Liquid-crystalline self-organization of isocyanide-containing dendrimers induced by coordination to gold(I) fragments

Article abstract of DOI:10.1021/ja909435e

Dendritic polyisocyanides can be considered as promising polytopic ligands to generate a great diversity of metallodendrimers due to the ability of the isocyanide moiety to bind to various transition metals. Here, new isocyanide-containing dendrimers and their corresponding polynuclear gold complexes have been prepared, [Gⁱ(NC)_Z] and [G ⁱ(NCAuR)_Z], respectively, where Gⁱ is a poly(phenyl ether) dendrimer, i is the generation number (i = 0, 1, or 2), Z is the number of peripheral groups (Z ) 3 x 2ⁱ), and AuR are the surface groups ([R ) Cl, C≡C-C₆H₄-OC₁₂H ₂₅, C≡CC₆H₂(OC₁₂H ₂₅)3]. The compounds are derived from a highly flexible phenyl ether-based dendritic core, Gi, having the general formula G⁰ = C₆H₃(OC₁₁H₂₂OC₆H _4-)₃, G¹ ) C₆H₃[OC ₁₁H₂₂OC₆H₃(OC₁₁H ₂₂OC₆H_4-)₂]₃, G ² = C₆H₃[OC₁₁H₂₂OC ₆H₃{OC₁₁H₂₂OC₆H ₃(OC₁₁H₂₂OC_6- H_4-) ₂}₂]₃), growing from the trivalent phloroglucinol and with undecylene aliphatic spacers between each branching benzene ring and end-functionalized by isocyanide groups. As in their monomeric model counterparts, stable liquid-crystalline phases are induced upon complexation of the AuR gold moieties at the branch termini. The nature of the anionic ligand R promotes the appearance of smectic or columnar mesophases, the formation of which are governed by steric and dipolar interactions. Based on X-ray diffraction experiments, models describing the supramolecular organization of these metallodendrimers into smectic and columnar mesophases are proposed: columnar phases result from the one-dimensional stacking of molecular disks made of self-assembled supermolecules in oblate cylindrical conformation, while the smectic phases form by the lateral two-dimensional registry of the supermolecules in antiparallel head-to-head prolate conformation.

Full text of DOI:10.1021/ja909435e

Published on Web 01/07/2010
Liquid-Crystalline Self-Organization of Isocyanide-Containing
Dendrimers Induced by Coordination to Gold(I) Fragments
Carlos Cordovilla,†,‡ Silverio Coco,† Pablo Espinet,† and Bertrand Donnio*,‡
IU CINQUIMA/Qu´ımica Inorga´nica, Facultad de Ciencias, UniVersidad de Valladolid, 47071
Valladolid, Castilla y Leo´n, Spain, and IPCMS, CNRS-UniVersite´ de Strasbourg (UMR 7504), 23
rue du Loess, BP43, 67034, Strasbourg Cedex 2, France
Received November 6, 2009; E-mail: bdonnio@ipcms.u-strasbg.fr
Abstract: Dendritic polyisocyanides can be considered as promising polytopic ligands to generate a great
diversity of metallodendrimers due to the ability of the isocyanide moiety to bind to various transition metals.
Here, new isocyanide-containing dendrimers and their corresponding polynuclear gold complexes have
been prepared, [Gi(NC)Z] and [Gi(NCAuR)Z], respectively, where Gi is a poly(phenyl ether) dendrimer, i is
the generation number (i ) 0, 1, or 2), Z is the number of peripheral groups (Z ) 3 × 2i), and AuR are the
surface groups ([R ) Cl, C≡C-C6H4-OC12H25, C≡CC6H2(OC12H25)3]. The compounds are derived from a
highly flexible phenyl ether-based dendritic core, Gi, having the general formula G0 ) C6H3(OC11H22OC6H4-)3,
G1 ) C6H3[OC11H22OC6H3(OC11H22OC6H4-)2]3, G2 ) C6H3[OC11H22OC6H3{OC11H22OC6H3(OC11H22OC6-
H4-)2}2]3), growing from the trivalent phloroglucinol and with undecylene aliphatic spacers between each
branching benzene ring and end-functionalized by isocyanide groups. As in their monomeric model
counterparts, stable liquid-crystalline phases are induced upon complexation of the AuR gold moieties at
the branch termini. The nature of the anionic ligand R promotes the appearance of smectic or columnar
mesophases, the formation of which are governed by steric and dipolar interactions. Based on X-ray
diffraction experiments, models describing the supramolecular organization of these metallodendrimers
into smectic and columnar mesophases are proposed: columnar phases result from the one-dimensional
stacking of molecular disks made of self-assembled supermolecules in oblate cylindrical conformation,
while the smectic phases form by the lateral two-dimensional registry of the supermolecules in antiparallel
head-to-head prolate conformation.
