Transcript Slide 1

Where are we with the discovery
and design of biaxial nematics?
Geoffrey Luckhurst
School of Chemistry, University of Southampton, UK
C12H25O
CH2O
C12H25O
CH2O
C12H25O
CH2O
OC6H13
O
CO2
CO2
OC12H25
C6H13O
O
H O
H
C6H13O
C6H13O
O
OC6H13
C6H13O
O
O
O
O
O
O
O
Praefcke, Kohne, Singer, Demus, Pelzl, Diele,
Liq. Cryst., 1990, 7, 589
O
Li, Percec and Rosenblatt, Phys. Rev. E, 1993, 48, R1
OC2H5
H
O
O Cu
C10H21
H25C12O
O
O
C10H21
OC12H25
H25C12O
H
OC2H5
OC12H25
H25C12O
OC12H25
V-shaped molecules: X-ray
scattering
B. R. Acharya, A. Primak, and S. Kumar Phys. Rev. Lett. 2004, 92, 145506
B. 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
Thermotropic Biaxial Nematic Liquid Crystals
● 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).
Features:
N N
O
O
C7H15
O
● core with high bisecting dipole
O
O
● rigid bent-core molecule (~140°)
C7H15
ODBP-Ph-C7
2H
Iso 222 N 173 SmX 166 SmY 148 Cr
N N
O
C12H25O
O
O
● biaxiality revealed in 2D powder
NMR and X-ray diffraction
Drawbacks:
● core with high dipole
● bend molecule with rigid core
O
O
OC12H25
ODBP-Ph-OC12
Iso 204 N 193 SmC 184 SmX 148 SmY 141 SmZ 104 Cr
● i.e. nematic at inexpediently
high temperatures
● materials degrade at these
high temperatures
Synthesis of Oxadiazoles
F
HO
F
O
F
O
F
F
O
F
OH
NHNH2
F
F
BnO
N
H
anhydr. DMF
F
BnO
1
HO
O
Pd/(C), H2,
THF / EtOH
BnO
O
N N
OH
O
pyridine
N N
O
3
R1
EDAC, DMAP
4 R1 = C12H25O
5 R1 = C7H15
R2
O
O
EDAC, DMAP
O
OH
R1
R4
O
OH
BnO
O
N N
R3
R3
SOCl2
O
O
6 R1 = C12H25O
7 R1 = C7H15
R4
H
N
2
R1
O
OH
O
HO
O
EDAC, DMAP,
DCM
BnO
F
O
O
R2
R1
O
8a - h
N N
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
O
C7H15
O
NN
O
O
O
schlieren texture of the
nematic phase at 202 °C
ODBP-P-C7
Iso 222 N 173 SmX 151 Cr
C7H15
texture of the nematic phase between slide and
coverslip at 222 °C observed by rotating the
analyser (a) anticlockwise (b) clockwise
despite the achiral molecular structure
chiral domains in the nematic phase!
Textures of the Nematic Phase
O
C9H19O
O
NN
O
O
O
OC12H25
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
● G. Pelzl, A.Eremin, S.Diele, H. Kresse, W. Weissflog, J.Mat.Chem. 12,2591 (2002).
Cl
O
O
O
O
O
O
C12H25O
O
O
Cr 98 °C (X 80 °C N 95 °C) I
OC12H25
● P19: M. Hird, K.M. Fergusson, Synthesis and Mesomorphic Properties of Novel
Unsymmetrical Banana-shaped Esters.
F
OF
O
O
O
C8H17O
FO
O
O
O
Cr 78.6 °C (B1 59.2 °C) N 97.2 °C I
OC12H25
Textures of the Nematic Phase
O
C5H11
O
NN
O
O
O
C7H15
C5-P-ODBP-P-C7 Iso 232 N 164 SmX 149 Cr
nematic
phase
in an
uncovered
region on a
glass slide
at 167 °C,
thinner
preparation
nematic
phase
in an
uncovered
region on a
glass slide
at 173 °C
O
C12H25O
O
NN
O
O
O
F
OC12H25
ODBP-P-OC12
Iso 203 N 192 SmC 184 SmX 143 SmY 138 SmZ
104 Cr
nematic
phase
in an
uncovered
region on a
glass slide
at 189 °C
C9H19O
F O
O
NN
O
O
O
OC12H25
C9O-2F3FP-ODBP-P-OC12
Iso 210 N 197 SmC 186 SmX 155 SmY 150 SmZ
