Influence of counter ions on structure, morphology, thermal stability of lanthanide complexes containing dipicolinic acid ligand.

Two kinds of lanthanide coordination polymers formed by dipicolinic acid with lanthanide ions were synthesized by varying the counter ions. And their crystal structures, morphology and thermal stabilities were measured and compared. X-ray single-crystal diffraction analysis reveals that Na3[Ln(DPA)3] (Ln = Tb or Eu) stretches to a rigid network by means of bridging Na+ ion. Moreover, Na3[Ln(DPA)]3 exhibits good thermal stability and luminescent properties, and its optical properties can be remained even after heating at 200 °C more than 3 days. However, when Na+ in Na3[Ln(DPA)3] was replaced with NH4+, i.e., (NH4)3[Ln(DPA)3] with a supramolecular structure based on π-π stacking and other weak interactions, shows relatively poor thermal stability which leads to deterioration of their luminescence properties after heating treatment. This result confirms that the rigid frame structure of Na3[Ln(DPA)]3 plays a crucial role in improving its thermal stability and keeping its highly luminescent quantum efficiency.