Self-assembling of tapered benzenesulfonate-based dendrons with bulky aromatic focal groups

Article abstract of DOI:10.1016/j.mencom.2015.03.023

Comparative analysis of the structure and phase behavior of the synthesized compounds on the basis of symmetrically and asymmetrically substituted benzenesulfonic acids with bulky aromatic focal groups revealed that their phase behavior is determined equa

Full text of DOI:10.1016/j.mencom.2015.03.023

Mendeleev
Communications
Mendeleev Commun., 2015, 25, 142–144
Self-assembling of tapered benzenesulfonate-based
dendrons with bulky aromatic focal groups
Maxim A. Shcherbina,*a,b Artem V. Bakirov,a,b Linglong Yan,c Uwe Beginn,c
Xiaomin Zhu,d Martin Möllerd and Sergey N. Chvaluna,b
a N. S. Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences, 117393 Moscow,
Russian Federation. Fax: +7 495 718 3404; e-mail: shcherbina@ispm.ru
b National Research Centre ‘Kurchatov Institute’, 123098 Moscow, Russian Federation
c Organic Materials Chemistry, Institute for Chemistry of New Materials, University of Osnabrück,
D-49069 Osnabrück, Germany
d Institute of Technical and Macromolecular Chemistry, RWTH Aachen and DWI e.V., D-52056 Aachen,
Germany
DOI: 10.1016/j.mencom.2015.03.023
Comparative analysis of the structure and phase behavior of the synthesized compounds on the basis of symmetrically and asym-
metrically substituted benzenesulfonic acids with bulky aromatic focal groups revealed that their phase behavior is determined equally
by the shape of the mesogenic groups as well as by specific interactions at the focal points of the monodendrons.
Exploiting the self-assembly of amphiphilic conic and tapered
dendrons for the rational design of functional supramolecular
ensembles is an important area of modern materials science.
Substantial interest in this field is caused by a wide variety of
lyotropic and thermotropic mesophases as observed in systems with
one-dimensional (smectic layers and bilayers), two-dimensional
(ordered and disordered columnar phase) and three-dimensional
(crystalline hexagonal and plastic crystals of different groups of
symmetry and bicontinuous cubic lattices1–7) long range order.
Despite of the apparent complexity of the observed phase behavior
of tapered and cone-shaped dendrons, theoretical models developed
so far allow one to explain the self-assembling of supramolecular
aggregates and mesophases.
First, a geometrical approach should be mentioned: the volume
radial distribution dV/dr8 of Voronoi polyhedrons constructed
for a crystal lattice of a particular group of symmetry is considered.
The shape of the dV/dr curve coincides with the cross-section of
a dendron; i.e., its projection on the plane perpendicular to its
axis, fitting in a best way to the considered crystal lattice with
minimal steric hindrances. Variations in temperature, pressure,
humidity, solution acidity, etc. differently affect various parts of
a molecule9–11 leading to essential changes in dendron shape
and, consequently, to a phase transition in the material. More-
over, the theory of microphase segregation developed for block
