Acene

In organic chemistry, the acenes or polyacenes are a class of organic compounds and polycyclic aromatic hydrocarbons made up of benzene (C₆H₆) rings which have been linearly fused.^[1][2] They follow the general molecular formula C_4n+2H_2n+4.

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 5 unsubstituted members are listed in the following table:

Name	Number of rings	Molecular formula	Structural formula
Anthracene	3	C14H10
Tetracene	4	C18H12
Pentacene	5	C22H14
Hexacene	6	C26H16
Heptacene	7	C30H18

Hexacene is not stable in air, and dimerises upon isolation. Heptacene (and larger acenes) is very reactive and has only been isolated in a matrix. However, bis(trialkylsilylethynylated) versions of heptacene have been isolated as crystalline solids.^[3]

Larger acenes

Due to their increased conjugation length the larger acenes are also studied.^[4] Theoretically, a number of reports are available on longer chains using density functional methods.^[5][6] They are also building blocks for nanotubes and graphene. Unsubstituted octacene (n=8) and nonacene (n=9)^[7] 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,^[8] an observation in violation of Kasha's rule. Subsequent work by others on different derivatives included crystal structures, with no such violations.^[9] The on-surface synthesis and characterization of unsubstituted, parent nonacene (n=9)^[10] and decacene (n=10)^[11] have been reported. In 2020, scientists reported about the creation of dodecacene (n=12)^[12] for the first time. Four years later, in the beginning of 2024, Ruan et al. succeeded in synthesizing unsubstitued tridecacene (n=13) on a (111)-gold surface. The acene was characterized by STM- and STS-measurements. ^[13]

Related compounds

The acene series have the consecutive rings linked in a linear chain, but other chain linkages are possible. The phenacenes have a zig-zag structure and the helicenes have a helical structure.

• Macromolecular forms consisting of seven fused benzene rings
• 
  Heptacene
• 
  [7]Phenacene
• 
  M-heptahelicene

Benz[a]anthracene, an isomer of tetracene, has three rings connected in a line and one ring connected at an angle.

References