Multi-stimulus responsive polyoxometalate–viologen hybrid ionic liquids for rewritable inkless printing and environmental monitoring

Abstract

Multi-stimulus responsive materials have attracted significant attention due to their ability to respond dynamically to multiple external stimuli, including light, temperature, and chemical signals. Achieving both rapid response times and precise selectivity is essential for advancing the functionality of such materials. In this study, five polyoxometalate-based viologen ionic liquids (ILs) were synthesized through ion exchange between H₃PMo₁₂O₄₀ and viologen ILs with varying substituents. These ILs demonstrated multi-stimulus responses to light and ammonia gas. Under Xe lamp irradiation, all ILs exhibited prominent photochromism, with C₁₂–PMo₁₂ undergoing a rapid change from light yellow to light green within just 5 seconds. Exposure to an H₂O₂ atmosphere for 3 hours resulted in the complete decolorization of C₁₂–PMo₁₂. Remarkably, after five cycles of coloring and decoloring, its absorbance remained virtually unchanged, highlighting its excellent reversibility and stability. Additionally, as the alkyl chain length of the viologen-based cation increased, the coloring rate accelerated, and the color contrast enhanced, likely due to the suppression of viologen radical quenching by longer carbon chains. These ILs also exhibited gas-chromic behavior in the presence of ammonia, showing high detection selectivity. Furthermore, the materials were successfully applied in inkless erasable printing and ultraviolet detection. The C₁₂–PMo₁₂-based carbon paper displayed clear and readable prints in only 10 seconds and could be bleached in an H₂O₂ atmosphere within 1.5 hours. These findings demonstrate that the synthesized ILs possess versatile multi-stimulus responsive properties, making them highly promising for applications in inkless printing, ultraviolet detection, and amine vapor sensing.