Steric Frustration at the Nanoscale: Self-Assembly, Chirality, and Fluctuations

包装奇形怪状的三维物体可能会导致立体障碍，这意味着很难填满空间。然而，如果对象是分子，热搅拌会采样所有的堆积可能性，要么找到意想不到的新排列，要么保持杂乱无章。最近发现的一个例子是螺旋向列相，这是一种液体，香蕉形状的分子通过将自己排列成分子尺度的螺旋结构来填充空间。在这个项目中，将设计和合成弯曲分子，并研究它们的相行为，目的是探索支配这种空间位阻如何在材料中适应的几何和统计原理。由此产生的设计标准将在基于控制和使用空间受挫的新材料的开发中有广泛的应用。该项目将为其初级科学家(本科生、研究生和博士后研究人员)提供机会，在一个令人兴奋的软材料科学领域参与广泛的跨学科研究。技术摘要棒状分子形成向列相液晶相，在三维流体中，棒的空间堆积在热平衡中诱导长程取向有序。但是，通过引入简单的化学变化来使棒子弯曲成形状，就会产生一种奇异的新相(螺旋向列相)，在这种相中，分子自发地组织成不同分子取向和纳米周期的手性螺旋晶格。该项目的假设是，这些独特的新结构是空间位阻的结果：弯曲的分子形状创造了难以填充的空间，从而吸引了波动最大的分子子成分。这一新形成的组织原则将在分子设计、化学合成、物理表征和螺旋向列相的计算机模拟的广泛、协调的程序中进行探索。特别是，共振软X射线散射将被用来表征它们的纳米级取向结构，并将被扩展到探测纳米级的分子取向涨落。所学知识将应用于各种软物质系统，包括液晶、聚合物和纳米材料，目的是创造广泛有用的、强大的手段来探索和理解纳米尺度的分子取向动力学。螺旋向列相具有很强的集体纳米手性，在手性客体分子对映体的分离和不对称合成的模板介质等方面具有广泛的应用前景。我们将探索螺旋向列相的这些应用。

NONTECHNICAL ABSTRACTThe packing of odd-shaped three dimensional objects can lead to steric frustration, meaning that it's hard to fill space. If the objects are molecules, however, thermal agitation samples all of the packing possibilities, either finding unexpected new arrangements or remaining disorganized. A recently discovered example of this is the heliconical nematic phase, a liquid in which banana-shaped molecules fill space by arranging themselves into molecular-scale helical structures. In this project bent molecules will be designed and synthesized and their phase behavior studied with the goal of exploring the geometrical and statistical principles that govern how such steric frustration is accommodated in materials. The resulting design criteria would have wide ranging applications in the development of new materials based on the control and use of steric frustration. This project will provide its junior scientists (undergraduate student, graduate student, and postdoctoral researchers) with opportunities to participate in broadly interdisciplinary research in an exciting area of soft materials science. TECHNICAL ABSTRACTRod-shaped molecules form nematic liquid crystal phases, three dimensional fluids in which steric packing of rods induces long-range orientational order in thermal equilibrium. But introducing a simple chemical change to make the rods bent in shape produces an exotic new (heliconical nematic) phase in which the molecules spontaneously organize into chiral helical lattices of varying molecular orientation and nanoscale periodicity. The project hypothesis is that these distinctive new structures are a result of steric frustration: the bent molecular shape creates pockets of space which are difficult to fill, thereby attracting the most strongly fluctuating molecular sub-components. This newly formulated organizing principle will be explored in a broad-based, coordinated program of molecular design, chemical synthesis, physical characterization, and computer simulation of heliconical nematic phases. In particular, resonant soft x-ray scattering will be used to characterize their nanoscale orientational structure, and will be extended to probe molecular orientational fluctuations at the nanoscale. What is learned will be applied to a variety of soft matter systems, including liquid crystals, polymers and nanofabrications, with the intention of creating broadly useful, powerful means of probing and understanding nanoscale molecular orientational dynamics. The strong collective nanoscale chirality of the heliconical nematic phase makes it attractive for application in separation of enantiomers of chiral guest molecules, and as a templating medium for asymmetric synthesis. These applications of the heliconical nematic phase will be explored.