Taking aryls for a walk

Taking aryls for a walk
15 October 2020 - Making it move XIX

He Swager aryls on graphene 2020.PNGThe last episode of Making it Move on this blog was in 2016? That has been a while and about time for a new episode. Maggie He and Timothy Swager are reporting mobility of 4-methoxyphenyl groups on a graphene surface (DOI).

With isotope labeling it can be demonstrated that if you add an acid to a biphenyl, the phenyl group will dance around the arenium ion, a phenomenon Swager had already noticed back in 1997 in work on certain polycyclic aromatic compounds (DOI). Can this work for graphene as well? It is clear from the paper that just sticking aryl groups on graphene and watch them move will not work, this is also not an AFM exercise. Instead, the observation of migrating aryls in this study relies on Raman spectroscopy which is able to differentiate between surface areas containing sp3 carbon atoms (aryl position anchors) from pristine graphite surfaces.

The team applied stencil lithography to a graphene source. By evaporating copper through a copper grid with 6 micron holes, a surface was created dotted with the metal as circles. Then the graphene was reduced in a solution of sodium naphthalenide and then bis(4-methoxyphenyl)iodonium tetrafluoroborate was added. Finally the copper was etched away by ammonium persulfate and yes, in subsequent Raman analysis the previously pristine surfaces (no longer covered in copper) and the aryl covered surrounding surface were indistinguishable as if a black Friday crowd had been set loose in a shopping mall.

The authors calculate it takes 26 thousand steps to traverse 3 microns to the center of a dot which translates to a speed of around 10 mm per minute. Control experiments: no sp3 bonds form without reducing the graphene first. Oxidation alone does not degrade the graphene. By replacing the methoxy group by a trifluoromethyl group no migration takes place at all, consistent with better ability of the methoxy group to accommodate a positive change during a hop.
Am I completely convinced? I would expect a time dependence in the aryl migration, by the time you get a sample from the oxidation bath to the Raman machine, all the aryls are done hopping. If you reduce the temperature enough and increase the size of the dots you should be able to watch a migration front?