Photoinduced electron transfer in supramolecular systems of fullerenes functionalized with ligands capable of binding to zinc porphyrins and zinc phthalocyanines

This review describes light induced energy or electron transfer reactions in self-assembled supramolecular zinc porphyrin/zinc phthalocyanine and fullerene bearing donor-acceptor systems. The self-assembled supramolecular dyads and triads are formed by using one or two types of the binding mechanisms including metal-ligand axial coordination. The photochemical properties of the metal porphyrin/phthalocyanine and fullerene moieties are shown to be tuned in a controlled manner upon coordination of metal center. The nature of the linker between the donor and acceptor entities influences the overall self-assembly process followed by the photochemical reactivity. In these self-assembled supramolecular systems, the photoinduced charge separation occurs mainly from the excited singlet state of the donor; and the back electron transfer rates generally occurs giving reversible systems. In some of the reported donor-acceptor conjugates, the predicted acceleration of the charge separation process and deceleration of the charge recombination process have been clearly observed, mainly due to the small reorganization energies of fullerenes in electron transfer reactions. Elegantly designed supramolecular triads to achieve sequential electron transfer to obtain the charge-separated states, and sequential energy transfer followed by electron transfer to mimic the photosynthetic 'antenna-reaction center' have also been developed and studied. The relations between structures and photochemical reactivities of these novel supramolecular systems are discussed in relation to the efficiency of forward electron transfer and slowing down the charge recombination process.