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Hockey-Puck Micelles from Oligo(p-benzamide)-b-PEG RodЦCoil Block Copolymers.

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Angewandte
Chemie
Block Copolymers
DOI: 10.1002/anie.200503514
Hockey-Puck Micelles from Oligo(p-benzamide)b-PEG Rod–Coil Block Copolymers**
Tobias W. Schleuss, Robert Abbel, Michael Gross,
Dieter Schollmeyer, Holger Frey, Michael Maskos,
Rdiger Berger, and Andreas F. M. Kilbinger*
Rod–coil block copolymers (RCP) have recently received
significant attention[1–3] as they provide a powerful means for
the preparation of stable supramolecular architectures in
solution and in the solid state. The conformational rigidity of
one of the blocks has been achieved through the use of helical
structures like polypeptides,[4] polyisocyanates,[5] polyisocyanides,[6] and polycarbodiimides[7] as well as p-conjugated
polymers, for example, poly(p-phenylene)s,[8, 9] poly(thiophene)s,[10] and poly(phenylquinoline)s.[11]
RCP with oligomeric rod block polymers (ORCP) are of
particular interest as they may be used to form supramolecular aggregates on the nanometer-length scale that are much
smaller than those achieved with RCPs consisting of polymeric rod blocks or coil–coil block copolymers.[12] Typical
ORCP structures consist of a p-conjugated oligomer attached
to a flexible solubilizing polymer chain. Stupp et al. have
reported the supramolecular organization of a variety of
structures in which the rod segment is based on oligomeric
aromatic esters. Depending on the rod structure, ribbons[13] as
well as mushroom-shaped aggregates[14] have been observed.
Lee et al. have described supramolecular rod bundles[15] and
supramolecular reactors[16] based on ORCP.
Chirally substituted electroactive p-conjugated oligomers
have been incorporated into rod–coil architectures by Meijer
et al. There, helical supramolecular organization in solution
was investigated by circular dichroism spectroscopy and other
[*] Dipl.-Chem. T. W. Schleuss, Dipl.-Chem. R. Abbel, Dr. D. Schollmeyer,
Prof. Dr. H. Frey, Dr. A. F. M. Kilbinger
Institut f/r Organische Chemie
Johannes Gutenberg-Universit4t Mainz
Duesbergweg 10–14, 55099 Mainz (Germany)
Fax: (+ 49) 6131-39-26138
E-mail: akilbing@uni-mainz.de
Dr. R. Berger
Max Planck Institut f/r Polymerforschung
Mainz (Germany)
Dipl.-Chem. M. Gross, Dr. M. Maskos
Institut f/r Physikalische Chemie
Johannes Gutenberg-Universit4t Mainz (Germany)
[**] The authors thank U. Rietzler for help with the SFM measurements,
Prof. H.-J. Butt (Max Planck Institute for Polymer Research, Mainz)
for kindly providing access to SFM equipment, and Prof. M.
Schmidt (University of Mainz) for helpful discussions. A.F.M.K. and
R.B. thank the German Research Foundation (DFG; SFB 625, “From
Single Molecules to Nanoscopically Structured Materials”) and the
Fonds der Chemischen Industrie (FCI) for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 2969–2975
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
spectroscopic methods.[17] Many different and, sometimes
unexpected, structures have been found for RCPs over the
last few years. Layered structures,[18] arrowhead or zigzag
structures,[18] and even double-hexagonal structures[12] have
been observed in the bulk phase. Micelles,[19] vesicles,[20–23] and
ribbonlike structures[24] have also been described. Most of the
reported ORCP aggregate through relatively weak p–p
interactions. To increase the association constant in solution
or the phase stability in bulk, stronger noncovalent interactions are desirable.
