Dendronized triangular oligo(phenylene ethynylene) amphiphiles: Nanoflbrillar self-assembly and dye encapsulation

Article abstract of DOI:10.1002/chem.200902894

Triangular-shaped oligo(phenylene ethynylene) amphiphiles 1a and 1 b decorated in their periphery with two- and four-branched hydrophilic triethyleneglycol dendron wedges, have been synthesized and their self-assembling properties in solution and onto surfaces investigated. The steric demand produced by the dendritic substituents induces a face-to-face rotated φ stacking of the aromatic moieties. Studies on the concentration and temperature dependence confirm this mechanism and provide binding constants of 1.2 xl0⁵ and 1.7 x10⁵ M¹ in acetonitrile for 1a and 1b, respectively. Dynamic and static light scattering measurements complement the study of the self-assembly in solution and demonstrate the formation of rod-like supramolecular structures in aqueous solution. The nanofibers formed in solution can be efficiently transferred onto surfaces. Thus, TEM images reveal the presence of strands of various thickness, with the most common being several micrometers long and with diameters of around 70 nm. Some of these nanofibers present folded edges that are indicative of their ribbon-like nature. Interestingly, compound 1b can also form thick filaments with a rope-like appearance, which points to a chiral arrangement of the fibers. AFM images under highly diluted conditions also reveal long fibers with height profiles that fit well with the molecular dimensions calculated for both amphiphiles. Finally, we have demonstrated the intercalation of the hydrophobic dye Disperse Orange 3 within the filaments and its subsequent release upon increasing the temperature.

Full text of DOI:10.1002/chem.200902894

DOI: 10.1002/chem.200902894
Dendronized Triangular Oligo(phenylene ethynylene) Amphiphiles:
Nanofibrillar Self-Assembly and Dye ACTHNUGRTENUNGEncapsulation
Fꢀtima Garcꢁa and Luis Sꢀnchez*[a]
Abstract: Triangular-shaped oligo(phe-
nylene ethynylene) amphiphiles 1a and
1b decorated in their periphery with
two- and four-branched hydrophilic
triethyleneglycol dendron wedges, have
been synthesized and their self-assem-
bling properties in solution and onto
surfaces investigated. The steric
demand produced by the dendritic sub-
stituents induces a face-to-face rotated
p stacking of the aromatic moieties.
Studies on the concentration and tem-
perature dependence confirm this
mechanism and provide binding con-
stants of 1.2ꢀ105 and 1.7ꢀ105 mꢀ1 in
acetonitrile for 1a and 1b, respectively.
Dynamic and static light scattering
measurements complement the study
of the self-assembly in solution and
demonstrate the formation of rod-like
supramolecular structures in aqueous
solution. The nanofibers formed in so-
lution can be efficiently transferred
onto surfaces. Thus, TEM images
reveal the presence of strands of vari-
ous thickness, with the most common
being several micrometers long and
with diameters of around 70 nm. Some
of these nanofibers present folded
edges that are indicative of their
ribbon-like nature. Interestingly, com-
pound 1b can also form thick filaments
with a rope-like appearance, which
points to a chiral arrangement of the
fibers. AFM images under highly dilut-
ed conditions also reveal long fibers
with height profiles that fit well with
the molecular dimensions calculated
for both amphiphiles. Finally, we have
demonstrated the intercalation of the
hydrophobic dye Disperse Orange 3
within the filaments and its subsequent
release upon increasing the tempera-
ture.
Keywords: amphiphiles · dendrim-
ers · drug delivery · nanostructures ·
self-assembly
Introduction
molecular architectures with different morphologies and di-
mensionalities.[4] The relative ratio of the hydrophilic/
-phobic groups in the p-conjugated amphiphilic systems, to-
gether with the influence of external factors (concentration,
temperature, and/or solvent polarity), finely tunes the deli-
cate balance between the noncovalent forces that finally de-
termines the “soft-matter polymorphism”[5] and, consequent-
ly, the change in the shape, size, physico-chemical properties,
and applications of the resulting supramolecular struc-
tures.[3,4]
Knowledge of a set of rules governing the self-assembly of
organic molecules and, in consequence, the properties and
function stemming from the self-assembly process, is crucial
in the quest for new organic materials that are useful for
semiconductor-based technologies.[1] p-Conjugated systems
are especially relevant for this purpose because the nonco-
valent forces, and more specifically p–p stacking aromatic
interactions[2] between their p-conjugated frameworks,
allows modulation of the optical and electronic properties of
the final materials.[3] The decoration of p-conjugated systems
with polar groups gives rise to monodisperse amphiphilic
molecules capable of precise self-organization into supra-
Calixarenes,[6] pyridine-based derivatives,[7] squaraines,[8]
perylenebisimides,[9] hexabenzocoronenes,[10] oligo-p-phenyl-
enes,[11] oligo(phenylene vinylenes),[12] and oligo(phenylene
ethynylenes) (OPE)[13] exemplify the variety of p-conjugated
moieties that have been used as hydrophobic cores in am-
phiphilic systems. Amphiphiles based on the organic com-
pounds mentioned above are able to self-organize into a va-
riety of well-ordered noncovalent architectures such as mi-
celles,[6,11d] fibers,[8b,9,11a,b] rolled lamellae,[10a,b,11a] and vesi-
cles.[8b,9b,12c,13a,b] Among these morphologies, 1D nanostruc-
tures[14] are being fully investigated due to their potential
application as nanowires and biomimetic macromolecules.[15]
In addition, amphiphiles that are able to form 1D fibers are
[a] F. Garcꢁa, Dr. L. Sꢂnchez
Departamento de Quꢁmica Orgꢂnica,
Facultad de Ciencias Quꢁmicas,
Ciudad Universitaria
s/n. 28040 Madrid (Spain)
Fax : (+34)913944103
E-mail: lusamar@quim.ucm.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902894.
