The effect of bis-carboxylic groups of squarylium dyes on the efficiency of dye-sensitized solar cells

Article abstract of DOI:10.1007/s11696-019-00978-5

Symmetric squarylium dye (SQIND1) with bi-carboxylic groups has been synthesized and photoelectrochemical properties have been studied in comparison with its non-functionalized analog groups (SQIND2). The experimental results showed that the introduction of anchor carboxylic groups in SQIND1 sensitizer provides a more intimate contact with nanoparticles TiO₂ which increases the number of charge carriers transferred from the SQIND1 to the semiconductor. The theoretical calculations and absorbance results show that the electron density of LUMO of SQIND1 is delocalized in the whole chromophore, leading to strong electronic coupling between SQIND1 sensitizer and conducting band of TiO₂, resulting in improved dye-sensitized solar cell efficiency compared to SQIND2. Hence, the SQIND1 sensitized exhibit better photovoltaic performance. Although, the absence of any linker groups in the SQIND1, then SQIND1 was perfect efficiently sensitized on porous TiO₂ with the long UV–Vis and NIR region up to 800?nm of the spectrum and showed higher remarkable performance of values, such as η of 3.3%, a J_sc of 7.6?mA/cm², a V_oc of 0.59, and FF of 0.73.

Full text of DOI:10.1007/s11696-019-00978-5

Chemical Papers
https://doi.org/10.1007/s11696-019-00978-5
ORIGINAL PAPER
The efect of bis‑carboxylic groups of squarylium dyes
on the efciency of dye‑sensitized solar cells
Sultan A. Al‑horaibi1,2 · Eman M. Garoon3 · Narendra A. Bhise1 · Suresh T. Gaikwad1 · Anjali S. Rajbhoj1
Received: 21 May 2019 / Accepted: 23 October 2019
© Institute of Chemistry, Slovak Academy of Sciences 2019
Abstract
Symmetric squarylium dye (SQIND1) with bi-carboxylic groups has been synthesized and photoelectrochemical properties
have been studied in comparison with its non-functionalized analog groups (SQIND2). The experimental results showed that
the introduction of anchor carboxylic groups in SQIND1 sensitizer provides a more intimate contact with nanoparticles TiO2
which increases the number of charge carriers transferred from the SQIND1 to the semiconductor. The theoretical calcula-
tions and absorbance results show that the electron density of LUMO of SQIND1 is delocalized in the whole chromophore,
leading to strong electronic coupling between SQIND1 sensitizer and conducting band of TiO2, resulting in improved dye-
sensitized solar cell efciency compared to SQIND2. Hence, the SQIND1 sensitized exhibit better photovoltaic performance.
Although, the absence of any linker groups in the SQIND1, then SQIND1 was perfect efciently sensitized on porous TiO2
with the long UV–Vis and NIR region up to 800 nm of the spectrum and showed higher remarkable performance of values,
such as η of 3.3%, a Jsc of 7.6 mA/cm2, a Voc of 0.59, and FF of 0.73.
Keywords Dye-sensitized solar cell · Symmetric squarylium dyes · Cyclic voltammetry · DFT · Photoelectrochemical
properties
Introduction
(Sawin and Janet 2013; Grätzel 2004). DSSC are one of the
alternative energy sources that have attracted widespread
interest by academic and economists researchers for the
conversion sunlight into electricity with high efciency and
low cost (Mishra et al. 2009; Wang et al. 2012). Excellent
photo-conversion efciencies for regenerative dye-sensitized
solar cells based on nonporous titanium dioxide (TiO2) flms
have been reported by squaraine dyes using iodide/triiodide
Renewable energy is increasingly important when talking
about the future. Investing in it represents an economic
and health dimension that is environmentally friendly. The
rate of renewable energy production has seen steady pro-
gress in recent times. In 2010, renewable energy provided
an estimated 16% of the world’s total energy consumption,
accounting for about half of the world’s new 194 gigawatts
−
(I−/I3 ) redox couple as the electrolyte (Mathew et al. 2014;
Yun et al. 2014a). Squarylium dyes (SQ) have attracted
importance in research for their unique spectral properties,
and they are eco-friendly, low cost, and give high perfor-
mance which results from photovoltaic and electrochemical
stability as well as high molar extinction coefcients in the
visible region (Gao et al. 2008; Yella et al. 2011).
This work was presented at Exploring New Horizons in Chemical
Sciences 2019, an international symposium held in Aurangabad,
India on 10–12 January, 2019.
* Anjali S. Rajbhoj
anjali.rajbhoj@gmail.com
Previous studies indicated that the absorption bands
of squaraine dyes could be easily tuned to get into NIR
region by structural modifcation, which closely matches
the spectral response of sunlight. In addition, theoretical
calculations on squaraine dyes revealed their suitability for
DSCC, as their LUMO energy lie above the LUMO energy
of TiO2 conduction band and thus ease electron injection
from the dye to TiO2 (Al-horaibi et al. 2018a; Al-horaibi
1
Department of Chemistry, Dr. Babasaheb Ambedkar
Marathwada University, Aurangabad, Maharashtra 431004,
India
2
Department of Chemistry, Al-Baydha University, Al-Baydha,
Yemen
3
Department of Environmental Science, Dr. Babasaheb
Ambedkar Marathwada University, Aurangabad,
Maharashtra 431004, India
Vol.:(0123456789)
1 3

Chemical Papers
et al. 2018b; Asiri et al. 2015a; Al-horaibi et al. 2018c;
Asiri et al. 2015b; Al-horaibi et al. 2019a; Al-horaibi et al.
2019b).
Synthesis and characterization
Synthesis of 2, 3, 3‑trimethyl‑3H‑indole (1)
The aim of this study is to fabricate and improve an ef-
cient photovoltaic system capable of absorption in the long
wavelength of the spectrum for squarylium dye (SQIND1)
which acts as photosensitizer for DSSC (Kakiage et al.
2014; Yum et al. 2007; Shivashimpi et al. 2012; Pandey
et al. 2013).
