Indenoquinaldine-Based Unsymmetrical Squaraine Dyes for Near-Infrared Absorption: Investigating the Steric and Electronic Effects in Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/chem.201803062

A series of near-infrared (NIR)-responsive unsymmetrical squaraine dyes (ISQ1–3) incorporating a fused indenoquinaldine-based donor have been designed and synthesized. C₁₂ alkyl chains were incorporated at the sp³-hybridized carbon center of the indene unit of the indenoquinaldine in an out-of-plane orientation to control dye aggregation on the surface of titanium dioxide, and indole (ISQ1), benzo[e]indole (ISQ2), and quinoline (ISQ3) moieties were included as the donor component bearing the anchoring carboxy group to extend the absorption in the NIR region and to systematically study the effect of the electronic modification on the performance of dye-sensitized solar cells (DSSC). All the dyes exhibit intense absorption (?≥10⁵ m^?1 cm^?1) in the NIR region, and the dye-adsorbed TiO₂ films exhibit broad panchromatic absorption. The incident photon-to-current efficiency (IPCE) spectrum of the ISQ3-based DSSC device displays a panchromatic IPCE response up to 880 nm. Additionally, the ISQ3-sensitized device provides the best efficiency of 4.15 % with a short circuit current density (J_SC) of 10.02 mA cm^?2, open-circuit voltage (V_OC) of 0.58 V, and fill factor (ff) of 72 % in the presence of 10 equivalents of 3α,7α-dihydroxy-5β-cholanic acid (CDCA). Electrochemical impedance spectroscopy analysis showed attenuated charge recombination in the ISQ3-sensitized DSSC, which contributes to its higher value of V_OC compared with the other dyes.

Full text of DOI:10.1002/chem.201803062

A Journal of
Accepted Article
Title: Indenoquinaldine Based Unsymmetrical Squaraine Dyes for
Near-Infrared Absorption: Investigating the Steric and Electronic
Effects in Dye-Sensitized Solar Cells
Authors: Nithyanandhan Jayaraj, Rajesh Bisht, and Munavvar Fairoos
Mele Kavungathodi
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To be cited as: Chem. Eur. J. 10.1002/chem.201803062
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Indenoquinaldine Based Unsymmetrical Squaraine Dyes for Near-
Infrared Absorption: Investigating the Steric and Electronic
Effects in Dye-Sensitized Solar Cells
Rajesh Bisht[a, b], Munavvar Fairoos Mele Kavungathodi , and Jayaraj Nithyanandhan[a, b]
*
[a]
Abstract: A series of near-infrared (NIR) responsive unsymmetrical
squaraine dyes (ISQ1-3), incorporating a fused indenoquinaldine
been explored. Ruthenium(II)polypyridyl complexes are among
the most studied sensitizers, and PCE up to 11.5% was
achieved[3c] whereas the DSSC based upon a panchromatic
porphyrin sensitizer, containing a benzothiadiazole (BTD) moiety,
yielded PCE up to 13%.[ Metal-free organic dyes have unique
advantages over metal-organic complex based sensitizers in
terms of low cost, non-toxicity, flexibility in molecular designing
and tunability in electronic properties.[5a] Several metal-free
sensitizers with donor--acceptor (D--A) and donor-acceptor--
acceptor (D-A--A) configuration have been explored.[ They
show high molar extinction coefficients due to intramolecular
charge transfer (ICT) which occurs within the molecule because
of the push-pull effect. The efficiencies over 10% have been
3
based donor, were designed and synthesized. The sp -C center
available on the indene unit of indenoquinaldine was employed to
incorporate twelve-carbon alkyl chains in an out-of-plane direction to
4b]
control dye aggregation on titanium dioxide (TiO
2
) surface. Indole
(ISQ1), benz[e]indole (ISQ2) and quinoline (ISQ3) moieties were
included towards anchoring group to extend the absorption in NIR
region and systematically study the effect of the electronic
modification on DSSC performance. All the dyes exhibited intense
6]
5
-1
-1
absorption ( ≥ 10 M cm ) in the NIR region, and dye-adsorbed
TiO films exhibited broad panchromatic absorption. The incident
2
photon-to-current efficiency (IPCE) spectrum of the ISQ3 based
device displayed panchromatic IPCE response up to 880 nm. ISQ3
sensitized device provided the best efficiency of 4.15 % with short
achieved for metal-free dyes[7] with I-/I -
redox mediator whereas
3
indenoperylene based dyes achieved PCE ≥ 12 % with Co(II/III)
-
2
[8]
circuit current density (JSC) of 10.02 mAcm , open-circuit voltage
OC) of 0.58 V and fill factor (ff) of 72% in the presence of 10
redox shuttle in a single sensitizer device configuration.
(V
Recently, an efficiency (PCE) of >14% was achieved by a metal-
free organic dye bearing silyl anchoring group through co-
sensitization.[9]
equivalent CDCA. Electrochemical impedance spectroscopy (EIS)
analysis showed attenuated charge recombination in ISQ3
sensitized DSSC which contributes to its higher VOC compared to
other dyes.
In DSSCs, near-infrared absorbing organic dyes are critically
needed as they are capable of generating high current density
which can improve the performance of the cell.[10] While there
has been remarkable progress in DSSC performance of metal-
free organic dyes in the last two decades, there is still ample
scope for improvement in terms of dye design, especially the
absorption in the near-infrared region (750-1000 nm). Squaraine
dyes are among the few sensitizers which have potential to
Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable
research attention as a promising alternative to inorganic solar
cells due to their ease of fabrication, economic viability, and
tunability of the various device components.[1] The performance
of DSSCs has been substantially improved due to significant
advances in the key components, such as molecular sensitizers,
n-type semiconductors, redox couples, and cathode materials in
the past two decades.[2] Sensitizing dyes play a crucial role in
directing the various photovoltaic parameters which eventually
affect the power conversion efficiency (PCE or ) of the DSSCs.
Several sensitizers, which includes ruthenium (II) complexes,[3]
zinc-porphyrin complexes[4] and metal-free organic dyes[5] have
[11]
absorb in this region other than porphyrin and phthalocyanine.
Squaraine dyes belong to the family of polymethine dyes and
[12]
comprise of resonance-stabilized zwitterionic structures.
