Electro-optical properties of new anthracene based organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2011.02.006

Dipolar compounds containing anthracene-based triarylamine donor and cyanoacrylic acid acceptor were synthesized and characterized by electro-optical measurements. The optical spectra of the dyes are dominated by a charge transfer transition. This band is

Full text of DOI:10.1016/j.dyepig.2011.02.006

Dyes and Pigments 91 (2011) 33e43
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Electro-optical properties of new anthracene based organic dyes
for dye-sensitized solar cells
,
a
a
b
K.R. Justin Thomas a , Prachi Singh , Abhishek Baheti , Ying-Chan Hsu ,
*
Kuo-Chuan Ho b, Jiann T’suen Lin c
a Organic Materials Lab, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
b Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
c Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Dipolar compounds containing anthracene-based triarylamine donor and cyanoacrylic acid acceptor
were synthesized and characterized by electro-optical measurements. The optical spectra of the dyes are
dominated by a charge transfer transition. This band is red-shifted and increased in intensity on elon-
gation of conjugation by the introduction of bithiophene or dithienylbenzothiadiazole moiety. Similarly,
the oxidation potentials of the dyes are shifted anodically on extension of the conjugation attributable to
the reduction in the interaction between the triarylamine and acceptor segments. Theoretical calcula-
tions revealed the charge transfer to occur between anthracene and triarylamine moieties. However, in
the benzothiadiazole containing dye the charge transfer is observed between the amine and the acceptor
segment. Dye-sensitized solar cells fabricated using these dyes showed moderate efficiency which is
highly dependent on the nature of the conjugation bridge.
Received 19 November 2010
Received in revised form
28 December 2010
Accepted 21 February 2011
Available online 8 March 2011
Keywords:
Anthracene
Triarylamine
Benzothiadiazole
Dye-sensitized solar cell
Absorption spectra
Theoretical calculation
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
development of organic dyes featuring advantageous electro-
optical properties. Organic dyes are attractive due to the availability
There is a growing demand for the development of alternate
technologies for the conservation of energy due to the expectation
that the fossil fuels may be consumed completely. Solar energy
can be converted to electricity by photovoltaic and photo-
electrochemical cells [1]. Photoelectrochemical cells commonly
termed as Grätzel cells or dye-sensitized solar cells (DSSCs) are
based on the sensitization of wide band gap semiconductors such
as nanocrystalline anatase TiO2 by inorganic or organic dyes
capable of absorbing sun light in the entire visible region. DSSCs are
considered cheap and robust alternatives to the conventional
silicon-based photovoltaic devices. The efficiency of the DSSCs
largely depends on the light absorbing capability of the dye while
the other parameters such as electron injection and dye regener-
ation are also crucial. Ruthenium-based organometallic complexes
developed by Grätzel and co-workers [2,3] are the most successful
dyes with promising device characteristics. However, the ruthe-
nium dyes are expensive and consequently a search is active for the
of facile synthetic methodologies and exhibition of large molar
extinction coefficients when compared to the dominant ruthe-
nium-based dyes. Organic dyes commonly feature donoreacceptor
architecture with the donor being a triarylamine and a cyanoacrylic
acid acceptor. Variation of the conjugating segment that links the
donor and acceptor fragments have led to a huge array of dyes
displaying promising energy conversion efficiencies and broader
and intense incident photon-to-current efficiency (IPCE) response.
Aromatic linkers such as phenyl [4,5], biphenyl [6], terphenyl [7],
fluorene [8e10], coumarin [11,12], thiophene [13e17], thienothio-
phene [18e20], furan [21], pyrrole [22] and benzothiadiazole
[23,24] have been exploited to develop dyes for DSSCs.
Anthracene-based molecular materials are known for displaying
bright blue electroluminescence [25e28]. Simple 9,10-diaryl
substituted anthracenes have been demonstrated as emitting
materials while the incorporation of arylamine moiety enhances
the hole-transporting ability of such compounds [28e31]. Even
though the anthracene containing compounds have been devel-
oped successfully for applications in light-emitting diodes [25e31],
thin film transistors [32e34] and constituents in bulk hetero-
junction solar cells [35] their use in DSSCs remain limited. Simple
* Corresponding author. Tel.: þ91 1332285376; fax: þ91 1332286202.
E-mail address: krjt8fcy@iitr.ernet.in (K.R. Justin Thomas).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.02.006

34
K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
anthracene-cyanoacrylic acid derivatives have been recently
reported as sensitizers for DSSCs [36,37]. In a recent paper by
Hagfeldt et al. [36] anthracene unit has been employed as a linker
for triphenylamine donors and cyanoacrylic acid acceptors.
However, to the best of our knowledge arylamines derived from
anthracene have not been exploited for the construction of dyes
suitable for dye-sensitized solar cells. In this paper, we present
three dyes (Fig. 1) featuring thiophene, bithiophene and dithie-
nylbenzothiadiazole linkers inserted between the anthracene-
based triarylamine donor and cyanoacrylic acid acceptor units.
Thiophene moiety is used to increase the electron-richness of the
linking segment, on the contrary the electron-deficient benzo-
thiadiazole unit is expected to shift the LUMO toward the cyanoa-
crylic anchoring segment and favour a facile charge transfer
transition between the triarylamine and cyanoacrylic acid units.
The tert-butyl groups were introduced in the anthracene nucleus
for two reasons: (a) incorporation of tert-butyl groups are expected
to improve the solubility of the dyes and (b) inhibit the aggregation
between the dyes which is more pronounced in the unsubstituted
anthracene derivatives [38]. It has been demonstrated in few
reports that the incorporation of substituents at the 2 and/or
7-positions of the anthracene nucleus reduce the aggregation
propensity [39,40]. Aggregation of dyes when anchored on the TiO2
surface may alter the absorption and electron transfer properties
which in turn may affect the performance of the DSSCs. Intermo-
lecular interactions via the anthracene moiety in the present design
will be inhibited not only due to the presence of tert-butyl groups
but also due to the anchoring of diarylamine segment. Trigonal
amine unit will also sterically forbid the approach of adjacent
molecules.
