Geometrical effect of stilbene on the performance of organic dye-sensitized solar cells

Article abstract of DOI:10.1039/c1jm11623a

New metal-free organic donor-bridge-acceptor dyes comprising a triphenylamine moiety as the electron donor (D), a cyanoacrylic acid moiety as the electron acceptor (A), and a conjugated chromophore between D and A as a spacer (S) were synthesized for the application of dye-sensitized solar cells (DSSCs). Two types of S were designed, one contains a normal stilbene backbone (dye 1) and the other contains a 2-phenylindene moiety (dye 2) with the central double bond locked in a transoid geometry. The performance of DSSCs made with these two types of dyes was compared, and it was found that the inhibition of trans/cis isomerization of the central double bond was advantageous to the efficiency of DSSCs. The phenomenon was investigated by a comparison of their photophysical properties, and rationalized by quantum mechanical calculations by using density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The electronic transitions were analyzed by time-dependent DFT, and the results were consistent with the experimental observations. The DSSCs were fabricated using both types of materials, and the performance was recorded under AM 1.5G irradiation (100 mW cm^-2) condition. The overall conversion efficiencies of dye 2 (5.14-5.67%) were higher than those of dye 1 (4.52-4.98%). Both the short-circuit photocurrent density (J_sc) and the open-circuit voltage (V_oc) were improved in 2 with respect to those of 1. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1jm11623a

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PAPER
Geometrical effect of stilbene on the performance of organic dye-sensitized
solar cells†
*
Yan-Duo Lin and Tahsin J. Chow
Received 15th April 2011, Accepted 19th July 2011
DOI: 10.1039/c1jm11623a
New metal-free organic donor–bridge–acceptor dyes comprising a triphenylamine moiety as the
electron donor (D), a cyanoacrylic acid moiety as the electron acceptor (A), and a conjugated
chromophore between D and A as a spacer (S) were synthesized for the application of dye-sensitized
solar cells (DSSCs). Two types of S were designed, one contains a normal stilbene backbone (dye 1) and
the other contains a 2-phenylindene moiety (dye 2) with the central double bond locked in a transoid
geometry. The performance of DSSCs made with these two types of dyes was compared, and it was
found that the inhibition of trans/cis isomerization of the central double bond was advantageous to the
efficiency of DSSCs. The phenomenon was investigated by a comparison of their photophysical
properties, and rationalized by quantum mechanical calculations by using density functional theory
(DFT) at the B3LYP/6-31G(d,p) level. The electronic transitions were analyzed by time-dependent
DFT, and the results were consistent with the experimental observations. The DSSCs were fabricated
using both types of materials, and the performance was recorded under AM 1.5G irradiation (100 mW
cmꢀ2) condition. The overall conversion efficiencies of dye 2 (5.14–5.67%) were higher than those of dye
1 (4.52–4.98%). Both the short-circuit photocurrent density (Jsc) and the open-circuit voltage (Voc) were
improved in 2 with respect to those of 1.
photo-excitation. In general, triarylamine (TPA) moieties have
Introduction
been widely used as electron donors due to their strong electron-
donating ability. Carboxylic acid, 2-cyanoacrylic acid, or rho-
danine moieties are usually employed as the electron acceptors as
well as the anchoring groups to attach onto the surface of
TiO2.1,14 Strategies toward high efficiency organic dyes are
developed to harvest photons mainly in the visible and near-IR
regions. In order to extend the absorption region, additional
double bonds were inserted into the main chromophores to
lengthen the p conjugation, so as to red-shift the absorption
spectrum. Among all p-bonds, ethylene is one of the most
commonly used units. However, the quantum efficiency of dyes
containing ethylene units is usually low due to a substantial
energy loss upon photoisomerization.15 The flexible conforma-
tion of ethylene could also lead to an undesired back-electron
transfer, therefore a reduction of open-circuit voltage.16 In this
regard an inhibition of trans / cis photoisomerization of
ethylene units along the main chromophore should be advanta-
geous to the efficiency of DSSCs.
The concern regarding global warming resulting from increasing
energy consumption and environmental pollution has led to
greater attention towards clean and renewable energy sources
over recent decades. Dye-sensitized solar cells (DSSCs) have
been intensively studied since their discovery by O’Regan and
1
2
€
Gratzel et al. in 1991. The Ru based sensitizers such as N3,
N719,3 and black dye4 exhibited impressive solar energy-to-
electricity conversion efficiencies up to ꢁ11%. On the other hand,
the search for metal-free organic sensitizers as an alternate to Ru
complexes is getting more intensive due to their low cost, envi-
ronmental friendliness, high molar absorption coefficient,
tunable spectral and electrochemical properties, etc. A variety of
organic dyes such as coumarin,5 merocyanine,6 cyanine,7 indo-
line,8 triphenylamine,9 dithieno[3,2-b:20,30-d]thiophene,10 carba-
zole,11 fluorene,12 and spirobifluorene,13 etc. have been
investigated.
The donor (D)–spacer (S)–acceptor (A) system is a basic
architecture for the design of organic sensitizers, which can
facilitate effective intramolecular charge transfer (ICT) upon
A compound with a stilbene backbone, i.e., 1P in Fig. 1, has
been reported to exhibit a remarkable solar-to-energy conversion
efficiency of 9.1%.9f However, stilbene derivatives are well-
known for their trans / cis isomerization as a major non-
radiative decay channel.17 For an excited trans-stilbene in fluid
solution, there are two major decay processes to dissipate the
energy, i.e., either through a radiative emission or through a
Institute of Chemistry, Academia Sinica, Taipei, 115, Taiwan. E-mail:
tjchow@chem.sinica.edu.tw
† Electronic supplementary information (ESI) available: The synthesis of
1Z, absorption spectra of dyes 1 and 2 in different solvents and TDDFT
calculated parameters of dyes 1 and 2. See DOI: 10.1039/c0jm03405c
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General procedures for Heck reaction. A heterogeneous
mixture of triethylamine (0.28 mL), DMF (1.00 mL), styrene
derivatives (1.05 mmol), Pd(OAc)2 (2 mol%), P(o-tolyl)3
ꢂ
(4 mol%), and aryl bromide (1.00 mmol) were heated at 80 C
under an argon atmosphere for 18 h. The solution was cooled, to
it was added CH2Cl2 (20 mL). The insoluble residue was filtered
off, and the filtrate was concentrated in vacuo to afford a crude
product. Further purification was performed by column
chromatography.
General procedures for Knoevenagel condensation reaction. A
mixture of aldehyde precursor (1.10 mmol), cyanoacetic acid
(1.30 mmol), and ammonium acetate (0.28 mmol) in acetic acid
(20 mL) was placed in a three-necked flask under a nitrogen
atmosphere and was stirred at 80 ꢂC for 18 h. After cooling, the
reaction was extracted with CH2Cl2. The organic layer was dried
over anhydrous MgSO4. The filtrate was concentrated under
reduced pressure. Purification was performed by recrystallization
in CH2Cl2/hexane.
