Study of derivatives of 2,6-diaminopyridine as fluorescence probe of transition metal ions

Three derivatives of 2, 6-diaminopyridine were synthesized with naphthyridine ring and -NH2, -OH and -NHCOCHCH2 groups. They are 2,4-dimethyl-7-hydroxyl-1,8-naphthyridine (DMHND), 2,4-Dimethyl-7-amino-1,8-naphthyridine (DMAND) and DMAAN. The binding ability of the derivatives of 2,6-diaminopyridine for metal ions in acetonitrile was investigated using UV-Vis and fluorescence spectrometry. The changes of the dyes spectra were observed in the presence of transition metal ions, but were not observed in the presence of K+, Na+, Mg2+, Ca2+ and Pd2+. Except for DMHND that has weak binding ability for Cu2+, two dyes can bind with Cu2+ at the ratios of DMAAN : Cu2+ = 1 : 1, DMAND : Cu2+ = 1 : 1. The resulting binding curves were analyzed by nonlinear regression methods, giving the association constant for DMAAN to be about 4. 4 X 10(5) L . mol(-1), and about 6. 3 X 10(6) L . mol(-1) for DMAND. DMAND contains an electron-donating group (amino group) conjugated to an electron-withdrawing group (the pyridine ring), and undergoes intramolecular charge transfer from the donor to the acceptor upon excitation by light. When Cu2+ interacting with the N of the pyridine ring enhances the electron-with-drawing character of this group, the spectrum is thus red-shifted and the intensity is increased. DMAND displayed a high sensitivity among the three derivatives of 2,6-diaminopyridine as the fluorescence probe of transition metal ions.