Facile synthesis of metal-free organic dyes featuring a thienylethynyl spacer for dye sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.01.004

In this article, we report the facile synthesis of metal-free dyes 6 and 7, their solution-based optical and redox properties and their use as sensitizers in dye-sensitized solar cells (DSSCs). Our studies indicate that the addition of the second thiophene unit in dye 7, decreases the oxidation and reduction potential and consequently the band gap of the molecule compared to 6. Furthermore, increasing the length of the conjugated spacer also affects on the properties of the DSSCs, with dye 7 providing a higher power conversion efficiency compared to 6 (η = 4.49 versus 3.23%).

Full text of DOI:10.1016/j.dyepig.2014.01.004

Dyes and Pigments 104 (2014) 197e203
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Facile synthesis of metal-free organic dyes featuring a thienylethynyl
spacer for dye sensitized solar cellsq
,
,
,
Manal Al-Eid a 1, SungHwan Lim b 1, Kwang-Won Park b 1, Brian Fitzpatrick a,
Chi-Hwan Han c, Kyungwon Kwak b , Jongin Hong
, Graeme Cooke
,
b,
a,
*
**
***
a Glasgow Centre for Physical Organic Chemistry, WestCHEM School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK
b Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea
c Photovoltaic Research Center, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In this article, we report the facile synthesis of metal-free dyes 6 and 7, their solution-based optical and
redox properties and their use as sensitizers in dye-sensitized solar cells (DSSCs). Our studies indicate
that the addition of the second thiophene unit in dye 7, decreases the oxidation and reduction potential
and consequently the band gap of the molecule compared to 6. Furthermore, increasing the length of the
conjugated spacer also affects on the properties of the DSSCs, with dye 7 providing a higher power
Received 7 November 2013
Received in revised form
19 December 2013
Accepted 3 January 2014
Available online 12 January 2014
conversion efficiency compared to 6 (
h
¼ 4.49 versus 3.23%).
Ó 2014 The Authors. Published by Elsevier Ltd. All rights reserved.
Keywords:
Dye-sensitized solar cells
Organic dye
Metal-free
Thiophene
Solvatochromic
Alkyne
1. Introduction
extinction coefficients [3e7]. One of the main goals in this area has
been focused upon improving the power-conversion-efficiency of
these systems through structural modification [8]. In general, these
systems feature donor (D) and acceptor (A) systems separated by a
Dye-sensitized solar cells (DSSCs) are attracting significant ac-
ademic and commercial interest as possible lower-cost alternatives
to conventional solid-state photovoltaic devices [1,2]. The nature of
the photosensitizer plays an important role in DSSCs, as it has a
profound influence on both the power conversion efficiency as well
as the stability of the cells. Organic dyes have attracted much in-
terest in recent years as they offer several advantages over their
ruthenium-based brethren including bespoke structural modifica-
tion through rich synthetic protocols, lower cost large-scale pro-
duction, reduced environmental and toxicity issues and high molar
p-conjugated bridge (DepeA). For example, triphenylamine de-
rivatives have been commonly utilized as the electron donor (D),
whilst a cyanoacrylic acid moiety acts as the electron acceptor (A)
and anchoring unit in the DepeA structure [9]. Various p-conju-
gated bridges, such as benzene, thiophene and benzothiadiazole,
have been introduced to broaden their absorption towards the
near-infrared region. Interestingly, studies have shown that in
addition to optimizing the optical properties of the dye, increasing
the number of thiophene
p-bridging units tends to result in an
increase in power conversion efficiency [10]. Another important
goal is to develop systems that may be accessed through short and
simple synthetic routes for more widespread implementation in
lower cost applications [11].
In this study, we report the facile synthesis of two organic dyes (6
and 7, Scheme 1), which can be conveniently prepared in two steps
from commercially available thiophene derivatives 1 and 2 and
alkyne 3. The dyes have been designed to investigate the effect of
increasing conjugation through the addition of thiopehene units has
on the properties of the dyes. Importantly, the acetylene linker
q
This is an open-access article distributed under the terms of the Creative
Commons Attribution-NonCommercial-No Derivative Works License, which per-
mits non-commercial use, distribution, and reproduction in any medium, provided
the original author and source are credited.
* Corresponding author. Tel.: þ44 141 3305500.
