Lanthanide complexes based on a conjugated pyridine carboxylate ligand: structures, luminescence and magnetic properties.

Three lanthanide compounds have been synthesized, namely, {[Dy 2 (bpda) 3 (H 2 O) 3 ] 4 ·2H 2 O}(Dy-1), {[Sm(bpda) 2 ·(H 2 O)]·H 2 O} n (Sm-2) and {[Tb 2 (bpda) 3 (H 2 O) 3 ] 4 ·2H 2 O} (Tb-3) (H 2 bpda = 2,2'-bipyridine-6,6'-dicarboxylic acid). Their structures were determined by single crystal X-ray diffraction and characterized by elemental analysis, infrared spectroscopy and thermogravimetric analysis. Dy-1 and Tb-3 are isostructural with a conjugate bimolecular four-nuclear cluster structure constructed with intramolecular hydrogen bonds and they form a 3D supramolecular structure with intermolecular hydrogen bonding. Sm-2 is a one-dimensional chain structure and is further connected by intricate hydrogen bonds into a three-dimensional supramolecular structure. These three compounds exhibit significant characteristic luminescence from the ligand to the central Ln(iii) ion, which is found by solid-state photoluminescence measurement. Sm-2 exhibits a long luminescence lifetime and high fluorescence quantum yield. A slow relaxation phenomenon is observed for the dysprosium compound by measuring the alternating-current susceptibility at low temperature and the underlying mechanism was further confirmed by theoretical calculations.