Ion-induced bicontinuous cubic and columnar liquid-crystalline assemblies of discotic block codendrimers

Article abstract of DOI:10.1002/chem.201001337

Discotic block codendrimers, consisting of two different nonmesogenic aliphatic polyether dendrons and a discotic linker, exhibited induced hexagonal columnar and bicontinuous cubic (unprecedented in dendrimers)liquidcrystalline (LC) phases by the addition of lithium salt (see figure). The interior and exterior of these morphologies can be altered by changing the majority dendritic block.

Full text of DOI:10.1002/chem.201001337

DOI: 10.1002/chem.201001337
Ion-Induced Bicontinuous Cubic and Columnar Liquid-Crystalline
Assemblies of Discotic Block Codendrimers
Jin-Woo Choi,[a] Mi-Hee Ryu,[a] Eunji Lee,[b] and Byoung-Ki Cho*[a]
Block codendrimers, sometimes known as hybrid or Janus
dendrimers, in which two constitutively different dendrons
are covalently bound at the center, have recently received
much attention as a new class of self-assembling soft materi-
als. Several block codendrimers displayed liquid-crystalline
(LC) properties in the melt. These contain mostly mesogenic
dendrons, for example, Percec-type poly(benzyl ether) den-
drons, as their basic building unit.[1] Meanwhile, despite sev-
eral reports suggesting the synthesis of block codendrimers
carrying nonmesogenic dendrons,[2] to date, only two exam-
ples have displayed a lamellar-type LC morphology (Lam)
in the melt.[3] Therefore, further research needs to be under-
taken to elucidate the bulk assembling behavior of the unex-
plored block codendrimer system.
Discotic liquid crystals, composed of a rigid aromatic core
and flexible alkyl chains, are another type of self-assembling
soft material. Due to the stacking of anisotropic disclike
cores, they are well known for forming columnar LC phases.
However, triphenylene discogens with ionic-liquid pendants,
as recently reported by the Aida group, frustrated this
common structure–LC morphology correlation.[4] In addition
to a hexagonal columnar LC phase (Colhex), the discogens
revealed a bicontinuous cubic (Cubbi) LC phase, consisting
of two continuous cylindrical networks. For the generation
of the Cubbi, the ionic interaction of the ionic-liquid pend-
ants was suggested as one of the major parameters.
These molecules are based on a rigid aromatic disc connect-
ed to two types of 2nd generation aliphatic polyether den-
drons: one (A) is made up of hydrophilic tri(ethylene oxide)
(TEO) coils, and the other (B) of hydrophobic tetradecyl
chains. Using the systematic combination of these two den-
drons, we prepared two discotic block codendrimers (A2B
and AB2). As a notable feature in the discotic block dendri-
mer design, we used a triazole-based aromatic disc to bridge
the dendrons. This is because, in terms of synthesis, the aro-
matic 1,2,3-triazole can be obtained by a straightforward
click reaction.[5] Herein, we report the ion-induced LC be-
havior of the discotic block codendrimers. In particular, we
highlight the generation of an unprecedented Cubbi LC
phase and the tuning of its microstructures (i.e., cylindrical
core and matrix).
In this context, we designed amphiphilic molecules, re-
ferred as “discotic block codendrimers”, which may be con-
sidered as either block codendrimers or discotic molecules.
The preparation of the discotic dendrimers began with the
convergent synthesis of the hydrophilic and -phobic 2nd
generation aliphatic polyether dendrons with the hydroxyl
focal group, as described in previous publications.[6] The hy-
droxyl group of each dendron was converted into an azide
group by sequential tosylation and azidation reactions
(Scheme S1 in the Supporting Information). The aromatic
core precursors, that is, mono-H and di-H, were prepared by
the Sonogashira reaction (Scheme S2 in the Supporting In-
formation).[7] Copper(I) bromide and 2,2’-dipyridyl were
used as reagents for the click reaction.[8] The azide-terminat-
[a] J.-W. Choi, M.-H. Ryu, Prof. B.-K. Cho
Department of Chemistry, Dankook University
Jukjeon 126, Gyeonggi, 448-701 (Korea)
Fax : (+82)31-8005-3153
E-mail: chobk@dankook.ac.kr
[b] E. Lee
Department of Chemistry, Yonsei University
Shinchon 134, Seoul 120-749 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001337.