In a round-bottom fask, phenylhydrazine (21.6 g, 200 mmol)
was taken dropwise isopropylmethyl ketone (17.30 g,
200 mmol) and added, and then, the solution was heated
with stirring at 90 °C for 12 h cooled to room temperature
and H2O (160 ml) was added extracted by Et2O (3×60 ml)
and it was dried over Na2SO4 for 30 min, and fltered and
washed by Et2O to get red liquid (97%) product. The prod-
uct liquid was dissolved in acetic acid (68 ml) and then the
mixture was heated with stirring at 90 °C for 2 h. After
cooling to room temperature, the mixture was neutralized
with saturated solution of Na2CO3 and extracted by ethyl
acetate (3×50 ml), dried over Na2SO4 for 1 h, and fltered
and washed with ethyl acetate. The solvent was removed
and the product was collected to give compound (1) as yel-
low oil in 95% yield. 1HNMR(400 MHz-CDCl3) spectrum
recorded at δ (ppm) 1.27 (s, 6H, 2CH3), 2.26 (s, 3H, CH3),
7.16 (t, 1H, J=0.88 Hz, Ar–H), 7.27 (d t, 2H, J=1.24 Hz,
Ar–H), 7.54 (d, 1H, J=7.6 Hz, Ar–H). 13C-NMR (CDCl3)
15.25 (CH3), 22.94 (2CH3), 53.45 (C3), 119.70, 121.19, 125,
127.46 (carbon phenyl aromatic), 145. 47, 153.41 (C7, C8
respectively), and 187.95 (C2).
Two symmetrical squarylium dyes were synthesized,
characterized, and studied the optical properties of new
SQIND1 sensitizer with bis-anchoring groups in com-
parison with the non-functionalized analog dye SQIND2.
The HOMOs and LUMOs of SQIND1 and SQIND2 were
calculated by Gaussian 09 package correlated with experi-
mental values for understanding the ground, excited state
electron density distribution, and band gap (Irfan et al.
2009; Torruellas et al.1990; Venkataraman et al.2006; Yen
et al. 2012; Al-Sehemi et al. 2012a; Irfan et al.2012; Al-
Sehemi et al. 2012b).
Experimental
Materials
Synthesis of 2, 3, 3‑trimethyl‑3H‑indole‑5‑carboxylic acid
(2)
4-Hydrazinobenzoic acid (97%), phenyl hydrazine
(97%), squaric acid (98%), sodium acetate anhydrous
(CH3COONa), acetic acid, sodium carbonate anhydrous
(Na2CO3), 3-methyl butan-2-one, chloroform, sodium
sulfate anhydrous, iodoethane, ethyl acetate, toluene,
n-butanol, diethyl ether, acetonitrile, methanol, ethanol,
petroleum ether 60/80, and silica gel 60–120 mesh for col-
umn chromatography from Sigma-Aldrich.
In a round-bottom fask, 4-hydrazinobenzoic acid hydro-
chloride (7.55 g, 0.04 mmol), 3-methylbutan-2-one (5.17 g,
0.06 mmol), sodium acetate anhydrous (6.56 g, 0.08 mmol),
and acetic acid (120 ml) were refuxed under N2 atmos-
phere for 25 h. It was then cooled to room temperature.
The solvent was removed and the crude residue was dis-
solved in MeOH/H2O (10:90 v/v), and fltered and washed
with distilled water to get compound (2) as orange solid in
64% yield. 1HNMR (600 MHz-CDCl3) spectrum recorded
at δ (ppm) 1.25(s, 6H, J = 19.8 Hz, 2CH3), 2.25(s, 3H,
CH3), 7.5(d, 1H, J=7.8 Hz, Ar–H), 7.90(dt,1H, J=1.8 Hz,
Ar–H), 7.98(d, 1H, J=1.2 Hz, Ar–H). 13C-NMR (CDCl3)
spectrum recorded at 15.4 (CH3), 22.3 (2 CH3), 53 (C3),
119.2–129.7(phenyl carbons), 146.18 (C9), 157.4 (C8), 167.6
(N=C), 191.84 (C=O).
Instrumental analysis
1
All HNMR spectra were recorded on Bruker AVANCE
III WM 400 and 600 spectrometers using DMSO-d6 and
CDCl3. The UV–Vis spectra were on a Perkin Elmer
Lambda UV-1800 spectroscopy in dimethyl sulfox-
ide (DMSO). The oxidation/reduction properties of the
SQIND1 and SQIND2 were examined by cyclic voltam-
metry (CV) on a Metrohm Autolab PGSTAT 128N in dried
dimethyl sulfoxide with three-electrode systems consist-
ing of a saturated Ag/AgCl reference electrode, a plati-
num wire auxiliary electrode, and Pt electrode as working
electrode. The scan rate is 100 mV/s. The optimization
geometries for SQIND1 and SQIND2 were calculated
at B3LYP/6-31(d, p) level using Gaussian 09 program
package.
Synthesis of 1‑ethyl‑2, 3, 3‑trimethyl‑3H‑indol‑1‑ium iodide
(3)
In a round-bottom flask 2, 3, 3-trimethyl-3H-indole
(15 g, 0.094 mmol), 1-butanol (25 ml), and 1-iodoethane
(36.65 g, 0.235 mmol) were refluxed at 82 °C for 18 h.
1 3

Chemical Papers
1-iodoethane (5 g, 0.032 mmol) was added to the mixture
and refluxed for 16 h. It was then cooled to room tem-
perature and diethyl ether (150 ml) was added with stir-
ring. The product precipitate was filtered and dissolved
in boiling 70 ml CH3Cl. 100 ml of acetone was added to
the mixture and the deposition was started. It was filtered
and washed with acetone to give compound (3) in 81%
Synthesis of SQIND1
A mixture of 5-carboxy-1-ethyl-2, 3, 3-trimethyl-3H-indol-
1-ium iodide (1.5 g, 0.0042 mmol) and 3,4-dihydroxycy-
clobut-3-ene-1, 2-dione (0.24 g, 0.0021 mmol) was taken,
40 ml of dry n-butanol/toluene 1:1 (v/v) was added, and
reaction mixture was refuxed for 13 h using Dean–Stark
trap for azeotropic removal of water. It was cooled to rt
and the solvent was removed. The product was fltered and
washed with hexane. The product was purifed by silica gel
60–120 mesh column chromatography with ethyl acetate and
methanol (10:1, 10:2, 10:3, 10:4, and 10:5 v/v, respectively).
