Bent-core compounds with two branched chains: Evidence of a new dark conglomerate mesophase

Article abstract of DOI:10.1039/c3sm27771b

Biphenyl-3,4′-diol derived bent-core compounds with branched side chains have been synthesized and characterized to study the influence of terminal branched groups on mesophase properties and transition temperatures of bent-core mesogens. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and X-ray scattering. Depending on the length of the chain, the size of molecular branches and the position of branched chains, either enantiotropic or monotropic rectangular columnar phases were observed. For one of the compounds combining two different branched side chains, a new type of optically isotropic dark conglomerate phase (M_Iso^[*]) was found. It is proposed to be a distorted version of a non-tilted or only slightly tilted B₆-like intercalated smectic phase with additional in plane order.

Full text of DOI:10.1039/c3sm27771b

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Volume 9 | Number 18 | 14 May 2013 | Pages 4555–4726
ISSN 1744-683X
PAPER
Hale Ocak, Carsten Tschierske et al.
Bent-core compounds with two branched chains: evidence of a new dark
conglomerate mesophase
1744-683X(2013)9:18;1-3

Soft Matter
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Bent-core compounds with two branched chains:
evidence of a new dark conglomerate mesophase†
Cite this: Soft Matter, 2013, 9, 4590
a
c
Hale Ocak, Belkız Bilgin-Eran,a Marko Prehmbc and Carsten Tschierske
*
*
Biphenyl-3,40-diol derived bent-core compounds with branched side chains have been synthesized and
characterized to study the influence of terminal branched groups on mesophase properties and
transition temperatures of bent-core mesogens. The liquid crystalline properties of the compounds were
investigated by differential scanning calorimetry, optical polarizing microscopy and X-ray scattering.
Depending on the length of the chain, the size of molecular branches and the position of branched
chains, either enantiotropic or monotropic rectangular columnar phases were observed. For one of the
compounds combining two different branched side chains, a new type of optically isotropic dark
conglomerate phase (MIso[*]) was found. It is proposed to be a distorted version of a non-tilted or only
slightly tilted B6-like intercalated smectic phase with additional in plane order.
Received 1st December 2012
Accepted 26th February 2013
DOI: 10.1039/c3sm27771b
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have identical or different branched chains at the terminals of
the rigid core. Three distinct types of chains were combined:
1 Introduction
(S)-2-methylbutyl-chains20 and racemic 2-ethyhexyl- or 3,7-
dimethyloctyl-chains,12,21 leading to optically active and inac-
tive bent-core mesogens, respectively. All compounds form
B1Rev type rectangular columnar phases (Colrec), independent
of the substitution pattern. Only one compound, combining a
2-ethyhexyl chain with a 3,7-dimethyloctyl chain, shows an
additional mesophase which is of special interest, as it forms a
new type of the so-called dark conglomerate phase composed
of macroscopic domains with opposite chirality.22–26 It is sug-
gested that this phase is a non-tilted or only slightly tilted
smectic phase with a B6-like intercalated structure27 and layer
distortion arising from the increased packing density of the
aromatic cores at the onset of in-plane order. This phase is
chiral if formed under an electric eld and obviously repre-
sents a new optically isotropic LC phase formed by bent-core
mesogens, besides the previously reported chiral B4[*]-type
Bent-core liquid crystals (LCs) represent a new sub-class of
thermotropic LCs with unusual physical properties when
compared with calamitic or discotic mesogens.1 A series of new
LC phases has been discovered with these bent-core mesogens,
among them unique LC phases showing polar order and
spontaneous achiral symmetry breaking.2–6 These materials are
also of signicant interest for applications, for example as NLO
active materials.7 However, the precise relationships between
the molecular structures and mesomorphism are far from being
completely understood for these types of mesogens.
