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Fig. 21
LC semiconductor containing an oxetane moiety, which retains highly ordered
smectic phase around room temperature
synthesized diphenylbithiophene derivatives containing a polymerizable 1,4-
pentadien-3-yloxy (
25
)oroxetane(
26
) group at the extremity of the alkyl
chains. Monomeric and polymerized diphenylbithiophene with an oxetane
moiety
26
and quaterthiophene with an oxetane moiety exhibited ambipo-
lar carrier transport in the ordered smectic phases. It was surprising that
the diphenylbithiophene derivative having the oxetane moiety in its side
chains exhibited quite high ambipolar carrier mobility (on the order of
10
-2
cm
2
V
-1
s
-1
) even in polymerized films. In this case, excellent carrier
transport properties (ambipolar and high mobility) were retained after pho-
topolymerization to stabilize the liquid-crystalline thin films. In addition,
the hole and electron mobility were independent of temperature and elec-
tric field, which is characteristic for ordered smectic phases of liquid crystals
with low molecular weight. These results suggest that molecular ordering
was not perturbed during the polymerization process in the smectic phases.
Photopolymerization in the isotropic phase produces thin films that do not
exhibit excellent carrier transport properties [108].
The carrier transport characteristics were analyzed using Bässler's dis-
order formalism and Holstein's small polaron theory; however, the tempera-
ture range in which the carrier mobility was measured precisely was not so
wide that the transport mechanism could be determined clearly [108].
Polymerization of reactive monomeric liquid crystals is one method for
stabilizing the liquid-crystalline thin films. Another approach is to form
chemical gels of liquid crystal molecules with low molecular weight by con-
struction of a polymer network. This method has been investigated for the
stabilization of ferroelectric liquid crystal displays. Guymon et al. reported
that a polymer network produced by photochemical cross-linking accumu-
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