Synthesis of new type of dendritic molecule and its columnar self-assembly having a potential as molecular cavity

Article abstract of DOI:10.1016/j.molstruc.2004.10.108

In this study, we synthesized the first generation aromatic polyamide dendron having one trioxyethylene chain at the top of the dendron and six octadecyl chains at the other terminals. This material forms two types of crystals, K₁ and K₂

Full text of DOI:10.1016/j.molstruc.2004.10.108

Journal of Molecular Structure 739 (2005) 125–130
www.elsevier.com/locate/molstruc
Synthesis of new type of dendritic molecule and its columnar
self-assembly having a potential as molecular cavity
K. Fu, S. Nakasugi, Y. Takagawara, L. Li, T. Hayakawa, M. Jikei, M. Tokita,
*
M. Kakimoto, J. Watanabe
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Received 10 June 2004; revised 27 September 2004; accepted 5 October 2004
Available online 11 January 2005
Abstract
In this study, we synthesized the first generation aromatic polyamide dendron having one trioxyethylene chain at the top of the dendron and
six octadecyl chains at the other terminals. This material forms two types of crystals, K1 and K2 crystals. The K1 crystal is obtained on cooling
from the isotropic melt at a normal rate, while the K2 crystal grows only when the isotropic melt is retained at around 70 8C for long duration,
i.e. 1 day. The K2 crystal melts at 78 8C which is higher than 59 8C of the K1 crystal, and has a well-defined packing structure with a huge
˚
hexagonal lattice (aZbZ69.8 and cZ4.24 A) that includes five molecules. Such a characteristic hexagonal structure is produced by a
columnar association of molecules, in such a way that five molecules aggregate to form a disk, the disks are stacked into a column and the
columns are packed into a hexagonal lattice. The microsegregation of chemically different components is responsible for the columnar
association of molecules since in this column aromatic groups construct the cylindrical core tube with hydrophilic trioxyethylene chains
inside and hydrophilic alkyl tail groups outside the tube.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Dendrimers; Hexagonal columnar phase; Trioxyethylene chain; Self assembly
1. Introduction
medical purpose [1]. Furthermore, these macromolecules
are unique in a sense that they have a good solubility in
many kinds of solvents and simultaneously, a low intrinsic
viscosity.
During the past 20 years, macromolecular chemists have
witnessed the growth of a new innovative area of research
dealing with highly branched polymers or cascade mol-
ecules known as dendrimers [1]. What makes such dendritic
compounds original and remarkable is that they exhibit
three-dimensional fractal structures with dimensions of the
nanoscopic scale and they have the possibility to carry a
large number of functional groups. The combination of
precise functionalities at the termini or in the cavities with
defined macromolecular structures offers many applications
in materials science as well as many fundamental aspects.
For example, these compounds can be used for the
construction of materials with new technological appli-
cations based on a wide variety of properties in self-
assembly, molecular recognition, and biomimetism for
Among this fascinating class of highly functional
compounds, thermotropic liquid crystalline dendrimers
(LC dendrimers) are of special interest because of the
possibility of creating controlled multifunctional macro-
molecular objects that are able to self-assemble into several
types of large organized assemblies. They are also of
academic interest because they combine two opposite
tendencies: the structural anisometric units, which will
preferentially produce anisotropic order (enthalpic gain),
and the flexible dendritic architecture from which the
branches tend to radiate isotropically (entropic gain),
leading to a pseudospherical morphology. LC dendrimers
can be regarded as block molecules, in analogy to block
copolymers, in which the dendritic core and the functiona-
lized terminal groups will tend to microphase separate
because of their incompatible chemical natures [2].
* Corresponding author.
E-mail address: jwatanab@polymer.titech.ac.jp (J. Watanabe).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.10.108

