Non-technical abstract Living matter can change its structure as it responds to changes in its environment. With support from the Solid-State and Materials Chemistry program in the Division of Materials Research, the goal of this research is to replicate this type of responsiveness using artificial materials that can reconfigure themselves in response to an external stimulus, in this case light. Reconfigurable matter consists of a structure composed of smaller elements whose properties can be switched by light. After switching, these elements can reassemble into a new composite structure that has different properties. Many workers in the field envision the switchable elements to be nanoparticles or colloids, but it is clear that the smallest switchable element is a molecule. A molecule that undergoes a photochemical reaction represents a switchable element. A crystal composed of photoreactive molecules can then restructure itself after exposure to light. This proposal is concerned with the development of molecular crystal materials that can undergo photoinduced shape changes as a route to reconfigurable matter. Materials that undergo structural changes in response to light have potential applications a wide range of disciplines ranging from engineering to medicine to cell biology. Participating students will receive advanced training in spectroscopy, materials characterization, and data analysis. Outreach efforts supported by this grant will improve science education at Taft Elementary, a Title I (>70% socioeconomically disadvantaged) school that is also >70% underrepresented minority.
This research program has three main goals. First, new materials and approaches to molecular crystal-based photoinduced motion will be developed. This motion usually originates from the presence of a mixture of reactant and product molecules within the crystal that form a bimorph structure. New halogenated anthracene derivatives will be synthesized to improve the robustness and reversibility of the [4+4] photodimerization reaction that powers the crystal mechanical response. New types of photochemical reactions based on the unimolecular cis-trans isomerization of divinyl(anthracene) derivatives will also be explored. The materials development will include efforts to develop novel micron-scale crystal morphologies that can give rise to new modes of mechanical response. A second area of focus will be to improve the physical understanding of the photomechanical response by studying its mechanism and limits. By measuring how light intensity, direction, wavelength, and crystal shape affect the rate and magnitude of crystal bending, different theories proposed to explain the mechanical response can be evaluated. Solid-state nuclear magnetic resonance experiments will measure spin diffusion dynamics to assess whether the reactions proceed homogeneously throughout the crystal or heterogeneously with domain formation. These experiments will provide information on both the size and shape of nanoscale domains that grow during the photochemical reaction. Third, we will prepare arrays of nanocrystals that can reshape themselves after exposure to light. Surface patterning followed by microdrop printing will generate separated nanocrystals composed of photoreactive molecules. These crystal arrays will be characterized by optical spectroscopy, scanning probe microscopy, and electron microscopy. Photoinduced shape changes will be measured and correlated with starting crystal shape and size, as well as light exposure.