** Corresponding author. Tel.: þ82 28205764.
*** Corresponding author. Tel.: þ82 28205869.
E-mail addresses: kkwak@cau.ac.kr (K. Kwak), hongj@cau.ac.kr (J. Hong),
Graeme.Cooke@glasgow.ac.uk (G. Cooke).
1
Equal contribution to this work.
0143-7208/$ e see front matter Ó 2014 The Authors. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2014.01.004

198
M. Al-Eid et al. / Dyes and Pigments 104 (2014) 197e203
O
O
S
(a)
H
S
H
+
Br
N
H
N
n
n
4, n=1
5, n=2
1, n=1
2, n=2
3
HOOC
H
CN
S
(b)
N
n
6, n=1
7, n=2
Scheme 1. Reagents and conditions: (a) Pd/C 10%, CuI, Ph3P, K2CO3, DME: H2O; (b) cyanoacetic acid, piperidine, CH3CN.
group has been included to provide a planar linker moiety [12e14]
and as a handle for further synthetic manipulation [15]. We have
investigated the influence the number of thiophene units has on the
optical, redox and DSSC properties of dyes 6 and 7. This study has
revealed that the addition of the second thiophene unit in com-
pound 7 plays an important role in modulating the optical and redox
properties of this system compared to 6. Furthermore, increasing
the conjugation between the D and A units in this way also results in
Gaussian ‘09 software package [17]. The geometry of dyes 6 and 7
was optimized using the B3LYP method with the 6e31 þ G(d) basis
set. The optimized structures have been classified as local minima
on their respective potential energy surfaces according to their
vibrational frequencies. None of the vibrational frequencies in the
optimized geometries generated negative frequencies in their
ground state. To introduce the effect of solvent, the electronic states
of both organic dyes in acetonitrile were also calculated by means
of the Tomasi’s Polarized Continuum Model (PCM).
improved efficiency of the DSSCs (6
h
¼ 3.23%; 7
h
¼ 4.49%).
2. Experimental
2.4. Synthesis
2.1. Materials
2.4.1. Compound 4
5-Bromo-2-thiophenecarboxaldehyde 1 (0.50 g, 2.30 mmol),
10% Pd/C (0.07 g, 0.07 mmol), CuI (0.13 g, 0.68 mmol), Ph3P (0.47 g,
1.80 mmol) and K2CO3 (0.50 g, 3.6 mmol) were dissolved at room
temperature in a 1:1 mixture of water (15 mL) and DME (15 mL).
The mixture was degassed with N2 for 5 min and stirred for 30 min
All reactions were undertaken under a nitrogen atmosphere.
Solvents were purified using a PureSolv solvent purifier system.
Compounds 1, 2 and 3 were purchased from either Aldrich or TCI,
and were used without further purification.
under N2. Then 4-ethynyl-N,N-dimethylaniline
3
(0.50 g,
3.40 mmol) was added to the mixture. The reaction mixture was
heated under reflux overnight. The mixture was then cooled and
DCM (80 mL) was added and the organic layers were separated,
dried over MgSO4 and filtered. The solvent was evaporated under
reduced pressure and the residue was purified by column chro-
matography (SiO2; toluene/DCM, 9:1) to give 4 as an orange solid
2.2. Characterization
NMR spectroscopy was undertaken on a Bruker AVIII (400 MHz)
spectrometer. Chemical shifts are reported in ppm and are relative
to tetramethylsilane. UVevis spectra were recorded on a Perkine
Elmer Lamda 25 instrument. Band gaps (Eg) were estimated using
(0.37 g, 63%); mp. 147e149 ꢁC; 1H NMR (400 MHz, CDCl3)
d 9.84 (s,
the absorption edge of the longest wavelength absorption (
l) using
1H), 7.64 (d, J ¼ 3.9 Hz, 1H), 7.41 (d, J ¼ 8.9 Hz, 2H), 7.22 (d,
Eg ¼ 1240/ . Cyclic voltammetry measurements were undertaken
l
J ¼ 3.9 Hz, 1H), 6.65 (d, J ¼ 8.9 Hz, 2H), 3.02 (s, 6H) ppm; 13C NMR
using a CH Instruments 440a electrochemical analyzer using a
platinum disc working electrode, a platinum wire counter electrode
and a silver wire pseudo-reference electrode. Ferrocene was used
as an internal standard and all redox couples are reported versus
the ferrocene/ferrocenium (Fc/Fcþ) redox couple adjusted to 0.0 V.