We recently described the synthesis[25] and aggregation
behavior[26] of a series of oligo(p-benzamide)-b-poly(ethylene
glycol)monomethyl ethers (OPBA-MPEG) in which strong
directional hydrogen bonds are responsible for the formation
of supramolecular structures. Within the series of OPBAMPEG rod–coil block copolymers, the block copolymer
carrying the longest OPBA block (1, a hepta(p-benzamide),
see Figure 1) showed the strongest aggregation; that is, the
Figure 1. Hepta(p-benzamide)-b-poly(ethylene glycol)s 1 and 2 synthesized according to reported procedures.[25]
ratio of aggregated to nonaggregated species observed in
GPC (chloroform) traces was the highest. Aggregation was
also determined by a characteristic blue shift in the UV/Vis
spectra (H aggregates)[27] and broadening of the aromatic
resonances in the 1H NMR spectra. In hydrogen-bonddisrupting solvents like N,N-dimethylformamide (DMF) and
N,N-dimethylacetamide (DMAc) no aggregation could be
observed by any of the above techniques, clearly indicating
that hydrogen-bond formation is crucially important in the
formation of supramolecular structures. While strong and
defined aggregation was evident in these experiments, a clear
picture of the aggregation process and the nature of the
interaction forces, as well as the overall shape and geometry
of the aggregate could not be obtained.
Here we present light-scattering data and scanning force
microscope (SFM) images of high quality that show for the
first time, to the best of our knowledge, the often proposed
hockey-puck micelle for rod–coil architectures in great detail.
Dynamic light-scattering (DLS) studies of a chloroform
solution of 1 (c = 0.6 g L1) containing 6.25 C 104 m tetrabutylammonium bromide (TBABr) gave a hydrodynamic radius
of Rh = 21 nm (see the Supporting Information) and a fairly
o
[28]
narrow size distribution of the aggregates (m 90
2 = 0.04).
Addition of TBABr was necessary to suppress residual
electrostatic interaction between the micelles in solution.
The residual electrostatic interaction is most likely due to
traces of ionic impurities such as tri(p-benzamide), which
bears a carboxylic acid group. Such trace impurities, while not
detectable by standard analytical procedures, can coaggregate
with the hepta(p-benzamide) rod–coil copolymers and cause
significant electrostatic interactions in a nonpolar solvent like
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chloroform, thereby rendering the light-scattering measurement in the absence of salt additives meaningless. Static lightscattering experiments gave a radius of gyration of Rg =
19.6 nm. The characteristic ratio 1 = Rg/Rh = 0.9 is indicative
of spherical aggregates with slightly ellipsoidal character.
A Zimm plot of 1 was recorded in chloroform and is
shown in Figure 2. The refractive index increment of 1 in
Figure 2. Zimm plot of 1 in chloroform. K: optical constant, c:
concentration of dissolved sample, Rq : Rayleigh ratio, q: scattering
vector. Four different concentrations were obtained by diluting the
sample inside the cuvette and subsequently each solution was
measured at 20 different scattering angles. The laser wavelength was
l0 = 632.8 nm. The data are plotted with an arbitrary refractive index
increment of dn/dc = 0.1000 mL g1; cTBABr = 6.25 J 104 m. Taking into
account the experimental refractive index increment[29] of 1 in chloroform of (dn/dc)exp = 0.0736 mL g1, one obtains the weight average of
the molecular mass Mw = 2.93 J 106 g mol1, the second virial coefficient of the osmotic pressure A2 = 5.2452 J 105 mol mL g2, and the zaverage of the squared radius of gyration hR2giz = 3.8497 J 1012 cm2
(Rg = 19.6 nm).
chloroform was determined to be dn/dc = 0.0736 mL g1.
From the Zimm plot in Figure 2 a weight average molar
mass for the aggregates of Mw = 2.93·106 g mol1 could be
calculated. This corresponds to an average aggregation
number of Nagg = 514.
Aggregates of such dimensions are very well suited for
visualization by scanning force microscopy (SFM). We chose
tapping-mode SFM for characterization of the supramolecular structures, since it allows nondestructive imaging of the
surface of soft polymer samples[30] and offers additional
information about sample properties. To visualize single
aggregates, highly dilute solutions of 1 (c = 5 mg L1) were
prepared in chloroform and left to equilibrate at room
temperature for at least twelve hours before film preparation.
One drop of the solution was cast onto a freshly cleaved sheet
of mica, the negatively charged surface of which offers good
adhesion for hydrophilic polymers like polyethylene glycol
(PEG). After evaporation of the solvent, the sample was
examined by SFM in soft tapping mode using a silicon SFM
tip.
