Metal Organic Frameworks

Meera - One recent, and fast developing, area of Chemistry is the design and use of Metal-Organic Frameworks. They have potential uses in Gas Storage and Catalysis, but can also provide greater insight into the fundamental workings of chemical reactions as Nottingham University's Neil Champness explains.

Neil Champness - So metal-organic framework is a material, always a solid, which is made by reacting metals with organic molecules which will act as a bridge between different metal centres. So, you react the 2 together and you make a 3 Dimensional structure, so it will look at bit like a climbing frame. And typically they will have holes within that structure. But more importantly, beyond that, metal-organic frameworks will have this subset of materials where you can, well react metals and ligands.

Meera - So, essentially it's a 3 dimensional, porous structure that you can encapsulate things into, but on a very, very small scale?

Neil - Yes, absolutely. The types of dimensions that we are talking about will be 2 nanometres and below. But that is the molecular scale.

Meera - What would you say is the general aims of these structures are and what is the purpose of them?

Neil - The beauty of metal-organic frameworks is that you can vary the metal that you use, so typically we use transition metals, things like copper and zinc, or you can use lanthanides. So there's a lot of variation there and you can have different properties that they bring with them, but also you can vary the organic molecule a lot as well. And therefore, you can have almost a limitless library of possible framework structures. A lot of the work is directed towards Gas Storage, this is a big theme, and our own research group work with Diamond in fact on developing Hydrogen Storage materials. You can use it for trapping Carbon Dioxide, is another possible direction, there is a lot of interest in developing them for pollutant separation, those types of things.

Meera - So essentially you can really design and tailor these to be specific to the kind of use or the things that you want to look into?

Neil - Absolutely, that's the great desire and in many ways that is more true for metal-organic frameworks than it is for anything else.

Meera - Tell me more about your specific use for these structures. You look into Photochemical and Photophysical reaction; reactions that really need the presence of light?

Neil - Yes. One of the interesting things in Photo-Chemistry is that you can shine light on a molecule and it becomes excited. It absorbs the light and becomes excited. And those excited states then undergo different reactions or use that energy, the energy of the excited state, in different ways. The actual properties of those excited states depends very much upon the environment in which they are sitting and also, how easily they can meet other excited molecules and react with them. What we have been doing is trying to incorporate those molecules which can be photo-excited within the metal-organic framework. You are controlling the environment of the excited state, of that excited molecule, and also you are stopping it meeting other excitable molecules, so therefore you are changing the properties of the molecule by putting it within the framework. Metal-organic frameworks are ideal for this because you can design the framework structure, as we've just been discussing.

Meera - What is the benefit then of keeping them in this rigid state and analysing what does happen to them?

Neil - So it gives you real insight into what is the nature of this photo-excited state. Probing those excited states is difficult because many of them are short lived, they can be less than a nano-second. It can react, it can undergo chemical transformations into all kinds of things, you need to understand that process, but also by putting it within the framework, we can prolong the lifetime of those excited states, we can change the actual properties in fact and actually we can probe them differently. Often, the way that you look at these excited states is by using spectroscopy. They give you lots of information, but it's not quite the same as the details that you can get from other techniques such as crystallography. This is where our work with Diamond comes in. At Diamond, we can get what are called crystal structures of single crystals of our frameworks, using the high energy x-rays that can be generated at the Diamond synchrotron, which we can't do in our own labs here. What this allows you to do is to actually get detailed information about the precise positions of the atoms within our molecules. This allows us to see what happens to the photo-excited state after it reacts.

Meera - Could you give an example of a particular reaction that you have been looking into at Diamond?

Neil - A particular case we can incorporate into our framework structure, a particular rhenium complex. Now, rhenium is a pretty unusual metal and this particular type of rhenium complex has been widely studied and is actually used, it's kind of a classic of photo-chemistry, and it has been used in all kinds of applications, studied in solutions, looked at how it interacts with DNA, used in photovoltaic applications. What we did was we incorporated this basic group within a framework, modified its properties and were then able to look at how it behaves after photo-excitation.

Meera - What would the next step be? With a really thorough understanding of the process, say for the rhenium complex and other examples and other reactions as well, what would it be hoped would be the next using this information?

Neil - In this process there are carbon monoxide molecules attached to the rhenium centre and actually on photo-excitation there is a short period of time where this carbon monoxide molecule comes away from the metal centre and there is what is called a 'Vacant Site'. Normally, and indeed in the studies we've done so far, what happens is that the carbon monoxide goes back on. What we really want to do is to prolong the lifetime of that situation where there are vacant sites on the metal and try to put different things onto it. There are 2 directions here, one is to try to put alkanes on to there - this is things like Methane, methane gas -and try to incorporate it and get a rhenium interaction and that leads to what is called 'Activation' of the alkane and make it react differently in chemical terms. But the one we'd love to do, is to try to put Xenon onto it, try to make a metal to xenon bond and actually characterise it so that we exactly where this xenon is sitting. The xenon is incredibly un-reactive. It's actually being able to identify and characterise a bond to xenon. It's almost one of the Holy Grails to chemistry.

Meera - So it would be quite an advancement in fundamental chemistry to try and get this bond.

Neil - Absolutely, it's fundamental understanding here. I mean, it gives us a far greater understanding of how these things can behave, it advances science all together. There is still a beauty in understanding how different atoms interact with each other, 'cause that's the basis of all chemistry.