Syntheses of multi-resonance frameworks towards narrowband organic light-emitting diodes

Multi-resonance (MR)-type organic emitters are highly attractive in the field of organic light-emitting diodes because of their narrowband emission and thermally activated delayed fluorescence (TADF) properties. Compared with conventional TADF emitters, MR-featured emitters have more complex chemical structures and building logics. The core structures of MR emitters are MR frameworks, i.e., polyaromatic frameworks, precisely embedded with electron-donating and electron-withdrawing atoms/groups. Generally, electron-donating units can be easily introduced through the dedicated design of the precursors/intermediates, while integrating electron-withdrawing units is the key point and bottleneck for the synthesis of the MR framework. In this review, we briefly summarize the synthetic strategies of MR frameworks, focusing on the means of introducing various electron-withdrawing atoms/groups, which will aid the further exploration of MR-TADF emitters and their applications in OLEDs.