Our aim is to study the role of anthracene in the donoreacceptor
architecture and compare the absorption and DSSC performance
parameters with the analogous dyes reported in the literature and
featuring phenylene, thienyl or fluorene moieties in place of
anthracene. TDDFT computations are used to rationalize the optical
properties and charge transfer probability. Initial TDDFT computa-
tions revealed the localization of LUMO orbitals over the anthra-
cene moiety and disfavouring the charge transfer necessary for
electron injection to the nanocrystalline TiO2. However, on inclu-
sion of benzothiadiazole unit in the conjugation pathway shifted
the LUMO toward the cyanoacrylic acid segment. In addition, the
electron-rich oligothiophene and electron-deficient benzothiadia-
zole segments are expected to reduce the band gap and red-shift
the absorption features. Such an improvement in optical properties
is hypothesized to benefit the light harvesting properties of the
dyes when used in the dye-sensitized solar cells.
2. Experimental details
2.1. Materials
All the reactions were carried out under nitrogen unless other-
wise mentioned. Solvents were dried by standard procedures. All
column chromatography separations were performed using silica gel
(230e400 mesh) as the stationary phase. The 1H and 13C NMR
spectra were measured by using Bruker AMX400 or AV400 spec-
trometer. Mass spectra were recorded on a JMS-700 double focusing
mass spectrometer (JEOL, Tokyo, Japan). Elemental analyses were
performed on a Perkin-Elmer 2400 CHN analyzer. Cyclic voltam-
metric experiments were performed with a CH Instruments elec-
trochemical analyzer. All measurements were carried out at room
temperature with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire
auxiliary electrode, and a nonaqueous Ag/AgNO3 reference elec-
trode. The E1/2 values were determined as 1/2(Epa þ Epc), where Ep
a
and Epc are the anodic and cathodic peak potentials, respectively. The
potentials are quoted against the ferrocene internal standard. The
solvent in all experiments was dichloromethane and the supporting
electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate.
Electronic absorption spectra were obtained on a Cary 50 Probe
UVeVisible spectrophotometer. The precursor compound 1 was
obtained according to the literature procedure [26].
2.2. 2-(10-Bromo-2,6-di-tert-butylanthracen-9-yl)thiophene (2)
A two-neck flask was charged with magnesium turnings (0.24 g,
10 mmol) and tetrahydrofuran (50 ml) and rapidly stirred under
nitrogen atmosphere. 2-Bromothiophene (1.63 g, 10 mmol) was
added through the side arm cautiously via a syringe. After stirring
for 1 h the Grignard reagent formed was transferred using
a cannula to a flask containing 9,10-dibromo-2,6-di-tert-butylan-
thracene (1) (5.38 g, 12 mmol) and bis(triphenylphosphine)palla-
dium dichloride (0.07 g). The mixture was heated to reflux for 24 h.
At the end it was poured into iced water and the organic products
formed were extracted with diethyl ether (3 ꢀ 50 ml). The
combined organic extracts were concentrated and adsorbed on
silica gel. It was purified by column chromatography by using
hexane/dichloromethane mixture (5:1) as eluant. Pale yellow solid.
Yield: 2.53 g (56%). 1H NMR (CDCl3, 500 MHz, ppm):
d 1.31 (s, 9 H),
1.46 (s, 9 H), 7.15e7.16 (m, 1 H), 7.27e7.30 (m, 1 H), 7.50 (dd, J ¼ 1.8,
9.2 Hz, 1 H), 7.59 (dd, J ¼ 1.5, 7.4 Hz, 1 H), 7.66 (dd, J ¼ 1.8, 9.2 Hz, 1
H), 7.72 (d, J ¼ 1.8 Hz,1 H), 7.77 (d, J ¼ 9.2 Hz,1 H), 8.45 (d, J ¼ 1.8 Hz,
1 H), 8.50 (d, J ¼ 9.2 Hz, 1 H).
2.3. 2,6-Di-tert-butyl-N-(naphthalen-1-yl)-N-phenyl-
10-(thiophen-2-yl)anthracen-9-amine (3)
A mixture of 2 (4.51 g, 10 mmol), N-phenylnaphthalen-1-amine
(2.63 g, 12 mmol), Pd(dba)2 (0.055 g, 0.02 mmol), (t-Bu)3P (0.051 g,
0.02 mmol), sodium t-butoxide (1.44 g, 15 mmol) and dry toluene
(25 ml) was heated at 80 ꢁC for 12 h in a Schlenk flask. After the
completion of the reaction it was evaporated to dryness in a rotary
evaporator and the residue was adsorbed on silica gel. It was purified
by column chromatography by using hexane/dichloromethane
mixture (4:1) as eluant. Yellow solid. Yield: 4.78 g (81%). 1H NMR
(CDCl3, 500 MHz, ppm): d 1.07 (s, 9 H), 1.26 (s, 9 H), 6.66e6.70 (m, 2
H), 6.77e6.81 (m, 1 H), 7.05e7.12 (m, 3 H), 7.18e7.22 (m, 2 H),
7.29e7.32 (m,1 H), 7.38e7.43 (m, 2 H), 7.45e7.52 (m, 2 H), 7.58e7.61
(m, 2 H), 7.77e7.83 (m, 2 H), 7.90 (dd, J ¼ 1.8, 9.2 Hz, 1 H), 8.16e8.20
(m, 2 H), 8.30 (d, J ¼ 8.2 Hz, 1 H). 13C NMR (CDCl3, 125 MHz, ppm):
d
150.9, 148.2, 147.5, 142.9, 135.2, 132.7, 129.4, 129.0, 128.3, 127.0,
Fig. 1. Structures of the anthracene-based dyes.
126.8, 126.6, 126.2, 125.7, 124.9, 124.3, 123.0, 121.5, 119.7, 118.6, 116.1,

K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
35
35.0, 34.9, 30.8, 30.7. FAB MS: m/z 589.2 [Mþ]. HRMS: Calcd. for
reaction was quenched by cooling to room temperature and
subsequently 20 ml methanol added to induce precipitation of the
product. The yellow solid thus obtained was dissolved in
dichloromethane and adsorbed with silica gel. It was purified by
column chromatography using hexane/dichloromethane mixture
(3:2) as eluant. Dark yellow solid. Yield: 1.04 g (74%). 1H NMR
C42H39NS 589.2803. Found: 589.2791 [Mþ].
