Substituent effects in discotic liquid crystals

Article abstract of DOI:10.1080/15421400490435305

A series of novel mesogens have been prepared by a five-fold Sonogashira reaction of terminal acetylenes with a functionalized pentabromophenol. The corresponding side chain polymers were prepared by a polymer analogous substitution reaction. The mesogens differ in the nature of the substituents, linking five hexyl tails to the aromatic core, i.e. CH₂, O, S, SO₂ and CONH groups. A wide range of mesophases and corresponding transition temperatures has been detected, ranging from low melting nematic phases to highly stable columnar phases. The widely variable phase behavior is described in terms of specific intermolecular interactions.

Full text of DOI:10.1080/15421400490435305

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Substituent Effects in Discotic
Liquid Crystals
Paul Kouwer a , Wolter Jager a , Wim Mijs a
Stephen Picken a
&
a Polymer Materials and Engineering, Delft University
of Technology, Delft, BL, The Netherlands
Published online: 18 Oct 2010.
To cite this article: Paul Kouwer , Wolter Jager , Wim Mijs & Stephen Picken (2004)
Substituent Effects in Discotic Liquid Crystals, Molecular Crystals and Liquid Crystals,
411:1, 305-312, DOI: 10.1080/15421400490435305
To link to this article: http://dx.doi.org/10.1080/15421400490435305
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 305=[1347]–312=[1354], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435305
SUBSTITUENT EFFECTS IN DISCOTIC
LIQUID CRYSTALS
Paul H. J. Kouwery, Wolter F. Jager, and Wim J. Mijs,
Stephen J. Pickenꢀ
Delft University of Technology, Polymer Materials and Engineering,
Julianalaan 136, 2628 BL Delft, The Netherlands
A series of novel mesogens have been prepared by a five-fold Sonogashira
reaction of terminal acetylenes with a functionalized pentabromophenol. The
corresponding side chain polymers were prepared by a polymer analogous
substitution reaction. The mesogens differ in the nature of the substituents,
linking five hexyl tails to the aromatic core, i.e. CH2, O, S, SO2 and CONH
groups. A wide range of mesophases and corresponding transition tempera-
tures has been detected, ranging from low melting nematic phases to highly
stable columnar phases. The widely variable phase behavior is described in
terms of specific intermolecular interactions.
Keywords: discotic; liquid crystals; mesomorphic properties; nematic; substituent; supramol-
ecular chemistry
INTRODUCTION
Supramolecular chemistry – ‘‘the chemistry beyond the molecule’’ – has
a large number of non-covalent interactions at its disposal [1]. These
secondary interactions include forces like (i) electrostatic interactions
(ion-dipole, dipole-dipole), (ii) hydrogen bonding, (iii) p-p stacking, (iv)
Van der Waals forces (dispersion and induction forces) and (v) hydrofobic
or, more general, solvofobic effects. A combination of these non-covalent
interactions is used in nature to build complex structures that can show
multiple levels of organisation. The interest in the self-organisation and
self-assembly in synthetic systems has grown considerably. This is dis-
played in numerous contributions in the literature, discussing the effects
of the wide range of secondary interactions available [2]. Liquid crystals
ꢀCorresponding author. Fax: þ 31 15 2787415, E-mail: s.j.picken@tnw.tudelft.nl
yCurrent address: University of Hull, Department of Chemistry, CottinghamRoad, Hull, HU6
7RX, UK. Fax: þ 44 1482 466411, E-mail: p.h.j.kouwer@hull.ac.uk
305=[1347]

306=[1348]
P. H. J. Kouwer et al.
are self-assembling by nature. The interactions mentioned above, can
improve or, oppositely, diminish the extent of organisation (i.e. the extent
of order) in the liquid crystalline phases. This makes liquid crystals
ideal models to study the effects of various interactions on the molecular
organisation.
Disc-shaped liquid crystals self-organise into nematic (N) or columnar
(ColX) mesophases, see Figure 1. In disc-shaped molecules specific interac-
tions can be conveniently introduced via the substituent that links the
flexible tail to the aromatic core. Here, we present the synthesis and the
liquid crystalline properties of a novel series of disc-shaped mesogens.
The mesogens vary in the substituents that links the aliphatic tail to the
aromatic core. The resulting differences in phase behaviour are explained
in terms of specific interactions.
RESULTS AND DISCUSSION
Synthesis of the Mesogens
The synthesis of the disc-shaped liquid crystals is outlined in Scheme 1.
Instead of more conventional numbering of the prepared mesogens, we
have chosen to include the functional groups into the numbering: In the
numbering nX–D, n represents the length of the tails (number of carbon
atoms); X represents the linking group and D stands for disc. Analogously,
the corresponding polymers are shown as: P(nX–D). Mesogens 6CH2-D,
6O-D, 6S-D and 6Am-D were prepared by a five-fold palladium catalysed
cross-coupling reaction (Sonogashira reaction) between a terminal acety-
lene and a pentabromo-phenol derivative [3]. The crude products were
obtained in reasonable yields, however, purification is tedious and is costly
in yield of final product. It is emphasised that acetylenes with electron
FIGURE 1 Commonly observed mesophases for disc-shaped liquid crystals: the
nematic (N) and an example of a columnar phase; here the columnar hexagonal
(Colh) phase is presented.