The solutions were prepared using dry dimethylformamide (DMF)
containing electrochemical grade tetrabutylammonium hexa-
fluorophosphate (0.1 M) as the supporting electrolyte. The solu-
tions were purged with nitrogen gas for 3e4 min prior to recording
the electrochemical data. The HOMO/LUMO energies were esti-
mated from the oxidation and reduction waves, respectively, using
a HOMO energy for ferrocene of ꢀ4.8 eV [16].
(100 MHz, CDCl3)
d 182.2 (C]O), 150.7 (CeN), 142.7 (CeH), 136.3
(CeH), 134.4 (CeC), 132.9 (2 ꢂ CeH), 131.2 (CeH), 111.6 (2 ꢂ CeH),
108.1 (CeC), 100.3 (C^C), 80.6 (C^C), 40.0 (2 ꢂ CeN) ppm; IR
(film)
y
2807, 2182, 1656, 1229 cmꢀ1; HRMS m/z (EIþ) calculated for
C15H13NOS 255.0718 found 255.0708.
2.4.2. Compound 5
5-Bromo-2,20-bithiophene-50-carboxaldehyde
2
(0.50 g,
1.83 mmol), 10% Pd/C (0.06 g, 0.06 mmol), CuI (0.10 g, 0.55 mmol),
Ph3P (0.40 g, 1.50 mmol) and K2CO3 (0.40 g, 2.90 mmol) were dis-
solved at room temperature in a 1:1 mixture of water (15 mL) and
DME (15 mL). The mixture was degassed with N2 for 5 min and
stirred for 30 min under N2. Then 4-ethynyl-N,N-dimethylaniline 3
(0.40 g, 2.75 mmol) was added to the mixture. The reaction mixture
was heated under reflux overnight. The mixture was then cooled
and DCM (80 mL) was added and the organic layers were separated,
2.3. Computation
Density functional theory (DFT) and time-dependent density
functional theory (TDDFT) calculations were performed using a

M. Al-Eid et al. / Dyes and Pigments 104 (2014) 197e203
199
dried over MgSO4 and filtered. Then the solvent was evaporated
under reduced pressure and the residue was then purified by col-
umn chromatography (SiO2; DCM/petroleum ether, 9:1) to give 5 as
an orange solid (0.37 g, 40%); mp. 200e203 ꢁC; 1H NMR (400 MHz,
heated hot plate. The screen printing and drying process was
repeated twice until a thickness of about 20 m was obtained. The
m
TiO2 thick films were gradually heated to 300 ꢁC over 30 min period
under an airflow, heated at 300 ꢁC for 1 h, heated to 575 ꢁC over
30 min period, and then finally sintered at 575 ꢁC for 1 h in a muffle
furnace and then cooled to room temperature. Active areas of the
electrodes were 0.25 cm2. The prepared TiO2 electrodes were
immersed in a 0.04 M TiCl4 solution at 70 ꢁC for 1 h [19]. They were
rinsed with water and ethanol and then sintered at 500 ꢁC for
30 min. The TiO2 electrodes were exposed to an O2 plasma and then
immersed into a 0.5 mM photosensitizer solution (ethanol) (N719,
compound 6, compound 7) for 24 h. Pt-counter electrodes were
prepared on the FTO glass by magnetron sputtering after two holes
were drilled in the glass. Both the photosensitizer-adsorbed TiO2
CDCl3)
d
9.86 (s, 1H), 7.66 (d, J ¼ 4.0 Hz, 1H), 7.39 (d, J ¼ 8.9 Hz, 2H),
7.23 (d, J ¼ 3.4 Hz,1H), 7.22 (d, J ¼ 3.4 Hz,1H), 7.13 (d, J ¼ 4.0 Hz,1H),
6.66 (d, J ¼ 8.9 Hz, 2H), 3.01 (s, 6H) ppm; 13C NMR (100 MHz, CDCl3)
d
182.4 (CeH), 150.4 (CeC), 146.6 (CeC), 141.6 (CeC), 137.3 (CeH),
135.8 (CeC), 132.7 (2 ꢂ CeH), 131.8 (CeH), 126.3(CeC), 126.0 (CeH),
124.2 (CeH), 111.7 (2 ꢂ CeH), 108.8 (CeC), 97.3 (C^C), 80.2 (C^C),
40.1 (2 ꢂ CH3eN) ppm; IR (film)
y ;
2787, 2185, 1654, 1228 cmꢀ1
HRMS m/z (EIþ) calculated for C19H15NOS2 337.0595 found
337.0591.
