Mechanoluminescent Mixed‐Anion Borate Sr ₂ B ₅ O ₉ Cl:Eu for Dynamic Anticounterfeiting

ABSTRACT Mechanoluminescence (ML)—the emission of light in response to mechanical stimuli—offers a direct and visually intuitive route for transducing mechanical information into optical signals. Herein, we report a mixed‐anion strategy to prepare mixed‐anion borate Sr 2 B 5 O 9 Cl:Eu phosphors through a facile, low‐temperature, and ambient‐atmosphere solid‐state synthesis. Benefiting from the intrinsic self‐reducing nature of the borate host, the phosphors demonstrate distinct blue and red photoluminescence under different excitation wavelengths. When embedded in a polydimethylsiloxane (PDMS) matrix, Sr 2 B 5 O 9 Cl:Eu exhibits bright blue ML centered at 423 nm without pre‐irradiation. The ML intensity scales monotonically with the applied strain and decays under repeated mechanical stimulation. Interfacial triboelectric analyses reveal that the ML originates from a triboelectricity‐driven electron‐bombardment process at the phosphor–polymer interface. Leveraging this mechanism, a dual‐mode secure label is fabricated by blending Sr 2 B 5 O 9 Cl:Eu with a conventional green ML phosphor (ZnS:Cu) in PDMS, enabling reversible ultraviolet authentication and irreversible mechanochromic tamper indication. This work establishes mixed‐anion borates as a versatile platform for self‐reduced ML materials and highlights interfacial charge engineering as a powerful strategy for advanced information security in the emerging Internet of Things ecosystem.