Degree of chain branching-dependent assemblies and conducting behavior in ionic liquid crystalline Janus dendrimers

Article abstract of DOI:10.1039/c0sm01435d

We prepared two Janus dendrimers 1 and 2 bearing a hydrophobic Percec-type minidendron (i.e., 3,4,5-tridodecyloxybenzyl group) and hydrophilic aliphatic ether dendrons with different degrees of branching via stepwise click reactions. The obtained Janus de

Full text of DOI:10.1039/c0sm01435d

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PAPER
Degree of chain branching-dependent assemblies and conducting behavior in
ionic liquid crystalline Janus dendrimers
*
Jin-Woo Choi and Byoung-Ki Cho
Received 7th December 2010, Accepted 28th January 2011
DOI: 10.1039/c0sm01435d
We prepared two Janus dendrimers 1 and 2 bearing a hydrophobic Percec-type minidendron
(i.e., 3,4,5-tridodecyloxybenzyl group) and hydrophilic aliphatic ether dendrons with different degrees
of branching via stepwise click reactions. The obtained Janus dendrimers have the same hydrophilic
volume fraction (f ¼ 0.34), but a different number of hydrophilic chains, i.e., tetrabranched and
dibranched oligo(ethylene oxide) coils for 1 and 2, respectively. Upon doping with lithium salt, liquid
crystalline (LC) ionic samples 1-Li+ and 2-Li+ were obtained. As characterized by polarized optical
microscopy (POM) and X-ray scattering techniques, 1-Li+ showed an optically isotropic cubic LC
phase, while 2-Li+ displayed a birefringent hexagonal columnar LC structure with a bilayered cross-
section. Interestingly, the conductivity values of the cubic phase were considerably higher than those of
the columnar phase, with which the cubic phase could be assigned as a bicontinuous cubic structure
rather than a micellar one. On the basis of the experimental observations, the morphological contrast
between two ionic Janus dendrimers is strongly attributed to the degree of chain branching, which is
one of the major parameters governing the morphological features in the Janus dendrimer assemblies.
consisting of hydrophobic benzyl ether dendrons and branched
Introduction
polyols. With the strong amphiphilicity between the two blocks,
the dendrimers were shown to self-organize into columnar and
micellar cubic LC phases by altering the hydrophilic volume
fraction.5
Janus dendrimers, so-called ‘‘block codendrimers’’, where
constitutively different dendron blocks are covalently attached at
the center have recently received much attention as a new class of
soft materials. Because of their esthetic chain topologies, earlier
studies were mostly focused on the synthetic ways to prepare
novel Janus dendrimer structures.1 In recent years, the focus of
study has shifted to the self-assembly of the Janus dendrimer
system. Several nanostructured examples, particularly in liquid
crystalline (LC) state, were reported by several groups. Although
some of these are based on aliphatic dendron building blocks,2
most of them consist of Percec-type minidendrons, e.g., meso-
genic benzyl or benzoyl groups with trisubstituted peripheral
chains. For example, Percec et al. synthesized benzamide Janus
dendrimers bearing two different kinds of peripheral chains that
are non-fluorinated and semifluorinated aliphatic chains. Due to
the incompatibility between two peripheral groups, the Janus
dendrimers formed pyramidal columnar LC phases.3 Addition-
ally, Lehmann et al. prepared a series of asymmetric oligo-
benzoates with semifluorinated and alkylated dendrons, and
investigated their mesomorphic behavior. They found an inter-
esting temperature-dependent morphological transformation
from columnar to lamellar LC phases.4 As another example, the
Donnio group studied the self-assembly of Janus dendrimers
Despite a few advanced reports, however, the correlation
between the chain structure and morphology in a Janus den-
drimer system is not completely understood. Therefore, further
research is required to gain more insight. Among several
parameters (e.g., volume fraction and core structure) governing
the dendrimer assembly, we thought that the degree of chain
branching could be an independent factor. To this end, Janus
dendrimers 1 and 2 were designed to have identical hydrophilic
volume fractions, f ¼ 0.34, but different branched hydrophilic
blocks, i.e., dibranched and tetrabranched dendrons, respectively
(Fig. 1). Therefore, their self-assembly behavior could be
understood with respect to the degree of chain branching of the
hydrophilic dendron block. Furthermore, in this study, we
investigated the ionic conductivities of ionic dendrimer samples,
through which the structural details could be demonstrated.
