Role of Solvents in the Adaptive Binding of Anions by Calix[4]pyrrole

By combining FTIR, ultrafast IR spectroscopy, and density functional theory calculations, we systematically investigate the anion recognition dynamics between calix[4]pyrrole (CP) and the thiocyanate (SCN – ) anion through noncovalent interactions, revealing how solvent polarity and viscosity regulate the structural dynamics of this complexation process. FTIR spectroscopy shows a distinct SCN – vibrational band at ∼2020 cm –1 and solvent-dependent anion recognition hosted by CP. Theoretical calculations reveal a stepwise conformational change in the CP receptor from its 1,3-alternate to a cone geometry upon anion binding in solution, where the transition-state route of the less stable intermediate of partial cone N was clearly identified. Furthermore, ultrafast IR spectroscopy shows that the reorientational dynamics of the bound anions are closely linked to solvent viscosity, rather than binding affinity, providing direct experimental evidence for a solvent-mediated mechanism in anion recognition. This study elucidates the importance of solvent polarity and viscosity in tuning host–guest interactions and proposes a new mechanism of anion recognition for designing adaptive anion receptors with potential applications in sensing, catalysis, and molecular recognition.