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Nematic liquid crystals Flashcards

1
Q

What is anisotropy?

A

Exhibiting properties with different values when measured along axes in different directions
l/b = measure of molecular anisotropy

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2
Q

What is molecular anisotropy?

A

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

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3
Q

What is crystal state

A

Molecules are all in fixed positions and unable to rotate

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4
Q

What is the Nematic phase

A

Molecules are roughly parallel/ pointing in same direction, determined by director, but still free to flow

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5
Q

What is an isotropic liquid

A

Molecules are free to rotate and tumble like a normal liquid. No intrinsic order.

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6
Q

What is the order parameter, S?

A

S = 1/2 (3Cos^2 ๐œƒ -1 )
Where ๐œƒ is the angle between long axis of molecule and director
S is temperature dependent
Nematic liquid crystal, typically S = 0.4 - 0.7

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7
Q

How are the molecules arranged if S = 1

A

The molecules are perfectly ordered, all molecules are aligned with the director

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8
Q

How are the molecules arranged if S = 0

A

The molecules are disordered, ie isotropic liquid

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9
Q

How can you measure the order parameter?

A

diamagnetism
NMR
Raman scattering

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10
Q

What is the effect of increasing the length of aliphatic chain R?

A

Can lead to microphase segregation due to separation of aromatic and aliphatic units

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11
Q

Why do cyanobiphenyls have a strong internal dipole moment

A

O = +M group
CN = -M group
๐œ‹ electrons are able to flow through aromatic system from the O to the CN group forming a strong internal dipole

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12
Q

How do quadrupolar interactions occur

A

Antiparallel pairs of molecules form with secondary dipoles between molecules stabilising the interaction
This cancels out the internal dipole

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13
Q

What is a dipole moment?

A

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.

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14
Q

Effect of chain length on behaviour of Alkylcyanobiphenyls

A

Short chain length = strong ๐œ‹-๐œ‹ interactions between aromatic units so higher mp
longer chain length alkyl interactions dominating raise mp again

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15
Q

Effect of size of ๐œ‹ unit on transition temp of nematic liquid crystal

A

As the length of the ๐œ‹ unit decreases the quadrupolar antiparallel overlap of ๐œ‹ units decreases in size and the transition temp of nematic liquid crystals increases

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16
Q

Effect of the cage ring unit on nematic liquid crystal transition temp

A

The introduction of a cage ring unit to the molecule increases the transition temp.
Raises the temp most when next to the aliphatic chain

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17
Q

What is the effect that Heteroatoms have on transition temp?

A

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

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18
Q

What effect do the size of heteroatoms have on the transition temp?

A

Larger heteroatoms e.g. Sulfur decrease linearity of molecule because theyโ€™re 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

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19
Q

What effect do linking groups between rings have?

A

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

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20
Q

How to add acyl group to benzene ring?

A

RCOCl and AlCl3

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21
Q

How to reduce acyl group to alkyl

A

NH2NH2 and KOH

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22
Q

How to reduce NO2 group

A

H2 and Pd/C

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23
Q

COCH3 => COOH

A

Br2 and NaOH

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24
Q

Boronic Acid Coupling

A
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25
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)
26
Add COOH to difluorobenzene
n-BuLi CO2 HCl
27
Add I to diflurorbenzene
N-iodosuccinimide H+
28
Add B(OH)2 to difluorobenzene
n-BuLi B(OMe)2 HCl
29
What are the 2 key physical properties that are important to liquid crystal display applications?
Birefringence Dielectric anisotropy
30
What is Birefringence, ฮ”n ?
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.
31
How to calculate Birefrigence?
ฮ”n = ne - no ne = perpendicular birefrigence no = parallel birefrigence
32
What is dielectric anisotropy, ฮ”๐œ€?
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)
33
How to calculate dielectric anisotropy?
ฮ”๐œ€ = ๐œ€parallel - ๐œ€perpendicular
34
What influences the birefringence of a molecule?
More ๐œ‹ electrons the more interactions the molecule will have with light therefore increasing the birefringence, ฮ”n The addition of a +M group to the ๐œ‹-system means it can donate electrons and increase birefringence More polarisable the molecule = higher birefringence
35
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
36
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-
37
Equation for order parameter, S?
S = ฮ”๐œ€/3(๐œ€bar-3)
38
Equation for mean permativity, ๐œ€bar.
๐œ€parallel + 2๐œ€perpendicular/3
39
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 ๐œ‹ electrons flowing towards perp dipole
40
How to calculate degrees of freedom of temp/press phase diagram
F = C - P +2 C = number of components P = number of phases
41
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
42
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
43
Physical property considerations
Dielectric anisotropy - ฮ”๐œ€ should have range -10 -> +50 Birefringence - ฮ”n should have range 0.03 - 0.5
44
Why is cyano group not suitable for modern liquid crystal displays?
Too high conductivity Instead have components that incorporate terminal or lateral fluoro subs
45
What is the arrangement of chiral nematic liquid crystals
Form a helical arrangement due to the chirality Gives an iridescent or coloured appearance
46
What are optical isomers
Optical isomers are superimposable mirror images of the same molecule that contains a chiral carbon
47
How to calculate the number of isomers of a molecule
2^(number of chiral centres)
48
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, ๐›ผ. Clockwise rotation (+) or (d) Anticlockwise rotations (-) or (l)
49
Equation for specific optical rotation
[๐›ผ] = ๐›ผ / c x l C = concentration (gml-1) L = pathlength dm
50
What does pitch mean?
Length of the helix of a chiral nematic Short pitch (blue) Long pitch (red)
51
Simplified Bragg equation for the reflection of light from a chiral nematic
๐›Œ = n x p x cos๐œƒ n = mean refractive index P = pitch ๐œƒ = angle from normal incidence
52
What is a chiral dopant?
Molecule which when added in small amounts can make a chiral nematic mixture - imparts chirality into system
53
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
54
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)
55
How to measure pitch in chiral nematics?
UV vis spectrometry Cano wedge cell
56
Formula to determine pitch using cano wedge
P = 2s(tan๐›ผ) s = distance between defect lines ๐›ผ = wedge angle
57
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
58
COCH3 to COOH
Br2 NaOH
59
NH2 to OH
NaNO2 H2SO4
60
Add B(OH)2 group
n-BuLi B(OMe)3 HCl
61
Add secondary alcohol
n-BuLi RCHO HCL
62
Add CHO to difluorobenzene
n-BuLi DMF
63
COOH to CONH2
SOCl2 NH3
64
CONH2 to CN
SOCl2
65
R-Br replaced with B(OH)2
Mg, THF B(OMe)3 HCl

Materials (5 decks)

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