Structure-Property Relationship
Discotic Liquid Crystals
CHM3T1
Lecture-3
M. Manickam
School of Chemistry
The University of Birmingham
M.Manickam@bham.ac.uk
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
Calamitic
Thermotropic
Polycatenar
Nematic (N)
Smectic (S)
Discotic
Banana-shaped
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
The existence of mesophases generated by disc-shaped
molecules was theoretically in 1970
OR
ROCO
ROCO
OR
OCOR
OCOR
RO
RO
OCOR
OCOR
OR
Benzene hexaester
By Chandrasekhar 1977
First Discotic core
OR
Triphenylene hexaether
Columnar phase
1 X10 -4cm2v-1s-1
Discotic LCs
SR
OR
ROCO
ROCO
OCOR
OCOR
OCOR
OCOR
OR
RO
Columnar phase
1 X10 -4cm2v-1s-1
RS
RS
RO
OR
OR
Benzene hexaester 1977
SR
Triphenylene
hexaether
SR
SR
Triphenylene
hexathioether
Helicoidal phase
1 X10 -1cm2v-1s-1
Photoconductors
OR
OR
RO
RO
-
A new class of
charge transporting
materials
e
-
e
-
e
-
e
OR
OR
supramolecular order
aromatic single
crystals
H-phase HHTT
10-1
Dh-phase H5T
Charge Carrier mobility  [cm2/Vs]
10-3
polymeric
photoconductors
10-6
Greater Supramolecular Order Means Higher Charge Carrier Mobility
Applications of Discotic Liquid Crystals
 One-dimensional conductors
Columnar phases as
electron transport system
 Photo-conducting systems
 One-dimensional energy transfer
properties
-
e
-
e
 Electro luminescence
-
e
 Light emitting diodes
-
e
 Optoelectrical switching
 Photovoltaic
 Electrically tuneable cholesteric
mirrors
Molecular wires
Classification of Discotic Mesophases
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 dimensional
lattice of columns and the order or the disorder of the molecular stacking within
the columns
Molecular arrangement
within Columns
Symmetry group
hexagonal
ordered
rectangular
disordered
oblique
Dho, Dhd, Drd, Dob.d
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
(a)
(b)
(c)
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
R
OR
A general structural template
for discotic liquid crystals
O
R
S
O
R
O
R
Discotic Core
O
Se
Si
O
O
O
O
X
(O)R
*
R
(O)R
R
Discotic Cores
There are more than 30 discotic cores are known
Two types of cores
1.
Aromatic cores
2.
Alicyclic cores
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
Some common Chains
O
O
ester
X
X
X
Y
Y
O
Y
dimethylene
methyleneoxy
X
Y
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
2
12
1
6
5
3
1
2
11
4
3
10
5
4
Benzene
9
8
6
7
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
OC5H11
O
C5H11
C5H11
O
C5H11
O
O
O
O
O
O
O
C5H11
O
C5H11
C5H11O
OC5H11
C5H11O
OC5H11
O
C5H11
O
OC5H11
C 68.3 Drd 86.0 I
Hexaalkanoyloxy
benzene (A)
(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)
C 68.0 Drd 97.0 I
Hexa (alkoxyphenyl)
Benzene (B)
Triphenylene Discotic
C 69.0 Dho 122.0 I
C5H11O
OC5H11
C5H11O
OC5H11
C5H11O
OC5H11
symmetrically
substituted hexaether
C 40.0 Dhd 79.0 I
C8H17O
OC6H13
C8H17O
OC6H13
C12H25O
OC12H25
unsymmetrically
substituted hexaether
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.
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.
Discotic Cores
C7H15
C7H15
O
O C7H15
O
O
C7H15
O O
O
O
O
C7H15
O
O
C7H15
C7H15
C7H15
C7H15
C7H15
C7H15
O
C7H15
C 66.0 Drd 126.0 I
Symmetrically
hexasubstituted ester
The ester possess higher mesophase
stability than for the simple alkoxysubstituted analogues, but they
exhibit a Drd phase.
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.
