New star-shaped mesogens with three different arms on a 1,3,5-benzene core

Article abstract of DOI:10.1080/15421400490435422

The convergent synthesis of new symmetrical and non-symmetrical star-shaped mesogens based on a phloroglucinol core is presented. Molecules with three different arms were derived via esterification of various 4-benzoylbenzoic acid derivatives, substituted

Full text of DOI:10.1080/15421400490435422

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New Star-shaped Mesogens
with Three Different Arms on a
1,3,5-Benzene Core
Matthias Lehmann a , Raluca Gearba b , Dimitri
Ivanov b & M.H. J. Koch c
a Laboratoire de chimie des polymères
b Laboratoire de physique des polymères , Université
Libre de Bruxelles , Boulevard du Triomphe, Belgium
c European Molecular Biology Laboratory , Hamburg,
Germany
Published online: 18 Oct 2010.
To cite this article: Matthias Lehmann , Raluca Gearba , Dimitri Ivanov & M.H.
J. Koch (2004) New Star-shaped Mesogens with Three Different Arms on a 1,3,5-
Benzene Core, Molecular Crystals and Liquid Crystals, 411:1, 397-406, DOI:
10.1080/15421400490435422
To link to this article: http://dx.doi.org/10.1080/15421400490435422
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 397=[1439]–406=[1448], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435422
NEW STAR-SHAPED MESOGENS WITH THREE
DIFFERENT ARMS ON A 1,3,5-BENZENE CORE
Matthias Lehmannꢀ
`
Laboratoire de chimie des polymeres, CP 206=1
Raluca I. Gearba and Dimitri A. Ivanov
`
Laboratoire de physique des polymeres, CP 223,
´
Universite Libre de Bruxelles, Boulevard du Triomphe,
1050 Bruxelles, Belgium
M. H. J. Koch
European Molecular Biology Laboratory, Hamburg Outstation,
EMBL c=o DESY, Notkestrasse 85,
D-22603 Hamburg, Germany
The convergent synthesis of new symmetrical and non-symmetrical star-
shaped mesogens based on a phloroglucinol core is presented. Molecules with
three different arms were derived via esterification of various 4-benzoyl-
benzoic acid derivatives, substituted with polar groups and n alkoxy chains
(n ¼ 1–3), by using a combination of protecting group techniques and stoichio-
metric reactions. The thermotropic properties were investigated by differential
scanning calorimetry, polarised optical microscopy and synchrotron X-ray
diffraction. It is found that molecules with more than seven lateral chains form
mesophases. The presence of strong polar groups (–NO2) enlarges the tempera-
ture range of the liquid crystal phase. For all studied materials the diffracto-
grams can be indexed according to a two-dimensional hexagonal lattice. The
lattice parameters indicate that the flexible chains are interdigitated.
Keywords: columnar liquid crystal; dipolar interaction; non-symmetric mesogen; star-shaped
molecule
ꢁ
´
We are grateful to the Communaute Francaise de Belgique for its financial support (ARC n
00=05-257), to Prof. Herbert Meier for the availability of FD Mass and elemental analysis at the
University of Mainz, to Prof. Yves Geerts for the generous support and helpful discussions. The
synchrotron radiation work was supported by the European Union through the HCMP Access
to Large Installation Project, Contract HPRI-CT-1999-00017 to the EMBL-Hamburg.
ꢀCorresponding author. E-mail: matthias.lehmann@ulb.ac.be
397=[1439]