Introduction
considered as intricate and versatile multicomponent building
blocks for self-assembly and, under particular conditions, can
Research in the field of dendrimers, as monodisperse
macromolecules with perfectly controlled cascadelike archi-
tectures, has substantially developed and matured from the
early pioneering works focused mainly on synthetic meth-
odologies and architectural challenges.1 Today, interests in
dendrimers are mostly driven by the molecular design of
novel arborescent functional structures and the subsequent
control of weak interactions to subtly modulate physical and
chemical properties toward the realization of function-specific
nanomaterials and the discovery of novel applications. Due
to specific intrinsic features, such as conformational flex-
ibility, leading to proteanlike molecular forms, multivalency,
and hyperfunctionality, responsible for the so-called dendritic
effect, dendrimers have since been successfully designed and
developed for applications in the fields of medicine, biology,
and biotechnology1–3 and are expected to have a huge impact
in materials science.1,4 These aesthetically appealing mac-
romolecules, when adequately functionalized, can be further
generate various types of periodically ordered networks.4,5
Both dendrimers6 and dendrons (the elementary dendritic
branches)7 are particularly versatile candidates as original
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† Universidad de Valladolid.
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Meijer, E. W. Chem. ReV. 1999, 99, 1665–1688. (c) Vo¨gtle, F.;
Gestermann, S.; Hesse, R.; Schwierz, H.; Windisch, B. Prog. Polym.
Sci. 2000, 25, 987–1041. (d) Grayson, S. M.; Fre´chet, J. M. J. Chem.
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‡ IPCMS, CNRS-Universite´ de Strasbourg.
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and Dendrons: Concepts, Synthesis and Applications; Wiley & Sons:
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10.1021/ja909435e  2010 American Chemical Society

Self-Organization of Isocyanide-Containing Dendrimers
A R T I C L E S
Chart 1. Schematic Structures of Metallodendrimersa
mentalization (alternated or segmented constitutive lobes) of
the dendrimeric structure and toplology. As a matter of fact,
concomitant with judicious molecular designs, metallodendrim-
ers have been found to be particularly efficient materials in
catalysis,10 as molecular devices,11 or as environmental tools
for site isolation and/or recyclability.12
In this study, we report on the design of novel metal-
containing dendrimers endowed with mesomorphic properties.
The overall core architecture of the dendrimers consists of a
highly flexible polyaryl ether-based core, with the trivalent
phloroglucinol as central node, and the branching benzene rings
at each generation are connected by undecylene aliphatic
spacers. The dendrimers are end-functionalized by isocyanide
groups, versatile moieties owing to their outstanding coordina-
tion ability to give very stable complexes with many transition
metals. Mesomorphism is induced in these systems upon
complexion of gold-containing fragments, and the type of
mesophases depends strongly on the molecular shape of the gold
complex connected at the dendritic extremities. Thus, smectic
phases are obtained after bonding of rodlike gold chloride and
gold acetylide fragments, whereas columnar hexagonal phases
are formed upon complexion with tapered shape gold-acetylide
fragments.
a With emphasis on the different locations of metal fragments within
the dendritic frame: at the center (I), at the branching points (II), and at
the periphery (III).
scaffoldings for the elaboration of new liquid-crystalline (LC)
multifunctional materials. Furthermore, as multicomponent
systems, they possess a high density of active groups per
unit volume, and it is anticipated that some of the incorpo-
rated functions can additionally be amplified within the
induced low-dimensional LC mesophases ordering by coop-
erative and synergistic effects.8
While the majority of dendrimers reported so far are purely
organic in nature, there is a rising interest in dendrimers
containing transition metal ions.1,9 These so-called metalloden-
drimers are particularly attractive as the metallic moieties can
be easily integrated into different parts of the dendritic scaffold,
including the nodal core, branching points, and periphery (Chart
1),9 and their specific properties (e.g., redox, electronic, optic,
magnetic, etc.) can be modulated accordingly by collective
effects through the hierarchization (generations) and compart-
Results and Discussion
Design and Synthesis. LC dendrimers have now reached a
high degree of sophistication, and the type and nature of the
mesophases can be predicted and controlled through dedicated
structural engineering at the molecular level by manipulating
the intrinsic dendritic connectivity or by modulating the core
flexibility and the nature and type of peripheral substitution.6,7
Surprisingly, however, very few dendritic metallomesogens13
(i.e., mesomorphic metallodendrimers) have been yet reported.
A simple and well-known synthetic route to prepare metallo-
dendrimers is to use metallic species as nodal linking groups
and coordinate protomesomorphic dendritic fragments that can
act as ligands (Chart 1, structure I). This is the most commonly
applied strategy for obtaining metallodendrimer-containing
LCs.14 In this case, the metallic fragment is shielded by the
dense organic dendritic shell, and mesomorphism is solely
governed by the structure and conformation of the dendritic
branches. Alternatively, functionalization of the dendritic pe-
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A R T I C L E S
Cordovilla et al.
Scheme 1a
a Reagents and conditions: (i) DIAD, PPh3, 1-bromoundecanol, THF, reflux; (ii) K2CO3, DMF, 110 °C; (iii) KOH, THF/H2O (1:1).
riphery with appropriate metal-biding sites represents a versatile
way to modulate the topology of the dendrimer, and thus their
self-assembly abilities, and to access to multivalent ligands
(“metals’ glue”) able to bind various metallic entities (Chart 1,
structure III); only a few such end-functionalized dendritic
metallomesogens have been reported.15,16 As far as we are
aware, and despite their potential topological and functional
diversity, mesomorphic systems with intermediate structures of
type II have not yet been considered.9
For this purpose, isocyanides are versatile functionalities in
organic synthesis and interesting ligands in organometallic
chemistry, owing to their outstanding coordination ability with
many transition metals.17 Isocyanide-metal complexes also
form stable liquid-crystalline materials,18 and on these grounds,
dendritic polyisocyanides can be used as promising polytopic
ligands to construct novel liquid-crystalline metallodendrimers.
