Polymerizable bent-core mesogens: Switchable precursors to ordered, polar polymer materials

Article abstract of DOI:10.1002/anie.200250680

Robust noncentrosymmetric polymer networks with a net polarization and bulk pyroelectric properties are achieved by aligning a polymerizable bent-core mesogen in the liquid-crystalline state and cross-linking under a static electric field (see schematic r

Full text of DOI:10.1002/anie.200250680

Angewandte
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Polymerizable Bent-Core Mesogen
Polymerizable Bent-Core Mesogens: Switchable
Precursors to Ordered, Polar Polymer Materials**
Alan C. Sentman and Douglas L. Gin*
Bent-core (that is, banana-shaped) liquid crystals (LCs) have
recently emerged as one of the most significant discoveries in
the area of ferroelectric LCs.[1–3] These polar, chevron-shaped
molecules have the ability to form switchable ferro- and
antiferroelectric liquid-crystalline phases, even though the
molecules are achiral.[1–3] Prior to the discovery of these
materials, ferroelectric and antiferroelectric liquid-crystalline
phases had only been possible with chiral, rodlike mesogens
that adopt tilted layered phases (that is, smectic C (SmC)
phases).[4] There is currently a great deal of interest in bent-
core mesogens because they exhibit complex phase behavior
not seen in other liquid-crystalline molecular architectures.
They also offer certain advantages over traditional rodlike
chiral SmC mesogens as switching materials in ferroelectric
LC displays[5] and as building blocks for ordered, noncen-
trosymmetric polar polymer materials (for example, for
nonlinear optical (NLO), piezoelectric, and pyroelectric
applications).[2,6–8] For example, bent-core LCs can be more
readily designed, synthesized, and modified since enantio-
merically pure or enriched starting materials are not required.
Unfortunately, bent-core mesogens generally form liquid-
crystalline phases at relatively high temperatures (greater
than ca. 1408C).[1,2] This not only limits their usefulness in
device applications, but also makes detailed analysis of their
mesophases more difficult. It has also not yet been possible to
synthesize a polymerizable derivative of a bent-core mesogen
that retains the desired liquid-crystalline properties. Polymer-
izable or cross-linkable bent-core LCs would afford the ability
to stabilize/trap the switchable liquid-crystalline mesophases.
This situation would not only aid in their structural character-
ization, but the resulting polar polymers could also be used for
[*] Prof. D. L. Gin
Department of Chemistry & Biochemistry, and
Department of Chemical Engineering
University of Colorado
Boulder, CO 80309 (USA)
Fax: (+1)303-492-8595
E-mail: gin@spot.colorado.edu
A. C. Sentman
Department of Chemistry
University of California
Berkeley, CA 94720 (USA)
[**] Financial support for this work was provided by the National
Science Foundation (DMR-0111193) and the NSF Ferroelectric
Liquid Crystal Materials Research Center at the University of
Colorado, Boulder (DMR-0213918). The authors also thank Dr. E.
Körblova, Dr. R. Zhao, Mr. A. Klittnick, and Profs. D. M. Walba and
N. A. Clark for their assistance in the analysis of the bent-core liquid-
crystalline phases.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200250680
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NLO and transducer applications if aligned.[2,9] Ikeda and co-
merizable tail units.[10,11] The detailed procedures for the
synthesis, characterization, and property testing of these
compounds are provided in the Supporting Information. The
one drawback with the 1,3-diene tail system is that it tends to
undergo spontaneous thermal polymerization at about 908C
when coupled to a rodlike mesogenic core.[10] However, the
effect of tail unsaturation on thermotropic liquid-crystalline
behavior is a complex function of both the nature of the
mesogenic core and the olefin units in the tails.[12] It was
hoped that the unprecedented combination of diene tails with
a bent core might afford lower phase-transition temperatures
and reduced susceptibility to thermal cross-linking.
The thermotropic liquid-crystalline behavior of com-
pounds 1a–c was studied by polarized light microscopy
(PLM), differential scanning calorimetry (DSC), and
powder X-ray diffraction (XRD; Table 1). Upon heating, 1a
(n = 8) begins to change from a crystalline solid to a liquid-
workers recently made a diacrylate derivative of a bent-core
mesogen for this purpose; however, this compound was found
to be completely nonmesogenic, which suggests that there is a
certain sensitivity or incompatibility of this bent-core plat-
form to polymerizable groups in the tails.[9] Blending with a
large fraction of a conventional bent-core mesogen was
required to obtain a switchable liquid-crystalline mixture that
could be gelled.
Herein, we report the first example of a family of
intrinsically polymerizable and cross-linkable bent-core
mesogens (1, see scheme 1). These bent-core LC monomers
are based on the 1,3-phenylenebis[4-(4-alkoxyphenylimino-
methyl)benzoate] structure,[1] but contain a pair of reactive
1,3-dienoxy tails.[10,11] These tails more closely resemble n-
alkyl and n-alkoxy tails than most other polymerizable tail
systems and thereby allow retention of the desired liquid-
crystalline properties. Some of the bent-core homologues
exhibit a switchable smectic (Sm) phase which appears to be
ferroelectric SmC in nature. They can also be thermally or
photochemically cross-linked with retention of the phase
microstructure. Ordered polymer films with pyroelectric
properties have been obtained by aligning and cross-linking
these mesogens under an electric field. This result demon-
strates the viability of these monomers as precursors to
ordered, polar polymer materials.
