Some ESR and time-resolved CIDEP studies of the photoreaction of benzoquinone in trifluoroacetic acid

Abstract

I . INTRODUCTION Thermal and photoredox reactions involving benzoquinones have long been the subject of considerable general interest [1,2]. The intermediates in many of these reactions are commonly referred to as "semiquinones" which include the neutral and charged forms. Of the charged semiquinone radicals, the radical anion is readily observed by both optical and ESR spectroscopy, the corresponding one electron oxidized species, have, however, been more d i f f i cu l t to observe and are thought to be stable only in strongly acidic media [3,4]. Thus, tr i f luoroacetic acid, TFA, generally recognized for i ts ab i l i t y to solvate and stabil ize organic cations due to i ts acidity and low nucleophilicity [5,6], has been used in this laboratory for the time-resolved CIDEP studies of organic radical cations [7-9]. Both a theoretical investigation [10] and experimental studies [7,11] have concluded that the major stabil ization effect arises from an interaction of the trifluoromethyl group and the organic cation. The photochemical formation of polarized benzoquinone radical cations in TFA has been used as a model system for'charge transfer process involving heterocyclic sulfur-containing compounds [8]. I t was confirmed that secondary polarization observed in the radical cations of heterocyclic sulfur compounds resulted direct ly from the rapid charge transfer process: * ~ . ~ BQ + + S ÷ BQ + S + I t was also clear that the primary polarized benzoquinone radical species is the radical cation, since neither a neutral, nor an anion radical could lead to a charge transfer process resulting in the formation of the sulfur radical