Synthesis and characterization of 5′-hexyl-2,2′-bithiophene based on organic dyes for dye-sensitized solar cell applications

Article abstract of DOI:10.1080/15421406.2014.935986

A series of new organic dyes, 2-(2,6-bis((E)-2-(5′-hexyl-2,2′-bithiophen-5-yl)vinyl)-pyran-4-ylidene)-2-cyanoacetic acid (OD-1) and 2,2′-(2-((E)-2-(5′-hexyl-2,2′-bithiophen-5-yl)vinyl)-1H-indene-1,3(2H)-diylidene)bis(2-cyanoacetic acid) (OD-2), were synthesized as a sensitizers for the application of dye-sensitized solar cells (DSSCs). The introduction of the 5′-hexyl-2,2′-bithiophene units as an electron donor group and 2,6-dimethyl-pyran-4-one or 1,3-indandione as a π-spacers units increased the conjugation length of the sensitizers and thus improved their molar absorption coefficient and light harvesting efficiency.

Full text of DOI:10.1080/15421406.2014.935986

Mol. Cryst. Liq. Cryst., Vol. 599: pp. 157–162, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.935986
Synthesis and Characterization
of 5ꢀ-Hexyl-2,2ꢀ-bithiophene Based on Organic
Dyes for Dye-Sensitized Solar Cell Applications
CHINNUSAMY SARAVANAN,1 WOOSUM CHO,1
YULHEE KIM,1 SUNG-HO JIN,1,∗ YEONG-SOON GAL,2
AND JAE WOOK LEE3
1Department of Chemistry Education, Graduate Department of Chemical
Materials, and Institute for Plastic Information and Energy Materials, Pusan
National University, Busan, Republic of Korea
2Department of Chemistry, Dong-A University, Busan, Republic of Korea
3Polymer Chemistry Lab, College of General Education, Kyungil University,
Hayang, Republic of Korea
A series of new organic dyes, 2-(2,6-bis((E)-2-(5ꢀ-hexyl-2,2ꢀ-bithiophen-5-yl)vinyl)-
pyran-4-ylidene)-2-cyanoacetic acid (OD-1) and 2,2ꢀ-(2-((E)-2-(5ꢀ-hexyl-2,2ꢀ-
bithiophen-5-yl)vinyl)-1H-indene-1,3(2H)-diylidene)bis(2-cyanoacetic acid) (OD-2),
were synthesized as a sensitizers for the application of dye-sensitized solar cells
(DSSCs). The introduction of the 5ꢀ-hexyl-2,2ꢀ-bithiophene units as an electron donor
group and 2,6-dimethyl-pyran-4-one or 1,3-indandione as a π-spacers units increased
the conjugation length of the sensitizers and thus improved their molar absorption
coefficient and light harvesting efficiency.
Keywords organic dyes; dye-sensitized solar cells; optical properties
Introduction
Increasing energy demands and concerns about global warming have led to a greater fo-
cus on renewable energy sources during the last years. The effective conversion of solar
energy into electricity has become a very important issue in the last few years due to
rising energy cost and demand [1]. In recent years, dye-sensitized solar cells (DSSCs)
based on nanocrystalline TiO2 (nc-TiO2) films have received significant attention because
of their high efficiency and low cost compared to those of silicon-based inorganic solar
cells [2]. In DSSCs, the sensitizing dye molecules must be chemically adsorbed on the
porous nc-TiO2 surfaces and be capable of harvesting solar light ranging from the visible
to the near-IR regions. Up to now, the efficient sensitizers are ruthenium (Ru) complexes,
which show effective charge separation at the metal to ligand (Ru-bpy) and ligand to metal
∗Address correspondence to Sung-Ho Jin, Department of Chemistry Education, Graduate De-
partment of Chemical Materials, and Institute for Plastic Information and Energy Materials, Pusan
National University, Busan 609-735, Korea. E-mail: shjin@pusan.ac.kr
[499]/157

