Heterotriangulene-based unsymmetrical squaraine dyes: synergistic effects of donor moieties and out-of-plane branched alkyl chains on dye cell performance

Article abstract of DOI:10.1039/c6ta08651a

Unsymmetrical squaraine sensitizers with two different donor moieties, triphenylamine (NSQR) and heterotriangulene (NSQ1-3), for dye-sensitized solar cells (DSSCs) have been designed and synthesized. These dyes utilize the indolium moiety to control charge recombination dynamics at the TiO₂-dye-electrolyte interface by connecting linear and branched alkyl functionalities. In the present study, an efficient heterotriangulene (HT) donor and a branched alkyl group at sp³-C atoms were strategically incorporated to increase the power conversion efficiency (PCE) of zwitterionic dyes by improving photo-current density (J_sc) and open-circuit potential (V_oc) of the cell. Among these four dyes, NSQ3 exhibited the highest efficiency of 6.73% with a J_sc of 18.74 mA cm^?2, V_oc of 0.53 V, and fill factor (ff) of 68.3%, without any co-adsorbent under an irradiance of 100 mW cm^?2 (simulated AM 1.5G sunlight). In the presence of 3α,7α-dihydroxy-5β-cholanic acid (CDCA), NSQ1, NSQ2 and NSQ3 showed an efficiency of 7.07%, 7.38% and 7.17%, respectively. Despite the low V_oc, far red light harvesting efficiency, reduced dye aggregation, long lifetime (τ) of injected electrons and high quantum efficiency of NSQ1-3 played constructive roles in achieving high PCE efficiency. Deceleration of charge recombination for NSQ dye cells was further studied by electrochemical impedance spectroscopy (EIS) and open-circuit photo-voltage decay (OCVD) measurements.

Full text of DOI:10.1039/c6ta08651a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Heterotriangulene-based unsymmetrical squaraine dyes:
synergistic effects of donor moiety and out-of-plane branched
alkyl chains on dye cell performance
Neeta Karjule†,‡, Munavvar Fairoos MK† and Jayaraj Nithyanandhan*,†,‡
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Unsymmetrical squaraine sensitizers with two different donor moieties, triphenylamine (NSQR) and heterotriangulene
(NSQ1-3), for dye-sensitized solar cell (DSSC) have been designed and synthesized. These dyes utilize the indolium moiety
to control charge recombination dynamics at TiO2-dye-electrolyte interface by connecting linear and branched alkyl
functionalities. In the present study, an efficient heterotriangulene (HT) donor and branched alkyl group at sp3-C atom
were strategically incorporated to increase the power conversion efficiency (PCE) of zwitterionic dyes by improving photo-
current density (Jsc) and open-circuit potential (Voc) of the cell. Among these four dyes, NSQ3 exhibited the highest
efficiency of 6.73% with a Jsc of 18.74 mA/cm2, Voc of 0.53 V, and fill factor (ff) of 68.3%, without any co-adsorbent under
irradiance of 100 mW/cm2 (simulated AM 1.5G sunlight). In presence of 3α,7α-dihydroxy-5β-cholanic acid (CDCA), NSQ1,
NSQ2 and NSQ3 showed efficiency of 7.07%, 7.38% and 7.17%, respectively. Despite the low Voc, far red light harvesting
efficiency, reduced dye aggregation, long lifetime (τ) of injected electron and high quantum efficiency of NSQ1-3 played
constructive role to achieve high PCE efficiency. Deceleration of charge recombination for NSQ dye cells were further
studied by electrochemical impedance spectroscopy (EIS) and open-circuit photo-voltage decay (OCVD) measurements.
www.rsc.org/
to inefficient power conversion.19,20 There are two types of
squaraine dyes known in the literature – symmetrical21,22 and
unsymmetrical23,24, and the directionality of electron injection
1. INTRODUCTION
DSSCs have been considered as a promising alternative to
conventional inorganic p-n junction silicon-based solar cells for
solar energy-to-electricity conversion.1,2 Since the first report
by O’Regan and Grӓtzel in 1991, dye cells have been received
considerable attention because of their low fabrication cost
and diversity in device components.3 Sensitizers used in dye
cell to absorb the photons are broadly classified into two
categories: 1) organometallic dyes (ruthenium-polypyridyl
complexes4,5 and zinc porphyrin dyes)6 and 2) metal-free
organic dyes7,8. Particularly, to harness the long wavelength
photons, near-infrared (NIR) light harvesting dyes like
phthalocyanines,9,10 polymethines,11 and porphyrins12,13 have
been explored and so far the best conversion efficiency was
obtained for porphyrin based dye (SM315) is 13% with
Co(II)/(III) redox couple.14
was realized better in the latter. The device efficiencies for
symmetrical and unsymmetrical dyes have been reached at
25 26
) and 8.9% (DTS-CA) respectively. Recently,
6.18% (VG10-C2
few triphenylamine (TPA) based unsymmetrical squaraine dyes
have been synthesized27,28 and the highest PCE of 6.29% was
achieved for JK216 dye.29 These sensitizers exhibited intense
absorption bands in the NIR region30 but they tend to form
either H- or J- type aggregates and reduce the excited state
electron lifetime.31 Also the propeller shaped TPA lacks rigidity
and poor extension of π -conjugation over the whole molecule
due to the large dihedral angles between the phenyl rings and
the plane of the N-bonded carbon atoms.32,33 Based on the
understanding from the previous studies, planar
heterotriangulene donor system was introduced in donor-π-
acceptor DSSCs to achieve fast electron injection,34 increases
the electronic coupling between donor and acceptor moieties
of the dye, and localize the positive charge resulting after
electron injection.32 By introducing a planar rigid moiety,35
dyes tend to assemble via π-π stacking while anchoring on TiO2
and to keep dyes separated for avoiding the aggregation-
induced decay of the excited state, indolium moiety of the
unsymmetrical squaraines was functionalized with in-plane
and out-of-plane alkyl functionalities.36
Squaraines, one of the polymethine dyes, have attracted
special interest because of their intense absorption in the far-
red and near-infrared (NIR) regions,15,16,17 and high molar
absorptivity (~105 M-1 cm-1) compared to zinc porphyrins.18
However, these zwitterionic molecules aggregate on metal
oxide surface that reduces the excited state lifetime, and leads
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Figure 1. Molecular structures of the unsymmetrical squaraine NSQR and NSQ1-3 sensitizers.
In this study, we report a new dye design to improve the voltage (I-V) measurements showed that HT based dyes
PCE of unsymmetrical squaraine by taking the advantage of a NSQ1-3) outperformed TPA based squaraine (NSQR) in terms
strong donor and an alkyl functionalized indolium unit (Figure of both Jsc and Voc. Although, NSQ2 and NSQR shows similar
). In both dyes, squaric acid unit is linked between either HT molecular structure apart from the different donors, it is found
(
1
or TPA donor and indolium moiety having a carboxylic acid as that, Jsc of NSQ2 significantly increased by 41.2% than NSQR
an anchoring group. Introduction of long alkyl chains on because of the effective conjugation. In-plane and out-of-
indolium unit is expected to increase the solubility, decrease plane alkyl groups of NSQ3 further increased the Jsc by 30.2%
the undesirable dye aggregation and suppress dye desorption and hence achieved the PCE of 6.73% without any co-
by reducing the infiltration of solvent to the TiO2 surface.37,38 adsorbent. Bulky, passivating indolium unit close to the
We systematically connected alkyl groups in the N- and sp3-C anchoring group suppressed the charge recombination, and
atoms of indolium unit to provide an in-plane and out-of-plane the Voc of NSQ3 increased by 118 mV without CDCA than
branching of hydrophobic chains, respectively.39 Current- NSQR
, which has been verified by photo-voltage decay
Scheme 1. a) Syntheses of indolium iodide (3a-3c), b) NSQ 1-3 dyes and c) NSQR dye.
