Fluoroquinolones boron complexes as promising antibiotics and fluorescent probes: single crystal, photophysical, theoretical and antimicrobial studies.

Fluoroquinolones are widely used antibiotics, yet despite decades of research on their metal complexes, the structural, photophysical, and biological properties of fluoroquinolone‑boron complexes remain relatively underexplored. In this study, three moxifloxacin‑boron complexes were synthesized and systematically characterized using mass spectrometry and NMR spectroscopy. X-ray single-crystal diffraction enabled the first structural characterization of 5a and 5c, confirming monoclinic crystallization in the space groups P21/n and P21/c, respectively. At the same time, we studied the UV-visible absorption and fluorescence emission properties of moxifloxacin and its boron complexes 5a-c in four solvents (DMF, ethanol, water, and dichloromethane). All complexes show typical π → π* and n → π* transitions in the UV spectrum, which shift with solvent changes. Due to the solute-solvent interaction, the fluorescence intensity in polar solvents decreases, while the complexation of boron significantly enhances the fluorescence. Among them, the difluoroboron complex 5c has the strongest emission in DCM, about 500 nm, and the quantum yield is as high as 50.30%. Quantum chemical calculations were conducted to elucidate the intermolecular interactions, energy gaps, and chemical stability of these complexes. Biological evaluation showed that all complexes exhibited inhibitory effects against the tested pathogens. Complex 5a (MIC = 230.76 nmol·L-1) showed the strongest inhibitory activity against methicillin-resistant Staphylococcus aureus, exceeding moxifloxacin hydrochloride (MIC = 279.34 nmol·L-1). In the primary in vitro LeuRS inhibition assay, complex 5b exhibited best inhibitory activity ith an inhibition rate of 78.4% at 1 mg·mL-1. These results provide an experimental basis for further research on their use as a topical antibacterial agent and a fluorescence probe.