Frontier Molecular Orbital Engineering of Aromatic Donor Fusion: Modularly Constructing Highly Efficient Narrowband Yellow Electroluminescence

The development of high-performance multiple resonance thermally activated delayed fluorescence (MR-TADF) materials with narrowband yellow emission is highly critical for various applications in industries, such as the automotive, aerospace, and microelectronic industries. However, the modular construction approaches to expeditiously access narrowband yellow-emitting materials is relatively rare. Here, a unique molecular design concept based on frontier molecular orbital engineering (FMOE) of aromatic donor fusion is proposed to strategically address this issue. Donor fusion is a modular approach with a "leveraging effect"; through direct polycyclization of donor attached to the MR parent core, it is facile to achieve red-shifted emission by a large margin. As a result, two representative model molecules, namely BN-Cz and BN-Cb, have been constructed successfully. The BN-Cz- and BN-Cb-based sensitized organic light-emitting diodes (OLEDs) exhibit bright yellow emission with peaks of 560 and 556 nm, full-width at half-maxima (fwhm's) of 49 and 45 nm, Commission Internationale de L'Eclairage coordinates of (0.44, 0.55) and (0.43, 0.56), and maximum external quantum efficiencies (EQEs) of 32.9% and 29.7%, respectively. The excellent optoelectronic performances render BN-Cz and BN-Cb one of the most outstanding yellow-emitting MR-TADF materials.