Chemistry Reference
In-Depth Information
Scheme 18 Synthesis of threefold HBC derivative 151 [
107
]
over macroscopic dimensions is essential to obtain optimized performance in
device applications. Furthermore, alkyl chain substituted nanosized graphenes are
able to self-heal structural defects due to their liquid crystalline (LC) character, and
can be processed from solution because of their high solubility. Inspired by these
merits, a great number of HBC derivatives have been developed with different
sizes, symmetries, and substitutions. Three major methods have been utilized to
tune their LC phase. The first is a well-developed way in which the length and
branching of the alkyl chains are adjusted. The long and branched chains tend to
decrease the phase transition temperature. The isotropic temperatures are higher
than 450
C for the
n
-alkyl substituted HBCs 152a [
108
] while the dove-tailed
chains substituted HBC 152b shows an isotropic temperature below 46
C[
109
]
(Fig.
29
). The second method is to tune the molecular size. A wide-range columnar
liquid crystalline phase from room temperature up to
400
C was observed in the
dodecylphenyl substituted HBC 152c [
110
] whereas further increasing the size of
the aromatic core results in broader columnar liquid crystalline phases which
cannot become isotropic melt below 600
C. The third method is to introduce
functional groups or additional non-covalent interactions at the side chains. For
instance, additional hydrogen bonding units were introduced in 154, which signifi-
cantly enhanced the self-assembling abilities of the HBC and thus endowed it with
excellent gelation ability [
111
]. Also interesting is compound 153, in which the
local dipoles are presented [
112
]. As a result, the solution processing onto a
substrate resulted in the formation of exceptionally long fibrous microstructures.
The rational control of the LC phase is beneficial to enhance the charge transporting
properties of materials, qualifying them as promising organic materials for organic
electronic devices such as FETs and solar cells.
It is well known that disc-like PAHs, such as HBCs, are able to self-assemble into
one-dimensional columnar structures; the random orientation of assembles, however,
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