Halogen-bonded Liquid Crystals

固体受热时，在一定温度下会融化成液体(如冰融化成水)。然而，有些化合物以介于固态和液态之间的状态存在，并具有这两种状态的某些性质。这种状态称为“液晶相”，形成这种状态的化合物称为液晶(LCS)。液晶是手机、计算器、笔记本电脑等中常见的液晶显示器。液晶在其他方面也很重要，众所周知，液晶在生物体细胞的工作中非常重要，也是肥皂和洗涤液清洁得如此之好的原因。构成液晶的分子有特殊的形状，要么又长又细(像铅笔)，要么薄而平(像披萨)，化学家们知道如何制造具有这些形状的分子。有时，用两部分而不是一部分来制造这些分子是可能的。因此，如果我们想要制造一个棒状分子，那么尽管这两部分都不够长，不足以显示LC相，但将它们结合在一起将得到一个确实显示液晶相的“超分子”。通常情况下，非常强的键将这些分子中的原子结合在一起，但在这里，这两个部分是通过一种非常特定但相当弱的相互作用保持在一起的。其中一种弱相互作用--氢键--已经知道很多年了，但还有一种非常类似的相互作用，它的强度更弱，也不太为人所知，称为“卤键”。最近，埃克塞特团队展示了它可以通过使用卤键将超分子结合在一起来制造液晶化合物。我们现在想做更多的例子，我们仔细地改变两个部分，以便我们对这些超分子有更多的了解。由于卤素键被用于化学的其他部分，我们所学到的对其他部分也很重要。顾名思义，卤素键涉及我们称为卤素的元素，两个部分中的一个(受体)将至少包含一个卤素；在这项工作中，卤素将是碘。在接受者中，碘带有一些正电荷，因为接受者的设计意味着负电荷被拉走。然后，它会很高兴地找到另一个带有一些负电荷的部分加入。这另一个部分被称为施主，我们的部分含有一个氮原子。在供体中，负电荷被推向氮，使其被正碘吸引。两者相互吸引，形成一个超分子。我们将首先通过形成几个相关的受体来研究碘必须有多大的正电性才能形成超分子。然后，如果我们在受体中加入两个碘，我们就可以结合两个供体--一种不同类型的超分子。我们还可以改变供体的形状，使这些2比1的超分子看起来更像披萨，而不是铅笔。接下来我们可以改变给体，因为如果我们在氮上加一个氧原子，那么氧就会被受体的碘吸引；氧被吸引得比氮更强，所以超分子的性质会发生变化。在我们制作这些新的体系后，我们需要观察给体-受体相互作用和LC行为；这需要特殊的实验。我们可以用X射线观察到碘和氮之间的距离，在超分子的固态中，我们可以看到给体和受体之间的相互吸引是多么强烈。我们用光来帮助我们理解液晶的行为(液晶会让我们看到漂亮的图案)，我们可以用不同的方法来估计液晶相中碘和氮的分离(供体吸收的光会随着碘离氮的距离而变化)。这项工作将给出一般情况下，特别是在液晶中卤素键的行为的精确图像，并展示它在未来可能如何有用。注意，这是在我14岁的女儿身上尝试过的！

When a solid is heated, at some temperature it will melt to form a liquid (eg the melting of ice to form water). However some compounds exist in a state that is intermediate between the solid and liquid states and has some properties of both. This state is known as a 'liquid crystal phase' and the compounds that form it are called liquid crystals (LCs). LCs are familiar to all as liquid crystal displays found in mobile phones, calculators, laptop computers etc. LCs are important in other ways, too and, it is known that they are important in the workings of the cells of living organisms and are the reason soaps and washing-up liquids clean so well.The molecules which make up liquid crystals have a special shape, being either long and thin (like a pencil), or thin and flat (like a pizza), and chemists knows how to make molecules with these shapes. Occasionally it is possible to make these molecules by using two parts instead of one. Thus, if we want to make a rod-shaped molecule, then while neither of the two parts would be long enough to show a LC phase, joining them together would give a 'supermolecule' that did show a liquid crystal phase. Normally, very strong bonds hold the atoms in these molecules together, but here the two parts are held together by a very specific but rather weak interaction.One of these weak interactions, the 'hydrogen bond', has been known for many years, but there is a very similar interaction that is weaker and much less well known called the 'halogen bond'. Recently the Exeter team showed that it could make LCs by using a halogen bond to hold together a supermolecule. We now want to make more examples where we vary the two parts carefully so that we learn more about these supermolecules. Because halogen bonding is used in other parts of chemistry, what we learn will be important for others, too.As the name suggests, halogen bonds involve the elements we call the halogens, and one of the two parts (the acceptor) will contain at least one halogen; in this work the halogen will be iodine. In the acceptor the iodine carries some positive charge because the acceptor design means negative charge is pulled away from it. It is then be happy to find another part to join with that carries some negative charge. This other part is called a donor and ours contain a nitrogen atom. In the donor, negative charge is pushed towards nitrogen allowing it to be attracted to the positive iodine. The two are attracted and a supermolecule is formed.We will first investigate how positive the iodine must be for the supermolecule to form by making several related acceptors. Then, if we put two iodines in our acceptor, we can bind two donors - a different type of supermolecule. We can also change the shape of the donor so that these 2-to-1 supermolecules look more like a pizza than a pencil. Next we can vary the donor, for if we add an oxygen atom to the nitrogen, it is then the oxygen that is attracted to the iodine of the acceptor; the oxygen is attracted more strongly than the nitrogen so the properties of the supermolecule will change.After we make these new systems, we need to look at the donor-acceptor interaction and the LC behaviour; this needs special experiments. We can see how strongly the donor and acceptor are attracted to each other in the solid state of the supermolecule using X-rays which 'see' the distance between the iodine and the nitrogen. We use light to help us understand the LC behaviour (the liquid crystal causes pretty patterns to be seen), and we can use light in a different way to estimate the separation of the iodine and nitrogen in the liquid crystal phase (the light absorbed by the donor changes depending how far the iodine is from the nitrogen).This work will give a precise picture of the behaviour of the halogen bond in general and, in particular, in liquid crystals, and show how it might usefully be used in the future.NB This was tried on my 14 year-old daughter!