Distinct Mechanical Properties and Photomechanical Response from Isostructural, yet Chemically Different Molecular Crystal Actuators

Deciphering the effect of small changes in crystal packing and intermolecular interactions on the type and magnitude of mechanical motion is central to the manipulation of the macrodynamic behavior of dynamic crystals as smart materials. Here, we describe a peculiar example where both the mechanical flexibility and the photomechanical motion in molecular crystals can be altered by a single small chemical substitution. Specifically, we demonstrate that the mechanical flexibility of an organic crystal switches from plastic bending to elastic/plastic bending by changing a single functional group of a thiophene ring from methyl (crystal 1) to ethyl (crystal 2), but they have an almost identical crystal structure. The different molecular packing also determines the rate of the [2 + 2] photocycloaddition reaction, which further reflects on the type of photomechanical effect: while upon UV radiation, crystals 1 undergo bending or coiling, depending on the crystal thickness, crystals 2 do not display macroscopic photomechanical response. The bending of crystals 1 can be repeated up to 30 times, and this deformation can be applied to move objects that are 10³-10⁴ times the mass of the crystals with an output force density of 10⁷-10⁸ N/m³ and output work density of 10²-10³ J/m³, surpassing a variety of traditional actuators.