Recrystallization of the dye was due in mixture of metha-
nol and hexane to give SQIND1 as blue dye solid in 46.5%
yield (see Scheme 1). 1HNMR (400 MHz–DMSO-d6) spec-
trum recorded at δ (ppm) 1.37 (s, 6H, 2CH3), 1.47(s, 6H,
2CH3), 2.31(s, 2H, N+–CH2), 4.06 (s, 2H,–CH2–N), 5.95(s,
2H,–CH=C), 6.95–7.8(m, 8H, Ar–H), 11.8(OH). 13C-NMR
(DMSO-d6) 14.73, 19.15, 24.67, 44.75, 49.59, 65.36, 88.16,
111.58, 117.64, 125.88, 131.66, 141.55, 169.65 and 175.47.
1
yield. HNMR(400 MHz-DMSO-d6) spectrum recorded
at δ (ppm) 1.48 (t, 3H, –CH2 CH3, J = 7.36 Hz), 1.56(s, 6
H, 2CH3), 2.86(s, 3H, –CH3), 4.53 (q, 2 H, J = 7.36 Hz,
–N–CH2–), 7.62 (t, d, 2H, J = 3.22 Hz, Ar–H), 7.83(t, 1H,
J = 3.2 Hz, Ar–H), 7.97(d, 1H, J = 3.84 Hz,Ar–H).13C-
NMR(DMSO-d6) 12.70(CH3), 13.89 (–CH2 CH3), 21.93(2
CH3), 43.12(C3), 54.09(N–CH2), 115.23, 123.45, 128.90,
129.35 (carbon phenyl aromatic), 140.65, 141.81 (C7, C8
respectively), 195.93 (C2).
Synthesis of 5‑carboxy‑1‑ethyl‑2, 3,
3‑trimethyl‑3H‑indol‑1‑ium iodide (4)
A mixture of 2, 3, 3-trimethyl-3H-indole-5 carbox-
ylic acid (3 g, 0.015 mmol) and 1-iodoethane (5.76 g,
0.037 mmol) was refluxed for 19 h under N2 atmosphere
in CH3Cl (20 ml). The output mixture was cooled to
room temperature and the solvent was removed. Diethyl
ether (100 ml) was added then, filtered, and washed with
diethyl ether to give compound (4) as an orange solid
in 78% yield.1HNMR (600 MHz-DMSO-d6) spectrum
recorded at δ (ppm) 1.6(t, 3H, J = 7.2 Hz, CH3), 1.65(s,
6H, 2CH3), 2.93(s, 3H, CH3), 4.60(q, 2H, J = 7.2 Hz,
CH2), 7.83(s, 1H, Ar–H), 7.95(d, 1H, J = 8.4 Hz, Ar–H),
8.28(d, 1H, J = 8.4 Hz, Ar–H), 8.32(s, 1H, Ar–H). 13C-
NMR (DMSO-d6) spectrum recorded at 12.9, 14.6(CH3),
22.7 (2CH3), 43.9(C3), 54.6(CH2–N), 116.7, 124.5, 130.9,
132.7, 141.7(carbons phenyl), 143.4(C–N), 166.4 (C=O),
198.3 (C = N).
Synthesis of SQIND2
A mixture of compounds 1-ethyl-2, 3, 3-trimethyl-3H-indol-
1-ium iodide (1.7 g, 0.0 054 mmol) and 3,4-dihydroxycy-
clobut-3-ene-1,2-dione (0.31 g, 0.0027 mmol) was taken
40 ml of dry n-butanol/toluene 1:1 (v/v). The reaction
was refuxed for 10 h using Dean–Stark trap for azeotropic
removal of water. It was cooled to rt and the solvent was
removed. The product was fltered and washed with hex-
ane. The dye was recrystallized with methanol and hex-
ane to give SQIND2 as green dye solid in 55% yield (see
Scheme 1). 1HNMR (400 MHz-CDCl3) spectrum recorded
at δ 1.20(t, 3H, J=6.6 Hz, CH3), 1.35(t, 3H, J=7.2 Hz,CH3),
1.78(s, 6H, 2CH3), 3.45(q, 2H, J=7.2 Hz, CH2), 5.9(s, 1H,
C=CH), 7.5(s, 1H, C=CH), 7.31–7.39(td, 1H, J=7.2 Hz,
Ar–H), 7.93(t, 1H, J = 7.8 Hz, Ar–H), 8.05(dd, 1H,
Scheme 1 Synthesis of squarylium sensitizers (SQIND1 and SQIND2)
1 3

Chemical Papers
−
J=5.4 Hz, Ar–H), 8.12(t, 1H, J=7.2 Hz, Ar–H), 8.2(d, 1H,
J = 8.4 Hz, Ar–H), 8.65(d, 1H, J= 8.4 Hz, Ar–H), 8.92(d,
1H, J=8.4 Hz, Ar–H), 9.3(d, 1H, J=4.8 Hz, Ar–H). 13C-
NMR (MHz-CDCl3) spectrum recorded at 12.09, 27.03,
38.46, 49.37, 86.30, 109.16, 122.42, 123.76, 127.86, 142.09,
142.36, 169.72.
(I−/I3 ) electrolyte was prepared as follows: a mixture of
iodine (0.03 M), lithium iodide (0.02 M), 1-butyl-3-meth-
ylimidazolium iodide (0.015 M), and 4-tertbutylpyridine
(0.5 M) was taken in acetonitrile. The photovoltaic perfor-
mance of the fabricated SQIND1 and SQIND2 based DSSC
was measured under sunlight (AM 1.5, 100 mW/cm2) solar
simulator (Dayton, Instruments, USA) with a source meter
(Keithley 2400) at 25 °C.
