By tethering molecular pumps onto the surface of a 2D metal-organic framework (MOF), chemists have created a new type of adsorption phenomenon. They call it mechanisorption because it uses mechanical bonds—in the form of molecular chains physically threaded through rings—to control the chemistry at surfaces and interfaces. Mechanisorption could someday be used to assemble molecules on surfaces or to concentrate compounds and release them on demand.
Researchers led by Northwestern University’s J. Fraser Stoddart, Liang Feng, and Yunyan Qiu created the mechanisorption system, which features functionalized chains attached to a MOF surface. Through repeated redox chemistry, the chains pull ring molecules from solution, threading up to five rings onto each chain (Science 2021, DOI: 10.1126/science.abk1391).
Mechanisorption expands the realm of surface chemistry beyond physisorption—where van der Waals interactions anchor substances to surfaces—and chemisorption—where covalent bonds moor molecules to surfaces, the researchers say. But unlike physisorption and chemisorption, which are phenomena observed at equilibrium, mechanisorption is a nonequilibrium phenomenon. By concentrating mechanically bonded rings to the MOF surface, the molecular pumps push the system out of equilibrium.
The synthetic system “is a step towards what living systems do,” says R. Dean Astumian, a physics professor at the University of Maine who collaborated on the project. Cellular ion pumps do the same thing, he explains. They “recruit materials to high concentration, put them in separate compartments, and use that to do nonequilibrium chemistry,” Astumian says.
V. Nicholas Vukotic, who studies materials at the University of Windsor, says in an email that synthetic mechanisorption systems developed by Stoddart and coworkers “lay the groundwork for the development of programmable materials able to store targeted substances away from equilibrium and deliver them on-demand, which would have a significant impact in drug release, energy storage, and many other exciting material science applications.”
Feng says that if the chemists can append an azide moiety to their rings, they could add virtually any other molecular entity to the system via click chemistry.
Adding molecular pumps to surfaces is an idea whose time has come, says David A. Leigh, an expert in molecular machines at the University of Manchester. Chemists in Leigh’s lab created a similar system, in which they attach molecular pumps to polystyrene beads—work that has not yet been peer-reviewed (ChemRxiv 2021, DOI: 10.33774/chemrxiv-2021-fl7tv). Until now, Leigh says, artificial molecular machines have performed only tasks that could also be done using simpler systems.