1,2,4-oxadiazole-based bent-core liquid crystals with cybotactic nematic phases

Article abstract of DOI:10.1002/cphc.201301070

Several series of bent-core mesogens derived from 3,5-diphenyl-1,2,4- oxadiazole with or without lateral groups and with different length terminal chains at both ends, and polycatenar molecules with three to six alkoxy chains are synthesized and their mes

Full text of DOI:10.1002/cphc.201301070

CHEMPHYSCHEM
ARTICLES
DOI: 10.1002/cphc.201301070
1,2,4-Oxadiazole-Based Bent-Core Liquid Crystals with
Cybotactic Nematic Phases
Govindaswamy Shanker,[a, d] Marko Prehm,[a] Mamatha Nagaraj,[b, e] Jagdish K. Vij,[b]
Marvin Weyland,[c] Alexey Eremin,[c] and Carsten Tschierske*[a]
Several series of bent-core mesogens derived from 3,5-diphen-
yl-1,2,4-oxadiazole with or without lateral groups and with dif-
ferent length terminal chains at both ends, and polycatenar
molecules with three to six alkoxy chains are synthesized and
their mesomorphic behaviour is investigated by polarizing mi-
croscopy, differential scanning calorimetry, X-ray diffraction
(XRD), dielectric, electro-optical and second-harmonic genera-
tion (SHG) experiments. Most compounds exhibit broad re-
gions of skewed cybotactic nematic (NcybC) and tilted smectic
(SmC) phases with a strong tilt of the aromatic cores (up to
638), but non-tilted SmA and NcybA phases are also observed
for a compound that has only one terminal chain. The XRD
patterns of the nematic phases of most of the compounds in-
vestigated indicate a 2D periodicity with short correlation
length in the magnetically aligned samples. This is of impor-
tance for the general interpretation of the small-angle XRD
splitting patterns typically observed for aligned samples of
bent-core nematic phases. In most nematic phases one current
peak is observed in the half period of an applied electric field,
though no coherent signal is found in the SHG experiments.
Based on additional electro-optical and dielectric results, the
nematic phases are considered to be cybotactic nematic
phases with local polar order, and show a dielectric reorienta-
tion of the polar domains. Only chiral nematic phases (NcybC*),
but not blue phases, are obtained for compounds with one or
two chiral (3S)-3,7-dimethyloctyloxy tail(s).
1. Introduction
Liquid crystals (LCs) represent a fascinating state of matter[1]
that combines features of anisotropic crystalline solids and iso-
tropic liquids[2] and have wide technical applications in the dis-
play industry[3] and optoelectronics,[4] as well as significant im-
portance for the development of new functional materials[5,6]
and biosensor applications.[7] In recent years, LC phases of achi-
ral bent-core molecules have received significant interest due
to their formation of polar ordered smectic phases[8] and the
mirror symmetry breaking observed in several mesophases of
these achiral molecules.[9–11] In most cases 1,3-substituted ben-
zenes, which provide a 1208 angle, were used as the bent
units, but five-membered heterocycles, such as 2,5-disubstitut-
ed 1,3,4-oxadiazoles[12–17] and 3,5-disubstituted 1,2,4-oxadia-
zoles have also been employed (see Scheme 1).[18–23] The re-
duced bend angle, compared with the related 1,3-disubstitut-
ed benzene derivatives, places these compounds on the bor-
derline between classical linear rod-like LCs and bent-core
mesogens.[24] Special attention has been paid to their nematic
phases, which could potentially provide access to biaxial nem-
atic[13,15,25–29] and “ferronematic” LCs.[21]
[a] Dr. G. Shanker, Dr. M. Prehm, Prof. Dr. C. Tschierske
Institute of Chemistry, Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Str. 2, 06120 Halle (Germany)
E-mail: Carsten.tschierske@chemie.uni-halle.de
In contrast to the symmetric 2,5-disubstituted 1,3,4-oxadia-
zoles, the slightly less bent[30] and, due to the positions of the
O and N atoms in the heterocycles, non-symmetric 3,5-disub-
stituted 1,2,4-oxadiazoles (see Scheme 1) have been signifi-
cantly less investigated. There has been recent interest in the
1,2,4-oxadiazoles as it has been claimed that ferroelectric
switching and biaxiality occur in the nematic phase formed by
one of these molecules.[21] This has been under controversial
[b] Dr. M. Nagaraj, Prof. Dr. J. K. Vij
Department of Electronic and Electrical Engineering
Trinity College, University of Dublin
Dublin 2 (Ireland)
[c] M. Weyland, Dr. A. Eremin
Institute for Experimental Physics
Otto-von-Guericke University Magdeburg
Universitꢀtsplatz 2, 39106 Magdeburg (Germany)
[d] Dr. G. Shanker
Present address: BMS R&D Centre
BMS College of Engineering
Bull Temple Road, Bangalore-560019 (India)
[e] Dr. M. Nagaraj
Present address: The School of Physics and Astronomy
The University of Manchester
Manchester, M13 9PL (UK)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.20130170.
Scheme 1. Bend angles provided by different structural units.
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discussion since then.[11d] Recent dielectric investigations sup-
ported the presence of polar clusters in these nematic
phases,[22] and further studies of a wider range of these com-
pounds therefore appeared to be attractive.[23] A major draw-
back of all mesogens incorporating the 1,2,4-oxadiazole unit is
that these compounds have extremely high transition temper-
atures, often in temperature ranges in which decomposition
becomes a serious problem. Moreover, the majority of nematic
phases of the known 1,2,4-oxadiazoles represent skewed cybo-
tactic nematic (NcybC) phases composed of tilted smectic (SmC)
cybotactic clusters with large tilt angles.[22] In the case of phase
biaxiality in the nematic (Nb) phases,[13a,25,26] this organisation
into SmC-type clusters would lead to the monoclinic type of
biaxial nematic phases (Nbm),[25,29] in which the molecules are
tilted with respect to the primary director n. The orthorhombic
type of biaxial nematic phases (Nbo), in which the molecular
long axis coincides with the primary director n, would be
much more useful for practical applications and also easier to
identify and confirm experimentally.[25] Therefore, the objec-
tives of this work were 1) to screen a variety of different molec-
ular structures in order to understand the general structure–
property relationship in this class of compounds, 2) to widen
the nematic phase ranges and to shift them to lower tempera-
tures, 3) to reduce or remove the tilt of the molecules in the
cybotactic clusters, 4) to check the influence of molecular chir-
ality on these nematic phases and 5) to investigate the field-in-
duced polarisation and possible ferroelectric switching by
means of dielectric and electro-optical investigations and
second-harmonic generation (SHG) experiments.