Therefore, a new end-functionalized polyisocyanide dendritic
system and several corresponding polynuclear gold complexes
(series I-III), hereafter abbreviated as [Gi(NC)Z] and [Gi(N-
CAuR)Z], where i ) 0, 1, or 2 is the generation number, Z ) 3
× 2i is the number of peripheral groups, and Gi are the highly
flexible poly(phenyl ether) dendrimers of generation 0, 1, and
2, that is, G0 ) C6H3(OC11H22OC6H4-)3, G1 ) C6H3[OC11H22-
OC6H3(OC11H22OC6H4-)2]3, and G2 ) C6H3[OC11H22OC6H3-
{OC11H22OC6H3(OC11H22OC6H4-)2}2]3, have been designed and
synthesized. Three types of gold fragments AuR have been
considered, namely, [AuCl], [AuC≡C-C6H4-OC12H25], and
[AuC≡CC6H2(OC12H25)3], to modulate the structures of the
formed mesophases. Moreover, for a complete evaluation of
the impact of the dendritic effect (i.e., generation) and the
dendrimerization (i.e., monomer f dendrimer) on the meso-
morphic properties, the monomers and several dendritic genera-
tion compounds were synthesized.
Synthesis and Characterization. The synthesis of the dendritic
ligands bearing nitro end groups [Gi(NO2)Z] was carried out
according to the divergent procedure represented in Scheme 1,
starting from the etherification of benzene-1,3,5-triol (phloro-
glucinol) or benzoic acid 3,5-dihydroxyphenyl ester by 1-bro-
moundecanol via the Mitsunobu method.19 The subsequent
Williamson etherification of the bromide derivatives (1, 3) with
4-nitrophenol (2) led directly to [G0(NO2)3] or, after hydrolysis,
to the precursory branch of [G1(NO2)6], 4, respectively. The
(15) (a) Deschenaux, R.; Serrano, E.; Levelut, A.-M. Chem. Commun. 1997,
1577–1578. (b) Dardel, B.; Deschenaux, R.; Even, M.; Serrano, E.
Macromolecules 1999, 32, 5193–5198.
(16) Barbera, J.; Marcos, M.; Omenat, A.; Serrano, J. L.; Martinez, J. I.;
Alonso, P. J. Liq. Cryst. 2000, 27, 255–262.
(17) (a) Malatesta, L.; Bonati, F. Isocyanide Complexes of Metals; Wiley:
Chichester, U.K., 1969. (b) Lentz, D. Angew. Chem., Int. Ed. Engl.
1994, 33, 1315–1331. (c) Tamm, M.; Hahn, F. E. Coord. Chem. ReV.
1999, 182, 175–209.
(18) (a) Espinet, P. Gold Bull. 1999, 32, 127–134. (b) Donnio, B.; Guillon,
D.; Bruce, D. W.; Deschenaux, R. In Metallomesogens; Crabtree,
R. H., Mingos, D. M. P., Eds.; Comprehensive Organometallic
Chemistry III: From Fundamentals to Applications; Elsevier: Oxford,
U.K., 2007; Vol. 12, Chapt. 12.05, pp 196-293.
(19) Mitsunobu, O. Synthesis 1981, 1–28.
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1426 J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010

Self-Organization of Isocyanide-Containing Dendrimers
A R T I C L E S
Scheme 2a
Mesomorphic Properties. The free isocyanide-containing
dendrimers are not liquid-crystalline and were obtained as room-
temperature oils, while induction of mesomorphism was detected
for all the dendritic gold complexes but one, that is, the alkynyl
complex [G0(NCAuC≡C-C6H4-OC12H25)3]. The morphology of
the mesophases is clearly governed by the nature of the end-
group mesogen. Thus, the two chloro-gold dendrocomplexes
(i ) 0 or 1, series I), [G0(NCAuCl)3] and [G1(NCAuCl)6], show
a smectic phase, in agreement with the behavior of the
monomeric model compound (the monomeric compound was
reported to display a smectic A phase).21 As for the three
generations of dendritic gold complexes (i ) 0, 1, or 2, series
III), [Gi{NCAuC≡CC6H2(OC12H25)3}Z], they all display a room-
temperature columnar mesophase; the model monomer,
H25C12OC6H4NCAuC≡CC6H2(OC12H25)3 (vide infra) also ex-
hibits such a two-dimensional (2D) mesophase. The thermal
behavior of the first-generation dendrimer bearing the one-chain
alkynyl derivative, series II [G1(NCAuC≡CC6H4OC12H25)6], is
less obvious, as it decomposed rapidly in the mesophase, thus
precluding unequivocal analysis; the model compound and the
zeroth generation are, however, not mesomorphic and decom-
posed in the crystalline phase, in contrast to the behavior of
some related alkynyl derivatives with alkyl groups.20 Thermal
and thermodynamic data of the metallodendrimers are collected
in Table 1, along with the thermal behavior of the precursor
monomeric gold complexes.
