Unraveling the Key Features of the Reactive State of Decatungstate Anion in Hydrogen Atom Transfer (HAT) Photocatalysis

Abstract

The decatungstate anion [W10O32]4- is a widely used photocatalyst for promoting hydrogen atom transfer (HAT) reactions. The mechanism implicated in the activation of organic substrates, however, still needs to be clarified and has been claimed to involve an unknown relaxed excited state of triplet multiplicity, tagged wO. A subpicosecond investigation allowed us to follow early events leading to the chemically reactive species. A hot singlet excited state (S1HOT) has been individuated through pump-probe experiments, yielding S1 by ultrafast decay (<1 ps). The reactive species wO arises from S1 in competition with decay to S0 (efficiency ca. 0.5) and has been detected spectroscopically by flash photolysis experiments, with peculiar absorption bands in the near-UV (370 nm) and visible (600-800 nm) regions. TD-DFT calculations demonstrated that excitation to S1 occurs through a ligand to metal charge transfer (LMCT) transition, involving a displacement of electron density from dicoordinated (bridging) oxygen to tungsten atoms. Population of wO ensues and involves a reorganization of the singly occupied orbital centered on oxygen (not tungsten) atoms. As a result, monocoordinated O centers acquire a partial radical character that well explains the known chemistry, essentially hydrogen atom transfer (HAT), and highlights the similarity with nπ∗ carbonyl triplets. This rationalization may help in devising other photocatalysts able to promote HAT processes from unactivated precursors.