Effect of extended π-conjugation on photovoltaic performance of dye sensitized solar cells based on unsymmetrical squaraine dyes

Tetrahedron 69 (2013) 2633e2639

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Effect of extended p-conjugation on photovoltaic performance of dye sensitized solar cells based on unsymmetrical squaraine dyes Shyam S. Pandey a, *, Rie Watanabe a, Naotaka Fujikawa a, Gururaj M. Shivashimpi a, Yuhei Ogomi a, Yoshihiro Yamaguchi b, Shuzi Hayase a a b

Graduate School of LSSE, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Japan Nippon Steel and Sumikin Chemical Company Limited, Nakabaru, Tobata, Kitakyushu, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 June 2012 Received in revised form 10 January 2013 Accepted 13 January 2013 Available online 29 January 2013

Aiming toward the far-red to near infra-red (NIR) photon harvesting, three new unsymmetrical squaraine dyes bearing direct ring carboxy functionalized indole as an anchoring moiety with varying donor groups with extended p-conjugation have been successfully synthesized and utilized for dye sensitized solar cell fabrication. Under simulated solar irradiation, dye SQ-8 gave a photoconversion efficiency of 3.3% mainly harvesting photons in the far-red region between 500 and 700 nm. By extending the p-conjugation of the donor moieties in the novel unsymmetrical squaraine dyes, it was possible to extend the light absorption from far-red to NIR wavelength region. In spite of good light absorption up to 900 nm and energetic matching, dye SQ-16 was found to exhibit the decreased photon harvesting, which was explained by the enhanced dye aggregation along with its difficulty in facile electron injection as indicated from electronic absorption spectroscopic and DFT calculation results, respectively. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Dye-sensitized solar cells Dye aggregation Unsymmetrical squaraine dyes p-Extended donors Nanocrystalline TiO2

1. Introduction Dye sensitized solar cells (DSSCs) based on mimicking the principle of natural photosynthesis are gaining an increased attention as one of the cheap alternatives to silicon solar cells due to high photoconversion efficiency (>10%) similar to amorphous silicon.1 It has been already demonstrated that efficient sensitizers used for DSSC fabrication exhibit nearly 100% photon harvesting in the visible region making an urgent need for the development of novel sensitizers absorbing in near infra-red (NIR)-IR wavelength region for panchromatic sensitization and further enhancement of the efficiency of DSSCs.2,3 In fact, covering a wide wavelength light absorption as well as photon harvesting by single dye in DSSCs was attempted in the recent past but overall efficiency was found to be reduced. Although they were successful in designing Osmium based complexes capable of light absorption from visible to IR region but along with the NIR-IR photon harvesting, the photon harvesting in visible reason was diminished leading to the decreased overall photoconversion efficiency.4 Similar situation was also observed for metal free organic dyes.5 To solve this problem, use of two or more dyes from their dyecocktail solution was also attempted but it quite often leads to the hampered efficiency due to the un-favorable inter-dye interactions.

* Corresponding author. Tel.: þ81 93 695 6230; fax: þ81 93 695 6005; e-mail address: [email protected] (S.S. Pandey). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.01.036

We have recently reported that the utilization of dye double layer architecture based DSSCs leads to the efficient photon harvesting from both of the dyes in the wide wavelength region.6,7 Our approach for the preparation of high efficiency DSSCs is based on the utilization of NIR dyes having sharp and intense absorption with the potential organic or inorganic based visible dyes to fabricate dye double layer DSSCs. In this context, squaraine dyes are one of the potential candidates amongst NIR dyes owing to their intense and sharp light absorption with narrow full width at half maximum (FWHM). They basically bear a donoreacceptoredonor zwitterionic molecular framework having squaric acid core as acceptor. The wavelength of squaraine dyes can be tailored from visible to IR wavelength region based on judicious molecular design using different donor moieties with extended p-conjugation.8 A perusal of squaraine sensitizers bearing carboxyl anchoring group clearly corroborates that dyes bearing carboxylic group directly substituted and in conjugation with aromatic ring are superior in performance as compared to that of their alkyl side chain carboxy substituted counterparts.9,10 It has also been reported that unsymmetrical squaraine dyes exhibit superior performance compared to that of symmetrical squaraine dyes due to unidirectional flow of electrons.11 Taking these reports into the considerations, Yum et al.12 have reported an unsymmetrical squaraine dye bearing carboxylic anchoring group directly substituted in the aromatic ring giving photoconversion efficiency of 4.5% with photon harvesting up to 700 nm. Such an efficiency given by squaraine sensitizer broke the

