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) undergoes rapid photocleavage to yield 2,2,6,6-tetramethylpiperidine N-oxide and benzyl radicals. Laser excitation (248 nm) of 3 in aqueous micellar solution leads to the formation of singlet radical pairs which decay via a combination of separation and geminate processes. The singlet radical pair produced from 3 undergoes moderately slow recombination and as a result allows for the competition between spin evolution and radical escape. Application of an external magnetic field leads to an increase in the rate constant for the geminate radical reaction, reflecting that the magnetic field prevents the interconversion of the singlet radical pair to the unreactive T-- and T-+ triplet sublevels. In contrast, when the radicals are produced directly on the triplet surface by sensitization, application of a field leads to a slowdown of radical recombination, as normally observed for tripler radical pairs in heterogeneous media. The magnetic field effects are ascribed to the manifestation of the HFI mechanism. The kinetic analyses for the various cases possible are presented in detail.