Due
to strong absorption ( ε > 105 M ) in far red and NIR region,
-1
squaraine dyes have found applications in wide range of areas
[13]
[14]
[15]
such
as
imaging,
sensors,
nonlinear
optics,
photodynamic therapy[16]
and photovoltaics.[17] There are
,
several successful DSSCs based on squaraine dyes which
absorb in the far-red region, however, only a few of them have a
spectral response over 800 nm (Figure 1). The extension of
indole based core-symmetric squaraine dyes by using a -
spacer in YR6[ and RSQ2 , leads to good incident photon to
current efficiency (IPCE) in NIR region (onset at 775-780 nm)
with efficiency ( of 6.7 and 6.8% respectively. The core-
dissymmetric squaraine dye JK-217, assembled using
18]
[19]
[
[
a]
b]
R. Bisht, M.F. M. Kavungathodi, Dr. J. Nithyanandhan
Physical and Materials Chemistry Division
CSIR-National Chemical Laboratory,
Dr. Homi Bhaba Road, Pune-411008, India.
E-mail: j.nithyanandhan@ncl.res.in
thiophenyl, pyrrolyl and indolium groups, exhibits
a
panchromatic light harvesting up to 850 nm to yield efficiency of
R. Bisht, Dr. J. Nithyanandhan
[20]
5.5%.
(Detailed explanation on squaraine dyes classification
Academy of Scientific and Innovative Research (AcSIR)
New Delhi 110025, India
is provided in the supporting information, Scheme S1). In order
to extend response further in the NIR region, the fused -
extended heterocyclic structures have been incorporated in
Supporting information for this article is given via a link at the end of
the document.
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FULL PAPER
squaraine dyes as they can induce strong intramolecular charge
transfer (ICT) as well as maintain planarity of the molecule.
assembly on the TiO
method involve co-adsorption of dyes with an optically
transparent substance bearing an anchoring group, such as
2
is very important and the most popular
3α,7α-dihydroxy-5β-cholanic acid (CDCA) which help to break
down the large aggregates to induce the favorable charge
injection.
Incorporation of bulky alkyl chains on dye skeleton is another
method to avoid aggregation in which π-stacking of molecules is
restricted
by increasing the bulk
surrounding the
chromophore.[ . Introducing in-plane and out-of-plane alkyl
groups helps controlling self-organization of dye molecules in
organic photovoltaics.[ In DSSCs, such introduction of out-of-
plane alkyl groups provides better control over aggregation at a
molecular level compared to CDCA and improves device
performance.[29c,29d,31] Besides, CDCA can reduce the dye
loading beyond optimal amount if used in excess, especially with
highly aggregating dyes, which makes the integration of out-of-
plane alkyl chains on dye preferable.
29]
30]
Controlling the aggregation of dyes on the TiO
has been a major challenge in NIR absorbing sensitizers. To this
end, we have designed and synthesized series of
2
surface
a
unsymmetrical squaraine dyes (ISQ1-3) for DSSC consisting of
a fused indenoquinaldine based donor which can help in
increasing the absorption in NIR region and also accommodates
3
long alkyl chains at its sp -C center to control dye aggregation
2
on TiO surface (Figure 2). Quinaldine is chosen to fuse with
indene as it has the ability to provide excellent absorption in the
NIR region in squaraine dyes.[
32]
While all the dyes retain
Figure 1. Examples for the best NIR responsive core-dissymmetric squaraine
dyes with IPCE onset greater than 800 nm.
indenoquinalidne as
a
donor, carboxyindolenine (ISQ1),
carboxybenz[e]indoleine (ISQ2) and carboxyquinaldine (ISQ3)
were used towards anchoring group to systematically investigate
their effect on electronic properties of dyes and DSSC
parameters.
Pyrylium, thiopyrylium, benzo[c,d]indolenine, quinolinium and
indole based heterocyclic components have been used to
synthesize NIR absorbing squaraine dyes which exhibit IPCE
onset at ca. 800-940 nm.[21] The dye WCH-SQ10, consisting of
triphenylamine as a donor and carboxyquinoline unit as acceptor,
extends the absorption further and IPCE up to 1050 nm is
obtained.[ In another approach, a fused dimeric squaraine dye
BSQ01[ shows IPCE response up to 800 nm whereas the
22]
23]
dimeric (LSQa)[
24]
and trimeric (TSQa-b)
[25]
squaraine dyes
demonstrate improved IPCE onsets at 900 nm and 980 nm
respectively. Despite the strong NIR absorption, the squaraine
dyes based on fused extended heterocyclic structures have
low efficiency (ca. 1-3 %) due to dye aggregation on TiO and
2
lack of directionality for charge injection from the photo-excited
state of the sensitizer.
2
The aggregation of dyes on TiO is a common feature in
squaraine based dyes which occur due to the interaction of
intermolecular dipoles.[26] Head-to-head dipole interaction forms
H-aggregates which appear as the blue-shifted peak, whereas
head-to-tail dipole interaction leads to and J-aggregates which
appear as red-shifted absorption compared to the absorption
peak of the monomeric dye[27]. While the aggregation of dyes on
Figure 2. (a) Design of steric and electronic effects incorporated
unsymmetrical squaraine dyes, (b) Structure of fused indenoquinaldine based
donor and (c) Unsymmetrical squaraine dyes ISQ1-3
TiO
charge injection into TiO
hampers the performance of DSSC.[28] Thus controlling the dye
2
broadens the absorption profile, it can also reduce the
2
through exciton quenching which
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Results and Discussion
described in supporting information (Scheme S2). In the final
step, the semi-squaric acid derivative P1 was refluxed with
indenoquinaldinium salt 5 in n-BuOH/PhMe mixture under
azeotropic distillation of water employing Dean−Stark apparatus
to give squaraine dyes ISQ1. Similarly, the dyes ISQ2 and ISQ3
were obtained by treating 5 with P2 and P3 respectively. All the
dyes were thoroughly characterized by 1H-NMR, 13C-NMR
spectroscopy, and mass spectrometric techniques.
Synthesis
The synthetic scheme for the dyes ISQ1-3 is illustrated in
scheme 1.
Optical characterization
To evaluate the optical properties of dyes, UV-Vis absorption
and emission spectra of ISQ1-3 were recorded in CHCl
3
solutions (Figure 3) and data are summarized in (Table 1). ISQ
dyes exhibited intense absorption bands in the far-red region
with high extinction coefficient (≥105 M cm ) due to strong
intramolecular charge transfer (ICT) process. ISQ1 showed the
absorption maximum (max) at 692 nm with onset at 734 nm due
to the strong donating effect of the indenoquinaldine unit. The
red-shifted max at 715 nm with onset at 760 nm was observed
for ISQ2 due to additional conjugation by carboxybenz[e]indole
unit towards anchoring group. In ISQ3, due to the presence of
quinaldine moiety on both ends of central squaraine unit, the
maximum bathochromic shift was observed, and max was
determined at 754 nm with onset at 800 nm (Figure 3a). The
same trend was observed in the emission spectra of the dyes
which displayed emission maxima at 726 nm, 745 nm and 781
nm for ISQ1, ISQ2 and ISQ3 respectively (Figure 3b). The band
gap (E0-0) was obtained from the intersection of absorption and
emission curve and found to be at 1.75 eV, 1.70 eV and 1.62 eV
for ISQ1, ISQ2, and ISQ3 respectively. Apart from the intense
-1
-1
Scheme 1. Syntheses of a) indenoquinaldinium iodide 5, (b) Indenoquinaldine
donor based unsymmetrical squaraine dyes (ISQ1-3).
