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ground and excited states are consistent with transient unfolding
during photoexcitation and the results are summarized in Table 5.2.
To complement the steady-state measurement results and
further probe the folding and unfolding dynamics of the excited
state, we collected time-resolved fluorescence decays to obtain
the lifetimes of the monomer-like and red emissive species, and to
estimate %-monomer-like emission across the linear dimer through
hexamer series. The perylene monomer consistently fit well to a
single exponential, yielding a lifetime of 4.00
±
0.01 ns. In contrast,
oligomer decays obtained over the full spectral window consistently
fit well to a biexponential to yield two lifetimes: A faster
4 ns
component attributed to transiently unfolded states and a slower
∼
∼
10 ns component corresponding to red emission from folded
π
-stacks. By analyzing the pre-exponential factors and accounting
for the differences in quantum yields, we calculate that up to 45%
of
5% of
linear trimer through hexamer display such emission. The measured
excited-state populations should obviously depend on the excitation
powers used in the experiments. Values for trimer through hexamer
agree well with the values obtained from steady-state spectra,
while
2A
fluorophores exhibit monomer-like emission while 7
−
exhibits approximately a twofold increase in the unfolded
population when compared with steady-state measurements.
Regardless of the method, there is an unambiguous increase in
the unfolded population between the ground and excited states as
summarized in Table 5.2; the difference is the most dramatic in the
least restricted architectures.
2A
Table 5.2
Percentage
of
unfolded
species
in
linear
and
cyclic
foldamers.
Foldamers
2A
2A
4A
2
x
2
Ground state
10%
<1%
<1%
<1%
a
Excited state
19.8%
3.4%
5.1%
1.7%
b
Excited state
23.8%
3.7%
5.8%
3.5%
c
−
−
Excited state
43.0%
6.1%
a
Determined by comparing green oligomer fluorescent intensity to monomer
fluorescent intensity.
b
Determined by comparing ratio of green and scaled red oligomer steady-state
emission.
c
Determined by time-resolved fluorescence.
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