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

Abstract

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 Tb_xGd_1−xPCFs. More significantly, upon combining a few percent of Nd³⁺ and Gd³⁺ with Tb³⁺, the resulting trimetallic Tb_0.4Gd_0.5Nd_0.1PCF shows dual emissions of both visible and near-infrared light.