(BAA) ions, respectively. Such work is considered to be a significant advance in the
construction of mechanically interlocked molecules with high structural complexity,
as well as a good supplement in the areas of multicomponent self-sorting and
noncovalent self-assembly.
11.3 Functionalization of Rotaxanes
The introduction of appropriate functional groups into rotaxanes is essential for the
synthesis of diverse structures capable of performing different tasks. Although there
are well-developed synthetic methodologies available for the construction of such
systems, new methods have to be introduced constantly according to the intended
application of the rotaxane. The unique shuttling motion on a single-molecular scale
of a rotaxane brings remarkable possibilities toward the development of functional
structures. Significant efforts have been concentrated into the functionalization of
rotaxanes, fabricating series of versatile systems bearing multiple functions, such as
fluorescent molecular switches, switchable rotaxane catalysts, molecular production
line, and molecular pumps [1i, 2f]. These functions can be considered as the
fundamental basis of potential applications of rotaxanes advancing their evolution
into molecular materials and devices.
11.3.1 Rotaxane-Based Fluorescent Molecular Switches
Molecular switches, a class of molecules that are able to alter their physical and/or
chemical properties between two or multiple states, have been considered as key
elements in many optical and electronic devices [40]. Although molecular switches
have been widely applied in the construction of smart materials, such as fluorescent
probes [41a], surfaces with controlled wettability [41b], stimuli-responsive polymers
[41c], or smart nanoparticles [41d], the expanding set of potential new applications
demands new molecular switches. Recently, some novel molecular switches based on
molecular machines, including molecular motors [42], rotaxanes, and catenanes [43],
have emerged and performed many unique switching-based tasks. In the case of rotaxane
molecular switches, one of the advantages lies in the precise control of the relative
position of the macrocycle moiety, including the precise control of the corresponding
distance between the macrocycle and the two stoppers. Hence, the integration of two
appropriate functional moieties, such as a fluorophore and an electron donor into the
stoppers and the macrocycle, respectively, would borrow the distance-dependent photoinduced electron transfer (PET) effect to the rotaxane system [44]. In such a system the
PET effect could be precisely controlled through the control of macrocycle motion.
Furthermore, this molecular motion could be also visual owing to the change in fluorescence. Hence, a fluorescent molecular switch could be constructed accordingly.
Pioneering work in the field of rotaxane-type fluorescent switches came from the
group of Sauvage [45], in which the PET effect could be switched through the
addition and removal of transitional metal ions. Later, our group developed a series
11 Functional Rotaxanes
287
construction of mechanically interlocked molecules with high structural complexity,
as well as a good supplement in the areas of multicomponent self-sorting and
noncovalent self-assembly.
11.3 Functionalization of Rotaxanes
The introduction of appropriate functional groups into rotaxanes is essential for the
synthesis of diverse structures capable of performing different tasks. Although there
are well-developed synthetic methodologies available for the construction of such
systems, new methods have to be introduced constantly according to the intended
application of the rotaxane. The unique shuttling motion on a single-molecular scale
of a rotaxane brings remarkable possibilities toward the development of functional
structures. Significant efforts have been concentrated into the functionalization of
rotaxanes, fabricating series of versatile systems bearing multiple functions, such as
fluorescent molecular switches, switchable rotaxane catalysts, molecular production
line, and molecular pumps [1i, 2f]. These functions can be considered as the
fundamental basis of potential applications of rotaxanes advancing their evolution
into molecular materials and devices.
11.3.1 Rotaxane-Based Fluorescent Molecular Switches
Molecular switches, a class of molecules that are able to alter their physical and/or
chemical properties between two or multiple states, have been considered as key
elements in many optical and electronic devices [40]. Although molecular switches
have been widely applied in the construction of smart materials, such as fluorescent
probes [41a], surfaces with controlled wettability [41b], stimuli-responsive polymers
[41c], or smart nanoparticles [41d], the expanding set of potential new applications
demands new molecular switches. Recently, some novel molecular switches based on
molecular machines, including molecular motors [42], rotaxanes, and catenanes [43],
have emerged and performed many unique switching-based tasks. In the case of rotaxane
molecular switches, one of the advantages lies in the precise control of the relative
position of the macrocycle moiety, including the precise control of the corresponding
distance between the macrocycle and the two stoppers. Hence, the integration of two
appropriate functional moieties, such as a fluorophore and an electron donor into the
stoppers and the macrocycle, respectively, would borrow the distance-dependent photoinduced electron transfer (PET) effect to the rotaxane system [44]. In such a system the
PET effect could be precisely controlled through the control of macrocycle motion.
Furthermore, this molecular motion could be also visual owing to the change in fluorescence. Hence, a fluorescent molecular switch could be constructed accordingly.
Pioneering work in the field of rotaxane-type fluorescent switches came from the
group of Sauvage [45], in which the PET effect could be switched through the
addition and removal of transitional metal ions. Later, our group developed a series
11 Functional Rotaxanes
287