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 of 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, 3 2+ exhibited an open-shell diradical character (y 0 = 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.
Bibliographical noteKAUST Repository Item: Exported on 2022-09-14
Acknowledgements: This work was supported by MOE Tier 1 grant (R-143-000-B62-114) and Tier2 grant (MOE-MOET2EP10120-0006). We also thank Ms. Tan Geok Kheng from National university of Singapore for the crystallographic analysis.
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