2.4. 5-(2,6-Di-tert-butyl-10-(naphthalen-1-yl(phenyl)amino)
anthracen-9-yl)thiophene-2-carbaldehyde (4)
Compound 3 (1.18 g, 2 mmol) was dissolved in dry tetrahydro-
furan (50 ml) and cooled to ꢂ70 ꢁC. To this mixture n-butyl lithium
(1.4 ml (1.6 M solution in n-hexane) 2.2 mmol) was added drop by
drop for 30 min with vigorous stirring. The reaction mixture was
stirred for another 90 min at this temperature and 2 ml dry dime-
thylformamide added at once. It was allowed to attain room
temperature overnight. The resulting yellow solution was poured
into ice-water and treated with saturated ammonium chloride
solution briefly. It was extracted with diethyl ether (3 ꢀ 25 ml) and
the combined organic extracts dried over anhydrous MgSO4.
Evaporation of the diethyl ether solution produced the crude
aldehyde which was further purified by column chromatography
on silica gel using 1:1 mixture of hexane and dichloromethane.
Dark yellow solid. Yield: 0.67 g (54%). 1H NMR (CDCl3, 500 MHz,
(CDCl3, 500 MHz, ppm): d 1.10 (s, 9 H), 1.31 (s, 9 H), 6.70e6.74 (m,
2 H), 6.84 (t, J ¼ 8.2 Hz, 1 H), 7.05e7.14 (m, 3 H), 7.18e7.27 (m, 2 H),
7.35 (d, J ¼ 4.0 Hz, 1 H), 7.40e7.56 (m, 4 H), 7.60e7.64 (m, 2 H), 7.71
(d, J ¼ 4.0 Hz, 1 H), 7.88e7.98 (m, 3 H), 8.21e8.26 (m, 2 H0, 8.32
(d, J ¼ 8.2 Hz, 1 H), 9.92 (s, 1 H). 13C NMR (CDCl3, 125 MHz, ppm):
d
182.5, 150.7, 148.4, 148.0, 147.7, 147.2, 142.8, 141.8, 141.6, 140.8,
137.4, 137.2, 135.2, 132.4, 131.5, 130.8, 130.3, 129.7, 129.0, 129.0,
128.3, 126.4, 126.3, 126.28, 126.1, 125.9, 125.8, 125.7, 125.6, 125.3,
125.1, 124.4, 124.2, 123.7, 123.2, 123.0, 121.0, 119.8, 119.4, 118.6, 35.0,
30.7. FAB MS: m/z 699.2 [Mþ]. HRMS: Calcd. for C47H41ONS2:
699.2630. Found: 699.2629 [Mþ].
2.7. (E)-2-Cyano-3-(50-(2,6-di-tert-butyl-10-(naphthalen-1-yl
(phenyl)amino)anthracen-9-yl)-2,20-bithiophen-5-yl)acrylic
acid (7)
ppm):
d 1.12 (s, 9 H), 1.31 (s, 9 H), 6.72e6.77 (m, 2 H), 6.84 (t,
J ¼ 7.4 Hz, 1 H), 7.11e7.27 (m, 5H), 7.37 (d, J ¼ 4.0 Hz, 1 H), 7.43e7.54
(m, 4 H), 7.62 (d, J ¼ 8.2 Hz,1 H), 7.75e7.85 (m, 2 H), 7.94 (dd, J ¼ 1.8,
9.2 Hz, 1H), 8.01 (d, J ¼ 4.0 Hz, 1 H), 8.25e8.32 (m, 2 H), 8.35
(d, J ¼ 8.2 Hz, 1 H), 10.08 (s, 1 H). 13C NMR (CDCl3, 125 MHz, ppm):
The title dye was prepared from the aldehyde 6 by following the
procedure described above for 5. Red solid. Yield: 77%. 1H NMR
(DMSO-d6, 500 MHz, ppm):
d 0.95 (s, 9 H), 1.17 (s, 9 H), 6.49
d
182.9, 150.7, 148.5, 148.3, 144.8, 136.5, 131.1, 131.0, 129.1, 129.0,
(d, J ¼ 7.4 Hz, 1 H), 6.56 (d, J ¼ 7.4 Hz, 1 H), 6.75e6.79 (m, 1 H), 6.96
(d, J ¼ 7.4 Hz,1 H), 7.03e7.07 (m, 2 H), 7.21 (t, J ¼ 7.4 Hz,1 H), 7.35 (d,
J ¼ 1.8 Hz, 1 H), 7.45e7.67 (m, 6 H), 7.79e7.82 (m, 3 H), 7.96e7.98
(m, 2 H), 8.07e8.14 (m, 3 H), 8.51 (s, 1 H). 13C NMR (DMSO-d6,
128.9, 126.1, 126.0, 125.9, 125.8, 125.7, 125.6, 125.1, 124.5, 123.0,
120.6, 119.9, 119.5, 118.6, 115.3, 35.0, 30.7.
2.5. (E)-2-Cyano-3-(5-(2,6-di-tert-butyl-10-(naphthalen-1-yl
125 MHz, ppm): d 164.0, 150.7, 148.9, 148.4, 146.6, 145.5, 142.6,
(phenyl)amino)anthracen-9-yl)thiophen-2-yl)acrylic acid (5)
141.5, 140.8, 140.7, 137.0, 135.3, 134.7, 132.2, 132.17, 131.3, 129.8,
129.6, 129.5, 128.7, 127.9, 127.7, 126.8, 126.4, 126.3, 125.73, 125.66,
125.2, 124.4, 122.9, 120.9, 120.5, 119.0, 118.5, 117.0, 99.2, 35.1, 30.8,
30.7. FAB MS: m/z 766.3 [Mþ]. HRMS: Calcd. for C50H42O2N2S2:
766.2688. Found: 766.2693 [Mþ].
A mixture of the aldehyde 4 (0.618 g, 1 mmol), 2-cyanoacetic
acid (0.102 g, 1.2 mmol), ammonium acetate (0.039 g, 0.5 mmol)
and acetic acid (5 ml) was refluxed for 8 h. The resulting dark red
solution was poured into water. The solid was filtered and thor-
oughly washed with water and hexane/diethyl ether mixture. It
was recrystallized from dichloromethane/hexane mixture. Red
powder. Yield: 0.47 g (69%). 1H NMR (DMSO-d6, 500 MHz, ppm):
2.8. 5-(7-(5-(2,6-Di-tert-butyl-10-(naphthalen-1-yl(phenyl)amino)
anthracen-9-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)
thiophene-2-carbaldehyde (8)
d
0.99 (s, 9 H), 1.21 (s, 9 H), 6.51 (d, J ¼ 7.6 Hz,1 H), 6.58 (d, J ¼ 7.6 Hz,
1 H), 6.82 (t, 7.4 Hz, 1 H), 6.96 (d, J ¼ 7.4 Hz, 1 H), 7.09e7.14 (m, 2 H),
7.29 (t, 7.4 Hz, 1 H), 7.52-7.73 (m, 8 H), 8.02 (d, J ¼ 8.2 Hz, 1 H),
8.08e8.18 (m, 3 H), 8.38 (d, J ¼ 4.0 Hz, 1 H), 8.69 (s, 1 H). 13C NMR
It was obtained in 72% yield as described above for the aldehyde
6, but 5-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-car-
baldehyde was used instead of 5-bromothiophene-2-carbaldehyde.
