4-Methylresorcinol based bent-core liquid crystals with azobenzene wings-a new class of compounds with dark conglomerate phases

Article abstract of DOI:10.1039/c4tc00533c

Stochastic achiral symmetry breaking in soft matter systems, leading to conglomerates of macroscopically chiral domains (so-called dark conglomerate = DC phases) is of contemporary interest from a fundamental scientific point of view as well as for numero

Full text of DOI:10.1039/c4tc00533c

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
4-Methylresorcinol based bent-core liquid crystals
with azobenzene wings – a new class of
Cite this: J. Mater. Chem. C, 2014, 2,
compounds with dark conglomerate phases†
5487
ab
a
a
a
*
*
Marko Prehm, Marcel Brautzsch and Carsten Tschierske
Mohamed Alaasar,
Stochastic achiral symmetry breaking in soft matter systems, leading to conglomerates of macroscopically
chiral domains (so-called dark conglomerate ¼ DC phases) is of contemporary interest from a fundamental
scientific point of view as well as for numerous potential applications in chirality sensing and non-
centrosymmetric materials. Herein we report the synthesis and investigation of first azobenzene
containing bent-core mesogens derived from 4-methylresorcinol forming DC phases with a new
structure, distinct from the known fluid sponge-like distorted smectic phases as well as from the helical
nano-filament phases (HNF phases, B4 phases). The effects of chain length and other structural
modifications on achiral symmetry breaking were investigated. Homologues with relatively short alkyl
chains form achiral intercalated lamellar LC phases (B6 phases), but on increasing the chains, these are
replaced by the chiral and optically isotropic DC phases. Compounds with the longest alkyl chains form
low birefringent crystalline conglomerates which represent less distorted versions of the optically
isotropic DC-phases. Introducing additional peripheral substituents at both outer rings removes the DC
phases. The DC phases were also removed and replaced by modulated smectic phases if the azo groups
were replaced by ester units, showing that azo groups favour DC phase formation with new
nanostructures, distinct from the previously known types.
Received 17th March 2014
Accepted 22nd April 2014
DOI: 10.1039/c4tc00533c
www.rsc.org/MaterialsC
into the liquid crystalline smectic phases (sponge phases)8–15
and the so crystalline helical nanolament phases (HNF
phases, also assigned as B4 phases).16–19 In the sponge type DC
1. Introduction
Since the discovery of polar order and chirality in the liquid
crystalline (LC) phases of compounds with a bent aromatic
core (bent-core liquid crystals ¼ BCLCs)1 this eld has become
a thrust area of liquid crystal research by providing a signi-
cant impact on the general understanding of molecular self-
assembly in so condensed matter.2,3 Moreover, these mate-
rials are also of signicant interest for numerous practical
applications, as ferroelectrica, exoelectrica and pyro-
electrica,4 for command surfaces and sensors,5 non linear
optical applications6 and in fast switching electrooptical
devices.7 A great fraction of contemporary interest is focussed
on the so-called dark conglomerate phases (DC phases).8–18
The common feature of these DC phases is the absence of
birefringence due to a local distortion of long range periodicity
and the inherent phase chirality indicated by stochastic achi-
ral symmetry breaking, leading to conglomerates of macro-
scopically chiral domains. These DC phases can be subdivided
phases, layers without in-plane order are strongly deformed in
a nonregular way, so that uniformly oriented regions become
smaller than the wavelength of light. In contrast, the HNF
phases are composed of helical laments with a crystalline
packing of the aromatic cores in twisted ribbons, here only the
chains are in a disordered state. As there is no preferred
direction for these laments these phases appear optically
isotropic. The major tool for distinguishing these two struc-
tures is XRD, where the wide angle scattering around d ¼ 0.45
nm is completely diffuse for the LC sponge phases whereas for
the so crystalline HNF phases a series of relatively sharp
scattering could be found in this region.16a,20,21 The reduced
symmetry as a result of the tilted organization of molecules in
polar layers (layer chirality),3,16 the collective segregation of
chiral molecular conformations and a combination of both are
considered as possible origins of the remarkable chirality and
mirror symmetry breaking in these so matter structur-
es.2a,b,8,17a,22 HNF phases might become interesting materials
for a number of applications, such as organic semiconductors
for thin-lm transistors and solar cells,23 as thin-lm polar-
izers,22 as nonlinear optical materials24 as well as for the
detection and amplication of chirality.25
aInstitute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2,
D-06120 Halle (Saale), Germany. E-mail: carsten.tschierske@chemie.uni-halle.de
bDepartment of Chemistry, Faculty of Science, Cairo University, Giza, Egypt. E-mail:
malaasar@sci.cu.edu.eg
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00533c
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5487

View Article Online
Journal of Materials Chemistry C
Paper
Interestingly, almost all of the known BCLCs with HNF pha-
ses incorporate the hydrolytically unstable benzylideneaniline
(Schiff base) moiety with the exception of only very few recent
examples.26–28 This calls for new materials with DC phases,
showing enhanced chemical stability and providing new phase
sequences involving the HNF phases. A broader variety of
different molecular structures could also lead to an improved
understanding of the molecular structural factors governing the
formation of distinct subtypes of DC phases, thus providing
rules for the directed design of such materials.
BCLCs incorporating azo (–N]N–) linkages represent
versatile materials due to their photochromic effects in addi-
tion to high birefringence and high polarizability, leading to
signicant nonlinear optical activity.6,29,30 The cis–trans iso-
merisation of the azo linkage in the presence of UV light could
be used, for example, in high-density data storage systems, for
sensors, photonic switches and molecular logic gates.31
Though a variety of azobenzene based BCLCs with interesting
properties have been reported,29,32–34 until recently no such
Scheme 2 Structures of the 4-methylresorcinol derived bent-core
BCLCs with DC phases have been available; only bent dimes- liquid crystalline compounds An–Gn (for An: n is an even number
except A9) and their synthesis; reagents and conditions: (i) Et3N,
pyridine, CH2Cl2, reflux, 6 h.
ogens combining two rod-like azobenzene units via an odd-
numbered alkylene spacer were reported.27 We have found that
iodine28 or bromine34a substitution in 4-position at the bent
central resorcinol unit of azobenzene based BCLCs leads to a
new type of DC-phases (Scheme 1).28 Hence, it appears that the
phases also
a birefringent crystalline conglomerate was
size of the halogens is of importance. Bulky halogen atoms like
bromine (cv ¼ 0.33 nm3)35 or iodine (cv ¼ 0.45 nm3),35 seem to
be favourable for formation of DC phases, whereas no DC
phases were found for BCLC with hydrogen32e or smaller
halogens in the same position (see Scheme 1; F: cv ¼ 0.13 nm3;
Cl: cv ¼ 0.27 nm3).34,35 Hence, the question arose if azobenzene
BCLCs with other bulky groups at the bent core, having a size
comparable to iodine/bromine, could also lead to azobenzene
based BCLCs showing DC phases. The methyl group has a size
(cv ¼ 0.32 nm3)35 comparable to bromine but it is almost non-
polar in contrast to the C-halogen bonds which introduce a
signicant dipole moment.
observed for a long chain compound. The formation of this
phase depends on the applied cooling rates and the transition
between the isotropic and birefringent types of conglomerate
phases was studied. In addition, the effect of lateral groups X
(see Scheme 2, X ¼ F, Br, CH3) at the outer rings of these BCLCs
(compounds Bn, C14 and D14, respectively) has been investi-
gated. The phase behaviour of these new compounds is
compared with related compounds incorporating terephthalate
or benzoate based wing groups instead of the azobenzenes,
either reported in the literature36 or synthesized herein (E14,
F14 and G14 see Section 3.6).
Here we report a series of new BCLCs derived from 4-meth-
ylresorcinol36,37 possessing two azobenzene side arms (An, see
Scheme 2). These compounds show B6 phases if the terminal
chains are short and a new type of dark conglomerate phases if
the chains are sufficiently long. Besides these isotropic DC
2. Experimental
Synthesis
The synthesis of the target BCLCs A/n–D/n with azobenzene
wings was performed as shown in Scheme 2 by acylation of 4-
methylresorcinol with 4-(4-n-alkoxyphenylazo)benzoyl chlorides
(X ¼ H) or their 3-substituted derivatives with X ¼ F, CH3, Br
(see Scheme S1†). For the synthesis of compounds E/14–G/14
appropriately substituted benzoyl chlorides (see Scheme S2†)
were used for the acylation reactions.37 Details of the synthesis
of the intermediates and nal compounds as well as analytical
data are reported in the ESI.†
Methods
Scheme 1 LC phases found for 4-halogenoresorcinol based bent-
core mesogens with azobenzene wings (abbreviations: Cr ¼ crystalline
The mesophase behaviour and transition temperatures were
measured using a Mettler FP-82 HT hot stage and a control unit
in conjunction with a Nikon Optiphot-2 polarizing microscope.
