Nematic liquid crystals Flashcards
What is anisotropy?
Exhibiting properties with different values when measured along axes in different directions
l/b = measure of molecular anisotropy
What is molecular anisotropy?
term used to describe the shape of a molecule when one axis is significantly longer than another axis e.g. molecules can be rod-like or disk-like
What is crystal state
Molecules are all in fixed positions and unable to rotate
What is the Nematic phase
Molecules are roughly parallel/ pointing in same direction, determined by director, but still free to flow
What is an isotropic liquid
Molecules are free to rotate and tumble like a normal liquid. No intrinsic order.
What is the order parameter, S?
S = 1/2 (3Cos^2 π -1 )
Where π is the angle between long axis of molecule and director
S is temperature dependent
Nematic liquid crystal, typically S = 0.4 - 0.7
How are the molecules arranged if S = 1
The molecules are perfectly ordered, all molecules are aligned with the director
How are the molecules arranged if S = 0
The molecules are disordered, ie isotropic liquid
How can you measure the order parameter?
diamagnetism
NMR
Raman scattering
What is the effect of increasing the length of aliphatic chain R?
Can lead to microphase segregation due to separation of aromatic and aliphatic units
Why do cyanobiphenyls have a strong internal dipole moment
O = +M group
CN = -M group
π electrons are able to flow through aromatic system from the O to the CN group forming a strong internal dipole
How do quadrupolar interactions occur
Antiparallel pairs of molecules form with secondary dipoles between molecules stabilising the interaction
This cancels out the internal dipole
What is a dipole moment?
The product of magnitude of charge and the distance separation between the charges.
Symmetry can reduce dipoles to 0 or dipoles can reinforce each other.
Effect of chain length on behaviour of Alkylcyanobiphenyls
Short chain length = strong π-π interactions between aromatic units so higher mp
longer chain length alkyl interactions dominating raise mp again
Effect of size of π unit on transition temp of nematic liquid crystal
As the length of the π unit decreases the quadrupolar antiparallel overlap of π units decreases in size and the transition temp of nematic liquid crystals increases
Effect of the cage ring unit on nematic liquid crystal transition temp
The introduction of a cage ring unit to the molecule increases the transition temp.
Raises the temp most when next to the aliphatic chain
What is the effect that Heteroatoms have on transition temp?
Heteroatoms make the molecule more linear as is reduces the steric clash of H atoms on the exterior of 2 connected benzene rings which usually causes the molecule to twist
Therefore increases the transition temp as molecules can pack closer together
What effect do the size of heteroatoms have on the transition temp?
Larger heteroatoms e.g. Sulfur decrease linearity of molecule because theyβre much large than the carbon atoms and therefore lower transition temp
If one O and one S atom = highly twisted structure = even lower transition temp
What effect do linking groups between rings have?
If linking group disrupts the 2 blocks of aliphatic and aromatic then it will decrease the transition temp.
However if introduces +M group into aromatic region will increase dipole moment and transition temp
How to add acyl group to benzene ring?
RCOCl and AlCl3
How to reduce acyl group to alkyl
NH2NH2 and KOH
How to reduce NO2 group
H2 and Pd/C
COCH3 => COOH
Br2 and NaOH
Boronic Acid Coupling
Why are boronic acids good for coupling reactions?
1) Easily purified
2) Stable in air and to moisture
3) Can be stored
4) High yields
5) No homocoupling (react w itself)
Add COOH to difluorobenzene
n-BuLi
CO2
HCl
Add I to diflurorbenzene
N-iodosuccinimide
H+
Add B(OH)2 to difluorobenzene
n-BuLi
B(OMe)2
HCl
What are the 2 key physical properties that are important to liquid crystal display applications?
Birefringence
Dielectric anisotropy
What is Birefringence, Ξn ?
A beam of light is split into two rays, which are orthogonally polarised with respect to each other. Birefringent materials essentially exhibit two refractive indexes. In the case of a liquid crystal these refractive indexes will vary with temperature.
How to calculate Birefrigence?
Ξn = ne - no
ne = perpendicular birefrigence
no = parallel birefrigence
What is dielectric anisotropy, Ξπ?
Related to the flow of polarisable electrons through the molecule. This can be referred to as a positive dielectric anisotropy (flow is along the long axis of the molecule for a rod-like molecule) or as a negative dielectric anisotropy (flow is towards the short axis of the molecule)
How to calculate dielectric anisotropy?