copolymers in the limiting cases of weak12 and strong13 segrega-
tion is also applicable to amphiphilic dendrons: any repulsion of
incompatible fragments is followed by their micro-segregation
under simultaneous minimization of the interphase area. A general
model incorporating two extreme cases was described,14 con-
sidering not only the enthalpy of segregation but also its entropy,
a tendency of the system to uniform space filling.
micelles packed into cubic lattices. With the bulky cations, the
shape of the dendron resembles a truncated cone. In such meso-
genes called cunitic (lat. cuneus = wedge) amphiphiles,18 ordered
and disordered columnar mesophases appear to be more stable.
Based on the experimental data, we concluded that the inter-
action of ionic pairs in the centers of supramolecular units defines
the structure of the formed supramers. For this reason
the compounds on the basis of benzenesulfonic acid possessing
large focal groups, which are capable of specific interactions with
an interaction energy comparable to that of ionic forces, such as
p–p stacking, are of special interest. We studied the structure
and temperature behavior of supramolecular aggregates formed
by pyridinium 2,3,4-tris(dodecyloxy)benzenesulfonate 1† and
symmetrically substituted sodium and pyridinium p-{(E)-p-[3,4,5-
tris(dodecyloxy)benzoyloxy]phenylazo}benzenesulfonates 2 and
3, respectively, by wide- and small-angle X-ray diffraction, dif-
ferential scanning calorimetry (DSC), polarizing optical micro-
scopy (TPOM) and molecular modeling. Note that the intro-
AlkO
AlkO
OAlk
O
HN
N
S
O
1
O
AlkO
AlkO
AlkO
O
S
O
O
X
O
N
O
2 X+ = Na+
3 X+ = pyridinium
Alk = C12H25
The synthesis of vast libraries of similar compounds with
incrementally varied parameters makes it possible to analyze
systematically the structure and phase behavior of amphiphilic
compounds and their self-assembling.15–18 Our recent studies of
the alkali metal salts of 2,3,4-tris(dodecyloxy)benzenesulfonic
acid19,20 revealed that a modification in the shape of the meso-
genic group due to variations in the size of its metal cation leads
to predictable changes in the phase behavior. Conic dendrons
with a small ion focal point tend to form smectic bilayers. With
increasing temperature, these bilayers transform to spherical
†
2,3,4-Tris(dodecyloxy)benzenesulfonate 1 was obtained by the sulfona-
tion of 1,2,3-tris(dodecyloxy)benzene [synthesized in 76% yield by the
alkylation of pyrogallol (1,2,3-trihydroxybenzene) with 1-bromodecane
in DMF, followed by recrystallization from acetone] with concentrated
sulfuric acid at room temperature for 2 h. The following recrystallization
from n-hexane provided 2,3,4-tris(dodecyloxy)benzenesulfonic acid in
87% yield. Then it was dissolved in pyridine by intense stirring for 24 h
under a nitrogen atmosphere. The samples were subsequently dried in a
rotary evaporator, recrystallized from methanol and dried in vacuo at
40°C. The yield of compound 1 was no less than 98%.18
© 2015 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 142 –

