Effect of UV Irradiation on Structural Stability and Aggregation Behavior of Imidazolium Ionic Liquids
Li Rao
School of Pharmaceutical Sciences and Innovative Drug Research Center, Chongqing Key Laboratory of Total Synthesis of Natural Products and Innovative Drug Research, Chongqing University, Chongqing 401331, PR China
Weili Wei
School of Pharmaceutical Sciences and Innovative Drug Research Center, Chongqing Key Laboratory of Total Synthesis of Natural Products and Innovative Drug Research, Chongqing University, Chongqing 401331, PR China
Zhining Xia
School of Pharmaceutical Sciences and Innovative Drug Research Center, Chongqing Key Laboratory of Total Synthesis of Natural Products and Innovative Drug Research, Chongqing University, Chongqing 401331, PR China
Feng Li
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, PR China
Fengqing Yang
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, PR China
Kai Zhou *
Analytical and Testing Center, Chongqing University, Chongqing 401331, PR China
*Author to whom correspondence should be addressed.
Abstract
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.
Keywords: Ionic liquids, imidazolium cation, chemical transformation, aggregation