Controlling the Outcome of Electron Transfer Reactions in Ionic Liquids

Abstract

In this article, we investigate the excited state intramolecular electron transfer (ET) reaction of crystal violet lactone (CVL) in the room temperature ionic liquid (RTIL) N-propyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide [Pr(31)(+)][Tf(2)N(-)]. This system was chosen in light of recent experimental observations by Maroncelli and co-workers (J. Phys. Chem. B 2007, 111, 13473), in which the kinetics of electron transfer between S(1) (commonly referred as LE) and S(2) (commonly referred as CT) emission states and, therefore, the ratio of emitting populations were shown to be absorption-wavelength-dependent. Our computational studies indicate that the kinetics of the intramolecular ET between S(1) and S(2) states of CVL in [Pr(31)(+)][Tf(2)N(-)] is local solvent-environment-dependent. Because emission time scales are smaller than solvent relaxation time scales, this behavior is characteristic of RTILs but uncommon in conventional solvents. Therefore, RTILs open a window of opportunity for manipulating the outcome of chemical reactions simply by tunning the initial excitation wavelength. Our studies show that when acetonitrile is used as a solvent instead of [Pr(31)(+)][Tf(2)N(-)] the ratio of populations of emission states is independent of excitation wavelength, eliminating the opportunity for influencing the outcome of reactions.