Selenophene π-Core Fusion for Simultaneous Fast Reverse Intersystem Crossing and Narrowband Long-Wavelength TADF.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising for ultra-high-definition organic light-emitting diode (OLED) displays because of their inherently narrowband emission. However, their generally sluggish reverse intersystem crossing (RISC) limits efficient triplet harvesting, thereby constraining device performance and stability. Here, a selenophene π-core fusion strategy is developed based on a prototypical MR scaffold, in which the central benzene unit is replaced with a selenium-embedded dibenzo[b,d]selenophene (DBSe) π-core. This structural transformation generates a redefined π-extended MR framework that simultaneously preserves narrowband emission and accelerates RISC through enhanced spin-orbit coupling, while extending the emission wavelength via para N-π-Se conjugation. Subsequent peripheral donor engineering further stabilizes the excited state, enabling stepwise bathochromic shifts from green to red emission (520-600 nm). The resulting DBSe-based emitters exhibit ultrafast RISC rates of up to 107 s-1 and afford narrowband green-to-red TADF-OLEDs with external quantum efficiencies exceeding 30%, accompanied with suppressed efficiency roll-off and prolonged operational lifetimes. These findings demonstrate that efficient triplet harvesting and long-wavelength narrowband emission can be integrated within a single emitter platform and establish DBSe π-core fusion as an effective design strategy for next-generation high-performance MR-TADF materials and OLED technologies.