Most known bent-core mesogens have linear terminal alkyl
chains.3,4,6 Those with branched chains are rare and in most
cases chain branching was used to introduce chirality into bent
core mesogens.8–12 Interestingly, for some of these compounds,
a coupling between molecular chirality and layer chirality was
observed.13,14 In addition, steric interlayer interactions can lead
[*]
helical lament phases,1,22,23 the smectic SmCPF[*], SmCPA
to
a change in phase structure and switching process
depending on the positions of chiral centers in the chains.15–18
In most cases only one or two identical branched chains were
used.
dark conglomerate type phases20a,25,26,28,29,30 and the achiral
cubic phases.31,32
Herein, we report new bent-core compounds with
a
biphenyl-3,40-diol based bent unit19 as the central core which
2 Results and discussion
2.1 Synthesis
aDepartment of Chemistry, Yildiz Technical University, Davutpaxsa Yerlexsim Birimi,
Esenler, Istanbul, TR-34220, Turkey. E-mail: ocak_hale@hotmail.com
bInstitute of Chemistry, Physical Chemistry, Martin Luther University Halle-
Wittenberg, Von-Danckelmann-Platz 4, D-06120 Halle, Germany
cInstitute 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
The bent-core compounds 1–9 were synthesized as shown in
Scheme 1. The synthesis starts with 40-benzyloxybiphenyl-
3-ol.28,33–35 4-(4-Alkyloxybenzoyloxy)benzoic acids AI–AIII with
branched alkyl chains were attached to the non-protected OH
group by esterication using dicyclohexylcarbodiimide (DCC)
and 4-dimethylaminopyridine (DMAP) as catalysts36 to yield
the benzylated intermediates BI–BIII.31,37 The required 4-(4-alkyl-
oxybenzoyloxy)benzoic acids AI–AIII were obtained by oxidation
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3sm27771b
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Scheme 1 Synthesis of bent-core compounds with branched side chains. Reagents and conditions: (a) cat. [Pd(PPh3)4], NaHCO3, H2O, glyme, reflux 9 h;28,33–35 (b)
A,13,14,39,40 DCC, DMAP, CH2Cl2, r.t.;36 (c) H2, Pd/C, THF, 40 ꢁC, 18 h;28 (d) A, DCC, DMAP, CH2Cl2, r.t.36
of appropriate 4-(4-alkyloxybenzoyloxy)benzaldehydes using
sodium chlorite as the oxidizing agent. These aldehydes
were prepared from the corresponding branched alcohols
(S)-2-methylbutanol (I), 2-ethylhexanol (II) and 3,7-dimethyl-
octanol (III),38 respectively, which were tosylated rst; these
tosylates were then used for etherication with ethyl
4-hydroxybenzoate and the obtained 4-alkyloxybenzoates were
hydrolyzed to give the branched 4-alkyloxybenzoic acids. Ester-
ication with 4-hydroxybenzaldehyde yielded the 4-(4-alkyloxy-
benzoyloxy)benzaldehydes.13,14,39,40 Aer catalytic hydrogenation
of compounds BI–BIII14,28 and acylation with one of the different
4-(4-alkyloxybenzoyloxy)benzoic acids AI–AIII,36 the bent-core
compounds 1–9 were obtained. All compounds with one (S)-2-
methylbutyl group represent mixtures of homogeneously chiral
diastereomers, those with two racemic chains are racemic
mixtures of two diastereomers and only compound (S,S)-7 with
two (S)-2-methylbutyl group is a pure enantiomer. Though
there is the possibility that in the mixtures of diastereomers one
diastereomer could be enriched during the purication
process, the properties of the diastereomers seem to be very
similar, as in TLC only one spot is found; we have no indication
of a signicant enrichment of one diastereomer. It is likely that
the fact that most compounds represent mixtures of diastereo-
mers contributes to the lowering of the melting points and
crystallization tendency, which is favorable for the observation
of the LC properties. Also the LC transition temperature of the
individual diastereomers could be slightly different from that of
the approximately 1 : 1 mixtures investigated here.
2.2 Liquid crystalline properties
The mesomorphic properties of the obtained compounds 1–9
were investigated by polarized light optical microscopy (PM),
differential scanning calorimetry (DSC), electro-optical studies
and some of them by X-ray diffraction (XRD). The transition
temperatures, corresponding enthalpy values and mesophase
type observed for these compounds are summarized in Table 1.
