Recent progress in photoinduced transitions between the solid, glass, and liquid states based on molecular photoswitches

Photoinduced transitions between the solid, glass, and liquid states based on molecular photoswitches promise a wide variety of applications. Photoswitchable adhesives are representative examples and are expected to contribute to material recycling for a sustainable future in the era of composite materials due to strong bonding and on-demand photo-induced debonding with minimal damage to the adherends. Only a few molecular photoswitches are known to undergo these transitions, but recent progress, mainly with azobenzene, has been remarkable. Here, we review the photoinduced transitions of small molecules and polymers over approximately a decade and systematically discuss the molecular designs, mechanisms, applications, merits and demerits, and future challenges in each photoswitch and the whole field. We hope this review provides useful information, inspiration, and ideas for the development of this field and the expansion of its applications. Photoinduced transitions between the solid, glass, and liquid states based on molecular photoswitches promise an enormous variety of applications, such as photoswitchable adhesives, which contribute to material recycling for a sustainable future in the era of composite materials. In this review, we highlight recent progress in the photoinduced transitions of small molecules and polymers and systematically discuss the molecular designs, mechanisms, applications, merits and demerits, and future challenges in each photoswitch and the whole field.