Mesogenic Groups Control the Emitter Orientation in Multi‐Resonance TADF Emitter Films**

Funding: Engineering and Physical Sciences Research Council - EP/P010482/1; Leverhulme Trust - RPG-2016-047; Royal Society - SRF\R1\201089; China Scholarship Council - 201603780001; Horizon 2020 Framework Programme - 838885; Fonds De La Recherche Scientifique - FNRS - 2.5020.11; Fonds De La Recherche Scientifique - FNRS - F.4534.21; Horizon 2020 Framework Programme - 101023743. The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL = 492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6%. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs. Publisher PDF