Squaraine dyes and applications thereof

Abstract

The present invention disclosed a squaraine dye of formula (I) and process for the preparation thereof. Further, the present invention disclosed to an electronic device comprising dye of formula (I).

Claims

1. A squaraine dye of formula (I): ##STR00006## wherein, R.sub.1 is selected from the group consisting of methyl, straight or branched chain —C.sub.5 to C.sub.20 alkyl, —C.sub.1 to C.sub.20 alkoxy, aryl, arylalkyl, fused aryl polyethylene glycol units, triethyleneglycol monomethylether, tetraethylene glycol monomethylether, halides, cyano, and trifluoromethyl; R.sub.2 is selected from the group consisting of straight or branched chain —C.sub.1 to C.sub.20 alkyl, —C.sub.1 to C.sub.20 alkoxy, aryl, arylalkyl, fused aryl polyethylene glycol units, triethyleneglycol monomethylether, tetraethylene glycol monomethylether, halides, cyano, and trifluoromethyl; R.sub.3-R.sub.13 are same or different and are selected from the group consisting of hydrogen, straight or branched chain —C.sub.2 to C.sub.20 alkyl, —C.sub.1 to C.sub.20 alkoxy, aryl, arylalkyl, fused aryl polyethylene glycol units, triethyleneglycol monomethylether, tetraethylene glycol monomethylether, C.sub.1 to C.sub.10 perfluoroalkyl chains, cyano, and trifluoromethyl; or each of R.sub.3-R.sub.6 independently is methyl; X is selected from the group consisting of —COOH and ##STR00007## and R.sub.14 and R.sub.15 are same or different and are selected from the group consisting of straight or branched chain —C.sub.1 to C.sub.20 alkyl, and —C.sub.1 to C.sub.20 alkoxy; provided that at least one of R.sub.1-R.sub.6 is straight or branched chain —C.sub.2 to C.sub.20 alkyl; and provided when R.sub.2 is C.sub.6 alkyl, R.sub.1 is methyl or C.sub.6 alkyl.

2. A squaraine dye, wherein said squaraine dye is 5-Carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ2), 5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium (SQ3), 5-Carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium (SQ4), 5-Carboxy-2-[[3-[(1,3-dihydro-1-hexyl-3-decyl-3-octyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ5), 5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1-hexyl-3-decyl-3-octyl-3H-indolium (SQ6), 5-Carboxy-2-[[3-[(1,3-dihydro-1-hexyl-3-decyl-3-octyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1-hexyl-3-decyl-3-octyl-3H-indolium (SQ7), 4-((5-(6-(2-Carboxy-2-cyanovinyl)-4,8-dimethoxybenzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (RSQ1), or 4-((5-(6-(2-carboxy-2-cyanovinyl)-4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate 20d (RSQ2).

3. A process for the preparation of squaraine dye of formula (I) as claimed in claim 1, said process comprising the steps of: (a) refluxing a reaction mixture of a hydrazine compound and 3-alkyl-2-alkanone dissolved in acetic acid at a temperature in the range of 80 to 100° C. for the period in the range of 14 to 16 hours to obtain a corresponding 2-methyl-3,3-dialkyl-3H-indole compound; (b) refluxing a reaction mixture of an alkyl halide and the compound as obtained in step (a) dissolved in acetonitrile at a temperature in the range of 100 to 110° C. for the period in the range of 14 to 16 hours to obtain a corresponding indolenium salt; (c) heating a reaction mixture of the compound obtained in step (b) and 3, 4-dibutoxycyclobut-3-ene-1,2-dione dissolved in a solvent in the presence of triethylamine at a temperature in the range of 60 to 70° C. for the period in the range of 1 to 2 hours to obtain a semisquaraine compound; (d) refluxing a reaction mixture of the compound obtained in step (c) with the compound obtained in step (b) dissolved in a solvent in the presence of pyridine at a temperature in the range of 110 to 115° C. for the period in the range of 20 to 24 hours to obtain a corresponding dye of formula (I); wherein X is —COOH; (e) stirring a reaction mixture of the squaraine dye of formula (I) obtained in step (d), a benzodithiophene compound, palladium (II) acetate [Pd(OAc).sub.2], tricyclohexylphosphine (PCy.sub.3), and pivalic acid (PivOH) in the presence of potassium carbonate (K.sub.2CO.sub.3) in toluene at a temperature of 110° C. for the period in the range of 14 to 16 hours to obtain an aldehyde compound; (f) adding cyanoacetic acid and piperidine to a reaction mixture of containing the compound as obtained in step (e) in a solvent followed by stirring the reaction mixture at a temperature in the range of 70 to 80° C. for the period in the range of 14 to 16 hours to obtain a dye of formula (I) wherein X is not —COOH.

4. The process as claimed in claim 3, wherein said hydrazine compound is 4-hydrazinobenzoic acid or phenyl hydrazine hydrochloride, said 3-alkyl-2-alkanone is 3-octyltridecan-2-one or 3-methylbut-2-one, and said alkyl halide is 1-iodohexane or iodomethane.

5. The process as claimed in claim 3, wherein said indolenium salt is 1-alkyl-2-methyl-3,3-dialkyl-3H-indol-1-ium iodide or 5-carboxy-1-alkyl-2-methyl-3,3-dialkyl-3H-indol-1-ium iodide.

6. The process as claimed in claim 3, wherein said semisquaraine compound is (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trimethyl-indoline (8), (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1-hexyl-3,3-dimethyl-indoline (9), (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trimethyl-indoline-5-carboxylic acid (10), or (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1-hexyl-3-decyl-3-octyl-indoline-5-carboxylic acid (11).

7. The process as claimed in claim 3, wherein said 2-methyl-3,3-dialkyl-3H-indole compound is 2-methyl-3,3-dialkyl-3H-indole or 2-methyl-3,3-dialkyl-3H-indole-5-carboxylic acid.

8. The process as claimed in claim 3, wherein said solvent in step (c), (d) and (f) is 1-butanol, toluene, chloroform, acetonitrile, or a mixture thereof and said process is carried out under nitrogen atmosphere.

9. An electronic device comprising a squaraine dye of formula (I) as claimed in claim 1.

10. A solar cell device comprising squaraine dye of formula (I) as claimed in claim 1, optionally along with at least one other dye, wherein said solar cell device is a dye-sensitized solar cell, wherein the dye is present in a solution form or in a film form and is chemisorbed to a photoactive semiconductor porous material, and the solar cell device has a solar cell efficiency in the range of 2.5 to 9.5%.

11. The process as claimed in claim 5, wherein the indolenium salt is 1,2,3,3-tetramethyl-3H-indolium iodide (5a); 1-hexyl-2,3,3-tetramethyl-3H-indolium iodide (5b); 5-Carboxy-1,2,3,3-tetramethyl-3H-indolium iodide (6a); 5-Carboxy-1-hexyl-2,3,3-trimethyl-3H-indolium iodide (6b); 1-hexyl-2-methyl-3-decyl-3-octyl-3H-indolium iodide (7a); or 5-carboxy-1-hexyl-2-methyl-3-decyl-3-octyl-3H-indolium iodide (7b).

12. The process as claimed in claim 7, wherein said 2-methyl-3,3-dialkyl-3H-indole compound is 2,3,3-trimethyl-3H-indole (3a), 2,3,3-trimethyl-3H-indole-5-carboxylic acid (4a), 2-methyl-3-decyl-3-octyl-3H-indole (3b), or 2-methyl-3-decyl-3-octyl-3H-indole-5-carboxylic acid (4b).

13. The squaraine dye of claim 1, wherein X is COOH.

14. The squaraine dye of claim 13, wherein R.sub.1 is methyl or hexyl and R.sub.2 is methyl or hexyl.

15. The squaraine dye of claim 14, wherein each of R.sub.3, R.sub.4, R.sub.5, and R.sub.6, independently, is methyl, octyl, or decyl.

16. The squaraine dye of claim 15, wherein each of R.sub.7, R.sub.8, R.sub.9, R.sub.10, R.sub.11, R.sub.12, and R.sub.13 is H.

17. The squaraine dye of claim 1, wherein X is ##STR00008##

18. The squaraine dye of claim 17, wherein R.sub.1 is methyl or hexyl and R.sub.2 is methyl or hexyl.

19. The squaraine dye of claim 18, wherein each of R.sub.3, R.sub.4, R.sub.5, and R.sub.6, independently, is methyl, octyl, or decyl.

20. The squaraine dye of claim 19, wherein each of R.sub.7, R.sub.8, R.sub.9, R.sub.10, R.sub.11, R.sub.12, and R.sub.13 is H.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: Normalized UV-Visible absorption and fluorescence spectra of SQ1-SQ7 in CH.sub.2Cl.sub.2.

(2) FIG. 2: Energy level diagram for SQ1-SQ7

(3) FIG. 3: I-V plots for the (top left without CDCA, top right with CDCA) and IPCE profile (down left without CDCA, down right with CDCA) for the dyes SQ1-SQ7

(4) FIG. 4: EIS study on the SQ5-SQ7 dyes (a) Nyquist plot and (b) Bode plot

(5) FIG. 5: (a) Absorption and emission spectra of RSQ dyes in CHCl3 solution. (b) Normalized absorbance of RSQ dyes adsorbed at the surface of 6 μm thick TiO.sub.2 film (Dye concentration=0.1 mM in CH2Cl2, dipping time 30 min).

(6) FIG. 6: (a) J-V curve and (b) IPCE spectrum and LHE spectrum of RSQ sensitizers without coadsorbent. ([Dye]=0.1 mM, in CH2Cl2, dipping time 5 h, TiO2 active area=0.22 cm.sup.2).

(7) FIG. 7: Synthesis of branched ketone

(8) FIG. 8: Synthesis of un-symmetrical squaraine dyes SQ1-SQ7.

(9) FIG. 9: Synthesis of RSQ1 and RSQ2

DETAILED DESCRIPTION OF THE INVENTION

(10) The present invention provides a novel squariane dye of formula (I) and process for the preparation of the same. Further, the present invention provides an electronic device comprising a squaraine dye of formula (I).

