Astumian on research team exploring mechanisorption, a fundamentally new mechanism for active surface adsorption

Researchers have made a significant advance in surface science, introducing a new active mechanism of adsorption based on rotaxane molecules with pumping cassettes attached on a surface, according to a new study in the journal Science.

University of Maine professor R. Dean Astumian, a theorist in the Department of Physics and Astronomy, is an author of the study, along with Sir James Fraser Stoddart, 2016 Nobel Laureate in Chemistry and Board of Trustees Professor of Chemistry at Northwestern University, and Omar Farha, an expert in metal-organic framework chemistry, a professor in the Department of Chemistry at Northwestern University.

The article, “Active Mechanisorption Driven by Pumping Cassettes,” describes the work, led by Northwestern University postdoctoral researchers Liang Feng and Yunyan Qiu that demonstrates how artificial molecular machines (AMMs) — artificially synthesized molecular components that produce machine-like movement — grafted on surfaces can be used to actively recruit molecules to a surface compartment at very high concentrations, thereby storing significant amounts of energy. In the present paper, the molecule brought to the surface was cyclo-bis-paraquat rings, but it is anticipated that the approach can be generalized to many other materials by functionalizing the rings.

The energy necessary for the active adsorption was provided by external modulation of the reduction-oxidation potential of the solution. The authors report increasingly large numbers of rings mechanisorbed at the surface in proportion to the number of cycles of the modulation.

“This research is the first example of utilizing artificial molecular pumps (AMPs) to actively recruit and adsorb molecules onto solid surfaces and opens the door to operating artificial molecular machines (AMMs) on the surfaces of a range of functional materials such as micellar nanoparticles and covalent organic framework,” according to Qiu.

Feng noted that the potential utility of mechanisorption in technology, such as chemical capacitors, will provide a new way to store and manipulate energy, information and matter on surfaces.

“The advent of the mechanical bond is sending major ripples through chemistry and materials science,” Feng said. “Given a little more time, the general area of sorption will witness an equally profound change in direction in a century since physisorption and chemisorption came on the scene.”

The research illustrates the synergy that results from combining theory with experiment. The idea of a pumping cassette arose from Astumian’s consideration of the effects of oscillating electric fields on membrane enzymes and was synthetically implemented in Stoddart’s laboratory using rotaxanes — long dumbbell shaped molecules — terminated on one or both ends with a recognition site surrounded by two groups to provide kinetic barriers between the bulk or the collecting chain and the recognition site.

Importantly, these barrier forming moieties can be designed to respond differently to changes in the environment. These modules can be incorporated on many types of polymer chains giving rise to numerous possible applications.

Mechanisorption has important implications for storage, and controlled release, of many materials. While the present paper focuses on recruitment of ring molecules to the surface, eventually it is anticipated that these rings can be functionalized to bring many different types of molecules at high concentration to surfaces.

The system bears some features in common with spray cans, where different materials are stored at high pressure, and then released at will by pressing a trigger. However, mechanisorbed substance remains in mechanical equilibrium even while being packaged far from thermodynamic equilibrium. The mechanism of triggered release involves only diffusion, a process which, while seemingly slow from a macroscopic perspective, is remarkably fast in these systems.

Astumian pointed out that the research also is important for understanding one of the deepest questions in chemistry: What are the principles by which simple matter becomes complex? A key point is that while thermodynamics determines the most likely structures near equilibrium, kinetics plays the dominant role in selecting structures when far from equilibrium, Astumian said.

“There’s good reason to believe that the concept of mechanisorption will command textbook attention one day,” said Stoddart, who supervises the lab at Northwestern where Feng and Qiu conducted the experiments. “If chemists can work out how mechanisorption can be incorporated in active structures, the storage of gases like hydrogen, carbon dioxide and methane will enter a whole new world and become a different ball game altogether. The challenge is essentially one of how do you create nanoconfinement away-from-equilibrium?”

Contact: Raymond Astumian,