How does the ligand affect the sensitivity of the luminescent thermometers based on Tb–Eu complexes

Abstract

Theoretical description of a four-level system for luminescent thermometry was proposed. Based on this description, the sensitivity was proved not to exceed S_r =|E₂₃ − E₁₂|/T², where T is the temperature and |E₂₃ − E₁₂| is the absolute energy difference between the excited state energy gaps. The analysis was verified for the terbium–europium coordination compounds, the most studied class of four-level thermometry systems, which revealed that most of the examples fall within either this theory or represent three-level systems, where the ligand is not involved in the thermally-activated processes. From this description, it follows that the highest S_r can be reached either when the triplet state is not involved or when the ligand triplet state is very high, which in the case of terbium–europium coordination compounds means exceeding 26 800 cm⁻¹.