autonomously due to the presence of DBA sites which can form metastable host-guest
structure with macrocycle 24. Interestingly, macrocycle 24 always shuttled into thread
25 from the side of trans-azobenzene because of the lower steric barrier of transazobenzene compared to that of the methylcyclopentyl moiety, which in fact has
realized the first “driving force” of the pump as discussed above. The other driving
force involved the photo-induced isomerization of azobenzene from the trans- to the
cis- form, resulting in a large steric barrier as a stopper for macrocycle 24, and then
macrocycle 24 would be released off the thread autonomously due to the dynamic and
metastable nature of the host-guest system formed by 24 and DBA sites. Significantly,
macrocycle 24 would shuttle out of the thread selectively from the methylcyclopentyl
side due to the presence of the cis-azobenzene stopper. Hence, in this process, the
cargo, macrocycle 24, is transported directionally by a light-powered molecular pump.
This work provides significant new strategy for the construction of (pseudo)rotaxanetype molecular pumps, advancing the evolution of machine-like motion of rotaxanes.
Stoddart and co-workers [57] have reported an artificial molecular pump based on
the radical cation template. As shown in Fig. 8b, macrocycle 26 cyclobis(paraquat-pphenylene) (CBPQT
4+ ), also well-known as blue box, was employed as the cargo,
and a thread 27 was designed as the molecular pump containing a pseudo-stopper
with multiple positive charges at one end and a sterically bulk stopper at the other
end. The molecular pump was operated by mixing thread 27 and macrocycle 26. In
the initial state, positive charge repulsion was dominated, generating a charge-based
energy barrier to inhibit the penetration of macrocycle 26. Upon addition of excess
chemical-reducing agent to reduce the viologen dication moieties on both of macrocycle 26 and thread 27 into viologen radical cation moieties, the strong radical cation
dimerization interaction drove the reduced macrocycle to shuttle into the viologen
radical cation sites on the thread. Meanwhile, the decreased charges of reduced
macrocycle (from tetracation to dication) also decreased the charge repulsion
between the macrocycle and the terminal pyridinium salt sites, making the entrance
of the pump more accessible for the macrocycle. The release of macrocycle 26
involved the subsequent chemical oxidation, converting the viologen radical cations
back into the viologen dication state, resulting in the removal of radical cation
dimerization interaction and the recovery of the strong positive charge repulsive
force. In this state, due to the higher positive charge repulsion between the tetracationic macrocycle 26 and the pyridinium salt compared to the steric barrier
generated by the isopropylphenyl units, the macrocycle spontaneously and autonomously crossed the isopropylphenyl moiety and resided at the stopper-containing
end of the thread on a PEG chain. The whole process shown in Fig. 8b represents the
motion of the first macrocycle cargo. The pumping of the second cargo (Fig. 8c)
could be performed in a similar fashion because the isopropylphenyl units acted as a
steric ratchet for macrocycle 26, meaning that the macrocycle could only pass the
ratchet directionally, from the viologen sites toward PEG chain. Hence, in theory,
the amount of the macrocycle (cargo) that can be pumped should be unlimited if the
PEG moiety is long enough to accommodate the pumped cargoes. This work
simulates the biological membrane transport phenomenon by carrier proteins and
thus introducing novel functions that a rotaxane can bear on a single-molecule level.
294
C.-X. Zhao et al.
structure with macrocycle 24. Interestingly, macrocycle 24 always shuttled into thread
25 from the side of trans-azobenzene because of the lower steric barrier of transazobenzene compared to that of the methylcyclopentyl moiety, which in fact has
realized the first “driving force” of the pump as discussed above. The other driving
force involved the photo-induced isomerization of azobenzene from the trans- to the
cis- form, resulting in a large steric barrier as a stopper for macrocycle 24, and then
macrocycle 24 would be released off the thread autonomously due to the dynamic and
metastable nature of the host-guest system formed by 24 and DBA sites. Significantly,
macrocycle 24 would shuttle out of the thread selectively from the methylcyclopentyl
side due to the presence of the cis-azobenzene stopper. Hence, in this process, the
cargo, macrocycle 24, is transported directionally by a light-powered molecular pump.
This work provides significant new strategy for the construction of (pseudo)rotaxanetype molecular pumps, advancing the evolution of machine-like motion of rotaxanes.
Stoddart and co-workers [57] have reported an artificial molecular pump based on
the radical cation template. As shown in Fig. 8b, macrocycle 26 cyclobis(paraquat-pphenylene) (CBPQT
4+ ), also well-known as blue box, was employed as the cargo,
and a thread 27 was designed as the molecular pump containing a pseudo-stopper
with multiple positive charges at one end and a sterically bulk stopper at the other
end. The molecular pump was operated by mixing thread 27 and macrocycle 26. In
the initial state, positive charge repulsion was dominated, generating a charge-based
energy barrier to inhibit the penetration of macrocycle 26. Upon addition of excess
chemical-reducing agent to reduce the viologen dication moieties on both of macrocycle 26 and thread 27 into viologen radical cation moieties, the strong radical cation
dimerization interaction drove the reduced macrocycle to shuttle into the viologen
radical cation sites on the thread. Meanwhile, the decreased charges of reduced
macrocycle (from tetracation to dication) also decreased the charge repulsion
between the macrocycle and the terminal pyridinium salt sites, making the entrance
of the pump more accessible for the macrocycle. The release of macrocycle 26
involved the subsequent chemical oxidation, converting the viologen radical cations
back into the viologen dication state, resulting in the removal of radical cation
dimerization interaction and the recovery of the strong positive charge repulsive
force. In this state, due to the higher positive charge repulsion between the tetracationic macrocycle 26 and the pyridinium salt compared to the steric barrier
generated by the isopropylphenyl units, the macrocycle spontaneously and autonomously crossed the isopropylphenyl moiety and resided at the stopper-containing
end of the thread on a PEG chain. The whole process shown in Fig. 8b represents the
motion of the first macrocycle cargo. The pumping of the second cargo (Fig. 8c)
could be performed in a similar fashion because the isopropylphenyl units acted as a
steric ratchet for macrocycle 26, meaning that the macrocycle could only pass the
ratchet directionally, from the viologen sites toward PEG chain. Hence, in theory,
the amount of the macrocycle (cargo) that can be pumped should be unlimited if the
PEG moiety is long enough to accommodate the pumped cargoes. This work
simulates the biological membrane transport phenomenon by carrier proteins and
thus introducing novel functions that a rotaxane can bear on a single-molecule level.
294
C.-X. Zhao et al.