Synthesis, Structure, and Thin Film Optical Properties of a Chiral Benzothiazole-Derived Squaraine

Through functionalization with enantiomerically pure substituents, semiconducting dye molecules such as squaraines can be directed into homochiral arrangements, in which molecular chirality is transferred to a supramolecular level resulting in excitonic circular dichroism. Replacing the sterically demanding CMe2 groups in indolenine-derived squaraines with sulfur atoms promotes aggregation and structural order in otherwise homogeneous thin films of benzothiazole-derived squaraines. Here, we investigate solvent-processed and thermally annealed thin films of a benzothiazole-derived squaraine functionalized with homochiral citronellyl groups. The crystalline arrangement of the thin films does not correspond to the single crystal structure found, which consists of a solvent-containing pseudo-polymorph. The spectral shape of the excitonic circular dichroism in spin-cast thin films is sensitive to the processing solvent and annealing temperature. Uniaxial anisotropy, evidenced by X-ray diffraction and spectroscopic ellipsometry, shows excitonic transitions that are exclusively polarized either in the plane or perpendicular to the plane of the thin film. We interpret this in terms of a Davydov splitting into spatially separated upper (in-plane) and lower (out-of-plane) Davydov components. This rarely documented arrangement changes our reading of optical spectra, including circular dichroism, recorded under standard normal incidence conditions, which probe only transitions with an in-plane dipole-moment component.