Rare-earth metal complexes with 4-methylbenzoic acid and 5,5′-dimethyl-2,2′-bipyridine: crystal structures, thermal behavior, luminescence, and DFT analysis

This article reports six new rare-earth complexes formed by coordinating 4-methylbenzoic acid combined with 5,5′-dimethyl-2,2′-bipyridine with six rare-earth metal ions (Dy, Ho, Er, La, Ce, Nd,). X-ray diffraction analysis of single crystals revealed two distinct structural types of complexes: the Dy, Ho, and Er complexes exhibit mononuclear structures, while the La, Ce, and Nd complexes form dinuclear structures. FT-IR and Raman spectra were employed to analyze the chemical bonds and vibrational modes of the complexes in detail, confirming the coordination between the ligands and metal ions. X-ray diffraction (XRD) further verified the crystal structures of the complexes and provided information on their unit cell parameters and space groups. The thermal decomposition behavior and pyrolysis products of six complexes were systematically analyzed by TG-DTG-DSC/FTIR/GC–MS combined methods. Furthermore, together with density functional theory (DFT) computations, the mechanism of energy migration from ligands towards rare-earth ions was elucidated, and the luminescence properties of the Dy³⁺ complex was examined. The results indicate that these complexes undergo multi-stage decomposition and release characteristic gas products when heated. Additionally, the luminescence characteristics of the Dy complex were explored: when excited by ultraviolet light, the complex displays unique Dy³⁺ emission bands at the specific wavelengths of 480 nm and 575 nm, and they are respectively attributed to the energy - level transitions of ⁴F₉/₂ → ⁶H₁₅/₂ and ⁴F₉/₂ → ⁶H₁₃/₂.