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

ABSTRACT 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 10 7 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.