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spectra are slightly shifted from those shown, or are composites of
multiple spectral types. These five spectral types are representative
and initially identified for linear trimer and hexamer [75].
The relative peak intensities and observed spectral positions
reflect the degree and strength of electronic coupling and therefore
the conformational state in which the perylene molecule exists on
the glass surface [58,75]. Integrating all emission wavelengths,
we typically count 5
−
emission cycles per second using the
experimental conditions described here. The fact that we see spectra
that are not completely representative of the ensemble spectra
suggests that the energy landscape and local energy minima (traps)
play important roles in the folding and unfolding.
Linear trimer and hexamer exhibit spectral trajectories that
are actively switching. Linear tetramer (
×
10
4
6
) further verifies these
observations, showing frequent switching from one spectral type
to another while displaying all spectral types. More importantly,
the concatenated tetramer (
4A
) reveals only minimal switching
trajectories, supporting a photoinduced folding and unfolding model.
Figure 5.24 shows portions of
2
x
2
4A
and
2
x
2
spectral trajectories,
where
4A
exhibits switching between different spectral types and
2
x
2
displays a marked and unambiguous decrease in spectral
switching and steadily emits one multicolored composite spectral
type. Although a small percentage of
molecules did show minor
spectral switching, Fig. 5.24 represents statistical emission of
2
x
2
.
The cyclic compounds mostly display composite spectra, having
simultaneous monomer-like and
2
x
2
-stack emission characteristics,
which imply that coupling and uncoupling dynamics occurs faster
than the experimental 2 s integration time.
Spectral trajectories for linear dimer
π
2A
and cyclic dimer
2A
compare somewhat differently than the tetramers. While being
excited,
preferentially and frequently adopts the unfolded
configurations, unlike
2A
, and thus shows mostly monomer-like
emission and only occasionally visits the folded state that fluoresces
red. In contrast to
4A
spends more time traveling between
folded and unfolded states and displays a smaller relative red peak.
After correcting the quantum yield and lifetime differences, we
find that
2
x
2
,
2A
spends more than three-fourth time in the folded state
and less than one-fourth time in unfolded states. The concatenated
tetramer, however, spends more than 90% time in folded states. The
difference is that
2A
2A
is not intercalated like
2
x
2
and thus allows
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