Self-assembly of bent-core liquid crystals: Formation of a modulated smectic phase with p2gg lattice to escape anticlinic tilt

Article abstract of DOI:10.1039/b907536d

A new silyl terminated bent-core mesogen was synthesized which shows a randomly tilted SmC_R phase and a columnar phase with p2gg lattice as new modes of self-assembly in this class of compounds.

Full text of DOI:10.1039/b907536d

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Self-assembly of bent-core liquid crystals: formation of a modulated
smectic phase with p2gg lattice to escape anticlinic tiltw
R. Amaranatha Reddy,z*a Ute Baumeisterb and Carsten Tschierske*a
Received (in Cambridge, UK) 15th April 2009, Accepted 18th May 2009
First published as an Advance Article on the web 9th June 2009
DOI: 10.1039/b907536d
A new silyl terminated bent-core mesogen was synthesized which
shows a randomly tilted SmCR phase and a columnar phase with
p2gg lattice as new modes of self-assembly in this class of
compounds.
Differential scanning calorimetry (DSC) of 1w indicates
three phase transitions, a melting point at 120 1C, a phase
transition at 127 1C (DH = 15 kJ molꢀ1) and the transition to
the isotropic liquid state at 135 1C (DH = 3.7 kJ molꢀ1). Upon
cooling under a polarizing microscope between crossed
polarizers, at 135 1C the formation of a weakly birefringent
fan-shaped texture can be observed (Fig. 1b). Shearing the
sample gives rise to an optically isotropic (dark) appearance,
as is typical for uniaxial smectic phases (Fig. S1c, ESIw).
On further cooling, at 127 1C, a striped pattern develops
perpendicular to the fan texture as shown in Fig. 1d. In a
homeotropically aligned sample a weak birefringence arises at
this temperature (see Fig. S1d, ESIw). Electrooptical investi-
gations indicate that both mesophases do not show any polar
(ferroelectric or antiferroelectric) switching.
Self-assembly enables chemists and physicists to construct
complex functional systems at different length scales. Self-
assembled soft matter is of special interest as it provides the
possibility to modify its properties and functionality by
external stimuli (electric, magnetic fields, light, etc.).1 Liquid
crystals (LCs) represent self-assembled soft matter structures
with long range order, and these materials are of growing
importance for electronic2 and optoelectronic applications3 as
well as being involved in biological structure formation.4
A relatively new blueprint for molecular tectons, useful for
the design of LC soft matter, is provided by molecules
incorporating a bent aromatic core, assigned as bent-core
molecules (banana molecules). This specific shape provides a
dense and directed packing of this kind of molecules which
leads to macroscopic polar order and new LC phase structures.5
Herein we report the self-assembly of a new bent-shaped
compound 1, having a rigid aromatic core combining terephthalate
based wings with a 2-methyl substituted bent resorcinol core6
and flexible end-chains composed of alkyl spacer units and
trisiloxane groups.7,8y The combination of attractive forces
(packing of terephthalate based cores) with nanoscale segregation
(aromatics vs. alkyl vs. siloxane) and with steric distortion
(2-methyl group, bulky siloxane units) leads to two new LC
phases. One is a special type of random tilt phase (SmCR) with
some relation to the deVries type smectic phases,9 the other
one is the first rectangular columnar phase with p2gg symmetry
(Colrec/p2gg) observed for bent-core mesogens.10,11
The X-ray diffraction pattern of a surface-aligned sample of
the smectic phase (Fig. 1a) exhibits first and third order layer
reflections on the meridian corresponding to a layer distance
d1 = 5.35 nm. This value is smaller than the molecular length
(L = 7.05 nm, see Fig. S2, ESIw) and this is the first hint of a
tilted organization of the molecules. In the wide-angle region
there are two diffuse maxima at d2 = 0.70 nmz and at
d3 = 0.50 nm, assigned to the mean distances between the
siloxane end groups and between the hydrocarbon segments
(alkyl chains and aromatics), respectively. The diffuse
character of both wide angle scatterings indicates a liquid-like
disorder within the layers, as is typical for LC phases. Both
form rings, but the scattering at d3 = 0.50 nm has two maxima
with equal intensity which make an angle with respect to the
equator, confirming a tilted organization of the molecules
within the layers. The maxima correspond to a tilt angle of
about 341.8 However, the proven tilted organization of the
molecules is in conflict with the optical uniaxiality of this
mesophase. Hence, the aromatic cores should be randomly
tilted, similar to the models proposed for deVries type smectic
phases9 (SmCR phase). This means that either the individual
molecules or—more likely—clusters of molecules have a
randomized tilt direction.**
In the following discussion, we mainly focus on the low
temperature phase developing upon cooling this SmCR phase.
At the transition to this mesophase the diffraction pattern is
completely changed (see Fig. 1c and e). The rather high
enthalpy of this transition indicates a significant increase of
order. However, a transition to a crystalline phase can be
excluded, because the wide angle scatterings remain diffuse.
