Research Article pubs.acs.org/acscatalysis
Unraveling the Key Features of the Reactive State of Decatungstate Anion in Hydrogen Atom Transfer (HAT) Photocatalysis Vincent De Waele,† Olivier Poizat,† Maurizio Fagnoni,‡ Alessandro Bagno,§,∥ and Davide Ravelli*,‡ †
Université de Lille, CNRS, UMR 8516, LASIR, Laboratoire de Spectrochimie et Raman, F59000 Lille, France PhotoGreen Lab, Department of Chemistry, University of Pavia, viale Taramelli 12, 27100 Pavia, Italy § Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy ‡
S Supporting Information *
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 (5% have been shown. cThis is the transition responsible for the well-known absorption band of the decatungstate anion, and its energy has been taken as the upper limit for the selection of the dark states to be considered. 7178
DOI: 10.1021/acscatal.6b01984 ACS Catal. 2016, 6, 7174−7182
Research Article
ACS Catalysis
minimum, not regenerating S0 (path b; compare Figure 1A, where it is shown that no recovery of the bleach occurred at this stage). On a slightly longer time scale (ca. 50 ps), the decay of the 375 nm band was simultaneous with the partial bleach recovery (ca. 50%, Figure 2) and the formation of a new long-lived red absorbing transient species. Thus, this decay characterized excited state S1 depopulation with a competition between recovery of the ground state (path c) and formation of a different state (path d), reasonably a new electronically excited state, such as the triplet state wO (path d). The fact that bonds as strong as aliphatic C−H could be cleaved homolytically by excited TBADT supported this hypothesis, as hinted also by previous literature reports.15−21 Accordingly, path d implied intersystem crossing (ISC) to wO but involved only a half of the precursor, the remaining part decaying to S0. On an even longer time scale (in the order of tens of nanoseconds), wO interacted with the solvent acetonitrile and led to the monoreduced form (H+)[W10O32]5−, as demonstrated by the evolution observed at 650−750 nm (path e; see also Figures 3 and 4). Time constants of the order of
