Effects of substituents on fluorometric detection of cyanide anions by indolium-coumarin dyads.

We synthesized an indolium–coumarin dyad (1) and its derivatives with –Cl (2), –N(CH3)2 (3), or –NO2 (4) substituent, and used them for fluorometric detection of cyanide anions (CN(-)) in aqueous media. All of the dyads exhibit fluorescence enhancement by CN(-) via a nucleophilic interaction of CN(-) with the indolium carbon atoms. Their fluorescence response and selectivity to CN(-), however, depend strongly on the substituents. Ab initio calculation and kinetic analysis were performed to verify the behaviors. Substitution of electron-withdrawing groups (2 and 4) increases the electrophilicity of the indolium carbon. This decreases the activation enthalpy for the nucleophilic interaction with CN(-) and facilitates rapid CN(-) sensing. Compound 4 with very high electrophilicity, however, also promotes nucleophilic interaction with OH(-) in the solution, resulting in decreased CN(-) selectivity. As a result of this, the –Cl-substituted compound 2 containing the indolium carbon with appropriate electrophilicity facilitates rapid (within 1 min) and selective CN(-) detection.