Hückel Molecular Orbital Analysis for Stability and Instability of Stacked Aromatic and Stacked Antiaromatic Systems

Face-to-face stacking of aromatic compounds leads to stacked antiaromaticity, while that of antiaromatic compounds leads to stacked aromaticity. This is a prediction with a long history; in the late 2000s, the prediction was confirmed by high-precision quantum chemical calculations, and finally, in 2016, a π-conjugated system with stacked aromaticity was synthesized. Several variations have since been reported, but essentially, they are all the same molecule. To realize stacked aromaticity in a completely new and different molecular system and to trigger an extension of the concept of stacked aromaticity, it is important to understand the origin of stacked aromaticity. The Hückel method, which has been successful in giving qualitatively correct results for π-conjugated systems despite its bold assumptions, is well suited for the analysis of stacked aromaticity. We use this method to model the face-to-face stacking systems of benzene and cyclobutadiene molecules and discuss their stacked antiaromaticity and stacked aromaticity on the basis of their π-electron energies. By further developing the discussion, we search for clues to realize stacked aromaticity in synthesizable molecular systems.