Novel coumarin-Schiff base derived electronically distinct fluorescent probes: Synthesis and comparative investigations of their unique selective sensing properties with Cu2+ and Cu+ ions

The present work describes the synthesis and comparative investigations of Cu²⁺ and Cu⁺ ions sensing properties of two novel π-extended coumarin-Schiff base-derived electronically distinct fluorescent probes (4 and 5). Due to the additional electron donating group substituent (-NEt₂) close proximate to the binding site of Schiff base in probe 5, it exhibits distinct electronic properties compared to probe 4. Our studies indicate that probe 5 displays efficient and highly selective fluorescence quenching behaviors with Cu²⁺ and Cu⁺ ions compared to other competitive metal ions (Ca²⁺, Mg²⁺, Al³⁺, Hg²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, Fe²⁺, Fe³⁺, Cr³⁺ and Zn²⁺). In our studies, Job's plot experiments revealed that the binding stoichiometry of the ligand-Cu²⁺ was 1:1 for both probes. The limit of detection (LOD) of 2.35 × 10^–5 M and 1.56 × 10^–5 M were obtained for the probes 4 and 5, respectively. Further, the Stren-Volmer equation has been used to calculate the binding abilities of probes with Cu²⁺ ions; it reveals the binding constant of 6.70 × 10⁴ M^-1 and 22.0 × 10⁴ M^-1 for probes 4 and 5, respectively. Also, in the present studies, our DFT computed results were comparatively presented to understand the electronic parameters (HOMO-LUMO energy band gaps, ESP and Mulliken atomic charge distribution) of compound 1, probes 4 and 5. We anticipate that the developed probes can be used to analyze Cu²⁺ and Cu⁺ ions in environmental and biological samples.