100 Cr
nematic
phase
in an
uncovered
region on a
glass 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 achiral
bent-core molecule can occur due to conical twist-bend deformations
Possible Explanations: Suggestion II
O
R
O
NN
O
O
O
R
possible twisted chiral conformer
helix-formation via self-assembly
of twisted conformers
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. Wilson
BLCS, Exeter, March 2005
Bananas are not really bananas!
• 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!
• 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 deg
Barrier 5 kJ/mol
Min 0/180 deg
Barrier 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. Meyer
Phys. Rev. Lett. 2004, 92, 145506
Orientational Order Parameters
XYZ phase principal axes
xyz molecular principal axes
Z
z
x
y
X
S  S zzZZ
Y
Major order parameter
ZZ
D  S xxZZ  S yy
Molecular biaxiality
P  S zzXX  S zzYY
Phase biaxiality
YY
C  (SxxXX  S yyXX )  (Sxx
 S YY
yy ) Molecular and phase
biaxiality
S S
ZZ
zz
DS
ZZ
xx
S
ZZ
yy
XX
zz
S
YY
zz
PS
C  (S
XX
xx
S
XX
yy
)  (S
YY
xx
S )
YY
yy
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. Mehl
Phys. Rev. Lett. 2005, 94, 107802
Molecular structure and organisation
NMR studies
~
~
~
~
~
  (qxx  qyy ) qzz
XX
YY
ZZ
 (S zz  S zz ) S zz
Molecular field theory of biaxial
nematics: Relation to molecular
structure
Potential of mean torque
Uniaxial molecule – uniaxial phase
z
Z phase director
z molecular symmetry axis
β
Z
U ( )  u200 P2 P2 (cos )
Derivation:
a) Truncated expansion of the pair potential
b) Variational analysis via dominant order parameter
Potential of mean torque
Biaxial molecule – uniaxial phase
z
Z phase director
xyz molecular symmetry axes
β
Z
y
x
U (  )    u2 mn C2 mC2 n (  )
m ,n
u 200
Molecular biaxiality
u220 (  u202 )
u220 u200
u 222
or
u222 u200
Potential of mean torque
Biaxial molecule – biaxial phase
X
XYZ phase directors
xyz molecular symmetry axes
z
β
Z
y
x
Y
U ()    u 2 pm D D
2
nm
m ,n , p
No new parameters
2
 np
( )
Parameters and molecular structure
Straley, Phys.Rev.A, 1974, 10, 1881
B
W
L
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.
Separability: Molecular field parameters
Relation to molecular properties
u2mn = u2mu2n
Geometric mean approximation
u220 = (u200u222)½
Principal axis system
u20 = (2uzz – uxx – uyy)/√6
u22 = (uxx – uyy)/2
Analogy to dispersion forces
contrast to excluded volume
(Luckhurst, Zannoni, Nordio and Segre, Mol Phys., 1975, 30, 1345)
Segmental interactions
z

y
x
Segmental anisotropy ua
u20
u22
= ua(1 – 3cos)/2
= (3/8)½ua(1 + cos)/2

= u22/u20
= (3/2)½(1 + cos)/(1 – 3cos)
Biaxiality parameter
General
Uniaxial segments
i
u2m   C2m (i )u20
i
Biaxial segments
2
u2m   Dnm
(i )u2i n
i ,n
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
u220
= u20u20
= u22u20
Landau point shifts from ~109º to 105º
Acknowledgements
John Goodby
Verena Görtz
Mark Wilson
Daniel Jackson