Tuning metal ion affinity in acyclic phenanthrene schiff bases: comparative study of ethylene and phenylene linkers

Abstract

We have successfully designed and synthesized two structurally simple salen-type Schiff base probes, designated as SB-1 and SB-2, for the selective detection of biologically and environmentally relevant metal ions. Fluorescence studies revealed that SB-1 exhibits a distinct “turn-on” fluorescence response in the presence of Zn²⁺, Mg²⁺, Na⁺, and K⁺ ions, while SB-2 demonstrated a selective fluorescence enhancement exclusively for Zn²⁺ ions. In addition to its fluorescence response, SB-1 displayed distinct colorimetric changes upon interaction with Zn²⁺, Cu²⁺, Mg²⁺, Na⁺, and K⁺ ions, highlighting its broad-spectrum sensing capability. In contrast, SB-2 exhibited selective colorimetric responses only toward Zn²⁺ and Cu²⁺ ions. These results underscore the dual-mode sensing potential of the probes, with SB-1 offering broader ion recognition and SB-2 demonstrating higher selectivity. To gain insights into the interaction mechanism and validate the spectral changes observed experimentally, density functional theory (DFT) calculations were performed. The computational results supported the experimental findings, confirming significant electronic transitions associated with metal ion binding and providing a detailed understanding of the coordination environment and binding modes of SB-1 and SB-2 with various metal ions. ¹H NMR titration studies further substantiated these results by revealing that the metal ions coordinate with the imine (CN) moiety at the core of the Schiff base structure, consistent with the UV-visible absorption and fluorescence spectroscopy data. The binding stoichiometry between the probes and metal ions (Zn²⁺, Cu²⁺, Mg²⁺, Na⁺, and K⁺) was determined to be 1 : 1, as confirmed through Job's plot analysis and supported by UV-Vis, fluorescence, DFT, and ¹H NMR (for Zn²⁺) studies. The binding affinities were quantified using the Benesi–Hildebrand method, with association constants (K_a) found to be in the range of 0.88–2.28 × 10³ M⁻¹. The limit of detection (LOD) for each metal ion was calculated to be in the micromolar (μM) range (0.1 to 0.05 μM), demonstrating the high sensitivity of the probes for practical sensing applications.