Molecular engineering and synthesis of novel metal-free organic sensitizers with D-π-A-π-A architecture for DSSC applications: The effect of the anchoring group

Article abstract of DOI:10.1016/j.dyepig.2018.05.011

Herein, we report the design and synthesis of two metal-free organic sensitizers (MR-5 & MR-6) with a D-π-A-π-A architecture based on a trimethoxy benzene donor core carrying two different withdrawing/anchoring groups including cyanoacetic acid and rhodan

Full text of DOI:10.1016/j.dyepig.2018.05.011

Accepted Manuscript
Molecular design and synthesis of novel metal-free organic sensitizers with D-π-A-π-
A architecture for DSSC application: The effect of the anchoring group
Mohamed R. Elmorsy, Rui Su, Ahmed A. Fadda, H.A. Etman, Eman H. Tawfik,
Ahmed El-Shafei
PII:
S0143-7208(18)30494-7
10.1016/j.dyepig.2018.05.011
DYPI 6740
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 6 March 2018
Accepted Date: 7 May 2018
Please cite this article as: Elmorsy MR, Su R, Fadda AA, Etman HA, Tawfik EH, El-Shafei A,
Molecular design and synthesis of novel metal-free organic sensitizers with D-π-A-π-A architecture
for DSSC application: The effect of the anchoring group, Dyes and Pigments (2018), doi: 10.1016/
j.dyepig.2018.05.011.
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ACCEPTED MANUSCRIPT
Molecular Design and Synthesis of Novel Metal-Free Organic Sensitizers
with D-π-A-π-A Architecture for DSSC Application: The Effect of the
Anchoring Group
a,b
a
b
b
b
Mohamed R. Elmorsy , Rui Su , Ahmed A. Fadda , H.A. Etman , Eman H. Tawfik , Ahmed
a,*
El-Shafei .
a
Polymer and Color Chemistry Program, North Carolina State University, Raleigh, NC, 27695,
United States.
b
Department of Chemistry, Faculty of Science, Mansoura University, El-Gomhoria Street,
35516 Mansoura, Egypt.
*
Corresponding author: E-mail: Ahmed_El-Shafei@ncsu.edu
GRAPHICAL ABSTRACT
Abstract:
Herein, we report design and synthesis of two metal-free organic sensitizers (MR-5 &
MR-6) with D-π-A-π-A architecture based on trimethoxy benzene donor core carrying two
different withdrawing/anchoring groups such as cyanoacetic acid and rhodanine-3-acetic acid

ACCEPTED MANUSCRIPT
via thiophene as π-spacer. Benzothiadiazole was incorporated as an auxiliary acceptor, which
greatly improved the molar extinction coefficient of the dyes. The optical and electrochemical
properties of two dyes were investigated. The photovoltaic studies revealed that the dyes (MR-
5) anchored with cyanoacetic acid showed significantly greater light harvesting (IPCE), which
resulted in maximum PCE with good photovoltaic parameter than MR-6. MR-5 achieved a
better total solar-to-electric conversion efficiency of 6.03% and showed IPCE of 65% at 420
nm. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT)
calculations were used to study equilibrium molecular geometries and frontier molecular
orbitals, and the data were consistent with experimental results.
Keywords: Trimethoxy benzene, D-π-A-π-A, Benzothiadiazole, DSSCs.
1.
Introduction
Dye-sensitized solar cells (DSSCs) have been widely used due to their environmental
friendliness, low-cost organic photovoltaics device with decent power conversion efficiency
PCE), as reported by O’Regan and Grätzel in 1991 [1]. Until now, the ruthenium complexes
(
are investigated more as photo-sensitizer in the field of DSSCs because of their relatively good
efficiency compared to normal silicon-based solar cells [2]. Two excellent dyes, N719
{
(Bu N) Ru[(4,4’-CO H) bipy] (NCS) } and N3 {[Ru[(4,4’-CO H) bipy] (NCS) ]} have
4 2 2 2 2 2 2 2 2 2
displayed maximum optimized PCE exceeding 11% [3-6]. In view of the low extinction
coefficient of ruthenium photosensitizers, high manufacturing costs, multiple purification steps
and rarity of the ruthenium metal, metal-free organic dyes attracted strong interest in the DSSCs
community due to their advantages over metal based sensitizers [7].
One of the most successful design strategies of metal-free organic dyes represented as
D-π-A architectures due to their easy synthesis and reliable performance [7-10]. Recent
activities have incorporated additional acceptor. In 2011, Zhu and Tian proposed a concept of a
D-A-π-A motif for designing a new generation of stable and efficient organic dyes [11-13].
From the literature reviews, some electron-withdrawing blocks, such as diketopyrrolopyrrole
[
14], benzotriazole [15] and benzothiadiazole, [16] have been employed to design D-A-π-A
framework. It is reported that additional acceptor is beneficial for extending absorption
wavelength, modulating the energy levels, and thus greatly improving photovoltaic
performances [13]. Furthermore, the evolution of π-bridge unit is essential in the molecular