(11) ##STR00004##

(12) Wherein,

(13) R.sub.1 and R.sub.2 are same or different are selected from straight or branched chain —C.sub.1 to C.sub.20 alkyl, —C.sub.1 to C.sub.20 alkoxy, aryl, arylalkyl, fused aryl polyethylene glycol units (triethyleneglycol monomethylether, tetraethylene glycol monomethylether), C.sub.1 to C.sub.10 perfluoroalkyl chains, halides, cyano, trifluoromethyl;

(14) R.sub.3-R.sub.13 are same or different and are selected from hydrogen, straight or branched chain —C.sub.2 to C.sub.20 alkyl, —C.sub.1 to C.sub.20 alkoxy, aryl, arylalkyl, fused aryl polyethylene glycol units (triethyleneglycol monomethylether, tetraethylene glycol monomethylether), C.sub.1 to C.sub.10 perfluoroalkyl chains, halides, cyano, trifluoromethyl;

(15) X is selected from —COOH,

(16) ##STR00005##

(17) R.sub.14 and R.sub.15 are same or different and are selected from straight or branched chain —C.sub.1 to C.sub.20 alkyl, —C.sub.1 to C.sub.20 alkoxy;

(18) provided when R.sub.1, R.sub.3 and R.sub.4 are methyl then R.sub.2 is C.sub.6 alkyl;

(19) when R.sub.2 is C.sub.6 alkyl then R.sub.1 is methyl or C.sub.6 alkyl.

(20) In preferred embodiment, said squaraine dye of formula (I) is selected from 5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ1), 5-Carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ2), 5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium (SQ3), 5-Carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium (SQ4), 5-Carboxy-2-[[3-[(1,3-dihydro-1-hexyl-3-decyl-3-octyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ5), 5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1-hexyl-3-decyl-3-octyl-3H-indolium (SQ6), 5-Carboxy-2-[[3-[(1,3-dihydro-1-hexyl-3-decyl-3-octyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1-hexyl-3-decyl-3-octyl-3H-indolium (SQ7), 4-((5-(6-(2-Carboxy-2-cyanovinyl)-4,8-dimethoxybenzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (RSQ-1) or 5-[2-Cyano-3-(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b]dithiophen-2-yl)acrylic acid]-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium 20d (RSQ2).

(21) The present invention further provides a process for the preparation of squaraine dye of formula (I), wherein said process comprising the steps of: a) refluxing the reaction mixture of hydrazine compound and 3-alkyl-2-alkanone dissolved in acetic acid at a temperature in the range of 80 to 100° C. for the period in the range of 14 to 16 h to afford corresponding 2-methyl-3,3-dialkyl-3H-indole derivative; b) refluxing the reaction mixture of alkyl halide and compound of step (a) dissolved in acetonitrile at a temperature in the range of 100 to 110° C. for the period in the range of 14 to 16 h to afford corresponding indolenium salt; c) heating the reaction mixture of compound of step (b) and 3, 4-dibutoxycyclobut-3-ene-1,2-dione dissolved in solvent in presence of triethylamine at a temperature in the range of 60 to 70° C. for the period in the range of 1 to 2 h to afford semisquaraine compound; d) refluxing the reaction mixture of compound of step (c) with compound of step (b) dissolved in solvent in presence of pyridine at a temperature in the range of 110 to 115° C. for the period in the range of 20 to 24 h to afford corresponding dye of formula (I), wherein X is —COOH. e) stirring the reaction mixture of squaraine dye of formula (I) of step (d), benzodithiophene derivative (ixa or ixb), palladium (II) acetate [Pd(OAc)2], tricyclohexylphosphine (PCy3), pivalic acid (PivOH) in presence of potassium carbonate (K2CO3) in toluene at a temperature in the range of 110 to 110° C. for the period in the range of 14 to 16 h to afford aldehyde derivative of SQ-BDT (xa or xb); f) adding cyanoacetic acid and piperidine to the reaction mixture of compound of step (e) in solvent followed by stirring the reaction mixture at a temperature in the range of 70 to 80° C. for the period in the range of 14 to 16 h to afford dye of formula (I) wherein X is not —COOH.

(22) In one embodiment, said reaction optionally comprises refluxing the semi-squariane compound of step (c) in acetone in presence Hydrochloric acid at temperature in the range of 60 to 80° C. for the period in the range of 6 to 8 h to afford semisqauraic acid.

(23) In preferred embodiment, said hydrazine compound is selected from 4-hydrazinobenzoic acid or phenyl hydrazine hydrochloride.

(24) In another preferred embodiment, said 3-alkyl-2-alkanone is selected from 3-octyltridecan-2-one or 3-methylbut-2-one.

(25) In yet another preferred embodiment, said alkyl halide is alkyl iodide selected from 1-iodohexane or iodomethane.

(26) In still another preferred embodiment, said compound indolenium salt is selected from 1-alkyl-2-methyl-3,3-dialkyl-3H-indol-1-ium iodide and 5-carboxy-1-alkyl-2-methyl-3,3-dialkyl-3H-indol-1-ium iodide such as 1,2,3,3-tetramethyl-3H-indolium iodide (5a); 1-hexyl-2,3,3-tetramethyl-3H-indolium iodide (5b); 5-Carboxy-1,2,3,3-tetramethyl-3H-indolium iodide (6a); 5-Carboxy-1-hexyl-2,3,3-trimethyl-3H-indolium iodide (6b); 1-hexyl-2-methyl-3-decyl-3-octyl-3H-indolium iodide (7a) or 5-carboxy-1-hexyl-2-methyl-3-decyl-3-octyl-3H-indolium iodide (7b).

(27) In yet still another preferred embodiment, said semisquaraine compound is selected from (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trimethyl-indoline (8), (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1-hexyl-3,3-dimethyl-indoline (9), (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trimethyl-indoline-5-carboxylic acid (10), (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1-hexyl-3-decyl-3-octyl-indoline-5-carboxylic acid (11).

(28) In yet still another preferred embodiment, said 2-methyl-3,3-dialkyl-3H-indole derivative is selected from 2-methyl-3,3-dialkyl-3H-indole such as 2,3,3-trimethyl-3H-indole (3a), 2,3,3-trimethyl-3H-indole-5-carboxylic acid (4a) or 2-methyl-3,3-dialkyl-3H-indole-5-carboxylic acid such as 2-methyl-3-decyl-3-octyl-3H-indole (3b) or 2-methyl-3-decyl-3-octyl-3H-indole-5-carboxylic acid (4b).

(29) In yet still another preferred embodiment, said process is carried out under nitrogen atmosphere.

(30) In yet still another preferred embodiment, said solvent in step (c) and (d) is selected from 1-butanol, toluene or mixture thereof.

(31) In yet still another preferred embodiment, said solvent in step (f) is selected from chloroform, acetonitrile or mixture thereof.

(32) A series of indole-based unsymmetrical squaraine (SQ) dyes that contain alkyl chains at N- and branched alkyl chains at sp.sup.3 C-atoms of indole moieties are synthesized. The optical and electrochemical properties of the SQ dyes are unchanged as there is no change in the conjugated π-surface unit, however, remarkable changes with respect to the power conversion efficiencies are observed. Introduction of alkyl groups on the indole unit that is far away from anchoring unit helps in more dye loading, avoiding the aggregation, increased charge transfer resistance, increased electron life time and hence more power conversion efficiency than the corresponding isomer in which the funtionalized indole unit contains the anchoring group. A DSSC device made out of SQ5 gave the Voc of 660 mV and Jsc of 19.82 mA/cm.sup.2, and efficiency 9.01%, respectively. This present investigation revealed the importance of position of alkyl groups in the squaraine based dyes for the better power conversion efficiency.

(33) The synthesis of un-symmetrical squaraine dyes requires semi-squariane and the indoline base. Suitably substituted indoline moiety with branched alkyl chain requires condensation of branched methyl ketone and the phenylhydrazine derivative. The branched methyl ketone is synthesized by adopting dithiane route, branched aldehyde is converted in to the corresponding dithiane by iodine catalyzed reaction with 1,3-propanedithiol, and the corresponding dithiane is methylated under n-BuLi reaction condition, subsequent mercuric perchlorate mediated deprotection provided the branched methylketone in moderate yield. The synthesis of branched ketone is as depicted in FIG. 7.

(34) The synthesis of squaraine dyes (SQ1-SQ7) with systematic variation in the position of alkyl groups is as depicted in FIG. 8.

(35) In preferred embodiment, the process for the preparation of 4-((5-(6-(2-Carboxy-2-cyanovinyl)-4,8-dimethoxybenzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (RSQ-1) and 5-[2-Cyano-3-(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b]dithiophen-2-yl)acrylic acid]-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium (RSQ2) is as depicted in FIG. 9.

(36) The UV-Vis spectrum of SQ dyes in CH.sub.2Cl.sub.2 showed absorption maximum at 640 nm for SQ1 with a distinct π-π* transition, and introducing alkyl groups on the sp.sup.3 carbon and N-atoms of the bottom (indoline moiety that contains the carboxylic acid group) and top or both indoline moieties causes 5-12 nm red shifted vibronic band for the dyes SQ2-SQ7 with the extinction coefficients of 1-2×10.sup.5 M.sup.−1 cm.sup.−1 (FIG. 1).

(37) Fluorescence spectrum showed the emission maximum at 650-660 nm. The fluorescent life times have been measured for the SQ dyes, and they are improved in the dyes SQ5-SQ7 (Table 1).