(DMSO-d6, 125 MHz, ppm):
d
162.5, 149.3, 147.6, 147.4, 147.2, 145.8,
Orange solid. 1H NMR (CDCl3, 500 MHz, ppm):
d 1.09 (s, 9 H), 1.29
141.3, 139.7, 139.3, 136.3, 134.0, 130.8, 130.5, 129.7, 128.5, 128.3,
128.1, 127.3, 126.6, 125.4, 125.1,124.6,124.4,123.9, 123.1, 121.6, 119.3,
117.7, 117.3, 115.6, 99.0, 33.8, 29.5, 29.4. FAB MS: m/z 684.2 [Mþ].
HRMS: Calcd for C46H40O2N2S: 684.2811. Found: 684.2811 [Mþ].
(s, 9 H), 6.70e6.74 (m, 2 H), 6.83 (t, J ¼ 7.4 Hz, 1 H), 7.09e7.27
(m, 4 H), 7.33e7.56 (m, 5 H), 7.63 (d, J ¼ 8.2 Hz, 1 H), 7.85
(d, J ¼ 4.0 Hz, 1 H), 7.92e8.06 (m, 5 H), 8.21e8.25 (m, 3 H), 8.33
(d, J ¼ 8.2 Hz, 1 H), 8.49 (d, J ¼ 4.0 Hz, 1 H), 10.0 (s, 1 H). 13C NMR
(CDCl3, 125 MHz, ppm):
d 183.0, 152.5, 152.45, 150.8, 148.7, 148.4,
2.6. 50-(2,6-Di-tert-butyl-10-(naphthalen-1-yl(phenyl)amino)
anthracen-9-yl)-2,20-bithiophene-5-carbaldehyde (6)
147.9, 143.4, 142.9, 142.3, 140.6, 140.1, 136.8, 135.2, 132.5, 131.6,
130.9, 129.7, 129.0, 128.98, 128.8, 128.3, 128.0, 127.9, 127.5, 127.1,
126.7, 126.2, 125.9, 125.8, 125.7, 125.2, 125.0, 124.9, 124.4, 124.3,
123.0, 121.3, 119.8, 119.4, 118.6, 35.0, 30.8. FAB MS: m/z 833.3 [Mþ].
HRMS: Calcd. for C53H43ON3S3: 833.2568. Found: 833.2575 [Mþ].
A tetrahydrofuran (50 ml) solution of compound 3 (1.17 g,
2.0 mmol) was cooled to ꢂ70 ꢁC and n-butyl lithium (1.4 ml (1.6 M
solution in n-hexane) 2.2 mmol) was added dropwise for 15 min
with vigorous stirring. The reaction mixture was stirred for another
60 min at this temperature and 0.72 g (2.2 mmol) of tributyl-
chlorostannane added at once. The resulting solution was allowed
to attain room temperature overnight. It was quenched by the
addition of ice-cold water and extracted with diethyl ether
(3 ꢀ 30 ml). The combined organic extracts was dried over anhy-
drous MgSO4 and evaporated to yield orange syrup.
2.9. (E)-2-Cyano-3-(5-(7-(5-(2,6-di-tert-butyl-10-(naphthalen-1-
yl(phenyl)amino)anthracen-9-yl)thiophen-2-yl)benzo[c][1,2,5]
thiadiazol-4-yl)thiophen-2-yl)acrylic acid (9)
It was prepared in 63% yield from the aldehyde 8 by following
a procedure similar to 5. Dark red solid. 1H NMR (DMSO-d6,
500 MHz, ppm):
d
0.98 (s, 9 H), 1.18 (s, 9 H), 6.52 (d, J ¼ 7.4 Hz,
The above syrup was mixed with 5-bromothiophene-2-carbal-
dehyde (0.38 g, 2.0 mmol), Pd(PPh3)2Cl2 (0.014 g, 0.02 mmol)
and dimethylformamide (10 ml) and heated at 70 ꢁC for 24 h. The
1 H), 6.59 (d, J ¼ 7.4 Hz, 1 H), 6.80 (t, J ¼ 7.4 Hz, 1 H), 6.98e7.12 (m,
3 H), 7.26e7.29 (m, 1 H), 7.47e7.57 (m, 5 H), 7.71 (d, J ¼ 4.0 Hz, 1 H),
7.85e7.89 (m, 2 H), 8.00e8.31 (m, 8 H), 8.45e8.53 (m, 2 H). 13C NMR

36
K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
(DMSO-d6, 125 MHz, ppm):
d
163.8, 151.8, 150.5, 148.7, 148.2, 147.4,
used. The vertical electronic excitations were calculated using the
time-dependent DFT (B3LYP) theory and the above mentioned
basis set. At least 10 excited states were calculated for each mole-
cule. Even though the time-dependent DFT method less accurately
describes the states with charge-transfer nature, the qualitative
trends in the TDDFT results can still offer correct physical insights.
146.5, 142.5, 141.6, 140.3, 140.0, 137.1, 135.2, 132.0, 131.2, 129.6,
129.4, 128.8, 128.6, 128.3, 127.8, 127.8, 127.1 126.8, 126.5, 126.3,
126.0, 125.5, 125.1, 124.1, 123.9, 122.7, 121.0, 120.4, 118.4, 116.8, 99.2,
35.0, 30.7, 30.6. FAB MS: m/z 900.2 [Mþ]. HRMS: Calcd. for
C56H44O2N4S3: 900.2626. Found: 900.2632 [Mþ].
