Zwitterions with Fluorinated Anions: Design Principles for Tuning Phase Behavior

Abstract Zwitterions (ZIs) have attracted interest across electrolyte, separation, and materials applications. Unlike ionic liquids (ILs), zwitterions have both charges covalently tethered to a single backbone, a constraint that removes translational entropy from melting and allows systematic study of how molecular design controls thermal behavior. Many conventional ZIs based on carboxylate- and sulfonate anions suffer from high melting points (often >250 °C) and thermal decomposition before melting. We report the synthesis, thermal characterization, structural investigation, and computational analyses of 12 fluorinated ZIs incorporating organotrifluoroborate (−BF3–) and sulfonyl(trifluoromethanesulfonyl)imide (−TfN–) anionic motifs, covalently tethered via alkyl spacers to six aromatic and nonaromatic N-heterocyclic cationic headgroups. The BF3–-bearing ZIs are reported here for the first time; the TfN–-bearing ZIs are synthesized via the Sulfur(VI) Fluoride Exchange (SuFEx) click chemistry route with higher efficiency. The BF3–-type ZIs melt at 111.5–249.7 °C, and TfN–-type ZIs melt at 77.4–251.8 °C. Notably, all ZIs exhibit substantially elevated melting points relative to their IL analogues, highlighting the strong influence of the loss of charge delocalization on solid-state packing. Thermodynamic analysis of ΔHfus and ΔSfus establishes that cation structure and spacer length dictate the dominant melting mechanism across both ZI series. All 12 ZIs dissolve readily in water, whereas the corresponding imidazolium ILs bearing analogous fluorinated anions are water-immiscible at comparable chain lengths. Single-crystal X-ray diffraction of four ZIs shows that C–H···F and C–H···O interactions, anchored by the imidazolium C2–H, template the crystal packing and correlate with enthalpy-driven melting in the imidazolium ZIs. Computational analysis indicates that headgroup identity and conformational rigidity modulate the conformational landscape, governing the entropy gain on melting. Collectively, thermal, thermodynamic, crystallographic, and computational analyses establish structure–melting point correlations across these fluorinated zwitterionic compounds, providing a quantitative framework for the rational design of this class of soft matter.