Electroclinic effect in a chiral carbosilane-terminated 5-phenylpyrimidine liquid crystal with 'de Vries-like' properties

Article abstract of DOI:10.1039/c5cc05212b

The chiral carbosilane-terminated liquid crystal 2-[(2S,3S)-2,3-difluorohexyloxy]-5-[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]pyrimidine (QL32-6) undergoes a smectic A?-smectic C? phase transition with a maximum layer contraction of only 0.2%. It exhibits an electroclinic effect (ECE) comparable to that reported for the 'de Vries-like' liquid crystal 8422[2F3] and shows no appreciable optical stripe defects due to horizontal chevron formation.

Full text of DOI:10.1039/c5cc05212b

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Electroclinic effect in a chiral carbosilane-terminated
5-phenylpyrimidine liquid crystal with ‘de Vries-like’
properties †
Cite this: DOI: 10.1039/x0xx00000x
Christopher P. J. Schubert,a Carsten Müller,b Michael D. Wand,c Frank Giesselmannb
and Robert P. Lemieux*,a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
distribution,15 although recent theoretical and experimental studies
The chiral carbosilane-terminated liquid crystal 2-[(2
2,3-difluorohexyloxy]-5-[4-(12,12,14,14,16,16-hexamethyl-
12,14,16-trisilaheptadecyloxy)phenyl]pyrimidine (QL32-6)
undergoes a smectic A*-smectic C* phase transition with a
maximum layer contraction of only 0.2%. It exhibits an elec-
troclinic effect (ECE) comparable to that reported for the ‘de
Vries-like’ liquid crystal 8422[2F3] and shows no appreciable
optical stripe defects due to horizontal chevron formation.
S,3S)-
suggest that ‘de Vries-like’ behavior generally results from an unu-
sual combination of high lamellar order and low orientational or-
der.16-20 Liquid crystals with ‘de Vries-like’ properties tend to have
unusually large electroclinic susceptibilities, with the field-induced
electroclinic tilt accompanied by a significant increase in birefrin-
gence that is consistent with an ordering of azimuthal and/or orien-
tational distribution(s).12, 14, 21 The ECE of two such materials with
first-order SmA*-SmC* transitions was recently explained using a
generalized Langevin-Debye model, which assumes a random azi-
muthal distribution of molecules on a fixed tilt cone of angle
θ
, with
The chiral smectic A (SmA*) liquid crystal phase is characterized by
an analog electro-optical effect known as the electroclinic effect that
makes it possible to generate a gray scale in display applications on a
much faster time scale than a nematic LCD.1 The SmA* phase has a
diffuse lamellar structure described by a density wave with a period
d corresponding to the layer spacing;2 the director n corresponding
to the average orientation of long molecular axes is coincident with
the layer normal z. Garoff and Meyer showed that an electric field E
applied parallel to the SmA* layers induces a uniform molecular tilt
relative to z in a direction orthogonal to E.3 This electroclinic effect
(ECE) is described by a phenomenological model derived from Lan-
dau theory that predicts a linear dependence of the induced tilt angle
an orientational distribution in which the tilt
θ is allowed to vary
with the applied field over a prescribed range.14
θ
on E at low field strengths; this relationship deviates from linearity
when the temperature approaches the transition point TAC from the
orthogonal SmA* phase to the tilted SmC* phase.4, 5
In conventional SmA* materials, the ECE causes a contraction
of the smectic layers that scales with the cosine of
θ and results in a
buckling of the layers into a horizontal chevron structure,6 which is
manifested optically by the appearance of periodic stripe patterns
that severely degrade the optical contrast.7 To solve this problem, a
number of groups have investigated the ECE of liquid crystals with
‘de Vries-like’ properties,8 which undergo a SmA*-SmC* transition
with minimal layer contraction.9-14 This behavior was explained by
de Vries using a ‘diffuse cone’ model in which mesogens in the
SmA* phase have a tilted orientation and a degenerate azimuthal
Three SmA* materials known to have large electroclinic suscep-
tibilities may be considered as bona fide ‘de Vries-like’, i.e., materi-
als that undergo a SmA*-SmC* phase transition with an increase in
birefringence and a maximum layer contraction of < 1%. These are
the siloxane-terminated mesogen TSiKN65 and its carbosilane-ter-
minated analogue W599,10, 14 and the 2-phenylpyrimidine mesogen
8422[2F3] from 3M with a chiral perfluoroether chain.12 The elect-
roclinic susceptibilities of TSiKN65 and W599 are remarkably high:
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Scheme 1. Reagents and conditions: (a) (–)-diethyl D-tartrate, Ti(OiPr)4, t-BuOOH, DCM;22 (b) p-TsCl, pyridine, THF; (c) 70% HF
Et3N 4, NaH, THF, (ii) 6.
