Transcript Document 7431210

Design and Properties of Molecular
Materials: Liquid Crystals
Review
Dr. M. Manickam
School of Chemistry
The University of Birmingham
[email protected]
Design and Properties of Molecular Materials:
Liquid Crystals
Third Year Course: CHM3T1
Special Chemical Topics
Weeks 12-22: Totally 11hrs
9 lecture
Review 1hr
Workshop 1hr
Design and Properties of Molecular
Materials: Liquid Crystals
Course synopsis
1.
Thermotropic Liquid Crystals
2.
Lyotropic Liquid Crystals
3.
Contemporary Research in Liquid Crystals
Lecture 1 Review
Anisotropic Liquid
An anisotropic liquid is a liquid, i.e. it has fluidity in much the same
way as solvent such as water or chloroform has.
However, unlike water or chloroform where there is no structural
ordering of the molecules in the liquid, molecules in anisotropic
liquid are on average structural order relative to each other along
their molecular axis.
What are Liquid Crystals?
Solidify
Melt
Liquid
Heat
Cool
Heat
Intermediate
Phase
Cool
Crystal
Liquid Crystals (LCs)
What is so special about liquid crystals?
LCs are orientationally ordered fluids with anisotropic properties
A variety of physical phenomena makes them one of the most interesting subjects
of modern fundamental science.
Their unique properties of optical anisotropy and sensitivity to external electric
fields allow numerous practical application.
Finally, liquid crystals are temperature sensitive since they turn
into solid if it is too cold, and into liquid if it is too hot.
This phenomenon can, for instance, be observed on
laptop screens when it is very hot or very cold.
LCD: Multi Disciplinary Area of Research
Preparation of various
types of liquid
crystalline
compounds and
characterisation
Organic and
Material
Chemists
Physicist
LCD based
Technological
application
Electrical&
Electronic
Engineering
Device (manufactures)
Technological application
Theory, law
and various
Physical
properties
Types of Liquid Crystals
Liquid crystals
Lyotropic
Calamitic
Thermotropic
Polycatenar
Nematic (N)
Smectic (S)
Discotic
Banana-shaped
Nematic Discotic(ND)
Columnar (Col)
Calamitic LCs
Calamitic or rod-like LCs are those mesomorphic compounds that
possess an elongated shape, responsible for the formanisotropy of the
molecular structure, as the result of the molecular length (l) being
significantly greater than the molecular breadth (b), as depicted in the
cartoon representation in the figure.
Cartoon representation of calamitic LCs, where l>>b
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
R
CN
Cartoon representation of N
Phase.
The molecules are oriented on average,
in the same direction referred to as the
directed, with on positional ordering with
respect to each other
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, of which are illustrated .
Cartoon representation of (a) the SA phases, and (b) the SC phase
Smectic hexagonal phases
The hexagonal smectic mesophase in addition to the molecular long axis and
layer organisation adopt within the layer hexagonally organised group of
molecules.
The positional order is greater than that of a smectic A or smectic C phases.
Cartoon representation of smectic hexagonal phase
Discotic LCs
Similarly to the calamitic LCs, discotic LCs possess a general structure
comprising a planar (usually aromatic) central rigid core surrounded by a
flexible periphery, represented mostly by pendant chains (usually four, six, or
eight), as illustrated in the cartoon representation.
As can be seen, the molecular diameter (d) is much greater than the disc
thickness (t), imparting the formanisotropy to the molecular structure.
Cartoon representation of
the general shape of
discotic LCs, where d >>t
Nematic Discotic LCs
Nematic discotic (ND) is the least ordered mesophase, where the molecules have
only orientational order being aligned on average with the director as
illustrated .
There is no positional order.
Cartoon representation of the ND
phase, where the molecule are
aligned in the same orientation, with
no additional positional ordering
Columnar phases
Columnar (Col) phases are more ordered. Here the disc-shaped cores have a
tendency to stack one on the top of another, forming columns.
Arrangement of these columns into different lattice patterns gives rise to a
number of columnar mesophases, namely columnar rectangular (Colr) and
columnar hexagonal (Colh) in the fashion described in Figure.
