Chemistry Reference
In-Depth Information
Fig. 31 Extended HBCs with different shapes and sizes [
117
-
121
]
which is beneficial to enhance its charge carrier mobility in terms of FET
applications. A general design concept involves preparation of appropriate
branched oligophenylenes by Diels-Alder reaction or cyclotrimerization, followed
by oxidative cyclodehydrogenation to fuse the phenyl rings for planarization. The
possibility of structures and the extension of conjugations seem infinite, but,
inevitably, there are restrictions and limitations. For example, incomplete cycliza-
tion happens when the size of oligophenylene precursors reaches a certain point,
and the ineluctable low solubility arising from extension of
-conjugation not only
precludes the reliable structural characterization in the solution but also makes the
applications for those systems unrealistic.
Despite all these obstacles, the pursuit of larger graphene fragments never stops
and a large number of extended all-benzenoid PAHs of different sizes, shape, and
symmetries were obtained from bottom-up synthetic methods and some of the
representative examples are listed in Fig.
31
. A triangle-shaped PAH 159 was
synthesized in 2008 by using FeCl
3
mediated cyclodehydrogenation reactions
[
117
], and the introduction of the swallow tailed substituents to the
D
3h
symmetric
core facilitated purification and control of the thermotropic properties and fabrica-
tion of efficient photovoltaic devices. Systematically increasing the molecular size
of HBC by fusion of three aromatic sextet rings in one direction produces a
homologue series 160a, b containing 60 and 78 carbon atoms. The synthesis of
these systems adopted a similar Diels-Alder reaction and cyclodehydrogenation
reaction sequence via a multistep procedure, and the alkyl-substituted derivatives
were soluble in organic solvents [
118
,
119
]. The C90 disc 161 containing five-
membered rings, together with the C150 disc 164, which is the largest molecule
p
