Highly sensitive and precise optical temperature sensors based on new luminescent Tb3+/Eu3+ tetrakis complexes with imidazolic counterions

Abstract

In the present work, new Tb³⁺/Eu³⁺ tetrakis(benzoyltrifluoroacetone) complexes containing imidazolic counterions were successfully prepared and characterized via elemental analysis, infrared spectroscopy, thermogravimetry, and X-ray powder diffraction. Photophysical features of the Eu³⁺ ion, such as the intrinsic emission quantum yields, radiative and non-radiative decay rates, and emission lifetimes were dramatically improved when compared with the data from the hydrated tris complex reported in the literature. The values found for the absolute emission quantum yields are up to 0.79 and ratiometric luminescent thermometers were built-up based on the ratio between the spectral areas of the ⁵D₀ → ⁷F₂ (Eu³⁺) and ⁵D₄ → ⁷F₅ (Tb³⁺) transitions. The best contactless temperature sensor operates in a wide temperature range (20–225 K) with a relative thermal sensitivity higher than 4% K⁻¹ (maximum value of ∼7.6% K⁻¹ at 20 K) and a temperature uncertainty lower than 0.1 K with a minimal lower than 0.01 K by combining excitation at the ligand (360 nm) and the Tb³⁺ ion (489 nm), illustrating the potential of lanthanide-based tetrakis complexes in the design of efficient luminescent thermometers.