Rotaxane charge-transfer

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. 

Figure B. Pictorial representation of the self-assembly of pseudorotaxa-nes based on (a) charge-transfer and C-H—O hydrogen-bonding interactions between 1,1 -diben-zyl-4,4 -bipyridinium dication and 1,5-dinaphtho[38] crown-10 (1/5DN38C10), and (b) hydrogen-bonding interactions between dibenzyl ammonium ion and dibenzo[24]crown-8 (DB24C8). A possible route towards the synthesis of rotaxanes and catenanes is also schematized.

In this chapter, for space reasons, only a few paradigmatic examples of rotaxanes and catenanes based on donor-acceptor (charge transfer (CT)) and/or hydrogen bonding interactions (systems based on metal-ligand bonding are reviewed in another... 

It can be concluded, as shown in the square scheme reported in Fig. 13.27, that in the deprotonated rotaxane (i) the first reduction of the bipyridinium weakens the charge transfer interactions and promotes the displacement of the ring far from the monoreduced unit, and (ii) the reoxidation of such a unit, restoring its electron acceptor power, causes the back movement of the ring. 

Rotaxane 40 (Figure 4.18) was synthesised since it was reasoned that it might be possible to prolong the lifetime of any redox intermediate by the incorporation of terminal redox-active stoppers. The presence of the latter may provide a means of facilitating spatial separation of the primary redox changes. Preliminary cyclic voltammetry studies indicated that the 7t-radical dialkoxybenzene cation, formed by excitation of the charge-transfer complex, should favour oxidation of one of the... 

As in the case of pseudorotaxanes (see Volume III, Part 2, Chapter 6), charge-transfer (CT) interactions between electron-donor and electron-acceptor units in rotaxanes and catenanes have several important consequences from the spectro-... 

In these cases 14 and 22 act as beads, threaded on stoppered polyether chains containing the Tr-electron-rich or 7r-electron-deficient stations. This noncovalent interaction simplifies the synthetic procedures necessary, as the unstoppered thread and its bead will self-assemble, and the resulting charge transfer complex can be stoppered with bulky terminal groups to give the functional rotaxane. Other sue-... 

The occupation of each 7i-donor station in the thread of rotaxane 24+ by the macrocyclic bead gives rise to characteristic charge transfer bands. Therefore,... 

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