Experimental and theoretical study on p-aminophenylthyil radical geminate recombination in ionic liquids; analysis using the Smoluchowski–Collins–Kimball equation

Recombination dynamics of geminate p-aminophenylthiyl (PAPT) radicals produced from the photodissociation of bis(p-aminophenyl) disulfide in ionic liquids (ILs) were investigated by transient absorption spectroscopy. ILs with various cationic species were used to examine the effect of viscosity and polarity on recombination dynamics. Experimentally obtained recombination yields and dynamics were found to be virtually independent of the cation species, despite the viscosity range of the solvent ILs being extensive, spanning from a few tens of mPa s to several hundred mPa s. We applied a theoretical analysis model based on the diffusion equation to the time profiles of the experimentally determined recombination yields of geminate PAPT radicals. The square well potential was incorporated into the diffusion equation to consider the concerted dynamics of solvent cage formation and recombination. A long-time asymptotic expression for the survival probability of the photodissociated products was derived and used to simulate the experimentally obtained time profile of the recombination yield. The time profiles in the range of 20–1000 ps and the final yield were successfully simulated by the asymptotic expression of the square well potential model. The optimized parameters used for the fit, including the mutual diffusion coefficient of the radical pairs, cage radius of the potential well, and well depth, were discussed in terms of the diffusion coefficient conventional theory and the potential mean force estimated from the molecular dynamics simulation for the photodissociation reaction in ILs.