Results and discussion
Synthesis
For the hydrophobic and hydrophilic dendron segments, we
employed
a hydroxy-terminated Percec-type minidendron
Department of Chemistry and Institute of Nanosensor and Biotechnology,
Dankook University, Gyeonggi-Do, 448-701, Korea. E-mail: chobk@
dankook.ac.kr; Fax: +82 31 8005 3148; Tel: +82 31 8005 3153
(i.e., 3,4,5,-tridodecyloxybenzyl alcohol) and aliphatic ether
dendrons with oligo(ethylene oxide) (OEO) coils, respectively.6
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The obtained Janus dendrimers were characterized by 1H and
13C NMR spectroscopy, elemental analysis, gel permeation
chromatography (GPC), and matrix-assisted laser desorption/
ionization time-of-flight mass spectrometry (MALDI-TOF MS).
All experimental data fitted well with the designed molecular
structure (see Experimental section). In both dendrimers, twelve
distinct carbon signals appeared in the aromatic region of the 13
C
NMR spectra. As shown in Fig. 2(a), the MALDI-TOF MS
spectrum of 1 exhibited a signal corresponding to the Na adduct
of the molecular ions. On the other hand, 2 showed a symmetric
and narrow mass distribution with a maximum of 2324.5 g molꢀ1
because the OEG coil (Mn ¼ 350 g molꢀ1) from Aldrich Corp.
does not have a single molecular weight. The polydispersity
values (Mn/Mw) of the codendrimers were measured to be 1.01,
indicative of high purity (Fig. 2(b)). From the mass and GPC
results, we could confirm that both dendrimers have nearly
identical molecular weights, with a gap of less than 5%.
Fig. 1 Molecular structures of Janus dendrimers 1 and 2.
Their hydroxyl ends were easily converted into azide groups by
sequential tosylation/or chlorination and azidation.7 The
aromatic core precursor, 1-bromo-3,4-diethynylbenzene, was
prepared by the Sonogashira reaction as described elsewhere.7
Two-step click reactions using these precursors produced the
final Janus dendrimers (1 and 2), as outlined in Scheme 1. Using
CuSO4$5H2O and sodium ascorbate as click reagents,8 the Per-
cec-type hydrophobic minidendrons were first-clicked into
a hexa-branched intermediate (A). This is because more polar A
can be easily isolated from the reactant in the column purifica-
tion. Then, the bromine end was changed into an ethynyl group.
Subsequently, by carrying out the second-click reaction, the
hydrophilic ether dendrons were attached to ethynyl-terminated
intermediate (C), resulting in Janus dendrimers 1 and 2 with
a Y-shaped aromatic core (Scheme 1). It is worth noting that the
extended Y-shaped aromatic core structure (including three
triazoles, central benzene and two benzyl units) via the click
reactions could be mesogenic, thus facilitating LC formation by
stacking together.
Liquid crystalline assemblies
According to the thermal analyses using polarized optical
microscopy (POM) and differential scanning calorimetry (DSC),
ꢁ
both dendrimers melted directly into isotropic liquid at 22.0 C
and 20.2 ꢁC, respectively, without passing through LC phases
(Fig. 3(a)). This thermal phenomenon suggests that the degree of
mixing between hydrophilic and hydrophobic segments is great
because of the large interfacial area associated with the den-
drimer architecture. This is consistent with a recent theoretical
report on the block codendrimer melt by Pickett and Rios.9
Meanwhile, selective ion-doping in the hydrophilic dendron
block may enhance incompatibility between the two blocks, by
which ordered structures in the melt can be induced.10 To this
end, ion-doped samples, 1-Li+ and 2-Li+ using lithium triflate
were prepared. In order to keep the same f, the lithium concen-
tration per ethylene oxide ([Li+]/[EO]) was chosen to be 0.05 for
both ionic samples. In contrast to the pristine Janus dendrimers,
the ionic samples exhibited ordered LC phases, as characterized
by the DSC, POM, small- and wide-angle X-ray scattering
(SAXS and WAXS) techniques.