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engineering of sensitizers, which should be inserted as planar-type building blocks of fused
heterocycles [17-19]. Many fused heterocycles such as benzene [19], thiophene [20],
bithiophene [21] have been incorporated into the ꢀ-bridge with broad and intense spectral
absorption in the visible-light region. So, extending the structure with π-spacer is beneficial to
the enhancement of the photocurrent for efficient charge transfer [22]. Moreover, the most
common electron withdrawing groups widely reported in the literature are cyanoacetic acid and
rodanine-3-acetic acid due to their strong electron withdrawing nature and effective binding on
the surface of mesoporous TiO through carboxylic acid anchoring groups [23].
2
In this regard, in this present work, we have designed two D-π-A-π-A dyes (MR-5 &
MR-6) based on trimethoxy benzene core attached with different electron withdrawing groups
such as cyanoacetic acid and rhodanine-3-acetic acid, respectively. In addition, thiophene ring
was introduced to the dyes while the π-conjugated extension is an effective way to improve the
short-circuit photocurrent density [24, 25]. Moreover, benzothiazole was also inserted into the
structures due to their high chemical and photophysical stability [26, 27].
The designed organic dyes were synthesized from the 2,4,6-trimethoxybenzene with
good yield following multi-step synthesis. The structures of target molecules MR-5 and MR-6
are shown in the Figure 1. The fabricated dyes and their intermediates were well-characterized
1
using spectral techniques such as MS, FTIR, and H NMR analysis. Optical, photovoltaic,
electrochemical and theoretical calculations of the synthesized dyes were studied. The results
are subsequently elucidated in the following sections.
Figure 1. Molecular structure of MR-5 and MR-6

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2. Experimental
2.1. Materials and Methods
The solvents and chemicals were purchased from Sigma-Aldrich, Fisher Scientific, TCI-
America or Ark Pharm and used as received. Sephadex LH-20 was purchased from Fisher
Scientific. The mass spectrometry analysis was carried out on a high-resolution mass
spectrometer, the Thermo Fisher Scientific Exactive Plus MS, a benchtop full-scan OrbitrapTM
mass spectrometer using heated electrospray ionization (HESI). Samples were dissolved in
methanol and sonicated for 15 min. They were then diluted 1:1 with 20 mM ammonium acetate
and analyzed via syringe injection into the mass spectrometer at a flow rate of 10 µL/min. The
mass spectrometer was operated in positive ion mode. Fourier transform infrared spectroscopy
(
FT-IR) (ATR) spectra were recorded on a Nicolet Nexus 470 FT-IR spectrometer (Thermo
Scientific, USA) and UV-visible spectra were measured by using Cary 300 spectrophotometer.
H NMR spectra were recorded using a Varian 400 MHz spectrometer. Photovoltaic
1
measurements of sealed cells were made by illuminating the cell through the conductive glass
from the anode side with a solar simulator (WXS-155S-10) at AM 1.5 illuminations (light
-
intensity: 100 mW cm 2). IPCE measurements were made on a CEP-2000 system (Bunkoh-
Keiki Co. Ltd.). EIS spectra were recorded over a frequency range of 100 mHz–200 kHz at 298
K. The applied bias voltage was set at the Voc of the DSSCs, with AC amplitude set at 10 mV.
The electrical impedance data were fitted using Z-Fit software (Bio-Logic). The procedure for
DSSC fabrication is given in supporting information. The DFT calculations were performed
using Guassian09 package [28, 29] package.
2.2 Synthesis
The synthesis procedures were carried out using the standard synthetic methods. The
synthetic procedure of the final compound and their intermediates with characterization data are
given below.
2.2.1 Synthesis of 5,5'-(benzo[c]thiophene-4,7-diyl) bis(thiophene-2-carbaldehyde) (1)
Compound 1 was synthesized with modification of the published procedure [30]. To a solution
of 4,7-dibromobenzo[c]thiophene (294 g, 1.0 mmol), tetrakis(triphenylphosphine)palladium
(115.53mg. 0.1mmol) in toluene (12 mL) were added ethanol (4 mL) solution of 5-

