Zinc(ii) ion detection and fluorescence emission properties of a diaminomaleonitrile-derived unsymmetrical Schiff base ligand

Abstract

A novel Schiff base ligand, 2-amino-3-(((Z)-2,5-dihydroxybenzylidene)amino)maleonitrile hemihydrate (C₁₁H₈N₄O₂·½H₂O or H₂L), was synthesized by a condensation reaction of diaminomaleonitrile and 2,5-dihydroxybenzaldehyde in ethanol. Its crystal structure, fluorescence (FL) emission and Zn(II)-detection properties were studied. The crystal structure of H₂L belongs to a face-centered orthorhombic space group (Fdd2), confirming its crystalline nature. H₂L emitted green-colored fluorescence in MeCN, EtOH, and DMF, with varying FL yields of 0.53, 0.68, and 0.95, respectively. This suggests that the solvent environment plays a crucial role in modulating the FL properties of H₂L. In DMF, the gradual addition of Zn(II) to the H₂L solution resulted in a color change from yellow to orange, accompanied by the appearance of new absorbance bands at 494 nm and two isosbestic points at 300 and 440 nm. These phenomena were not observed either in MeCN or EtOH, indicating that Zn(II)-H₂L binding was more pronounced in DMF. Thus, H₂L demonstrated sensitive Zn(II) binding, with a 1 : 1 binding stoichiometry, a high binding constant (3.745 × 10⁷ μM⁻¹), and a low limit of detection (3.455 × 10⁻⁶ μM). Overall, the findings highlight the potential of the novel crystalline H₂L for efficient FL emission and sensitive Zn(II) detection in DMF, suggesting its potential utilization for fabricating light-emitting organic devices and metal-ion sensors.