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 pi-electron-deficient rings, and tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene units as the pairs of pi-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 tripodal 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 observed 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.