Super-Molecular Stereochemistry inLiquid CrystalsQ: What’s a Molecule? A: Stereochemistry.Q: What’s a Liquid Crystal?A: Supermolecular StereochemistryChem 6321, Spring 2002David M. WalbaDepartment of Chemistry and Biochemistry, University of Colorado atBoulderANDThe Ferroelectric Liquid Crystal Materials Research Centerhttp://anini.colorado.eduThe Twisted Nematic CellCharles Mauguinu Mauguin, C. "Sur les cristaux liquides de Lehmann," B.Soc. Franc. Miner. 1911, 34, 71.u Mauguin, C. "Orientation of Liquid Crystals by MicaSheets," Compt. Rend. 1913, 156, 1246-7.Mauguin FollowingThe output polarization follows the orientation ofthe top platePhoto-Switching in a Hybrid CellWhat’s a Molecule? What’s a Liquid Crystal?u At a fundamental level, a mole-cule is simply a collection ofatoms caught in a well on theconfigurational hypersurface,with a time-average structure andsymmetry.u These systems are inherentlydynamic in nature.GConfigurationn-decaneIso-decaneC10H22Liquid Crystals are Super-MoleculesGTslopes = -SMore disorderedmolecule/phaseMore ordered molecule/phaseu Phases and molecules are basically the same thing.u Phases are super-molecules; a large collection of atomscaught in a well on the hypersurface with a "time average"structure and symmetry, but in this case not all of theatoms are covalently bonded.u Key difference: Phases have a collective free energy!K = e-ÎG/RTG = H - TSG = free energy/molecule * some # of molecules (the “collective volume”).There are Many LC “Thermotropic Isomers”GTNIslopes = -STANTCAn^n^^zn^^zIsoNSmASmCIsoNSmASmCu The classic INAC phasesu The SmC is the most ordered,with C2h symmetryBasic Geometry of the SmCznC2hThe projection of thedirector n on the layerinterface below it hasnematic order. Thispolar vector is termedthe c-director.cLayer Plane ≡ Normal to electrondensity modulation (X-ray)Tilt Plane ≡ Normal to C2Potpourri of LC MoleculesC NNNOOOOONO2ORORORORROROCyanobiphenyls (Nematics)Phenylpyrimidines (Tilted Smectics)o-Nitrophenyl-biphenylcarboxylates (Chiral Tilted Smectics for NLO)Discotics (Columnar)OOOONNORROBanana Mania!N and SmAINACIACI-N-“SmB”The SmC Phase is NonpolarC∞v or lowerMolecules with polar symmetry can exhibit adipole moment.The SmC phase has a two foldsymmetry axis perpendicular to thetilt plane, and a mirror plane parallelto the tilt plane giving C2h symmetry,which is nonpolar.n^^zSmCTrans-2-butene is a simplemolecule with C2h