Bridging the gap between Molecular Dynamics and EPR spectroscopy: Application to Liquid Crystal systems

液晶（lc）是物质的一种中间状态，它结合了晶体中发现的长期有序和流体特征的无序。这种性质的组合产生了独特的光学和电学性质，使lc能够通过电场快速切换。这些特性为它们的广泛应用奠定了基础，例如，在平面电视屏幕和数字手表中看到的显示。分子在LC相中的排列可以是，例如，作为平行棒（称为向列）或作为成堆的盘（称为柱状盘）。后者可以在柱上和柱下传导电荷。这为发光二极管（led）、一维导体等器件中的有机电荷传输材料提供了潜在的应用，以提供新型光导体和光伏太阳能电池。底层的分子组织必须经过工程设计，以产生发光或分离太阳能电池的电子-空穴重组。分子组织可以通过真空沉积或薄膜的形成。对具有新性能的液晶的需求不断增长，导致了新的液晶混合物的发展，全息视频投影的液晶层对齐的新方法，以及新型聚合物液晶的生产。自组织LCs的开发是当前研究的一个重要途径。为了设计具有理想性能的新材料，有必要从分子水平上描述和预测它们的行为。近年来，在理论和实验领域都取得了重大进展，有望在复杂的部分有序分子系统（如LCs）的研究中取得进展。首先，自旋标签是一种特殊设计的化学试剂，它携带一个稳定的未配对电子，可以在复杂的分子系统中引入，以报告周围分子的秩序和动力学。因为电子有磁矩，所以它可以与外部磁场相互作用。电子顺磁共振（EPR）以谱线形状的形式测量这种相互作用。自旋标签对磁场的方向对这条线的形状有很大的影响，因此可以研究分子的迁移率、动力学和分布。EPR是一种技术，可以作为非常快速的分子运动的快照，并且可以在比十亿分之一秒短的时间内解决引入的自旋探针的分子重新定向动力学。EPR仪器的最新进展是，使用不同频率的自旋标记和探针已经成为研究复杂相（如lc）、蛋白质及其复合物、DNA/RNA、聚合物、脂质和纳米结构的结构和动力学的重要方法。其次，在过去十年中，计算机能力的巨大增长导致分子动力学（MD）模拟的使用增加，作为理解复杂化学系统的工具，具有预测复杂自组织系统（如LC， LC混合物和复合系统）的各种特性的潜力。然而，目前还没有通用的方法和用户友好的计算工具，能够将复杂分子系统的最先进的MD模拟与EPR光谱的模拟和分析直接联系起来。本提案的目的是弥合MD模拟和EPR光谱之间的差距，并开发方法，这将使人们能够获得详细的描述，并得出关于复杂分子系统中分子排列和相互作用的明确结论。在建议的工作中，这种方法将应用于不同类型的LCs，混合物和混合系统。这项建议的成果将以用户友好软件的形式提供给国际科学界。

Liquid crystals (LCs) are an intermediate state of matter that combines both long range order, as found in crystals, and disorder, a characteristic of fluids. This combination of properties gives rise to unique optical and electrical properties that allow LCs to be switched rapidly by electric fields. These properties underlay their widespread application, for example, in displays as seen in flat TV screens and digital watches. The ordering of molecules in LC phases can be, for example, as parallel rods (termed nematic) or as stacks of discs (termed columnar discotics). The latter can conduct charge up and down the columns. This offers potential applications as organic electronic charge transport materials in devices such as light emitting diodes (LEDs), one-dimensional conductors, to provide new types of photoconductors, and photovoltaic solar cells. The underlying molecular organisation must be engineered to produce electron-hole recombination for luminescence or separation for solar cells. Molecular organisation can be by vacuum deposition or the formation of films. The growing demand for LCs with new properties is leading to the development of new mixtures of liquid crystals, new ways to align LC layers for holographic video projection, and the production of new polymeric liquid crystals. The exploitation of self-organising LCs is an important avenue of current research. In order to design novel materials with desired properties it is necessary to describe and predict their behaviour from the molecular level. Recently, major advances in both theoretical and experimental areas have emerged which promise progress in the studies of complex partially ordered molecular systems such as LCs. Firstly, spin labels, specially designed chemical agents that carry a stable unpaired electron, can be introduced within complex molecular systems in order to report on the order and dynamics of surrounding molecules. Because an electron has a magnetic moment it can interact with an external magnetic field. Electron Paramagnetic Resonance (EPR) measures this interaction in the form of spectral line shape. The orientation of the spin label to the magnetic field has a dramatic effect on this line shape and therefore molecular mobility, dynamics and distribution can be studied. EPR is a technique that acts as a snapshot of very fast molecular motions and can resolve molecular re-orientational dynamics of the introduced spin probe over times shorter than a billionth of a second. Recent advances in EPR instrumentation, using different frequencies with spin labels and probes has become an important method for studying structure and dynamics of complex phases such as LCs, of proteins and their complexes, DNA/RNA, polymers, lipids and nanostructures. Secondly, the huge increase in computer power over the last decade has led to an increase in the use of molecular dynamics (MD) simulations as a tool to understand complex chemical systems with the potential to predict various properties of complex self-organising systems such as LCs, LC mixtures and composite systems. Yet there is no general methodology and user-friendly computational tools which are able to link directly state-of-the art MD simulations of complex molecular systems with the simulation and analysis of EPR spectra.The aim of this proposal is to bridge the gap between MD simulations and EPR spectroscopy and to develop methodology, which would enable one to obtain a detailed description and reach unambiguous conclusions about molecular arrangement and interactions within complex molecular systems. In the proposed work, this methodology will be applied to different types of LCs, mixtures and hybrid systems. The output of this proposal will be available to the international scientific community in the form of user-friendly software.