The design of smectic liquid crystals with an axially chiral biphenyl core: In search of a proper ferroelectric liquid crystal phase

Article abstract of DOI:10.1039/c7tc05426b

We report the results of a study in which we introduce a tricarbosilane end-group in a mesogenic scaffold derived from an axially chiral biphenyl with a large transverse dipole moment. This structural modification is intended to promote the formation of a chiral smectic C (SmC?) phase, with the aim of discovering a mesogen with a virtual transition temperature to a proper ferroelectric phase (T_o) that approaches the crystal-SmC? melting point. We show herein that the combination of a 4-pentylbicyclo[2.2.2]octane-1-carboxylate chain and a tricarbosilane-terminated 4-alkoxybenzoate chain gives a mesogen (R,S)-WL43 forming an enantiotropic orthogonal smectic A (SmA) phase and a higher order tilted smectic phase. Substitution of the 4-pentylbicyclo[2.2.2]octane-1-carboxylate group with a trans-4-pentylcyclohexane-1-carboxylate group gives a mesogen (R,S)-WL45 forming monotropic SmA and SmC phases. The SmC? phase formed by the enantiomerically pure (R)-WL45 exhibits Goldstone-mode switching characteristic of a surface-stabilized FLC, but its spontaneous polarization P_S is too low to be measured by conventional methods.

Full text of DOI:10.1039/c7tc05426b

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The design of smectic liquid crystals with an
axially chiral biphenyl core: in search of a proper
Cite this:DOI: 10.1039/c7tc05426b
ferroelectric liquid crystal phase†
Ziauddin Ahmed,a Carsten Muller,b Marcel Holzwarth,b Christian Haege,b
¨
b
a
Frank Giesselmann
and Robert P. Lemieux
*
We report the results of a study in which we introduce a tricarbosilane end-group in a mesogenic
scaffold derived from an axially chiral biphenyl with a large transverse dipole moment. This structural
modification is intended to promote the formation of a chiral smectic C (SmC*) phase, with the aim of
discovering a mesogen with a virtual transition temperature to a proper ferroelectric phase (To) that
approaches the crystal-SmC* melting point. We show herein that the combination of a 4-pentylbicyclo-
[2.2.2]octane-1-carboxylate chain and
a tricarbosilane-terminated 4-alkoxybenzoate chain gives a
mesogen (R,S)-WL43 forming an enantiotropic orthogonal smectic A (SmA) phase and a higher order
tilted smectic phase. Substitution of the 4-pentylbicyclo[2.2.2]octane-1-carboxylate group with a trans-
4-pentylcyclohexane-1-carboxylate group gives a mesogen (R,S)-WL45 forming monotropic SmA and
SmC phases. The SmC* phase formed by the enantiomerically pure (R)-WL45 exhibits Goldstone-mode
switching characteristic of a surface-stabilized FLC, but its spontaneous polarization PS is too low to be
measured by conventional methods.
Received 27th November 2017,
Accepted 22nd January 2018
DOI: 10.1039/c7tc05426b
rsc.li/materials-c
Introduction
Ferroelectric liquid crystals (FLC) define a class of polar materials
forming a chiral tilted smectic phase such as the SmC* phase in
which rod-like molecules with a rigid aromatic core and two alkyl
chains are self-organized in diffuse layers and tilted at an angle y
with respect to the layer normal z. The chiral SmC* phase forms a
helical structure that unwinds between glass slides with a rubbed
polymer substrate to give a surface-stabilized FLC film with a
spontaneous polarization (PS) oriented along the C2 symmetry
axis (Fig. 1).1 By coupling PS to an electric field E, the FLC film can
be switched between opposite tilt orientations (Goldstone mode)
to give a light shutter used in high-resolution color microdisplays
that switch on a microsecond time scale.2,3
Unlike proper ferroelectrics (e.g., crystalline LiNbO3), in Fig. 1 Schematic representation of hard spherocylinders forming a SmC*
phase as a surface-stabilized ferroelectric liquid crystal film in a bookshelf
geometry. The SmC* phase is characterized by a uniform tilt y of the
average direction of all molecular axes (the director n) with respect to the
layer normal z.
which PS originates from non-centrosymmetric dipole–dipole
coupling, chiral SmC* liquid crystals are improper or pseudo-
proper ferroelectrics because PS originates from the rotational
order of the chiral mesogens about the director n, which is
imposed by the molecular tilt y.4,5 Hence, coupling between PS
a Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada.
and y normally results in PS(y) being invariant of temperature.
