Controlled self-assembly manipulated by charge-transfer interactions: From tubes to vesicles

Article abstract of DOI:10.1002/anie.200803361

Transfer request: A self-assembled-supramolecular charge-transfer complex of 1-(11-oxo-11-pyren-1-ylmethoxy)undecyl)-pyridinium bromide (PYR) and ethane-1,2-diyl bis(3,5-dinitrobenzoate) (DNB) is shown to form vesicular aggregates in aqueous solution, in contrast to the tubular aggregates of pure PYR (see picture). A curvature-dependent mechanism for this change is proposed. (Figure Presented)

Full text of DOI:10.1002/anie.200803361

Angewandte
Chemie
DOI: 10.1002/anie.200803361
Controlled Self-Assembly
Controlled Self-Assembly Manipulated by Charge-Transfer
Interactions: From Tubes to Vesicles**
Chao Wang, Shouchun Yin, Senlin Chen, Huaping Xu, Zhiqiang Wang, and Xi Zhang*
Molecular self-assembly is an attractive and powerful strategy
[1]
for fabricating supramolecular architectures. Based on this
concept, nanoscale structures can potentially be precisely
manipulated by rational design and modification of the self-
assembling building blocks at the molecular level. Nature has
provided infinite examples which perfectly elucidate this
concept. In the endoplastic reticulum, for example, the
membrane structures can be elegantly regulated from tubes
to vesicles by adjusting the concentration of curvature-
[2]
shaping proteins in the multicomponent lipid bilayer. To
elucidate this unique feature of the natural self-assembly
process, a variety of artificial building blocks have been
designed. Among them, amphiphiles have proven to be an
important type of building block, with the ability to bridge
molecules and form nanostructures stabilized by different
[
3]
driving forces, such as charge-transfer interactions, which
have been extensively exploited to produce liquid crystal
[
4]
structures with high charge mobility. Moreover, the shape
and amphiphilicity of the building blocks can be readily
tailored by forming supramolecular complexes, thus further
[
5]
manipulating the self-assembly architectures. In this con-
text, the structural regulation of tubes and vesicles by
molecular coassembly, which is crucial to numerous cellular
activities, has rarely been realized in artificial amphiphile
Figure 1. Schematic representation of the transformation from tubes
to vesicles, andthe molecular structures of PYR andDNB.
[
6]
[7]
systems.
PYR and the dinitrobenzene groups of DNB. In this way,
Herein, we report a successful manipulation of self-
assembling nanostructures by noncovalent modification of
amphiphiles, manipulated by charge-transfer interactions,
which result in a transformation from fluorescent tubes to
vesicles in aqueous media. For this purpose, 1-[11-oxo-11-
the amphiphilicity and the shape of the building units can be
altered, further influencing the self-assembly behavior in
aqueous media.
Typically, the PYR–DNB complexwas prepared by
mixing PYR and DNB, in a molar ratio of 2:1, in THF, a
good solvent for both PYR and DNB. The solvent was then
removed under reduced pressure. The dissolution–evapora-
tion procedure was repeated three times, to ensure complete
complexation, and the resultant complex was dried at room
temperature under reduced pressure. Interestingly, the pre-
assembled complexwas readily soluble in water, yielding a
yellow transparent solution which was stable for several
months. As uncomplexed DNB is insoluble in water, solubil-
ity can be attributed to its charge-transfer-driven complex-
ation with PYR. Notably, PYR forms a colorless solution in
water. The color change after addition of DNB is also a result
of the complexation process. Further evidence for the
formation of a stable charge-transfer complexwas obtained
from UV/Vis absorption and fluorescence emission spectros-
copy. As is shown in Figure 2, the complexe xh ibits a broad
absorption between 400 nm and 550 nm which corresponds to
the characteristic absorption of the charge-transfer com-
(
pyren-1-ylmethoxy)-undecyl]pyridinium bromide (PYR),
containing an electron-rich pyrenyl group, and ethane-1,2-
diyl bis(3,5-dinitrobenzoate) (DNB), with two electron-defi-
cient dinitrobenzene units utilized as electron acceptors, were
chosen as the two components (Figure 1). PYR and DNB can
initially coassemble into a supramolecular complexdriven by
a charge-transfer interaction between the pyrenyl group of
[*] C. Wang, Dr. S. C. Yin, S. L. Chen, Dr. H. P. Xu, Prof. Z. Q. Wang,
Prof. X. Zhang
Key Lab of Organic Optoelectronics & Molecular Engineering
Department of Chemistry, Tsinghua University
Beijing 100084 (P.R. China)
Fax: (+86)10-62771149
E-mail: xi@mail.tsinghua.edu.cn
[
**] This work was financially supportedby the National Basic Research
program (2007CP808000), the NSF (20574040, 50703022), and
theNSFC-DFG joint grant (TRR61). We also acknowledge Prof.
