Mechanically interlocked molecules
have been created, such as rotaxanes with alternating alkyne groups (which are linear, and therefore rigid) where the macrocycle acts as an insulator, or polyrotaxanes where the rings are designed to interact with each other intermolecularly, to ensure tha t the ‘wire’ remains firm. Outside of molecular electronics, rotaxane-based machines have seen promise as molecular muscles. This is done by creating
‘daisy chain’ rotaxanes (as seen in the image to the right), which are constructed by interweaving two rotaxanes and covalently bonding each macrocycle to the end of the other rotaxane’s axle. They can then be contracted/extended like a muscle through similar methods to a molecular switch (i.e. through a chemical input that attracts a macrocycle to ‘the other ’ docking site). These daisy chains can then be polymerized, usually through metal ions which bond to the stoppers and attach two of them together, allowing for quite large contractions, as demonstrated by a French team of chemists, who formed a chain of 3000 rotaxanes, which
contracted from 15.8 to 9.4 micrometres. If a method is found to bunch these together, they could see usage in an engineering or pseudo-biological context. 8 Another bioinspired implementation was developed by Tian and colleagues, who created rotaxanes which ‘mimicked the function of channel proteins between lipid bilayers’, funnelling ions across the impermeable membranes which act as regulators for salt and pH levels. Furthermore, rotaxanes have potential as automatic synthesis machines, as shown by Leigh and colleagues, who created a ‘rotaxane based tripeptide synthesizer’, which worked by having three amino acids already attached, before allowing the macrocycle to shuttle along down the axle, picking up each one in in turn, before falling off an unblocked end with the tripeptide attached, allowing for it to be easily cleaved off. While this does hold promise, in that you could form a molecular factory of sorts, it is still far off from the efficiency and speed of traditional synthesis. 9 Catenanes and molecular knots have seen much less research into potential applications, but they still hold a lot of value. Of course, catenanes can be used as molecular switches if synthesized with one ring much larger than the other, so the smaller macrocycle can shuttle around the big ring to different docking sites (this is of course equally as exciting as in rotaxanes, but I have already written in length about it). In addition, they have usage as switchable dyes, as depending on which docking site the catenanes are attached to, the solution they are in can change colour. Finally, both catenanes and molecular knots have seen use as catalysts, with the molecular knot’s distinctive shape helping in regulating catalysed reactions, and the catenane’s mecha nical bonds proving more useful in asymmetric organocatalysis (reactions where you want to form a specific enantiomer) in place of their noninterlocked counterparts. 10
8 Patrick 2017: 152-156. 9 Sluysmans and Stoddart 2019: 8-11. 10 Ibid.: 8-9.
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