Solid structures of the stepwise self-assembled copillar[5]arene-based supramolecular polymers

Full text of DOI:10.1002/bkcs.10930

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DOI: 10.1002/bkcs.10930
BULLETIN OF THE
Y. S. Park et al.
KOREAN CHEMICAL SOCIETY
Solid Structures of the Stepwise Self-assembled Copillar[5]arene-based
Supramolecular Polymers
*
Yeon Sil Park, Seong Min Hwang, Jaeyeon Shin, and Kyungsoo Paek
Department of Chemistry, Soongsil University, Seoul 156-743, Korea. *E-mail: kpaek@ssu.ac.kr
Received July 22, 2016, Accepted August 16, 2016
Keywords: Supramolecular polymers, Solid structure, Atomic force microscopy, Copillar[5]arene, Cati-
onic guests, Quadruple hydrogen bonds, Cation-π interaction
Development of supramolecular polymer has attracted
much interest because of their interesting properties such as
stimuli-responsiveness, recycling, self-healing and degrada-
bility, and their consequential applications.1 The essential
feature of this class of polymers is the self-assembly of dis-
crete monomeric subunits via non-covalent interactions or
dynamic covalent bonds. Among the many monomeric sub-
units, pillar[n]arenes2 have been ideal building blocks for
the fabrication of polymeric supramolecules because of
their intrinsic characteristics and properties such as facile
preparation and flexible modification, π-rich cavities, as
well as peculiar guest complexation capability.3
solid state, atomic force microscopy (AFM) studies were
performed (Figure 3). Samples of each stepwise self-
assembly confirmed by 1H NMR experiment were spin-
coated onto a Si-wafer surface. The high-resolution AFM
images revealed the formation of relatively regular isolated
nano-ring structures. According to the stepwise increase
from P1 to P4, the diameters of the typical nano-ring grad-
ually increased from 2 μm to 29 μm (P1 = 2 μm,
P2 = 6 μm, P3 = 15 μm, P4 = 29 μm). But, supramolecu-
lar polymers P maintained its average wall thickness of
2 μm regardless of the stepwise additions.
The ring-shaped morphologies in supramolecular poly-
mer P are probably due to the tendency of the end-to-end
connection6 in the solid state of long flexible supramolecu-
lar chains. The size increase of nano-rings as the stepwise
addition increases might be due to the fact that the linear
supramolecular polymer P in solution seems to be main-
tained until the nano-ring formation by solidification.
On the other hand, for one pot addition, the high-
resolution AFM image revealed relatively irregular form of
nano-structures (see Supporting Information, Figure S13).
In our previous report,4 we have reported that ureidopyri-
midinonyl copillar[5]arene 1, dicationic 2 or tricationic
guest 3 can assemble linear or dendritic supramolecular
polymers P or D in solution, which were assembled
through quadruple hydrogen bonds and cation–π interac-
tions. We herein report the solid-state morphologies of their
stepwise self-assemblies.
Ureidopyrimidinone-functionalized
copillar[5]arene
1 was prepared by reaction of 2-(1-imidazolylcarbonyla-
mino)-6-methyl-4(1H)-pyrimidinone with amino-copillar[5]
arene which was synthesized. The repeating guest units,
dicationic guest 2 and tricationic guest 3, have ethynylben-
zene moiety and pyridinium salt which are linked together
by a flexible C8 alkyl chain, and were synthesized in good
yields by a four step process (Figure 1).4
Copillar[5]arene 1 with a ureido-4[1H]-pyrimidinone unit
forms a stable dimer 12 (Kdimm > ~ 107 M−1)5 by quadruple
hydrogen bonding in nonpolar conditions, and the dimer 12
was applied to form a self-assembled supramolecular poly-
mers P and D through cation–π interactions with dicationic
2 or tricationic guests 3 [Ka of pillar[5]arene-pyridium cation:
7.61 × 102 M−1 in CDCl3-CD3CN (5:1, v/v)].4 The dimeri-
zation of host 1 and the successive assembly of dimer 12 with
dicationic guest 2 or tricationic guests 3 were observed by 1H
NMR experiments as well as 2D-DOSY NMR experiments
through stepwise addition of host and guest (Figure 2).4
The combination of copillar[5]arene 1 and dicationic
guest 2 could form linear supramolecular polymers P -
(Figure 2(a)). To visualize the successive formation of each
stepwise self-assembly of supramolecular polymer P in
H
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H
N
H
O
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O
H
N
O
O
O
O
N
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O
O
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12
O
O
O
H
N+
CH2
(CH2)7
-
PF6
N+
-
PF6
CH2
(CH2)7
2
3
(CH2)7
(CH2)7
(CH2)7
CH2
N+
CH2
CH2
-
N+
N+
PF6
-
-
PF6
PF6
Figure 1. Chemical structures of 12, dicationic guest 2, and trica-
tionic guest 3.
