Accepted Manuscript

Title: Effect of Nature OF anchoring groups onphotosensitization behavior in Unsymmetrical squaraine dyes

Author: Gururaj M. Shivashimpi Shyam S. Pandey RieWatanabe Naotaka Fujikawa Yuhei Ogomi YoshihiroYamaguchi Shuzi Hayase

PII: S1010-6030(13)00403-6DOI: http://dx.doi.org/doi:10.1016/j.jphotochem.2013.09.004Reference: JPC 9514

To appear in: Journal of Photochemistry and Photobiology A: Chemistry

Received date: 31-5-2013Revised date: 6-9-2013Accepted date: 9-9-2013

Please cite this article as: G.M. Shivashimpi, S.S. Pandey, R. Watanabe,N. Fujikawa, Y. Ogomi, Y. Yamaguchi, S. Hayase, Effect of Nature OFanchoring groups on photosensitization behavior in Unsymmetrical squarainedyes, Journal of Photochemistry and Photobiology A: Chemistry (2013),http://dx.doi.org/10.1016/j.jphotochem.2013.09.004

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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EFFECT OF NATURE OF ANCHORING GROUPS ON

PHOTOSENSITIZATION BEHAVIOR IN UNSYMMETRICAL

SQUARAINE DYES

Gururaj M. Shivashimpi*1, Shyam S. Pandey

1, Rie Watanabe

1, Naotaka Fujikawa

1,

Yuhei Ogomi1, Yoshihiro Yamaguchi

2, Shuzi Hayase

1

1 Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Japan.

2 Nippon Steel and Sumikin Chemical Company Limited, Nakabaru, Tobata, Kitakyushu,

Japan.

Fax: 81-93-695-6005; Tel: 81-93-695-6044; E-mail: shivashimpi@life.kyutech.ac.jp

Abstract

A series of indole based unsymmetrical squaraine dyes bearing various anchoring groups

such as carboxylate (SQ-A), cyanoacrylate (SQ-B) and thiophene-bridged cyanoacrylate

(SQ-C) were synthesized and evaluated for their performance in dye sensitized solar cells

(DSSCs) under similar experimental conditions. Electronic absorption spectral

investigation on thin films of these dyes adsorbed on nanoporous TiO2 reveals relatively

enhanced spectral broadening upon the incorporation of cyanoacrylate functionality as

anchoring group. Incorporation of thiophene -bridge between the main chromophore and

anchoring group (SQ-C) although resulted in to bathochromic shift in the far-red region

but hampered photon harvesting due to relatively enhanced dye aggregation.

Unsymmetrical squaraine dye (SQ-B) bearing cyanoacrylate anchoring group directly

substituted and in conjugation with aromatic chromophore exhibited the best photovoltaic

performance giving photoconversion efficiency of 5.03% under simulated solar irradiation.

Keywords: Dye-sensitized solar cells, Dye aggregation, Unsymmetrical squaraine dyes, -

extended donors, Nanocrystalline TiO2

Revised manuscript

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1. Introduction

Ruthenium based dye sensitized solar cells (DSSCs) [1–3] with the conversion efficiency

over 10% had provided a new hope for the realization of relatively low cost solar cells.

Relatively high cost, rarity of Ru metal and photon harvesting mainly in the visible region

limits the use of ruthenium complexes as a single sensitizer to fabricate high efficiency

DSSCs. To further enhance the efficiency there is a critical need for design and synthesis of

suitable sensitizers having good photon harvesting in near infrared (NIR) to IR wavelength

region, so that such metal free organic sensitizers can be utilized with potential visible

photon harvesting sensitizers through dye bilayer architecture in hybrid or tandem solar

cells [4, 5]. Squaraine dyes showing intense and sharp absorption in the NIR region and

high molar extension co-efficient have recently been used as potential NIR sensitizers for

DSSCs [6-8]. Report of 4.5% photoconversion efficiency by Yum and et al [9] on

unsymmetrical squaraine dye bearing carboxylic acid anchoring group directly attached to

its indole chromophore opened up the new idea that dyes bearing carboxylic group directly

linked to aromatic chromophore are superior in performance as compared to that of their

alkyl side chain carboxy substituted dye counterparts [10]. Afterward, several research

groups followed the same idea and reported many squaraine dyes bearing carboxylic acid

anchoring group attached to various heterocyclic and aromatic donors as potential NIR

sensitizers for DSSCs [11-15].

