18-member nanoring pushes the boundaries of global aromaticity

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.

The findings were published in the Journal of the American Chemical Society.

Aromatic compounds are in high demand across industries, from solvents and plastics to life-saving pharmaceuticals, thanks to their unique blend of stability and reactivity.

These unique properties arise from aromaticity (or its opposite, antiaromaticity) in ring-shaped molecules with delocalized π-electrons, where the electron wave function resembles that of a particle on a ring. This phenomenon can shift thermodynamic stability, making aromatic molecules more stable and antiaromatic molecules less stable.

The researchers set out to determine the size limit of global aromaticity—how large a macrocyclic π-system can be while still sustaining an aromatic ring current around its circumference.

They designed c-P18, composed of 18 zinc porphyrin units linked by butadiyne (conjugated acetylene) linkers to form a large cyclic structure approximately 8 nm in diameter. The porphyrins either carry bulky trihexylsilyl or octyloxy side chains. The nanoring’s large size and flexibility make molecular templates (T18A or T18B) essential for maintaining a circular geometry suitable for NMR studies of global aromaticity.

These templates are 18-legged ligands with pyridyl binding sites that coordinate to zinc centers on the porphyrins, locking the ring in place and preventing rotation—with T18A containing meta-pyridyl substituents and T18B featuring all-para pyridyl substituents.

The experiments showed that c-P18 supports detectable global aromaticity, with aromatic and antiaromatic ring currents appearing at the 10+ and 12+ oxidation states. The ring current in c-P18 was only about half as strong as that in the 12-porphyrin ring and could be observed only at the extremely low temperature of −40 °C.

Moreover, only a subset of the molecules adopted conformations that could support global aromaticity. Despite these limitations, the findings still offer a glimpse of the upper size limit for global aromaticity in this class of molecules.

Defining the upper size limit of global aromaticity is key for molecular design and theory, offering a blueprint for creating systems with tailored electronic properties for applications in molecular electronics, photonics, and nanotechnology.

© 2025 Science X Network