CHM 6365 Chimie supramoléculaire Partie 8
Liquid crystals: Fourth state of matter Discovered in 1888 by Reinitzer, who observed two melting points for a series of cholesterol derivatives Subsequent studies by Lehmann led to the notion that the cloudy phase upon initial melting constitutes a new phase of matter Vorländer prepared a large number of liquid crystals (LC) in the early part of the 20th century Mostly of academic interest until the 1960 s, when LCs were shown to be useful as electrooptical switches (for displays)
Liquid crystals: Fourth state of matter Liquid crystalline phases (or mesophases) exhibit properties intermediate between those of crystalline solids and liquids Molecules show significant mobility (rotational and translational) Fluid rientational and (sometimes) positional order Anisotropic properties (molecular shape, birefringence, dielectric anisotropy) This combination of anisotropic optical and electric properties and the mobility of the phase is what makes LCs useful: molecules can reorient themselves in response to stimuli
Liquid crystalline phases Thermotropic mesophases (bserved upon heating and cooling) Lyotropic mesophases (bserved upon solvation)
Calamitic (rod-like) liquid crystals T 1 T 2 T 3 T 1 T 2 T 3 crystalline phase (Cr) smectic phases (S x ) nematic phase (N) isotropic liquid phase (I) Increasing temperature, decreasing order Examples: CN N N
Discotic liquid crystals Rectangular columnar (Col d ) Hexagonal columnar (Col h ) Nematic Isotropic liquid Decreasing order R R Examples: C 6 H 13 C 6 H 13 C 6 H 13 N N N C 6 H 13 R H N N C 6 H 13 R N H N N R R C 6 H 13 R R
Characterization of mesophases Mesophases are typically characterized using the following techniques: Polarized optical microscopy: Provides information on phase transitions and on the phase identity, based on the observed texture Differential scanning calorimetry (DSC): Quantifies phase transitions Powder X-ray diffraction: provides information on symmetry of phases, periodicity (e. g. layer spacing, intercolumnar distances), and orientation Solid-state NMR: Provides information on the degree of order and orientation
Polarized optical microscopy
LC phases by polarized optical microscopy Nematic columnar Smectic A Smectic C Hexagonal (N) (SmA) (SmC) (Col h )
Differential scanning calorimetry Determination of phase-transition temperatures and enthalpies
X-ray diffraction } layer spacing Intercolumnar distances
Polymorphism in liquid crystal phases Given mesogen can exhibit one or more mesophases rder of appearance of different phases as a function of temperature can be predicted based on the degree of order of the phase Impossible to predict how many phases as well as which phases will be present C 5 H 11 CN C 12 H 25 CN C 4 H 9 N N C 8 H 17 Nematic phase Smectic A phase Nematic, smectic A, and smectic C phases
Polymorphism in liquid crystal phases C 10 H 21 Cr 35 SmC 70.5 SmA 72 N 75 I Smectic C Smectic A Nematic
Directed assembly of liquid crystals Rational creation of a mesogen by the association of molecules that are not mesogens Hydrogen bonding to create aggregate with rod-like geometry For a review, see: Angew. Chem., Int. Ed. 2006, 45, 38 (Kato)
Hydrogen-bonded liquid crystals H N Nematic and smectic phases J. Am. Chem. Soc. 1989, 111, 8533 (Kato & Fréchet) R N H N H N R'' R' N N N H H R''' R' R Columnar phase Chem. Commun. 1989, 1868 (Lehn)
Hydrogen-bonded mesophases
Folic acid derivatives Folic acid derivatives can form either hydrogen-bonded ribbons or tetrameric aggregates J. Mater. Chem. 2001, 11, 2875
Folic acid derivatives Studied by X-ray diffraction and polarized optical microscopy
Modulating hydrogen-bonded assemblies Adding cations such as Na + can induce change from smectic to columnar phases by creating ion-dipole interactions
Chirality in LC phases: Cholesteric/ferroelectric liquid crystals Chiral dopant Surface stabilization helical pitch S C phase P S SS-FLC P S
Cholesteric liquid crystals Chiral dopant 5CB 5CB + (S)-binol CN Helical twisting power: Ability of a chiral solute (dopant) to induce a helical twist in the nematic phase
Cholesteric liquid crystals Sign of dopant helicity correlates with helical twist of cholesteric phase Long axis of dopant is oriented with the long axis of the liquid crystal host Proposed mechanism of induction: Subsequent studies have shown that the relationship between dopant structure and twisting power is complicated
Summary and conclusions Liquid crystals are cool Liquid crystallinity arises from a combination of factors, including anisotropic molecular shape, microsegregation of incompatible groups, and a combination of rigid and flexible components Non-covalent interactions such as hydrogen bonding can be exploited to produce new materials whose phase behaviour is controlled by self-assembly Solute molecules (dopants) in liquid crystalline solvents can have a large impact on the properties of the liquidcrystal phase, and the study of these phenomena has analogies with host-guest chemistry
References The Handbook of Liquid Crystals (4 volumes!) (Demus, ed., Wiley-VCH, 1998) Introduction to Liquid Crystals (Collings and Hird, Taylor & Francis Publishers, 1997) Website: http://plc.cwru.edu/tutorial/enhanced/main.htm (Polymers and Liquid Crystals Virtual Textbook)
Supramolecular Chemistry Hall of Fame Jean Fréchet Takashi Kato