Exceptionally Water‐Stable Hybrid Metal Halides via Divide‐and‐Conquer Strategy for Anti‐Counterfeiting and X‐Ray Imaging

The lattice of organic–inorganic hybrid metal halides (OIMHs) is predominated by ionic interactions, which leads to spontaneous hydrolysis and structural collapse in water. Most prevailing stabilization approaches tend to either compromise photophysical performance to some extent or involve relatively tedious fabrication routes, failing to reconcile intrinsic water resistance stability with outstanding optoelectronic performance. Herein, we present a “divide‐and‐conquer” molecular interface engineering paradigm based on two typical types of Sn 2+ ‐ and Cu + ‐based OIMHs. For Sn 2+ ‐based OIMHs, we propose for the first time a counterintuitive “water‐locking” mechanism that exploits lattice water to build a protective hydrogen‐bond network, maintaining structural integrity in water for over one year. For Cu + ‐based OIMHs, a “covalent armor” strategy switches the bonding mode from ionic to covalent, anchoring a hydrophobic organic molecular layer onto the inorganic framework to prevent water erosion. Benefiting from these water‐resistant designs, we demonstrate multi‐level information encryption and X‐ray imaging applications, enabling non‐encapsulated underwater X‐ray imaging with a record spatial resolution of 15.5 lp/mm, thereby laying a firm foundation for their practical use in underwater and high‐humidity scenarios.