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Why is electronic transport in conjugated organic materials rather inefficient? Already several years ago it has been shown that for molecular dimers the electronic coupling between neighboring molecules strongly depends on their relative arrangement.[1,2] When considering semiconducting molecular crystals this relative arrangement of neighboring molecules then plays a crucial role for charge transport, as in virtually all transport models for organic semiconducting materials the inter-molecular electronic couplings are essential ingredients. However, when calculating these couplings usually only the molecular dimer is considered while the whole crystalline environment is basically ignored. In some cases this can lead to an insufficient description of the anisotropy of the electronic transport within a certain material, as we will show based on the example of $alpha$-quinacridone by using dispersion-corrected density functional theory.[3] Furthermore, based on this material we will construct an orthorhombic model system which allows us to study the interplay between the inter-molecular electronic couplings and this system’s total energy as a function of the molecular arrangement. We will find that molecular arrangements with high inter-molecular electronic couplings are typically energetically unfavorable, which can be traced back to Pauli exchange repulsion of neighboring π -systems.[4] A major consequence of our findings is that without blocking relevant degrees of freedom regarding the relative arrangement of neighboring molecules, such materials will always adopt structures with comparably poor electronic transport properties. A brief outlook on potential strategies how this blocking could be achieved will finalize this talk. |