Celine Leonhardt1,Stefan Bräse1,Eli Zysman-Colman2
KIT1,University of St Andrews2
Celine Leonhardt1,Stefan Bräse1,Eli Zysman-Colman2
KIT1,University of St Andrews2
Thermally activated delayed fluorescence (TADF) emitters are considered as being the most promising emitter materials for organic light-emitting diodes (OLEDs).<sup>[1]</sup> This work focuses on the design and synthesis, as well as the analytical, photophysical and optoelectronic characterization of blue emitting TADF materials. We chose the twisted intramolecular charge transfer transition approach for the design of our emitters. Electron-accepting and electron-donating groups are combined using N-heterocycles, inducing a charge transfer effect, which is crucial for TADF. To achieve a suitable HOMO-LUMO overlap and enhance the dihedral angle between donor and acceptor, bulky spacing units as an interconnection were used.<br/>In 2020, our group developed the novel, star-shaped tris(triazolo)triazine (TTT) acceptor core and decorated it with carbazole- and acridine derivatives, to behave as donating groups, such as 9,9-dimethyl-9,10-dihydroacridine and 3,6-ditert-butylcarbazole. A combinatoric synthetic route was established using three tetrazole-donor-groups reacting with cyanuric chloride and 2,6-lutidine.<br/>These emitters displayed blue emission and a delayed fluorescence with delayed lifetimes on the order of milliseconds. High photoluminescence quantum yields (PLQY) of 79% and 80% were observed in CzSi for the 9,9-dimethyl-9,10-dihydroacridine-TTT and 3,6-ditert-butylcarbazole-TTT materials, respectively. The solution-processed OLED devices performed with an EQE<sub>max </sub>of 11.0% and 5.8%, respectively.<sup>[2]</sup><br/>The monotriazolotriazine (MTT) and bistriazolotriazine (BTT) were developed as a follow up to the TTT family. Instead of using three tetrazole units, only one unit for MTT and two units for BTT were used. These new acceptors, unlike the TTT core, are not symmetrical star-shaped. Achieved were two novel acceptors in combination with commonly known donors. The effect on the TADF properties, such as the emission color and efficiency, using different donor types and donor counts was investigated. The same synthetic protocol was applied, using the triazine core linked with one and two tetrazole-donor-groups to obtain MTT and BTT, respectively.<br/>[1] G. Hong, X. Gan, C. Leonhardt, Dr. Z. Zhang, J. Seibert, Dr. J. M. Busch, S. Bräse, <i>Adv. Mater.</i> <b>2021</b>, <i>33</i>, 2005630.<br/>[2] F. Hundemer, E. Crovini, Y. Wada, H. Kaji, S. Bräse, E. Zysman-Colman, <i>Mater. Adv.</i> <b>2020</b>, <i>1</i>, 2862.