2.10. Computational details
2.11. DSSC fabrication and characterization
The ground state geometries of the compounds at the gas phase
were optimized by employing density functional theory with Becke
three parameter hybrid functional and correlation functional of Lee,
Yang, and Parr (B3LYP) and the basis set 6-31G with additional
d polarization functions on first row element and p polarization
functions on hydrogens (6-31G**) as implemented in the Gaussian
09 package [41]. The default options for the self-consistent field
(SCF) convergence and threshold limits in the optimization were
The TiO2 thin film serving as the photoanode in this work was
prepared through the general solegel method. The precursor
solution was made according to the following procedure: 430 ml of
0.1 M nitric acid solution under vigorous stirring was slowly
combined with 72 ml Ti(C3H7O)4 to form a mixture. After the
hydrolysis, the mixture was heated at 85 ꢁC in a water bath and
stirred vigorously for 8 h in order to achieve the peptization. When
the mixture was cooled to room temperature, the resultant colloid
Scheme 1. Synthesis of the thiophene containing dyes 5 and 7.

K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
37
was filtered, and the filtrate was then heated in an autoclave at
a temperature of 240 ꢁC for 12 h to grow the TiO2 particles. When
the colloid was cooled to room temperature, it was ultrasonically
vibrated for 10 min. The TiO2 colloid was concentrated to 13 wt %,
followed by the addition of 30 wt % (with respect to TiO2 weight) of
poly(ethylene glycol) (PEG, MW) 20,000 g/mol) to prevent the film
from cracking while drying.
6 and 9 were accessed by condensing the stannylene derivative of
3 with either 2-bromo-5-thiophene aldehyde or 5-(7-bromobenzo
[c][1,2,5]thiadiazol-4-yl)thiophene-2-carbaldehyde by following
the Stille coupling protocol and Pd(PPh3)2Cl2/dmf reagent system
[45]. In the final step, the aldehyde precursors (4, 6 and 8) were
successfully converted to the desired dyes (5, 7 and 9) on treatment
with cyanoacetic acid in the presence of ammonium acetate cata-
lyst in acetic acid. The dyes are deep red in colour and soluble in
common organic solvents such as dichloromethane, tetrahydro-
furan and methanol.
The TiO2 paste was then deposited on a FTO glass substrate by
the glass rod method. The TiO2-coated FTO was heated to 500 ꢁC at
a heating rate of 20 ꢁC/min. and maintained for 30 min before being
cooled to room temperature. The thickness of TiO2 film was
controlled by repeating the procedure described above. The thick-
3.2. Optical properties
ness and the area of TiO2 film were controlled at 18 m
m and 3 cm2,
respectively. The TiO2 film thickness was measured by a profil-
ometer (Dektak3, Veeco/Sloan Instruments Inc., USA). The illumi-
nation area was constrained using a cardboard mask. A platinized
FTO with a layer of Pt 100 nm-thick applied by sputtering was used
as a counter electrode. The active area was controlled at a dimen-
sion of 0.5 ꢀ 0.5 cm2 by adhered polyester tape (3 M) with a thick-
A dye suitable for application in dye-sensitized solar cell must
possess a broad absorption profile covering the entire visible region.
In order to determine the optical absorption of the dyes we have
ness of 60 mm on the Pt electrode. After heating of the TiO2 thin film
to 80 ꢁC, the film was taken out from the oven and dipped into the
THF solution containing 3 ꢀ10ꢂ4 M dye sensitizers for at least 12 h.
Use of THF solution for dipping was meant to ensure enough
uptakes of the sensitizers due to the higher solubility of the
sensitizers in THF. After being rinsed with CH3CN, the photoanode
was placed on top of the counter electrode and tightly clipped to it
to form a cell. Electrolyte was then injected into the space and the
cell sealed with Torr Seal cement (Varian, MA). The electrolyte was
composed of 0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M
4-tert-butylpyridine dissolved in acetonitrile. Note that there was
no mask used to constrain the illumination area of the device in this
study; therefore, the power conversion efficiency and the corre-
sponding measurements might be overestimated [42]. However,
our results were compared with that of N3 dye (cis-bis (4,40-
dicarboxy-2, 20-bipyridine) dithiocyanato ruthenium (II)) [43],
which acted as a reference for each measurement, and the condi-
tions for each device were identical. Therefore, we believe that the
difference of the performance among the devices made in this
study was mainly attributed to the intrinsic properties of the
sensitizers.
The photoelectrochemical characterizations on the solar cells
were carried out using a modified light source, 300W Xe lamp
(Oriel, 6258) equipped with a water-based IR filter and AM 1.5 filter
(Oriel, 81080 kit). Light intensity, attenuated by neutral density
filter (Optosigma, 078-0360) at the measuring (cell) position, was
estimated to be ca. 100 mW/cm2 according to the reading from
a radiant power meter (Oriel, 70310) connected to a thermopile
probe (Oriel, 71964). Photoelectrochemical characteristics of the
DSSCs, were recorded through the potentiostat/galvanostat
(CHI650B, CH Instruments, Inc., USA).
3. Results and discussion
3.1. Synthesis and characterization
The synthetic methods employed to obtain the dyes (Fig. 1) are
shown in Schemes 1 and 2. In the first step the dibromo derivative
of anthracene 1 was treated with thienylmagnesium bromide in the
presence of Pd(PPh3)2Cl2 in tetrahydrofuran to obtain the thio-
phene derivative 2 in 56% yield. It was converted to the amine
substituted compound 3 by treatment with N-phenylnaphthalen-
1-amine under the palladium catalyzed (Pd(dba)2/(t-Bu)3P) C-N
cross coupling conditions [44] in 81% yield. Compound 3 was then
converted to the corresponding aldehyde, 4 by reacting with
n-butyl lithium followed by dry dimethylformamide. The aldehydes
Scheme 2. Synthesis of the benzothiadiazole containing dye 9.

38
K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
Table 1
Optical and electrochemical data of the dyes.
