Embracing Cooperativity - Spin-Crossover Compounds with Functional Dopants

自旋交叉化合物的合成、固体化学和物理是目前国际上非常关注的问题。这些材料在热、光或其他物理刺激的作用下，磁矩会发生可逆变化。这通常对应于过渡金属中心的高自旋到低自旋的d电子跃迁，因此伴随着颜色的变化。像这样的材料，其颜色可以可逆和快速切换，在显示设备和光学计算等方面有潜在的应用。在过去的几年里，该领域的研究越来越多地集中在多功能自旋交叉开关上，这种开关利用自旋转变来控制材料的另一种特性。例如，通过在由自旋交叉阳离子和导电阴离子构成的盐中围绕自旋跃迁循环来调节分子材料的导电性。该方案的主要目的是利用自旋交叉来控制另一种功能，即荧光。其他人先前已经表明，自旋跃迁可以对溶液中化合物的发光强度产生强烈影响，但在固态中从未显示过这种作用，我们打算通过两种方式来实现。一种是形成与上述导电盐类似的自旋交叉阳离子和荧光阴离子的混合盐。另一种方法是基于我们最近在自己的工作中所做的观察。我们研究2,6-二吡唑基吡啶的铁化学已有一段时间了。这些配体的铁（II）配合物通常在室温附近或低温激光照射下发生自旋交叉转变。我们的几种材料所显示的转变异常相似和一致，这与它们晶体中的分子相互联系的方式有关。我们发现这种特殊的结构类型是非常宽容的，因为我们可以清晰地形成一种自旋交叉开关和一种完全不同的化合物的固溶体，这种化合物在相同的晶体包装基序的不同版本中结晶。这些分子合金保留了它们的自旋交叉功能，尽管通常随着掺杂量的增加转变变得更加缓慢。这项提议最重要的目标是用新化合物合成这样的固溶体，这些化合物的形状正确，可以与我们的自旋交叉开关共结晶，并且在室温下具有荧光性（因此能够被它们切换）。我们将比较所有荧光开关在单晶和粉末样品以及纳米薄膜中的效率。这是生产可用于设备的功能材料的第一步。同时，我们还将在自旋交叉晶格中掺杂另外两个具有电化学或光谱活性的功能中心。通过观察自旋跃迁过程中单个分子如何受到结构变化的干扰，监测这些性质是如何变化的，可以深入了解这种跃迁是如何在材料中传播的。这些数据将帮助我们通过设计生产量身定制的自旋交叉材料，而不是目前主要使用的试错方法。

The synthesis, and solid state chemistry and physics, of spin-crossover compounds is of great international interest at present. These are materials that can undergo a reversible change in magnetic moment upon application of heat, light or some other physical stimulus. This most commonly corresponds to a high-spin-to-low-spin d-electron transition at a transition metal centre, and so is accompanied by a colour change. Materials like these, whose colours can be reversibly and rapidly switched, have potential applications in display devices and in optical computing, among other things. In the past few years, research in the area has increasingly concentrated on multifunctional spin-crossover switches, that use a spin-transition to control another property of the material. For example, the electrical conductivity of a molecular material has been modulated by cycling around a spin-transition in a salt built from spin-crossover cations, and conducting anions. The bulk of this proposal aims to use spin-crossover to control another functionality, namely fluorescence. Others have previously shown that a spin-transition can have a strong effect on the intensity of light emission from a compound in solution, but it has never been shown to work before in the solid state, We intend to do that in two ways. One is to form mixed salts of spin-crossover cations and fluorescent anions, comparable with the conducting salts described above. The other approach is based on a recent observation we have made in our own work.We have been studying the iron chemistry of 2,6-dipyrazolylpyridines for some time. Iron(II) complex compounds of these ligands often undergo spin-crossover transitions near room temperature or under laser irradiation at low temperatures. The transitions shown by several of our materials are unusually similar and consistent, which is related to the way the molecules in their crystals associate with each other. We have found that this particular structure type is remarkably forgiving, in that we can cleanly form solid solutions of one of our spin-crossover switches and a completely different compound, which crystallises in a different version of this same crystal packing motif. These molecular alloys retain their spin-crossover functionality, although as usual the transition becomes more gradual as the amount of dopant increases. The most important goal of this proposal to synthesise such solid solutions with new compounds that are the right-shape to co-crystallise with our spin-crossover switches, and are fluorescent at room temperature (and so competent to be switched by them). We will compare the efficiency of all our fluorescent switches in single crystal and powder samples, and also as nanometre thin films. That is the first step towards producing functional materials that can be used in devices.At the same time, we will also dope our spin-crossover lattice with two other functional centres, that are electrochemically or spectroscopically active. Monitoring how those properties change as the spin-transition progresses will give insight into how the transition propagates through the material, by showing how individual molecules are perturbed by the structural changes going on. Those data will help us poduce tailor-made spin-crossover materials by design, rather than the trial-and-error methods that are still mostly used at the moment.