Highly Stable Rare Earth Metal–Organic Frameworks for Fluorescence Recognition of Folic Acid, Proton Conduction, and Magnetic Refrigeration

Four new isostructural rare earth metal–organic frameworks (RE-MOFs) were synthesized and full characterized, namely, {[(CH) 2 NH 2 ] 3 [RE 2 (BTDBA) 2 (HCOO)]·5H 2 O·2DMF} n (H 4 BTDBA = (4′,4′′′-(benzo[ c ][1,2,5]thiadiazole-4,7-diyl)bis([1,1′-biphenyl]-3,5-dicarboxylic acid); RE = Eu ( JXUST-34 ), Gd ( JXUST-35 ), Tb ( JXUST-36 ), and Dy ( JXUST-37 )). The single-crystal structures analysis shows that JXUST-34 – 37 are chain-based three-dimensional structures. Importantly, JXUST-34 exhibits excellent water, organic solvents, and acid–base stability, which can be used as a fluorescence sensor for folic acid and Al 3+ with detection limits of 0.02 mM and 0.05 μM, respectively. The presence of free [(CH) 2 NH 2 ] + cations in the channels can engage the proton carrier during proton conduction. JXUST-34 – 37 display good proton conductivity, and the conductivities vary with relative humidity and temperatures, among which JXUST-37 has the highest conductivity of 9.66 × 10 –3 S·cm –1 at 60 °C and 98% RH. The magnetic studies show that the −Δ S m of JXUST-35 reaches 16.13 J kg –1 K –1 at 2 K and Δ H = 7 T. JXUST-34 – 37 show multifunctional properties of fluorescence sensing, high proton conductivity, and magnetic refrigeration, which provides a new clue for the development of fluorescent-responsive, magnetic-refrigerant, and proton-conductive RE-MOF materials.