Specific Position of Halogen in Crystalline TADF Scintillators Enables Efficient Triplet Harvesting Through Vibrational Modulation of Spin–Orbit Coupling

Thermally activated delayed fluorescence (TADF) offers a powerful route for harvesting triplet excitons in organic scintillators, yet achieving simultaneously small singlet–triplet gaps and efficient spin–orbit coupling (SOC) in rigid molecular crystals remains a fundamental challenge. Here we demonstrate that targeted halogen substitution can activate vibrationally assisted spin‐flip channels that dramatically enhance triplet harvesting in crystalline donor‐acceptor emitters, tailoring them for scintillators with higher light yield and faster response times. Using DMAC‐TRZ derivatives bearing fluorine or chlorine substituents, we combine single‐crystal structural analysis, temperature‐resolved photoluminescence, radioluminescence spectroscopy, and quantum‐chemical calculations to reveal how subtle changes in halogen chemistry control excited‐state dynamics. Fluorination lowers the rISC activation barrier to 7.1 meV, consistent with an almost degenerate emissive singlet–triplet manifold, whereas chlorine additionally introduces dynamic SOC enhancement mediated by Cl‐atom vibrations within the crystal lattice. As a result, the chlorinated crystal exhibits an exceptionally small E a of 3.9 meV, (sub)microsecond‐scale delayed fluorescence, and a scintillation yield of 26 000 photons MeV −1 . These results reveal a powerful, targeted approach to achieving near S 1 ‐T 1 degeneracy combined with vibrationally activated heavy‐atom effects, enabling high‐performance TADF scintillators in organic crystals.