Synthesis of D-(π-A)2 organic chromophores for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.02.016

New bi-anchoring donor-π-acceptor (D-π-A) metal-free organic dyes based on diphenylamine, carbazole as donor, cyano vinyl biphenyl as a π-linker and cyanoacrylic acid, rhodanine-3-acetic acid as acceptor/anchor are synthesized, characterized and used for the dye-sensitized solar cell (DSSC) applications. Among the fabricated DSSCs, device based on the carbazole as donor, cyano vinyl biphenyl as a π-linker and cyanoacrylic acid as acceptor exhibits a high power conversion efficiency of 2.37% with a short-circuit current density of 5.18 mAcm^-2, an open-circuit photo voltage of 0.766 V and a fill factor of 50.8% under AM1.5 illumination (85 mWcm ^-2) compared to other devices.

Full text of DOI:10.1016/j.dyepig.2012.02.016

Dyes and Pigments 94 (2012) 503e511
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of D-(
p-A)2 organic chromophores for dye-sensitized solar cells
*
Sekar Ramkumar, Subbaiah Manoharan, Sambandam Anandan
Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e i n f o
a b s t r a c t
Article history:
New bi-anchoring donor-
as donor, cyano vinyl biphenyl as a
p
-acceptor (D-
-linker and cyanoacrylic acid, rhodanine-3-acetic acid as acceptor/
anchor are synthesized, characterized and used for the dye-sensitized solar cell (DSSC) applications.
Among the fabricated DSSCs, device based on the carbazole as donor, cyano vinyl biphenyl as a -linker
p-A) metal-free organic dyes based on diphenylamine, carbazole
Received 6 January 2012
Received in revised form
25 February 2012
Accepted 27 February 2012
Available online 13 March 2012
p
p
and cyanoacrylic acid as acceptor exhibits a high power conversion efficiency of 2.37% with a short-
circuit current density of 5.18 mAcmꢀ2, an open-circuit photo voltage of 0.766 V and a fill factor of 50.8%
under AM1.5 illumination (85 mWcmꢀ2) compared to other devices.
Keywords:
DSSC
Ó 2012 Elsevier Ltd. All rights reserved.
Bi anchoring molecules
Carbazole
Diphenylamine
Fill factor
Planarity
1. Introduction
deeply depend on the molecular structures (i.e.) donor (D) and
acceptor (A) groups of the organic molecules and the amount of
Dye-sensitized solar cells (DSSCs) have drawn the attention of
many researchers due to their ability of converting sunlight to
electricity at a low cost fabrication technique with moderate effi-
ciencies compared to conventional pen junction solar cells [1e3]. A
typical DSSC is constructed with dye-adsorbed wide band gap
oxide semiconductor electrode (TiO2), electrolyte containing Iꢀ/I3ꢀ
redox couples, and a platinum counter electrode [4]. In the DSSCs
system, electrons are injected into the conduction band (CB) of the
TiO2 from the excited dye molecules under light illumination, while
the ground state of the dye is regenerated by the electrolyte to give
efficient charge separation [5]. For this, the photosensitizers (dyes)
namely metaleorganic complexes and metal-free organic dye
molecules, is one of the critical components in DSSC to improve the
solar energy-to-electricity conversion efficiency [6]. Even though
the metaleorganic complexes (Ru dyes such as N3 and N719) have
been used to achieve high power conversion efficiencies above 11%
under AM 1.5 irradiation [7,8], the metal-free organic dyes have
recently become noticeable due to their many advantages such as
high molar extinction coefficient, simple synthesis procedure, and
environmental friendliness, despite to their slightly lower effi-
ciencies in the range of 6e9% [9e11]. The photoconversion effi-
ciencies of DSSCs fabricated using metal-free organic dye molecules
adsorbed molecules on the TiO2 surface [12]. The D-p-A ambipolar
system is the basic feature for most metal-free organic dyes due to
the effective photoinduced intramolecular charge transfer property
[13e15], and these dyes are firmly grafted on to the semiconductor
oxide surface through anchoring groups such as carboxylate or
phosphonate. Among the various types of metal-free organic dye
molecules, in which most of the organic dyes contain only one
anchoring group [16], however organic dyes containing bi-
anchoring groups [17e21] has been received much attention from
the researchers and industrialists to design as an efficient photo
sensitizer for DSSCs. The reason for much attention has been paid to
these materials is due to (i) increasing the photoinduced intra-
molecular charge transfer (ICT) from donor to acceptor [22] (ii)
enhancing the electron injection from the LUMO level of the dye
molecule to conduction band (CB) of TiO2 surface [23] and (iii)
enhancing the optical density and absorbing the maximum solar
spectrum due to extended p-conjugated framework.
