Axle Molecules Threaded Through Macrocycles
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Interlocked structures exploit a mode of interaction that is old beyond history with everyday objects, but is relatively new in the molecular sciences. Just as interlocking iron rings make chains for pulling or suspending heavy loads and as two-dimensional arrays of interlocked iron rings were used to make chain mail for the armor of knights of yore, one-dimensional and two-dimensional interlocking molecular rings may provide exciting, new materials for the future. If chains can be made, then why not molecular braiding, knitting, or weaving? A molecular cloth composed of linear polymer molecules, woven at the molecular level, would certainly have unanticipated properties. For example, the behavior might appear at first to resemble a live object. At room temperature, each molecular strand in the cloth would likely have some of its vibrational modes activated, so that the cloth would constantly be subject to motions that would tend to unravel it. This unraveling could be prevented, probably by having large groups on the ends of each strand, so that they could not pass through the openings in the weave, a subject of this article.
One reason interlocked molecular structures are not yet available in multitudes of shapes and sizes, including the familiar forms just described, is that they are difficult to make. Unlike ordinary thread, string, or rope, it is difficult to get a hold on a molecular strand, so it is more difficult to put them through the motions that would create a needle work product—and that is precisely what this brief missive is about. It is about one of the simplest of tasks that has to be overcome if one wants to make chains, or knit, or weave on the molecular level. The subject is the threading of a, more or less, linear molecule through a molecular ring. One of the two simplest interlocked molecules is the rotaxane and it is composed of an axle molecule, inserted through a ring molecule and held there by its own end groups that are simply too big to permit the axle to be propelled easily out of the ring.