However, we recently found that the chiral mesogen E6 forms a
E-mail: rplemieux@uwaterloo.ca
b Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55,
SmC* phase with a PS(T) profile showing an atypical convex
D-70569 Stuttgart, Germany
curvature that does not correlate with the y(T) profile.6 Accord-
† Electronic supplementary information (ESI) available: Detailed experimental
and synthetic procedures. See DOI: 10.1039/c7tc05426b
ing to a molecular-statistical theory of ferroelectric ordering in
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Scheme 2 Chiral mesogens WL43–46 prepared as racemic mixtures;
WL45 was also prepared as the pure (R) enantiomer (correct stereo-
chemistry shown).
Scheme 1 Chiral mesogens E6 and 2(n), and non-mesomorphic chiral
dopant 1.
behave similarly to siloxane end-groups, but are hydrolytically
and electrochemically far more stable.22 Tricarbosilane-terminated
mesogens tend to form very stable SmC phases, which may be due
to a suppression of out-of-layer fluctuations that reduces the
entropic cost of molecular tilt. In this paper, we report the results
of a study in which we introduce tricarbosilane end-groups in the
2(n) molecular scaffold to promote the formation of a chiral SmC*
phase, and explore further structural modifications with the aim of
discovering a mesogen with a virtual transition temperature to a
proper ferroelectric phase (To) that approaches the crystal-SmC*
melting point (Scheme 2).
the SmC* phase, this behavior may be explained by an increase
in the virtual transition temperature to a proper ferroelectric
phase (To), which is normally well below the crystal-SmC*
melting point. In the case of E6, an unusually high To of
152 K was ascribed to stronger dipole–dipole coupling of the
chiral epoxy units due to the high lamellar ordering imposed by
the siloxane end-group, and the biaxial ordering of the epoxy unit
(Scheme 1). These findings suggest that one might further increase
To in a siloxane-terminated SmC* mesogen featuring a chiral unit
with a larger transverse dipole moment and higher biaxiality.
Axially chiral biaryl compounds have proven to be very
effective in the induction of chiral bulk properties in calamitic
liquid crystals, including the helical pitch of the chiral nematic
Results and discussion
Synthesis
(N*) phase or the spontaneous polarization of the SmC* The first structural modification to the 2(n) scaffold was to
phase.7,8 For example, we have shown that the dopant 1, which replace the alkoxy chain with a tricarbosilane-terminated unde-
has an axially chiral biphenyl core with a transverse dipole cenyloxy chain to give the new derivative WL43. Two additional
moment of 6.9 D, induces a remarkably high PS in the SmC structural modifications were investigated: (i) substitution of
phase of 2-phenylpyrimidine liquid crystal hosts as a result of the benzoate group with a 2,3-difluorobenzoate group, which
chirality transfer via core–core interactions.9 Yet, despite the is known to promote the formation of a SmC phase,23 and
ubiquitous qualities of these molecules as strong chiral indu- (ii) substitution of the laterally bulky 4-pentylbicyclo[2.2.2]octane-
cers, there are only a few examples of axially chiral biaryl 1-carboxylate, which is a strong nematogenic unit,17,18 with a
mesogens.10–13 Compounds such as 1 are not liquid crystalline trans-4-pentylcyclohexane-1-carboxylate unit.