The key step in the synthetic route of ISQ dyes is the synthesis
of indenoquinalinium salt 5, which begins with functionalization
of 2-nitrofluorene (1) with 12 carbon long chains by treatment
with NaOH in the presence of dodecyliodide. The resultant
compound 2 was reduced to 2-amino-9,9-didodecylfluorene (3)
by hydrazine hydrate in the presence of catalyst Pd/C. The
amine derivative 3 was treated with crotonaldehyde under
Skraup-Doebner-Von Miller reaction condition to afford
indenoquinaldine derivative 4 which was subsequently reacted
with iodomethane to give N-methyl indenoquinaldinium salt 5.
The synthesis of semi-squaric acid derivatives of carboxyindole
P1, carboxybenz[e]indole P2, and carboxyquinoline P3 is
3
-1
-1
absorption in NIR region, a weak absorption ( ≈ 10 M cm ) in
the visible region was observed between 400 to 500 nm for
ISQ1-3 which can be beneficial to absorb the high energy
photons as well. To understand the self-assembly of dyes on
TiO
dye solution for 15 minutes. Exposing the TiO
for short duration also gives information about the self-
2
film, the titania coated glass was immersed in a 0.1 mM
2
film to the dyes
2
organization of dyes on the TiO surface.
1
1
1
4 (a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
ISQ1
ISQ2
ISQ3
2
0
8
6
4
2
0
ISQ1
ISQ2
ISQ3

6
00 650 700 750 800 850 900
Wavelength (nm)
4
00
500
600
700
800
900
Wavelength (nm)
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(
d)
(
c)
ISQ1
ISQ2
ISQ3
1
0
0
0
0
0
.0
.8
.6
.4
.2
100
80
60
40
20
0
ISQ1
ISQ2
ISQ3
.0
4
00 500 600 700 800 900
Wavelength (nm)
400 500 600 700 800 900 1000
Wavelength [nm]
Figure 3. a) The absorption spectra of ISQ1-3 in CHCl
3
([dye] = 6 µM)
,
(b) Normalized emission spectra ([dye] = 6 µM in CHCl
3 ,
) λexc = 600nm (ISQ1), 700 nm
(ISQ2) and 720 nm (ISQ3) , (c) Normalized absorption spectra of ISQ dyes immobilized on TiO
2
, and (d) Light harvesting efficiency of the ISQ dyes.
The absorption spectra of dye anchored on TiO
2
film displayed
conduction band of the TiO
electrolyte (Figure 4).
2
and the dye regeneration by the
large spectral broadening compared to the spectra in solution
due to the interaction of anchoring group (-COOH) with TiO
.[33]
and the formation of H-type (blue-shifted) aggregates
2
[
34]
predominantly (Figure 3c). The formation of aggregates were
confirmed by comparing the absorption spectra of dyes on TiO
2
in the absence and the presence of CDCA (Figure S2). Addition
of CDCA leads to a decrease in the extent of H-aggregate with a
concomitant increment of monomeric dyes on the surface. All
the dyes indicated the extensive formation of H-aggregates with
blue-shifted peaks of high-intensity observed at 645, 661 and
620 nm for ISQ1, ISQ2 and ISQ3 respectively. The shoulder
peaks were also observed at 693, 714 and 753 nm for ISQ1,
ISQ2, and ISQ3 nm respectively, which belongs to the
absorption by monomers. ISQ3 exhibited maximum peak
broadening with absorption ranging from 550 nm to 900 nm. The
-A
light harvesting efficiency (LHE = 1-10 ) was calculated by
measuring absorption in transmittance mode of dye adsorbed
films which was obtained by immersing the TiO
solution for a long period of 6 h (Figure 3d). The LHE of dyes
anchored on TiO indicates the ability of dyes to absorb the
2
film in the dye
2
fraction of photons of different energy across the solar spectrum.
LHE profiles showed that ISQ1-3 dyes are able to harvest the
photons from visible as well as NIR region with LHE ≥ 90% in
400-500 nm and 550-850 nm range, which makes these
sensitizers suitable candidates for panchromatic absorption. The
dyes ISQ1, ISQ2, and ISQ3 showed LHE onset at 850 nm, 880
nm, and 1000 nm respectively which lie well in the NIR region.
Electrochemical characterization
2 2
Cyclic voltammetry was carried out in CH Cl solutions to
evaluate the redox properties of the dyes in order to determine
the feasibility of charge injection from the excited dye into the
Figure 4. (a) Cyclic voltammogram of ISQ dyes measured in CH
Energy level diagram for ISQ dyes with DSSC device components.
2 2
Cl (b)
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Table 1. Optical and electrochemical properties of ISQ dyes
absmax/CH
(
emimax/CH
(nm)
abs

max/TiO
ε 104
(M cm )
E
DFT
E
E
opt
0-0
(eV)g
E
LUMO
(V vs NHE)
2
Cl
2
2
Cl
2
2
g
HOMO
(V vs NHE)f
Dyes
nm)a
b
(nm)c
-1
-1
d
(eV)e
h
ISQ1
ISQ2
ISQ3
462, 692
480, 715
487, 754
726
745
781
645, 693
661, 714
0.62, 12.9
0.56, 11.1
0.43, 13.5
2.09
1.97
1.77
0.62
0.55
0.47
1.75
1.70
1.62
-1.13
-1.15
-1.15
620, 682, 753
*
See SI for experimental details of measurements and calculations
HOMO levels of the dyes ISQ1-3 were obtained from the first
oxidation onset and calculated to be at 0.62 V, 0.55V, and 0.47
V respectively (Vs. NHE). The upward shift of HOMO in case of
ISQ2 and ISQ3 can be credited to the stronger donating effect of
benz[e]indole and quinoline respectively compared to indole.
