Broadband-Light-Induced [2+2] Cycloaddition and Thermoinduced Cycloreversion-Powered Dynamic Molecular Crystals

Photomechanically responsive dynamic molecular crystals are central to developing efficient, rapid, and robust materials capable of conversion of light energy to mechanical work. However, unlike some other, mainly photochromic molecular solar thermal energy storage (MOST) systems, solids that undergo photoinduced [2+2] cycloaddition have not been thoroughly explored for powering reversible actuation, despite that this reaction system carries potential in the heavily strained bonds of the cyclobutane ring. In this study, we propose that broadband-light-induced [2+2] cycloaddition can be used to store energy and actuate dynamic organic crystals by irradiation with visible light. The prototypical material, pyrenylvinylpyrylium tetrafluoroborate (1-PVPyL), undergoes a topochemical [2+2] cycloaddition induced not only by ultraviolet radiation (365 nm) but also by monochromatic green light (532 nm), red light (620 nm) and broadband visible light in a single-crystal-to-single-crystal (SCSC) manner, causing its crystals to bend. The crystals effectively act as energy depots, where the reverse deformation can be initiated by heating and the stored energy is released via thermal cycloreversion reaction. Given the ubiquity of the [2+2] cycloaddition in the solid state, the current study invites the development of new dimeric MOST architectures that utilize sunlight for energy storage and thermal triggers for energy release. Within a broader scope, this approach provides a platform for fabrication of visible-light-driven crystal actuators capable of harnessing sunlight.