Dual-Emitting Ratiometric Luminescent Thermometers Based on Lanthanide Metal-Organic Complexes with Brønsted Acidic...

By introducing Brønsted acidic ionic liquids (ILs), two dual-emitting lanthanide metal-organic complexes (Ln-MOCs) of {[Ln(imdc)(CH₃OH)(H₂O)₃]Cl₂} (LnIMDC, Ln = Eu and Tb, imdc = 1,3-bis(carboxymethyl)imidazolium ion) were first prepared under solvothermal conditions. The crystal structures of LnIMDC were measured by single-crystal X-ray diffraction, and the crystal growth process of LnIMDC was carefully studied. It is found that the crystals are three-dimensional supramolecular structures built up by Ln-O coordination bonds and supramolecular forces. In the temperature range of 303-403 K, LnIMDC show good temperature-dependent emission properties with maximum relative thermal sensitivities of 3.29% K^-1 at 375 K and 2.08% K^-1 at 303 K for EuIMDC and TbIMDC, respectively. It is worth mentioning that both EuIMDC and TbIMDC exhibit good linear ratiometric emission-temperature response in temperature ranges of 353-403 and 323-373 K, respectively, which render EuIMDC and TbIMDC good ratiometric thermometers in high temperature range. Computational studies on the energy level of ILs [H₂imdc]Cl and [imdc]^- ion were performed, which validated the high energy transfer efficiency between the [imdc]^- and Ln³⁺ ions and the unique solution concentration- and wavelength-dependent fluorescence properties of [H₂imdc]Cl. The high fluorescence performance opens up new opportunities for practical applications of ILs in optical sensing.