One‐Electron Redox Processes during Polyoxometalate‐Mediated Photocatalytic Reactions of TiO 2 Studied by Two‐Color Two‐Laser Flash Photolysis

The one‐electron redox processes of several compounds during polyoxometalate (POM)‐mediated photocatalytic reactions of TiO 2 were investigated using the two‐color two‐laser flash‐photolysis technique. The efficiency of the one‐electron oxidation of aromatic sulfides by the trapped hole (h tr + ) or the surface‐bound OH radical (OH s . ) is found to be significantly enhanced due to electron transfer from the conduction band (CB) of TiO 2 to the POM. The efficiency of the electron transfer from the CB of TiO 2 to the POM decreases in the order H 2 W 12 O 40 6− < SiW 12 O 40 4− < PW 12 O 40 3− , that is, it depends on the reduction potential ( E red ) of the POMs. Electron injection from PW 12 O 40 4− in the excited state (PW 12 O 40 4−* ) to the CB of TiO 2 was clearly observed using the two‐color two‐laser flash‐photolysis technique. Storage of electrons in the TiO 2 /PW 12 O 40 3− /methyl viologen (MV 2+ ) ternary system was also achieved upon two‐color two‐laser irradiation.