Fluorescent “Turn-Off” Detection of Fluoride and Cyanide Ions Using Zwitterionic Spirocyclic Meisenheimer Compounds

Stable zwitterionic spirocyclic Meisenheimer compounds were synthesized using a one-step reaction between picric acid and diisopropyl (ZW1) or dicyclohexyl (ZW3) carbodiimide. A solution of these compounds displays intense orange fluorescence upon UV or visible light excitation, which can be quenched or “turned-off” by adding a mole equivalent amount of F(−) or CN(−) ions in acetonitrile. Fluorescence is not quenched in the presence of other ions such as Cl(−), Br(−), I(−), NO(2)(−), NO(3)(−), or H(2)PO(4)(−). These compounds can therefore be utilized as practical colorimetric and fluorescent probes for monitoring the presence of F(−) or CN(−) anions.