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

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.