Highly efficient triarylene conjugated dyes for sensitized solar cells

Article abstract of DOI:10.1039/c1jm10107b

A new series of organic dyes containing a triarylamine donor group, a triarylene-linked bridging moiety, and a cyanoacrylic acid acceptor group were synthesized through a simple procedure in high yields. A selected set of substituents were added onto the

Full text of DOI:10.1039/c1jm10107b

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PAPER
Highly efficient triarylene conjugated dyes for sensitized solar cells†
a
Yuan Jay Chang and Tahsin J. Chow
b
*
Received 8th January 2011, Accepted 28th February 2011
DOI: 10.1039/c1jm10107b
A new series of organic dyes containing a triarylamine donor group, a triarylene-linked bridging
moiety, and a cyanoacrylic acid acceptor group were synthesized through a simple procedure in high
yields. A selected set of substituents were added onto the phenyl group ortho to the cyanoacrylic acid in
order to examine their influences on the performance of dye-sensitized solar cells (DSSCs). Their
photochemical behaviors were examined under AM1.5 solar condition (100 mW cmꢀ2). A typical device
made with a compound containing a -CF3 substituent (PSP-CF3) displayed a short-circuit current (Jsc)
15.16 mA cmꢀ2, an open-circuit voltage (Voc) 0.68 V, a fill factor (ff) 0.68, corresponding to an overall
conversion efficiency of 7.0% and a maximal monochromatic incident photon-to-current conversion
efficiency (IPCE) 78%. Their photophysical and electrochemical properties were analyzed with the aid
of a time-dependent density functional theory (TDDFT) model with the B3LYP functional. Their
HOMO and LUMO energy levels are verified by both electrochemical measurements and theoretical
calculations.
selected substitutions on their triarylene bridge are made.12 Five
Introduction
new organic dyes, i.e., PSP–H, PSP–F, PSP-CN, PSP-CF3 and
PSP-OCH3 (Chart 1) were synthesized in this regard. The tuning
of electron flow by the addition of a substituent can be viewed
from two sides, i.e., the presence of an electron-withdrawing
group located on the acceptor side can stabilize the charge-
separated excited state; whilst an electron-donating group may
improve the charge injection efficiency toward the TiO2 con-
ducting electrode. Nevertheless, the absorption wavelength will
be red-shifted with respect to the parent structure, as a result of
more extended electronic resonance induced by the substituents.
A broader absorption range reveals that more photons can be
harvested by DSSCs.13 In all the five structures, the central thio-
phenylene moiety is replaced by a 3,4-ethylenedioxythiophylene
(EDOT) group in order to enhance the absorbance of the
dyes.14,15 Their device characteristics made with these dyes are
examined in this report.
Dye-sensitized solar cells (DSSCs) have been considered as
a promising alternative energy source.1 The ruthenium-based
complexes, i.e., N3, N719, and black dye reported by Michael
€
Gratzel, have achieved maximal power conversion efficiencies
over 11%.2 Compared with metallic materials, organic dyes are
indispensable due to their advantages of environmental friend-
liness, higher structural flexibility, lower cost, easier preparation
and purification, etc. A wide variety of organic dyes have been
reported in the literature including the derivatives of coumarin,3
indoline,4 cyanine,5 merocyanine,6 hemicyanine,7 perylene,8
dithiensilole,9 and porphyrin,10 etc., whilst most of them exhibi-
ted energy-to-electricity conversion efficiency in a range 5ꢁ9%.
In our previous studies we have developed a series of organic
dipolar dyes composed of an aromatic amine donor (D) and
a cyanoacrylic acid acceptor (A), which were separated by
a linearly connected phenylene-thiophenylene-phenylene bridge
(PSP). The performance of DSSCs made with these dyes can
achieve a maximal quantum efficiency of 7%, considerably close
to that of N719.11 To further improve their performance and also
to investigate their structure/performance relationship, certain
Experimental
General information
All reactions and manipulations were carried out under
a nitrogen atmosphere. Solvents were distilled freshly according
to standard procedures. 1H and 13C NMR spectra were recorded
on Bruker (AV 400/AV 500 MHz) spectrometer in CDCl3, THF-
d8, and DMSO-d6 as a solvent. Chemical shifts are reported in
d scale downfield from the peak for tetramethylsilane. Absorp-
tion spectra were recorded on a Jasco-550 spectrophotometer.
Emission spectra and photoluminescence quantum yield were
obtained from a Hitachi F-4500 spectrofluorimeter. The emis-
sion spectra in solutions were measured in spectral grade solvent
aDepartment of Chemistry, National Taiwan University, No.1, Sec. 4,
Roosevelt Rd., Da’an Dist., Taipei, 10617, Taiwan. E-mail: d93223006@
ntu.edu.tw; Fax: +886-2-27884179; Tel: +886-2-27898641
bInstitute of Chemistry, Academia Sinica, No.128, Sec. 2, Academia Rd.,
Nangang Dist., Taipei, 11529, Taiwan. E-mail: tjchow@chem.sinica.edu.
tw; Fax: +886-2-27884179; Tel: +886-2-27898552
† Electronic Supplementary Information (ESI) available: 1H and 13C
NMR, infrared, and EIS spectra. Experimental for the synthesis of 1.
TDDFT simulated data. Loading amount of the dyes. See DOI:
10.1039/c1jm10107b/
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J. Mater. Chem., 2011, 21, 9523–9531 | 9523

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Chart 1 The structures of organic dyes.
by a 90ꢂ angle detection, and those of thin film were recorded by
front-face detection. The redox potentials were measured by
using cyclic voltammetry on a CHI 620 analyzer. All measure-
ments were carried out in THF solutions containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as sup-
porting electrolyte at ambient condition after purging 10 min
with N2. The conventional three electrode configuration was
employed, which consists of a glassy carbon working electrode,
a platinum counter electrode, and a Ag/Ag+ reference electrode
calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal
reference. Mass spectra were recorded on a VG70-250S mass
spectrometer.
a commercial dye N719 under the same condition (3 ꢃ 10ꢀ4 M,
Solaronix S.A., Switzerland) was used as a reference. The cell
parameters were obtained under an incident light with intensity
100 mW cmꢀ2 measured by a thermopile probe (Oriel 71964),
which was generated by a 300 W Xe lamp (Oriel 6258) passing
through a AM 1.5 filter (Oriel 81088). The current–voltage
parameters of DSSCs were recorded by a potentiostat/galvano-
stat model CHI650B (CH Instruments, USA). The light intensity
was further calibrated by an Oriel reference solar cell (Oriel
91150) and adjusted to be 1.0 sun. The monochromatic quantum
efficiency was recorded through a monochromator (Oriel 74100)
at short circuit condition.
