Impacts of β-diketonate terminal ligands on slow magnetic relaxation and luminescence thermometry in dinuclear DyIII single-molecule magnets†

Lanthanide-based Single-Molecule Magnets (SMMs) with optical and magnetic properties provide a means to understand intrinsic energy levels of 4f ions and their influence on optical and magnetic behaviour. Fundamental understanding of their luminescent and slow relaxation of the magnetization behaviour is critical for targeting and designing SMMs with multiple functionalities. Herein, we seek to investigate the role of Dy^III coordination environment and fine electronic structure on the slow magnetic relaxation and luminescence thermometry. Our findings are illustrated through two distinct Dy^III complexes, [Dy₂(bpm)(hexd)₆] (1) and [Dy₂(bpm)(hpd)₆] (2), (bpm = 2,20-bipyrimidine, hexd = 2,4-hexanedione, hpd = 3,5-heptanedione), by comparing their features with a family of Dy^III dinuclear species bridged by bpm. These findings highlight that the hexd⁻ and hpd⁻ ligands exhibit a similar effective barrier to the reversal of magnetization (280–290 K). The values are among the highest for dinuclear Dy^III complexes bridged by bpm, due to the low distortion of the Dy^III coordination polyhedra and the long Dy–N equatorial bonds. Furthermore, the luminescence performance is affected by the triplet state energy of the terminal ligand, influencing ligand-to-Dy^III energy transfer. The hpd⁻ ligand's higher T₁ state energy leads to poor ligand-to-Dy^III energy transfer, limiting the use of 2 for luminescence thermometry. Conversely, this issue is absent in 1, which offers a relative thermal sensitivity of 0.1 to 0.7% K⁻¹ (10 to 60 K) with a temperature uncertainty below 1 K. These findings contribute to our understanding of lanthanide-based SMMs and facilitate the design of multifunctional materials with tailored magnetic and luminescent properties for molecular electronics and beyond.