Three BTF solutions of 1 at different concentrations (1024 M,
1025 M and 1026 M) were investigated by cryo-SEM. The
investigation of a highly concentrated solution (1024 M) revealed
the presence of large bundles of laterally aggregated HBC columns
with an average length of about 20 mm and a cross section of
typically 500 6 200 nm, representing an architecture consisting of
approx. 10,000 laterally aggregated columns (see Fig. 1a). The
remarkable bending observed for the long axis of these structures
implies considerable non-crystallinity in the lateral aggregation of
these supramolecular stacks. At an intermediate concentration
(1025 M), thinner bundles of HBC columns were observed, having
a cross section of about 250 6 50 nm, extending still over 20 mm in
length (approx. 400 aggregated columns, see Fig. 1b and 1c). At
low concentration (1026 M), only isolated HBC fibres were found
with an apparent diameter of 25 nm and 3 mm typical length,
indicating
a freestanding monocolumnar linear architecture
(Fig. 1d). As the observed diameter of 25 nm is due essentially
to the applied platinum coating (15 nm), we assume that this
diameter, uniform for all filaments, nevertheless proves the
existence of laterally non aggregated structures.
Fig. 2 Optical spectroscopy: (a) Excitation spectrum (ldet 5 509 nm)
and fluorescence spectrum (lex 5 360 nm) of the solid precipitated from a
1023 M solution of HBC-[(CH2)4(CF2)6F]6 (1) in BTF. (b) Excitation
spectrum (ldet 5 485 nm), fluorescence spectrum (lex 5 360 nm), and
absorption spectrum (dotted line) of a solution of 1024 M 1 in BTF. (c)
Same as in (b) but for a solution of 1026 M 1 in BTF. Note that the
intensities of the excitation and fluorescence spectra of (b) can be
compared with those of (c). Note also the different absorbance scales of (b)
and (c). The luminescence spectra are arbitrarily scaled.
BTF solutions of 1 ranging from 1024 M to 10210
M
were investigated by optical spectroscopy. At low concentrations
(1026 M to 10210 M) excitation, fluorescence and absorption
spectra reveal typical bands of HBC derivatives16 (Fig. 2). By
increasing the concentration to 1025 M or 1024 M, intense and
broad red-shifted luminescence bands appear which can be
attributed to HBC columns and aggregates by comparison to
the data collected by cryo-SEM. Moreover, the solid state
luminescence spectrum of the precipitate collected from the
1024 M solution reveals similar red shifted bands. Nevertheless,
the observed differences of the solid state emission in spectrum 2a)
emission spectroscopy provides a powerful tool for monitoring the
formation of such structures.
Notes and references
as compared to the emission spectrum in solution at 1024
M
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2 S. Sharma, H. Li, H. Chandrasekaran, R. C. Mani and M. K. Sunkara,
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suggest the presence of at least two different types of aggregates in
solution. Cryo-SEM observations support this assumption,
showing that large laterally aggregated fibres are observed at
medium to high concentration in addition to monostranded stacks
(Fig. 1a).
In conclusion, cladding HBC with ‘‘Teflon’’ like side chains
makes it possible to control the self-assembly into highly ordered
architectures. Cryo-SEM correlated to optical excitation and
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Fig. 1 Micrographs obtained by cryo-SEM of HBC-[(CH2)4(CF2)6F]6
nanostructures in BTF at different concentrations: (a) 1024 M; (b), (c)
1025 M; (d) 1026 M.
4222 | Chem. Commun., 2006, 4221–4223
This journal is ß The Royal Society of Chemistry 2006