The ultimate goal of such studies is to produce artificial, linear molecular motors that move directionally along polymeric tracks to transport cargoes and perform complex tasks at the nanoscale, says Professor Leigh in the paper 'A Synthetic Small Molecule that can Walk down a Track', by Max von Delius, Edzard M Geertsema, and David A Leigh, published online today in Nature Chem., DOI:10.1038/NCHEM.481.

Future Outlook
Chemists have previously made small-molecule rotary motors, but this is the first example of a synthetic small-molecule linear motor. Even this simple first generation system possesses some of the important characteristics (processive and directional walker transport) that are a feature of biological motor proteins.

The figure illustrates the large difference in molecular size between this first small-molecule walker and the motor protein kinesin (DNA walkers [2] are comparable in size to kinesin).

Future work will go into systems with other origins of directional bias, systems with more rigid and extended tracks, and walkers that can capture, transport and release molecular cargoes.
The ultimate goal is to construct small-molecule walkers that can perform work by migrating autonomously and unidirectionally along polymeric, surface-bound tracks.

NEMS portfolio buildup
In 2009 the work Hybrid Organic-Inorganic Rotaxanes and Molecular Shuttles’, by Chin-Fa Lee, David A Leigh, Robin G Pritchard, David Schultz, Simon J Teat, Grigore A Timco and Richard E P Winpenny, Nature.’dealt with the tetravalency of carbon and its ability to form covalent bonds with itself and other elements are the cornerstones of organic chemistry, enabling large and complex (in terms of structure, function and dynamics) carbon-based molecules to be constructed.

The electronic configurations and multivalency of metals and other inorganic elements, on the other hand, can impart diverse and useful electronic, magnetic, catalytic and other chemical and physical properties on molecular level structures.

A team of chemists at the University of Manchester and the University of Edinburgh described the first discrete rotaxane molecules in which inorganic and organic components are linked together mechanically at the molecular level.

An essentially inorganic wheel is assembled around a linear organic axle terminated with bulky ‘stoppers’ to form a hybrid rotaxane structure with dynamic properties that hitherto have been the preserve of organic molecules, such as the large amplitude thermal movement of the macrocycle up and down the axle (a ‘molecular shuttle’, a motion of interest for synthetic molecular machine systems.

The threaded molecular architecture also ensures that the electronic, magnetic and paramagnetic characteristics of the heterometallic nuclear cages, which are part of a family of inorganic clusters viewed as potential components for qubits for quantum information processing[5], are intrinsically associated with—and can potentially influence and be influenced by—the organic portion of the structure.

And back in 2007 work on Exercising Demons: A Molecular Information Ratchet’, paid homage to James Clerk Maxwell’s contributions to the kinetic theory of gases, which for the first time explained real-world properties in terms of the statistical behaviour of atoms and molecules becoming a cornerstone of modern physical science.

'Maxwell’s Demon’, an offshoot of his work on the kinetic theory of gases, had had its own extraordinary impact. It has captured the imagination and interest of scientists in different fields, profoundly influencing the development of statistical and quantum physics, information theory, computer science and cybernetics. 

Some 140 years after its conception, it was the inspiration for a new motor mechanism for nanomachine systems when chemists at the University of Edinburgh actually made a molecular machine that performs the sorting task envisaged for Maxwell’s pressure demon.

But crucially, it requires an input of external energy to do so and so does not challenge the Second Law of Thermodynamics. Using light energy, the molecule is able to transmit information about the position of a molecular fragment in a manner that allows transport of the same fragment in a particular direction. This information-based system represents a fundamentally new type of motor-mechanism for synthetic nanomachines.

The Macroscopic Transport by Synthetic Molecular Machine paper published in 2005 by Leigh (University of Edinburgh), Rudolf (University of Groningen) and Zerbetto (University of Bologna) groups describe the first demonstration of the stimuli-induced motion in synthetic molecular machines being used to transport an object and do macroscopic work, using the same mechanism that biology does, namely biasing Brownian motion.

A single layer of light-driven molecular shuttles attached to a self-assembled monolayer of thiols on gold was able to transport microlitre droplets of diiodomethane along the surface - a mechanical macroscopic response from a mechanical molecular event.

More detailed descriptions.