Application of benz[c,d]indolenine-based unsymmetrical squaraine dyes to near-infrared dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2017.02.024

Novel benz[c,d]indolenine-based unsymmetrical near-infrared squaraine dyes having germinal butyl (η?=?0.55%) and octyl (η?=?0.58%) groups at another indolenine ring exhibited higher conversion efficiency than the usually used methyl derivative (η?=?0.38%)

Full text of DOI:10.1016/j.dyepig.2017.02.024

Accepted Manuscript
Application of benz[c,d]indolenine-based unsymmetrical squaraine dyes to near-
infrared dye-sensitized solar cells
Masaki Matsui, Yuki Haishima, Yasuhiro Kubota, Kazumasa Funabiki, Jiye Jin, Taek
Hyeon Kim, Kazuhiro Manseki
PII:
S0143-7208(16)31455-3
10.1016/j.dyepig.2017.02.024
DYPI 5802
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 December 2016
Revised Date: 8 February 2017
Accepted Date: 13 February 2017
Please cite this article as: Matsui M, Haishima Y, Kubota Y, Funabiki K, Jin J, Kim TH, Manseki K,
Application of benz[c,d]indolenine-based unsymmetrical squaraine dyes to near-infrared dye-sensitized
solar cells, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.02.024.
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ACCEPTED MANUSCRIPT
Graphical abstract
R
O
O
R
R
Novel benz[c,d]indolenine-based
unsymmetrical near-infrared squaraine
dyes having germinal butyl and octyl
groups at another indolenine moiety
exhibited higher conversion efficiency
than the usually used methyl derivative,
owing to less aggregate formation and
inhibition of approaching I3 species on
titanium oxide.
COOH
7a
CH3
7b C4H9
7c C8H17
N
N
C6H13
C2H5
4
3
2
1
7
a
7b
7c
0
0
0.1
0.2
Voltage / V
0.3
0.4
0.5

ACCEPTED MANUSCRIPT
Application of benz[c,d]indolenine-based unsymmetrical squaraine
dyes to near-infrared dye-sensitized solar cells
a,
a
a
b
Masaki Matsui, * Yuki Haishima, Yasuhiro Kubota, Kazumasa Funabiki, Jiye Jin,
c
a
Taek Hyeon Kim, and Kazuhiro Manseki
a
Department of Chemistry and Biochemistry, Faculty of Engineering, Gifu University,
Yanagido, Gifu 501-1193, Japan
b
Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi,
Matsumoto, Nagano 390-8621, Japan
c
Department of Chemical Engineering, Chonnam National University, Gwangju
5
00-757, South Korea
_
_____________________________________________________________________
Abstract
Novel benz[c,d]indolenine-based unsymmetrical near-infrared squaraine dyes
having germinal butyl ( = 0.55 %) and octyl ( = 0.58 %) groups at another indolenine
ring exhibited higher conversion efficiency than the usually used methyl derivative (
.38 %). The germinal butyl and octyl groups can inhibit aggregation formation and
η
η
η
=
0
‒
approaching I species to improve the conversion efficiency.
3
Keywords: Dye-sensitized solar cell; Squaraine dye; Near-infrared;
Benz[c,d]indolenine; Aggregation.
_
*
_____________________________________________________________________
Corresponding author: TEL: +81(0)58 293 2601, Fax: +81 (0)58 293 2794
E-mail address: matsuim@gifu-u.ac.jp
1
. Introduction
1