cent to a terminal alkyl chain. Further increasing the number
of alkyloxy chains at one or both ends of the molecules provid-
ed a transition from bent-core-type to polycatenar-type 1,2,4-
oxadiazoles (compounds 6–9).[31]
Whereas broad regions of NcybC phases with tilted organisa-
tion of the molecules in the clusters were observed for all com-
pounds with two or three terminal chains, non-tilted SmA and
NCybA phases were obtained for one compound with only one
terminal chain. A single current peak was typically observed in
the nematic phases by electro-optical investigations under an
applied triangular wave field. Though dielectric investigations
confirm the presence of relatively large polar domains, the cor-
relation length of the polar domains appears to be too small
to give a coherent SHG signal. Hence, SHG cannot confirm
long-range polar order or ferroelectric switching for these
nematic phases. Therefore, the nematic phases are considered
to be cybotactic nematic phases with local polar order and
show a dielectric reorientation of the polar domains.
Experimental Section
Investigation Methods
The obtained compounds (for synthesis see Section 2.1, synthetic
procedures and analytical data are reported in the Supporting In-
formation) were investigated by polarising microscopy (Optiphot 2,
Nikon) in conjunction with a heating stage (FP82HT, Mettler) and
by differential scanning calorimetry (DSC) using a DSC-7 (Perkin
Elmer). The assignment of the mesophases was based on the com-
bined results of optical textures and X-ray diffraction (XRD). Investi-
gations on oriented samples were performed using a 2D detector
(Hi-Star, Siemens AG) and uniform orientation was achieved by
alignment in a magnetic field (Bꢀ1 T) using thin capillaries. Elec-
tro-optical experiments were carried out using a home-built elec-
tro-optical setup in commercially available indium tin oxide (ITO)-
coated glass cells (E.H.C., Japan) with a parallel rubbed polyimide
(PI) coating and measuring area of 1 cm2. Dielectric investigations
were performed using a Novocontrol Alpha High Resolution Die-
lectric Analyser and ZG4 dielectric interface (Novocontrol GmbH,
Hundsagen, Germany). The real and imaginary parts of the com-
plex permittivity of the sample were recorded in the frequency
range of 1 Hz to 10 MHz on slow cooling of the sample and the
measuring triangular wave voltage was 0.1 V peak to peak (Vpp).
Compounds 5c and 5d were investigated over wide temperature
ranges of 100 to 2008C and 60 to 2308C, respectively, for both
planar and homeotropic anchoring conditions in 15 mm cells. The
planar alignment was achieved by an aligning agent, whereas the
homeotropic alignment was achieved by applying a magnetic field
of B=1.4 T during the measurements. The data were fitted to the
Havriliak–Negami (H–N) equation;[33] the asymmetric distribution
parameter was fixed as unity and the symmetric distribution pa-
rameter was found to lie between 0.9 and 1.0 on fitting the experi-
mental data to the H–N equation. Dielectric strengths and the fre-
quency of the peaks were determined using the Winfit program
purchased from Novocontrol GmbH Germany. The SHG of selected
compounds was performed using a pulsed NdYAG laser in TEM00
mode at l=1064 nm, pulse width 9 ns and pulse energy 11 mJ.
The compounds under investigation represent five-ring mol-
ecules, mostly esters of 3,5-bis(4-hydroxyphenyl)-1,2,4-oxadia-
zole, but also include examples of compounds in which one
COO group was reversed or replaced by a CH2O group, and
one compound with two CH2O groups (compound 12) (see
Figure 1). In some cases branched alkoxy chains were used to
reduce transition temperatures and to introduce chirality. Fur-
thermore, small (F, Cl, Br), medium size (CH2CH=CH2) and
large groups (OR with R=CnH2n+1) were attached laterally to
the bent aromatic core in one of the peripheral positions adja-
Figure 1. Major types of bent 1,2,4-oxadiazoles under investigation.
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2. Results and Discussion
2.1. Synthesis
The synthesis started with 4-alkyl(oxy)benzoyloxybenzoic acids
B1–B3 (Scheme 2). These were condensed with 4-benzyloxy-
benzamidoxime A[32] using N,N’-dicyclohexylcarbodiimide
Scheme 3. Synthesis of compounds 13–15 and 17 derived from 4-[5-(4-hy-
droxyphenyl)-1,2,4-oxadiazol-3-ly]benzoic acid, and compounds 8, 9, 12 and
(S)-19 with identical wing groups. Reagents and conditions: i) A, DCC, 1,4-di-
oxane, reflux, 8 h; ii) THF: EtOAc (3:7), 10% Pd/C, 100 kPa, 6 h; iii) DCC,
DMAP, CH2Cl2, 10 h; iv) K2CO3, butanone, 10 h; the precise chemical formulae
of the distinct compounds are shown in the heads of the Tables 3–5 and in
Figure 2.
ates and final compounds are given in the Supporting
Information.
2.2. Liquid Crystalline Phases
2.2.1. Effects of Structural Variation on Phase Type and
Transition Temperature
Scheme 2. Synthesis of the target compounds 1–7, 10, 11 and (S)-18. Re-
agents and conditions: i) DCC, 1,4-dioxane, reflux, 8 h; ii) THF: EtOAc (3:7),
10% Pd/C, 100 kPa, 6 h; iii) DCC, DMAP, CH2Cl2, 10 h; iv) K2CO3, butanone,
10 h; the precise chemical formulae of the distinct compounds are shown in
the heads of the Tables 1,2,4 and 5.
