Toroidal nanoobjects from rosette assemblies of melamine-linked oligo(p-phenyleneethynylene)s and cyanurates

Article abstract of DOI:10.1002/anie.200800417

(Figure Presented) Nanodonuts! A hydrogen-bonded cyclic assembly (rosette, left) of a melamine featuring π-conjugated oligo(p-phenyleneethynylene) hierarchically self-organizes with a cyanurate in decane under dilute conditions to form toroidal nanostructures (right) with a diameter of 40 nm.

Full text of DOI:10.1002/anie.200800417

Angewandte
Chemie
DOI: 10.1002/anie.200800417
Self-Assembly
Toroidal Nanoobjects from Rosette Assemblies of Melamine-Linked
Oligo(p-phenyleneethynylene)s and Cyanurates**
Shiki Yagai,* Sankarapillai Mahesh, Yoshihiro Kikkawa, Kanako Unoike, Takashi Karatsu,
Akihide Kitamura, and Ayyappanpillai Ajayaghosh*
Spontaneous organization of small molecular building blocks
into complex superstructures by concerted action of various
noncovalent interactions is abundant in nature. The precision,
elegance, complexity, and functions of these structures pose
significant challenge to scientists. The quest to translate the
salient features of natureꢀs self-assembly principles has lead to
the design of a variety of artificial objects useful in the field of
advanced materials research.[1] In this context, the fabrication
of well-defined, discrete nanoobjects of photonically and
electronically active molecular components with controlled
tures.[5] Reports on superstructures hierarchically organized
from cyclic supramolecular assemblies are limited to
extended columnar architectures.[6,7] Herein we report an
unprecedented self-organization of hydrogen-bonded rosette
assemblies of oligo(p-phenyleneethynylene) (OPE)[8]
attached melamine 1 and the cyanurate 2 in aliphatic solvent,
leading to the formation of toroidal objects of nanometer
dimension (Figure 1).[9–11]
size, shape, and function is of paramount importance.[2]
A
major focus on this area is the self-assembly of linear p-
conjugated molecules because of their inherent optical and
electronic properties, which are vulnerable to intermolecular
interactions.[3] Such interactions can be controlled by direc-
tional multiple hydrogen-bonding modules, leading to supra-
molecular objects of various dimensionalities.[4]
Among various multiple H-bonding modules, the com-
plementary interaction between melamines and cyanurates
has been of great value for chemists to the design of rosette
(macrocyclic) and tape-like (linear or crinkled) architec-
[*] Dr. S. Yagai, K. Unoike, Prof. Dr. T. Karatsu, Prof. Dr. A. Kitamura
Department of Applied Chemistry and Biotechnology
Faculty of Engineering, Chiba University
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
Fax: (+81)43-290-3039
http://www.eng.chiba-u.ac.jp/outProfile.tsv?no=1228
E-mail: yagai@faculty.chiba-u.jp
Figure 1. Self-assembly of melamine-linked OPE 1 and cyanurate 2.
S. Mahesh, Dr. A. Ajayaghosh
Photosciences and Photonics Group
Chemical Science and Technology Division
National Institute for Interdisciplinary Science and Technology
(NIIST)
Trivandrum-695019 (India)
Fax: (+91)471-249-0186
http://w3rrlt.csir.res.in/photo/people/draajayaghosh/Homepage.
htm
E-mail: ajayaghosh62@gmail.com
The melamine-linked OPE derivative 1 was prepared by
Sonogashira coupling[12a] of N,N’-di(p-iodophenyl)triamino-
triazine with 3,4,5-tridodecyloxybenzyloxy-substituted ethy-
1
nyltolan, which was characterized by H NMR spectroscopy
and MALDI-TOF mass spectrometry.[13] Coassembly of 1
with the cyanurate 2[12b] resulted in discrete rosettes (13·23).
