More on spontaneous reflection symmetry breaking
•
Starting with achiral materials, you get...
• A conglomerate (mixture of macroscopic chiral domains) - the Pasteur result
in organic crystals, and the B2 story in smectic fluids
• Phenomena or materials that are chiral, and all the same handedness a.k.a. parity violation.
The Standard Model incorporates parity violation by expressing the weak interaction as
a chiral gauge interaction. Only the left-handed components of particles and righthanded components of antiparticles participate in weak interactions in the Standard
Model. This implies that parity is not a symmetry of our universe, unless a hidden mirror
sector exists in which parity is violated in the opposite way.
•
Suddenly, after publication of the B2 story, people were looking for,
and finding spontaneous reflection symmetry breaking all over the
place!
Flood of reports of SRSB
•
Evidence for novel spontaneous reflection symmetry breaking
• Kishikawa 2005 JACS (conformational chirality hypothesis extended to achiral
calamitics: A simple phenylbenzoate gives a SmC conglomerate!)
• Niori 2004 MCLC (nematic conglomerate from an achiral bent-core mesogen)
• Takezoe and Watanabe [Thisayukta 2002] JACS (doping an achiral bent-core
mesogen into a chiral nematic tightens the pitch)
• Takezoe [Takanishi 1999] Angew. Chem. Internat. Ed. (spontaneous deracemization of enantiomers - a SmC conglomerate)
• Torgova 1998 Liq. Cryst. (nematic conglomerate from achiral mesogen)
• Takezoe and Watanabe [Sekine 1997] Jpn. J. Appl. Phys. (conformational chirality
hypothesis for reflection symmetry breaking in bananas)
•
Evidence for parity violation
• Goodby [Cowling 2005] Adv. Mater. (unichiral FLC EO from a racemate)
• Goodby [Cowling 2005] Chem. Commun. (unichiral FLC EO from a racemate)
• Goodby [Hird 2001] J. Mater. Chem. (unichiral FLC EO from an achiral mesogen)
Weird Observation from the ‘90s
P
1) DEAD, Ph3P, HO
BzO
C6H13
OH
C6 H1 3
HO
O
2) LiOH, H2O
NO2
P
NO2
O
C1 0H21O
OH
DCC, DMAP
• We were shocked to discover that samples of
racemic 2-octanol led to W314 samples
showing unichiral electro-optics in SSFLC cells!
• Response equivalent to ~0.3% ee
• The unichiral EO disappeared when authentic
racemic 2-octanol was prepared by NaBH4
reduction of 2-octanone
O
C6 H1 3
C1 0H21O
O
(S)-W314
O
NO2
P ~ -550 nC/cm2
~ -1.5 D/molecule
SSFLC switching is a very sensitive detector of chirality
3
Flood of reports of SRSB
•
Evidence for novel spontaneous reflection symmetry breaking
• Kishikawa 2005 JACS (conformational chirality hypothesis extended to achiral
calamitics: A simple phenylbenzoate gives a SmC conglomerate!)
• Niori 2004 MCLC (nematic conglomerate from an achiral bent-core mesogen)
• Takezoe and Watanabe [Thisayukta 2002] JACS (doping an achiral bent-core
mesogen into a chiral nematic tightens the pitch)
• Takezoe [Takanishi 1999] Angew. Chem. Internat. Ed. (spontaneous deracemization of enantiomers - a SmC conglomerate)
• Torgova 1998 Liq. Cryst. (nematic conglomerate from achiral mesogen)
• Takezoe and Watanabe [Sekine 1997] Jpn. J. Appl. Phys. (conformational chirality
hypothesis for reflection symmetry breaking in bananas)
•
Evidence for parity violation
• Goodby [Cowling 2005] Adv. Mater. (unichiral FLC EO from a racemate)
• Goodby [Cowling 2005] Chem. Commun. (unichiral FLC EO from a racemate)
• Goodby [Hird 2001] J. Mater. Chem. (unichiral FLC EO from an achiral mesogen)
Spontaneous deracemization in a calamitic SmC*
POL
ANA
n(dark)
z
n
E⊗
E⊙
•
Looks like ferroelectric chiral
EO from the racemate
•
Proposed to be due to
spontaneous partial resolution
of the racemate
Takanishi, Y.; Takezoe, H.; Suzuki, Y.; Kobayashi, I.; Yajima, T.; Terada, M.; Mikami, K. "Spontaneous enantiomeric
resolution in a fluid smectic phase of a racemate," Angew. Chem. Int. Ed. 1999, 38, (16), 2353-2356.
