Threading rings on strings
Bethany Halford · C&EN Global Enterprise · 2020
Making a macroscale version of a polyrotaxane—a string threaded through multiple rings—is a fairly simple task. Young children commonly craft them, for example, by threading yarn through tube-shaped pasta. But making such structures on a molecular scale is a different matter. It’s tough to craft a molecule composed of a precise number of rings on a string when the only tools for assembly are fleeting chemical interactions. But chemists in Fraser Stoddart’s lab at Northwestern University have done just that. They report a molecular machine that can precisely build polyrotaxanes by using redox chemistry to thread rings onto a polymer chain two at time. The system could be the foundation for information storage systems, with the sequence of rings encoding data, Stoddart says. Two molecular pumps help synthesize the polyrotaxane. Each pump is made of a bipyridinium unit sandwiched between a 2,6-dimethylpyridinium end group and an isopropylphenylene group that’s attached