A tetra-stable donor–acceptor rotaxane 1⋅4PF6 has been synthesized. The dumbbell component is comprised of an oxyphenylene (OP), a tetrathiafulvalene (TTF), a monopyrrolo-TTF (MPTTF), and a hydroquinone (HQ) unit, which can act as recognition sites (stations) for the tetra-cationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+). The TTF and the MPTTF stations are located in the middle of the dumbbell component and are connected by a triethylene glycol (TEG) chain in such a way that the pyrrole moiety of the MPTTF station points toward the TTF station, while the TTF and MPTTF stations are flanked by the OP and HQ stations on their left hand side and right hand side, respectively. The rotaxane was characterized in solution by 1H NMR spectroscopy and cyclic voltammetry. The spectroscopic data revealed that the majority (77 %) of the tetra-stable rotaxane 14+ exist as the translational isomer 1⋅MPTTF4+ in which the CBPQT4+ ring encircles the MPTTF station. The electrochemical studies showed that CBPQT4+ in 1⋅MPTTF4+ undergoes ring translation as result of electrostatic repulsion from the oxidized MPTTF unit. Following tetra-oxidation of 1⋅MPTTF4+, a high-energy state of 18+ was obtained (i.e., 1⋅TEG8+) in which the CBPQT4+ ring was located on the TEG linker connecting the di-oxidized TTF2+ and MPTTF2+ units. 1H NMR spectroscopy carried out in CD3CN at 298 K on a chemically oxidized sample of 1⋅MPTTF4+ revealed that the metastable state 1⋅TEG8+ is only short-lived with a lifetime of a few minutes and it was found that 70 % of the positively charged CBPQT4+ ring moved from 1⋅TEG8+ to the HQ station, while 30 % moved to the much weaker OP station. These results clearly demonstrate that the CBPQT4+ ring can cross both an MPTTF2+ and a TTF2+ electrostatic barrier and that the free energy of activation required to cross MPTTF2+ is ca. 0.5 kcal mol−1 smaller as compared to TTF2+.
- charge-transfer complex
- molecular machines