Azulene-Fused Acenes

Non-alternant non-benzenoid π-conjugated polycyclic hydrocarbons (PHs) are expected to exhibit very different electronic properties from the all-benzenoid PHs. Herein, we report the synthesis and physical properties of three azulene-fused acene molecules (1, 2 and 3), which are isoelectronic to the pentacene, hexacene and heptacene, respectively. X-ray crystallographic analysis, NMR spectra, and theoretical calculations reveal a localised aromatic backbone comprising all the six- and five-membered rings while the seven-membered ring remains non-aromatic. They display properties of both azulene and acenes and are much more stable than the respective acenes. The dications of 1, 2 and 3 were formed by chemical oxidation. Notably, 3²⁺ exhibited an open-shell diradical character (y₀ =30.2 %) as confirmed by variable-temperature NMR and ESR measurements, which can be explained by recovery of aromaticity of an 2,6-anthraquinodimethane unit annulated with two aromatic tropylium rings.