2.4.3. Dye 6
electrode and the Pt-counter electrode were sealed with 60 mm-
To a mixture of compound 4 (0.40 g, 1.80 mmol) and cyanoacetic
acid (0.30 g, 3.60 mmol) in CH3CN (20 mL) was added piperidine
(0.2 mL). The reaction mixture was heated under reflux overnight
under nitrogen. After cooling to room temperature, the mixture
was then poured into (1 N) HCl and chloroform (200 mL) was
added. The organic layer was separated and washed with brine,
water and dried over MgSO4 and filtered. Then solvent was evap-
orated and the crude product was purified by column chromatog-
raphy (SiO2: chloroform/acetic acid, 9:1) to afford 6 as a red solid
thick Surlyn (Solaronix). An ionic liquid electrolyte (0.60 M butyl-
methyl imidazolium iodide (BMIM-I), 0.03 M I2, 0.50 M 4-tert-
butylpyridine (TBP) and 0.10 M guanidinium thiocyanate (GTC) in
acetonitrile/valeronitrile 85/15 (v/v), No. ES-0004, io∙li∙tec, Ger-
many) was filled through the holes in the backside of the counter
electrode. The photovoltaic characteristics of the devices under AM
1.5 global one sun illumination (100 mW/cm2) were investigated by
a solar cell IeV measurement system (K3000 LAB, McScience, Ko-
rea). Photocurrent density (Jsc), open-circuit voltage (Voc), fill factor
(0.18 g, 31%); mp. 210e220 ꢁC; 1H NMR (500 MHz, DMSO)
d
8.48 (s,
(FF) and power-conversion-efficiency (h) were simultaneously
1H), 7.96 (d, J ¼ 4.0,1H), 7.45 (d, J ¼ 4.0,1H), 7.42 (d, J ¼ 9.1, 2H), 6.73
measured. The incident monochromatic photon-to-current con-
version efficiency (IPCE) was recorded as a function of excitation
wavelength (l) by a spectral IPCE measurement system (K3100,
McScience, Korea).
(d, J ¼ 9.1, 2H), 2.98 (s, 6H) ppm; 13C NMR (125 MHz, DMSO)
d
163.2
(CeOOH), 150.9 (CeN phen), 145.6 (CeH), 139.7 (CeH), 135.5 (CeC),
132.8 (2 ꢂ CeH), 132.0 (CeH), 131.8 (CeC), 116.5 (C^N), 111.9
(2 ꢂ CeH), 106.7 (CeC), 101.1 (C]CN,COOH), 81.3 (C^C), 80.7
(C^C), 39.9 (2 ꢂ CeN) ppm; IR (film)
y 3200, 2177, 1680,
3. Results and discussion
1600 cmꢀ1; HRMS (FAB (M þ Na)þ) calculated for C18 H14 N2 O2 S Na
345.0674 found 345.0676.
3.1. Characterization
2.4.4. Dye 7
Dyes 6 and 7 were conveniently prepared in a two-step protocol
from commercially available precursors 1 to 3 (Scheme 1). Sono-
gashira coupling reactions of compound 1 or 2 with 3 provided
compounds 4 and 5, which upon Knoevenagel condensation with
cyanoacetic acid provided dyes 6 and 7. The UVevis spectra of
compounds 6 and 7 are provided in Fig. 1. The maximum wave-
length absorption corresponds to an intramolecular charge-
transfer (ICT) between the donor and acceptor groups, whilst the
To a mixture of compound 5 (0.40 g, 1.20 mmol) and cyanoacetic
acid (0.20 g, 2.40 mmol) in CH3CN (20 mL) was added piperidine
(0.2 mL). The reaction mixture was heated under reflux overnight
under nitrogen. After cooling to room temperature, the mixture
was then poured into (1 N) HCl and the chloroform (200 mL) was
added. The organic layer was separated and washed with brine,
water, dried over MgSO4 and filtered. Then solvent was evaporated
and the crude product was purified by column chromatography
(SiO2: chloroform/acetic acid, 9:1) to afford 7 as a red solid (0.10 g,
absorption band around 300 nm is likely due to the
pep*
28%); mp. 235e245 ꢁC; 1H NMR (400 MHz, DMSO)
d 8.40 (s, 1H),
7.92 (d, J ¼ 4.0, 1H), 7.58 (d, J ¼ 4.0, 1H), 7.54 (d, J ¼ 4.0, 1H), 7.37 (d,
J ¼ 8.9, 2H), 7.33 (d, J ¼ 4.0, 1H), 6.73 (d, J ¼ 8.9, 2H), 2.97 (s, 6H)
ppm; IR (film)
y
3308, 2177, 1633, 1604 cmꢀ1; HRMS (FAB (M-H)ꢀ)
calculated for C22H16N2O2S2 403.0574 found 403.0561.