Mendeleev Commun., 2015, 25, 142–144
4
3
80
60
40
20
0
100 µm
iso
1
2
1
2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3
T/°C
50
100
T/°C
150
Figure 2 Small-angle X-ray scattering patterns of compound 1 at (1) room
temperature, at (2) 80, (3) 110 and (4) 130°C.
Figure 1 DSC heating scans of compounds (1) 1, (2) 2 and (3) 3. For
clarity, curves for 1 and 2 are shifted along the vertical axis. The insert is a
polarizing optical micrograph of compound 1 at 100°C. The arrow marks
the isotropization peak of compound 1.
4.24 nm
Table 1 Thermal transitions of compounds 1–3.a
Compound
Tmeso/°C DHmeso/kJ mol–1 Tiso/°C
DHiso/kJ mol–1
1
2
3
82
55
95
101
34
129
1.5
b
—
—
68
200
< 0.5
a Subscripts ‘meso’ refer to the transition in a high-temperature mesophase,
while subscripts ‘iso’ refer to the isotropization. b Isotropization was not
observed up to 300°C.
Figure 3 Molecular modeling of a smectic bilayer formed by 1. Atomic
sizes are estimated from van der Waals potentials.
duction of an E/Z-photoisomerable azobenzene group into the
structure of a dendron can open a new way of controlling the
structure of supramolecular ensembles by means of light.21–23
It is important to note that the electrophilic aromatic substitution
always leads to the sulfonation of 1,2,3-tris(dodecyloxy)benzene
in the 4- and 6-positions of the benzene ring. Hence, the synthesis
of compounds 2 and 3 based on the symmetrically substituted
3,4,5-tris(dodecyloxy)benzoic acid was performed as described
in detail elsewhere.24 The synthesized materials were white disperse
crystalline powders, which became waxy on heating. The DSC
scans of all the compounds are characterized by a strong endo-
thermal effect which was attributed to an order–disorder meso-
morphic transition according to the X-ray data. Transition tempe-
ratures and enthalpies measured by DSC (Figure 1) are given in
Table 1. The transitions of both compounds 1 and 3 to an isotropic
state are characterized by an extremely low heat effect testifying
the low degree of an order of their high-temperature mesophases
(see below). In case of compound 2, an isotropization was not
observed up to T = 300°C when the samples started to decompose.
Figure 2 shows the temperature evolution of small-angle
X-ray patterns of compound 1. At room temperature, at least
six orders of the smectic reflection are observed. The measured
layer thickness was dL = 42.7 Å, while the length of the molecule
with completely extended aliphatic tails calculated from mole-
cular modeling was 22.5Å. Thus, the bilayer structure of smectic
mesophase can be concluded. The results of the molecular
modeling of the layer structure of compound 1 (Figure 3) show
that mutual arrangement and local order formation of the meso-
genic groups are facilitated by the p–p-stacking interaction of
the pyridinium fragments. On heating to 80°C, this ordering is
improved: the small-angle reflections become narrower and shift
to the wide-angle region (dL = 42.4 Å). Two orders of the new
reflection corresponding to the interplanar distance of d = 30.6 Å
also developed.
The intensities of the reflections of the low-temperature
smectic mesophase fall sharply on further heating, disappearing
completely at 110°C. Note that the characteristic fan-shaped
texture of the samples observed in polarized light (Figure 1)
changed insignificantly up to sample isotropization. Therefore,
one can assume that a smectic–smectic transition occurs at 85°C.
As its heat effect is high, a high degree of ordering in the low-
temperature phase can be expected. The isotropization taking
place at 129°C is characterized by a low heat of transition, as
the ordering of a high-temperature smectic mesophase is poor.
Isotropization of compound 1 is observed at 129°C, while its
decomposition starts at 150°C, accompanied by the emission of
gaseous products. However, the phase transitions of compound 1
are reversible at temperatures up to 150°C.
The extension of the focal groups of mesogenic groups due
to the introduction of azobenzene units (compounds 2 and 3)
leads to considerable changes in phase behavior. In compound 2,
a layer structure is observed at room temperature [dL = 67.6 Å,
Figure 4(a)]. Wide-angle reflection corresponding to an inter-
planar distance of 10.0 Å and a disordered columnar phase were
detected. The latter phase is characterized by five reflections
with the ratio of d-spacing squares d12 : d22 : d32 : d42 : d52 : d62 : d72
=
= 1:3:4:7:9:13:16. The measured diameter of the cylinders
of the columnar mesophase was D = 55.6 Å. Due to the specific
packing of azobenzene rings, an unusual electron density distri-
bution inside the columns was revealed, manifesting itself in
substantially high relative intensities of reflections 10 and 21.
As the column diameter is much smaller than two molecular
lengths of 2 (2·37.7 Å = 75.4 Å), we can assume that the aliphatic
tails of dendrons belonging to the neighboring columns, inter-
digitate considerably. Heating of compound 2 was accompanied
by the further improvement of a 2-D hexagonal lattice, with
the shift of corresponding reflections to wide angles (the linear
coefficient of temperature expansion b = –3.3×10–4 K–1). Note
– 143 –

Mendeleev Commun., 2015, 25, 142–144
10
(a)
(a)
1
21
30
11 20
3
31
5
40
100°C
30°C
(b)
2
4
7
0.1
0.2
0.3
0.4
s/nm–1
0.5
0.6
0.7
(b)
10
Figure 5 Molecular modeling of supramolecular ensembles formed by
the dendrons of compound 3: (a) nascent smectic chevron bilayers and (b)
smectic mesophase after cooling and sustained annealing.
11
This work was supported by the Russian Foundation for Basic
Research (grant no. 14-03-00975).
1
20
125°C
7
3
2
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Received: 13th May 2014; Com. 14/4383
– 144 –