Polar and chiral mesophases formed by m-terphenyl derived bent-core molecules with lateral F-substituents

Article abstract of DOI:10.1039/b705499h

A new series of bent-core molecules based on fluorinated terphenyl cores has been synthesized. The influence of the position of the polar fluorine substituent on the mesomorphic properties was studied. The mesophases under discussion were investigated by means of polarizing microscopy, differential scanning calorimetry, X-ray diffraction as well as electrooptical experiments. Depending on the position of the fluorine substituent, polar smectic or columnar mesophases were formed. Remarkable textures could be observed for the smectic phases. For some compounds regular arrays of dots can be found which are interpreted as layer deformation on a macroscopic scale. Of special interest is the coexistence of helical filaments and a dark conglomerate texture with domains of opposite handedness found for one of the polar smectic phases. The smectic phases under investigation show antiferroelectric switching behaviour, whereas the rectangular columnar phases do not show polar switching. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b705499h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Polar and chiral mesophases formed by m-terphenyl derived bent-core
molecules with lateral F-substituents{
Gert Dantlgraber,a Christina Keith,a Ute Baumeisterb and Carsten Tschierske*a
Received 11th April 2007, Accepted 15th May 2007
First published as an Advance Article on the web 29th May 2007
DOI: 10.1039/b705499h
A new series of bent-core molecules based on fluorinated terphenyl cores has been synthesized.
The influence of the position of the polar fluorine substituent on the mesomorphic properties was
studied. The mesophases under discussion were investigated by means of polarizing microscopy,
differential scanning calorimetry, X-ray diffraction as well as electrooptical experiments.
Depending on the position of the fluorine substituent, polar smectic or columnar mesophases
were formed. Remarkable textures could be observed for the smectic phases. For some
compounds regular arrays of dots can be found which are interpreted as layer deformation on a
macroscopic scale. Of special interest is the coexistence of helical filaments and a dark
conglomerate texture with domains of opposite handedness found for one of the polar smectic
phases. The smectic phases under investigation show antiferroelectric switching behaviour,
whereas the rectangular columnar phases do not show polar switching.
phases also assigned as columnar phases).13 Layer deformation
1. Introduction
can also be non-regular, leading to strongly distorted layer
Polar order and chirality are important materials’ properties,
structures, similar to sponge phases, which appear optically
isotropic.12,14–16 Within the layers, the tilted organization of
required for several applications. Liquid crystalline (LC)
materials are of increasing interest, not only for display
applications where they dominate the today’s market;1,2 they
the bent-core molecules with a uniform polar direction leads to
a further reduction of the symmetry of these layers to C2 which
are also of interest for self-assembly of semiconducting organic
lacks a mirror plane and hence these layers are inherently
chiral though the molecules themselves are achiral.10 This is a
materials, as used for electroluminescent and photoconducting
devices and for organic transistors and solar cells.3,4 Chiral
new source of supramolecular chirality which is distinct from
and polar ordered LC materials are useful for tuneable lasers,5
helical superstructures observed in most other cases.17
non-linear optical applications,6 microdisplays and phase
Changing one of the parameters, either tilt direction or polar
modulators in optical communication devices.7 Until 1996,
direction reverses the chirality sense.10 Details of these basic
concepts are summarized and discussed in recent reviews.12
chiral molecules organized in tilted smectic (SmC*) or
columnar phases were used exclusively as polar ordered LC
materials.8 The discovery that achiral molecules with a bent
aromatic core also show polar order9 and chirality10,11 in their
mesophases had a huge impact on worldwide LC research
activities.12 The fundamental principle is that the bent shape of
the aromatic core restricts rotation around the molecular long
axis and this leads to a directed packing of the molecules in
layers, which gives rise to polar order in these layers. Escape
from polar order can cause layer frustration, leading to regular
layer deformation (undulations = wavy deformed layers) or
Especially interesting properties were found for bent-core
layer modulation (breaking of the layers into ribbon-like
molecules with a fluorinated aromatic core. The influence of
segments), giving rise to mesophases with a 2D lattice (ribbon
F-substituents at the aromatic core was mostly investigated for
