Reversible Phase Transitions of all Inorganic Copper-Based Perovskites: Water-Triggered Fluorochromism for Advanced Anticounterfeiting Applications

Recently, several lead halide perovskites have been demonstrated as functional materials with hydrochromic, thermochromic, and photochromic properties for anticounterfeiting applications. However, their lead toxicity and relatively poor stability are still drawbacks for large-scale practical applications. In this work, we report the reversible water-triggered fluorochromism of the copper-based perovskites, which utilize the crystalline phase transition between Cs 3 Cu 2 I 5 and CsCu 2 I 3 . The custom-designed images printed using the Cs 3 Cu 2 I 5 ink are invisible under daylight but emit intense blue fluorescence under UV light. This fluorescence undergoes a blue-to-orange color change upon exposure to water, which can be recovered upon removing water. Furthermore, the fluorochromism of Cs 3 Cu 2 I 5 is tunable to blue, blue-green, and cyan via anion-exchange reactions in various halide salt solutions. These multicolor- and multistimuli-responsive benefits make Cs 3 Cu 2 I 5 a promising chromic material for high-level information security applications.