Femtosecond dynamics of geminate recombination of radicals upon photodissociation of aromatic disulfides

The dynamics of the geminate recombination of thiyl radicals formed upon photodissociation of aromatic disulfides and the effect of the intramolecular relaxation on this recombination are studied using pico-and femtosecond kinetic spectroscopy. It is shown that, in terms of a phenomenological model, the geminate recombination of phenylthiyl radicals in neutral solvents can be satisfactorily described by a biexponential dependence. The model suggests the occurrence of primary geminate recombination in a solvent cage formed around an original molecule and secondary recombination controlled by the diffusional motion of the radicals of a pair. The primary geminate recombination, whose characteristic time (9 ps) is close to the characteristic times of intermolecular vibrational relaxation of complex molecules in solvents at room temperatures, masks the manifestation of thermalization processes in the time kinetics. The direct geminate recombination of aminophenylthiyl radicals with the formation of original molecules virtually does not occur because of the intramolecular charge transfer. In connection with this, the intermolecular vibrational relaxation manifests itself in the kinetics of the induced optical density in the region of the absorption band maximum of radicals as a growing component with a characteristic time of 6 ps.