Morphology control of fluorescent nanoaggregates by co-self-assembly of wedge- and dumbbell-shaped amphophilic perylene bisimides

Article abstract of DOI:10.1021/ja070994u

Wedge- and dumbbell-shaped amphiphilic perylene bisimides PBI 1-4 were synthesized. The wedge-shaped PBI 1 and PBI 2 and dumbbell-shaped PBI 4 self-assemble into micelles, and rod aggregates in aqueous solution, respectively. Interestingly, the co-self-assembly of wedge-shaped PBI 1 with dumbbell-shaped PBI 3 generates hollow vesicles with bilayer structures because of the curvature changes during the self-assembly process. The bilayer vesicles could be stabilized by in situ photopolymerization. Copyright

Full text of DOI:10.1021/ja070994u

Published on Web 04/03/2007
Morphology Control of Fluorescent Nanoaggregates by Co-Self-Assembly of
Wedge- and Dumbbell-Shaped Amphiphilic Perylene Bisimides
Xin Zhang, Zhijian Chen, and Frank Wu¨rthner*
UniVersita¨t Wu¨rzburg, Institut fu¨r Organische Chemie and Ro¨ntgen Research Center for Complex Material Systems,
Am Hubland, D-97074 Wu¨rzburg, Germany.
Received February 12, 2007; E-mail: wuerthner@chemie.uni-wuerzburg.de
Perylene bisimides (PBIs) have been extensively studied as
building blocks for functional supramolecular architectures in
nonpolar solvents through hydrogen bonding, metal ion coordina-
tion, and π-π stacking interaction.1 In contrast, little attention was
paid to the self-assembly behavior of PBIs in aqueous solution.2
Amphiphilic molecules, consisting of hydrophobic and hydrophilic
moieties, self-assemble into highly organized aggregates with
various morphologies such as spherical micelles, wormlike micelles,
spherical and hollow vesicles, planar bilayers, nanotubes, and
others.3 The formation of these morphologies depends on solvent
environments, molecular structures and shapes, as well as the
relative fraction of hydrophilic and hydrophobic parts.4 On the other
hand, the stabilization of self-assembled superstructures through
covalent linkage, for example, by polymerization, is of great interest
for the practical application of supramolecular chemistry.5 By
covalent stabilization, the aggregate dimension and shape can be
captured, and aggregate strength can be increased.6 Here, we report
Figure 1. Fluorescence spectra of PBI 1 in THF and THF-containing water,
λex ) 490 nm, [PBI 1] ) 2 × 10-6 M. The inset shows a photograph of
for the first time that the nanoaggregates of perylene bisimides with
particular morphology can be obtained by self-assembly of differ-
PBI 1 in THF and THF-containing water under UV-light irradiation.
ently shaped amphiphilic PBIs in aqueous solution.
high degree of curvature due to the conical structure. The same
aggregation behavior was also observed for PBI 2. In contrast,
dumbbell-shaped PBI 4 self-assembled into rod aggregates with a
diameter of 4 nm (Figure 2b). The formation of well-defined
aggregates of these PBIs may be attributed to the π-π stacking
interaction between the perylene cores along one-dimensional long
axis without curvature, and stabilized by surrounding hydrophilic
chains as shown in Scheme 1(bottom).
Chart 1
More interestingly, hollow vesicles were observed for the co-self-
assembled system of PBI 1 and PBI 3 ([PBI 1]/[PBI 3] ) 8/1 in
molar ratio) in THF-containing water (2%, v/v) as shown in Figure
2c. Spherical vesicles were formed with the average diameter of 94
nm and the wall thickness of 7-8 nm, which is approximately twice
the length of a single optimized molecule PBI 1 (3.3 nm) or PBI 3
(4.0 nm) in water. The aggregation number for the inner and outer
layers were calculated to be approximate 2.9 × 105 and 3.2 × 105,
respectively, from the bilayer volume divided by the volume of the
single molecule. For the co-self-assembled system with higher PBI
3 content ([PBI 1]/[PBI 3]) 4/1), bilayer vesicles were observed
with a larger average diameter of 133 nm in THF-containing water
(2%, v/v) as shown in Figure 2e. The size distribution of these co-
aggregates also became broader at higher PBI 3 content as measured
from TEM (Figure 2g,h) and dynamic light scattering (DLS) (Figure
S-24), and these large vesicles emit red fluorescence (Figure 2f).
