New 2D Lanthanide MOFs Constructed from Bis(imide) Pyromellitic Alanine Ligands with Enhanced Fluorescence toward Activation and Modulation of Microstructure

Abstract

Five isostructural 2D lanthanide (Ln) MOFs were synthesized using 2,2′-(1,3,5,7-tetraoxo-5,7-dihydropyrrolo[3,4-f]isoindole-2,6(1H,3H)-diyl)dipropionic acid (PBIA ligand) semiflexible ligand and Pr/Eu/Tb/Er/Tm hydrated nitrates in DMF leading to PBIA–Pr/PBIA–Eu/PBIA–Tb/PBIA–Er/PBIA–Tm materials. New MOFs were characterized employing Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), single crystal and powder X-ray diffractions (SCXRD/PXRD), photoluminescence, and scanning electron microscopy (SEM). MOFs crystallized in P1̅ triclinic space group, developing 2D structures through different ligand coordination-conformation schemes toward dinuclear Ln–metal clusters (SBU). Materials were thermally stable ca. 400 °C, being activated partially removing DMF → Ln molecules giving PBIA–PrACT/PBIA–EuACT/PBIA–TbACT/PBIA–ErACT/PBIA–TmACT phases reorganizing the PBIA ligand toward isobidentate mode, releasing ca. two of six-coordinated DMF per SBU. Just Eu/Tb materials could be studied in the 200–800 nm range showing characteristic emission bands, and the PBIA ligand was not sensitization-suitable for other Ln emissions. Band shape modification and emission intensity enhancement of 6.7(Eu) and 30.6(Tb) resulted in Eu/Tb-activated compounds. Excitation of PBIA–Eu/PBIA–EuACT directly in Eu3+ resulted efficient, but not via the PBIA ligand. In PBIA–Tb/PBIA–TbACT compounds, PBIA ligand excitation is more efficient and PBIA–TbACT could be directly excited in Tb3+. Hence nonradiative processes are playing decisive roles in luminescent properties. Isostructural materials resulted, but different Ln-metals provided different morphologies/sizes, modulating microstructures, and luminescence properties. These materials would be candidates for molecular/ionic sensing, through emission modulation, mainly when Eu and Tb are employed as Ln.