All chemicals, i.e., o-tolunitrile, 1,4-dibromo-2-fluorobenzene,
4-bromo-2-methoxyphenylmethanol, n-butyllithium (1.6 M in
hexane), trans-dichlorobis(triphenylphosphine) palladium(II), N,
N-dimethylformamide, sulfuric acid, nitric acid, zinc powder, N-
bromosuccinimide, trimethylamine N-oxide, pyridinium chloro-
chromate, cyanoacetic acid, ammonium acetate, and triethyl
amine, were purchased from ACROS, Merck, Lancaster, TCI,
Sigma-Aldrich, separately, and purified while necessary before
use. The synthetic procedures of compound 1 is shown in ESI.
Chromatographic separations were carried out by using silica gel
from Merk, Kieselgel si 60 (40–63 mm).
4-Bromo-2-trifluoromethylbenzaldehyde (2)
To a solution of 1,4-dibromo-2-trifluoromethylbenzene (5 g,
19.85 mmol) in THF at ꢀ78 ꢂC was added dropwise n-BuLi
(12.4 mL, 19.85 mmol, 1.6 M in hexane). The mixture was stirred
for 1 h, then to it was added DMF (2.3 mL, 29.78 mmol). The
reaction was stirred with a magnetic bar for 1 h, then was
quenched by adding water and extracted with ethyl acetate. The
organic layer was dried over anhydrous MgSO4 and evaporated
under vacuum. The products were purified by silica gel column
chromatograph eluted with dichloromethane–hexane (1/9). A
colorless liquid of 2 was obtained in 40% yield (2.0 g, 7.94 mmol).
Spectroscopic data of 2: dH (400 MHz, CDCl3) 10.36 (s, 1H), 8.02
(d, 1H, J ¼ 8.4 Hz), 7.96 (s, 1H), 7.88 (d, 1H, J ¼ 8.4 Hz); dC (100
MHz, CDCl3) 187.6, 135.6, 132.3, 130.5, 129.4, 128.7, 124.1,
121.3; m/z (FAB) 251.9392 (M+, C8H4BrF3O requires 251.9398).
Fabrication and characterization of DSSCs
The FTO conducting glass (FTO glass, fluorine doped tin oxide
over-layer, transmission <90% in the visible, sheet resistance 8 U
squareꢀ1), titania-oxide pastes of Ti-Nanoxide T/SP and Ti-
Nanoxide R/SP were purchased from Solaronix. A thin film of
TiO2 (16ꢁ18 mm thick) was coated on a 0.25 cm2 FTO glass
substrate. It was immersed in a THF solution containing 3 ꢃ
10ꢀ4 M dye sensitizers for at least 12 h, then rinsed with anhy-
drous acetonitrile and dried. Another piece of FTO with sput-
tering 100 nm thick Pt was used as a counter electrode. The active
area was controlled at a dimension of 0.25 cm2 by adhering
a 60 mm thick polyester tape on the Pt electrode. The photo-
cathode was placed on top of the counter electrode and was
tightly clipped together to form a cell. Electrolyte was then
injected into the seam between two electrodes. An acetonitrile
solution containing LiI (0.5 M), I2 (0.05 M) and 4-tert-butyl-
pyridine (0.5 M) was used as the electrolyte. Devices made of
4-Bromo-2-methoxybenzaldehyde (3)
A mixture of 4-bromo-2-methoxyphenylmethanol (2.00 g, 9.21
mmol) and pyridinium chlorochromate (1.97 g, 9.21 mmol) in
dry CH2Cl2 was stirred with a magnetic bar for 4 h. The reaction
was quenched by adding water, and then was extracted with
dichloromethane. The organic layer was dried over anhydrous
MgSO4 and evaporated under vacuum. The products were
purified by silica gel column chromatograph eluted with
dichloromethane–hexane (1/1). White solids of 3 were obtained
in 92% yield (1.81 g, 8.47 mmol). Spectroscopic data of 3: dH
(400 MHz, CDCl3) 10.38 (s, 1H), 7.67 (d, 1H, J ¼ 8.0 Hz), 7.17
(d, 1H, J ¼ 8.4 Hz), 7.15 (d, 1H, J ¼ 1.6 Hz); dC (100 MHz,
9524 | J. Mater. Chem., 2011, 21, 9523–9531
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4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylenedioxy-
thiophen-2-yl)-2-(trifluoromethyl)benzaldehyde (4-CF3)
CDCl3) 188.7, 161.9, 130.5, 129.7, 124.2, 123.7, 115.3, 56.0; m/z
(EI) 213.9634 (M+, C8H7BrO2 requires 213.9629).
Compound 4-CF3 was synthesized according to the same
procedure as that of 4-H, giving orange solids of 4-CF3 in 55%
yield. dH (400 MHz, CDCl3) 10.3 (s, 1H), 8.16 (s, 1H), 8.11 (d,
1H, J ¼ 8.4 Hz), 7.96 (t, 2H, J ¼ 8.4 Hz), 7.92 (d, 1H, J ¼ 8.4 Hz),
7.82 (d, 1H, J ¼ 8.4 Hz), 7.61 (d, 2H, J ¼ 8.8 Hz), 7.53–7.47 (m,
2H), 7.39 (t, 1H, J ¼ 8.4 Hz), 7.37 (d, 1H, J ¼ 7.6 Hz), 7.28–7.23
(m, 2H), 7.13 (d, 2H, J ¼ 8.0 Hz), 7.03–6.99 (m, 3H), 4.40–4.38
(m, 2H), 4.33–4.31 (m, 2H); dC NMR (100 MHz, CDCl3) 188.2,
147.8, 147.7, 142.9, 141.6, 138.8, 137.8, 135.3, 131.6, 131.3, 131.1,
130.1, 129.5, 129.2, 128.4, 127.8, 127.3, 127.2, 126.8, 126.5, 126.3,
126.2, 125.1, 124.9, 124.1, 122.6, 122.4, 120.8, 119.7, 111.4, 64.8,
64.3; m/z (FAB) 607.1428 (M+, C36H24F3NO3S requires
607.1429).
p-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylenedioxy-
thiophen-2-yl)benzaldehyde (4-H)
To a mixture of 4-bromobenzaldehyde (1.22 g, 6.60 mmol),
PdCl2(PPh3)2 (139 mg, 0.20 mmol), and 2-tributylstannyl-5-
(p-(naphthylphenylamino)phenyl)-3,4-ethylenedioxythiophene
(7.18 g, 9.9 mmol) in a three-necked flask was added DMF
(20 mL). The mixture was stirred at 90 ꢂC for 24 h. After cooling,
the reaction was quenched by adding MeOH and saturated KF
(15 mL). It was extracted with CH2Cl2 and the organic layer
dried over anhydrous MgSO4. Evaporation of the solvent gave
a crude product, which was purified by silica gel with hexane as
the eluent. The orange solid of 4-H was obtained in 78% yield
(2.77 g, 5.15 mmol). Spectroscopic data for 4-H: dH (400 MHz,
CDCl3) 9.96 (s, 1H), 7.97 (d, 1H, J ¼ 8.4 Hz), 7.93ꢀ7.84 (m, 5H),
7.81 (d, 1H, J ¼ 8.4 Hz), 7.61 (d, 2H, J ¼ 8.8 Hz), 7.52ꢀ7.47 (m,
2H), 7.40 (d, 1H, J ¼ 9.6 Hz), 7.38 (d, 1H, J ¼ 7.2 Hz), 7.28ꢀ7.23
(m, 2H), 7.13 (d, 2H, J ¼ 8.0 Hz), 7.03 (d, 2H, J ¼ 8.8 Hz), 6.99
(d, 1H, J ¼ 7.6 Hz), 4.39ꢀ4.37 (m, 2H), 4.34ꢀ4.32 (m, 2H); dC
(100 MHz, CDCl3) 191.4, 147.9, 147.5, 143.1, 140.7, 139.3, 137.8,
135.3, 133.7, 131.2, 130.1, 129.2, 128.4, 127.3, 127.1, 126.7, 126.5,
126.3, 126.2, 125.6, 125.4, 124.1, 122.5, 122.3, 121.1, 118.5, 112.7,
64.7, 64.4; m/z (FAB) 540.1644 ((M + H)+, C35H25NO3S requires
540.1633).
