Nematic Liquid Crystals Flashcards
What is anisotropy?
A difference in appearance or properties depending on which angle you look at it
What is the typical length-to-breadth ratio of a nematic liquid crystal?
Greater than 4:1
What is the nematic phase?
A phase in between a solid and a liquid where all of the molecules are roughly parallel with respect to a director
What is a typical order parameter (S) for a nematic liquid crystal?
Between 0.4 and 0.7
What does an order parameter of 0 mean?
It is completely disordered i.e. we have an isotropic liquid
What does an order parameter of 1 mean?
It is completely ordered i.e. molecules are aligned with the director and each other
What happens to the order as temperature decreases?
The order increases as they have less energy to deviate from the director
What is the main interaction in NLCs when the chain length is short?
Pi-pi interactions are dominant
What is the main interaction in NLCs when the chain length is long?
Aliphatic interactions begin to dominate and they are arranged into layers (smectic A or smectic C)
What interactions hold two NLCs together?
A quadrupolar interaction formed from two anti-parallel pairs
What effect does ring structure have on the N-Iso transition temperatures?
Having large sigma blocks and large pi blocks increases the transition temperature
Having increased rigid cyclohexyl rings (by a cage system) maintains the linearity and increases the transition temperature
Why does having large sigma blocks increase the N-Iso transition temperature?
The quadrupole range is smaller, leading to an effective increased length to breadth ratio
What effect do heteroatoms have on the N-Iso transition temperature?
They can increase as there is less steric interaction, which helps to maintain planarity
If they are the same size as carbon, the difference is minimal
If there are larger than carbon, the ring starts to kink and decreases the N-Iso transition temperature
Why are boronic acids in coupling reactions?
Easily purified
Stable in air
Stable to moisture
Can be stored
What catalyst is used in a Suzuki coupling reaction?
A Pd (0) catalyst e.g. palladium tetratriphenylphosphine
Na₂CO₃, water and alcohol/ether are generally used too
What are the benefits of Suzuki coupling reactions?
Excellent yields
No homocoupling
Tolerates many functional groups
How can we synthesise a boronic acid?
(1) nBuLi / -78 degrees
(2) B(OMe)₃
(3) HCl
How can we convert a boronic acid into an OH?
Hydrogen Peroxide
How can we add an aldehyde onto a difluorinated benzene ring?
(1) nBuLi / -78 degrees
(2) DMF
How can we add a carboxylic acid onto a difluorinated benzene ring?
(1) nBuLi / -78 degrees
(2) Solid carbon dioxide
(3) HCl
How can we add a secondary alcohol to a difluorinated benzene ring?
(1) nBuLi / -78 degrees
(2) RCHO
(3) HCl
How do we add iodine onto a difluorinated benzene ring?
(1) N-iodosuccinimide
(2) H+
How do we add bromine onto a difluorinated benzene ring?
Br₂, FeBr₃
How do we convert a carboxylic acid into an amide?
(1) SOCl₂
(2) NH₃
How do we convert an amide into a nitrile?
SOCl₂
What do we convert an amide into an amine?
LiAlH₄
How do we convert a secondary alcohol into an alkyl chain?
(1) p-TSA
(2) H₂, Pd/C
How do we reduce a ketone into an alkyl chain?
(1) NH₂NH₂
(2) KOH
How do we maintain a trans-arrangement when linking a cyclohexyl group onto a benzene ring?
(1) Cyclohexanone + Grignard + THF + PTSA (alcohol then reduction to alkene)
(2) BF₃.THF, PCC, DCM (adds alcohol and then oxidises to ketone)
(3) ethane-1,2-dithiol, BF₃.2HOAc (creates thioacetal, ensures trans arrangement)
(4) Raney Ni/H₂, Ethanol (removes thioacetal)
What is birefringance?
Due to the anisotropy of the NLCs, they exhibit two refractive indices (parallel and perpendicular)
Birefringence is calculated by the perpendicular refractive index - the parallel refractive index
What do high birefringence values lead to?
Coloured compounds (not good for NLCs)
What impacts the birefringence of a material?
They reflect the polarisability of the material; the larger the number of pi electrons, the more polarisable the compound
What is dielectric anisotropy?
It is related to the flow of polarisable electrons through the molecule
What is the equation for dielectric anisotropy?
Δε = εparallel - εperpendicular
What leads to a negative dielectric anisotropy?
Having a greater electron flow across the short axis compared to the long axis e.g. in difluorinated alkyl biphenyls
How does the quadrupolar interfere with dielectric anisotropy?
Due to there being an antiparallel arrangement, the εparallel values are smaller than expected (as there is partial cancelling out)
This leads to lower dielectric anisotropy than expected
How does having a pair splitter (e.g. a fluorine on a cyanobiphenyl) impact the dielectric anisotropy?
It breaks up the quadrupolar interaction, meaning there is no cancelling out of the εparallel value hence leading to a larger dielectric anisotropy
What is the equation for ε bar (the mean permittivity in a liquid)?
ε bar = (εparallel + 2εperpendicular) / 3
What is the equation to calculate the order parameter?
S = Δε / 3(ε bar - 3)
How many degrees of freedom are there at the eutectic point?
3 phases, 2 components
Degrees of Freedom = 2 - 3 +2 = 1
This degree of freedom is pressure, but we fix this at 1 atm, leading to 0 degrees of freedom
What is pitch?
The distance until a full 360-degree rotation around the director occurs
What does a low refractive index mean?
We have a pure colour
What does a high refractive index mean?
The same contains more colours
What is the pitch at high temperatures?
The pitch is short
Why do we use chiral dopants?
We can induce the chiral nematic phase into a NLC host by using a chiral dopant
How can we modify the pitch of a chirally doped NLC?
(1) Add more host (reduces the [dopant] -> longer pitch)
(2) Add the opposite enantiomer of the dopant
(3) Add the racemate of the dopant (dilute the effect of the dopant down)
(4) Add a new dopant (e.g. chain extension compared to the original dopant)
What two methods can we use to measure the pitch of a chiral molecule?
UV-Vis Spectroscopy
A Cano wedge cell
How does a Cano wedge cell work?
The chiral NLC is aligned in a planar geometry (molecules are aligned parallel to the bottom substrate)
The Cano wedge cell is arranged in such a way that the upper plate is orientated at a defined angle from the lower plate
This angle is defined by ‘alpha’
How can we calculate pitch from a cano wedge cell?
P = 2s(tan(‘alpha’))
where s = distance between defect lines
How can we calculate the composition of a mixture to give a defined pitch?
Plotting pitch against 1/concentration gives us a straight-line plot where:
y = pitch
x = 1/concentration
We can use this to calculate the pitch for any given concentration, or the concentration required for a specific pitch
What is the equation to calculate the order parameter?
1/2 x <3cos²Θ - 1>
where Θ is the angle between the long axis of each rod-like molecule and the director
How do we increase the carbon chain length by 1 for a chiral nematic?
1) HBr, sulfuric acid (brominate)
2) Mg, carbon dioxide, HCl (Grignard to form carboxylic acid)
3) Lithium Aluminium Hydride, then HCl (reduction to alcohol)
4) HBr, sulfuric acid (brominate)
How do we increase the carbon chain length by 2 for a chiral nematic?
1) TsCl, Py (add tosylate leaving group)
2) NaOEt, DEM (enolate chemistry)
3) KOH, heat, HCl (single carboxylic acid formed)
4) Lithium Aluminium Hydride, then HCl (reduction to alcohol)
5) HBr, sulfuric acid (brominate)