ACCEPTED MANUSCRIPT
Much attention has been paid to find near-infrared (NIR) sensitizers in
dye-sensitized solar cells (DSSCs). As squaraine (SQ) dyes exhibit the absorption
maximum (λmax) in NIR to infrared region, many derivatives have been proposed as the
sensitizers[1-16]. Especially, benz[c,d]indolene-based SQ dyes are very bathochromic
among the heteroaromatic SQ derivatives [17]. Benz[c,d]indolene-based unsymmetrical
SQ dyes having an anchor carboxyl group on another indolenine ring have been
reported to show the λmax at around 800 nm and exhibit the conversion efficiency of
0
.10 [18] to 1.1 % [19,20]. When the carboxyl group was introduced on the
benz[c,d]indolenine ring, the value has been reported to be 0.3 % [21]. SQ dyes have
η
strong intermolecular interactions originate from the conjugated wide π-plane and polar
cyclobutene-1,3-dione moiety to easily aggregate. We consider that the intermolecular
interactions are inhibited by introducing bulky substituent(s) in the SQ molecule. We
report herein the application of novel benz[c,d]indolenine-based unsymmetrical SQ
dyes having germinal alkyl group(s) on another indolenine core to NIR DSSCs.
2
. Experimental
2
.1 Instruments
Melting points were measured with a Yanaco MP-J3 micro-melting-point apparatus.
NMR spectra were taken with a JEOL JNM-ECX 400P spectrometer. FABMS spectra
were taken on a JEOL MStation 700 spectrometer using m-nitrobenzylalcohol (NBA) as
a matrix. UV-vis absorption and fluorescence spectra were taken on Hitachi U-4100 and
JASCO FP-8600 spectrophotometers, respectively. Electrochemical measurement was
carried out using an EG&G Princeton Applied Research Potentiostat/Galvanostat
(Model 263A) driven by the M270 software package.
2
.2. Materials.
2

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3
,4-Diethoxy-3-cyclobutene-1,2-dione, ethyl iodide, 2, 4, and 5a were purchased
from Tokyo Chemical Industry Co., Ltd. Compound 5a was obtained from Nakalai
Tesque, Inc. Compounds 1 [22], 5b [23], 5c [23], and 6a [24] were prepared as
described in literature.
2
.3. Synthesis of 3
An ethanol solution (15 mL) of 1 (2.47 g, 7.0 mmol), 2 (1.50 g, 8.0 mmol), and
triethylamine (1.77 g, 17.5 mmol) was refluxed for 10 h. After the reaction was
completed, the solvent was evaporated in vacuo. The resulting precipitate was dissolved
in ethanol (60 mL) and 10% aqueous sodium hydroxide (1 mL) was added. The mixture
was heated at 60°C for 5 min and neutralized with 2N hydrochloric acid. The solvent
was evaporated in vacuo. The resulting crude product was purified by column
+
chromatography (SiO , CHCl : MeOH = 4 : 1) as a pink solid. No MH ion peak was
2
3
1
observed by FABMS. Yield 413 mg (17 %); mp >300 °C; H NMR (DMSO-d ) δ = 0.85
6
(t, J = 6.6 Hz, 3H), 1.10‒1.50 (m, 6H), 1.60‒1.80 (m, 2H), 3.92 (t, J = 7.1 Hz, 2H), 5.84
(s, 1H), 6.74 (d, J = 7.8 Hz, 1H), 7.18 (d, J = 7.8 Hz, 1H), 7.35 (t, J = 7.8 Hz, 1H), 7.57
(t, J = 7.8 Hz, 1H), 7.68 (d, J = 7.8 Hz, 1H), 8.05 (d, J = 7.8 Hz, 1H).
2
.4 Synthesis of 6b and 6c
An ethanol solution (30 mL) of 4-hydrazinobenzoic acid 4 (1.52 g, 10 mmol), 5
(10.9 mmol), and conc. sulfuric acid (0.3 mL, 5 mmol) was refluxed for 40 h. After the
reaction was completed, the mixture was filtered. The filtrate was neutralized with
saturated aqueous sodium hydrogencarbonate and washed with water. The product was
extracted with dichloromethane. The aqueous layer was acidified to pH 4 with 1N
hydrochloric acid. The product was extracted with dichloromethane. The
dichloromethane layer was combined and evaporated in vacuo. The product was
purified by column chromatography (SiO , C H : MeCOMe = 3 : 1). 4b and 4c are
2
6
14
+
yellow solid. No MH ion peak was observed for 6c by FABMS.
3

