Bioinspired Electron-Transfer Systems and Applications

Abstract Bioinspired electron-transfer systems including artificial photosynthesis and respiration are presented herein together with some of their applications. First, multi-step electron-transfer systems composed of electron donor–acceptor ensembles have been developed, mimicking functions of the photosynthetic reaction center. However, a significant amount of energy is lost during the multi-step electron-transfer processes. Then, as an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer within redox cascades, simple donor–acceptor dyads have been developed to attain a long-lived and high energy charge-separated state without significant loss of excitation energy, by fine control of the redox potentials and of the geometry of donor–acceptor dyads that have small reorganization energies of electron transfer. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications including construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion. An efficient four-electron reduction of dioxygen to water by one-electron reductants such as ferrocene derivatives as well as by an NADH analog has also been achieved as a respiration model by using a cofacial dicobalt porphyrin that can form the μ-peroxo Co(III)–O2–Co(III) complex. The catalytic mechanism of the four-electron reduction of dioxygen has been clarified based on the detailed kinetic study and the detection of the intermediate.