Dye
labs, nm (3max, ꢀ 10þ4 Mꢂ1 cmꢂ1
)
lem
,
a nm (Ff
)
Stokes
Eox, V (
DEp, mV)
HOMO, eV
LUMO, eV
E0-0, eV
E0-0*, eV
a
shift, cmꢂ1
THF
CH2Cl2
3
5
7
9
e
425 (1.11), 357 (1.09)
435(1.28), 350 (2.72)
442 (4.10), 364 (sh)
491 (4.49), 365 (2.12)
e
e
0.466 (81), 0.554 (120)
0.604 (73), 0.770 (83)
0.496 (63)
5.266
5.404
5.296
5.251
2.628
2.949
2.912
3.150
2.638
2.455
2.384
2.101
e
434 (1.31), 344 (2.50)
436 (3.58), 366(sh)
486 (3.94), 366 (2.26)
508 (0.12)
565 (0.07)
633 (0.02)
3356
5237
4778
ꢂ1.08
ꢂ1.12
ꢂ0.88
0.451 (60)b
a
Recorded for tetrahydrofuran solutions. Coumarin-6 or Rhodamine-6G (for 7 and 9) was used as standards in the quantum yield measurements.
An additional reversible reduction couple was observed at ꢂ1.612 (66) which is attributable to the reduction of the benzothiadiazole segment.
b
performed the electronic spectral measurements in dichloro-
intensity of the peak arising due to the charge transfer transition
[20]. However, the enormous increase in the molar extinction coef-
ficient for the dye 7 when compared to 5 is probably pointing the
methane and tetrahydrofuran. The optical parameters deduced
from the absorption studies are listed in Table 1 and the absorption
spectra recorded for the dichloromethane solutions of the dyes (5, 7,
and 9) and the selected precursors (3 and 6) are displayed in Fig. 2.
The absorption profile of the parent amine (3) displays two
peaks centred at 425 and 357 nm respectively (Fig. 2). On intro-
ducing the acceptor group the higher wavelength band grows in
intensity with a significant red-shift while the higher energy peak
remains unaltered except for a slight hike in molar extinction
coefficient. Presence of higher wavelength electronic transitions in
the parent amine (3) overrules the possibility of pure charge
transfer character to the second absorption observed for the dyes.
However, it is likely that the peak at 425 nm in the precursor
compound (3) originates from the charge transfer from amine
presence of a significant
pep* contribution to this profile or an
enhanced donoreacceptor interaction mediated through the
conjugation bridge. Alternatively, an increment in the donor
strength due to the addition of thiophene unit may attenuate the
donoreacceptor interaction. A broadening observed for this transi-
tion on increasing the spacer length also suggests the involvement of
additional electronic transitions rather than a straightforward donor
to acceptor-based transition.
It is interesting to compare the optical properties of the dyes
with the related compounds containing fluorene [10,22] or phenyl
[5] segments in place of anthracene (Table 2). Firstly, the anthracene
derivative, 5 possesses red-shifted absorption when compared to
the analogous fluorene (10, þ14 nm) and phenyl (12, þ31 nm)
derivatives, however, with the moderate optical density. Such
a lowering of absorption energy indicates the involvement orbital
based on anthracene in the absorption rather than an orbital based
on cyanoacrylic acid segment. Relatively, anthracenes are expected
to exert more stabilizing effect on the unoccupied molecular orbitals
when compared to the phenyl and fluorene segments due to the
functionality to the p* orbitals of the anthracene segment. Such an
intramolecular charge transfer (ICT) electronic excitation is
commonly observed in amine derivatives of polyaromatics [46,47].
In the dyes, the higher wavelength (434e486 nm) absorption
may also possess contribution from a charge transfer from the amine
to the acceptor segment. This peak is slightly red-shifted for the
bithiophene derivative (7) and moved to 486 nm for the dye (9)
containing the dithienylbenzothiadiazole bridge. This clearly points
that benzothiadiazole being a low-band gap chromophore signifi-
cantly alters the electronic structure of the dye and reduces the band
gap tremendously by lowering the LUMO energy level (vide infra)
[24]. The molar extinction coefficient of the dye 7 is three times
larger than that observed for the compound 5. The extension of the
linker conjugation in dipolar compounds is expected to diminish the
donoreacceptor interaction and consequently reduce the absorption
p-acceptor ability. Secondly, in the fluorene and phenyl derivatives
[5,7,10,22] (see Table 2) the absorption peak is bathochromically
shifted on increasing the conjugation by the insertion of the thio-
phene moiety (compare 10 and 12 with 11 and 13 respectively).
However, in the present compounds addition of thiophene exerts
only a slight effect on the absorption position (compare 5 with 7).
This clearly indicates that the orbitals involved in the electronic
transitions corresponding to this peak are localized within anthra-
cene and are less influenced by the thiophene unit.
In addition, all the dyes exhibit a lower wavelength absorption
peak at ca. 350 nm which is insensitive to the alternations in the
structure of the dyes. It is likely to originate from the anthracene
localized pep* transition.
In order to gain more insights into the nature of electronic
transitions and the orbitals contributing to the transitions we have
performed the TDDFT [48] computations on the DFT [49] optimized
structures. The low energy vertical transitions predicted by the
TDDFTcomputations and their orbital contributions are collected in
Table 3. In order to access the impact of the electron-withdrawing
cyanoacrylic acid on the electronic structure we have also per-
formed calculations on the precursors 3 and 4. The HOMO and
LUMO are localized on the triarylamine and anthracene segments
respectively in these compounds. In the compound 4, the LUMO þ 1
is mainly composed by the thiophene and aldehyde segments.
However, in the dyes 5, 7 and 9 the HOMO and LUMO orbitals are
localized on the triarylamine and acceptor segments respectively
(Fig. 3). The HOMO-1 in these dyes is again composed of the tri-
arylamine unit. The LUMO þ 1 is located on the anthracene
segment for the dyes 5 and 7 and is contributed by the acceptor unit
in the dye 9. The segment wise contributions to the construction of
the orbitals, LUMO and LUMO þ 1 clearly establish the electronic
Fig. 2. Absorption spectra of the dyes 5, 7, and 9 along with the precursors 3 and 6
recorded in dichloromethane.

K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
39
Table 2
Optical and electrochemical parameters of the related dyes.
