Understanding and engineering function in switchable molecular crystals

晶体材料的物理性质取决于其原子或分子的空间排列，也取决于分子本身。通常，相同的分子可以通过不同的方式堆积在一起，产生两种或两种以上不同的晶体，从而产生物理上不同但化学成分(晶型)相同的材料。在不同的温度或压力条件下，化合物往往会采用不同的晶体多晶型结构。因此，当温度改变时，晶格可以重新排列成新的三维结构--相变。这一点很重要，例如，在制药工业中，药物化合物的不同晶型可能具有不同的溶解度，较不容易溶解的形式活性较低。晶体相变也可能对导电、磁性和光子材料的性能产生巨大影响，在这些材料中，原子的微小重新排列对其电子的行为产生重大影响。我们已经研究了一段时间的一种相变是自旋交叉，这是原子中电子对温度变化的反应重新排列。这在某些类型的过渡金属化合物中很常见，在铁化学中尤其普遍。当一种材料中的分子分别经历自旋交叉时，它会导致它们的大小和形状发生很大的变化，这些变化在固体状态下通过材料传播。当一个分子经历跃迁并改变其大小时，它会引起晶格中压力的变化，进而促进其邻近分子的跃迁。这些效应以不同的速率通过晶格，这取决于分子之间相互作用的强度。因此，一种特定的材料是突然地还是逐渐地经历自旋交叉，随着温度或时间的变化，取决于它的晶体堆积。就材料结构的变化而言，自旋交叉是晶体相变的一个相当极端的例子。但它可以作为其他更一般类型的晶体相行为的模型。这个项目代表了一个协调一致的计划，以提高我们对晶体材料中的相变的理解，使用自旋交叉化合物作为试验台。我们将为分子晶体的工程相变建立新的基本原理，这些相变发生在预定的条件下(温度和/或光照射)，以不同的速率发生，并且具有滞后特性。除了合成这些新材料之外，这个看似简单的目标还需要最先进的方法来测量这些结构变化。这将使用新的X射线衍射技术来实现，用于在受控条件下以高产量诱导晶体中的相变，并用于解释这些实验产生的数据(例如，从材料的起始和产品相中解卷的贡献)。我们还将开发使用计算机模型模拟相变事件的改进方法，以提供新的见解，了解晶体的设计如何影响相变通过其主体的传播。我们联合体中的专业知识的结合将在解决问题方面取得真正的进展，到目前为止，这个问题还只是通过反复试验才成功解决的。

The physical properties of a crystalline material depend on the spacial arrangement of its atoms or molecules, as much as on the molecules themselves. Quite often the same molecules can generate two or more different kinds of crystal, by packing together in different ways, leading to materials that are physically distinct but with the same chemical composition (polymorphs). A compound can often prefer to adopt different crystal polymorph structures under different conditions of temperature or pressure. Thus, when the temperature is changed, the crystal lattice can rearrange itself into a new three-dimensional structure - a phase transition. This is important, for example, in the pharmaceutical industry, for example, where different crystal polymorphs of drug compounds can have different solubilities, with the less soluble form being less active. Crystal phase transitions can also have drastic effects on the properties of conducting, magnetic and photonic materials, where small rearrangements of the atoms in a material have large consequences for how their electrons behave. One type of phase change that we have been studying for some time is spin-crossover, which is a rearrangement of the electrons in an atom in response to a change in temperature. This is common in some types of transition metal compound, being particularly prevalent in iron chemistry. While the molecules in a material undergo spin-crossover individually, it leads to large changes in their size and shape which are propagated through the material in the solid state. As one molecule undergoes the transition and changes its size, it causes a change in pressure in the crystal lattice that in turn promotes the transition in its nearest neighbours. These effects are transmitted through a crystal lattice at differing rates, depending on the strength of the interactions between molecules. Hence, whether a particular material undergoes spin-crossover abruptly or gradually, with temperature or with time, is controlled by its crystal packing. Spin-crossover is a rather extreme example of a crystallographic phase change, in terms of the changes involved to the structure of the material. But it can serve as a model for other, more general types of crystal phase behaviour.This project represents a concerted program to improve our understanding of phase changes in crystalline materials, using spin-crossover compounds as a test-bed. We will establish new fundamental principles for engineering phase changes into molecular crystal, that occur under pre-defined conditions (of temperature and/or light irradiation), at different rates, and with the property of hysteresis. As well as synthesising these new materials, this apparently simple objective requires state-of-the-art methods for measuring these structure changes. This will be achieved using new X-ray diffraction techniques, for inducing phase changes in crystals in high yields under controlled conditions, and for interpreting the data that result from these experiments (to deconvolute contributions from the starting and product phases of the material, for example). We will also develop improved methods for simulating the phase change events using computer models, to provide new insight into how the design of the crystal affects the propagation of the phase change through its bulk. The combination of expertise in our consortium will achieve real advances towards solving a problem, that has only been successfully addressed up to now by trial-and-error.