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Tilted and polar smectic phases are removed, but biaxial order is
retained, thus providing a new way to non-polar SmAb phases.
These SmAb phases are additionally modulated, thus giving rise to
unique modes of LC self-assembly, leading to increased structural
complexity in these self-assembled soft matter systems.
This work was supported by the EU within the FP7 funded
Collaborative project BIND (grant No. 216025).
Notes and references
‡ Though there are numerous 2D modulated LC phases of bent-core
mesogens, to the best of our knowledge there is presently only one
report about a long range 3D modulated LC structure formed by bent-
core molecules.19b However in the reported cases there is long range
periodicity in all three dimensions, whereas in our case there is reduced
coherence length in at least one of the direction.
§ There are also clear differences to the known B6-type intercalated
smectic phases. The formation of a Schlieren texture upon shearing is
usually not observed for B6 phases. Moreover, though in B6 phases a
diffuse XRD scattering can sometimes be observed as 11 scatterings
between equator and meridian, a diffuse small angle scattering was
never observed on the equator.18
Fig. 4 Models showing the proposed organization of the molecules in the
distinct SmAb# phases: (a) frozen rotation of the molecules around the long axis,
leading to biaxiality; (b) mixing of terminal and lateral chains stabilizes the
staggered packing on the c2mm lattice; (c) biaxial organization of the molecules
in the batches; (d) stacking of the batches in ribbons and organization of the
#
ribbons on a c2mm lattice leads to the SmAb/c2mm and Colrec/c2mm#
˜
phases; the space between the batches is filled by the terminal and the lateral
alkyl chains; the periodicity d2 has only short coherence length in all phases.
¶ The effect of a triangular wave electric field on the mesophases was
investigated for compounds 1/6 and 1F/22 as representatives. No polar
switching could be detected under an applied field up to 60 V mmꢁ1, and
therefore, there should be no macroscopic polar order in these LC phases.
#
˜
In the SmAb/c2mm phases of the long chain compounds the
correlation length of the c2mm lattice is increased, thus providing the
additional diffuse 11 scatterings. As shown in Table 2, with rising
length of the lateral chain the parameters a (related to the diameter of
the ribbons) as well as d2 (modulation along the ribbons) increase,
whereas the parameter b remains nearly constant and is exclusively
determined by the molecular length Lmol (b B Lmol/2). At reduced
temperature this c2mm lattice could become long range, as in the
Colrec/c2mm# phase of compound 1F/22.
As indicated in Fig. 4d the short range periodicity d2 results from a
modulation of the ribbons which is most likely caused by a fracturing
of the ribbons into strings of patches with enhanced concentration of
the aromatic cores (Fig. 4c and d). The reason is that the lateral alkyl
chains segregate from the aromatic cores, thus providing a periodicity
with short coherence length along the ribbon long axis. These patches
have an average size of B12 and B22 molecules for 1/18 and 1F/22,
respectively (for calculations, see Table S3†). It is hypothesized that in
the patches the rotation of the molecules around their long axes is
restricted, leading to biaxiality but no long range polar order is
achieved,¶ as the lateral chains distort a polar packing. In analogy
to the organization in B6 and B1Rev phases of bent-core mesogens
without lateral chains,6,18,19a a preferred alignment of the biaxial
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Overall, introduction and elongation of a lateral chain in the
bay position of bent-core mesogens has significant influence on
their self-assembly, giving rise to new LC phase structures.
9906 | Chem. Commun., 2014, 50, 9903--9906
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