Artificial photosynthetic reaction center

The objective of this chapter is to describe the recent development of model systems of the photosynthetic reaction center, which is composed of light harvesting and charge-separation units. High quantum yields and long-lived charge separation, which mimic the multi-step electron transfer processes in the photosynthetic reaction center, have been achieved by electron donor-acceptor ensembles linked by covalent bonding. During the multi-step electron transfer processes, however, a significant amount of energy is lost to attain the final long-lived charge-separated state. Simple electron donor-acceptor dyads linked by covalent or non-covalent bonding have been developed to attain a long-lived and highenergy charge-separated state without significant loss of excitation energy. More sophisticated supramolecular complexes composed of light harvesting and chargeseparation units have been readily constructed by using non-covalent bonding such as Π- Π interaction, coordination bonds, and hydrogen bonds to attain long-lived charge-separated states with a high light harvesting efficiency. Single crystal of a supramolecular complex formed between a diprotonated porphyrin and an electron donor exhibit direction-dependent photocurrent generation toward the crystallographic axis along which the intermolecular Π- Π interaction between diprotonated porphyrins is the main conduction pathway. Such single crystals of supramolecular electron donor-acceptor ensembles will pave a way to the development of molecular scale photovoltaics.