Heterogeneous water oxidation photocatalysis based on periodic mesoporous organosilica immobilizing a tris(2,2′-bipyridine)ruthenium sensitizer

Abstract

A periodic mesoporous organosilica (PMO) containing 2,2′-bipyridine groups (BPy-PMO) has been shown to possess a unique pore wall structure in which the 2,2′-bipyridine groups are densely and regularly packed. The surface 2,2′-bipyridine groups can function as chelating ligands for the formation of metal complexes, thus generating molecularly-defined catalytic sites that are exposed on the surface of the material. We here report the construction of a heterogeneous water oxidation photocatalyst by immobilizing several types of tris(2,2′-bipyridine)ruthenium complexes on BPy-PMO where they function as photosensitizers in conjunction with iridium oxide as a catalyst. The Ru complexes produced on BPy-PMO in this work were composed of three bipyridine ligands, including the BPy in the PMO framework and two X₂bpy, denoted herein as Ru(X)-BPy-PMO where X is H (2,2′-bipyridine), Me (4,4′-dimethyl-2,2′-bipyridine), t-Bu(4,4′-di-tert-butyl-2,2′-bipyridine) or CO₂Me (4,4′-dimethoxycarbonyl-2,2′-bipyridine). Efficient photocatalytic water oxidation was achieved by tuning the photochemical properties of the Ru complexes on the BPy-PMO through the incorporation of electron-donating or electron-withdrawing functionalities. The reaction turnover number based on the amount of the Ru complex was improved to 20, which is higher than values previously obtained from PMO systems acting as water oxidation photocatalysts.