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

Abstract

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.

Spotlight on Charge-Transfer Excitons in Crystalline Textured n-Alkyl Anilino Squaraine Thin Films

“…32 In contrast, for linear n-alkyl substitution triclinic crystal structures have been reported with only a single molecule per unit cell. 11,[33][34][35][36] This excludes Davydov splitting as the reason for the absorbance spectra consisting of two broad spectral features. In this case short intermolecular molecular distances based on the slipped π-stacking allows for a pronounced intermolecular charge transfer (ICT).…”

“…Here, we provide the crystal structure of nOSQ, n-octyl SQ with eight carbon atoms. Together with the recently published crystal structure of nPSQ, 36 (n-pentyl SQ, five carbon atoms) the impact of the ICT on the optical spectra can be quantified for the series nBSQ, nPSQ, nHSQ, and nOSQ. Upon spincoating, these compounds were shown to form extended pseudo-uniaxial thin films with the crystallographic {0 0 1} plane parallel to the substrate but effectively random in-plane orientation.…”

“…Upon spincoating, these compounds were shown to form extended pseudo-uniaxial thin films with the crystallographic {0 0 1} plane parallel to the substrate but effectively random in-plane orientation. 36 Crystallites of sizes suitable for spectro-microscopy could also be obtained for nHSQ and nOSQ via dropcasting or dipcoating, and these samples exhibited an interesting polarization dependence of the high and low energy peaks. A previously established essential state model that includes ICT 35,37 is used to account for the distinct linear dichroism of textured, micro-crystalline samples.…”

Determining the Dielectric Tensor of Microtextured Organic Thin Films by Imaging Mueller Matrix Ellipsometry

“…However, a minimal systematic deviation from the unit cell parameters is possibly present due to the thin film nature of the samples. 23 …”

“…The transmission measurement was crucial for parameter decorrelation. 23 Reflection scans at variable AOIs and Theta Stage positions enabled the determination of the full dielectric tensor. 24 Owing to the orthorhombic symmetry, this was possible from only a single crystallographic orientation.…”