As shown in Figure 3 a, the particles are mainly spherical
or slightly oval-shaped and have almost consistent dimensions, measuring 20–35 nm across and approximately 2 nm in
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2969 –2975
Angewandte
Chemie
Figure 3. A. Tapping-mode SFM image showing spherical micelles of 1 on mica. The sample was prepared by drop-casting of a solution of 1 in
chloroform (c = 5 mg L1) onto freshly cleaved mica. a) Topography image, b) phase-contrast. c) Left and bottom axes (black and white bars):
statistical analysis based on 62 micelles of 1; for nonspherical shapes, the shorter axis was defined as the width and the longer axis as the length;
top and right axes (&): a strong correlation between the length and width of the particles is observed. B. Tapping-mode SFM image showing
rodlike micelles of 2 on mica. The sample was prepared by drop-casting of solution of 2 in chloroform (c = 5 mg L1) onto freshly cleaved mica.
d) Topography image, e) phase-contrast image. f) Left and bottom axes (black and white bars): statistical analysis based 64 micelles of 2; top and
right axes (&): length and width of the particles are not correlated.
height. The aggregates can be seen particularly well in the
SFM phase-contrast image (Figure 3 b). The phase-contrast
image was also used to determine the in-plane particle
dimensions. Besides the globular ones, few elongated aggregates were found. A statistical representation of the particle
dimensions (Figure 3 c) shows that the width and length
distribution of the particles almost coincide. The narrow size
distribution observed by SFM correlates well with the results
obtained by DLS. The size of the aggregates was independent
of solution concentration over a wide range of concentrations
examined by SFM (2–50 mg L1).
Once the particles are deposited on mica, they are
extremely stable; they can be clearly visualized by SFM
even after being heated to 100 8C for several hours. In order to
exclude false representation of the micelles by interactions
Angew. Chem. Int. Ed. 2006, 45, 2969–2975
between the mica surface and the aggregate, additional SFM
experiments were carried out on amorphous carbon as well as
SiO2 (oxidized Si afer) and similar results were obtained (see
the Supporting Information). Furthermore, the hydrodynamic radius of about 21 nm obtained by DLS measurements
is in good agreement with the dimensions observed by SFM
(r = 10–17.5 nm).
In addition, more concentrated solutions of 1 (c = 0.05–
1 g L1) were examined. For the highest concentrations
investigated (c = 1 g L1) we observed dendritic structures
several hundred micrometers in length. Such structures are
well-known to be caused by diffusion-limited aggregation (see
the Supporting Information).[31]
Rod–coil block copolymers are believed to form hockeypuck micelles in coil-selective solvents;[32] however, there has
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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been little experimental evidence in support of such unusual
micelles.[33] To the best of our knowledge, this is the first time
that such structures have been evidenced in great detail.
In analogy to micelles from low-molecular-weight amphiphiles the shape of the micelles is strongly influenced by the
volume of the solvated head group, here, the hydrophilic
PEG. To gain further insight into the formation of OPBAPEG superstructures, rod–coil copolymer 2 was prepared in
analogy to 1 but bearing a shorter PEG head group.
Interestingly, the block copolymer bearing a tetra(p-benzamide) block, MPEG2k-Ar4-NH2, an intermediate in the synthesis of 2, showed strong aggregation in chloroform solution
as determined by GPC, whereas for the analogous compound
bearing a longer polymer chain, MPEG5k-Ar4-NH2, no
aggregation was observed.[26] This clearly shows that the
length of the rod as well as the length of the coil segment
strongly influence the overall ability of these rod–coil
copolymers to form aggregates in nonpolar solvents.
Dynamic light-scattering (DLS) measurements of a solution of 2 in chloroform (c = 0.3 g L1; 104 m TBABr) revealed
o
a broader particle size distribution (m 90
2 > 0.1) than that of 1
90o
(m 2 = 0.04, see above). The hydrodynamic radius of the
aggregates of 2 (35 nm) was also larger than that observed for
1 (21 nm, see above). Static light-scattering (SLS) experiments with the same solution gave a radius of gyration of
57 nm and a 1 ratio Rg/Rh of 1.6, which indicates that the
aggregates have an anisotropic shape.[34, 35]
SFM measurements of thin films of 2 (c = 5 mg L1) were
carried out in the same way as described above for 1. As can
be seen in Figure 3 d,e, the aggregates observed are no longer
spherical in shape but rodlike and elongated. The lengths of
these rods differ over a range of ca. 80 nm, while the widths
are very narrowly distributed (Figure 3 f). These results are in
very good agreement with the data obtained from DLS.
