Six-Core Ln-MOF Materials (Ln = La, Eu, Tb) Constructed from the Building Blocks [Ln ₄ (COO) ₁₀ ] and [Ln ₄ (COO) ₁₀ (μ-H ₂ O)] Based on <i>In Situ</i> Reactions and Their Multifunctional Fluores

La-MOF complex with six metal cores was synthesized by an in situ reaction of terephthaloyl chloride and La(NO3)3·6H2O under solvothermal conditions. During the reaction process, it was surprisingly found that the terephthaloyl chloride was hydrolyzed into 1,4-H2BDC and then coordinated with La(NO3)3·6H2O to form a six-core metal–organic framework, complex 1, [La6(1,4-BDC)9(μ-H2O)(H2O)(DMF)6]·H2O·3DMF, which was constructed by two 4-centered building blocks [La4(COO)10] and [La4(COO)10(μ-H2O)] and the ligand 1,4-BDC2–. Complex 1 was characterized by infrared spectroscopy (IR), fluorescence spectra, thermogravimetric (TG) analysis, PXRD, and UV–vis spectra, etc. Moreover, a fluorescence sensing experiment was carried out with complex 1 to detect harmful substances (TNP, DNP, ONP, Fe3+, and Cr2O72–). The results indicate that the fluorescence intensity can be reduced to varying degrees by five kinds of detection compounds. Especially, the fluorescence quenching constant KSV of complex 1 for TNP detection is as high as 1.18 × 105 M–1. In the above fluorescence sensing experiment, it was found that the detection wavelength in the ultraviolet region was due to the contribution of the ligand, which is not conducive to future practical applications. Therefore, we successfully prepared another two complexes (2 and 3) containing rare earth elements europium and terbium by regulating the ligand and rare earth ions. The experimental results showed that complexes 2 and 3 also have detection effects on the five harmful substances mentioned above. However, their detection wavelengths have undergone a significant redshift, which is more advantageous for future visual fluorescence detection.