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Design and Properties of Molecular Materials: Liquid Crystals Third Year Course: CHM3T1 Special Chemical Topics Weeks 12-22: Totally 11hrs Totally 9 lectures and 1 hr Review 1 hr Workshop Course Content Thermotropic Liquid Crystals (3hrs) Lyotropic Liquid Crystals (1hr) Contemporary Research in Liquid Crystals (1hr) Design, Synthesis, Properties and Applications of Liquid Crystalline Materials (3hrs) Liquid Crystal Technology (LCT) (1hr) Course Main Learning Objectives What does the term Liquid Crystal Mean? What is the molecular structure of a liquid crystal? What is the bulk structure of a liquid crystal? How do liquid crystals interact with light? How do liquid crystals interact with electric fields? How do liquid crystals interact with surfaces? The Twisted Nematic liquid Crystal Display (TNLCD) Liquid Crystals Display In Our Everyday Life Introduction LCD (Liquid Crystal Display) is used as display panel of To calculators, Liquid Crystals watches, word-processors, monitors, Note PCs, CHM3T1 TVs, and Lecture-1 many household electric appliances. Dr. M. Manickam School of Chemistry With it's light weight,The it has become of a necessity for many mobile University Birmingham [email protected] items as dry batteries have become smaller and longer in life. Outline of Lecture Introduction Historical background Classification of Liquid Crystals Examples of Liquid Crystals Guide to the Nomenclature and General Definitions References 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. What are liquid crystals? What does anisotropic mean? What does mesophase mean? The term mesogens. The different types of liquid crystal and their arrangement General Definitions Liquid crystal state: symbol LC - A mesomorphic state having long range orientational order and either partial positional order or completed positional disorder. Mesomorphic state: a state of matter in which the degree of molecular order is intermediate between the perfect three-dimensional, long-range positional and orientational order found in solid crystals and the absence of long-range order found in isotropic liquids, gases and amorphous solids. Liquid crystal: a substance in the liquid crystal state. Mesophase or Liquid crystal phase: phase that does not possess long-range positional ordering, but does have long-range orientational order. A phase occurring over a defined range of temperature or pressure or concentration within the mesomorphic state. Mesogen (mesomorphic compound): A compound that under suitable conditions of temperature, pressure and concentration can exist as a mesophase. Nomenclature Cr or C or K: crystalline phase N: nematic phase S: smectic phase n: director N*: chiral nematic (cholesteric) Tg : glass-transition temperature I: isotropic ND: nematic discotic phase Colh: hexagonal discotic TN-I for a nematic-isotropic transition Cr 34 N 56 I :designates a compound melting at 34 0C into the nematic phase; at 56 0C it changes into the isotropic phase; normal behaviour Anisotropic Liquid An anisotropic liquid is a liquid, i.e. it has fluidity in much the same way as solvent such as water or chloroform has. However, unlike water or chloroform where there is no structural ordering of the molecules in the liquid, molecules in anisotropic liquid are on average structural order relative to each other along their molecular axis. Liquid Crystals 1. What is a liquid crystal? 2. What is so special about liquid crystals? A liquid crystals is a phase between solid and liquid states (phases) Solidify Melt Liquid Heat Cool Heat Intermediate Phase Cool Crystal Examples Example of a compound that shows no LCs phase Ice Cube heat Water heat liquid water 0 degrees of order solid crystalline water; 3- (dimensional) degrees of order Steam gaseous water 0 degrees of order Example of a compound that shows LCs phases Crystals of a solid organic compound heat 3 degrees of order in solid form heat Looks like milk 1 degree of order 3 degrees of order Crystals of a solid organic compound Nematic liquid crystals phase heat Smectic liquid crystal phase gooey material 2 degrees of order Isotropic liquid 0 degrees of order heat Isotropic liquid 0 degrees of order Liquid Crystals (LCs) What is so special about liquid crystals? LCs are orientationally ordered fluids with anisotropic properties A variety of physical phenomena makes them one of the most interesting subjects of modern fundamental science. Their unique properties of optical anisotropy and sensitivity to external electric fields allow numerous practical application. Finally, liquid crystals are temperature sensitive