Structure, morphology, and optical properties of highly ordered films of para-sexiphenyl

We have investigated the properties of highly textured films of para-sexiphenyl $\mathrm{}\left(6P\right).$ The films are obtained by evaporation in high vacuum with different deposition rates and substrate temperatures. The crystal structure is analyzed by x-ray and elastic electron diffraction. Depending on the evaporation conditions and on the substrate pretreatment we find a marked texturing of the films. To better understand the growth conditions of $6P$ we have analyzed the film morphology by atomic force microscopy. The optical properties in the ultraviolet and infrared range are studied as a function of the orientation of the molecules. This enables us to show a strong anisotropy of absorption, luminescence, and luminescence excitation spectra. In order to gain a better understanding of the optical-absorption spectrum of the $6P$ molecular crystals we have analyzed the properties of films with chains oriented parallel and perpendicular to the substrate surface at low temperatures using a He cryostat. Both orientation and temperature-dependent absorption experiments clearly show the very complex nature of the low-energy absorption spectrum of $6P.$ To distinguish between intrinsic molecular properties and the effects of the three-dimensional crystal structure we have compared the experimental results to highly correlated post Hartree-Fock and to density functional theory (DFT) calculations for isolated $6P$ molecules as well as to angular dependent absorption spectra derived from DFT band structures.