488 (20,635)
379 (76,105)
498 (19,680)
376 (1,16,745)
0.90
2.36
ꢀ1.46
ꢀ1.41
ꢀ1.57
ꢀ4.90
ꢀ3.67
1
H12
0.90
0.94
2.31
2.51
ꢀ4.77
ꢀ4.74
ꢀ3.27
ꢀ3.00
1.2
1.3
H13
a
Maximum absorption wavelengths of dyes in 2.0ꢁ10ꢀ5 M DMF solution.
b
Measured in DMF with 0.1 M TBAPF6. The oxidation potential was determined from the onset via cyclic voltammetry at a scan rate of 100 mV sꢀ1. Potentials were
calibrated against an internal ferrocene/ferrocenium (Fc/Fcþ) standard.
c
E0e0 values were derived from the cross peaks of the normalized absorption spectra and photoluminescence spectra.
d
Calculated by E
*
.
0-0
e
Obtained by DFT calculations using the B3LYP/6-31G* level in the Gaussian 09 package.
obtained with H13 and 10 mM CDCA, with a Voc of 0.71 V, a Jsc of
purified by silica gel column chromatography (eluent; MC/Hexane
12.69 mA cmꢀ2, and a FF of 0.71.
(Hex)¼1:5). 1H NMR (300 MHz, acetone-d6):
d (ppm) 7.79 (s, 2H),
7.60e7.58 (d, J¼5.2 Hz, 2H), 7.18e7.16 (d, J¼5.2 Hz, 2H), 2.73e2.68
(t, J¼7.7 Hz, 4H), 1.67e1.57 (m, 4H), 1.27e1.22 (m, 12H), 0.80e0.76
4. Experimental
(t, J¼6.6 Hz, 6H); 13C (75 MHz, CDCl3):
d (ppm) 154.3, 141.7, 132.2,
129.9,127.5, 125.9, 31.6, 30.7, 29.4, 29.1, 22.6, 14.1; MALDI-TOF MS, m/
4.1. General information
z: calcd for C26H32N2S3: 468.173, found: 468.631 [M]þ.
All organic chemicals were purchased from Aldrich and TCI.
Column chromatography was conducted on Merck silica gel 60
(70e230 mesh). All solvents and reagents were commercially
available and used without further purification unless otherwise
noted. 1H and 13C NMR spectra were recorded using an Advance
300 MHz Bruker spectrometer. UVevis spectra were recorded on
a Beckman DU 650 spectrophotometer. Mass spectra were obtained
using a gas chromatographyemass spectrometry instrument from
4.2.3. 5-(7-(3-Thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thio-
phene-2-carbaldehyde (3). This intermediate was prepared as de-
scribed in a previous report.20
4.2.4. 4-Hexyl-5-(7-(3-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-
4-yl)thiophene-2-carbaldehyde (4). This compound was prepared
according to the synthetic procedure for 3, using 2 instead of 1.

W. Lee et al. / Tetrahedron 69 (2013) 9175e9182
9179
Fig. 2. Frontier molecular orbitals of H11eH13.
Table 2
Photovoltaic parameters of DSCs based on H11eH13 in full sunlight (AM 1.5G,
100 mW cmꢀ2
)
Dye
Voc (V)
Jsc (mA/cm2)
FF
h
(%)
Maximum
efficiency
H11
H12
0.54ꢃ0.00
0.55ꢃ0.00
0.70ꢃ0.02
0.72ꢃ0.01
0.73ꢃ0.01
9.14ꢃ1.18
10.01ꢃ0.60
11.11ꢃ0.59
12.07ꢃ0.60
15.81ꢃ0.66
0.67ꢃ0.01
0.68ꢃ0.01
0.66ꢃ0.02
0.72ꢃ0.01
0.67ꢃ0.02
3.32ꢃ0.47
3.70ꢃ0.27
5.11ꢃ0.16
6.19ꢃ0.13
7.80ꢃ0.22
3.79
3.97
5.27
6.32
8.02
H13
H13a
N719
a
The device was fabricated with 10 mM CDCA.
