With support from the Division of Materials Research from the National Science Foundation, this research is aimed at the development of a new class of materials, known as amphidynamic crystals, which have switchable physical properties based on the reorientation and rotational motion of groups contained within their constituent molecules. With well-selected electric, magnetic and optical properties, these crystalline molecular rotors will be able to respond to external fields and change the physical properties of the material. The structures proposed for this project can be viewed as an ordered array of compasses, where every compass needle is held within a box, and it is able to affect the orientation of its closest neighbors. Since every rotating needle has positive and negative charged poles, each needle is referred to as a "dipole". It is predicted that some specific molecular compass arrangements will cause all the dipoles to point in the same direction, while others will cause adjacent dipoles to point in opposite ways (these opposite arrangements are known, respectively, as ferroelectric and antiferroelectric). Furthermore, the reorientation of a single dipole can be transferred from one to another, as in a chain of falling dominoes, such that changes in orientation and rotational motion is expected to be transferable over long distances. Under some conditions, all the dipoles are expected to spontaneously rotate in the same direction, such that the transmission of linearly polarized light would make the crystal blink, from very bright to dark, in a spontaneous manner. By contrast, it is expected that the application of a strong homogenous field will result in the alignment of all dipoles along the of direction of the electric field, which can be used to change the color of transmitted light from red to blue (a nonlinear optical effect). While these new materials have the potential of displaying interesting electric, magnetic, and elastic capabilities (so-called multiferroic properties) that can be controlled with external fields, in this proposal, experiments are outlined to analyze their interaction with light, including changes in refractive index, polarization, and nonlinear optical properties that may result from changes in the orientation of the constituent dipoles. These materials are expected to be among the fastest, most efficient optical switches. With molecules designed to maintain a state rotation in the solid state, the UCLA group will be able to control the speed of sound and create devices that will help control the rate of signal transmission. Research on amphidynamic crystals based on inertial dipolar arrays 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. Active collaborations for the project have also been established with Faculty at local Community Colleges.
Engineered rotation relies on structural elements that combine a set of relatively static, lattice-forming units, with dipolar components that possess the ability to rotate and reorient about structurally established lattice directions. With components that display motion at the two ends of the dynamic spectrum, these materials are said to be "amphidynamic". One of the most intriguing aspects of engineered rotation comes from the fact that it offers the potential for controlling the properties of the thermal bath, potentially helping modulate a variety of quantum effects that rely on phonon coupling. Depending on the structure and components, engineered rotation can have a range of frequencies that vary from a few Hertz to several Teraherz, with activation energies (thermal coefficients) that range from nearly zero up to ca. 15-20 kcal/mol. Notably, one can have faster rotation in well-designed crystalline solids than in non-viscous liquids. While the ultimate goal of crystalline rotor design is the precise control of dynamics properties over a wide range of time scales in order to interface them with various materials properties, the realization of inertial polar molecular rotors described in this proposal offers some of the most interesting possibilities. Inertial polar rotors constitute a new frontier of chemical and crystallographic design. Inertial polar rotators will have rotational barriers that are lower than thermal energy, such that under some conditions they will be able maintain a constant angular velocity for as long as there are no forces acting on them. They will also be able to reorient very rapidly in the presence of internal and strong external influences, such that their emergent polarization will make it possible to study dipolar self-organization to permit the emergence of a new class of designer ferroic materials with electric, magnetic, and elastic switching capabilities. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, this project will center on the design, synthesis and dynamic characterization of inertial polar rotators. To test their properties, an emphasis will be placed on measurements that will help disclose inner dipolar order in the form of temperature-dependent transitions between paraelectric and ferroelectric or antiferroelectric states. Experiments are also envisioned to analyze their dynamic correlations resulting from rotational motion and dipole-dipole interactions, which are predicted to follow either conrotatory (ferroelectric arrays) or disrotatory (antiferroelectric arrays) trajectories. Studies are also formulated to analyze rotational correlations (gearing) based on mechanical (steric) forces.
