INSIGHT INTO PHOTOCATALYTIC WATER OXIDATION BY A TETRARUTHENIUM MOLECULAR CATALYST AND RUTHENIUM POLYPYRIDINE SENSITIZERS

Water oxidation 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 towards water oxidation: (i) a typical biomimetic pathway, involving first oxidation of the excited sensitizer by the sacrificial acceptor and subsequent hole transfer to the catalyst, and (ii) a less usual (“anti-biomimetic”) one, involving first oxidation of the catalyst by the excited sensitizer followed by electron shift to the sacrificial agent. Herein we show a comparison between two photochemical systems involving persulfate as the sacrificial acceptor, a tetraruthenium polyoxometalate as the oxygen evolving catalyst, and either the standard Ru(bpy)32+ complex [3] or a tetranuclear ruthenium polypyridine dendrimer [4] as the photosensitizer. With these sensitizers, different photochemical pathways, (i) or (ii), are followed. Moreover, while in the former case the molecular interactions between the sensitizing and catalytic moieties are detrimental for the photocatalytic mechanism, in the latter case they are of fundamental importance to drive the electron flow towards the right direction in view of an efficient water oxidation catalysis.