macrocycles shuttle dynamically and freely along the PEG chains. Upon mixing the
rotaxane chains in high concentration, the noncovalent intermolecular dimerization
between two α-CD macrocycles would work as a crosslink in the system, resulting in
a transparent polymeric gel network with high mechanical strength and chemical
stability. Upon addition of external stress by a spiky tip, the surface would produce
microscopic deformation and subsequent visual scratch. However, in contrast to
conventional polymer coatings (Fig. 14b), the directional shuttling movement of
α-CD macrocycles would occur away from the deformation instead of the irreversible breaking of covalent backbone: the macrocycles at the stressed area move to the
sides for decreasing the space volume of the stressed area (Fig. 14c). After removal
of the stress, the dynamic shuttling movement of α-CD macrocycles would become
unidirectional and uniform again smoothing the surface, resulting in self-recovery of
the scratched state (Fig. 14d). Obviously, this system has limitation regarding the
degree of damage because too large deformation generated by too high stress would
not be met with the directional movement of α-CD macrocycles, leading to irreversible breaking of covalent backbone. In any case, this work ingeniously combined
self-healing behavior of polymeric materials with the dynamic shuttling movement
of macrocycles on a molecular scale, providing a significant method for the amplification of rotaxane molecular motion and the application of rotaxanes toward
functional self-healing materials.
11.5 Conclusions
In the past 20 years, rotaxane, as a luminous member in the family of artificial
molecular molecules, has evolved remarkably in terms of synthetic methodology,
functionalization, and materials applications. In a recent work, our group reported
[2]rotaxane composed of an amphiphilic molecular thread with three binding stations would be inserted into lipid bilayers as a unimolecular vehicle to perform
passive ion transport through its stochastic shuttling motion [70]. Nowadays
rotaxanes are considered as a core part of chemistry and materials design [71]. In
structural design and construction of rotaxanes, several novel and reliable synthetic
strategies, such as self-sorting, have been highlighted in this chapter. Novel strategies have also been combined with conventional methods toward efficient syntheses
of rotaxanes with high structural complexity. The functionalization of rotaxanes to
increase structural complexity is the most developed for the class of bistable [2]
rotaxanes, due to their widely applied unique stimuli-responsive capability in the
construction of functional molecular switches and devices. Meanwhile, rotaxanes
also have the potential of mimicking the functions of biological machines like
enzyme catalysis, pumping, or peptide synthesis. Inspired by the evolution of living
systems in nature from sea to land, the working platform of rotaxanes has also
evolved from solution to surfaces and interfaces, resulting in a forthcoming period
full of challenges but also opportunities. The challenging issues lie on the unexpected operation modes when numbers of rotaxanes are immobilized on a surface,
because the steric and orientation effects could alter the operation mode on a surface
302
C.-X. Zhao et al.
rotaxane chains in high concentration, the noncovalent intermolecular dimerization
between two α-CD macrocycles would work as a crosslink in the system, resulting in
a transparent polymeric gel network with high mechanical strength and chemical
stability. Upon addition of external stress by a spiky tip, the surface would produce
microscopic deformation and subsequent visual scratch. However, in contrast to
conventional polymer coatings (Fig. 14b), the directional shuttling movement of
α-CD macrocycles would occur away from the deformation instead of the irreversible breaking of covalent backbone: the macrocycles at the stressed area move to the
sides for decreasing the space volume of the stressed area (Fig. 14c). After removal
of the stress, the dynamic shuttling movement of α-CD macrocycles would become
unidirectional and uniform again smoothing the surface, resulting in self-recovery of
the scratched state (Fig. 14d). Obviously, this system has limitation regarding the
degree of damage because too large deformation generated by too high stress would
not be met with the directional movement of α-CD macrocycles, leading to irreversible breaking of covalent backbone. In any case, this work ingeniously combined
self-healing behavior of polymeric materials with the dynamic shuttling movement
of macrocycles on a molecular scale, providing a significant method for the amplification of rotaxane molecular motion and the application of rotaxanes toward
functional self-healing materials.
11.5 Conclusions
In the past 20 years, rotaxane, as a luminous member in the family of artificial
molecular molecules, has evolved remarkably in terms of synthetic methodology,
functionalization, and materials applications. In a recent work, our group reported
[2]rotaxane composed of an amphiphilic molecular thread with three binding stations would be inserted into lipid bilayers as a unimolecular vehicle to perform
passive ion transport through its stochastic shuttling motion [70]. Nowadays
rotaxanes are considered as a core part of chemistry and materials design [71]. In
structural design and construction of rotaxanes, several novel and reliable synthetic
strategies, such as self-sorting, have been highlighted in this chapter. Novel strategies have also been combined with conventional methods toward efficient syntheses
of rotaxanes with high structural complexity. The functionalization of rotaxanes to
increase structural complexity is the most developed for the class of bistable [2]
rotaxanes, due to their widely applied unique stimuli-responsive capability in the
construction of functional molecular switches and devices. Meanwhile, rotaxanes
also have the potential of mimicking the functions of biological machines like
enzyme catalysis, pumping, or peptide synthesis. Inspired by the evolution of living
systems in nature from sea to land, the working platform of rotaxanes has also
evolved from solution to surfaces and interfaces, resulting in a forthcoming period
full of challenges but also opportunities. The challenging issues lie on the unexpected operation modes when numbers of rotaxanes are immobilized on a surface,
because the steric and orientation effects could alter the operation mode on a surface
302
C.-X. Zhao et al.