ELECTROLYTE EFFECTS ON THE ENERGETICS AND DYNAMICS OF INTERMOLECULAR ELECTRON-TRANSFER REACTIONS

Picosecond absorption spectroscopy is used to examine the effects of added electrolytes on the electron transfer reaction between the trans-stilbene radical cation and the radical anion of fumaronitrile in acetonitrile and ethyl acetate solutions formed by photoexcitation of the corresponding ground state complex. The salts studied are LiClO4, NaClO4, and Bu4NClO4. Contrary to previous reports, the yield of ion-pair separation following photolysis of the ground state complex is small. Greater than 90% of the photogenerated ion-pairs undergo back electron transfer. The electron transfer rates are dependent on the particular electrolyte present in solution. Thus, the driving force of the reaction cannot be quantified by any general models which solely consider the concentration of dissolved salt. A model is proposed which parametrizes the energetics of the electron transfer reaction in terms of the photophysical behavior of 3-aminofluoren-9-one in the salt solutions studied. Using this approach, the dependence of the reaction rate on exothermicity is quantitatively determined. For the salt solutions studied, the driving force changes by approximately 15 kcal/mol. Excellent agreement between the experimental data and the predictions of the Marcus inverted region are observed. The influence that added salts have on the outer-sphere reorganization energy and solvent dynamic contributions to the electron transfer rate are also discussed.