2.4.5. Fabrication of DSSCs and photovoltaic measurements
In order to form artificial pores in the TiO2 photoanode films,
acetylene-black TiO2 pastes were prepared using a paste-blending
method. First, a TiO2 colloidal solution, which was synthesized by
the hydrothermal growth method, was used for the starting ma-
terial. Both ethylene carbonate and terpineol were added in the
solution and all were mixed by using a paste blender. Then,
acetylene-black powder (Chevron Philips Chemical Company) was
blended. The mixed paste was concentrated at 80 ꢁC for 2 h using a
rotary evaporator to achieve appropriate viscosity for screen
printing. More details about its preparation can be found in
Ref. [18]. The acetylene-black TiO2 paste was screen-printed on
transparent fluorine-doped SnO2 (FTO) conducting glass (TEC 8,
Fig. 1. UVevis spectra for dyes 6 (black line) and 7 (red line) recorded in DMF
(1 ꢂ 10ꢀ5 M). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
sheet resistance ¼ 8
U/sq) which were purchased from Pilkington.
The resultant layer was dried for 10 min at 300 ꢁC in air on a pre-

200
M. Al-Eid et al. / Dyes and Pigments 104 (2014) 197e203
Table 1
transition. It is clear that the addition of the second thiophene
moiety in compound 7 induces a bathochromic shift in the longest
wavelength absorption of around 18 nm compared to compound 6,
presumably a consequence of extending the conjugation. In addi-
tion, the molar extinction coefficient of this absorption is signifi-
cantly higher for compound 7 (52,000 Mꢀ1 cmꢀ1) than compound 6
(37,000 Mꢀ1 cmꢀ1). The optically determined band gaps were
2.56 eV and 2.44 eV for compounds 6 and 7, respectively, indicating
that the addition of the second thiophene unit in the latter is
responsible for this decrease. Cyclic voltammetry was used to es-
timate the HOMO and LUMO energies of the dyes from the oxida-
tion and reduction waves, respectively (Fig. 2). Upon oxidation,
both molecules show a reversible oxidation wave (þ0.41 V for 6
and þ0.36 V for 7), assigned to the oxidation of the phenylamine
unit. Upon reduction, both compounds display an irreversible wave
(ꢀ2.09 V for 6 and ꢀ1.99 V for 7). The addition of the second
thiophene unit in compound 7 lowers the half-wave potential of
the oxidation and reduction waves, resulting in a lower band gap
for compound 7 (2.36 eV) compared to 6 (2.51 eV). The electro-
chemically determined band gaps of 6 and 7 are in reasonable
accordance with those determined optically. Optical and electro-
chemical data are summarized in Table 1.
Redox and UVevis properties of dyes 6 and 7.
E
(V)
(ox) (red) EHOMO ELUMO Eg (e-chem) lmax lonset Eg (optical)
E
½
½
(V)
(eV)
(eV)
(eV)
(nm) (nm) (eV)
Dye 6 0.41
Dye 7 0.36
ꢀ2.09 ꢀ5.22 ꢀ2.71 2.51
ꢀ1.99 ꢀ5.17 ꢀ2.81 2.36
408 484 2.56
426 508 2.44
structure and as shown in Fig. 5, the energy difference between these
conformers corresponds to 1.08 kcal/mol, which indicates that
around 10% of the cis form exists at equilibrium.
