Lanthanide-Dependent Switching between Magnetic and Optical Thermometry in Structurally Analogous Complexes

Reliable temperature sensing is crucial for understanding thermal relaxation processes and operational limits in single-molecule magnets (SMMs). However, the development of luminescence-based molecular thermometers is often limited by inefficient ligand-to-metal energy transfer, restricting their applicability in many lanthanide SMMs. Here, we investigate two structurally analogous dinuclear lanthanide complexes, [Ln²(TTA)⁶(BPTZ)]·2S (Ln = Dy³⁺ for 1, Eu³⁺ for 2; TTA = 2-thenoyltrifluoroacetone, BPTZ = 3,6-di(2-pyridyl)-1,2,4,5-tetrazine; S = CHCl³ or CH²Cl²), whose thermometric behavior is governed solely by the choice of metal ion. The Dy³⁺ complex exhibits SMM behavior and functions as a magnetic thermometer in 15-100 K using inverse magnetic susceptibility as the sensing parameter, compensating for the lack of luminescence thermometry. It exhibits a maximum relative thermal sensitivity (S_max) of 6.65% K^-1 at 15 K and maintains S_r > 1% K^-1 throughout 15-100 K. In contrast, Eu³⁺ complex functions as a luminescent thermometer with an S_max of 2.15% K^-1 at 350 K. The divergent responses arise from differences in energy-level alignment between ligand donor and lanthanide acceptor states. This work introduces a metal-dependent approach for modulating thermometric function between optical and magnetic within a constant ligand framework, expanding the utility of lanthanide complexes beyond luminescence-based sensing.