4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylenedioxy-
thiophen-2-yl)-2-methoxybenzaldehyde (4-OCH3)
Compound 4-OCH3 was synthesized according to the same
procedure as that of 4-H, giving orange solids of 4-OCH3 in 75%
yield. dH (400 MHz, CDCl3) 10.45 (s, 1H), 8.00 (d, 1H, J ¼ 8.4
Hz), 7.92 (d, 1H, J ¼ 8.0 Hz), 7.82 (t, 1H, J ¼ 8.0 Hz), 7.63 (d,
2H, J ¼ 8.8 Hz), 7.52–7.47 (m, 2H), 7.42–7.38 (m, 4H), 7.26 (t,
2H, J ¼ 7.2 Hz), 7.15 (d, 2H, J ¼ 7.6 Hz), 7.06–6.99 (m, 2H),
4.35–4.34 (m, 2H), 4.30–4.29 (m, 2H); dC (100 MHz, CDCl3)
188.8, 162.0, 147.9, 147.6, 143.1, 140.8, 140.7, 137.8, 135.3, 131.2,
129.2, 128.8, 128.5, 127.3, 127.1, 126.8, 126.5, 126.4, 126.2, 125.4,
124.1, 122.5, 122.4, 122.3, 121.1, 118.4, 117.9, 113.0, 108.1, 64.8,
64.3; m/z (FAB) 569.1666 (M+, C36H27NO4S requires 569.1661).
4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylenedioxy-
thiophen-2-yl)-2-fluorobenzaldehyde (4-F)
(E)-2-(4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylene-
dioxythiophen-2-yl)phenyl)cyanoacrylic acid (PSP–H)
Compound 4-F was synthesized according to a similar procedure
as that of 4-H, giving orange solids of 4-F in 60% yield. dH (400
MHz, CDCl3) 10.3 (s, 1H), 7.96 (d, 1H, J ¼ 8.4 Hz), 7.92 (d, 1H,
J ¼ 8.0 Hz), 7.96 (d, 1H, J ¼ 8.4 Hz), 7.84 (d, 1H, J ¼ 8.4 Hz),
7.82 (d, 1H, J ¼ 8.0 Hz), 7.64ꢀ7.47 (m, 6H), 7.39 (t, 1H, J ¼ 6.8
Hz), 7.38 (d, 1H, J ¼ 7.6 Hz), 7.28ꢀ7.23 (m, 2H), 7.13 (d, 1H, J ¼
7.6 Hz), 7.04ꢀ6.99 (m, 3H), 4.41ꢀ4.39 (m, 2H), 4.35ꢀ4.33 (m,
2H); dC (100 MHz, CDCl3) 187.0, 166.2, 163.7, 147.8, 143.0,
141.5, 141.4, 137.7, 135.3, 131.2, 129.2, 129.1, 128.8, 128.4, 127.3,
127.2, 126.8, 126.5, 126.3, 126.2, 125.0, 124.3, 124.1, 122.6, 122.4,
122.0, 121.8, 121.6, 121.3, 121.2, 121.1, 120.9, 119.5, 64.8, 64.3;
m/z (FAB) 557.1458 (M+, C35H24FNO3S requires 557.1461).
A mixture of 4-H (1 g, 1.85 mmol), cyanoacetic acid (189 mg,
2.22 mmol), and ammonium acetate (36 mg, 0.46 mmol) in acetic
acid was placed in a three-necked flask under a nitrogen atmo-
sphere and was stirred at 120 ꢂC for 12 h. After cooling, the
reaction was quenched by adding water, then was extracted with
CH2Cl2. The organic layer was dried over anhydrous MgSO4 and
evaporated under vacuum. The products were purified by silica
gel column chromatograph eluted with CH2Cl2/acetic acid (19/
1). The deeply red solid was isolated in 72% yield (807 mg, 1.33
mmol), mp: 293ꢁ295 ꢂC. Spectroscopic data of PSP–H: dH (400
MHz, DMSO-d6) 8.21 (s, 1H), 8.02 (d, 3H, J ¼ 8.0 Hz), 7.92 (d,
1H, J ¼ 8.4 Hz), 7.82 (d, 3H, J ¼ 8.4 Hz), 7.60ꢀ7.50 (m, 4H),
7.43 (d, 1H, J ¼ 8.0 Hz), 7.26 (t, 2H, J ¼ 8.0 Hz), 7.00 (d, 3H, J ¼
8.0 Hz), 6.99 (d, 2H, J ¼ 8.8 Hz), 4.42 (d, 2H, J ¼ 3.6 Hz), 4.35 (d,
2H, J ¼ 3.6 Hz); dC (100 MHz, DMSO-d6) 163.9, 153.2, 147.6,
147.5, 142.7, 141.7, 138.6, 137.4, 135.4, 131.8, 130.9, 129.9, 129.5,
129.1, 127.7, 127.4, 127.2, 127.1, 126.8, 125.6, 125.2, 123.7, 123.0,
122.5, 120.9, 117.4, 117.1, 112.1, 103.1, 65.3, 64.8; m/z (FAB)
607.1699 ((M + H)+, C38H26N2O4S requires 607.1691).
4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylenedioxy-
thiophen-2-yl)-2-cyanobenzaldehyde (4-CN)
Compound 4-CN was synthesized according to the same proce-
dure as that of 4-H, giving orange solids of 4-CN in 58% yield. dH
(400 MHz, CDCl3) 10.3 (s, 1H), 8.24 (s, 1H), 7.94–7.89 (m, 4H),
7.80 (d, 1H, J ¼ 8.4 Hz), 7.57 (d, 2H, J ¼ 8.4 Hz), 7.50–7.45 (m,
2H), 7.37 (t, 1H, J ¼ 8.0 Hz), 7.35 (d, 1H, J ¼ 7.6 Hz), 7.23 (t, 2H,
J ¼ 8.0 Hz), 7.11 (d, 2H, J ¼ 7.6 Hz), 7.01–6.97 (m, 3H), 4.46–
4.44 (m, 2H), 4.38–4.36 (m, 2H); dC (100 MHz, CDCl3) 187.4,
148.0, 147.6, 142.9, 142.1, 139.6, 137.8, 135.3, 133.0, 131.1, 130.2,
129.8, 129.2, 126.8, 126.5, 126.3, 126.2, 124.6, 124.0, 122.7, 122.6,
120.7, 116.3, 114.4, 110.3, 64.9, 64.3; m/z (FAB) 564.1506 (M+,
C36H24N2O3S requires 564.1508).
