Tuning the frustration between SmA- and SmC-promoting elements in liquid crystals with 'de Vries-like' properties

Article abstract of DOI:10.1039/c1cc10344j

Smectic liquid crystals with 'de Vries-like' properties are characterized by a maximum layer contraction of ≤1% upon transition from the non-tilted SmA phase to the tilted SmC phase. We show herein that one can systematically increase the 'de Vries-like'

Full text of DOI:10.1039/c1cc10344j

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COMMUNICATION
Tuning the frustration between SmA- and SmC-promoting elements in
liquid crystals with ‘de Vries-like’ propertiesw
Qingxiang Song,a Dorothee Nonnenmacher,b Frank Giesselmannb and Robert P. Lemieux*a
Received 18th January 2011, Accepted 1st March 2011
DOI: 10.1039/c1cc10344j
Smectic liquid crystals with ‘de Vries-like’ properties are
characterized by a maximum layer contraction of r1%
upon transition from the non-tilted SmA phase to the tilted
SmC phase. We show herein that one can systematically
increase the ‘de Vries-like’ character of a smectic liquid
crystal by tuning the frustration between SmA- and SmC-
promoting elements according to established structure–property
relationships.
To solve this problem, recent studies have focused on a
unique class of chiral and achiral materials with maximum
layer contractions of r1% upon cooling from the SmA to
the SmC phase.8 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 this
model, the SmA–SmC transition is described as an ordering
of the azimuthal distribution that results in zero layer
contraction.9 In fact, the structure of this unusual phase has
yet to be fully elucidated, but recent theoretical studies suggest
that materials combining low orientational order and high
lamellar order are likely to exhibit this behavior.10,11 We
recently reported results of the first rational design strategy
for achiral ‘de Vries-like’ liquid crystals based on a concept
of frustration12 between a structural element promoting the
formation of a SmA phase and one promoting the formation
of a SmC phase.13–15 For example, we have shown that a
mesogen 1 with a 2-phenylpyrimidine core that combines a
trisiloxane-terminated alkoxy side-chain (SmC-promoting)
with a chloro-terminated alkoxy side-chain (SmA-promoting)16
undergoes a SmA–SmC phase transition with a maximum
layer contraction of 1.3%,15 which is significantly less
than that of 7.1% observed with the parent compound
2-PhP8.5 Despite this improvement, the reduction factor
R (eqn (1))17 of compound 1 at 10 K below the SmA–SmC
transition temperature TAC (R = 0.47) is still well above
those reported for the best ‘de Vries-like’ materials
(R = 0.14–0.20).15,18,19
Ferroelectric liquid crystals (FLC) define a class of polar
materials forming a chiral smectic C* (SmC*) phase in which
rod-like molecules are self-organized in diffuse layers described
by a density wave with a period d corresponding to the layer
spacing,1 and tilted at an angle y with respect to the layer
normal. Alignment of a SmC* liquid crystal between glass
slides with rubbed polymer alignment layers gives a surface-
stabilized FLC film with a spontaneous polarization (PS)
oriented perpendicular to the alignment layers.2–4 By coupling
the polarization to an electric field, the FLC film can be
switched between opposite tilt orientations to give an electro-
optical shutter that is currently used in high-resolution reflective
color microdisplays. FLC materials used in display applications
normally consist of a mixture of achiral liquid crystals
(e.g., 2-PhP8)5 and a chiral dopant with an appropriate
polarization power.6 The achiral liquid crystal components
of FLC mixtures normally have a phase sequence with a
nematic phase and a non-tilted 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, a significant problem with surface-stabilized
FLC films is the layer contraction (B7–10%) that occurs upon
cooling from the SmA* to the SmC* phase, which results in
buckling of the smectic layers into a chevron structure, and the
formation of zigzag defects that reduce the optical quality of
the film (vide infra).7
R = d(T)/yopt(T) = cosÀ1[dC(T)/d(TAC)]/yopt(T)
(1)
(2)
b
y p |T À TAC
|
In this communication, we report small angle X-ray
scattering (SAXS) and optical tilt angle (yopt) measurements
on isometric analogues of compound 1 which suggest that
the frustration between the trisiloxane-terminated 2-phenyl-
pyrimidine element and the chloro-terminated alkoxy element
in 1 is unbalanced towards the latter, and that one can
‘tune’ the frustration—and further reduce R—by increasing
the SmC-promoting character of the former. This is
achieved by replacing the 2-phenylpyrimidine core in 1 with
one of two cores known to be stronger SmC-promoters, either
6-phenylpyridazine (2) or 2-phenyl-1,3,4-thiadiazole (3).20
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
w Electronic supplementary information (ESI) available: Synthetic
procedures for compounds 2 and 3, DSC and POM characterization.
