TOWARD CONTROLLABLE MOLECULAR SHUTTLES

A number of nanometer-scale molecular assemblies, based on rotaxane-ty pe structures, have been synthesized by means of a template-directed s trategy from simple building blocks that, on account of the molecular recognition arising from the noncovalent interactions between them, ar e able to self-assemble into potential molecular abacuses. In all the cases investigated, the pi-electron-deficient tetracationic cyclophane cyclobis(paraquat-p-phenylene) is constrained mechanically around a d umbbell-shaped component consisting of a linear polyether chain interc epted by at least two, if not three, K-electron-rich units and termina ted at each end by blocking groups or stoppers. The development of an approach toward constructing these molecular abacuses, in which the te tracationic 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 position ed within a polyether chain terminated by triisopropylsilyl ether bloc king groups. In this first so-called molecular shuttle, the tetracatio nic cyclophane oscillates in a degenerate fashion between the two pi-e lectron-rich hydroquinone rings. Replacement of one of the hydroquinon e rings-or the insertion of another pi-electron-rich ring system betwe en the two hydroquinine rings-introduces the possibility of translatio nal isomerism, a phenomenon that arises because of the different relat ive positions and populations of the tetracationic cyclophane with res pect to the pi-donor sites on the dumbbell-shaped component. In two su bsequent [2]rotaxanes, one of the hydroquinone rings in the dumbbell-s haped component is replaced, first by a p-xylyl and then by an indole unit. Finally, a tetrathiafulvalene (TTF) unit is positioned between t wo hydroquinone rings in the dumbbell-shaped component. Spectroscopic and electrochemical investigations carried out on these first-generati on molecular shuttles show that they could be developed as molecular s witches.