(E)-2-(4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylene-
dioxythiophen-2-yl)-2-fluorophenyl)cyanoacrylic acid (PSP–F)
Compound PSP–F was synthesized according to the same
procedure as that of PSP–H, giving black solids of PSP–F in
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 9523–9531 | 9525

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(E)-2-(4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylene-
53% yield, mp: 261ꢁ263 ꢂC. dH (400 MHz, THF-d8) 8.47 (t, 1H,
J ¼ 8.4 Hz), 8.44 (s, 1H), 7.95 (d, 2H, J ¼ 7.6 Hz), 7.85 (d, 1H, J
¼ 8.4 Hz), 7.71 (d, 2H, J ¼ 8.8 Hz), 7.64 (d, 2H, J ¼ 8.8 Hz), 7.52
(t, 1H, J ¼ 8.0 Hz), 7.48 (t, 1H, J ¼ 7.6 Hz), 7.38 (t, 2H, J ¼ 8.8
Hz), 7.23 (t, 2H, J ¼ 7.6 Hz), 7.11 (d, 2H, J ¼ 8.0 Hz), 6.99ꢀ6.97
(m, 3H), 4.47 (d, 2H, J ¼ 3.6 Hz), 4.39 (d, 2H, J ¼ 3.6 Hz); dC
(100 MHz, THF-d8) 164.4, 161.8, 149.0, 148.9, 144.2, 143.2,
139.4, 136.6, 132.3, 130.1, 129.88, 129.4, 128.2, 127.7, 127.3,
127.2, 127.1, 126.5, 125.0, 123.6, 123.4, 122.2, 121.8, 112.6, 112.3,
66.1, 65.5; m/z (FAB) 625.1605 ((M + H)+, C38H25FN2O4S
requires 625.1597).
dioxythiophen-2-yl)-2-methoxyphenyl)cyanoacrylic acid (PSP-
OCH3)
Compound PSP-OCH3 was synthesized according to the same
procedure as that of PSP–H, giving black solids of PSP-OCH3
in 70% yield, mp: 294ꢁ296 ꢂC. dH (400 MHz, DMSO-d6) 8.42 (s,
1H), 8.09 (d, 1H, J ¼ 8.4 Hz), 7.96 (d, 1H, J ¼ 8.0 Hz), 7.86 (d,
1H, J ¼ 8.0 Hz), 7.78 (d, 1H, J ¼ 8.4 Hz), 7.51 (t, 1H, J ¼ 7.2 Hz),
7.46 (d, 3H, J ¼ 7.6 Hz), 7.37 (t, 1H, J ¼ 7.2 Hz), 7.29 (d, 3H, J ¼
7.2 Hz), 7.21 (t, 2H, J ¼ 7.2 Hz), 6.95 (d, 3H, J ¼ 7.2 Hz), 6.81 (d,
2H, J ¼ 8.0 Hz), 4.35 (d, 2H, J ¼ 3.6 Hz), 4.28 (d, 2H, J ¼ 3.6
Hz), 3.85 (s, 3H); dC NMR (100 MHz, DMSO-d6) 164.3, 159.4,
147.5, 147.4, 146.7, 142.7, 141.7, 139.4, 138.5, 135.3, 130.9, 129.8,
129.0, 127.6, 127.3, 127.2, 127.1, 126.8, 125.2, 123.7, 122.9, 122.5,
120.8, 118.2, 117.7, 117.5, 117.4, 112.3, 107.7, 102.2, 65.2, 64.7,
56.2; m/z (FAB) 637.1806 ((M + H)+, C39H28N2O5S requires
637.1797).
(E)-2-(4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylene-
dioxythiophen-2-yl)cyanophenyl)cyanoacrylic acid (PSP-CN)
A mixture of 4-CN (0.8 g, 1.42 mmol), cyanoacetic acid (145 mg,
1.70 mmol), and excess triethylamine in toluene was placed in
a three-necked flask under a nitrogen atmosphere and was stirred
Quantum chemistry computations
ꢂ
at 120 C for 12 h. After cooling, the reaction was quenched by
The structure of dyes was optimized by using a B3LYP/6-31G*
hybrid functional. For the excited states, a time-dependent
density functional theory (TDDFT) with the B3LYP functional
was employed. All analyses were performed under Q-Chem 3.0
software. The frontier orbital plots of HOMO and LUMO were
drawn using Gaussian 03.
adding water, then was extracted with CH2Cl2. The organic layer
was dried over anhydrous MgSO4 and evaporated under
vacuum. The products were purified by silica gel column chro-
matograph eluted with CH2Cl2/acetic acid (19/1). The black
solids of PSP-CN were isolated in 10% yield (89 mg, 0.14 mmol),
mp: 212ꢁ214 ꢂC. Spectroscopic data of PSP-CN: dH (400 MHz,
DMSO-d6) 8.26 (d, 1H, J ¼ 8.4 Hz), 8.21 (s, 1H), 8.13 (s, 1H),
8.00 (d, 1H, J ¼ 8.4 Hz), 7.97 (d, 1H, J ¼ 8.4 Hz), 7.91 (d, 1H, J ¼
8.4 Hz), 7.82 (d, 1H, J ¼ 8.4 Hz), 7.59ꢀ7.49 (m, 4H), 7.42 (t, 1H,
J ¼ 7.6 Hz), 7.35 (d, 1H, J ¼ 7.2 Hz), 7.26 (t, 2H, J ¼ 7.6 Hz),
7.01ꢀ6.97 (m, 3H), 6.87 (d, 2H, J ¼ 8.8 Hz), 4.44 (d, 2H, J ¼ 3.2
Hz), 4.34 (d, 2H, J ¼ 3.2 Hz); dC (100 MHz, DMSO-d6) 163.3,
147.6, 147.5, 143.3, 142.6, 142.1, 138.5, 135.8, 135.4, 132.4, 130.9,
129.9, 129.4, 129.2, 129.1, 128.9, 127.7, 127.4, 127.2, 127.1, 126.8,
125.0, 123.7, 123.0, 122.6, 120.8, 118.2, 118.0, 117.2, 114.0, 110.2,
65.4, 64.8; m/z (FAB) 632.1654 ((M + H)+, C39H25N3O4S
requires 632.1643).