Cartoon representation of (a) the general structure of Col phases, where the
molecules are aligned in the same orientation and, in addition, form columns,
(b) representation of Colr, and (c) representation of Colh.
Lyotropic LCs
Lyotropic LCs are two-component systems where an amphiphile is dissolved in a
solvent. Thus, lyotropic mesophases are concentration and solvent dependent.
The amphiphilic compounds are characterised by two distinct moieties, a
hydrophilic polar“ head” and a hydrophobic “tail”.
Examples of these kinds of molecules are soaps (Figure-a) and various
phospholipids like those present in cell membranes (Figure-b).
[a]
[b]
Questions
1. What do you understand by the term anisotropic liquid.
2. Discuss with the aid of diagrams the structures of a nematic phase,
a smectic C phase and a smectic hexagonal phase.
Your answer should include reference to the director, the positional order, and
the unit lattice vector.
Lectures 2 and 3 Review
General Structural Template for Calamitic
LCs
Representation of calamitic
LCs, where l >>b
R’ and R’’
: flexible terminal units; alkyl, alkoxy chains, CN, NO2
A, B, C and D : ring systems; phenyl, cyclohexyl, heteroaromatics and hetrocycles
L
: linking units; CH=N, COO, N=N, COS, C=C,
General Structural Template for Nematic
Phase
Representation of calamitic
LCs, where l >>b
 Polar groups
 Not a longer alkyl/alkoxy chains
 Rigid core with conjugation
But not must
General Structural Template for Smectic
Phase
Representation of calamitic
LCs, where l >>b
 Polar groups
 Longer alkyl/alkoxy chains
 Rigid core with conjugation
 Lateral substituents
But not must
Lateral Substituents
The important issues when considering lateral substitution
Lateral Substitution
Size
Small
Large
Polarity
Polar
Non-polar
Position
Inner-core, outer-edge
On terminal chain
On linking group
Effects of Aromatic Core on Transition
Temperatures
N
C5H11
CN
N
C 71.0 (N 52.0) I
17.0
C5H11
91.5
95.0
CN
C 24.0 N 35. 0 I
C5H11
CN
C 84.0 N 126.5 I
204.0
3.5
C5H11
CN
112.5
109.0
C5H11
CN
C 130.0 N 239.0 I
C 68.0 N 130.0 I
Effects of Lateral Fluoro substitution
Stability of LC phases
F
F
OC8H17
C5H11
C 89.0 SC 155.5 SA 165.0 N 166.0 I
F
C7H15
F
C5H11
C 56.0 SC 105.5 SA 131.0 N 136.0 I
Position
High smectic phase stability of both
compounds are largely due to the effect
of the outer-edge fluoro substituent,
which fills a void and so enhances the
intermolecular attractions and hence
the lamellar packing of the molecules
Discotic LCs
Similarly to the calamitic LCs, discotic LCs possess a general structure
comprising a planar (usually aromatic) central rigid core surrounded by a
flexible periphery, represented mostly by pendant chains (usually four, six, or
eight), as illustrated in the cartoon representation in figure below.
As can be seen, the molecular diameter (d) is much greater than the disc
thickness (t), imparting the form anisotropy to the molecular structure.
Cartoon representation of the general shape of discotic LCs, where d >>t
Discotic Liquid Crystals
OR
OR
RO
RO
-
A new class of
charge transporting
materials
e
-
e
-
e
-
e
OR
OR
supramolecular order
aromatic single
crystals
10-1
H-phase HHTT
Dh-phase H5T
Charge Carrier mobility  [cm2/Vs]
10-3
polymeric
photoconductors
10-6
Greater Supramolecular Order Means Higher Charge Carrier Mobility
A General Structural Template
R
OR
A general structural template
for discotic liquid crystals
O
R
S
O
R
O
R
Discotic Core
O
O
O
O
O
X
(O)R
*
R
(O)R
R
Classification of Discotic Mesophases
Two basic types of discotic mesophases have been widely recognised, these are
1. Columnar; 2. Nematic
Several different types of columnar mesophases exhibited by discotic materials;
these arise because of the different symmetry classes of the two dimensional
lattice of columns and the order or the disorder of the molecular stacking within
the columns
Molecular arrangement
within Columns
Symmetry group
hexagonal
ordered
rectangular
disordered
oblique
Dho, Dhd, Drd, Dob.d
Questions
3. How would you modify the structure of a calamatic liquid crystalline material,
which adopts a smectic mesophase, such that it will adopt a nematic mesophase?
