Acenes
The acenes or polyacenes are a class of organic compounds and polycyclic aromatic hydrocarbons made up of linearly fused benzene rings. The larger representatives have potential interest in optoelectronic applications and are actively researched in chemistry and electrical engineering. Pentacene has been incorporated into organic field-effect transistors, reaching charge carrier mobilities as high as 5 cm2/Vs.
The first 7 unsubstituted members are listed in table 1.
The last members, hexacene and heptacene, are very reactive and have only been isolated in a matrix. However, bis(trialkylsilylethynylated) versions of hexacene and heptacene have been isolated as crystalline solids.
Due to their increased conjugation length the larger acenes are also studied. Theoretically, a number of reports are available on longer chains using density functional methods. They are also building blocks for nanotubes and graphene. Unsubstituted octacene (n=8) and nonacene (n=9) have been detected in matrix isolation. The first reports of stable nonacene derivatives claimed that due to the electronic effects of the thioaryl substituents the compound is not a diradical but a closed-shell compound with the lowest HOMO-LUMO gap reported for any acene, an observation in violation of Kasha's rule. Subsequent work by others on different derivatives included crystal structures, with no such violations. The on-surface synthesis and characterization of unsubstituted, parent nonacene (n=9) and decacene (n=10) have been reported.
A related group of compounds with 1,2-fused rings and with helical not linear structures are the helicenes. Compounds in which the fused rings form a zigzag pattern are phenacenes. Polyquinanes and quinenes are fused cyclopentane rings.
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