Tuning 'de Vries-like' properties in binary mixtures of liquid crystals with different molecular lengths

Article abstract of DOI:10.1039/c3cc45207g

Smectic liquid crystals with 'de Vries-like' properties are characterized by a maximum layer contraction of ≤1% upon transition from the orthogonal SmA phase to the tilted SmC phase. We show that binary mixtures of 'de Vries-like' and conventional SmC mesogens with a molecular length ratio of 1.34 undergo a SmA-SmC phase transition with a maximum layer contraction ranging from 1.0 to 1.9% depending on the mixture composition.

Full text of DOI:10.1039/c3cc45207g

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COMMUNICATION
Tuning ‘de Vries-like’ properties in binary mixtures of
liquid crystals with different molecular lengths†
Qingxiang Song,a Andreas Bogner,b Frank Giesselmannb and Robert P. Lemieux*a
Cite this: Chem. Commun., 2013,
49, 8202
Received 10th July 2013,
Accepted 1st August 2013
DOI: 10.1039/c3cc45207g
www.rsc.org/chemcomm
Smectic liquid crystals with ‘de Vries-like’ properties are characterized this model, the SmA–SmC transition is described as an ordering
by a maximum layer contraction of r1% upon transition from the of the azimuthal distribution that results in zero layer contrac-
orthogonal SmA phase to the tilted SmC phase. We show that binary tion.6 In fact, the structure of this unusual phase has yet to be fully
mixtures of ‘de Vries-like’ and conventional SmC mesogens with a elucidated, but recent theoretical studies suggest that materials
molecular length ratio of 1.34 undergo a SmA–SmC phase transition combining low orientational order and high lamellar order are
with a maximum layer contraction ranging from 1.0 to 1.9% depending likely to exhibit this behavior.7,8 We have developed a rational
on the mixture composition.
design strategy for achiral ‘de Vries-like’ liquid crystals based on a
concept of frustration9 between a structural element promoting
Ferroelectric liquid crystals (FLC) are ordered fluids forming a the formation of a SmA phase and one promoting the formation
chiral smectic C (SmC*) phase in which rod-like molecules are of a SmC phase.10–12 For example, we recently showed that
organized in diffuse layers with a period d corresponding to the a mesogen with a phenylthiadiazole core (QL6-6) that combines
layer spacing, and are tilted at an angle y with respect to the layer a carbo-silane-terminated alkoxy chain (SmC-promoting) with a
normal. Alignment of a SmC* liquid crystal between glass slides chloro-terminated alkyl chain (SmA-promoting) undergoes a
with a rubbed alignment substrate gives a surface-stabilized FLC SmA–SmC phase transition with a maximum layer contraction
film with a spontaneous polarization oriented perpendicular to of only 0.4%. The compound forms a tilt angle of 211 at 10 K
the glass slides.1–3 By coupling the polarization to an electric field, below the SmA–SmC transition temperature TAC, which gives a
the FLC film can be switched between opposite tilt orientations to reduction factor R of 0.20 (eqn (1))13 that is comparable to those
give an electro-optical shutter that is used in high-resolution calculated for the best ‘de Vries-like’ materials.11,14,15
reflective color microdisplays. The components of FLC mixtures
R = d(T)/yopt(T) = cosÀ1[dC(T)/d(TAC)]/yopt(T)
(1)
normally have a phase sequence with an orthogonal smectic A
(SmA) phase above the tilted SmC phase in order to achieve a high
quality alignment upon cooling the material from the isotropic
liquid phase. However, the layer contraction (B7–10%) that
typically occurs upon cooling from the SmA to the SmC phase
results in buckling of the smectic layers and the formation of
zigzag defects that reduce the optical quality of the SSFLC film.4
To solve this problem, recent studies have focused on a unique
class of chiral and achiral materials with layer contractions of
r1% upon SmA–SmC transition.5 The SmA phase formed by
these so-called ‘de Vries-like’ liquid crystals was originally
described as a lamellar structure in which molecules have a tilted
orientation and a degenerate azimuthal distribution; according to
The layer spacing d of materials like QL6-6 is characterized by a
pronounced negative thermal expansion in the SmA phase that
persists in the SmC phase (Fig. 1) and compensates for the layer
contraction due to molecular tilt to such an extent that d expands
well beyond the layer spacing at the SmA–SmC transition (dAC).16
a Department of Chemistry, Queen’s University, Kingston, Ontario, Canada.
