Breathing Europium–Terbium Co-doped Luminescent MOF as a Broad-Range Ratiometric Thermometer with a Contrasting Temperature–Intensity Relationship

Solvothermal reactions of lanthanide salts and a semirigid tripodal H₃tatab (4,4′,4″-s-triazine-1,3,5-triyltri-p-aminobenzoic acid) ligand in mixed water and N-methyl pyrrolidone (NMP) generated novel breathing MOFs [Ln­(tatab)]·solvent (Ln = Eu in 1-Eu, Tb in 1-Tb, Eu_0.015Tb_0.985 in 1-Eu_0.015Tb_0.985). The framework of 1 was contracted upon removal of guests to form partly desolvated [Ln (tatab)]·3.7H₂O·2.5NMP (1′). Single-crystal X-ray analyses demonstrated that 1 has the breathing ability to spontaneously release guests and maintain the same topology. In contrast to as-synthesized 1, the cell volume of 1′ decreased markedly upon removal of the guests. Different from linear dicarboxylates, the semirigid tripodal tatab ligand is bridged to an inorganic Ln–O chain, limiting the rotation around the O–O-axis of carboxylate. The breathing mechanism is based on the flexible C–N–C angles of amide bonds in the tatab ligand, causing a change in the solvent-accessible volume in the framework. Interestingly, the luminescence color of breathing co-doped lanthanide MOF 1′-Eu_0.015Tb_0.985 is blue-shifted and turned from orange to green with an increase in temperature, which can be attributed to a change in the relative intensity of Tb and Eu emissions, and is quite different from that observed for the reported related compounds. The breathing co-doped lanthanide MOF 1′-Eu_0.015Tb_0.985 can be applied as a high-sensitivity ratiometric thermometer in a broad temperature from 90 to 300 K.