Stacked antiaromaticity in the π-congested space between the aromatic rings in the anthracene dime

Substances containing two pi-congested aromatic systems are attractive targets in synthetic studies as well as efforts designed to explore the unique properties that originate from pi-congestion. Since the time of the computational studies by Schleyer and Warner, the concept of stacked aromaticity created by the encounter of two antiaromatic rings has received much attention. A structure containing pi-congestion between two antiaromatic norcorrole rings was prepared by Shinokubo et al. In contrast, questions about what happens when two aromatic rings are closely stacked have remained unanswered. Specifically, the electronic consequences of stacking of two aromatic pi-conjugated systems have not been fully evaluated. To the best of our knowledge, only one computational study has been performed by Herges, the results of which suggest that [2.2]paracyclophane, possessing short distance (<3.00 angstrom) between two aromatic ring planes, has a paratropic ring current between two the benzene rings. This observation is consistent with the conclusion that stacked antiaromaticity exists within the pi-congested space between aromatic rings. Herein, we investigated the stacked antiaromaticity of highly pi-congested anthracene dimers using anthracenophane, which possesses a short interplane carbon-carbon distance of ca. 2.80 angstrom. The absorption spectrum of this substance contains a weak broad band from 450 to 550 nm that is attributed to the HOMO-LUMO transition. These properties exist in planar cyclooctatetraene derivatives that have 8 pi-electron antiaromaticity. The results of nucleus-independent chemical shift and anisotropy of the induced current density calculations indicate that a relatively weak antiaromatic character exists between the central six-membered rings of the two anthracene moieties in anthracenophane. In addition, an attempt to enhance the stacked antiaromaticity of anthracenophane using pressure to enhance pi-congestion was unsuccessful.