Dye
lmax, nm
(
E
ox, V
HOMO/LUMO, eV
E0-0, eV
E0-0*, eV
Ref
3max, ꢀ 10þ4 Mꢂ1 cmꢂ1
)
R
Aromatic linker
n
10
11
1-naphthyl
9,9-diethyl
fluorene
9,9-diethyl
fluorene
1
421 (5.29), 333 (1.39)a
470 (5.00), 363 (3.00)a
0.51
5.31/2.81
5.26/3.07
2.51
2.19
ꢂ1.23
10
6
1-naphthyl
2
0.46, 0.75
0.96
12
13
Phenyl
Phenyl
Benzene
Benzene
1
2
404 (2.50)
1.21 vs NHE
0.49
2.64
2.20
ꢂ1.43
ꢂ0.96
5
14
473 (4.35), 351 (1.79),
5.29/3.09
302 (2.15)a
461(2.71)b
0.79b
0.43, 0.57
5.29/2.99
5.23/3.06
2.30
2.17
7
14
14
Phenyl
Benzene
3
480 (4.62), 386 (2.24),
ꢂ0.97
302 (2.07)a
15
16
17
1-naphthyl
Phenyl
1-naphthyl
Benzene
Thiophene
Thiophene
2
2
2
461 (3.13)b
0.81b
0.58b
0.53b
5.31/2.99
5.08/2.84
5.03/2.87
2.32
2.24
2.16
e
e
e
7
7
7
468 (2.25)b
480 (2.22)b
a
Recorded for dichloromethane solutions.
Recorded for tetrahydrofuran solutions.
b
impact of the acceptor segments. Introduction of the electron-
accepting cyanoacrylic acid first shifts the LUMO from the anthra-
cene segment (compare 3, 4, 5 and 7) to the cyanoacrylic acid unit.
However, its accepting ability is not enough to adopt the LUMO þ 1.
Addition of benzothiadiazole alters the electronic structure and
pushes the LUMO þ 1 to the acceptor end.
3.3. Electrochemical properties
For an efficient electron injection to the TiO2 electrode, the
dyes must possess excited state potential lower than the conduc-
tion-band-edge energy of the photoanode. Such an electronic
arrangement will benefit a favourable down-hill electron conduc-
tion process. The dye regeneration is essential to achieve high
stability and operational lifetime for the DSSCs. Efficient dye
regeneration is possible only if the ground state redox potential of
the dye is more positive than that of the electrolyte. As the excited
state oxidation potential can be grossly estimated from the ground
state oxidation potential and the energy gap, we have evaluated the
redox behaviour of the dyes in dichloromethane using the
conventional three-electrode cell assembly and cyclic voltammetric
technique. It is expected to throw information about the nature of
oxidation process resulting in the electron injection to the TiO2
The higher wavelength vertical transitions predicted by the
theory for the precursors 3 and 4 possess major contribution
from the HOMO to LUMO electronic excitation and possess very
large oscillator strength. But in the dyes 5, 7 and 9 HOMO to
LUMO transitions have very low oscillator strength (<0.1) and
are not observable in the experimental spectra. However, the
peaks appearing at the next lower wavelengths (493, 499 and
555 nm) with reasonable intensity reveal major contributions
from HOMO-1 to LUMO and HOMO to LUMO þ 1 transitions for
the dyes 5 and 7 while it is originating from the HOMO-2 to
LUMO transition for the dye 9. From the nature of the orbitals, it
is clearly evident that the absorption peaks of the dyes 5 and 7
occurring at 435 and 442 nm respectively in dichloromethane
Table 3
Computed (TDDFT/6-31G(d,p)/B3LYP) vertical transitions, their compositions,
orbital energies and dipole moments.
solutions are originating from the
pep* and charge transfer
transitions. On the contrary in the compound 9 the peak at
491 nm is attributed to the charge transfer transition involving
the triarylamine donor and benzothiadiazole based acceptor
moieties. The wavelength order of the red region absorption for
the compounds is 3 < 4 < 5 w 7 < 9, which is in keeping with the
theoretical predictions. The trend in the absorption intensity
realized for the dyes (5 < 7 < 9) is also roughly matching with
the proposed trend. Thus, the highly intense and red-shifted
absorption profile observed for the dye 9 is quite expected from
the theoretical computations. However, a slight overestimation
(þ50 nm) of absorption wavelengths for the dyes at the TDDFT
level is attributed to the flaws in modelling the long range
charge transfer interactions.
The dyes are weakly emissive in tetrahydrofuran but the
fluorescence is completely quenched when measured in dichloro-
methane. This is probably due to the intramolecular electron-
transfer quenching or dipole-dipole interactions present in the
more polar solvent. The fluorescence behaviour of the dyes in
nonpolar solvents such as toluene or hexane could not be deter-
mined due to their poor solubility.
Compound labs, f
Assignment
HOMO, eV LUMO, mg
,
nm
eV Debye
3
4
480 0.13 HOMO / LUMO (89%)
HOMO-1 / LUMO (10%)
489 0.13 HOMO / LUMO (90%)
HOMO-1 / LUMO (8%)
ꢂ4.869
ꢂ5.009
ꢂ1.797 0.38
ꢂ1.970 3.83
5
7
9
634 0.003 HOMO / LUMO (96%)
505 0.007 HOMO-1 / LUMO (86%)
HOMO / LUMO þ 1 (9%)
ꢂ5.136
ꢂ5.055
ꢂ5.025
ꢂ2.768 8.93
ꢂ2.750 7.50
ꢂ3.123 8.50
499 0.12 HOMO / LUMO þ 1 (84%)
HOMO-1 / LUMO (9%)
HOMO-1 / LUMO þ 1 (5%)
628 0.001 HOMO / LUMO (98%)
493 0.18 HOMO / LUMO þ 1 (58%)
HOMO-1 / LUMO (34%)
HOMO-1 / LUMO þ 1 (5%)
492 0.11 HOMO-1 / LUMO (64%)
HOMO / LUMO þ 1 (32%)
772 0.003 HOMO / LUMO (98%)
581 0.008 HOMO-1 / LUMO (98%)
555 1.05 HOMO-2 / LUMO (94%)

40
K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
Fig. 3. Molecular orbital diagrams of the dyes calculated by TDDFT (B3LYP-6-31G(d,p)) method.
photoanode and the regeneration feasibility by electron acceptance
from the electrolyte.
All the dyes exhibited a quasi reversible oxidation process in the
potential range 0.60e0.45 V vs. the internal ferrocene/ferrocenium
couple (Fig. 4). Firstly, for the dyes reported here, the oxidation
potentials lie in the range 1.22e1.37 V vs NHE. These values are
anodically shifted to that of the electrolyte (Iꢂ/I3ꢂ) redox couple
(w0.4 V vs NHE) and suggest facile dye regeneration in the pres-
ence of electrolyte. Secondly, the quasi-reversibility observed for
the redox potential arising due to the oxidation of the amine
functionality attests the electrochemical robustness of the dyes.
