Transcript Chiral liquid crystals

Chiral liquid crystals-art and
science
CHM3T1
Lecture-5
Dr. M. Manickam
School of Chemistry
The University of Birmingham
[email protected]
Outline of Lecture
•
Introduction
•
Structure-Property Relations
•
Synthesis of Chiral liquid crystals Strategies and Methods
•
Final comments
Learning Objectives
After completing this lecture you should have an understanding of, and
be able to demonstrate, the following terms, ideas and methods.
 Understand helical order in chiral nematic or cholesteric phases
 Understand what you mean chiral nematic phase
 Understand what you mean chiral smectic phase
 Importants fo chiral nematic and smectic phases in devices systems
 General structure of chiral materials
 Physical properties of chiral molceules.
 Understand ferroelectric, antiferroelectric and ferrielecric structures
 Understand achiral ferroelectric liquid crystals
 Chiral discotic and chiral polymeric mesogens
Nomenclature
Cr or C or K: crystalline phase
N*: chiral nematic phase or cholesteric phase
S*: chirl smectic phase
n: director
Ps: spontaneous polarisation
P: polarisation
Tg : glass-transition temperature
I: isotropic
ND*: chiral nematic discotic phase
B: banana phase (B1 to B7)
TGBA*: twist-grain boundary phase
Introduction
An object or a molecule that is not superimposable on its mirror image is termed
chiral. For example, a glove is chiral but a coffee cup is achiral.
However, the term chiral is usually employed in chemistry to denote that
a molecular structure is asymmetric.
Chirality is generated by there being four different structural moieties attached
to a tetrahedral, sp3 carbon atom (often described as a chiral centre).
Four such different units can be arranged in two ways, generating two compounds
(enantiomers) that are mirror images of each other.
One stereochemical configuration is termed S and the other enantiomer is termed
R.
For example, 2-octanol is chiral and is commonly used in the synthesis of chiral
liquid crystals; one isomer is (S)-2-octanol and the other isomer is (R)-2-octanol.
C6H13
H
H
C6H13
(R)-2-octanol
(S)-2-octanol
H3C
OH
HO
CH3
Enantiomers
Both enantiomers are identical except for the way in which the structural
moieties are arranged in space.
The different arrangement of functional groups in space is responsible for
optical activity,
i.e., an enantiomer will rotate the of plane polarised light;
Its mirror image will produce the same magnitude of rotation but in the
opposite direction; a 1:1 mixture of enantiomers is termed racemic and
will not generate a rotation of plane polarised light.
The chiral liquid crystalline molecules organise into an asymmetric, chiral
structure that takes the form of a helix.
The helical structure is handed; for example, one enantiomer will
generated a left-handed helix and the other enantimer will produce a
right-handed helix.
C6H13
H3C
H
OH
H
HO
C6H13
CH3
Cholesteryl Benzoate
The very first thermotropic liquid crystalline
material (cholesteryl benzoate) discovered in
1888 by the Austrian botanist Reinitzer,
exhibits what is now known as the chiral
nematic (N*) phase.
Cholesteryl benzoate
C 146 N* 178 I
*
O
O
*
* *
* ** *
Cholesterol, has an asymmetric molecular
structure and it is chiral and optically
active.
Cholesteryl benzoate has 8 chiral centres
giving a total of 256 stereisomers;
however, only one is produced in nature
Historically, the chiral nematic phase was
called the cholesteric phase because the
first materials exhibiting this phase were
in cholesterol derivatives
Physical Phenomenon of The Helical
Structures
Attractive physical phenomenon connected to these helical structures
is that when plane polarised light interacts with them, its plane of
polarisation will be rotated in the same direction of the helix.
When the pitch of the helix corresponds to a wavelength in the visible region
of the spectrum (400-800 nm) the chiral mesophase is coloured.
Additionally, the pitch of the helix is temperature-sensitive and, therefore,
colour changes result as the temperature varies.
Furthermore, helical structures can be unwound by the application of an
electric field, which drives the reorientation of the molecular axis along
the field direction.
Helical Order in Chiral Nematic Phases
The chief of a mesophase formed by chiral mesogens with a predominant
concentration of one enantiomer, is the organisation of the molecules into bulk
helical structures. The helical ordering can be defined as left- or right-handed by
the rotation of the helical pitch
Helical Pitch
The pitch is the distance for one full director rotation
If the molecules that form a liquid
crystals phase are chiral (lack of
inversion symmetry), then chiral phases
exist in place of certain non-chiral phases.
