Coexisting hydrophobic compartments through self-sorting in rod-like micelles of bisurea bolaamphiphiles

Article abstract of DOI:10.1021/ja101872x

Multiple, coexisting hydrophobic compartments have been created in water using molecular self-sorting among mixtures of bolaamphiphiles with differently spaced urea groups in their hydrophobic parts. The selective incorporation of bisurea functionalized f

Full text of DOI:10.1021/ja101872x

Published on Web 05/20/2010
Coexisting Hydrophobic Compartments through Self-Sorting in Rod-like
Micelles of Bisurea Bolaamphiphiles
Asish Pal, S. Karthikeyan, and Rint P. Sijbesma*
Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513,
5600 MB EindhoVen, The Netherlands
Received March 5, 2010; E-mail: r.p.sijbesma@tue.nl
The creation of multiple, coexisting hydrophobic compartments
in water is an important step toward systems that emulate the
structural complexity of biological cells. Discrete hydrophobic
compartments can keep mutually incompatible reagents or catalysts
apart and are, therefore, essential in developing complex chemical
systems. When amphiphilic polymers are employed to create
multiple compartments, use can be made of the incompatibility
between chemically different segments.1 However, when small
molecule amphiphiles are used, phase separation is limited to
combinations of highly incompatible molecules such as hydrocar-
bons and fluorocarbons.2 In small molecule systems, a more
versatile process is molecular self-sorting. Self-sorting entails the
ability to differentiate between self and nonself and distinguishes
itself from microphase separation by the specificity and direction-
ality of the recognition process.3 Specificity can be based on any
of a number of supramolecular interactions,4 including metal-ligand
interaction, hydrogen bonding, and ion-dipole interactions. In
aqueous systems, self-sorting via specific supramolecular interac-
tions offers an attractive alternative to microphase separation for
creation of separate, coexisting aggregates. For instance, self-sorting
via hydrogen bonding has been used in the orthogonal formation
of self-assembled fibers and micelles or vesicles.5,6 However, the
formation of multiple hydrophobic compartments by means of self-
sorting has not been reported. Here, we demonstrate that self-sorting
among mixtures of bisurea bolaamphiphiles (UnU) results in rod-
like micelles that form multiple hydrophobic environments. In
hydrogen bond mediated self-assembly, the urea group is one of
the most reliable motifs, which has been used in self-sorting of
small molecules and in self-assembly of gels,7 surfaces,8 sol-gel
materials,9 and thermoplastic elastomers.10,11 Bisurea motifs in the
hard block of the elastomers were shown to allow selective
incorporation of matching bisurea guest molecules10 and to allow
the self-sorting of dissimilar hard segments in a mixed polymer
matrix.11 Self-sorting in these mixed matrices was studied by
making use of the fluorescent probes Py-UnU and DMA-UnU,
which display exciplex emission when they are in molecular contact.
Recently, we have shown that the bisurea based bolaamphiphile
U7U self-assembles in water to form rod-like micelles.12 The
bisurea motif in U7U was shown to specifically bind probe
molecules with an identical hydrogen bonding unit and to lead to
unfolding of the bisurea probe molecule, which is present in a folded
conformation in the absence of U7U. We now use the highly
selective incorporation of Py-UnU and DMA-UnU probes in bisurea
bolaamphiphiles to show that the micelles dynamically coexist in
solution, each micelle specifically binding to its correspondingly
functionalized guests.
determined from the ratio I1/I3 of the vibronic bands of pyrene.
With increasing spacer chain length, the cmc’s decrease from 3.22
× 10-5 M to 9.1 × 10-6 M, 5.6 × 10-6 M, and 3.6 × 10-6 M for
U3U, U4U, U6U, and U7U respectively.
Cryo-TEM images of 1 wt % micellar solutions of the three stable
amphiphiles showed rod-like structures (dcore ≈ 8 nm), although
the detailed behavior of the rods depended on the spacer length.
U4U formed rods in a compact arrangement (Figure 1a) whereas
the solution of U7U showed a random arrangement of rods (Figure
1b).
Figure 1. Cryo TEM images of 1 wt % micellar solutions of (a) U4U and
(b) U7U.
The behavior of U4U rod-like micelles was studied further by
fluorescence spectroscopy using the pyrene probe Py-U4U and the
dimethylaniline probe DMA-U4U. The pyrene probe molecules
were shown to be more or less randomly dispersed in the micelles
of corresponding U4U amphiphiles, since a band due to excited
state pyrene dimers (typical λmax ≈ 480 nm) was absent when 0.01
equiv (relative to U4U) or less of Py-U4U was added. The dynamics
of these micelles was studied by monitoring the intensity of the
exciplex band that formed when the DMA-U4U probe was added
Oligoethyleneglycol-bisurea bolaamphiphiles U3U, U4U, U6U,
U7U formed stable micelles upon dissolution in water (up to ∼3%
w/v), whereas U12U was insoluble and was not studied further.
Critical micellar concentrations (cmc’s) of the amphiphiles were
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10.1021/ja101872x  2010 American Chemical Society