The product was obtained as an orange liquid (450 mg, 64%). 1H
NMR (300 MHz, DMSO-d6): (ppm) 9.97 (s, 1H), 7.80 (s, 1H),
d
7.68e7.74 (m, 2H), 7.46e7.47 (d, J¼4 Hz, 1H), 7.12e7.13 (d, J¼4.3 Hz,
1H), 2.67e2.73 (m, 4H), 1.62e1.67 (m, 4H), 1.18e1.26 (m, 12H),
0.81e0.85 (t, J¼6.2 Hz, 6H); 13C (75 MHz, CDCl3):
d (ppm) 182.9,
154.2,135.6,143.1,142.9,142.5,142.1,137.8,131.8,130.4,129.6,129.4,
129.1, 126.3, 125.9, 31.6, 31.5, 30.7, 30.4, 29.4, 29.3, 29.1, 29.0, 22.5,
22.4, 14.1, 14.0; MALDI-TOF MS, m/z: calcd for C27H32N2OS3:
496.168, found: 496.639 [M]þ.
4.2.5. 5-(7-(5-Iodothiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thio-
phene-2-carbaldehyde (5).19 N-Iodosuccinimide (NIS, 1.03 g,
4.57 mmol) was slowly added to a solution of 3 (500 mg,
1.52 mmol) in MC (30 mL) and acetic acid (30 mL) at room tem-
perature, and the mixture was stirred overnight. After the reaction
was complete, the mixture was filtered, and washed with water
(590 mg, 91.4%). Mp 263e265 ꢂC; 1H NMR (300 MHz, DMSO-d6):
d
(ppm) 9.99 (s, 1H), 8.28e8.34 (m, 2H), 8.12e8.18 (m, 2H),
7.88e7.89 (d, J¼3 Hz, 1H), 7.49e7.51 (d, J¼6 Hz, 1H); 13C NMR
Fig. 3. (a) JeV curves and (b) IPCE spectra of H11eH13.
(150 MHz, DMSO-d6): d (ppm) 184.5, 151.7, 151.6, 147.0, 143.6, 138.1,

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W. Lee et al. / Tetrahedron 69 (2013) 9175e9182
153.9, 153.5, 143.9, 143.2, 143.0, 142.3, 130.2, 129.4, 127.7, 126.4, 75.1,
31.5, 31.5, 30.6, 30.4, 29.3, 29.1, 29.0, 22.5, 22.5,14.0,14.0; MALDI-TOF
MS, m/z: calcd for C27H31IN2OS3: 622.064, found: 622.646 [M]þ.
4.2.7. 4,40-(1E,10E)-2,20-(4,40-(9H-Carbazole-3,6-diyl)bis(4,1-
phenylene))bis(ethene-2,1-diyl)bis(N,N-diphenylaniline)
(7). This
intermediate was prepared as described in a previous report.21
4.2.8. 3,6-Bis-{4-[2-(4-diphenylamino-phenyl)-vinyl]-phenyl}-9-4-
bromophenyl-carbazole (8). A mixture of 7 (500 mg, 0.583 mmol),
1-bromo-4-iodobenzene (110 mg, 0.388 mmol), Cu powder (74 mg,
1.165 mmol), K2CO3 (215 mg, 1.554 mmol), and 18-crown-6
(103 mg, 0.388 mmol) in o-dichlorobenzene was refluxed for 36 h.
After the reaction was complete, the mixture was poured into EtOH
and the solid was filtered. The solvent was evaporated to give a pale
yellow solid (240 mg, 61%), which was purified by silica gel column
chromatography (eluent; ethyl acetate (EA)/Hex¼1:30). Mp
196e198 ꢂC; 1H NMR (300 MHz, CDCl3):
d (ppm) 8.44 (s, 2H),
7.72e7.78 (m, 8H), 7.62e7.65 (m, 4H), 7.52e7.55 (m, 2H), 7.43e7.49
(m, 6H), 7.32 (m, 4H), 7.03e7.16 (m, 24H); 13C (75 MHz, CDCl3):
d
(ppm) 147.6, 147.4, 140.5, 136.2, 133.5, 133.2, 131.6, 129.3, 128.5,
128.0, 127.4, 127.3, 127.0, 126.8, 126.7, 125.6, 124.5, 124.2, 123.6,
123.0, 118.7, 110.0; MALDI-TOF MS, m/z: calcd for C70H50BrN3:
1011.319, found: 1011.484 [M]þ.
4.2.9. 3,6-Bis-{4-[2-(4-diphenylamino-phenyl)-vinyl]-phenyl}-9-4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl-carbazole (9). A
mixture of
8 (230 mg, 0.227 mmol), bis(pinacolato)diboron
(144 mg, 0.567 mmol), PdCl2(dppf)$CH2Cl2 (37 mg, 0.045 mmol),
and potassium acetate (89 mg, 0.908 mmol) in 1,4-dioxane (25 mL)
was refluxed for 24 h. After the reaction was complete, the solvent
was removed under high vacuum and the residue was extracted
with MC and water. The solvent was evaporated to give a pale
yellow solid (100 mg, 42%), which was purified by silica gel chro-
matography (eluent; MC/Hex¼1:1). Mp 186e188 ꢂC; 1H NMR
Fig. 4. (a) Nyquist plots and (b) Bode plots of H11eH13-sensitized solar cells under
open-circuit.
