Liquid Crystals as a Stereochemical Artefact: Toward a New ...
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board members and 3.5 million followers to its credit. OMICS Group has organized 500 conferences, workshops and national symposiums across the major cities including San Francisco, Las Vegas, San Antonio, Omaha, Orlando, Raleigh, Santa Clara, Chicago, Philadelphia, Baltimore, United Kingdom, Valencia, Dubai, Beijing, Hyderabad, Bengaluru and Mumbai. Liquid Crystals as a Stereochemical Artefact: Toward a New Paradigm Sosale Chandrasekhar Department of Organic Chemistry Indian Institute of Science Bangalore 560 012 India Classification Liquid crystals Thermotropic Lyotropic
Current View of Liquid Crystals Intermediate state of matter (mesophase) Possess some of the characteristics of both solids and liquids Order and orientation (solids) Fluidity (liquids) Molecular level interpretation of properties in terms of phases (nematic, smectic, etc.) A Hierarchy of Order Crystalline solid Smectic A Smectic C Cholesteric Nematic Isotropic Liquid Electro-optic Effects
Liquid crystalline thin films respond to an electrical field This changes the consistency of the film: more or less dispersed medium Affects light transmission: transparent vs. cloudy DEVICE DISPLAYS! PROBLEMS! Liquid crystals (LCs) are not easy to study They are neither liquids nor solids They are also unstable in dilute solution So, normal spectroscopic and related techniques not directly applicable Current view of LCs based on theoretical speculation and indirect experimental evidence! An Early Break! Cholesteryl benzoate Ph
O O Fortuitously the first LC! 1888 Two mp's: 145 & 178 oC Let There Be (Polarized) Light! Cholesteryl benzoate stimulated interest in the effects of polarized light on LCs, whether cholesteric or not! MeO O N N NC OMe
Crossed-polarizers & LCDs V P P S TRANSPARENT V CLOUDY M Twisted Nematics Prepared by mechanical action (rubbing) RUB Once generated thus, resulting chirality stable
over time Stable to electrical and magnetic effects which are also reversible The Rub! Can mechanical action generate chirality? Even if so, would it be stable to thermal effects over time? What, indeed, is the origin of the Maugin effect? Dealing with an enormous level of molecular order far exceeding even that of solids! Not easy to grow single crystals of comparable size Twisted Nematics Liquid crystals for twisted nematic display devices G. W. Gray and S. M. Kelly J. Mater. Chem., 1999, 9, 20372050 What?..Why?..How?...
The Physical Organic Chemist as Devils Advocate! Stereochemistry to the Rescue! Alternative Possibility Are liquid crystals derived from conglomerates? d c b a b d c b d c
b a a a a a b c d b c d a
a d c b CRYSTALLINE R A C E M A T E c d b c d Conglomerates E.L. Eliel, S.H. Wilen, L.N. Mander, Stereochemistry of Organic Compounds, John Wiley, New York, 1994, p 159 Spontaneous Resolution
Achiral molecules can form chiral crystals! O Ph Ph H NaClO3 O O H O Benzil Sodium chlorate O
H 3N O Glycine O O O Succinic anhydride Hydrogen peroxide PhOH Phenol A Nanoparticle Dispersion. A partially molten conglomerate a
liquid n a n c d b a liquid b a b p c d a
o a r t c d b d c l i c e b liquid liquid
a c d Compatibility Effect on polarized light + + + + + + + + + + + + _ _ _ _ _ _ _ _ CRYSTAL
* CLOUDY Coagulation to minimize surface charge Twisting??... A nanoparticle dispersion : a twilight zone . . . . . . . . . . . . MACROSCOPIC ~ 10-1 m
TWILIGHT ZONE ~ 10-7 m SUBMICROSCOPIC ~ 10-10 m Molecular level order in the bulk LC not compatible with fluidity! Supplemental Chirality * * * * * *
* * * * * * * * * * * *
* * * * * * * * * * * * * * *
* * * Twisting imparts helicity to the suspension ??? A LIQUID Crystal! What imparts fluidity to it? Is fluidity compatible with molecular order extending in the bulk liquid? Extended order would exceed that in a crystalline sample! A nano-dispersion can be both fluid and amenable to manual twisting CONCLUSIONS Chiroptical properties of LCs demand a new
approach Conventional view of molecular order extending in the bulk seems unlikely LCs possibly represent a symmetry-broken state Nanoparticle dispersion model compatible with observed optical & electrical phenomena ACKNOWLEDGMENTS OMICS & Conference Organizers Indian Institute of Science THANK YOU! Let Us Meet Again We welcome you all to our future conferences of OMICS Group International Please Visit: http://materialsscience.conferenceseries.com/ Contact us at
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