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fold
0
SA
∆
H
0.12
kcal/mol. This indicates that both folding and self-assembling of
perylene molecules are exothermic processes in organic solvents.
Self-organization requires more ordering; therefore, it should
have a larger entropy change than that of folding into similar final
structures. Experimentally, we observed
=
−
3.01
±
0.06 kcal/mol and
∆
H
(dimer) =
−
2.44
±
fold
∆
S
=
−
6.60
±
0.40 cal/
0
SA
11
mol•K and
indicating that
the folding is favored by entropy contribution. Thus, folding precedes
self-assembly.
∆
S
(mono) =
−
17.5
±
0.7 cal/mol•K,
5.3.4
Photoluminescence Properties of Folded Polymers
The remarkable characteristics of chromophoric folded polymers
are that the folded states or partially folded states emit completely
different colors than that of the unfolded states or free monomers.
Free monomers emit green fluorescence, but folded or partially
folded oligomers can emit green, yellow, orange, or red fluorescence
(Fig. 5.6). The excited states can be classified as either localized on a
single chromophore or delocalized among multiple chromophores.
When the excited state is localized on a single chromophore,
the emission is green if the chromophore is not electronically
coupled to other chromophores significantly. If the chromophore
is significantly coupled to other chromophores as discussed in the
optical absorption section, such localized excited states should emit
fluorescence following the Franck
−
Condon principle. Both folded
and self-assembled
π
-stacks promote the perylene absorption shift
from 0
1 transition (500 nm) as shown
in Fig. 5.6a. This is attributed to shifts of the maximum overlap of
Franck
→
0 transition (530 nm) to 0
→
−
Condon integral from <
c
′
v
c
v
> in free monomers (
1A
)
'=0
=0
to <
). As the folded
chromophoric oligomers become larger, the photoluminescence
favors emissions to higher vibronic ground states (
c
′
c
> in the folded nanostructures (
2A
-
11
v'=0
v=1
= 0,
1, 2, 3), thereby red shifting the spectra. Representative examples
of folded perylene oligomers are shown in Schemes 5.3 and 5.4. The
concentration in Fig. 5.6b is at or below the critical concentration
for self-assembly (
v
'= 0
→
v
,
v
) and therefore the photoluminescence shifts
are largely due to folded chromophores.
C
C
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