In scheme 1, dicarboxylic-substituted squaraine sensitizer
SQIND1 was synthesized in three steps from 4-hydrazinyl
benzoic acid with 3-methyl butan-2-one to give compound
(2) which was subsequently converted to heterocyclic qua-
ternary salt (4); the subsequent condensation of two equiva-
lents of compound (4) and one equivalent of squaric acid
under reflux to give symmetrical squarylium sensitizer
SQIND1 in 46.5% yield. The SQIND2 was synthesized by
similar method to SQIND1 as depicted in scheme 1. In the
frst step, phenyl hydrazine was treated with 3-methyl butan-
2-one to aford compound (1) which was subsequently con-
verted to heterocyclic quaternary salt (3) and condensation
of two equivalent of compound (3) with one equivalent of
squaric acid under refux to aford the symmetrical squary-
lium sensitizer SQIND2 in 55% yield.
Results and discussion
Synthesis and spectro‑electrochemical properties
of the SQIND‑1 and SQIND‑2 dyes
The SQIND1 sensitizer-based indoline have two anchoring
groups to facilitate electron injection to the conduction band
(CB) of porous TiO2. Fig 1 shows the normalized UV–Vis
absorption spectra of SQIND1 and SQIND2 dissolved in
dimethyl sulfoxide. The bands maxima of SQIND1 and
SQIND2 were observed at 664 and 652 nm, respectively, and
their corresponding high molar extinction coefcient values
1.53× 105 and 1.49× 105 M−1cm−1 are in agreement with
π–π* transitions which could be observed in the SQIND1
and SQIND2. Figure 3 shows that the normalized UV–Vis
absorption spectra on TiO2 along with the emission and
electrochemical for SQIND1 and SQIND2 are summarized
in Table 1.
Preparation, fabrication, and measurements
of DSSC
A mixture of TiO2 powder≤25 nm (3 g), nitric acid aqueous
solution 0.5 ml in dimethyl sulfoxide 3 ml, and deionized
water 2 ml was taken, and reagent of Triton X–100 (2 ml)
was added. The TiO2 paste was stirred by a rod glass for
10 min and then left it for 5 min and stored in a closed bot-
tle for later use. The conducting glasses’ substrate indium
tin-oxide glass slide, square (FTO glasses 8 Ω/sq and area
about 2 cm2) were cleaned in a detergent solution for 10 min
by sonication and with acetone for 10 min, then with deion-
ized water for 10 min, and fnally with ethanol for 10 min.
The conducting glass was dried by air and then in oven at
80 °C for 10 min. The glass square (~8 Ω) was measured by
a multi-meter set to examine the conductive side, and then,
strips of Scotch Magic tape on were put on the edges of the
glass square by the doctor-blade technique. The TiO2 paste
was deposited in the middle of the glass square. The con-
ducting glass TiO2 porous flm was leaved in the air for 5 min
and then placed on hotplate for 30 min at 70 °C. The fnal
step, it was placed in furnace for 1 h at 450 °C and immersed
in squarylium dyes solution (SQIND1 and SQIND2) 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, H2PtCl6 0.3 g
were dissolved in deionized water (2 ml) under stirring and
dropwise NaBH4 solution was added to the mixture. The
color of the mixture PVP coated immediately changed from
orange to black, and fnally, the counter electrode PVP was
placed in furnace at 450 °C for 1 h. The iodine/triiodide
In Fig. 2 and Fig. 4, the absorption spectra of SQIND1
and SQIND2 sensitizers on TiO2 semiconductor electrode
are broad which strongly refer to the increased interaction
between COOH groups for SQIND1 sensitizer and nanopo-
rous TiO2. The SQIND1 and SQIND2 showed fuorescence
Fig. 1 The absorption spectrum in DMSO solution for SQIND1 and
SQIND2
1 3

Chemical Papers
emission at 680 and 668 nm in DMSO solution, respectively.
The λmax of SQIND1 shifts from 664 to 680 nm (Δλshift
SQIND1= 16 nm), while the λmax of SQIND2 shifts from
652 to 666 nm (Δλshift, SQIND2 = 14 nm). The Δλshif
,
,
SQIND1 is 2 nm higher than SQIND2. This refers which
SQIND1 sensitizer shows interaction electronic coupling
with the TiO2 better than SQIND2. SQIND1 sensitizer has
two COOH groups, resulting in improved dye-sensitized
solar cell efciency compared to SQIND2. The UV–Vis
spectrum has been used to determine the optical energy
gap (Eg) of SQIND1 and SQIND2 on thin flm (Fig. 3) by
plotting (Tauc’s plot) the graph between (ahv)^(1/n) versus
photon energy (hv).
where a is the optical absorption coefcient, which can
be calculated from absorbance(A), and thickness of the
SQIND1 and SQIND2 (t) using (a = 2.303A/t) and (hv)
can be calculated form wavelength using (hv=1240/wave-
length). The values of Eg (Table 1) were evaluated 1.81 and
Fig. 2 The absorption spectrum of SQIND1 and SQIND2 loaded on
TiO2, respectively
Fig. 3 The Tauc plot for (αE)2 vs photon energy (E) of SQIND1 and SQIND2 on thin flm
Table 1 Experimental of photophysical and electrochemical properties for SQIND1 and SQIND2
Dye
λamax (nm)
Ɛamax (M−1 cm−1
)
λmax on
λaem (nm)
Eb0–0 (eV)
Ecoxd (NHE)
HOMO (eV)
LUMO (eV)
Edg (eV)
TiO2 (nm)
SQIND1
SQIND2
664
652
1.53×105
1.49×105
666
655
680
666
2.15
2.2
0.86
0.84
−6.03
−6.01
−3.88
−3.81
1.81
1.88
aMeasured in DMSO solution
bThe E0–0 was determined from CV in the DMSO solution
cThe Eoxd was measured on 0.1 M TBAPF6 in DMSO [counter electrode (Pt), reference electrode (Ag/AgCl), working electrode (Pt)], and cali-
brated with ferrocene/ferrocenium (Fc/Fc+)
dThe optical band gap (Eg) was determined from Tauc’s plot between (ahv)^(1/n) versus photon energy (hv)
1 3

Chemical Papers
sensitizers and ferrocene/ferrocenium (Fc/Fc+) as internal
standard at room temperature. The potential scan was taken
from − 0.25 to 1.0 V at scan rate 100 mV/s (see Fig. 5).
