Stoichiometric-like ion–dipole coordination saturation in an isolated polymer–ion system: Single-molecule force spectroscopy and theoretical insights

Abstract

Ion–dipole interactions are well defined in small-molecule systems, but in macromolecular systems, their behaviors remain insufficiently understood due to structural complexity and binding heterogeneity. Herein, the ion–dipole interactions between a single poly[trifluoropropyl(methyl)siloxane] (PMTFPS) chain and 1-allyl-3-methylimidazolium chloride (AMIMCl) have been systematically investigated using single-molecule force spectroscopy combined with complementary techniques. For the first time, the isolated polymer–ion system is shown to reach a state with stoichiometric-like saturated ion–dipole bridges at an AMIMCl-to-PMTFPS repeating unit molar ratio of 0.33:1, in which one AMIMCl molecule bridges the two terminal trifluoropropyl groups within each three-unit segment. Further increasing the AMIMCl content allows for additional association with the intermediate trifluoropropyl group, but steric and electrostatic constraints prevent the formation of new bridges. The ion–dipole binding energy is calculated to be 9.32 ± 0.11 kJ/mol. These results reveal the distinctive ion–dipole combining pattern of a single polymer chain under molecular confinement. This work is expected to bridge the gap between the well-defined binding behaviors of small-molecule systems and the complex landscape of bulk macromolecular systems, offering mechanistic insights into how ion–dipole interactions can be tuned within macromolecular materials.