Morphology-tunable architectures constructed by supramolecular assemblies of α-diimine compound: Fabrication and application as multifunctional host systems

Article abstract of DOI:10.1039/c1jm13081a

An α-diimine compound (DC) bearing multiple hydroxyl and amine groups presents excellent self-assembly behavior, yielding DC self-assemblies with tunable morphologies ranging from solid spheres, nanotubes and capsules via hydrogen bonds and π-π stacking i

Full text of DOI:10.1039/c1jm13081a

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PAPER
Morphology-tunable architectures constructed by supramolecular assemblies
of a-diimine compound: fabrication and application as multifunctional host
systems†
Haiqing Li,ab Bijal K. Bahuleyan,ac Renjith P. Johnson,a Yury A. Shchipunov,a Hongsuk Suh,d Chang-Sik Haa
a
*
and Il Kim
Received 4th July 2011, Accepted 2nd September 2011
DOI: 10.1039/c1jm13081a
An a-diimine compound (DC) bearing multiple hydroxyl and amine groups presents excellent self-
assembly behavior, yielding DC self-assemblies with tunable morphologies ranging from solid spheres,
nanotubes and capsules via hydrogen bonds and p–p stacking interactions. These DC self-assemblies
provide promising multifunctional hosts for varied metal species. As a typical example, Au
nanoparticles are in situ generated and accommodated into both solid and hollow DC self-assemblies
by one-pot and two-step fabrication processes, respectively, resulting in the formation of solid and
hollow DC/Au hybrid nanostructures. Both solid and hollow DC self-assemblies also enable host Ni(II)
ions to generate DC/Ni(II) catalysts for the efficient production of porous polyethylene (PE) beads
consisting of numerous PE microspheres. Moreover, the yielded PE beads replicate the textural
morphologies of the original DC/Ni(II) catalysts. These DC self-assemblies also might be further
utilized to host varied metal species to fabricate versatile DC/metal nanoparticle (Ag, Pt, Pd, etc.)
hybrids and porous polyolefin beads with desired morphologies.
such as p–p interaction, hydrogen bonding, electrostatic
attraction, metal–ligand coordination and their combinations to
hold the subunits together.
Introduction
Significant efforts have been devoted to the fabrication of diverse
host systems with micro- and nanoscale size, as they provide
some of the best means to obtain multifunctional materials with
distinct and desired properties.1 Supramolecular chemistry offers
a powerful and convenient approach to prepare such fascinating
host materials with well-defined architectures from small
molecular building blocks, since the synthetic assemblies gener-
ally possess enormous internal cavities ranging from a few cubic
angstroms to over a cubic nanometre.2 Such supramolecular
assemblies often rely on a variety of noncovalent interactions
Recent studies have demonstrated that the efficiency for guest-
encapsulation strongly depends on the molecular composition
and structure of the small building blocks owing to the following
reasons: (1) functional components act as media to induce the
formation of intermolecular non-covalent interactions; (2) the
stereostructure of molecules together with (1) direct the stacking
and growing fashions of building blocks into supramolecular
assemblies, which not only determine the morphologies of final
self-assembled architectures, but leads to the formation of
internal cavities to facilitate guest species accommodation,
transportation and exchange;3 (3) functional moieties provide
‘‘active sites’’ to catch guest matter, forming novel hybrids with
promising properties. Therefore, suitable molecular design of
building blocks is of enormous significance to create promising
supramolecular hosts. To date, based on diverse molecular
compositions and structures of building blocks such as porphy-
rins,4 bis(imidazol-1-ylmethyl)benzene,5 p-phenylene-vinylene,6
naphthalene,7,8 etc., a wide range of supramolecular host systems
have been created and exhibit extensive catalytic, bio-imaging
and photoelectrical functions by encapsulation of various func-
tional guest species. However, most of those supramolecular
hosts tend to lack multifunctional encapsulating capacity
towards diverse guest matter, owing to the limited functions of
building blocks. Especially for the multifunctional host systems
aThe WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials,
Department of Polymer Science and Engineering, Pusan National
University, Busan, 609 735, Korea. E-mail: ilkim@pusan.ac.kr; Fax: +82
51-513-7720; Tel: +82 51 510 2466
bAustralian Institute for Bioengineering and Nanotechnology, The
University of Queensland, Brisbane, QLD, 4072, Australia. E-mail: h.
li8@uq.edu.au
cDepartment of Chemistry, Tokyo Metropolitan University, Tokyo,
1920397, Japan. E-mail: kbijal@gmail.com
dDepartment of Chemistry and Chemistry Institute for Functional
Materials, Pusan National University, Busan, 609 735, Korea. E-mail:
hssuh@pusan.ac.kr
† Electronic supplementary information (ESI) available: Synthesis of
compounds, molecular dynamics simulations, Raman spectra of hollow
DC and DC/Au particles, the EDX spectrum of DC/Ni(II) assemblies,
and the plots of ethylene polymerization rate versus time. See DOI:
10.1039/c1jm13081a
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which not only can accommodate in situ generated metal nano-
particles, but allow coordination of late-transition metals (such
as nickel, cobalt, etc.) to form bulky catalysts for olefin poly-
merizations, few explorations have been involved. Targeted to
that, herein, a novel type of multifunctional supramolecular
hosts has been fabricated using a-diimine compound (DC) as
building blocks (Fig. 1).
numerous building blocks can incorporate stoichiometric
amount of metals, forming bulky catalysts towards olefin
polymerizations.