2.2.1 Colrec-phases of compounds 1–6. All bent-core
compounds 1–9, containing two branched chains, exhibit
enantiotropic or monotropic Colrec phases as indicated by the
typical textures observed by polarizing microscopy and
conrmed by XRD experiments. Fig. 1 shows the texture and
XRD pattern for a representative example. In the XRD patterns,
beside the diffuse scattering in the wide angle region at d ¼
0.46–0.47 nm, which conrms the uid nature of the phase,
there are two sharp reexes in the small angle region, which can
be indexed to the 11 and 02 reexes of a body centered rect-
angular lattice (c2mm). The lattice parameters a ¼ 3.3–3.9 nm
and b ¼ 4.3–4.9 nm are very similar for the investigated
compounds (see Table 2). Fig. 1b shows the proposed model of
molecular organization in this Colrec phase. Accordingly, the
molecules are organized in ribbons with antipolar organization.
In line with recent investigations it is assumed that the polar
direction is perpendicular to the 2D lattice (B1Rev type
phases).41,42
The parameters b are related to the molecular lengths (b ꢀ L,
see Table 2) and the d/L ratios are about 0.88–0.94. The
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Table 1 Phase transition temperatures and transition enthalpies of the bent-core compounds 1–9a
Comp.
R1
R2
T/ꢁC [DH kJ molꢂ1
]
(S)-1
(S)-2
(S)-3
4
(S)-5
6
I
I
II
II
III
III
II
III
I
III
I
Cr 122 [46.1] (Colrec 120 [10.5]) Iso
Cr 122 [23.0] Colrec 134 [14.9] Iso
Cr 126 [26.5] (Colrec 97 [6.6]) Iso
Cr 118 [38.3] (Colrec 96 [12.7]) Iso
Cr 129 [17.6] Colrec 141 [12.4] Iso
[*]
II
Heating: MIso 110 [16.2] Iso
[*]
Cooling: Iso 101 [7.3] Colrec 85–65 [8.9] MIso
(S,S)-7
8
9
I
II
III
I
II
III
Cr1 127 [19.9] Cr2 157 [34.1] (Colrec 148 [8.6]) Iso
Cr 101 [22.4] Iso
Cr 90 [12.0] Colrec 122 [13.0] Iso
a
Perkin-Elmer DSC-7; heating rates 10 K minꢂ1; peak temperatures; data from the 1st heating scan, monotropic transitions were determined in the
rst cooling scan; enthalpy values are given aer the phase transition temperatures in square brackets; monotropic transitions are indicated by
parentheses; abbreviations: Cr ¼ crystalline solid; Colrec ¼ rectangular columnar phase with antipolar packing in adjacent ribbons (B1Rev
[*]
phase); MIso ¼ optically isotropic intercalated smectic phase representing a deformed Colrec phase with short range density modulation along
the layers and Iso ¼ isotropic liquid phase.
molecular lengths were determined for a V-shaped conforma-
tion with an assumed bending angle of 120ꢁ and all-trans
conformation of the longest branch in each terminal alkyl
chain. This deviation of d/L from being exactly one is typical for
LC phases and is mainly due to chain folding and the reduced
order parameter of the aromatic cores in the LC state. Thus the
d/L ratio is in line with a non-tilted organization of the mole-
cules in the ribbons. The non-tilted organization is also evident
from textural observations where the extinction crosses of
Table 2 XRD data of the Colrec phases and molecular dimensions of compounds
(S)-2, 6 and 9 and comparison with related data determined for the linear
compounds C/8 and C/10 (ref. 19)
Lattice
Compound
R1
R2
parameters/nm
Lmol/nm
nmol
(S)-2
6
I
III
III
C8H17
C10H21
III
II
III
C8H17
C10H21
a ¼ 3.3; b ¼ 4.4
a ¼ 3.6; b ¼ 4.3
a ¼ 3.8; b ¼ 4.6
a ¼ 3.3; b ¼ 4.6
a ¼ 3.9; b ¼ 4.9
4.7
4.9
5.1
5.1
5.5
4.9
4.8
5.1
5.1
5.4
9
C/8a
C/10a
Fig. 1 (a) Powder diffraction patterns of the Colrec phase of compound (S)-2 at
T ¼ 133 ꢁC; (b) model of the molecular organization in the Colrec phase and (c)
typical texture of the Colrec-phases ((S)-3 at 93 ꢁC); the orientation of the polarizer
and analyzer is indicated by the arrows.
a
The transition temperatures of compounds C/8 and C/10 with linear
chains have already been reported in ref. 19, but these compounds
were studied herein by XRD.
