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.