Hydrogen sulfide (H 2 S) plays a crucial signalling role in a variety of physiological systems, existing as the hydrosulfide anion (HS À ) at physiological pH. Combining the potency of halogen bonding (XB) for anion recognition in water with coumarin fluorophore incorporation in acyclic host structural design, the first XB receptors to bind and, more importantly, sense the hydrosulfide anion in pure water in a reversible chemosensing fashion are demonstrated. The XB receptors exhibit characteristic selective quenching of fluorescence upon binding to HS À . Computational DFT and molecular dynamics simulations in water corroborate the experimental anion binding observations, revealing the mode and nature of HS À recognition by the XB receptors.

Hydrogen sulfide (H 2 S) plays a crucial signalling role in a variety of physiological systems, existing as the hydrosulfide anion (HS À ) at physiological pH. Combining the potency of halogen bonding (XB) for anion recognition in water with coumarin fluorophore incorporation in acyclic host structural design, the first XB receptors to bind and, more importantly, sense the hydrosulfide anion in pure water in a reversible chemosensing fashion are demonstrated. The XB receptors exhibit characteristic selective quenching of fluorescence upon binding to HS À . Computational DFT and molecular dynamics simulations in water corroborate the experimental anion binding observations, revealing the mode and nature of HS À recognition by the XB receptors.