Hence, we report the synthesis, exhaustive structural charac-
terization, and investigation of the electrochemical, photophysical
and photovoltaic properties of a series of four novel organic dyes
[Car-CN ((2E,20E)-3,30-(40,400-(1Z,10Z)-2,20-(9-hexyl-9H-carbazole-
3,6-diyl)bis(1-cyanoethene-2,1-diyl)bis(biphenyl-40,4-diyl))bis(2-
cyano acrylic acid)), Car-Rh (2,20-(5E,50E)-5,50-(40,400-(1Z,10Z)-2,20-
(9-hexyl-9H-carbazole-3,6-diyl)bis(1-cyanoethene-2,1-diyl)bis
(biphenyl-40,4-diyl))bis(methan-1-yl-1-ylidene)bis(4-oxo-2-thio-
xothiazolidin-3-yl-5-ylidene)diacetic acid), Dpa-CN ((2E,20E)-3,30-
* Corresponding author. Tel.: þ91 431 2503639; fax: þ91 431 2500133.
E-mail addresses: sanand@nitt.edu, sanand99@yahoo.com (S. Anandan).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.02.016

504
S. Ramkumar et al. / Dyes and Pigments 94 (2012) 503e511
(40,400-(1Z,10Z)-2,20-(4,40-(hexylazanediyl)bis(4,1-phenylene))bis
(1-cyanoethene-2,1-diyl)bis(biphenyl-40,4-diyl))bis(2-cyanoacrylic
acid)) and Dpa-Rh (2,20-(5E,50E)-5,50-(40,400-(1Z,10Z)-2,20-(4,40-
(hexylazanediyl)bis(4,1-phenylene)) bis(1-cyanoethene-2,1-diyl)
bis(biphenyl-40,4-diyl))bis(methan-1-yl-1-ylidene)bis(4-oxo-2-thio-
xothiazolidin-3-yl-5-ylidene)diacetic acid)] (Fig. 1) based on
amine (carbazole and diphenylamine) framework as donor con-
taining bi electron acceptor (either cyano acrylic acid or rhoda-
nine-3-acetic acid) and the bridging cyanovinyl biphenyl unit to
provide conjugation between the donor amine and the anchoring
acid group (acceptor). In order to reduce the aggregation and
enhance the solubility of the sensitizer, the hydrophobic hexyl
groups [24,25] are attached to the donor group.
2.2. Synthesis
2.2.1. Synthesis of 9-hexyl-9H-carbazole (1a) [26]
1-Bromohexane (3.356 g, 0.02033 mol), carbazole (2.0 g,
0.01196 mol) and sodium hydroxide (4.0 g, 0.10046 mol) were added
in dimethylsulfoxide (DMSO) (30 mL), followed by heating at 110 ꢂC
for 12 h. After cooling to room temperature the resulting mixture
was extracted with ethyl acetate (EA)/water and then dried with
Na2SO4. The solvent was evaporated and the resulting crude solid
was purified by column chromatography on neutral alumina by
using hexane as solvent to give white solid with yield 90.4% (3.1 g).
1H NMR (CDCl3, ppm):
2H), 7.52e7.46 (m, 2H), 7.31 (t, 2H, J ¼ 7.2 Hz), 4.33 (t, 2H, J ¼ 7.2 Hz),
1.96e1.89 (m, 2H), 1.54e1.37 (m, 6H), 0.96 (d, 3H, J ¼ 6.8 Hz).
d
8.18 (d, 2H, J ¼ 7.6 Hz), 7.56e7.54 (m,
2. Experimental section
2.2.2. Synthesis of 9-hexyl-9H-carbazole-3,6-dicarbaldehyde (1b)
[26,27]
2.1. Material and methods
Freshly distilled POCl3 (23.1 ml, 25 eq) was added drop wise to
DMF (17.6 ml, 23 eq) under an atmosphere of N2 at 0 ꢂC, and then it
was stirred for 1 h. Compound 1a (2.5 g, 9.9 m mol) was added to
the above solution, and the resulting mixture was stirred for 4 h at
95 ꢂC. After cooling to room temperature, the mixture was poured
into a beaker containing ice-cube, and basified with 4 M NaOH.
Filtered the solid and extracted with EA/brine. After evaporating
the organic solvent the crude product was purified by column
chromatography on neutral alumina using a mixture of ethyl
acetate/hexane (1:4, v/v), to give a white solid (1.5 g, yield ¼ 49%).
1H and 13C NMR spectra were obtained on Bruker 400 MHz and
500 MHz NMR Spectrometer using tetramethylsilane as internal
standard. Mass spectra (ESI-MS) were recorded on a Micromass
QUATTRO 11 spectrometer coupled to a Hewlett Packard series
1100 degasser. UVevis and fluorescence spectra were recorded on
a T90þ UV/VIS spectrometer and Shimadzu RF-5301 PC spectro-
fluorophotometer respectively. The cyclic voltammograms repor-
ted here were recorded with a computer controlled AUTOLAB-
potentiostat/galvanostat with a conventional 3-electrode system
such as platinum working electrode, Ag/AgNO3 reference electrode
and glassy carbon counter electrode in deoxygenated and anhy-
drous DMF at room temperature. The potentials are reported vs
ferrocene as standard using a scan rate of 0.1 Vsꢀ1 and the sample
solutions contained 3.0 ꢁ 10ꢀ4 M sample and 0.1 M tetrabuty-
lammonium hexafluorophosphate as a supporting electrolyte.
Argon was bubbled for 10 min before each measurement. All
reactions were monitored using TLC plates. All chromatographic
separations were carried out on silica gel (60e130 mesh).
