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

Pyrene- and anthracene-stoppered bistable [2]rotaxanes, comprising [23]- and [24]crown ethers, were obtained in high yield by a click reaction. All four [2]rotaxanes 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 [23]crown ether (X23C7) than for the [24]crown ether (DB24C8) in both pyrene- and anthracene-stoppered bistable [2]rotaxanes. 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 [2]rotaxanes. An impressive nanomolar limit of detection (LOD) for the cyanide anion was observed with all the [2]rotaxanes. Portable cyanide anion investigation with a fixed concentration of anthracene-terminated [2]rotaxanes 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.