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myth of poor power conversion efficiency of squaraine dyes reported by various research groups.13,14 We have recently reported our systematic study on the role of substituents on the sensitization behavior of model squaraine dyes bearing variable alkyl chain length as well as fluoroalkyl substituents.15 It has been observed that it is possible to systematically control the energetics, i.e., energy of their highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) by mere alkyl chain substitution. Based on minimum energy barrier needed for electron injection and dye regeneration, it was demonstrated that it is possible to design new squaraine dyes up to the maximum wavelength of 950 nm for DSSCs based on  16 nanoporous TiO2 and I 3 /I redox electrolyte. Present article deals with the development of new unsymmetrical squaraine dyes having the ability of light absorption in the far-red to NIR wavelength region by judicious selection of suitable donor moieties having extended p-conjugation. Efforts have also been made to investigate the effect of extended p-conjugation on the photosensitization behavior and photovoltaic performance of DSSCs based on these dyes in terms of structureeproperty correlation. 2. Result and discussion 2.1. Synthesis of the materials Structure of the unsymmetrical squaraine (SQ)-dyes with varying extent of p-conjugation utilized in the present investigation has been shown in Fig. 1. Direct ring carboxy functionalized indole derivative 2,3,3-trimethyl-3H-indole-5carboxylic acid used as common anchoring group for all of the unsymmetrical squaraine dyes was synthesized following the methodology reported by Pham et al.17 Unsymmetrical squaraine dye SQ-8 was synthesized as per our earlier publication.9 Quinoline donor moiety based unsymmetrical squaraine dye (SQ-12) and Benzo(c.d)indole donor based unsymmetrical squaraine dye (SQ16) have been synthesized following the method adopted by Oswald et al.18 as shown in Schemes 1 and 2, respectively.

Fig. 1. Structure of unsymmetrical squaraine dyes.

Scheme 1. Synthesis of unsymmetrical squaraine dye SQ-12.

2.2. Electronic absorption spectra of unsymmetrical squaraine dyes Electronic absorption spectra of unsymmetrical squaraine dyes in ethanol solution and dye adsorbed on the thin films of nanoporous TiO2 have been shown in Fig. 2. In the ethanol solution unsymmetrical squaraine dye SQ-8 shows a sharp absorption peak at 635 nm associated with pep* electronic transition with narrow FWHM of 27 nm, a typical characteristic of the squaraine dyes. At the same time, this pep* electronic transition have been found to exhibit the bathochromic shift appearing at 656 nm and 767 nm along with the increasing FWHM of 65 nm and 103 nm for unsymmetrical squaraine dyes SQ-12 and SQ-16, respectively. This bathochromic shift could be attributed to the increase in the extent of p-conjugation of the corresponding donor moieties, such as quinoline (SQ-8) and benzo(c,d) indole (SQ-16) as compared that of indole (SQ-8). Similar behavior pertaining to the bathochromic

Scheme 2. Synthesis of unsymmetrical squaraine dye SQ-16.

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TiO2, respectively, to maintain the energetic cascade. Suitability of a particular dye for NIR sensitization depends on the minimization of the energy gap for electron injection to the CB of the TiO2 and energy required for dye regeneration. Therefore, increasing the wavelength of the dyes needs relatively more careful control of the energetics due to their decreased energy band gap. The energy band diagram of various unsymmetrical squaraine dyes along with the energy level of the TiO2 and redox species has been shown in  Fig. 3. Redox potential of I 3 /I redox couple has been reported to be 0.44 V versus NHE,21 which can be translated intod4.9 eV with respect to the vacuum level. At the same time, CB of TiO2 wasd4.0 eV considering the most negative quasi Fermi level corresponding to the flat band potential of TiO2 (0.7 V vs SCE) as reported by Ogomi et al.22

