Amphiphilic folded dendrimer discs and their thermosensitive self-assembly in water

Article abstract of DOI:10.1002/chem.201200414

Folded dendrimers with peripheral ether side chains show a thermally induced hierarchical aggregation process, in which the transition temperature and the dimensions of the aggregates can readily be tuned via the generation number (see figure). Copyright

Full text of DOI:10.1002/chem.201200414

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DOI: 10.1002/chem.201200414
Amphiphilic Folded Dendrimer Discs and Their Thermosensitive Self-
Assembly in Water
Tamer El Malah,[a, b] Simone Rolf,[c] Steffen M. Weidner,[c]
Andreas F. Thꢀnemann,*[c] and Stefan Hecht*[a]
Self-assembly is a major construction principle in nature
and takes advantage of specific non-covalent interactions
between custom-designed building blocks.[1] The size as well
as the shape of these building blocks is crucial for the result-
ing assembly structure[2] and impressive examples of super-
structure formation have been reported over the years.[3] In
this context, shape-persistent scaffolds, in particular large
(hetero)aromatic discs[4] and macrocycles[5] have been very
attractive building blocks owing to their high tendency to
form cofacial aggregates, that is, p-stacks,[6] either in solution
or in the bulk, for example in discotic liquid crystalline
phases. While these structures are rendered rigid by strong
covalent bonds giving rise to annelated or macrocyclic sys-
tems, an alternative approach to construct objects with a de-
fined conformation and hence shape relies in the use of
non-covalent bonds between neighboring repeat units. Such
foldamers[7] have been heavily investigated in the context of
designing three-dimensional, mostly helical structures but
can also lead to flat two-dimensional structures such as
tapes.[8] Introducing regular branching into these folded
tapes gives rise to an extension to molecular discs and hence
2D, ideally shape-persistent, folded dendrimers.[9] This ap-
proach to construct large molecular discs constitutes a non-
covalent version of the covalent annelation approach of
Mꢀllen and co-workers to nanographenes, which are ac-
cessed via planarization of 3D dendritic precursors.[10] The
great advantage of the folded dendrimer structure is its pos-
sible shape change in response to chemical stimuli and the
ease of tuning size and amount of peripheral substitution by
varying the generation.[11] In recent years, we[12] and
others[13] have successfully utilized click chemistry[14] to pre-
pare triazole connected pyridines that adopt either helically
folded or zigzag tape structures. By connecting the 2,6-
bis(1,2,3-triazol-4-yl)pyridine (BTP) moieties not in a linear
fashion but using them as branch points this folding motif
can be utilized to construct the desired dendrimer discs.
Herein, we report on the synthesis and characterization of
these 2D amphiphilic shape-persistent folded dendrimers
and their thermosensitive self-assembly behavior in water.[15]
To efficiently construct the target dendrimer structures,
a convergent synthetic route[16] was devised (Scheme 1). In
our synthesis 2,6-diethynyl-4-nitropyridine serves as the key
A’B2 monomer, which carries two free terminal alkyne units
(B), ready to couple to azide functionalities via the click re-
action, and one protected focal point functionality (A’),
which can be converted into an azide group. Convergent
dendron growth was achieved by repetition of a sequence of
twofold click reactions to the alkyne termini using previous-
ly established protocols (see the Supporting Information)[12]
followed by activation of the focal point functionality via
nucleophilic aromatic substitution with sodium azide. The
thus prepared first three generation dendrons carrying
single azide groups at their focal points were subsequently
coupled to
a
trifunctional core, that is, 1,3,5-tri-
AHCTUNGTREG(NNUN ethynyl)benzene to yield the target dendrimers with molec-
ular weights exceeding 9000 D.
