Syntheses, structures and luminescence properties of two novel lanthanide metal-organic frameworks based on a rigid tetracarboxylate ligand

Solvothermal reaction of Ln(NO ₃) ₃•6H ₂O (M = Eu, Tb) with a rigid tetracarboxylate organic ligand, 5,5′-(1,2-ethynediyl)bis(1,3-benzenedicarboxylic acid) (H ₄EBDC), affords two novel isostructural lanthanide-based metal-organic frameworks (MOFs): [Ln(HEBDC)(DMA)(H ₂O) ₂] (Ln = Eu (1) and Tb (2), DMA = N,N′-dimethylacetamide). X-ray single-crystal structural analyses reveal that both compounds crystallize in the triclinic space group P-1. The crystal structures constructed from HEBDC ^{3 -} ligand and the binuclear Ln ₂ building unit. Each of the noncentrosymmetric Ln atoms is bridged through carboxyl groups to generate binuclear Ln building unit, then the binuclear building units are linked together by the organic ligands to give rise to a binodal (3,6)-connected network with a rare kgd topology. The layers further extend into 3D supramolecular frameworks via aromatic π-π stacking interactions. The compounds discussed herein exhibit strong fluorescence in the visible region at room temperature and promising luminescent properties.