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the disulfide. Regardless of explanation, the self-assembly molecular
code is significant, orchestrating unambiguous reaction pathways
that yield distinct products.
5.7
Single-Molecule Studies of Chromophoric
Foldamers
5.7.1
Ensemble Absorption and Emission Spectra of
Foldamers
It is instructive to first review the solution phase ensemble spectra
before studying single-molecule properties. In dilute CH
Cl
/MeOH
2
2
(4:1) solvent, the vibronic 0
1 bands in the absorption
spectra reveal that all four compounds have diagnostic intensity
reversal (Fig. 5.22) in the
→
0 and 0
→
−
−
ratio when compared with non-
self-assembled PDI monomer (superimposed). All PDI foldamers
(dimer
A
0
0
/
A
0
1
2
through undecamer
11
) exhibit this intensity reversal,
which indicates that weak
molecular orbital overlap and
solvophobic interactions drive cofacial stacking of perylene units
[18]. Linear and macrocyclic dimers have similar 0
π−π
−
0/0
−
1 vibronic
−
−
−
−
= 0.73, indicating
that the folding equilibrium and configurations for both dimers are
quite similar in the
peak ratios:
2A
:
A
0
0
/
A
0
1
= 0.84 and
2A
:
A
0
0
/
A
0
1
, with approximately 90% of the
linear dimers and essentially all of the cyclic dimers completely
coupled. Likewise, the ratios for
ground state
(0.63) reveal
that tetramers are also stacked almost identically and that all
four chromophores are preferably folded together and not as two
separately stacked dimers [58]. For all foldamers, the equilibrium
strongly favors the folded state and the absorbance ratios persist
even as solutions are diluted to nanomolar concentrations used to
prepare samples for single-molecule studies.
Although absorbance spectra indicate that the linear and cyclic
foldamers are similarly folded in the
4A
(0.66) and
2
x
2
, fluorescent spectra
reveal that the model oligomers behave significantly different in the
ground state
excited state
. With the emission spectra in Fig. 5.22 normalized to
the 0
0 monomer-like peak, it is apparent that cyclic species have
relatively higher
′→
-stack red emission than the linear species, with
tetrameric species showing much more prominent red emission
π
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