C O M M U N I C A T I O N S
particle sizes of 4.2 and 2.3 nm, respectively, at 10-5 M concentra-
tion in toluene. This result suggests that, while the didendron 5
could act as a unimolecular container, the monodendron 3 seems
to afford an aggregated particle. Considering that these dendrimers
have similar molecular weights, the reason for the difference in
aggregation state is not clear at this time.
In aqueous solutions, we found that even the third-generation
dendrimer 6 formed an aggregate with average particle size of 25
nm (10-4 M) or 22 nm (10-5 M). Similarly, dendrimers 3 and 5
afforded aggregates of about 42 and 28 nm, respectively. Note that
the particle sizes in micelle-type structures are larger for all
dendrimers compared to the inverse micelle-type assemblies
obtained in toluene. This result could be due to the difficulty in
packing the bigger decyl chains in the interior in this case, compared
to the smaller carboxylic acid functionalities in the toluene solution.
Also, it is likely that the smaller dendrimers formed aggregates to
exhibit the container properties. However, we experienced difficul-
ties in obtaining unambiguous DLS measurements with these
dendrimers.
In summary, we have shown that the amphiphilic dendrimers
reported here are capable of forming either hydrophobic or
hydrophilic nanocontainers, depending on the solvent environment.
Whether these containers are unimolecular or an aggregate seems
to depend on the size of the functionality in the interior as well as
the size of the dendrimer itself. Ultimately, we are interested in
arriving at structures where diverse functionalities could be
converged in the hydrophobic interiors of the dendrimer in water.
Such multifunctional macromolecules are the first step toward
structurally complex biomimetic assemblies.
Figure 2. Absorption spectra of (Left) Reichardt’s dye in H2O. (Right)
Proflavine‚HCl in toluene in the presence of dendrimers 1-6 (10-4 M).
Using the extinction coefficient of Reichardt’s dye in glycerol,
the number of these guest molecules incorporated in the dendrimers
was estimated. There is clear generation dependence in the
capability of these dendrimers to sequester guests (Figure 2, left).
The third-generation dendrimer 6 was able to encapsulate about
8.5 molecules of Reichardt’s dye. The second-generation dendrimer
5 and the third-generation monodendron 3 were able to acquire
about 3.2 and 2.8 dye molecules, respectively. Note that even the
first-generation monodendron 1 was able to sequester 0.6 dye
molecules in its interior, which is likely to be the result of an
aggregation.
To investigate the inverted micelle-like behavior of these macro-
molecules, we incorporated a hydrophilic dye within the dendritic
interiors in an apolar solvent. The dendrimers 1-6 were all soluble
in most of the common organic solvents. We solubilized the
dendrimers in toluene and added the hydrophilic dye, proflavine.10
Proflavine is not soluble in toluene by itself in the absence of the
dendrimers. In the presence of any of the dendrimers 1-6,
proflavine was solubilized in toluene (Figure 2, right). We attribute
this to the combination of the fact that the dendritic interior is polar
and the possibility of specific acid-base interaction between the
carboxylic acid units of the dendrimer and the amine units of the
guest molecule. Interestingly, the number of molecules incorporated
in the hydrophilic dendritic interiors is much smaller than those
observed with the Reichardt’s dye in a hydrophobic container. For
example, the third-generation dendrimer 6 sequestered an average
of only 1.5 molecules. It is possible that the size of the nano-
container is dictated by the size and nature of the functionality that
is directed toward the interior. In toluene, the smaller and self-
complementary carboxylic acid units seem to form small containers
that acquire fewer guest molecules. While these macromolecules
are clearly capable of providing containers with polar or apolar
interiors, we have not unambiguously identified the spatial disposi-
tion of the amphiphilic functionalities within each layer of the
dendrimer. This is a subject of current investigation in our
laboratories.
Acknowledgment. Support from the NIGMS of the National
Institutes of Health is gratefully acknowledged.
Supporting Information Available: Synthetic and other experi-
mental details. This material is available free of charge at http://
pubs.acs.org.
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It is also interesting to investigate whether these container
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concentrations at which the dye studies above were carried out. At
10-5 M concentration toluene solution of the third-generation
dendrimer 6 exhibited particles with an average size of about 2.8
nm, whereas a 10-4 M solution afforded particles with an average
size of about 44 nm. Note that this dendrimer exhibits the ability
to sequester dye molecules at 10-5 M concentration also. Therefore,
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hydrophilic container in toluene. Dendrimers 3 and 5 exhibited
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