High‐Temperature, Reversible, and Robust Thermochromic Fluorescence Based on Rb₂MnBr₄(H₂O)₂ for Anti‐Counterfeiting

Abstract Thermochromic fluorescent materials (TFMs) characterized by noticeable emission color variation with temperature have attracted pervasive attention for their frontier application in stimulus‐response and optical encryption technologies. However, existing TFMs typically suffer from weak PL reversibility as well as limited mild operating temperature and severe temperature PL quenching. PL switching under extreme conditions such as high temperature will undoubtedly improve encryption security, while it is still challenging for present TFMs. In this work, high‐temperature thermochromic fluorescence up to 473 K and robust structural and optical reversibility of 80 cycles are observed in Rb₂MnBr₄(H₂O)₂ and related crystals, which is seldom reported for PL changes at such a high temperature. Temperature‐driven nonluminous, red and green light emission states can be achieved at specific temperatures and the modulation mechanism is verified by in situ optical and structural measurements and single particle transition. By virtue of this unique feature, a multicolor anti‐counterfeiting label based on a broad temperature gradient and multidimensional information encryption applications are demonstrated. This work opens a window for the design of inorganic materials with multi‐PL change and the development of advanced encryption strategies with extreme stimuli source.