This proposal describes a research plan centered on the design of a novel class of functional materials based on the motion of their molecular units. These materials are said to be amphidynamic in reference to the fact that they have architectural elements that exist at the two extremes of the molecular dynamics spectrum. The long-term vision of this project is to create dense multi-component assemblies with the complexity and addressability that is expected of smart materials and artificial molecular machines. By form and function, the building units selected for this study resemble macroscopic compasses or gyroscopes, with the designation of choice depending on their state of motion and the presence of dipoles that respond to external fields. Their structures consist of a rotary mass, or rotator, that is connected by an axle to a rigid frame that holds the assembly together and plays the role of a stator. Amphidynamic materials based on inertial rotors and dipolar arrays are expected to have addressable mechanic, dipolar, and optical properties for electro-optic and dielectric applications. In analogy to liquid crystals, the new materials will possess birefringence, dichroism, and second-order non-linear optical responses that can be switched on and off with external fields. However, molecular compasses will not require the entire molecule and the bulk domain to reorient so that dipolar lattices with suitable symmetries will have a ferroelectric ground state and response times approaching those given by the moment of inertia of the reorienting units, on the order of 1012 s-1 (THz). With the support of the Solid-State and Materials Chemistry program in the Division of Materials Chemistry and the Macromolecular, Supramolecular, and Nanochemistry program in the Division of Chemistry , the UCLA team will test structures design to display inertial rotation in the solid state, a new family of molecular compasses and gyroscopes with rotary motion that responds to thermal and photochemical stimuli, and will investigate a new class of electrochromic materials based on changes in orientation between linearly conjugated chromophores. Hoping to make an impact on the number of underrepresented minority students who pursue materials research careers, the PI has established a collaboration with Faculty at Trade Technical College, which is located in downtown Los Angeles and has a very large Hispanic and African American population. The PI and his students have outlined plans to disseminate their research to the general public.
NON-TECHNICAL SUMMARY The primary objective of this project, supported by the Solid-State and Materials Chemistry program in the Division of Materials Chemistry and the Macromolecular, Supramolecular, and Nanochemistry program in the Division of Chemistry, is to develop a new class of responsive materials that can be switched on-and-off to manipulate light and electrical signals. This will be accomplished by altering the orientation and motion of the molecules that constitute the material. Notably, molecular motion is not the first thing that comes to mind when thinking about solids. However, recent advances in the PI laboratory have shown that such materials can be built with molecular rotors linked to rigid frameworks by molecular axles and bearings akin to those found in macroscopic machines. The specific structures studied in this project are evocative of a macroscopic compass. They have a bar needle, or "dipole", that rotates about an axle in order to point towards the strongest magnetic or electric field. By collectively changing their orientation in the presence of strong external fields many such molecular compasses within the material can change the intensity, color, and polarization of transmitted light. Using elements designed to change the speed of rotation, the UCLA group aims at changing the speed of signal transmission in order to improve technologies currently used in cell phones and other devices. Research on these so-called "amphidynamic" materials will provide the PI with opportunities to educate and train talented materials chemists and to carry out research activities aimed at increasing the number of students from underprivileged backgrounds that enter the field of materials chemistry. The PI has been very successful attracting women and minority students to his research group by maintaining a highly supportive and creative environment that fosters careers in materials science and in science education. As part of this project, the PI has established several outreach activities in collaboration with Faculty at Trade Technical College, which is located in downtown Los Angeles and has a very large Hispanic and African American population. It is hoped that the most talented students will transfer to UCLA and other research universities to pursue careers in chemistry and materials sciences.
Materials properties can be traced to a combination of specific molecular features and the way they are assembled. When all the molecules that make up a macroscopic object are arranged in specific orientations, as is the case of crystalline solids and liquid crystals, their physical attributes depend on the direction of measurement. The object is said to be anisotropic. This includes the interaction of materials and light. In fact, with crystals made up of the appropriate molecular units, it is possible to control the intensity, color, polarization, direction, and velocity of incident light, which can be advantageous for a wide range of communication technologies, displays, and many other applications. One can achieve many of those effects by simply turning the material towards different directions with respect to the direction of an incident beam. With that in mind, a very attractive alternative that has been supported by this grant involves the design of crystalline molecular machines where each molecule in the ensemble has the ability to rotate and reorient when exposed to controlled electric or magnetic fields. The design of crystalline molecular machines has required the development of advanced molecular design and synthetic strategies to obtain ordered ensembles with components that make up the rigid architecture of a solid, but that also have components that move and interact with each other. A few years ago we proposed the term "amphidynamic" to describe the characteristics of these new solids. The Greek word "amphi" means "both sides" and is generally used to denote the presence of two incompatible characteristics. Thus, amphidynamic crystals can have components that occupy the two ends of the dynamic spectrum: some components are very static while others are very mobile. Many scientists have joined our efforts. With funds provided by this grant we continued the development of amphidynamic materials built with a collection of molecules that emulate the properties of macroscopic compasses and gyroscopes. These consist of a central rotary component that is linked by a barrierless axle to an encapsulating structure that acts as a stator. We designed amphidynamic materials based on solids that are held together by weak interactions and materials based on extended solids, which are assembled with the help of transition metals such as Zink, Copper and Manganese. We succeeded in the preparation of the first crystalline solid with an inertial rotator, and we demonstrated that rotation in the solid state could be as fast as rotation in the gas phase with a limit that is set by rotational inertia - in the order of a trillion rotations per second. In another important benchmark, we established the characteristics needed for materials to change their color in the presence of strong electric fields as a result of changes in the nature of their light-absorbing units by rotation of their components. We also showed that the speed of rotation can be altered by using chemical reaction in appended photochromic units. In another promising discovery, we found that mestranol, a steroid that has been used for 50 years for birth control, turns out to be an ideal molecular component for the formation of helical arrays of nested molecular rotors, which have been shown to display mechanical gearing. We begun an exploration on the effects of high frequency rotation on organic conductivity we characterized the first two-dimensional rotary glass-to-rotary liquid phase transition in an ordered silicate. We also reported progress towards the implementation of strategies to prepare more complex, larger structures, which will allow the use of much larger rotators capable of bearing very large dipole moments. Students involved in this project receive a rigorous training in the most current areas of materials science and future technologies. Students learn how to bridge the gap between molecular and materials design and their implementation in new devices. This project has attracted talented American women and men from all cultural and ethnic backgrounds, many of whom are now contributing to the economy of the country by joining top technology companies and educational institutions.
