Discotic multipodes with nematic mesophases

Article abstract of DOI:10.1080/15421400490435413

Various core materials have been introduced at the terminal, end of a discotic mesogen (based on a pentakis(methylphenylethynyl) phenoxy core), resulting in the formation of liquid crystalline 'monomers' and 'dimers.' The mesomorphic properties of this se

Full text of DOI:10.1080/15421400490435413

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Discotic Multipodes with
Nematic Mesophases
Paul Kouwer a , Georg Mehl a & Stephen Picken b
a Department of Chemistry , University of Hull ,
Hull, UK
b Polymer Materials and Engineering , Delft
University of Technology , Delft, BL, The
Netherlands
Published online: 18 Oct 2010.
To cite this article: Paul Kouwer , Georg Mehl & Stephen Picken (2004) Discotic
Multipodes with Nematic Mesophases, Molecular Crystals and Liquid Crystals, 411:1,
387-396, DOI: 10.1080/15421400490435413
To link to this article: http://dx.doi.org/10.1080/15421400490435413
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 387=[1429]–396=[1438], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435413
DISCOTIC MULTIPODES WITH NEMATIC
MESOPHASES
Paul H. J. Kouwer and Georg H. Mehlꢀ
University of Hull, Department of Chemistry, Cottingham Road,
Hull, HU6 7RX, UK
Stephen J. Picken
Delft University of Technology, Polymer Materials and Engineering,
Julianalaan 136, 2628 BL Delft, The Netherlands
Various core materials have been introduced at the terminal end of a discotic
mesogen (based on a pentakis(methylphenylethynyl) phenoxy core), resulting
in the formation of liquid crystalline ‘monomers’ and ‘dimers.’ The meso-
morphic properties of this series of derivatives were investigated as a function
of the attached core material. It is shown that the clearing temperatures of the
mesogens are dependent on: (i) the fraction of rigid mesogen, (ii) the presence
of any linkages between the mesogens and (iii) the presence of any H-bonding
interactions. In some materials extra stabilisation of the phase or, indeed,
higher ordered mesophases were observed due to the end-group interactions.
In addition, we report the investigations to prepare DAB-Am based dendri-
mers.
Keywords: dendrimer; dimer; discotic; liquid crystals; mesogen; mesomorphic properties
INTRODUCTION
One of the major steps forward in research on discotic liquid crystals [1]
was made when well-defined mono-functional materials were obtained
[2]. Derivatisation at the terminal end of a single functional spacer of the
usually highly symmetric mesogens allowed the preparation of new materi-
als – with new liquid crystalline properties – without the synthesis of new
mesogens. Well-known examples include side-chain polymers, wherein the
crystallisation process is fully suppressed, resulting in materials with broad
We like to thank the Ramsay Memorial Fellowship Trust and the European Union (contract
HPRN-CT2000-00016) for financial support.
ꢀCorresponding author. Fax: þ 44 1482 466411, E-mail: g.h.mehl@hull.ac.uk
387=[1429]

388=[1430]
Paul H. J. Kouwer et al.
mesophases, often still at room temperature. Disadvantages of the use of
side-chain polymers are their intrinsic high viscosity and the ill-defined
chemical structure. Alternatively, multipodes can be used, which are built
op from a well-defined core, substituted with a known number of meso-
gens, giving rise to the formation of dimers, trimers, up to heptamers. In
addition, the core can be selected to introduce special chemical properties.
Dendrimers can be considered as a special group of multipodes, which
show a low viscosity as well as suppression of crystallisation due to a suf-
ficient molecular weight, combining the advantages of multipodes and poly-
mers. Furthermore, interesting properties are expected from dendrimers
with substituted discotics, since in the corresponding materials with rod-
shaped end-groups remarkable behaviour has been observed as a function
of the generation number [3]. As far as we know, no attempts have been
made to attach disc-shaped mesogens to dendrimers in order to investigate
the role of the dendritic core on the phase behaviour of discotics.
Here, we present the synthesis of mesogen with a pentakis(methyl-
phenylethynyl) phenoxy core and equipped with a carboxylic acid group
1, see Figure 1.
The carboxylic group at the terminal end of the spacer has been used to
introduce various end-groups with different functionalities. By using
bi-functional spacers, the corresponding dimers have been obtained. The
mesomorphic properties of the monomers and dimers are mutually com-
pared and the phase transitions are explained in terms of the fraction of
rigid mesogen and other stabilising effects, occurring from the various
end-groups and spacers. In addition, we were interested in coupling this
mesogen to a series of dendrimers, which would result in new materials
with a fascinating phase behaviour could be obtained, particularly, since
the side chain polymers of this mesogen showed both the ND and the NCol
phase [4]. Attempts to synthesise these systems are discussed.
RESULTS AND DISCUSSION
Synthesis of the Mesogens
The synthesis of the mesogen with the carboxylic end-group is outlined in
Scheme 1. Pentabromophenol is substituted with a spacer containing a
terminal protected carboxylic moiety [5a]. A five-fold palladium catalysed
cross-coupling reacting with 4-ethynyltoluene [4a,5b], followed by a depro-
tection under basic conditions affords the mesogen in reasonable yields.
Ester derivatives of 1 were prepared under standard esterification
conditions (DCC or DIC, DMAP, pTSA). To prepare the dimers a slight
excess of mesogen was employed. Amides were prepared from the
corresponding pentafluorophenyl esters.

