Supra-amphiphiles formed by complexation of azulene-based amphiphiles and pyrene in aqueous solution: From cylindrical micelles to disklike nanosheets

Article abstract of DOI:10.1039/c3cc00059a

We have fabricated supra-amphiphiles by charge-transfer complexation of azulene-based amphiphiles and pyrene in water, and the self-assembled nanostructures can reversibly change between cylindrical micelles and disklike nanosheets in response to interact

Full text of DOI:10.1039/c3cc00059a

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Supra-amphiphiles formed by complexation of
azulene-based amphiphiles and pyrene in aqueous
solution: from cylindrical micelles to disklike
nanosheets†
Cite this: DOI: 10.1039/c3cc00059a
Received 5th December 2012,
Accepted 9th January 2013
DOI: 10.1039/c3cc00059a
Fei Li, Qiao Song, Liulin Yang, Guanglu Wu and Xi Zhang*
www.rsc.org/chemcomm
We have fabricated supra-amphiphiles by charge-transfer com- introduced as a core of the Y-shaped aromatic segment to increase
plexation of azulene-based amphiphiles and pyrene in water, the hetero-association with pyrene. It should be noted that azulene,
and the self-assembled nanostructures can reversibly change a bicyclic aromatic system fused with an electron-rich five-
between cylindrical micelles and disklike nanosheets in response membered and an electron-poor seven-membered ring, exhibits a
to interaction with guest molecules.
dipole moment of 1.08 D and a higher CT association constant
than naphthalene.4 In addition, it has been widely used as a basic
Self-assembly involves spontaneous formation of well-defined unit of conjugated polymers and oligomers5 with conductive6 and
structures from various chemical building blocks. Among the optical7 properties. In this work, we have designed and synthesized
building blocks for self-assembly, amphiphiles that contain both a Y-shaped bola-amphiphile bearing an azulene moiety (PAL) (see
hydrophilic and hydrophobic parts are widely used building ESI†).8 As shown in Scheme 1, pyrene, a water-insoluble molecule,
blocks.1 In contrast to conventional amphiphiles that are formed can be mixed with PAL in aqueous media which is expected to form
by covalent bonding, supra-amphiphiles refer to amphiphiles supra-amphiphiles driven by hydrophobic and CT interactions,
that are formed by different noncovalent interactions.2 The major allowing for preparation of the complexes directly in water. More-
advantage of supra-amphiphiles is that different functional over, the self-assemblies may be changed reversibly in response to
groups with stimuli-responsiveness can be easily attached to interaction with guest molecules. In contrast to the external stimuli
the amphiphiles by noncovalent synthesis, thus avoiding tedious such as pH, light, redox and temperature, introduction of pyrene
chemical synthesis to some extent. In the meantime, it may and its interaction with PAL allows us to provide a tool for
endow the assemblies with desirable functions, thus allowing structural evolution of the assemblies, which is promising for
for the fabrication of adaptive supramolecular nanostructures. application in smart nanoscale materials.9
The study of supra-amphiphiles with diversified topologies can
The formation of supra-amphiphiles is confirmed by UV-visible
not only enrich the traditional field of colloid and interface and fluorescence spectroscopy. Upon adding pyrene in THF to
science, but also bring a new horizon to supramolecular science. an aqueous solution of PAL (1 : 1 molar ratio of PAL/pyrene),
Herein, for the first time we attempted to fabricate supra- the resulting mixture was initially turbid, which then yielded
amphiphiles with azulene-based oligomers of amphiphiles and a transparent solution with color changed from light green
pyrene as building blocks by charge-transfer (CT) interactions.3 In to green after 36 hours. The change of the solution color
this regard, we wanted to load directly one equivalent of water-
insoluble pyrene into the hydrophobic interior of the amphiphiles
without use of organic solvent. Technically, this is difficult,
because the existing strong homo-association of the aromatic
segments of amphiphiles will lead to the precipitation of pyrene
in aqueous media. To solve this problem, Y-shaped aromatic
segments of the amphiphiles are designed in order to avoid the
strong homo-association between them, and the azulene moiety is
Key Lab of Organic Optoelectronics & Molecular Engineering Department of
Chemistry, Tsinghua University, Haidian District, Beijing 100084, China.
E-mail: xi@mail.tsinghua.edu.cn; Fax: +86-10-6277-1149; Tel: +86-10-62796283
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc00059a
Scheme 1 Amphiphiles consisting of azulene-based oligomer complexes with
pyrene through CT interactions. The self-assembled structure is transformed from
cylindrical micelles to nanosheets by adding pyrene.
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Fig. 1 (a) Photographs of PAL, initial addition of pyrene, and the equilibrated
complex; (b) UV/Vis absorption spectra of PAL, pyrene in THF, the resulting solution
before (0 h) and after complexation (36 h); (c) emission spectra of the pyrene and
resulting solution before (0 h) and after complexation (36 h) (excitation wavelength:
335 nm). The PAL solution concentration is 5 Â 10À4 M; the concentration of pyrene
in THF is 2 Â 10À2 M. The molar ratio of PAL and pyrene in the solution is 1 : 1.
Fig. 2 (a) TEM image of PAL assembly with negative staining (1.5 wt% uranyl
acetate); (b) vector-dependent scattering intensities of the PAL assemblies; (c) TEM
image of the PAL–pyrene co-assembly; (d) tapping-mode AFM image of the PAL–
pyrene complexes adsorbed on a carbon-coated grid; (e) section analysis of (d).
was also confirmed by lyophilization (Fig. 1a). It was supposed of I and that of q and the slope value was À1.01, which is
