Liquid-Crystalline Octopus Dendrimers: Block Molecules with Unusual Mesophase Morphologies

Article abstract of DOI:10.1021/ja031506v

The synthesis and the mesomorphic properties of several new main-chain liquid-crystalline dendrimers, thereafter designated as octopus dendrimers in accordance with their eight sidearms, are reported. In these dendritic systems, the arborescence is ensured by anisotropic segments, acting as branching cells with a double multiplicity, which are incorporated at every node of the dendritic architecture. In such a way, these compounds radically differ from the classical end-functionalized liquid-crystalline dendrimers, the most commonly reported systems. Following our previous report on purely homolithic systems, that is, the building blocks constituting the dendritic matrix are all identical, several heterolithic systems made of different anisotropic blocks have been prepared. The dendritic branches and corresponding dendrimers were synthesized using a modular construction. Polarized optical microscopy and X-ray diffraction studies showed that all of these new octopus dendrimers exhibit either smectic-like or columnar phases with novel morphologies, the nature of the mesophases depending on the number of terminal chains attached to the peripheral groups. The mesomorphism of these heterolithic dendrimers is discussed in terms of their intrinsic architecture and compared to the analogous homolithic octopus systems. Models for the molecular organizations within both the smectic and the columnar phases are proposed on the basis of small Bragg angle X-ray diffraction studies and are supported by molecular modelizations. Moreover, this study showed that the mesophase stability is very sensitive to the nature and to the mutual arrangement (the spatial location) of the mesogenic segments within the dendritic matrix, illustrating the intimate relationships existing between the mesomorphic properties and the molecular architecture of these dendrimers.

Full text of DOI:10.1021/ja031506v

Published on Web 03/09/2004
Liquid-Crystalline Octopus Dendrimers: Block Molecules with
Unusual Mesophase Morphologies
Lionel Gehringer, Cyril Bourgogne, Daniel Guillon, and Bertrand Donnio*
Contribution from the Institut de Physique et Chimie des Mate´riaux de Strasbourg - IPCMS,
Groupe des Mate´riaux Organiques - GMO, UMR 7504 - CNRS/UniVersite´ Louis Pasteur, 23,
rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France
Received December 4, 2003; E-mail: bdonnio@ipcms.u-strasbg.fr
Abstract: The synthesis and the mesomorphic properties of several new main-chain liquid-crystalline
dendrimers, thereafter designated as octopus dendrimers in accordance with their eight sidearms, are
reported. In these dendritic systems, the arborescence is ensured by anisotropic segments, acting as
branching cells with a double multiplicity, which are incorporated at every node of the dendritic architecture.
In such a way, these compounds radically differ from the classical end-functionalized liquid-crystalline
dendrimers, the most commonly reported systems. Following our previous report on purely homolithic
systems, that is, the building blocks constituting the dendritic matrix are all identical, several heterolithic
systems made of different anisotropic blocks have been prepared. The dendritic branches and corresponding
dendrimers were synthesized using a modular construction. Polarized optical microscopy and X-ray
diffraction studies showed that all of these new octopus dendrimers exhibit either smectic-like or columnar
phases with novel morphologies, the nature of the mesophases depending on the number of terminal chains
attached to the peripheral groups. The mesomorphism of these heterolithic dendrimers is discussed in
terms of their intrinsic architecture and compared to the analogous homolithic octopus systems. Models
for the molecular organizations within both the smectic and the columnar phases are proposed on the
basis of small Bragg angle X-ray diffraction studies and are supported by molecular modelizations. Moreover,
this study showed that the mesophase stability is very sensitive to the nature and to the mutual arrangement
(the spatial location) of the mesogenic segments within the dendritic matrix, illustrating the intimate
relationships existing between the mesomorphic properties and the molecular architecture of these
dendrimers.
Introduction
supermolecules possessing a regular and controlled branched
topology,2 with an exponential rate of growth as the generation
Since their discovery in the late 1970s,1 and the adjustments
of perfectly controlled synthetic processes, dendrimers have led
to the most impressive developments and rapidly expanding
areas of current science.2 This extraordinary enthusiasm was
primarily caused by the intrinsic and unique molecular features
of the dendrimers2 and the possibility to generate numerous and
original chemical architectures, offering new synthetic concepts
and challenges for chemists3 as well as raising several interesting
theoretical questions.4 Indeed, dendrons and dendrimers are a
class of aesthetic, compartmentalized, practically monodisperse,
number increases.5 These features are the result of sophisticated
genealogical directed syntheses consisting of controlled iterative
methods involving successive and specific elementary steps
(convergent or divergent directed sequential construction). The
control of the ultimate molecular architecture (size and shape)
can be modulated by the generation growth, the multiplicity of
the branches, and the connectivity of the focal core. Research
in this area has been further boosted by the appreciation of their
uses as potentially interesting candidates in widespread applica-
tions.6 Dendrimers may be used, when suitably functionalized,
in biology as drug or gene delivery devices7 due to their rough
resemblance to some living components.8 Or, alternatively, as
ideal oligomeric substances, they may be designed as materials
with precise functionalities in which molecular level information
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Liquid-Crystalline Octopus Dendrimers
A R T I C L E S
emanating from a single multivalent initiator core and mesogenic
units attached at the termini of the branches.17-22 Induction of
liquid-crystalline properties may simply be achieved by respect-
ing these criteria,17-22 although a few LCDs with nonmesogenic
end-groups have been reported too.23-25 Mesomorphism results
essentially from both the enthalpic gain provided by anisotropic
interactions and the strong tendency for microphase separation
due to the chemical incompatibility between the flexible
dendritic core and the terminal groups as in AB-block copoly-
mers.26 The structure of the mesogen as well as the topology
of attachment to the core (end-on and side-on, Figure 1)
determine the mesomorphism of the entire compound. Other
subclasses of LCDs also include supramolecular dendrome-
sogens,27 shape-persistent,28 metallodendrimers,29 polypedic,30
and fullerene-containing LCDs.31 Let us note that the dendritic
systems so far mentioned are distinct from hyperbranched
polymers, often wrongly referred to as dendrimers, and are
Figure 1. Schematic 2D representation of an end-group dendrimer of
second generation with a 4-fold core connectivity (NC ) 4), and a ternary
branch multiplicity (NB ) 3). The mesogen can be attached terminally or
laterally to yield (a) end-on and (b) side-on LCDs.
