Photoactuators can directly transform light into mechanical motion. The photochemical reactions are the “engines” of these photoactuators. One such reaction is the [4+4] photodimerization of crystalline 9-anthracene carboxylic acid (9AC), a molecule that shows a reversible photomechanical response in its crystal form. In our pursuit of improved derivatives of 9AC, four fluorinated derivatives of 9AC have been synthesized and characterized. The spectroscopic properties and crystal structures of 4-fluoro-9-anthracene carboxylic acid (4F-9AC), 2-fluoro-9-anthracene carboxylic acid (2F-9AC), 10-fluoro-9-anthracene carboxylic acid (10F-9AC) and 2,6-difluoro-9-anthracene carboxylic acid (2,6dF-9AC) are all very similar to those of 9AC. However, their photomechanical properties vary widely. Both the photodimer dissociation rate and the mechanical recovery varied by more than an order of magnitude, with 4F-9AC exhibiting the most rapid recovery time, on the order of 30 s. Microneedles of 4F-9AC can undergo more than 100 mechanical bending cycles. Given the similarity of the crystal packing, the improved photomechanical properties must arise from subtle changes in intermolecular interactions or possibly differences in disorder. These results demonstrate that it is possible to significantly improve the properties of photomechanical materials through small modifications of the molecular structure. Furthermore, we discovered that it is possible to make a branched structure of 4F-9AC that undergoes ratchet-like motion. This work should motivate further studies into the growth of complex crystal shapes that can do useful photomechanical work.

Light powered ratchet

251st National Meeting (2016)

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