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Our research program focuses on the study of structure-property
relationships in liquid crystal phases using a multidisciplinary
approach that bridges the diverse fields of organic synthesis,
physical organic chemistry and condensed matter physics.
Our group is one of only a few chemistry groups in the world
with the expertise necessary to synthesize new liquid crystal
materials, characterize their physical properties, and evaluate
their potential as active components of optoelectronic and
photonic devices. Liquid crystals are one- and two-dimensionally
ordered fluids formed by molecules with anisometric shapes such
as rod-like (calamitic) or disk-like (discotic). The
materials we are primarily interested in form nematic (N)
smectic A (SmA) and smectic C (phases) as a function of
temperature (see Fig. 1).
Fig. 1. Liquid crystal phase
sequence for calamitic molecules, and the textures observed for
each mesophase by polarized optical microscopy.
The overarching theme of our research program is chirality,
and how it propagates in liquid crystal phases via
intermolecular interactions. The induction of a chiral smectic
C* (SmC*) liquid crystal phase gives rise to a ferroelectric
polarization (PS) that can be coupled to an
electric field to produce fast switching, bistable ON-OFF light
shutters for display applications (Fig. 2). High-resolution
displays based on ferroelectric liquid crystal (FLC) films are
emerging in commercial applications such as color viewfinders
and high-definition projection TV. The magnitude of PS
depends on the molecular structure and mole fraction of the
chiral constituent of the FLC mixture, and a detailed
understanding of this structure-property relationship is crucial
to the development of high-performance FLC materials.
Fig. 2. A ferroelectric
liquid crystal light shutter and microdisplay device
(Displaytech Inc, Longmont, CO)
An ongoing challenge in terms of achieving defect-free FLC
films for display devices has been the formulation of FLC
mixtures with a minimum content of chiral component, which
requires the development of chiral materials with high
polarization powers
(dp).
Whereas the conventional wisdom in developing chiral materials
with high
dp
had been to design molecules with ready-made, chiral side-chains
that are coupled to polar functional groups, our group has
focused on the design of molecules with axially chiral polar
cores which can propagate their handedness to an achiral SmC
liquid crystal host via core-core interactions. This led
to the discovery of a new class of chiral dopants with
atropisomeric biphenyl cores, one of which gives the highest
polarization power on record in a phenylpyrimidine SmC host (top
molecules in Fig. 3) [1]. We have shown that the chiral
perturbations exerted by the dopant on the liquid crystal host
environment has a feedback effect that amplifies
dp
,
which was coined the ‘Chirality Transfer Feedback effect’, and
reviewed in invited articles in Accounts of Chemical Research
and Chemical Society Reviews [2,3]. More recent work
has focused on a different class of axially chiral dopants with
a 2,2’-spirobiindan-1,1’-dione core (bottom of Fig. 3), and the
use of 2H NMR spectroscopy to detect short range
chiral perturbations exerted by these dopants on the achiral
smectic C environment [4].
Fig. 3. Examples of axially
chiral dopants for the induction of ferroelectric SmC* liquid
crystal phases.
Another issue
pertaining to the formulation of FLC mixtures is the layer
contraction caused by the tilting of molecules upon transition
from the orthogonal SmA phase to the SmC phase, which results in
a buckling of the smectic layers into a chevron geometry and the
formation of ‘zigzag’ defects that reduce the optical quality of
FLC displays. To solve this problem, several groups have
focused on an unusual class of liquid crystal materials with
layer contractions on the order of 1% or less at the SmA-SmC
transition [5]. These so-called ‘de Vries-like’ materials
exhibit strong lamellar ordering, which is often promoted by
nanosegregating elements such as siloxane end-groups or
fluorinated side-chains. The minimal layer contraction of
these materials may be explained by the diffuse cone model
proposed by de Vries (Fig. 4). According to this model,
molecules in the SmA phase are tilted, but have a random
azimuthal distribution described by a ‘diffuse cone’, which
becomes biased in one direction upon transition to the SmC
phase. This is in contrast to the classic rigid rod model,
in which the layer contraction scales with the cosine of the
tilt angle
q.
Fig. 4. Schematic
representation of the SmA-SmC phase transition according to (a)
a classic rigid-rod model and (b) the ‘de Vries’ diffuse cone
model.
Research in my group has recently focused on developing a
rational design strategy for liquid crystal materials with de
Vries-like properties [6]. In collaboration with Frank
Giesselmann at the University of Stuttgart, we have shown that
combining a structural element that promotes the formation of a
SmC phase (trisiloxane-terminated side-chain) with one that
promotes the formation of a SmA phase (either a
chloro-terminated side-chain in 1 or a 5-phenylpyrimidine
core in 2) results in layer contractions on the order of
1% at the SmA-SmC transition, which is significantly smaller
than the layer contraction of 7% observed with non-siloxane
parent compounds. Additional evidence for de Vries-like
properties is a pronounced change in interference color observed
by polarized microscopy upon cooling from the SmA to the SmC
phase (Fig. 5). The color change corresponds to an increase in
birefringence of ~20%, which is consistent with the increase in
orientational order associated with a de Vries-like transition.