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Dynamic light scattering (DLS) experiments were also
carried out to investigate the size distributions of the supra-
molecular nanoparticles in each stepwise self-assembly. For
supramolecular polymer P, the average hydrodynamic
radius (Dh) of each stepwise self-assembly appeared
P1 = 6, 160 nm (≈6 μm), P2 = 10, 276 nm (≈10 μm),
P3 = 16, 057 nm (≈16 μm), and P4 = 29, 242 nm
(≈29 μm), respectively. The measured average diameters of
stepwise self-assemblies, especially of P1 and P2, are larger
than those of solid-state AFM results, owing to the swelling
of the nanoparticles in solution state.
The combination of copillar[5]arene 1 and tricationic
guest 3 could form dendritic supramolecular polymers D -
(Figure 2(b)). AFM measurements of each stepwise self-
assembly in supramolecular polymer D were performed on
mica by spin-coating method (Figure 4). It is found that
supramolecular polymer D formed nano-ring at first, and
then aggregated to clods as the stepwise addition proceeds.
The average diameter of these aggregates gradually
increased from 2 μm to 8 μm. (D1 = 2 μm, D2 = 5 μm,
and D3 = 8 μm).
supramolecular polymers P
(a)
12
12
12
12
2
2
2
2
12
P3
P4
P1
P2
supramolecular polymers D
(b)
(c)
12
3
12
3
12
D2
3
D3
D1
supramolecular polymers S
12
12
12
12
2
2
2
S4
S2
S3
3
S1
The combination of copillar[5]arene 1, tricationic guest
3 as core guest, and dicationic guest 2 as expended guest
can assemble supramolecular polymers S (Figure 2(c)).
The morphologies of supramolecular polymers S were also
investigated by AFM measurement (Figure 5). Interestingly
Figure 2. Schematic illustrations of supramolecular polymers
(a) P, (b) D, and (c) S by stepwise self-assemblies of monomeric
subunit 12 and 2 or 3.
Figure 3. AFM height images of each stepwise self-assembly in supramolecular polymers P prepared by spin coating of a chloroform-
acetonitrile (5:1, v/v) solution (c = 1.0 × 10−5 M) onto Si-wafer: (a) P1, (b) P2, (c) P3, (d) P4.
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Figure 4. AFM height images of each stepwise self-assembly in supramolecular polymers D prepared by spin coating of a chloroform-
acetonitrile (5:1, v/v) solution (c = 1.0 × 10−5 M) onto mica: (a) D1, (b) D2, (c) D3.
the supramolecular polymers S of all the generations
self-assembled to nano-rings of the similar diameter
(24 – 26 μm) and wall thickness (2 μm). Also branched
non-circular structures were surrounded by these nano-rings.
It is presumable that the growing polymer S was surrounded
by a nano-ring formed by supramolecular polymer P before
completion of its distinct self-assembled final structure,
which means the growing polymer S acted as a template
for the formation of a nano-ring as the solvent evaporated.
In summary, the solid states of self-assemblies of supra-
molecular polymers P, D, and S by stepwise additions of
dimer 12 and guest 2 or 3 were investigated by AFM and
dynamic light scattering experiments. AFM measurements
in solid state showed that the linear supramolecular poly-
mers P formed nano-ring and its size increased as the step-
wise addition increased. Supramolecular polymers D which
contains dimer 12 and tricationic guest 3 gave cloddish
aggregates due to its dendritic assembling process. Supra-
molecular polymer S which contains dimer 12, tricationic
guest 3 as core guest, and dicationic guest 2 as expended
guest could not complete its distinct structure, but sur-
rounded by nano-ring type suprapolymer. The studies for
Figure 5. AFM height images of each stepwise self-assembly in supramolecular polymers S prepared by spin coating of a chloroform-
acetonitrile (5:1, v/v) solution (c = 1.0 × 10−5 M) onto Si-wafer: (a) S1, (b) S2, (c) S3, (d) S4.
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structures of supramolecular polymer in solution and its
solid states will accelerate their applications.
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
Experimental
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1
assembly confirmed by H NMR experiment were prepared
by spin coating of a chloroform-acetonitrile (5:1, v/v) solu-
tion (c = 1.0 × 10−5 M) onto cleaned Si-wafer or freshly
cleaved mica.
Acknowledgment.
This research was supported by
Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology (NRF-
2012R1A1A2039112).
Bull. Korean Chem. Soc. 2016
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
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