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circular domains are parallel to the polarizer and analyzer (see exclusively forms the Colrec phase. However, for the isomeric
Fig. 1c and 3c). compound 6 having the longer 3,7-dimethyloctyl chain at the
The parameter a corresponds to twice the width of the shorter wing of the bent core, an isotropic mesophase is
ribbons. The number of molecules in the cross section of the observed besides the Colrec phase (see below). It is also
ribbons (nmol), calculated simply by assuming a mean distance remarkable that no crystallization could be observed for
between the molecules of 0.46 nm (position of the diffuse compound 6. Even aer prolonged storage for more than three
scattering maximum in the wide angle range), is about 5 for all months at room temperature, no melting peak could be
compounds (Table 2). As typical for Colrec phases, there is no observed by DSC. In contrast, compound 4 has a melting point
ꢁ
response to an external electric eld up to an applied voltage of at 118 C and readily crystallizes on cooling.
340 Vpp in a 5 mm cell.
As observed previously with other bent-core compounds,43
As expected, the transition temperatures are reduced by there is apparently no effect of molecular chirality on the
chain branching compared with related non-branched formation of Colrec phases and also the phase structure seems
compounds with the same alkyl chain length; compare for to be unaffected by molecular chirality.
[*]
example compounds 9 and C/8 (ref. 19) with an identical length
2.2.2 The MIso phase of compound 6. Compound 6 was
of alkyl chains or compounds 9 and C/10 (ref. 19) with identical investigated in more detail. On cooling, the birefringent texture
numbers of carbon atoms in the chains (Fig. 2). Surprisingly, of the Colrec phase disappears and a completely isotropic phase
chain branching cannot remove the Colrec phases; neither it can is formed. This isotropic phase is assigned here as MIso[*]. The
modify the density wave length in the modulated smectic pha- superscript [*] is used to indicate that spontaneous formation of
ses. In all cases the same rectangular lattice (c2mm) can be chiral domains can be observed in this mesophase aer appli-
found, even the lattice parameters are very similar to each other cation of an electric eld (see below; as compound 6 represents
and to those of the related n-alkyl substituted compounds a racemic mixture of diastereomers it is optically inactive in
(Table 2).
solution or in the isotropic liquid state and hence chirality is the
The size of the branches in the terminal chains has a strong result of symmetry breaking in the LC phase, which is assigned
effect on the phase stability which decreases in the sequence with [*]). This mesophase is so, but not uid, similar to the
2-methybutyl (R ¼ I) > 3,7-dimethyloctyl (R ¼ III) > 2-ethylhexyl dark conglomerate phases.25,26 However, no indication of chiral
(R ¼ II). This means that compounds with an ethyl branch in domains could be observed if the phase is formed in the
the 2-position of a hexyloxy chain have the lowest Colrec–Iso absence of an electric eld. On heating there is no transition
transition temperatures (compounds 1, 3, 4) and compound 8
with two such chains has no mesophase at all. In contrast,
methyl branching in the 3- and 7-positions of the octyloxy chain
gives rise to the widest Colrec phase ranges (compare
compounds 7–8) among the branched compounds considered
herein. As expected, compounds 1–6 with different chains have
a tendency to show reduced melting points compared with
compounds 7–9 with identical chains. The exchange of the
branched alkyl chains at both ends of the molecules can have
quite a signicant effect, especially if compounds 4 and 6,
combining 3,7-dimethyloctyl with 2-ethylhexyl chains, are
compared. Compound 4 with the shorter and more bulky
2-ethylhexyl chain at the shorter wing of the bent core
Fig. 3 Mesophases of compound 6: (a and b) powder diffraction patterns (a) in
[*]
the Colrec phase at T ¼ 97 ꢁC and (b) in the MIso phase at T ¼ 60 ꢁC; insets
show the diffuse scattering in the wide angle region; (c–f) textures observed on
Fig.
2
Comparison of the thermal behaviour of symmetrical bent-core
cooling; the orientation of the polarizer and analyzer is indicated by the arrows in
[*]
compounds with branched side chains 7–9 and their linear analogues
C/8, C/10.19
(f) and is the same in (c–f); (c) Colrec phase at T ¼ 100 ꢁC, (d) Colrec–MIso tran-
[*]
sition at T ¼ 93 ꢁC, (e) at T ¼ 90 ꢁC and MIso phase at T ¼ 60 ꢁC.
