Synthesis of Dicyanovinyl-Substituted 1-(2-Pyridyl)pyrazoles: Design of a Fluorescent Chemosensor for Selective Recognition of Cyanide.

A fluorescence "turn-off" probe has been designed and successfully applied to detect cyanide (CN-) based on a Michael-type nucleophilic addition reaction and intramolecular charge transfer (ICT) mechanism. For this research, a family of 3-aryl-4-(2,2-dicyanovinyl)-1-(2-pyridinyl)pyrazoles as donor-π-acceptor (D-π-A) systems have been synthesized in 58-66% overall yield, by a three-step synthesis sequence starting from p-substituted acetophenones. The substituted p-methoxyphenyl showed good fluorescence emission and large Stokes shifts in different solvents due to its greater ICT. Likewise, this probe evidenced high selectivity and sensitivity and fast recognition for CN- with a detection limit of 6.8 μM. HRMS analysis, 1H NMR titration experiments, and TD-DFT calculations were performed to confirm the mechanism of detection and fluorescence properties of the chemodosimeter of CN-. Additionally, fluorescent test paper was conveniently used to detect cyanide in aqueous solution.