Time-resolved electron spin resonance (TR ESR) has been used to investigate the chemically induced dynamic electron polarization (CIDEP) generated by the interaction of stable free radicals with the triplet states of benzophenone, benzil, and 2-acetylnaphthalene. The stable radicals were mono-, di-, tri-, and tetranitroxyl free radicals possessing the 2,2,6,6-tetramethylpiperidineN-oxyl moiety. All of the stable radical systems investigated were found to be emissively polarized by interaction with the triplet states, and the phase of polarization was independent of the sign of zero-field splitting (D) of the interacting triplet molecule. Possibleand likely mechanisms of polarization transfer (creation) resulting from the interaction of photoexcited triplet molecules with nitroxyls in the strong electron exchange are discussed. The emissive CIDEP of nitroxyls observed in the interactions with triplet benzil, which has D > 0, provides strong support for the operation of the radical-triplet pair mechanism. Within the time scale of T R ESR experiments (-lO-'-lO" s) no significant variation in the shape of the CIDEP spectra of the nitroxyls was observed, either in viscous media or in micelles. It is concluded that intramolecular spin exchange (or conformational change) of polynitroxyls occurs much faster than the time resolution of the experiment,

modeling of diffuse scattering; indeed, the analysis of local correlations in these strongly disordered crystals seems as important as the structure refinement. Then, it will hopefully be possible to think about microscopic models to interpret the distribution of proton tunnel splitting since that fascinating observation did in fact occasion the whole work.Acknowledgment. We thank José Dianoux and Werner Press for very useful discussions or comments on the manuscript.

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