Halogen-bonded Liquid Crystals

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!