The observed variation of the aggregates’ shape and size can be
rationalized by considering spontaneous curvature. When dumbbell-
shaped PBI 3 is co-self-assembled with wedge-shaped PBI 1, the
average hydrophobic part increases, and the interface between the
hydrophilic and hydrophobic parts changes from a curved interface
to a more flat one. The resulting decrease in spontaneous curvature
releases the strain in the initially formed micelles with a high
curvature, leading to micelle growth and a transition to vesicles as
In this work, the wedge- and dumbbell-shaped amphiphilic
perylene bisimides PBI 1-4 (Chart 1) were synthesized and fully
characterized (see details in Supporting Information). The absorption
and fluorescence spectra of molecularly dissolved PBI 1-4 display
a mirror-image relationship with a small Stokes’ shift (5-6 nm) in
“good” solvents such as CH2Cl2 and THF (Figure S-15 in
Supporting Information). Upon addition of the “bad” solvent water
into the THF solution, a gradual decrease in fluorescence intensity
at 500-600 nm was observed as exemplified for PBI 1 (Figure 1),
and a new broad and structureless fluorescence band appears at
600-800 nm owing to excimer formation.7 These observations
imply that monomeric PBI 1 self-assembles into fluorescent
multimolecular aggregates in aqueous solution.
For the wedge-shaped PBI 1, Israelachvili’s critical packing
parameter (Pc)8 was calculated to be 0.252, implying that spherical
micelles should be favorably formed in polar solvents (Pc < 1/3 for
micelle formation).8a The theoretical prediction was confirmed by
transmission electron microscopic (TEM) studies. A large number
of spherical micelles with the diameter of 4-6 nm were observed
for PBI 1 in THF-containing water (2%, v/v) as shown in Figure
2a. The micelles were formed with a narrow polydispersity and a
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J. AM. CHEM. SOC. 2007, 129, 4886-4887
10.1021/ja070994u CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Schematic Illustration for the Formation of Micelles
from Wedge-Shaped PBI 1 (top), Bilayer Vesicles from the
Co-self-assembly of PBI 1 and Dumbbell-Shaped PBI 3 (middle),
and Rod Aggregates from Dumbbell-Shaped PBI 4 (bottom).
In summary, the aggregate morphologies of amphiphilic PBIs
are dependent on their molecular shapes. The co-self-assembly of
wedge-shaped PBI 1 and dumbbell-shaped PBI 3 generated hollow
vesicles owing to the changes in spontaneous curvature. The bilayer
structures of the vesicles could be stabilized by in situ photopo-
lymerization. The morphology changes by co-self-assembly of
different molecular architectures provide a guideline for the rational
design of particular morphology such as vesicles.
Acknowledgment. We thank the Alexander von Humboldt
Foundation (fellowship for Xin Zhang) and Theo Kaiser, Elisabeth
Meyer-Natus, and Dr. Georg Krohne for their kind help.
Supporting Information Available: Synthetic details, spectroscopic
characterization, UV-vis absorption, fluorescence and 2D NMR spectra
of amphiphilic perylene bisimides, TEM images, and DLS data of
aggregates. This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 2. TEM images of self-assembled PBI 1 (a), PBI 4 (b), and before
(c) and after (d) photopolymerization of co-aggregates of PBI 1 and PBI 3
in a 8:1 molar ratio; TEM (e) and confocal fluorescence images (f) of co-
aggregates of PBI 1 and PBI 3 in a 4:1 molar ratio in THF-containing water
(2%, v/v); and size distribution obtained from 548 co-aggregates ([PBI
1]/[PBI 3] ) 8/1) (g), and from 402 co-aggregates ([PBI 1]/[PBI 3] ) 4/1)
(h). [PBI 1] ) 0.5 mg/mL (4.32 × 10-4 M); [PBI 4] ) 0.5 mg/mL.
