Understanding the remarkable difference in liquid crystal behaviour between secondary and tertiary amides: The synthesis and characterisation of new benzanilide-based liquid crystal dimers

Article abstract of DOI:10.1039/d1cp01125a

A number of liquid crystal dimers have been synthesised and characterised containing secondary or tertiary (N-methyl) benzanilide-based mesogenic groups. The secondary amides all form nematic phases, and we present the first example of an amide to show the twist-bend nematic (N_TB) phase. Only two of the correspondingN-methylated dimers formed a nematic phase and with greatly reduced nematic-isotropic transition temperatures. Characterisation using 2D ROESY NMR experiments, DFT geometry optimisation and X-ray diffraction reveal that there is a change in the preferred conformation of the benzanilide core on methylation, fromZtoE. The rotational barrier around the N-C(O) bond has been measured using variable temperature¹H NMR spectroscopy. This dramatic change in shape accounts for the remarkable difference in liquid crystalline behaviour between these secondary and tertiary amide-based materials.

Full text of DOI:10.1039/d1cp01125a

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Understanding the remarkable difference in liquid
crystal behaviour between secondary and tertiary
amides: the synthesis and characterisation of new
benzanilide-based liquid crystal dimers†
Cite this: Phys. Chem. Chem. Phys.,
2021, 23, 12600
Grant J. Strachan,
Corrie T. Imrie
William T. A. Harrison,
John M. D. Storey and
*
A number of liquid crystal dimers have been synthesised and characterised containing secondary or
tertiary (N-methyl) benzanilide-based mesogenic groups. The secondary amides all form nematic
phases, and we present the first example of an amide to show the twist-bend nematic (NTB) phase. Only
two of the corresponding N-methylated dimers formed a nematic phase and with greatly reduced
nematic–isotropic transition temperatures. Characterisation using 2D ROESY NMR experiments, DFT
geometry optimisation and X-ray diffraction reveal that there is a change in the preferred conformation
of the benzanilide core on methylation, from Z to E. The rotational barrier around the N–C(O) bond has
been measured using variable temperature 1H NMR spectroscopy. This dramatic change in shape
accounts for the remarkable difference in liquid crystalline behaviour between these secondary and
tertiary amide-based materials.
Received 12th March 2021,
Accepted 12th April 2021
DOI: 10.1039/d1cp01125a
rsc.li/pccp
inclusion of amide groups has a detrimental effect on liquid
Introduction
crystallinity.2 It was found, however, that 21 amides could form
The great majority of low molar mass liquid crystals consist of liquid crystal phases, although these materials showed high
molecules containing a single semi-rigid, mesogenic core attached melting points which were attributed to the presence of strong
to which are one or two alkyl chains.1 In essence, the interactions hydrogen bonding.3 Careful molecular design using a selection
between the mesogenic units promote liquid crystal behaviour of suitable lateral substituents and terminal groups attached to
whereas the role of the chains is to reduce the melting point, so the mesogenic units allowed the melting point to be manipulated
making the liquid crystal behaviour accessible. The mesogenic and a range of calamitic4–13 and bent-core14–18 liquid crystalline
unit normally consists of phenyl rings connected through short materials based around secondary amide groups have been
unsaturated linkages, often ester or imine groups. By comparison, reported. These systems tend to predominantly form smectic
the amide group is relatively uncommon in low molar mass liquid phases, again attributed to intermolecular hydrogen bonding
crystals (LCs) although it is found in technologically important between the amide groups. Indeed, the use of amide groups in
liquid crystal polymers including, most notably, Kevlar.1 In this structures such as benzene tricarboxamides (BTA),19–22 hydrazine
paper we address the fundamental question, why is the amide derivatives,23–28 or peptide residues29–31 promotes columnar
group not used more widely in the design of mesogenic units liquid crystal phases in which hydrogen bonding stabilises the
found in low molar mass liquid crystals?
columns.