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formylthiophene-2-ylbronic acid (400mg, 2.5mmol) and aqueous 2M sodium carbonate
solution(5 mL) at room temperature under argon. After the mixture was heated at 50 °C for 4h,
the reaction mixture was cooled, and the precipitate formed was filtered off, washed with water,
methanol, and methylene chloride. The precipitate was further dried and collected as orange
solid. The solid was purified by recrystallization with chloroform to give compound 1 with
yield 60 %.
FT-IR: 1641.32 cm-1 (–CHO stretch); ESI mass spectrum: C H N O S [M+H] + Theo. M/Z =
1
6
8
2
2 3
356.698 [M+H] +, Found. M/Z 356.981.086, Error = -1.132 ppm.
Synthesis of (2,4,6-trimethoxyphenyl) methanol (2):
To a solution of NaBH (160 mg, 4.2 mmol) in 30 ml dry dichloromethane and 10 mL
4
anhydrous ethanol, 2,4,6-trimethoxybenzaldehyde (0.82 g, 4.2 mmol) was added rapidly and the
bath was stirred at room temperature for 2h. The solution was poured into 50 ml water under
vigorous stirring and then extracted with dichloromethane (15 mL). The organic layer was dried
with anhydrous sodium sulfate and then rotary evaporated to remove the solvent and
recrystallized using hexanes, to give 2 as a yellow solid.
1
H NMR: (400 MHz, DMSO-d ) δ 6.24 (s, 2H), 4.80 (s, 2H), 3.82 (s, 6H), 3.75 (s, 3H). FT-IR:
6
3
440.43 cm-1 (–OH stretch); 2940.77, 2839.12 cm-1 (-CH stretch). ESI mass spectrum:
3
C H O [M+H] + Theo. M/Z = 181.086 [M+H] +, Found. M/Z 181.086, Error = 0.0145 ppm.
10
12
3
Synthesis of bromotriphenyl(2,4,6-trimethoxybenzyl)-l5-phosphane (3)
Compound 2 (0.75 g, 3.8 mmol) and PPh3·HBr (1.37 g, 4 mmol) were dissolved in 20 ml
chloroform and refluxed for 2h. After removing the solvent, the residue was solidified with
ether and filtrated to obtain compound 3 as a white solid.
1
H NMR: (400 MHz, DMSO-d ) δ 7.46(dd, 6H), 7.35 (m, 9H), 6.13 (s, 2H), 3.82 (s, 6H), 3.78
6
(
s, 3H), 2.60 (s, 2H). FT-IR: 2986.85, 2936.54 cm-1 (-CH stretch). ESI mass spectrum:
3
C H O P [M+] Theo. M/Z = 443.177 [M+], Found. M/Z 443.178, Error = 0.305 ppm.
28
28
3
Synthesis of (E)-5-(7-(5-(2,4,6-trimethoxystyryl) thiophen-2-yl) benzo[c]thiophen-4-yl)
thiophene-2-carbaldehyde (4)

