Hexacene


Hexacene is an aromatic molecule consisting of six linearly-fused benzene rings. It is one of a series of linear polycyclic molecules created by such aromatic ring fusions, a series termed acenes—the previous in the series being pentacene and the next being heptacene. The CAS registry number for hexacene is, and it has a molecular weight of 328 g/mol. It and other acenes and their derivatives have been investigated in potential applications related to organic semiconductors.

Reactivity

Hexacene is very unstable, being highly reactive in the 6 and 15 positions, due to pi electron localization. Its spectra must be collected on freshly purified samples handled in an inert atmosphere such as nitrogen or argon. Only with very large substituents it is possible to isolate an hexacene for instance the compound 6,15-bishexacene, which melts with decomposition at 96 °C.

Syntheses

The first reported synthesis of hexacene dates to 1939. In 1955, the compound was reportedly synthesized by dehydrogenation of hexacosahydrohexacene by palladium on carbon. In 1982, hexacene was reported to be a bluish green compound that decomposed at 380 °C. In 2007, a research group claimed the first reproducible synthesis of unsubstituted hexacene—purportedly invalidating previous claims—based on photochemical decarbonylation of a diketone precursor:
The compound synthesized could not be isolated: it dimerized at concentrations as low as 10−4 M, and reacted with oxygen in solution to form an organic peroxide. In a poly polymer matrix such side-reactions were limited, and the compound survived up to 12 hours. The next homologue heptacene also studied by this group, and was even more unstable, decomposing within 4 hours.
The synthesis of hexacene has also been reported to yield a crystalline solid from a monoketone precursor, by solid state synthesis at 180 °C. The solid was reported stable in the dark for up to a month, stability attributed to its "herringbone" crystalline packing that prevented dimerization.
In December 2016, the groups of Diego Peña and Francesca Moresco reported synthesis of hexacene in-situ, via deoxygenation of an air-stable precursor, and used scanning tunneling microscopy to produce orbital resonance images of a single hexacene molecule.