Structure and Dielectric Properties of Anisotropic n-Alkyl Anilino Squaraine Thin Films

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.