Fig. 2. Electronic absorption spectra of unsymmetrical squaraine dyes (a) in ethanol solution and (b) thin film adsorbed on TiO2.

shift of absorption maxima as a function of increasing p-conjugation in squaraine dyes have also been reported by Yagi et al.19 Upon adsorption of these dyes on the nanoporous TiO2, they undergo spectral broadening along with the red shift of absorption maximum as compared to their spectral behavior in the ethanol solution. Electronic absorption parameters in terms of absorption maximum and FWHM have been summarized in Table 1. A perusal of Table 1 and Fig. 2 clearly corroborates that there is a bathochromic shift and increase in the respective FWHM for the unsymmetrical squaraine dyes as a function of the increasing extent of p-conjugation of the donor moieties. This bathochromic shift and spectral broadening upon dye adsorption onto thin TiO2 film could be attributed to the interaction between carboxyl functionality of squaraine dyes with the TiO2 surface. Table 1 Electronic absorption parameters for unsymmetrical squaraine dyes Sensitizing dye

Absorption maximum

FWHM

SQ-8 in solution SQ-8 on TiO2 SQ-12 in solution SQ-12 on TiO2 SQ-16 in solution SQ-16 on TiO2

635 647 656 664 767 787

27 nm 50 nm 65 nm 95 nm 103 nm 166 nm

nm nm nm nm nm nm

It is noteworthy that in the solution state, SQ-12 and SQ-16 exhibit a very small peak in the 400e500 nm wavelength regions, which get pronounced after adsorption upon the nanoporous TiO2. Interestingly this behavior is not observed for the SQ-8. It is well known that dyes like squaraine and cyanine are prone to the dye aggregate formation, such as red-shifted J-aggregates and blueshifted H-aggregates depending on the molecular structure of the dyes. It seems that SQ-12 and SQ-16 are relatively more susceptible to the blue-shifted H-aggregate formation due to presence of enhanced p-extended molecular framework. At the same time, this effect get more pronounced upon the adsorption to the solid surface due to enhanced inter molecular interactions. Kim et al.20 have also observed the formation of blue-shifted H-aggregates formed by squaraine dyes on SnO2 surface absorbing in the lower wavelength regions. 2.3. Energy band diagram for squaraine dyes Design of a novel sensitizer for DSSC is based on the judicious control of energy level of HOMO and LUMO of dyes with respect to the energy level of the redox couple and conduction band (CB) of

Fig. 3. Energy band diagram for unsymmetrical squaraine dyes along with the energy levels TiO2 and redox electrolyte.

It can be clearly seen from Fig. 3 that all of the unsymmetrical squaraine dyes utilized as sensitizer for DSSC in the present work possess the energetic cascade with respect to the CB of the TiO2 and energy level of the iodide/triiodide redox electrolyte allowing them to work thermodynamically for the proper functioning of the DSSCs. At the same time, increase in the p-conjugation of the sensitizers, which is in the order of SQ-8
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DSSC based on SQ-16 leads to some increase in the Jsc leading to the enhancement in the photoconversion efficiency from 0.1 % to 0.35% suggesting that the small driving force for electron injection is also one of the possible reasons for its lower efficiency.

Fig. 4. Currentevoltage characteristics of DSSCs based on unsymmetrical squaraine dyes.

2.4.2. Photo-action spectra. To elucidate the decrease in the Jsc, incident photon to current conversion efficiency (IPCE) also known as photocurrent action spectrum was measured. In principle, IPCE should exhibit the symbatic photo response with the electronic absorption spectrum since the observed photocurrent in the DSSCs arises from the injection of electrons from the LUMO of the dye to the CB of the TiO2 after the photo-excitation. A perusal of the photoaction spectra of the DSSCs as shown in Fig. 5 clearly indicates a symbatic photo response with the electronic absorption spectra shown in Fig. 2. SQ-12 although exhibits the bathochromic shift in the photo response as compared to that of SQ-8 due to the presence of extended p-conjugation but the magnitude of the IPCE is decreased in the far-red region, which could be responsible for the lower Jsc value. At the same time, it exhibits another peak in the wavelength region of 400e500 nm, which can be attributed to the H-type dye aggregate formation as reported by Kim et al.20 also which might be responsible for the hindered electron injection and lower observed Voc.

