Shaping Crystals with Light: Crystal-to-Crystal Isomerization and Photomechanical Effect in Fluorinated Azobenzenes

Unusually long thermal half-lives of perhalogenated cis-azobenzenes enabled their structural characterization and the first evidence of a crystal-to-crystal cis -> trans azobenzene isomerization. Irradiation with visible light transforms a perhalogenated cis-azobenzene single crystal into a polycrystalline aggregate of its trans-isomer in a photomechanical transformation that involves a significant, controllable, and thermally irreversible change of crystal shape. This is the first demonstration of permanent photomechanical modification of crystal shape in an azobenzene.