In calamitic liquid crystals, the nematic
phase is replaced by the chiral nematic
phase, in which the director rotates in
helical fashion about an axis perpendicular
to the director.
The pitch of a nematic phase is the
distance along the helix over which the
director rotates by 360o
Chiral Nematic Phase
The structure of the chiral nematic phase.
The views represent imaginary slices through the
Structure and do not imply any type of layered
structure
Nematic phase
The least ordered mesophase (the closest to the isotropic liquid state) is
the nematic phase, where the molecules have only an orientational
order. The molecular long axis points on average in one favoured
direction referred to an the director. The classical examples of LC
displaying a nematic mesophase in the cynobiphenyl
Cartoon representation of N phase.
The molecules are oriented on
average, in the same direction
referred to as the director, with on
positional ordering with respect to
each other
Chiral Nematic or Cholesteric Phase
(a) Helical structure of the
chiral nematic phase;
(b) The director lies in the xy
plane, perpendicular to the
direction of the helix (z), and
rotates in the plane that
defines the helical structure.
The simplest chiral mesophase is the chiral nematic where the
local molecular ordering is similar to that of the nematic phase
(only orientation order), and additionally the molecules pack to
form helical macrostructures in the direction perpendicular to the director.
The helicity depends on the absolute configuration (enantiomer R or S) of
the molecules.
Chiral Smectic Phases
Chiral smectics are also found.
There are many different types of smectic liquid crystals.
The form of chirality of all of these chiral smectic mesophases takes the form of a
helical structure
Types of smectic liquid crystal phases (SC*, SI*, and SF*)
Crystal smectic mesophases (J*, G*, and K*, H*) which could generate
form chirality as a direct result of the molecular chirality of the constituent
molecules
In the chiral crystal smectic mesophases the helix is in effect unwound
by the crystal structure and the form chirality is suppressed.
There are also other chiral mesophases of the smectic type
(e.g., TGBA* and SC* anti phases)
Smectic phases
The next level of organisation is classified as smectic
(S), where in addition to the orientational order the
molecules possess positional order, such that the
molecules organise in layered structures. The S
phase has many subclasses, which are illustrated
below.
Cartoon representation of (a) the SA phases, and (b) the SC phase
Chiral Smectic Phase
The chiral smectic C phase is by far the most important phase.
The tilted director rotates layer to layer forming a helical structure.
These systems can be surface stabilised in this case the helix may be
decreased by liquid crystal in cell.
This means the material is effectively trapped between two glass plates.
Once the helix is suppressed and the directors in each layer are forced
to lie in the plane of the glass plates, this creates spontaneous polarisation
within each layer because of the chiral nature of the molecules.
This is the basis of ferroelectric display devices.
These ferroelectric liquid crystal displays and antiferroelectric liquid crystal
displays operate by the application of an electric field which couples with the
spontaneous polarisation and switches the director in the layers.
Chiral Smectic Phase
Helical macrostructure of the chiral smectic C (SC*) phase; (b) chiral molecule
represented in its layer plane (xy) with its polarisation (P) due to the inherent
asymmetry. The layers precess around the normal (z) to the layers, forming a helical
macrostructure.
A General Structure of Chiral Materials for
Ferroelectric Mixture
Y Chiral Chain
Terminal Chain
X *C
Core
R
Z
Common units found within the core
Common terminal chain units
(F)
C6H13O, C8H17O,
C9H19, C5H11OCH2
(F)
N
N
Common chiral centre unit combinations
R=
O
X=
O
O
O
O
C2H5, C6H13, C6H13O
O
O
O
CH2
Y and Z = H, CH3, CN, F, CF3, Cl
O
O
Transition temperatures
C10H21O
CH N
CH
CH COOR*
Cinnamate Ester Schiff’s Bases
Transition Temperatures (0C)
[ PS (nC cm-2)]
R*
CH3
C2H5 CHCH2
(a) C 76.0 (SI* 63.0) SC* 95.0 SA 117.0 I
*
Cl
(b) C 65.0 (J* 63.0) SI*74.5 SC* 81.0 SA 136.0 I
C2H5 CHCH2
*
CH3
Cl
C2H5 CHCH2-CH-CH2
CH3
C2H5 CH
*
*
(c) C 49.0 SC* 80.0 SA 94.0 I
(d) C 82.0 (SI* 61.0) SC*91.0 SA 106.0 I
Compound (a)
photochemically unstable,
relatively high viscosity,
very low Ps,
It not suitable
for use in devices
Ferroelectric Host: Physical Properties
Problem: Chiral materials are often difficult and expensive to synthesise
A low melting point (below room temperature)
A wide SC range with no underlying ordered smectic phases
A cooling phase sequence of I-N-SA-SC
A low viscosity
A tilt angle (θ) of 22.50
A low to moderate optical anisotropy
A negative dielectric anisotropy
A high dielectric biaxiality
A high chemical and photochemical stability
Chiral Dopant: Physical properties
A reasonably high spontanous polarisation (PS)
A long nematic pitch
Good solubility and compatibility beween host and dopant
Ferroelectric LCs
Schematic representation
of the helical macroscopic
superstructure of Sc*
unwound to the ferroelectric
state, when an external
electric is applied.
