Photoinduced curling of organic molecular crystal nanowires.

Materials that transform light into mechanical motion are interesting from both fundamental and technological perspectives. Photomechanical materials usually consist of photoreactive molecules assembled in an ordered matrix, for example a liquid crystal polymer. The use of molecular crystals as photomechanical materials is challenging, since strain between reacted and unreacted crystal phases often leads to fracture and crystal disintegration. While there are examples of macroscopic crystals that can support lightinduced shape changes, a more general approach is to shrink at least one of the structure s dimensions to the nanometer scale so that interfacial strain can be dissipated at the surface, preventing fracture. Such small-scale photomechanical structures could be useful in fields like cell biology, but generating complex motions remains a challenge. In most photomechanical actuators, directional motion, for example, bending, is induced by illumination of one side of the crystal, creating a bimorph structure consisting of a layer of reacted molecules adjacent to a layer of unreacted molecules. The interfacial strain between the two phases drives directional motion like bending. But illuminating only one side of a nanostructure becomes impractical once the size of the structure falls below the diffraction limit of the excitation light. One goal of our research has been to develop molecular crystal nanostructures where the directional response is “built in” by the crystal shape andmolecular packing. Previously, we showed that microribbons composed of 9-anthracene carboxylic acid could reversibly twist under uniform illumination, providing an example of how complex motion could be initiated by nondirectional light. This motion was driven by the [4+4] photodimerization of the anthracene rings, resulting in a temporary two-phase (monomer and dimer) system whose interaction provided strain energy to twist the crystal. Photomechanical materials based on dimerization reactions are not ideal since this bimolecular reaction requires two molecules to be in close proximity, placing severe constraints on the crystal structure of potential materials. We have recently begun synthesizing anthracene-9-(1,3-butadiene) derivatives with the goal of developing photomechanical materials powered by a unimolecular E!Z reaction that can be initiated by visible light (l> 430 nm). An added requirement is that nanostructures composed of this molecule should be able to execute complex motions under uniform illumination conditions. We have recently synthesized a novel anthracene-9-(1,3-butadiene) derivative, dimethyl-2(3-(anthracen9-yl)allylidene)malonate (DMAAM). Here, we study its solid-state structure and reactivity and show that a pulse of visible light can induce a curling motion in crystalline nanowires. Considerable effort has been devoted to making static coiled nanowires, usually by changing extrinsic factors like the nanowire s surface chemistry or solvent environment. The coiling observed in this study is qualitatively different, since it relies on intrinsic chemical changes taking place inside the wire itself, namely the E$Z photoisomerization. This motion occurs under uniform illumination conditions and illustrates how a molecular crystal nanostructure can undergo a nontrivial and possibly useful geometry change after photoexcitation. The synthesis of (E)and (Z)-DMAAM isomers is outlined in Scheme 1 and described in detail in the Supporting Information. Both molecules are stable at room temperature in the dark and can be crystallized from various organic solvents. The E and Z isomers have overlapping absorption spectra in the solid state, as shown in Figure 1, that are very similar to those in dilute solution, suggesting that strong intermolecular interactions are absent in both crystals. The detailed crystal structures for both molecules are given in the Supporting Information and are discussed below. Two types of nanowires, composed of either pure (E)or pure (Z)-DMAAM, were made using solvent-annealed crystallization inside the channels of commercially available anodic aluminum oxide (AAO) templates. After dissolution of the AAO template in phosphoric acid, the 200 nm [*] Dr. T. Kim, Prof. Dr. C. J. Bardeen Department of Chemistry, University of California, Riverside 501 Big Springs Rd., Riverside, CA 92521 (USA) E-mail: [email protected] Homepage: http://www.faculty.ucr.edu/~ christob/