ISQ1 shows high potential offset (-Greg = 220 mV) with respect
Computational studies
To deeply understand the structural, electronic, and optical
properties of ISQ dyes, quantum mechanical calculation were
carried out using density functional theory (DFT) and time-
dependent DFT (TD-DFT) at B3LYP/6-31G (d,p) level in
-
-
[35]
to redox level of I /I
3
(0.4 V) which provides large driving force
Gaussian 09. . Suitable spatial distributions of the frontier
for effective dye regeneration whereas ISQ2 (-Greg = 150 mV)
and ISQ3 (-Greg = 70 mV) shows moderate driving force (Figure
molecular orbitals (FMO) are crucial for facilitating the electron-
transfer processes. Ideally, the highest occupied molecular
orbitals (HOMO) should be away from the TiO2, and lowest
unoccupied molecular orbitals (LUMO) should be as close as
4b). The LUMO levels were calculated by subtracting E0−0 from
EHOMO and found to be at -1.13 V, -1.15 V and -1.15 V (vs NHE)
for ISQ1, ISQ2, and ISQ3 respectively which gives enough
2
possible to the TiO for efficient electron injection. Isodensity
energy offset (-Ginject ≈ 700 mV) for charge injection on
surface plots of ISQ1-3 dyes show that HOMO are
conduction band (CB) of TiO
2
(-0.5 V). While the HOMO levels
predominantly confined over central squaric acid units (Figure 5).
varied significantly, the LUMO levels were essentially the same
for all the dyes.
Figure 5. Isodensity surface plots of ISQ1-3 dyes (alkyl chains are removed for clarity).
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The HOMO and LUMO overlap for the most part of ISQ1-3 dyes,
however, there is slight delocalization of LUMO towards the
anchoring group in case of ISQ3 LUMO+1 and LUMO+2 are
prominently delocalized towards anchoring group for the ISQ3
dye which suggests that it can exhibit better charge injection
compared to ISQ1-2 and can counterbalance the effect of low
driving force for dye regeneration. Simulated absorption spectra
were plotted using TD-DFT (Figure S1), and spectra were found
to be consistent with the experimental spectra. By comparing the
two spectra it can be predicted that the peaks of high oscillator
strength at longer wavelength originates exclusively from
HOMO-LUMO vertical energy transition (> 99 % contribution) in
all the dyes whereas the high energy peaks with low oscillator
strength between 350 and 500 nm are associated with the
transition to higher excited states, i.e., LUMO+1, LUMO+2
Photovoltaic characterization
The photovoltaic measurements of the DSSCs fabricated using
ISQ1-3 dyes were done under irradiance of 100 mWcm-2
simulated AM 1.5G sunlight. The current density-voltage (J-V)
plot and IPCE spectra of the best cells are shown in Figure 7
and summarized in table 1. The DSSC based on ISQ1 gave the
PCE of 3.24 % whereas ISQ2 and ISQ3 showed PCE of 2.35 %
and 1.85% respectively, without using any co-adsorbent. The
major difference in their performance comes from the variation in
the short circuit current density (JSC) which is highest for ISQ1
-2
-2
(8.99 mAcm ), followed by ISQ2 (6.85 mA cm ) and ISQ3 (5.23
-2
mA cm ). The low short circuit current density could be ascribed
to the strong aggregation of dye on TiO which is maximum in
2
(
Table S1). The optimized geometry in Figure 6 shows that the
case of ISQ3. To minimize the negative effect of aggregation on
cell performance, the dye was co-adsorbed in the presence of
CDCA to reduce the self-quenching of excitons and charge
recombination. All the ISQ dyes showed a different response
with the addition of CDCA. The efficiency of ISQ1 dyes reduced
to 2.63 % mainly due to a reduction in JSC (6.86 mA cm ), even
though the slight improvement in VOC was observed (0.558 V). In
contrast, the performance of ISQ2 and ISQ3 sensitized cells
improved when co-adsorbed with CDCA. ISQ2 achieved the
maximum efficiency of 3.68% on addition of 5 equiv CDCA with
distance between branched alkyl chains and oxygen atoms of
anchoring unit d is about 20.5 Å, 22.Å and 19.8 Å for ISQ1,
ISQ2, and ISQ3 respectively.
-2
V
OC of 0.558 V and JSC of 9.62 mA cm-2 whereas ISQ3 reached
-2
the best efficiency of 4.15% with JSC of 10.02 mA cm , VOC of
.576 V, and ff of 72%, in presence 10 equiv of CDCA. Further
0
addition of CDCA led to a reduction in device efficiency of all the
dyes, primarily due to substantial loss of JSC (Figure S3 and
Table S2). It was suspected that the decrease in efficiency of
ISQ1 on the addition of CDCA could be due to reduced
2
adsorption of dyes on the TiO surface due to competitive
binding of CDCA. To confirm our hypothesis, the dye loading
was calculated by desorption of dye-sensitized photo-anodes in
the
presence of
2M
ethanolic
HCl
followed by
spectrophotometric measurement of dye concentration (Table 2).
In the case of ISQ1, the addition of CDCA, as low as 5
equivalents, led to the drastic decrease of 68% in dye content.
-7
The dye loading of ISQ2 with 5 equiv of CDCA (1.22 × 10 mol
-2
cm ) was significantly higher than the dye content of ISQ1
devices with 5 equiv. CDCA (7.56 × 10-8 mol cm ). ISQ3
sensitized cells, even in the presence of 10 equiv CDCA,
showed higher dye content (1.05 x 10-7 mol cm ) compared to
-2
-2
ISQ1 (5 equiv CDCA). This shows that dye-TiO
poor in case of ISQ1 which allows the dyes to be replaced by
CDCA on TiO rather easily compared to ISQ2 and ISQ3 (Figure
b (ii).
2
interaction is
Figure 6. Optimized geometry of ISQ1-3 dyes, a) Lateral view, b) Top view
2
8
Thus, the variable effect of CDCA on performance of DSSC
could be ascribed to competitive binding of ISQ1-3 dyes with
CDCA, where on the one hand CDCA helps in improving JSC
and VOC by reducing exciton quenching induced by aggregation
on TiO surface, on the other hand, it may replace the dye
2
molecules to reduce the effective dye content on the photo-
anode leading to poor performance.[19]
The dihedral angle between indenoquinalidne and squaraine
unit (θ
between squaraine units and anchoring unit is different for all the
dyes. The dye ISQ1 is almost planar with θ = 4.5° and θ =0°,
ISQ2 is non-planar with θ =11.7°, and planarity is further
reduced in ISQ3 with θ = 21.2°. The distance between the
1
) is the same for all the dyes whereas the dihedral angle
1
2
2
2
terminal carbons of the two out of plane alkyl chains on the dyes
is about 27 Å which helps in controlling the dye-dye interactions
2
on the TiO surface (Figure 6b).
