Crystal Structures and Photophysical Properties of 9-Anthracene Carboxylic Acid Derivatives for Photomechanical Applications

Molecular crystals composed of 9-anthracene carboxylic acid (9AC) can undergo reversible light-induced mechanical motions driven by a [4 + 4] photodimerization reaction. This paper explores the structure, photophysics, and photomechanical response of a family of anthracene carboxylic acid derivatives, with the goal of finding materials that have comparable or improved photomechanical properties. We find that methyl or phenyl substitution at the 10-position leads to a complete loss of photoreactivity due to changes in crystal packing. A series of halogen (F, Cl, Br) 10-substituted 9AC molecules all showed a similar stacked packing motif, but only the fluoro-substituted molecule was photoreactive in the solid. Its photomechanical response was similar to that of 9AC but with a much longer recovery time. Extending the carboxylic acid by adding a vinylene group at the 9-position resulted in crystals that showed good photoreactivity and a lack of fracture but no reversibility. Attempts to self-consistently rationalize observed trends in terms of excited state lifetimes or steric effects were only partly successful. Balancing factors such as electronic relaxation, steric interactions, and crystal packing present a challenge for engineering photoactive solid-state materials based on molecular crystals.