Reversible Photoinduced Shape Changes of Crystalline Organic Nanorods

Mechanical devices that function on sub-micrometer length scales are expected to find applications in fields ranging from medicine to manufacturing. Biology provides many examples of nanoscale machines, for example, the adenosine triphosphate (ATP)-fueled motion of myosin along the actin filament. In such systems, the motion is fueled by random encounters with high-energy chemical species in the surrounding medium and occurs stochastically, because there is no way to externally control diffusive encounters at the molecular level. An alternative is to use external energy sources (e.g., photons) to activate motion at the nanoscale. Photochemically powered mechanical motion is attractive because it does not require the presence of a second chemical species and external control of the motion can be achieved by manipulating the illumination conditions. These advantages have propelled research into photoactivated mechanical motion on the molecular level, and there have been many examples of photoactivated molecular switching, translation, and geometrical transformations. In general, these schemes rely on intramolecular events—bond formation, cyclization, cis–trans isomerization—to drive a structural change on the order of 1 Å or less. This molecular-level change can be amplified by coupling multiple molecules together using a liquid-crystal polymeric host. Recent progress in the development of photodeformable polymer fibers and strips has led to micrometer-scale materials that exhibit light-induced shape memory effects, reversible bending under anisotropic illumination conditions, and photocontrol of the crosslink density. Recently, we demonstrated the use of an intermolecular photochemical reaction to achieve large physical displacements in a different type of structure: organic molecular crystal nanorods. By taking advantage of the well-known anthracene [4+4] photocycloaddition reaction in 9-tertbutyl-anthracene ester (9-TBAE) molecular crystal nanorods, a large (15 %) change in rod length could be generated. In most cases, the photochemical changes drive a reconstructive crystal-to-crystal phase transition that leads to crystal fragmentation and disintegration. Apparently, the high surface-to-volume ratio of the nanorods provides sufficient strain relief to avoid cracking and fragmentation during this transition. Although the expansion of the 9-TBAE rods was ca. 100 times larger than that observed in other photoisomerizable molecular crystals, it was largely irreversible. If a reversible system could be found, significant amounts of useful work could be generated by repeatedly inducing such mechanical motion. In the following, we demonstrate that molecular crystal nanorods composed of a related molecule, 9-anthracene carboxylic acid (9-AC), can undergo reversible photoinduced cycling between well-defined shapes after spatially localized excitation. The kinetics of recovery, its dependence on illumination conditions, and the long-term stability of the effect over the course of multiple photocycles are all characterized. In our search for anthracene derivatives that undergo reversible photodimerization reactions, we examined 9-AC, which crystallizes in a head-to-head “cis” arrangement rather than the head-to-tail “trans” arrangement common to most 9-substituted anthracenes. The 9-AC monomer bulk crystal structure (Fig. 1a and b) consists of neighboring stacks of molecules that interact via hydrogen bonding between the carboxyl groups. This crystal structure is retained in the nanorods, as can be seen from the transmission electron microscopy (TEM) image of a single 9-AC nanorod (Fig. 1c) and its electron diffraction pattern (Fig. 1c, inset). Although the cis arrangement is often assumed to prevent the [4+4] cycloaddition reaction in the solid state due to “topochemical” factors, solid state NMR measurements showed that 9-AC does in fact undergo the [4+4] cycloaddition reaction characteristic of anthracenes in the solid state (Fig. 1d). This photodimer is unstable at room temperature and spontaneously reverts back to the monomer state within a few minutes. We have confirmed the reversible nature of the solid-state photodimerization in our samples by monitoring the decrease and subsequent recovery of the monomer crystal fluorescence, which peaks at 505 nm in both nanorods and other crystal types. Although solid state NMR data has confirmed the molecular structure of the photodimer, it does not provide information on changes in the crystal structure or unit cell dimensions. Unfortunately, attempts to obtain the crystal structure of the photodimer were unsuccessful. Even though it was possible to stabilize the photodimer using a liquid-nitrogencooled X-ray diffractometer mount, every attempt to irradiate a single 9-AC crystal of sufficient size to obtain high quality diffraction data resulted in cracks and fissures, which made the crystal unsuitable for diffraction. C O M M U N IC A TI O N