Investigating the reduction characteristics of transient free radicals by laser-pulse electron photo-injection—mechanism diagnostic criteria and determination of reactivity parameters from time-resolved experiments

Capture of electrons photo-injected from an electrode into a solution by an appropriate substrate is a convenient way of generating transient free radicals and investigating their reduction at the same electrode. In most cases however, the half-wave potentials of the radical ‘polarogram’ thus obtained do not have a simple direct thermodynamic meaning. It is a reflection of the thermodynamics and kinetics of the various coupled reactions in which the radical (electron transfer, dimerization, disproportionation, reaction with the medium, adsorption onto the electrode surface) and the resulting anion (back electron transfer, reactions with Bronsted and Lewis acids) are engaged. The theory describing quantitatively the effect of these reactions on the reduction potential is presented with particular emphasis on the establishment of criteria for mechanism diagnosis and of procedures for extracting the various kinetic and thermodynamic parameters of interest from the raw data. The observation time is one of the most important parameters in this respect. In this connection, one valuable feature of the laser pulse electron-photo-injection method is that the time window available in current practice extends over three orders of magnitude, from ca. 1 μs to 1 ms