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studies.40 41
,
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization
The molecular structure of sensitizers and their synthetic
scheme are illustrated in Figure 1 and Scheme 1, respectively.
Synthesis of unsymmetrical squaraine sensitizers, NSQR and
NSQ1-3, requires semi-squaraine and an indolium salt with
either N- or sp3-C or both atoms functionalized with alkyl
groups. To reduce the number of steps involved in the
synthesis, we have chosen TPA and HT derived semi-squaric
acid and suitably functionalized indolium salt as the
precursors. For the indolium moiety contains sp3-C atom
functionalized with alkyl groups, suitably substituted branched
methyl ketone is required and we have chosen judiciously 3-
octyltridecan-2-one. The branched methyl ketone was
synthesized starting from oxidation of branched alcohol to the
corresponding aldehyde. Further reaction with MeMgI
afforded the corresponding secondary alcohol which was
oxidized to give the required precursor in moderate yield and
reacted with 4-caboxyphenylhydrazine to provide the
intermediate (2a-2b). The corresponding indolium salts (3a-3c
)
were synthesized by reacting with alkyl halide in moderate
yield. The semi-squaraine derivatives 4c and 5c were prepared
by reaction of 4a and 5a with squaryl chloride followed by acid
hydrolysis of the semi-squaryl chloride derivatives.42
A
Figure 2. (a) Absorption (solid line) and emission (dot line)
spectra of NSQ dyes in CH3CN solution (b) Absorption spectra
of NSQ sensitizers (0.1 mM in CH3CN) adsorbed on the
transparent TiO2 film (6 ꢀm), dipped for 10 min.
condensation reaction between semi-squaryl chloride
derivatives of 4c or 5c and 5-carboxy-2,3,3-trialkyl-1-alkylindol-
ium units
(3a-3c) afforded the NSQ sensitizers. All the
sensitizers have good solubility in MeCN, CH2Cl2, CHCl3, DMSO
and limited solubility in MeOH. All the intermediates and final
the presence of out-of-plane alkyl segments on indolium sp3-C
atom, which effectively suppresses dye aggregation. It is
1
dye molecules were characterized by H- and 13C- NMRs and
mass spectrometry (HRMS-ESI).
observed that emission λmax of NSQ sensitizers are shifted to
longer wavelength and maximum stokes shift of ~70 nm is
observed for NSQ1-3 sensitizers, which is relatively small for
NSQR (~50 nm). This finding qualitatively supports the larger
dipole moments in excited state for NSQ1-3 in solution (Figure
2a) due to additional conjugation from HT unit.
2.2. Photophysical Properties
The UV-Vis absorption spectra of NSQR and NSQ1-3 in CH3CN
solution, on transparent mesoporous TiO2 film, and the
emission properties are shown in Figure 2 and summarized
photo-physical properties are provided in Table
1. The
absorption spectra of both NSQR and NSQ1-3 were exhibited
an intense absorption band centered around 643 nm and 674
nm, respectively, corresponding to internal charge transfer
(ICT) transition, with a high molar absorption coefficient (ε) in
the range of 104 M-1 cm-1.
2.3. Electrochemical Properties
Cyclic voltammetry measurements were carried out in CH3CN
to understand the feasibility of electron transfer from the
excited state of the sensitizers to the conduction band of the
TiO2 and the regeneration of oxidized dyes by the electrolyte.
Redox potentials of NSQR and NSQ1-3 were measured (Figure
Apart from the ICT band, NSQ1-3 shows π-π* transition
between 400-500 nm with lower ε
values (~103 M-1 cm-1). After
3
and Table 1), and exhibited the quasi-reversible first ground-
substituting the propeller triphenylamine donor with planar
triphenylamine, i.e. heterotriangulene, absorption maxima
state oxidation potential of NSQR at 0.78 and for NSQ1-3 at
0.79 (NSQ1), 0.81 (NSQ2) and 0.81 (NSQ3) V vs NHE. Eox of
NSQs was low enough to get regenerated by iodide (0.4 V vs
NHE). Highest occupied molecular orbital (HOMO) of the NSQR
was not affected after substituting with fused triarylamine,
NSQ1, as listed in Table 1. The lowest unoccupied molecular
orbital (LUMO) of NSQ dyes were calculated by subtracting E0-0
from EHOMO and the values are found to be -1.08, -0.98, -0.95
(λmax) of NSQ1-3 is red-shifted around 30 nm relative to that of
NSQR, due to extended conjugation. The absorption spectra of
NSQ1-3 are broadened on TiO2 thin film due to the interaction
of carboxylic acid groups with the surface titanium ions and
λmax is blue shifted by 32 and 38 nm for NSQ1 and NSQ2
,
respectively, because of H-type of aggregation.43 However,
NSQ3 shows only 8 nm shift towards higher energy because of
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Figure 3. (a) Cyclic voltammograms of NSQR and NSQ1-3
measured in CH3CN, with TBAClO4 (0.1 M) at the scan rate of
50 mV s−1 (b) Energy level diagram for the NSQ sensitizers vs
NHE.
Figure 4. HOMO and LUMO isosurface plots for NSQR (left) and
NSQ2 (right), respectively.
and -0.96 V vs NHE for NSQR, NSQ1, NSQ2 and NSQ3,
(NSQR, NSQ2) and Figure S47. In NSQ1-3 sensitizers, the
electron density in HOMO and HOMO-1 was more delocalized
on donor part than NSQR and also well distributed throughout
squaric acid, whereas the LUMO was delocalized towards
indolium unit contains electron withdrawing carboxylic acid.
This HOMO and LUMO overlap facilitates the effective charge
transfer from donor to anchoring unit and hence shows a red-
shifted λmax in the absorption spectrum. The LUMO+1 for all
sensitizers were mainly located near to anchoring group. On
the other hand, in NSQR the HOMO-1 was located on squaric
acid unit, which indicates the inability of triarylamine to
regenerate the dye after injection of electron to TiO2
compared to other NSQ dyes. These overall DFT results
respectively, which lie above the conduction band edge (CB) of
TiO2
injection into the conduction band of TiO2 as shown in Figure
3b NSQR and NSQ1-3 showed a band gap (Eg) in the range of
(-
0.5 V vs NHE)44 that allows an effective electron
.
1.7-1.9 eV, which are in good accordance to the band gaps
determined from the DFT studies.
