Structure-Property Relationship Discotic Liquid Crystals M. Manickam School of Chemistry The University of Birmingham 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 D ho : discotic hexagonal ordered phase D hd : discotic hexagonal disordered phase D rd : discotic rectangular disordered phase D ob.d : oblique n: director N D : nematic discotic phase Col h : hexagonal discotic

Types of Liquid Crystals

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 1977 First Discotic core Triphenylene hexaether The existence of mesophases generated by disc-shaped molecules was theoretically in 1970

Discotic LCs Benzene hexaester 1977 Triphenylene hexaether Triphenylene hexathioether

supramolecular order aromatic single crystals H-phase HHTTD h -phase H5Tpolymeric photoconductors A new class of charge transporting materials Greater Supramolecular Order Means Higher Charge Carrier Mobility Charge Carrier mobility  [cm 2 /Vs] 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 Columnar phases as electron transport system 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

Discotic nematic phase Nematic discotic (N D ) 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 N D 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 (Col r ) and columnar hexagonal (Col h ) in the fashion described in the above figure. 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 Col r, (c) representation of Col h (a) (b) (c)

A General Structural Template 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 esterdimethylenemethyleneoxy ethylene acetylene cinnamate azo Imine (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 ester dimethylene methyleneoxy acetylene Some common Polar Groups NO 2, 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 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 C 68.3 D rd 86.0 I C 68.0 D rd 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) Hexaalkanoyloxy benzene (A) Hexa (alkoxyphenyl) Benzene (B)

Triphenylene Discotic 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 69.0 D ho I symmetrically substituted hexaether unsymmetrically substituted hexaether C 40.0 D hd 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 Dh d phase is exhibited.

Discotic Cores C 66.0 D rd I Symmetrically hexasubstituted ester The ester possess higher mesophase stability than for the simple alkoxy- substituted analogues, but they exhibit a D rd phase. C 98.2 N D 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 90 o 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 Strict Effects

Truxene Discotic C 67.0 D ho I C 68.0 N D 85.0 D rd D ho 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 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 D ho phase up to C Ester compounds exhibits an inverted phase sequence where the N D phase is exhibited at a lower temperature than the D rd and the D ho 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.

Phthalocyanines Discotic Phthalocyanines with eight peripheral moieties show wide-range columnar mesophases of the D ho and D hd types. 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 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 D rd type.

Unusual Discotic R= C 9 H 19 C 53.5 D I R= C 7 H 15 COO 2 C (D 95) D 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 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 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 I

Macrocyclic Discotic Core R= OC 7 H 15 C C N D 192 I R= OCOC 7 H 15 C C N D 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 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.

Discotic Oligomer R= C 5 H 11 : g? D h 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 triphenylene units to a central discotic core in a star-like manner. 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 monofunctionalised difunctionalised trifunctionalised Precursors for dimers, oligomers, polymers and networks nitration halogenations core expansion mono 2,62,7 3,6 2,32,6,10 2,7,10

Direct Core Functionalisation First Synthesis of 1, 2, 3, 6, 7, 10, 11- heptaalkoxytriphenylenes

Direct Core Functionalisation Functionalisation of Nitro Group R = C 4 H 9 to C 7 H 15

Advantages - Good yield Limitations - Acid needed Not easy purification Side products 50-75% FeCl 3 / Organic Solvent / Acid Method Literature Method

Oxidative Trimerisation of o-Dialkoxybenzene to Hexaalkoxytriphenylene Symmetrically Substituted Hexaalkoxytriphenylenes New Method Molybdenum (V) chloride as a novel Reagent R = CH 3 to C 10 H 21

Unsymmetrical and Monofunctionalised Triphenylenes Unsymmetrical Advantages: No acid Easy purification High yield 74-95% Selective derivatisation monohepta

Organometallic Method Another method for preparation of unsymmetrical substituted triphenylene discotic derivatives

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 N D 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-1 Write down a detailed mechanism for the reaction below?