Structure-Property Relationship Discotic Liquid Crystals
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Transcript of Structure-Property Relationship Discotic Liquid Crystals
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Structure-Property Relationship
Discotic Liquid Crystals
M. ManickamSchool of Chemistry
The University of [email protected]
CHM3T1
Lecture-3
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Outline of Lecture
Introduction
Structure-Property Relationship of Discotic LCs
Synthesis of Discotic LCs
Final comments
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Learning Objectives
After completing this lecture you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods.
Be aware of the fundamental principles and general structures of Discotic Lcs
Understand different types of molecular arrangement within columns
Understand the hexagonal columnar phase
How do the different types of cores influence the mesophases?
How to design and synthesis discotic liquid crystalline materials?
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Nomenclature
Dho: discotic hexagonal ordered phase
Dhd: discotic hexagonal disordered phase
Drd: discotic rectangular disordered phase
Dob.d: oblique
n: director
ND: nematic discotic phase
Colh: hexagonal discotic
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Types of Liquid Crystals
Liquid crystals
Lyotropic Thermotropic
Calamitic Polycatenar Discotic Banana-shaped
Nematic (N)
Smectic (S)
Nematic Discotic(ND)
Columnar (Col)
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Discotic LCs
Similarly to the calamitic LCs, discotic LCs possess a general structure comprising a planar (usually aromatic) central rigid core surrounded by a flexible periphery, represented mostly by pendant chains (usually four, six, or eight), as illustrated in the cartoon representation in figure below.
As can be seen, the molecular diameter (d) is much greater than the disc thickness (t), imparting the form anisotropy to the molecular structure.
Cartoon representation of the general shape of discotic LCs, where d >>t
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Discotic LCs
Benzene hexaester By Chandrasekhar 1977First Discotic core
RO
RO
OROR
OROR
Triphenylene hexaether
OCOROCOR
OCOROCOR
ROCO
ROCO
1 X10 -4cm2v-1s-1
Columnar phase
The existence of mesophases generated by disc-shaped molecules was theoretically in 1970
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Discotic LCs
RS
RS
SRSR
SRSR
Benzene hexaester 1977
RO
RO
OROR
OROR
Triphenylene hexaether
Triphenylene hexathioether
OCOROCOR
OCOROCOR
ROCO
ROCO
1 X10 -1cm2v-1s-1
Helicoidal phase
1 X10 -4cm2v-1s-1
Columnar phase
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e-
e-
e-
e-
supramolecular order
aromatic singlecrystals
H-phase HHTT Dh-phase H5T polymericphotoconductors
A new class of charge transporting
materials
10-1 10-3 10-6
Greater Supramolecular Order Means Higher Charge Carrier Mobility Greater Supramolecular Order Means Higher Charge Carrier Mobility
Charge Carrier mobility [cm2/Vs]
OROR
OROR
RO
RO
Photoconductors
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Applications of Discotic Liquid Crystals
One-dimensional conductors
Photo-conducting systems
One-dimensional energy transfer properties
Electro luminescence
Light emitting diodes
Optoelectrical switching
Photovoltaic
Electrically tuneable cholesteric mirrors
e-
e-
e-
e-
Columnar phases as electron transport system
Molecular wires
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Classification of Discotic Mesophases
hexagonal
rectangular
oblique
ordered
disordered
Dho, Dhd, Drd, Dob.d
Symmetry group
Molecular arrangementwithin Columns
Two basic types of discotic mesophases have been widely recognised, these are
1. Columnar; 2. Nematic
Several different types of columnar mesophases exhibited by discotic materials;these arise because of the different symmetry classes of the two dimensionallattice of columns and the order or the disorder of the molecular stacking within the columns
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Discotic nematic phase
Nematic discotic (ND) is the least ordered mesophase, where the molecules have only orientational order being aligned on average with the director as illustrated in the figure.
There is no positional order.
Figure: Representation of the ND phase, where the molecules are aligned in the same orientation, with no additional positional ordering
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Columnar phases
Columnar (Col) phases are more ordered.
Here the disc-shaped cores have a tendency to stack one on the top of another, forming columns.
Arrangement of these columns into different lattice patterns gives rise to a number of columnar mesophases, namely columnar rectangular (Colr) and columnar hexagonal (Colh) in the fashion described in the above figure.
