Photophysical study of a polyoxo ethylene linked naphthalene-based fluorescent chemosensor for Mg2+ and Ca2+ detection

A naphthalene-based bichromophoric fluorescent sensor 2,2'-[oxy-bis(2-oxatetramethyleneoxy)]bis[N-(2-naphthyl)-benzamide)] (1) was synthesized and characterized. Fluorescence decay for 1 in alcoholic solvents in the region of 415-460 nm revealed bi-exponential behavior. The faster component of the decay can be attributed to the formation of dimers. Above 480 nm, besides the dimer, there is also a little excimer formation and this excimer emits at longer wavelengths than the dimer. The observation of the change of the fluorescence emission spectra upon addition of water in EtOH-water mixtures is in line with the formation of water-bridged complexes preventing excimer formation. The sensor shows an increase in fluorescence intensity upon increasing Mg2+ or Ca2+ concentration in EtOH because the formation of the excimer can be hindered upon complexation with Mg2+ or Ca2+ ions. Because of the competition between hydrated metal ions and the water-bridged complex, spectral changes by complexation with Mg2+ or Ca2+ in EtOH-H2O (9 : 1 v/v) are quite different from those in neat ethanol. The ground-state dissociation constant K-d estimated for the complex with Mg2+ or Ca2+ was found to be around 2.0 mM in EtOH-H2O (9 : 1 v/v), which makes it suitable for the measurement of the concentrations of these ions in physiologically relevant concentration ranges.