The introduction of π-π stacking moieties for fabricating stable micellar structure: Formation and dynamics of disklike micelles

Article abstract of DOI:10.1002/anie.200500980

(Figure Presented) Enhancing the intermolecular interactions between micelles by introducing a strong π-π stacking moiety and flexible spacers of appropriate length results in the formation of disklike micelles that can maintain their monomolecule-layered

Full text of DOI:10.1002/anie.200500980

Angewandte
Chemie
Self-Assembly
The Introduction of p–p Stacking Moieties for
Fabricating Stable Micellar Structure: Formation
and Dynamics of Disklike Micelles**
Bo Song, Zhiqiang Wang, Senlin Chen, Xi Zhang,*
Yu Fu, Mario Smet, and Wim Dehaen
Amphiphiles can self-organize into micellar structures with
defined sizes and shapes[1,2] that may be used in applications
such as templates for making nanostructured materials and
mimicking biomineralization processes. Engineering robust-
ness is required to make these applications practical, as the
micellar structures are inherently dynamic and fluid. Enhanc-
ing the intermolecular interactions among the aggregates is
one possible method for stabilizing the micellar structures.
This approach may rely on generating a strong association
among its assembling units through a combination of different
interactions, including van der Waals interactions and hydro-
gen bonding.[3–5] Herein, we report the introduction of a
strong p–p stacking moiety into bolaamphiphiles[6] so as to
increase the robustness of the self-organized entities, thus
providing a new approach to fabricating organic nanostruc-
tured materials.
The diaryldiketopyrrolopyrrole (DPP) dye is a recently
developed technical pigment with remarkable properties such
as a high fade-proof nature, extraordinary thermal stability,
and very low solubility as a consequence of strong intermo-
lecular interactions.[7] We attempted to introduce a DPP dye
as a strong p–p stacking moiety into bolaamphiphiles, and
hoped that such a bolaamphiphile might give rise to a stable
micellar nanostructure with light-emitting properties. For this
[*] Dr. Z. Wang, S. Chen, Prof. X. Zhang
Key Lab of Optoelectronics andMolecular Engineering
Department of Chemistry
Tsinghua University
Beijing 100084 (China)
Fax: (+86)010-6277-1149
E-mail: xi@mail.tsinghua.edu.cn
B. Song
Key Lab of Supramolecular Structure andMaterials
Jilin University
Changchun, 130023 (China)
Dr. Y. Fu, Dr. M. Smet, Prof. W. Dehaen
Department of Chemistry
Katholieke Universiteit Leuven
Celestijnenlaan 200F, 3001 Leuven (Belgium)
[**] We thank the major state basic research development program
(G2000078102) andthe National Natural Science Foundation of
China (20334010, 20473045) for financial support. The Flemish and
Belgian governments, as well as the University of Leuven, supported
this work with a bilateral grant (BIL02/03), the grant IAP-V-4, anda
postdoctoral fellowship to Y.F., respectively. M.S. thanks F. W. O.
Vlaanderen for a postdoctoral fellowship. Professor Lei Jiang and
Huan Liu of the Institute of Chemistry, Chinese Academy of
Sciences are thankedfor their help with FE-SEM measurements.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 4731 –4735
DOI: 10.1002/anie.200500980
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4731

Communications
purpose, a series of bolaamphiphiles bearing diaryldiketopyr-
rolopyrrole units with different spacer lengths were synthe-
sized, namely, 2,5-bis(n-(1-pyridinium)alkyl)-3,6-bis-(2-
thienyl)-1,4-dioxopyrrolo[3,4-c]pyrrole ditosylate salts,
abbreviated as DPP-n, where n (n = 7, 11, 15) corresponds
to the number of methylene units in the alkyl chain spacer.
For DPP-11, for example, there exists a critical micelle
concentration (cmc) at 5.5 ” 10À5 m, as indicated by the plot of
the conductivity as a function of the concentration (Figure 1).
between the water and the pyridinium groups is stronger than
that with the thienyl residues, thus leading to increased local
mobility and less line broadening of the NMR signals.
