Nematic Liquid Crystals

Question 1

What is anisotropy?

Answer 1

A difference in appearance or properties depending on which angle you look at it

Question 2

What is the typical length-to-breadth ratio of a nematic liquid crystal?

Answer 2

Greater than 4:1

Question 3

What is the nematic phase?

Answer 3

A phase in between a solid and a liquid where all of the molecules are roughly parallel with respect to a director

Question 4

What is a typical order parameter (S) for a nematic liquid crystal?

Answer 4

Between 0.4 and 0.7

Question 5

What does an order parameter of 0 mean?

Answer 5

It is completely disordered i.e. we have an isotropic liquid

Question 6

What does an order parameter of 1 mean?

Answer 6

It is completely ordered i.e. molecules are aligned with the director and each other

Question 7

What happens to the order as temperature decreases?

Answer 7

The order increases as they have less energy to deviate from the director

Question 8

What is the main interaction in NLCs when the chain length is short?

Answer 8

Pi-pi interactions are dominant

Question 9

What is the main interaction in NLCs when the chain length is long?

Answer 9

Aliphatic interactions begin to dominate and they are arranged into layers (smectic A or smectic C)

Question 10

What interactions hold two NLCs together?

Answer 10

A quadrupolar interaction formed from two anti-parallel pairs

Question 11

What effect does ring structure have on the N-Iso transition temperatures?

Answer 11

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

Question 12

Why does having large sigma blocks increase the N-Iso transition temperature?

Answer 12

The quadrupole range is smaller, leading to an effective increased length to breadth ratio

Question 13

What effect do heteroatoms have on the N-Iso transition temperature?

Answer 13

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

Question 14

Why are boronic acids in coupling reactions?

Answer 14

Easily purified

Stable in air

Stable to moisture

Can be stored

Question 15

What catalyst is used in a Suzuki coupling reaction?

Answer 15

A Pd (0) catalyst e.g. palladium tetratriphenylphosphine

Na₂CO₃, water and alcohol/ether are generally used too

Question 16

What are the benefits of Suzuki coupling reactions?

Answer 16

Excellent yields

No homocoupling

Tolerates many functional groups

Question 17

How can we synthesise a boronic acid?

Answer 17

(1) nBuLi / -78 degrees

(2) B(OMe)₃

(3) HCl

Question 18

How can we convert a boronic acid into an OH?

Answer 18

Hydrogen Peroxide

Question 19

How can we add an aldehyde onto a difluorinated benzene ring?

Answer 19

(1) nBuLi / -78 degrees

(2) DMF

Question 20

How can we add a carboxylic acid onto a difluorinated benzene ring?

Answer 20

(1) nBuLi / -78 degrees

(2) Solid carbon dioxide

(3) HCl

Question 21

How can we add a secondary alcohol to a difluorinated benzene ring?

Answer 21

(1) nBuLi / -78 degrees

(2) RCHO

(3) HCl

Question 22

How do we add iodine onto a difluorinated benzene ring?

Answer 22

(1) N-iodosuccinimide

(2) H+

Question 23

How do we add bromine onto a difluorinated benzene ring?

Answer 23

Br₂, FeBr₃

Question 24

How do we convert a carboxylic acid into an amide?

Answer 24

(1) SOCl₂

(2) NH₃

Question 25

How do we convert an amide into a nitrile?

Answer 25

SOCl₂

Question 26

What do we convert an amide into an amine?

Answer 26

LiAlH₄

Question 27

How do we convert a secondary alcohol into an alkyl chain?

Answer 27

(1) p-TSA (2) H₂, Pd/C

Question 28

How do we reduce a ketone into an alkyl chain?

Answer 28

(1) NH₂NH₂ (2) KOH

Question 29

How do we maintain a trans-arrangement when linking a cyclohexyl group onto a benzene ring?

Answer 29

(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)

Question 30

What is birefringance?

Answer 30

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

Question 31

What do high birefringence values lead to?

Answer 31

Coloured compounds (not good for NLCs)

Question 32

What impacts the birefringence of a material?

Answer 32

They reflect the polarisability of the material; the larger the number of pi electrons, the more polarisable the compound

Question 33

What is dielectric anisotropy?

Answer 33

It is related to the flow of polarisable electrons through the molecule

Question 34

What is the equation for dielectric anisotropy?

Answer 34

Δε = εparallel - εperpendicular

Question 35

What leads to a negative dielectric anisotropy?

Answer 35

Having a greater electron flow across the short axis compared to the long axis e.g. in difluorinated alkyl biphenyls

Question 36

How does the quadrupolar interfere with dielectric anisotropy?

Answer 36

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

Question 37

How does having a pair splitter (e.g. a fluorine on a cyanobiphenyl) impact the dielectric anisotropy?

Answer 37

It breaks up the quadrupolar interaction, meaning there is no cancelling out of the εparallel value hence leading to a larger dielectric anisotropy

Question 38

What is the equation for ε bar (the mean permittivity in a liquid)?

Answer 38

ε bar = (εparallel + 2εperpendicular) / 3

Question 39

What is the equation to calculate the order parameter?

Answer 39

S = Δε / 3(ε bar - 3)

Question 40

How many degrees of freedom are there at the eutectic point?

Answer 40

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

Question 41

What is pitch?

Answer 41

The distance until a full 360-degree rotation around the director occurs

Question 42

What does a low refractive index mean?

Answer 42

We have a pure colour

Question 43

What does a high refractive index mean?

Answer 43

The same contains more colours

Question 44

What is the pitch at high temperatures?

Answer 44

The pitch is short

Question 45

Why do we use chiral dopants?

Answer 45

We can induce the chiral nematic phase into a NLC host by using a chiral dopant

Question 46

How can we modify the pitch of a chirally doped NLC?

Answer 46

(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)

Question 47

What two methods can we use to measure the pitch of a chiral molecule?

Answer 47

UV-Vis Spectroscopy A Cano wedge cell

Question 48

How does a Cano wedge cell work?

Answer 48

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'

Question 49

How can we calculate pitch from a cano wedge cell?

Answer 49

P = 2s(tan('alpha')) where s = distance between defect lines

Question 50

How can we calculate the composition of a mixture to give a defined pitch?

Answer 50

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

Question 51

What is the equation to calculate the order parameter?

Answer 51

1/2 x <3cos²Θ - 1> where Θ is the angle between the long axis of each rod-like molecule and the director

Question 52

How do we increase the carbon chain length by 1 for a chiral nematic?

Answer 52

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)

Question 53

How do we increase the carbon chain length by 2 for a chiral nematic?

Answer 53

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)