Solvent-Separated Radical Ion Pairs and Free Ion Yields. 1. Effect of Temperature on Free Ion Formation in Solution

Free ion yields from geminate ion pairs formed after photoinduced electron transfer are measured by the transient photocurrent method in three moderately polar solvents. Photoexcited 9,10-dicyanoanthracene (DCA) is used as the electron acceptor and alkyl-substituted benzenes as donors. It was found that, generally, there is no significant change in free ion yield as the temperature is increased. On the basis of a theoretical model developed under Collins−Kimball boundary conditions, several factors are analyzed, including dielectric constants and viscosities of solvents, initial separation distance distribution of geminate ion pairs, and temperature-induced changes in recombination rates. By comparing escape probabilities of geminate ion pairs calculated at different initial formation and recombination/separation distances with measured free ion yields, we show that free ions are mainly produced from the solvent-separated ion pairs that are initially formed after electron transfer quenching of photoexcite...