Spotlight on Excitonic Coupling in Polymorphic and Textured Anilino Squaraine Thin Films

Abstract

Structural diffraction analysis of an anilino squaraine with branched isobutyl side chains shows crystallization into two polymorphic structures in the bulk and in spin-casted thin films. We observe multipeaked and pleochroic absorption spectra being blue-(red)-shifted for the monoclinic (orthorhombic) polymorph. We understand the packing as Coulombic molecular H-(J)-aggregates supporting Davydov splitting. Pictures of projected Davydov components in oriented thin films fit well to polarization resolved spectro-microscopy and crossed-polarized light microscopy investigations. By comparison with literature on anilino squaraines with linear alkyl side chains, we point out a general trend for steering the thin film excitonic properties by simple side chain and/or processing condition variation. Combined with the ability to locally probe the direction of transition dipole moments, this adds value to the rational design of functional thin films for optoelectronic applications, especially envisioning ultrastrong light–matter interactions.

Nanoscale Polarization‐Resolved Surface Photovoltage of a Pleochroic Squaraine Thin Film

“…This, in turn, means that the UDC runs parallel to the crack direction. Looking at the crystallography of the platelets, the UDC direction is perpendicular to the molecular stacking direction [1 −1 0] in the (110) plane (Figure S1a, Supporting Information) . This relation between crack direction and UDC direction also holds true for the long hole axes of the SQIB:PCBM blend.…”

“…An optical reflection microscopy image between crossed polarizers (Figure a) shows fern‐like features of the monoclinic polymorph with a (011) contact face to the substrate as well as platelets of the orthorhombic polymorph with a (110) contact plane . This is analogous to neat SQIB thin films . For the platelets, several shades of green‐golden color mark different orientational domains.…”

“…For the platelets, several shades of green‐golden color mark different orientational domains. The presence of two (monoclinic) or four (orthorhombic) translationally invariant molecules per unit cell entails a characteristic splitting into an upper (UDC) and lower (LDC) Davydov component in the absorbance spectra . This is also observed in polarized reflection microscopy images.…”

“…The investigated photovoltaic layer blend consists of a phase‐separated squaraine‐fullerene layer blend, which has been shown to stimulate neuronal model cells by local, light‐induced buildup of surface charge . The anisotropy of the sample is determined by the distinct polycrystalline texture of the donor material 2,4‐bis[4‐( N,N ‐diisobutylamino)‐2,6‐dihydroxyphenyl]squaraine (SQIB) (Figure ) and its characteristic Davydov splitting (see also Figure S1, Supporting Information). This Davydov splitting is with 0.3 eV in the red spectral range large compared with other organic semiconductors being only on the order of 0.1–0.2 eV .…”

Photoluminescence of Squaraine Thin Films: Spatial Homogeneity and Temperature Dependence

“…The squaraines used here, referred to as SQIB thereafter, are terminated by isobutyl chains, being added to steer molecular packing in the solid state. Two crystal structures are known for SQIB, a monoclinic polymorph in which the molecules adopt a classical herringbone packing and an orthorhombic structure with molecules arranged in a cofacial herringbone motif (Figure b,c) . With increasing temperature, the monoclinic polymorph changes to the orthorhombic one, although both structures coexist in a wide temperature range.…”