Para-thioalkyl-substituted arylazo-3,5-dimethylisoxazoles show high degree of forward and reverse isomerization accompanied by reversible solid-liquid phase transition under visible light, and allow storage of a high energy density (visible-light energy+latent heat) of 190–196 J/g in the liquid cis-state. The stored energy can be readily released as heat by green light illumination.

Abstract

The photoswitchable MOlecular Solar Thermal (MOST) energy storage systems that are capable of exhibiting high energy storage densities are found to suffer from the poor cyclability, the use of less abundant UV light of the solar spectrum, or reduced charging/discharging rates and poor photoconversions in solid states. Herein, we have designed and readily synthesized a novel set of para-thioalkyl substituted arylazoisoxazoles, that undergo high trans-cis and cis-trans photoconversions under visible light, and show fast charging/discharging and impressive cyclability. Remarkably, the presence of C6-or C10-thioalkyl chainin photochromes permitted reversible solid-liquid phase transition with the formation of cis-enriched charged states by 400 nm light irradiation and trans-enriched discharged states by 530 nm light at various temperatures (10–35 °C). The solid-to-liquid phase transition enabled storage of the latent heat in addition to the isomerization energy, resulting in a high net energy storage density of 189–196 J/g, which are substantially higher than that of many recently reported azobenzene-based MOST compounds (100–161 J/g). Using a high-resolution infrared camera, we further demonstrated that a brief irradiation of green light can be employed to readily release the trapped photon energy as heat. Our results suggest that the arylazoisoxazole with C6-thioalkyl chain at para-position can serve as an effective and eco-friendly photoliquefiable MOST material.