Representation of (a) the general structure of Col phases, where the molecules arealigned in the same orientation and, in addition, form columns,
(b) representation of Colr, (c) representation of Colh
(a) (b) (c)
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A General Structural Template
(O)RR*
OO
(O)R
O
O X
O R
O
S R
ORO
R
O
R
Discotic CoreR
SeSi
A general structural template for discotic liquid crystals
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Discotic Cores
Two types of cores
1. Aromatic cores
2. Alicyclic cores
There are more than 30 discotic cores are known
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Linking Groups
XO Y
XY
X
O Y
O
H
Y
X
HYX
OY
H
X H
O
N N NX
Y
X
H Y
ester dimethylene methyleneoxy
ethylene acetylene cinnamate
azoImine (Schiff’s base)
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
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Some common Chains
XO Y
XY
X
O Y
O
YX
ester dimethylene
methyleneoxy
acetylene
Some common Polar Groups
NO2, Cl, Br, F, OH
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Terminal Moieties
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
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Discotic Cores
12
3
4
5
67
89
10
11121
2
34
5
6
Benzene
Triphenylene
Triphenylene isolated from the pyrolytic products of benzene. Also it was synthesized from cyclohexanone.
Six peripheral for substitution
Its various physical and chemical properties were studied.
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Benzene Discotic
O
O
O
O
O
O
C5H11
OC5H11
C5H11
C5H11
C5H11
C5H11
O
O
O
O
O
OC5H11
C5H11O
C5H11O
OC5H11
OC5H11
OC5H11
C 68.3 Drd 86.0 I
C 68.0 Drd 97.0 I
(B) Six directly attached benzenerings to a central benzene ringwhich provides a highly conjugated central core
Mesophase stability muchgreater than that of compound (A)
Hexaalkanoyloxybenzene (A) Hexa (alkoxyphenyl)
Benzene (B)
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Triphenylene Discotic
C5H11O
C5H11O OC5H11
OC5H11
OC5H11C5H11O
Triphenylene core consists of three benzene ringsconjugatively joined to give a plannar aromatic unitthat enables six peripheral units to be symmetricallyattached, and because the core is much larger thanbenzene, the mesomorphic tendency of such compounds is much higher.
Ether showed hexagonal ordering with the molecules ordered within the columns, probably because the polaroxygens combined with the large core facilitate a very ordered packing and the absence of any bulky units allows for ordered packing within the columns.
C 69.0 Dho 122.0 I
C8H17O
C8H17O OC6H13
OC6H13
OC12H25C12H25O
symmetrically substituted hexaether
unsymmetrically substituted hexaether
C 40.0 Dhd 79.0 I
Three different sets of peripheral chains and this results of the reduction of melting point.
This unsymmetrical nature of the molecular structureis no longer truly disc-like and this is the reason why thestability of the hexagonal mesophase is much reducedand why the less ordered Dhd phase is exhibited.
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Discotic Cores
O
O O
O
OO
C7H15
C7H15
C7H15
C7H15
O
OO
O
C7H15
O
C7H15
O
C 66.0 Drd 126.0 ISymmetrically hexasubstituted ester
The ester possess higher mesophase stability than for the simple alkoxy-substituted analogues, but theyexhibit a Drd phase.
C7H15
C7H15
C7H15
C7H15
C7H15
C7H15
C 98.2 ND 131.2 ISymmetrically hexasubstituted
Benzene core structure with six peripheral acetylene-linked benzenering units attached; the incorporationof the acetylene linkages removes the steric interactions between the aryl rings and allows the rings to be twisted at 90o with respect to each other. This arrangement of benzene rings prevents the molecules from aggregating in a columnar fashion.
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Transition Temperature and Phase Behaviours of Triphenylenes
OO
OO
O
O O
O
O
OO
O
OC10H21
OC10H21
OC10H21
OC10H21
C10H21O
C10H21O
a b
a
ba
b
ab
a
ba
b
x y
x
yx
y
xy
x
yx
y
Compound a b x y
1 H H H H
Transition Temperatures
C 142 D rd 191 ND 212 I
2 CH3 CH3 H H C 157 D hd 167 ND 182 I
3 H H CH3 H C 108 ND 134 I
Strict Effects
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Truxene Discotic
C10H21O
C10H21OOC10H21
OC10H21
C10H21O OC10H21
O
OO
O
O O
C9H19
C9H19
C9H19
C9H19
C9H19
C9H19
O
O
OO
O
O
C 67.0 Dho 260.0 I
C 68.0 ND 85.0 Drd 138.0 Dho 280.0 I
Truxene core is even larger than the triphenylene core and consists of four benzene rings.
Three radial rings are symmetrically attached to the central ring in two ways; firstly by a conjugative single bond, and secondly through a methylene spacer thatlocks in an approximately planar structure by preventing inter-annular twisting.