(38) TABLE-US-00001 TABLE 1 Photophysical and electrochemical characterization of SQ1 to SQ7 at rt..sup.a ε λ.sub.max (10.sup.5 M.sup.−1 λ.sub.max Life time.sup.c E.sub.HOMO E.sub.LUMO E.sub.g Dye (nm).sup.a cm.sup.−1) (nm) Φ.sup.b τ.sub.1 (ns) τ.sub.2 (ns) χ.sup.2 (eV) (eV).sup.d (eV) SQ-1 640 1.4 651 0.1 0.44 (79%) 0.89 (21%) 1.04 −4.84 −2.99 1.85 SQ-2 641 2.1 650 0.09 0.43 (65%) 0.82 (35%) 1.00 −4.82 −2.97 1.85 SQ-3 643 1.84 652 0.1 0.51 (72%) 1.03 (28%) 1.26 −4.83 −2.99 1.84 SQ-4 646 2.09 658 0.12 0.58 (58%) 1.16 (42%) 1.17 −4.85 −3.02 1.83 SQ-5 644 2.1 652 0.15 0.67 (43%) 1.23 (57%) 1.02 −4.83 −2.99 1.84 SQ-6 647 2.2 654 0.15 0.57 (47%  1.23 (53%) 1.18 −4.82 −2.98 1.84 SQ-7 652 1.0 660 0.3 0.29 (10%) 1.78 (90%) 1.07 −4.82 −2.99 1.83 .sup.aIn CH.sub.2Cl.sub.2, .sup.bexcitation wavelength 610 nm and relative method using a SQ based dye.sup.ref, .sup.cTCSPC method excitation wavelength 635 nm, .sup.dE.sub.Lumo = E.sub.Homo + E−00

(39) The oxidation potentials correspond to HOMO level of SQ dyes (0.71-0.74 vs NHE) are significantly more positive than the liquid electrolyte I.sup.−/I.sub.3.sup.− redox potential LUMO energy level is estimated by HOMO and E.sub.0-0 levels that is calculated from the intersecting point in which normalized absorption and emission spectra are overlapped. The LUMO level of SQ dyes are (−1.09 to −1.14) also more negative than the E.sub.CB for thermodynamically favoured electron injection (FIG. 2). The cyclic voltametric studies showed that the HOMO and LUMO level for the SQ1 is −0.72 eV and −1.12 eV and the energy levels are not affected for the various substituted SQ2-SQ7 dyes.

(40) In still another embodiment, the present invention provides an electronic device comprising a squaraine dye of formula (I).

(41) In preferred embodiment, said device is a solar cell.

(42) In another preferred embodiment, said solar cell is a dye-sensitized solar cell, and said dye is chemisorbed to a photoactive semiconductor porous material in said dye-sensitized solar cell.

(43) In yet another preferred embodiment, said dye is present in solution or wherein said dye is present in a film.

(44) In yet another preferred embodiment, the solar cell efficiency of above solar cell device is in the range of 2.5 to 9.5%.

(45) In one embodiment, said device further comprises at least one other dye.

(46) The photovoltaic device performances of SQ1-7 dyes under standard conditions (1.5 G, 100 mW/cm.sup.2) are measured using iodine (I.sup.−/I.sub.3.sup.−) liquid electrolyte. The DSSC device parameters of SQ based dyes are summarized in Table 3. A DSSC device based on SQ2 with a N-hexyl chain far away from TiO.sub.2 surface gave a short-circuit photocurrent density (Jsc) of 12.56 mA/cm.sup.2, an open-circuit photo-voltage (V.sub.oc) of 0.649 V, a fill factor of 71.5% and a PCE of 5.8%. When the N-hexyl group placed near to TiO.sub.2 surface, i.e., N-hexyl chain in indolidine unit that contains the TiO.sub.2 anchoring carboxylic acid group as in SQ3 showed significant reduction in Voc, Jsc and the overall device efficiency (Jsc 9.05 mA/cm2, Voc 0.61 V, ff-70.1% and η=3.85%). When both the top (which is away from the TiO.sub.2 surface) and bottom (near to the TiO.sub.2 surface possessing carboxylic acid groups) indolidine units alkylated with hexyl groups, the resultant dye SQ4 showed an improvement of Voc, slight reduction of Jsc, and gave a better efficiency of 4.36% in compared to SQ3 over SQ2. In dyes SQ2-4, in-plane N-alkylation of indolidine moieties may inhibit the electrolyte to reach TiO.sub.2 surface besides partly avoiding the dye aggregation.

(47) TABLE-US-00002 TABLE 2 Photovoltaic parameters of SQ1-SQ7 Jsc Cell Specification.sup.a Voc [V] [mA/cm.sup.2] ff [%] η [%] τ (ms) SQ-1 0.571 7.44 68.6 2.91 0.16 SQ-1 (CDCA 0.582 8.72 70.8 3.59 0.30 20 equiv.) SQ-2 0.610 10.44 69.0 4.39 1.05 SQ-2 0.649 12.5 71.5 5.80 2.26 (CDCA, 20 equiv.) SQ-3 0.594 8.61 68.3 3.49 0.41 SQ-3 (20 equiv.) 0.607 9.05 70.1 3.85 1.66 SQ-4 0.623 8.94 69.3 3.85 0.42 SQ-4 (10 equiv.) 0.623 10.16 69 4.36 2.26 SQ-5 0.636 18.05 67.4 7.74 2.64 SQ-5 (5 equiv.) 0.660 19.82 68.9 9.01 3.59 SQ-6 0.633 12.41 67.7 5.32 1.94 SQ-6 (5 equiv.) 0.647 14.23 68.6 6.31 2.26 SQ-7 0.649 14.06 68.2 6.22 2.64 SQ-7 (5 equiv.) 0.650 16.88 69.7 7.64 3.59 .sup.a8 + 4 mm TiO.sub.2 thickness

(48) The dyes SQ5-SQ7 possess the branched alkyl groups in either one or both the indolidine units, and DSSC device showed SQ5 (7.74%), SQ6 (5.32%) and SQ7 (6.22%) showed moderate PCE without any CDCA and the Voc is significantly increased (FIG. 3, and FIG. 4) in the presence of co-adsorbent. Again, it is observed that out-of-plane branching at the top indolidine moiety plays an important role as SQ5 gave a Jsc of 19.82 mA/cm2, an open-circuit photovotage of 660 mV, Whereas SQ6 dye showed Jsc and Voc of 12.41 mA/cm2 and 0.63 V, respectively. In the case of SQ7, significant improvement of PCE from 6.22% to 7.64% upon co-adsorbed with CDCA.

(49) The IPCE response for the dyes SQ1-7 are studied in the presence and absence of the co-adsorbent CDCA, and presented in FIG. 3. In the absence of CDCA, the observed IPCE responses are broad and have a maximum in 540-555 nm, 570-620 nm and 638-668 nm regions. Dyes other than SQ1, SQ2 and SQ5, contributed to 570-620 nm and 638-668 nm regions. The regions 540-555 nm, 570-620 nm are correspond to the H-aggregated assembly of the dyes on TiO.sub.2 surface and the region 638-668 nm corresponds to the monomeric dye. Dyes SQ5, SQ2 and SQ1 contributed 66.2% (5556 nm), 43.32% (552 nm) and 35% (541 nm), respectively, which are higher than the contribution from aggregated and monomeric dye. Between SQ2 and SQ5, introducing a sp3-branching unit helps in increasing the efficiency about 1.5 times higher for SQ5 (9.1%) than SQ2 (5.8%).

(50) EIS analysis data were acquired under 1 sun illumination (100 mW/cm.sup.2) to emphasise the effect of linear and branched alkyl group's position on the performance of SQ series (SQ1 to 7). The second semicircle at the intermediate frequency region of Nyquist plot FIG. 4 (a) ascribed to charge recombination resistance between CB.sub.TiO2 and electrolyte, and corresponding peak frequency (f) observed in Bode phase plot FIG. 4 (b) represents the electron life time (τ) in TiO.sub.2 film and it was derived from equation τ=(2πf).sup.−1. For clear comparison, SQ2 and SQ5, SQ3 and SQ6, and SQ4 and SQ7 are classified as top-alkyl, bottom-alkyl, and top and bottom alkyl respectively. Among SQ series, SQ5 and SQ7 showed maximum V.sub.OC and long electron life time (τ=3.59 ms), and least for SQ1 (t=0.16 ms). Interestingly, dye cell without any co-adsorbent, τ value of top-alkyl SQ2 and SQ5 were showed superior result than SQ3 and SQ6 (the bottom alkyl counterpart), and for all SQ dyes with and without co-adsorbent the result summarized in Table 2. In case of N-alkylated molecules without CDCA the values of ƒ decreased in the order of SQ2 (151.99 Hz)<SQ3 (383.64 Hz)=SQ3 (383.64 Hz) and with CDCA SQ4 (60.25 Hz)<SQ2 (70.29 Hz)<SQ3 (95.76 Hz), and SQ2 showed maximum τ value of 1.05 ms in the first case and 2.64 ms (SQ4) for the later. As shown in FIG. 4b, for sp.sup.3-alkylated without CDCA, τ increased in the order of SQ5=SQ7 (2.64 ms) >SQ6 (1.94 ms), and with CDCA SQ5=SQ7 (3.59 ms)>SQ6 (2.26 ms). After all, the life time of injected electron at TiO.sub.2/Iodolyte interface is governed by varying the position of alkyl groups on simple squaraine structure and for the highest efficiency cell (SQ5 with 5 eqv. CDCA) V.sub.OC and τ are 0.66 V and 3.59 ms, respectively, with PCE of 9.1%. Apparently, EIS analysis of SQ dyes helped to deduce the correlation between alkyl group's position on the SQ backbone and cell potential by measuring the characteristics frequency of Bode phase plot that represents impedance due to electron transfer from CB.sub.TiO2 to triiodide ions at the interface.

(51) The UV-vis absorption and emission spectra of RSQ1 and RSQ2 in CHCl.sub.3 solution are shown in FIG. 5a, and UV-vis spectra of RSQ dyes adsorbed on transparent mesoporous TiO2 film is shown in FIG. 5b. The absorption spectra of both the dyes in solution exhibit intense absorption band in the range of 500 nm to 700 nm. They have λmax at 664 nm corresponding to intramolecular charge transfer (ICT) transition, with a high molar absorption coefficient (ε) of 2.18×105 and 2.39×105 M.sup.−1 cm.sup.−1 for RSQ1 and RSQ2 respectively.

(52) The photovoltaic performance of DSSC based on RSQ1-2 dyes are evaluated under simulated AM 1.5 G illumination (100 mW cm.sup.−2). The current density-voltage (J-V) characteristics of DSSCs are shown in FIG. 6a and device performance data with and without coadsorbent are summarized in Table 3. RSQ2 sensitized cells showed better performance in comparison to RSQ1 without CDCA and achieved an overall power conversion efficiency (i) of 6.72% with a Jsc of 18.53 mAcm.sup.−2, fill factor (f f) of 67.4% and Voc of 0.538 V. RSQ1 sensitized solar cells gave PCE of 5.43%, Jsc of 19.03 mAcm.sup.−2, ff of 58.3% and Voc of 0.490 V. The greater PCE of RSQ2 is due to better Voc and fill factor (ff) in comparison to RSQ1. Both enhancement in Voc and ff in RSQ2 could be attributed to controlled assembly of these dyes on TiO.sub.2 surface.