3HF, Et3N, DCM;23 (e) SmI2, THF;24 (f) 1-bromo-12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecane, Cs2CO3, acetone; (g) (i)
•
pyridine, DCM; (d) XtalFluor-E,
•
tilt angles
θ of 31° and 25°, respectively, are induced by an elec-
crystal 53 (10) SmC* 62 SmA* 77 (7.4) isotropic
tric field of 5 V ꢀm-1 at T–TAC = +1 K. This ECE is accompanied
by an increase in birefringence (ꢁꢁn = 0.025 for TSiKN65)21
that is consistent with ‘de Vries-like’ behavior. Despite the high
electroclinic susceptibilities of these compounds, there are struc-
tural limitations to their applicability in electro-optical devices.
The siloxane end-group in TSiKN65 is hydrolytically and elec-
trochemically labile,25 and the lateral nitro group in both com-
pounds is associated with higher rotational viscosities and higher
levels of ionic impurities. The electroclinic susceptibility of
8422[2F3] is somewhat lower than those of TSiKN65 and W599
(vide infra), but it is chemically inert and the only ‘de Vries-like’
SmA* material reported thus far that may be suitable for electro-
optical devices based on the electroclinic effect.
The second-order SmA*-SmC* transition was observed by POM
as the appearance of a Schlieren texture in the homeotropic do-
main of the SmA* phase (see Fig. S1 in ESI). The fan texture of
the SmA* phase turned to a characteristic broken fan texture with
a pronounced interference color change that is consistent with an
increase in birefringence, as shown in Fig. 1.8 The SmC* phase
persisted on cooling down to 47 °C.
We recently developed a molecular design of ‘de Vries-like’
liquid crystals that combines a nanosegregating carbosilane end-
group as SmC-promoting element with either a chloro-terminated
alkyl chain or a 5-phenylpyrimidine core as SmA-promoting
26
element.20,
For example, the 5-phenylpyrimidine mesogen
QL16-6 undergoes a SmA-SmC transition with a maximum layer
contraction of only 0.5% and has a reduction factor R of 0.23 at
the point of maximum layer contraction ( T–TAC = –6 K),27 which
makes it one of the best ‘de Vries-like’ liquid crystals reported
heretofore. The profile of relative layer spacing d/dAC vs. T–TAC
exhibited by these materials typically shows a pronounced nega-
tive thermal expansion in the SmA phase that persists on cooling
into the SmC phase and counteracts the layer contraction caused
by tilting to such an extent that the layer spacing at the SmA-
SmC transition is restored. In the case of QL16-6, we recently
showed that ‘de Vries-like’ behavior is due to the combined ef-
fect of an increase in orientational order and a decrease in bilayer
interdigitation with decreasing temperature.20 In this commu-
nication, we report a chiral variant of QL16-6 that includes a
(S,S)-2,3-difluorohexyloxy chain, which is known to induce high
spontaneous polarizations in FLC mixtures without increasing
rotational viscosity.28 This new mesogen (QL32-6) forms SmA*
and SmC* phases and is comparable to 8422[2F3] in terms of
electroclinic susceptibility and chemical inertness, but is unprece-
dented in terms of ‘de Vries-like’ properties.
