Conformational effects on iodide binding: a comparative study of flexible and rigid carbazole macrocyclic analogs

To our knowledge, this work represents one of the earliest comparative studies on the anion-binding behaviors of carbazole-based structural analogs, demonstrating that a flexible macrocycle markedly improves iodide binding affinity via an induced-fit mechanism. The flexible analog <b>PBG</b> exhibits a 22.78-fold higher fluorescence quenching efficiency upon iodide binding compared to the rigid <b>WDG</b> (<i>K</i> <b>_PBG</b> /<i>K</i> <b>_WDG</b> = 22.78), demonstrating its potential as a highly sensitive optical probe and offering a novel strategy for engineering dynamic supramolecular receptors. Two carbazole-based macrocyclic probes, <b>PBG</b> (flexible benzene ring) and <b>WDG</b> (rigid fluorene backbone), were synthesized via Friedel-Crafts reactions. Their iodide (I^-) recognition properties were systematically explored using ¹H NMR, UV-vis absorption, and fluorescence spectroscopy. Quantitative analysis via the Benesi-Hildebrand equation and nonlinear fitting demonstrated that flexible <b>PBG</b> achieves superior I^- binding (<i>K</i> <b>_PBG</b> = 1.387 × 10⁵ M^-1) through induced-fit conformational adjustments, whereas rigid <b>WDG</b> (<i>K</i> <b>_WDG</b> = 6.089 × 10³ M^-1) is constrained by preorganized cavity geometry, adhering to a conformational selection mechanism. This work elucidates the synergistic interplay between conformational dynamics and localized structural adaptations governing anion recognition. The findings advance the rational design of tunable, high-affinity anion receptors and deepen the understanding of conformational regulation in supramolecular systems.