3138
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FULL PAPER
also relevant due to that fact that: 1) their hierarchical as-
sembly can induce a gelation process to form gels with ap-
plicability in food, cosmetics, or advanced materials,[16–18]
and 2) the incorporation of bioinspired moieties results in
chiral assemblies that are structurally comparable to those
of natural peptide systems.[19]
In our research group, we have recently reported the
preparation of radial amphiphilic OPEs as building blocks
for the construction of supra-
Results and Discussion
Synthesis: Dendronized amphiphiles 1a and 1b were ob-
tained in 36 and 32% yield, respectively, by utilizing the So-
nogashira cross-coupling reaction[23] between 1,3,5-triethy-
nylbenzene (3)[24] and the corresponding iodoaryl derivatives
endowed with two (5a) or four (5b) dendritic TEG wedges
(Scheme 1). Unlike for compound 2, which is endowed with
molecular structures such as
vesicles,[20] toroids,[21] and mi-
crocrystalline lamellae.[22] The
different molecular geometries
of these radial OPEs (triangular
or rectangular), together with
the variable number of polar or
paraffinic chains, makes them
useful benchmarks for the study
and quantification of the non-
covalent forces participating in
the self-assembly of small or-
ganic amphiphiles and also ena-
bles an evaluation of their ca-
pacity to transfer onto surfaces
as
ordered
supramolecular
structures.
Herein, we report the synthe-
sis of dendronized, triangular
Scheme 1. Synthesis of the dendronized amphiphilic OPEs 1a and 1b.
OPEs (1a and 1b) and a study
of their self-assembling features
in solution and onto surfaces.
We compare the synthesized
OPEs with those of our previously reported triangular am-
phiphilic equivalent, which was endowed with only three
triethyleneglycol (TEG) chains (2).[20a] The attachment of
TEG polar dendritic wedges in the periphery of the radial
OPE skeleton induces a rotated stacking of the amphiphiles
that alleviates the steric repulsion of the bulky substituents.
The twisted packing of the aromatic moieties provokes a 1D
growth that finally generates nanofibers with lengths of sev-
eral micrometers. We have also investigated the capability
of these supramolecular fibers to intercalate and release hy-
drophobic dye molecules.
only three TEG chains,[20a] we used tris(dibenzylideneace-
tone)dipalladium(0) [Pd2A(dba)3] as a catalyst in the Sonoga-
G
CHTUNGTRENNUNG
shira reaction to overcome the steric demand exerted by the
dendritic TEG substituents, which could significantly de-
crease the yield of the reaction, and avoid the Glaser-type
homocoupling reaction.[25] It is well established that the
presence of the dibenzylideneacetone ligand controls the
concentration and the reactivity of the active Pd0 species
within the catalytic cycle and results in higher yields.[25] The
peripheral polyether dendrons 4a and 4b, and compounds
5a and 5b, which allow the modulation of the ratio of hy-
drophilic to hydrophobic groups, were prepared by following
previously reported procedures.[26] All of the compounds re-
ported herein were fully characterized through a wide varie-
ty of spectroscopic techniques (see the Supporting Informa-
tion).
Unambiguous confirmation of the proposed structure for
both amphiphiles 1a and 1b was obtained from the
1H NMR spectra. Thus, whereas 1a shows only one isochro-
nous quintuplet at dꢁ2.5 ppm, which is ascribable to the
ꢀ
ꢀ
CH group supporting the two-branched dendron wedge
(filled circle in Scheme 1), compound 1b shows two quintu-
plets at dꢁ2.2 and 2.1 ppm (filled and empty circles in
Scheme 1) corresponding to three and six protons, respec-
tively. At the same time, the presence of six aromatic reso-
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3139

L. Sꢂnchez and F. Garcꢁa
nances and two signals diagnostic of the triple bonds in the
13C NMR spectra, confirm the C3 symmetry of the investi-
gated amphiphiles.
of amphiphile 2, which is endowed with only three peripher-
al TEG chains.[20a] This difference was the first indication of
the face-to-face rotated p stacking of the aromatic frame-
work of amphiphiles 1a and 1b, which results in a lower Ka
value. In addition, the Ka value determined for 1b was
higher than that calculated for 1a, despite the higher
volume of the four-branched peripheral dendrons. The
direct correlation between a higher hydrophobicity—and
consequently a higher tendency to form supramolecular
structures in polar solvents—and the generation of dendrim-
ers explain this phenomenon,[28] which was also previously
observed for related amphiphilic structures endowed with
dendritic substituents.[29] The relevance of the solvophobic
component in the self-assembly of these amphiphiles has
been quantified by using apolar benzene as a solvent in con-
centration-dependent UV/Vis spectroscopy experiments. In
this solvent, the coiling effect experienced by the dendritic
TEG chains, together with a better solvation of the aromatic
fragments, results in weaker p–p interactions and, ultimate-
ly, in lower values for Ka (4.7ꢀ104 and 6.5ꢀ104 mꢀ1 for 1a
and 1b, respectively; see Figures S3 and S4 in the Support-
ing Information).
The MALDI-TOF mass spectrum of amphiphile 1a shows
a weak set of peaks corresponding to the isotopic distribu-
tion at m/z 1566, together with a highly intense set of peaks
at m/z 1589 and a further weaker set at m/z 1605, which
have been assigned to the [M+Na]+ and [M+K]+ molecular
ions (Figure 1). The mass spectra for compound 1b exhibits
only one set of peaks at m/z 2994 and a second at m/z 3010,
which correspond to the [1b+Na]+ and [1b+K]+ molecular
ions (Figure S1 in the Supporting Information). This behav-
ior has also been observed for other, related amphiphilic
dendritic block molecules.[26b] Nevertheless, the MALDI-
TOF spectra of compounds 1a and 1b also provides a first
indication of their ability to self-assemble, and peaks corre-
sponding to the dimer and trimer are noticeable for both
amphiphiles (Figure S2 in the Supporting Information).
Attempts to determine the Ka values with water as the
solvent demonstrated the extremely high value for this ther-
modynamic parameter under these conditions; spectroscopic
features corresponding to the monomeric species were not
observed until concentrations were as low as around 10ꢀ7 m.
To further investigate the self-assembly mechanism, temper-
ature-dependent UV/Vis spectroscopy experiments in aque-
ous solutions were conducted (Figure 2 and Figure S5 of the
Supporting Information). In aqueous solution, less energetic
wavelength absorption appears as a shoulder, which is in
contrast to the profile observed in less polar acetonitrile and
benzene where this band is the most intense (Figure 2 and
Figure 1. MALDI-TOF (ditranol) mass spectrum of amphiphile 1a show-
ing the molecular ion peaks corresponding to [M]+, [M+Na]+, and
[M+K]+ species.