Fig. 1 Molecular structures of dyes 1 and 2.
non-radiative path of rotating about the central double bond.
There is a thermal barrier for the non-radiative process but not
for the fluorescence emission. Trans / cis isomerization in the
excited state reaches to a minimum on the potential energy
surface with
a perpendicular geometry, corresponding to
a conical intersection with the ground-state energy surface.
Stilbene derivatives are suitable tools for analyzing the influence
of photoisomerization on the quantum efficiency of DSSCs.
In this report, two types of organic dyes 1 and 2 (Fig. 1) are
prepared, and their performance on DSSCs compared. For dye
2, the p-conjugated spacer contains an indene moiety in order to
inhibit trans / cis photoisomerization,18 and to compare with
the parent stilbene derivatives 1. Detailed structural and elec-
tronic properties were computed by using density function theory
(DFT) and time-dependent DFT. The photovoltaic performance
of these dyes was analyzed with the aid of the theoretical results.
General procedures for reduction reaction with DIBAL-H. To
a solution of CN-substituted precursor (1.00 mmol) in THF (10
mL) at ꢀ78 ꢂC was added DIBAL-H (diisobutylaluminium
hydride) (20% in cyclohexane, 1.9 mL, 2.00 mmol) under an
ꢂ
argon atmosphere. The mixture was stirred and kept at ꢀ20 C
for 16 h. The reaction was quenched by dropwise addition of
concentrated HCl(aq) (3.0 mL). The solution was then extracted
with water. The organic layer was dried over anhydrous MgSO4,
and the filtrate was concentrated under reduced pressure.
Further purification of the CHO-substituted product was per-
formed by column chromatography.
Experimental section
3-[4-(2-{4-[Bis(4-methoxyphenyl)amino]phenyl}vinyl)phenyl]-2-
cyanoacrylic acid (1M). Compound 1M was obtained according
to the standard Knoevenagel condensation reaction as described
above. Purification was performed by recrystallization in
CH2Cl2/hexane to afford red solid (0.12 g, 82% yield). Mp 236–
237 ꢂC. Spectral data of 1M: (found: C, 76.28, H, 5.32, N, 5.46.
C32H26N2O4 requires C, 76.48, H, 5.21, N, 5.57%). dH (400 MHz,
d6-DMSO) 8.28 (s, 1H), 8.03 (d, J ¼ 8.4 Hz, 2H), 7.73 (d, J ¼ 8.4
Hz, 2H), 7.45 (d, J ¼ 8.7 Hz, 2H), 7.38 (d, J ¼ 16.3 Hz, 1H), 7.12–
7.04 (m, 5H), 6.95–6.92 (m, 4H), 6.75 (d, J ¼ 8.7 Hz, 2H), 3.75 (s,
6H) ppm; dC (100 MHz, d6-DMSO) 164.43, 157.00, 154.54,
149.65, 143.59, 140.47, 132.81, 132.27, 130.65, 128.94, 127.98,
127.49, 124.94, 119.46, 117.38, 115.93, 102.64, 56.16 ppm. IR
(KBr): 3031, 2819, 2225, 1543, 1265, 955, 694 cmꢀ1; m/z (FAB)
502.1886 (M+. C32H26N2O4 requires 502.1893).
Materials
1H and 13C NMR spectra were recorded on a Bruker 400 MHz
spectrometer. Fast atom bombardment (FAB) mass spectra were
recorded on a Jeol JMS 700 double-focusing spectrometer.
Elemental analyses were determined by the Instrumentation
Center of National Cheng-Kung University. UV spectra were
measured on a Jasco V-530 double beam spectrophotometer.
Fluorescence spectra were recorded on a Hitachi F-4500 fluo-
rescence spectrophotometer. The optical density (OD) of all
solutions was about 0.1 at the wavelength of excitation. A N2-
bubbled solution of anthracene (Ff ¼ 0.27 in hexane)19 was used
as standards for the fluorescence quantum yield determinations.
An error of 10% is estimated for the fluorescence quantum yields.
Fluorescence lifetimes were also measured at room temperature
by the use of an Edinburgh FLS920 spectrometer. The instru-
ment response function was calibrated by a scatter solution using
a gated hydrogen arc lamp. The goodness of the nonlinear least-
squares fit was judged by the reduced c2 value (<1.2 in all cases),
the randomness of the residuals, and the autocorrelation func-
tion. Solvents of reagent grade were used for syntheses, and those
of spectroscopy grade for spectra measurements. Compounds
purchased from commercial sources were used as received. The
syntheses of compounds 1P,9f 3,20 4M,9d 4N,9d 4P,9d 4T,9d 7M,22
7N,21 7P,9f 7T,23 8M,24 8P,9f 8T,25 and 9P9f have been reported
previously. The synthetic procedure of compound 1Z is included
in the ESI†.
2-Cyano-3-(4-{2-[4-(naphthalen-1-ylphenylamino)phenyl]vinyl}
phenyl)acrylic acid (1N). Compound 1N was obtained according
to the standard Knoevenagel condensation reaction as described
above. Purification was performed by recrystallization in
CH2Cl2/hexane to afford red solid (1.00 g, 85% yield). Mp 201–
ꢂ
202 C. Spectral data of 1N: dH (400 MHz, d6-DMSO) 8.28 (s,
1H), 8.04–8.01 (m, 3H), 7.93 (d, J ¼ 8.2 Hz, 1H), 7.84 (d, J ¼ 8.4
Hz, 1H), 7.73 (d, J ¼ 8.3 Hz, 2H), 7.61–7.37 (m, 7H), 7.30–7.26
(m, 2H), 7.13 (d, J ¼ 16.3 Hz, 1H), 7.05–6.99 (m, 3H), 6.86
(d, J ¼ 8.6 Hz, 2H) ppm; dC (100 MHz, d6-DMSO) 171.89,
163.44, 153.56, 148.14, 147.04, 142.42, 142.23, 134.90, 131.62,
14908 | J. Mater. Chem., 2011, 21, 14907–14916
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131.28, 130.49, 129.89, 129.42, 128.57, 128.11, 127.30, 126.96,
126.66, 126.32, 124.86, 123.29, 122.67, 122.40, 120.04, 116.37,
101.93 ppm. IR (KBr): 3035, 2817, 2224, 1544, 1262, 965, 694
cmꢀ1; m/z (FAB) 493.1916 ((M + H)+, C34H25N2O2 requires
493.1928).
ppm; dC (100 MHz, d8-THF) 164.48, 155.27, 152.86, 152.43,
149.42, 148.61, 144.86, 132.49, 130.34, 129.41, 128.11,
126.13,125.86, 125.65, 125.39, 124.41, 123.87, 121.89, 117.12,
101.63, 39.83 ppm. IR (KBr): 3033, 2821, 2221, 1560, 1259, 749,
695 cmꢀ1; m/z (FAB) 455.1767 ((M + H)+, C31H23N2O2 requires
455.1759).
