The fluorescence of rare earth complexes is an ideal fluorescence sensing material due to its long life, high color purity, strong luminosity, and high sensitivity. It has broad application prospects in environmental monitoring, hazardous materials detection, homeland security and other fields. However, rare earth ions have the characteristics of high coordination number, uncertain coordination configuration and easy hydrolysis. Therefore, it is challenging to achieve effective construction of mononuclear and multinuclear rare earth complexes with good luminescence properties. In-depth research on the relationship between the structure and luminescence properties of multinuclear rare earth complexes has important theoretical and practical significance. 

Bichen Yuan, a year-four undergraduate student studying chemistry major at the College of Chemistry & Materials Engineering, synthesized two new flexible Schiff base ligands containing (CH2)2NH(CH2)2 framework based on the preliminary experimental research of the research group. Using these two flexible ligands with good coordination properties, two mononuclear rare earth complexes [LnL1(NO3)(H2O)DMF] (Ln = Nd (1) and Yb (2)) and two dinuclear rare earth complexes were synthesized [Ln2(L2)2(OAc)2] (Ln = Nd (3) and Yb (4)). The crystal structures of these four complexes were determined by X-ray crystallography. The Schiff base ligands have a "folded" structure in complexes 1-4, and their functional nitrogen and oxygen atoms are coordinated with rare earth ions. The length of the Schiff base ligand is about 18 Å, which helps to form large-scale metal complexes. Interestingly, the binuclear complexes 3 and 4 have nano-sized structures, and their molecular size is about 7 × 11 × 16 Å. Energy dispersive X-ray spectroscopy analysis shows that their Nd:Br:O molar ratio is consistent with the crystal structure. Luminescence performance studies have shown that the Schiff base ligands in these complexes can effectively transfer energy to rare earth ions, making them show the near-infrared luminescence of rare earth ions. At the same time, studies have shown that the near-infrared emission quantum yields of complexes 3 and 4 are higher than those of complexes 1 and 2, due to their higher intramolecular energy transfer effect. This research has a good theoretical significance for designing and synthesizing new flexible organic ligands to regulate the structure and properties of different rare earth complexes. The results were published in the International Journal of Rare Earths (SCI Second-class Journal, DOI: 10.1016/j.jre.2019.02.014, 2020, 38, 143-147).

Figure 1. The crystal structures of rare earth complexes 1 (left) and 3 (right).

Figure 2. Excitation and near-infrared emission spectra of rare earth complexes 1-4.