Molecular Rotors and Materials Properties of Rotary Dipolar Arrays

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Non-technical Abstract:Based on the use of advanced molecular design, synthetic chemistry, and molecular self-assembly, researchers at UCLA are pursuing the control of molecular motion in the solid state to control the physical properties of a new class of materials known as amphidynamic crystals. Equipped with rotary components bearing positive and negative groups in structures that resemble macroscopic compasses, these materials are capable of responding to the presence external electric and magnetic fields to change their physical properties. While some specific molecular compass arrangements are expected to cause all the dipoles to point in the same direction, others are expected cause adjacent dipoles to point in opposite ways (these arrangements are known, respectively, as ferroelectric and antiferroelectric). While these new materials have the potential of displaying interesting electric, magnetic, and elastic capabilities, the so-called multiferroic properties that can be controlled with external fields, this work includes experiments that measure their interaction with light as a result of 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 switch between states rotation in the solid state, the UCLA group intends 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 provides a unique opportunity to educate and train talented materials chemists from a wide range of backgrounds. The PI has been successful attracting women and students for underprivileged backgrounds to his research group by maintaining a supportive and creative environment that fosters careers in materials science and in science education. Technical Abstract:The realization of freely reorienting dipolar molecular rotors pursued in this project is a new frontier in materials chemistry design. Engineered rotation in these "amphidynamic crystals," relies on structural elements that combine a set of relatively static, lattice-forming units, and dipolar components that possess the ability to reorient about established lattice directions in response of internal dipolar interactions and external fields. To determine internal rotational dynamics in the solid state this project takes advantage of multinuclear (1H, 19F, 15N, 13C) Nuclear Magnetic Resonance (NMR) techniques. These include spin-lattice (T1) relaxation and quadrupolar echo 2H NMR line shape analysis as a function of temperature. Variations in temperature between ca. 4K and 500K make it possible to determine rotational motion over a range that covers from a few kilohertz to the terahertz regime and determine activation energies from close to zero up to ca. 15-20 kcal/mol. Inertial polar rotators with barriers that are lower than thermal energies are expected to reorient very rapidly, such that their emergent polarization below their corresponding Curie-Weiss temperatures will make it possible to study dipolar self-organization that will lead to the emergence of a new class of designer ferroic materials with electric, magnetic, and elastic switching capabilities. To test their properties, an emphasis is placed on measurements that help disclose inner dipolar order in the form of temperature-dependent transitions between paraelectric and ferroelectric or antiferroelectric states. It is predicted that dynamic correlations resulting from rotational motion and dipole-dipole interactions follow either conrotatory (ferroelectric) or disrotatory (antiferroelectric) trajectories and studies are in progress to analyze rotational correlations (gearing) based on mechanical (steric) forces.