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

Abstract

A new series of five lanthanide(III) metal–organic frameworks with a general formula of [Ln₃(bcpb)₄(μ-HCOO)(μ-H₂O)(H₂O)₂(DEF)]_n (1–5) was generated by solvothermal reactions from Ln(NO₃)₃·6H₂O {Ln = Eu (1), Tb (2), Gd (3), Dy (4), and Sm (5)} and 3,5-bis(3-carboxyphenyl)pyridine (H₂bcpb) 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 μs. Compound 2 shows an intense green luminescence upon excitation at 336 and its lifetime is 981 μs. The triplet state (³ππ*) of bcpb²⁻ studied by using the Gd(III) derivative 3 demonstrated that the ligand effectively populates Tb(III) emission (Φ = 70.96%), whereas the corresponding Eu(III) derivative 1 shows a weak luminescence efficiency (Φ = 15.81%) as the triplet state of bcpb²⁻ has a poor match with the ⁵D₀ 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, [Tb_3(1−x)Eu_3x(bcpb)₄(μ-HCOO)(μ-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.