The chiral mesogen QL32-6 was prepared by a modification
of the synthesis reported for QL16-6, as shown in Scheme 1.20
The chiral (S,S)-2,3-difluorohexan-1-ol (4) was derived from the
chiral epoxide 2, which was obtained from E-2-hexen-1-ol via a
Sharpless asymmetric epoxidation reaction in 95% ee.22 The
chiral alcohol 4 and the carbosilane-terminated 2-chloro-5-phe-
nylpyrimidine precursor 6 were then combined via a nucleophilic
aromatic substitution reaction to give QL32-6 (see ESI for all
synthetic details). Characterization by polarized optical micros-
copy (POM) and differential scanning calorimetry (DSC) showed
that QL32-6 forms chiral SmA* and SmC* phases at the fol-
lowing temperatures (measured on heating in °C; enthalpies of
transition in kJ mol-1 are in parentheses):
Fig 1. Polarized photomicrographs of QL32-6 at 63
(left) and at 61 C in the SmC* phase (right).
°C in the SmA* phase
°
Measurements of the layer spacing d as a function of temper-
ature were carried out by small angle X-ray scattering (SAXS).
The profile of relative layer spacing d/dAC vs. T–TAC for QL32-6
is compared to that of QL16-6 in Fig. 2, and shows the same
degree of negative thermal expansion in the SmA* phase, but a
less pronounced layer contraction in the SmC* phase, to the ex-
tent that the layer spacing at TAC is restored at T–TAC = –7 K; the
maximum layer contraction of QL32-6 upon SmA*-SmC* phase
transition is only 0.2%. The two compounds have comparable
tilt angles (θ = 25° vs. 27° at T–TAC = –10 K, see Fig. S2 in ESI),
which suggests that QL32-6 is more ‘de Vries-like’ than QL16-
6. Indeed, at the point of maximum layer contraction (T–TAC = –
3 K), QL32-6 has a R value of only 0.17. By comparison, the 3M
material 8422[2F3] has a maximum layer contraction of 0.8%
and a R value of 0.36 at the point of maximum layer contraction
(T–TAC = –10 K).12
The compound QL32-6 was aligned in an ITO glass cell with
a rubbed nylon alignment substrate and a cell gap of 3 ꢀm. The
optical birefringence ꢁn and electroclinic tilt
θ were measured as
a function of temperature with a rotating analyzer setup described
by Langhoff and Giesselmann.29 The sample was switched by
applying alternately a positive and negative electric dc field and
the corresponding optical signals were evaluated in terms of
phase shift and amplitude to determine the tilt angle and the
birefringence simultaneously (see ESI for details).
The birefringence was first measured at E = 0 over a relati-
vely narrow temperature range about the SmA*-SmC* phase
transition in order to precisely set TAC as reference point for all
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the experiments. As shown in Fig. 3, the birefringence is ap-
proximately invariant in the SmA* phase and increases sharply at
TAC, which is consistent with the interference color change ob-
S4 in ESI). Further heating of the sample up to T–TAC = +10 K
resulted in the gradual appearance of a stripe pattern at E = 0,
which may be ascribed to a contraction of the smectic layers, viz.
Fig. 2. Application of a high field of 15 V ꢀm-1 caused the stripe
pattern to disappear (see Fig. S5 in ESI).30
served by POM. Measurements of the electroclinic tilt
θ in the
SmA* phase were performed at five different temperatures above
TAC, from T–TAC = +0.2 to +5 K, with an electric field E ranging
from 0.1 to 20 V ꢀm-1. As shown in Fig. 4, the
θ
(E) plot is linear
at T–TAC = +5 K, but increasingly deviates from linearity as T
approaches TAC; at T–TAC = +0.2 K, reaches values close to
θ
21°. The electroclinic susceptibility of QL32-6 is about half that
of TSiKN65 but it is comparable to that of 8422[2F3]: both
compounds give a tilt of 15° at 3 V ꢀm-1 and T–TAC = +0.2 K.12
Fig 4. Electroclinic tilt angle
formed by QL32-6 at T–TAC = +0.2 K (
K ( ).