due to the unfavorable lateral bulk of the biphenyl core caused
The axially chiral core 4,40-dihydroxy-3,30-dinitro-2,20,6,60-
by the four ortho substituents, which maintain the configura- tetramethylbiphenyl (3) was synthesized as a racemic mixture
tional integrity of the core by imposing a torsional angle of according to Hartley et al.,24 and converted to the THP-protected
ca. 901 between the phenyl rings.9,14,15 However, we have shown derivative 4 in 29% yield, as shown in Scheme 3 (see ESI† for
that substituting one 4-alkoxyphenyl benzoate chain with a 4- details). Esterification of 4 with either 4-pentylbicyclo[2.2.2]-
pentylbicyclo[2.2.2]octane-1-carboxylate chain results in the octane-1-carboxylic acid or trans-4-pentylcyclohexane-1-carboxylic
formation of an enantiotropic N* phase (e.g., 2(8)–2(14)),16 acid using DIC and DMAP followed by deprotection gave the
which has been attributed to the bicyclo[2.2.2]octane segment precursors 5 and 6 in 80% and 63% yield, respectively. The
effectively ‘shielding’ the unfavorable lateral bulk of the axially target compounds were then obtained as racemic mixtures
chiral biphenyl core.17,18
((R,S)-WL43–WL46) by esterification of 5 and 6 with the appro-
We and others have also shown that carbosilane end-groups priate tricarbosilane-terminated 4-undecyloxybenzoic acid (7 or 8)
promote the lamellar ordering of calamitic mesogens due to using DIC and DMAP in yields ranging from 56 to 87% yield.
their tendency to nanosegregate from hydrocarbon segments Compound (R)-WL45 was obtained in enantiomerically pure form
into distinct sub-layers, which normally results in the formation by resolution of 3 using preparative chiral phase HPLC (Chiralpak
of partially interdigitated smectic bilayers.19–21 As such, they AS column) according to Hartley et al.24
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Scheme 3 Reagents and conditions: (a) 3,4-dihydro-2H-pyran, pyridinium p-toluenesulfonate, CH2Cl2; (b) 4-pentylbicyclo[2.2.2]octane-1-carboxylic
acid, DIC, DMAP, CH2Cl2; (c) TsOH, 1 : 1 MeOH/CH2Cl2; (d) trans-4-pentylcyclohexane-1-carboxylic acid, DIC, DMAP, CH2Cl2; (e) 7 or 8, DIC, DMAP,
CH2Cl2; (f) 1-bromo-12,12,14,14,16,16-hexamethyl-12,14,16-trisilylheptadecane, K2CO3, acetone, reflux; (g) NaOH, MeOH/THF/H2O, reflux.
Mesomorphic characterization. Compounds (R,S)-WL43–46 in intensity whereas the azimuthal intensity distribution of the
were first analyzed by differential scanning calorimetry (DSC) wide-angle scattering evens out, which is inconsistent with the
and polarized optical microscopy (POM), but characterization formation of a nematic phase.
of the mesophases formed by these compounds proved difficult
The X-ray scattering pattern of the lower temperature phase
due to our inability to produce textures with unambiguous obtained prior to heating to the SmA phase shows sharp funda-
characteristics (see Fig. S1 in ESI,† for example). We therefore mental and higher-order scattering peaks at small-angles and a
relied on additional evidence from small angle X-ray scattering sharp peak at wide-angle, which is consistent with a smectic
(SAXS) and 2D X-ray scattering to confirm the mesomorphic phase with hexagonal ordering of the hydrocarbon sub-layer,
properties of these new compounds. As shown in Table 1, the while a diffuse inner ring corresponding to the carbosilane sub-
addition of a tricarbosilane end-group to the 2(n) scaffold layer persists (see Fig. 2b). We were unable to obtain a 2D X-ray
resulted in suppression of the nematic (N) phase, and the scattering pattern of this phase due to persistent supercooling
formation of an orthogonal smectic A (SmA) phase and a higher of the SmA phase, which precluded observation of any azimuthal
order tilted smectic phase, with no evidence of a crystalline shift of the wide-angle scattering. However, a decrease in layer
phase upon cooling down to ꢀ20 1C. The assignment of a SmA spacing measured by SAXS, as well as the corresponding change
phase is based on a X-ray scattering pattern that shows a sharp from homeotropic to Schlieren textures observed by POM (see
scattering peak at small-angle corresponding to the periodicity Fig. S1 in ESI†) are consistent with the formation of a tilted SmF
along the layer normal z, and a diffuse wide-angle scattering or SmI phase (Fig. 3).
