Photoinduced Reversible Solid‐to‐Liquid Transitions for Photoswitchable Materials

Heating and cooling can induce reversible solid‐to‐liquid transitions of matter. In contrast, athermal photochemical processes can induce reversible solid‐to‐liquid transitions of some newly developed azobenzene compounds. Azobenzene is photoswitchable. UV light induces trans‐to‐cis isomerization; visible light or heat induces cis‐to‐trans isomerization. Trans and cis isomers usually have different melting points (T_m) or glass transition temperatures (T_g). If T_m or T_g of an azobenzene compound in trans and cis forms are above and below room temperature, respectively, light may induce reversible solid‐to‐liquid transitions. In this Review, we introduce azobenzene compounds that exhibit photoinduced reversible solid‐to‐liquid transitions, discuss the mechanisms and design principles, and show their potential applications in healable coatings, adhesives, transfer printing, lithography, actuators, fuels, and gas separation. Finally, we discuss remaining challenges in this field.