The electronic density distributions of the HOMO and LUMO of
dyes 6 and 7 are illustrated in Fig. 3. The electron density of the
HOMO for the two dyes is localized mainly on the phenylamine
moiety (D) and is extended along the alkyne/thiophene bridge to
the cyanoacrylic acid (A) moiety. Under light illumination, the
intramolecular charge transfer likely occurs from electron donor to
electron acceptor. This implies that the HOMO-LUMO excitation
would cause a net electron-transfer from the donor to the acceptor
group which anchors to the TiO2 surface [20]. Accordingly, we think
that the position of the LUMO close to the anchoring group would
enhance the orbital overlap with the Ti 3d orbitals and thus excited
electrons can be easily injected into TiO2 via the cyanoacrylic acid
unit. Importantly, the introduction of the second thiophene unit in
dye 7 resulted in a more efficient electron transfer from the HOMO
of the phenylamine unit to the LUMO of the acrylic acid unit. The
molecular orbital (MO) energy levels of the two organic dyes are
shown in Fig. 6.
3.2. Computation
To gain insight on the electronic structure and optical properties
of our metal-free dyes, DFTand TDDFTcalculations were conducted.
Ground state optimized geometries gave rise to planar structures for
both dyes (Fig. 3). This planarity is likely to contribute to the charge-
transfer or separation between the D and A moieties in the molecule
as the acetylene and thiophene linker groups help maintain a planar
structure whilst increasing the distance between donor and
acceptor groups. Moreover, the acetylene linker between phenyl-
amine and thiophene units eliminates the steric hindrance between
the hydrogen atoms of these rings. The specific role of acetylene as a
As the number of thiophene
LUMO energy gap is decreased, due to an increase in the
p
-bridges is increased, the HOMO-
-conju-
p
gation length, which is in accordance with our spectroscopic data
described above. In addition to moving the absorption maximum to
longer wavelength, the insertion of the second thiophene in-
troduces larger separation between donor and acceptor unit, which
may cause the increase of transition dipole strength, the square of
which is proportional to the absorption coefficient. From the TDDFT
calculations, we can obtain the transition dipole strength for the
HOMO-LUMO transition (Table 2). The absorption spectrum in Fig.1
shows that the ratio of maximum absorbance is 0.72, which is quite
close to the calculated value from the TDDFT. The absorption
strength is proportional to the square of the transition dipole which
is 0.75. In the above section, charge transfer character is qualita-
tively described with the electron density map of HOMO and LUMO
state. Here, we quantitatively calculate the charge transfer magni-
tude with the help of TDDFT calculation. The extent of intra-
molecular charge transfer can be estimated from the difference of
dipole moment between ground and excited state [21]. The dipole
moment in ground state is obtained by DFT calculation, whereas
the excited state by TDDFT calculations. The calculated values for
dipole and transition dipole moments are listed in Table 2. The
comparison between the dipole moment difference of dye 6 and 7
directly indicates that the insertion of thiophene units
enhances the intermolecular charge transfer upon the absorption of
light (Fig. 3).
p-linker in dyes 6 and 7 was investigated with DFT calculations.
Firstly, the acetylene was removed from compounds 6 and 7 and the
structures were optimized using the same basis set used above. The
resulting energy minimum structures show that the dihedral angle
between phenylamine ring and thiophene moieties is 17.2ꢁ for 6 and
23.2ꢁ for 7 (Fig. 4). The second thiophene group adopts a trans-like
geometry with respect to the thiophene group nearest to the phe-
nylamine unit. We performed potential energy surface scan upon
adjustment of the dihedral angle between two thiophene groups to
estimate the relative stability of trans- form compared to cis-
We have also investigated the dye protonation/deprotonation in
acetonitrile (the solvent used for deposition of dyes) solution.
Recent DFT calculations and FT-IR experiments provides evidence
for the coordination of the carboxylate (COO-) form of the cya-
noacrylic acid fragment to titania [22]. Deprotonation of the cya-
noacrylic moiety resulted in an increase in the HOMO-LUMO
energy gap, suggesting that deprotonation leads to the destabili-
zation of the LUMO and thus the reduction of the electron-acceptor
ability of the cyanoacrylic group (Fig. 6). Interestingly, the data
obtained for the deprotonated dyes was more in accordance with
the HOMO and LUMO energies determined experimentally.
Fig. 2. Cyclic voltammetry of dyes 6 (black line) and 7 (red line) recorded in DMF
(5 ꢂ 10ꢀ4 M). Scan rate ¼ 100 mV/s. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)

M. Al-Eid et al. / Dyes and Pigments 104 (2014) 197e203
201
Fig. 3. Optimized structure and frontier molecular orbitals (HOMO and LUMO) of protonated organic dyes: (a) dye 6 and (b) dye 7.
