Excited-state properties and photosensitization behaviour of bis(2,4-dihydroxyphenyl)squaraine

Abstract

Two protonation equilibria (pKa at 3.7 and 7.4) have been identified for the dye, bis(2,4-dihydroxyphenyl)-squaraine in the ground state. These various protonation forms exhibit widely different absorption properties. The singlet excited-state behaviour of this dye has been characterized by picosecond laser flash photolysis. The fluorescence quantum yield and the excited singlet lifetime of the unprotonated dye (φ_f= 0.092, T_s= 740 ps) are considerably higher than that of the neutral (φ_f= 0.037, T_s= 130 ps) or the protonated (φ_f= 0.01, T_s≤ 100 ps) forms of the dye. The triplet excited state has been characterized by both direct excitation and T-T energy transfer methods. The triplet excited state has an absorption maximum around 430 nm and a lifetime of ca. 100 μs in ethanol. Photosensitization of a thin microporous TiO₂ semiconductor film has been carried out by adsorbing the squaraine dye from an ethanolic solution and exciting it with visible light. An incident photon-to-current conversion efficiency of 0.05% at 500 nm has been obtained for the dye-modified TiO₂ electrode.