switching behavior could be used to design smart materials, whose properties could
be controlled by employing external stimuli. For example, when a bistable rotaxane
whose two co-conformers have remarkable difference in electric conductivity, it has
the potential application in the design of molecular computer [48], i.e., the two
co-conformers could represent “0” and “1,” respectively, for information storage.
Second, switching between different co-conformers allows the components of a
MIM to undergo intramolecular machinery mechanical movement, which could
perform work on the surroundings and influence their properties. This potential
ability opens up the opportunities for human to precisely control the microscopic
world. The early trials include the nanoelectromechanical systems [49], in which the
switching behavior of a MIM is employed to control the shapes and properties of an
inorganic component. Using molecular switches to develop mechanized silica nanoscopic particles [50] for precisely targeted drug delivery represents another example
of their potential applications.
In the year of 2016, the Nobel Prize in Chemistry was awarded to three chemists,
including Jean-Pierre Sauvage [19a], Sir Fraser Stoddart [19b], as well as Bernard
Feringa, on account of their contributions in the field of molecular machines. The
former two chemists, namely, Jean-Pierre Sauvage and Sir Fraser Stoddart,
employed MIMs in the design of molecular machines.
In order to shed light on the underneath mechanism how a molecular switch or
machine works, we use a bistable rotaxane as a model compound. In the dumbbell
component, two binding stations are introduced. This bistable rotaxane thus has two
co-conformations, determined by which station the ring encircles. The ring is doing
random Brownian movement along the dumbbell between the two stations, even
although the energy barrier could be controllable by introducing a steric or electronic
“speed bump” in the dumbbell between the two stations. When the macrocycle
encircles the stronger or primary binding station, the rotaxane adopts a
co-conformation that has been called ground state co-conformation (GSCC). In
contrast, the co-conformation in which the ring sits on the weaker or secondary
binding station has been called metastable-state co-conformation (MSCC). The ratio
of the two co-conformations could be determined by the energy gap (ΔG) between
the two co-conformations, as claimed by Boltzmann distribution, i.e., ÀΔG = RTlnK
(K is ratio of GSCC to MSCC). A molecular switch and machine based on a bistable
catenane has a similar working mechanism, except that the dumbbell in a rotaxane is
replaced by a macrocycle containing two binding stations in a catenane. When an
external stimulus is introduced, which either weakens the binding between the
primary binding station (i.e., station A) and the ring or strengthens the binding
provided by the secondary binding station (i.e., station B), the preference of the
macrocyclic component to encircle the two stations would change. That is, more
macrocycles would prefer to encircle the station B, after addition of the external
stimuli, which implies that it is a net effect that the ring is “moving” from station A to
station B. This mechanism is different from its macroscopic counterparts, in which a
macroscopic object undergoes direct change of its physical position. Removing the
stimuli might recover the noncovalent bonding, and therefore the ring might move
back to station A as a net effect.
3 Host-Guest Chemistry of a Tetracationic Cyclophane, Namely, Cyclobis. . .
69
be controlled by employing external stimuli. For example, when a bistable rotaxane
whose two co-conformers have remarkable difference in electric conductivity, it has
the potential application in the design of molecular computer [48], i.e., the two
co-conformers could represent “0” and “1,” respectively, for information storage.
Second, switching between different co-conformers allows the components of a
MIM to undergo intramolecular machinery mechanical movement, which could
perform work on the surroundings and influence their properties. This potential
ability opens up the opportunities for human to precisely control the microscopic
world. The early trials include the nanoelectromechanical systems [49], in which the
switching behavior of a MIM is employed to control the shapes and properties of an
inorganic component. Using molecular switches to develop mechanized silica nanoscopic particles [50] for precisely targeted drug delivery represents another example
of their potential applications.
In the year of 2016, the Nobel Prize in Chemistry was awarded to three chemists,
including Jean-Pierre Sauvage [19a], Sir Fraser Stoddart [19b], as well as Bernard
Feringa, on account of their contributions in the field of molecular machines. The
former two chemists, namely, Jean-Pierre Sauvage and Sir Fraser Stoddart,
employed MIMs in the design of molecular machines.
In order to shed light on the underneath mechanism how a molecular switch or
machine works, we use a bistable rotaxane as a model compound. In the dumbbell
component, two binding stations are introduced. This bistable rotaxane thus has two
co-conformations, determined by which station the ring encircles. The ring is doing
random Brownian movement along the dumbbell between the two stations, even
although the energy barrier could be controllable by introducing a steric or electronic
“speed bump” in the dumbbell between the two stations. When the macrocycle
encircles the stronger or primary binding station, the rotaxane adopts a
co-conformation that has been called ground state co-conformation (GSCC). In
contrast, the co-conformation in which the ring sits on the weaker or secondary
binding station has been called metastable-state co-conformation (MSCC). The ratio
of the two co-conformations could be determined by the energy gap (ΔG) between
the two co-conformations, as claimed by Boltzmann distribution, i.e., ÀΔG = RTlnK
(K is ratio of GSCC to MSCC). A molecular switch and machine based on a bistable
catenane has a similar working mechanism, except that the dumbbell in a rotaxane is
replaced by a macrocycle containing two binding stations in a catenane. When an
external stimulus is introduced, which either weakens the binding between the
primary binding station (i.e., station A) and the ring or strengthens the binding
provided by the secondary binding station (i.e., station B), the preference of the
macrocyclic component to encircle the two stations would change. That is, more
macrocycles would prefer to encircle the station B, after addition of the external
stimuli, which implies that it is a net effect that the ring is “moving” from station A to
station B. This mechanism is different from its macroscopic counterparts, in which a
macroscopic object undergoes direct change of its physical position. Removing the
stimuli might recover the noncovalent bonding, and therefore the ring might move
back to station A as a net effect.
3 Host-Guest Chemistry of a Tetracationic Cyclophane, Namely, Cyclobis. . .
69