Unlocking Organic Cation Diversity in Metal Halides via Concurrent Crystal Growth and Menshutkin Reaction With Demonstrated Scintillation

Organic‐inorganic hybrid metal halides (OIMHs) have garnered great attention for optoelectronics applications. However, the synthesis of OIMHs has been largely confined to the use of pre‐synthesized, commercial organic cations, severely limiting structural diversity and functional exploration. Herein, we present a groundbreaking in situ synthetic paradigm that concurrently executes the Menshutkin reaction and crystal growth in a single step. This one‐pot strategy seamlessly couples the on‐site custom quaternization of organic precursors with the crystallization of metal halide frameworks (such as Mn/Zn/Sb halides). By decoupling the synthesis from commercial cation sources, this method provides unparalleled freedom to incorporate a vast array of tailored cations, including those with long‐chain, multi‐aryl, and sophisticated geometric motifs (e.g., boat, chair, wing, bowl, and cyclic architectures). The power of this approach is exemplified by a novel Mn‐based OIMH, Py‐2 ([B(3‐Br‐PyCH 2 )Ph] 3 [MnBr 4 ] 2 ·2Br), which acts as an exceptional X‐ray scintillator with a high light yield of 60,000 photons/MeV—2.4 times that of commercial LuAG:Ce. A flexible scintillator screen fabricated from Py‐2 further demonstrates a high spatial resolution of 12.77 lp mm − 1 , enabling high‐resolution X‐ray imaging. This work establishes a transformative in situ crystallization platform that opens a new dimension for the bespoke design of functional hybrid materials.