An unprecedented roll-off ratio in high-performing red TADF OLED emitters featuring 2,3-indole-annulated naphthalene imide and auxiliary donors

The capability of organic emitters to harvest triplet excitons via a thermally activated delayed fluorescence (TADF) process has opened a new era in organic optoelectronics. Nevertheless, low brightness, and consequently an insufficient roll-off ratio, constitutes a bottleneck for their practical applications in the domain of organic light-emitting diodes (OLEDs). To address this formidable challenge, we developed a new design of desymmetrized naphthalimide (NMI) featuring an annulated indole with a set of auxiliary donors on its periphery. Their perpendicular arrangement led to minimized HOMO-LUMO overlap, resulting in a low energy gap (Δ E ST = 0.05-0.015 eV) and efficient TADF emission with a photoluminescence quantum yield (PLQY) ranging from 82.8% to 95.3%. Notably, the entire set of dyes ( NMI-Ind-TBCBz , NMI-Ind-DMAc , NMI-Ind-PXZ , and NMI-Ind-PTZ ) was utilized to fabricate TADF OLED devices, exhibiting yellow to red electroluminescence. Among them, red-emissive NMI-Ind-PTZ , containing phenothiazine as an electron-rich component, revealed predominant performance with a maximum external quantum efficiency (EQE) of 23.6%, accompanied by a persistent luminance of 38 000 cd m −2 . This results in a unique roll-off ratio (EQE 10 000 = 21.6%), delineating a straightforward path for their commercial use in lighting and display technologies. The annulation of an NMI scaffold at positions 3 and 4 with indole, which has a set of twisted electron-rich substituents at its periphery, allows for the development of high-performance and persistent red-TADF OLED emitters.