E-mail: lemieux@chem.queensu.ca
b
¨
Institute of Physical Chemistry, Universitat Stuttgart, Pfaffenwaldring 55, D-70569
Stuttgart, Germany. E-mail: f.giesselmann@ipc.uni-stuttgart.de
† Electronic supplementary information (ESI) available: Synthetic procedures for
compounds QL13-6 and QL14-8/8, POM textures, tilt angle measurements and dA
vs. x13-6 plot. See DOI: 10.1039/c3cc45207g
Fig. 1 Relative smectic layer spacing d/dAC vs. reduced temperature T À TAC for
compound QL6-6. From ref. 12.
c
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In order to dampen this excessive layer expansion, we are inves-
tigating the effect of mixing ‘de Vries-like’ liquid crystals with
conventional smectic liquid crystals by small angle X-ray scattering
(SAXS). In this Communication, we report that mixtures of two
SmC mesogens, QL13-6 and QL14-8/8, form SmA and SmC phases
in the mole fraction range 0.1 r x13-6 r 0.6, with a maximum layer
contraction in the range of 1.0–1.9% upon SmA–SmC transition.
More importantly, we provide the first proof-of-principle that the
reduction factor R can be tuned, and excessive negative thermal
expansion of d can be dampened by mixing ‘de Vries-like’ and
conventional smectic liquid crystals.
Fig. 2 Phase diagram of the binary mixture QL13-6–QL14-8/8. The phase
transition temperatures were measured by POM on heating.
SmA layer spacing and the mole fraction, which is consistent
with Diele’s additivity rule.23 Furthermore, the SmA–SmC transi-
tion of these mixtures showed a contraction of the layer spacing
that is consistent with a classic model of hard spherocylinders,24
and thus presented no evidence of ‘de Vries-like’ properties.21
In the present case, measurements of the layer spacing d as a
function of temperature were carried out by SAXS for the binary
mixtures with x13-6 = 0.3, 0.4, 0.5 and 0.6 as well as for the pure
compounds QL13-6 and QL14-8/8. The relative layer spacing
d/d* is plotted as a function of the reduced temperature T À T*,
as shown in Fig. 3; d* corresponds either to dAC for the binary
mixtures (T* = TAC), or to the layer spacing dCI measured just
below the clearing point for the pure compounds (T* = TCI). The
d/d*(T) profile of QL13-6 shows a pronounced negative thermal
expansion that is characteristic of ‘de Vries-like’ smectogens,
whereas the profile of QL14-8/8 shows a layer contraction that is
characteristic of conventional tilted smectogens. The negative
thermal expansion is evident in the d/d*(T) profiles of mixtures
with higher proportions of QL13-6 (x13-6 = 0.5, 0.6) as it appears to
compensate for the increasing molecular tilt, but it is not evident
in mixtures with lower proportions of QL13-6 (x13-6 = 0.3, 0.4),
either in the SmA or SmC phase. The maximum layer contraction
is lowest for the mixture with x13-6 = 0.4 (1.0%) and d/d* appears to
level off with decreasing temperature, which suggests that a
balance between the layer expansion and contraction behaviors
of the two components is achieved around x13-6 = 0.4.