Most of the mesophases could be recognized by optical
microscopy. The smectic C mesophase of the chloro-gold
dendrimers was identified by a fluid and homogeneous texture
and the absence of homeotropic regions or Brownian flashes
under mechanical stress. Moreover, grayish textures could be
observed by mechanical displacement. No birefringent texture
could be observed for [G1(NCAuC≡CC6H4OC12H25)6] in the
explored temperature range (110-130 °C) but only the forma-
tion of large and black (homeotropic texture) areas on increasing
temperature. However, upon mechanical shear, weakly bire-
fringent, transient dendritic patterns with homeotropic contours
were observed under microscope, which disappeared when the
pressure was released. These indications suggest the formation
of the orthogonal smectic A phase, although complete phase
assignment could not be carried out further. The last series of
complexes, [Gi{NCAuC≡CC6H2(OC12H25)3}Z], displayed a fan-
like texture when viewed with a polarizing microscope on
cooling from the isotropic melt down to room temperature,
suggesting the formation of a columnar mesophase. The data
of the monomeric gold complexes [H25C12OC6H4NCAuCl],21
[H25C12OC6H4NCAuC≡CC6H4OC12H25], and [H25C12OC6-
H4NCAuC≡CC6H2(OC12H25)3] are also included here for
comparison.
a Reagents and conditions: (i) hydrazine monohydrate, graphite, EtOH,
reflux; (ii) formic acid, toluene, reflux; (iii) triphosgene[bis(trichlorometh-
yl)carbonate], NEt3, CH2Cl2, reflux.
Scheme 3a
a Reagents and conditions: (i) CH2Cl2 (i ) 0, 1; Z ) 3, 6); (ii) THF, R
) p-C6H4-OC12H25, (i ) 0, 1; Z ) 3, 6); R ) p-C6H2-3,4,5-(OC12H25)3 (i
) 0, 1, 2; Z ) 3, 6, 12).
dendrimer of the first generation, [G1(NO2)6], was immediately
obtained after ether coupling of 3 equiv of 4 with 1. Etherifi-
cation of 4 with 3, followed by hydrolysis and then a subsequent
etherification with 1, yielded the dendrimer of the second
generation, [G2(NO2)12].
Conventional procedures were also used to transform the nitro
groups into the isocyanide functionality (Scheme 2). The
synthesis involved the reduction with hydrazine of the nitro-
containing dendrimers, [Gi(NO2)Z], into the corresponding
anilines, [Gi(NH2)Z], which were further treated with formic acid
to give the formamides, [Gi(NHC(O)H)Z]. Finally, in the last
step, dehydration of the formamides with triphosgene[bi-
s(trichloromethyl)carbonate] yielded the desired isocyanides
[Gi(NC)Z].
Depending on the final metallodendrimers, two methods were
used for the gold complexation. Reaction of the dendrimers
[Gi(NC)Z] with the appropriate amount of [AuCl(tht)] afforded
the chloro-gold complexes [Gi(NCAuCl)Z], (i ) 0, 1) by
displacement of the ligand tetrahydrothiophene (tht) for the
isocyanide groups (Scheme 3). The gold-acetylide complexes
[Gi(NCAuR)Z] were obtained by reaction of the isocyanide
dendrimers with [AuR]n following procedures previously re-
ported for related compounds (Scheme 3).20
In order to evaluate the dendrimerization effects on the
mesomorphic properties, the two monomeric acetylide com-
plexes, not previously described, were synthesized according
to the procedures given in the Experimental Section (see
Supporting Information). The purity of all the complexes was
confirmed by C, H, and N elemental analysis. IR and 1H NMR
spectra of the complexes, very similar to each other due to the
high symmetry of the molecules and due to the fact that the
aromatic rings are almost isolated electronically, confirmed their
structures. The IR spectra showed one ν(C≡N) absorption at
2120 cm-1 for the free isocyanides and at 2227 cm-1 for the
chloro complexes and at 2212 cm-1 for alkynyl derivatives,
confirming complete complexation. The 1H NMR spectra
displayed one singlet at 6.06 ppm for the protons of the 1,3,5-
trialkoxybenzenic central and internal rings and two resonances
for the Hortho and Hmeta of the C6H4NC groups (AA′XX′ spin
system). In the case of the alkynylcomplexes the typical
resonances for the R groups were also observed (see Supporting
Information).
A net decrease of the transition temperatures (melting and
clearing) as well as an overall increase of the mesophase
stability (corresponding to an increase of the mesophase
temperature range) were observed upon dendrimerization of
the mononuclear compounds [H25C12OC6H4NCAuR] [R ) Cl,
C≡CC6H4OC12H25, and C≡CC6H2(OC12H25)3]. In addition,
melting and clearing temperatures were found to decrease
steadily with increasing generations. Note that for the hairy
alkynyl derivatives, [Gi{NCAuC≡CC6H2(OC12H25)3}Z], an im-
portant supercooling effect is observed with the generation. No
phase formation was detected above 25 °C upon cooling at 10
(21) Coco, S.; Espinet, P.; Falaga´n, S.; Mart´ın-Alvarez, J. M. New. J. Chem
1995, 19, 959–964.
(20) Alejos, P.; Coco, S.; Espinet, P. New J. Chem. 1995, 19, 799–805.
