Photoreduction of Chloranil by Benzhydrol and Related Compounds. Hydrogen Atom Abstraction vs Sequential Electron−Proton Transfer via Quinone Triplet Radical Ion−Pairs

The photoreduction of chloranil (Q) to the hydroquinone (QH2) in benzene by benzhydrols and by related arylmethanols has been investigated. The products of photooxidation of the benzhydrols are benzophenones, in lieu of formation of benzpinacols. Three distinct mechanisms of oxidation−reduction have been identified from quantum yield determinations and laser flash photolysis experiments, including quinone triplet quenching via H-atom and electron transfer paths. Direct excitation of ground state quinone complexes has also been investigated. The quenching of the triplet state of the quinone by benzhydrol proceeds normally (k q = 1.3 × 106 M-1 s-1) and gives semiquinone radical (QH•, λmax = 435 nm) and the benzhydryl radical (λmax = 535 nm). The latter intermediate decays by pseudo-first-order kinetics through hydrogen atom transfer with ground state quinone (Q). Triplet quenching by bis(4-methoxyphenyl)methanol proceeds at a more rapid rate (k q = 5.5 × 109 M-1 s-1) leading to an intermediate that is identified as the chloranil radical anion (λmax = 450 nm). A similar intermediate is observed on Q quenching by 1-naphthylmethanol and acenapthenol with the appearance of an accompanying naphthalene radical cation absorption (ca. 670 nm). The radical ion transients, which are assigned to contact ion-pairs (triplet excited complexes) of the quinone and the various electron donors, decay to semiquinone radicals (QH•) by first-order processes occurring in the 100 ns time regime. The transient behavior is interpreted in terms of a hydrogen atom transfer mechanism for photoreduction with benzhydrol and, for the more robust electron donors, a mechanism involving electron transfer followed by proton transfer between geminate radical ions. For the electron transfer donors, ground state charge-transfer (CT) complexes can be observed (λmax ca. 500 nm). Selective CT excitation leads to quinone photoreduction with reduced quantum yield. The results are discussed in terms of the time resolution of sequential electron/proton transfer steps for photogenerated ion-pairs, the occurrence of one photon−two electron transfer photoredox mechanisms, and the kinetically distinct pathways for decay of singlet and triplet intimate radical ion-pairs.