Reversible Ion-Conducting Switch by Azobenzene Molecule with Light-Controlled Sol–Gel Transitions of the PNIPAm Ion Gel

Exploring a simple, on-demanding method of manipulating ionic conduction of ionic liquids with large amplitudes is a challenging task. Here, a reversible ion-conducting switch was obtained based on photoswitchable sol-gel transitions. The device was successfully applied in an electronic circuit to switch it on/off. The ion gel was prepared by directly mixing following individual components: azobenzene (Azo), poly(N-isopropylacrylamide) (PNIPAm), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]). The mixture is denoted as Azo/PNIPAm/[C2mim][NTf2]. The framework of this gel structure was particularly designed as an analogue to the physical mode of control theory: sensor/amplification/action. Light-induced isomerization of Azo acts as the light sensor to trigger the macroscopic sol-gel transition of PNIPAm assemblies. Such transition works as the amplification, which significantly affects the ionic movements, resulting in high-amplitude switching behavior. A photoswitchable ionic conductive device was demonstrated as action in this paper. Under UV irradiation, the sol-like state of Azo/PNIPAm/[C2mim][NTf2] provided a higher ion conduction (around 1 mS/cm) while being exposed to visible light, and a lower ion conduction (0.04 mS/cm) was observed in the gel state. This photoswitchable ion conductivity device was integrated to a well-designed logic gate to switch circuits on or off. This confirms the possible practical application of the sol-gel device, which outputs stable and detectable electrical signals. The research here demonstrates a simple but effective strategy to control the ionic movements, which can be applied in optoelectronic devices. The principle can be used to design different types of molecular optoelectronic switches.