Azaindolo[3,2,1-jk]carbazole based donor-acceptor materials as potential TADF emitters

Abstract

Throughout the last decades, the field of organic optoelectronics gained constantly more importance in our everyday world and thereby research and development in this sector exhibits a need to improve these materials. Organic Light Emitting Diodes (OLEDs) are already in use for many electronical applications, including commercial TV screens and smartphones of everyday usage. Their biggest advantage lies in the fact, that organic semiconductors are soft materials. This results in flexible, thin and lightweight devices, which show good luminous efficacies, small contrast rates, as well as low production costs and no toxic containments. These properties arise also big interest in the use as lighting sources. Towards this development, different strategies of OLED designs can be achieved, including many factors, that need improvement. Latest research in our group introduced novel bipolar host materials for Phosphorescent Organic Light Emitting Diodes (PhOLEDs), based on oxadiazole electron acceptors and planarized triarylamines as donors. It was observed, that an increase of triarylamine planarization, leads to a decrease of the donor strength in the molecule as a consequence. Furthermore, fully planarized indolo[3,2,1 jk]carbazole (ICz) even showed weak acceptor character. The introduction of electron withdrawing nitrogen atoms in the ICz scaffold induced an increase of acceptor strength. In addition to the lowered HOMO and LUMO levels, high triplet energies were preserved. The aim of this work is the synthesis of novel donor-acceptor systems as potential Thermally Activated Delayed Fluorescence (TADF) emitters. Theoretical calculations for several bipolar systems using N-incorporated ICz acceptors with different nitrogen amounts and varying its position showed promising electro-luminescent properties, such as small singlet-triplet-energy gaps (ST < 0.2 eV), as well as a broad spectral range to cover. Hence, not only the amount, but also the position of the additional nitrogen in the structure has influence on the HOMO/LUMO levels, a comparison of acceptors with alternating strength in the final materials is also possible. Potential bipolar TADF-emitters In the course of this thesis a comprehensive synthetic approach towards the bipolar materials, including the required building blocks, was established. Furthermore, electrochemical and photophysical properties of the target materials were studied in detail.