Radical-cation dimerization overwhelms inclusion in [N]pseudorotaxanes

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril (CB7) in water is a well-known phenomenon. Herein, two counter-examples are presented. Two viologen-containing thread molecules were designed, synthesized, and thoroughly characterized by ¹H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square-wave voltammetry, and chronocoulometry: BV⁴⁺, which contains two viologen subunits, and HV¹²⁺, which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]- and [7]pseudorotaxanes that form on complexation with CB7, that is, BV ⁴⁺⊂(CB7)₂ and HV¹²⁺⊂(CB7)₆, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen (MV²⁺) and benzyl methyl viologen (BMV²⁺), as well as their [2]pseudorotaxane complexes with CB7 (MV²⁺⊂CB7 and BMV²⁺⊂CB7) were also investigated. As expected, the control pseudorotaxanes remained intact after one-electron reduction of their viologen-recognition stations. In contrast, analogous reduction of BV ⁴⁺⊂(CB7)₂ and HV¹²⁺⊂(CB7)₆ led to host-guest decomplexation and release of the free threads BV ^2(.+) and HV^{6(. +)}, respectively. ¹H DOSY NMR spectrometric and chronocoulometric measurements showed that BV^{2(. +)} and HV^6(.+) have larger diffusion coefficients than the corresponding [3]- and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical-cation dimerization. These results demonstrate that radical-cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.