1999 年 57 巻 10 号 p. 856-866
This review deals with two important kinetic effects based on concentration changes of radical and radical ion intermediates, which in some cases dominate the efficiency and selectivity of photoinduced electron transfer (PET) reactions. One is the persistent radical ion effect suggesting the predominant reaction of a more stable radical ion rather than that of less stable, i. e., more reactive, intermediate, just as the Ingold-Fischer persistent radical one which predicts high yields of the cross-reaction product from two radicals with markedly different self-termination constants. In PET reactions between D and A, reactions under continuous irradiations are in principle governed by the steady-state concentrations of radical ion intermediates, 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 irradiation. Such a contrast was demonstrated for the redox between diphenylmethyl radical and simultaneously generated aromatic radical cation/anion, and for the photosensitized redox reaction of carboxylates. Another important effect is based on abortive equilibria, indicating that a reactive intermediate exists in an equilibrium with stable species not leading to products. Kinetic consequences of this effect are described for the deprotonation of diarylmethane radical cations with pyridines, and for the PET oxidative C-C cleavage of 1, 1, 2, 2-tetraphenylethane.