Substituent Effects in Discotic Liquid Crystals
307=[1349]
SCHEME 1 Synthesis of the mesogens.
withdrawing groups (SO2, NO2, etc.) can not be coupled to the central core
in a satisfactory way. Hence, 6SO2-D was prepared by selective oxidation
of 6S-D using 3-chloroperoxybenzoic acid (m-CPBA), giving the final
product in high yields.
Synthesis of the Polymers
Mesogens 6CH2-D and 6O-D were incorporated in a polymer backbone.
The polymers were synthesised by a polymer analogous reaction (see
SCHEME 2 Synthesis of the polymers P(6CH2-D) and P(6O-D).

308=[1350]
P. H. J. Kouwer et al.
Scheme 2) between the mesogens and the reactive poly(acryloyl chloride)
[3b]. At the final stages of the reaction, the remaining acyl groups were con-
verted to methyl esters by addition of an excess of dry methanol. This pre-
vents the formation of (crosslinking) anhydride groups or carboxylic acid
groups, which are expected to have a major influence on the phase behav-
iour, in particular the glass transition temperature. The clear advantage of
this approach is that polymers with a sufficient chain length can be pre-
pared relatively easily, taking into account that the triple bonds of the
mesogens are expected to hinder a (free-radical) polymerisation reaction.
Liquid Crystalline Properties
The mesophase behaviour of the low molar mass mesogens as well as the
polymers of 6CH2-D and 6O-D is summarised in Table 1. Mesogen
6CH2-D shows a monotropic N phase at low temperatures. In the corre-
sponding polymer, all mesogenic properties disappear and P(6CH2-D) is
characterised as an amorphous material with a low glass transition
temperature. Changing the linking group from methylene to oxygen results
in a stabilisation of the nematic phase. The nematic phase of compound
6O-D shows the characteristics of a nematic texture during microscopy
experiments, see Figure 2a. Moreover, at elevated temperature, the phase
aligns rapidly in an homeotropic organisation between two unprepared
microscope slides. The photograph, shown in Figure 2a is taken at 80ꢁC
after only 30 seconds annealing, resulting already in a very low birefrin-
gence. Further annealing (ꢂ5 minutes at 80ꢁC) resulted in almost
complete homeotropic alignment. The corresponding polymer P(6O-D)
TABLE 1 Thermal Behaviour of the Mesogens, Measured with OPM and DSC
Material
Thermal behavioura,b
6CH2-D
6O-D
6S-D
Cr
Cr
[39 (34)
N
52 (1.2)]b
57 (36)
118 (0.15)
167 (0.21)
>260
I
I
71 (33)
56 (65)
115
N
Cr
Colhd
Colhd
I
6SO2-D
6Am-D
GCol
Cr
d
I=d
215 (16)
P(6CH2-D)
P(6O-D)
GI
GN
ꢃ15
I
I
24
N
113 (0. 18)
a Transition temperatures are shown in [ꢁC] and latent heat values ( between the brackets)
in [kJ molꢃ1].
bCr ¼ Crystalline; GCol=N=I ¼ Glassy (Columnar=Nematic=Isotropic) phase frozen in);
N ¼ Nematic; Columnar hexagonal, disordered; I ¼ Isotropic; d ¼ decomposition. bMonotropic
phase transition.

Substituent Effects in Discotic Liquid Crystals
309=[1351]
FIGURE 2 Optical textures of (a) low birefringent homeotropic N phase of 6O-D at
80ꢀC; (b) N phase of P(6O-D) at 100ꢀC; (c) Colhd phase of 6S-D at 100ꢀC; (d) Colhd
phase of 6SO2-D at 255ꢀC (freshly formed columnar phase from the isotropic
phase). All pictures were taken with crossed polarizers. (See COLOR PLATE VI)
shows a ND phase as well, with a clearing temperature roughly equal to
that of the monomer. The presence of the polymer backbone prevents
crystallisation and a glass transition at 24ꢀC is observed, resulting in a
material with a nematic discotic phase close to room temperature. Only
recently, the first example of a room temperature nematic phase obtained
from disk-shaped liquid crystals was reported. Contrary to the monomer,