It is well known that anchoring group plays an important role in attaching the dye

on the surface of titania nanoparticles and also electron injection from excited dye molecule

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to conduction band (CB) of TiO2 [15]. Carboxylic acid, cyanoacrylic acid and rhodanine-3-

acetic acid are well known electron acceptors and have been widely employed as anchoring

groups for the attachment of the visible wavelength absorbing D--A dyes on TiO2 surface

[16, 17]. Amongst them cyanoacrylic acid as an anchoring group was prefered exhibiting

pronounced spectral broadening and enhancement in the photoconversion efficiency as

compared to its carboxylic acid [18] and rohodanine-3-acetic acid [19] counterparts. Since

development of NIR sensitizer is crucial to attain the panchromatic photon harvesting, it is

also necessary to investigate the role of such anchoring groups on their photon harvesting

behavior. In this context, efforts have been directed to study the influence of anchoring

groups like carboxylic acid and cyanoacrylic acid for porphyrin [20] as well as

phthalocyanine [21] class of NIR dyes, where introduction of cyanoacrylate anchoring

group was reported to have the hampered photon harvesting as compared to its carboxylate

anchoring group counterparts. To further investigate the effect of anchoring groups on the

photophysical behavior in solution as well as after their attachment on the thin nanoporous

TiO2 films, three indole based unsymmetrical squaraine dyes (Fig. 1) with varying

anchoring groups like carboxylate (SQ-A), cyanoacrylate (SQ-B) and thiophene bridged

cyanoacrylate (SQ-C) were synthesized and their photovoltaic characterstics as well as

photon harvesting behavior were investigated under similar experimental conditions.

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2. Experimental

2.1. Materials, instruments and methods.

All the chemicals for synthesis or solvents are of analytical or spectroscopic grade and used

as received without further purification. Flash chromatography was performed using silica

gel 60 (230–400 mesh) eluting with solvents as indicated. Purity of all compounds

including intermediates and final products were confirmed by high performance liquid

chromatography (JASCO). Mass of the intermediates as well as final dyes was confirmed

by MALDI-TOF mass (Applied Biosystems) or fast ion bombardment mass (FAB-MS) and

high resolution mass (HR-MS) spectra on a JEOL JMS-SX 102A instrument. Nuclear

magnetic resonance (NMR) spectra were recorded on a JEOL JNM A (500 MHz)

spectrometer in CDCl3 or DMSO-d6 solvents with reference to tetramethyl silane (TMS) for

structural elucidation. All proton NMR signal shifts are given in parts per millions

(s = singlet; d = doublet; t = triplet; m = multiplet). Electronic absorption spectroscopic

investigations in solution and thin film adsorbed on TiO2 surface were conducted using

UV–visible spectrophotometer (JASCO model V550). The highest occupied molecular

orbital (HOMO) energy level was measured using photoelectron spectroscopy in air (Riken,

model AC3). The lowest unoccupied molecular orbital (LUMO) energy level was

determined from the edge of optical absorption considering it as optical band gap (Eg)

using the relation LUMO = HOMO + Eg.

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2.2. Synthesis of SQ-dyes and dye intermediates.

Detailed synthesis of all the three unsymmetrical squaraine dyes under present investigation

has been shown in the schemes 1, 2 and 3 respectively. The dyes SQ-A and SQ-B bearing

carboxylate and cyanoacrylate functionality were synthesized following the methods

developed in our earlier reports [10, 23]. SQ-C bearing thiophene bridged cyanoacrylate

has been synthesized by modifying the method developed by Shi et al [22].

The

intermediates 2,3,3-trimethyl-3H-indole-5-carboxylic acid (4a) and 3-(5-Bromo-1-butyl-

3,3-dimethyl-2,3-dihydroindol-2-ylidenemethyl)-4-hydroxy cyclobut-3-ene-1,2-dione (10)

were synthesized according to the reported procedures [9, 24].