In terms of molecular design, it would appear desirable to
Amides are classified as primary (11), secondary (21), or
tertiary (31), depending on the nature of the N-substituents, eliminate the strong hydrogen bonding between the amide
as shown in Fig. 1. Our focus is the use of the amide link in groups, with a view to reducing melting temperatures and to
constructing the mesogenic unit and hence, the discussion is promote nematic behaviour. This may be achieved by replacing
restricted to 21 and 31 amides. Early work recognised that the the hydrogen atom on the nitrogen with a methyl group, giving
a tertiary amide (Fig. 1). Surprisingly, there have been very few
examples of liquid crystals containing tertiary amides. Examples of
Department of Chemistry, University of Aberdeen, Aberdeen, Scotland, UK.
these include discotic liquid crystals with amide groups incorpo-
E-mail: c.t.imrie@abdn.ac.uk
rated into a fused ring structure32–34 and mesogens featuring an
† Electronic supplementary information (ESI) available. CCDC 2016868 and
N,N-dialkyl amide terminal group,35 as well as the use of N-methyl
2016869. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d1cp01125a
amides as linking groups to attach a chiral aliphatic tail to an
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Table 1 The structures of the amide-based dimers reported
Fig. 1 The structure of primary (11), secondary (21) and tertiary (31) amides.
aromatic core.36,37 Critically, we are not aware of any reports of
tertiary amides being used in the construction of calamitic or
bent core mesogenic units. Conventional wisdom attributes this
surprising observation to the loss of hydrogen bonding between
the molecules.14 This intuitively pleasing assumption appears
not to have been questioned. Tertiary benzanilides are known,
however, to show a marked preference for adopting the E
conformation38–41 although this does not appear to have been
taken into consideration when considering the absence of
liquid crystalline behaviour.
Compound
A
B
n
X
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
Br
Br
Br
Br
CN
CN
Ph–CN
Ph–CN
Ph–CN
Ph–CN
Ph–CN
Ph–CN
Br
Br
CN
CN
CN
CN
CN
CN
CN
CN
Ph–CN
Ph–CN
6
6
6
6
6
6
6
6
10
10
10
10
H
CH3
H
CH3
H
CH3
H
CH3
H
CH3
H
CH3
In this work we investigate the difference in phase behaviour
between liquid crystal dimers containing 21 and 31 benzanilide-
based mesogenic units to establish the effect of N-alkylation on
liquid crystalline behaviour. A liquid crystal dimer is a molecule
containing two mesogenic units attached by a flexible spacer.42,43
In a symmetric dimer, the two mesogenic units are the same
whereas they are different in a non-symmetric dimer. The
advantage of using this molecular architecture to address our
central question is that we know that moving from 21 to 31
amides we will see a marked reduction in the stability of the
liquid crystal phase, but this may be counteracted in the dimer by
careful design of the second mesogenic unit and by varying the
length of the spacer. Furthermore, if an odd-membered spacer is
chosen, these dimers, on average, will have a bent molecular
shape and the potential to exhibit the highly topical twist-bend
nematic, NTB, phase.42,44–47 In the NTB phase the director forms a
conical helix and is tilted with respect to the helical axis. This
chiral structure forms spontaneously and so the induced twist
may be either left- or right-handed and equal amounts of both
types of helix are expected. This chirality is observed even though
the constituent molecules are achiral. The double degeneracy
may be removed by the inclusion of intrinsic molecular chirality
and the chiral twist-bend nematic phase is observed.48 The
general structure of the non-symmetric dimers reported here is
shown in Table 1. Each contain an odd-membered spacer linked
to one mesogenic unit via an ether link and to the other by a
methylene group. This ensures the necessary molecular curvature
to potentially exhibit the NTB phase.49–52 The structural changes
shown in Table 1 are intended to change the shape anisotropy of
the molecule and allow the transition temperatures to be
manipulated. The role of the lateral methyl group is to disrupt,
at least to some extent, lateral interactions in an attempt to
Experimental
Schemes 1–4 provide an overview of the synthetic routes used to
prepare the benzanilide-based dimers (Fig. 2). The syntheses
were carried out according to literature methods, and full
experimental details and structural characterisation data are
given in the ESI.†
Thermal behaviour
The transitional behaviour of the materials was studied using
differential scanning calorimetry (DSC) with a Mettler Toledo
DSC3 differential scanning calorimeter equipped with a TSO
801RO sample robot and calibrated with indium and zinc
standards. The heating and cooling rates were 10 1C minꢀ1 and
the transition temperatures and their associated enthalpy changes
were extracted from heating traces unless otherwise noted.