is transferred from the initiator core to the periphery (or vice
versa) with the expectation of complementary and synergic
phenomena (i.e., induction of new properties) and/or cooperative
effects (i.e., amplification of the existing properties).9 Several
excellent comprehensive review articles have emphasized many
of the interesting assets of dendrimers.2,4,5,10-12
Molecular engineering of liquid crystals is also an important
issue for controlling the self-assembling ability and the self-
organizing process of single moieties into controlled nanostruc-
tures.13 It was thus logical to functionalize such supermolecules
to obtain liquid-crystalline materials14 with the possibility to
discover new types of mesophases and original morphologies.15
So far, most studies have focused on side-group liquid-
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Mater. 1995, 7, 578-581. (b) Seitz, M.; Plesnivy, T.; Schimossek, K.;
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J. L. Chem.-Eur. J. 1999, 5, 1834-1840. (b) Marcos, M.; Gime´nez, R.;
Serrano, J. L.; Donnio, B.; Heinrich, B.; Guillon, D. Chem.-Eur. J. 2001,
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Marcos, M.; Serrano, J. L. Macromoelcules 2002, 35, 370-381.
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1996, 8, 414-416. (c) Ryumtsev, E. I.; Evlampieva, N. P.; Lezov, A. V.;
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G.; Butz, T.; Sellner, H. Top. Curr. Chem. 1998, 197, 125-164. (b)
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1080. (b) Venturi, M.; Serroni, S.; Juris, A.; Campagna, S.; Balzani, V.
Top. Curr. Chem. 1998, 197, 193-228. (c) Gorman, C. AdV. Mater. 1998,
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(16) This terminology is used by analogy to side-chain liquid crystal polymers.
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A R T I C L E S
Gehringer et al.
as a function of the number of peripheral alkoxy chains, and,
due to the multicomponent nature of the compounds, the effects
of the anisotropic moieties as well as their spatial location and
mutual arrangement within the dendritic scaffolds will be
emphasized. A model accounting for the molecular organization
of these octopus dendrimers into the smectic and columnar
phases is proposed on the basis of X-ray diffraction studies and
molecular modelizations.
Synthesis
Synthetic Methodology. The anisotropic units selected for
this work were stilbene- and tolane-based moieties that were
chosen not only for the versatility of their chemistry, their ease
of derivatization, and their good thermal stability, but also for
the poor mesogenic character of the parent di-4,4′-alkoxy
homologues.37 This latter point was important to test whether
mesomorphism could be induced solely by the self-assembling
process of the dendrimers, on one hand, and to be able to
monitor with a great sensitivity the effects of small structural
variations on the mesomorphic properties of the resultant
dendrimers, on the other hand.
Figure 2. Schematic 2D representation of (a) homolithic and (b) heterolithic
main-chain liquid-crystalline octopus dendrimers.
characterized by randomly branched structures with a high
degree of branching and broad molecular weight distributions.32
Adequately functionalized, they have also been found to form
original mesomorphic systems.33,34
We have recently reported on a new family of LCDs,
affectionately named octopus LCDs35 in accordance with their
eight arms (Figure 2), for which the arborescence is ensured
by anisometric segments acting as branching cells, incorporated
at every node of the dendritic architecture by modular construc-
tion.36 Due to the presence of anisotropic groups at every level
of the dendritic hierarchy (and thus of additional intermesogen
interactions), the dendrimers are forced to adopt constrained
and regular structures, radically differing in this respect from
their side-chain homologues. For this study,35 the building blocks
constituting the dendritic matrix were stilbene groups, and as
such these octopus dendrimers were described as homolithic.
The principles of the modular construction can be applied for
the preparation of co-dendrimers made of two basic building
bricks (heterolithic systems), which can be arranged indepen-
dently in a controlled, alternated manner (Figure 2). The ability
of such polyfunctional and discrete dendritic structures to self-
assemble into mesophases may be an attractive strategy in the
field of materials science for the elaboration of multicomponent
nanosize objects. In this paper, the design and the synthetic
methodology of new homolithic and heterolithic octopus den-
drimers are presented. The mesomorphic behavior is discussed
All of these new main-chain dendrimers were prepared by
modular construction (Schemes 1-3).35 The dendritic branches
were constructed in a two-stage procedure. The first stage
involved the simultaneous preparation of the various constitutive
building bricks, including both the difunctionalized AB2-type
stilbene (4) and tolane (10) derivatives (Scheme 1) and the
terminal functional “stilbenol” (13) and “tolanol” groups (16a,b)
(Scheme 2). In the second stage of the procedure, these building
blocks were assembled selectively together to produce the
desired focal point functionalized dendrons (Scheme 3, acids
17a-e). As for the final dendrimers, they were obtained by a
convergent method consisting of the chemical coupling of these
acidic branches, through amino linkage, to a small tetravalent
core unit, tetra-amino core N,N,N′,N′-tetrakis(3-aminopropyl)-
1,4-butanediamine (DAB, Generation 1.0) (Scheme 3, 19a-e).
Preparation of the Internal Branching Subunits, 4 and
10. The internal branching units, 4 and 10, were prepared in
several steps using standard chemical reactions as shown in
Scheme 1. For the preparation of 4, the commercially available
starting material, 3,5-dimethoxy benzaldehyde, was almost
quantitatively converted into the corresponding styrene (1) by
a Wittig reaction. The coupling of this styrene derivative with
the appropriate ethyl 4-iodobenzoate via a palladium-catalyzed
Heck reaction then produced the dimethoxy stilbene ester
derivative, 2, also in good yields. The dihydroxy compound 3
was obtained in two successive steps including first the
demethylation with BBr3, immediately followed by the re-
esterification of the acid function because the ester group did
not survive the demethylation. Finally, bromoundecanol was
grafted to the free hydroxyl groups of 3 by a Mitsunobu
etherification to yield the prefunctionalized internal core moiety,
4. The analogous tolane compound 10 was prepared in seven
steps. It first consisted of the conversion of the commercially
available 3,5-dimethoxy aniline into 3,5-dimethoxy iodobenzene
(5) by a Sandmeyer reaction. Such an iodo derivative was then
coupled with trimethylsilylacetylene via a Sonogashira coupling
reaction to yield 6, which was deprotected to give the true
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Ithaca, NY, 1953. (b) Kim, Y. H. J. Polym. Sci., Part A: Polym. Chem.