Changyou Gao at Zhejiang University for CLSM measurements.
[
7]
plex. Concomitantly, drastic fluorescence quenching was
also detected after complexation. A series of PYR–DNB
complexes with different donor/acceptor ratios were also
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803361.
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above the CAC, the rods exhibited the same shape and similar
size distribution. After complexation, no images for the PYR–
DNB complexcould be visualized due to the aforementioned
fluorescence quenching effect. The rodlike structure of PYR
was further confirmed by optical microscopy. As is shown in
Figure 3d, the shape and size of the rods were consistent with
those displayed by fluorescence microscopy and CLSM
images. After complexation, however, spherelike aggregates
were observed (Figure 3e), drastically different from the
rodlike structures constructed by PYR alone.
A clearer indication of the self-assembly structures before
and after complexation was provided by transmission electron
microscopy (TEM). Analysis of PYR samples on carbon-
coated grids revealed that the rodlike aggregates were tubular
in structure, as evidenced by the clear contrast between the
dark periphery and the lighter central part (Figure 4a). The
Figure 2. a) UV/Vis absorption spectrum andb) fluorescence emission
spectrum of PYR andPYR–DNB complex. The solvent is water andthe
À4
10À5
concentrations of PYR andDNB are 1.0”10 m and5”
m,
respectively.
prepared. As the DNB-content increased, the emission
intensity decreased, without any change in the spectral
pattern (see Supporting Information, Figure S4). No exciplex
emission was detectable in the spectral region under these
experimental conditions, indicating that the quenching pro-
cess occurred by charge transfer from the pyrenyl units to the
dinitrobenzoate units.
To investigate higher-order self-assembled aggregates in
aqueous media, aqueous solutions of PYR and PYR–DNB
complex(2:1 ratio) were prepared. The concentration of PYR
À4
in both of these solutions was 1 ” 10 m, approximately two
times higher than the critical aggregation concentration
(
CAC).
As indicated by the fluorescence microscopy images and
confocal laser scanning microscopy (CLSM) images (Fig-
ure 3a–c), PYR self-assembled into rods of up to 20 micro-
meters in length (Figure 3b). At different concentrations
Figure 4. TEM images of a) PYR aggregates, b) one of the broken
tubes and c) PYR–DNB aggregate; d) DLS data of PYR–DNB
aggregate.
hollow nature of the assemblies could also be visualized for
the broken aggregate shown in Figure 4b. Conversely, after
complexation, the PYR–DNB complex displayed a typical
vesicular structure. Optical microscopy revealed that the
spheres were actually hollow (Figure 4c). The average size of
the vesicles shown in the TEM images was about 300 nm,
consistent with the dynamic laser scattering (DLS) results,
which gave an average size of 279.5 nm.
A remarkable increase in stability is also apparent upon
the formation of the charge-transfer interaction. Prior to
complexation, the tubular structure was relatively unstable
and gradually decomposed into amorphous aggregates over a
period of two weeks. After complexation, however, the
vesicular structure of PYR–DNB is maintained for several
months.
UV/Vis spectroscopy and X-ray diffraction (XRD) were
used to clarify the packing of PYR molecules in the tubular
aggregates. Higher dilution leads to a blue shift (Figure 5a),
[
8]
indicating an H-aggregation form, which suggests that
adjacent pyrene aromatic rings undergo considerable overlap.
The bilayer nature of the PYR membrane was revealed by
XRD and small-angle X-ray scattering (SAXS) results, as
Figure 3. a,b) Fluorescence microscopy images of PYR aggregates;
c) confocal laser scanning microscopy (CLSM) image of the PYR
aggregates; optical microscopy images of d) PYR and e) PYR–DNB.
