Non-symmetric chiral liquid crystal dimers. Preparation and characterisation of the (S)-(benzylidene-4′-substitutedaniline)-2″-methylbutyl-4?(4″″-phenyloxy)-benzoateoxy)hexanoates

Article abstract of DOI:10.1016/j.molstruc.2016.01.007

The properties of six chiral non-symmetric dimers belonging to the family of (S)-(benzylidene-4′-substitutedaniline)-2″-methylbutyl-4?(4″″-phenyloxy)benzoate-oxy)hexanoates are described. The terminal substituents on the 4-benzylidene-4′-substituted fragm

Full text of DOI:10.1016/j.molstruc.2016.01.007

Accepted Manuscript
Non-symmetric chiral liquid crystal dimers. Preparation and characterisation of
the (S)-(benzylidene-4’-substitutedaniline)-2’’-methylbutyl-4’’’-(4’’’’-phenyloxy)-
benzoateoxy)hexanoates
Guan-Yeow Yeap, Faridah Osman, Corrie T. Imrie
PII:
S0022-2860(16)30007-2
10.1016/j.molstruc.2016.01.007
MOLSTR 22123
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 16 November 2015
Accepted Date: 4 January 2016
Please cite this article as: G.-Y. Yeap, F. Osman, C.T. Imrie, Non-symmetric chiral liquid crystal dimers.
Preparation and characterisation of the (S)-(benzylidene-4’-substitutedaniline)-2’’-methylbutyl-4’’’-
(4’’’’-phenyloxy)-benzoateoxy)hexanoates, Journal of Molecular Structure (2016), doi: 10.1016/
j.molstruc.2016.01.007.
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ACCEPTED MANUSCRIPT
Non-symmetric chiral liquid crystal dimers. Preparation
and
characterisation
of
the
(S)-(benzylidene-4’-
substitutedaniline)-2’’-methylbutyl-4’’’-(4’’’’-phenyloxy)-
benzoateoxy)hexanoates
Guan-Yeow Yeapa*, Faridah Osmana and Corrie T. Imrieb
aLiquid Crystal Research Laboratory, School of Chemical
Sciences, Universiti Sains Malaysia, 11800 Minden, Penang,
Malaysia
bDepartment of Chemistry, School of Natural and Computing
Sciences, University of Aberdeen, Meston Walk, Aberdeen
AB24 3UE, United Kingdom
*Corresponding author G-Y.Yeap (Email: gyyeap@usm.my and
fax: +60-4-6574854)
1

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Abstract
The properties of six chiral non-symmetric dimers belonging to the family of (S)-
(benzylidene-4’-substitutedaniline)-2’’-methylbutyl-4’’’-(4’’’’-phenyloxy)benzoate-
oxy)hexanoates are described. The terminal substituents on the 4-benzylidene-4’-
substituted fragment are H, Cl, Br, I, CH3 and C2H5. All six dimers exhibit an
enantiotropic chiral nematic phase. The relationship between the chiral nematic-
isotropic transition temperature and the terminal substituent is discussed in terms of
the size of the substituent and its effect of the molecular shape, and its ability to
interact with mesogenic group to which it is attached. The C2H5-substituted dimer
also shows an enantiotropic monolayer chiral smectic C phase, the formation of which
is attributed to the enhanced molecular inhomogeneity arising from the longer alkyl
chain. The weak smectic tendencies seen for the halogen-substituted dimers are
attributed to potentially repulsive interactions between the ester links that would occur
in an intercalated smectic arrangement which would be expected for dimers of this
type.
Keywords: liquid crystal dimers; non-symmetric dimers; chirality
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1.
Introduction
Liquid crystal dimers consist of molecules containing two mesogenic units
connected via a flexible spacer, and continue to be the focus of significant research
interest [1-3]. If the two mesogenic units in the molecule are identical, then the dimers
are referred to as being symmetric, whereas if they differ, they are termed non-
symmetric. Initial interest in liquid crystals dimers arose from their role as model
compounds for both main chain [4] and side chain liquid crystal polymers [5].
