Arrangement effect of the di-μ-oxo dimanganese catalyst and Ru(bpy)32+ photoexcitation centers adsorbed on mica for visible-light-derived water oxidation

Abstract

A photosynthetic photosystem II (PS II) model was developed by adsorbing [(OH₂)(terpy)Mn^III(μ-O)₂Mn^IV(terpy)(OH₂)]³⁺ (1, terpy = 2,2′:6′,2′′-terpyridine) as an oxygen evolving center and Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine) as a photoexcitation center onto mica. The mica adsorbates were prepared by three different methods; Adsorbate A was prepared by adsorption of 1 followed by Ru(bpy)₃²⁺ on mica, and Adsorbates B and C were prepared by the opposite adsorption order and co-adsorption of 1 and Ru(bpy)₃²⁺, respectively. The UV-visible diffuse reflectance (DR) spectroscopic data and emission decay measurements of the photoexcited Ru(bpy)₃²⁺ suggested the different arrangements of 1 and Ru(bpy)₃²⁺ among the three adsorbates in a mica interlayer. For Adsorbate A, Ru(bpy)₃²⁺ could be adsorbed near the mica surface, being shallowly intercalated relative to 1. For Adsorbate B, 1 is adsorbed near the mica surface, Ru(bpy)₃²⁺ being deeply intercalated relative to 1. For Adsorbate C, either 1 or Ru(bpy)₃²⁺ could be randomly intercalated relative to the other adsorbates. In photochemical water oxidation experiments, a significant amount of O₂ was evolved when visible light was used to irradiate an aqueous suspension of Adsorbate A containing a S₂O₈²⁻ electron acceptor in a liquid phase, whereas O₂ was not evolved under the same conditions when using Adsorbates B and C. 1 is considered to work for photochemical water oxidation in Adsorbate A due to an efficient electron transport from deeply intercalated 1 to S₂O₈²⁻ ions in a liquid phase via Ru(bpy)₃²⁺ photoexcitation near the mica adsorbate surface.