Hidden Structural Colors from Bistable, Electrically Driven COF Photonic Assemblies for Secure Optical Encoding

Optical encryption based on nanostructured materials offers an innovative approach to secure data encoding. Here, we report an electrically addressable, colloidal photonic platform using covalent organic framework (COF) particles that enables dynamic, bistable image encryption. Through the electrophoretic self-assembly of size-controlled COF particles within patterned cells, we achieve spatially resolved Bragg reflections visible only under bright-field (BF) microscopy. The encrypted optical states remain hidden to the naked eye due to strong broadband scattering caused by nanoscale surface roughness—a feature tunable via synthesis time. Notably, the system exhibits bistability, maintaining the encoded state even after the removal of the electric field (EF) and enabling full reversibility by field polarity switching. The combination of electrical control, conditional visibility, and algorithmic decoding results in a multi-factor encryption strategy within a single material system. This work positions colloidal COF dispersions as a new class of multifunctional photonic materials for secure displays, anti-counterfeiting, and adaptive optical technologies.