Cryogenic Luminescent Tb/Eu-MOF Thermometer Based on a Fluorine-Modified Tetracarboxylate Ligand

Abstract

A fluorine-modified tetracarboxylic acid ligand, namely, 2'-fluoro-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid (H₄FTPTC), with suitable triplet energy excited state, was designed and applied to construct the luminescent lanthanide metal-organic frameworks (LnMOFs) for cryogenic temperature sensing. With the lanthanides codoping strategy, we developed a new Tb³⁺/Eu³⁺ mixed LnMOF system Tb_{1- x}Eu _xFTPTC ( x = 0.05, 0.1, 0.2), which feature excellent linear responses to temperature with high relative sensitivity in the cryogenic range of 25-125 K. It was found that the relative sensitivity of such mixed LnMOF could readily be tuned by adjusting the incorporation amount of Eu³⁺ ions in the host framework. In addition, the energy transfer efficiency between the Tb³⁺ and Eu³⁺ ions in the framework with different Tb³⁺/Eu³⁺ ratios are also investigated and discussed.