Nuclear Spin Polarization Effects in Radical Ion Pair Reactions. Recombination in the Singlet and Triplet State

The nuclear spin polarization effects observed during photoinitiated electron transfer reactions of olefins with suitable reaction partners provide insights into electron spin effects on radical ion pair reactivity and serve to illustrate two spinsorting principles. The mechanisms differ in the electron spin multiplicity dependence of the ion-pair reactions. In reactions involving the radical cation of phenyl vinyl ether or phenylpropene and the radical anion of chloranil, electron return is efficient for singlet pairs but energy forbidden for triplet pairs. In contrast, the pair phenylpropene radical anion-tetramethylphenylenediamine radical cation undergo recombination with similar efficiency in the singlet and triplet state because the pair energy lies above the triplet energy of phenylpropene. Finally, pairs such as stilbene cation-phenanthrene anion undergo triplet recombination more efficiently than singlet recombination suggesting that, in this system, the small gap between the pair energy and the triplet energy of stilbene allows faster electron return than does the larger gap between pair and ground state in qualitative agreement with the Marcus theory.