Synthesis and characterization of new squaraine dyes with bis-pendent carboxylic groups for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.molstruc.2019.06.056

In the present study, we introduce a new plan to boost the charge transfer between squaraine sensitizers and the conduction band of TiO₂ by increasing the anchoring number. Two new symmetric squaraine sensitizers (SQIND-1 and SQIND-2) derived from indoline and having pendant bis-COOH as anchor groups to inject the electron into the TiO₂ nanoparticles were tested as DSSC. The theoretical calculations and absorbance results showed that the electron density of LUMO of SQIND-1 is delocalized in the whole carboxylic group resulting in strong electronic interaction between SQIND-1 sensitizer and the conducting band of TiO₂. Also, the relatively longer alkyl chain present in SQIND-1 would lower the aggregation of the dye on the TiO₂ surface compared to SQIND-2. Hence, the SQIND-1 sensitized exhibit better photovoltaic performance. Dye-sensitized solar cell base fabricated in SQIND-1 was on TiO₂ with the long wavelength and NIR region up to 750 nm of the spectrum and showed higher noteworthy performance properties, such as energy conversion efficiency (η) of 6.2% (AM 1.5 G), a short-circuit photocurrent density of 6.64 mA/cm², an open-circuit photovoltage of 0.53, a fill factor of 0.72 and an incident photon-to-current conversion efficiency of 45% at 668 nm.

Full text of DOI:10.1016/j.molstruc.2019.06.056

Accepted Manuscript
Synthesis and characterization of new squaraine dyes with bis-pendent carboxylic
groups for dye-sensitized solar cells
Sultan A. Al-horaibi, Abdullah M. Asiri, Reda M. El-Shishtawy, Suresh T. Gaikwad,
Anjali S. Rajbhoj
PII:
S0022-2860(19)30780-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.06.056
MOLSTR 26698
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 March 2019
Revised Date: 25 May 2019
Accepted Date: 14 June 2019
Please cite this article as: S.A. Al-horaibi, A.M. Asiri, R.M. El-Shishtawy, S.T. Gaikwad, A.S.
Rajbhoj, Synthesis and characterization of new squaraine dyes with bis-pendent carboxylic groups
for dye-sensitized solar cells, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/
j.molstruc.2019.06.056.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and characterization of new squaraine dyes with bis-
pendent carboxylic groups for dye-sensitized solar cells
a,b
c,d
c,e
a
a,
Sultan A. Al-horaibi , Abdullah M. asiri , Reda M. El-Shishtawy , Suresh T. Gaikwad , Anjali S. Rajbhoj *
a
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Maharashtra 431001, India
Department of Chemistry, Al-Baydha University,Yemen.
b
c
Department of Chemistry, King Abdulaziz University, P.O BOX 80203, Jeddah, Saudi Arabia.
d
Center of Excellence for Advanced Materials Research.
Dyeing, Printing and Textile Auxiliaries Department, Textile Research Division, National Research Center, Dokki, Cairo, 12622,
e
Egypt.
*
Corresponding author e-mail address: anjali.rajbhoj@gmail.com
Abstract: In the present study, we introduce a new plan to boost the charge transfer between
squaraine sensitizers and the conduction band of TiO by increasing the anchoring number. Two new
2
symmetric squaraine sensitizers (SQIND-1 and SQIND-2) derived from indoline and having pendant
bis-COOH as anchor groups to inject the electron into the TiO nanoparticles were tested as DSSC.
2
The theoretical calculations and absorbance results showed that the electron density of LUMO of
SQIND-1 is delocalized in the whole carboxylic group resulting in strong electronic interaction
between SQIND-1 sensitizer and the conducting band of TiO . Also, the relatively longer alkyl chain
2
present in SQIND-1 would lower the aggregation of the dye on the TiO surface compared to SQIND-
2
2
. Hence, the SQIND-1 sensitized exhibit better photovoltaic performance. Dye-sensitized solar cell
base fabricated in SQIND-1 was on TiO with the long wavelength and NIR region up to 750 nm of
2
the spectrum and showed higher noteworthy performance properties, such as energy conversion
2
efficiency (η) of 6.2 % (AM 1.5 G), a short-circuit photocurrent density of 6.64 mA/cm , an open-
circuit photovoltage of 0.53, a fill factor of 0.72 and an incident photon-to-current conversion
efficiency of 45 % at 668 nm.
Keywords: Dye-sensitized solar cell, Symmetric squaraine dyes, Photoelectrochemical properties,
DFT
1
. Introduction
The replacement of fossil fuels is currently an essential global issue not only due to their adverse
impact on the environment but as well to their last shortage by time where the energy demand is
continuously increasing. Thus, the search for alternative renewable resources would be of great
interest. Photovoltaic technology is the best choice to supply us plenty of renewable energy by
converting solar energy into electricity [1, 2]. Dye-sensitized solar cells (DSSC) are an important
source of renewable clean energy sources [3]. There have been tremendous growth and a great deal of

