MOF-Directed Synthesis of Crystalline Ionic Liquid with Enhanced Proton Conduction.

Arranging ionic liquids (ILs) with long-range order can not only enhance their performances in a desired application, but can also help elucidate the vital relationship between structure and properties. However, achieving long-range ordered ILs is still a formidable challenge and no successful example has been reported to date. In this work, we report a feasible strategy to achieve a crystalline IL via coordination self-assembly based reticular chemistry. The first crystalline IL bearing a long-range ordered framework, IL 1 MOF , was prepared by designing an IL bridging ligand and then connecting them with metal clusters. IL 1 MOF possesses a very unique structure, where the IL ligands are arranged on a long-range ordered framework but have a labile ionic center. This structure enables IL 1 MOF to break through the typical limitation where the solid ILs have lower proton conductivity than their counterpart bulk ILs. IL 1 MOF shows 2-4 orders of magnitude higher proton conductivity than its counterpart IL monomer across a wide temperature range. Its conductivity is also an order magnitude higher than the highest reported values for solid protic IL derivatives and MOF-based anhydrous proton conductors at room temperature. Moreover, by confining the IL within ultramicropores (< 1 nm), IL 1 MOF suppresses the liquid-solid phase transition temperatures to lower than -150 o C, allowing it to function with high conductivity in a subzero temperature range. This work is expected to open a toolbox for designing new-generation IL materials for important applications.