Controlled self-assembly behavior of an amphiphilic bisporphyrin- bipyridinium-palladium complex: From multibilayer vesicles to hollow capsules

Article abstract of DOI:10.1002/anie.200600554

(Figure Presented) Gas-filled: Porphyrin-based amphiphiles can self-assemble into multibilayer vesicles. These vesicles can be converted into hollow capsules and wormlike structures (see picture) by controlling the release rate of methanol encapsulated th

Full text of DOI:10.1002/anie.200600554

Angewandte
Chemie
Porphyrins
recent years.[10,11] The introduction of molecular recognition
motifs into the porphyrin building blocks, such as hydrogen
bonding, p-p stacking, electrostatic interactions, and metal–
ligand bonds offers an easy way to access well-defined arrays
such as fibers, sheets, grids, cubes, wheels, and rings.[12–14]
Amphiphilic porphyrins have been exploited in the prepara-
tion of simple micelles,[15] fibers,[16] and vesicles.[17] Their
optoelectronic features are strictly related to their aggrega-
tion states and strongly depend on the microstructural
environment.[18]
DOI: 10.1002/anie.200600554
Controlled Self-Assembly Behavior of an
Amphiphilic Bisporphyrin–Bipyridinium–
Palladium Complex: From Multibilayer Vesicles
to Hollow Capsules**
Yongjun Li, Xiaofang Li, Yuliang Li,* Huibiao Liu,
Shu Wang, Haiyang Gan, Junbo Li, Ning Wang,
Xiaorong He, and Daoben Zhu*
Herein, we describe the aggregation behavior of a new
zinc porphyrin derivative 1 (Scheme 1) in which two zinc
The construction of supramolecular assemblies with well-
defined nano-structures is of great interest owing to their
potential applications in diverse fields such as molecular
electronics, light-energy conversion, and catalysis.[1,2] The
ability to control the specific shapes, dimensions, and
pattern formation of supramolecular organization by
nonconvalent interactions is still a challenging task in the
materials field.[3] Nanoscale hollow capsules represent an
important class of materials because their unique struc-
tural, optical, and surface properties may lead to a wide
range of applications, such as capsule agents for drug
delivery, filters, coatings, chemical catalysis, or templates
for functional architectural composite materials.[4] Various
efforts have been made to prepare inorganic[5] and
organic[6] hollow capsules. Hollow polymer spheres were
produced by McDonald et al. through the encapsulation of
hydrocarbon solvents within polymer particles during an
emulsion polymerization.[7] C60-derived amphiphiles have
a tendency to form closed submicrospheres with a bilayer
shell below the critical aggregation concentration and
multibilayer vesicles above the critical aggregation con-
centration.[8] Meanwhile, 1D nanotubes and nanowires
have attracted much attention because of their unique
optical, electronic, and mechanical properties, which result in
promising applications in electrical and optoelectronic nano-
devices.[9]
Scheme 1. Synthesis of bpy–ZnP and 1. a) toluene/pyridine, RT, 60%; b) Zn(OAc)2/
CH3OH, CHCl3, 98%; c) PdCl2(CH3CN)2, CHCl3/CH3CN.
porphyrins were attached to the 4,4’-position of the 2,2’-
bipyridyl group, and the 2,2’-bipyridine (bpy) was complexed
with palladium (ii) dichloride. The precursor of 1, bpy–ZnP,
has one structural feature: the two porphyrin units can rotate
freely around the central bipyridine bond and as such, the
relative position of two porphyrins can be controlled by the
addition/elimination equilibrium of metal ions.[19] Bipyridyl
groups are strong ligands for various transition metal ions, 1 is
an amphiphile because the bipyridine–PdCl2 units exhibit
hydrophilic properties and the porphyrin units show hydro-
phobic interactions under specific solvent environments.[20,21]
Compound 1 was synthesized according to Scheme 1.
Characterization was performed through NMR spectroscopy
and MALDI-TOF MS. The MALDI-TOF mass spectra of 1
exhibited two signals that could be assigned as [MÀ2Cl] and
[MÀPdCl2].
Porphyrins have remarkable photo-, catalytic-, electro-,
and biochemical properties, and so the self-assembly of
porphyrin derivatives has attracted considerable attention in
[*] Y. J. Li, Dr. X. F. Li, Prof. Y. L. Li, Dr. H. B. Liu, Prof. S. Wang,
H. Y. Gan, J. Li, N. Wang, X. R. He, Prof. D. B. Zhu
CAS Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100080 (P.R. China)
Fax: (+86)10-8261-6576
E-mail: ylli@iccas.ac.cn
zhudb@iccas.ac.cn
[**] This work was supported by the National Natural Science
Foundation of China (20531060, 10474101, 20418001, 20571078,
and 20421101), and the Major State Basic Research Development
Program (2005CB623602). This project is partly supported by
National Center for Nanoscience and Technology, China.
