Synergistic Modulation of Triplet Density and Heavy-Atom Effect Accelerates Reverse Intersystem Crossing for...

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 × 10⁶ s^-1. Theoretical studies and control experiments jointly reveal a dense manifold of triplet states near S₁ 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.