Simple Charged Rotaxanes

The realisation that the polyether chains attached to the hydroquinone fragments in systems such as these must be at least diethyleneglycol (or longer) units also served to confirm the finely balanced nature of the assembly process. 

As in the above system, the triisopropylsilyl group has proved to be a convenient stopper for the ends of polyether chains incorporating hydroquinone derivatives even though other bulky groups, such as adamantoyl, have also been used. Indeed, by fine-tuning the size of the stoppers, in specific instances the cyclic com- 

Preliminary investigations suggested that it would be possible to form a related poly-pseudorotaxane built up from a linear component incorporating linked linear polyether strings. Thus, reaction of the linear component incorporating three hydroquinone residues with methylenebis(4-phenylisocyanate) yielded the 

A template-directed synthesis of [2]-rotaxanes, with a yield in one case of 72%, has been reported. The experiments employed dumb-bell-shaped components incorporating terminal triisopropylsilyl stoppers connected to a central 1,5-dioxynaph-thalene recognition site by [-CH2CH20-] spacers (n = 1-3). - These components were used as templates for the synthesis ( clipping reaction ) of the corresponding rotaxanes incorporating cyclobis(paraquat-p-phenylene) as the ring component. The 

It is noted that further reports have appeared describing other molecular shuttles exhibiting a variety of molecular architectures.  

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In other studies, the above research group synthesised a number of related one-and two-station, photoactive [2]-rotaxanes incorporating either ferrocene or anthracene stoppers or a combination of each type. In these systems, the terminal stoppers associate with the exterior of the bis(4-4 -bipyridinium)cyclophane tetracation via 7U-stacking. Such proximity has obvious implications for the required rapid electron-transfer to a terminal (ferrocenyl) group in order to be competitive with the simple charge recombination process discussed previously. The relationship between the photoactivity, electron-transfer behaviour and concomitant configurational charges in the above systems has now been described in some detail. " ... 

The basic character of benzidine affords an alternate mechanism to influence the average position of the bead. Protonation of the amine functional groups by simple addition of trifluoroacetic acid (TFA) to the solution also generates positive charges on the benzidine unit and forces the bead to encircle the biphenol station. Neutralization with base (pyridine) returns the system to its initial state by removing the positive charges on the benzidine station (Scheme 6). These phenomena were verified in careful JH-NMR spectroscopic experiments [7], Thus, chemical (proton transfer) reactions are also useful to control the sliding motion of the macrocyclic bead in this rotaxane. 

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