resorcinol derivatives18 and isophthalates.19 For example, the
aInstitute of Chemistry, Organic Chemistry, Martin-Luther-University
Halle-Wittenberg, Kurt-Mothes-Str. 2, D-06120 Halle, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: +49 345 5527 346;
antiferroelectric switching smectic phases, usually observed for
non-fluorinated bent-core molecules were replaced by ferro-
electric switching smectic and columnar phases if
F-substituents were introduced in a peripheral position at the
bent aromatic core, adjacent to the terminal alkyloxy
chains.18,19 Also so-called dark conglomerate phases, char-
acterized by the spontaneous breaking of achiral symmetry
and formation of macroscopic regions with uniform chirality
sense,16,18e and different types of columnar phases with
ferroelectric and antiferroelectric switching behaviour were
Tel: +49 345 5525 664
bInstitute of Chemistry, Physical Chemistry, Martin-Luther-University
Halle-Wittenberg, Mu¨hlpforte 1, D-06108 Halle, Germany
{ Electronic supplementary information (ESI) available: Table with
X-ray diffraction data; additional figures showing typical textures and
X-ray diffraction patterns; scheme explaining the distinct subtypes of
polar smectic phases; calculations of the number of molecules per unit
cell; experimental procedures for the synthesis of compounds 1–3 and
the intermediates together with the analytical data. See DOI: 10.1039/
b705499h
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observed for some representatives. F-substitution at the
central bent unit also has a distinct effect on the mesophases
as it can, for example, induce polarization modulated smectic
phases.18l
A main effect of the replacement of hydrogen by fluorine is a
reduction of the electron density of the conjugated p-systems,
which has a large impact on the molecular self-assembly. Such
electron deficient bent-core molecules sometimes form special
textures characterized by helical filaments and spirals, devel-
oping during the growing process of the LC phase from the
disordered liquid state.19,20
In recent papers we have introduced the 3,49-disubstituted
biphenyl unit as a new bent central unit for construction of the
aromatic cores of bent-core molecules21 and conducted studies
on fluorinated bent-core molecules based on these biphenyl
cores.22
Herein we report a new class of core-fluorinated bent-core
LC materials incorporating an 4,40-disubstituted 1,19;3910-
terphenyl (m-terphenyl) core (see compounds 1–3).23 The
influence of fluorine substitution at the central terphenyl unit
(substituents a–c) as well as at the periphery (substituent X) on
the properties was investigated.
Generally, the LC phases are very similar to those of related
molecules with a biphenyl core, i.e. SmCPA phases and Colr
phases are dominating in most cases.22 The major difference is
provided by the strongly increased transition temperatures due
to the additional benzene ring. The most interesting observa-
tions were made for some smectic phases which show very
unusual optical textures. One of them is characterized by a
hexagonal dot pattern, the other one is characterized by helical
filaments, which coexist with chiral dark conglomerate type
textures. Interestingly, there is a correlation between helix
sense and chirality sense of these optically isotropic domains.
The observations are important for the discussion of the origin
of spontaneous achiral symmetry breaking in the mesophases
of bent-core mesogens in a more general sense.
Scheme 1 Synthesis of 1a (R = C12H25, X = H), 2a (R = OC12H25
,
X = H), and 3a (R = OC12H25, X = F). Reagents and conditions: (i)
Pd(PPh3)4, NaHCO3, glyme, reflux, 8h, (ii) 1. PMDTA, THF; 2.
n-BuLi, 278 uC; 3. B(OCH3)3, 278 uC, 4. H+, H2O, 0 uC, (iii) 1.
4-Methoxybromobenzene, Pd(PPh3)4, NaHCO3, glyme, reflux, 8 h; 2.
BBr3, CH2Cl2, 0 uC; 3. H2O, 0 uC, (iv) CMC, DMAP, CH2Cl2, 25 uC,
24 h; PMDTA
= N,N,N9,N0,N0-pentamethyldiethylene triamine,
CMC = N-cyclohexyl-N9-[2-(N0-methylmorpholino)ethyl]carbodiimide
4-toluene sulfonate, DMAP = 4-(N,N-dimethylamino)pyridine.
The observed mesophase types, the transition temperatures
and corresponding enthalpy values of the fluorinated com-
pounds 1–3 together with the related non-fluorinated m-ter-
phenyl derivatives 1H and 2H,21c which have been synthesized
previously21b are summarized in Table 1. For comparison, also
2. Results and discussion
2.1 Synthesis
The synthetic pathways to the target compounds 1–3 are
described in Scheme 1 and 2. The key-steps of the multi-step
syntheses were ortho-metallation reactions24 and Suzuki cross-
coupling reactions25 to build up the F-substituted m-terphenyl-
4,40-diols used for the DMAP catalyzed carbodiimide
mediated esterification reactions26 with fluorinated or non-
fluorinated 4-[4-dodecyl(oxy)benzoyloxy]benzoic acids.21,22
The crude products were purified by chromatography (silica
gel, eluent: CHCl3) followed by crystallisation from ethyl
acetate.