ACCEPTED MANUSCRIPT
interest from scientists and economic researchers because of their low cost, eco-friendly and ease of
fabrication compared to conventional silicon cells based on solar energy. Improving the performance
of dye-sensitized solar cells in terms of broadband gap mesoporous semiconductor, potential dye-
sensitizers, redox electrolyte, and counter electrodes have resulted in photovoltaic conversion
efficiency of more than 10% compared to non-crystalline silicon solar cells [4, 5]. A perusal revision
to photoconversion by potential dye-sensitizer used in dye-sensitized solar cells confirms that
photoreceptor efficiency can be achieved upto 12% even with the photon harvest mainly in the visible
absorption area of 400-700 nm of the solar spectrum [6–9]. Therefore, an urgent need to improve
photovoltaic efficiency by developing high-capacity sensitizers dyes to harvest photons in NIR-IR
wavelength region [10–13]. The squaraine dyes (SQ) are particularly important in scientific research
due to strong absorption in the visible region, high extinction coefficients, electrochemical and
photoelectric stability, so they are used in many applications such as bioimaging applications [14],
sensors [15] and semiconducting devices such as laser technology [6], organic field effect transistors
(
OFETs), and photovoltaic cells [16–18]. Non-linear optical behavior [19] and efficient electron
transport properties [20]. SQ dyes are also being investigated as dye-sensitized solar cells (DSSC)
21], organic solar cells [22], organic-inorganic hybrid solar cells [23]. Squaraine dyes are considered
the best candidates between the different organic dyes to be used in applications of dye-sensitized
[
5
−1
−1
solar cells. They are low cost and have higher molar extinction coefficients ∼3×10 M cm , also
squaraine dyes absorb the photons in the range 400 to 800 nm [23–25].
Our previous studies indicated that the absorption bands of SQ dyes could be easily tuned to get
into NIR region by structural modification, which closely matches the spectral response of sunlight. In
addition, theoretical calculations on SQ dyes reveal their suitability for DSCC, as their LUMO energy
lie above the LUMO energy of TiO conduction band and thus ease electron injection from the dye to
2
the TiO [21, 24, 26–28].
2
The purpose of the present study is to fabricate and get an efficient photovoltaic system able of
absorption in the long wavelength region of the spectrum through the application of two types of
symmetrical squaraine dyes. The indoline-based squaraine dyes (SQIND-1 and SQIND-2) were
synthesized and compared with those previously reported indoline-based squarylium (dye 1 & dye 2)
sensitizers [29]. The results of Jsc, Voc, FF, and η values of SQIND-1 and SQIND-2 showed higher
photovoltaic performance compared to dye1 & dye2.
The newly synthesized SQIND-1 and SQIND-2 were used for the first time as photosensitizers in
DSSC. The state of art of this type of symmetric indoline-based squaraine dyes that have been
reported as yet for DSSC reveals only one example in which short chain containing bis-pendent butyl

ACCEPTED MANUSCRIPT
eater was evaluated. Most other dyes are containing monocarboxylic groups either as pendent alkyl or
as substituted in the aromatic ring [30].
The HOMOs and LUMOs of SQIND-1 and SQIND-2 were calculated by Gaussian 09 package to
have a detailed understanding of the ground, excited state electron density distribution and band gap
[
24, 31–36]. The design concept of these dyes was to fulfill the need of less dye aggregation that was
persuaded by having long alkyl chain on N-atom and by having pendent two carboxyl groups
connected with the dye via long alkyl chain as in SQIND-1 and short alkyl chain for comparison as in
SQIND-2. Besides, having two pendent carboxyl groups would be beneficial for better electron
injection into the TiO surface.
2
2
. Materials, Instruments and Methods
2
.1 Materials
All chemicals, 4-Bromophenyl hydrazine hydrochloride (98%), Phenyl hydrazine (97%),
Squaric acid (98%), Sodium Acetate Anhydrous, Sodium carbonate anhydrous were from Sigma–
Aldrich. Isopropyl methyl ketone (98%), Sodium sulphate anhydrous (99%), 3-Bromopropionic acid
(97%), Ethyl acetate (99%), Toluene (99%), 6-Bromohexanoic acid were from Alfa Aesar, n-Butanol
(
SISCO research laboratories PVT-LTD), Diethyl ether (Et O, 98%, Molychem), Acetonitrile (98%,
2
Molychem) were from Alfa Aesar, Methanol, 99.8%, Ethanol, 99.9% were from Sigma-Aldrich,
petroleum ether 60/80 was from Alfa Aesar, Silica gel 60-120 mesh for column chromatography
(Vetec) was from Sigma-Aldrich. Synthesis processes of SQIND-1 and SQIND-2 are showed in
scheme 1.
2
.2 Instrumental analysis
1
13
All HNMR spectra were recorded on Bruker 400 and 600 MHz spectrometer and C NMR
(
150 MHz) using DMSO-d and CDCl . UV-vis spectra were recorded on a Perkin Elmer UV-1800
6 3
spectroscopy in dichloromethane (DCM). The oxidation/reduction properties of the SQIND-1 and
SQIND-2 were examined by cyclic voltammetry (CV) on a Metrohm Autolab PGSTAT 128N in dried
dimethyl sulfoxide (DMSO) with three electrode systems consists of a saturated Ag/AgCl reference
electrode, a platinum wire auxiliary electrode and Pt electrode as working electrode. The scan rate was
1
00 mV/s. The optimization geometries for SQIND-1 and SQIND-2 were calculated at B3LYP/3-21
(d, p) level of theory using Gaussian 09 program.
2
.3 Synthesis and charactrizations
2
.3.1 Synthesis of 2, 3, 3-trimethyl-3H-indole (1)
In a round bottom flask phenylhydrazine (10.80 g, 100 mmol) was taken and isopropylmethyl
ketone (8.80 g, 100 mmol) was added dropwise to the solution and then the solution was heated with
◦
stirring at 90 C for 12 h, cooled to RT. H O (80 ml) was added and was extracted by Et O (3×30 ml),
2
2

