pH-dependent reversible switching of fluorescence of water-soluble porphyrin adsorbed on mesoporous TiO2 film

Changes of the fluorescence spectra of tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and tetrakis(4-carboxyphenyl)porphyrin (TCPP) chemisorbed on TiO2 film in pH-adjusted water have been investigated. TPPS on TiO2 film in pH 6.5 water showed strong fluorescence, while it hardly emitted in pH 1.6 water, demonstrating a fluorescence switching with dramatic intensity changes in the TPPS-TiO2 system. On the other hand, only small changes in the fluorescence were observed for TCPP adsorbed on TiO2 instead of TPPS, although the changes were reversible. Fluorescence switching observed for TPPS and TCPP only in the adsorbed states on TiO2 are discussed on the basis of quenching of the excitation energy caused by molecular arrangements on TiO2 and photoinduced electron injection of excited porphyrin to TiO2.