Halogen-bonded charge-transfer co-crystal scintillators for high-resolution X-ray imaging

The development of high-quality organic scintillators encounters challenges primarily associated with the weak X-ray absorption ability resulting from the presence of low atomic number elements. An effective strategy involves the incorporation of halogen-containing molecules into the system through co-crystal engineering. Herein, we synthesized a highly fluorescent dye, 2,5-di(4-pyridyl)thiazolo[5,4-d]thiazole (Py₂TTz), with a fluorescence quantum yield of 12.09%. Subsequently, Py₂TTz was co-crystallized with 1,4-diiodotetrafluorobenzene (I₂F₄B) and 1,3,5-trifluoro-2,4,6-triiodobenzene (I₃F₃B) obtaining Py₂TTz-I₂F₄ and Py₂TTz-I₃F₃. Among them, Py₂TTz-I₂F₄ exhibited exceptional scintillation properties, including an ultrafast decay time (1.426 ns), a significant radiation luminescence intensity (146% higher than Bi₃Ge₄O₁₂), and a low detection limit (70.49 nGy s^-1), equivalent to 1/78th of the detection limit for medical applications (5.5 μGy s^-1). This outstanding scintillation performance can be attributed to the formation of halogen-bonding between I₂F₄B and Py₂TTz. Theoretical calculations and single-crystal structures demonstrate the formation of halogen-bond-induced rather than π-π-induced charge-transfer cocrystals, which not only enhances the X-ray absorption ability and material conductivity under X-ray exposure, but also constrains molecular vibration and rotation, and thereby reducing non-radiative transition rate and sharply increasing its fluorescence quantum yields. Based on this, the flexible X-ray film prepared based on Py₂TTz-I₂F₄ achieved an ultrahigh spatial resolution of 26.8 lp per mm, underscoring the superiority of this strategy in developing high-performance organic scintillators.