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 Summary Machines 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.
Intellectual Merit. The research supported by this grant has centered on investigating organic materials that turn light into mechanical work using solid-state photochemical reactions. In order to transform photons into mechanical motion, we have developed molecular crystals composed of photochemically reactive anthracene derivatives. We have shown that these molecular crystal structures can be formed into nanorods that bend, microribbons that reversibly twist, and nanowires that rapidly coil into balls. An example of the twisting is shown in Figure 1. The chemistry that drives the reversible photomechanical motions involves the dimerization of two anthracene molecules within the crystal. Molecular crystals are generally assumed to be static, rigid structures, so it was somewhat surprising that they could undergo such dramatic motions. One key is that the crystals must be small – large crystals tend to shatter. The molecular-level mechanism that drives their dynamic motion arises from reversible photodimerization reactions that occur within stacks of molecules. The strain energy between reacted and unreacted molecules causes the elastic deformation of the crystal. To better understand these photomechanical motions at the molecular lengthscale, we have used a combination of x-ray diffraction, solid-state NMR, and theory to determine the molecular-level mechanism of expansion. The research supported by this proposal can be regarded as a first step toward creating ultrasmall structures that can be remotely controlled. This is a central goal of nanotechnology, with photon-fueled nanomachines having many potential applications, for example in medical treatments at the single cell level. We recently demonstrated that it is also possible to go in the reverse direction and use mechanical force to "undo" photochemical reactions. By making a highly strained anthracene photodimer, we demonstrated for the first time that molecular carbon-carbon bonds could be broken using mild, isotropic pressure in a polymer matrix. These molecules have potential applications as nanoscopic sensors that can be placed inside carbon materials that are subject to strain. Broader Impacts. The technological advances made possible by this research could eventually enable the construction of new devices with applications in surgery, manufacturing, and national defense. In terms of scientific outreach, during this grant we have developed several outreach modules for local elementary schools that conform to the California State standards for science education. Working with local teachers, we coordinate the presentations with the grade-specific curriculum and incorporate it into class assignments. We have visited 1st and 3rd grade students at predominantly Hispanic local elementary schools in Riverside, CA and have directly impacted more than 1000 pre-college students over the past three years. In addition to visiting local schools, we have now collaborated with the Entomology Department at UC Riverside to bring students onto campus so that they can be introduced to the college environment at an early age. Figure 2 shows the P.I. doing an experiment for third grade students from the predominantly Hispanic local elementary school Rosemarie Kennedy Elementary School as part of the "Bugs and Booms" program on the UCR campus. In collaboration with an undergraduate student organization at UC Riverside, the Young Science Scholars, we are starting a new outreach effort where college students go into classrooms to demonstrate the scientific method and provide mentoring for elementary school students.
