Hydrogen bonding and the design of twist-bend nematogens

Article abstract of DOI:10.1016/j.molliq.2020.112630

The phase properties of equimolar mixtures consisting of a hydrogen bond donor, a 4-alkoxybenzoic acid (nOBA), and one of two different stilbazole-based hydrogen bond acceptors, either 4-[(E)-2-(4-{[6-(4′-methoxy[1,1′-biphenyl]-4-yl)hexyl]oxy}phenyl)-ethenyl]-pyridine (1OB6OS) or 4-[(E)-4′-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)-[1,1′-biphenyl]-4-carbonitrile (CB6OS) are reported. Neither hydrogen bond acceptor exhibits liquid crystal behaviour whereas the nOBA compounds show smectic and/or nematic behaviour depending on the length of the alkyloxy chain. For the complexes of an nOBA with n = 1–5, both conventional nematic and twist-bend nematic phases were observed, while for n ≥ 6 smectic phases emerged and the twist-bend nematic phase was extinguished. The CB6OS-nOBA mixtures may exhibit the heliconical smectic C_TB phase. The local molecular arrangement in the two sets of mixtures are similar and changes on increasing n but this is not reflected in the nematic-isotropic transition temperatures. Birefringence studies of the mixtures are reported. In general the behaviour of the hydrogen-bonded mixtures is similar to that of their covalently bonded counterparts.

Full text of DOI:10.1016/j.molliq.2020.112630

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Hydrogen bonding and the design of twist-bend nematogens
Rebecca Walker, Damian Pociecha, Catriona Crawford, John
M.D. Storey, Ewa Gorecka, Corrie T. Imrie
PII:
S0167-7322(19)36729-7
DOI:
https://doi.org/10.1016/j.molliq.2020.112630
MOLLIQ 112630
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
12 December 2019
30 January 2020
31 January 2020
Please cite this article as: R. Walker, D. Pociecha, C. Crawford, et al., Hydrogen bonding
and the design of twist-bend nematogens, Journal of Molecular Liquids(2020),
https://doi.org/10.1016/j.molliq.2020.112630
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Hydrogen bonding and the design of twist-bend nematogens
Rebecca Walker1*, Damian Pociecha2, Catriona Crawford1, John MD Storey1, Ewa Gorecka2
and Corrie T Imrie1
1Department of Chemistry, King’s College, University of Aberdeen, Aberdeen, AB24 3UE,
UK. *E-mail: rebecca.walker@abdn.ac.uk
2Department of Chemistry, University of Warsaw, ul. Zwirki i Wigury 101, 02-089 Warsaw,
Poland.
Abstract
The phase properties of equimolar mixtures consisting of a hydrogen bond donor, a 4-
alkoxybenzoic acid (nOBA), and one of two different stilbazole-based hydrogen bond
acceptors,
either
4-[(E)-2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)-
ethenyl]-pyridine (1OB6OS) or 4-[(E)-4'-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)-
[1,1'-biphenyl]-4-carbonitrile (CB6OS) are reported. Neither hydrogen bond acceptor
exhibits liquid crystal behaviour whereas the nOBA compounds show smectic and / or
nematic behaviour depending on the length of the alkyloxy chain. For the complexes of an
nOBA with n=1-5, both conventional nematic and twist-bend nematic phases were observed,
while for n ≥ 6 smectic phases emerged and the twist-bend nematic phase was extinguished.
The CB6OS-nOBA mixtures may exhibit the heliconical smectic CTB phase. The local
molecular arrangement in the two sets of mixtures are similar and changes on increasing n
but this is not reflected in the nematic-isotropic transition temperatures. Birefringence studies
of the mixtures are reported. In general the behaviour of the hydrogen-bonded mixtures is
similar to that of their covalently bonded counterparts.
Introduction
The spontaneous emergence of chirality in systems composed of achiral molecules is of
fundamental importance in both physical and biological sciences and thought to play a
pivotal role in the origin of biological homochirality. In this context the study of liquid
1
crystalline systems has greatly enhanced our understanding of symmetry breaking in fluids .
