Probing Anion - π interactions between fluoroarene and carboxylate anion in aqueous solutions

Despite the much progress in developing π-conjugated fluoroarene moieties based functional materials in which anion - π interactions are commonly involved, it remains challenging to quantitatively characterize the nanomechanical interaction mechanism of these anion - π systems, particularly in aqueous solutions. In this study, we reported the first experimental quantification of the nanomechanics of anion - π interactions between π-conjugated fluoroarene moieties and carboxylate anions in aqueous solutions through direct molecular force measurements, with a special focus on the impact of the anion species, concentration and of the substitution effect of aromatic side group. The results using surface forces apparatus (SFA) and single-molecule force spectroscopy (SMFS) provide complementary evidences to demonstrate that the robust and reversible adhesion measured between the fluoroarene π systems and carboxylate anions was mainly attributed to anion - π interaction. Moreover, their nanomechanical properties were also systematically scrutinized, with the interaction strength being found to be significantly determined by the contact time, the type of fluoroarene systems (PFST > DFST) and the type of anions and ion concentration (HPO₄^2- > CO₃^2- > I^- > Cl^- ≈ NO₃^- > F^-).