ACCEPTED MANUSCRIPT
after that it was dried over Na SO for 30 minutes then filtered and washed by Et O to get a red liquid
2
4
2
in 98% yield. The obtained liquid was dissolved in CH COOH (34 ml), and then heated with stirring
3
◦
at 90 C for 2 h. After cooled to RT, the mixture was neutralized with saturated solution of Na CO
2
3
was extracted by ethyl acetate (3×50 ml) and the dried over Na SO for 1hour, then filtered and
2
4
washed with ethyl acetate. The solvent was removed and the liquid residue was collected to give
1
yellow compound (1) in 95% yield. HNMR (400MHz-CDCl ) spectrum was recorded at δ (ppm)
3
1
7
1
2
.27(s, 6H, 2CH ), 2.26(s, 3H, CH ), 7.16(t, 1H, J=0.88Hz, Ar-H), 7.27(d, t, 2H, J=1.24 Hz, Ar-H),
3 3
13
.54(d, 1H, J=7.6 Ar-H). C-NMR (CDCl ) 15.25(CH ), 22.94(2CH ), 53.45(C ), 119.70, 121.19,
3
3
3
3
25, 127.46 (carbon phenyl aromatic), 145.47, 153.41 (C , C respectively), 187.95(C ).
7
8
2
.3.2 Synthesis of 5-bromo 2, 3, 3-trimethyl-3H-indolenine (2)
In a round bottom flask p-Bromophenyl hydrazine hydrochloride (7.55 g, 33.85 mmol),
isopropylmethyl ketone (5.85 g, 67.5 mmol) CH COOH (60 ml) were refluxed under N atmosphere
3
2
for 7 h and stirred at RT for 19 h. The solvent was removed and the residue was dissolved in Et O/
2
H O (3×50 ml) and dried over MgSO for 1h, then filtered and washed with diethyl ether to give
2
4
1
compound (2) as brownish oil in 97% yield. HNMR(400MHz-DMSO-d ) spectrum recorded at
6
δ(ppm) 1.21(s, 3H, CH ), 1.9(s, 6H, 2CH ), 7.45(d,1H, J=8.2Hz, Ar-H), 7.69(d, 1H, J=8.68Hz , Ar-H)
3
3
1
3
,
7.74(s, 1H, Ar-H). C-NMR(CDCl ) 21.68, 21.76(CH 2CH ), 54.52(C ), 118.70, 119.43, 125.6,
3 3, 3 3
1
39.90(carbon phenyl aromatic), 146.17, 153.26(C , C respectively), 178.98(C ).
8 9 2
2
.3.3 Synthesis of 1-(carboxymethyl)-2, 3, 3-trimethyl-3H-indolium iodide (3)
A mixture of 2, 3, 3-trimethyl-3H-indole (1.61 g, 10 mmol) and 3-iodopropanoic acid (4 g, 20
◦
mmol) was heated at 115 C for 15 h under N atmosphere in 1,2-dichloro -benzene (20 ml), cooled to
2
1
RT and filtered with ethyl acetate to give compound (3) as pinkish white colour in 48% yield. HNMR
(
400 MHz-DMSO-d ) spectrum recorded at δ(ppm) 1.57(s, 6H, 2CH ), 2.88(s, 3H, CH ), 2.98(t, 2H,
6
3
3
J=6.8 Hz, CH -CO), 4.66(t, 2H, J=6.72Hz, N-CH ), 7.80(d, 1H, J=7.04 Hz, Ar-H), 7.98(d, 1H, J= 8.6
2
2
1
3
Hz,Ar-H),8.14(s,1H,Ar-H). C-NMR(DMSO-d )14.70(CH ),21.72(2CH ),30.94(CH - CO),43.80(C ),
6
3
3
2
3
5
4.47(N-CH ),117.47, 122.81, 126.77,131.75(carbon phenyl aromatic), 140.04, 143.85(C , C respec
2 8 9
-tively), 171.41(C=O), 198.28(C2).
2
.3.4 Synthesis of 5-bromo-1-(5-carboxypentyl)-2, 3, 3-trimethyl-3H-indolium bromide (4)
A mixture of 5-bromo-2, 3, 3-trimethyl-3H-indole (2.5 g, 0.0105 mmol) and 6-bromohexanoic
acid (2.5 g, 0.005 mmol) was heated to reflux for 24 h under N atmosphere in nitromethane (20 ml),
2
cooled to RT, then diethyl ether was added and left for 1 h. Then it was filtered and washed with
1
diethyl ether to give compound (4) as light brown colour in 78% yield. HNMR(400 MHz-DMSO-d )
6
spectrum recorded at δ(ppm) 1.45(pent, 2H, CH ), 1.61( pent, s, 8H, CH , 2CH ), 1.89(pent, 2H,
2
2
3
+
CH ), 2.26(t, 2H, CH -CO), 4.47(t, 2H, CH -N ), 7.78(d, 1H, J=6.84Hz, Ar-H), 7.87(d,1H, J=8.56Hz,
2
2
2

