Understanding and engineering function in switchable molecular crystals

晶体材料的物理性质既取决于分子本身，也取决于其原子或分子的空间排列。通常，相同的分子可以通过以不同的方式聚集在一起产生两种或更多种不同的晶体，从而产生物理上不同但具有相同化学成分的材料（多晶型物）。化合物在不同的温度或压力条件下通常可以优选采用不同的晶体多晶型物结构。因此，当温度改变时，晶格可以重新排列成一个新的三维结构-相变。例如，这在制药工业中是重要的，例如，其中药物化合物的不同晶体多晶型物可以具有不同的溶解度，其中溶解度较低的形式活性较低。晶体相变也会对导电、磁性和光子材料的性质产生巨大影响，其中材料中原子的微小重排会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。这在某些类型的过渡金属化合物中很常见，在铁化学中特别普遍。当材料中的分子单独进行自旋交叉时，它会导致它们的尺寸和形状发生很大的变化，这些变化在固态材料中传播。当一个分子经历转变并改变其大小时，它会引起晶格中压力的变化，从而促进其最近邻居的转变。这些效应通过晶格以不同的速率传递，这取决于分子之间相互作用的强度。因此，一种特定的材料是随着温度还是随着时间突然地还是逐渐地经历自旋交叉，是由其晶体堆积控制的。自旋交叉是晶体学相变的一个相当极端的例子，涉及材料结构的变化。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相变的理解，使用自旋交叉化合物作为试验台。我们将建立新的基本原则工程相变成分子晶体，发生在预定义的条件下（温度和/或光照射），在不同的速率，并与滞后的属性。除了合成这些新材料外，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率诱导晶体中的相变，并用于解释从这些实验中得到的数据（例如，从材料的起始相和产物相解卷积贡献）。我们还将开发使用计算机模型模拟相变事件的改进方法，以提供晶体设计如何影响相变通过其体积传播的新见解。我们联合体的专业知识的结合将在解决一个问题方面取得真实的进展，这个问题到目前为止只能通过试错法成功解决。

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.