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

Abstract

Large conjugated rings give rise to novel promising structures that can sustain persistent currents at low temperatures even in the presence of strong magnetic fields. One of the most interesting such molecules was recently synthesized [Anderson et al., Nature, 2017, 541, 3512] in the form of a six-porphyrin nanoring structure, which, according to the authors, in its +6-oxidation state (c-P66+) sustained an aromatic ring current involving 78π electrons; one of the largest aromatic rings ever produced. In this paper, we have provided compelling evidence that this molecule is not aromatic, as it was incorrectly inferred from computational calculations that suffer from large delocalization errors. A thorough analysis of four oxidation states of the six-porphyrin nanoring re-veals that the main reason behind the poor aromaticity of 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 an adequate computational method to study large conjugated molecules and the appropriate aromaticity descriptors to identify the part of the molecule that is responsible for the loss of aromaticity. We believe the strategy here employed will be helpful in designing new large aromatic molecular nanorings. File list (2)download file view on ChemRxiv Nanoring_manuscript.pdf (3.59 MiB) download file view on ChemRxiv SI_nanoring.pdf (7.39 MiB)

Aromaticity rules

“…The (anti)aromatic character of annulenes has also been shown to fade away quite rapidly as their ring size increases 50 . Annulenes with large rings have negligible aromatic stabilization energies irrespective if they follow the Hückel rule or not 51,52,53 .…”