Luminescent properties and ratiometric optical thermometry of Ln-BDC-F₄ compounds

A series of luminescent lanthanide compounds with 2, 3, 5, 6-tetrafluoro-1, 4-benzenedicarboxylate (H₂BDC-F₄) acting as the organic ligand have been synthesized by solvothermal reaction. The terbium compound (C1: Tb-BDC-F₄) and europium compound (C7: Eu-BDC-F₄) exhibit intense green and red luminescence respectively upon excitation of the ligand at room temperature, which suggests that H₂BDC-F₄ ligand can sensitize Tb³⁺ and Eu³⁺ ions effectively. The emitting color of the mixed lanthanide compounds is tunable from green to red in the mixed lanthanide compounds Tb_xEu_1-x-BDC-F₄ (x = 0.3%, 0.5%, 0.7%, 1%, and 2%), which is caused by energy transfer from Tb³⁺ to Eu³⁺. The self-reference ratiometric optical thermometry was realized by fluorescence intensity ratio between Tb³⁺ green emission (∼544 nm) and Eu³⁺red emission (∼ 619 nm). The maximum relative sensitivity S_R,max is 0.76%K^-1@50 K. Furthermore, the emitting color of C5: Eu_0.01Tb_0.99-BDC-F₄ could be tuned from green to orange by temperature in the range of 50 to 300 K. It allows the mixed Eu/Tb-BDC-F₄ compounds to be potentially used as sensitive luminescent materials to directly imagine the temperature distribution.