Unanticipated Emergence of the Photo-Switchable Cs⁺-Selective Ion Channels From One-Dimensional 18-Crown-6 Arrays

A photo-regulated cesium ion channel-the first artificial ion channel of its kind-has been successfully constructed using a small-molecule self-assembly strategy. This breakthrough emerged from an initial effort to design a light-responsive potassium ion channel, incorporating azobenzene-modified 18-crown-6 units, amino acid residues, and side chains. Unexpectedly, the azobenzene-functionalized 18-crown-6 units displays a surprising and previously unrecognized capability for transporting Cs⁺ ions. Among the six channels studied, trans-isomers t-F and t-L exhibit the highest Cs⁺ transport rates. They also demonstrate pronounced ion selectivity, with Cs⁺/K⁺ selectivity ratios of 2.66 and 2.03, and markedly higher Cs⁺/Na⁺ selectivity ratios of 32.1 and 31.5, respectively. Moreover, by alternating exposure to ultraviolet and visible light, the opening and closing of these channels can be effectively and reversibly controlled, resulting in a 4-fold difference in ion transport activity between the trans- and cis-configured channels. This work establishes, for the first time, that the 18-crown-6 motif can be rationally engineered for selective Cs⁺ transport, thereby broadening its potential applications in ion-transport systems and separation technologies.