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

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 Cs₃MnBr₅ 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 [MnBr₄] 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 Cs₃MnBr₅ 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 Cs₃MnBr₅ 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 Cs₃MnBr₅ NCs. Furthermore, Morse code information carried by Cs₃MnBr₅ 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.