(300 MHz, CDCl3):
d
(ppm) 8.44 (s, 2H), 8.10e8.12 (d, J¼7.9 Hz, 2H),
7.71e7.77 (t, J¼8.8 Hz, 6H), 7.62e7.68 (t, J¼9.1 Hz, 6H), 7.53e7.55 (d,
J¼8.5 Hz, 4H), 7.43e7.46 (d, J¼8.5 Hz, 4H), 7.32 (s, 2H), 7.03e7.16 (m,
24H); 13C (75 MHz, CDCl3):
d (ppm) 147.6, 147.3, 140.7, 140.5, 136.5,
136.1,133.3, 131.7,129.3, 127.9, 127.4, 127.3,126.8, 126.7,125.9, 125.5,
124.5, 124.2, 123.7, 123.0, 118.6, 110.3, 84.1, 83.5, 25.0, 24.9;
MALDI-TOF MS, m/z: calcd for C76H62BN3O2: 1059.494, found:
1059.564 [M]þ.
4.2.10. 5-(7-(5-(3,6-Bis(4-(4-(diphenylamino)styryl)phenyl)-9H-car-
bazol-9-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-
carbaldehyde (10). A mixture of 7 (400 mg, 0.466 mmol), 5 (318 mg,
0.699 mmol), Cu powder (21 mg, 0.326 mmol), potassium acetate
(258 mg, 1.864 mmol), and 18-crown-6 (37 mg, 0.139 mmol) in
o-dichlorobenzene (45 mL) was refluxed for 48 h. After the reaction
was complete, the solvent was removed by high vacuum, extracted
with MC, and dried over sodium sulfate. The solvent was evapo-
rated to give a reddish brown solid (416 mg, 75%), which was pu-
rified by silica gel chromatography (eluent; MC/Hex¼2:1). Mp
Fig. 5. JeV curve of H13 and H13 with CDCA based cell.
138.0, 129.2, 128.2, 128.0, 127.9, 127.0, 125.5, 123.5; MALDI-TOF MS,
m/z: calcd for C15H7IN2OS3: 453.877, found: 454.189 [M]þ.
194e197 ꢂC; 1H NMR (300 MHz, CDCl3):
d (ppm) 9.92 (s,1H), 8.26 (s,
4.2.6. 4-Hexyl-5-(7-(3-hexyl-5-iodothiophen-2-yl)benzo[c][1,2,5]
thiadiazol-4-yl)thiophene-2-carbaldehyde (6). This compound was
prepared according to the synthetic procedure of 5, using 4 instead of
3. After the reaction, the mixture was extracted with MC and water,
and the organic layer was dried over Na2SO4. The solvent was
evaporated to give an orange liquid product (436 mg, 90%), which
2H), 8.09e8.03 (dd, J¼3.7 Hz, 12.9 Hz, 2H), 7.78e7.71 (m, 2H),
7.64e7.53 (m, 13H), 7.42e7.40 (d, J¼8.4 Hz, 4H), 7.31e7.26 (m, 6H),
7.21e7.13 (m, 10H), 7.08e7.01 (m, 13H); 13C (75 MHz, CDCl3):
d
(ppm) 183.0, 158.2, 152.5, 147.6, 147.4, 141.4, 140.7, 140.4, 136.8,
136.3, 134.2, 131.6, 129.6, 129.3, 128.0, 127.5, 127.4, 126.8, 126.6,
126.0, 124.5, 123.6, 123.1, 118.6, 110.8; MALDI-TOF MS, m/z: calcd for
C79H53N5OS3: 1183.341, found: 1183.199 [M]þ.