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COMMUNICATION
ed dendron (B-N3) was first coupled with the core precur-
sors, because the resulting compounds (B1-I and B2-I) could
then be clearly distinguished from the reactant dendron in
the column chromatography (Scheme S3 in the Supporting
Information). The iodine groups of the first clicked inter-
mediates were changed into ethynyl ends. Subsequently, the
hydrophilic dendrons (A-N3) were attached, producing dis-
cotic block codendrimers A2B and AB2. The obtained den-
1
drimers were characterized by H, and 13C NMR spectrosco-
py, elemental analysis, gel permeation chromatography
(GPC), and MALDI-TOF MS. All experimental data fit
well with the designed molecular structure (see the Support-
ing Information).
The thermal properties of the discotic block dendrimers
were analyzed by polarized optical microscopy (POM) and
differential scanning calorimetry (DSC), and are summar-
ized in Table S1 in the Supporting Information. No LC
phase was observed in any of the block codendrimers. From
the thermal data, we may consider that the degree of mixing
between hydrophilic and -phobic blocks in the dendrimer ar-
chitecture is greater than that in linear–linear block copoly-
mers. This speculation is consistent with recent theoretical
work by Pickett and Rios, which predicted that there would
be difficulty in obtaining a strongly segregated regime in
block codendrimers.[9]
It is well known that ion-doping in PEO-containing block
copolymers enhances microphase separation,[10] and we also
wanted to test this. Therefore, we prepared ion-doped sam-
ples with A2B and AB2. For each block codendrimer, we
made two samples with different lithium concentrations per
ethylene oxide ([Li+]/[EO]). All ion-doped samples from
the block codendrimers revealed ordered structures in the
melt, as supported by the DSC, POM, small- and wide-angle
X-ray scattering (SAXS and WAXS) techniques.
Figure 1. SAXS spectra for a) A2B-1, b) A2B-2, c) AB2-1, and d) AB2-2 at
different temperatures plotted against the scattering wave vector, q (=
4psinq/l).
was missing, presumably due to a structural factor. In addi-
tion, a fanlike texture from the POM observation at 1008C
also suggests a hexagonal columnar LC phase (Figure S4 in
the Supporting Information). From the primary (100) reflec-
tion, the intercolumnar distance (a) was calculated to be
6.96 nm. On the basis of the interpretation of the hexagonal
columnar structure, we can speculate that the cubic LC
phase in lower temperatures is a Cubbi morphology. As the
temperature increases, the steric repulsion between the ma-
jority TEOs would be greater than those between the minor-
ity tetradecyl chains, leading to the Colhex. Therefore, we
suggest that the cubic phase at lower temperatures is a
Cubbi rather than a micellar structure.
As the lithium concentration increased to [Li+]/[EO]=
0.4, A2B-2 with f=0.74 also exhibited a Colhex, which persist-
ed up to the experimentally accessible temperature of
2208C. In Figure 1b, the SAXS spectrum at 308C exhibited
five reflections, which can be indexed as the (100), (110),
(200), (210), and (300) planes of a Colhex. In comparison to
the Colhex of A2B-1, the intercolumnar distance (7.28 nm) of
A2B-2 is somewhat larger, which is attributed to the more
added lithium salts.
