Artificial photosynthesis is expected as one of promising clean energy-providing systems in future. Development of an efficient catalyst for water oxidation to evolve O2 is a key task to yield a breakthrough for construction of an artificial photosynthetic device. Recently, significant progress has been reported in development of the molecular catalysts for water oxidation based on manganese, ruthenium and iridium complexes. The molecular aspects of the catalysts in chemical, electrochemical, and photo- or photoelectro-driven water oxidation were reviewed to provide hints to design an efficient catalyst. This review will mainly cover the related progress in the last 5 or 6 years.
