Investigation of the complete miscibility of disc-rod mesogens in the nematic phase

Article abstract of DOI:10.1080/15421400600582523

The synthesis and the phase behaviour of nematic dimers, built up from rod and disc shaped mesogens, separated by extended spacers is described. Miscibility studies with the rod as well as disc shaped mesogens were performed and OPM data of contact sample

Full text of DOI:10.1080/15421400600582523

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Investigation of the Complete
Miscibility of Disc-rod Mesogens
in the Nematic Phase
a b
a
Daniela Apreutesei
& Georg Mehl
a
Department of Chemistry, University of Hull, Hull,
UK
b
Faculty of Industrial Chemistry, Gh. Asachi
Technical University, Iasi, RO
Published online: 31 Aug 2006.
To cite this article: Daniela Apreutesei & Georg Mehl (2006): Investigation of the
Complete Miscibility of Disc-rod Mesogens in the Nematic Phase, Molecular Crystals
and Liquid Crystals, 449:1, 107-115
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Mol. Cryst. Liq. Cryst., Vol. 449, pp. 107–115, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600582523
Investigation of the Complete Miscibility of Disc-rod
Mesogens in the Nematic Phase
Daniela Apreutesei
Department of Chemistry, University of Hull, Hull, UK and Faculty
of Industrial Chemistry, Gh. Asachi Technical University, Iasi, RO
Georg Mehl
Department of Chemistry, University of Hull, Hull, UK
The synthesis and the phase behaviour of nematic dimers, built up from rod and
disc shaped mesogens, separated by extended spacers is described. Miscibility
studies with the rod as well as disc shaped mesogens were performed and OPM
data of contact samples and the results of detailed DSC experiments of mixtures
are presented. Full miscibility in the nematic phase was observed.
Keywords: biaxial nematic; disc-rod mixtures; liquid crystal; nematic
INTRODUCTION
The interest in the thermotropic biaxial nematic phase has intensified
recently [1], with the focus of research either concentrated on side-
chain polymers, bent core mesogens or soft supermolecular multi-
podes. All of these systems are based on the concept that below a
certain temperature the rotation about a molecular long axis, the
primary nematic director is hindered and a second ordering is formed
[
1–4]. The alternative approach of achieving a biaxial nematic by the
mixing of rods and discs has attracted considerable theoretical inter-
est [5–8]. Common in all of the theoretical models is that full misci-
bility of rod and discs is required and that biaxiality is associated
with a minimum at the latent heat of the nematic-isotropic phase
transition positioned at the lowest point in nematic-isotropic phase
transition temperature in the phase diagram. A large range of disc
and rod shaped systems have been investigated [9–11]. Of particular
Address correspondence to Georg Mehl, Department of Chemistry, University of
Hull, Hull, HU6 7RX, UK. E-mail: g.h.mehl@hull.ac.uk
107

1
08
D. Apreutesei et al.
interest have been rod-disc amphiphiles, materials, which contain rod
and disc shaped mesogens in one molecule. For such a system full mis-
cibility of rods and discs could be observed in the nematic phase. The
miscibility of these systems has been attributed mainly to a similar
aspect ratio of rods and the disc. In this presentation we report our
result of the investigation of a system containing one rod and one disc,
mixed with rods and disc. Crucially different to previously reported sys-
tems is, that the length of the alkyl chain connecting the rod and disc
has been extended significantly, when compared to previous systems.
RESULTS AND DISCUSSION
Following the previous investigations, we synthesized new systems
miscible in the nematic phase containing disc-rod structures connected
directly. Compared to previously synthesised systems the length of the
hydrocarbon chain linking rods and disc has been increased [13]. The
structures of the disc D, the rod, R, and the rod-disc system DR, are
shown in Figure 1. Contact microscopy studies of the disc-rod system
DR with the rod, R, and the disc, D, show a complete miscibility over
the whole composition range. Various attempts to detect regions with
a miscibility gap have failed. A nematic phase, characterized by a
typical schlieren and=or marbled texture, is observed over the full
width of the phase diagram of either DR=D and or DR=R. The OPM
micrographs are shown in Figure 2. The pure materials were placed
on the sides of the microscopy slides and flowed at high temperature
FIGURE 1 The structure of the investigated compounds.