Experimental
Materials
DC contains a conjugated acenaphthylene-1,2-diimine center
substituted by bis(2,6-diisopropylaniline), four hydroxyl, two
amine and two methoxy groups, which may facilitate the
formation of promising self-assembled architectures through
intermolecular p–p stacking interaction and hydrogen bonds.
Multiple hydroxyl group containing compounds, such as
ethylene glycol,9 glucose10 and hyperbranched polyglycidol,11 can
be used as effective reducing agents towards various metal ions,
yielding metal nanoparticles. Analogously, DC self-assemblies
contain a large number of phenolic hydroxyl groups, which may
provide promising platforms for in situ generation and accom-
modation of metal nanoparticles (NPs). In addition, since the
a-diimine center of DC provides reactive sites for easily coordi-
nating late-transition metals,12,13 each DC assembly composed of
The polymerization grade of ethylene (SK Co., Korea) was
purified by passing it through columns of Fisher RIDOXꢀ
ꢀ
catalysts and molecular sieve 5 A/13ꢀ. All other reagents used in
the current study were purchased from Aldrich Chemical Co.
The organic solvents for synthesis of DC and ethylene poly-
merization were purified using known procedures and stored
ꢀ
over molecular sieves (4 A). The other chemicals were used as
received without further purification.
Synthesis of DC
The synthetic procedures13 for DC are described in the ESI†.
Fabrication of DC self-assemblies with tunable morphologies
To fabricate solid spheres, 2.0 mL of premixed solvent (THF : DI
water ¼ 1 : 3) was added to a vial containing 1 mg of DC with
vigorous stirring at room temperature. Subsequently, the
homogeneous solution was transferred to a cellulose acetate
dialysis tube (MWCO 12 000 g molꢁ1) and subjected to dialysis
treatment in deionized (DI) water for 24 h. The obtained turbid
solution was collected and the solid assembled spheres with 604
nm in average diameter were prepared. Following similar
procedures, the solid assembled spheres with 183 and 115 nm in
mean size were fabricated by using 0.7 and 0.4 mg of DC,
respectively. To fabricate DC assembled tubes, 1.5 mL of DI
water was added to a vial containing 1 mg of DC. After soni-
cating treatment for 30 seconds, 0.5 mL of THF was added with
vigorous stirring. Thereafter, an additional 0.5 mL of DI water
was injected with magnetic stirring for 5 min. Followed by
dialysis treatments for 24 h, turbid assembled tubes solution was
obtained. If the employed amount of additional DI water was 1.0
mL, the hollow assembled spheres can be generated following the
same procedures. Note that all the DC self-assemblies were
finally collected and redispersed in 2 mL of DI water.
In situ synthesis of DC/Au NPs hybrids
1.5 mL of DI water was added to a vial containing 1 mg of DC.
After sonicating treatment for 30 seconds, 0.5 mL of THF was
added with vigorous stirring. Thereafter, an additional 1.0 mL of
HAuCl4 aqueous solution (1 mM) was injected with magnetic
stirring for 3 h. Followed by dialysis treatments in DI water for
24 h, solid DC/Au hybrid nanospheres dispersed in 2 mL of DI
water were obtained. To fabricate hollow DC assemblies
encapsulating Au NPs, hollow assembled spheres were firstly
prepared as described above. Thereafter, 1.0 mL of HAuCl4
aqueous solution (1 mM) was injected into the as-prepared
turbid solution. After stirring at room temperature for 3 h, Au
NPs were in situ generated and encapsulated into the matrices of
hollow architectures. The resulting hybrids were purified by
three-circles of centrifugation and redispersion in DI water.
Fig. 1 (a) Molecular structure of a-diimine compound of this study; (b)
its geometrical energy minimized structure generated by the Forcite
model with the Dreiding 2.21 force field system and Gasteiger charges
using Materials Studioꢀ release 4.3 (see ESI† for details); and (c) pre-
ꢀ
ꢀ
dicted crystal structure (space group ¼ P1, a ¼ 9.752 A, b ¼ 11.597 A, c ¼
ꢂ
ꢂ
ꢂ
37.867 A, a ¼ 9.752 , b ¼ 11.597 , g ¼ 77.5 , density ¼ 1.046 g cmꢁ3) by
Polymorph Predictor embedded in the same package, showing
a hydrogen-bonded chain (dotted line).
ꢀ
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Eventually, the hollow DC/Au hybrids were collected by
centrifugation and then redispersed in 2 mL of DI water.
collected in the 2q range 15–90ꢂ in steps of 0.02ꢂ and a counting
time of 2 s per step.