Polarised optical microscopy (POM) was used to identify the
liquid crystal phases observed using an Olympus BH2 polarising
optical microscope equipped with a Linkam TMS 92 hot stage.
The samples were prepared using untreated glass slides.
Rotational barriers
Measurement of rotational energy barriers were carried out
using variable temperature H NMR spectroscopy on a Bruker
Ascend 600 MHz spectrometer with a cryoprobe and VT unit.
Lineshape analysis was performed using the DNMR module in
the Bruker TOPSPIN package.
1
X-ray crystallography
reduce the melting temperature. The key question to address is Single crystals of compounds 4b and 5b in the forms of
how does N-methylation change the shapes of these molecules, colourless blocks were grown from a 2 : 1 solution of toluene
and specifically, in terms of the conformation of the amide? and chloroform, and intensity data were collected on a Rigaku
This has been studied using a combination of 2D NOESY and AFC11 CCD diffractometer (Cu Ka radiation, l = 1.54184 Å) at
ROESY NMR experiments; single crystal X-ray diffraction; and 100 K. The structures were solved by ‘dual space’ methods
DFT geometry optimisation. We also investigate the effect of using SHELXT and the structural models were completed and
N-methylation on the rotational barrier around the N–C(O) bond optimised by refinement against |F|2 with SHELXL-2014.53 The
using variable temperature 1H NMR spectroscopy.
benzene rings were modelled as rigid hexagons (C–C = 1.39 Å)
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Scheme 1 Synthesis of compounds 1a, 1b, and 3b.
and the H atoms were placed geometrically (C–H = 0.95–0.98 Å) c = 28.98487 (19) Å, b = 110.2699 (7)1, V = 8119.69 (9) Å3, Z = 8,
and refined as riding atoms. The structure of 4b refined well R(F) = 0.186 [24423 reflections with I 4 2s(I)], wR(F2) = 0.455
but the data and refinement quality for 5b are very poor with (26446 reflections), CCDC deposition number = 2016869.†
unresolvable disorder in the decyl chains possibly indicative
Geometry optimisations
of an unmodelled supercell and only qualitative conclusions
may be drawn for the latter structure. Crystal data for 4b: Geometry optimisation was carried out using DFT calculations at
C
47H41N3O2, Mr = 679.83, monoclinic, space group Cc (No. 9), the B3LYP-6-31G(d) level of theory with Gaussian09 software.54
a = 31.5211 (4), b = 14.7867 (2), c = 33.4555 (5) Å, b = 112.551 (2)1, Space-filling models were generated from the optimised geo-
V = 14401.1 (4) Å3, Z = 16, R(F) = 0.078 [22766 reflections with metries using the QuteMol package.55 Although ether-linked
I 4 2s(I)], wR(F2) = 0.222 (26665 reflections), CCDC deposition spacers have been found to have a gauche linkage in their lowest
number = 2016868.†
energy state,56 the energy difference between this and the
Crystal data for 5b: C51H49N3O2, Mr = 735.93, monoclinic, all-trans conformation is small (ca. 1 kJ molꢀ1) and it is expected
space group Pn (No. 7), a = 21.61054 (14), b = 13.81870 (7), that the greater linearity of the all-trans state will be more favourable
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Scheme 2 Synthesis of compounds 2a, 2b, and 3a.