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2
.4.1. 3,3-Dibutyl-2-methyl-3H-indole-5-carboxylic acid (6b)
1
Yield 1.29 g (45 %); mp 87–88 °C; H NMR (CDCl ) δ = 0.50‒0.61 (m, 2H),
3
0
2
8
.62‒0.70 (m, 2H), 0.72 (t, J = 7.6 Hz, 6H) 1.13 (sex, J = 7.6 Hz, 4H), 1.74‒1.81 (m,
H), 1.93‒2.00 (m, 2H), 2.32 (s, 3H), 7.68 (d, J = 8.2 Hz, 1H), 7.98 (d, J = 1.4 Hz, 1H),
+
.16 (dd, J = 8.2 and 1.4 Hz, 1H), 9.90 (br s, 1H); FABMS (NBA) m/z 288 (MH ).
2
.4.2. 2-Methyl-3,3-dioctyl-3H-indole-5-carboxylic acid (6c)
1
Yield 1.24 g (31 %); mp 80–81 °C; H NMR (CDCl ) δ = 0.49‒0.63 (m, 2H),
3
0
.64‒0.77 (m, 2H), 0.83 (t, J = 7.1 Hz, 6H) 1.00‒1.40 (m, 20H), 1.73‒1.80 (m, 2H),
.92‒1.99 (m, 2H), 2.29 (s, 3H), 7.63 (d, J = 8.2 Hz, 1H), 7.96 (d, J = 1.4 Hz, 1H), 8.15
1
(dd, J = 8.2 and 1.4 Hz, 1H).
2
.5 Synthesis of 7
An acetonitrile solution (25 mL) of 6 (2.14 mmol) and ethyl iodide (7a, 7b: 1.70 g,
0.7 mmol, 7c: 3.40 g, 21.4 mmol) was refluxed (7a: 22 h, 7b: 40 h, 7c: 55 h). After the
1
reaction was completed, the solvent was evaporated in vacuo. The resulting precipitate
was washed with ether. Then, to the residue were added 3 (1.0 mmol) and
1
-butanol-toluene (1 : 1) mixed solvent (10 mL). The mixture was refluxed using a
dean-stark tube (7a: 14 h, 7b: 14 h, 7c: 17 h). After the reaction was completed, the
solvent was evaporated in vacuo. The product was purified by column chromatography
+
(
SiO , CHCl : MeOH = 15 : 1). 7a and 7b are brown solid. 7c is green solid. No MH
2 3
ion peak was observed for 7c by FABMS.
2
.5.1. 7a
1
Yield 204 g (17 %); mp 259 °C (dec); H NMR (CDCl ) δ = 0.87 (t, J = 7.3 Hz 3H),
3
1
8
7
.13‒1.52 (m, 11 H), 1.85 (s, 6H), 4.18 (br s, 4H), 6.12 (s, 1H), 6.38 (s, 1H), 7.04 (d, J =
.5 Hz, 1H), 7.07 (d, J = 7.3 Hz, 1H), 7.50 (t, J = 7.3 Hz, 1H), 7.57 (d, J = 7.3 Hz, 1H),
.85 (t, J = 7.3 Hz, 1H), 7.96 (d, J = 7.3 Hz, 1H), 8.09 (s, 1H), 8.13 (d, J = 8.5 Hz, 1H),
4

