Synthesis, Structures, optical properties and DFT studies of neodymium complexes containing octanoyl amino carboxylic acids

Amino acid rare-earth complexes are widely used due to their unique physical and chemical properties. In this study, three kinds of neodymium (III) complexes with octanoyl-aminocarboxylate ligands (abbr. Nd(oct-ala)3, Nd(oct-phe)3, Nd(oct-ser)3) were prepared by a chemical reaction method in methanol as the solvent. The chemical formulas and coordination modes of the neodymium complexes were determined by elemental analysis, 1H NMR and FT-IR. Additional analyses by X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the neodymium complexes have lamellar irregular shapes and long-range disordered amorphous structures. The thermal performance of the complexes was determined by differential scanning calorimetry (DSC) and thermal gravimetry-differential thermal analysis (TG-DTA). Nd(oct-phe)3 and Nd(oct-ser)3 appeared glassy when heated. The solubility of the complexes in various organic reagents was also measured. The optical properties of the complexes were examined using UV–Vis absorption, polarizing microscopy observations, fluorescence spectra, quantum yield, and fluorescence lifetime. The luminescence properties of the complexes as a function of temperature were further observed by analysis of temperature-dependent emission spectra. The polarizing microscopy observations show that the three neodymium complexes exhibited the characteristic properties of thermotropic liquid crystals. UV–Vis, fluorescence spectroscopy, and quantum yield results show that they have excellent light absorption and luminescence properties. Temperature-dependent emission spectra show that the complexes have good luminescence properties in the temperature range of 25–160 °C. The powdered complexes and the complexed solutions in methanol were irradiated with 380 nm UV light, resulting in blue-violet light emission. A warm white-light device was constructed to further verify the luminescence of the complexes after combining with three kinds of rare-earth complexes (Nd(oct-phe)3, Eu(oct-phe)3, and Tb(oct-phe)3. The color rendering index (CRI) and color temperature (CCT) were measured. In addition, the density functional theory (DFT) was used to determine the geometric structures (Fig S1), and obtain 1H NMR, FT-IR spectra of the neodymium complexes (Fig S2, S3). Their structures were further examined by comparing the calculated with the experimental spectra. The results show that the ligands coordinate to the central Nd3+ through the bidentate chelation mode.