Copper(I) Clusters With Different Nuclear Structures Featuring Thermally Activated Delayed Fluorescence for Highly Efficient X‐Ray Scintillator

ABSTRACT Cu(I) clusters are molecular aggregates consisting of several Cu centers consolidated by organic ligands. They typically exhibit significant metal‐to‐ligand charge transfer (MLCT) characteristics, enabling thermally activated delayed fluorescence (TADF) emission, making them promising candidates for next‐generation X‐ray scintillators. However, the low triplet exciton utilization rate is a critical obstacle to their further practical application. To address this challenge and establish key structure‐property relationships, we designed a series of Cu(I) clusters featuring a diversity of nuclear structures (the number and arrangement of Cu) and coordination environments. Our investigation reveals that the PPh 3 ‐coordinated mononuclear clusters (CuI(PPh 3 ) 2 Py) exhibit high structural rigidity, suppressing excited‐state distortion and thereby forming a narrow singlet‐triplet energy gap (Δ E ST = 0.09 eV). This enables efficient reverse intersystem crossing (RISC), maximizing triplet exciton utilization. Moreover, the mononuclear structure maximally suppresses non‐radiative relaxation pathways, achieving near‐unity photoluminescence quantum yields (99.5%). The scintillator films exhibit imaging quality comparable to that of commercial BGO, achieving a spatial resolution of 17.5 lp/mm. Furthermore, high‐definition imaging of low‐brightness images through digital image processing algorithms was realized. This work establishes key structure‐property relationships in Cu(I) clusters, paving the way for the rational design of advanced X‐ray scintillators.