Light-Responsive Solid–Solid Phase Change Materials for Photon and Thermal Energy Storage

[Image: see text] We report a series of adamantane-functionalized azobenzenes that store photon and thermal energy via reversible photoisomerization in the solid state for molecular solar thermal (MOST) energy storage. The adamantane unit serves as a 3D molecular separator that enables the spatial separation of azobenzene groups and results in their facile switching even in the crystalline phase. Upon isomerization, the phase transition from crystalline to amorphous solid occurs and contributes to additional energy storage. The exclusively solid-state MOST compounds with solid–solid phase transition overcome a major challenge of solid–liquid phase transition materials that require encapsulation for practical applications.