from both carbosilane and hydrocarbon sub-layers (Fig. 2a).20
Substitution of the benzoate group in (R,S)-WL43 with a
The orthogonal structure of the SmA phase was confirmed by 2,3-difluorobenzoate group did not affect the mesomorphic
2D X-ray scattering on a sample aligned on cooling, which properties other than cause an increase in transition tempera-
shows an azimuthal intensity distribution of the wide angle tures. On the other hand, substitution of the 4-pentylbicyclo-
scattering with maxima perpendicular to that of the small-angle [2.2.2]octane-1-carboxylate group in (R,S)-WL43 with a trans-4-
scattering (see inset in Fig. 2a). On heating towards the clearing pentylcyclohexane-1-carboxylate group to give (R,S)-WL45 com-
point, the small-angle scattering peak broadens and decreases pletely suppressed the enantiotropic mesomorphism, although
Table 1 Transition temperatures (1C) and enthalpies of transitions (kJ molꢀ1, in parentheses) on heating
Compound
Cr
SmX
SmC
SmA
N
I
2(12)a
ꢁ
108 (31)
ꢁ
156 (1.6)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(R,S)-WL43
(R,S)-WL44
(R,S)-WL45
(R,S)-WL46
(R)-WL45
ꢁ
69 (9.4)
81 (7.8)
28 (3.5))b
45 (5.9))b
61 (4.5)
ꢁ
95 (2.6)
ꢁ
ꢁ
100 (3.0)
66 (3.3))b
74 (3.8))b
ꢁ
ꢁ
67 (13)
77 (16)
(ꢁ
(ꢁ
ꢁ
(ꢁ
(ꢁ
60)b,c
(ꢁ
(ꢁ
59 (4.0))b
a
b
c
From ref. 16. Monotropic phase transition; transition temperature measured on cooling. Second order phase transition; transition
temperature measured by polarized microscopy.
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Fig. 2 X-ray scattering profiles of (R,S)-WL43 (a) at 82 1C in the SmA phase (left, inset shows the azimuthal intensity of the wide-angle scattering relative
to the small-angle scattering peak) and (b) at 29 1C in the SmF/I phase prior to first heating (right).
between axially chiral cores of the same handedness are stronger
in a lamellar organization, which would favor the tilted SmC*
phase.25 As a consequence of enantiomeric resolution, Goldstone-
mode switching was observed in the SmC* phase and the optical
tilt angle yopt was measured as a function of temperature on a
sample aligned between rubbed polyimide ITO glass slides
(1.6 mm spacing), as shown in Fig. 5. The sharp rise in tilt
angle and relative temperature invariance of yopt on cooling is
consistent with a first-order transition from the isotropic phase
and confirms the absence of a SmA* phase above the SmC*
phase. However, we were unable to measure a spontaneous
polarization via the triangular wave method (detection limit of
B0.5 nC cmꢀ2) over a broad temperature range below the
SmC*–I transition point (see ESI† for details). The absence of
Fig. 3 Layer spacing d versus temperature T for (R,S)-WL43 measured
by SAXS on heating; the open symbols correspond to the diffuse peak a measurable PS may be ascribed to a lack of biaxial ordering of
maxima in the isotropic phase.
the ‘stereo-polar unit’ by virtue of the orthogonal geometry of
the axially chiral biphenyl core, as well as the subtle asymmetric
conformational bias about the ester C–O bonds of the biphenyl
core, which is not enhanced by chiral perturbations from core–
core interactions with mesogenic host molecules that are dyna-
mically racemic, such as those featuring a 2-phenylpyrimidine
both SmA and SmC phases formed on cooling below the
melting point. The derivative (R,S)-WL46 featuring both struc-
tural modifications formed only a monotropic SmA phase. The
SmC phase in (R,S)-WL45 was characterized by the formation of
contrasting domains of opposite tilt orientation observed by
POM in a sample aligned between rubbed polyimide glass
slides on cooling from the SmA phase, as shown in Fig. 4.
