Optical discrimination of fluoride and cyanide ions by coumarin-salicylidene based chromofluorescent probes in organic and aqueous medium

Two chromofluorescent coumarin-functionalized salicylidene based probes (3a and 3b) have been synthesized and evaluated for selective detection of fluoride and cyanide ions. Probes 3a and 3b selectively detect fluoride ions in acetonitrile medium via H-bond interaction and subsequent deprotonation to elicit a distinct visual colour change from colourless to deep red with a significant enhancement in their emission intensity to “turn on” a yellow fluorescence. In addition, probe 3a could optically discriminate the presence of cyanide ions over other anions by a colour change from colourless to deep yellow with an enhancement of green fluorescence in aqueous-acetonitrile medium (1:1 v/v). Job’s plot experiments revealed a 1:2 binding stoichiometry between the probes and fluoride or cyanide ions. A detailed analysis of the binding characteristics of probe 3a with fluoride and cyanide ions have been further carried out by 1H NMR titration studies which indicates a good correlation between colorimetric, UV–vis and 1H NMR observations. These experimental results were further corroborated from the theoretical models using quantum chemical calculations.