Indeed, the first example of spontaneous chiral symmetry breaking in a fluid with no spatial
ordering was provided by the twist-bend nematic, NTB, phase in which the director exhibits
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periodic twist and bend deformations forming a conical helix, and is tilted with respect to the
2
helical axis . In this system consisting of achiral molecules, chirality forms spontaneously
and so an equal number of left- and right-handed helices would be expected. The pitch of the
helix is strikingly small, of the order of 10 nm, which corresponds to just a few molecular
3
lengths . The introduction of intrinsic or molecular chirality removes the degeneracy of the
helices and the chiral NTB phase is observed 4–6. Prior to its experimental discovery, the NTB
7
8
phase had been predicted first by Meyer and later and independently by Dozov . In their
models the director modulations arose from flexoelectric couplings, or from an elastic
instability equivalent to a sign change of the bend elastic constant (K33), respectively. Within
Dozov’s framework, bent molecules are required to obtain the necessary low values of K33
and indeed the vast majority of twist-bend nematogens are odd-membered liquid crystal
9–17
dimers
i.e. molecules containing two mesogenic units attached through a flexible alkyl
spacer. If the parity of the spacer is odd then the molecule, on average, is bent 18. The NTB
phase is most often preceded by a conventional nematic phase with uniform director structure
but having an anomalously low bend elastic constant, and such materials can be exploited in
new applications 19,20. However, direct NTB-isotropic transitions have also been observed 6,21
In addition, Dozov predicted that bent achiral molecules would form chiral twist-bend
smectic phases 8 and these have also been observed experimentally 13,22,23
.
.
The NTB phase has been found for a range of other structures including bent-core liquid
crystals 24, and higher liquid crystal oligomers such as trimers and tetramers 22– 24. In each of
these systems molecular curvature is thought to be the essential structural requirement for the
formation of the NTB phase. Hydrogen bonding has also been employed in the design of
twist-bend nematogens, and the first example was 4-[6-(4’-cyanobiphenyl-4-yl)hexyloxy]-
benzoic acid (CB6OBA), see Figure 1.28 The observation of NTB behaviour was attributed to
the formation of hydrogen-bonded complexes between pairs of acids yielding bent
supramolecular complexes 28. In this system, however, there is no element of molecular
recognition given that two identical molecules form the complexes. In order to extend this
design approach and build in an aspect of molecular recognition, we reported recently the
behaviour of mixtures containing unlike hydrogen bond acceptors and donors, namely 4-[(E)-
2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]-pyridine
(1OB6OS)
and alkoxybenzoic acids (nOBA) with n = 4 and 5 29, respectively, see Table 1. We have
showed that these systems do indeed exhibit the NTB phase providing examples of the
formation of spontaneous chirality driven by hydrogen bonding between unlike and achiral
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components. To further investigate this fascinating phenomena, here we report the behaviour
of mixtures containing a wider range of the hydrogen bond donors nOBA, in which the length
of the terminal chain has been varied, n=1-10 and hydrogen bond acceptor 1OB6OS, and also
mixtures containing a different stilbazole-based hydrogen bond acceptor, 4-[(E)-4'-(6-{4-
[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1'-biphenyl]-4-carbonitrile (CB6OS), see
Table 1. These particular systems have been chosen for study because the hydrogen bond
acceptors both contain a hexyloxy spacer which has been shown to impart the required
9
molecule curvature for the observation of twist-bend behaviour and both the methoxy and
cyano terminal groups have been shown to promote the NTB phase 30–32. Furthermore, the
hydrogen bond between pyridyl-based fragments and benzoic acids are known to be
sufficiently strong to promote liquid crystalline complexes 33–35. By analogy with covalently
assembled systems 12,13, the extension of the terminal chain associated with the hydrogen
bond donor may drive the formation of smectic phases.
CB6OBA
Figure 1. 4-[ω-(4’-Cyanobiphenyl-4-yl)alkyloxy]-benzoic acid (CB6OBA) 28.
Table 1. Hydrogen bond acceptors and hydrogen bond donors used in this study.
H-bond Acceptors
H-bond Donors
1OB6OS
CB6OS
nOBA (n = 1-10)
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Experimental
Synthesis
nOBAs
The synthesis of the n-alkoxybenzoic acids (nOBAs) followed a standard procedure adapted from that
36
reported by Jensen et al. and is shown in Scheme 1. Full synthetic details and analytical data for
nOBA with n = 1-10 are given in the ESI.
Scheme 1. Synthesis of the n-alkoxybenzoic acid (nOBA) series, where R = CnH2n+1 (n = 1-10).