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The mixture of compound 1 (0.354g, 1 mmol), 18-crown-6 (15 mg, 0.05 mmol), K CO (276
2
3
mg, 2 mmol) was charged sequentially in a three-necked flask and then dried in vacuo for 30
min. A mixture of adduct 3 (0.523 g, 1.00 mmol) in dried DMSO (30 mL) was added dropwise
to the mixture of over 3 h at 70 °C under N atmosphere. The reaction mixture was stirred for a
2
further 5 h at 70 °C, then was quenched with water and extracted with CH Cl . The organic
2
2
layer was dried with anhydrous MgSO . Silica gel chromatography with CH Cl gave aldehyde
4
2
2
(4).
1
H NMR: (400 MHz, DMSO-d ) δ 10.05 (s, 1H), 7.98 (d, 1H), 7.90 (d, 1H), 7.75 (d, 1H), 7.65
6
(d, 1H), 7.25 (d, 1H), 7.08 (d, 1H), 6.79 (d, 1H), 6.25 (s, 2H), 3.95 (s, 6H), 3.80 (s, 3H). FT-IR:
2
960.25, 2927.73.13 cm-1 (-CH stretch), 1660.78 cm-1 (–CHO stretch); ESI mass spectrum:
3
C H O S [M+H] + Theo. M/Z = 521.0658 [M+H] +, Found. M/Z 521.0652, Error = -
24
20
4 3
0.987ppm.
(
Z)-2-cyano-3-(5''-((E)-2,4,6-trimethoxystyryl)-[2,2':5',2''-terthiophen]-5-yl) acrylic acid
MR-5)
(
A mixture of aldehyde 4 (1.4g, 2.69mmol), cyanoacetic acid (0.265g, 3.12 mmol), ammonium
acetate (2.2g, 2.85mmol), and glacial acetic acid (30 mL) was heated at 118 °C for 3h under N2
atmosphere. The reaction mixture was allowed to cool to room temperature and then was
quenched with water and extracted with CH Cl . The organic layer was washed with water and
2
2
dried with anhydrous MgSO and purified by silica gel chromatography with CH Cl : CH OH
4
2
2
3
(8:1) to yield MR-5 as a deep red colored solid.
1
H NMR (400 MHz, DMSO-d ) δ = 12.40 (s, 1H), 8.32 (s, 1H), 8.18 (s, thio-H), 8.16 (m, 2H,
6
Ar-H), 7.63 (d, 1H, thio-H), 7.60 (d, 1H, thio-H) , 7.28 (d, 1H, thio-H), 7.21 (d, 1H, thio-H),
.31 (s, 2H, Ar-H), 3.91 (s, 6H,-CH3), 3.84 (s, 3H, -CH ) ; FT-IR: 2941.04, 2836.55 cm-1 (-
6
3
CH stretch), 2213.35 cm-1 (-CN stretch), 1706.27(-conjugated C=O stretch of -COOH); ESI-
3
FTMS for C H N O S [M+H]+ Theo. M/Z = 587.071 [M+H]+, Found. M/Z 588.071, Error
29
21
3
5 3
=
-1.695 ppm.
2-((Z)-4-oxo-2-thioxo-5-((5''-((E)-2,4,6-trimethoxystyryl)-[2,2':5',2''-terthiophen]-5-yl)
methylene) thiazolidin-3-yl) acetic acid (MR-6)

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A mixture of aldehyde 4 (1.4 g, 2.69 mmol), rhodanine-3-acetic acid (0.59 g, 3.12 mmol), and
ammonium acetate (2.2 g, 2.85 mmol), dissolved in 30 mL of glacial acetic acid. The mixture
was refluxed for 12 h under argon atmosphere. The progress of the reaction was monitored by
TLC technique. After completion of the reaction, the content was cooled to room temperature,
then it was poured into 100 g of crushed ice and the obtained red solid was filtered and dried.
The product was purified by column chromatography using silica gel and CHCl : CH OH (5:1)
3
3
as mobile phase to yield MR-6 as pink colored solid
H NMR (400 MHz, DMSO-d ) δ = 12.85 (s, 1H, -COOH), 8.48 (s, 1H, =CH), 8.19 (s, 1H,
1
6
thio-H), 8.16 (m, 2H, Ar-H), 7.64 (d, 1H, thio-H), 7.31 (d, 1H, thio-H) , 7.28 (d, 1H, thio-H),
.21 (d, 1H, thio-H), 5.21 (s, 1H, -CH2), 6.32 (s, 2H, Ar-H), 3.91 (s, 6H,-CH3), 3.80 (s, 3H, -
CH3); FT-IR: 2965.99, 2938.87, 2835.18 cm-1 (-CH stretch), 1701.17( C=O stretch) cm-1 ,
7
3
1
723.59 (conjugated C=O stretch of -COOH); ESI-FTMS for C H N O S [M+H]+ Theo.
31 23 3 6 5
M/Z = 694.026 [M+H]+, Found. M/Z = 694.025, Error = -1.532 ppm.
3. Results and Discussions
The synthetic pathways of two novel metal-free organic dyes MR-5 and MR-6 are
depicted in Scheme 1. 5,5'-(benzos[c]thiophene-4,7-diyl) bis(thiophene-2-carbaldehyde) (1) was
synthesized by Suzuki coupling of 4,7-dibromobenzo[c]thiophene with (4-formylthiophen-2-yl)
boronic acid. After that compound 1 treated with a triphenyl phosphonium ylide adduct 3
through witting reaction to form the key precursor aldehyde 4 which synthesized in good yield.
In the last step, the target sensitizers MR-5 and MR-6 were obtained in good yield by following
Knoevenagel condensation of precursor 4 with cyanoacetic acid and rhodanine-3-acetic acid
respectively. The target organic photosensitizers and their intermediates were purified using
recrystallization and column chromatography techniques. The structures of the newly
synthesized molecules and their intermediates were confirmed by various spectroscopic
analysis.