We found that if the concentration of the DPP-11 solution
was above the cmc the solution turned from bright red to dark
purple. We anticipated that the color change was indicative of
the formation of micelles, which was further supported by
spectroscopic studies: We first identified the UV/Vis spectral
absorption bands of the DPP-11 monomer and aggregate. As
shown in Figure 3, there are two absorption bands around 514
Figure 1. Dependence of the solution conductivity of DPP-11 on
concentration; the cmc is around5.5”10 À5 m.
Figure 3. Concentration-dependent UV/Vis spectra of DPP-11: in meth-
anol at 1.0”10À4 m (dashed line); in aqueous solution at 1.0”10À4 m,
above the cmc (solidline); in aqueous solution at 1.0”10 À5 m, below
the cmc (dotted line).
When the concentration is above this critical value the
bolaamphiphiles self-organize into micelles in aqueous solu-
tion. The formation of a micellar structure is also supported
1
by H NMR spectroscopic analysis. As seen in Figure 2, the
and 540 nm in dilute aqueous solution (below the cmc, dotted
curve) or in the good solvent, methanol (dashed curve). These
bands are assigned to the monomer of DPP-11. In concen-
trated solution (above the cmc), a new absorption band
appears around 601 nm (solid curve), which is attributed to
the aggregate state of DPP-11. The bathochromic shift of the
absorption bands is caused by the strong p–p stacking of the
diketopyrrolopyrrole units in the amphiphiles upon self-
assembly into micellar structures. Interestingly, in contrast
with the normal formation of micelles that reaches equilib-
rium in a very short time, we found that DPP-11 requires a
relatively long period to self-assemble, thus allowing the
dynamic process to be monitored (Figure 4). Only absorption
bands at around 514 and 540 nm corresponding to the
monomer absorption exist at the beginning. The monomer
absorption becomes weaker and weaker over time and
concomitantly the absorption band of the aggregate around
601 nm appears and then becomes greater and greater, thus
indicating the formation of micelles and the increase in their
numbers. There is an isosbestic point around 560 nm.
Figure 4b shows that the absorption change of DPP-11
almost reaches a plateau after about 10 minutes, thus
suggesting an equilibrium situation.
Figure 2. 1H NMR spectra of DPP-11 in D2O with concentrations lower
andhigher than its cmc.
1H NMR signals of the aromatic rings from the two terminal
pyridinium groups as well as from the rigid unit are well
1
resolved below its cmc. However, all the H NMR signals
become broad and weak when the concentration is above its
cmc. The change in the 1H NMR signals of the thienyl
residues in the rigid moiety is even greater than that of the
two terminal pyridinium groups. This change can be ration-
alized by the restricted motion of the amphiphilic molecule in
the associated state.[3,8] The segregation of the hydrophobic
and hydrophilic parts of the molecule during the self-
assembly in water results in the thienyl groups residing in
the inner part of the micelles and the terminal pyridinium
groups at the outer part of the micelles. Therefore, interaction
To determine whether the aggregation is temperature-
dependent in solution we carried out systematic experiments
of the variation of the UV/Vis spectrum with temperature. As
shown in Figure 5, the UV/Vis absorption experiment indeed
shows that the aggregate can disassemble as the temperature
increases and reassemble as the temperature decreases. The
increase and decrease of the characteristic absorption bands
of the aggregates (601 nm) and monomers of the DPP-11
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molecules (514 nm) show aggregation is a reversible process.
At the same time, the reversible phenomena also indicate the
DPP-11 molecules experience self-assembly through supra-
molecular interactions.
The micelles formed in aqueous solution could be
adsorbed on freshly cleaved mica sheets and visualized by
ex situ atomic force microscopy (AFM). As exhibited in
Figure 6, DPP-11 forms disklike micelles with diameters
Figure 4. a) Time-resolvedabsorbance of DPP-11 in aqueous solution
at a concentration of 1.0”10À4 m. b) The change in the absorbance
with time at 601 nm (solidline) and514 nm (dashedline).
Figure 6. Ex situ AFM images of DPP-11 micellar structures adsorbed
on a mica sheet: a) large area image; b) section analysis of (a); c) and
d) magnified images.
ranging from several hundreds of nanometers to one micro-
meter. Adjacent disklike micelles partially overlap, and the
overlapping parts stack together to form double layers, triple
layers, or even multilayers. The section analysis (Figure 6b)
shows that the disk has an average thickness of about 3.7 nm.
