EXAFS Spectroscopic Analysis of Heterobinuclear TiOMn Charge-Transfer Chromophore in Mesoporous Silica

Analysis of extended X-ray absorption fine structure (EXAFS) measurements of binuclear Ti IV OMn II visible light charge-transfer chromophores anchored on silica nanopore surfaces reveals an oxo-bridged structure of a Ti and a Mn center. For TiMn-SBA-15 samples with 1 mol % for each metal, Ti and Mn K-edge EXAFS curve fitting indicates a common TiMn distance of 3.3 Å (Ti edge: 3.36 ± 0.05 Å; Mn edge: 3.25 ± 0.07 Å) and a bond angle of 111°. The first sphere coordination of the two metal centers, a distorted tetrahedral for Ti and a pseudo octahedral for Mn, is largely preserved upon formation of the linkage of the binuclear unit. Increasing the Ti loading from 1 to 3% does not introduce titania clusters, whereas loading of Mn beyond 2% leads to some Mn oxide cluster formation. The binuclear unit retains its structural integrity upon prolonged exposure to air or heating at high temperature (350 °C) in the presence of oxygen. The oxidation state increase of the Mn center upon calcination is accompanied by a shortening of the oxo bridge. The results provide the first detailed structural information on the Ti IV OMn II MMCT unit, which is a promising candidate as a visible light charge-transfer chromophore for driving multielectron catalysts for artificial photosynthesis.