1H NMR (CDCl3, ppm):
d
10.13 (s, 2H), 8.67 (d, 2H, J ¼ 1.5 Hz),
8.10e8.08 (m, 2H), 7.55 (d, 2H, J ¼ 8.5 Hz), 4.39 (t, 2H, J ¼ 7.5 Hz),
1.94e1.88 (m, 2H),1.41 (d, 2H, J¼ 7Hz),1.38e1.30(m, 4H)0.86(m, 3H).
2.2.3. Synthesis of (2Z,20Z)-3,30-(9-hexyl-9H-carbazole-3,6-diyl)
bis(2-(4-bromophenyl) acrylonitrile) (1c) [26]
The obtained compound 1b (1.0 g, 3.25 mmol), 4-
bromophenylacetonitrile (2.55 g, 13.01 mmol), were dissolved in
dried methanol (34 mL). A catalytic amount of potassium tert-
Fig. 1. Molecular structures of the synthesized dyes.

S. Ramkumar et al. / Dyes and Pigments 94 (2012) 503e511
505
butoxide was added into this mixture at room temperature. A
bright yellow solid was filtered after 12 h reflection. It was
recrystallized in methylene chloride and methanol to give the
product 1.3 g (yield ¼ 60.2%).
1H NMR (DMSO-d6, ppm):
d
8.75e8.69 (m, 2H), 8.31e8.27 (m,
2H), 8.21e8.15 (m, 2H), 8.0e7.82 (m, 14H), 7.76e7.72 (m, 6H), 4.71
(s, 4H), 4.48 (s, 2H), 1.82 (s, 2H), 1.3e1.24 (m, 6H), 0.83e0.81 (m,
3H). 13C NMR (DMSO-d6 and CDCl3, ppm)
d 192.38, 167.52, 165.43,
1H NMR (CDCl3, ppm):
d
8.65 (d, 2H, J ¼ 1.5 Hz), 8.18e8.2 (m,
147.42, 145.73, 144.59, 143.95, 131.48, 131.00, 127.39, 125.97, 121.61,
120.42, 108.19, 106.48, 53.44, 47.63, 28.02, 27.72, 26.35, 22.05, 13.83.
Anal. Calcd for C60H45N5O6S4: C, 67.97; H, 4.28; N, 6.61. Found: C,
67.91; H, 4.39; N, 6.66. m/z (ESI) Anal. Calcd. for C60H45N5O6S4:
2H), 7.72 (s, 2H), 7.57 (d, 8H, J ¼ 19 Hz), 7.53 (d, 2H, J ¼ 9 Hz), 4.35 (t,
2H, J ¼ 7.5 Hz), 1.89e1.92 (m, 2H), 1.40 (d, 2H), 1.29e1.35 (m, 4H),
0.89 (m, 3H).
1059.23. Found: 1077.23 [M þ NH4] þ
.
2.2.4. Synthesis of (2Z,20Z)-3,30-(9-hexyl-9H-carbazole-3,6-diyl)
bis(2-(40-formylbiphenyl-4-yl)acrylonitrile) (1d)
2.2.7. Synthesis of N-hexyl-N-phenylaniline (2a) [26]
It was synthesized according to the procedure of 1a. The product
obtained was colorless liquid (Yield ¼ 80.2%).
A
mixture of compound 1c (0.500 g, 0.7536 mmol), 4-
Formylphenylboronic acid (0.26 g, 2.3 eq), 0.05 eq. Pd(OAc)2
(0.0195 g, 0.0866 mmol) and 0.1 eq. P(O-tolyl)3 (0.053 g,
0.1733 mmol) were dissolved in a distilled, degassed THF (45 ml) at
N2 atmosphere. To this K2CO3 (0.59 g, 2.5 eq) was added and
refluxed for 12 h. After cooling to room temperature, distilled out
the solvent under vacuum. The resulting solid was purified by
column chromatography using hexane:ethyl acetate as the eluent,
to generate the sandal color solid which was dried in vacuum. The
resulting product yield was 41% (0.22 g).
1H NMR (CDCl3, ppm): 7.27e7.23 (m, 4H), 6.98e6.96 (m, 4H),
6.94e6.91 (m, 2H), 3.67 (t, 2H, J ¼ 8 Hz), 1.66e1.62 (m, 2H),
1.35e1.26 (m, 6H), 0.88e0.85 (m, 3H).
2.2.8. Synthesis of 4, 40-(hexylazanediyl)dibenzaldehyde (2b)
[26,27]
It was synthesized according to the procedure of 1b. The product
is Brown color liquid (Yield ¼ 94%).
1H NMR (CDCl3, ppm):
d
10.07 (s, 2H), 8.67 (d, 2H, J ¼ 1.6 Hz),
1H NMR (CDCl3, ppm):
d
9.85 (s, 2H), 7.78 (d, 4H, J ¼ 8.4 Hz), 7.13
8.21 (dd, 2H, J ¼ 8.4 Hz), 7.99e7.92 (m, 4H), 7.84e7.79 (m, 6H),
7.78e7.69 (m, 4H), 7.60e7.58 (m, 2H), 7.57e7.48 (m, 2H), 7.26 (s,
2H), 4.33 (s, 2H), 1.62e1.92 (m, 3H), 1.34e1.28 (m, 4H), 1.27e1.25
(d, 4H, J ¼ 8.4 Hz), 3.82 (t, 2H, J ¼ 8.0 Hz), 1.67 (d, 4H, J ¼ 8.4 Hz),
1.33e1.26 (m, 6H), 0.85 (t, 3H, J ¼ 6.4 Hz).
