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

This work reports on the structural and photophysical characterization of -symmetrical octanuclear circular Ln helicates, [()- or ()-Pr-Pybox](Ln)(THP) (where Ln = Eu and Tb, THP = trianionic tris-β-diketonate, and Pr-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 [()- or ()-Pr-Pybox]Ln(THP) asymmetric units arranged in a closed ring form. Ligand-to-ligand interactions between the THP and the Pr-Pybox ligands have successfully directed formation of enantiopure, homoconfigurational (Δ,Δ,Δ,Δ,Δ,Δ,Δ,Δ)- and (Λ,Λ,Λ,Λ,Λ,Λ,Λ,Λ)- isomers. All of the nonacoordinated Ln 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 (red, D → F, = 0-4) or Tb (green, D → F, = 6-3) core f-f luminescence. The overall emission quantum yields of the circular Eu and Tb helicates are 0.145 and 0.0013, respectively, in chloroform. The Eu and Tb complexes exhibit remarkable circularly polarized luminescence (CPL) activity at their magnetic dipole transition with observed luminescence dissymmetry factors || of 1.25 (D → F, λ = 592 nm) and 0.25 (D → F λ = 541 nm), respectively. Exceptional || values of the circular Eu helicates highlight the visible intensity difference between left and right circularly polarized emissions of and isomers in chloroform and PMMA thin film.