All dendrons and dendrimers contain polar enantiomeri-
cally pure tetra(ethylene glycol) chains[17] to provide for suf-
ficient solubility in water and to induce circular dichroism
(CD) in the course of aggregate formation, therefore facili-
tating analysis of the aggregation process by CD spectrosco-
py. Most importantly, oligo(ethylene glycol)-derived archi-
tectures[18] commonly exhibit a lower critical solution tem-
perature (LCST),[19] rendering our dendrimers temperature-
sensitive. For comparison purposes, the respective dendrim-
ers, terminated by free carboxylic acid groups, were pre-
pared by complete saponification of the ester side chains.
All compounds were characterized by standard analytical
techniques (see the Supporting Information) and investigat-
ed with regard to their aggregation behavior as pure sub-
stances. Based on some initial molecular modeling studies
using molecular mechanics (MM2), due to the significant
size of the systems, we anticipated the dendrimers to adopt
folded two-dimensional structures giving rise to flat disc-like
shapes with increasing diameter (Scheme 1, bottom). The
[a] Dr. T. El Malah, Prof. Dr. S. Hecht
Department of Chemistry, Humboldt-Universitꢁt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax : (+49)30-2093-6940
E-mail: sh@chemie.hu-berlin.de
[b] Dr. T. El Malah
Present address: National Research Centre
El Buhouth St., Dokki, Cairo (Egypt)
[c] S. Rolf, Dr. S. M. Weidner, Prof. Dr. A. F. Thꢀnemann
BAM Federal Institute for Materials Research and Testing
Unter den Eichen 87, 12205 Berlin (Germany)
E-mail: andreas.thuenemann@bam.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200414.
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Scheme 1. Convergent synthesis of first, second, and third generation dendrons and dendrimers (G-ng and G-na). Molecular models (MM2) shown were
used to estimate the approximate diameters of the flat heteroaromatic dendrimer core.
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Chem. Eur. J. 2012, 18, 5837 – 5842

Amphiphilic Folded Dendrimer Discs
COMMUNICATION
dendrimers therefore resemble an amphiphilic architecture
in which a rigid and extended hydrophobic heteroaromatic
core with a tunable surface area is surrounded by a polar
corona of flexible ether side chains. These structural features
should induce self-assembly in aqueous systems and hence
aggregation was studied in water to evaluate critical struc-
tural parameters (generation number) and environmental
conditions (concentration, temperature).
ability of the aggregates for the higher generation, hysteresis
suggests furthermore an increased kinetic stability, which is
nicely revealed by monitoring the kinetics of disassembly
(Figure 2b). By analysis of the rates it was found that the
disassembly is 34 times slower in the case of G-3g as com-
pared to G-1g (see Figure S2 in the Supporting Information).
To gather more experimental evidence for the solution
structure of the aggregates, small-angle X-ray scattering
(SAXS) measurements were carried out on diluted aqueous
solutions of the ester- as well as carboxylate-terminated den-
drimer series G-ng and G-na, respectively, at temperatures
ranging from 208C to 808C. Note that the carboxylic acids
G-na were not soluble at pH 7 and hence measured at pH 10
as their corresponding sodium salts. The scattering patterns
of all dendrimers are typical for nanoparticles (Figure 3),
Initially, clear aqueous solutions of the dendrimers over
a
broad concentration range (8.5·10ꢀ6 mꢁcꢁ1.6·10ꢀ2 m)
became turbid upon heating above 608C and regained their
transparency after cooling. Closer inspection of the respec-
tive absorption spectra revealed hypochromic shifts of the
bands associated with the aromatic cores upon heating (see
Figure S1 in the Supporting Information), thus indicating
p,p-stacking. For all generations, a strong decrease of the
absorbance signal was observed above 458C. The associated
CD spectra further support these findings as strong Cotton
effects, caused by chirality transfer from the side chains to
the aggregate, emerge with increasing temperature. These
sharp temperature-induced transitions monitored by UV/Vis
and CD spectroscopy (Figure 1) are characteristic for ther-
mosensitive polymers and show a clear dependence on the
generation number and hence molecular weight.[20] Thereby,
the largest dendrimer exhibits the lowest LCST, indicating
the most stable aggregate in the case of G-3g.
