Directed 1D assembly of a ring-shaped inorganic nanocluster templated by an organic rigid-rod molecule: An inorganic/organic polypseudorotaxane

Article abstract of DOI:10.1002/anie.200705308

(Figure Presented) Facing up to matters: Template-assisted cofacial assembly of a ring-shaped polyoxomolybdate cluster (MC) by an oligomeric p-phenylenebutadiynylene rigid rod molecule bearing ammonium ion pendant groups (PB_n) allowed for the formation of an inorganic/organic one-dimensional (1D) nanoobject. Studies indicate that the 1D object most likely has a polypseudorotaxane structure (see picture), in which multiple MC rings are threaded by the PB_n rod.

Full text of DOI:10.1002/anie.200705308

Communications
DOI: 10.1002/anie.200705308
Supramolecular Chemistry
Directed 1D Assembly of a Ring-Shaped Inorganic Nanocluster
Templated by an Organic Rigid-Rod Molecule: An Inorganic/Organic
Polypseudorotaxane**
Md. Akhtarul Alam, Yeong-Sang Kim, Saho Ogawa, Akihiko Tsuda,* Noriyuki Ishii, and
Takuzo Aida*
Polyoxometalates (POMs) have attracted a great deal of
attention because of their multielectronic redox activities and
organic nanocomposite MCꢀTAP1–3. On the basis of this
observation, PBn was designed with the expectation that it
unique photochemical properties.[1] To enhance their expe- may connect multiple MC rings in a cofacial manner through
diency for materials science, the controlled assembly of POMs
with nanometric precision is one of the important goals.[2,3]
We took notice of the ring-shaped polyoxomolybdate (MC)
developed by Müller et al.,[4] since MC is expected to display
interesting physical properties that originate from its mixed-
valent electronic structure. Polarz et al. have reported that
coassembly of MC with a cationic surfactant results in the
formation of a hexagonal array of MC rings.[5] Herein we
report that MC coassembles with a rigid p-phenylenebuta-
diynylene polymer (PBn, Scheme 1) bearing pendant ammo-
nium ion groups to form a novel one-dimensional (1D)
tubular assembly of cofacially connected MC rings (Figure 1).
The MC contains 176MoO3 units and adopts a 1.3-nm-
thick ring-shaped structure, with external and internal
diameters of roughly 4.1 and 2.3 nm, respectively.[4] Since
the MC has many acidic OH (OÀ···H+) groups on its surface, it
electrostatic interactions to form a one-dimensional struc-
ture.[7–11]
For the synthesis of PBn, a 1,4-diethynylbenzene deriva-
tive with four tert-butoxycarbonyl-protected amino groups
(
BocPB1) was subjected to CuII-mediated Glaser–Hey cou-
pling. The high-molecular-weight fraction of the resultant
polymer (BocPBn) was isolated by preparative size-exclusion
chromatography (SEC) and then deprotected with trifluoro-
acetic acid (TFA).[12,13] By using the analytical SEC profile of
an oligomeric fraction of the coupling product as a calibration
standard, the average number of repeating PB units (n) of the
isolated BocPBn and its polydispersity were estimated as 14 and
1.5, respectively.
