From Fluorine to Chlorine: How Atomic Vibrations Enable Efficient Triplet Harvesting in Crystalline TADF Scintillators

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. While fluorination reduces sharply the singlet-triplet energy gap, chlorine additionally introduces dynamic SOC enhancement mediated by the vibrations of Cl atoms within the crystal lattice. As a result, the chlorinated crystal exhibits an exceptionally small activation energy (Ea = 3.9 meV), (sub)microsecond-scale delayed fluorescence, and a scintillation yield of 26 000 photons MeV-1. These results reveal a powerful, atom-economical approach to achieving near S1-T1 degeneracy combined with vibrationally activated heavy-atom effects, enabling high-performance TADF scintillators in organic crystals.