Interactions and Transitions in Imidazolium Cation Based Ionic Liquids

Abstract

Ionic liquids (ILs) raise considerable research interest not only as promising new solvents for the replacement of conventional solvents in synthesis, but also as new liquid materials.[1-2] It appears from the recent breakthroughs that the novel properties of various ILs are of more interest than mere application as ‘green solvent’ in traditional organic chemistry. ILs are now rediscovered to be whole new materials with many wonderful properties, much of it are yet to be discovered. The defining characteristic of ILs is of their constitutions molecular ions as their building blocks as opposed to molecules in the traditional solvents. In other words, ILs or molten salts in general are defined as liquids composed of ions only, either at room temperature or at elevated temperatures (below 100C). ILs are rather unique in the sense that in addition to ionic and covalent interactions, there are relatively weaker interactions such as H-bondings, and π-stacking, which are not commonly found in conventional solvents.[3-4] The nature of the forces in different ILs may however differ from one another and mainly control their physical properties. As the properties of any material depends on the structure of molecules in different phases, it is very important to understand the structural features of ILs in depth. Researchers have paid considerable attention towards pyridinium, imidazolium and pyrrolidinium based ILs in addition to ammonium and phosphonium ILs. Pyridinium based ILs are heterocyclic aromatic compounds proven to have great potential in organic synthesis and biocatalyst. Compared with the imidazolium based ILs, few studies have examined the biodegradability of pyridinium based ones.[5] Due to biodegradable property of these pyridinium and pyrrolidinium ILs, it has been extensively studied.[6-12] On the other hand, pyrrolidinium ILs are mainly involved in dye sensitized solar cell and batteries.[ 11] Since structure plays the vital role for any application, recently detailed x-ray scattering studies have been reported on the pyrrolidinium cations based ILs while varying the length of the alkyl chain attached with the ring.[12] Interestingly, diffraction pattern shows signature of intermediate range ordering similar to that of in imidazolium based ILs. Among all ionic liquids, imidazolium cation based ILs are the most extensively studied ILs, and therefore our discussion will mainly be confined with imidazolium cation based ILs. As of true ‘designer solvent’, it has been observed that a small variation in imidazolium cation (such as increase or decrease in alkyl chain length) alters their physical properties