Chemistry Reference
In-Depth Information
Fig. 1 The smaller acene
molecules 1-5
1
2
3
4
5
1
Introduction
Acenes are polycyclic aromatic hydrocarbons that consist of linearly fused benzene
rings. The smaller members of the acene series are well-known organic compounds.
For example, benzene 1 was discovered by Michael Faraday in 1825, and the next
highest homologue, naphthalene 2, was isolated in 1819 from coal tar. Anthracene 3,
tetracene 4 (also known as naphthacene), and pentacene 5 follow as the next
highest homologues (Fig.
1
). Pentacene was first synthesized by Clar and John in
1930 [
1
]. The name “pentacene” was introduced by Philippi, who claimed its
synthesis in 1929 [
2
]. Clar and John showed that Philippi's hydrocarbon was a
dihydropentacene that could be turned into the blue pentacene by dehydrogenation
[
1
]. Clar noticed the high radical character of his novel deep blue hydrocarbon and
considered it to be the most stable radical known at that time [
1
]. As the name
“pentacene” was taken by Philippi's hydrocarbon, Clar named his blue hydrocarbon
2,3,6,7-dibenzoanthracene-9,10-diyl in view of its radical character [
1
]. Clar later
suggested that the polycyclic aromatic compounds consisting of linearly fused
benzene units be called “acenes” [
3
].
To a large extent, current interest in the acene series is due to the fact that these
molecules are valuable compounds in organic electronics. In particular, pentacene
is at the focus of organic materials research [
4
,
5
]. In contrast to the extremely well
investigated pentacene, longer acenes are considerably less well studied. This is due
to their low solubility and high reactivity. For example, the rate constant for the
Diels-Alder reaction with maleic anhydride increases strongly with acene length
(Table
1
)[
6
]. The sites of addition in all cases are the hydrogen carrying carbon
atoms of the central ring(s). These represent the most reactive sites of any given
acene.
The enormous interest in pentacene has naturally led to the question of whether
larger acenes could have even more favorable properties for materials applications.
This has led to a revival of the chemistry of higher acenes in the last decade.
The strong interest in the acenes as potential semiconducting materials has led to
a number of reviews of this field [
7
-
11
]. The purpose of the present account is to
review the properties of the higher acenes, which we define for the purpose of this
account as those that are larger than pentacene (Fig.
2
).