1998, 36, 1685-1698. (c) Frey, H.; Ho¨lter, D. Acta Polym. 1999, 50, 67-
76. (d) Hult, A.; Johansson, M.; Malmstro¨m, E. AdV. Polym. Sci. 1999,
143, 1-34.
(33) Main-chain systems: (a) Percec, V.; Kawasumi, M. Macromolecules 1992,
25, 3843-3850. (b) Bauer, S.; Fischer, H.; Ringsdorf, H. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1589-1592. (c) Percec, V.; Chu, P.; Kawasumi,
M. Macromolecules 1994, 27, 4441-4453. (d) Hanh, S. W.; Yun, S. Y.
K.; Jin, J. I.; Han, O. H. Macromolecules 1998, 31, 6417-6425.
(34) Side-chain systems: Sunder, A.; Quincy, M. F.; Mu¨lhaupt, R.; Frey, H.
Angew. Chem., Int. Ed. 1999, 38, 2928-2930.
(35) Gehringer, L.; Guillon, D.; Donnio, B. Macromolecules 2003, 36, 5593-
5601.
(36) The concept and the compounds remain very different from the so-called
willow-like LCDs reported some years (a) Percec, V.; Chu, P.; Ungar, G.;
Zhou, J. J. Am. Chem. Soc. 1995, 117, 11441-11454. (b) Li, J. L.; Crandall,
K. A.; Chu, P.; Percec, V.; Petschek, R. G.; Rosenblatt, C. Macromolecules
1996, 29, 7813-7819.
(37) Demus, D.; Demus, H.; Zaschke, H. Flu¨ssige Kristalle in Tabellen; VEB
Deutscher Verlag fu¨r Grundstoffindustrie: Leipzig, 1974.
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3858 J. AM. CHEM. SOC. VOL. 126, NO. 12, 2004

Liquid-Crystalline Octopus Dendrimers
A R T I C L E S
Scheme 1. Preparation of the Internal Branching Units 4 and 10
Scheme 2. Preparation of the Terminal Groups 7a-d
acetylene derivative 7. The tolane species 8 was obtained by a
second Sonogashira coupling between 7 and ethyl 4-iodoben-
zoate. Compounds 9 and 10 were finally obtained using exactly
the same procedures as were used for the preparation of 3 and
4, respectively.
substituted benzaldehydes. Mono- and 3,4-dialkoxytolanoide-
like moieties (16a,b) were prepared by identical synthetic
methods, including the prior conversion of the appropriate
alkoxybenzaldehydes into the corresponding dibromo vinylic
compounds (14a,b) by the Corey-Fuchs procedure, which
under basic conditions was transformed into the true acetylenic
derivatives 15a,b. 16a,b were obtained by a Sonogashira
Preparation of the Terminal Groups, 13 and 16. Com-
pounds 13 and 16 were prepared from the same alkoxy-
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A R T I C L E S
Gehringer et al.
Scheme 3. Preparation of the Dendritic Branches (Acids 17a-f) and the Corresponding Octopus Dendrimers 19a-f
coupling between 15a,b and iodophenol. The 3,4-dialkoxystil-
benoide-like moiety, 13, was obtained by the conversion of the
dialkoxybenzaldehyde (11) into the corresponding styrene (12),
followed in the next step by its coupling to iodophenol under
palladium-catalyzed Heck reaction conditions (Scheme 2).
Preparation of the Precursory Acid Dendrons 17 and
Corresponding Octopus 19. The esters, precursors of the
various acidic branches 17, were obtained by straightforward
double alkylation reactions between the internal unit 4 or 10
and the appropriate stilbene (13) or tolane derivatives (16a,b);
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3860 J. AM. CHEM. SOC. VOL. 126, NO. 12, 2004

Liquid-Crystalline Octopus Dendrimers
A R T I C L E S
their subsequent hydrolysis yielded the desired acids 17a-f
(Scheme 3). The targeted octopus dendrimers (19a-f) were
obtained by reaction of the acid 17a-f with DAB, Generation
1.0, the acid function being activated by diphenyl(2,3-dihydro-
2-thioxo-3-benzoxazolyl)phosphonate, 18.38 Note that the den-
dron 17f, and the corresponding dendrimer 19f, were already
prepared.35 Unfortunately, due to solubility reasons, the acid
monodendrons and corresponding dendrimers having a single-
chain stilbene as end-groups (17, 19: Y ) CdC, X ) CdC,
CtC, R ) H) could not be synthesized.
All of the final compounds were isolated as air-stable
crystalline solids (except 19a obtained as a waxy material) and
were soluble in most organic solvents. The acidic arms were
purified by repeated crystallizations in petroleum ether, and the
dendrimers were purified by flash chromatography.
Structural Characterization of the Octopus Dendrimers.
The purity and characterization of the precursory dendritic acids
were determined by a combination of thin-layer chromatography
(TLC), 1H and 13C NMR spectroscopy, and MALDI-TOF mass
spectroscopy, and the final dendrimers were analyzed by
additional elemental analysis and SEC. The purity of all
intermediary compounds was only checked by 1H NMR
spectroscopy and TLC. In all cases, the results were in good
agreement with the proposed structures. The absolute molecular
weights were given by MALDI-TOF MS, which showed the
correct molecular peaks with additional lower mass fragments,
in small amount, which did not correspond to intermediate
compounds. The SEC analysis showed a monomodal molecular
weight distribution, with polydispersity indices close to unity,
meaning that the dendrimers were monodisperse. All of these
data, as well as the detailed synthetic procedures, have been
deposited in the Supporting Information.
Figure 3. Representative DSC traces: (a) 19a and (b) 19e.
mesophase at higher temperatures (ca. 80-90 °C) and cleared
at or above 100 °C; only one compound (19b) exhibited a
monotropic mesophase (the mesophase appeared on cooling
from the isotropic liquid). Comparing octopus having identical
dendritic cores, 19a,b and 19c,d, respectively, we observed an
important reduction of the mesophase stability with the doubling
of the terminal aliphatic chains number.
