Enhancing emission performance of red TADF emitters via the introduction of electronically inert pendant

In addition to emission efficiency and colors, emission characteristics such as preferentially horizontal emitting dipole orientation and resistance against concentration quenching are also highly desired for high-performance thermally activated delayed fluorescence (TADF) emitters. In this work, we report a simple and yet effective strategy for enhancing such emission performance of the pure red-to-near infrared (NIR) TADF emitter by introducing bulky aryl derivatives (m-xylene) to the efficient emission core. Incorporating m-xylene groups as the electronically inert pendants to the acceptor unit of the TADF emitter TPA-CN-N4-CH3 successfully retains favorable emission properties (e.g., pure red color chromaticity, nearly unitary photoluminescence quantum yield etc.) of the parent compound TPA-CN-N4 and yet also renders significantly higher horizontal emitting dipole ratio Theta// of similar to 85 % with the more extended molecular configuration and reduced concentration quenching at very high concentrations in thin films. As a result, OLEDs adopting TPA-CN-N4-CH3 can demonstrate rather high EQEs of 29.8-32.1 % for pure red emission (625-650 nm) at low doping concentrations and decent EQEs of 18.9-27.5 % in the deep red to NIR range (650-700 nm) at higher doping concentrations. The results of this work shall provide a useful reference for development of efficient TADF emitters.