Photoregulated release of noncovalent guests from dendritic amphiphilic nanocontainers

Article abstract of DOI:10.1002/anie.201006193

Supramolecular disassembly: Phototriggered release of noncovalently sequestered lipophilic guest molecules from dendritic supramolecular assemblies has been demonstrated. Facially amphiphilic dendrimers have been shown to provide a unique opportunity to f

Full text of DOI:10.1002/anie.201006193

Communications
DOI: 10.1002/anie.201006193
Dendritic Micelles
Photoregulated Release of Noncovalent Guests from Dendritic
Amphiphilic Nanocontainers**
Volkan Yesilyurt, Rajasekharreddy Ramireddy, and S. Thayumanavan*
Dedicated to Professor Peter Beak on the occasion of his 75th birthday.
The design of molecular systems with stimuli-sensitive
properties is of great interest for applications ranging from
drug delivery to gene transfection.[1] The use of these
molecular systems in drug-delivery applications has been
enhanced by imparting amphiphilic character to these
designs, as these systems are capable of self-assembling into
various supramolecular architectures such as micelles and
vesicles, and thus provide interiors that can encapsulate guest
molecules noncovalently.[2] In this context, significant effort
has been devoted to amphiphilic polymers with stimuli-
sensitive elements because they are able to 1) form stable
micelles, thus providing interiors that can sequester lipophilic
guest molecules noncovalently, and 2) release guest molecules
in response to both external and internal stimuli such as
light,[3] pH,[4] temperature,[5] and reduction/oxidation.[6] Since
dendrimers can be obtained with a high degree of control over
their polydispersity and size,[7] it is fundamentally interesting
to investigate stimuli-sensitive characteristics in these
branched macromolecules. Incorporation
esting, small-molecule-based micelles exhibit rather high
critical aggregate concentrations (CACs) and low inherent
stabilities. Therefore, light-induced guest release has also
been performed with polymer-based micelles.[3,12] To the best
of our knowledge, there is no prior report on the light-induced
disassembly of dendrimer-based micelles, accompanied by the
release of a lipophilic guest molecule.
Recently, we reported a unique class of amphiphilic biaryl
dendrimers in which every repeating unit in the dendritic
backbone contains both lipophilic and hydrophilic functiona-
lities.[8f,13] We have shown that these facially amphiphilic
biaryl dendrimers form micelle-type aggregates in water and
inverted micelle-type aggregates in apolar solvents such as
toluene. The micellar aggregates from our dendrimers are
formed through aggregation of several dendrimer molecules
and can sequester lipophilic guest molecules. The ability of
these molecules to assemble and bind guest molecules is
dependent on their hydrophilic–lipophilic balance (HLB).[14]
of stimuli-sensitive characteristics into
dendrimers has been relatively underex-
plored.[8] In particular, amphiphilic dendri-
mers[8f,9] with stimuli-responsive properties
would significantly expand the scope of
these molecules in a variety of applications.
Herein, we describe the design and syn-
theses of dendritic micelles that can release
their guest molecules in response to a light
stimulus.
Light-induced release of guest mole-
cules is interesting, because it provides a
pathway for releasing a molecule with a
remote control, that is, an external physical
stimulus.[3,10] Light-induced release of lip-
ophilic guest molecules from small-mole-
Figure 1. Schematic representation of the light-induced disassembly of dendritic micellar
assembliles.
cule surfactant aggregates has been
reported previously.[3c,11] While this is inter-
We hypothesized that a change in the HLB in response to
light would result in alterations in the amphiphilic assembly,
which should concurrently effect release of guest molecules
(Figure 1).