2.4. Theoretical Investigations
The molecular geometries of NSQ sensitizers were optimized
using density functional theory (DFT) at B3LYP/6-31G** level
with the Gaussian 09 program45 and the isosurface plot for
HOMO-1, HOMO, LUMO, and LUMO+1 are shown in Figure 4
4
E0-0 (eV)d
SQ Dyes
Eg/DFT (eV)
Eox/onset (V vs
+
Ag/Ag )
EHOMO
ELUMO
(V vs NHE)e
λ
λ
λ
max /em
ε ×10
max
max
c
a
-1
-1
(M cm )
a
CH CN
/TiO2
/
(V vs NHE)
(nm)
3
a
b
(nm)
643
(nm)
NSQR
NSQ1
NSQ2
NSQ3
624
641
636
666
696
746
746
743
5.5
2.13
2
0.28
0.29
0.31
0.31
0.78
0.79
0.81
0.81
1.86
1.77
1.76
1.77
-1.08
-0.98
-0.95
673, 458
674, 464
674, 464
6.1, 0.86
6.9, 0.73
6.5, 0.92
2.01
2.01
-0.96
c
aAbsorption and emission spectra, molar extinction coefficients (ε) were measured in CH3CN. bOn 6 ꢀm transparent TiO2 film. The
oxidation potentials were measured in CH3CN solutions with tetrabutylammonium perchlorate (TBAClO4) as supporting electrolyte,
d
ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 0.63 V. Optical energy gaps (E0-0) were
e
deduced from the intersection of absorption and emission spectra, E0-0 (eV) = 1240/λ. ELUMO was calculated from ELUMO (V vs NHE) =
EHOMO (V vs NHE) - E0–0
.
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Figure 5. Selected dihedral angles of NSQR and NSQ3 calculated from the optimized ground state geometry using density functional
theory (DFT) at B3LYP/6-31G** level with the Gaussian 09 program (hydrogen atoms are removed for better clarity).
suggest that the HOMO-LUMO or HOMO-LUMO+1 excitations respectively. But for NSQR, IPCE response around 750 nm is
can afford electron flow from the donor unit to the anchoring 15% at the same condition.
group.
The current density−voltage (J−V) characteristics of DSSCs
Degree of conjugation between donor and acceptor could based on NSQR and NSQ1-3 were evaluated under simulated
be influenced by dihedral angle between benzene and AM 1.5G solar light (100 mW cm-2), and the curves are shown
nitrogen atom of TPA or HT donor (θ1), donor and squaric acid in Figure 6a and 7a. In the case of NSQR, NSQ1 and NSQ2, the
unit (θ2), and squaric acid and indolium unit θ3.46 θ1 for NSQR moderate performance is achieved without any co-absorbent,
is found to be more deviated than NSQ1-3 dyes, indicates a because of the aggregation of dyes in the absence of CDCA.
better co-planarity of donor-acceptor parts (Figure 5). The θ1 Among the NSQs, NSQ3 sensitized cells gives better
value of NSQR is 47.06°, whereas for NSQ1-3 it is close to 0o performance without CDCA and achieved an overall power
(
Table S1). Bulky alkyl functionalities of indolium unit exhibits conversion efficiency (η
) of 6.73% with a Jsc of 18.74 mA cm−2
extended conformation with θ4 of 176.6°, which helps to and Voc of 0.53 V. In particular, among the four dyes the NSQ3
improve the photo-physical properties by reducing the
aggregation as well as extending the conjugation. Distance
between (i) sp3-C (methylene group of HT) to –O atom of
carboxylic acid, (ii) terminal carbon atoms of sp3 -branched
alkyl chain and (iii) sp3-C (indoline) to –O atom of carboxylic
acid were 19.6 Å, 23.1 Å, and 6.2 Å, respectively for NSQ3
(Figure S48). Such structural features may lead to better
exposed HOMO for efficient dye regeneration, reduced dye
aggregation and charge recombination due to extended
conformation of alkyl groups, and the presence of long
branched hydrophobic alkyl chains near to the surface,
respectively.
2.5. Photovoltaic Performance of DSSCs
Incident photon-to-current conversion efficiency (IPCE) of
NSQR and NSQ1-3 DSSCs are shown in Figure 6b and 7b. The
increased JSC value of NSQ1-3 sensitizers are reflected in the
IPCE spectra, and shows broader and higher IPCE in the range
of 400 to 800 nm, reaching a maximum of 73% and 69% at 420
and 635 nm, respectively, in the absence of CDCA for NSQ3
.
NSQ3 dye converted 50% of the photons at 750 nm, whereas
NSQR shows only 8% IPCE response at the same wavelength.
NIR photons are effectively converted when the TiO2 sensitized
with NSQ3 than any other dyes in the series without CDCA,
with the IPCE of 30% at 780 nm, where NSQ1 and NSQ2 shows
below 10%. After the addition of CDCA, NSQs shows further
increase in IPCE at higher wavelength region, and at 750 nm
NSQ1, NSQ2 and NSQ3 exhibits IPCE of 50%, 58% and 62%,
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Results of five cells with deviation for optimized dye/ CDCA
are shown in Figure S49 and photovoltaic performances are
summarized in Table 2. As expected, the addition of CDCA has
a positive impact on improving the Voc and Jsc values of NSQ
sensitizers based DSSCs, and hence after the addition of 10
equivalents of CDCA, the NSQ2 sensitized cells shows an
overall power conversion efficiency (
20.57 mA cm−2, Voc of 0.53
and ff of 0.68, while
triphenylamine based NSQR sensitized cells were exhibited a
η) of 7.4% with a Jsc of
V
relatively poor performance with an overall η of 4.63% at the
same amount of co-adsorbent due to lower Jsc (14.95 mA cm-2)
and Voc (0.47 V) values. The NSQ1 sensitized cell was achieved
a
Jsc value of 21.32 mA cm-2, Voc of 0.49 V, and ff of 0.68,
corresponding to
η of 7.07%. Device made with NSQ3 reached
at a Jsc of 20.11 mA cm-2, Voc of 0.54 V, ff of 0.66 and overall
conversion efficiency of 7.17% with the addition of minimum
amount of CDCA (1 equivalent), and the curves are shown in
Figure 7a. It is found that the Jsc and Voc of NSQ3 dyes
decreases with further addition of CDCA due to competitive
binding of CDCA over dyes. These results indicate that the
addition of large amount of CDCA is not beneficial for NSQ3 to
enhance the photovoltaic performance (Table S2 in SI). Dye
desorption results indicate the modulated adsorption of dyes
on TiO2 with respect to their structure (Table 2 and Figure S50
). Dye loading ability decreases in the order of NSQR
NSQ2 NSQ3, due to progressive increase of alkyl groups near
TiO2 surface which blocks the dye anchoring to the
pentacoordinated Ti center However, addition of 10
> NSQ1 >
≈
.
equivalents of CDCA abruptly changes the quantity of
adsorbed dyes in the case of NSQ3 over other three dyes that
leads to decreased device efficiency (Table S2). All the dyes
with HT donor showed high Jsc with and without CDCA than
triphenylamine donor. Herein, considering the very similar
configuration between NSQ1-3 and NSQR, the incorporated
HT donor and branching at chromophore paved the way to
extend the light harvesting ability and reducing the charge
recombination property. The distinct enhancement of Jsc and
Figure 7. (a) J–V curve and (b) IPCE spectra of NSQR, and
NSQ1-3 in the presence of CDCA.
