Liquid crystal

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Slide 1:

Dr. I. Navis Paul Sriganth, Ph.D

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Cell Biology Physical Chemistry, Organic chemistry Microbiology, Molecular Biology Medicinal Chemistry Instrumental methods of analysis, Heat and Mass transfer Biochemical Engineering, Pharmaceutical analysis Pharmaceutics, Pharmacognogy and Pharmacokinetics B.Tech Pharmaceutical Technology

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Cell Biology Physical Chemistry, Organic chemistry Microbiology, Food Chemistry Food Microbiology, and Nutrition Instrumentation methods of analysis, Heat and Mass transfer, Molecular Biology, Biochemical Engineering Food Processing and Preservation integration Process plant equipment and design in food industry B.Tech Food Technology

Specialist:

Specialist Covers areas not covered by other specialty programs: • Organic and Inorganic Synthesis • Organometallic Chemistry & Catalysis • Medicinal & Pharmaceutical Chemistry • Analytical Chemistry • Molecular Devices

Physical Chemistry Specialist:

Physical Chemistry Specialist Explaining the behavior of polymers, fluids and solids, self-assembly of complex systems. Study of biophysical systems: enzyme catalysis, protein structure and dynamics, packing of DNA. Properties of nanoscale systems. Molecular collisions and scattering theory, chemical processes at surfaces. The quantum structure of molecules, photochemistry, heterogeneous kinetics and reaction dynamics.

Why study physical chemistry?:

Why study physical chemistry? Understanding chemical waves in cardiac tissue

Why study physical chemistry?:

Why study physical chemistry? Chemistry of molecular recognition in enzymes

Why study physical chemistry?:

Why study physical chemistry? Molecular dynamics of membrane bilayers

Why study physical chemistry?:

Why study physical chemistry? Drug Interactions

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Liquid Crystals

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Normally, matter to have three distinct states: solid, liquid, and gas. Mesophases (liquid crystals) are a state of order between crystals and liquids. They have imperfect long range orders of orientation and/or position. Thus, the can be fluid like a liquid and they can have anisotropic properties like crystals. The liquid crystals have a predominating orientational order.

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What are liquid crystals ? phenomena was discovered in 1888 by Austrian chemist Frederich Reinitzer.

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Compounds of liquid crystals : lecithin, DNA, cellulose, cholesterol esters, gangliosids, paraffins, graphite. Biologist - cell membrane as a liquid crystal with respect to intercellular transport, drug delivery, and communications. Physicist - measuring the unique properties, e.g. ferroelectricity, piezoelectricity, phase transitions, etc. and developing new ways of electrooptics. Engineers - creating electrooptical displays (LCD), optical information storage, surfactants and lubricants etc. Artists are using the color effects of liquid crystals for special effects.

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Typical applications are: LCD displays research Dyes (cholesterics) Biomembrans Template synthesis of nano materials Temperature measurement (by changing colors) Solvents for GC, NMR, reactions, etc. Cosmetics Drug delivery effect colors and many other research areas

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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.

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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 .

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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) Liquid Crystals

Examples:

Examples Example of a compound that shows LCs phases 3 degrees of order 3 degrees of order in solid form Looks like milk 1 degree of order 0 degrees of order gooey material 2 degrees of order 0 degrees of order Example of a compound that shows no LCs phase solid crystalline water; 3- (dimensional) degrees of order liquid water 0 degrees of order gaseous water 0 degrees of order

LCD: Multi Disciplinary Area of Research:

LCD: Multi Disciplinary Area of Research LCD based Technological application Preparation of various types of liquid crystalline compounds and characterisation Theory, law and various Physical properties Device (manufactures) Technological application

A Brief History of LCs:

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 o C into a cloudy fluid, which upon further heating to 178.5 o C became clear. 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”.

A Brief History of LCs:

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.

Types of LCs:

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

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Types of Liquid Crystals

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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. Thermotropic LCs

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Calamitic LCs Calamitic or rod-like LCs are those mesomorphic compounds that possess an elongated shape, responsible for 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

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Calamitic LCs can exhibit two common types of mesophases : Nematic and Smectic Types of Calamitic LCs

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Nematic Phase Molecules in this phase are polarizable long and rod-like of 20 Angstroms in length. They are free to move in space. The molecules are oriented on average, in the same direction referred to as the directed, with on positional ordering with respect to each other

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Have tendency to organize themselves in a parallel fashion, they demonstrate interesting optical properties, (the digital watch functioned using nematic liquid crystals). The Nematic molecules are free to flow Their center of mass positions are randomly distributed as in a liquid Most nematics are uniaxial

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The Nematics can be easily aligned by an external magnetic or electric field. Aligned nematics have the optical properties of uniaxial crystals and this makes them extremely useful in liquid crystal displays (LCD) However, some are biaxial nematics , (in addition to orienting their long axis, they also orient along a secondary axis) The classical examples of LC displaying a nematic mesophase in the cynobiphenyl

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Chiral Nematic This phase is composed of nematic molecules in a helical structure about the layer normal. The distance that it takes for the molecules to complete one turn is called the pitch.

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Smectic phases Found at lower temperatures than the nematic Form well-defined layers that can slide over one another The smectics are positionally ordered along one direction Smectic A phase - the molecules are oriented along the layer Smectic C phase - tilted away from the layer Many different smectic phases - different types and degrees of positional and orientational order These phases are liquid-like within the layers

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Example: 4,4’’’ – Bis-nonyloxy -[1,1’;1’’;4’’,1’’’] quaterphenyl (2) exhibiting S A and S C phases . The presence of 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 (S A and S C )

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Chiral Smectic In a similar way to chiral nematics there are chiral forms of smectic phases. Figure shows a chiral smectic C material, denoted by smectic C*. The tilted director rotates from layer to layer forming a helical structure.

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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

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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

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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 Types of Discotic LCs

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Nematic Discotic LCs 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 . Cartoon representation of the N D phase, where the molecule are aligned in the same orientation, with no additional positional ordering Least ordered mesophase Molecules have only orientational order There is no positional order.

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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 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 Col r , and (c) representation of Col h .

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Discotic Liquid Crystals Molecular structure of some discotic mesogens 2,3,6,7,10,11-hexakishexyloxy- triphenylene Porphyrin metallomesogen 70 o C 100 o C 184 o C 273 o C

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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 half- discs 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.

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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

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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]