A hydrazone Schiff base as a Fe(iii) sensor and precursor to a violet-emissive Mn(ii) complex: synthesis, crystal structure, luminescence, supramolecular and DFT studies

In this study, a novel diacylhydrazone Schiff base butanedihydrazide-bridged bis(4-methoxysalicylaldehyde) ligand (HL) was synthesized by the condensation of 2-hydroxy-4-methoxybenzaldehyde and succinic dihydrazide. UV-vis and fluorescence spectroscopy were used to methodically examine how HL binds to common monovalent, divalent, and trivalent metal ions. According to the results, HL showed strong interaction with the majority of divalent and trivalent ions and indicated clear fluorescence selectivity toward Fe(iii), while demonstrating minimal interaction with monovalent metal ions (Li(i), Na(i), and K(i)). The HL ligand displayed excellent Fe(iii) recognition ability, as evidenced by its low detection limit (0.76 µM) and relatively strong binding constant (2.74 × 10⁵ M⁻¹), underscoring its potential as an effective Fe(iii) sensing probe. Fe(iii) is detected by the HL ligand using a fluorescence quenching method. The Mn–L complex crystallizes in the monoclinic crystal system with space group C2/c, as proven by single-crystal X-ray diffraction analysis, which also provides structural support for the stable coordination framework of the complex. C–H⋯π contacts and π⋯π stacking interactions enhance the Mn–L crystal packing, improving structural stability. The optical band gap of the Mn–L complex was determined to be 2.55 eV from Tauc plot analysis, in excellent agreement with the DFT calculated value of 2.50 eV, highlighting the strong consistency between experimental and theoretical results. Photophysical studies revealed that HL and its Mn–L complex are highly luminescent, emitting violet light upon excitation at 365 nm and exhibiting fluorescence quantum yields of 0.66 and 0.61, respectively, indicating that coordination to Mn only marginally reduces the emissive efficiency of the ligand.