Table 2 Photovoltaic performance of DSSCs fabricated with various unsymmetrical squaraine dyes under irradiation of AM 1.5 G simulated solar light (100 mW/cm2) Dye

Jsc (mA/cm2)

Voc (V)

FF

Efficiency

SQ-8 SQ-12 SQ-16 SQ-16 without TBP

7.22 6.61 0.29 1.23

0.63 0.57 0.47 0.46

0.72 0.61 0.72 0.62

3.28% 2.29% 0.10% 0.35%

efficiency of about 3.3% was obtained for SQ-8 having the lowest pconjugation. Increasing the p-conjugation, although squaraine dyes exhibit the bathochromic shift but at the same time leads to the decrease in the photoconversion efficiency. This decrease in the efficiency with the extended p-conjugation is associated with the decrease in both of the short circuit current density (Jsc) and open circuit voltage (Voc) but the decrease of Jsc was drastically pronounced for SQ-16. It can be clearly seen from Table 2 that increasing the p-conjugation in the donor moieties for the unsymmetrical dyes leads to consequent decrease in the Voc. This could be explained considering the fact that increasing the p-conjugation leads to increased planer aromatic framework, which may promote the p-stacked dye aggregate formation. Khazrazi et al.23 have also emphasized that dye aggregates leads to the hindered electron injection as compared to the monomeric dyes, which facilitates the electron recombination leading to the decreased Voc. It is important to note that in spite of thermodynamically favorable energy level and good light absorption especially in case of SQ-16, there is tremendous decrease in the Jsc. This indicates that most likely either electron injection from the photoexcited dye is getting hampered or there is serious recombination due to the dye aggregate formation. It can also be thought that the energy difference between the LUMO of the SQ-16 and CB of TiO2 is smallest amongst the dyes used in this work leading to the smallest driving force for the electron injection. To clarify this, DSSC was fabricated with SQ-16 with electrolyte having no tert-butyl pyridine (TBP). Incorporation of TBP in the electrolyte has been reported to shift the CB of TiO2 positively, which is responsible for the observation increased Voc as compared to electrolyte without TBP.24 In the present case, removal of TBP from electrolyte is expected to decrease the CB of TiO2 and increase the driving force for the electron injection. It can be seen from Fig. 4 and Table 2 that removal of TBP from electrolyte for

Fig. 5. Photocurrent action spectra for various unsymmetrical squaraine dyes after the DSSC fabrication under monochromatic light irradiation.

Interestingly, SQ-16 shows very small photo response in the wavelength region of 600e900 nm as shown in the inset of Fig. 5 with a maximum of IPCE between the 0.15e0.2 percent, which is responsible for the very low Jsc. This clearly indicates that in spite of maintaining the energetic cascade and good light absorption, it is unable to show the good photovoltaic performance. This could only be attributed to the lack of efficient electron injection from the photoexcited dye to the conduction band of nanoporous TiO2 or large electronic recombination between the injected electron and oxidized dye or electrolyte. Removal of TBP from the electrolyte solution led to the increase in the photon harvesting from 0.15% to about 2% in the wavelength region of 600e900 nm, which is in accordance with increased Jsc as shown in Fig. 4. This suggests that the hindered electron injection due small driving force for the electron injection. Since even after removal of the TBP to enhance the driving force, although there was an enhanced photon harvesting as compared to that of the DSSC fabricated with electrolyte having TBP, but the magnitude of the overall photocurrent is still much lower as compared to that of the performance of DSSCs

