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) 2 Cl 3 ] (Ln 3+ = Sm 3+, Eu 3+, Gd 3+, Tb 3+, Dy 3+, and Y 3+; MeDPQ─2-methyldipyrido-[3,2-f:2′,3′-h]-quinoxaline) with good thermal stability up to 446 °C. Statistical substitution of Sm 3+, Tb 3+, Gd 3+, and Dy 3+ with a second Ln 3+ ion led to [Ln 1– x Ln′ x (MeDPQ) 2 Cl 3 ] 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– x Eu x (MeDPQ) 2 Cl 3 ], 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 (Eu 3+ ) = 0.01, 0.05, and 0.1, respectively. The compositions [Dy 1– x Eu x (MeDPQ) 2 Cl 3 ] and [Tb 1– x Sm x (MeDPQ) 2 Cl 3 ] also showed significant performance. For the pair Dy 3+ –Eu 3+, 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 Sm 3+ –Tb 3+, 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 0.8 Tb 0.18 Eu 0.02 (MeDPQ) 2 Cl 3 ]. In addition, patterns of thermometric performance are bound to the energy transfer efficiency Tb 3+ → Eu 3+, Dy 3+ → Eu 3+ → Dy 3+, and Tb 3+ → Sm 3+, as this characteristic is strongly temperature-dependent in the studied range.