AIE-active Schiff base compounds as fluorescent probes for the highly sensitive and selective detection of Fe3+ ions

Abstract

Aggregation-induced emission (AIE)-active compounds are fascinating because of their unique property of restricted intramolecular rotation, and they can be exploited for manifold applications. Herein, we have designed and synthesized fluorescent salicylaldehyde derivatives, viz. 4-(tert-butyl)-2-((2-hydroxybenzylidene)amino)phenol (L1) and 4-(tert-butyl)-2-((2-hydroxy-5-methylbenzylidene)amino)phenol (L2), through facile Schiff base condensation. The photophysical properties of the as-synthesized ligands were investigated via UV-vis absorption and fluorescence emission spectral analysis. The ligands L1 and L2 acquired maximum AIE in THF : H₂O (10% : 90%) and THF : H₂O (40% : 60%), respectively, and showed exclusive AIE properties and high quantum yields (Φ) in the aggregated state. The fluorescence of the ligands L1 and L2 in the AIE state was selectively quenched by ferric (Fe³⁺) ions. This fluorescence quenching is attributed to the inhibition of photoinduced electron transfer (PET) via complexation between the ligands and Fe³⁺ through ligand-to-metal charge transfer (LMCT). Job's plot revealed the formation of complexes between L1/L2 and Fe³⁺ in 1 : 1 stoichiometry. Further, the binding constants (K_a) were calculated using Benesi–Hildebrand plots and were found to be 1.31 × 10⁵ M⁻¹ and 2.93 × 10⁵ M⁻¹ for L1 and L2, respectively. The proposed mechanism of interaction has been defined using density functional theory (DFT) and also through electrochemical investigations using differential pulse voltammetry (DPV). Ligands L1 and L2 demonstrated very good and selective response towards the detection of Fe³⁺ ions; they exhibited detection limits of 0.163 μM and 3.99 μM, respectively.