Chain Length
It is apparent from Table 1 that all of the synthesised bisben-
zoates 1–4 display enantiotropic nematic phases over broad
temperature ranges; these were observed down to the crystal-
lisation temperature without formation of smectic phases in
almost all cases. Only compound 3b, which bears a much
longer dodecyloxy chain, shows an additional SmC phase. The
N–Iso transition temperatures of all compounds are very high,
in the range between 254 and 3058C, so that in some cases
partial decomposition occurs at the phase transition; this is in-
dicated by a decrease in the transition temperatures in subse-
quent heating and cooling cycles. The clearing temperatures
decrease with rising alkyl chain length. The lowest melting
points and best overcooling of the crystallisation was observed
for compounds combining an alkyl and an alkyloxy chain.
There is also a tendency for the melting points to decrease
with increasing difference in chain length at both ends of the
molecule (compare 1c/1a and 2c/2a in Table 1).
(DCC) as the dehydrating agent in dry dioxane to give benzyl
ethers C1–C3.[23] Hydrogenation led to phenols D1–D3.[23] Start-
ing from phenols D, DCC coupling led to the series of ben-
zoates 1–7 and (S)-18, whereas etherification with 4-alkylben-
zylbromides yielded benzylethers 10 and 11 (Scheme 2). For
the synthesis of polycatenar compounds 8 and 9, and com-
pound (S)-19, which has two (3S)-3,7-dimethyloctyloxy chains,
4,4’-(1,2,4-oxadiazole-3,5-diyl)biphenol E[19c] (Scheme 3) was
used as the starting material for the DCC coupling with two
equivalents of 3,4-dialkoxybenzoic acid, 3,4,5-tridecyloxybenzo-
ic acid or (3S)-4-(3,7-dimethyloctyloxy)benzoic acid, respective-
ly. Dibenzylether 12 was also obtained from E by benzylation
with 4-heptylbenzylbromide. Compounds 13–15 and 17 with
reversed COO groups were synthesised in an analogous way
to that shown in Scheme 2 by condensing 4-(4-n-hexylphenox-
ycarbonyl)benzoic acid B4 (Scheme 3) with A, followed by acy-
lation with benzoic acids (for compounds 14 and 17) or trans-
4-propylcyclohexane carboxylic acid (for compound 15), or by
benzylation with benzylbromides to yield compound 13b. De-
tailed synthetic procedures and analytical data of all intermedi-
Lateral Substituents and Polycatenar Compounds
The effect of additional lateral substituents was investigated
for compounds derived from 3b (compounds 5a–d in Table 1).
The melting points are not significantly affected by the addi-
tional halogen substituent, but the stability of the smectic
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Table 1. Phase transition temperatures [8C], associated enthalpy values
(in square brackets, DH [kJmolꢁ1]) and crystallisation temperature, Tcr [8C],
on cooling of compounds 1–5.[a]
Table 2. Phase transition temperatures [8C], associated enthalpy values
(in square brackets, DH [kJmolꢁ1]) and crystallisation temperature, Tcr [8C],
on cooling of the tri- and tetracatenar compounds 6 and 7.[a]
Compd. R1 =R3
R4
Phase transitions
Tcr
Compd. R1
R2
R3 Phase transitions
Tcr
6a
6b
OC4H9
OC10H21
H
H
Cr 121 [16.6] NcybC 214 [1.3] Iso
Cr 122 [55.7] (SmC 114 [3.6]) NcybC
155 [0.6] Iso
95
82
˜
1a
1b
1c
2a
C4H9
OC6H13
OC6H13
OC6H13
C6H13
H
H
H
H
Cr 125 [26.6] NcybC 305 (dec) Iso 88
Cr 130 [21.6] NcybC 297 (dec) Iso 85
Cr 140 [31.8] NcybC 294 (dec) Iso 90
Cr 146 [22.6] NcybC 294 [1.0]
Iso
˜
C5H11
C6H13
C4H9
˜
˜
7
OC10H21 OC10H21 Cr 78 [47.4] Iso
63
[b]
126
[a] DSC peak temperatures obtained in the first heating cycles at
10 Kminꢁ1; for abbreviations see Table 1.
2b
2c
C5H11
C6H13
C6H13
C6H13
H
H
Cr 154 [35.6] NcybC 285 (dec) Iso 118
Cr 160 [35.1] NcybC 282 (dec)
Iso
˜
[b]
133
137
86
[b]
2d
3a
3b
4
C7H15
C6H13
H
H
H
H
F
Cr 153 [36.3] NcybC 275 [1.2]
Iso
Cr 143 [35.5] NcybC 290 [1.7]
Iso
Cr 110 [19.1] SmC 129 [1.0]
NcybC 254 [0.6] Iso
Cr 108 [30.3] NcybC 260 [1.4]
[b]
OC6H13 C6H13
OC12H25 C6H13
Table 3. Phase transition temperatures [8C], associated enthalpy values
(in square brackets, DH [kJmolꢁ1]) and crystallisation temperature, Tcr [8C],
on cooling of the tetra- and hexacatenar compounds 8 and 9.[a]
94
[b]
OUnd
C6H13
90
Iso
5a
5b
5c
5d
OC12H25 C6H13
OC12H25 C6H13
OC12H25 C6H13
OC12H25 C6H13
Cr 107 [29.3] SmC159 [1.4] NcybC 98
243 [0.9] Iso
Cl Cr 106 [63.5] SmC 152 [1.7]
NcybC 229 [0.9] Iso
Br Cr 110 [62.3] SmC 139 [1.4]
NcybC 220 [0.7] Iso
75
Compd.