The enantiomerically pure (R)-WL45 forms only a SmC* phase
on cooling, together with a higher order smectic phase. It is not
entirely clear why the orthogonal SmA* phase is suppressed upon
resolution, although it is possible that non-covalent interactions
Fig. 5 Optical tilt angle yopt measured as a function of temperature T for
Fig. 4 Photomicrographs of (R,S)-WL45 as a thin film between rubbed
polyimide glass slides (4 mm spacing) in the SmA phase at 65 1C (left) and in
the SmC phase at 55 1C (right).
(R)-WL45 by applying a field of 6 V mmꢀ1 across an aligned film between
rubbed polyimide ITO glass slides with a spacing of 1.6 mm; the optical tilt is
saturated at this applied field.
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core.8,26 Another possible explanation may be that any biaxial
ordering of the polar core imposed by the SmC* phase may cause
the transverse dipole moment to lie in the n, z tilt plane, which
would minimize its contribution to PS.
5 D. M. Walba, in Advances in the Synthesis and Reactivity of
Solids, ed. T. E. Mallouck, JAI Press, Ltd, Greenwich, CT,
1991, vol. 1, p. 173.
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Conclusions
7 S. Pieraccini, A. Ferrarini and G. P. Spada, Chirality, 2008,
20, 749–759.
In this study, we have shown that the introduction of a tri-
carbosilane end-group in a nematogenic scaffold with an axially
chiral biphenyl core does induce smectogenic properties, albeit
not to the extent that it does in more conventional mesogenic
scaffolds, which may be ascribed to unfavorable steric repulsion of
the laterally bulky biphenyl cores in a lamellar organization.19–21
Indeed, we were unable to produce a material forming an enantio-
tropic SmC phase, although one did form a monotropic SmC
phase. Interestingly, the SmC* phase formed by the enantiomeri-
cally pure (R)-WL45 does not exhibit a measurable spontaneous
polarization. These results suggest that the design features of a
SmC* mesogen with proper ferroeletric properties should include
a large transverse dipole moment and exhibit strong biaxial
ordering. Further investigation will therefore focus on mesogenic
structures derived from axially chiral biphenyl cores with a bridge
structure such as 11-dimethyl-5,7-dihydrodibenz[c,e]thiepin,27
which restricts the dihedral angle formed by the two phenyl
rings to ca. 65 degrees and reduces the unfavorable lateral bulk
that may hinder the formation of a SmC phase. Such a core
structure, appropriately derivatized to give a large transverse
dipole moment, should also exhibit enhanced biaxial ordering
in the SmC* phase.
8 R. P. Lemieux, Chem. Soc. Rev., 2007, 36, 2033–2045.
9 D. Vizitiu, C. Lazar, B. J. Halden and R. P. Lemieux, J. Am.
Chem. Soc., 1999, 121, 8229–8236.
10 J. Rokunohe, A. Yamagushi and A. Yoshizawa, Liq. Cryst.,
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11 M. Goh and K. Akagi, Liq. Cryst., 2008, 35, 953–965.
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15 D. J. Byron, G. W. Gray and B. M. Worrall, J. Chem. Soc.,
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16 K. Ayub, M. Moran, C. Lazar and R. P. Lemieux, J. Mater.
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17 W. Piecek, J. M. Kaufman and P. Kaszynski, Liq. Cryst., 2003,
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18 B. Ringstrand, J. Vroman, D. Jensen, A. Januszko,
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Conflicts of interest
There are no conflicts to declare.
21 K. M. Mulligan, A. Bogner, Q. Song, C. P. J. Schubert,
F. Giesselmann and R. P. Lemieux, J. Mater. Chem. C,
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P. Raynes, Wiley-VCH, Weinheim, 2014, vol. 8, ch. 8,
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Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada and the Deutsche Forschungsgemeinschaft
(NSF/DFG Materials World Network program DFG Gi 243/6) for
support of this work.
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