We have also estimated the energies of frontier orbitals
(HOMO–LUMO) in SQIND1 and SQIND2 sensitizers to
justify its candidatures as sensitizers in DSSC (Ooyama and
Harima 2009).
Table 1 shows the experimental of photophysical and
electrochemical properties of SQIND1 and SQIND2. The
Eoxd of SQIND1 and SQIND2 was 0.86 and 0.84 V, respec-
tively, which was estimated from the oxidation potential of
dyes molecules in CV graph using Eq. (1) (Wei et al. 2012):
EHOMO = −(Eooxnset − EFoncs∕eFtc+) − 4.8(eV).
(1)
The values of SQIND1 and SQIND2 are converted on an
absolute scale of −5.66 and −5.64 eV, respectively, while
LUMO energies were estimated using Eq. (2):
Fig. 4 The fuorescence spectrum of SQIND1 and SQIND2 sensitiz-
ers in DMSO solution
LUMO = HOMO−Eg.
(2)
1.88 eV, respectively, from the intersection of the linear part
of the above extrapolation, assuming that α2 = 0 (Fig. 3)
(Mardare and Rusu 2002; Valencia et al. 2009.
Geometries
To investigate the electrochemical properties of squar-
aine sensitizers, cyclic voltammetry (CV) measurement
was performed on a Metrohm Autolab PGSTAT 128N with
three-electrode systems consists of a saturated Ag/AgCl ref-
erence electrode, a platinum wire auxiliary electrode, and Pt
electrode as working electrode (Fig. 5). Cyclic voltammetry
measurement was performed in DMSO solution using 0.1 M
tetrabutylammonium hexafuorophosphate (TBAPF6) as sup-
porting electrode with 0.001 M of SQIND1 and SQIND2
We have presented geometrical parameters of two indoline-
based SQIND1 and SQIND2 sensitizers in Table 2. The
C1–C2, C2–C3, C4–N1, C7–O2, and C8–O1 bond lengths
lengthened, while C1–N1, C3–C4 shortened compared to
SQIND2. In SQIND2, the C4–C5–C6 bonds angles short-
ened, while C1–N1–C4, C2–C3–C4, C6–C7–O2, and C6–C8–O1
lengthened compared to SQIND2 sensitizer. When –COOH
group is substituted at –R position, then C1–N1 and C3–C4
usually shortened, while C4–N1, C2–C3, C1–C2, C7–O2, and
C8–O1 bond distances lengthened than SQIND2. It is might
be due to the moderate electron withdrawing groups which
are in the same side which infuence geometry more than
isomer without carboxylic groups. Thus, in geometric form,
variation in SQIND1 sensitizer is smaller than the SQIND2.
The electron charge density pull/donating by electron pull/
Table 2 The geometrical parameters, bond lengths (Å), and bond
angles (°) of indole-based SQIND1 and SQIND2 sensitizers opti-
mized at B3LYP/6-311G (d, p) level of theory
SQIND1 SQIND2
Bond length
SQIND1 SQIND2
Bond angles
C1–C2 1.385
1.383
1.483
1.541
1.537
1.379
1.234
1.230
C1–N1–C4 111.99
111.98
101.44
131.89
137.41
132.70
C1–N1 1.476
C2–C3 1.542
C3–C4 1.536
C4–N1 1.380
C7–O2 1.235
C8–O1 1.231
C2–C3–C4 101.45
C4–C5–C6 131.78
C6–C7–O2 137.46
C6–C8–O1 132.75
Fig. 5 Cyclic voltammograms of SQIND1 and SQIND2 in DMSO
solution at scan rate 100 mV/s
1 3

Chemical Papers
electron donating groups results in shortening/lengthening
of bond lengths which are in good agreement with the pre-
vious studies (Asiri et al. 2015a; Al-horaibi et al. 2018a).
et al. 2005; Qin et al. 2014). In the charge density dis-
tribution patterns of the ground state highest occupied
molecular orbitals (HOMOs and HOMOs-1), lowest
unoccupied molecular orbitals (LUMOs and LUMOs + 1)
of SQIND1 and SQIND2 indole-based squarylium dyes
which are illustrated in Fig. 6 (Kalyanasundaram 1982).
