Preparation and characterization of microscopic photomechanical molecular crystals

University Of California-Riverside, Riverside CA

Investigators

Abstract

TECHNICAL SUMMARY Photochemical reactions within molecular crystals provide a way to transform light energy into nanoscale mechanical motion. Progress in nanoscale photomechanical materials requires an interdisciplinary approach that combines materials chemistry and physical characterization. In this joint proposal, supported by the Solid State and Materials Chemistry program, the Bardeen research group's expertise in molecular crystalline materials, optical spectroscopy and microscopy will be complemented by the Mueller group's strengths in solid-state nuclear magnetic resonance (NMR) and computational chemistry, in order to pursue a research program with two main thrusts: 1) New photomechanical materials. We will concentrate on thermally reversible photomechanical systems that can operate with a single light source. To make photomechanical elements that can function on the nanoscale, we will look for systems where changes in the crystal shape can be induced intrinsically without relying on specialized irradiation conditions. Two new types of photochemical "engines" will also be investigated: the unimolecular Dewar isomerization of anthracene, and thermally induced polymorphic phase changes. We will also develop the capability to make uniform arrays of molecular crystal plates as a step toward making devices based on photomechanical molecular crystals. 2) Physical characterization of the solid-state reaction dynamics and their connection to crystal deformation. This part of the project will involve using optical, x-ray, NMR and computational methods to understand how molecular-level reactions drive large-scale crystal shape changes in model systems. After the photochemical reaction has been initiated, the structure of the photoproduct and how the reaction affects the crystal packing will be determined using solid-state NMR, x-ray diffraction, and computational methods. We will also use optical and scanning probe microscopy to map out how the crystal geometry changes as a function of these parameters. Information from these experiments will be combined to provide a holistic picture of the photomechanical process on multiple length scales and timescales. NON TECHNICAL SUMMARY Machines that function on length scales 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. Our approach involves self-assembling photochemically reactive molecules within a crystal, whose shape and size is controlled by the preparation conditions. Because the molecules are organized within a crystal, they move in concert to expand or bend the overall nanostructure. The research in this proposal will assess whether these nanoscale photochemical "engines" can be used to manipulate objects on nanometer to micron length scales. We also want to gain a predictive understanding of how molecular-level chemical changes can combine together to create much larger shape changes in the crystals. 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 strong connections to the surrounding public schools. We are currently designing and implementing outreach modules for 1st, 2nd and 4th grade classes that are consistent with the California State Standards for science education. With undergraduate volunteers, we are bringing these modules to local elementary schools, like Taft Elementary, a local Title I school whose student body is more than 50% Hispanic.