Discotic Multipodes with Nematic Mesophases
389=[1431]
FIGURE 1 Structure of the investigated materials.
Synthesis of the Dimers
Some dimers with varying spacers were synthesised to study the effect
of the nature of the core. They were prepared from a straightforward
esterification reaction from 1 with the commercially available alcohol-
terminated spacers, see Scheme 3. Diisopropyl carbodiimide (DIC) was
used as coupling agent, since it is slightly more reactive than the standard

390=[1432]
Paul H. J. Kouwer et al.
SCHEME 1 Synthesis of the mesogen with a carboxylic acid functional group.
dicyclohexyl carbodiimide (DCC). after the work-up (column chromato-
graphy), the mesogen were obtained in rather good yields.
Attempted Synthesis of the Dendrimers
To attach the mesogens to the dendritic cores, the pentafluorophenyl ester
was stirred with commercially available polypropylenimine (DAB-Am) den-
drimers of the first and second generation (see Scheme 2), according to
literature procedures [6a]. Although stirring was continued for 3 to 10 days,
it has been impossible to isolate a fraction with solely four (first gener-
ation) amide groups. Instead GPC experiments indicated an inseparable
mixture of components with an average degree of substitution of about
three mesogens. Another approach employed the use of a diphenyl (2,3-
dihydro-2-thioxo-3-benzoxazolyl)phosphonate activator [6b], prepared
from 2-benzoxazolethiol and phosphorochloridate. The activated ester
was formed in-situ by reacting 1 with an excess of activator. Surprisingly,
subsequent addition of the dendrimer did not result in fully substituted
dendrimers either.

Discotic Multipodes with Nematic Mesophases
391=[1433]
SCHEME 2 Synthesis of the monomers 1-6 and dimers 7-11.
We believe that the lack of complete substitution is likely to be caused
by the limited solubility of the intermediate products in the reaction mix-
ture. This might be due to the combination of the amide groups forming
hydrogen bonds and the strong aromatic character of the mesogens that
is lacking multiple flexible tails. Therefore, future research will focus on
SCHEME 3 Schematic representation of the attempted substitution of the
mesogen to the first generation (DAP-Am-4) dendrimer.