The associated enthalpies were obtained from DSC-thermo-
solid, USmC ¼ undulated smectic phase; N ¼ nematic phase, B6
¼
intercalated smectic phase formed by ribbons with only short range
register; DC ¼ dark conglomerate phase; phases in brackets were
observed only for one homologue in a small temperature range).28,34a grams which were recorded on a Perkin-Elmer DSC-7, the
5488 | J. Mater. Chem. C, 2014, 2, 5487–5501
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
heating and cooling rate was 10 K minꢀ1. Electro-optical investigations. According to the present understanding, this is a
switching characteristics were examined in 6 mm polyimide frustrated smectic phase with layers broken into ribbons and
coated ITO cells (EHC Japan) using the triangular-wave method. adopting a staggered organization with only short range peri-
XRD patterns were recorded with a 2D detector (Vantec-500, odicity between the ribbons (see Fig. 1d); the layer frustration
Bruker). Ni ltered and pin hole collimated CuKa radiation was takes place in the direction perpendicular to the molecular
used. The exposure time was 15 min and the sample to detector bending plane (Fig. 1d).39,40
distance was 8.95 and 26.7 cm for small angle and wide angle
The wide angle scattering is diffuse and has a maximum at
scattering experiments, respectively. Samples were aligned by 0.43 nm, in line with the LC state of this phase. For an aligned
slow cooling (rate: 1–0.1 K minꢀ1) of a small droplet of the sample this scattering is split into four maxima indicating the
compound on a glass plate and takes place at the sample–air presence of a tilt of about 26ꢁ (Fig. 1b and S12†). This tilt could
interface. The samples were held on a temperature-controlled arise either from a tilt of the molecular bending planes, or alter-
heating stage.
natively, from the inherently tilted orientation of the individual
DFT computation was carried out with the Gaussian 09 rod-like wings of the bent-core mesogens, similar to anticlinic
package.38 Geometry optimization was performed with the SmC phases.41 The latter would be in line with recent microbeam
B3LYP functional and the LAN L2 DZ basis set. The solvation XRD results obtained for B6 phases of Schiff base compounds.39a
model was IEFPCM with solvent chlorobenzene.
In this case the orientation of the molecules would be on average
orthogonal and the splitting is related to the angle of the molec-
ular bent of 128ꢁ which is in good agreement with the geometry
provided by the molecular structure. This would also be in line
with the optical textures which have the extinction brushes strictly
parallel to the polarizers (see Fig. 1a). However, the measured d
value is a bit shorter than half the molecular length (2d/Lmol ꢂ 0.9)
which in this case must be attributed to partial intercalation and
conformational disorder of the alkyl chains. On cooling A6 from
the isotropic state the B6 phase appears at T ¼ 129 ꢁC and remains
on cooling down to room temperature without visible crystalli-
zation of the sample (for DSC traces, see Fig. 2a), but the sample
becomes glass-like solid without any change of the texture at ꢂ40
ꢁC. This might indicate a transition to a glassy state of the B6
phase or an isomorphous crystallization. As no clear DSC peak
(only a small hump between 40 and 50 ꢁC, see Fig. 2a) was
observed in the cooling traces and crystallization is observed >35
ꢁC on heating, a glassy B6 phase (gB6) appears more likely.
In electrooptical investigations using a triangular wave
voltage no current peak could be observed in the whole
temperature range of this mesophase up to a voltage of 200 Vpp
3. Results and discussion
3.1 Overview over the LC phases formed by compounds An–
Dn
The thermal behaviour of all synthesized compounds was
studied by polarizing optical microscopy (POM) and differential
scanning calorimetry (DSC). The transition temperatures (ꢁC),
the associated phase transition enthalpies (kJ molꢀ1) and the
phase sequences of compounds An, Bn, C14 and D14 are given
in Table 1. All compounds are thermally stable as conrmed by
the reproducibility of thermograms in several heating and
cooling cycles. Related compounds E/14–G/14 with phenyl-
benzoate wing groups instead of the azobenzenes, synthesized
for comparison purposes, are described and discussed in
Section 3.6.
3.2 B6-type intercalated smectic phases of the short chain
compounds A6 and A8
Compounds An without lateral substituents at the outer rings of in a 6 mm ITO cell. These observations, together with the rela-
the bent-core structure exhibit two different types of meso- tively high transition enthalpy value of DH ¼ 12.3 kJ molꢀ1 for
phases. On heating the shortest homologue (A6, with n ¼ 6) a the Iso–LC transition at T ¼ 129 ꢁC are in line with the sug-
transition from the crystalline state to a focal-conic fan-shaped gested B6 phase. With increasing chain length (n ¼ 8 and 9) the
texture is observed at T ¼ 124 ꢁC (Fig. 1a). The dark extinctions B6 phase is retained but the Iso–B6 transition temperature
are parallel to a polarizer and an analyzer, indicating an average decreases to 96 ꢁC for A8 and to 87 ꢁC for A9, and the B6 phase
orthogonal or anticlinic tilted smectic phase. No homeotropic becomes monotropic (metastable) for these two compounds.
alignment with pseudoisotropic areas or with birefringent
Schlieren texture, as typical for SmA or SmC phases, respec-
tively, could be achieved by shearing the sample. Instead, a ne
3.3 Competition between B6 and DC phases in A9
fan-texture is immediately reformed aer releasing shear stress On cooling the B6 phase of A9 a highly viscous, almost solid-
ꢁ
(see Fig. S2†), as typical for intercalated smectic phases of bent- like optically isotropic mesophase starts to grow at T ¼ 85 C
core mesogens, assigned as B6 phases.
(Fig. 3a and b). Uncrossing the polarizers by a small angle
The XRD pattern (Fig. 1b and c) shows only one sharp peak in leads to the appearance of dark and bright domains.
the small angle region corresponding to a d-value of 2.0 nm (T ¼ Uncrossing the polarizer in the opposite direction reverses the
80 ꢁC), which is smaller than half of the molecular length (Lmol
)
dark and bright domains (Fig. 3c and d). Rotating the sample
in a conformation with a 120ꢁ bent aromatic unit and the alkyl between crossed polarizers does not lead to any change and
chains in the most stretched all-trans conformation. For this indicates that the distinct regions represent chiral
compound A6 Lmol is between 4.3 and 4.5 nm, depending on the domains with opposite handedness, as typical for dark
assumed conformation shown in Fig. S1.† This conrms the conglomerate phases (DC phases). The distribution of the
intercalated B6-type smectic phase deduced from optical areas of domains with opposite chirality is on average 1 : 1.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5489

View Article Online
Journal of Materials Chemistry C
Paper
Table 1 Phase transition temperatures (T/ꢁC), mesophase types, and transition enthalpies (DH/kJ molꢀ1, values in square brackets) of
compounds An, Bn, C14 and D14a
Comp.
n
X
Heating
Cooling
A6
A8
A9
6
8
9
H
H
H
Cr 124 [41.0] B6 131 [11.9] Iso
Cr 111 [35.9] Iso
Iso 129 [12.3] B6
Iso 96 [11.1] B6 62 [9.1] Cr
Iso 87 [9.4] B6 67 [16.5] Crf
Iso 87 [9.4] B6 ꢂ 85 DC [$9.1] 67 Crf
Iso 87 [24.1] DC
Cr 110 [37.4] Iso
DC 100 [22.7] Isob
Cr 107 [37.4] Iso
A10
A12
10
12
H
H
DC 100 [25.5] Isoc
Cr 109 [38.1] Iso
Iso 87 [26.8] DC
DC 102 [27.3] Isob
DC 101 [31.4] Iso
DC 102 [40.8] Iso
DC 103 [49.6] Iso
Cr[*] 99 DC 104 [68.0]d Iso
DC 104 [56.7] Isoe
Cr[*] 105 [73.2] Iso
Cr 112 [41.3] Iso
A14
A16
A18
A20
14
16
18
20
H
H
H
H
Iso 88 [33.9] DC
Iso 90 [43.9] DC
Iso 93 [51.7] DC
Iso 92 [70.8] Cr[*]
Iso 95 [59.0] DCg
A22
B6
B14
B20
C14
D14
22
6
14
20
14
14
H
F
F
F
Br
CH3
Iso 96 [76.5] Cr[*]
Iso 88 [8.6] SmCaPA 67 [10.5] Crh
Iso 90 [15.2] SmCaPA 74 [39.8] Cr
Iso 92 [16.0] SmCaPA 82 [77.7] Cr
Iso 30 [53.2] Cr
Cr 90 [32.9] SmCaPA 95 [12.8] Iso
Cr 95 [93.0] Iso
Cr 78 [77.2] Iso
Cr 79 [47.9] Iso
Iso 33 [0.9] SmC < 20 Cr
a
Before measurement the compounds were melted and heated to 150 ꢁC to remove traces of enclosed solvent, aerwards they were cooled with 10 K
minꢀ1 to room temperature; the phase transition temperatures (peak temperatures) were taken from the following heating and cooling scans at 10 K
minꢀ1, if not otherwise specied; abbreviations: SmC ¼ synclinic tilted birefringent smectic phase; SmCaPA ¼ anticlinic tilted antiferroelectric SmC
phase; Iso ¼ isotropic liquid; Cr[*] ¼ crystalline phase composed ofꢀa1 conglomerate of chiral domains, for other explanations, see Schemeꢀ11.
b
d
c
Obtained on heating aer previous cooling to 75 ꢁC with 10 K min
.
