Stacked Antiaromaticity in the π-Congested Space Between the Aromatic Rings in the Anthracene Dimer

Substance containing two π-congested aromatic systems are attractive targets in synthetic studies as well as efforts designed to explore the unique properties that originate from π-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. In contrast, questions about what happens when two aromatic rings are closely stacked have remained unanswered. 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 Å) 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 π-congested space between aromatic rings. Herein, we investigated the antiaromaticity of highly π-congested anthracene dimers using anthracenophane, which possesses a short inter-plane carbon-carbon distance of ca. 2.80 Å. The absorption spectrum of this substance contains a weak broad band from 450 to 550 nm that is attributed from HOMO-LUMO transition. These properties exist in planar cyclooctatetraene derivatives that having 8π-electron antiaromaticity. The results of nucleus-independent chemical shift and anisotropy of the induced current density calculations indicate that 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 π-congestion using was unsuccessful.