shows the maximum Voc of 0.54 V and 0.53 V with and without
CDCA, respectively, proving the strong ability of branched and
long alkyl side chain on chromophore that prevents surface
aggregation and reduces the charge recombination even
without any co-adsorbent.47
NSQ Dye Cell
Voc [ V ]
Jsc [ mA/cm2 ]
ff [ % ]
η [ % ]
Amount of adsorbed dyes
(×10-7 mol cm-2)b
NSQR
NSQR/CDCA (1:10)
NSQ1
0.406±0.002
0.467±0.004
0.465±0.004
0.486±0.002
0.474±0.002
0.526±0.001
0.521±0.004
0.540±0.001
7.4±0.29
14.64±0.31
11.93±0.38
20.77±0.55
12.90±0.18
20.35±0.22
18.38±0.36
20±0.11
66.8±0.8
64.4±1.3
68.7±0.5
67.6±0.4
65.1±1.0
66.9±1.2
68.1±0.20
65.7±0.25
2.01±0.11
4.40±0.23
3.81±0.18
6.82±0.25
3.98 ±0.13
7.16±0.22
6.53±0.20
7.09±0.08
2.12
1.56
1.81
1.01
1.23
0.87
1.13
0.94
NSQ1/CDCA (1:10)
NSQ2
NSQ2/CDCA (1:10)
NSQ3
NSQ3/CDCA (1:1)
aPhotovoltaic performance of NSQR and NSQ1-3 with deviation of 5 cells, thickness of electrode: 8+4 ꢀm (transparent+ scattering)
layer of TiO2, Electrolyte: 0.5 M DMII, 0.1 M LiI, 0.1 M I2 and 10 mM TBP in CH3CN. Dipping time was 6 h. Active area of 0.22 cm2 and
measurements were carried out under 1 sun intensity (100 mW/cm2), bby dye desorption method, carried out in 2M ethanolic HCl at
rt.
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Voc of NSQ1-3 suggests that the incorporation of fused donor Thus, through Nyquist and Bode plots of EIS analysis,
unit and branched alkyl groups into the dye structure would be corresponding charge transfer resistance (Rct), Cμ and τ is
a meaningful approach to improve the PCE of metal-free
squaraine dyes.
2.6. Electrochemical Impedance Spectroscopy
To understand the effect of fused donor and out-of-plane
elucidated to explain the charge transfer process at
TiO2/dye/electrolyte interface.
branched alkyl groups on photovoltaic performance of NSQ
dye cells, EIS analysis was carried out in the dark with applied
potential. Generally, Voc and Jsc of dye cells are known to be
affected by the charge recombination dynamics at the
interface of TiO2/dye/electrolyte.48 Voc of dye cell is
determined by the difference between quasi-Fermi level of
Nyquist plot of NSQR and NSQ1-3 dye cells is shown in
Figure 8a, the second semi-circle representing the charge
transfer reaction at the TiO2/dye/electrolyte interface for
NSQ3 was larger than other dyes, in the order of
NSQ3>NSQ2>NSQ1>NSQR. The peak frequency of Bode plot
has shifted towards the lowest value for NSQ3 as shown in
Figure 8b. In general, a long electron life time implies slower
electron recombination rate at the interface, which increases
the free electron density and these observation points the
effective passivation of surface by hydrophobic alkyl chains
near to TiO2 surface through van der Waals interaction. Figure
-
TiO2 (EF,n) and Fermi level (EF,redox) of the redox couple (I-/I3 in
this study), as expressed in Equation 1. The uncertainty of the
photo-voltage originates from a shift of the TiO2 electron
quasi-Fermi-level, Equation 2, where kB is the Boltzmann
constant,
T is the absolute temperature (293 K), nc the free
electron density, and Nc the density of accessible states in the
conduction band, which may be ascribed to two major
reasons: i) a shift in the TiO2 conduction band edge (ECB), which
can be inferred from the chemical capacitance (Cμ), and ii) a
fluctuation of electron density, which is related to the electron
lifetime (τ) determined by the charge recombination rate.26
8c and 8
after fitting the Nyquist plot using an equivalent circuit (inset
Figure 8c). The value of NSQ3 is longer than NSQR NSQ1 and
NSQ2, and which is consistent with observed improvement in
Voc. Strong interaction and better screening of oxide
d shows observed Rct and τ respectively, extracted
τ
,
π-π
Figure 8. Plots of EIS analysis of NSQ dye cells. (a) Nyquist plot (b) Bode plot, (c) and (d) Rct and τ as a function of voltage, respectively.
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rate of recombination in NSQ3 sensitized dye cell was
attributed to the effective passivation of mesoporous surface
by introducing hydrophobic alkyl functionalities near to the
anchoring group. The observed 120 mV difference in the Voc of
NSQ3 compared to NSQR without any co-adsorbent indicates
the slow recombination of injected electron with triiodide.
Results of EIS experiment in the dark and OCVD measurement
in the presence of light in turn supports the deceleration of
charge recombination, via dark reaction and back electron
transfer, for NSQ3 dye compared to other NSQs
.
3. CONCLUSION
The effects of HT and TPA donor moieties and types of alkyl
functionalities in unsymmetrical squaraine dyes for dye-
sensitized solar cell were investigated. Heterotriangulene
based NSQ1-3 sensitizers have shown significant improvement
Figure 9. OCVD profile of DSSCs based on NSQR and NSQ1-3
sensitizers.
in PCE than NSQR, NSQ3 sensitized device achieved an
efficiency of 6.73% without co-adsorbent, and in presence of
CDCA NSQ2 and NSQ3 exhibited 7.4% and 7.17% PCE,
respectively. Higher and broader IPCE of NSQ1-3, due to
extended conjugation and strong donor-acceptor coupling,
shows the better NIR light harvesting efficiency of these metal-
free dyes than NSQR. The EIS and OCVD studies were proved
that alkyl-functionalized indolium unit attached to carboxylic
acid suppresses the undesirable charge recombination and
prolongs the electron lifetime, and hence significantly
enhancing the PCE. As a result, NSQ1-3 exhibited an efficiency
of more than 70% over reference sensitizer NSQR, PCE of
4.6%. This study suggests that introducing an out-of-plane alkyl
chain on indolium group of unsymmetrical squaraine increases
the Jsc and Voc, and avoiding the co-adsorbent from dye cell
fabrication. Hydrophobic alkyl functionalities on squaraine
chromophore near to the surface of TiO2 leads to better
control over the recombination process between electrolyte
and TiO2.
surface showed maximum τ around 52 ms at applied bias of -
0.3 V. Under same condition, NSQ1 and NSQ2 showed τ of
13.8 and 29.5 ms, respectively, indicating increase in charge
recombination rates. NSQs increased Rct and Cμ
(Figure S51)
further supported the high Jsc and Voc values than NSQR
.
Increase in rate of electron injection and reduced interfacial
recombination could bring the difference in electron density in
conduction band of metal oxide.49 Overall, the successful
approach towards increasing the efficiency of unsymmetrical
squaraine dyes discussed here could improve further by
50
focusing on Voc
.
Overall performance of NSQ3 shows an
approach to design co-adsorbent free unsymmetrical
squaraine dye cells with high efficiency.
2.7. Open-circuit voltage decay measurement
Furthermore, transient recombination kinetics of NSQs were
studied after illuminating the device. Open-circuit voltage
decay-time profile reflects the electron recombination kinetics
after generating triiodide at TiO2/dye/electrolyte interface.51
Equation 3 was used to evaluate the correlation between Voc
4. EXPERIMENTAL SECTION
4.1. Materials and Characterization
All reagents were purchased from commercial sources.
