Molecular Rotors and Materials Properties of Rotary Dipolar Arrays

摘要：基于先进的分子设计、合成化学和分子自组装技术，加州大学洛杉矶分校的研究人员正在研究控制固态中的分子运动，以控制一类被称为两动力晶体的新材料的物理性质。这些材料在类似于宏观指南针的结构中装有承载正负极组的旋转元件，能够对存在的外部电场和磁场做出反应，从而改变其物理性质。虽然某些特定的分子罗盘排列预计会导致所有的偶极子指向同一个方向，但其他排列预计会导致相邻的偶极子指向相反的方向（这些排列分别被称为铁电和反铁电）。虽然这些新材料有潜力显示出有趣的电、磁和弹性能力，即所谓的多铁性，可以用外场控制，但这项工作包括测量它们与光的相互作用，因为组成偶极子的方向发生了变化。这些材料有望成为速度最快、效率最高的光开关之一。加州大学洛杉矶分校的研究小组设计了能够在固态中在不同状态之间旋转的分子，他们打算控制声速，并制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动力晶体的研究为教育和培养具有广泛背景的材料化学家提供了一个独特的机会。PI通过保持支持和创造性的环境，培养材料科学和科学教育领域的职业，成功地吸引了女性和贫困背景的学生加入他的研究小组。技术摘要：本课题所追求的自由重定向偶极分子转子的实现是材料化学设计的新前沿。在这些“双动力晶体”中，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组件，这些组件具有响应内部偶极相互作用和外部场而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，该项目利用了多核（1H, 19F, 15N, 13C）核磁共振（NMR）技术。这些包括自旋晶格（T1）弛豫和四极性回声2H核磁共振线形分析作为温度的函数。在大约4K和500K之间的温度变化使得可以确定从几千赫兹到太赫兹范围内的旋转运动，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性旋转器有望非常迅速地重新定向，因此它们的涌现极化低于相应的居里-魏斯温度，这将使研究偶极自组织成为可能，这将导致具有电、磁和弹性开关能力的新型设计铁材料的出现。为了测试它们的性质，重点放在有助于揭示内部偶极序的测量上，以温度依赖的形式在准电态和铁电态或反铁电态之间转变。据预测，由旋转运动和偶极子-偶极子相互作用产生的动态相关性遵循控制（铁电）或畸变（反铁电）轨迹，并且正在进行研究以分析基于机械（立体）力的旋转相关性（传动）。

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.