David M. Walba and Noel A. ClarkDepartment of Chemistry and Biochemistry, Department of Physics,

and the Liquid Crystal Materials Research CenterUniversity of Colorado at Boulder

Supermolecular Stereochemistry inLiquid Crystals

Stereochemical Aspects of Novel MaterialsUCSB, August 2005

Introduction to thermotropic liquid crystals

– What’s a liquid crystal?

– What’s a liquid crystal texture (birefringence)?

Some applications of ferroelectric liquid crystals

– LC electro-optics

– The SSFLC light valve and LCOS microdisplays

Molecules can be Very Dynamic

Lots of molecules are veryFLOPPY in the gas or liquidphase - time-average structureand symmetry.

The structure of these moleculesin the liquid phase is inherentlydynamic in nature.

G

Configuration

n-decaneIso-decane

C10H22

G = H - TS

N

N

C9H19 OC6H13 PYP906

The classic INAC phasesThe SmC is the most ordered, with

C2h symmetry ⇒ Nonpolar

C v or lower

What’s a Liquid Crystal?

Collective free energy

Slope = -S

N IsoSmASmC

50 71 7234X

G

TNITANTCA

Potpourri of LC Mesogens

C N

N

NO

OO

OO

NO2

OR

OR

OR

OR

RO

ROCyanobiphenyls (Nematics)

Phenylpyrimidines (Tilted Smectics)

o-Nitrophenyl-biphenylcarboxylates (Chiral Tilted Smectics for NLO)

Discotics (Columnar)

O

O

O

O

NN

ORRO

Banana Mania!

N and SmA

INAC

IAC

I-N-“Bn”

Nematic and Smectic LCs

C N

5CB (nematic)

Cr 24.0 N 35.3 IN

Nematic Schlieren texture(Photo by Mary Neubert)

Smectic A focal conic texture

Cr 63.5º SmC* 93.7º SmA* 116º I

Cr (20°) SmC* 93.7° SmA* 116° I

C10H21OO O

C5H11

O NO2

SmC SmA

W314 (SmC* - SmA*)

Columnar Phases

OR

OR

OR

OR

RO

RO

Discotic Mesogens(columnar phases)

O

O

O

O

NN

ORRO

Bent-core Mesogens(smectic and columnar banana phases)

The B1 columnar banana phase(Photo by Renfan Shao)

Long-range positional orderin two dimensions

Some Techniques used to Study LCs

Polarized light microscopy (PLM)

Electro-optics

Cross-polarization magic angle spinning13C NMR (CP-MAS)

Dielectric spectroscopy

X-ray diffraction (XRD)

Depolarized reflected light microscopyfrom freely suspended films (DRLM)

Freeze fracture transmission electronmicroscopy (FFTEM)

Powder XRD

DRLM from freely suspended films

FFTEMZasadzinskiand Clark

A Polarized Light Microscope

Interference Colors

Water

Oil

Air

These waves interfere

Since the speed of light (refractive index) in airand oil are different, a thin film of oil causes aretardation of one wave with respect to the other

Birefringent Materials Give InterferenceColors in the PLM

All birefringent materials have something in common: Theyare all anisotropic

– Piece of stretched plastic (LC polymers are a special kind of plasticwhich can have very special properties, like kevlar)

– Mica crystal

– Organic crystals

– Liquid crystals - the only birefringent liquids

The color you see is related to the birefringence ( n) and thethickness of the sample (d)

2nd order green 1st order purple

Birefringence Color and Extinction Brushes

When the optic axis of the birefringent sample isparallel or perpendicular to the polarizer in the PLM,you see dark features known as extinction brushes

Electro-optics - Birefringence Changes and Brush Rotation

Robert Meyer - 1974Tilt & Chirality

Polar axis (C2)Collective dipole

Chiral molecules

zn

The SmC* Polar Fluid

Polarity

Between Glass Plates the SmC* is Ferroelectric

=P E

“Single Molecule in a Binding Site” Model for P

C-phase “binding site”

Pi = μi cos

Pi * Dii conformations

P =

Pc

alc

(n

C/c

m2)

Pexp (nC/cm2)

O O

O

F

P > 0 (R) configuration

SSFLC Electro-optics

POL

ANA

2 n

Dark State Bright State

≅ 2μm!

E = Electric field

SSFLC EO is Unichiral

nz

POL ANA

E

nz

E

P > 0 (right-handed)

2 mP

P

SSFLC Switching

Bright and Dark ferroelectric domains

Chiral EO response to anapplied triangular drivingwaveform (the material hasa very small P)

FLC on Silicon (FLCOS) Microdisplays

Unichiral Electro-optics can be Useful

“Display Product of the Year

Gold Award:

Minolta’s DiMAGE 7 Digital Still Camera

In its 5-Mpixel DiMAGE 7, Minolta Corp.has pioneered the use of a liquid-crystal-on-silicon (LCoS) microdisplay…”

Curved PolarizingBeam Splitter (3M)

FLCOS(Displaytech)

8 Millionth shipped (Feb, 2005)

Displaytech/Samsung FLCOS HDTV

One 0.78”Displaytech

FLCOS Chip foreach color channel

Boulder SoundTrack brochure, Fall 2000Failed due to manufacturing problems

Sony VAN-LCOS HDTV - 2005

70” Rear-Projection 1080 p (!) HDTV @ ~$10KVertically-Aligned Nematic LCOS

Apparently shipping in US as of June 2005

SXRD = LCOS

Next time - the liquid crystal version of the most famousexperiment in organic stereochemistry!

A Liquid Crystal Conglomerate?

Sodium Ammonium Racemate

Quartz