Invited talk

Aromaticity reversals in molecules and materials
Igor Rončević^1,2

1 Department of Chemistry, Oxford University, Oxford, United Kingdom
2 Institute of Organic Chemistry and Biochemistry of the CAS, Prague, Czechia

Aromaticity, most commonly defined as the ability of a molecule to sustain a diatropic ring current, is often correlated with electron delocalisation, stability, energy gaps, and other properties. A sign change in the ring current, corresponding to an aromaticity reversal, usually requires a significant perturbation in the electronic structure (e.g. electronic excitation or the removal of two electrons) and results in an equally dramatic change in properties. Here, we will discuss aromaticity reversals in cyclocarbons and metalloporphenes. Cyclocarbons are small all-carbon rings with two orthogonal π systems [1]. When these two π systems sustain opposing currents (Fig. 1 left), small changes in the molecular geometry can drastically change the overall current, resulting in an aromaticity reversal. Using a combination of multireference methods and density functional theory, we investigate such aromaticity reversals in low-lying excited states of the recently synthesised C₁₆ [3]. These results are rationalised using a minimal qualitative model based on orbital contributions. Zinc porphene, a two-dimensional material made of fully fused zinc porphyrins, has recently been synthesized on a water surface [3]. Zinc porphene is a member of a larger family of metalloporphenes, fully conjugated materials which can be tuned by changing the metal inside the porphyrin macrocycles. We predict the properties of metalloporphenes carrying first transition row metals, finding that different metals affect the number of electrons in the π system, thus changing the metalloporphene band structure from antiaromatic to aromatic [4].

[1] H. L. Anderson et al., Bull. Chem. Soc. Jpn. 94, 798 (2021)
[2] Y. Gao et al., ChemRxiv (2023); DOI: 10.26434/chemrxiv-2023-w124b-v2
[3] T. Magnera et al. “Porphene and Porphite: Porphyrin Analogs of Graphene and Graphite” Nat. Commun., in press (2023)
[4] I. Pavlak et al. ChemRxiv (2023); DOI: 10.26434/chemrxiv-2023-j0bv8