Enantioselective segregation in achiral nematic liquid crystals

Article abstract of DOI:10.1039/b503846d

The elusive biaxial nematic liquid crystal phase was recently discovered in a family of substituted oxadiazoles. Our investigations of these materials show that the achiral biaxial nematic phase can segregate into chiral domains of opposite handedness, th

Full text of DOI:10.1039/b503846d

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Enantioselective segregation in achiral nematic liquid crystals{
Verena Go¨rtz and John W. Goodby
Received (in Cambridge, UK) 15th March 2005, Accepted 18th April 2005
First published as an Advance Article on the web 13th May 2005
DOI: 10.1039/b503846d
substituted compounds in comparison to the symmetric parent
systems 8a and 8g; the nematic to smectic phase transitions occur
at lower temperatures, and the melting points are reduced. The
phase transition temperatures given in Table 1 are the onset
temperature values for transitions observed in first DSC cooling
cycles at a cooling rate of 10 uC min21 (first values are reported as
the materials undergo thermal degradation). Although we are
mainly concerned with the nematic phase, Table 1 also lists the
higher ordered, lower temperature, phases which have undefined
structures. Following the terminology used by Samulski et al.,2,4
we denote the unidentified phases as smectic X, Y, and Z, however
we also note the lettering is not consistent with the phase identity
across the series of compounds.
The elusive biaxial nematic liquid crystal phase was recently
discovered in family of substituted oxadiazoles. Our
a
investigations of these materials show that the achiral biaxial
nematic phase can segregate into chiral domains of opposite
handedness, thereby demonstrating that the liquid-like nematic
phase exhibits the properties of a conglomerate.
The possibility of the occurrence of a biaxial nematic phase was
highlighted, in 1970, in an article by Freiser.1 However, it was only
recently that Samulski et al.2 and Kumar et al.3 reported on low
molecular mass materials, based on the oxadiazole motif, which
exhibited thermotropic biaxial nematic phases. The biaxial order
parameter however was found to be relatively small, with a value
of approximately 0.1. The materials themselves have molecular
structures that are bent; the bend being associated with the
bis(phenyl)oxadiazole moiety located at the centre of the aromatic
core, as shown by structure I.
Upon examination of compound 8g, which was reported
previously by Samulski et al.2,4 and Kumar et al.,3 pristine samples
sandwiched between a slide and a cover slip gave unusual textures
when examined by thermal, polarized light optical microscopy
(POM), as shown in Fig. 1 (a and b). Under crossed polarizers, an
apparently normal nematic phase was formed first on cooling from
the isotropic liquid. The nematic phase separated in the form of a
schlieren texture exhibiting Brownian motion. However, the
mesophase also exhibited domains, with walls separating one
domain from another. Upon rotation of the analyser of the
microscope, the domains which were dark became light and vice
versa, as shown in the figure. This result indicates the domains are
helical, and that they have the opposite handedness.
The novel, dissimilarly substituted, bis(phenyl)oxadiazoles were
also examined by thermal, polarized light optical microscopy to see
if they too displayed helical domains. Fig. 2 shows that the nematic
mesophase of compound 8b, sandwiched between a slide and a
The structures and accompanying phase transitions of these first
examples of low mass materials possessing thermotropic biaxial
nematic phases are shown in Scheme 1 and Table 1 (compounds
8a and 8g).
Although the materials have very high temperature nematic
phases, we nevertheless repeated their syntheses. In addition, we
also prepared dissimilarly substituted bis(phenyl)oxadiazole mate-
rials with differing aliphatic chain lengths, and fluoro-substituents
incorporated in the outer phenyl rings in order to moderate and
reduce melting points and clearing points, thereby making nematic
phases accessible at lower temperatures. Scheme 1 shows the route
we established to synthesize the materials. Further details of the
syntheses are given in the supplementary information.{
With the exception of compound 8f we found the nematic phase
to have a broader temperature range for the dissimilarly
{ Electronic supplementary information (ESI) available: detailed synthetic
procedures, NMR data, mass spectra, and elemental analyses of final
compounds. See http://www.rsc.org/suppdata/cc/b5/b503846d/
Scheme 1 Synthesis of similarly and dissimilarly substituted bis(phenyl)-
oxadiazoles.
