Luminescent\nThermometer Systems Dy³⁺/Eu³⁺ and Tb³⁺/Sm³⁺ Based on Coordination\nCompounds: New Pairs to the Approved Tb³⁺/Eu³⁺?

Abstract

This work addresses a comprehensive study of six new\ncomplexes\nof the constitution [Ln(MeDPQ)₂Cl₃] (Ln³⁺ = Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, and Y³⁺; MeDPQ2-methyldipyrido-[3,2-f:2′,3′-h]-quinoxaline)\nwith good thermal stability up to 446 °C. Statistical substitution\nof Sm³⁺, Tb³⁺, Gd³⁺, and Dy³⁺ with a second Ln³⁺ ion led to [Ln_1–<i>x</i>Ln′_<i>x</i>(MeDPQ)₂Cl₃] solid solutions, which exhibit temperature-dependent\nluminescent properties. Their visible emission and intensity ratios\nof transitions vary with temperature in the range of 253–353\nK. In the case of the composition [Tb_1–<i>x</i>Eu_<i>x</i>(MeDPQ)₂Cl₃], the maximum relative thermal sensitivity <i>S</i>_r values were determined as 3.77% K^–1, 3.97%\nK^–1, and 3.97% K^–1 for <i>x</i>(Eu³⁺) = 0.01, 0.05, and 0.1, respectively.\nThe compositions [Dy_1–<i>x</i>Eu_<i>x</i>(MeDPQ)₂Cl₃] and [Tb_1–<i>x</i>Sm_<i>x</i>(MeDPQ)₂Cl₃] also showed significant performance. For the pair\nDy³⁺–Eu³⁺, the <i>S</i>_r values were determined as 3.88%K^–1, 3.91%\nK^–1, and 3.80% K^–1 for <i>x</i>(Eu) = 0.01, 0.05, and 0.1, respectively. For the pair\nSm³⁺–Tb³⁺, the <i>S</i>_r values are 3.28% K^–1 and 3.82% K^–1 for <i>x</i>(Sm) = 0.9 and 0.1, respectively. The largest\nthermal sensitivity value <i>S</i>_r of 4.11%\nK^–1 was achieved for the composition [Gd_0.8Tb_0.18Eu_0.02(MeDPQ)₂Cl₃]. In addition, patterns of thermometric performance are bound to\nthe energy transfer efficiency Tb³⁺ → Eu³⁺, Dy³⁺ → Eu³⁺ → Dy³⁺, and Tb³⁺ → Sm³⁺, as this characteristic\nis strongly temperature-dependent in the studied range.