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Structure-Property Relationship Discotic Liquid Crystals. CHM3T1 Lecture-3. M. Manickam School of Chemistry The University of Birmingham [email protected] Outline of Lecture. Introduction Structure-Property Relationship of Discotic LCs Synthesis of Discotic LCs Final comments.

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structure property relationship discotic liquid crystals

Structure-Property RelationshipDiscotic Liquid Crystals



M. Manickam

School of Chemistry

The University of Birmingham

[email protected]

outline of lecture
Outline of Lecture
  • Introduction
  • Structure-Property Relationship of Discotic LCs
  • Synthesis of Discotic LCs
  • Final comments
learning objectives
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?

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

discotic lcs
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

discotic lcs1
Discotic LCs

The existence of mesophases generated by disc-shaped

molecules was theoretically in 1970

Benzene hexaester

By Chandrasekhar 1977

First Discotic core

Triphenylene hexaether

discotic lcs2
Discotic LCs








A new class of

charge transporting


supramolecular order

aromatic single


H-phase HHTT

Dh-phase H5T






Charge Carrier mobility  [cm2/Vs]

Greater Supramolecular Order Means Higher Charge Carrier Mobility

applications of discotic liquid crystals
Applications of Discotic Liquid Crystals

Columnar phases as

electron transport system

  • One-dimensional conductors
  • Photo-conducting systems
  • One-dimensional energy transfer properties
  • Electro luminescence
  • Light emitting diodes
  • Optoelectrical switching
  • Photovoltaic
  • Electrically tuneable cholesteric mirrors

Molecular wires

classification of discotic mesophases
Classification of Discotic Mesophases
  • Two basic types of discotic mesophases have been widely recognised, these are
  • Columnar; 2. Nematic
  • Several different types of columnar mesophases exhibited by discotic materials;
  • these arise because of the different symmetry classes of the two dimensional
  • lattice of columns and the order or the disorder of the molecular stacking within
  • the columns
discotic nematic phase
Discotic nematic phase

Figure: Representation of the ND phase, where the molecules are aligned in the

same orientation, with no additional positional ordering

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.

columnar phases
Columnar phases




Representation of (a) the general structure of Col phases, where the molecules are

aligned in the same orientation and, in addition, form columns,

(b) representation of Colr,

(c) representation of Colh

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.

a general structural template
A General Structural Template

A general structural template

for discotic liquid crystals

discotic cores
Discotic Cores

There are more than 30 discotic cores are known

Two types of cores

  • Aromatic cores
  • Alicyclic cores
linking groups
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







Imine (Schiff’s base)


some common chains
Some common Chains





Some common Polar Groups

NO2, Cl, Br, F, OH

terminal moieties
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

discotic cores1
Discotic Cores



  • 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.
benzene discotic
Benzene Discotic

C 68.3 Drd 86.0 I

C 68.0 Drd 97.0 I


benzene (A)

Hexa (alkoxyphenyl)

Benzene (B)

(B) Six directly attached benzene

rings to a central benzene ring

which provides a highly

conjugated central core

Mesophase stability much

greater than that of compound (A)

triphenylene discotic
Triphenylene Discotic

C 69.0 Dho 122.0 I

Triphenylene core consists of three benzene rings

conjugatively joined to give a plannar aromatic unit

that enables six peripheral units to be symmetrically

attached, and because the core is much larger than

benzene, the mesomorphic tendency of such

compounds is much higher.

Ether showed hexagonal ordering with the molecules ordered within the columns, probably because the polar

oxygens combined with the large core facilitate a very ordered packing and the absence of any bulky units allows for ordered packing within the columns.


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 structure

is no longer truly disc-like and this is the reason why the

stability of the hexagonal mesophase is much reduced

and why the less ordered Dhd phase is exhibited.


substituted hexaether

discotic cores2
Discotic Cores

C 66.0 Drd 126.0 I


hexasubstituted ester

C 98.2 ND 131.2 I

Symmetrically hexasubstituted

Benzene core structure with six peripheral acetylene-linked benzene

ring units attached; the incorporation

of 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.

The ester possess higher mesophase

stability than for the simple alkoxy-

substituted analogues, but they

exhibit a Drd phase.

truxene discotic
Truxene Discotic

Truxene hexaether

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 that

locks 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.

C 67.0 Dho 260.0 I

Truxene hexaester

C 68.0 ND 85.0 Drd 138.0 Dho 280.0 I

phthalocyanines discotic
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.
phthalocyanines discotic1
Phthalocyanines Discotic

Phthalocyanines with eight peripheral moieties show

wide-range columnar mesophases of the Dho and Dhd


These materials are of interest because of their potential

as electron carriers for use in electronic devices. This

core is able to hold metal ions in the centre which is often

copper 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 clearing


This core also has eight non-peripheral sites available

for substitution; such materials have been prepared and

these also exhibit columnar mesophases, often of the Drd


unusual discotic
Unusual Discotic

R= C9H19

C 53.5 D 171.5 I

This compound unusually exhibited columnar

mesophase over a wide temperature range despite

the presence of only four peripheral units.

The presence of oxygens in the high polarisable

central core is probably an important factor which,

in part, offsets the small number of peripheral units

R= C7H15COO2

C 107.5 (D 95) D 127 .5 I

This compund is also unusual because it exhibits

columnar mesophases even though the molecular

structure is not quite disc-like; again the high polarity

of the oxygen units (carbonyl in this case) within the

central core aid in the generation of the necessary

intermolecular forces of attraction

alicyclic discotic
Alicyclic Discotic

Disc-shaped molecules can be generated

from alicyclic core structures.

A cyclohexane ring is a simple example and

this compound shows that mesophases are

exhibited by such systems.

The transition temperatures of this compound

reveal the cyclohexane core to be better at

generating columnar mesophases that the

analogous benzene systems.

C 68.5 D 199.5 I

macrocyclic discotic core
Macrocyclic Discotic Core

Phenylacetylene macrocycles

Acetylene-linking units have been employed in the

construction of a conjugated ring to give a discotic


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 as

the peripheral moieties.

These materials were designed to exhibit columnar

mesophases that would self-organise into molecular

channels which could be used for transportation

of electrons in applications such as molecular wires

and membranes.

R= OC7H15

C1 144 C2 168 ND 192 I


C1 104 C2 121 ND 241 I

discotic oligomer
Discotic Oligomer

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 triphenylene

units to a central discotic core in a star-like


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.

R= C5H11: g? Dh 137 I


functionalised triphenylene derivatives
Functionalised Triphenylene Derivatives





core expansion








Precursors for dimers,

oligomers, polymers and networks



Literature Method

FeCl3 / Organic Solvent / Acid Method


Advantages - Good yield

Limitations - Acid needed

Not easy purification

Side products

oxidative trimerisation of o dialkoxybenzene to hexaalkoxytriphenylene

New Method

Oxidative Trimerisation of o-Dialkoxybenzene to Hexaalkoxytriphenylene

Molybdenum (V) chloride as a novel Reagent

Symmetrically Substituted Hexaalkoxytriphenylenes

R = CH3 to C10H21

unsymmetrical and monofunctionalised triphenylenes




Unsymmetrical and Monofunctionalised Triphenylenes


No acid

Easy purification

High yield 74-95%

Selective derivatisation

organometallic method
Organometallic Method

Another method for preparation of

unsymmetrical substituted triphenylene

discotic derivatives

final comments
Final Comments

One aspect of the structure property relationships of discotic materials is that

the 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.

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.

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).

exercise 11

Write down a detailed mechanism for the reaction below?