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, ¹ H NMR, ¹³ C 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 ⁺ , Mg ²⁺ , Ca ²⁺ , Zn ²⁺ , Cd ²⁺ , Cu ²⁺ , Ni ²⁺ , Ag ⁺ , Fe ³⁺ and Al ³⁺ 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 Al ³⁺ , generating an emissive complex formed by three units of 1 per metal ion. On the other hand compound 3, shows a ratiometric effect with Al ³⁺ 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 Zn ²⁺ and Cu ²⁺ , 0.48 ppm of Ni ²⁺ , 0.061 ppm of Fe ³⁺ and 0.047 ppm of Al ³⁺ . 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 Al ³⁺ 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.