Catenation of Self-Assembled Nanorings
COMMUNICATION
of the p conjugation length has a significant impact on the
solution phase organization, as described below.
Nanostructures of 2 and 3 were prepared by dissolving
small amounts of the compounds in MCH with heating, and
then cooling the resulting homogeneous solution to room
temperature. Temperature-dependent UV/Vis spectroscopy
showed the growth of red-shifted absorption bands upon
cooling from 90 to 208C (Figure S2 in the Supporting Infor-
mation); this indicates the formation of p–p stacked aggre-
gates. DLS measurements of 2ꢁ10ꢀ5 m solutions indicated
only small aggregates (or monomers) with hydrodynamic di-
ameters (DH) less than 4 nm (Figure 2d and e). Upon in-
creasing the concentrations above 1ꢁ10ꢀ4 m, both com-
pounds showed the formation of nanostructures with aver-
age DH of 30 nm for 2 and 20 nm for 3. Interestingly, the DH
of these compounds did not change when the concentrations
were increased to 1ꢁ10ꢀ3 m. These results are in sharp con-
trast to that of 1, which displayed a significant increase of
DH when the concentration was increased from 2ꢁ10ꢀ5
m
(ca. 60 nm, nanoring) to 1ꢁ10ꢀ4 m (ca. 300 nm, open-ended
nanostructures).
The nanostructures formed by 2 and 3 in MCH were spin-
coated onto highly oriented pyrolytic graphite (HOPG) and
imaged by using AFM. Although the specimens prepared by
drop-casting displayed nanostructures identical to those pre-
pared by spin-coating, we avoided the former preparation
method to prevent unfavorable agglomeration of nanostruc-
tures. The specimens prepared from a 1ꢁ10ꢀ4 m solution of 2
displayed a large number of nanorings with high uniformity
of size and shape (Figure 3a and Figure S3 in the Supporting
Information). Open-ended structures, such as curved fibrils,
spirals and stiff rods, which were all observed for 1 at the
same concentration, were rarely found in repeated imaging
of the specimens. The typical width of nanorings (edge-to-
edge distance) was 36 nm, which is slightly shorter than that
for 1 by 4 nm (Figure 1d).
A considerable amount of nanorings (ca. 60% of all nano-
structures) was imaged even for specimens prepared from
5ꢁ10ꢀ4 m solution of 2 (Figure 3b). This situation is compa-
rable to the specimen prepared from a 4ꢁ10ꢀ5 m solution of
1. Closer inspection of the AFM images revealed the pres-
ence of interlocked structures of two nanorings (circled
areas in Figure 3b).[14] Figure 3c–e show magnified images
of the catenated nanostructures. One of the two contact
parts of the interlocked nanorings, for example, the contact
point x of rings A and B in Figure 3d, has identical height to
the main body of nanorings (ca. 1.8 nm). At such a point,
the upper-lying nanoring is clearly discernible from the
AFM image (i.e., ring A). Another contact point, that is,
point y, exhibits a higher height (2.6 nm) than the main
body of nanorings. For such a point, cross-sectional analyses
along to ring A (Figure 3 f) and B (Figure 3g) gave different
height profiles, and revealed that ring B is the upper-lying
nanostructure at this contact point. It can be concluded
from these analyses that the two nanorings are interlocked
with each other and not simply overlapped nanostructures.
Approximately one or two catenanes were found in every
Figure 3. AFM height images of nanostructures of
2 spin-coated
(3000 rpm) from MCH solutions at concentrations of: a) 1ꢁ10ꢀ4, and
b) 5ꢁ10ꢀ4 m onto HOPG (z scale: 20 nm). c)–e) Magnified images of the
catenated nanorings found in the circled areas in (b); scale bar: 20 nm.
f), g) Cross-sectional analysis of the catenane in (d).
200ꢁ200 nm area (Figure S4 in the Supporting Information).
Because single nanorings are composed of approximately
1300 molecules of 2,[15] this finding indicates an unprece-
dented type of supramolecular catenanes that consists of
more than 2000 molecules of the SMBBs.
Further reduction of the p segments in the present “BAR-
p-wedge” system resulted in the almost exclusive formation
of nanorings. Thus, specimens of 3 exhibited a number of
nanorings, despite being prepared from 5ꢁ10ꢀ4 m solutions
(Figure 4a). Although other open-ended structures are
rarely found, the presence of small spherical nanostructures
should be noted. Such nanostructures can be produced
Figure 4. a) AFM height image of a sample prepared by spin-coating of a
MCH solution of 3 (5ꢁ10ꢀ4 m) onto HOPG; z scale: 20 nm. b), c) Magni-
fied images of the square areas indicated in (a).
Chem. Eur. J. 2011, 17, 13657 – 13660
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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