Ξπ = πparallel - πperpendicular
What influences the birefringence of a molecule?
More π electrons the more interactions the molecule will have with light therefore increasing the birefringence, Ξn
The addition of a +M group to the π-system means it can donate electrons and increase birefringence
More polarisable the molecule = higher birefringence
What is a disadvantage of high birefringence?
The higher the polarisability/ birefringence the more likely the compound will be coloured - not favourable for liquid crystal displays
What Increases the dielectric anisotropy of a molecule?
Heteroatoms which make molecule more planar so electron flow along parallel axis is higher
Halogens substituted onto benzene ring decrease quadrupolar interaction increasing parallel flow of e-
Equation for order parameter, S?
S = Ξπ/3(πbar-3)
Equation for mean permativity, πbar.
πparallel + 2πperpendicular/3
What decreases the dielectric anisotropy of a molecule?
Halogens or CN substituted onto benzene ring increase perpendicular e- flow so decrease dielectric anisotropy
Antiparallel pairing (quadrupolar interactions) decreases parallel flow of electons and decreases dielectric anisotropy
Replacing -M CN group with +M group = more electron density towards perp dipole
Increasing number of π electrons flowing towards perp dipole
How to calculate degrees of freedom of temp/press phase diagram
F = C - P +2
C = number of components
P = number of phases
For a 2 component system how many degrees of freedom are there at the eutectic point?
1 degree of freedom according to the phase rule but for a liquid crystal device fix pressure at 1 atm so F = 0
What considerations must be made for component selection for liquid crystal devices?
Stability - chemically
Appearance - colourless
Temperature range - wide
Low cost
Safe - device could break
Low viscosity - respond faster
Physical property considerations
Dielectric anisotropy - Ξπ should have range -10 -> +50
Birefringence - Ξn should have range 0.03 - 0.5
Why is cyano group not suitable for modern liquid crystal displays?
Too high conductivity
Instead have components that incorporate terminal or lateral fluoro subs
What is the arrangement of chiral nematic liquid crystals
Form a helical arrangement due to the chirality
Gives an iridescent or coloured appearance
What are optical isomers
Optical isomers are superimposable mirror images of the same molecule that contains a chiral carbon
How to calculate the number of isomers of a molecule
2^(number of chiral centres)
What effect do chiral molecules have on polarised light and how do we measure this?
Chiral molecules can rotate the plane of polarised light
We can measure the observed optical rotation, πΌ.
Clockwise rotation (+) or (d)
Anticlockwise rotations (-) or (l)
Equation for specific optical rotation
[πΌ] = πΌ / c x l
C = concentration (gml-1)
L = pathlength dm
What does pitch mean?
Length of the helix of a chiral nematic
Short pitch (blue)
Long pitch (red)
Simplified Bragg equation for the reflection of light from a chiral nematic
π = n x p x cosπ
n = mean refractive index
P = pitch
π = angle from normal incidence
What is a chiral dopant?
Molecule which when added in small amounts can make a chiral nematic mixture - imparts chirality into system
When making a chiral nematic mixture what do we need to consider?
Compatibility with the host
Helical twisting power of the dopant
Bend or kink in molecule = greater HTP
Longer chain molecule = greater HTP
How to modify the pitch of a chiral mixture once dopant added?
Add more host
Add opposite enantiomer of dopant
Add racemate dopant
Add new dopant (larger HTP)
How to measure pitch in chiral nematics?
UV vis spectrometry
Cano wedge cell
Formula to determine pitch using cano wedge
P = 2s(tanπΌ)
s = distance between defect lines
πΌ = wedge angle
What happens if you plot pitch against 1/C
straight line y = mx + c
Can be used to determine what concentration is required for a specific pitch
COCH3 to COOH
Br2
NaOH
NH2 to OH
NaNO2
H2SO4
Add B(OH)2 group
n-BuLi
B(OMe)3
HCl
Add secondary alcohol
n-BuLi
RCHO
HCL
Add CHO to difluorobenzene
n-BuLi
DMF
COOH to CONH2
SOCl2
NH3
CONH2 to CN
SOCl2
R-Br replaced with B(OH)2
Mg, THF
B(OMe)3
HCl
