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interactions of the chromophores. Under this circumstance,
the chromophores can interact either intramolecularly or
intermolecularly. If the chromophores interact intramolecularly, the
polymer will fold first (Scheme 5.2). However, if the chromophores
interact intermolecularly, the polymer will self-assemble first. The
central question is illustrated in Scheme 5.2: Will the polymer fold
first or will it self-assemble first? The consequence of this outcome
could be significant. If folding prevails, one can design individual
foldable molecular machinery for probing mechanisms of interest
or use folded polymers as nanoscale building blocks for advanced
materials. However, if self-assembly prevails [3], foldable polymer
will form highly ordered molecular assemblies that cause loss of the
individual character of the folded polymer.
Here, we present simple thermodynamic arguments about folding
versus self-assembling. Since folding is a unimolecular process and
self-assembling is a multimolecular process, one cannot directly
compare the equilibrium constants of
to determine
which process is dominant. For example, assuming that the
concentrations of
K
and
K
fold
SA
folded, unfolded, and self-assembled structures
were 1 mM, we would have
all
−
K
= 1 and
K
(dimer) = 1000 M
1
or
fold
SA
−
2
K
, etc. Apparently, larger self-assembling
equilibrium constants do not reflect larger concentrations of self-
assembled products. Experimentally, we have measured
(trimer) = 1,000,000 M
SA
K
= 6.2
fold
−
1
and
for the perylene-based foldable polymer in 1,1,2,2-
tetrachloroethane.
To understand which process is favored thermodynamically, we
examine the simplest case: the folding or self-assembling of dimers.
To the first-order approximation, we can assume that the interaction
forces of both folding and self-assembling are of the same origin
[11,16]. In other words, the enthalpy due to chromophore units of
the folding process is approximately the same as the enthalpy of the
self-assembling process,
K
= 31 M
SA
. However, the entropies of
folding and self-assembling are quite different because in the case of
folding the two chromophores are covalently bound together within
close proximity, whereas the two chromophores could be far apart
in the case of self-assembly. Consider a folding process of A
∆
H
fold
≈
∆
H
0
SA
,
where A is the monomeric unit, the change of entropy is expressed
as
−
A
↔
A
2
∆
S
fold
=
S
(A
)
−
S
(A-A). Similarly, consider a self-assembly process
2
of A + A
↔
A:A, where A is the monomer (e.g.,
1A
), the change of
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