However, it soon became clear that their transitional properties differed significantly
from those of conventional low molar mass materials consisting of molecules having
just a single mesogenic core, and thus, quickly became of fundamental interest in their
own right [6]. In particular, the study of non-symmetric dimers led to the discovery of
the intercalated smectic phases.[7-10] This created interest in higher oligomers such
as trimers and tetramers consisting of molecules containing either three or four
mesogenic units, respectively. [11-15] More recently, research involving dimers has
included, for example, new non-symmetric dimers [16-18], discotic dimers [19, 20],
H-shaped dimers [21-23], bent-core dimers [24, 25], disulfide-linked spacers [26, 27],
carbohydrate-based non-symmetric dimers [28], their elastic and viscoelastic
behaviour [19, 29, 30], hydrogen bonded dimers [31, 32] as well as continued interest
in higher oligomers. [33-37] However, the most exciting research area involving
dimers is the recently discovered twist bend nematic phase (see, for example, [38-
48]).
The structural requirement for a liquid crystal dimer to exhibit the twist bend
nematic phase is a bent molecular shape, and this is largely controlled by the number
of atoms connecting the two mesogenic units, and the chemical nature of the groups
linking the spacers to the mesogenic units. In the majority of dimers reported in the
literature, ether links are used to connect the spacer to the liquid crystal groups. [1-3]
The considerable interest focussed on the twist bend nematic phase has led to
significant interest in replacing the ether links by methylene groups as these give rise
to a more bent structure in odd-membered dimers (see, for example, [38, 39, 42, 49]).
We have recently considered the effects of using ester links to connect the
spacer to the mesogenic units. [10, 50, 51] Here we extend these studies to chiral
materials and report the liquid crystalline behaviour of the (S)-(benzylidene-4’-
substitutedaniline)-2’’-methylbutyl-4’’’-(4’’’’-phenyloxy)benzoateoxy)hexanoates,
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We refer to these dimers using the acronym (S)MB-6E-X in which (S)MB denotes the
(S)-2-methylbutyl group, 6 is the number of carbon atoms linking the two mesogenic
units, E indicates the ester linkage in the spacer, and X the terminal substituent on the
Schiff’s base moiety.
2.
Experimental
2.1
Synthesis
The syntheses of the (S)-(benzylidene-4’-substituted-aniline)-2’’-methylbutyl-
4’’’-(4’’’’-phenyloxy)benzoateoxy)hexanoate dimers, (S)MB-6E-X, were carried out
according to the method summarized in Scheme 1. The preparation of the
intermediates 1-X and 2-X have been described in detail elsewhere [51]. 4-(4-
hydroxyphenyl)benzoic acid reacted with (S)-methyl-1-butanol in acetone in the
presence of a catalytic amount of hydrochloric acid to give 3. A Williamson ether
reaction between 2-X and 3 in the presence of anhydrous potassium carbonate and
potassium iodide in acetone gave (S)MB-6E-X. The method used for each member of
the series was identical, and so a representative synthesis and characterisation data are
described in detail for just one final compound, namely (S)MB-6E-Br.
2.1.1 Synthesis of (S)-4-(4-hydroxyphenyl)benzoate-2-methyl-1-butyl (3)
3 was prepared according to the method described in our earlier study [50].
Thus, 4-(4-hydroxyphenyl)benzoic acid (2.0g, 9.34 mmol) was added to (S)-2-methyl-
1-butanol (25ml) which served both as a reactant and the solvent medium. As the
temperature was increased to 90 °C, a catalytic amount of concentrated HCl was
added drop wise, and the reaction mixture was refluxed for 24h. The mixture was
evaporated under reduced pressure. The white precipitate obtained was recrystallized
from chloroform.
Yield 80%. Elemental analysis (%): Found, C 75.98, H 7.11; calculated C18H20O3, C
76.03, H 7.09. IR (KBr) ν/cm-1, 3410 (OH), 2968-2875 (C-H alkyl), 1690 (C=O
1
ester). H-NMR (CDCl3) δ/ppm, 0.93 (t, 3H, CH3), 0.99 (d, 3H, CH3), 1.34 (m, 1H,
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CH2), 1.57 (m, 1H, CH2), 1.91 (m, 1H, CH), 4.12-4.18 (2dd, 2H, COOCH2), 6.89 (d,
2H, Ar), 7.59 (d, 2H, Ar), 7.75 (d, 2H, Ar), 7.99 (d, 2H, Ar), 9.72 (s, 1H, OH).
