Hydrogen-bonded block mesogens with fluorinated molecular fragments

Article abstract of DOI:10.1080/15421400903051069

An alkoxyphenyl substituted melamine derivative has been investigated in binary mixtures with a two-chain semiperfluorinated benzoic acid. The two components are non-liquid crystalline in their own right. Equimolar mixtures of the melamine with the complementary benzoic acid form discrete hydrogen-bonded heterodimers. The dimeric supermolecules exhibit an induced rectangular columnar mesophase with two-dimensional c2 mm lattice symmetry. The columnar phase represents a ribbon phase resulting from the collapse of smectic bilayers. Docking of two or three equivalents of the two-chain benzoic acid to the melamine core leads to the induction of hexagonal columnar mesophases. Copyright Taylor & Francis Group, LLC.

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Hydrogen-Bonded Block
Mesogens with Fluorinated
Molecular Fragments
Dietmar Janietz a & Alexandra Kohlmeier a
a Fraunhofer Institute for Applied Polymer
Research , Potsdam-Golm, Germany
Published online: 05 Oct 2009.
To cite this article: Dietmar Janietz & Alexandra Kohlmeier (2009) Hydrogen-Bonded
Block Mesogens with Fluorinated Molecular Fragments, Molecular Crystals and Liquid
Crystals, 509:1, 39/[781]-46/[788], DOI: 10.1080/15421400903051069
To link to this article: http://dx.doi.org/10.1080/15421400903051069
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Mol. Cryst. Liq. Cryst., Vol. 509, pp. 39=[781]–46=[788], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903051069
Hydrogen-Bonded Block Mesogens with
Fluorinated Molecular Fragments
Dietmar Janietz and Alexandra Kohlmeier
Fraunhofer Institute for Applied Polymer Research, Potsdam-Golm,
Germany
An alkoxyphenyl substituted melamine derivative has been investigated in binary
mixtures with a two-chain semiperfluorinated benzoic acid. The two components
are non-liquid crystalline in their own right. Equimolar mixtures of the melamine
with the complementary benzoic acid form discrete hydrogen-bonded hetero-
dimers. The dimeric supermolecules exhibit an induced rectangular columnar
mesophase with two-dimensional c2 mm lattice symmetry. The columnar phase
represents a ribbon phase resulting from the collapse of smectic bilayers. Docking
of two or three equivalents of the two-chain benzoic acid to the melamine core leads
to the induction of hexagonal columnar mesophases.
Keywords: columnar phases; hydrogen bonding; mesophase induction; nanosegregation
INTRODUCTION
The formation of thermotropic and lyotropic liquid crystalline phases
mainly results from nanoscale segregation of incompatible moleculer
segments [1,2]. Thereby, the mesophase morphologies, lamellar,
columnar or cubic, are predominantly determined by the volume
fractions of the chemically distinct molecular fragments such as rigid=
flexible or polar=nonpolar which separate into different subspaces.
The intramolecular contrast can be reinforced if a lipophilic chain is
replaced by a perfluorinated segment. (Semi)perfluorinated molecular
This work was supported by the Land Brandenburg, Germany and the Fonds der
Chemischen Industrie. We are very grateful to B. Smarsly for the kind support in per-
forming X-ray measurements at the Max Planck Institute of Colloids and Interfaces,
Potsdam-Golm, Germany.
Address correspondence to Dietmar Janietz, Fraunhofer Institute for Applied
Polymer Research, Geiselbergstr. 69, Potsdam-Golm D-14513, Germany. E-mail:
dietmar.janietz@iap.fraunhofer.de; dietmar.janietz@iap.fhg.de
39=[781]

40=[782]
D. Janietz and A. Kohlmeier
FIGURE 1 Chemical structures of the decyloxyphenyl substituted melamine
1 and of the semiperfluorinated benzoic acid 2.
fragments have been successfully introduced into calamitic [3],
discotic [4] and polycatenar [5] mesogens as well as in liquid crystals
without a pronounced anisometric molecular shape such as taper-
shaped [6] and tetrahedral [7] compounds. It could be shown that
the fluorophobic effect may lead to a significant stabilization and even
to modifications of smectic, columnar and cubic mesophases.
A powerful tool towards designing highly organised soft matter
arises from strong cohesive forces such as hydrogen bonding between
identical or complementary molecules [8]. For example, mesomorphic
complexes have been reported emerging from double-hydrogen bond-
ing of 2,6-diacylaminopyridines with aromatic carboxylic acids [9,10]
and of amino substituted 1,3,5-triazine derivatives with benzoic acids
grafted either with lipophilic alkyl tails [11,12] or with semiperfluori-
nated chains [13–15].
We report here on mesophase inductions by combining molecular
recognition between two complementary molecular species and fluoro-
phobic effect. We present mesomorphic structure formation of the
alkoxyphenyl substituted melamine derivative 1 in mixtures with
the two-chain semiperfluorinated benzoic acid 2 (Fig. 1). The two
components are non-liquid crystalline by themselves.
EXPERIMENTAL
The decyloxyphenyl substituted melamine 1 was prepared by reaction
of 6-chloro-2,4-diamino-1,3,5-triazine with 4-decyloxyaniline. The two-
fold semiperfluorinated benzoic acid 2 was obtained by etherification
of methyl 3,5-dihydroxybenzoate with 1-bromo-1H,1H,2H,2H,3H,3H,
4H,4H-perfluorodecane [16] in the presence of potassium carbonate
followed by alkaline ester cleavage. The binary mixed systems of the
arylmelamine 1 with the benzoic acid 2 were prepared by dissolving
the components separately in THF, combining the solutions and
evaporating the solvent. Further investigations were performed after