2.2 Mesomorphic properties—overview
The mesomorphic properties of the discussed compounds
were investigated by polarized light microscopy on a hot
stage (Mettler FP 82 HAT), differential scanning calorimetry
(DSC-7, Perkin-Elmer), X-ray-diffraction on powder-like and
partially surface-aligned samples as well as by electrooptical
experiments using a home built electrooptical setup.
Scheme 2 Synthesis of 1b,c (R = C12H25, X = H), 2b,c (R = OC12H25
,
X = H), and 3b (R = OC12H25, X = F). Reagents and conditions: (i)
4-methoxybenzene boronic acid (2 eq.), Pd(PPh3)4, NaHCO3, glyme,
reflux, 8h, (ii) BBr3, CH2Cl2, 0 uC, 3. H2O, 0 uC, (iii) CMC, DMAP,
CH2Cl2, 25 uC, 24 h.
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Table 1 Mesophases, phase transition temperatures and transition
enthalpies (values in square brackets) of the m-terphenyl derived
compounds 1–3a
Table 2 Mesophasesa of biphenyl derived bent-core molecules 4, 5c,a
and 6.b
Compd
X
a
b
R
T/uC
Compd X a
b
c
R
T/uC [DH/kJ mol21
]
4H
4a
4b
5H
5a
5b
6H
6a
6b
a
H
H
H
H
H
H
F
H
F
H
H
F
H
H
F
H
H
F
H
H
F
H
H
F
C12H25
C12H25
C12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
Cr 78 SmCPA 156 Iso
Cr 95 SmCPA 148 Iso
Cr 99 SmCPA 135 Iso
Cr 106 SmCPA 159 Iso
Cr 120 Colr 158 Iso
1Hb
1a
1b
1c
H H H H C12H25 Cr 169 SmCPA 203 Iso
[44.5] [24.5]
H F H H C12H25 Cr 143 SmCPA 196 Iso
[30.1] [22.8]
H H F H C12H25 Cr 153 SmCPA 199 Iso
Cr 110 Colr 136 Iso
Cr 101 SmCPA 170 Iso
Cr 120 ColobPFE 165 Iso
Cr 114 SmCPA 152 Iso
[38.7]
H H H F C12H25 Cr 192 Iso
[26.5]
F
F
H
[69.9]
H H H H OC12H25 Cr 165 Colr
[40.8]
H F H H OC12H25 Cr 130 Colr
[19.6]
H H F H OC12H25 Cr 127 Colr
[23.2]
H H H F OC12H25 Cr 181 Colr
[40.9]
F H H H OC12H25 Cr 169 SmCPA 186 MPA 220 Iso
2Hb
2a
2b
2c
203 Iso
[21.8]
199 Iso
[20.2]
197 Iso
[24.1]
191 Iso
[22.0]
Abbreviations as in Table 1,
which is in the usual range found for polar smectic phases [B2
phases, see Fig. 1(a)] of bent-core mesogens. Only for
compound 1c could no liquid crystalline properties be
observed.
3H
3a
3b
a
Replacement of the alkyl chains by alkoxy chains with the
same number of C-atoms (compound 2, see Table 1) leads to
the formation of rectangular columnar phases [Colr, often also
assigned as B1 phases, see Fig. 1(b)] characterized by very
typical textures (see Fig. S3{). The X-ray diffraction pattern of
a powder-like sample of compound 2a, as a representative
example, shows two reflections which can be indexed as 11 and
02, respectively according to a centred rectangular 2D lattice
with the parameters a = 4.7 nm, b = 5.4 nm (T = 180 uC, see
Table S1{). The 2D pattern of a partially surface-aligned
sample in the small-angle region is in line with this indexing
(Fig. S4{). A number, n, around 7–8 molecules, can be
calculated for a 3D unit with the cross section of the centred
2D unit cell and a height of the average stacking distance of
the bent-core molecules normal to this plane of about
0.52 nm27 (see Table S2{). This number and the lattice
parameters correspond to those of other B1-type mesophases
and confirm the phase assignment.13 As is typical for these
[38.9]
F F H H OC12H25 Cr 144 Colr9
[27.4]
[—]
219 Iso
[18.5]
[24.7]
F H F H OC12H25 Cr 151 SmCPA[*] 210 Iso
[30.2]
[25.5]
Values from 2nd DSC heating scans (peak temperatures), heating
rate: 10 K min21; abbreviations: Cr = crystalline solid; Colr = non-
switchable centered rectangular columnar phase (B1 phase); Colr9 =
non-switchable non-centered rectangular columnar phase which is
most probably an undulated smectic phase; SmCPA
=
antiferroelectric switching polar SmC phase (B2 phase); SmCPA[*] =
antiferroelectric switching mesophase characterized by the
coexistence of birefringent textures and optically isotropic textures
with chiral domains (dark conglomerate texture) as well as helical
filaments;
MPA
=
antiferroelectric
switching
characterized by the formation of helical filaments in addition to
mesophase
b
textural features of columnar phases; Iso = isotropic liquid. See
ref. 21c.
