Fluorescent anion sensing by bisquinolinium pyridine-2,6-dicarboxamide receptors in water

Dicationic N-methylated at quinolyl moieties derivatives of three isomers of N,N′-bis(quinolyl)pyridine-2,6-dicarboxamide, and respective N-methyl quinolinium benzamides as reference compounds, have been prepared and characterized by crystal structures, spectral and acid–base properties in water. First pKa values of dicarboxamides between 8.1 and 9.3 determined spectrophotometrically are unusually low for amides. Dicarboxamide derivatives of 3- (1) and 6-aminoquinoline (2) undergo efficient fluorescence quenching by halide, acetate, pyrophosphate and nucleotide anions but the derivative of 5-aminoquinoline (3) shows very small quenching effects. The shape of Stern–Volmer plots for dicarboxamides indicates the existence of ground state complexation with anions, which is absent for related benzamides. Association constants, KA, with anions were calculated from analysis of concentration profiles of the quenching effects on the fluorescence of 1 and 2. Quenching by nucleoside triphosphates is much more efficient than by inorganic anions. Efficient binding of even simple inorganic anions by neutral amide N–H donors in water is attributed to high acidity of amides and preorganized rigid structure of the receptors.