Towards Switchable Systems

Further progress was achieved with the subsequent synthesis of the bis(2-oxypropylenedithio)tetrathiafulvalene-containing [2]-rotaxane 33. An electron-rich tetrathiafulvalene derivative appeared an attractive candidate for incorporation into the linear component. The parent tetrathiafulvalene is of comparable bulk to a hydroquinone group and has also been demonstrated to exhibit reversible redox behaviour at low potentials. Further, the green 1 1 complex between tetrathiafulvalene and the above cyclophane tetracation had been characterised previously - 

Despite the fact that none of the three non-symmetrical [2]-rotaxanes just described fully meet the goal of having the cyclophane tetracation solely occupying the 7i-electron-donor site with the lowest oxidation potential, in two cases this isomer is the preferred one - provided that an appropriate choice of solvent is used. The results of these studies are still of considerable value in a general sense since they serve to delineate some of the subtleties inherent in attempting to use this approach to obtain a controllable shuttle.  

Additional experiments were aimed at investigating other Ti-electron-rich site types that might prove suitable for incorporation into systems capable of undergoing controlled switching. Thus both benzidine 34 and 4,4 -biphenol 35 were demonstrated to form stable host-guest complexes in acetonitrile with the eyclo-bis(paraquat-/7-phenylene) tetracation. Although these hosts are structurally related, they have different redox properties, with 34 being much more readily oxidised than 35. 

The blue complex of 34 has a formation constant of 1044 dm mok while the corresponding value for the complex of 35 is 140 dm mok. In both cases the NMR spectra confirm that these species are true inclusion complexes. The com-plexation of 34 was also investigated by voltammetry the half-wave potential for the single electron oxidation of benzidine in acetonitrile is shifted anodically after the addition of excess host to the guest. 

The related linear derivatives 36 and 37 have also been synthesised. Each of these species binds to the cyclophane tetracation with an approximately similar 

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