Unveiling the Anti‐Heavy Atom Effect in Organic Ionic Host‐Guest Phosphorescent Scintillator

ABSTRACT Organic phosphorescent scintillators, offering superior processability and triplet exciton utilization, hold substantial potential for sensitive X‐ray detection. Yet, their practical application is critically constrained by inadequate X‐ray stopping power and intrinsically forbidden radiative decay. Although high‐Z halogen incorporation is frequently employed to enhance radioluminescent intensity, the precise mechanism governing this enhancement remains ambiguous. Here, we reveal a pronounced “anti‐heavy atom effect (anti‐HAE)” in a novel ionic host‐guest system, which defies conventional heavy‐atom principles. Using F/Cl‐substituted potassium benzoate (F/Cl‐BAK) as the ionic host for the phosphorescent ionic guest potassium coronene‐tetracarboxylate (CotA‐K) yields significantly stronger photoluminescence and radioluminescence and better thermal stability than using Br/I‐BAK. Notably, the meta‐ and para‐ halogenated derivatives consistently outperform their ortho‐ counterparts. This anti‐heavy atom effect is ascribed to the reduced basicity of BAK induced by halogen substitution, which suppresses nonradiative decay and thus prevents triplet exciton quenching. Consequently, the CotA‐K@p‐Cl‐BAK system exhibits the highest RL emission intensity and exceptional thermal stability ( > 250°C), enabling high‐performance X‐ray imaging under elevated temperatures. Overall, this work goes beyond the heavy‐atom effect and opens up a new avenue for the rational design of phosphorescent scintillators.