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.全卤代顺式偶氮苯（perhalogenated cis-azobenzenes）具有异常长的热半衰期，这为其结构表征提供了可行条件，并首次获得了单晶到单晶型顺式→反式偶氮苯异构化的实验证据。通过可见光辐照，单块全卤代顺式偶氮苯单晶可转化为其反式异构体的多晶聚集体；该光机械转变（photomechanical transformation）过程中，晶体形貌会发生显著、可控且热不可逆的变化。这是偶氮苯类材料领域首次实现晶体形貌永久性光机械改性的研究实例。