Transcript Structure-Property Relations

Structure-Property Relations:
Calamitic Liquid Crystalline
Materials
CHM3T1
Lecture - 2
M. Manickam
School of Chemistry
The University of Birmingham
[email protected]
Outline of Lecture
 Introduction
 Structure-Property Relations of Calamity LCs
 Synthesis of calamitic LCs
 Final Comments
 Exercises
Learning Objectives
After completing this lecture you should have an understanding of, and be able
to demonstrate, the following terms, ideas and methods.
What are the general structures for calamitic liquid crystalline materials?
How do the different types of cores influence the mesophases?
How hydrogen bonds can be incorporated as the core to form rod like
structure?
How does the aromatic and alicyclic cores influence the transition
temperatures?
How does terminal moieties, linking groups, lateral substituents affect the
liquid crystal properties?
How to design and synthesis calamitic liquid crystalline materials?
Types of Liquid Crystals
Liquid crystals
Lyotropic
Calamitic
Thermotropic
Polycatenar
Nematic (N)
Smectic (S)
Discotic
Banana-shaped
Nematic Discotic(ND)
Columnar (Col)
Physical Properties For LCD Display
A low melting point (below room temperature)
A wide SC range with no underlying ordered smectic
phases
A cooling phase sequence of I-N-SA-SC
A low viscosity
A tilt angle (θ) of 22.50
A low to moderate optical anisotropy
A negative dielectric anisotropy
A high dielectric biaxiality
A high chemical and photochemical stability
General Structural Template for Calamitic
LCs
Representation of calamitic
LCs, where l >>b
R’ and R’’
: flexible terminal units; alkyl, alkoxy chains, CN, NO2
A, B, C and D : ring systems; phenyl, cyclohexyl, heteroaromatics and hetrocycles
L
: linking units; CH=N, COO, N=N, COS, C=C,
Calamitic LCs
Calamitic or rod-like LCs are those mesomorphic compounds that
possess an elongated shape, responsible for the form anisotropy of
the molecular structure, as the result of the molecular length (l) being
significantly greater than the molecular breadth (b), as depicted in the
cartoon representation in the figure.
Cartoon representation of calamitic LCs, where l>>b
General Structural Template for Nematic
Phase
Representation of calamitic
LCs, where l >>b
 Polar groups
 Not a longer alkyl/alkoxy chains
 Rigid core with conjugation
General Structural Template for Smectic
Phase
Lateral
substituents
Representation of calamitic
LCs, where l >>b
 Polar groups
 Longer alkyl/alkoxy chains
 Rigid core with conjugation
 Lateral substituents
Core structures: Selected Aromatic Core Units
The most fundamental structural feature of a liquid crystal material is the
so-called core. The core can, in fact, be difficult to define absolutely, but it is
usually defined as the rigid unit which is constructed from linearly linked ring
units
N
N
1,4-phenyl
4, 4-biphenyl
2,5-pyrimidinyl
2,6-naphthyl
terphenyl
The core is often defined to include any linking groups and any lateral
substituents that are connected to the rings.
Most calamitic liquid crystals possess aromatic rings usually 1,4 -phenyl
because of the relative ease of synthesis, but heteroaromatics and heterocycles
are also common.
Core structures: Selected Alicyclic Core
Units
1, 4-bicyclo[2.2.2] octyl
trans-1,3-cyclobutyl
trans-1, 4-cyclohexyl
trans-2, 5-dioxanyl
trans-2, 6-decalinyl
O
O
The nematic phase, like the smectic phase, is generated by many alicyclic
materials where the cores are constructed solely of alicyclic rings.
Hence, the relationship between polarisability and liquid crystal phase stability
is not so clear cut.
Hydrogen Bond: Acyclic Nematogen
C6H13
O
OH
HO
C6H13
O
C 32.0 N 62.5 I
Acyclic nematogen
1. Hydrogen bonding is responsible for giving an elongated unit with a rigid
central core (ring)
2. Two flexible terminal chains (saturated hexyl units)
3. Conjugated di-alkenic units
Hydrogen Bond: Nematic Phase
O
C5H11
OH
HO
C5H11
O
C 88.0 N 126.5 I
The molecular species is certainly not long and lath-like and
would not be expected to exhibit mesomorphism;
however, dimerisation through hydrogen bonding creates a long lath-like
structure with a three- ring core and two flexible terminal pentyl chains
Why nematic phase and not a smectic phase?