On the basis of the hydrophilic volume fractions of A2B-1
and A2B-2, the outer matrix in the observed columnar struc-
tures must be composed of hydrophilic species, while hydro-
phobic dendrons occupy the inner cylindrical cores. In this
An ion-doped sample, A2B-1 with [Li+]/[EO]=0.15,
melted at 15.78C, after which temperature two distinct LC
phases were observed upon heating, followed by isotropiza-
tion near 1438C. The POM observation of the first entering
LC phase showed no birefringence and high viscosity in the
melt from the POM observation, reminiscent of a cubic
phase. In contrast, upon heating, the second LC phase
showed a birefringence, although this birefringence did not
cover the whole area. To clarify the identities of both LC
phases, we investigated their SAXS patterns. The SAXS
data of A2B-1 detected at 308C displayed two strong and
four weak reflections, which can be indexed as the (211),
(220), (420), (332), (422), and (431) planes of a cubic struc-
ture with space group Ia3d (Figure 1a). From the observed d
spacing of the (211) reflection, the best-fit value for the
cubic lattice parameter was estimated to be 16.16 nm. On
the other hand, above 808C, another LC phase was re-
vealed. Its SAXS pattern detected at 1008C showed two re-
flections with a q-spacing ratio of 1:2 (Figure S3 in the Sup-
porting Information). By taking into account the hydrophilic
volume fraction (f=0.68) and the wedge-type molecular
structure, however, it could be considered that the (110) re-
way, the structural transformation from the Cubbi to Colhex
,
as a function of temperature or f, seems to be very reasona-
ble. It can be explained that the expanded hydrophilic parts
break up the interconnected hydrophobic cylindrical net-
works into cylinders as temperature or ion concentration in-
creases.
By considering the stretched molecular length of 5.80 nm,
the intercolumnar distances (6.96 and 7.28 nm for A2B-1
and A2B-2, respectively) and the spacing-filling requirement
in the 2D columnar structure, a bilayered columnar assem-
bly can be suggested as a suitable packing model because it
flection of a 2D hexagonal columnar morphology (Colhex
)
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B.-K. Cho et al.
avoids undesirable contact between the hydrophilic and
-phobic regions (Figure 2c). By using the basis of the 4.6 ꢁ
thick column stratum (from the WAXS data in Figure 3a)
which persisted up to 1608C and 1978C for AB2-1 and AB2-
2, respectively.
From the POM observations, sample AB2-1 with [Li+]/
[EO]=0.2 showed a birefringent fanlike texture taken at
1408C, which is typically found in the columnar LC phase of
discotic liquid crystals (Figure 3b).[14] This texture suggests
that the aromatic discs form an anisotropic stacking, from
which our block codendrimers share the self-assembling
character of discotic LCs. Consistent with the POM results,
the SAXS data of AB2-1 at 708C displayed a Colhex pattern
p
p
where three reflections with q-spacing ratios of 1: 3:
4
(Figure 1c). From the (100) plane, the intercolumnar dis-
tance was estimated to be 6.48 nm. The comparison of this
intercolumnar distance with the AB2 stretched molecular
length of 5.80 nm,[15] suggests a bilayered packing in the col-
umnar cross-section, as proposed for the ion-doped A2B
samples (Figure 4a). Taking into account the wedgelike mo-
Figure 2. Schematic illustrations of a) the molecular model of A2B, b) the
bicontinuous cubic structure of A2B-1, and c) the hexagonal columnar
structure with the bilayered cross-section of A2B-1 and A2B-2. For rea-
sons of clarity, the red hydrophilic matrix is not shown in (b), and lithium
salts are omitted in the columnar cross-section in (c). The gray and red
colors in (c) represent the hydrophobic and -philic parts, respectively.
Figure 4. Schematic illustrations of a) the inverted-hexagonal columnar
structure (with a bilayered cross-section) of AB2-1, and b) the inverted-
bicontinuous cubic LC structure of AB2-2. For reasons of clarity, lithium
salts are omitted in the columnar cross-section in (a), and the gray hydro-
phobic region is not shown in (b). The gray and red colors represent the
hydrophobic and -philic parts, respectively.
lecular structure (i.e., the higher grafting density of tetradec-
yl chains in AB2) and the hydrophilic volume fraction (f=
0.47) of AB2-1, however, the configuration of hydrophilic
and -phobic parts must be inverted in comparison with the
bilayered Colhex of the ion-doped A2B samples. By using the
4.6 ꢁ thick column stratum from Figure 3a and the relevant
densities, the number of AB2 per column cross-section was
calculated to be approximately 2.4.
Figure 3. a) WAXS spectra in the Colhex of A2B-1, A2B-2, and AB2-1, and
b) the optical texture of AB2-1.