Crystal structure prediction was performed using the Poly-
morph Predictor (PP) module in Materials Studioꢀ release 4.3.
The DC molecule was first energetically optimised in the
Dreiding 2.21 force field by using the Forcite model. The details
are described in the ESI†.
DC self-assemblies hosting nickel ions for ethylene
polymerization
Following the same procedures described above, 11.5 mg of
hollow and solid DC self-assemblies were firstly prepared and
redispersed in absolute ethanol, respectively. Subsequently, 2.2
mg of NiBr2 (1 : 1 equivalent DC) was added into the as-
prepared colloid solution. After continuous stirring at 30 ꢂC for
24 h, the Ni(II) coordinated DC self-assemblies were purified by
three-circles of centrifugation and redispersion in absolute
ethanol. The solid sample was collected by centrifugation and
The molecular weight and polydispersity index (PDI) of PE
were determined by gel permeation chromatography (GPC,
PL-GPC220/FTIR, 135 ꢂC) in 1,2,4-trichlorobenzene using
polystyrene columns as the sꢂtandard. The intrinsic viscosity was
measured in decalin at 135 C using an Ubbelohde viscometer
and the viscosity average molecular weight (Mv) was calculated.
Differential scanning calorimetry (DSC, Perkin-Elmer) analysis
then dried under vacuum at 60 C overnight. Since Ni(II) coor-
was carried out at a 10 C minꢁ1 heating rate under a nitrogen
ꢂ
ꢂ
dinated DC is paramagnetic, a high resolution NMR spectro-
scopic analysis was not feasible. Anal. Calcd for hollow Ni(II)
coordinated self-assemblies: C, 66.72; H, 6.89; N, 3.99. Found C,
66.68; H, 6.91; N, 4.05%. MS (FAB+): m/z ¼ 1323 (M+ ꢁ Br).
Anal. Calcd for solid Ni(II) coordinated self-assemblies: C, 66.72;
H, 6.89; N, 3.99. Found C, 66.70; H, 6.91; N, 4.07%. MS (FAB+):
m/z ¼ 1323 (M+ ꢁ Br).
atmosphere. The branching numbers for PE were determined by
1H NMR spectroscopy using the ratio of the number of methyl
groups to the overall number of carbons and are reported as
branches per thousand carbons.
Results and discussion
The ethylene polymerizations were carried out under a purified
nitrogen atmosphere using the Schlenk technique.13 Briefly, Ni(II)
complex (2.5 mmol) and dry toluene (80 mL) were added into
a 250 mL round-bottom Schlenk flask equipped with a magnetic
stirrer and a thermometer outside. After the solvent was satu-
rated with ethylene at 1.3 atm, methylaluminoxane (MAO,
MAO/Ni ¼ 250) was injected into the reaction system. The
polymerization reaction was performed at 50 ꢂC for 30 min. The
reaction was stopped by the addition of ethanol. After precipi-
tation of reaction solution in 10% HCl–EtOH and subsequent
filtration, polyethyleneꢂ(PE) was collected by filtration and then
dried in vacuum at 40 C for 20 h.
DC self-assemblies with tunable morphologies
Inspired by the fabrication of DC supramolecular hosts, bare
organic matrices in the form of micro- and nano-architectures
were initially investigated and prepared by nonsolvent (DI water)
induction phase separation from homogeneous a-diimine solu-
tion (in tetrahydrofuran (THF)), followed by dialysis treatments
in DI water. More interestingly, the morphologies of resulting
architectures could be effectively tuned from solid spheres, tubes
to capsules by simply changing the fabrication conditions. To
obtain solid spheres, DC was directly dissolved in premixed
solvent of THF and DI water (v/v, 1 : 3, 0.55 mM of DC) with
vigorous stirring at room temperature, followed by dialysis
treatment in DI water. The resulting colloids were subjected to
SEM and TEM analysis. It was observed that the yielded DC self-
assemblies are solid spheres with a rather broad size distribution
ranging from 200 to 2080 nm in diameter (604 nm in mean size,
Fig. 2a). Following the similar fabrication procedures, when the
employed concentration of DC was further decreased to 0.33 and
0.11 mM, both yielded DC assemblies exhibited solid and spher-
ical architectures (Fig. 2b and c) with diameter in the range of
78–413 nm (183 nm in mean size) and 74–185 nm (115 nm in mean
size), respectively (Fig. 2d). It can be seen that the lower employed
DC concentration is favorable for the formation of DC self-
assemblies with smaller size and narrower size distribution.
