Abstracts

Single-molecule magnets (SMMs) have been anticipated as promising candidates for ultra-compact information storage in the next generation molecular devices. Nowadays, Dy^III complexes are amongst the state-of-the-art SMMs. In these SMMS, the magnetic dynamic is influenced by the local temperature, which can be self-probed by luminescence thermometry. In the multifunctional SMMS, luminescence thermometry is mainly achieved by taking advantage of the thermal population of Kramers duplets (KDs) arising from the Dy^{III 4}F_9/2 emitting level. Yet, Dy^III SMMs with luminescence thermometry capabilities are far away in magnetic performance than the highest-development Dy^III SMMs. That is because small energetic gaps between the KDs lead to superior relative thermal sensitivity, but this condition is favoured by distorted Dy^III pseudo-symmetries. Such condition contrasts the high-symmetry of the Dy^III polyhedron envisaged to enhance the magnetic performance. To tackle this exciting challenge, herein, we investigate the compromise between Dy^III polyhedron pseudo-symmetry, slow relaxation of magnetization, fine electronic structure, and luminescence to shed light on the synergistic improvement of the magnetic and luminescent features of SMMs. For that, the electronic features and the distortion of the Dy^III coordination polyhedron were tuned by employing different terminal ligands in the [Dy₂bpm(hexd)₆] (1) and [Dy₂bpm(hpd)₆] (2) dinuclear complexes (bpm = 2,20-bipyrimidine, hexd = 2,4-hexanedione, hpd = 3,5-Heptanedione). In these complexes, the DyO₆N₂ polyhedron is described by a distorted square antiprismatic (SAP) configuration (D_4d pseudo-symmetry). Yet, 2 displays a slightly larger distortion degree than 1. The different terminal ligand scaffold also changes the skew angle of the SAP symmetry, which induces different axial and equatorial crystal field components acting on Dy^III. Upon a static external magnetic field, the less distorted Dy^III coordination polyhedra in 1 lead to a larger effective barrier (U_eff) to the slow relaxation of magnetization (335 K) than in 2 (288 K). Moreover, at zero static field, the U_eff of both complexes is quite similar (280 – 290 K). From the luminescence perspective, the two complexes display the typical Dy^III emission at 10 K. However, 2 presents a poor ligand-to-Dy^III energy transfer due to a larger triplet state energy, which limits its use for luminescence thermometry. This is not an issue for 1, which presents relative thermal sensitivity of 0.1-0.7% K^-1 within the SMM operation range (up to 30 K). Therefore, the hexd^- ligand with a larger electronic donating capability than hpd^- leads to a lower polyhedron distortion degree and slightly larger U_eff. The luminescence performance, in its turn, is controlled by the terminal ligand triplet state energy and its impact on the energy transfer. The lessons learned from this study help to draw a picture on how slight structural changes and the electronic structure can play synergistically to enhance the magnetic and luminescent performance of luminescent SMMs.