Microwave-assisted synthesis of lanthanide 2,6-naphthalenedicarboxylates: Thermal, luminescent and sorption characterization

Abstract

The series of lanthanide complexes with 2,6-naphthalenedicarboxylic acid and DMF molecules was synthesized by the microwave-assisted solvothermal method. The compounds of the general formula Ln2ndc3·nDMF, where Ln=lanthanides from La(III) to Lu(III) (except Pm(III)); ndc2−=C10H6(COO)22− and n=1.5–3 were characterized by the elemental analysis, X-ray diffraction patterns, thermal analysis methods (TG–DSC, TG–FTIR) and infrared spectroscopy. The departure of DMF molecules from the structures occurs in one (Nd), two (Sm–Lu) or several (La–Pr) stages. The photoluminescence properties of the selected complexes in the visible (Eu, Tb) and near-infrared (Nd, Er) spectral regions were investigated. The Eu complex displays intensive red luminescence upon excitation by UV radiation with a long lifetime 1.39 (±0.03) ms whereas the compounds of Nd and Er emit light in the NIR spectral region with the lifetimes of 1.66 (±0.06); 10.13 (±0.08) μs and 2.53 (±0.26); 9.38 (±0.29) μs, respectively. For the Tb complex characteristic metal-based emissions were not observed. Some structural changes were noticed after removal of solvent molecules from the Nd and Er complexes. Porosity of Nd, Tb and Er complexes was investigated on their desolvated forms by nitrogen sorption experiments at 77K. The complexes of Tb and Er displayed real porosity with the BET surface area of 127 and 198m2g−1, respectively whereas nitrogen molecules were not adsorbed by the Nd complex.