In contrast to AB2-1, AB2-2 with [Li+]/[EO]=0.4 was op-
tically isotropic over the entire LC temperatures, indicative
of a cubic phase. The SAXS data of AB2-2 detected at
1208C displayed a similar reflection pattern to the Cubbi of
A2B-1. It displayed the (211), (220), (321), (400), (420),
(332), and (422) planes of a cubic structure with space group
Ia3d (Figure 1d). The cubic lattice parameter was estimated
to be 14.68 nm. Considering the phase sequence with in-
creasing lithium concentration, this cubic phase can be con-
sidered an “inverted” bicontinuous cubic (Cubbi) structure.
On going from AB2-1 to AB2-2 (i.e., increasing f), more
added lithium salts expand the minority cylindrical core by
which the cubic phase of AB2-2 must be more continuous
than the Colhex of AB2-1.[16] Accordingly, only the cubic can-
and the relevant densities, the numbers of codendrimers per
column cross-section of the Colhex were calculated to be ap-
proximately 3.1 and 2.8 for A2B-1 and A2B-2, respective-
ly.[1b,13] The bilayered columnar structure differs from the or-
thogonal stacking of conventional discotic LCs, and this is
attributed to the amphiphilic nature of the discotic block co-
dendrimer.
Contrary to A2B, the majority of AB2 is the hydrophobic
dendron block. Therefore, we expected a certain phase-in-
verted morphology consisting of hydrophobic matrix and hy-
drophilic cores, in contrast to the ionic samples of A2B. We
chose [Li+]/[EO]s to be 0.2 and 0.4 for AB2-1 and AB2-2,
respectively. In the melt, both samples showed LC phases,
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Liquid-Crystalline Assemblies
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didate is a Cubbi consisting of a hydrophobic matrix and hy-
drophilic cylindrical networks (Figure 4b). Remarkably, the
Cubbi phase of AB2-2 persisted over a wide temperature
range (from 15 to almost 1978C). This is in marked contrast
to linear block copolymers in which bicontinuous cubic net-
works change into hexagonal cylinders with the addition of
ionic species.[17]
As described above, the ion complexation of the discotic
block codendrimers showed the induced LC phases with
hexagonal columnar and bicontinuous cubic architectures.
Notably, to date, cubic LC phases observed from dendrimer
architectures have all been revealed to be discrete micellar
structures.[1b,18] To the best of our knowledge, the Cubbi
phases of A2B-1 and AB2-2 are the first bicontinuous cubic
example in LC dendrimers, although only a few dendron–
polymer conjugates showed this cubic phase.[19] Further-
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Along with the generation of the unique Cubbi LC phase,
it is worth noting that the local properties in the Cubbi mor-
phology (i.e., the inversion of hydrophilic and -phobic prop-
erties between cylindrical cores and matrix) could be fine-
tuned; in other words, we can endow an exterior matrix
with either hydrophilic or -phobic character by the simple
alteration of the majority dendritic block.
In summary, we prepared two discotic block codendrimers
by a stepwise click reaction, and investigated the LC-form-
ing behavior of their ion-doped samples. Thermal and X-ray
analyses revealed the generation of the Colhex and Cubbi LC
phases as well as their phase-inverted homologues, as a
function of hydrophilic volume fraction. Interestingly, the
Cubbi phase shown herein could be utilized as a promising
platform applicable to organic electronics, such as LC semi-
conductors or electrolytes, when we consider the 3D connec-
tivity of its cylindrical core domain, including aromatic discs
and ions.
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Acknowledgements
This research was supported by Basic Science Research Program through
the National Research Foundation of Korea (NRF) funded by the Minis-
try of Education, Science and Technology (2009-0070798), Mid-career
Research Program through NRF grant funded by the MEST (2009-
0084501) and a grant from the Fundamental R&D Program for Core
Technology of Materials funded by the Ministry of Commerce, Industry
and Energy, Republic of Korea. We also appreciate the Graduate Re-
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Keywords: click chemistry · dendrimers · liquid crystals ·
morphologies · self-assembly
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Received: May 17, 2010
Published online: July 6, 2010
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