the related biphenyl derived bent-core molecules 4,521a,c,22a
and 6,22b are collated in Table 2.
2.3 LC phases of the core-fluorinated terphenyl derivatives 1 and
2
The influence of F-substitution on mesophase behaviour is
minor if the fluoro substituent is located at the central
m-terphenyl core. For example, the alkyl substituted com-
pounds 1a and 1b (Table 1), fluorinated only at the
m-terphenyl core, show antiferroeletric switching polar smectic
phases (SmCPA) just as was found for the non-fluorinated
analogue 1H.21c These mesophases are characterized by a
typical birefringent texture (see Fig. S2 in the ESI{) and the
occurrence of two current response peaks in a half period of
the applied triangular wave voltage. The corresponding
polarization values (Ps) are in the range 500–1000 nC cm22
Fig. 1 Structures of the polar smectic B2 phase (SmCPA) and the
non-switchable rectangular columnar phase (B1) formed by bent-core
molecules, in (a) the view is perpendicular to the polar direction and
the molecules are additionally tilted, in (b) the view is along the polar
axis and the molecules are not tilted.
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conglomerate textures (SmCPA *] phase) which were observed
[
mesophases no polar switching can be observed up to a peak-
to-peak voltage of 400 Vpp, the maximum voltage attainable
with our experimental setup. For the non-fluorinated analogue
2H, reported in an earlier paper,21c a columnar rectangular B1-
type mesophase was found as well. This leads to the
assumption that the phase structure is dominated by the
nature of the side chain (alkyl vs. alkoxy) in these cases. In
comparison to the biphenyl analogues shown in Table 2,21c,22a,b
the terphenyl moiety enhances the transition temperatures
significantly (by about 50 K), but the effect on the mesophase
type seems to be minor. Replacing the biphenyl core by a
terphenyl core slightly increases the tendency for formation of
B1 phases. Usually, B1 phases are replaced by SmCPA phases
on elongation of the terminal alkyl chains and this transition
depends on the ratio of core size to chain length.21c For the
terphenyl derivatives with a larger rigid aromatic core longer
alkyl chains are required for this transition than for the
biphenyl derivatives having a shorter aromatic core. This
explains that for the alkoxy substituted terphenyl derivative
2H the SmCPA phase of the biphenyl derivative 5H is replaced
by a Colr phase.
for compounds 3H and 3b, respectively.14,19,20,29
2.4 Smectic and modulated/undulated smectic phases of the
m-terphenyl derivatives 3 with peripheral F-substitution
In the series of compounds 3 with F-substituents at the
periphery of the bent aromatic core, other interesting phases
were observed. Fluoro substitution at the outer phenyl rings of
the side wings of the bent-core molecules influences the
mesophase structure significantly. For the mesophases of 3H
and 3b we found antiferroelectric switching behaviour
determined by two well separated peaks in the current response
to a sufficiently high triangular wave field (threshold ca.
120 Vpp for a 6 mm cell gap). The corresponding polarization is
about Ps = 700 nC cm22 for 3H and Ps = 1100 nC cm22 for 3b.
