π-complex formation in electron-transfer reactions of porphyrins

π-Complex formation between porphyrins and their radical cations plays an important role in self-exchange electron transfer between neutral porphyrins and the radical cations, leading to negative activation enthalpies when the stabilization energy of the π-complex is larger than the activation energy for the intracomplex electron transfer in the π-complex. A number of porphyrin molecules are self-organized on three-dimensional gold nanoclusters to form monolayer-protected gold nanoclusters (MPCs) that act as an efficient photocatalyst for the uphill reduction of HV²⁺ by BNAH to produce l-benzylnicotinamidinium ion (BNA⁺) and hexyl viologen radical cation (HV^.+). Such three-dimensional architectures of porphyrin MPCs with large surface area allow supramolecular π-complexation between MPCs and HV⁺², resulting in fast electron transfer from the singlet excited state of porphyrin to HV²⁺ on MPCs. The π-π interaction is exploited to develop efficient photovoltaic devices consisting of porphyrin and fullerene assemblies which have an enhanced light-harvesting efficiency throughout the solar spectrum together with a highly efficient conversion of the harvested light into electrical energy.