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Figure 2. (A) Fluorescence emission spectra of binary micellar solution
containing bisurea guest molecules. Black, red, and blue lines indicate
Py-USU, Py-USU with 15 mol % of DMA-USU, and Py-USU with 15
mol % of DMA-URU in mixed micelles of USUÀURU (2 mM each).
The excitation wavelength used was 347 nm, and spectra are normalized
to the peak at 377 nm for comparison. (B) Intensity of the exciplex band
at 490À510 nm as a function of time upon mixing of micellar solutions
containing Py-USU with micellar solutions containing DMA-USU or
DMA-URU. Data have been fitted with first-order kinetics (see text).
Figure 3. (A) FRET of binary micellar solution containing bisurea guest
molecules. Emission spectra of 0.25 mol % of Py-USU and 0.5 mol % of
Naph-USU and Naph-URU in mixed micelles of USUÀURU. The
excitation wavelength used was 290 nm, and spectra are corrected for
direct pyrene excitation. (B) Comparison of the ratios of integrated
intensities between 375À430 nm and 320À345 nm of the USUÀURU
binary micelles containing matching and nonmatching naphthalene and
pyrene probes molecules.
indicative of considerably lower amount of energy transfer in the
15 mol % of DMA-URU to the mixture did not result in exciplex
formation (Figure 2A, see also SI). Therefore, we conclude that
the probes recognize their matching amphiphiles which are self-
sorted to form separate coexisting micelles. The extent of probe
self-sorting was quantified by analyzing the exciplex band in-
tensities in different binary mixtures and is given on a scale of 0%
(for the exciplex intensity in a fully mixed system of matching
probes) to 100% (when the exciplex band is absent). A value of
80% of self-sorting between USU and URU micelles was
determined under the conditions described above (see SI).
In 2 mM aqueous solutions of USU containing either 0.5 mol %
of Py-USU or 15 mol % of DMA-USU an exciplex band was
absent. However, upon mixing these solutions, a broad band at
490À510 nm appeared and grew over time with a first order rate
constant of 7.1 Â 10À4 sÀ1 (Figure 2B, red squares), demonstrat-
ing the dynamic nature of the micelles. However, when nonmatch-
ing micelles USU (containing 0.5 mol % of Py-USU) and URU
(containing 15 mol % of DMA-URU) were mixed, hardly any
exciplex formation was observed (black squares). These observa-
tions suggest that USU and URU bolaamphiphiles form separate
rodlike micelles and that the probes are confined to their matching
micelles.
nonmatching system. The corresponding FRET ratios (I375À430
/
I320À345) for the matching and nonmatching FRET pairs are
shown in Figure 3B.
In summary we have demonstrated the formation of segre-
gated enantiomeric compartments in water, from the self-sorting
of chiral trans-1,2-bisureido cyclohexane-based bolaamphiphiles.
The use of both exciplex and FRET is shown to be a versatile
methodology to probe chiral self-sorting. Additionally, being
more sensitive, FRET allows a large reduction in loading of the
micelles with probe molecules. We are currently investigating the
use of FRET probes as an alternative method to quantitatively
determine the degree of self-sorting in this and similar systems.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
b
compound characterization data, TEM images, and fluorescence
data. This material is available free of charge via the Internet at
’ AUTHOR INFORMATION
Although exciplex formation between DMA and pyrene
probes strongly supports self-sorting of micelles, the high loading
with 15% DMA probes may perturb the system. Therefore, we
resorted to the use of fluorescence resonance energy transfer
(FRET) between naphthalene probe Naph-USU (as donor) and
Py-USU (as acceptor) {F€orster radius (Ro) is 2.86 nm}. Fluor-
escence emission spectra of binary solutions of USU and URU
containing probes were recorded with an excitation wavelength
of 290 nm where the donor naphthalene absorbs. Figure 3A
shows that for the matching system (Py-USU and Naph-USU)
the intensity of characteristic pyrene fluorescence in the wave-
length region 375À430 nm increases with the increase in amount
of Naph-USU (see also SI). Increased energy transfer is attrib-
uted to the increased fraction of pyrene that is within a few
nanometers of naphthalene. In contrast to this, only a minimal
increase in intensity of pyrene emission with increasing concen-
tration of naphthalene donor occurs in the nonmatching system
of Py-USU and Naph-URU (Figure 3A, also see SI). Moreover,
the absolute intensity of characteristic naphthalene fluorescence
in the wavelength region 320À345 nm is higher in the non-
matching system than in the matching system, which is also
Corresponding Author
’ ACKNOWLEDGMENT
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, Agriculture
and Innovation. The financial contribution of the Nederlandse
Hartstichting is gratefully acknowledged. We thank NWO for
beam time at the ESRF and G. Portale, M. Gillissen, T. Mes, P.
Stals, and M. Koenigs for their help in measurement and analysis
of the SAXS data.
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dx.doi.org/10.1021/ja205345e |J. Am. Chem. Soc. 2011, 133, 12987–12989