Targeted modifications: conformational flexibility and design of ionic liquids

The versatility of ionic liquids is an extraordinary challenge and chance for the future of chemistry. To efficiently harness this huge potential, our understanding of ionic liquids must grow to a point where application-oriented design becomes possible. However, structure-property relationships and thus design elements are often obscured by confounding variables. Here we show how useful design elements can be extracted with a systematic approach combining theory and experiment. To this end, we used targeted modifications of ionic liquids to isolate the effect of one variable at a time, with a focus on the influence of ion conformational space. Preparatory ab initio simulations on a large number of cations and ions revealed excellent model systems which were then synthesised and characterised experimentally. Physicochemical measurements and experimental crystal structures were in excellent agreement with ab initio results, thus building a consistent picture of the conformational space of ions in ionic liquids. In particular, conformational space emerged as a key design element for macroscopic properties, with an impact comparable to fluorination and ion volume. The link between conformational flexibility on a molecular level and the macroscopic transport properties was further investigated by means of classical molecular dynamics simulations. Additional experimental studies on uncharged analogues of ionic liquids revealed the need for a dual approach to the design of ionic liquids as well as an upper limit to such simple design elements. Thus, this work not only provides a set of specific design elements, but also a versatile strategy for the design of ionic liquids in general.