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

This work reports
on the structural and photophysical characterization of D4-symmetrical octanuclear circular LnIII helicates,
[(R)- or (S)-iPr-Pybox]8(LnIII)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 [(R)- or (S)-iPr-Pybox]2LnIII2(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)-R and (Λ,Λ,Λ,Λ,Λ,Λ,Λ,Λ)-S isomers. All of the nonacoordinated LnIII 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 EuIII (red, 5D0 → 7FJ, J = 0–4) or TbIII (green, 5D4 → 7FJ, J = 6–3) core f–f luminescence.
The overall emission quantum yields of the circular EuIII and TbIII helicates are 0.145 and 0.0013, respectively,
in chloroform. The EuIII and TbIII complexes
exhibit remarkable circularly polarized luminescence (CPL) activity
at their magnetic dipole transition with observed luminescence dissymmetry
factors |glum| of 1.25 (5D0 → 7F1, λ = 592 nm) and
0.25 (5D4 → 7F5, λ = 541 nm), respectively. Exceptional |glum| values of the circular EuIII helicates
highlight the visible intensity difference between left and right
circularly polarized emissions of R and S isomers in chloroform and PMMA thin film.