was purified by silica gel column chromatography (eluent; MC/
1
Hex¼1:1); H NMR (300 MHz, CDCl3):
d (ppm) 9.96 (s, 1H), 7.79 (s,
1H), 7.70e7.72 (d, J¼7.3 Hz, 1H), 7.63e7.65 (d, J¼7.3 Hz, 1H), 7.26 (s,
4.2.11. 5-(7-(5-(4-(3,6-Bis(4-(4-(diphenylamino)styryl)phenyl)-9H-
carbazol-9-yl)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)
thiophene-2-carbaldehyde (11). This compound was prepared
1H), 2.63e2.70 (m, 4H), 1.59e1.67 (m, 4H), 1.21e1.28 (m, 12H),
0.80e0.84 (t, J¼6.5 Hz, 6H); 13C (75 MHz, CDCl3):
d (ppm) 182.9,

W. Lee et al. / Tetrahedron 69 (2013) 9175e9182
9181
according to the same synthetic procedure as 2, using 5 instead of
4,7-dibromobenzo[c][1,2,5]thiadiazole and instead of 2-(3-
compound was prepared according to the same synthetic pro-
cedure as H11, using 12 instead of 10. The product was obtained as
an orange solid (80 mg, 77%). Mp 299e302 ꢂC; 1H NMR (300 MHz,
9
hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The
product was obtained as a dark red solid (124 mg, 42%). Mp
DMSO-d6):
d
(ppm) 8.77 (s, 2H), 8.12 (s, 1H), 8.02e8.05 (d, J¼8 Hz,
252e254 ꢂC; 1H NMR (300 MHz, CDCl3):
d
(ppm) 10.00 (s, 1H), 8.46
2H), 7.81e7.87 (m, 8H), 7.68e7.76 (m, 8H), 7.53e7.56 (d, J¼8.56 Hz,
6H), 7.30e7.35 (t, J¼7.8 Hz, 8H), 7.24e7.26 (d, J¼6.5 Hz, 2H),
6.96e7.10 (m, 18H), 2.63e2.73 (m, 4H), 1.49e1.65 (m, 4H), 1.11e1.15
(s, 3H), 8.26 (s, 2H), 7.98e8.05 (m, 6H), 7.88 (s, 2H), 7.65e7.70 (d,
J¼1.4 Hz, 5H), 7.59e7.62 (d, J¼1.1 Hz, 5H), 7.43e7.60 (d, J¼0.8 Hz,
6H), 7.06e7.16 (m, 27H); 13C NMR (75 MHz, CDCl3):
d
(ppm) 183.0,
(m, 12H), 0.72e0.76 (m, 6H); 13C NMR (150 MHz, CDCl3):
d (ppm)
152.3, 147.6, 147.3, 140.5, 136.1, 133.3, 131.6, 129.6, 129.3, 127.9, 127.4,
127.3, 127.2, 126.8, 126.7, 124.9, 124.7, 124.5, 124.2, 123.6, 123.0,
118.6, 110.2; MALDI-TOF MS, m/z: calcd for C85H57N5OS3: 1259.373,
found: 1259.394 [M]þ.
153.7,147.5,147.2,143.8,143.3,140.4,136.8,135.9,133.0,131.6,129.7,
129.3,127.7,127.4,126.8,126.3,125.1,124.5,124.1,123.6,123.0,118.4,
115.2, 113.8, 110.0, 31.6, 31.4, 29.9, 29.3, 22.6, 14.1; HRMS, m/z: calcd
for C100H82N6O2S3: 1494.5664, found: 1494.5661.
4.2.12. 5-(7-(5-(4-(3,6-Bis(4-(4-(diphenylamino)styryl)phenyl)-9H-
carbazol-9-yl)phenyl)-3-hexylthiophen-2-yl)benzo[c][1,2,5]thiadia-
zol-4-yl)-4-hexylthiophene-2-carbaldehyde (12). This compound
was prepared according to the same synthetic procedure as 2, using
6 instead of 4,7-dibromobenzo[c][1,2,5]thiadiazole and 9 instead of
2-(3-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane.
The product was obtained as an orange solid (70 mg, 52%). Mp
4.3. Device fabrication
Fluorine-doped tin oxide (FTO) glass plates (Pilkington, 8
U ,
sqꢀ1
2.3 mm thick) were cleaned in a detergent solution in an ultrasonic
bath for 15 min, and then rinsed with water and EtOH. The washed
FTO glass plates were immersed in aqueous 10 mM TiCl4 at 70 ꢂC for
30 min. A transparent nanocrystalline layer was deposited on the
FTO glass by the doctor blade printing method using TiO2 paste
(Solaronix, TieNanoxide T/SP, and ENB Korea, TTP-20N) and
a scattering layer paste (Dyesol Timo,18NR-AO, Batch 314). The TiO2
layer was gradually sintered according to a programmed procedure.
