Solution-processed
and thermally annealed thin films from a series
of n-alkyl terminated anilino squaraines are systematically
investigated regarding their structural and optical properties by
means of X-ray diffraction (XRD) and spectroscopic ellipsometry (SE).
Their characteristic intense double-hump-shaped absorbance spectra
consisting of coupled H-aggregate and intermolecular charge transfer
(ICT) resonance bands make them appealing for fundamental light–matter
interaction studies, and their environmental sustainability invites
for consumer optoelectronic applications. Now, the single-crystal
structure of the n-pentyl anilino squaraine (nPSQ) provides a missing link to identify potential odd–even
effects with respect to the terminal alkyl chain for bulk crystals.
While all single crystals adopt a triclinic unit cell with biaxial
dielectric properties, the thin films condense into effectively uniaxial
anisotropic thin films. Here, the inherently low sensitivity to the
out-of-plane complex refractive index of SE in reflection is reasonably
compensated by adding transmission SE and transmission intensity data.
With that, a general picture of the structural and dielectric properties
of n-alkyl anilino squaraine thin films is launched,
revealing that the terminal alkyl chain length affects the structural
and optical properties in a twofold way: we observe a steady change
which is superimposed by an anisotropic odd–even effect.
The honeycomb compound α-RuCl 3 is widely discussed as a proximate Kitaev spin-liquid material. This scenario builds on spin-orbit entangled j = 1/2 moments arising for a t 5 2g electron configuration with strong spin-orbit coupling λ and a large cubic crystal field. The actual low-energy electronic structure of α-RuCl 3 , however, is still puzzling. In particular, infrared absorption features at 0.30, 0.53, and 0.75 eV seem to be at odds with a j = 1/2 scenario. Also the energy of the spin-orbit exciton, the excitation from j = 1/2 to 3/2, and thus the value of λ, are controversial. Combining infrared and Raman data, we show that the infrared features can be attributed to single, double, and triple spin-orbit excitons. We find λ = 0.16 eV and = 42(4) meV for the observed noncubic crystal-field splitting, supporting the validity of the j = 1/2 picture for α-RuCl 3. The unusual strength of the double excitation is related to the underlying hopping interactions, which form the basis for dominant Kitaev exchange.
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