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

## TL;DR

This study compares flexible and rigid carbazole structures to show that flexibility improves iodide binding, offering a new approach for designing sensitive chemical sensors.

## Contribution

The work demonstrates that flexible macrocycles enable superior iodide binding through induced-fit mechanisms, providing a novel strategy for supramolecular receptor design.

## Key findings

- Flexible PBG shows 22.78-fold higher fluorescence quenching efficiency for iodide compared to rigid WDG.
- PBG achieves higher iodide binding affinity (1.387 × 10⁵ M⁻¹) via conformational adjustments, while WDG is limited by its rigid structure.
- The study reveals the role of conformational dynamics in enhancing anion recognition in supramolecular systems.

## Abstract

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 PBG exhibits a 22.78-fold higher fluorescence quenching efficiency upon iodide binding compared to the rigid WDG (KPBG/KWDG = 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, PBG (flexible benzene ring) and WDG (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 PBG achieves superior I− binding (KPBG = 1.387 × 105 M−1) through induced-fit conformational adjustments, whereas rigid WDG (KWDG = 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.