Visible Light-Driven Water Oxidation with Porphyrin Sensitizers and Water Oxidation Catalysts

Abstract

Finding renewable energy sources is a crucial task in making our society sustainable. In this regard, exploitation of sunlight as an infinite energy source is attractive. Specifically realizing artificial photosynthesis, i.e., integration of light-harvesting, multi-step electron and proton transfer, and water oxidation for the efficient production of fuels, is a great challenge in chemistry. For the purpose, dye-sensitized photoelectrosynthesis cells (DSPSC) have been investigated, as the heterogeneous water splitting on inorganic semiconductor is promising for the upcoming large scale device operation. In DSPSC a molecular sensitizer adsorbed on a semiconducting electrode harvests visible light and injects an electron from the sensitizer excited-state (S*) to a conduction band (CB) of the electrode. Then, the sensitizer radical cation (S• +) extracts an electron from a water oxidation catalyst (WOC) to regenerate the sensitizer and one-electron oxidized WOC. After repeating the cycle, high oxidation states of the WOC are generated, eventually transforming two water molecules into four protons and one oxygen molecule. As the sensitizer bis(2,2’-bipyridine)(4,4’-diphosphonato-2,2’-bipyridine)ruthenium(II) (RuP) has been frequently employed for the construction of molecular artificial photosynthetic systems, due to its sufficient first oxidation potential for water oxidation and a long lifetime of its excited state for electron injection. However, the light-harvesting ability of RuP is rather low in visible region beyond 500 nm. Given that yellow to red photons mainly shower down on the earth from sun, utilization of photons in visible region is important for efficient chemical conversion by sunlight. Along this line, porphyrins are promising as the sensitizer because of their excellent light-harvesting in visible region and facile tuning of their excited-states and redox properties by their peripheral functionalization. Nevertheless, molecular artificial photosynthetic systems with porphyrins as the sensitizer are so far limited owing to their low performance. One plausible reason is the occurrence of fast charge recombination (CR) between the electron injected into the CB of TiO2 (denoted as TiO2(e−)) and S• +. CR from TiO2(e−) to the oxidized WOC is also reported to take place within a few microsecond. Undesirable CR from TiO2(e−) to water is also suggested. Therefore, to overcome the drawbacks, it is crucial to modulate the electron transfer (ET) properties at the interfaces. In this talk, I will give an overview of our recent initiatives on visible light-driven water oxidation with porphyrin sensitizers and water oxidation catalysts. [1] M. Yamamoto, L. Wang, F. Li, T. Fukushima, K. Tanaka, L. Sun and H. Imahori, Chem. Sci., 7, 1430-1439 (2016). [2] M. Yamamoto, Y. Nishizawa, P. Chábera, F. Li, T. Pascher, V. Sundström, L. Sun, and H. Imahori, Chem. Commun., 52, 13702-13705 (2016).