158/[500]
C. Saravanan et al.
(SCN-Ru) absorption bands in the visible light region. With regard to the Ru sensitiz-
ers such as cis-dithiocyanato bis(4,4ꢀ-dicarboxy-2,2ꢀ-bipyridine)ruthenium(II) (N3) [3], its
bis(tetrabutylammonium) salt (N719) [4], and trithiocyanato (4,4ꢀ,4ꢀꢀ-tricarboxy-2,2ꢀ:6ꢀ,2ꢀꢀ-
terpyridine)ruthenium(II) (black dye) [5], the zinc porphyrin dye (YD2-o-C8) gave the
best power conversion efficiency (PCE) over 12 % measured at the AM1.5G condition [6].
Recently, metal-free organic dyes have recently been stimulating intensive research efforts
because of their advantages as sensitizers for DSSCs due to the high molar absorption
coefficients than that of Ru sensitizers, which increase the light-harvesting capabilities over
the wide spectral region of sunlight. Organic dyes need to overcome disadvantages such as
narrow absorption band, π-stacked aggregation, and stability. Thiophene and its derivatives
have high thermal stability, being ordinarily easy for structural design and modifications.
In addition, they are very easy to prepare and solubilized by wide range of organic sol-
vents. Here, we introduced n-hexyl chains on the terminal thiophene unit to enhance the
solubility, to suppress charge recombination, improve the open-circuit voltage, suppress the
aggregation of the sensitizers resulting in increase the power conversion efficiency. Finally,
the hydrophobic n-hexyl chains could also enhance the long-term stability of the solar cell
by preventing water induced dye desorption from the TiO2 surface. The carboxylic acid
group is the most employed group for attachment of the sensitizers to the semiconductor
surface. And the cyanoacrylic acid is the most commonly used unit in the dyes for DSSCs
due to its strong electronic withdrawing properties. These anchoring groups are increasing
the conjugation length of dye that increases the light harvesting efficiency.
Herein, we report two types organic dyes using a 5ꢀ-hexyl-2,2ꢀ-bithiophene unit as
an electron donor, the 2,6-dimethyl-pyran-4-one and 1,3-indandione as the π-spacer and
cyanoacrylic acid as an anchoring group. The thermal, photophysical, and electrochemical
properties have been investigated.
Experimental
1H-NMR spectra were recorded in CDCl3 on a Varian Mercury 300, and chemical shifts were
recorded in ppm. The absorption spectra were measured using a Jasco V-570 UV-visible
spectrometer. Thermal analyses were carried out on a Mettl◦er Toledo TGA/SDTA 851e
analyzer under a nitrogen atmosphere at a heating rate of 8 C/min. Cyclic voltammetry
(CV) was carried out with a Bioanalytical Systems CV-50W voltametric analyzer at a
potential scan rate of 100 mV/s in a 0.1 M solution of tetrabutylammonium perchlorate in
anhydrous CHCl3. A platinum wire was used as the counter electrode and an Ag/AgNO3
electrode was used as the reference electrode. All of the electrochemical experiments
were performed in a glove box under an N2 atmosphere at room temperature. 5-Formyl-
5ꢀ-hexyl-2,2ꢀ-bithiophene (3) was synthesized according to the previously literature [7].
2-(2,6-Dimethyl-pyran-4-ylidene)-2-cyanoacrylic acid (4) and 2,2ꢀ-(1H-indene-1,3(2H)-
diylidene)bis(2-cyanoacetic acid) (6) were synthesized according to the reported literature
with slightly modification [8].
Synthesis of 2-(2,6-bis((E)-2-(5ꢀ-hexyl-2,2ꢀ-bithiophen-5-yl)vinyl)-pyran-4-ylidene)-2-
cyanoacetic acid (OD-1) (1)
5-Formyl-5ꢀ-hexyl-2,2ꢀ-bithiophene (3) (1.5 g, 5.23 mmol) and 2-(2,6-dimethyl-pyran-4-
ylidene)-2-cyanoacrylic acid (4) (0.5 g, 2.62 mmol) were allowed to react in acetonitrile in
the presence of piperidine (0.13 ml, 1.3 mmol). The solution was refluxed at 80◦C for 18 h.