2-Cyano-3-(4-{2-[4-(di-p-tolylamino)phenyl]vinyl}phenyl)acrylic
acid (1T). Compound 1T was obtained according to the standard
Knoevenagel condensation reaction as described above. Purifi-
cation was performed by recrystallization in CH2Cl2/hexane to
afford red solid (0.54 g, 87% yield). Mp 231–232 ꢂC. Spectral
data of 1T: dH (400 MHz, CDCl3) 8.26 (s, 1H), 8.02 (d, J ¼ 8.3
Hz, 2H), 7.59 (d, J ¼ 8.3 Hz, 2H), 7.34 (d, J ¼ 8.6 Hz, 2H), 7.22
(d, J ¼ 16.2 Hz, 1H), 7.10–7.01 (m, 4H), 7.00–6.95 (m, 7H), 2.33
(s, 6H) ppm; dC (100 MHz, CDCl3) 167.18, 155.84, 148.88,
144.70, 143.88, 133.31, 132.57, 132.22, 130.01, 129.59, 129.09,
127.91, 126.80, 125.18, 124.46, 121.49, 115.51, 99.91, 20.84 ppm.
IR (KBr): 3029, 2817, 2224, 1542, 1261, 953, 693 cmꢀ1; m/z
(FAB) 471.2082 ((M + H)+, C32H27N2O2 requires 471.2072).
2-Cyano-3-{2-[4-(di-p-tolylamino)phenyl]-3H-inden-5-yl}acrylic
acid (2T). Compound 2T was obtained according to the standard
Knoevenagel condensation reaction as described above. Purifi-
cation was performed by recrystallization in CH2Cl2/hexane to
afford red solid (0.14 g, 81% yield). Mp 265–266 ꢂC. Spectral
data of 2T: dH (400 MHz, d8-THF) 8.25 (s, 2H); 7.89 (d, J ¼ 7.8
Hz, 1H), 7.58–7.564 (m, 2H), 7.45 (d, J ¼ 8.0 Hz, 1H), 7.23 (s,
1H), 7.10–7.08 (m, 4H), 7.00–6.96 (m, 6H), 3.91 (s, 2H), 2.30 (s,
6H); dC (100 MHz, d8-THF) 164.51, 155.28, 153.09, 152.60,
149.83, 146.11, 144.80, 134.09, 132.54, 130.91, 129.33, 127.97,
127.65, 126.11, 125.91, 124.90, 123.00, 122.61, 121.75, 117.15,
37.80, 21.03; m/z (FAB) 483.2083 ((M + H)+, C33H27N2O2
requires 483.2072).
3-(2-{4-[Bis-(4-methoxyphenyl)amino]phenyl}-3H-inden-5-yl)-
2-cyanoacrylic acid (2M). Compound 2M was obtained accord-
ing to the standard Knoevenagel condensation reaction as
described above. Purification was performed by recrystallization
in CH2Cl2/hexane to afford red solid (0.29 g, 86% yield). Mp
282–283 ꢂC. Spectral data of 2M: dH (400 MHz, d8-THF) 8.24 (s,
2H); 7.88 (d, J ¼ 7.6 Hz, 1H), 7.57–7.49 (m, 2H), 7.43 (d, J ¼ 8.0
Hz, 1H), 7.19 (s, 1H), 7.08–7.03 (m, 4H), 6.88–6.84 (m, 6H), 3.89
(s, 2H), 3.76 (s, 6H); dC (100 MHz, d8-THF) 157.84, 153.34,
152.77, 150.49, 144.72, 141.63, 141.42, 132.59, 129.02, 128.10,
127.82, 127.57, 126.00, 125.03, 124.32, 124.01, 121.60, 120.87,
120.51, 115.73, 55.78, 39.77 ppm. IR (KBr): 3029, 2821, 2224,
1564, 1261, 754, 697 cmꢀ1; m/z (FAB) 515.1983 ((M + H)+,
C33H27N2O4 requires 515.1970).
2-{4-[Bis(4-methoxyphenyl)amino]phenyl}-3H-indene-5-carbon-
itrile (5M). Compound 5M was synthesized via the Heck reaction
as described above. Further purification was performed by
column chromatography using a mixed solvent CH2Cl2/hexane
(1/5) as the eluent to provide the desired product (1.08 g, 38%
yield). mp 195–196 ꢂC. Spectral data of 5M: (found C, 81.12; H,
5.59; N, 6.23, C30H24N2O2 requires C, 81.06; H, 5.44; N, 6.30%).
dH (400 MHz, CDCl3) 7.92 (t, J ¼ 7.5 Hz, 2H), 7.81 (d, J ¼ 8.3
Hz, 2H), 7.64 (s, 1H), 7.57 (s, 1H), 7.54–7.36 (m, 4H), 7.10–7.02
(m, 5H), 6.87 (d, J ¼ 3.0 Hz, 2H), 6.84 (d, J ¼ 3.3 Hz, 2H), 3.81
(s, 6H), 3.78 (s, 2H) ppm. dC (100 MHz, CDCl3) 156.33, 151.02,
150.52, 149.26, 142.96, 140.24, 131.31, 128.00, 127.02, 126.93,
126.79, 126.57, 123.01, 120.64, 119.60, 114.82, 110.39, 106.63,
55.50, 38.86 ppm. IR (KBr): 3009, 2225, 1573, 1431, 1318, 1286,
763, 699 cmꢀ1. m/z (FAB) 445.1927 ((M + H)+, C30H25N2O2
requires 445.1916).
