Hidden Structural Colors from Bistable, Electrically Driven Covalent Organic Framework Photonic Assemblies for Secure Optical Encoding.

Optical encryption using nanostructured materials provides a powerful route for secure data encoding. In this work, an electrically reconfigurable colloidal photonic platform based on covalent organic framework (COF) particles is described, enabling dynamic and bistable data encryption. Spatially controlled electrophoretic assembly of monodisperse COF particles within patterned cells produces Bragg reflections that are visible only under bright-field (BF) microscopy as strong broadband scattering from nanoscale particle surface roughness conceals the encoded states from the naked eye. By tuning the synthesis time, the particle surface roughness and, thus, the degree of concealment can be precisely controlled. Unlike conventional optical systems, where scattering degrades visibility, we report it as an intrinsic security feature, transforming a loss mechanism into a tool for optical masking. The demonstrated platform combines electrical addressability, conditional optical visibility, and algorithmic decoding to deliver a compact, multifactor encryption system. These results demonstrate colloidal COF dispersions as a versatile class of photonic materials for secure displays, anticounterfeiting, and adaptive optical communication technologies.