1OB6OS
The synthesis of 4-[(E)-2-(4-{[6-(4'-methoxy[1,1'-biphenyl]-4-yl)hexyl]oxy}phenyl)-ethenyl]pyridine
(1OB6OS) followed the procedure outlined in Scheme 2. A Friedel-Crafts acylation and subsequent
ketone reduction gave 1OB6Br.31 The alcoholic stilbazole 4-[(E)-2-(pyridin-4-yl)ethenyl]phenol
(HOS) was prepared through the base-catalysed condensation of 4-hydroxybenzaldehyde and 4-
methylpyridine. 37 1OB6Br and HOS were combined in a Williamson ether synthesis to give the title
compound. Full synthetic details and analytical data for 1OB6OS and its intermediates are given in
the ESI.
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Scheme 2. Synthesis of 1OB6OS.
CB6OS
The synthesis of CB6OS followed the procedure outlined in Scheme 3. The synthesis of 4'-[6-(4-
formylphenoxy)hexyl][1,1'-biphenyl]-4-carbonitrile, CB6OAH, and its intermediates was performed
as described in an earlier work.13 The condensation of CB6OAH and 4-methylpyridine gave the title
compound. Full synthetic details and analytical data for CB6OS and its intermediates are given in the
ESI.
Scheme 3. Synthesis of CB6OS.
Materials and Methods
Materials All reagents and solvents available commercially were purchased from Sigma Aldrich,
Alfa Aesar, ACROS Organics or TCI Chemicals and used as received unless stated otherwise.
Purity Analysis The proposed structures of all final products and their intermediates were
1
1
characterised using a combination of H and 13C NMR, and FTIR spectroscopies. H and 13C NMR
spectra were recorded on either a 400 MHz Varian Unity INOVA, or a 300 MHz Bruker Ultrashield
NMR spectrometer. Infrared spectra were recorded on a Thermal Scientific Nicolet IR100 FTIR
spectrometer with an ATR diamond cell. The purities of the final products were verified using C, H,
N microanalysis performed by the Micro Analytical Laboratory in the School of Chemistry at the
University of Manchester or Sheffield Analytical and Scientific Services Elemental Microanalysis
Service at the University of Sheffield.
Polarised Optical Microscopy Phase characterisation was performed using an Olympus BH2
polarising light microscope equipped with a Linkam TMS 92 hot stage (University of Aberdeen) or a
Zeiss AxioImager A2m polarizing microscope equipped with Linkam heating stage (University of
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Warsaw). Glass cells were obtained from Warsaw Military University of Technology (WAT) having
1.6 or 3 micron thicknesses, and ITO conducting and polymer aligning layers.
Differential Scanning Calorimetry The phase behaviour of the complexes was studied by
differential scanning calorimetry using a Mettler-Toledo DSC1 fitted with an intracooler and
calibrated using indium and zinc as standards. Heating and cooling rates were 10 ˚C min-1 and all
samples were measured under a nitrogen atmosphere. Transition temperatures and associated enthalpy
changes were extracted from the second heating trace unless otherwise noted. For each sample, two
aliquots were measured and the data listed are the average of the two sets of data.
X-ray Diffraction Wide angle diffractograms (WAXRD) were obtained with a Bruker D8 GADDS
system (CuKα line, Goebel mirror, point beam collimator, Vantec2000 area detector). Samples were
prepared as droplets on a heated surface.
Birefringence Birefringence was measured with a set-up based on a photoelastic modulator (PEM-90,
Hinds) working at a modulation frequency f = 50 kHz; as a light source, a halogen lamp (Hamamatsu
LC8) was used equipped with a narrow bandpass filters (633 nm and 690 nm). The signal from a
photodiode (FLC Electronics PIN-20) was deconvoluted with a lock-in amplifier (EG&G 7265) into
1f and 2f components to yield a retardation induced by the sample. Knowing the sample thickness, the
retardation was recalculated into optical birefringence. The samples were prepared in 3-micron-thick
glass cells with planar anchoring. The alignment quality was checked prior to measurement by
inspection under the polarising optical microscope.
Binary Mixture Studies Binary mixtures were prepared by co-dissolving pre-weighed amounts of
each compound in dichloromethane or chloroform and allowing the solvent to evaporate slowly at
room temperature. The mixtures were further dried in a vacuum oven at 50 °C for ~16 h.
