Organic Crystal Packing Is Key to Determining the Photomechanical Response

Organic photomechanical crystals have great promise asmolecularmachines, but their development has been hindered by a lack of cleartheoretical design principles. While much research has focused onthe choice of the molecular photochrome, density functional theorycalculations here demonstrate that crystal packing has a major impacton the work densities that can be produced by a photochrome. Examinationof two diarylethene molecules reveals that the predicted work densitiescan vary by an order of magnitude across different experimentallyknown crystal structures of the same species. The highest work densitiesoccur when molecules are aligned in parallel, thereby producing ahighly anisotropic photomechanical response. These results suggestthat a greater emphasis on polymorph screening and/or crystal engineeringcould improve the work densities achieved by photomechanical engines.Finally, an inherent thermodynamic asymmetry is identified that biasesphotomechanical engines to exhibit higher work densities in the forwardstroke direction.