(53) TABLE-US-00003 TABLE 3 Photovoltaic Performance of RSQ Dyes with CDCA and without CDCA under 1 Sun Illumination SQ dyes Voc (V) Jsc (mA/cm.sup.2) ff (%) η (%).sup.a RSQ1 0.490 19.03 58.3 5.43 RSQ1/CDCA (1 equiv) 0.488 18.25 59.4 5.29 RSQ2 0.538 18.53 67.4 6.72 RSQ2/CDCA (1 equiv) 0.539 18.77 67.7 6.84 .sup.aPhotovoltaic performance of RSQ cells, thickness of electrode: 8 + 4 μm (transparent + scattering) layer of TiO2, Electrolyte: 0.5M DMII, 0.1M LiI, 0.1M I2 and 10 mM TBP in CH3CN. [Dye] = 0.1 mM in CH.sub.2Cl.sub.2, dipping time was 5 h, Active area of 0.22 cm.sup.2 and measurements were carried out under 1 sun intensity (100 mW/cm.sup.2).

EXAMPLES

(54) Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

(55) Required precursors 3a,.sup.S1 4a,.sup.S2 5a,.sup.S3 5b,.sup.S4 6a,.sup.S2 6b.sup.S5 and 8.sup.S1 were synthesized according to the reported literature procedure.

Example 1: Synthesis of 3-octyltridecan-2-one (Branched Ketone)

a) 2-Octyldodecanal (A)

(56) 2-Octyl-1-dodecanol (5 g, 16.7 mmol) was taken in a 100 mL round bottomed flask, pyridiniumchlorochromate (10.8 g, 50.24 mmol) was added to it and the mixture was dissolved in anhydrous CH.sub.2Cl.sub.2 (120 mL). The reaction mixture was stirred at room temperature (27° C.) for 3 h and filtered through a short pad of silica gel to provide the required aldehyde as a colourless liquid. 4.68 g, Yield: 94%. .sup.1H NMR (400 MHz, CDCl.sub.3) δ: 9.54 (d, J=3.2 Hz, 1H), 2.26-2.14 (m, 1H), 1.68-1.53 (m, 2H), 1.42 (dd, J=14.2, 5.6 Hz, 2H), 1.25 (broad s, 28H), 0.87 (t, J=6.8 Hz, 6H); .sup.13C NMR (100 MHz, CDCl.sub.3) δ: 211.9, 182.8, 161.1, 77.3, 77.2, 76.7, 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].sup.+ calcd for C.sub.20H.sub.40O: 296.3079; found: 296.2517.

b) 2-(Nonadecan-9-yl)-1,3-dithiane (B)

(57) 2-Octyldodecanal (5 g, 16.8 mmol) was dissolved in 25 mL of chloroform in a 100 mL round bottomed flask and cooled to 0° C. Propane-1,3-dithiol (2 mL, 20.2 mmol) and I.sub.2 (25 mg, cat.) were added and reaction mixture was stirred for 20 min at room temperature (27° C.). The reaction mixture was quenched with 0.1 M solution of sodium thiosulphate (50 mL), diluted with CH.sub.2Cl.sub.2 (100 mL), washed with 10% NaOH (20 mL) followed by H.sub.2O (3×200 mL) and then dried with Na.sub.2SO.sub.4. The crude product was purified using silica gel column and CH.sub.2Cl.sub.2: hexane as eluents. 4.4 g, Yield: 72%. .sup.1H NMR (400 MHz, CDCl.sub.3) δ: 4.22 (d, J=3.4 Hz, 1H), 2.93-2.74 (m, 4H), 2.14-2.04 (m, 1H), 1.67-1.51 (m, 3H), 1.25 (broad s., 30H), 0.86 (t, J=6.4 Hz, 6H); .sup.13C NMR (100 MHz, CDCl.sub.3) δ: 77.3, 76.7, 54.1, 43.7, 31.9, 31.2, 31.2, 29.7, 29.6, 29.6, 29.5, 29.5, 29.3, 29.3, 27.5, 26.5, 22.6, 14.1; MALDI-TOF (m/z): [M].sup.+ calcd for C.sub.23H.sub.47S.sub.2: 387.3110; found: 387.1503.

c) 2-Methyl-2-(nonadecan-9-yl)-1,3-dithiane(C)

(58) 2-(Nonadecan-9-yl)-1,3-dithiane (1.12 g, 2.8 mmol) was dissolved in dry THF (15 mL) in a 50 mL two necked round bottomed flask and cooled to −5° C. n-BuLi (1.4 mL, 2.5 M solution in hexane, 3.47 mmol) was added and stirred for 1 h. Methyl iodide (0.22 mL, 3.48 mmol) was added drop wise and stirred for 30 min at −5° C. The reaction mixture was brought to room temperature (27° C.) and stirred further for 15 h. The reaction was quenched with saturated NaHCO.sub.3 solution (50 mL), extracted with CH.sub.2Cl.sub.2 (3×20 mL) and then dried with Na.sub.2SO.sub.4. The reaction mixture was purified by silica gel column and CH.sub.2Cl.sub.2: hexane as eluents to afford the required product as colourless oil. 1.09 g, Yield: 94%. H NMR (400 MHz, CDCl.sub.3) δ: 2.96-2.81 (m, 2H), 2.81-2.55 (m, 2H), 2.03-1.79 (m, 3H), 1.66 (d, J=7.3 Hz, 1H), 1.58 (s, 2H), 1.25 (broad s., 32H), 0.87 (t, J=5.6 Hz, 6H); .sup.13C NMR (100 MHz, CDCl.sub.3) δ: 77.3, 76.7, 60.2, 55.3, 54.1, 46.7, 44.5, 43.8, 35.4, 32.7, 32.0, 31.9, 31.7, 31.6, 31.4, 31.2, 31.2, 30.1, 30.0, 29.8, 29.6, 29.5, 29.5, 29.3, 29.0, 27.7, 27.5, 27.0, 26.5, 26.4, 26.3, 25.9, 25.5, 25.4, 24.1, 23.1, 22.7, 22.5, 14.1 MALDI-TOF (m/z): [M].sup.+ calcd for C.sub.24H.sub.49S.sub.2: 401.3275; found: 401.1729.

d) 3-Octyltridecan-2-one(D)

(59) 2-Methyl-2-(nonadecan-9-yl)-1,3-dithiane (1.3 g, 0.28 mmol) was dissolved in acetonitrile/water (9:1, 10 mL) in a 100 mL round bottomed flask and Hg(ClO.sub.4).sub.2.H.sub.2O (1.56 g, 3.9 mmol) was added into it and stirred for 12 h. The reaction mixture was filtered through Whatman filter paper and 5% aqueous NaHCO.sub.3 solution (50 mL) was added to the filtrate, extracted with CH.sub.2Cl.sub.2 (3×20 mL). The organic layer was dried over Na.sub.2SO.sub.4, concentrated under reduced pressure, and purified by silica gel column using CH.sub.2Cl.sub.2: hexane as eluents to afford the required product as colourless oil. 0.58 g, Yield: 58%. .sup.1H NMR (400 MHz, CDCl.sub.3) δ: 2.41 (m, 1H), 2.09 (s, 3H), 1.61-1.48 (m, 2H), 1.43-1.33 (m, 2H), 1.23 (br. S., 28H), 0.86 (t, J=6.8 Hz, 6H); .sup.13C NMR (100 MHz, CDCl.sub.3) δ=213.2, 77.3, 76.7, 53.3, 31.9, 31.8, 31.7, 31.6, 29.7, 29.6, 29.4, 29.4, 29.3, 29.2, 28.6, 27.4, 22.6, 22.6, 14.1; MALDI-TOF (m/z): [M+Na].sup.+ cald for C.sub.24H.sub.49S.sub.2: 333.32; found: 333.2532.

Example 2: General procedure for the synthesis of 2-methyl-3,3-dialkyl-3H-indole (3a, 4a) and 2-methyl-3,3-dialkyl-3H-indole-5-carboxylic acid (3b, 4b)

(60) 4-Hydrazinobenzoic acid (1 equiv.) or phenyl hydrazine hydrochloride, corresponding 3-alkyl-2-alkanone (2 equiv.) were dissolved in acetic acid (50 mL) in a 100 mL round bottom flask. The reaction mixture was heated to reflux at 100° C. for 16 h under nitrogen atmosphere. The reaction mixture was cooled and the solvent acetic acid was removed under reduced pressure and washed with petroleum ether to provide the required compound as a brown color solid.

(61) 3b: 0.3 g, Yield: 82%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 7.51 (dd, J=8 Hz, 2 Hz, 1H), 7.33-7.27 (m, 1H), 7.18 (d, J=8 Hz, 2H), 2.20 (s, 3H), 1.94-1.62 (m, 4H), 1.16 (b, 26H), 0.89-0.83 (m, 6H), 0.70-0.5 (m, 2H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ: 186.5, 154.3, 141.9, 128.6, 127.1, 124.7, 121.3, 119.1, 62.3, 53.0, 41.6, 36.7, 31.6, 31.5, 31.4, 31.3, 29.4, 29.3, 29.2, 29.1, 29.0, 28.9, 28.8, 28.7, 28.3, 27.2, 27.1, 23.5, 23.2, 22.3, 22.2, 15.7, 13.8, 13.7; HRMS (m/z): [M−H].sup.+ calcd for C.sub.27H.sub.44N: 382.3468; found: 382.3471.

(62) 4b: 0.56 g, Yield: 85%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.15 (d, J=8.4 Hz, 1H), 7.96 (s, 1H), 7.64 (d, J=8.4 Hz, 1H), 2.29 (s, 3H), 1.99-191 (m, 2H), 1.80-1.73 (m, 2H), 1.29-1.21 (b, 26H), 0.88-0.84 (m, 6H), 0.75-0.5 (m, 2H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ: 191.1, 171.1, 142.2, 130.8, 123.1, 119.1, 53.0, 31.6, 31.5, 31.4, 31.3, 30.6, 29.4, 29.3, 29.1, 29.0, 28.9, 28.8, 28.4, 28.3, 27.2, 27.1, 25.3, 24.3, 24.3, 24.1, 23.6, 23.3, 22.3, 22.2, 22.1, 15.9, 13.8, 13.6; HRMS (m/z): [M+H].sup.+ calcd for C.sub.28H.sub.46NO.sub.2:428.3523; found: 428.3527.

