Tuning Luminescent Thermometry in Ytterbium N3O2 Field-Induced Single-Molecule Magnets via Auxiliary Ligands

The reactions of ytterbium(III) bromide or iodide with the N3O2 Schiff base 2,6-bis(2-hydroxyphenyliminomethyl)pyridine (H2L) afford the complexes [Yb(L)Br(CH3CH2OH)]·CH3OH (1·CH3OH) and [Yb(L)(H2O)3]I·CH3CH2OH·H2O (2·CH3CH2OH·H2O), respectively, as single crystals. X-ray diffraction reveals markedly different geometries around the Yb3+ ions. Thus, 1 features a seven-coordinate pentagonal-bipyramidal arrangement, whereas 2 exhibits a triangular dodecahedral environment. Magnetic studies indicate that both compounds exhibit field-induced single-ion magnet (SIM) behavior, with magnetization relaxation governed by quantum tunneling of the magnetization (QTM) and Raman processes. Furthermore, temperature-dependent luminescence measurements demonstrate that both complexes can function as nanoscale luminescent thermometers, resulting in bifunctional magnetic–thermometric molecular materials. Notably, the auxiliary ligands exert a pronounced influence on the thermometric performance while only moderately affecting the magnetic behavior. Complex 1 displays superior thermometric properties, reaching a relative sensitivity of 3.7% K–1 at 60 K, which represents the highest value reported to date for mononuclear Yb3+ field-induced single-ion magnets, together with a minimum temperature uncertainty of 0.2 K. Ab initio calculations provide a theoretical interpretation of both the observed magnetic relaxation dynamics and the luminescence properties.