Recent Progress in Strategies to Enhance the Spin‐Orbit Coupling of Multiresonance Thermally Activated Delayed Fluorescent Materials for Narrowband Organic Light‐Emitting Diodes

Due to their remarkable performance and exceptional color purity, the multiresonance thermally activated delayed fluorescent (MR‐TADF) materials are prioritized in the research of narrowband organic light‐emitting diodes (OLEDs). However, the atomic numbers of elements such as carbon (C), boron (B), and nitrogen (N) used for B–N‐based MR‐TADF system are relatively small, which offers limited spin‐orbit coupling (SOC) strength. Furthermore, the singular short‐range charge‐transfer (SRCT) characteristics of MR‐TADF emitters also result in the small SOC matrix elements between singlet and triplet excited states. Both lead to a slow reverse intersystem crossing (RISC) process, which constrains their further development in OLED devices. This review focuses on the molecular design strategies aimed at enhancing the SOC, including heavy atom and long‐range charge‐transfer (LRCT) strategies, with the objective of systematizing knowledge in this domain to facilitate the prosperous development of MR‐TADF emitters with high RISC rates. Finally, the challenges and prospects in this area are discussed comprehensively at the end of the review.