Redox-induced ring shuttling and evidence for folded structures in long and flexible two-station rotaxanes

Two dumbbell-shaped components with tetraarylmethane-type stoppers - one hydrophobic and one hydrophilic - and a rod-like section containing a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety as electron-donating units, and their [2]rotaxanes, incorporating the cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) cyclophane as their electron-accepting ring component, have been synthesized, the latter using template-directed protocols. The two amphiphilic [2]rotaxanes, which differ from each other only in the lengths of the polyether chains associated with their hydrophilic stoppers, were designed in order (i) to have them exhibit enhanced amphiphilicities and, by altering the lengths of polyether chains, (ii) to improve the qualities of their Langmuir-Blodgett films, and by removing the phenolic residues, (iii) to increase the oxidative stabilities of these switchable molecules, and so extend the lifetimes of electronic devices fabricated from amphiphilic hysteretic molecular switches of this type. UV-VIS absorption and ¹H NMR spectra, as well as electrochemical measurements, show that both [2]rotaxanes exist to all intents and purposes in solution as the translational isomer in which the CBPQT⁴⁺ cyclophane surrounds the TTF unit. Evidence has also been obtained for the presence in solution of folded conformations of these [2]rotaxanes. While ox/red stimulation of the TTF unit causes shuttling of the CBPQT⁴⁺ cyclophane between the TTF and DNP stations, reduction of CBPQT⁴⁺ causes unfolding of the [2]rotaxane molecules.