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, 1H NMR, 13C 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 (ϕ = 0.009) and 1 (ϕ = 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.