Results and discussion
Synthesis of the dyes
Five organic dyes, i.e., PSP–H, PSP–F, PSP-CN, PSP-CF3 and
PSP-OCH3 were synthesized as outlined in Scheme 1. All
compounds are modified at the ortho-position of the phenylene
group near the A terminal. All syntheses utilized 2-substituted 4-
bromobeznaldehydes as the key intermediates. For example, the
preparation of 5-bromo-2-formylbenzonitrile (1) from o-cyano-
toluene took 5 steps with a total yield of 21% (ESI†).16 4-Bromo-
2-(trifluoromethyl)benzaldehyde (2) was prepared by the
formylation of 1,4-dibromo-2-(trifluoromethyl)benzene with n-
BuLi and DMF. 4-Bromo-2-methoxybenzaldehyde (3) was made
form (4-bromo-2-methoxy)phenylmethanol by oxidation with
pyridinium chlorochromate.17 The presence of a formyl group in
(E)-2-(4-(5-(p-(Naphthylphenylamino)phenyl)-3,4-ethylene-
dioxythiophen-2-yl)-2-(trifluoromethyl)phenyl)cyanoacrylic acid
(PSP-CF3)
1
these compounds was verified using H NMR by a downfield
signal at d 10.30ꢁ10.38 ppm. The donor moiety was synthesized
from N-phenyl-1-naphthylamine, onto which the third aromatic
group was added by a Buchwald–Hartwig coupling reaction with
the aid of Pd(OAc)2.18 The resulting triarylamine was then
attached onto a 3,4-ethylenedioxythiophene (EDOT) group by
a Stille coupling reaction. The reaction was repeated again by
attaching it to a 2-substituted 4-bromobenzaldehyde to elongate
the chain length. The final step was a Knoevenagel condensation
between compound 4 and cyanoacetic acid in the presence of
ammonium acetate.19 This procedure was quite successful for all
the products except PSP-CN, for which an apparent decarboxy-
lation happened as a side reaction.20 The synthesis of PSP-CN
was finally achieved in low yield by using triethylamine as a base
under a refluxed temperature in toluene. All products were
confirmed by their corresponding spectroscopic characteristics.
Compound PSP-CF3 was synthesized according to the same
procedure as that of PSP–H, giving black solids of PSP-CF3 in
ꢂ
55% yield, mp: 269ꢁ271 C. dH (400 MHz, DMSO-d6) 8.43 (s,
1H), 8.14 (d, 1H, J ¼ 8.4 Hz), 8.10 (s, 1H), 7.99 (d, 1H, J ¼ 8.0
Hz), 7.96 (d, 1H, J ¼ 8.8 Hz), 7.90 (d, 1H, J ¼ 8.4 Hz), 7.81 (d,
1H, J ¼ 8.4 Hz), 7.58ꢀ7.48 (m, 4H), 7.41 (t, 1H, J ¼ 8.0 Hz), 7.35
(d, 1H, J ¼ 7.2 Hz), 7.25 (t, 2H, J ¼ 7.6 Hz), 7.01ꢀ6.97 (m, 3H),
6.87 (d, 2H, J ¼ 8.8 Hz), 4.43 (d, 2H, J ¼ 3.6 Hz), 4.34 (d, 2H, J ¼
3.6 Hz). dC (100 MHz, DMSO-d6) 162.9, 162.9, 148.6, 147.7,
147.5, 142.6, 142.2, 138.5, 136.6, 135.4, 131.2, 130.9, 129.9, 129.0,
128.8, 128.4, 127.7, 127.4, 127.2, 127.1, 126.8, 125.2, 124.9, 123.7,
123.1, 122.7, 122.5, 120.7, 118.2, 115.9, 110.6, 109.3, 65.4, 64.7.
m/z (FAB) 675.1571 ((M + H)+, C39H25F3N2O4S requires
675.1564).
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Scheme 1 Synthetic routes of organic dyes (PSP-X, X ¼ H, OCH3, F, CF3, CN).
Absorption spectra
possessing electron withdrawing groups, i.e., PSP–F and PSP-
CF3, displayed slightly lower values. It is noteworthy that
compound PSP-CN exhibited the lowest absorptivity (3 ¼ 1.06
ꢃ 104 Mꢀ1 cmꢀ1), although its longest wavelength tailed to
a region comparable to those of PSP–F and PSP-CF3. A longer
wavelength absorption is favorable to the performance of
DSSCs, as more photons with lower energy can be harvested.
The absorption spectra of all dyes on the surface of TiO2 is
shown in Fig. 2. All spectra were blue-shifted about 30–36 nm
with respect to those in the solution, except PSP-CN which
seemed to be red-shifted. The blue-shift on TiO2 is a common
The UV-Vis absorption spectra of organic dyes in THF solution
are displayed in Fig. 1 and the parameters are listed in Table 1.
Each of these compounds exhibits three major absorption bands,
appearing at 250–260 nm, 330–340 nm, and 440ꢁ460 nm,
respectively. The first two bands were assigned to localized
aromatic p–p* transitions, and the last one to a charge-transfer
(CT) transition. The high intensity of this band is well-correlated
with the large value of oscillator strength, which was calculated
by a TDDFT/B3LYP modelling study (Table 1). The molar
extinction coefficients of both PSP–H and PSP-OCH3 were
quite high, e.g. exceeding 4 ꢃ 104 Mꢀ1 cmꢀ1 (Fig. 1), while those
Fig. 2 Absorption spectra of organic dyes adsorbed on TiO2. The
spectra were subtracted from that of pure TiO2, while the peak minima at
ca. 300 nm was assumed to be zero.
Fig. 1 Absorption spectra of organic dyes in THF.
Table 1 Calculated (TDDFT/B3LYP), absorption, and electrochemical parameters for organic dyesa
b
c
d
Eox (V)
Dye
HOMO/LUMOb (eV)
Band gapb
f
labs (nm)/(3 (Mꢀ1 cmꢀ1))
labs (nm) on TiO2
E0-0 (eV)
ELUMOe (V)
N-PSP
PSP–H
PSP–F
PSP-CN
PSP-CF3
PSP-OCH3
ꢀ5.12/ꢀ2.63
ꢀ4.96/ꢀ2.47
ꢀ5.02/ꢀ2.55
ꢀ5.10/ꢀ2.84
ꢀ5.02/ꢀ2.69
ꢀ4.96/ꢀ2.36
2.49
2.28
2.25
2.06
2.13
2.37
0.77
0.89
0.88
0.72
0.84
0.99
434(37700)
456(42300)
460(34000)
442(10600)
458(35800)
458(41900)
400
426
430
457
426
422
2.52
2.39
2.40
2.36
2.35
2.39
0.82
0.71
0.72
0.75
0.72
0.67
ꢀ1.70
ꢀ1.68
ꢀ1.68
ꢀ1.61
ꢀ1.63
ꢀ1.72
a
f: Oscillator Strength for the lowest energy transition; 3: absorption coefficient; Eox: oxidation potential; E0-0: 0ꢀ0 transition energy measured at the
intersection of absorption and emission spectra; Jsc: short-circuit photocurrent density; Voc: open-circuit photovoltage; FF: fill factor; h: total power
b
c
d
conversion efficiency. TDDFT/B3LYP calculated values. Absorption in THF. Oxidation potential in THF (10ꢀ3 M) containing 0.1 M (n-
C4H9)4NPF6 with a scan rate 100 mV sꢀ1 (vs. NHE). e ELUMO was calculated by Eox ꢀ E0-0
.
