Flash-photolysis electron spin resonance in solution studies of free radicals

Flash-photolysis electron spin resonance spectra obtained using the time-integration spectroscopy technique usually exhibit electron spin polarization and so have unusual intensities. Here the analysis of the spectra is discussed, as is the recognition of the type of polarization which occurs and the information which can be obtained from the spectra. Emphasis is placed on the earliest spectra after radical formation, although some indication of the extra information available in a time-dependent study is given. A brief theoretical description of the effect of radical-pair mechanism (RPM) polarization on a pair of simple radicals precedes a discussion of the simultaneous action of the triplet mechanism (TM) as well. Original examples exhibit the extremes of polarization behaviour which occur. Pure TM effects are shown by radicals derived from phthalimide and 5-nitrofuroic acid, whilst reaction of excited singlet methylpyridazine yields a radical which displays only RPM characteristics. Mixed behaviour is demonstrated by radicals in the cycloheptanone + triethylamine and 1,4-dichlorophthalazine + di-t-butylphenol reaction systems. Time-dependent behaviour is exemplified by degenerate electron exchange involving the anion of benzene-1,2:4,5-tetracarboxylic acid dianhydride and the parent molecule.