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limitation and one is that synthetic polymers do not have a reporter
to signal when they fold or unfold [69,70]. Sequence-controlled
foldamers made of fluorophores, however, offer promising insight
into the intrinsic folding dynamics and folding behavior. Furthermore,
probing synthetic foldamers at the single-molecule (SM) level should
allow observation of complex folding dynamics normally obscured
in ensemble measurements. Toward this end, we have a series of
well-defined foldamers with a specific sequence of alternating rigid
perylene chromophores and flexible ethylene glycol chains that exist
as free
π
:MeOH
solution. Using the 488 nm line of an argon ion laser and the
perylene monomer (
-stacked folded nanostructures in dilute 4:1 CH
Cl
2
2
) as the control, we interrogated individual
foldamers from monomer to undecamer (
1
1 11
), emphasized on
monomer (
1A
), dimer (
2A
), macrocyclic dimer (
2A
), trimers (
3A
),
tetramer (
4A
), concatenated tetramer (
2 x 2
), pentamer (
5A
), and
hexamers (
), and monitored both single-foldamer emission spectra
and intensity temporal profiles at room temperature.
6
Because monomer
1A
displays minor spectral fluctuations, it
was used
as a reference against which linear foldable trimer
3A
and
hexamer
6
were compared. The unique feature exhibited by
3A
and
6
is the degree of dynamics in their spectral trajectories. Figure 5.23a
e
shows the five most common spectral types observed, though some
spectra were either slightly shifted (<10 nm) or intermediary in
shape between those shown, or a combination of one or more types.
All spectra of
1A
(103 molecules, 1000 spectra) are represented by
”type a” (
64%
) or a variant of “type b” (
36%
) where the peak max
is 540
is much more complex.
Switching between different spectral types (Fig. 5.23f-g) occurred
for 38% (40/104) and 62% (81/130) of all trimer and hexamer
molecules, respectively. In addition, spectral switching from frame
to frame for a single trimer or hexamer could be quite dramatic both
in terms of spectral shape as well as intensity. Differences in peak
maxima of up to 110 nm for
580 nm. The situation for
3A
and
6
were observed.
While interaction with the local environment can explain minor
spectral fluctuations as seen in
3
and 140 nm for
6
, they cannot account entirely
for the observed unusual color shifts of
1A
. The flexibility of
perylene foldamers is in contrast to perylene-based photonic wires
[71] and nanofibers [72], in which the relative degree of motional
freedom of individual perylenes is limited by chemical structure. We,
therefore, attribute the rich dynamics to a photoinduced stochastic
3A
and
6
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