Visible Circularly Polarized Luminescence of Octanuclear Circular Eu(III) Helicate

Abstract

This work reports on the structural and photophysical characterization of <i>D</i>₄-symmetrical octanuclear circular Ln^III helicates, [(<i>R</i>)- or (<i>S</i>)-ⁱPr-Pybox]₈(Ln^III)₈(THP)₈ (where Ln = Eu and Tb, THP = trianionic tris-β-diketonate, and ⁱPr-Pybox = chiral bis(4-isopropyl-2-oxazolinyl)pyridine). X-ray crystallographic analysis revealed that the octanuclear circular helicate possesses square antiprism architecture and consists of four [(<i>R</i>)- or (<i>S</i>)-ⁱPr-Pybox]₂Ln^III₂(THP)₂ asymmetric units arranged in a closed ring form. Ligand-to-ligand interactions between the THP and the ⁱPr-Pybox ligands have successfully directed formation of enantiopure, homoconfigurational (Δ,Δ,Δ,Δ,Δ,Δ,Δ,Δ)-<i>R</i> and (Λ,Λ,Λ,Λ,Λ,Λ,Λ,Λ)-<i>S</i> isomers. All of the nonacoordinated Ln^III ions are identical and exhibit a distorted capped square antiprism (CSAP) geometry. Upon excitation of the ligand absorption band (λ = 360 nm), the circular helicates display characteristic Eu^III (red, ⁵D₀ → ⁷F_J, <i>J</i> = 0-4) or Tb^III (green, ⁵D₄ → ⁷F_<i>J</i>, <i>J</i> = 6-3) core f-f luminescence. The overall emission quantum yields of the circular Eu^III and Tb^III helicates are 0.145 and 0.0013, respectively, in chloroform. The Eu^III and Tb^III complexes exhibit remarkable circularly polarized luminescence (CPL) activity at their magnetic dipole transition with observed luminescence dissymmetry factors |<i>g</i>_lum| of 1.25 (⁵D₀ → ⁷F₁, λ = 592 nm) and 0.25 (⁵D₄ → ⁷F_5, λ = 541 nm), respectively. Exceptional |<i>g</i>_lum| values of the circular Eu^III helicates highlight the visible intensity difference between left and right circularly polarized emissions of <i>R</i> and <i>S</i> isomers in chloroform and PMMA thin film.