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 Ea 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 S1-T1 degeneracy combined with vibrationally activated heavy-atom effects, enabling high-performance TADF scintillators in organic crystals.