Solvents were dried and distilled immediately prior to use by
standard procedures. All reactions were carried out under an
argon atmosphere. Characterizations of all compounds are
provided in supporting information. 1H NMR and 13C NMR
were recorded in, CDCl3, MeOH-d4 or DMSO-d6 on 200 MHz
NMR, 400 MHz NMR and 500 MHz NMR spectrometers. High-
resolution mass spectrometric measurements were carried out
using the ESI method and an ion-trap mass analyzer.
Absorption spectra were recorded at room temperature in
decay and electron lifetime (τn), where kBT the thermal energy
and e is is the positive elementary charge:
According to this equation, the electron lifetime of the
devices can be extracted from the slope of the photovoltage
decay curves. Voc decay traces were measured in the dark after
illuminating the cell for 10 s at 1 sun intensity. Figure 9 shows
the decay profile of NSQR and NSQ1-3 dye cells, NSQ3 dye cell
shows the slowest decay (78% at 50 s) and NSQR displays the
fast voltage decay (95% at 50 s). Remarkably, the minimum
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quartz cuvette
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using
Analytik
Jena
UV−Visible liquid. 1H NMR (200 MHz, CDCl3)
δ
: 2.45 - 2.32 (m, 1 H), 2.06 (s,
spectrophotometer. Electrochemical measurement was 3 H), 1.57 - 1.48 (m, 2 H), 1.42 - 1.30 (m, 2 H), 1.21 (br. s, 28 H),
carried out using a Bio-Logic potentiostat (model no: SP300). 0.88 - 0.78 (m, 6 H). 13C NMR (100 MHz, CDCl3)
: 212.9, 53.3,
δ
The cyclic voltammetric analysis (CV) was carried out in dry 31.8, 31.8, 31.7, 29.7, 29.5, 29.4, 29.4, 29.3, 29.2, 29.1, 28.5,
acetonitrile solvent by using 0.1 M tetrabutylammonium 27.4, 22.6, 14.0. HRMS (ESI): m/z calcd for C21H43O ([M+H]+):
perchlorate as supporting electrolyte and Fc/Fc+ as internal 311.3314. Found: 311.3308.
reference. The experiments were performed at room
2,3,3-Trimethyl-3H-indole-5-carboxylic acid (2a)
temperature in nitrogen atmosphere with a three-electrode
cell consisting of a platinum foil as counter electrode, an
Ag/Ag+ reference electrode, and a platinum wire as working
electrode.
4-Hydrazinobenzoic acid hydrochloride (4 g, 21.2 mmol) and 3-
methyl-2-butanone (3.92 mL, 36.69 mmol) were dissolved into
glacial acetic acid (40 mL) and refluxed under nitrogen for 16 h.
The mixture was cooled and the solvent was removed under
reduced pressure. The residue was dissolved by adding CH2Cl2,
and then washed with saturated solution of NaHCO3 and dried
4.2. Synthetic Procedure
4,4,8,8,12,12-Hexamethyl-4H,8H,12H-benzo[1,9]quinolizino
[3,4,5,6,7,-defg]acridine (HBQA,4a)52,53 was synthesized using a
over Na2SO4. The organic layer was concentrated to afford 2a
1
(3.1 g, 72%) as a light red solid. H NMR (200 MHz, CDCl3)
δ
:
procedure of previous references.
10.56 (br. s., 1 H), 8.15 (dd,
Hz, 1 H), 7.69 (d,
= 8.1 Hz, 1 H), 2.39 (s, 3 H), 1.37 (s, 6 H). 13
NMR (100 MHz, CDCl3) : 192.5, 170.9, 156.9, 145.4, 130.7,
J = 8.1, 1.6 Hz, 1 H), 8.07 (d, J = 1.6
2-Octyldodecanal (1a)54
J
C
δ
To a solution of 2-octyldodecan-1-ol (4 g, 13.4 mmol) in CH2Cl2
(30 mL), pyridinium chlorochromate (7.2 g, 33.5 mmol) was
added and stirred at room temperature for 3 h. The crude
mixture was filtered through silica gel column
chromatography, washed with pet ether, solvent was
evaporated and dried under reduced pressure to afford 1a (2.2
127.2, 123.2, 119.5, 53.8, 22.8, 15.4. HRMS (ESI): m/z calcd for
C12H14NO2 ([M+H]+): 204.1025. Found: 204.1019.
3-Decyl-2-methyl-3-octyl-3H-indole-5-carboxylic acid (2b)
4-Hydrazinobenzoic acid hydrochloride (0.6 g, 3.18 mmol) and
3-octyltridecan-2-one (1.18 g, 3.82 mmol) were dissolved into
glacial acetic acid (12 mL) and refluxed under nitrogen for 12 h.
The mixture was cooled and the solvent was removed under
vacuum. The crude product was extracted with CH2Cl2, washed
with saturated solution of NaHCO3, dried over Na2SO4 and
solvent was evaporated. The reaction mixture was purified by
g, 55%) as pale yellow liquid. 1H NMR (400 MHz, CDCl3)
(d, = 3.2 Hz, 1 H), 2.40 - 2.29 (m, 1 H), 1.62 - 1.56 (m, 2 H),
1.47 - 1.43 (m, 2 H), 1.25 (br. s., 28 H), 0.89 - 0.87 (m, 6 H). 13
NMR (100 MHz, CDCl3) : 211.9, 182.8, 161.1, 77.3, 77.2, 76.7,
δ: 9.55
J
C
δ
74.6, 72.1, 45.5, 42.8, 37.4, 34.0, 32.2, 31.9, 31.8, 29.7, 29.6,
29.6, 29.5, 29.5, 29.3, 29.3, 29.3, 29.1, 27.4, 25.6, 25.2, 23.9,
22.7, 14.1. MALDI-TOF (m/z): [M]+ calcd for C20H40O: 296.3079;
found: 296.2517.
silica gel chromatography to afford 2b (0.74 g, 54%) as a light
1
red liquid. H NMR (400 MHz, CDCl3)
δ
= 8.0, 1.6 Hz, 1 H), 7.99 (s, 1 H), 7.70 (d,
: 11.78 (br. s., 1 H), 8.17
= 8.0 Hz, 1 H),
(dd,
J
J
2.33 (s, 3 H), 2.02 - 1.88 (m, 2 H), 1.83 - 1.71 (m, 2 H), 1.25 -
1.07 (m, 24 H), 0.85 - 0.80 (m, 6 H), 0.75 - 0.65 (m, 2 H), 0.61 -
3-Octyltridecan-2-ol (1b)
2-Octyldodecanal (2 g, 6.74 mmol) was dissolved in dry THF
(20 mL) at 0 oC, and then MeMgI (3.37 mL, 10.1 mmol) was
slowly added. The mixture was stirred at 0 oC for 2 h, then
quenched by adding saturated aqueous solution of ammonium
chloride, extracted with ethyl acetate and dried over Na2SO4.