[*] V. Yesilyurt, R. Ramireddy, Prof. S. Thayumanavan
Department of Chemistry, University of Massachusetts
Amherst, MA 01003 (USA)
Fax: (+1)413-545-4490
To test this hypothesis we have designed and synthesized
amphiphilic dendrimers with a photolabile 2-nitrobenzyl ester
moiety as the lipophilic unit (Scheme 1). 2-Nitrobenzyl esters
have been widely used as photolabile groups.[15] The hydro-
philic part of these facially amphiphilic dendrons is based on
oligoethylene glycol units. Our molecule is designed in such a
E-mail: thai@chem.umass.edu
Homepage: http://www.umass.edu/thaigroup/
[**] We thank the NIGMS of the National Institutes of Health, U.S Army
Research Office, and NSF-MRSEC for support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006193.
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fashion that the light-induced cleavage of the nitrobenzyl
ester disconnects a significant part of the lipophilic chain from
the dendrimer. Moreover, the functionality—a carboxylic
acid—in the product, generated on the dendron side of the
molecule, is significantly hydrophilic (Figure 1). We envisaged
that this transformation should result in a significant change
in the HLB and thus cause the supramolecular assembly to
release its guest molecules.
The structures of the targeted light-sensitive G1 and G2
dendrons are shown in Scheme 1. The dendrons were
constructed from a biaryl monomer (3 in Scheme 2), which
was synthesized from the arylstannane 1 and bromoarylester 2
by using Stille coupling as the key step (Scheme 2). Reaction
between the peripheral unit 4 and the biaryl building block
unit 3 in the presence of potassium carbonate afforded the
dendron 5 in 80% yield. Similarly, the corresponding G2
acetylene dendron was synthesized from 3 and the bromi-
nated version of the G1 dendron 5. Attachment of the
photolabile nitrobenzyl moiety 8 by a Huisgen 1,3-dipolar
cycloaddition reaction (click chemistry) led to the targeted
G1 and G2 dendrons.[16] All dendrons were characterized by
1H NMR and 13C NMR spectroscopy as well as MALDI-TOF
mass spectrometry; details of the synthesis and character-
ization data are outlined in the Supporting Information.
First, we investigated the micellar behavior of these
dendrons in the aqueous phase by encapsulating a hydro-
phobic dye, Nile red. Nile red is not soluble in water, unless it
is accomodated in a hydrophobic pocket of micellar aggre-
gates. Emission spectra of Nile red in the presence of various
concentrations of G1 and G2 dendrons were used to calculate
the CACs of these dendrons,[16] as about 18 and 20 mm,
respectively. Dynamic light-scattering (DLS) experiments
Scheme 1. Structures of the photocleavable G1 and G2 dendrons.
Scheme 2. Synthetic scheme for the photolabile G1 dendron (18-cr-6=[18]crown-6; MOM=methoxymethyl; PEG-OTs=tosylate of pentaethlylene-
glycol monomethyl ether).
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further verified the formation of micellar aggregates from the
G1 and G2 dendrons. The micellar aggregates formed by G1
and G2 dendrons are about 80 and 85 nm in diameter,
respectively, thus indicating that these dendrons are able to
form micelle-like aggregates in water. The light-triggered
disassembly of dendritic micellar aggregates was first inves-
tigated by monitoring the change in the emission spectrum of
Nile red. When a 55 mm solution of Nile red encapsulated G1
dendron was irradiated at a wavelength of 365 nm we
observed a systematic decrease in the emission intensity of
Nile red over time, thus indicating disassembly of the micelle
and the concomitant release of Nile red from the interiors of
the dendritic micelles (Figure 2a). The total amount of dye
Figure 3. a) Size evolution of 55 mm solutions of the G1 and G2
dendrons upon irradiation with UV light, b) comparision of dye release
with the photolabile G1 and G1-control dendrons, c) structure of the
G1-control dendron, d) sizes of the G1-control dendron before and
after UV irradiation.
Next, we were interested in evaluating the size evolution
of the dendritic micellar aggregates by using DLS (Figure 3a).
