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Dr. M. ManickamSchool of Chemistry
The University of [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
Design and Properties of Molecular Materials: 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.
Liquid Crystal
Melt
Solidify
Intermediate Phase
Heat
Cool
Heat
Cool
What are Liquid Crystals?
Liquid Crystals (LCs)
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.
What is so special about liquid crystals?
LCD: Multi Disciplinary Area of Research
LCD based Technologicalapplication
Physicist
Electrical&Electronic
Engineering
Organic and Material
Chemists
Preparation of varioustypes of liquid crystallinecompounds and characterisation
Theory, lawand variousPhysicalproperties
Device (manufactures)Technological application
Types of Liquid Crystals
Liquid crystals
Lyotropic Thermotropic
Calamitic Polycatenar Discotic Banana-shaped
Nematic (N)
Smectic (S)
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
Cartoon representation of NPhase.
The molecules are oriented on average, in the same direction referred to as the directed, with on positional ordering with respect to each other
CNR
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 LCsSimilarly 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 arealigned in the same orientation, withno 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 LCsLyotropic 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
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,
Representation of calamitic LCs, where l >>b
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
Lateral Substitution
Size Polarity Position
SmallLarge
PolarNon-polar
Inner-core, outer-edgeOn terminal chainOn linking group
The important issues when considering lateral substitution
N
NCNC5H11
CNC5H11
C5H11 CN
C5H11
CN
CNC5H11
C 71.0 (N 52.0) I
C 24.0 N 35. 0 I
C 130.0 N 239.0 IC 68.0 N 130.0 I
C 84.0 N 126.5 I
17.0
204.03.5
91.595.0
112.5109.0
Effects of Aromatic Core on Transition Temperatures
Effects of Lateral Fluoro substitution
Stability of LC phases
C5H11 OC8H17
C7H15 C5H11
F F
F F
C 89.0 SC 155.5 SA 165.0 N 166.0 I
C 56.0 SC 105.5 SA 131.0 N 136.0 I
High smectic phase stability of bothcompounds are largely due to the effectof the outer-edge fluoro substituent,which fills a void and so enhances the intermolecular attractions and hencethe lamellar packing of the molecules
Position
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
e-
e-
e-
e-
supramolecular order
aromatic singlecrystals
H-phase HHTT Dh-phase H5T polymericphotoconductors
A new class of charge transporting
materials
10-1 10-3 10-6
Greater Supramolecular Order Means Higher Charge Carrier Mobility Greater Supramolecular Order Means Higher Charge Carrier Mobility
Charge Carrier mobility [cm2/Vs]
OROR
OROR
RO
RO
Discotic Liquid Crystals
A General Structural Template
(O)RR*
OO
(O)R
O
O X
O R
O
S R
ORO
R
O
R
Discotic CoreR
A general structural template for discotic liquid crystals
Classification of Discotic Mesophases
hexagonal
rectangular
oblique
ordered
disordered
Dho, Dhd, Drd, Dob.d
Symmetry group
Molecular arrangementwithin Columns
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 dimensionallattice of columns and the order or the disorder of the molecular stacking within the columns
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
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 toform helical macrostructures in the direction perpendicular to the director. The helicity depends on the absolute configuration (enantiomer R or S) of the molecules.
(a) Helical structure of thechiral 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.
Figure – 4 a
Figure -4b
Chiral Smectic Phase
Figure- 5 (a) Helical macrostructure of the chiral smectic C (SC*) phase; (b) chiral moleculerepresented in its layer plane (xy) with its polarisation (P) due to the inherentasymmetry. The layers precess around the normal (z) to the layers, forming a helical
macrostructure.
Figure-5(b)Figure -5(a)
Antiferroelectric LCsAntiferroelectric 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”.
The chevrons represent the banana-shaped molecules
The block arrows represent the polarisation P of the layer
Ferroelectric Antiferroelectric Ferrielectric phase
Figure-8
Lecture 6 Review
Structure of micelles formed by amphiphilic molecules
micelle
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.
Micelles are aggregates of molecules that form such thatthe non-polar chains aggregate together and are effectivelyremoved from the water solvent by the surrounding polar head groups.
Such micelles occur when the solution is relatively diluteand the solution behaves as an isotropic fluid.
Micelles are stable in water provided that the concentrationof surfactant is above the critical micelle concentration.
micelle cross-section
Figure -2
The Liquid Crystalline Structure of Biological Membranes
OO
OO
OPO OO
N CH3
CH3
H3CPolar region
Non-polar region
Phospholipid (11)
Plasma membranes of cells, are constructed of phospholipids.
Phospholipids all have a structure that closely resembles the structure of the soaps and detergentsurfactants 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 non-polar region is hydrophobic (lipophilic).
Phospholipids are composed of glycerol where twoadjacent 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 amino-alcohol 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
Refelected Beam
Ex, Ey Ex, Ey
Refracted BeamsEyEx
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 or Double Refraction
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 refractedlight beams, as illustrated in the figure.
Refelected Beam
Ex, Ey Ex, Ey
Refracted BeamsEyEx
Differential Scanning Calorimetry (DSC)
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 (MI) transition at 95.7oC. The endothermic peaks go up, and exothermic ones go down:y, heat flow (mW); x, temperature (oC)
Figure-a
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 N
RO C N
..
+ -
Resonance structure of analkoxycyanobiphenyl,producing a permanent electric dipole
Figure-3
Interaction with Electric FieldsIf no electric field is present, the permanent electric dipole on the liquid crystal molecules are not aligned, although the molecules themselves are aligned withrespect 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.
+ -
+ -+ -
+ -+ -- +- + - +
- + -+
-+
-+
-+
-+
-+
-+
-+
-+
-+
applying an electric field
Diagram to illustrate the effect of an applied electric field on the alignment of liquid crystal molecules.
Figure-4
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).
N C N
O
= dipole
K N106 128
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.
RR
No permanent dipole RO
RO
N
C N
permanent dipole
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 rangeof 5 0C – 50 0C, which is quite suitable for an LCD.
Questions7. 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?
NC5H11O
A
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.
Questions
12. What is birefringence?
13. With reference to planar alignment and homeotropic alignment layers, discuss alignment layers, and their technological importance in display devices.
