Novel tetrakis lanthanide β-diketonate complexes: Structural study, luminescence properties and temperature sensing

In this study, we report the synthesis and some new crystal structures of tetrakis lanthanide β-diketonate complexes throughout the lanthanide series for homonuclear (Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Er³⁺ and Yb³⁺) as well as for heteronuclear (Eu³⁺-Tb³⁺ and Tb³⁺-Sm³⁺) complexes. The well-known 1,1,1-trifluoro-2,4-pentadione (Htfac) ligand has been used in the synthesis. Here, we show that when employing the same synthesis conditions, we can obtain complexes with different coordination environments of the lanthanide ions. This is strongly linked to the ionic radius of the lanthanide ion. The luminescence properties of the visible emitting complexes (Ln³⁺ = Eu³⁺, Tb³⁺, Sm³⁺ and Dy³⁺) were investigated in solution as well as solid state. The near-infrared emitting complexes (Ln³⁺ = Pr³⁺, Nd³⁺, Er³⁺ and Yb³⁺) were recorded in the solid state. The heteronuclear complexes ([Eu_1-xTb_x(tfac)₈]^2-Na₂⁺ (x: 0.59 (1) and 0.47 (2), Tb_1-ySm_y(tfac)₈]^2-Na₂⁺ (y: 0.1 (3) and 0.2 (4)) and homonuclear complex [Dy(tfac)₄)]⁻Na⁺ (DyL₄) exhibited temperature-dependent luminescence properties in the physiological range, with complex DyL₄ showing the highest relative sensitivity S_r = 3.45% K⁻¹ (280 K). Complex 2 also showed a high S_r = 2.70% K⁻¹ (353 K), which makes them promising for application as physiological luminescence thermometers. To the best of our knowledge until now no lanthanide β-diketonate complexes have been reported for use as good luminescence thermometers operating in the physiological range.