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−C 60 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)ClO 4, 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 k cs and k cr 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−C 60 dyads, and this has been attributed to the electronic and geometric effects caused by the axial coordinated ligand.