2-Cyano-3-{2-[4-(naphthalen-1-ylphenylamino)phenyl]-3H-inden-
5-yl}acrylic acid (2N). Compound 2N was obtained according to
the standard Knoevenagel condensation reaction as described
above. Purification was performed by recrystallization in
CH2Cl2/hexane to afford red solid (0.12 g, 73% yield). Spectral
data of 2N: Mp 282–283 ꢂC. dH (400 MHz, d8-THF) 8.24 (s, 2H);
7.94–7.82 (m, 4H), 7.57–7.43 (m, 5H), 7.37–7.33 (m, 2H), 7.23–
7.18 (m, 3H), 7.10–7.06 (m, 2H), 6.98–6.94 (m, 3H), 3.89 (s, 2H)
ppm; dC (100 MHz, d8-THF) 164.47, 155.31, 153.04, 152.56,
150.10,149.02, 144.80, 144.24, 136.71, 132.53, 131.85, 130.20,
129.48, 129.33, 128.33, 128.08, 127.82, 127.30, 127.16, 126.07,
125.05, 124.96, 124.67, 123.97, 123.64, 123.36, 122.81, 122.04,
121.70, 117.12, 101.43, 39.78. IR (KBr): 3031, 2820, 2225, 1561,
2-[4-(1-Naphthylphenylamino)phenyl]-3H-indene-5-carbonitrile
(5N). Compound 5N was synthesized via a standard procedure of
the Heck reaction as described above. Further purification was
performed by column chromatography, using a mixed solvent
CH2Cl2/hexane (1/10) as eluent to provide the desired product
(1.14 g, 44% yield). mp 178–179 ꢂC. Spectral data of 5N: (found
C, 88.39; H, 5.20; N, 6.33, C32H22N2 requires C, 88.45; H, 5.10;
N, 6.45%). dH (400 MHz, CDCl3) 7.92 (t, J ¼ 7.5 Hz, 2H), 7.81
(d, J ¼ 8.3 Hz, 2H), 7.64 (s, 1H), 7.53–7.35 (m, 8H), 7.26–7.22 (m,
2H), 7.14–6.97 (m, 6H), 3.78 (s, 2H) ppm. dC (100 MHz, CDCl3)
150.82, 150.36, 148.94, 148.70, 147.58, 142.99, 135.31, 131.30,
129.27, 128.48, 128.12, 127.65, 127.34, 126.90, 126.72, 126.59,
126.34, 124.02, 123.50, 122.95, 122.78, 122.63, 120.75, 120.53,
120.17, 110.41, 106.81 ppm. IR (KBr): 3011, 2224, 1568, 1434,
1322, 1289, 767, 693 cmꢀ1. m/z (FAB) 435.1870 ((M + H)+,
C32H23N2 requires 435.1861).
; m/z (FAB) 505.1927 ((M +
1258, 750, 696 cmꢀ1 H)+,
C35H25N2O2 requires 505.1916).
2-Cyano-3-[2-(4-diphenylaminophenyl)-3H-inden-5-yl]acrylic
acid (2P). Compound 2P was obtained according to the standard
Knoevenagel condensation reaction as described above. Purifi-
cation was performed by recrystallization in CH2Cl2/hexane to
afford red solid (0.34 g, 75% yield). Mp 305–306 ꢂC. Spectral
data of 2P: dH (400 MHz, d8-THF) 8.25 (s, 2H), 7.98 (d, J ¼ 7.8
Hz, 1H), 7.61 (d, J ¼ 8.68 Hz, 2H), 7.46 (d, J ¼ 7.8 Hz, 1H), 7.28–
7.23 (m, 5H), 7.30–7.15 (m, 4H), 7.11–7.02 (m, 4H), 3.92 (s, 2H)
2-(4-Diphenylaminophenyl)-3H-indene-5-carbonitrile (5P).
Compound 5P was synthesized via a standard procedure of the
Heck reaction as described above. Purification was performed by
silica gel column chromatograph eluted with a mixture of
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J. Mater. Chem., 2011, 21, 14907–14916 | 14909

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CH2Cl2/hexane (1/5) to provide product 5P (2.06 g, 40% yield).
mp 206–207 ꢂC. Spectral data of 5P: dH (400 MHz, CDCl3) 7.67–
7.60 (m, 1H), 7.56–7.41 (m, 3H), 7.44–7.40 (m, 1H), 7.30–7.26
(m, 4H), 7.14–7.06 (m, 9H), 3.81 (s, 2H) ppm. dC (100 MHz,
CDCl3) 150.70, 147.21, 143.05, 131.33, 129.39, 128.36, 126.66,
124.91, 124.80, 123.89, 123.54, 122.92, 122.72, 120.87, 116.66,
106.46, 38.89 ppm. IR (KBr): 3030, 2213, 1590, 1487, 1329, 1283,
765, 696 cmꢀ1. m/z (FAB) 384.1626 (M+, C28H20N2 requires
384.1634).
ethyl acetate/hexane (1/10) afforded the yellow solids of 6P
ꢂ
(1.27g, 78% yield). mp 199–200 C. Spectral data of 6P: (found
C, 86.84; H, 5.52; N, 3.51C28H21NO requires C, 86.79; H, 5.46;
N, 3.61%). dH (400 MHz, CDCl3) 9.99 (s, 1H), 7.94 (s, 1H), 7.78
(d, J ¼ 7.8 Hz, 1H), 7.52 (d, J ¼ 7.8 Hz, 2H), 7.46 (d, J ¼ 7.7 Hz,
2H), 7.30–7.15 (m, 4H), 7.13–7.09 (m, 5H), 7.07 (d, J ¼ 6.8 Hz,
4H), 3.84 (s, 1H) ppm. dC (100 MHz, CDCl3) 192.08, 152.30,
151.49, 148.27, 147.25, 143.17, 133.10, 130.48, 129.38, 128.74,
126.92, 124.88, 124.29, 123.69, 123.49, 122.79, 120.69, 38.89
ppm. IR (KBr): 3030, 2732, 1686, 1585, 1486, 1268, 750, 696
cmꢀ1. m/z (FAB) 388.1709 ((M + H)+, C28H22NO requires
388.1701).
2-[4-(Di-p-tolylamino)phenyl]-3H-indene-5-carbonitrile (5T).
Compound 5T was synthesized via the Heck reaction as
described above. Further purification was performed by column
chromatography, using a mixed solvent CH2Cl2/hexane (1/10) as
the eluenꢂt to provide the desired product (2.00 g, 42% yield). mp
205–206 C. Spectral data of 5T: dH (400 MHz, d8-THF) 7.70–
7.52 (m, 4H), 7.44–7.41 (m, 1H), 7.18–7.16 (m, 1H), 7.09–7.07
(m, 4H), 6.99–6.96 (m, 6H), 3.86 (s, 2H), 2.29 (s, 6H) ppm. dC
(100 MHz, CDCl3) 152.12, 151.30, 149.69, 146.00, 144.56,
133.95, 131.86, 130.78, 128.99, 127.79, 127.41, 125.95, 124.32,
122.53, 121.65, 120.21, 108.27, 39.63, 20.91 ppm. IR (KBr): 3014,
2224, 1571, 1429, 1313, 1281, 760, 696 cmꢀ1. m/z (FAB) 413.2026
((M + H)+, C30H25N2 requires 413.2018).
2-[4-(Di-p-tolylamino)phenyl]-3H-indene-5-carbaldehyde (6T).
