Supramolecular Chemistry

Meera – An exciting area emerging from research at Diamond is the synthesis of new, specialist molecules, known as supramolecular chemistry and the scientist pioneering this field is Harry Anderson from the University of Oxford.

Harry – I’m interested in organic molecules with strongly coupled electrons and controlling their properties often using non-covalent, weak, reversible interactions.

Meera – And what kinds of molecules are these?

Harry – Well they tend to be coloured compounds, a bit like dyes, a lot of the molecules we work with are made from porphyrins, which are naturally occurring pigments. But the ones we work with are not naturally occurring, I mean we make them ourselves from scratch but they’re related to the compounds that make blood red and grass green.

Meera – So are you essentially building molecules and then working with them from there?

Harry – Yes, we do synthesis from very simple starting components and we make compounds and design the compounds. So we are making compounds that have never existed anywhere ever before and so we’re interested in learning about their properties and relating the properties to the molecular structure.

Meera – The actual molecules you’re making are quite big, so how do you actually set about constructing these from scratch?

Harry – It’s a classical, organic synthesis except we also use templates quite a lot. So a template is a molecule around which you can build another molecule which determines the shape of the components in the right places as you link them together and then you can remove the template afterwards. So we’ve developed new ways of using templates to make bigger molecules.

Meera – And is it then hoped that they can have some new applications as well?

Harry – Yeah, of course. So the sort of molecules we work with have strongly coupled electrons which means they are often semiconductors. Organic semiconductors have one important area of application in devices like, well, display devices, electroluminescent displays, but also their interaction with light. They are often fluorescent and have unusual ways of interacting with light that can be used in optical switching devices. And some of the materials which we work with generate singlet oxygen which can be useful in medical applications.

Meera – and could you give an example of a particular molecule that you’ve worked on recently? So how you’d set about making it, why you wanted to make it and maybe some of the uses that it could now have?

Harry – One of the sorts of molecules that we’re excited about are molecular wires that are in rings rather than being straight. So we’ve worked on, what we call, molecular wires which are straight molecules down which you can transfer electrons, but recently we’ve realised that it’s possible to make rings of these molecules. So part of the challenge is just to make a molecular wire into a ring and then look at the charge mobility around the ring but for this particular class of molecules, then it’s not that easy to identify applications here, but we think that it may have unusual properties, maybe magnetic properties, than can get them to behave like little loops of magnetic wire. So we’re just investigating them to see how they behave.

Meera – What are the applications of the first set of molecular wires that were created, the linear ones?

Harry – Yes, potentially in molecular electronics, that means molecular scale electronics, transistors or electronic devices on the molecular scale, the ultimate in miniaturisation as integration circuits get smaller and smaller. So a lot of people are working in that area but it’s still a very long way from being useful. On the other hand in devices made on a larger scale, using larger amounts of material, like solar cells and maybe also organic transistors, it’s useful to have materials which can transfer charge efficiently. The molecular wires might be components in a material, perhaps in a solar cell.

Meera – So would the circular forms have different applications? So you’ve mentioned that they could perhaps alter magnetic fields and so on. So changing the shape of them could give them different properties and other uses?

Harry – Yes, just like a bit of wire, if it’s in a solenoid shape it behaves quite differently to if it’s in a straight shape. That’s one of the amazing things about molecules that just changing the shape, thinking about the shape, often relates to the properties.

Meera – And how do you use Diamond in all of this? How do you use synchrotron radiation to perhaps aid this process?

Harry - For structurally characterising the compounds that we make, proving that they are actually the compounds which we think they are, and for getting information on the shapes. So there are two techniques that we use at Diamond; small angle x-ray scattering of solutions, is very useful for getting low resolution information on the shapes of the molecules. It hasn’t been used much before for synthetic molecules, it has been used a lot for proteins, and it’s useful because you don’t have to grow crystals, you can take a compound and take a dilute solution and just from the small angle x-ray scattering you can get information on the 3-dimensional shape of the molecule. And the other thing that we use at Diamond is x-ray crystallography, and it gives much higher resolution information than the solution phase scattering.

Meera – Is there a particular molecule that has perhaps been your most successful to date in terms of how far you’ve managed to go with it?

Harry – Well, I suppose we’ve worked most on porphyrin-based molecular wires. Perhaps they’ve been most successful because they’re so versatile. That a similar group of molecules can have potential applications in medicine and photodynamic therapy and also in molecular electronics and there are lots of other potential applications too, they have such a rich range of properties.

Meera - Harry Anderson, Professor of Chemistry from the University of Oxford.