Colorimetric Fluoride‐Anion Sensor Based on Intramolecular Hydrogen Bonding and Enol–Keto Tautomerization of a Phenothiazine Derivative

Abstract A colorimetric sensor for fluoride ions based on a new sensing mechanism is reported. The colorimetric sensor contains an isomerizable enol–keto moiety as the recognition site and phenothiazine as chromogenic center. A color change visible to the naked eye is observed upon addition of fluoride ions to the solution of sensor 1 in aprotic solvents such as CHCl3 and MeCN. The sensor shows no colorimetric response for other halide ions. Enol‐keto tautomerization is proposed to be responsible for the anion sensing of 1, based on UV/VIS absorption, 1H‐NMR, and single‐crystal structure analysis.