Kinetic and Mechanistic Aspects of Photocatalysis by Polyoxotungstates: A Laser Flash Photolysis, Pulse Radiolysis, and Continuous Photolysis Study

Abstract

Illumination of polyoxotungstates generates a short-lived excited state which decays to form the reactive intermediate wO that is responsible for subsequent transformation of organic substrates. It is shown by laser flash photolysis that wO is formed with a quantum yield of 0.6 and 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 spectroscopy and as confirmed by pulse radiolysis. Oxygen intercepts the initially-formed radicals so that this methodology may be used to follow the course of the reaction. Under continuous photolysis, propan-2-ol is converted into acetone and hydrogen peroxide but with adamantane as substrate the corresponding hydroperoxides are formed. Quantum yields of oxygen 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 counterions may be observed. All these observations are consistent with hydrogen-atom abstraction from an organic substrate by wO, followed by rapid deprotonation of •wOH and/or trapping by O2. A concise reaction scheme involving coupling between oxyradicals is proposed that adequately describes the overall chemistry.