Anion-Induced Shuttling of Macrocycles in Pyrene- and Anthracene-Stoppered 2Rotaxane-Based Molecular Switches: X23C7 Shuttling-Induced Superior Fluorescence Enhancement

Pyrene- and anthracene-stoppered bistable 2rotaxanes, comprising 23- and 24crown ethers, were obtained in high yield by a click reaction. All four 2rotaxanes exhibited anion-induced shuttling of crown ethers from the ammonium to the methyl triazolium moiety, accompanied by sharp fluorescence enhancements in an acetonitrile/water (v/v = 99:1) medium. The magnitude of fluorescence enhancement, however, has been much greater in the case of shuttling for 23crown ether (X23C7) than for the 24crown ether (DB24C8) in both pyrene- and anthracene-stoppered bistable 2rotaxanes. Anions such as cyanide, fluoride, acetate, and sulfate affected the switching in acetonitrile/water (v/v = 99:1) medium, while only the cyanide anion induced switching in a more hydrated environment, such as acetonitrile/water (v/v = 92:8), where, yet again, greater fluorescence enhancement was observed upon X23C7 shuttling. Reversibility with trifluoroacetic acid was maneuvered toward reproducibility of the fluorescence output and consequent recyclability of these 2rotaxanes. An impressive nanomolar limit of detection (LOD) for the cyanide anion was observed with all the 2rotaxanes. Portable cyanide anion investigation with a fixed concentration of anthracene-terminated 2rotaxanes enabled a naked eye "turn-on" response only when incorporated with the X23C7 macrocycle, thereby manifesting a visually discernible difference with respect to the closely related DB24C8 macrocycle.