Results and Discussion
The transitional properties of the equimolar 1OB6OS:nOBA complexes are listed in Table 2,
and those of the individual components are given in Tables S1-S3 in the ESI. 1OB6OS melts
into the isotropic phase at 143 °C, and on cooling crystallisation precluded any observable liquid
crystallinity, while nOBA compounds are mesogenic for n > 2. All ten complexes show an
enantiotropic conventional nematic, N, phase assigned on the basis of the observation of a
characteristic schlieren texture containing two- and four-point brush singularities in untreated
cells, and a uniform texture in cells with planar anchoring, see Figure 2. For the
1OB6OS:nOBA complexes with n = 1-5, a transition to a lower temperature, monotropic
nematic phase was apparent by the development of a striped texture, characteristic of the NTB
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phase, see Figure 2. The N-NTB phase transition was accompanied by the cessation of optical
flickering associated with director fluctuations in the higher temperature N phase. For the
1OB6OS:5OBA complex, the temperature range of the NTB phase narrows to only ca. 4 K,
insufficient for the striped texture to develop prior to a transition to a smectic A, SmA, phase.
The N-NTB transition was detected using DSC as a very small jump in the heat capacity, in
accord with the general observation that as the temperature range of the preceding nematic
phase increases, the N-NTB transition tends towards becoming second order in nature, i.e. ΔH
≈ 0 32.
In the equimolar 1OB6OS:nOBA complexes with n > 5, the NTB phase is no longer observed
and instead the nematic phase cools directly into a SmA phase. The SmA phase is assigned
on the basis of the observation of a truncated focal conic fan texture, indicative of smectic
layer ordering, in coexistence with homeotropic regions indicating that the director is
orthogonal to the layer planes. The entropy change associated with the SmA-N transition
increases as the temperature range of the N phase decreases in accord with McMillan theory
and the values observed are consistent with the phase assignment.38 In thin cells sample
crystallisation is suppressed, and cooling from either the NTB phase (n = 3-4) or the SmA
phase (n = 5-10) revealed a reversible transition to another smectic phase, SmX, having a
mosaic-like texture, see Figure 2. The monotropic nature of the SmX phase precluded its
identification using X-ray diffraction but given the large enthalpy change associated with its
formation, it is presumably a highly ordered smectic phase.
Table 2. Transition temperatures and associated entropy changes for the 1OB6OS:nOBA equimolar
mixtures. All data extracted from DSC heating traces unless stated otherwise. *Temperature obtained
from DSC cooling trace. †Temperature obtained using POM. ‡ Peak overlapped with crystallisation
exotherm.
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TCr- TSmASmX TSmAN TNTB
TNI ΔSCr-
/ °C
/
ΔSSmASmX
/
ΔSSmAN
/
ΔSNTB
/
ΔSNI /
R
N
N
n
/ °C
/ °C
/ °C
R
R
R
/ °C
R
132
120
115
122
112
119
97
101
95
106
104†
98†
99†
102†
98†
173 15.59
168 10.94
164 10.94
166 14.22
158 12.26
≈ 0
≈ 0
≈ 0
≈ 0
≈ 0
0.70
0.66
0.63
0.65
0.57
0.59
0.73
0.65
0.71
0.71
1
2
3
4
5
6
7
8
9
77*
86*
85*
86*
89*
96*
92*
99*
‡
2.42
1.41
1.40
1.10
1.61
1.32
2.17
94*
102*
114
127
127
138
0.08
0.08
0.04
0.11
0.15
0.21
155
8.54
152 10.31
155
149
153
8.82
5.82
9.71
10
N
NTB
SmA
SmX
Figure 2. POM textures of the 1OB6OS:3OBA equimolar mixture in a cell with planar anchoring in
the N (131 ºC) and NTB (89 ºC) phases, and for 1OB6OS:5OBA in the SmA (91 ºC) and SmX (83 ºC)
phases.
The phase behaviour of the 1OB6OS:nOBA complexes was studied further using X-ray
diffraction. The N and NTB phases show essentially identical X-ray diffraction patterns with
broad signals indicative of the liquid-like arrangement of the molecules. Table 3 lists the
periodicities measured along the director in the liquid crystal phases of these mixtures along
with the estimated all-trans molecular lengths of the complexes. The very weak reflections
seen in the XRD patterns of the complexes with n < 4 did not allow for the molecular
distance along the director to be estimated. For 1OB6OS:nOBA complexes with n = 4 and 5,
the X-ray patterns contained small angle signals corresponding to approximately 1 and 1/3 of
the complex’s length, indicating an unusual head-to-tail arrangement of the complexes
discussed in detail elsewhere 29. The layer periodicity in the SmA phase for the
1OB6OS:nOBA complexes with n = 7-10 corresponds to the estimated complex length
indicating a monolayer arrangement, see Table 3.
