Completely miscible disc and rod shaped molecules in the nematic phase

Article abstract of DOI:10.1039/b512120e

Disc and rod shaped molecules which are completely miscible in the nematic phase were synthesised and the miscibility behaviour was investigated and confirmed by POM, DSC and XRD studies. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512120e

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Completely miscible disc and rod shaped molecules in the nematic
phase{
Daniela Apreuteseiab and Georg H. Mehl*a
Received (in Cambridge, UK) 26th August 2005, Accepted 13th December 2005
First published as an Advance Article on the web 13th January 2006
DOI: 10.1039/b512120e
Experimentally research has been hampered by the lack of fully
miscible rod and disc shaped molecules.10–12 Only recently a
combined system containing rods and discs in a molecule has been
reported, which is miscible at all concentrations with rods and
discs.13
Disc and rod shaped molecules which are completely miscible
in the nematic phase were synthesised and the miscibility
behaviour was investigated and confirmed by POM, DSC and
XRD studies.
Molecules which exhibit uniaxial nematic phase behaviour are the
basis of the current LCD technology; their biaxial nematic relatives
have so far proven to be rather elusive. The biaxial nematic phase
(Nb) was first predicted in 1970, and is characterised by two
directors (compared to only one in the uniaxial variant), which are
orthogonal to each other. It has been investigated intensively in
theoretical studies.1,2 Experimental evidence for lyotropic systems
and very recently for the first time several thermotropic systems
has been reported.3,4
In this report we present, to the best of our knowledge the first,
systems where rod shaped molecules are completely miscible in the
nematic phase with disc shaped molecules. As rod shaped
mesogens the systems R1, and R2 were prepared, as they are
known to exhibit nematic phase behaviour over
a wide
temperature range and additionally, as they contain a high
hydrocarbon content, when compared to many other mesogens
with similar transition temperatures.13 The disc shaped mesogens
D2, D3, and D4 were synthesised by simple esterification of D1
with undecyloxy benzoic acid, 10-undecenoic acid and octanoic
acid and are shown in Scheme 1.14,15
Great theoretical and computational effort has been concen-
trated on mixtures of nematic rod and disc shaped molecules,
which are individually uniaxial nematic (Nu), where in a certain
region in the phase diagram biaxial nematic phase behaviour
should occur, as shown schematically in Fig. 1.5–10 Common for
all of these models, is the occurrence of the Nb phase in the
minimum of the rod–disc phase diagram. Associated is a strong
decrease of the latent heat of the N to I transition upon
approaching the tricritical point. The nature of the Nu–Nb
transition is however model dependent and hence still discussed.
The transition temperatures of R1, R2, D2, D3 and D4 are
summarised in Table 1. All materials exhibit a nematic mesophase,
monotropic for D2 and enantiotropic for R1, R2, D3 and D4.The
mixing behavior was investigated by studying contact samples with
optical microscopy and by preparation of discrete mixtures, which
were studied by polarized optical microscopy (POM) and DSC
and for selected examples by XRD.
Fig. 1 Schematic phase diagram containing the Nb phase for mixtures of
rods and discs. x represents the fraction of discs. The shape of the phase
diagram, especially the transitions between the Nb and the Nu phases, is
actually dependent on the model used. n and m refer to the two nematic
directors in the biaxial phase.
aDepartment of Chemistry, University of Hull, Hull, UK HU6 7RX.
E-mail: g.h.mehl@hull.ac.uk
bFaculty of Industrial Chemistry, ‘‘Gh. Asachi’’ Technical University,
700050 Iasi, Romania
{ Electronic supplementary information (ESI) available: Experimental
procedures, 1H NMR spectra and DSC data, contact microscopy images
and phase diagrams of the disc–rod mixtures. See DOI: 10.1039/b512120e
Scheme 1 Structures of disc and rod shaped molecules used for
mixtures.
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Chem. Commun., 2006, 609–611 | 609