For the mesophase of both compounds special textural
features could be observed. On cooling compound 3H from the
isotropic liquid, helical filaments and highly birefringent
spherulitic textures grew [see Fig. 3(a)]. The appearance of
helical filaments is very similar to B7 phases,20a whereas the
spherulitic texture is typical for a columnar phase. At T =
186 uC the texture becomes more smooth and less specific [see
Fig. S6(a,b) in the ESI{], but no peak can be detected in the
Some unusual textural features were found for the smectic
phases of compounds 1a and 1b. On slow cooling (1 K min21
)
for both the compounds a very unusual texture, as shown in
Fig. 2, was observed. This texture is formed in regions with
homeotropic alignment (smectic layers are more or less parallel
to the substrate surfaces) and it is characterized by a regular
pattern of spots with an apparently local hexagonal order. It
seems that this pattern is due to a layer distortion where the
flat layers change their direction from parallel to the surfaces
to perpendicular to the surfaces. This pattern is more regular
for compound 1a than for 1b (see Fig. S5{). The reason for the
appearance of this pattern is not clear, but it could be due to a
layer instability which gives rise to a spontaneous curvature of
the layers. These layer instabilities are known from lyotropic
systems28 and could have steric reasons (different areas
required by the aromatic cores and the aliphatic chains at
the aromatic–aliphatic interfaces) or it might arise due to an
escape from polar order within these layers.12a Probably, this
pattern can be regarded as a kind of intermediate state at the
transition from non-distorted smectic phases to smectic phases
with spiral textures (MPA phase of compound 3H) and dark
Fig. 3 Compound 3H: (a) texture (crossed polarizers) at T = 219 uC
(MPA phase), showing a helical filament and spherulites, dark areas
are residues of the isotropic liquid; (b) 2D X-ray diffraction pattern of
a partially surface-aligned sample at 170 uC on heating (SmCPA
phase).
Fig. 2 Texture of compound 1a at 190 uC.
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DSC curves at this temperature. After application of a dc
electric field (.1.5 Vmm21) over the whole investigated
temperature range (T = 165–205 uC) the birefringent textures
were replaced by an optically isotropic texture composed
of domains with opposite handedness, as typically seen for
dark conglomerate smectic phases [see Fig. S6(c,d) in the
ESI{].30
X-Ray investigations indicate a simple layer structure at T
between 170 and 180 uC (d = 4.7 nm) without indications of a
layer undulation or layer modulation. The d-value is in
agreement with a single layer structure formed by a tilted
arrangement of the bent-core molecules. From the position of
the maxima of the outer diffuse scattering in the 2D X-ray
pattern of a partially surface-aligned sample the tilt angle t can
be estimated to 32u [see Fig. 3(b) and S7{]. The effective
molecular length is Leff = 5.5 nm, calculated according to
Leff = d/cost from these values. It is in very good agreement
with the effective molecular length Leff = 5.4 nm derived for 2a
in the Colr (B1-type) phase, which is assumed to be equal to
the length of the b-axis.31 As our experimental setup only
allows X-ray measurements up to a maximum temperature of
180 uC the mesophase occurring above 186 uC could not be
investigated. It seems that the mesophase of compound 3H,
occurring at a high temperature (assigned as MPA) represents
an undulated or modulated smectic phase and that these layer
undulations/modulations are removed at a lower temperature.
Also the fact that under a relatively weak electric field these
modulations/undulations can be removed and a smectic phase
is induced is in line with a relatively weak layer undulating/
modulating force within this mesophase.
Helical filaments could also be observed for compound 3b
with fluoro substituents at the outer rings as well as at the
inner position b. X-Ray scattering of the mesophase of 3b
indicates a simple smectic phase without in-plane order
(diffuse wide angle scattering) and a layer distance of d =
4.7 nm, indicating a tilted arrangement of the molecules with
apparently the same tilt angle of about 30u as found for 3H.
This compound does not only show the texture with helical
filaments but also a variety of different textural features
known from smectic phases of other bent-core mesogens.
Birefringent spherulitic and lancet-like patterns as well as leaf-
like textures were found next to optically isotropic regions
composed of domains with opposite handedness (dark
conglomerate type) and helical filaments and germs. The
predominating type of texture is independent of the substrate
(simple glass plates, ITO coated and polyimide coated cells
give the same textures), but it strongly depends on the cooling
rates. At relatively high cooling rates (10 K min21) the
birefringent textures dominate [see Fig. 4(a)], whereas at very
low cooling rates (,1 K min21) only the dark conglomerate
texture is observed [see Fig. 4(d,e)]. Intermediate cooling rates
(around 1 K min21) provide helical filaments coexisting with
optically active dark textures, as shown in Fig. 4(b,c). The
coexistence of dark conglomerate textures with helical
filaments was recently also reported for a resorcinol derivative
with Schiff’s base units as connections and fluoro substituents
at the outer wings.18k However, it is not completely clear from
this reference if there is any relation between helix sense and
chirality of the dark domains.
