Molecular conformation engineering for enhanced performance of violet multi-resonance thermally activated delayed fluorescence OLEDs

Achieving violet emission with both high efficiency and excellent color purity remains a major challenge for organic light-emitting diodes (OLEDs). In this study, we report two isomeric multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters based on 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene and carbazole frameworks. Phenyl groups were strategically introduced adjacent to the carbazole units to induce molecular twisting, thereby increasing the dihedral angles and enhancing the TADF properties. Among them, 9,9'-(4,10-diphenyl-DOBNA-3,11-diyl)bis(9H-carbazole) (BO-2CzPh-2) exhibited a narrowband violet emission at 408 nm with a full width at half maximum of 27 nm in toluene. In a 10 wt% doped film using bis[2-(diphenylphosphino)phenyl]ether oxide as the host, the emitter also has a near unity photoluminescence quantum yield of 99% and a high degree of horizontal emitting dipole orientation of 88.8%. The OLEDs with BO-2CzPh-2 showed violet emission at 432 nm, an exceptionally high maximum external quantum efficiency of 25.6%, and CIE coordinates of (0.156, 0.035), showing minimal spectral shift at high doping concentrations. These results highlight the effectiveness of simple molecular twisting in improving both TADF performance and outcoupling efficiency, providing a promising pathway for efficient violet OLED applications.