Photochemical Electron Transfer Reactions in Clay-porphyrin Complexes(Characterization of Clays,Proceedings of the 13^<th> International Clay Conference)

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Photochemical electron transfer reactions of cationic porphyrins on an anionic-type clay (Smecton SA) surface were investigated. Two types of complexes were formed between the cationic porphyrin and the anionic-type clay depending on the conditions. In the first type, the porphyrin molecules are adsorbed on surfaces of the individual dispersed clay sheets or layers (exfoliated complex). In the second type, the porphyrin molecules are intercalated between the stacked clay sheets (intercalated complex). For the exfoliated complex, we have found recently that the porphyrin molecules adsorb in a flat orientation on the clay sheets as monolayers, without discernible aggregation. The high packing density is determined by the fact that the positive charges on the porphyrin precisely neutralize the negative charges on the clay surface. Fluorescence quenching experiments of porphyrins in exfoliated and intercalated complexes by hydroquinone and ascorbate as electron donors were carried out. The quenching rate constant was affected by the charges of the electron donor in solution. When hydroquinone was used as a neutral electron donor, an efficient electron transfer was observed both in exfoliated and intercalated complexes. The fluorescence quenching experiment for the intercalated complex indicated that the diffusion of hydroquinone is restricted in the interlayer space.
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