Probing the Electrostatic Barrier of Tetrathiafulvalene Dications using a Tetrastable Donor−Acceptor [2]Rotaxane

A tetrastable [2]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 stations for the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+). The [2]rotaxane was characterized by 1H NMR spectroscopy and cyclic voltammetry. The spectroscopic data revealed that the majority (77%) of the tetrastable [2]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+ undergo ring translation as result of electrostatic repulsion from the oxidized MPTTF unit. Following tetraoxidation of 1*MPTTF4+, a high-energy state of 18+ was obtained (i.e., 1*TEG8+) in which CBPQT4+ was located on the TEG linker connecting the dioxidized TTF2+ and MPTTF2+ units. 1H NMR spectroscopy carried out in CD3CN at 298 K on a chemically oxidized sample of 1*MPTTF4+ revealed that 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+.