A Spin-Crossover Module for Monolayers and Supramolecular Architectures - Cooperativity in Two Dimensions

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或磁性材料的性能产生剧烈的影响。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响应。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。这是一个基础性的项目，其主要目的是研究二维晶格中的自旋交叉，这种晶格是由功能性铁中心单层结合到金表面形成的。在这些条件下，我们可以测量整个单层中跃迁的传播，或者通过单独监测小分子簇来近距离测量。通过测量层的不同位置处的跃迁，我们可以在原子水平上绘制跃迁如何进行。最近有人提出，自旋交叉事件在晶格结构中的缺陷处开始，然后传播到体中。我们希望能够在实验中观察到这一点。该基金的第二个目标是制备一种新型的可转换表面活性剂化合物，这种化合物在溶液中组装成纳米结构。这些结构可能是囊泡或膜。我们用于单层化学的分子设计也适用于表面活性剂，因此我们也将在资助期间研究这方面。我们的目标是使弱关联的空心球或管可逆地组装和拆卸，或改变它们的大小或形状，因为它们的分子进行自旋交叉。像这样的结构，在不同的温度或pH值下改变它们的聚集，可以在它们的结构转变后释放化学有效载荷。因此，它们作为药物递送的载体被大量研究。在此期间，我们不会获得新的药物输送剂，但一种转换胶束结构的新方法可能会导致类似的应用。与传统设计相比，我们的新胶束的一个优点是，它们的结构重排将伴随着金属头部基团的颜色变化，这可以用肉眼监测。

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 or magnetic materials.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 is a fundamental project, whose main aim is to study spin-crossover in two-dimensional lattices, formed from monolayers of functional iron centres bonded to a gold surface. Under these conditions we can measure the propagation of the transition in the monolayer as a whole, or in close-up by individually monitoring small clusters of molecules. By measuring the transition at different positions of the layer, we can map how the transition proceeds at the atomic level. It has recently been proposed, that a spin-crossover event is initiated at flaws in the lattice structure, before propagating into the bulk. We hope to be able to observe that experimentally.A second goal of the grant, is to prepare a new type of switchable surfactant compound, that assembles itself into nanostructures in solution. These structures might be vesicles, or membranes. The molecular design we are using for the monolayer chemistry also lends itself to being used in surfactants, so we will also examine this aspect during the grant. Our aim is to make weakly associating hollow spheres or tubes that reversibly assemble and disassemble, or change their size or shape, as their molecules undergo spin-crossover. Structures like this, that change their aggregation at different temperatures or pHs, can be made to release a chemical payload following their structural transformation. As such, they are being heavily studied as vehicles for drug delivery. We will not achieve a new drug delivery agent during this grant, but a new method of switching micellar structures could lead to comparable applications down the line. One advantage our new micelles could have over conventional designs, is that their structural rearrangement will be accompanied by a change in colour from the metal head group, that can be monitored with the naked eye.