In situ electrochemical ESR studies of reactive radicals: The reductions of bromo-anthraquinone and methyl viologen

The application of channel electrodes for quantitative in situ electrochemical ESR studies of unstable radicals is discussed with reference to two systems. First, the reduction of I-bromo-anthraquinone in acetonitrile solution in the presence of laser light of wavelength 488 nm is shown to lead to the loss of bromide and the formation of the anthraquinone radical anion via a photo-ECE process. The kinetics of this reaction as determined by channel electrode ESR studies are found to be in good agreement with those measured by independent methods. Second, the one-electron reduction of the methyl viologen dication in aqueous solution is shown to lead to the formation of viologen radicals which are known to dimerise. However, it is shown that since this reaction is rapid and reversible, measurements of the ESR signal as a function of electrolyte flow rate and current are essentially blind to this reaction. Lower estimates for the dimerisation rate constant are deduced.