The mesomorphic tendency of the compouns based on the hexa-substituted truxene core is very high.
Simple ether exhibits a wide-range Dho phase up to 260 0C
Ester compounds exhibits an inverted phase sequence where the ND phase is exhibited at a lower temperature than the Drd and the Dho mesophases.
Normally this type of behaviour relates to a changing molecular packing ability with temperature, often caused by the conformational arrangements of the peripheral chains.
Truxene hexaether
Truxene hexaester
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Phthalocyanines Discotic
• Phthalocyanines have been targeted for a wide variety of applications including colour, dyes.
• Electrochromics, detection of conductivity changes (sensors),
• nonlinear optic and photodynamic therapy for the destruction of cancer cells.
N
NH
N
HN
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Phthalocyanines Discotic
N
N
N
NM
RR
R
R
RR
R
R
M= H2, Cu, Ni
R = alkyl, e.g., C8H17
R= alkoxy, e.g., C12H25O
R= alkoxymethyl, e.g., C12H25OCH2
Phthalocyanines with eight peripheral moieties showwide-range columnar mesophases of the Dho and Dhd
types.
These materials are of interest because of their potentialas electron carriers for use in electronic devices. This core is able to hold metal ions in the centre which is oftencopper or nickel.
The metal has the effect of increasing the columnar mesophase stability, but this usually results in the materials decomposing before they reach their clearingpoint.
This core also has eight non-peripheral sites availablefor substitution; such materials have been prepared and these also exhibit columnar mesophases, often of the Drd
type.
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Unusual Discotic
O O
RR
R R
O
O
R
R
R
R
R
R
R= C9H19
C 53.5 D 171.5 I
R= C7H15COO2
C 107.5 (D 95) D 127 .5 I
This compound unusually exhibited columnar mesophase over a wide temperature range despitethe presence of only four peripheral units.
The presence of oxygens in the high polarisablecentral core is probably an important factor which,in part, offsets the small number of peripheral units
This compund is also unusual because it exhibits columnar mesophases even though the molecularstructure is not quite disc-like; again the high polarityof the oxygen units (carbonyl in this case) within thecentral core aid in the generation of the necessaryintermolecular forces of attraction
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Alicyclic Discotic
C5H11
C5H11
C5H11
C5H11
C5H11
O
O
OO
O
C5H11
O
Disc-shaped molecules can be generatedfrom alicyclic core structures.
A cyclohexane ring is a simple example and this compound shows that mesophases areexhibited by such systems.
The transition temperatures of this compound reveal the cyclohexane core to be better atgenerating columnar mesophases that theanalogous benzene systems.
C 68.5 D 199.5 I
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Macrocyclic Discotic Core
R
R
R
R
R
R
R= OC7H15
C1 144 C2 168 ND 192 I
R= OCOC7H15
C1 104 C2 121 ND 241 I
Phenylacetylene macrocycles
Acetylene-linking units have been employed in the construction of a conjugated ring to give a discotic architecture.
This core is not of the usual type but has a hollow centre surrounded by alternating benzene rings and acetylene-linking groups;
Conventional ether and ester units have been used asthe peripheral moieties.
These materials were designed to exhibit columnarmesophases that would self-organise into molecularchannels which could be used for transportation of electrons in applications such as molecular wiresand membranes.
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Discotic Oligomer
OO
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OROR
OROR
O
RO
OROR
OROR
RO
O
OROR
OROR
RO
O
O
OR
OROR
RO
RO
RO
RO
OROR
ORO
OROR
RO
RO
ORO
6
6 6
6
6
6
R= C5H11: g? Dh 137 I
Triphenylene
Centre triphenylene core with six peripheral triphenylene units exhibit columnar mesophases, and these are commonly called star-like liquid crystals.
It is a very large molecule that uses flexible spacers to attach peripheral triphenyleneunits to a central discotic core in a star-likemanner.
Hexagonal columnar phase of this compound has been identified as hexagonal. This structures are oligomeric and could almost be considered polymeric.
Such a large discotic compound are a recent development, and this type of architecture offer much possibility for future development.
