Ionic radius-dependent self-assembly of lanthanide organic polyhedra: structural diversities and luminescent properties

Abstract

The synthesis of nonclassical polyhedra is at the forefront of supramolecular research because of their unique anisotropic interior cavities. However, due to the difficulty in controlling the topology of Ln supramolecular systems, the preparation of nonclassical lanthanide organic polyhedrals (LOPs) remains a challenge. Herein, we explore the ionic radius-dependent self-assembly of LOPs using a rectangular tetra-tropic ligand L. Owing to the rectangular geometry of the ligand panels (rather than square), its assembly with lanthanide ions located in the middle of the Ln series afforded an irregular tetragonal antiprismatic Ln₈L₄ (Ln = Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺ and Ho³⁺) with two faces unoccupied with L ligands. Interestingly, this tetragonal antiprism possessed an oblate internal cavity that binds to four THF molecules in the solid-state structure. With an increase in radius, the larger La³⁺ and Nd³⁺ ions produced Ln₄L₂ with a distinct sandwich square architecture. In contrast, the smaller Er³⁺ and Lu³⁺ ions gave rise to a mixture of both Ln₈L₄ and Ln₆L₃. On adding excess Ln³⁺ ions, a structural transformation from Ln₈L₄ to Ln₆L₃ occurred. Structural comparisons of La₄L₂ and Sm₈L₄ revealed that the differences in architecture within these systems were governed by both the ionic radii of the lanthanides and conformational flexibility of the ligands. Photophysical investigations revealed that the ligand L exhibited a sensitizing ability toward Sm³⁺, Tb³⁺ and Dy³⁺ ions, displaying their characteristic luminescence emission, with a new record-setting luminescent quantum yield of 92.74% observed for Tb₈L₄. This work provides new insights into the effect of lanthanide size on the resulting assemblies and opens new avenues to develop nonclassical LOPs.