Chemistry Reference
In-Depth Information
An example of such a supramolecular-aided function is the growing area
of supramolecular LC materials. In such systems, molecular shape anisotropy
is important for the formation of an LC phase. For example, rigid and semi-
rigid molecules which have a high aspect ratio often exhibit LC behavior.
Therefore, the self-assembly of small (semi)rigid units into a larger linear ar-
ray, with a high axial ratio, can result in the appearance of an LC phase. The
induction of a LC phase in such a case is a macroscopic expression of the mo-
lecular recognition designed into the molecules. Furthermore, the formation
of such an LC phase, with its long range order, can also aid the formation of
higher molecular weight aggregates.
Rowan, Mather et al. [63-65] have examined the use of complementary nu-
cleobase interactions to allow the formation of supramolecular LC materials
of the main type shown in Fig. 3a and b. In these cases, a rod-like mesogen-
bis(phenylethynyl)benzene-was functionalized symmetrically (homoditopi-
cally) with thymine (
T
)or
N
6
-anisoyl protected adenine (
A
An
)usingalkoxy
spacers of varying length to yield molecules of the type:
B
P
-1-B
P
,where
B
P
depicts the variable nucleobase moiety,
A
An
or
T
(Fig. 5).
It was found that the single-component systems featured quite high crys-
tal melting transitions, presumably owing to the high symmetry of the
bis(phenylethynyl)benzene mesogen combined with minimal supramolecular
polymerization. These high-melting transitions had the effect of narrowing
the range of liquid crystallinity to ca. 10
◦
C. Interestingly, however, it was
observed [63, 64] that when homoditopic molecules featuring complemen-
tary nucleobase pairs were mixed in 1 : 1 stoichiometry (
T-1-T
+
A
An
-1-A
An
)
a significant broadening of temperature range for liquid crystallinity was wit-
nessed. The broadening of liquid crystallinity was attributed to the formation
of large, linear supramolecular aggregates only possible when complemen-
tary nucleobase pairs are mixed. This conclusion was further substantiated by
the report of fiber-forming capacity from the LC phase of the same mixtures.
Using X-ray diffraction, it was found that the LC phase formed was generally
smectic-C, with layer spacings of 40-50A (depending on spacer length) and
Fig. 5
Structures of the homoditopic (A-A, B-B) nucleobase terminated bis(phenyl-
ethynyl)benzene monomers (
B
P
1B
P
) investigated by Rowan, Mather et al. [63-65]
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