MAGNETIC-FIELD EFFECTS ON THE DYNAMICS OF NITROXIDE-BASED SINGLET RADICAL PAIRS IN MICELLES

The nitroxide adduct 1-benzyloxy-2,2,6,6-tetramethylpiperidine (3) und ergoes rapid photocleavage to yield 2,2,6,6-tetramethylpiperidine N-ox ide and benzyl radicals. Laser excitation (248 nm) of 3 in aqueous mic ellar solution leads to the formation of singlet radical pairs which d ecay via a combination of separation and geminate processes. The singl et radical pair produced from 3 undergoes moderately slow recombinatio n and as a result allows for the competition between spin evolution an d radical escape. Application of an external magnetic field leads to a n increase in the rate constant for the geminate radical reaction, ref lecting that the magnetic field prevents the interconversion of the si nglet radical pair to the unreactive T-- and T-+ triplet sublevels. In contrast, when the radicals are produced directly on the triplet surf ace by sensitization, application of a field leads to a slowdown of ra dical recombination, as normally observed for tripler radical pairs in heterogeneous media. The magnetic field effects are ascribed to the m anifestation of the HFI mechanism. The kinetic analyses for the variou s cases possible are presented in detail.