(Keynote) One-Electron Initiated Two-Electron Oxidation of Water Catalyzed By Aluminum Porphyrins, Incorporating Earth

Abstract

One of the most promising solutions to the current global problems of energy and the environment would be to develop a clean system for the chemical conversion of solar energy, for example, photochemical hydrogen evolution and carbon dioxide fixation. In both cases, the water molecule should be the electron source, as is the case with natural photosynthesis. The point of research in this area is to discover how the electrons of water can best be utilized. The oxidation of water can be approached through (a) one-electron (b) two-electron and (c) four-electron processes. In spite of remarkable progress of molecular catalysts, the visible light-induced oxidation of water to molecular oxygen via the four-electron process has been one of the bottle-necks in the field, mostly due to the photon-flux density problem of sunlight, with its intrinsically limited intensity. A photochemical method for the four-electron oxidation of water requires four photons to excite sensitizers and catalysts in a stepwise manner in order to reach the higher oxidized states. Under typical sunlight conditions, the time interval between the arrivals of photons to a single molecule is on the order of seconds, which is far longer than the usual timescale for the survival of the highly unstable oxidized states needed to elicit the catalytic activity. Alternative approaches thus are clearly required. In this context, we have been studying the one-electron initiated two-electron oxidation of water. The latest results on the oxidation of water catalyzed by earth-abundant metalloporphyrins will be reported.