4. Compound A displays a smectic liquid crystalline phase, and no nematic phase.
Discuss brieifly the factors which promote the smectic mesophase, over the
nematic mesophase.
C5H11O
OC5H11
Compound A
Identify two modifications to compound A which would promote the nematic phase
over the smectic phase, and explain (a) the rational behind your chemical
modification, and (b) what the effect these modifications have on the
clearing temperature (Tc).
Lecture 5 Review
Chiral Nematic or Cholesteric Phase
Figure – 4 a
Figure -4b
(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 (figure-4) 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 Phase
Figure -5(a)
Figure-5(b)
Figure- 5 (a) 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.
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
Figure-8
The chevrons represent the
banana-shaped molecules
The block arrows represent the
polarisation P of the layer
Lecture 6 Review
Structure of micelles formed by amphiphilic
molecules
Figure -2
Amphiphilic molecules are usually depicted as circles
(polar head group) with an attached chain (non-polar unit)
as shown in figure-2, and often have more than one
non-polar unit.
These amphiphilic materials are either insoluble or
the molecules dissolve to form a miccellar solution.
micelle
Micelles are aggregates of molecules that form such that
the non-polar chains aggregate together and are effectively
removed from the water solvent by the surrounding
polar head groups.
Such micelles occur when the solution is relatively dilute
and the solution behaves as an isotropic fluid.
micelle crosssection
Micelles are stable in water provided that the concentration
of surfactant is above the critical micelle concentration.
The Liquid Crystalline Structure of Biological
Membranes
Plasma membranes of cells, are constructed of
phospholipids.
CH3
H3C N CH3
O O
P
O
O
O
O O
Polar
region
O
Phospholipids all have a structure that closely
resembles the structure of the soaps and detergent
surfactants discussed above in that the constituent
molecules have an amphiphilic nature.
This nature arises from the presence of both polar and
non-polar regions within the same molecule.
Polar region is hydrophilic (lipophobic) and the nonpolar region is hydrophobic (lipophilic).
Non-polar
region
Phospholipid (11)
Phospholipids are composed of glycerol where two
adjacent hydroxyl functions are esterified with large,
long chain fatty acid units.
Remaining terminal hydroxyl function is esterified with
a phosphoric acid unit that has an attached aminoalcohol moiety.
The Liquid Crystalline Structure of Biological
Membranes (Fluid mosaic model)
Compound (11) is a typical example of a phospholipid, where one fatty acid is
partially unsaturated and choline is employed as the nitrogenous phase.
Accordingly, phospholipid materials have two non-polar chains in their structure
and the polar head group is composed of the glycerol ester unit, the phosphate
ester unit, and the amino-alcohol unit.
Figure- 9; The Liquid Crystalline Structure of the cell membrane (fluid mosaic model)
Questions
5. What are the principle differences between a thermotropic liquid crystal and a
lyotrpopic liquid crystal?
6. What is meant by the terms lyotropic liquid crystal and the fluid mosaic model
of the cell membrane?
Lectures 7 and 8 Review
Reflection and Refraction of Light at the
Surface of an Isotropic Materials
Refelected Beam



Refracted Beam
The path of the reflected or refracted light is independent
of the polarization of light
Reflection and Refraction of Light at the Surface of
an Anisotropic Materials
Birefringence or Double Refraction
Refelected Beam
Ex, Ey
Ex, Ey


 
Ex Ey
Refracted Beams
The path of the reflected light is indepenent of the polarization
(Ex or Ey) of light
The path of the refracted light is dependent on the polarization of
light
Birefringence
Birefringence is the term applied to the double refraction of
nonpolarised light as it passes through an anisotropic material.
This phenomenon occurs because the x-polarised and y- polarised
component of the light interact differently with the anisotropic
material, giving rise to two refractive indices, and therefore two refracted
light beams, as illustrated in the figure.