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Scheme 1: Synthesis of photosensitizer’s MR-5 and MR-6.

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3
.1.Photophysical Properties
-5
The absorption and emission spectra of the synthesized dyes were recorded in DMF (2 × 10
M) solutions are depicted in Figure 2 and corresponding results are summarized in Table 1.
Table 1 Absorption and emission properties for MR-5 and MR-6.
4
-1
-1
Sensitizer Absorption λmax (nm)
ε (10 M cm ) Emission λmax (nm)
MR-5
MR-6
382; 545
368; 558
5.05; 4.43
3.45; 4.64
700
708
From the absorption spectra, all dyes possess two distinctive bands. The band in the region of
3
60 to 400 nm are ascribed to the π-π* electron transitions of the conjugated molecules. The
absorption band from (400-600 nm) is assigned to an intramolecular charge transfer (ICT)
between the central unit and the anchoring moieties. The observed red shifted for both dyes due
to the addition of the benzothiadiazole moiety. Whereas, compared to MR-5, MR-6 showed a
red shifted absorption, which can be attributed to the different structures of the anchoring group.
From literature reviews, organic dyes with the same electron donor and π-conjugated spacer
with rhodanine-3-acetic acid as electron acceptor compared to cyanoacetic acid anchoring
moieties have better optical absorption properties [31, 32]. The molar extinction coefficient of
-1
-1
dyes was found to be 4.43 and 4.64 M cm , respectively indicating good light harvesting
capability. Further, the emission spectra of MR-6 was more red-shifted than that of MR-5.
1
.2
1
6
5
4
3
2
1
0
0
0
0
0
.8
.6
.4
.2
0
MR-5
MR-6
MR-5
MR-6
2
80
380
480
580
680
780
880
Wave length/nm
Figure 2. UV-Vis absorption (solid-line) and emission spectra (dashed-line) of MR-5 and MR-
-5
6
measured in DMF (2 × 10 M).

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The absorption spectra of MR-5 and MR-6 dyes adsorbed on TiO film are displayed in Figure
2
3
. As dyes adsorbed on the TiO nanoparticle surface, the absorption spectra were red-shifted
2
and broadened compared to the absorption spectra of the dye, indicating that majority of the dye
molecules absorbed on the TiO surface. This broadening of the absorption spectra is due to an
2
interaction between the dye and TiO [33]. Moreover, MR-5 found to be more adsorbed on
2
TiO surface compared to MR-6, which is an indication of the better performance of this dye.
2
Figure 3. Absorption spectra of MR-5 and MR-6 adsorbed on nanoporous TiO2.
3.2.Electrochemical Characterization
Cyclic voltammetry (CV) studies were measured in DMF with the electrolyte 0.1 M
-1
[
TBA][PF6] at a scan rate of 50 mV s . The working electrode used is the Glassy carbon, Pt
wire represented the counter electrode and the reference electrode is Ag/Ag+ in ACN. Fc/Fc+
was introduced as an internal reference. The voltage measured was converted to NHE by
addition value of 0.63 V. Fig. 1(b) shows the CV profile of MR-5 and MR-6. The attained CV
data were used to calculate the GSOP/HOMO energy level from onset oxidation potential of
oxidation peak using the equation [34] (1)
oxd
onset
GSOP = −
[
E
+ 4.7eV
………. . . (1)
]