The diffuse ring positioned at d2 = 0.69 nm, assigned to the
packing of the disordered siloxane units, is not changed, but
the positions of the maxima of the other diffuse scattering at
d3 = 0.49 nm (aromatics and aliphatics) were shifted (Fig. 1c).
a Martin-Luther University Halle-Wittenberg, Institute of Chemistry
Organic Chemistry, Kurt-Mothes Str. 2, D-06120 Halle, Germany.
E-mail: Carsten.tschierske@chemie.uni-halle.de;
Fax: +49 (0)345 5527346; Tel: +49 (0)345 5525664
b Martin-Luther University Halle-Wittenberg, Institute of Chemistry
Physical Chemistry, Muhlpforte 1, D-06108 Halle, Germany.
¨
E-mail: Ute.baumeister@chemie.uni-halle.de;
Fax: +49 (0)345 5527157; Tel: +49(0)345 5525827
w Electronic supplementary information (ESI) available: Synthesis,
analytical data, additional textures and X-ray data. See DOI:
10.1039/b907536d
z Present address: VVi Bright Inc., 2450 Central Avenue, Boulder,
CO 80301, USA.
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b
corresponds approximately to the molecular length
(L = 7.05 nm) which is in line with an arrangement of the
molecules on average parallel to the shorter direction b of this
lattice (Fig. 1g). The maxima of the outer diffuse scattering,
corresponding to the average lateral distances of the molecular
cores, are in directions perpendicular to the b-axes of the three
distinct orientations (see dotted lines in Fig. 1c, the colour is
the same as that of the corresponding lattice in Fig. 1f, see also
Fig. S4b, ESIw). This additionally supports the p2gg type of
organization. A number of about 9 molecules in the cross
section of each ribbon can be calculated from the lattice
parameters and the molecular volume.
The three distinct lattice orientations with a B601 angle
between them (Fig. 1f) could result from different surface
alignments of domains either parallel to the molecular long
axes (blue lattice) or parallel to the ribbons (green and black
lattice).ww In this case the tilt angle of the molecules in the
ribbons should be about 301 which is in line with the proposed
model (Fig. 1g) and it is also close to the tilt angle of 341 in the
SmCR phase. Remarkably, in different experiments the same
diffraction pattern with the same relative intensities of the
reflections, as shown in Fig. 1e and f, was always obtained.
Based on this observation and the relatively diffuse character
of the small angle scatterings, it is concluded that this
diffraction pattern is intrinsic and the mesophase has only
short range p2gg symmetry which is space averaged with
preferred directions corresponding to those seen for surface
alignment.12
Accordingly, this columnar phase can be regarded as a
modulated smectic phase consisting of ribbons with alter-
nating tilt of the molecules (tilt in the ribbons). Any organization
with alternating tilt along the ribbon stacks (SmCa) is
forbidden due to the huge steric frustration occurring at the
inter-ribbon interfaces (see Fig. 2). However, in the p2gg phase
with the proposed herring-bone structure the tilt direction
changes perpendicular to the stacking direction which leads
to an overall parallel arrangement of the molecules and thus
avoids unfavourable clashing of the ribbons. Hence, at the
phase transition SmCR-to-Colrec/p2gg the randomly tilted
domains of the SmCR phase fuse and as a consequence of
the denser packing the layers get broken into ribbons with
opposite tilt direction which organize on a rectangular lattice.
The special feature of this ribbon phase is that a p2gg lattice is
formed instead of the c2mm type rectangular or the oblique
lattices as usually observed for the nontilted (SmA) or
synclinic tilted (SmCs, see Fig. 2) modulated smectic phase
of bent-core mesogens.5,7,8
Fig. 1 Mesophases of compound 1: (a) X-ray diffraction pattern
(surface-aligned sample, X-ray beam nearly parallel to the substrate)
and (b) texture of the SmCR phase at 130 1C; (c) X-ray diffraction
pattern and (d) texture of the Colrec/p2gg phase at 115 1C; the optical
textures actually shown were obtained under a triangular wave electric
field (400 Vpp in a 5 mm cell at 100 Hz), but they are very similar to the
textures of the virgin samples shown in Fig. S1a and b, ESIw; (e) small
angle X-ray diffraction pattern of the Colrec/p2gg phase at 115 1C;
(f) same diffraction pattern showing the lattices of three distinct,
strongly scattering domains in the multi-domain sample with indexa-
tion; axes and indices labelled for the black lattice (see also Fig. S3, S4
and Tables S1, S2 (ESIw) for additional supporting X-ray data); all
patterns are fibre-like disordered around an axis normal to the aligning
interface and the scattering of the isotropic liquid was subtracted;
(g) model of the organization of the bent-core molecules in the
Colrec/p2gg phase (the molecules are rotationally disordered around
the long axis and therefore shown as black lines) and (h) in a columnar
mesophase with antiferroelectric polar order (dots and crosses indicate
opposite polar directions; there are actually 9 molecules in each
ribbon, though a smaller number of molecules is shown).