Synthesis and Characterization of 5ꢀ-Hexyl-2,2ꢀ-bithiophene
[501]/159
After removal of acetonitrile in vacuo, the crude product was extracted with methylene
chloride and water. The crude product was purified by column chromatography using
chloroform as an eluent to give a 2-(2,6-bis((E)-2-(5ꢀ-hexyl-2,2ꢀ-bithiophen-5-yl)vinyl)-
1
pyran-4-ylidene)-2-cyanoacetic acid (OD-1) (0.38 g, 20%). H-NMR (CDCl3, δ ppm)
8.13(s, 1H), 7.54(m, 2H), 7.20 (d, 2H), 7.14(d, 2H), 6.83 (s, 1H), 6.68 (d, 2H), 6.58 (d, 2H),
6.51(d, 2H), 2.79(t, 4H),1.70 (m, 4H), 1.29 (m, 12H), 0.86 (s, 6H). 13C-NMR (CDCl3, δ
ppm) 174.5, 163.9, 142.5, 140.6, 137.5, 134.2, 128.7, 127.9, 125.4, 120.1, 97.5, 91.0, 42.9,
32.8, 31.8, 22.4, 14.6
Synthesis of 2,2ꢀ-(2-((E)-2-(5ꢀ-hexyl-2,2ꢀ-bithiophen-5-yl)vinyl)-1H-indene-1,3(2H)-diy
lidene)bis(2-cyanoacetic acid) (OD-2) (2)
5-Formyl-5ꢀ-hexyl-2,2ꢀ-bithiophene (3) (0.55 g, 1.96 mmol) and (2Z, 2ꢀZ)-2,2ꢀ-(1H-indene-
1,3(2H)-diylidene)bis(2-cyanoacetic acid) (6) (0.5 g, 1.78 mmol) were allowed to react in
acetonitrile in the presence of piperidine (0.09 ml, 0.89 mmol). The solution was refluxed
at 80 ◦C for 18 h. After removal of acetonitrile in vacuo, the crude product was extracted
with methylene chloride and water. The crude product was purified by column chro-
marography using chloroform as an eluent to give 2,2ꢀ-(2-((E)-2-(5ꢀ-hexyl-2,2ꢀ-bithiophen-
5-yl)vinyl)-1H-indene-1,3(2H)-diylidene)bis(2-cyanoacetic acid) (OD-2) (0.26 g, 27%).
1H-NMR (CDCl3, δ ppm) 7.95 (m, 2H), 7.85 (d, 1H), 7.76(m, 2H), 7.32(d, 1H), 7.21(d,
1H), 6.77 (d, 1H) 6.65 (d, 1H), 6.57 (d, 1H), 3.52 (s, 1H) 2.82 (t, 2H) 1.78(m, 2H) 1.37
(m, 6H), 0.92 (m, 3H). 13C-NMR (CDCl3, δ ppm) 141.9, 141.5, 137.5, 134.4, 128.7, 128,4,
127.6, 126.4, 125.8, 95.6, 43.2, 32.9, 31.9, 31.8, 22.7, 14.2
Result and Discussion
The synthetic routes for the OD-1 and OD-2 are described in Scheme 1. 2-Bromo-5-
formylthiophene (1) was synthesized by bromination of 2-thiophene aldehyde with NBS in
DMF solvent. After synthesized the 2-(tri-n-butylstannyl)-5-hexylthiophene (2), 5-formyl-
5ꢀ-hexyl-2,2ꢀ-bithiophene (3) was synthesized by Stille coupling reaction between com-
pounds (1) and (2). 2,6-Dimethyl-pyran-4-one coupled with cyanoacetic acid in acetic anhy-
dride to give a 2-(2,6-dimethyl-pyran-4-ylidene)-2-cyanoacrylic acid (4). 2,2ꢀ-(1H-indene-
1,3(2H)-diylidene)bis(2-cyanoacetic acid) (6) was also synthesized with a similar reaction
condition for the synthesis of (4). 2-(2,6-Dimethyl-pyran-4-ylidene)-2-cyanoacrylic acid
(4) and 2,2ꢀ-(1H-indene-1,3(2H)-diylidene)bis(2-cyanoacetic acid) (6) were coupled with
5-formyl-5ꢀ-hexyl-2,2ꢀ-bithiophene (3) by the Knoevenagel condensation reaction to give a
OD-1 and OD-2, respectively. The structure and thermal properties of the OD-1 and OD-2
were identified by 1H-NMR, 13C-NMR, TGA, and CV.
The 1H-NMR spectra of the OD-1 and OD-2 indicated doublet peaks by vinylic bond,
which confirmed the double bond between thiophene moiety and 2,6-dimethyl-pyran-4-one
and 1,3-indandione as the π-spacer by Knoevenagel reaction.
The bithiophene unit was introduced n-hexyl groups to suppress the charge recombina-
tion and/or aggregation of the sensitizers and hence to increase the photovoltaic performance
[9]. The hydrophobic n-hexyl aliphatic chains will be enhanced the long-term stability of
the DSSCs through preventing water induced dye desorption from the TiO2 surface [10–11].
Finally, we used 2,6-dimethyl-pyran-4-one and 1,3-indandione as a π-spacers and intro-
duced the cyanoacrylic acid as an anchoring group. The cyanoacrylic acid group is by far
the most employed group for attachment of the sensitizers to the semiconductor surface.