See DOI: 10.1039/c1cc10344j
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4781–4783 4781

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Compound 2 was obtained by a Pd-catalyzed cross-coupling
of 3-chloro-6-iodopyridazine21 with the MOM-protected
4-hydroxyphenylboronic acid, followed by a nucleophilic
aromatic substitution reaction with the sodium salt of
7-chloroheptanol, deprotection, and alkylation via a Mitsunobu
reaction with 11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)undecanol.
Compound 3 was obtained by acylation of 4-methoxybenzo-
hydrazide with 9-chlorononanoyl chloride,22 followed by
reaction with Lawesson’s reagent to give 2-(8-chlorooctyl)-5-
(4-methoxyphenyl)-1,3,4-thiadiazole, which was demethylated
with BBr3 and alkylated via a Mitsunobu reaction with
11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)undecanol (see ESIw
for details). The mesophases formed by compounds 2 and 3
were characterized by polarized optical microscopy (POM)
and differential scanning calorimetry (DSC). Both compounds
form SmA and SmC phases (Table 1), as shown by the
characteristic fan and homeotropic textures of the SmA phase
that turn into broken fan and Schlieren textures upon transition
to the SmC phase (ESIw). Unlike 1, compounds 2 and 3
undergo SmA–SmC phase transitions with measurable enthalpies
of transition DHAC that are consistent with weakly first-order
transitions (vide infra).
Fig. 1 Relative smectic layer spacing d/dAC versus reduced temperature
T À TAC for compounds 1 (K), 2 (J) and 3 (n). Data for 1 are from
ref. 15.
Fig. 2 Polarized photomicrographs (400Â) of a 5 mm film of 3 doped
with a chiral additive (1 mol%)23 in an ITO glass cell with rubbed Nylon
alignment layers. The film is cooled at 10 K minÀ1 while applying a 10 V
dc field across the cell: (a) T À TAC = +1 K, (b) T À TAC = À15 K,
(c) T À TAC = À25 K and (d) T À TAC = À32 K.
Accurate measurements of the smectic layer spacing d as a
function of temperature were carried out by SAXS. As shown
in Fig. 1, the normalized d/dAC(T) profiles of 2 and 3 are very
similar to that of 1, showing a negative thermal expansion in
the SmA phase that persists in the SmC phase and counteracts
the layer contraction caused by tilting in the SmC phase. Other
‘de Vries-like’ materials have been reported to show a similar
negative thermal expansion in their d(T) profiles.8,13–15
Maximum layer contractions of 1.1% and 1.3% are observed
at ca. 10 K below TAC for 2 and 3, respectively, and the layer
spacing at the SmA–SmC transition dAC is restored upon
further cooling to ca. 30 K below TAC. This behavior is also
manifested by the appearance and disappearance of zigzag
defects on cooling either material from the isotropic liquid
phase to T À TAC = À32 K in ITO glass cells with rubbed
Nylon alignment layers and a spacing of 5 mm, as shown in
Fig. 2.
Although there is no appreciable change in the normalized
d/dAC(T) profiles relative to that of 1, the introduction of
stronger SmC-promoting cores in 2 and 3 has the effect of
increasing the optical tilt angle yopt, as shown in Fig. 3, which
results in significantly lower reduction factors R (Table 1).
