Post on 29-Jan-2016
description
Where are we with the discovery and design of biaxial
nematics?
Geoffrey Luckhurst
School of Chemistry, University of Southampton, UK
CH2C12H25O
CH2C12H25O O
O
CH2OC12H25O CO2 CO2 OC12H25
C10H21
OO
H
OC2H5
C10H21
O
O
H
OC2H5
Cu
OC12H25
OC12H25H25C12O
OC12H25
H25C12O
H25C12O
C6H13O
C6H13OOC6H13
O
O H
C6H13O
C6H13OOC6H13
O
OH
OO
OO
O
O
O
O
Praefcke, Kohne, Singer, Demus, Pelzl,
Diele, Liq. Cryst., 1990, 7, 589 Li, Percec and Rosenblatt, Phys. Rev. E, 1993, 48, R1
V-shaped molecules: X-ray scattering
B. R. Acharya, A. Primak, and S. Kumar Phys. Rev. Lett. 2004, 92, 145506B. R. Acharya, A. Primak, T. J. Dingemans, E. T. Samulski, S. Kumar, Pramana, 2003, 61, 231
The molecules
The scattering patterns
Calculated scattering patterns
V-shaped molecules: structure and optical studies
V. Görtz and J.W. Goodby
BLCS, Exeter March 2005
N N
OO O
C7H15
OO
C7H15
N N
OO O
OC12H25
OO
C12H25O
ODBP-Ph-C7
ODBP-Ph-OC12
Iso 204 N 193 SmC 184 SmX 148 SmY 141 SmZ 104 Cr
Iso 222 N 173 SmX 166 SmY 148 Cr
Thermotropic Biaxial Nematic Liquid Crystals
Features:● core with high bisecting
dipole
● rigid bent-core molecule
(~140°)
● biaxiality revealed in 2D
powder
2H NMR and X-ray diffraction
Drawbacks:● core with high dipole
● bend molecule with rigid
core
● i.e. nematic at
inexpediently
high temperatures
● materials degrade at these
high temperatures
● L.A. Madsen, T.J. Dingemans, M. Nakata, E.T. Samulski, Phys. Rev. Lett. 92, 145505 (2004).
● B.R. Acharya, A. Primak, S. Kumar, Phys. Rev. Lett. 92, 145506 (2004).
Synthesis of Oxadiazoles
OH
O
BnO
F
F
F
F
F
HO
O
O
BnO
F
F
F
F
F NHNH2
O
HONH
O
BnO
HN
O
OH
N N
OBnO OH
OH
O
R1
N N
OBnO O
R1
ON N
OHO O
R1
O
OH
O
R2N N
OO O
SOCl2
EDAC, DMAP,DCM
1
anhydr. DMF
2 3
pyridine
EDAC, DMAP
4 R1 = C12H25O5 R1 = C7H15
Pd/(C), H2,THF / EtOH
EDAC, DMAP
O OR2
R3R4
R1
R3
R4
6 R1 = C12H25O7 R1 = C7H15
8a - h
No R1 R2 R3 R4 Phase Transitions [°C]
8a C12H25O C12H25O H H Iso 203 N 192 SmC 184 SmX 143 SmY 138 SmZ 104 Cr
8b C12H25O C9H19O H H Iso 210 N 182 SmX 157 SmY 149 SmZ 91 Cr
8c C12H25O C8H17O H H Iso 213 N 176 SmX 162 SmY 152 SmZ 77 Cr
8e C12H25O C9H19O H F Iso 205 N 168 SmX 135 SmY 125 SmZ 72 Cr
8f C12H25O C9H19O F F Iso 210 N 197 SmC 186 SmX 155 SmY 150 SmZ 100 Cr
8g C7H15 C7H15 H H Iso 222 N 173 SmX 151 Cr
8h C7H15 C5H11 H H Iso 232 N 164 SmX 149 Cr
8d C12H25O C5H11 H H Iso 215 N 160 SmX 91 Cr
Textures of the Biaxial Nematic Phase
despite the achiral molecular structure
chiral domains in the nematic phase!
