Ratiometric Fluorescent/Colorimetric Cyanide-Selective Sensor Based on Excited-State Intramolecular Charge Transfer−Excited-State Intramolecular Proton Transfer Switching

A novel salicylideneaniline-based fluorescent sensor, SB1, with a unique excited-state intramolecular charge transfer–excited-state intramolecular proton transfer (ESICT–ESIPT) coupled system was synthesized and demonstrated to fluorescently sense CN^– with specific selectivity and high sensitivity in aqueous media based on ESICT–ESIPT switching. A large blue shift (96 nm) was also observed in the absorption spectra in response to CN^–. The bleaching of the color could be clearly observed by the naked eye. Moreover, SB1-based test strips were easily fabricated and low-cost, and could be used in practical and efficient CN^– test kits. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations further support the cyanide-induced ESICT–ESIPT switching mechanism. The results provide the proof of concept that the colorimetric and ratiometric fluorescent cyanide-selective chemodosimeter can be created based on an ESICT–ESIPT coupled system.