Fitting of the yopt(T) profiles to the power law shown in
eqn (2), where b is the order parameter related to the nature
of the SmA–SmC phase transition, suggests that the phase
transition is weakly first-order for 2 (b = 0.18) and 3 (b = 0.19),
which is consistent with the DHAC measurements.24 These
results are also consistent with theoretical models associating
‘de Vries-like’ behavior to smectic phases with a high lamellar
and low orientational order, which are predicted to undergo
a SmA–SmC transition that is either tricritical or weakly first-
order.10,11
Table
transitions (kJ molÀ1, in parentheses),a optical tilt angles (1) and
reduction factors at T À TAC = À10 K for compounds 1–3
1 Phase transition temperatures (1C) and enthalpies of
cpd Cr
SmC
SmA
I
yopt
R
1b
2
ꢀ
ꢀ
ꢀ
26 (37)
44 (14)
30 (25)
ꢀ
ꢀ
ꢀ
84 (o0.1)c
78 (0.16)
72 (0.48)
ꢀ
ꢀ
ꢀ
98 (9)
90 (8)
77 (10)
ꢀ
ꢀ
ꢀ
20
26
29
0.47
0.32
0.32
In summary, the results reported herein strongly suggest
that one can systematically increase the ‘de Vries-like’
character of a smectic liquid crystal by tuning the frustration
between SmA- and SmC-promoting elements according to
3
a
b
c
Measured on heating by DSC. From ref. 15. Measured by
polarized optical microscopy.
c
4782 Chem. Commun., 2011, 47, 4781–4783
This journal is The Royal Society of Chemistry 2011

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11 (a) K. Saunders, D. Hernandez, S. Pearson and J. Toner, Phys.
Rev. Lett., 2007, 98, 197801; (b) K. Saunders, Phys. Rev. E: Stat.,
Nonlinear, Soft Matter Phys., 2008, 77, 061708; (c) K. Saunders,
Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2009, 80,
011703.
12 R. D. Kamien and J. V. Selinger, J. Phys.: Condens. Matter, 2001,
13, R1–R22.
13 L. Li, C. D. Jones, J. Magolan and R. P. Lemieux, J. Mater.
Chem., 2007, 17, 2313–2318.
14 J. C. Roberts, N. Kapernaum, F. Giesselmann and R. P. Lemieux,
J. Am. Chem. Soc., 2008, 130, 13842–13843.
15 J. C. Roberts, N. Kapernaum, Q. Song, D. Nonnenmacher,
K. Ayub, F. Giesselmann and R. P. Lemieux, J. Am. Chem.
Soc., 2010, 132, 364–370.
16 J. W. Goodby, I. M. Saez, S. J. Cowling, V. Gortz, M. Draper,
¨
A. W. Hall, S. Sia, G. Cosquer, S.-E. Lee and E. P. Raynes, Angew.
Chem., Int. Ed., 2008, 47, 2754–2787.
17 The reduction factor R is a measure of ‘de Vries-like’ character
defined as the ratio of the tilt angle d(T) required to give the
Fig. 3 Optical tilt angles yopt versus reduced temperature T À TAC for
compounds 1 (K), 2 (J) and 3 (n). The solid lines represent the fits to
eqn (2) (1: b = 0.22, R2 = 0.947; 2: b = 0.18, R2 = 0.996; 3: b = 0.19,
R2 = 0.994). The data for 1 are from ref. 15.
observed layer spacing contraction dC(T)/dAC at
temperature T below the SmA–SmC transition temperature TAC
a given
,
assuming a conventional rigid-rod model, 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. Y. Takanishi, Y. Ouchi, H. Takezoe,
A. Fukuda, A. Mochizuki and M. Nakatsuka, Jpn. J. Appl. Phys.,
1990, 29, L984–L986.
established structure–property relationships. This new rational
approach represents a powerful means of optimizing FLC
molecular design and mixture formulation to produce chevron-
free FLC displays. A detailed study of structure–property
relationships for homologous series of liquid crystals based
on 2 and 3, including the measurements of orientational and
lamellar order parameters, is under way and will be reported in
due course.
18 M. D. Radcliffe, M. L. Brostrom, K. A. Epstein, A. G. Rappaport,
B. N. Thomas, R. Shao and N. A. Clark, Liq. Cryst., 1999, 26,
789–794.
19 M. S. Spector, P. A. Heiney, J. Naciri, B. T. Weslowski, D. B. Holt
and R. Shashidhar, Phys. Rev. E: Stat. Phys., Plasmas, Fluids,
Relat. Interdiscip. Top., 2000, 61, 1579–1584.