Upon feeding 0.5 mL of additional DI water into 2 mL of DC
solution in premixed solvent of THF and DI water (v/v, 1 : 3,
0.55 mM of DC) with vigorous stirring, the homogeneous solu-
tion suddenly became turbid, indicating a rapid formation of DC
self-assemblies. After dialysis against DI water, surprisingly,
uniform nanotubular architectures with 100 nm of average
diameter were obtained (Fig. 3a). A detailed inspection of the
SEM images showed the presence of cracks in some of these
nanotubes (Fig. 3b), most likely caused by the rapid evaporation
of the encapsulated solvent.14,15 The average wall thicknesses of
the nanotubes were measured to be ꢃ25 nm. Following the
similar procedures for fabricating DC nanotubular architectures,
Characterization and molecular dynamics simulations
1
The H-NMR, 13C-NMR spectra of ligands were recorded on
a Varian Gemimi-2000 (300 MHz, 75 MHz) spectrometer. The
1H-NMR spectra of polyethylene (PE) were taken in C6H4Cl2 at
135 ꢂC on a Varian Unity Plus (300 MHz) spectrometer. Samples
dispersed in DI water for scanning electron microscopy (SEM)
were mounted onto a silicon plate and dried in air. After osmium
coating was carried out with an Edwards S150B sputter coater,
SEM analysis was carried out using a JSM-6700F microscope at
an accelerating voltage of 10 kV. Samples dispersed in DI water
for transmission electron microscopy (TEM) were deposited
onto carbon-coated copper electron microscope grids and dried
in air. The size, shape and fine structures of hybrids were inves-
tigated using a MODEL H-7600 (HITACHI) low resolution and
a JEOL 1200 EX high resolution TEM. UV-vis absorption
spectra of the samples were recorded at room temperature on
a UV-1650PC apparatus (SHIMADZU). Fourier transform
(FT)/Raman spectra were recorded at a resolution of 4 cmꢁ1 with
a Bruker IFS 66 interferometer coupled to a Bruker FRA 106
Raman module equipped with a continuous He/Ne laser. The
structures of the products were examined by X-ray diffraction
(XRD) with an automatic Philips powder diffractometer using
a nickel-filtered CuKa radiation. The diffraction pattern was
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Fig. 4a, the homogeneous DC solution shows a broad absor-
bance band centered at around 510 nm. Upon self-assembly, the
absorbance peaks of the DC nanotubes, capsules and solid
spheres are red-shifted to 550, 552 and 610 nm, respectively.
These significant red shifts evidence the presence of strong
intermolecular p–p stacking interactions within the DC assem-
blies.16,17 In addition, compared with the shoulder band of DC
solution at shorter wavelength, the DC assemblies exhibit rela-
tively enhanced absorbance intensity, which results from the
delocalization of the excited state due to p–p interaction.16 Also,
a line broadening of the absorbance bands is visualized for the
DC assemblies, reflecting the strong intermolecular electronic
interactions of the close-packed molecules.18
Further insights into the intermolecular interactions are
obtained by means of Raman spectra (Fig. 4b). DC solution in
the mixed solvent of THF and water shows strong bands at 2968
and 2881 cmꢁ1, corresponding to CH3 and CH stretching
vibrations, respectively. Upon self-assembly, both bands signif-
icantly red-shift and evolve into a relatively broad band situated
at 3330 cmꢁ1. This red-shift of the band position and the band-
width broadening could be related to the combined effects of the
intermolecular hydrogen bondings and p–p stacking interac-
tions.19–21 However, the specific mechanism still remains unclear
so far. Additionally, in the region of 1680–1300 cmꢁ1, DC solu-
Fig. 2 SEM and TEM images of DC assemblies prepared with different
concentrations of DC solutions in premixed solvent of THF and DI water
(v/v, 1 : 3): (a) 0.55, (b) 0.33, (c) 0.11 mM and (d) particle size distribu-
tions of DC assemblies obtained at 0.55 (top), 0.33 (middle) and 0.11 mM
(bottom).
tion exhibits strong bands ranging from 1491 to 1454 cmꢁ1
,
Fig. 3 (a) and (b) SEM images of DC assemblies obtained by using 0.55
mM of DC solution in THF and water (v/v, 1 : 4); (c) and (d) SEM and
TEM images of hollow DC spheres prepared with 0.55 mM of DC
solution in THF and water (v/v, 1 : 5).
when the employed amount of additional DI water was increased
to 1.0 mL, hollow DC microspheres were achieved (Fig. 3c and
d). The outer and inner diameters of these hollow spheres were
430 and 205 nm on average. In addition, the presence of a hole in
some hollow spheres was also visualized as in the case of
nanotubes.
As described above, the self-assembled DC architectures with
tunable morphologies ranging from solid spheres, tubes to
capsules can be generated by simply controlling the composition
of mixed solvent and the involvement of a suitable amount of DI
water. The intriguing behavior of these DC self-assemblies leads
us to get insight into the key intermolecular interactions that
resulted in the final self-assembled architectures. As is shown in
Fig. 4 (a) UV-vis and (b) Raman spectra of DC solution in the mixed
solvent (THF and water, v/v, 1 : 1) and DC self-assembly colloids in
water.
