Enhanced Luminance of Pentaazaphenalene‐Based Delayed Fluorescence Emitters by Breaking Forbidden Transition

1,3,4,6,9b‐pentaazaphenalene (5AP) derivatives are of growing interest because of their potential for exhibiting thermally activated delayed fluorescence and inverted singlet‐triplet excited state properties. However, a major challenge has been the non‐emissive nature of 5AP. This study reports a donor‐5AP‐acceptor‐type molecular design for converting non‐emissive 5AP into highly emissive molecules. The newly designed molecules, 2,5‐di(1‐pyrrolidino)‐7,9‐bis(4‐(trifluoromethyl)phenyl)‐1,3,4,6,9b‐pantaazaphenalene (Pyr‐5AP‐CF3) and 2,5‐di(1‐pyrrolidino)‐7,9‐bis(4‐benzonitrile)‐1,3,4,6,9b‐pantaazaphenalene (Pyr‐5AP‐CN), exhibited delayed fluorescence and achieved high photoluminescence quantum yields of 83.5% and 90.6%, respectively, in solid films. These values dramatically exceed those of previously reported 5AP derivatives with only 8% or less. Furthermore, Pyr‐5AP‐CF3 and Pyr‐5AP‐CN exhibited the fastest radiative decays and the narrowest emission spectra among all the 5AP based materials reported to date. This study provides a promising solution to the non‐emissive nature of 5AP, leading to the development of a class of highly luminescent materials for future organic light‐emitting diodes.