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J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010 1427

A R T I C L E S
Cordovilla et al.
Table 1. Thermal Transitions and Corresponding Enthalpy Changes for the Three Series of Compoundsa
compound
transition temperatures, °C (∆H, kJ·mol-1
)
Series I
Cr 123.9 (33.2) SmA 172.0 (8.2) I
H25C12OC6H4NCAuClb
I 171.0 (-15.6) SmA 117.7 (-33.2) Cr
Crc 124.0 (32.4) SmA 171.7 (7.8) I
[G0(NCAuCl)3]
[G1(NCAuCl)6]
Cr 86.4 (9.0) Cr′ 97.2 (11.6) SmC 178.0 (13.9) I
I 172.6 (-26.7) SmC 67.7 (-23.1) Cr
Crc 80.8 (30.9) Cr′ 105.3 (12.2) SmC 172.1 (13.9) I
Cr 35.7 (4.3) SmC 132.1 (45.5) I
I 130.0 (-45.2) SmC
Crc 35.0 (4.2) SmC 132.6 (45.6) I
Series II
H25C12OC6H4NCAuC≡CC6H4OC12H25
[G0(NCAuC≡CC6H4OC12H25)3]
[G1(NCAuC≡CC6H4OC12H25)6]
Cr 47.4 (6.2) Cr′ 58.8 (12.8) Cr′′ 94.1 (9.8) Cr′′′ 150.6 (24.3) dec
Crd 150 (-) dec
Cr 112.2 (121.3) Sm 127.5 (4.2) I
I 123.2 (-5.3) Sm 48.5 (-1.5) Cr
Crc 109.5 (7.2) Sm 121.2 (13.4) I
Series III
H25C12OC6H4NCAuC≡CC6H2(OC12H25)3
[G0{NCAuC≡CC6H4(OC12H25)3}3]
[G1{NCAuC≡CC6H4(OC12H25)3}6]
[G2{NCAuC≡CC6H4(OC12H25)3}12]
Cr 47.2 (80.1) Colh 125.2 (4.7) I
I 120.5 (-3.4) Colh 0 (-26.5) Cr
Crc 5.9 (25.6) Colh 114.5 (4.3) I
Colh 115.9 (13.8) I
I 112.6 (-11.6) Colh
c
Colh 111.6 (13.8) I
Colh 72.7 (13.9) I
I 59.6 (-13.5) Colh
c
Colh 73.3 (15.9) I
Colh 35.6 (13.1) I
a Abbreviations: Cr-Cr′′′, crystalline phases; SmA, smectic A phase; SmC, smectic C phase; Sm, unidentified smectic phase; Colh, columnar
hexagonal phase; I, isotropic liquid; dec, decomposition. b From ref 21. c Second heating. d Measured by thermogravimetric and thermodifferential
analysis (TGA-TDA).
°C/min from isotropic liquid for the second-generation com-
pound. Nevertheless, the same differential scanning calorimetry
(DSC) traces on heating can be achieved if the sample is kept
at room temperature for several hours. This fact suggests that
the I-Col transition is slow, due to the high viscosity of the
compounds, but nevertheless is reversible.
The mesogenic behavior of the one-chain alkynyl derivatives
is clearly limited by the thermal instability of the Au-C(alkynyl)
bond. Related mixed isonitrile-acetylyde complexes, with alkyl
groups instead of alkoxy ones, consistently exhibited a SmA
phase between 100 and 200 °C and also showed a strong
tendency to decompose in the mesophase.20 It can be speculated
thus that the absence of mesomorphic properties in
[G0(NCAuC≡CC6H4OC12H25)3] and in the monomeric com-
pound is due to decomposition starting before melting, since
the next generation exhibits a smectic phase.
Temperature-dependent, small-angle X-ray diffraction experi-
ments were systematically carried out in order to identify
unequivocally the nature of the mesophase. Their structural data
and the mesophases’ characteristics are collected in Table 2.
All the mesophases can be considered as disordered, as they
show only one broad and diffuse signal in the wide-angle region
of the X-ray diffraction (XRD) patterns, centered at around
4.5-4.6 Å, distance reflecting the liquidlike state of the molten
alkyl chains. In the low-angle region, sharp signals reflecting
the 1D lamellar structures [Gi(NCAuCl)Z] or the 2D arrangement
of the columns [Gi{NCAuC≡CC6H4(OC12H25)3}Z] series were
detected.
periodicity measured for the SmA phase of the monomer is
about twice its estimated molecular length in agreement with a
head-to-head disposition in dimers stabilized by strong dipolar
interactions associated with the Au-Cl bond, as already
proposed.18a The lamellar periodicity of the phase of dendrimer
G0 has the same magnitude, suggesting that the organization is
driven by such an antiparallel disposition of the mesogenic end
groups. To have a more precise description of the molecular
arrangement in the smectic layers, a quantitative discussion
based on the measured periodicities and calculated partial
molecular volumes is now proposed. Determination of the
molecular cross-section area is usually obtained from the ratio
of the molecular volume and the layer periodicity. Given the
proposed alternated arrangement of the polar mesogenic units,
it is therefore necessary to consider two molecules to calculate
the area occupied by the mesogenic units at the interface
between the smectic layers, that is, Amol ) 2Vmol/d; a density of
1.222 was considered for all these gold-enriched dendrimers.