This value agrees well with the length of a single DPP-11
molecule, thus suggesting that the disks form a monolayer
structure with the thickness of a single molecule. Although
the self-assembly of the amphiphiles is mainly driven by the
hydrophobic effect, once the disklike micelle forms, the p–p
stacking interaction becomes another dominating factor that
enhances the stability of the disks. This is the reason why we
can readily observe the micellar structure by ex situ AFM: the
micellar structure formed in this way is stable enough toward
the drying process. When looking more closely at the disks we
found that there were uniform stripes with a width of about
10 nm (Figure 6c, d). These stripes have a preferred orienta-
tion within one disk, although the orientation can be different
in different disks. Thus, the surface disklike micelle is a
combination of micro- and nanostructures, which can be
explained in terms of a compromise between the different
intermolecular interactions. The intermolecular p–p stacking
in the middle part of the aggregate forces the molecules to
pack closely, while the repulsive interaction of the positively
charged head groups has the reverse effect. We believe that
the repulsion between the charged head groups may be
enhanced as a consequence of water evaporation during the
drying process. A compromise between these two types of
Figure 5. Temperature-dependent UV/Vis spectra of DPP-11 in
aqueous solution at 1.0”10À4 m. The temperature increase (a) and
decrease (b) are in intervals of 58C.
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was dissolved in a minimum quantity of methanol (a good solvent for
DPP-11) and then the solution injected into a certain volume of water.
The UV/Vis spectrum was then recorded immediately by capturing
interaction could lead to crystallization of the amphiphiles,
which is responsible for the formation of the observed
stripelike structure.
1
one spectrum every 44 s. H NMR spectroscopic analysis (600 MHz)
The disklike micellar structure in the dry state as revealed
by ex situ AFM should also reflect its associated structure in
solution. This speculation is supported by the following
experimental facts. First, in situ AFM studies demonstrate the
presence of similar disks as those observed by ex situ AFM,
thus excluding the possibility that the disks are formed during
the air-drying process. Second, the disks can also be observed
when adsorbed onto hydrophilic quartz slides. This fact
indicates that formation of the disks is not induced by the
mica substrate, and that the disklike micelles formed in the
aqueous solution are robust enough to resist the effect of the
substrate. Third, the UV/Vis absorption bands for the
adsorbates on both a mica sheet and quartz slide are almost
the same as that for the micellar suspension, which indicates
the micelles are resistant to the transferring and drying
processes. Fourth, the enhanced stability is also supported by
AFM studies at elevated temperature. The disks undergo no
apparent changes until the substrate is heated to 808C, after
which they gradually melt as the temperature is further
increased.
In addition, an appropriate length of the flexible spacers is
also necessary for such bolaamphiphile to self-organize into
the disklike micellar structure. We performed similar studies
on DPP-7 and DPP-15, but failed to obtain self-assembled
structures for these two bolaamphiphiles where the spacers
are shorter and longer than DPP-11, respectively. Neither the
concentration/conductivity plot nor the UV/Vis spectra for
DPP-7 show any distinct phase transition, thus indicating that
this compound can not form micellar structures in aqueous
solution. Analysis of the adsorbate on a hydrophilic substrate
by AFM showed no regular structures were formed. The self-
assembly of DPP-15 is inhibited by its poor solubility, since
the increase in the spacer length decreases its solubility in
water significantly.
was performed in deuterium oxide. The substrates used in the AFM
studies were commercial mica and quartz slides. Before each use, the
mica was freshly cleaved, and the quartz slides treated with piranha
solution (concentrated sulfuric acid and 30% H2O2 (7:3)) to obtain a
hydrophilic surface.
The three bolaamphiphiles DPP-7, DPP-11, and DPP-15 were
synthesized in an analogous way. The synthesis of DPP-11 is
presented as an example. 3,6-Bis-(2-thienyl)-1,4-dioxopyrrolo[3,4-
c]pyrrole (DPP) was synthesized as reported before.[9] 7-Bromo-1-
heptanol was obtained by monobromination of 1,7-heptanediol with
HBr, and 15-bromo-1-pentadecanol was prepared by cleaving w-
pentadecalactone with H2SO4/HBr, followed by reduction with
borane·methyl sulfide complex (2m solution in THF).
