Photochemically Driven Shape Changes of Crystalline Organic Nanorods

Nanorods composed of 9-<i>tert</i>-butylanthroate (9-TBAE) are synthesized using an Al<sub>2</sub>O<sub>3</sub> template and solvent annealing. The rods consist of micron-scale crystalline domains, and UV light induces a [4 + 4] photodimerization that results in a uniform 15% expansion along the rod axis. This is in contrast to random 9-TBAE crystals, which disintegrate under the same conditions. Transmission electron microscopy, atomic force microscopy, and comparison of the X-ray crystal structures of the monomer and photodimer all provide evidence for a mechanism based on a crystal-to-crystal photoreaction leading to an increase in molecular volume. It is likely that the high surface-to-volume ratio in the nanorods provides a strain relief pathway that is absent in larger crystals. Preliminary attempts to reverse the reaction using shorter wavelength light to photodissociate the dimers were only partly successful. These results suggest that crystalline organic nanostructures may provide an efficient way to transform photochemical energy into mechanical motion on the nanometer scale.