(m, 2H), 0.9e0.85 (m, 2H). 13C NMR (CDCl3, ppm)
d
191.96, 146.05,
2.2.9. Synthesis of (2Z,20Z)-3,30-(4,40-(hexylazanediyl)bis(4,1-
phenylene))bis(2-(4-bromophenyl) acrylonitrile) (2c) [26]
The compound 2c (Dark yellow solid) was prepared from 2b by
following the procedure similar to 1c. (Yield ¼ 44%).
1H NMR (CDCl3, ppm): 7.86 (d, 4H, J ¼ 8.8 Hz), 7.82e7.50 (m, 8H),
7.43 (s, 2H), 7.32e7.30 (m, 2H), 7.13e7.05 (m, 2H), 3.81 (t, 2H,
J ¼ 7.6 Hz), 1.74e1.64 (m, 2H), 1.38e1.29 (m, 6H), 0.89 (t, 3H,
J ¼ 6.8 Hz).
143.28, 142.35, 139.89, 135.59, 135.26, 130.5, 130.44, 129.83, 128.32,
128.04, 127.84, 127.66, 126.44, 126.39, 125.8, 123.3, 123.13, 118.94,
109.86,107.55, 52.35, 31.62, 29.07, 27.04, 22.65,14.13. Anal. Calcd for
C50H39N3O2: C, 84.12; H, 5.51; N, 5.89. Found: C, 84.26; H, 5.47; N,
5.79. m/z (ESI) Anal. Calcd. for C50H39N3O2: 713.86. Found: 714.39
[M þ H] þ, 731.42 [M þ NH4] þ
.
2.2.5. Synthesis of (2E,20E)-3,30-(40,400-(1Z,10Z)-2,20-(9-hexyl-9H-
carbazole-3,6-diyl)bis(1-cyanoethene-2,1-diyl)bis(biphenyl-40,4-
diyl))bis(2-cyanoacrylic acid) (1e)
2.2.10. Synthesis of (2Z,20Z)-3,30-(4,40-(hexylazanediyl)bis(4,1-
phenylene))bis(2-(40-formyl biphenyl-4-yl)acrylonitrile) (2d)
It was synthesized according to the procedure of 1d. The final
product is light red color solid (Yield ¼ 42%).
A chloroform solution of 1d (0.25 g, 0.3502 mmol) and cyano-
acetic acid (0.12 g, 1.4 mmol) was refluxed in the presence of
piperidine (0.083 ml) for 12 h. After cooling to room temperature,
the solvent was removed by distillation. The residue was purified
by column chromatography using silica gel and CHCl3:CH3OH (v:v,
10:1) mixed as the eluent to give the dye as Dark sandal color solid
(0.147 g, yield ¼ 49.5%).
1H NMR (CDCl3, ppm): 10.06 (s, 2H), 7.99e7.96 (m, 4H),
7.92e7.85 (m, 4H), 7.80e7.77 (m, 4H), 7.75e7.67 (m, 4H), 7.58e7.50
(m, 4H), 7.26e7.06 (m, 4H), 6.92 (d, 2H, J ¼ 8.8 Hz), 3.81 (t, 2H,
J ¼ 7.6 Hz), 1.60 (m, 2H), 1.34e1.28 (m, 6H), 0.90e0.85 (m, 3H). 13C
NMR (CDCl3, ppm):
d 191.94, 149.02, 146.06, 144.08, 141.86, 141.77,
1H NMR (DMSO-d6, ppm):
d
9.30e9.25 (bs, 2H), 8.70 (s, 2H), 8.17
140.44, 139.99, 135.67, 135.61, 135.16, 132.29, 131.69, 131.26, 131.19,
130.51, 128.17, 128.04, 127.77, 127.68, 127.35, 127.23, 126.45, 126.42,
120.93, 52.41, 31.65, 27.64, 26.81, 22.76, 14.15. Anal. Calcd. for
C50H41N3O2: C, 83.89; H, 5.77; N, 5.87. Found: C, 83.78; H, 5.81; N,
5.94. m/z (ESI) Anal. Calcd. for C50H41N3O2: 715.86. 716.91
[M þ H] þ.
(d, 4H, J ¼ 4.5 Hz), 8.04 (d, 6H, J ¼ 7.8 Hz), 7.82e7.84 (m, 12H), 7.62
(d, 2H, J ¼ 8.1 Hz), 4.41 (s, 2H), 1.87e1.64 (m, 4H), 1.32 (m, 4H), 0.86
(m, 3H). 13C NMR (DMSO-d6 and CDCl3, ppm)
d 166.20, 150.23,
142.28, 141.66, 138.70, 134.28, 131.45, 130.62, 127.16, 126.85, 125.89,
125.36, 122.36, 106.20, 43.65, 30.95, 26.18, 22.19, 22.00, 13.71. Anal.
