EXCITED-STATE PROPERTIES AND PHOTOSENSITIZATION BEHAVIOR OF BIS(2,4-DIHYDROXYPHENYL)SQUARAINE

Two protonation equilibria (pK(a) 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 (phi(f) = 0.092, tau(s) = 740 ps) are considerably higher than that of the neutral (phi(f) = 0.037, tau(s) = 130 ps) or the protonated (phi(f) = 0.01, tau(s) less-than-or-equal-to 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 mus 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 TiO, electrode.