Scheme 3 Synthesis of compounds 4a, 4b, 5a, and 5b.
within a liquid crystalline phase.57 Based on this, the geometry all-trans conformation. This assumption is in accord with similar
optimisations were carried out with the central spacer in the studies.58
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Scheme 4 Synthesis of compounds 6a and 6b.
containing two- and four-brush defects and which flashes when
subjected to mechanical stress. A representative texture is given
in Fig. 2. The scaled nematic–isotropic entropy changes are
Results and discussion
Phase behaviour
The first set of dimers to be discussed contain two biphenyl consistent with this assignment.56,59
units joined by a hexyloxy spacer and are divided into three
The methylation of the amide nitrogen to form 31 amides in
groups based on their terminal groups. For each the N-H and all three cases led to a loss of liquid crystalline behaviour. All
N-methyl versions are reported. Structures 1a and 1b have
three 31 benzanilides melted directly into the isotropic phase
bromobiphenyl units at each end, 2a and 2b have one bromo- on heating and formed a glass on cooling (Tg 59 1C, 69 1C and
biphenyl and one cyanobiphenyl unit, and 3a and 3b have two 51 1C for compounds 1b, 2b, and 3b, respectively). This
cyanobiphenyl units. The transitional properties of this group represents a reduction in the value of TNI by at least 61 1C for
of dimers are listed in Table 2. The three secondary amides 1b and 2b compared to the corresponding 21 benzanilides, and
(1a, 2a and 3a) display a conventional nematic, N, phase. of 101 1C for 3b. These reductions appear too large to be
Throughout this study nematic phases have been assigned on associated with the anticipated change in molecular shape
the basis of the observation of a schlieren optical texture associated with the N-methyl group.60
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Fig. 3 POM texture of compound 4a in (left) the nematic phase at 136 1C
and (right) the twist-bend nematic phase at 96 1C.
Fig. 2 POM texture of compound 1a in the nematic phase at 190 1C.
Table 2 The transition temperatures, transitional enthalpies and scaled
entropies of 21 compounds 1a, 2a and 3a and 31 compounds 1b, 2b, and 3b
liquid crystalline behaviour. The non-methylated material 5a
again showed a nematic phase, with a schlieren texture containing
2- and 4-point brush defects, and in addition a twist-bend nematic
phase on cooling, for which the schlieren texture changed to
include rope-like defects (Fig. 4). The value of TNI for 5a
(Table 4) is slightly lower than that of compound 4a, and this
may be attributed to the dilution of the mesogenic units arising
TCrN/1C
TNI/1C
DHNI/kJ molꢀ1
DSNI/R
TCrI/1C
1a
2a
3a
212
188
186
187a
199
224
0.77a
0.66
1.11
0.20a
0.17
0.27
1b
2b
3b
126
138
123
a
Taken from DSC cooling trace.
from increasing the spacer length. The value of TN
for 5a is
TBN
also slightly lower than that seen for 4a, which may be attributed
to an increased molecular flexibility which reduces molecular
curvature.56 Of particular significance is the effect molecular
flexibility has on the tendency to crystallise; thus, 5a can be
cooled to 101 1C prior to crystallisation, whereas 4a crystallises at
208 1C. The very extensive supercooling of the isotropic phase of
5b is now sufficiently large to reveal a nematic phase which
crystallised at 81 1C. The entropy change associated with the
nematic-isotopic transition (Table 3) is very small and we will
return to this observation later. The reduction in TNI on methylat-
ing 5a to give 5b is 219 1C.
As a result of the dramatic loss of liquid crystallinity on
N-methylation seen for the compounds listed in Table 2,
changes to the basic dimer structure were made with the aim
of increasing the potential transition temperatures to offset the
effect of introducing the N-methyl group. Firstly, one of the
cyanobiphenyl groups was replaced with a cyanoterphenyl unit to
give 4a, which in a conventional low molar mass liquid crystal
can typically increase TNI by around 150 1C.61 The inclusion of the
extra phenyl ring increased TNI (Table 3) of 4a by 94 1C compared
to that of 3a. In addition, on cooling the N phase of 4a, a
transition to the twist-bend nematic phase was seen at 130 1C.
This was most clearly observed for isolated droplets, for which
the schlieren texture developed a blocky appearance (Fig. 3).
