Effects of Sensitizer/Catalyst Interactions on the Photocatalytic Water Oxidation by a Tetraruthenium Polyoxometalate Catalyst

Water oxidation to dioxygen is a key step common to most artificial photosynthetic reaction schemes.[1,2] Photocatalytic water oxidation can be accomplished in homogeneous systems upon irradiation of aqueous solutions containing a sensitizer, a sacrificial electron acceptor, and a catalyst. In such systems two photochemical mechanisms are in principle available for the stepwise one-electron oxidations of the catalyst eventually leading to water oxidation. These include the typical biomimetic pathway, involving first oxidation of the excited sensitizer by the sacrificial acceptor and subsequent hole transfer to the catalyst, and a less usual (“anti-biomimetic”) one, involving first oxidation of the catalyst by the excited sensitizer followed by electron shift to the sacrificial agent. In such a scenario, the molecular interactions between the sensitizing and the catalytic moieties are of particular importance in determining the actual photochemical pathway as well as the efficiencies of the electron transfer processes and of the overall catalysis. Herein we show three photochemical systems involving persulfate as the sacrificial acceptor, a tetraruthenium polyoxometalate as the oxygen evolving catalyst, and (i) the standard Ru(bpy)32+ complex,[3] (ii) a water-soluble tetracationic zinc porphyrin,[4] or (iii) a tetranuclear ruthenium polypyridine dendrimer [5] as the photosensitizer. Particular attention has been deserved to the evaluation of the molecular interactions between the sensitizers and catalyst and the resulting photochemical processes. The water oxidation performances of the three-component photochemical systems will be thus discussed accordingly.