Efficient Harvesting of Triplet Excitons via a T₁-Blocked TADF Mechanism in Series MOFs for Optimal X-ray Detection and Imaging

Scintillators play vital roles in fields such as medical imaging, high-energy physics, astronomy, and radiation monitoring. Their operational principle, rooted in the excitation of high-energy radiation, underscores that luminescence efficiency in scintillators is fundamentally limited by their capacity to harness triplet excitons. In this context, thermally activated delayed fluorescence (TADF) molecules present a promising avenue, enabling the efficient utilization of triplet excitons through thermally activated up-conversion, thereby advancing the development of superior scintillators. Our investigation reveals a T₁-blocked TADF mechanism in H₄TCPE, where efficient singlet-triplet exciton transfer arises from the minimized S₁-T₂ energy gap (0.18 eV). Unlike conventional TADF molecules, H₄TCPE features carboxylic acid groups that enable heavy metal coordination to enhance X-ray attenuation. Using tetravalent metals (Zr, Hf, and Th) as nodes and H₄TCPE as linkers, we fabricated metal-organic frameworks (MOFs) that synergize H₄TCPE's TADF properties with metal-enhanced radiation absorption. The resulting MOFs show X-ray detection and imaging performances superior to pure H₄TCPE (20.0 lp mm⁻¹ and 1.15 µGy s⁻¹ for Th-TCPE vs. <14.3 lp mm⁻¹ and 5.01 µGy s⁻¹ for H₄TCPE), with efficacy correlating to metal atomic number. This work not only broadens TADF molecular diversity through a new energy transfer mechanism and pioneers TADF-MOF integration for advanced radiation detection.