Sensitive water-soluble fluorescent chemosensor for chloride based on a bisquinolinium pyridine-dicarboxamide compound

Abstract

The new fluorescent dicationic compound, N,N′-bis(N-benzyl quinolinium)pyridine-2,6-dicarboxamide triflate 1, was synthesized and studied as a chemosensor for inorganic anions in water. At pH 5.0 chloride quenches the blue fluorescence of 1 (the Stern–Volmer constant KSV=380) and forms a very stable complex with receptor (KA=5050M−1). A photoinduced electron transfer quenching mechanism with simultaneous receptor-chloride complexation both in the excited and the ground state is proposed. On basis of 1H NMR and UV–vis titrations, ESI-MS experiments, crystal structure and DFT calculations, the binding mode of Cl− is proposed involving two hydrogen bonding interactions N–H···Cl− with simultaneous formation of two short C–H···Cl− contacts inside the cleft of 1 formed by pyridine-dicarboxamide motif; these interactions are favored by complementarity of the cleft to chloride anion. The efficient binding is attributed to strong acidification of NH and CH donors by quaternization of quinolyl groups. The receptor 1 allows the fluorescence detection of chloride in water with a detection limit of 33μmolL−1 and good selectivity over other common anions such as bromide, phosphate, pyrophosphate and acetate.