126
K. Fu et al. / Journal of Molecular Structure 739 (2005) 125–130
The mesomorphic properties (phase structure and transition
temperatures) thus depend on a balance of enthalpic and
entropic gains, the degree of chemical incompatibility, and
the sizes of the different building blocks, as well as the
structure of the mesogenic unit itself and its location in the
dendrimer.
2.2. Synthesis of PEO3-G1-3T18
The synthetic route of the dendron, PEO3-G1-3T18, is
illustrated in Scheme 1 [13].
[Compound 3 from 3,5-dinitrobenzoylchloride and
triethyleneglycol monomethyl ether]. In a flask, 0.82 g
(5.0 mmol) of triethyleneglycol monomethyl ether 2 was
dissolved in 10 mL of DMAc, and the solution was frozen
using dry ice bath. To the solidified solution, 1.16 g
(5.0 mmol) of solid 3,5-dinitrobenzoylchloride 1 was
added at once. The bath was changed to the ice–water bath,
and the reaction solution was stirred for 8 h. After 50 mL of
water was added to the reaction mixture, the solution was
extracted with chloroform. The chloroform layer was dried
with anhydrous magnesiumsulfate. Ester 3 was obtained as a
viscous liquid after the solvent was evaporated. Yield was
69%. IR (KBr): 2881 cmK1, 1546, 1346 cmK1 (nitro), 1105–
1171 cmK1 (ether C–O–C). 1H-NMR (d ppm, CDCl3): 9.18
(s, 1H, Ar), 9.13 (s, 2H, Ar), 4.57 (s, 2H, COOCH2CH2), 3.85
(br, 2H, COOCH2CH2), 3.67 (m, 8H, COOCH2CH2O(C2
H4O)4), 3.33 (s, 3H, OCH3).
The field of LC dendrimers has now properly expanded,
and when suitably designed, several structures [3–12] have
been found to be compatible with the formation of liquid
crystals. The structurally perfect dendrimers exhibit a well-
defined architecture, with a controlled number of functional
moieties as a consequence of their layer-wise construction.
They are built outward from (or inward toward) a central
core moiety by an iterative type of synthesis consisting of
successive and controlled elementary steps. Such a
sequential mode of construction thus leads to practically
monodisperse systems. A perfect control of the number of
terminal groups is therefore achieved, and more accurate
structural comparisons are then made possible.
The work reported in this article is a part of a research
program on the design of new dendritic materials with a
mesomorphic nature. We synthesized a dendritic molecule,
PEO3-G1-3T18, with following structure,
[Compound 4 by reduction of 3]. In the flask connected
with gas storage filled with hydrogen, 1.23 g (3.4 mmol) of
3 and 0.5 g of 10% Pd/C was dissolved in 40 mL of ethanol,
and the mixture was stirred for 48 h at 25 8C. Diamino
compound 4 was obtained as viscous liquid after filtration of
Pd/C and evaporation of the solvent. Yield was 98%. IR
(KBr): 3449, 3361 cmK1 (NH amine), 1707 cmK1 (CaO
1
ester). H NMR (d ppm, CDCl3): 6.76 (s, 2H, Ar), 6.15
(s, 1H, Ar), 4.36 (t, 2H, COOCH2CH2, JZ7.0 Hz), 3.75 (m,
2H, COOCH2CH2), 3.76–3.48 (m, 8H COOCH2CH2O(C2
H4O)4), 3.32 (s, 3H, OCH3).
which has one trioxyethylene chain at the top of molecule
and six octadecyl chains at the other terminals. It is
interesting that this material forms two types of crystal
phases and one of them has a well-defined hexagonal
packing structure with a huge lattice including five
molecules. The preparation method of such a crystal
structure and its structural feature will be described.
[3,4,5-Trioctadecyloxybenzoylchloride]. In a flask, 1.84 g
(10 mmol) of methyl-3,4,5-trihydroxybenzoate 5 and
20.97 g (150 mmol) of potassium carbonate were dissolved
in the mixture of 40 mL of DMSO and 40 mL of toluene. The
mixture was heated up to 150 8C with distillation of toluene.
After the mixture was cooled to 25 8C, 12.00 mL (33 mmol)
of 1-bromooctadecane was added. The mixture was stirred
for 24 h at 60 8C. Solid of 3,4,5-Tris-octadecyloxy-benzoic
acid methyl ester 7 was obtained by pouring the reaction
solution into the mixture of water and acetone (1:1). Yield
was 98%. IR (KBr): 3449, 3361 cmK1 (NH amine),
2. Experimental section
1
1707 cmK1 (CaO ester). H NMR (d ppm, CDCl3): 6.76
(s, 2H, Ar), 6.15 (s, 1H, Ar), 4.36 (t, 2H, COOCH2CH2, JZ
7.0 Hz), 3.75 (m, 2H, COOCH2CH2), 3.76–3.48 (m, 8H
COOCH2CH2O(C2H4O)4), 3.32 (s, 3H, OCH3).
2.1. Materials
Reagents and solvents. N,N-Dimethylacetamide
(DMAc), dimethylsulfoxide (DMSO), dimethylformamide
(DMF), methylene chloride (CH2Cl2), and triethylamine
were distilled from calcium hydride. Thionylchloride, 10%
palladium–carbon (Pd/C), 3,5-dinitrobenzoylchloride,
triethyleneglycol monomethyl ether, 3,4,5-trihydroxy ben-
zoic acid methyl ester, and 1-bromooctadecane were used as
received. Solvents such as chloroform, toluene, ethanol, and
hexane were used without purification.
In a flask, 8.51 g (9.0 mmol) of 7 and 6.0 g of potassium
hydroxide were dissolved in 100 mL of ethanol. After the
mixture was refluxed for 2 h, the solvent was evaporated.
The residue was dissolved in 200 mL of water, and the
solution was acidified by adding conc. hydrochloric acid.
The obtained solid was filtered and dried under reduced
1
pressure. Yield: 98%. IR (KBr): 1686 cmK1 (CaO). H
NMR (d ppm, CDCl3): 3.84–4.13 (br, 6H, OCH2),

