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.
Structure-Property RelationshipDiscotic Liquid Crystals
School of Chemistry
The University of Birmingham
Dho: discotic hexagonal ordered phase
Dhd: discotic hexagonal disordered phase
Drd: discotic rectangular disordered phase
ND: nematic discotic phase
Colh: hexagonal discotic
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
The existence of mesophases generated by disc-shaped
molecules was theoretically in 1970
By Chandrasekhar 1977
First Discotic core
A new class of
Charge Carrier mobility [cm2/Vs]
Greater Supramolecular Order Means Higher Charge Carrier Mobility
Columnar phases as
electron transport system
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.
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
for discotic liquid crystals
There are more than 30 discotic cores are known
Two types of cores
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 Polar Groups
NO2, Cl, Br, F, OH
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
C 68.3 Drd 86.0 I
C 68.0 Drd 97.0 I
(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 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.
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.
C 66.0 Drd 126.0 I
C 98.2 ND 131.2 I
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 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
C 68.0 ND 85.0 Drd 138.0 Dho 280.0 I
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
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
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
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
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
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
C1 144 C2 168 ND 192 I
C1 104 C2 121 ND 241 I
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
Precursors for dimers,
oligomers, polymers and networks
R = C4H9 to C7H15
FeCl3 / Organic Solvent / Acid Method
Advantages - Good yield
Limitations - Acid needed
Not easy purification
Molybdenum (V) chloride as a novel Reagent
Symmetrically Substituted Hexaalkoxytriphenylenes
R = CH3 to C10H21
High yield 74-95%
Another method for preparation of
unsymmetrical substituted triphenylene
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.
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.
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).
Write down a detailed mechanism for the reaction below?