P represents the polarisation
On reversal of the polarity of
the applied field, the precession
of the molecules around the normal
to the layer (z) allows a very fast
switching driven by the interaction of
P and the electric field
Ferroelectric LCs
The switching time in ferroelectric materials is very fast, relative to the nematic
materials in the twisted nematic displays, because the energy required for the
molecular reorientation is small.
In fact, the polar chiral molecules simply rotate about the normal to the layers
(z) in the so-called cone-like fashion mode, driven by the tendency of P to align
with the external field.
The cone-like fashion mode is represented in the figure where the chiral
molecule is symbolised by a “fish”.
This powerful macroscopic property of net polarisation is combined with the
processability of fluids in ferroelectric LCs, which, therefore, represent a
highly attractive class of materials for technological display application.
Cone-like fashion switching by
precession of the director about the
normal to the layers (z).
The chiral molecule is symbolised
by a “fish” representation, where the “eye”
represents the direction of P
(black forwards, and white backwards)
Ferroelectric LCs
Generally a material is ferroelectric when it possesses spontaneous polarisation
(Ps), which confers to the materials the property of being switched between two
states of polarisation, by reversing the direction of an applied electric field.
Solids such as NaNO2, Li2Ge7O15, and (CH3NH3)5Bi2Br11 can be ferroelectric
as well as chiral LCs, as they allow appropriate symmetry-breaking elements in
order to generate Ps.
S C* phase can be driven towards a ferroelectric state, by applying an external
electric field. Each single polarised layer reorients in such a way to position the
direction of polarisation P with the electric field.
As a consequence, the S C* helix is unwound and all the layers will be oriented in
the same direction, driven by the interaction with the electric field (Figure-6).
The overall result is a polarised phase (ferroelectric), which has a net dipole
alignment along the electric field and can be readily further switched between the
opposite states of polarisation (P1 and P2), by reversing the applied electric field
(Figure-7)
Antiferroelectric LCs
Antiferroelectric liquid crystals are similar to ferroelectric liquid crystals,
although the molecules tilt in an opposite sense in alternating layers.
In consequence, the layer-by-layer polarization points in opposite directions.
These materials are just beginning to find their way into devices, as they are fast,
and devices can be made “bistable”.
Ferroelectric
Antiferroelectric
Ferrielectric phase
The chevrons represent the
banana-shaped molecules
The block arrows represent the
polarisation P of the layer
Ferrielectric Structure
The ferrielectric chiral smectic C (SC* freei) phase also has an alternating
tilted structure expcept that the alternation is not symmetrical and more
‘layers’are tilted in one direction than the other.
According, the ferrielectric phase generates a PS which depends upon the
degree of alternation of tilt direction.
Ferroelectric
Antiferroelectric
Ferrielectric
The chevrons represent the
Banana-shaped Molecules
The block arrows represent the
polarisation P of the layer
Achiral Ferroelectric LCs
The presence of a chiral centre in liquid crystalline molecules is not necessary the
only method to introduce an element of asymmetry in a LC phase.
Symmetry can also be broken by achiral molecules, such as bent molecules
(banana-or bow-shaped) or bowl –shaped molecules, which lead to a net
symmetry-breaking and chiral bulk structure.
Bowl-shaped molecules
Banana-shaped molecules
Both bent and bowl-shaped molecules can pack in a form an isotropic fashion,
generating sequences of sheets or columns with a polar axis.
Thus, there will be sheets or columns either pointing all in the same direction
or in an anti-parallel fashion.
In the first case, the arrangement of the molecules will be parallel and have an
overall noncentrosymmetric structure, hence the phase is ferroelectric.