No conclusive and unequivocal mesophase assignment could
be given by this technique of characterization because no typical
and recognizable optical textures were obtained. However, as
compared to the homolithic stilbene dendrimers reported
recently,35 it is safe to assign a columnar phase for compounds
19b, 19d,e, consistent with the number of terminal chains grafted
to the peripheral groups, and to predict a smectic-like behavior
for 19a and 19c. Some optical textures have been deposited in
the Supporting Information.
Confirming the results of the microscopic observations,
several first-order transitions were detected by DSC at corre-
sponding temperatures. Most of the dendrimers exhibited
complicated and nonexploitable DSC traces during the first
heating scan, a common situation in macromolecular systems.
However, on subsequent heating-cooling cycles, more simple
and perfectly reproducible thermograms were obtained as shown
by two representative dendrimers of each group (Figure 3). This
good thermal stability was also confirmed by thermogravimetry
showing a loss-weight of less than 5% detected above 200 °C,
with no more degradation observed up to 300 °C, well above
the transition into the isotropic liquid. In agreement with POM,
the DSC traces consisted of crystal-to-mesophase transforma-
tions (19b, 19c-e) and of mesophase-to-isotropic liquid transi-
tions (Figure 3, Table 1). In the case of 19a and 19c, an
additional enthalpy change was seen, likely corresponding to a
mesophase-to-mesophase transformation (Figure 3a). The transi-
tion temperatures of the acids and dendrimers are gathered in
Table 1. The fairly large values of the transitions’ enthalpies
were at first surprising, but were probably due to long and
complicated reorganization processes, to the numerous π-π
intermolecular interactions, and to the important volume of
aliphatic chains per mesogenic group and dendrimer. This
thermodynamic feature will be discussed later. On cooling,
Results and Discussion
The thermal behavior of the acid monodendrons and of the
dendrimers was investigated by three complementary tech-
niques: polarized optical microscopy (POM), differential scan-
ning calorimetry (DSC), and small-angle X-ray diffraction
(XRD).
Optical and Thermal Studies by POM, TGA, and DSC.
The liquid-crystalline behavior of all of the dendrimers was
probed by the observation under polarized microscope of optical
textures showing homogeneous, birefringent, and fluid domains,
coalescing on an increase in the temperature; the presence of
large homeotropic domains pointed either to orthogonal-type
smectic phases (SmA or SmB) or to a columnar hexagonal phase
(Colh). In contrast, none of the precursor acids, except 17e, was
mesomorphic. Some general trends can be immediately deduced
from these observations. On the basis of cylindrical domains
or focal-conic-like textures being observed in some cases and
not in other cases, and of the differences in viscosity, two groups
of compounds could be distinguished, one set consisting of 19a
and 19c, and the other set made of 19b, 19d,e. Dendrimers with
peripheral groups having one terminal chain only (19a, 19c)
were mesomorphic at or near room temperature, up to 121 °C
(19a) or 140 °C (19c); the crystalline phase of 19c was not
recovered after the first heating. In contrast, the compounds with
two terminal chains per end-groups (19b, 19d,e) melted into a
(38) Ueda, M.; Kameyama, A.; Hashimoto, K. Macromolecules 1988, 21, 19-
24.
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A R T I C L E S
Gehringer et al.
Table 1. Thermal Behavior of the Dendritic Branches and Octopus Dendrimers and X-ray Characterization of the Mesophasesa
mesophase
parameters
molecular
volume
indexation
00l hk
compound
transition temperatures/
°
C
dmeas/Å
I
dcalc/Å
measured at T
at T (Vmol
)
N
17a
17b
17c
17d
17e
Cr 150 I
Cr 103 I
Cr 155 I
Cr 125 I
Cr 111 (137.8b) Colh 124 (6.1b) Ic
58.9
33.7
29.7
4.5
VS
S
S
10
11
20
h
58.9
34.0
T ) 115 °C
a ) 68.0 Å
Vmol ) 3000 Å3
Vmol ) 9545 Å3
Vmol ) 9750 Å3
6.0
29.45 S ) 4005 Å2
br
Vcell ) 18 090 Å3
17f
19a
Cr 146 I
SmB 101 (49.0b) SmA 121 (127.4b) Ic
107.1
53.5
35.5
4.5
VS 001
S
S
br
sh
VS 001
S
S
br
VS
M
S
106.9
53.45 d ) 106.9 Å
35.6
T ) 80 °C
002
003
h
AM ) 89.3 Å2
am ) 22.3 Å2
4.3
97.4
48.7
32.4
4.5
78.4
44.3
29.6
4.5
97.3
T ) 110 °C
002
003
h
48.65 d ) 97.3 Å
32.45 AM ) 100.2 Å2
am ) 25.05 Å2
77.8
44.9
29.4
19b
19c
Cr 78 (Colh 75) Id
10
11
21
h
T ) 75 °C
Vmol ) 12 055 Å3 2.6
a ) 89.8 Å
S ) 6990 Å2
Vcell ) 31 450 Å3
T ) 90 °C
br
Cr 65 (165.7b) SmB 110 (50.5b) SmA 140 (144.5b) I 104.0
52.1
VS 001
S
S
br
sh
VS 001
S
S
br
VS
M
M
S
br
VS
S
S
S
M
M
br
104.0
52.0
34.7
Vmol ) 9625 Å3
002
003
h
a ) 104.0 Å
AM ) 92.6 Å2
am ) 23.15 Å2
SmB 109 (53.1b) SmA 132 (127.3b) Ie
34.6
4.5
4.3
102.0
50.9
33.9
4.5
84.0
48.9
42.3
31.6
4.5
82.0
47.2
40.7
31.0
23.5
22.7
4.5
101.8
50.9
33.9
T ) 120 °C
Vmol ) 9830 Å3
002
003
h
a ) 101.8 Å
AM ) 96.6 Å2
am ) 24.15 Å2
T ) 90 °C
19d
19e
Cr 82 (200.4b) Colh 98 (44.0b) Ie
Cr 88 (249.4b) Colh 110 (52.6b) Ie
10
11
20
21
h
10
11
20
21
22
31
h
10
11
20
21
22
h
84.2
48.6
42.1
31.8
Vmol ) 12 200 Å3 3.0
a ) 97.2 Å
S ) 8170 Å2
Vcell ) 36 750 Å3
81.7
47.2
T ) 105 °C
a ) 94.3 Å
Vmol ) 12 345 Å
2.8
40.85 S ) 7705 Å2
30.9
23.6
22.65
Vcell ) 34 600 Å3
19f
Cr 95 (227b) Colh 132 (17.8b) Ie,f
82.45 VS
82.45 T ) 100 °C
Vmol ) 12 300 Å3 2.9
47.55
41.4
31.1
23.8
4.5
S
M
S
M
br
47.6
41.2
a ) 95.2 Å
S ) 7850 Å2
31.15 Vcell ) 35 320 Å3
