Novel asymmetrical phenothiazine for fluorescent detection of cyanide anions

Abstract

Novel asymmetrical phenothiazine (PTZ) bearing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile (OIM) moiety as the cyanide receptor was synthesized. The sensor fluorophore displayed an intramolecular charge transfer (ICT) electronic absorption peak at 500 nm and fluorescence peak at 576 nm in CH3CN:H2O (90:10) solution. When running a titration of the prepared phenothiazine fluorophore with a range of concentrations of cyanide, the electronic absorption and fluorescence peaks diminish in a ratio-metric mode. This can be attributed to the nucleophilic Michael attack of cyanide anion at the α-position of the oxoindenylidenemalononitrile vinyl moiety of the fluorophore. This results in switching such electron acceptor to anionic electron donor and thus breaking the conjugated molecular system, which eventually influence the ICT and the emission peak position of the fluorophore. It was found that the fluorescent sensor displayed high sensitivity with a rapid responding time as low as than 50 s. A high selectivity was also detected toward cyanide ion at a recognition limit as low as 3.2 × 10−9 mol L−1.