The sintered layers (0.2 cm2) consisted of a transparent layer
197e199 ꢂC; 1H NMR (300 MHz, CDCl3):
d (ppm) 9.99 (s, 1H),
7.94e7.97 (d, J¼8.2 Hz, 2H), 7.74e7.78 (m, 8H), 7.63e7.70 (m, 8H),
7.55e7.58 (d, J¼8.6 Hz, 2H), 7.44e7.48 (m, 4H), 7.30e7.32 (m, 4H),
6.99e7.19 (m, 24H), 2.69e2.81 (m, 4H), 1.63e1.78 (m, 4H),
1.25e1.28 (m, 12H), 0.86e0.90 (m, 6H); 13C NMR (75 MHz, CDCl3):
d
(ppm) 183.0, 153.6, 147.6, 147.3, 143.2, 140.6, 137.8, 136.1, 133.3,
(12 mm thick) and a scattering layer (10 mm thick). The TiO2 elec-
131.6, 130.4, 129.3, 127.9, 127.4, 127.2, 126.9, 126.7, 126.1, 125.6,
124.5, 124.2, 123.7, 123.0, 118.6, 110.3, 31.6, 31.5, 30.7, 30.4, 29.7,
29.4, 29.2, 29.0, 22.6, 22.5, 14.1; MALDI-TOF MS, m/z: calcd for
C97H81N5OS3: 1427.560, found: 1427.679 [M]þ.
trodes were again treated with 10 mM TiCl4 at 70 ꢂC for 30 min and
sintered at 450 ꢂC. Subsequently, the TiO2 electrodes were im-
mersed in solutions of the dyes (0.3 mM in THF/EtOH (2:1)) at 50 ꢂC
for 8 h in the dark. The FTO plates for the counter electrodes were
cleaned in a detergent solution in an ultrasonic bath for 15 min.
Then, the counter electrodes were prepared by coating each FTO
plate with a drop of H2PtCl6 solution (2 mg Pt in 1 mL EtOH) and
heating the plate to 450 ꢂC, causing the dyes adsorbed on the TiO2
electrode to adhere to the counter electrode by coverage with a hot-
melt polymer film (Solaronix, Meltonix 1170-60). A drop of elec-
trolyte solution was injected through a hole in the counter elec-
trode and driven into the cell. Finally, the hole was sealed using the
hot-melt polymer film and a cover glass (0.1 mm thick).
4.2.13. (E)-3-(5-(7-(5-(3,6-Bis(4-(4-(diphenylamino)styryl)phenyl)-
9H-carbazol-9-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thio-
phen-2-yl)-2-cyanoacrylic
acid
(H11). Piperidine
(51
mL,
0.51 mmol) was added to a mixture of 10 (200 mg, 0.17 mmol) and
cyanoacetic acid (43 mg, 0.51 mmol) in acetonitrile (20 mL) and
THF (20 mL). The mixture was refluxed overnight and then cooled
to room temperature. The mixture was extracted with MC and
water, and the organic phase was dried over sodium sulfate. The
solvent was evaporated to give a dark brown solid (98 mg, 46%),
which was purified by silica gel chromatography (eluent; MC/
MeOH/triethylamine (TEA)¼10:1:0.1). Mp 208e211 ꢂC; 1H NMR
Acknowledgements
(300 MHz, DMSO-d6):
d (ppm) 9.36 (s, 1H), 8.82 (s, 1H), 8.38e8.29
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
for the Center for Next Generation Dye-sensitized Solar Cells
(No. 2013004800), and by a grant from the New & Renewable
Energy Technology Development Program of the KETEP (No.
20113020010070), funded by the Ministry of Knowledge Economy.
(m, 4H), 7.94e7.92 (m, 2H), 7.89e7.86 (d, J¼8.5 Hz, 5H), 7.73e7.67
(m, 6H), 7.57e7.55 (d, J¼7.9 Hz, 3H), 7.36e7.23 (m, 14H), 7.17e7.04
(m, 13H), 6.99e6.96 (d, J¼8.1 Hz, 5H); 13C NMR (75 MHz, DMSO-
d6):
d (ppm) 150.8, 147.5, 147.4, 132.1, 130.1, 128.2, 127.4, 124.7,
123.8; HRMS, m/z: calcd for C82H54N6O2S3: 1250.3470, found:
1250.3466.
4.2.14. 3-(5-(7-(5-(4-(3,6-Bis(4-(4-(diphenylamino)styryl)phenyl)-
9H-carbazol-9-yl)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-
yl)thiophen-2-yl)-2-cyanoacrylic acid (H12). This compound was
prepared according to the same synthetic procedure as H11, us-
ing 11 instead of 10. The product was obtained as a dark red solid
(55 mg, 48%). Mp 317e319 ꢂC; 1H NMR (300 MHz, CDCl3):
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