3262 | Chem. Commun., 2005, 3262–3264
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Table 1 Phase transition temperatures of compounds 8a–h. Reported are the onset temperature values for transitions observed in DSC cooling
cycles at 10 uC min21
Nu
R1
R2
R3
R4
Phase transitions (uC)
8a
8b
8c
8d
8e
8f
C
12H25
O
O
O
O
O
O
C12H25
O
H
H
H
H
H
F
H
H
H
H
F
F
H
H
Iso 203 N 192 SmC 184 SmX 143 SmY 138 SmZ 104 Cr
Iso 210 N 182 SmX 157 SmY 149 SmZ 91 Cr
Iso 213 N 176 SmX 162 SmY 152 SmZ 77 Cr
Iso 215 N 160 SmX 91 Cr
Iso 205 N 168 SmX 135 SmY 125 SmZ 72 Cr
Iso 210 N 197 SmC 186 SmX 155 SmY 150 SmZ 100 Cr
Iso 222 N 173 SmX 151 Cr
C12H25
C12H25
C12H25
C12H25
C12H25
C7H15
C7H15
C9H19
C8H17
C5H11
C9H19
C9H19
C7H15
C5H11
O
O
O
O
8g
8h
H
H
Iso 232 N 164 SmX 149 Cr
Fig. 1 The schlieren texture of the nematic phase of compound 8g at
222 uC on cooling from the isotropic, where (a) shows an anticlockwise
rotation and (b) a clockwise rotation of the analyser (6100).
Fig. 3 The texture of the nematic phase of compound 8f in an uncovered
region on a glass slide at 189 uC (6100).
In investigations of the textures exhibited by compound 8h we
observed intense and extensive dynamical motion in uncovered
regions of the nematic phase. Fig. 4 shows lines in the nematic
phase which occur well above the transition to the smectic X
phase. At constant temperature well within the temperature range
of the nematic phase, defect lines appear and flow rapidly across
the schlieren texture of the phase, rather like Williams domains or
possibly Rayleigh Be´nard thermal instabilities.
In conclusion the results appear to indicate the formation and
separation of enantiomeric species during the generation of the
nematic phase. Furthermore the observation of what appears to be
different pitch lengths in different domains for compound 8f
indicates that the proposed enantioselective segregation is not
homogeneous, suggesting that the process is kinetically driven.
Differential scanning calorimetry of the compounds for the first
cooling, at slow scanning rates of 0.2 uC min21 (Mettler DSC
822e), indicates that there is a thermal event in the liquid state at a
temperature slightly above that of the clearing point. Although the
continuous drop in transition temperatures on further cycles shows
Fig. 2 Defect texture of the nematic phase of compound 8b at 202 uC,
where (a) shows an anticlockwise rotation and (b) a clockwise rotation of
the analyser (6100).
cover slip, exhibited domains that also reversed colour upon
rotation of the analyser of the microscope. The walls separating
the domains from one another are clearly visible in the figure.
Remarkably, uncovered regions of the nematic phase of
compound 8f showed fingerprint textures similar to the chiral
nematic phase as depicted in Fig. 3. The photomicrograph of the
nematic phase was taken at a temperature of 189 uC, which is
lower than the transition to the smectic C phase obtained on first
cooling due to slight sample decomposition. In this texture the
equidistant parallel lines could be manifestations of the helical
structure of a chiral mesophase with the distance between the lines
being equivalent to half the pitch of the helix. Unlike conventional
chiral nematic phases where the pitch is constant at constant
temperature, the line-separation in this case varied from one
domain to the next. Typically it varied from approximately 5 to
12 mm in different domains.
Fig. 4 Lines in the nematic phase of compound 8h in an uncovered
region on a glass slide at 173 uC (6100).
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This structural speculation would satisfy the criterion that the
process is kinetically driven; that the energy barrier to conforma-
tional flipping is raised through the self-assembly, and that the
pitch length of the helix could vary from one domain to another.
In addition, helix formation can be suppressed by external forces
such as surface interactions. Furthermore, the length scales of the
self-organized structures are such that diffusion probably does not
easily occur thereby stabilising the formation of helical domains.
Thus we propose that the bis(phenyl)oxadiazole materials
studied could be examples of self-assembling-self-organizing
nematogens where the conformational forms can spontaneously
segregate into chiral domains.
