Angewandte
Chemie
the lengths of the nanofiber are nearly the same, ranging from
about several micrometers to about 20 micrometers.
The formation of nanofibers relies on appropriate molec-
ular packing and specific directional noncovalent interactions,
such as p–p interactions and hydrogen bonding.[7] We used
different characterization techniques to help elucidate how
the nanofibers are formed. We specifically wished to identify
whether Coulombic attractions and charge-transfer interac-
tions were the dominant driving force in the formation of the
complex between viologen groups and PYR molecules. We
also wanted to examine the viologen–PYR packing motif that
favors the one-dimensional nanostructures. Considering the
low solubility of the PYR–RV complex, we synthesized
methyl viologen (MV) as a model compound to investigate
the complex formation between the viologen group and the
PYR molecule. MV (an electron acceptor) and PYR (an
electron donor) were mixed in water in a 1:1 ratio, and the
resulting spectroscopic changes were observed. Both the
fluorescence quenching in the emission spectra and the red
shifts of the peaks in the UV/Vis spectra are indicative of
charge-transfer complex formation (Figure S4 in the Support-
ing Information). Furthermore, mass spectroscopy supports
the formation of charge-transfer complex between MV and
PYR. A peak at m/z 641.02 is observed, which corresponds to
the PYR3À–MV2+ complex (Figure S5 in the Supporting
Information), and indicates that PYR and MV indeed form
a charge-transfer complex with 1:1 stoichiometry, thus
supporting the previously reported assumption.[6]
Figure 1. TEM images of a) RV and b,c) PYR–RV in pH 9 buffer
solution; d) TEM image of PYR–RV in pH 10 buffer solution. The
concentration of RV is 1ꢀ10À4 m in each case.
(DLS) experiments (Figure S1 in the Supporting Informa-
tion).
It should be noted that the vesicles transformed into one-
dimensional nanofibers when an equivalent amount of PYR
was added into the RV solution. As shown in Figure 1b, the
PYR–RV supramolecular amphiphile self-assembles into a
one-dimensional wormlike structure. From the magnified
image of this structure, no clear contrast between the edge
and central part is observed, thus indicating that the wormlike
structures are solid nanofibers. Moreover, the nanofibers
have a large length/diameter ratio: they exhibit a uniform
diameter of about 14 nm, and their length reaches tens of
micrometers.
1H NMR spectroscopy was used to provide information
on the intermolecular interactions that occur during complex
formation. As shown in Figure 2a, upon formation of the
charge-transfer complex, it was observed that all the reso-
nances of the protons on MVand PYR were shifted upfield. It
is noted that the protons at the center of the molecule
exhibited larger shifts than those at the edges of molecules
(Figure S7 in the Supporting Information). This effect arises
from the aromatic p electrons that are situated above or
below the respective protons, thus strongly suggesting the
face-to-face packing of PYR and MV. Interestingly, for PYR,
the central protons adjacent to the hydroxy group exhibit
larger shifts than those near the sulfonate groups. This effect
possibly occurs because the sulfonate groups are electron-
withdrawing and thus have a local effect as well as reducing
the electron-donating properties of the PYR unit as a whole.
This effect could lead to reduced interaction with the MV
units at these positions, and therefore the reduced shift. The
packing fashion could be further confirmed by nuclear
Overhauser effect spectroscopy (NOESY), since the typical
distance between the charge-transfer donor and acceptor is 3–
5 ꢁ, which is in the detectable range of NOE signals. As
shown in Figure 2b, the cross-peaks m2–p4 and m1–p6 were
observed, which are consistent with our packing model in
which PYR and MV adopt a one-dimensional face-to-face
packing feature.
To understand the mechanism of the transformation from
vesicles to nanofibers, a series of complexes with different
PYR/RV ratios were prepared. When the PYR/RV ratio was
1:10, the vesicles broke into pieces of membranes (Figure S2
in the Supporting Information). As the PYR component
increased, the membranes became longer and narrower. At a
PYR/RV ratio of 2:3, wormlike nanowires appeared, which
coexisted with the membranes. When the PYR/RV ratio
reached 1:1, uniform wormlike nanostructures were observed.
It should be pointed out that further increases in the PYR/RV
ratio did not bring any significant changes to the nanofibers.
A plausible explanation is that when the PYR/RV molar ratio
is larger than 1:1, all the RV molecules were complexed and
therefore no further structural changes could be generated.
Since PYR is itself pH-responsive, we wondered if this
property could be brought to the supramolecular amphiphile,
and a 1.0 ꢀ 10À4 m solution of the PYR–RV complex in pH 10
buffer was prepared. Figure 1d shows the self-assembled
structures of the PYR–RV complex. Notably, the curly
wormlike nanofibers were straightened. Contrary to the
random wormlike nanofibers, the straight nanofibers prefer
to hierarchically aggregate into bundles (Figure S3 in the
Supporting Information). The diameter of the nanofibers is
still about 14 nm, which is the same as that of the curly
nanofibers, and suggests that the molecular packing frame-
work should remain unchanged. When the pH value was
changed to 9, the straight nanofibers became curly again, thus
indicating that the process is reversible. At pH 9 and pH 10,
To more directly understand the packing structure on the
basis of intermolecular interactions, a single crystal of the
complex was successfully grown. As shown in Figure 3, the
asymmetric unit contains one PYR3À ion, one MV2+ ion, one
Na+ ion, and six water molecules. The overall PYR3À /MV2+
Angew. Chem. Int. Ed. 2009, 48, 8962 –8965
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