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apparent mesogen tilting within the layers from 27
◦
to 30
◦
.Furtherworkby
the same group investigated the influences of complementary mixture stoichi-
ometry and thermal history on LC phase behavior [65]. There it was found
that departure from 1 : 1 stoichiometry hampered liquid crystallinity. Fur-
ther, the precise thermal history of such a 1 : 1 blend had a large impact on
phase behavior, a finding attributed by a propensity for the high-melting
N
6
-
anisoyl-adenine functionalized mesogen to phase separate from the mixture
under certain conditions.
A commonly studied motif for the main-chain SLCPs involves chains of hy-
drogen bonding between dicarboxylic acids and bipyridyls, building on some
early work by Griffin et al. in this area [66]. Such SLCPs tend to be of the
mesogenic supramolecular type shown in Fig. 3d, where the non-covalent in-
teraction is part of the mesogen. More recently, Griffin et al. [67] found that
a stoichiometric mixture of tetraethyleneglycoxy-bis(2,6-dimethyl-4-benzoic
acid) (
2
)and4,4
-(
p
-phenylenedi-1,2-ethenediyl)bipyridyl (
3
)(Fig.6)ledto
a nematic LC material with polymer-like fiber-forming capacity. In particular,
the stoichiometric mixture featured an enantiotropic nematic phase sequence
of (K 150 N 175 I) on heating and (I 175 N 88 K) on cooling. In contrast, the
two starting compounds (bisbenzoic acid and bipyridyl compounds) under-
went direct melting from the crystalline to isotropic phases. For this system
it was argued that the liquid crystallinity is afforded by enhancement in the
mesogen length/diameter (L/D) ratio from 3.3 (15.5
A
/5
A
) for the bipyridine
derivative to 6 (31
A
/5
A
) for the hydrogen-bonded mesogenic unit.
Fig. 6
Structures of the non-mesogenic bis-pyridyl (
2
) and bis-benzoic acid (
3
) monomers
studied by Griffin et al. [67] and the proposed structure of the main chain liquid crys-
talline supramolecular polymer (
2
·
3
) highlighting the length of supramolecular mesogen
He et al. [68] examined the effect of adding flexibility to the hydrogen
bond acceptor bis-pyridyl units. They attached stilbazole derivatives to ei-
ther end of differently sized oligo(ethylene oxide) spacers (
4
) (Fig. 7). Mixing
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