Stable Lanthanide Metal–Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties

Four new isostructural lanthanide metal–organic frameworks (MOFs), namely {[Ln(DMTP-DC) 1.5 (H 2 O) 3 ]·DMF} n [H 2 DMTP-DC = 2′,5′-dimethoxytriphenyl-4,4″-dicarboxylic acid; Ln III = Eu III ( 1 ), Gd III ( 2 ), Tb III ( 3 ), and Dy III ( 4 )], have been synthesized and characterized. Single-crystal structure analysis reveals that 1 – 4 are three-dimensional Ln-MOFs with rich H-bonding of coordinated H 2 O molecules in the network channels. The X-ray diffraction patterns indicate that Ln-MOF 1 displays good stabilities in organic solvents and aqueous solutions with distinct pH values. Both 1 and 3 show characteristic emission of Ln III ions. Ln-MOF 1 can be used as a ratiometric fluorescence sensor for arginine and lysine in aqueous solution, and the detection limits are 24.38 μM for arginine and 9.31 μM for lysine. All 1 – 4 show proton conductivity related to relative humidity (RH) and temperature, and the maximum conductivity values of 1 – 4 at 55 °C and 100% RH are 9.94 × 10 –5, 1.62 × 10 –4, 1.71 × 10 –4, and 2.67 × 10 –4 S·cm –1, respectively. The value of σ increases with the decrease in ionic radius, indicating that the radius of the Ln III ions can regulate the proton conductivity of these MOFs. Additionally, 2 exhibits a significant magnetocaloric effect (MCE) with a magnetic entropy change (−Δ S m ) of 18.86 J kg –1 K –1 for Δ H = 7 T at 2 K, and 4 shows weak field-induced slow relaxation of magnetization. The coexistence of good fluorescence sensing capability, attractive proton conductivity, and relatively large MCE in Ln-MOFs is rare, and thus, 1 – 4 are potentially multifunctional MOF materials.