Porphyrin ring pushes the size limit for fully aromatic molecules

A nanoring with a diameter of eight nanometres is the largest molecule yet to show global aromaticity. While aromaticity is usually seen in smaller rings, the new structure pushes the size limit for the phenomenon.

Five years ago, a molecular wheel made from 12 porphyrins linked together by butadiyne units became the world’s largest aromatic ring. That structure was produced by a team led by the University of Oxford’s Harry Anderson. Now, Anderson’s lab has gone bigger, constructing an larger ring that the researchers say ‘defines the upper size limit to global aromaticity’ in this kind of structure.

This new nanoring consists of 18 zinc porphyrins arranged cyclically and connected by butadiyne linkages. The team designed radial 18-legged templates that act like spokes to hold the ring in shape.

The nanoring’s circular structure was confirmed by scanning tunnelling microscopy, while¹⁹F NMR spectroscopy and computational modelling revealed measurable ring currents that relate to electron delocalisation at certain oxidation states. When the molecule is in the 10+ state, 242 π electrons participate in the aromatic system.

However, the ring currents are at least two times weaker than those seen in the 12-porphyrin ring, with only a fraction of molecules adopting the right conformation to sustain delocalisation around the entire circumference.

According to Anderson’s team, the weakness of the ring current suggests that the nanorings have reached the upper size limit for global aromaticity. However, they note that more rigid macrocycles like fused porphyrin nanobelts may enable ring currents to persist in even larger structures.