K. Fu et al. / Journal of Molecular Structure 739 (2005) 125–130
127
Scheme 1. Synthetic routes of PEO3-G1-3T18.
1.58–1.81 (br, 6H, CH2CH3), 1.01–1.43 (br, 84H, (CH2)14
CH2CH3), 0.68–0.93 (br, 3H, CH2CH3).
8.50 (br, 2H, amide NH), 8.46 (br, 1H, Ar), 7.80 (s, 1H, Ar),
7.15 (s, 4H, Ar), 4.41 (br, 2H, COOCH2CH2), 4.00 (br, 6H,
OCH2CH2), 3.62 (m, 8H, (C2H5O)4CH3), 3.45 (br, 2H,
COOCH2CH2), 3.27 (br, 3H, CH2OCH3), 1.76 (br,
6H,OCH2CH2), 1.45(br, 6H, CH2CH3), 1.24 (br, 148H,
(CH2)14CH2CH3), 0.86 (t, 9H, CH2CH3, JZ3.0 Hz).
In a flask, 3.71 g (4.00 mmol) of 8 was dissolved in
10 mL of thionylchloride. After one drop of DMF was
added to the reaction mixture, it was stirred for 6 h at 50 8C.
3,4,5-Trioctadecyloxy-benzoyl chloride 9 was obtained by
distillation of the excess thionylchloride. Yield: 98%. IR
(KBr): 1768 cmK1 (CaO).
2.3. Methods
[PEO3-G1-3T18, 10]. To the solution of 3.78 g
(4.0 mmol) of 9 in 18 mL of CH2Cl2, the solution of
0.56 mL (4.0 mmol) of triethylamine and 0.536 g
(1.8 mmol) of 4 in 1 mL of CH2Cl2 was added at 0 8C.
After the solution was stirred at 40 8C for 3 h, the reaction
mixture was poured into 300 mL of water. The aqueous
mixture was extracted by chloroform. The obtained crude
product was charged to the silicagel column chromatog-
raphy (eluent: hexane/chloroformZ1/1) to isolate the pure
dendron 10. Yield: 67%. IR(KBr): 3337, 1584 cmK1 (amide
Optical microscopic observation of the liquid crystal-
line textures was made with an Olympus BX50
polarizing microscope equipped with a Mettler FP-90
hot stage. Differential scanning calorimetric (DSC)
measurements were performed with a Perkin Elmer
DSC-II at a scanning rate of 10 8C minK1. Wide- and
small-angle X-ray patterns were observed using a
Rigaku-Denki X-ray generator with Ni-filtered Cu Ka
radiation and a flat imaging plate (IP). Temperature of
the sample was controlled by a Mettler FP-82 hot stage
1
NH) 1645 cmK1 (amide CaO), H NMR (d ppm, CDCl3):

