Long Distance Photoinduced Electron Transfer in Solutions: A Mechanism for Producing Large Yields of Free Ions by Electron Transfer Quenching

Abstract

Free ion and fluorescence quantum yields from the geminate ion pairs formed by electron-transfer quenching of the excited acceptor 9,10-dicyanoanthracene by aromatic electron-donors are measured in different solvents. Theeffects of solvent polarity, energetic driving force, and steric substitution on free ion yields are studied. From the comparison of the effect of driving force on free ion yields and fluorescence quantum yields in different solvents, especially those of moderate polarity, it is concluded that free ion yields are controlled by both recombination rate constants and the separation distance distribution of the initially formed geminate radical ion pairs. In dichloromethane (DCM), the recombination and free ion formation processes are directly observed by fluorescence lifetime and transient photocurrent measurements. The time-resolved results indicate that free ion formation is faster than the recombination process. This means that the geminate radical ion pairs that form free ions and those that are neutralized by electron-transfer recombination have different histories with different separation distances. From studies of steric effects on free ion yields in different solvents, it is concluded that, in polar solvents such as acetonitrile and butyronitrile, the main effect of steric hindrance is to decrease the recombination rate constant and increase the escape probability, whereas in moderately polar solvents such as tetrahydrofuran, DCM and 1,2-dichlorobenzene, the main effect of steric bulk is to change the initial separation distance distribution of the geminate radical ion pairs formed by electron-transfer quenching. As an example, we compare donors such as durene (DUR) with those of greater steric bulk like 1,2,4,5-tetra-iso-propylbenzene (TIPB), for which the driving force for electron transfer is similar. The free radical ion yield for TIPB is more than 40 times greater than it is for DUR in DCM. This is the first example from our work in which the infinite rate boundary condition for ion recombination used by Onsager is not adequate, because there is no perfect sink at the origin. The free ion yield data are analyzed by the theory of Hong and Noolandi under the Collins and Kimball boundary condition.