Abstract
Photomechanical molecular crystals can undergo a variety of light‐induced motions, including expansion, bending, twisting, and jumping. The use of more complex crystal shapes may provide ways to turn these motions into useful work. To generate such shapes, pH‐driven reprecipitation has been used to grow branched microcrystals of the anthracene derivative 4‐fluoroanthracenecarboxylic acid. When these microcrystals are illuminated with light of λ=405 nm, an intermolecular [4+4] photodimerization reaction drives twisting and bending of the individual branches. These deformations drive a rotation of the overall crystal that can be repeated over multiple exposures to light. The magnitude and direction of this rotation vary because of differences in the crystal shape, but a typical branched crystal undergoes a 50° net rotation after 25 consecutive irradiations for 1 s. The ability of these crystals to undergo ratchet‐like rotation is attributed to their chiral shape.
In a spin: Slow pH‐driven reprecipitation of 4‐fluoroanthracenecarboxylic acid from aqueous solution results in the growth of branched microcrystals. The twisting of the branches under illumination drives a rotation of the overall crystal, as seen by optical microscopy, and can be repeated by repeated pulses of light. In the example shown, an X‐shaped molecular crystal undergoes a net clockwise rotation of 50° after 25 irradiation cycles.