since they turn into solid if it is too cold, and into liquid if it is too hot. This phenomenon can, for instance, be observed on laptop screens when it is very hot or very cold. Technological Application Where are liquid crystals used? Liquid crystals can be found in the following devices: Digital watches Pocket TVs Gas pumps Parking meters Telecommunications Cell phones and pagers High-speed computing Digital signs Electronic games Personal digital assistants Electronic books Calculators Digital cameras and camcorders Fishfinders Thermometers LCD: Multi Disciplinary Area of Research Preparation of various types of liquid crystalline compounds and characterisation Organic and Material Chemists Physicist LCD based Technological application Electrical& Electronic Engineering Device (manufactures) Technological application Theory, law and various Physical properties A Brief History of LCs In 1888 the Austrian botanist Reinitzer found that cholesteryl benzoate showed two melting points each. The crystal of this material melted at 145.5 oC into a cloudy fluid, which upon further heating to 178.5 oC became clear. Cholesteryl benzoate heat 0 145.5 C heat 178.5 0C What Reinitzer was observing was a liquid crystal phase. Further, investigation were carried out by the German physicist O. Lehmann who observed and confirmed, by using polarised optical microscope “crystal [which] can exist with a softness… that one could call them nearly liquid”. * O R O FLEXIBLE R RIGID O FLEXIBLE O R= CH3 or C6H5 or C13H27 * * * * ** * A Brief History of LCs In 1922 the French scientist G. Friedel produced the classification scheme of LCs, dividing them into three different types of mesogens (materials able to sustain mesophases), based upon the level of order the molecules possessed in the bulk materials. Nematic (from the Greek word nematos meaning “thread”) Smectic (from the Greek word smectos meaning “soap”) Cholesteric (better defined as chiral nematic) In 1973 the discovery of the most technologically and commercially important class of liquid crystals to date: the 4-alkyl-4-’cyanobiphenyl of which an example shown below. This material which constitute the simple common displays found in calculators or mobile phones. NC K C5H11 24oC N 35oC I Types of LCs Different types of molecules can form LCs phases. the common structural feature is that these molecules are anisotropic: one molecular axis is much longer Or wider than another one. The two major categories are: Thermotropic LCs, whose mesophase formation is temperature (T) dependent Lyotropic LCs, whose mesophase formation is concentration dependent Types of Liquid Crystals Liquid crystals Lyotropic Calamitic Thermotropic Polycatenar Nematic (N) Smectic (S) Discotic Banana-shaped Nematic Discotic(ND) Columnar (Col) Thermotropic LCs The essential requirement for a molecule to be a thermotropic LC is a structure consisting of a central rigid core (often aromatic) and a flexible peripheral moiety (generally aliphatic groups). This structural requirement leads to two general classes of LCs. Calamitic LCs and Discotic LCs Both of which have molecular subclasses. Calamitic LCs Calamitic or rod-like LCs are those mesomorphic compounds that possess an elongated shape, responsible for the formanisotropy 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 Types of Calamitic LCs Calamitic LCs can exhibit two common types of mesophases: Nematic and Smectic. Nematic phase The least ordered mesophase (the closest to the isotropic liquid state) is the nematic phase, where the molecules have only an orientational order. The molecular long axis points on average in one favoured direction referred to an the director . The classical examples of LC displaying a nematic mesophase in the cynobiphenyl R CN Molecules possess Orientational order Least ordered mesophase The molecules are oriented on average, in the same direction referred to as the directed, with on positional ordering with respect to each other Cartoon representation of N Phase. Smectic phases The next level of organisation is classified as smectic (S), where in addition to the orientational order the molecules possess positional order, such that the molecules organise in layered structures. The S phase has many subclasses, of which are illustrated . Molecules possess Orientational order Positional order Layered structures Cartoon representation of (a) the SA phases, and (b) the SC phase Smectic phases Example: 4,4’’’ – Bis-nonyloxy-[1,1’;1’’;4’’,1’’’] quaterphenyl (2) exhibiting SA and SC phases. The presence of such an extended aromatic core, characterised by a large pi system, is responsible for