Six Lanthanide Complexes: Crystal Structures, Spectroscopic Properties, and Pyrolysis Characteristics Analysis by Thermogravimetry-Differential Scanning Calorimetry/Fourier Transform Infrared/Mass Spectrometry

Six dinuclear lanthanide complexes were synthesized: [Ln(4-MOBA) 3 (4,4′-DM-2,2′-bipy)] 2 ·(H 2 O)·(C 2 H 5 OH) (Ln = Eu ( 1 )) and [Ln(4-MOBA) 3 (4,4′-DM-2,2′-bipy)] 2 ·2(H 2 O) (Ln = Sm ( 2 )) (where 4-MOBA = 4-methoxybenzoate, 4,4′-DM-2,2′-bipy = 4,4′-dimethyl-2,2′-bipyridine); [Ln(3-MBA) 3 (5,5′-DM-2,2′-bipy)] 2 (Ln = Tb ( 3 )), [Ln(3-MBA) 3 (5,5′-DM-2,2′-bipy)] 2 ·(5,5′-DM-2,2′-bipy) (Ln = Dy ( 4 ), Eu ( 5 )) and [Ln(3-MBA) 4 (5,5′-DM-2,2′-bipy)] 2 (Ln = Ce ( 6 )) (where 3-MBA = 3-methylbenzoate, 5,5′-DM-2,2′-bipy = 5,5′-dimethyl-2,2′-bipyridine). Single-crystal X-ray diffraction analysis determined that the structure of complex 6 contains eight 3-MBA ligands, which adopt monodentate, bridging bidentate, and bridging tridentate coordination modes, respectively, a feature that is relatively rare in the study of such complex structures. In addition, the pyrolysis characteristics of the six complexes were studied using coupled TG-DSC/FTIR/MS techniques. The decomposition initially involves the loss of free substances at relatively low temperatures, followed by the elimination of neutral ligand molecules. Subsequently, the organic carboxylate ligands are progressively removed at higher temperatures. Based on the fluorescence spectra of complexes 1 - 5, we further investigated the fluorescence decay behavior and quantum yields of complexes 1, 3, and 4, observing significantly long fluorescence lifetimes. DFT calculations unveiled an optimal energetic match wherein the triplet state of the neutral ligands aligns favorably with the excited states of the Ln 3+ .