Chemistry Reference
In-Depth Information
3 Diaryl[
a,e
]pentalenes ........................................................................ 162
3.1 Recent Synthetic Developments in Diaryl[
a,e
]pentalenes ........................... 163
3.2 Electrochemical and Optical Properties of Diaryl[
a,e
]pentalenes ................... 169
3.3 Diaryl[
a,e
]pentalene Ladder Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
4 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
1
Introduction
The concepts of aromaticity and antiaromaticity have long been employed to
explain certain chemical phenomena; however there is still much to be explored
regarding the effect of these simultaneously opposing and intertwined theories
on the properties of materials. Aromatic molecules are generally characterized by
several properties including unexpectedly high thermal stability, nonalternate bond
lengths, and diamagnetic anisotropy [
1
,
2
]. Since it was first proposed by H¨ckel in
the 1930s, the theory that these properties belong to cyclic arrays containing 4
n
þ
2
p
-electrons
later came to be classified as antiaromatic, and can be high energy and difficult to
isolate [
4
-
7
].
Organic molecules consisting of multiple fused aromatic rings are commonly
known as polycyclic aromatic hydrocarbons (PAHs). Due to the extended conjuga-
tion and ring current inherent in these materials, they make a class of important
candidates for use in organic electronics [
8
]. Antiaromatic compounds, on the other
hand, are appealing candidates for optoelectronic applications due to small HOMO-
LUMO gaps relative to analogous aromatic systems, and theoretically high polar-
izability, which could improve intermolecular interactions. Although attempts have
been made to predict computationally the structure and solid-state properties of
polycyclic antiaromatic hydrocarbons (PAAHs), there is still much that is not well
understood [
9
]. In this regard, one area of interest is to study the interplay of these
two concepts in polycyclic systems consisting of alternating aromatic and
antiaromatic character [
10
].
-electrons has held true [
3
]. Alternatively, those species containing 4
p
2
[N]Phenylenes and Related Materials
The most basic example of antiaromaticity is found in the 4
-electron system of
cyclobutadiene (Fig.
1a
)[
11
]. There has been a great deal of theoretical interest in
the antiaromaticity of cyclobutadiene; however, experimental analysis has been
difficult as a result of the very high reactivity of the parent hydrocarbon.
The [N]phenylenes are a family of PAHs in which benzene rings are fused
together by cyclobutadiene rings, resulting in a ladder structure with alternating
aromatic and antiaromatic character (Fig.
1b, c
)[
12
]. The simplest member of this
family, biphenylene
p
2), was first synthesized in 1941 from 2,2
0
-
dibromobiphenyl after a fairly long history of eluding synthetic chemists [
13
].
(
n
¼