ACCEPTED MANUSCRIPT
1
3
Ar-H), 8.04(s,1H, Ar-H). C-NMR(DMSO-d ) 21.83(2CH ), 23.78, 25.27, 26.83(3CH ) 33.17(CH -
6
3
2
2
+
CO), 47.74(C ), 54.32(CH –N ), 117.14, 123.11, 126.59, 13 1.94(carbon phenyl aromatic), 139.97,
3
2
1
43.74(C , C respectively), 174.54 (CO), 196. 6(C ).
8 9 2
2
.3.5 Synthesis of SQIND-1
A mixture of 5-bromo-1-(5-carboxypentyl)-2, 3, 3-trimethyl -3H-indolium bromide (1.5 g,
.0036 mmol) and 3, 4-dihydroxycyclobut-3-ene-1, 2-dione (0.205 g, 0.0018 mmol) was taken and 40
0
ml of dry BuOH/toluene 1:1(v/v) was added. Reaction mixture was refluxed for 4 h using Dean–Stark
trap for azeotropic removal of water. The mixture was cooled to RT and the solvent was evaporated.
The blue product was filtered and washed with hexane. The product was purified by silica gel column
chromatography with ethyl acetate and methanol (9:1, 9:2, 9:3, 9:4 and 9:5 (v/v) respectively).
Recrystallization of the product with ethyl acetate gave SQIND-1 as blue colour dye in 89.74% yield.
1
HNMR(400MHz-DMSO-d ) spectrum recorded at δ(ppm) 1.37(pent, 4H, 2CH ), 1.47(pent, 4H, 2
6
2
CH ), 1.58(pent, 4H, 2CH ), 1.79(pent, 4H, 2CH ), 1.81(s, 12H, 4CH ), 2.31(t, 4H, CH -CO), 4.06(t,
2
2
2
3
2
1
3
4
H, CH -N), 5.95(s, 2H, CH=C), 6.83(dd, 2H, J=8.36Hz, Ar-H), 7.4(ds, ds, 4H, J=6.6 Hz, Ar-H). C-
2
NMR(DMSO-d ) 13.73(2CH ), 19.15(2CH 2CH ) 24.61(2CH ), 26.61, 26.50, 26.71, 27.06(4CH ),
6
3
2,
3
2
2
+
3
0.66(2CH -CO), 34.00(C ), 43.65(N-CH ), 49.39(N -CH ), 64.31(C ), 87.15(C=CH), 110.68,
2 3 2 2 3
1
16.74, 12578(carbon phenyl aromatic), 130.76, 141.45(C , C respectively), 169.55 (C ), 173.43
8
9
2
(CO).
5
.2.6 Synthesis of SQIND-2
A mixture of compounds 1-(2-carboxyethyl)-2, 3, 3-trimethyl-3H-indolium iodide (1.7 g,
.0048 mmol) and 3, 4-dihydroxycyclobut-3-ene-1, 2-dione (0.273 g, 0.0012 mmol) was taken in 40
0
ml of dry n-BuOH/ toluene 1:1 (v/v). Reaction mixture was refluxed for 5 h using Dean–Stark trap for
azeotropic removal of water. The mixture was cooled to RT and solvent was evaporated. The product
was filtered and washed with hexane. The product blue was purified by silica gel column
chromatography with ethyl acetate and methanol (9:1, 9:2, 9:3, 9:4 and 9:5 (v/v) respectively).
Recrystallization of the product with ethyl acetate gave SQIND-2 as blue colour dye in 56% yield.
1
HNMR(400MHz-DMSO-d ) spectrum recorded at δ(ppm) 1.37(s, 6H, 2CH ), 1.51(s, 6H, 2CH ),
6
3
3
+
2
.25(t, 2H, 2CH -CO), 2.31(t, 2H, 2CH -CO), 3.9(t, 2H, CH -N), 4.09(t, 4H, CH -N ), 5.79(s, H,-
2 2 2 2
1
3
CH=C), 6.55(s, H, CH=C), 6.94-7.5(m, 8H, Ar-H),11.7(OH). C-NMR(DMSO-d ) 13.73(2CH ),
6
3
1
4
9.15 (2CH 2CH ) 24.61(2CH ), 26.61, 26.50, 26.71, 27.06(4CH ), 30.66(2 CH -CO), 34(C ),
2, 3 2 2 2 3
+
3.65(N-CH ), 49.39(N -CH ), 64.31(C ), 87.15(C=CH), 110.68, 116.74, 125.78(carbon phenyl
2
2
3
aromatic), 130.76, 141.45(C , C respectively), 169.55(C ), 173.43 (CO).
8
9
2