160/[502]
C. Saravanan et al.
n-BuLi, Bu3SnCl
NBS
DMF
Br
CHO
CHO
S
Bu3Sn
C6H13
S
C6H13
S
S
THF
(1)
(2)
S
C6H13
NC
COOH
Pd(PPh3)4
DMF
CHO
(1)
(2)
S
+
(3)
O
S
S
NC
COOH
O
O
S
S
piperidine
NC
COOH
OD-1
+
(3)
acetonitrile, 18h, 80
(CH3CO)2O, 20h, 120
O
(4)
NC
HOOC
CN
COOH
O
O
piperidine
acetonitrile, 18h, 80
S
C6H13
NC
COOH
+
CHO
S
(CH3CO)2O, 24h, 120
(3)
(6)
COOH
COOH
NC
NC
S
S
C6H13
OD-2
Scheme 1. Synthetic Route for OD-1 and OD-2.
TGA thermograms of the OD-1 and OD-2 revealed a 5% weight loss at 162◦C under a
N2 atmosphere. Thermal stability of the OD-1 and OD-2 based on 5-hexylbithiophene
prevents degradation of the active layer during the operation of DSSCs. The ther-
mal, electrochemical, and optical properties of OD-1 and OD-2 are summarized in
Table 1.
Fig. 1 shows the normalized UV-visible absorption spectra of OD-1 and OD-2 in
chloroform solution. The OD-1 and OD-2 show the absorption maximum at 528 and
491 nm, respectively, indicate that the dyes are absorbed at the medium visible range. By
comparing these absortpoin spectra with other reported dyes, the absorption coverage must
be improved.
To investigate the charge carrier injection properties of the OD-1 and OD-2 and evaluate
their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels, we carried out redox measurements using CV. The HOMO values
were calculated from Eox as −5.08 eV for OD-1 and −5.34 eV for OD-2. The LUMO
energy level of OD-1 and OD-2 was approximated by subtracting the optical band gap from
Table 1. Thermal, optical and electrochemical properties of OD-1 and OD-2
dye
λabs (nm)
DSC (Tg)a
TGAb
EHOMOc (V)
ELUMOd (V)
OD-1
OD-2
528
491
74
75
162
162
2.52
2.27
−1.20
−0.74
aDetermined by DSC at a heating rate of 10◦C/min under N2 atmosphere.
bTGA was measured at temperature of 5% weight loss for the polymer.
cHOMO: Determined by onset oxidation of CV curves.
dLUMO = HOMO + Eg.

Synthesis and Characterization of 5ꢀ-Hexyl-2,2ꢀ-bithiophene
[503]/161
OD-1
OD-2
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
Wavelength(nm)
Figure 1. UV-visible absorption spectra of OD-1 and OD-2.
600
700
800
the HOMO value and these values are listed in Table 1. The energy band diagrams of OD-1
and OD-2 are displayed in Fig. 2. The results confirmed the thermodynamic favorability of
electron injection from the excited OD-1 and OD-2 into the conduction band of TiO2 and
also oxidized OD-1 and OD-2 are easily reduced by accepting the electrons from I− ions,
generating the ground state of the OD-1 and OD-2 [12].
Figure 2. Energy band diagram of OD-1 and OD-2 dyes.

162/[504]
C. Saravanan et al.
Conclusion
In this study, we have successfully designed and synthesized a new series of 5ꢀ-hexyl-2,2ꢀ-
bithiophene based on OD-1 and OD-2 as a sensitizer for DSSCs. The synthesized OD-1
1
and OD-2 were characterized by H-NMR, 13C-NMR, TGA, UV-visible, and CV. Their
optical, thermal and electrochemical properties were measured. Optical and electrochemical
measurements exhibited that OD-1 and OD-2 confirmed the thermodynamic favorability
of electron injection from the excited OD-1 and OD-2 sensitizers into the conduction band
of TiO2.
Acknowledgment
This work was financially supported by the 2013 Post-Doc. Development Program of Pusan
National University.
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