C O M M U N I C A T I O N S
(Scheme 1). Typically, such a band is observed if the pyrene and
dimethylaniline chromophores are in molecular contact.11
U6U micelles (98 and 94%, respectively), whereas the mixture of
U4U and U3U showed moderate self-sorting (55%).
Finally, a mixture of three bisurea amphiphiles U4U-U6U-U7U
was used to probe the possibility of creating more than two
coexisting micellar compartments in a single solution by self-sorting.
For these measurements, one Py-UnU probe and two nonmatching
DMA-UnU probes were added to the ternary mixture of micelles.
Py-U4U exhibited the highest self-sorting (97%), while Py-U6U
and Py-U7U showed 83% and 73% self-sorting respectively (see
SI, Figure S14).
Scheme 1. Cartoon of the Use of Exciplex Fluorescence To Probe
Self-Sorting in the Bisurea Rod-Like Micelles
In summary, oligoethyleneglycol-bisurea bolaamphiphiles, dif-
fering only in the spacing between urea groups, show remarkably
effective self-sorting in aqueous solution, leading to coexisting rod-
like micelles with up to 98% demixing of probes in binary solution.
The high specificity of the self-sorting even allows the formation
of more than two coexisting micellar phases in a single solution. It
holds great potential for chiral self-sorting, in keeping apart
incompatible catalysts or reagents, and for the suppression of
backfolding in supramolecularly cross-linked gels.
In 4 mM aqueous solutions of U4U containing either 0.01 equiv
of Py-U4U or 0.15 equiv of DMA-U4U and 2% of DMSO or DMF
used to add the probes,13 an exciplex band was absent. However,
upon mixing these solutions, a band at 520 nm appeared and grew
over time with a first-order rate constant of 1.8 × 10-3 s-1 (Figure
2a, black line), demonstrating the dynamic nature of the micelles.
Interestingly, when the same experiment was performed with a
solution in which the DMA-U4U probe was taken up in U6U
micelles, the rate constant of exciplex formation was the same, but
the final emission intensity was twice as high (Figure 2a, red line),
showing that the probes are confined to half the micellar space.
However, when nonmatching micelles U4U (containing 0.01 equiv
of Py-U4U) and U6U (containing 0.15 equiv of DMA-U6U) were
mixed, hardly any exciplex formation was observed (blue line).
These observations suggest that U4U and U6U bolaamphiphiles
form separate rod-like micelles and that the probes are confined to
their matching micelles.
Acknowledgment. The authors thank Laura Brylka and Dr.
Nico Sommerdijk for their kind help with cryo-TEM measurements.
This research forms part of the Project P1.04 SMARTCARE of
the research program of the BioMedical Materials Institute,
cofunded by the Dutch Ministry of Economic Affairs. The financial
contributionoftheNederlandseHartstichtingisgratefullyacknowledged.
Supporting Information Available: All the experimental proce-
dures, compound characterization data, TEM images, and fluorescence
data. This material is available free of charge via the Internet at http://
pubs.acs.org.
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