Supramolecular approach towards light-harvesting materials based on porphyrins and chlorophylls

Photosynthesis is one of the most important processes that supplies the energy required for the survival of all living things. The initial process is the absorption of sunlight by a supramolecular organization called a light-harvesting antenna. Sophisticated natural energy-conversion devices have inspired chemists to produce artificial counterparts. This review presents the status quo of research on artificial light-harvesting antenna based on noncovalent supramolecular assemblies of porphyrin and chlorophyll derivatives. The electronic structures of chlorophylls and porphyrins can be understood qualitatively on the basis of the four-orbital model. The electronic energy transfer (EET) processes are described by the Forster and Dexter theories and beyond. The light-harvesting antennas found in nature are sophisticated supramolecular organizations of chlorophyll molecules that achieve efficient light collection via EET processes. To artificially reproduce the antenna function, supramolecular self-assembled porphyrin/chlorophyll organizations have been constructed and studied, both for discrete assemblies and extended assemblies. Porphyrin and chlorophyll molecules have also been organized on biological and nonbiological scaffolds, such as peptides and clay, respectively. Proof-of-principle demonstrations of enhancing the performance of solar cells have also been made by incorporating light-harvesting antenna systems. This research enriches our understanding on the organization of molecules and the correlation with the photophysical processes therein, possibly leading to the realization of new types of solar power conversion devices incorporating an antenna function.