2.1.2 Synthesis of (S)-(benzylidene-4’-bromoaniline)-2’’-methylbutyl-4’’’-(4’’’’-
phenyloxy)benzoateoxy)hexanoate ((S)MB-6E-Br)
(S)MB-6E-Br was prepared as described in detail elsewhere [51].
Yield 45%. Elemental analysis (%): Found, C 67.34, H 5.87, N 2.15; calculated
C37H38BrNO5, C 67.68, H 5.83, N 2.13. IR (KBr) ν/cm-1, 2875-2960 (C-H alkyl),
1
1760 (C=O ester), 1713 (C=O ester), 1621 (C=N), 1253 (O-CH2). H-NMR (CDCl3)
δ/ppm, 0.97 (t, 3H, CH3), 1.03 (t, 3H, CH3), 1.34 (m, 1H, CH), 1.56-1.95 (m, 8H,
CH2), 2.65 (t, 2H, COCH2), 4.08 (t, 2H, OCH2), 4.14-4.28 (2dd, 2H, COOCH2), 6.98
(d, 2H, Ar), 7.08 (d, 2H, Ar), 7.18 (d, 2H, Ar), 7.48 (d, 2H, Ar), 7.62 (dd, 4H, Ar),
7.83 (d, 2H, Ar), 8.10 (d, 2H, Ar), 8.34 (s, 1H, CH=N).
2.2
Characterization
The Fourier transform infrared (FTIR) spectra of the intermediate and target
compounds were obtained using a Perkin-Elmer 2000 FTIR spectrophotometer. The
samples were mixed with KBr and the spectra recorded in the frequency range 4000-
1
400 cm-1. H-NMR spectra were obtained using a Bruker 300 MHz UltrashieldTM
spectrometer. The samples were dissolved in CDCl3 with tetramethylsilane (TMS) as
the internal standard. Carbon, hydrogen and nitrogen (CHN) microanalyses were
performed using a Perkin Elmer 2400 LS Series CHNS/O analyser. The liquid
crystalline textures were observed using a Carl Zeiss Axioskop 40 polarizing
microscope equipped with a Linkam LTS350 hot stage and TMS94 temperature
controller. The phase transition temperatures and associated enthalpy changes of the
dimers were determined with a Mettler Toledo DSC822e differential scanning
calorimeter equipped with a TSO 801RO sample robot and calibrated using indium
and zinc standards. The heating and cooling rates used were ±5 °C min-1.
3.
Result and discussion
3.1
Transitional behaviour of the (S)MB-6E-X dimers
The transitional properties of the (S)MB-6E-X dimers are listed in Table 1,
and all six dimers show an enantiotropic chiral nematic phase. Throughout chiral
nematic phases have been identified on the basis of the observation of characteristic
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fan and oily streak textures when viewed through the polarised microscope;
representative examples are shown as Figures 1 and 2, respectively. The initial DSC
heating trace for (S)MB-6E-H contains an endothermic peak corresponding to the
crystal-chiral nematic transition which overlaps a peak associated with a crystal-
crystal transition, and at higher temperatures an endotherm associated with the chiral
nematic-isotropic transition, see Figure 3(a). On cooling, the sample does not
crystallise and a new exotherm is observed at 28˚C (∆H = 1.89 kJ mol-1), see Figure
3(b). On subsequent reheating, a glass transition is seen with a mid-point value of
-25˚C (see Figure 4) followed by cold crystallisation, which appears to be triggered by
the onset of the phase transition observed on cooling at 28˚C, crystal-crystal and
melting endotherms and finally the endotherm associated with the chiral nematic-
isotropic transition, see Figure 3(c). The exotherm at 28˚C seen in the cooling trace
(Figure 3(b)) may be associated with a smectic-chiral nematic transition, but
crystallisation precluded its detailed study using polarised light microscopy.