The HOMO-1 is distributed on the indoline moieties in
SQIND1 and SQIND2. The charge density of HOMO is
distributed on center of squaric acid and only on carbox-
ylic groups of indoline moieties in SQIND1 compares
SQIND2. The charge densities of HOMOs are localized
on squarylium moieties. The energies of HOMO (EHOMOs),
LUMO (ELUMOs), and LUMOs + 1 (ELUMOs+1), and energy
gaps (Eg) of SQIND1 and SQIND2 indoline-based squar-
ylium dyes are computed at B3LYP/6-31(d, p) level of
theory. In Fig. 6, the charge density distribution on LUMO
is expected on SQIND1 to be more stable after anchoring
with the TiO2 surface. Furthermore, –COOH would be a
Frontier molecular orbital calculations
We optimized the ground-state geometries of the sen-
sitizers SQIND1 and SQIND2 at B3LYP/6-31G (d, p),
PBEBE/6-31G (d, p), and WB97XD/6-31G (d, p) (Anders-
son and Uvdal 2005) level theories in the vacuum using
DFT method (Becke 1993) (Table 3). Also, we performed
the frequency calculation at the same level of theory to
confrm the stable state for the sensitizers. The computed
energies of HOMO (EHOMOs), LUMO (ELUMOs), and
LUMOs + 1 (ELUMOs+1) and energy gaps (Eg) of SQIND1
and SQIND2 in the excited state were optimized at the
TD- B3LYP/6-31G (d, p) level of theories with a polariz-
able continuum model (PCM) in DMSO solvent (Tomasi
Fig. 6 The charge density distribution of the frontier molecular orbitals (0.02 contour value) of indole-based SQIND1 and SQIND2 at B3LYP/6-
31(d, p) level of theory
Table 3 Calculated highest occupied molecular orbital energies
(EHOMO), lowest unoccupied molecular orbital energies (ELUMO),
EHOMO−1, EHOMO+1, and energy gaps (Eg) of SQIND1 and SQIND2
in DMSO solution at B3LYP/6-31G (d, p), PBEBE/6-31G (d, p), and
WB97XD/6-31G(d, p)
Compounds
SQIND1
Diferent functionals
EHOMO (eV)
ELUMO (eV)
EHOMO−1 (eV)
ELUMO+1 (eV)
Eg (eV)
Transition
B3LYP/6-31G (d, p)
PBEBE/6-31G(d, p)
WB97XD/6-31G(d, p)
B3LYP/6-31G (d, p)
PBEBE/6-31G(d, p)
WB97XD/6-31G(d, p)
−3.39
−4.92
−6.84
−4.64
−3.07
−6.32
−0.67
−2.43
−1.43
−2.43
−0.83
−2.09
−6.35
−5.45
−9.11
−4.85
−4.11
−8.12
−1.5
2.72
2.28
5.41
2.29
2.24
4.23
H→L
−1.15
−0.92
−0.59
−1.63
−2.11
SQIND2
H→L
1 3

Chemical Papers
Fig. 8 The IPCE spectrum for DSSC based on SQIND1 and SQIND2
Fig. 7 Photocurrent density–voltage (J–V) characteristics for
SQIND-1 and SQIND-2, respectively, under AM 1.5G illumination
(100 mWcm−2) on DSSC for examination
Table 4 Photo-electrochemical characteristics for DSSC based on
SQIND1 and SQIND2
DSSC
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
SQIND1
SQIND2
7.6
0.63
0.59
0.50
0.73
0.83
3.3
0.26
favorable site to transfer electrons from dyes to the TiO2
(Niyazbek et al. 2017).
Photovoltaic characterized of the SQIND‑1
and SQIND‑2
Fig. 9 Nyquist plots for SQIND-1 and SQIND-2-based DSSC under
The photocurrent short-circuit density–photovoltage (J–V)
characteristics were tested for SQIND1 and SQIND2 of
illumination AM1.5 (100 mW/cm2)
−
DSSC having iodine/tri-iodide (I−/I3 ) as redox electrolyte
under illumination with AM 1.5G solar showed in Fig. 7 and
summarized in Table 4.
better performance than SQIND2. In SQIND1 sensitizer, the
enhancement was due to the high charge transfer rate in the
interfacial region which was verifed by J–V characterization
and electrochemical impedance spectral (EIS) (Ooyama and
Harima 2009; Hara et al. 2004; Grisorio et al. 2013). It can
be concluded that the strong electronic coupling between
SQIND1 and TiO2 leads to a marked improvement in pho-
tovoltaic performance under solar radiation.
Table 4 lists the J–V characteristics of DSSC-fabri-
cated indoline-based squarylium sensitizers (SQIND1 and
−
SQIND2) on TiO2 flm with redox electrolyte I/I3 . The
SQIND1 sensitizer dye showed a better photovoltaic per-
formance than SQIND2. The SQIND1 sensitized exhibits a
JSC of 7.6 mA/cm2, VOC of 0.59 V, FF of 0.73, and η of 3.3,
while the low values of SQIND2 give a JSC of 0.63 mA/cm2,
VOC of 0.50, FF of 0.83, and η of 0.26 due to the absence
of carboxyl groups that inject the electron to the surface of
TiO2.
Figure 9 depicts Nyquist plots of DSSC based on
SQIND1 and SQIND2 measured under AM 1.5G illumina-
tions (100 mW/cm2). The SQIND1 showed that the major
semicircle in the electrochemical impedance spectral (EIS)
Nyquist plot corresponds to the resistance of electron trans-
port at the TiO2/dye/electrolyte interface (Yun et al. 2014b;
Halme et al. 2010). It is clear from the Nyquist plot that
Figure 8 shows the incident photon-to-current conver-
sion efciency (IPCE) spectrum of DSSC based on SQIND1
and SQIND2. The IPCE spectrum for SQIND1 exhibited
1 3

Chemical Papers
cells. J Mol Struct 1195:850–858. https://doi.org/10.1016/j.
molstruc.2019.06.056
SQIND1 showed a lesser recombination resistance because
of the lower driving force from the electron injection. The
radius larger semicircle on the left, which contains R1 from
the charge transfer at the electrode and R2 from the electron
transfer in the TiO2/dye/electrolyte interface, decreased in
the order of SQIND1>SQIND2 (Zhang and Li 2017). This
result indicates that electron generation and transport in
DSSC-based SQIND1 sensitizer is larger than the SQIND2
(Xia and Wang 2017).
Al-Sehemi AG, Irfan A, Asiri AM, Ammar YA (2012a) Synthesis,
characterization and DFT study of methoxybenzylidene con-
taining chromophores for DSSC materials. Spectrochim Acta
A Mol Biomol Spectrosc 91:239–243. https://doi.org/10.1016/j.
saa.2012.01.016
Al-Sehemi A, Irfan A, Asiri A (2012b) The DFT investigations of the
electron injection in hydrazone-based sensitizers. Theor Chem
Acc 131:1–10. https://doi.org/10.1007/s00214-012-1199-6
Andersson MP, Uvdal P (2005) New scale factors for harmonic
vibrational frequencies using the B3LYP density functional
method with the triple-ζ basis set 6–311+G (d, p). J Phys Chem
A 109:2937–2941. https://doi.org/10.1021/jp045733a
Asiri AM, Al-Horaibi SAH, Irfan A, Basaif SA, El-Shishtawy RM
(2015a) Cis/trans geometric efect on the electro-optical prop-
erties and electron injection in indole-based squaraine sensi-
tizers: quantum chemical investigations. Int J Electrochem Sci
10:1529–1542
Conclusions
Synthesis, characterization, and optical properties of two
symmetric indoline-based squarylium dyes (SQIND1 and
SQIND2) were tested to use in DSSC with nanoporous
TiO2. The results have shown that the introduction of car-
boxylic groups in the indoline of SQIND1 sensitizer leads to
bathochromic shifts of the absorption and fuorescence band.
