11.4 Rotaxane-Based Functional Materials
Although the functional rotaxanes discussed above have performed well as novel
molecular switches and biomimetic molecules, most of these rotaxanes are used in
solution, which is a limitation in several practical aspects in the construction of
functional materials [16]. Hence, functional rotaxanes should be evolved further
toward immobilization on solid surfaces and interfaces. Macroscopic surfaces could
act as a platform on which molecular-scale rotaxane motions and functions could be
amplified into macroscopic changes in properties, such as optics, sizes, morphology,
dispersion, or catalytic activity [16h]. However, considering the differences between
solution phase and solid phase, several challenges has to be considered when a
rotaxane is immobilized on a surface or inside a solid material, but also new
opportunities are emerging given the unexpected performance of an immobilized
rotaxane and the mutual communication (such as PET effect, FRET effect, SPR
effect) between the rotaxane moieties and the inorganic surfaces. Herein, we focus
on the general and reliable strategies of rotaxane immobilization on solid surfaces.
Synergistic effects between rotaxanes and surfaces are also highlighted in the recent
examples of novel functional materials, including solid-state molecular switches,
controlled drug release systems, or self-healing materials.
11.4.1 Rotaxane Molecular Switches on Surfaces
As demonstrated in Fig. 5, rotaxanes could be developed into a series of molecular
switches with tuneable fluorescence, responding to external stimuli [46]. However,
the practicability of a fluorescent switch in solution is limited in portable functional
materials applications. To operate rotaxane switches on a solid surface is essential for
the development of novel switchable fluorescent materials. Surfaces [16h], porous
metal organic frameworks [20], and polymers [35] have been proved reliable
platforms for the effective functioning of molecular switches or molecular machines.
Effective operation of rotaxanes on these platforms needs enough free space for the
stimuli-responsive motion of the macrocycle, which is not an issue in a dilute
solution. However, confinement effects dominate on a surface, interface, or solidstate polymer, meaning that crowding of rotaxanes would affect the operation of
rotaxanes by steric hindrance. Hence, it is necessary to decrease to some extent the
concentration of rotaxanes on the solid platform [58]. Another challenge is finding
general and reliable strategies to attach rotaxanes to different surfaces. Although
there are many approaches developed for the effective functionalization of surfaces,
the structural complexity of rotaxanes requires specific and effective methods for the
immobilization. Click reactions provide unique specificity and efficiency compared
to other organic coupling reactions, and we also demonstrated their advantages in the
construction of rotaxanes with high structural complexity [26].
Employing the CuAAC click reaction, the effective immobilization of the [2]
rotaxane fluorescent switches have been realized by our group using the a linear
polymer platform [59] (Fig. 9a) and inorganic silica NPs [60] (Fig. 9b). The common
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295
Although the functional rotaxanes discussed above have performed well as novel
molecular switches and biomimetic molecules, most of these rotaxanes are used in
solution, which is a limitation in several practical aspects in the construction of
functional materials [16]. Hence, functional rotaxanes should be evolved further
toward immobilization on solid surfaces and interfaces. Macroscopic surfaces could
act as a platform on which molecular-scale rotaxane motions and functions could be
amplified into macroscopic changes in properties, such as optics, sizes, morphology,
dispersion, or catalytic activity [16h]. However, considering the differences between
solution phase and solid phase, several challenges has to be considered when a
rotaxane is immobilized on a surface or inside a solid material, but also new
opportunities are emerging given the unexpected performance of an immobilized
rotaxane and the mutual communication (such as PET effect, FRET effect, SPR
effect) between the rotaxane moieties and the inorganic surfaces. Herein, we focus
on the general and reliable strategies of rotaxane immobilization on solid surfaces.
Synergistic effects between rotaxanes and surfaces are also highlighted in the recent
examples of novel functional materials, including solid-state molecular switches,
controlled drug release systems, or self-healing materials.
11.4.1 Rotaxane Molecular Switches on Surfaces
As demonstrated in Fig. 5, rotaxanes could be developed into a series of molecular
switches with tuneable fluorescence, responding to external stimuli [46]. However,
the practicability of a fluorescent switch in solution is limited in portable functional
materials applications. To operate rotaxane switches on a solid surface is essential for
the development of novel switchable fluorescent materials. Surfaces [16h], porous
metal organic frameworks [20], and polymers [35] have been proved reliable
platforms for the effective functioning of molecular switches or molecular machines.
Effective operation of rotaxanes on these platforms needs enough free space for the
stimuli-responsive motion of the macrocycle, which is not an issue in a dilute
solution. However, confinement effects dominate on a surface, interface, or solidstate polymer, meaning that crowding of rotaxanes would affect the operation of
rotaxanes by steric hindrance. Hence, it is necessary to decrease to some extent the
concentration of rotaxanes on the solid platform [58]. Another challenge is finding
general and reliable strategies to attach rotaxanes to different surfaces. Although
there are many approaches developed for the effective functionalization of surfaces,
the structural complexity of rotaxanes requires specific and effective methods for the
immobilization. Click reactions provide unique specificity and efficiency compared
to other organic coupling reactions, and we also demonstrated their advantages in the
construction of rotaxanes with high structural complexity [26].
Employing the CuAAC click reaction, the effective immobilization of the [2]
rotaxane fluorescent switches have been realized by our group using the a linear
polymer platform [59] (Fig. 9a) and inorganic silica NPs [60] (Fig. 9b). The common
11 Functional Rotaxanes
295