ACCEPTED MANUSCRIPT
+
9
.13 (br s, 1H); FABMS (NBA) m/z 561 (MH ).
2
.5.2. 7b
1
Yield 276 g (20 %); mp 271 °C (dec); H NMR (CDCl ) δ = 0.43‒0.60 (m, 2H),
3
0
1
4
7
8
.71 (t, J = 7.3 Hz, 6H), 0.72‒0.83 (m, 2H), 0.88 (t, J = 6.9 Hz, 3H), 1.12‒1.21 (m, 4H),
.22‒1.51 (m, 9H), 1.89 (quint, J = 7.8 Hz, 2H), 2.05‒2.11 (m 2H), 3.04‒3.10 (m, 2H),
.15 (br s, 4H), 6.22 (s, 1H), 6.37 (s, 1H), 6.90‒7.10 (m, 2H), 7.50 (t, J = 8.2 Hz, 1H),
.57 (d, J = 8.2 Hz, 1H), 7.84 (t, J = 8.2 Hz, 1H),7.96 (d, J = 8.2 Hz, 1H), 8.03 (s, 1H),
+
.16 (dd, J = 8.2 and 1.4 Hz, 1H), 9.09 (br s, 1H); FABMS (NBA) m/z 645 (MH ).
2
.5.3. 7c
1
Yield 275 mg (17 %); mp 106–107 °C; H NMR (CDCl ) δ = 0.53 (br.s, 2H),
3
0.70‒0.84 (m, 8H), 0.88 (t, J = 6.9 Hz, 3H), 1.00‒1.22 (m, 20H), 1.24‒1.50 (m, 9H),
1.82‒1.94 (m, 2H), 2.04‒2.09 (m, 2H), 3.05‒3.10 (m, 2H), 4.14 (br s, 4H), 6.22 (s, 1H),
6.37 (s, 1H), 6.90‒7.10 (m, 2H), 7.50 (t, J = 8.1 Hz, 1H), 7.56 (d, J = 8.1 Hz, 1H), 7.84
(t, J = 7.9 Hz, 1H), 7.95 (d, J = 7.9 Hz, 1H), 8.03 (s, 1H), 8.16 (dd, J = 8.7 and 1.4 Hz,
1
H), 9.13 (br s, 1H).
2
.6 Preparation of TiO /Dye hybrid films and device assembly
2
‒
1
F-doped tin oxide coated glass substrates (4 mm thickness, 13 Ω □ , Nippon Sheet
Glass) were used to prepare dyed-titanium oxide electrodes. The titanium oxide plate
was immersed into the 40 mM aqueous titanium tetrachloride solution at 70 °C for 30
min. A commercially available titanium oxide paste (PST-18NR, JGC Catalysts and
Chemicals Ltd.) was coated onto the FTO glass by doctor blade technique and
subsequently sintered at 500 °C for 15 min as described in the paper to prepare a
mesoporous titanium oxide layer (8 µm thickness) [25]. After immersing titanium oxide
plates into the 40 mM aqueous titanium tetrachloride solution at 70 °C for 30 min, the
substrates were sintered at 500 °C for 30 min. Thus obtained titanium oxide electrodes
were immersed in the 0.12 mM dye solution (solvent: acetonitrile/t-butyl alcohol = 1 : 1
5

ACCEPTED MANUSCRIPT
(v/v)) in the presence of chenodeoxycholic acid (6.0 mM) and kept in a dark at 25 °C
for 15 h. Pt-sputtered FTO glass plates were used as counter electrodes. The
dyed-titanium oxide electrode and counter electrode were then assembled into a
sandwich-type cell (5 mm × 4 mm area), for which the electrolyte solution composed of
the
acetonitrile-chloroform
(1:1
(v/v))
mixed
solution
of
1
,2-dimethyl-3-propylimidazolium iodide (0.5 M), I (0.05 M), and LiI (0.5 M) was
2
filled with a vacuum backfilling method [26].
2
.7 Photoelectrochemical Measurements
Action spectrum was obtained under monochromatic light with a constant photon
1
6
‒2 ‒1
number (0.05 x 10 photon cm s ). I-V characteristics were measured under
‒
2
illumination with AM 1.5 simulated sun light (100 mW cm ) by using a Bunko-Keiki
CEP-2000 system.
3
. Results and discussion
3
.1 Synthesis
New benz[c,d]indolenium-based SQ dyes 7a, 7b, and 7c were synthesized as shown
in Scheme 1. 2-Hexylbenz[c,d]indolenium perchlorate 1 was allowed to react with
diethoxysquarate (2) to give a semi-squaric acid 3. 4-Carboxyphenylhydrazine (4) was
allowed to react with methyl ketones 5 to give 3,3-dialkyl-5-carboxy-2-methylindoles 6
followed by the reaction with ethyl iodide and 3 to afford 7.
[
Scheme 1 ]
3
.2 UV-vis absorption and fluorescence spectra
The UV-vis absorption and fluorescence spectra of 7a, 7b, and 7c are shown in Fig.
6

