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 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)-1,3,4,6,9b-pentaazaphenalene (Pyr-5AP-CF₃) and 2,5-di(1-pyrrolidino)-7,9-bis(4-benzonitrile)-1,3,4,6,9b-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-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 nonemissive nature of 5AP, leading to the development of a class of highly luminescent materials for future organic light-emitting diodes.