Reversible Single-Crystal-to-Single-Crystal Isomerization of the Azo Photoswitch

The reversible <i>E</i> ⇄ <i>Z</i> photoisomerization of azo molecules is of fundamental significance for the development of photoresponsive materials, but it is typically hindered in crystals due to restricted molecular motion. A longstanding challenge has been the achievement of single-crystal-to-single-crystal (SCSC) isomerization, in which isomerization occurs in bulk molecular crystals while preserving the perfect single-crystalline order. In this work, SCSC <i>E</i> ⇄ <i>Z</i> isomerization of an azobispyrazole molecule is directly observed by single-crystal X-ray diffraction. The crystal lattice similarity between the <i>E</i> and <i>Z</i> single crystals favors such a transformation. The single crystal structures of two key intermediate states are captured, revealing a three-step transformation process. The process involves not only conformation change and isomerization but also molecular motions including interlayer gliding and molecular distance adjustment that proceed with minimal displacements. This discovery redefines the long-held perception that azo isomerization inevitably disrupts single crystallinity, thereby opening new avenues for the development of light-responsive crystalline materials and devices.