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make future significant impact in many other areas of chemistry and
biology. Studying the weak secondary forces, we find that folding
occurs before self-assemblies because intramolecular interactions
hardly pay the entropy penalty that is required for intermolecular
interactions. For the same reason, molecular self-assemblies require
a critical concentration region, below which molecules remain free
and above which self-association occurs.
disassembling trigger
actuation at the nanoscale. At the molecular scale, molecules
are brought together and separated from each other. Because
photophysical properties such as FRET are very sensitive to the
structural change at the nanoscale, biosensors based on molecular
recognition can be fabricated using foldamers. Using complementary
recognition, DNA and protein biosensors have been constructed.
With deeper understanding of molecular self-assembly
processes, we begin to appreciate that formation of strong covalent
bonds is intrinsically linked to weak secondary interactions, such as
ion pairing, dipolar interaction, hydrogen bonding, or solvophobic
effects. While molecular self-assembly employs weak interactions
to form various nanostructures, their potential functions to control
specific reaction pathways have not been widely exploited. This
chapter demonstrates that molecular self-assembly promotes novel
reactions that yield exquisite products, which would be difficult to
obtain otherwise. Conceptually, such directing ability of molecules
is quantified as molecular codes, which add a new chapter to
fundamental organic chemistry: Reaction between functional
groups is favored if the molecules carrying those functional groups
have complementary molecular codes.
Single-molecule studies clearly reveal the structure
Folding
−
unfolding
and
assembling
−
−
property
relationship. Linear foldamers have some restraints on the
chromophores because of primary linear linkage. As a result,
chromophoric foldamers emit nearly rainbow colors from green
to red, whereas corresponding chromophoric unit only emit green
fluorescence. As the primary structures change from linear linkage
to a circular structure, this structural constraint enhances
π
-stack
emission in the red. Furthermore, when the primary structure
consists of two concatenated rings, dynamic switching of the single-
molecule spectra emerges as steady emission of colorful composite
spectra. As such, each new single-molecule spectrum roughly
reproduces the previous spectrum in time-lapse trajectories.
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