Obtained on heating aer previous cooling to 80 ꢁC with 20 K min
.
Total enthalpy for both transitions. Obtained on heating aer previous cooling to 80 ꢁC with 2 K minꢀ1
.
The actual phase sequence
e
f
strongly depending on conditions, see DSC traces in Fig. 2b; transition enthalpy value could not be determined for the B6-DC transition (see
g
h
Section 3.3). Cooling with 2 K minꢀ1
.
Partial crystallization.
The formation of the DC phase is slow and on further cooling mesophase and it is directly formed upon cooling the isotropic
the remaining B6 phase rapidly crystallizes with formation of a liquid (see Fig. S4† for textures observed for compound A14).
ꢁ
birefringent crystalline phase (see Fig. 3b) at T ¼ 67 C, while The growth process from the isotropic liquid, as observed
the DC phase only very slowly crystallizes. On heating the areas between slightly uncrossed polarizers, is shown for compound
ꢁ
of the DC phase go to the isotropic liquid state at T ¼ 100 C, A10 in Fig. 4. The distinct chiral domains grow with a circular
while the birefringent crystalline regions of the sample melt to shape (Fig. 4a–c) and nally coalesce to a mosaic-like pattern
ꢁ
a clear liquid at T ¼ 110 C. This indicates that the B6-phase, with sharp linear boundaries, representing the Voronoi cells
formed rst, is thermodynamically unstable and upon separating the germs of circular domain growth (Fig. 4d). In this
annealing is replaced by the isotropic DC phase and a bire- respect the formation of the DC phases reported herein is
fringent crystalline phase. Formation of the DC phase is in distinct from previously reported cases, showing fractal
competition with fast crystallization, but the DC phase once growth.43
formed crystallizes only slowly (see Fig. S3†). However, aer
The melting of the DC phases (DC–Iso-transition) takes place
slow cooling (2 K minꢀ1) to ꢂ70 ꢁC, followed by immediate around 100–104 ꢁC and the crystallization of the DC phase (Iso–
heating, a uniform DC phase can be obtained (see 3rd heating DC transition) is found between 87 and 95 ꢁC for all compounds
curve in Fig. 2b).42
A10–A20 and both transition temperatures increase slightly
with rising alkyl chain length (see Table 1 and Fig. 5a). Hence,
there is a supercooling of this phase transition by about 10 K
(peak temperatures at scanning rates of 10 K minꢀ1). The
formation of the DC phases is associated with relatively high
3.4 DC phases of the long chain compounds A10–A22
3.4.1 Optical and calorimetric investigations. For the next
homologues with n ¼ 10–18 the DC phase is the only observed
5490 | J. Mater. Chem. C, 2014, 2, 5487–5501
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
Fig. 2 DSC heating and cooling traces of compounds (a) A6 and (b) A9
in heating and cooling runs with a rate of 10 K minꢀ1 (the 3rd heating
was recorded after slow cooling with 2 K minꢀ1 to 80 ꢁC followed by
immediate heating with a rate of 10 K minꢀ1).
Fig. 3 Optical micrographs observed for A9 between crossed polar-
izers: (a) B6 phase at T ¼ 86 ꢁC; (b) growth of the DC phase and the
birefringent crystalline phase from the B6 phase; (c) DC phase as
grown from B6 (T ¼ 83 ꢁC, different region) after rotating one polarizer
by 10ꢁ from the crossed position in the anticlockwise direction and (d)
in the clockwise direction, showing the chiral domains (on a larger field
of view the stochastically distributed chiral domains occupy equal
areas); the texture related to (c), (d), but between 90ꢁ crossed polar-
izers is shown in Fig. S3b†; additional textures are shown in Fig. S3a, c
and d.†
Fig. 1 Compound A6. (a) Texture (crossed polarizers) as observed for
the B6 phase at T ¼ 120 ꢁC (on cooling, textures observed after applying
shear stress are shown in Fig. S2†); (b) XRD pattern of an aligned sample
of the B6-phase at 80 ꢁC; (c) 2q-scan over this pattern and (d) the
proposed model of the molecular organization in the B6 phase, left:
view along the bending direction, right: side view perpendicular to the
molecular bending plane (in this direction the ribbons are quasi infinite)
and the assumed electron density modulation profile in the middle.41
transition enthalpies, ranging between DH ꢂ 24 and 59 kJ molꢀ1
Compounds A10 and A12 easily crystallize on cooling or on
(measured in cooling scans), strongly rising with growing alkyl reheating, but the DC phases of compounds with longer chains
chain length from A10 to A20 (see Table 1 and Fig. 5a).
(n ¼ 14–20) do not crystallize, once the DC phase is formed, as
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5491

View Article Online
Journal of Materials Chemistry C
Paper
molꢀ1 on heating and ꢂ59 kJ molꢀ1 on cooling. However, on
cooling with a faster rate of $5 K minꢀ1 (Fig. 5d, curve d) a
crystalline phase with a weakly birefringent spherulitic texture
(see Fig. 6b) is formed at T ¼ 92 ꢁC and the transition enthalpy is
now signicantly larger, around ꢂ71 kJ molꢀ1 on cooling.44 The
spherulitic texture (Fig. 6b) is similar to those found for
columnar or (modulated) smectic LC phases, however, this
phase is solid-like and does not ow on applying shear forces,
and therefore, this is a crystalline phase.
Interestingly, also this crystalline phase is chiral and
composed of domains with opposite handedness, i.e. dark and
bright domains become visible if the polarizers are slightly
uncrossed (see Fig. 6a and c), hence, this birefringent
conglomerate phase is assigned as Cr[*]. Each of the spherulites
appears to have uniform chirality, so it seems that the chirality
is determined by each nucleus and is preserved through the
growth process.45 On further cooling the Cr[*] phase is retained
and does not change down to room temperature, but on heating
to T ¼ 99 ꢁC it transforms into the DC phase. At this transition
the spherulitic texture becomes uniformly isotropic and the
chirality as well as the sign of chirality in the distinct domains is
retained (see Fig. 6e–g). This temperature corresponds to the
position of the exotherm II in the heating curve c in Fig. 5d. The
unusual shape of the DSC heating curve c is characterized by a
signicant tailing which rises up to a local maximum I, before it
abruptly goes through an exothem II, and nally the peak
maximum III is reached. This kind of DCS curve is always found
for the heating curves aer cooling the sample with rates $5 K
minꢀ1, independent of the used heating rates (see Fig. S9†). The
Fig. 4 a–d) Growth of the circular chiral domains of the DC phase of
A10 upon cooling from the isotropic liquid state at T ¼ 87 ꢁC as
observed between slightly uncrossed polarizers (see arrows); the
textures of the DC phase of A14 are shown in Fig. S4.†
shown in Fig. 5b for compound A18 as an example (for addi-
tional DSC curves, see Fig. S7 and S8). In the case of the long
chain compound A20 a different behaviour is observed which is
discussed here in some more detail. For this compound there is
a strong effect of the cooling rate on the phase sequence and the
phase structure (see Fig. 5c and d). On cooling from the
isotropic liquid phase with a low rate of #2 K minꢀ1 (Fig. 5c,
ꢁ
ꢁ
endothermic tailing I, starting at T ꢂ 85 C, is interpreted as a
curve b) the DC phase is formed at T ¼ 95 C and it does not
result of a chain melting process which leads to a soening of
the Cr[*] phase. This obviously allows a denser packing (crys-
tallization) of other molecular parts, giving rise to the exotherm
II in the heating curve. At this temperature the Cr[*] phase
transforms into the DC phase, i.e. this transition is accompa-
nied by a layer deformation which leads to the formation of the
crystallize on further cooling, and on heating only a single peak
is observed at T ¼ 104 ꢁC (Fig. 5c, curve a), similar to
compounds A14–A18 (Fig. 5b). The transition enthalpy is ꢂ57 kJ
ꢁ
optically isotropic DC phase at T ¼ 99 C. On further heating
this DC phase melts into the isotropic liquid at the endothermic
peak III at T ¼ 104 ꢁC. Once formed (either on cooling the
preformed DC phase or on slow cooling from the isotropic
liquid sate), the DC phase is stable and it is retained down to
room temperature; only a single peak at T ¼ 104 ꢁC is observed
on heating (curve e in Fig. 5d). Even aer storage at room
temperature for more than 8 weeks the same transition
temperatures and enthalpies were obtained. The persistence of
the DC phases might be the result of the freezing of disordered
alkyl chain segments into an immobilized, probably a glassy
state, which, once formed, inhibits the transformation into the
chiral Cr[*] phase or any other crystalline phase.
