Investigation of the Variation of Near-Circular Polarization in Scarabaeoidea Beetles

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

This work reports on the structural and photophysical characterization of D₄-symmetrical octanuclear circular Ln^III helicates, [(R)- or (S)-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 [(R)- or (S)-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)-R and (Λ,Λ,Λ,Λ,Λ,Λ,Λ,Λ)-S 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, J = 0-4) or Tb^III (green, ⁵D₄ → ⁷F_J, J = 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 |g_lum| of 1.25 (⁵D₀ → ⁷F₁, λ = 592 nm) and 0.25 (⁵D₄ → ⁷F_5, λ = 541 nm), respectively. Exceptional |g_lum| values of the circular Eu^III helicates highlight the visible intensity difference between left and right circularly polarized emissions of R and S isomers in chloroform and PMMA thin film.

Photonic Crystal Characterization of the Cuticles of Chrysina chrysargyrea and Chrysina optima Jewel Scarab Beetles

A unified description involving structural morphology and composition, dispersion of optical constants, modeled and measured reflection spectra and photonic crystal characterization is devised. Light reflection spectra by the cuticles of scarab beetles (Chrysina chrysargyrea and Chrysina optima), measured in the wavelength range 300-1000 nm, show spectrally structured broad bands. Scanning electron microscopy analysis shows that the pitches of the twisted structures responsible for the left-handed circularly polarized reflected light change monotonically with depth through the cuticles, making it possible to obtain the explicit depth-dependence for each cuticle arrangement considered. This variation is a key aspect, and it will be introduced in the context of Berreman's formalism, which allows us to evaluate reflection spectra whose main features coincide in those displayed in measurements. Through the dispersion relation obtained from the Helmholtz's equation satisfied by the circular components of the propagating fields, the presence of a photonic band gap is established for each case considered. These band gaps depend on depth through the cuticle, and their spectral positions change with depth. This explains the presence of broad bands in the reflection spectra, and their spectral features correlate with details in the variation of the pitch with depth. The twisted structures consist of chitin nanofibrils whose optical anisotropy is not large enough so as to be approached from modeling the measured reflection spectra. The presence of a high birefringence substance embedded in the chitin matrix is required. In this sense, the presence of uric acid crystallites through the cuticle is strongly suggested by frustrated attenuated total reflection and Raman spectroscopy analysis. The complete optical modeling is performed incorporating the wavelength-dependent optical constants of chitin and uric acid.