The DSC traces for (S)MB-6E-Cl and (S)MB-6E-Br are similar to those
discussed for (S)MB-6E-H. On cooling, again extensive crystallisation does not occur
but instead a number of weak exotherms are seen beginning at about 40˚C for (S)MB-
6E-Cl and at 49˚C for (S)MB-6E-Br. Difficulties associated with slow crystallisation
prevented the study of these thermal events using polarised light microscopy. On
reheating, both dimers also exhibit a glass transition, followed by a cold
crystallisation exotherm, and melting and clearing endotherms. The first heating DSC
trace for S-MB-6E-I shows overlapping endotherms associated with a crystal-crystal
and crystal-chiral nematic transition and at higher temperature an endotherm
corresponding to the chiral nematic-isotropic transition. On cooling the sample
undergoes crystallisation, and the subsequent reheating trace is similar to that
observed on initial heating.
Figure 5 shows the DSC traces for S-MB-6E-CH3, and on initial heating a
crystal-crystal transition overlaps the melting endotherm and at higher temperature the
chiral nematic-isotropic endotherm is seen. The focal conic fan and oily streak optical
textures seen through the polarised light microscope for the chiral nematic phase are
shown as Figures 1 and 2, respectively. On cooling, the sample does not crystallise,
revealing a transition at 60˚C, see Figure 5(b). The DSC peak shape suggests that this
is in fact two overlapping phase transitions although this was not evident optically. At
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this transition, a mosaic optical texture, see Figure 6, develops from the focal conic
fan texture shown in Figure 1, and an essentially identical texture was formed from
regions of the oily streak texture. This texture is similar to the natural optical texture
of the G/J phase, and the associated entropy change is consistent with this view [52].
The monotropic nature of this phase precluded the possibility of its further study. On
reheating the sample (Figure 5(c)), a glass transition was observed with a mid-point
value of -22˚C and an associated small enthalpy overshoot, a weak endotherm was
observed at ca 5˚C, followed by a cold crystallisation exotherm, and endotherms
associated with the melting and clearing transitions.
The initial heating DSC trace for (S)MB-6E-C2H5 shown as Figure 7(a)
consists of two endothermic peaks corresponding to the melting and chiral nematic-
isotropic transitions. On cooling, see Figure 7(b), crystallisation does not occur but
instead a new peak is seen at 74˚C, and the reheating DSC trace contains endotherms
associated with this new event and the clearing transition, see Figure 7(c). On closer
examination, both traces contain two additional and very weak endotherms (Figure 8)
and a glass transition is seen in the reheat trace with an associated enthalpy overshoot.
When viewed under the polarised light microscope, on cooling, the fan texture of the
chiral nematic phase becomes crossed suggesting the formation of a smectic C phase,
and on further cooling a filamentary texture coexisting with homeotropic regions
forms which subsequently transforms into a mosaic texture. This sequence is shown
in Figures 9-12.
The phase behaviour of (S)MB-6E-C2H5 was further studied using X-ray
diffraction, and Figure 13 shows patterns collected at 154, 103 and 76 ˚C. The pattern
corresponding to 154 ˚C shows a very weak reflection in the low angle region
corresponding to 39.1 Å, while in the wide angle region, broad diffuse scattering is
seen, centred at 4.5 Å, characteristic of a liquid-like arrangement of the molecules.
This is a typical pattern for a chiral nematic phase. At 103 ˚C, a sharper first order and
also a weak second order reflection are seen in the low angle region while in the wide
angle a broad reflection is still observed. The sharp first order reflection in the low
angle implies a layered structure, while the diffuse nature of the wide-angle scattering
indicates a liquid-like arrangement of the molecules with the layer. This is consistent
with the assignment of a smectic C phase. The periodicity of the phase is 38.9 Å
which is similar to the estimated all-trans molecular length of the dimer, 39.1 Å, and
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strongly suggests that the smectic C phase has a monolayer arrangement with a small
tilt angle. On further cooling to 76˚C, the reflections in the low angle become
stronger, and the band in the wide angle narrows considerably implying the presence
of much longer range ordering within the layers. The layer spacing is 35.0 Å
suggesting that the molecules are tilted within the layers. This pattern is consistent
with the assignment of a G phase based on the optical texture.