For the coassembly of MC with PB14, a solution of PB14 in
MeOH ([PB unit] = 6.0 ” 10À5 m) was mixed with a solution of
MC in MeCN (0.5 ” 10À5 m)[14] at [PB unit]/[MC] = 3:1
(MeCN/MeOH = 4:1 v/v), and the resulting mixture was
stirred for 10 minutes at 208C. Dynamic light scattering
(DLS) analysis indicated that the mixture contains large
objects with sizes ranging from 50 to 3500 nm (average radius;
347 nm).[13] As shown in Figure 2c,d, transmission electron
microscopy (TEM) analysis of an air-dried sample of the
solution clearly displayed the presence of one-dimensional
(1D) objects with a high aspect ratio. While most of the 1D
objects visualized by TEM are much longer than PB14 (which
has an average length of 14 nm, see below), they are
characterized by a uniform diameter of 4 nm, which is
nearly identical to that of MC.[4] In sharp contrast, TEM
analysis of MC alone under identical conditions but without
PB14 showed only a great number of discrete nanodots with
diameters of 3–5 nm (Figure 2b),[4] while PB14 could not be
visualized regardless of the presence or absence of MC
(Figure 2a). From these contrasting observations, it is clear
that the 1D objects in Figure 2c,d are composed of MC rings
cofacially connected to one another. In this nanoscale
aggregate, the rigidity of PB14 likely plays an important role,
since the mixing of MC with protonated polylysine, a rather
+
can interact with NH2 and NH3 groups through hydrogen-
bonding and electrostatic interactions, respectively. In fact, as
reported previously,[6] MC can accommodate 1–3 molecules of
a metalloporphyrin with aminophenyl side groups (TAP,
Scheme 1) within its cavity, thereby forming the inorganic/
[*] Dr. M. A. Alam, Dr. Y.-S. Kim, S. Ogawa, Dr. A. Tsuda, Prof. Dr. T. Aida
Department of Chemistry and Biotechnology
School of Engineering, and Center for NanoBio Integration
The University of Tokyo, 7-3-1 Hongo
Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7310
E-mail: tsuda@macro.t.u-tokyo.ac.jp
aida@macro.t.u-tokyo.ac.jp
Homepage: http://macro.chemt.u-tokyo.ac.jp/
Dr. A. Tsuda
PRESTO, Japan Science Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
Dr. N. Ishii
(Responsible for TEM microscopy)
Biological Information Research Center, AIST, Tsukuba Central 6
1-1-1 Higashi Tsukuba, Ibaraki 305-8566 (Japan).
+
[**] This work was sponsored by a Grants-in-Aid for Scientific Research
(no. 17350044) and Encouragement of Young Scientists
(no. 18750113) from the Ministry of Education, Science, Sports, and
Culture (Japan). M.A.A. and Y.-S.K. made an equal contribution to
the work and thank the JSPS Postdoctoral Fellowships for Foreign
Researchers.
flexible polymer having NH3 groups, resulted in the for-
mation of an amorphous agglomerate, as observed by
TEM.[13]
When the MC was added to a solution of PB14 in MeCN/
MeOH (4:1 v/v) ([PB unit]/[MC] = 3:1), the visible absorp-
tion band of PB14 became less intense and broadened.[13]
Furthermore, the addition of MC efficiently quenched the
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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that PB14 likely loses its conformational freedom
when deposited onto the MC surface.[13]
Two important issues need to be considered
in regard to the mechanism of the 1D coassembly
of MC with PB14: 1) Does the doughnutlike
structure of MC play a role and 2) why is the 1D
coassembled structure much longer than the
average length of PB14? To address question (1),
we attempted the coassembly of guest-included
MCꢀTAP1–3 ([TAP]/[MC]= 3:1)[6] with PB14.
The MCꢀTAP1–3 coassembly showed only a
slight red fluorescence at 600–750 nm originating
from TAP (Figure 4), as a consequence of the
photochemical quenching of the singlet excited
state of included TAP by MC. However, when
MCꢀTAP1–3 was mixed with PB14 in MeCN/
MeOH (4:1 v/v), the TAP recovered its red
fluorescence (Figure 4), thus indicating that
TAP binds MC less strongly than PB14 and is
“kicked out” of the MC cavity upon mixing
MCꢀTAP1–3 with PB14 (Scheme 2). We also
prepared MCꢀHPB by mixing MC with hexa-
phenylbenzene (HPB) which carried six pendant
ammonium ion groups at its periphery ([HPB]/
[MC] = 3:1). This experiment was based on the
expectation that HPB can bind MC more
strongly than TAP and even PB14. In fact,
mixing MCꢀTAP1–3 with HPB resulted in the
recovery of the fluorescence of TAP and quench-
ing of the HPB fluorescence,[13] which indicates
that the TAP in the MC cavity can be kicked out
by HPB. On the other hand, when MCꢀHPB
was titrated with PB14, the fluorescence of HPB
Scheme 1. Compounds used in the study.
Figure 1. Schematic illustration of an inorganic/organic polypseudoro-
taxane derived from MC and PBn.
photoexcited state of PB14 (Figure 3a): In the absence of MC,
excitation of PB14 at 440 nm resulted in a blue fluorescence
centered at 480 nm. When MC was titrated with PB14, the
fluorescence emission from PB14 did not occur until [PB unit]/
[MC] exceeded 11:1 (Figure 3b; filled circles). These results
likely reflect that MC and PB14 coassemble to form a complex.
