Conjugation Pathway Management in Hybrid Multi-resonance Thermally Activated Delayed Fluorescent Emitters for Horizontal Emitting Dipole Orientation

The development of deep-blue emitters with both high efficiency and excellent color purity remains a challenge in organic light-emitting diode (OLED). Herein, we report a novel hybrid multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitter, BN- o -OICz, with a boron-based MR core conjugated to indolo[3,2,1-jk]carbazole which introduces a zigzag conjugation pathway that extends the MR framework and enhances charge-transfer delocalization. BN- o -OICz shows a small singlet–triplet energy gap, high oscillator strength, and large transition dipole moment alignment through conjugation pathway management, resulting in increased horizontal dipole orientation and reduced device efficiency roll-off. The OLED based on BN- o -OICz demonstrates a deep-blue emission at 456 nm with a fwhm of 25 nm and a high external quantum efficiency of 31.0%. These results underscore the importance of tuning the conjugation pathway to optimize MR-TADF properties and provide an effective strategy for high-performance deep-blue OLEDs.