Insight into Integrated Multi-Stimuli Chromism in Viologen-Based Composite Hydrogels

Viologen-based materials are widely used in electrochromic (EC) systems because of their reversible redox properties. However, their large energy gap makes it difficult to achieve efficient photochromism (PC) at the molecular level. This limitation restricts their further development into multifunctional responsive materials. Therefore, improving their photoresponses and integrating multiple stimuli into one system remain important challenges. Here, we report multi-stimuli-responsive composite hydrogels prepared by integrating viologen derivatives and TiO 2 nanoparticles (NPs) into a poly(vinylpyrrolidone) (PVP) polymer network. Under UV irradiation, the viologen composite hydrogels exhibit obvious photochromic behavior, showing a distinct purple coloration. The colored state can be maintained for more than one month under an N 2 atmosphere. Interestingly, the incorporation of TiO 2 enables thermochromic (TC) behavior in the viologen-based composite hydrogels. After heating at 80 °C for 10 min, a distinct blue coloration was observed. This work provides a practical strategy for the design of multifunctional chromic materials.