Siloxane-terminated phenylpyrimidine liquid crystal hosts

Article abstract of DOI:10.1039/b700972k

We report the synthesis and characterization of trisiloxane-terminated liquid crystals with 2-phenylpyrimidine cores that form partially bilayered SmA and SmC phases. Variable temperature measurements of smectic layer spacings by powder X-ray diffraction

Full text of DOI:10.1039/b700972k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Siloxane-terminated phenylpyrimidine liquid crystal hosts{
Li Li,a Christopher D. Jones,b Jakob Magolana and Robert P. Lemieux*a
Received 22nd January 2007, Accepted 5th March 2007
First published as an Advance Article on the web 21st March 2007
DOI: 10.1039/b700972k
We report the synthesis and characterization of trisiloxane-terminated liquid crystals with
2-phenylpyrimidine cores that form partially bilayered SmA and SmC phases. Variable
temperature measurements of smectic layer spacings by powder X-ray diffraction combined with
measurements of optical tilt angles and observations of birefringence changes by polarized optical
microscopy reveal that the trisiloxane end-group causes the SmA–SmC transition of these
compounds to be more ‘de Vries-like’ when compared to a non-siloxane analogue. One
such compound, 2-(4-(11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)undecyloxy)phenyl)-5-(1-
chlorooctyloxy)pyrimidine (3), is characterized by a maximum layer shrinkage of only 1.6%
and may be considered a bona fide de Vries material.
An interesting aspect of amphiphilic molecules such as
Introduction
siloxane-terminated smectogens is the reduced degree of
out-of-layer fluctuations, and the added control of core–core
correlation in the smectic layer that is afforded by siloxane
nanosegregation. Such control is particularly relevant to our
ongoing studies of chirality transfer via core–core interactions
in ferroelectric SmC* liquid crystals induced by chiral dopants
with atropisomeric biphenyl cores.16,17 We have shown that
the chiral perturbations exerted by these atropisomeric
dopants are propagated most effectively in an achiral liquid
crystal host with a complementary 2-phenylpyrimidine core
structure, leading to enhanced ferroelectric polarizations in
the chiral SmC* phase. Hence, the ability to tune core–core
interactions by varying the length of the side-chain linking
siloxane end-groups to the cores of both dopant and
phenylpyrimidine host would enable one to probe the effect
of chirality transfer via core–core interactions more system-
atically. More recently, we have shown that one particular
atropisomeric dopant induces a remarkably high electroclinic
effect in the chiral SmA* phase using the same phenyl-
pyrimidine liquid crystal host.18
Nanosegregation plays a key role in the self-organization of
liquid crystal phases. In the condensed phase, amphiphilic
molecules containing two or more incompatible segments self-
organize due to the segregation of these segments into distinct
domains on the nanometer scale with topographies dictated by
the molecular shape and the amphiphilic interface curvature.1
Conventional calamitic (rod-shaped) molecules consisting of
a rigid aromatic core and two paraffinic side-chains form
lamellar smectic phases due, in part, to the difference in
conformational rigidity between aromatic and paraffinic
segments. Whereas the amphiphilicity of conventional
calamitic mesogens is rather subtle, the formation of lamellar
mesophases can be strongly promoted by the design of
calamitic mesogens with more pronounced amphiphilicity
combining, for example, hydrophilic and lipophilic segments,
fluorocarbon and hydrocarbon segments, or siloxane and
hydrocarbon segments. Amphiphilicity and nanosegregation
can also be used to compatibilize non-mesogenic dopants with
liquid crystal hosts. Coles and co-workers demonstrated this
concept with a combination of non-mesogenic dichroic dye
and smectic liquid crystal host molecules, both terminated with
short siloxane oligomers.2–4 The addition of a siloxane end-
group to a calamitic mesogen has been shown to promote
lamellar organization and the formation of smectic phases
due to the tendency of siloxane and paraffinic groups to
nanosegregate into distinct sublayers,5–14 forming a so-called
‘virtual siloxane backbone’ that enables dopant compatibiliza-
tion in a way similar to what is achieved with side-chain liquid
crystalline copolymers.15
Yet another interesting aspect of some amphiphilic com-
pounds, including both siloxane-terminated mesogens and
mesogens with perfluorinated side-chains, is the formation
of ‘de Vries’ SmA* phases, which are characterized by a
tilted molecular orientation with random azimuthal distribu-
tion.19–25 These materials experience minimal layer shrinkage
upon transition to the ferroelectric SmC* phase, which
minimizes the formation of chevrons and zigzag defects that
severely degrade the quality of electro-optic devices based on
ferroelectric liquid crystal materials.26 Surprisingly, despite the
ubiquitous nature of phenylpyrimidine liquid crystals in
ferroelectric liquid crystal (FLC) mixture formulations,
examples of siloxane-terminated phenylpyrimidine mesogens
have yet to be reported. In this paper, we report the synthesis
and characterization of trisiloxane-terminated liquid crystals
with 2-phenylpyrimidine cores that form partially bilayered
SmA and SmC phases (1–8), and present evidence suggesting
that the SmA phase formed by some of these compounds has
de Vries character.
