Visible-Light Responsive Azobenzene and Cholesterol Based Liquid Crystals as Efficient Solid-State Solar–Thermal Fuels

Solar–thermal fuels (STFs) based on photoresponsive molecules, which can harvest and store solar energy by the configurational change of molecules and can release it in the form of heat on demand, have been investigated recently. So far, azobenzene has been the most widely studied molecule for these applications. However, until now, only solid and liquid derivatives of azobenzene have been explored which suffer from several limitations. Here, we are reporting for the first time visible-light responsive STFs based on liquid crystals (LCs). We have prepared a series of compounds based on azobenzene and cholesterol with varying spacer and ortho-substitution on azobenzene. All of these derivatives exhibit enantiotropic chiral nematic (N*) mesophases. Moreover, they showed excellent photostability, photocyclability, and long half-life times of cis-states. We have further evaluated the thin films of these compounds for charging, i.e., trans to cis conversion under solar irradiation using various bandpass filters, and also studied the kinetics of the conversion. These compounds exhibited a maximum charging of up to 70% for the derivative with ortho-fluoro azobenzene. The charged thin films were further evaluated for their heat release properties by infrared (IR) thermal imaging. The maximum heat release observed was around 5.4 °C from the surrounding temperature which was significantly higher as compared to similarly substituted azobenzene-based derivative without a LC phase.