Self-assembly in solution: The self-assembly of amphiphiles
1a and 1b was initially investigated by UV/Vis spectroscopy
experiments that looked at the concentration dependence in
two solvents of distinct polarity (acetonitrile and benzene)
and by considering the isodesmic model[27] as the mechanism
by which these compounds aggregate (Figures S3 and S4 in
the Supporting Information). Both amphiphiles 1a and 1b
show identical spectroscopic features in which the depletion
of the high energy band at approximately 260 nm is accom-
panied by an increase of two lower energy bands at approxi-
mately 300 and 314 nm; this observation is similar to those
reported for triangular OPE 2 and is diagnostic of p stack-
ing.[20] The calculated values for the binding constant (Ka) in
acetonitrile are 1.2ꢀ105 and 1.7ꢀ105 mꢀ1 for 1a and 1b, re-
spectively. These values are approximately half the Ka value
Figure 2. Temperature-dependent UV/Vis absorption spectra of 1b (H2O,
1ꢀ10ꢀ4 m, 278–348 K). Arrows indicate the spectroscopic changes with in-
creasing temperature. The isosbestic point occurs at 296 nm (a). Inset b)
depicts the nonlinear variation of the absorbance as a function of temper-
*
^
ature at 305 ( ) and 259 nm ( ).
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Nanofibrillar Self-Assembly
FULL PAPER
Figures S3 and S4 of the Supporting Information). In these
experiments, the decrease of the absorption maxima at
305 nm and also the shoulder at 319 nm mentioned above
occurs in concert with an increasing absorbance at 259 nm,
which can be ascribed to the formation of the monomeric
form. The isosbestic points observed at 281 and 296 nm thus
stem from amphiphiles 1a and 1b, respectively (Figure 2
and Figure S5 of the Supporting Information).
The nonlinear behavior of the spectroscopic changes with
temperature (insets in Figure 2 and Figure S5 of the Sup-
porting Information) cannot be used to calculate the mole
fraction of aggregates at each temperature [aagg(T)], because
most of the sample is aggregated under these conditions;[9a]
determination of aagg(T) would require recording UV/Vis
spectra at higher temperatures. However, the dendritic pe-
ripheral wedges do play a relevant role in these tempera-
ture-dependence experiments. Thus, increasing temperatures
resulted in turbid dispersions due to the lower critical solu-
tion temperature (LCST) transition in aqueous media.
Above this LCST, the TEG chains are dehydrated to yield
new supramolecular ensembles that are less soluble in the
aqueous solution.[30] The LCST for amphiphiles 1a and 1b
have been determined to be 318 and 338 K, respectively, by
plotting the absorbance at 600 nm versus temperature (Fig-
ure S6 of the Supporting Information).
tion was approximated to a simpler dimerization mecha-
nism, the data obtained by the temperature-dependent
1H NMR spectroscopy experiments allowed the DH and DS
parameters that control the thermodynamics of the assembly
to be determined. To calculate these thermodynamic values,
we have utilized Equation (1), which was reported by Lee
et al.:[33]
ꢄ
ꢂ
ꢀ ꢁ
ꢃ
ꢅ
DH
R
1
T
DS
R
ð1Þ
dobs ¼ ðdA ꢀ dBÞ exp ꢀ
þ
þ 1 ꢀ1þdB
in which dA and dB are the chemical shift values at lower
and higher temperatures, respectively. The nonlinear fit of
the observed chemical shift changes versus temperature for
amphiphile 1b, results in DH=(ꢀ30.3ꢂ1.5) kJmolꢀ1 and
DS=(ꢀ91.9ꢂ4.8) Jmolꢀ1 (inset in Figure 3). The calculated
DH and DS values indicate that the self-assembly process is
enthalpically driven and entropically disfavored and, hence,
lowering temperatures enhances the self-association proc-
ess.[20b,34]
The aggregation of 1a and 1b in aqueous solution
(ꢁ10ꢀ4 m) has also been investigated by dynamic (DLS) and
static (SLS) light-scattering measurements. CONTIN analy-
sis of the correlation functions obtained in the former shows
a major contribution centered at hydrodynamic radii (RH) of
approximately 130 and 90 nm for amphiphiles 1a and 1b, re-
spectively (Figure 4a), which is in good agreement with pre-
viously reported examples of organic amphiphiles that self-
assemble into cylindrical micelles.[29] By plotting the inverse
of time versus the square of the scattering vector (Fig-
ure S10 in the Supporting Information), it is possible to ac-
curately determine the diffusion coefficient (D) for the
supramolecular structures formed under these experimental
conditions. Such measurements gave D values of 1.870ꢀ10ꢀ8
The aggregation behavior in solution has also been stud-
ied by temperature-dependent 1H NMR spectroscopy ex-
periments in different deuterated solvents. In deuterated
acetonitrile, there is a fast exchange between the monomeric
and aggregated species and relatively sharp resonances
appear that slightly shield upon increasing temperature (Fig-
ures S7 and S8 of the Supporting Information).[31,32] Plotting
the variation of the chemical shift against temperature
shows a linear relationship for both para aromatic protons
*
^
*
(
and ) and the aliphatic ( ) resonances (insets in Fig-
ure S7 and S8 of the Supporting
Information). The 1H NMR
spectra for 1a and 1b in D2O
are significantly broader and
the upfield shift of most reso-
nances are more pronounced
than those observed in CD3CN
due to the strong propensity of
OPEs 1a and 1b to stack in
aqueous solution and also due
to the LCST effect (Figure 3
and Figure S9 of the Supporting
Information).
As stated before, the high Ka
values for 1a and 1b in aque-
ous solution impede an accu-
rate determination of the ther-
modynamic parameters associ-
ated with the self-assembly pro-
cess by a vanꢄt Hoff analysis
under these conditions. Howev-
Figure 3. Partial 1H NMR spectra (D2O, 300 MHz, 1 mm) of 1b at different temperatures. The inset shows
er, when the isodesmic aggrega- plots of dobs versus T and the fit of the data to Equation (1).
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L. Sꢂnchez and F. Garcꢁa
PðqÞ ¼ P0=½1ꢀx2=9þ2x4=225ꢃ, x ¼ ql=2
ð3Þ
in which RG is the radius of gyration and l is the length of
the rod. The random coil model [Eq. (2)] failed in fitting the
scattering data. However, fitting these scattering data to the
form factor for a rod model [Eq. (3)] provided correlation
factors (R2) of 0.999 and 0.992 for 1a and 1b, respectively,
which is diagnostic of the presence of supramolecular struc-
tures with a rod shape. In addition, the application of Equa-
tion (2) allows the calculation of length values of 907 and
718 nm for the rods formed from 1a and 1b, respectively.