Calcd for C56H41N5O4: C, 79.32; H, 4.87; N, 8.26. Found: C, 79.26; H,
4.81; N, 8.38. m/z (ESI) Anal. Calcd. for C56H41N5O4: 847.32. Found:
2.2.11. Synthesis of (2E,20E)-3,30-(40,400-(1Z,10Z)-2,20-(4,40-
(hexylazanediyl)bis(4,1-phenylene))bis (1-cyanoethene-2,1-diyl)
bis(biphenyl-40,4-diyl))bis(2-cyanoacrylic acid) (2e)
It was synthesized according to the procedure of 1e. The product
is orange red color solid (Yield ¼ 39%).
865.35 [M þ NH4] þ
.
2.2.6. Synthesis of 2,20-(5E,50E)-5,50-(40,400-(1Z,10Z)-2,20-(9-hexyl-
9H-carbazole-3,6-diyl)bis(1-cyanoethene-2,1-diyl)bis(biphenyl-
40,4-diyl))bis(methan-1-yl-1-ylidene)bis(4-oxo-2-thioxothiazolidin-
3-yl-5-ylidene)diacetic acid (1f)
The compound 1d (0.17 g, 0.2381 mmol), rhodanine-3-acetic
acid (0.273 g, 1.4288 mmol), ammonium acetate (0.46 g), and
glacial acetic acid (10 mL) were mixed together and refluxed for
12 h. The clear red solution was cooled to room temperature to
yield a red colored precipitate. It was filtered and washed with
cooled acetic acid and water mixture. The obtained solid was
further purified by crystallization from the dichloromethane/
hexane mixture to give saffron color solid (0.133 g, yield ¼ 53%).
1H NMR (DMSO-d6, ppm):
d 8.78e8.73 (m, 4H), 8.33 (s, 2H), 8.23
(d, 4H, J ¼ 8.5 Hz), 8.18 (s, 4H), 7.95e7.91 (m, 12H), 7.85 (d, 2H,
J ¼ 8.5 Hz), 4.49 (s, 2H), 1.65e1.63 (m, 2H), 1.56e1.55 (m, 2H),
1.32e1.22 (m, 4H), 0.84e0.83 (m, 3H). 13C NMR (DMSO-d6 and
CDCl3, ppm)
d 161.26, 148.96, 141.87, 133.93, 132.53, 132.23, 131.15,
131.12, 127.28, 127.01, 125.89, 122.89, 120.84, 118.38, 107.25, 52.43,
52.35, 31.63, 27.62, 26.75, 26.71, 22.72, 14.12. Anal. Calcd. for
C56H43N5O4: C, 79.13; H, 5.10; N, 8.24. Found: C, 79.21; H, 5.14; N,
8.94. m/z (ESI) Anal. Calcd. for C56H43N5O4: 849.32. Found: 850.34
[M þ H]þ.

506
S. Ramkumar et al. / Dyes and Pigments 94 (2012) 503e511
H
N
C6H13
N
C6H13
N
(i)
(ii)
(iii)
CHO
OHC
1b
1
1a
C6H13
N
C6H13
N
(iV)
CN
NC
CN
NC
1c
Br
Br
1d
OHC
CHO
(Vi)
(V)
C6H13
N
C6H13
N
CN
NC
CN
NC
1e
1f
O
O
NC COOH
S
S
HOOC CN
N
N
S
S
COOH
HOOC
H
N
C6H13
N
C6H13
N
(i)
(ii)
(iii)
OHC
CHO
2b
2
2a
C6H13
N
C6H13
N
(iV)
CN
NC
CN
NC
2c
Br
Br
OHC
2d
CHO
(Vi)
(V)
C6H13
N
C6H13
N
CN
NC
CN
NC
2e
2f
O
O
S
S
N
HOOC CN
NC COOH
N
S
S
HOOC
COOH
Scheme 1. Synthetic procedure of the organic dyes. (i) 1-Bromohexane, NaOH, DMSO, 110 ꢂC. (ii) PoCl3, DMF, 95 ꢂC. (iii) 4-bromophenyl acetonitrile, t-BuOK, CH3OH, refluxed. (iv) 4-
Formylphenylboronic acid, Pd(OAc)2, P(O-tolyl)3, K2CO3, THF, refluxed. (v) Cyanoaceticacid, piperidine, CHCl3, refluxed. (vi) Rhodanine-3-acetic acid, ammonium acetate, glacial
CH3COOH, refluxed.

S. Ramkumar et al. / Dyes and Pigments 94 (2012) 503e511
507
2.2.12. Synthesis of 2,20-(5E,50E)-5,50-(40,400-(1Z,10Z)-2,20-(4,40-
(hexylazanediyl)bis(4,1-phenylene)) bis(1-cyanoethene-2,1-diyl)
bis(biphenyl-40,4-diyl))bis(methan-1-yl-1-ylidene)bis(4-oxo-2-
thioxothiazolidin-3-yl-5-ylidene)diacetic acid (2f)
35000
30000
25000
20000
15000
10000
5000
0
a
Car-CN
Dpa-CN
Car-Rh
Dpa-Rh
The compound 2f (Dark red solid) was prepared from 2d by
following the procedure similar to synthesis of 1f (Yield ¼ 44%).
