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Chemistry In Pictures

Chemistry in Pictures: Kinks in molecular chains

by Manny I. Fox Morone
June 14, 2021

Two atomic force microscopy images showing hexagonal molecules in a line; the molecules in the top image look lined up side-to-side; the molecules in the bottom row are also lined up side-to-side but look tilted slightly clockwise.
Credit: Aleš Cahlík/John Hellerstedt

If you look closely at these micrographs of molecular chains, you can see that the top set of molecules is lined up straight, while the bottom set is just a little slanted to the right. Yet these chains are made with the same “links,” molecules called quinonediimines. After detecting two electronic states of the chains with scanning-tunneling microscopy, researchers in the labs of Pavel Jelinek of the Institute of Physics of the Czech Academy of Sciences and Olivier Siri of Aix-Marseille University decided to get a better look at the individual molecules using atomic force microscopy (shown).

In one state, this slight tilt in the chain links has to do with the molecules squirming around in an effort to better align the hydrogen bonds that connect them: generally, the better the hydrogen bond donor can line up with the acceptor, the stronger the hydrogen bond will be. But in the chains with straight links, the researchers noticed proton tunneling, by which a hydrogen atom that bridges two chain links can move between the chain links easily and be shared equally. Because the links are all equivalent in when tunneling happens, each molecule acts as both an acceptor and a donor to the left and the right, so they don’t look like they’re leaning farther to one side or the other. They just stand straight up.

A structure of a molecular chain of benzoquinonediimines lined up symmetrically side-by-side labeled "Molecular chains with proton tunneling" and a structure of structure of a molecular chain of benzoquinonediimines that are each slightly tilted clockwise labeled "Molecular chains without proton tunneling".

Credit: Aleš Cahlík/John Hellerstedt. Read the paper in ACS Nano (2021, DOI: 10.1021/acsnano.1c02572)

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