Stoichiometric-like ion-dipole coordination saturation in an isolated polymer-ion system: Single-molecule force...

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.