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characteristic of aromatic stretching modes, while the DC self-
assembled colloids demonstrate more abundant bands. In the
case of DC hollow spheres, the aromatic stretching vibrations
red-shift to the region of 1505–1477 cmꢁ1 together with newly
appeared bands at 1659 and 1597 cmꢁ1, suggesting a rearranged
orientation in close-packing manners of aromatic rings in the DC
assemblies.22 The bands at 1642 and 1628 cmꢁ1, 1426 and 1327
cmꢁ1 are attributed to the hydrogen-bonded NH2 and OH
deformation, respectively.23 The bands at 1383 and 1313 cmꢁ1
correspond to CH3 symmetric deformation and COH bending
vibrations, respectively.23 These results demonstrate the presence
of strong hydrogen bonds in the DC assemblies, which play
important roles in directing the self-assembly of DC molecules
(Fig. 1). In comparison, the other DC assemblies exhibit similar
Raman spectra to the DC solid spheres, although there exist
slight differences in the Raman shifts and intensities, which could
be caused by differences in the stacking manners of DC mole-
cules and thus result in the formation of DC assemblies with
varied morphologies.
DC/Au NPs hybrids
In order to demonstrate the efficiency of the in situ generation
and encapsulation of metal NPs in assembled DC architectures,
we first identified DC capsules as typical hosts to encapsulate Au
NPs via a two-step process. Aqueous HAuCl4 solution was
added in the as-prepared self-assembled DC capsules (Fig. 3c)
with sustained stirring at room temperature for 3 h in the absence
of any other additional reagents. After three circles of centrifu-
gation and redispersion in DI water, the resulting colloids were
collected and subjected to microscopy examination. As is shown
in Fig. 5a, the as-prepared colloids exhibit slightly rougher
surfaces compared with the original DC capsules, while the size
remains unchanged. The Au NPs with 9.0 nm in average diam-
eter are clearly distinguished and randomly distributed on the
shell of DC capsules (Fig. 5b). Powder XRD measurements
(Fig. 5c) further evidence the successful generation of face-
centered cubic structured Au NPs (JCPDS 4-784), while DC
capsules exhibit a typical amorphous structure. The formation of
amorphous DC assemblies is caused by the non-ordered packing
of DC molecules during the self-assembly process, where the self-
assembly occurs so rapidly that DC molecules are unable to
adjust their configurations to orderly pack into a well-defined
crystalline structure.
Fig. 5 (a) SEM and (b) TEM images of hollow DC/Au nanostructures;
(c) XRD patterns of DC and DC/Au hollow spheres; (d) SEM and (e)
TEM images of solid DC/Au nanospheres; (f) UV-vis spectra of
as-prepared DC assemblies and DC/Au nanostructures.
formed Au NPs is ꢃ5.5 nm in average diameter, which is smaller
than the Au NPs in DC/Au capsules generated by the two-step
process.
The optical properties of the DC/Au NPs hybrids were
investigated. In comparison with the UV-vis spectra of bare solid
and hollow DC spheres, solid and hollow DC/Au NPs hybrid
colloids exhibit a newly broad absorbance band centered at
around 590 and 540 nm (Fig. 5f), respectively, corresponding to
the surface plasma resonance (SPR) band of encapsulated Au
NPs. Generally, free spherical Au NPs with <20 nm of average
diameter show UV-vis absorbance band at around 520 nm.
These significant red-shifts are characteristic of Au NPs inter-
acted in string-like fashions,24 evidencing the encapsulation of
Au NPs within the DC self-assemblies.
To further simplify the preparation procedures, we have
explored a one-pot protocol to encapsulate Au NPs in the matrix
of DC self-assemblies. In a typical procedure, 1.0 mL of aqueous
HAuCl4 solution (1 mM) was fed to 2 mL of DC solution in
a premixed solvent of THF and DI water (v/v, 1 : 3, 0.55 mM of
DC) with continuous stirring at room temperature for 3 h, fol-
lowed by dialysis treatment in DI water. The resulting colloids
exhibit uniform nanospheres with 100 nm in average diameter
(Fig. 5d). More interestingly, these as-prepared nanospheres
exhibit solid structures, in which numerous Au NPs are in situ
generated. In addition, there exists a uniform shell with ꢃ6.0 nm
in thickness around the as-formed Au NPs (Fig. 5e), directly
visualizing the successful encapsulation of Au NPs within the
matrices of DC self-assemblies. These facts evidence the forma-
tion of DC/Au NPs solid hybrid nanospheres. The size of in situ
In both two-step and one-pot fabrication processes, Au NPs in
DC/Au NPs hybrid are in situ obtained in the absence of any
additional reducing agents; thus DCs serve as both reducing and
stabilizing agents for the in situ formed Au NPs. The formation
of such Au NPs could follow an analogous polyol process. The
polyol process has been widely explored for the synthesis of metal
NPs although there is insufficient understanding of their
formation mechanism.25–27 Previous reports have revealed that
the reduction of metal ions in a typical polyol process occurs only
when the employed alcohols are present in high concentrations
above a threshold value due to their limited reducing capability.28
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From this point of view, in the matrix of DC capsules, there are
lots of hydroxyl group aggregates derived from adjacent assem-
bled DC molecules. Each aggregate provides a ‘‘domain’’ with
a certain alcohol concentration, which mimics an analogous
polyol environment. When the metal precursor is added to self-
assembled DC colloids solution, there exist non-covalent, elec-
trostatic (ion dipole) interactions among the electropositive
metal ions and electron-rich oxygen atoms in those
‘‘domains’’.11,29 Such interactions cause the metal ions to be
tightly trapped within the matrix of DC assemblies, forming
DC/metal ions complexes. After a polyol process, these trapped
metal ions were in situ reduced to their zero-valent metallic states,
and then coalesce into metal NPs.
