Chemistry Reference
In-Depth Information
Fig. 20
The chemical structure of a thermotropic folic acid derivative
44
and its proposed
self-assembly into smectic or columnar phases
the size of the side chains through attachment of dendritic oligo(glutamic
acid) derivatives to the folic acid result in the formation of cubic as well as
columnar phases [110, 111]. A number of other related supramolecular tape
side-chain LCPs has also been reported and include binding motifs such as
aromatic amides [112, 113] and barbituric acid dye-melamine [114] tapes,
both of which form columnar phases.
Rather than having the hydrogen bonding group at the end of the
molecule, the placement of a hydrogen-bonding motif in the middle of
themesogencanhaveaprofoundaffectonthenatureandstabilityofthe
mesophase. For example, there have been a number of reports through the
years of using lateral hydrogen bonding to help to stabilize smectic phases
(Fig. 21a). In a recent example, Snieckus and Lemieux compared a series of
fluorenone derivatives (
45a
), with no lateral hydrogen bond donors, with
fluorenol derivatives (
45b
), which contain a hydrogen bonding group in a lat-
eral position (Fig. 21b) [115]. They found that the fluorenones formed both
smectic and nematic phases (K 74 SmC
∗
77 N 87 I for
n
=8),whilethefluo-
renols only formed smectic phases which were also much more stable (K 70
SmC
∗
116 I for
n
= 8). Kishikawa et al. showed [116] with a series of aromatic
ester mesogens that replacing one ester with an amide not only increases
the stability of the material but changes the type of the mesogenic phase
formed (Fig. 21c). In the case of the ester derivative
46a
both smectic A and
nematic phases (K 124 SmA 237 N 250 decomp) are formed while the amide-
containing compound
46b
exhibits two smectic C phases and smectic A phase
which is stable at much higher temperatures (K 202 SmC
x
206 SmC
y
280
SmA 237 decomp). FT-IR data suggest that in the SmC
x
phase there are
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