However, the N-methylated material 4b again did not show
any mesogenic behaviour. The material melted directly into the
isotropic phase at 145 1C on the initial heat. On cooling, the
sample formed a glass at 57 1C which indicates a reduction in
the nematic–isotropic transition temperature of at least 261 1C
compared to the corresponding 21 benzanilide.
In an attempt to increase TNI for the 31 amide 5b the remaining
cyanobiphenyl unit was replaced with a second cyanoterphenyl
fragment. The corresponding 21 amide 6a melts into the nematic
Given the change from a biphenyl to terphenyl end group
was not sufficient to promote liquid crystalline behaviour in the
N-methylated dimer 4b, the length of the central spacer was
increased to ten carbon atoms. The increase in the chain length
was anticipated to suppress crystallisation possibly revealing
Fig. 4 POM texture of compound 5a in (left) the nematic phase at 140 1C
and (right) the twist-bend nematic phase at 114 1C.
Table 3 The transition temperatures and transitional enthalpies of com-
pounds 4a and 4b
Table 4 The transition temperatures and transitional enthalpies of com-
pounds 5a and 5b
TCrN/1C
TN -N/1C
TNI/1C
DHNI/kJ molꢀ1
DSNI/R
TCrN/1C TN -N/1C DHN -N/kJ molꢀ1 TNI/1C DHNI/kJ molꢀ1 DSNI/R
TB
TB
TB
4a
4b
208
145
130ab
—
329
—
4.08
—
0.82
—
5a 195
5b 146
123b
—
B0a
—
303
84b
4.02
0.84
0.07
0.22b
a
b
a
b
Taken from DSC cooling trace. DSC transition appeared as a step
change in the baseline.
DSC transition appeared as step change in baseline. Taken from
DSC cooling trace.
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Table
5
The transition temperatures and transitional enthalpies of
compounds 6a and 6b
TCrN/1C
TNI/1C
DHNI/kJ molꢀ1
DSNI/R
6a
6b
186
177
386
5.6
0.68
1.0
0.20
142a
Fig. 7 E and Z conformations of benzanilide.
a
Taken from DSC cooling trace.
methyl group is added. The central C–N bond of an amide group
has partial double bond character as a result of delocalisation of
the lone pair from the nitrogen, and this leads to the possibility
of E and Z isomers, see Fig. 7. For most amides, the Z isomer is
lower in energy and hence is the preferred conformer.
To determine the amide conformations adopted by the 21
and 31 amides reported here, 2D NOESY and ROESY NMR
spectra were collected for each compound. Unlike most NMR
experiments, these experiments show interactions that occur
between atoms that are close together in space, rather than
connected by chemical bonds. The spectra of all the 21 amides
should show one set of correlations, see Fig. 8(A), whereas the
spectra of the 31 amides should show a different set of signals,
see Fig. 8(B). The ROESY NMR spectra of 3a and 3b are shown
in Fig. 9 and 10, respectively. For the 21 amides, interactions are
seen between the N–H and the aryl methyl, shown in Fig. 8(A).
In addition, there is a cross-peak between the N–H and the
aromatic protons adjacent to the carbonyl group. This cross-peak
would be expected for the Z-conformation. Based on these inter-
actions, the 21 amides are assigned as being in the Z conformation.
The ROESY spectra of the 31 amide compounds, on the other
hand, show a different set of interactions. The N-methyl has
ROE cross-peaks with the aryl methyl, as well as the aromatic
proton next to the nitrogen. However, it has no cross-peak with
the aromatic protons next to the carbonyl. Instead, these
aromatic protons now have a cross-peak with the aromatic
methyl. This shows that the amide has switched conformations
to the E-isomer.
