Solar Thermal Energy Storage Systems Based on Discotic Nematic Liquid Crystals That Can Efficiently Charge and Discharge below 0 °C

Abstract Solid‐state solar thermal fuels (SSTFs) serve as efficient means of storing solar energy as chemical potential energy in a closed loop system and releasing it as heat on‐demand. An ideal SSTF requires photoswitchability in visible‐region without any external heating as well as extended storage times. However, existing systems often rely on ultraviolet (UV) light or heating for photoswitching, or suffer from low storage times. Addressing this, a novel strategy is presented to obtain visible‐light responsive SSTFs designed to operate effectively at room‐temperature or sub‐zero temperatures by innovatively integrating a tetra‐ortho‐fluoro/chloro azobenzene arm in triphenylene based liquid crystal (LC) moiety. The resulting compounds exhibit discotic nematic (N D ) mesophases till −6.5 °C. These compounds exhibit excellent photocyclability, photostability, and sufficient half‐lives of cis states. Achieving up to 77% charging under sunlight with a bandpass filter and 62.4% without it, these systems uniquely demonstrate efficient chargeability and dischargeability at sub‐zero temperatures. Upon discharging, temperature rise of up to 6.5 and 29.5 °C occur at room‐temperature (25 °C) and sub‐zero temperatures (around −6 to −7 °C), respectively. This efficacy is attributed to less‐ordered N D phases providing conformational freedom for photoisomerization at low temperatures.