Compound 6T was synthesized via a standard reduction proce-
dure as described above. Column chromatography using ethyl
acetate/hexane (1/10) as eluent afforded the yellow solid of 6T
(0.47 g, 85% yield). Spectral data of 6T: mp 200–201 ꢂC. dH (400
MHz, d8-THF) 9.94 (s, 1H), 7.91 (s, 1H), 7.75–7.73 (m, 1H),
7.59–7.52 (m, 2H), 7.46–7.44 (m, 1H), 7.22 (s, 1H), 7.09–7.07 (m,
4H), 6.99–6.96 (m, 6H), 3.87 (s, 2H), 2.30 (s, 6H). dC (100 MHz,
d8-THF) 191.48, 153.04, 152.58, 149.60, 146.04, 144.42, 134.69,
133.89, 130.77, 129.36, 127.77, 127.50, 125.92, 124.72, 124.26,
122.61, 121.32, 39.60, 20.91. IR (KBr): 3035, 2714, 1692, 1588,
1479, 1277, 750, 699 cmꢀ1. m/z (FAB) 416.2025 ((M + H)+,
C30H26NO requires 416.2014).
2-{4-[Bis-(4-methoxyphenyl)amino]phenyl}-3H-indene-5-car-
baldehyde (6M). Compound 6M was synthesized via a standard
reduction procedure as described above. Column chromatog-
raphy using ethyl acetate/hexane (1/10) as eluent afforded the
yellow solids of 6M (0.40 g, 80% yield). mp 180–181 ꢂC. Spectral
data of 6M: (found: C, 80.46; H, 5.60; N, 3.39, C30H25NO3
requires C, 80.51; H, 5.63; N, 3.13%). dH (400 MHz, CDCl3) 9.98
(s, 1H), 7.92 (s, 1H), 7.77 (d, J ¼ 7.9 Hz, 1H), 7.48–7.43 (m, 3H),
7.10–7.07 (m, 5H), 6.93–6.91 (m, 2H), 6.88–6.85 (m, 4H), 3.81 (s,
6H), 3.78 (s, 2H). dC (100 MHz, CDCl3) 192.10, 152.59, 151.87,
149.20, 148.80, 143.09, 140.48, 140.30, 132.91, 130.54, 127.00,
123.62, 123.42, 120.46, 119.98, 119.67, 114.83, 55.50, 38.86. IR
(KBr): 3026, 2728, 1681, 1581, 1484, 1271, 749, 697 cmꢀ1. m/z
(FAB) 448.1926 ((M + H)+, C30H26NO3 requires 448.1913).
1-Naphthylphenyl(4-vinylphenyl)amine
(8N).
Methyl-
triphenylphosphonium iodide (9.37 g, 23.2 mmol) was dissolved
in THF (80 mL). To it was added NaH (2.78 g, 69.6 mmol) slowly
in an ice-water bath, and then the mixture was stirred for another
2 h at ambient temperature. Compound 7N (3.75 g, 11.6 mmol)
in THF (30 mL) was added dropwise to this reaction mixture by
using a pressure-equalizing addition funnel. After the completion
of reaction, methanol was added to quench excess NaH in an ice-
water bath and then THF was removed by rotary evaporation.
The reaction mixture was extracted with ethyl acetate. The
organic layer was dried over anhydrous MgSO4 and filtered. The
filtrate was concentrated under reduced pressure. Column
chromatograph eluted with ethyl acetate/hexane (1/25) afforded
the desired product as white solids (3.39 g, 91% yield). mp 93–
95 ꢂC. Spectral data of 8N: dH (400 MHz, CDCl3) 7.93–7.87
(m, 2H), 7.77 (d, J ¼ 8.2 Hz, 1H), 7.49–7.43 (m, 2H), 7.37–7.31
(m, 2H), 7.22–7.18 (m, 4H), 7.05 (d, J ¼ 8.0 Hz, 2H), 6.96–
6.93 (m, 3H), 6.67–6.59 (m, 1H), 5.59 (d, J ¼ 17.6 Hz, 1H), 5.11
(d, J ¼ 10.9 Hz, 1H). dC (100 MHz, CDCl3) 148.30, 143.49,
136.45, 135.46, 132.22, 131.38, 131.08, 129.30, 128.58, 127.39,
127.17, 126.73, 126.60, 126.51, 126.33, 124.37, 122.39, 122.17,
121.49, 111.82. m/z (FAB) 322.1600 ((M + H)+, C24H20N
requires 322.1596).
2-[4-(1-Naphthylphenylamino)phenyl]-3H-indene-5-carbalde-
hyde (6N). Compound 6N was synthesized via the standard
reduction procedure as described above. Column chromatog-
raphy eluted ethyl acetate/hexane (1/10) afforded the yellow
solids of 6N (0.86 g, 82% yield). mp 190–191 ꢂC. Spectral data of
6N: (found: C, 87.95; H, 5.42; N, 3.20. C32H23NO requires C,
87.84; H, 5.30; N, 3.20%). dH (400 MHz, CDCl3) 9.98 (s, 1H),
7.94–7.90 (m, 3H), 7.82–7.76 (m, 2H), 7.52–7.36 (m, 7H), 7.26–
7.22 (m, 2H), 7.14–7.11 (m, 3H), 7.03–6.97 (m, 3H), 3.80 (s, 2H)
ppm. dC (100 MHz, CDCl3) 192.08, 152.40, 151.63, 148.87,
147.63, 143.11, 142.92, 135.31, 133.00, 131.17, 130.48, 129.27,
128.48, 127.82, 127.34, 126.93, 126.59, 126.35, 126.25, 124.03,
123.89, 123.65, 122.92, 122.72, 122.48, 120.88, 120.58, 38.84
ppm. IR (KBr): 3028, 2731, 1683, 1581, 1483, 1270, 751, 695
cmꢀ1. m/z (FAB) 438.1862 ((M + H)+, C32H24NO requires
438.1858).
trans-4-Formyl-40N-N-bis(4-methoxyphenyl)aminostilbene (9M).
Compound 9M was synthesized via a standard Heck reaction
procedure as described above. Column chromatography eluted
with ethyl acetate/hexane (1/10) afforded the yellow solid of 9M
(0.36 g, 42% yield). mp 172–173 ꢂC. Spectral data of 9M: dH (400
MHz, CDCl3) 9.96 (s, 1H), 7.83 (d, J ¼ 8.2 Hz, 2H), 7.60 (d, J ¼
8.2 Hz, 2H), 7.34 (d, J ¼ 8.6 Hz, 2H), 7.19 (d, J ¼ 16.3 Hz, 1H),
7.16–6.98 (m, 4H), 6.96 (d, J ¼ 16.3 Hz, 1H), 6.90 (d, J ¼ 8.6 Hz,
2-(4-Diphenylaminophenyl)-3H-indene-5-carbaldehyde (6P).
Compound 6P was synthesized via a standard reduction proce-
dure as described above. Column chromatography eluted with
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2H), 6.87–6.84 (m, 4H), 3.81 (s, 6H) ppm. dC (100 MHz, CDCl3)
191.55, 156.25, 149.18, 144.14, 140.38, 134.80, 132.01, 130.23,
128.39, 127.73, 126.934, 126.48, 124.31, 119.83, 114.80, 55.49. IR
(KBr): 2805, 1929, 1683, 1279, 962, 695 cmꢀ1. m/z (FAB)
435.1830 (M+, C29H25NO3 requires 435.1834).
