Electron spin polarization (C.I.D.E.P.) in photosensitized reactions

Abstract

The sensitized and unsensitized photoreductions of biacetyl with triethylamine have been studied using time-resolved e.s.r. spectroscopy. It is confirmed that triethylamine quenches both singlet and triplet biacetyl. However, only the reaction of the triplet leads to efficient radical production, and this is not sufficiently rapid for significant primary electron polarization to be induced by the triplet mechanism in the systems investigated. Polarized e.s.r. spectra of biacetyl radical anions that can be obtained from benzophenone-sensitized reactions are shown to arise via a chemical sensitization mechanism: polarization in the primary radical MeĊHNEt₂ results from the rapid reaction of triplet benzophenone with triethylamine and is transferred via reaction with ground state biacetyl to the secondary radical anions. The initial polarizations of both primary radicals (MeĊHNEt₂ and Ph₂ĊOH) have been estimated and are shown to be equal within experimental error, providing the first quantitative evidence for this requirement of the triplet mechanism for electron polarization.