Mononuclear aluminum complex derived from 1,1,1,1-tetrakis[(2-salicylaldiminomethyl)]methane acting as a zinc sensor: Crystal structure, emission and lifetime studies

The mononuclear complex [AlHL] (1) has been prepared using AlCl3, H4L and triethylamine in a 1:1:3 ratio (H4L = 1,1,1,1-tetrakis[(2-salicylaldiminomethyl)]methane). [Al(HL)] crystallizes in the monoclinic space group P2(1)/c. Although H4L does not emit at room temperature, a remarkable CHEF (Chelation Enhancement of Fluorescence Emission) effect is observed upon the formation of the complex [Al(HL)] (1). When excited at a wavelength of 350 nm, complex 1 emits at 433 nm. Blue emitting aluminum complexes find special importance as far as OLEDs are concerned and complex 1 is one of the best blue emitting compounds to the best of our knowledge. Emission titration of a solution of complex 1 in acetonitrile with Zn2+, Cd2+, Hg2+ and Pb2+ shows selectivity towards the Zn2+ ion. Unlike Cd2+, Hg2+ and Pb2+, which quench the emission, Zn2+ enhances the fluorescence intensity by 1.64-folds. The quenching process follows the Stern–Volmer equation and in order to get an insight of whether the quenching is static or dynamic in nature, lifetime measurements were carried out. Interestingly, mercury was found to follow static quenching, while cadmium, lead and nickel adopted dynamic quenching pathways.

Graphical abstractThe fluorescence intensity of the aluminium complex derived from the tetrapodal ligand 1,1,1,1-tetrakis[(2-salicylaldiminomethyl)]methane increases in the presence of Zn2+, but is quenched by all other M2+ metal ions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Chemistry of a tetrapodal ligand. ► A blue emitting aluminium complex. ► Fluorescence. ► Zinc sensor. ► Lifetime.