Metal organic frameworks

Metal organic frameworks (MOFs) are molecular structures, constructed from metal cations linked by organic molecules. They have recently shown considerable promise in a wide range of applications, including hydrogen storage, catalysis and drug delivery. MOFs can be built systematically for desired applications, including particular configurations that are stable and porous and which can be used to trap other molecules in a cage-like structure.

Because these structures can be constructed systematically, they can be designed to trap unusual species in specific environments – for example short-lived molecular “intermediates” that arise part way through a process – that allows them to be studied. Scientists, Neil Champness and Mike George, from the University of Nottingham have been using Diamond to study MOFs containing photoactive complexes – complexes which change their physical or chemical properties when exposed to light.

In detail

Fast time-resolved crystallography is a rapidly growing technique for studying photophysical and photochemical processes. However, there can be limitations in this technique, such as the low conversion to form the excited state or intermediates and analysis can be further hindered by the possibility that the crystal is under strain as the structure changes. Building the photoactive component into a MOF provides a stable framework in a specific environment where the reaction can take place without damage to the crystal. In addition, reactive intermediates may not have the potential to recombine, and so possibly become stabilised within the structure. Furthermore, the MOF structure is free from solvents, and so will potentially mimic the gas phase of the trapped species.

The group from Nottingham demonstrated the use of MOFs to examine the photoactive units of metal diimine complexes. They investigated the nature of excited states and the formation of reactive intermediates of Re(diimine)(CO)₃Cl and Mn(diimine)(CO)₃X (X=Br, Cl) species. The MOFs were designed so that the localised centre of the complex could be defined before and potentially during and after excitation, and so that any localised changes around the complex would not affect the overall framework structure.

“We were able to observe photoisomerization of Mn(diimine)(CO)3Cl despite initially poor crystal quality which was degraded even further by the photo-excitation process. This is important because it has much wider ramifications in studying what happens when molecules in a crystal lattice are excited, providing a full three dimensional picture of the excited molecule. We have shown that this approach can be applied to a wide range of small molecules, and it opens possibilities of using MOFs to host short-lived species, prolonging their lifetimes and potentially giving enhanced insights into the nature of intermediate species.”
Professor Sandy Blake, the Nottingham-based crystallographer and part of the research team.

This work has been published in Nature Chemistry
Published online: 30 May 2010 | doi:10.1038/nchem.681

Photoreactivity examined through incorporation in metal−organic frameworks
Alexander J. Blake, Neil R. Champness, Timothy L. Easun, David R. Allan, Harriott Nowell, Michael W. George, Junhua Jia & Xue-Zhong Sun

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Metal organic frameworks

Metal organic frameworks

In detail