Isoreticular Lanthanide Metal-Organic Frameworks: Syntheses, Structures and Photoluminescence of a Family of 3D Phenylcarboxylates

Five isoreticular lanthanide (Ln) metal-organic frameworks (MOFs) Er(bdc)(HCOO) (1a), Dy(bdc)(HCOO) (1b), Tb(bpdc)(HCOO) (2a), Gd(bpdc)(HCOO) (2b) and Tb(bpydc)(HCOO) (3) (bdc = 1,4-benzenedicarboxylate, bpdc = 4,4'-biphenyldicarboxylate and bpydc = 2,2'-bipyridine-5,5'-dicarboxylate) were synthesized under solvothermal conditions. The complexes were characterized by powder X-ray diffraction, elemental analyses, infrared spectroscopy and single-crystal X-ray crystallography. Each LnIII ion is eight-coordinated and forms a binuclear pseudo-paddlewheel secondary building unit (SBU), [Ln2(CO2)2(CO2)2]. The SBUs are interconnected into 2D Lncarboxylate nets, which act as tertiary building units (TBUs) that are linked by the rigid organic linkers to form 3D pillared frameworks. Preliminary luminescence studies reveal that energy transfer from Tb3+ to Eu3+ trace impurities is active in crystals of 2a and 3 and contributes to interesting temperature-dependent photoluminescence. At 298 K, under UV excitation, 2a is lilac and 3 glows bright red, whereas at 77 K both 2a and 3 emit green light. Together with the previously published Tb(bdc)(HCOO), Eu(bdc)(HCOO), Gd(bdc)(HCOO) and Ln(bpedc)(HCOO) (bpedc = biphenylethene-4,4'-dicarboxylate), these complexes form the first examples of isoreticular LnMOFs.