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shown in Figure 5b and c. The thickness of the bilayer was
calculated to be 4.5 nm, in agreement with the length of two
PYR molecules with antiparallel packing and overlapped
pyrene rings. We then investigated whether the bilayer
framework was maintained after complexation. According
[4,9]
to the classical packing form,
the dinitrobenzyl group is
inserted between adjacent pyrene groups to form an interca-
lated donor–acceptor structure, without changing the thick-
ness of the bilayer. This structure was further confirmed by
XRD results, with a bilayer thickness of approximately 4.5 nm
retained.
A mechanism is proposed to explain why the shape of
PYR aggregates transforms from tubes to vesicles after
complexation with DNB. The shapes of the aggregates,
formed by the two distinct lipid bilayers, are determined by
[
10]
the spontaneous curvature of the membrane. Typically, the
formation of tubular structures is dependent on high mem-
brane curvature while a low-curvature membrane favors the
[11]
formation of a vesicular structure. Prior to complexation
with DNB, PYR self-assembles in aqueous solution to form a
highly ordered bilayer of packed pyrenyl groups with a strong
tendency to stack together through p-electronic interactions,
generating a high curvature and forming a tubular structure
(Figure 5d). After complexation, the dinitrobenzyl groups are
inserted into the PYR arrays. As a result of the polar nature of
the dinitrobenzyl groups and the steric repulsion generated
upon insertion, the straight PYR arrays along the axis become
[12]
curved.
As a result, the high curvature for shaping the
tubular structure is destroyed, topologically facilitating the
formation of vesicles. Moreover, the DNB molecule serves as
a “cross-linker” between the stacking arrays to enhance the
stability of the vesicular aggregates.
To test the proposed mechanism, we undertook several
control experiments. Firstly, the self-assembled structures of
charge-transfer complexes with different PYR/DNB molar
ratios were studied by TEM and DLS. TEM results revealed
that the complexes with PYR/DNB ratios of 10:1 and 4:1 both
exhibit vesicle structures but with differing size distributions
(see Supporting Information, Figure S7a,b). DLS tests gave
the average sizes of these vesicles as 519 nm and 374 nm,
respectively, in contrast to the average size of 279.5 nm with a
ratio of 2:1. These results indicated that the size of the vesicles
decreased as the proportion of DNB in the mixture was
increased. This trend can be attributed to the increase of
intercalation, which leads to a more curved bilayer, and thus
producing vesicles with smaller size.
Secondly, the electron-acceptor molecule was replaced by
C H (NO ) CH OCOCH (SDNB), in which dinitrobenzoate
6
3
2
2
2
3
groups were not covalently bonded to each other. A complex
of PYR and SDNB in a 1:1 ratio (PYR–SDNB) was prepared.
Formation of the complexwas confirmed by the UV/Vis and
the fluorescence emission spectra (see Supporting Informa-
tion, Figure S6). The PYR–SDNB complex, with curvature
induced by the intercalation of dinitrobenzene groups, was
assumed to form vesicular aggregates, and this was revealed
by TEM results (see Supporting Information, Figure S7c).
According to TEM data, the vesicles have an average size of
Figure 5. a) UV/Vis absorption spectra of aqueous solutions of PYR at
different concentrations; b) XRD and (inset) SAXS scans of PYR;
c) XRD scan of PYR–DNB; d) Schematic representation of the curva-
ture-dependent mechanism.
2
50 nm, similar to that of PYR–DNB. However, the vesicles
of PYR–SDNB are less stable than those of PYR–DNB.
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Communications
Separate solutions of the two complexes were kept at room
Keywords: aggregation · amphiphiles · charge transfer ·
self-assembly · vesicles
.
temperature for two weeks, after which time the remaining
aggregates were inspected by TEM. The majority of the
vesicles fabricated by PYR–SDNB collapsed into spherical
micelles and branched tubes with many three-way junctions
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NMR spectra were recorded on a JEOL JNM-ECA300 spec-
trometer; ESI–MS was carried out on a PE SciexAPI 3000
spectrometer. UV/Vis spectra were obtained using a Hitachi U-
[
3
010 spectrophotometer. TEM was performed on a JEMO 2010
electron microscope, operating at an acceleration voltage of 110 kV.
The samples were prepared by drop-coating the aqueous solution on
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Received: September 8, 2008
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Published online: October 20, 2008
12384.
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