Fig. 4. Geometry of (a) dye 6, (b) dye 7, (c) dye 6 (without acetylene spacer) and (d) dye 7 (without acetylene spacer) optimized with Gaussian 09 at the DFT-B3LYP 6-31 þ G(d) level
of theory.
3.3. Photovoltaic measurements
Fig. 7 shows JeV characteristics of DSSCs fabricated from dyes 6
and 7 compared to DSSCs produced from dye N719 fabricated and
measured using the same conditions. The photovoltaic parameters
are summarized in Table 3. The addition of the second thiophene
bridging unit in dye 7 resulted in better photovoltaic performance:
Jsc from 7.5 mA/cm2 to 10.50 mA/cm2, Voc from 0.61 V to 0.63 V and
power
h from 3.23% to 4.49%. The increase in h of about 39% can be
ascribed to the improvement of the electron-donating ability of this
dye compared to 6. Fig. 8 shows IPCE spectra for DSSCs fabricated
from dyes 6, 7 and N719. The IPCE of dye 7 displays a broader band
between 300 and 800 nm than that of dye 6. It is worth noting that
dye 7 shows higher maximum IPCE values between 300 and
450 nm than 6 and N719. The aforementioned results demonstrate
that the addition of the second p-bridge thiophene between the D
and A groups broaden the visible absorption spectrum both in so-
lution and upon deposition onto TiO2 which may be responsible for
Fig. 5. Energy difference between trans- and cis-conformers of compound 7 (without
acetylene).

202
M. Al-Eid et al. / Dyes and Pigments 104 (2014) 197e203
Table 3
Photovoltaic performance of DSSCs based on dye 6, dye 7 and N719.
Voc (V)
Jsc (mA/cm2)
FF (%)
h (%)
Dye 6
Dye 7
N719
0.61
0.63
0.75
7.5
10.5
16.4
70.7
67.7
61.9
3.23
4.49
7.65
Fig. 6. Comparison between the molecular orbital energy levels of the two organic
dyes in acetonitrile solution: (a) dye 6 and (b) dye 7.
Table 2
Summary of calculated values of dipole and transition dipole moments.
Ground state
(D)
Excited state
(D)
Transition dipole
moment (a.u.)
Difference
(D)
Fig. 8. IPCE spectra of the DSSCs based on dye 6, dye 7 and N719.
Dye 6
Dye 7
9.63
14.56
9.97
12.36
18.60
21.38
0.34
3.29
structural, electronic and optical properties. The calculations show
that the cyanoacrylic acid moiety is coplanar with respect to the
acetylene/thiophene bridge, reflecting effective conjugation be-
tween these units. Although the efficiencies of dye 6 and 7 were
lower than that of the DSSCs fabricated from N719 (7.65%), we think
that through optimization of the device structure (e.g. film thick-
ness, electrolyte, addition of co-additives), improvements in the
performance of the DSSCs will result. Nevertheless, the convenient
synthesis of these materials and acceptable power conversion ef-
ficiencies when incorporated into DSSCs, paves the way for their
further development in lower cost photovoltaic applications. Our
work towards these goals will be published in due course.
Dmge ¼ ½ðmgx
ꢀ
mexÞ2 þ ðmgy
ꢀ
meyÞ2 þ ðmgz
ꢀ
mezÞ2ꢃ1=2
the significant enhancement of DSSC performance (3.23% of dye 6
to 4.49% of dye 7).
4. Conclusion
In this article, we report the facile synthesis of dyes 6 and 7 in
two steps from commercially available materials that have the
propensity to form DSSCs with respectable power conversion effi-
ciencies. UVevis spectroscopy and cyclic voltammetry measure-
ments have shown that the addition of the second thiophene
bridge unit in 7 improves both the optical properties of the dye by
providing a higher wavelength absorption and consequently a
lower band gap than compound 6. DFT and TDDFTcalculations have
been performed on the dyes to gain better insight into their
Acknowledgements
MA-E thanks the Ministry of Higher Education (Saudi Arabia) for
a scholarship. BF and GC thank the EPSRC. JH acknowledges Basic
Science Research Program (2013-026989) and Mid-career Research
Program (2012-047815) through the National Research Foundation
(NRF) funded by the Ministry of Science, ICT & Future Planning
(MSIP) of Korea. KW acknowledges the Chung-Ang University
Excellent Student Scholarship. KK thanks for the support from Basic
Science Research Program through the NRF funded by the MSIP of
Korea (2009-0093817).
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