the cyclopentadienyl ring of ferrocene unit and complexed by dibenzo-24-crown8 which was connected to the other cyclopentadienyl ring of the ferrocene unit.
Then they investigated the relative mechanical motion of the chain part to the crown
ether. When pH value of the system was changed, the dibenzo-24-crown-8 unit
would slide on the axis of rotaxane, thus changing the electrochemical behavior
of the molecular system. When in acidic condition, the macrocycle bound with
the secondary ammonium salt unit, and the pseudo[1]rotaxane system was electrochemically reversible; while upon the addition of DBU, the triazole rings fell into
the cavity of the dibenzo-24-crown-8, resulting with an electrochemically irreversible system. In 2013, a continuous work was reported by them [22]. In this work,
they introduced a fluorescent naphthenyl imide group at the end of the [1]rotaxane
system, so that a fluorescent emission “active” or “silent” mode of the system could
be controlled by simply changing the pH (Fig. 9b). These systems laid a
solid foundation for the construction of advanced logic circuits with memories or
sequential functions.
As shown in Fig. 10a, in 2014, Qu et al. [23] also synthesized a class of
[1]<3>rotaxane with star-shaped structure by using some bifunctional end-sealing
agents. In these systems, when external base or acid was added, the crown
ether would do an uniform relative mechanical movement along with the arms
of the [1]rotaxanes. Then they investigated the energy-minimized structure of
the [1]rotaxanes in acetone via molecular dynamics simulations. The results showed
that a complexed molecular structure which resembled the muscle’s extension
and contraction was successfully constructed (Fig. 10a). Similarly, Coutrot [24]
also designed and synthesized a star-shaped [1]<3>rotaxane through a covalent
template strategy in 2015 (Fig. 10b), which could not be constructed by classical
straightforward strategies. In this system, the “macrocycle transporter” played three
key roles: first to bind a dibenzo-24-crown-8, second to link a triazolium-containing
axle temporarily, and third to deliver the dibenzo-24-crown-8 around the newly
formed axle as a molecular machine. Finally, the extended encircled thread was
cut off and a [1]<3>rotaxane was obtained.
As shown in Fig. 11, Mayer et al. [25] connected the crown ether ring containing
1,10-phenanthroline unit to a long alkyl chain which also contained 1,10phenanthroline group. This compound could dynamically change between the selfentangled and disentangled conformations. The change of the morphology could
efficiently influence the size and length of the pseudo[1]rotaxane molecules,
suggesting a potential application in the area of molecular actuators’ construction.
Amazingly, upon the addition of metal ions, this pseudo[1]rotaxane could keep its
conformation as self-entangled due to the complexation of a copper ion with two
phenanthroline units on both the macrocycle and alkyl chain in the same molecule.
While treated with an excess of KCN, the copper ion could be cleared from
the system, thus recovering to the equilibrium of a self-entangled and disentangled
state. In this way, they obtained a pseudo[1]rotaxane structure that could response to
chemical stimuli.
As shown in Fig. 12, Takata et al. [26] used a polymer chain (loaded with terminal
agent) to connect with a pseudo[1]rotaxane structure which contained crown ether
4 Mechanically Self-Locked Molecules
91
Then they investigated the relative mechanical motion of the chain part to the crown
ether. When pH value of the system was changed, the dibenzo-24-crown-8 unit
would slide on the axis of rotaxane, thus changing the electrochemical behavior
of the molecular system. When in acidic condition, the macrocycle bound with
the secondary ammonium salt unit, and the pseudo[1]rotaxane system was electrochemically reversible; while upon the addition of DBU, the triazole rings fell into
the cavity of the dibenzo-24-crown-8, resulting with an electrochemically irreversible system. In 2013, a continuous work was reported by them [22]. In this work,
they introduced a fluorescent naphthenyl imide group at the end of the [1]rotaxane
system, so that a fluorescent emission “active” or “silent” mode of the system could
be controlled by simply changing the pH (Fig. 9b). These systems laid a
solid foundation for the construction of advanced logic circuits with memories or
sequential functions.
As shown in Fig. 10a, in 2014, Qu et al. [23] also synthesized a class of
[1]<3>rotaxane with star-shaped structure by using some bifunctional end-sealing
agents. In these systems, when external base or acid was added, the crown
ether would do an uniform relative mechanical movement along with the arms
of the [1]rotaxanes. Then they investigated the energy-minimized structure of
the [1]rotaxanes in acetone via molecular dynamics simulations. The results showed
that a complexed molecular structure which resembled the muscle’s extension
and contraction was successfully constructed (Fig. 10a). Similarly, Coutrot [24]
also designed and synthesized a star-shaped [1]<3>rotaxane through a covalent
template strategy in 2015 (Fig. 10b), which could not be constructed by classical
straightforward strategies. In this system, the “macrocycle transporter” played three
key roles: first to bind a dibenzo-24-crown-8, second to link a triazolium-containing
axle temporarily, and third to deliver the dibenzo-24-crown-8 around the newly
formed axle as a molecular machine. Finally, the extended encircled thread was
cut off and a [1]<3>rotaxane was obtained.
As shown in Fig. 11, Mayer et al. [25] connected the crown ether ring containing
1,10-phenanthroline unit to a long alkyl chain which also contained 1,10phenanthroline group. This compound could dynamically change between the selfentangled and disentangled conformations. The change of the morphology could
efficiently influence the size and length of the pseudo[1]rotaxane molecules,
suggesting a potential application in the area of molecular actuators’ construction.
Amazingly, upon the addition of metal ions, this pseudo[1]rotaxane could keep its
conformation as self-entangled due to the complexation of a copper ion with two
phenanthroline units on both the macrocycle and alkyl chain in the same molecule.
While treated with an excess of KCN, the copper ion could be cleared from
the system, thus recovering to the equilibrium of a self-entangled and disentangled
state. In this way, they obtained a pseudo[1]rotaxane structure that could response to
chemical stimuli.
As shown in Fig. 12, Takata et al. [26] used a polymer chain (loaded with terminal
agent) to connect with a pseudo[1]rotaxane structure which contained crown ether
4 Mechanically Self-Locked Molecules
91