Investigation of the Complete Miscibility
109
FIGURE 2 Optical textures of contact samples of 1. DR and D at
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a) T ¼ 60.4 C, b) T ¼ 58.7 C, c) T ¼ 56.8 C d) T ¼ 56 C and 2. DR and R at
ꢀ
a) T ¼ 82.2 C, b) T ¼ 74.5 C, c) T ¼ 70.7 C d) T ¼ 69.9 C; The isotropic area
in between disappears on cooling and a continuous nematic phase is formed.
All pictures taken with crossed polarisers.

1
10
D. Apreutesei et al.
FIGURE 3 Phase diagrams of a) DR=R (. Iso-N (cooling) & Tg —~— N-Cr
(
(
cooling) –ꢁ^ꢁ– Cr-l (heating)) and b) DR=D (—^—Iso-N (cooling) & Tg ~ Cr-l
ꢀ
heating)) as measured by DSC (cooling 10 C=min).

Investigation of the Complete Miscibility
111
SCHEME 1 Synthesis of disc-rod compounds.
together and mixed. On cooling the nematic regions increase and meet
at a minimum temperature, forming a continuous nematic phase.
The phase diagrams of both mixtures are presented in Figure 3. The
transitions temperatures of mixtures between DR and R show a linear
dependence of the clearing temperature with concentration of the
rod. The phase diagram of DR=D mixtures has a minimum of the
TABLE 1 Phase Behaviour of Disc and Rods
ꢀ
ꢂ1
)
Sample
Transition temperatures ( C)
DH (kJ mol
a
DR
C
C
C
N
22.5
87.1
137
N
N
N
69.1(ꢂ1.11)
105.9(ꢂ11.9)
246 (0.2)
I
I
I
a
R
r
r
b
D
a
monotropic phase transition.
enantiotropic phase transition.
b

1
12
D. Apreutesei et al.
transition temperatures at ꢃ20% disc due to the destabilisation of
the nematic phase on mixing.
The synthesis of the investigated compounds is presented
in Scheme 1. The disc D1 has been prepared according to the
literature reports [12]. The rod R has been prepared by etherifi-
0
cation of 4-hydroxy-4 -cyanobiphenyl and 1,10-dibromodecane under
Williamson conditions and a further etherification with methyl
4
-hydroxybenzoate. Ester deprotection and esterification with D
provided the disc-rod mesogen DR. The phase behaviour of the pure
compounds is summarized in Table 1.
CONCLUSIONS
The presented results confirm that complete miscibility in the nematic
phase of molecules which contain one rod and one disc with either rod
or disc shaped molecules can be achieved. The length of the alkyl
spacer linking rod and disc shaped groups is crucial in enhancing
the miscibility of the system, when compared to related materials.
The construction of such a phase diagram is a significant step forward
in the design and development of future materials exhibiting a
nematic biaxial phase. As theory for the NB phase predicts, the
mixtures exhibit a minimum in the transition enthalpies when
approaching the minimum nematic to isotropic transition.
EXPERIMENTAL
Materials
All materials were used as purchased unless mentioned otherwise.
The synthesis and properties of the discotic mesogen D has been
published before [12].
INSTRUMENTATION
Nuclear magnetic resonance (NMR) spectra were recorded on a Jeol
JNM-ECP 400 M Hz FT-NMR spectrometer. Chemical shifts are
reported in ppm relative to TMS. Thermal properties were investi-
e
gated using a a Mettler Toledo differential calorimeter (DSC 822 ) in
nitrogen against an indium standard. Transition temperatures were
determined as the onset of the maximum in the endotherm or
exotherm. The mesophases were studied on an Olympus BH-2 optical
polarising microscope, equipped with a Mettler FP82 HT hot stage and
a Mettler FP90 central processor. Pictures of the mesophases were