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back to the Colrec phase, instead there is a direct transition from
the viscous MIso[*] phase to the more uid isotropic liquid state
ꢁ
at T ¼ 110 C. Only on cooling from the isotropic liquid is the
birefringent Colrec phase formed again at T ¼ 101 ꢁC, indicating
a signicant hysteresis of the phase transitions involving the
MIso[*] phase and that the Colrec phase seems to be a metastable
phase in this case. This is in line with DSC results (see Table 1)
which indicate two transitions on cooling with comparable
enthalpy values, a relatively sharp one at T ¼ 101 ꢁC for the Iso–
Colrec transition and another very broad one between 85 and
Fig. 4 Texture obtained by applying an electric field to compound 6 (T ¼ 89 ꢁC,
voltage ¼ 124 Vpp, resistance ¼ 5 kU, f ¼ 100 Hz) between polarizers slightly
decrossed from 90ꢁ by a small angle (ꢃ1ꢁ).
[*]
ꢁ
65 C for the slow formation of the MIso phase. On heating,
only one relatively sharp peak is observed at T ¼ 110 ꢁC for the
MIso[*]-to-Iso transition and the transition enthalpy on heating observed.13,14 In the case of a B6-like intercalated structure, the
corresponds to the total of both transitions on cooling.
increase in d compared to b/2 (in the Colrec phase) at the tran-
[*]
The phase transitions are also evident from XRD investiga- sition from Colrec to MIso could be the result of a denser
tions. The diffraction pattern of the Colrec phase (Fig. 3a) packing of the chains in the low temperature phase, leading to
changes into one where the diffuse wide angle scattering chain stretching. Hence, this non-tilted organization appears
becomes a bit sharper. This is associated with a change in shape more likely and this would be in line with the occurrence of this
which becomes less symmetric and seems to be the result of the phase beside a Colrec phase and also with the absence of chiral
[*]
overlapping of several distinct maxima (Fig. 3b). Overall, the domains in the MIso phase formed without an applied eld.
[*]
changes in the wide angle XRD pattern indicate a transition to a
However, the formation of the MIso phase is strongly
phase with an increased in-plane order of the molecules. The dependent on the actual conditions. In 5 mm ITO cells, for
appearance of the wide angle scattering has some similarities example, it does not appear on cooling, but only aer applica-
with that of the B5 phases consisting of three maxima caused by tion of a triangular wave voltage of $7.5 Vpp. Once formed, the
density waves perpendicular to the layer normal, indicating a dark phase is stable without an applied eld and the phase does
hexatic order of the cores in the layer planes.2,44 However, due to not change upon cooling down to room temperature and
the overlapping of the wide angle diffractions, it is not possible heating up to the transition to the isotropic liquid; also an
to determine the kind of short range lattice in the MIso[*] phase application of increased voltage up to 340 Vpp, the maximal
of compound 6.
applicable voltage, does not induce any textural change. Only
In the small angle region, the position of the rst very sharp aer heating to the isotropic liquid state, followed by cooling
and intense reection is slightly shied. In total there are two without an applied eld to the phase transition temperature, is
reections in the small angle region with the ratio of their the birefringent texture of the Colrec phase formed again.
d-values corresponding to 1 : 1/2. This indicates the transition Interestingly, in contrast to the dark texture obtained in the
from a modulated smectic phase with a 2D lattice (the Colrec absence of an external eld which appears to be achiral, a
phase) to a simple smectic phase with a layer thickness corre- conglomerate of chiral domains is clearly visible in the eld
sponding approximately to half of the parameter b in the rect- induced texture aer slightly decrossing the polarizers by ca.
angular lattice of the Colrec phase. This is very similar to the 1–2ꢁ in either direction (see Fig. 4). This could mean that there
[*]
transition Colrec–B6, where the B6 phase can be considered as a is a small tilt in the MIso phase, which is difficult to exclude.
