Role of Polar Protic Solvents in the Dissociation and Reactivity of Photogenerated Radical Ion Pairs.

The UV photolysis of bimolecular charge transfer complexes is employed to yield reactive radical ions in their solvent-equilibrated electronic ground state. In polar protic media, noncovalent complexes of 1,2,4,5-tetracyanobenzene and toluene undergo efficient, ultrafast dissociation to ion pairs and equilibrate with their solvent environment before the resulting radical ions engage in electron transfer and proton abstraction on subnanosecond time scales. Solvent molecules play a critical role in these reactive pathways and in the dissociation and relaxation processes that precede them. We report a clear separation of time scales for these relaxation and reactive processes, which implies that solvent-solute interactions can be used as a tool for tuning the reaction pathways of equilibrated radical ions in solution.