Angewandte
Communications
Chemie
[48,54]
band and the occurrence of an H-band (243 nm).
This
suggests that oligomer 1 forms supramolecular polymers in
which the phenanthrenes are arranged in extended H-
aggregates. Heating of the aqueous solution to 808C leads
to disassembly of the polymers, with concomitant disappear-
ance of the H-band.
Trimer 1 forms tubular structures in aqueous solution.
Atomic force microscopy (AFM, Figure 3A) reveals elon-
gated objects reaching a length of several micrometers. The
measured height of the polymers after deposition on NiCl -
2
[
55]
treated mica is in the range of 4–4.5 nm. This corresponds
Figure 4. A) Fluorescence and B) excitation spectra of poly1. Condi-
tions: 1 mm in 10 mm sodium phosphate buffer pH 7.0, 10% ethanol;
excitation slit: 5 nm, emission slit: 5 nm; (lex and lem as indicated).
[
57]
to solvation effects. The fluorescence quantum yield (F)
also depends significantly on the excitation wavelength. For
excitation at 316 nm, a value of 7% was obtained (see the
Supporting Information for details), whereas excitation at
2
43 nm (H-band) renders a value of only 3%. A red-shifted
fluorescence and a low quantum yield are well-known
[58]
features of H-aggregates. Excitation spectra (Figure 4B)
further demonstrate the strong dependence of the fluores-
cence on the excitation wavelength. The spectrum obtained
by recording the emission at 391 nm resembles the absorption
spectrum of oligomer 1 dissolved in ethanol. Monitoring of
the emission at 412 nm gives a spectrum with a peak at
2
43 nm, which coincides with the H-band in the absorption
spectrum of poly1. Finally, the excitation spectrum recorded
at 437 nm is comparable to the absorption spectrum of poly1
obtained in aqueous medium exhibiting again the character-
istic band at 243 nm and a comparatively reduced intensity at
Figure 3. A) AFM image of supramolecular polymers (poly1) assem-
bled in aqueous solution and deposited on mica using NiCl as
2
surface binding agent. Formation of the nanotubes is accomplished by
slow cooling of a 1 mm solution of trimer 1 from 808C to room
temperature in water (10 mm sodium phosphate buffer, pH 7.0)
containing 10% v/v of ethanol. B) TEM image of a phenanthrene
nanotube. C) Illustration of the self-assembly of phenanthrene oligo-
mers 1. Hydrophobic phenanthrene units are arranged in a sheet-like
manner with hydroxyl-groups (red) and phosphates (yellow) located on
its inner and outer surfaces.
2
75 and 316 nm. Thus, H-aggregation leads to a red-shifted
fluorescence.
We previously observed efficient energy transfer from 3,6-
disubstituted phenanthrenes to acceptor pyrenes in light-
[
43]
harvesting supramolecular polymers fibers. To test if the
present nanotubes formed of 2,7-dialkynyl phenanthrenes
also possess light-harvesting properties, polymerization of
1 was carried out in the presence of small quantities of the
pyrene containing oligomer 2. This leads to the formation of
pyrene-doped supramolecular nanotubes, as illustrated in
Figure 5.
to a double-layer of p-stacked phenanthrenes formed by the
[46]
deposition of supramolecular tubes on the surface.
diameter of the nanotubes varies in the range of 50–150 nm
Supporting Information). The tubular morphology of the
The
(
nanostructures was confirmed by transmission electron mi-
croscopy (TEM) experiments (Figure 3B). Notably, polymer-
ization of oligomer 1 exclusively results in the formation of
tubular objects. This is in contrast to findings with trimers of
the iso-structural 2,7-pyrene, which leads to the simulta-
neous formation of nanosheets and nanotubes.
The fluorescence spectrum of poly1 measured in aqueous
medium shows a structured emission band with maxima at 371
and 391 nm when excited at 316 nm (Figure 4A). Excitation
at 243 nm (H-band) leads to changes in the shape of the
emission band. The maxima are shifted by 5 nm to the red
A comparison of the fluorescence spectra of nanotubes
consisting exclusively of phenanthrenes and nanotubes con-
taining 1.2% of pyrene molecules is displayed in Figure 6A.
Already small quantities of pyrene acceptor lead to a sub-
stantial increase in the fluorescence intensity upon irradiation
at 243 nm, which corresponds to the H-band in the spectrum
of the phenanthrene polymer. The effect of increasing pyrene
content in the nanotubes on the intensity of pyrene fluores-
cence is displayed in Figure 6B. The maximum intensity is
reached with a pyrene content of about 7%. This confirms
that the supramolecular nanotube functions as a light-harvest-
ing antenna in which the excitation energy is transferred from
phenanthrenes to pyrene acceptor moieties. Phenanthrene
excitation at 316 nm also leads to a steady increase of pyrene
emission, although not as much as excitation at the H-band
[
46]
(
4
376 and 396 nm) and a broad band newly appears around
25 nm, which most likely arises from excimer formation.
[56]
The excitation wavelength dependence of phenanthrene
fluorescence has been reported previously and was attributed
9
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Angew. Chem. Int. Ed. 2016, 55, 9961 –9964