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.