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( E )-4-Fluoro-cinnamaldehyde malononitrile (( E )- 4FCM ) is a new phenylbutadiene derivative that undergoes a [2+2] photocycloaddition in the crystal form. Optical absorption and proton nuclear magnetic resonance ( 1 H-NMR) measurements demonstrate that the solid-state ( E )- 4FCM photodimerization is a negative photochromic reaction that proceeds to 97% completion. The large geometry change and full conversion allow bulk crystals of ( E )- 4FCM to show strong photosalient effects when exposed to 405 nm ultraviolet light. When ( E )- 4FCM nanowires are grown in an anodic alumina oxide (AAO) template, they maintain a high degree of crystallinity and orientation, as determined by X-ray diffraction measurements. When illuminated, ( E )- 4FCM nanowire bundles exhibit a rapid expansion, during which they spread by as much as 300% in the lateral direction. This lateral expansion is at least partially due to a photoinduced crystal expansion along the diameter of the nanowires. When ( E )- 4FCM nanowires are confined inside the AAO template, the photoinduced expansion can be harnessed to deform the template, causing it to bend under UV light irradiation. The bending motion due to 2.0 mg of 4FCM in a template can cause the template to bend by up to 1.0 mm and lift up to 200 g. These results represent a significant improvement in work output relative to previous composite actuator membranes based on diarylethene photochromes. 

Crystals composed of photoreactive molecules represent a new class of photomechanical materials with the potential to generate large forces on fast timescales. An example is the photodimerization of 9- tert -butyl-anthracene ester ( 9TBAE ) in molecular crystal nanorods that leads to an average elongation of 8%. Previous work showed that this expansion results from the formation of a metastable crystalline product. In this article, it is shown how a novel combination of ensemble oriented-crystal solid-state NMR, X-ray diffraction, and first principles computational modeling can be used to establish the absolute unit cell orientations relative to the shape change, revealing the atomic-resolution mechanism for the photomechanical response and enabling the construction of a model that predicts an elongation of 7.4%, in good agreement with the experimental value. According to this model, the nanorod expansion does not result from an overall change in the volume of the unit cell, but rather from an anisotropic rearrangement of the molecular contents. The ability to understand quantitatively how molecular-level photochemistry generates mechanical displacements allows us to predict that the expansion could be tuned from +9% to −9.5% by controlling the initial orientation of the unit cell with respect to the nanorod axis. This application of NMR-assisted crystallography provides a new tool capable of tying the atomic-level structural rearrangement of the reacting molecular species to the mechanical response of a nanostructured sample.