Compound A22 with the longest chains is a crystalline solid
with a melting point at T ¼ 105 ꢁC. No formation of a DC phase
Fig. 5 a) Dependence of DC–Iso transition temperatures (on heating could be detected at any cooling rate. Also in this case the
and cooling scans with 10 K minꢀ1) and transition enthalpy values (on
cooling from the isotropic liquid) of compounds A10–A20 on alkyl
crystalline phase formed on cooling consists of a conglomerate
of chiral crystals (see Fig. S5†), though the chiral domains
cannot be observed with the same clarity as for A20, because the
birefringence is higher and no uniform texture is obtained. In
the following focus is on the DC phases of compounds A10–A20.
chain length n; (b–d) DSC heating and cooling curves of (b) compound
A18 at 10 K minꢀ1, (c) compound A20 at 2 K minꢀ1 and (d) compound
A20 with a rate of 10 K minꢀ1 (curves c and d); curve e was recorded
after cooling with 2 K minꢀ1
.
5492 | J. Mater. Chem. C, 2014, 2, 5487–5501
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
alignment could be achieved and therefore in the 2D patterns,
all scattering form closed rings with uniform intensity distri-
bution (see Fig. 7 and S13–S17†). This is due to the disordered
meso-scale structure, which is an inherent feature of all DC
phases and leads to their optically isotropic appearance as well
as to a broadening of all XRD reections. The 2q scans of the
other compounds An are collated in Fig. 8.
In all cases the d-value is signicantly larger than Lmol/2, but
also smaller than the single molecular length, indicating a tilted
monolayer-like organization of the molecules in the DC phases.
The d-value of the layer reection grows, as expected, with rising
molecular length (Fig. 9a). The ratio d/Lmol is ꢂ0.73–0.75 for all
investigated compounds which would, according to d/Lmol
¼
cos b, lead to a tilt angle around 40ꢁ.46 This relatively large
difference between d and Lmol distinguishes these DC phases
from the previously reported HNF phases where d is usually
close to the molecular length.16,17
Fig. 6 Textures of compound A20 as observed between crossed and
slightly uncrossed polarizers. (a–c) Cr[*] phase at T ¼ 93 ꢁC as obtained
upon cooling with a rate of 5 K minꢀ1 and (e–g) DC phase as obtained
on heating to 100 ꢁC; (b and f) between crossed polarizers and (a and
e) between slightly uncrossed polarizers with the analyzer rotated 8ꢁ
clockwise and (c and g) rotated 8ꢁ anticlockwise; (d and h) show
models of the molecular organization; (d) in the Cr[*] phase the alkyl
chains are fully crystallized, the aromatics are more disordered and
have a stronger helical twist than in the DC phase; (h) in the DC phase
where the chains are disordered, allowing a denser and more ordered
packing of the aromatic cores (“core crystallization”); this leads to a
reduced molecular twist (reduced optical activity) and a stronger layer
deformation, giving rise to optical uniaxiality.
3.4.2 Electrooptical investigations of the DC phases. In
electrooptical experiments no current peak could be observed in
the DC phases and also no birefringence is induced under an
applied triangular wave voltage up to 200 Vpp in a 6 mm ITO cell.
These features are similar to those known for so crystalline
HNF phases (B4 phases) and distinct from most uid sponge
phases.
3.4.3 Investigation of the DC phases by XRD. XRD investi-
gations of A14 in the DC phase (see Fig. 7) indicate an intense
layer reection with its weak second to h order harmonic in
Fig. 7 XRD pattern of the DC phase of compound A14 at T ¼ 70 ꢁC, (a)
complete diffraction pattern, (b) small angle region and (c) 2q-scans at
the small and medium angle region (d ¼ 4.47 nm, T ¼ 70 ꢁC). No T ¼ 70 ꢁC and T ¼ 90 ꢁC.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5493

View Article Online
Journal of Materials Chemistry C
Paper
In the medium angle region of the diffraction patterns of chain lengths. An indexing of the wide angle diffraction
compounds A14–A20 there are additional distinct scattering patterns was not attempted, because of the limited number and
maxima on top of a broad diffuse scattering covering the range the diffuse character of the reections. Due to the overlapping
between 2q ¼ 5ꢁ and 2q ¼ 15ꢁ (see Fig. 8), only for A10 exclu- of several scattering maxima the precise assignment of the
sively the diffuse scattering could be observed. These reections positions of the maxima is difficult and the presence of addi-
can be indexed as higher harmonics of the layer reection (see tional scattering in this region cannot be excluded. In any case,
Fig. 7c), as also found for the HNF phases. In the wide angle the scattering in this region should result from the packing of
range there is a broad feature which could be tted to 4 maxima the crystallized (mainly aromatic) segments on a 2D in-plane
(w1–w4) in the 2q range between 19 and 21ꢁ (see Fig. 7c). This lattice. The signicant line broadening is thought to be due to
pattern excludes uid sponge phases, showing exclusively a very the limited correlation length of the crystalline micro-domains.
diffuse wide angle scattering besides the layer reection.
Major changes can be observed for the scattering w3 occur-
However, it is also distinct from the typical patterns of HNF ring around 2q ꢂ 19–20ꢁ which decreases in intensity (compare
phases (B4 phases), where the wide angle reections have other Fig. 8a–e) and is shied to smaller d-values (see Fig. 9b) from
positions and appear as signicantly sharper separate A14 to A20. Based on its d-value in the range between 0.46 and
peaks.9,20,21
0.43 nm and its strong intensity dependence on temperature
With growing chain length the intensity of the second (see discussions below) it is thought that this scattering is most
harmonics of the small angle scattering decreases and it has likely due to the mean distance between less ordered (not
completely disappeared for compound A18 (Fig. 8), whereas the crystallized) segments of the alkyl chains. The observation that
(50) and (40) reections increase in intensity, most probably with increasing chain length the position of w3 is shied to
due to the changing electron density modulation resulting from smaller d-values would be in line with a denser chain packing
the changing thickness of the aliphatic layers and other struc- for longer chains.
tural modications. There is also a nearly continuous change of
The temperature effect on the XRD pattern of compound A14
the positions of the wide angle scattering (Fig. 8 and 9), indi- in the DC phase is shown in Fig. 7c. There is no signicant
cating continuous structural transformations depending on the inuence on the intensities and positions of the scattering
Fig. 8 Comparison of 2q-scans of the XRD patterns at the indicated temperatures for (a) A10, (b) A14, (c) A16, (d) A18 and (e) A20 in the DC phase
(cooling rate 0.5 K minꢀ1 in all cases) and (f) A20 in the Cr[*] phase (after cooling with rate 10 K minꢀ1).
5494 | J. Mater. Chem. C, 2014, 2, 5487–5501
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
the suggested core crystallization as origin of the exotherm (II)
occurring on heating at the transition from the birefringent
Cr[*] phase to the optically isotropic DC phase (see curve c in
Fig. 5d). Because chirality is observed in the birefringent Cr[*]
phase as well as in the isotropic DC phase, the aromatic cores
should in both phases adopt chiral helical conformations with
uniform helix sense, which is opposite in the distinct chiral
domains. At the Cr[*]–DC transition the contrast between the
dark and bright domains decreases (compare Fig. 6a, e and c, g).
