MPa-Level Piezochromic Excimer Switching for Self-Erasing Visual Encryption.

Piezochromic fluorescence usually requires high pressure and often suffers from intensity quenching, limiting its utility in encryption and optical security. Here we uncover a previously unrecognized MPa-level dynamic excimer-switching mechanism in 9-phenylanthracene (An-Ph) microcrystals, enabled by pressure-induced reorganization within pre-associated J-type columns. Under increasing pressure (0-40 MPa), the emission evolves from blue → white → intense cyan, accompanied by enhanced rather than quenched brightness. Upon pressure release, unstable excimer states dissipate, yet the material still exhibits bright white emission with a high quantum yield (67.2%) from a mixed monomer-excimer output. A 3D chromaticity trajectory is introduced to precisely visualize monomer-excimer conversions, revealing reversible optical pathways inaccessible to conventional CIE mapping. Single-crystal analysis and molecular dynamics jointly elucidate the formation and relaxation of multiple Y- and X-type excimer conformations under MPa-level stresses. Finally, we demonstrate a recyclable, mechanically triggered self-erasing QR/Morse encryption system, highlighting the material's potential for dynamic, time-gated security. This work establishes a general design framework for low-pressure excimer activation and mechanically adaptive photonic materials.