self-assemblies (hollow DC/Au, Fig. 5a and b) also aroused the
similar changes in the Raman spectrum (Fig. S3 in the ESI†).
Hence, the in situ reduction of Au(III) ions also results in the
formation of Au0-O bonds (1596 cmꢁ1) at the cost of the
destruction of partial hydrogen bonds. Such Au0-O bonds are
beneficial for the tight trapping of in situ formed Au NPs within
the matrix of DC assemblies.
Ni(II) coordinated DC self-assemblies for ethylene
polymerization
The a-diimine center in DC molecules provides efficient ‘‘active
sites’’ for coordinating late transition metals such as Ni(II) and Pd
(II), forming metal complexed DCs, which are well-known effi-
cient catalysts towards the polymerizations of diverse olefins.12,13
Analogously, by coordinating late transition metals to the
assembled DC hosts, efficient catalysts for olefin polymerizations
are expected. Bulky Ni(II) a-diimine catalysts (BNDC; Fig. 7a)
were fabricated by coordinating NiBr2 to pre-formed hollow and
solid spherical DC assemblies. Energy-dispersive X-ray analysis
evidenced the presence of Ni elements in DC assemblies (Fig. S4
in the ESI†). Elemental and MS (FAB+) analyses further
confirmed the successful coordination of Ni(II) to the matrixes of
DC assemblies (see Experimental section). The BNDCs keep the
hollow structures and no obvious changes in morphologies after
metallation reactions (Fig. 7b). These hollow BNDCs showed
Note that the one-pot reaction for fabricating DC/Au NPs solid
nanospheres is exactly same as the one for preparing DC capsules
(Fig. 3c and d) except for the involvement of HAuCl4. This can be
related to the DC-assisted reducing processes of Au(III) ions,
which alter the self-assembly behavior of DC molecules. This
effect is evidenced by the Raman scattering measurements shown
in Fig. 6. Compared with DC capsules, DC/Au solid spheres show
a significantly weakened intensity of Raman bands in the region of
1800–1200 cmꢁ1, including hydrogen-bonded NH2, aromatic
stretching vibrations, hydrogen-bonded OH, CH3 symmetric
deformation and COH bending vibrations. It indicates that the
number of hydrogen bonds in the DC/Au solid spheres is greatly
reduced. In contrast, the band at 1596 cmꢁ1 is relatively enhanced,
which can be derived from the formation of Au0–O bonds.30 In
current self-assembly processes, the hydrogen bonds serve as
critical media to manipulate the intermolecular stacking, leading
to the formation of various assembled architectures. However,
when the aqueous HAuCl4 solution is involved in the homoge-
neous DC solution in THF and DI water, metal ions rapidly
complexed with DC molecules through the non-covalent, elec-
trostatic interactions among the electropositive metal ions and
electron-rich oxygen atoms.29,30 This unavoidably hinders the
formation of hydrogen bonds and leads to the reduction in the
number of hydrogen bonds. These effects together with p–p
stacking interactions direct DC to self-assemble in certain
manners, resulting in the formation of DC/Au solid spheres after
the reduction of complexed Au(III) ions.
In comparison with solid DC/Au particles, the incorporation
of in situ generated Au NPs into the shell of hollow DC
Fig. 7 (a) Molecular structure of BNDC; (b) SEM and TEM images of
BNDC hollow spheres; (c and d) SEM images of PE beads polymerized
by using BNDC hollow spheres as bulky catalysts; (e) SEM and TEM
images of BNDC solid spheres; (f) SEM images of PE beads obtained by
using BNDC solid spheres as bulky catalysts.
Fig. 6 Raman spectra of self-assembled DC hollow spheres and DC/Au
solid spheres obtained by one-pot reaction.
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excellent catalytic reactivity towards the ethylene polymerization
(6.06 ꢀ 105 g PE mol Niꢁ1 hꢁ1 barꢁ1; Table 1, also see Fig. S5 in
the ESI†) combined with MAO as a cocatalyst. More interest-
ingly, the yielded polyethylene (PE) exhibits spherical structures
with diameters in the range of 3.1–24.3 mm (7.4 mm in mean size,
Fig. 7c), which is significantly larger than BNDCs (430 nm in
average diameter). The PE beads exhibit solid structures formed
by an agglomerate of large numbers of uniform PE microspheres
with around 200 nm of average diameter (Fig. 7d). This fact is
responsible for the rough surface of polyethylene spheres.
