Structure-Property Relationship

Discotic Liquid Crystals

M. ManickamSchool of Chemistry

The University of BirminghamM.Manickam@bham.ac.uk

CHM3T1

Lecture-3

Outline of Lecture

Introduction

Structure-Property Relationship of Discotic LCs

Synthesis of Discotic LCs

Final comments

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?

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

Types of Liquid Crystals

Liquid crystals

Lyotropic Thermotropic

Calamitic Polycatenar Discotic Banana-shaped

Nematic (N)

Smectic (S)

Nematic Discotic(ND)

Columnar (Col)

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 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

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

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

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

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

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

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)

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

Discotic Cores

Two types of cores

1. Aromatic cores

2. Alicyclic cores

There are more than 30 discotic cores are known

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

Some common Chains

XO Y

XY

X

O Y

O

YX

ester dimethylene

methyleneoxy

acetylene

Some common Polar Groups

NO2, Cl, Br, F, OH

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 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.

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)

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.

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.

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

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

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

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.

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

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

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.

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.

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

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

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

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

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

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

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

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.

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

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).

Exercise-1

RO

RO

OROR

OROR

OR

OR

FeCl3, DCMconc.H2SO4

Write down a detailed mechanism for the reaction below?