11.1 Introduction
Artificial molecular machines have attracted considerable attention from synthetic
chemists due to their unique ability to mimic many aspects of biological phenomena
[1]. Rotaxanes, consisting of one or more macrocycles and an encircled dumbbelllike thread component, have been extensively studied and developed and thus
became one of the major classes of artificial molecular machines [2]. Since the
first [2]rotaxane was reported in the 1990s [2a], the structural complexity of
rotaxanes has increased rapidly along with the development of synthetic methodology and host-guest chemistry [3]. Various rotaxanes have been designed and
constructed based on versatile macrocycles, such as cyclodextrins [2b, 4], cucurbit
[n]uril [5], crown ethers [2c, 6], pillarenes [7], cyclophanes [8], and benzylic amide
macrocycles [9]. Recently, some novel types of rotaxanes, such as [1]rotaxane [10],
daisy chain [11], and hetero[n]rotaxane [12], are rising and being developed extensively due to their peculiar structure and unique molecular motion. Consequently, the
development of novel synthetic methodology is crucial for the further advancement
of these new classes of rotaxanes.
Their stimuli-responsive nature is one of the most attractive features of rotaxanes,
which makes them capable of producing molecular-scale motion upon external
stimuli [2f] including pH [2c, 11b, c], redox [8, 11h], light [3d, 4a–c, 11a], and
microenvironmental changes [7, 11i]. The motion is mostly driven by the relative
difference in binding constants between the recognition stations and the macrocycles: the macrocycles tend to be close to the stations with higher binding constants;
and dynamic shuttling motion would occur when two or more stations are recognized by the macrocycles with comparable binding strength [13]. Rotaxanes can be
developed into effective and versatile molecular switches only if reversibility can be
combined with their stimuli-responsive behavior [14].
Considering the nature of the output signal, functional molecular switches based
on the typical [2]rotaxane framework have been designed and fabricated to modulate
physical properties [15], such as fluorescence change [15c] or wettability [15b], and
chemical properties, such as catalytic activity [15d].
Hence, we can call rotaxanes as “versatile” species within the family of artificial
molecular machines.
Although functional switches have been constructed based on [n]rotaxane systems, most of the reported examples operate in solution. This facilitates their
characterization, however, limiting potential applications of rotaxanes due to the
difficulty of controlling dissolved material in many practical aspects [2f].
Inspired by the transition of life on earth from sea to land, with the transition of
rotaxanes from solution (sea) to solid surfaces/interfaces (land), the emergence of
new properties is expected to form the basis of new functional materials [16]. However, new and challenging problems are in the way of this “evolution”: the development of reliable covalent/noncovalent interactions to bind the organic rotaxane
component to the inorganic platform, the unexpected changes in the working manner
of immobilized rotaxanes [17], and the difficulties in detecting single-molecular
motion in a solid sample.
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C.-X. Zhao et al.
Artificial molecular machines have attracted considerable attention from synthetic
chemists due to their unique ability to mimic many aspects of biological phenomena
[1]. Rotaxanes, consisting of one or more macrocycles and an encircled dumbbelllike thread component, have been extensively studied and developed and thus
became one of the major classes of artificial molecular machines [2]. Since the
first [2]rotaxane was reported in the 1990s [2a], the structural complexity of
rotaxanes has increased rapidly along with the development of synthetic methodology and host-guest chemistry [3]. Various rotaxanes have been designed and
constructed based on versatile macrocycles, such as cyclodextrins [2b, 4], cucurbit
[n]uril [5], crown ethers [2c, 6], pillarenes [7], cyclophanes [8], and benzylic amide
macrocycles [9]. Recently, some novel types of rotaxanes, such as [1]rotaxane [10],
daisy chain [11], and hetero[n]rotaxane [12], are rising and being developed extensively due to their peculiar structure and unique molecular motion. Consequently, the
development of novel synthetic methodology is crucial for the further advancement
of these new classes of rotaxanes.
Their stimuli-responsive nature is one of the most attractive features of rotaxanes,
which makes them capable of producing molecular-scale motion upon external
stimuli [2f] including pH [2c, 11b, c], redox [8, 11h], light [3d, 4a–c, 11a], and
microenvironmental changes [7, 11i]. The motion is mostly driven by the relative
difference in binding constants between the recognition stations and the macrocycles: the macrocycles tend to be close to the stations with higher binding constants;
and dynamic shuttling motion would occur when two or more stations are recognized by the macrocycles with comparable binding strength [13]. Rotaxanes can be
developed into effective and versatile molecular switches only if reversibility can be
combined with their stimuli-responsive behavior [14].
Considering the nature of the output signal, functional molecular switches based
on the typical [2]rotaxane framework have been designed and fabricated to modulate
physical properties [15], such as fluorescence change [15c] or wettability [15b], and
chemical properties, such as catalytic activity [15d].
Hence, we can call rotaxanes as “versatile” species within the family of artificial
molecular machines.
Although functional switches have been constructed based on [n]rotaxane systems, most of the reported examples operate in solution. This facilitates their
characterization, however, limiting potential applications of rotaxanes due to the
difficulty of controlling dissolved material in many practical aspects [2f].
Inspired by the transition of life on earth from sea to land, with the transition of
rotaxanes from solution (sea) to solid surfaces/interfaces (land), the emergence of
new properties is expected to form the basis of new functional materials [16]. However, new and challenging problems are in the way of this “evolution”: the development of reliable covalent/noncovalent interactions to bind the organic rotaxane
component to the inorganic platform, the unexpected changes in the working manner
of immobilized rotaxanes [17], and the difficulties in detecting single-molecular
motion in a solid sample.
278
C.-X. Zhao et al.