1H NMR (DMSO-d6, ppm):
d 7.98 (s, 2H), 7.95e7.90 (m, 10H), 7.87
(d, 4H, J ¼ 8 Hz), 7.81 (d, 4H, J ¼ 8.4 Hz), 7.74 (d, 4H, J ¼ 8 Hz), 7.20
(m, 4H) 4.73 (s, 2H), 3.85 (s, 4H), 1.61 (s, 2H), 1.33e1.25 (m, 6H),
0.86e0.82 (m, 3H). 13C NMR (DMSO-d6 and CDCl3, ppm)
d 190.57,
161.39, 160.79, 157.69, 151.99, 144.52, 143.34, 141.39, 131.65, 130.64,
120.79,118.34, 111.95,110.64, 98.36, 52.61, 43.51, 31.54, 30.98, 27.52,
26.65, 22.63, 14.03. Anal. Calcd. for C60H47N5O6S4: C, 67.84; H, 4.46;
N, 6.59. Found: C, 67.91; H, 4.34; N, 6.64. m/z (ESI) Anal. Calcd. for
C60H47N5O6S4: 1061.24. Found: 1079.24[M þ NH]þ.
3. Results and discussion
300
350
400
450
500
550
600
Wavelength (nm)
3.1. Synthesis of bi-anchoring dyes
b
PL Dpa-CN
PL Car-CN
PL Car-Rh
PL Dpa-Rh
The synthetic pathways of organic dyes Car-CN, Car-Rh, Dpa-CN
and Dpa-Rh were depicted in Scheme 1. Alkylation of amine
compounds using 1-bromohexane in dimethylsulfoxide afforded
compounds 1a and 2a. The VilsmeiereHaack formylation in alky-
lated carbazole and diphenylamine gave compounds 1b and 2b.
The aldehydes (1b, 2b and 1d, 2d) and active methylene
compounds such as 4-bromophenylacetonitrile, cyano acetic acid
and rhodanine-3-acetic acid were condensed by Knoevenagel
Condensation to gave different products (1c, 2c; 1e, 2e; and 1f, 2f)
respectively. Suzuki coupling of dibromo compounds (1c, 2c) with
4-formylphenylboronicacid in the presence of Pd(OAc)2 gave
compounds 1d and 2d. Synthesized compounds were well
confirmed by various analytical tools (See Supplementary data).
1.0
0.8
0.6
0.4
0.2
0.0
3.2. Optical properties
500
600
700
The UVevis absorption and normalized emission spectra of all
dyes in DMF solution are depicted in Fig. 2a and b respectively and
the corresponding data are summarized in Table 1. Two major
absorption features were observed in the spectra of all the mole-
cules: one relatively weak band in the near-UV region
Wavelength (nm)
Fig. 2. Absorption (a) and normalized emission (b) spectra of dyes recorded in DMF.
of all compounds in DMF (Fig. 3), and their data are summarized in
Table 1. We find that all the dyes which have electron donating/
withdrawing groups show one quasi-reversible peak arising from
the oxidation potential which were determined from the inter-
section of two tangents drawn at the rising current and background
(330e400 nm) corresponding to the pep* electron transition, and
the other strong absorption in the visible region (415e460 nm) that
can be assigned to an intramolecular charge transfer (ICT) between
the donor unit and the acceptor end group [28]. For the similar
structural architecture, the rhodanine acetic acid acceptor
compounds (Dpa-Rh and Car-Rh) display red-shifted and induced
intramolecular charge transfer absorption band when compared to
the cyano acetic acid analogs (Dpa-CN and Car-CN) due to the
stronger electron-withdrawing nature of rhodanine acetic acid
substituents [17,29]. However, their molar extinction coefficients
are lower than the latter. Upon comparing all the compounds, Dpa-
CN and Dpa-Rh shows red-shifted absorption peaks, due to low
ionization potential of diphenylamine [30]. Whereas in the case of
carbazole derivative conjugation is disallowed between the two
aryl segments may be due to its planar nature and hence it exhibits
shorter wavelength maxima (lmax) [31]. The fluorescence emission
spectra of all the dyes exhibited strong luminescence maxima in
the wavelength region 490e550 nm recorded upon their excitation
of absorption maximum value.
