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

摘要

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.