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) (CBPQT(4+)) 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 center dot+ radical cation dimerization. In fact, two types of TTF dimers, namely, the radical cation dimer [TTF center dot+], and the mixed-valence one [(TTF)(2)](center dot+), have been observed at room temperature for the tripodal dumbbell, whereas, in the case of the [4]rotaxane, only the radical cation dimer [TTF center dot+](2) is formed. This anomaly can be explained if it is accepted that most of the neutral TTF units in the [41rotaxane are encircled by CBPQT(4+) rings, which renders the formation of the mixed-valence dimer [(TTF)(2)](center dot+) highly unfavorable.