Study of Imidazolium Ionic Liquids: Temperature-dependent Fluorescence and Molecular Dynamics Simulation

The steady-state fluorescence spectra and molecular dynamics simulations were explored to investigate the temperature dependent organization in some imidazolium ionic liquids: 1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]), 1-ethyl-3-methylimidazolium ethylsulfate([emim][EtSO4]) and 1-butyl-3-methylimidazolium tetrafluoroborate([bmim][BF4]). The pure room temperature ionic liquids(ILs) exhibit a large red shift at more than an excitation wavelength of around 340 nm, which demonstrates the heterogeneous nature of the liquids. Furthermore, the fluorescence spectra of the ionic liquids were found to be temperature-dependent. The emission intensity gradually decreased with increasing temperature for the neat ionic liquids and the mixed solutions of [bmim][BF4]-H2O, which was the special phenomena induced by not only the local structure but also the viscosity. The molecular dynamics simulation further confirms that the structures of ionic liquids are sensitive to the surrounding environment because of the aggregation degree of ILs.