Understanding and Controlling Molecular Orientation in Organic Guest-Host Systems

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Organic semiconductors are nowadays frequently used in optoelectronic devices, particularly in organic light emitting diodes (OLEDs). In spite of their amorphous nature, these thin film structures can exhibit a pronounced anisotropy of their optical and (di)electric properties, resulting from orientational order of the molecules with respect to the substrate normal. With this renewal proposal we plan to broaden the scientific focus in several aspects with the goal to achieve a deeper understanding of the driving forces for molecular orientation in organic guest-host systems. Most importantly, we will correlate optical and electrical dipole moments by structuring our work program into two complementary, but equally important experimental parts to be conducted in the Bruetting group at U Augsburg. Additionally, a third work package by the Wenzel group at KIT will address these topics by molecular simulation. From this holistic approach, we expect detailed insights into the layer morphology and significant added value into the mechanistic understanding. Moreover, regarding the class of emitter materials, we will mainly focus on thermally acitvated delayed fluorescence (TADF) emitters, i.e. donor-acceptor systems with charge-transfer (CT) character in the excited state. On the one hand, owing to their extended molecular structures, one can expect strong alignment effects in thin films, and on the other hand, due to their CT character we anticipate strong guest-host interactions, specifically with polar host materials. Finally, we plan a substantial extension of our experimental capabilities for measuring emitter orientation towards time-resolved techniques. This is particularly interesting for TADF emitters with their prompt and delayed fluorescence acting over several orders of magnitude in time. Depending on the time scale, the emission can originate from different electronic states on the emitter molecule and often stems from dye molecules with different local environments.