θ
vs. electric field E measured in the SmA* phase
ꢀ), +0.8 K (ꢁ), +2 K (ꢁ), +3 K () and +5
ꢀ
Fig 2. Relative layer spacing d/dAC vs. reduced temperature T–TAC for com-
pounds QL32-6 (ꢀ) and QL16-6 (, from ref. 20) measured on cooling from
the isotropic phase.
Fig 5. Birefringence
phase formed by QL32-6 at T–TAC = +0.2 K (
and +5 K ( ).
ꢁ
n vs. electroclinic tilt angle
θ
), +0.8 K (
measured in the SmA*
), +2 K ( ), +3 K ()
ꢀ
ꢁ
ꢁ
ꢀ
Fig 3. Birefringence ꢁn vs. reduced temperature T–TAC for QL32-6 measured
at zero-field.
The electroclinic tilt observed in the SmA* phase of QL32-6
is accompanied by an increase in birefringence (see Fig. S3 in
ESI). As shown in Fig. 5, the birefringence increases non-line-
arly with the electroclinic tilt angle
θ and is invariant of tem-
perature, so that the ꢁn( ) data sets recorded at five different
θ
temperatures fall approximately on the same curve; this behavior,
as well as the low absolute values of ꢁn, were observed for the
other three ‘de Vries-like’ materials described herein.12, 14, 21 The
unprecedented ‘de Vries-like’ character of QL32-6 is reflected
by the optical quality of the ECE when compared to 8422[2F3].
Optical stripe patterns were generated under the same conditions
at T–TAC = +0.2 K by applying a 105 Hz square wave ac field
across 3 ꢀm films at a voltage giving rise to the most intense pat-
terns (7.3 V ꢀm-1 for QL32-6 and 3.7 V ꢀm-1 for 8422[2F3]).30
As shown in Fig. 6, the optical stripe pattern generated by the
ECE of QL32-6 is very subtle, and even less pronounced than the
pattern generated by the ECE of 8422[2F3]. Similar results were
obtained on heating the sample to T–TAC = +1 and +2 K (see Fig.
Fig 6. Polarized photomicrographs of QL32-6 (top) and 8422[2F3] (bottom) in
m ITO glass cells with a rubbed nylon alignment substrate at T–TAC = +0.2
K at zero-field (left) and with an applied 105 Hz square wave ac field of 7.3 V
m-1 m-1
QL32-6) and 3.7 V 8422[2F3]).
3
ꢀ
ꢀ
(
ꢀ
(
In conclusion, we have shown that a chiral variant of the ‘de
Vries-like’ carbosilane-terminated mesogen QL16-6 exhibits an
electroclinic susceptibility that is comparable to that of the 3M
material 8422[2F3]. Measurements of smectic layer spacing as a
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C5CC05212B
function of temperature revealed that it undergoes a SmA*-SmC*
phase transition with a maximum layer contraction of 0.2 %,
which corresponds to an R value of only 0.17. This is reflected
by an ECE with improved optical quality over that of 8422[2F3],
showing no appreciable optical stripe patterns over prolonged
electroclinic switching. Given its chemical inertness and lack of
a polar nitro group, QL32-6 is currently the best ECE material
suitable for device applications and represents a promising new
lead in the development of materials for fast analog electro-opti-
cal applications. Ongoing efforts are focused on increasing the
electroclinic susceptibility of QL32-6 via structural modifica-
tions and mixture formulation that can broaden the temperature
range of the SmA* phase,31 and will be reported in due course.
We thank the Natural Sciences and Engineering Research
Council of Canada and the Deutsche Forchungsgemeinschaft
(NSF/DFG Materials World Network program DFG Gi 243/6)
for support of this work. We also thank Prof. Andy Evans for the
use of his chiral phase HPLC instrument.
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a
Chemistry Department, Queen’s University, Kingston, Ontario,
Canada.
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b
Institute of Physical Chemistry, University of Stuttgart,
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c LC Vision, LLC, 4150 Darley Avenue, Suite 10, Boulder, CO, USA.
†
Electronic Supplementary Information (ESI) available: Synthetic
procedures for compound QL32-6, POM textures, tilt angle and
birefringence measurements. See DOI: 10.1039/b000000x/
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