Colrec phase with only short range density modulation along the
There is no current response observed under an applied
direction of the modulated layers,27 and therefore, it appears triangular wave voltage, independent of the temperature and
like an intercalated smectic phase. In the B6 phases the mole- even under an applied voltage of up to 340 Vpp. This is in line
cules are tilted in most cases, but non-tilted versions have also with the observation that the layer deformation, inhibiting
been observed.45
polar switching, could not be removed under an applied eld.14
In the MIso phase reported here, all diffractions are However, for this MIso phase the proposed B6-like structure
smeared out to form closed rings, very similar to the appearance could also be the reason for the absence of a response to an
of the XRD patterns of the dark conglomerate phases,14,30 indi- electric eld.
[*]
[*]
cating a strong distortion. The absence of alignment in XRD
experiments and the absence of birefringence in optical exper-
iments do not allow one to distinguish if there is a tilt or not,
3 Summary and conclusions
neither by XRD nor by optical methods. The d-value d ¼ 2.5 nm A series of biphenyl-3,40-diol based bent-core compounds
[*]
in the MIso phase corresponds approximately to half the incorporating a total of 6 benzene rings in the core structure
molecular length (L ¼ 4.9 nm), which could indicate either an and with two branched side chains has been synthesized and
extraordinary strong tilt of the molecules of around 60ꢁ in a investigated. All compounds 1–9, with the exception of
monolayer smectic phase or a non-tilted organization in a B6- compound 8, show enantiotropic or monotropic rectangular
like intercalated smectic phase. A strong tilt is unlikely to occur, columnar phases (Colrec phases) of the B1rev type. The same
because for similar bent-core compounds with branched Colrec phase was also observed for related bent-core molecules
chains, orthogonal or only slightly tilted phases were with linear terminal alkyl chains having the same chain length
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distortion also leads to the optically isotropic appearance of this
[*]
phase. Hence, the MIso phase appears to represent a new
subtype of the dark conglomerate phase (DC[*]) formed by bent-
core mesogens. However, in contrast to most other DC[*] phases
which have chiral domains in the ground state, only the eld
induced isotropic phase of compound 6 shows a conglomerate
of chiral domains, whereas no chiral domains can be detected
before applying an electric eld. This electro-gyration effect was
sometimes also observed for other low birefringent smectic
phases where chiral domains became clearly visible only aer
application of an electric eld.3,30a,47 It appears that in the
absence of a eld, chiral segregation is incomplete or the size of
the domains is too small to be visible. An electric eld seems to
support chiral segregation, probably due to the improved pre-
organization of the molecules by alignment of the molecular
dipoles.
To the best of our knowledge the MIso[*] phase reported here
is the rst DC[*] phase developing adjacent to a Colrec phase. In
this new phase the molecules are non-tilted or only slightly
tilted, have additional in-plane order and are arranged in a B6-
like intercalated and distorted lamellar phase, composed of
Fig. 5 Possible models of the Colrec and MIso[*] phases observed for compound 6:
(a) Colrec phase (cut perpendicular to the ribbons; dots and crosses indicate the
polar direction in the ribbons), (b) view from the top indicating that no “in-plane”
order is present (view along the molecular long axes of the rotationally disordered
ribbons with a signicant distribution of the ribbon diameter
[*]
molecules); (c) B6-like intercalated smectic phase resulting from a non-uniform and only short range correlation.27 The MIso phase reported
diameter of the ribbons; (d) 2D in-plane lattice resulting from the denser packing
here appears to be intermediate between the so crystalline
of the aromatic cores (the c2mm lattice is arbitrarily chosen, but is also the most
B4-type helical lament phases with crystallized aromatic cores
likely); (e) twist of the two rod-like aromatic wings of the bent core takes place
organized in helical ribbons24 and the uid DC phases formed
collectively with development of
a uniform sense of the resulting chiral
by sponge-like deformed layers of polar smectic phases.26
conformers. This gives rise to a deformation of the ribbons which causes the loss
of the 2D-lattice of the Colrec phase and a deformation of the ribbons leading to Accordingly, wide angle XRD scattering (see Fig. 3b) is neither
[*]
the optical isotropic appearance of the MIso phase.
completely diffuse as in the uid DC phases nor does it show
the distinct sharp three peak pattern of the B4 phases.
