Chemistry Reference
In-Depth Information
Fig. 12
Chemical structures of mesogenic 3 : 1 discotic complexes based on complexes of
benzoic acid derivatives (
21
or
24
)with
a
a
tris
(imidazoline)
22
or
b
a benzotri(imidazole)
23
core
Interestingly, the 3 : 1 complex with the less symmetrical 3,4-substituted ben-
zoic acid shows a more stable mesophase (K 79 M 244 I) than the derivative
formed from the 3,4,5 substituted benzoic acid (K 63 M 214 I). A related
system has also been reported which uses a benzotri(imidazole) (
23
)asthe
trifunctional core unit (Fig. 12b) along with benzoic acid derivatives (
24
).
This 1 : 3 complex exhibits a hexagonal columnar mesophase between 23
◦
C
and 75
◦
C. The acrylate moieties on the alkoxyl chains could be photopoly-
merized to covalently “fix” the LC phase. Removal of the supramolecularly
bound benzotri(imidazole) core yields a nanoporous polymer film containing
hexagonally-ordered channels-opening the door to the potential application
of these materials in such areas as separation, nano-composites, and catalysis.
Interactions other than hydrogen bonding have been utilized in the con-
struction of columnar structures. The aromatic nature of most of the com-
pounds used in discotic systems enables the tailoring of the
interactions
in order to aid the formation and stability of the desired polymeric stacks. It
has been known for a number of years that “doping” electron-rich discotics
with electron-poor aromatics leads to mesophases with enhanced stability.
For example, the electron rich 2,3,6,7,10,11-hexakis(pentyloxy)triphenylene
(
25
,
n
=4) exhibits a Col
h
phase (hexatic columnar) between 69-122
◦
C,
while its 1 : 1 mixture with 2,4,7-trinitrofluoren-9-one (
26
)hasamesophase
between 49-232
◦
C. The complementary interaction between the two aro-
matic components results in the formation of alternating AB stacks. Park and
Hamilton introduced the use of the C
3
-symmetric mellitic triimide (
27
)as
the electron-deficient component which also forms columnar phases when
mixed with derivatives of
25
[84, 85]. It is interesting to note that the stacking
of aromatic units often yields offset stacks, in agreement with the Sanders-
Hunter model of
π
-
π
π
-
π
stacking, which is of course not ideal for the formation
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