Dual-Emitting Ratiometric Luminescent Thermometers Based on Lanthanide Metal–Organic Complexes with Brønsted Acidic Ionic Liquids

By introducing Brønsted acidic ionic liquids (ILs), two dual-emitting lanthanide metal–organic complexes (Ln-MOCs) of {[Ln(imdc)(CH3OH)(H2O)3]Cl2}n (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