The current–voltage parameters of DSSCs were recorded by
a potentiostat/galvanostat model CHI650B (CH Instruments,
USA).
Results and discussion
trans-4-Formyl-40N-naphthyl-N-phenylaminostilbene (9N).
Compound 9N was synthesized via a standard Heck reaction
procedure as described above. Column chromatography eluted
with ethyl acetate/hexane (1/10) afforded the yellow solids of 9N
(0.79 g, 38% yield). mp 162–163 ꢂC. Spectral data of 9N: dH (400
MHz, CDCl3) 9.97 (s, 1H), 7.91 (d, J ¼ 8.0 Hz, 2H), 7.85–7.79
(m, 3H), 7.60 (d, J ¼ 8.2 Hz, 2H), 7.48 (d, J ¼ 7.7 Hz, 2H), 7.39–
7.35 (m, 4H), 7.35–7.21 (m, 3H), 7.16–7.00 (m, 2H), 7.00–6.95
(m, 4H): dC (100 MHz, CDCl3) 191.52, 148.88, 147.73, 143.97,
143.01, 135.32, 134.93, 131.87, 131.17, 130.22, 129.39, 129.25,
128.47, 127.84, 127.32, 126.83, 126.54, 126.34, 126.24, 124.93,
124.08, 122.86, 122.64, 120.78. IR (KBr): 2807, 1933, 1680, 1281,
961, 693 cmꢀ1. m/z (FAB) 426.1867 ((M + H)+, C31H24NO
requires 426.1858).
Synthesis
Scheme 1 shows the synthesis of series 2 from compound 3.
Compound 3 was prepared according to literature as a pair of
tautomers, i.e., 5- and 6-cyanoindene isomers, which could not
be isolated.20 The mixture was used together to react with
compounds 4P, 4N, 4M, and 4T, respectively by a Heck-type
reaction to obtain compounds 5P, 5N, 5M, and 5T. Compounds
5P, 5N, 5M, and 5T were reduced to a formyl group with
DIBAL-H according to a known procedure28 and afforded
compounds 6P, 6N, 6M, and 6T, which were finally converted to
2P, 2N, 2M, and 2T through the Knoevenagel condensation
reaction with 2-cyanoacrylic acid. The syntheses of 1N, 1M, and
1T are shown in Scheme 2. The synthesis of dye 1P has been
recently reported,9f and that of 1Z in the ESI†. Preparation of
compounds 8N, 8M, and 8T was achieved through a methyle-
nation of the aldehyde groups via the Wittig reaction. Subse-
quently, 8N, 8M, and 8T were then coupled with
p-bromobenzaldehyde by a Heck-type reaction to afford 9N,
9M, and 9T, respectively. Subsequent Knoevenagel condensa-
tion reaction with 2-cyanoacetic acid in the presence of ammo-
nium acetate afforded the dyes 1N, 1M, and 1T, respectively.
trans-4-Formyl-40N-N-bis(4-methylphenyl)aminostilbene (9T).
Compound 9T was synthesized via a standard Heck reaction
procedure as described above. Column chromatography with
ethyl acetate/hexane (1/10) as eluent afforded the yellow solids of
9T (0.29 g, 36% yield). mp 144–145 ꢂC. Spectral data of 9T:
(found: C, 86.10; H, 6.20; N, 3.41. C29H25NO requires C, 86.32;
H, 6.24; N, 3.47%). dH (400 MHz, CDCl3) 9.97 (s, 1H), 7.85 (d, J
¼ 8.0 Hz, 2H), 7.61 (d, J ¼ 8.0 Hz, 2H), 7.34 (d, J ¼ 8.6 Hz, 2H),
7.37 (d, J ¼ 8.6 Hz, 2H), 7.20 (d, J ¼ 16.2 Hz, 2H), 7.11–7.04 (m,
4H), 7.02–6.97 (m, 7H), 2.34 (s, 6H). dC (100 MHz, CDCl3)
191.52, 148.64, 144.78, 144.00, 134.86, 133.16, 131.90, 130.22,
129.98, 129.32, 127.72, 126.50, 125.08, 124.77, 121.65, 20.83. IR
(KBr): 2802, 1932, 1682, 1283, 960, 694 cmꢀ1. m/z (FAB)
404.2021 ((M + H)+, C29H26NO requires 404.2014).
Absorption and electrochemical properties
The absorption spectra of dyes 1 and 2 in THF solution are
shown in Fig. 2 and the data are listed in Table 1. All dyes dis-
played two major absorption bands: i.e., a short wavelength one
at <350 nm corresponds to a p–p* transition, and a long
wavelength one at about 430–460 nm, except 1Z at 365 nm, is
Computation method. Computations were performed using the
Gaussian 03 program package.26 The geometry was optimized by
using B3LYP (Becke three parameters hybrid functional with
Lee–Yang–Parr correlation functionals) with the Pople 6-31G(d,
p) atomic basis set.27
Device fabrication. A 0.25 cm2 thin film of TiO2 which is 16–18
mm in thickness and consists of nanoparticles ꢁ20 nm in diam-
eter was coated on the FTO glass substrate. It was immersed in
a THF solution containing 3 ꢃ 10ꢀ4 M dye sensitizers for 12 h,
then rinsed with anhydrous acetonitrile and dried. Another piece
of FTO with sputtering 100 nm thick Pt was used as a counter
electrode. The active area was controlled at a dimension of
0.25 cm2 by adhering 60 mm thick polyester tape on the Pt elec-
trode. The photocathode was placed on top of the counter
electrode and was tightly clipped together to form a cell. Elec-
trolyte was then injected into the seam between two electrodes.
An acetonitrile solution containing LiI (0.5 M), I2 (0.05 M), and
4-tert-butylpyridine (0.5 M) was used as the electrolyte. Devices
made of a commercial dye N719 under the same condition was
compared as a reference. The cell parameters were obtained
under an incident light with intensity 100 mW cmꢀ2, which was
generated by a 300 W Xe lamp passing through an AM 1.5 filter.
Scheme 1 Synthetic scheme for the dye 2.
Scheme 2 Synthetic scheme for the dye 1.