392=[1434]
Paul H. J. Kouwer et al.
the use of mesogens with flexible tails laterally attached to the mesogen as
well as other methods to attach the mesogens to the dendritic core.
Liquid Crystalline Properties
The liquid crystalline properties of the mesogens were investigated by dif-
ferential scanning calorimetry (DSC), optical polarising microscopy (OPM)
and X-ray diffraction (XRD). All materials exhibit the nematic discotic
phase at high temperatures, easily characterised by their characteristic op-
tical textures. Their phase behaviour is summarised in Tables 1 and 2.
Rigid Mesogen Fraction
Various factors that are responsible for diversity in the phase behaviour can
be enumerated: (i) the dilution the fraction rigid mesogen (ii) the
reduction of the mobility of the mesogens by a chemical linkage and
(iii) the introduction of specific stabilising intermolecular interactions,
such as hydrogen bonding.
It has been shown that, for a series of side chain polymers with methyl-
substituted mesogens attached, the clearing temperature TNI shows a
virtually linear dependence with the rigid mesogen fraction [M] [7]. We have
calculated the rigid mesogen fraction for all derivatives of 1 (see Tables 1
and 2) and drawn them in a master phase diagram, see Figure 2. Mutual
comparison of the materials gives insight into the various interactions
between the mesogens and=or the end-groups or spacers.
The phase diagram clearly shows the strong dependence of the clearing
temperature with the rigid mesogen fraction. In addition, the stabilising
effects of hydrogen bonding as well as chemically linking the mesogens
TABLE 1 Liquid Crystalline Properties of the Mesogens
Endgroup ꢁR
Phase behavioura
[M]=%-wtb
1
2
3
4
5
6
7
ꢁOH
Cr
Cr
Cr
Cr
Cr
Cr
Cr
187
N
N
N
244
201
196
I
I
I
I
I
I
I
78.2
76.7
65.4
62.4
56.2
74.6
75.9
ꢁOCH3
167
150
151
165
168
202
ꢁOPhF5
ꢁ(aza-5-crown-15)ꢁ
ꢁCH2C6F13
ꢁNH(CH2)2CH3
ꢁ[NHCH2CH2]2ꢁ
N
N
N
176
189
230
a Transition temperatures in [ꢂC], K ¼ crystalline, N ¼ nematic, I ¼ isotropic.
b [M] is the mesogen weight fraction.

Discotic Multipodes with Nematic Mesophases
393=[1435]
[M]=%-wtb
TABLE 2 Liquid Crystalline Properties of the Dimers
Spacer –R0–
Phase behavioura
8
9
ꢁO(CH2)12Oꢁ
Cr
Cr
Cr
Cr
159
167
166
158
N
N
N
N
180
197
196
172
I
I
I
I
71.2
71.5
59.7
70.6
ꢁOCH2CH2(OCH2CH2)2Oꢁ
ꢁOCH2(CF2)10CH2Oꢁ
ꢁ(diaza-5-crown-15)ꢁ
10
11
a Transition temperatures in [ꢂC], K ¼ crystalline, N ¼ nematic (discotic), I ¼ isotropic.
b [M] is the mesogen weight fraction.
can be observed [8]. Interestingly, while the clearing temperature shows a
nearly linear dependence with [M] the melting temperature is for all mate-
rials rather constant (apart from the materials with strong H-bonding, the
Tm’s are between 150 and 170ꢂC). This explains why materials with a too
low rigid mesogen fraction (such as the mono-crownether 4) show no
FIGURE 2 Clearing temperature TNI of the studied mesogens as a function of the
rigid mesogen fraction [M]. The solid symbols the mesogens discussed in this con-
tribution: dimers (triangles); strong H-bonding groups (solid circles); strong
H-bonding dimers (diamond) and other (squares). Note that some compounds
based on the same mesogen with an undecanol spacer, have been added to the dia-
gram [4]. The open symbols represent a series of side chain polymers bearing the
same mesogen [4]. The solid line represents the nearly linear dependence of the
clearing temperature of the polymers with the rigid mesogen fraction [M]: TNI
695ꢃ[M]0.98, obtained after fitting the phase diagram [7].
¼

394=[1436]
Paul H. J. Kouwer et al.
mesophase behaviour. The corresponding dimer with an increased [M] by
attaching two mesogens per crown ether unit, shows again the nematic
phase. It was anticipated that by the introduction of metal ions in the crown
ethers, the mesophase could be stabilised. However, we observed a desta-
bilisation of the nematic phase and a slight stabilisation of the crystalline
phase, resulting in a loss of the liquid crystalline properties: 11:Naþ Tf ꢁ
complexe (1:1):K 161 I. No mesophase could be induced by complexing
4 with one equivalent of Naþ Tf–: 4:Naþ Tf ꢁ complexe (1:1):K 135 I.
Two materials show anomalous behaviour, indicating a strong stabilising
effect. For the pentafluorophenyl ester 3 stabilising quadrupolar interac-
tions are believed to increase the clearing temperature [9]. The largest
deviation, observed for the dimer with the fluorinated spacer 8, is most
probably due to microphase separation.
It is stressed that only after constructing the phase diagram, shown in
Figure 2, we realised that nematic mesophase was stabilised because of
specific interactions, in particular the nanophase separation process in
the fluoroalkyl-substituted materials. Note that these materials (5 and
10) have similar mesophases and phase transitions to all other materials
and are therefore not easily selected as the odd ones out. With Figure 2
so clearly indicating mesophase stabilisation, we performed X-ray diffrac-
tion to look for any higher extent in order.
X-ray Diffraction
The dimers with the alkyl and glycol spacers (8 and 9) show the expected
diffraction patterns of the nematic (discotic) phase: two (weakly aligned)
diffuse reflections of the disc-dimensions and one (non-aligned) diffuse re-
flection of the alkyl tails, see Figure 3 and Table 3. The pattern of the dimer
with the fluorinated spacer 10 looks quit different. The sample is better
˚
aligned and shows in the small angle area an extra reflection of at 34 A.
FIGURE 3 Two-dimensional X-ray diffraction pattern of the dimers: (a) dimer 8,
alkyl spacer; (b) dimer 9, glycol spacer and (c) dimer 10, fluorinated spacer.

