Crystal structure of the meta-stable intermediate in the photomechanical, crystal-to-crystal reaction of 9-tert-butyl anthracene ester

Abstract

The photodimerization of 9-tert-butyl-anthracene ester in molecular crystal nanorods can cause expansions of up to 15%. This expansion results from the formation of a metastable crystalline intermediate, the solid-state reacted dimer (SSRD). In this paper, a combination of powder X-ray diffraction, solid-state nuclear magnetic resonance, and computational modeling is used to determine the crystal structure of the SSRD intermediate. An ensemble of six possible structures is obtained, which differ only in small packing details that lead to different crystal space groups. The structure with the best overall agreement with the experimental data belongs to the Pccn space group and, like the other members of the ensemble, retains a packing motif close to that of the monomer crystal. This crystal structure allows us to conclude that the SSRD crystal density is slightly greater than that of the monomer crystal and that gross changes in the volume or unit cell parameters of the SSRD are not responsible for the photoinduced expansion.