Table 1: Thermotropic liquid-crystalline phases and transition temper-
atures of compounds 1a–c.[a]
Compound
n
Transition
T [8C]
1a
1b
8
10
K!LC[b]
K!B4
90–95
107
141
145
143
128
101
119
135
136
125
B4!SmC
SmC!I
I!SmC
SmC!B4
B4!K
Three homologues (1a–c) containing an even number of
methylene units in the tails were initially synthesized to
investigate the properties of a system comprised of a bent-
core and 1,3-diene tails. These diene tail units were originally
developed by our group as a means of making polymerizable
analogues of LCs which were sensitive to or incompatible
with conventional polar or bulky polymerizable moieties in
the tails.[10,11] Compounds 1a–c were synthesized as shown in
Scheme 1, using w-bromoalkan-1,3-dienes as modular poly-
1c
14
G!SmC
SmC!I
I!SmC
SmC!G
[a] K=crystalline phase, LC=unknown liquid-crystalline phase,
G=ordered glassy phase, I=isotropic melt. [b] Cross-links upon enter-
ing the liquid-crystalline phase.
a)
(CH2)nBr
n = 8, 10, 14
+
(CH2)nO
NH2
HO
NH2
(2 equiv)
+
O
O
O
HO
O
H
b)
+
2
O
O
HO
OH
O
O
H
H
c)
O
O
O
O
N
N
1a: n = 8
1b: n = 10
1c: n = 14
(CH2)nO
O(CH2)n
Scheme 1. Synthesis of cross-linkable bent-core mesogens 1a–c containing 1,3-diene tails. a) K2CO3, methyl ethyl ketone, D, b) 4-dimethylamino-
pyridine (DMAP), N,N’-dicyclohexylcarbodiimide (DCC), CH2Cl2, c) molecular sieves, CH2Cl2.
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crystalline phase at about 90–958C. However, the monomer
spontaneously cross-links in a matter of seconds as it begins to
enter the liquid-crystalline phase, which makes phase identi-
fication difficult. In contrast, 1b (n = 10) exhibits well-defined
enantiotropic liquid-crystalline behavior without premature
diene polymerization. Thermally induced polymerization of
1b only occurs if samples are held at temperatures above
908C for several hours. Upon cooling from the isotropic melt
at 58Cminꢀ1, 1b initially enters a stable liquid-crystalline
phase with a fan-type optical texture at 1438C, which persists
down to 1288C (see Supporting Information). XRD analysis
of this mesophase revealed a single diffraction peak at 49.6 Š,
consistent with a lamellar phase with a layer spacing of that
magnitude (Figure 1a). Based on the calculated length of 1b
of 54 Š[13] and assuming there is little or no tail interdigitation,
the observed layer spacing is consistent with a SmC phase
with a tilt angle of 238. Upon cooling below 1288C, 1b forms
an anisotropic phase with a blue fan-type texture that does
not respond to applied pressure, which is consistent with the
so-called solid B4 phase.[1] Below 1018C, 1b forms a solid
crystalline phase which has the same optical texture as the “B4
phase” but lacking the blue color. The longest homologue 1c
(n = 14) is a glassy solid at ambient temperature and also
forms a reversible SmC phase upon heating, but the phase
transitions are weaker and much harder to observe. In
addition, 1c partially thermally polymerizes upon initial
heating. These diene mesogens exhibit very different trends
with respect to their initial melting and final clearing temper-
atures as a function of tail length compared to n-alkoxy and n-
alkenoxy analogues with the same bent core.[1,14] As a
consequence of the susceptibility of 1a and 1c to thermal
polymerization in the liquid-crystalline state, all subsequent
studies were performed with 1b since it has the most stable
and most well-defined liquid-crystalline behavior.
Bent-core mesogens can adopt four general SmC packing
configurations (not including enantiomeric forms), depending
on the nature of the layer interface (namely, synclinic (S) or
anticlinic (A)), and whether the mesogens are parallel or
antiparallel in adjacent layers (that is, ferroelectric (PF) or
antiferroelectric (PA)).[3] Only two of these four possible
geometries (the SmCSPF and SmCAPF phases) are ferroelec-
tric (Figure 2).[3] To help identify the SmC phase observed in
Polar plane view
(Side view)
Tilt plane view
(Front view)
P
–
–
–
–
Tilt plane view
(Front view)
Polar plane view
(Side view)
P
+
–
+
–
OR
+
+
+
SmCAPF
+
(enantiomeric forms)
SmCSPF
Figure 2. Schematic representations of the ferroelectric SmCAPF and
SmCSPF bent-core phases. The symbols + and ꢀ indicate the enantio-
meric layer configurations, and P is the layer polarization.