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Functionalised Triphenylene Derivatives
RO
RO
OROR
OROR
RO
RO
OHOR
OROR
RO
RO
OHOR
OHOR
RO
RO
OHOR
OROH
RO
RO
OROH
OHOR
RO
RO
OHOH
OROR
monofunctionalised
difunctionalised
trifunctionalised
Precursors for dimers,oligomers, polymers and networks
nitration
halogenations
core expansion
mono
2,6 2,7
3,6 2,3 2,6,10
2,7,10RO
HO
OHOR
OROH
RO
HO
OHOR
OHOR
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Direct Core Functionalisation
First Synthesis of 1, 2, 3, 6, 7, 10, 11- heptaalkoxytriphenylenes
RO
RO
OHOR
OROR
RO
RO
OR
OROR
RO
RO
OAcOR
OROR
RO
RO
OR'OR
OROR
OO
AcOR'O
Ceric ammoniumnitrate, CH3CN
RT, 5-10 mins,85-90%
Zn, Ac2oEt3N, Rf., 3h90-95%
DMSO, KOH,
R'Br, 55oC,12h
80-90%
R R' -C4H9 -C4H9 -C8H17 -C8H17 -C3H7 -C12H25 -C4H9 -C7H15 -C5H11 -C6H13 -C8H17 -C10H21
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Direct Core Functionalisation
Functionalisation of Nitro Group
RO
RO
OROR
OROR
RO
RO
OROR
OROR
RO
RO
OROR
OROR
RO
RO
OROR
OROR
NO2
O2N
NO2
O2N
O2N
NO2
HNO3DCM-CH3NO2
HNO3DCM-CH3NO2
HNO3DCM-CH3NO2
RO
RO
OROR
OROR
RO
RO
OROR
OROR
NN
H2N
THF-MeOHNiCl2.6H2ONaBH4
DCM-AcOHNaNO2
RO
RO
OROR
OROR
H2N
NH2
THF-MeOHNiCl2.6H2ONaBH4
R = C4H9 to C7H15
RO
RO
OROR
OROR
H2N
NH2NH2
THF-MeOHNiCl2.6H2ONaBH4
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RO
RO
OROR
OROR
OR
OR
FeCl3, DCMconc.H2SO4
Advantages - Good yield
Limitations - Acid needed
Not easy purification
Side products
50-75%
FeCl3 / Organic Solvent / Acid Method
Literature Method
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Oxidative Trimerisation of o-Dialkoxybenzene to Hexaalkoxytriphenylene
RO
RO
OROR
OROR
OR
OR
MoCl5, DCMr.t., 20min
74-95%
OH
OH
RBr, DMSOKOH
Symmetrically Substituted Hexaalkoxytriphenylenes
New Method
Molybdenum (V) chloride as a novel Reagent
R = CH3 to C10H21
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Unsymmetrical and Monofunctionalised Triphenylenes
OH
OH
OR
OR
OR
ORIOR
ORRO
RO
I2, con.H2SO4 HIO3
H2O, AcOH
Cu
RBr, KOH
DMSO
OR'
OR
OH
OR
OR
OROR
MoCl5,CH2Cl2,RT,30min con.H2SO4 50-70%
MoCl5,CH2Cl2,RT,30min no Acid 60-90%
OR'OR
OROR
RO
RO
Unsymmetrical
Advantages:
No acid
Easy purification
High yield 74-95%
Selective derivatisation
mono
OHOR
OROR
RO
RO
hepta
OROR
OROR
RO
RO
RO
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Organometallic Method
Y1
Y2
X1
X2ZnX
RO
RO
+
ORRO
ORRO
Y1
Y2
OROR
OROR
RO
RO
FeCl3/DCM
H2SO4
Pd2 (dba)
PPh3
X1 = X2 = I
Y1 = Y2 = OR
Another method for preparation ofunsymmetrical substituted triphenylenediscotic derivatives
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Final CommentsOne aspect of the structure property relationships of discotic materials is thatthe mesophase exhibited are much more sensitive to slight changes in molecular
structure than are their calamitic analogues.
Columnar phases are far more common within the discotic family than is the ND phase.
Research into discotic liquid crystals has not been very extensive because of the perceived lack of applications for such materials and mesophases;
Perhaps the lack of ready applications for discotic liquid crystals results from the relative novelty of the discotic mesophase structure.
Applications in traditional liquid crystal display devices, so important for calamitic liquid crystals, are not appropriate for discotic liquid crystals because of the inherently high viscosity of the phases.
A few applications have been suggested throughout this lecture, notably those which utilse columnar phases as electron transport systems (molecular wires).
Accordingly, there is much valuable research to be performed and discotic liquid crystals have a bright future, especially in the biological area of ion channels and artificial membranes.
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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.
OC9H13C9H13O
C9H13O OC9H13
CNC10H21O
A
B
C
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Exercise-2
OC9H13C9H13O
C9H13O OC9H13
CNC10H21O
A
B
C
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).
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Exercise-1
RO
RO
OROR
OROR
OR
OR
FeCl3, DCMconc.H2SO4
Write down a detailed mechanism for the reaction below?