The combination of the concentration-dependent UV/Vis
and light-scattering experiments unambiguously demon-
strate the isodesmic mechanism followed by these amphi-
philes to self-assemble.[27d]
Self-assembly onto surfaces: The studies performed in solu-
tion demonstrate the presence of elongated aggregates that
could be effectively transferred onto a substrate. The rotat-
ed stacking of the aromatic moieties of amphiphiles 1a and
1b frustrates the three-dimensional growth of the supra-
molecular structures into vesicles observed for 2[20a] and, in
consequence, induces the formation of 1D aggregates. This
is indeed what we have observed upon depositing aqueous
solutions (ꢁ10ꢀ4 m) of amphiphilic OPEs 1a or 1b onto
carbon-coated copper grids and registering the correspond-
ing transmission electron micrographs. Micrographs of both
amphiphilic OPEs 1a and 1b recorded by using this tech-
nique, show their organization into long fibrillar structures
(Figures 5 and 6 and Figure S12 of the Supporting Informa-
tion).
Compound 1a forms fibers with average widths of around
70 nm (Figure 5) that can be entangled into thicker fila-
ments (Figure 5a). Considering the molecular dimensions of
OPE 1a (around 4.5 nm in height), the fibers observed in
the TEM images would be formed by the interconnection of
unimolecular wires through the hydrophilic dendritic periph-
eral substituents (see Scheme 2 below and Scheme S1 of the
Supporting Information). A closer examination of some of
the TEM micrographs reveals a ribbon-like shape for some
of the fibers that, in addition, exhibit folded edges with a
thickness of around 10 nm (inset in Figure 5b). This dimen-
sion is in good agreement with a bimolecular packing of
molecules of 1a.
*
Figure 4. a) Distribution of hydrodynamic radii of aggregates of 1a (
)
^
and 1b ( ). The inset shows the angular dependence of the apparent dif-
fusion coefficient vs. the angle q; b) Static light scattering intensity data
for 1b superimposed on the theoretically derived curves for a random
coil (gray) and a rod (black). All measurements were carried out with
ꢁ10ꢀ4 m aqueous solutions.
and 2.801ꢀ10ꢀ8 cm2 sꢀ1 for 1a and 1b, respectively. In addi-
tion, the slope obtained by plotting the angular dependence
of D is lower than 0.03 (0.01 for 1a and 0.02 for 1b), sug-
gesting the presence of elongated aggregates (inset in Fig-
ACHTUNGTRENNUNG
ure 4a).[35]
Additional information on the shape of the supramolec-
Similar images were recorded when amphiphile 1b was
used for TEM imaging. Drop-casting a ꢁ10ꢀ4 m aqueous so-
lution of this four-branched dendronized amphiphile onto
carbon-coated copper grids allowed the observation of
fibers several micrometers long (Figure 6a and Figure S12b
of the Supporting Information). As for the previous amphi-
phile, a closer examination of the TEM micrographs also re-
veals the presence of ribbon-like strands that can be folded
(Figure 6b). Interestingly, the TEM images also demonstrate
that large ribbon-like fibers around 300 nm wide can
become entangled into coiled rope-like supramolecular
structures (Figure 6c) that resemble those observed for
other, related p-conjugated oligomers decorated with chiral
ular structures present in aqueous solution has been ob-
tained by studying the corresponding particle scattering
factor, also known as the form factor [P(q)], determined by
SLS techniques. Figure 4b (and Figure S11 of the Support-
ing Information) shows a plot of the dependence of the
form factor P(q) on the scattering data. Considering the an-
gular dependence of D, we have fitted the P(q) values by
applying the equations corresponding to random coils
[Eq. (2)] and rods [Eq. (3)]:[36]
2
PðqÞ ¼ 2½eꢀxꢀ1þxꢃ=x2, x ¼ ðqRGÞ
ð2Þ
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Nanofibrillar Self-Assembly
FULL PAPER
substituents.[12b] This supramolecular organization would
imply a helical arrangement of amphiphile 1b, despite the
lack of any chiral center in the molecule. The formation of
chiral objects is diagnostic of a hierarchical packing of 1b in
which van der Waals contacts between the dendritic wedges,
and also p-stacking interactions between the aromatic
moiety, play a pivotal role in the final self-assembly process.
Although both amphiphiles 1a and 1b are able to form
thick strands, the induced chirality observed only for 1b
could be attributed to the steric constraints imposed by the
four-branched TEG dendrons.
To gain a deeper insight into the self-assembly mechanism
followed by amphiphiles 1a and 1b, we have carried out
AFM imaging in highly diluted (ꢁ10ꢀ6 m) aqueous solutions
utilizing highly ordered pyrolytic graphite (HOPG) as the
substrate. AFM images of both radial OPEs 1a and 1b
under these conditions showed long fibrous structures
(Figure 7 and Figure S13 of the Supporting Information).
Figure 5. TEM images, stained with uranyl acetate, of the fibers formed
from the self-assembly of 1a in aqueous solution (ꢁ10ꢀ4 m). a) Fibers ap-
proximately 70 nm wide entangled into thicker filaments. b) Ribbon-like
shape of some regions (the insets shows a twisted ribbon).
Figure 7. Height-tapping-mode AFM images (air, 298 K) of drop-cast 1b
on HOPG from ꢁ10ꢀ6 m aqueous solution. a) z scale=19 nm; b) z scale=
10 nm. The inset in b) shows the height profile along the white line.
The observed fibers are around 4.5 and 4.8 nm in height for
1a and 1b, respectively, as demonstrated by their corre-
sponding profiles. These dimensions are in good agreement
with those calculated for these dendronized amphiphiles
(see Scheme 2) and are diagnostic of unimolecular packing
to form the fibers. The AFM images also allowed the effec-
tive fiber width to be calculated by subtracting the tip
broadening factor.[37] Thus, amphiphile 1a forms fibers of
(30ꢂ2) nm in width and, analogously, the fibers formed
from 1b present a diameter of (25ꢂ2) nm. These width
values imply the interaction of several single wires through
their peripheral dendritic substituents to form thicker fibers.
