Microstructures of the Porphyrin/Viologen Monolayer on the Clay Surface: Segregation or Integration?

Microstructures of the porphyrin/viologen monolayer on an anionic clay surface (synthetic saponite) were investigated by the observation of photochemical behavior of porphyrin. Fluorescence behaviors of porphyrin–viologen–clay complexes were observed by steady state and time-resolved fluorescence spectroscopy. Although fluorescence of porphyrin was effectively quenched by coadsorbed viologen on the clay surface, a part of the fluorescence of porphyrin was not quenched and remained even at high loading level of viologen. According to time-resolved fluorescence measurement, the decay profile of excited singlet porphyrin can be analyzed by two- or three-component fitting for porphyrin–viologen–clay complexes. These results indicate that porphyrin and viologen adsorb with island (segregation) structure on the clay surface. The size of the island formed by porphyrin was quantitatively estimated for two kinds of porphyrins. It turned out that the porphyrin molecular structure affects the size of the island. It has been believed that electron transfer on a clay surface is inefficient due to segregation of dyes between clay sheets (A. J. Bard et al., J. Phys. Chem. 1984, 88, 5519). Our results indicate that segregation behavior sensitively depends on the structure of the dye, and it is possible to construct an efficient electron transfer system on the clay surface.