Catalytic effects of dioxygen on intramolecular electron transfer in radical ion pairs of zinc porphyrin-linked fullerenes

Dioxygen accelerates back electron transfer (BET) processes between a fullerene radical anion (C₆₀) and a radical cation of zinc porphyrin (ZnP) in photolytically generated ZnP^•+-C₆₀^•- and ZnP^•+-H₂P-C₆₀^•- radical ion pairs. The rate constant of BET increases linearly with increasing oxygen concentration without, however, forming reactive oxygen species, such as singlet oxygen or superoxide anion. When ferrocene (Fc) is used as a terminal electron donor moiety instead of ZnP (i.e., Fc-ZnP-C₆₀), no catalytic effects of dioxygen were, however, observed for the BET in Fc⁺-ZnP-C₆₀^•-, that is, from C_•60- to the ferricenium ion. In the case of ZnP-containing C₆₀ systems, the partial coordination of O₂ to ZnP^•+ facilitates an intermolecular electron transfer (ET) from C₆₀^•- to O₂. This rate-determining ET step is followed by a rapid intramolecular ET from O₂^•- to ZnP^•+ in the corresponding O₂^•--ZnP^•+ complex and hereby regenerating O₂. In summary, O₂ acts as a novel catalyst in accelerating the BET of the C₆₀^•--ZnP^•+ radical ion pairs.