Fusion of Selenium-Embedded Multi-Resonance Units Toward Narrowband Emission and Fast Triplet-Singlet Exciton Conversion

Developing multi-resonance thermally activated fluorescence (MR-TADF) emitters with both fast reverse intersystem crossing (RISC) rate and narrow emission bandwidth still remains a formidable challenge. Herein, a design strategy of fused MR skeleton containing heavy chalcogen (sulfur or selenium) for high-performance MR-TADF molecules is developed. Impressively, Se-embedded emitter (DSeBN) shows extremely narrow full width at half maximum (FWHM) value of 16 nm and ultrafast RISC rate constant up to 2.0 × 10⁶ s⁻¹. The organic light-emitting diode (OLED) based on this emitter exhibits excellent performance parameters with extremely narrow FWHM of 17 nm and high external quantum efficiency (EQE) of 35.31%. Significantly, much suppressed efficiency roll-off is achieved, in which the EQE still stayed at 32.47% and 25.05% at the luminance of 100 and 1000 cd m⁻², respectively. These results represent the state-of-the-art device performance in terms of efficiency and FWHM, shedding new light on the development of practical MR-TADF emitters.