The theoretical calculations and absorbance results showed
that the electron density of LUMO of SQIND1 sensitizer
is delocalized in the whole chromophore, leading to strong
electronic coupling between SQIND1 and conduction band
of TiO2 and improved dye-sensitized solar cell efciency.
Therefore, the SQIND1 sensitized exhibits a better photo-
voltaic performance.
Asiri AM, Al-Horaibi SAH, Irfan A, Basaif SA, El-Shishtawy RM
(2015b) Comparative study of the structural, electronic and charge
transport properties of benzothiazole-and indole-based squaraine
sensitizers. Int J Electrochem Sci 10:1822–1832
Becke AD (1993) Density-functional thermochemistry. III. The role
of exact exchange. J Chem Phys 98:5648–5652. https://doi.
org/10.1063/1.464913
Gao F, Wang Y, Shi D, Zhang J, Wang M, Jing X, Humphry-Baker R,
Wang P, Zakeeruddin SM, Grätzel M (2008) Enhance the optical
absorptivity of nanocrystalline TiO
2 flm with high molar extinc-
tion coefcient ruthenium sensitizers for high performance dye-
sensitized solar Cells. J Am Chem Soc 130:10720–10728. https
://doi.org/10.1021/ja801942j
Grätzel M (2004) Conversion of sunlight to electric power by nanocrys-
talline dye-sensitized solar cells. J Photochem Photobiol A 164:3–
14. https://doi.org/10.1016/j.jphotochem.2004.02.023
Acknowledgements The authors gratefully acknowledge Department
of Chemistry, UGC SAP DRS II and DST FIST for fnancial assis-
tance and Sophisticated Analytical Instrumentation Facility (SAIF)
Chandigarh.
Grisorio R, De Marco L, Allegretta G, Giannuzzi R, Suranna GP,
Manca M, Mastrorilli P, Gigli G (2013) Anchoring stability and
photovoltaic properties of new D (-π-A) 2 dyes for dye-sensi-
tized solar cell applications. Dyes Pigm 98:221–231. https://doi.
org/10.1016/j.dyepig.2013.02.012
Halme J, Vahermaa P, Miettunen K, Lund P (2010) Device phys-
ics of dye solar cells. Adv Mater 22:E210–E234. https://doi.
org/10.1002/adma.201000726
References
Hara K, Dan-oh Y, Kasada C, Ohga Y, Shinpo A, Suga S, Sayama K,
Arakawa H (2004) Efect of additives on the photovoltaic per-
formance of coumarin-dye-sensitized nanocrystalline TiO2 solar
cells. Langmuir 20:4205–4210. https://doi.org/10.1021/la0357615
Irfan A, Cui R, Zhang J (2009) Fluorinated derivatives of mer-Alq3:
energy decomposition analysis, optical properties, and charge
transfer study. Theor Chem Acc 122:275–281. https://doi.
org/10.1007/s00214-009-0506-3
Al-horaibi SA, Alghamdi MT, Gaikwad ST, Rajbhoj AS (2018a) Com-
parison and determine characteristics potentials of HOMO/LUMO
and relationship between Ea and Ip for squaraine dyes (SQ1, SQ2)
by using cyclic voltammetry and DFT/TD-DFT. Moroc J Chem
6:404–413
Al-horaibi SA, Gaikwad ST, Rajbhoj AS (2018b) Symmetrical and
unsymmetrical squaraine sensitizers for dye-sensitized solar cells:
present day advances and future challenges. Adv Mate Lett 9:353–
362. https://doi.org/10.5185/amlett.2018.1740
Irfan A, Hina N, Al-Sehemi A, Asiri A (2012) Quantum chemical
investigations aimed at modeling highly efcient zinc porphyrin
dye sensitized solar cells. J Mol Model 18:4199–4207
Al-horaibi SA, Gaikwad ST, Rajbhoj AS (2018c) Synthesis, charac-
terizations, and comparative study of electro-optical properties
of indole-based squaraine sensitizers as efciency to enhancing
dye-sensitized solar cells. Adv Mater Lett 9:275–283. https://doi.
org/10.5185/amlett.2018.1457
Kakiage K, Aoyama Y, Yano T, Kakiage Kenji, Aoyama Yohei,
Yano Toru, Otsuka T, Kyomen T, Unno M, Hanaya M (2014)
An achievement of over 12 percent efciency in an organic dye-
sensitized solar cell. Chem Commun 50:6379–6381. https://doi.
org/10.1039/C4CC02192D
Al-horaibi SA, Asiri AM, El-Shishtawy RM, Gaikwad ST, Rajbhoj
AS (2019a) Indoline and benzothiazole-based squaraine dye-sen-
sitized solar cells containing bis-pendent sulfonate groups: syn-
thesis, characterization and solar cell performance. J Mol Struct
1195:591–597. https://doi.org/10.1016/j.molstruc.2019.05.068
Al-horaibi SA, Asiri AM, El-Shishtawy RM, Gaikwad ST, Rajbhoj
AS (2019b) Synthesis and characterization of new squaraine
dyes with bis-pendent carboxylic groups for dye-sensitized solar
Kalyanasundaram K (1982) Photophysics, photochemistry and
solar energy conversion with tris (bipyridyl) ruthenium (II)
and its analogues. Coord Chem Rev 46:159–244. https://doi.
org/10.1016/0010-8545(82)85003-0
Mardare D, Rusu GI (2002) The infuence of heat treatment on the opti-
cal properties of titanium oxide thin flm. Mater Lett 56:210–214.
https://doi.org/10.1016/S0167-577X(02)00441-X
1 3

Chemical Papers
Wang M, Grätzel C, Zakeeruddin SM, Grätzel M (2012) Recent
developments in redox electrolytes for dye-sensitized solar cells.