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fabricated using dyes SQ-12 and SQ-8. This indicates that there are some other factors also which affect the electron injection apart from the small driving force, which has been discussed in the next section. 2.5. Effect of extended p-conjugation on electronic distribution in HOMO and LUMO: implication on new dye design Molecular orbital (MO) calculation for the design of novel sensitizers has been proved their potentiality in the recent past. Amongst MO calculation techniques, density functional theory (DFT) has emerged as reliable and standard tool for the theoretical treatment of suitability of dye molecular structure. Its time dependent extension also known as TD-DFT has been proved to give reliable values for excitation energies with the standard exchange correlation functional, which has been extensively used in the recent past for the design and development of novel sensitizers for the DSSCs.25,26 In order to have a further insight about the nature of the excited states of the unsymmetrical squaraine dyes, DFT/TDDFT computational investigations have also been performed using B3PW91 exchange functional and 6-311G basis set as implemented in the Gaussian (G09) program package. Results of the MO calculation for the unsymmetrical squaraine dyes have been shown in Fig. 6. For reducing the computation cost pertaining to the long alkyl substituents in SQ-12 and SQ-16, only ethyl group has been used as their analogous structure for the calculation. A perusal of Fig. 6(a) pertaining to the dye SQ-8, it can be clearly seen that HOMO is mainly centralized at squaraine core, which is associated with the p-framework of the squaraine dye. On the other hand LUMO associated with p* MO is also delocalized over the entire molecular framework with reduction of electron density distribution at squaric acid core and sufficient electron density at the eCOOH anchoring group present in the indole ring. Excited state TD-DFT calculation on this dye (see supporting Fig. S3) reveals an intense absorption at 557 nm (f¼1.4970) associated with the pep* electronic transition consisting of excitations from HOMOeLUMO responsible for the facile charge flow from the squaraine core to the anchoring group. This unidirectional electron flow is actually responsible for

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enhanced photovoltaic performance of unsymmetrical squaraine dyes as compared to their symmetrical dye counterpart. In the case of SQ-12 and SQ-16, HOMO associated with p-molecular framework is not only localized at squaric acid core but there is sufficient electron density contribution at quinoline and benzo(c,d) indole donor moieties, which in contrast to the SQ-8. At the same time, the electron density contribution in their LUMO at eCOOH anchoring group is in the order of SQ-8>SQ-12>SQ-16. Excited state TD-DFT calculations performed on the SQ-12 (Fig. S4) and SQ-16 (Fig. S5) reveal the intense absorption at 603 nm (f¼1.4286) and 662 nm (f¼1.3172), respectively, associated with electronic transitions originating from HOMOeLUMO excitations. Calculated results also exhibit the bathochromic shift along with the decrease in the molar extinction coefficient as a function of increasing p-conjugation, which is in accordance with experimental results. Therefore, lack of sufficient unidirectional electron flow along with the very small electron density at the anchoring group in SQ-16 could be responsible for the hindrance in the electron injection after the photo-excitation of the dye. From these results it could also be inferred that it is not a good idea to incorporate a relatively large p-extended framework as donor, which hinders the unidirectional flow of electrons rather they should be located near the anchoring group. 3. Conclusions Unsymmetrical squaraine dyes bearing direct carboxy functionalized indole as anchoring group and various donor moieties with extended p-conjugation have been synthesized and utilized as sensitizer for DSSC fabrication. Extension in p-conjugation leads to the lowering of the band gap of the dyes along with bathochromic shift in the absorption maxima as well as enhancing the photon harvesting window toward extended wavelength region. With the increasing extent of conjugation length of squaraine dyes, although increases the photon harvesting window in far-red to NIR wavelength region but at the same time it led to the decrease in the IPCE. Introduction of flat aromatic donor species led to the decrease in the relative extent of the electron density at the anchoring eCOOH group in the LUMO resulting into the hindered electron injection

Fig. 6. Electron density distribution in molecular orbitals of unsymmetrical squaraine dyes for SQ-8(a), SQ-12(b) and SQ-16(c) after structural optimization at B3PW91/6-311G level of calculation.

