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-mu-oxo manganese compound [H2O(terpy)Mn-III(mu-O)(2)Mn-IV(terpy)H2O](NO3)(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 I onto nanoparticulate TiO2 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 TiO2 conduction band We characterize the electronic and structural properties of the 1-L-TiO2, 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-TiO2 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 TiO2 NPs is essential for promoting photooxidation of the adsorbates by photoinduced charge separation when the TiO2 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, TiO2 and Mn complexes) that arc robust under aqueous and oxidative conditions