128
K. Fu et al. / Journal of Molecular Structure 739 (2005) 125–130
10 8C minK1. Here, to eliminate the thermal prehistory,
the sample was first heated upto the isotropic melt at
80 8C. On cooling from the isotropic melt, one significant
peak appears at 49 8C and another small peak at 36 8C.
On heating, the corresponding peaks are observed at 48
and 59 8C. As far as the normal heating and cooling rates
of 5–10 8C/minK1 are adopted, this transition behaviour
is completely reproducible.
The optical microscopic observation shows that some
fan-like texture grows up at 49 8C on cooling from the
isotropic liquid as shown in Fig. 2a. The resulting phase is
not fluid under shearing so that it looks like crystal. It is
called K1 crystal. On the lower temperature transition at
36 8C, no significant change in the texture can be detectable
through a microscopic observation. Possibly this small
transition is due to a crystal–crystal transformation that
includes only a small structural change. The K1 crystal
indicates the poor X-ray diffraction pattern as shown in
Fig. 3a. Only two reflections with the spacings of 4.2 and
Fig. 1. DSC thermograms of PEO3-G1-3T18 observed on heating and
cooling cycle at 10 8C minK1
.
mounted along the beam path. The IP detector to
specimen distance was determined by calibration with
silicon powder pattern.
˚
30 A are included, which do not allow further discussion on
the structure.
3. Results and discussion
Interesting is that another crystal is formed when the
isotropic melt is retained at around 70 8C which is higher
than the melting temperature of the K1 crystal. The
growth of crystal is very slow; it takes more than 12 h for
the birefringent phase to fulfill the whole part of
3.1. Transition behavior of PEO3-G1-3T18
Fig. 1 shows the DSC thermograms of PEO3-G1-3T18
observed at normal cooling and heating rate of
Fig. 2. Optical microscopic photographs for (a) K1 and (b) K2 crystal phases.
Fig. 3. Wide-angle X-ray patterns of PEO3-G1-3T18: (a) K1 crystal phase and (b) K2 crystal phase.