2.2.1. Synthesis of 2,3,3-trimethyl-1-ethyl-3H-indolium iodide [2].

3.17 g (20 mmol) of 2,3,3-trimethyl-3H-indole (1) and 4.68 g (30 mmol) of 1-iodoethane

were dissolved in 100 mL of dehydrated acetonitrile and reaction mixture was refluxed for

18 h. After completion of the reaction, solvent was evaporated and the crude product was

washed with ample diethyl ether giving 5.7 g of titled compound as whitish powder in 91%

yield having 99% purity as confirmed by HPLC. FAB-mass, observed [M-Iodine]+ 188.0

for C13H18IN (calcd 315.04.13).

2.2.2. Synthesis of 3-butoxy-4-[(1-ethyl-1,3-dihydro-3,3- dimethyl-2H-indole-2-

ylidene)methyl]-3-cyclobutene-1,2-dione [3].

In a round bottom flask fitted with condenser, 1.26 g (4 mmol) of compound 2, 900mg (4

mmol) of 3,4-dibutoxy-3-cyclobutene-1,2-dione and 0.8 mL of triethylamine were

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dissolved in 6 mL butanol. Reaction mixture was heated at 70 0C for 1 h leading to green

solution. Solvent was removed at rotary evaporator and product was purified by column

chromatography (Silica gel) with ethyl acetate and hexane as eluent giving 920 mg of titled

compound in 50% yield and 99% purity as confirmed by HPLC. Compound was confirmed

by MALDI-TOF-mass, observed [M+H]+ 340.60 for C21H25NO3 (calcd 339.18).

2.2.3. Synthesis of 5-carboxy-2,3,3-trimethyl-1-ethyl-3H-indolium iodide [4a].

820 mg (4 mmol) of 2,3,3-trimethyl-3H-indole-5-carboxylic acid and 3.21 g (20 mmol) of

1-iodoethane were dissolved in 60 ml of dehydrated acetonitrile and reaction mixture was

refluxed for 18 h under nitrogen. After the completion of the reaction, solvent was

evaporated and the crude product was washed with ample diethyl ether giving 1.14 g of

titled compound as off white powder in 79% yield having 98% purity as confirmed by

HPLC. FAB-mass, observed [M-Iodine]+ 232.0 for C14H18INO2 (calcd 359.0382).

2.2.4. Synthesis of unsymmetrical squaraine dye SQ-A.

Unsymmetrical squaraine dye SQ-A was synthesized using semi-squaraine ester (3) and

compound (4a) as follows: In a round bottom flask fitted with condenser, 466 mg (1 mmol)

of compound (3) was dissolved in 10 mL ethanol followed by 0.54 mL (1.5 mmol) of

aqueous NaOH. Reaction mixture was refluxed for 30 min which was then cooled followed

by addition of 1N HCl (3.6 ml) giving compound 4. Solvent was then removed at rotary

evaporator followed by addition of 359 mg (1 mmol) of compound (4a) and 20 ml of 1-

butanol:toluene mixture (1:1, v/v). Reaction mixture was refluxed for 18 h using Dean-

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Stark trap. Reaction mixture was cooled, solvent was evaporated and product was purified

by silica gel column chromatography using chloroform : methanol as eluting solvent. 350

mg of final titled compound was obtained as blue solid in 97% purity as confirmed by

HPLC in 64% yield. MALDI-TOF-mass, observed [M]+ 497.76 for C31H32N2O4 (calcd

497.24) confirms the successful synthesis of the unsymmetrical dye SQ-A. 1H NMR (500

MHz, DMSO-d6): δ = 1.29 (m, 2H), 170 (s, 12H), 4.01 (m, 2H), 4.20 (m, 2H), 5.80 (s, 1H),

5.89 (s, 1H), 7.23 (t, 1H), 7.33 (d, J = 8.5 Hz, 1H), 7.41 (m, 2H), 7.57 (d, J = 9.5 Hz, 1 H),

7.94 (dd, J = 1.5, 1.5 Hz, 1H), 7.98 (d, J = 1.5 Hz, 1H).

2.2.5. 5-cyano-2,3,3-trimethyl-3H-indole [5].

