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　Cu4I4　cubes　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　Cu4I4　scintillators　in　achieving　a　lowest　X-ray　detection　limit　of　77　nGy　s−1　and　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.　©　2023,　The　Author(s).