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

Abstract 1,3,4,6,9 b ‐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 nonemissive nature of 5AP. This study reports a donor‐5AP‐acceptor‐type molecular design for converting nonemissive 5AP into highly emissive molecules. The newly designed molecules, 2,5‐di(1‐pyrrolidino)‐7,9‐bis(4‐(trifluoromethyl)phenyl)‐1,3,4,6,9 b ‐pentaazaphenalene ( Pyr‐5AP‐CF 3 ) and 2,5‐di(1‐pyrrolidino)‐7,9‐bis(4‐benzonitrile)‐1,3,4,6,9 b ‐pentaazaphenalene ( 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‐CF 3 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 nonemissive nature of 5AP, leading to the development of a class of highly luminescent materials for future organic light‐emitting diodes.