23.8
admeas and dcalc are the measured and calculated diffraction spacings (dcalc is deduced from the following mathematical expressions: d001 ) 1/Nl (∑l d00l‚l),
2
2
x
where Nl is the number of 00l reflections for the SmA and SmB phases; d10 ) 1/Nhk(∑h,k dhk‚ h +k +hk), where Nhk is the number of hk reflections for
the Colh phase); I is the intensity of the reflection (VS, very strong; S, strong; M, medium; br, broad; sh, sharp); 00l and hk are the indexations of the
reflections corresponding to the smectic and Colh phases, respectively; d is the smectic periodicity (d ) d001 ); AM is the molecular area (AM ) Vmol/d); and
x
a
m is the area of one mesogenic unit (am ) AM/4); a is the lattice parameter of the Colh phase (a ) 2/ 3 d10 ); S is the lattice area (S ) a d10 ); Vcell is the
volume of the hexagonal cell (h‚S), that is, a slice of column h-thick; the molecular volume is defined as Vmol ) (M/F0.6022)(VCH (T)/VCH (T0)); M is the
2
2
molecular weight; VCH (T) ) 26.5616 + 0.02023T (T in °C, T0 ) 22 °C); F is the volume mass (1 g cm-3); N is the number of molecules per Vcell (N )
2
V
cell/Vmol); Cr, crystalline phase; I, isotropic liquid; SmB and SmA, smectic B and smectic A phases; Colh, hexagonal columnar phase. b ∆H (kJ mol-1).
c Identical first and second heatings. d Determined on cooling. e Second heating. f Data reported in ref 35.
crystallization was systematically observed for 19b, 19d,e (as
for 19f), whereas it was absent for 19a and 19c, which retained
the low-temperature mesophase down to room temperature.
Structural Investigation of the Mesophases by Small-Angle
X-ray Diffraction. Identification and unequivocal assignment
of the mesophases were finally achieved by small-angle X-ray
diffraction on powder samples. Qualitatively similar X-ray
patterns were obtained for structurally related dendrimers, that
is, 19a and 19c, on one hand, and 19b, 19d,e, on the other hand.
(a) The Smectic-Like Phases. The smectic nature of the
mesophases suggested by POM for 19a and 19c was eventually
confirmed by X-ray diffraction; both compounds exhibited
exactly the same X-ray patterns. The powder X-ray patterns
recorded in the temperature intervals delimited by DSC exhibited
a set of three sharp and equidistant small-angle reflections in
the ratio 1:2:3 (Table 1), corresponding to the smectic layering
of the molecules, and a diffuse scattering halo in the wide angle
region, centered around 4.5 Å, associated to the liquidlike order
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Liquid-Crystalline Octopus Dendrimers
A R T I C L E S
the area per mesogenic units, am (i.e., area occupied by one
terminal unit), were calculated from X-ray and volumetric data
(Table 1). The molecular area is directly deduced from the
molecular volume39 and the layer periodicity, and the area per
mesogenic unit was calculated considering as above that half
of the mesogenic units lie on one side, and the other half lie on
the other side of the dendritic core (i.e., 4am ) AM) (Table 1).
It was found that in the low-temperature phase, AM ) 90-92
Å2, and in the high-temperature phase, AM ) 96-100 Å2,
leading to the corresponding area per mesogenic unit, am
)
22.5-23 Å2 and am ) 24-25 Å2, respectively. The values of
am, for each dendrimer in both mesophases, deduced from these
calculations correspond very precisely to the expected cross
sections of molten alkyl chain in both the smectic B and A
phases, respectively. This result supports the correct mesophase
assignments (vide supra), as well as confirms the prolate
conformation of the dendrimers in both smectic phases with
the peripheral anisotropic units being almost perpendicular to
the layer normal direction. In this case, due to geometric
constraints, the mesogenic groups of the first row of the
arborescence, located in the smectic sublayer, should be tilted
to match the molecular area of roughly two mesogenic groups,
to occupy a larger apparent surface area (Figure 6). Of course,
the rigid units contained in these aromatic slabs are tilted with
respect to the layer normal, but the tilt is not correlated, that is
the tilt order is short-range. In other words, the relative disor-
dered distribution of the rigid parts yields an apparent zero tilt
angle, and then a uniaxial smectic sublayer. This model is con-
sistent with the molecular conformation deduced from MD calc-
ulations and also explains the difference between the periodicity
and the molecular length. The SmB-to-SmA transformation
corresponds to the loss of the hexatic order consequent to the
lateral disorganization of the anisotropic units of the outer slabs.
Therefore, the morphology of the smectic phases generated
by such multiblock molecules is quite unique in that it possesses
a two-level molecular organization, each being dependent on
the other one. It consists of an internal sublayer made of tilted
rigid segments with no correlation of the tilt, flanked by outer
slabs inside which the mesogenic groups are arranged perpen-
dicular to the layer (Figure 6). Molecular modelization supports
this view of strongly segregated multilayer structures, with
interfaces between the various molecular parts (Figure 5).
Obviously, these interfaces are not so well defined due to
thermal fluctuations. Nevertheless, let us point out that, due to
this peculiar structural feature, such layered mesophases cannot
exactly be described as purely SmA or SmB phases.
