The
integration of mechanophores, motifs that transduce mechanical
forces into chemical reactions, allows creating materials with stress-dependent
properties. Typical mechanophores are activated by cleaving weak covalent
bonds, but these reactions can also be triggered by other stimuli,
and this renders the behavior unspecific. Here we show that this problem
can be overcome by extending the molecular-shuttle function of rotaxanes
to mechanical activation. A mechanically interlocked mechanophore
composed of a fluorophore-carrying macrocycle and a dumbbell-shaped
molecule containing a matching quencher was integrated into a polyurethane
elastomer. Deformation of this polymer causes a fluorescence turn-on,
due to the spatial separation of fluorophore and quencher. This process
is specific, efficient, instantly reversible, and elicits an easily
detectable optical signal that correlates with the applied force.
The introduction of mechanophores into polymers makes it possible to transduce mechanical forces into chemical reactions that can be used to impart functions such as self-healing, catalytic activity, and mechanochromic response. Here, an example of mechanically induced metal ion release from a polymer is reported. Ferrocene (Fc) was incorporated as an iron ion releasing mechanophore into poly(methyl acrylate)s (PMAs) and polyurethanes (PUs). Sonication triggered the preferential cleavage of the polymers at the Fc units over other bonds, as shown by a kinetic study of the molar mass distribution of the cleaved Fc-containing and Fc-free reference polymers. The released and oxidized iron ions can be detected with KSCN to generate the red-colored [Fe(SCN) (H O) )] complex or reacted with K [Fe(CN) ] to afford Prussian blue.
This review summarizes the opportunities offered by internal and external confinement of single-chain nanoparticles (SCNPs) in catalysts, drug delivery, sensing, and other emerging potential applications.
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