Luminescent Thermometer Systems Dy3+/Eu3+ and Tb3+/Sm3+ Based on Coordination Compounds: New Pairs to the Approved Tb3+/Eu3+?

This work addresses a comprehensive study of six new complexes of the constitution [Ln(MeDPQ)₂Cl₃] (Ln³⁺ = Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, and Y³⁺; MeDPQ─2-methyldipyrido-[3,2-f:2′,3′-h]-quinoxaline) with good thermal stability up to 446 °C. Statistical substitution of Sm³⁺, Tb³⁺, Gd³⁺, and Dy³⁺ with a second Ln³⁺ ion led to [Ln_1–xLn′_x(MeDPQ)₂Cl₃] solid solutions, which exhibit temperature-dependent luminescent properties. Their visible emission and intensity ratios of transitions vary with temperature in the range of 253–353 K. In the case of the composition [Tb_1–xEu_x(MeDPQ)₂Cl₃], the maximum relative thermal sensitivity S_r values were determined as 3.77% K^–1, 3.97% K^–1, and 3.97% K^–1 for x(Eu³⁺) = 0.01, 0.05, and 0.1, respectively. The compositions [Dy_1–xEu_x(MeDPQ)₂Cl₃] and [Tb_1–xSm_x(MeDPQ)₂Cl₃] also showed significant performance. For the pair Dy³⁺–Eu³⁺, the S_r values were determined as 3.88%K^–1, 3.91% K^–1, and 3.80% K^–1 for x(Eu) = 0.01, 0.05, and 0.1, respectively. For the pair Sm³⁺–Tb³⁺, the S_r values are 3.28% K^–1 and 3.82% K^–1 for x(Sm) = 0.9 and 0.1, respectively. The largest thermal sensitivity value S_r of 4.11% K^–1 was achieved for the composition [Gd_0.8Tb_0.18Eu_0.02(MeDPQ)₂Cl₃]. In addition, patterns of thermometric performance are bound to the energy transfer efficiency Tb³⁺ → Eu³⁺, Dy³⁺ → Eu³⁺ → Dy³⁺, and Tb³⁺ → Sm³⁺, as this characteristic is strongly temperature-dependent in the studied range.