aChemistry Department, Queen’s University, Kingston, Ontario,
K7L 3N6, Canada
bDepartment of Physics, University of Colorado at Boulder, Boulder,
CO, 80309, USA
{ Electronic supplementary information (ESI) available: synthetic
procedures and spectral characterization data (1H and 13C NMR,
low- and high-resolution MS) for all new compounds, thermal analysis
data and optical tilt angle plots. See DOI: 10.1039/b700972k
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Fig. 1 Phase transition temperatures for compounds 1a–e measured
Results and discussion
by DSC on heating.
Synthesis
broken fan and Schlieren textures by POM, which are
characteristic of a tilted SmC phase (Fig. 1, and Table S1 in
ESI{). DSC analysis revealed, in some cases, a second melting
transition (Cr–Cr9) on heating, which has been attributed
previously to the melting of siloxane domains separate from
that of paraffinic domains.7,10 The phase transition tempera-
tures in series 1a–e show a relatively subtle odd–even effect
with respect to the alkoxy chain length. The powder XRD
profiles of these compounds are consistent with a disordered
smectic phase; they show first and second-order Bragg diffrac-
tion peaks in the small-angle region corresponding to the
smectic layer structure, together with a pair of diffuse bands in
the wide-angle region corresponding to the disordered lateral
All siloxane-terminated phenylpyrimidine liquid crystals
reported herein were prepared by sequential alkylations
of 2-(4-hydroxyphenyl)-5-pyrimidinol (10). As shown in
Scheme 1, the synthesis of compounds 1 and 2 began with
the selective alkylation of 10 under basic conditions to give
the 2-(4-hydroxyphenyl)-5-alkoxypyrimidine precursors 11a–e
in 61–65% yield. The siloxane-terminated alcohols 9a and b
were prepared from 10-undecen-1-ol and 5-hexen-1-ol, respec-
tively, via a platinum-catalyzed hydrosilation reaction using
1,1,1,3,3,5,5-heptamethyltrisiloxane and either dicyclopenta-
dienylplatinum(II) chloride or Karstedt’s catalyst in 80–95%
yield, and then combined with 11a–e via a Mitsunobu reaction
to give 1a–e and 2a–e in yields ranging from 46 to 52%. The
‘inverted’ phenylpyrimidine liquid crystals 5 and 6 were
prepared by simply reversing the alkylation sequence shown
in Scheme 1. The terminal chlorooctyloxy analogues 3 and 4
were prepared by alkylation of 10 via a Mitsunobu reaction
with 8-chloro-1-octanol in 52% yield, followed by alkylation
with 9a and 9b, respectively. The inverted analogues 7 and 8
were prepared by reversing the alkylation sequence.