As part of an ongoing effort to tune ‘de Vries-like’ properties in
chloro-terminated carbosilane mesogens,12 we sought to prepare
QL13-6 as an analogue of QL6-6 based on our observation that the
model compound QL14-8/8 forms a broad SmC phase (Table 1).17
Both QL13-6 and QL14-8/8 were derived from 2,5-dibromo-1,3,4-
thiadiazole18 using a modification of the synthesis of 2-alkoxy-5-
(4-cyanophenyl)-1,3-thiazole by Grubb et al., as shown in Scheme 1
for QL14-8/8 (see ESI† for details).19,20 Interestingly, QL13-6
forms only a SmC phase (Table 1), which suggests that the
SmC-promoting effect of the dialkoxy 5-phenyl-1,3,4-thiadiazole
core completely overrides the SmA-promoting effect of the
chloro-terminated chain in this new scaffold. Models of QL13-6
and QL14-8/8 minimized at the B3LYP/6-31G* level (Spartan’10,
Wavefunction Inc.) give molecular lengths of 41.1 Å and 30.6 Å,
respectively, and a molecular length ratio of 1.34.
Mixing the SmC mesogens QL13-6 and QL14-8/8 in varying
proportions gave binary mixtures forming both SmA and SmC
phases in the mole fraction range 0.1 r x13-6 r 0.6, as shown in
Fig. 2. This unusual phenomenon was previously observed by
Kapernaum et al. for binary mixtures of 5-alkoxy-2-(4-alkoxy-
phenyl)pyrimidine mesogens with molecular length ratios ran-
ging from 1.4 to 1.8, and was attributed to a compensation of
free volume by out-of-layer translational fluctuations, which are
less hindered in the SmA phase.21,22 SAXS experiments on these
‘bidisperse’ mixtures showed a linear correlation between the
Table
1
Phase transition temperatures (1C) and enthalpies of transitions
(kJ molÀ1, in parentheses)a,b
Cpd
Cr
SmC
SmA
I
QL6-6c
QL13-6
QL14-8/8
ꢀ
ꢀ
ꢀ
14 (22)
24 (18)
60 (25)
ꢀ
ꢀ
ꢀ
61 (o0.1)
ꢀ
71 (10)
78 (13)
94 (14)
a
Measured on heating by differential scanning calorimetry (DSC).
The SmC phase was identified by polarized optical microscopy (POM)
b
based on the appearance of broken fan and schlieren textures on cooling
c
from the isotropic liquid phase (see Fig. S1 in ESI). From ref. 12.
Fig. 3 Relative smectic layer spacing d/d* vs. reduced temperature T À T* for
the binary mixtures QL13-6–QL14-8/8 with x13-6 = 0.3, 0.4, 0.5 and 0.6 (open
symbols) and for the pure compounds QL13-6 and QL14-8/8 (filled symbols);
d* for the mixtures is the layer spacing dAC measured at the SmA–SmC transition
(T* = TAC) and d* for the pure compounds is the layer spacing dCI measured just
below the clearing point (T* = TCI).
Scheme 1 Reagents and conditions: (a) Br2, NaOAc, AcOH; (b) HBr, NaNO2,
CuBr; (c) CH3(CH2)7OH, NaH, CuO, KI, THF; (d) 1-bromo-4-octyloxybenzene,
n-BuLi, ZnCl2(TMEDA), Pd(PPh3)4, THF.
c
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We thank the Natural Sciences and Engineering Research
Council of Canada (Discovery and CREATE grants) and the
Deutsche Forschungsgemeinschaft (NSF/DFG Materials World
Network program DFG Gi 243/6) for support of this work.
Notes and references
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Fig. 4 Reduction factors R vs. reduced temperature T À TAC for the mixtures
QL13-6–QL14-8/8 with x13-6 = 0.3 (J), 0.4 (K), 0.5 (&) and 0.6 (m).
Optical tilt angles yopt were measured as a function of
temperature in the absence of an electric field by measuring
the angle of rotation between dark states in domains of
opposite tilt orientation in 4 mm films aligned in glass cells
with rubbed polyimide alignment layers (see Fig. S2 in ESI†).
