The dynamics of electron transfer reactions depends strongly on the polarity and dielectric response of the environment. Whereas these reactions are being widely studied in dipolar solvents, much less is known about their dynamics in room temperature ionic liquids (RTILs). Several investigations on bimolecular photoinduced charge separation (CS) in RTILS have reported behaviors departing strongly from those usually observed in dipolar liquids, such as CS faster than diffusion and a bell-shaped dependence of the CS rate constant on the driving force. We have compared the dynamics of bimolecular CS reactions in RTILS and in dipolar solvents of the same viscosity (17 to 130 cP) using femtosecond-resolved fluorescence spectroscopy and found that, because of the high viscosity of the medium, the reactions dynamics is dominated by the static and transient stages of the quenching. Analysis of the data with a theoretical model of diffusion-assisted reaction that takes these effects into account reveals that, for the same viscosity, CS is not faster in RTILs than in dipolar solvents.1 The apparently faster rate is due to a neglect of the static and transient stages of the quenching when applying a simple Stern-Volmer analysis. The same neglect is at the origin of the above-mentioned observation of the Marcus inverted regions in RTILs and viscous media.2Whereas the ionic nature of RTILs seems to have little influence on the CS dynamics, a large effect can be a priori expected on the fate of the ensuing geminate ion pair. Recent results on the ion pair dynamics in RTILs and dipolar solvents of similar viscosity will also be presented.1) Koch, M.; Rosspeintner, A.; Angulo, G.; Vauthey, E., J. Am. Chem. Soc. 2012, 134, 3729.2) Rosspeintner, A.; Koch, M.; Angulo, G.; Vauthey, E., J. Am. Chem. Soc. 2012,  134, 11396

Bimolecular photoinduced electron transfer reaction in room-temperature ionic liquids

248th National Meeting (2014)

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