Energy Environ Sci 5:9394–9405. https://doi.org/10.1039/C2EE2
3081J
Mathew S, Yella A, Gao P, Humphry-Baker R, Curchod FEB, Ashari-
Astani N, Tavernelli I, Rothlisberger U, Nazeeruddin MdK,
Grätzel M (2014) Dye-sensitized solar cells with 13% efciency
achieved through the molecular engineering of porphyrin sensitiz-
ers. Nat Chem 6:242
Wei H, Bin S, Hany A, Won-Tae P, Yong YN, Yuning L (2012) A con-
jugated polyazine containing diketopyrrolopyrrole for ambipolar
organic thin flm transistors. Chem Commun 48:8413–8415. https
://doi.org/10.1039/c2cc33998f
Mishra A, Markus KRF, Peter B (2009) Metal-free organic dyes for
dye-sensitized solar cells: from structure: property relationships
to design Rules. Angew Chem Int Ed 48:2474–2499. https://doi.
org/10.1002/anie.200804709
Xia G, Wang H (2017) Squaraine dyes: the hierarchical synthesis and
its application in optical detection. J Photochem Photobiol C
31:84–113. https://doi.org/10.1016/j.jphotochemrev.2017.03.001
Yella A, Lee H-W, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MdK,
Diau EW-G, Yeh C-Y, Zakeeruddin SM, Grätzel M (2011) Por-
phyrin-sensitized solar cells with cobalt (II/III) based redox elec-
trolyte exceed 12 percent efciency. Science 334:629–634. https
://doi.org/10.1126/science.1209688
Niyazbek Kh, Ibrayev Evgeniya V, Seliverstova Alexander A, Ish-
chenko Margarita A (2017) The efect of sulfonate groups on
spectral-luminescent and photovoltaic properties of squary-
lium dyes. J Photochem Photobiol A 346:570–575. https://doi.
org/10.1016/j.jphotochem.2017.06.029
Ooyama Y, Harima Y (2009) Molecular designs and syntheses of
organic dyes for dye-sensitized solar cells. Eur J Org Chem
18:2903–2934. https://doi.org/10.1002/ejoc.200900236
Pandey SS, Watanabe R, Fujikawa N, Shivashimpi GM, Ogomi Y,
Yamaguchi Y, Hayase S (2013) Efect of extended π-conjugation
on photovoltaic performance of dye sensitized solar cells based on
unsymmetrical squaraine dyes. Tetrahedron 69:2633–2639. https
://doi.org/10.1016/j.tet.2013.01.036
Yen Y-S, Chou H-H, Chen Y-C, Hsu C-Y, Lin JT (2012) Recent devel-
opments in molecule-based organic materials for dye-sensitized
solar cells. J Mater Chem 22:8734–8747. https://doi.org/10.1039/
C2JM30362K
Yum J-H, Walter P, Huber S, Rentsch D, Geiger T, Nüesch F, Angelis
FD, Grätzel M, Nazeeruddin MK (2007) Efcient far red sen-
sitization of nanocrystalline TiO2
Qin C, Numata Y, Zhang S, Yang X, Islam A, Zhang K, Chen H, Han
L (2014) Novel near-infrared squaraine sensitizers for stable and
efcient dye-sensitized solar cells. Adv Funct Mater 24:3059–
3066. https://doi.org/10.1002/adfm.201303769
flms by an unsymmetrical
squaraine dye. J Am Chem Soc 129:10320–10321. https://doi.
org/10.1021/ja0731470
Yun HJ, Paik T, Edley ME, Baxter JB, Murray CB (2014a) Enhanced
charge transfer kinetics of CdSe quantum dot-sensitized solar cell
by inorganic ligand exchange treatments. ACS Appl Mater Inter-
faces 6:3721–3728. https://doi.org/10.1021/am500026a
Yun S, Hagfeldt A, Ma T (2014b) Pt-free counter electrode for dye-sen-
sitized solar cells with high efciency. Adv Mater 26:6210–6237.
https://doi.org/10.1002/adma.201402056
Sawin, Janet L et al (2013) Renewables 2010-Global status report
Shivashimpi GM, Pandey SS, Watanabe R, Fujikawa N, Ogomi Y,
Yamaguchi Y, Hayase S (2012) Novel unsymmetrical squaraine
dye bearing cyanoacrylic acid anchoring group and its photosen-
sitization behavior. Tetrahedron Lett 53:5437–5440. https://doi.
org/10.1016/j.tetlet.2012.07.125
Tomasi J, Mennucci B, Cammi R (2005) Quantum mechanical con-
tinuum solvation models. Chem Rev 105:2999–3094. https://doi.
org/10.1021/cr9904009
Zhang J, Li T (2017) Properties of electronically excited states of four
squaraine dyes and their complexes with fullerene C70: a theoreti-
cal investigation. Spectrochim Acta Part A 184:82–88. https://doi.
org/10.1016/j.saa.2017.04.086
Torruellas WE, Neher D, Zanoni R, Stegeman GI, Kajzar F, Leclerc
M (1990) Dispersion measurements of the third-order nonlin-
ear susceptibility of polythiophene thin flms. Chem Phys Lett
175:11–16. https://doi.org/10.1016/0009-2614(90)85510-J
Valencia S, Marin JM, Restrepo G (2009) Study of the band gap of syn-
thesized titanium dioxide nanoparticles using the sol–gel method
and a hydrothermal treatment. Open Mater Sci J 4:9–14. https://
doi.org/10.2174/1874088X01004010009
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
Venkataraman L, Klare JE, Nuckolls C, Hybertsen MS, Steigerwald
ML (2006) Dependence of single-molecule junction conductance
on molecular conformation. Nature 442:904–907
1 3