Although the growth of a single crystal from bpy–ZnP in
different solvents was not successful, we succeeded in growing
the single crystal of the bpy–ZnP–pyridine complex in n-
hexane (the crystal data are shown in the Supporting
Information). The single-crystal X-ray structure analysis
showed that the porphyrin rings are distorted. Each zinc
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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porphyrin coordinates with one pyridine molecule that
alternates above and below the plane (Figure 1). The distance
between the nitrogen atom with the coordinated pyridine and
the zinc atom is 2.113Š. The two nitrogen atoms of bipyridine
Figure 1. Crystal structure of the bpy–ZnP–pyridine complex. Hydrogen
atoms, 3,4,5-trimethoxyphenyl, and solvent molecules are omitted for
clarity.
face in the opposite direction, whereas the bipyridine and two
amides are nearly planar (the mean deviation from the plane
is 0.0321). The two benzene rings connecting the porphyrin
and the amide are parallel (the dihedral angel is 08). The
dihedral angel between these benzene rings and the bipyr-
idine plane is 30.18. From the crystal-packing mode of the
bpy–ZnP–pyridine complex, we can see that the bipyridine of
one molecule overlaps with the porphyrin part of another
molecule.
Figure 2. Space-filling models of bpy–ZnP from a top view (a), and a
top view (b) and side view (c) of complex 1. The MM2 force field was
used to calculate the minimium-energy conformation.
The growth of a single crystal of 1 in a variety of different
solvents was not successful. Computer simulations were used
to gain insight into the structure.[6a] The simulation results
indicated that bpy–ZnP showed linear conformers similar to
the X-ray crystal structure, which indicated that this simu-
lation method was fit for our porphyrin dimer system. The
simulation results showed that 1 was a V-shaped structure
(Figure 2), which was confirmed by NMR spectroscopic-
titration experiments. After the addition of 2.2 equivalents of
[PdCl2(CH3CN)2], palladium complex 1 was completely
formed along with the [PdCl2(pyridine)2] complex.[19] The
large downfield shift of 1-H (+ 1.5 ppm) reflected the
development of a d + charge upon complexation with
PdCl2. The most-sensitive shielding effect of the porphyrin-
ring current appeared at the meso phenyl groups. According
to symmetry, there were two sets of meso phenyl groups in
bpy–ZnP, namely, four 3,4,5-trimethoxyphenyl groups adja-
cent to, and two opposite to the bipyridyl groups. Before
complexation, these groups were not distinguishable from one
bpy–ZnP from a freely rotating conformation to a cofacial V-
shaped one.
The aggregation behavior of 1 was subsequently studied
by injecting a solution of 1 in CHCl3 (at a concentration of 1 ”
10À4 m) into CH3OH to give a final chloroform/methanol ratio
of 1:1 (v/v). After the solution was allowed to equilibrate over
one day, one drop of the solution was evaporated on silicon
slides to observe the aggregate behavior in the solid state. The
morphology of molecule 1 on the substrate was examined by
scanning electron microscopy (SEM; Figure 3). In SEM
images, spherical particles with a uniform diameter of about
200 nm were observed. Energy-dispersive X-ray spectroscopy
(EDX) indicated the presence of C, N, O, Cl, Pd, and Zn, and
the atom ratio of Cl/Pd/Zn was about 2:1:2. The majority of
the spherical particles were larger than the typical micelle size
of 20–30 nm in diameter as they were derived from small
molecular amphiphiles.[22] A few of the particles started to
form opening holes on their surfaces, which showed that they
had a hollow interior. To confirm this possibility and
substantiate whether these vesicles are hollow in nature, the
perfect spherical vesicles (Figure 3a) prepared at a 5.0 ”
10À5 m were subjected to heat treatment for one hour (heating
pattern I). This then allowed us to see that the holes were
formed on the particles surface (Figure 3b); one example of
the typical open vesicles is shown in the inset of Figure 3b.
TEM images also showed that some capsulelike structures
with holes were formed (Figure 3c). According to the
irregularity of the size and shape of the hole and the location
of defective sites at the surface, we suggested that these holes
are not a result of the density–gradient image of the spherical
1
another in the H NMR spectrum because of free and rapid
rotation around the bond connecting the two pyridyl units.
Thus, the protons of the meso phenyl groups (meso-Ph-H)
appeared as a single peak near d = 7.4 ppm; the para-methoxy
and meta-methoxy groups in these meso phenyl groups
appeared as a single peak near d = 4.06 and 3.89 ppm,
respectively. When palladium complex 1 was formed, each
of the single peaks split into a doublet. Peak separations
between the meso-Ph-H as well as between the methoxy
groups in the two sets of meso phenyl groups became
detectable owing to differences in the distances from the
facing porphyrin plane. These results showed that the
addition of 2.2 equivalents of PdII completely converted the
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Figure 3. a) SEM of 1 derived vesicles prepared in CHCl3/CH3OH (1:1)
at room temperature. b) SEM and c) TEM of compound 1 vesicles
derived from the heat treatment (pattern I at 708C) of the vesicles
prepared as in a). The inset of b) shows a close up of a vesicle, and
the inset of c) shows the membrane thickness, indicated by arrows.
d) Schematic representation of compound 1. e) Schematic representa-
tion of a compound 1 vesicle formed in methanol with a close up of
the vesicle membrane showing the proposed multibilayer structure.
