RESUMO
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(trifluoromethylsulfonyl)imide ([Py4][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 (-ΔGET) to be varied over a 0.7 V range, leading to electron transfer rates that increase with driving force over the range 1010-1012 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 [Py4][Tf2N] were found to be highly non-exponential functions of time, including both subpicosecond components and components in the 102-103 ps range. Such broadly distributed emission dynamics were not observed in prior studies. Emission decays in [Py4][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.
RESUMO
Ionic liquids are being tested as potential replacements for current electrolytes in energy-related applications. Electron transfer (ET) plays a central role in these applications, making it essential to understand how ET in ionic liquids differs from ET in conventional organic solvents and how these differences affect reaction kinetics. A new intramolecular electron donor-acceptor probe was synthesized by covalently linking the popular photoacceptor coumarin 152 with the donor dimethylaniline to create the dyad "C152-DMA" for potential use in probing dynamical solvent effects in ionic liquids. Molecular dynamics simulations of this dyad show the considerable conformational flexibility of the linker group but over a range of geometries in which the ET rate parameters vary little and should have minimal effect on reaction times >100 ps. Steady-state and time-resolved fluorescence methods show the spectra of C152-DMA to be highly responsive to solvent polarity, with ET rates varying over the range of 108 to 1012 s-1 between nonpolar and high-polarity conventional solvents. The sensitivity to hydrolysis in the presence of acidic impurities limits the dyad's use to ionic liquids of high purity. The results in the few ionic liquids examined here suggest that in addition to solvent polarity, electron transfer in C152-DMA also depends on solvent fluidity or solvation times.
