(Invited) Visible Light-Driven Water Oxidation with Porphyrin Sensitizers and Water Oxidation Catalysts
Authors: Imahori Hiroshi 
Published: January 2022

Exporing renewable energy sources is an important task in making our society sustainable. In this regard, use of sunlight as an infinite energy source is fascinating. Specifically, realizing artificial photosynthesis, i.e., integration of light-harvesting, multi-step electron and proton transfer, and water oxidation for the efficient production of solar 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 semiconductors 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 excited-state of the sensitizer (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 reiterating the cycle, high oxidation states of the WOC are produced, 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 molecule-based artificial photosynthetic systems, owing 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. Considering that yellow to red photons mainly shower down on the earth from sun, use of photons in visible region is essential for efficient chemical conversion by sunlight. In this context, porphyrins are attractive as the sensitizer due to their excellent light-harvesting in visible region and facile tuning of their excited-states and redox properties by their chemical functionalization. Nevertheless, molecule-based artificial photosynthetic systems with porphyrins as the sensitizer have been very limited as the result of their poor 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 would also take place within a few microsecond. Undesirable CR from TiO2(e−) to water is indicated. Thus, to overcome the disadvantages, it is crucial to optimize the electron transfer (ET) processes at the interfaces. In this talk, I will give an overview of our recent initiatives on visible light-driven water oxidation with novel porphyrin sensitizers and water oxidation catalysts.
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