Figure 4. (a) X-ray pattern of 19c at 90 °C (SmB) and 120 °C (SmA). (b)
Decomposition of the wide angle signal into a sum of two functions.
of the molten aliphatic chains and rigid parts (Figure 4a). As
for the lower-temperature phase, an additional sharp peak (4.3
Å) was seen, still in association with the diffuse scattering
(Figure 4b), pointing to a long-range, hexatic order within the
smectic layer. Thus, on the basis of these X-ray patterns, the
high-temperature phase can be assigned as a disordered smectic
phase (i.e., SmA or SmC), whereas the other, low-temperature,
phase due to the extra in-layer order can be assigned as a hexatic
smectic phase (i.e., SmB, SmF, or SmI). The observation by
POM of large homeotropic areas indicates orthogonal smectic
phases, and they can safely be assigned as SmA and SmB phases
for the high- and low-temperature mesophases, respectively.
In a first approximation, one can realistically assume an
elongated conformation for the dendrimer, where the rigid parts
are collinear to the layer normal. Indeed, to allow the formation
of lamellar mesophases, the overall molecular structure should
adopt such a parallel conformation, with the elementary me-
sogenic units arranged in a pseudo-parallel fashion, with
necessarily half the mesogenic units (i.e., four in the present
case) extending up and the other half extending down the
molecular center.18 As such, the molecular model of the smectic
layer consists of a tetragonal periodic cell in which the cross
section was set to match roughly the molecular area of four
mesogens in a SmA phase (10 × 10 Å2). The third parameter
of the cell was fixed to 150 Å, much longer than the length of
the fully extended dendrimer to simulate a single layer and to
allow the molecules to expand or shrink freely. A molecular
dynamics (MD) simulation was then performed on this model
to evaluate the dendrimer conformation within the smectic layer.
The result of these calculations is represented by the molecular
snapshot in Figure 5, and the estimated molecular length (ca.
90-100 Å) was found to be in very good agreement with the
periodicities measured by XRD (ca. 100 Å).
(b) The Columnar Phase. As for the other set of dendrimers,
they all showed a Colh phase, consistent with the previous
described system 19f.35 Four (19d) and up to six (19e) sharp
reflections were observed in the small-angle region, with the
x
x
x
x
reciprocal spacings in the ratios 1, 3, 4, 7, 12, and
x
13, and they were indexed in a 2D hexagonal lattice as (hk)
) (10), (11), (20), (21), (22), and (31), respectively (Figure 7).
In the case of 19b, despite the fact that the columnar mesophase
was monotropic and existed over a narrow temperature range
of only a few degrees, it was still possible to obtain a meaningful
X-ray pattern on cooling from the isotropic liquid, allowing also
(39) The molecular volumes, Vmol, were estimated considering a density of 1
for the molecules in the mesophase, and then were corrected by a
temperature factor because the volume increases with temperature (see Table
1).
To verify the pertinence of this molecular conformation and
to identify the mesophases, the molecular area, AM, as well as
9
J. AM. CHEM. SOC. VOL. 126, NO. 12, 2004 3863

A R T I C L E S
Gehringer et al.
Figure 5. Snapshot of the molecular conformation of 19c in the smectic phases obtained by molecular dynamics simulation. The parts in gray and black
represent the aliphatic and aromatic parts, respectively.
consequence of the mismatch between the surface areas of the
aromatic cores and the cross section of the aliphatic chains,
resulting in the curvature of all of the interfaces. In the present
case, to compensate the discrepancy between the cross sections
of both the anisometric segments and the chains, one can also
imagine the former to be tilted and distributed in a “splay”
fashion, with respect to the columnar axis, also resulting in the
curvature of the interfaces.42 Indeed, the parameters of the
hexagonal lattices obtained experimentally, a ) 90-100 Å,
correspond fairly well to the diameter of the dendrimers in a
flattened conformation, ranging between 100 and 110 Å as
estimated by MD simulation (vide infra). It is therefore highly
probable that the octopus preferably adopts an oblate shape
within the columns that is a flattened or a wedge-like conforma-
tion with the anisotropic blocks lying more or less in the 2D
hexagonal lattice plane, rather than a prolate conformation
(cylindrical) as in the smectic systems. The small discrepancy
between the estimated molecular diameters (100-110 Å) and
Figure 6. Model for the octopus conformation and organization within
the “supersmectic” phases.
the measured lattice parameters (90-100 Å) of the Colh
mesophases, corresponding to a molecular contraction of about
10-20%, was ascribed to thermal fluctuations (the system is
fluid and heated), that is, to the liquidlike order of the molten
aliphatic chains, and to some degree of tilt of the rigid segments
with respect to the columnar axis (in and out-off plane tilts)
and within the lattice plane (in-plane splay or bent orientations),
that is, no preferential direction and no correlation of the
molecular tilt.
To propose a coherent explanation for the self-assembling
of these compounds within the columns, the number of octopus
able to fill a columnar slice 4.5 Å thick (h) was first calculated.
This distance h is considered realistic and corresponds to the
average liquidlike correlations between the peripheral mesogenic
groups in liquid-crystalline phases.18b Let us remark that, in
Figure 7. X-ray pattern of 19e at 105 °C (Colh).
nondiscoid systems, a columnar slice does not have a particular
the assignment of the phase as Colh. Note that, apart from the
significant meaning, and it does not necessarily imply that, in
fundamental reflection (10), the second most intense one is
the present case, the columns result simply from the stacking
always the (21) reflex and is probably connected to the structure
of flat dendrimeric or supramolecular dendritic discs on top of
factor and must be the signature of a particular type of molecular
each other, as in purely discotic materials. However, this
arrangement within the columns (vide infra).40 Similarly, the
approach permits the calculation of a linear density of occupation
mesomorphic dendron 17e displayed the same kind of X-ray
pattern, and the same Colh phase was assigned. The results of
along the columnar axis. The molecular volumes, Vmol, were
calculated as previously described (vide supra). The volume of
these investigations are summarized in Table 1.
one columnar stratum with a thickness (h) of 4.5 Å can be
The formation of columnar mesophases in nondiscotic
calculated from S‚h, where S is the columnar cross section
systems, and particularly with polycatenar mesogens,41 is a
(Table 1). The number of molecules per slice can thus be directly
(40) This peculiarity is likely connected to the onion structure of the columns,
and experiments to prove this hypothesis are underway.
