Photoresponsive amphiphiles based on azobenzene-dendritic glycerol conjugates show switchable transport behavior

Article abstract of DOI:10.1039/c1cc11673h

The synthesis and aggregation behavior of photo-switchable, nonionic dendritic amphiphiles was investigated with regard to transport and release of guest molecules. The correlation between the critical micelle concentration and the switching ability is shown.

Full text of DOI:10.1039/c1cc11673h

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COMMUNICATION
Photoresponsive amphiphiles based on azobenzene-dendritic glycerol
conjugates show switchable transport behaviorw
¨
Christian Kordel, Chris S. Popeney and Rainer Haag*
Received 23rd March 2011, Accepted 21st April 2011
DOI: 10.1039/c1cc11673h
The synthesis and aggregation behavior of photo-switchable,
nonionic dendritic amphiphiles was investigated with regard to
transport and release of guest molecules. The correlation
between the critical micelle concentration and the switching
ability is shown.
molecule, which is irreversible or can lead to other undesired
effects. Another promising approach is the light-induced
isomerization of azobenzenes.9 This photo-switch has been
used for controlling the expression of genes or the function of
proteins10 as well as in liquid crystals11 or for molecular
switches on surfaces.12
The thermodynamically favored azobenzene trans isomer
can be switched to the cis isomer within seconds by light,
usually at UV wavelengths of 350 nm. Back-switching to the
trans isomer can be achieved either thermally or by light at
longer wavelengths. Abe et al. reported on the light-triggered
change in micelle formation of an azobenzene derivative
substituted with ammonium bromide as a polar head group
and a short aliphatic chain.13 However, the use of ionic
amphiphiles as a polar group is problematic for potential
applications like additives for drugs or in solution with various
pH values. In this communication, a new class of switchable
non-ionic amphiphiles based on glycerol dendrons have been
employed, which form highly defined micelles.14 Furthermore,
by choosing different generations of the dendrimer the polarity
and the size of the headgroup can be tuned. Due to the large
change in geometry of the azobenzene–dendritic glycerol
conjugates upon irradiation of light, the aggregation behavior
is expected to change significantly, which should lead to lower
aggregation (higher cmc) of the amphiphiles in the cis state as
compared to the trans isomer (Fig. 1). For comparison we
synthesized a polymeric amphiphile made from polyethylene
glycol with a molecular weight of 500 g molÀ1 (mPEG500).
Nowadays the concept of drug delivery is of great interest
because the application of many drugs suffers from low
water-solubility and short half-life in the bloodstream.
Because these drugs are only applicable in low concentrations,
they have low pharmaceutical potential. The development of
new drugs that do provide water-solubility and an adequate
residence time is often not practical due to the time and
resources needed in development and testing. The use of
additives is an alternative to improve the solubility of existing
drugs. For example, the hydrophobic antitumor drug Taxols
is used in combination with a polyethoxylated castor oil called
Cremophors. These additives, however, produce unexpected
effects, such as hypersensitivity reactions1 and peripheral
neuropathy.2 Thus, recent research has concentrated on the
development of biocompatible micelles,3 which are able
to incorporate guest molecules and improve their overall
solubility in pharmaceutical preparations. Grinstaff and
co-workers have reported on dendritic amphiphiles from
glycerol, succinic acid and myristic acid.4a,b Our group has
recently developed new dendritic amphiphiles based on second
generation glycerol dendrons, which form monodisperse
micelles and show
a significantly higher solubilization
compared to other nonionic surfactants.5 These dendritic
glycerol amphiphiles show exceptionally low critical micelle
concentrations (cmc) of B10À6 M. In addition to the enhancement
of solubility, a triggered release of the guest molecules is highly
desired because the pharmaceutical potential of drugs can be
further increased and side-effects decreased. Attempts have
been made to disrupt micellar structures with different external
stimuli like temperature,6 pH,7 or redox processes.8 Most of
these methods involve the degradation of the amphiphilic
Freie Universitat Berlin, Institut fur Chemie und Biochemie,
¨
Takustrasse 3, 14195 Berlin, Germany.
