Carbonyl lock enables multi‐resonant organoboron framework with substantially enhanced spin‐flip and reduced emission bandwidth

Abstract Enhancing molecular rigidity of multi‐resonant (MR) frameworks is a popular strategy for improving their performance, yet the convoluted interplay between structural and electronic effects has obscured a rational design path. Herein, these effects are systematically decoupled via a three‐stage comparative study progressing from a twisted prototype (B2) to a planarized, non‐conjugated control (AcB2), and finally to a carbonyl‐integrated target (COB2). This investigation reveals that simple planarization (B2→AcB2) can lead to narrower and blueshifted emission but contribute little to triplet exciton harvesting and device efficiency enhancement. In contrast, the subsequent electronic engineering via the methylene bridge oxidation (AcB2→COB2) acts as a powerful orbital engineering tool. This modulation not only induces a significant bathochromic shift without spectral broadening, but also activates a dominant, ultra‐efficient higher‐triplet‐mediated reverse intersystem crossing (RISC) channel, resulting in an accelerated RISC rate of 1.7 × 10 6 s −1 . Consequently, the non‐sensitized organic light‐emitting diode based on COB2 achieves an outstanding external quantum efficiency of 37.3% with mild efficiency roll‐off. This work establishes a new design paradigm, demonstrating that active electronic modulation, rather than passive structural rigidification, is the critical point for developing next‐generation MR emitters.