Phosphonium Iodide Featuring Blue Thermally Activated Delayed Fluorescence for Highly Efficient X‐Ray Scintillator

Abstract Organic scintillators are praised for their abundant element reserves, facile preparation procedures, and rich structures. However, the weak X‐ray attenuation ability and low exciton utilization efficiency result in unsatisfactory scintillation performance. Herein, a new family of highly efficient organic phosphonium halide salts with thermally activated delayed fluorescence (TADF) are designed by innovatively adopting quaternary phosphonium as the electron acceptor, while dimethylamine group and halide anions (I−) serve as the electron donor. The prepared butyl(2‐[2‐(dimethylamino)phenyl]phenyl)diphenylphosphonium iodide (C4‐I) exhibits bright blue emission and an ultra‐high photoluminescence quantum yield (PLQY) of 100 %. Efficient charge transfer is realized through the unique n‐π and anion‐π stacking in solid‐state C4‐I. Photophysical studies of C4‐I suggest that the incorporation of I accounts for high intersystem crossing rate (kISC) and reverse intersystem crossing rate (kRISC), suppressing the intrinsic prompt fluorescence and enabling near‐pure TADF emission at room temperature. Benefitting from the large Stokes shift, high PLQY, efficient exciton utilization, and remarkable X‐ray attenuation ability endowed by I, C4‐I delivers an outstanding light yield of 80721 photons/MeV and a low limit of detection (LoD) of 22.79 nGy ⋅ s−1. This work would provide a rational design concept and open up an appealing road for developing efficient organic scintillators with tunable emission, strong X‐ray attenuation ability, and excellent scintillator performance.