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after photo-excitation ultimately leading to the decreased photoconversion efficiency. 4. Experimental section 4.1. General All the chemicals for synthesis or solvents are of analytical or spectroscopic grade and used as received without further purification. Synthesized SQ-dyes and dye intermediates were analyzed by high performance liquid chromatography (HPLC) for purity, fast ion bombardment (FAB)-mass spectrometry in positive ion monitoring mode and nuclear magnetic resonance spectroscopy (NMR, 500 MHz) for structural elucidation. Electronic absorption spectroscopic investigations in solution and thin film adsorbed on TiO2 surface were conducted using UVevisible spectrophotometer (JASCO model V550). HOMO energy level was measured using photoelectron spectroscopy in air (Riken, model AC3). The LUMO energy level was determined from the edge of the optical absorption considering it as optical band gap (Eg) using the relation LUMO¼HOMOþEg. Optimized geometries of final dyes along with the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were calculated without any symmetry restriction at B3PW91/6-311G level of calculation using Gaussian 09 program package.27 The 1-ethyl-2-methyl quinolinium iodide (1),28 5-carboxy-2,3,3-trimethyl-1-octyl-3H-indolium iodide (3),9 5-carboxy-2,3,3-trimethyl-1-ethyl-3H-indolium iodide (4),9 and 1-hexyl-benz[c,d]indolium perchlorate (7)29,30 were synthesized according to the literature methods. 4.2. Synthetic details and characterization 4.2.1. Synthesis of 3-butoxy-4-[(1-ethyl-quinoline-2-ylidene)methyl]3-cyclobutene-1,2-dione [2]. In a round bottom flask fitted with condenser, compound 1 (2.99 g, 10 mmol), 3,4-dibutoxy-3cyclobutene-1,2-dione (2.24 g, 10 mmol) and 2 mL of triethylamine were dissolved in butanol (10 mL). Reaction mixture was heated at 70  C for 1 h leading to green solution. Solvent was removed under vacuum and product was purified by column chromatography (silica gel) with ethyl acetate and hexane as eluent giving 1.73 g orange colored solid compound in 53% yield and 99% purity as confirmed by HPLC. Mp 248  C (dec). HR-FAB-mass (calculated 324.1555 for (Mþ1); measured 324.1605). 1H NMR (500 MHz, CDCl3): dH 7.54 (t, 1H), 7.48 (dd, J¼1.5, 1.5 Hz, 1H), 7.36 (dd, J¼10.5, 8.5 Hz, 3H), 7.23 (t, 1H), 5.28 (s, 1H), 4.82 (t, 2H), 4.21 (m, 2H), 1.85 (m, 2H), 1.50 (m, 5H), 1.00 (t, 3H). 13C NMR (500 MHz, CDCl3): d 185.0, 173.8, 150.4, 139.13, 132.9, 131.1, 128.8, 124.2, 123.9, 113.9, 85.6, 73.5, 42.7, 32.2, 18.7, 13.8, 11.5. FTIR (KBr, cm1): 2956, 2938, 2872, 1762, 1753, 1697, 1628, 1536, 1491, 1457, 1406, 1323, 1186, 1172, 1063, 916, 824, 743. Anal. calcd for C20H21NO3: C, 74.28%; H, 6.55%; N, 4.33; found: C, 74.31%; H, 6.51%; N, 4.29. 4.2.2. Synthesis unsymmetrical squaraine dye SQ-12. Unsymmetrical squaraine dye SQ-12 was synthesized using semi-squaraine ester (2) and compound (3) as follows: in a round bottom flask fitted with condenser, compound 2 (550 mg, 1.7 mmol) was dissolved in ethanol (30 mL) followed by 1 mL of 10% NaOH. Reaction mixture was refluxed for 30 min, which was then cooled followed by addition of 2 N HCl (0.5 mL) giving compound (3). Solvent was then removed at rotary evaporator followed by addition of compound 3 (753 mg, 1.7 mmol) and 1-butanol/toluene mixture (1:1, v/v) (40 mL). Reaction mixture was refluxed for 18 h using DeaneStark trap. Reaction mixture was cooled, solvent was evaporated and product was purified by silica gel column chromatography using chloroform: methanol as eluting solvent. 650 mg of final titled compound was obtained as blue colored solid in 98% purity as confirmed by HPLC in

