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 T 2 and S 1 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 cm 2 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.