Figure 3. SAXS curves of the dendrimers (solid lines) and model curve
fits (dashed lines). Upper curves are multiplied by 100 for better visibili-
ty. The inset (left bottom) shows an approximation of G-1a (as Na+ salt)
by a homogeneous flat disk (R=0.35 nm, L=0.15 nm). Measurements
were performed at 208C and concentrations were 1.6·10ꢀ3 molLꢀ1 for all
dendrimers (except G-1a: 1.6·10ꢀ2 molLꢀ1).
Figure 1. a) UV/Vis and b) CD spectral evolution as a function of temper-
except the one for G-1a being characteristic of single mole-
cules. While the SAXS patterns of the carboxylates G-na are
insensitive to temperature variation, the patterns of the oli-
go(ethylene glycol) terminated dendrimers G-ng are con-
stant up to a critical temperature, above which an abrupt
phase separation into a water and a dendrimer phase is ob-
served. The latter one subsequently precipitates as an in-
soluble white solid. This phase transition temperature is at-
tributed to a lower critical solution temperature (LCST),[19]
comparable to that observed in numerous water-soluble
polymeric systems, such as poly(ethylene glycol)-based ther-
mogels.[20] For accurate determination of the LCST transi-
tions, temperature-dependent dynamic light scattering
(DLS) experiments were carried out. The LCST transitions
(Table 1) are nicely reflected by an abrupt increase of the
hydrodynamic radii of G-ng nanoparticles from ca. 10 nm to
ca. 800 nm (see Figure S3 in the Supporting Information).
The subsequent rapid additional increase in the sizes up to
the micrometer range is accompanied by precipitation, ex-
plaining the decreasing scattering intensity.
ature to determine the LCST for G-ng (c=8.5·10ꢀ6 m).
During subsequent heating-cooling cycles monitored by
CD, pronounced hysteresis was observed, in particular in
the case of the higher generation dendrimers (Figure 2a).
While the LCST indicates an enhanced thermodynamic sati-
Figure 2. a) Heating–cooling cycles at 0.5 Kminꢀ1 and b) disassembly ki-
netics upon rapid cooling to 558C for G-ng, as observed by CD spectros-
copy (c=8.5·10ꢀ6 m).
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S. Hecht, A. F. Thꢀnemann et al.
Table 1. Structural parameters and dimensions derived from curve fits of
the SAXS measurements (see Figure 3) at 208C, below LCST, independ-
ently measured by different techniques (DLS, UV, CD).
nanorods are composed of flat dendrimer discs (see above)
spaced by a typical p,p-stacking distance of 3.5 ꢄ,[22] the ob-
served lengths correlate with increasing aggregation num-
bers for higher generation dendrimers, that is, 42, 48, and
165 for G-1g, G-2g, and G-3g, respectively. While the aggre-
gation numbers of the lower two generations are typical for
poly(ethylene oxide) based surfactants,[3,15,23,24] the third gen-
eration dendrimer forms rather large aggregates. The signifi-
cant increase in the aggregation number, when going from
the second to the third generation, is reflected in a much
higher aspect ratio, that is, length: radius ratio, for G-3a (22)
and G-3g (25). Assuming a simple isodesmic model for ag-
gregate growth,[3d,6] the association constant rises from K=
1.4·105 mꢀ1 (for G-2g) to K=1.7·107 mꢀ1 (for G-3g), approach-
ing the values of other heteroatom-containing extended p-
systems, such as perylene bisimides, in water.[6,25] This
strongly enhanced aggregation constant can be correlated to
the significant increase of the dendrimer coreꢃs hydrophobic
surface area from about 2ꢅ800 ꢄ2 (for G-2) to about 2ꢅ
1520 ꢄ2 (for G-3).[6]
To test our proposed structural model, a model-free data
evaluation procedure for the calculation of the radial densi-
ty profile developed by Glatter,[26] was used. In a first step,
we determined the cross-sectional pair distributions func-
tions with the established indirect Fourier transforma-
tion.[27,28] The radial electron density profiles of the cylindri-
cal micelles 1(r) were calculated in a second step using the
convolution square root method implemented in
DECON.[29–31] The resulting continuous density profiles, as
measured from the center of the cylinders (Figure 4), show
Dendrimer Shape
Length [nm] Radius [nm] LCST [8C]
DLS UV CD
[a]
[a]
[a]
G-1a
G-1g
G-2a
G-2g
G-3a
G-3g
disc
ca. 0.15
ca. 0.35
–
–
–
column 14.7ꢂ0.2
column 12.3ꢂ0.5
column 16.8ꢂ0.7
column 58.0ꢂ0.8
column 57.7ꢂ1.0
1.41ꢂ0.04
2.20ꢂ0.02
1.86ꢂ0.01
2.62ꢂ0.01
2.31ꢂ0.01
51
54
53
[a]
[a]
[a]
–
–
–
52
56
55
[a]
[a]
[a]
–
–
–
57
58
57.5
[a] No LCST Transition was detected for the carboxylate salt (pH 10).
