Molecular Photocatalytic Water Splitting by Mimicking Photosystems I and II.

In nature, water is oxidized by plastoquinone to evolve O and form plastoquinol in Photosystem II (PSII), whereas NADP is reduced by plastoquinol to produce NADPH and regenerate plastoquinone in Photosystem I (PSI), using homogeneous molecular photocatalysts. However, water splitting to evolve H and O in a 2:1 stoichiometric ratio has yet to be achieved using homogeneous molecular photocatalysts, remaining as one of the biggest challenges in science. Herein, we demonstrate overall water splitting to evolve H and O in a 2:1 ratio using a two liquid membranes system composed of two toluene phases, which are separated by a solvent mixture of water and trifluoroethanol (HO/TFE, 3:1 v/v), with a glass membrane to combine PSI and PSII molecular models. A PSII model contains plastoquinone analogs [-benzoquinone derivatives (X-Q)] in toluene and an iron(II) complex as a molecular oxidation catalyst in HO/TFE (3:1 v/v), which evolves a stoichiometric amount of O and forms plastoquinol analogs (X-QH) under photoirradiation. On the other hand, a PSI model contains nothing in toluene but contains X-QH, 9-mesityl-10-methylacridinium ion (Acr-Mes) as a photocatalyst, and a cobalt(III) complex as an H evolution catalyst in HO/TFE (3:1 v/v), which evolves a stoichiometric amount of H and forms X-Q under photoirradiation. When a PSII model system is combined with a PSI model system with two glass membranes and two liquid membranes, photocatalytic water splitting with homogeneous molecular photocatalysts is achieved to evolve hydrogen and oxygen with the turnover number (TON) of >100.