Synthesis, Structures, and Properties of Dehydrobiphenyleno[12]Annulenes: Carbon-Rich Compounds Comprising Two Different Antiaromatic Units.

In this work we report the synthesis, structure, and electronic properties of carbon-rich compounds dehydrobiphenyleno[12]annulenes (DBP[12]As) comprising antiaromatic four-membered rings (4MR) and 12-membered ring (12MR). Ultraviolet-visible absorption spectra and electrochemical behaviors of DBP[12]As confirmed their relatively narrow highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap values and high HOMO energy levels, which were supported by density functional theory simulations. Parent DBP[12]A adopts a slipped herringbone structure in a crystalline state, with the molecules forming 1D stacks via π-π interactions. The experimentally derived bond lengths, bonding analyses using the Wiberg bond indices, and localized orbital locator calculation support a stronger double bond character for the 12MR bonds than the 4MR bonds in the inner six-membered ring. The chemical shifts of hydrogens in ¹H NMR spectra, as well as magnetically induced ring current analyses using quantum chemical calculations, indicate that the 4MRs have stronger antiaromatic character than the 12MR. The present information is useful for a fundamental understanding of carbon-rich compounds with different antiaromatic units as well as designing novel molecules with unique electronic properties.