The solution dynamics of photoinduced geminate radical ion pairs: free ion formation

For geminate radical ion pairs (RIPs) formed by electron transfer quenching of excited 9,10-dicyanoanthracene (DCA) using benzene donors substituted with different bulky groups in several moderately polar solvents, the recombination rates at a variety of separation distances are measured. We used photocurrent techniques together with single photon timing measurements. For the first time, increased recombination rate constants with increasing RIP separation distance are observed. It is concluded that the dominant mechanism for the recombination of solvent separated radical ion pairs (SSRIP) involves a direct return electron transfer from SSRIPs by tunneling, thus by-passing the contact radical ion pairs (CRIP).