Example 3: General procedure for the synthesis of 1-alkyl-2-methyl-3,3-dialkyl-3H-indol-1-ium iodide and 5-carboxy-1-alkyl-2-methyl-3,3-dialkyl-3H-indol-1-ium iodide (5a, 5b, 6a, 6b, 7a and 7b)

(63) Alkyl iodide (2 equiv.) and corresponding 2-methyl-3,3-dialkyl-3H-indole derivative (1 equiv.) were dissolved in MeCN (60 mL) in a 100 mL round bottom flask and refluxed at 100° C. for 16 h under inert atmosphere. The reaction mixture was cooled to room temperature (27° C.); the solvent was removed under reduced pressure. The precipitate was washed with diethyl ether (4×5 mL) to afford the required compound as a red color liquid in case of 7.

(64) 7a: 0.22 g, Yield: 60%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 7.73-7.60 (m, 3H), 7.53-7.47 (m, 1H), 4.89 (t, J=7.6 Hz, 2H), 3.15 (s, 3H), 2.25-2.05 (m, 4H), 2.01-1.69 (m, 2H), 1.68-1.45 (m, 2H), 1.41-1.29 (m, 4H), 1.28-1.03 (b, 26H), 0.85 (m, 9H), 0.64 (m, 2H); HRMS (m/z):[M−H].sup.+ calcd for C.sub.33H.sub.58IN: 594.3530; found: 594.3533.

(65) 7b:0.18 g, Yield: 26%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.36 (d, J=8.4 Hz, 1H), 8.19 (s, 1H), 7.87 (d, J=8.4 Hz, 1H), 4.94 (t, J=8 Hz, 2H), 3.19 (s, 3H), 2.30-2.01 (m, 4H), 1.96-1.82 (m, 2H), 1.56-1.46 (m, 2H), 1.42-1.09 (b, 30H), 0.87-0.83 (m, 9H), 0.70 (m, 2H); HRMS (m/z):[M−H].sup.+ calcd for C.sub.34H.sub.58INO.sub.2:638.3512; found: 638.3445.

Example 4: Synthesis of (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trialkyl-indoline (8, 9, and 11) and (E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trialkyl-indoline-5-carboxylic acid (10)

(66) The corresponding indolium iodide (1 equiv.) and 3,4-dibutoxycyclobut-3-ene-1,2-dione (1 equiv) were dissolved in 1-butanol in a 50 mL two necked round bottomed flask and triethylamine (1.2 equiv) was added into the reaction mixture. The reaction mixture was heated at 70° C. for 1 h under nitrogen atmosphere. The reaction mixture cooled to room temperature, and the solvents were removed under reduced pressure. The reaction mixture was purified by column chromatography (SiO.sub.2, 100-200 mesh) 5% EtOAc and 95% petroleum ether to afford the required compound as a yellow solid.

(E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trimethyl-indoline (8)

(67) 1.6 g, Yield: 74%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 7.29-724 (m, 2H), 7.07 (t, J=7.4 Hz, 1H), 6.88 (d, J=7.24 Hz, 1H), 5.36 (s, 1H), 4.85 (t, J=6.6 Hz, 2H), 3.37 (s, 3H), 1.95-1.79 (m, 2H), 1.61 (s, 6H), 1.58-1.42 (m, 2H), 1.01 (t, J=7.2 Hz, 3H); .sup.13C NMR (CDCl.sub.3, 50 MHz) δ: 192.7, 187.5, 173.5, 168.3, 142.6, 140.9, 127.7, 122.6, 121.9, 108.4, 81.2, 73.7, 47.9, 43.0, 32.1, 31.4, 26.9, 26.6, 26.2, 22.4, 18.7, 13.9, 13.7; HRMS (m/z): [M+H].sup.+ calcd for C.sub.20H.sub.24NO.sub.3: 326.1751; found: 327.1570; [M+Na].sup.+ calcd for C.sub.20H.sub.23NO.sub.3Na: 348.1570; found: 348.1569.

(E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1-hexyl-3,3-dimethyl-indoline (9)

(68) 2.75 g, Yield: 77%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 7.3-7.23 (m, 2H), 7.08 (t, J=7.0 Hz, 1H), 6.87 (d, J=8.0 Hz, 1H), 5.41 (s, 1H), 4.86 (t, J=6.4 Hz, 2H), 3.81 (t, J=7.4 Hz, 2H), 1.9-1.62 (m, 4H), 1.61 (s, 6H), 1.60 (s, 6H), 1.60-1.32 (m, 8H), 1.00 (t, J=7.2 Hz, 3H), 0.9 (t, J=7.0 Hz, 3H); .sup.13C NMR (CDCl.sub.3, 50 MHz) δ: 192.6, 187.8, 173.5, 169.0, 143.0, 140.7, 127.7, 122.7, 121.9, 108.1, 81.4, 73.8, 63.1, 47.8, 32.1, 29.9, 26.9, 18.7, 13.7; HRMS (m/z): [M+H].sup.+ calcd for C.sub.25H.sub.34NO.sub.3:396.2533; found: 396.2533.

(E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1,3,3-trimethyl-indoline-5-carboxylic acid (10)

(69) 0.53 g, Yield: 50%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.09 (dd, J=12.4, 1.6 Hz, 1H), 7.98 (d, J=1.6 Hz, 1H), 6.92 (d, J=8.4 Hz, 1H), 5.46 (s, 1H), 4.87 (t, J=6.6 Hz, 2H), 3.41 (s, 3H), 1.94-1.80 (m, 2H), 1.65 (s, 6H), 1.57-1.42 (m, 2H), 1.26 (t, J=7.2 Hz, 2H), 1.00 (t, 3H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ:192.2, 189.1, 188.8, 173.4, 171.5, 168.0, 147.9, 140.8, 131.6, 123.0, 107.5, 83.5, 74.2, 51.1, 47.2, 32.1, 30.2, 27.0, 18.7, 13.4; HRMS (m/z): [M+H].sup.+ calcd for C.sub.21H.sub.24NO.sub.5: 370.1649; found: 370.1647.

(E)-2-((2-butoxy-3,4-dioxocyclobut-1-en-1-yl)methylene)-1-hexyl-3-decyl-3-octyl-indoline-5-carboxylic acid (11)

(70) 20 mg, Yield: 11%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 7.20 (m, 2H), 7.08 (t, J=6.6 Hz, 1H), 6.85 (d, J=8.0 Hz), 5.50 (s, 1H), 4.87 (t, J=6.4 Hz, 2H), 3.82 (t, J=7.2 Hz, 2H), 2.56-2.40 (m, 2H), 2.02-1.69 (m, 8H), 1.29-1.00 (b, 32H), 0.85 (t, J=1.4 Hz, 12H), 0.51 (m, 2H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ: 192.8, 187.4, 186.9, 173.0, 166.2, 144.5, 137.3, 127.6, 122.5, 122.0, 108.0, 81.7, 73.6, 57.1, 43.0, 39.6, 32.1, 31.8, 31.4, 29.6, 29.4, 29.3, 29.1, 26.8, 26.2, 22.5, 18.8, 14.0, 13.9, 13.7; MALDI-TOF (m/z):[M+H].sup.+ calcd for C.sub.41H.sub.66NO.sub.3: 620.4964; found: 620.5691.

Example 5: Synthesis of Unsymmetrical SQ Compounds

(71) Indolium iodide derivatives (5a, 6a, 6b, 7a, 7b; 1 equiv.) and semi-squaraine derivatives (8, 9, 10, 11; 1 equiv.) were dissolved in 1-butanol and dry toluene (1:1, 3 mL each) in a 50 mL two necked round bottomed flask, dry pyridine (1.5 equiv.) was added to it and charged with Dean-Stark apparatus according the FIG. 8. The reaction mixture was refluxed for 24 h under inert atmosphere. The reaction mixture was cooled to room temperature and the solvents were removed under reduced pressure. The reaction mixture was subjected to column chromatography (SiO.sub.2, 100-200 mesh, 5% CH.sub.3OH and 95% CH.sub.2Cl.sub.2) to afford the required dye as green coloured solids. In case of 18, pet.ether and EtOAc were used as eluents.

5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ-1)

(72) 0.17 g, Yield: 87%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.11 (dd, J=8.1, 1.4 Hz, 1H), 8.03 (s, 1H), 7.47-7.32 (m, 3H), 7.09 (d, J=7.6 Hz, 1H), 6.98 (d, J=8.6 Hz, 1H), 6.09 (s, 1H), 5.99 (s, 1H), 3.67 (s, 3H), 3.53 (s, 3H), 1.80 (b, 12H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ: 181.4, 176.6, 172.8, 169.4, 168.8, 146.8, 145.1, 142.2, 141.6, 141.3, 130.7, 128.2, 127.6, 124.4, 123.4, 121.9, 109.6, 107.9, 87.6, 87.3, 61.5, 49.4, 47.9, 33.3, 31.1, 30.2, 26.8, 26.3; HRMS (m/z):[M+H].sup.+ calcd for C.sub.29H.sub.29N.sub.2O.sub.4: 469.2128; found: 469.2118.

5-Carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ-2)

(73) 0.54 g, Yield: 79%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.11 (dd, J=8.3, 1.4 Hz, 1H), 8.05 (s, 1H), 7.42-7.21 (m, 3H), 7.06 (d, J=7.6 Hz, 1H), 6.98 (d, J=8.6, 1H), 6.12 (s, 1H), 5.89 (s, 1H), 4.08 (t, J=7.8 Hz, 2H), 3.52 (s, 3H), 1.82 (broad s, 12H), 1.62-1.09 (m, 8H), 0.86 (t, J=6.8 Hz, 3H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ: 182.2, 176.6, 172.6, 170.1, 168.9, 147.3, 142.6, 142.0, 141.8, 131.1, 127.9, 124.7, 124.4, 123.9, 122.4, 110.1, 108.0, 87.9, 87.7, 49.9, 48.2, 44.1, 31.4, 30.9, 30.5, 27.2, 27.2, 26.7, 26.6, 22.4, 13.9; HRMS (m/z): [M].sup.+ calcd for C.sub.34H.sub.38N.sub.2O.sub.4: 538.2832; found: 538.2831.