To a flask fitted with reflux condenser 10 g (58 mmol) of 4-cyano hydrazine hydrochloride

in 100 mL ethanol was added 20 mL (174 mmol) 3-methyl-2-butanone. The mixture was

refluxed under nitrogen for 2 hours. After the complete consumption of 4-cyano hydrazine

hydrochloride solvent and the excess of 3-methyl-2-butanone were removed by evaporation

under vacuum. The residue was dissolved in 100 mL glacial acetic acid and refluxed for 12

hours. After the completion of reaction acetic acid was removed under vacuum and the

crude product was purified by column chromatography using ethyl acetate and hexane as

eluent giving 11 g of desired product in 56% yield and 98% purity as confirmed by HPLC.

FAB-MS observed [M+H]+ 185.11 for C12H12N2 (calcd 184.10).

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2.2.6. 1-butyl-5-cyano-2,3,3-trimethyl-3H-indolium iodide [6].

This compound was prepared from 3.2 g (17.4 mmol) of compound 5 and 10 mL (88

mmol) of 1-iodobutane in acetonitrile (40 mL) according to the procedure reported for

compound 2. Yield 5.5 g (86%). FAB-mass, observed [M-Iodine]+ 241.21 for C16H21IN2

(calcd 368.02).

2.2.7. Synthesis of tert-butyl-3,3-dimethyl-2-methyleneindoline-5-cyanoacrylate [8].

After hydrolyzing 5 g (13.5 mmol) of indolium iodide salt (6), using 20 mL of 2N NaOH to

its methylene base, the resulting base was isolated by extracting in toluene, drying and

evaporating. To a solution of 3.2 g (13 mmol) methylene base in 30 mL dichloromethane at

0 °C, was added slowly 11 mL (15.6 mmol, 1.5 M solution in toluene) of DIBAL-H. After

stirring for 10 hours under nitrogen atmosphere, the reaction mixture was quenched with 2

mL of dilute HCl and the contents were refluxed for 30 min. The crude product was

extracted into chloroform, washed with water and dried over Na2SO4. After evaporating the

solvent under vacuum, 2.8 g (11.5 mmol, 88%) of 5-formyl-1-butyl-3,3-dimethyl-2-

methylene indole (7) thus obtained was dissolved in 30 mL of acetonitrile. To this solution

was added 3.4 mL (23 mmol) of tert-butyl-cyanoacetate and 2 mL (17.3 mmol) of

piperdine. After refluxing the reaction mixture for 8 hours, solvent was evaporated and the

crude product was passed through small silica gel column using hexane:ethylacetate (6:1)

as eluting solvent to obtain compound 4. Yield 3.5 g (83%) and purity 96% as confirmed

by HPLC. FAB-MS observed [M+H]+ 367.0 for C23H30N2O2 (calcd 366.23).

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2.2.8 Synthesis of cyanoacrylic acid tert-butyl ester of unsymmetrical squaraine dye [9].

To a solution of 500 mg (1.4 mmol) of compound 4 in 40 mL of n-butanol:toluene mixture

(1:1, v/v) was added 400 mg (1.4 mmol) of semisquaric acid 5. Reaction mixture was

refluxed for 18 hours using Dean-Stark trap. Reaction mixture was cooled, solvent was

evaporated and product was purified by silica gel column chromatography using

chloroform:methanol as eluting solvent. 600 mg of final titled compound was obtained as

blue solid in 98% purity as confirmed by HPLC and 68% yield. HR-FAB MS, observed

[M]+ 631.3424 for C40H45N3O4 (calcd 631.3410).

2.2.9 Synthesis of unsymmetrical squaraine dye SQ-B.

500 mg (0.8 mmol) tert-butyl ester of unsymmetrical squaraine dye (6) was dissolved in 3

mL of trifluoroacetic acid (TFA) and stirred for 4 hours at room temperature. After

completion of the reaction as monitored by TLC, TFA was evaporated and the crude

product was purified by silica gel chromatography using chloroform:methanol (98:2)

solvent system as eluent to give 450 mg of target compound 7 as blue solid with 98% purity

as confirmed by HPLC in 98% yield. HR-FAB MS, observed [M]+ 575.2861 for

C36H37N3O4 (calcd 575.2784). 1H NMR (500 MHz, DMSO-d6): δ = 0.949 (t, 3H), 1.331 (t,

3H), 1.401 (m, 2H), 1.694 (m, 2H), 1.711 (s, 12H), 4.048 (m, 2H), 4.251 (m, 2H). 5.853 (s,

1H), 5.969 (s, 1H), 7.277 (t, 1H), 7.426 (m, 2H), 7.485 (d, J = 8.0 Hz, 1H), 7.610 (d, J = 7.5

Hz, 1H), 8.088 (dd, J = 1.0, 1.5 Hz, 1 H), 8.142 (s, 1H), 8.291 (s, 1H), 13.628 (s, 1H).

