A photoswitchable macrocyclic ion-pair receptor

Abstract

Achieving photoresponsive heteroditopic receptors capable of simultaneously binding and releasing ion-pairs under light irradiation remains a significant challenge, due to the difficulty in incorporating two distinct types of binding sites within a single cavity. Herein, we report the design and synthesis of a convergent heteroditopic azo-macrocycle that functions as a photoswitchable ion-pair receptor. This macrocycle incorporates an integrated cavity with amide-based cation- and anion-binding sites, bridged by photoisomerizable azobenzene units. Structural characterization, NMR titration studies, and DFT calculations reveal that the trans-isomer exhibits strong, cooperative binding for LiCl, with significant enhancement in affinity compared to individual ions. Upon UV-induced isomerization to the Z-form, binding is substantially weakened, enabling reversible capture and release of the ion-pair. Crucially, the receptor demonstrates highly selective solid–liquid extraction of LiCl from mixtures of alkali metal chlorides in a light-controlled manner. This work provides a strategic design for photoregulated ion-pair receptors and advances the development of smart extraction systems for lithium resource recovery.