The assumption that the rod–coil block copolymers form
micelles in chloroform is supported by the fact that the shape
of these aggregates changes from spherical to rodlike when
the solvated head group is smaller; this is in analogy to the
behavior of classical low-molecular-weight amphiphiles. The
way these hockey-puck micelles are formed is most likely
identical for compounds 1 and 2. It is merely the size of the
coil block that determines whether the micelle formed is more
spherical or more elongated. The way these micelles are held
together in the core, however, is fundamentally different from
classical micelles. For a rod–coil hockey-puck micelle, the
micellar core is believed to consist of a stack of rod segments
arraged either as a monolayer or as a bilayer.[24, 31, 32, 36]
As polymers 1 and 2 are amphiphilic, it should be possible
to discriminate between the rigid hydrophobic micellar core
and the softer hydrophilic corona by tapping-mode SFM. The
SFM image in Figure 3 b reveals some fine structure at the
core of the micelles. To investigate this further, the same
sample was re-examined at higher resolution. To minimize
possible interactions between the SFM tip and the polar PEG,
SFM images of single micelles were recorded using hydrophobized SiO2 cantilevers.[37] Samples of 1 were prepared in
the same way as described above. The SFM phase-contrast
image (Figure 4 b) clearly shows distinct micelles consisting of
an anisotropic core approximately 10 nm in width and
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Figure 4. a) SFM tapping-mode image of 1 on mica. The sample was
prepared by drop-casting of a solution of 1 in chloroform
(c = 5 mg L1) onto freshly cleaved mica and recorded with a plasmacleaned and hydrophobized SFM tip; b) STM phase-contrast image;
c) representative height profile through two of the micelles; d)
corresponding phase profile (see bars across SFM images).
different lengths. The SFM image shows that the core is
surrounded by a broad corona, presumably PEG. The hydrophobic tip leads to significantly fewer interactions with the
PEG corona, resulting in a more distinct determination of the
micelle core. The functionalized tip further offers the benefit
of hydrophobic–hydrophobic interactions with the micelle
core without being disturbed by nearby PEG. As a result, a
more positive phase shift between the driving and oscillating
frequency of the SFM cantilever at resonance frequency
(Figure 4 d) can be achieved. To further minimize intermixing
of hydrophobic/hydrophilic properties with the materialFs
mechanic properties, the images were usually recorded in
soft-tapping mode.[29, 38] The size of the micelle core could be
confirmed by applying hard tapping on the same sample. In
the hard-tapping mode the phase-contrast image is dominated
by the mechanical properties of the material. The heights of
the micelles were constant within each sample. They varied,
however, in the range of 2 to 4 nm depending on sample
preparation and tapping parameters (Figure 4 a,c). Slight
fluctuations of micelle height within one sample are most
likely caused by differences of the PEG coils above and below
the micelle core. To exclude artifacts in determining the width
of the micelle core, a high-resolution SFM tip with a tip radius
of curvature of < 1 nm was used. This measurement confirmed the previous width measurement of 10 nm.
Based on this data together with the data obtained from a
single-crystal X-ray structure of tri(p-benzamide) 3[25]
(Figure 5) we propose a structural model for the hockeypuck micelles formed from OPBA-MPEG block copolymers.
The dimensions of the micellar core (10 nm) determined by
SFM cannot result from a monolayer of hepta(p-benzamide),
the length of which is approximately 4.4 nm (extrapolated
from the X-ray crystal structure of the trimer shown in
Figure 5). It can, however, be explained by a bilayer model, in
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 6. a) SFM tapping-mode images of 1; b) a model of a bilayer
hockey-puck micelle. The micellar core measures roughly 10 nm
across, while the overall micelles have an average diameter of 35 nm.
The height of the micelles is about 2 nm, corresponding to a stack of
six bilayers formed by oligo(aramide)s.
