Metal ion complexation in acetonitrile by upper-rim benzyl-substituted, di-ionized calix[4]arenes bearing two dansyl fluorophores

The influence of Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, Ag+, Cd2+, Co2+, Fe2+, Hg2+, Mn2+, Pb2+, Zn2+ and Fe3+ on the spectroscopic properties of two dansyl (1-dimethylaminonaphthalene-5-sulfonyl) groups linked to the lower rims of a series of three, structurally related, di-ionized calix[4]arenes is investigated by means of emission spectrophotometry. Di(tetramethylammonium) salts of the di-ionized ligands, L, L1 and L2, which differ in having no, two and four benzyl groups, respectively, on the upper rim of the calix[4]arene scaffold, are utilized for the spectrofluorimetric titration experiments in MeCN. On complexation by alkaline earth metal cations, the emission spectra undergo marked red shifts. Alkali metal cations cause fluorescence enhancement of benzyl-substituted di-ionized calix[4]arenes L1 and L2 with red shifts, except for Li+ and Cs+. Transition metal cations and Pb2+ interact strongly with the ligands. In particular, Fe3+, Hg2+ and Pb2+ cause greater than 99 % quenching of the dansyl fluorescence for all three ligands.