This reduced optical activity indicates a reduction of the helical
twist of the molecular conformations at the transition from the
crystalline to the DC phase. Molecular entities with uniform
helix sense cannot be densely packed in nondistorted layers and
thus induce twist and curvature. This distortion increases with
growing packing density of the helical entities, and therefore
the denser core packing leads to a stronger layer deformation
force, giving rise to the formation of the optically isotropic DC
phase. The birefringent Cr[*] phase, formed on rapid cooling
appears to be dominated by optimized alkyl chain packing
(chain crystallization), which allows a non-twisted lamellar or
modulated lamellar organization of the molecules, but without
optimized core-packing. The aromatic cores remain in a
strongly twisted helical conformation, leading to a high optical
activity, but the strong molecular twist also leads to a disruption
of the aromatic layers into smaller domains (see Fig. 6d). Upon
heating the Cr[*] phase, the alkyl chain order decreases and a
growing fraction of these chains becomes disordered. This
allows a denser core packing, simultaneously leading to a
reduced helicity of the molecular conformations (reduced
Fig. 9 Dependence of the XRD reflex position in the DC phases of
compounds A10–A20 on chain length, (a) d-values of the layer
reflections with the observed higher harmonics and (b) wide angle
scattering maxima, the assignment of the reflections is shown in
Fig. 7c, see also Fig. S13–S17.†
maxima with exception of w3 which is strongly reduced at optical activity) and, giving rise to a strong layer distortion and
decreased temperature, though its position is retained. formation of the DC phase with an increased alkyl chain
Reducing the temperature has obviously a similar effect on the disorder. This means that the isotropic DC phases are the result
intensity of this scattering like increasing the alkyl chain length. of the optimization of core packing and represent disordered
This supports the assumption that this scattering maximum versions of the birefringent crystalline conglomerates (Cr[*]
does not belong to the 2D in-plane lattice and could be attrib- phases), providing improved alkyl chain packing. With rising
uted to the mean distance between the disordered alkyl chain alkyl chain length the chain crystallization becomes increas-
segments. The fraction of the more disordered alkyl chain ingly more important. For compound A20 formation of the
segments becomes smaller as temperature is reduced and as the optical isotropic DC phase can still be achieved by using suffi-
alkyl chain length is increased, because longer chains provide a ciently slow cooling rates, so that the slow core crystallization
higher tendency for chain crystallization, as mentioned above. can take place and determine the structure, leading to the DC
This corresponds with the observed strong rise of the DC-Iso phase. On fast cooling, however, densest core packing obviously
transition enthalpy from 24.1 kJ molꢀ1 for A/10 to 59 kJ molꢀ1 cannot be achieved and the fast chain crystallization leads to a
for A/20 (see Table 1 and Fig. 5a; values on heating). In line with non-twisted lamellar organization, giving rise to the birefrin-
increasing chain crystallization by chain elongation, the opti- gent conglomerate phase Cr[*] with reduced core order and
cally isotropic DC phase of compound A20 is in competition enhanced optical activity. For compound A22 chain crystalli-
with a low birefringent crystalline conglomerate phase Cr[*] and zation dominates at all cooling rates, and hence, exclusively a
exclusively a Cr[*] phase is formed by A22 with the longest crystalline phase is formed.
chains.
Low birefringent crystalline conglomerate phases were also
For compound A20 the XRD patterns were recorded at the observed for compounds A14–A18, but for these compounds
same temperature (T ¼ 90 C) aer slow cooling (0.5 K min
very high cooling rates ([20 K minꢀ1) are required for their
ꢀ1
ꢁ
Fig. 8e) in the DC phase as well as aer fast cooling (10 K minꢀ1 formation and there is apparently always a coexistence of these
Fig. 8f) in the Cr[*] phase. Surprisingly, the two XRD patterns birefringent Cr[*] phases with the isotropic DC phases, which
look very similar, only all scattering appear to be a bit broader makes the investigations difficult. Fig. 10 shows representative
and have slightly lower intensity in the birefringent Cr[*] phase, textures and DSC curves as obtained for compound A16 aer
though the sample and exposure time were identical. This is rapid cooling (30 K minꢀ1 for the texture and 50 K minꢀ1 before
reproducibly observed and would suggest a reduced local order recording the DSC trace a). For these compounds core crystal-
in the Cr[*] phase compared to the DC phase. This is in line with lization is sufficiently fast compared to alkyl chain
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5495

View Article Online
Journal of Materials Chemistry C
Paper
crystallization and therefore DC phases can be observed for
moderate cooling rates. The Cr[*] phases are absent for all
shorter homologues starting with A12, which have the smallest
contribution of alkyl chain crystallization to the total transition
enthalpy value (Fig. 5a); here self-assembly is dominated by
optimized core packing, which leads to DC phase formation.
3.4.4 Molecular conformation and structure property rela-
tions. DFT calculations of the model compound A1 with n ¼ 1
(Gaussian 09 package, B3LYP functional and the LAN L2 DZ
basis set) indicate a bent and helically twisted chiral molecular
conformation as the minimum energy conformation shown in
Fig. 11. The 4-methyl group at the resorcinol core unit does not
induce a signicant deviation from the 120ꢁ bending angle
between the two azobenzene wings. Also, the azobenzene units
themselves are linear and the benzene rings in these wings as
well as the C]O groups are nearly coplanar. However, the
methyl group at the 4-position at the resorcinol core induces a
signicant twist of the adjacent carboxyl group together with
the coplanar azobenzene wing out of the plane of the resorcinol
unit by 72.4ꢁ, which is much larger than that of the azobenzene
wings adjacent to H-6 (53.6ꢁ). This strong twist leads to an
overall helical molecular conformation and this is assumed to
be responsible for a high tendency for chiral segregation and
distorted packing in layers. Though the understanding of the
general structure property relations with respect to formation of
DC phases is still at the infancy it appears that the presence of
relatively at and rigid aromatic p-systems, as provided by
Fig. 11 Energy minimized molecular conformer of the model
compound A1 (n ¼ 1) as observed under two distinct perspectives (a)
perpendicular to the bending plane and (b) along the azobenzene wing
adjacent to the methyl group.
azobenzenes and to some extent also by Schiff base units is appears that p-stacking favours DC phase formation. In the case
favorable for formation of so crystalline DC phases. So, it of the azobenzenes the presence of an additional relatively
bulky substituent, such as I, Br or CH3 seems to be required,
probably by providing a stronger molecular twist. In contrast,
formation of LC sponge type DC phases is favoured for bent-
core compounds with more exible phenylbenzoate wings and
having long and exible or bulky end groups at the termini.8
However, nal conclusions cannot be drawn here and require
further investigations.
3.4.5 Investigation of mixtures with 5-CB. It is known that
HNF phases can be diluted by nematic LC hosts to a high degree
(>95%) without the loss of the chirality.47 This is a direct conse-
quence of the helical nanolament structure of these phases,
allowing a swelling of the laments by the nematic LC and
transfer of chirality from the nanolaments to the nematic LC.
This effect is not observed for the sponge type DC phases formed
by distorted uid smectic phases. Therefore, investigation of
mixtures with 5-CB could provide additional information con-
cerning the microstructure of the DC phases of compounds An.
The 1 : 1 mixture of A14 with 40-n-pentyl-4-cyanobiphenyl (5-CB)
does not show any DC phase. On heating this mixture only a direct
transition from the crystalline material to the isotropic liquid state
Fig. 10 Compound A16: (a) DSC traces (10 K minꢀ1): curve a, heating
occurs at 85 ꢁC which is below the DC–Iso transition temperature
after previous cooling with 50 K minꢀ1; curves b and c, heating/cooling
of pure A14 (T ¼ 101 ꢁC). On cooling this mixture a transition from
with 10 K minꢀ1; (b) low birefringent texture of the Cr[*] phase as
obtained after fast cooling (30 K minꢀ1); (c) same texture between the isotropic liquid to the nematic phase takes place at T ꢂ 54 ꢁC,
slightly uncrossed polarizers and (d) after reversal of the direction of
one polarizer, indicating the presence of a chiral conglomerate;
remarkable is the presence of a domain with higher contrast in the
center of the circular domain at the top right; probably, the birefrin-
gent textures develop as surface layers and in these regions goes
through the complete sample.
but no chiral domains can be observed. The nematic phase
rapidly crystallizes with formation of a highly birefringent crys-
talline phase at T ꢂ 53 ꢁC (see Table 2 and Fig. S19†).
The longer compounds A16–A20 behave differently; for the
1 : 1 mixtures the DC phases were retained and their
5496 | J. Mater. Chem. C, 2014, 2, 5487–5501
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
temperature ranges increase with growing chain length (Table 2 Fig. S10†). The presence of two polarization reversal peaks in
and Fig. S20†). However, at only slightly higher 5-CB concen- each half period of an applied triangular wave voltage (160 Vpp
,
tration, as for example, in the 4 : 6 mixtures of compounds A16– see Fig. 12a) indicate an antiferroelectric switching process. The
A20 with 5-CB the chiral DC phase is lost and instead a heter- spontaneous polarization value was calculated to be Ps ¼ 650 nC
ogenous mixture is obtained indicating that the amount of 5-CB cmꢀ2 at 85 ꢁC. On cooling the compound in a 6 mm ITO coated
which can be mixed into the DC phases of compounds An is cell under an external applied eld circular domains are formed
strongly limited. So, compared with the HNF phases, the DC (Fig. 12c). At 0 V the position of the extinction crosses is parallel
phases of compounds An can only be retained if the alkyl chains to the polarizers, indicating an anticlinic tilted SmCaPA phase.
are sufficiently long, but also in this case much less 5-CB is Under an applied electric eld the extinction crosses in the
tolerated. These observations, in conjunction with the XRD circular domains become inclined to the directions of the
studies, conrm that the DC phases of compounds An should polarizers, indicating a eld induced synclinic SmCsPF phase.
be different from the classical HNF phases. It appears that these By changing the direction of the applied electric eld three
phases are formed by smaller nano-domains instead of long distinct states (see Fig. 12b–d) can be clearly observed con-
helical laments, in line with the results discussed in the rming a tristable switching process by rotation on a cone.
previous sections. Because no extended laments are formed, Based on these results it is concluded that this is a B2-type polar
no gel-like networks of these bers are possible and therefore SmC phase with an anticlinic antiferroelectric SmCaPA ground
these compounds can take up only a limited amount of 5-CB, state structure.
which is directly incorporated between the bent-core molecules
thus reducing the stability of the DC phases.
