High‐Triplet‐Energy Ancillary‐Ligand Regulation of Dual‐Antenna Eu(III) Scintillators: Near‐Unity PLQY for 3D X‐Ray Videography

Eu(III) complex scintillators, benefiting from efficient triplet‐exciton utilization and strong x‐ray absorption, are promising candidates for advanced x‐ray detection and imaging. However, current design strategies for dual‐antenna Eu(III) complex scintillators primarily prioritize low‐triplet‐energy primary ligands, whereas the energetic and mechanistic roles of high‐triplet‐energy ancillary ligands remain insufficiently explored. Herein, we present a systematic study on ancillary‐ligand‐regulated scintillation in dual‐antenna Eu(III) complex scintillators. Using dibenzoylmethane (DBM) as the primary ligand and three high‐triplet‐energy arylphosphine oxides as ancillary ligands, a series of Eu(DBM) 3 (L) complexes (L = Dpepo, Xpo, Dppbo) were constructed. Among them, Eu(DBM) 3 (Dppbo) exhibits a near‐unity photoluminescence quantum yield of 98.5% and an ultrahigh relative light yield of 61737 photons MeV −1 . This outstanding performance arises from triplet‐energy matching between the Dppbo and DBM that enhances ligand‐ligand charge transfer, together with increased ligand rigidity and ordered π–π stacking that suppresses nonradiative decay and facilitates charge‐carrier transport. Furthermore, Eu(DBM) 3 (Dppbo) was embedded into a styrene‐ethylene‐butylene‐styrene elastomer to afford a large‐area stretchable scintillation film, delivering > 30 lp mm −1 in static x‐ray imaging and enabling advanced x‐ray videography, including 3D dynamic, underwater, and stretchable imaging. These findings establish ancillary‐ligand engineering as a general molecular design strategy for achieving highly efficient Eu(III) complex scintillators.