Photophysical and Photomechanical Properties of Molecular Crystal Nanorods

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Technical Summary. Photochemical reactions within molecular crystals provide a way to transform light energy into nanoscale mechanical motion. The development of nanoscale molecular crystal actuators requires knowledge about how molecular-scale events combine to produce micron-scale motions and deformations in crystalline nanostructures. Furthermore, there is much room for material optimization and the development of tools to control the motion of these nanostructures. Thus the proposed research has two main goals:1) To determine the physical mechanisms that give rise to the photomechanical response of molecular crystal nanorods. A variety of methods will be used to characterize the structure and dynamics of the nanorods over a range of lengthscales, from Angstroms to microns. The goal is to correlate the dynamic properties of the nanorods (photochemical reaction rates, crystal deformation rates, and force generation) with structural properties (crystal packing and orientation within the rod, size and shape, and surface treatment). These correlations will result in a more quantitative and predictive understanding of how molecular-level photochemical events give rise to microscopic motions.2) To develop improved, size-tunable nanoscale actuators. Material parameters like reversibility (both light-induced and thermally-induced), and photoinduced volume changes will be optimized. New optical techniques for selective control of motion, like two-photon excitation, will be investigated. Practical applications of these structures will also be developed, with an emphasis on two devices: a simple linear actuator based on rod expansion, and a synthetic analog to biological cilia based on reversible nanorod bending. The combination of improved physical understanding and improved materials should allow us to assess the ultimate utility of molecular crystal nanostructures as photomechanical actuators. Nontechnical SummaryMachines that function on lengthscales smaller than biological cells could lead to revolutionary advances in fields like medicine and defense. But there are many questions that must be answered before this goal can be achieved, including how to produce such structures, how to provide them with power, and how to control their motion. In the proposed research, templating methods are used to mass produce organic nanorods. When exposed to light, the molecules within these rods undergo photochemical reactions that change their structure. Because the molecules are organized within a crystal, they move in concert to expand or bend the overall nanostructure. The location and amount of the motion can be controlled by laser exposure conditions. In this way, photons provide both the power source and control mechanism for such photomechanical nanostructures. The research in this proposal will assess whether these nanoscale machines can be used to manipulate objects on nanometer to micron lengthscales. In addition, outreach programs based on this research will be used to increase the participation of underrepresented minorities in science. U.C. Riverside is a Hispanic Serving Institution, with connections to the surrounding middle and high schools that are more than 50% Hispanic. Experimental modules that permit direct visualization of microscopic phenomena like Brownian motion and nanorod photomechanics will be integrated into the curriculum of local schools to help teach aspects of the California State Standards for science education in the 7th and 8th grades.