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Figure 3 shows the dependence of the transition temperatures on the length of the terminal
alkyloxy chain, n, for the 1OB6OS:nOBA complexes. TNI falls on increasing the terminal
chain length reflecting that the interactions between the mesogenic units responsible for
formation of the nematic phase become weakened. TNTBN shows only a weak dependence on
n reflecting the predominantly shape-driven nature of this transition, for complexes having
similar values of n as the molecular curvature is determined mainly by the spacer chain. On
increasing the terminal chain length microphase separation is promoted and TSmAN increases
rapidly. We will discuss this later together with the behaviour of the CB6OS:nOBA
complexes.
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Table 3. Layer thickness (in smectic phases) or end-to-end separation between molecules (in nematic
phases) obtained from integrated X-ray signals for the equimolar 1OB6OS:nOS mixtures. For some
mixtures and phases, the observed pattern was too weak to determine the signal position.
Approximate molecular length was estimated from a sketch of the complex in its all-trans
conformation using Avogadro.39
Molecular
Length, l /Å
37.9
Phase
Layer Thickness/Longitudinal
d/l
n
Distance, d /Å
38.6, 12.8
39.0, 13.0
39.0, 13.0
42.9
NTB
N
NTB
SmA
SmA
SmA
SmA
SmA
1.02, 0.34
0.99, 0.33
0.99, 0.33
1.09
1.03
1.04
4
39.4
5
42.5
44.0
45.6
47.1
43.8
45.6
47.2
48.2
7
8
9
1.04
1.02
10
180
170
160
150
140
130
120
110
100
90
I
N
SmA
NTB
SmX
80
70
1
2
3
4
5
6
7
8
9
10
n
Figure 3. Dependence of phase behaviour of the equimolar 1OB6OS:nOBA complexes on the length
of the terminal alkyl chain, n, where X represents TNI,  TSmAN, ▲ TNTBN and ● TNTB
/SmASmX.
The transitional properties of the equimolar CB6OS:nOBA mixtures are listed in Table 4.
CB6OS melts directly into the isotropic phase at 129 ˚C and on cooling, crystallisation
precludes the observation of liquid crystallinity. Similar to the behaviour seen for the
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1OB6OS:nOBA mixtures (Table 2), CB6OS:nOBA complexes with n = 1-5 exhibit two
nematic phases, an enantiotropic conventional N phase and a monotropic NTB phase. The N
and NTB phases were assigned as described earlier: specifically, in planar cells, the N phase
has a uniform texture with a strong degree of flickering and on cooling to the NTB phase a
distinct stripe texture develops and the flickering ceases.
CB6OS:nOBA complexes with n>5 do not show the NTB phase and instead, they exhibit up
to three smectic phases below an enantiotropic conventional N phase. The highest
temperature smectic phase for complexes with n = 6-10 showed a uniform texture with no
optical flickering in planar cells (Figure 4b) and in one-surface-free samples, a non-
birefringent (homeotropic) texture was seen (Figure 4f). This phase is therefore assigned as a
SmA phase. On further cooling a schlieren texture containing two- and four-point brush
defects (Figure 4g) develops from homeotropic texture of SmA phase, suggesting formation
of a tilted smectic phase. Two-brush defects are characteristic of phases in which the
mesogenic units are arranged in an anticlinic fashion, thus the phase is probably of the SmCA
type. In the cells with planar anchoring, a striped texture is seen in SmC phase (Figure 4c)
40
and such stripes are indicative of a low bend elastic constant . This texture resembles that
reported for the heliconical smectic phase denoted the SmCTB phase 22,23. However, the
emergence of a schlieren texture is not consistent with a simple heliconical structure in which
director in consecutive layers rotate according to an ideal clock model. In such a system the
molecular orientations are averaged over the helix and thus a homeotropic texture is expected
even though the phase is tilted. Recently, however, we have shown that a variant of the
heliconical smectic C phase may have a distorted clock structure, in which the molecular
orientations are not fully averaged over the helix and hence a schlieren texture can be
observed. To distinguish between a conventional SmCA phase and a distorted-clock type
smectic CTB phase one should perform detailed resonant X-ray diffraction studies but the
monotropic nature of the phase precludes this possibility. On cooling from the SmC phase,
the schlieren texture transforms into a somewhat smudged, plate-like texture (Figure 4h)
38
resembling that of the SmI phase . In the planar cells, the striped texture of the SmC phase
is replaced with a mosaic texture containing domains with ‘arrowheads’ structures pointing in
opposite directions (Figure 4d). The phase has been denoted SmX.