Transition Temperature and Phase
Behaviours of Triphenylenes
OC10H21
b
a
x
b
y
y
O
O
Strict Effects
x
O
O
C10H21O
y
x
x
y
O
C10H21O
b
a
O
OC10H21
O
O
a
OC10H21
O
O
a
b
a
O
x
O
y
b
y
b
x
a
OC10H21
a
b
x
y
Transition
Temperatures
1
H
H
H
H
C 142 D rd 191 ND 212 I
2
CH3
CH3
H
H
3
H
CH3
H
Compound
H
C 157 D
hd
167 ND 182 I
C 108 ND 134 I
Truxene Discotic
C10H21OTruxene hexaether
Truxene core is even larger than the triphenylene core
OC10H21
and consists of four benzene rings.
C10H21O
OC10H21
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.
OC10H21
C10H21O
C 67.0 Dho 260.0 I
C9H19
O
O
O
O
O
C9H19
O
O
C9H19
C9H19
O
O
C9H19
O
O
O
C9H19
Truxene hexaester
C 68.0 ND 85.0 Drd 138.0 Dho 280.0 I
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.
Phthalocyanines Discotic
• Phthalocyanines have been
targeted for a wide variety of
applications including
colour, dyes.
• Electrochromics, detection
of conductivity changes
(sensors),
N
HN
NH
• nonlinear optic and
photodynamic therapy for
the destruction of cancer
cells.
N
Phthalocyanines Discotic
Phthalocyanines with eight peripheral moieties show
wide-range columnar mesophases of the Dho and Dhd
types.
M= H2, Cu, Ni
R
R
N
R
M
N
R
N
R
R
N
R
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.
R
R = alkyl, e.g., C8H17
R= alkoxy, e.g., C12H25O
R= alkoxymethyl, e.g., C12H25OCH2
The metal has the effect of increasing the columnar
mesophase stability, but this usually results in the
materials decomposing before they reach their clearing
point.
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
type.
Unusual Discotic
R= C9H19
C 53.5 D 171.5 I
R
R
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
O
O
This compound unusually exhibited columnar
mesophase over a wide temperature range despite
the presence of only four peripheral units.
R
R
R= C7H15COO2
C 107.5 (D 95) D 127 .5 I
O
R
R
R
R
R
R
O
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
C5H11
O
C5H11
Disc-shaped molecules can be generated
from alicyclic core structures.
C5H11
O
C5H11
O
A cyclohexane ring is a simple example and
this compound shows that mesophases are
exhibited by such systems.
O
O
O
C5H11
C 68.5 D 199.5 I
C5H11
The transition temperatures of this compound
reveal the cyclohexane core to be better at
generating columnar mesophases that the
analogous benzene systems.
Macrocyclic Discotic Core
R
Phenylacetylene macrocycles
Acetylene-linking units have been employed in the
construction of a conjugated ring to give a discotic
R architecture.
R
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
R the peripheral moieties.
R
R= OC7H15
R
C1 144 C2 168 ND 192 I
R= OCOC7H15
C1 104 C2 121 ND 241 I
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.
Discotic Oligomer
OR
OR
RO
OR
RO
OR
O
6
6
O
RO
O
O
OR
OR
6
RO
O
OR
OR
RO
O
O
O
O
O
OR
O
OR
O
6 O
O
OR
O
O
O
6
O
O
O
RO
OR
O
RO
OR
6
OR
It is a very large molecule that uses flexible
spacers to attach peripheral triphenylene
units to a central discotic core in a star-like
manner.
OR
O
O
OR
RO
Centre triphenylene core with six
peripheral triphenylene units exhibit
columnar mesophases, and these are
commonly called star-like liquid crystals.
O
OR
OR
Hexagonal columnar phase of this
compound has been identified as
hexagonal. This structures are oligomeric
and could almost be considered polymeric.
OR
RO
OR
R= C5H11: g? Dh 137 I
Triphenylene
OR
Such a large discotic compound are a
recent development, and this type of
architecture offer much possibility for
future development.