The ionic sample (1-Li+) with the tetrabranched block melted
ꢁ
at 23.3 C, after which an LC phaꢁse was shown upon heating,
followed by isotropization at 34.3 C (Fig. 3(b)). According to
Scheme 1 Synthetic route of Janus dendrimers 1 and 2 via stepwise click
reactions.
Fig. 2 (a) MALDI-TOF MS spectra, and (b) GPC elugrams of 1 and 2.
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The SAXS pattern displayed four reflections, which can be
interpreted as the (100), (110), (200), and (210) planes of a typical
2-dimensional hexagonal structure (Fig. 4(d)). The inter-
columnar distance (b) was estimated to be 6.20 nm. On the basis
ꢀ
of the SAXS data, the 4.4 A-thick column stratum (from the
WAXS data in Fig. 4(b)), and the relevant densities, the number
of molecules occupying a column cross-section was calculated to
be approximately 3.45.
Conductivity behavior in the LC phases
Besides the structural characterization using the X-ray tech-
niques, the measurement of the conductivities of ionic samples
could be a useful tool to understand morphological features,
e.g., domain connectivity.13 The ionic conductivity values of both
ionic samples were examined by the AC impedance method using
a cell with a radius and thickness of 1.9 mm and 1.2 mm,
respectively. The heating rate during the measurements was set at
Fig. 3 DSC thermograms of (a) 1 and 2, and (b) 1-Li+ and 2-Li+.
the POM observation, the LC phase displayed no birefringence,
and maintained a high viscosity before the isotropization. These
optical and mechanical features strongly suggest a cubic LC
structure.11 To substantiate the identity of the LC phase, X-ray
scattering experiments were carried out. In the LC phase, the
WAXS data displayed a broad halo (Fig. 4(b)), while the SAXS
data detected at 30.0 ꢁC showed two strong and six weak
reflections. Interestingly, they can be indexed consistently as the
(211), (220), (321), (400), (420), (332), (422), and (431) planes of
a cubic structure with space group Ia3d (Fig. 4(c)). From the
observed d-spacing of (211) reflection, the best fit value for the
cubic lattice parameter (a) was estimated to be 16.16 nm.
On the other hand, 2-Li+ bearing the dibranched hydrophilic
dendron revealed a different thermotropic property. From the
POM study in the melt, 2-Li+ showed a birefringent fan-like
texture at 37.0 ꢁC, which is commonly found in columnar LC
phases of disc-like liquid crystals (Fig. 4(a)).12 This birefringent
texture is proof of an anisotropic stacking of the Y-shaped
aromatic cores within columns. The X-ray analyses confirmed
that the LC phase is a hexagonal columnar morphology (Colhex).
0.3 C minꢀ1. Considering the cell size and the heating rate, we
ꢁ
assumed that conductivity values in this study were obtained
from the polydomain samples. Fig. 5(a) is the impedance spectra
of 1-Li+ and 2-Li+ measured at 28 ꢁC (LC phases in both
samples). From the values of the semicircles on the real X-axis,
the resistance (R) values were estimated.14 Then, from the
product of 1/R and the cell constant, the DC conductivity values
were obtained to be 2.8 ꢂ 10ꢀ7 S cmꢀ1 and 5.2 ꢂ 10ꢀ8 S cmꢀ1 for
1-Li+ and 2-Li+, respectively. Remarkably, the former value
(in the Cubbi) is 5.4 times higher than the latter (in the Colhex).
