Effect of nature of anchoring groups on photosensitization behavior in unsymmetrical squaraine dyes

• Three unsymmetrical squaraine dyes with varying anchoring groups were synthesized.
• Dye aggregation behavior and NIR photon harvesting was investigated.
• The SQ-C dye showed pronounced H-aggregation upon adsorption on TiO2.
• The SQ-B dye with direct ring substituted cyanoacrylate moiety exhibited best photovoltaic performance.

A series of indole based unsymmetrical squaraine dyes bearing various anchoring groups such as carboxylate (SQ-A), cyanoacrylate (SQ-B) and thiophene-bridged cyanoacrylate (SQ-C) were synthesized and evaluated for their performance in dye sensitized solar cells (DSSCs) under similar experimental conditions. Electronic absorption spectral investigation on thin films of these dyes adsorbed on nanoporous TiO2 reveals relatively enhanced spectral broadening upon the incorporation of cyanoacrylate functionality as anchoring group. Incorporation of thiophene π-bridge between the main chromophore and anchoring group (SQ-C) although resulted in to bathochromic shift in the far-red region but hampered photon harvesting due to relatively enhanced dye aggregation. Unsymmetrical squaraine dye (SQ-B) bearing cyanoacrylate anchoring group directly substituted and in conjugation with aromatic chromophore exhibited the best photovoltaic performance giving photoconversion efficiency of 5.03% under simulated solar irradiation.

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