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Table 4. Transition temperatures and associated entropy changes for the CB6OS:nOBA complexes.
All data are extracted from heating DSC traces unless stated otherwise. *Temperature obtained from a
DSC cooling trace. †Temperature obtained using POM.
TCr-/°C
TSmX-/ °C
TSmCSmA/ °C
TSmAN/ °C
TNTBN / °C
TNI / °C
n
126
104
110
107
104
105
103
122
129
110†
106†
102†
95†
178
179
169
167
161
158
155
155
152
151
1
2
3
4
5
6
7
8
9
72*
75*
77*
78*
76*
94†
86†
95*
101†
114
122
91*
90†
99*
117
78*
102*
10
ΔSCr-/ R
ΔSSmX-/ R
ΔSSmCSmA/ R
ΔSSmAN/ R
ΔSNTBN / R ΔSNI / R
n
15.09
12.64
8.38
≈ 0
≈ 0
≈ 0
≈ 0
≈ 0
0.61
0.79
0.78
0.79
0.66
0.59
0.47
0.47
0.66
0.59
1
2
3
4
5
6
7
8
9
9.08
11.98
10.17
10.99
13.76
13.54
11.84
1.87
1.19
0.93
0.50
0.94
0.55
-
0.03
-
0.03
0.02
0.004
-
0.03
0.04
10
The assignments of the nematic phases were confirmed by the observation of broad, diffuse
signals in the small- and wide-angle regions in their X-ray diffraction patterns, indicating
liquid-like positional ordering of the complexes, see Figure 5. The periodicities in the
nematic phases estimated from the position of the broad signal in the small angle region, d,
are listed in Table 5 along with the calculated molecular length, l, for each complex. For the
complex with n=1, d/l≈0.5 suggesting a locally intercalated arrangement of the complexes.
On increasing n, for the nematic phases of CB6OS:5OBA two diffraction signals are
observed at d/l≈1 and 0.33 similarly as for 1OB6OS:5OBA, suggesting a local head-to-tail
arrangement of the complexes 29. For CB6OS:6OBA, the diffraction signal at d/l≈0.9
indicates a local monolayer packing of molecules in the nematic phase. Thus the local
structure of the nematic phases changes as n is increased and we will return to this
observation later.
The XRD patterns of the SmA and SmC phases contain a sharp reflection in the low angle
region due to their lamellar structure, and diffuse wide-angle scattering characteristic of
liquid-like ordering within the layers (Figure 5). The values of d/l in these phases suggest
monolayer packing, see Table 5. On cooling into the SmX phase, the reflection in the wide-
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angle region narrows, following an increase in the molecular translational ordering within the
smectic layer. The similarities in textures between this phase and that found at low
temperatures for the 1OB6OS:5OBA complexes suggest these are the same phases.
SmC
c. SmX
d.
N
SmA
a.
b.
78 °C
g.
74 °C
h.
130 °C
e.
98 °C
f.
SmC
SmX
SmA
N
Figure 4. POM textures of CB6OS:9OBA in the N, SmA, SmC and SmX phases in a planar aligned
cell (top) and untreated glass slides (bottom).
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Figure 5. XRD patterns obtained for CB6OS:10OBA in (left to right) the N (126 °C), SmA (107 °C),
SmC (85 °C) and SmX (45 °C) phases.
The temperature dependence of the layer spacings, d, in the SmA, SmC, and SmX phases was
measured for the n = 10 complex, see Figure 6. In the SmA phase, the layer spacing increases
slightly with decreasing temperature, (negative temperature layer expansion). At the
transition to the SmC phase d begins to sharply decrease as the molecules tilt. There is no
measurable discontinuity in layer spacing at the transition and this is consistent with the weak
first-order nature of the transition. The maximum tilt angle, estimated from the decrease in
푑푆푚퐶
the layer thickness using: cos 휃 =
⁄
was found to be approximately 20°. A slight
푑푆푚퐴
increase in layer thickness is observed at the transition to the SmX phase. The lower value of
the layer thickness measured in the SmX phase than in the SmA suggests that the SmX phase
is a tilted phase. Cooling the sample into the Cr phase results in a marked decrease in layer
spacing, although it is not clear whether this is the transition to a reversible soft crystal-type
phase or crystallisation.