In addition to determining the LCST transition, the
SAXS data provide valuable information about the structure
of the aggregates in solution. At 208C the form of the
curves of dendrimers G-ng are typical for a highly elongated
shape of the particles where the scattering intensity in the
q region from about 0.3 to 0.9 nmꢀ1 scales with qꢀ1 as indi-
cated by a straight line in Figure 3. At lower q values,
a Guinier region is visible, demonstrating that the total size
of the nanoparticles is attainable from the data. Finally, the
particle cross-section becomes visible in the high q range.
The obtained SAXS curves of dendrimers G-ng are most ap-
propriately described by assuming a nanorod structure
model. Reasonable curve fits for other simple shapes, such
as spherical or disk-like nanoparticles, could not be found.
By proper curve fitting(see the Supporting Information), the
dimensions of the nanorods, that is, lengths and radii, could
be determined (Table 1).
The radii of the nanorods increase with increasing genera-
tion number, nicely reflecting the larger size of the higher
generation dendrimers. The radii of the dendrimer cores of
1.0 nm, 1.6 nm, and 2.2 nm derived from molecular modeling
represent the lower limits of the radii, which upon addition
of the extended chiral oligo(ethylene glycol) chainsꢃ length
of ca. 0.8 nm give 1.8 nm, 2.4 nm, and 3.0 nm as upper limits
for G-1g, G-2g, and G-3g, respectively. Hence, the experimen-
tally determined radii are in agreement with a structural
model assuming that the cylinders are composed of stacks of
flat dendrimer discs. Note that at the first glance, it is sur-
prising that the radius of the carboxylate-terminated den-
drimers is larger than their corresponding oligo(ethylene
glycol)-terminated dendrimers in both the second and third
generations. The relatively large radii of G-2a and G-3a are
most likely a result of the sodium ions, which are condensed
to the surface of the nanorods and therefore contribute sig-
nificantly to electron density of the corona, rendering them
to appear bigger while the solvated ether side chains are
partially “invisible” due to their electron density matching
their surroundings.[21] This interpretation is supported by the
Figure 4. a) Cross-section density profiles of the nanorod as calculated
with DECON.[33] The arrow traverses the curves in the order of G-1g!
G-2g!G-2a!G-3g!G-3a (solid lines indicate oligo(ethylene glycol)
chain functionalized dendrimers, dashed lines indicate carboxylate-func-
tionalized dendrimers). b) Structural model of G-3g dendrimer aggregates
with dimensions as given in Table 1.
a clear decrease of electron density towards the periphery.[32]
The shift of the inflection points towards larger distances at
higher generations is in agreement with the increasing
radius of the dendrimer discs. Note that again the carboxyl-
ate-terminated dendrimers G-na appear to be more extend-
ed than the oligo(ethylene glycol) terminated dendrimers
G-ng due to the higher electron density of the sodium ions
fact that the differences of the radii, that is, r
2g)=0.34 nm and r (G-3g)=0.31 nm, are the same
(G-3a)ꢀr
within experimental error.
