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) pyridine2,6- 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 pK(a) 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, K-A, 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.