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phenomenon for most organic dyes, that is regarded as the result
of a strong interaction between the dye and the semiconductor
surface. A similar trend has been demonstrated while the dyes
were deprotonated in the presence of base. Upon deprotonation
the electron density in the cyanoacrylate moiety is increased so
that the energy level of the LUMO is elevated, consequently the
band gap of CT transition is enlarged. However, such an effect is
partially annihilated by the presence of a CN group at the ortho-
position across a phenyl ring, which attracts the extra electron
density from the cyanoacrylate through an efficient p-conjuga-
tion (Fig. 4).
introduction of an electron-withdrawing group in p-bridge
lowered the HOMO level, thus increased the oxidation potential.
The potential energy of ionization decreased slightly in the order
of PSP-CN (0.75) > PSP–F (0.72) z PSP-CF3 (0.72) > PSP–H
(0.71) > PSP-OCH3 (0.67). The HOMO levels thus obtained
were within the range 0.75ꢁ0.67 eV, which were sufficiently
ꢀ
lower than that of electrolyte pair Iꢀ/I3 (ca. 0.40 V), thus
ensured a smooth electron flow from the electrolyte to the dyes.
The LUMO levels were estimated by the values of Eox and the 0-
0 band gaps, which was estimated at the intersection of absorp-
tion and emission spectra. A trend appeared as PSP-CN (ꢀ1.61)
> PSP-CF3 (ꢀ1.63) > PSP–H (ꢀ1.68) z PSP–F (ꢀ1.68) > N-
PSP (ꢀ1.70) > PSP-OCH3 (ꢀ1.72). All these levels were
considerably higher than the conductive band level of TiO2 (ca.
ꢀ0.5 V).
Electrochemical properties
The oxidation potentials (Eox), corresponding to the HOMO
level of dyes, were measured by cyclic voltammetry (CV) in THF
solutions, and the results were shown in Fig. 3 and Table 1. The
Computational analysis
The structures of the dyes were analyzed using a B3LYP/6-31G*
hybrid functional for full geometrical optimization. In the
ground state all compounds possess a nearly co-planar confor-
mation to achieve a maximal extent of p-delocalization.
Compared to the parent structure of N-PSP, the addition of
a dioxane ring in the EDOT moiety did not increase the dihedral
angle between the two adjacent aryl groups (Fig. 4). The EDOT
moieties may have a function of preventing the formation of
aggregates on the surface of TiO2.21
The charge distribution in the frontier molecular orbitals are
depicted in Fig. 5. As shown in the graphs, the HOMO level is
localized mostly at the triarylamine (D) moiety, while the LUMO
is mostly at the cyanoacrylic acid (A). The electron distributions
of both D and A are heavily coupled with the orbitals in the
central triarylene bridge (B). The geometry of the bridges must
have a critical influence on the transition probability of photo-
excitation. The calculated potential energy levels of HOMOs and
LUMOs compared well with the experimental values of Eox and
Fig. 3 Oxidative voltammograms of PSP-series organic dyes.
Fig. 4 The optimized structure by TDDFT on B3LYP/6-31G* of
N-PSP, PSP–H, PSP–F, and PSP-CF3.
Fig. 5 Computed frontier orbitals of dyes. The left graphs are the
HOMOs, and the right ones are the LUMOs.
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Table 2 Photovoltaic parameters for DSSCs of PSP-seriesa
ELUMO estimated by CV. The electron-withdrawing substituents
on the acceptor moiety should have an effect of stabilizing the
charge-separated species upon photo-excitation. As indicated in
Fig. 5, the electron density on the LUMOs of PSP–F and PSP-
CN can spread more widely onto the substituents, cf. that of
PSP–H.
Dye
Jsc (mA cmꢀ2
)
Voc (V)
ff
he (%)
N-PSP
PSP–H
PSP–F
PSP-CN
PSP-CF3
PSP-OCH3
N719
14.96
15.44
15.48
8.32
15.16
17.04
16.84
0.70
0.68
0.69
0.58
0.68
0.65
0.705
0.64
0.64
0.65
0.70
0.68
0.61
0.62
6.71
6.76
6.87
3.37
7.00
6.72
7.32
The transition probability was estimated by time-dependent
density functional theory (TDDFT) with the B3LYP functional,
and the calculated oscillator strength (f) is shown in Table 1. The
f values for the lowest energy transitions, i.e., the CT transitions,
are all relatively high, e.g., ranging from 0.72ꢁ0.99. The results
are consistent with the absorptivity measured in solutions, except
for PSP-CN. The absorption coefficient (3) of all dyes measured
by experiment was significantly lower than expected (Fig. 1 and
Table 1).
a
Performance of DSSC measured in a 0.25 cm2 working area on a FTO
(8U/square) substrate.
recombination rate in the nanocrystallite/dye/redox electrolyte
interface. The order of open current voltages matched with the
trend of reducing dark current, i.e., PSP-CN (0.58) < PSP-
OCH3 (0.65) < PSP–H ¼ PSP–F (0.68) < PSP-CF3 (0.69). It is
noteworthy that the fill factor increased with the strength of the
electron-withdrawing ability, particularly in the cases of PSP-
CN and PSP-CF3 (ff ¼ 0.68ꢁ0.70).
Photovoltaic performance of DSSCs
The photocurrent–voltage (J–V) plots of DSSCs fabricated with
these dyes, along with that of N719 for comparison, are shown in
Fig. 6. The detailed parameters, i.e., short circuit current (Jsc),
open-circuit photovoltage (Voc), fill factor (ff), and solar-to-
electrical photocurrent density (h) measured under AM 1.5 solar
light (100 mW cmꢀ2) are summarized in Table 2. All dyes with an
EDOT group exhibit a higher values of Jsc (15.2ꢁ17.0) than the
parent compound N-PSP without it (Jsc ¼ 15.0), presumably due
to a more extended delocalization. The absorption range of the
former is apparently longer than that of the latter (Fig. 2).
The only exception is PSP-CN, which yielded a low value for the
short circuit current, i.e., 8.3 mA cmꢀ2 only about half of the
others. It was mainly ascribed to its low molar absorptivity
extinction coefficient, as indicated in Fig. 1 and Table 1. All the
new dyes, except PSP-CN, performed better that N-PSP
exhibiting a higher quantum efficiency.