ACCEPTED MANUSCRIPT
Scheme 1. Synthesis of squaraine dyes (SQIND-1 and SQIND-2)
2
.4 Preparation, Fabrication and measurements of DSSC
A mixture of TiO powder ≤ 25 nm (3 g), nitric acid aqueous solution 0.5 ml in
2
dimethyl sulfoxide 3ml, and deionized water 2ml was taken and reagent of Triton X–100
2ml) was added. The TiO paste was stirred by a rod glass for 10 min and then left it for 5
(
2
min and stored in a closed bottle for later use. The conducting glasses substrate indium tin oxide
glass slide, square (ITO glasses 8Ω/ Sq.m, thickness 0.15 mm and area about 0.25 mm) were
cleaned in a detergent solution for 10 min by sonication and with acetone for 10 min, then with
deionized water for 10 min and finally with ethanol for 10 min. The conducting glass was
dried by air and then in oven. The glass square (~8ꢀ) was measured by a multi-meter set to
examine the conductive side, then strips of Scotch Magic tape on were put on the edges of the
glass square by the doctor blade technique. The TiO paste was deposited in the middle of the
2
glass square. The conducting glass TiO porous film was left in the air for 5 min, and then
2
◦
placed on hotplate for 30 min at 70 C. In the final step it was placed in furnace for 1 h at 450
◦
C and immersed in squaraine dyes solution (SQIND-1 and SQIND-2) for 25 h in the dark in
ethanol with chenodeoxycholic acid (CDCA) (0.001 M). Counter electrode was prepared as follows:
A mixture of polyvinylpyrrolidone (PVP) 1 g, H PtCl 0.3 g was dissolved in deionized water (2 ml)
2
6
under stirring and NaBH solution was added to the mixture. The color of the mixture was changed
4
◦
from orange to black, finally the counter electrode PVP-coated was placed in furnace at 450
C for 1
-
-
h. The solution iodide/tri-iodide (I /I ) electrolyte was prepared as follows: A mixture of iodine (0.03
3
M), lithium iodide (0.02 M), 1-butyl-3-methylimidazolium iodide (0.015 M), and 4-tertbutylpyridine
(
0.5 M) were taken in CH CN.
3

ACCEPTED MANUSCRIPT
The photovoltaic performance of the fabricated SQIND-1 and SQIND-2 based DSSC were
2
measured under sunlight (AM 1.5, 100 mW/cm ) solar simulator (Dayton, Instruments, USA) with a
source meter (Keithley, 2400) at 25 °C.
3
Results and Discussion
3
.1 Synthesis and spectroelectrochemical properties of the SQIND-1 and SQIND-2 sensitizers
The synthesis details of squaraine sensitizers (SQIND-1 and SQIND-2) are showed in Scheme
. SQIND-1 and SQIND-2 based indoline have two anchor groups in each dye to facilitate electron
1
injection to the conduction band of titanium dioxide porous.
Fig. 1 shows the normalized UV-Vis absorption spectra of SQIND-1 and SQIND-2 dissolved
in methanol. The band maxima of SQIND-1 and SQIND-2 dissolved in methanol solution are
observed at 649 and 642 nm, respectively, and their corresponding high molar extinction coefficient
5
5
−1
−1
values 1.84×10 and 2.19×10 M cm are in agreement with π-π* transitions which could be
observed in the SQIND-1 and SQIND-2. The optical band gap energy is estimated for SQIND-1 and
SQIND-2 to be 1.76 and 1.82 eV, respectively, from the intersection of the absorption and emission
spectra in the MeOH solution.
To investigate the electrochemical property of dye, cyclic voltammetry (CV) measurement
was performed on a Metrohm Autolab PGSTAT 128N (Metrohm B.V., Utrecht, Netherlands) with
three electrode system consisting of a saturated Ag/AgCl reference electrode, a Pt wire auxiliary
electrode and Pt electrode as working electrode. Cyclic voltammetry measurements were performed in
DMSO using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF ) as supporting electrode and
6
0
1
.0001 M of SQIND-1 and SQIND-2 at room temperature. The potential scan was taken from -2.0 to
.4 V at scan rate 100 mV/s (see Fig. 2). We have also estimated the energies of frontier orbitals
(HOMO-LUMO) in SQIND-1 and SQIND-2 sensitizers in order to justify its candidatures as
sensitizers in DSSC.

ACCEPTED MANUSCRIPT
Fig. 1 Absorption spectrum of SQ1ND-1 and SQIND-2 in MeOH and loaded on TiO2.
Fig. 2 Cyclic voltammogram of SQIND-1 and SQIND-2.