Interestingly, it is visualized that there exists a round ‘‘trap’’ on
the surfaces of some PE beads, exactly replicating the structures
of some hollow BNDCs (Fig. 6a).
the growing PE microspheres would occupy the interior space of
BNDCs, yielding the final solid PE beads. Additionally, the
hollow structures of BNDCs are more favorable for the
substance diffusion, giving slightly higher catalytic activity of
hollow BNDCs than that of solid BNDCs. Note that the resul-
tant PE beads possess textural pores derived from the inter-
particle spacing, which not only lower the density of PE mate-
rials, but also provide promising applications in the field of
substance transportation. In addition, it could be feasible to
generate PE beads with desired surface morphologies by using
BNDCs with designed topologies. To the best of our knowledge,
this is the first report concerning the achievement of PE beads
with porous textures and possibly tunable surface
morphologies.32
In addition, the solid BNDCs exhibit solid and spherical
structures (Fig. 7e), which are similar to the original DC hosts
(Fig. 2a). These BNDCs also showed good catalytic activity for
ethylene polymerization assisted with MAO (1.22 ꢀ 105 g PE mol
Niꢁ1 hꢁ1 barꢁ1, Table 1, also see Fig. S5 in the ESI†). The yielded
PE beads show solid structures with mean sizes ranging from 2.4
to 17.8 mm (7.0 mm in average, Fig. 7f). Each PE bead consists of
numerous uniform PE microspheres with 190 nm in average
diameter. Interestingly, in comparison with the PE beads
obtained by hollow BNDCs, no round ‘‘trap’’ is observed on the
surfaces of yielded PE beads. This suggests that the produced PE
beads replicate the surface morphologies of original solid
BNDCs.
Conclusions
A facile route to fabricate DC self-assemblies with tunable
morphologies ranging from solid spheres, nanotubes and
capsules via p–p stacking and hydrogen bond interactions has
been demonstrated. The DC assemblies are excellent multifunc-
tional hosts for in situ generation and encapsulation of Au
nanoparticles, yielding novel solid and hollow DC/Au hybrid
nanospheres by one-pot or two-step fabrication process. Solid
and hollow DC assemblies were also used to host Ni(II) ions to
generate DC/Ni(II) hybrid particles, resulting in bulky and highly
active catalysts for ethylene polymerizations to produce porous
PE beads consisting of numerous PE microspheres. Moreover,
the PE beads replicate the surface morphologies of original bulky
catalysts. The formation mechanism of the PE beads is eluci-
dated as an analogous ‘‘multigrain model’’. Note that these DC
assemblies can be used to host various metal species to fabricate
a variety of DC/metal NPs (Ag, Pt, Pd, etc.) hybrid nano-
materials and polymer (PE, polypropylene, etc.) beads with
porous textures.
The formation of these fascinating PE structures can be
explained by an analogous ‘‘multigrain model’’ mechanism.31
Each building block, the Ni(II) a-diimine complex, in BNDCs
acts as an active site. Since BNDCs possess textural pores formed
in the self-assembly processes of DC molecules, ethylene mono-
mers and co-catalysts are allowed to diffuse to the active sites and
in situ polymerize, producing a large number of grain-sized
polymer spheres. Accompanied with the successive feeding of
monomers, polymerizations continuously occur on the living
chains surrounded on the surfaces of as-formed grain-sized
polymer spheres. These newly formed polymer chains push the
previous formed polymer layer, thus leading to the growth of the
grain-sized polymer spheres into microspheres and consequently
the formation of the final PE beads. Since each building block
serves as a separated active site to catalyze the production of a PE
microsphere, the uniform in situ growth of these PE microspheres
is responsible for the yield of final PE beads with replicating
topology of original BNDCs. If hollow interiors in BNDCs exist,
This work was supported by the World Class University
Program (no. R32-2008-000-10174-0), the National Core
Research Center Program from MEST (no. R15-2006-022-01001-
0), and the Brain Korea 21 program (BK-21).
Notes and references
€
1 A. Walther, J. Yuan, V. Abetz and A. H. E. Muller, Nano Lett., 2009,
9, 2026.
2 M. Schmittel and V. Kalsani, Top. Curr. Chem., 2005, 245, 1.
3 M. D. Pluth and K. N. Raymond, Chem. Soc. Rev., 2007, 36, 161.
4 C. M. Drain, A. Varotto and I. Radivojevic, Chem. Rev., 2009, 109,
1630.
Table 1 Selected ethylene polymerization resultsa
5 I. Imaz, J. Hernando, D. Ruiz-Molina and D. Maspoch, Angew.
Chem., Int. Ed., 2009, 48, 2325.
6 A. Ajayaghosh and V. K. Praveen, Acc. Chem. Res., 2007, 40,
644.
