Chiral Eu(iii) tetrahedral cage: circularly polarized luminescence induced by an ancillary chiral terpyridyl ligand

Abstract

Chiral luminescent lanthanide-organic polyhedra (LOPs) have attracted great attention due to their potential applications in circularly polarized luminescence (CPL) materials. However, constructing predefined LOPs with effective CPL activity remains challenging due to the diverse coordination numbers and geometries of lanthanide(III) ions and unsuitable chiral organic ligands. In this work, we developed a pair of chiral Eu³⁺-based tetrahedral LOPs, (Eu₄L₄)·(+/−)tpy₄ [L = 4,4′,4′′-tris(4,4,4-trifluoro-1,3-dioxobutyl)-triphenylamine, (+/−)tpy = (+/−)-‘dipineno’-[4,5:4′′,5′′]-fused 2,2′:6,2′′-terpyridine], exhibiting strong CPL activity resulting from successful chirality transfer from chiral terpyridyl ligand (+/−)tpy to the achiral tetrahedron. Single crystal analysis confirms the effective formation of the anticipated chiral tetrahedral cage, with the C₃-symmetric tri-β-diketonate ligand spanning the face and the assistant terpyridyl ligand serving as a chiral resource at the vertice. Additionally, ¹H NMR, IR, and MALDI-TOF-MS confirmed the formation of chiral complexes. The mirror circular dichroism spectra revealed that an enantiomeric pair of chiral tetrahedral cages formed in solution, demonstrating potential chiral activity. Notably, the chiral tetrahedral cages exhibited a high quantum yield (68%) in CHCl₃ solution, surpassing most previously reported Eu³⁺-based tetrahedral cages. Furthermore, the CPL measurements indicated that the luminescence dissymmetry factor and the CPL brightness of chiral cages were 0.014 and 59.1 M⁻¹ cm⁻¹, respectively. Our chiral Eu³⁺-based tetrahedral LOPs demonstrate a high overall CPL efficiency, indicating valuable potential application as functional chiral luminescent LOPs.