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 LCswere 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– Orientational 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

(Observed upon heating and cooling)

Lyotropic mesophases

(Observed upon solvation)

Calamitic (rod-like) liquid crystals

crystalline phase(Cr)

smectic phases(Sx)

nematic phase(N)

isotropic liquid phase(I)

T1

T1

T2

T2

T3

T3

Increasing temperature, decreasing order

N

NOO

CNExamples:

Discotic liquid crystals

Rectangular columnar (Cold)

Hexagonal columnar (Colh)

Nematic Isotropic liquid

OC6H13C6H13O

C6H13OOC6H13

OC6H13

OC6H13

N N

NH

HNN N

N N

R R

R

R

R R

R

R

Decreasing order

Examples:

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) (Colh)

Differential scanning calorimetry

Determination of phase-transition temperatures and enthalpies

X-ray diffraction

} layer spacing

Intercolumnardistances

Polymorphism in liquid crystal phases

• Given mesogen can exhibit one or more mesophases• Order 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

N

NOC8H17C4H9OCNC5H11 CNC12H25O

Nematic phase Smectic A phase Nematic, smectic A, and smectic C phases

Polymorphism in liquid crystal phases

C10H21OO

O

O

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

OO

O H NO

OO

Nematic and smectic phases

J. Am. Chem. Soc. 1989, 111, 8533 (Kato & Fréchet)

N

N

N

RO

RO

OO

O

O

R'

R'O

NN

H

H

O

ON

R'''O

R''

H

H

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 derivativesStudied 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

PS

helicalpitch

PSSC phase

Chiral dopant

Surfacestabilization

SS-FLC

CholestericCholesteric liquid crystalsliquid 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 cholestericphase

• 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, microsegregationof 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 liquid-crystal 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

Takashi KatoJean Fréchet