(41) (a) Maltheˆte, J.; Nguyen, H. T.; Destrade, C. Liq. Cryst. 1993, 13, 171-
187. (b) Nguyen, H. T.; Destrade, C.; Maltheˆte, J. AdV. Mater. 1997, 9,
375-388.
(42) (a) Fazio, D.; Mongin, C.; Donnio, B.; Galerne, Y.; Guillon, D.; Bruce, D.
W. J. Mater. Chem. 2001, 11, 2852-2863. (b) Smirnova, A. I.; Fazio, D.;
Iglesias, E. F.; Hall, C. G.; Guillon, D.; Donnio, B.; Bruce, D. W. Mol.
Cryst. Liq. Cryst. 2003, 396, 227-240.
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Liquid-Crystalline Octopus Dendrimers
A R T I C L E S
Figure 8. Schematic representation of the two possible molecular conformations of the dendrimers, (a) and (b), and their self-assembling and self-organization
processes into the columns of the Colh phase, (i) and (ii).
obtained by N ) h‚S/Vmol. It showed that, whatever the system
considered, three dendrimers are necessary to fill such a
columnar stratum (and six acid dendrons for 17e as it could
have been expected from our previous work35). Thus, because
more than one molecule is needed to satisfy the dimensions of
a column, the formation of the mesophase results from the self-
assembling process of octopus molecules necessarily adopting
predefined shapes. As the supramolecular dendrimers described
by Percec et al.,27 columnar structures are generated from the
self-assembling of the most stable molecular conformations
having either a wedge-like or a half-disc shape. The overall
molecular conformations of the dendrimers in the mesophase
are driven by the steric congestion of the terminal aliphatic
chains and depend on the segregation between the different
constitutive blocks. In the present case, two molecular confor-
mations likely predominate to satisfy the geometrical require-
ments, the wedge-like (a in Figure 8) and the flattened (b in
Figure 8) conformations. Therefore, the formation of supramo-
lecular discs and columns results from the molecular association
of these two types of dendritic conformations, as shown in
Figure 8. One possibility consists of assembling three dendrimers
with solely the wedge-like conformation (i in Figure 8) and
another one of associating three octopus with both wedge-like
and flattened conformations in a 2:1 mixture (ii in Figure 8).
Clearly, one has to bear in mind that these various arrangements
must coexist in the columns because it is not possible to privilege
one over the other. The resultant columns then further self-
organize into a 2D hexagonal lattice. Considering the diblock,
alternated chemical nature of these octopus dendrimers, an onion
morphology for the columns is most likely probable.
enhancement of the micro segregation over the entire simulation
experiment time was also observed, contributing to the stabiliza-
tion of the onion structure. Furthermore, the compensation of
the molecular areas at the various interfaces at every level of
the arborescence, which implies the tilt of the internal and
external rigid segments with respect to the radial directions, was
also shown in the modelization (Figure 9).
The case of the acid dendron 17e is a bit puzzling. Indeed,
the volume of one octopus is exactly 4 times larger than that of
the precursory dendritic branch, whereas the corresponding cell
volumes are in a 2-to-1 ratio (see Table 1), instead of the
expected 4-to-1 ratio; a simple correlation rule clearly does not
apply here. This corresponds to an important contraction (ca.
25%) of the cell parameter of the Colh formed by the octopus
and that formed by the dendron. This diminution of the cell
dimension may be due to the reduction of the molecular rigidity
passing from the dendromesogen to the dendrimer; the rigidity
of the precursory dendron would be due to cohesive and
cooperative H-bondings that are due to the presence of free acid
functions. Similar observations were made on an increase in
the generation number (and thus the molecular volume) of the
supramolecular dendrimers studied by Percec at al.27 They did
not observe a linear rule either, but on the contrary they noticed
the reduction of the number of monodendrons per cell. Assum-
ing that the molecular conformation of such dendromesogens
forming the columnar structure is described as flat tapered, they
explained this phenomenon by a decrease of the cone angle,
and therefore by the decrease of the curvature, with increasing
generation. This decrease of the curvature implicates an increase
of the disc diameter (and thus of the columnar cross section),
and thus more molecules to fill the disc. Here, the rigidifying
of the dendron likely reduced the degree of conformational
freedom, forcing them to assemble very close together (increase
of the cone angle, θd), resulting in the reduction of the hexagonal
cell, as well as an augmentation of the interfacial curvature:
the tendency to pack in larger cells is then reduced (Figure 10).
Once attached to the tetravalent core, each of the four branches
This proposed model was further justified by MD simula-
tion: a periodic molecular model for 19d was built from the
experimental X-ray data, that is, a hexagonal lattice with a 97
Å parameter and a thickness of 4.5 Å, paved with three
molecules in a flattened wedge conformation. The result of the
calculation (Figure 9) evidenced a good filling of the available
volume, acknowledged by a calculated density of 0.95. An
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A R T I C L E S
Gehringer et al.
Figure 9. Snapshot of the molecular conformation in the Colh mesophase for 19d obtained by a MD calculation. The parts in gray and black represent the
aliphatic and aromatic parts, respectively.
Figure 10. Effect of the cone angle on the curvature and columnar diameter.
of the dendrimers is more confined than the free branch
(reduction of the cone angle, θo, decrease of the curvature,
Figure 10), and thus a higher number of these individual
branches is needed to be in accordance with the consequent
larger perimeter of the columnar cross section.
Molecular Structure and Mesophase Morphology. All of
the new compounds synthesized, with the exception of most of
the acid precursors (except 17e), are mesomorphic, showing
either smectic-like phases or self-assembling into a columnar
mesophase with a hexagonal 2D symmetry. An important result
is therefore the induction of the mesomorphism upon “den-
drimerization”, an interesting illustration of the so-called “den-
dritic effect” because none of the low-molar weight monomeric
components nor most of the acid precursors were mesomorphic.
This result is consistent with our previous report on homolithic
dendrimers.35
will adopt either a parallel (prolate) or a flat (oblate) conforma-
tion. The formation of the smectic lamellar phases is the result
of the parallel disposition of the mesogenic groups on both sides
of the focal tetravalent core, the dendrimer adopting the shape
of a giant elongated multipode, and then organizing into layers.
