Nearly Unity Intersystem Crossing in Organometallic Emitters Enabling High‐Performance X‐Ray Scintillators

Abstract Advanced X‐ray imaging scintillators hold great potential for medical diagnostics, security screening, and aerospace applications. However, organic materials encounter significant challenges in achieving high X‐ray absorption and efficient exciton utilization due to their low atomic number and weak intersystem crossing (ISC), resulting in limited triplet emission when exposed to light excitation. Here, organometallic materials incorporating heavy‐atom metal centers and organic linkers are developed as highly efficient X‐ray imaging scintillators, exhibiting near‐unity ISC and remarkable phosphorescence efficiency. Their optimized triplet‐state properties and enhanced exciton utilization enable superior performance in X‐ray imaging, offering improved sensitivity and spatial resolution. Notably, these materials achieve an ultralow X‐ray detection limit of 84 nGy s −1 , an impressive imaging resolution of 26.9 lp mm −1 , and an outstanding light yield of 70.5 k photons MeV −1 , substantially outperforming conventional organic and inorganic scintillators available in the X‐ray imaging market. Moreover, these organometallic scintillators demonstrate nearly a threefold improvement in X‐ray sensitivity at low temperatures, highlighting their significant potential for low‐temperature X‐ray imaging.