Synergistic Modulation of Triplet Density and Heavy‐Atom Effect Accelerates Reverse Intersystem Crossing for Narrowband Multi‐Resonance TADF Emitters

ABSTRACT Combining rapid triplet‐to‐singlet spin conversion with BT.2020‐relevant color purity in a single purely organic emitter remains a major challenge for OLED development. Here, we report a synergistic strategy that couples increased triplet density with a modest heavy‐atom effect, in which an energetically matched sulfur‐containing fragment is fused into a multi‐resonance thermally activated delayed fluorescence (MR‐TADF) skeleton to accelerate reverse intersystem crossing (RISC) while preserving narrowband emission. The resulting emitter exhibits pure‐green emission at 514 nm with a full width at half‐maximum of 17 nm, together with an ultrafast RISC rate constant of 5.1 × 10 6 s −1 . Theoretical studies and control experiments jointly reveal a dense manifold of triplet states near S 1 and show that the sulfur atom enhances spin–orbit coupling between states of distinct electronic character, opening multiple efficient RISC pathways. Benefiting from these features, the corresponding non‐sensitized devices deliver a maximum external quantum efficiency (EQE) of 34.6% with minimal efficiency roll‐off (25.2% at 10 000 cd m −2 ) and Commission Internationale de l’Éclairage (CIE) coordinates of (0.20, 0.74), ranking among the best‐performing green devices with a binary emitting layer. These results demonstrate a general design principle for overcoming the trade‐off between ultrafast RISC and color purity in MR‐TADF systems.