The organic layer was concentrated under reduced pressure to
afford 1b (2.03 g, 97%) as pale yellow liquid. This product was
0.52 (m, 2 H). 13C NMR (100 MHz, CDCl3)
δ: 191.4, 171.2, 158.4,
142.2, 130.7, 127.1, 123.3, 119.2, 63.0, 36.9, 31.8, 31.7, 29.7,
29.5, 29.5, 29.4, 29.3, 29.3, 29.2, 29.1, 29.1, 23.5, 22.6, 22.5,
16.0, 14.0, 14.0. HRMS (ESI): m/z calcd for C28H46NO2 ([M+H]+):
428.3529. Found: 428.3523.
5-Carboxy-1,2,3,3-tetramethyl-3H-indol-1-ium iodide (3a)
1
used further without purification. H NMR (200 MHz, CDCl3) δ:
In a 50 mL round bottom flask, 2,3,3-trimethyl-3H-indole-5-
carboxylic acid (1.5 g, 7.38 mmol) and 1-iodomethane (1.15
mL, 18.45 mmol) were dissolved in MeCN (12 mL), and
refluxed under nitrogen for 16 h. The solvent was evaporated
3.81 (q,
(d,
= 6.3 Hz, 3 H), 0.94 - 0.84 (m, 6 H). 13C NMR (100 MHz ,
CDCl3) : 69.7, 44.8, 37.4, 32.2, 31.9, 31.9, 30.2, 30.1, 30.1,
J = 5.8 Hz, 1 H), 1.70 (br. s., 1 H), 1.26 (br. s, 33 H), 1.12
J
δ
30.1, 29.7, 29.6, 29.6, 29.5, 29.5, 29.4, 29.3, 29.3, 27.4, 27.4,
25.6, 22.7, 19.8, 14.1.
and the crude product was washed with diethyl ether to afford
1
3a (1.2 g, 75%) as red solid. H NMR (400 MHz, DMSO-d6
)
δ
:
8.37 (s, 1 H), 8.18 (d,
3.99 (s, 3 H), 2.81 (s, 3 H), 1.57 (s, 6 H). 13C NMR (100 MHz,
DMSO-d6 : 199.0, 166.5, 145.3, 141.9, 131.6, 130.4, 124.2,
J = 8.2 Hz, 1 H), 8.02 (d, J = 8.2 Hz, 1 H),
3-Octyltridecan-2-one (1c)
To a solution of 3-octyltridecan-2-ol (2 g, 6.34 mmol) in CH2Cl2
(30 mL), pyridinium chlorochromate (2.76 g, 12.68 mmol) and
celite (PCC: celite, 1:1, 2.76 g) were added and stirred at room
temperature for 4 h. The reaction mixture was filtered through
)
δ
115.4, 54.3, 35.0, 21.5, 14.6. HRMS (ESI): m/z calcd for
C13H16NO2 (M+): 218.1176. Found: 218.1176.
silica
gel
column
chromatography,
washed
with
5-Carboxy-1-hexyl-2,3,3-trimethyl-3H-indol-1-ium iodide (3b)
dichloromethane, solvent was evaporated and purified by
column chromatography to afford 1c (1.1 g, 56%) as colorless
In a 50 mL round bottom flask, 2,3,3-trimethyl-3H-indole-5-
carboxylic acid (1.5 g, 7.38 mmol) and 1-iodohexane (2.72 mL,
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18.45 mmol) were dissolved in MeCN (12 mL) and refluxed residue was dissolved in hexane and kept in refrigerator to
under nitrogen for 16 h. The solvent was evaporated and the isolate pale yellow crystals of the required 3,4-
crude product was washed with diethyl ether to afford 3b (1.7 dichlorocyclobut-3-ene-1,2-dione. (Yield: 0.56 g, 42%).
g, 80%) as red solid. 1H NMR (200 MHz, DMSO-d6
s., 1 H), 8.25 - 8.04 (m, 2 H), 4.54 - 4.35 (m, 2 H), 2.89 (br. s., 3
H), 1.57 (br. s., 6 H), 1.32 (d, = 7.7 Hz, 8 H), 0.93 - 0.83 (m, 3
H). 13C NMR (100 MHz, DMSO-d6
: 192.9, 167.4, 155.6, 145.7,
) δ: 8.39 (br.
3-chloro-4-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-
benzo[9,1]quinolizino[3,4,5,6,7-defg]acridin-2-yl)cyclobut-3-ene-
1,2-dione (4b)
J
)
δ
129.7, 127.7, 122.9, 118.7, 53.6, 22.2, 21.9, 15.4, 13.8. HRMS
(ESI): m/z calcd for C18H26NO2 (M+): 288.1958. Found:
288.1958.
A
mixture of 4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-
benzo[9,1]quinolizino[3,4,5,6,7-defg] acridine (4a) (1 g, 2.74
mmol) and 3,4-dichlorocyclobut-3-ene-1,2-dione (0.45 g, 3
mmol) in benzene (15 mL) was heated at 80 °C for 24 h in a 50
mL round bottom flask. The resultant reaction mixture was
cooled and solvents were removed under reduced pressure
and purified by column chromatography to give 3-chloro-4-
(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1]
quinolizino[3,4,5,6,7-defg]acridin-2-yl)cyclobut-3-ene-1,2-
dione (4b) (0.46 g, 35%) as yellow solid. 1H NMR (200 MHz,
CDCl3) δ: 8.26 (s, 2 H), 7.48 - 7.37 (m, 4 H), 7.24 - 7.17 (m, 2 H),
1.67 (s, 6 H), 1.64 (s, 12 H). 13C NMR (100 MHz, CDCl3) δ: 195.4,
190.3, 186.9, 175.0, 138.5, 131.2, 130.6, 130.2, 130.1, 124.9,
124.8, 124.3, 123.4,121.1, 35.6, 35.4, 34.3, 32.2. HRMS (ESI):
m/z calcd for C31H26ClNO2Na ([M+Na]+): 502.1550. Found:
502.1544.
5-Carboxy-3-decyl-1-hexyl-2-methyl-3-octyl-3H-indol-1-ium iodide
(3c)
3-Decyl-2-methyl-3-octyl-3H-indole-5-carboxylic acid (0.7 g,
1.64 mmol) and 1-iodo-hexane (0.48 mL, 3.27 mmol) were
dissolved in MeCN (10 mL) in a 25 mL round bottom flask and
refluxed at 100 oC under nitrogen for 72 h. The solvent was
evaporated and the crude product was purified by silica gel
chromatography to afford 3-decyl-1-hexyl-3-methyl-2-
methyleneindoline-5-carboxylic acid (0.35 g, 42%) as a light red
1
liquid. H NMR (200 MHz, CDCl3)
δ
: 7.96 (dd,
= 1.5 Hz, 1 H), 6.50 (d, = 8.3 Hz, 1 H), 4.12 (s, 1
H), 3.87 (s, 1 H), 3.53 (t, = 7.1 Hz, 2 H), 1.85 - 1.44 (m, 6 H),
J = 1.5, 8.3 Hz, 1
H), 7.68 (d,
J
J
J
1.40 - 1.02 (m, 35 H), 0.86 (dd,
(m, 2 H).
J = 2.7, 6.0 Hz, 6 H), 0.73 - 0.58
3-(4,4,8,8,12,12-Hexamethyl-8,12-dihydro-4H-benzo [9,1]
In
a
25 mL RB flask, 3-decyl-1-hexyl-3-methyl-2- quinolizino[3,4,5,6,7-defg]acridin-2-yl)-4-hydroxycyclobut-3-ene-
methyleneindoline-5-carboxylic acid (0.25 g, 0.49 mmol) was
added in dry diethyl ether (5 mL). Then hydroiodic acid (57wt%
in water, 0.20 mL, 0.98 mmol) was added dropwise. The
mixture was stirred at room temperature for 2 h. The diethyl
ether was removed in vacuum to give 3c (0.22 g, 88%) as
1,2-dione (4c)
A
mixture of 3-chloro-4-(4,4,8,8,12,12-hexamethyl-8,12-
dihydro-4H-benzo[9,1]quinolizino
[3,4,5,6,7-defg]acridin-2-
yl)cyclobut-3-ene-1,2-dione (4b) (0.4 g, 0.83 mmol), acetic acid
(5 mL), water (5 mL) and 2 N HCl (2 mL) was refluxed for 16 h
in a 50 mL round bottom flask, the resultant mixture was
concentrated under reduced pressure, and washed with aq.