68% yield. Mp 268  C (dec). High resolution (HR)-FAB-mass (calculated 564.2988 and observed 564.2987) confirms the successful synthesis of the unsymmetrical squaraine dye SQ-12. 1H NMR (500 MHz, CDCl3): dH 9.50 (d, J¼9.5 Hz, 1H), 8.19 (d, J¼9 Hz, 1H), 8.05 (d, J¼9 Hz, 1H), 7.93 (d, J¼7.5 Hz, 1H), 7.86 (m, 3H), 7.57 (t, 1H), 7.16 (d, J¼8.5 Hz, 1H), 6.02 (s, 1H), 5.62 (s, 1H), 4.60 (m, 2H), 3.91 (m, 2H), 1.67 (s, 6H), 1.47 (t, 3H), 1.33 (m, 2H), 1.23 (m, 10H), 0.84 (t, 3H) [see supporting Fig. S1 for NMR chart]. 13C NMR (500 MHz, DMSO-d6): d 181.5, 180.3, 167.2, 162.5, 160.1, 152.0, 149.3, 146.8, 140.5, 138.2, 136.8, 132.7, 130.6, 130.1, 129.2, 123.4, 122.7, 120.1, 116.6, 107.9, 104.7, 94.2, 88.4, 76.4, 46.7, 31.0, 28.6, 26.9, 26.1, 21.9, 13.8, 12.4. FTIR (KBr, cm1): 3042, 2958, 2924, 2953, 1722, 1695, 1616, 1562, 1498, 1346, 1298, 1260, 1159, 1105, 984, 933, 825, 756. Anal. Calcd for C36H40N2O4: C, 76.57%; H, 7.14%; N, 4.96; found: C, 76.48%; H, 7.34%; N, 4.83. 4.2.3. Synthesis of 3-butoxy-4-[(1-ethyl-1,3-dihydro-3,3-dimethyl2H-indole-2-ylidene)methyl]-3-cyclobutene-1,2-dione [6]. In a round bottom flask fitted with condenser compound 4 (1.50 g, 4 mmol), 3,4-dibutoxy-3-cyclobutene-1,2-dione (5) (900 mg, 4 mmol) and triethylamine (1.0 mL) were dissolved in butanol (20 mL). Reaction mixture was heated at 70  C 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 740 mg of orange colored solid compound in 48% yield and 98% purity as confirmed by HPLC. Mp 244  C (dec). HR-FAB-mass (calculated 384.1766 for (Mþ1); measured 384.1790). 1 H NMR (500 MHz, CDCl3): dH 8.10 (dd, J¼1.5, 1.5 Hz, 1H), 7.98 (d, J¼2 Hz, 1H), 6.90 (d, J¼8.5 Hz, 1H), 5.50 (s, 1H), 4.90 (m, 2H), 3.91 (q, 2H), 3.50 (t, 2H), 1.66 (s, 6H), 1.54 (m, 2H), 1.36 (t, 3H), 1.21 (t, 3H). 13 C NMR (500 MHz, CDCl3): d 192.4, 192.2, 188.79, 188.63, 188.5, 173.7, 173.4, 171.2, 167.0, 166.9, 146.9, 146.9, 141.0, 140.9, 131.6, 123.9, 123.1, 107.5, 82.9, 82.8, 74.2, 70.31, 65.8, 47.4, 47.3, 37.9, 31.1, 29.7, 26.9, 26.9, 18.7, 15.9, 15.3, 13.7, 11.3. FTIR (KBr, cm1): 3054, 2965, 2580, 2513, 1773, 1715, 1680, 1582, 1540, 1363, 1296, 1207, 1118, 1053, 934, 818, 778, 668, 626. Anal. Calcd for C22H25NO5: C, 68.91%; H, 6.57%; N, 3.65; found: C, 68.84%; H, 6.49%; N, 3.69. 4.2.4. Synthesis of unsymmetrical squaraine dye [16]. Unsymmetrical squaraine dye SQ-16 was synthesized using semi-squaraine ester (6) and compound (7) as follows: in a round bottom flask fitted with condenser, semi-squaraine ester 6 (575 mg, 1 mmol) was dissolved in ethanol (20 mL) followed by 10% NaOH (1.5 mL) solution. Reaction mixture was refluxed for 30 min, which was then cooled followed by addition of 2 N HCl (1 mL). Solvent was then removed at rotary evaporator followed by addition of 550 mg (1.5 mmol) of compound 7 and 50 mL of 1-butanol/toluene mixture (1:1, v/v). Reaction mixture was refluxed for 18 h using DeaneStark trap. Reaction mixture was cooled, solvent was evaporated and product was purified by silica gel column chromatography using chloroform/ methanol as eluting solvent. 230 mg of final titled compound was obtained as blue colored solid in 97% purity as confirmed by HPLC in 27% yield. Mp 249  C (dec). HR-FAB-mass (calculated 560.2675; measured 560.2701) confirms the successful synthesis of the unsymmetrical dye SQ-16. 1H NMR (500 MHz, CDCl3): dH 8.98 (br, 1H), 8.09 (d, J¼8 Hz, 2H), 7.99 (dd, J¼1.5, 1.5 Hz, 1H), 7.84 (t, 1H), 7.66 (d, J¼8.5 Hz, 1H), 7.59 (t, 1H), 7.49 (d, J¼8.0 Hz, 1H), 7.42 (d, J¼8 Hz, 1H), 6.19 (s, 1H), 5.98 (s, 1H), 4.28 (m, 2H), 4.23 (m, 2H), 1.78 (m, 3H), 1.75 (s, 6H), 1.41 (m, 2H), 1.33 (m, 6H), 0.85 (t, 3H). See supporting Fig. S2 for NMR chart. 13C NMR (500 MHz, DMSO-d6): d 181.0, 179.1, 178.0, 170.0, 166.9, 149.3, 145.2, 141.8, 141.3, 130.2, 129.2, 128.9, 126.3, 124.5, 123.2, 120.8, 110.3, 107.6, 90.5, 88.1, 79.1, 48.8, 43.0, 30.9, 27.9, 26.0, 21.9, 13.7, 11.8. FTIR (KBr, cm1): 3058, 2957, 2926, 2858, 1718, 1707, 1603, 1559, 1496, 1434, 1355, 1265, 1197, 1065, 934, 818, 775. Anal. Calcd for C36H36N2O4: C, 77.12%; H, 6.47%; N, 5.00; found: C, 77.20%; H, 6.55%; N, 4.85.