The Amol value found for the model compound (ca. 37 Å2, Table
2) is compatible with a SmA phase. In this case, either the
sublayers containing the supramolecular dimers are partially
interdigitated, as in the case of the SmAd phase,23 or the
mesogenic groups are randomly tilted (without any correlation
in the tilt direction, as in the SmA phase of the de Vries type)
within the layer to produce an average “zero” tilt. In such a
phase, the molecules are assumed to be tilted as in the SmC
phase resulting from either tilted layers being stacked in a
The chloro-gold dendrimers (series I) showed up to three
sharp and intense small-angle diffraction peaks in the 1:2:3 ratio
that were indexed as (00l) ) (001), (002), (003) of a lamellar
structure. In addition to their optical texture, the smectic phase
can therefore be safely assigned as a SmC phase. The lamellar
(22) (a) Lee, K. M.; Lee, C. K.; Lin, I. J. B. Angew. Chem., Int. Ed. Engl.
1997, 36, 1850–1852. (b) Barbera, J.; Elduque, A.; Gimenez, R.;
Lahoz, F. J.; Lopez, J. A.; Oro, L. A.; Serrano, J. L. Inorg. Chem.
1998, 37, 2960–2967. (c) Benouazzane, M.; Coco, S.; Espinet, P.;
Marty´n-Alvarez, J. M.; Barbera, J. J. Mater. Chem. 2002, 12, 691–
696.
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Self-Organization of Isocyanide-Containing Dendrimers
A R T I C L E S
Table 2. Mesophases Structural Parameters Extracted from X-ray Diffraction Experiments
indexationa
compound
dmeas, Å
00l, hk
dcalc, Å
parametersb
Series I
H25C12OC6H4NCAuCl, SmA
39.0 (VS, sh)
19.45 (S, sh)
4.6 (VS, br)
001
002
38.95
19.47
hch
T ) 140 °C
d ) 38.95 Å
Vmol ) 720 Å3
Amol ) 36.9 Å2
T ) 135 °C
[G0(NCAuCl)3], SmC
[G1(NCAuCl)6], SmC
37.95 (VS, sh)
18.88 (S, sh)
12.47 (M, sh)
4.6 (VS, br)
001
002
003
37.71
18.85
12.57
hch
d ) 37.71 Å
Vmol ) 2260 Å3
Amol ) 120.0 Å2
Amu ) 40.0 Å2
T ) 115 °C
58.30 (VS, sh)
29.29 (S, sh)
19.18 (M, sh)
4.6 (VS, br)
001
002
003
58.14
29.07
19.38
hch
d ) 58.14 Å
Vmol ) 5510 Å3
Amol ) 189.5 Å2
Amu ) 31.6 Å2
Series III
30.21 (VS, sh)
17.46 (S, sh)
15.11 (S, sh)
11.4 (M, sh)
10.1 (W, sh)
8.77 (VW, sh)
8.38 (VW, sh)
4.5 (S, br)
30.09 (VS, sh)
17.31 (S, sh)
15.14 (S, sh)
11.39 (W, sh)
4.6 (S, br)
30.32 (VS, sh)
17.63 (S, sh)
15.25 (S, sh)
11.48 (W, sh)
4.7 (VS, br)
31.8 (VS, sh)
18.41 (M, sh)
15.92 (M, sh)
4.7 (S, br)
H25C12OC6H4NCAuC≡CC6H2(OC12H25)3, Colh
10
11
20
21
30
22
31
30.24
17.46
15.12
11.43
10.08
8.73
T ) 60 °C
a ) 34.92 Å
S ) 1056 Å2
Vmol ) 1575 Å3
〈N〉 ) 3.0
8.39
hch
[G0{NCAuC≡CC6H4(OC12H25)3}3], Colh
[G1{NCAuC≡CC6H4(OC12H25)3}6], Colh
[G2{NCAuC≡CC6H4(OC12H25)3}12], Colh
10
11
20
21
30.11
17.38
15.06
11.38
hch
30.43
17.57
15.22
11.50
hch
T ) 75 °C
a ) 35 Å
S ) 1047 Å2
Vmol ) 4835 Å3
〈N〉 ) 1.0
10
11
20
21
T ) 50 °C
a ) 35.1 Å
S ) 1069 Å2
Vmol ) 10 650 Å3
〈N〉 ) 0.5
10
11
20
31.84
18.38
15.92
hch
T ) 20 °C
a ) 35 Å
S ) 1171 Å2
Vmol ) 22 280 Å3
〈N〉 ) 0.25
a dmeas and dcalc are the measured and calculated diffraction spacings, and distances are given in angstroms. VS (very strong), S (strong), M (medium),
W (weak), and VW (very weak) stand for the peak intensity, while br (broad) and sh (sharp) denote peak line shape. 00l and hk are the Miller indices
for the reflections (lamellar and columnar systems, respectively). b Molecular volume Vmol ) MW/(NAF); where NA is Avogadro’s number and density F
) 1.2 g ·mL-1. For smectic phases: d is the smectic periodicity, d ) 1/Nhk(Σld00l), where Nhk is the number of reflections; molecular cross-section area
Amol) 2Vmol/d; cross-section area per mesogenic unit Amu) Amol/Z. For columnar hexagonal phases: a is the columnar phase lattice parameter, a )
Σdhkꢀ(h2 + k2 + hk)]/ꢀ3Nhk, where Nhk is number of reflections; columnar cross-section area S ) a2ꢀ3/2;·NVmol ) Sh, where N is the number of
molecules (or molecular equivalent) per columnar elementary repeat unit of height h (h ) hch) 4.5-4.7 Å).
