Mononuclear lanthanide(III)-based complexes with β-thiooxoketone and β-diketone as ligands: effects of soft (S) vs. hard (O) donor substitution on magnetic and luminescent properties.

Lanthanide(III) complexes combine unique magnetic and photophysical features that are highly sensitive to their coordination environment. While β-diketonate ligands with oxygen donors are widely employed in designing single-molecule magnets and luminescent complexes, the incorporation of sulfur donors remains underexplored. Here, we present a direct comparison between conventional oxygen-based β-diketonate complexes and their sulfur-substituted analogues, incorporating β-thiooxoketonate ligands. Structural characterization confirms the efficient coordination of sulfur to Ln3+ centers, introducing significant modifications to the local environment. Photophysical investigations demonstrate that the presence of sulfur alters ligand-to-metal energy transfer pathways and affects emission spectra and lifetimes. Together, these results establish β-thiooxoketonates as promising ligands for the design of multifunctional Ln3+ complexes, where magnetic and luminescent properties can be tuned in synergy via fine structural variation of the coordination environment.