We then investigated the conductivity curves as a function of
temperature. In Fig. 5(b), the conductivity gap between Cubbi
and Colhex persists while maintaining the LC phases. Upon
entering the structureless liquid phases (I), the conductivity
values gradually merge. From these observations, the conduc-
tivity contrast in the LC phases is obviously attributed to the
connectivity of the ion-conducting hydrophilic domain in each
LC phase.15 Consequently, the cubic LC phase can be a ‘‘gyroid’’-
like bicontinuous cubic structure (Cubbi) consisting of contin-
uous channels rather than a micellar one. It is well-known that
the bicontinuous cubic structure has a less-tapered core region in
comparison with the columnar one.16 Considering the degrees of
branching in 1 and 2, the morphological results suggest that the
tetrabranched dendron of 1 has more laterally expanded
conformations than the dibranched dendron of 2 in the hydro-
philic core region, due to the larger steric repulsion arising from
the high grafting density.
Fig. 4 (a) Fan-like optical texture (ꢂ100) of 2-Li+ taken at 37.0 ꢁC on
cooling, (b) the WAXS and (c) SAXS spectra of 1-Li+ and 2-Li+ at 30 ꢁC.
Fig. 5 (a) Impedance spectra obtained at 28 ꢁC, and (b) the DC
conductivity values of 1-Li+ and 2-Li+ as a function of temperature.
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4.08 mmol), CuSO4$5H2O (0.1 g, 0.4 mmol) and sodium ascor-
bate (0.1 g, 0.5 mmol) were dissolved in 30 mL of acetonitrile,
30 mL of THF and 2 mL of deionized water. The reaction
mixture was stirred for 24 h at room temperature. After termi-
nating the reaction, the solvent was removed by a rotary evap-
orator. The resulting mixture was extracted with deionized water
and dichloromethane. The dichloromethane layer was washed
with deionized water, and dried over MgSO4. After removing
Conclusions
In conclusion, we prepared two Janus dendrimers 1 and 2 with
a Y-shaped aromatic core consisting of three triazoles, a central
benzene and two benzyl units via stepwise click reactions. Janus
dendrimers 1 and 2 have the same hydrophilic volume fraction
(f), but the tetrabranched and dibranched hydrophilic dendritic
blocks, respectively. The morphological analyses of their ion-
doped samples using the POM, X-ray scatterings, and ionic
conductivity techniques proved that the ionic block coden-
drimers self-assembled into the bicontinuous cubic and hexag-
onal columnar LC phases. Despite the same f, the observed
morphological contrast can be explained by the degree of
branching of the hydrophilic block as a major driving force.
Notably, with respect to the structural connectivity, the observed
Cubbi phase could be an excellent platform for advanced elec-
trolyte materials.
dichloromethane by
a rotary evaporator, the resulting
compound was purified by sequential silica gel column chroma-
tographies from dichloromethane to hexane : ethyl acetate ¼
4 : 1 mixture as the eluent, to yield 1.8 g (60%) of a dark brown
solid. 1H NMR (CDCl3, d, ppm): 8.17 (s, Ar–H), 7.93 (s, Ar–H),
7.75 (s, H-triazole), 6.51 (s, benzyl-H), 5.46 (s, CHCH2N), 3.93–
3.97 (m, CH2OAr), 1.75–1.81 (m, CH2CH2OAr), 1.43–1.49
(m, CH2(CH2)8CH3), 1.27–1.43 (m, CH2(CH2)8CH3), 0.89 (t, J ¼
7.0 Hz, (CH2)8CH3). Anal. Calcd for C96H163BrN6O6: C, 73.10;
H, 10.42; N, 5.33, Found: C, 73.14; H, 10.37; N, 5.29%. Mw/
Mn ¼ 1.01 (GPC). MALDI-TOF MS: 1577.3 [A + H]+, 1600.7
[A + Na]+, 1616.8 [A + K]+.
