关键词: 构筑, 锌-稀土纳米簇, 席夫碱配体, 结构, 荧光

Abstract:

Heterometallic d-4f nanoclusters are currently of interest due to their potential use in material science and as probes in biology. Self-assembly by metal-ligand coordination is one of the most efficient processes that organize individual molecular components into nanosized species. However, for lanthanide-based self-assemblies, their stoichiometries and structures are difficult to control during synthesis, because the Ln(Ⅲ) ions often display high and variable coordination numbers. As a result, the structures of lanthanide complexes are commonly influenced by a variety of factors, such as the type of metal ions, the formation of ligands, and the nature of counter anions. In this article, two Zn-Ln nanoclusters [Ln₂Zn₂L₂(OAc)₆] (Ln = Yb (1) and Er (2)) were prepared using a new long Schiff base ligand with a Ph(CH₂)Ph backbone. These nanoclusters show interesting rectangular-like structures. The long Schiff base ligand displays a "stretched" configuration and is bound to the metal ions through its N and phenoxide and methoxy O atoms. As a result, large clusters (0.7 nm × 1.1 nm × 2.2 nm for 1) were formed. In the crystal structures of 1 and 2, each Ln³⁺ ion and its closer Zn²⁺ ion are linked by one OAc^- anion and phenolic oxygen atoms of two long Schiff base ligands, forming a ZnLn unit. Two such ZnLn units are then bridged by two Schiff base ligands to form the nano-rectangular structures. Energy dispersive X-ray spectroscopy (EDX) analyses of the clusters indicate that the molar ratio of Zn : Ln is about 1 : 1, in agreement with their crystal structures. Thermogravimetric analyses show that the clusters lose about 5% of the weight when heated to below 100 ℃. Melting point measurements show that the clusters are thermodynamically stable. Upon excitation of the ligand-centered absorption bands, 1 and 2 exhibit the NIR luminescence of Yb³⁺ and Er³⁺, respectively. The clusters show two excitation bands from 250 to 500 nm, in agreement with their absorption spectra, confirming that energy transfer occurs from the Zn/L centers to Ln³⁺ ions. These results indicate that the chromogenic Zn/L components in these nanoclusters can act as efficient sensitizers for lanthanide luminescence. The efficiencies of the energy transfer from Zn/L-centers to Yb³⁺ is higher than that to Er³⁺, being 75.71% and 25.00% for 1 and 2, respectively. These results provide new insights into the design of polynuclear nanoclusters with interesting luminescence properties.

Key words: Construction, Zn-Ln nanocluster, Schiff base ligand, Structure, Luminescence