3.2
Comparison of the liquid crystal behaviour of the (S)MB-6E-X dimers
The chiral nematic-isotropic transition temperatures of the (S)MB-6E-X
dimers increase in the order of
H < C2H5 < I < Cl < CH3 < Br
although the differences in TN*I between four of the dimers (I, Cl, CH3, Br) are very
small. As would be expected, this is essentially the same trend as seen for the
corresponding racemic materials, and may be accounted for in terms of the size and
shape of the substituent, and its ability to interact with the mesogenic unit [53]. Figure
14 shows the dependence of TN*I on the van der Waals volume of each substituent
when bound to a phenyl ring, [54] and reveals a linear dependence of TN*I on the size
of the substituent with the exception of (S)MB-6E-H. This relationship is illustrated in
Figure 14 by the line of best fit for the data points but excluding that for (S)MB-6E-H.
It is interesting to note that in our previous studies of similar materials, we
also observed such behaviour but the line of best fit had a positive gradient [10, 51].
The physical significance of this differing behaviour is unclear, although we note that
the differences between the transition temperatures are small. It is clear that TN*I for
(S)MB-6E-H is considerably lower than would be predicted on the basis of the
relationship revealed by Figure 14. Indeed, the percentage increase in TN*I on
exchanging H for a Cl substituent is 9.6% which far exceeds the associated increase in
the van der Waals molecular volume. Such behaviour has been seen for other liquid
crystal dimers and attributed to a change of shape on substituting the mesogenic unit
rather than from any significant enhancement of the shape anisotropy [8].
The entropy changes associated with the chiral nematic-isotropic transition are
also listed in Table 1 and expressed as the dimensionless quantity ∆SN*I/R. These
show a weak dependence on the nature of the substituent with the values ranging from
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0.96 to 1.31. These values are rather small for even-membered liquid crystal dimers
containing small terminal substituents [49, 55]. The low values of ∆SN*I/R may be
accounted for in terms of the enhanced molecular biaxiality arising from the branched
terminal chain, and similarly low clearing entropies have been observed for dimers
containing branched terminal chains [56] or mesogenic units having enhanced
biaxiality including, for example, cholesteryl-based and pyrene-based units [57, 58].
Smectic behaviour is unambiguously observed for just two of the (S)MB-6E-X
dimers, namely (S)MB-6E-CH3 and (S)MB-6E-C2H5. The halogen-substituted dimers
may show strongly monotropic smectic behaviour but it was not possible to confirm
this by structural studies because of crystallisation. The corresponding halogen-
substituted members of a closely related series of dimers in which the ester link
between the spacer and the mesogenic unit in the (S)MB-6E-X dimers has been
replaced by an ether link were shown to exhibit smectic A behaviour in which the
molecules pack in an intercalated arrangement [10]. The driving force for the
formation of intercalated smectic phases by non-symmetric dimers is most often
attributed to a specific favourable interaction between the unlike mesogenic groups
[9] which has been suggested to be an electrostatic quadrupolar interaction between
groups having quadrupole moments of opposite signs [59]. The lower smectic phase
stabilities seen for the ester-linked materials may be attributed to a repulsive dipolar
interaction between the ester group linking the spacer to the mesogenic unit and that
connecting the terminal chain that would destabilise an intercalated smectic
arrangement. By comparison, (S)MB-6E-C2H5 shows an enantiotropic chiral smectic
C phase. As we have seen, this is a conventional monolayer chiral smectic C phase
and is presumably stabilised by the anisotropic interactions between the mesogenic
groups, while the driving force for its formation is the enhanced molecular
inhomogeneity arising from the longer terminal chain. (S)MB-6E-CH3 shows a much
lower smectic-chiral nematic transition temperature because there is now a mismatch
in the lengths of the chains which creates packing difficulties within the smectic
layers.
4.
Conculsions
The dependence of the chiral nematic-isotropic transition temperatures on the
chemical nature of the terminal substituent of the six new chiral non-symmetric
dimers reported here, reinforces the view that it is predominantly the size of the
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substituent and how this affects the shape of the molecule that influences the clearing
temperature. However, the ability of the substituent to interact with the mesogenic
group to which it is attached should also be taken into account. By contrast the
smectic forming tendencies of these dimers appear to be controlled by repulsive
dipolar interactions between the ester linking groups for the halogen-substituted
materials that destabilise the expected intercalated packing arrangement. The driving
force for the monolayer enantiotropic chiral smectic C phase observed for (S)MB-6E-
C2H5 appears to be the molecular inhomogeneity arising from the longer terminal
group and the phase stabilised by the interactions between the mesogenic units.