398=[1440]
M. Lehmann et al.
INTRODUCTION
The classical columnar phases are formed by planar aromatic mesogens,
e.g. triphenylene, hexabenzocoronene, segregating from flexible chains,
which are uniformly surrounding the rigid core. Such a supramolecular
organisation can be also obtained by non-conventional multi-arm systems
[1–3]. They may be built by linking three or more calamitic or non-
mesogenic units to a central building block. Up to sixty-four mesogenic
or promesogenic arms are attached in the case of starburst dendrimers,
where the flexibility of the central dendritic scaffolds lead to self-assembly
in lamellar, columnar or cubic phases, depending on the number of peri-
pheral alkyl chains [4]. The simplest multi-arm mesogen is a structure
containing three branches. Recently, thermotropic liquid crystal properties
have been observed for various star-shaped, three-armed molecules with a
1,3,5-benzene core [2,6–14]. The supramolecular columnar arrangement
was tuned by introduction of polar groups [10,11] and hydrogen bonds
[12,13]. These molecules are non conventional discotic mesogens, where
the single core can display cavities [6b]. In the condensed phase, molecules
should assemble in uniformly space filling way. Consequently, the arms
of the mesogens should deviate from coplanar stacking. In principle, space
filling and specific interactions of star-shaped molecules, with attached
polar groups at three different arms could lead to appearance of intra-
or inter-columnar superstructures, e.g. staggered arrangement for anti-
parallel dipoles, or helical displacement along the column. Theoretically,
the equilibrium organisation of N polar molecules cannot be reliably
predicted [15]. However, recent theoretical calculations envisage for dis-
cotic mesogens with transverse dipole moments potential application as
biaxial switches [16].
In this contribution the synthesis of new star-shaped mesogens 1
(Chart 1) based on the phloroglucinol core is presented. Earlier publica-
tions reveal that direct attachment of alkyloxy substituted benzoic acid
derivatives lead only to monotropic phase behaviour [14]. Elongation of
CHART 1 General structure of the star-shaped mesogens 1.

New Star-shaped Mesogens with Three Different Arms
399=[1441]
the arms could stabilise the supramolecular liquid crystal organisation.
Derivatives of 4-benzoylbenzoic acid seem to be appropriate for this pur-
pose, because peripheral alkoxy chains and internal polar substituents
can be easily attached.
RESULTS AND DISCUSSION
Synthesis
The synthesis was carried out following a convergent strategy outlined in
Schemes 1 and 2. According to literature procedures, [17] the polycatenar
arms 5a-g were prepared by esterification of 4-hydroxy-(benzylbenzoate)
derivatives 4a-g [18] with benzoic acid building blocks bearing 1–3 periph-
eral dodecyloxy chains 6 and subsequent reductive cleavage of the protect-
ing group (Scheme 1). The symmetrical star-shaped mesogen 1a (Table I)
were directly obtained by threefold coupling of arm 5a (X, Y ¼ H,
R1–3 C12H25) to the phloroglucinol core.
=
For the synthesis of non-symmetrical mesogens with three different
arms, a combination of protecting group technique and stoichiometric
reaction was chosen. In the first step, coupling of the arm 5a (X, Y ¼ H;
R1–3 OC12H25) to the twofold benzylether protected phloroglucinol 7 [19]
=
and selective reductive cleavage gave the dihydroxy derivative 3. Stoichio-
metric esterification of arms 5 with 3 resulted in up to 34% of 2 after
exhaustive purification. As a side product, mesogens 1e,g with twofold
symmetry were isolated. In a final reaction a third different arm 5 were
attached to form the target molecules 1b-d, f. Yields of exhaustively
purified materials 1 are given in Table I. The compounds 1 were charac-
1
terised by H and 13C NMR and elemental analysis [20].
SCHEME 1 Synthesis of various arms 5 with lateral dodecyloxy chains.

400=[1442]
M. Lehmann et al.
SCHEME 2 Synthesis of non-symmetrical star-shaped molecules 1.
Thermotropic Properties
The thermotropic behaviour of the star-shaped molecules 1 was studied
by means of differential scanning calorimetry (DSC), polarised optical
microscopy (POM) and synchrotron X-ray diffraction (XRD). The DSC
and POM results are collected in Table II. It is found that molecules with
less than eight lateral dodecyloxy chains (1b,g) crystallise from the

