New emissive dopamine derivatives as fluorescent chemosensors for metal ions: A CHEF effect for Al(III) interaction

Three new Schiff-base ligands 1-3 derivatives from neurotransmitter dopamine, bearing an indazole (1), methylimidazole (2) or hydroxynaphthaldehyde (3) units were successfully synthesized and characterized by elemental analysis, H-1 NMR, C-13 NMR, IR (infrared), ESI-TOF (electrospray ionization-time of flight-mass), UV-Vis absorption and emission spectroscopy. The interaction with the metal ions Li+, Na+, Mg2+, Ca2+, Zn2+, Cd2+, Cu2+, Ni2+, Ag+, Fe3+ and Al3+ has been explored by absorption and fluorescence emission spectroscopy. Compound 3 shows to be the most emissive one, with a quantum yield of 0.022, followed by compound 2 (phi = 0.009) and 1 (phi = 0.001). Compound 1 reveals quite selective for Al3+, generating an emissive complex formed by three units of 1 per metal ion. On the other hand compound 3, shows a ratiometric effect with Al3+ appearing as a new emission band centred at 530 nm. With the other transition and post-transition metal ions, a quenching in the emission was observed, being 3 the most sensitive one, being quenched by 0.34 ppm of Zn2+ and Cu2+, 0.48 ppm of Ni2+, 0.061 ppm of Fe3+ and 0.047 ppm of Al3+. In most cases a stoichiometry of three ligands by one metal ion was postulated. In order to study the fluorescence properties in the solid state, the Al3+ metal complexes of 1 (4) and 3 (5) were synthesized as new emissive materials. In both cases a very strong emissive systems were obtained, with a quantum yield of 0.01 and 0.04 for 4 and 5. (C) 2011 Elsevier B. V. All rights reserved.