Replacing the uorine substituents in Bn by methyl groups
(D14) leads to a monotropic SmC phase (for texture see
Mixing the longest homologue A22, which does not show any Fig. S6b†) without polar order, also indicated by the much
DC phase, with 5-CB in 1 : 1 ratio induces a DC phase as an smaller SmC–Iso transition enthalpy values (0.9 kJ molꢀ1) and
enantiotropic phase between T ¼ 59 ꢁC and 87 ꢁC (see Fig. S21†). the observation that no current peak could be observed up to a
This shows that 5-CB can reduce the chain crystallization and voltage of 200 Vpp in a 6 mm ITO cell. It appears that increasing
thus allow the formation of the DC phase even if the pure the size of the peripheral substituents X reduces the packing
compound itself does not show any DC phase. However, no DC density signicantly, thus leading to strong reduction of mes-
phase can be induced in the uorinated compounds Bn (X ¼ F, ophase stability in the order H ꢂ F > CH3 > Br, at rst leading to
1 : 1 mixtures with 5-CB) that show polar smectic phases and a loss of polar order for X ¼ CH3 and nally to a complete
are discussed in Section 3.5.
3.5 Polar and non-polar smectic phases formed by
compounds Bn–Dn with additional peripheral substituents
In order to investigate the effect of introducing a lateral
substitution on the outer two benzene rings in the periphery of
the aromatic core of the bent-core liquid crystals An, related
molecules Bn, C14 and D14 were synthesized (Scheme 2). In the
series of compounds Bn with X ¼ F all compounds show the
same type of mesophase, irrespective of the chain length (see
Table 1). As an example, on heating B14 in a homeotropic cell a
ꢁ
schlieren texture is observed at T ¼ 90 C (see Fig. S6a†) indi-
cating the presence of a SmC phase. The relatively high transi-
tion enthalpy (DH ꢂ 12.8 kJ molꢀ1) is in the range as usually
found for polar SmC phases (for representative DSC traces, see
Table 2 Phase transition temperatures and mesophase types of 1 : 1
mixtures of 5-CB and compounds A14–A22 and B14a
Mixture
Heating T/ꢁC
Cooling T/ꢁC
A14–5-CB
A16–5-CB
A18–5-CB
A20–5-CB
A22–5-CB
B14–5-CB
Cr1 39 Cr2 85 Iso
Cr 64 DC 73 Iso
DC 77 Iso
Iso 54 N 53 Cr
Iso 66 DC
Iso 72 DC
Fig. 12 Electrooptical investigation of compound B14. (a) Switching
current response curve recorded by applying a triangular wave voltage
(160 Vpp, 10 Hz, 5 kU) to a 6 mm coated ITO cell with a homogeneous PI
alignment layer at 85 ꢁC in the SmCaPA phase; (b–d) optical textures as
obtained at T ¼ 79 ꢁC under an applied DC voltage between crossed
polarizers (parallel rubbing), rubbing direction parallel to the polarizer,
direction of the polarizers is shown in (b); (b) field induced SmCsPF
state at +10 V, (c); SmCaPA ground state after switching off the field (0
V) and (d) the field induced SmCsPF state at ꢀ10 V.
DC 80 Iso
Iso 78 DC
Cr[*] 59 DC 87 Iso
Cr 63 Iso
Iso 80 DC 37 Cr[*]
Iso 61 Cr
a
Transition temperatures were taken from the observed textures using
polarized optical microscopy; abbreviations: N ¼ nematic phase, for
other abbreviations please see Table 1.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5497

View Article Online
Journal of Materials Chemistry C
Paper
suppression of all mesophases for X ¼ Br. As the electronegative
In XRD measurements of compound E14 (Fig. S18†) several
F atoms increase the polarity of the core region, and hence equally spaced sharp reections can be found in the small angle
increase the segregation of the core from the lipophilic alkyl region, indicating a lamellar structure with a d-value of 4.83 nm.
chains, there is no signicant mesophase destabilization or a The diffuse scattering in the wide angle region shows four
slight stabilization if H is replaced by F (compare A14 and B14 intensity maxima beside the equator, from which a tilt angle of
in Table 1). But nevertheless, chiral segregation is removed. The the molecules of ꢂ28ꢁ could be calculated. As the extinction
dominance of steric effects of Br and CH3 leads to the loss of crosses are approximately parallel to the polarizers (Fig. S6c†)
polar order and to the lowest LC phase stability for these and an equal intensity of the wide angle scattering is found
compounds.
(Fig. S18a and b†) the tilt correlation should be anticlinic.
Though there are indications of layer modulations or undula-
tions from other experimental data, these are not visible in the
XRD patterns. Either the modulation wavelength is very long, so
that the layer reections are not resolved into separate reec-
tions, or the reections with h and k s 0 are too weak to be
detected with the used XRD setup.
Replacing the terminal alkoxy chains in E14 by alkyl chains
in compound F14 (see Scheme 3, for DSC traces see Fig. S11a†)
leads to a compound with about 20 K lower clearing tempera-
3.6 Effects of replacing the azo groups by ester groups
Aiming to investigate the effects of replacing the azo linking
groups in compounds A14 by ester groups, compounds E14, F14
and G14 were synthesized. As shown in Scheme 3, the meso-
phase exhibited by E14 is enantiotropic with a relatively wide LC
range and much higher transition temperatures than found for
A14. The texture of the LC phase (Fig. S6c†) is similar to those
observed for modulated smectic phases of bent-core meso-
gens.48 On applying a triangular wave electric eld of 160 Vpp to
E14 in a 6 mm ITO cell, two polarization reversal peaks can be
clearly observed in the switching current response curve (see
Fig. S22†) indicating antiferroelectric switching. The sponta-
~
ture which can also be assigned as SmCaPA based on the
observed textures and measured Ps values (Ps ¼ 300 nC cmꢀ2 at
95 ꢁC, see Fig. S24†). The major difference between the two
compounds is the inclination of the dark extinctions upon
applying an electric eld (Fig. S25†), indicating that the
switching process in this case takes place on a cone; so probably
the modulation wavelength of F14 is larger than for E14.
Inverting the direction of the terminal ester group in
compound G14 (see Scheme 3) further reduces the transition
temperatures, which are now similar to those of the azobenzene
derivative A14 (for DSC traces see Fig. S11b†). Also in this case
the texture is similar to those of some modulated smectic
phases (Fig. S6d†). The position of the extinction crosses is
inclined with the orientation of the polarizers and thus indi-
cates a synclinic tilted organization of the molecules. In elec-
trooptic studies (160 Vpp in a 6 mm ITO cell) no current response
can be detected. This might be due to a shorter modulation
wavelength than in the case of E14, which is known to suppress
polar switching or would require very high voltages. Also the
observation of a uniform synclinic tilt would be in line with a
neous polarization value, Ps ¼ 310 nC cmꢀ2 at 120 C is only
ꢁ
about one half of that usually observed for SmCPA phases
(ꢂ600–800 in the B2 phases) and is in the typical range as
observed for modulated SmCPA phases. By changing the direc-
tion of the applied electric eld (Fig. S23†), no change in the
texture could be observed, conrming that the switching
process in this mesophase takes place by collective rotation
around the long axis of the molecules. This mode of switching is
also in line with a modulated structure of this SmCPA phase
~
(SmCaPA); for a nondistorted at SmCaPA phase a switching on a
cone would be more likely.
B1rev-like modulated SmC phase with an oblique lattice.
However, it was not possible to further conrm the proposed
phase structure by XRD due to the rapid crystallization. Overall,
replacing the azobenzene wings by terephthalates (E14, F14) or
benzoates (G14) removes the DC phases and leads to polar and
apolar smectic phases with a signicant tendency for layer
modulation or undulation.
4. Conclusions and summary
New bent-core liquid crystalline materials combining 4-meth-
ylresorcinol with azobenzene wings were synthesized and
investigated. The short homologues (n ¼ 6–8) exhibit interca-
Scheme 3 Chemical structures of the synthesized 4-methylresorcinol
based terephthalates E14, F14 and benzoate G14 with transition lated smectic phases (B6 phases) which upon increasing the
temperatures (T/ꢁC) and corresponding enthalpies (DH/kJ molꢀ1, given
chain length (n ¼ 9–20) convert to DC phases and for the longest
in square brackets) on heating (top lines) and cooling (bottom lines, both
homologue (n ¼ 22) only a crystalline phase was observed. For
recorded at a rate of 10 K minꢀ1); abbreviations: SmCaPA ¼ modulated
~
compound A20 formation of the optically isotropic DC phase is
in competition with formation of a low birefringent crystalline
conglomerate (Cr[*]) and the observed phase type is strongly
~
antiferroelectric switching anticlinic SmC phase; SmC ¼ non-switchable
modulated SmC phase with synclinic tilt correlation, for other abbrevi-
ations, see Scheme 1 and Table 1; for DSC traces, see Fig. S11.†
5498 | J. Mater. Chem. C, 2014, 2, 5487–5501
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
dependent on the cooling rate. It appears that formation of the
Cr[*] phase is favored by chain crystallization, whereas in the
isotropic DC phases core crystallization is dominating. Based
on the XRD patterns and high viscosity the DC phases of
compounds An belong to the so crystalline DC phases, but
they are distinct from the previously reported B4-type HNF
phases by the absence of extended laments as indicated by the
results of their mixtures with 5-CB.