The rather high enthalpy of the transition from the smectic
to the modulated smectic phase (DH = 15 kJ molꢀ1) indicates
In addition, though this wide angle scattering remains diffuse
it is much less diffuse than that of the smectic phase, indicating
a more ordered packing of the molecules (see Fig. S3 and
Table S2, ESIw). In the small angle region there are several
reflections which can be assigned to a rectangular p2gg lattice
(a = 12.0 nm, b = 6.9 nm) disordered in a multi-domain
sample with three distinct main orientations of the 2D
lattice, as shown in Fig. 1f. According to this indexation
(see also Fig. S4 and Table S1, ESIw) the lattice parameter
Fig. 2 Molecular organizations in the different types of ribbon
phases; the SmCa phase is not possible because of the disturbance of
the molecular packing at the unfavourable inter-ribbon interfaces in
this organization.
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a significant increase of order at this transition which is in line
with the observed less diffuse wide angle scattering. Hence, it is
likely that the rotational disorder about the long molecular
axis is reduced at the SmCR-to-Colrec/p2gg transition and
polar order arises in the ribbons. A structure with synpolar
order along the ribbon stacks and antipolar order in the other
direction, leading to an overall antiferroelectric organization,
corresponding to the layer group p21212 as shown in Fig. 1h,13
is most likely, because this structure can be stabilized by the
parallel alignment of the rod-like aromatic wings at the inter-
ribbon interfaces (see Fig. S5a, ESIw).5a,8zz This stabilizes the
antipolar organization with respect to the related synpolar
(ferroelectric) structure (where the aromatic wings would not
be parallel, see Fig. S5b, ESIw) and this could lead to a very
high threshold voltage for polar switching which cannot
be achieved under the available experimental conditions
(Vpp,max = 400 V at 5 mm cell gap). This is in line with
observations in other B1 type Colrec phases of bent-core
mesogens, which also do not show polar switching despite a
polar order in the ribbons.5a,b
dendrimer;16a smectic phases of bent-core molecules with a random
direction of polar order were also reported for non-silylated molecules.16b
ww All the domains are rotationally disordered around an axis normal
to the aligning surface. Hence, all orientations parallel to the molecular
long axes contribute to the diffuse scattering on the meridian of the
pattern, but only two of those parallel to the ribbons with opposite tilt
directions to that rotated by B601 out of the meridian which explains
the different intensities for both types of the outer diffuse scattering.
zz Splay of polar direction might also be involved in the proposed
structure composed of synpolar stacks of ribbons.11a,17
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In this way the missing case of a modulated smectic phase
with rectangular lattice, anticlinic tilt and interdigitated layer
defects was observed for the first time in a LC phase formed by
bent-core molecules. The interesting point is that in this p2gg
structure the unfavourable clashing of the ribbons in an
anticlinic organization of the molecules in adjacent ribbons
is avoided by a staggered zigzag modulation of the layers. This
shows once again that segregation and steric effects provided
by oligo(siloxane) units, combined with the packing
restrictions of bent rigid cores, is a powerful tool to program
molecules for self-assembly into new soft matter structures.
The work was supported by the Fonds der Chemischen
Industrie; R. A. Reddy is grateful to the Alexander von
Humboldt Foundation for the research fellowship; we thank
G. Ungar (University of Sheffield, UK) for discussions.
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Notes and references
y The related compound with reversed peripheral ester groups has the
phase sequence Cr 103 1C Colobl 111 1C Iso (Colobl = ribbon phase
with oblique lattice).8b This indicates a strong effect of the direction of
the ester group on mesophase stability and mesophase type, as no
smectic phase is observed and the ribbons adopt a synclinic tilted
arrangement in this isomeric compound. Similar effects on mesophase
stability were seen for the change of the direction of the inner ester
groups (resorcinol bisbenzoates vs. isophthalates).14
z The diffuse scattering at d2
= 0.7 nm can be explained
by a segregation of the siloxane units to form distinct sublayers
(see Fig. S3, ESIw), the ring-like shape indicates the liquid-like disorder
in these layers.8 The segregation is mainly due to the distinct shape
of alkyl chains and oligosiloxanes, rather than due to a chemical
incompatibility.
B. Luhmann, H. Lang and C. Tschierske, J. Am. Chem. Soc.,
¨
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J. Mieczkowski, K. Gomola and H. Takezoe, Phys. Rev. Lett.,
2006, 97, 113901.
8 The two distinct wide angle scatterings at d3 = 0.5 nm could not be
due to the intrinsic tilt of the rod-like wings of the bent-core molecules.
This was proven for aligned samples of SmAP phases of bent-core
molecules where the layer reflection appears on the meridian and the
diffuse wide angle scattering is centered on the equator.8a,15
** This could be regarded as a non-polar variant of the ferroelectric
SmCRPFE phase, recently reported for a carbosilane-based bent-core
17 D. A. Coleman, J. Fernsler, N. Chattham, M. Nakata,
Y. Takanishi, E. Korblova, D. R. Link, R.-F. Shao, W. G. Jang,
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4238 | Chem. Commun., 2009, 4236–4238