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 OMnII 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 angstrom (Ti edge: 3.36 +/- 0.05 angstrom; Mn edge: 3.25 +/- 0.07 angstrom) and a bond angle of 111 degrees. 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 degrees 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 (TiOMnII)-O-IV MMCT unit, which is a promising candidate as a visible light charge-transfer chromophore for driving multielectron catalysts for artificial photosynthesis.