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Additionally, E0-0 (energy between the ground and excited states) which is the difference
between HOMO and the first excited singlet was determined from the intersection of the
absorbance and emission spectra. The optical band-gap E0-0 and GSOP values were employed to
calculate their excited state oxidation potential (ESOP); the values in volts (V) against NHE
were converted to electron volt (eV) according to the equation (2):
ESOP =
[
GSOP− E0−0 eV………. (2)
]
Figure 4 depicts the energy level diagram of the dyes. The calculated GSOP level of
dyes MR-5 (-5.47 eV) and MR-6 (-5.57 eV) were found to be lower than that of the redox
−
−
potential of I /I electrolyte system (-5.2 eV), demonstrating that there is enough
3
thermodynamic driving force to regenerate the dye [35]. Moreover, estimated ESOP levels were
found to be in order of MR-5 (-3.54 eV) and MR-6 (-3.56 eV) which were found to be higher
than the conduction band edge of TiO (-4.2 eV), which shows that the electron injection from
2
the synthesized dyes into the conduction band edge of TiO2 semiconductors is
thermodynamically favorable [36]. Furthermore, MR-5 has a larger electron injection potential
than MR-6, indicating that the light-excited electrons are injected more efficiently in case of
MR-6. Therefore, it is expected that MR-5 carrying cyanoacetic acid as anchoring group will be
more efficient for electron injection into the CB of TiO than the MR-6. Both MR-5 and MR-6
2
dyes meet the thermodynamic requirements for electron injection and dye regeneration process
in the fabricated devices [37].

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Figure 4. Energy level diagram and comparison between the GSOP and the ESOP of MR-5 and
MR-6.
3
.4. TiO electrode preparation and device fabrication
2
TiO electrode preparation and DSSC devices fabrications with detailed procedure are provided
2
in supplementary information
3.5.Photovoltaic and Electron impendences studies
The photo-voltaic performance of the DSSCs device fabricated using dyes MR-5 and MR-6 on
mesoporous TiO electrode in presence of 10 mM CDCA co-adsorbent were studied using
2
-2
standard AM 1.5 illuminations (light intensity: 100 mW cm ) [38-40]. The procedure
employed for the fabrication of devices are well explained in the electronic supplementary
section. Figure 5 reveals the J-V characteristic curve of the photovoltaic devices and their
detailed photovoltaic parameters were summarized in Table 2.

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Table 2. I-V characteristics of solar devices based on MR-5 and MR-6
−2
Sensitizer CDCA (mM) J (mA⋅cm ) VOC (V)
FF (%)
65.12
64.65
η (%)
6.03
3.2
SC
MR-5
MR-6
10
10
15.27
8.7
0.61
0.56
2
Conditions: sealed cells; coadsorbate, CDCA 10 mM; photoelectrode, TiO (15 µm thickness and 0.18 cm );
2
−2
electrolyte, 0.6 M DMPII, 0.1 M LiI, 0.05 I 2 in HI-30; irradiated light, AM 1.5 solar light (100 mW cm ). J ,
SC
short-circuit photocurrent density; V , open-circuit photovoltage; FF, fill factor; η, total power conversion
OC
efficiency.
The cell based on the dye MR-5 showed the highest efficiency (η) of 6.03 % (JSC = 15.27
2
mA/cm , V = 0.606 V, FF = 65.12) than MR-6. However, where MR-6 showed an efficiency
OC
2
(
η) of 3.2 % (J = 8.7 ma/cm , V = 0.56 V, FF = 64.65). It was observed that both the short-
SC
OC
circuit photocurrent (J ) and the open-circuit voltage (V ) for DSSC based on MR-5 dye are
SC
OC
higher than that of MR-6 under the same experimental conditions. This improvement in VOC
contributed to the increase in electron lifetime in the DSSC by preventing the dark current,
which will be discussed later in the impedance studies section. The higher ESOP level of MR-5
is also supportive for the higher value of VOC [41]. The increase in JSC may be related to the
higher amount of MR-5 dye adsorbed on the TiO nanocrystalline surface as compared to MR-
2
6
as shown in Figure 3. This increased surface area affords additional absorption sites for the
dye, which correlates with better light harvesting efficiency of the photoelectrode resulting in
an improvement in JSC.