˚
arrangement of paraffinic chains (4.5 A) and siloxane groups
˚
(6.9 A) within each layer (for a representative example, see
Fig. 2).4,9,10 In the case of compound 1b, the Bragg angle 2h in
the small-angle region corresponds to a layer spacing d of
˚
43.5 A, which is slightly longer than the molecular length of 1b
(see Table 1) and therefore inconsistent with a tilted lamellar
structure. Such a discrepancy between the X-ray data and POM
observations has been previously reported for other siloxane-
terminated liquid crystals, and rationalized by the formation of
a partially bilayered SmC structure.9,10 We attempted to
measure the optical tilt angle of the SmC phase by POM using
a sample of 1b doped with the chiral siloxane Br11-Si3
(1 mol%),10 but were unable to achieve a uniform alignment
of the SmC* mixture in ITO cells (ca. 1 mm spacing) with either
rubbed polyimide or rubbed Elvamide1 alignment layers.
Mesophase characterization
The mesophases formed by compounds 1–8 were characterized
by polarized optical microscopy (POM), differential scanning
calorimetry (DSC) and powder X-ray diffraction (XRD).
Compounds 1a–e form a single mesophase showing both
Scheme 1
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Shortening the alkyl spacer linking the siloxane end-group
to the phenylpyrimidine core (series 2a–e) resulted in the
formation of a very narrow SmA phase along with the broader
SmC phase (Fig. 3). These results are consistent with normally
observed trends showing a stabilization of the SmC phase at
the expense of the SmA phase with increasing chain length.
The SmA phase was distinguished from the SmC phase by
POM observations of coexisting domains with fan and
homeotropic textures (SmA) which turned into broken fan
and Schlieren textures upon transition to the SmC phase.
Accurate measurements of smectic layer spacings as a function
of temperature by powder XRD were carried out for
compound 2b. As shown in Fig. 4, the SmA–SmC phase
transition results in a maximum layer shrinkage of 7.6%, which
is within the range of conventional materials with this phase
sequence.26 The d spacing in the SmA phase of compound 2b is
Fig. 3 Phase transition temperatures for compounds 2a–e measured
by DSC on heating.
˚
longer than the calculated molecular length by 3.9 A (see
Table 1), which is consistent with the assignment of a partially
bilayered structure. Unlike the previous case, we were able to
achieve a uniform alignment of a 1 mol% mixture of Br11-Si3
Fig. 4 Smectic layer spacing d vs. temperature T for compounds 2b
(circles), 3 (filled triangles) and 4 (open triangles).
in 2b in an ITO cell with rubbed Elvamide1 alignment layers,
which enabled the measurement by POM of an optical tilt
angle of 36u at T 2 TC = 210 K.
We sought to modify the molecular structure of these
compounds to broaden the range of the SmA phase because of
the proven potential of siloxane-terminated mesogens to form
de Vries SmA phases.19–23 Based on a recent report by
Cowlings et al. that the inclusion of a terminal halogen atom in
the side-chain of a calamitic mesogen promotes the formation
of a SmA phase,27 we modified the side-chains of both 1b and
2b to give the chloro-terminated mesogens 3 and 4. As shown
in Fig. 5, the inclusion of a terminal chloro substituent resulted
in narrower SmC temperature ranges, and broader SmA
temperature ranges of 9 K in both cases. As observed with
series 1 and 2, layer spacings in the orthogonal and tilted
smectic phases are longer than the corresponding molecular
lengths (Table 1), which is consistent with the assignment of
partially intercalated bilayer structures. The SmA–SmC phase
transition results in a maximum layer shrinkage of 4.2% for
compound 4 and only 1.6% for compound 3, although the dC
spacing of 3 increases with decreasing temperature to such an
Fig. 2 Powder X-ray diffraction profile for compound 1b in the
SmC phase.