The blue dots represent the ligated CH3OH. f) Schematic representa-
tion of an interdigitated bilayer structure.
Figure 4. Morphology transition of the vesicle heated a) at 608C for
0.5 h (inset 2.6” enlargement), b) at 808C for 0.5 h (inset 2.6”
enlargement), and wormlike aggregates of 1 derived from the heat
treatment (heating pattern II) of the vesicles prepared as Figure 3a,
c) SEM and d) TEM.
close to each other could fuse together during this process.
When the vesicles were heated at 50–608C, the methanol
escaped and evaporated quickly from the aperture of the
membrane without resulting in obvious changes in morphol-
ogy. However, when heated at 70–808C (higher than the
boiling point of methanol), the evaporating methanol
bumped and the gas broke quickly through the membrane.
This resulted in deformation and the hollow spherical vesicles
were formed. When heated at 908C for 0.5 h, in addition to
the escape of methanol (some hollow capsules were visible),
the molecular motion of 1 was excited and rearrangement was
achieved during the cooling process. Some fused vesicles were
found in which the hollow capsules were empty. Although
heated with pattern II, a wormlike morphology was observed
by using SEM as shown in Figure 4c, and TEM images
(Figure 4d) also confirmed the wormlike structure. These
results indicated that heating temperature and pattern are
able to control the rate of methanol release and lead to a
controllable process for producing hollow capsules and
wormlike structures from these vesicles.
The UV/Vis spectra of the aggregated species were
significantly different from that of the corresponding porphy-
rin solutions. The absorption spectra of the films resulting
from a chloroform/methanol mixture (v/v, 1:1) are shown in
Figure 5. Together with the expected bands at 440 nm and the
Q bands in the range from 500–700 nm, there was one
additional band at 680 nm. Both the Soret and Q absorption
bands of the films were broadened and red shifted when
compared with the organic solution phase. These vesicular
supramolecular structures suggest “J-type” (edge-to-edge)
interaction in the vesicles.[21,25]
vesicles.[22] The edge of these holes should be measurable to
reveal the thickness of the vesicle shell. As shown in the inset
of Figure 3c, we found that the morphology of the slightly
“tilted” shell membrane on the top was surrounded by a dark
ring area with an irregular wall width and shape. Measuring
the membrane-wall thickness at the tilted-membrane site
(perpendicular to the view as marked by the arrow) gave an
edge width of about 15–20 nm. These distances across the
membrane wall fit approximately with 2–3bilayers with a
molecular dimension of 12–18 nm, estimated by the 3D
molecular structural modeling of 1. These results indicate that
the most likely membrane structure of the hollow spherical
vesicles in methanol was a shell-like complex multilayer
structure resembling that of a liposome at the vesicle surface
(Figure 3c). Our vesicles were very stable on the solid surface,
even when heated at 50–608C (Figure 4a) or stored for
12 days (see the Supporting Information). Furthermore, they
were unlike other vesicles that showed an immediate collapse
on solid surfaces;[23] dried samples of egg-yolk lecithin vesicles
showed only planar circles and bolaamphiphile vesicles
crystallized quickly upon drying to form circular platelets of
double-layer thickness.[24]
Figure 4 and the Supporting Information show SEM
images of the vesicles on a silica slide after heating at
different temperatures for 0.5 h and then cooling to room
temperature. The volume of encapsulated methanol
expanded when the vesicles were heated at 408C (lower
than the boiling point of methanol) and as such, the methanol
that flowed from the aperture of the membrane couldnꢀt
evaporate quickly. The nearby molecules then flowed with the
methanol and formed the “tail” morphology. “Tails” located
In selective solvents, block copolymer systems or amphi-
philic molecules like liposomes form micellar or vesicular
supramolecular structures that would normally belong to the
superstrong segregation limit (SSSL). This is mainly owing to
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SEM images were taken by using a field emission scanning
electron microscope (JEOL JSM 6700F and Hitachi S4300) operated
at an acceleration voltage of 5–15 kV. TEM images were taken by
using a JEOL-100 microscope operated at 50 kV. The samples were
prepared by transferring them from the silicon slides by carbon–
copper grids dampened with methanol.
Received: February 10, 2006
Revised: March 15, 2006
Published online: April 27, 2006
Keywords: amphiphiles · porphyrinoids · self-assembly ·
.
vesicles
Figure 5. UV/Vis spectra of solutions of 1 in chloroform (dotted line),
chloroform/methanol (1:1; solid line), and the film cast from the
chloroform/methanol (1:1) solution (dashed line).
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like aggregates shown in Figure 4c,d.
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composed of two porphyrins connected with 2,2’-bipyridyl
group, in which 2,2’-bipyridine was complexed with palladi-
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experiments indicated that the Pd complex showed a V-
shaped conformer. A simple and controllable process for
producing the hollow capsules from vesicles was described.
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CHCl3/CH3OH. These vesicles could assemble into hollow
capsules and wormlike structure on demand.
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