K. Fu et al. / Journal of Molecular Structure 739 (2005) 125–130
129
formed only when the isotropic melt is retained at around
70 8C just below its melting temperature for a long time.
Melting enthalpy of the K2 crystal is 126.8 kJ molK1
which is larger than 86.7 kJ molK1of the K1 crystal.
,
3.2. Characteristic self-assembly of molecules in K2 crystal
X-ray pattern of the K2 crystal is shown in Fig. 3b. It
includes many reflections. Spacing data are listed in Table 1.
Although the powder pattern does not allow a precise
determination of the structure, it offers two characteristic
features suggesting a specific structure. First, the spacing of
˚
most inner reflection is around 60 A. This is fairly larger
than the molecular size, showing that the molecules in the
K2 crystal should associate to form some superstructure.
˚
Secondly, all the reflections with spacings of 60–5 A can be
Fig. 4. DSC heating thermogram of K2 crystal observed at a heating rate of
10 8C minK1. After the K2 crystal melts at 78 8C, the DSC thermograms
measured at cooling and heating rates of 10 8C minK1 become same as
observed in Fig. 1.
satisfactorily assigned into (h k 0) reflections from the two-
dimensional hexagonal lattice with the lattice parameters of
˚
aZbZ69.8 A. One can find the good correspondence
between the observed and calculated ones as given in
Table 1.
the viewing area. The optical micrscopic texture is shown
in Fig. 2b. The crystal thus prepared is called K2 crystal.
Fig. 4 shows the heating DSC thermogram of the K2
crystal. There is a main transition, the melting of K2
crystal, at 78 8C. Some small peaks are also observed in
lower temperature region, which may be due to the
transitions related to a small quantity of the K1 crystal
coexisting with the K2 crystal. Once the K2 crystal is
melt, the similar transition behavior as in Fig. 1
is observed at normal heating and cooling rates of
10 8C/minK1. Thus, we know that the K2 crystal is
It should be noticed that the two other reflections with
˚
spacings of 4.24 and 4.06 A are observed with relatively
strong intensity. The spacings approximate to the van der
Waal thickness of the aromatic group so that these
reflections may be indexed to (h k 1) ones (refer to
Table 1). Thus, we reach the three-dimensional hexagonal
˚
lattice of aZbZ69.8 and cZ4.24 A. Considering that the
density of K2 crystal is around 1 g/cmK3, the number of
molecules included in a lattice can be calculated as five.
Table 1
X-ray data of K2
a
a
hkla
hkla
540
˚
dobsd (A)
˚
dcalcd (A)
˚
dobsd (A)
˚
dcalcd (A)
59.4
36.2
28.9
100
110
200
210
300
220
310
400
320
410
500
330
420
510
600
430
520
610
440
530
700
620
710
60.0
34.7
30.0
22.7
20.0
17.3
16.6
15.0
13.8
13.1
12.0
11.6
11.3
10.8
10.0
9.87
9.61
9.15
8.66
8.57
8.57
8.32
7.95
7.68
7.56
7.50
7.33
7.02
6.93
6.88
6.75
6.67
6.55
6.29
6.29
6.22
6.16
6.00
5.91
5.78
630
800
720
810
550
640
730
900
820
910
650
740
830
1000
920
660
7.44
7.03
20.1
14.9
13.7
13.1
12.1
6.66
6.28
10.7
9.83
9.16
8.54
5.87
4.24
4.06
001
401
321
411
501
4.24
4.08
4.06
4.04
4.00
8.25
7.86
a
˚
Based on a hexagonal lattice of aZbZ69.3 and cZ4.24 A.

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K. Fu et al. / Journal of Molecular Structure 739 (2005) 125–130
Fig. 5. Self-assembly structure proposed in the K2 crystal. The molecules (a) aggregate with each other to form a disk (b), disks are stacked into a column (c),
and columns are packed into a hexagonal lattice (d). In the resulting superstructure, the aromatic groups construct the cylindrical tube, with the trioxyethylene
chains inside the tube and the alkyl chains outside the tube.
The structure speculated is illustrated in Fig. 5, where
five molecules are assembled to form a disk (Fig. 5b)
and the disks are stacked into a column (Fig. 5c). In this
column, then, aromatic groups construct the cylindrical
core tube, accommodating trioxyethylene groups at a
central part of the tube and alkyl tail groups outside the
tube. The hexagonal packing of columns is well
promised by a microsegregation of three components
[11], as illustrated in Fig. 5d. Such a complicated
microsegregation may be a reason for the slow-rate
formation of the K2 crystal. The precise determination of
this structure and its application will be done after the
preparation of highly oriented crystal.
as one-dimensional membranes [11], ionic [14,15] and
electronic conductors [16], photoconductors [17], etc.
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