Solvent-controlled intramolecular electron transfer in ionic liquids

The rates of excited-state intramolecular electron transfer in 9-(4-biphenyl)-10-methylacridinium (BPAc), crystal violet lactone (CVL), and bianthryl have been measured in a variety of ionic liquids using time-correlated single-photon counting. All three of these reactions had previously been studied in conventional dipolar solvents and their reaction rates shown to be controlled by solvation dynamics. The main focus of this work is to ask whether the same relationships between reaction and solvation times already established in dipolar solvents also apply in ionic liquids. In BPAc⁺ where reaction conforms to a simple two-state kinetic scheme and reaction rates are easily measured, the result is a clear "yes". In the case of bianthryl, whose spectra reflect the more complex kinetics of a barrierless process, the answer is also yes. In contrast to other recent studies of bianthryl, the present results demonstrate that the same equality between (integral) reaction times and solvation times observed in conventional solvents also applies in ionic liquids. Finally, the case of CVL is less clear due to the greater uncertainty associated with the data afforded by this weak fluorophore, combined with a lack of data in conventional solvents having large solvation times. But the CVL reaction can also be reasonably interpreted as exhibiting a common behavior in dipolar and ionic solvents.