Photoinduced electron transfer in pinacol cleavage with quinones via highly labile cation radicals. Direct comparison of charge-transfer excitation and photosensitization

Benzopinacol and related diphenylethane-like donors (D) form electron donor-acceptor (EDA) complexes with chloranil and similar benzoquinones (A), in which the deliberate irradiation of the charge-transfer absorption band (hνCT) leads to oxidative cleavage (retropinacol)via electron transfer. Photosensitization by excitation of the π–π* band of chloranil and diffusive quenching also effects the C–C bond cleavage of the same donors. The photoefficiencies of both photochemical processes are quantitatively compared with respect to the lifetimes of the pinacol cation radicals (D+˙), as determined by the competition from back electron transfer and diffusion. These photoinduced processes are considered in the context of electron-transfer for an equivalent thermal reaction which occurs in the dark with high-potential quinones and electron-rich pinacols.