KINETIC AND MECHANISTIC ASPECTS OF PHOTOCATALYSIS BY POLYOXOTUNGSTATES - A LASER FLASH-PHOTOLYSIS, PULSE-RADIOLYSIS, AND CONTINUOUS PHOTOLYSIS STUDY

Illumination of polyoxotungstates generates a short-lived excited stat e which decays to form the reactive intermediate wO that is responsibl e for subsequent transformation of organic substrates. It is shown by laser flash photolysis that wO is formed with a quantum yield of 0.6 a nd that it survives for ca. 55 ns in deoxygenated acetonitrile. In the presence of a substrate such as propan-2-ol wO reacts to produce the one-electron-reduced species, as characterized by transient spectrosco py and as confirmed by pulse radiolysis. Oxygen intercepts the initial ly-formed radicals so that this methodology may be used to follow the course of the reaction. Under continuous photolysis, propan-2-ol is co nverted into acetone and hydrogen peroxide but with adamantane as subs trate the corresponding hydroperoxides are formed. Quantum yields of o xygen consumption have been measured for a variety of other substrates and range from ca. 0.05 for pure acetonitrile to ca. 0.5 for propan-2 -ol. In the absence of other substrates, reaction of organic counterio ns may be observed. All these observations are consistent with hydroge n-atom abstraction from an organic substrate by wO, followed by rapid deprotonation of wOH and/or trapping by O-2. A concise reaction scheme involving coupling between oxyradicals is proposed that adequately de scribes the overall chemistry.