The weak spin-orbit coupling (SOC) in metal-free organic molecules poses a challenge in achieving phosphorescent emission. To attain pure phosphorescence in RTP organic emitters, a promising molecular design concept has been proposed. This involves incorporating n → π* transition and leveraging heavy atomic effect within the spin-orbit charge transfer-induced intersystem crossing (SOCT−ISC) mechanism of the bipolar molecules. Following this design concept, two bipolar metal-free organic molecules (PhSeB and PhSeDB) with donor-acceptor (D−A) and acceptor-donor-acceptor (A−D−A) configurations have been synthesized. When the molecular configuration changes from D−A to A−D−A, PhSeDB exhibits stronger electron coupling and n → π* transitions, which can further enhance the spin-orbit coupling (SOC) together with the heave atom effect from selenium atom. By the advanced synergism among enhanced n → π* transition, heavy atom effect and magnified electron coupling to efficiently promote phosphorescent emission, PhSeDB can achieve pure RTP emission in both solution and dope solid film. Thanks for the higher spin-orbit coupling matrix elements (SOCMEs) for T1 ↔ S0, PhSeDB attains the highest phosphorescent quantum yield (ca. 0.78) among all the RTP organic emitters reported. Consequently, the purely organic phosphorescent light-emitting diodes (POPLEDs) based on PhSeDB achieve the highest external quantum efficiencies of 18.2% and luminance of 3000 cd m-2. These encouraging results underscore the significant potential of this innovative molecular design concept for highly efficient POPLEDs.

This article is Open Access

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