Dye intercalation experiments: One of the most important
potential applications of self-assembled structures is their
capability to transport and release different species. The en-
capsulation of substances by supramolecular structures
could be utilized as drug-delivery containers capable of con-
trolling the concentration of the released drug and also of
enabling site-specific delivery of a drug to the desired point
of action.[11b,29,38] We have evaluated the molecular encapsu-
lation and release of the hydrophobic dye 4-(4-nitrophenyl-
Figure 6. TEM images of the fibers obtained from ꢁ10ꢀ4 m aqueous solu-
tion of 1b, stained with uranyl acetate, onto carbon-coated copper grids.
a) Long fibers; b) expanded view of the black rectangle in a) showing a
folded edge. c) Left-handed rope-like strand obtained by the hierarchical
assembly of 1b. The inset in c) shows the density profile along 600 nm,
indicated by the black line.
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3143

L. Sꢂnchez and F. Garcꢁa
1a, which implies a tight bind-
ing between this amphiphile
and the dye and that, therefore,
a
higher temperature is re-
quired to completely liberate
the dye. In addition, the higher
temperatures reached in these
experiments, in comparison to
those performed with pristine
1a and 1b, could be considered
to be indirect proof of the for-
mation of different supramolec-
ular structures.
Conclusion
We have synthesized two trian-
Scheme 2. Molecular dimensions and schematic representation of the self-assembly of compounds 1a and 1b
into nanofibrillar structures.
gular-shaped,
amphiphilic
OPEs decorated in their pe-
riphery with hydrophilic TEG-
based dendron wedges (com-
A
pounds 1a and 1b) that allows their dissolution in aqueous
media. All of the studies carried out, both in solution and
on surfaces, support the idea of a face-to-face rotated p
stacking of the aromatic units, which avoids the steric
demand caused by the hydrophilic dendron wedges and
brings about the generation of the fibers through the plausi-
ble mechanism detailed above. This stacking mode decreas-
es the value of the binding constant in comparison to our
previously synthesized, triangular OPE-based amphiphile
2.[20a] At low concentrations in water, the initial p–p interac-
tions between amphiphilic molecules grow only in one di-
mension to create unimolecularly stacked aggregates that
can ultimately interact by means of the van der Waals con-
tacts, solvophobic interactions, and/or hydrogen bonding ex-
erted by the hydrophilic chains to form bundles of fibers. By
using more concentrated aqueous solutions, it was possible
to generate thicker and longer fibers, with thicknesses of
around 70 nm, through efficient interaction between thinner
aggregates (Scheme 2). Some of these fibers also present
folded edges that suggest a ribbon-like nature. Compound
1b was also able to self-associate into thick strands with
rope-like features, despite the lack of any chiral element in
its chemical makeup. This helical arrangement implies a hi-
erarchical self-assembly to form the helical supramolecular
structures.
phiphiles 1a and 1b in aqueous solution by means of tem-
perature-dependent UV/Vis experiments (Figure 8).
To carry out the temperature-dependent UV/Vis encapsu-
lation studies we have utilized a ꢁ10ꢀ5 m solution of both
amphiphiles in water to evaluate the spectroscopic changes
brought about by the release of dye molecules. To efficiently
intercalate the hydrophobic dye molecules, we added ten
equivalents of Disperse Orange 3, sonicated the resulting
mixtures for three hours and filtered them off with 0.45 mm
filters. Upon heating the mixtures, the diagnostic band for
Disperse Orange 3 at around 400 nm increased gradually,
while the maxima at around 284 and 303 nm decreased.
However, the lack of defined isosbestic points in the UV/Vis
spectra obtained during these experiments is indicative of
the coexistence of aggregates with different stoichiome-
tries.[39] These findings suggest the dye molecules are inter-
calated in the aggregated structure previously formed by the
amphiphiles and can be considered to be indirect proof of
the encapsulation of hydrophobic guests within the supra-
molecular structure formed by the artificial organic amphi-
philes.[29] Plotting the absorbance at 400 nm, which corre-
sponds to the dye, versus temperature showed a nonlinear
dependence that is diagnostic of dye release (insets in
Figure 8). The increased absorbance as a function of temper-
ature can be fitted to the Boltzmann function with correla-
tion factors of 0.99. For both amphiphiles, release of the dye
molecules started at around 310 K; below this temperature,
the change in the absorbance at 400 nm was negligible. At
higher temperatures, the changes in the absorbance were
slightly different for compounds 1a and 1b. In the latter
case, at high temperature (>350 K) the absorbance at
400 nm remained practically unaltered, suggesting that most
of the dye was already released. However, the absorbance
at 400 nm continues to increase above 350 K for compound
Finally, temperature-dependent UV/Vis experiments have
demonstrated the ability of the aggregates formed by the
new amphiphilic OPEs reported to intercalate and release
dye molecules in aqueous solution. These features suggest
that these materials can be exploited for drug-delivery pur-
poses. In addition, work is in progress to synthesize new
chiral dendronized congeners that should be useful to pre-
pare helical supramolecular structures.
3144
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Chem. Eur. J. 2010, 16, 3138 – 3146

Nanofibrillar Self-Assembly
FULL PAPER
Amphiphile 1b: Compound 5b (1.17 g, 1.02 mmol),
3
(0.045 g,
0.30 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.02 g, 0.03 mmol)
and triphenylarsine (0.004 g, 0.01 mmol) were dissolved in triethylamine
(10 mL). The mixture was subjected to several vacuum/argon cycles and
heated at reflux for 24 h. After cooling to RT, the solvent was evaporated
under reduced pressure. The residue was purified by column chromatog-
raphy (silica gel; CHCl3/methanol, 10:1) affording 1b as a brown oil
(0.28 g, 32%). 1H NMR (CDCl3, 300 MHz): d=7.51 (s, 3H; Ha), 7.38 (d,
J=8.8 Hz, 6H; Hb), 6.82 (d, J=8.8 Hz, 6H; Hc), 3.96 (d, J=5.4 Hz, 6H;
Hd), 3.58–3.39 (br, 192H; Hf–o), 3.30 (s, 36H; Hp), 2.28 (q, J=5.4 Hz, 3H;
He), 2.10 ppm (q, J=5.8 Hz, 6H; Hh); 13C NMR (CDCl3, 75 MHz): d=
159.4, 133.3, 133.1, 124.2, 114.7, 114.6, 90.4, 87.7, 76.7, 71.9, 70.9, 70.6,
70.57, 70.52, 70.4, 69.6, 66.4, 69.1, 66.1, 59.0, 40.1, 39.9 ppm; FTIR (neat):
n˜ =687, 772, 840, 1030, 1104, 1248, 1291, 1356, 1462, 1508, 1577, 2860,
2921 cmꢀ1; MALDI-TOF: m/z: 2994 [M+Na]+; HRMS: m/z calcd for
[C150H258NaO57]+: 2994.7199; found: 2994.7222.