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1
1
2
0
8
6
4
2
0
80
ISQ1
ISQ2
ISQ3
ISQ1
ISQ2
ISQ3
(
a)
(
b)
6
0
40
2
0
0
4
00
500
600
700
800
900
0
.0
0.2
0.4
Voltage (V)
0.6
Wavelength (nm)
1
2
80
(
c)
(d)
ISQ1/CDCA (1:5)
ISQ2/CDCA (1:5)
ISQ3/CDCA (1:10)
1
0
8
6
4
2
0
6
0
40
2
0
ISQ1/CDCA (1:5)
ISQ2/CDCA (1:5)
ISQ3/CDCA (1:10)
0
4
00
500
600
700
800
900
0
.0
0.2
0.4
Voltage (V)
0.6
Wavelength [nm]
Figure 7. J−V and IPCE characterization of ISQ dyes. (a, b) J−V and IPCE curves for ISQ1-3 in the absence of CDCA. (c, d) J−V and IPCE curves for ISQ1-3 in
the presence of CDCA.
Table 2. Photovoltaic parameters for DSSCs employing the I-/I - electrolyte under the optimized conditionsa
3
Dyes
J
SC
INTJSCb
(mA cm )
V
(V)
OC
ff
(%)

(%)
Dye loadingc
(mol cm )
-
2
-2
-2
(mA cm )
ISQ1
8.99
6.86
6.85
9.62
5.23
10.02
8.46
6.23
6.52
9.37
4.93
11.16
0.544
0.558
0.537
0.558
0.538
0.576
68.4
68.9
64.1
68.7
65.7
72.0
3.34
2.63
2.35
3.68
1.85
4.15
2.33 × 10-7
7.56 × 10-8
2.82 × 10-7
1.22 × 10-7
2.54 × 10-7
1.05 × 10-7
ISQ1/CDCA(1:5)
ISQ2
ISQ2/CDCA (1:5)
ISQ3
ISQ3/CDCA (1:10)
aSee SI for experimental details of measurements and calculations. bCalculated by integrating IPCE over the standard AM1.5G emission spectrum (ASTM G173-
3).
0
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The variation in JSC can be further explained through a detailed
investigation of IPCE spectra as the JSC is directly related to
IPCE and can be expressed as eq 1, where q is the charge of
the electron, and ϕ is the photon flux of solar spectrum (AM 1.5
G).
distinct humps on the curve which are contributed by the H-type
aggregates (39 % at 650 nm), monomers (40 % at 710 nm) and
J-type aggregates (34 % at 800 nm). In the case of ISQ3 (10
equiv CDCA), the contribution by J- aggregates was more
prominent with IPCE over 50% at 800 nm whereas IPCE of 30-
4
0 % was obtained in the range of 600-750 nm which was
contributed mainly by the monomers, dimers and H-type dye
aggregates on the TiO surface. The results indicate that for
J
SC = q ∫ IPCE (λ) ϕ (λ) dλ
(1)
2
In the absence of CDCA, ISQ1 showed best IPCE response with
a maximum of 46% at 600 nm which is primarily contributed by
the H-type aggregates (Figure 6b). The peak with low IPCE
ISQ2 and ISQ3, the addition of CDCA facilitated the formation of
smaller aggregates in a controlled fashion which helped to inject
electron more efficiently while maintaining the broad spectrum in
comparison to the devices without CDCA (Figure 8b (ii)).
Remarkably, a substantial IPCE response of over 40 % in the
visible region (400-550 nm) was also observed for ISQ3. The
origin of this response can be traced to the high concentration of
electron density on anchoring group in higher excited states
(LUMO, LUMO+1, and LUMO+2) which led to strong electronic
(
12%) at 810 nm can be attributed to current generation from J-
aggregates. When CDCA is added, the aggregation was
reduced effectively which can be observed as the
disappearance of the peaks at 600 nm and 810 nm (Figure 6d).
The reduction in aggregation, though improved the VOC from
0.544V to 0.558 V in ISQ1 based cell, the severe decrease in
dye amount led to low IPCE which in turn led to reduced JSC. In
the case of ISQ2, the H-aggregates (23 % at 640 nm) and J-
coupling and good charge injection into the TiO
oscillator strength. Hence, with the help of panchromatic IPCE,
2
despite low
aggregates (25 % at 790 nm) contributed almost equally to IPCE, ISQ3 attained better JSC compared to other dyes and eventually
however, the overall response was much lower compared to
ISQ1. Similarly, for ISQ3 the IPCE response was quite low with
equal contribution from H- (16 % at 660 nm) and J- aggregates
higher efficiency.
Though ISQ dyes have similar indenoquinaline based donor on
the non-anchoring side, the anchoring unit containing different
donor units plays a vital role in determining the stability of the
(
16 % at 780 nm). The reason behind the low IPCE for ISQ2 and
ISQ3, in the absence of CDCA, could be the formation of closely
2
monolayer thus formed on the TiO surface. For example, the
stacked large-sized aggregates which could not inject an
nature of IPCE profile of ISQ3 was almost similar with and
without CDCA indicating the contribution from monomers, H-
and J-aggregates (Figure S3(f)). Whereas for ISQ2, the IPCE
profile showed the contribution from monomers, H- and J-
aggregates up to 5 equiv. of CDCA, and further addition of
CDCA leads to contribution mainly from monomers and dimers
(Figure S3(d)). In the case of ISQ1, without CDCA, the obtained
photocurrent is due to both monomer and aggregates (H- and J-
).
electron into the TiO (Figure 8a). The extended -surface of
2
ISQ2 and ISQ3 due to benz[e]indole and quinoline moieties
respectively may induce such close intermolecular packing, and
the absence of out-of-plane two methyl group on
carboxyquinoline group may cause the ISQ3 to be aggregate
excessively. When CDCA was added, both ISQ2 and ISQ3
showed improved IPCE response. ISQ2 (5 equiv CDCA)
showed IPCE of over 30% in between 650 to 800 nm with three
Figure 8. Cartoon representation of the formation of dye aggregate and variation in dye loading in the (a) absence of CDCA and the (b) presence of CDCA.
