A Fluorescent Ditopic Rotaxane Ion-Pair Host.

We report a rotaxane based on a simple urea motif that binds Cl- selectively as a separated ion pair with H+ and reports the anion binding event through a fluorescence switch-on response. The host selectively binds Cl- over more basic anions, which deprotonate the framework, and less basic anions, which bind more weakly. The mechanical bond also imparts size selectivity to the ditopic host.