Metal–organic macrocycles with tunable pore microenvironments for selective anion transmembrane transport

Abstract

The development of artificial ion channels is critically important not only for understanding the natural processes but also for many applications. Exploring how to finely tune the pore microenvironments is the key to achieving highly efficient synthetic ion channels. Here we report the self-assembly of a distinct type of four-membered metallacycles with tunable pore microenvironments capable of selective anion transmembrane transport. By elaborately designing three 1,1′-bi-2-naphthol-based ligands, we prepare three cationic Zn₄L₄ metallacycles with sub-nanosized apertures. In this system we can finely tune the pore sizes and binding sites through the deployment of pre-designed ligands with hydroxyl, ethoxyl, and pentaethylene glycol groups. We found that the metallacycle with hydrophilic hydroxyl moieties is unable to transport anions owing to its low lipophilicity, whereas the one bearing pentaethylene glycol units can transport the anions but with low selectivity. In contrast, the analogous metallacycle featuring ethoxyl groups and hydrophobic pore surfaces displays a significantly higher transport efficiency for iodide ions over other monovalent anions, with I⁻/X⁻ selectivities of up to 38. Our finding unambiguously reveals that the transport abilities of self-assembled complexes are highly dependent on their pore microenvironments.