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2.2.1. Synthesis of compound 11.

To a solution of 2.7 g (8.5 mmol) of compound 2 in 60 mL of n-butanol:toluene mixture

(1:1, v/v) was added 3.4 g (8.5 mmol) of semisquaric acid 10. Reaction mixture was

refluxed for 18 hours using Dean-Stark trap. The reaction mixture was cooled, solvent was

evaporated and the product was purified by silica gel column chromatography using

chloroform:methanol as eluting solvent. 2.44 g of the final titled compound was obtained as

blue solid in 99% purity as confirmed by HPLC and 52% yield. HR-FAB MS, observed

[M]+ 558.1862 for C32H35BrN2O2 (calcd 558.1882).

2.2.2. Synthesis of compound 12.

A solution of 560 mg (1.0 mmol) of above synthesized unsymmetrical squaraine [11], 156

mg (1.0 mmol) of 5-formylthiophen-2-ylboronic acid and 2 mL of 2M Na2CO3 in 10 mL

dry THF was degassed using argon. To this mixture was added 150 mg (0.1 mmol) of

Pd(PPh3)4 and reaction mixture was heated at reflux under argon for 18 h. Progess of the

reaction was monitored by TLC. After the completion of reaction, solvent was evaporated

and the crude product was extracted with ethylacetate. The organic layer was collected,

dried over anhydrous Na2SO4 and finally solvent was evaporated. The precipitate was

purified by column chromatography on silica gel using chloroform:methanol as eluting

solvent yielding 600 mg (96% yield) of desired product with 98% purity as confirmed by

HPLC. HR-FAB MS, observed [M]+ 590.2575 for C37H38N2O3S (calcd 590.2603).

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2.2.3. Synthesis unsymmetrical squaraine dye SQ-C.

In a round bottom flask fitted with condenser, 500 mg (0.85 mmol) of compound 3, 151

mg (1.7 mmol) of cyanoacetic acid and 0.095 mL piperidine were dissolved in 15 mL of

dry THF. The reaction mixture was refluxed for 4-5 h under argon atmosphere. After

completion of the reaction, the reaction mixture was cooled and solvent was removed by

rotary evaporator. The residue was purified by silica gel column chromatography using

chloroform: methanol as eluting solvent to yield 345 mg of final titled compound as blue

solid in 98% purity as confirmed by HPLC and in 62% yield. HR-FAB MS, observed [M]+

657.2701 for C40H39N3O4S (calcd 657.2661). 1H NMR (500 MHz, DMSO-d6): δ = 0.96 (t,

3H), 1.31 (t, 3H), 1.42 (m, 2H), 1.69 (s, 12H), 1.74 (m, 2H), 4.08 (m, 2H), 4.18 (m, 2H),

5.83 (s, 1H), 5.88 (s, 1H), 7.22 (m, 1H), 7.39 (m, 1H), 7.56 (d, J = 7 Hz, 1H), 7.74 (dd, J =

8.5, 2.0 Hz, 1H), 7.77 (d, J = 4.0 Hz, 1H), 7.98 (dd, J = 7.5, 4.0 Hz, 2H), 8.43 (s, 1H).

2.3. DSSC fabrication and measurement of cell performance

DSSCs were fabricated using Ti-Nanoxide D paste (Solaronix SA) which was coated on

F-doped SnO2 substrate (Nippon Sheet Glass Co., Ltd.) by a doctor blade. The substrate

was then baked at 450 0C to fabricate TiO2 layers of about 10 m thickness. The

substrate was dipped in the ethanolic solution of the respective dyes in the presence of

chenodeoxycholic acid (CDCA) for 4 hours. The dye concentration was fixed to be 0.25

mM while CDCA concentration was 25 mM. A Pt sputtered SnO2/F layered glass

substrate (Nippon Sheet Glass Co., Ltd.) was employed as the counter electrode.