In contrast, the grafting of additional terminal chains at the
periphery prevents such a parallel disposition of the pro-
mesogenic groups, which are forced to be radially arranged
around the central moiety: the dendrimers can adopt the shape
of a flat-tapered objects which will self-arrange into supramo-
lecular discs. Although the expected mesomorphism crossover
from lamellar to columnar structures was observed with an
increase in the number of terminal chains,18 the two-phase
behavior or the induction of intermediate mesophases (such as
the bicontinuous cubic or rectangular columnar phase,43 for
instance) is nevertheless absent within a single dendrimer.
Molecular Structure and Mesophase Stability. The stability
of these mesophases results in a large part from the formation
of layer-block structures being driven by microphase separation
The morphology of the mesophase is determined by the
number of alkyl chains grafted on the peripheral mesogenic
group. Indeed, the change in the number of terminal chains per
end group (one or two) modifies the relationships between the
hard parts and the soft parts, and consequently the molecules
(43) Rueff, J. M.; Barbera´, J.; Donnio, B.; Guillon, D.; Marcos, M.; Serrano, J.
L. Macromolecules 2003, 36, 8368-8375.
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Liquid-Crystalline Octopus Dendrimers
A R T I C L E S
stability were found to be influenced by the nature and the
relative disposition of the anisotropic segments within the
dendritic matrix.
As previously demonstrated, the high density of aliphatic
chains likely imposes curved interfaces at all hierarchical levels
of the dendrimers, that is, between the pro-mesogenic units, the
aliphatic spacers, and the terminal chains, respectively, forcing
the molecules to adopt a wedge-like conformation, and thus
promoting their self-assembling toward a columnar organization
with an onion internal morphology. As for the smectic-like
phases, the dendrimers adopt a parallel conformation, and, due
to the particular nature of the molecules, the segments of the
first generation, which are located in the smectic sublayer, are
tilted, leading to a two-level molecular organization.
Figure 11. Evolution of the mesophase stability from tolane-rich (19b,
19d) to stilbene-rich (19e, 19f) octopus LCDs. The brackets indicate that
the mesophase is metastable (monotropic).
This structural concept proves to be innovative and versatile
as it offers many new opportunities in the design of a wide
range of multicomponent systems with specific properties for
potential novel applications. Indeed, it appears that large
flexibility and freedom are allowed in the choice of the
elementary anisotropic bricks forming the dendritic skeletons
of these LCDs, without the suppression of the mesomorphic
properties. Additionally, such bricks can be independently
interchanged, and the stability of the mesophases accordingly
modulated, probing the good sensitivity of the dendritic scaffold
with the nature and the mutual arrangement of the mesogenic
segments (spatial location), and the intimate relationships with
the mesomorphic properties. The high sensitivity of such AB-
block co-dendrimers to the surrounding environment (properties
versus molecular structure) could be, in principle, beneficial to
access to some kinds of molecular sensors, that is, to use such
supermolecules as tools to test how properties in general may
be altered or modulated upon delicate external stimuli.
Future studies should examine the synergic and/or cooperative
effects upon the insertion of various types of functional bricks.
The effects of the branching cell multiplicity as well as the
nature of the core (connectivity, structure) are currently under
investigation and will be reported in due course. We have now
showed that the concept works for homolithic as well as for
alternated heterolithic systems. It is now envisaged to prepare
segmented “block-type” co-dendrimers, where each of the two
dendritic branches (of same or different generation) contains
different mesogenic units in different locations, with different
terminal chain substitution patterns and topologies of attachment
to the core. However, an important effort should be made toward
the selective chemistry of the core to access to such systems.
between the noncompatible constituting segments of the mol-
ecule, which is at each hierarchical level reinforced by the
specific anisotropic interactions between the rigid mesogenic
units. An octopus with one chain per end group has been found
to be more stable than those with two chains per end group
probably due to an optimization of the lateral interactions for
the former.
Another interesting observation is the continuous increase of
the thermodynamic stability of the mesomorphism in relation
to the increasing number of stilbene units in the dendrimer at
the expense of tolane groups. Indeed, whatever the system being
considered, neither the type of the building blocks (tolane or
stilbene) nor their spatial disposition (alternated in the multi-
component heterolithic systems) affects the nature of the
mesophase, but does modify substantially the mesophase
stability. In the case of the smectic systems, generalization is
not made possible because only two molecules (vide supra) were
synthesized, although this tendency is nevertheless perceptive.
However, in the case of the columnar systems, the impact of
the octopus topology on the mesophase stability is obvious
(Figure 11). Indeed, 19b, containing 100% of tolane units,
exhibits a monotropic Colh phase, whereas, for 19d (66.6%
tolane-rich), 19e (33.3% tolane-rich), and 19f (100% stilbene-
rich), both transition temperatures increase, the clearing tem-
perature increasing at a faster rate than the melting temperature,
resulting in a net enhancement of the mesomorphic temperature
range. This is likely a consequence of the greater flexibility of
the stilbene units, favoring more efficient and less constrained
intermesogen interactions than the tolane species, which is too
rigid. It is therefore possible to tune and control the mesomor-
phic properties of these octopus dendrimers by a subtle modular
construction.
Experimental Section and Method of Characterization
The synthesis of the elementary bricks, monodendrons, and final
dendrimers, their structural analysis, and the various techniques used
for their analytical characterization and for the structural determination
of the mesophases structures have been deposited as Supporting
Information.
Conclusions
Several novel liquid-crystalline octopus dendrimers, with both
homolithic and heterolithic structures, have been synthesized,
containing stilbene and/or tolane groups as basic elementary
subunits. They were obtained by a modular construction,
consisting of the preparation of the acid branches and their
subsequent grafting onto a tetravalent core through amido
linkages. They showed smectic or columnar phases with unusual
morphologies (smectic with a two-level ordering and onion-
like columnar phases) depending on the substitution pattern of
the terminal groups. The transition temperatures and mesophase
Acknowledgment. The authors would like to thank Dr. Benoıˆt
Heinrich, M. Nicolas Beyer, and Ms. Laurence Oswald for their
general assistance.
Supporting Information Available: Experimental section
containing techniques, materials, the synthesis and analytical
characterization for all organic precursors, monodendrons, and
dendrimers, and optical textures (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA031506V
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