Although it is thus apparently possible to have enantioselective
segregation in the nematic phase of achiral bent-core systems, it is
interesting that Strigazzi et al.6,7 also evoke a comparable model to
explain their observation of chiral domains in the nematic phases
of achiral 4-alkyl(oxy)benzoic acids. In their case twisted open
dimers formed via hydrogen bonding are the suggested source for
the formation of helical structures.6
Takezoe et al.8 also showed that the addition of an achiral bent-
core solute to a conventional chiral nematic phase reduces the
pitch, thereby indicating that the achiral dopant has a strong chiral
effect on the local helical packing of the molecules.
Lastly we also note that Pelzl et al.9 reported on the formation
of chiral domains in the nematic phase of another achiral bent-core
system. Thus with our work on biaxial nematogens we can state
that it seems to be a more general phenomenon that nematic
phases of certain achiral materials are capable of exhibiting chiral
ordering. Pelzl et al. also refer to a computer simulation study by
Memmer10 that suggests the helical superstructure occurs due to
conical twist-bend deformations, which as a result reduces the
overall flexoelectric effect. However it is also possible to devise a
chiral twisted conformer similar to II for the bent-core molecule
studied by Pelzl et al. This emphasises again the possibility of self-
assembly in a quasi-liquid phase driving enantioselective separa-
tions similar to those found in the solid state.
Fig. 5 Cartoon of the proposed formation of a helix via the self-assembly
of twisted conformers of the bent core molecules.
that the materials undergo decomposition, these results suggest
that the liquid is structured at temperatures just above the
formation of the liquid-crystalline state, and consequently it is
possible that the molecules undergo a process of self-assembly
before self-organization into the mesophase occurs.
The results described for the nematic phases of the materials
studied strongly suggest the formation of chiral domains
characterized by the generation of a helical macrostructure. In
each case the formation of helical domains was dependent on the
thermal and mechanical history of the sample, indicating that
domain formation was kinetically based rather than thermody-
namic. Such results are of relevance to the observation made that
compound 8g exhibits a biaxial nematic phase.2,3 In addition the
results challenge our very understanding of the concept of liquid
crystallinity, where molecular diffusion, rotation and flipping
would suppress the formation of helical domains of opposite
handedness in a fluid-like nematic phase.
It is possible to speculate on the results in the following way. For
the bis(phenyl)oxadiazole molecules, chiral conformational iso-
mers such as II and its mirror image might be expected to exist in
the bond rotational profile of a molecule. At the clearing point the
conformers could self-select to give the most stable self-assembled
structure. This means that the nucleation of the nematic phase
could occur through a process of self-assembly where conformers
of one hand pack together to give a helical macrostructure which
in turn stabilizes conformer formation. Such helical structures
assemble into spiralling ribbons, which then self-organize into
chiral nematic phases, and because of the sizes of the ribbons,
domains are formed by segregation.
We wish to thank Andrew Stipetic, Kenneth M. Fergusson, and
Alan W. Hall for the supply of alkyl(oxy)benzoic acids and the
European research training network SAMPA for financial
support. We also acknowledge the input of a referee.
Verena Go¨rtz and John W. Goodby
Liquid Crystal Group, Department of Chemistry, University of York,
Heslington, York, UK YO10 5DD. E-mail: vg501@york.ac.uk;
jwg500@york.ac.uk; Tel: +44 (0)1904 432539
Notes and references
Fig. 5 shows how the self-assembly of chiral conformers such as
II might occur. The chirality of the conformational structure II is
generated from the two ester linkages being rotated in opposite
directions with respect to the bis(phenyl)oxadiazole core. When
molecules in this twisted conformation are packed one on top of
another a helical self-assembled structure results. The self-
organization of the self-assembled structures would result in the
formation of a chiral nematic phase. An inverted rotation of the
two ester linkages produces the enantiomeric form of conformer
II, which would assemble to give a helix of opposite handedness.
Such self-assembled structures would be dynamically fluctuating
with up and down domains as a function of time and external
influences such as surface pinning and mechanical disturbances.
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3264 | Chem. Commun., 2005, 3262–3264
This journal is ß The Royal Society of Chemistry 2005