S.S. Pandey et al. / Tetrahedron 69 (2013) 2633e2639

4.3. DSSC fabrication and measurement of cell performance DSSCs were fabricated using Ti-Nanoxide D paste (Solaronix SA), which was coated on a Low E glass (Nippon Sheet Glass Co., Ltd.) by a doctor blade. The substrate was then baked at 450  C to fabricate TiO2 layers of about 12 mm thickness. The substrate was dipped in the ethanolic solution of the respective dyes in the presence of chenodeoxycholic acid (CDCA) for 4 h. The dye concentration was fixed to be 0.25 mM while CDCA concentration was 25 mM. A Pt sputtered SnO2/F layered glass substrate was employed as the counter electrode. Electrolyte containing LiI (500 mM), iodine (50 mM), tert-butylpyridine (580 mM), MeEtIm-DCA (ethyl methylimidazolium dicyanoimide) (4:6, w/w) (600 mM) in acetonitrile was used to fabricate the DSSC. A Himilan film (Mitsui-DuPont Polychemical Co., Ltd.) of 25 mm 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). Supplementary data The details of quantum chemistry computation pertaining to the calculation of electronic absorption spectra in gas phase along with the 1H NMR spectra are also available. Supplementary data related to this article can be found at http://dx.doi.org/10.1016/ j.tet.2013.01.036. References and notes 1. Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Prog. Photovolt: Res. Appl. 2011, 19, 565e572. 2. Gratzel, M. Inorg. Chem. 2005, 44, 6841e6851. 3. Yela, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; €tzel, M. Science 2011, 334, Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Gra 629e634. 4. Altobello, S.; Argazzi, R.; Caramori, S.; Contado, C.; Da-Fre, S.; Rubino, P.; Chone, C.; Larramona, G.; Bignozzi, C. A. J. Am. Chem. Soc. 2005, 127, 15342e15343. 5. Tian, H.; Yang, X.; Chen, R.; Hagfeldt, A.; Sun, L. Energy Environ Sci. 2009, 2, 674e677. 6. Noma, Y.; Iizuka, K.; Ogomi, Y.; Pandey, S. S.; Hayase, S. J. Appl. Phys. 2009, 48, 020213(01)e020213(03).

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