Fig. 5. Polarized
photomicrographs (500´)
of compound 2 between untreated glass slide and cover
slip at 75.0 °C
(SmA, left), 74.6 °C
(SmC, center) and 74.0 °C
(SmC, right)
Another facet
of our research program is the development of photochromic
chiral dopants that enable the photomodulation of PS
in FLC films without destabilization of the SmC* liquid crystal
phase. Our most notable achievement in this area has been
the design of an ‘ambidextrous’ thioindigo dopant which features
two competing chiral side-chains that induce polarizations of
opposite signs [7, 8]. Upon trans-cis photoisomerization,
the sign of induced polarization switches from positive to
negative, which enables the ON-OFF switching of FLC light
shutters using visible light (Fig. 6). Such technology
enables the reversible writing of images, diffraction gratings
and waveguides on FLC films on a microsecond timescale, which
holds significant potential in the areas of photonics and
fiber-optic telecommunication.
Fig. 6. Photoinduced
polarization inversion in a FLC film via trans-cis
photoisomerization of an ambidextrous chiral thioindigo dopant.
In collaboration with Prof. Cathy Crudden, we are applying the
principle of chirality transfer previously shown to amplify PS
in ferroelectric liquid crystals to the development of a new
class of chiral periodic mesoporous organosilicates (PMO, see
Fig. 7). These materials hold significant potential as
recoverable heterogeneous catalysts in the industrial production
of chiral drugs, and as stationary phases in chiral
chromatography. The connection with liquid crystals is
that PMO materials are obtained via acid- or base-catalyzed
condensation of building blocks such as
4,4’-bis-(triethoxysilyl)biphenyl (3) in the presence of
a templating surfactant that forms a lyotropic liquid
crystalline phase in solution. We have recently shown using
solid-state circular dichroism spectroscopy that the templated
co-condensation of 3 with the atropisomeric biphenyl 4
in a 85:15 ratio produces a chiral PMO in which the chirality of
4 is propagated to the bulk material via core-core
interactions (Fig. 8) [9]. Using ferroelectric liquid crystals
as model systems, we are currently exploring different
combinations of complementary chiral/achiral biaryl building
blocks to maximize chirality transfer in the solid state.
.
Fig. 7. Synthesis of periodic
mesoporous organosilicates via surfactant-templated
condensation.
Fig. 8. Chiral induction on
PMO using an atropisomeric biphenyl dopant.
By virtue of
the interdisciplinary nature of our research program, each
graduate student in the group has the opportunity to develop
expertise in synthetic, computational and physical methods
related to materials characterization. Some of the
specialized physical methods used in our laboratory for
characterization of materials properties include differential
scanning calorimetry, polarized optical microscopy, powder
X-ray diffraction, circular dichroism spectroscopy, and
polarization reversal current methods. In a nutshell, our
research program can provide graduate students with a
broad-based training in materials science that is in high demand
in today’s job market.
References
(1)
D. Vizitiu, C. Lazar,
B.J. Halden and R.P. Lemieux “Ferroelectric Liquid
Crystals Induced by Atropisomeric Dopants: Dependence of the
Polarization Power on the Core Structure of the Smectic C Host,”
J. Am. Chem. Soc., 1999, 121, 8229.
(2)
R.P. Lemieux
“Molecular Recognition in Chiral Smectic Liquid Crystals: The
Effect of Core-Core Interactions and Chirality Transfer on Polar
Order,” Chem. Soc. Rev., 2007, 36,
2033.
(3)
R.P. Lemieux
"Chirality Transfer in Ferroelectric Liquid Crystals," Acc.
Chem. Res.,
2001, 34, 845.
(4)
C.J. Boulton, J.G.
Finden, E. Yuh, J.J. Sutherland, M.D. Wand, G. Wu and R.P.
Lemieux “Ferroelectric Liquid Crystals Induced by Dopants
with a 2,2’-Spirobiindan-1,1’-dione Core,” J. Am. Chem.
Soc.,
2005, 127, 13656.
(5)
J.P.F. Lagerwall and
F. Giesselmann “Current Topics in Smectic Liquid Crystal
Research,” ChemPhysChem. 2006, 7, 20.
(6)
L. Li, C.D. Jones, J.
Magolan and R.P. Lemieux “Siloxane-Terminated
Phenylpyrimidine Liquid Crystal Hosts,” J. Mater. Chem.,
2007,
17, 2313.
(7)
J.Z. Vlahakis, M.D.
Wand and R.P. Lemieux “Photoswitching of Ferroelectric
Liquid Crystals Using Unsymmetrical Chiral Thioindigo Dopants:
Photoinduced Inversion of the Sign of PS,”
Adv. Funct. Mater., 2004, 14, 637; J.Z.
Vlahakis, M.D. Wand and R.P. Lemieux “Photoinduced
Polarization Inversion in a Ferroelectric Liquid Crystal Using
an Ambidextrous Chiral Thioindigo Dopant,” J. Am. Chem. Soc.,
2003, 125, 6862.
(8)
R.P. Lemieux
“Photoswitching of Ferroelectric Liquid Crystals Using
Photochromic Dopants,” Soft Matter, 2005, 1,
348.
(9)
S. MacQuarrie, M.
Thompson, A. Blanc, N. Mosey, R.P. Lemieux and C.M. Crudden
“Chiral Periodic Mesoporous Organosilicates Based on Axially
Chiral Monomers.” J. Am. Chem. Soc., 2008,
130, 14099.
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