Tetrathiafulvalene radical cation dimerization in a bistable tripodal [4]rotaxane

The template-directed synthesis of a bistable tripodal [4]rotaxane, which has cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) as the π-electron-de-ficient rings, and tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene units as the pairs of π-electron-rich recognition sites located on all three legs of the tripodal dumbbell, is described. The chemical and electrochemical oxidation of the [4]rotaxane and its tripo dal dumbbell have allowed us to unravel an unprecedented TTF⁺ radical cation dimerization. In fact, two types of TTF dimers, namely, the radical cation dimer [TTF⁺]₂ and the mixed-valence one [(TTF) ₂]⁺, have been ob served at room temperature for the tripodal dumbbell, whereas, in the case of the [4]rotaxane, only the radical cation dimer [TTF⁺]₂ is formed. This anomaly can be explained if it is accepted that most of the neutral TTF units in the [4]rotaxane are encircled by CBPQT⁴⁺ rings, which renders the formation of the mixed-valence dimer [(TTF)₂]⁺ highly unfavorable.