Tailoring the Spin Flipping Process of Multi‐Resonance Thermally Activated Delayed Fluorescence Emitter via Symmetry‐Breaking Interaction for Ultralow‐Power Photopolymerization

Abstract Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials are renowned for their excellent color purity; however, their functional versatility and potential for interdisciplinary integration are constrained by the inherently poor spin‐flipping and intersystem crossing (ISC) efficiencies. Herein, this challenge is overcome through an innovative strategy of leveraging symmetry‐breaking π – π interactions by tethering two MR scaffolds into a dimer ( DBNCz ), enabling the precise control over excited‐state properties. It is showed that dimer design alters the orbital nature of selective high‐lying excited states, generating densely packed, degenerate hybrid locally‐excited (LE)/charge transfer (CT) states, resulting in the enhanced spin‐orbit coupling (SOC) and significantly improvement in the spin‐flipping process. Compared to monomeric MR ( BNCz ), which exhibited inefficient ISC (Φ Δ = 3%; k ISC = 2.14 × 10⁷ s⁻¹), the dimer DBNCz demonstrated nearly ten‐folds enhancement (Φ Δ = 40%; k ISC = 12.7 × 10⁷ s⁻¹) while retaining its narrowband emission. Utilizing these MR‐emitters, a novel photoinitiating system is developed, achieving 98% olefin polymerization under low‐energy visible‐light irradiation (450 nm) within 40 s, surpassing commercial benchmarks. This work not only enhances the fundamental photophysical processes of MR‐TADF emitters but also paves the way for their advancement in photocatalytic applications, unlocking new opportunities for the development of high‐performance functional materials.