Controlling the Performance of Light-Activated Color-Changing Coatings Using Semiconductor Crystal Structure, Redox-Active Colorants, and Bulk Matrix Composition

Abstract

Nonemissive display technologies, such as e-ink and e-paper, have emerged as reconfigurable alternatives to single-use printed materials that become solid waste. These draw considerably less power than emissive displays but rely on traditional electronic circuitry and computer control to create displayed information. Reversible photochromic systems, in which displayed images are generated by patterned irradiation, enable fabrication of scalable, reversible media from single material formulations, prepared as conformal thin films, without traditional electronic hardware. These systems can comprise combinations of redox-active chromophores and light-sensitive semiconductor particles formulated in polymer matrices. The accessible colors and activation time scales of reversible photoreduction of the embedded dyes, facilitated by ultraviolet stimulation of proximal semiconductor particles, is controlled by the convergence of the optical (e.g., color), electrical (e.g., band gap, redox potential) and physical (e.g., permeability) properties of the components of these formulations. To address this, we investigate the catalytic behaviors of the two most abundant titanium dioxide polymorphs, anatase and rutile, and their effect on the magnitude, rate, and reversibility of photoinduced color change of two redox-active colorants, xanthommatin and methyl viologen. We demonstrate that these formulations can be tuned to provide a diverse color range across a variety of activation time scales by controlling particle size, loading density, and particle–matrix interactions. We establish semiconductor crystal structure and formulation design as critical determinants of optical performance and provide a framework for engineering tunable, low-energy optical materials for adaptive displays and reusable visual communication technologies.