Pushing the limits of size constraints in chemistry, an 8-nanometer 18-porphyrin nanoring (c-P18) becomes the largest known cyclic molecule to exhibit detectable global aromaticity. This phenomenon, where π-electrons are delocalized not just over individual aromatic units but around the entire macrocyclic ring, is mostly seen in smaller aromatic molecules but rarely found in macrocyclic entities.
Researchers from the University of Oxford and the University of Nottingham confirm that the c-P18 nanoring carries a circuit of 242 π-electrons, setting the current upper size limit for global aromaticity in butadiyne-linked systems. Using highly sensitive Fluorine-19 NMR spectroscopy, they tracked ring currents while charging the nanoring via oxidation.
The experiments uncovered faint magnetic shoulder signals—the telltale signature of electrons flowing globally between aromatic and antiaromatic states. This pushes beyond the benchmark set by the 12-member porphyrin nanoring, which had previously been the largest in this class, to show clear global aromaticity.