connected with only one of the two pyridinium units. The consequence is that the
central biphenyl units do not have good π-electron-accepting ability. Instead, the
cavity of Ex
2
box
4+ has a dual feature. In the terminal part, the cavity is rather
electron-poor and can recognize electron-rich guest, which is reminiscent of
CBPQT
4+ ring. In the middle part, the cavity of Ex
2
box
4+ is able to recognize
electron-deficient guest. Ex
2
box
4+ is even able to accommodate two trichlorobenzene guests.
A few other extended boxes, including TVBox
8+ [65], Ex
2.2
Box
4+ [66], as well
as ExBox2
4+ [67], were also synthesized, whose structures are illustrated in Fig. 21.
In addition, now cage-shaped versions of CBPQT
4+ , namely, Excage
6+ [68] and
BlueCage
6+ [69], were also obtained. These prism-shaped cages are composed of
two triangular π-electron acceptors that are bridged by three spacers. Given that the
triangular π-electron acceptors are triscationic and have better π-electron-accepting
ability, these two cages often have larger binding affinity for π-electron-rich guests,
compared to the ring counterparts (Figs. 23 and 24).
3.8
Conclusions and Outlook
In sum, the CBPQT
4+ ring represents one of the most important macrocyclic hosts in
the field of supramolecular chemistry and host-guest chemistry. It can recognize a
variety of aromatic guests, most of which are π-electron-rich. These capabilities are
based on (i) the rigid macrocycle framework of CBPQT
4+ , resulting in little to no
entropy loss during guest accommodating, (ii) the distances between the two
pyridinium moieties are optimal to allow the guests to undergo donor-acceptor
interactions with both of them in either A-D-A or A-D-D-A manner. This hostguest recognition ability allows CBPQT
4+ ring to be used as the building block to
synthesize a variety of supramolecular or mechanically interlocked architectures.
The driving forces for CBPQT
4+ to recognize guests including donor-acceptor
interactions, hydrogen bonding, electrostatic forces, C–H•••π interaction, as well as
hydrophobic effect are expressed in water. In the condition of reduction, the
CBPQT
2(•+) ring becomes attractive to BIPY
•+ guests, driven by radical-pairing
interactions. By tuning these driving forces, the host-guest complexation could be
controlled, in the form of either dissociation of pseudorotaxanes or co-conformation
switching in the case of bistable rotaxanes or catenanes. These switching behaviors
of the CBPQT
4+ ring based on supramolecular or mechanically interlocked architecture are taken advantage of in the design of molecular switches and machines,
which could potentially be used in developing smart materials. The dream of human
beings to precisely control the microscopic world might come true.
In order to allow the recognition of some larger guests, or realize multiple guest
recognition, extended versions of CBPQT
4+ ring were developed. These macrocycles demonstrate different guest recognition behavior from that of CBPQT
4+
ring. Their recognition abilities enable more complex architectures to be
synthesized.
76
H. Li et al.
central biphenyl units do not have good π-electron-accepting ability. Instead, the
cavity of Ex
2
box
4+ has a dual feature. In the terminal part, the cavity is rather
electron-poor and can recognize electron-rich guest, which is reminiscent of
CBPQT
4+ ring. In the middle part, the cavity of Ex
2
box
4+ is able to recognize
electron-deficient guest. Ex
2
box
4+ is even able to accommodate two trichlorobenzene guests.
A few other extended boxes, including TVBox
8+ [65], Ex
2.2
Box
4+ [66], as well
as ExBox2
4+ [67], were also synthesized, whose structures are illustrated in Fig. 21.
In addition, now cage-shaped versions of CBPQT
4+ , namely, Excage
6+ [68] and
BlueCage
6+ [69], were also obtained. These prism-shaped cages are composed of
two triangular π-electron acceptors that are bridged by three spacers. Given that the
triangular π-electron acceptors are triscationic and have better π-electron-accepting
ability, these two cages often have larger binding affinity for π-electron-rich guests,
compared to the ring counterparts (Figs. 23 and 24).
3.8
Conclusions and Outlook
In sum, the CBPQT
4+ ring represents one of the most important macrocyclic hosts in
the field of supramolecular chemistry and host-guest chemistry. It can recognize a
variety of aromatic guests, most of which are π-electron-rich. These capabilities are
based on (i) the rigid macrocycle framework of CBPQT
4+ , resulting in little to no
entropy loss during guest accommodating, (ii) the distances between the two
pyridinium moieties are optimal to allow the guests to undergo donor-acceptor
interactions with both of them in either A-D-A or A-D-D-A manner. This hostguest recognition ability allows CBPQT
4+ ring to be used as the building block to
synthesize a variety of supramolecular or mechanically interlocked architectures.
The driving forces for CBPQT
4+ to recognize guests including donor-acceptor
interactions, hydrogen bonding, electrostatic forces, C–H•••π interaction, as well as
hydrophobic effect are expressed in water. In the condition of reduction, the
CBPQT
2(•+) ring becomes attractive to BIPY
•+ guests, driven by radical-pairing
interactions. By tuning these driving forces, the host-guest complexation could be
controlled, in the form of either dissociation of pseudorotaxanes or co-conformation
switching in the case of bistable rotaxanes or catenanes. These switching behaviors
of the CBPQT
4+ ring based on supramolecular or mechanically interlocked architecture are taken advantage of in the design of molecular switches and machines,
which could potentially be used in developing smart materials. The dream of human
beings to precisely control the microscopic world might come true.
In order to allow the recognition of some larger guests, or realize multiple guest
recognition, extended versions of CBPQT
4+ ring were developed. These macrocycles demonstrate different guest recognition behavior from that of CBPQT
4+
ring. Their recognition abilities enable more complex architectures to be
synthesized.
76
H. Li et al.