Supramolecular triads formed by axial coordination of fullerene to covalently linked zinc porphyrin-ferrocene(s): Design, syntheses, electrochemistry, and photochemistry

Supramolecular triads have been constructed by using covalently linked zinc porphyrin-ferrocene(s) dyads, self-assembled via axial coordination to either pyridine- or imidazole-appended fulleropyrrolidine. These triads were characterized by optical absorption, computational, and electrochemical methods. The calculated binding constants (K) revealed stable complexation and suggested the existence of intermolecular interactions between the ferrocene and fullerene entities. Accordingly, the optimized geometry obtained by ab initio B3LYP/3-21G(*) methods revealed closely spaced ferrocene and fullerene entities in the studied triads. Photoinduced charge-separation and charge-recombination processes were examined in the dyads and triads by means of time-resolved transient absorption and fluorescence lifetime measurements. In the case of zinc porphyrin-ferrocene(s) dyads, upon photoexcitation, efficient (Φ _CS = 0.98) to moderate (Φ _CS = 0.54) amounts of electron transfer from the ferrocene to the singlet excited zinc porphyrin occurred depending upon the nature of the spacer, resulting in the formation of the Fc ⁺-ZnP ^•- radical pair. Upon formation of the supramolecular triads by axial coordination of fulleropyrrolidines, the initial electron transfer originated either from or to the singlet excited zinc porphyrin, resulting ultimately in the formation of the charge-separated states of Fc ⁺-ZnP:C ₆₀ ^•- with high quantum efficiency. The calculated ratio of k _CS/k _CR. from the kinetic data was found to be ∼100, indicating a moderate amount of charge stabilization in the studied supramolecular triads.