2,5-Bis-(11-hydroxyundecyl)-3,6-bis-(2-thienyl)-1,4-dioxopyrrolo-
[3,4-c]pyrrole (1): 11-Bromo-1-undecanol (1.7 g, 6.8 mmol), K2CO3
(1 g, 7.2 mmol), and NaI (1 g, 6.7 mmol) were added to a solution of
DPP (1 g, 3.3 mmol) in dry DMF (40 mL). The mixture was stirred at
608C for 18 h. After cooling the mixture to room temperature, it was
poured into water and extracted with ethyl acetate. The organic phase
was dried over MgSO4 and the solvent was evaporated in vacuo. The
product (0.6 g, 28%) was obtained by column chromatography (SiO2,
CH2Cl2). 1H NMR (300 MHz, CDCl3, 258C, tetramethylsilane
(TMS)): d = 8.92 (d, 3J(H,H) = 4.2 Hz, 2H,), 7.64 (d, 3J(H,H) =
4.2 Hz, 2H), 7.28 (t, 3J(H,H) = 4.2 Hz, 2H), 4.07 (t, 3J(H,H) =
7.2 Hz, 4H), 3.64 (t, 3J(H,H) = 3.6 Hz, 4H), 1.29–1.74 ppm (m, 36H).
2,5-Bis-(11-tosyloxyundecyl)-3,6-bis-(2-thienyl)-1,4-dioxopyrrolo-
[3,4-c]pyrrole (2): Tosyl chloride (TosCl; 0.6 g, 3 mmol) and 4-
dimethylaminopyridine (0.6 g, 4.9 mmol) were added to a solution
of 1 (0.6 g, 0.9 mmol) in dichloromethane (140 mL). The mixture was
stirred at room temperature for 24 h, and then the solvent evaporated
in vacuo. The product (0.16 g, 19%) was obtained by column
1
chromatography (SiO2, CH2Cl2). H NMR (300 MHz, CDCl3, 258C,
TMS): d = 8.92 (d, 3J(H,H) = 4.2 Hz, 2H), 7.80 (d, 3J(H,H)=8.7 Hz,
3
3
4H), 7.64 (d, J(H,H) = 4.2 Hz, 2H), 7.34 (d, J(H,H) = 8.7 Hz, 4H),
7.28 (t, 3J(H,H)=4.2 Hz, 2H), 3.99–4.09 (m, 3J(H,H) = 7.2 Hz, 8H),
2.44 (s, 6H), 1.29–1.74 ppm (m, 36H).
DPP-11: Dry pyridine (4 mL) was added to a solution of 2
(0.165 g, 0.17 mmol) in dry chloroform (10 mL). The solution was
refluxed under nitrogen for 20 h. After cooling the mixture to room
temperature, it was added dropwise to toluene (150 mL). The
resulting red precipitate (0.18 g, 93%) was purified by two precip-
itations by adding a solution of the product in methanol to diethyl
ether. 1H NMR (300 MHz, DMSO, 258C, TMS): d = 9.07 (d, 3J-
(H,H) = 6.0 Hz, 4H; 2-H, 6-H Py), 8.80 (d, 3J(H,H) = 4.2 Hz, 2H; 3-H
Th), 8.58 (t, 3J(H,H) = 7.2 Hz, 2H; 4-H Py), 8.14 (t, 3J(H,H) = 7.2 Hz,
In summary, interesting disklike micelles have been
obtained by self-assembly of a novel bolaamphiphile, and
the dynamics of disk formation have been investigated. The
disklike micelles can maintain their monomolecule-layered
structures even when being transferred to hydrophilic sub-
strates, thus showing their high stability as a result of the
introduction of a strong p–p stacking moiety into the
amphiphile. Although micelles are inherently dynamic and
fluid, they can be stabilized by enhancing the intermolecular
interactions, thus providing a new approach for the fabrica-
tion of organic nanostructured materials.