Refelected Beam
Ex, Ey
Ex, Ey


 
Ex Ey
Refracted Beams
Differential Scanning Calorimetry (DSC)
Figure-a
Figure-a: DSC trace showing the typical pattern of a LC exhibiting a crystal to
mesophase (K M) transition at 65.8oC, and a mesophase to isotropic liquid (MI)
transition at 95.7oC. The endothermic peaks go up, and exothermic ones go down:
y, heat flow (mW); x, temperature (oC)
Alignment at Surfaces
Figure -2: Schematic to show a single alignment layer of liquid crystal molecules
a; parallel to a surface
Homotropical alignment
b; perpendicular to a surface
Heterotopical alignment
Permanent Electric Dipole
Many liquid crystals molecules are composed of neutral atoms and not charged.
However, it is possible for the bonding between the atoms of a molecule to be
such that a permanent electric dipole is produced.
The result is that the molecule bears a positive charge at one end and a negative
charge at the other.
One example is a common calamitic liquid crystals template, the
alkoxycyanobiphenyls (Figure-3).
..
RO
C
Resonance structure of an
alkoxycyanobiphenyl,
producing a permanent
electric dipole
N
+
RO
C
N
Figure-3
-
Interaction with Electric Fields
If no electric field is present, the permanent electric dipole on the liquid crystal
molecules are not aligned, although the molecules themselves are aligned with
respect to one another.
When direct current is applied the molecule will orient themselves along the field
(Figure-4).
This property is unique to liquid crystals, in a liquid the fast, disordered motion
of the molecules prevents the same orientation from occurring, and in solids, the
bonding between molecules means they are unable to change their positions.
The principle features of liquid crystals enabling this interaction with electric
fields are their freedom of movement, like isotropic liquids, and their maintenance
of orientation order, like crystalline solids.
Figure-4
+ -+ - - +
+ - + - + - + - + - +
applying an
electric field
- - - - + + + + +
- + +
- - + + +
Diagram to illustrate the
effect of an applied electric
field on the alignment of
liquid crystal molecules.
Operation Principles of Twisted Nematic
Displays
The nematic materials used in these devices are characterised by a positive
dielectric anisotropy, as a consequence of the presence of highly polar
terminal groups, resulting in the molecular dipole being oriented along the
molecular director and the long axis (Figure-8).
permanent dipole
= dipole
No permanent
dipole
C
N
R
R
N
RO
N
RO
C N
O
K
106
128
N
Figure-8: N-(4-ethoxybenzylidene)-4’-aminobenzonitrile is a typical example of
one of the first nematic liquid crystal used in TNDs, with positive dielectric anisotropy.
Twisted Nematic Display
Eutectic Mixtures
Eutectic mixtures are mixtures of liquid crystalline materials, typically 4
to 10 materials, which have been blended in a specified proportions to
achieve a desired mesophases working range.
For example: for a mixture of p-n-pentyl-p’-cyanobiphenyl (5CB) and
p-n-octyloxy-p’-cyanobiphenyl (8OCB).
The compound 5CB has a nematic range of 24 0C – 35 0C while 8OCB has
a nematic range of 67 0C – 89 0C, neither of which is satisfactory for display
purposes.
However, a mixture of roughly 35% 5CB and 65% 8OCB has a nematic range
of 5 0C – 50 0C, which is quite suitable for an LCD.
Questions
7. What are alignment layers?
8. With reference to liquid crystal displays what are eutectic mixtures and why
are they important?
9. Molecular structure A has a positive dielectric anisotropy. What is dielectrtic
anisotropy and what is the molecular basis for it with reference to compound
A? Why is it important that compounds used in the twisted nematic display
have a positive dielectric anisotropy?
C5 H11O
N
A
Questions
10.
What is required in molecular terms for a compound to display good
material properties in the twisted nematic display.
11.
What is meant by the terms induced electric dipole, permanent electric
dipole, dielectric anisotropy, and electric polarisation.
12.
What is birefringence?
13. With reference to planar alignment and homeotropic alignment layers,
discuss alignment layers, and their technological importance in display devices.