Photo-mechanical azobenzene cocrystals and in situ X-ray diffraction monitoring of their optically-induced crystal-to-crystal isomerisation

We demonstrate the first supramolecular cocrystallisation strategy to generate crystalline azobenzene materials with a range of photo-mechanical and thermochemical properties: from those that exhibit isomerisation without any change in crystal shape to those that undergo a crystal-to-crystal cis–trans isomerisation accompanied by large scale bending. The latter permitted the use of variable temperature single crystal X-ray diffraction for the first in situ monitoring of structural changes behind the cis–trans isomerisation in the solid-state, which revealed a topotactic process mediated by an amorphous phase. While the design of photo-mechanical azobenzene solids has so far focused on polymer matrices and gels, the herein presented cocrystallisation approach represents the first methodology for generating new photo-mechanical azobenzene crystals from a limited number of photo-active building blocks, opening a route to potential bio-mimetic and light-harvesting materials based on crystalline solids.