random fashion24 or the molecules being tilted without long-
range order in the direction of tilt within the layer.24,25 For the
supermolecules, if it is reasonably assumed, as in other related
systems, that they adopt a cylindrical conformation in the
smectic phase,26 resulting from the antiparallel pairing of
neighbored dendrimers, then these calculation performed for the
G0 and G1 dendrimers lead to molecular and mesogenic unit
cross-section values compatible with a SmC phase (Amu ) Amol/3
) 40 Å2 for G0 and Amu ) Amol/6 ) 31.6 Å2 for G1, Table 2).
The decrease of these values concomitant with the increasing
lamellar periodicity between G0 and G1 is due to the increasing
size of the dendritic core but also to a reduction of the average
tilt of the mesogens, consequent to a tighter packing of both
species (mesogens and dendrimer). These results validate the
above hypothesis: the dendrimers, having three sets of arms,
adopt an elongated prolate conformation and associate into
supramolecular cylinders required for the formation of the
lamellar mesophases, where the rigid parts are quasi-collinear
to the layer normal and distributed homogeneously on either
side of the molecular center (Figure 1). To achieve such a
homogeneous distribution, the dendrimers are arranged in an
alternating antiparallel manner within the layers in order to
compensate the uneven number of mesogens (G0) or pairs of
mesogens (G1). A representation of this arrangement into layers
is shown below (Figure 1).
(23) (a) Seurin, P.; Guillon, D.; Skoulios, A. Mol. Cryst. Liq. Cryst. 1981,
71, 37–49. (b) Guillon, D.; Skoulios, A. Mol. Cryst. Liq. Cryst. 1983,
91, 341–352. (c) Hardouin, F.; Levelut, A. M.; Achard, M. F.; Sigaud,
G. J. Chim. Phys. 1983, 80, 53–64. (d) Hardouin, F. Physica A 1986,
140, 359–367.
(24) De Vries, A. Mol. Cryst. Liq. Cryst. 1977, 41, 27–31.
(25) De Vries, A. J. Chem. Phys. 1979, 71, 25–31.
(26) (a) Donnio, B.; Barbera´, J.; Gime´nez, R.; Guillon, D.; Marcos, M.;
Serrano, J.-L. Macromolecules 2002, 35, 370–381. (b) Lenoble, J.;
Campidelli, S.; Maringa, N.; Donnio, B.; Guillon, D.; Yevlampieva,
N.; Deschenaux, R. J. Am. Chem. Soc. 2007, 129, 9941–9952. (c)
Wolska, J.; Mieczkowski, J.; Pociecha, D.; Buathong, S.; Donnio, B.;
Guillon, D.; Gorecka, E. Macromolecules 2009, 42, 6375–6384.
As for the acetylene dendrocomplexes of series III, up to
four sharp reflections (and seven for the monomer, Figure 2)
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J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010 1429

A R T I C L E S
Cordovilla et al.
dendritic systems28 and is explained by the great flexibility of
the dendritic arborescence, which can easily be deformed
because of the steric constraints and confined due to unidirec-
tional stretching along the columnar axis. If a density22 of
1.1-1.2 g ·cm-3 is assumed and a thickness for the repeating
columnar slice along the column h ∼ 4.5-4.7 Å is chosen,
corresponding to the average separation between molten alkyl
chains and thus to the short-range stacking distance between
consecutive layers of aryl ether dendrons,7,14-16,26 then the
average number of dendrimers (or dendritic equivalents), N, that
associate per such an elementary columnar unit can be estimated
according to N ) Sh/Vmol (Vmol ) molecular volume). For this
series, it was found that 1 G0, 1/2 G1, and 1/4 G2 supermolecule
is required to fulfill the available volume of such a defined
columnar slice. To do so, the overall supermolecules ought to
adopt an oblate conformation (flat cylinder) resulting from the
uniaxial stretching of the dendritic skeleton over several repeat
units (two for G1 and four for G2) and from the radial
arrangement of the end mesogens (3 per slice for G0 f G2). As
for the monomeric model compound, three complexes, also in
such a radial arrangement, are needed to fulfill the geometrical
and volume requirements to stabilize the columnar mesophase.
In all cases, the perfect paving of the 2D lattice is achieved.
Thus, the total number of peripheral alkyl chains radiating from
the columnar interface per repeat unit remains constant through-
out (ca. nine chains), independent of the generation number.
Despite the huge increase of the molecular volume with
generation, the surface area of the hexagonal cell for all of
these dendrimers and the monomer is quite similar regardless
of the generation number, suggesting that the packing is
governed by the tapered molecular shape7 of the sole
mesogenic unit. A plausible model to explain the self-
assembling into columns is thus to consider that one, two,
and four of these elemental discs are necessary to accom-
modate the metallodendrimer of the zeroth, first, and second
generation, respectively, as schematically shown in the
graphical representation below (Figure 3).