The size of the aggregates was found to decrease from about
80 to 37 nm upon irradiation. This result indicates that there is
some residual nanoscale assembly in the aqueous phase, even
after the photochemical reaction. The change in size, how-
ever, shows that there is certainly a change in the nature of the
supramolecular assembly. The DLS data, combined with the
fact that we have effected a significant release of lipophilic
guest molecules, suggest that the dendrimer has been
converted from an amphiphilic into a significantly hydrophilic
structure. Double hydrophilic macromolecules have been
observed previously to assemble into core–shell structures
and vesicles.[17] The 37 nm diameter of the residual hydro-
philic dendrimer can be rationalized on the basis of similar
arguments. However, the precise nature of the assembly could
not be readily discerned at this time.
Ultimately, we were interested in testing whether the
decrease in the emission intensity of Nile red is solely due to
the release of the dye molecules from the micellar interior
upon exposure to light. For this purpose, we synthesized a
first-generation control dendrimer, G1-control, which lacks
the photocleavable functionalities (Figure 3c). We hypothe-
sized that there should not be any change in the fluorescence
of Nile red upon irradiation with UV light at a wavelength of
365 nm if the light does not have any effect on the electronic
properties of the dye molecule that cause a change in the
emission spectrum. We were gratified to find that exposure of
a 55 mm solution of Nile red containing G1-control dendron to
UV light of a wavelength of 365 nm caused less than 5% guest
release, compared to 88% release with the photolabile G1
dendron (Figure 3b). This finding supports our hypothesis
that the decrease in fluorescence obtained with the photo-
Figure 2. a) Release of Nile red from a 55 mm micellar solution of the
G1 dendron upon irradiation with UV light for different time intervals
(0–200 s), b) release of Nile red from the G1 and G2 dendrons upon
irradiation with UV light, c) UV/Vis spectra of the G1 dendron upon
irradiation with UV light for different time intervals (0–380 s), d) plot
of the absorbance at 320 nm, which illustrates cleavage of the photo-
labile ester bond.
released, after 200 seconds, was about 88%. When a similar
experiment was carried out with the G2 dendron, a 72%
release of the guest molecules was observed (Figure 2b). The
smaller amount of Nile red released from the G2 dendron
compared to that from the G1 dendron is likely due to the
more tightly packed nature of the assembly generated from
the cleaved form of the higher generation G2 dendron. The
difference in the slopes of the lines in Figure 2b also indicates
that a generation-dependant controlled release of the guest
molecules can be obtained with these dendrons. The cleavage
of photolabile ester groups was further verified by UV/Vis
spectroscopy. It is known that cleavage of 2-nitrobenzyl esters
leads to the formation of a by-product, 2-nitrosobenzalde-
hyde, which weakly absorbs at 360 nm and hence can be
detected with absorption spectroscopy. Irradiation of the G1
and G2 dendrons with UV light at a wavelength of 365 nm
resulted in a decrease in the intensities of the absorption at
320 nm and a concomitant increase at 360 nm over time, thus
indicating cleavage of the photolabile ester bond and the
formation of the by-product (Figure 2c and 2d).
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labile G1 and G2 dendrons is indeed due to release of Nile
red. Moreover, we carried out DLS studies with the G1-
control dendron to determine whether UV light has any effect
on the size of the aggregate, which may result in leakage of the
guest molecule. We indeed found that exposure of the G1-
control dendron to UV light did not cause any change in the
size of the aggregate (Figure 3d), which indicates that UV
light does not cause our biaryl dendrimers to undergo any
structural change.
In summary, we have designed and synthesized light-
sensitive facially amphiphilic dendrimers that can form
micellar aggregates in water. The hydrophobic part of these
dendrons consists of photolabile ester groups, which are
susceptible to cleavage by UV light. We have shown that
light-induced cleavage of the hydrophobic ester groups
caused the dendrimers to lose their HLB, thereby resulting
in dissociation of the micellar aggregates. Since the facially
amphiphilic dendrimers provide the opportunity to sequester
lipophilic guest molecules noncovalently, the light-induced
supramolecular disassembly provides an opportunity to
demonstrate photosensitive release of noncovalently seques-
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Received: October 3, 2010
Revised: December 30, 2010
Published online: February 25, 2011
Keywords: amphiphiles · dendrimers · micelles ·
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supramolecular disassembly
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