3 D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark,
¨
E. Korblova and D. M. Walba, Science, 1997, 278, 1924.
4 (a) J. Harden, B. Mbanga, N. Eber, K. Fodor-Csorba,
´
S. Sprunt, J. T. Gleeson and A. Jakli, Phys. Rev. Lett., 2006,
97, 157802; (b) A. C. Charif, N. Diorio, K. Fodor-Csorba,
´
´
J. E. Puskasad and A. Jakli, RSC Adv., 2013, 3, 17446.
5 (a) W. Iglesias, T. J. Smith, P. B. Basnet, S. R. Stefanovic,
´
C. Tschierske, D. J. Lacks, A. Jakli and E. K. Mann, So
Introduction of additional substituents at the outer rings of
the bent-core molecules An removes the DC phases and replaces
them rst by polar smectic phases (X ¼ F, SmCaPA). Upon
further enlargement of these substituents (X ¼ CH3) also polar
order is removed, yielding a non-polar SmC phase which is then
removed for the compound with the largest substituent X ¼ Br.
Replacing the azo groups by ester groups in the related tere-
phthalates or benzoates also removes the DC phases and leads
to different types of modulated SmC phases which in most cases
show polar switching.
Overall, new modications of isotropic as well as birefrin-
gent conglomerate phases were observed and the relationship
between molecular structure and formation of these phases was
identied. It appears that the azobenzene unit is a specially
powerful tool for introducing new types of chiral conglomerate
phases, distinct from the typical B4 type phases found for
benzylidene imines. There seems to be a whole zoo of different
types of these DC phases, ranging from the uid B2-like sponge
phases to several distinct types of so crystalline phases,
including modulated subtypes;49–51 obviously only a small frac-
tion of the potential structures has been explored yet. Moreover,
the possibilities provided by introduction of the photo-
isomerizable azobenzene units into BCLC forming DC phases
could lead to interesting perspectives for chirality switching and
phase modulation, leading to unique and potentially useful
multifunctional chiral materials for application in chiral
discrimination of chiral physical forces and molecular species22
and as non-centrosymmetric materials.
Matter, 2011, 7, 9043; (b) W. Iglesias, N. L. Abbott,
´
E. K. Mann and A. Jakli, ACS Appl. Mater. Interfaces, 2012,
4, 6884.
6 J. Etxebarria and M. B. Ros, J. Mater. Chem., 2008, 28, 2919.
7 M. Nagaraj, Y. P. Panarin, J. K. Vij, C. Keith and
C. Tschierske, Appl. Phys. Lett., 2010, 97, 213505.
8 (a) G. Dantlgraber, A. Eremin, S. Diele, A. Hauser, H. Kresse,
G. Pelzl and C. Tschierske, Angew. Chem., Int. Ed., 2002, 41,
2408; (b) G. Dantlgraber, S. Diele and C. Tschierske, Chem.
Commun., 2002, 2768; (c) C. Tschierske and G. Dantlgraber,
Pramana, 2003, 61, 455; (d) C. Keith, R. A. Reddy,
A. Hauser, U. Baumeister and C. Tschierske, J. Am. Chem.
Soc., 2006, 128, 3051; (e) C. Keith, R. A. Reddy,
U. Baumeister, H. Hahn, H. Lang and C. Tschierske, J.
Mater. Chem., 2006, 16, 3444; (f) H. Hahn, C. Keith,
H. Lang, R. A. Reddy and C. Tschierske, Adv. Mater., 2006,
18, 2629; (g) C. Keith, G. Dantlgraber, R. A. Reddy,
U. Baumeister, M. Prehm, H. Hahn, H. Lag and
C. Tschierske, J. Mater. Chem., 2007, 17, 3796; (h) C. Keith,
G. Dantlgraber, R. A. Reddy, U. Baumeister and
C. Tschierske, Chem. Mater., 2007, 19, 694; (i) Y. Zhang,
U. Baumeister, C. Tschierske, M. O'Callaghan and
C. Walker, Chem. Mater., 2010, 2869; (j) H. Ocak, B. Bilgin-
Eran, M. Prehm and C. Tschierske, So Matter, 2012, 8, 7773.
9 L. E. Hough, M. Spannuth, M. Nakata, D. A. Coleman,
C. D. Jones, G. Dantlgraber, C. Tschierske, J. Watanabe,
¨
E. Korblova, D. M. Walba, J. E. Maclennan, M. A. Glaser
and N. A. Clark, Science, 2009, 325, 452.
10 G. Heppke, D. D. Parghi and H. Sawade, Liq. Cryst., 2000, 27,
313.
11 J. Thisayukta, Y. Nakayama, S. Kawauchi, H. Takezoe and
J. Watanabe, J. Am. Chem. Soc., 2000, 122, 7441.
Acknowledgements
M. Alaasar is grateful to the Alexander von Humboldt Founda-
tion for the research fellowship at the Martin-Luther University 12 (a) R. A. Reddy and B. K. Sadashiva, Liq. Cryst., 2003, 30,
Halle-Wittenberg, Germany.
1031; (b) A. Roy, M. Gupta, S. Radhika, B. K. Sadashiva and
R. Pratibha, So Matter, 2012, 8, 7207.
13 (a) J. Ortega, C. L. Folcia, J. Etxebarria, N. Gimeno and
M. B. Ros, Phys. Rev. E: Stat., Nonlinear, So Matter Phys.,
2003, 68, 11707; (b) N. Gimeno, A. Sanchez-Ferrer,
N. Sebastian, R. Mezzenga and M. B. Ros, Macromolecules,
2011, 44, 9586.
References
1 T. Sekine, T. Niori, J. Watanabe, T. Furukawa, S. W. Choi and
H. Takezoe, J. Mater. Chem., 1997, 7, 1307.
2 (a) R. A. Reddy and C. Tschierske, J. Mater. Chem., 2006, 16,
¨
907; (b) H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., Part 14 M. Nagaraj, K. Usami, Z. Zhang, V. Gortz, J. W. Goodby and
1, 2006, 45, 597; (c) G. Pelzl, S. Diele and W. Weissog, Adv.
H. F. Gleeson, Liq. Cryst., 2014, 41, 800.
Mater., 1999, 11, 707; (d) C. Tschierske, Angew. Chem., Int. 15 (a) A. Jakli, Y.-M. Huang, K. Fodor-Csorba, A. Vajda, G. Galli,
´
¨
Ed., 2013, 52, 8828; (e) V. Gortz and J. W. Goodby, Chem.
S. Diele and G. Pelzl, Adv. Mater., 2003, 15, 1606; (b)
¨
Commun., 2005, 3262; (f) G. Pelzl and W. Weissog, in
Thermotropic Liquid Crystals, ed. A. Ramamoorthy,
W. Weissog, M. W. Schroder, S. Diele and G. Pelzl, Adv.
Mater., 2003, 15, 630.
¨
Springer, The Netherlands, 2007, pp. 58–83; (g) I. Dierking, 16 (a) L. E. Hough, H. T. Jung, D. Kruerke, M. S. Heberling,
Angew. Chem., Int. Ed., 2010, 49, 29; (h) A. Eremin and M. Nakata, C. D. Jones, D. Chen, D. R. Link, J. Zasadzinski,
´
¨
A. Jakli, So Matter, 2013, 9, 615.
G. Heppke, J. P. Rabe, W. Stocker, E. Korblova,
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5499

View Article Online
Journal of Materials Chemistry C
Paper
D. M. Walba, M. A. Glaser and N. A. Clark, Science, 2009, 325,
J. Etxebarria, J. Mater. Chem., 2007, 17, 2219; (h)
J. Etxebarria and M. B. Ros, J. Mater. Chem., 2008, 18, 2919;
(i) I. C. Pintre, J. L. Serrano, M. B. Ros, J. M. Perdiguero,
I. Alonso, J. Ortega, C. L. Folcia, J. Etxebarria, R. Alicante
and B. Villacampa, J. Mater. Chem., 2010, 20, 2965; (j)
M. Vijaysrinivasan, P. Kannan and A. Roy, Liq. Cryst., 2012,
39, 1465.
