Honing Up a Genre of Amphiphilic Bistable [2]Rotaxanes for Device Settings

In response to molecular electroactive device requirements, a molecular shuttle in the shape of an amphiphilic bistable [2]rotaxane has been designed, synthesized, and characterized. It contains a hydrophobic, tetraarylmethane and a hydrophilic, dendritic stopper and two π-electron-rich stations—a (monopyrrolo)tetrathiafulvalene unit and a 1,5-dioxynaphthalene moiety — on the rod section of its dumbbell component, which can act as recognition sites for the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), to reside around. In contrast to a previously reported “slow” amphiphilic bistable [2]rotaxane, which has an SMe group attached directly to the (monopyrrolo)tetrathiafulvalene unit, this “new and improved” bistable [2]rotaxane has a much less bulky hydrogen atom attached to the (monopyrrolo)tetrathiafulvalene unit. This seemingly small difference in the substituents on the (monopyrrolo)tetrathiafulvalene unit leads to profound changes when comparing the physical properties of the two bistable [2]rotaxanes. Two hydrophilic semidumbbell compounds, comprising only a (monopyrrolo)tetrathiafulvalene unit, and one hydrophobic semidumbbell compound comprising only a 1,5-dioxynaphthalene moiety on its rod section have also been prepared and used to form their corresponding cyclobis(paraquat-p-phenylene) complexes. The kinetics of the complexation/decomplexation process occurring in these complexes have been investigated, using both absorption and 1H NMR spectroscopy. In addition, these model complexes have been used to investigate the strength of the noncovalent bonding interactions occurring between the cyclobis(paraquat-p-phenylene) ring component and the semidumbbell component at different temperatures, allowing the free energies, enthalpies, and entropies for the formation of these complexes to be determined. The outcome of these investigations can be used to explain the fact that the “slow” [2]rotaxane exists as a 1:1 mixture of its two possible translational isomers, whereas the “new and improved” [2]rotaxane exists as a 4:1 mixture in favor of the translational isomer where the cyclobis(paraquat-p-phenylene) tetracation encircles the (monopyrrolo)tetrathiafulvalene unit, and also to explain the thermochromic behavior of the “new and improved” [2]rotaxane. In this amphiphilic bistable [2]rotaxane, shuttling of the cyclobis(paraquat-p-phenylene) tetracation can be driven by both chemical and electrochemical oxidation of the (monopyrrolo)tetrathiafulvalene unit rendering it a good candidate for incorporation into two-terminal molecular switch tunnel junctions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)