Structure-Property Relations: Calamitic Liquid Crystalline Materials M. Manickam School of Chemistry The University of Birmingham CHM3T1 Lecture - 2

Outline of Lecture  Introduction  Structure-Property Relations of Calamity LCs  Synthesis of calamitic LCs  Final Comments  Exercises

Learning Objectives After completing this lecture you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods. What are the general structures for calamitic liquid crystalline materials? How do the different types of cores influence the mesophases? How hydrogen bonds can be incorporated as the core to form rod like structure? How does the aromatic and alicyclic cores influence the transition temperatures? How does terminal moieties, linking groups, lateral substituents affect the liquid crystal properties? How to design and synthesis calamitic liquid crystalline materials?

Types of Liquid Crystals

A low melting point (below room temperature) A wide S C range with no underlying ordered smectic phases A cooling phase sequence of I-N-S A -S C A low viscosity A tilt angle (θ) of A low to moderate optical anisotropy A negative dielectric anisotropy A high dielectric biaxiality A high chemical and photochemical stability Physical Properties For LCD Display

General Structural Template for Calamitic LCs R’ and R’’ : flexible terminal units; alkyl, alkoxy chains, CN, NO 2 A, B, C and D : ring systems; phenyl, cyclohexyl, heteroaromatics and hetrocycles L : linking units; CH=N, COO, N=N, COS, C=C, Representation of calamitic LCs, where l >>b

Calamitic LCs Calamitic or rod-like LCs are those mesomorphic compounds that possess an elongated shape, responsible for the form anisotropy of the molecular structure, as the result of the molecular length (l) being significantly greater than the molecular breadth (b), as depicted in the cartoon representation in the figure. Cartoon representation of calamitic LCs, where l>>b

General Structural Template for Nematic Phase Representation of calamitic LCs, where l >>b  Polar groups  Not a longer alkyl/alkoxy chains  Rigid core with conjugation

General Structural Template for Smectic Phase Representation of calamitic LCs, where l >>b  Polar groups  Longer alkyl/alkoxy chains  Rigid core with conjugation  Lateral substituents Lateral substituents

Core structures: Selected Aromatic Core Units 1,4-phenyl 2,5-pyrimidinyl2,6-naphthyl 4, 4-biphenyl terphenyl The most fundamental structural feature of a liquid crystal material is the so-called core. The core can, in fact, be difficult to define absolutely, but it is usually defined as the rigid unit which is constructed from linearly linked ring units The core is often defined to include any linking groups and any lateral substituents that are connected to the rings. Most calamitic liquid crystals possess aromatic rings usually 1,4 -phenyl because of the relative ease of synthesis, but heteroaromatics and heterocycles are also common.

Core structures: Selected Alicyclic Core Units trans-1, 4-cyclohexyl 1, 4-bicyclo[2.2.2] octyltrans-1,3-cyclobutyl trans-2, 5-dioxanyltrans-2, 6-decalinyl The nematic phase, like the smectic phase, is generated by many alicyclic materials where the cores are constructed solely of alicyclic rings. Hence, the relationship between polarisability and liquid crystal phase stability is not so clear cut.

Hydrogen Bond: Acyclic Nematogen Acyclic nematogen 1. Hydrogen bonding is responsible for giving an elongated unit with a rigid central core (ring) 2. Two flexible terminal chains (saturated hexyl units) 3. Conjugated di-alkenic units C 32.0 N 62.5 I

Hydrogen Bond: Nematic Phase C 88.0 N I The molecular species is certainly not long and lath-like and would not be expected to exhibit mesomorphism; however, dimerisation through hydrogen bonding creates a long lath-like structure with a three- ring core and two flexible terminal pentyl chains Why nematic phase and not a smectic phase? 1.The molecules must have sufficient lateral attractions to enable packing 2.Terminal chains may be short relative to the length of the rigid core

Hydrogen Bond: Smectic and Nematic Phases C S C N I C 97.0 S C N I Introduction two long terminal alkoxy chains, exhibits a smectic C phase In the second compound the terminal chains are longer so the smectic phase stability increases further and the nematic phase stability is reduced

Effect of Terminal Chain Length on the Transition Temperature Alkylcyanobiphenyl homologues

C 71.0 (N 52.0) I C 24.0 N I C N I C 68.0 N I C 84.0 N I Effects of Aromatic Core on Transition Temperatures

Effects of Alicyclic Core Changes on Transition Temperatures C 47.5 N 61.0 I C 24.0 N 35.0 I C 31.0 N 55.0 I C 62.0 N I C 35.0 (N 5.0) I C 56.0 (N 52.0) I C 74.0 (N 19.0) I C 98.0 I (super cools to 28.0)

Terminal Moieties Cyano group, nitro group Fairly long, straight hydrocarbon (Usually alkyl or alkoxy) 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 R’ and R’’ terminal moieties

Effect of Terminal Moieties on the Transition Temperature Cyano group Fairly long, straight hydrocarbon (Usually alkyl or alkoxy)

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

Examples of linking groups No linking group Imine (schiff’s base) azo ester methyleneoxy dimethylene

Lateral Substituents Lateral Substitution Size PolarityPosition Small Large Polar Non-polar Inner-core, outer-edge On terminal chain On linking group The important issues when considering lateral substitution

Size of some common lateral units X= lateral units

Effects of Lateral Fluoro substitution Physical Properties

Naphthoic Acid: Lateral Substitution Large lateral substituents cause an increase in clearing point because of the efficient filling of space which enhances intermolecular attractions. The smectic tendency of the naphthoic acid with the lateral substituents is very high This high smectic tendency is generated because of favourable lamellar packing afforded by the polarity of the lateral substituent combined with the space-filling effect.

Effects of Lateral Fluoro substitution Stability of LC phases High smectic phase stability of both compounds are largely due to the effect of the outer-edge fluoro substituent, which fills a void and so enhances the intermolecular attractions and hence the lamellar packing of the molecules Position

Polarisability and Stability It was a long held theory that the nematic phase, for example, was generated because of the anisotropy of the polarisability resulting from the conjugated core unit and that the higher the polarisability anisotropy the higher the nematic phase stability. However, the nematic phase, like the smectic phase, is generated by many alicyclic materials where the cores are constructed solely of alicyclic rings. Hence, the relationship between polarisability and LC phase stability is not so clear cut. Based on the above comments, what type of core combinations give a nematic liquid crystals phase? It is difficult to generalise. Nematic phase

Introduction of Alkyl Terminal Chain

Cynopentylbiphenyl from Biphenyl

Terphenyl from Arylboronic Acid

Lateral Fluoro Substitution

Summary The molecular structure of liquid crystalline materials are indeed very varied while at the same time being very similar in principle and style. The generation of liquid crystal phases is possible in a wide range of structures. However, if a specific type of liquid crystal phase is required to exist over a specific range of temperature, than molecular structure needs to be more carefully considered. Structural, considerations become even more involved when additional requirements in terms of physical properties are added to the list of essential features. Liquid crystals are often said to have simple structures and in some respects this is true. However, liquid crystals are very special materials in terms of their unique combination of properties. Research into liquid crystals is still in its infancy and as new, technically advanced applications for liquid crystals are realised, the materials will require much more complex combinations of structural features. The knowledge of structure-property relationships acquired so far will be invaluable in the design of new materials to satisfy new, advanced applications.

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).