R1–R3, R5
R4 =R6
Phase transitions
Tcr
68
8a
8b
8c
9
OC4H9
H
H
H
Cr 162 [50.1] Iso
Cr 133 [52.6] Iso
Cr 127 [68.5] Iso
Cr 68 [76.0] Iso
123
119
114
33
All Cr 93 [61.2] (SmC 66 [4.7]) NcybC 29
˜
OC6H13
OC10H21
OC10H21
215 [1.3] Iso
OC10H21
[a] Peak temperatures in the DSC thermograms obtained during the
first heating scans at 10 Kminꢁ1; abbreviations: Und=-(CH2)9CH=CH2;
All=-CH2-CH=CH2; Cr=crystalline solid (often a rich polymorphism is ob-
served; in all cases only the highest observed melting transition is given,
additional Cr–Cr transitions and melting–crystallisation processes are not
considered); NcybC =nematic phase composed of SmC-type cybotactic
clusters; NcybC˜ =NcybC phase with local 2D lattice formed by SmC ribbons;
Iso=isotropic liquid; dec=decomposition, these transitions were deter-
mined by polarising microscopy. [b] These compounds have not been in-
[a] DSC peak temperatures obtained in the first heating cycles at
10 Kminꢁ1; for abbreviations see Table 1.
Effect of Linking Groups
Replacing one of the benzoyloxy groups with the more flexible
benzyloxy group (compounds 10 and 11, Table 4) reduces the
clearing and melting points. In this way, replacing COO with
CH2O provides an alternative access to materials with relatively
low melting points and broad nematic ranges. Replacing both
benzoyloxy groups with benzylether groups (compound 12)
further reduces the transition temperatures.
vestigated by XRD, therefore an assignment to NcybC or NcybC is not possi-
ble though it is likely that most of these nematic phases represent NcybC
phases.
˜
˜
phases is enhanced and the N–Iso transition temperatures are
reduced if one peripheral H-atom is replaced with a halogen
(compounds 5a–c). With increasing size of the laterally at-
tached halogen the SmC–N and N–Iso transition temperatures,
and the crystallisation tendency were reduced. For the allyl-
substituted compound 5d a further reduction in the melting
point, SmC–N transition temperature and clearing temperature
was observed compared to compounds 5a–c. This compound
shows the lowest transition temperatures and a broad nematic
phase range. However, a further increase in the lateral chain
length (Table 2) and an increase in the number of chains
(Table 3) appears to be disadvantageous, such that only crystal-
line phases were observed for polycatenar compounds 7–9
(Tables 2 and 3).
Reversal of the direction of carboxyl groups is also known to
have a strong effect on mesophase stability and mesophase
type.[34] As shown for the pairs of compounds 14/4, 15/16 and
17/5c in Figure 2, reversal of the direction of one carboxyl
group leads to the stabilisation of the SmC phase.
In contrast to all other compounds, benzylether 13a with
R1 =H shows a non-tilted SmA phase instead of the usually ob-
served SmC phase, and the nematic phase is therefore a NcybA
phase (see Table 4; for an explanation of the distinct subtypes
of nematic phases, see Section 2.2.2). In this case, the removal
of tilt in the smectic phase should be mainly due to the reduc-
tion in the number of chains. This reduces the interfacial area
required by the conformationally disordered alkyl chains,
which removes an important driving force for the tilted ar-
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Table 4. Phase transition temperatures [8C], associated enthalpy values
(in square brackets, DH [kJmolꢁ1]) and crystallisation temperature, Tcr [8C],
on cooling of compounds C1 and 10–13 incorporating benzyl ether link-
ing groups.[a]
Compd. R1
R2
Y
Phase transitions
Tcr
C1
H
C6H13 OOC Cr 136 [51.9] N 205 [1.3] Iso
55
89
84
69
[b]
10a
10b
11 a
C3H7 C6H13 OOC Cr 105 [16.0] NcybC 241 [0.9] Iso
C7H15 C6H13 OOC Cr 99 [6.4] NcybC 211 [0.9] Iso
C3H7 Oi-
Hex
C7H15 Oi-
Hex
C7H15 C7H15 OCH2 Cr 98 [29.6] SmC 134 [0.3]
NcybC161 [1.6] Iso
H
Figure 3. Oily streak texture of the cholesteric phase (NcybC*) of (S)-19 at
1408C (see Figure S18 for a colour version).
OOC Cr 93 [17.5] NcybC 228 [1.1] Iso
˜
[b]
11 b
12
OOC Cr 98 [16.6] NcybC 207 [0.9] Iso
79
72
111
97
ples of 1,2,4-oxadiazoles with chiral chains were synthesised
and investigated. Oily streak textures typical for cholesteric
phases (N* phases, see Figure 3) were observed in the whole
mesomorphic temperature range for compounds (S)-18 and
(S)-19, which have either one or two chiral (3S)-3,7-dimethyloc-
tyloxy tail(s) (see Table 5), but no blue phase was observed at
13a
13b
C6H13 COO Cr 130 [27.6] SmA [<0.01] 161
NCybA 218 [0.6] Iso
C7H15 C6H13 COO Cr 108 [36.6 ] SmC 192 [0.3]
[b]
NCybC 219 [1.3] Iso
[a] DSC peak temperatures obtained in the first heating cycles at
10 Kminꢁ1; abbreviations: i-Hex =-(CH2)3CH(CH3)2; for other abbreviations
see Table 1. [b] These compounds have not been investigated by XRD,
and therefore an assignment to NcybC or NcybC is not possible.
˜
Table 5. Phase transition temperatures [8C], associated enthalpy values
and crystallisation temperature, Tcr [8C], on cooling of the chiral com-
pounds (S)-18 and (S)-19.[a]
Compd. R2
Phase transitions
Tcr
(S)-18
(S)-19
C6H13
(S)-O(CH2)2CH(CH3)(CH2)3CH(CH3)2
Cr 110 [28.2] NcybC
240 [1.2] Iso
Cr 126 [39.9] NcybC
205 [0.6] Iso
*
106
*
107
[a] DSC peak temperatures obtained in the first heating cycles at
10 Kminꢁ1
abbreviations: NCybC*=chiral nematic phase composed of
;
SmC*-type clusters; for other abbreviations see Table 1.
