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.