Assembly and upconversion luminescence of lanthanide–organic frameworks with mixed acid ligands

Three new lanthanide coordination polymers based on mixed acid ligands [Ln(oba)(ox)0.5(H2O)2]n (Ln = Y (1); Er (2); Yb (3). H2oba = 4,4′-oxybis (benzoic acid); H2ox = oxalic acid) were prepared by hydrothermal reactions and characterized by single-crystal X-ray diffraction. In these complexes, lanthanide ions are bridged by oba ligands to form 1D double-stranded chains, which are further connected by ox ligands, resulting in the formation of 2D (4,4) grids. The upconversion emission of the Y:Er–Yb co-doped coordination polymer was studied and the unusual blue emission for the Er(III) complexes was observed, which arises from the 2H9/2 → 4I15/2 transition and can be explained by three-photon excitation mechanism which is mostly phonon-dependent. The introduction of the oxalate anion without high-energy vibrational groups is beneficial to the increasing intensity of upconversion fluorescence. The magnetic properties of complexes 2 and 3 were investigated. The decrease of χMT over the temperature range of 300–2 K and the negative value of θ are due primarily to the splitting of the ligand field of the ErIII and YbIII ions together with the possible weak antiferromagnetic coupling between the rare earth ions.

Under hydrothermal conditions, four two-dimensional coordination polymers of lanthanide have been obtained by mixed acid ligands 4,4′-oxybis (benzoic acid) and oxalic acid. Upconversion properties of the co-doped coordination polymer were determined and discussed.Figure optionsDownload as PowerPoint slide