Bendable Crystals

Crystals are generally rigid and brittle, but this paper describes microcrystals of dimethylamino trans azobenzene that bend into a semicircle when a light is shined on them. That the azobenzene molecule responds to light is no real surprize, but the fact that the whole crystal changes shape, and reversibly to boot, was pretty cool.

Take a look at the video's of this that are provided, for free, in the supplemental information for the paper. The movie 002 shows the bending motion most clearly. This crystal is about 0.5 mm by 0.28 mm and 0.005 mm thick, so it is really tiny. A larger crystal probably would not do this.

So what's going on? The N=N double bond can be in either the trans orientation or the cis orientation. The trans version is preferred - the cis version suffers from steric crowding as the two benzene rings bump into one another. By shining light with the proper wavelength on the molecule, you can convert the trans into the cis molecule. Turn off the light, and the molecule can revert back the the trans version.

The authors were able to confirm that when they shine light on their crystals they convert about 1% of the molecules from the trans form to the cis form. This was readily apparent in proton NMR, but evidence could also be seen in changes in the UV-vis spectrum and the melting point of the crystals. After turning off the light source, the molecules in the crystal return slowly to all-trans.

So, how does this cause the crystal to bend almost in half? It seems to have to do with the way the molecules arrange themselves in the crystal lattice. The trans molecules are flat and stack into a very regular herring-bone type of pattern. The cis molecules don't fit this pattern - in addition to the U-shape, the cis molecules have a twist in them to reduce some of the steric strain between the benzene rings. The twist make the molecules much bulkier than the flat trans form As a result the cis molecules don't fit into the crystal lattice and cause the unit cell to be longer than the unit cell for only the trans molecules.

When the light source converts some of the molecules to the cis form, the side of the crystal nearest the light source appears to expand - because of the larger unit cell of the cis molecules. The side of the crystal away from the light source does not experience this and stays the same size. To accommodate the expanding surface facing the light source, the whole crystal bends away from the light source. When the light is turned off, the cis molecules slowly revert to the (more stable) trans form and the crystal un-bends.

Koshima, H., Ojima, N., & Uchimoto, H. (2009). Mechanical Motion of Azobenzene Crystals upon Photoirradiation Journal of the American Chemical Society, 131 (20), 6890-6891 DOI: 10.1021/ja8098596