The first spontaneous resolution of a fluid racemate?
The first spontaneous resolution of a fluid racemate?
Behavior of authentic racemic W314 is similar
u
For W314 P ∝ ee
The EO response of authentic
rac-W314 is complex
– After a few hours under drive, chiral
Pexp (nC/cm2)
domains can be observed
– After a month under drive, the entire
sample segregates into a pair of
heterochiral domains
ee
e
Hetereochiral domains
formed in rac-W314
Determination of ee in
domains from careful
risetime measurements
assume η doesn’t change with ee
eess ~ 4%
1 ⎛ P ⎞
= ⎜ ⎟ E
τ ⎝ η ⎠
Kane 2007
8
PE can drive partial deracemization (~4% ee max)
C6H13
P>0
O
NO2
O
G = -PS (ee) E
near ee = 0
“extras” in a smectic C
z
G
entropy of
mixing
ee
l E
ee > 0
O
P<0
z
Pink - tilt left
Blue - tilt right
With field UP
Electrostatic
for P>0
G
l E
ee < 0
OC10H21
eess = pE/ kBT
p = PS /molecule
The electrostatic free lowering from deracemization is quantitatively equal to
the free energy increase from the entropy cost of demixing ➨
there is no enthalpic advantage to deracemization, and no
measurable tendency for conglomerate formation in this SmC*
Kane 2007
9
What happens if you take the field off? Diffusive broadening
Summary
• For W314, E field drives partial
deracemization
• The electrostatic free energy
gain from polarization in the field
exactly balances the entropic
free energy loss of separating the
enantiomers
• Takezoe’s TFMHPOBC with two
stereocenters similar? If so, the
enantiomers separate much
faster in the field
Kishikawa’s evidence that dioctyloxyphenylbenzoate give a SmC*
•
In a very dramatic interpretation of experimental data using the
conformational chirality hypothesis, Keiki Kishikawa published in JACS that
a simple dialkoxyphenylbenzoate (8̅O8̅) gives a chiral SmC* phase
O
O
O
8̅O8̅
O
On heating: X — 63.3 — SmC — 73.8 — N — 93.1 — I
On cooling: X — 51.2 — SmC — 72.4 — N — 90.5 — I
•
•
Parallel-aligned cells show SmC* helical pitch lines
•
Homeotropic cells show circular dichroism in the
range 300-310 nm
Optically active domains in homeotropicallyaligned cells
•
Doping of 8̅O8̅ into an achiral host causes the
helix pitch to increase in parallel aligned cells
•
Doping of 8̅O8̅ with a chiral dopant enhances the
CD signal, and increases the are of one
handedness of the optically active domains
observable in homeotropic cells
Kajitani, T.; Masu, H.; Kohmoto, S.; Yamamoto, M.; Yamaguchi, K.; Kishikawa, K. "Generation of a chiral
mesophase by achiral molecules: Absolute chiral induction in the smectic c phase of 4-octyloxyphenyl 4octyloxybenzoate," J. Am. Chem. Soc. 2005, 127, (4), 1124-1125.
Walba, D. M.; Korblova, E.; Huang, C. C.; Shao, R. F.; Nakata, M.; Clark, N. A. "Reflection symmetry breaking in
achiral rod-shaped smectic liquid crystals?," J. Am. Chem. Soc. 2006, 128, (16), 5318-5319.
•
Optically active domains in homeotropically-aligned cells. Sometimes you see them, some times you don’t.
• According to Kishikawa, clean glass slides give homeotropic alignment
(water contact angle ~ 0°). We found this irreproducible (we get parallel
alignment).