A
ACHTUNGERTN(NUNG G-
A
ACHTUNGTRENNUNG
The lengths of the nanorods reflect the aggregation
number, that is, how many dendrimer discs are stacked in
one cylindrical column. Based on the assumption that the
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Amphiphilic Folded Dendrimer Discs
COMMUNICATION
located at the periphery.[21] On a more quantitative basis, the
integrated electron density (98% for G-1g, 97% for G-2a,
95% for G-2g, 98% for G-3a, and 94% for G-3g) is given
within the radii determined by the above used simple model
function. While Glatterꢃs model-free data evaluation pro-
vides additional information on the cross-sectional structure,
it nicely confirms the radii derived from applying the simple
cylinder model.
ample the organization of inorganic semiconductors or cata-
lysts and modification of their surfaces.
Acknowledgements
T.E.M. acknowledges the Egyptian Government for providing a doctoral
fellowship. BASF AG, Bayer Industry Services, and Sasol Germany are
thanked for generous donations of chemicals.
Above the LCST, distinct reflections became visible in the
SAXS curves (see Figure S3 in the Supporting Information),
indicating that the aggregation process proceeds to structure
formation on the next hierarchical level. Apparently, the
nanorods pack into a 2D hexagonal columnar lattice, colh,
with increasing lattice constants at higher generations, that
is, 3.36 nm (G-1g), 5.10 nm (G-2g), and 5.66 nm (G-3g). If we
further assume a dense packing of the nanorods, their radii
are given by Rc =a/2, that is, 1.68 nm G-1g), 2.55 nm (G-2g),
and 2.83 nm (G-3g). When comparing these values with the
radii of the nanorods found below LCST (see Table 1), we
find that the radii of the nanorods in the solid state are
larger than in solution. This apparent increase of the radii is
most likely a consequence of the dehydrated chiral oligo
(ethylene glycol) chains, which provide stronger scattering
contrast in the solid as compared to their hydrated state in
solution. This leads to the observed slight underestimation
of the radii when using a homogeneous core nanorod model
for the solution structure. The coherently scattering domain
sizes of the colh phase are 311 nm (G-1g), 15 nm (G-2g), and
13 nm (G-3g), as derived from the width of the (1.0) peak.
The comparatively large value of the domain sizes of the
first generation dendrimer is probably due to a rapid paral-
lel alignment of its thin nanorods during precipitation. How-
ever, detailed investigations will be necessary to correlate
precipitation conditions and colh ordering in the bulk and on
surfaces.
Our work shows how shape-persistent flat dendrimers can
be realized using non-covalent interactions in poly(triazole-
pyridine) dendrimers prepared via click chemistry. Function-
alization of the dendrimer periphery provides access to disc-
like amphiphiles, which assemble into cylindrical stacks in
water and furthermore undergo a thermosensitive phase
transition from a nanorod to a colh phase. Modulation of the
hydrophobic surface area of the dendrimer discs by growing
different generations allows for fine-tuning of the dimen-
sions of the nanorods and resulting aspect ratio as well as
adjustment of the associated phase transition temperatures.
With increasing generation number and hence association
strength, the dissociation kinetics are slowed down signifi-
cantly, thereby leading to a pronounced hysteresis of the
CD signal, that is, chiral memory effect. Studies to elucidate
the role of chirality in the assembly process are currently in
progress. The demonstrated control over colh structure for-
mation should provide new opportunities for creating well-
defined and highly ordered self-assembled water-borne
nanostructures. Incorporating additional function in combi-
nation with their inherent chirality renders these assemblies
potentially useful templates for aqueous processes, for ex-
Keywords: click chemistry · Cotton effect · dendrimers ·
hierarchical self-assembly · thermochemistry
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Received: February 8, 2012
Published online: March 29, 2012
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