any temperature. It is likely that these cholesteric phases repre-
sent skewed cybotactic NcybC* phases composed of SmC* clus-
ters. The absence of blue phases could be due to elastic con-
stants which are less favourable for twist deformation. This is
in line with the observation that the formation of conglomer-
ates of chiral domains was not observed in any of the nematic
phases of the achiral molecules although it has been reported
for some isomeric 1,3,4-oxadiazoles.[15]
Figure 2. Comparison of the phase transition temperatures (T/8C) and associ-
ated enthalpy values (in square brackets, DH/kJmolꢁ1) of pairs of com-
pounds with different orientations of the carboxylate groups (DSC peak tem-
peratures obtained in the first heating cycles at 10 Kminꢁ1; for abbreviations
see Table 1);[a] reported in ref. [22].
rangement of the molecules. In contrast to most SmC–NCybC
transitions no transition enthalpy could be observed for the
SmA–NcybA transition (see Figure S19 in the Supporting
Information).[35]
2.2.2. Investigation of the Nematic Phases
Textures of the Nematic Phases
As a representative example, Figure 4 shows the typical tex-
tures obtained for the nematic phase of compound 1b be-
tween crossed polarisers and using microscopy glass plates
without specific surface treatment. Typically, the nematic
phases occur with a birefringent Schlieren texture and there is
Effects of Chain Branching and Chirality
It is known that chiral bent-core mesogens tend to form blue
phases[36–38] instead of the cholesteric phases usually found for
chiral rod-like molecules. Therefore, two representative exam-
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Figure 4. Microphotographs of the textures observed for the nematic phase
of compound 1b between crossed polarisers on cooling at a) T=2958C,
b) T=2908C with predominately planar alignment, c) T=2808C showing the
anchoring transition to a predominately homeotropic alignment,
d) T=1408C showing a region with co-existing planar (red area) and homeo-
tropic alignment (black area).
a strong increase in birefringence with decreasing temperature
close to the phase transition temperature (Figure 4a,b). On fur-
ther cooling, an anchoring transition to a homeotropic align-
ment was often observed (Figure 4c,d) as a result of the
growth of the cybotactic clusters.[40] Close to the N–Iso transi-
tion the alignment is determined by the preference of the indi-
vidual molecules to align parallel to the surface. As the clusters
increase further in size, their shape becomes dominating and
these larger clusters tend to align with their “SmC layer
planes” parallel to the surfaces, which gives rise to a homeo-
tropic alignment at the anchoring transition. The presence of
optically isotropic regions (Figure 4d) indicates the uniaxiality
of the nematic phases.
X-Ray Scattering of the Nematic Phases
The cluster structure of all the nematic phases under investiga-
tion is evident from the XRD studies that were performed for
representative compounds from each series. The XRD patterns
observed for magnetically aligned samples of compound 1a at
T=1408C and for compound 6b at T=1308C are shown as
representative examples in Figure 5, and the results obtained
for the other compounds are shown in Figures S1–S16 and
summarised in Tables S1 and S2. In the XRD patterns of the
nematic phases diffuse wide-angle maxima are observed at
d=0.46 nm, which corresponds to the mean lateral distance
between the molecules and indicates fluid LC phases. These
wide-angle scatterings are centred on the equator (Figure 5a),
which indicates that the alignment of the molecules is along
the long molecular axis. For compound 1a the small-angle
scattering has a maximum at d=2.4 nm. This d value is signifi-
cantly smaller than the molecular length (Lmol =3.7 nm in a V-
shaped conformation with a bend angle of 1408 and all-trans
alkyl chains, see Table S1).
Figure 5. Representative XRD patterns of magnetic field oriented samples of
the NcybC phases: of a–c) compound 1a at T=1408C; a) complete pattern,
b) small-angle region and c) q-scans over the complete XRD pattern (solid
line) and over the meridian (ꢂ258; dotted line); d–f) compound 6b at
T=1308C, d) complete pattern, e) small-angle region and f) q-scan scans
over the complete XRD pattern (solid line) and over the meridian (ꢂ258;
dotted line); the scans over the meridian were smoothened using a Savitz-
ky–Golay function; additional XRD patterns are shown in Figures S1–S16.
The intensity of the small-angle scattering is higher than
that of the wide-angle scattering, as is typical for cybotactic
nematic phases (for models of the different types of cybotactic
nematic phases see Figure 6).[39] Moreover, the small-angle
scattering is split into four diffuse spots with clear maxima
beside the meridian and smeared out to two dumbbell-shaped
streaks parallel to the equator (see Figure 5b). This pattern is
typically observed for cybotactic nematic phases formed by
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SmC-like clusters (NcybC phase).[16,25,40,41] The splitting of the
maxima of the small-angle scatterings Dc in the nematic phase
of compound 1a is about 1088. The calculated value Dc/2=
548 is close to the tilt angle of bcal =508 calculated according
to the equation cosbcal =d/Lmol from the d values of the layer
reflections (see Table S1).
