Dipolar Correlations in Amphidynamic Crystalline Rotor Arrays

非技术总结：结晶固体的所有分子都以精确的周期性方式排列。人们可以想象分子聚集在一起形成有序的链和层，紧密堆叠在一起构成三维晶体。虽然我们知道任意形状的分子紧密地聚集在一起，它们基本上是静态的，但已经表明，一些分子形状和框架结构导致了两动力晶体的形成。这些相对较新的结构拥有静态的、晶体形成的元素，与高度可移动的分子单位相连。与该项目特别相关的一类两动力晶体形成器包含具有玩具指南针结构元素的分子：它们具有针状，电和/或磁旋转单元，被屏蔽在一个盒子里，称为偶极子，它们自发地重新定向以相互作用，并与强大的外部磁场（或电场）相互作用。这些偶极-偶极相互作用，或偶极相关性，提供了探索材料特性的机会，这些特性是基于单个偶极子以依赖于它们组织方式的方式相互作用的能力。一些有序的偶极阵列具有对称性，所有的偶极子都指向同一个方向，而另一些则形成所有的偶极子相互抵消的结构。第一种排列方式产生宏观极性晶体，第二种排列方式导致宏观非极性材料的形成。在美国国家科学基金会材料研究部固态与材料化学项目的支持下，加州大学洛杉矶分校的研究人员探索了两动力晶体转子阵列中的偶极相关性，该阵列可用于制备可寻位的极性和非极性材料。他们研究了偶极相关、自发极化和一组相互作用的偶极子的集体重定向是否具有控制许多热、光学和介电特性的潜力，同时为智能材料的开发提供了一个有前途的平台，例如用于清洁能源或半导体应用，以及人工分子机器。有效地利用宏观玩具圆规的大阵列来说明分子偶极子阵列的行为，已发展成为高中学生的课堂演示，可以帮助化学教师介绍与分子结构，分子间力和紧急现象相关的概念。技术概述：在美国国家科学基金会材料研究部固态与材料化学项目的支持下，加州大学洛杉矶分校的研究人员在四边形、六边形和Kagome晶格结构的金属有机框架的基础上合成了晶体偶极阵列，以探索紧急秩序的开始。四边形电偶极子阵列有望采用反铁电排列，而六边形和Kagome晶格上的电偶极子有望采用铁电排列和宏观极化。要克服的结构挑战不仅仅是创造适当的晶格对称：众所周知，集体偶极重定向和有效极化要求旋转能垒小于取向相关的偶极-偶极相互作用能，而旋转能垒需要大于热能才能避免单独的布朗运动。设计在环境温度（例如300 K）下满足这些特性的材料需要具有旋转偶极子的两动力晶体，(1)没有固有的旋转电子势垒，(2)存在于大于其旋转体积的空隙中以避免空间势垒，以及(3)具有足够近的大电偶极矩以相互作用而不干扰其旋转运动。合成的结构包括具有Zn和Zr节点的金属有机框架，避免相互渗透的致密羧酸层，具有大电偶极矩的连接体以及一组具有TEMPO基自由基旋转阵列的结构。研究了具有“自由”旋转偶极子的新材料，以确定它们是否在广泛的应用交流场频率范围内具有大的介电常数，这是由于它们的偶极子与外场对齐的能力。然而，对于取向为铁电或反铁电的样品，观察到不同的弛豫性质。虽然铁电样品具有较大的磁化率和宏观极化，但具有反铁电取向的样品需要克服其倾向以保持其反铁电基态的场。研究人员还研究了介电常数的温度依赖性，这对于无障偶极阵列来说是很弱的。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。通过这个项目，PI支持他的研究小组中来自贫困背景的妇女和学生。通过保持一个支持性和创造性的环境，他因此促进了材料科学和科学教育的职业发展。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARY: Crystalline solids have all their molecules aligned in a precise periodic manner. One can visualize molecules coming together to form ordered chains and layers that stack closely together to make up a three-dimensional crystal. While it is known that molecules with arbitrary shapes pack so tight together that they become essentially static, it has been shown that some molecular shapes and framework architectures result in the formation of amphidynamic crystals. These relatively new architectures possess static, crystal-forming elements, linked to molecular units that can be highly mobile. A class of amphidynamic crystal-former of particular relevance to this project contains molecules that have the structural elements of a toy compass: they have needle-like, electric and/or magnetic rotary units shielded in a box, known as dipoles, which spontaneously reorient to interact with each other and with strong external magnetic (or electric) fields. These dipole-dipole interactions, or dipolar correlations, offer opportunities to explore materials properties based on the ability of individual dipoles to interact with each other in manners that depend on the way they are organized. While some ordered dipolar arrays have symmetries where all the dipoles point in the same direction, others form structures where all the dipoles cancel each other. The first alignment mode generates macroscopically polar crystals, and the second one leads to the formation of macroscopically non-polar materials. With support from the Solid State and Materials Chemistry program in NSF’s Division of Materials Research, researchers at the University of California Los Angeles explore dipolar correlations in amphidynamic crystalline rotor arrays that can be used for the preparation of addressable polar and non-polar materials. They investigate whether dipolar correlations, spontaneous polarization and the collective reorientation of a set of interacting dipoles have the potential for controlling a number of thermal, optical, and dielectric properties while providing a promising platform for the development of smart materials, for example for clean energy or semiconductor applications, and artificial molecular machines. The effective use of large arrays of macroscopic toy compasses to illustrate the behavior of molecular dipole arrays is developed into a classroom demonstration for high school students that can help chemistry instructors introduce concepts related to molecular structure, intermolecular forces, and emergent phenomena. TECHNICAL SUMMARY: With support from the Solid State and Materials Chemistry program in NSF’s Division of Materials Research, researchers at the University of California Los Angeles synthesize crystalline dipolar arrays based on metal organic frameworks with tetragonal, hexagonal and Kagome lattice architectures to explore the onset of emergent order. While tetragonal electric dipole arrays are expected to adopt antiferroelectric order, electric dipoles on hexagonal and Kagome lattices are expected to adopt ferroelectric alignment and macroscopic polarization. The structural challenges to surmount go beyond the creation of the proper lattice symmetry: It is well known that collective dipole reorientation and efficient polarization require rotational energy barriers to be smaller than the orientation-dependent dipole-dipole interaction energies, which need to be greater than thermal energy to avoid individual Brownian motion. Designing materials that meet these characteristics at ambient temperature (e.g., 300 K) require amphidynamic crystals with rotary dipoles that (1) have no intrinsic electronic barriers for rotation, (2) exist in voids that are greater than their volumes of revolution to avoid steric barriers, and (3) possess large electric dipole moments that are close enough to interact with each other without interfering with their rotational motion. The synthesized structures include metal organic frameworks with Zn and Zr nodes, dense carboxylate layers to avoid interpenetration, and linkers with large electric dipole moments plus a set of structures with TEMPO base free radical rotary arrays. The new materials with “freely” rotating dipoles are investigated to determine if they have large dielectric constants over a broad range of applied AC field frequencies as a result of the ability of their dipoles to align with an external field. Different relaxation properties, however, are observed for samples where the alignment is ferroelectric or antiferroelectric. While ferroelectric samples have large susceptibilities and large macroscopic polarization, samples with antiferroelectric alignment require fields that overcome their tendency to maintain their antiferroelectric ground state. The researchers also study the temperature dependence of the dielectric constant, which is expected to be weak for barrierless dipolar arrays. 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. Through this project, the PI supports women and students from underprivileged backgrounds in his research group. By maintaining a supportive and creative environment he thereby fosters careers in materials science and in science education.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.