Table 1 Smectic layer spacings in the SmA phase (dA) and SmC
phase (dC) at T 2 TC = 210 K, molecular lengths derived from AM1
molecular modeling (L), optical tilt angles (hopt) measured by POM,
and de Vries coefficients (CdV) at T 2 TC = 210 K
b
˚
dA/A
˚
dC/A
˚
L/A
Cpd
h
opt/dega
CdV
1b
2b
3
4
5
—
43.5
36.9
47.7
38.9
35.8
28.3
42.3
35.7
42.8
36.2
42.3
35.7
—
36
24
26
—
—
39.6
48.4
40.4
—
0.64
0.83
0.63
6
—
a
Measured with samples containing
Calculated using eqn (3) and maximum dA value at TC.
1
mol% of Br11-Si3.
b
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in a ‘de Vries’ phase transition from random to ordered
molecular tilt (vide infra).26
To assess whether the decrease in layer shrinkage achieved
by introducing a terminal chloro substituent in the side-chain
of 2b is due to an increase in ‘de Vries’ character, one must
consider the difference in tilt angle in the SmC phases formed
by these materials. The description of any SmA–SmC phase
transition may be found somewhere along a continuum
between two limiting cases, the classic rigid rod model
(Fig. 8a) and the diffuse cone model proposed by de Vries
(Fig. 8b).26,28 According to the first model, in which rigid rod
molecules are uniformly oriented along a director n and
tilted at an angle h with respect to the layer normal z, the
relationship between dC and dA is expressed by eqn (1), and the
layer shrinkage (dA 2 dC)/dA is a function of the tilt angle h
according to eqn (2). In order to attribute any decrease in layer
shrinkage to an increase in ‘de Vries’ character based on the
diffuse cone model, we normalize the layer shrinkage with
respect to h by dividing the observed (dA 2 dC)/dA by 1 2 cosh,
where h is approximated as the optical tilt angle hopt measured
by POM (eqn (3)). Using this expression, the ‘de Vries’
coefficient CdV of any given material can range from 0 (classic
rigid rod model with maximum layer shrinkage) to 1 (pure ‘de
Vries’ SmA–SmC transition with no layer shrinkage).
Fig. 5 Phase transition temperatures for compounds 3–8 measured
by DSC on heating.
extent that dC = dA near the SmC–Cr transition temperature
(Fig. 4). The d(T) profile for compound 3 is very similar to that
reported for one of the perfluorinated 3M materials with a ‘de
Vries’ SmA*–SmC* transition that results in a maximum
layer shrinkage of 0.8%.24 The effect of the terminal chloro
substituents in promoting orthogonal layer structures is
reflected in smaller optical tilt angles of 24 and 26u at
T 2 TC = 210 K in the SmC phases of 3 and 4, respectively.
We also observed significant changes in interference colors
in the fan/broken fan textures of both 3 and 4 upon cooling
from the SmA to the SmC phase (Fig. 6 and 7). These color
changes indicate an increase in birefringence that is expected
dC = dA cosh
(1)
(2)
(3)
(dA 2 dC)/dA = 1 2 cosh
CdV = 1 2 [(dA 2 dC)/(dA(1 2 cosh))]
Fig. 6 Polarized photomicrographs of compound 3 on cooling: in the SmA phase at 91 uC (left), just below the SmA–SmC phase transition at
86 uC (middle), and in the SmC phase at 82 uC (right).
Fig. 7 Polarized photomicrographs of compound 4 taken on cooling: in the SmA phase at 82 uC (left), just below the SmA–SmC phase transition
at 73 uC (middle), and in the SmC phase at 62 uC (right).
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compound 3 may be considered a bona fide ‘de Vries’ material
with a SmA–SmC transition approaching that described by the
diffuse cone model.19–25 However, given the partially bilayered
structure of the smectic phases formed by these compounds,
one cannot rule out a variation in the degree of bilayer
intercalation as a contributor to the observed reductions in
layer shrinkage.
