Geminate Recombination of the p-Aminophenylthiyl Radical Pair Produced by the Photodissociation of p-Aminophenyl Disulfide in Nonpolar Solvents

Abstract

Photodissociation of p-aminophenyl disulfide and the following geminate recombination of the p-aminophenylthiyl radical pair as well as radical dimer formation in nonpolar solvents have been investigated by using picosecond transient absorption spectral measurement techniques. Temperature effects of the geminate recombination and the sandwich dimer formation were measured and compared with the results of the molecular orbital calculation. The spectral broadening of the radical band observed in the short delay times was considered to be an indication of the radical−radical interaction in the geminate pair and was analyzed to obtain the time dependence of the separation distance between the radicals. The exponential narrowing in time was found for the p-aminophenylthiyl radical band in n-hexane, which should carry information about the diffusion process in the solvent cage and is compared with the results of the simulation. The speed of the increase in the radical−radical separation distance shows a larger temperature effect than for the geminate radical recombination to form the parent molecule. The characteristic feature of the dynamic behavior of the p-aminophenylthiyl radical pair, the formation of the sandwich dimer, and the slow recombination of the linear dimer should be due to the large dipole moment of the radical, which not only prevents the recombination but keeps the radicals in close proximity with each other.