Understanding the Impact of Halogenation on the Crystalline Photomechanical Response Properties of 9‑Anthracene Carboxylic Acid from First-Principles

Photomechanical
organic crystals transform light into mechanical
work via solid-state photochemical reactions. While many examples
of photomechanical systems have been demonstrated, significant gaps
in understanding the relationships between the chemical structure
of the photochrome molecule, its crystal structure, and the observed
photomechanical response remain. Focusing on the case of [4 + 4] photodimerization
of 9-anthracene carboxylic acid, this density functional theory study
examines the impact of replacing hydrogen atoms with fluorine or chlorine
in various positions on the photomechanical response. The results
demonstrate how the interplay of intramolecular deformations and intermolecular
crystal packing interactions means that even simple chemical substitutions
that preserve the overall crystal packing motif can impact the photochemical
response in ways that are not obvious a priori. In addition, the changes
in work density obtained from the halogenated species here are an
order of magnitude smaller than what previous work found can be obtained
through varying the crystal packing motif. This suggests that crystal
engineering should take precedence when trying to increase the work
density of a system, while additional minor molecular modifications
can be used to refine the photomechanical response further or to tune
other properties of the material.