Functionalised Triphenylene Derivatives
nitration
monofunctionalised
OR
OR
halogenations
RO
difunctionalised
RO
OR
OR
core expansion
mono
OH
OH
OH
OR
OR
RO
RO
RO
RO
2,6
OR
RO
OH
OR
OH
OH
OR
OR
OR
OH
OR
OH
OH
RO
2,3
RO
3,6
2,7
RO
OR
RO
trifunctionalised
2,6,10
HO
RO
RO
OH
OR
OH
OR
OR
OR
OH
OR
RO
2,7,10
HO
OR
OH
Precursors for dimers,
oligomers, polymers and
networks
Direct Core Functionalisation
First Synthesis of 1, 2, 3, 6, 7, 10, 11- heptaalkoxytriphenylenes
OH
OR
RO
Ceric ammonium
nitrate, CH3CN
RO
RT, 5-10 mins,
85-90%
O
O
OR
RO
RO
OR
OR
OR
OR
Zn, Ac2o
Et3N, Rf., 3h
90-95%
OR'
OR
R'O
RO
RO
OR
OR
DMSO, KOH,
OAc
OR
AcO
R'Br, 55oC,12h
RO
80-90%
RO
OR
OR
R
-C4H9
-C8H17
-C3H7
-C4H9
-C5H11
-C8H17
R'
-C4H9
-C8H17
-C12H25
-C7H15
-C6H13
-C10H21
Direct Core Functionalisation
Functionalisation of Nitro Group
OR
OR
O2N
OR
OR
OR
O2N
RO
RO
HNO3
DCM-CH3NO2
RO
RO
HNO3
DCM-CH3NO2
RO
OR
HNO3
DCM-CH3NO2
OR
O2N
NO 2
RO
NO 2
OR
OR
THF-MeOH
NiCl2.6H2O
NaBH4
OR
OR
OR
OR
OR
OR
H2N
OR
THF-MeOH
NiCl2.6H2O
NaBH4
RO
RO
NO 2
RO
RO
OR
OR
OR
OR
THF-MeOH
NiCl2.6H2O
NaBH4
DCM-AcOH
NaNO2
OR
OR
RO
H2N
OR
N
OR
N
H2N
OR
RO
RO
OR
RO
RO
NH 2
RO
OR
OR
OR
OR
NH 2
NH 2
OR
OR
R = C4H9 to C7H15
Literature Method
FeCl3 / Organic Solvent / Acid Method
OR
OR
OR
FeCl3, DCM
conc.H2SO4
RO
OR
50-75%
RO
OR
OR
Advantages - Good yield
Limitations - Acid needed
Not easy purification
Side products
New Method
Oxidative Trimerisation of o-Dialkoxybenzene
to Hexaalkoxytriphenylene
Molybdenum (V) chloride as a novel Reagent
Symmetrically Substituted Hexaalkoxytriphenylenes
OR
OH
OH
RBr, DMSO
KOH
OR
OR
OR
MoCl5, DCM
r.t., 20min
RO
74-95%
RO
R = CH3 to C10H21
OR
OR
Unsymmetrical and Monofunctionalised
Triphenylenes
OH
RBr, KOH
OR
OH
DMSO
OR
RO
OR
I
I2, con.H2SO4
HIO3
H2O, AcOH
RO
Cu
OR
Advantages:
OR
OR
MoCl5,CH2Cl2,RT,30min
no Acid
60-90%
No acid
Easy purification
MoCl5,CH2Cl2,RT,30min
con.H2SO4
50-70%
High yield 74-95%
OR'
OH
OR
OR
OR
OR
Selective derivatisation
OR
OR
OH
OR'
RO
OR
OR
RO
RO
RO
RO
RO
RO
OR
OR
Unsymmetrical
OR
OR
OR
OR
mono
OR
hepta
Organometallic Method
OR
RO
RO
RO
+
ZnX
X1
Y1
Pd2 (dba)
Y1
X2
Y2
PPh3
Y2
RO
X1 = X2 = I
OR
Y1 = Y2 = OR
FeCl3/DCM
H2SO4
OR
Another method for preparation of
unsymmetrical substituted triphenylene
discotic derivatives
OR
RO
RO
OR
OR
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.
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
Exercise-1
Write down a detailed mechanism for the reaction below?
OR
OR
OR
FeCl3, DCM
conc.H2SO4
OR
RO
RO
OR
OR