Hydrogen-Bonded Block Mesogens
41/[783]
annealing the samples in the isotropic state. Texture observations
were made using an Olympus BHS polarizing microscope in conjunc-
tion with a Linkam TMH=S 600 hot stage and a Linkam TP 92 control
unit. Photo micrographs were obtained with an Olympus E20 digital
mirror reflex camera. Calorimetric investigations were performed
with a Netzsch DSC 200. The X-ray studies (CuKa) were carried out
with a Nenius PDS 120 (Bruker) equipped with a detector from Inel.
RESULTS AND DISCUSSION
The phase transition temperatures of the aryl modified melamine deri-
vative 1, of the semiperfluorinated benzoic acid 2 and of binary mixed
systems 1=2 are collected in Table 1. The pure compounds 1 and 2
show a direct transition from the crystalline state to the isotropic
liquid. Each investigated binary mixture 1=2 displays an enantiotropic
mesophase. The clearing temperatures are higher than the melting
temperature of the pure carboxylic acid 2. The mixtures show a strong
tendency towards supercooling. The isotropization temperature is
lower for the 1:2 complex in comparison to the 1:1 mixed system. A
further decrease of the clearing temperature is observed for the
mixture containing three equivalents of the acid 2.
Fernlike optical textures develop on cooling the equimolar composi-
tion of the triazine 1 with the two-chain acid 2 from the isotropic liquid
which collate to form spherulitic textures (Fig. 2). The mesophase
textures of the investigated (1:2) and (1:3) compositions are identical
and can be characterized by the observation of large homeotropic
aligned regions containing birefringent filaments (Fig. 3). Therefore,
it seems that the mixtures exhibit different mesophases depending
on the molar content of the semiperfluorinated acid.
TABLE 1 Phase Transition Temperatures (ꢀC) of the Pure Compounds 1 and
2 and of Binary Mixtures of the Melamine 1 with the Complementary
Semiperfluorinated Benzoic Acid 2 (DSC 2nd Heating; Phase Transition
Enthalpies (kJ=mol) in Parenthesis)
˚
Sample
Transition temperatures
Lattice constants (A)
1
2
Cr
Cr
Cr
Cr
Cr
206.0
58.8
–
I
I
I
I
I
(1:1) 1=2
(1:2) 1=2
(1:3) 1=2
Colr
Colh
Colh
108.3 (2.2)
83.1 (2.8)
73.3 (4.3)
a ¼ 65.1, b ¼ 71.1
ahex ¼ 38.5
ꢁ37.0 (1.7)
–
ahex ¼ 40.3
Cr: crystalline; Colr: rectangular columnar; Colh: hexagonal columnar; I: isotropic.

42/[784]
D. Janietz and A. Kohlmeier
FIGURE 2 Optical texture of the induced mesophase of the equimolar
composition 1=2 between crossed polarizers.
Figure 4 displays the wide angle X-ray diffractogram obtained
for the mesophase of the equimolar 1=2 mixture. The small angle
reflections can be indexed on the basis of a two-dimensional rectangu-
lar unit cell with a centered c2 mm lattice symmetry. The lattice para-
˚
˚
meters are a ¼ 65.1 A and b ¼ 71.1 A.
FIGURE 3 Microphotograph of the columnar mesophase of the melamine 1 in
mixture with two equivalents of the benzoic acid 2 between crossed polarizers.

Hydrogen-Bonded Block Mesogens
43/[785]
FIGURE 4 Wide angle X-ray diffractogramm for the rectangular columnar
phase of the equimolar complex of the triazine 1 with the benzoic acid 2.
The WAXS diffractogram for the (1:2) 1=2 complex is given in
Figure 5. The X-ray diffraction pattern of the mesophase of the mixed
system 1=2 at a 1:2 molar ratio exhibits three sharp reflections in
the small angle region and a diffuse halo in the wide angle region.
The ratio of positions of the reflections is 1:31=2:2, proving the exis-
tence of a hexagonal columnar (Colh) mesophase. The hexagonal
FIGURE 5 Wide angle X-ray diffractogram obtained for the Colh phase of the
(1:2) mixture 1=2.

