2D lanthanide MOFs driven by a rigid 3,5-bis(3-carboxy-phenyl)pyridine building block: solvothermal syntheses, structural features, and photoluminescence and sensing properties

A new series of five lanthanide(III) metal-organic frameworks with a general formula of [Ln(3)(bcpb)(4)(mu-HCOO)( mu-H2O)(H2O)(2)(DEF)](n) (1-5) was generated by solvothermal reactions from Ln(NO3)(3).6H(2)O {Ln = Eu (1), Tb (2), Gd (3), Dy (4), and Sm (5)} and 3,5-bis(3-carboxyphenyl) pyridine (H(2)bcpb) in an aqueous N, N-diethylformamide (DEF) medium. The obtained products were characterized by elemental analysis, FT-IR spectroscopy, thermogravimetric analysis (TGA), and powder and single crystal X-ray diffraction. The latter reveals that all compounds (1-5) are isostructural and feature very intricate 2D metal-organic frameworks. These were topologically analyzed, revealing a very complex hexanodal underlying net that can be simplified further to a binodal 3,5-connected layer with the 3,5L52 topology. Solid-state photoluminescence properties of 1-5 were studied in detail. Compound 1 exhibits a strong red luminescence upon excitation at 338 nm and its lifetime is 532 mu s. Compound 2 shows an intense green luminescence upon excitation at 336 and its lifetime is 981 mu s. The triplet state ((3)pi pi*) of bcpb(2-) studied by using the Gd(III) derivative 3 demonstrated that the ligand effectively populates Tb(III) emission (Phi = 70.96%), whereas the corresponding Eu(III) derivative 1 shows a weak luminescence efficiency (Phi = 15.81%) as the triplet state of bcpb(2-) has a poor match with the D-5(0) energy level of Eu(III). A notable feature of 1 is its remarkable sensing ability for Cu2+ ions and 2,4,6-trinitrophenol (TNP). Besides, a series of heterobimetallic Tb-based MOFs, [Tb3(1-x)Eu3x(bcpb)(4)(mu-HCOO)(mu-H2O)(H2O)(2)(DEF)](n) {x = 0.001, 0.002, 0.003, 0.01, 0.02, 0.03, 0.04, 0.10, 0.15, 0.20} (compounds 2a-2j, respectively), was prepared. The luminescence studies reveal that the simultaneous presence of the characteristic sharp emission bands of Eu3+ and Tb3+ allows the tuning of the photoluminescence colors of such materials by adjusting the doping concentration of the Eu3+ ions.