• But, with Matsunami Glass Ind., LTD sides used by Kishikawa (water
contact angle ~45° out of the box) we get homeotropic alignment
sometimes showing a schlieren texture, and sometimes showing optically
active domains
• Hypothesis - this is due to pinning of the c-director at the surface, which is
seen on parallel-rubbed SAMs (the Charles Maughan experiment)
• A helical LC structure is well known to show CD as well
Kishikawa’s twist domains
•
Parallel-aligned cells show SmC* helical pitch lines. Ah yes, stripes must mean a helix - NOT
• These stripes are easily reproduced with
8̅O8̅ at a free surface with air (in
the paper, it’s air bubbles in the cell
giving rise to the stripes
• These are splay stripes, which have long
been known to occur at SmC free
surfaces at air
• 8̅O8̅ shows especially prominent splay
stripes - this is likely a factor in the missassignment
LC
Air
•
Doping of 8̅O8̅ into an achiral host causes the helix pitch to increase in parallel aligned cells
•
•
The “helix pitch” is the pitch of the splay stripes. I’m not sure if this has anything to do with whether the
host is achiral
Doping of 8̅O8̅ with a chiral dopant enhances the CD signal, and increases the are of one handedness of the
optically active domains observable in homeotropic cells
•
Adding the chiral dopant creates a SmC* phase, which, in a homeotropic cell, should have a spontaneous
helix without the surface anchoring. Perhaps this helix is fairly tight, e.g. more than a 180° twist in a cell.
Splay stripes in freely suspended films of 8̅O8̅
• 8̅O8̅ shows especially prominent splay stripes
- a Schlieren texture is seen - note the 2π wall.
Michi Nakata photomicrographs
• As an independent test for chirality, the EO
behavior of 8̅O8̅ was studied
• A dielectric effect was seen - but no chiral EO
• The cell shows nominal SmC behavior with higher field
• Due to the chevron layer interface and negative dielectric anisotropy, application
of a high field causes boat wake defects, which evolve into quasi-bookshelf stripes
Clark, N. A.; Rieker, T. P.; Maclennan, J. E. "Director and layer structure of ssflc cells," Ferroelectrics 1988, 85, 79-97.
Quasi-bookshelf alignment
Chevrons
Skarp, K.; Andersson, G.; Hirai, T.; Yoshizawa, A.; Hiraoka, K.; Takezoe,
H.; Fukuda, A. "Investigations of soft-mode and electroclinic response in
a ferroelectric liquid-crystal with ps-approximate-to-5 mc/m2," Jpn. J.
Appl. Phys. Pt 1 1992, 31, (5A), 1409-1413.
Quasi-bookshelf
Sato, Y.; Tanaka, T.; Kobayashi, H.; Aoki, K.; Watanabe, H.; Takeshita, H.; Ouchi, Y.;
Takezoe, H.; Fukuda, A. "High quality ferroelectric liquid crystal display with quasibookshelf layer structure," Jpn. J. Appl. Phys., Part 2 1989, 28, (3), L483-L486.
To understand quasi bookshelf, you need to understand the chevron layer structure
χ
Zig-zag walls in an SSFLC cell
χ
Proof of the chevron layer structure by X-ray
z
n
SmA
SmC
Tilt increases with
decreasing temperature
Rieker, T. P.; Clark, N. A.; Smith, G. S.; Parmar, D. S.; Sirota, E. B.; Safinya, C. R. ""Chevron" local layer
structure in surface-stabilized ferroelectric smectic-c cells," Phys. Rev. Lett. 1987, 59, (23), 2658-61.
Zig-zag walls, C1 and C2 chevron alignment
•
•
the director tilts up toward the rubbing direction
C1
C2
High pretilt favors C1
Low pretilt favors C2
Zig-zag walls, coneheads, and C2 chevron alignment
•
•
High pretilt favors C1
Low pretilt favors C2
C1
C2
n sticks at the green circles in the chevron layer
interface, providing the ferroelectric response
Quasi bookshelf alignment (a third way to get stripes)
Layers “stand up” and kink in the plane of the cell
P
E
P
Quasi bookshelf alignment
SmC*
Ferroelectric
Quasi-bookshelf alignment
SmC
Dielectric (∆n < 1)
Quasi-bookshelf stripes
polarizer parallel to n in one of the stripe domains