Efficient and Fast X-Ray Luminescence in Organic Phosphors Through High-Level Triplet-Singlet Reverse Intersystem Crossing

Organic scintillators that efficiently generate bright triplet excitons are of critical importance for high-performance X-ray-excited luminescence in radiation detection. However, the nature of triplet-singlet spin-forbidden transitions in these materials often result in long-lived phosphorescence, which is undesirable for ultrafast X-ray detection and imaging. Here we demonstrate that the effect of hybridized local and charge-transfer (HLCT) excited states enables organic scintillators to exhibit highly efficient and fast radioluminescence (RL) in response to X-ray irradiation. Our experimental and theoretical investigation shows that the oxidized 1,8-naphthalimide-phenothiazine dyad (OMNI-PTZ 2) with HLCT-excited states has an enhanced overlap integral of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) on MNI π-orbitals, and moderate donor–acceptor electron interactions. As a result, the RL of these crystals exhibits a 61-fold increase and its monoexponential decay lifetime is three orders of magnitude faster compared to its corresponding thermally activated delayed fluorescence (TADF) molecule MNI-PTZ 1. We further demonstrate the practical utility of the OMNI-PTZ 2 (G) in high-performance X-ray detection and imaging, achieving an X-ray dose sensitivity of 97 nGy s⁻¹ and an exceptional spatial resolution of 20 lp/mm. Our study provides a promising molecular design principle for utilizing triplet excitons to develop high-efficiency and fast X-ray scintillators for the development of next-generation flexible and stretchable X-ray imaging detectors.