A significant advancement in molecular chemistry has been realized with the development of a nanoring featuring a diameter of eight nanometres, representing the largest molecule to demonstrate global aromaticity to date. Aromaticity, typically seen in smaller molecular rings, is redefined with this groundbreaking structure.
Five years ago, a record was established with a molecular wheel made up of 12 porphyrins linked by butadiyne units as the largest aromatic ring, designed by a team under the leadership of Harry Anderson from the University of Oxford. Anderson’s lab has now surpassed this achievement by creating an even larger ring, believed to establish the upper boundary for global aromaticity in these systems.
The newly designed nanoring consists of 18 zinc porphyrins, linked in a cyclic manner through butadiyne connections. To ensure its structural stability, radial templates with 18 legs serve similarly to spokes, preserving the ring’s shape.
Confirmation of the nanoring’s circular structure was carried out using scanning tunnelling microscopy. Furthermore, ^19F NMR spectroscopy combined with computational modeling revealed detectable ring currents, suggesting electron delocalisation at specific oxidation states. In the 10+ oxidation state, 242 π electrons participate in the aromatic framework.
Despite this considerable electronic involvement, the ring currents are significantly reduced, at least halved, when compared to those seen in the previously noted 12-porphyrin ring, with only a few molecules reaching the required conformation for complete delocalisation throughout the ring.
The results indicate, as stated by Anderson’s team, that this nanoring represents the maximum size limit for sustaining global aromaticity. Nonetheless, they propose that more rigid macrocycles, perhaps akin to fused porphyrin nanobelts, could sustain ring currents, paving the way for even more extensive aromatic structures in upcoming studies.