Insights into structure-property relationships in ionic liquids using cyclic perfluoroalkylsulfonylimides

Area of Science:

• Materials Science
• Electrochemistry
• Physical Chemistry

Background:

• Ionic liquids (ILs) are salts with low melting points, offering tunable properties for various applications.
• Cyclic sulfonimide anions present a novel structural motif for IL design, potentially altering physicochemical characteristics.
• Understanding structure-property relationships in ILs is crucial for optimizing their performance.

Purpose of the Study:

• To synthesize and characterize room temperature ionic liquids (ILs) featuring cyclic sulfonimide anions (ncPFSI, ring size n = 4-6).
• To investigate the influence of cyclic sulfonimide anion ring size on the physicochemical properties of ILs.
• To elucidate the ion diffusion mechanisms and structural features of these novel ILs.

Main Methods:

• Synthesis of ILs with [EMIm]⁺, [BMIm]⁺, and [BMPL]⁺ cations and ncPFSI anions.
• Solid-state structure determination via single-crystal X-ray diffraction.
• Physicochemical property assessment: thermal behavior, stability, viscosity, conductivity.
• Ion diffusion studies using pulsed field gradient stimulated echo (PFGSTE) NMR spectroscopy.
• Molecular dynamics (MD) simulations for property comparison and interaction analysis.

Main Results:

• The ring size of cyclic sulfonimide anions decisively influences IL properties, differentiating them from non-cyclic TFSI anions.
• The rigidity of cyclic anions, particularly the 6-membered 6cPFSI, leads to distinct thermal and conductive behaviors.
• MD simulations and X-ray diffraction reveal significant π⁺-π⁺ interactions between [EMIm]⁺ cations in both liquid and solid states.

Conclusions:

• Cyclic sulfonimide anions offer a pathway to tune IL properties through conformational locking and ring size variation.
• The observed differences in properties are directly linked to the rigidity and structural constraints imposed by the cyclic anions.
• π⁺-π⁺ cation-cation interactions play a crucial role in the structure and behavior of imidazolium-based ILs.