1
The H NMR spectrum of an equimolar mixture of 1 and 2
Dr. Y. Kikkawa
([1] = [2] = 5 ” 10À3 m) in CDCl3 showed resonances of the
hydrogen-bonded imide protons of 2 at d = 14.3 ppm and the
secondary amino protons of 1 at d = 9.58 ppm. Such values are
characteristic of the formation of well-defined rosette assem-
blies.[13,14] The absorption spectrum of the equimolar mixture
of 1 and 2 in CHCl3 (1 ” 10À2 m) showed a maximum at l =
334 nm, which is slightly blue-shifted from that of 1 alone in
CHCl3 (lmax = 339 nm).[13] This indicates H-type aggregation
of OPE segments upon conformational fixation of 1 to form a
H-bonded rosette assembly with 2.[8] The emission of the OPE
coassembly showed a red shift of ca. 30 nm compared to that
Nanoarchitectonics Research Center
National Institute of Advanced Industrial Science and Technology
(AIST)
1-1-1 Higashi, Tsukuba, Ibaraki 305-8562 (Japan)
[**] S.Y., S.M., A.A., and A.K. thank Department of Science and
Technology, New Delhi and the Japan Society forPromotion of
Science (Japan) forfinancial support. A.A. is a Ramanna Fellow of
DST at NIIST. S.M. acknowledges the University Grants Commis-
sion, New Delhi, fora fellowship. This is manuscript No. PPG-263
from NIIST.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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of the monomer (lem–max = 378 to 407 nm), which is in good
agreement with the H-type excitonic coupling.
Dynamic light scattering (DLS) experiments with solu-
tions of equimolar mixtures of 1 and 2 in chloroform and
decane are shown in Figure 2. In chloroform, aggregates with
Figure 2. Dynamic light scÀa3ttering analysis of an equimolar mixture of
1 and 2 in CHCl3 ( 1”10 m) and in decane ( 5”10À5 m).
*
*
an average hydrodynamic diameter of 8 nm were obtained.
This size is in line with the gyration diameter of rosette
assemblies with extended alkyl chains.[13] However, in decane
(5 ” 10À5 to 5 ” 10À4 m) large aggregate species with average
hydrodynamic diameter of 50 nm were obtained, indicating
the formation of nanoobjects hierarchically organized from
rosettes. Such nanoobjects seem to have a discrete shape, as
the hydrodynamic diameter does not change over the above
concentration regime. The optical properties of the mixture in
decane (1 ” 10À5 m) revealed a further excitonic interaction of
the OPE segments. The absorption maximum emerged at
324 nm, whereas the fluorescence maximum appeared at
436 nm. These maxima are blue- and red-shifted from the
mixture in CHCl3 (1 ” 10À2 m, see above), respectively.[13] Thus,
further extended H-type aggregation of OPE segments takes
place in nonpolar solvent, which drives hierarchical organ-
ization of rosettes. Remarkably, both spectra showed only
marginal changes upon heating the solution up to 708C.[13]
Accordingly, the rosette–rosette interaction as well as the
hydrogen bonding interaction within rosette are significantly
strong, which may render the hierarchically-organized assem-
blies considerably stable.
Insight into the morphology of the hierarchical structures
formed from the rosette 13·23 in decane is obtained from
tapping-mode AFM images. To our surprise, AFM images of a
decane solution (5 ” 10À5 m), spin-coated (3000 rpm) onto
freshly cleaved highly-oriented pyrolytic graphite (HOPG),
displayed a large number of toroidal nanoobjects (Figure 3a).
The outer diameters of toroids (c in Figure 3c) are uniform at
around 40 nm, which is in line with Dh detected by DLS. The
average cross-sectional diameter a is 20 nm, whereas the
height a’ is (3.2 Æ 0.3) nm. The actual cross-sectional diameter
was estimated to be (8 Æ 2) nm after subtracting the tip
broadening parameters.[15] This value matches the gyration
diameter of a rosette from molecular modeling, implying that
toroids are formed by the stacking of the individual rosettes.
The discrepancy between the AFM height and the diameter
of rosette might be caused by the tilting of rosettes on the
Figure 3. a) AFM phase image of an equimolarmixture of 1 and 2
spin-coated from decane solution (5”10À5 m) on HOPG. Inset: image
obtained by low tapping force.[13] b) Cross-sectional analysis of an
isolated toroid. Red arrows show the body-to-body distance b. c) Rep-
resentation of the toroidal organization of rosettes (red disks).
substrate and/or the deformation of the aliphatic chains by
high local forces exerted by the AFM tip. The body-to-body
distance (b in Figure 3c) given by cross-sectional analysis is
(21 Æ 2) nm (Figure 3b), thus the circumference of toroids is
ca. 67 nm. Assuming that the periodic distance of rosettes is in
the range of 3.5–5 Š,[6b] the number of rosettes in a single
toroid and the angle of inclination between the adjacent
rosette disks are estimated to be 191–134 and 1.9–2.78,
respectively.
