Engineering Flexible Photonic Composites From Cellulose Nanocrystals and Poly(Vinyl Alcohol) With Multi‐Stimuli Responsive Optical Properties

Counterfeiting poses serious threats to global security and economic integrity, driving the demand for advanced anti‐counterfeiting materials with multi‐modal optical responses. Here we report a flexible, multi‐responsive photonic crystal composite that integrates structural color, pigmentary color and fluorescence as a unified triple‐modal platform for information encryption. Cellulose nanocrystals (CNCs) were co‐assembled with poly(vinyl alcohol) (PVA) to form robust left‐handed chiral nematic architectures via evaporation‐induced self‐assembly, while laccaic acid (LA) was introduced as a natural fluorescent chromophore. The fabricated CNC/PVA/LA n composite films retained their chiral architecture, producing vivid structural coloration, and simultaneously exhibited tunable photoluminescence. By overcoming the inherent brittleness of pristine CNC films, the incorporation of PVA substantially reinforces the composite's toughness and imparts exceptional solvent resistance. The CNC/PVA/LA n film fluorescence output is strongly modulated by solvent and pH, enabling reversible on/off switching and multi‐state optical coding. Importantly, spectral overlap between the photonic bandgap and LA emission generates pronounced circularly polarized luminescence (CPL) with a dissymmetry factor of −0.21, which is further enhanced to −0.33 and −0.56 after DMSO and DMF treatment, respectively. By integrating multiple responsive optical properties into writable patterns, the CNC/PVA/LA n composite enables dynamic, multi‐level anti‐counterfeiting platforms. These customizable features provide an intelligent photonic strategy for high‐security and rewritable encryption applications.