This difference in the preferred conformation between the
21 and 31 amides is also predicted by quantum mechanical DFT
calculations. Geometry optimisations of compound 4a in the
E- and Z-forms suggest that the Z conformer is 15 kJ molꢀ1
lower in energy than the E conformer, as would be expected. For
4b, however, the E-isomer is 9.8 kJ molꢀ1 lower in energy than the
Z-isomer. This view is consistent with the extremely small value of
DSNI/R seen for 5b (Table 4). The kinked amide fragment increases
the molecular biaxiality which reduces the change in orientational
order at the nematic–isotropic transition (Fig. 11).63,64
Fig. 5 POM texture of compound 6b in the nematic phase at 190 1C.
phase at 186 1C and clears at 388 1C (Table 5). The inclusion of the
second cyanoterphenyl group promotes crystallisation and pre-
cludes the observation of the twist-bend nematic phase.62 The
N-methylated dimer 6b melted directly into the isotropic phase
at 179 1C, and on cooling formed a nematic phase at 161 1C
(Fig. 5), and vitrified at 44 1C (Table 5). On reheating, a glass
transition occurred at 47 1C, followed by cold crystallisation at
88 1C, a crystal-crystal transition at 103 1C, and the sample
melted into the isotropic phase at 177 1C. The reduction in TNI
on methylating 6a to give 6b was 244 1C. The value of TNI of 6b
was 77 1C higher than that of 5b, which is consistent with the
inclusion of an additional phenyl ring (Table 5).
It is very clear that the structural change from a 21 to a 31
amide is associated with very dramatic changes in liquid
crystalline behaviour (Tables 2–5). The reduction observed in
TNI cannot be accounted for simply in terms of the addition of a
lateral methyl group. In itself this is not a sufficiently large
difference in shape to account for such dramatic decreases in
TNI, see Fig. 6. As noted earlier, conventional wisdom accounts
for the change in terms of the loss of hydrogen bonding. This
seems unlikely, however, because if compared to similar ester-,
and imine-based compounds for which hydrogen bonding is
not an issue, the transition temperatures are much lower than
would be expected.
This preference for the E-isomer is found for all the 31 amides.
This view was confirmed using X-ray diffraction. The crystal
In an attempt to understand this remarkable difference in
behaviour between the 21 and 31 amides we need to establish if
the molecular configuration about the nitrogen changes when a
Fig. 6 Space filling models of 3a and 3b showing the effect of N-
methylation on molecular shape.
Fig. 8 (A) The ROE interactions seen in the 21 amides. (B) The ROE
interactions seen for the 31 amides.
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Fig. 9 2D ROESY Spectrum of compound 3a in DMSO.
Fig. 10 2D ROESY Spectrum of compound 3b in DMSO.
structures of compounds 4b and 5b show that the amide group is
entirely present as the E-conformer in the solid state. There are
four unique C47H41N3O2 molecules in the asymmetric unit of 4b,
with an example molecule illustrated in Fig. 12a, which shows its
overall kinked shape, with the main twist occurring in the vicinity
of the amide link. The O1a–C13a–N1a–C14a amide torsion angle
Fig. 11 Left: DFT optimised Z-amide geometry of compound 4a. Right:
of 0.3(9)1 shows that these atoms are statistically coplanar in a cis The optimised E-amide geometry of compound 4b.
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Fig. 12 (a) The molecular structure of the C1a molecule in 4b showing 50% displacement ellipsoids. (b) Overlay plot of the four molecules in the
asymmetric unit of 4b. (c) Overlay plot of the C1a molecule (red) in 4b and its mirror image (blue) as generated by c-glide crystal symmetry with the atoms
of the C1a–C6a benzene rings overlapped. (d) The wave-like unit-cell packing for 4b viewed approximately down [100] with the four asymmetric
molecules and their symmetry-generated clones shown in red, blue, green and yellow (H atoms omitted for clarity).
conformation and the N1a–C15a–C16a–C21a torsion angle is adjacent to the amide link lies to both the left- and right-hand
11.6(6)1. The hexyl chain adopts an extended (all anti) conformation sides of the molecules. When viewed down the [100] direction,
and the dihedral angles subtended by the adjacent benzene rings the packing for 4b could be described as ‘wave-like’ (Fig. 12d).
