Triplet Excimers and the Heavy-Atom Effect Enable Bright Room-Temperature Phosphorescence and X-ray Scintillation in an Yttrium-Phenothiazine Coordination Polymer

Abstract The design of room-temperature phosphorescent materials that simultaneously offer long lifetimes and high exciton utilization remains challenging. Herein, through a solvothermal reaction between 5-(5,5-dioxido-10H-phenothiazin-10-yl)isophthalic acid (H2PTZO) and Y(NO3)3·6H2O, an yttrium-based coordination polymer, {Y(NO3)(PTZO)(DMA)2}n (Y-PTZO), was constructed. Single-crystal X-ray diffraction reveals T-shaped π–π stacking between the adjacent ligands, providing a structural basis for ground-state dimers that, upon excitation, form triplet excimers. Y-PTZO exhibits bright room-temperature phosphorescence (RTP) with an average phosphorescence lifetime of 148 ms and a total quantum yield of 18.7%. Density functional theory calculations confirm the intermolecular charge-transfer character within the π-stacked dimer. Y-PTZO displays outstanding stability under various storage conditions. Furthermore, Y-PTZO shows a light yield of ∼23,800 photons/MeV under X-ray irradiation and an X-ray detection limit of 2940 nGy·s–1. This work demonstrates a cooperative strategy that combines the heavy-atom effect of Y(III) with a rigid-framework component to achieve excellent RTP and X-ray scintillation performance, offering new insights into triplet excimer engineering in coordination polymers.