(DBA) station. The stopper unit was attached by a CuAAC click reaction [26] via a
triazole linkage resulting in the interlocked [1]rotaxane 1-H. By the N-methylation
of the triazole moiety, a secondary recognition station, N-methyltriazolium (MTA),
was introduced. Owing to the presence of the MTA station, upon addition of
1 equivalent of diazabicyclo[5.4.0]undec-7-ene (DBU) to deprotonate the DBA
station, DB24C8 macrocycle would move toward the MTA station, leading to a
contracted [1]rotaxane 1 due to the integrated structure. On the other hand, the
addition of excess trifluoroacetic acid (TFA) could initiate the re-protonation process
and drive [1]rotaxane 1 back to [1]rotaxane 1-H.
For further amplification of the contraction and stretching motion, tri- and tetrabranched [1](n)rotaxanes (n = 3: compound 2-H; n = 4: compound 3-H) were also
synthesized via a similar template-directed click reaction [31]. [1]rotaxanes 2-H and
3-H were able to undergo similar acid/base responses as well, and significantly the
molecular size upon contraction from 2-H to 2 in acetone solution was calculated to
change from 30.7 to 21.9 Å, respectively, while that for the contraction from 3-H to
3 was calculated to change from 30.9 to 22.5 Å, respectively. These percentage
changes in size for 2-H and 3-H are 28.7% and 26.9%, respectively, which are both
close to the value (27%) observed in the contraction of human muscle [31].
11.2.3 Daisy Chains and Poly[n]daisy Chain-Based Rotaxane Muscles
Recently, significant synthetic effort has been focused on the preparation and study
of biomimetic systems. Creating and understanding of such systems is crucial, as
they could serve as models for biological phenomena and could also lead to the
emergence of new properties [32]. Rotaxane-based molecular muscles have the
unique capability of mimicking the muscle-like contraction and stretching motion
on a single-molecular level [11, 26d]. In this field, [c2]daisy chain type rotaxanes are
key structures that are fabricated from the dimerization of AB-type linear monomers
containing two self-complementary moieties, A (host) and B (guest). Pioneering
work on such systems was reported by the group of Sauvage in 2000, in which the
contraction and stretching motion of the individual molecular muscle could be
switched reversibly via addition of metal ions [33]. Since then, rotaxane muscles
based on versatile macrocycles have been designed and constructed successfully
[11]. The available synthetic methods are limited mostly to the template-directed
pre-assembly and then stopping route. Importantly, in these systems the distance
between the macrocycle and the recognition site should be small enough to prevent
the formation of [an]daisy chain polymers as unwanted byproducts [34].
A typical representation of bistable [c2]daisy chain rotaxanes is shown in Fig. 3a.
Muscle-like contraction and stretching motion can be achieved upon applying
external stimuli (Fig. 3a, A and B) such as light, pH, or redox changes, solvent
effect, and metal ion coordination [11g]. The stimuli input can be seen as the energy
input for driving the muscles to produce mechanical work. Although the rotaxanebased molecular muscles can perform single-molecular muscle-like motion, obviously, the single-molecular scale is too small to be used to control macroscopic
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triazole linkage resulting in the interlocked [1]rotaxane 1-H. By the N-methylation
of the triazole moiety, a secondary recognition station, N-methyltriazolium (MTA),
was introduced. Owing to the presence of the MTA station, upon addition of
1 equivalent of diazabicyclo[5.4.0]undec-7-ene (DBU) to deprotonate the DBA
station, DB24C8 macrocycle would move toward the MTA station, leading to a
contracted [1]rotaxane 1 due to the integrated structure. On the other hand, the
addition of excess trifluoroacetic acid (TFA) could initiate the re-protonation process
and drive [1]rotaxane 1 back to [1]rotaxane 1-H.
For further amplification of the contraction and stretching motion, tri- and tetrabranched [1](n)rotaxanes (n = 3: compound 2-H; n = 4: compound 3-H) were also
synthesized via a similar template-directed click reaction [31]. [1]rotaxanes 2-H and
3-H were able to undergo similar acid/base responses as well, and significantly the
molecular size upon contraction from 2-H to 2 in acetone solution was calculated to
change from 30.7 to 21.9 Å, respectively, while that for the contraction from 3-H to
3 was calculated to change from 30.9 to 22.5 Å, respectively. These percentage
changes in size for 2-H and 3-H are 28.7% and 26.9%, respectively, which are both
close to the value (27%) observed in the contraction of human muscle [31].
11.2.3 Daisy Chains and Poly[n]daisy Chain-Based Rotaxane Muscles
Recently, significant synthetic effort has been focused on the preparation and study
of biomimetic systems. Creating and understanding of such systems is crucial, as
they could serve as models for biological phenomena and could also lead to the
emergence of new properties [32]. Rotaxane-based molecular muscles have the
unique capability of mimicking the muscle-like contraction and stretching motion
on a single-molecular level [11, 26d]. In this field, [c2]daisy chain type rotaxanes are
key structures that are fabricated from the dimerization of AB-type linear monomers
containing two self-complementary moieties, A (host) and B (guest). Pioneering
work on such systems was reported by the group of Sauvage in 2000, in which the
contraction and stretching motion of the individual molecular muscle could be
switched reversibly via addition of metal ions [33]. Since then, rotaxane muscles
based on versatile macrocycles have been designed and constructed successfully
[11]. The available synthetic methods are limited mostly to the template-directed
pre-assembly and then stopping route. Importantly, in these systems the distance
between the macrocycle and the recognition site should be small enough to prevent
the formation of [an]daisy chain polymers as unwanted byproducts [34].
A typical representation of bistable [c2]daisy chain rotaxanes is shown in Fig. 3a.
Muscle-like contraction and stretching motion can be achieved upon applying
external stimuli (Fig. 3a, A and B) such as light, pH, or redox changes, solvent
effect, and metal ion coordination [11g]. The stimuli input can be seen as the energy
input for driving the muscles to produce mechanical work. Although the rotaxanebased molecular muscles can perform single-molecular muscle-like motion, obviously, the single-molecular scale is too small to be used to control macroscopic
282
C.-X. Zhao et al.