Dethreading

More recently, Kim et al. synthesized dendritic [n] pseudorotaxane based on the stable charge-transfer complex formation inside cucurbit[8]uril (CB[8j) (Fig. 17) [59]. Reaction of triply branched molecule 47 containing an electron deficient bipyridinium unit on each branch, and three equiv of CB[8] forms branched [4] pseudorotaxane 48 which has been characterized by NMR and ESI mass spectrometry. Addition of three equivalents of electron-rich dihydrox-ynaphthalene 49 produces branched [4]rotaxane 50, which is stabilized by charge-transfer interactions between the bipyridinium unit and dihydroxy-naphthalene inside CB[8]. No dethreading of CB[8] is observed in solution. Reaction of [4] pseudorotaxane 48 with three equiv of triply branched molecule 51 having an electron donor unit on one arm and CB[6] threaded on a diaminobutane unit on each of two remaining arms produced dendritic [ 10] pseudorotaxane 52 which may be considered to be a second generation dendritic pseudorotaxane. 

Dethreading/rethreading of the wire and ring components of a pseudorotaxane reminds the movement of a piston in a cylinder. We have shown that, in suitably designed systems, the movement of such a rudimentary molecular machine can be driven by chemical energy or electrical energy and, most importantly, by light... 

Figure 6. Light-driven dethreading of (a) a pseudorotaxane by excitation of an external photosensitizer, (b) a pseudorotaxane incorporating a photosensitizer as a stopper in...

Chemically induced dethreading/rethreading in similar pseudorotaxane systems has been shown to behave according to the XOR logic function.1251... 

Photochemically-driven dethreading and rethreading of an azobenzene-based pseudorotaxane in acetonitrile occurs when (5) threads with (6) (Figure 12.17). 

The cis isomer of (5) does not form a pseudorotaxane with (6), a fact that is exploited to obtain photoinduced (365nm) dethreading of the pseudorotaxane by trans —> cis isomerisation of (5). When the cis isomer is converted back to the trans form by light irradiation (436 nm), the pseudorotaxane is obtained once again. 

Figure 21. Synthesis of macrocycle 51 via rotaxanation followed by dethreading by Leigh et al.

Different types of polyrotaxanes, depending on how the cyclic and the linear units are connected, have been conceived [6-8, 12], According to the location of the rotaxane unit, polyrotaxanes can be defined as main-chain systems, Types 4, 5, 6, 7, and 8 (rows one and two in Table 1), and side-chain systems, Types 9, 10, 11, and 12 (rows three and four in Table 1). In main-chain polyrotaxanes the rotaxane unit is part of the main chain. In side-chain polyrotaxanes, the rotaxane moiety is located in the side chain as a pendant group. Polyrotaxanes can also be classified as polypseudorotaxanes and true polyrotaxanes, depending on their thermal stability toward dethreading. Polypseudorotaxanes are those without BG (column one in Table 1), in which the rotaxane components can be disassociated from each other by external forces. True polyrotaxanes are those with BG at the chain ends or as in-chain units (column two in Table 1), in which the rotaxane units are thermally stable unless one or more covalent bonds is/are broken. 

Similar to that in copoly(ester rotaxane)s 64, min for these poly(urethane rotax-ane)s increased with increasing BG, i.e. higher x values [116,117]. However, compared with the copoly(ester rotaxane), the dethreading occurred to lesser extent in these polyrotaxanes this is attributed to the fact that the NH groups retard dethreading by hydrogen bonding with the threaded crown ether as in structure 67. A linear relationship between min values and x was revealed. 

Fig. 6.24 Synthesis of a rotaxane (schematic) with dendritic stopper (cone) and tri-tylphenol stopper (sphere). SN = nucleophilic substitution. The grey arrows indicate dethreading of the wheel (red ellipse) - to the left or the right according to choice - from...

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