charging current of
a cyclic voltammogram [32] at around
0.60e1.19 V and two irreversible peaks arising from reduction
potential at around ꢀ0.22 to ꢀ0.95 V and are very close to each
other. This was attributed due to the sp3-type donor nitrogen
center [33] and electron withdrawing group contained aromatic
linker, acceptor groups. The HOMO energy levels were calculated
ox
based on the relationship of HOMO (eV) ¼ ꢀe(E
V(vs Fc/
onset
Fcþ) þ 4.8 V) by assuming the Ferrocene (Fc) energy level to
be ꢀ4.8 eV below the vacuum level [34] and the LUMO levels of the
ox
dyes were calculated by E
ꢀ E0e0, where E0e0 is the zeroezero
onset
energy of the dyes estimated from the solution absorption edge
[35,36]. In our work, the potential of Fc was measured to be 0.45 V
against Ag electrode. The oxidation onset potentials of Car-CN, Car-
Rh, Dpa-CN and Dpa-Rh were unequivocally determined to be 0.90,
0.63, 0.67 and 0.75 V respectively. The reason for high oxidation
potential of Car-CN compared to other dyes may be due to the
weaker electron donating nature of Carbazole moiety [37]. The
HOMO and LUMO energy levels of all the dyes (Table 1), compared
to the levels of the conduction band of TiO2 (ꢀ4.0 eV vs vacuum)
and the Iꢀ/Iꢀ3 redox electrolyte (ꢀ4.6 eV vs vacuum), are suitable for
3.3. Electrochemical measurements
To understand the electronic structures of the bi anchoring D-
(p-A)2 molecules, we performed cyclic voltammetry measurements

508
S. Ramkumar et al. / Dyes and Pigments 94 (2012) 503e511
Table 1
Optical, electrochemical data and HOMO, LUMO energy levels of Car-CN, Dpa-CN, Car-Rh and Dpa-Rh.
a
Dye
labs/nm (
3
)
lem/nma,b
Eogpt (eV)c
Eox/V (vs. Fc)
EHOMO (eV)d
ELUMO(eV)d
HOMOe
LUMOe
Ege
Car-CN
Dpa-CN
Car-Rh
Dpa-Rh
419 (25000)
446 (32579)
421 (22313)
458 (28617)
487
536
506
544
2.68
2.46
2.61
2.41
0.90
0.67
0.63
0.75
5.25
5.02
4.98
5.1
2.57
2.56
2.37
2.69
ꢀ5.72
ꢀ5.46
ꢀ5.55
ꢀ5.44
ꢀ2.77
ꢀ2.73
ꢀ2.81
ꢀ2.84
2.95
2.73
2.74
2.56
a
Absorption and emission spectra were recorded in DMF solution (3.8 ꢁ 10ꢀ5 M) at room temperature.
b
c
Dyes were excited at their absorption maximum value.
Optical band gap calculated from absorption edge.
The values of EHOMO and ELUMO were calculated with the following formula: HOMO (eV) ¼ ꢀe(E
d
ox
ox
V(vs Fc/Fcþ) þ 4.8 V); LUMO(eV) ¼ E
ꢀ E0e0, where E0e0 is the
onset
onset
absorption onset estimated from the electronic absorption spectra of the sensitizers.
e
B3LYP/6-31G(d) calculated values.
electron injection and dye regeneration thermodynamically
[19,38]. However the potential is further improved negatively about
0.3 V by adding additives such as 4-tert-butylpyridine (TBP) to the
Iꢀ/Iꢀ3 redox electrolyte [39].
principle, the overlapping of both HOMO and LUMO levels by the p-
bridged cyano vinyl biphenyl units enhance the electronic transi-
tion dipole moments between vibrational energy levels [40]. This
suggests that such strong intramolecular charge transfer (ICT)
nature of Car-CN and Dpa-CN dyes upon illumination results the
electron injection from the excited dyes to TiO2 conduction band
[41]. Nevertheless, in Car-Rh and Dpa-Rh dyes, the HOMO levels
were entirely populated on anchoring Rhodanine moiety especially
on the sulfur and thiocarbonyl group and LUMO levels are also
3.4. Ab initio molecular orbital (MO) calculations
To gain more insight into the electronic structures of dyes,
density functional theory (DFT) calculations were performed at
a B3LYP/6-31G(d) level for the geometry optimization. As shown in
Fig. 4, the HOMO levels of Car-CN and Dpa-CN dyes are mainly
delocalized on the central electron donating amine moieties, the
neighboring cyano vinyl phenyl groups with considerable contri-
bution to the former, whereas LUMO levels are delocalized through
the aromatic bridge and cyanoacrylic acid fragments however
sizable contribution from the latter. According to Franck Condon
delocalized on the anchoring units as well as on the
p-bridged
biphenyl units. Hence, the encumbrances in the electronic transi-
tions between the HOMO to LUMO energy levels had been antici-
pated in Car-Rh and Dpa-Rh dyes. These results indicate that dyes
with rhodanineeacetic acid units cannot give effective and fast
electron injection from LUMO level to TiO2 conduction band while
comparing to the dyes with cyanoacrylic acid units [29].
0.00008
0.00014
Car-CN
0.00012
Car-Rh
0.00006
0.00010
0.00008
0.00006
0.00004
0.00002
0.00000
-0.00002
-0.00004
-0.00006
-0.00008
0.00004
0.00002
0.00000
-0.00002
-1.5 -1.0 -0.5
0.0
Potential (V)
0.5
1.0
1.5
2.0
-1.5
-1.0
-0.5
0.0
Potential(V)
0.5
1.0
1.5
0.00003
0.00002
0.00001
0.00000
-0.00001
-0.00002
-0.00003
-0.00004
0.00008
0.00006
0.00004
0.00002
0.00000
-0.00002
-0.00004
-0.00006
Dpa-Rh
Dpa-CN
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Potential (V)
-2
-1
0
Potential (V)
1
2
Fig. 3. Cyclic volammogram of dyes were measured in DMF solutions with TBAPF6 (0.1 M) as the electrolyte (working electrode: Pt; reference electrode: Ag/Agþ; counter electrode:
glassy carbon; calibrated with Fc/Fcþ as a standard reference; scan rate: 0.1 Vsꢀ1).