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18
16
14
12
10
8
6
4
2
0
MR-6
MR-5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage/V
Figure 5. I-V characteristics of solar devices based on MR-5 and MR-6.
Furthermore, the IPCE of MR-5 was 57% at 350 nm and 65% in the range from 430 to 520 nm
as shown in Figure 6. However, MR-6 displayed a red-shifted expanding to 600nm but the top
value reached only 49%. The observed low PCE value of MR-6 may be related to the
separation of LUMO level from the anchoring group of the dye due to the presence of
methylene group which causes breakage in the π -conjugation and so, causing an inefficient
electron injection from the photoexcited dye into the conduction band (CB) edge of TiO2,
resulting lower value of J . Withthe aforementioned in mind, it is clear that the photovoltaic
SC
performances of the DSSCs are affected by the nature of the acceptor groups on the designed
dyes. This agrees with the results of phenothiazine-based dyes reported by Tian et al. [42, 43].
70
60
50
40
30
20
10
0
MR-6
MR-5
3
00
350
400
450
500
550
600
650
700
750
800
Wave length/nm

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Figure 6. Photocurrent action spectra (IPCE) of solar devices based on MR-5 and MR-6.
Furthermore, electrochemical impedance spectroscopy (EIS) is used to evaluate the
charge transfer, electron lifetime and chemical capacitance at the interface of
TiO /dye/electrolyte and pt/electrolyte in the fabricated DSSC under practical conditions [44-
2
4
6]. In general, the electrons are transported through the TiO nanoparticles to the electrolyte.
2
The smaller the radius of the intermediate frequency semicircle in the Nyquist plot, the lower
the electron transport resistance. On the other hand, the higher the charge recombination
resistance in the Nyquist plot, the higher the photovoltic obtained from the fabricated solar cells,
resulting in a higher eTiO lifetime [47, 48]. Figure 7 shows the Nyquist plots of DSSCs
2
sensitizers with metal-free dyes-based cells, which reflects the electron recombination
resistance, was found to be in order of MR-5 > MR-6 which is in good agreement with the Voc
of the devices.
9
8
7
6
5
4
3
2
1
0
MR-6
MR-5
0
10
20
30
40
50
60
Re(Z)/Ohm
Figure 7. EIS Nyquist plots for solar devices based on MR-5 and MR-6.
Moreover, the Bode plots, shown in Figure 8 can be used to evaluate eTiO (injected
2
electrons in the conduction band of TiO ) by using the relation (τ = 1/2πf), where τ is the
2
eff
eff
lifetime of electrons in TiO and fis the midpoint frequency from Bode plot. Higher eTiO leads
2
2
to improved Voc. The lifetime in CB edge of TiO of the fabricated devices was found to be in
2

ACCEPTED MANUSCRIPT
the order of MR-5 > MR-6. From the later results, it is observed that MR-5 has the highest
eTiO , which means the highest V .
2
OC
40
35
30
25
20
15
10
5
0
MR-5
MR-6
-2
-1
0
1
2
3
4
5
6
log (freq/Hz)
Figure 8. EIS Bode plots for solar devices based on MR-5 and MR-6.
3.6 Molecular modeling
DFT calculations, implemented in Guassian09 package [28, 29], were performed on MR-
and MR-6. First, the ground-state geometries were optimized using DMF as a solvent, with
5
B3LYP as energy functional and 6-31g (d, p) basis set. These optimized geometries were
successively introduced to the TD-DFT energy calculation using the same energy functional and
basis set. E , the optical energy gap, the ground and excited state oxidation potentials were
0
-0
calculated and are shown in Table 3, which were in good agreement with the experimental data.
Table 3: Excited state oxidation potential, and the lowest electronic transitions (E ) for MR-5
0
-0
and MR-6.
d
Experimental (eV)
Theoretical (eV), TD-DFT
Sensitizer
E0-0
=
b
a
c
E0
-0
GSOP (HOMO) E* (ESOP)
EHOMO
ESOPsinglet
(
S – S )
0
1
MR-5
MR-6
1.93
1.89
–5.47
–5.49
–3.54
–3.60
1.85
1.74
–5.48
–5.68
–3.63
–3.88