Experimental
General methods
1H and 13C NMR spectra were recorded from a CDCl3 solution
using Varian 200 and Bruker AM 500 spectrometers. The purity
of the products was checked by thin-layer chromatography
(TLC; Merck, silica gel 60). Gel permeation chromatography
(GPC) measurements were conducted in THF and N,N0-dime-
thylacetamide (99.9%) (98 : 2 volume ratio) using a Waters 401
instrument equipped with KF-802, KF-803, AT-G and AT-804S
Shodex columns at a flow rate of 1.0 mL minꢀ1. A Perkin Elmer
DSC-7 was used to determine thermal transitions. In all cases,
Synthesis of compound B. Compound A (1.8 g, 1.18 mmol),
trimethylsilyl acetylene (0.76 mL, 5.54 mmol), copper(I) iodide
(0.0067 g, 0.035 mmol), and PdCl2(PPh3)2 (0.012 g, 0.017 mmol)
were dissolved in 20 mL of dry triethylamine. The mixture was
heated at 120 ꢁC for 60 h. After cooling to room temperature, the
solvent was removed by a rotary evaporator. Then the mixture
was extracted with deionized water and dichloromethane. The
dichloromethane layer was washed with deionized water several
times, and dried over MgSO4. The solvent was removed by
a rotary evaporator, and the crude product was then purified by
sequential silica gel column chromatographies from dichloro-
methane to ethyl acetate : hexane ¼ 1 : 4 mixture as the eluent, to
heating and cooling scans were measured at a rate of 10 ꢁC minꢀ1
.
X-Ray scattering measurements were performed in a trans-
mission mode with synchrotron radiation at the 10C1 beam line
of the Pohang Accelerator Laboratory (PAL), Korea. The
sample was held in an aluminium sample holder with films on
both sides. Microanalyses were performed with a Perkin Elmer
240 elemental analyzer at the Organic Chemistry Research
Center, Sogang University, Korea. The compounds were purified
by column chromatography (silica gel) and prep-HPLC (Japan
Analytical Instrument). Matrix-assisted laser desorption ioniza-
tion mass spectra were obtained on a Perceptive Biosystems
Voyager-DE STR system, Korea. Ionic conductivities were
measured by the complex impedance method using a Schlum-
berger Solartron 1260 impedance analyzer (frequency range:
10 Hz–10 MHz, applied voltage: 1.0 V) with a temperature
controller (heating rate: 0.3 ꢁC minꢀ1). The radius and thickness
of the cell in the measurements were 1.9 mm and 1.2 mm,
respectively. DC conductivities were practically calculated to be
the product of 1/R (Uꢀ1) times cell constants (cmꢀ1). The densities
of the Janus dendrimers were measured to be approximately
1.07 g mLꢀ1 using an aqueous sodium chloride solution. The
densities of 3,4,5-tridodecyloxybenzyl azide and the hydrophilic
dendrons were used to be 1.00 g mLꢀ1 and 1.05 g mLꢀ1, respec-
1
yield 1.6 g (91%) of a dark brown solid. H NMR (CDCl3, d,
ppm): 8.22 (s, Ar–H), 7.86 (s, Ar–H), 7.73 (s, H-triazole), 6.49
(s, benzyl-H), 5.44 (s, CHCH2N), 3.93–3.97 (m, CH2OAr), 1.23–
1.78 (m, ArOCH2(CH2)10CH3), 0.89 (t, J ¼ 7.0 Hz, (CH2)10CH3),
0.24 (s, Si(CH3)3). Mw/Mn ¼ 1.01 (GPC).