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Fig. 2 Absorption spectra of (a) dye 1 and (b) dye 2 in the THF.
attributed to an intramolecular charge transfer (ICT) transition
from the TPA donor to the cyanoacrylic acid acceptor. The ICT
band was characterized by strong negative solvatochromic shifts
in different solvents (Fig. S1 and S2†). This phenomenon can be
ascribed to a partial deprotonation of the carboxylic acid group
in more polar solvents, which weakens its electron-withdrawing
ability, therefore elevating the potential energy level of the
acceptor.5c,13a The absorption spectra of dyes P and N are similar,
while those of dyes M and T are slightly red-shifted as a result of
an electron-donating effect. The molar extinction coefficient of
these dyes was ranging from 2.45–3.04 ꢃ 104 Mꢀ1 cmꢀ1, which
was higher than that of N719 (1.41 ꢃ 104 Mꢀ1 cmꢀ1), revealing
a good light harvesting nature.
p-chromophore out of plane (ESI, Fig. S4†). The absorptivity of
this band is also weaker than others, as shown in Fig. 2(a), which
is consistent with the smaller oscillator strength calculated by
time-dependent density function theory (ESI, Table S1†). The
blue-shift of the spectrum is expected to result in a lower light-
harvesting efficiency, which is also evidenced by its relatively low
absorbance on the surface of TiO2 (Fig. 3(a)).
The absorption spectra of dyes 1 and 2 on the surface of TiO2
are shown in Fig. 3, and the corresponding data are collected in
Table 1. The absorption maxima of the dyes exhibit an apparent
blue-shift in comparison with those in THF solution. It was
ascribed to the deprotonation of carboxylic acid upon attaching
to the surface of TiO2, as a similar effect to the negative sol-
vatochromic shift in polar solvents.5c,13a
The ICT band of 1Z is substantially blue-shifted comparing
with other compounds. The presence of a methyl substituent on
the double bond of the central stilbene moiety twisted the
The redox potentials of all compounds were measured by
cyclic voltammetry (CV) in THF solution containing 0.1 M
Table 1 Photophysical and electrochemical data for dyes 1 and 2
b
d
E0–0 (nm)/(eV)
Compd
labsa/nm
lmax (3 ꢃ 10ꢀ4/dm3 molꢀ1 cmꢀ1
)
lflc/nm
Eoxe/V
ELUMOf/V
1P
1N
1M
1T
1Z
2P
2N
2M
2T
433 (400)
433 (406)
455 (424)
437 (417)
305 (323)
426 (411)
437 (413)
455 (426)
447 (417)
2.71
2.73
3.04
2.72
3.17
2.82
2.95
2.61
2.45
607
611
567
552
526
592
589
557
605
482 (2.57)
500 (2.48)
514 (2.41)
500 (2.48)
433 (2.86)
487 (2.55)
500 (2.48)
507 (2.45)
496 (2.50)
1.15
1.20
0.88
1.03
1.16
1.09
1.20
0.90
1.02
ꢀ1.48
ꢀ1.28
ꢀ1.53
ꢀ1.45
ꢀ1.70
ꢀ1.46
ꢀ1.31
ꢀ1.55
ꢀ1.50
a
Maximum of absorption bands in THF, and that of TiO2 films in parentheses. b Extinction coefficient of the absorption band in THF. c Maximum of
fluorescence spectra in THF. d Estimated from the onset of the absorption spectra in THF. e Oxidation potential measured vs. Fc+/Fc in THF converted
f
to normal hydrogen electrode (NHE) by adding + 0.63 V. Estimated by Eox ꢀ E0–0
.
Fig. 3 Absorption spectra of (a) dye 1 and (b) dye 2 on the TiO2 films.
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tetrabutylammonium hexafluorophosphate (TBAPF6) as a sup-
porting electrolyte, Ag/AgNO3 as a reference electrode, and
a standard ferrocene/ferrocenium redox system as an internal
standard. The potential level of the highest occupied molecular
orbital (HOMO) was estimated from the first oxidative wave
(Eox) as listed in Table 1. The HOMO levels of all dyes were more
positive than the reducing potential of the iodine/iodide (0.4 V
versus NHE) pair, ensuring sufficient driving force for the
oxidized dyes to recapture an electron from the electrolyte. The
first oxidation potentials of dye 1M and 2M were shift negatively
by ca. 200 mV in comparison with those of 1P and 2P, as
a consequence of the stronger electron-donating nature of
methoxy group. The oxidation potentials of dyes 1 and 2 were
similar, revealing that the structure change on the bridges did not
affect the potential energy level of the TPA (Table 1). The lowest
unoccupied molecular orbital (LUMO) levels of the dyes were
deduced from the first oxidation potential and the 0–0 transition
energy (E0–0) estimated at the intersection of normalized
absorption and fluorescence spectra. The LUMO level of all the
dyes was higher than the TiO2 conductive band (ꢀ0.5 V), thus
warranted the feasibility of electron injection.
than those of dye 2. It complies well with our expectation, that
the trans / cis isomerization pathway is prohibited in dye 2. The
rate constants of fluorescence (kf ¼ Ff/sf) of 1 and 2 are close to
each other as evidenced by their structural similarity. The slow
fluorescence rates of the methoxy derivatives 1M and 2M are
noteworthy. Although their emission quantum yields are excep-
tionally low, their non-radiative decay rates are comparable to
others. It is therefore anticipated that depressing the rate of non-
radiative decay should have a profound effect of enhancing the
efficiency of solar cells. The fluorescence lifetimes of all dyes are
found in a range of 2.11–2.95 ns, significantly longer than the
time needed for effective electron transfers from the dyes to the
conductive band of TiO2.29
Performance of DSSCs
Solar cells were fabricated by coating these dyes on the surface of
TiO2 as sensitizers. The open-circuit photovoltage (Voc), short-
circuit photocurrent density (Jsc), fill factor (ff), and solar-to-
electrical energy conversion efficiencies (h) are summarized in
Table 3, where N719 was included for a comparison. The opti-
mized incident photo-to-current conversion efficiencies (IPCE)
and J–V curves of all dyes are shown in Fig. 4 and 5, respectively.
The amount of dye absorbed on the TiO2 films was also esti-
mated. The absorbed dye density on TiO2 is about equal for all
dyes, therefore it is not regarded as a significant parameter for
consideration.
Fluorescence quantum yield and lifetime
It is well known that the major decay channels of a photo-excited
trans-stilbene are either through fluorescence or via a rotation
about the p-bond.17 In dye 2, while the rotation about the central
double bond is restricted, their physical properties such as fluo-
rescence quantum yields, lifetimes, and the nonradiative decay
rate (knr ¼ (1 ꢀ Ff)/sf) would be different from those of dye 1. A
comparison of the kinetic parameters between dyes 1 and 2 is
therefore in demand.
As shown in Fig. 4, the IPCE spectra of dye 2 are broader than
those of corresponding dye 1, which is consistent with their
absorption spectra. For example, the IPCE maxima of dye 2 are
approximately 80% in the range of 400–500 nm, while at a similar
IPCE level the spectral width of dye 1 is much narrower. For
both types of dyes, the methoxy derivatives (e.g., 1M and 2M)
displayed the widest wavelength range among all, as a result of
a stronger resonance effect induced by the substituent.
