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 [Ln1–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 [Tb1–xEux(MeDPQ)2Cl3], the maximum relative thermal sensitivity Sr 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 [Dy1–xEux(MeDPQ)2Cl3] and [Tb1–xSmx(MeDPQ)2Cl3] also showed significant performance. For the pair Dy3+–Eu3+, the Sr 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 Sr 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 Sr of 4.11% K–1 was achieved for the composition [Gd0.8Tb0.18Eu0.02(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.