Influence of the Monodentate Coordination of Biphenyl Carboxylic Acid Isomers on the Luminescent Properties of Mononuclear Europium Complexes

This study presents the synthesis, crystal structure determination and luminescence properties of three novel mononuclear europium (III) complexes constructed from sodium benzoate and the three different isomers of biphenylcarboxylic acid (2-, 3-, and 4-). All complexes share a common structural core; consisting of a single europium (III) ion coordinated by three bidentate benzoate ligands and two monodentate biphenylcarboxylate ligands. The synthesis followed a two-step strategy: first, a benzoate-based precursor complex was prepared by displacing the chloride and water molecules from europium (III) chloride hexahydrate with sodium benzoate by chelating process. The remaining water ligands were substituted with each biphenylcarboxylic acid isomer yielding solvent-free luminescent complexes. The structural characterization involved thermogravimetric analysis (TGA) to confirm the complete elimination of water molecules. The crystal structures of the europium (III) complexes were solved from X-ray powder diffraction data using the EXPO2014 software; and were deposited in the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers 2477668, 2477667 and 2476992. The photoluminescence properties, including excitation, emission, decay time, and color purity, were studied. The influence of each biphenylcarboxylate isomer on the symmetry of the europium coordination sphere was assessed by calculating the asymmetry ratio, R = (I5D0 → 7F2/I5D0 → 7F1), revealing clear differences in emission intensity and symmetry distortion, directly correlated to the ligand isomer used. These findings demonstrate the antenna effect and tunable luminescence enabled by ligand design, offering potential applications in optoelectronics, bio-imaging and others.