In most cases, more detailed inspection of the XRD patterns
of the NcybC phases reveals the presence of an additional weak
diffuse and streak-like scattering parallel to the equator with
a maximum located on the meridian (Figure 5a–c). Only for
compounds with long alkyl chains (compounds 5a–c, 6b, 10b
and 14) is this scattering very weak and almost invisible (see,
for example, Figure 5d–f). The diffraction pattern in Fig-
ure 5a,b could be indexed to a c2mm lattice with short corre-
lation length and lattice parameters a=3.1, b=3.6 nm. Re-
markably, the value of b is in excellent agreement with the es-
timated molecular length Lmol =3.7 nm. This c2mm lattice most
probably arises from the space- and time-averaging of cybo-
tactic clusters formed by SmC ribbon fragments on an oblique
lattice. In these clusters the molecules are aligned parallel to
the field direction, but with different tilt-orientations in distinct
clusters, as shown in Figure 6d. This would mean that the four
small-angle scatterings arise from the (11) reflections of the re-
sulting time- and space-averaged pseudo-rectangular lattice
(Figure 6e). It is possible that this organisation on a 2D lattice
with relatively short correlation length is stabilised or induced
by the alignment of the molecules in the magnetic field, which
provides an ordering effect on the aromatic cores.[42] In
Tables 1–4 and Figure 2 the nematic phases showing this addi-
tional small-angle scattering are assigned as NcybC˜. As the four
small-angle scatterings represent the (11) reflections of the
pseudo-rectangular lattice, the actual tilt of the molecules with
respect to the normal of the layers in the cybotactic clusters
(b) can deviate from the measured Dc/2 splitting in these NcybC
˜
phases (see Figure 6d). This needs to be considered during in-
terpretation of the diffuse small-angle diffraction patterns of
magnetically aligned cybotactic nematic phases. Comparison
of the XRD tilt angles in Tables S1 and S2 indeed shows that
the splitting of the small-angle scattering Dc/2 in the nematic
phases is, in most cases, slightly larger than the tilt bcal, which
is calculated using the relation cosb=d/Lmol, whereas in the
SmC phases the tilt calculated from Dc/2 is in all investigated
cases smaller than bcal.[43]
2.2.3. Investigation of the Smectic Phases
Figure 6. Models of the nematic phases formed by the reported bent-core
mesogens; the bent aromatic cores are simplified and shown as simple rods,
that is, effects of molecular biaxiality and the bent shape (polar order) on
the phase structure[25,29] are not considered here: a) “ordinary” nematic
phase (N); b) cybotactic nematic phase composed of SmA-type clusters
(NcybA); c) skewed cybotactic nematic phase composed of SmC-type clusters
(NcybC) with the molecules arranged parallel to the magnetic field direction,
leading to the four spot SAXS pattern usually observed (see Figure 5e);
d) local ordering of the SmC ribbon segments on an oblique lattice (p2) with
different alignment directions and e) space- and time-averaging of the dis-
tinct orientations of the p2 lattice leading to a nematic phase with a c2mm
pseudo-lattice with short correlation length (NcybC˜), giving rise to the diffrac-
tion pattern shown in Figure 5b. In c) the splitting of the small-angle scat-
tering Dc/2 corresponds to the tilt angle b, whereas in (d) and (e) this is not
necessarily the case.
For compounds with longer aliphatic chains there is a transition
from cybotactic nematic to smectic phases at lower tempera-
ture (see Tables 1–4 and Figure 2). This transition is associated
with a significant texture change to fan-like (Figure S17).
As shown in the representative DSC curves in Figure 7, the
N
cybC–SmC transitions are associated with relatively low transi-
tion enthalpy values (<0.01–1.8 kJmolꢁ1), which are compara-
ble to the Iso–N transition enthalpy values (0.6–1.5 kJmolꢁ1).
This is in line with a cybotactic cluster structure in these nem-
atic phases, that is, layer fragments are already present in the
nematic phase and fuse to infinite layers at the phase transi-
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tic phase (SmC). The distinct intensities of these small-angle re-
flections on the right and left indicate a synclinic tilt of the
molecules and together confirm the transition from NcybC
phases to synclinic tilted smectic C phases. The tilt angles (see
Tables S1 and S2) are between 42 and 568 in most cases; they
are smaller (15–408) only for compounds 13–15, which have
reversed ester groups. An SmA phase without tilt was only
found for compound 13a, which has just one terminal alkyl
chain (see Figure S12). The optical tilt (bopt), as estimated from
investigation of homogeneously aligned samples of the SmC
phases in polyimide coated ITO cells with parallel rubbing di-
rection, indicates a strong tilt of between 50 and 638 of the ar-
omatic cores in all investigated SmC phases (see Tables S1, S2).
Hence, the bent 3,5-diphenyl-1,2,4-oxadiazole units have a ex-
traordinarily high tendency to form tilted organisations.
2.2.4. Electro-Optical Investigations
In previous work the nematic phases of 3,5-diphenyl-1,2,4-oxa-
diazole-based molecules were reported to show a single cur-
rent peak during each half period of an applied triangular
wave voltage.[21,22] Dielectric studies support the presence of
polar domains with appreciable size.[22] The same behaviour
was also found for most of the nematic phases of the com-
pounds reported herein, as well as in some of the smectic
phases. Examples from each major class of compounds were
chosen for electro-optical investigation. Table 6 gives an over-
view over the investigated compounds and Figures 9, 10 and
Figure 7. Representative DSC heating and cooling traces for: a) 5c and
b) 5d at a rate of 10 Kminꢁ1
.
Table 6. Compounds investigated in electro-optical studies.[a]
tion. No measurable enthalpy change could be detected for
the NcybA–SmA transition of compound 13a (see Table 4 and
Figure S19). Only for tricatenar compound 6b (Table 2) and lat-
erally allyl-substituted compound 5d (Table 1) are the N–SmC
transition enthalpies larger (3.6 and 4.7 kJmolꢁ1, respectively).
The N–SmC transition is also indicated by clear changes in the
small-angle XRD patterns. At this transition the diffuse scatter-
ings are replaced by sharp layer reflections whereas there is no
change in the position and shape of the diffuse wide-angle
scattering (see Figure 8).
Compd.
NcybC
SmC
1a
3b
5a
5b
+ +
+ +
+ +
+
–
+ +
+ +
+
5c
5d
6a
+ +
+ +
0
+ +
–
–
0
–
6b
0
11 a
12
13a[b]
14
+ +
0
+
0
+
In almost all cases the position of the layer reflections is
beside the meridian, indicating the transition to a tilted smec-
0
0
15
(S)-18
(S)-19
+ +
+ +
0
+ +
–
0
[a]+ + indicates that there is a clear single peak, similar to those shown
in Figure 9a and 10a; + means that there is a relatively small single
peak; 0 indicates that there is no peak; – indicates the absence of a smec-
tic phase. [b] Non-tilted NcybA and SmA phases were observed instead of
tilted phases in this case.
S20 show the switching curves recorded for compounds 3d,
5c and 5b as representative examples.
The investigations were carried out in a 10 mm polyimide
coated ITO cell with planar alignment layers, antiparallel rub-
bing direction, and a measuring area of 1 cm2. A triangular
Figure 8. XRD patterns of a magnetically aligned sample of compound 6b:
a) in the NcybC phase at T=1308C (inset SAXS) and b) in the SmC phase at
T=908C (inset SAXS).