Synthesis of compound C. Compound B (1.6 g, 1.00 mmol) and
K2CO3 (1.38 g, 10.0mmol)weredissolvedin20 mL of dry THFand
dry MeOH. The reaction mixture was stirred for 4 h at room
temperature under N2 atmosphere. The solvent was removed by
a rotary evaporator. Then the mixture was extracted with deionized
water and dichloromethane, and dried over MgSO4. The solvent
was removed by a rotary evaporator, and the crude product was
then purified by sequential silica gel column chromatographies
from dichloromethane to ethyl acetate : hexane ¼ 1 : 4 solvent
mixture as the eluent, to yield 1.5 g (98%) of a yellowish solid. 1H
NMR (CDCl3, d, ppm): 8.24 (s, Ar–H), 7.88 (s, Ar–H), 7.74
(s, H-triazole), 6.49 (s, benzyl-H), 5.44 (s, CHCH2N), 3.93–
3.97 (m, CH2OAr), 3.10 (s, Ar–C^CH), 1.23–1.78 (m,
ArOCH2(CH2)10CH3), 0.89 (t, J ¼ 7.0 Hz, (CH2)10CH3).
tively. And the density of lithium triflate was taken as 1.9 g mLꢀ1
.
Synthesis of Janus dendrimer 1. Compound C (0.5 g,
0.33 mmol), the tetrabranched hydrophilic dendron (0.35 g,
0.39 mmol), CuSO4$5H2O (0.1 g, 0.4 mmol) and sodium ascor-
bate (0.1 g, 0.5 mmol) were dissolved in 20 mL of acetonitrile,
20 mL of THF and 2 mL deionized water. The reaction mixture
Synthesis
Synthesis of compound A. 1-Bromo-3,5-diethylnylbenzene
(0.38 g, 1.85 mmol), 3,4,5-tridodecyloxybenzyl azide (2.8 g,
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was stirred for 13 h at room temperature. After terminating the
reaction, the solvent was removed by a rotary evaporator. The
resulting mixture was extracted with deionized water and
dichloromethane. The dichloromethane layer was washed with
deionized water several times, and dried over MgSO4. The
solvent was removed by a rotary evaporator, and the remaining
hydrophilic dendrons were removed by an HPLC (Japan
Analytical Industry), and the crude product was then purified by
(NRF) funded by the Ministry of Education, Science and
Technology (MEST), Republic of Korea. We acknowledge
the Pohang Accelerator Laboratory (Beamline 10C1),
Korea.
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(m,
(CH2)2CHCH2O),
1.26–1.83
(br,
ArOCH2(CH2)10CH3), 0.88 (t, J ¼ 7.0 Hz, (CH2)10CH3). 13C
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Synthesis of Janus dendrimer 2. The synthetic procedure of 2 is
nearly identical to that of 1. The final purification yielded 1.3 g
(76.3%)of a yellowish wax. 1H NMR (CDCl3, d, ppm): 8.28 (s, Ar–
H), 8.26 (s, H-triazole), 7.89 (s, H-triazole), 6.52 (s, benzyl-H),
5.46 (s, ArCHCH2N), 4.57 (d, J ¼ 6.0 Hz, (CH2)2CHCH2N),
3.93–3.97 (m, CH2OAr), 3.30–3.67 (br, CH2OCH2, OCH3), 2.55–
2.59 (m, (CH2)2CHCH2N), 1.25–1.82 (br, ArOCH2(CH2)10CH3),
0.87 (t, J ¼ 7.0 Hz, (CH2)10CH3). 13C NMR (125 MHz, CDCl3, d,
ppm): 153.7, 147.5, 146.7, 138.5, 132.0, 131.7, 129.2, 122.4, 122.3,
122.0, 120.2, 106.7, 73.5–69.2 (carbon atoms next to oxygen
except 69.21), 59.04 (terminal methoxy), 54.76, 40.52, 31.95,
31.94, 30.38, 29.78–29.39, 26.14, 26.12, 22.70, 14.14. Anal. Calcd
for C132H233N9O22: C, 68.98; H, 10.22; N, 5.48, Found: C, 68.84;
H, 10.12; N, 5.48%. Mw/Mn ¼ 1.01 (GPC).
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Acknowledgements
This work was supported by the Core Research Program
(2009-0084501) through the National Research Foundation
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This journal is ª The Royal Society of Chemistry 2011
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