STUDY OF THERMALLY ACTIVATED RADICAL PAIR SEPARATION IN THE ANTHRAQUINONE TRIETHYLAMINE PHOTOREDOX SYSTEM BY FOURIER-TRANSFORM EPR

The generation of free radicals in the photoreduction of anthraquinone (AQ) by triethylamine in alcoholic solutions is monitored with FT-EPR with a time resolution of 10 ns. Spin-polarized AQ.- radicals, emissively polarized by the triplet mechanism, are generated with typical time constants of 100 ns, whereas the primary photoreduction step proceeds with a time constant of approximately 1 ns, only. This delay can be attributed to the generation of a Coulomb-stabilized intermediate radical pair, from which FT-EPR observable free radicals are escaping by thermal activation. The strong solvent dependence of the escape rate can be accounted for mainly by considering the solvent's dielectric constant, shielding the attractive potential. A quantitative analysis results in a fictitious binding potential of 2.6 eV.