ACCEPTED MANUSCRIPT
1
7
7
9
. The results are listed in Table 1. These dyes showed two absorption bands at around
17 and 770 nm with a shoulder peak at 650 nm. The molar absorption coefficients at
17 and 770 nm were calculated to be in the range of 83,800 to 90,400 and 92,400 to
3
‒1
‒1
4,400 dm mol cm , respectively. SQ dyes 7a, 7b, and 7c were very weak
fluorescent compounds, showing the fluorescence maximum (Fmax) at around 800 nm
with fluorescence quantum yield (Φf) less than 0.01.
[
Fig. 1 ]
[
Table 1 ]
3
.3 Electrochemical measurements
Fig. 2 shows the cyclic voltammogram of 7a, 7b, and 7c vs. Ag quasi reference
electrode (QRE) in DMF in the presence of ferrocene (Fc). The oxidation potential (Eox)
of Fc and 7b and the reduction potential (E ) of 7b were observed at +0.75, +0.86, and
red
‒
‒
+
0.42 V, respectively. Therefore, the E and E of 7b were calculated to be +0.11 and
ox red
+
1.17 V vs. Fc/Fc , respectively. Therefore, the E and E of 7b were calculated to be
ox
red
0.74 and ‒0.54 V vs. NHE, respectively [27]. In a similar way, the E and Ered levels
ox
of 7a and 7c were calculated. The results are also listed in Table 1. The conduction band
‒
‒
level of titanium oxide and the I /I redox level vs. NHE have been reported to be at
3
about ‒0.5 and +0.4 V, respectively [28,29]. Thus, the electrochemical measurements
suggest that dyes 7a, 7b, and 7c can thermodynamically act as the sensitizers for
‒
‒
titanium oxide in I /I redox electrolyte.
3
[
Fig. 2 ]
3
.4 DFT calculations
The optimized structures of 7a, 7b, and 7c were obtained by the DFT calculations
(B3LYP/3-21G). The germinal alkyl groups at the indolenine core are perpendicularly
7

ACCEPTED MANUSCRIPT
located against the planar SQ chromophore. The distance between the terminal
methyl-carbons in the germinal alkyl group are calculated to be 2.54, 10.3, and 20.5 Å
for 7a, 7b, and 7c, respectively. Thus, the alkyl groups can inhibit π-π and dipole-dipole
interactions in the planar SQ chromophore.
[
Fig. 3 ]
The single-point energy calculations performed by the B3LYP/6-31G(d,p) level are
shown in Table 2. The DFT calculations suggest that the first and second absorption
bands are observed at 658 and 589 nm with f values of 1.27 and 0.0007, respectively.
Thus, the f value of the second absorption band is very small. There are 218 nm
differences between first and third absorption bands. Therefore, the peaks observed at
7
70 and 717 nm are assigned to the vibration peaks of S0,0 to S1,0 and S0,0 to S1,1
transitions, respectively.
[
Table 2 ]
3
.5 Photoelectrochemical properties
The photoelectrochemical properties of 7a, 7b, and 7c are shown in Fig. 4. Fig. 4a
depicts the UV-vis absorption spectra on titanium oxide. The figure indicates that
similar amount of SQ dyes are adsorbed on titanium oxide. Fig. 4b shows the IPCE
spectra of 7a, 7b, and 7c. They showed similar shape. The maximum IPCE values of 7b
and 7c were larger than that of 7a. Fig. 4c indicates the I-V curve. The J value of 7c is
sc
highest followed by 7b and 7a. The V values of 7b and 7c were larger than that of 7a,
oc
‒
suggesting that the long alkyl groups can protect approaching I species to titanium
3
oxide surface [30]. Consequently, the conversion efficiencies (η) of 7b and 7c were
larger than that of 7a.
[
Fig. 4 ]
8

