Organic semiconductor rubrene: crystal chemistry of derivatives and high-pressure polymorphism

Electronic devices based on organic molecules, displaying interesting semiconducting properties, have recently become commercially available after being widely studied for more than twenty years. The focus of the present thesis is the organic semiconductor rubrene, a very promising material in the field of organic electronics, due to its outstanding charge transport properties. The crystal structure of the rubrene derivatives synthesized up to now, to improve the poor solubility of the pristine molecule and its low stability towards oxidation, were analyzed; this was done in order to identify a possible synthetic strategy to obtain novel rubrene derivatives with improved chemical properties while preserving the favorable crystal packing of the molecules in the solid state. This analysis was carried out by a systematic evaluation of the Hirshfeld surface properties of the polymorphs of rubrene and of those rubrene derivatives whose structure has been deposited at the Cambridge Structural Database. As a result, the 4-position of the peripheral phenyl rings was identified as a suitable position for the introduction of different chemical modifications in the rubrene molecule, without affecting the intermolecular contacts between first neighbors in the (100) layer of orthorhombic rubrene, the layer mainly involved in the semiconduction process. Following this crystal engineering approach, a number of new rubrene derivatives, properly tailored and synthesized in order to mimic the crystal structure of orthorhombic rubrene while potentially displaying different chemical and electronic properties, were synthesized and characterized: as a consequence of the different nature of the functionalizations introduced, these derivatives were proved to be more stable than the parent rubrene, displaying very different rates of oxidation. The transport properties of the crystals were probed by conductive AFM; the results showed strong variations in the semiconducting behavior of the different derivatives, suggesting the existence of a link between the interaction with oxygen, the transport properties of these molecules and their oxidation potential; on the contrary, a direct role of rubrene endoperoxide in the enhancement of the semiconducting properties of the material when exposed to air, seems to be excluded. Along with this work, the high-pressure behavior of the polymorphs of rubrene was investigated: aiming at the achievement of a deeper understanding of the interplay of intermolecular interactions inside crystalline rubrene, changes in the intermolecular contacts and in the crystal packing of the different crystal forms were monitored as a function of the applied pressure: a single-crystal to single-crystal reversible transition was identified between 6.0 and 7.2 GPa for the triclinic polymorph and the new phase was investigated from a crystallographic point of view, with a particular attention to the energetic contribution of short molecular contacts to the packing energy of the crystal, calculated by means of the Coulomb - London - Pauli (CLP) model of intermolecular interaction. This research work opens some interesting and promising perspectives: the increase in the ensemble of rubrene derivatives suitable for full physical characterization allows a deeper insight not only in the transport properties of these systems, but also regarding their optical properties and possible related applications. The elucidation of the relationship between the nature of a single specific substituent and the enhancement of the stability of the rubrene molecule could allow to tail and design additional rubrene derivatives with even more specific properties. In this sense, the analysis of the high-pressure behavior of the polymorphs of rubrene may also provide a valuable contribution, while at the same time providing useful structural information for the construction and validation of intermolecular potentials to be used for computational purposes and crystal structure prediction.