Functional Liquid-Crystalline Polymers for Ionic and Electronic Conduction - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 168

Fig. 18 Electroluminescence device based on a uniaxially aligned liquid-crystalline semi-
conductor layer emitting linearly polarized light
and branched alkyl chains exhibits a higher hole mobility (on the order
of 10 -4 cm 2 V -1 s -1 ) in the glassy nematic than does an amorphous organic
semiconductor [104]. Although the 1,4-pentadien-3-yloxy moiety was pho-
topolymerizable and the polymerization rate was slower than those of acry-
lates and methacrylates, the photopolymerization reaction was controllable,
resulting in suppression of formation of defects and disorder.
In 2000, the group reported the fabrication of electroluminescence devices
that emitted green polarized light with a dichroic ratio of ten based on com-
pound 19 . In general, uniaxial molecular alignment can be easily realized
using rubbed polyimide films in a nematic phase. However, the rubbing pro-
cess usually degrades the surface morphology of the alignment layers and
generates dust, which is not suitable for thin film devices. In the photo-
alignment layer, structural anisotropy induced by photochemical reaction is
utilized and therefore the process is dustless, exhibiting good surface morph-
ology. One problem is that conventional photo-alignment polymers are insu-
lators, which inhibits carrier injection from an electrode to a semiconductor
layer. In the devices, a photo-alignment layer was used that consisted of
polyacrylate polymer with a photoreactive coumarine moiety and a tripheny-
lamine derivative, which promoted hole injection. At high concentrations of
the triphenylamine derivative, hole injection was surely promoted but mo-
lecular alignment severely degraded [105].
In 2005, Kelly and coworkers achieved full color polarized light-emitting
displays. They synthesized three kinds of liquid crystals containing a bithi-
enylfluorene moiety in their central core and photopolymerizable 1,4-penta-
dien-3-yloxy moieties in alkyl side chains ( 20 , 21 ,and 22 ) as shown in Fig. 19.
These three compounds emitted red, green, and blue fluorescence, respec-
tively, and exhibited a nematic glass state around room temperature [103]. As
a photo-alignment layer, they synthesized polyacrylate copolymer containing
a liquid-crystalline semiconductor moiety with a bithienylfluorene as well as
a photoreactive coumarine moiety in the side chains ( 23 ) as shown in Fig. 20.