Electron Transfer Kinetics between an Electron-Accepting Ionic Liquid and Coumarin Dyes

Study of electron transfer in ionic liquids is of interest for what it may reveal about the effects of solvent dynamics on electron transfer as well as for helping to inform current efforts to employ ionic liquids as electrolytes in energy-related applications. The present report describes time-resolved fluorescence quenching measurements of electron transfer between electronically excited 7-aminocoumarin dyes and a redox-active pyridinium ionic liquid, 1-butylpyridinium bis(trifluoromethylsulfonypimide ([Py-4][Tf2N]). Comparable measurements of fluorescence quenching in conventional dipolar solvents were made over 20 years ago, primarily in aromatic amine liquids. Like these prior experiments, use of commercially available coumarin dyes allowed the driving force for electron transfer (-Delta G(ET)) to be varied over a 0.7 V range, leading to electron transfer rates that increase with driving force over the range 10(10)-10(12) s(-1). These rates are similar to rates previously measured in aromatic amine solvents, despite the much greater polarity of the ionic liquid, which increases the driving force by more than 0.5 eV. Fluorescence decays of most of the fluorophores in [Py-4][Tf2N] were found to be highly non-exponential functions of time, including both subpicosecond components and components in the 10(2)-10(3) ps range. Such broadly distributed emission dynamics were not observed in prior studies. Emission decays in [Py-4][Tf2N] resemble the broadly distributed solvation response characteristic of ionic liquids, suggesting that solvent motions may control the rate of electron transfer, at least in the more slowly reacting dyes. This similarity could be interpreted either in terms of solvent motions being responsible for varying the energy gap or the electronic coupling between the reactant and product states.