Luminescence clusters composed of organic ligands and metals have gained significant interests as scintillators owing to their great potential in high X-ray absorption, customizable radioluminescence, and solution processability at low temperatures. However, X-ray luminescence efficiency in clusters is primarily governed by the competition between radiative states from organic ligands and nonradiative cluster-centered charge transfer. Here we report that a class of Cu4I4cubes exhibit highly emissive radioluminescence in response to X-ray irradiation through functionalizing biphosphine ligands with acridine. Mechanistic studies show that these clusters can efficiently absorb radiation ionization to generate electron-hole pairs and transfer them to ligands during thermalization for efficient radioluminescence through precise control over intramolecular charge transfer. Our experimental results indicate that copper/iodine-to-ligand and intraligand charge transfer states are predominant in radiative processes. We demonstrate that photoluminescence and electroluminescence quantum efficiencies of the clusters reach 95% and 25.6%, with the assistance of external triplet-to-singlet conversion by a thermally activated delayed fluorescence matrix. We further show the utility of the Cu4I4scintillators in achieving a lowest X-ray detection limit of 77 nGy s−1and a high X-ray imaging resolution of 12 line pairs per millimeter. Our study offers insights into universal luminescent mechanism and ligand engineering of cluster scintillators.