Table 5. Layer thickness (smectic phases) or end-to-end separation between molecules (nematic
phases) and lateral spacings obtained from X-ray diffraction patterns of the CB6OS:nOBA series. For
some complexes and phases the observed pattern was too weak to determine the signal position.
Phase
Molecular
Length / l
33.0
End-to-end separation/Layer
LateralDistance
BetweenMesogens / Å
d/l
n
1
5
6
7
8
Thickness, d / Å
NTB
N
NTB
15.6
4.5
4.5
4.5
4.9
4.3
4.3
4.3
4.4
4.3
4.3
0.47
0.95, 0.31
0.95, 0.32
0.88
0.95
1.01
1.03
1.03
1.00
0.98
37.2, 12.0
37.1, 12.5
36.0
39.9
42.5
45.3
46.8
45.4
44.5
39.2
40.7
42.2
43.8
N
SmC
SmX
SmA
SmA
SmC
SmX
45.3
10
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Figure 6. Dependence of the layer spacing on temperature for CB6OS:10OBA. The maximum
relative error in d value is 0.03 Å (mainly due to fluctuations of the incident X-ray beam position and
intensity), the maximum possible absolute error (the whole curve vertical shift) can be ~0.5 Å due to
the uncertainty in the position of the sample in z direction.
The dependence of the transition temperatures on n for the CB6OS:nOBA complexes is shown in
Figure 7. The behaviour observed is qualitatively the same as that seen for the 1OB6OS:nOBA
complexes in Figure 3. The principal difference is the insertion of the SmC phase for long terminal
chain lengths. It would be tempting to attribute this difference in behaviour to a stabilisation of the
anticlinic arrangement of the molecules by antiparallel associations of the cyanobiphenyl fragments.
Such a view, however, is not supported by the similar values of d/l observed in the SmA phase for
these two sets of materials. It is apparent that both sets of complexes show a progression in terms of
the local arrangement of the complexes in the nematic phases but this is not reflected in the transition
temperatures which show a regular dependence on n. Similar behaviour has been observed for
covalently assembled liquid crystal dimers in which the length of the terminal chain is varied 12,13 and
thought to suggest that the importance of the local arrangement of the molecules in the nematic phase,
specifically their tendency to intercalate, has been overstated in considering the stability of the NTB
phase. Both sets of complexes, 1OB6OS:nOBA and CB6OS:nOBA, show a strong tendency to
exhibit smectic phases rather than the NTB phase on increasing n, and this is seen also for an
analogous series of covalently assembled non-symmetric dimers containing a cyanobiphenyl unit and
a three-ring mesogenic moiety.22 By comparison, non-symmetric dimers consisting of a
cyanobiphenyl unit and a two-ring mesogenic moiety have a much weaker tendency to show smectic
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phases over the same range of terminal chain lengths 12,13. This behaviour may be attributed to the
enhanced anisotropic interactions between the mesogenic units on increasing the length of one. Thus
it appears that the hydrogen-bonded complexes behave similarly to their covalently assembled
counterparts.
180
I
160
140
N
120
100
SmC
80
NTB
SmX
60
40
1
2
3
4
5
6
7
8
9
10
n
Figure 7. Dependence of phase behaviour of the CB6OS:nOBA on the length of the terminal alkyl
chain, n, where X represents TN*I,  TSmAN, ▲ TNTBN, ◆TSmCSmA, and O TSmXSmC.