S. Ramkumar et al. / Dyes and Pigments 94 (2012) 503e511
509
3.5. Fabrication of DSSCs and photovoltaic measurements
photovoltage (VOC), short-circuit photocurrent density (JSC), fill
factor (FF), and solar-to-electric conversion efficiencies ( ) are lis-
h
The nanocrystalline TiO2 [42] and Pt counter electrode [43,44]
were prepared by the known method reported in the literature
for fabrication of dye-sensitized solar cells. The prepared titania
paste was deposited onto an F-doped tin oxide (FTO) coated con-
ducting glass by doctor blade technique (around 1 cm2 area) and it
was immersed in the dye solution (3 ꢁ 10ꢀ4 M in DMF) for 24 h at
room temperature. 0.05 M I2/0.5 M KI/0.5 M 4-tert-butyl pyridine
(TBP) in 2-methoxypropionitrile was used as an electrolyte. The
photovoltaic properties of the solar cells fabricated from the
prepared electrodes under simulated AM 1.5 irradiation
(85 mW cmꢀ2) are shown in Fig. 5. The measured open-circuit
ted in Table 2. It is clear from Fig. 5 that the efficiency of the
fabricated device decrease in the order Car-CN > Car-Rh > Dpa-
CN > Dpa-Rh. This indicates that carbazole moiety may act as
a good light harvester due to its planar nature [45] and in addition
its lowered HOMO level allows efficient regeneration of the
oxidized dye and as well as minimize the charge recombination
[46]. The observed VOC values of Car-Rh and Dpa-Rh were much
lower than that of the other two dyes, showing rhodanineeacetic
acid has the low tendency to inject electrons from the LUMO to
TiO2 conduction band due to the presence of the eCH2e group [47].
The incident photon to current conversion efficiency (IPCE) as
Fig. 4. The frontier molecular orbitals of the HOMO and LUMO levels calculated with B3LYP/6-31G (d) of synthesized dyes.

510
S. Ramkumar et al. / Dyes and Pigments 94 (2012) 503e511
35
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
A
CAR-CN
CAR-Rh
30
Dark
Car-CN
DPA CN
CAR RH
DPA-Rh
25
20
15
10
5
0
0
100
200
300
400
500
600
700
400
500
600
Voltage (mV)
Wavelength (nm)
Dark
B
Fig. 6. IPCE spectra of Car-CN, Car-Rh, Dpa-CN and Dpa-Rh.
Dpa-CN
Dpa-Rh
2.0
1.5
1.0
0.5
0.0
H2O (1:1) mixture [29]. The surface concentration (G) of the dyes
was spectrophotometrically measured [48] (see Table 2) indicates
that cyano acetic acid analogs have adsorbed better on TiO2 surface
compared to rhodanine acetic acid analogs which also supports the
observed IPCE (%) order of the fabricated devices.
4. Conclusion
In this study, we have synthesized a series of new bi-anchoring
metal-free organic dyes for dye-sensitized solar cell applications. It
was found that both the absorption and emission peaks of diphe-
nylamine with rhodanine moieties are red-shifted compared to the
carbazole with cyanoacrylic acid moieties. However, their DSSC
performances are lower than the latter due to the bad LUMO
overlap with the conduction band of TiO2. Car-CN based device
exhibits a power conversion efficiency of 2.37%. This result indi-
cates that compared to diphenylamine and rhodanine-3-acetic
acid, carbazole and cyanoacetic acid based device open up a new
way for researchers toward DSSCs.
0
200
400
600
800
Voltage (mV)
Fig. 5. Current densityevoltage characteristics for (A) Car-CN, Car-Rh, and (B) Dpa-CN
Dpa-Rh based devices for DSSCs under illumination of simulated solar light (AM 1.5,
85 mW cmꢀ2).
a function of wavelength was measured for all the fabricated device
(Fig. 6) with an Oriel 300 W Xe Arc lamp in combination with an
Oriel Cornerstone 260¼ monochromator. The number of incident
photons was calculated for each wavelength using a calibrated
monocrystalline silicon diode as reference. The IPCE (%) of fabri-
cated devices shows a broader response between 400 and 600 nm
with a maximum efficiency of about 33.4% for Car-CN followed by
DPA-CN (20.0%), Car-RH (16.6%) and DPA-RH (16.5%) at its wave-
length maximum. In order to identify whether surface adsorbed
dye may play any vital role in the device fabrication, desorption of
the dye from the TiO2 surface was done using 0.1 M NaOH in DMF/
Acknowledgments
Authors thank DST, New Delhi for sanctioning major research
project (SR/S1/PC-49/2009) and India-Spain joint collaborative
project (DST/INT/Spain/P-37/11).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2012.02.016.
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