Ligand-dependent ultrasonic-assistant self-assemblies and photophysical properties of lanthanide nicotinic/isonicotinic complexes

Two structural series, including two isomorphous homodinuclear complexes Ln₂(H₂O)₄(C₆NO₂H ₄)₆ (Ln = Tb (1) and Er (2)) and four isostructural one-dimensional (1-D) chain-like assemblies [Ln(H₂O) ₄(C₆NO₂H₄)₂]_n · nCl (Ln = Sm (3), Eu (4), Tb (5), and Dy (6)), have been rationally prepared through a facile ultrasonic synthesis and have been characterized by X-ray diffraction and photophysical measurements. Both complexes, 1 and 2, feature a homodinuclear structure, based on two 8-fold coordination lanthanide atoms bridged by four nicotinic acid ligands. Complexes 3-6 are characterized by a 1-D polycationic chain-like structure, containing eight-coordinated lanthanide ions and bridging isonicotinic acid ligands. The 1-D polycationic chains and the isolated chloride anions are interconnected via hydrogen bonds and π-π interactions to form a three-dimensional supramolecular network. The effect of nicotinic/isonicotinic acid ligands on the structures and the photoluminescence properties, as well as the relationship between the photoluminescence properties and the structures, was investigated based on IR, UV-vis absorption spectra, low temperature phosphorescent spectra, excitation, and emission spectra. The fluorescence quantum yields of complexes 1 and 2 were determined to be 44% and 21%, respectively.