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 μm 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 molecular-level boundaries into the macroscopic
motion that propels the entire crystal specimen.