New Coumarin-Schiff Bases: Synthesis, Electrochemical Behavior, Anion/Cation Sensing Capacities, Spectroscopic...

This study reports the design, synthesis, and comprehensive experimental and theoretical investigation of a series of novel, cost-effective fluorescent Schiff base chemical sensors based on a high-emission 7-(N, N-diethylamino)coumarin fluorophore core. Three different derivatives containing pyrimidine (1a), 4-fluorophenyl (1b), and 2,4-difluorophenyl (1c) moieties were designed to systematically evaluate ion detection performance, intrinsic stability, and the effects of electronic modulations on electrochemical pathways. Within the series, sensor 1a demonstrated exceptional selectivity and sensitivity to Cu²⁺ and Ni²⁺ via a visible fluorescence shutdown mechanism. Importantly, the 1a-Cu²⁺ ensemble acts as a reversible cascade sensor for the highly toxic cyanide anion via a metal substitution mechanism, recovering its original intense fluorescence with a noticeable yellow-to-orange color shift visible to the naked eye. In DMSO, the LOD values were 1.51 µM for Cu²⁺ and 2.76 µM for Ni²⁺. The sensing performance of 1a was also retained in highly aqueous media containing 90% distilled or tap water. The corresponding LOD values were 1.22 and 4.16 µM for Cu²⁺ and 4.22 and 1.93 µM for Ni²⁺ in distilled and tap water, respectively. The Cu²⁺/CN^- turn-off/turn-on response was also preserved in both aqueous media. Cyclic voltammetric (CV) analysis revealed irreversible, diffusion-controlled redox properties. Experimental HOMO/LUMO energy gap values closely correlated with trends predicted by DFT calculations. Furthermore, the calculated hyperpolarizations (β) for 1(a-c) are 91, 30, and 9 times higher, respectively, than those of the reference urea standard. These findings demonstrate that the synthesized coumarin-based Schiff bases have enormous potential not only as selective "naked-eye" detectable chemical sensors but also as high-performance candidate materials for advanced nonlinear optics (NLO) applications.