How Aromatic Are Molecular Nanorings? The Case of a Six‐Porphyrin Nanoring**

Abstract

Large conjugated rings with persistent currents are novel promising structures in molecular‐scale electronics. A six‐porphyrin nanoring structure that allegedly sustained an aromatic ring current involving 78π electrons was recently synthesized. We provide here compelling evidence that this molecule is not aromatic, contrary to what was inferred from the analysis of ¹H‐NMR data and computational calculations that suffer from large delocalization errors. The main reason behind the absence of an aromatic ring current in these nanorings is the low delocalization in the transition from the porphyrins to the bridging butadiyne linkers, which disrupts the overall conjugated circuit. These results highlight the importance of choosing a suitable computational method to study large conjugated molecules and the appropriate aromaticity descriptors to identify the part of the molecule responsible for the loss of aromaticity.

Abstract

The correct assessment of the aromaticity of a 78π‐electron six‐porphyrin nanoring hinges upon the choice of the computational method. Density functional approximations with a low percentage of long‐range Hartree–Fock exchange commit large delocalization errors and suggest the presence of an aromatic ring current. The transition from the porphyrins to the bridging butadiyne linkers is responsible for the absence of an aromatic ring current.