In the metal-organic framework developed by Stephen Loeb's group at the University of Windsor, circular crown ether 'wheels' (represented by yellow toruses) are able to spin around organic 'axles' connected by complexes of copper (brown spheres). This is the first time a rotating mechanically interlocked molecule has been synthesized in the solid state.
By Tyler Irving
Posted July 2012
To ‘spin one’s wheels’ usually means a failure to make progress, but last month a group of researchers from the University of Windsor spun themselves onto the cover of Nature Chemistry. They’ve created the first metal-organic framework (MOF) with rotating dynamic components; the innovation could bring us one step closer to molecular computing.
Stephen Loeb’s group in Windsor’s Department of Chemistry specializes in rotaxanes, a type of mechanically interlocked molecule where a cyclical ‘wheel’ freely rotates around a straight ‘axle.’ Large functional groups on either end of the axle prevent the wheel from slipping off. The group has even created rotaxanes where a single wheel can jump between two discrete locations on an axle, acting as a molecular switch that could encode digital information. Until now such molecules had only been made in solution. “If you want to make random access memory using these things, then you’ve got to organize them in some way,” says Loeb.
In the paper, the group describes a simple rotaxane in which the functional groups on the end have been modified into carboxylate groups. These groups interact with copper-based metal-organic complexes to form a solid-state, three-dimensional MOF. Experiments using NMR with deuterium labelling conducted by Robert Shurko’s groupin the same department demonstrated that the wheels were indeed spinning, at speeds above 10 MHz.
Loeb’s group has made MOFs with switchable rotaxanes as well, but proving that the wheel can not only rotate but translate along the axle has proven difficult. Another huge challenge lies in determining how to trigger individual molecular switches by either electrical or photochemical means, something that Loeb admits is still “science fiction.” Still, if it could be done it would yield a material with a density of switches over a billion times higher than today’s most advanced devices.
Photo credit: Stephen Loeb
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