5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium (SQ-3)

(74) 0.13 g, Yield: 35%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.03 (dd, J=8.2, 1.6 Hz, 1H), 7.98 (s, 1H), 7.34-7.13 (m, 3H), 7.00 (d, J=8.0 Hz, 1H), 6.90 (d, J=8.6 Hz, 1H), 5.98 (s, 1H), 5.95 (s, 1H), 4.04 (broad t, 2H), 3.58 (s, 3H), 1.72 (broad s, 12H), 1.42-1.18 (m, 8H), 0.81 (t, J=6.2 Hz, 3H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ: 181.2, 177.4, 172.9, 171.0, 170.6, 168.9, 146.9, 145.5, 142.7, 142.0, 131.7, 131.1, 130.8, 127.9, 126.8, 124.6, 124.0, 123.2, 122.3, 109.8, 108.5, 87.9, 87.5, 53.9, 49.7, 48.5, 43.7, 31.5, 27.2, 26.9, 26.8, 26.7, 26.6, 22.8, 22.4, 15.5, 13.9; HRMS (m/z): [M].sup.+ calcd for C.sub.34H.sub.38N.sub.2O.sub.4: 538.2832; found:538.2829.

5-Carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-hexyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-hexyl-3H-indolium (SQ-4)

(75) 95 mg, Yield: 62%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ:8.12 (dd, J=8.4, 1.8 Hz, 1H), 8.06 (s, 1H), 7.45-7.20 (m, 3H), 7.05 (d, J=7.8 Hz, 1H), 6.96 (d, J=8.6 Hz, 1H), 6.11 (s, 1H), 6.02 (s, 1H), 4.22-3.86 (b, 4H), 1.85-1.78 (b, 12H), 1.52-1.17 (m, 16H), 0.87 (t, J=6.8 Hz, 6H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ: 182.0, 177.4, 172.6, 168.8, 147.3, 142.8, 142.6, 142.4, 142.3, 139.3, 131.4, 128.3, 128.2, 124.9, 124.6, 124.3, 122.7, 110.3, 108.6, 88.1, 87.8, 50.2, 50.0, 48.7, 48.2, 44.4, 44.0, 31.7, 30.0, 27.5, 27.4, 27.2, 27.1, 27.0, 22.8, 14.4, 14.2; HRMS (m/z): [M].sup.+ calcd for C.sub.39H.sub.48N.sub.2O.sub.4: 608.3614; found: 608.3608.

5-Carboxy-2-[[3-[(1,3-dihydro-1-hexyl-3-decyl-3-octyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1,3,3-trimethyl-3H-indolium (SQ-5)

(76) 111 mg, Yield: 45%; .sup.1H NMR (CDCl.sub.3, 400 MHz) δ:8.10 (dd, J=8.4, 1.6 Hz, 1H), 8.04 (d, J=1.6 Hz, 1H), 7.38-7.21 (m, 3H), 7.05 (d, J=8.4 Hz, 1H), 6.96 (d, J=8.8 Hz, 1H), 6.16 (s, 1H), 5.97 (s, 1H), 4.09 (b, 2H), 3.51 (s, 3H), 3.01 (b, 2H), 2.01 (m, 2H), 1.83 (s, 6H), 1.53-1.39 (m, 2H), 1.37-1.28 (m, 4H), 1.23-0.97 (b, 28H), 0.90-0.84 (m, 9H), 0.46 (m, 2H); .sup.13C NMR (CDCl.sub.3, 100 MHz) δ:182.6, 176.5, 171.5, 170.7, 168.6, 147.7, 144.2, 142.0, 131.4, 128.1, 124.9, 124.2, 122.7, 110.1, 108.1, 88.6, 88.3, 59.6, 48.4, 44.5, 40.2, 32.1, 32.0, 31.7, 29.8, 29.7, 29.7, 29.5, 29.3, 27.6, 27.1, 22.3, 22.8, 14.3, 14.2; HRMS (m/z): [M].sup.+ calcd for C.sub.50H.sub.70N.sub.2O.sub.4: 762.5330; found: 762.5334.

5-Carboxy-2-[[3-[(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1-hexyl-3-decyl-3-octyl-3H-indolium (SQ-6)

(77) 121 mg, Yield: 53%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.11 (dd, J=8.6, 1.4 Hz, 1H), 7.97 (d, J=1.4 Hz, 1H), 7.42-7.20 (m, 3H), 7.06 (d, J=8.0 Hz, 1H), 6.96 (d, J=8.6 Hz, 1H), 6.10 (s, 1H), 6.03 (s, 1H), 3.98 (t, J=6.6 Hz, 2H), 3.63 (s, 3H), 2.99 (t, J=9.4 Hz, 2H), 2.05 (m, 2H), 1.87-1.69 (b, 6H), 1.50-1.24 (b, 6H), 1.23-1.0 (b, 28H), 0.95-0.72 (m, 9H), 0.46 (m, 2H); .sup.13C NMR (CDCl.sub.3, 125 MHz) δ: 182.6, 181.9, 178.6, 172.6, 170.9, 167.3, 149.1, 143.0, 139.1, 131.3, 128.2, 124.7, 124.4, 124.2, 124.1, 122.6, 109.9, 108.4, 89.0, 87.8, 58.2, 49.9, 44.0, 40.4, 32.1, 32.0, 31.8, 29.9, 29.8, 29.7, 29.6, 29.5, 29.3, 27.1, 22.8, 14.3, 14.2; HRMS (m/z): [M].sup.+ calcd for C.sub.50H.sub.70N.sub.2O.sub.4: 762.5330; found: 762.5334.

5-Carboxy-2-[[3-[(1,3-dihydro-1-hexyl-3-decyl-3-octyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-1-hexyl-3-decyl-3-octyl-3H-indolium (SQ-7)

(78) 20 mg, Yield: 34%; .sup.1H NMR (CDCl.sub.3, 200 MHz) δ: 8.10 (d, J=8.4 Hz, 1H), 7.95 (s, 1H), 7.40-7.14 (m, 3H), 7.02 (d, J=8.2 Hz, 1H), 6.85 (d, J=8.6 Hz, 1H), 6.23-5.93 (b, 2H), 4.18-3.82 (b, 4H), 3.12-2.89 (b, 4H), 2.12-1.89 (b, 6H), 1.87-1.67 (b, 6H), 1.5-0.7 (b, 78H), 0.46 (m, 4H); .sup.13C NMR (CDCl.sub.3, 125 MHz) δ: 183.3, 179.9, 179.2, 170.9, 170.4, 166.4, 149.1, 144.0, 138.8, 131.1, 127.8, 124.4, 123.9, 123.2, 122.4, 109.6, 107.8, 89.5, 88.2, 59.2, 57.6, 43.6, 40.2, 37.1, 31.8, 31.5, 29.7, 29.5, 29.3, 29.2, 29.1, 26.9, 22.6, 14.1, 14.0; MALDI-TOF (m/z): [M].sup.+ calcd for C.sub.71H.sub.112N.sub.2O.sub.4: 1056.8622, found: 1056.6190.

Example 6: Synthesis of SQ-BDT Dye, RSQ1 and RSQ2

a) 1-Hexyl-2,3,3-trimethyl-3H-indol-1-ium iodide (ii)

(79) A mixture of 2,3,3-trimethylindolenine (i) (2 g, 12.56 mmol) and n-hexyl iodide (3.2 g, 15.07 mmol) was stirred and heated at 100° C. for 12 h. The reaction mixture was cooled to room temperature (27°) after the completion of reaction. The contents were dissolved in minimum amount of dichloromethane and poured over 100 mL of diethyl ether and filtered under vacuum. The precipitate obtained was washed with diethyl ether (20 mL×3) to give pure compound ii (4.3 g, 92%) as brown solid. Mp 135-137° C. .sup.1H NMR (200 MHz, CDCl.sub.3) δ 7.71-7.49 (m, 4H), 4.76-4.54 (m, 2H), 3.10 (s, 3H), 2.04-1.82 (m, 2H), 1.64 (s, 6H), 1.51-1.18 (m, 6H), 0.86 (t, J=6.9 Hz, 3H). .sup.13C NMR (101 MHz, MeOH-d.sub.4) δ 197.6, 143.4, 142.5, 131.2, 130.5, 124.7, 116.6, 55.9, 49.5, 32.4, 28.9, 27.4, 23.5, 22.8, 14.3. HRMS (ESI) m/z: [M].sup.+ Calcd for C.sub.17H.sub.26N.sup.+ 244.2060: Found 244.2053.

b) 3-Butoxy-4-((1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)cyclobut-3-ene-1,2-dione (iii)

(80) To solution of compound ii (3.5 g, 9.43 mmol) in 25 mL of n-butanol, 3,4-dibutoxycyclobut-3-ene-1,2-dione (2.13 g, 9.43 mmol) was added. To the stirring mixture triethylamine (1.34 g, 13.2 mmol) was added dropwise. The resultant mixture was stirred at room temperature (27°) for 12 h followed by heating at 70° C. for 1 h. Solvents were evaporated after the completion of reaction and crude product was purified by column chromatography by silica gel to give compound iii (2.9 g, 77%) as yellow solid. Mp 85-87° C. .sup.1H NMR (400 MHz, CDCl.sub.3) δ 7.31-7.24 (m, 2H), 7.12-7.00 (m, 1H), 6.88 (dd, J=7.1, 1.4 Hz, 1H), 5.41 (s, 1H), 4.86 (t, J=6.6 Hz, 2H), 3.87-3.75 (m, 2H), 1.93-1.80 (m, 2H), 1.74 (d, J=7.4 Hz, 2H), 1.63 (d, J=4.5 Hz, 6H), 1.52 (dd, J=15.0, 7.5 Hz, 2H), 1.46-1.38 (m, 2H), 1.35 (ddd, J=7.3, 4.5, 2.5 Hz, 4H), 1.01 (t, J=7.4 Hz, 3H), 0.90 (t, J=7.1 Hz, 3H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 192.8, 187.7, 187.6, 173.7, 168.5, 142.8, 141.0, 127.8, 122.8, 122.1, 108.5, 81.4, 73.9, 48.1, 43.1, 32.3, 31.5, 27.1, 26.8, 26.4, 22.60, 18.9, 14.1, 13.8. HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.25H.sub.34NO.sub.3 396.2539; Found 396.2530.