Figure 5. Intermolecular hydrogen bonding as the dominating force
determining the aggregation of oligo(p-benzamide)s. a) Schematic
representation of hydrogen bonds in trimer 3.[25] b) Single-crystal X-ray
structure of 3.[39] Intermolecular hydrogen bonds are represented as
dashed lines.
which the C-termini of the oligoamide form the outside of the
micellar core while the N-termini face each other. This type of
aggregation is further supported by the X-ray crystal structure
of the trimer 3 shown in Figure 5. A cross-section through the
crystal structure shows that the C- and N-termini are on the
side within an infinite hydrogen-bonded stack of rods. The
uniform height of the micelles of roughly 2 nm suggests that
about six hepta(p-benzamide) bilayers are stacked on top of
each other by means of p–p interactions. The proposed model
of the bilayer hockey-puck micelle is shown in Figure 6. In
this model, two types of noncovalent interactions are present:
one type (H bonds) is significantly stronger than the other (p–
p interactions). Therefore, aggregation along the axis of
stronger noncovalent interaction (length of the micelle) is
more pronounced.
Analyzing the SFM image in Figure 4, we calculate an
average number of ca. 90 hepta(p-benzamide)s in one pstacked bilayer within the core of a micelle. Together with the
data from the SFM height measurements, the overall
aggregation number per micelle can be estimated to be
Nagg,SFM = 540. This value is in very good agreement with that
determined in static light-scattering experiments (Nagg,LS =
514).
As can be seen in Figures 4 and 6, the shape anisotropy of
the micellar core is more pronounced than that of the overall
micelle. This is most likely a result of the stretching of the
Angew. Chem. Int. Ed. 2006, 45, 2969–2975
PEG at the rod–coil boundary, resulting in stretched linear
PEG in the vicinity of the core. The initial linear PEG
becomes more coiled with increasing distance from the core,
thereby canceling out the large axial ratio of the core itself to
some degree.
We have shown that for large coil blocks, spherical
micelles, and for smaller coil blocks, rodlike micelles, are
predominantly formed. We are currently investigating the
effect of further reducing the size and varying the chemical
properties of the coil block on formation of supramolecular
structures.
In conclusion, we have synthesized oligoaramide-PEG
rod–coil block copolymers with different PEG lengths but
identical rod lengths. These block copolymers aggregate
strongly in nonpolar solvents. The aggregation was investigated by dynamic and static light scattering, and the
aggregates were visualized by high-resolution SFM. Depending on the size of the PEG coil block, spherical or rodlike
micelles were obtained and characterized. The core-shell
structure of individual micelles was visualized by highresolution SFM, and strong experimental evidence was
found for the often proposed hockey-puck micelle. For the
data obtained, a bilayer hockey-puck micelle model was
proposed, in which the micellar core is stabilized by means of
strong hydrogen bonds and p–p interactions. Such very stable
noncovalent micelles with predictable geometry will play an
important role in the preparation of well-defined functionalized nanoparticles. Owing to the high thermal and chemical
stability of the deposited micelles, surface patterning, coating,
and functionalization are also envisaged. Microphase-separated noncovalent block copolymers based on oligoaramides
are currently being investigated.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Experimental Section
All reactions were carried out in solvents of analytical quality (p.a.)
purchased from Fischer Scientific. Poly(ethylene glycol) monomethyl
ether (Mw = 2000 g mol1, 5000 g mol1) was obtained from Fluka. All
further chemicals were purchased from Acros Organics. All block
copolymers were purified on a preparative GPC system eluting with
DMF (Knauer HPLC Pump 64, MZ-GPC 250 C 30 mm GPC column,
MZ-Gel SDplus, 103 J 10mm (MZ-Analysentechnik); Knauer variable-wavelength monitor (UV detector at 254 nm); Shodex RI-71
refractive index detector) followed by freeze-drying from benzene
solution.