Electrolyte containing LiI (500 mM), iodine (50 mM), tert-butylpyridine (580 mM),

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MeEtIm-DCA (ethylmethylimidazolium dicyanoimide) (600mM) in acetonitrile was

used to fabricate the DSSC. A Himilan film (Mitsui-DuPont Polychemical Co., Ltd.) of

25 m thickness was used as a spacer. The cell area was 0.25 cm2 which was precisely

defined using a black metal mask. Solar cell performance was measured with solar

simulator (CEP-2000, Bunko Keiki, Japan) equipped with xenon lamp for the light

exposure. The spectrum of the solar simulator and its power were adjusted to be 100

mW/cm2 at AM 1.5 using a spectroradiometer (LS-100, Eiko Seiki, Japan).

3. Results and discussion

3.1 Electronic absorption spectra

The optical absorption spectra of unsymmetrical squaraine sensitizers SQ-A, SQ-B and

SQ-C in ethanol solution are shown in the Fig. 2. SQ-A exhibited an absorption maximum

(max) at 636 nm associated with the π-π* electronic transition originating from HOMO to

LUMO, a typical characteristic of the squaraine dyes. Under similar conditions, *

electronic transitions for dyes SQ-B and SQ-C bearing cyanoacrylate and thiophene-

cyanoacrylate anchoring groups have been found to be red-shifted and exhibiting the max

at 653 nm and 663 nm, respectively. Further, dye SQ-C shows comparatively larger red

shift in the max and onset absorption edge than dyes SQ-A and SQ-B which could be

attributed to the increase in the extent of -conjugation due to incorporation of thiophene-

cyanoacrylate anchoring group.

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The electronic absorption spectra of unsymmetrical squaraine dyes adsorbed on the

thin film of nanoporous TiO2 are shown in Fig. 3. It can be seen that, upon adsorption of

the dyes onto nanoporous TiO2, there is a broadening of absorption spectra on both sides

(higher and lower wavelength) as well as a pronounced red shift in absorption maximum

and onset absorption edge as compared to their behavior in the ethanol solution. The

distinctive observation of red shift and the spectral broadening is a clear indication of

interaction of anchoring groups of these unsymmetrical squaraine dyes with the surface of

nanoporus titania. The main -* electronic transition at 660 nm for SQ-B and SQ-C while

at 640 nm for SQ-A was observed in the solid-state absorption spectra. Apart from this, a

prominent absorption shoulder around 620 nm was observed for dyes SQ-B and SQ-C

while it was observed at 600 nm for dye SQ-A. This peculiar absorption was found to be

blue shifted as compared to main absorption associated to the monomeric species. This has

been ascribed to the H-aggregate formation. This conclusion was based on the fact that flat

squaraine dyes are succesptible to aggregate formation while commonly used dye de-

aggregating agent CDCA has been reported to suppress the dye aggregation [25, 26]. In an

interesting report, Yum. et al [27] have reported that, H-aggregation of the squaraine dyes

co-adsorbed with the CDCA onto the TiO2 surface can be controlled and are highly affected

by the relative concentration of CDCA in the dye solution. In the same report authors have

emphasized that ratio of optical density of the aggregate band (AH0) and the monomer band

(AQ0) can be visualized as indicator of the aggregation and higher is the value of ratio

AH0/AQ0 larger is the extent of H-aggregation. Accordingly, we evaluated the ratio of AH0 /

AQ0 for the three squaraine dyes as follows. The optical density of the aggregation band

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(AH0) of SQ-A dye observed at 600 nm (0.5) was divided by the optical density of

monomer band (AQ0) observed at 620 nm (1.0) in the normalized absorption spctra (Fig. 3)

to obtain the AH0/AQ0 ratio to be 0.50. Similarly the AH0/AQ0 ratio for dyes SQ-B and SQ-C

was calculated as 0.65 and 0.90 respectively. A perusal of these numerical values indicates

that extent of H-aggregate formation is more pronounced in the SQ-C dye as compared to

other two dyes which is clearly related to the extended -conjugation in SQ-C due to the

insertion of thiophene spacer between cyanoacrylate anchoring group and the aromatic

chromophore. Interestingly dyes SQ-B and SQ-C also show characteristic absorbance in

the 400-500 nm wavelength region when adsorbed on TiO2, which was weak in the solution

state (Fig. 2), but similar behavior was not observed for the dye SQ-A in the lower

wavelength region. This behavior is probably related to the cyanoacrylate functionality as

an anchoring group on unsymmetrical squaraine class of dyes which is possibly associated

with HOMO to LUMO+1 electronic transition [23].