NaHCO3(50 mL) and brine (50 mL) to give 3-(4,4,8,8,12,12-
hexamethyl-8,12-dihydro-4H-benzo[9,1] quinolizino [3,4,5,6,7-
brown liquid. 1H NMR (500 MHz, CDCl3)
H), 8.20 (s, 1 H), 7.95 (d, = 8.2 Hz, 1 H), 4.96 - 4.75 (m, 2 H),
δ: 8.36 (d, J = 8.2 Hz, 1
J
3.15 (s, 3 H), 2.30 - 2.18 (m, 2 H), 2.17 - 2.10 (m, 2 H), 1.99 -
1.91 (m, 2 H), 1.60 - 1.45 (m, 2 H), 1.42 - 1.26 (m, 6 H), 1.25 -
1.15 (m, 11 H), 1.13 (br. s., 13 H), 0.88 - 0.81 (m, 9 H), 0.72 -
0.63 (m, 2 H). 13C NMR (100 MHz, CDCl3)
δ: 198.8, 168.3, 145.6,
139.1, 132.1, 131.5, 125.0, 115.9, 64.1, 50.9, 37.3, 31.8, 31.6,
31.3, 29.6, 29.4, 29.2, 29.0, 28.9, 28.6, 26.6, 24.2, 22.6, 22.5,
22.4, 17.5, 14.1, 14.0, 13.9. HRMS (ESI): m/z calcd for
C34H58NO2 (M+): 512.4462. Found: 512.4462.
defg]acridin-2-yl)-4-hydroxycyclobut-3-ene-1,2-dione
(4c)
(0.22 g, 57%) as brown solid. 1H NMR (400 MHz, CDCl3) δ: 8.28
(s, 2 H), 7.48 - 7.40 (m, 4 H), 7.27 - 7.20 (m, 2 H), 1.69 (s, 6 H),
1.65 (s, 12 H). 13C NMR (100 MHz, CDCl3) δ: 195.4, 190.4, 186.9,
175, 138.5,131.2, 130.6, 130.2, 130.1, 124.9, 124.7, 124.3,
123.4, 121.1, 35.6, 35.4, 34.3, 32.1. HRMS (ESI): m/z calcd for
C31H28NO3 ([M+H]+): 462.2069. Found: 462.2064.
3, 4-Dichlorocyclobut-3-ene-1, 2- dione55
Squaric acid (1 g, 8.76 mmol) was dissolved in 15 mL of
benzene in a 50 mL round bottom flask. Thionyl chloride (1.27
mL, 17.53 mmol) was added dropwise, after half of the
addition was over, a drop of dry DMF was added and then
continued the dropwise addition of thionyl chloride at room
temperature. The mixture was refluxed for 6 h and cooled to
room temperature. Excess of solvent was removed and the
General procedure for synthesis of NSQ dyes
A
mixture of 3-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-
benzo[9,1]quinolizino[3,4,5,6,7-defg]acridin-2-yl)-4-hydroxy
cyclobut-3-ene-1,2-dione (4c) (150 mg, 0.32 mmol) and 2a-2c
(0.36 mmol, 1.1 eq.) in benzene (3 mL), 1-butanol (3 mL) and
pyridine (1mL) was added in 25 mL round bottom flask and
refluxed with a Dean-Stark apparatus for 5 h. The resultant
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mixture was concentrated under reduced pressure, then In a 50 mL round bottom flask, a mixture of triphenylamine
diluted with dichloromethane, dried over Na2SO4, and (5a) (0.8 g, 3.3 mmol) and 3, 4-dichlorocyclobut-3-ene-1, 2-
concentrated under vacuum to give a residue, which was dione (0.54 g, 3.6 mmol) in benzene (15 mL) was heated at 80
purified with dichloromethane/ methanol to give NSQ1-NSQ3 °C for 24 h. The resultant reaction mixture was evaporated
as green solid.
under reduced pressure to give residue, which was purified by
column chromatography to give 3-chloro-4-(4-(diphenylamino)
5-Carboxy-2-[[3-[(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-
benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-2-yl)]-4-oxo-2-
cyclobuten-2-olate-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium
(NSQ1)
phenyl) cyclobut-3-ene-1, 2-dione (5b) (0.45 g, 38%) as yellow
1
solid. H NMR (500 MHz, CDCl3) δ: 8.07 (m,
J
=9.16 Hz, 2 H),
=9.16 Hz, 2
: 195.3, 190.4, 186.7, 174.6,
7.42 - 7.35 (m, 4 H), 7.27 - 7.18 (m, 6 H), 7.02 (m,
H). 13C NMR (100 MHz, CDCl3)
J
δ
Yield: 19%. 1H NMR (200 MHz, DMSO-d6
8.05 (m, 1 H), 7.78 (d, =8.46 Hz, 1 H), 7.44 - 7.44 (m, 5 H), 7.27
- 7.12 (m, 2 H), 6.26 (s, 1 H), 3.92 (s, 3 H), 1.81 (s, 6 H), 1.60 (br.
s., 18 H). 13C NMR (100 MHz, DMSO-d6
δ: 197.2, 191.7, 183.0,
179.5, 178.8,168.3, 145.5, 142.9, 133.8, 130.3, 130.0, 129.9,
129.8, 126.4, 124.8, 124.3, 124.2, 123.8, 123.7, 123.5, 123.5,
121.9, 119.1, 113.1, 91.7, 50.9, 48.6, 35.2, 35.1, 34.0, 33.5,
32.8, 32.8, 32.4, 25.04. HRMS (ESI): m/z calcd for C44H41N2O4
([M+H]+): 661.3066. Found: 661.3061.
) δ: 8.24 (s, 2 H), 8.15 -
153.8, 145.2, 130.8, 129.9, 126.7, 126.0, 118.6, 118.0. HRMS
(ESI): m/z calcd for C22H14ClNO2Na ([M+Na]+): 382.0611.
Found: 382.0605.
