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ability reduces the electron density on the iodine atom promoting the N-I
halogen bond. Interestingly, the flexibility in
4
appears important for form-
ing LC material with
11
, as complexes of
11
with rigid bispyridine derivatives
(e.g., 4,4
-bipyridine) formed only crystalline compounds that melted to an
isotropic liquid. However, when both the halogen “donor” compound and
bispyridyl compound included flexible spacers,
4
11
, monotropic (unstable
to crystallization) nematic phases were observed upon cooling. It should
be noted that while these supramolecular materials did form liquid crystal
phases, they were generally not as stable as those materials formed from the
benzoic acid/pyridine supramolecular motif.
·
2.2
Supramolecular Columnar Structures
There has been a great amount of work in recent years focused on the for-
mation of 1-dimensional polymers aggregates (Fig. 3e) that form columnar li-
quid crystal phases. Most of these aggregates utilize
-stacking to aid their
assembly. For the purpose of this paper we will focus in this section only on
systems which utilize specific, tailored supramolecular interactions that have
been designed into the monomer to aid the formation of the desired colum-
nar aggregates. A key example of such a system which has attracted some
attention is the benzene-1,3,5-tricarboxamide moiety (Fig. 10a). Early work
by Matsunaga et al. demonstrated that simple alkyl group derivatives (
12
)of
these compounds do form mesophases (e.g., for
n
= 12, K 88 M 212 I) [75].
In order for such molecules to form hydrogen bonded columnar aggregates,
the three amide moieties have to distort out of conjugation with the aro-
matic core (Fig. 10b). Nuckolls et al. showed that long chain alkoxy- [76, 77]
or alkyne-substituents [78] on the 2,4 and 6 positions of the benzene-1,3,5-
tricarboxamide unit (
13
and
14
, respectively) resulted in the formation of
hydrogen-bonded polymeric aggregates that form more stable columnar LC
phases (K 98 M 294 I) than the phases observed with the materials without the
2,4,6-substitution (K 88 M 212 I) [75]. The benzene-1,3,5-tricarboxamide core
appears to be a flexible platform allowing access to a wide range of LC ma-
terials [79, 80]. From the examples reported it appears that while increasing
the bulk on the amide positions (as in
15
, Fig. 10c) yields wider LC tempera-
ture ranges, primarily as a consequence of reducing the compounds melting
point, the clearing temperature is reduced (K - 4 M 178 I). This would be
consistent with bulkier groups reducing packing efficiency and destabilizing
molecular aggregation. The same benzene-1,3,5-tricarboxamide core has also
been used by Meijer et al. to access much larger discotic materials, such as
16
(Fig. 10c), in which the amides are involved in intramolecular hydrogen
bonding, rigidifying and preorganizing the mesogenic disc [79, 81].
A number of supramolecular discotic systems have been investigated in
which hydrogen bonding has been used to form or stabilize the disc rather
π
-
π
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