Chemistry Reference
In-Depth Information
Micellar cubic (0D), hexagonal columnar (1D), lamellar (2D), and bicontin-
uous cubic (3D) nanostructures are formed by self-assembly of
13
.Forthe
complexes with LiClO
4
, the ionic conductivities show discontinuous changes
following the phase transitions with change of temperature or molecular
structure of the dendritic moiety. For example, the conductivity of the com-
plex of
13
with LiClO
4
drops from 4.6
10
-9
Scm
-1
,alongthe
phase transition from crystalline lamellar to micellar cubic phases.
10
-6
×
to 1.2
×
3
Electron-Conductive Side-Chain Liquid-Crystalline Polymers
3.1
Side-Chain Polymers and Low Molecular Weight Liquid Crystals
Not only conjugated polymers but also side chain type polymers, such as
polyvinylcarbazole, have been applied to electronics devices. In particular,
this kind of polymer has played an important role in xerographic photore-
ceptors utilizing their photoconductivity [78]. In this type of polymer, the
dominant process of carrier transport is charge carrier hopping between
chromophores containing
-conjugated aromatic system connected to the
side chains. In this mechanism, charge carriers are localized on the chromo-
phores and transferred between them via
π
-molecular orbital overlaps as-
sisted by thermal activation and the electric field. Conventional side chain
type photoconductive polymers are amorphous and, therefore, the positions
of the chromophores as well as their HOMO and LUMO levels are disordered
as shown in Fig. 13. These disorders and small intermolecular orbital overlaps
cause low carrier mobility and strong dependence on temperature and elec-
tric field, compared to molecular crystals. This behavior is well described by
the Gaussian disorder model proposed by Bässler [16].
Based on this model, we can expect that reduction of the energetic and po-
sitional disorders should increase carrier mobility and decrease dependence
on temperature and electric field. One of the methods to reduce the disorders
is introduction of mesogenic groups into the side chains of this kind of semi-
conductive polymer. The introduced mesogenic groups induce liquid crystal
molecular order in which chromophores are aligned, reducing energetic and
positional disorders as shown in Fig. 13b.
In general, for side chain liquid-crystalline polymers, macroscopic mo-
lecular alignment is not easy and therefore clear evidence of electronic charge
carrier transport was confirmed first in liquid crystals with low molecu-
lar weight. In the 1990s, fast electronic conduction was verified in discotic
columnar phases of triphenylene derivatives [79, 80] and hexabenzocoronene
derivatives [81, 82] as well as smectic phases of 2-phenylbenzothiazole [83,
84] and 2-phenylnaphthalene derivatives [85], as shown in Fig. 14. Carrier
π
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