3.2 Energy band diagram for squaraine dyes

Fig. 4. shows the energy band diagram of unsymmetrical squaraine dyes bearing various

anchoring groups used in the present investigation along with the energy level of the

oxide semiconductor and redox species. It can be clearly seen from Fig. 4 that the

LUMO of all of the three dyes are higher than that of the CB edge of the TiO2, where

the CB of TiO2 was considered as - 4.0 eV reported by Ogomi et al [28]. Also, the

energy of the HOMO for all of the dyes are lower than that of I3-/I

- redox energy level,

which has been reported to be 0.44 V vs. NHE [29], and can be translated into - 4.9 eV

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with respect to the vacuum level. These results indicate that, favorable electron

injection should occur from LUMO of the dyes to the CB of TiO2 and at the same time,

dye regeneration is also eneregetically possible, proving that these sensitizers bearing

various anchoring groups should work thermodynamically for the proper functioning of

the DSSCs.

3.3 Photovoltaic performance

DSSCs were fabricated using unsymmetrical squaraine dyes SQ-A, SQ-B and SQ-C

bearing various anchoring groups as sensitizer along with the use of nanoporous TiO2 as

electron transporter and I-/I3

- electrolyte as hole transporter followed by photovoltaic

measurements to investigate the photon harvesting behavior of these dyes. Current density-

voltage characteristics of the DSSCs fabricated using all of the three unsymmetrical

squaraine dyes are shown in Fig. 5. Detailed photovoltaic parameters of these dyes are

listed in the Table 1. It is clearly revealed by Fig. 5 and Table 1 that the dye SQ-B with its

cyanoacrylate anchoring group directly substituted on its aromatic chromophore shows best

efficiency of about 5.03% with the short circuit current density (JSC) of 11.53 mA/cm2,

open circuit voltage (VOC) of 0.63 V and fill factor (ff) of 0.69 amongst the unsymmetrical

dyes used in the present work. Interestingly, the dye SQ-C having the extended

conjugation due to the presence of thiophene spacer between cyanoacrylate and indole

chromophore, although exhibited good light absorption behavior and bathochromic shift as

compared to other two dyes but finally led to the much decreased photoconversion

efficiency of 2.7 % (JSC = 6.93 mA/cm2, VOC = 0.58 V, ff = 0.66), which is almost equal to

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the efficiency of dye SQ-A bearing carboxylate anchoring group. This decrease in the

efficiency was associated with the decrease in both of the JSC as well as VOC.

Fig. 6 shows the photocurrent action spectrum, also known as incident photon to

current conversion efficiency (IPCE), which was measured to elucidate the observed

differential JSC values for unsymmetrical squaraine dyes in their DSSCs performance.

The photocurrent action spectrum resembled with the optical absorption spectrum for

the different sensitizers because after photo-excitation, the photocurrent in the DSSCs

arises from the injection of electrons from the LUMO of the dye to the CB of the TiO2.

A perusal of the photocurrent action spectra clearly reveals that the onset IPCE edge of

the SQ-C is although shifted bathochromically than SQ-A but at the same time it is blue

shifted towards SQ-B along with the decrease in the magnitude of IPCEmax. Thus the

lack of photon harvesting in 400-500 nm (SQ-A) and diminished photon harvesting in

far-red region (SQ-C) could be responsible for the lower JSC values exhibited by these

two dyes as compared to direct ring cyanoacrylate functionalized squaraine dye (SQ-B).

