Synthesis, crystal structure, luminescence properties, silica nanoparticle encapsulation, and antimicrobial activity of an Eu(III) β-diketonate complex with 4,7-dimethyl-1,10-phenanthroline

A novel luminescent europium(III) β-diketonate complex, Eu(btfa)₃(dmphen) [btfa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione; dmphen = 4,7-dimethyl-1,10-phenanthroline], was synthesized and investigated as a potential multifunctional material for optical and biological applications. The complex was prepared from the precursor Eu(btfa)₃(H₂O)₂ and structurally characterized by single-crystal X-ray diffraction. Crystallographic analysis revealed an eight-coordinate Eu(III) center adopting a distorted square-antiprismatic geometry, coordinated by six oxygen atoms from three btfa ligands and two nitrogen atoms from a chelating dmphen ligand. Photophysical studies showed that the complex exhibits intense Eu(III)-centered red emission dominated by the hypersensitive 5D₀ → 7F₂ transition at 614 nm. Replacement of coordinated water molecules with the dmphen ligand significantly enhanced the luminescence performance, resulting in a quantum yield of 21% and a luminescence lifetime of 0.813 ms in dichloromethane solution, compared with 7.5% and 0.470 ms, respectively, for the precursor aqua complex. To improve aqueous dispersibility and potential biocompatibility, Eu(btfa)₃(dmphen) was incorporated into silica nanoparticles using a modified Stöber sol–gel process. The resulting nanoparticles exhibited spherical morphology with average diameters of approximately 40–45 nm and remained well dispersed in water without visible aggregation. The silica-encapsulated materials retained the characteristic Eu(III) emission at 614 nm, with luminescence quantum yields dependent on complex loading, reaching a maximum value of 11.58% for nanoparticles prepared with 40 mg of Eu(btfa)₃(dmphen). Antimicrobial studies performed against Escherichia coli, Staphylococcus epidermidis, and Staphylococcus aureus demonstrated that Eu(btfa)₃(dmphen) exhibited substantially higher antibacterial activity than the precursor complex and starting materials. The enhanced activity is attributed to the incorporation of the hydrophobic dmphen ligand, which may facilitate cellular uptake and interactions with intracellular biomolecular targets. These results demonstrate that Eu(btfa)₃(dmphen) and its silica nanoparticle formulations are promising candidates for luminescent bioimaging and antimicrobial applications.