The scientists examined how much twisting the chemical bonding in an aromatic ring could endure before it broke. They accomplished this by creating overcrowded aromatic rings, utilizing tropylium instead of benzene, which shares electrons around a ring of seven carbon atoms.
Each of these carbon atoms can be functionalized and having seven attachment points in the ring, rather than the six carbon atoms of benzene, allowed the researchers to cram more groups around the edge of the aromatic ring, causing more strain. The researchers found that low levels of overcrowding made the ring twist, but without breaking its aromatic bonding.
By adding progressively larger groups around the edge of the ring, the team twisted the ring further, eventually causing the aromatic bonding to break.
The electrons no longer circle the seven carbon atoms and instead, the ring pinches across its middle to form two smaller flat rings. Surprisingly, the researchers found there is a balance point, where the ring jumps back and forth between the aromatic structure and the two smaller rings. One molecule made in this study spends 90% of its time as the pinched structure and 10% of its time as a larger aromatic ring.
Full study results have been published in the journal Nature Chemistry.
Reflecting on the study results, Dr. Paul McGonigal of the University of York, said: “In these overcrowded molecules, strain and aromatic bonding are delicately balanced. The structure, properties, and potential applications of a material are ultimately determined by this balance. The precise control over the twisting of our molecules is unprecedented. We were not only able to twist an aromatic molecule up to the maximum amount of strain it can tolerate but also to discover what happens when we push beyond that limit. We hope this investigation is a step towards us being able to more routinely turn aromatic bonding ‘off’ and ‘on’ in a controlled manner.”
Project lead investigator, Promeet Saha of Durham University, said: “The reversible pinching and reopening of an aromatic ring are truly remarkable. Aromatic bonding is such a powerful stabilizing force that we usually think of it being a constant presence. However, our findings demonstrate that it can be surprisingly dynamic.”
Chemical bonding in aromatic molecules is key to the structure, stability, and function of chemicals such as drugs and plastics.
Reference: “Rupturing aromaticity by periphery overcrowding” by Promeet K. Saha, Abhijit Mallick, Andrew T. Turley, Aisha N. Bismillah, Andrew Danos, Andrew P. Monkman, Alyssa-Jennifer Avestro, Dmitry S. Yufit, and Paul R. McGonigal, 6 March 2023, Nature Chemistry.
The study was funded by the Engineering and Physical Sciences Research Council (EPSRC).