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[
Table 3 ]
Fig. 5 indicates the UV-vis absorption spectra of 7a, 7b, and 7c in solution. SQ dyes
a, 7b, and 7c can form aggregate in the water-ethanol mixed solvent. When water was
7
added to the ethanol solution, the absorption band became broad. The absorbance at
around 770 nm decreased and that at around 720 nm became relatively larger than that
at 770 nm.
[
Fig. 5 ]
The normalized UV-vis absorption spectra of 7a, 7b, and 7c in ethanol and on
titanium oxide are shown in Fig. 6. Fig. 6(a) indicates that the absorption band on
titanium oxide is broad compared with that in ethanol. Furthermore, the absorbance at
around 720 nm is larger or similar than that at around 790 nm, whereas the absorbance
at 717 nm is smaller than that at 770 nm in ethanol. Especially, the absorbance of 7a at
7
20 nm on titanium oxide is very large compared with those of 7b and 7c, indicating
that the methyl derivative can easily aggregate. Therefore, the butyl and octyl
derivatives 7b and 7c could improve the Jsc value compared with the methyl derivative
7
a.
[
Fig. 6 ]
4
. Conclusions
New unsymmetrical NIR SQ dyes based on benz[c,d]indolenine core having
germinal methyl, butyl, and octyl groups on another indolenine ring have been
synthesized. These dyes showed two absorption maxima at around 717 and 770 nm in
ethanol, which are assigned to the S to S and S to S transition, respectively. The
0
,0
1,1
0,0
1,0
conversion efficiencies of octyl (0.58 %) and butyl (0.55 %) derivatives were larger than
that of usually used methyl derivative (0.38 %). The germinal butyl and octyl groups
‒
could inhibit aggregate formation and approaching I3 species to improve the cell
9

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performance.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http//0000000.
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Nazeeruddin MK, Péchy P, Takata M, Miura H, Uchida S, Grätzel M.
1
2

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Highly-efficient
organic-dye-sensitized
solar
cells
controlled
by
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Figure captions
Scheme 1. Reagents and conditions: i) 1 (1.0 molar amount), 2 (1.1 molar amounts),
triethylamine (2.5 molar amounts), C H OH, reflux, 10 h, ii) 4 (1.0 molar
2
5
amount), 5 (1.1 molar amounts), C H OH, conc. sulfuric acid (0.5 molar
2
5
amount), reflux, 40 h, iii) 6 (1.0 molar amount), C H I (5.0‒10.0 molar
2
5
amounts), CH CN, reflux, then, 3 (0.47 molar amount), 1-butanol-toluene
3
(1 : 1) mixed solvent, reflux, 14‒17 h.
‒
5
Fig. 1.
UV-vis absorption and fluorescence spectra of 7a, 7b, and 7c ( 1.0 ᵡ 10
‒3
mol dm ) in ethanol. Solid and dotted lines represent UV-vis absorption
1
3

ACCEPTED MANUSCRIPT
and fluorescence spectra, respectively.
Fig. 2.
Cyclic voltammogram of 7a, 7b, and 7c measured vs. Ag quasi reference
‒
1
electrode in DMF in the presence of ferrocene (scan rate: 50 mV sec ).
Fig. 3.
Fig. 4.
Fig. 5.
Optimized structures of 7a, 7b, and 7c.
Photoelectrochemical measurements of 7a, 7b, and 7c.
‒
5
UV-vis absorption spectra of (a) 7a, (b) 7b, and (c) 7c (1.0 x 10 mol
‒3
dm ) in ethanol-water mixed solvent.
Fig. 6.
Normalized UV-vis absorption spectra of 7a, 7b, and 7c (a) in ethanol and
(b) on titanium oxide.
Table 1
Table 2
Physical properties of 7a, 7b, and 7c.
a
−5
−3
Measured on 1.0 x 10 mol dm of substrate in ethanol.
b
vs. NHE
Absorption maxima (λmax), main orbital transition, and oscillator strengths
(f) of 7b calculated by TDDFT at the B3LYP/6-31G(d,p) level.
a
in vacuum.
b
Oscillator strength.
a
Table 3
Photoelectrochemical measurements of 7a, 7b, and 7c .
a
−2
On titanium oxide under AM 1.5 irradiation (100 mW cm ).
1
4