[
]
Fig. 4 Textures of the SmCPA
* phase of compound 3b: (a)
birefringent texture at 206 uC after cooling at a rate of 10 K min21
(crossed polarizers); the encircled circular domain indicates a synclinic
tilted organization in this mesophase; (b, c) at 209 uC after cooling at a
rate of 2 K min21 (slightly decrossed polarizers): coexistence of chiral
domains and helical filaments, the direction of the polarizers is
indicated by the arrows; dark areas which do not change the brightness
by changing the position of the polarizers are residues of the isotropic
liquid state; (d,e) dark conglomerate texture at 205 uC after cooling at
a rate of 0.5 K min21, slightly decrossed polarizers with opposite
direction of decrossing in (d) and (e).
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In the case of compound 3b more detailed information was
gained. Upon slow cooling the helices grow simultaneously
with the grainy optically isotropic texture. Upon slightly
decrossing the polarizers some of these domains appear dark
whereas others became bright. Changing the direction of the
analyzer reverses all bright and dark domains, indicating the
chirality of these domains. As seen in Fig. 4(b,c), the chirality
sense of the dark conglomerate texture is uniform around each
of the helices, whereas it can be opposite for different helices. It
seems that the chirality sense of the dark conglomerate
domains is correlated with the chirality sense of the helices,
i.e. helices of one helix sense are surrounded by dark
conglomerate domains of one handedness, whereas helices
with opposite chirality are surrounded by domains with
opposite handedness. There is no reversal of the chirality
sense of the dark texture along straight helix strands, even after
crossing a helix with opposite helicity (see Fig. S8{). Only at
kinks where the direction of the helix suddenly reverses
direction, can a reversal of the chirality of the isotropic LC
domains be observed [see Fig. 4(b,c) at the left]. It appears that
the helical filaments change their direction during the growing
process if they collide with a previously formed optically
isotropic LC domain. In this way helical zig zag type filaments
are formed in addition to the linear strands.
structure of the dark chiral domains. However, the phase
structure in the birefringent areas could possibly be different
from that in the dark conglomerate texture and hence the
situation is not completely clear. In any case, helix formation
itself does not require any chirality,17 but it is likely that the
helix sense is initially determined by a slight local fluctuation
of the distribution of chiral (helical) molecular conformations
in the isotropic liquid.33 The chirality once developed is
maintained during the growing of the helices. These helical
filaments determine the chirality sense also at their periphery,
where the optically isotropic mesophase develops.
Compound 3a, where the fluoro substituent at the m-ter-
phenyl core is shifted into the bay position a, behaves
completely different from 3H and 3b as no smectic phase is
found for this compound. The liquid crystalline phase of 3a is
characterized by a well developed highly birefringent spheru-
litic texture, as shown in Fig. 5(a). The powder-like X-ray
diffraction pattern of the mesophase of this compound shows
five reflections in the small-angle region, which are not
multiple-order reflections of each other and confirm
a
mesophase with a 2D lattice [see Fig. 5(b) and Table S1 in
the ESI{). The diffuse scattering around d = 0.47 nm indicates
a fluid liquid crystalline phase. A unique determination of
It seems that there is a layer distorting effect of steric or
polar nature which destabilizes flat layers. This layer
destabilization leads to the formation of helical filaments
upon slow growing from the isotropic liquid as long as helix
formation is not disturbed, i.e. if the helical filaments are
surrounded by the isotropic liquid. A dendritic growth, leading
to the optically isotropic chiral domains, takes place simulta-
neously. There is a stochastic distribution of the helix sense of
the helical filaments and the chirality sense of the isotropic
mesophase in this growth process. However, there is obviously
no branching of the helical filaments and further growth of the
mesophase takes place by formation of strongly disordered
and optically isotropic areas, starting from the surfaces of
these helices. The helical filaments determine the chirality sense
at their periphery, where the optically isotropic mesophase
develops. At the end of the growing process space is divided
into areas with uniform positive or negative chirality sense.
