Regulating charge transfer of copper( i ) coordination compounds via conformation engineering for highly efficient radioluminescence and 3D X-ray imaging

Copper(i) coordination compounds have emerged as promising candidates for X-ray scintillators because of their superior X-ray absorption capacity and tunable radioluminescence. However, a strategy that realizes efficient radioluminescence by varying the competition between radiative ligand-related transitions and nonradiative cluster-centered charge transfer is yet to be clearly demonstrated. Here, via conformation engineering, a series of copper(i) iodide coordination clusters have been designed and synthesized, and named (POPy)₄Cu₂I₂, (POPy)₄Cu₄I₄-α and (POPy)₄Cu₄I₄-β (POPy = 4-phenoxypyridine). Mechanistic studies reveal that the radioluminescence in isomeric Cu₄I₄ clusters originates from triplet metal/halide-to-ligand charge transfer (³M/XLCT) and cluster-centered (³CC) excited states, respectively. The experimental results disclose that intramolecular charge transfer is desirable for radioluminescence as it can transfer excitons generated by absorbing radiation ionizations to ligands to form thermal electrons. Due to the large X-ray absorption cross-section of the Cu₄I₄ cube and efficient ³M/XLCT emissions, the scintillation film fabricated by using (POPy)₄Cu₄I₄-α realizes a high resolution of 20.2 lp mm^-1, and further enables a 3D X-ray imaging demonstration. Our work provides a comprehensive comparison of the radioluminescence characteristics between ³M/XLCT and ³CC excited states in isostructural Cu₄I₄-based clusters, providing a template for enhancing scintillation properties through conformation engineering.