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Figure 10. Electro-optical investigations of compound 5c in a 10 mm PI-
coated ITO cell with homogenous alignment layer (100 Vpp, 10 Hz, 5 kW):
a) Current response (black line) in the NcybC phase at T=1608C, the dotted
line indicates the triangular wave voltage; b–d) textures in the nematic
phase at T=1208C under a DC voltage of b) 0 V, c)+40 V and d)+80 V
showing the absence of electro-convection patterns; e) peak area as a func-
tion of temperature.
Figure 9. Electro-optical investigations of compound 3b in a 10 mm PI-
coated ITO cell with planar alignment (80 Vpp, 10 Hz, 5 kW): a) Current re-
sponse in the NcybC phase at T=1608C; the dotted line indicates the triangu-
lar wave voltage; b) peak area as a function of temperature and c) frequency
dependence of the measured peak area at T=1608C.
ously by Francescangeli et al.[21] in the following regard: 1) the
switching current peak is more distinct, 2) its fall-off at higher
fields is faster than at lower fields as shown in Figure 9a, 3)
the peak is found at higher frequencies than those by Fran-
cesscangli et al.[21] and 4) the area of the peak decreases with
frequency as seen in Figure 9c. We also find that the polarisa-
tion results are supported by dielectric spectroscopy (see
Section 2.2.5).
wave voltage of frequency 10 Hz and a voltage of 8–
10 Vpp mmꢁ1 was applied (see Figures 9a, 10a and S20a as ex-
amples). A clear symmetric single peak was observed for the
compounds indicated with + + in Table 6. The shape of the
switching current shown in Figure 9a, the dependence of the
area of the curve on the frequency of the applied field seen in
Figure 9c and the use of a cell with a cell thickness of 10 mm,
excludes the effects of the surface on the polarisation results.
The maximum peak areas were observed at a distinct tem-
perature between 100 and 1608C and the peak areas de-
creased gradually towards higher and lower temperatures (see
Figure 9b, 10e and S20b). Assuming a spontaneous polarisa-
tion (PS) as the origin of this current peak, PS values between
25 and 80 nCcmꢁ2 were calculated for the maximum current
values. Both the shape of the current response curve and the
temperature dependence are nearly identical for all com-
pounds with this peak and similar to those reported previous-
ly.[21,22] Our results are more detailed than those reported previ-
This single peak can be found for all investigated com-
pounds 1–5 with X=COO, Y=OOC, irrespective of whether or
not there is a lateral substituent (Table 6). There is no peak for
polycatenar compounds 6, and the presence of branched
chains at both ends results in the disappearance of the peak
(compound (S)-19), whereas one branched chain is tolerated
(compounds 11 b and (S)-18). The replacement of one ben-
zoate group with a benzylether unit (X=CH2O) results in reten-
tion of the peak (compound 11 b), whereas the peak is not ob-
served for compound 12, which incorporates two of these flex-
ible linkages (X=CH2O, Y=OCH2). This is in line with the as-
sumption that a local polar order is present in the cybotactic
clusters of the nematic phases, and is distorted by steric effects
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Figure 11. Dielectric spectroscopy of compound 5c: a), b) Frequency dependence of a) de’?/d(lnf) and b) de’k/d(lnf) at different temperatures. P1 and P2 are
different relaxation processes. c), d) Temperature dependence of the dielectric strength (de) and relaxation frequency (f) of P1 and P2, respectively. In c) the
frequency scale coincides with that of de.
(lateral groups or chain branching) or increased conformational
flexibility (benzylethers).
transition temperature. The frequency of the P1 peak increases
in compound 5c by a factor of three over a 708C rise in tem-
perature. Had the clusters not been present in the nematic
phase, a dramatic decrease in this frequency would have been
expected. Peak P1 is present in both the planar and homeo-
tropic cells with cell thicknesses of 15 mm. This shows that the
overall permanent dipole moment is neither parallel nor per-
pendicular to the long molecular axis; this is based on an as-
sumption that alignment in the two cells is almost perfect. The
effect of surfaces on the behaviour is excluded. The permanent
dipole moment due to the central oxadiazole group is normal
to the long molecular axis, but an additional uncompensated
dipole moment is directed towards the end of one of the
wings of the molecule in both compounds 5c and 5d. Peak P1
in the nematic phase shows a clear dielectric dispersion in the
frequency range of 130 Hz to 1 kHz. This peak is predominant
and compares favourably with peak P1 of 1,2,4-oxadiazole-
based bent-core liquid crystals with SmC-type clusters reported
previously.[22] The frequency at which the dielectric loss is
a maximum for the mode increases from 200 to 700 Hz with
temperature, in contrast to the previously studied oxadiazole,
for which this frequency lies between 50 and 250 Hz. This is
almost a factor of three higher due to the reduced correlations
within the clusters in these compounds. The dielectric strength
of peak P1 in the nematic phase is almost independent of tem-
perature and increases slightly in the SmC phase. It appears
that this peak is due to the polar clusters in the nematic
phase. The dielectric strength reduces in amplitude at the N–
The effect of the reversal of one ester group (Y=COO, com-
pounds 14, 15, 17) is not so clear. Whereas cyclohexane car-
boxylate 15 with X=Y=COO exhibits a clear switching peak,
compound 14 shows no peak and compound 17 could not be
investigated due to its high transition temperatures.
2.2.5. Dielectric Investigations
The plots of the relative permittivity (e’) and the dielectric loss
(e’’) versus frequency for compounds 5c and 5d clearly show
two maxima marked as P1 and P2 in both planar and homeo-
tropic alignments (Figure 11a–d). For better resolution of the
peaks, the derivative of the real part of permittivity, de’?/
djln fj, was also analysed for a planar cell, and this was com-
pared with a corresponding plot of e’’?. The two curves com-
pare favourably with each other though the differential of e’?
with respect to the log of frequency is expected to have given
better details in the spectra. The two peaks P1 and P2 are ob-
served in the dielectric spectra of both cell configurations,
planar and homeotropic. Peak P1 is assigned to the collective
motion of clusters since the dielectric strength de is approxi-
mately 110 for 5c and approximately 200 for 5d. These values
are exceptionally large in the nematic phase and could be only
due to the ferroelectric domains in this phase. The dielectric
strength de increases at the Iso–N phase transition and increas-
es slightly further with a reduction in temperature in the N
phase, but there is no observable discontinuity at the N–SmC
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gests that the clusters increase in size with a reduction in tem-
perature. P2 is assigned to the rotation around the bow axis or
short axis as stated above. At lower temperatures the dielectric
strength for P2 decreases significantly, presumably due to the
fact that the sample converts partly to a glassy state. The high
frequency peak P3 is assigned to the ITO coating of the cells.
