Rational Control of Ionic Conduction of Encapsulated Ionic Liquid by Fluorination of Isoreticular Metal-Organic Frameworks

Abstract

Hybrid materials have attracted much attention in solid-state conductors because of their great advantage in various combinations of components. In this study, the control of ionic conductivity of ionic liquid (IL), which is encapsulated in micropores of metal-organic frameworks (MOFs), has been realized by the chemical modification of the ligands of MOFs for the first time. When IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ((EMI)(TFSA)), is encapsulated into the pores with a 50% volumetric filling level, it shows the highest ionic conductivity when encapsulated in the fluorous MOF among the three isoreticular zinc-based microporous MOFs; the ionic diffusivity is 1 order of magnitude higher than that of (EMI)(TFSA)-encapsulated non-fluorous MOF. Computational simulations indicate that the mobility of (EMI)(TFSA) in a non-fluorous framework is significantly restricted due to abundant C-H···F interactions, whereas the introduction of fluorine atoms on the ligands suppresses the C-H···F interactions, thereby showing the highest ionic conductivity. On the other hand, the ion migration of encapsulated (EMI)(TFSA) is not greatly affected by the fluorination of the ligands at a 100% filling level, because the possible different IL distributions with a 50% filling level are largely eliminated for the fully occupied (EMI)(TFSA).