Abstract
A number of nanometer‐scale molecular assemblies, based on rotaxane‐type structures, have been synthesized by means of a template‐directed strategy from simple building blocks that, on account of the molecular recognition arising from the noncovalent interactions between them, are able to self‐assemble into potential molecular abacuses. In all the cases investigated, the π‐electron‐deficient tetracationic cyclophane cyclobis(paraquat‐p‐phenylene) is constrained mechanically around a dumbbell‐shaped component consisting of a linear polyether chain intercepted by at least two, if not three, π‐electron‐rich units and terminated at each end by blocking groups or stoppers. The development of an approach toward constructing these molecular abacuses, in which the tetracationic cyclophane is able to shuttle back and forth with respect to the dumbbell‐shaped component, begins with the self‐assembly of a [2]rotaxane consisting of two hydroquinone rings symmetrically positioned within a polyether chain terminated by triisopropylsilyl ether blocking groups. In this first so‐called molecular shuttle, the tetracationic cyclophane oscillates in a degenerate fashion between the two π‐electron‐rich hydroquinone rings. Replacement of one of the hydroquinone rings—or the insertion of another π‐electron‐rich ring system between the two hydroquinine rings—introduces the possibility of translational isomerism, a phenomenon that arises because of the different relative positions and populations of the tetracationic cyclophane with respect to the π‐donor sites on the dumbbell‐shaped component. In two subsequent [2]rotaxanes, one of the hydroquinone rings in the dumbbell‐shaped component is replaced, first by a p‐xylyl and then by an indole unit. Finally, a tetrathiafulvalene (TTF) unit is positioned between two hydroquinone rings in the dumbbell‐shaped component. Spectroscopic and electrochemical investigations carried out on these first‐generation molecular shuttles show that they could be developed as molecular switches.
An approach to the construction of molecular switches based on the self‐assembly of development [2]rotaxanes is taken through all four development phases In all four models, the π‐electron‐deficient tetracationic cyclophane cyclobis(paraquat‐p‐phenylene) is constrained mechanically around a dumbbell‐shaped component based on a linear polyether chain terminated by stoppers and containing at least two π‐blocking groups between which it can shuttle (right).