SFM measurements were carried out on a Dimension 3100
instrument equipped with a Nanoscope IV Controller or a MultiMode
connected to a NanoScope IIIa Controller using an E scanner. For
SFM measurements, a standard solution (1 g L1) was prepared and
subsequently filtered through a 0.45-mm polytetrafluoroethylene
(PTFE) syringe filter. This solution was diluted as required. Samples
were prepared by drop-casting or spin-coating the dilute solution
(5 mg L1) onto freshly cleaved mica. Both film-casting techniques
gave identical results. To achieve an equilibrium state, solutions were
prepared at least 12 h before casting. Olympus tapping-mode silicon
cantilevers (OMCL-AC 160 TS-W2, RTip < 10 nm opening angle,
spring constant KF = 42 N m1, Fres = 300 kHz) were used for measurements. For high-resolution imaging, Micro Mesh cantilevers with
hydrophobic tip (DP15/HI’RES/AIBS/15, RTip < 1 nm opening angle,
spring constant KF = 40 N m1, Fres = 325 kHz) were used.
All cantilevers were cleaned with Ar plasma immediately prior to
use. Cantilever functionalization was carried out according to a
general procedure: decalin, carbon tetrachloride, and chloroform
were mixed in a 7:2:1 ratio and 1 vol % trimethylsilyl chloride was
added. The solution was placed in a closed desiccator, and the plasmacleaned SiO2 cantilevers were placed in the gas phase above the
solution for four days. Contact-angle measurements on the chip
support were used to prove functionalization.
Dynamic and static light scattering experiments were performed
with a Uniphase He/Ne laser (l = 632.8 nm), an ALV-SP 86 goniometer, an ALV/High QE APD-Avalanche photodiode with fiberoptic detection, an ALV300 correlator, and a Lauda RC-6 thermostatization unit at angles between 30 and 1508 in steps of 208 (DLS) or
58 (SLS). The temperature was kept constant at 293 K. The samples
were filtered through Millex-LG filters (0.2 mm). The DLS data were
analyzed by a cumulant fit and by a biexponential fit utilizing the
simplex algorithmto yield the angular-dependent diffusion coefficient
Dq. The extrapolation to zero-scattering angle gives the apparent
diffusion coefficient D, which is translated into the sphere-equivalent
apparent hydrodynamic radius (Rh) by application of the Stokes–
Einstein relation.[42] The concentration and angular dependence of
the Zimm plot of the static light-scattering data gives the weight
average of the molecular mass Mw, the second virial coefficient of the
osmotic pressure A2, and the z-average of the squared radius of
gyration hRg2iz.
Received: October 4, 2005
Revised: January 7, 2006
Published online: March 28, 2006
.
Keywords: aggregation · block copolymers · micelles ·
nanostructures · scanning probe microscopy
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2969 –2975
Angewandte
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A detailed analysis of the light-scattering data will be published
elsewhere.
H. Wang, W. You, P. Jiang, L. Yu, H. H. Wang, Chem. Eur. J.
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See, for example: R. LuginbOhl, A. Szuchmacher, M. D.
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In particular, we have observed for lower setpoint ratios of
around 0.6–0.7 that the micelles are deformed and that topography is altered. We observed that this deformation is
reversible and that the micelle returned to its initial shape
after the setpoint ratio was set again to 0.98.
Crystal data for 3: C24H22N4O7; Mr = 478.45; 0.02 C 0.13 C
0.19 mm3 ; colorless; monoclinic; space group P21/c, Z = 4; a =
24.540(5), b = 5.363(2), c = 17.078(3) J, b = 99.050(7)8; V =
2219.8(8) J3 ; 1calcd = 1.432 g cm3 ; CuKa radiation (l =
1.54056 J); 2qmax = 1488; w/2q-scans on a Turbo-CAD4; T =
298 K; 5140 reflections measured (all 4512 unique used);
30 h 0, 6 k 0, 21 l 21; Lorentz factor and absorption correction applied, no absorption correction (m =
0.90 mm1) applied; structure solved with direct methods and
refined with full-matrix least-squares on F2 with SHELXL-97;[40]
4512 data and 316 parameters; R(1180 reflections > 2s(I), all
data) = 0.1274, 0.3538; wR2(all data) = 0.4172; S = 0.963; hydrogen atoms riding; greatest final electron density difference
excursions of + 0.41, 0.46 e J3.[41]
G. M. Sheldrick, SHELXL-97, Program for Structure Refinement, University of GPttingen, Germany, 1997.
CCDC-283302 (3) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
B. J. Berne, R. Pecora, Dynamic Light Scattering, J. Wiley, New
York, 1976.
Angew. Chem. Int. Ed. 2006, 45, 2969–2975
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2975
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