Reversible visible-light photooxidation of an oxomanganese water-oxidation catalyst covalently anchored to TiO2 nanoparticles.

Several polynuclear transition-metal complexes, including our own dinuclear di-μ-oxo manganese compound H(2)O(terpy)Mn(III)(μ-O)(2)Mn(IV)(terpy)H(2)O(3) (1, terpy = 2,2':6',2''-terpyridine), have been reported to be homogeneous catalysts for water oxidation. This paper reports the covalent attachment of 1 onto nanoparticulate TiO(2) surfaces using a robust chromophoric linker L. L, a phenylterpy ligand attached to a 3-phenyl-acetylacetonate anchoring moiety via an amide bond, absorbs visible light and leads to photoinduced interfacial electron transfer into the TiO(2) conduction band. We characterize the electronic and structural properties of the 1-L-TiO(2) assemblies by using a combination of methods, including computational modeling and UV-visible, IR, and EPR spectroscopies. We show that the Mn(III,IV) state of 1 can be reversibly advanced to the Mn(IV,IV) state by visible-light photoexcitation of 1-L-TiO(2) nanoparticles (NPs) and recombines back to the Mn(III,IV) state in the dark, in the absence of electron scavengers. Our findings also indicate that a high degree of crystallinity of the TiO(2) NPs is essential for promoting photooxidation of the adsorbates by photoinduced charge separation when the TiO(2) NPs serve as electron acceptors in artificial photosynthetic assemblies. The reported results are particularly relevant to the development of photocatalytic devices for oxidation chemistry based on inexpensive materials (e.g., TiO(2) and Mn complexes) that are robust under aqueous and oxidative conditions.