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 fluorescent switch-on response. The host selectively binds Cl- over more basic anions, which deprotonate the framework and less basic anions that bind more weakly. The mechanical bond imparts size selectivity which alters the relative preferences for larger anions.