Scintillator‐based X‐ray imaging has attracted great attention from industrial quality inspection and security to medical diagnostics. Herein, a series of lanthanide(III)‐Cu4I4 heterometallic organic frameworks (Ln‐Cu4I4 MOFs)‐based X‐ray scintillators are developed by rationally assembling X‐ray absorption centers ([Cu4I4] clusters) and luminescent chromophores (Ln(III) ions) in a specific manner. Under X‐ray irradiation, the heavy inorganic units ([Cu4I4] clusters) absorb the X‐ray energy to populate triplet excitons via halide‐to‐ligand charge transfer (XLCT) combined with the metal‐to‐ligand charge‐transfer (MLCT) state (defined as the X/MLCT state), and then the 3X/MLCT excited state sensitizes Tb3+ for intense X‐ray‐excited luminescence via excitation energy transfer. The obtained Tb‐Cu4I4 MOF scintillators exhibit high resistance to humidity and radiation, excellent linear response to X‐ray dose rate, and high X‐ray relative light yield of 29 379 ± 3000 photons MeV−1. The relative light yield of Tb‐Cu4I4 MOFs is ≈3 times higher than that of the control Tb(III) complex. X‐ray imaging tests show that the Tb‐Cu4I4 MOFs‐based flexible scintillator film exhibits a high spatial resolution of 12.6 lp mm−1. These findings not only provide a promising design strategy to develop lanthanide‐MOF‐based scintillators with excellent scintillation performance, but also exhibit high‐resolution X‐ray imaging for biological specimens and electronic chips.

non-destructive testing and medical theranostic applications. [1] Conventional scintillators containing heavy atoms (e.g., CsI:Tl, Bi 4 Ge 3 O 12 , PbWO 4 , YAlO 3 :Ce) have been commercially utilized in X-ray imaging, however, almost all of them relied on costly bulk crystals and were grown by the Czochralski method under harsh preparation conditions. [2] Recently, lead halide perovskite-based scintillators exhibited outstanding performance, but the self-absorption, hygroscopicity and elemental toxicity severely limit their practical applications. [3] Numerous attempts have been made to address these issues by developing lead-free perovskite analogs, trying to replace lead with other elements such as silver, [4] bismuth, [5] antimony, [6] manganese [7] and copper. [8] Lanthanide-doped inorganic oxide or fluoride nanoscintillators [9] and organic scintillators [10] have also been reported. Despite enormous efforts, the development of novel scintillators with low cost, high performance, and good solution processability remains a formidable challenge.The general scintillation process mainly consists of three stages: conversion, transport, and luminescence. [11] Under X-ray irradiation, electrons and holes were mainly generated in the inner layers of atoms through the photoelectric effect and Compton scattering. Subsequently, the cascaded secondary electrons migrated to the conduction band and valence band or generated low-energy excitations for luminescence. In this sense, most of the light emission in scintillators was excited by electronic transitions caused by electron impact, which was similar to the electroluminescence in some respects. [12] A large number of triplet excitons and singlet excitons can be generated in a ratio of 3:1. Thus, we can naturally think that those molecules that have been successfully utilized in organic light-emitting diodes (OLEDs) are likely to be good candidate for X-ray excited luminescence (XEL) materials. Yang and coworkers reported three organic thermally activated delayed fluorescence (TADF) scintillators for high-resolution X-ray imaging, which demonstrated the importance of harvesting both singlet and triplet excitons for efficient X-ray scintillation. [13] Although these organic scintillators showed high relative light yields, in fact, their XEL intensity Scintillators have attracted tremendous attention due to their great potential in radiation detection, industrial non-destructive testing, and medical theranostic applications. Thermally activated delayed fluorescence (TADF) based scintillators can simultaneously harvest radiation-induced singlet and triplet excitons for high-efficient X-ray excited luminescence. However, pure organic TADF materials composed of light elements exhibit low X-ray absorption coefficients, resulting in relatively weak X-ray excited luminescence. Herein, a series of novel high-performance X-ray scintillators based on TADF mononuclear Cu(I)-halide complexes are successfully developed. Together with high X-ray absorption coefficient of heavy h...

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