Achiral Ferroelectric LCs
In the second arrangement, the molecules will result in either a symmetric
antiferroelectric structure,
if the same number of layers are pointing in the two opposite directions, or
a noncentrosymmetric ferrielectric structure, if one direction is preferred
to the other
Ferroelectric phase
Anti-Ferroelectric phase
Ferrielectric phase
The chevrons represent the
Banana-shaped Molecules
The block arrows represent the
polarisation P of the layer
Banana – Shaped LCs
Banana shaped LCs are generally composed of three molecular units:
An angular central core (commonly 1,3-disubstituted benzene ring) to make the
bend and two linear rod like units (often containing Schiff’s base)
Terminal chains such as alkoxy groups
Banana phases are numbered according to their chronological
discovery, from B1 to the most recent B7
In 1996, the first observation of ferroelecitricty B2 exhibited by
achiral banana-shaped mesogens caught the attention
of the whole LCs community, opening worldwide intense research
and discussion on this new and exciting field of LCs, most of all towards
the exploitation of novel types of LCs-based technological devices.
Bend unit
Terminal chain
Linear rod like unit
Banana – Shaped LCs
The main feature of molecules such as is their symmetry C2v in
this case and therefore its polarisation in the direction of the C2 axis.
Subsequently interchangeable ferroelectric states may be induced.
RO
N
O
O
O
O
N
C2V
RO
Banana-shaped LCs have a much
faster switching time, as these types of LCs
can reorientate in an electric field, not by a
90o turn, but, by precessing around an axis
to realign themselves (figure- 10c). This
process does not require as much energy
and can occur much faster.
B2 Phase
The most investigated banana mesophase is the B2 phase shown by the
series of previous compound.
The interesting feature of the B2 phase is that it is a layered phase with a
C2v symmetry and spontaneous polarisation Ps in the direction of the
C2 (two-fold) symmetry axis.
Therefore, a switchable ferroelectric state can be induced.
X-ray, NMR, and electro-optical studies have led to a detailed structural
model of the B2 phase and its ferroelectric switching mode.
Chiral Discotic Phases
Discotic system can be made chiral by
incorporating a chiral unit into one or
more of the periperal units that
surround the discotic core
This compound exhibits solely a chiral
nematic discotic phase (ND*) phase
because the steric effect of the
branched chains at the chiral centre
disrupt the ability of the molecules to
pack in columns
The liquid crystal tendency depends
critically on the type of chiral
peripheral chain
*
O
O
*
O
O
O
O
*
O
O
O
O
O
O
O
O
O
*
O
*
O
O
*
C 192.5 ND* 246.5 I
Chiral Polymeric Mesogens
Liquid crystal polymers are usually made chiral in the same manner as for
low molar mass liquid crystals, i.e., by incorporating a chiral moiety within
the structure (usually as part of the terminal chain because of simplicity).
Chiral liquid crystal polymers are commonly designed and synthesised
to exhibit the chiral smectic C (SC*) phase because of their potential use as
non-linear optical materials or as pyroelectric detectors.
Such ferroelectric polymers have a spontaneous polarisation and can be
switched in the same manner as the analogous low molar mass materials.
However, polymers are extremely viscous and switching times are quite long.
However, ferroelectric switching is much less affected by polymerisation than
nematic switching.
Accordingly, ferroelectric polymers will probably find use in ferroelectric
displays (e.g., storage type displays)
CH
O
g 56 SF* 80 SC*X 148 SC*Y 197 SA 216 I
O
n H2C O
6
O
O
O *
Typical ferroelectric liquid crystals
poly(acrylate with simple chiral
terminal chiral chain
Several chiral phases
Two SC* phases
Final Comments
Liquid crystalline materials are fascinating fluids that are widely recognised
for their use in devices.
Chiral liquid crystalline materials are even more fascinating and have an array
of special properties enabling them to be used in new technological application
(thermochromics and ferroelectric displays).
In some case chiral liquid crystalline materials generate intriguing new liquid
crystalline phase structures, for example, the antiferoelectric phase which has
tremendous potential in a fast-switching display device.
Other phase structure generated by chiral materials remain a curiosity without
applications (e.g., blue phase and twist grain boundary phases).
Chirality in liquid crystals is currently the subject of intense research; however,
the topic is still in its infancy.
As synthetic routes to novel and more pure chiral materials become available,
then a wider range of chiral liquid crystals will be generated.
Such chiral liquid crystals are expected to become technologically important
in the coming years.