Addressable molecular machines arranged in a porous crystal

Schematic of the approach to organizing rotaxanes inside the channels of a metal–organic framework. Credit: (c) 2015 PNAS doi: 10.1073/pnas.1514485112

The artificial molecular machines overview we pointed to last week included several significant recent developments that we had missed, including this about molecular machines organized within a metal-organic framework, from the research group of 2007 Foresight Feynman Prize winner for the Experimental category J. Fraser Stoddart and his collaborators. Two months ago we described here a similar advance by a Canadian group immobilizing a molecular shuttle in a metal organic framework. The difference in the approach taken by the Loeb research group at the University of Windsor and the Stoddart research group at Northwestern University and their collaborators is very nicely explained in an article by Heather Zeiger at “Solid-state molecular switches using redox active molecules in a porous crystal“:

A group of researchers have provided a proof-of-concept procedure for making a solid-state molecular-sized switch. They combined a mechanically interlocked molecule with a pre-synthetized metal-organic framework (MOF).

Mechanically interlocked molecules have several features that make them ideal candidates for molecular switches. These interlocked molecules, known as rotaxanes or catenanes, typically involve two molecular components that have distinct orientations based on interactions between them. Scientists can control which orientation the interlocked molecules take using stimuli, such as electrochemical potential or light. Redox active interlocked molecules are compelling candidates for a molecular switch. However, in solution, these molecules are unpredictable, and when it comes to designing circuitry, controlling the molecular switch is vital.

A more precise description than “unpredictable” would be to point out that molecular switches in solution are randomly oriented so any net switching movement would average to zero. Returning to Zeiger at Physorg:

This is where MOFs can be helpful. MOFs are highly porous materials. Other molecules can be trapped within the pores, and these pores can be chemically tailored to select for certain molecules. A group of researchers from Northwestern University in collaboration with Intel Labs and King Abdulaziz University have successfully captured a redox-active rotaxane within the pores of a premade MOF via post-synthetic building block replacement. Their paper is published in the Proceedings of the National Academy of Sciences.

“As chemists, we have become proficient at manipulating molecular switches and rudimentary molecular machines in solution during the past quarter of a century,” said co-author Dr. Fraser Stoddart, “What we would really like to be able to do now is mount these switches inside porous solids in a rapid and changeable manner. This task has turned out to be easier said than done!” …

Zeiger continues to explain that previous efforts had involved building the metal organic frameworks around the molecular machines, which is technically demanding, while this most recent work designed a molecular machine to fit within the pores of a pre-made metal organic framework.

The research paper [“Electrochemically addressable trisradical rotaxanes
organized within a metal–organic framework
“] begins with a summary of 25 years of research on bistable mechanically interlocked molecules (MIMs) as molecular switches, starting with early work creating two dimensional arrays in a crossbar architecture using the Langmuir–Blodgett technique. Limitations of this approach include lack of scalability of the Langmuir–Blodgett technique, and lack of stability of the molecular switches after no more than a few hundred cycles.

The paper continues by describing how, about five years ago, attention turned to incorporating bistable mechanically interlocked molecules into the organic struts, or attaching them to the metal oxide joints, of MOFs. An important advance was incorporating a donor-acceptor molecular recognition pair in a MOF. The rest of the paper describes their initial difficulties with extending this concept of molecular switch arrays inside a MOF, the molecular shuttle inside a MOF results from Loeb’s group cited above, and their current strategy “for the organization of MIMs within the channels of a premade MOF through postsynthetic building block replacement.” Their conclusion:

… The results establish proof-of-concept for the application of postsynthetic transformations of porous crystalline frameworks in the creation of solid-state molecular switches … and molecular machines. …

Like several other technologies that Foresight has been following and advocating for nearly thirty years, progress with integrating and controlling artificial molecular machine systems seems to be accelerating. What will be the next major steps with metal organic frameworks and molecular machines, and how will they fit into the larger picture of atomically precise nanosystems integrated to achieve high throughput atomically precise manufacturing?
—James Lewis, PhD

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