New Star-shaped Mesogens with Three Different Arms
401=[1443]
TABLE I Yields of the Different Target Molecules 1 for the Final Reaction Step
ꢀ
Compound
n(R1)ꢀ
n(R2)ꢀ
n(R3)
X
Y
Z
Yield [%]
1a
1b
2
1
2
2
2
2
2
2
2
2
2
2
2
2
2
0
2
2
2
1
0
H
H
H
H
H
H
H
H
H
Br
H
79
46
84
28
11
52
13
1c
Br
Cl
Cl
H
Cl
1d
NO2
Cl
1e#
1fx
1g#
Br
H
H
ꢀn(R) ¼ number of dodecyloxy chains (n ¼ 0 i.e. R ¼ H).
x Three chains at the dibromo substituted arm.
# Side product obtained by the synthesis of 2.
isotropic liquid and are not forming mesophases. All other molecules with
eight and nine lateral chains organise in enantiotropic liquid crystal phases
M. Most of them, i.e. 1c-f, solidify into the glassy state close to room
temperature. At lower temperatures an additional broad peak is observed,
which can be probably attributed to partial crystallisation of the alkyl
chains. The LC phase of the mesogen with the largest expected dipole
moment owing to the nitro group, 1d, is considerably more stable than
the symmetrical non-substituted molecule 1a. Halogen substituents do
not seem to have a significant influence on the mesophase stability.
POM of 1a and 1d (Figs. 1 and 2) reveal a pseudo-focal-conic texture
with homeotropic domains. These textures are frequently found for hexag-
onal columnar structures [21]. Homeotropic aligned domains are evidence
for a uniaxial phase.
TABLE II Phase Transitions Determined by DSC and POM
DSC results of the second heating,
rate 10ꢁC=min (Onset [ꢁC]=DH [kJ=mol])
Compound
1a
1b
1c
1d
1e
1f
Cr ꢂ 25.9=21.5 M 50.7=3.3 I
Cr 43.7=16.7 I
g1 ꢂ 26.6=20.2 g2 23.4 (Tg) M 50.3=3.6 I
g1 ꢂ 35.6=32.6 g2 29.4 (Tg) M 77.8=3.6 I
g1 ꢂ 27.0=19.4 g2 20.0 (Tg) M 41.2=3.7 I
g 29.7 (Tg) M 44.6=3.6 I
1g
Cr 53.7=28.6 I
Cr crystal, g glassy phase, M mesophase, I isotropic liquid.

402=[1444]
M. Lehmann et al.
FIGURE 1 Pseudo-focal-conic texture of 1a upon cooling between crossed
polarisers at 47ꢀC with homeotropic domains. (See COLOR PLATE XII)
Star-shaped molecules 1 are not typical discotic mesogens, since the
three arms cannot fill the space belonging to the core region. They should
rather be regarded as link between discotic and phasmidic compounds.
Previous studies on mesogens with a phloroglucinol core, revealed only
columnar and nematic mesomorphism [14]. In principle, smectic phases
could also be formed as shown in Figure. 3. Alternately directing two or
one arms to one side of the layer, molecules can form a smectic-like
FIGURE 2 Pseudo-focal-conic texture of 1d upon cooling between crossed polarisers
at ambient temperature with homeotropic domains. (See COLOR PLATE XIII)

New Star-shaped Mesogens with Three Different Arms
403=[1445]
FIGURE 3 Model of possible mesophases: lamellar, lamellar columnar and
hexagonal columnar phases.
arrangement, but no homeotropic alignment would be expected in this case.
Homeotropic alignment can be achieved if the molecules assemble in
columns which may then arrange in a lamellar or hexagonal superstructure.
The mesophase structures of compounds 1a,c,d,e were studied by X-ray
diffraction. The measurements were performed on the X33 camera of
the European Molecular Biology Laboratory at the storage ring DORIS III
of the Deutsches Electronen Synchrotron (DESY), Hamburg using two
linear position sensitive detectors connected in series [22,23]. The X-ray
diffraction pattern of the star-shaped mesogen 1d(Fig. 4) shows a set of
three reflections in the small angle region with reciprocal spacings in the
p
ratio 1 : 3 : 2. These peaks are assigned to the (10), (11), (20) reflections
of a hexagonal columnar mesophase. In the wide-angle region (Inlet, Fig. 4),
X-ray diffraction data displays an amorphous halo reflecting the mean
distance between the aliphatic side chains (0.43 nm). Superimposed on
the small-angle region an additional halo is observed and could originate
from a superstructure along the column. This has to be proven by recording
two-dimensional X-ray scattering patterns from oriented samples. All other
molecules 1a,c,e yield analogous diffraction patterns.
Calculated cell parameters a for the Colhd phases (a ¼ d10 ¼ sin 60ꢁ)
vary from 4.16 nm (1e), 4.23 nm (1b), 4.25 nm (1a) up to 4.32 nm for 1d.
All cell parameters are significantly smaller than the molecular diameter,
derived from a model with all trans configuration of the peripheral chains.
The radius of the semi-flexible core were estimated to rcore ¼ 1.52 nm,
which is the distance between the centre of the molecule and the peri-
pheral oxygen in para position, connecting an aliphatic side chain.
The length of the alkyl chain in all trans configuration is calculated by