c) 3-((1-Hexyl-3,3-dimethylindolin-2-ylidene)methyl)-4-hydroxycyclobut-3-ene-1,2-dione (iv)

(81) To a solution of compound iii (2.45 g, 6.194 mmol) in 15 mL of acetone, 5 mL of 2N HCl was added. Resultant mixture was refluxed for 8 h, and solvents were removed under reduced pressure after the completion of reaction. The crude compound iv (1.98 g, 94%), obtained as dark yellow solid, was used further without purification. Mp 170-172° C. .sup.1H NMR (400 MHz, CDCl.sub.3) δ 10.01 (s, 1H), 7.31 (dd, J=10.2, 7.9 Hz, 2H), 7.13 (t, J=7.4 Hz, 1H), 6.96 (d, J=7.8 Hz, 1H), 5.68 (s, 1H), 3.91 (t, J=6.6 Hz, 2H), 1.82-1.74 (m, 2H), 1.67 (s, 6H), 1.43 (d, J=6.1 Hz, 2H), 1.39-1.31 (m, 4H), 0.90 (t, J=6.8 Hz, 3H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 189.9, 187.6, 176.9, 170.7, 142.5, 141.4, 128.0, 123.6, 122.2, 109.2, 82.4, 48.7, 43.5, 31.5, 29.8, 27.0, 26.7, 26.6, 22.6, 14.0. HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.21H.sub.25NO.sub.3 340.1913; Found 340.1903.

d) 5-Bromo-1-hexyl-2,3,3-trimethyl-3H-indol-1-ium iodide (vi)

(82) A mixture of 5-bromo-2,3,3-trimethyl-3H-indole v (1.7 g, 7.14 mmol) and n-hexyliodide (1.82 g, 8.56 mmol) was heated at 100° C. for 4 h. Reaction mixture was cooled to room temperature after (27° C.) the completion of the reaction. The contents were dissolved in minimum amount of dichloromethane and precipitated by pouring in 100 mL of diethyl ether. The precipitate obtained was washed with diethyl ether (20 mL×3) and dried under vacuum to give compound vi (1.7 g, 53%) as dark brown solid. Mp 208-210° C. .sup.1H NMR (200 MHz, DMSO-d.sub.6) δ 8.20 (s, 1H), 7.96 (d, J=8.5 Hz, 1H), 7.85 (d, J=8.7 Hz, 1H), 4.43 (t, J=7.4 Hz, 2H), 2.84 (s, 3H), 1.80 (s, 2H), 1.55 (s, 6H), 1.30 (s, 6H), 0.86 (s, 3H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 198.1, 145.5, 141.7, 133.7, 128.3, 128.2, 125.3, 118.3, 56.1, 49.8, 32.4, 28.8, 27.4, 23.5, 22.7, 14.3. HRMS (ESI) m/z: [M].sup.+ Calcd for C.sub.17H.sub.25BrN.sup.+ 322.1165; Found 322.1160.

e) 4-((5-Bromo-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (viii)

(83) A mixture of compound 6 (0.3 g, 0.88 mmol) and compound iv (0.478 g, 1.06 mmol) in 16 mL of toluene/n-butanol (1:1) was refluxed under dean-stark apparatus for 24 h. After the completion of reaction the solvent were removed under reduced pressure and crude product was purified by column chromatography by silica gel using ethyl acetate/dichloromethane as eluent to yield compound vii (0.32 g, 56%) as blue solid. Mp 172-173° C. .sup.1H NMR (400 MHz, CDCl.sub.3) δ 7.46-7.28 (m, 4H), 7.17 (t, J=7.4 Hz, 1H), 7.01 (d, J=7.9 Hz, 1H), 6.82 (dd, J=13.3, 8.4 Hz, 1H), 6.06-5.83 (m, 2H), 4.01 (d, J=7.1 Hz, 2H), 3.91 (s, 2H), 1.81 (s, 2H), 1.78 (d, J=5.6 Hz, 12H), 1.72 (s, 2H), 1.45-1.28 (m, 12H), 0.89 (t, J=6.5 Hz, 6H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 181.1, 171.2, 168.6, 142.5, 130.8, 130.7, 128.0, 125.9, 125.8, 124.2, 122.5, 116.7, 116.2, 110.5, 109.8, 87.1, 86.9, 49.7, 49.2, 44.0, 31.6, 31.6, 29.8, 27.3, 27.2, 27.1, 27.0, 26.9, 26.8, 22.66, 22.65, 22.6, 14.1. HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.38H.sub.48BrN.sub.2O.sub.2 643.2899; Found 643.2885.

f) 4,8-Dimethoxybenzo[1,2-b:4,5-b′]dithiophene-2-carbaldehyde (ixa)

(84) In a two necked round bottom flask fitted with reflux condenser, viiia (1.2 g, 5.20 mmol) was taken. It was dissolved in 20 mL of 1,2-dichloroethane and N,N-dimethylformamide (8 mL, 104 mmol) was added to the mixture. The flask was cooled to 0° C. and POCl.sub.3 (9.5 mL, 104 mmol) was added to it dropwise and refluxed for 24 h. After completion of reaction the reaction mixture was poured in ice cold solution of ammonium chloride and extracted by dichloromethane. The organic layer was dried over sodium sulphate and solvents were removed under reduced pressure. The crude product was purified by column chromatography over silica gel with ethyl acetate/pet ether as eluent to afford ixa (1.2 g, 80%) as light yellow solid. Mp 140-143° C. .sup.1H NMR (200 MHz, CDCl.sub.3) δ 10.10 (s, 1H), 8.23 (s, 1H), 7.52 (s, 2H), 4.22 (s, 3H), 4.13 (s, 3H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 184.6, 148.0, 145.4, 143.0, 135.1, 131.6, 131.3, 130.1, 129.6, 128.9, 120.4, 61.5, 61.2. HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.13H.sub.11O.sub.3S.sub.2 279.0150; Found 279.0140.

g) 4,8-Bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2-carbaldehyde (ixb)

(85) In a two necked round bottom flask fitted with reflux condenser, viiib (2.8 g, 6.27 mmol) was taken. It was dissolved in 20 mL of 1,2-dichloroethane and N,N-dimethylformamide (9.76 mL, 125.4 mmol) was added to the mixture. The flask was cooled to 0° C. and POCl.sub.3 (11.7 mL, 125.361 mmol) was added to it dropwise and refluxed for 48 h. After completion of reaction the reaction mixture was poured in ice cold solution of ammonium chloride and extracted by dichloromethane. The organic layer was dried over sodium sulphate and solvents were removed under reduced pressure. The crude product was purified by column chromatography over silica gel with ethyl acetate/pet ether as eluent to afford ixb ((2 g, 67%) as yellow viscous oil. .sup.1H NMR (400 MHz, CDCl.sub.3) δ 10.10 (s, 1H), 8.17 (s, 1H), 7.49 (s, 2H), 4.27 (d, J=5.4 Hz, 2H), 4.18-4.15 (m, 2H), 1.82 (dd, J=12.0, 6.0 Hz, 2H), 1.75-1.65 (m, 2H), 1.62-1.56 (m, 4H), 1.54-1.47 (m, 2H), 1.41-1.35 (m, 8H), 1.05-0.99 (m, 6H), 0.97-0.90 (m, 6H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 184.7, 147.4, 144.7, 142.7, 135.3, 131.9, 131.5, 130.3, 129.9, 128.6, 120.6, 76.7, 76.4, 40.8, 30.5, 29.3, 24.0, 23.2, 14.3, 11.4.

(86) HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.27H.sub.39P.sub.3S.sub.2 475.2341; Found 475.2333.

h) General Synthetic Procedure for Direct Arylation of Squaraine and BDT

(87) In a Schlenk tube corresponding bromo-squaraine (vii) and BDT aldehydes (ixa and ixb) were taken. The Shclenk tube is evacuated and refilled with nitrogen three times. Pd(OAc).sub.2 (5 mol %), PCy.sub.3 (10 mol %), PivOH (30 mol %) and K.sub.2CO.sub.3 (2.5 eq.) were added to it followed by 4 mL of anhydrous toluene. The mixture was stirred at 110° C. for 24 h. After completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The organic layer was then washed with brine, dried over sodium sulfate and concentrated under vacuum. Crude product was purified by column chromatography to give of pure compounds.

i) (Z)-4-((5-(6-Formyl-4,8-dimethoxybenzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-(((Z)-1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (xa)

(88) From bromo-squaraine 7 (0.250 g, 0.388 mmol) and aldehyde ixa (0.432 g, 1.55 mmol), the compound xa (0.2 g, 61%) was obtained as green solid. Mp 251-253° C. .sup.1H NMR (500 MHz, CDCl.sub.3) δ 10.10 (s, 1H), 8.22 (s, 1H), 7.71 (d, J=1.7 Hz, 1H), 7.69 (s, 1H), 7.69 (s, 1H), 7.39 (d, J=7.3 Hz, 1H), 7.33 (td, J=7.8, 0.9 Hz, 1H), 7.18 (t, J=7.3 Hz, 1H), 7.02 (dd, J=10.3, 8.5 Hz, 2H), 6.03 (s, 1H), 6.00 (s, 1H), 4.26 (s, 3H), 4.18 (s, 3H), 4.06-4.01 (m, 2H), 4.00-3.94 (m, 2H), 1.88 (s, 6H), 1.81 (s, 6H), 1.50-1.40 (m, 6H), 1.37-1.31 (m, 10H), 0.92-0.88 (m, 6H). .sup.13C NMR (126 MHz, CDCl.sub.3) δ 184.5, 181.3, 178.4, 171.4, 168.5, 147.6, 146.5, 145.0, 142.6, 142.4, 136.4, 131.8, 131.6, 130.2, 129.0, 128.5, 128.0, 126.9, 124.3, 122.5, 120.6, 114.8, 109.9, 109.5, 87.2, 61.4, 61.2, 49.8, 49.1, 44.1, 43.9, 31.63, 31.61, 29.8, 27.5, 27.33, 27.25, 27.13, 27.05, 26.9, 22.7, 14.1. HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.51H.sub.57N.sub.2O.sub.5S.sub.2 841.3709; Found 841.3701.

j) (Z)-4-((5-(4,8-Bis((2-ethylhexyl)oxy)-6-formylbenzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-(((Z)-1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (xb)