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Addition of 5 equiv of CDCA disturbed the self-assembly of ISQ1,
and consequently, the photocurrent is derived mainly from
monomer and dimer (Figure S3(b)). Hence the nature of the
anchoring unit containing donor units not only modulate the
electronic property of ISQ dyes but also controls the self-
V
OC= Eredox – E
F
(2)
(3)
E
F
= ECB + k T ln (n /N )
B c c
According to eq 3, the variation in E
electrons on the TiO (n
where k is Boltzmann constant, N
state, and T is the absolute temperature (293 K). Thus, VOC is
intimately susceptible to shift in the TiO conduction band edge
CB), which can be deduced from the chemical capacitance
F
is linked to the number of
2
c
) and conduction band (ECB) of TiO
2
,
assembly of dyes on the TiO
2
surface.
c
is the effective density of
Electrochemical Impedance Spectroscopy (EIS).
2
(
E
To understand the slight difference in VOC values of ISQ
sensitized cell electrochemical impedance spectroscopy (EIS)
was carried out under dark conditions. Referring to the energy
band diagram and the carrier transfer processes in Figure 4, the
(C) and fluctuation of electron density, which is associated to
the electron lifetime(s) determined by the charge recombination
rate.[ Typical EIS Nyquist plots for DSSCs based on the ISQ1-
3 dyes were measured in the dark under applied bias −0.5, -0.47,
37]
VOC is calculated from the potential difference between the
S 
-0.44 and -0.41 V and are fitted using R + Rpt/Cpt + Rct/C circuit
quasi-Fermi level of TiO
2
(E
F
) and the chemical potential of
model (Figure 9). The larger semicircle at lower frequencies in
Nyquist plots represents the interfacial charge-transfer
redox species (Eredox) in an electrolyte (eq2).[
36]
resistance (Rct) at the TiO
2
-dye/electrolyte interface.
40
35
30
25
20
15
10
5
(b)
ISQ1
ISQ2/CDCA (1:5)
ISQ3/CDCA (1:10)
0
0
.40 0.42 0.44 0.46 0.48 0.50
Applied Potential (V)
20
15
10
5
ISQ1
ISQ2/CDCA (1:5)
ISQ3/CDCA (1:10)
1
0
0
0
.2 (c)
ISQ1
ISQ2/CDCA (1:5)
ISQ3/CDCA (1:10)
(d)
.9
.6
.3
0
.40 0.42 0.44 0.46 0.48 0.50
0
.40 0.42 0.44 0.46 0.48 0.50
Applied Potential (V)
Applied Potential (V)
μ
Figure 9. (a) The Nyquist plot at an applied bias of 0.5 V, (b) Charge transfer resistance (Rct), (b) Rct vs. applied potential, (c) C vs. applied potential, and d) τ vs.
applied potential.
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The fitted recombination resistance (Rct) value of ISQ sensitized
devices (under an applied bias of -0.5 V) increases in the order
of ISQ1 (7.25 Ω) < ISQ2 (10.08 Ω) < ISQ3 (13.13 Ω), which is
consistent with the sequence of VOC values (Figure 8b and Table
Details on the reagents, equipments utilized to characterize the
synthesized precursors, final dye molecules and evaluating the
Photophysical, electrochemical, photovoltaics parameters,
electrochemical impedance spectroscopy and procedure for the
DSSC device fabrication is provided in the supporting
information.
3
). Larger the Rct value, slower is the recombination of electrons
from the conduction band of TiO species in
to the oxidized I -
the electrolyte. The electron lifetime(s) on TiO were calculated
from Rct and chemical capacitance C using  = Rct × C . The
2
3
2
µ
µ
Synthetic procedure
longer electron lifetime in ISQ3 sensitized cell further supports
the higher VOC for ISQ3 (10.24 ms) compared to ISQ1(2.39 ms)
and ISQ2 (5.94 ms) based cells (Figure 9d and Table 3).
9,9-Didodecyl-2-nitro-9H-fluorene (2). 2-Nitrofluorene (3.0 g, 14.20
mmol), 1-iodododecane (12.6 g, 42.61 mmol) and tetrabutyl ammonium
bromide (0.3 g, 0.710 mmol) were taken in 70 mL toluene in two-necked
round bottom flask. An aqueous solution of NaOH (23 mL, 50 % in
weight) was added to the mixture rapidly with constant stirring. The
resultant mixture was stirred at 60 ºC for 8 h under argon atmosphere.
Reaction mixture was cooled to room temperature and 1 M HCl (100 mL)
was added to it followed by addition of water (100 mL). The reaction
mixture was extracted with ethyl acetate, washed with brine and dried
over anhydrous sodium sulphate. Solvents were removed under reduced
pressure and the residue was purified by column chromatography
Table 3. EIS Parameters of ISQ Dye Cells at an Applied Potential of −0.5 V in
the Dark.
R
s
ISQ Dyes
R
ct (ohm)
C

(mF)
 (ms)
(ohm)
ISQ1
15.80
14.56
14.41
7.25
0.33
0.59
0.78
2.39
5.94
10.24
(
6
EtOAc-pet ether) to afford pure compound 2 as pale yellow oil ( 5.13 g,
6 %). 1H NMR (500 MHz, CDCl
3
) δ 8.26 (dd, J = 8.3, 2.0 Hz, 1H), 8.20
ISQ2/CDCA 5 eqv
ISQ3/CDCA 10 eqv
10.08
13.13
(
d, J = 2.0 Hz, 1H), 7.83 – 7.75 (m, 2H), 7.45 – 7.36 (m, 3H), 2.08 – 1.96
m, 4H), 1.27 – 1.12 (m, 24H), 1.08 – 1.00 (m, 12H), 0.86 (t, J = 7.0 Hz,
(
6H), 0.68 – 0.46 (m, 4H). 13C NMR (125 MHz, CDCl ) δ 152.5, 152.1,
3
1
47.8, 147.3, 138.9, 129.4, 127.5, 123.4, 123.436, 121.3, 119.9, 118.4,
7.2, 55.8, 40.2, 32.0, 30.0, 29.7, 29.7, 29.6, 29.5, 29.4, 23.9, 22.8, 14.3.
7
HRMS (ESI) m/z: [M+H]+ Calcd for C37
H58NO 548.4468, found 548.4456.
2
2
-Amino-9,9-didodecylfluorene (3): To a mixture of compound 2 (2.0 g,
Conclusions
3.65 mmol) in 30 mL of EtOH, 0.2 g of 10% Pd/C was added. The
resultant mixture was stirred at room temperature under argon
atmosphere for 15 min. Hydrazine monohydrate (1 mL) was added
dropwise over 15 min. The reaction mixture was stirred at 80 ºC for 10 h.
