Electron Transfer Properties and Control of Active Oxygen Species

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Formation of a variety of superoxide-metal complexes and the catalytic function have been investigated. The gzz-values of ESR spectra of superoxide-metal ion complexes vary significantly depending on the type of metal ions. From the deviation of the gzz-value from the free spin value are determined the energy splitting values (ΔE) of π_g levels due to the complex formation.The ΔE values are correlated well with the catalytic reactivities of metal ions in the electron transfer reduction of oxygen. Such a complex formation between superoxide ion and metal ions has been shown to play an essential role in the enzymatic function of superoxide dismutase, formation of active oxygen-opper complexes, and the novel catalytic effect of O_2 in back electron transfer of zinc porphyrin-fullerene linked molecules. The catalytic oxygenation of substrates including water has also been achieved using manganese porphyrins and dendrimers as catalysts and the catalytic mechanisms have been revealed based on the detection of active oxygen species and detailed kinetic studies. The catalytic mechanism of four-electron reduction of oxygen to water has also been elucidated using dicobalt porphyrins with an appropriate spacer.