Simultaneous Achievement of Efficient Triplet Harvesting and Ultrafast Radiative Decay in D-A-D Hot-Exciton Scintillators for High-Resolution X-Ray Imaging.

Organic scintillators play a crucial role in medical imaging, radiotherapy, and nondestructive inspection. However, their development is hindered by the difficulty of simultaneously achieving efficient triplet exciton utilization and ultrafast radiative decay. Herein, we propose a molecular design strategy based on donor-acceptor-donor (D-A-D) hybridized local and charge-transfer (HLCT) dyes to overcome this intrinsic limitation. The designed molecule 2,2'-(2,5-difluoro-1,4-phenylene)bis[9,9-dimethyl-9H-fluorene] (DFLu2F), composed of 9,9-dimethylfluorene donor units and a 1,4-difluorophenylene acceptor unit, exhibits a moderate donor-acceptor interaction, which stabilizes an HLCT excited state and enables efficient triplet exciton utilization through high-level triplet-singlet reverse intersystem crossing (HL-RISC). In contrast, the reference molecule DPTZ2F, offering a much stronger 10-phenyl-10H-phenothiazine donor unit, suffers from excessive charge-transfer character that leads to severe emission quenching, highlighting the necessity of balancing local excitation and charge transfer for efficient scintillation. As a result, crystalline DFLu2F powder delivers outstanding scintillation performance, including an ultrafast decay time of 1.29 ns, a radioluminescence intensity 2.4 times higher than that of Bi4Ge3O12, and a low detection limit of 30.7 nGy s-1. Furthermore, flexible scintillation films based on DFLu2F achieve a high spatial resolution of 31.655 lp mm-1, demonstrating the effectiveness of HLCT engineering for high-performance organic scintillators.