Direct Singlet‐Triplet Transition Enables Efficient X‑Ray‐Activated Organic Room‐Temperature Phosphorescent Scintillators

Organic x‐ray scintillators (OXSTs) hold promising prospects in x‐ray imaging and detection by virtue of their low cost, superior mechanical flexibility, and facile large‐scale fabrication. However, their inferior x‐ray absorption capacity and low exciton utilization efficiency, particularly of triplet excitons, greatly hinder their widespread deployment. Herein, we propose a facile strategy to harness x‐ray‐generated excitons for pure room‐temperature phosphorescent (RTP) emission through direct singlet‐triplet transition enabled by heavy‐atom engineering, which efficiently populate triplet excited states and significantly boost RTP luminescence. Experimental results verify that intramolecular and intermolecular heavy‐atom effects can prominently boost x‐ray absorption capability, and modulate triplet exciton utilization through direct singlet‐triplet transition within aggregated states, thereby promoting the scintillation performance of RTP scintillators under x‐ray excitation. The as‐prepared scintillators achieve a high light yield of 43649 photons MeV −1 , a low x‐ray detection limit of 294 nGy s −1 and an excellent x‐ray imaging resolution exceeding 20.0 lp mm −1 . These values highlight the innovative route toward the rational design of high‐efficiency OXSTs, offering profound guidance for advancing triplet‐involved organic optoelectronic devices and technologies.