Sir J. Fraser Stoddart, Northwestern University
Artificial Molecular Machines Going from Solution to Surfaces
Written by Don Monroe
Fraser Stoddart has been a pioneer in molecular machines, as recognized by sharing the 2016 Nobel Prize for Chemistry. A useful feature for these structures is the “mechanical bond,” such as that which holds together interlocking molecules, such as a ring-shaped molecule surrounding a dumbbell-shaped one. Among chemistry advances, “a new chemical bond is extremely rare,” he noted.
In his Kavli lecture, Stoddart focused on artificial molecular pumps that exploit this feature and add extra elements to achieve unidirectional motion. But he stressed that these pumps “don’t operate like the mechanical ones” that humans have used for millennia. “It’s a world of difference.”
In the nanomolecular pumps, the free-energy terrain is changed, allowing the molecules to jump around between different accessible states. “It’s all about kinetics,” rather than thermodynamics, he said. The kinetics of association and dissociation can be modulated by changing the charge state of radicals, for example by changing oxidizing or reducing conditions chemically, or electrochemically with an applied voltage.
Many of the structures Stoddart described use a “pumping cassette” that loads a charged ring-shaped radical onto a “collecting chain” where it is mechanically bound. This process can be repeated to load additional rings, with little increase in the free-energy cost. His research team has loaded as many as 80 rings onto a star polyethylene glycol, incorporating 344 positive charges.
Attaching pumping cassettes to both ends of a chain can double the loading. Stoddart noted that this technique can create a symmetrical loading of molecules, which could in principle be used to make palindromic polymers of the rotaxane ring molecules.
Moving away from solution chemistry, Stoddart illustrated the tethering of molecular pumps to a metalorganic-framework membrane. The result is what he termed “mechanisorption” to the membrane. Unlike the well-known physisorption and chemisorption, driven by van der Waals or chemical bonding, respectively, this process is intrinsically far from equilibrium, and is made possible by mechanical bonding.
Stoddart also mentioned the potential for molecular nanotopology (formerly called chemical topology) to form various interlocking ring-like structures, including knots, belts, and Möbius strips. (The linear molecules employed for his molecular pumps do not satisfy this description.) “There are eight million knots, so we can keep chemists and materials scientists occupied for centuries,” he said, since only about a dozen have been made so far.
Although Stoddart admitted that he is “not an applications scientist,” he expressed the hope that the tools and techniques his group has developed could be helpful for battery technology and hydrogen storage as well as capture of CO2 and methane. He also expects that there will be huge opportunities in medical science, in view of the profound importance of biological molecular pumps.
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