Dimerization of xanthene dyes in sol–gel titania films

Abstract

Dye-dispersing titania gel films were prepared by the sol–gel method using titanium alkoxide solutions containing xanthene dyes, i.e., fluorescein, eosin Y, rhodamine 110, and rhodamine 123. Dimerization of the xanthene dyes in the films, depending on their concentrations, and their photoinduced electron transfer processes to the titania gels were investigated using spectroscopic and photoelectric techniques. The dye having a higher dimerization constant in solution formed a higher amount of the dimer in the titania gel. There was only a slight difference between the photoelectric conversion efficiencies of the monomer and dimer. The titanium species were coordinated to the carboxylate of the dyes. The dimers as well as the monomers were encapsulated in the narrow spaces of the gel films and strongly interacted with the surrounding titania matrix in the dye-dispersing titania gel films. The dye having a more nucleophilic carboxylate oxygen could easily form the coordination complex with the titanium species. The dye–titania interaction played an important role in the electron injection from the dye to the titania conduction band.