The origin of chirality in the mesophases of bent core
or the same chain volume.19 It is surprising that the Colrec phase molecules is usually assigned to the layer chirality resulting
– which requires a mixing of alkyl chains and aromatic cores – is from the combination of tilt and polar order,26,48,49 and the
not suppressed by chain branching. However, for one of the coupling between layer chirality and molecular conformational
[*]
compounds combining two different branched racemic side chirality is likely to contribute to optical activity. As the MIso
chains, a 2-ethylhexyl chain and a 3,7-dimethyloctyl chain, an phase appears to be non-tilted or only slightly tilted, the denser
additional optically isotropic LC phase, assigned as MIso[*], was packing of the molecules in the MIso phase and hence the
[*]
observed.
stronger segregation of helical twisted conformers of the bent-
From the obtained data we conclude the following model of core mesogens appear in this case to be of signicant or even
self-assembly in the MIso[*] phase (see Fig. 5c). Starting with the dominating importance for chiral segregation.50 It is thought
Colrec phase (Fig. 5a and b), upon reducing the temperature a that with increasing packing density of the aromatic cores,
denser packing of the aromatic cores becomes possible; this chiral segregation of the twist conformers becomes more
leads to a packing of the aromatics on a hexatic in-plane lattice dominating and the importance of tilt (layer chirality) for chiral
(Fig. 5d). Due to this short range in-plane lattice and the bent segregation decreases. Though we presently cannot answer the
molecular shape, there is a stronger restriction of the rotation of question of whether chiral conformers alone could be sufficient
the aromatics. If preferred chiral molecular conformers result for spontaneous achiral symmetry breaking in the MIso[*] phase,
from the helical twist of the two aromatic wings, then the denser due to the uncertainty concerning a possible small tilt, this
packing in this phase could lead to a selection and collective phase adds a new variant to the family of dark conglomerate
packing of conformers with uniform chirality into distinct phases which contributes to the ongoing discussion of the
domains (Fig. 5e).46 Because chiral molecules cannot pack origins of achiral symmetry breaking in liquid crystalline
exactly parallel to each other, chirality induces a twist, giving phases.3,23,50–52
rise to frustration of the packing in straight ribbons. This cau-
ses a loss of the long range character of the density modulation
4 Experimental
4.1 Characterization methods
along the modulated layers at the transition from the Colrec
phase to the MIso[*] phase. Hence, the long range correlation of
the ribbons is removed, leading to a B6-like intercalated smectic The characterization of the synthesized compounds is based on
structure as shown in Fig. 5c. The chirality induced layer/ribbon 1H-NMR, 13C-NMR (Varian Unity 500 and Varian Unity
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Paper
400 spectrometers, in CDCl3 solutions, with tetramethylsilane voltage: 4.53 kV, sheath gas ow rate: 4.54, capillary voltage:
ꢁ
as an internal standard), and MS [AMD 402 (electron impact, 20.34 V, capillary temp.: 319.90 C, source fragmentation colli-
70 eV) and Thermo Finnigan LCQ Advantage Max (electrospray sion energy: 90 V). C58H62O10 (919.12): Anal. calc.: C, 75.79; H,
ionization)]. Microanalysis was performed using a Leco CHNS- 6.79. Found: C, 75.48; H, 6.59%.
932 elemental analyzer.
The analytical data of all other compounds are collated in
Transition temperatures were measured using a Mettler FP- the ESI.†
82 HT hot stage and control unit in conjunction with a Leica
polarizing microscope. The associated enthalpies were obtained
Acknowledgements
from DSC-thermograms which were recorded on a Perkin-Elmer
DSC-7 (heating and cooling rate: 10 C minꢂ1).
H. Ocak is grateful to the Scientic and Technological Research
ꢁ
The X-ray diffraction patterns of the compounds were Council of Turkey (TUBITAK) for a research fellowship at Martin
recorded with a 2D detector (HI-STAR, Siemens). Ni ltered and Luther University, Halle, Germany. B. Bilgin-Eran is grateful to
pin hole collimated CuKa radiation was used. The exposure the Alexander von Humboldt Foundation for nancial support
time was normally 60 min. The sample to detector distance was toward liquid crystal research. M. Prehm acknowledges the
8.8 cm and 26.9 cm for the wide angle and small angle support by the Cluster of Excellence “Nanostructured Materials”.
measurements, respectively. The samples were held on a
temperature-controlled heating stage.
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