44/[786]
D. Janietz and A. Kohlmeier
lattice constants ahex for the mixtures of the melamine 1 with two and
three acid equivalents are given in Table 1.
IR-spectroscopic investigations previously have proven that associa-
tion of phenyl substituted diamino-1,3,5-triazines with complemen-
tary benzoic acids is related to double-hydrogen bonding between
the nitrogen heterocycle substituted with two exocyclic primary amino
groups and the carboxylic group of the acid [13]. Thereby, in equimolar
compositions, preferentially heterodimers are formed with a rod-like
shape of the polar H-bonded core [13,15]. Hence, it is reasonable to
suggest that docking of one equivalent of the benzoic acid 2 to the
melamine heterocycle 1 preferably occurs opposite from the bulky
phenyl group leading to heterodimeric supermolecules 1=2 with a
pronounced rod-like core geometry as well.
The linear pre-organization of the extended hydrogen-bonded cores
should enable sufficiently strong attractive interactions to favour a
parallel organization. As it is evident from CPK models the length of
˚
H-bonded dimers 1=2 amounts to approximately 45 A considering
almost stretched conformations of the flexible alkyl segments. The
lattice parameter b matches nearly twice the molecular length. There-
fore, it is most likely that hydrogen-bonded bilayer aggregates are
present in the rectangular lattice.
The cross sectional area of the terminal semiperfluorinated chains
exceeds the cross-section of the closely aligned central H-bonded aro-
matic and the lipophilic regions. The smectic bilayers should break
up into infinite ribbon-like segments with curved interfaces due to
the different space filling of the fluoroaliphatic and aromatic regions.
FIGURE 6 Schematic presentation of the induced mesophases of the mixed
systems 1=2. Left: rectangular columnar phase (c2 mm lattice symmetry) of
the 1:1 complex; Right: hexagonal columnar phase (two-dimensional p6 mm
space group) of the 1:2 and 1:3 complexes.

Hydrogen-Bonded Block Mesogens
45/[787]
The ribbon-like aggregates organize to a columnar mesophase on a
two-dimensional rectangular lattice (Fig. 6).
The proposed ribbon model seems reasonable since it enables the
segregation of fluorophilic and lipophilic moieties into separate
regions whereby the parallel organization of the polar H-bonded cores
is maintained.
CPK models reveal that docking of two or three molecules of the
semiperfluorinated benzoic acid 2 at the melamine core leads to
H-bonded associates with a pronounced circular geometry. Cylindrical
aggregates are formed that consist of three distinct molecular regions,
the polar hydrogen-bonded aromatic region located in the centre sur-
rounded by the lipophilic alkyl segments and, finally, the fluorinated
blocks distributed arround the cylindrical core region. The parallel
alignment of these aggregates leads to the two-dimensional hexagonal
lattice symmetry (Fig. 6).
The diameter of the (1:3) mixture of the melamine 1 and the acid 2,
for example, evaluated from the CPK model amounts to approximately
˚
44 A. This value is in excellent agreement with the hexagonal lattice
˚
constant (40 A) determined by X-ray diffraction.
CONCLUSIONS
The hydrogen-bonded complexes of the alkoxyphenylmelamine with the
complementary semiperfluorinated benzoic acid can be considered as
non-covalent analogues of block mesogens composed of three different
and incompatible molecular segments. Docking of one equivalent of
the acid preferentially leads to heterodimers with elongated aromatic
cores. Microsegregation along with the linear pre-organization of the
extended H-bonded polar segments favours a parallel bilayer-like
arrangement. However, the stabilization of layered mesophases is in
competition with the disturbance provided by the different space
requirements of the fluorinated and lipophilic molecular blocks. A
bilayer ribbon phase with two-dimensional c2 mm plain group is formed.
A further approach towards controlling the molecular topology
arises from the molar content of the acid. Hydrogen-bonding with
two or three acid molecules defines a cylindrical molecular geometry
of the melamine=benzoic acid associates which, consequently, organize
to hexagonal columnar mesophases.
Hence, the cooperative interplay of hydrogen bonding, nanoscale seg-
regation of incompatible molecular block and controlling the interface
curvature of assembled aggregates by the volume fractions of molecular
fragments segregated in different sub-spaces accounts for tailoring the
mesophase morphologies of the complementary molecular species.

46/[788]
D. Janietz and A. Kohlmeier
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