ACCEPTED MANUSCRIPT
In table 1 the photophysical and electrochemical properties of SQIND-1 and SQIND-2. The
Eoxd of SQIND-1 and SQIND-2 are 0.81 and 0.79 V, respectively which was estimated from the
oxidation potential of dyes molecules in CV graph using question (1). The values of SQIND-1 and
SQIND-2 are converted on an absolute scale of -5.24 and -5.20 eV respectively. The HOMO energy
levels were calculated using the equation (5.1) [37].
onset
onset
where Eox and
E
are the onset oxidation potentials for the squaraine sensitizers and
Fc/Fc+
Fe(C H ) (ferrocene) against the Ag/AgCl reference electrode, while the value -4.8 eV is the HOMO
5
5 2
energy level of ferrocene against vacuum [37].
onset
onset
Fc/Fc+
EHOMO = −(Eox − E
)−4.8(eV)...(1)
While LUMO energies were estimated by using following equation (2).
LUMO = HOMO – E … …… …… … .......(2)
g
In Fig.1 the absorption spectrum of SQIND-2 on TiO shift toward a longer wavelength
2
compared to the SQIND-1. The absorption spectra of SQIND-1 and SQIND-2 sensitizers Fig.1 on
TiO semiconductor electrode are broad which strongly refer to the increased interaction between
2
COOH groups for SQIND-1 and SQIND-2 and TiO . Also, Fig.1, the absorption maxima showed a
2
red-shift because of the removal of a proton between COOH group and TiO semiconductor electrode.
2
The SQIND-1 and SQIND-2 showed fluorescence emission at 680 and 688 nm, respectively in
MeOH (see Fig. 3). The Stokes shifts (ꢁλshift nm) observed for SQIND-1 and SQIND-2 are 39 and 36
nm, respectively. The ꢁλshift, SQIND-1 is relatively higher than ꢁλshift, SQIND-2. This result may
indicate that SQIND-1 sensitizer has a better electronic coupling interaction with the TiO than
2
SQIND-2. The λmax of SQIND-1 shifts from 649 to 688 nm (ꢁλshift, SQIND-1= 39 nm), while the λmax
of SQIND-2 shifts from 642 to 678 nm (ꢁλshift, SQ-4=36 nm). The ꢁλshift, SQIND-1 is one time higher
than ꢁλshift, SQIND-2. This refers to SQIND-1 sensitizer which shows interaction electronic coupling
with the TiO better than SQIND-2.
2
The SQIND-1 sensitizer has alkyl group with a long chain in N-position which decreases the
possibility of aggregation on TiO surface as compared to SQIND-2. In Table 1, the data of
2
experimental reveal that the indoline-based squaraine dyes (SQIND-1 and SQIND-2) have superior
absorption wavelengths (λmax), fluorescence emission spectrum (λ ) and bands gap (E ) compared to
em
g
dye 1 & dye 2. The λmax of SQIND-1 and SQIND-2, are red shifted by 11, 4 nm and 20, 13 nm
compared to dye 1 & dye 2 respectively. SQIND-1 and SQIND-2 are red shifted more than dye1 &
dye 2. The squaraine sensitizer that contains bromo shows a larger bathochromic shift than the non
bromo derivative, this can be attributed to the electron-donating nature of the bromo substituent in
SQIND-1.

ACCEPTED MANUSCRIPT
Fig. 3 The fluorescence emission of SQIND-1 and SQIND-2.
Table. 1 The comparative study of photophysical and electrochemical properties of SQIND-1 and
SQIND-2 with squaraine dyes (dye1 & dye2).
a
a
a
b
c
λ max
Ɛ
λ max on
TiO2
λem
Eg
Eoxd
(V
HOMO LUM
max
-
1
-1
Dye
(nm) (M cm )
(nm)
(eV)
(eV)
O
(nm)
vs.NHE)
(eV)
SQIND-1
SQIND-2
649
184000
659
688
1.49
0.81
-5.24
-3.75
642
638
219000
155000
644
-
678
648
1.53
2.02
0.79
0.91
-5.20
-5
-3.67
-2.98
d
Dye 1
d
Dye 2
629
287000
-
643
2.18
0.72
-4.98
-2.80
a
c
b
Measured in MeOH solution. The band gap was determined from the intersection of the emission spectra and absorption.
The Eox was measured on 0.1 M TBAPF in DMSO (counter electrode (Pt), reference electrode (Ag/AgCl), working
6
+
d
electrode (Pt)) and calibrated with ferrocene/ ferrocenium (Fc/Fc ). [27].

ACCEPTED MANUSCRIPT
3
.2 Frontier molecular orbitals calculations
We optimized the ground state geometries of the sensitizers SQIND-1 and SQIND-2 at
B3LYP/3–21 G (d, p) [38] level theories in the vacuum using DFT method [39]. Also, we performed
the frequency calculation at the same level of theory to confirm the stable state for the sensitizers. The
computed energies of HOMO (EHOMOs), LUMO (ELUMOs), and LUMOs+1 (ELUMOs+1) and energy gaps
(Eg), oscillator strengths, and absorption maxima wavelengths of SQIND-1 and SQIND-2 in the
excited state were optimized at the TD-B3LYP/3-21G (d, p) level of theories with a polarizable
continuum model (PCM) [40] in methanol solvent [41]). In the charge density distribution patterns of
the ground state highest occupied molecular orbitals (HOMOs and HOMOs-1), lowest unoccupied
molecular orbitals (LUMOs and LUMOs+1) of SQIND-1 and SQIND-2 indole-based squaraine dyes
are illustrated above in Fig 4 and summarized in table 2. In SQIND-2 sensitizer, the HOMOs-1 is
distributed on indoline moieties and center of squaric acid while it is distributed on indoline moieties
in SQIND-1. The charge of HOMO-LUMO is distributed on the whole structure of the molecule in
the squaraine dyes (SQIND-1 and SQIND-2) [42]. The charge of LUMO+1 is distributed on center of
squaric acid in both of SQIND-1 and SQIND-2. The charge densities of HOMOs are localized on
squaraine moieties. The charge density distribution on LUMO is expected on SQIND-1 to be more
stable after anchoring with the TiO surface. Furthermore, bis-COOH groups would be a favorable
2
site to transfer electrons from dyes to the TiO surface [24, 26, 35, 43]. The electron distributions for
2
HOMO-1, HOMO-LUMO and LUMO+1 are shown in Fig 4.
Table 2. Calculated highest occupied molecular orbital energies (EHOMO), lowest unoccupied
molecular orbital energies (ELUMO), EHOMO-1, EHOMO+1, energy gaps (E ), oscillator strengths, and
g
absorption maxima wavelengths of SQIND-1 and SQIND-2 in methanol solution at B3LYP/3-21G(d,
p) and TD-B3LYP/3-21G (d, p) level .
EHOMO ELUMO EHOMO-1
EHOMO+1
(eV)
Eg
(eV)
λmax
(nm)
b
Compounds
a
f
Transition
(eV)
(eV)
(eV)
SQIND-1
SQIND-2
-4.61 -2.56
-4.54 -2.37
-5.59
-6.33
-0.168
-0.043
2.05
2.17
591
582
1.721
1.634
H- > L
H- > L
a
Experimental λa in methanol = 649 and 642 nm for SQIND-1 & SQIND-2 respectively.
b
Oscillator strengths
Because the electron density distribution of SQIND-1 HOMO is delocalized on the entire
molecular skeleton. Also, lower band gap and higher hole mobility can be expected in the SQIND-1
as compared to the SQIND-2. Therefore, deduced from the HOMO energy level and the
corresponding band gap, the LUMO energy level of SQIND-1 is computed to be −2.56 eV. This value
is 0.19 eV lower than that of SQIND-2 (−2.37 eV), leading to a lower band gap (see Table 2). Thus, in
the SQIND-1 sensitizer, lower energy loss is expected during the electron transfer process from the

