Ketyl-phenoxyl triplet radical pair in glycerol: Magnetic field effect and cage kinetics in terms of the two-state model.

The geminate recombination kinetics of the triplet pairs of neutral radicals formed as a result of hydrogen atom transfer from 4-methylphenol to 4,4'-dimethylbenzophenone in the triplet excited state was studied by laser flash photolysis in the temperature range of 263-323 K. The pair included aromatic ketyl and 4-methylphenoxyl radicals produced in a viscous (∼10 P) solution of the initial reagents in glycerol. The geminate recombination kinetics demonstrate a "tail" c(t) ∼ 1/t, with contribution of magnitude An, which increases with temperature. The two-state (TS) model, which takes into account the potential well caused by the interaction between radicals at short distances and the radicals that escaped from a well but returned to it after free diffusion outside the well, was used to fit the data. The application of an external magnetic field (MF) (0.2 T) gave rise to the retardation of the geminate recombination and to the increase of fraction of radicals in the solvent bulk. The bell-shaped dependence of the magnetic field effect (MFE) value upon temperature (with a maximum near 300 K) was found. This dependence correlates with An and its dependence on the MF. At temperatures lower than 270 K, both the contribution of the nonexponential part of geminate recombination kinetics and the MFE were found to be small. The TS model can quantitatively describe the effects of solvent viscosity and MF on the kinetics of geminate recombination of triplet RP.