The yopt(T) profiles are virtually identical for the mixtures x13-6
=
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the mixture x13-6 = 0.3 levels off at ca. 201; these values are near
the optimum yopt of 22.51 for SSFLC display applications. The
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and plotted as a function of T À TAC, as shown in Fig. 4. The
13 The reduction factor R is a measure of ‘de Vries-like’ character
R(T) profiles for the four binary mixtures reveal that the
defined as the ratio of the tilt angle d(T) required to give the observed
‘de Vries-like’ character of the binary mixtures is optimized at
x13-6 = 0.4, with a R value of 0.31 at T À TAC = À10 K that remains
more or less constant on further cooling.
layer spacing contraction dC(T)/dAC at a given temperature T below
the SmA–SmC transition temperature TAC, assuming a classic model
of hard spherocylinders, over the tilt angle yopt(T) measured directly
by polarized optical microscopy. According to eqn (1), a SmA–SmC
transition would approach the idealized de Vries model as R - 0.
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of x13-6 gives a linear correlation that is consistent with Diele’s
additivity rule based on a model of hard spherocylinders (see
Fig. S3 in ESI†).23 Extrapolation of the dA(x13-6) plot to x13-6 = 0
and 1 gives d values of 31.8 Å and 49.9 Å, which correspond to
the layer spacings of the virtual SmA phases of pure QL14-8/8
and QL13-6, respectively. The former is near the molecular
length of QL14-8/8 (vide supra), whereas the latter is ca. 1.2 times
the molecular length of QL13-6, which may be consistent with an
intercalated bilayer structure with relatively low orientational
ordering.16 However, it is unclear at this juncture how one
can reconcile this apparent adherence to Diele’s rule with
‘de Vries-like’ behavior.
14 M. D. Radcliffe, M. L. Brostrom, K. A. Epstein, A. G. Rappaport,
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16 Recent SAXS analyses of smectic monodomains have shown that
similar ‘de Vries-like’ organosiloxane liquid crystals exhibit unusually
large molecular tilt fluctuations, and that the layer contraction due to
molecular tilt at the SmA–SmC transition is almost fully compensated
by an increase in orientational order as the tilt fluctuations decrease
with decreasing temperature. D. Nonnenmacher, S. Jagiella, Q. Song,
R. P. Lemieux and F. Giesselmann, ChemPhysChem, DOI: 10.1002/
cphc.201300358.
17 By comparison, the model compound corresponding to QL6-6,
2-nonyl-5-(4-octyloxyphenyl)-1,3,4-thiadiazole, forms a SmC phase
with a narrower temperature range: Cr 77 SmC 90 I. Ref. 12.
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We have shown that binary mixtures of a SmC mesogen with
‘de Vries-like’ character (QL13-6) and a shorter conventional
SmC mesogen (QL14-8/8) form SmA and SmC phases in the
mole fraction range 0.1 r x13-6 r 0.6, which is consistent with
the behavior of bidisperse liquid crystal mixtures previously
19 A. M. Grubb, C. Zhang, A. Jakli, P. Sampson and A. J. Seed, Liq.
Cryst., 2012, 39, 1175–1195.
reported by Kapernaum et al.21 More importantly, we have 20 Z. M. Hudson, X.-Y. Liu and S. Wang, Org. Lett., 2011, 13, 300–303.
shown that the tuning of ‘de Vries-like’ properties can be
achieved by varying x13-6, and provided the first proof-of-principle
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R. P. Lemieux and F. Giesselmann, ChemPhysChem, 2010, 11, 2099–2107.
that the excessive negative thermal expansion characteristic of a 23 According to Diele’s additivity rule, the layer spacing d of a binary
mixture of smectogens A and B is calculated as d = dAxA + dBxB,
where dA, dB are the layer spacings of the pure smectogens. S. Diele,
Ber. Bunsen-Ges. Phys. Chem., 1993, 97, 1326–1336.
pure ‘de Vries-like’ liquid crystal can be dampened by mixing
with a conventional smectic liquid crystal. A detailed study of the
scope of this formulation strategy will be reported in due course. 24 L. Onsager, Ann. N. Y. Acad. Sci., 1949, 51, 627–659.
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8204 Chem. Commun., 2013, 49, 8202--8204
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