Tailored Mn2+-Activated Zinc Halides for Enhanced X-ray Imaging via Ligand Modulation.

Lead-free organic-inorganic metal halides have emerged as promising candidates for X-ray imaging applications owing to their intrinsic radioluminescence, solution processability, and economic viability. However, their development remains largely empirical, with limited understanding of the structure-property relationships governing radioluminescence efficiency and stability. Herein, we address this challenge using two zero-dimensional (0D) mixed-metal Mn-Zn bromide systems with distinct organic spacers (T = C9H14BrN and D = C6H12N2), enabling controlled modulation of organic spacer length and metal-metal separation. Although both systems exhibit near-unity photoluminescence quantum yields (PLQYs), they display markedly different scintillation responses. The larger metal-metal separation in Mn0.5Zn0.5Br4T, induced by the longer organic cation T, suppresses nonradiative interactions, leading to enhanced radioluminescence efficiency and operational stability. The optimized system, Mn0.5Zn0.5Br4T, achieves a high light yield of 76,000 photons MeV-1, excellent stability, and an ultralow detection limit of 112 nGy s-1-approximately 50 times lower than standard medical diagnostic dose rate (5.5 μGy s-1). Flexible large-area scintillators further demonstrate high-resolution X-ray imaging capability. This work establishes a structure-property relationship linking organic spacer modulation, mixed-metal composition, and scintillation efficiency, enabling a rational design framework for scalable, high-performance X-ray scintillators.