Moisture‐Induced Reversible Phase Conversion of Cesium Copper Iodine Nanocrystals Enables Advanced Anti‐Counterfeiting

Hydrochromic materials have attracted widespread attention in the fields of anti‐counterfeiting because of their ability of the reversible light absorption and/or emission properties in response to water. Here, for the first it is demonstrated that the ternary copper halides Cs 3 Cu 2 I 5 nanocrystals (NCs) possess excellent hydrochromic properties. The prepared Cs 3 Cu 2 I 5 NCs films can dynamically extract and insert CsI by exposing/removing water to realize the reversible conversion between blue‐emissive Cs 3 Cu 2 I 5 and yellow‐emissive CsCu 2 I 3 . Interestingly, polymethyl methacrylate (PMMA) coated Cs 3 Cu 2 I 5 can effectively avoid the extraction of CsI and maintain long‐term stability in the water. Further, the hydrochromic Cs 3 Cu 2 I 5 and water‐resistant Cs 3 Cu 2 I 5 @PMMA are used as the inks to synergistically act on anti‐counterfeiting information to achieve multiple encryption effects, which can clearly identify and authenticate the effective information after moisture decryption. Importantly, the pattern can be re‐encrypted to the invalid pattern after water evaporation. In addition, the anti‐counterfeiting pattern has excellent stability during repeated encryption/decryption conversion cycles, which can not only balance the accessibility of anti‐counterfeiting information but also effectively improve the security of information. This new discovery may not only deepen the understanding of Cs 3 Cu 2 I 5 but also provide new options for the design of hydrochromic materials for anti‐counterfeiting information.