Wheel-like {Ln6} luminescent lanthanide complexes covering the visible and near-infrared domains

Abstract

The syntheses of pure wheel-like 4f lanthanide complexes are challenging because of the variable and high coordination numbers as well as poor directionality of the lanthanide ions. Therefore, a multifunctional ligand, salicylhydroxamic acid (H₃shi), with various coordination sites is deliberately selected for the construction of lanthanide complexes. At the same time, it is found that the ligand can successfully sensitize the lanthanide centers, exhibiting their characteristic emission. A series of wheel-like complexes are synthesized by using metal to ligand stoichiometry (Ln : H₃shi : 2-pyca) with a ratio of 1 : 1 : 2, namely, [Ln₆(H₂shi)₆(2-pyca)₁₂(CH₃CN)₄] ({Ln₆}; Ln = Nd, Sm–Er, and Yb). Single-crystal X-ray diffraction shows that the {Eu₆} complex is formed as a hexatomic core with dimensions of approximately 18 × 18 × 13 Å. The powder X-ray diffraction analysis demonstrates that all other isolated complexes are isostructural. The analysis of the luminescence properties of these complexes shows typical metal-centered emission bands from the visible (Sm³⁺, Eu³⁺, Tb³⁺ and Dy³⁺) to near-infrared (Nd³⁺, Ho³⁺, Er³⁺ and Yb³⁺) ranges. {Sm₆} and {Tb₆} exhibit orange and bright-green emission in the visible region, respectively. {Eu₆} and {Dy₆} exhibit co-luminescence of the corresponding Ln³⁺ ions and the ligands. {Nd₆}, {Ho₆}, {Er₆} and {Yb₆} exhibit characteristic sharp emission bands in the NIR range.