KINETICS AND MAGNETIC-FIELD EFFECT IN GEMINATE RECOMBINATION OF TRIPLET RADICAL PAIRS ADSORBED ONTO POROUS-GLASS STUDIED BY LASER FLASH TECHNIQUE

Geminate recombination kinetics of radical pairs (RP) formed by electron or hydrogen atom transfer from triphenylamine, tri (4-bromophenyl)amine, 4-phenylaniline or 4-phenylphenol to triplet 9,10-anthraquinone, benzophenone or 4-bromobenzophenone both adsorbed onto an optically transparent SiO2 porous glass have been studied, using the laser flash technique with spectrophotometric registration. The kinetics are adequately described by the sum of two exponentials, ascribed to the existence of different kinds of "supercages" on the surface. At the same time, the simplest approximation by only one exponential is fair in many cases, because the contribution of a "slow" exponential is comparatively low and is masked by the slow component because of the decay of the escaped radicals. Introduction of a heavy Br atom leads to the acceleration of the geminate recombination; application of an external magnetic field results in retardation. The heavy-atom effect displays the contribution of the intersystem backwards electron transfer or intersystem recombination in the contact states of a triplet RP owing to the spin-orbit coupling. The magnetic field effect is the result of the significant contribution of the recombination route through the separated RP, where the hyperfine coupling and relaxation mechanisms of the RP spin evolution are active.