Construction, Thermochemistry, and Fluorescence Properties of Novel Lanthanide Complexes Synthesized from Halogenated Aromatic Carboxylic Acids and Nitrogen-Containing Ligands

Abstract

In this study, new lanthanide complexes were synthesized via the volatilization method in solution at room temperature. The general molecular formulas for the lanthanide complexes are as follows: [Ln(2,4-DFBA)3(phen)]2 (Ln = Sm 1, Eu 2, and Er 3; 2,4-DFBA = 2,4-difluorobenzoate; and phen = 1,10-phenanthroline), as well as [Ln(2-Cl-6-FBA)2(terpy)(NO3)(H2O)]2 (Ln = Tb 4 and Dy 5; 2-Cl-6-FBA = 2chloro-6-fluorobenzoate; and terpy = 2,2':6'2''-tripyridine). Based on single-crystal Xray analysis, the five complexes exhibited a monoclinic crystal structure belonging to the space group P21/n. Even though complexes 1, 2 (I), and 3 (II) share a general molecular formula, their coordination modes were different. For example, complexes 1 and 2 formed a muffin-like structure with nine coordinated atoms, while complex 3 formed a double hat triangular geometry with eight coordinated atoms. The twodimensional (2D) polyhedral structures of complexes 1 and 2 were formed via weak π-π stacking interactions, whereas complex 3 exhibited a 2D faceted supramolecular structure through C-H•••F hydrogen bonds. Complexes 4 and 5 were isostructural, with the presence of nitrate ions in their structure. This occurred through the C-H•••F hydrogen bonds and π-π stacking of the molecules to form a faceted supramolecular crystal structure. A series of characterizations, such as elemental analysis, infrared and Raman spectroscopy, as well as powder X-ray diffraction, were performed on the five complexes. Thermogravimetry-derivative thermogravimetry-differential scanning calorimetry were performed between 299.25 and 1073.15 K to investigate the mechanism for the thermal decomposition of complexes 1-5. The analysis of the escaping gas stacking maps of the five complexes using thermogravimetric and 3D infrared coupling techniques further confirmed the correctness of the thermal decomposition mechanism of each complex. The results obtained revealed that similar structured complexes follow a similar thermal decomposition mechanism, and the end solid products for all complexes were their corresponding metal oxides. During the irradiation of the Xe lamp, the solid fluorescence of complexes 1, 2, 4, and 5 were measured. The characteristic transition peaks were located at 4 G5/2 → 6 H5/2, 4 G5/2 → 6 H7/2, and 4 G5/2 → 6 H9/2 (1); 5 D0 → 7 F0, 5 D0 → 7 F1, 5 D0 → 7 F2, 5 D0 → 7 F3, and 5 D0 → 7 F4 (2); 5 D4 → 7 F6, 5 D4 → 7 F5, 5 D4 → 7 F4, and 5 D4 → 7 F3 (4); and 4 F9/2 → 6 H15/2, 4 F9/2 → 6 H13/2 (5). The peaks observed indicated the characteristic transitions of Ln(III). The lanthanide complexes exhibited characteristic fluorescence due to this fact, which also explained their characteristic color. Furthermore, the fluorescence lifetimes of complexes 2 and 4 were measured, and their fluorescence decay curves indicated fluorescence lifetimes of 1.288 and 0.648 ms, respectively.