Modellsysteme organischer Halbleiter: Präparation, Charakterisierung und Interpretation

This cumulative thesis summarizes articles dealing with development and application of model systems of several organic semiconductors like pentacene (PEN), newly synthesized PEN-derivatives, Perfluoropentacene (PFP), Perylen or fluorinated rubrene as well as hexabenzocoronene (HBC). Based on the high charge carrier mobility and thriving processability of PEN, new derivatives with static dipole moments of variable strength were investigated. For this purpose, thin films were grown on varying substrates by using organic molecular beam deposition (OMBD). These samples were characterized by means of x-ray-diffraction (XRD), UV-Vis and x-ray-absorption techniques (NEXAFS). In particular an estimation of exciton binding energies on the basis of a dipole approach points out the significance of changes within the crystal structures for the properties of solids. Beside a good processability, the growth of some of the compounds can be controlled by the substrate, which allows the choice of molecular orientation. This finding is of general interest for directional resolved investigations. In addition to PEN, rubrene also stands out as a high-performance semiconductor, which however is hampered by oxidation. In this context fluorination is introduced as a strategy for chemical stabilization and the effectiveness is proven by NEXAFS-data among others. Apart from chemical stability, fluorination also influences packing motives, which has been exemplarily analyzed for partially fluorinated HBC. Thereby a hirshfeldanalysis revealed that F-H-hydrogenbonds already mediate the observed slipped-stacked-motiv for just a small number of fluorine atoms. Furthermore, the hirshfeldanalysis was applied to crystal structures of a selection of pentacene-like molecules within another study. Owing to the geometric similarity of the molecules, electrostatic forces could be identified as driving force for the packing motives, leading to a prediction of herringbonemotives by examination of molecular electrostatic potentials (MEP). Within another paper polymorphselective growth of perylene is described. By applying in-plane-measurements (XRD) to single crystals, crystallographic directions were assigned to the habitus. This enables an alignment of crystals using a microscope, which is of special importance for polarization resolved spectroscopic methods. While these studies aim for the development and improvement of model systems, there are also publications dealing with the utilization of such systems in the field of NEXAFS-spectroscopy. For the interpretation of spectra a theoretical description is essential, which is realized with the DFT-code StoBe. Based on the NEXAFS-signature of anthracene in the range of the carbon 1s absorption edge, the accuracy of the calculations using a slater-transition- respectively a ΔSCF-approach was determined and the relevance of chemical shifts and relaxation effects has been discussed. On this basis the dichroism of PFP at the F1s-edge was studied. By preparing lying molecules on pyrolytic graphite (HOPG) and standing ones on SiO2, complementary dichroisms were recorded. Together with a theoretical analysis these data allowed for a detailed assignment of underlying excitations. In the course of this, a strong shift of σ-type resonances due to relaxation was observed for fluorinated, conjugated hydrocarbons, leading to a superposition with π-type resonances in the region of energetically low lying signals. Moreover, the spectrum is dominated by σ-type resonances since unoccupied π-orbitals show little overlap with the F1s-initial states which in turn yields small oscillator strength. An elaborate analysis of the angle dependence showed that differences between the dichroism at the carbon and fluorine edge cannot be described by easy rules as it was stated in literature before. In fact, a complete consideration of all relevant transition dipole moments is needed instead. The procedure, as well as the possibilities and challenges of dichroism evaluation in the case of NEXAFS-measurements on thin films of conjugated molecules have been summarized in a review-like publication.