Aromaticity and Magnetic Properties in Molecular Systems

Aromaticity describes a class of cyclic, conjugated molecules that exhibit enhanced thermodynamic stability and a characteristic response to magnetic fields. In systems following Hückel’s rule, a delocalised π electron network circulates under an applied magnetic field, generating a diatropic ring current that leads to shielding effects observable by nuclear magnetic resonance. Conversely, anti-aromatic compounds contain 4n π electrons, which induce paratropic currents and destabilise the ring. Beyond planar rings, three-dimensional frameworks and large macrocycles challenge traditional criteria by sustaining complex current patterns and novel electronic states. Advances in spectroscopy, crystallography and first-principles calculations have revealed that by tuning ring size, oxidation state or substituents, it is possible to switch between aromatic and anti-aromatic behaviour, to access multiple aromatic circuits in a single molecule, and to exploit these properties in molecular electronics, magnetic materials and catalysis.

Research from Nature Portfolio

Recent studies have re-examined the archetype [18]annulene system by isolating its dianion and further transforming it into a tetraanion. High-resolution NMR and X-ray crystallography reveal that while the dianion is formally anti-aromatic, the tetraanion re-establishes a 4n+2 π-electron circuit, displaying clear diatropic ring currents and forming metallocene-like sandwich structures upon complexation.

Investigations into furan-acetylene macrocycles of varying ring size demonstrate that odd-membered assemblies exhibit global aromatic ring currents, whereas even-membered analogues show anti-aromatic behaviour. Spectroelectrochemical and DFT studies correlate these alternations with changes in oxidation potentials, emission spectra and chemical shifts, defining the limits of Hückel’s rule in neutral, extended π systems.

Foundational work on double aromaticity in a hexakis(phenylselenyl)benzene dication has shown that σ- and π-electron circuits can independently satisfy aromatic criteria within the same molecule. Structural and magnetic analyses confirm two concentric aromatic rings, each adhering to the 4n+2 rule, opening pathways for design of multi-circuit aromatic materials.

Aromaticity and Magnetic Properties in Molecular Systems publication trend

The graph below shows the total number of articles in aromaticity and magnetic properties in molecular systems across all publications each year (not limited to Nature Index journals).

Technical terms

Aromaticity: Enhanced stability and characteristic magnetic response of cyclic conjugated molecules with (4n+2) π electrons.

Anti-aromaticity: Destabilisation and paratropic magnetic behaviour of cyclic conjugated systems containing 4n π electrons.

Ring current: Electron circulation induced in a conjugated ring by an external magnetic field, observed as shielding or deshielding effects.

Hückel’s rule: A criterion predicting aromatic stability in planar rings possessing (4n+2) π electrons.

3D-aromaticity: Delocalised electron networks in three-dimensional molecular frameworks that generate aromatic currents across polyhedral or spherical circuits.

Magnetizability: A measure of a molecule’s tendency to become magnetised in an applied magnetic field, reflecting its global magnetic susceptibility.

References

1. The anti-aromatic dianion and aromatic tetraanion of [18]annulene. Nature Chemistry (2024).
2. SingleNot Double3D-Aromaticity in an Oxidized Closo Icosahedral Dodecaiodo-Dodecaborate Cluster. Journal of the American Chemical Society (2023).
3. Global Aromatic Ring Currents in Neutral Porphyrin Nanobelts. ACS Nano (2024).
4. Alternating behavior in furan-acetylene macrocycles reveals the size-dependency of Hückel’s rule in neutral molecules. Communications Chemistry (2023).
5. Current density and molecular magnetic properties. Chemical Communications (2021).
6. Double aromaticity arising from σ- and π-rings. Communications Chemistry (2018).