Key to the formation of the grossly warped nanographene was the addition of five seven-membered rings into the hexagonal lattice by way of a previous method of building larger graphene subunits from the ground up. Thanks to earlier success with polycyclic aromatic hydrocarbons (PAHs) the team decided to likewise arylate the bowl-shaped PAH corannulene by C–H activation. Instead of stopping at the expected five-fold annulated product they found that the reaction continued all the way to the grossly warped nanographene comprising 80 carbon atoms joined together in a network of 26 rings. ‘This is purely a discovery,’ explains Japanese team leader Kenichiro Itami. ‘Initially we were not expecting that the seven-membered rings could be nicely formed under mild conditions.’
While the positive curvature of five-membered rings is well known in the likes of fullerenes, the seven-membered rings introduce a negative curvature that not only causes the graphene sheets to warp but also alters their properties from their planar counterparts.
Jay Siegel at the Univeristy of Zurich, Switzerland, agrees. ‘Bringing this to a real chemical example is remarkable given that such surfaces can be prepared from corannulene, which itself is available now in kilogram quantities,’ he says. ‘One can be confident that new electrochemical and electro-optic properties of these will become engineering materials to be exploited in OLED, solar cell or photovoltaic applications.’
Lawrence Scott from the US half of the team says that there is more to come now that they have the first warped nanographene. ‘As we learn nature's rules about how to build seven-membered rings into these carbon lattices, we hope to design and synthesise more members of this fascinating new family of nanocarbons.’