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.