Design of Ultraviolet Multiple-Resonance TADF Materials Enabled by Phosphine Oxide/Sulfide-Based Polycyclics with Accelerated Reverse Intersystem Crossing

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) materials have attracted more attention owing to their theoretical 100% exciton utilization capability and narrowband emission. However, the development of ultraviolet (UV) narrowband MR-TADF emitters remains challenging, as such materials are still scarce and exhibit inadequate reverse intersystem crossing (RISC) efficiency. Herein, we employ a theoretical investigation of the design of high-performance UV MR-TADF materials based on phosphine oxide/sulfide polycyclic aromatics using acceptor modifications and peripheral fusion strategies. A series of UV emitters exhibit small reorganization energies and short-range charge transfer characteristics upon excitation, enabling narrowband emission. Notably, the designed molecules featuring bilateral P═S units or 2-position sulfur peripheral locking exhibit ultrahigh total RISC rates (k_toRISC) of ∼10⁵ s^–1 via efficient high-lying triplet-mediated RISC channels, where the significantly reduced energy difference between the first and second triplet states (ΔE_T1-T2) enhances the T₁ ↔ T₂ → S₁ transitions and leads to the accumulation of T₂ excitons. These findings provide deep physical insights into structure–property relationships and offer valuable design principles for high-performance optoelectronic materials.