Control of coordination and luminescence properties of lanthanide complexes using octadentate oligopyridine-amine ligands

Lanthanide complexes with linear and cyclic octadentate oligopyridine-amine ligands were synthesized, and their molecular structures were determined by single-crystal X-ray crystallography. All of the complexes had a distorted capped square antiprism (CSAP) geometry, and the coordination environments of lanthanide complexes were more distorted for the complexes with the linear ligand than those with the cyclic ligand. The Eu3+ and Tb3+ complexes showed intense luminescence due to energy transfer from the ligand to the metal center (antenna effect). The Eu3+ complexes with the linear ligand showed more intense emissions, which were attributed to the D-5(0) -> F-7(2) transition, than the complex with the cyclic ligand in acetonitrile, which can be attributed to the distortion in the coordination environments. In contrast, the coordination of water molecules to Eu3+ and Tb3+ ions was strongly prevented because the metal ions were surrounded by the cyclic ligand, resulting in intense luminescence in water. These results indicate that the coordination environments of lanthanide complexes, and thus the luminescence properties, can be controlled by tuning the geometrical structures of polydentate ligands.