3
4H; 3-H 5-H Py), 8.10 (d, J(H,H) = 4.2 Hz, 2H; 5-H Th), 7.47 (d,
3J(H,H) = 8.1 Hz, 4H; 2-H, 6-H Tos), 7.40 (t, 3J(H,H) = 4.2 Hz, 2H; 4-
H Th), 7.10 (d, 3J(H,H) = 8.1 Hz, 4H; 3-H, 5-H Tos), 4.57 (t,
3J(H,H) = 7.2 Hz, 4H; CH2Py), 3.99 (t, 3J(H,H) = 7.2 Hz, 4H;
CH2DPP), 2.50 (t, 3J(H,H) = 1.7 Hz, 6H; CH3 Tos), 1.88 (m, 4H;
CH2CH2Py), 1.62 (m, 4H; CH2CH2DPP), 1.20 ppm (m, 28H;
CH2CH2(CH2)7CH2CH2).
DPP-7: 1H NMR (300 MHz, DMSO, 258C, TMS): d = 9.07 (d,
3J(H,H) = 6.0 Hz, 4H; 2-H 6-H Py), 8.80 (d, 3J(H,H) = 4.2 Hz, 2H; 3-
H Th), 8.58 (t, 3J(H,H) = 7.2 Hz, 2H; 4-H Py), 8.14 (t, 3J(H,H) =
7.2 Hz, 4H; 3-H 5-H Py), 8.10 (d, 3J(H,H) = 4.2 Hz, 2H; 5-H Th), 7.47
(d, 3J(H,H) = 8.1 Hz, 4H; 2-H, 6-H Tos), 7.40 (t, 3J(H,H) = 4.2 Hz,
Experimental Section
A commercial multimode Nanoscope IVAFM was used to character-
ize the surface structure. In situ AFM: The substrate was mounted
under a liquid cell and the solution injected into the cell. After leaving
the solution to stand for at least 30 minutes to reach equilibrium, it
was scanned in the tapping mode. Ex situ AFM: The substrate was
incubated in the solution for at least 30 minutes, before a sample was
removed from the solution and air-dried. The sample was then
scanned in the tapping mode. UV/Vis spectra were measured with a
HITACHI U-3010 spectrophotometer. A weighed amount of DPP-11
3
2H; 4-H Th), 7.10 (d, J(H,H) = 8.1 Hz, 4H; 3-H; 5-H Tos), 4.57 (t,
3J(H,H) = 7.2 Hz, 4H; CH2Py), 3.99 (t, 3J(H,H) = 7.2 Hz, 4H;
CH2DPP), 2.50 (t, 3J(H,H) = 1.7 Hz, 6H; CH3 Tos), 1.90 (m, 4H;
CH2CH2Py), 1.62 (m, 4H; CH2CH2DPP), 1.37 ppm (m, 12H;
CH2CH2(CH2)7CH2CH2).
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1
DPP-15: H NMR (300 MHz, DMSO, 258C, TMS): d = 9.07 (d,
3J(H,H) = 6.0 Hz, 4H; 2-H, 6-H Py), 8.80 (d, 3J(H,H) = 4.2 Hz, 2H; 3-
H Th), 8.58 (t, 3J(H,H) = 7.2 Hz, 2H; 4-H Py), 8.14 (t, 3J(H,H) =
7.2 Hz, 4H; 3-H 5-H Py), 8.10 (d, 3J(H,H) = 4.2 Hz, 2H; 5-H Th), 7.47
(d, 3J(H,H) = 8.1 Hz, 4H; 2-H, 6-H Tos), 7.40 (t, 3J(H,H) = 4.2 Hz,
3
2H; 4-H Th), 7.10 (d, J(H,H) = 8.1 Hz, 4H; 3-H; 5-H Tos), 4.57 (t,
3J(H,H) = 7.2 Hz, 4H; CH2Py), 3.99 (t, 3J(H,H) = 7.2 Hz, 4H;
CH2DPP), 2.50 (t, 3J(H,H) = 1.7 Hz, 6H; CH3 Tos), 1.90 (m, 4H;
CH2CH2Py), 1.62 (m, 4H; CH2CH2DPP), 1.20 ppm (m, 44H;
CH2CH2(CH2)7CH2CH2).
Received: March 17, 2005
Revised: May 20, 2005
Published online: July 1, 2005
Keywords: bolaamphiphiles · micelles · nanostructures ·
.
noncovalent interactions · self-assembly
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