Effect of UV Irradiation on Structural Stability and Aggregation Behavior of Imidazolium Ionic Liquids

Ionic liquids (ILs) have attracted great attention in both industry and academic researches. They usually exposed under ultraviolet light (UV) irradiation when they were used as solvents for photochemical reactions, additives of chromatographic separation medium with UV detection. However, the photostability of ILs still remained unexplored. Herein, the photostability of imidazolium IL was studied. We selected 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) as the model because of its wide applications and remarkable properties. After UV irradiation, with maximal absorption between 250-350 nm was dramatically enhanced. To clarify the underling mechanism, nuclear magnetic resonance, UV-vis spectrum, fluorescence spectum, high performance liquid chromatography equipped diode array detector and electrospray ionization mass spectrometry were intensively applied. The results showed that UV irradiation induced both chemical transformation and physical aggregation of the imidazolium cations. In addition, we found out that only protonated N-methylimidazolium underwent similar chemical transformation as imidazolium cations. This was indicative that all the imidazolium cation-based ILs were chemically and physically unstable under UV irradiation.