Both of the SQ-B and SQ-C although bear similar cyanoacrylate anchoring

group but there was much diminished photon harvesting for SQ-C (44 % and 29 %) as

compared to that of SQ-B (54 % and 41 %) in both of the far-red (550-750 nm) as well

as visible (400-500 nm) wavelength regions, respectively. This could be attributed to

the enhanced H-aggregate formation by SQ-C as compared to that of SQ-B which is in

accordance with the solid-state electronic absorption of these dyes adsorbed on thin

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nanoporous TiO2 (Fig. 3). Formation of H-aggregates not only leads to the blue-shifted

light absorption but hampering in the electron injection also. Khazraji et al [30] have

also emphasized that as compared to dye aggregates, monomeric dyes more efficiently

inject electron to the CB of TiO2. The hampered electron injection in the dye aggregates

could be attributed to the aggregate induced energy transfer which competes with the

interafcial electron transfer. In an intersting report, Luo et al [31] systematically

investigated the role of dye aggregation by molecular engineering of porphyrin dyes

using steady state and transient fluorescence measuremnt techniques. They

demonstrated that electron injection yield decreases as a function of increasing extent of

dye aggregation associated with the enhanced -stacking. Miguel et al [32] have also

demonstrated that monomeric squaraine dyes adsorbed on TiO2 nanoparticles exhibit

about 5 times higher electron injection rate as compared to its H-aggergate counterparts

based on their femto-second transient absorption studies. Thus enhanced H-aggregate

assisted blue shift in the photon harvesting as well as hampered electron injection in the

SQ-C could be responsible for observed small JSC value as compared to that of SQ-B.

Therefore, incorporation of thiophene bridge between indole and cyanoacrylate

anchoring group does not offer any beneficial effect, rather hampers the overall photon

harvesting behavior. Based on these results most potential way would be the design and

development of such NIR squaraine dyes with direct ring functionalization of

cyanoacrylate anchoring groups with other -extended donors.

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Conclusions

Three indole based unsymmetrical squaraine dyes with varying anchoring groups have been

designed and synthesized to utilize them as metal free organic sensitizers for DSSCs. All of

the dyes exhibited intense light absorption in the far-red to NIR wavelength region.

Squaraine dye SQ-C has been found to exhibit pronounced H-aggregation when adsorbed

on to thin nanoporous TiO2 surface. Spectral broadening in the visible wavelength region

was also observed for dyes SQ-B and SQ-C, which is characteristic of cyanoacrylate

functionality. Direct ring substitution of cyanoacrylate anchoring group and in conjugation

with aromatic squaraine chromophore (SQ-B) leads not only to the red-shift in absorption

spectra but pronounced spectral broadening as well as photon harvesting (= 5.03%) as

compared to that of –COOH anchoring group (SQ-A) having 2.82 % photoconversion

efficincy.

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Figure captions

1. Structure of unsymmetrical squaraine dyes.

2. Electronic absorption spectra of unsymmetrical squaraine dyes in ethanol solution.

3. Electronic absorption spectra of unsymmetrical squaraine dyes adsorbed on thin

films of nanoporous TiO2.

4. Energy band diagram for unsymmetrical squaraine dyes along with the TiO2 and

redox electrolyte.

5. Current-voltage characteristics of DSSCs based on unsymmetrical squarine dyes

after simulated solar irradiation under AM 1.5 condition.

6. Photo-current action spectra of various unsymmetrical squaraine dyes after the

DSSC fabrication under monochromatic light irradiation.

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Table-1: Photovoltaic parameters for DSSCs fabricated using various unsymmetrical

squaraine dyes after simulated solar irradiation.

Dye Jsc (mA/cm2) Voc (V) FF Efficiency

SQ-A 6.71

(6.39±0.31)

0.58

(0.58±0.02)

0.72

(0.73±0.01)

2.82 %

(2.72±0.16)

SQ-B 11.53

(11.56±0.17)

0.63

(0.62±0.01)

0.69

(0.69±0.01)

5.03 %

(4.96±0.08)

SQ-C 6.93

(6.84±0.18)

0.58

(0.58±0.01)

0.66

(0.67±0.01)

2.67 %

(2.65±0.14)

Values shown in the parentheses indicate the average values for the four independent

DSSCs with their standard deviations for the solar cell parameters

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Figure-1

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Figure-2

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Figure-3

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Figure-4

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Figure-5

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Figure-6

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Scheme-1

Scheme-2

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Scheme-3

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Highlights

Three unsymmetrical squaraine dyes with varying anchoring groups were

synthesized.

Dye aggregation behavior and NIR photon harvesting was investigated.

The SQ-C dye showed pronounced H-aggregation upon adsorption on TiO2.

The SQ-B dye with direct ring substituted cyanoacrylate moiety exhibited best

photovoltaic performance.

*Highlights (for review)

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Graphical Abstract