texture of the nematic phase between slide and coverslip at 222 °C observed by
rotating the analyser (a) anticlockwise (b) clockwise
N N
OO O C7H15
OO
C7H15
ODBP-P-C7 Iso 222 N 173 SmX 151 Cr
schlieren texture of thenematic phase at 202 °C
Textures of the Nematic Phase N N
OO O OC12H25
OO
C9H19O
C9O-P-ODBP-P-OC12
Iso 210 N 182 SmX 157 SmY 149 SmZ 91 Cr
texture of the nematic phase between slide and coverslip at 202 °C observed by
rotating the analyser (a) anticlockwise (b) clockwise
Cr 98 °C (X 80 °C N 95 °C) I
FF
F
OO
O O
O O
O O
C8H17O OC12H25Cr 78.6 °C (B1 59.2 °C) N 97.2 °C I
Cl
OO
O O
O O
O O
C12H25O OC12H25
● G. Pelzl, A.Eremin, S.Diele, H. Kresse, W. Weissflog, J.Mat.Chem. 12,2591 (2002).
● P19: M. Hird, K.M. Fergusson, Synthesis and Mesomorphic Properties of Novel Unsymmetrical Banana-shaped Esters.
Textures of the Nematic Phase N N
OO O C7H15
OO
C5H11
nematicphasein an
uncovered
region on a
glass slide
at 173 °C
C5-P-ODBP-P-C7 Iso 232 N 164 SmX 149 Cr
N N
OO O OC12H25
OO
C9H19O
FF
C9O-2F3FP-ODBP-P-OC12 Iso 210 N 197 SmC 186 SmX 155 SmY 150 SmZ
100 Cr
nematic phasein anuncoveredregion on aglass slideat 167 °C,thinnerpreparation
nematic phasein an uncoveredregion on aglass slideat 189 °C
N N
OO O OC12H25
OO
C12H25O
ODBP-P-OC12 Iso 203 N 192 SmC 184 SmX 143 SmY 138 SmZ
104 Cr
nematic phasein an
uncoveredregion on aglass slide
at 189 °C
Possible Explanations: Suggestion I
● G. Pelzl, A.Eremin, S.Diele, H. Kresse, W. Weissflog, J. Mat. Chem. 12, 2591 (2002).
R. Memmer, Liq. Cryst. 29, 483 (2002).
helical superstructure in a nematic phase of an achiralbent-core molecule can occur due to conical twist-bend deformations
possible twisted chiral conformer
Possible Explanations: Suggestion II
helix-formation via self-assemblyof twisted conformers
N N
OO O R
OO
R
Questions
● Are pitch lines really observed in the nematic?
● Are similar effects to be expected for all achiral bent-core materials that have a nematic phase?
● Is there a connection between these observations and the biaxiality of a nematic phase?
V-shaped molecules: atomistic simulations
M. WilsonBLCS, Exeter, March 2005
• 4 key dihedrals with low barriers where rotation leads to conformations with radically different structures at a cost of < 2.5 kcal/mol
Bananas are not really bananas!
Bananas are not really bananas!
• 4 key dihedrals with low barriers were rotation leads to conformations with radically different structures at a cost of < 2.5 kcal/mol
Min 90/-90 degBarrier 5 kJ/mol
Min 0/180 degBarrier kJ/mol Min 90/-90 deg
Barrier 5 kJ/mol
Bulk phase – biaxial?• Fully atomistic
simulation of biaxial phase at 468 K
• 256 molecules, 3 ns• Colour coding (according
to direction of dipole across central ring)
(Red + along short axis director
blue – along short axis director)
• Looks like the formation of biaxial domains but not biaxial phase?
