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in chloroform were spread
on carbon-coated grids or glass surfaces and allowed to evaporate,
similar rings as observed for the porphyrin dimer
When micromolar solutions of
2
or
3
were observed,
but in the case of the hexamers and dodecamers generally rings were
formed with a larger diameter, even up to 27
1
m. It was generally
observed that upon an increase in diameter, the thickness of the
rings became larger. The observation that hexamer
µ
2
on average
self-assembles into larger rings than dimer
is believed to be
directly related to stronger intermolecular interactions between the
molecules of the former compound. Although the rings are formed
by dewetting of a solution, the hexamers apparently already have
a preference to form columnar stacks when the thin film of the
solution is concentrated upon evaporation of the solvent.
To investigate the degree of internal molecular order in the rings,
NSOM was used to study simultaneously their morphology and optical
properties (Fig. 8.4A) [4]. The fluorescence was observed to correlate
strongly with the topography, in the sense that an increase in the ring
thickness was accompanied by an equal increase in the fluorescence
intensity. To gain more detailed insight into the optical properties
of the rings, polarized fluorescence experiments were carried out
on rings formed by
1
. Sets of fluorescence images were acquired
using linearly polarized light in both the horizontal and the vertical
directions with respect to the image, and with a polarizer in the
emission path oriented parallel and perpendicular to the excitation
direction. It was expected that differences in fluorescence in the
rings would point to differences in the orientation of the porphyrin
hexamers within them. Initially, no such polarization effects were
observed within the rings, but after they had been annealed at
80
2
C for two days, apparently the internal ordering had changed
because now fluorescence anisotropy could be observed (Fig. 8.4B).
It is believed that the annealing step rearranges the molecules into
better-ordered stacks, which in turn is accompanied with a more
pronounced response in the polarized fluorescence experiments.
When vertically polarized excitation light and vertical detection
was used, the fluorescence intensity at the left and right parts of the
ring was higher than when horizontally polarized excitation light
was used. The reverse fluorescence behaviour was observed for
the top and bottom parts of the ring. These results indicate that the
°
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