Studies on Covalently Linked Porphyrin−C60 Dyads: Stabilization of Charge-Separated States by Axial Coordination

Abstract

The effect of axial ligation on the photoinduced charge separation and charge recombination of a series of covalently linked porphyrin−C60 dyads is investigated. Toward this, meso-tetraphenylporphyrin and its zinc(II) derivative are functionalized at the ortho or para positions of one of the aryl groups to bear a fulleropyrrolidine entity through a flexible ethylene dioxide bridge to probe the effect of intramolecular electron transfer phenomena. In o-dichlorobenzene, 0.1 M (TBA)ClO4, the synthesized dyads exhibit seven one electron reversible redox reactions within in the potential window of the solvent and the measured redox potentials and UV−visible absorption spectra reveal charge−transfer interactions between the electron donor, porphyrin, and the electron acceptor, fullerene entities. The geometric and electronic structures of the dyads probed by ab initio B3LYP/3-21G(*) methods also revealed the existence of charge−transfer interactions. The excited state photochemical events are monitored by both steady state and time-resolved emission as well as transient absorption techniques. In o-dichlorobenzene or in benzonitrile, the main quenching pathway involves charge separation from the excited porphyrin to the moiety. The kcs and kcr are found to depend on the type of substitution (ortho or para) and the metal ion in the porphyrin cavity. Relatively long-lived charge-separated states are observed upon coordinating pyridine axial ligands to the central metal ion of the zinc porphyrin−C60 dyads, and this has been attributed to the electronic and geometric effects caused by the axial coordinated ligand.