20 The following isometric analogues of 2-PhP8 form only a SmC
phase: 3-octyloxy-6-(4-octyloxyphenyl)pyridazine (Cr 95 SmC 117
I) and 2-nonyl-5-(4-octyloxyphenyl)-1,3,4-thiadiazole (Cr 77 SmC
90 I). (a) T. Hegmann, M. R. Meadows, M. D. Wand and
R. P. Lemieux, J. Mater. Chem., 2004, 14, 185–190;
(b) G. W. Gray, R. M. Scrowston, K. J. Toyne, D. Lacey,
A. Jackson, J. Krause, E. Poetsch, T. Geelhar, G. Weber and
We thank the Natural Sciences and Engineering Research
Council of Canada and the Deutsche Forschungsgemeinschaft
for support of this work.
Notes and references
1 (a) W. L. McMillan, Phys. Rev. A: At., Mol., Opt. Phys., 1971, 4,
1238–1246; (b) W. L. McMillan, Phys. Rev. A: At., Mol., Opt.
Phys., 1972, 6, 936–947.
A. Wachtler, WO Patent, 8,808,019, October 20, 1988.
¨
21 A. J. Goodman, S. P. Stanforth and B. Tarbit, Tetrahedron, 1999,
55, 15067–15070.
22 (a) A. Schulze and A. Giannis, Synthesis, 2006, 257–260;
(b) P. Perlmutter, W. Selajerern and F. Vounatsos, Org. Biomol.
Chem., 2004, 2, 2220–2228.
2 N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 1980, 36, 899–901.
Principles,
3 Ferroelectric
Liquid
Crystals:
Properties
and Applications, ed. J. W. Goodby, R. Blinc, N. A. Clark,
S. T. Lagerwall, M. A. Osipov, S. A. Pikin, T. Sakurai,
K. Yoshino and B. Zeks, Gordon & Breach, Philadelphia, 1991.
4 S. T. Lagerwall, Ferroelectric and Antiferroelectric Liquid Crystals,
Wiley-VCH, Weinheim, 1999.
5 C. S. Hartley, N. Kapernaum, J. C. Roberts, F. Giesselmann and
R. P. Lemieux, J. Mater. Chem., 2006, 16, 2329–2337.
6 (a) R. P. Lemieux, Chem. Soc. Rev., 2007, 36, 2033–2045;
(b) R. P. Lemieux, Acc. Chem. Res., 2001, 34, 845–853.
7 T. P. Rieker, N. A. Clark, G. S. Smith, D. S. Parmar, E. B. Sirota
and C. R. Safinya, Phys. Rev. Lett., 1987, 59, 2658–2661.
8 J. P. F. Lagerwall and F. Giesselmann, ChemPhysChem, 2006, 7,
20–45.
23 J. C. Roberts, N. Kapernaum, F. Giesselmann, M. D. Wand and
R. P. Lemieux, J. Mater. Chem., 2008, 18, 5301–5306.
24 According to the generalized mean-field theory of phase transitions
(Landau theory), a b value of 0.5 is expected in the case of a pure
second-order SmA–SmC transition, whereas a b value of 0.25 is
expected in the case of a transition at the crossover (tricritical)
point from second- to first-order. R. J. Birgeneau, C. W. Garland,
A. R. Kortan, J. D. Litster, M. Meichle, B. M. Ocko,
C. Rosenblatt, L. J. Yu and J. Goodby, Phys. Rev. A: At., Mol.,
Opt. Phys., 1983, 27, 1251–1254. In the present study, there was no
detectable electroclinic effect observed above TAC under the
conditions used for optical tilt angle measurements (1 mol% of
chiral dopant, 2 V mmÀ1 applied field), and we therefore assume
that the applied electric field has no significant effect on b values
derived from the yopt(T) fits. N. Kapernaum, D. M. Walba,
E. Korblova, C. Zhu, C. Jones, Y. Shen, N. A. Clark and
F. Giesselmann, ChemPhysChem, 2009, 10, 890–892.
9 A. de Vries, J. Chem. Phys., 1979, 71, 25–31.
10 (a) M. V. Gorkunov, M. A. Osipov, J. P. F. Lagerwall and
F. Giesselmann, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys.,
2007, 76, 051706; (b) M. V. Gornukov, F. Giesselmann, J. P.
F. Lagerwall, T. J. Sluckin and M. A. Osipov, Phys. Rev. E: Stat.,
Nonlinear, Soft Matter Phys., 2007, 75, 060701.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4781–4783 4783