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Chemistry - Year One > Nematic Liquid Crystals > Flashcards

Nematic Liquid Crystals Flashcards

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1
Q

What is anisotropy?

A

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

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2
Q

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

A

Greater than 4:1

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3
Q

What is the nematic phase?

A

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

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4
Q

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

A

Between 0.4 and 0.7

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5
Q

What does an order parameter of 0 mean?

A

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

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6
Q

What does an order parameter of 1 mean?

A

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

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7
Q

What happens to the order as temperature decreases?

A

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

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8
Q

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

A

Pi-pi interactions are dominant

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9
Q

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

A

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

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10
Q

What interactions hold two NLCs together?

A

A quadrupolar interaction formed from two anti-parallel pairs

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11
Q

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

A

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

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12
Q

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

A

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

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13
Q

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

A

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

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14
Q

Why are boronic acids in coupling reactions?

A

Easily purified

Stable in air

Stable to moisture

Can be stored

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15
Q

What catalyst is used in a Suzuki coupling reaction?

A

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

Na₂CO₃, water and alcohol/ether are generally used too

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16
Q

What are the benefits of Suzuki coupling reactions?

A

Excellent yields

No homocoupling

Tolerates many functional groups

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17
Q

How can we synthesise a boronic acid?

A

(1) nBuLi / -78 degrees

(2) B(OMe)₃

(3) HCl

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18
Q

How can we convert a boronic acid into an OH?

A

Hydrogen Peroxide

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19
Q

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

A

(1) nBuLi / -78 degrees

(2) DMF

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20
Q

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

A

(1) nBuLi / -78 degrees

(2) Solid carbon dioxide

(3) HCl

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21
Q

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

A

(1) nBuLi / -78 degrees

(2) RCHO

(3) HCl

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22
Q

How do we add iodine onto a difluorinated benzene ring?

A

(1) N-iodosuccinimide

(2) H+

23
Q

How do we add bromine onto a difluorinated benzene ring?

A

Br₂, FeBr₃

24
Q

How do we convert a carboxylic acid into an amide?

A

(1) SOCl₂

(2) NH₃

25
Q

How do we convert an amide into a nitrile?

A

SOCl₂

26
Q

What do we convert an amide into an amine?

A

LiAlH₄

27
Q

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

A

(1) p-TSA

(2) H₂, Pd/C

28
Q

How do we reduce a ketone into an alkyl chain?

A

(1) NH₂NH₂

(2) KOH

29
Q

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

A

(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)

30
Q

What is birefringance?

A

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

31
Q

What do high birefringence values lead to?

A

Coloured compounds (not good for NLCs)

32
Q

What impacts the birefringence of a material?

A

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

33
Q

What is dielectric anisotropy?

A

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

34
Q

What is the equation for dielectric anisotropy?

A

Δε = εparallel - εperpendicular

35
Q

What leads to a negative dielectric anisotropy?

A

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

36
Q

How does the quadrupolar interfere with dielectric anisotropy?

A

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

37
Q

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

A

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

38
Q

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

A

ε bar = (εparallel + 2εperpendicular) / 3

39
Q

What is the equation to calculate the order parameter?

A

S = Δε / 3(ε bar - 3)

40
Q

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

A

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

41
Q

What is pitch?

A

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

42
Q

What does a low refractive index mean?

A

We have a pure colour

43
Q

What does a high refractive index mean?

A

The same contains more colours

44
Q

What is the pitch at high temperatures?

A

The pitch is short

45
Q

Why do we use chiral dopants?

A

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

46
Q

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

A

(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)

47
Q

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

A

UV-Vis Spectroscopy

A Cano wedge cell

48
Q

How does a Cano wedge cell work?

A

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’

49
Q

How can we calculate pitch from a cano wedge cell?

A

P = 2s(tan(‘alpha’))

where s = distance between defect lines

50
Q

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

A

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

51
Q

What is the equation to calculate the order parameter?

A

1/2 x <3cos²Θ - 1>

where Θ is the angle between the long axis of each rod-like molecule and the director

52
Q

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

A

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)

53
Q

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

A

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)

Chemistry - Year One (16 decks)

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