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a
a
a
b
b
Dye λ max
(
ε
) / nm
F max / nm Φ f
E ox / V E red / V
7
a
650 (sh), 716 (83,800), 769 (92,400 799
650 (sh), 717 (90,400), 770 (94,400 801
650 (sh), 718 (88,000), 771 (92,700 802
< 0.01
< 0.01
< 0.01
0.74
0.74
0.74
–0.54
–0.54
–0.54
7b
7
c

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a
b
Transition λ max / nm
Main orbital transition
HOMO to LUMO (0.72)
HOMO‒1 to LUMO (0.70)
f
S0 to S1
S0 to S2
S0 to S3
658
589
1.27
7E-04
440 HOMO‒2 to LUMO (0.49), HOMO to LUMO+1 (0.50 0.06

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-2
Run Dye Maximum IPCE / % J / mA cm V / mV ff
η / %
sc
oc
1
2
3
7a
7b
7c
10 (790 nm)
13 (790 nm)
14 (790 nm)
2.49
2.84
3.05
260
330
320
0.59
0.59
0.65
0.38
0.55
0.58

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OC H
2
5
i)
O
OC2H5
O
O
O
2
CH3
N
N
ClO4
OH
C6H
13
C H
1
6
13
3
R
ii)
CH
R
3(
iii) C H I, CH CN,
2 5 3
1.4 eq)
O
O
R
R
O
reflux, 22 55 h
R
R
COOH
5
COOH
COOH
H C
3
N
H NHN
2
N
N
conc. H SO , C H OH,
O
2
4
2 5
C H
2 5
4
C H
6
6
13
Reflux, 40 h
O
7
N
R
OH
C H
6
13
3
a
b
c
CH3
1-butanol-toluene (1:1),
C H
4
9
reflux, 14 17 h
C H
8
17

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1
.8
.6
.4
.2
0
7
7
7
7
7
7
a
b
c
a
b
c
0
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0
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1
5
0
0
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Wavelength / nm

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0
.2
.1
0
0
7b/7b
-0.1
-0.2
-0.3
7
a
b
c
7
7
Fc/Fc+
b/7b+
7
1
0
-1
Potential / V vs. Ag (QRE)

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top view
top view
top view
20.5 A
10.3 A
2.54 A
side view
side view
side view
7a
7b
7c

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(
a)
(b)
1
.5
1
1
5
0
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0
7
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7
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b
c
7a
7b
7c
1
0
.5
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5
00
600
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900 1000
400
500
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900
Wavelength / nm
Wavelength / nm
(
c)
4
3
2
1
0
7a
7b
7c
0
0.1
0.2
0.3
0.4
0.5
Voltage / V

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(a)
(b)
1
.5
1
1.5
EtOH:H 2O
0:0
EtOH:H 2O
1
10:0
8:2
6:4
8:2
6:4
4:6
2:8
1:9
1
4:6
2:8
1:9
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.5
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Wavelength / nm
(c)
1
.5
1
EtOH:H 2O
10:0
8:2
6:4
4:6
2:8
1:9
0.5:9.5
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(a)
(b)
7
7
7
a
b
c
7
7
7
a
b
c
1
1
0.5
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.5
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Highlight
Novel benz[c,d]indolenine-based unsymmetrical NIR squaraine dyes having germinal
butyl and octyl groups on another indolenine core.
Inhibition of aggregation formation on titanium oxide.
‒
Inhibition of approaching I species to titanium oxide.
3
Improvement of conversion efficiency.