ACCEPTED MANUSCRIPT
-
SQIND-1 to Br. The resulting band gap for SQIND-1 is 2.05 eV, which is 0.12 eV lower than that of
SQIND-2 (2.17 eV), enabling SQIND-1 to harvest more sunlight. The calculated λa of SQIND-1 upon
the transition from H ->L was observed to be 591 nm, which is in good agreement with the
experimentally measured value of 649 nm. The computed λa of SQIND-1 is red shifted by 9 nm
compared to SQIND-2.
SQIND-1
SQIND-2
HOMO-1
HOMO
LUMO
LUMO+1
Fig. 4 The charge density distribution of the frontier molecular orbitals (0.02 contour value) of indoline-based SQIND-1
and SQIND-2 at B3LYP/3-21(d, p) level of theory.
3
.3 Photovoltaic characterized of the SQIND-1 and SQIND-2
The photocurrent short-circuit density-photovoltage (J-V) characteristics were tested for
-
-
SQIND-1 and SQIND-2 of DSSC having I /I as redox electrolyte under illumination with AM 1.5 G
3
solar is showed in Fig 5 summarized in Table 3. In Table 3 lists the J-V characteristics with various
concentrations of chenodeoxycholic acid (CDCA) on each solar cell based on SQIND-1 and SQIND-
2
. The SQIND-1 showed better photovoltaic performance than SQIND-2. The sensitizer SQIND-1
2
exhibits JSC at 3.77 mA/cm , V at 0.48 V, FF at 0.68 and η at 2.56. The SQIND-2 sensitizer gives
OC
2
JSC of 3.57 mA/cm , V of 0.48, FF of 0.73, and η of 2.61. The low conversion efficiency in DSSC
OC
refer to charge π-aggregates on squaraine sensitizers, causing charge recombination on TiO electrode
2
leading to reduced cell efficiency [25]. So, a certain amount of chenodeoxycholic acid (CDCA)
enhances the dye sensitized solar cell reducing the π-aggregates of the squaraine sensitizers, thus
increasing the process of electron injection to the surface of titanium dioxide and increasing cell
efficiency [23, 44, 45].

ACCEPTED MANUSCRIPT
Table. 3 The comparative study photo-electrochemical characteristics at different concentrations (0,
, 6, and 9 mM) of chenodeoxycholic acid (CDCA) for each cell based on SQIND-1 and SQIND-2
3
2
under the illumination AM 1.5 (100 mW/ cm ) with dye1 & dye2.
Jsc
Voc
FF
η
DSSC
2
(
mA/cm )
(V)
(%)
(%)
SQIND-1 CDCA 0 mM
SQIND-1 CDCA 3 mM
SQIND-1 CDCA 6 mM
SQIND-1 CDCA 9 mM
SQIND-2 CDCA 0 mM
SQIND-2 CDCA 3 mM
SQIND-2 CDCA 6 mM
SQIND-2 CDCA 9 mM
3.82
0.48
0.48
0.53
0.53
0.50
0.48
0.51
0.48
0.27
0.03
0.67
0.74
0.69
0.71
0.72
0.77
0.78
0.77
0.47
0.10
1.24
2.1
5.88
6.41
2.32
2.46
1.27
1.66
1.87
1.94
1.03
0.008
6.62
3.56
4.51
4.48
5.21
a
Dye 1
0.62
a
Dye 2
0.03
a
Details can found [29
]
2
Table. 3 shows the values of J =6.62 mA/ cm , V =0.53 V, FF=0.71, η=2.46% and J =5.21
sc
oc
sc
2
mA/ cm , V =0.48 V, FF =0.77, η =1.94% respectively, for squaraine sensitizers (SQIND-1 and
oc
2
SQIND-2) which are superior to squarylium dyes (dye1 & dye 2), J =0.62 mA/ cm , V =0.27 V, FF
sc
oc
2
=
0.47, η =1.03% and J = 0.03 mA/ cm , V =0.03V, FF =0.10, η=0.008 % respectively revealing
sc oc
that the SQIND-1 and SQIND-2 sensitizers study of in the present article would be efficient squaraine
sensitizers for DSSC and improved electron injection to TiO [29].
2

