Scale-Dependent Photosalience and Topotactic Reaction of Microcrystalline Benzylidenebutyrolactone Determined by Electron Microscopy and Electron Diffraction

The macroscopic motion of crystals induced by a solid-state photochemical reaction, also known as photosalience, is of interest for the development of micromechanical actuators and crystalline molecular machines. Using microcrystal electron diffraction, we report evidence for a minimum crystal size threshold below which photosalience is not observed for benzylidenebutyrolactone. We confirm our observations by solving the crystal structure of micron-scale specimens before and after their topotactic single crystal-to-single crystal reaction and by collecting transmission electron microscopy images that reveal the absence of photosalient effects for crystals that are below ca. 10 mu m in size. These results indicate that photosalience depends not only on the ability of crystals to support a photochemical reaction but also on a size threshold where the accumulated product phase can form a layer that is separate from that of the reactant, such that their different packing dimensions are able to transduce the collective strain accumulated at the molecularlevel boundaries into the macroscopic motion that propels the entire crystal specimen.