Among all the devices, except PSP-CN, the ones with strong
electron-withdrawing groups performed the best, e.g., the solar-
to-electricity conversion efficiency for PSP–F was 6.87%, and
that for PSP-CF3 was 7.00%. Their IPCE values in the region of
380ꢁ620 nm were all >50% (Fig. 7). For the abnormal behaviour
of PSP-CN, it may be compared to previous reports revealing
that a strong electron-withdrawing cyano group located at the
proximity of the acceptor might jeopardise the injection of
electrons toward the conducting electrode.20 The best perfor-
mance in this work was observed for the device made with PSP-
CF3, which exhibited a Jsc value 15.16 mA cmꢀ2, Voc value of
0.68 V, and ff value of 0.68. The overall conversion efficiency (h)
was estimated to be 7.0%, which was quite comparable to the
well-known ruthenium complex N-719. The incident photon to
current conversion efficiency (IPCE) was higher than 70% in the
range 420ꢁ540 nm. It is worth noting that in a recent report,
Chou and Chi has found that adding a fluorine substituent at the
meta position to the cyanoacrylate can induce a profound effect
on the quantum efficiency of DSSC.12b In our comparative
analysis, the trifluoromethyl substituent (PSP-CF3) works even
better then the fluorine (PSP–F).
It is known that reducing the amount of dark current can
retard charge recombination, thus increase the efficiency of the
solar cells.22 The dark currents were found to be in the order of
PSP-CN > PSP-OCH3 > PSP–H > PSP–F > PSP-CF3 as
shown in the bottom of Fig. 6. The amount of open current
voltage (Voc) can be benefited by reducing the charge
Fig. 6 J–V curves of the DSSC devices made with the dyes. The plots in
the upper section were measured under the light intensity of 1.0 sun. The
plots in the bottom half were taken in the dark.
Fig. 7 IPCE curves of PSP-series organic dyes.
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central thiophene group of N-PSP by an EDOT group have also
promoted slightly the performance of devices, presumably due to
the prevention of aggregation on the surface of TiO2.
An exception was found for compound PSP-CN, which
exhibited a low absorption on the CT transition which led to
a small value of Jsc (8.32). Its overall quantum efficiency was
found to be 3.37% only, despite its high ff factor (0.70). The low
absorptivity may be ascribed to the electronic configuration in
the ground state, which does not favour a CT transition from the
D (triarylamine) to A (cyanoacrylate). Shifting the CN group to
other site of the phenyl group in future designs may alter the
selection rule of the CT transition.
Fig. 8 EIS Nyquist plots of electrochemical impedance of the DSSCs
made with the dyes at ꢀ0.73 V bias in the dark.
Acknowledgements
Electrochemical impedance spectroscopy
Financial supports from the National Science Council of Taiwan
and Academia Sinica are gratefully acknowledged. Special
thanks to Professor C.-P. Hsu at the Institute of Chemistry for
her assistance on the quantum chemistry computations.
Electrochemical impedance spectroscopy (EIS) analysis was
performed to further elucidate the photovoltaic properties. Fig. 8
shows the electrochemical impedance spectra for the DSSCs
made with TiO2 electrodes, dipped with the PSP-series sensi-
tizers under a forward bias of ꢀ0.73 V in the dark. Three semi-
circles were observed in the Nyquist plots. The first one is
attributed to the high-frequency peak (>102 Hz) is ascribed to the
charge-transfer process at the interfaces between the redox
couple and the platinized counter electrode. The middle-
frequency peak (in the 10–100 Hz region) is related to the
transport process of the injected electrons at the interfaces
between TiO2 and the electrolyte/dye, and the low frequency one
(<1 Hz) is associated with the Nernst diffusion of I3ꢀ within the
electrolyte (EIS Bode plots in ESI).23
References
€
1 B. O’Regan and M. Gratzel, Nature, 1999, 352, 737.
2 (a) M. K. Nazeeruddin, A. Key, I. Rodicio, R. Humphry-Baker,
€
€
E. Muller, P. Liska, N. Vlachopoulous and M. Gratzel, J. Am.
Chem. Soc., 1993, 115, 6382; (b) M. K. Nazeeruddin,
S. M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska,
€
N. Vlachopoulous, V. Shklover, C. H. Fischer and M. Gratzel,
ꢀ
Inorg. Chem., 1999, 38, 6298; (c) M. K. Nazeeruddin, P. Pechy,
T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte,
P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia,
€
G. B. Deacon, C. A. Bignozzi and M. Gratzel, J. Am. Chem. Soc.,
2001, 123, 1613; (d) M. Gratzel, Inorg. Chem., 2005, 44, 6841.
The charge recombination resistance at the TiO2 surface
(Rrec), it can be deduced by fitting curves from a range of inter-
mediate frequencies using Z-view software. It is related to the
charge recombination rate, e.g., a smaller Rrec indicates a faster
charge recombination and therefore a larger dark current
(Fig. 6). The radius of the biggest semicircle increases in the order
PSP-CN (11.3 ohm) < PSP-OCH3 (12.0 ohm) < PSP–H (13.3
ohm) < PSP–F (18.0 ohm) < PSP-CF3 (20.7 ohm). This trend
appears to be consistent with both the lower Voc values of PSP-
CN (0.58 V) and PSP-OCH3 (0.65 V), as well as the higher values
of PSP–F (0.69 V) and PSP-CF3 (0.68 V). It seems reasonable to
conclude that the better performance of the latter can be ascribed
to a reduced rate of charge recombination, which led to higher
values of Voc.
€
3 (a) Z.-S. Wang, Y. Cui, K. Hara, Y. Dan-oh, C. Kasada and
A. Shinpo, Adv. Mater., 2007, 19, 1138; (b) K. Hara, K. Miyamoto,
Y. Abe and M. Yanagida, J. Phys. Chem. B, 2005, 109, 23776.
4 D. Kuang, S. Uchida, R. Humphry-Baker, S. M. Zakeeruddin and
€
M. Gratzel, Angew. Chem., Int. Ed., 2008, 47, 1923.
5 (a) A. Ehret, L. Stuhl and M. T. Spitler, J. Phys. Chem. B, 2001, 105,
9960; (b) K. Sayama, K. Hara, Y. Ohga, A. Shinpou, S. Suga and
H. Arakawa, New J. Chem., 2001, 25, 200.
6 K. Sayama, S. Tsukagoshi, K. Hara, Y. Ohga, A. Shinpou, Y. Abe,
S. Suga and H. Arakawa, J. Phys. Chem. B, 2002, 106, 1363.
7 (a) Q.-H. Yao, L. Shan, F.-Y. Li, D.-D. Yin and C.-H. Huang, New J.
Chem., 2003, 27, 1277; (b) Q.-H. Yao, F.-S. Meng, F.-Y. Li, H. Tian
and C.-H. Huang, J. Mater. Chem., 2003, 13, 1048.
8 (a) S. Ferrere and B. A. Gregg, New J. Chem., 2002, 26, 1155; (b)
€
T. Edvinsson, C. Li, N. Pschirer, J. Schoneboom, F. Eickemeyer,
R. Sens, G. Boschloo, A. Herrmann, K. Mullen and A. Hagfeldt, J.
€
Phys. Chem. C, 2007, 111, 15137; (c) Y. Shibano, T. Umeyama,
Y. Matano and H. Imahori, Org. Lett., 2007, 9, 1971; (d)
Conclusions
ꢀ
J. Fortage, M. Severac, C. H. Rassin, Y. Pellegrin, E. Blart and
F. Odobel, J. Photochem. Photobiol., A, 2008, 197, 156; (e)
H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42, 1809.
