Spatial Molecular Decoupling Design for High-Z and Fast Organic Scintillators

Organic scintillators are pivotal for flexible and low-cost radiation detection; however, they face a long-standing "absorption-speed" trade-off: incorporating high-Z elements to enhance X-ray attenuation typically triggers strong spin-orbit coupling (SOC), which quenches prompt fluorescence and diverts excitation energy into slow, microsecond-scale triplet pathways. Here, we present a molecular decoupling strategy to overcome this dilemma, realized in a donor-acceptor-donor (D-A-D) hybridized local and charge-transfer (HLCT) molecule, 4,7-bis(4-(bis(4-iodophenyl)methyl)phenyl)benzo[c][1,2,5]thiadiazole (TPBI). By terminally tethering iodine atoms to the triphenylamine units, we achieve robust X-ray attenuation while preserving a fast "hot-exciton" emission channel. The intrinsic nonplanar geometry and strategic spatial arrangement of the molecule successfully isolate the heavy-atom effect from the emissive core, maintaining the crucial quasi-degeneracy between the T₂ and S₁ states. This configuration facilitates rapid high-lying reverse intersystem crossing (hRISC), enabling near-unity exciton utilization without compromising nanosecond-scale decay kinetics. Our TPBI-based scintillator demonstrates a synergized performance profile of potent X-ray interaction (attenuation efficiency 5.672 cm² g^-1 at 28 keV) and a fast temporal response (2.96 ns), providing a general molecular design paradigm for next-generation, high-performance organic radiation detectors.