Molecular Photocatalytic Water Splitting by Mimicking Photosystems I and II

Abstract

In nature, water is oxidized by plastoquinone to evolve O2 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 H2 and O2 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 H2 and O2 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 (H2O/TFE, 3:1 v/v), with a glass membrane to combine PSI and PSII molecular models. A PSII model contains plastoquinone analogs [p-benzoquinone derivatives (X-Q)] in toluene and an iron­(II) complex as a molecular oxidation catalyst in H2O/TFE (3:1 v/v), which evolves a stoichiometric amount of O2 and forms plastoquinol analogs (X-QH2) under photoirradiation. On the other hand, a PSI model contains nothing in toluene but contains X-QH2, 9-mesityl-10-methylacridinium ion (Acr+-Mes) as a photocatalyst, and a cobalt­(III) complex as an H2 evolution catalyst in H2O/TFE (3:1 v/v), which evolves a stoichiometric amount of H2 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.

Reaction Intermediates in Artificial Photosynthesis with Molecular Catalysts

“…340 As shown in Figure 57, the molecular models of PSI and PSII consist of two phases, a toluene and a water/trifluoroethanol mixture (H 2 O/TFE, 3:1 v/v) phase, respectively, and the two phases are separated by a liquid membrane. 340 Then the toluene phase of the PSII model was connected to the same phase of the PSI model, and the water/trifluoroethanol phase of the PSII model was also connected to the same phase of the PSI model by two glass membranes, respectively. 340 The PSII model contains plastoquinone analogues [p-benzoquinone derivatives (X-Q)] in toluene and an iron(II) complex ([(N4Py)Fe II ] 2+ ) as a molecular water oxidation catalyst in H 2 O/TFE (3:1 v/v), which resulted in evolution of the stoichiometric amount of O 2 and generation of hydroquinone derivatives (X-QH 2 ).…”

How Much Technological Progress is Needed to Make Solar Hydrogen Cost‐Competitive?

“…These characteristics point to the potential of homogenous photocatalysts, which can be used at low concentrations due to high molar absorptivity. [ 27 ] Furthermore, it was recently shown that it is theoretically possible for homogeneous photocatalysts to achieve dual absorber STH efficiencies (>20%) with a single catalyst. [ 28 ] With progress on catalyst concentration and STH efficiency, there is a pathway for PC to cost‐competitive hydrogen production.…”

Photoexcited charge manipulation in conjugated polymers bearing a Ru(ii) complex catalyst for visible-light CO2 reduction

“…Therefore, simultaneous multi-electron reduction and oxidation reactions, e.g., overall water splitting [19][20][21][22] and CO 2 reduction using water as the electron source, 16 have been accomplished using a number of semiconductor photocatalysts, while such reactions have hardly been reported using a molecular photocatalyst system. 23 In terms of CO 2 reduction, however, semiconductor photocatalysts frequently suffer from the low selectivity of CO 2 reduction by competing with efficient proton reduction. 16 Recently, molecular-semiconductor hybrid photocatalysts have been developed to maximize their advantages, although it still remains a challenge for efficient photocatalytic CO 2 reduction.…”