Bulk phase – biaxial?• Fully atomistic
simulation of biaxial phase at 468 K
• 256 molecules, 3 ns• Colour coding (according
to direction of dipole across central ring)
(Red + along short axis director
blue – along short axis director)
• Looks like the formation of biaxial domains but not biaxial phase?
Tetrapodes: The orientational order parameters from IR
spectroscopy
K. Merkel, A. Kocot, J. K. Vij, R. Korlacki, G. H. Mehl and T. MeyerPhys. Rev. Lett. 2004, 92, 145506
Orientational Order Parameters
XYZ phase principal axes xyz molecular principal axes
Major order parameter
Molecular biaxiality
Phase biaxiality
Molecular and phase
biaxiality
ZZzzSS
ZZyy
ZZxx SSD
YYzz
XXzz SSP
)()( YYyy
YYxx
XXyy
XXxx SSSSC
Y
X
Z
x
y
z
ZZzzSS
ZZyy
ZZxx SSD
YYzz
XXzz SSP
)()( YYyy
YYxx
XXyy
XXxx SSSSC
Order Parameters
S
P/√6
D/√6
C/6
Tetrapodes: NMR studies
J. L. Figueirinhas, C. Cruz, D. Filip, G. Feio, A. C. Ribeiro, Y. Frère and T. Meyer, G. H. MehlPhys. Rev. Lett. 2005, 94, 107802
Molecular structure and organisation
NMR studies
zzyyxx qqq ~)~~(~ ZZzz
YYzz
XXzz SSS )(
~
Molecular field theory of biaxial nematics: Relation to
molecular structure
Potential of mean torque
Uniaxial molecule – uniaxial phase
Derivation:
a)Truncated expansion of the pair potential
b)Variational analysis via dominant order
parameter
z
Z
Z phase director z molecular symmetry axis
β
)(cos)( 22200 PPuU
Potential of mean torque
Biaxial molecule – uniaxial phase
Molecular biaxiality or
n,m
n2m2mn2 )(CCu)(U x
y
z
Z
Z phase director xyz molecular symmetry axes
β
200u )u(u 202220 222u
200220 uu 200222 uu
Potential of mean torque
Biaxial molecule – biaxial phase
No new parameters
pnm
npnmpm DDuU,,
222 )()(
XYZ phase directors xyz molecular symmetry axes
x
y
z
Z
Y
X
β
Parameters and molecular structure
Straley, Phys.Rev.A, 1974, 10, 1881
u200 = {– 2B(W2 – L2) – 2W(L2 + B2) + L(W2 + B2) + 8WBL}/3
u220 = (L2 – BW)(B –W)/√6
u222 = – L(W – B)2/2
n.b. Does not obey the geometric mean rule.
L
B W
Separability: Molecular field parameters
Relation to molecular properties
u2mn = u2mu2n
Geometric mean approximationu220 = (u200u222)½
Principal axis system u20 = (2uzz – uxx – uyy)/√6u22 = (uxx – uyy)/2
Analogy to dispersion forces contrast to excluded volume
(Luckhurst, Zannoni, Nordio and Segre, Mol Phys., 1975, 30, 1345)
Segmental interactions
Segmental anisotropy ua
u20 = ua(1 – 3cos)/2u22 = (3/8)½ua(1 + cos)/2
Biaxiality parameter
= u22/u20
= (3/2)½(1 + cos)/(1 – 3cos)GeneralUniaxial segments
Biaxial segments
x
y
z
i
iimm uCu 2022 )(
ni
ininmm uDu
,2
22 )(
Surface tensor model
u20 = (2LB – B2)(1 – 3cos)/2 + B2cos(/2)(1 + sin(/2)u22 = (3/8)½ (2LB – B2)(1 + cos) – 2B2cos(/2)(1 – sin(/2))
n.b.
u200 = u20u20
u220 = u22u20
Landau point shifts from ~109º to 105º
Acknowledgements
John Goodby
Verena Görtz
Mark Wilson
Daniel Jackson