Pressure-Responsive Color-Changing Films Using Mechanochromic Fluorenylidene-Acridane

Abstract

In recent years, mechanochromic materials—whose color changes reversibly in response to external physical stimuli—have attracted considerable attention due to their potential applications in pressure sensors, data storage media, and visual interfaces. This review outlines the molecular design, physicochemical properties, and application development of mechanochromic materials based on fluorenylidene-acridane (FA) derivatives. FA exhibits significant changes in visible absorption spectra depending on its conformational state, specifically between folded and twisted forms, and demonstrates reversible chromism in response to a variety of stimuli including mechanical pressure, heat, solvents, vapor, and protonation. Thermodynamic analyses using theoretical calculations and temperature-dependent UV–Vis spectroscopy clarified the equilibrium and energy differences between the two conformers, providing a quantitative understanding of the relationship between structural isomerization and optical properties. On the application side, FA has been successfully incorporated into cellulose nanofibers (CNFs), acrylic elastomers, organic thin films, and polyvinyl alcohol, resulting in pressure-responsive materials in the form of paper, films, and vapor-deposited layers. In particular, FA/CNF composite films displayed reversible color changes from yellow to green under pressure and returned to the original color upon heating or ethanol treatment. These films also demonstrated quantifiable pressure response through nanoimprinting and image-based analysis. Furthermore, FA-based materials were extended to vacuum-deposited films and inkjet-printed patterns, suggesting their potential for high-resolution pressure sensing and smart device applications. Through the integration of molecular chromism design, film fabrication processes, and device engineering, FA-based mechanochromic systems hold promise for practical implementation in next-generation visual pressure sensors, wearable devices, and smart interior technologies.