Figure 1. Schematic representation of the self-organization of the
chloro-gold mesogens and dendrocomplexes in smectic phases: (a)
monomer, (b) G0, and (c) G1.
Thus, many properties of the materials described here are
consistent with those observed previously for other dendrim-
eric mesogens.6 The change in the shape of the end groups,
monitored by the number of terminal chains per mesogen,
modifies the relationships between the hard and the soft parts,
and consequently the molecules adopt either a cylindrical
(prolate, series I) or a disklike (oblate, series III) conforma-
tion, and smectic and/or columnar mesophases are induced,
respectively. Compounds with no terminal chain per end
group (series I), for which the compatibility of spatial
requirement between the aliphatic dendritic matrix and
mesogenic cores is immediately satisfied, self-organize into
noncurved smectic layers resulting from the lateral two-
dimensional registry of cylindrical antiparallel molecular
pairs. In contrast, the grafting of hemipolycatenar mesogens29
at the extremity of the dendrimer prevents such a parallel
disposition of the promesogenic groups, since the cross-
sectional area of the terminal chains is larger than the area
occupied by the mesogenic units (the mismatch between the
surface areas of the aromatic cores and the cross-section of
the aliphatic chains results in the curvature of the interface);
therefore, the metallodendrimers adopt a cylindrical confor-
Figure 2. X-ray patterns of the monomeric and dendritic generations in
the Colh phase at 60, 75, 50, and 20 °C for H25C12OC6H4-
NCAuC≡CC6H2(OC12H25)3 and [Gi{NCAuC≡CC6H4(OC12H25)3}Z], respec-
tively.
were observed, with reciprocal d-spacings in the ratio 1:ꢀ3:
ꢀ4:ꢀ7 (ꢀ9:ꢀ12:ꢀ13), and most readily assigned to the (10),
(11), (20) and (21) [(30), (22), and (31)] of a 2D hexagonal
lattice with p6mm planar group (Colh phase).27 One can notice
a substantial widening and intensity decrease of the small-angle
diffraction peaks upon generation increase, which is associated
with less sharp interfaces and an increasing difficulty toward
ordering, a common behavior for liquid-crystalline macromol-
ecules. A remarkable feature within this series of compounds
is the quasi-invariance of the lattice parameter starting from the
monomer and up to the second generation (Figure 2, Table 2),
suggesting a similar mode of packing within the column for all
the members of the series. Such a generation-independent
behavior has previously been observed in other mesomorphic
(27) (a) International Tables for Crystallography, Vol. A, 4th ed.; Hahn,
T., Ed.; International Union of Crystallography; Kluwer Academic:
Dordrecht, The Netherlands, 1995. (b) Hammond, C. In The Basics
of Crystallography and Diffraction, 2nd ed.; International Union of
Crystallography; Oxford Science Publications: Oxford, U.K., 2001.
(28) Marcos, M.; Gime´nez, R.; Serrano, J. L.; Donnio, B.; Heinrich, B.;
Guillon, D. Chem.sEur. J. 2001, 7, 1006–1013.
(29) Nguyen, H.-T.; Destrade, C.; Maltheˆte, J. AdV. Mater. 1997, 9, 375–
388.
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1430 J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010

Self-Organization of Isocyanide-Containing Dendrimers
A R T I C L E S
Figure 3. Schematic model representing the supramolecular arrangement in columnar hexagonal mesophase of monomeric compound (a) and dendrimers
[Gi{NCAuC≡CC6H2(OC12H25)3}Z] as a function of the generation, G0 (b) f G1 (c) f G2 (d).
mation, allowed by virtue of their great flexibility, and the
end mesogens are radially arranged around the dendritic
moiety; the columnar phase results from the one-dimensional
periodic stacking of these supramolecular discs made of
molecules or portions of molecules.
columnar mesophase, directly from room temperature, as does
the model monomer.
Acknowledgment. The work was supported by ESF/2007/03
grant. We thank the CNRS and the Universite´ de Strasbourg for
support and funding. P.E. and S.C. thank the DGI (Project
CTQ2008-03954/BQU) and INTECAT Consolider Ingenio-2010
(CSD2006-0003) and the Junta de Castilla y Leo´n (Projects
VA012A08 and GR169).
Conclusions
New liquid-crystalline metallodendrimers have been syn-
thesized. Their mesomorphic properties are largely influenced
by the nature of the peripheral gold fragments, while the
increase of the dendritic generation contributes to a strong
stabilization of the mesophases. The chloro-gold complexes
(series I), [G0(NCAuCl)3] and [G1(NCAuCl)6], show a
smectic C phase, in agreement with the behavior of the
monomeric model compound. As for the three generations
of dendritic gold complexes bearing hemipolycatenars (series
III), [Gi{NCAuC≡CC6H2(OC12H25)3}Z], they all display a
Note Added after ASAP Publication. In the version published
ASAP on January 7, 2010, Scheme 3 and the paragraph describing
it contained errors; the corrected version reposted on January 11,
2010.
Supporting Information Available: Complete experimental
section and synthesis and characterization of the precursors and
final dendrimers. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA909435E
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J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010 1431