Optical activity of these isotropic areas could arise if there
would be a non-regular helix growth, leading to a strongly
distorted structure composed of randomly disordered helix-
fragments with small size and uniform helicity.16e However, it
is not necessary that helical fragments are retained in the
optically isotropic regions. The chirality in these regions could
also arise from the layer chirality, determined by the
combination of tilt-direction and polar direction.14,32 In this
case the phase structure has to be homogeneously chiral, either
SmCsPF or SmCaPA (see Fig. S1{), whereas for a disordered
organization of helical fragments there would be no strict
restriction to a special phase structure. As an antiferroelectric
switching is observed and the dark extinction crosses are
inclined with the directions of the polarizer and analyzer in the
birefringent texture [see Fig. 4(a), domain in the cycle] an
antiferroelectric and synclinic SmCsPA structure seems to be
most likely for compound 3b. This phase structure is racemic
(see Fig. S1{) and this would point to a disordered helix
Fig. 5 Investigation of compound 3a: (a) texture at 190 uC; (b) theta-
scan (h-values are used to assign the reflections, for d-values see
Table S1{) with the maximum of the outer diffuse scattering at 2h =
˚
19.0u, d = 4.7 A; the additional scattering around 2h = 6.9u is assigned
to the 24, 34 and 44 reflections (see Table S1{).
3424 | J. Mater. Chem., 2007, 17, 3419–3426
This journal is ß The Royal Society of Chemistry 2007

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2 P. Kirsch and M. Bremer, Angew. Chem., Int. Ed., 2000, 39, 4130.
3 M. O’Neill and S. M. Kelly, Adv. Mater., 2003, 15, 1135.
4 M. Kastler, W. Pisula, F. Laquai, A. Kumar, R. J. Davies,
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5 H. Finkelmann, S. T. Kim, A. Muæoz, P. Palffy-Muhoray and
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61, 334.
7 M. J. O’Callaghan, M. D. Wand, C. M. Walker and M. Nakata,
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597.
13 Examples of mesophases with 2D lattices formed by bent-core
molecules: (a) T. Sekine, T. Niori, M. Sone, J. Watanabe,
S. W. Choi, Y. Takanishi and H. Takezoe, Jpn. J. Appl. Phys.,
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W. Weissflog, U. Baumeister and S. Diele, Liq. Cryst., 2003, 30,
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Chem. Mater., 2004, 16, 4050; (g) J. Mieczkowski, K. Gomola,
J. Koseska, D. Pociecha, J. Szydlowska and E. Gorecka, J. Mater.
Chem., 2003, 13, 2132; (h) S. Kang, J. Thisayukta, H. Takezoe,
J. Watanabe, K. Ogino, T. Doi and T. Takahashi, Liq. Cryst.,
2004, 31, 1323; (i) E. Gorecka, D. Pociecha, J. Mieczkowski,
J. Matraszek, D. Guillon and B. Donnio, J. Am. Chem. Soc., 2004,
126, 15946; (j) C. Keith, R. A. Reddy, U. Baumeister and
C. Tschierske, J. Am. Chem. Soc., 2004, 126, 14312; (k)
E. Gorecka, N. Vaupotic, D. Pociecha, M. Cepic and
J. Mieczkowski, ChemPhysChem, 2005, 6, 1087; (l) V. Kozmik,
M. Kuchar, J. Swoboda, V. Novotna, M. Glogarova,
U. Baumeister, S. Diele and G. Pelzl, Liq. Cryst., 2005, 32, 1151;
(m) C. L. Folcia, I. Alonso, J. Ortega, J. Etxebarria, I. Pintre and
M. B. Ros, Chem. Mater., 2006, 18, 4617; (n) H. Takezoe,
K. Kishikawab and E. Gorecka, J. Mater. Chem., 2006, 16, 2412;
(o) V. Kozmik, A. Kovarova, M. Kuchar, J. Swoboda, V. Novotna,
M. Glogarova and J. Kroupa, Liq. Cryst., 2006, 33, 41; (p)
M. G. Tamba, B. Kosata, K. Pelz, S. Diele, G. Pelzl,
Z. Vakhovskaya, H. Kresse and W. Weissflog, Soft Matter,
2006, 2, 60; (q) Y. Takanishi, H. Takezoe, J. Watanabe,
Y. Takahashib and A. Iida, J. Mater. Chem., 2006, 16, 816; (r)
C. L. Folcia, J. Etxebarria, J. Ortega and M. B. Ros, Phys. Rev. E,
2006, 74, 031702; (s) J. Matraszek, J. Mieczkowski, D. Pociecha,
E. Gorecka, B. Donnio and D. Guillon, Chem.–Eur. J., 2007, 13,
3377; (t) F. Vera, R. M. Tejedor, P. Romero, J. Barbera, M. B. Ros,
J. L. Serrano and T. Sierra, Angew. Chem., Int. Ed., 2007, 46, 1873;
(u) B. Mettout, Phys. Rev. E, 2007, 75, 011706; ref. 18b; ref. 21c.
