Highly selective and sensitive fluorescent “TURN-ON” furan-based Schiff base for zinc(ii) ion probing: chemical synthesis, DFT studies, and X-ray crystal structure

Abstract

A simple and efficient fluorescent probe (E)-4-bromo-2-((((5-methylfuran-2-yl)methyl)imino)methyl)phenol (BFMP) for the detection of Zn²⁺ ions was synthesized by the condensation of 5-methyl-furfurylamine and 5-bromosalicylaldehyde. Elemental analysis, LC-MS, ATR-IR spectroscopy, and NMR (¹H and ¹³C) spectroscopy were performed to confirm the structures of the Schiff base ligand and its Zn(II) complex. Further, the molecular structure of BFMP was confirmed by single-crystal XRD. It is noteworthy that BFMP displayed favourable behaviour of the florescent active molecule. The BFMP solution was yellow, which after the addition of the Zn²⁺ solution became colourless, as observed by the naked eye. Moreover, under ultraviolet (UV) radiation, a maximum absorption at 425 nm and a large fluorescence enhancement at 560 nm were observed. BFMP was very selective to Zn²⁺ ions, even in the presence of other metal ions. This includes Cd²⁺, which has a binding constant (K_a) of 1.74 × 10⁶ M⁻¹, and a detection limit of this finding suggested that a fluorescence probe (BFMP) can be used to detect Zn²⁺ by colorimetric detection. The detection limit can reach up to 0.249 × 10⁶ M⁻¹. Furthermore, the efficiency of the ligand was tested using real water samples (lake and tap water), and the results revealed that BFMP acted more efficiently in both the tested water samples. The geometries of BFMP and BFMP–Zn²⁺ were investigated by the density functional theory (DFT) method using the B3LYP method with 6-311++G(d,p) and LAN2LZ basis sets, respectively. The optimized geometrical parameters obtained by the DFT calculation are in good agreement with the X-ray crystal structure data.