Chemistry Reference
In-Depth Information
spectively [77]. As mentioned,
15
containing the PPO coils self-organizes
into a 3D body-centered tetragonal lattice. In contrast,
18
shows significantly
distinct self-assembly behavior. The optical microscopic observation of an
arced pseudo-focal conic texture and the small-angle X-ray diffraction meas-
urement both indicate that the supramolecular structures in mesophases
are honeycomb-like lamellar structures where hexagonally perforated layers
(
P
6
3
/
mmc
symmetry) are stacked in ABAB order.
The different self-assembly behavior of
15
and
18
with identical coil vol-
ume fraction points out the significance of coil cross-sectional area for the
packing of rod segments. It can be rationalized by the consideration of coil
density at the rod/coil interface as dependent upon coil cross-section. For
a given space at the rod/coil junction, the coils with larger cross-sectional
area cause more space crowding. The steric repulsion resulting from the space
crowding leads to the stretched conformation of coils, leading to the coil
stretching penalty [9c]. The morphological transition from continuous (the
honeycomb-like lamellar structure of
18
) into discrete rod packing structures
(the tetragonal structure of
15
) allows coils enough room to lower the coil
conformational energy. Finally, self-assembly of rods can be fine-tuned in 3D
nanospace since, in addition to coil volume fraction, coil cross-section is an
independent parameter in building a variety of supramolecular structures.
A method for manipulating the size of the discrete nanostructures as-
sembled from conjugated rod building blocks may be accessible by attaching
chemically dissimilar, flexible dendritic wedges to their ends. Dumbbell-
shaped molecules consisting of three biphenyls connected through vinyl
linkages as a conjugated rod segment and aliphatic polyether dendritic
wedges with different cross-sections (i.e., dibranch (
19
), tetrabranch (
20
)
and hexabranch (
21
)) self-assemble into discrete bundles that organize into
3D superlattices [79]. Molecule
19
, based on a dibranched dendritic wedge,
organizes into primitive monoclinic-crystalline and body-centered, tetrago-
nal liquid crystalline structures, while molecules
20
and
21
, based on tetra-
and hexabranched dendritic wedges, respectively, form only body-centered,
tetragonal liquid crystalline structures (Fig. 13). X-ray diffraction experi-
ments and density measurements showed that the rod-bundle cross-sectional
area decreases with increasing cross-section of the dendritic wedges. The
number of molecules per bundle decreases systematically with increasing
cross-section of the dendritic wedge. Consequently, the size of the rod-bundle
in cross-sectional area decreases in nanoscale dimension from 17.0 to 11.5
to 9.6 nm
2
for
19
,
20
,and
21
, respectively. The variation of rod bundle size
in cross-section can be rationalized by considering both the steric repulsion
between the bulky dendritic wedges and the nanophase separation between
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