Theoretical study on aromatic characters in π-stacked multimers composed of antiaromatic molecules

Abstract Corminboeuf et al. theoretically predicted that closely π-stacked dimers of antiaromatic molecules tend to exhibit aromatic characters in the sense of magnetic and structural criteria due to the strong intermolecular orbital interactions. This phenomenon is usually called stacked-ring aromaticity. Several experimental and theoretical studies have been conducted to elucidate the aromaticity switching mechanism in the π-stacked dimers and proposed molecular systems realizing this type of phenomenon. So far, most of these studies focused on the aromatic characters of dimers. This paper aims to examine aromatic characters of π-stacked multimers of antiaromatic molecules where the number of monomers, N, is more than 2. We constructed theoretical and calculation models of π-stacked N-mers consisting of cyclobutadienes with different stacking distances d. Applying the theoretical analyses based on the Hückel theory, we clarified the mechanisms of N-, d-, and monomer position dependences of aromatic characters that are evaluated from the density functional theory calculations of these models. It is found that the terminal monomers in the π-stacked N-mer (N > 2) exhibit the most significant antiaromatic character, whereas the second terminal monomers show the most reduced antiaromatic characters at small d. These results demonstrate that, in addition to the stacking distance and configuration, the number and the positions of monomers can be useful factors in controlling the aromaticity switching in π-stacked assemblies of antiaromatic molecules.