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

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 P1(-) triclinic space group, developing 2D structures through different ligand coordination-conformation schemes toward dinuclear Ln-metal clusters (SBU). Materials were thermally stable ca. 400 degrees C, being activated partially removing DMF -> Ln molecules giving PBIA-Pr-ACT/PBIA-Eu-ACT/PBIA-Tb-ACT/PBIA-Er-ACT/PBIA-Tm-ACT 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-Eu-ACT directly in Eu3+ resulted efficient, but not via the PBIA ligand. In PBIA-Tb/PBIA-Tb-ACT compounds, PBIA ligand excitation is more efficient and PBIA-Tb-ACT 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.