Compared with PB14, PB1 appears to be have much less
affinity toward MC, as judged from its fluorescence titration
profile (Figure 3b; open circles), where the characteristic
fluorescence of PB1 started to appear at a [PB1]/[MC] ratio of
8:1 (which is smaller than in the case of PB14).[13] This
tendency indicates the importance of a multivalent interac-
tion between PBn and MC for the complexation.[15] The
1H NMR signals corresponding to PB14 disappeared com-
pletely upon mixing the solution with MC, which indicates
Figure 2. TEM micrographs of air-dried MeCN/MeOH (4:1 v/v) solu-
tions of a) PB14, b) MC, and c),d) a mixture of MC and PB14 ([PB unit]/
[MC]=3:1), deposited on a specimen grid covered with a thin carbon
support film. [PB unit]=1.2”10À5 m, [MC]=0.4”10À5 m.
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Communications
Scheme 2. Schematic illustration of the possible liberation of TAP from
MCꢀTAP3 upon threading of PB14.
remained quenched, even upon addition of a large excess of
PB14.[13] Therefore, HPB indeed binds MC much more
strongly than PB14. Quite interestingly, while the mixing of
MCꢀTAP1–3 with PB14 (Figure 4) resulted in the formation of
fibrous (1D) objects, as observed by TEM, only aggregated
dots formed when MCꢀHPB was mixed with PB14.[13] These
contrasting results allow us to conclude for question (1) that
the threading of the MC rings with PB14 is essential for their
controlled 1D coassembly. As for question (2), the fluores-
cence profiles of PB14 in the titration experiments showed an
interesting possibility in regard to the tube dimensions. In the
competition experiment of MCꢀTAP1–3 with PB14 (Figure 4),
the fluorescence of PB14 was hardly visible until the [PB unit]/
[MC] ratio exceeded 10:1 (Figure 4b, blue filled circles).
Since a similar trend was observed for the titration of guest-
free MC with PB14 (Figure 3b), we initially thought that it
must be simply due to the threading interaction of MC with
PB14. However, despite no 1D coassembly and no threading
interaction upon mixing PB14 with MCꢀHPB, PB14 showed an
analogous fluorescence quenching profile.[13] Therefore, PB14
likely adheres to MC, irrespective of whether the MC cavity is
occupied by a guest molecule or not. Nevertheless, for the
controlled 1D coassembly of MC and PB14, MC must be
threaded by PB14. We assume that the threaded MC units are
“stitched” together by the surface adhesion of PB14, and such
short-chain 1D objects are occasionally connected to one
another. Consequently, they become much longer than
expected from the average length (14 nm) of the PB14 used
as a template (Figure 2c,d). From these observations, the 1D
structure formed from MC and PB14 may be called an
inorganic/organic polypseudorotaxane.[7–11]
Figure 3. a) Fluorescence spectra of PB14 (lext: 440 nm) in MeCN/
MeOH (4:1 v/v) at 208C upon titration of MC with PB14. b) Plots of
the fluorescence intensities of PB14 at 472 nm (filled circle) and
reference PB1 (lext: 357 nm) at 450 nm (open circles)[13] versus [PB
unit]/[MC]. [MC]=6.1”10À7 m.
In conclusion, we have demonstrated the formation of the
first inorganic/organic polypseudorotaxane by the template-
assisted cofacial assembly of a ring-shaped molybdenum
cluster (MC) with a rigid-rod molecule having a high affinity
toward the MC surface. Since the MC is a mixed-valent
inorganic cluster with chromophoric characteristics, explora-
tion of the optoelectronic properties of this novel 1D
nanocomposite material is one of the subjects worthy of
further investigation.[16]
Received: November 19, 2007
Published online: February 6, 2008
Figure 4. a) Fluorescence spectra of PB14 (lext; 435 nm) in MeCN/
MeOH (4:1 v/v) at 208C upon titration of MCꢀTAP1–3 (1:3 mixture of
MC and TAP) with PB14. b) Plots of the fluorescence intensities of PB14
at 478 nm (blue filled circles) and TAP at 624 nm (red filled circles)
versus [PB unit]/[MC]. [MC]=6.1”10À7 m.
Keywords: organic–inorganic hybrid composites ·
polyoxometalates · polypseudorotaxanes ·
supramolecular chemistry · transmission electron microscopy
.
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