Tuning Luminescence Quenching in Dinuclear TbIII Complexes through β-Diketonate Sensitizers for Cryogenic Lifetime Thermometry

Cryogenic temperature measurement is essential for quantum-information platforms, where ultralow temperatures preserve qubit coherence. Lanthanide(III)-based luminescence thermometry combines high thermal sensitivity, submicrometric spatial resolution, and rapid acquisition. Here, two dinuclear Tb^III complexes, [Tb₂(bpm)(bzac)₆] (1) and [Tb₂(bpm)(dpm)₆] (2), were synthesized to shift Tb^III luminescence quenching into the cryogenic regime through ligand-mediated multiphonon relaxation. Both complexes exhibit Tb^III emission at 80 K; however, complex 2 also shows ligand-centered emission, consistent with less efficient energy transfer. Complex 1 displays strong temperature-dependent luminescence quenching between 80 and 150 K and a sigmoidal decrease in the Tb^{III 5}D₄ lifetime from 0.68 to 0.08 ms. It operates as a lifetime-based thermometer over this range, reaching a maximum relative thermal sensitivity of 5.35% K^-1 at 140 K. These results demonstrate that bzac effectively enables cryogenic Tb^III luminescence thermometry.