Self-assembled Tetranuclear Eu^III Complexes with D₂- and C_2h-Symmetrical Square Scaffold

Three new tetranuclear europium(III) alternating circular helicates, (2-RRRR/2-SSSS) [(R)- or (S)-Ph-Pybox]₄Eu^III₄(BPP)₆, (2-RRRR/2-SSSS) [(R)- or (S)-ⁱPr-Pybox]₄Eu^III₄(BPP)₆, and (3-RRRR/3-SSSS) [(R)- or (S)-Ph-Pybox]₄Eu^III₄(BHP)₆, are presented with their structural and chiroptical characterization (BPP and BHP = bis-β-diketonates; Pybox = chiral bis(oxazolinyl) pyridine ligand). X-ray crystallographic analysis revealed that different extents of interligand-contacting interactions between bis-β-diketonates and Pybox ligands produce different bis-β-diketonates arrangements around four Eu^III ions and, thus, their specific symmetry in the final tetranuclear complexes. 2-RRRR/2-SSSS and 3-RRRR/3-SSSS are recognized as D₂-symmetry, while 2-RRRR/2-SSSS self-assemblies possess pseudo C_2h-symmetry. Due to different molecular symmetry, 2-RRRR/2-SSSS and 2-RRRR/2-SSSS crystals display different ligands orientation in their Eu^III coordination spheres. The presence of pseudo σ_h-mirror symmetry in 2-RRRR/2-SSSS promotes a pair of distinguishable Eu^III geometries. All the chiral self-assemblies exhibit almost identical photoluminescence emission spectra patterns of f-f transitions in the Eu^III core. The Eu^III self-assemblies exhibit intense CPL with different observed |g_lum| values (2-RRRR/2-SSSS ∼ |0.31|, 2-RRRR/2-SSSS ∼ |0.08|, 3-RRRR/3-SSSS ∼ |0.31| in chloroform).