Multimetal lanthanide phosphonocarboxylate frameworks: structures, colour tuning and near-infrared emission.

A series of isostructural lanthanide phosphonocarboxylate frameworks {(H3O)3[Ln7(pbpdc)6(DMF)4(H2O)3]·4H2O}n (named LnPCF, Ln = Tb, Eu and Gd, H4pbpdc = 4'-phosphono-[1,1'-biphenyl]-3,5-dicarboxylic acid) were solvothermally synthesized and characterized by the single crystal X-ray diffraction technique. By combining lanthanide cations with a phosphonocarboxylate ligand, a heptametallic lanthanide phosphonate [Ln7(PO3)6(COO)12] core was obtained. This core exhibited as a rare highly 18-connected node and was linked by the 3-connected pbpdc4- ligand, forming a (3,18)-connected framework with a novel topology of {43}6{438·676·839}. This LnPCF structure is an ideal platform for accommodating various lanthanide ions. The TbPCF and EuPCF show efficient luminescence emission due to the "antenna effect" and incorporating Gd3+ into the TbPCF results in a drastic luminescence enhancement. Fine colour tuning between green and red can be easily achieved in bimetallic TbxGd1-xPCFs. More significantly, upon combining a few percent of Nd3+ and Gd3+ with Tb3+, the resulting trimetallic Tb0.4Gd0.5Nd0.1PCF shows dual emissions of both visible and near-infrared light.