Chromophore Aggregation Associated with the Increase of Photoactivity at Merocyanine-coated Electrodes

Abstract

Absorption spectra and photoelectrochemical properities were studied for thin films of several merocyanine dyes, each of which consists of a dihydrobenzothiazole or dihydrobenzoselenazole part having an octadecyl group (as a hydrophobic group) and of a rhodanine part. The change in quantum efficiency of the photocurrent at the electrodes coated with the dye films was investigated in connection with the formation of dye aggregates which showed visible absorptions in a low-energy region. Only the dyes having a carboxymethyl group at the rhodanine ring showed the absorptions of dye aggregates when the dye films were soaked in neutral aqueous solutions; the absorption bands located at 600 and 630 nm were assigned to the dye tetramer and hexamer, respectively. The extent of the aggregation was affected by the chemical structure of dye, the ionic strength of aqueous solutions, and the thermal isomerization of the dye molecules, etc. All the electrodes coated with the thin films of the merocyanine dyes behaved as the photocathodes. The aggregate formation gave an important effect on the photoelectrochemical energy-conversion, that is, the photocurrent increased remarkably with increasing the visible absorption of the dye tetramer or hexamer.