supermolecular stereochemistry in liquid crystals david m ...bent-core mesogens (smectic and...
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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