References
(1) Wu¨rthner, F. Chem. Commun. 2004, 1564-1579.
(2) (a) Abdalla, M. A.; Bayer, J.; Ra¨dler, J. O.; Mu¨llen, K. Angew. Chem.,
Int. Ed. 2004, 43, 3967-3970. (b) Wang, W.; Wan, W.; Zhou, H. H.;
Niu, S. Q.; Li, A. D. Q. J. Am. Chem. Soc. 2003, 125, 5248-5249.
(3) (a) Percec, V.; Dulcey, A. E.; Balagurusamy, V. S. K.; Miura, Y.;
Smidrkal, J.; Peterca, M.; Nummelin, S.; Edlund, U.; Hudson, S. D.;
Heiney, P. A.; Hu, D. A.; Magonov, S. N.; Vinogradov, S. A. Nature
2004, 430, 764-768. (b) Antonietti, M.; Fo¨rster, S. AdV. Mater. 2003,
15, 1323-1333. (c) Hill, J. P.; Jin, W. S.; Kosaka, A.; Fukushima, T.;
Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T.
Science 2004, 304, 1481-1483. (d) Hartgerink, J. D.; Beniash, E.; Stupp,
S. I. Science 2001, 294, 1684-1688.
illustrated in Scheme 1 (top and middle). It is interesting to note
that such mechanism for the control of self-assembly by spontane-
ous curvature is present in biology. A typical example is the
deformation of flat lipid membranes by protein coating into transport
spherical vesicles.9
(4) (a) Uzun, O.; Sanyal, A.; Nakade, H.; Thibault, R. J.; Rotello, V. M. J.
Am. Chem. Soc. 2004, 126, 14773-14777. (b) Arnt, L.; Tew, G. N. J.
Am. Chem. Soc. 2002, 124, 7664-7665. (c) Percec, V.; Ahn, C. H.; Ungar,
G.; Yeardley, D. J. P.; Mo¨ller, M.; Sheiko, S. S. Nature 1998, 391, 161-
164.
(5) Mu¨ller, A.; O’Brien, D. F. Chem. ReV. 2002, 102, 727-757.
(6) (a) Yamamoto, T.; Fukushima, T.; Yamamoto, Y.; Kosaka, A.; Jin, W.;
Ishii, N.; Aida, T. J. Am. Chem. Soc. 2006, 128, 14337-14340. (b) Zhang,
X.; Li, Z. C.; Li, K. B.; Lin, S.; Du, F. S.; Li, F. M. Prog. Polym. Sci.
2006, 31, 893-948.
(7) Wu¨rthner, F.; Chen, Z.; Dehm, V.; Stepanenko, V. Chem. Commun. 2006,
1188-1190.
(8) (a) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic
Press: San Diego, CA, 1991. (b) Pc ) V/al, where V is the effective
molecular volume, a is the occupied area by hydrophilic chains, and l is
optimal molecular length.
(9) (a) Bigay, J.; Gounon, P.; Robineau, S.; Antonny, B. Nature 2003, 426,
563-566. (b) Lippincott-Schwartz, J.; Liu, W. Nature 2003, 426,
507-508.
To stabilize these nanoaggregates, the in situ photopolymerization
was performed in an organized state using 2,2-dimethoxy-2-
phenylacetophenone as photoinitiator under UV irradiation at 350
nm. This wavelength was chosen because the photoinitiator has a
maximum absorption at 350 nm, where perylene chromophores have
a very weak absorption.1 After photopolymerization, the vesicles
from co-self-assembly retained their original shapes as shown in
Figure 2d. The photopolymerization was confirmed by infrared (IR)
studies. The vibration band at 968.1 cm-1 (characteristic vibration
of δ(dCH2) wagging mode) disappears after photopolymerization
(Figure S-23). These vesicles do not change their morphology after
the addition of PBI 3, confirming that the vesicle structures are
locked by photopolymerization.
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