Visible light photocatalysis by metal halide complexes containing titania as a semiconductor ligand

Reaction of an aqueous suspension of titania with H2[PtCl6] or RhX3 (X = Cl, Br) affords halogenometalate complexes of composition {[TiO2]OMXnL}z−; z = 1, 2, L = H2O, OH−, M = Pt: n = 4, M = Rh, n = 3. In these hybrid materials, titania can be viewed as a semiconducting “inorganic ligand” bound via oxygen to a platinum(IV) or rhodium(III) center. The quasi-Fermi level of electrons in the titania ligand is shifted anodically by 0.2–0.3 V as compared to the free “ligand.” In the case of the platinum complex, visible light irradiation results in a platinum centered excitation followed by homolytic Pt–Cl bond cleavage and electron injection of Pt(III) into the titania conduction band. Thus, the reducing and oxidizing surface sites can be described as a trapped electron, that is, a Ti(III) center, and a loosely bound chlorine atom. In the case of the rhodium surface complex, the excitation is of rhodium-to-titanium charge-transfer type resulting in a Ti(III) center and a Rh(IV) species. These heterosupramolecular metal complexes in the presence of air photocatalyze the exhaustive visible light oxidation of pollutants like halocarbons and atrazine. They are active even in diffuse indoor daylight.