A Bionispired System for Light-Driven Water Oxidation with a Porphyrin Sensitizer and a Tetrametallic Molecular Catalyst

Abstract

Inspired by natural photosynthesis, light driven water splitting aims at the production of renewable fuels exploiting solar radiation. In such systems where sustained hydrogen production is desirable, the oxidation of water to oxygen is currently recognized as the bottleneck of the entire process. Therefore, novel solutions aimed at solving this difficult task retain a fundamental interest. In this paper, we present a bioinspired, three-component system for water oxidation, constituted by a tetracationic Zn(II) porphyrin as the photosensitizer, a tetraruthenium water oxidation catalyst and sodium persulfate as the electron acceptor. An in-depth photophysical study reveals the photogeneration of a pentacation radical of the porphyrin (quantum yield up to = 1.01), upon oxidative quenching of the triplet excited state by persulfate. Electron transfer from the water oxidation catalyst to the pentacation radical (hole scavenging) is slow (bimolecular rate, k < 4×107 M-1s-1), and is likely the main reason for the low efficiency of the system in photocatalytic tests for water oxidation. Perspectives for improvements of the system and for the development of a light-activated device for water splitting are discussed