In order to establish to what extent other physical properties of the hydrogen bonded complexes
resemble those of covalent dimers, optical birefringence measurements were performed for a selection
of materials from the CB6OS:nOBA series. Representative plots of the temperature dependence of the
birefringence for those complexes exhibiting nematic to twist-bend nematic phase transitions, n = 1-5,
and those for longer homologues with a sequence of smectic phases, n = 6-10, are shown in Figures 8
and 9, respectively. The measured optical birefringence, Δn, sharply rises at the transition from the
isotropic to the nematic phase and on further cooling follows a critical dependence,
훽
푇푁퐼 − 푇
∆푛0 = ∆푛푚푎푥
(
)
푇푁퐼
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where TNI is the nematic-isotropic transition temperature and Δnmax is the theoretical birefringence for
the nematic phase with an ideal orientational order (order parameter, S = 1).41 The departure of the
birefringence from the critical dependence several degrees above the N-NTB transition temperature is
attributed to the presence of strong fluctuations of the director in the nematic phase caused by the
instantaneous formation of local heliconical states, a pre-transitional effect on approaching the helical
twist-bend nematic phase (Figure 8). Such behaviour is consistent with previous reports on the optical
42
birefringence at the N-NTB transition in covalently bonded liquid crystal dimers . In the NTB phase,
we observe a decrease in birefringence which relates to the tilting and averaging of the molecular
positions around the helical axis. Furthermore, the birefringence data were also used to determine the
conical angle  in the NTB phase (inset Figure 8), following a procedure described by Meyer et al. 43 in
which the tilt was calculated using,
∆푛
∗ 2 + 1
√∆푛0
휃 = 푐표푠−1
(
3
)
An essentially linear increase of the conical angle on decreasing temperature was found below the N-
NTB phase transition, and the maximum conical angle value,  sat was calculated as approximately 20°.
Figure 8. Birefringence vs. temperature for CB6OS:5OBA. The red line is the fitted curve obtained
assuming critical temperature dependence. Inset shows a plot of conical angle vs. temperature. The
maximal error in the absolute value of birefringence is 0.005, which is mainly due to uncertainty of
the cell thickness. Relative errors in birefringence (noise) are negligible, error bars would be smaller
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than points on the graph. Maximal absolute errors in birefringence could translate to errors in the tilt
angle in the NTB phase being less than 1 deg.
For longer homologues, the SmA-N transition is marked by a small increase in birefringence, due to
the increase of orientational order at the onset of layer formation (Figure 9). The subsequent transition
to the tilted anticlinic SmC phase lowers the birefringence. The tilt angle in the SmC phase was
∆푛
calculated by the simple estimation: cos 2휃 = 푆푚퐶 ; its temperature behaviour can be fitted to the
∆푛
푆푚퐴
훽
푇
critical dependence: 휃 = 휃푠푎푡
(
− 푇) . Maximum tilt angle measured in the SmC phase just
푆푚퐶−푆푚퐴
푇
푆푚퐶−푆푚퐴
before transition to SmX phase was ~20°. This value is completely in accord with the magnitude of
SmCA
SmX
SmA
N
0.16
20
10
SmCA
0.08
SmA
0
76
78
80
82
86
88
90
92
T8/4°C
0.00
60
90
120
150
T / °C
the tilt angle found by X-ray measurements.
Figure 9. Birefringence vs. temperature for CB6OS:10OBA. The red line is the fitted curve obtained
assuming critical temperature dependence of birefringence in SmA phase. Inset shows a plot of tilt
angle in anticlinic SmC phase vs. temperature.
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Conclusion
In summary, we report the synthesis, thermal and optical characterisation of two series of
supramolecular liquid crystal dimers formed by molecular recognition between unlike H-bond donors
and acceptors. The hydrogen bonded complexes of 1OB6OS or CB6OS and nOBA (n = 1-5) exhibit
twist-bend nematic phases providing further examples of the spontaneous formation of chirality in
mixtures of achiral compounds. Mixtures of CB6OS and nOBA with n=7-10 may exhibit the
heliconical smectic C phase. In general, the properties of these hydrogen bonded complexes strongly
resemble those of their covalently bonded counterparts.
Acknowledgements
The work was supported by the National Science Centre (Poland) under the grant no.
2016/22/A/ST5/00319.
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Declaration of competing interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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Author Statement
Rebecca Walker: Investigation, Formal analysis, Validation, Visualisation, Writing – Original Draft,
Writing – Reviewing and Editing Damian Pociecha: Visualisation, Investigation, Formal analysis
Catriona Crawford: Investigation, Formal analysis John MD Storey: Resources, Supervision Ewa
Gorecka: Investigation, Formal analysis, Resources, Supervision Corrie T Imrie: Conceptualization,
Supervision, Resources, Writing – Reviewing and Editing
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Highlights


Supramolecular H-bonded liquid crystals can form the twist-bend nematic
phase
Observation of spontaneous symmetry breaking through mixing of achiral
components


Local molecular arrangement changes on increasing terminal chain length
Hydrogen-bonded complexes behave similarly to their covalently-bonded
counterparts
24