Molecular Rotors and Materials Properties of Rotary Dipolar Arrays

非技术摘要：加州大学洛杉矶分校的研究人员基于先进的分子设计、合成化学和分子自组装，致力于控制分子在固态中的运动，以控制一种被称为两栖动力学晶体的新材料的物理性质。这些材料配备了旋转部件，在类似宏观罗盘的结构中带有正负基团，能够对外部电场和磁场的存在做出反应，以改变其物理性质。虽然一些特定的分子罗盘排列预计会使所有偶极指向同一方向，但另一些排列预计会使相邻的偶极指向相反的方向(这些排列分别称为铁电和反铁电)。虽然这些新材料有可能显示出有趣的电、磁和弹性能力，即所谓的可以通过外场控制的多铁性性质，但这项工作包括测量由于组成偶极的取向变化而与光相互作用的实验。这些材料有望成为最快、最高效的光开关之一。加州大学洛杉矶分校的研究小组设计了在固态状态下在不同状态之间旋转的分子，他们打算控制声速，并创造出有助于控制信号传输速率的设备。基于惯性偶极阵列的两端动力学晶体的研究为教育和培训来自广泛背景的有才华的材料化学家提供了一个独特的机会。通过维持一个支持性和创造性的环境，促进材料科学和科学教育的职业生涯，PI成功地吸引了贫困背景的女性和学生加入他的研究小组。技术摘要：本项目所追求的实现偶极分子转子自由重定向是材料化学设计的新前沿。这些“两栖晶体”中的工程旋转依赖于结构元素，这些结构元素结合了一组相对静态的晶格形成单元和偶极成分，这些偶极成分具有根据内部偶极相互作用和外部磁场重新定向既定晶格方向的能力。为了确定固态中的内部旋转动力学，该项目利用多核(1H、19F、15N、13C)核磁共振(核磁共振)技术。其中包括自旋晶格(T1)弛豫和作为温度函数的四极回波2H核磁共振线形分析。在大约4K和500K之间的温度变化使得有可能在从几千赫兹到太赫兹的范围内确定旋转运动，并确定从接近零到大约15-20千卡/摩尔的活化能。具有低于热能的势垒的惯性极旋转体有望非常迅速地重新定向，这样它们的出现极化低于相应的居里-魏斯温度将使研究偶极自组织成为可能，这将导致一类新的设计型铁性材料的出现，这些材料具有电、磁和弹性开关能力。为了测试它们的性质，重点放在有助于揭示顺电和铁电或反铁电态之间随温度变化的内部偶极有序性的测量上。据预测，旋转运动和偶极-偶极相互作用产生的动力学关联遵循旋转(铁电)或不旋转(反铁电)轨迹，分析基于机械(立体)力的旋转关联(齿轮)的研究正在进行中。

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.