Reproducible Pressure‐Sensitive Fluorescence Switch for Anti‐Counterfeiting and Information Encryption

Abstract Pressure‐responsive smart materials with rapid response capabilities are promising candidates for sensing and information security. Here, a reversibly operable photoluminescence on‐off switch activated by external pressure is first achieved in metal halide Cs 3 MnBr 5 nanocrystals (NCs). The triggered pressure is as low as 0.43 GPa that can be easily accessible through manual squeeze. First‐principles calculations reveal that the approaching [MnBr 4 ] tetrahedral units with off‐centering distortion facilitate cross‐relaxation, energy migration and trap states activation, ultimately quenching the luminescence. Meanwhile, the all‐inorganic and rigid framework of Cs 3 MnBr 5 NCs contributes significantly to their stability after undergoing pressure cycles. Such reversible low‐pressure‐caused quenching (RLPCQ) enables the photoluminescence (PL) decay of butterfly patterns created with Cs 3 MnBr 5 NCs to undergo darkening and recovery in response to manual pressing and release. Through the introduction of softer materials, a slight force applied at a specific point can induce a localized PL to monitor the pressure gradient within the film fabricated from Cs 3 MnBr 5 NCs. Furthermore, Morse code information carried by Cs 3 MnBr 5 NCs can remain concealed beneath a green luminescent substrate under normal pressure, but will become distinctly visible when subjected to artificial pressing. The work represents a significant breakthrough to intelligent materials design for applications in anti‐counterfeiting, pressure alarm, and information encryption.