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Binuclear TiOMn charge-transfer chromophore in mesoporous silica.

An all-inorganic heterobinuclear chromophore consisting of Ti(IV) oxo-bridged to a Mn(II) center has been assembled on the surface of silica pores of MCM-41 material. The key step of covalent attachment on the pore surface is the reaction of a Mn(II) precursor featuring weakly held CH3CN ligands with the OH group of a previously anchored titanol site. The optical diffuse reflectance spectrum reveals a Ti(IV)OMn(II) --> Ti(III)OMn(III) metal-to-metal charge-transfer (MMCT) absorption extending from the UV throughout to visible into the red spectral region. FT-IR, FT-Raman and optical spectroscopy confirm that the material is free of Mn oxide clusters, while EPR and Mn K-edge X-ray absorption spectra indicate that the donor center is predominantly in oxidation state +2. In situ FT-IR spectroscopy allowed detection of visible light-induced redox chemistry of the MMCT unit using O2 (18O2) and methanol as acceptor and donor probe molecules, respectively. Formate and water were observed as primary products, with methyl formate emerging as a secondary condensation product. The observed photochemistry demonstrates that excitation of the Ti(IV)OMn(II) --> Ti(III)OMn(III) results in complete transfer of an electron from donor to acceptor center, with the charge separation sufficiently long lived for initiation of redox chemistry to occur. With donor and acceptor redox potentials appropriate for driving multi-electron catalysts for water oxidation, proton or CO2 reduction, the TiOMn(II) unit is an attractive candidate as a charge-transfer chromophore in a solar fuel generating system.