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

We have investigated the properties of highly textured films of para-sexiph enyl (6P). The films are obtained by evaporation in high vacuum with differ ent deposition rates and substrate temperatures. The crystal structure is a nalyzed by x-ray and elastic electron diffraction. Depending on the evapora tion conditions and on the substrate pretreatment we find a marked texturin g of the films. To better understand the growth conditions of 6P we have an alyzed the film morphology by atomic force microscopy. The optical properti es in the ultraviolet and infrared range are studied as a function of the o rientation 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 m olecular crystals we have analyzed the properties of films with chains orie nted parallel and perpendicular to the substrate surface at low temperature s using a He cryostat. Both orientation and temperature-dependent absorptio n experiments clearly show the very complex nature of the low-energy absorp tion 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 den sity functional theory (DFT) calculations for isolated 6P molecules as well as to angular dependent absorption spectra derived from DFT band structure s.