Persistent intermediate effect and abortive equilibrium: Important kineticeffects in photoinduced reactions

This review deals with two important kinetic effects based on concentration changes of radical and radical ion intermediates, which in some cases domi nate the efficiency and selectivity of photoinduced electron transfer (PET) reactions. One is the persistent radical ion effect suggesting the predomi nant reaction of a more stable radical ion rather than that of less stable, i.e., more reactive, intermediate, just as the Ingold-Fischer persistent r adical one which predicts high yields of the cross-reaction product from tw o radicals with markedly different self-termination constants. In PET react ions between D and A, reactions under continuous irradiations are in princi ple governed by the steady-state concentrations of radical ion intermediate s, the ratios of D+. and A(-.) being quite different from 1:1 depending on their relative stabilities. The kinetics are more simple and the ratio of D +. and A(-.) becomes 1 : 1 when intermediates are generated by pulsed irrad iation. Such a contrast was demonstrated for the redox between diphenylmeth yl radical and simultaneously generated aromatic radical cation/anion, and for the photosensitized redox reaction of carboxylates. Another important e ffect is based on abortive equilibria, indicating that a reactive intermedi ate exists in an equilibrium with stable species not leading to products. K inetic consequences of this effect are described for the deprotonation of d iaryl-methane radical cations with pyridines, and for the PET oxidative C-C cleavage of 1,1,2,2-tetraphenylethane.