Counter-Anion Tuning of Aqueous Carboxylate Recognition by Imidazolium Poly(Ionic Liquids)

Abstract

Achieving strong and general anion recognition in pure water remains a major unresolved challenge in supramolecular chemistry due to intense competitive solvation. Here we demonstrate that counteranion identity can be used as a tunable design element to control aqueous carboxylate binding in poly(ionic liquid) receptors. A benzyl methyl imidazolium polymer scaffold was prepared and systematically converted into Cl–, Br–, I–, PF6–, NTf2–, TFA–, and Ac– forms, enabling direct evaluation of counteranion effects under identical structural conditions. The acetate-functionalized polymer displays markedly enhanced binding affinity and capacity in 90 to 100% water, with apparent association constants spanning 103–106 M–1 across structurally diverse carboxylates. Spectroscopic analysis (1H and 19F NMR) and quantitative ion-release assays establish a stoichiometric counterion displacement mechanism that drives analyte capture. Benchmarking against pyridine-, pyridinium-, and trimethylammonium-based analogues confirms that imidazolium–acetate pairing uniquely amplifies aqueous binding performance. These results identify counteranion tuning as a powerful and generalizable strategy for engineering high-affinity polymeric receptors for polar analytes in water.