9 (a) S. Ko, H. Choi, M.-S. Kang, H. Hwang, H. Ji, J. Kim, J. Ko and
Y. Kang, J. Mater. Chem., 2010, 20, 2391; (b) L.-Y. Lin, C.-H. Tsai,
K.-T. Wong, T.-W. Huang, L. Hsieh, S.-H. Liu, H.-W. Lin, C.-
C. Wu, S.-H. Chou, S.-H. Chen and A.-I. Tsai, J. Org. Chem.,
2010, 75, 4778.
10 (a) S. Eu, S. Hayashi, T. Umeyama, A. Oguro, M. Kawasaki,
N. Kadota, Y. Matano and H. Imahori, J. Phys. Chem. C, 2007,
111, 3528; (b) J.-J. Cid, J.-H. Yum, S.-R. Jang, M. K. Nazeeruddin,
The presence of a substituent on the phenyl group ortho to the
cyanoacrylate acceptor of the dyes can effectively elongate the
absorption wavelength. A wider wavelength of absorption is
favourable to the light-harvesting efficiency of DSSCs. The
compound PSP-OCH3 thus exhibited the highest Jsc value of all.
However, an electron-withdrawing substituent was capable of
stabilizing the negative charge developed in the CT state, and
thus reduced the rate of charge recombination. Such an effect can
be evidenced by the high Voc values of both compounds PSP–F
and PSP-CF3. A combined effect of the above two factors
resulted in the highest ff value for PSP-CF3, which yielded the
best performance with a h value 7.0%. The replacement of the
ꢀ
€
E. Martınez-Ferrero, E. Palomares, J. Ko, M. Gratzel and
T. Torres, Angew. Chem., Int. Ed., 2007, 46, 8358; (c) H.-P. Lu, C.-
L. Mai, C.-Y. Tsia, S.-J. Hsu, C.-P. Hsieh, C.-L. Chiu, C.-Y. Yeh
and E. W.-G. Diau, Phys. Chem. Chem. Phys., 2009, 11, 10270; (d)
C.-L. Mai, W.-K. Huang, H.-P. Lu, C.-W. Lee, Y.-R. Liang, E. W.-
G. Diau and C.-Y. Yeh, Chem. Commun., 2010, 46, 809.
9530 | J. Mater. Chem., 2011, 21, 9523–9531
This journal is ª The Royal Society of Chemistry 2011

View Article Online
11 (a) Y.-J. Chang and T. J. Chow, Tetrahedron, 2009, 65, 4726; (b) Y.-
J. Chang and T. J. Chow, Tetrahedron, 2009, 65, 9626.
12 (a) J.-H. Yum, D. P. Hagberg, S.-J. Moon, K. M. Karlsson,
T. Marinado, L. Sun, A. Hagfeldt, M. K. Nazeeruddin and
B.-S. Chen, D.-Y. Chen, C.-L. Chen, C.-W. Hsu, H.-C. Hsu, K.-
L. Wu, S.-H. Liu, P.-T. Chou and Y. Chi, J. Mater. Chem., 2011,
21, 1937.
15 W. Zeng, Y. Cao, Yu. Bai, Y. Wang, Y. Shi, Min. Zhang, F. Wang,
C. Pan and P. Wang, Chem. Mater., 2010, 22, 1915.
16 C.-L. Li, S.-J. Shieh, S.-C. Lin and R.-S. Liu, Org. Lett., 2003, 5, 1131.
€
M. Gratzel, Angew. Chem., Int. Ed., 2009, 48, 1576; (b) H. Im,
S. Kim, C. Park, S.-H. Jang, C.-J. Kim, K. Kim, N.-G. Park and
ꢀ
17 M. Da˛browski, J. Kubicka, S. Lulinski and Janusz Serwatowski,
Tetrahedron, 2005, 61, 6590.
18 S. Urgaonkar, M. Nagarajan and J. G. Verkade, J. Org. Chem., 2003,
68, 452.
19 E. Knoevenagel, Angew. Chem., Int. Ed. Engl., 1922, 35, 29.
20 H. Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt and L. Sun, J.
Phys. Chem. C, 2008, 112, 11023.
21 G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu and P. Wang,
Chem. Commun., 2010, 2198.
C. Kim, Chem. Commun., 2010, 46, 1335; (c) G. Zhang, Y. Bai,
€
R. Li, D. Shi, S. Wenger, S. M. Zakeeruddin, M. Gratzel and
P. Wang, Energy Environ. Sci., 2009, 2, 92; (d) D. P. Hagberg, J.-
H. Yum, H. Lee, F. D. Angelis, T. Marinado, K. M. Karlsson,
€
R. Humphry-Baker, L. Sun, A. Hagfeldt, M. Gratzel and
M. K. Nazeeruddin, J. Am. Chem. Soc., 2008, 130, 6259; (e)
C. Kim, H. Choi, S. Kim, C. Baik, K. Song, M.-S. Kang,
S. O. Kang and J. Ko, J. Org. Chem., 2008, 73, 7072; (f) M. Xu,
R. Li, N. Pootrakulchote, D. Shi, J. Guo, Z. Yi,
€
S. M. Zakeeruddin, M. Gratzel and P. Wang, J. Phys. Chem. C,
2008, 112, 19770; (g) D. Shi, Y. Cao, N. Pootrakulchote, Z. Yi,
22 W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long and H. Tian, J.
Mater. Chem., 2010, 20, 1772.
€
M. Xu, S. M. Zakeeruddin, M. Gratzel and P. Wang, J. Phys.
23 (a) K. R. Justin Thomas, Y.-C. Hsu, J.-T. Lin, K.-M. Lee, K.-C. Ho,
C.-H. Lai, Y.-M. Cheng and P. T. Chou, Chem. Mater., 2008, 20,
1830; (b) Q. Li, L. Lu, C. Zhong, J. Huang, Q. Huang, J. Shi,
X. Jin, T. Peng, J. Qin and Z. Li, Chem.–Eur. J., 2009, 15, 9664; (c)
N. Cho, H. Choi, D. Kim, K. Song, M.-S. Kang, S. O. Kang and
J. Ko, Tetrahedron, 2009, 65, 6236; (d) J. Tang, J. Hua, W. Wu,
J. Li, Z. Jin, Y. Long and H. Tian, Energy Environ. Sci., 2010, 3, 1736.
Chem. C, 2008, 112, 17478; (h) R. Li, X. Lv, D. Shi, D. Zhou,
Y. Cheng, G. Zhang and P. Wang, J. Phys. Chem. C, 2009, 113, 7469.
13 J. Song, F. Zhang, C. Li, W. Liu, B. Li, Y. Huang and Z. Bo, J. Phys.
Chem. C, 2009, 113, 13391.
14 (a) W.-H. Liu, I.-C. Wu, C.-H. Lai, C.-H. Lai, P.-T. Chou, Y.-T. Li,
C.-L. Chen, Y.-Y. Hsu and Y. Chi, Chem. Commun., 2008, 5152; (b)
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J. Mater. Chem., 2011, 21, 9523–9531 | 9531