Femtosecond UV Excitation in Imidazolium-Based Ionic Liquids

Femtosecond pump-probe absorption spectroscopy was employed to investigate ultrafast dynamics in various room temperature ionic liquids (RTILs) based on imidazolium cations, i.e., 1,3-dimethylimidazolium iodide ([DMIM]I), 1-butyl-3-methylimidazolium iodide ([BMIM]I), 1-hexyl-3-methylimidazolium iodide ([HMIM]I), 1-hexyl-3-methylimidazolium chloride ([HMIM]CI), and 1-methyl-3-octylimidazolium chloride ([MOIM]CI). Immediately after photoexcitation, an induced absorption was observed at various probe wavelengths (555-1556 nm). Afterward, the decay of the induced absorption was found to be independent of the alkyl chain length and viscosity of the ionic liquids. Two alternative mechanisms were proposed to explain the dynamics. In a first scenario excess electrons are generated through one-photon photodetachment of halides analogous to aqueous halide photodetachment. The dynamics in this case were analyzed with the help of a competing kinetic model proposed for geminate recombination in aqueous chloride photodetachment. Alternatively, imidazolium cations may be subject to photoionization. The transient NIR absorption can then be assigned to imidazolium dimer radical cations and/or excess electrons which may be formed upon association of imidazolium radicals with their parent cations. Both scenarios suggest that a thorough explanation of the ultrafast dynamics probably requires the implication of cooperative effects in the ionic liquids upon photoexcitation.