Photoreactions of fluorenone with electron-rich alkenes: radical ion pair formation in polar and non-polar solvents

Abstract

Flash photolysis studies showed that triplet fluorenone reacts with electron-rich olefins like tetramethyl-1,3-dioxole and tetraethoxyethene to give radical ions and radical ion pairs. The fraction of contact ion pairs and solvent-separated ion pairs was determined in various solvents. Ion pair formation was detected even in non-polar solvents like benzene and cyclohexane, although the formation of solvent-separated ion pairs is endothermic according to Weller and Mataga. The application of the special salt effect confirms our observations of an equilibrium between contact ion pairs and solvent-separated ion pairs. The same technique was used to show that excited fluorenone reacts with triethylamine to give both solvent-separated ion pairs and contact ion pairs in polar solvents or only contact ion pairs in non-polar solvents.