404=[1446]
M. Lehmann et al.
FIGURE 4 SAXS and WAXS X-ray diffractograms of 1d recorded simultaneously
with two position sensitive detectors connected in series, k ¼ 0.154 nm.
the formula rchain ¼ (0.189 þ 0.1265 ꢃ n) nm given in reference [24], where
n is the number of carbon atoms and thus, rchain (C12H25) ¼ 1.707 nm.
The diameter of the molecule is consequently dmolecule ¼ 2(rcore þ rchain
)
6.45 nm. If the flexibility of the lateral chains is taken into account the
diameter diminishes to dflex ¼ 5.54 nm. This value is still larger than the
experimental measured columnar distances. Consequently, a model
with interdigitated chains is proposed. On the basis of fully interdigitated
flexible chains, a minimum value dcolumn ¼ 4.28 nm for columns can be
estimated, which is closer to experimental values (4.16–4.32 nm). Note
that the distances vary within the series of different mesogens. The largest
intercolumnar distance is found for the molecule with the largest estimated
dipole, thus suggesting repulsive dipolar interactions between neighbour-
ing columns. On the other hand, molecule 1d shows the highest mesophase
stability, thus attractive intracolumnar dipole interactions should be
present. This can be explained by the semi-flexibility of the benzoylester
core. Various conformations can be adapted by rotation about the C-O
single bonds. Thus molecules can possess not only a transversal but also
an axial dipole moment.
CONCLUSIONS
New star-shaped molecules 1 were synthesised with up to three different
arms, bearing various polar groups on the central benzoyl ring and one

New Star-shaped Mesogens with Three Different Arms
405=[1447]
to three dodecyloxy chains on the peripheral benzoyl unit. Mesogens with
more than seven flexible chains form mesophases. The columnar hexagonal
LC phases are stabilised by dipolar interactions and interdigitation of the
lateral chains. The largest unit cell parameter is found for the molecule
with the largest dipole indicating the existence of repulsive dipolar inter-
action between neighbouring columns. Moreover, a diffuse halo observed
in the small-angle region could be related to superstructure along the
columns.
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406=[1448]
M. Lehmann et al.
1a and 1c. Analytical data for 1a: 1H NMR (300 MHz, CDCl3) d ¼ 0.92 (m, 27H, CH3),
1.26–1.55 (m, 162H, CH2), 1.81 (m, 18H, CH2), 4.06, 4.08 (2t, 18H, OCH2), 7.20
(s, 3H, ArH), 7.42 (s, 6H, ArH), 7.37, 8.35 (AA0BB0, 12H, ArH). 13C NMR (75 MHz, CDCl3)
d ¼ 14.0, 22.6, 26.0, 26.8, 29.2, 29.3, 29.5, 29.6, 30.3, 31.8, 69.2, 73.5 (CH3, CH2, OCH2),
108.6, 113.5, 122.2, 123.2, 126.4, 132.8, 143.3, 151.4, 152.9, 155.5, 163.6, 164.3 (Caromat
,
C¼O). EA (C156H246O21) calculated %C 76.24, %H 10.09, found %C 76.12, %H 10.03.
Analytical data for 1c : 1H NMR (300 MHz, CDCl3) d ¼ 0.92 (m, 27H, CH3), 1.26-1.55 (m,
162H, CH2), 1.81 (m, 18H, CH2), 4.06, 4.08 (2t, 18H, OCH2), 7.21 (3H, ArH), 7.36-7.48
(10H, ArH), 8.18 (dd, 3J ¼ 8.6 Hz, 4J ¼ 2 Hz, 1H, ArH), 8.22 (dd, 3J ¼ 8.6 Hz, 4J ¼ 2 Hz,
1H, ArH), 8.29 (AA0BB0, 2H, ArH), 8.35, 8.51 (2d, 4J ¼ 2 Hz, 2H, ArH). 13C NMR
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(CH3, CH2, OCH2), 108.7, 108.8, 113.0, 113.5, 122.3, 124.2, 124.3, 129.9, 130.6, 131.9,
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143.6, 151.3, 151.6, 153.1, 155.6, 162.6, 163.5, 164.4 (quat-Caromat
,
C¼O). EA
(C156H244BrClO21) calculated %C 72.88, %H 9.57, found %C 73.17, %H 9.34.
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