As shown in Fig. 5, inverting the orientation of the
phenylpyrimidine core in compounds 5–8 resulted in a
complete suppression of the SmA phase and a reduction in
layer spacing dC (Table 1), which is remarkable considering the
fact that both side-chains are linked to the core via alkoxy
groups, and that their conformational hypersurfaces should
therefore remain unaffected by the inversion. The reduced
layer spacing may reflect an increase in tilt angle, although we
were unable to measure hopt with samples doped with Br11-Si3
due to extremely poor alignment of the SmC* phases in ITO
cells with either rubbed polyimide or rubbed Elvamide1 align-
ment layers. It may also reflect specific core–core interactions
in the intercalated bilayer structure that are controlled
primarily by p-stacking of the electron-poor pyrimidine rings,
thus causing a shift in offset upon inverting the orientation of
the cores.
Fig. 8 Schematic representations of the SmA–SmC phase transition
according to (a) a classic rigid-rod model and (b) a ‘de Vries’ diffuse
cone model.28
Optical tilt angles hopt were measured as a function of
temperature by POM using 1 mol% mixtures of Br11-Si3 in 2b,
3 and 4, and used to calculate CdV values over the SmC
temperature range of each mesogen.29 As shown in Fig. 9,
the CdV of compound 2b shows little variance with tempera-
ture, with values ranging from 0.63 to 0.67. By comparison,
the non-siloxane analogue PhP1 has a CdV value of 0.40 at
T 2 TC = 210 K,18 which suggests that the trisiloxane end-
group does impart some ‘de Vries’ character to the SmA–SmC
transition of compounds in series 2. The CdV(T 2 TC) profile
of compound 4 is steeper than that of 2b, although CdV
values are fairly similar in the T 2 TC range of 25 to 215 K.
At T 2 TC = 210 K, the two compounds have nearly the same
CdV value despite the large difference in layer shrinkage (6.8%
vs. 3.7%, respectively) because of the difference in tilt angle.
The CdV(T 2 TC) profile of compound 3 is similar to that of 4,
but features significantly higher CdV values. With a maximum
layer shrinkage of 1.6% and a CdV of 0.89 at T 2 TC = 210 K,
Summary
The addition of a trisiloxane end-group on one side-chain of a
dialkoxyphenylpyrimidine mesogen results in the formation of
a broad SmC phase and, in some cases, a narrower SmA
phase. The temperature range of the SmA phase may be
broadened by the addition of a terminal chloro substituent on
the other alkoxy side-chain. Measurements of d spacings and
optical tilt angles as a function of temperature, combined with
observations of changes in birefringence by polarized optical
microscopy, revealed that the trisiloxane end-groups cause the
SmA–SmC transitions of these compounds to be more ‘de
Vries-like’. This is consistent with the notion that nanophase
segregation plays a significant role in the manifestation of
‘de Vries’ behavior, whether it is achieved with amphiphilic
mesogens,19–25 or with side-chain siloxane copolymers.30
One
compound,
2-(4-(11-(1,1,1,3,3,5,5-heptamethyltri-
siloxanyl)undecyloxy)phenyl)-5-(1-chlorooctyloxy)pyrimidine
(3), is characterized by a maximum layer shrinkage of only
1.6% and may be considered a bona fide de Vries material with
the potential of exhibiting interesting ferroelectric and electro-
clinic properties upon doping with chiral materials. The use of
this compound as liquid crystal host in the induction of
ferroelectric and electroclinic properties is currently under
investigation.
Acknowledgements
We are grateful to the Natural Sciences and Engineering
Research Council of Canada, the Canada Foundation for
Fig. 9 De Vries coefficient CdV vs. reduced temperature T 2 TC for
compounds 2b (filled circles), 3 (filled triangles) and 4 (open triangles).
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14 J. Z. Vlahakis, K. E. Maly and R. P. Lemieux, J. Mater. Chem.,
2001, 11, 2459.
15 J.-C. Dubois, P. Le Barny, M. Mauzac and C. Noel, in Handbook
of Liquid Crystals, ed. D. Demus, J. W. Goodby, G. W. Gray,
H.-W. Spiess and V. Vill, Wiley-VCH, Weinheim, 1998, vol. 3,
p. 207.
Innovation and the National Science Foundation (MRSEC
grant DMR-0213918) for support of this work.
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