The reaction mixture was cooled to the room temperature after the
completion and filtered over celite pad to collect the filtrate. The solvents
were removed to give the desired compound 3 as white solid (1.8 g,
To address the dearth of efficient NIR absorbing metal-free dyes
in DSSC, a series of unsymmetrical squaraine dyes (ISQ1-3)
featuring a fused indenoquinoline as a -extended donor have
been successfully designed and synthesized. All the dyes exhibit
intense absorption in the NIR region with onset at 820 nm for
95 %). m.p. 55-57 oC. 1H NMR (400 MHz, CDCl
ISQ3 and spectra broadened after absorption on TiO
2
surface
3
) δ 7.54 (d, J = 7.5 Hz,
1
1
–
H), 7.47 (d, J = 8.4 Hz, 1H), 7.27 – 7.22 (m, 2H), 7.18 (d, J = 7.2 Hz,
H), 6.69 – 6.62 (m, 2H), 1.97 – 1.79 (m, 4H), 1.28 – 1.14 (m, 24H), 1.10
1.01 (m, 12H), 0.87 (t, J = 6.8 Hz, 6H), 0.69 – 0.55 (m, 4H). 13C NMR
which extends up to 1000 nm. The variable photovoltaic
response was observed after the addition of CDCA where the
efficiency of ISQ1 dyes decreased in contrast to improvement in
efficiency of ISQ2 and ISQ3 due to competitive binding of CDCA
against the dye. The best devices of ISQ2 and ISQ3 dyes
displayed panchromatic IPCE response which ranges from 400
nm to 880 nm and efficient electron injection by the both H-type
and J-type aggregates along with the monomers was also
observed. DFT and TDFT suggested the delocalization of
LUMO+1 and LUMO+2 towards anchoring group which
facilitated charge injection by high energy excitations as well
especially for ISQ3. As a result, ISQ3 displayed the best
photovoltaic properties among the dyes in the presence of 10
(
1
2
100 MHz, CDCl
3
) δ 152.8, 149.9, 146.0, 141.7, 132.7, 126.7, 125.5,
22.7, 120.6, 118.4, 114.1, 110.0, 77.2, 54.9, 40.8, 32.1, 30.3, 29.8, 29.5,
3.9, 22.8, 14.3. HRMS (ESI) m/z: [M+H]+ Calcd for C37
60N 518.4726,
H
found 518.4718.
10,10-Didodecyl-2-methyl-10H-indeno[1,2-g]quinoline (4). In a two-
necked round bottom flask, compound 3 (1.9 g, 3.60 mmol), acetic acid
(0.2 mL, 3.6 mmol) and 20 mL HCl (6 M) were added. The mixture was
refluxed at 100 ºC for 1 h, followed by addition of iodine (42 mg, 0.18
mmol), potassium iodide (60 mg, 0.36 mmol) and PhMe (4 mL).
Crotonaldehyde (0.6 mL, 7.2 mmol) was dissolved in PhMe (2 mL) and
added to the reaction flask dropwise over 30 min. The resulting mixture
was refluxed for another 6 h and cooled to the room temperature after
completion of the reaction. The ammonia solution 10 mL was added to
neutralize the reaction. 50 mL water was added to the reaction mixture,
extracted by ethyl acetate and organic phase was dried over sodium
sulphate. The solvents were removed under reduced pressure and the
residue was purified by column chromatography (EtOAc-Pet ether) to
-2
equiv CDCA with JSC of 10.02 mAcm , VOC of 0.58 V and ff of
2% which led to the PCE of 4.15%. The results present an
7
effective approach for developing efficient and NIR responsive
sensitizers for DSSC application.
1
afford pure compound 4 as dark brown oil (1.23 g , 60 %). H NMR (400
Experimental Section
MHz, CDCl
J = 4.5 Hz, 1H), 7.41 – 7.33 (m, 3H), 7.27 (d, J = 8.5 Hz, 1H), 2.76 (s,
H), 2.13 – 1.97 (m, 4H), 1.25 – 1.07 (m, 23H), 1.07 – 0.98 (m, 12H),
3
) δ 8.11 (d, J = 8.4 Hz, 1H), 8.01 (s, 1H), 7.95 (s, 1H), 7.82 (d,
3
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Chemistry - A European Journal
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FULL PAPER
0
.85 (t, J = 6.9 Hz, 6H), 0.68 – 0.58 (m, 4H). 13C NMR (100 MHz, CDCl
δ 158.3, 153.4, 151.4, 148.2, 140.2, 140.0, 136.5, 128.3, 127.2, 126.3,
3
)
– 0.45 (m, 4H). 13C NMR (100 MHz, DMSO-d
/CDCl (1:1)) δ 176.5,
6 3
170.8, 170.2, 156.1, 151.8, 150.5, 149.1, 143.6, 140.2, 139.2, 138.5,
135.2, 132.0, 131.3, 129.9, 129.5, 128.3, 127.5, 126.7, 125.0, 124.0,
123.1, 120.5, 119.1, 114.7, 110.3, 98.9, 94.5, 93.7, 79.2, 56.0, 37.5, 36.1,
31.8, 29.7, 29.5, 29.4, 29.2, 29.1, 23.8, 22.6, 14.3. HRMS (ESI) m/z: [M]+
1
2
23.3, 122.2, 121.5, 120.5, 117.0, 55.2, 41.4, 32.0, 30.2, 29.8, 29.7, 29.7,
9.4, 29.4, 25.5, 24.1, 22.8, 14.3. HRMS (ESI) m/z: [M+H]+ Calcd for
41
C H62N 568.4882, found 568.4877.
72 2 4
Calcd for C58H N O 860.5492, found 860.5474.
1
0,10-Didodecyl-1,2-dimethyl-10H-indeno[1,2-g]quinolin-1-ium
iodide (5). The compound 4 (0.68 g, 1.2 mmol), MeI (0.85g, 6 mmol) and
mL acetonitrile were taken in a sealed tube and heated at 100 ºC for 48
2
Acknowledgments
h. After the completion of the reaction the solvents were removed to give
compound 5 as yellow gum which was used without further purification
0.767 g, 90%). 1H NMR (200 MHz, CDCl
This work is financially supported by the Council of Scientific and
Industrial Research (CSIR) Network Project NWP0054 (CSIR-
TAPSUN), and SERB-EMR/2016/007114, India. R.B. thanks
CSIR, New Delhi for research fellowships. J.N. thanks Dr.
Kothandam Krishnamoorthy, Polymer Science Engineering
Division, CSIR-National Chemical Laboratory, Pune, India, for
his support, help with device fabrication and characterization.
Authors also thank Mr. Ananthan Alagumalai for help with
making a precursor.
