Tailoring Energy Transfer in Mixed Eu/Tb Metal-Organic Frameworks for Ratiometric Temperature Sensing

Eu/Tb metal-organic frameworks (Eu/Tb-MOFs), exhibiting Eu³⁺ and Tb³⁺ emissions, stand out as some of the most fascinating luminescent thermometers. As the relative thermal sensitivity model is limited to its lack of precision for fitting ratio of Eu³⁺ and Tb³⁺ emissions, accurately predicting the sensing performance of Eu/Tb-MOFs remains a significant challenge. Herein, we report a series of luminescent Eu/Tb-MOF thermometers, Eu_xTb_1-xL, with excellent thermal sensitivity around physiological levels, achieved through the tuning energy transfer from ligands to Eu³⁺ and Tb³⁺ and between the Ln ions. It was found that the singlet lowest-energy excited state (S₁) of the ligand and the higher triplet energy level (T) are crucial in the energy transfer processes of ligand→Tb³⁺ and ligand→Eu³⁺. This enables Eu_xTb_1-xL to serve as an effective platform for exploring the impact of these energy transfer processes on the temperature-sensing properties of luminescent Eu/Tb-MOF thermometers. The relative thermal sensitivity is comparable to that of dual-center MOF-based luminescent thermometers operating at physiological levels. This study provides valuable insights into the design of new Eu/Tb thermometers and the accurate prediction of their sensing performance.