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

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.