Investigation of the Complete Miscibility
113
taken using a JVC digital video camera connected to a PC. Software
Studio Capture, supplied by Studio86 Designs was used for image
capturing.
0
Synthesis of 4 -(10-bromodecyloxy) biphenyl-4-carbonitrile (3)
A mixture of 1 (1eq), dibromoalcane 2 (5eq), K CO (2eq), KI (0.2 eq)
2
3
and butanone was refluxed overnight. The mixture was allowed to cool
to room temperature, the solids were filtered off and the solvent was
evaporated under reduced pressure. The oily residue was precipitated
in methanol (five times the volume) and the crude product was
filtered. Purification was carried out by crystallisation from acetone:
1
MeOH and CH Cl :hexane. 79% Yield of a white powder. H-NMR
2
2
dH (CDCl ):7.66, 7.61, 7.51, 6.97 (8H, Ar), 3.99 (t, 2H, ꢂCH ꢂOꢂ),
3
2
13
3
.40 (t, 2H, ꢂCH ꢂBr), 1.92-1.78, 1.66-1.57 (m, ꢂCH ꢂ). C-NMR
2
2
d
(CDCl ):159.64; 145.18; 132.51; 131.28; 128.28; 127.02; 127.02;
C
3
1
19.06; 115.01; 109.99; 67.83; 32.60; 33.75; 28.98; 27.85; 25.23.
0
Synthesis of 4-methyl-[10-(4 -cyanobiphenyl-4-
yloxi)decyloxi]benzoate(R)
A mixture of 3 (1eq), methyl 4-hydroxybenzoate (1.1eq), K CO (1.5
2
3
eq), KI (0.5 eq) and butanone was refluxed overnight. The mixture was
allowed to cool to room temperature, the solids were filtered off,
washed thoroughly with warm acetone and the solvent was evapo-
rated under reduced pressure. Purification was achieved using column
cromatography column using a mixture of DCM=hexane 3:1 as eluent.
Recristalization from methanol gives a white powder (87% yield).
1
H-NMR dH (CDCl ):7.07 (d, 2H, Ar), 7.69 (d, 2H, Ar), 7.63 (d, 2H,
3
Ar), 7.52 (d, 2H, Ar), 6.99 (d, 2H, Ar), 6.90 (d, 2H, Ar), 4.00 (t, 4H,
ꢂCH ꢂ), 3.88 (s, 3H, ꢂCH ), 1.81 (m, 4H, ꢂCH ꢂ), 1.34–1.47
2
3
2
1
3
(
1
1
m, 12H, ꢂCH ꢂ);
C-NMR dC (CDCl ):167.02; 163.02, 159.88;
3
2
45.37; 132.67; 131.66; 128.42; 127.16; 122.41; 119.24; 115.15;
14.14; 110.11; 68.25; 51.95; 29.56; 29.44; 29.30; 29.19; 26.11; 26.07.
0
Synthesis of 4-[10-(4 -cyanobiphenyl-4-yloxi)decyloxi]benzoic
acid 11-[pentakis(4-metoxyphenyletynyl)phenoxy]undecyl
ester (DR)
The free carboxylic acid R1a was obtained by refluxing R1 in etha-
nol and aqueous KOH (4 N) until the TLC plates indicate complete
consumption of the starting material (30 min). The reaction mixture
was neutralised with con HCl, cooled and the precipitated product
was filtered from the mixture. The products were used directly for
the esterification with D1. A mixture of D1 (1 eq) and R1a (1.2 eq),

1
14
D. Apreutesei et al.
DIPC (1.5 eq), DMAP (0.5 eq) and pTSA (0.5 eq) in CH Cl was stirred
2
2
for 6 days at room temperature under inert atmosphere. Subsequently
the solvent was removed under reduced pressure and the residue was
applied to a column for chromatography (silicagel, DCM). Recristaliza-
1
tion from acetone. Yield 34%, pale yellow powder. H-NMR dH
(
CDCl ):7.97 (d, 2H, Ar); 7.68 (d, 2H, Ar); 7.63 (d, 2H, Ar); 7.53 (m,
3
1
4
1
1
1
1
9
3
2
0H, Ar); 6.98 (d, 2H, Ar); 6.89 (m, 10H, Ar); 4.33 (t, 2H, ꢂCH ꢂ);
2
.26 (t, 2H, ꢂCH ꢂ); 3.99 (t, 4H, ꢂCH ꢂ), 3.84 (s, 3H, ꢂCH ), 1.25-
2
2
3
1
3
.93 (m, 34 H, ꢂCH ꢂ);
C-NMR dC (CDCl ): 166.61; 164.72;
3
2
62.94; 160.12; 152.91; 145.36; 133.46; 133.38; 133.22; 132.66;
31.59; 131.31; 128.48; 128.41; 127.15; 123.94; 122.77; 119.99;
19.24; 115.64; 115.59; 115.4; 114.21; 114.16; 114.09; 110.07; 99.26;
9.13; 97.02, 86.73; 86.21; 83.65; 77.44; 77.12; 76.80; 68.22; 55.45;
1.05; 30.69; 29.73; 29.66; 29.57; 29.43; 29.30; 29.19; 28.86; 26.47;
6.18; 26.11; 26.07.
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