Solvatochromism based on structural color: Smart polymer composites for sensing and security

We report a convenient and scalable strategy to achieve large-area transparent photonic crystal (TPC) films that can be fully reversibly switched between an initial transparent state and a structurally-colored state. This photonic material is based on an ordered colloidal crystal structure of polymer core-shell particles with an engineered refractive index balance of the core and the shell components. Highly transparent viscoelastic quasi-solid films are created that can be spatially UV-cross-linked, giving invisible encrypted photonic patterns with different solvent response compared to non-cross-linked regions, which appear after immersion in solvent media, thus demonstrating clear and fully reversible/switchable solvatochromic properties. Our intuitive simulation models demonstrate how the emergence of structural color occurs due to increasing refractive-index contrast, whereas wavelength shifting originates from commensurate solvent swelling of the shell medium. In addition to the very clear revelation of optical patterns (large fractional changes in reflectance, from transparency to a resonant reflectivity of ~10%), these materials and thin-film generation methods are inherently scalable, robust and versatile. The demonstrated stimuli-responsive photonic films may thus give rise to new practicable applications in the fields of environmental sensing and product security