Studies on intra-supramolecular and intermolecular electron-transfer processes between zinc naphthalocyanine and imidazole-appended fullerene

Spectroscopic computational redox, and photochemical behaviour of a self-assembled donor-acceptor dyad formed by axial coordination of zinc napthalocyanine, ZnNc, and fulleropyrrolidine bearing an imidazole coordinating ligand(2-(4-imidazolylphenyl)-fullereropyrrolidine, C(60)Im) was investigated in noncoordinating solvents, toluene and o-dichlorobenzene, and the results were compared to the intermolecular electron transfer processes in a coordinating solvent, benzonitrile. The optical absorption and ab initio B3LYP/3-21G(*) computational studies revealed self-assembled supramolecular 11 dyad formation between the ZnNc and C(60)Im entities. In the optimized structure, the HOMO was found to be entirely located on the ZnNc entity while the LUMO was found to be entirely on the fullerene entity. Cyclic voltammetry studies of the dyad exhibited a total of seven one-electron redox process in o-dichlorobenzene, with 0.1M tetrabutylammonium perchlorate. The excited state electron-transfer process were monitored by both optical-emission and transient-absorption techniques. Direct evidence for the radical-ion-pair (C(60)Im(.-) :ZnNc(.+)) formation was obtained from picosecond transient-absorption spectral studies, which indicated charge separation from the singlet-excited ZnNc to the C(60)Im moiety. The calculated rates of charge separation and charge recombination were 1.4 x 10(10) s(-1) and 5.3 x 10(7) s(-1) in toluene and 8.9 x 10(9) s(-1) and 9.2 x 10(7) s(-1) in o-dichlorobenezene, respectively. In benzonitrile, intermolecular electron transfer from the excited triplet state of ZnNc to C(60)Im occurs and the second-order rate constant (k(q)(triplet)) for this quenching process was 5.3 x 10(8) M-1 s(-1).