Acknowledgements
We are thankful to Centro de Microscopꢁa y Citometrꢁa (UCM) for mi-
croscopy imaging, Unidad de Espectrocopꢁa de Infrarrojo-Ramꢂn-Corre-
laciꢅn (UCM) for DLS experiments, and CAI de Espectrometrꢁa de
Masas (UCM) for recording MALDI-TOF spectra. This work was sup-
ported by UCM (UCM-SCH-PR34/07-15826). The authors gratefully ac-
knowledge Prof. N. Martꢁn, Prof. G. Orellana, and Dr. J. Lꢅpez for their
generous help, and especially to Dr. G. Fernꢂndez for helpful discussions.
[1] a) C. Kim, A. Facchetti, T. J. Marks, Science 2007, 318, 76–80; b) V.
Burtman, G. Hukic, A. S. Ndobe, T. Drori, Z. V. Vardeny, J. Appl.
Phys. 2007, 101, 54502–54507.
[2] a) C. A. Hunter, J. K. M. Sanders, J. Am. Chem. Soc. 1990, 112,
5525–5534; b) C. A. Hunter, K. R. Lawson, J. Perkins, C. J. Urch, J.
Chem. Soc. Perkin Trans. 2 2001, 651–669; c) C. H. Hunter, Angew.
Chem. 2004, 116, 5424–5439; Angew. Chem. Int. Ed. 2004, 43, 5310–
5324.
[3] a) J. Wu, W. Pisula, K. Mꢆllen, Chem. Rev. 2007, 107, 718–747;
b) A. Ajayaghosh, V. K. Praveen, Acc. Chem. Res. 2007, 40, 644–
656; c) F. J. M. Hoeben; P. Jonkheijm, E. W. Meijer, A. P. H. J.
Schenning, Chem. Rev. 2005, 105, 1491–1546; P. Jonkheijm, E. W.
Meijer, A. P. H. J. Schenning, Chem. Rev. 2005, 105, 1491–1546; P.
Jonkheijm, E. W. Meijer, A. P. H. J. Schenning, Chem. Rev. 2005,
105, 1491–1546.
Figure 8. Temperature-dependent UV/Vis spectra of aqueous solutions
A
(ꢁ10ꢀ5 m) of: a) 1a and b) 1b containing ten equivalents of Disperse
Orange 3. Arrows indicate the spectroscopic changes upon increasing
temperature. The insets in a) and b) depict the changes in the absorbance
at 400 nm as a function of temperature.
[4] a) J.-H. Ryu, D.-J. Hong, M. Lee, Chem. Commun. 2008, 1043–
1054; b) E. Lee, Y.-H. Jeong, J.-K. Kim, M. Lee, Macromolecules
2007, 40, 8355–8360.
[5] N. Arai, K. Yasuoka, X. C. Zeng, J. Am. Chem. Soc. 2008, 130,
7916–7920.
Experimental Section
[6] a) M. Kellermann, W. Bauer, A. Hirsch, B. Schade, K. Ludwig, C.
Bçttcher, Angew. Chem. 2004, 116, 3019–3022; Angew. Chem. Int.
Ed. 2004, 43, 2959–2962; b) A. Hirsch, Pure Appl. Chem. 2008, 80,
571–587; c) M. S. Becherer, B. Schade, C. Bçttcher, A. Hirsch,
Chem. Eur. J. 2009, 15, 1637–1648.
[7] a) R. M. Meudtner, M. Ostermeier, R. Goddard, C. Limberg, S.
Hecht, Chem. Eur. J. 2007, 13, 9834–9840; b) L. Piot, R. M. Meudt-
ner, T. El Malah, S. Hecht, P. Samorꢇ, Chem. Eur. J. 2009, 15, 4788–
4792.
Amphiphile 1a: Compound 5a (0.65 g, 1.08 mmol), 3 (0.04 g, 0.30 mmol),
tris(dibenzylideneacetone)dipalladium(0) (0.04 g, 0.04 mmol), and triphe-
nylarsine (0.005 g, 0.01 mmol) were dissolved in triethylamine (20 mL)
and dry THF (5 mL). The mixture was subjected to several vacuum/
argon cycles and heated at reflux for 48 h. After cooling to RT, the sol-
vent was evaporated under reduced pressure. The residue was purified by
column chromatography (silica gel; ethyl acetate/EtOH, 5:1) affording
1
1a as a brown oil (0.17 g, 36%). H NMR (CDCl3, 300 MHz): d=7.58 (s,
3H; Ha), 7.45 (d, J=8.8 Hz, 6H; Hb), 6.90 (d, J=8.8 Hz, 6H; Hc), 4.07
(d, J=5.8 Hz, 6H; Hd), 3.78–3.59 (br, 84H; Hf–l), 3.39 (s, 18H; Hm),
2.52 ppm (q, J=5.8 Hz, 3H; He); 13C NMR (CDCl3, 75 MHz): d=159.8,
133.7, 133.5, 124.6, 115.1, 115.0, 90.8, 87.1, 72.3, 71.0, 71.0, 70.9, 70.8, 69.6,
66.5, 59.4, 40.2 ppm; FTIR (neat): n˜ =684, 756, 837, 876, 948, 1028, 1107,
1248, 1290, 1355, 1464, 1509, 1576, 1606, 1676, 1731, 1954, 2209, 2867,
[8] a) S. Sreejith, K. P. Divya, A. Ajayaghosh, Angew. Chem. 2008, 120,
8001–8005; Angew. Chem. Int. Ed. 2008, 47, 7883–7887; b) A.
Ajayaghosh, P. Chithra, R. Varghese, Angew. Chem. 2007, 119, 234–
237; Angew. Chem. Int. Ed. 2007, 46, 230–233.
[9] a) X. Zhang, S. Rehm, M. M. Safont-Sempere, F. Wꢆrthner, Nat.
Chem. Biol. 2009, 1, 623–629; b) S. Ghosh, X.-Q. Li, V. Stepanenko,
F. Wꢆrthner, Chem. Eur. J. 2008, 14, 11343–11357; c) X. Zhang, Z.
Chen, F. Wꢆrthner, J. Am. Chem. Soc. 2007, 129, 4886–4887.