The fluorescence quantum yields (Ff) and lifetimes (sf) of
trans-stilbene in THF were measured (Table 2), and the sf values
were determined in THF with excitation and emission wave-
lengths around the spectral maximum, while all decay curves can
be well fitted into a single-exponential equation. Values of fluo-
rescence quantum yields (Ff) for dye 1 are slightly smaller than
those of the corresponding dye 2 in THF, and also the fluores-
cence lifetimes (sf) are shorter. The nonradiative decay rate
constants were estimated by the equation knr ¼ ((1 ꢀ Ff)/sf), and
the result shows that the values of dye 1 are substantially larger
The current–voltage plots are displayed in Fig. 5. In these plots
it is clear that the Jsc values of dye 2 are apparently greater than
those of dye 1. The trend can be rationalized by the more planar
geometry of 2, which leads to a better p-resonance. The Voc
values of both types of dyes are not much different from each
other, nore are the field factors (ff) of the two. As a consequence,
the overall conversion efficiency of dye 2 is higher than those of
Table 3 DSSCs performance parameters of dyesa
Table 2 Fluorescence quantum yields (Ff), fluorescence decay times (sf),
Dye
Rel. loadingb
Voc/mV
Jsc/mA cmꢀ2
ff
h (%)
rate constants for fluorescence decay (kf), and nonradiative decay (knr
for dye 1 and 2 in THF
)
N719
1P
1N
1M
1T
1Z
2P
2N
2M
2T
0.88
1.01
0.97
0.96
0.98
0.95
1.00
0.97
0.99
0.95
0.77
0.65
0.66
0.66
0.66
0.62
0.69
0.70
0.68
0.66
14.47
10.50
10.55
11.12
9.96
0.62
0.68
0.65
0.68
0.70
0.63
0.67
0.67
0.65
0.66
6.95
4.52
4.46
4.98
4.55
3.86
5.14
5.27
5.67
5.36
d
Compd
Ffa
sfb/ns
kfc/108 sꢀ1
knr /108 sꢀ1
1P
1N
1M
1T
2P
2N
2M
2T
0.22
0.21
0.05
0.13
0.24
0.23
0.07
0.16
2.29
2.12
2.52
2.11
2.59
2.43
2.95
2.74
0.96
0.99
0.20
0.62
0.93
0.95
0.24
0.58
3.41
3.73
3.77
4.12
2.93
3.17
3.15
3.07
9.93
11.14
11.34
12.89
12.37
a
Experiments were conducted with TiO2 photoelectrodes with
approximately 16 mm thickness and 0.25 cm2 working area on the FTO
Fluorescence quantum yield in THF. b Value of sf was determined with
a
excitation and emission around the spectral maxima. c kf ¼ Ff/sf. d knr
(1 ꢀ Ff)/sf.
¼
b
(8 U sqꢀ1) substrates. Dye absorbed on TiO2 relative to 2P, which is
2.1 ꢃ 10ꢀ6 mol cmꢀ2
.
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Fig. 4 The IPCE plots of DSSCs made with (a) dyes 1 and (b) dyes 2.
Fig. 5 Current–voltage plots for the DSSCs made with (a) dyes 1 and (b) dyes 2.
dye 1 in an amount of ca. 16%. The restriction of central C]C
bond rotation does exhibit an effect of depressing the energy
dissipation pathway, therefore enhances the efficiency of light
harvesting.
1N, revealing a longer lifetime of the former. In both plots the
information complies well with what was observed in the kinetic
experiments. The longer electron lifetime of 2N is believed to be
a result of inhibiting the trans / cis photoisomerization about
the central double bond.
To get a deeper understanding on the difference of Voc between
1 and 2, electrochemical impedance spectral (EIS) analyses were
performed in the dark under a forward bias. The EIS analysis
provides information about interfacial charge transfer processes.
The Nyquist and Bode plots of the DSSCs 1N and 2N are
compared in Fig. 6. In the Nyquist plots the larger semicircle at
lower frequencies corresponds to the charge-transfer process at
the TiO2/dye/electrolyte interface. The radius of the semicircle
for 2N is larger than that of 1N, indicating a larger charge-
transfer resistance at the interface. In the Bode plots, the peaks at
higher frequencies correspond to the charge-transfer at the
counter electrode, which is the same for both 1N and 2N. The
peaks at lower frequency are related to the charge recombination
rate, or to the reciprocal of electron lifetime.30 The peak
maximum of device 2N is shifted to lower frequency than that of
Molecular orbital calculations
To get a basic understanding on the electronic structure of the
dyes, molecular orbital calculations were performed by density
functional theory (DFT) at the B3LYP/6-31G(d,p) level with full
geometry optimization. The excited states were computed by
time-dependent DFT (TDDFT) with the B3LYP functional to
compare with the experimental results. It reveals that the electron
density of the HOMOs are localized mainly on the TPA moiety
and extended into the stilbene moiety, while the LUMOs are
localized on the cyanoacrylic acid acceptor site and the adjacent
phenyl group (Fig. 7). The dipolar locations of the frontier
orbitals suggest that a significant charge separation is produced
Fig. 6 The electrochemical impedance spectra of (a) Nyquist plots and (b) Bode phase plots for the DSSCs made with 1N and 2N.
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Notes and references
€
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2 M. Gratzel, J. Photochem. Photobiol., A, 2004, 164, 3–14.
€
3 M. K. Nazeeruddin, A. Kay, L. Rodicio, R. Humphry-Baker,
€
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The TDDFT excited state calculations were then performed
on the lowest 10 singlet–singlet excitations for dyes 1 and 2,
neglecting the influence of solvent. The parameters of electronic
transitions, i.e., energy, composition, and oscillator strength
above 0.1 are summarized in Table S1†. In all cases, the lowest
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nature, as reflected by the electron distribution of the frontier
molecular orbitals. The oscillator strength of all ICT transitions
is quite large, indicating they are formally allowed (0.82–1.06).
The computed wavelength for the ICT transitions is red-shifted
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Both types of dyes 1 and 2 displayed effective DSSCs charac-
teristics with quantum efficiency in a range of 4.5–5.7%. The
performance of dyes 2 behaved better than dye 1, as a result of
restricting the trans / cis isomerization of the C]C bond as
a major channel of non-radiative decay. The rate constant of
isomerization of dyes 1 was found to be substantially larger than
that of dyes 2. The effect can also be verified by observing a low
frequency shift in the Bode phase plot of electrochemical
impedance spectroscopy. As a result the DSSCs made with dyes 2
displayed a higher value of Jsc and a broader spectrum of IPCE
than those made with dyes 1. In a comparison among
compounds with different substituents, the ones with methoxy
groups are worthy of a special note, i.e., 1M and 2M. Although
these compounds displayed the lowest fluorescence quantum
yields, they were able to perform the highest conversion effi-
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Acknowledgements
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Financial supports were provided partly by the National Science
Council. We thank the assistance of Professor J.-S. Yang in
National Taiwan University for the lifetime measurements.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14907–14916 | 14915

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