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Table 1 Phase behaviour of discs and rods
Sample
Temperatures (uC) [enthalpies (DH) J g21)]
a
D2
D3
Cr
Cr
Cr
Cr
Cr
92
80
95
54
93
(N
N
N
N
N
81
[20.08])
[0.12]
[0.11]
[1.75]
[1.77]
I
I
I
I
I
b
140
157
73
c
D4
R1
d
d
R2
a
141
b
Monotropic phase transition. Enantiotropic, crystallisation on
cooling at 11 uC. Enantiotropic, crystallisation on cooling at 49 uC.
On cooling both rods exhibit liquid crystal properties at room
Fig. 3 X-Ray diffractograms of (a) R2 at T = 100 uC, (b) D2 at T = 75 uC
(on cooling) and (c) D2–R2 mixture at the minimum of the transition
temperature c = 15.87% (R2) and at T = 33 uC (on cooling); direction of
the magnetic field B.
c
d
temperatures and crystallise slowly below 0 uC.
Contact studies of D2 and R1 and D2 and R2 showed complete
miscibility over the whole composition range. Various attempts to
detect regions with a miscibility gap have failed. A nematic phase,
characterised by a typical schlieren and/or marbled texture, is
observed over the full width of the phase diagrams of both D2–R1
and D2–R2. The POM micrographs are shown in Fig. 2. On
cooling the samples, the nematic regions increase and meet at a
minimum temperature, forming a continuous nematic phase,
ranging from pure disc (left) to pure rod (right). Destabilisation of
the nematic phase, due to mixing, causes a minimum in the
clearing temperature at y18 uC in the D2–R1 mixture at D2 #
80% and at y28 uC in the D2–R2 mixture at D2 # 84%. The shift
in the minimum transition temperatures to a lower content of rods
in the phase diagram for the D2–R2 mixture is likely to be
associated with the larger aromatic ring system in R2, when
compared to R1.
Fig. 4 Radially integrated XRD patterns of the pure disc (D2), pure rod
(R2) and mixture D2–R2 at the minimum of the transition temperature (c =
15.87%).
The phase structures of D2, R2 and the mixture D2–R2 at the
minimum of the transition temperature were further investigated
by X-ray diffraction (XRD) of samples in capillaries. Fig. 3 shows
the diffractograms and Fig. 4 the radially integrated intensities.
The diffraction pattern of R2 is characterised by diffuse crescent
for bow-shaped molecules it was possible to assign such a pattern
to the formation of the biaxial nematic phase.4c Fig. 3b shows the
non-oriented pattern of D2, typical for this class of molecule with
˚
intensities at y16.78 A (in line with the diameter of the aromatic
˚
disc), and 4.31 A. The diffraction pattern of mixture D2–R2,
˚
shaped reflections (y4.63 A, corresponding to the lateral distance
of the hydrocarbon groups) and four small off-meridian intensities
containing only 15.87% of R2 in the nematic phase, is characterised
˚
by four weak off-meridian intensities at 23.8 A and wide angle
˚
reflections at 4.45 A. The small angle reflections are very weak and
˚
(y25.5 A, corresponding approximately to the length of an
aromatic mesogen), and attributed to the formation of cybotactic
clusters, typical for such systems. However it should be noted that
the pattern is similar to that of R2, thus excluding high ordered LC
Fig. 2 Optical textures of contact samples of 1: D2 and R1 at (1a) T = 25.5 uC, (1b) T = 22.8 uC, (1c) T = 18.0 uC and 2: D2 and R2 at (2a) T = 31.0 uC,
(2b) T = 29.5 uC, (2c) T = 27.9 uC. The isotropic area in between the nematic regions disappears on cooling and a continuous nematic phase is formed. All
pictures taken with crossed polarisers.
610 | Chem. Commun., 2006, 609–611
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Fig. 5 Phase diagrams of (a) D2–R1, (b) D2–R2. The upper plots refer to the mixtures of discs and rods as determined by DSC measurements. The
compositions are plotted as molar concentrations of the rods. Data shown in magenta refer to enantiotropic N–I transitions obtained on heating; data
shown in blue refer to N–I transitions recorded on cooling. Data shown in red indicate crystallisation temperatures recorded on cooling. Glass transitions
are omitted. The sketched lines are guides for the eyes only. The lower plots refer to the enthalpy changes associated with the N–I transitions recorded on
cooling.
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The results of detailed DSC investigations of the phase diagrams
for the mixtures of D2 and R1 and D2 and R2 are summarised in
Fig. 5 and details for the systems D3–R2 and D4–R2 are provided
in the supporting information.14 In all phase diagrams the latent
heat of the nematic to isotropic transition decreases on approach-
ing the transition minima, see Fig. 5.
Minima of y0.02 kJ mol21 for both mixture systems are
recorded at the detected minimum N–I temperatures, which are
low compared to 0.09 kJ mol21 for D2, 1.30 kJ mol21 for R1 and
1.52 kJ mol21 for R2. In conclusion, the preparation of a phase
diagram with a continuous phase consisting of mixtures of rod and
disc shaped molecules in the nematic phase was shown. The
position of the N–I minimum in the phase diagram is dependent
on the size of the rod shaped mesogen and it is associated with the
minimum of the transition enthalpy. This is in line with theoretical
models for the formation of nematic biaxiality. For the nematic
miscibility the size of the hydrocarbon chains plays a crucial role.
This has so far not yet been recognised fully for theoretical and
experimental studies. For further studies on nematic biaxiality the
investigation of enantiotropic nematic rod–disc mixtures is
required.
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D. A. acknowledges support by the EU project ‘‘DesignLC’’
(HPMT-CT2000-0322).
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