Overall, the dielectric investigations support the presence of
local polar order in the cybotactic SmC clusters of the nematic
phases under investigation.
2.2.6. SHG Studies
SHG studies with compounds 3b, 5c and 5d gave no evidence
of any SHG activity in the absence of a field or after applying
an electric field of up to 50 Vmmꢁ1, neither in the nematic nor
in the smectic phases. The measurements were performed at
different incidence angles and polariser positions. However,
though no SHG activity could be detected in the temperature
range of the nematic phase of compound 3b, a weak, but
clear SHG signal was found for the SmC phase under an ap-
plied electric field (Figure 13).
Figure 12. Dielectric spectroscopy of compound 5d: a), b) Frequency de-
pendence of a) e’? and b) e’’? at different temperatures. P1, P2 and P3 are
different relaxation processes; c) temperature dependence, dielectric
strength (de) and relaxation frequency (f) of the P1 process.
Figure 13. SHG intensity depending on temperature as measured for com-
pound 3b (black squares on cooling, open circles on reheating) under
a square wave field with Epp =20 Vmmꢁ1 at f=10 Hz.
Iso transition but remains very large. The dipolar correlations
persist in the isotropic phase.
This indicates that this smectic phase shows an induced po-
larisation and thus represents a paraelectric SmC phase,[44,45] in
which local polar domains are present. Thus we can conclude
that it is possible that the peaks observed in the nematic
phases under the triangular wave field result from a kind of di-
electric reorientation of polar domains. The size of the polar
domains was estimated to be about 100 molecules, based on
dielectric results for related 1,2,4-oxadiazoles incorporating cy-
clohexane rings (similar to compound 16, shown in
Figure 2),[22] and appears to be a bit smaller for the compounds
investigated herein. It seems that under an applied electric
field of reasonable strength the correlation length of the polar
domains can be increased, but in most smectic phases and in
the investigated nematic phases the coherence length cannot
reach the size required for the observation of a coherent SHG
signal (01 mm). Therefore, in this case it is not possible to cor-
roborate a ferroelectric switching in the nematic phases by
Peak P2 is assigned to a rotation about the short axis. If the
peak were due to the ITO coatings of the cell, the frequency of
this peak would not be temperature dependent. Its strength
follows the 1+2S dependence (see Figure 11d). Again the
peak is significant in both planar and homeotropic cell config-
urations; this implies that the overall permanent dipole
moment lies at a large angle with respect to the long molecu-
lar axis. Its frequency is a factor of ten larger than the frequen-
cies reported for the oxadiazoles studied in reference [22].
Peak P1 is also very predominant in 5d (Figure 12a–c, see
also Figures S21 and S22) and shows dispersion in the frequen-
cy range 5–100 Hz (see Figure 12c). P3 is also present but its
frequency lies in the MHz region, whereas the frequencies for
P1 and P2 are drastically reduced at lower temperatures (see
Figures 12c and S22). The behaviour of P1 in particular sug-
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SHG experiments. However, the fact that a SHG signal was
found for the SmC phase of compound 3b (Figure 13) con-
firms that 3,5-diphenyl-1,2,4-oxadiazoles are in principle capa-
ble of polar ordering. It seems that for compound 3b, which
has no lateral substituent and relatively long terminal chains,
the polar clusters reach a larger coherence length than the
other investigated compounds. However, due to the disruption
of the layers into smaller clusters, the coherence length of the
polar domains in this compound is reduced and, at the transi-
tion to the nematic phase, becomes smaller than the critical
value such that no coherent SHG signal could be observed in
these nematic phases. Based on these observations and con-
sidering the dielectric results, the switching in the nematic
phases could be related to a kind of dielectric reorientation of
polar domains and does not represent a true ferroelectric
switching.
cybotactic nematic phases is not necessarily related to the tilt
angle of the molecules in the clusters.[43]
Acknowledgements
This work was supported by the European Union (EU) within the
FP7 funded Collaborative Project BIND (Grant No. 216025); A.E.
acknowledges the German Research Foundation (DFG, Grant No.
ER 467/2) for financial support.
Keywords: bent-core mesogens
· ferroelectricity · liquid
crystals · oxadiazoles · phase transitions
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The nematic phases are uniaxial, but show local polar order
as indicated by dielectric investigations; this is reported herein
and in a previous paper,[22] and means that there is also local
biaxiality. However, as the polar domains have coherence
lengths that are smaller than approximately 1 mm no SHG
signal could be observed in the nematic phases, even under
a high applied electric field. Therefore, we must conclude that
the peak typically observed in the nematic phases of these
compounds under a triangular wave field most probably re-
sults from dielectric reorientation of polar domains.[46–48] A truly
ferroelectric switching cannot be confirmed for the nematic
phases of this kind of compound.
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Compounds with one or two chiral (3S)-3,7-dimethyloctyloxy
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[43] This also provides a possible explanation for the observed jump in
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Received: November 15, 2013
Published online on && &&, 2014
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ARTICLES
G. Shanker, M. Prehm, M. Nagaraj,
J. K. Vij, M. Weyland, A. Eremin,
C. Tschierske*
Between order and disorder: Variation
of the molecular structure of bent 1,2,4-
oxadiazoles (see picture) leads to in-
sights into the molecular organization
in their special nematic phases. These
nematic phases have cybotactic cluster
structures with strongly tilted aromatic
cores and local polar and biaxial order.
This gives rise to dielectric reorientation
of the polar domains under an applied
electric field and local periodic order on
a 2D lattice in a magnetic field.
&& – &&
1,2,4-Oxadiazole-Based Bent-Core
Liquid Crystals with Cybotactic
Nematic Phases
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