¨
E-mail: haag@chemie.fu-berlin.de; Fax: +49 3083853357
w Electronic supplementary information (ESI) available: Experimental
procedures, NMR data, DLS, SLS and UV/Vis spectra. See DOI:
10.1039/c1cc11673h
Fig. 1 Proposed micelle disruption by light at a concentration
between cmc (trans) and cmc (cis).
c
6584 Chem. Commun., 2011, 47, 6584–6586
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Table 1 Characterization of the amphiphiles
PSSa Particle
(%)
cmc ratio
cis/trans
Compound
G2azoC11
sizeb
PDI cmc in mol lÀ1
50
9.2 nm
0.2
trans: 4 Â 10À5
2
cis: 8 Â 10À5
G3azoC11
G2azoC16
G3azoC16
70
20
50
7.2 nm
0.08 trans: 7 Â 10À6 11
cis: 8 Â 10À5
10.2 nm 0.02 trans: 6 Â 10À6
cis: no change
—
9.4 nm
0.1
trans: 4.4 Â
2
10À6
cis: 8.5 Â 10À5
mPEGazoC11 10
205 nm 0.28 trans: 3.1 Â
—
10À5
cis: 3.0 Â 10À5
a
b
Photostationary state. DLS by intensity, for spectra see ESI.w
Fig. 2 Structures of the four glycerol amphiphiles.
number of amphiphiles with a similar structure: glycerol
dendrons connected to an aliphatic chain by an aromatic
moiety.14 The resulting spherical micelles of [G3] contained
only a third of the molecules compared to the micelles of the
[G2] amphiphile, which indicates [G3] amphiphiles are more
loosely packed due to the larger size of the headgroups.
The [G3] micelles are more defined even though the
amphiphile head group is larger, which leads to more space
in the interior.14 This phenomenon led, in our case, to less
steric hindrance of azobenzene switching. Thus, the ability to
switch from trans to the cis isomer was higher. The same
phenomenon occurred with the examined C16 amphiphiles,
where the pss increased from 20% for [G2] to 50% for [G3].
The pss was lower than for the amphiphiles with a C11 chain
as a hydrophobic unit, which can be explained by a higher
aggregation number and inhibition due to more densely
packed micelles (Table 1).
In this case, due to the flexible linear arrangement, less change
in the aggregation behavior (cmc) is expected upon photo-
isomerization for the PEG-based amphiphile.
The photoresponsive amphiphiles were synthesized starting
from 4-aminophenol, which was treated under standard
diazotation conditions and then reacted with phenol under
basic conditions to yield 4,4-dihydroxyazobenzene (see ESIw).
The hydrophobic building block was attached to the azobenzene
by etherification with the chosen aliphatic chain (C11 and
C16). The hydrophilic units, glycerol dendrons of Generation
[G2] and [G3], were synthesized as reported previously.15
Their attachment to the azo switch was carried out by
etheration with mesylated acetal-protected glycerol dendrons
under basic conditions. Subsequent removal of the acetal
protecting groups resulted in the four azobenzene-containing
amphiphiles (Fig. 2). The same procedure was carried out for
the ether formation of mPEG(500) with the hydrophobic
building block.
These observations were further supported by the particle
size measurements from dynamic light scattering (DLS). The
micelles formed by G2azoC11 had a diameter of 9.2 nm,
whereas compound G3azoC11 with the higher PG dendron
formed more defined aggregates with 7.2 nm. The size of
the G3azoC11 micelles is in excellent agreement with the
theoretical value of 7.3 nm of two molecules in a row for the
diameter. In addition, the size distributions revealed
well-defined particles because the intensity curve showed a
monomodal behavior. The molecular weights of the resulting
micelles can also be measured by static light scattering. For the
trans-G3azoC11 an average molecular weight of the micelles
of 98.1 Æ 6.8 kDa was determined, which indicated there were
66 Æ 5 molecules per micelle. After switching to the cis isomer
by light a higher molecular weight was measured. According
to the SLS measurements the aggregates were then composed
of 97 Æ 14 molecules. Taking into account the resulting
mixture of cis and trans isomers which form at the pss, the
higher aggregation number suggests a formation of ill-defined,
mixed micelles.16 The change in geometry during the switching
process should also affect micelle stability and, accordingly,
the critical micelle concentration (cmc), which was determined
by the pendant drop method. The cmc for G2azoC11 was
doubled to 8 Â 10À5 M upon irradiation with UV light. The
effect of isomerization was much higher for the G3azoC11,
where the cmc increased by a factor of ten. The large influence
The photoisomerization was examined using simple UV/Vis
adsorption spectroscopy. The UV/Vis spectra of the compound
G2azoC11 illustrate the light-induced switching (Fig. 3). The
absorbance at 350 nm decreased after 60 minutes to 50% of
the former intensity, which means approximately half of the
trans isomers were switched to the cis compound. For the
compound bearing the next higher glycerol generation,
G3azoC11, the photo-stationary state (pss) was increased to
70%. The higher ability to switch can be explained by the
lower aggregation number of the micelles bearing [G3] as
compared to [G2]. Trappmann et al. determined the aggregation
Fig. 3 UV/Vis spectrum of the trans (black) and the partially
switched (grey) G2azoC11.