¨
456; (b) D. M. Walba, L. Eshat, E. Korblova and
R. K. Shoemaker, Cryst. Growth Des., 2005, 5, 2091; (c)
D. Chen, J. E. Maclennan, R. Shao, D. K. Yoon, H. Wang,
¨
E. Korblova, D. M. Walba, M. A. Glaser and N. A. Clark, J.
Am. Chem. Soc., 2011, 133, 12656.
17 (a) J. Thisayukta, H. Takezoe and J. Watanabe, Jpn. J. Appl.
Phys., Part 1, 2001, 40, 3277; (b) H. Niwano, M. Nakata, 33 (a) M. Alaasar, M. Prehm, M. Nagaraj, J. K. Vij and
J. Thisayukta, D. R. Link, H. Takezoe and J. Watanabe, J.
Phys. Chem. B, 2004, 108, 14889; (c) H. Kurosu,
M. Kawasaki, M. Hirose, M. Yamada, S. Kang,
C. Tschierske, Adv. Mater., 2013, 25, 2186; (b) M. Alaasar,
M. Prehm, K. May, A. Eremin and C. Tschierske, Adv.
Funct. Mater., 2013, 24, 1703.
J. Thisayukta, M. Sone, H. Takezoe and J. Watanabe, J. 34 (a) M. Alaasar, M. Prehm and C. Tschierske, Liq. Cryst., 2013,
Phys. Chem. A, 2004, 108, 4674.
18 H. Kresse, J. Saltetnokova, H. Nadasi, W. Weissog and
A. Hauser, Liq. Cryst., 2001, 28, 1017.
19 V. Prasad, Liq. Cryst., 2001, 28, 1115.
20 H. Nadasi, C. Lischka, W. Weissog, I. Wirth, S. Diele,
40, 656; (b) M. Alaasar, M. Prehm and C. Tschierske, Liq.
Cryst., 2014, 41, 126.
35 cv ¼ crystal volumes, calculated using the increments of A.
Immirzi: A. Immirzi and B. Perini, Acta Crystallogr., Sect. A:
Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1977, 33, 216.
G. Pelzl and H. Kresse, Mol. Cryst. Liq. Cryst., 2003, 399, 69. 36 Very few examples of BCLCs based on 4-methylresorcinol
21 J. Martinez-Perdiguero, I. Alonso, C. L. Folcia, J. Etxebarria
and J. Ortega, J. Mater. Chem., 2009, 15, 5161.
have been reported previously: (a) U. Dunemann,
¨
M. W. Schroder, R. A. Reddy, G. Pelzl, S. Diele and
22 H. Takezoe, Top. Curr. Chem., 2012, 318, 303.
W. Weissog, J. Mater. Chem., 2005, 15, 4051; (b)
W. Weissog, G. Pelzl, H. Kresse, U. Baumeister, K. Brand,
¨
23 D. K. Yoon, Y. Yi, Y. Shen, E. D. Korblova, D. M. Walba,
¨
I. I. Smalyukh and N. A. Clark, Adv. Mater., 2011, 23, 1962.
24 J. Ortega, N. Pereda, C. L. Folcia, J. Etxebarria and M. B. Ros,
Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip.
Top., 2000, 63, 011702.
25 G. Lee, R. J. Carlton, F. Araoka, N. L. Abbott and H. Takezoe,
Adv. Mater., 2013, 25, 245.
M. W. Schroder, M. G. Tamba, S. Findeisen-Tandel,
U. Kornek, S. Stern, A. Eremin, R. Stannarius and
J. Svoboda, J. Mater. Chem., 2010, 20, 6057.
37 C. Keith, A. Lehmann, U. Baumeister, M. Prehm and
C. Tschierske, So Matter, 2010, 6, 1704.
38 M. J. Frisch, et al., Gaussian 09, Revision A.1, Gaussian Inc.,
Wallingford CT, 2009.
´
26 E. Bialecka-Florjanczyk, I. Sledzinska, E. Gorecka and
J. Przedmojski, Liq. Cryst., 2008, 35, 401.
39 (a) S. Kang, M. Tokita, Y. Takanishi, H. Takezoe and
J. Watanabe, Phys. Rev. E: Stat., Nonlinear, So Matter
Phys., 2007, 76, 042701; (b) S. Kang, S. K. Lee, M. Tokita
and J. Watanabe, Phys. Rev. E: Stat., Nonlinear, So Matter
Phys., 2009, 80, 042703.
27 (a) S.-W. Choi, T. Izumi, Y. Hoshino, Y. Takanishi,
K. Ishikawa, J. Watanabe and H. Takezoe, Angew. Chem.,
Int. Ed., 2006, 45, 1382; (b) G. Lee, R. J. Carlton, F. Araoka,
N. L. Abbott and H. Takezoe, Adv. Mater., 2013, 25, 245.
28 M. Alaasar, M. Prehm and C. Tschierske, Chem. Commun., 40 Also dielectric studies conrm the similarity of the B6
2013, 49, 11062.
phases having short-range ordered ribbons with the B1
phases formed by a long range 2D lattice of ribbons:
H. Kresse, H. Schmalfuss and W. Weissog, Liq. Cryst.,
2001, 28, 799.
29 I. C. Pintre, J. L. Serrano, M. B. Ros, J. M.Perdiguero,
I. Alonso, J. Ortega, C. Folcia, J. Etxebarria, R. Alicantee
and B. Villacampa, J. Mater. Chem., 2010, 20, 2965.
30 Y. Zang, J. Martinez-Perdiguero, U. Baumeister, C. Walker, 41 The electron density modulation occurs between the regions
J. Etxebirra, M. Prehm, J. Ortega, C. Tschierske,
M. O'Calleghan, A. Harant and M. Handschy, J. Am. Chem.
Soc., 2009, 131, 18366.
where alkyl chains are mixed with the central units of the
bent cores (low electron density) and the regions with
overlapping parts of the rod-like wings (high electron density).
31 (a) H. G. Heller and J. R. Langan, J. Chem. Soc., Perkin Trans. 42 Due to this overlapping of phase transition and
1, 1981, 2, 341; (b) P. Schmitz, H. Gruler and M. Eberhardt,
Mol. Cryst. Liq. Cryst., 1995, 262, 129.
crystallization the transition enthalpy values could not be
determined for the B6–DC transition.
32 (a) V. Prasad, S.-W. Kang and S. Kumar, J. Mater. Chem., 43 I. Dierking, Liq. Cryst. Today, 2003, 12, 1.
´
2003, 13, 1259; (b) V. Prasad and A. Jakli, Liq. Cryst., 2004, 44 The difference of the transition temperatures in the cooling
31, 473; (c) V. Prasad, S.-W. Kang, X. Qi and S. Kumar, J.
curves b and d in Fig. 5c and d is mainly due to the very
´
Mater. Chem., 2004, 14, 1495; (d) A. Jakli, V. Prasad,
different cooling rate used.
D. S. S. Rao, G. Liao and I. Janossy, Phys. Rev. E: Stat., 45 A similar texture was recently described for mixtures of B4
Nonlinear, So Matter Phys., 2005, 71, 02170911; (e)
N. G. Nagaveni, A. Roy and V. Prasad, J. Mater. Chem.,
2012, 22, 8948; (f) C. L. Folcia, I. Alonso, J. Ortega,
J. Etxebarria, I. Pintre and M. B. Ros, Chem. Mater., 2006,
18, 4617; (g) I. C. Pintre, N. Gimeno, J. L. Serrano,
M. B. Ros, I. Alonso, C. L. Folcia, J. Ortega and
phases with nematic LC and was interpreted as an
alignment of the helicalnanolaments by the nematic
director, we think that in our case this texture has
different origin and indicates a crystalline conglomerate
phase: F. Araoka, G. Sugiyamam, K. Ishikawa and
H. Takezoe, Adv. Funct. Mater., 2013, 23, 2701.
5500 | J. Mater. Chem. C, 2014, 2, 5487–5501
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
46 The molecular length in the Chevron shaped conformation 49 H. Ocak, B. Bilgin-Eran, M. Prehm and C. Tschierske, So
(see Fig. S1†) was used for calculations; partial Matter, 2013, 9, 4590.
intercalation of the alkyl chains, leading to a reduced tilt 50 I. Miyake, Y. Takanishi, N. V. S. Rao, M. K. Paul,
a
angle, cannot be excluded.
K. Ishikawa and H. Takezoe, J. Mater. Chem., 2005, 15,
47 T. Otani, F. Araoka, K. Ishikawa and H. Takezoe, J. Am. Chem.
Soc., 2009, 131, 12368.
4688.
¨
51 E. Tsai, J. M. Richardson, E. Korblova, M. Nakata, D. Chen,
48 R. A. Reddy, U. Baumeister, C. Keith and C. Tschierske, J.
Mater. Chem., 2007, 17, 62.
Y. Shen, R. Shao, N. A. Clark and D. M. Walba, Angew.
Chem., Int. Ed., 2013, 52, 5254.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5487–5501 | 5501