that addition of water-insoluble pyrene into aqueous solution consistent with that predicted for one-dimensional nanostructures
quickly led to a suspension of particles in water, which then slowly (À1.0) (Fig. 2b).13 After complexation, the CT complex displayed a
formed complexes with PAL. There was no clear absorption of disklike structure (Fig. 2c). The adjacent disklike micelles partially
pyrene when it is added initially into the PAL solution (Fig. 1b). overlapped and stacked together with diameters ranging from a
After 36 hours, the transparent solution showed absorption bands few hundreds of nanometers to several micrometers, much larger
belonging to pyrene, indicating that pyrene and PAL have formed than those obtained for PAL alone. The selected area electron
some complexes in the hydrophobic region. Notably, the complex diffraction (SAED) pattern was obtained for the multiple-layered
exhibits a broad absorption band between 430 and 550 nm which stacking nanosheets (Fig. S10, ESI†). It displayed two Debye–
corresponds to the characteristic absorption of the CT com- Scherrer ring patterns around the discrete diffraction spots, which
plexes.10,11 Concomitantly, significant quenching of the emission confirms that the PAL–pyrene assemblies are crystalline. The outer
between 400 nm and 600 nm which is ascribed to the excimer of ring corresponded to a distance of 0.39 nm, which should be
pyrene was detected after complexation (Fig. 1c).12 It is important attributed to the p–p stacking distance between PAL and pyrene.
to note that the CT interactions are influenced by temperature. The stacking of the disklike structure is clearly observed by atomic
When temperature increased, the absorbance above 430 nm force microscopy (AFM, Fig. 2d). Section analysis revealed that the
weakened and the absorption bands peaked at 369 nm and total thickness of three nanosheets was 10.55 nm, therefore the
395 nm also decreased and exhibited blue-shifts, indicating the average thickness of a single layer was 3.52 nm (Fig. 2e). This value
gradual destruction of the CT complexes (Fig. S4, ESI†). Therefore, is a little bit smaller than that of the molecule at full stretch
the combination of UV-visible and fluorescence spectroscopy (ca. 4.13 nm by CPK modeling studies), indicating that the disklike
supports the formation of CT complexes indeed. Removal of the aggregates are single-layered nanosheets.
pyrene from the aqueous phase was performed by addition of
X-ray diffraction (XRD) was used to provide structural informa-
ethyl ether, leading to restoration of the solution colour (Fig. S5, tion before and after complexation. Before complexation, the
ESI†). In addition, adding more than one equivalent of pyrene led sharp reflection peak that was indexed to the (020) reflection of
to a white precipitate of pyrene at the bottom of the solution and PAL corresponded to a layered structure with a d-spacing of
no further increase of that absorption, further supporting that the 3.56 nm in the bulk state (Fig. 3a), which was further proved by
complex has a molar ratio of 1 : 1 (Fig. S6, ESI†).
the small angle X-ray diffraction (SAXD) experiment (Fig. S11a,
Different methods were employed to study the self-assembling ESI†). Therefore, this indicates that the cylindrical micelles formed
structures before and after CT complexation. As indicated by by PAL in water are not stable against the drying process (see ESI,†
transmission electron microscopy (TEM), PAL molecules self- part 6). The cylindrical micelles become thin films in the bulk
assemble in aqueous media to form cylindrical micelles of state. However, after complexation, the disklike structures formed
3–5 nm in diameter (Fig. 2a). Static light scattering (SLS) measure- by self-assembly of PAL–pyrene complexes are rather stable not
ments of angular dependent scattering intensities further provided only in solution but also in the bulk state. SAXD measurement of
evidence. A linear dependence between the Napierian logarithm the d-spacing of single-layered disklike nanosheets afforded a
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(Institute of Chemistry, CAS), Mr Kai Liu, and Mr Yu Yi
(Tsinghua University) for helpful discussion.
Notes and references
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cell lattice because the characteristic Y-shaped aromatic segments
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fabrication of supra-amphiphiles without using organic solvent.3a
In conclusion, we have successfully fabricated supra-
amphiphiles based on a CT complex between PAL and pyrene in
aqueous media. The formation process of the supra-amphiphiles
is well monitored, and it is found that the self-assembled
nanostructures can reversibly change between cylindrical micelles
and disklike nanosheets in response to interaction with guest
molecules. Compared with cylindrical micelles of PAL in aqueous
solution, self-assembly of supra-amphiphiles leads to disklike
micelles with crystalline order. Thus, it is highly anticipated that
this line of research may enrich the realm of supramolecular
engineering and provide a new avenue for fabricating self-
assembling materials for organic electronics.
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¨
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We acknowledge the financial support from the National Basic
Research Program (2013CB834502), the Foundation of Innovative
Groups of NSFC (21121004), and an NSFC-DFG joint grant (TRR
61) as well as the Tsinghua University Initiative Scientific
Research Program (2009THZ02230). We thank Ms Ling Hu
(Tsinghua University) for TEM and electron diffraction
measurements. We also acknowledge Prof. Zhibo Li, Mr Yu Liu
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