310=[1352]
P. H. J. Kouwer et al.
the ND phase hardly aligns between microscope slides, see Figure 2b. The
approach of suppression of crystallisation by chemically linking mesogens
proves to be successful in obtaining low temperature nematic discotic
phases. It is anticipated that the use of an even more flexible backbone
(such as polyether or polysiloxane) could lower the glass transition
temperature even further, opening up the way towards real room tem-
perature nematic discotic mesophases.
The fan-shaped optical texture of 6S-D (see Figure 2c) is characteristic
of a columnar phase. At high temperatures, a transition with a small latent
heat to the isotropic phase is observed. The mesophase of 6SO2-D could be
characterised by fast cooling from the isotropic phase at elevated
temperatures, although the optical textures remained rather difficult to
interpret, see Figure 2d. The growing hexagons and six-leafed ‘‘oak-leaf’’
textures are characteristic for a Colh mesophase. Compound 6SO2-D did
not show any melting or crystallisation phenomena, but rather showed a
second order transition at lower temperatures, attributed to the formation
of a columnar plastic (Colp) phase. Due to degradation at elevated
temperatures (>250ꢁC), no clearing temperature (and obviously no latent
heat) could be determined. The penta-amide 6Am-D cleared at high
temperatures and neither microscopy experiments nor DSC analysis could
distinguish between a highly ordered columnar or a ill-defined crystalline
phase at lower temperatures.
Substituent Effects
A wide range of liquid crystalline properties is observed for the series of
mesophases. The (monotropic) nematic phase of 6CH2-D can be well
understood by taking the fraction of rigid mesogen into account. A series
of methyl substituted mesogens, both molar mass and high molecular
weight, has been described in literature. It was shown that the clearing
temperature TNI was strongly dependent on the rigid mesogen fraction
[M]. Both the heptyl-substituted materials as well as the pentyl substituted
material (and a corresponding side chain polymer) fit perfectly on the same
line, see Figure 3. In other words, regardless of the length and the position
(attached to the mesogen or to the polymer backbone) of any alkyl group,
the ND to isotropic transition temperature can simply be predicted, just by
calculating its fraction of rigid mesogens [M]. It is striking to see that this
relation holds for such a variety of materials and over a wide temperature
range.
The mesophases of 6S-D, 6SO2-D and 6Am-D are strongly stabilised,
resulting in columnar mesophases or even a crystalline phase for 6Am-D
which remain stable up to high temperatures. It still is unclear why the
thio-ether 6S-D shows such stabilisation with respect to ether 6O-D. This

Substituent Effects in Discotic Liquid Crystals
311=[1353]
FIGURE 3 Clearing temperature TNI as a function of the rigid mesogen fraction [M]
for various alkyl-substituted mesogens: LLM (low molar mass) materials and poly-
mers. In the schematic picture (right) all the variables in the molecular structure for
this series of materials are illustrated. The materials differ in size of the laterally
attached tails and whether a polymer backbone is present or not. The polymers also
show differences in the degree of substitution and the presence (type and concen-
tration) of any side groups attached to the backbone. The solid line represents the
power-law equation: TNI ¼ 695.[M]0.98 which was determined before [6].
is opposite to the commonly observed effect. The strong additional stabil-
isation of the 6SO2-D can be explained by the five sulphon groups introdu-
cing multiple dipole-dipole interactions, resulting in a columnar mesophase
up to 280ꢁC. A further increase in the intracolumnar order is found in the
penta-amide, where the formation of attractive hydrogen bonds completely
prevent any mesophase formation.
CONCLUSIONS
We have synthesised a novel series of disc-shaped mesogens with different
substituents linking the five hexyl tails to the aromatic core. The liquid
crystalline behaviour was investigated with DSC and OPM and it was
observed that the stability of the mesophase was strongly dependent on
the linking group, ranging from a monotropic ND at low temperatures
(6CH2-D) to a columnar phase, stable to 280ꢁC (6SO2-D). By attaching
6O-D to a polyacrylate backbone, crystallisation was suppressed, making
room temperature nematic discotic liquid crystals within arms reach.
Further modification of the mesophase behaviour is possible using non-
covalent charge transfer interactions, which will be investigated in the near
future.

312=[1354]
P. H. J. Kouwer et al.
REFERENCES AND NOTES
[1] Lehn, J. M. (1995). Supramolecular Chemistry, VCH, Weinheim: New York.
[2] For example, read: (a) Lawrence, D. S., Jiang, T., & Levett, M. (1995). Chem. Rev., 95,
2229. (b) Philip, D. & Stoddart, J. F. (1996). Angew. Chem. Int. ed. Engl., 35, 1154.
[3] (a) Janietz, D., Praefcke, K., & Singer, D. (1993). Liq. Cryst., 13, 247–253. (b) Kouwer,
P. H. J., Jager, W. F., Mijs, W. J., & Picken, S. J. (2000). Macromolecules, 33, 4336.
[4] (a) The room temperature nematic phase was observed in a discotic liquid crystal based on
a hexakis(phenylethynyl)benzene core and substituted with a rac-4-methylnonyl tails,
resulting in a broad distribution of stereo isomers. (b) Kumar, S. & Varhney, S. K.
(2000). Angew. Chem. Int. Ed. Engl., 39, 3140.
[5] By fast heating in an OPM experiment a clearing temperature of ꢂ280ꢁC could be esti-
mated.
[6] Kouwer, P. H. J., Mijs, W. J., Jager, W. F., & Picken, S. J. (2001). J. Am. Chem. Soc., 123,
4645.