J
)
3-(4-(Diphenylamino) phenyl)-4-hydroxycyclobut-3-ene-1, 2-dione
(5c)
In a 50 mL round bottom flask, a mixture of 3-chloro-4-(4-
(diphenylamino) phenyl) cyclobut-3-ene-1, 2-dione (5b) (0.4 g,
1.1 mmol), acetic acid (7 mL), water (7 mL) and 2 N HCl (2 mL)
was refluxed for 16 h, the resultant mixture was concentrated
under vacuum to give residue. The residue was washed with
NaHCO3 aq. and brine to give 3-(4-(diphenylamino) phenyl)-4-
hydroxycyclobut-3-ene-1, 2-dione (5c) (0.23 g, 61%) as brown
5-Carboxy-2-[[3-[(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-
benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-2-yl)]-4-oxo-2-
cyclobuten-2-olate-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-
solid. 1H NMR (400 MHz, CDCl3) δ: 8.07 (d,
- 7.34 (m, 4 H), 7.25 - 7.18 (m, 6 H), 7.02 (d,
J
=8.80 Hz, 2 H), 7.42
=8.80 Hz, 2 H). 13
indolium (NSQ2)
1
J
C
Yield: 29%. H NMR (400 MHz, DMSO-d6
)
δ: 8.24 (s, 2 H) 8.13 -
=8.31 Hz, 1 H), 7.56 - 7.36 (m, 5 H), 7.22
=8.31 Hz, 2 H), 6.28 (s, 1 H), 4.39 (t, =6.60 Hz, 2 H), 1.81 (s,
6 H), 1.61 (s, 6 H), 1.59 (s, 12 H), 1.40 (br. s., 2 H), 1.35 - 1.18
(m, 6 H), 0.84 (t,
=6.60 Hz, 3 H). 13C NMR (100 MHz, DMSO-d6
NMR (100 MHz, CDCl3) δ: 195.3, 190.4, 186.8, 174.7, 153.8,
145.2, 130.8, 129.9, 126.7, 126.0, 118.6, 118.0. HRMS (ESI):
m/z calcd for C22H16NO3 ([M+H]+): 342.1130. Found: 341.1125.
8.05 (m, 1 H), 7.79 (d,
(t,
J
J
J
J
)
5-carboxy-2-[[3-[N,N-diphenyl-anilin-4-yl]-4-oxo-2-cyclobuten-2-
olate-1-ylidene] methyl]-3, 3-dimethyl-1-hexyl-3H-indolium
(NSQR)
δ: 192.3, 182.8, 179.3, 178.1, 169.2, 166.8, 144.7, 143.1,133.9,
130.3, 129.9, 129.9, 129.8, 128.9, 126.3, 124.8, 124.3, 123.7,
123.7, 121.9, 113.1, 91.2, 50.9, 44.8, 35.2, 34.0, 32.4, 30.8,
27.2, 25.7, 25.2, 21.9, 13.8. HRMS (ESI): m/z calcd for
C49H51N2O4 ([M+H]+): 731.3849. Found: 731.3843.
A mixture of 3-(4-(diphenylamino) phenyl)-4-hydroxycyclobut-
3-ene- 1, 2-dione (5b) (0.15 g, 0.44 mmol) and 2b (0.14 g, 0.48
mmol, 1.1 eq.) in benzene (5 mL), 1-butanol (5 mL) and
pyridine (0.5 mL) was added in 25 mL RB flask and refluxed
with a Dean-Stark apparatus for 5 h. The resultant mixture was
concentrated under reduced pressure, washed with 2 N HCl,
extracted with dichloromethane and dried over Na2SO4, then
concentrated under vacuum to give a residue, which was
5-Carboxy-2-[[3-[(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-
benzo[9,1]quinolizino [3,4,5,6,7-defg]acridine-2-yl)]-4-oxo-2-
cyclobuten-2-olate-1-ylidene]methyl]-3-decyl-1-hexyl-3-octyl-3H-
indolium (NSQ3)
Yield: 38%. 1H NMR (400 MHz, CDCl3)
δ: 8.46 (s, 2 H), 8.24 (d,
J
purified with dichloromethane/ methanol to give NSQR as blue
= 8.8 Hz, 1 H), 8.12 (s, 1 H), 7.45 - 7.40 (m, 5 H), 7.21 (d,
J
= 8.8
1
solid (67 mg, 25%). H NMR (200 MHz, DMSO-d6
)
δ: 8.22 (s, 1
=8.84 Hz, 1 H), 7.45 - 7.31 (m,
4 H), 7.24 - 7.08 (m, 6 H), 6.92 (d, =8.84 Hz, 2 H), 6.24 (s, 1 H),
Hz, 2 H), 6.48 (s, 1 H), 4.28 (br. s., 2 H), 3.14 (br. s., 2 H), 2.18 -
2.12 (m, 2 H), 1.86 (t, = 7.3 Hz, 2 H), 1.70 (s, 12 H), 1.67 (s, 6
H), 1.47 - 1.44 (m, 2 H), 1.11 (br. s., 14 H), 1.05 (br. s., 10 H),
0.88 (t, = 6.8 Hz, 6 H), 0.79-0.73 (m, 11.7 Hz, 9 H), 0.46 (br. s.,
H), 8.11 – 7.95 (m, 3 H), 7.78 (d,
J
J
J
4.39 (br. s., 2 H), 1.76 (s, 6 H), 1.45 - 1.16 (m, 8 H), 0.87 - 0.79
(m, 3 H). 13C NMR (100 MHz, CDCl3) δ: 192.67, 182.7, 179.0,
178.0, 169.8, 166.8, 149.8, 145.9, 144.7, 143.0, 130.3, 129.9,
128.9, 128.3, 125.8, 125.0, 123.9, 123.6, 120.1, 113.0, 91.2,
50.9, 30.8, 27.2, 25.7, 25.2, 21.9, 13.8. HRMS (ESI): m/z calcd
for C40H39N2O4 ([M+H]+): 611.2910. Found: 611.2904.
J
2 H). 13C NMR (100 MHz, CDCl3) δ: 185.3, 180.1, 175.8, 170.0,
147.4, 136.0, 131.1, 130.9, 130.6, 130.3, 127.3, 126.1, 124.4,
124.3,124.1, 123.6, 110.4, 91.7, 60.3, 45.1, 40.2, 39.9, 35.6,
35.4, 34.0, 32.7, 31.8, 31.7, 31.4, 29.7, 29.5, 29.4, 29.3, 29.2,
29.1, 29.0, 27.8, 26.7, 24.8, 24.1, 22.5, 22.5, 22.4, 14.0, 13.9 .
HRMS (ESI): m/z calcd for C65H83N2O4 ([M+H]+): 955.6353.
Found: 955.6347.
4.3. Solar cells preparation and characterization
FTO (F-doped SnO2 glass; 6 - 8 Ω /sq; Pilkington TEC 7) was
cleaned by mucasol (2 % in water), deionised water, and
3-Chloro-4-(4-(diphenylamino) phenyl) cyclobut-3-ene-1, 2-dione
(5b)
ethanol, successively. To grow
a TiO2 under layer, the
substrate was immersed in freshly prepared 50 mM TiCl4
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aqueous solution at 70 oC for 30 min, and washed with
deionised water before drying at 100 oC for 10 min. A paste of
TiO2 nanocrystal (< 20 nm, Ti-Nanoxide T/SP, Solaronix) was
deposited by the doctor-blade technique on TiO2 buffer layer
coated FTO substrate, kept in air for 5 min and then annealed
at 125 oC in air for 15 min. The films were about 6 - 8 μm thick.
The annealed films were coated with scattering layer TiO2
ACKNOWLEDGMENT
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. This work is financially supported by CSIR-
Network Project NWP0054 (CSIR-TAPSUN). N. K. thanks UGC
for SRF fellowship. The authors also thank Mr. Manik Chandra
Sil for help with DFT studies.
o
paste (WER2-O, Dyesol) and annealed at 125 C in air for 15
min. The annealed films were sintered at 325 oC for 5 min, 375
oC for 5 min, 450 oC for 15 min and 500 oC for 15 min with
o
heating rate of 5 C per min in air. After reaching the furnace
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Graphical Abstract