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

We have investigated the properties of highly textured films of para-sexiphenyl (Formula presented) 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 (Formula presented) 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 (Formula presented) 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 (Formula presented) 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 (Formula presented) molecules as well as to angular dependent absorption spectra derived from DFT band structures.