C1a/C7a, C28a/C34a and C34a/C40a are 32.2(3), 23.6(3) and 18.2(2)1,
The asymmetric unit of 5b also contains four unique
respectively. The four asymmetric molecules have very similar
C
51H49N3O2 molecules, which adopt similar but not identical
conformations for their cyanobiphenyl ends, but their conforma- conformations (Fig. 13), which may be described as L-shaped:
tions increasingly diverge along the hexyl chains and the terphenyl again the cyanobiphenyl ends overlap well but there is increasing
units (Fig. 12b) as shown in an overlay plot generated with QMOL.65 divergence along the decyl chains with some of the C–C–C–C links
Crystal symmetry in space group Cc generates a 50 : 50 mix of mirror in these chains adopting gauche conformations. As with 4b,
images of each asymmetric molecule (Fig. 12c), where it may be crystallographic symmetry (an n-glide in this case) generates equal
seen that the C21 pendant methyl group of the benzene ring numbers of mirror images in the crystal. The packing for 5b
(Fig. 13) has a ‘grid-like’ appearance when viewed down [101].
Although the 31 materials preferentially adopt the E-conformation
in the solid state and in solution, the 1H NMR spectra show an
additional set of minor peaks which indicates the presence of a
small amount of the Z-amide conformer. The ratio of major:
minor peaks is dependent on the NMR solvent. To determine
the rotational barrier around the N–C(O) bond, the VT NMR
experiment was repeated in deuterated DMF from 10-55 1C,
using compound 5b as a representative example, and the
spectra are shown in Fig. 14.
Lineshape analysis was carried out using the DNMR option
within TOPSPIN software to determine the rotational constant
krot at each temperature, and the free energy of rotation was
determined using the Eyring equation.66 The rotational barrier
was found to be 71.4 ꢁ 1.0 kJ molꢀ1. This is consistent with
reported amide rotational barriers, which tend to fall between
63–84 kJ molꢀ1, and the error is in line with similar work.67
Conclusions
We have shown that there is a marked preference for the E
conformer in 31 benzanilide dimers and this has been reported
5b. Below: The unit-cell packing for 5b viewed approximately down [101]
Fig. 13 Top: Overlay plot of the four molecules in the asymmetric unit of
in the literature but not for liquid crystalline materials. The
dramatic change in shape on methylation of the corresponding
with the four asymmetric molecules and their symmetry-generated clones
shown in red, blue, green and yellow (H atoms omitted for clarity).
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Fig. 14 Stacked VT NMR spectra of 5b in DMF showing coalescence of the minor peaks.
7 T. Kajitani, S. Kohmoto, M. Yamamoto and K. Kishikawa,
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21 amide readily accounts for the dramatic difference seen in
the liquid crystalline behaviour of the 21 and 31 dimers. Although
intuitively pleasing, the disruption in hydrogen bonding need not
be invoked to explain the thermal behaviour of these amides,
although we do not exclude the possibility that it has some role to
play in the reduction seen in TNI. Future attempts to incorporate
benzanilides into new mesogenic moieties will require the
structures to be designed based on the E-amide conformation.
9 Q. C. Zhang, T. Takeda, N. Hoshino, S. I. Noro, T. Nakamura
and T. Akutagawa, Liquid Crystalline Phase Induced by
Molecular Rotator and Dipole Fluctuation, Cryst. Growth
Des., 2015, 15, 5705–5711.
10 R. A. Vora and A. K. Prajapati, Liquid Crystalline Homologous
Series with Biphenyl Nucleus, Mol. Cryst. Liq. Cryst. Sci. Technol.
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11 M. L. Parra, E. Y. Elgueta, V. Jimenez and P. I. Hidalgo,
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viour, Liq. Cryst., 2009, 36, 301–317.
12 M. L. Parra, C. G. Saavedra, P. I. Hidalgo and E. Y. Elgueta,
Novel chiral liquid crystals based on amides and azo
compounds derived from 2-amino-1,3,4-thiadiazoles: synthesis
and mesomorphic properties, Liq. Cryst., 2008, 35, 55–64.
Conflicts of interest
There are no conflicts to declare.
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