How transparent are the imidazolium ionic liquids? A case study with 1-methyl-3-butylimidazolium hexafluorophosphate, [bmim][PF6]

Imidazolium ionic liquids are generally regarded as optically transparent materials. Herein, we report on the optical behaviour of a typical imidazolium ionic liquid that not only suggests that [bmim][PF6] does have significant absorption in the UV region, but it also brings into light a very interesting fluorescence behaviour. The fluorescence peak position of [bmim][PF6] is found to be very strongly dependent on the excitation wavelength. The unusual fluorescence response of [bmim][PF6] is attributed to the presence of several energetically different associated forms of the imidazolium ions in the ionic liquid.