Discotic Multipodes with Nematic Mesophases
395=[1437]
TABLE 3 Results of the X-ray Diffraction Measurements of the Dimers 8–10
T=ꢂC
Phase
˚
d100=A
˚
d001=A
˚
dalkyl=A
Dimer
8
175
175
175
N
N
N
15.7
15.4
14.2
4.2
4.2
4.0
5.3
5.2
5.2
9
10a
a
˚
The fluorinated dimer 10 shows an extra reflection at d ¼ 34.2 A.
Although the assignment of this extra reflection is at present not com-
pletely clear, it clearly indicates an increased order, even stronger pointed
˚
out by the high correlation length of this reflection: n ¼ 255 A [10]. This is
in good agreement with the stabilisation of the nematic phase, observed in
the phase diagram in Figure 3. Similarly, the corresponding ‘monomer’ 5
showed a sharper small angle reflection at similar angels. these interesting
materials are currently under detailed investigation.
CONCLUSIONS
We have synthesised a disc-shaped and carboxylic acid-functionalised
mesogen. Several attempts to attach the mesogen to commercially available
DAB-Am dendrimers did not afford the desired products, presumably due
to the limited solubility of the intermediate products. The mesogen has
been attached to a series of end-groups, of which some were bi-functional,
resulting in dimeric species. All members of this series of mesogens display
the nematic discotic phase at high temperatures. The clearing tempera-
tures of the individual materials could be well understood by taking into
account: (i) their fraction of rigid mesogen, (ii) the presence of any lin-
kages between the mesogens and (iii) the presence of any non-covalent
interactions. We would like to stress the large value of constructing a phase
diagram as shown in Figure 2 in comparing a series of derivatives in order
to easily select ‘exceptional’ materials. Special cases of stabilisation of the
nematic phase were observed for 3 (caused by quadrupolar interactions)
and 5 and 10 (caused by microphase segregation). Detailed investigation
of the latter is still in progress.
REFERENCES
[1] Cammidge, A. N. & Bushby, R. J. (1998). For a review on discotic liquid crystals, read. In:
Handbook of Liquid Crystals, Demus, D., Goodby, J. W., Gray, G. W., Spiess, H. W., & Vill,
V. (Eds.), Wiley VCH: New York, Vol. 2B, 693–748.

396=[1438]
Paul H. J. Kouwer et al.
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¨
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(b) The rigid mesogen fraction is defined as the weight fraction of the rigid (aromatic)
part of the mesogen. In this specific mesogens, the peripheral methyl groups are
included in the rigid part of the molecule.
[8] Note that the clearing temperatures of the polymer samples are all slightly higher because
of the far-reaching degree of linking the mesogens.
[9] (a) Blatch, A. E., Fletcher, I. D., & Luckhurst, G. R. (1995). Liq. Cryst., 18, 801–809.
(b) Weck, M., Dunn, A. R., Matsumoto, K., Coates, G. W., Lobkovsky, E. B., & Grubbs, R.
H. (1999). Angew. Chem. Int. ed. Engl., 38, 2741–2745.
[10] The correlation length of a reflection is determined from the width (at half maximum)
from the radially integrated diffraction pattern with the Scherrer equation. It is a measure
for the extent of positional order in the material. Typical values, measured for these meso-
˚
˚
˚
gens in the ND phase are: n100 ¼ 50 A, n001 ¼ 12 A and nalkyl ¼ 10 A.