(
8
3
) δ 8.87 (d, J = 8.5 Hz, 1H),
.39 (s, 1H), 8.22 (s, 1H), 8.01 – 7.87 (m, 2H), 7.52 – 7.40 (m, 3H), 4.77
(
s, 3H), 3.33 (s, 3H), 2.25 – 2.07 (m, 4H), 1.22 – 1.02 (m, 35H), 0.85 (t, J
=
1
1
2
6.4 Hz, 6H), 0.63 – 0.44 (m, 4H). 13C NMR (100 MHz, CDCl
3
) δ 161.7,
59.0, 151.3, 145.4, 143.9, 140.1, 137.4, 130.6, 128.8, 128.1, 124.8,
23.6, 121.8, 119.4, 112.1, 57.0, 54.5, 41.6, 41.1, 32.0, 30.0, 29.71,
+
9.65, 29.4, 24.7, 24.2, 22.8, 14.2. HRMS (ESI) m/z: [M] Calcd for
+
C
42
H
64N 582.5033, found 582.5031.
General procedure for synthesizing ISQ dyes: Indenoquinaldinium
iodide, 5 (1 equiv) and respective semi-squaric acid derivatives P1, or P2
or P3 (1.2 equiv) were dissolved in 10 mL of n-BuOH/PhMe (1:1) in a
two-necked round bottom flask and fitted with Dean-Stark apparatus.
Quinoline (1 mL) was added to the reaction mixture and refluxed for 24 h
under argon atmosphere. The reaction mixture was cooled to room
temperature, and the solvents were removed under reduced pressure.
Keywords: unsymmetrical squaraine dyes, Indenoquinaldine,
H- and J-type aggregation, dye-sensitized solar cells, NIR active
dyes, CDCA
2 2
The residue was purified by column chromatography (MeOH-CH Cl ) to
afford the desired dye as dark green solid.
[
1]
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[2]
a) B. E. Hardin, H. J. Snaith, M. D. McGehee, Nat. Photonics 2012, 6,
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o
1
Yield: 0.165 g, 67 %. m.p. 200-202 C. H NMR (400 MHz, CDCl
3
) δ 9.57
(d, J = 9.3 Hz, 1H), 8.09 (d, J = 8.4 Hz, 1H), 8.03 (s, 1H), 7.98 – 7.87 (m,
2
8
1
0
1
1
1
2
H), 7.85 – 7.72 (m, 1H), 7.54 (s, 1H), 7.44 – 7.34 (m, 3H), 6.87 (d, J =
.4 Hz, 1H), 6.30 (s, 1H), 5.84 (s, 1H), 4.13 (s, 3H), 3.44 (s, 3H), 2.15 –
.96 (m, 4H), 1.83 (s, 6H), 1.24 – 1.02 (m, 36H), 0.84 (t, J = 6.9 Hz, 6H),
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.65 – 0.49 (m, 4H). 13C NMR (100 MHz, CDCl
3
) δ 184.7, 181.8, 180.8,
70.5, 168.1, 165.4, 156.9, 153.7, 150.6, 148.1, 141.6, 139.8, 138.9,
36.8, 131.3, 128.8, 127.6, 125.7, 125.6, 124.0, 123.3, 123.2, 120.7,
19.2, 109.7, 107.3, 96.0, 87.8, 56.2, 47.5, 41.1, 37.8, 32.00, 30.4, 30.1,
378; g) P. Pinpithak, A. Kulkarni, H.-W. Chen, M. Ikegami, T.
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Segawa, Y. Murata, A. Wakamiya, Bull. Chem. Soc. Jpn. 2017, 90,
+
9.7, 29.6 29.4, 29.4, 27.6, 24.0, 22.8, 14.2. HRMS (ESI) m/z: [M] Calcd
441–450.
4
for C59H76N2O 876.5805, found 876.5793.
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343–1361; c) C.-Y. Chen, M. Wang, J.-Y. Li, N. Pootrakulchote, L.
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o
1
Yield: 0.121 g, 50 %. m.p.180-182 C. H NMR (500 MHz, CDCl
3
) δ 9.54
(d, J = 9.1 Hz, 1H), 8.70 (s, 1H), 8.19 (d, J = 11.0 Hz, 2H), 7.93 (d, J =
[4]
8
1
.0 Hz, 1H), 7.87 (d, J = 12.9 Hz, 2H), 7.77 (d, J = 4.4 Hz, 1H), 7.49 (s,
H), 7.38 (d, J = 6.1 Hz, 3H), 7.27 (s, 1H), 6.25 (s, 1H), 5.89 (s, 1H), 4.08
(s, 3H), 3.55 (s, 3H), 2.10 (s, 6H), 2.06 – 1.97 (m, 4H), 1.23 – 1.11 (m,
2
4H), 1.06 – 1.01 (m, 12H), 0.84 (t, J = 7.1 Hz, 6H), 0.64 – 0.55 (m, 4H).
) δ 184.9, 181.9, 179.0, 168.8, 168.1, 156.6,
53.2, 150.6, 139.9, 139.4, 139.0, 136.1, 133.7, 133.1, 131.4, 131.2,
1
3C NMR (126 MHz, CDCl
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ISQ3: Started with 5 (0.2 g, 0.28 mmol) and P3 (0.124 g, 0.34 mmol).
Yield: 0.16 g, 60%. m.p. 142-144 oC. 1H NMR (400 MHz, [D
]DMSO
(1:1)) δ 9.29 (d, J = 9.3 Hz, 1H), 9.05 (d, J = 9.5 Hz, 1H), 8.01 (s,
H), 7.97 (s, 2H), 7.82 (d, J = 9.4 Hz, 1H), 7.75 (d, J = 4.5 Hz, 1H), 7.69
6
/
CDCl
3
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(s, 1H), 7.41 (d, J = 8.9 Hz, 1H), 7.37 – 7.33 (m, 2H), 7.32 – 7.29 (m, 2H),
5
1
.84 (s, 1H), 5.57 (s, 1H), 3.97 (s, 3H), 3.64 (s, 3H), 2.08 – 1.96 (m, 4H),
.17 – 1.05 (m, 24H), 1.00 – 0.95 (m, 12H), 0.79 (t, J = 6.8 Hz, 6H), 0.52
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Three indenoquinaldine based
unsymmetrical squaraine dyes were
synthesized for dye-sensitized solar
cells. The dye ISQ3 exhibited strong
absorption in NIR region,
Rajesh Bisht, Munavvar Fairoos Mele
Kavungathodi, and Jayaraj
Nithyanandhan*
Page No. – Page No.
panchromatic IPCE and efficiency up
to 4.15 %.
Indenoquinaldine Based
Unsymmetrical Squaraine Dyes for
Near-Infrared Absorption:
Investigating the Steric and
Electronic Effects in Dye-sensitized
Solar Cells
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