our bent-core monomers, the electro-optic switching behavior
of 1b at 1358C was examined in a 4-mm thick ferroelectric test
cell using the triangular wave method.[2,3,11] The resulting
current versus applied voltage curve (Figure 3) exhibits a
single peak during voltage ramp-up and ramp-down, with a
maximum polarization of 210 ꢁ 30 nCcmꢀ2 and a rise-time of
190 ms (averaged from five runs). This single-peak response is
characteristic of ferroelectric behavior.[3,12,15] No signs of a
double peak response indicative of antiferroelectric switching
were observed, even when the frequency was varied from 10
to 1000 Hz,[3,12] so there is a high degree of confidence that the
phase is ferroelectric in nature. Unfortunately, the magnitude
of the observed polarization and the switching behavior
steadily decrease over several minutes until a nonswitching
sample with a fixed optical texture is formed. This behavior is
Figure 1. XRD profiles of 1b taken at a) 1338C in the SmC phase
before polymerization; b) 1808C after thermal polymerization; and
c) 1608C after photopolymerization.
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tion of the optical texture of the LC even down to room
temperature. Pyroelectric measurements were performed on
the sample to confirm the presence of a permanent net
polarization.[18] As can be seen in Figure 4, the polymer
exhibits a well-defined electrical response to changes in
temperature, indicative of an aligned material with a perma-
Figure 3. Triangular wave switching curve for 1b in the SmC phase at
1358C. The blue curve is the initial switching response, and the green
curve is the switching response after 15 min.
consistent with accelerated thermal cross-linking under the
oscillating electric field. It also accounts for the unusual
broadness of the switching curve (which complicates phase
identification), since the sample is polymerizing and becom-
ing more viscous as switching proceeds. It should also be
noted that the current response appears to lead the applied
voltage by a small amount. This phenomenon is a character-
istic artifact of an electro-optic switching system with little or
no hysteresis, which has been observed before in surface-
stabilized ferroelectric LC cells.[16] Only small changes in
optical texture were observed upon switching,[17] so it was not
possible to unequivocally determine whether the observed
phase has a SmCSPF or SmCAPF configuration. Nevertheless,
1b does form a switchable Sm phase that can be aligned for
subsequent polymerization.
Compound 1b was subjected to both thermal and radical
photopolymerization experiments to investigate the polymer-
ization behavior of the bent-core dienes and whether they can
be cross-linked with retention of the liquid-crystalline micro-
structure. Samples of 1b were thermally cross-linked by
maintaining the samples in the SmC phase at 1358C for 16 h,
which resulted in completely insoluble samples that no longer
responded to pressure. XRD analysis of the resulting material
shows retention of the phase order even at temperatures well
above the clearing point of the original monomer (Figure 1b),
and even down to room temperature. Only a slight decrease in
the primary XRD peak occurs upon cross-linking. The optical
textures of the materials before and after thermal polymer-
ization were essentially unchanged. Similar results were
observed for samples of 1b that were mixed with 2 wt% of
a radical photoinitiator and irradiated with 365-nm UV light
for 30 min (Figure 1c). The extent of 1,3-diene conversion in
both polymerization cases was estimated to be approximately
50% using FTIR analysis as previously described.[10,11]
To demonstrate that the bent-core monomer can be
aligned and cross-linked to form an ordered, polar non-
centrosymmetric network, a sample of 1b was placed in a 4-
mm thick ITO cell and thermally cross-linked in the SmC
phase for 3 h under a static 10 V electric field. Retention of
order in the polymerized sample was confirmed by observa-
Figure 4. Pyroelectric response curve for 1b thermally cross-linked in
the SmC phase under a 10 V electric field.
nent net polarization.[18] In contrast, a sample of 1b similarly
cross-linked in the absence of an applied field yields no
pyroelectric response. The calculated pyroelectric coefficient
for the material is 4.8 mCmꢀ2 Kꢀ1, which is similar to values
observed for noncentrosymmetric Langmuir–Blodgett films
(10–20 mCmꢀ2 Kꢀ1
)
and
poled
polymers
(30–
40 mCmꢀ2 Kꢀ1).[18,19] The optical texture of 1b aligned and
cross-linked in the field shows that it is still a polydomain
sample (see Supporting Information). Samples with more
uniform alignment under applied field and better bulk
noncentrosymmetric properties might be possible if methods
can be developed for aligning bent-core mesogens at the
surface.
In conclusion, we have designed and synthesized the first
example of a cross-linkable bent-core mesogen. This achiral
mesogen adopts a SmC phase with switching behavior
consistent with a ferroelectric phase. When aligned and
cross-linked in the liquid-crystalline state under a static
electric field, ordered polymer networks with a net polar-
ization are generated that exhibit pyroelectric properties.
Studies are currently underway to identify the exact structure
of the liquid-crystalline phase formed and to examine the
noncentrosymmetric properties of the resulting networks in
more detail.
Received: December 2, 2002 [Z50680]
Keywords: liquid crystals · mesogens · nanostructures ·
.
polymerization · polymers
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