Organic Crystal Packing Is Key to Determining the Photomechanical Response

Organic photomechanical crystals have great promise as
molecular
machines, but their development has been hindered by a lack of clear
theoretical design principles. While much research has focused on
the choice of the molecular photochrome, density functional theory
calculations here demonstrate that crystal packing has a major impact
on the work densities that can be produced by a photochrome. Examination
of two diarylethene molecules reveals that the predicted work densities
can vary by an order of magnitude across different experimentally
known crystal structures of the same species. The highest work densities
occur when molecules are aligned in parallel, thereby producing a
highly anisotropic photomechanical response. These results suggest
that a greater emphasis on polymorph screening and/or crystal engineering
could improve the work densities achieved by photomechanical engines.
Finally, an inherent thermodynamic asymmetry is identified that biases
photomechanical engines to exhibit higher work densities in the forward
stroke direction.