In the AFM image of the assemblies (Figure 3a), short
wormlike and “opened” objects are also visualized, which
might arise from deformation of the toroids by AFM tip force.
Imaging with lower tapping force allowed almost all the
nanoobjects to retain the toroidal shape (inset in Figure 3a).
The fragile nature of toroids is more pronounced in sparse
regions than in crowded regions, making visualization of
isolated toroids by AFM difficult.[13] Spontaneous formation
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of the toroidal nanostructures on different substrates was
confirmed by AFM measurements on HOPG, mica, and
silicon substrates,[13] which all showed the same nanostructur-
es.[6c]
The toroidal nature of the supramolecular assembly of
13·23 was further confirmed by transmission electron micro-
scopy (TEM) measurements. Figure 4 shows the TEM image
H-type aggregation of OPE segments occurs for each
adjacent rosettes by tilting their planes. Extended association
of rosettes with such a tilted stacking fashion could lead to the
formation of columns with a curvature, and eventually to the
formation of toroidal architectures in solution.
The results shown herein are remarkable, as toroidal
nanoobjects, hierarchically constructed from small molecular
building blocks by discrete cyclic supramolecules, were
hitherto unknown. This result is in contrast to the previ-
ously-reported toroidal nanostructures that are directly
formed from building blocks, such as amphiphilic poly-
mers,[9a,c,f] dumbbell-shaped molecules,[9b,d,g] proteins,[9e]
DNA, or biopolymers.[10] The present assembly is novel as
p-electronic segments are spatially arranged in closed circular
structures, which is reminiscent of the light-harvesting sys-
tems of purple photosynthetic bacteria, in which the circular
arrays of chlorophyll pigments are crucial for efficient
excitation energy migration.[16] Therefore, intriguing applica-
tions, such as the creation of artificial light-harvesting nano-
devices with ring-shape morphology, might be possible based
on the present system.
Received: January 26, 2008
Revised: April 2, 2008
Figure 4. TEM image of an equimolarmixture of 1 and 2 from decane
with concentration 5”10À5 m and stained by RuO4. Inset (scale bar
10 nm) shows an enlarged view of the upper-left toroid in the main
image.
Published online: May 21, 2008
Keywords: p conjugation · hydrogen bonds · self-assembly ·
.
supramolecular chemistry · toroids
of a dip-coated sample from a decane solution at a concen-
tration of 5 ” 10À5 m after negative staining by RuO4 vapor.
Isolated toroids are clearly seen with sizes of around 40 nm,
which is in good agreement with those of the AFM images.
The cross-sectional diameter (a in Figure 3c) is approximately
10 nm, which is close to the diameter of rosettes from
molecular modeling (8 nm). These results highlight the
stability of toroids, which remain intact with staining,
subsequent vacuum drying, and noncontact imaging process-
es. Furthermore, DLS and AFM analyses showed no evidence
of morphological transformation in decane dispersions over a
year, or after heating to 708C implying that the toroids are the
thermodynamically stable product.
The above observations unveil a unique property of the
rosette 13·23 hierarchically organizing into discrete toroidal
nanostructures in decane. However, the end-to-end binding of
a short columnar nanostructure with persistent length of
67 nm (average circumference of toroids, see above) seems to
be energetically disfavored because of the stiffness of such
columns consisting of stacked disk-shaped supramolecules.
Accordingly, a unique stacking property might be evolved for
the present rosette, for which OPE p-electronic segments play
an important role. As shown by the absorption measurements,
a major driving force for the association of rosettes is H-type
aggregation between OPE segments. Molecular modeling
shows, however, that OPE p planes are rotated out of the
hydrogen-bonded plane by 458.[13] This structural feature
indicates that inter-rosette face-to-face stacking (H-type
aggregation) between OPE segments does not occur if the
rosette stack on top of each other without tilting. Thus, biased
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