Conformational isomers of a dinuclear Zn(ii) complex with a flexible diacylhydrazone Schiff base: crystal structure, luminescence, and DFT studies

Diacylhydrazone-derived Schiff-bases are multi-functional ligands with several donor groups and flexibility that can favor the stabilization of a variety of coordination topologies. In this work, we describe the synthesis of a diacylhydrazone Schiff-base ligand and its Zn–L complex. Single-crystal X-ray diffraction (SCXRD) reveals two crystallography-independent dinuclear Zn(ii) units in the asymmetric unit with distinct torsion angles (57.58° and 64.91°) indicating conformational isomers in the solid state. Photophysical investigations demonstrate that the free ligand exhibits relatively weak emission. Meanwhile, coordination with Zn(ii) results in a significant fluorescence enhancement, enabling a selective fluorescence “turn-on” sensing response toward Zn(ii) ions. The observed fluorescence enhancement may be tentatively attributed to a chelation-enhanced fluorescence effect arising from coordination-induced restriction of intramolecular rotations and vibrations. However, detailed excited-state calculations and time-resolved photophysical measurements are required to establish the underlying emission mechanism conclusively. The great sensitivity and selectivity of the ligand toward Zn(ii) in solution are further supported by spectroscopic investigations. The probe exhibits a strong binding affinity toward Zn(ii) with a binding constant of K_b = 4.80 × 10⁴ M⁻¹ and a low limit of detection of 2.6 × 10⁻⁷ M, indicating excellent sensing performance. This study demonstrates the versatility of flexible diacylhydrazone Schiff base systems in creating multinuclear zinc assemblies while also acting as excellent fluorescent probes for Zn(ii) detection.