[23]Crown Ether Incorporating Metastable [2]Rotaxanes and Pseudo[2]rotaxanes: Selective and Sensitive Fluoride Anion Detection with 4‐Isopropylphenyl Stoppered [2]Rotaxane

Two metastable [2]rotaxanes incorporating [23]crown-7-ether, with cyclohexyl and 4-isopropylphenyl slippage stoppers, are explored. The axle component of metastable rotaxanes is comprised of a secondary ammonium station and an anthracene fluorophore. In the presence of fluoride anion, the deslippage of [23]crown-7-ether over the slippage stoppers is induced by ion-pair formation with fluoride anion, followed by neutralization. Both the rotaxanes displayed highly selective fluoride anion responsiveness, however, 4-isopropylphenyl being less bulky, exhibited more sensitive fluoride anion detection in acetonitrile-water medium. The addition of excess TBACl, TBABr, and TBAI did not invoke any switching response in acetonitrile-water medium (v/v = 99:1, 98:2, and 97:3). Furthermore, the metastable [2]rotaxane with a 4-isopropylphenyl slippage stopper could successfully detect ionic fluoride anions such as potassium fluoride and cesium fluoride well below the W.H.O. permissible limit. In addition, [23]crown-7-ether generated pseudo[2]rotaxane complexes with a series of ammonium axles comprised of variable terminal groups.