Amphiphilic bistable rotaxanes

Abstract

Two molecular shuttles/switches - a slow one and a fast one - in the shape of amphiphilic, bistable [2]rotaxanes have been synthesized and characterized. Both [2]rotaxanes contain a hydrophobic, tetraarylmethane and a hydrophilic, dendritic stopper. They are comprised of two π-electronrich stations - a monopyrrolotetrathiafulvalene unit and a 1,5-dioxynaphthalene moiety - which can act as recognition sites for the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), to reside around. In addition, a model [2]rotaxane, incorporating only a monopyrrolotetrathiafulvalene unit in the rod section of the amphiphilic dumbbell component and cyclobis(paraquat-p-phenylene) as the ring component, has been investigated. The dumbbell-shaped components were constructed using conventional synthetic methodologies to assemble 1) the hydrophobic, tetraaryl-methane stopper and 2) the hydrophilic, dendritic stopper. Next, 3) the hydrophobic stopper was fused to the 1,5-dioxynaphthalene moiety and/or the monopyrrolotetrathiafulvalene unit by appropriate alkylations, followed by 4) attachment of the hydrophilic stopper, once again by alkylation to give the dumbbell-shaped compounds. Finally, 5) the [2]rotaxanes were self-assembled by using the dumbbells as templates for the formation of the encircling cyclobis-(paraquat-p-phenylene) tetracations. The two [2]rotaxanes differ in their arrangement of the π-electron-rich units, one in which the SMe group of the monopyrrolotetrathiafulvalene unit points toward the 1,5-dioxynaphthalene moiety (2.4PF6) and another in which it points away from the 1,5-dioxynaphthalene moiety (3·4PF6). This seemingly small difference in the orientation of the monopyrrolotetrathiafulvalene unit leads to profound changes in the physical properties of these rotaxanes. The bistable [2]rotaxanes were both isolated as brown solids. 1H NMR and UV-visible spectroscopy, and electrochemical investigations, reveal the presence of both possible translational isomers at ambient temperature. As a consequence of the existence of both possible translational isomers in these bistable [2]rotaxanes, they exhibit a complex electrochemical behavior, which is further complicated by the presence of folded.