(89) From bromo-squaraine 7 (0.200 g, 0.310 mmol) and aldehyde ixb (0.589 g, 1.24 mmol), 0.150 g of compound xb (0.15 g, 46%) was obtained as green sticky gum. .sup.1H NMR (400 MHz, CDCl.sub.3) δ 10.10 (s, 1H), 8.17 (s, 1H), 7.69 (d, J=8.3 Hz, 1H), 7.66 (s, 1H), 7.63 (s, 1H), 7.39 (d, J=7.3 Hz, 1H), 7.33 (t, J=7.6 Hz, 1H), 7.18 (t, J=7.4 Hz, 1H), 7.02 (dd, J=7.5, 5.4 Hz, 2H), 6.03 (s, 1H), 6.00 (s, 1H), 4.32 (d, J=5.4 Hz, 2H), 4.21 (d, J=5.2 Hz, 2H), 4.06-3.95 (m, 4H), 1.87 (s, 6H), 1.81 (s, 6H), 1.73-1.53 (m, 10H), 1.47-1.39 (m, 14H), 1.36-1.25 (m, 10H), 1.05 (t, J=7.3 Hz, 6H), 0.97-0.89 (m, 12H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 184.6, 181.3, 178.5, 171.3, 168.5, 147.1, 146.1, 144.4, 142.4, 136.6, 132.0, 131.9, 130.3, 129.2, 128.7, 128.0, 126.8, 124.329, 122.5, 120.6, 115.0, 109.9, 87.2, 76.6, 76.3, 49.8, 49.1, 44.1, 40.8, 40.8, 31.6, 30.6, 30.5, 29.3, 27.4, 27.3, 27.13, 27.07, 26.9, 24.0, 23.3, 22.7, 14.3, 14.1, 11.5. HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.65H.sub.85N.sub.2O.sub.5S.sub.2 1037.5900; Found 1037.5876.

k) General Procedure for Knoevenagel Condenstation of Aldehyde to Cyanoacetic Acid

(90) Corresponding aldehydes (xa and xb) were dissolved in 5 mL of chloroform and 5 mL of acetonitrile. To this 5 eq. cyanoacetic acid was added followed by 40 μL of piperidine. The resultant solution was stirred at 80° C. for 12 h. Solvents were removed under rotavap after completion of reaction and dissolved in 50 mL of dichloromethane. The organic layer was washed with water followed by brine and dried over sodium sulphate. The solvents were removed under reduced pressure and purified by column chromatography by silica gel using MeOH/CHCl.sub.3 as an eluent.

l) 4-((5-(6-(-2-Carboxy-2-cyanovinyl)-4,8-dimethoxybenzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((-1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (RSQ-1)

(91) From xa (0.15 g, 0.178 mmol), pure compound RSQ-1 (0.1 g, 62%) was obtained as dark green solid. Mp 281-283° C. .sup.1H NMR (400 MHz, DMSO-d.sub.6+CDCl.sub.3) δ 8.30 (s, 1H), 7.95 (s, 1H), 7.67 (d, J=9.7 Hz, 2H), 7.62 (d, J=8.1 Hz, 1H), 7.33 (d, J=7.3 Hz, 1H), 7.27 (t, J=7.6 Hz, 1H), 7.11 (t, J=7.4 Hz, 1H), 7.05 (t, J=8.4 Hz, 2H), 5.88 (s, 1H), 5.84 (s, 1H), 4.12 (s, 3H), 4.07 (s, 3H), 4.04-3.89 (m, 4H), 1.75 (s, 6H), 1.69 (s, 6H), 1.40-1.16 (m, 16H), 0.81 (t, J=6.5 Hz, 6H). .sup.13C NMR (101 MHz, DMSO-d.sub.6+CDCl.sub.3) δ 181.3, 180.1, 170.2, 167.7, 145.6, 145.1, 143.8, 143.1, 142.8, 142.3, 141.7, 141.5, 135.7, 134.4, 130.7, 129.2, 128.9, 128.4, 127.9, 127.5, 126.3, 123.8, 121.8, 119.7, 117.6, 114.3, 109.6, 109.4, 86.3, 63.0, 60.7, 60.6, 48.9, 48.3, 43.1, 30.9, 29.0, 26.7, 26.5, 26.3, 26.0, 21.9, 13.5. HRMS (ESI) m/z: [M].sup.+ Calcd for C.sub.54H.sub.57N.sub.3O.sub.6S.sub.2 907.3689; Found 907.3683.

m) 4-((5-(6-(-2-carboxy-2-cyanovinyl)-4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophen-2-yl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((-1-hexyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (RSQ-2)

(92) From xb (0.1 g, 0.097 mmol), compound RSQ-2 (0.065, 61%) was obtained as dark green solid. Mp 242-243° C. .sup.1H NMR (400 MHz, DMSO-d.sub.6+CDCl.sub.3) δ 8.38 (s, 1H), 8.08 (s, 1H), 7.63 (d, J=7.0 Hz, 2H), 7.58 (s, 1H), 7.35 (d, J=7.3 Hz, 1H), 7.28 (t, J=7.6 Hz, 1H), 7.11 (dd, J=14.5, 7.6 Hz, 3H), 5.88 (s, 1H), 5.84 (s, 1H), 4.21 (d, J=4.9 Hz, 2H), 4.13 (d, J=4.2 Hz, 3H), 4.05-3.93 (m, 4H), 1.77 (s, 6H), 1.70 (s, 6H), 1.65-1.43 (m, 10H), 1.42-1.32 (m, 14H), 1.30-1.16 (m, 10H), 0.98 (dd, J=16.7, 7.6 Hz, 6H), 0.91-0.87 (m, 6H), 0.83 (t, J=6.2 Hz, 6H). .sup.13C NMR (101 MHz, CDCl.sub.3) δ 181.1, 180.2, 177.4, 170.2, 167.5, 145.4, 145.3, 143.0, 142.9, 142.1, 141.7, 141.4, 135.1, 134.8, 131.3, 128.9, 128.3, 128.1, 127.5, 126.3, 123.7, 121.7, 119.5, 116.0, 115.9, 114.3, 109.7, 109.6, 86.5, 86.3, 78.2, 75.9, 75.4, 49.5, 48.9, 48.2, 43.2, 30.8, 29.9, 29.7, 28.6, 28.5, 26.6, 26.5, 26.3, 26.0, 23.2, 22.49, 22.46, 21.9, 13.7, 13.5, 10.9. HRMS (ESI) m/z: [M+H].sup.+ Calcd for C.sub.68H.sub.86N.sub.3O.sub.6S.sub.2 1104.5958; Found 1104.5946.

Example 7: Solar Cells Fabrication and Characterization

(93) FTO (F-doped SnO.sub.2 glass; 6-8 Ω/sq; Pilkington TEC 7) was cleaned by diluted mucasol solution in water, deionized water, and ethanol, successively. To grow a TiO.sub.2 blocking layer, the substrate was immersed in freshly prepared 50 mM aqueous TiCl.sub.4 solution at 70° C. for 30 min, and washed with deionized water before drying at 125° C. for 10 min. A paste of TiO.sub.2 nanocrystal (<20 nm, Ti-Nanoxide T/SP, Solaronix) was deposited by the doctor-blade technique on TiO.sub.2 buffer layer coated FTO substrate for transparent layer of TiO2, kept in air for 5 min and then annealed at 125° C. in air for 15 min. The films were about 6-8 μm thick. The annealed films were coated with scattering layer TiO.sub.2 paste (WER2-O, Dyesol) and annealed at 125° C. in air for 15 min. The annealed films were sintered at 325° C. for 5 min, 375° C. for 5 min, 450° C. for 15 min and 500° C. for 15 min with heating rate of 5° C. per min in air. After reaching the furnace temperature at 50° C., sintered films were immersed in freshly prepared 50 mM aqueous TiCl.sub.4 solution at 70° C. for 30 min. After sintering the TiCl.sub.4-treated TiO.sub.2 films at 500° C. for 30 min, they were immediately immersed in 0.1 mM RSQ dye solution in dichloromethane for 5 h, washed and dried at 80° C. In case of CDCA added experiments, different ratio of CDCA added to 0.1 mM dye solution and photoanode dipped for 5 h. Sandwich type cell configuration was completed using platinum as cathode, 0.5 M DMII, 0.1 M LiI, 0.1 M 12 and 10 mM TBP in CH.sub.3CN was used as electrolyte and 25 m spacer. I-V characteristics of the cells were measured using Keithley digital source meter (2420, Keithley, USA) controlled by a computer and standard AM 1.5 solar simulator (PET, CT200AAA, USA). To measure the photocurrent and voltage, an external bias of AM 1.5G light was applied using a xenon lamp (450 W, USHIO INC, Philippines) and recorded. The action spectra of monochromatic incident photon-to-current conversion efficiency (IPCE) for the solar cell were performed by using a commercial setup. Electrochemical impedance spectra (EIS) were obtained by the Biologic potentiostat, equipped with an FRA2 module, with applied potential of −0.45 V in the dark. The frequency range explored was 1 Hz to 1 MHz with an ac perturbation of 10 mV. The impedance spectra were analyzed using an equivalent circuit model of R1+R2/C2+R3/C3. The loading amount of the dyes was assessed by UV-vis spectrophotometry as follows: Photoanodes were sensitized in same dye solutions which were used for photovoltaic characterization. The photoanodes were taken out and dyes were desorbed by dipping in 2 M solution of HCl in EtOH. The resultant dye solution was used to evaluate the dye concentration by UV-vis study, which allows the determination of the amount of dye adsorbed in terms of number of moles per unit area of TiO2 film.

(94) Light Harvesting Efficiency was Obtained by
LHE=1-10.sup.−εΓ=1-10.sup.−A  (1)
Where ε is the molar extinction coefficient of the dye sensitized on TiO.sub.2 film, Γ is the dye molar concentration per projected surface area of the film, and A is the absorbance of the dye-sensitized film (equal to the product of ε and Γ).

Advantages of Invention

(95) There were very few families of dyes, porphyrins, phthalocyanines and polymethine dyes (sqauraines) absorb in the NIR regions of the solar spectrum. In the present investigation, self assembling nature of SQ dyes were systematically studied and showed the importance of branching units in SQ dyes for the high efficient device cell.