ACCEPTED MANUSCRIPT
Fig. 5 Photocurrent density-voltage (J-V) characteristics with different concentrations (0, 3, 6, and 9 mM) of CDCA for
-
2
SQIND-1 and SQIND-2 under AM 1.5 G illumination (100 mW cm ) on each cell for examination.
The Eq. (3) was used to measure the IPCE from the measurements.
µ
A
1
240 × Jsc (
)
2
cm
Wavelength(nm ) × Pin (
IPCE (% ) =
........(3)
watt
m
)
2
So, the IPCE of SQIND-1 and SQIND-2 based DSSC are 34% and 27% respectively.
The addition of CDCA with increased concentrations up to 9 mM lead to increase values of J , V ,
SC
OC
and cell efficiency for both SQIND-1 and SQIND-2 showed the highest conversion efficiency than
SQIND-2, (see table. 3), because SQIND-1 sensitizer has alkyl group with a long chain in N-position
of indoline which hinders the recombination of charge on TiO surface compared to SQIND-2, also,
2
cause stronger electronic coupling between SQIND-1 and TiO . Thus, the increase of the injection of
2
photogenerated electrons in SQIND-1 into CB of nanoparticle TiO with high charge transfer results
2
from strong electronic coupling between SQIND-1 and TiO2.
In Fig. 6 show the IPCE spectra for SQIND-1 and SQIND-2 at various concentrations of
CDCA. The IPCE spectra for SQIND-1 exhibited better performance than SQIND-2. The SQIND-1
and SQIND-2 photoanodes showed a poor photocurrent without adding CDCA because of the large
amount of charge losses caused by charge recombination on the π-aggregated squaraine sensitizers.
The additions of CDCA acts to enhance the photocurrent generation significantly in DSSC based on
SQIND-1 and SQIND-2. In this regard, the DSSC based SQIND-1 sensitized solar cell generate larger
photocurrents than the SQIND-2 sensitized ones through the whole visible wavelength [43, 45–48].
In SQIND-1 sensitizer, the enhancement is due to the high charge transfer rate in the
interfacial region which is verified by J-V characterization and EIS [49]. The strong electronic
coupling between squaraine sensitizers (SQIND-1 and SQIND-2) and TiO leads to a marked
2
improvement in photovoltaic performance under solar radiation.

ACCEPTED MANUSCRIPT
Fig. 6 The IPCE spectra for DSSC based on SQIND-1 and SQIND-2.
Fig. 7 Depicts the Nyquist plots of DSSC based on SQIND-1 and SQIND-2 measured under
2
AM 1.5 G illuminations (100 mW/cm ). The SQIND-1 and SQIND-2 showed that the major
semicircle in the electrochemical impedance spectral (EIS) Nyquist plot corresponds to the resistance
of electron transport at the TiO /dye/ electrolyte interface [47, 50]. The larger the semicircle slower
2
the recombination kinetics. It is clear from the Nyquist plot that SQIND-1 even with hexyl group on
N-position showed a lesser recombination resistance because of the lower driving force from the
electron injection. The smaller semicircle diameters on the right are attributed to the diffusion of
electrolytes which do not differ significantly between SQIND-1 and SQIND-2.The radius larger half a
circle on the left, which contain R1 from the charge transfer at the electrode and R2 from the electron
transfer in the TiO /dye/ electrolyte interface, SQIND-2 less than the SQIND-1. This result indicates
2
that electron generation and transport in DSSC based SQ-sensitizers are similar but SQIND-2 shows
slightly less than the SQIND-2 sensitizer.

ACCEPTED MANUSCRIPT
-2.5
-2.0
-1.5
-1.0
-0.5
SQIND-1
SQIND-2
0.0
0
1
2
3
4
5
Z'(ohm)
Fig. 7 Nyquist plots for SQIND-1 and SQIND-2 based DSSC under AM 1.5 (100 mW/ cm ).
2
4
Conclusions
Synthesis, characterization, and photovoltaic properties of two new symmetric squaraine
sensitizers (SQIND-1 and SQIND-2) carrying based indoline that have pendant bis-COOH as anchor
groups to injected the electron into nanoparticles TiO were test in DSSC. The theoretical calculations
2
and absorbance results showed that the electron density of LUMO of SQIND-1 is delocalized in the
whole carboxylic group resulting strong electronic interaction between SQIND-1 sensitizer and
conduction band of TiO . In addition, the relatively longer alkyl chain present in SQIND-1 would
2
lower the aggregation of the dye on TiO surface compared to SQIND-2. Dye sensitized based solar
2
cells have been fabricated in SQIND-1 & SQIND-2 were efficiently sensitized on porous TiO with
2
the long wavelength of the spectrum and showed higher noteworthy performance properties, such as
2
energy conversion efficiency (η) of 6.2 %, a J of 6.64 mA/cm , a V of 0.53, a ff of 0.72 and an
sc
oc
IPCE of 45% at 668 nm.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge Department of Chemistry, UGC SAP DRS II and DST FIST for
financial assistance and Sophisticated Analytical Instrumentation Facility (SAIF) Chandigarh.
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Two new squaraine dyes (SQIND-1 and SQIND-2) with bis-pendent.
carboxylic groups for better electron injection into TiO were synthesized.
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Both theoretical calculations experimental data indicated the suitability of
the HOMO and LUMO levels of the dyes as DSSC.
Compared with SQIND-2, SQIND-1 sensitizer exhibited better
photovoltaic performance owing to its red-shift absorption and the de-
aggregation effect of its long alkyl chain.