Fast Triplet-Exciton Harnessing in Organic X-Ray Scintillators for Dynamic Radiography

Organic scintillators that combine high triplet-exciton utilization with rapid radioluminescence are vital for achieving high-resolution dynamic X-ray imaging. Thermally activated delayed fluorescence (TADF) scintillators offer an attractive route to efficient triplet harvesting, yet conventional systems face an intrinsic trade-off between radioluminescence efficiency and decay lifetime. This limitation originates from inherently slow reverse intersystem crossing (RISC), which is difficult to accelerate because minimizing the singlet-triplet energy gap (ΔEST) often reduces spin-orbit coupling (SOC). Here, we present a molecular design strategy that overcomes this constraint by simultaneously reducing ΔEST and enhancing SOC through strengthened hybridization between charge-transfer (CT) and locally excited (LE) states, leading to a hybridized ³LE/³CT triplet manifold coupled to a predominantly ¹CT singlet state. Guided by this principle, we developed three TADF scintillators by pairing a 2-chloro-7-fluoroquinazoline acceptor with phenoselenazine, phenothiazine, or phenoxazine donors. These scintillators exhibit a high photoluminescence quantum yield of 96.3% and an accelerated RISC rate approaching 10⁷ s⁻¹. Impressively, under X-ray irradiation, they achieve a light yield of 26508 photons M eV⁻¹ and a fast radioluminescence lifetime of 3 μs, enabling both high-resolution static imaging and real-time dynamic radiography. This work provides a generalizable molecular design strategy for developing high-performance TADF scintillators for X-ray imaging.