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[
a] Oxidation potentials of dyes were measured in DMF with 0.1m [TBA][PF6] and at a scan rate of 50 mVs-1.
They were calibrated with Fc/Fc+ as internal reference and converted to the NHE scale by addition of 0.63 V. [b]
E0–0 was determined from the intersection of the absorption and tangent of the emission peak in DCM [25]. [c]
E*was calculated as E*ox-E0–0. GSOP=ground state oxidation potential= EHOMO. Theoretical calculations based
on B3LYP/ 6-31g (d, p) level DFT and TD-DFT calculations were given in parenthesis
The isodensity plots of frontier molecular orbitals (FMO) of MR-5 and MR-6 are displayed in
Figure 9. As evidenced from their HOMO energy level, MR-5 dye show push−pull electron
effects, where the HOMO and the LUMO are mainly composed of donor and acceptor moieties,
respectively. The HOMO is mainly localized on the trimethoxy benzene units (donors) and
thiophene rings, and the LUMO is effectively delocalized on the benzothiadiazole and
cyanoacetic acid units (acceptors). In the case of MR-6, the LUMO isosurface is mainly
delocalized on the rhodanine-3-acetic acid moiety, particularly on the thiocarbonyl and carbonyl
and thus resulting in the separation of LUMO from the anchoring moiety and, thus, preventing
better injection of electrons into CB edge of TiO [49]. Additionally, in MR-6, the dihedral
2
angle between the -COOH acceptor/anchoring group and the rhodanine moiety is 75.66°,
meaning that the anchoring group and the D-π-A-π-A scheme of the dye is not coplanar. As a
consequence, the conjugation system is obstructed, and the electrons are partially delocalized,
leading to inefficient photoexcitation upon light explosion, which decreases the electron
injection from MR-6 into the semiconductor and the final photovoltaic performance. However,
it is coplanar in the case of MR-5, favoring efficient electron injection into TiO (Figure 9b, d),
2
which is also good evidence of the deficient performance of the dye MR-6 bearing rhodanine-3-
acetic acid as an anchoring group.

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Figure 9 Calculated isodensity of frontier occupied and unoccupied molecular orbitals for MR-
5
and MR-6.
4. Conclusion
We have successfully synthesized two novel D-π-A-π-A architecture metal-free organic
dyes for DSSC application. Addition of benzothiazole unit as electron-acceptor next to the
anchoring moiety provided strong electronic coupling between HOMO/LUMO, which
translated to better DSSCs performance. MR-6 showed a red shifted absorption, which can be
attributed to the greater conjugation of the 4-oxo-2-thioxothiazolidine ring compared with the
cyanoacetic unit. All the dyes possess the thermodynamical requirements for electron injection

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into the conduction band edge of TiO and dye regeneration of the electrolyte species. The
2
device fabricated with MR-5 carrying cyanoacetic acid as anchoring moiety showed higher PCE
2
of 6.03 %. Its J , V , and IPCE values were found to be 15.27 mA/cm , 0.606 V, and 65 %,
SC
OC
respectively. The higher JSC value indicated that, the dyes bearing cyanoacetic acid moiety as
anchoring groups in the designed dyes bind efficiently on the surface of TiO than the one
2
containing rhodamine-3 acetic acid. Furthermore, from the electrochemical impedance
spectroscopy (EIS), MR-5 has longer electron lifetime (eTiO ), which means the highest V .
2
OC
indicating good agreements with photovoltaic measurements. DFT studies indicate that
rhodanine-3-acetic acid is poor anchoring group as in MR-6 because the −COOH
acceptor/anchoring group is almost at a 75.66° plane relative to the rhodanine ring which lowers
the electron injection from MR-6 into the CB edge of TiO . Hence, the geometry of the
2
anchoring group with respect to the chromophore is key in the efficiency of dye as a sensitizer
for DSSCs.
5. Acknowledgements
The authors are thankful to the Department of Textile Engineering, Chemistry, and Science
Department at NC State and the Egyptian Cultural and Educational Bureau, Washington D.C.,
for the financial support.
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