Photomechanical molecular crystals can undergo a variety of light-induced motions, including expansion, bending, twisting, curling and coiling. They have potential applications as photo-induced actuators that can drive motion on submicron lengthscales. They can be thought of as larger analogs of molecular machines; which were the subject of the 2016 Nobel Prize. Anthracene derivatives provide a robust platform to make a variety of crystals that show strong photomechanical response under light illumination. To date, however, the fundamental understanding of the mechanism and how these photoreactions affect their mechanical responses is still unclear. We have developed an extended Finke-Watzky model and derived rigorous equations to describe the photomechanical kinetics in solid-state molecular crystals of 9-methlyanthracene (9MA) in a quantitative way. This reaction kinetics analysis provides a systematic framework for understanding how reaction rates change during the photoreaction process. 2-(3-Anthracen-9-yl-allylidene) malononitrile (9DVAM), another anthracene derivative, shows reversible trans to cis isomerization when exposed to visible light. This switching between two isomers can cause strong reversible shape changes like bending and twisting in microneedles and microribbons composed of this molecule. We can also grow differently shaped crystals that can show interesting motion. Branched microcrystals of 4-fluoroanthracenecarboxylic acid (4F-9AC) were made from pH-driven reprecipitation method. These microcrystals show twisting of the branches under illumination, which will drive a rotation of the overall crystal that can be repeated over the course of many exposures to light, thereby leading to ratchet-like motion. In summary, the combination of synthesis, crystal engineering, novel reaction kinetics, and crystal shape control provide a route for designing new photomechanical materials based on organic molecules.

Photomechanical molecular crystals based on anthracene derivatives

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.