1. The molecules must have sufficient lateral attractions to enable packing
2. Terminal chains may be short relative to the length of the rigid core
Hydrogen Bond: Smectic and Nematic Phases
O
C8H17O
OH
HO
OC8H17
O
C 101.0 SC 108.0 N 147.0 I
O
C10H21O
OH
HO
OC10H21
O
C 97.0 SC 122.0 N 142.0 I
Introduction two long terminal alkoxy chains, exhibits a smectic C phase
In the second compound the terminal chains are longer so the smectic phase
stability increases further and the nematic phase stability is reduced
Effect of Terminal Chain Length on the
Transition Temperature
Alkylcyanobiphenyl homologues
R
CN
Transition temperatures (oC)
compounds
R
C
SA
N
CH3
109.0
-
45.0
C2H5
75.0
-
22.0
C 3H 7
66.0
-
25.5
C 4H 9
48.0
-
16.5
C5H11
24.0
-
35.0
C6H13
14.5
C7H15
30.0
C8H17
21.5
33.5
40.5
C9H19
42.0
48.0
49.5
C10H21
44.0
50.5
-
-
29.0
-
43.0
I
Effects of Aromatic Core on Transition
Temperatures
N
C5H11
CN
N
C 71.0 (N 52.0) I
17.0
C5H11
91.5
95.0
CN
C 24.0 N 35. 0 I
C5H11
CN
C 84.0 N 126.5 I
204.0
3.5
C5H11
CN
C 130.0 N 239.0 I
112.5
109.0
C5H11
C 68.0 N 130.0 I
CN
Effects of Alicyclic Core Changes on
Transition Temperatures
CN
C7H15
C 35.0 (N 5.0) I
CN
C5H11
CN
C7H15
C 47.5 N 61.0 I
26.0
20.0
56.0
O
17.0
C5H11
C 24.0 N 35.0 I
CN
O
C 56.0 (N 52.0) I
O
CN
C5H11
3.0
36.0
C5H11
CN
C5H11
C 62.0 N 100.0 I
S
C 74.0 (N 19.0) I
C 31.0 N 55.0 I
45.0
CN
S
C5H11
CN
S
C 98.0 I (super cools to 28.0)
Terminal Moieties
R'
Cyano group, nitro group
R''
R’ and R’’ terminal
moieties
Fairly long, straight hydrocarbon (Usually alkyl or alkoxy)
The role of the terminal units in the generation of liquid crystal phases is still not
yet fully understood.
However, the long alkyl/alkoxy chains add flexibility to the rigid core structure that
tends to reduce melting points and allow liquid crystal phases to be exhibited.
Additionally the alkyl/alkoxy chains are believed to be responsible for stabilising
the molecular orientations necessary for liquid crystals phase generation.
Polar groups, do not necessarily reducing the melting points, but stabilise the
molecular orientation.
Physical properties are also strongly dependent upon the choice of terminal unit
Effect of Terminal Moieties on the Transition
Temperature
Cyano group
Fairly long, straight hydrocarbon (Usually alkyl or alkoxy)
CN
CN
C 66.0 (N 25.5) I
O
C 78.0 (N 75.5) I
C 48.0 (N16.5) I
CN
CN
O
C 48.0 (N 68.0) I
Linking Groups
Linking groups are normally those structural units, other than a direct
bond, that connect one part of a core to another
Selected examples of linking groups in liquid crystals
O
ester
X
X
X
O
Y
dimethylene
Y
X
X
H
X
H
Y
X
N
azo
Y
methyleneoxy
H
H
acetylene
ethylene
Y
O
O
O
cinnamate
Y
X
N
N
Y
H
Imine (Schiff’s base)
Y
Examples of linking groups
C5H11
C5H11
C 24.0 N 35.0 I
CN
C5H11
CH N
N
N
CN
CN
C 46.4 N 75.0 I
C 89.0 (N 86.5) I
No linking group
Imine (schiff’s base)
azo
O
C5H11
O
CN
C 64.5 (N 55.5) I
ester
C5H11
O
CN
C 49.0 (N - 20) I
methyleneoxy
CN
C 62.0 (N-24)I
C5H11
dimethylene
Lateral Substituents
The important issues when considering lateral substitution
Lateral Substitution
Size
Small
Large
Polarity
Polar
Non-polar
Position
Inner-core, outer-edge
On terminal chain
On linking group
Size of some common lateral units
Lateral substituent
X
Size (A0)
H
1.20
F
1.47
Cl
1.75
X
R
R
X
X
X= lateral units
Br
1.85
I
1.98
C
1.70
N
1.55
O
1.52
Effects of Lateral Fluoro substitution
Physical Properties
O
C5H11
A
O
CN
B
A
B
H
H
F
H
F
F
Viscosity
47 cP
33 cP
-
Dipole
Dielectric
anistropy
5.9 D
40.1
6.6 D
41.0
7.2 D
61.0
Naphthoic Acid: Lateral Substitution
C8H17O
COOH
X
X=H
C 161.5 N 190.0 I
X = Cl
C169.0 SC 183.0 N 199.0 I
X = Br
C164.0 SC 174.5 N 196.0 I
X=I
C161.0 (SC 144.0) N 185.5 I
Large lateral substituents cause an increase in clearing point because of the
efficient filling of space which enhances intermolecular attractions.