2921 cmꢀ1
;
MALDI-TOF: m/z: 1566 [M]+; HRMS: m/z calcd for
[C84H128O27]2+: 784.4322; found: 784.4316.
Chem. Eur. J. 2010, 16, 3138 – 3146
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3145

L. Sꢂnchez and F. Garcꢁa
[10] a) J. P. Hill, W. S. Jin, A. Kosaka, T. Fukushima, H. Ichihara, T. Shi-
momura, K. Ito, T. Hashizume, N. Ishii, T. Aida, Science 2004, 304,
1481–1483; b) Y. Yamamoto, T. Fukushima, Y. Suna, N. Ishii, A.
Saeki, S. Seki, S. Tagawa, M. Taniguchi, T. Kawai, T. Aida, Science
2006, 314, 1761–1764; c) M. Yin, J. Shen, W. Pisula, M. Liang, L.
Zhi, K. Mꢆllen, J. Am. Chem. Soc. 2009, 131, 14618–14619.
[11] a) E. Lee, J.-K. Kim, M. Lee, Angew. Chem. 2009, 121, 3711–3714;
Angew. Chem. Int. Ed. 2009, 48, 3657–3660; b) J.-K. Kim, E. Lee, Y.
Lim, M. Lee, Angew. Chem. 2008, 120, 4740–4744; Angew. Chem.
Int. Ed. 2008, 47, 4662–4666; c) D.-J. Hong, E. Lee, J.-K. Lee, W.-C.
Zin, M. Han, E. Sim, M. Lee, J. Am. Chem. Soc. 2008, 130, 14448–
14449; d) J.-K. Kim, E. Lee, Z. Huang, M. Lee, J. Am. Chem. Soc.
2006, 128, 14022–14023.
[12] a) J. van Herrikhuyzen, S. J. George, M. R. J. Vos, N. A. J. M. Som-
merdijk, A. Ajayaghosh, S. C. J. Meskers, A. P. H. J. Schenning,
Angew. Chem. 2007, 119, 1857–1860; Angew. Chem. Int. Ed. 2007,
46, 1825–1828; b) A. Ajayaghosh, R. Varghese, S. J. George, C. Vi-
jayakumar, Angew. Chem. 2006, 118, 1159–1162; Angew. Chem. Int.
Ed. 2006, 45, 1141–1144; c) S. J. George, A. Ajayaghosh, P. Jonk-
heijm, A. P. H. J. Schenning, E. W. Meijer, Angew. Chem. 2004, 116,
3504–3507; Angew. Chem. Int. Ed. 2004, 43, 3422–3425; d) F. J. M.
Hoeben, L. M. Herz, C. Daniel, P. Jonkheijm, A. P. H. J. Schenning,
C. Silva, S. C. J. Meskers, D. Beljonne, R. T. Phillips, R. H. Friend,
E. W. Meijer, Angew. Chem. 2004, 116, 2010–2013; Angew. Chem.
Int. Ed. 2004, 43, 1976–1979.
[13] a) A. Ajayaghosh, R. Varghese, S. Mahesh, V. K. Praveen, Angew.
Chem. 2006, 118, 7893–7896; Angew. Chem. Int. Ed. 2006, 45, 7729–
7732; b) A. Ajayaghosh, R. Varghese, V. K. Praveen, S. Mahesh,
Angew. Chem. 2006, 118, 3339–3342; Angew. Chem. Int. Ed. 2006,
45, 3261–3264.
[14] L. C. Palmer, S. I. Stupp, Acc. Chem. Res. 2008, 41, 1674–1684.
[15] a) J. H. K. Ky Hirschberg, L. Brunsveld, A. Ramzi, J. A. J. M. Veke-
mans, R. P. Sijbesma, E. W. Meijer, Nature 2000, 407, 167–170;
b) M. Enomoto, A. Kishimura, T. Aida, J. Am. Chem. Soc. 2001,
123, 5608–5609; c) S. Kawano, N. Fujita, S. Shinkai, J. Am. Chem.
Soc. 2004, 126, 8592–8593.
[22] G. Fernꢂndez, F. Garcꢁa, F. Aparicio, E. Matesanz, L. Sꢂnchez,
Chem. Commun. 2009, 7155–7157.
[23] Metal-Catalyzed Cross-Coupling Reactions (Eds.: F. Diederich, P. J.
Stang), Wiley-VCH, Weinheim, 1998.
[24] W. Uhl, H. R. Bock, F. Breher, M. Claesener, S. Haddadpour, B.
Jasper, A. Hepp, Organometallics 2007, 26, 2363–2369.
[25] a) C. Amatore, A. Jutand, Coord. Chem. Rev. 1998, 178–180, 511–
528; b) R. W. Wagner, T. E. Johnson, F. Li, J. S. Lindsey, J. Org.
Chem. 1995, 60, 5266–5273; c) C. Amatore, A. Jutand, F. Khalil,
M. A. MꢄBarki, L. Mottier, Organometallics 1993, 12, 3168–3178.
[26] a) M. Jayaraman, J. M. J. Frꢈchet, J. Am. Chem. Soc. 1998, 120,
12996–12997; b) C.-J. Jang, J.-H. Ryu, J.-D. Lee, D. Sohn, M. Lee,
Chem. Mater. 2004, 16, 4226–4231; c) K.-S. Moon, E. Lee, M Lee,
Chem. Commun. 2008, 3061–3063.
[27] a) R. B. Martin, Chem. Rev. 1996, 96, 3043–3064; b) D. Zhao, J. S.
Moore, Org. Biomol. Chem. 2003, 1, 3471–3491; c) Z. Chen, A.
Lohr, C. R. Saha-Mçller, F. Wꢆrthner, Chem. Soc. Rev. 2009, 38,
564–584; d) T. F. A. De Greef, M. M. J. Smulders, M. Wolffs,
A. P. H. J. Schenning, R. P. Sijbesma, E. W. Meijer, Chem. Rev. 2009,
109, 5687–5754.
[28] a) Y. Haba, C. Kojima, A. Harada, K. Kono, Angew. Chem. 2007,
119, 238–241; Angew. Chem. Int. Ed. 2007, 46, 234–237; b) S. V. Aa-
thimanikandan, E. N. Savariar, S. Thayumanavan, J. Am. Chem. Soc.
2005, 127, 14922–14929.
[29] K.-S. Moon, H.-J. Kim, E. Lee, M. Lee, Angew. Chem. 2007, 119,
6931–6934; Angew. Chem. Int. Ed. 2007, 46, 6807–6810.
