Color-Changing Paints Enabled by Photoresponsive Combinations of Bio-Inspired Colorants and Titanium Dioxide Semiconductors

For thousands of years, humans have incorporated natural materials into paints and coatings for decorative and protective purposes. Over time, these formulations have been further engineered to provide long lasting and a wide variety of colors, as well as optimized for modern functions ranging from fine art to extraterrestrial thermal control. Recently, there has been a push to design materials that can undergo transient visual changes in response to external stimuli without the need for advanced fabrication strategies or external power sources. In this work, we design photochromic paint formulations that utilize xanthommatin, a redox-active small molecule colorant found throughout biology, and titanium dioxide, a semiconductor and ubiquitous whitening pigment in commercial coatings. This combination of materials triggers the reversible photoreduction of xanthommatin when exposed to sunlight, shifting the coating from a yellow (oxidized) to red (reduced) color. We observe that the degree of photoreduction is dependent on the density, size, and polymorph structure of the titanium dioxide particles, as well as the type of paint matrix. These coatings can be mixed with non-photoresponsive colorants to increase the range of accessible colors and can be applied to both rigid and flexible substrates. Additionally, these coatings can be selectively irradiated using masks to create transient disappearing artwork. Overall, this work demonstrates the first step in using natural colorants for dynamic photoresponsive coatings and highlights new opportunities for consumer goods and low power optical materials.<br/><br/><i>Approval for public release pending</i>