The smectic tendency of the naphthoic acid with the lateral substituents is very
high
This high smectic tendency is generated because of favourable lamellar packing
afforded by the polarity of the lateral substituent combined with the space-filling
effect.
Effects of Lateral Fluoro substitution
Stability of LC phases
F
F
OC8H17
C5H11
C 89.0 SC 155.5 SA 165.0 N 166.0 I
F
C7H15
F
C5H11
C 56.0 SC 105.5 SA 131.0 N 136.0 I
Position
High smectic phase stability of both
compounds are largely due to the effect
of the outer-edge fluoro substituent,
which fills a void and so enhances the
intermolecular attractions and hence
the lamellar packing of the molecules
Polarisability and Stability
It was a long held theory that the nematic phase, for example, was
generated because of the anisotropy of the polarisability resulting from
the conjugated core unit and that the higher the polarisability anisotropy
the higher the nematic phase stability.
However, the nematic phase, like the smectic phase, is generated by many
alicyclic materials where the cores are constructed solely of alicyclic
rings. Hence, the relationship between polarisability and LC phase
stability is not so clear cut.
RO
N
Nematic phase
RO
C N
Based on the above comments, what type of core combinations give a
nematic liquid crystals phase?
It is difficult to generalise.
Introduction of Alkyl Terminal Chain
O
C4H9
C4H9COCl
hydrazine
hydrate
AlCl3
KOH
C5H11
C5H11
CH3COCl
AlCl3
COCH3
HNO3
H2SO4
Br2, NaOH
C5H11
C5H11
C5H11
NaNO2
H2SO4
C5H11
H2, Pd/C
NO2
NH2
OH
COOH
(O)R
C5H11
DCC
DMAP
in CH2Cl2
DCC
DMAP
in CH2Cl2
COOH
C5H11
O
OH
(O)R
O
C5H11
O
(O)R
O
Cynopentylbiphenyl from Biphenyl
Br2
Br
C4H9COCl
AlCl3
C5H11
Br
KOH
CuCN
DMF
C5H11
CN
O
NH2NH2
C4H9
Br
Terphenyl from Arylboronic Acid
R(O)
Br
1. Mg or n-BuLi, THF
2. (MeO)3B, THF
3. HCl, H2O
R(O)
B(OH)2 + I
Br
Pd(PPh3)4, NaCO3,
CH3OCH2CH2OCH3,
H2O
R(O)
R'(O)
R(O)
Br
B(OH)2
Pd(PPh3)4, NaCO3,
CH3OCH2CH2OCH3,
H2O
(O)R'
Terphenyl from Arylboronic Acid
Lateral Fluoro Substitution
R(O)
Br
F
1. Mg or n-BuLi, THF
2. (MeO)3B, THF
3. HCl, H2O
R(O)
B(OH)2 + I
Br
Pd(PPh3)4, NaCO3,
CH3OCH2CH2OCH3,
H2O
F
R(O)
Br
F
R'(O)
B(OH)2
Pd(PPh3)4, NaCO3,
CH3OCH2CH2OCH3,
H2O
F F
R(O)
(O)R'
Summary
The molecular structure of liquid crystalline materials are indeed very varied while at the
same time being very similar in principle and style.
The generation of liquid crystal phases is possible in a wide range of structures.
However, if a specific type of liquid crystal phase is required to exist over a specific range of
temperature, than molecular structure needs to be more carefully considered.
Structural, considerations become even more involved when additional requirements in
terms of physical properties are added to the list of essential
features.
Liquid crystals are often said to have simple structures and in some respects this is true.
However, liquid crystals are very special materials in terms of their unique combination
of properties.
Research into liquid crystals is still in its infancy and as new, technically advanced
applications for liquid crystals are realised, the materials will require
much more complex combinations of structural features.
The knowledge of structure-property relationships acquired so far will be invaluable in the
design of new materials to satisfy new, advanced applications.
Exercise-1
Compounds A, B and C displays a smectic liquid crystalline phase, and
no nematic phase. Discuss brieifly the factors which promote the
smectic mesophase, over the nematic mesophase.
CN
C10H21O
A
OC9H13
C9H13O
B
OC9H13
C9H13O
C
Exercise-2
Identify two or three modifications to compounds A, B and C which would
promote the nematic phase over the smectic phase, and explain (a) the
rational behind your chemical modification, and (b) what the effect these
modifications have on the clearing temperature (Tc).
CN
C10H21O
A
OC9H13
C9H13O
B
OC9H13
C9H13O
C