[30] a) G. D. Smith, D. Bedrov, J. Phys. Chem. B 2003, 107, 3095–3097;
b) S. V. Aathimanikandan, E. N. Savariar, S. Thayumanavan, J. Am.
Chem. Soc. 2005, 127, 14922–14929; c) Z. Jia, H. Chen, X. Zhu, D.
Yan, J. Am. Chem. Soc. 2006, 128, 8144–8145; d) T. Hirose, K. Mat-
suda, Chem. Commun. 2009, 5832–5834.
[31] W. Wang, J. J. Han, L.-Q. Wang, L.-S. Li, W. J. Shaw, A. D. Q. Li,
Nano Lett. 2003, 3, 455–458.
[32] a) Y. Zhou, D. Yan, W. Dong, Y. Tian, J. Phys. Chem. B 2007, 111,
1262–1270; b) F. Hua, X. Jiang, B. Zhao, Macromolecules 2006, 39,
3476–3479.
[16] a) J. H. van Esch, B. L. Feringa, Angew. Chem. 2000, 112, 2351–
2354; Angew. Chem. Int. Ed. 2000, 39, 2263–2266; b) N. M. Sangee-
tha, U. Maitra, Chem. Soc. Rev. 2005, 34, 821–836; c) A. Ajaya-
ghosh, V. K. Praveen, C. Vijayakumar, Chem. Soc. Rev. 2008, 37,
109–122.
[17] a) J. C. Tiller, Angew. Chem. 2003, 115, 3180–3183; Angew. Chem.
Int. Ed. 2003, 42, 3072–3075; b) L. A. Estroff, A. D. Hamilton,
Chem. Rev. 2004, 104, 1201–1217; c) K. J. C. van Bommel, A. Frig-
geri, S. Shinkai, Angew. Chem. 2003, 115, 1010–1030; Angew. Chem.
Int. Ed. 2003, 42, 980–999.
[18] a) S. Srinivasan, P. A. Babu, S. Mahesh, A. Ajayagosh, J. Am. Chem.
Soc. 2009, 131, 15122–15123; b) C. Vijayakumar, V. K. Praveen, A.
Ajayaghosh, Adv. Mater. 2009, 21, 2059–2063; c) S. S. Babu, S.
Mahesh, K. K. Kartha, A. Ajayaghosh, Chem. Asian J. 2009, 4, 824–
829; d) S. S. Babu, V. K. Praveen, S. Prasanthkumar, A. Ajayaghosh,
Chem. Eur. J. 2008, 14, 9577–9584; e) A. Ajayaghosh, V. K.
Praveen, S. Srinivasan, R. Varghese, Adv. Mater. 2007, 19, 411–415;
f) S. J. George, A. Ajayaghosh, Chem. Eur. J. 2005, 11, 3217–3227.
[19] a) T. Muraoka, C.-Y. Koh, H. Cui, S. I. Stupp, Angew. Chem. 2009,
121, 6060–6063; Angew. Chem. Int. Ed. 2009, 48, 5946–5949; b) T.
Muraoka, H. Cui, S. I. Stupp, J. Am. Chem. Soc. 2008, 130, 2946–
2947; c) H. A. Behanna, K. Rajangam, S. I. Stupp, J. Am. Chem.
Soc. 2007, 129, 321–327.
[33] X. Jiang, Y.-K. Lim, B. J. Zhang, E. A. Opsitnick, M.-H. Baik, D.
Lee, J. Am. Chem. Soc. 2008, 130, 16812–16822.
[34] a) M. M. J. Smulders, A. P. H. J.; Schenning, E. W. Meijer, J. Am.
Chem. Soc. 2008, 130, 606–611; Schenning, E. W. Meijer, J. Am.
Chem. Soc. 2008, 130, 606–611; Schenning, E. W. Meijer, J. Am.
Chem. Soc. 2008, 130, 606–611; b) E. M. Garcꢁa-Frutos, B. Gꢅmez-
Lor, J. Am. Chem. Soc. 2008, 130, 9173–9177.
[35] M. Schmidt, W. H. Stockmayer, Macromolecules 1984, 17, 509–514.
[36] a) Dynamic Light Scattering, (Eds.: J. Berne, R. Pecora) Wiley, New
York, 1976; b) for a recent example, see: M. Yuasa, K. Oyaizu, A.
Yamaguchi, M. Kuwakado, J. Am. Chem. Soc. 2004, 126, 11128–
11129.
[37] a) H.-J. Butt, R. Guckenberger, J. P. Rabe, Ultramicroscopy 1992,
46, 375–393; b) P. Samorꢁ, V. Francke, T. Mangel, K. Mꢆllen, J. P.
Rabe, Opt. Mater. 1998, 9, 390–393. The radius of the tip (Veeco
probes, MPP-11100-10) has been considered to be 12.5 nm. The cal-
culated tip broadening factors are of (19.2ꢂ2) and (19.5ꢂ1) nm for
1a and 1b, respectively.
[38] a) E. Soussan, S. Cassel, M. Blanzat, I. Rico-Lattes, Angew. Chem.
2009, 121, 280–295; Angew. Chem. Int. Ed. 2009, 48, 274–288; b) M.
Kellermann, W. Bauer, A. Hirsch, B. Schade, K. Ludwig, C. Bçttch-
er, Angew. Chem. 2004, 116, 3019–3022; Angew. Chem. Int. Ed.
2004, 43, 2959–2962; c) J.-H. Ryu, E. Lee, Y. Lim, M. Lee, J. Am.
Chem. Soc. 2007, 129, 4808–4814.
[20] a) G. Fernꢂndez, F. Garcꢁa, L. Sꢂnchez, Chem. Commun. 2008,
6567–6569; b) F. Garcꢁa, F. Aparicio, G. Fernꢂndez, L. Sꢂnchez,
Org. Lett. 2009, 11, 2748–2751.
[39] G. Fernꢂndez, L. Sꢂnchez, E. M. Pꢈrez, N. Martꢁn, J. Am. Chem.
Soc. 2008, 130, 10674–10683.
[21] F. Garcꢁa, G. Fernꢂndez, L. Sꢂnchez, Chem. Eur. J. 2009, 15, 6740–
6747.
Received: October 20, 2009
Published online: January 29, 2010
3146
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Chem. Eur. J. 2010, 16, 3138 – 3146