single-molecular system. A recent work reported the unique performance of hetero
[4]rotaxane in a novel tuneable solid-state fluorescent material [36], even as a
pigment for artwork painting, showing the promise of hetero[n]rotaxanes in diverse
applications. Herein, we focus on the synthetic methodology for the formation of
hetero[n]rotaxanes, especially the crown ether-based species due to the multiple
types of this macrocycle and its wide employment in the construction of rotaxanes.
Template-directed pre-self-assembly and then stopping is a reliable route for the
construction of rotaxanes [25]. However, in the case of hetero[n]rotaxanes, the
presence of two or more different macrocycles increase the number of possible
assembly routes, which ultimately results in a nonselective formation of a mixture
of products. Hence, a “programmed” advanced self-assembly route should be
developed and employed for the selective template-directed pre-self-assembly process. The programmed self-assembly should bear the capability of selective dimerization in a multiple component mixture, just like in DNA base pairing (A for T, and
G for C). Self-sorting can be a reliable strategy for the selective pre-self-assembly in
a complex mixture [26c, d, 37]. As shown in Fig. 4a, Schalley and co-workers [38]
demonstrated for the first time that integrative self-sorting of two kinds of crown
ether macrocycles, D24C8 and B21C7, can be used for the effective construction of
a hetero[3]rotaxane. The key feature is the introduction of a phenyl group between
two ammonium sites, which brings a steric barrier for the smaller macrocycle B21C7
rather than for D24C8. Hence, in the apolar solution, stoichiometric mixture of
thread 9 and D24C8 and B21C7 macrocycles could self-assemble into one single
species, semi[3]rotaxane, and the following stopping by phenyl units afforded the
target compound, hetero[3]rotaxane 10 in high yield. The same group utilized this
self-sorting system to fabricate many elegant hetero(pseudo)[n]rotaxane with high
structural complexity, showing that this strategy can be a general approach for the
construction of hetero[n]rotaxanes.
Liu and co-workers [12a] assembled twin-axial hetero[7]rotaxane 15 via self-sorting
strategy, as shown in Fig. 4b, which involved a self-sorting system of four components,
thread 11, thread 12, macrocycle bis(p-phenylene-34-crown-10) (BPP34C10), and
macrocycle B21C7. The key design element also lies on the steric barrier formed by a
phenyl moiety for the smaller macrocycle B21C7. Through the efficient click reaction
between the two intermediates, pseudo[3]rotaxane 13 and semi[2]rotaxane 14, hetero[7]
rotaxane 15 could be afforded with a high yield (42%). This work demonstrated the
synthesis of a more complex hetero[7]rotaxane in a one-pot fashion, significantly
advancing the application of self-sorting in the synthesis of rotaxanes.
As an attempt to integrate daisy chain rotaxanes and hetero[n]rotaxanes to further
increase structural complexity, our group has reported a novel daisy chain-containing
hetero[4]rotaxane 20, synthesized via an improved self-sorting strategy [39a]. In our
system, shown in Fig. 4c, self-sorting was initiated in the mixture of a daisy chain
monomer 16, thread 17, and B21C7. Although the previously reported work [38] has
accomplished self-sorting process involving D24C8 and B21C7 on a thread with two
ammonium sites, however, the daisy chain monomer 16 underwent an interpenetration
motion in its self-assembly accompanied by the formation of the semi[2]rotaxane 19. As
a result, only two intermediate species could be detected in the apolar solution: daisy
11 Functional Rotaxanes
285
[4]rotaxane in a novel tuneable solid-state fluorescent material [36], even as a
pigment for artwork painting, showing the promise of hetero[n]rotaxanes in diverse
applications. Herein, we focus on the synthetic methodology for the formation of
hetero[n]rotaxanes, especially the crown ether-based species due to the multiple
types of this macrocycle and its wide employment in the construction of rotaxanes.
Template-directed pre-self-assembly and then stopping is a reliable route for the
construction of rotaxanes [25]. However, in the case of hetero[n]rotaxanes, the
presence of two or more different macrocycles increase the number of possible
assembly routes, which ultimately results in a nonselective formation of a mixture
of products. Hence, a “programmed” advanced self-assembly route should be
developed and employed for the selective template-directed pre-self-assembly process. The programmed self-assembly should bear the capability of selective dimerization in a multiple component mixture, just like in DNA base pairing (A for T, and
G for C). Self-sorting can be a reliable strategy for the selective pre-self-assembly in
a complex mixture [26c, d, 37]. As shown in Fig. 4a, Schalley and co-workers [38]
demonstrated for the first time that integrative self-sorting of two kinds of crown
ether macrocycles, D24C8 and B21C7, can be used for the effective construction of
a hetero[3]rotaxane. The key feature is the introduction of a phenyl group between
two ammonium sites, which brings a steric barrier for the smaller macrocycle B21C7
rather than for D24C8. Hence, in the apolar solution, stoichiometric mixture of
thread 9 and D24C8 and B21C7 macrocycles could self-assemble into one single
species, semi[3]rotaxane, and the following stopping by phenyl units afforded the
target compound, hetero[3]rotaxane 10 in high yield. The same group utilized this
self-sorting system to fabricate many elegant hetero(pseudo)[n]rotaxane with high
structural complexity, showing that this strategy can be a general approach for the
construction of hetero[n]rotaxanes.
Liu and co-workers [12a] assembled twin-axial hetero[7]rotaxane 15 via self-sorting
strategy, as shown in Fig. 4b, which involved a self-sorting system of four components,
thread 11, thread 12, macrocycle bis(p-phenylene-34-crown-10) (BPP34C10), and
macrocycle B21C7. The key design element also lies on the steric barrier formed by a
phenyl moiety for the smaller macrocycle B21C7. Through the efficient click reaction
between the two intermediates, pseudo[3]rotaxane 13 and semi[2]rotaxane 14, hetero[7]
rotaxane 15 could be afforded with a high yield (42%). This work demonstrated the
synthesis of a more complex hetero[7]rotaxane in a one-pot fashion, significantly
advancing the application of self-sorting in the synthesis of rotaxanes.
As an attempt to integrate daisy chain rotaxanes and hetero[n]rotaxanes to further
increase structural complexity, our group has reported a novel daisy chain-containing
hetero[4]rotaxane 20, synthesized via an improved self-sorting strategy [39a]. In our
system, shown in Fig. 4c, self-sorting was initiated in the mixture of a daisy chain
monomer 16, thread 17, and B21C7. Although the previously reported work [38] has
accomplished self-sorting process involving D24C8 and B21C7 on a thread with two
ammonium sites, however, the daisy chain monomer 16 underwent an interpenetration
motion in its self-assembly accompanied by the formation of the semi[2]rotaxane 19. As
a result, only two intermediate species could be detected in the apolar solution: daisy
11 Functional Rotaxanes
285