Catalysts
(BNDC)
Rp,avg
104
ꢀ
Mv
104
ꢀ
Mn
103
ꢀ
Branchesf/
1000 C
b
c
d
e
Tm /
PDId ꢂC
7 M. D. Pluth, R. G. Bergman and K. N. Raymond, Acc. Chem. Res.,
2009, 42, 1650.
8 S. M. Biros, R. G. Bergman and K. N. Raymond, J. Am. Chem. Soc.,
2007, 129, 12094.
9 Y. Sun and Y. Xia, Science, 2002, 298, 2176.
10 P. Raveendran, J. Fu and S. L. Wallen, J. Am. Chem. Soc., 2003, 125,
13940.
11 H. Li, J. K. Jo, L. Zhang, C.-S. Ha, H. Suh and I. Kim, Langmuir,
2010, 26, 18442.
Hollow DC sphere 60.6
Solid DC spheres 12.2
10
12
43
48
3.8
4.4
125 37
127 30
a
Conditions: toluene ¼ 80 mL, PC2H4 ¼ 1.3 bar, catalyst ¼ 2.5 mmol, time
¼ 30 min, temperature ¼ 50 ꢂC, and MAO/Ni ¼ 250. b Average rate of
c
polymerization as
g
PE mol Niꢁ1 hꢁ1 barꢁ1
.
Viscosity-average
molecular weight characterized with viscometry at 135 ꢂC in decalin
(Ubbelohde viscometer). Number-average molecular weight (Mn) and
d
e
polydispersity index (PDI) determined by GPC. Melting point
f
determined by DSC. Branches per 1000 carbon atoms determined by
12 G. G. Hlatky, Chem. Rev., 2000, 100, 1347.
13 B. K. Bahuleyan, G. W. Son, D. W. Park, C. S. Ha and I. Kim, J.
Polym. Sci., Part A: Polym. Chem., 2008, 46, 1066.
1H NMR.
17944 | J. Mater. Chem., 2011, 21, 17938–17945
This journal is ª The Royal Society of Chemistry 2011

View Article Online
14 H. Li, C.-S. Ha and I. Kim, Langmuir, 2008, 24, 10552.
ꢁ
15 I. Alfonso, M. Bru, M. I. Burguete, E. Garcıa-Verdugo and S. V. Luis,
23 G. Socrates, in Infrared and Raman Characteristic Group Frequencies:
Tables and Charts, ed. G. Socrates, John Wiley and Sons Ltd, 3rd edn,
2001, pp. 51, 97–99 and 107.
24 R. Djalali, Y.-F. Chen and H. Matsui, J. Am. Chem. Soc., 2002, 124,
13660.
ꢁ
25 L. Poul, N. Jouini and F. Fievet, Chem. Mater., 2000, 12, 3123.
Chem.–Eur. J., 2010, 16, 1246.
16 M. Huang, U. Schilde, M. Kumke, M. Antonietti and H. Colfen, J.
€
Am. Chem. Soc., 2010, 132, 3700.
17 H. Huo, K. Li, Q. Wang and C. Wu, Macromolecules, 2007, 40,
ꢁ ꢁ
26 G. Viau, P. Toneguzzo, O. Acher, F. Fievet-Vincent and F. Fievet,
6692.
18 Y. Luo, J. Lin, H. Duan, J. Zhang and C. Lin, Chem. Mater., 2005,
17, 2234.
Scr. Mater., 2001, 44, 2263.
27 S. E. Skrabalak, B. G. Wiley, M. Kim, E. V. Formo and Y. Xia, Nano
Lett., 2008, 8, 2077.
ꢁ
ꢁ
19 T. P. Ruiz, M. F. Gomez, J. J. L. Gonzalez and A. E. Koziol, Chem.
Phys., 2006, 320, 164.
20 Y. Yu, K. Lin, X. Zhou, H. Wang, S. Liu and X. Ma, J. Phys. Chem.
C, 2007, 111, 8971.
28 T. Sakai and P. Alexandridis, J. Phys. Chem. B, 2005, 109, 7766.
29 C.-H. Lai, I.-C. Wu, C.-C. Kang, J.-F. Lee, M.-L. Ho and P.-T. Chou,
Chem. Commun., 2009, 1996.
21 D. Baskaran, J. W. Mays and M. S. Bratcher, Chem. Mater., 2005, 17,
3389.
22 R. S. Armstrong, L. M. Atkinson, E. Carter, M. S. Mahinary,
B. W. Skelton, P. Turner, G. Wei, A. H. White and L. F. Lindoy,
Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4987.
30 J.-P. Sylvestre, S. Poulin, A. V. Kabashin, E. Sacher, M. Meunier and
J. H. T. Luong, J. Phys. Chem. B, 2004, 108, 16864.
31 T. F. McKenna and J. B. P. Soares, Chem. Eng. Sci., 2001, 56, 3931.
32 B. K. Bahuleyan, B. C. Son, Y. S. Ha, S. H. Lee, H. Suh and I. Kim, J.
Mater. Chem., 2010, 20, 7150.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 17938–17945 | 17945