The photodimerization of 9-tertbutyl-anthracene ester in molecular crystal nanorods can cause expansions of up to 15%. This expansion results from the formation of a metastable crystalline intermediate termed the solid-state reacted dimer (SSRD). Photoreaction of bulk single crystal monomer invariably leads to strain that shatters the crystal, precluding direct characterization with single crystal X-ray diffraction. Here, the combination of powder X-ray diffraction, solid-state nuclear magnetic resonance, and first principles computational modeling is used to determine the crystal structure of the SSRD intermediate and establish a microscopic model for the macroscopic expansion. We find that the SSRD crystal unit cell and volume are quite similar to those of the monomer crystal, leading to the conclusion that gross changes in the volume or unit cell parameters of the SSRD are not responsible for the expansion. At the same time, solid-state NMR of the aligned monomer nanorods and the photoreacted product shows the generation of new lattice orientations within the nanorod. Based on our observations, the nanorods expand not due to a change in the volume of the unit cell, but rather due to a rotation of the cells. While most photomechanical materials rely on the generation of a mixed phase bimorph structure, our results demonstrate how reconfiguration of the product phase can also generate a large mechanical response.

NMR crystallography: Bridging atomic-level structural rearrangement and macroscopic motion in molecular organic crystals

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.