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can be explained by the increasing effectiveness of switching.
The cmc values for the two C16 compounds in the trans state
were 2.5 Â 10À5 M and 4.4 Â 10À6 M for G2azoC16 and
G3azoC16, respectively, which are lower than the corresponding
C11 amphiphiles. For the [G2] moiety there was no detectable
difference in cmc upon irradiation of light (350 nm).
Corresponding UV/Vis adsorption measurements revealed
not only the above-mentioned low pss of 20%, but also a fast
thermal back-switching with a half-life of two hours. Thus, the
resulting change was too small to be measurable. In the case of
the G3azoC16 the cmc doubled upon photo-switching,
which corresponds, perhaps noncoincidentally, to the same
pss as G2azoC11 of 50%.
spectrum because of isomerization to the cis isomer, which
resulted in an increase in the cmc above the chosen concentration
of 1 Â 10À5 M. We conclude, based on these observations, that
a light attenuated uptake of the dye has occurred. To exclude
the possibility of photo-bleaching of the dye, a solution of Nile
Red in methanol was kept under 350 nm irradiation for the
same time and resulted in no decrease of Nile Red adsorption.
In this work we have shown the synthesis of a new class
of light switchable dendritic amphiphiles. All four of the
PG-based amphiphiles formed well-defined spherical micelles
and offered a higher ability to undergo trans–cis photo-
isomerization than the corresponding linear mPEG amphiphile,
which formed rather large ill-defined aggregates. The size of
the micelles influenced the photo-stationary state: the smaller
the micelles the higher the pss. These findings are based on the
larger empty space between the molecules in smaller micelles
which improve the photo response better than densely packed
larger aggregates. Because the critical micelle concentration is
effectively changed by light, these amphiphiles can be used for
controlling the solubilization and encapsulation of a guest
molecule. The effectiveness of the switching in modulating the
aggregation behavior of the amphiphiles is highly influenced
by the pss. A high pss led to a larger change in the cmc and a
larger overall effect on the solution behavior. In summary, a
new nonionic switchable delivery system has been developed,
which will be investigated for controlled release of active
agents.
We also tested azobenzenes substituted with a linear
polyether function, namely mPEG with a molecular weight
of 500 g molÀ1 that was chosen because the molecular weight
is similar to the [G2] PG dendrons and should provide similar
solubilization properties. We expected no change in cmc due
to the long flexible PEG chain which could more easily
compensate the isomerization of the neighboring azobenzene.
In fact, the ability to switch was very weak: roughly 10% of
the molecules were switched to the cis state. The cmc was also
negligibly affected upon irradiation. DLS measurements
showed large and presumably ill-defined aggregates with
sizes of ca. 200 nm. We suppose that the packing of the
mPEGazoC11 molecules is too dense for efficient switching
because of their linear geometry, which is also reflected by
their tendency to form large ill-defined aggregates that
are known for nonionic surfactant micelles.17 Again, steric
hindrance restricted the isomerization.
Notes and references
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The solubilization of a water-insoluble dye, Nile Red, was
also undertaken to further illustrate the effect of switching. At
an amphiphile concentration slightly above the cmc of the
trans G3azoC11, solid Nile Red was added to two vials. One
was kept in the dark, the other was irradiated with 350 nm
light for 24 hours with continuous stirring. After removal of
the excess Nile Red by filtration, the resulting solution of the
sample which was kept in dark remained red indicating the
solubilization of the dye (Fig. 4). The UV/Vis spectrum of the
solution exhibited typical adsorption bands for Nile Red,
whereas the other sample, which was kept under 350 nm
was the normal color of the azobenzene moiety. Here no
adsorption bands for Nile Red were seen in the UV/Vis
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Fig. 4 Left: UV/Vis spectra of the light-attenuated solubilization of
Nile Red. An uptake of the dye can be seen in the sample which was
kept in dark (grey), whereas the switched sample showed no adsorption
of the dye (black). Right: solubilization of Nile Red in a 1 Â 10À5
M
solution of G3azoC11. The left vial was kept in the dark, right vial was
kept under 350 nm irradiation.
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c
6586 Chem. Commun., 2011, 47, 6584–6586
This journal is The Royal Society of Chemistry 2011