the establishment of lateral stacking interaction between adjacent molecule, resulting in a layered organisation (SA and SC) 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. As can be seen, the molecular diameter (d) is much greater than the disc thickness (t), imparting the formanisotropy to the molecular structure. Cartoon representation of the general shape of discotic LCs, where d >>t Discotic LCs In 1977, a second type of mesogenic structure, based on discotic (dis-shaped) molecular structure was discovered. The first series of discotic compounds to exhibit mesophase belonged to the hexa-substituted benzene derivatives synthesised by S. Chandrasekhar et al Molecular structure of the first series of discotic LCs discovered: the benzene hexa-n-alkanoate derivatives Types of Discotic LCs Discotic LCs, as well as calamitic LCs, can show several types of mesophases, with varying degree of organisation The two principle mesophases are: nematic discotic, and columnar Nematic Discotic LCs Nematic discotic (ND) is the least ordered mesophase, where the molecules have only orientational order being aligned on average with the director as illustrated . Least ordered mesophase Molecules have only orientational order There is no positional order. Cartoon representation of the ND phase, where the molecule 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 Figure. Cartoon 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, and (c) representation of Colh. Discotic Liquid Crystals Molecular structure of some discotic mesogens 2,3,6,7,10,11-hexakishexyloxytriphenylene 70 oC K Porphyrin metallomesogen 100 oC Colh I K 184 oC Colh 273 oC I Polycatenar Liquid Crystals Polycatenar mesogens represent a hybrid class of thermotropic LCs, which can be described with intermediate molecular features between classic rod-like and disc-like mesogens. The central core of polycatenar LCs comprises a calamitic region, with halfdiscs on the extremities. This hybrid molecular structure allows both calamitic and columnar phases to be generated, depending on the molecular structure of the compounds. Polycatenar molecules possess a number of flexible alkyl chain substituents, which varies from two to six. Metallomesogens Metal containing liquid crystals are called metallomesogens. First of all, the possibility to obtain mesomorphism in compounds that exhibit geometries which are forbidden for pure organic structures, i.e. squar planar, octahedral, etc. Secondly, the possibility to find new technological applications induced by the presence of metal ions in the complexes such as colour, polarizability, paramagnetism, etc. Moreover, the development of bimetallic mesogens constitutes a further step in the field of magnetic and conductive materials since these types of compounds can exhibit paramagnetism and/or mixed oxidation states properties. Phthalocyanine Porphyrins Lyotropic LCs Lyotropic LCs are two-component systems where an amphiphile is dissolved in a solvent. Thus, lyotropic mesophases are concentration and solvent dependent. The amphiphilic compounds are characterised by two distinct moieties, a hydrophilic polar“ head” and a hydrophobic “tail”. Examples of these kinds of molecules are soaps (Figure-a) and various phospholipids like those present in cell memberanes (Figure-b). [a] [b] Suggestions For Further Reading Bahadur, B., editor. Liquid Crystals: Applications and Uses. Volumes 1, 2, and 3. World Scientific Publishing, 1990. Chandrasekhar, S. Liquid Crystals, 1 and 2nd edition. Cambridge University Press, 1992. Collings, P. J., and M. Hird. Introduction to Liquid Crystals: Chemistry and Physics. Collings, P. J., and J. S. Patel, editors. Handbook of Liquid Crystal Research. Oxford University Press, 1997. R. J. Bushby and O. R. Lozman, Current Opinion in Colloid and Interface Science (2002) 7, 343 – 354. S. Kumar, Liquid Crystals, (2004), 31, 1037 – 1059. Final Comments Any study of liquid crystals must concern itself with both the chemistry and physics of this state of matter. The most important areas of liquid crystal chemistry are the relationship between molecular structure and the properties of the various liquid crystal phases, the many methods for synthesising liquid crystalline compounds and the behaviour of amphiphilic molecules and polymers that from liquid crystal phases. Liquid crystal physics concentrates on the mechanical, electrical, magnetic, and optical properties of the various liquid crystalline phases, the different theoretical models that describe both liquid crystal phases and the transition between them, and the behaviour of liquid crystals in technical devices.