Understanding Thermally Activated Delayed Fluorescence in Cu(I) Complexes and Metal-Free Donor-Acceptor Systems: A Quantum Chemical Approach

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Thermally activated delayed fluorescence (TADF) is looked upon as a significant emerging technology for generating highly performant electroluminescent devices for displays and lighting systems. The main objective of the proposed research is to gain a better insight into the factors that determine the probability of TADF as this is a key step towards the design and optimization of third-generation OLED emitters. Despite intensive research on this topic in the latest years, a complete and consistent rationalization of TADF is still missing. A small singlet-triplet splitting of the electronically excited emitter states alone is not sufficient for TADF to take place. Rather, the molecular parameters that steer the relative probabilities of excited-state processes such as intramolecular charge and energy transfer, intersystem crossing, reverse intersystem crossing, fluorescence, phosphorescence and nonradiative deactivation have to be understood. Quantum chemistry can substantially contribute to this understanding. In particular, it can provide detailed information about spectroscopically dark states and their coupling to the luminescent state, information that is difficult or even impossible to obtain from experimental data alone. Moreover, starting from a lead structure, quantum chemistry can easily assess the effects of chemical substitution on the photophysics of the emitter. After having validated our theoretical methods, we will develop strategies to enhance the electroluminescence efficiency of d10 transition metal complexes and purely organic TADF emitters, be it by varying the chemical structure or by increasing the electronic or vibrational density of states. In the first three years, we will focus on intrinsic TADF emitter properties. This includes environment effects but not the excitonic coupling of the TADF emitter to host molecules. The latter topic shall be addressed at a later stage of the project.