Acknowledgements
The author (G.Y.Yeap) would like to thank Universiti Sains Malaysia for providing
the research facilities and financial support under RU Research Grant (Account No.
203/PKIMIA/6711192) and PRGS Postgraduate Research Grant (Account No.
1001/KIMIA/835010). Authors also thank Ms Anna Zep and her supervisor Prof E.
Gorecka for the powder X-ray diffraction measurement.
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Table 1. Phase transition temperatures, associated enthalpy changes and entropy
changes of the (S)MB-6E-X dimers. Cr crystal, Sm smectic, N* chiral nematic phase,
I isotropic phase. ( ) denote monotropic transitions.
Dimer
Tg
TCr- TSmN TN*I
∆HCr-
∆HSmN
∆HN*I
∆SSmN ∆SN*I
/˚C
-25
-20
/˚C
88
/˚C
/˚C
/kJ mol-1 /kJ mol-1 /kJ mol-1 /R
/R
0.96
(S)MB-6E-H
(S)MB-6E-Cl
124
162
164
160
31.4
33.9
32.6
31.9
35.2
3.18
4.14
4.27
4.64
4.74
86
1.14
(S)MB-6E-Br -23
(S)MB-6E-I
90
1.18
112
99
1.29
(S)MB-6E-
-22
(63) 163
(4.05)
0.32
(1.45) 1.31
CH3
(S)MB-6E-
-30
89
106 156
28.6
4.25
0.10
1.19
a
C2H5
a G-Sm 78˚C, 5.31 kJ mol-1, ∆S/R = 1.82; Sm-Sm 89˚C (see text).
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Figure 1
The fan texture of the chiral nematic phase shown by S-MB-6E-CH3 at
140˚C.
Figure 2
The oily streak texture of the chiral nematic phase shown by S-MB-6E-
CH3 at 120˚C.
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Figure 3 DSC traces for S-MB-6E-H on (a) first heating, (b) cooling and (c) second
heating cycles. The traces have been arbitrarily shifted on the y-axis in order to
separate them for the sake of clarity.
Figure 4
Magnified section of the DSC reheating trace for (S)MB-6E-H shown as
Figure 1(c) showing the glass transition.
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Figure 5
DSC traces for S-MB-6E-CH3 on (a) first heating, (b) cooling and (c)
second heating cycles. The traces have been arbitrarily shifted on the y-axis in order
to separate them for the sake of clarity.
Figure 6
The mosaic texture of the lower temperature phase shown by S-MB-
6E-CH3 at 56˚C obtained on cooling the fan texture sown in Figure 1.
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Figure 7
DSC traces for (S)MB-6E-C2H5 on (a) first heating, (b) cooling and (c)
second heating cycles. The traces have been arbitrarily shifted on the y-axis in order
to separate them for the sake of clarity.
Figure 8
Magnified sections of the DSC cooling and reheating traces for (S)MB-
6E-C2H5 showed as Figure 17(b) and (c) showing the weak phase transitions.
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Figure 9
The fan texture of the chiral nematic phase shown by (S)MB-6E-C2H5 at
113˚C.
Figure 10
The fan texture of the chiral smectic C phase shown by (S)MB-6E-
C2H5 at 98˚C obtained on cooling the preparation shown in Figure 9.
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Figure 11
The filament texture shown by (S)MB-6E-C2H5 at 88˚C obtained on
cooling the preparation shown in Figure 10.
Figure 12
The mosaic texture of the G phase shown by (S)MB-6E-C2H5 at 68˚C
obtained on cooling the preparation shown in Figure 11.
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a
b
38.86
19.07Å
39.05
4.5Å
4.5Å
c
4.5
Å
17.55
35.04
Figure 13 X-ray diffraction patterns obtained for (S)MB-6E-C2H5 at (a) 154 ˚C, (b)
103 ˚C and (c) 76 ˚C.
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Figure 14
The dependence of the chiral nematic-isotropic transition temperature,
TN*I, on the calculated van der Waals volume of the terminal substituent when bound
to a phenyl ring [54] for the (S)MB-6E-X dimers. The line shows the best fit through
the data points excluding (S)MB-6E-H.
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Scheme 1. Synthetic route used to prepare the (S)MB-6E-X dimers.
1