14 (a) L. E. Hough and N. A. Clark, Phys. Rev. Lett., 2005, 95,
107802; (b) L. E. Hough, C. Zhu, M. Nakata, N. Chattham,
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indices for the reflections was not possible in this case, because
only powder-like and poorly aligned samples were obtained,
but a tentative indexing of the small-angle reflections for a
partially surface-aligned sample points to a non-centred
rectangular 2D lattice with parameters a = 12 nm, b = 5.3 nm
(Colr9 phase).34 The parameter b is close to that in the B1-type
phase of 2a, that means the effective molecular length would
be approximately the same for an arrangement of the
molecules parallel to b. This would be in line with an
undulated smectic phase where the molecules adopt a non-
tilted organization. The non-tilted organization is in line with
the position of the extinction brushes in the textures, which
under all conditions are parallel to the polarizer and analyzer
[see Fig. 5(b)]. The periodicity a is quite large and about
19 molecules would be organized along an undulation period
(see Table S2 in the ESI{).13
No switching can be observed up to a high voltage of
400 Vpp. Hence 3a is not only distinct from the other
compounds of series 3 which predominately show polar
smectic phases, it is also distinct from the related biphenyl
derivative 6a with the same fluorination motif (see Table 2),
which shows synclinic tilt and ferroelectric switching.22b
3. Conclusions
Herein we reported a new class of fluorinated bent-core LC
materials incorporating a m-terphenyl core. The influence of
fluorine substitution at the m-terphenyl core, as well as at the
periphery, on the properties was investigated. There are some
similarities to related molecules with a biphenyl core, i.e.
SmCPA phases and Colr phases are dominant for the molecules
fluorinated only at the centre of the bent unit. A major
difference to related biphenyl derivatives is provided by the
strongly increased transition temperatures due to the addi-
tional benzene ring and an enhanced tendency for layer
distortion and formation of rectangular columnar phases. The
most interesting observations were made for the smectic phases
of the alkyl substituted compounds 1a and 1b which show a
regular dot pattern in the textures, and for 3H and 3b, where
helical superstructures were observed. These textural features
can be explained by layer deformation on a macroscopic
length scale. Interestingly, in the textures of 3b the helical
superstructures are accompanied by an optically isotropic
mesophase and there is a relation between the chirality of the
macroscopic helical filaments and the bulk mesophase (dark
conglomerate texture). It seems that the layer destabilizing
forces increase from compound 1H and 1c (typical SmCPA
phases) via compounds 1a and 1b (array of dots in the textures)
to 3H and 3b (helical filaments and dark conglomerate
textures). Nevertheless there are still numerous open questions
and the complex phase behaviour of bent-core molecules is not
yet understood in depth.12 However, the empirical observa-
tions reported herein could contribute to improvement of
theoretical concepts of polar and chiral self-assembly in
smectic phases formed by achiral molecules.
15 C. Keith, R. A. Reddy, A. Hauser, U. Baumeister and
C. Tschierske, J. Am. Chem. Soc., 2006, 126, 3051.
16 (a) J. Thisayukta, Y. Nakayama, S. Kawauchi, H. Takezoe and
J. Watanabe, J. Am. Chem. Soc., 2000, 122, 7441; (b) J. Thisayukta,
H. Kamee, S. Kawauchi and J. Watanabe, Mol. Cryst. Liq. Cryst.,
2000, 346, 63; (c) G. Dantlgraber, A. Eremin, S. Diele, A. Hauser,
H. Kresse, G. Pelzl and C. Tschierske, Angew. Chem., Int. Ed.,
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Cryst., 2002, 29, 1223; (e) J. Ortega, C. L. Folcia, J. Etxebarria,
N. Gimeno and M. B. Ros, Phys. Rev. E, 2003, 68, 011707; (f)
A. Jakli, Y.-M. Huang, K. Fodor-Csorba, A. Vajda, G. Galli,
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3426 | J. Mater. Chem., 2007, 17, 3419–3426
This journal is ß The Royal Society of Chemistry 2007