Thirdly, the excited state oxidation potential deduced for the dyes
(ꢂ0.88 to ꢂ1.12 V vs NHE) are more negative than the conduction-
band-edge energy level of the TiO2 electrode (ꢂ0.5 V vs NHE) [12].
This suggests that an electron injection from the dyes to TiO2
photoanode is favourable.
The oxidation potential of the dyes decreases in the order,
5 > 7 > 9. In compound 7, incorporation of additional thiophene
unit in the conjugation pathway may increase the electron-richness
of the aromatic linker which in turn facilitates the oxidation of the
end-capped amine unit [8]. On the contrary, an anodically shifted
redox potential observed for the dye containing the electron-
accepting benzothiadiazole segment (9) in the conjugation is
interesting. It may be argued that the extension of conjugation
length between the amine donor and the cyanoacrylic acid acceptor
Table 4
Performance characteristics of the dye-sensitized solar cells.
Dye
Jsc (mA/cm2)
Voc (V)
FF
h (%)
5
7
9
N3
6.00
8.28
9.80
0.565
0.535
0.520
0.741
0.648
0.657
0.633
0.589
2.20
2.91
3.23
7.87
18.04
Fig. 4. Cyclic voltammograms recorded for the dyes 5, 7, and 9 in dichloromethane.

K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
41
Fig. 5. IeV plots of the DSSCs constructed using the dyes 5, 7 and 9.
Fig. 7. Absorption spectra of the dyes 5, 7, and 9 anchored on nanocrystalline anatase
TiO2.
beyond a saturation limit may downgrade the electronic commu-
nication between them. An amine not so much influenced by an
acceptor segment may oxidize at a lower potential than the amine
affected by an acceptor group. In fact, the oxidation potential
observed for this dye (9) is lesser than that observed for the parent
amine, 3 (Table 1). This clearly points that the elongation of
conjugation plays a critical role in facilitating the oxidation of the
molecule rather than the effect due to the insulation of amine and
acceptor units by linker extension.
The orbital energies computed (Table 3) for the dyes (5, 7 and
9) are comparable to those deduced from the electrochemical
data measured in the dichloromethane solutions. From the orbital
energies it is evident that the incorporation of acceptor segment
lowers the LUMO level and the extension of conjugation raises
the HOMO level. Thus, the variation in the structural elements
helps the fine tuning of the orbital energies which is expected to
benefit the electron injection to the TiO2 layer and the dye
regeneration by the electrolyte. Orbital energies of the related
dyes containing fluorene or phenyl segments in place of anthra-
cene are close to those observed for the present dyes [5,7,10,22]
(see Table 2).
3.4. DSSC characteristics
The dye-sensitized solar cells were constructed by using the
dyes 5, 7 or 9 as the sensitizer for nanocrystalline anatase TiO2 in
a conventional Grätzel photoelectrochemical cell. Typically, the
effective light exposure area of the DSSCs was maintained at
0.25 cm2, and a liquid electrolyte composed of 0.05 M I2/0.5 M LiI/
0.5 M tert-butylpyridine in acetonitrile solution used. The device
performance statistics under AM 1.5 illumination are collected in
Table 4. Fig. 5 shows the photocurrentevoltage (IeV) curves of the
cells. The incident photon-to-current conversion efficiencies (IPCE)
of the dyes on TiO2 are plotted in Fig. 6. The devices of the dyes
exhibited moderate conversion efficiencies, ranging from 28% to
41% of the standard device fabricated using the ruthenium dye N3
and evaluated under identical conditions.
The order of efficiency observed for the dyes studied in this
work roughly correlates well with the trend in the molar extinction
coefficients for the absorption peaks realized for the dyes. The dye 5
is possessing blue-shifted and hypsochromic absorption among the
series while the compound 9 displays a broad, red-shifted and
hypochromic absorption. The open-circuit voltages (VOC) observed
for the DSSCs are in the order 5 > 7 > 9. However, the short-circuit
current (Jsc) measured for the DSSCs using the dyes are in the
reverse order (9 > 7 > 5). Larger short-circuit current and open-
circuit voltage and smaller overall fill factor (FF) are responsible for
the improved efficiency realized for the N3 dye [50]. All the dyes
adsorbed on TiO2 displayed a blue shift in the absorption spectra
(w25 to 40 nm, see Fig. 7) which probably indicates the deproto-
nation of cyanoacrylic acid during the interaction with the semi-
conductor [6] or H-aggregation of the dyes on the surface of the
semiconductor [51,52].
A comparison of the solar cell performance of the present dyes
with the related compounds [5,7,10,22] suggests that the perfor-
mance of these dyes are significantly influenced by the absorption
features. In the dyes 5 and 7, as the prominent absorption is
occurring between the triarylamine and the anthracene based
orbitals it is probable that the charge is trapped in anthracene
segment. This will inhibit the electron injection into the conduction
band of the TiO2 as the anthracene segment is further away from
the anchoring unit.
Fig. 6. IPCE plots of the DSSCs fabricated using the dyes 5, 7, and 9.

42
K.R. Justin Thomas et al. / Dyes and Pigments 91 (2011) 33e43
[13] Li R, Lv X, Shi D, Zhou D, Cheng Y, Zhang G, et al. Dye-sensitized solar cells
4. Conclusions
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In summary, in this paper we have presented the synthesis,
electro-optical properties and dye-sensitized solar cell perfor-
mance of the new class of metal free organic dyes containing
anthracene based triarylamine donor which is linked to the cya-
noacrylic acid acceptor through a conjugation pathway composed
of thiophene and benzothiadiazole units. The optical absorption of
the dyes is red-shifted on extension of the conjugation pathway by
the introduction of electron-rich oligothiophene or electron-defi-
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Acknowledgements
This research was supported by funding from Department of
Science and Technology (DST/TSG/ME/2010/27) at New Delhi and
the Indian Institute of Technology Roorkee (Initiation Grant) to
KRJT.
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Appendix. Supplementary information
Absorption and emission spectra of the dyes recorded in tetra-
hydrofuran, optimized structures of the dyes and their atomic
coordinates are provided in the Supporting Information.
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.dyepig.2011.02.006.
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