The structure, synthesis, and potential applications of mechanically interlocked molecules
George Croally
Mechanically Interlocked Molecules (MIMs) are two or more distinct structures that are not bonded chemically, and are instead connected by mechanical bonds, which are defined as an entanglement in space between two or more molecular entities (component parts) such that they cannot be separated without breaking or distorting chemical bonds between atoms. 1 Comprised of rotaxanes, catenanes, and molecular knots, MIMs were first discovered in the 1960s, when Edel Wasserman synthesized the first catenane, yet they were only considered a ‘laboratory curiosity’, as obtaining them was wildly expensive and inefficient. Their potential became realized only after J. F. Stoddart et al. demonstrated the first ‘template directed’ synthesis of a rotaxane in the 1990s, which allowed for far greater yields, and paved the way for a wave of research into the field. Nowadays, the possibilities of MIMs seem almost infinite, and they have prospective applications in a broad range of fields, including catalysis, medicine, ultra-stable dyes, and molecular computing. This essay aims to both introduce the concept of MIMs, as well as gauge whether they will have any truly realistic and useful applications.
Structure
Simplified diagram of a rotaxane
The most well-known type of MIM is the rotaxane, which is (at its most basic) comprised of an axle-like molecule, with a stopper at each end, interconnected with a large cyclic molecule (macrocycle). The axle allows the macrocycle to rotate around it and ‘shuttle’ along its length ; yet the stoppers (usually a bulky silicon group/a group of aromatic rings) prevent it from ‘slipping off’, as they are larger than the diameter of the ring. Many rotaxanes also contain a number of ‘docking sites’, which interact
with the macrocycle through some intermolecular force (usually π - π inte ractions, which occur between aromatic rings facing each other). These forces are fairly weak, and so while they keep the macrocycle in place for some time, the rotaxane can still shuttle to other available docking sites. Some rotaxanes have been synthesized which can act as molecular switches, as the macrocycle will bond to one docking site under basic conditions, and the other under acidic conditions. Of course, it is possible to synthesize rotaxanes with more than one ring, and they are known as polyrotaxanes (or [n]rotaxane, where n is the total number of separate structures). 2
1 Sluysmans and Stoddart 2019: 2. 2 Patrick 2017: 150-151.
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