Turn-On Fluorescence Detection of Cyanide in Water: Activation of Latent Fluorophores through Remote Hydrogen Bonds That Mimic Peptide β-Turn Motif

A molecular probe was prepared that selectively responds to cyanide in aqueous solutions by fluorescence enhancement. Using the peptide beta-turn as a structural template, we designed a series of diphenylacetylene derivatives in which the pi-conjugated backbone was functionalized with an aldehyde group to render the molecule nonfluorescent. The N-H center dot center dot center dot O hydrogen bond across the 2,2'-functionalized diphenylacetylene turn motif activates the carbonyl group toward nucleophilic attack, and chemical transformation of this internal quencher site by reaction with CN- elicits a rapid (k = 72 M-1 s(-1)) enhancement in the emission at lambda(max) = 375 nm. Tethering of an ammonium group to the hydrogen bond donor fragment significantly increased both the response kinetics and the intensity of the fluorescence signal. In addition to providing electrostatic attraction toward the CN- ion, this positively charged R-NH3+ fragment can engage in a secondary hydrogen bond to facilitate the formation of the cyanohydrin adduct responsible for the signaling event. The structurally optimized molecular probe 3 responds exclusively to mu M-level cyanide in neutral aqueous solutions, with no interference from other common anions including F- and AcO-.